Simultaneous photoelectrochemical and visualized immunoassay of β-human chorionic gonadotrophin

Progress in Electrochemical Immunosensors with Alkaline Phosphatase as the Signal Label

Electrochemical immunosensors have shown great potential in clinical diagnosis, food safety, environmental protection, and other fields. The feasible and innovative combination of enzyme catalysis and other signal-amplified elements has yielded exciting progress in the development of electrochemical immunosensors. Alkaline phosphatase (ALP) is one of the most popularly used enzyme reporters in bioassays. It has been widely utilized to design electrochemical immunosensors owing to its significant advantages (e.g., high catalytic activity, high turnover number, and excellent substrate specificity). In this work, we summarized the achievements of electrochemical immunosensors with ALP as the signal reporter. We mainly focused on detection principles and signal amplification strategies and briefly discussed the challenges regarding how to further improve the performance of ALP-based immunoassays.

Stimulus-Responsive Metal-Organic Framework Signal-Reporting System for Photoelectrochemical and Fluorescent Dual-Mode Detection of ATP

Dual-mode bioanalysis integrating photoelectrochemical (PEC) and other modes is emerging and allows signal cross-checking for more reliable results. Metal-organic frameworks (MOFs) have been shown to be attractive materials in various biological applications. This work presents the utilization of MOF encapsulation and stimuli-responsive decapsulation for dual-mode PEC and fluorescence (FL) bioanalysis. Photoactive dye methylene violet (MV) was encapsulated in zeolitic imidazolate framework-90 (ZIF-90) to form an MV@ZIF-90 hybrid material, and MV could be released by adenosine triphosphate (ATP)-induced ZIF-90 disintegration. The released MV not only had FL emission but also had a sensitization effect on the ZnIn₂S₄ (ZnInS) photoanode. Based on the MV-dependent sensitization effect and FL emission characteristic, a dual-mode PEC-FL strategy was established for ATP detection with low detection limits, that is, 3.2 and 4.1 pM for PEC and FL detection, respectively. This study features and will inspire the construction and implementation of smart MOF materials for dual-mode bioanalysis.

Functionalization of 2D MoS2 Nanosheets with Various Metal and Metal Oxide Nanostructures: Their Properties and Application in Electrochemical Sensors

Two-dimensional transition metal dichalcogenides (2D TMDs) have gained considerable attention due to their distinctive properties and broad range of possible applications. One of the most widely studied transition metal dichalcogenides is molybdenum disulfide (MoS₂). The 2D MoS₂ nanosheets have unique and complementary properties to those of graphene, rendering them ideal electrode materials that could potentially lead to significant benefits in many electrochemical applications. These properties include tunable bandgaps, large surface areas, relatively high electron mobilities, and good optical and catalytic characteristics. Although the use of 2D MoS₂ nanosheets offers several advantages and excellent properties, surface functionalization of 2D MoS₂ is a potential route for further enhancing their properties and adding extra functionalities to the surface of the fabricated sensor. The functionalization of the material with various metal and metal oxide nanostructures has a significant impact on its overall electrochemical performance, improving various sensing parameters, such as selectivity, sensitivity, and stability. In this review, different methods of preparing 2D-layered MoS₂ nanomaterials, followed by different surface functionalization methods of these nanomaterials, are explored and discussed. Finally, the structure-properties relationship and electrochemical sensor applications over the last ten years are discussed. Emphasis is placed on the performance of 2D MoS₂ with respect to the performance of electrochemical sensors, thereby giving new insights into this unique material and providing a foundation for researchers of different disciplines who are interested in advancing the development of MoS₂-based sensors.

Dual-Modal Photoelectrochemical and Visualized Detection of Copper Ions

Copper is one of the extensively utilized heavy metals in modern industry and can be easily released into the environment due to high solubility of copper ions (Cu²⁺). Its percolation in water and accumulation along the food chain pose a serious threat to human health. Hence, it is of great significance to explore a novel, facile, and sensitive detection method for Cu²⁺. Based on the intriguing photo-to-electricity conversion process of CdS QDs, as well as desirable electrochromic property of WO₃ NFs, a dual-modal photoelectrochemical (PEC) and visualized detection platform for Cu²⁺ is fabricated. The electrochromic WO₃ NFs act as a display for the Cu²⁺ concentration, of which the color change could be observed directly by the naked eye, while the PEC signal provides accurate data for further analysis. In this work, a sensitive detection of Cu²⁺ in the range of 1 × 10^-5 to 5 × 10^-4 M is achieved, with a detection limit of 3.2 × 10^-6 M. The dual-modal analysis gives more choices for signal readouts with enhanced quantification reliability, which is adaptive for diverse application scenarios, especially for on-site investigation. This protocol offers a prototype for quick and reliable detection of the Cu²⁺ concentrations, and is promising for other environmental pollutants.

Photoelectrochemical Enzyme Biosensor Based on TiO2 Nanorod/TiO2 Quantum Dot/Polydopamine/Glucose Oxidase Composites with Strong Visible-Light Response

Although photoelectrochemical (PEC) enzyme biosensors based on visible-light detection would have a high practical value, their development has been limited by the weak visible-light response of available photoactive substrates. Here, to enhance the visible-light response of a photoelectric substrate, a TiO₂ nanorods (NRs)/TiO₂ quantum dots (QDs)/polydopamine (PDA)/glucose oxidase nanocomposite was prepared via hydrothermal synthesis, followed by photopolymerization. TiO₂ QDs with strong light absorption and excellent photocatalytic activity were introduced between the TiO₂ NRs and the PDA. An efficient electron transport interface that formed as a result of the combination of the TiO₂ NRs, TiO₂ QDs, and the PDA could not only transfer electrons quickly and orderly, but also substantially improve the response of the TiO₂ NRs under visible light. Through a series glucose detection, a sensor based on the nanocomposite was found to exhibit superior sensing performance under visible light with a sensitivity of 4.63 μA mM^-1 cm^-2, a linear response over the concentration 0.1-4 mM, and a detection limit of 8.16 μM. This work proposes a biosensor that can detect under visible light, thereby expanding the application range of PEC enzyme biosensors.

