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Continuous monitoring of pH level in flow aqueous system by using liquid crystal-based sensor device
Accepted Manuscript Continuous monitoring of pH level in flow aqueous system by using liquid crystal-based sensor device
Wei-Long Chen, Tsung Yang Ho, Jhih-Wei Huang, Chih-Hsin Chen PII: DOI: Reference:
S0026-265X(17)31316-4 doi:10.1016/j.microc.2018.03.020 MICROC 3093
To appear in:
Microchemical Journal
Received date: Revised date: Accepted date:
23 December 2017 9 March 2018 10 March 2018
Please cite this article as: Wei-Long Chen, Tsung Yang Ho, Jhih-Wei Huang, ChihHsin Chen , Continuous monitoring of pH level in flow aqueous system by using liquid crystal-based sensor device. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Microc(2017), doi:10.1016/ j.microc.2018.03.020
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ACCEPTED MANUSCRIPT Continuous Monitoring of pH Level in Flow Aqueous System by Using Liquid Crystal-based Sensor Device
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Wei-Long Chen, Tsung Yang Ho, Jhih-Wei Huang and Chih-Hsin Chen*
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Department of Chemistry, Tamkang University, New Taipei City 25137, Taiwan To whom correspondence should be addressed.
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Phone: +886-2-26215656
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Email: [email protected]
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ACCEPTED MANUSCRIPT Abstract In this work, we report a liquid crystal (LC)-based sensor to determine pH level of aqueous solutions. This sensor was fabricated by filling the LCs doped with a pH-sensitive molecule into a cupper grid on a glass substrate. Theoretically, the dopants are neutral and disperse freely in the LCs. When the sensor was immersed in the aqueous solution, the increases of pH can lead to the
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dissociation of the dopants, allowing them to align at the LC/aqueous interface. This
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phenomenon causes the reorientation of the LC molecules and therefore a bright-to-dark
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transition of the LC image was observed simply through naked-eye. In our research, we found
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that the critical pH value for the optical transition of LC sensors can be adjusted from 6.8 to 8.2 through the selection of dopants, while it can be adjusted from 6.2 to 7.0 through the selection of
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dopant concentrations. By arranging four individual sensors in a device with inlet and out
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channels, we demonstrated that the pH level of an aqueous flow system can be determined by the number of bright LC sensors shown in the device. Based on this strategy, we developed a device
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to monitor the pH safety level for drinking water ranging from pH 6.5 to 8.5. This device
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demonstrated fast response (~1 s), good stability, reversibility and its capability to measure the pH change in tap water and pond water, which make it suitable for real-time and continuous
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monitoring the pH change in various flow aqueous systems.
Keywords:
Liquid crystal-based sensor; Reversible pH sensor; Continuous monitoring of pH in flow system; pH monitoring for drinking water Naked-eye detection
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ACCEPTED MANUSCRIPT 1. Introduction pH value, the indication of hydronium ion (H3O+) concentration in the solution, is regarded as the critical parameter in a variety of fields such as life science, medical science, food industry, and environmental protection. The most well-established and widely applied method to retrieve the pH value is through the electrochemical process, which requires an electrode to
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report the change in potential as the pH changes [1]. Although the use of pH electrodes are
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considered a simple, fast, and precise approach, however, pH electrodes are not ideal of long-
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term and continuous monitoring of pH. This is due to its susceptibility of electrical interference,
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which would cause the signal drifting from the electrodes [2]. In order to ensure the accuracy of pH electrodes, calibrations must be carried out periodically. Besides, the use of pH electrodes
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normally requires relatively larger quantity of sample volumes due to the size of electrodes.
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Although various types of microfabricated electrochemical pH sensors have been developed recently to circumvent these disadvantages, there are still rooms for their stability and
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repeatability to be improved [3-6]. Alternatively, optical chemical sensors, which apply a
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chemical indicator covalently attached to solid matrix, provide another simple strategy to study the pH of a solution [7-10]. Typically, an ideal chemical indicator should possess both
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protonated form and deprotonated form with distinct absorption or fluorescence properties. The
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change of pH value in the solution changes the composition of the probes and therefore the optical properties exhibited by the solution changes accordingly. The results of this type of pH sensor can be precisely quantified by spectrometer-based instruments. Nevertheless, considering the photo-stability and the immobilization stability of the probes, its results are often affected by the time and temperature at which the test is made. In recent years, the development of liquid crystal (LC)-based sensor system provided a new approach to study the pH value in aqueous solution [11-17]. One of the most significant feature of LC-based sensor system is that its results can be simply interpreted through the 3
ACCEPTED MANUSCRIPT colorful signals created by the LC molecules. As a result, it is readily recognizable to general users under ambient conditions and has been considered as a simpler and cheaper sensor system comparing to those requiring electrical instrumentation to report the intensity of the signals [1823]. The principle of LC-based pH sensor applies an amphiphilic molecule, normally a weak acid or water soluble polymer with a hydrophobic tail to assemble at LC/aqueous interface. Under
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acidic condition, the amphiphilic molecules are protonated and distributed randomly in the LC
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phase. In this case, the orientation of LC is planar and the LC image is bright cross-polarized
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light. On the contrary, the amphiphilic molecules are deprotonated under basic condition such that they tend to align at the LC/aqueous interface with hydrophobic tails perpendicular to the
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interface, which induces the homeotropic alignment of the LC molecules and result in a dark LC
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image under cross-polarized light (Scheme 1). By using this principle, a small change in pH (~0.1) can be distinguished from dark-to-bright transition of LC image with the naked-eye
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immediately. In past studies, this principle has been used to establish the sensors for various
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targets which are able to change the pH of solution under specific conditions. For example, Zhong et al. reported a LC-based pH sensor to detect glucose through the H+ released by the
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oxidation of glucose [13]. In their system, nematic LCs 4-cyano-4’-pentylbiphenyl (5CB) treated
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by UV exposure, which partially converts 5CB to its oxidized product 4-cyano-4’biphenylcarboxylic acid (CBA), was used as the pH sensitive LC. This system was able to detect
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glucose concentration as low as 1 pM with high selectivity. Later, this system was also applied to detect cholesterol molecules by the same research group [11]. However, the potential problem of this system is that the dopant concentration could be affected by the time, distance and power efficiency of the UV exposure, therefore, the critical pH value for optical transition may vary from batch to batch. On the other hand, Bi et al. reported a LC pH sensor which is able to monitor the activity of penicillinase in real-time [15]. In their system, the pH sensitive LC was composed of 5CB doped with 4’-pentyl-biphenyl-4-carboxylic acid (PBA). Because the dopant concentration was fixed, the critical pH value for optical transition was stable. However, the 4
ACCEPTED MANUSCRIPT critical pH value for optical transition in this system depends on the pKa value of the dopant, making this system can only tell whether the pH level exceeds 7.0 or not. The dynamic range of this type of pH sensor was limited such that it is not applicable in various environmental conditions. In general, the dynamic range of a pH sensor is closely related to the pKa value of the
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indicator applied in this sensor. In a recent review paper discussing the characteristics of optical
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chemical pH sensors, it has been reported that the dynamic range of an optical pH sensor is
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limited to approximately pKa ± 1.5 of the indicator and its highest sensitivity is usually observed
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around the pKa of the indicator [24]. In the same optical pH sensor, extended dynamic range can be achieved by applied a series of indicators with different pKa values. Considering to this point,
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it can be estimated that the dynamic range of the LC-based pH sensor can be extended by using
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the dopant molecules with different pKa values. To the best of our knowledge, this strategy has
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never been used to develop the LC-based pH sensor before. In this paper, we applied four different weak acids as the dopants to develop the LC-
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based pH sensor. We investigated the effect of the pKa of the dopants on the dynamic range of
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the LC-based pH sensor. By using different dopants in the LCs, the critical pH value for optical transition of the LC-based pH sensor could be adjusted. In addition, we also studied the effect of
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dopant concentration and salt concentration on the results of the sensor. To determine the specific pH level, we arranged four LC-based pH sensors with different dopants in a device with inlet and outlet channels and studied the correlation between the pH level of the solutions in the device and the number of the bright LC sensors shown in the device. Finally, the response time, stability and reversibility of the reported device was also tested for its applicability for continuous monitoring of pH level in the flow aqueous system.
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ACCEPTED MANUSCRIPT 2. Materials and Methods 2.1 Materials
Glass slides were obtained from Fisher Scientific (U.S.A). N,N-Dimethyl-n-octadecyl-3aminopropyltrimethoxysilyl chloride (DMOAP), 4'-n-hexylbiphenyl-4-carboxylic acid (6CBA),
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4'-n-octyloxybiphenyl-4-carboxylic acid (8OCBA), NaCl, CaCl2 and MgCl2 were purchased
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from Sigma Aldrich (U.S.A). Tris buffer was purchased from J.T. Baker (U.S.A). Liquid crystal
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4-cyano-4’-pentylbiphenyl (5CB) was purchased from TCI (Taiwan). 4'-n-decyloxybiphenyl-4carboxylic acid (10OCBA) and 4'-n-dodecyloxybiphenyl-4-carboxylic acid (12OCBA) were
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synthesized following reported procedures [25] and characterized by 1H-NMR (Figure S1).
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Water was purified by using a Milli-Q system (Millipore). Tap water and pond water were
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2.2 Determination of the pKa of dopants
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collected in the campus of Tamkang University.
