Demonstration of the Concept Using Methylene Blue and Fer

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A Simultaneous Multiselective Spectroelectrochemical Fiber Optic Sensor; A Demonstration of the Concept Using Methylene Blue and Ferrocyanide Kenichiro Imai, Takuya Okazaki, Noriko Hata, Shigeru Taguchi, Kazuharu Sugawara, and Hideki Kuramitz Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 21 Jan 2015 Downloaded from http://pubs.acs.org on January 24, 2015

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Analytical Chemistry

A Simultaneous Multiselective Spectroelectrochemical Fiber Optic Sensor; A Demonstration of the Concept Using Methylene Blue and Ferrocyanide Kenichiro Imai,† Takuya Okazaki,† Noriko Hata,† Shigeru Taguchi,† Kazuharu Sugawara,‡ and Hideki Kuramitz*,† Department of Environmental Biology and Chemistry, Graduate School of Science and Engineering for Research, University of Toyama, Gofuku, Toyama 930-8555, Japan, and Maebashi Institute of Technology, 371-0816 Maebashi, Gunma, Japan ABSTRACT: Herein, we present a novel spectroelectrochemical fiber optic sensor that combines electrochemistry, spectroscopy, and electrostatic adsorption in three modes of selectivity. The proposed sensor is simple and consists of a gold mesh cover on a multimode fiber optic that uses attenuated total reflection (ATR) as the optical detection mode. The sensing is based on changes in the attenuation of the light that passes through the fiber optic core accompanying the electrochemical oxidation-reduction of an analyte at the electrode. Methylene blue and ferrocyanide were used as model analytes to evaluate the performance of the proposed sensor. The optical transmission changes generated by electrochemical manipulation showed a good linear relationship with the concentration and the limits of detection (3σ) for methylene blue and ferrocyanide at 2.0×10-7 M and 1.6×10-3 M, respectively. The sensor responses were successfully enhanced with an additional level of selectivity via an electrostatically adsorbed, self-assembled monolayer (SAM), which consisted of a silane coupling layer, a poly anion, and a poly cation. The improvement observed in the sensitivity of a SAM-modified fiber optic sensor was rather encouraging. The optimized sensor had detection limits (3σ) for 8.3×10-9 M of methylene blue and 7.1×10-4 M of ferrocyanide. The developed sensor was successfully applied to the detection of ferrocyanide in simulated nuclear waste.

Spectroelectrochemical analysis has been employed for more than five decades in the investigation of a wide variety of inorganic, organic, and biological redox systems, for which the main purpose has been to determine the mechanisms of electrode reactions.1-5 For example, it has been used in the analysis of ultra-thin films of a conductive polymer 6-7 as well as in the electrochemical processing of adsorbed protein films.8-9 In general, the spectroelectrochemical measurements were carried out by using a thin-layer spectroscopic cell incorporated with optically transparent electrodes (OTEs) composed of indium-tin oxide (ITO), platinum, or gold mesh. The superior advantage of spectroelectrochemistry is the crosscorrelation of information that is attainable from simultaneous electrochemical and optical spectroscopy measurements.10 The optical responses generated by the electrochemical stimulation of OTEs can be detected by either transmission or attenuated total reflection (ATR). If the information obtained from the optical spectroscopy and electrochemical responses can be applied in the sensor, the overall sensor selectivity would be significantly enhanced. However, spectroelectrochemistry has a critical detection-sensitivity problem, which is caused by the use of thin-layer spectroscopy cells that accelerates the rapid progression of an electrochemical redox reaction. The short length of the optical path in thin-layer spectroscopy cells results in a low degree of photochemical sensitivity for spectroelectrochemical measurements. Heineman et al. have developed spectroelectrochemical methods based on multimode selectivity, which can be simul-

