High performance optical ratiometric sol–gel-based pH sensor

Sensors and Actuators B 139 (2009) 208–213

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

High performance optical ratiometric sol–gel-based pH sensor D. Wencel ∗ , B.D. MacCraith, C. McDonagh Optical Sensors Laboratory, National Centre for Sensor Research, School of Physical Sciences, Dublin City University, Collins Avenue, Glasnevin, Dublin 9, Ireland

a r t i c l e

i n f o

Article history: Available online 20 January 2009 Keywords: Optical pH sensor Sol–gel Ratiometric measurements

a b s t r a c t We present an optical sol–gel-based pH sensor which compares well with the current state-of-the-art in terms of the combination of resolution, stability, response time and leaching characteristics. The pHsensitive dye used is the fluorescent indicator 8-hydroxy-1,3,6-pyrene trisulfonic acid (HPTS) which has been ion-paired with hexadecyltrimethylammonium bromide (CTAB). The sol–gel matrix is a composite of the precursors 3-glycidoxypropyltrimethoxysilane (GPTMS) and ethyltriethoxysilane (ETEOS) in which the dye is completely physically entrapped with no leaching. A referenced excitation ratiometric sensor detection system is used which capitalises on the dual excitation bands of the dye. The sensor has a dynamic range from pH 5.0 to pH 8.0, a resolution of 0.02 pH units and exhibits excellent reproducibility, reversibility, temporal stability and a short response time of 12 s. © 2009 Elsevier B.V. All rights reserved.

1. Introduction pH is a commonly measured parameter in many applications such as environmental monitoring, bioprocessing and biomedical diagnostics. Optical pH sensing has many advantages over conventional electrochemical techniques for example, immunity to electrical interference, ease of miniaturisation and the possibility of remote sensing [1–3]. Optical fluorescence-based pH sensing is one of the most widely used optical techniques and offers advantages such as high sensitivity and versatility with respect to detection schemes. Despite the inherent sensitivity of fluorescence, unreferenced intensity-based sensing is hampered by effects such as fluctuations in excitation source, detector drift and changes in light path through the sensor film. These effects can be largely overcome by using referenced detection, for example, lifetime-based sensing [4–6] or ratiometric detection [7–10]. Most optical pH sensors consist of a proton-permeable solid matrix in which the pH indicator is encapsulated such that it is accessible to the analyte while being impervious to leaching effects. pH is measured as a function of reversible changes in the fluorescence or lifetime of the indicator, which are often influenced by the matrix–indicator interaction. Polymers have been widely used as immobilisation matrices for pH-sensitive fluorescent indicators [11–14]. However, in recent years, sol–gel materials have emerged as very versatile immobilisation matrices for the optical detection of pH [15–18] and other analytes such as oxygen

∗ Corresponding author. E-mail address: [email protected] (D. Wencel). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.12.066

[19] and carbon dioxide [20]. Inorganic and ORganically MOdified SILicates (ORMOSILs) sol–gel matrices have many advantages over their organic polymer counterparts such as superior chemical and mechanical stability, high optical transparency, material tailorability and versatility with regard to coating and deposition techniques [21]. Both polymer and sol–gel materials facilitate the immobilisation of analyte-sensitive indicators either physically or covalently. The indicator can be covalently bound to the polymer or sol–gel matrix [22,23] which completely eliminates leaching effects. However, often this process involves complex chemistry and the sensor response of covalently bound indicators is sometimes reduced compared to their performance when physically entrapped. It is possible to achieve complete entrapment thereby eliminating leaching using sol–gel matrices because of their unique tailorability. Process parameters such as precursor type, aging times and heat treatment can be adjusted to achieve optimum pH response while eliminating leaching. This approach is used in the work presented here. A wide range of fluorescent indicators have been used in optical pH sensing, for example, fluorescein and related dyes [24,25], ruthenium complexes [15,26], SNARF [27], SNAFL [28], Rhodamine 6G [29] and HPTS [30–32]. In this paper, we present an optical sol–gel-based pH sensor, which is based on ratiometric detection of the pH-dependent fluorescence of HPTS, which has been ion-paired with CTAB, (HPTSIP, Fig. 1) and which has been physically entrapped in a novel hybrid sol–gel film. The microstructure of the film, which is a composite of the precursors 3-glycidoxypropyltrimethoxysilane and ethyltriethoxysilane (GPTMS–ETEOS), has been tailored to completely encapsulate the dye thereby eliminating leaching. The polar GPTMS precursor provides hydrophilic matrix which promotes proton permeability while ETEOS precursor improves the adhesion

