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A Universal Design of Selectivity-Enhanced Photoelectrochemical Enzyme Sensor: Integrating Photoanode with Biocathode Zhen Song, Gao-Chao Fan, Zimeng Li, Fengxian Gao, and Xiliang Luo Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02651 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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Analytical Chemistry

A Universal Design of Selectivity-Enhanced Photoelectrochemical Enzyme Sensor: Integrating Photoanode with Biocathode Zhen Song, Gao-Chao Fan*, Zimeng Li, Fengxian Gao, and Xiliang Luo*

Key Laboratory of Sensor Analysis of Tumor Marker, Ministry of Education; Shandong Key Laboratory of Biochemical Analysis; Key Laboratory of Analytical Chemistry for Life Science in Universities of Shandong; College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, P. R. China

* To whom correspondence should be addressed.

* E-mail: [email protected] (G.-C. Fan)

* E-mail: [email protected] (X. Luo)

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Abstract: Previous works on photoelectrochemical (PEC) biosensors have demonstrated that photoanode-based type possesses satisfying sensitivity, because photoanode utilize electrons as the majority charge carriers and a distinct photocurrent can be generated when electron donors are furnished. However, as hole-oxidation reaction occurs at the photoanode interface, photoanode-based PEC sensor has inferior anti-interference capacity to reductive substances coexisting in the biological sample, leading to a challenged selectivity. Herein, a universal design on selectivity-enhanced PEC enzyme sensor was proposed by integrating photoanode with biocathode. Specifically, CuInS2 sensitization layer and ZnS passivation layer were deposited in sequence on the TiO2 film modified indium−tin oxide (ITO) electrode mainly by successive ionic layer adsorption and reaction (SILAR) means, forming the hybrid ZnS/CuInS2/TiO2/ITO photoanode. A carbon fiber paper (CFP) electrode was modified with biocatalysts of enzymes via the assistance of chitosan (CS) to fabricate the biocathode. Utilizing glucose oxidase (GOx) and horserdish peroxidase (HRP) as biocatalysts, a selectivity-enhanced PEC sensor for glucose was developed. The PEC sensing platform integrating photoanode with biocathode not only inherits distinct photocurrent of the photoanode-based sensor; but also possesses enhanced selectivity, because just biocathode was incubated in the biological sample and there is on interaction between photoanode and coexisting reductive substances.

Keywords: Photoelectrochemical; Enzyme sensing; Selectivity-enhanced; Photoanode; Interface

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INTRODUCTION As a promising and powerful analytical method, photoelectrochemical (PEC) technique has attracted increasing attention resulting from its fascinating features of simple device, low-cost, easy operation, high sensitivity, and low background signals.1 Tremendous developments recently have witnessed to use the PEC sensors for the detection of various targets, including enzymatic sensing,2,3 immunoassay,4,5 DNA analysis,6,7 cell-based analysis,8,9 etc. Photoactive material and biocatalyst are the two necessary elements for the construction of a PEC enzyme sensor. Thereinto, PEC properties of the photoactive material play crucial roles in analytical performances of the enzyme sensor. In terms of the difference of the main charge carrier, the applied semiconductors in PEC sensors can be divided into n-type and p-type.10 Concisely, n-type ones use electrons as the majority charge carriers, whereas p-type ones use holes as the majority charge carriers. The photoanode is developed on the n-type PEC matrix, while the photocathode is constructed on the p-type PEC matrix.11 The popular n-type semiconductors refer to TiO2, ZnO, CdSe, CdS, CdTe, Bi2S3,12-17 etc., and the p-type ones include BiOI, NiO, Cu2O, CuS, PbS,18-23 etc. From an extensive survey on the reported PEC enzyme sensors, the photoanode-based ones have satisfactory sensitivity, because electrons serve as majority charge carriers in the system and a distinct photocurrent arise when electron donor exists. However, as hole-oxidation reaction occurs at the photoanode interface, photoanode-based enzyme sensors has inferior anti-interference nature to reductive agents (such as glutathione, dopamine and ascorbic acid) coexisting in the biological sample,24-27 bringing about a challenged selectivity. To pursue an enhanced selectivity, the PEC enzyme sensors fabricated on photocathode appear, because 3



