Sensing with an Optically Transparent Electrode

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Simultaneous Multiselective Spectroelectrochemical FiberOptic Sensor: Sensing with an Optically Transparent Electrode Takuya Okazaki, Eri Shiokawa, Tatsuya Orii, Takamichi Yamamoto, Noriko Hata, Akira Taguchi, Kazuharu Sugawara, and Hideki Kuramitz Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03957 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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Analytical Chemistry

Simultaneous Multiselective Spectroelectrochemical Fiber-Optic Sensor: Sensing with an Optically Transparent Electrode Takuya Okazaki,† Eri Shiokawa,† Tatsuya Orii,† Takamichi Yamamoto,† Noriko Hata,† Akira Taguchi,‡ Kazuharu Sugawara,§ and Hideki Kuramitz*,† †

Department of Environmental Biology and Chemistry, Graduate School of Science and Engineering for Research, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan ‡ Hydrogen Isotope Science Research Center, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan § Maebashi Institute of Technology, Maebashi, Gunma 371-0816, Japan ABSTRACT: We present a spectroelectrochemical fiber-optic sensor with an optically transparent electrode. The sensor was fabricated by coating indium tin oxide (ITO) onto the surface of fiber-optic core chips using a polygonal barrel-sputtering method. The ITO-coated fiber-optic probe can be simply and cheaply mass-produced and used as a disposable probe. The sensing is based on changes in an attenuated total reflection signal accompanying the electrochemical oxidation–reduction of an analyte at the electrode. The properties of an ITO-coated fiber-optic probe as an optically transparent electrode were investigated for varying thicknesses of ITO. The sensor responses were successfully enhanced with an additional level of selectivity via an electrostatically adsorbed, selfassembled monolayer, which comprised a polyanion and polycation.

Spectroelectrochemistry has been used for more than five decades in the investigation of a wide variety of inorganic, organic, and biological redox systems.1 In general, spectroelectrochemical measurements were conducted using a thin-layer spectroscopic cell incorporating optically transparent electrodes (OTEs) composed of indium tin oxide (ITO), carbon, platinum or gold mesh. An advantage of spectroelectrochemistry is the cross-correlation of information that is attainable from simultaneous electrochemical and optical spectroscopy measurements.2 Heineman et al. developed spectroelectrochemical methods based on multimode selectivity, which can be simultaneously achieved in a single device based on attenuated total reflection (ATR).3-22 This concept combines the fundamentals of electrochemistry, optical spectroscopy, and selective partitioning in a single device. Application of the ATR method in spectroelectrochemical measurements 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 a 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. Recently, the photochemical and fiber-optic evanescent wave-based sensing techniques have been popular for analyzing chemical and biochemical properties.23-26 Fiber-optic sensing offers many advantages for measurement equipment: high sensitivity, simplicity, low cost, and real-time performance for remote monitoring. We reported fiber-optic sensors based on changes in attenuation of light that passes through the fiber core for scale formation, biofilms, and anionic surfactants.27-32 Beam et al. reported a fully integrated multimode fiber-opticcoupled spectroelectrochemical internal reflection element

platform.33 The electroactive fiber-optic probe consists of a side-polished fiber-optic 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 sidepolishing procedure to form a planar sensing region. Our group has presented a spectroelectrochemical fiber-optic sensor.34 That sensor consists of a gold mesh cover on a multimode fiber-optic that uses ATR as the optical detection mode. The simple design, in which the electrode and optical element are spatially separated, provides great flexibility in terms of the material that can be utilized as the electrode. In this study, we demonstrate a spectroelectrochemical fiberoptic sensor with an optically transparent thin-film working electrode consisting of ITO, which was coated on the fiber-optic core by a polygonal barrel-sputtering method.35,36 ITO has the advantages of excellent optical transparency, wide electrochemical potential windows, and stable chemical and physical properties. Generally, it is difficult to coat the entire surface of a cylindrical object such as a fiber-optic. In the polygonal barrel-sputtering method, a polygonal barrel was placed in the vacuum chamber to hold exposed fiber core chips. The barrel could be rotated using a motor device during sputtering, making it possible to coat ITO onto the entire surface of the fiber chips. This method offers cheap and simple mass production for disposable probes. Figure 1 shows a schematic of the spectroelectrochemical setup and sensor concept. Sensing was based on changes in the attenuation of an evanescent wave generated on the surface of the ITO-coated fiber probe, which accompanied electrochemical analyte oxidation or reduction. The spectroelectrochemical performance of the ITO-coated fiber probe was evaluated using methylene blue as the redox indicator. In addition, the enhancement of selectivity and sensitivity was demonstrated using a self-assembled monolayer