taneously achieved in a single devices based on ATR.11-31 This concept combines the fundamentals of electrochemistry, optical spectroscopy, and selective partitioning into a single device. The application of the ATR method in spectroelectrochemical measurement is an excellent strategy that overcomes the problems caused by the short length of the optical path. The functions of this sensor are as follows: (1) preconcentration of the analyte by charge-selective film on the ITO-coated glass, (2) electrochemical redox reaction of the analyte, and (3) monitoring of the oxidized or reduced species by ATR measurement. This sensor has been demonstrated using a series of metal ion complexes such as Fe(CN)63-/411,12,17,19-21,28 and Ru(bipy)32+ 13-16,18,26-27 that are electrochemically reversible and have shown a substantial difference in molar absorptivity during spectroelectrochemical modulation. Over the past few decades, photochemical and fiber optic evanescent wave-based sensing techniques have been popular for use in the determination of chemical and biochemical materials.32-39 Fiber optic sensing offers many advantages for measurement equipment: high sensitivity, simplicity, low cost, and real time performance for remote sensing. Brewster et al. demonstrated with fiber optic to couple a thin-layer spectroelectrochemical cell, with a long optical path length.40 This technique was applied to titrations of cytochrome c and ferricyanide in the spectropotentiostatic mode. On the other hand, Van Dyke and Cheng have reported on fabrication and characterization of a spectroelectrochemical fiber optic probe. The probe was implemented from fused silica optical fibers em-

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bedded in electrically conductive graphite/epoxy material with the optical end face and the working electrode active surface in a coplanar arrangement.41 They also demonstrated that spectroelectrochemical approach can be extended to fiber optic fluorescence and chemiluminescence measurements as the bioanalytical sensor.42 Beam et al. have reported a fully integrated multi-mode fiber optic-coupled spectroelectrochemical internal reflection element platform.32 The electro-active fiber optic probe consists of a side-polished fiber optic that is coated with a thin film of ITO as the working electrode. This fiber optic features a region where about half the fiber core and cladding is removed by a side-polishing procedure to form a planar sensing region. This technology was demonstrated by electrochemically driven changes in absorbance for methylene blue as a redox indicator. They also investigated the enhancement of spectroelectrochemical sensitivity by modifying the conductive polymer on the ITO-deposited fiber optic. In the present study, we developed a new and simple type of spectroelectrochemical fiber optic sensor based on ATR that utilizes the evanescent wave and the electrochemical reaction. The fiber optic electrode sensor was prepared by using a gold mesh to cover the core portion where the plastic cladding had been removed. Sensing was based on the changes in the light attenuation that passed through the fiber optic core that accompanied an electrochemical analyte oxidation or reduction, as illustrated by Table of contents graphic. The advantages of this sensor are as follows: (1) flexibility allows adjustment of the sensing length, i.e., the optical path length; (2) real-time remote sensing can be achieved; (3) it is suitable for a low sample volume. The spectroelectrochemical performance of the fiber optic electrode sensor was evaluated via redox indicators such as methylene blue and ferrocyanide. Furthermore, the improvement in selectivity and sensitivity were investigated using a self-assembled monolayer, which can be pre-concentrated based on the electrostatic adsorption of an analyte to the sensing region on the fiber surface.

LUS-363). A portable spectrophotometer (JASCO Co, Model MV-3100) was used as a detector for the analyte by the ATR signal obtained from the fiber optic electrode sensor. All the voltammetric measurements and applied potential were carried out using an electrochemical analyzer Model 620CZ (Bioanalytical Systems, Inc. (BAS), IN, USA). The three-electrode system used a gold mesh as the working electrode (100 mesh, 10 mm × 100 mm, 0.35 mm thickness), a platinum mesh as the counter electrode (100 mesh, 10 mm × 100 mm, 0.35 mm thickness), and Ag/AgCl as the reference electrode (model No. 11-2020, BAS). The gold mesh and platinum mesh were purchased from Sanwa (Saitama, Japan).

EXPERIMENTAL SECTION

Figure 1. (A) Schematic of the ATR-based spectroelectrochemical fiber optic cell: (a) gold mesh working electrode, (b) Pt counter electrode, (c) Ag/AgCl reference electrode, (d) sample inlet (3 mm inner diameter), (e) sample outlet (3 mm inner diameter), (f) exposed fiber optic core (8 cm length, φ200 µm core diameter), (g) contacts to potentiostat. (B) Scale bar representation of the spectroelectrochemical cell in the sensing region.