The solution of the dopants (6CBA, 8OCBA, 10OCBA and 12OCBA, 10-4 M in THF :
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H2O = 3:1) were titrated with tetrabutylammonium hydroxide (TBAOH). During the titration, the
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UV-vis spectra and pH value of each solution were recorded until no more change on the UV-vis spectra was observed. As the dopants have a protonated and a deprotonated form, the absorbance
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of the dopants follows the modified Henderson-Hasselbalch equation: pH = log[(Ai - A)/(A - Af)] + pKa
where Ai and Af are the initial and final absorbance at selected wavelength of the dopant, while A is the absorbance of the dopant at different pH values. By plotting log[(Ai - A)/(A - Af)] vs. pH values, the pKa of the dopants can be determined as the intercept of each plot. 2.3 Preparation of DMOAP-coated slides
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ACCEPTED MANUSCRIPT DMOAP-coated slides were prepared following reported procedures in the literature [26]. To clean the surface, glass slides were immersed in a 5% Decon-90 solution for 2 h followed by sonicated in DI water for 15 min and rinsed thoroughly with DI water twice. After this, the slides were dried under a stream of nitrogen. The cleaned glass slides were immersed in an aqueous solution containing 0.1% (v/v) DMOAP for 5 min, and then rinsed with copious amounts of DI
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water. DMOAP-coated slides were dried under a stream of nitrogen and heated in a 100°C
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vacuum oven for 15 min.
2.4 Preparation of LC-based pH sensor
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TEM copper grids (75 mesh, Ted Pella, Inc., U.S.A) were cleaned in methanol, ethanol,
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and acetone (sonicated in each solvent for 15 min), and then heated overnight at 100 °C to evaporate residual solvent. The pH responsive LC was prepared by doping 6CBA, 8OCBA,
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10OCBA or 12OCBA into 5CB and sonicated for 1 min before use. To fabricate the LC-based
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sensing platform, a piece of copper grid was placed on a DMOAP-coated slide (size: 5 mm × 5 mm), followed by dispensing 0.25 μL of 5CB doped with various dopants onto the grid.
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Excessive LC was removed by using a capillary tube. Finally, the slide together with the grid
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containing the LC was immersed in 300 μL of Tris buffer solution with different pH values. The optical appearances of these samples were observed by using a polarizing optical microscope
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(Canon EOS D650, Japan) in the transmission mode. Each image was captured with a digital camera mounted on the microscope with an exposure time of 1/80 sec.
2.5 Fabrication of LC-based pH sensor device for flow system
Polydimethylsiloxane (PDMS) films were prepared by casting Sylgard 184 (Dow Corning) on a petri dish placed by a glass slide and two capillaries to form a groove dimension (10 × 25 × 2 mm) with inlet and outlet channels. After that, four individual LC-based pH sensors were aligned in the groove dimension of PDMS film. The whole system was then sandwiched by 7
ACCEPTED MANUSCRIPT two glass slides with cross-polars and fixed by two binder clips to form the LC-based pH sensor device for flow system. 3. Results and Discussion 3.1 Effect of the dopant on the critical pH value for optical transition of LC sensors
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Aiming a larger dynamic range for LC-based pH sensor, our first goal is to study how the pKa of the dopant affects the critical pH value for optical transition in the system. Considering
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the compatibility of the dopants with LC molecules, we selected biphenyl-4-carboxylic acid,
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which is structurally similar to 5CB, as the main backbone of the dopants. The molecular structures of the dopants are shown in Scheme 2, which includes 4'-n-hexylbiphenyl-4-
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carboxylic acid (6CBA), 4'-n-octyloxybiphenyl-4-carboxylic acid (8OCBA), 4'-n-
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decyloxybiphenyl-4-carboxylic acid (10OCBA) and 4'-n-dodecyloxybiphenyl-4-carboxylic acid (12OCBA). When these molecules were applied as the dopants in the LC-based sensor, we found
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that the critical pH values for the bright-to-dark transition of the LC images are different. As
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shown in Figure 1, the critical pH values for the bright-to-dark transition of the LC image in 6CBA-, 8OCBA-, 10OCBA-, and 12OCBA-doped system were 7.0, 7.2, 7.6, and 8.2,
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respectively. According to previous report, the bright-to-dark transition of the LC image can be
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attributed to the protonation and deprotonation equilibrium of dopants occurred at the LC/aqueous interface of the system. As the mechanism shown in Scheme 1, protonated dopants distributed randomly in the LC phase and result in planar orientation of LCs, while deprotonated dopants aligned at the LC/aqueous interface and result in homeotropic orientation of LCs [15]. Therefore, it can be hypothesized that the pKa of the dopants, which decides the extent of the deprotonation under a specific pH value, should affect the critical pH value for optical transition of the LC sensor. To further investigate this phenomenon, the pKa of 6CBA, 8OCBA, 10OCBA and 12OCBA were measured by using UV-Vis titration experiments to determine the
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ACCEPTED MANUSCRIPT concentration ratio for the protonated and deprotonated form of the acid. The results revealed that the pKa of 6CBA, 8OCBA, 10OCBA and 12OCBA were calculated as 7.68, 7.66, 9.77 and 8.74, respectively [27]. The critical pH value for optical transition of the LC sensor was not increased as the pKa of the dopants increased, which suggested that this value was not only determined by the pKa of the dopants. The critical pH value for optical transition of the LC could
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also be affected by the solubility of the dopant in the LCs. In the case of 12OCBA-doped system,
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we found that 12OCBA was not completely dissolved in the LCs. To obtain a more accurate
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concentration for the dopant, we analyzed the composition of 12OCBA doped 5CB by using HPLC. The results showed that the real concentration of 12OCBA in 5CB is 0.14%, not 0.3%
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(Figure S2). Therefore, in the case of 12OCBA, higher pH was required for producing the
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deprotonated dopants that are sufficient to align the LCs in homeotropic orientation at the LC/aqueous interface. Additional factors to affect the critical pH value for optical transition of
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the LC sensor could be the ability of the dopants to align the LCs at the LC/aqueous interface, as
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we found that the pKa of 10OCBA (9.77) is significantly larger than that of 8OCBA (7.66) while the critical pH value for optical transition in 10OCBA-doped system (7.6) was only slightly
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higher than that in 8OCBA-doped system (7.2). Previous study has demonstrated that 5CB are
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prone to align homeotropically at aqueous/LC interface using the surfactant with longer alkyl chain because the penetration of 5CB into surfactant monolayers [28]. This phenomenon implies
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that less deprotonated dopants are required to induce the optical transition of LCs in the LCbased pH sensor. As a result, the optical transition for the LC sensor using the dopants with longer alkyl-chain could occur at a relative lower pH value. In addition, the structural similarity between the dopants and the LCs may also affect the ability of dopants to align LCs at aqueous/LC interface, as we found that the pKa values of 6CBA and 8OCBA are comparable while the critical pH values for optical transition of LC image for 8OCBA and 6CBA are different. Comparing to 8OCBA, which has an additional oxygen atom on the alkyl chain, the molecular structure of 6CBA is more similar to 5CB (the LCs) and shows better ability to align 9
ACCEPTED MANUSCRIPT LCs at aqueous/LC interface. Although we are unable to predict the critical pH value for optical transition when different dopant was chose, we demonstrated that it could be adjusted through the selection of dopants for the system. Considering the different safety pH levels for various environmental conditions, this feature make it possible to develop the LC-based pH sensor suitable for monitoring the specific pH change in different environments. To test the reversibility
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of the sensor, we conducted a series of experiments by alternatively exposing the LC-based pH
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sensor using 6CBA as dopant to the aqueous solution at pH 6.5 or 7.0. The results showed that
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all the LC image were bright at pH 6.5, while they were bright at pH 7.0 in five cycles (Figure S3a). In addition, plotting the average gray-scale intensity of LCs as a function of pH changes
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also demonstrated that the bright-to-dark transition of LC signals were reversible in five cycles
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(Figure S3b). Both results suggest the good reversibility of the LC-based pH sensor.
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3.2 Effect of dopant concentrations on the critical pH values for optical transition of LC sensors
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Next, we study how the dopant concentration of this system affects the critical pH value for optical transition of LC sensor. Based on the abovementioned results, the amount of
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deprotonated dopants is an important factor to affect the orientation of LC at LC/aqueous
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interface. We selected 6CBA as the dopant because it is highly soluble in 5CB, and doped different concentrations of 6CBA into 5CB to develop the LC-based pH sensor. The results
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indicated that the critical pH values for the bright-to-dark transition of the LC image in 0.3%, 0.4%, 0.5% and 0.6% 6CBA-doped system were 7.0, 6.8, 6.6, and 6.4, respectively (Figure 2). This means the critical pH value for optical transition of LC decreased as the dopant concentration increased. Assuming that the amount of deprotonated dopant required for the reorientation of LC is constant in the same dopant system, such trend could be explained by that the amount of deprotonated dopant required for the reorientation of LC was reached at lower pH when the dopant concentration was higher. This finding provides another strategy to adjust the critical pH value for optical transition of the LC for the pH values for the sensor system by using 10
ACCEPTED MANUSCRIPT only one dopant. Moreover, we have successfully extended the critical pH value for optical transition of LC in the system to a lower pH value by changing the dopant concentrations. 3.3 Effect of salt concentration on the critical pH values for optical transition of LC sensors We further studied the effect of salt concentration on the critical pH value for optical
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transition of LC sensors because it is also an important environmental variable to the water
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samples. Figure 3 shows the results for 0.3% 6CBA doped LC-based pH sensors in the solution
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containing different concentrations of NaCl. It reveals that the critical pH values for the brightto-dark transition of LC sensors in the solutions containing 0 mM, 50 mM and 400 mM of NaCl
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were 7.0, 6.8 and 6.6, respectively. That is, the critical pH values for optical transition of LC
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lowered as the NaCl concentration increased. In previous studies, it has been found that the homeotropic anchoring of LCs at LC/aqueous can be stabilized by the high ionic strength in the
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aqueous phase [29, 30]. Therefore, the lowered critical pH values for the bright-to-dark transition
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of LC sensors could be attributed to the more stabilized homeotropic anchoring of LCs in the
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environment with higher ionic strength.