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(SAM), which can be accumulated by the electrostatic adsorption of an analyte on the sensor surface.

3 mm × 10 mm, 0.35 mm thickness), and Ag/AgCl as the reference electrode (RE-1B, BAS). Both ends of the ITO-coated fiber-optic probe were connected to the FT400EMT fibers, which were connected to a light source and a detector (Figure 1A). The sample stage comprised a 2.5 cm × 5.0 cm glass plate covered with Parafilm. Before measurement, hydroxylation of an optical fiber was accomplished by immersing the ITO fiber in 1.0 M sodium hydroxide for 60 min, which was followed by rinsing with Milli-Q water.

Preparation of a SAM on the Fiber-Optic

Figure 1. Schematic of (A) the experimental setup of the spectroelectrochemical fiber-optic sensor with an OTE and (B) the detection principle of the sensor for electrochemical reduction– oxidation of the analyte.

EXPERIMENTAL SECTION

The surface of the ITO fiber was activated by soaking in 1.0 M sodium hydroxide for 1 h. The SAM was then applied to the ITO-coated fiber probe according to the method reported by Van Dyke and Cheng.37 The surface was dipped in 0.5% (v/v) PDDA for 15 min at room temperature. In these experiments, PAH and PCBS were chosen as the polycation and polyanion, respectively. The combination of one monolayer of PAH and one of PCBS is denoted herein as a bilayer. The PCBS layer was formed on the PDDA layer by immersing it in a solution containing 10 mM PCBS for 15 min at room temperature. The core-immobilized PCBS/PDDA was then soaked in 10 mM PAH solution for 15 min. Under this condition, one bilayer (PCBS/PAH) of the self-assembled monolayer film was ∼1.5 nm in thickness.37

Chemicals and Materials

RESULTS AND DISCUSSION

Methylene blue, Na2HPO4, NaH2PO4, acetone, rhodamine B, and sodium hydroxide were obtained from Wako Pure Chemical Industries, Ltd. Poly(diallyldimethylammonium chloride) (PDDA), poly(allylamine hydrochloride) (PAH), and poly{1-[4-((3-carboxy-4-hydroxyphenyl)azo)benzenesulfonamido]-1,2-ethanediyl, sodium salt} (PCBS) were purchased from Sigma-Aldrich. All the chemicals were used as received without further purification, and Milli-Q water was used to prepare the solution. The supporting electrolyte for the spectroelectrochemical measurements was 0.1 M phosphate buffer solution (PBS; pH 7, 0.044 M NaH2PO4, 0.056 M Na2HPO4).

Cyclic Voltammetry of Methylene Blue

Apparatus A step index multimode plastic-clad silica fiber-optic (FT400EMT) with a 400 μm core diameter was purchased from Thorlabs. The fiber-optic was cut to a length of approximately 10 cm, and acetone was used to remove the plastic cladding from the fiber. An ITO layer was coated on the surface of the fiber-optic chips by a hexagonal barrel-sputtering method.35, 36 ITO was deposited at 40 sccm Ar under 10 Pa for 8, 25, and 53 min. The thicknesses of ITO on the fiber core were estimated by scanning electron microscopy observation of a cross section, and this can be controlled by adjusting the sputtering time. The spectroelectrochemical sensing based on the ITO-coated fiberoptic probe consisted of the following instrumental setup. A deuterium halogen and tungsten lamp was used as the light source (JASCO, model LUS-363). A portable spectrophotometer (JASCO, model MV-3100) was used as a detector for the analyte via the ATR signal obtained from the fiber-optic electrode sensor. All the voltammetric and applied potential measurements were conducted using an electrochemical analyzer, model 620CZ (ALS). The threeelectrode system used an ITO-coated fiber as the working electrode, a platinum mesh as the counter electrode (100 mesh,