electron-reduction reaction occurs at the photocathode interface and has no interaction with reductive agents coexisting in the biological sample.19,28 Unfortunately, photocathode-based sensors utilize holes as the majority charge carriers and need electron acceptors to neutralize the excited electrons at the photocathode interface, which makes the electron donors out of action, thus causing evidently decreased photocurrent signal and poor sensitivity comparing to photoanode-based ones.11 In short, the photocathode-based enzyme sensor sacrifices the sensitivity to enhance the selectivity. It is thus very desirable and necessary to develop PEC enzyme sensors with enhanced selectivity on the premise of retaining its sensitivity. We propose herein a universal design of selectivity-enhanced PEC enzyme sensor without sensitivity sacrifice, by an inspired strategy of integrating photoanode with biocathode, as exhibited in Scheme 1. Glucose was used as a target model to exhibit and describe the proposal. The biocatalysts of glucose oxidase (GOx) and horserdish peroxidase (HRP) were immobilized together on the carbon fiber paper (CFP) electrode via the assistance of chitosan (CS) to build the biocathode. The CuInS2 sensitization layer and ZnS passivation layer were coated in order on the TiO2 film modified indium−tin oxide (ITO) electrode mainly by successive ionic layer adsorption and reaction (SILAR) strategy, forming the hybrid ZnS/CuInS2/TiO2/ITO photoanode to offer an evident photocurrent output. The target of glucose was detected by the remarkable decrease of the photocurrent signal rooting from the GOx&HRP-induced biocatalytic precipitation on the biocathode surface.

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Scheme 1. The Proposed PEC Enzyme Sensor Integrating Photoanode with Biocathode.

EXPERIMENTAL SECTION The materials and reagents, apparatus, preparation of the hybrid ZnS/CuInS2/TiO2/ITO photoanode, preparation of the biocathode, and conditions of the PEC measurement were described in the Supporting Information.

RESULTS AND DISCUSSION

Design principle. The photogenerated electron-hole transfer process of the PEC enzyme sensor integrating photoanode with biocathode was illustrated in Scheme 2. The hybrid ZnS/CuInS2/TiO2/ITO photoanode had excellent PEC properties of evident photocurrent output and high stability. (i) For evident photocurrent output, CuInS2 as a narrow-band-gap (~1.5 eV) semiconductor coupling with wide-band-gap (~3.2 eV) of TiO2 can increase the harvest of short-wavelength light notably, and meanwhile the stepwise band-edge levels of TiO2 and CuInS2 benefits the injection of excited electrons from CuInS2 to TiO2 and the holes from CuInS2 to TiO2,29 which effectively inhibited the recombination of electron-hole pairs. 5



(ii) For high stability, the wide-band-gap semiconductor of ZnS (~3.6 eV) as a passivation layer was applied on the surface of the PEC electrode, which could lessen surface defects of the narrow-band-gap of CuInS2 and thus blocked charge recombination effectively.30 Different from traditional photoanode-based PEC enzyme sensor, we used just biocathode without photoactive materials as the probe electrode to enhance the selectivity of the enzyme sensor. This strategy perfectly avoided the interaction between photoanode and reductive agents coexisting in the biological sample, because only biocathode was incubated in the target-related sample, making coexisting reductive agents just access the biocathode interface and have no contact with the photoanode. When biocathode was incubated in glucose with 4-chloro-1-naphthol (4-CN), the biocatalyst of GOx could catalyze the oxidation of glucose to generate H2O2, and meanwhile, HRP catalyze H2O2 to oxidize 4-CN to produce insoluble and insulating product of benzo-4-chlorohexadienone on the biocathode surface,31,32 causing obvious decrease in the photocurrent signal during the PEC measurement, and thus the target of glucose could be detected with high sensitivity and enhanced selectivity.

Scheme 2. The Photogenerated Electron-Hole Transfer Process of the Enzyme Sensor.