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3.4 Development of LC-based pH sensor device to determine the pH level of aqueous solution Based on the findings above, we conclude that the critical pH value for optical transition
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of the LC-based pH sensor could be adjusted through the selection of dopants or dopant concentrations. For each sensor, however, it could only report whether the pH of the aqueous solution exceeds a specific value or not. The exact pH level of the aqueous solution is still not uncertain. To solve this issue, we integrated four LC-based sensors with different dopants (i.e., 6CBA, 8OCBA, 10OCBA, and 12OCBA) into one device. Because the critical pH value for dark-to-bright transition of each sensor is different, we anticipate that the number of the bright LC-based pH sensor should be correlated with the pH level of the aqueous solution. To simplify the operation and observation of the device, we arranged the LC-based sensors in array format on 11
ACCEPTED MANUSCRIPT a concave dimension of a PDMS film, followed by sandwiching this film with two glass slides. The schematic illustration (side view) and real image (top view) of the LC-based pH sensor device was shown in Scheme 3. We embedded two capillaries in the PDMS film as the inlet and outlet channels for the device. In addition, we pasted two crossed polarizers on the top and bottom surface of the device such that the optical signals of the device can be directly observed
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under ambient light without using polarized microscope. Aqueous solutions with different pH
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values were injected into the device with the flow rate of 6 mL/min. Figure 4 show the images of
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the device at different pH values through naked-eye. It was found that four LC-based pH sensors were all bright at pH 6.8. In contrast, three LC-based pH sensors were bright at pH 7.0. The one
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doped with 6CBA turned dark because the critical pH value for its optical transition was reached.
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Accordingly, the number of bright LC-based system in the device at pH 7.0, 7.2, 7.6 and 8.2 were 3, 2, 1, and 0, respectively. Apparently, the number of bright LC-based sensor in the device
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decreased as the pH level of the aqueous solution increased. Therefore, we can simply determine
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the pH level of the aqueous solution through the number of bright LC-based sensors in the device. For example, the pH level is 7.0~7.2 when three bright LC-based sensors showed, while it is
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7.2~7.4 when two bright LC-based sensors showed. Furthermore, we fabricated a device
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composing of four LC-based sensors doped with 0.3%, 0.4, 0.5 and 0.6% of 6CBA and studied how the pH affects the number of bright LC sensor in the device. Figure 5 shows that the number
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of bright LC-based sensor in the device at pH 6.2, 6.4, 6.6, 6.8, and 7.0 were 4, 3, 2, 1, and 0, respectively. Similarly, the number of bright LC-based sensor shown in the device decreased as the pH level of the aqueous solution increased. Therefore, we could also determine the pH level through the number of bright LC-based sensor in the device. It is worth to note that because the critical pH value for optical transition of the LC-based pH is adjustable, it is possible to fabricate the LC-based pH sensor device with desired pH range and interval through controlling the number and the critical pH value of the LC-based pH sensor.
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ACCEPTED MANUSCRIPT 3.5 Reversibility and applicability of the LC-based pH sensor device in flow system In order to evaluate the feasibility of the LC-based pH sensor device that could monitor the pH level in a continuous flow system, we conducted series of experiments to investigate the reversibility of the LC-based pH sensor device. These experiments were executed by continuous injection of the aqueous solution with different pH values into the device. Figure S4a shows that
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when we injected the aqueous solutions of pH 6.8, 7.0, 7.2, 7.6 and 8.2 into the device
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sequentially, the number of bright LC sensors in the device was 4, 3, 2, 1 and 0, respectively.
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Immediately after the first cycle of injection, we performed the second cycle of injection into the
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same device again. Figure S4b shows that the number of bright LC sensors in the device was consistent with the first cycle, which means the result of the LC-based pH sensor device is
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reversible. The results are consistent at least for ten consecutive cycles (data not shown). Besides,
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we also noticed the LC signals in the device were stable if no further injection was performed. These characteristics, when combined, make this device suitable to monitor the pH level in the
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continuous flow system. To make this work more practical, we designed a LC-based pH sensor
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device to monitor the pH change in the range from 6.5 to 8.5 in a flow system and it can be accomplished by using 6CBA, 8OCBA, 10OCBA and 12OCBA as the dopants for the four LC
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sensors in the device. This is because the U.S. Environmental Protection Agency (EPA)
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recommends that public and drinking water systems maintain pH levels of between 6.5 and 8.5 [31]. Besides, the Canadian government also recommends the optimum pH level for drinking water in the range of 6.5 to 8.5 [32]. Because the temperature of nematic phase for the host LCs, 5CB, in our system ranges from 18~35 oC, it is expected that the LC-based pH sensor device can be applied in normal drinking water. The results for the continuous injection of the aqueous solutions with pH from 6.5 to 8.5 into this device are shown in Figure 6 and corresponding video can be found in the Supporting Information. This device showed four bright LC sensors when the pH level was equal or lower than 6.5, while it showed none bright LC sensor when the pH level
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ACCEPTED MANUSCRIPT was equal or higher than 8.5. At pH 7.0, 7.5 and 8.0, this device showed three, two and one bright LC sensors, respectively. As a result, the safety level of the aqueous solution can be identified easily through number of bright LC sensors in the device. Moreover, the video also demonstrated the reversibility and fast response (~1 s) for the device, which makes it suitable for continuous and real-time monitoring. We want to note that this video was captured under
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ambient light without using any electrical instrumentation and measurement, and its results were
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readily understandable to general users, which demonstrate its potential for on-site applications.
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3.6 Measuring the pH change in real water samples by using the LC-based pH sensor device
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Finally, we tested the capability of this device in real water samples, such as tap water
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and pond water. As shown in Figure 7a, after we injected the tap water (pH = 6.9 determined by pH meter) into the device, the device showed three bright LC sensors. After that, we adjusted the
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pH of tap water to 8.0 and the device showed only one bright LC sensor (Figure 7b). When we
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adjusted the pH of tap water again to 6.0, the device showed four bright LC sensors (Figure 7c). The bright/dark signals of the individual LC sensor shown in the device is in accordance with the
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results using buffer system shown in Figure 1. Same phenomenon was observed when we did
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this experiment using pond water (Figure 7d-7f). Both results demonstrated the capability of the LC-based pH sensor device to measure the pH change in real water samples. According to the
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data announced by local water supplier, the total hardness of the tap water we used is 32 ppm [33]. Therefore, we believe that divalent cations, such as Ca2+ and Mg2+, in normal tap waters would not affect the result of our system. Moreover, we found that the bright LC signals in the device did not change within 24 hours as long as the pH of the samples was unchanged (Figure S5). Based on the findings above, it is believed that the LC-based pH sensor device developed in this work may suitable be used as a pH tag for continuous monitoring the pH level in the flow system such as water pipe and aquarium.
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ACCEPTED MANUSCRIPT 4. Conclusions In this work, we studied the effect of dopants in LCs on the results of LC-based pH sensor and found that the critical pH value for the bright-to-dark transition of the LC sensors can be adjusted through the selection of dopants and the dopant concentrations. By arranging four LC-based pH sensors with different critical pH values in between two glass slides pasted with
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crossed polarizers, we developed a LC-based sensor device to determine the pH level of an
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aqueous flow system. In this device, the pH level can be simply determined through the number
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of bright LC-based pH sensors observed through naked-eye. For practical applications, we
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developed a LC-based sensor device to monitor the pH change ranging from 6.5 to 8.5, which was assumed as the safe range for public and drinking water systems. Moreover, we
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demonstrated the capability of this device to measure the pH change in tap water and pond water.
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The results of this device were reversible, with fast response and readily understandable to general users. Therefore, we believe that it can be utilized as the pH tag for continuous, real-time
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Acknowledgement
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and on-site monitoring of the pH levels in aqueous flow system.
We would like to thank Ministry of Science and Technology (103-2113-M-032-003-MY2 and
support.
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105-2113-M-032-008) and Department of Chemistry, Tamkang University for the funding
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ACCEPTED MANUSCRIPT Appendix A. Supplementary data
Supplementary data associated with this article, including the NMR and HPLC data for the dopants and LC mixtures, the reversibility, stability and interference study of the device, as well as the video showing the continuous injection of the device can be found in the online version, at
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doi:
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References:
[1] H. Sigel, A.D. Zuberbühler, O. Yamauchi, Comments on potentiometric pH titrations and the
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relationship between pH-meter reading and hydrogen ion concentration, Anal. Chim. Acta 255
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(1991) 63-72.
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[2] Y. Qin, H.J. Kwon, M.M.R. Howlader, M.J. Deen, Microfabricated electrochemical pH and
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free chlorine sensors for water quality monitoring: Recent advances and research challenges, RSC Adv. 5 (2015) 69086-69109.
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[3] H. Suzuki, H. Shiroishi, S. Sasaki, I. Karube, Microfabricated liquid junction Ag/AgCl
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5069-5075.
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reference electrode and its application to a one-chip potentiometric sensor, Anal. Chem. 71 (1999)
[4] V.V. Cosofret, M. Erdösy, T.A. Johnson, R.P. Buck, R.B. Ash, M.R. Neuman, Microfabricated sensor arrays sensitive to pH and K+ for ionic distribution measurements in the beating heart, Anal. Chem. 67 (1995) 1647-1653. [5] N.F. Sheppard Jr, M.J. Lesho, P. McNally, A. Shaun Francomacaro, Microfabricated conductimetric pH sensor, Sens. Actuators, B 28 (1995) 95-102.
