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Selective fluorescence sensors for p-phenylenediamine using formyl boronate ester with an assistance of micelles
Sensors and Actuators B 173 (2012) 682–691
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Selective fluorescence sensors for p-phenylenediamine using formyl boronate ester with an assistance of micelles Kessarin Ngamdee a , Surangkhana Martwiset a , Thawatchai Tuntulani d , Wittaya Ngeontae a,b,c,∗ a Materials Chemistry Research Unit, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand b Research Center for Environmental and Hazardous Substance Management, Khon Kaen University, Khon Kaen 40002, Thailand c Center of Excellence for Environmental and Hazardous Waste Management (EHWM), Bangkok 10330, Thailand d Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e
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Article history: Received 29 May 2012 Received in revised form 15 July 2012 Accepted 17 July 2012 Available online 27 July 2012 Keywords: p-Phenylenediamine Micelle Fluorescence sensors Chemical sensors
a b s t r a c t A selective fluorometric approach for detection of p-phenylenediamine (PPD) was fabricated using the alizarin–boronic acid adduct with an assistance of micelles. When Alizarin Red S (ARS) reacted with 4formylphenylboronic acid (4-FPBA) and yielded the boronate ester adduct (ARS/4-FPBA), the fluorescence emission turned on. However, upon the addition of PPD, the fluorescence intensity of the ARS/4-FPBA adduct decreased as a linear function of PPD concentrations. Enhancements in sensitivity and selectivity of this fluorescence sensor were studied by incorporating the sensor molecule into the hydrophobic core of micelles. The results showed significant improvement in sensitivity and selectivity comparing to the sensor in the buffer solution. Parameters affecting the fluorescence quenching of boronate ester adducts by PPD such as solution pH, 4-FPBA concentration, types of surfactants and surfactant concentrations were investigated. The optimum conditions for the determination of PPD was 2.0 mM cetyltrimethylammonium bromide (CTAB) in 50 mM phosphate buffer solution pH 7.0. The fluorescence intensity of the ARS/4-FPBA adduct in the presence of CTAB was remarkably 11 times as high as that in the buffer solution. The linear working concentration range was found to be 0.03–0.40 mM and the calibration sensitivity was 48 times higher than that from the system containing only buffer solution. Moreover, PPD exhibited the highest calibration sensitivity among the studied primary amines, confirming that the proposed sensor provided high sensitivity and good selectivity for PPD. The proposed sensor was used to determine PPD in spiked water samples satisfactorily. © 2012 Elsevier B.V. All rights reserved.
1. Introduction p-Phenylenediamine (PPD) is widely used in industries as a precursor to common hair dyes and aramid plastics and fibers such as Kevlar. A number of allergic reactions such as acute inflammatory reactions, eczematous hypersensitivity reactions, photoaggravated reactions, granulomatous reactions, lichenoid reactions and pseudolymphomatous reactions were caused by PPD [1–6]. PPD detection was carried out using many analytical methods including UV spectrophotometric detection [7], gas chromatography (GC) [8] or hyphenated techniques such as gas chromatography–mass spectrometry (GC–MS) [4,6,9–12], high performance liquid chromatography (HPLC) [5,13–16], capillary
∗ Corresponding author at: Materials Chemistry Research Unit, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand. Tel.: +66 432 02222x12243; fax: +66 432 02373. E-mail address: [email protected] (W. Ngeontae). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.07.077
isotachophoresis (CE) [17,18] and micellar electrokinetic capillary chromatography (MEKC) [16,19,20]. However, such techniques have some limitations, for example, the restrictions of hydrophilic substances determination in GC, and the need of using 1,8diaminooctane or triethylamine as an amine modifier and sodium heptane sulfonate as counter ion to solve asymmetric and tailing peaks in the mobile phase of HPLC [15]. Therefore, suitable methods for determination of PPD are still under development to improve the aforementioned problems and the robustness of determination methods. Fluorescence sensing is a promising approach due to the advantages of high sensitivity, ease of operation, high stability, continuous monitoring and easy manufacturing [21]. Boronic acid derivatives play a very important role in current researches in chemical sensors due to a specific reaction towards some interested species [22–25]. Among boronic acid adducts employed in chemical sensing approaches, Alizarin Red S (ARS)–boronic acid adduct is one of the most interesting molecules for fabricating fluorescence sensors. Mostly, this adduct has been used to fabricate sensors for the determination of cis-diol compounds such as glucose [26–28]. Commercially available phenyl
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Scheme 1. The reaction between boronic acid derivatives and ARS for PPD sensing.
boronic acid derivatives with formyl group can be chosen as a potential amine sensing molecule due to the well known reaction with primary amines yielding imines or Schiff bases [29]. However, the reaction in water is undesirable [30]. Recently, our group successfully enhanced the selectivity and sensitivity of the glucose sensor using surfactants [31–33]. Therefore, we expected that with an assistance of surfactants, imine formation in water would be eligible. The aim of this study was to develop a new fluorescence sensor using alizarin–boronic acid adduct in aqueous micelles for the determination of PPD. The 2-step mechanism of the proposed PPD sensor was shown in Scheme 1. In the first step, fluorescence was turned on by mixing ARS with 4formylphenylboronic acid (4-FPBA) to provide the boronate ester. When the boronate ester was formed, the fluorescence emission of ARS was observed. Upon the addition of PPD to the ester solution, a decrease in fluorescence intensity would be expected. Moreover, parameters that were likely to affect the quenching efficiency such as surfactant types, surfactant concentrations, types of boronic acid, boronic acid concentration, and solution pH were investigated. 2. Experimental 2.1. Reagents All chemicals were of analytical grade and used without further purification. Alizarin Red S (ARS), p-tolylboronic acid (TBA), 3-bromophenylboronic acid (BrPBA), 4-formylphenylboronic acid (4-FPBA), and 1,3-phenylenediamine were obtained from Aldrich. Sodium dodecyl sulphate (SDS) was purchased from BDH. Ethanolamine was received from Carlo Erba. 1,4-Phenylenediamine (PPD), 2-formylphenyl boronic acid (2-FPBA), phenylboronic
acid (PBA), melamine, ethylenediamine, n-butylamine, diethylenetriamine, phenylamine, benzylamine, 1,2-phenylenediamine, triton X-114, cetyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DTAB) and trimethyltetradecylammonium bromide (TTAB) were received from Fluka. Potassium dihydrogen phosphate was obtained from Scharlau. Dipotassium hydrogen phosphate was purchased from UNIVAR. All aqueous solutions were prepared from deionized water (DI) with the specific resistivity of 18.2 M cm from RiOs TM Type I Simplicity 185 (Millipore water). 2.2. Instrumentation Emission spectra were recorded using a RF-5301PC spectrofluorometer (Shimadzu). Excitation and emission spectra were measured using the slit width of 5 mm. The pH of solution was measured using UB-10 UltraBasic pH meter (Denver Instrument). 2.3. General procedures 2.3.1. Fluorescence measurements The fluorescence quenching of the ARS/boronic acid adduct by PPD was performed in the presence and in the absence of surfactants. To a 10.00 mL volumetric flask, 400 L of 0.5 mM ARS, 500 L of 1.0 mM of boronic acid derivatives and 500 L of 1.0 M phosphate buffer solution (PBS) pH 7.0 were sequentially added. In the study that required a surfactant, 1.00 mL of the stock solution of the surfactant was then introduced. An appropriate volume of 5.0 mM PPD was added into the mixture to make the desired concentrations. The mixture was diluted to the mark with DI water. After standing at room temperature for 15 min, emission spectra were recorded at a particular wavelength: ex = 460 nm for 2-FPBA and ex = 475 nm for other boronic acid derivatives.