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Fig. 1. Chemical structure of HPTS-IP.

and mechanical stability of the resulting sol–gel-derived layers. The sensor displays a reproducible and reversible response, a resolution of 0.02 pH units and a response time of <12 s. The response is stable for >1 month. 2. Materials and methods 2.1. Chemical reagents and materials The sol–gel precursors ethyltriethoxysilane (ETEOS), (3-glycidoxypropyl)trimethoxysilane (GPTMS), 1-methylimidazole (MI) and potassium chloride were purchased from Fluka (Ireland). The pH indicator 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS), 0.1N hydrochloric acid (HCl), hexadecyltrimethylammonium bromide (CTAB), hydrogen phosphate (K2 HPO4 ) and dihydrogen phosphate (KH2 PO4 ) were purchased from Aldrich Chemicals (Ireland). Absolute ethanol (EtOH), standard buffer solutions (pH 4.0, 7.0 and 10.0) for pH meter calibration and glass slides were purchased from VWR International (Ireland). Polystyrene bottomless 96-well microplates were obtained from Cruinn Ltd (Ireland). All chemicals were of analytical grade and used without further purification. Aqueous solutions were prepared from deionised (DI) water. 10 mM phosphate buffer solutions of defined pH and ionic strength (IS) were prepared from K2 HPO4 and KH2 PO4 with potassium chloride as the background electrolyte. 2.2. Synthesis of HPTS-IP The ion pair was synthesised by dissolving 0.76 mmol of CTAB in 25 ml of DI water at 50 ◦ C. Subsequently, 0.38 mmol of HPTS which had been previously dissolved in 25 ml of DI water was added to the CTAB solution. The precipitate of ion pair (HPTS-IP) was subsequently filtered and dried in the oven at 70 ◦ C for 12 h. 2.3. Fabrication of pH sensor films The sensor films were prepared from a mixture of ETEOS- and GPTMS-derived sols combined in 1:1 molar ratio. The ETEOS-based sol was prepared by mixing ETEOS, 0.1 M aqueous HCl and EtOH in a 1:0.007:6.25 molar ratio. The GPTMS-based sol was prepared by mixing GPTMS, MI, DI water and EtOH in 1:0.69:4:6.25 molar ratio. The GPTMS–ETEOS hybrid sol was prepared by mixing the two separate sols in equal molar ratios. The HPTS-IP doped solutions were fabricated by mixing an ethanolic solution of HPTS-IP with the prepared hybrid sol to give a final silane/dye ratio of 10−3 . The final mixture was aged for 72 h under ambient conditions. Sensor films were fabricated by dip-coating onto glass slides using a dip-speed of 3 mms−1 in a controlled environment using a computer-controlled dipping apparatus resulting in a sensor layer thickness of 1 ␮m. The glass slides were first treated with 30% nitric

acid for 24 h and then rinsed with copious amount of DI water and EtOH. After deposition, the films were cured at 140 ◦ C for 4 h. All sensor films were very uniform and crack-free. Samples were prepared in sextuplicate. All films were stored in the dark under ambient conditions or in buffer solutions at pH 7.0 for the temporal stability studies. All experiments were performed at room temperature. 2.4. Absorbance and fluorescence measurements Absorption spectra were recorded using a two-channel UV–Vis spectrophotometer (Cary® 50, Varian, USA) equipped with a xenon flash lamp as a light source. Fluorescence emission spectra were acquired using a FluoroMax-2 spectrofluorometer (Jobin Yvon, USA) using a continuous wave 150 W xenon lamp as light source. Sensor films dip-coated on glass slides were mounted in a home-made glass flow cell, into which buffer solutions at varying pH were pumped using a Minipuls-3 peristaltic pump (Gilson, France) at a rate of 8 ml min−1 . For ionic strength experiments a Safire II microplate reader from Tecan Systems Inc., Austria was employed. 2.5. Other characterisation techniques A digital Orion Benchtop 420 A+ pH meter was used to measure pH values. This was calibrated with three standard buffers of pH 4.00, 7.00 and 10.00 at room temperature. Film thickness measurements were obtained using a white light interferometer (WYCO N1100 Optical Profiling System, Veeco, USA). 3. Results and discussion 3.1. Optical properties of HPTS-IP entrapped in a sol–gel-derived matrix HPTS is a very photostable, highly fluorescent pH indicator with a pKa of ∼7.30. In this study HPTS-IP was used as it is more hydrophobic than HPTS and displays poor water solubility. By using such a dye, one can expect minimised leaching and improved sensor film stability. It exhibits two different pH-dependent excitation bands, corresponding to the protonated (acidic, 405 nm) form and the deprotonated (basic, 460 nm) form, and a single emission band (515 nm). The presence of the dual excitation bands facilitates the use of referenced, excitation ratiometric detection. Its pKa of ∼7.4 makes it suitable for pH determination in the physiological pH range. In addition, large Stokes shift and compatibility with the blue LED excitation make it a very attractive pH indicator for practical applications. Fig. 2 shows the pH-dependent excitation and emission spectra of the ion-paired indicator, HPTS-IP in the hybrid ETEOS-GPTMS sol–gel matrix, measured over the pH range 4.0–10.0. The spectra were recorded using 10 mM phosphate buffer solutions of IS = 150 mM at room temperature. It can be seen from Fig. 2(a) that