The electrochemical behavior of the ITO coated on the fiber was investigated by cyclic voltammetry. Methylene blue was used as a representative redox indicator to evaluate the electrochemical performance of the sensor. Cyclic voltammograms obtained from sensors coated with 42, 104, and 208 nm of ITO are shown in Figure 2. The voltammograms had well-defined oxidation and reduction peaks that corresponded to redox coupling with methylene blue. Thus, methylene blue on the surface of the fiber-optics could be maintained in oxidized and reduced forms by applying potentials of approximately +0.4 and −0.6 V, respectively. The peak separations in all experiments linearly increased with the scan rate. The slopes of the regression lines decreased with increasing ITO thickness on the sensor surface; these were 0.053, 0.040, and 0.020 for 42, 104, and 208 nm of ITO, respectively (R2 = 0.9987, 0.9587, and 0.9788). The peak separation of 141 mV, which was the smallest of all the results, was much larger than the theoretical value of 29 mV for a twoelectron transfer for methylene blue/leucomethylene blue. The results indicate that the large peak separation is due to the electrode resistance of the ITO, which is caused by an increasing IR drop. For 42, 104, and 208 nm of ITO, the effective resistance, according to an Ohm’s law plot of peak current versus peak separation, was calculated as 31, 24, and 12 kΩ, respectively.

Performance of the Sensor for the Optical Determination of Methylene Blue The spectroelectrochemical determination of methylene blue was investigated via potential-step chronoabsorptometry experiments, as shown in Figure 3. All measurements were performed as follows: (1) The potential was first held at a value (+0.4 V) that maintained methylene blue in its colored state to obtain a stable background signal for 100 s. (2) The potential


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Figure 2. Cyclic voltammograms of spectroelectrochemical fiber-optic sensors coated with 42, 104, and 208 nm of ITO, using 0.1 mM methylene blue, as a function of scan rate. The scan rate was varied from 5 to 100 mV/s.

was then stepped to reduce the methylene blue to a colorless state and held for 100 s. (3) The potential was stepped back to regenerate and to allow absorption of the methylene blue. When an adequate positive potential was applied, oxidized methylene blue appeared colored with maximum absorption. When a negative potential was applied, however, the methylene blue was reduced electrochemically to leucomethylene blue (colorless). The modulations of the absorbance that accompanied the electrochemical oxidation and reduction during the potential steps were successfully monitored. The modulations increased as the thickness of ITO coated on the surface of the fiber core decreased. This could be due to the loss of effective optical path length penetrating the evanescent wave into the sample. In the case of the optical fiber-based ATR sensor, the light transmitted through the fiber sensor after interacting with the absorbing solution was measured with a spectroscopy detector. This technique is based on the penetration of an evanescent wave through the sensing sample when the light that is traveling within the waveguide undergoes total internal reflection. The maximum of the electric field amplitude is located at the interface but decays exponentially in a direction outward normal to the interface. The electric field amplitude, E, at a distance x is given by

deviation of the blank (3σ) for methylene blue, were 2.8 × 10−6 M. The sensitivity of the methylene blue was better than that of a previous electroactive fiber-optic chip sensor coated with a thin film of ITO.33 However, the sensitivity of the proposed sensor for methylene blue was approximately 10 times lower than that of the spectroelectrochemical fiber-optic sensor using gold mesh in our previous work. This was due to the loss of optical path length resulting from the thickness of the ITO coating mentioned above. Moreover, the sensor proposed in this study consists of a 400 μm diameter fiber core to improve the strength and electrical conductivity, whereas another sensor had a 200 μm diameter. It is known that a larger fiber-optic diameter decreases the sensitivity of the fiber-optic sensor based on ATR.27