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2.3.2. Effect of types and tail length of surfactants Cationic, anionic and nonionic surfactants were used to study the effect of surfactant types. The effect of surfactant tail length was investigated by using three cationic surfactants: CTAB, DTAB and TTAB. The concentrations of ARS and 4-FPBA were 0.05 mM and 0.10 mM, respectively. The concentration of each surfactant was twice its critical micelle concentration (cmc): 0.5 mM for Triton X-114, 16.4 mM for SDS, 7.4 mM for TTAB, 32.0 mM for DTAB and 2.0 mM for CTAB [34]. 2.3.3. Effect of CTAB concentrations Concentrations of ARS and 4-FPBA were 0.02 mM and 0.05 mM, respectively. The final concentrations of CTAB were varied from 0.5 to 5.0 mM in the absence and in the presence of 0.25 mM PPD. 2.3.4. Effect of types of boronic acids Five boronic acid-derivatives, PBA, TBA, BrPBA, 2-FPBA, and 4FPBA were used. Final concentration of boronic acids and ARS were 0.05 and 0.10 mM, respectively. Each type of boronic acids was prepared in the absence and in the presence of 2.0 mM CTAB, following by addition of 500 L of 5.0 mM PPD. 2.3.5. Effect of the concentration of boronic acid The 0.02 mM ARS solution was mixed with appropriated volume of stock 4-FPBA to make the final concentration in the range of 0.001–0.10 mM. The fluorescence quenching was investigated in the absence and in the presence of 2.0 mM CTAB. 2.3.6. Effect of pH The final concentrations of ARS and 4-FPBA were 0.02 and 0.05 mM, respectively, in the absence and in the presence of 2.0 mM CTAB. Solution pHs were controlled by varying the pHs of the buffer. Acetic-acetate buffer was used for pH 4.0, and phosphate buffer was used for the rest. 2.3.7. Selectivity of the PPD sensor In this study, other amines including butylamine, ethanolamine, ethylenediamine, diethylenetriamine, phenylamine (aniline), benzylamine, melamine, 1,2-phenylenediamine, and 1,3-phenylenediamine were also studied as a substitute of PPD. Final concentrations of ARS and 4-FPBA were 0.02 and 0.05 mM, respectively. The fluorometric titrations of PPD sensor with each amine were performed in the absence and in the presence of 2.0 mM CTAB. 2.3.8. Calibration curve To a 10-mL volumetric flask, a solution of the following components, 0.02 mM ARS, 0.05 mM FPBA solution, and 2.0 mM CTAB was prepared. The solution pHs were controlled by 50 mM phosphate buffer solution pH 7.0. Appropriate volumes of 5.0 mM PPD standard were added into the mixture to make final concentrations ranging from 0 to 0.80 mM. After mixing for 15 min, the fluorescence spectra were recorded. 2.3.9. Application to real water samples In order to demonstrate the applicability of the proposed sensor, the treated waste water, natural pond water and natural river water were used as representative environmentally contaminated samples. The water samples were collected from three sources: Khon Kaen University waste water treatment pond, local pond water, and local river water. All water samples were used immediately without storage. The water samples were filtered through filter paper. The accuracy and precision of each measurement were evaluated by spiking 3 different concentrations of PPD (0.10, 0.20 and 0.30 mM) into the samples under the optimized condition.
3. Results and discussion 3.1. Emission and absorption spectra To study sensing properties of ARS/4-FPBA for the determination of PPD, emission characteristics of ARS and ARS/4-FPBA before and after adding PPD were studied using the excitation wavelength at 475 nm. The emission spectra were recorded in two systems; phosphate buffer solution pH 7.0 in the absence (buffer system) and in the presence of CTAB (micelle system). The fluorescence spectra were shown in Fig. 1(A) and (B) for buffer and micelle systems, respectively. In both systems, ARS did not fluoresce; however, upon the addition of 4-FPBA, the emission maxima appeared at the wavelength of 566 and 556 nm for solutions without and with CTAB, respectively. This is due to a reversible ester formation between ARS and 4-FPBA. The observed blue shift of emission maxima from 566 to 556 nm may be due to the aggregation/disaggregation of dye molecules [33]. Moreover, the fluorescence intensity of the ARS/4FPBA adduct in the micelle system was 11 times as great as that in the buffer system. The increase in fluorescence intensity is probably due to the assistance of the surfactant in protecting the excited state of the adduct from collision with solvent molecules [33]. Upon the addition of PPD, fluorescence intensity of the adduct decreased because of the formation of Schiff base compound from a reaction between amino group in PPD and aldehyde moiety of 4-FPBA [29]. The remaining of ordinary pattern of the position and shape of the spectrum suggested that the Schiff base molecule is not fluorescent active. In addition, a dramatic decrease in emission properties of the adducts upon an addition of PPD was observed in the micelle system. In the buffer system, fluorescence intensity of the adduct slightly decreased upon adding PPD. It implied that PPD and the adduct could dissolve in the micelle core, giving a better imine formation compared to the reaction in aqueous solution. In the presence of micelle, PPD and the adduct may be incorporated in the hydrophobic core of micelle due to the hydrophobicity of both species. The incorporation may also enhance the rate of the reaction between the adduct and PPD due to relatively concentrated sensoring molecules inside micelles [32]. To understand the optical properties of the recognition and sensoring molecule, ARS/4-FPBA adduct was examined by measuring the absorption spectra before and after adding PPD. The absorption spectra of ARS/4-FPBA in the presence of various PPD concentrations were shown in Fig. 2(A) and (B) for the buffer system and the micelle system, respectively. In both systems, a linear increase in absorption at the wavelength of 370 nm was observed when PPD was introduced, with a higher value in micelle system. The increasing of absorption at 370 nm in micelle system indicated that PPD could form Schiff base with aldehyde group of 4-FPBA. The inset showed the plot between the concentration of PPD and the increasing of absorption peak at 370 nm. The slope of the plot between absorbance at 370 nm and concentration of PPD in micelle system was approximately six times higher than that in the buffer system, suggesting a preference of imine formation between PPD and ARS/4-FPBA. 3.2. Types of surfactants To obtain a suitable surfactant system, three types of surfactants, i.e. non ionic (Triton X-114), anionic (SDS), and cationic (CTAB), were examined. Two additional cationic surfactants, DTAB and TTAB, were used to study the surfactant tail length effect on the emission characteristic of the sensing molecule. The final concentration of each surfactant was twice of critical micelle concentration [33]. Fluorescence intensities of the adduct in the absence (F0 ) and
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Fig. 1. Emission spectra (ex = 475 nm) of ARS (····), ARS/4-FPBA (—) and ARS/4-FPBA in the presence of 0.30 mM PPD (- - - -) in 50 mM PBS pH 7.0 (A) and 50 mM PBS, pH 7.0 with 2.0 mM CTAB solution (B). Concentrations: ARS 0.02 mM, 4-FPBA 0.05 mM.