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Fig. 3. Calibration plot for a 5 pH sensor elements. IS of buffer solutions = 150 mM, temperature = 25 ◦ C.

is the relevant range for bioprocessing and for clinical applications. Fig. 4 shows the response of the sensor over 7 measurement cycles, demonstrating the reversibility of the response. The sensor resolution, measured under the conditions of IS = 150 mM and T = 25 ◦ C, was calculated to be 0.02 pH units in the pH range from 5.0 to 8.0. This value compares well with literature reports [35,36].

Fig. 2. Normalised (a) excitation spectra (em = 515 nm) and (b) emission spectra (exc = 460 nm) of pH sensor film at various pH values. IS of a buffer solution = 150 mM, room temperature.

the entrapped HPTS-IP exhibits 2 pH-dependent excitation bands, one at 405 nm (acidic form) and one at 460 nm (basic form) while the emission band, as seen in Fig. 2(b), occurs at 515 nm. For ratiometric detection, the excitation intensity ratio (R) is defined here as the emission intensity at 515 nm with 460 nm excitation divided by the emission intensity with 405 nm excitation (I460 nm /I405 nm ). The apparent pKa (pKa ) value for the indicator in the sol–gel matrix, calculated from the data in Fig. 3 was measured to be 6.28 ± 0.02. This is significantly lower than the pKa measured in solution (pKa = 7.3). This shift is attributed to the influence of the matrix on the pH response of the indicator [33,34] and not to the presence of the IP as the pKa of HPTS in sol–gel matrix was measured by us to be similar to that of HPTS-IP (data not shown).

3.2.2. Response time We define the sensor response time as so-called t90 time, the time taken for the intensity to achieve 90% of the final value when the pH is changed from pH 5.0 to pH 7.0. These pH values were chosen as they are on either side of the pKa value of 6.28. The response time is dependent on film thickness and the flow rate/injection time of the buffer solutions. In order to eliminate the fill time of the flow cell, buffer solutions at pH 5.0 and 7.0 were injected directly into the flow cell through a short section of tubing. A typical response under these conditions for films of thickness ∼1 ␮m is shown in Fig. 5 from which data a response time of 12 s was measured. The response time could be further improved by using thinner sol–gel films. The choice of film thickness usually represents a compromise between signal-to-noise ratio and response time. The signal-to-noise ratio in our experiments is sufficiently high to facilitate thinner films and hence achieve shorter response times.

3.2. pH sensor performance 3.2.1. Reproducibility and reversibility The pH-dependent ratiometric response for 5 different films, over the pH range 3.0–10.0, is shown in Fig. 3. The calculated standard deviation of pKa for these films was 0.02. It is clear from the data in Fig. 3 that the error bars are very small (<0.02) which indicates the high level of reproducibility in pH response for these films. The films respond to pH over the range pH 4.0–9.0 with the most sensitive dynamic range occurring between pH 5.0 and 8.0 which

Fig. 4. Reversibility of the pH sensor films. IS of buffer solutions = 150 mM.

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of the dye leached from the TEOS matrix in 24 h (data not shown). Other authors have reported complete loss of dye within 12–48 h from a TEOS matrix. [37]. The complete encapsulation of the dye in our films is partly attributed to the decreased water solubility of the HPTS-IP compared to that of the pure dye. However, it is considered that the predominant effect is the dense sol–gel network resulting from the inorganic-organic polymerisation and cross-linking which occur in the GPTMS–ETEOS matrix and form interconnected epoxy and silica network compared to the somewhat less dense, more open structure formed by pure organically modified alkoxides such as ETEOS [38].