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PEC properties of the ZnS/CuInS2/TiO2/ITO photoanode. As an n-type semiconductor, TiO2 has a wide-band-gap (~3.2 eV), which can absorb only the ultraviolet light. Yet, it has the fascinating features of environmentally friendly, high stability and good biocompatibility, making it very suitable for serving as the PEC matrix.33,34 As another environmentally friendly semiconductor, CuInS2 has a narrow band gap of 1.5 eV and is a promising alternative to the toxic photoactive species currently in use.35,36 Besides, the conduction band edge of CuInS2 is higher than that of TiO2, contributing to the injection of excited electrons from CuInS2 to TiO2.29 Thus, incorporation of CuInS2 with TiO2 can increase light harvest and enhance photocurrent intensity evidently. The wide-band-gap semiconductor of ZnS (~3.6 eV) was normally introduced as a passivation layer for PEC electrode,37,38 which could effectively blocked charge recombination and lessen surface defects of the narrow band-gap of CuInS2. The hybrid ZnS/CuInS2/TiO2/ITO photoanode was therefore first applied to boost the photocurrent signal of the elaborated enzyme sensor. And the fabrication parameters were optimized. Figure 1A exhibits photocurrent output of the TiO2/ITO electrode with changed concentrations (0.5, 0.75, 1.0, 1.25, 1.5, 1.75, and 2.0 mg/mL) of TiO2 suspension. With the increase in the concentration, the photocurrent output of the TiO2/ITO increased gradually in the beginning and then started to decrease after the concentration reached 1.25 mg/mL. It was because the increased TiO2 concentration could supply more amounts of TiO2 to absorb the ultraviolet light at first resulting in the increased photocurrent intensity, and then the excessive TiO2 produced more and more surface recombination centers to block the charge transfer causing the decrease in photocurrent intensity. 7



Figure 1B displays photocurrent output of the CuInS2/TiO2/ITO electrode obtained with varied SILAR cycles (0, 3, 6, 9, 12, and 15) of CuS intermediate. It was found that the CuInS2/TiO2/ITO electrode obtained with nine SILAR cycles of CuS intermediates owns the optimal photocurrent intensity. Along with the increase of the SILAR cycle, the amounts of CuInS2 on the electrode gradually raised, leading to increased light harvest and photocurrent intensity. After the optimal SILAR cycle researched, the excessive amount of CuInS2 served as surface recombination centers to increase the diffusion resistance for electron motion, and the photocurrent intensity decreased. Figure 1C shows photocurrent output of the hybrid ZnS/CuInS2/TiO2/ITO photoanode with varied SILAR cycles of ZnS. It could be obtained that two SILAR cycles of ZnS presented the optimal photocurrent intensity. Thus, 1.25 mg/mL of TiO2 suspension, nine SILAR cycles of CuS intermediates, and two SILAR cycles of ZnS were applied to fabricate of the hybrid ZnS/CuInS2/TiO2/ITO photoanode. Figure 1D exhibits time-varying photocurrent output of the hybrid ZnS/CuInS2/TiO2/ITO photoanode. As could be found, the photocurrent intensity of each irradiation period nearly stay the same and without obvious attenuation when the photoanode encountered repeated light illumination, indicating high stability of the ZnS/CuInS2/TiO2/ITO photoanode. Besides, the light-absorption of the ZnS/CuInS2/TiO2/ITO photoanode was characterized by UV-visible diffuse reflectance spectroscopy (UV-vis DRS), as displayed in Figure S1. After CuInS2 layer was deposited on the TiO2 film, an evident red-shift in absorption edge from ultraviolet to visible region was clearly observed, which reflected the small band-gap value of CuInS2 and also confirmed sensitization action of the CuInS2 layer. After ZnS layer 8


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further deposition, the light-absorption has no further red-shift, indicating wide band-gap value of ZnS and passivation action of the ZnS layer.