in the presence of 0.25 mM PPD (F) in different micelle systems were shown in Fig. 3(A). The differences in fluorescence intensities of the adduct before and after adding PPD (F0 − F) were shown in Fig. 3(B). A small increase in fluorescence intensity of the adduct was observed in the presence of non ionic and anionic surfactants, while a dramatic increase was seen upon the addition of cationic surfactants. The negative charge of sulfonate group in the adduct favorably interacts with positive head of cationic surfactant via electrostatic interactions. In the tail length study, all three cationic surfactants, CTAB, DTAB and TTAB, provided the enhancement in the fluorescence intensity of the adduct. The maximum fluorescence quenching of the adduct in the presence of PPD was obtained when CTAB was used. Moreover, CTAB has the lowest cmc among the three cationic surfactants. Therefore, CTAB was chosen to investigate other parameters. 3.3. Concentrations of CTAB To study the effect of CTAB concentrations, the fluorescence intensities of the adduct in the absence and in the presence of 0.25 mM PPD were studied using CTAB concentrations between 0 and 5.0 mM. The fluorescence intensities of the adduct before and after adding PPD in the presence of different CTAB concentraitons were shown in Fig. 4(A). The fluorescence intensities of the adduct increased with increasing CTAB concentration when CTAB concentrations were in the range of 0.5–2.0 mM, and remained the same at the concentrations above 2.0 mM. In addition, differences in fluorescence intensities of the adduct before and after adding PPD (F0 − F) were shown in Fig. 4(B). The fluorescence quenching (F0 − F) after adding PPD increased with the increment of CTAB concentration from 0.5 to 2.0 mM, while decreased at the concentrations above 2.0 mM. The result suggested that the number of micelle spheres increased with increasing CTAB concentration. When the concentration of CTAB was greater than 2.0 mM, there
would be micelle spheres that were not occupied by the sensing molecules; thus, PPD that was introduced to the system would be incorporated and protected in those empty micelles. Therefore, the fluorescence quenching was decreased with increasing CTAB concentration because PPD could not react with ARS/4-FPBA. Since highest fluorescence quenching ability was observed at CTAB concentrations of 1.0 and 2.0 mM, the concentration of 2.0 mM was chosen to study other parameters. 3.4. Types of boronic acids Five derivatives of phenylboronic acids with different substituted groups, i.e. PBA, TBA, BrPBA, 2-FPBA, and 4-FPBA, were used to study the effect of boronic acid types on fluorescence emission. Fluorescence intensities of ARS/boronic acids adducts before and after adding PPD in buffer and micelle systems were recorded, and shown in Fig. 5(A) and (B). The result showed that ARS/boronic acids could emit fluorescence, suggesting the adduct formation of ARS with all boronic acids. The boronate esters from ARS and phenyl boronic acids with electron withdrawing groups, i.e. BrPBA, 2-FPBA, and 4-FPBA, showed higher fluorescence intensities than that from ARS and phenylboronic acids with electron donating group, TBA. Moreover, the fluorescence intensities of ARS/boronic acids in the presence of CTAB provided higher intensities than that in the absence of CTAB. The significant fluorescence quenching by adding PPD was obtained when the phenylboronic acids contained only formyl moiety. However, the quenching abilities of PPD on the adducts from two formyl phenylboronic acids, i.e. 2-FPBA and 4-FPBA, in buffer and micelle systems were different. The addition of PPD resulted in a decrease in fluorescence intensity of the ARS/2-FPBA adduct whether CTAB was present or not, while PPD could quench the ARS/4-FPBA adduct effectively in the micelle system only. Moreover, the highest fluorescence quenching of the adduct was observed when 4-FPBA was used, which may be due to the less steric hindrance of the
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Fig. 2. Absorption spectra of (····) 2.0 mM CTAB, (- - - -) 0.20 mM PPD and (—) colorimetric titration of ARS/4-FPBA with 0, 0.10, 0.20, 0.30, 0.40 and 0.50 mM PPD, respectively. All solutions were prepared in 50 mM PBS pH 7.0. Inset: plots of the absorbance of ARS/4-FPBA (at 370 nm) against PPD concentrations in buffer (A) and micelle system (B). Concentrations: ARS 0.02 mM, 4-FPBA 0.05 mM.
formation of Schiff base at the para position of ARS/4-FPBA compared to the ortho position of ARS/2-FPBA. Therefore, 4-FPBA was chosen to fabricate the PPD sensor in further studies. 3.5. Concentrations of boronic acid The optimization of 4-FPBA concentration was studied by fixing the final concentration of ARS at 0.02 mM and varying the concentration of 4-FPBA from 0.001 to 0.20 mM. Fluorescence intensities of the resulting adducts before and after adding PPD were recorded and shown in Fig. 6(A) and (B) for buffer and micelle systems, respectively. The differences in intensities of the adduct before and after the addtion of PPD (F0 − F) were shown in Fig. 6(C). In both systems, the fluorescence intensities of the adduct increased with increasing the concentration of 4-FPBA. However, the fluorescence quenching abilities of PPD in the buffer and in the micelle systems were different. In the buffer system, upon PPD
addition, a small change in fluorescence intensities was observed with increasing of 4-FPBA concentration. In micelles, fluorescence quenching increased with the increment of 4-FPBA concentration in the range of 0.001–0.10 mM. The different behavior may stem from the limit of imine formation in the buffer solution. When the concentrations of 4-FPBA were greater than 0.10 mM, the quenching ability remained constant. Therefore, 4-FPBA concentration of 0.10 mM was chosen as the optimum concentration to fabricate the PPD sensor. 3.6. Effect of pH Since ARS is a pH indicator dye, the effect of pH, in the range of 4.0–10.0, on the fluorescence intensity of the ARS/4-FPBA adduct was investigated. The fluorescence intensities of the adduct in different pHs before and after adding PPD were shown in Fig. 7(A) and (B) for buffer and micelle systems, respectively.
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Fig. 3. The effect of surfactant types on the fluorescence intensity of ARS/4-FPBA in the absence and in the presence of 0.25 mM PPD in 100 mM PBS pH 7.0 (A). Fluorescence quenching of ARS/4-FPBA by 0.25 mM PPD in buffer and surfactant systems (B). Concentrations: ARS 0.05 mM, 4-FPBA 0.10 mM.
Fig. 4. The effect of CTAB concentration on fluorescence intensity of ARS/4-FPBA in the absence and in the presence of 0.25 mM PPD in 50 mM PBS pH 7.0 (A). Fluorescence quenching of ARS/4-FPBA by 0.25 mM PPD at different concentrations of CTAB (B). Concentrations: ARS 0.02 mM, 4-FPBA 0.05 mM.
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Fig. 5. The effect of boronic acid types on the fluorescence intensity of ARS/boronic acid in the absence and in the presence of 0.25 mM PPD in 100 mM PBS pH 7.0 (A) and 2.0 mM CTAB in 100 mM PBS pH 7.0 (B). The comparison of fluorescence quenching of different ARS/boronic acid adducts by 0.25 mM PPD in buffer and micelle systems (C). Concentrations: ARS 0.05 mM, boronic acids 0.10 mM.
In the buffer system, at pHs 4.0–6.0, the fluorescence intensity of the adduct before adding PPD (F0 ) increased with increasing pH; however, at pHs above 7.0, a decrease in intensity was observed. In the micelle system, the fluorescence intensities of the adduct stayed almost the same between pHs 4.0–7.0, and decreased at pHs above 7.0. This observation is due to the effect of pH on the formation of the ARS/4-FPBA adduct. The formation of the ARS/boronic adduct required a pH that is equal to the average value between pKa of ARS (pKa ∼4 [35]) and boronic acid [36]. The highest binding constant was observed when the solution pH was around the pKa of 4-FPBA (7.6–7.8) [36,37]. Therefore, the highest fluorescence intensity of the adduct was obtained when pH of the solution was around 6.0–7.0. Moreover, the fluorescence quenching by PPD (pKa ∼ 6.0 [18]) in buffer and micelle systems were also explored at pHs ranging from 4.0 to 10.0. In the buffer system, no change in flluorescence
intensity was observed, indicating no formation of the Schiff base. In the micelle system, a drop in fluorescence intensity was observed when pHs were in the range of 4.0–10.0. In addition, the highest fluorescence quenching (F0 − F) was obtained when the solution pH was 7.0. Therefore, the pHs of standard solutions were controlled at 7.0 in further studies. 3.7. Selectivity To study the selectivity of the proposed sensor, the sensor was titrated with several amines including butylamine, ethanolamine, ethylenediamine, diethylenetriamine, phenylamine, benzylamine, melamine, o-phenylenediamine and m-phenylenediamine. The response of the proposed assay was studied by comparing the calibration sensitivity for PPD and the previously mentioned amines.