Fig. 5. Real time response of pH sensor films. IS of buffer solutions = 150 mM.

3.2.3. Dye leaching Dye leaching, while not an issue in the case of covalently bound indicators is a significant problem for pH sensors where the dye is physically entrapped either in polymer or sol–gel matrices. In this investigation, two different types of leaching experiment were carried out. In each case, the experiment was done under conditions where the more soluble deprotonated form of the dye was dominant. In the first experiment, the films were immersed in pH 7.0 buffer solutions (IS = 150 mM) for 1 month. Optical properties of the films and of the phosphate buffer solutions were monitored weekly. There was no change in optical absorption of the films over the 1month period and there was no dye fluorescence detected in the buffer solution. In the second experiment, pH 7.0 buffer solutions were pumped for 24 h with a flow rate of 1 ml min−1 through the flow cell containing the film. The fluorescence intensity of the film was recorded at time intervals of 1 h, 3 h, 6 h and 24 h. The results of this experiment are shown in Fig. 6. Clearly, no leaching effects are observed and the dye is completely physically entrapped in the GPTMS–ETEOS hybrid sol–gel matrix. In contrast to these results, significant leaching was observed by us for the HPTS-IP in other sol–gel matrices such as pure ETEOS and TEOS. For example, ∼40%

Fig. 6. Fluorescence intensity of the pH sensor film recorded after 0 h, 3 h, 6 h and 24 h in phosphate buffer. IS of buffer solution = 150 mM, room temperature.

3.2.4. Temporal stability In order to test sensor temporal stability two pH sensor elements were stored for 1 month under two different conditions, one in ambient conditions in the dark (in air) and the other in phosphate buffer solution at pH 7.0. The sensor response was monitored weekly and, after 1 month, the standard deviation of the pKa value was 0.01 for the sensor stored in air and 0.04 for the film stored in buffer (results not shown). This indicates that there was no significant pKa change over the 1-month period and that pH sensor films display very good temporal stability when stored both in dry and aqueous conditions. 3.2.5. Photostability Of the dyes typically used for fluorescence-based pH sensing, HPTS is regarded as being one of the most photostable, compared, for example, to fluorescein which undergoes considerable photobleaching under conditions of continuous illumination [39]. The photostability of the sensor films developed here was tested in pH 10.0 as at this pH HPTS-IP exists only in the deprotonated form yielding maximum fluorescence signal. The film was placed in a quartz cuvette filled with buffer solution and was continuously illuminated with exc = 460 nm using a 150 W xenon lamp. After 1 h of illumination the fluorescence intensity decreased by 3%. However, the ratiometric signal changed insignificantly (less than 1%) after the photostability experiment which demonstrates the benefits of using ratiometric detection. 3.2.6. Interferences Sensitivity to IS is a common issue in the case of optical pH sensors, because it results in a shift of pKa values and therefore causes errors in pH determination [40]. This cross-sensitivity is due to the measurement of the concentration of the hydrogen ions, where the pH value is related to their activity. In this study, the influence of IS was investigated by monitoring the sensor response at different ionic strengths. The films were attached to the bottomless plates of a microplate reader. 10 mM phosphate buffers at IS of 50 mM, 100 mM, 150 mM, 200 mM and 300 mM were used and potassium chloride was employed as a background electrolyte. Fig. 7 clearly shows that there is a dependence of the pH response on the IS. The largest variation, when IS is varied from 50 mM to 300 mM, yields a pH error of 0.5 pH units. A change from 150 mM to 200 mM (relevant in bioprocess monitoring) yielded a maximum pH error of 0.15 pH units. The impact of this interference on pH performance is dependent on the application i.e. on the variation of IS in the sample. Such a response would have to be corrected, for example, by carrying out simultaneous IS measurements using conductivity techniques. However, it still would give only approximate values of the IS. Effect of temperature on pH response was also investigated. A negligible temperature effect was measured when temperature was varied between 25 ◦ C and 42 ◦ C. A pH error of 0.06 pH units (averaged over 5 independent measurements) was measured for a temperature change between 25 ◦ C and 37 ◦ C and the response shifted by 0.11 pH units when temperature was changed from 25 ◦ C

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Fig. 7. Influence of IS on sensor response at temperature = 25 ◦ C.