Chemicals and Materials. Methylene blue, potassium ferrocyanide, Na2HPO4, NaH2PO4, acetone, and sodium hydroxide were obtained from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). Poly (allylamine hydrochloride) (PAH) and Poly {1-[4-(3-carboxy-4-hydroxyphenylazo) benzensulfonamido]-1,2-ethanediyl, sodium salt} (PCBS) were purchased from Sigma Aldrich (St Louis, USA). 3-Amino propyl triethoxysilane (APTES) was obtained from the Tokyo Chemical Industry Co. (Tokyo, Japan). All other reagents were of analytical grade. All the chemicals were used as received without further purification, and Milli-Q water was used in the preparation of the solution. The supporting electrolyte for the spectroelectrochemical measurements was provided by 0.1 M phosphate buffer solution (PBS: 0.044 M NaH2PO4, 0.056 M Na2HPO4). Apparatus. Lengths of multimode plastic-clad silica fiber optic (FT200EMT, FT200UMT) with a 200 µm step index for the core diameter were purchased from ThorLabs (Newton, NJ, USA). The fiber optic was cut to length of approximately 1 m, and acetone was used to remove 8.0 cm of the plastic cladding from the center of the fiber. The spectroelectrochemical sensing based on fiber optic electrodes consisted of the following instrument setup. A deuterium halogen and tungsten lamp was used as the light source (JASCO Co., Tokyo, Japan, Model

Preparation of a Self-assembled Monolayer on the Fiber Optic. The APTES-immobilized fiber optic was prepared using a silane coupling reaction. The exposed core (8 cm) was activated in preparation for silanization by soaking in 1.0 M sodium hydroxide for 1 hour, which was followed by rinsing with Milli-Q water. The core surface was subsequently dipped in acetone containing 2% (v/v) APTES for 1 min at room temperature, which accomplished the silanization. After silanization, the self-assembled monolayer was applied onto the core of the fiber optic. In our experiments, PAH and PCBS were chosen as the polycation and polyanion, respectively. The combination of one monolayer of PAH and PCBS is denoted herein as a bilayer. We applied the modifed ionic selfassembled monolayer onto the fiber optic immobilized APTES, as previously reported.43 After rinsing with Milli-Q water, the PCBS layer was formed on the APTES layer by immersing it in a solution containing 10 mM PCBS for 15 min at room temperature. Then, the core immobilized PCBS/APTES were soaked in 10 mM of PAH solution for 15 min.44-46 Under these


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Figure 2. (b) A series of cyclic voltammograms for 5.0×10-5 M methylene blue were recorded by the fiber optic holding a gold mesh electrode (Scan rate 2-500 mV/sec, E/V vs Ag/AgCl). (a) Absorbance spectrum of the bare fiber optic sensor at a positive potential limit of +0.5 V. (c) Absorbance spectrum of the bare fiber optic sensor at a negative potential limit of -0.8 V.

conditions, one bilayer (PCBS/PAH) of the self-assembled monolayer film was ~1.5 nm in thickness.41 Spectroelectrochemical Measurement. The glass fiber optic spectroelectrochemical cell consisted of a glass tube with 3.0 mm diameter as the main configuration that is connected to three electrodes and inlet-outlet holes for sample solution as shown in Figure 1A. Figure 1B is an illustration of the fiber optic electrode represented in a scale bar. The fiber optic electrode was fabricated by covering the 8cm length of the exposed core of the fiber optic with 10×100 mm gold mesh. Both ends of the glass cell were sealed by parafilm®. The sample inlet was filled with various concentrations of methylene blue or ferrocyanide in 0.1 M PBS buffer to make up the sensing portions. The sample volume was approximately 2 mL. The ATR signal and electrochemical measurements were performed simultaneously using either cyclic voltammetry or chronoamperometry. The spectroscopic properties were monitored over time from the optical absorbance spectrum of the analyte, which attenuated the evanescent wave generated during total reflection. Cyclic voltammetry and chronoamperometry were recorded in a range of -0.8 to +0.5 V. The subsequent measurements were done by simply replacing each used sample solution with a new one.