Fig. 6. The effect of 4-FPBA concentrations on fluorescence intensity of 0.02 mM ARS in the absence and the presence of 0.25 mM PPD in 50 mM PBS pH 7.0 (A) and 2.0 mM CTAB in 50 mM PBS pH 7.0 (B). The comparison of fluorescence quenching of ARS/4-FPBA by 0.25 mM PPD at different concentrations of 4-FPBA in buffer and micelle systems (C).
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Fig. 7. The effect of pH on the fluorescence intensity of ARS/4-FPBA in the absence and in the presence of 0.25 mM PPD in 50 mM PBS pH 7.0 (A) and 2.0 mM CTAB in 50 mM PBS pH 7.0 (B). The comparison of fluorescence quenching of ARS/4-FPBA by 0.25 mM PPD at different pHs in buffer and micelle system (C). Concentrations: ARS 0.02 mM, 4-FPBA 0.05 M.
The slopes of calibration curves of different amines in buffer and micelle systems were shown in Fig. 8. PPD in micelle showed a maximum value for slope of calibration curve compared with other amines. This result confirmed that the sensor exhibited the highest sensitivity to a small change of PPD concentration. Moreover, it should be noted that when comparing with the equivalent buffer system, the sensitivity in micelle was improved for most amines. The superior selectivity may be attributed to a suitable hydrophobicity of PPD which results in a better incorporation of PPD into the hydrophobic micelle core. In addition, the presence of amino group at the para position can promote the electron density of the opposite amine group. Therefore, the imine formation of PPD is more favorable than other amines in this study. It can be concluded that the addition of CTAB not only enhanced the sensitivity but also improved the selectivity of this sensor.
3.8. Fluorometric titrations The sensing ability of PPD sensor was studied by following the fluorescence intensity of the adduct upon the addition of different concentrations of PPD. At the optimized condition, PPD concentrations ranging from 0 to 0.80 mM were added to the adduct in buffer and micelle systems. Flourescence spectra of the adduct after adding different concentrations of PPD were shown in Fig. 9(A) and (B) for buffer and micelle systems, respectively. A comparison of sensing sensitivity in both systems was shown in Fig. 9(C). Fluorescence intensities of the adduct were nearly constant when performing the sensor in buffer. On the other hand, fluorescence intensities of the ARS/4-FPBA adduct decreased as a function of the increment of PPD concentration in the micelle system. The unchanged peak position and spectrum shape suggested that the drop in intensity was due to the decrease in fluorophore (adduct) concentration since the Schiff base adduct is fluorescent inactive. In the micelle system, the linear working concentration range was found to be 0.03–0.40 mM with correlation coefficient (r2 ) of 0.9972. The calibration slopes indicated that the sensor response to PPD was more sensitive in the micelle system and the calibration sensitivity was enhanced by 48 folds compared to the equivalent buffer system. The precision level of the proposed assay was expressed by the relative standard deviations (RSD, n = 3) from a measurement of 0.25 mM PPD. The result showed RSD of 1.02% and confirmed that the proposed assay provided a good precision. Moreover, the limit of detection (LOD), calculated by the concentration of PPD which gave the intensity equal to F0 – 3× standard deviation of F0 [38], of 0.03 mM was obtained. 3.9. Application to real water samples
Fig. 8. Slope of calibration curves of different amines in buffer and micelle systems. Amines (a–j) butylamine, ethanolamine, ethylenediamine, diethylenetriamine, phenylamine, benzylamine, melamine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine.
The results obtained from the representative samples were summarized in Table 1. It was found that PPD presented in the water samples was not detectable by the proposed sensor. However, after spiking three different concentrations of PPD standards into the water samples, the % recovery values of 99–120% were obtained, confirming the accuracy of the sensor. Regarding the precision aspect, %RSD values of five measurements were less than 6%. These
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Fig. 9. Emission spectra of ARS/4-FPBA in the presence of different concentrations of PPD in 50 mM PBS pH 7.0 (A) and 2.0 mM CTAB in 50 mM PBS pH 7.0 (B). The comparison of calibration curves of PPD in buffer and micelle systems (C). Concentrations: ARS 0.02 mM, 4-FPBA 0.05 mM. Table 1 Determination of PPD in real water samples. Type of water samples Natural pond water
Natural river water
Treatment waste water
a b
Spiked (mM) – 0.10 0.20 0.30 – 0.10 0.20 0.30 – 0.10 0.20 0.30
Found (mM)a b
n.d. 0.12 ± 0.004 0.20 ± 0.002 0.30 ± 0.005 n.d. 0.11 ± 0.007 0.23 ± 0.003 0.30 ± 0.004 n.d. 0.11 ± 0.004 0.21 ± 0.004 0.31 ± 0.004
Recovery (%)a
RSD (%)
– 120 ± 5 100 ± 1 101 ± 2 – 110 ± 7 116 ± 1 99 ± 1 – 113 ± 4 104 ± 2 102 ± 2
– 3.9 1.1 1.6 – 6.0 1.1 1.2 – 3.9 2.0 1.3
Mean ± SD (n = 5). n.d. = not detectable (less than 0.03 mM).
results showed that the proposed sensor provided good precisions, and could be potentially used to determine PPD in real water samples. 4. Conclusion In this work, the selective PPD fluorescene sensor has been sucessfully fabricated. The formyl boronate ester from ARS and 4-FPBA was used as sensing molecule. The sensing approach showed high selectivity to PPD over other amine-containing compounds tested. The fluorescence intensity of the sensing molecules decreased with increasing of PPD concentration. Moreover, the presence of cationic surfactants helped improving both sensitivity and selectivity of the PPD sensor. The effect of various parameters has been investigated and the optimized condition was found to be 2.0 mM CTAB in 50 mM phosphate buffer solution pH 7.0. In the presence of cationic micelles, the working concentration range was 0.03–0.4 mM with detection limit of 0.03 mM. The calibration sensitivity was 48 times higher compared to the system containing only buffer solution. The proposed sensor was applied to determine PPD in spiked real water samples with satisfactory results. Acknowledgements This research was financially supported by the Higher Education Research Promotion and National Research University Project
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Biographies
Kessarin Ngamdee is currently a Ph.D. student in Chemistry under the supervision of Assistant Professor Wittaya Ngeontae, Faculty of Science, Khon Kaen University, Thailand. She got her Master Degree in Analytical Chemistry in 2011 from Faculty of Science, Khon Kaen University, Thailand. Her research interests are the optical and electrochemical sensors.
Surangkhana Martwiset is currently a lecturer at the Department of Chemistry, Faculty of Science, Khon Kaen University, Thailand. She obtained her B.A. in Chemistry from Mount Holyoke College, USA, and M.Sc. and Ph.D. in Polymer Science and Engineering with Prof. E. Bryan Coughlin from University of Massachusetts Amherst, USA. Her research interests include polymer syntheses for fuel cell and sensor applications.
Thawatchai Tuntulani is currently a professor in the Department of Chemistry, Chulalongkorn University, Thailand. He earned his PhD in Chemistry with Prof. Marcetta Y. Darensbourg at Texas A&M University, USA after obtaining a Bachelor Degree of Engineering from Chiang Mai University, Thailand. He did his postdoctoral studies with Prof. Jean-Marie Lehn before joining Chulalongkorn University in 1995. His research interests include syntheses of macrocyclic compounds and development of electrochemical and optical sensing materials for ions and biomolecules.