− x  E = E0 exp d   p 

(1) where E0 is the electric field at the interface (in this case only, E and E0 are not indicated as the electrochemical potential) and dp is the penetration depth of the evanescent wave.38 It is suggested that the loss of penetration depth into the sample by modification of ITO on fiber optic core surface decreased the effective optical path length based on ATR. Therefore, the absorbance of methylene blue decreased exponentially as the thickness of the ITO coated on the fiber core increased. From these results, the optimal thickness of ITO for the spectroelectrochemical sensor was 42 nm. For a sensor with less than 42 nm of ITO, the electrical conductivity of the fiber as the working electrode was insufficient for the electrochemical measurement. The modulation intensity for the optical transmission at 580 nm generated by electrochemical manipulation in potential steps from +0.4 to −0.6 V showed a good linear relationship with the concentration range from 10 μM to 40 μM. The slope of the curve was 1.92 mM-1 (R2 = 0.995). The limits of detection, which were determined to be 3 times the standard

Figure 3. Potential-step chronoabsorptometry of 0.1 mM methylene blue in 0.1 M PBS. The measurements were performed on sensors coated with 42, 104, and 208 nm of ITO in potential steps from +0.4 to −0.6 V four times at a pulse width of 100 s.

Figure 4(A) shows the absorption spectra of 25 μM methylene blue for a series of increasingly negative applied potentials that incrementally converted methylene blue to leucomethylene blue. An absorption band of methylene blue was observed at 580 nm, that is assigned to the trimer of methylene blue.39 This shape of the spectrum was not analogous to that of methylene blue monomer at 660 nm obtained from the solution by mean of spectrophotometer. These results indicate the adsorption of methylene blue onto the ITO surface of this sensor.32,40 Each potential was held until the solution redox equilibrated with the applied potential, as confirmed by no further changes in the spectrum. It took about 200 s for 25 μM methylene blue to reach equilibrium. The absorbance of methylene blue decreased with potential. Figure 4(B) shows the



absorbance at 580 nm as a function of applied potential. The absorbance profile shows a continuous sigmoid curve. From the curve, E0 determined using the spectroelectrochemistry version of the Nernst equation is −340 mV versus Ag/AgCl.

However, the absorbance change was approximately 10% at 670 nm within 400 s when using the applied potential for methylene blue reduction.34 This could be due to the slower diffusion of methylene blue in the charge-selective film than in solution.

Figure 5. Absorption spectra of 40 μM methylene blue in 0.1 M, pH 7 PBS containing 50 μM rhodamine B while applying a constant potential first at +0.6 V and then at −0.6 V for 100 s (solid line) and the absorption spectrum of 50 μM rhodamine B (dashed line). The spectra were obtained every 2 s after applying the potential at −0.6 V.

Figure 4. (A) Absorption spectra of 25 μM methylene blue in 0.1 M, pH 7 PBS for different applied constant potentials. (B) Absorbance at 580 nm as a function of applied potential.

Performance of the Sensor for Selective Detection of Methylene Blue in Rhodamine B To demonstrate the selectivity of the fiber sensor in the detection of methylene blue redox, it was tested in 0.1 M PBS containing rhodamine B. Rhodamine B does not indicate any electrochemical response with the potential range in this investigation. The cyclic voltammograms of MB, rhodamine B, and their mixture are shown in Figure S-1. Figure 5 shows the absorption spectra of a solution containing 40 μM methylene blue and 50 μM rhodamine B under an applied potential from +0.6 to −0.6 V. Only modulation of the absorbance that accompanied the reduction of methylene blue in the mixture was successfully obtained. The calibration curves were obtained from the samples with and without 50 μM rhodamine B in the range of 10 to 40 μM methylene blue at a wavelength of 600 nm. The slopes of the curves with and without rhodamine B were 1.86 and 1.92 mM-1 (R2 = 0.998 and 0.995), respectively. Hence, calibration plots for methylene blue coexisting with non-electroactive dye can be obtained with reasonable linearity to determine the presence of methylene blue.