to 42 ◦ C. Therefore, small temperature changes should have a negligible effect on pH response. 4. Conclusions A high performance optical ratiometric, sol–gel-based pH sensor has been developed which, when a combination of key performance parameters is considered, compares very well to similar sensor systems which have been reported in the literature. The sensor is based on the ratiometric detection of the pH-dependent fluorescence of HPTS which has been ion-paired with CTAB to reduce its solubility in water and hence minimise dye leaching. Furthermore, the indicator has been completely physically encapsulated in a novel hybrid GPTMS–ETEOS sol–gel matrix which, due to the degree of organic–inorganic polymerisation and cross-linking, results in a dense microstructure which eliminates dye leaching while allowing ingress of H+ ions. The sensor is reproducible and reversible, has a resolution of 0.02 pH units in the pH range from 5.0 to 8.0, a response time of 12 s for a film thickness of 1 ␮m and displays long-term stability of at least 1 month. Self-referenced ratiometric detection ensures that the sensor is immune to drifts such as photobleaching effects. The dynamic range of the sensor is from pH 5.0 to 8.0 and the dye is completely encapsulated in the sol–gel matrix. The dynamic range and performance of the sensor are compatible with a range of applications such as bio-processing and clinical measurements. While many reported optical pH sensor studies present optimum sensor performance for selected parameters, this study reports high performance in all of the key sensor parameters, namely, reproducibility, resolution, stability, response time and leaching characteristics. Acknowledgement The authors wish to acknowledge funding from the European Commission for this work under Project IPS-2000-0079 MAST. References [1] O.S. Wolfbeis (Ed.), Fiber Optic Chemical Sensors and Biosensors, vol. 1, CRC Press, Boca Raton, 1991, pp. 1–23 (Chapter 1). [2] M.J.P. Leiner, Luminescence chemical sensors for biomedical applications – scope and limitations, Anal. Chim. Acta 255 (1991) 209–222. [3] J. Lin, Recent development and applications of optical and fiber-optic pH sensors, Trends Anal. Chem. 19 (2000) 541–552. [4] S.B. Bambot, J. Sipior, J.R. Lakowicz, G. Rao, Lifetime-based optical sensing of pH using resonance energy-transfer in sol–gel films, Sens. Actuators B: Chem. 22 (1994) 181–188.

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Biographies Dorota Wencel received her M.Sc. degree by research in analytical chemistry from Warsaw University in 2001 with a thesis entitled “Prussian blue-based optical glucose biosensor in flow-injection analysis”. In 2004 she embarked on a Ph.D. programme in the Optical Sensors Laboratory at Dublin City University under the supervision of Prof. Colette McDonagh. She is currently finishing her Ph.D. with a thesis entitled “Development of sol–gel-derived optical oxygen, pH and dissolved carbon dioxide sensors”. Her work involved the development, optimisation and characterisation of novel fluorometric optical sensors for industry and environmental applications.

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Brian D. MacCraith is Director of the Biomedical Diagnostics Institute (BDI) at DCU. The BDI is a Science Foundation Ireland (SFI) Centre for Science, Engineering & Technology (CSET) focused on developing the underpinning science leading to next-generation biomedical diagnostics. Established in October 2005, the BDI is an academic-industry partnership involving 6 industrial and 4 academic partners and is funded for 5 years in the first instance. The funding awarded to the BDI includes over D 6m from its industry partners and D 16.5m from SFI. Prof. MacCraith was founding Director of the National Centre for Sensor Research at Dublin City University (DCU) and held this position from its establishment in October 1999 until the establishment of the BDI. Currently, the NCSR comprises over 200 full-time researchers working on the fundamental science and applications of chemical sensors and biosensors. With a strong track record and international reputation in the field of optical chemical sensors and biosensors, Prof. MacCraith has published widely (over 150 publications) on these topics as well as developing significant Intellectual Property. Colette McDonagh studied undergraduate physics at the National University of Ireland in Galway and was awarded a Ph.D. in Physics from Trinity College, Dublin in 1980. After postdoctoral work at Trinity College and at the Department of Applied Science at the University of California, Davis, she took up an academic position in the School of Physical Sciences at Dublin City University. She currently holds the position of Associate Professor and her research interests include development of sol–gel-based optical sensors for environmental monitoring, luminescence-based optical biosensors and development of strategies for luminescence enhancement in biochips including metal-enhanced luminescence and high-brightness nanoparticles.