Wittaya Ngeontae is currently an assistant professor in the Department of Chemistry at Khon Kaen University, Khon Kaen, Thailand. He earned his Ph.D. in analytical chemistry with Assistant Professor Wanlapa Aeungmaitrepirom and Professor Thawatchai Tuntulani at Chulalongkorn University under the Development and Promotion of Science and Technology Talent project (DPST). In 2006, he was a visiting scholar with Professor Eric Bakker before joining Khon Kaen University in 2008. His research interests include chemical sensors based nano materials. The sensitivity and selectivity enhancement strategy for the optical sensors are also focused. Moreover, he is interested in the new ionophores based on the calixarene platform for fabrication and applications of ISEs.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Selective fluorescence sensors for p-phenylenediamine using formyl boronate ester with an assistance of micelles Kessarin Ngamdee a , Surangkhana Martwiset a , Thawatchai Tuntulani d , Wittaya Ngeontae a,b,c,∗ a Materials Chemistry Research Unit, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand b Research Center for Environmental and Hazardous Substance Management, Khon Kaen University, Khon Kaen 40002, Thailand c Center of Excellence for Environmental and Hazardous Waste Management (EHWM), Bangkok 10330, Thailand d Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e
i n f o
Article history: Received 29 May 2012 Received in revised form 15 July 2012 Accepted 17 July 2012 Available online 27 July 2012 Keywords: p-Phenylenediamine Micelle Fluorescence sensors Chemical sensors
a b s t r a c t A selective fluorometric approach for detection of p-phenylenediamine (PPD) was fabricated using the alizarin–boronic acid adduct with an assistance of micelles. When Alizarin Red S (ARS) reacted with 4formylphenylboronic acid (4-FPBA) and yielded the boronate ester adduct (ARS/4-FPBA), the fluorescence emission turned on. However, upon the addition of PPD, the fluorescence intensity of the ARS/4-FPBA adduct decreased as a linear function of PPD concentrations. Enhancements in sensitivity and selectivity of this fluorescence sensor were studied by incorporating the sensor molecule into the hydrophobic core of micelles. The results showed significant improvement in sensitivity and selectivity comparing to the sensor in the buffer solution. Parameters affecting the fluorescence quenching of boronate ester adducts by PPD such as solution pH, 4-FPBA concentration, types of surfactants and surfactant concentrations were investigated. The optimum conditions for the determination of PPD was 2.0 mM cetyltrimethylammonium bromide (CTAB) in 50 mM phosphate buffer solution pH 7.0. The fluorescence intensity of the ARS/4-FPBA adduct in the presence of CTAB was remarkably 11 times as high as that in the buffer solution. The linear working concentration range was found to be 0.03–0.40 mM and the calibration sensitivity was 48 times higher than that from the system containing only buffer solution. Moreover, PPD exhibited the highest calibration sensitivity among the studied primary amines, confirming that the proposed sensor provided high sensitivity and good selectivity for PPD. The proposed sensor was used to determine PPD in spiked water samples satisfactorily. © 2012 Elsevier B.V. All rights reserved.
1. Introduction p-Phenylenediamine (PPD) is widely used in industries as a precursor to common hair dyes and aramid plastics and fibers such as Kevlar. A number of allergic reactions such as acute inflammatory reactions, eczematous hypersensitivity reactions, photoaggravated reactions, granulomatous reactions, lichenoid reactions and pseudolymphomatous reactions were caused by PPD [1–6]. PPD detection was carried out using many analytical methods including UV spectrophotometric detection [7], gas chromatography (GC) [8] or hyphenated techniques such as gas chromatography–mass spectrometry (GC–MS) [4,6,9–12], high performance liquid chromatography (HPLC) [5,13–16], capillary
∗ Corresponding author at: Materials Chemistry Research Unit, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand. Tel.: +66 432 02222x12243; fax: +66 432 02373. E-mail address: [email protected] (W. Ngeontae). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.07.077
isotachophoresis (CE) [17,18] and micellar electrokinetic capillary chromatography (MEKC) [16,19,20]. However, such techniques have some limitations, for example, the restrictions of hydrophilic substances determination in GC, and the need of using 1,8diaminooctane or triethylamine as an amine modifier and sodium heptane sulfonate as counter ion to solve asymmetric and tailing peaks in the mobile phase of HPLC [15]. Therefore, suitable methods for determination of PPD are still under development to improve the aforementioned problems and the robustness of determination methods. Fluorescence sensing is a promising approach due to the advantages of high sensitivity, ease of operation, high stability, continuous monitoring and easy manufacturing [21]. Boronic acid derivatives play a very important role in current researches in chemical sensors due to a specific reaction towards some interested species [22–25]. Among boronic acid adducts employed in chemical sensing approaches, Alizarin Red S (ARS)–boronic acid adduct is one of the most interesting molecules for fabricating fluorescence sensors. Mostly, this adduct has been used to fabricate sensors for the determination of cis-diol compounds such as glucose [26–28]. Commercially available phenyl
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Scheme 1. The reaction between boronic acid derivatives and ARS for PPD sensing.
boronic acid derivatives with formyl group can be chosen as a potential amine sensing molecule due to the well known reaction with primary amines yielding imines or Schiff bases [29]. However, the reaction in water is undesirable [30]. Recently, our group successfully enhanced the selectivity and sensitivity of the glucose sensor using surfactants [31–33]. Therefore, we expected that with an assistance of surfactants, imine formation in water would be eligible. The aim of this study was to develop a new fluorescence sensor using alizarin–boronic acid adduct in aqueous micelles for the determination of PPD. The 2-step mechanism of the proposed PPD sensor was shown in Scheme 1. In the first step, fluorescence was turned on by mixing ARS with 4formylphenylboronic acid (4-FPBA) to provide the boronate ester. When the boronate ester was formed, the fluorescence emission of ARS was observed. Upon the addition of PPD to the ester solution, a decrease in fluorescence intensity would be expected. Moreover, parameters that were likely to affect the quenching efficiency such as surfactant types, surfactant concentrations, types of boronic acid, boronic acid concentration, and solution pH were investigated. 2. Experimental 2.1. Reagents All chemicals were of analytical grade and used without further purification. Alizarin Red S (ARS), p-tolylboronic acid (TBA), 3-bromophenylboronic acid (BrPBA), 4-formylphenylboronic acid (4-FPBA), and 1,3-phenylenediamine were obtained from Aldrich. Sodium dodecyl sulphate (SDS) was purchased from BDH. Ethanolamine was received from Carlo Erba. 1,4-Phenylenediamine (PPD), 2-formylphenyl boronic acid (2-FPBA), phenylboronic
acid (PBA), melamine, ethylenediamine, n-butylamine, diethylenetriamine, phenylamine, benzylamine, 1,2-phenylenediamine, triton X-114, cetyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DTAB) and trimethyltetradecylammonium bromide (TTAB) were received from Fluka. Potassium dihydrogen phosphate was obtained from Scharlau. Dipotassium hydrogen phosphate was purchased from UNIVAR. All aqueous solutions were prepared from deionized water (DI) with the specific resistivity of 18.2 M cm from RiOs TM Type I Simplicity 185 (Millipore water). 2.2. Instrumentation Emission spectra were recorded using a RF-5301PC spectrofluorometer (Shimadzu). Excitation and emission spectra were measured using the slit width of 5 mm. The pH of solution was measured using UB-10 UltraBasic pH meter (Denver Instrument). 2.3. General procedures 2.3.1. Fluorescence measurements The fluorescence quenching of the ARS/boronic acid adduct by PPD was performed in the presence and in the absence of surfactants. To a 10.00 mL volumetric flask, 400 L of 0.5 mM ARS, 500 L of 1.0 mM of boronic acid derivatives and 500 L of 1.0 M phosphate buffer solution (PBS) pH 7.0 were sequentially added. In the study that required a surfactant, 1.00 mL of the stock solution of the surfactant was then introduced. An appropriate volume of 5.0 mM PPD was added into the mixture to make the desired concentrations. The mixture was diluted to the mark with DI water. After standing at room temperature for 15 min, emission spectra were recorded at a particular wavelength: ex = 460 nm for 2-FPBA and ex = 475 nm for other boronic acid derivatives.