RESULTS AND DISCUSSION Spectroelectrochemical Behavior of Methylene Blue Using a Bare Fiber Optic Electrode Sensor. The spectral property changes of the methylene blue at the bare fiber optic electrode sensor were first investigated using ATR and cyclic voltammetry. The electrochemical reaction of methylene blue is illustrated in Figure 2. The potential sweep rate was investigated in the range of 2-500 mV/sec, and the spectral change in absorbance was monitored by ATR. Figure 2(b) shows the cyclic voltammograms of 5.0×10-5 M of methylene blue obtained from the bare fiber optic electrode sensor for different

potential sweep rates. The voltammograms had well-defined oxidation and reduction peaks that corresponded to the coupling with methylene blue redox. Thus, methylene blue on the surface of the fiber optics could be maintained in a fully oxidized and reduced form by applying a potential of +0.5 V and -0.8 V, respectively. The effective resistance of the spectroelectrochemical cell according to the ohm’s law plot of ΔIpeak vs. ΔEpeak, was calculated to be 136 Ω. The concomitant spectral changes found during the electrochemical cycling of 5.0×10-5 M of methylene blue are shown in Figures 2(a) and (c). By applying an adequate positive potential, the oxidation of methylene blue appears colored at 580 nm with maximum absorption (Figure 2(a)). When negative potential was applied (e.g., -0.8 V), however the methylene blue was reduced electrochemically to leuco-methylene blue (colorless), as indicated by the flat absorbance spectrum in the visible region (Figure 2(c)). This spectroelectrochemical behavior showed that when the fiber optic-core was covered with a gold mesh electrode, it exhibited traits that were favorable for spectroelectrochemical sensor performance. The effective optical path length calculated from Lambert-Beer law (ε= 110,000) was ca. 0.17 mm. However, this value is not considered as the adsorption of methylene blue on the fiber optic surface. Figure 3 shows the peak current plots and the changes in absorbance vs. a sub duplicate of the potential sweep rates for 2-500 mV/s. The peak current changes in the cyclic voltammetry and absorbance at different potential sweep rates were measured using a fiber optic electrode sensor with 5×10 -5 M of methylene blue. The peak currents for methylene blue increased linearly with the square root of the scan rate (curve b). This indicated that the electrode processing of methylene blue was controlled by diffusion. The changes in absorbance were measured simultaneously with the decreases in cyclic voltammetry as the scan rate increased (curve a). At higher scan rates, the optical signal modulation was decreased because of in-



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complete electrolysis in the sensing region of the fiber optic surface. At these faster scan rates, the electrochemical diffusion layer generated by the gold mesh was decreased and the thickness was insufficient to reach the evanescent field on the fiber optic surface. As a result, only the methylene blue molecules that were near the gold mesh surface were cycled through the redox process. At slow scan rates, however, the diffusion layer exceeded the thickness required to reach the evanescent field on the fiber surface, and thus complete electrochemical redox cycling was accomplished.

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to obtain a stable background signal for 1 min; (2) the potential then was stepped to reduce either the methylene blue or the ferrocyanide to a colorless state and was held there for 1 or 2 min; and, (3) finally, the potential was stepped back to its oxidant state for another 1 or 2 min to regenerate, and to allow the absorption of either the methylene blue or the ferricyanide. The changes in absorbance vs. time for several concentrations of methylene blue and ferrocyanide obtained from the potential-step chronoabsorptometry are shown in Figure 5. The modulations of the absorbance that accompanied the electrochemical oxidation and reduction by the potential steps were successfully monitored. Modulation intensity for the optical transmission generated by electrochemical manipulation showed a good linear relationship with the concentration (R2 = 0.9985 for methylene blue and R2 = 0.9931 for ferrocyanide). The limits of detection, which were determined to be 3-fold the standard deviation of the blank (3σ), for the methylene blue and the ferrocyanide were 2.0×10-7 M and 1.6×10-3 M, respectively. The sensitivity for the methylene blue was better than that of the previous electroactive fiber optic chip sensor. 35

Figure 3. Changes in (a) absorbance at 580 nm, (b) peak current obtained from measurements of 5.0×10-5 M methylene blue at various scan rates (2-500 mV/sec). The measurements were done on the bare fiber optic electrode using cyclic potential scanning in a range of from 0.5 to -0.8 V.