16
ACCEPTED MANUSCRIPT [6] A.K. Dengler, R.M. Wightman, G.S. McCarty, Microfabricated collector-generator electrode sensor for measuring absolute pH and oxygen concentration, Anal. Chem. 87 (2015) 1055610564. [7] J. Qi, D. Liu, X. Liu, S. Guan, F. Shi, H. Chang, H. He, G. Yang, Fluorescent pH sensors for
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broad-range pH measurement based on a single fluorophore, Anal. Chem. 87 (2015) 5897-5904.
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[8] G. Mistlberger, K. Koren, S.M. Borisov, I. Klimant, Magnetically remote-controlled optical
CR
sensor spheres for monitoring oxygen or pH, Anal. Chem. 82 (2010) 2124-2128.
US
[9] E. Wang, K.F. Chow, V. Kwan, T. Chin, C. Wong, A. Bocarsly, Fast and long term optical
AN
sensors for pH based on sol-gels, Anal. Chim. Acta 495 (2003) 45-50. [10] Z. Jin, Y. Su, Y. Duan, Improved optical pH sensor based on polyaniline, Sens. Actuators, B
M
71 (2000) 118-122.
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[11] Y. Wei, C.H. Jang, Detection of cholesterol molecules with a liquid crystal-based pH-driven
PT
sensor, J. Mater. Sci. 50 (2015) 4741-4748.
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[12] D. Liu, C.H. Jang, A new strategy for imaging urease activity using liquid crystal droplet
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patterns formed on solid surfaces, Sens. Actuators, B 193 (2014) 770-773. [13] S. Zhong, C.H. Jang, Highly sensitive and selective glucose sensor based on ultraviolettreated nematic liquid crystals, Biosens. Bioelectron. 59 (2014) 293-299. [14] C.H. Chen, K.L. Yang, A liquid crystal biosensor for detecting organophosphates through the localized pH changes induced by their hydrolytic products, Sens. Actuators, B 181 (2013) 368-374.
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ACCEPTED MANUSCRIPT [15] X.Y. Bi, D. Hartono, K.L. Yang, Real-time liquid crystal pH sensor for monitoring enzymatic activities of penicillinase, Adv. Funct. Mater. 19 (2009) 3760-3765. [16] M.I. Kinsinger, B. Sun, N.L. Abbott, D.M. Lynn, Reversible control of ordering transitions at aqueous/liquid crystal interfaces using functional amphiphilic polymers. Adv. Mater. 19 (2007)
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4208-4212.
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[17] D.Y. Lee, J.M. Seo, W. Khan, J.A. Kornfield, Z. Kurji, S.Y. Park, pH-responsive
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aqueous/LC interfaces using SGLCP-b-polyacrylic acid block copolymers. Soft Matter 6 (2010)
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1964-1970.
[18] J.M. Brake, M.K. Daschner, Y.Y. Luk, N.L. Abbott, Biomolecular interactions at
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phospholipid-decorated surfaces of thermotropic liquid crystals. Science 302 (2003) 2094-2098.
M
[19] Z. Hussain, F. Qazi, M.I. Ahmed, A. Usman, A. Riaz, A.D. Abbasi, Liquid crystals based
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sensing platform-technological aspects, Biosens. Bioelectron. 85 (2016) 110-127.
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[20] D. Wang, S.Y. Park, I.K. Kang, Liquid crystals: Emerging materials for use in real-time
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detection applications, J. Mater. Chem. C 3 (2015) 9038-9047. [21] A.M. Lowe, N.L. Abbott, Liquid crystalline materials for biological applications, Chem.
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Mater. 24 (2012) 746-758.
[22] J. W. Goodby, P. J. Collings, T. Kato, C. Tschierske, H. F. Gleeson, P. Raynes, Biosensors and Liquid Crystals, Handbook of Liquid Crystals, 8 Volume Set, Second Edition. [23] Y. C. Dong, Z. Q. Yang, Beyond displays: The recent progress of liquid crystals for bio/chemical detections, Chin Sci Bull, 58 (2013) 2557-2562.
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ACCEPTED MANUSCRIPT [24] D. Wencel, T. Abel, C. McDonagh, Optical chemical pH sensors, Anal. Chem. 86 (2014) 15-29. [25] S. Coco, P. Espinet, E. Marcos, Mesomorphic trigonal bipyramidal iron(0) isocyanide complexes, J. Mater. Chem. 10 (2000) 1297-1302.
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[26] C.H. Chuang, Y.C. Lin, W.L. Chen, Y.H. Chen, Y.X. Chen, C.M. Chen, H.W. Shiu, L.Y.
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Chang, C.H. Chen, C.H. Chen, Detecting trypsin at liquid crystal/aqueous interface by using
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surface-immobilized bovine serum albumin. Biosens. Bioelectron. 78 (2016) 213-220.
US
[27] J.M. Hamilton, J.R. Whitehead, N.J. Williams, M. El Ojaimi, R.P. Thummel, R.D. Hancock, Metal ion complexing properties of dipyridoacridine, a highly preorganized tridentate
AN
homologue of 1,10-phenanthroline, Inorg. Chem. 50 (2011) 3785-3790.
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[28] J.M. Brake, A.D. Mezera, N.L. Abbott, Effect of surfactant structure on the orientation of
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liquid crystals at aqueous-liquid crystal interfaces. Langmuir 19 (2003) 6436-6442.
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[29] C.H. Chen, Y.C. Lin, H.H. Chang, A.S.Y. Lee, Ligand-doped liquid crystal sensor system
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for detecting mercuric ion in aqueous solutions, Anal. Chem. 87 (2015) 4546-4551. [30] R.J. Carlton, J.K. Gupta, C.L. Swift, N.L. Abbott, Influence of simple electrolytes on the
31-36.
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orientational ordering of thermotropic liquid crystals at aqueous interfaces, Langmuir 28 (2012)
[31] U.S. Environmental Protection Agency (EPA). Secondary drinking water standards. Retrieved August 31, 2016 from https://www.epa.gov/dwstandardsregulations/secondarydrinking-water-standards-guidance-nuisance-chemicals [32] Health Canada, Guidelines for Canadian drinking water quality. Retrieved August 31, 2016 from http://www.hc-sc.gc.ca/ewh-semt/pubs/water-eau/sum_guide-res_recom/index-eng.php 19
ACCEPTED MANUSCRIPT [33] Taipei Water Department. International Comparison of Water Quality. Retrieved July 20,
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Scheme 1. The working principle for LC-based pH sensor.
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Scheme 2. Molecular structures of LCs and dopants used in this study.
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Scheme 3. (a) Schematic (side view) and (b) real image (top view) of the LC-based pH sensor
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device for flow system.
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Figure 1. Polarized images of the LC-based sensor using 0.3% of (a) 6CBA, (b) 8OCBA, (c)
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10OCBA and (d) 12OCBA as dopants at different pH values. It was found that the critical pH
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Figure 2. Polarized images of the LC-based sensor using (a) 0.3%, (b) 0.4%, (c) 0.5% and (d)
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0.6% 6CBA as the dopant at different pH values. It was found that the critical pH values for
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dopants.
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optical transition of LC-based sensor can be adjusted through changing the concentrations of
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Figure 3. Polarized images of the LC-based sensor using 0.3% 6CBA as the dopant in the
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aqueous solutions containing (a) 0 mM, (b) 50 mM and (c) 400 mM of NaCl at different pH values. It was found that the critical pH values for optical transition of LC-based sensor lowered
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Figure 4. Polarized images of the device composing of four LC-based sensors using 12OCBA,
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10OCBA, 8OCBA and 6CBA as the dopants (from left to right) at pH (a) 6.8, (b) 7.0, (c) 7.2, (d) 7.6 and (e) 8.2. It was found that the pH level of the aqueous solutions can be determined
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Figure 5. Polarized images of the device composing of four individual LC-based sensors using
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0.3%, 0.4%, 0.5% and 0.6% of 6CBA (from left to right) as the dopants at pH (a) 6.2, (b) 6.4, (c) 6.6, (d) 6.8 and (e) 7.0. It was found that the pH level of the aqueous solutions can be determined
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Figure 6. Real images of the LC-based pH sensor device by continuous injection of aqueous
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solutions with pH (a) 6.5, (b) 7.0, (c) 7.5, (d) 8.0, (e) 8.5, and (f) 6.5 again. In the device, the dopants applied in the sensors (from left to right) were 12OCBA, 10OCBA, 8OCBA and 6CBA,
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Figure 7. Polarized images of the device composing of four individual LC-based sensors using
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6CBA, 8OCBA, 10OCBA and 12OCBA in tap water and pond water. In (a-c), the device was
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injected by tap water (pH 6.9), then the pH of tap water was adjusted to 8.0 and 6.0. In (d-f), the device was injected by pond water (pH 6.7), then the pH of pond water was adjusted to 8.0 and
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Graphical Abstracts
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ACCEPTED MANUSCRIPT Highlights
Continuous Monitoring of pH Level in Flow Aqueous System
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by Using Liquid Crystal-based Sensor Device
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Wei-Long Chen, Tsung Yang Ho, Jhih-Wei Huang and Chih-Hsin Chen*
To whom correspondence should be addressed.
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Phone: +886-2-26215656
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Department of Chemistry, Tamkang University, New Taipei City 25137, Taiwan
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Email: [email protected]
The liquid crystal (LC)-based sensor device was developed for pH determination.