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2.3.2. Effect of types and tail length of surfactants Cationic, anionic and nonionic surfactants were used to study the effect of surfactant types. The effect of surfactant tail length was investigated by using three cationic surfactants: CTAB, DTAB and TTAB. The concentrations of ARS and 4-FPBA were 0.05 mM and 0.10 mM, respectively. The concentration of each surfactant was twice its critical micelle concentration (cmc): 0.5 mM for Triton X-114, 16.4 mM for SDS, 7.4 mM for TTAB, 32.0 mM for DTAB and 2.0 mM for CTAB [34]. 2.3.3. Effect of CTAB concentrations Concentrations of ARS and 4-FPBA were 0.02 mM and 0.05 mM, respectively. The final concentrations of CTAB were varied from 0.5 to 5.0 mM in the absence and in the presence of 0.25 mM PPD. 2.3.4. Effect of types of boronic acids Five boronic acid-derivatives, PBA, TBA, BrPBA, 2-FPBA, and 4FPBA were used. Final concentration of boronic acids and ARS were 0.05 and 0.10 mM, respectively. Each type of boronic acids was prepared in the absence and in the presence of 2.0 mM CTAB, following by addition of 500 L of 5.0 mM PPD. 2.3.5. Effect of the concentration of boronic acid The 0.02 mM ARS solution was mixed with appropriated volume of stock 4-FPBA to make the final concentration in the range of 0.001–0.10 mM. The fluorescence quenching was investigated in the absence and in the presence of 2.0 mM CTAB. 2.3.6. Effect of pH The final concentrations of ARS and 4-FPBA were 0.02 and 0.05 mM, respectively, in the absence and in the presence of 2.0 mM CTAB. Solution pHs were controlled by varying the pHs of the buffer. Acetic-acetate buffer was used for pH 4.0, and phosphate buffer was used for the rest. 2.3.7. Selectivity of the PPD sensor In this study, other amines including butylamine, ethanolamine, ethylenediamine, diethylenetriamine, phenylamine (aniline), benzylamine, melamine, 1,2-phenylenediamine, and 1,3-phenylenediamine were also studied as a substitute of PPD. Final concentrations of ARS and 4-FPBA were 0.02 and 0.05 mM, respectively. The fluorometric titrations of PPD sensor with each amine were performed in the absence and in the presence of 2.0 mM CTAB. 2.3.8. Calibration curve To a 10-mL volumetric flask, a solution of the following components, 0.02 mM ARS, 0.05 mM FPBA solution, and 2.0 mM CTAB was prepared. The solution pHs were controlled by 50 mM phosphate buffer solution pH 7.0. Appropriate volumes of 5.0 mM PPD standard were added into the mixture to make final concentrations ranging from 0 to 0.80 mM. After mixing for 15 min, the fluorescence spectra were recorded. 2.3.9. Application to real water samples In order to demonstrate the applicability of the proposed sensor, the treated waste water, natural pond water and natural river water were used as representative environmentally contaminated samples. The water samples were collected from three sources: Khon Kaen University waste water treatment pond, local pond water, and local river water. All water samples were used immediately without storage. The water samples were filtered through filter paper. The accuracy and precision of each measurement were evaluated by spiking 3 different concentrations of PPD (0.10, 0.20 and 0.30 mM) into the samples under the optimized condition.
3. Results and discussion 3.1. Emission and absorption spectra To study sensing properties of ARS/4-FPBA for the determination of PPD, emission characteristics of ARS and ARS/4-FPBA before and after adding PPD were studied using the excitation wavelength at 475 nm. The emission spectra were recorded in two systems; phosphate buffer solution pH 7.0 in the absence (buffer system) and in the presence of CTAB (micelle system). The fluorescence spectra were shown in Fig. 1(A) and (B) for buffer and micelle systems, respectively. In both systems, ARS did not fluoresce; however, upon the addition of 4-FPBA, the emission maxima appeared at the wavelength of 566 and 556 nm for solutions without and with CTAB, respectively. This is due to a reversible ester formation between ARS and 4-FPBA. The observed blue shift of emission maxima from 566 to 556 nm may be due to the aggregation/disaggregation of dye molecules [33]. Moreover, the fluorescence intensity of the ARS/4FPBA adduct in the micelle system was 11 times as great as that in the buffer system. The increase in fluorescence intensity is probably due to the assistance of the surfactant in protecting the excited state of the adduct from collision with solvent molecules [33]. Upon the addition of PPD, fluorescence intensity of the adduct decreased because of the formation of Schiff base compound from a reaction between amino group in PPD and aldehyde moiety of 4-FPBA [29]. The remaining of ordinary pattern of the position and shape of the spectrum suggested that the Schiff base molecule is not fluorescent active. In addition, a dramatic decrease in emission properties of the adducts upon an addition of PPD was observed in the micelle system. In the buffer system, fluorescence intensity of the adduct slightly decreased upon adding PPD. It implied that PPD and the adduct could dissolve in the micelle core, giving a better imine formation compared to the reaction in aqueous solution. In the presence of micelle, PPD and the adduct may be incorporated in the hydrophobic core of micelle due to the hydrophobicity of both species. The incorporation may also enhance the rate of the reaction between the adduct and PPD due to relatively concentrated sensoring molecules inside micelles [32]. To understand the optical properties of the recognition and sensoring molecule, ARS/4-FPBA adduct was examined by measuring the absorption spectra before and after adding PPD. The absorption spectra of ARS/4-FPBA in the presence of various PPD concentrations were shown in Fig. 2(A) and (B) for the buffer system and the micelle system, respectively. In both systems, a linear increase in absorption at the wavelength of 370 nm was observed when PPD was introduced, with a higher value in micelle system. The increasing of absorption at 370 nm in micelle system indicated that PPD could form Schiff base with aldehyde group of 4-FPBA. The inset showed the plot between the concentration of PPD and the increasing of absorption peak at 370 nm. The slope of the plot between absorbance at 370 nm and concentration of PPD in micelle system was approximately six times higher than that in the buffer system, suggesting a preference of imine formation between PPD and ARS/4-FPBA. 3.2. Types of surfactants To obtain a suitable surfactant system, three types of surfactants, i.e. non ionic (Triton X-114), anionic (SDS), and cationic (CTAB), were examined. Two additional cationic surfactants, DTAB and TTAB, were used to study the surfactant tail length effect on the emission characteristic of the sensing molecule. The final concentration of each surfactant was twice of critical micelle concentration [33]. Fluorescence intensities of the adduct in the absence (F0 ) and
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Fig. 1. Emission spectra (ex = 475 nm) of ARS (····), ARS/4-FPBA (—) and ARS/4-FPBA in the presence of 0.30 mM PPD (- - - -) in 50 mM PBS pH 7.0 (A) and 50 mM PBS, pH 7.0 with 2.0 mM CTAB solution (B). Concentrations: ARS 0.02 mM, 4-FPBA 0.05 mM.