Improvement in Sensitivity by Modification of the SAM The sensitivity can be improved by utilizing a SAM, which can accumulate the analyte into the sensing portion of the fiber on the basis of electrostatic adsorption. In our previous study, this consisted of a gold mesh cover on a fiber-optic core, and the absorbance of methylene blue was drastically increased when this charge-selective film was coated onto the fiber surface.34

Figure 6 shows the absorption spectra obtained from an ITO fiber probe with three bilayers of PAH and PCBS for 1.0 × 10−5 M methylene blue, to which was applied a potential of −0.6 V for 150 s. The inset shows the absorbance changes as a function of time after applying the potential. The maximum absorption was observed at 670 nm, whereas that was 580 nm in the absence of SAM modification. The absorption peak at 670 nm was similar to monomer band observed in the bulk solution using spectrophotometer.39 The change in the wavelength of maximum absorbance suggests the dissolution of methylene blue into SAM film rather than the adsorption onto the sensor surface. The absorbance decreased to a constant value within 40 s for applying a potential at −0.6 V. The SAM-modified sensor showed 11 times the sensitivity of that without SAM from the slopes of the calibration curves obtained in the range of 0.5 μM to 1 μM at a wavelength of 670 nm (R2 = 0.971). An improvement in sensitivity was achieved using the negatively charged PCBS-coated ITO fiber probe. The limit of detection (3σ) was estimated to be 1.7 × 10−7 M. From the results, the ITO coating on the fiber core improved the electrolysis efficiency of methylene blue in the SAM film as compared to covering the sensor with gold mesh. In the case of the sensor with gold mesh, the reduction of methylene blue occurred on the surface of the mesh, which was located outside the surface of the fiber core with the SAM film. The improvement in electrolysis efficiency for the proposed sensor was due to the reduction of methylene blue in the evanescent field of the SAM-modified ITO fiber probe. However, complete electrolysis was not achieved within 40 s. It might be possible that methylene blue was supplied from the bulk solution more rapidly than leucomethylene blue was produced by the ITO electrode in the evanescent field.

CONCLUSIONS We proposed a spectroelectrochemical sensor based on fiberoptics. This sensor consists of a core-exposed fiber-optic coated


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Analytical Chemistry with ITO by a polygonal barrel-sputtering method. The proposed sensor was demonstrated for absorption of methylene blue/leucomethylene blue based on spectroelectrochemistry. In addition, the accumulation of methylene blue using a SAM film was achieved. The advantages of the sensor proposed in this study are cheap and simple mass production for disposable probes, capacity for real-time and remote monitoring, a wide potential and optical window, small sample volume, small size, and stable chemical and physical properties. On the other hand, the sensitivity of the proposed sensor for methylene blue was slightly lower than that of the spectroelectrochemical fiber sensor described in our previous work, which could be due to the loss of optical path length owing to the thickness of the ITO coating. This problem can be overcome by using a material with much higher electrical conductivity as an OTE. The OTEcoated fiber sensor could be applied to chemical or biochemical analysis in future studies.

Figure 6. Absorption spectra of 1.0 × 10−5 M methylene blue with an ITO electrode fiber-optic sensor coated with three bilayers of PAH and PCBS for applying a potential at −0.6 V. Inset: the absorbance changes at 680 nm as a function of time after applying the potential.

ASSOCIATED CONTENT Supporting Information Supporting Information Available: [The cyclic voltammograms of MB, rhodamine B, and their mixture]

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel. & Fax: +81 76 445 6669

ORCID Takuya Okazaki: 0000-0002-1008-7037 Hideki Kuramitz: 0000-0002-9102-3667

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the Japan Society for the Promotion of Science (JSPS) via a Grant-in-Aid for Scientific Research (JP16H02976).

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Sensing with an Optically Transparent Electrode

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