The electrochemical and spectral properties of the bare fiber optic electrode sensor were investigated based on the potential step methods by chronoamperometry and ATR i.e., potential-step chronoabsorptometry. This measurement was made by first applying a potential of 0.5 V to obtain an initial spectrum of the methylene blue and then stepping to -0.8 V to monitor the reduction in the methylene blue. In the previous experiments using cyclic voltammetry, a reversible cyclic voltammogram was observed with an anodic peak at 0 V, and a cathodic peak at -0.3 V. Therefore, the selected optimal potentials for potential-step chronoabsorptometry were 0.5 and 0.8 V, respectively, for the electrochemical oxidationreduction process to occur. Absorbance spectra for 5×10-5 M of methylene blue was obtained from the fiber optic electrode sensor by applying a 0.5 V potential followed by -0.8 V for 400 sec, as shown in Figure 4(A). The absorbance at 580 nm decreased significantly with increasing time at -0.8 V as the accumulated methylene blue on the fiber surface was electrochemically reduced. The optimum reaction time to reach equilibrium was estimated to be approximately 2 min, as shown in Figure 4(B). The demonstrated potential-step chronoabsorptometry was suitable for evaluating the spectroelectrochemical performance of a fiber optic electrode sensor. Performance of the Bare Fiber Optic Electrode Sensor in the ATR Mode for the Determination of Methylene Blue and Ferrocyanide. The spectroelectrochemical determination of methylene blue and ferrocyanide was investigated via potential-step chronoabsorptometry experiments, as shown in Figure 5. All measurements were performed as follows: (1) The potential was first held at a value (0.5 V) that maintained methylene blue and ferricyanide in their colored states in order

Figure 4. (A)Absorbance spectra of 5.0×10-5 M methylene blue due to the attenuation of an evanescent wave at the bare fiber optic electrode. The measurements used absorbance spectroelectrochemistry by applying potentials at +0.5 V and then at -0.8 V for 400 sec. (B) The plots of absorbance at 580 nm with time for applying potential at -0.8 V.


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Figure 5. Spectroelectrochemical behavior of (A) methylene blue, (a) 5.0×10-6, (b) 1.0×10-5, (c) 2.5×10-5, and (d) 5.0×10-5 M. (B) ferrocyanide (a) 4.0×10-2, (b) 8.0×10-2, (c) 1.0×10-1, and (d) 2.5×10-1 M. The measurements were done on a bare fiber optic electrode sensor in potential steps from 0.5 V to -0.8 V for a total of 6 times at a pulse width of 60 sec for ferrocyanide and 120 sec for methylene blue.

However, further sensitivity improvement is required for species with a low molar absorptivity, such as with ferrocyanide For example, the detection limit of the spectroelectrochemical sensor reported by Heineman et al. was 8.0×10-6 M.11 Improvement in Sensitivity via Modification of the Self-Assembled Monolayer. The sensitivity and selectivity were improved by utilizing a self-assembled monolayer (SAM), which is capable of accumulating the analyte into the sensing portion of the fiber based on electrostatic adsorption. The spectroelectrochemical measurements for methylene blue and ferrocyanide were demonstrated using a charge-selective SAM coated onto the fiber optic electrode. Figure 6(A) shows the absorbance spectra obtained from the bare fiber optic electrode (dash line) and the electrode coated with three layers of PCBS (solid line) for 1.0×10-5 M of methylene blue when applied with a potential of -0.8 V for 400 sec. The absorbance was drastically increased when charge-selective film was coated onto the fiber surface. However, complete absorbance recovery was not achieved within 400 sec when using the applied potential for methylene blue reduction. These results showed that PCBS modified on the fiber surface accumulated methylene blue via electrostatic interaction. An electrochemical reduction of the methylene blue molecules that had accumulated in the charge-selective film required a longer period of time. This could be due to slower diffusion of methylene blue in the charge-selective film than in solution. The change in absorbance caused by the number of PCBS layers is shown in Figure 6(B). The absorbance change increased with increases in the number of poly anion layers and became a constant value after three layers. About 0.2 of the absorbance changes were obtained from the fiber optic electrode coated by PCBS at 1.0×10-5 M of methylene blue, although only ca. 0.02 of the changes were shown for bare fiber sensors. This indicated that methylene blue molecules were successfully accumulated by the negatively charged selective film.