The pH levels were interpreted through the number of bright and dark LC signals.
A portable device was developed to monitor the water safety level (pH 6.5~8.5).
It is suitable for real-time and continuous pH monitoring in a flow aqueous system.
This sensor can be applied in tap water and pond water.
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Wei-Long Chen, Tsung Yang Ho, Jhih-Wei Huang, Chih-Hsin Chen PII: DOI: Reference:
S0026-265X(17)31316-4 doi:10.1016/j.microc.2018.03.020 MICROC 3093
To appear in:
Microchemical Journal
Received date: Revised date: Accepted date:
23 December 2017 9 March 2018 10 March 2018
Please cite this article as: Wei-Long Chen, Tsung Yang Ho, Jhih-Wei Huang, ChihHsin Chen , Continuous monitoring of pH level in flow aqueous system by using liquid crystal-based sensor device. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Microc(2017), doi:10.1016/ j.microc.2018.03.020
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ACCEPTED MANUSCRIPT Continuous Monitoring of pH Level in Flow Aqueous System by Using Liquid Crystal-based Sensor Device
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Wei-Long Chen, Tsung Yang Ho, Jhih-Wei Huang and Chih-Hsin Chen*
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Department of Chemistry, Tamkang University, New Taipei City 25137, Taiwan To whom correspondence should be addressed.
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Phone: +886-2-26215656
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Email: [email protected]
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ACCEPTED MANUSCRIPT Abstract In this work, we report a liquid crystal (LC)-based sensor to determine pH level of aqueous solutions. This sensor was fabricated by filling the LCs doped with a pH-sensitive molecule into a cupper grid on a glass substrate. Theoretically, the dopants are neutral and disperse freely in the LCs. When the sensor was immersed in the aqueous solution, the increases of pH can lead to the
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dissociation of the dopants, allowing them to align at the LC/aqueous interface. This
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phenomenon causes the reorientation of the LC molecules and therefore a bright-to-dark
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transition of the LC image was observed simply through naked-eye. In our research, we found
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that the critical pH value for the optical transition of LC sensors can be adjusted from 6.8 to 8.2 through the selection of dopants, while it can be adjusted from 6.2 to 7.0 through the selection of
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dopant concentrations. By arranging four individual sensors in a device with inlet and out
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channels, we demonstrated that the pH level of an aqueous flow system can be determined by the number of bright LC sensors shown in the device. Based on this strategy, we developed a device
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to monitor the pH safety level for drinking water ranging from pH 6.5 to 8.5. This device
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demonstrated fast response (~1 s), good stability, reversibility and its capability to measure the pH change in tap water and pond water, which make it suitable for real-time and continuous
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monitoring the pH change in various flow aqueous systems.
Keywords:
Liquid crystal-based sensor; Reversible pH sensor; Continuous monitoring of pH in flow system; pH monitoring for drinking water Naked-eye detection
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ACCEPTED MANUSCRIPT 1. Introduction pH value, the indication of hydronium ion (H3O+) concentration in the solution, is regarded as the critical parameter in a variety of fields such as life science, medical science, food industry, and environmental protection. The most well-established and widely applied method to retrieve the pH value is through the electrochemical process, which requires an electrode to
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report the change in potential as the pH changes [1]. Although the use of pH electrodes are
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considered a simple, fast, and precise approach, however, pH electrodes are not ideal of long-
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term and continuous monitoring of pH. This is due to its susceptibility of electrical interference,
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which would cause the signal drifting from the electrodes [2]. In order to ensure the accuracy of pH electrodes, calibrations must be carried out periodically. Besides, the use of pH electrodes
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normally requires relatively larger quantity of sample volumes due to the size of electrodes.
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Although various types of microfabricated electrochemical pH sensors have been developed recently to circumvent these disadvantages, there are still rooms for their stability and
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repeatability to be improved [3-6]. Alternatively, optical chemical sensors, which apply a
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chemical indicator covalently attached to solid matrix, provide another simple strategy to study the pH of a solution [7-10]. Typically, an ideal chemical indicator should possess both
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protonated form and deprotonated form with distinct absorption or fluorescence properties. The
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change of pH value in the solution changes the composition of the probes and therefore the optical properties exhibited by the solution changes accordingly. The results of this type of pH sensor can be precisely quantified by spectrometer-based instruments. Nevertheless, considering the photo-stability and the immobilization stability of the probes, its results are often affected by the time and temperature at which the test is made. In recent years, the development of liquid crystal (LC)-based sensor system provided a new approach to study the pH value in aqueous solution [11-17]. One of the most significant feature of LC-based sensor system is that its results can be simply interpreted through the 3
ACCEPTED MANUSCRIPT colorful signals created by the LC molecules. As a result, it is readily recognizable to general users under ambient conditions and has been considered as a simpler and cheaper sensor system comparing to those requiring electrical instrumentation to report the intensity of the signals [1823]. The principle of LC-based pH sensor applies an amphiphilic molecule, normally a weak acid or water soluble polymer with a hydrophobic tail to assemble at LC/aqueous interface. Under
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acidic condition, the amphiphilic molecules are protonated and distributed randomly in the LC
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phase. In this case, the orientation of LC is planar and the LC image is bright cross-polarized
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light. On the contrary, the amphiphilic molecules are deprotonated under basic condition such that they tend to align at the LC/aqueous interface with hydrophobic tails perpendicular to the
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interface, which induces the homeotropic alignment of the LC molecules and result in a dark LC
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image under cross-polarized light (Scheme 1). By using this principle, a small change in pH (~0.1) can be distinguished from dark-to-bright transition of LC image with the naked-eye
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immediately. In past studies, this principle has been used to establish the sensors for various
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targets which are able to change the pH of solution under specific conditions. For example, Zhong et al. reported a LC-based pH sensor to detect glucose through the H+ released by the
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oxidation of glucose [13]. In their system, nematic LCs 4-cyano-4’-pentylbiphenyl (5CB) treated
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by UV exposure, which partially converts 5CB to its oxidized product 4-cyano-4’biphenylcarboxylic acid (CBA), was used as the pH sensitive LC. This system was able to detect
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glucose concentration as low as 1 pM with high selectivity. Later, this system was also applied to detect cholesterol molecules by the same research group [11]. However, the potential problem of this system is that the dopant concentration could be affected by the time, distance and power efficiency of the UV exposure, therefore, the critical pH value for optical transition may vary from batch to batch. On the other hand, Bi et al. reported a LC pH sensor which is able to monitor the activity of penicillinase in real-time [15]. In their system, the pH sensitive LC was composed of 5CB doped with 4’-pentyl-biphenyl-4-carboxylic acid (PBA). Because the dopant concentration was fixed, the critical pH value for optical transition was stable. However, the 4
ACCEPTED MANUSCRIPT critical pH value for optical transition in this system depends on the pKa value of the dopant, making this system can only tell whether the pH level exceeds 7.0 or not. The dynamic range of this type of pH sensor was limited such that it is not applicable in various environmental conditions. In general, the dynamic range of a pH sensor is closely related to the pKa value of the
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indicator applied in this sensor. In a recent review paper discussing the characteristics of optical
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chemical pH sensors, it has been reported that the dynamic range of an optical pH sensor is
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limited to approximately pKa ± 1.5 of the indicator and its highest sensitivity is usually observed
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around the pKa of the indicator [24]. In the same optical pH sensor, extended dynamic range can be achieved by applied a series of indicators with different pKa values. Considering to this point,
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it can be estimated that the dynamic range of the LC-based pH sensor can be extended by using
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the dopant molecules with different pKa values. To the best of our knowledge, this strategy has
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never been used to develop the LC-based pH sensor before. In this paper, we applied four different weak acids as the dopants to develop the LC-
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based pH sensor. We investigated the effect of the pKa of the dopants on the dynamic range of
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the LC-based pH sensor. By using different dopants in the LCs, the critical pH value for optical transition of the LC-based pH sensor could be adjusted. In addition, we also studied the effect of
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dopant concentration and salt concentration on the results of the sensor. To determine the specific pH level, we arranged four LC-based pH sensors with different dopants in a device with inlet and outlet channels and studied the correlation between the pH level of the solutions in the device and the number of the bright LC sensors shown in the device. Finally, the response time, stability and reversibility of the reported device was also tested for its applicability for continuous monitoring of pH level in the flow aqueous system.
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ACCEPTED MANUSCRIPT 2. Materials and Methods 2.1 Materials
Glass slides were obtained from Fisher Scientific (U.S.A). N,N-Dimethyl-n-octadecyl-3aminopropyltrimethoxysilyl chloride (DMOAP), 4'-n-hexylbiphenyl-4-carboxylic acid (6CBA),
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4'-n-octyloxybiphenyl-4-carboxylic acid (8OCBA), NaCl, CaCl2 and MgCl2 were purchased
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from Sigma Aldrich (U.S.A). Tris buffer was purchased from J.T. Baker (U.S.A). Liquid crystal
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4-cyano-4’-pentylbiphenyl (5CB) was purchased from TCI (Taiwan). 4'-n-decyloxybiphenyl-4carboxylic acid (10OCBA) and 4'-n-dodecyloxybiphenyl-4-carboxylic acid (12OCBA) were
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synthesized following reported procedures [25] and characterized by 1H-NMR (Figure S1).
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Water was purified by using a Milli-Q system (Millipore). Tap water and pond water were
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2.2 Determination of the pKa of dopants
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collected in the campus of Tamkang University.
