Ultrasensitive photoelectrochemical immunosensor for the detection of amyloid β-protein based on SnO₂/SnS₂/Ag₂S nanocomposites

Highlights

• A label-free PEC immunosensor was developed based on SnO₂/SnS₂/Ag₂S nanocomposites.

• SnO₂/SnS₂/Ag₂S nanocomposites exhibited excellent photoelectrochemical performance.

• The PEC immunosensor displayed highly sensitive detection toward amyloid β-protein.

Abstract

An ultrasensitive label-free photoelectrochemical (PEC) immunosensor with high visible-light activity was developed for quantitative detection of amyloid β-protein (Aβ) by cross-linking anti-Aβ antibody onto the Ag₂S sensitized SnO₂/SnS₂ nanocomposites. Specifically, SnO₂ with flower-like porous nanostructure was innovatively applied in PEC immunosensor as a basal material. It could form a heterostructure with SnS₂, which brought about the sensitization of SnO₂ and enhanced the separation of photogenerated electrons and holes. Moreover, Ag₂S was in-situ growth on the surface of SnO₂/SnS₂, which further enhanced the photocurrent response significantly. Therefore, SnO₂/SnS₂/Ag₂S could form stepwise band-edge structure, which benefited the light harvesting and provided a good foundation for sensor construction and detection. Under optimal conditions, the PEC immunosensor was used to detect the content of Aβ and exhibited a wide linear concentration range from 0.5 pg mL⁻¹ to 100 ng mL⁻¹, with low limit of detection (0.17 pg mL⁻¹) and limit of quantification (0.56 pg mL⁻¹). Additionally, the designed PEC immunosensor exhibited good reproducibility, specificity, and stability which may find potential applications in the biosensor, biomedicine, clinical diagnosis, photocatalysis and other related fields.

Introduction

Alzheimer's disease (AD) is an insidious and chronic neurodegenerative disorder causing memory decline and dysfunction of cognitive functions, with no effective disease-modifying therapy available currently (Cummings, 2004, Thapa et al., 2016). The amyloid-β protein (Aβ) has been proposed as the trigger for a cascade of events in the brain that lead to AD for 27 years (Selkoe and Hardy, 2016). Although the precise neurotoxic mechanism remains to be controversial, genetic, biochemical, molecular biological and pathological evidence strongly supported the amyloid hypothesis, positing that the excessive accumulation of Aβ is the primary pathological event leading to neurofibrillary tangles, neurodegeneration and neuroinflammation in AD. In recent years, a growing number of amyloid β-protein antibody (anti-Aβ) treatments have been developed to short-circuit this cascade, and several are currently being evaluated in people who have already developed or are at risk of developing symptoms of Alzheimer's (Reiman, 2016). But there are no treatments available that prevent this condition. Actually, Aβ is deposited early in the disease process decades prior to symptoms. Specifically, Aβ levels are detected in preclinical disease stages and can predict future cognitive decline and neurodegeneration. An Aβ concentration of < 500 pg mL⁻¹ is indicative that Aβ is accumulating in the brain (Carneiro et al., 2017). Thus, Aβ is a very useful biomarker for confirming whether the patient has a good or bad prognosis for AD and ultralow concentration detection of Aβ is of great importance. Moreover, ultralow concentration detection of Aβ is required, which is because that the low limit of detection (LOD) could undoubtedly improve the detection accuracy and reduce the amount of sample required. Therefore, there is a continuing demand for fast and simple analytical methods for the determination of Aβ.

Many studies have been reported on the detection of carcinoembryonic antigen (CEA), prostate specific antigen (PSA), α-fetoprotein (AFP), squamous cell carcinoma antigen (SCCA), insulin and other disease markers by photoelectrochemical (PEC) sensor (Fan et al., 2015, Han et al., 2018, Lv et al., 2018, Qiu et al., 2018, Wang et al., 2017b, Wang et al., 2017c, Zhao et al., 2012). As far as we know, few researches have been reported for detecting Aβ by label-free PEC method. Compared with conventional optical methods, the PEC immunoassay is particularly attractive in the field of diagnosis because it can unite the specificity and affinity of the antibody-antigen reaction with the inherent characteristics of electrochemical techniques such as high sensitivity, low cost, easy miniaturization, and so on. Furthermore, no researches have been done by using PEC method for the detection of Aβ based on SnO₂/SnS₂/Ag₂S modified ITO electrode.