Figure 6. (A) Absorbance spectra of 1.0×10-5 M methylene blue with bare fiber optic electrode sensor (dash line) and fiber optic electrode sensor coated PCBS (solid line). The measurements were done using spectroelectrochemistry by applying a potential at 0.5 V and then at -0.8 V for 400 sec. (B) Changes in absorbance responses for 1×10-5 M methylene blue for increasing number of PCBS layers.

Figure 7 shows the calibration curves for methylene blue obtained from the bare fiber optic electrode sensor (curve a), the PCBS-coated fiber optic electrode (curve b), and the APTES-coated fiber optic electrode (curve c). A drastic improvement in sensitivity was achieved by using the negatively charged PCBS-coated fiber optic electrode. In this case, the limits of detection (3σ) were estimated to be 8.3×10-9 M (R2=0.9796). The exponentially shape of curve (b) is due to the saturation of the charge-selective film at a higher concentration of methylene blue. In the case of the modified APTES, which was positively charged, the change in absorbance was scant, compared with that of the bare fiber optics. This was due to the electrostatic repulsion between the methylene blue and the APTES that modified the fiber surface. This suggested that methylene blue was successfully accumulated and enhanced the signal via the charge-selective film.



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Ni2+ (pH = 9.5) was prepared to mimic the waste solution at the Hanford site.21 Figure 9 shows the spectroelectrochemical response for 0.2 M ferrocyanide on an APTES-coated fiber optic electrode sensor. The five consecutive sample measurements were first performed by applying a potential of -0.8 V and then at 0.5 V for 400 sec. The average optical modulation obtained from the Hanford simulated solution (blue line) and 0.2 M of ferrocyanide in PBS showed almost no differences with a recovery of 92% at 0.2 M of ferrocyanide. The slopes of the calibration curves obtained from the simulated waste and pure samples were 0.413 and 0.425 (absorbance change at 420 nm / concentration of ferrocyanide (M)), respectively. Hence, calibration plots for ferrocyanide under high ionic strength and alkaline conditions (pH 9.5) can be obtained with reasonable linearity in order to determine the presence of ferrocyanide.

Figure 7. Calibration curves for the methylene blue obtained from (a) a bare fiber optic sensor, (b) a PCBS-modified fiber optic sensor, and, (c) an APTES-modified fiber optic sensor. The measurements were performed by a double potential step ranging from 0.5 V to -0.8 V at a pulse width of 400 sec.