in the presence of 0.25 mM PPD (F) in different micelle systems were shown in Fig. 3(A). The differences in fluorescence intensities of the adduct before and after adding PPD (F0 − F) were shown in Fig. 3(B). A small increase in fluorescence intensity of the adduct was observed in the presence of non ionic and anionic surfactants, while a dramatic increase was seen upon the addition of cationic surfactants. The negative charge of sulfonate group in the adduct favorably interacts with positive head of cationic surfactant via electrostatic interactions. In the tail length study, all three cationic surfactants, CTAB, DTAB and TTAB, provided the enhancement in the fluorescence intensity of the adduct. The maximum fluorescence quenching of the adduct in the presence of PPD was obtained when CTAB was used. Moreover, CTAB has the lowest cmc among the three cationic surfactants. Therefore, CTAB was chosen to investigate other parameters. 3.3. Concentrations of CTAB To study the effect of CTAB concentrations, the fluorescence intensities of the adduct in the absence and in the presence of 0.25 mM PPD were studied using CTAB concentrations between 0 and 5.0 mM. The fluorescence intensities of the adduct before and after adding PPD in the presence of different CTAB concentraitons were shown in Fig. 4(A). The fluorescence intensities of the adduct increased with increasing CTAB concentration when CTAB concentrations were in the range of 0.5–2.0 mM, and remained the same at the concentrations above 2.0 mM. In addition, differences in fluorescence intensities of the adduct before and after adding PPD (F0 − F) were shown in Fig. 4(B). The fluorescence quenching (F0 − F) after adding PPD increased with the increment of CTAB concentration from 0.5 to 2.0 mM, while decreased at the concentrations above 2.0 mM. The result suggested that the number of micelle spheres increased with increasing CTAB concentration. When the concentration of CTAB was greater than 2.0 mM, there
would be micelle spheres that were not occupied by the sensing molecules; thus, PPD that was introduced to the system would be incorporated and protected in those empty micelles. Therefore, the fluorescence quenching was decreased with increasing CTAB concentration because PPD could not react with ARS/4-FPBA. Since highest fluorescence quenching ability was observed at CTAB concentrations of 1.0 and 2.0 mM, the concentration of 2.0 mM was chosen to study other parameters. 3.4. Types of boronic acids Five derivatives of phenylboronic acids with different substituted groups, i.e. PBA, TBA, BrPBA, 2-FPBA, and 4-FPBA, were used to study the effect of boronic acid types on fluorescence emission. Fluorescence intensities of ARS/boronic acids adducts before and after adding PPD in buffer and micelle systems were recorded, and shown in Fig. 5(A) and (B). The result showed that ARS/boronic acids could emit fluorescence, suggesting the adduct formation of ARS with all boronic acids. The boronate esters from ARS and phenyl boronic acids with electron withdrawing groups, i.e. BrPBA, 2-FPBA, and 4-FPBA, showed higher fluorescence intensities than that from ARS and phenylboronic acids with electron donating group, TBA. Moreover, the fluorescence intensities of ARS/boronic acids in the presence of CTAB provided higher intensities than that in the absence of CTAB. The significant fluorescence quenching by adding PPD was obtained when the phenylboronic acids contained only formyl moiety. However, the quenching abilities of PPD on the adducts from two formyl phenylboronic acids, i.e. 2-FPBA and 4-FPBA, in buffer and micelle systems were different. The addition of PPD resulted in a decrease in fluorescence intensity of the ARS/2-FPBA adduct whether CTAB was present or not, while PPD could quench the ARS/4-FPBA adduct effectively in the micelle system only. Moreover, the highest fluorescence quenching of the adduct was observed when 4-FPBA was used, which may be due to the less steric hindrance of the
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Fig. 2. Absorption spectra of (····) 2.0 mM CTAB, (- - - -) 0.20 mM PPD and (—) colorimetric titration of ARS/4-FPBA with 0, 0.10, 0.20, 0.30, 0.40 and 0.50 mM PPD, respectively. All solutions were prepared in 50 mM PBS pH 7.0. Inset: plots of the absorbance of ARS/4-FPBA (at 370 nm) against PPD concentrations in buffer (A) and micelle system (B). Concentrations: ARS 0.02 mM, 4-FPBA 0.05 mM.
formation of Schiff base at the para position of ARS/4-FPBA compared to the ortho position of ARS/2-FPBA. Therefore, 4-FPBA was chosen to fabricate the PPD sensor in further studies. 3.5. Concentrations of boronic acid The optimization of 4-FPBA concentration was studied by fixing the final concentration of ARS at 0.02 mM and varying the concentration of 4-FPBA from 0.001 to 0.20 mM. Fluorescence intensities of the resulting adducts before and after adding PPD were recorded and shown in Fig. 6(A) and (B) for buffer and micelle systems, respectively. The differences in intensities of the adduct before and after the addtion of PPD (F0 − F) were shown in Fig. 6(C). In both systems, the fluorescence intensities of the adduct increased with increasing the concentration of 4-FPBA. However, the fluorescence quenching abilities of PPD in the buffer and in the micelle systems were different. In the buffer system, upon PPD
addition, a small change in fluorescence intensities was observed with increasing of 4-FPBA concentration. In micelles, fluorescence quenching increased with the increment of 4-FPBA concentration in the range of 0.001–0.10 mM. The different behavior may stem from the limit of imine formation in the buffer solution. When the concentrations of 4-FPBA were greater than 0.10 mM, the quenching ability remained constant. Therefore, 4-FPBA concentration of 0.10 mM was chosen as the optimum concentration to fabricate the PPD sensor. 3.6. Effect of pH Since ARS is a pH indicator dye, the effect of pH, in the range of 4.0–10.0, on the fluorescence intensity of the ARS/4-FPBA adduct was investigated. The fluorescence intensities of the adduct in different pHs before and after adding PPD were shown in Fig. 7(A) and (B) for buffer and micelle systems, respectively.
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Fig. 3. The effect of surfactant types on the fluorescence intensity of ARS/4-FPBA in the absence and in the presence of 0.25 mM PPD in 100 mM PBS pH 7.0 (A). Fluorescence quenching of ARS/4-FPBA by 0.25 mM PPD in buffer and surfactant systems (B). Concentrations: ARS 0.05 mM, 4-FPBA 0.10 mM.
Fig. 4. The effect of CTAB concentration on fluorescence intensity of ARS/4-FPBA in the absence and in the presence of 0.25 mM PPD in 50 mM PBS pH 7.0 (A). Fluorescence quenching of ARS/4-FPBA by 0.25 mM PPD at different concentrations of CTAB (B). Concentrations: ARS 0.02 mM, 4-FPBA 0.05 mM.
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Fig. 5. The effect of boronic acid types on the fluorescence intensity of ARS/boronic acid in the absence and in the presence of 0.25 mM PPD in 100 mM PBS pH 7.0 (A) and 2.0 mM CTAB in 100 mM PBS pH 7.0 (B). The comparison of fluorescence quenching of different ARS/boronic acid adducts by 0.25 mM PPD in buffer and micelle systems (C). Concentrations: ARS 0.05 mM, boronic acids 0.10 mM.
In the buffer system, at pHs 4.0–6.0, the fluorescence intensity of the adduct before adding PPD (F0 ) increased with increasing pH; however, at pHs above 7.0, a decrease in intensity was observed. In the micelle system, the fluorescence intensities of the adduct stayed almost the same between pHs 4.0–7.0, and decreased at pHs above 7.0. This observation is due to the effect of pH on the formation of the ARS/4-FPBA adduct. The formation of the ARS/boronic adduct required a pH that is equal to the average value between pKa of ARS (pKa ∼4 [35]) and boronic acid [36]. The highest binding constant was observed when the solution pH was around the pKa of 4-FPBA (7.6–7.8) [36,37]. Therefore, the highest fluorescence intensity of the adduct was obtained when pH of the solution was around 6.0–7.0. Moreover, the fluorescence quenching by PPD (pKa ∼ 6.0 [18]) in buffer and micelle systems were also explored at pHs ranging from 4.0 to 10.0. In the buffer system, no change in flluorescence
intensity was observed, indicating no formation of the Schiff base. In the micelle system, a drop in fluorescence intensity was observed when pHs were in the range of 4.0–10.0. In addition, the highest fluorescence quenching (F0 − F) was obtained when the solution pH was 7.0. Therefore, the pHs of standard solutions were controlled at 7.0 in further studies. 3.7. Selectivity To study the selectivity of the proposed sensor, the sensor was titrated with several amines including butylamine, ethanolamine, ethylenediamine, diethylenetriamine, phenylamine, benzylamine, melamine, o-phenylenediamine and m-phenylenediamine. The response of the proposed assay was studied by comparing the calibration sensitivity for PPD and the previously mentioned amines.
Fig. 6. The effect of 4-FPBA concentrations on fluorescence intensity of 0.02 mM ARS in the absence and the presence of 0.25 mM PPD in 50 mM PBS pH 7.0 (A) and 2.0 mM CTAB in 50 mM PBS pH 7.0 (B). The comparison of fluorescence quenching of ARS/4-FPBA by 0.25 mM PPD at different concentrations of 4-FPBA in buffer and micelle systems (C).