As is known to all, metal oxide nanomaterials have attracted tremendous attention in the field of PEC immunosensor, which have shown excellent capability to detect trace concentrations of various environmental pollutants and tumor markers. In recent years, various semiconductor materials have been exploited as the substrate for the PEC sensor. Tin dioxide (SnO₂), a typical representative of metal oxide semiconductor nanomaterials, has received considerable attentions owning to its controllable morphology, environmental friendliness, large surface area, excellent photoelectronic properties and low cost (Jiang et al., 2018). Different external environmental parameters and reaction conditions can change the morphology of SnO₂ to a certain extent, such as SnO₂ nano-microspheres, SnO₂ nanoparticles, SnO₂ nanoflowers and other microstructures. Therefore, SnO₂ as a stable n-type wide band-gap semiconductor (Eg = 3.6 eV) has been intensively explored for various applications in transparent electrode, gas sensor, electrochemical luminescence sensor, rechargeable lithium-ion battery and solar cell (Elangovan and Ramamurthi, 2003, Huang et al., 2018, Shi and Lin, 2011, Wang et al., 2018a, Wang et al., 2006, Yan et al., 2018, Yin et al., 2018). However, very few reports of SnO₂ in PEC sensors were shown, especially when the ultrathin SnO₂ nanosheets aggregated to form flower-like nanostructure with better sensing properties.

It is now accepted that using semiconductors composites can improve photocatalytic activity compared with using a single semiconductor, which is due to the effective separation of photogenerated electrons (e^-) and holes (h⁺) (Cao et al., 2011, Ma et al., 2014, Xie et al., 2017, Zhang et al., 2009, Zhou et al., 2010). Stannic sulfide (SnS₂) has been known as good semiconductor photocatalytic materials with its narrow band-gap (~ 2.3 eV) and has been used for photoelectric sensor under visible light irradiation because of its low toxicity, low relative cost and wide spectral response (Banerjee et al., 2008). SnO₂ can form a heterostructure with SnS₂, which brings about the sensitization of SnO₂ and enhances the separation of photogenerated electrons and holes (Yao et al., 2014, Zhang et al., 2018, Zhou et al., 2012). Moreover, SnO₂ possessed superior electron mobility and the very positive valence band position which can prevent hole back-transfer through the SnO₂ layer and forms a ‘hole mirror’ (Liang et al., 2011, Zhang et al., 2011).

Meanwhile, semiconducting metal sulfides such as CdS, Ag₂S and MoS₂ have relatively narrow band-gaps and thus are capable of harvesting photons in the visible range, which are widely used in photocatalysts (Liu et al., 2011, Yang et al., 2012, Yun et al., 2016). Recently, Ag₂S nanoparticles have attracted many interests owing to their narrow band-gap (~ 1.0 eV), negligible toxicity, good electron transfer efficiency and relatively high visible-light absorption (Liu et al., 2015, Park et al., 2015). Ag₂S nanoparticles can be applied to construct a stepwise structure of band-edge levels in the SnO₂/SnS₂ electrode because of their harmonious band-gaps, which can promote ultrafast transfer charge and effectively inhibited the e^-/h⁺ pair recombination.

In this study, we synthesized pure flower-like hierarchical SnO₂ nanosheets by combining a facile hydrothermal and calcination method. At the same time, the hierarchical nanosheet structure exhibited large surface area, strong adsorption capacity, good biocompatibility and excellent sensing properties. In fact, as a basal material, SnO₂ showed good load performance and can firmly attach to the electrode surface. Moreover, Ag₂S nanoparticles can be applied to construct a stepwise structure of band-edge levels in the SnO₂/SnS₂ electrode because of their harmonious band-gaps. Ag₂S nanoparticles were loaded on the surface of SnO₂/SnS₂ by in-situ growth via strong adsorption interactions between SnS₂ and Ag₂S, which enhanced the photocurrent response in the visible-light region. It was found that the SnO₂/SnS₂/Ag₂S nanocomposites could greatly enhance the photocurrent with visible light irradiation in comparison with single SnO₂, SnS₂ or SnO₂/SnS₂. Meanwhile, an ultrasensitive label-free PEC immunosensor based on Ag₂S sensitized on SnO₂/SnS₂ with high photocurrent response was fabricated by layer-by-layer (LBL) method for the first time for the quantification of Aβ, with the goal of performing a clinical diagnosis and monitor biochemical effects of AD treatments. Ascorbic acid (AA) was selected as an electron donor for scavenging photogenerated holes to suppress photogenerated e^-/h⁺ recombination. The fabricated label-free PEC immunosensor exhibited a wide line arrange (0.0005–100 ng mL⁻¹) with low LOD (0.17 pg mL⁻¹) and limit of quantification (LOQ, 0.56 pg mL⁻¹), and good selectivity, reproducibility and stability, which exhibited its potential applications in many disease markers, environmental pollutants and other biological targets detection, clinical diagnosis, photocatalysis and other fields.