Figure 1. Photocurrent outputs of (A) the TiO2/ITO electrode with varied concentrations of TiO2 suspension, (B) the CuInS2/TiO2/ITO electrode with varied SILAR cycles of CuS intermediates, and (C) the ZnS/CuInS2/TiO2/ITO electrode with varied SILAR cycles of ZnS. (D) Time-varying photocurrent output of the ZnS/CuInS2/TiO2/ITO photoanode. SEM characterization of the hybrid ZnS/CuInS2/TiO2/ITO photoanode. The surface topography of the ZnS/CuInS2/TiO2/ITO photoanode was observed by scanning electron microscopy (SEM). As could be seen in Figure 2A, a large number of indium-tin oxide nanoclusters scattered on bare ITO electrode. After TiO2 coating, as shown in Figure 2B, plenty of small nanoparticles with a size distribution of 23−30 nm were well covered the electrode and formed mesoporous ﬁlm, which increased the specific surface area for more CuInS2 nanoparticles adhesion. After CuInS2 coating, as revealed in Figure 2C, the increase 9



in nanoparticle sizes and decrease in pore sizes were observed. The elemental mapping analysis in the inset of Figure 2C also pointed the deposition of CuInS2 on the electrode. After ZnS coating (Figure 2D), further increased nanoparticle sizes, further decreased pore sizes, and the elemental mapping analysis jointly reflected the deposition of ZnS on the electrode. Thus the SEM characterization illustrated the successful fabrication of the hybrid ZnS/CuInS2/TiO2/ITO photoanode. Besides, the X-ray photoelectron spectroscopy (XPS) characterizations including full-scan and high-resolution XPS spectra on the photoanode fabrication were performed (see Figure S2 and Figure S3), which further convinced the successful development of the ZnS/CuInS2/TiO2/ITO photoanode.

Figure 2. SEM images of the electrodes: (A) bare ITO, (B) TiO2/ITO, (C) CuInS2/TiO2/ITO, and (D) ZnS/CuInS2/TiO2/ITO. Insets in panels C and D were related to elemental mapping analysis of (Cu, In, and S) and (Cu, In, S, and Zn), respectively. SEM characterization of the biocathode. The surface topography of the biocathode during the modification process was monitored by SEM, as show in Figure S4. The pristine CFP electrode consisted of abundant carbon fibers (~7 µm in diameter) with very smooth 10


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surfaces (Figure S4A). After CS modification, the carbon fibers were embedded with gel-like film and the outline became blurred (Figure S4B). Figure S4C presents SEM image with high magnification after CS modification. Compared with Figure S4C, it could be clearly observed from Figure S4D, after GOx&HRP anchoring, the carbon fibers were covered with plenty of substances and the surfaces of the carbon fibers become rough. Hence, the change on surface morphology illustrated successful fabrication of the biocathode. EIS and PEC characterizations of the biocathode. The fabrication of the biocathode was also characterized by electrochemical impedance spectroscopy (EIS). As exhibited in Figure 3A, each impedance spectrum consisted of a semicircle at higher frequencies indicating electron-transfer resistance and a linear part at low frequencies pointing diffusion process. The semicircle diameter equals electron-transfer resistance (Ret), which points restricted diffusion of the redox probe approaching the interface layer. For the bare CFP electrode, the impedance spectrum presented a very small Ret (curve a, inset in Figure 3A). After CS coating and GOx&HRP anchoring in order, gradual increase in Ret was observed due to poor conductivity of CS layer and insulating effect of protein molecules (curves b and c). After the biocatalysts modified electrode was then incubated in 10 mM glucose with 10 mM 4-CN, the

Ret further increased (curve d), indicating biocatalytic precipitation with poor conductivity produced on the biocathode. The EIS characterization thus suggested successful fabrication of the biocathode. Figure 3B displays photocurrent characterization of the biocathode, with the hybrid ZnS/CuInS2/TiO2/ITO electrode as the photoanode. The bare CFP cathodic electrode had an evident photocurrent output (curve a). After the modification of CS and GOx&HRP on the 11



electrode in order, the photocurrent output decreased gradually (curves b and c) due to relatively weak electron-transfer of CS and steric hindrance of protein molecules. After the incubation of the electrode in 10 mM glucose with 10 mM 4-CN, a further photocurrent decrease was produced (curve d), implying the insoluble and insulating biocatalytic product was formed on the biocathode surface. The variation trend of the photocurrent output therefore further proved successful building of the biocathode.

