Near-Infrared Light-Excited Photoelectrochemical Biosensing Platform for MC-LR in Aquatic Products Based on Ag2s Cubes

Abstract

Introduction Photoelectrochemical (PEC) analysis is a sensing technology that combines photochemistry and electrochemistry effectively. Nowadays, most of PEC analysis use ultraviolet/visible light as a light source [1]. Compared with visible/ultraviolet light, near-infrared (NIR) owns a strong penetrating ability and can directly penetrate tissue, glass and plastic packaging, thus has a good prospect in practical application. However, only a few researchers use NIR light as a light source for the PEC sensing platforms. The biggest problem is difficult to find a photovoltaic material that can absorb NIR efficiently while having high efficiency of PEC conversion due to the longer wavelength and less energy of NIR. Thereby, exploring some new NIR PEC materials is conducive to the development of advanced PEC sensing platforms. NIR Response of PEC Biosensor for MC-LR Silver sulfide (Ag2S) is a narrow-band semiconductor material with high chemical stability. Its band gap is about 1.1 eV, thus is capable absorb light in the NIR region [2]. AuNPs have good electrical conductivity and SPR effect. This paper developed an unlabeled PEC biosensor of MC-LR based on AuNPs/Ag2S/FTO. The sensor utilized an antibody as a capture probe immobilized on the surface of the AuNPs/Ag2S/FTO to recognize the target selectively. After adding MC-LR to the detection system, the fabricated composite electrode absorbed the inactive MC-LR, which would cause steric effects and limit the surface electron transfer, resulting in a decrease in photocurrent. Therefore, the MC-LR can be detected quantitatively based on the decrease in the intensity of the photocurrent. The sensor reveals a large detection range, high sensitivity, good selectivity and excellent stability. Method For the preparation of AuNPs/Ag2S/FTO, polyvinylpyrrolidone was dissolved in ethylene glycol first, and then added with potassium iodide and sodium sulfide under stirring to obtain Ag2S cubes. The above Ag2S suspension was then dropped onto the FTO surface and dried at room temperature [3]. After that, 5 μL of AuNPs were added to the surface of Ag2S/FTO and dried at 60 ℃ to obtain AuNPs/Ag2S/FTO. For determination of MC-LR, chitosan and glutaraldehyde were used to immobilize antibodies. After activation, 50 μL of 50 μg L–1 MC–LR Ab was dropped onto the electrode and then incubated at 37 °C for 4 h. After incubation, the Ab/AuNPs/Ag2S/FTO was rinsed with phosphate buffer solution containing 0.05 % Tween–20 solution and then incubated in 1 % (w/v) BSA solution to block the unbound sites. The final electrode was incubated into 50 μL of 5 μg L–1 MC–LR solutions for 1 h at 37 °C, followed by washing with the phosphate buffer solution for three times. Results and Conclusions The typical SEM images were employed to characterize the morphology and microstructure of the as-prepared AuNPs/Ag2S/FTO electrodes. As showed in Fig. 1A, the Ag2S on the surface of the electrode exhibited unique cubic arrangement with a size of 4.5 µm (inset of Fig. 1A). After modification of AuNPs, the SEM image also showed clearly that the original cubic morphology of Ag2S, and the PEI-stabilized Au NPs were spherical in shape with diameters of 20-45 nm (inset of Fig. 1B), indicating that AuNPs solution only forms a film on the surface of Ag2S, which does not destroy the original cubic shape of Ag2S. To evaluate the PEC behaviors of the modified electrodes, the I-t curves were recorded by our homemade NIR responsive PEC system under a bias potential of 0.6 V with the excitation light at 980 nm (Fig. 2). We found that by immobilizing Ab, BSA blocking and incubation with the target analyte MC-LR, the photocurrent was gradually reduced, which can be attributed to the inhibition of interface electrons transfer by these modified substances on the BSA/Ab/AuNPs/Ag2S/FTO electrodes. Based on the reduction effect of this signal, the quantitative analysis of MC-LR can be achieved.