An improvement in sensitivity was attempted for ferrocyanide in the fiber optic electrode coated with APTES, as shown in Figure 8. The spectral changes in ferrocyanide from a potential of +0.5 V, for the bare fiber optic electrode (dash line) and the APTES-modified fiber optic electrode (solid line), are shown in Figure 8(A). Although a significant improvement did not occur when compared with methylene blue, a decrement in the absorbance was observed by APTES modification. Figure 8B compares the calibration curves of these sensors. The bare fiber optic electrode contributed approximately 0.05 to the absorbance change for 0.25 M of ferrocyanide (curve a). However, the APTES coated fiber optic electrode sensor provided 0.1 of the absorbance change at the same concentration, which indicated better sensitivity (curve b). The optimal time for electrolysis was similar to that for the bare fiber optic electrode. The detection limit (3σ) improved to 7.1×10-4 M (R2=0.9967). These results highlight the potential of PCBS and APTES as a good strategy to improve the sensitivity of a spectroelectrochemical fiber optic sensor. Sensing Ferrocyanide in a Hanford Tank-Simulated Solution. Spectroelectrochemical sensing for ferrocyanide was investigated using simulated radioactive waste solution in the form of Cs2NiFe(CN)6 from tank mixtures containing NO3and NO2- salts, K4Fe(CN)6, Na4Fe(CN)6, and NiSO4, all of which were used to precipitate radio-cesium from the subsurface sediments at the Hanford Site, USA. Tank leakage is one of the most threatening ecological problems, and to overcome it liquids are pumped out from the tanks leaving residue that contains ferrocyanide precipitates. The mixtures of ferrocyanide and nitrate are potentially explosive at elevated temperatures under dry conditions. In order to resolve the safety concerns of ferrocyanide waste, e.g., at the Hanford Site, a realtime remote sensor which incorporates three modes of selectivity is critical for the evaluation of radio-active waste tanks before and during the disposal process.20,21 In the present study, a solution containing 0.00938 M to 0.2 M K4Fe(CN)6, 2.0 M NO3-, 1.0 M NO2-, 0.2 M SO42-, 0.15 M PO43-, and 0.005 M

Figure 8. (A) Absorbance spectra of 2.5×10-1 M ferrocyanide with a bare fiber optic electrode sensor (dash line) and a fiber optic electrode sensor coated with APTES (solid line). The measurements were done using spectroelectrochemistry by applying a potential at -0.8 V and then at 0.5 V for 1 min. (B) Calibration curves for ferrocyanide obtained from (a) a bare fiber optic electrode sensor and (b) an APTES-modified fiber optic electrode sensor.


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Analytical Chemistry †

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‡

University of Toyama Maebashi Institute of Technology

REFERENCES (1) (2)

Figure 9. Sensor responses for 0.2 M ferrocyanide in the simulated radio-active waste sample (blue line) and in a pure sample (red line). Changes in the absorbance at 420 nm of the sensor induced by chronoamperometry between the limits of -0.8 V and 0.5 V at a pulse width of 400 sec.

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CONCLUSIONS This paper introduces a novel approach for a spectroelectrochemical sensor based on fiber optics. The approach was demonstrated via the detection of methylene blue and ferrocyanide as model analytes. The spectroelectrochemical sensor consists of a multimode core exposed fiber covered with gold mesh as a working electrode with a core surface coated by a charged selective film for the accumulation of an analyte. ATR cells for spectroelectrochemical measurements typically employ a transparent electrode integrated into a waveguide. However, the simple design proposed in this study in which the electrode and optical element are spatially separated provides a great flexibility in terms of material that can be utilized as electrode. On the other hand, a long equilibrium time required is for this sensor when compared to the previous works reported by Heineman et al.23 This disadvantage could be overcome by using a much smaller volume of spectroelectrochemical cell. Trimodal selectivity based on absorbance changes, electrolysis potential, and electrostatic adsorption by a SAM-modified fiber core surface was simultaneously demonstrated using this new fiber optic sensor. The performance of this sensor was further proven by the high recovery ratio obtained from the detection of ferrocyanide contained by a Hanford simulated solution. The major advantages of the spectroelectrochemical fiber optic sensor are as follows: 1) fiber optics can be successfully utilized based on the spectroelectrochemical method to further improve sensitivity; and, 2) this sensor could be applied to real-time remote sensing. By simultaneously using the three modes of selectivity, this sensor can be applied to other analytes, and the effectiveness of the fiber optic sensor will be further investigated in future work.

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ACKNOWLEDGEMENT

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This research was supported by the Japan Society for the Promotion of Science (JSPS), and a Grant-in-Aid for Scientific Research (No. 24550095) is gratefully acknowledged.

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AUTHOR INFORMATION (28)

Corresponding Author * To whom correspondence should be addressed. E-mail: [email protected]

Present Addresses

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“For TOC graphic only”


Demonstration of the Concept Using Methylene Blue and Fer

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