The solution of the dopants (6CBA, 8OCBA, 10OCBA and 12OCBA, 10-4 M in THF :
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H2O = 3:1) were titrated with tetrabutylammonium hydroxide (TBAOH). During the titration, the
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UV-vis spectra and pH value of each solution were recorded until no more change on the UV-vis spectra was observed. As the dopants have a protonated and a deprotonated form, the absorbance
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of the dopants follows the modified Henderson-Hasselbalch equation: pH = log[(Ai - A)/(A - Af)] + pKa
where Ai and Af are the initial and final absorbance at selected wavelength of the dopant, while A is the absorbance of the dopant at different pH values. By plotting log[(Ai - A)/(A - Af)] vs. pH values, the pKa of the dopants can be determined as the intercept of each plot. 2.3 Preparation of DMOAP-coated slides
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water. DMOAP-coated slides were dried under a stream of nitrogen and heated in a 100°C
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2.4 Preparation of LC-based pH sensor
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TEM copper grids (75 mesh, Ted Pella, Inc., U.S.A) were cleaned in methanol, ethanol,
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and acetone (sonicated in each solvent for 15 min), and then heated overnight at 100 °C to evaporate residual solvent. The pH responsive LC was prepared by doping 6CBA, 8OCBA,
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10OCBA or 12OCBA into 5CB and sonicated for 1 min before use. To fabricate the LC-based
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sensing platform, a piece of copper grid was placed on a DMOAP-coated slide (size: 5 mm × 5 mm), followed by dispensing 0.25 μL of 5CB doped with various dopants onto the grid.
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Excessive LC was removed by using a capillary tube. Finally, the slide together with the grid
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containing the LC was immersed in 300 μL of Tris buffer solution with different pH values. The optical appearances of these samples were observed by using a polarizing optical microscope
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(Canon EOS D650, Japan) in the transmission mode. Each image was captured with a digital camera mounted on the microscope with an exposure time of 1/80 sec.
2.5 Fabrication of LC-based pH sensor device for flow system
Polydimethylsiloxane (PDMS) films were prepared by casting Sylgard 184 (Dow Corning) on a petri dish placed by a glass slide and two capillaries to form a groove dimension (10 × 25 × 2 mm) with inlet and outlet channels. After that, four individual LC-based pH sensors were aligned in the groove dimension of PDMS film. The whole system was then sandwiched by 7
ACCEPTED MANUSCRIPT two glass slides with cross-polars and fixed by two binder clips to form the LC-based pH sensor device for flow system. 3. Results and Discussion 3.1 Effect of the dopant on the critical pH value for optical transition of LC sensors
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Aiming a larger dynamic range for LC-based pH sensor, our first goal is to study how the pKa of the dopant affects the critical pH value for optical transition in the system. Considering
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the compatibility of the dopants with LC molecules, we selected biphenyl-4-carboxylic acid,
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which is structurally similar to 5CB, as the main backbone of the dopants. The molecular structures of the dopants are shown in Scheme 2, which includes 4'-n-hexylbiphenyl-4-
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carboxylic acid (6CBA), 4'-n-octyloxybiphenyl-4-carboxylic acid (8OCBA), 4'-n-
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decyloxybiphenyl-4-carboxylic acid (10OCBA) and 4'-n-dodecyloxybiphenyl-4-carboxylic acid (12OCBA). When these molecules were applied as the dopants in the LC-based sensor, we found
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that the critical pH values for the bright-to-dark transition of the LC images are different. As
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shown in Figure 1, the critical pH values for the bright-to-dark transition of the LC image in 6CBA-, 8OCBA-, 10OCBA-, and 12OCBA-doped system were 7.0, 7.2, 7.6, and 8.2,
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respectively. According to previous report, the bright-to-dark transition of the LC image can be
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attributed to the protonation and deprotonation equilibrium of dopants occurred at the LC/aqueous interface of the system. As the mechanism shown in Scheme 1, protonated dopants distributed randomly in the LC phase and result in planar orientation of LCs, while deprotonated dopants aligned at the LC/aqueous interface and result in homeotropic orientation of LCs [15]. Therefore, it can be hypothesized that the pKa of the dopants, which decides the extent of the deprotonation under a specific pH value, should affect the critical pH value for optical transition of the LC sensor. To further investigate this phenomenon, the pKa of 6CBA, 8OCBA, 10OCBA and 12OCBA were measured by using UV-Vis titration experiments to determine the
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ACCEPTED MANUSCRIPT concentration ratio for the protonated and deprotonated form of the acid. The results revealed that the pKa of 6CBA, 8OCBA, 10OCBA and 12OCBA were calculated as 7.68, 7.66, 9.77 and 8.74, respectively [27]. The critical pH value for optical transition of the LC sensor was not increased as the pKa of the dopants increased, which suggested that this value was not only determined by the pKa of the dopants. The critical pH value for optical transition of the LC could
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also be affected by the solubility of the dopant in the LCs. In the case of 12OCBA-doped system,
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we found that 12OCBA was not completely dissolved in the LCs. To obtain a more accurate
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concentration for the dopant, we analyzed the composition of 12OCBA doped 5CB by using HPLC. The results showed that the real concentration of 12OCBA in 5CB is 0.14%, not 0.3%
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(Figure S2). Therefore, in the case of 12OCBA, higher pH was required for producing the
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deprotonated dopants that are sufficient to align the LCs in homeotropic orientation at the LC/aqueous interface. Additional factors to affect the critical pH value for optical transition of
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the LC sensor could be the ability of the dopants to align the LCs at the LC/aqueous interface, as
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we found that the pKa of 10OCBA (9.77) is significantly larger than that of 8OCBA (7.66) while the critical pH value for optical transition in 10OCBA-doped system (7.6) was only slightly
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higher than that in 8OCBA-doped system (7.2). Previous study has demonstrated that 5CB are
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prone to align homeotropically at aqueous/LC interface using the surfactant with longer alkyl chain because the penetration of 5CB into surfactant monolayers [28]. This phenomenon implies
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that less deprotonated dopants are required to induce the optical transition of LCs in the LCbased pH sensor. As a result, the optical transition for the LC sensor using the dopants with longer alkyl-chain could occur at a relative lower pH value. In addition, the structural similarity between the dopants and the LCs may also affect the ability of dopants to align LCs at aqueous/LC interface, as we found that the pKa values of 6CBA and 8OCBA are comparable while the critical pH values for optical transition of LC image for 8OCBA and 6CBA are different. Comparing to 8OCBA, which has an additional oxygen atom on the alkyl chain, the molecular structure of 6CBA is more similar to 5CB (the LCs) and shows better ability to align 9
ACCEPTED MANUSCRIPT LCs at aqueous/LC interface. Although we are unable to predict the critical pH value for optical transition when different dopant was chose, we demonstrated that it could be adjusted through the selection of dopants for the system. Considering the different safety pH levels for various environmental conditions, this feature make it possible to develop the LC-based pH sensor suitable for monitoring the specific pH change in different environments. To test the reversibility
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of the sensor, we conducted a series of experiments by alternatively exposing the LC-based pH
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sensor using 6CBA as dopant to the aqueous solution at pH 6.5 or 7.0. The results showed that
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all the LC image were bright at pH 6.5, while they were bright at pH 7.0 in five cycles (Figure S3a). In addition, plotting the average gray-scale intensity of LCs as a function of pH changes
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also demonstrated that the bright-to-dark transition of LC signals were reversible in five cycles
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(Figure S3b). Both results suggest the good reversibility of the LC-based pH sensor.
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3.2 Effect of dopant concentrations on the critical pH values for optical transition of LC sensors
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Next, we study how the dopant concentration of this system affects the critical pH value for optical transition of LC sensor. Based on the abovementioned results, the amount of
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deprotonated dopants is an important factor to affect the orientation of LC at LC/aqueous
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interface. We selected 6CBA as the dopant because it is highly soluble in 5CB, and doped different concentrations of 6CBA into 5CB to develop the LC-based pH sensor. The results
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indicated that the critical pH values for the bright-to-dark transition of the LC image in 0.3%, 0.4%, 0.5% and 0.6% 6CBA-doped system were 7.0, 6.8, 6.6, and 6.4, respectively (Figure 2). This means the critical pH value for optical transition of LC decreased as the dopant concentration increased. Assuming that the amount of deprotonated dopant required for the reorientation of LC is constant in the same dopant system, such trend could be explained by that the amount of deprotonated dopant required for the reorientation of LC was reached at lower pH when the dopant concentration was higher. This finding provides another strategy to adjust the critical pH value for optical transition of the LC for the pH values for the sensor system by using 10
ACCEPTED MANUSCRIPT only one dopant. Moreover, we have successfully extended the critical pH value for optical transition of LC in the system to a lower pH value by changing the dopant concentrations. 3.3 Effect of salt concentration on the critical pH values for optical transition of LC sensors We further studied the effect of salt concentration on the critical pH value for optical
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transition of LC sensors because it is also an important environmental variable to the water
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samples. Figure 3 shows the results for 0.3% 6CBA doped LC-based pH sensors in the solution
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containing different concentrations of NaCl. It reveals that the critical pH values for the brightto-dark transition of LC sensors in the solutions containing 0 mM, 50 mM and 400 mM of NaCl
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were 7.0, 6.8 and 6.6, respectively. That is, the critical pH values for optical transition of LC
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lowered as the NaCl concentration increased. In previous studies, it has been found that the homeotropic anchoring of LCs at LC/aqueous can be stabilized by the high ionic strength in the
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aqueous phase [29, 30]. Therefore, the lowered critical pH values for the bright-to-dark transition
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of LC sensors could be attributed to the more stabilized homeotropic anchoring of LCs in the
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environment with higher ionic strength.