Figure 3. (A) EIS and (B) Photocurrent outputs of (a) the CFP cathodic electrode, (b) after CS coating, (c) after GOx&HRP anchoring, and (d) after incubation with 10 mM glucose coexisting with 10 mM 4-CN. (C) Calibration curve of the PEC enzyme sensor for glucose detection from 0.1 µM to 5 mM. (D) Photocurrent signals of the enzyme sensor toward 1 mM glucose (Glu) without interfering species, or in the presence of 1 mM sucrose (Suc), ascorbic acid (AA), cysteine (Cys), uric acid (UA), dopamine (DA), and all their mixture (Mix), respectively. 12


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Detection performance. The PEC detection of glucose was based on GOx&HRP-induced biocatalytic precipitation on the biocathode surface, which gave rise to evident decease in photocurrent signal due to insulating effect of the insoluble precipitation. Being accompanied by the increased concentration of glucose, the insoluble product accumulated on the biocathode surface to block the electron transfer, and thereupon the photocurrent signal decreased gradually. As shown in Figure 3C, within the wide range of the concentration from 0.1 µM to 5 mM, the photocurrent signal lowered linearly with increase of logarithm of the glucose concentration. The regression equation was I = −19.72 + 1.51 log C (mM), with a correlation coefﬁcient of 0.9988. The limit of detection (LOD) was calculated to be 0.035 µM (S/N= 3), which was lower than those of previous PEC detection of glucose.3,19, 28, 39-41 Selectivity inspection. To solve the problem of inferior selectivity of the photoanode-based enzyme sensor, the tactics via integrating photoanode with biocathode was proposed. This design effectively avoided the contact between photoanode and reductive agents coexisting in the biological sample, resulting in enhanced-selectivity. Additionally, as no illumination on biocathode, the designed enzyme sensor also has anti-interference to exciting light. To verify the selectivity of the enzyme sensor, some potential interfering species in the biological sample including sucrose, ascorbic acid, cysteine, uric acid, and dopamine were inspected. As could be found from Figure 3D, these interferents had almost no negative effects on the photocurrent signal. Thus, as a new proof-of-concept, the acquired results convinced a great promising of the elaborated PEC enzyme sensor with enhanced selectivity. Reproducibility and preliminary application. The reproducibility of the PEC enzyme sensor was inspected by analyzing five groups of enzyme sensors fabricated separately under 13



the same experimental conditions. The relative standard deviations (RSDs) against the detection of glucose at the concentrations of 100 µM and 1 mM were 2.3% and 3.4%, respectively. The results thus reflected a satisfactory precision and repeatability of the enzyme sensor. The preliminary application of the enzyme sensor was assessed by recovery experiment carried out with medical glucose injections. The real sample was first diluted to a certain concentration with PBS (pH 7.0), and then a standard concentration of H2O2 and 10 mM 4-CN were spiked into the sample solution. After the biocathode was incubated in the sample solution, the photocurrent signal was tested and recorded. The recoveries for 100 µM and 1 mM glucose was measured to be 95.8 ± 3.6 and 97.2 ± 4.5%, respectively, illustrating the feasibility of this enzyme sensor in real biological sample assay.

CONCLUSIONS

A universal selectivity-enhanced PEC enzyme sensor with integrated photoanode and biocathode was developed and inspected. The hybrid ZnS/CuInS2/TiO2/ITO photoanode had excellent PEC properties of evident photocurrent response and high stability. The enzyme modified biocathode substituted for photoande to incubate in target-related biological sample, making the coexisting reductive substances generate no negative effect on the interface reaction of the photoanode, and thus effectively enhanced the selectivity of the enzyme sensor. On the whole, the photoanode and biocathode can be fabricated and employed separately with no-interference of each other, which realized high flexibility, good sensitivity and high selectivity of the sensor synchronously. As a result, using GOx&HRP as biocatalysts,

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glucose can be detected sensitively and selectively. The inspired design of the PEC enzyme sensor by integrating photoanode with biocathode can also be applied for the building of other PEC sensors such as DNA and cell sensors.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

Experimental details, UV-vis DRS of the photoanode, full-scan XPS of the photoanode, high-resolution XPS of the photoanode, and SEM characterization of the biocathode.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.-C. Fan)

*E-mail: [email protected] (X. Luo)

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (21603099 and 21675093), the Taishan Scholar Program of Shandong Province of China (ts20110829), and State Key Laboratory of Analytical Chemistry for Life Science (SKLACLS1802).

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