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Fig. 7. The effect of pH on the fluorescence intensity of ARS/4-FPBA in the absence and in the presence of 0.25 mM PPD in 50 mM PBS pH 7.0 (A) and 2.0 mM CTAB in 50 mM PBS pH 7.0 (B). The comparison of fluorescence quenching of ARS/4-FPBA by 0.25 mM PPD at different pHs in buffer and micelle system (C). Concentrations: ARS 0.02 mM, 4-FPBA 0.05 M.
The slopes of calibration curves of different amines in buffer and micelle systems were shown in Fig. 8. PPD in micelle showed a maximum value for slope of calibration curve compared with other amines. This result confirmed that the sensor exhibited the highest sensitivity to a small change of PPD concentration. Moreover, it should be noted that when comparing with the equivalent buffer system, the sensitivity in micelle was improved for most amines. The superior selectivity may be attributed to a suitable hydrophobicity of PPD which results in a better incorporation of PPD into the hydrophobic micelle core. In addition, the presence of amino group at the para position can promote the electron density of the opposite amine group. Therefore, the imine formation of PPD is more favorable than other amines in this study. It can be concluded that the addition of CTAB not only enhanced the sensitivity but also improved the selectivity of this sensor.
3.8. Fluorometric titrations The sensing ability of PPD sensor was studied by following the fluorescence intensity of the adduct upon the addition of different concentrations of PPD. At the optimized condition, PPD concentrations ranging from 0 to 0.80 mM were added to the adduct in buffer and micelle systems. Flourescence spectra of the adduct after adding different concentrations of PPD were shown in Fig. 9(A) and (B) for buffer and micelle systems, respectively. A comparison of sensing sensitivity in both systems was shown in Fig. 9(C). Fluorescence intensities of the adduct were nearly constant when performing the sensor in buffer. On the other hand, fluorescence intensities of the ARS/4-FPBA adduct decreased as a function of the increment of PPD concentration in the micelle system. The unchanged peak position and spectrum shape suggested that the drop in intensity was due to the decrease in fluorophore (adduct) concentration since the Schiff base adduct is fluorescent inactive. In the micelle system, the linear working concentration range was found to be 0.03–0.40 mM with correlation coefficient (r2 ) of 0.9972. The calibration slopes indicated that the sensor response to PPD was more sensitive in the micelle system and the calibration sensitivity was enhanced by 48 folds compared to the equivalent buffer system. The precision level of the proposed assay was expressed by the relative standard deviations (RSD, n = 3) from a measurement of 0.25 mM PPD. The result showed RSD of 1.02% and confirmed that the proposed assay provided a good precision. Moreover, the limit of detection (LOD), calculated by the concentration of PPD which gave the intensity equal to F0 – 3× standard deviation of F0 [38], of 0.03 mM was obtained. 3.9. Application to real water samples
Fig. 8. Slope of calibration curves of different amines in buffer and micelle systems. Amines (a–j) butylamine, ethanolamine, ethylenediamine, diethylenetriamine, phenylamine, benzylamine, melamine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine.
The results obtained from the representative samples were summarized in Table 1. It was found that PPD presented in the water samples was not detectable by the proposed sensor. However, after spiking three different concentrations of PPD standards into the water samples, the % recovery values of 99–120% were obtained, confirming the accuracy of the sensor. Regarding the precision aspect, %RSD values of five measurements were less than 6%. These
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Fig. 9. Emission spectra of ARS/4-FPBA in the presence of different concentrations of PPD in 50 mM PBS pH 7.0 (A) and 2.0 mM CTAB in 50 mM PBS pH 7.0 (B). The comparison of calibration curves of PPD in buffer and micelle systems (C). Concentrations: ARS 0.02 mM, 4-FPBA 0.05 mM. Table 1 Determination of PPD in real water samples. Type of water samples Natural pond water
Natural river water
Treatment waste water
a b
Spiked (mM) – 0.10 0.20 0.30 – 0.10 0.20 0.30 – 0.10 0.20 0.30
Found (mM)a b
n.d. 0.12 ± 0.004 0.20 ± 0.002 0.30 ± 0.005 n.d. 0.11 ± 0.007 0.23 ± 0.003 0.30 ± 0.004 n.d. 0.11 ± 0.004 0.21 ± 0.004 0.31 ± 0.004
Recovery (%)a
RSD (%)
– 120 ± 5 100 ± 1 101 ± 2 – 110 ± 7 116 ± 1 99 ± 1 – 113 ± 4 104 ± 2 102 ± 2
– 3.9 1.1 1.6 – 6.0 1.1 1.2 – 3.9 2.0 1.3
Mean ± SD (n = 5). n.d. = not detectable (less than 0.03 mM).
results showed that the proposed sensor provided good precisions, and could be potentially used to determine PPD in real water samples. 4. Conclusion In this work, the selective PPD fluorescene sensor has been sucessfully fabricated. The formyl boronate ester from ARS and 4-FPBA was used as sensing molecule. The sensing approach showed high selectivity to PPD over other amine-containing compounds tested. The fluorescence intensity of the sensing molecules decreased with increasing of PPD concentration. Moreover, the presence of cationic surfactants helped improving both sensitivity and selectivity of the PPD sensor. The effect of various parameters has been investigated and the optimized condition was found to be 2.0 mM CTAB in 50 mM phosphate buffer solution pH 7.0. In the presence of cationic micelles, the working concentration range was 0.03–0.4 mM with detection limit of 0.03 mM. The calibration sensitivity was 48 times higher compared to the system containing only buffer solution. The proposed sensor was applied to determine PPD in spiked real water samples with satisfactory results. Acknowledgements This research was financially supported by the Higher Education Research Promotion and National Research University Project
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Biographies
Kessarin Ngamdee is currently a Ph.D. student in Chemistry under the supervision of Assistant Professor Wittaya Ngeontae, Faculty of Science, Khon Kaen University, Thailand. She got her Master Degree in Analytical Chemistry in 2011 from Faculty of Science, Khon Kaen University, Thailand. Her research interests are the optical and electrochemical sensors.
Surangkhana Martwiset is currently a lecturer at the Department of Chemistry, Faculty of Science, Khon Kaen University, Thailand. She obtained her B.A. in Chemistry from Mount Holyoke College, USA, and M.Sc. and Ph.D. in Polymer Science and Engineering with Prof. E. Bryan Coughlin from University of Massachusetts Amherst, USA. Her research interests include polymer syntheses for fuel cell and sensor applications.
Thawatchai Tuntulani is currently a professor in the Department of Chemistry, Chulalongkorn University, Thailand. He earned his PhD in Chemistry with Prof. Marcetta Y. Darensbourg at Texas A&M University, USA after obtaining a Bachelor Degree of Engineering from Chiang Mai University, Thailand. He did his postdoctoral studies with Prof. Jean-Marie Lehn before joining Chulalongkorn University in 1995. His research interests include syntheses of macrocyclic compounds and development of electrochemical and optical sensing materials for ions and biomolecules.
Wittaya Ngeontae is currently an assistant professor in the Department of Chemistry at Khon Kaen University, Khon Kaen, Thailand. He earned his Ph.D. in analytical chemistry with Assistant Professor Wanlapa Aeungmaitrepirom and Professor Thawatchai Tuntulani at Chulalongkorn University under the Development and Promotion of Science and Technology Talent project (DPST). In 2006, he was a visiting scholar with Professor Eric Bakker before joining Khon Kaen University in 2008. His research interests include chemical sensors based nano materials. The sensitivity and selectivity enhancement strategy for the optical sensors are also focused. Moreover, he is interested in the new ionophores based on the calixarene platform for fabrication and applications of ISEs.