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3.4 Development of LC-based pH sensor device to determine the pH level of aqueous solution Based on the findings above, we conclude that the critical pH value for optical transition
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of the LC-based pH sensor could be adjusted through the selection of dopants or dopant concentrations. For each sensor, however, it could only report whether the pH of the aqueous solution exceeds a specific value or not. The exact pH level of the aqueous solution is still not uncertain. To solve this issue, we integrated four LC-based sensors with different dopants (i.e., 6CBA, 8OCBA, 10OCBA, and 12OCBA) into one device. Because the critical pH value for dark-to-bright transition of each sensor is different, we anticipate that the number of the bright LC-based pH sensor should be correlated with the pH level of the aqueous solution. To simplify the operation and observation of the device, we arranged the LC-based sensors in array format on 11
ACCEPTED MANUSCRIPT a concave dimension of a PDMS film, followed by sandwiching this film with two glass slides. The schematic illustration (side view) and real image (top view) of the LC-based pH sensor device was shown in Scheme 3. We embedded two capillaries in the PDMS film as the inlet and outlet channels for the device. In addition, we pasted two crossed polarizers on the top and bottom surface of the device such that the optical signals of the device can be directly observed
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under ambient light without using polarized microscope. Aqueous solutions with different pH
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values were injected into the device with the flow rate of 6 mL/min. Figure 4 show the images of
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the device at different pH values through naked-eye. It was found that four LC-based pH sensors were all bright at pH 6.8. In contrast, three LC-based pH sensors were bright at pH 7.0. The one
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doped with 6CBA turned dark because the critical pH value for its optical transition was reached.
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Accordingly, the number of bright LC-based system in the device at pH 7.0, 7.2, 7.6 and 8.2 were 3, 2, 1, and 0, respectively. Apparently, the number of bright LC-based sensor in the device
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decreased as the pH level of the aqueous solution increased. Therefore, we can simply determine
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the pH level of the aqueous solution through the number of bright LC-based sensors in the device. For example, the pH level is 7.0~7.2 when three bright LC-based sensors showed, while it is
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7.2~7.4 when two bright LC-based sensors showed. Furthermore, we fabricated a device
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composing of four LC-based sensors doped with 0.3%, 0.4, 0.5 and 0.6% of 6CBA and studied how the pH affects the number of bright LC sensor in the device. Figure 5 shows that the number
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of bright LC-based sensor in the device at pH 6.2, 6.4, 6.6, 6.8, and 7.0 were 4, 3, 2, 1, and 0, respectively. Similarly, the number of bright LC-based sensor shown in the device decreased as the pH level of the aqueous solution increased. Therefore, we could also determine the pH level through the number of bright LC-based sensor in the device. It is worth to note that because the critical pH value for optical transition of the LC-based pH is adjustable, it is possible to fabricate the LC-based pH sensor device with desired pH range and interval through controlling the number and the critical pH value of the LC-based pH sensor.
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ACCEPTED MANUSCRIPT 3.5 Reversibility and applicability of the LC-based pH sensor device in flow system In order to evaluate the feasibility of the LC-based pH sensor device that could monitor the pH level in a continuous flow system, we conducted series of experiments to investigate the reversibility of the LC-based pH sensor device. These experiments were executed by continuous injection of the aqueous solution with different pH values into the device. Figure S4a shows that
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when we injected the aqueous solutions of pH 6.8, 7.0, 7.2, 7.6 and 8.2 into the device
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sequentially, the number of bright LC sensors in the device was 4, 3, 2, 1 and 0, respectively.
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Immediately after the first cycle of injection, we performed the second cycle of injection into the
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same device again. Figure S4b shows that the number of bright LC sensors in the device was consistent with the first cycle, which means the result of the LC-based pH sensor device is
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reversible. The results are consistent at least for ten consecutive cycles (data not shown). Besides,
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we also noticed the LC signals in the device were stable if no further injection was performed. These characteristics, when combined, make this device suitable to monitor the pH level in the
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continuous flow system. To make this work more practical, we designed a LC-based pH sensor
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device to monitor the pH change in the range from 6.5 to 8.5 in a flow system and it can be accomplished by using 6CBA, 8OCBA, 10OCBA and 12OCBA as the dopants for the four LC
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sensors in the device. This is because the U.S. Environmental Protection Agency (EPA)
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recommends that public and drinking water systems maintain pH levels of between 6.5 and 8.5 [31]. Besides, the Canadian government also recommends the optimum pH level for drinking water in the range of 6.5 to 8.5 [32]. Because the temperature of nematic phase for the host LCs, 5CB, in our system ranges from 18~35 oC, it is expected that the LC-based pH sensor device can be applied in normal drinking water. The results for the continuous injection of the aqueous solutions with pH from 6.5 to 8.5 into this device are shown in Figure 6 and corresponding video can be found in the Supporting Information. This device showed four bright LC sensors when the pH level was equal or lower than 6.5, while it showed none bright LC sensor when the pH level
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ACCEPTED MANUSCRIPT was equal or higher than 8.5. At pH 7.0, 7.5 and 8.0, this device showed three, two and one bright LC sensors, respectively. As a result, the safety level of the aqueous solution can be identified easily through number of bright LC sensors in the device. Moreover, the video also demonstrated the reversibility and fast response (~1 s) for the device, which makes it suitable for continuous and real-time monitoring. We want to note that this video was captured under
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ambient light without using any electrical instrumentation and measurement, and its results were
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readily understandable to general users, which demonstrate its potential for on-site applications.
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3.6 Measuring the pH change in real water samples by using the LC-based pH sensor device
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Finally, we tested the capability of this device in real water samples, such as tap water
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and pond water. As shown in Figure 7a, after we injected the tap water (pH = 6.9 determined by pH meter) into the device, the device showed three bright LC sensors. After that, we adjusted the
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pH of tap water to 8.0 and the device showed only one bright LC sensor (Figure 7b). When we
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adjusted the pH of tap water again to 6.0, the device showed four bright LC sensors (Figure 7c). The bright/dark signals of the individual LC sensor shown in the device is in accordance with the
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results using buffer system shown in Figure 1. Same phenomenon was observed when we did
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this experiment using pond water (Figure 7d-7f). Both results demonstrated the capability of the LC-based pH sensor device to measure the pH change in real water samples. According to the
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data announced by local water supplier, the total hardness of the tap water we used is 32 ppm [33]. Therefore, we believe that divalent cations, such as Ca2+ and Mg2+, in normal tap waters would not affect the result of our system. Moreover, we found that the bright LC signals in the device did not change within 24 hours as long as the pH of the samples was unchanged (Figure S5). Based on the findings above, it is believed that the LC-based pH sensor device developed in this work may suitable be used as a pH tag for continuous monitoring the pH level in the flow system such as water pipe and aquarium.
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ACCEPTED MANUSCRIPT 4. Conclusions In this work, we studied the effect of dopants in LCs on the results of LC-based pH sensor and found that the critical pH value for the bright-to-dark transition of the LC sensors can be adjusted through the selection of dopants and the dopant concentrations. By arranging four LC-based pH sensors with different critical pH values in between two glass slides pasted with
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crossed polarizers, we developed a LC-based sensor device to determine the pH level of an
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aqueous flow system. In this device, the pH level can be simply determined through the number
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of bright LC-based pH sensors observed through naked-eye. For practical applications, we
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developed a LC-based sensor device to monitor the pH change ranging from 6.5 to 8.5, which was assumed as the safe range for public and drinking water systems. Moreover, we
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demonstrated the capability of this device to measure the pH change in tap water and pond water.
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The results of this device were reversible, with fast response and readily understandable to general users. Therefore, we believe that it can be utilized as the pH tag for continuous, real-time
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Acknowledgement
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and on-site monitoring of the pH levels in aqueous flow system.
We would like to thank Ministry of Science and Technology (103-2113-M-032-003-MY2 and
support.
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105-2113-M-032-008) and Department of Chemistry, Tamkang University for the funding
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ACCEPTED MANUSCRIPT Appendix A. Supplementary data
Supplementary data associated with this article, including the NMR and HPLC data for the dopants and LC mixtures, the reversibility, stability and interference study of the device, as well as the video showing the continuous injection of the device can be found in the online version, at
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doi:
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References:
[1] H. Sigel, A.D. Zuberbühler, O. Yamauchi, Comments on potentiometric pH titrations and the
US
relationship between pH-meter reading and hydrogen ion concentration, Anal. Chim. Acta 255
AN
(1991) 63-72.
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[2] Y. Qin, H.J. Kwon, M.M.R. Howlader, M.J. Deen, Microfabricated electrochemical pH and
ED
free chlorine sensors for water quality monitoring: Recent advances and research challenges, RSC Adv. 5 (2015) 69086-69109.
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[3] H. Suzuki, H. Shiroishi, S. Sasaki, I. Karube, Microfabricated liquid junction Ag/AgCl
AC
5069-5075.
CE
reference electrode and its application to a one-chip potentiometric sensor, Anal. Chem. 71 (1999)
[4] V.V. Cosofret, M. Erdösy, T.A. Johnson, R.P. Buck, R.B. Ash, M.R. Neuman, Microfabricated sensor arrays sensitive to pH and K+ for ionic distribution measurements in the beating heart, Anal. Chem. 67 (1995) 1647-1653. [5] N.F. Sheppard Jr, M.J. Lesho, P. McNally, A. Shaun Francomacaro, Microfabricated conductimetric pH sensor, Sens. Actuators, B 28 (1995) 95-102.
16
ACCEPTED MANUSCRIPT [6] A.K. Dengler, R.M. Wightman, G.S. McCarty, Microfabricated collector-generator electrode sensor for measuring absolute pH and oxygen concentration, Anal. Chem. 87 (2015) 1055610564. [7] J. Qi, D. Liu, X. Liu, S. Guan, F. Shi, H. Chang, H. He, G. Yang, Fluorescent pH sensors for
T
broad-range pH measurement based on a single fluorophore, Anal. Chem. 87 (2015) 5897-5904.
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[8] G. Mistlberger, K. Koren, S.M. Borisov, I. Klimant, Magnetically remote-controlled optical
CR
sensor spheres for monitoring oxygen or pH, Anal. Chem. 82 (2010) 2124-2128.
US
[9] E. Wang, K.F. Chow, V. Kwan, T. Chin, C. Wong, A. Bocarsly, Fast and long term optical
AN
sensors for pH based on sol-gels, Anal. Chim. Acta 495 (2003) 45-50. [10] Z. Jin, Y. Su, Y. Duan, Improved optical pH sensor based on polyaniline, Sens. Actuators, B
M
71 (2000) 118-122.
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[11] Y. Wei, C.H. Jang, Detection of cholesterol molecules with a liquid crystal-based pH-driven
PT
sensor, J. Mater. Sci. 50 (2015) 4741-4748.
CE
[12] D. Liu, C.H. Jang, A new strategy for imaging urease activity using liquid crystal droplet
AC
patterns formed on solid surfaces, Sens. Actuators, B 193 (2014) 770-773. [13] S. Zhong, C.H. Jang, Highly sensitive and selective glucose sensor based on ultraviolettreated nematic liquid crystals, Biosens. Bioelectron. 59 (2014) 293-299. [14] C.H. Chen, K.L. Yang, A liquid crystal biosensor for detecting organophosphates through the localized pH changes induced by their hydrolytic products, Sens. Actuators, B 181 (2013) 368-374.
17
ACCEPTED MANUSCRIPT [15] X.Y. Bi, D. Hartono, K.L. Yang, Real-time liquid crystal pH sensor for monitoring enzymatic activities of penicillinase, Adv. Funct. Mater. 19 (2009) 3760-3765. [16] M.I. Kinsinger, B. Sun, N.L. Abbott, D.M. Lynn, Reversible control of ordering transitions at aqueous/liquid crystal interfaces using functional amphiphilic polymers. Adv. Mater. 19 (2007)
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4208-4212.
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[17] D.Y. Lee, J.M. Seo, W. Khan, J.A. Kornfield, Z. Kurji, S.Y. Park, pH-responsive
CR
aqueous/LC interfaces using SGLCP-b-polyacrylic acid block copolymers. Soft Matter 6 (2010)
US
1964-1970.
[18] J.M. Brake, M.K. Daschner, Y.Y. Luk, N.L. Abbott, Biomolecular interactions at
AN
phospholipid-decorated surfaces of thermotropic liquid crystals. Science 302 (2003) 2094-2098.
M
[19] Z. Hussain, F. Qazi, M.I. Ahmed, A. Usman, A. Riaz, A.D. Abbasi, Liquid crystals based
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sensing platform-technological aspects, Biosens. Bioelectron. 85 (2016) 110-127.
PT
[20] D. Wang, S.Y. Park, I.K. Kang, Liquid crystals: Emerging materials for use in real-time
CE
detection applications, J. Mater. Chem. C 3 (2015) 9038-9047. [21] A.M. Lowe, N.L. Abbott, Liquid crystalline materials for biological applications, Chem.
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Mater. 24 (2012) 746-758.
[22] J. W. Goodby, P. J. Collings, T. Kato, C. Tschierske, H. F. Gleeson, P. Raynes, Biosensors and Liquid Crystals, Handbook of Liquid Crystals, 8 Volume Set, Second Edition. [23] Y. C. Dong, Z. Q. Yang, Beyond displays: The recent progress of liquid crystals for bio/chemical detections, Chin Sci Bull, 58 (2013) 2557-2562.
18
ACCEPTED MANUSCRIPT [24] D. Wencel, T. Abel, C. McDonagh, Optical chemical pH sensors, Anal. Chem. 86 (2014) 15-29. [25] S. Coco, P. Espinet, E. Marcos, Mesomorphic trigonal bipyramidal iron(0) isocyanide complexes, J. Mater. Chem. 10 (2000) 1297-1302.
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[26] C.H. Chuang, Y.C. Lin, W.L. Chen, Y.H. Chen, Y.X. Chen, C.M. Chen, H.W. Shiu, L.Y.
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Chang, C.H. Chen, C.H. Chen, Detecting trypsin at liquid crystal/aqueous interface by using
CR
surface-immobilized bovine serum albumin. Biosens. Bioelectron. 78 (2016) 213-220.
US
[27] J.M. Hamilton, J.R. Whitehead, N.J. Williams, M. El Ojaimi, R.P. Thummel, R.D. Hancock, Metal ion complexing properties of dipyridoacridine, a highly preorganized tridentate
AN
homologue of 1,10-phenanthroline, Inorg. Chem. 50 (2011) 3785-3790.
M
[28] J.M. Brake, A.D. Mezera, N.L. Abbott, Effect of surfactant structure on the orientation of
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liquid crystals at aqueous-liquid crystal interfaces. Langmuir 19 (2003) 6436-6442.
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[29] C.H. Chen, Y.C. Lin, H.H. Chang, A.S.Y. Lee, Ligand-doped liquid crystal sensor system
CE
for detecting mercuric ion in aqueous solutions, Anal. Chem. 87 (2015) 4546-4551. [30] R.J. Carlton, J.K. Gupta, C.L. Swift, N.L. Abbott, Influence of simple electrolytes on the
31-36.
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orientational ordering of thermotropic liquid crystals at aqueous interfaces, Langmuir 28 (2012)
[31] U.S. Environmental Protection Agency (EPA). Secondary drinking water standards. Retrieved August 31, 2016 from https://www.epa.gov/dwstandardsregulations/secondarydrinking-water-standards-guidance-nuisance-chemicals [32] Health Canada, Guidelines for Canadian drinking water quality. Retrieved August 31, 2016 from http://www.hc-sc.gc.ca/ewh-semt/pubs/water-eau/sum_guide-res_recom/index-eng.php 19
ACCEPTED MANUSCRIPT [33] Taipei Water Department. International Comparison of Water Quality. Retrieved July 20,
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Scheme 1. The working principle for LC-based pH sensor.
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Scheme 2. Molecular structures of LCs and dopants used in this study.
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Scheme 3. (a) Schematic (side view) and (b) real image (top view) of the LC-based pH sensor
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device for flow system.
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Figure 1. Polarized images of the LC-based sensor using 0.3% of (a) 6CBA, (b) 8OCBA, (c)
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10OCBA and (d) 12OCBA as dopants at different pH values. It was found that the critical pH
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Figure 2. Polarized images of the LC-based sensor using (a) 0.3%, (b) 0.4%, (c) 0.5% and (d)
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0.6% 6CBA as the dopant at different pH values. It was found that the critical pH values for
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dopants.
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optical transition of LC-based sensor can be adjusted through changing the concentrations of
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Figure 3. Polarized images of the LC-based sensor using 0.3% 6CBA as the dopant in the
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aqueous solutions containing (a) 0 mM, (b) 50 mM and (c) 400 mM of NaCl at different pH values. It was found that the critical pH values for optical transition of LC-based sensor lowered
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Figure 4. Polarized images of the device composing of four LC-based sensors using 12OCBA,
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10OCBA, 8OCBA and 6CBA as the dopants (from left to right) at pH (a) 6.8, (b) 7.0, (c) 7.2, (d) 7.6 and (e) 8.2. It was found that the pH level of the aqueous solutions can be determined
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Figure 5. Polarized images of the device composing of four individual LC-based sensors using
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0.3%, 0.4%, 0.5% and 0.6% of 6CBA (from left to right) as the dopants at pH (a) 6.2, (b) 6.4, (c) 6.6, (d) 6.8 and (e) 7.0. It was found that the pH level of the aqueous solutions can be determined
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Figure 6. Real images of the LC-based pH sensor device by continuous injection of aqueous
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solutions with pH (a) 6.5, (b) 7.0, (c) 7.5, (d) 8.0, (e) 8.5, and (f) 6.5 again. In the device, the dopants applied in the sensors (from left to right) were 12OCBA, 10OCBA, 8OCBA and 6CBA,
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Figure 7. Polarized images of the device composing of four individual LC-based sensors using
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6CBA, 8OCBA, 10OCBA and 12OCBA in tap water and pond water. In (a-c), the device was
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injected by tap water (pH 6.9), then the pH of tap water was adjusted to 8.0 and 6.0. In (d-f), the device was injected by pond water (pH 6.7), then the pH of pond water was adjusted to 8.0 and
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Graphical Abstracts
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ACCEPTED MANUSCRIPT Highlights
Continuous Monitoring of pH Level in Flow Aqueous System
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by Using Liquid Crystal-based Sensor Device
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Wei-Long Chen, Tsung Yang Ho, Jhih-Wei Huang and Chih-Hsin Chen*
To whom correspondence should be addressed.
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Phone: +886-2-26215656
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Department of Chemistry, Tamkang University, New Taipei City 25137, Taiwan
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Email: [email protected]
The liquid crystal (LC)-based sensor device was developed for pH determination.
The pH levels were interpreted through the number of bright and dark LC signals.
A portable device was developed to monitor the water safety level (pH 6.5~8.5).
It is suitable for real-time and continuous pH monitoring in a flow aqueous system.
This sensor can be applied in tap water and pond water.
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