Nanostructure-based photoelectrochemical sensing platforms for biomedical applications

As a newly developed and powerful analytical method, the use of photoelectrochemical (PEC) biosensors opens up new opportunities to provide wide applications in the early diagnosis of diseases, environmental monitoring and food safety detection. The properties of diverse photoactive materials are one of the essential factors, which can greatly impact the PEC performance. The continuous development of nanotechnology has injected new vitality into the field of PEC biosensors. In many studies, much effort on PEC sensing with semiconductor materials is highlighted. Thus, we propose a systematic introduction to the recent progress in nanostructure-based PEC biosensors to exploit more promising materials and advanced PEC technologies. This review briefly evaluates the several advanced photoactive nanomaterials in the PEC field with an emphasis on the charge separation and transfer mechanism over the past few years. In addition, we introduce the application and research progress of PEC sensors from the perspective of basic principles, and give a brief overview of the main advances in the versatile sensing pattern of nanostructure-based PEC platforms. This last section covers the aspects of future prospects and challenges in the nanostructure-based PEC analysis field.

A label-free photoelectrochemical immunosensor for detection of the milk allergen β-lactoglobulin based on Ag2S -sensitized spindle-shaped BiVO4/BiOBr heterojunction by an in situ growth method

Food allergies have become a nonnegligible food safety issue, and milk allergies are one of the most common food allergies, that has attracted large consumer attention. In this work, a novel label-free photoelectrochemical (PEC) immunosensor for the detection of the allergen β-lactoglobulin (β-LG) in dairy products was designed that used the specific recognition of allergen β-LG and antibodies in dairy products in combination with biosensing technology. Here, Ag₂S-sensitized spindle-shaped BiVO₄/BiOBr heterojunction was fixed on the surface of the ITO electrode as an excellent photoactive substrate and effectively improved the photocurrent responses and sensitivity. Thioglycolic acid (TGA) was used as a linker to immobilize the β-LG antibody on the surface of the electrode. The photocurrent was detected at different antigen concentrations, which realized the quantitative testing of β-LG. Under the optimal experimental conditions, the PEC immunosensor proved an ideal linear relationship ranging from 10 pg/mL to 100 ng/mL, with a low detection limit of 3.7 pg/mL. The designed immunosensor showed good stability, a wide linear range, high sensitivity and good reproducibility and could be used for the detection of actual samples. The PEC immunosensor had a strong ability to specifically recognize β-LG, which was not affected by other proteins in the milk without pretreatment. Meanwhile, the developed immunosensor provided a promising PEC detection platform and reference idea for the detection of other proteins in milk.

Boosting the biocatalytic precipitation with enzyme-loaded liposomes: Toward a general platform for amplified photoelectrochemical immunoassay

Liposome-assisted photoelectrochemical (PEC) bioanalysis represents one of the latest frontiers in the arena of PEC bioanalysis. This work reports a general enzyme-amplified liposomal PEC bioanalysis protocol via the use of enzyme-loaded liposomes to boost the biocatalytic precipitation (BCP) effect. In the representative system, the horseradish peroxidase (HRP)-loaded liposome (HRPLL) and the Au nanoclusters (NCs)/Au nanoparticles (NPs)/TiO₂ nanotubes (NTs) framework (AATF) were used as liposomal label and photoelectrode, respectively. In the detection, the sandwich immunocomplex reaction was accomplished in a 96-well plate to confine the HRPLL label, which was then lysed to release the HRP molecules to initiate the BCP process. Due to the amplified formation of HRP-induced BCP on the AATF scaffold, the photo-current response correlated closely with the immunorecognition process and the analyte could be detected very sensitively. This work features the first integration of enzyme-loaded liposomes and the BCP for sensitive PEC bioanalysis, which to our knowledge has not been reported. With the use of various other enzymes, this work could serve as a general basis for the PEC bioanalysis of numerous other target of interest.

Strip-shaped Co3O4 as a peroxidase mimic in a signal-amplified impedimetric zearalenone immunoassay

An impedimetric immunoassay was designed for ultrasensitive determination of zearalenone (ZEN). It is making use of the peroxidase-like activity of strip-shaped Co₃O₄ (ssCo₃O₄) which catalyzes the oxidation of 4-chloro-1-naphthol to produce an insoluble precipitate in the presence of H₂O₂. The precipitate is electrically nonconductive and accumulates on the electrode, thereby retarding the electron transfer from the redox probe ferro/ferricyanide to the surface of electrode. This amplifies the impedimetric signal in accordance with logarithm of the concentration of ZEN. The electrode was further modified with TiO₂ mesocrystals (TiO₂ MCs) which improve the capture of more analytes and increase the performance of the immunoassay. Under optimized experimental condition, the impedimetric signal increased linearly with the logarithm of the ZEN concentration range between 0.1 fg·mL^-1 to 10 pg·mL^-1. The detection limit is of 33 ag· mL^-1. Graphical abstractThis work describes an impedimetric immunoassay based on the use of strip-shaped Co3O4 that catalyzes the production of an insoluble precipitate in the presence of H2O2 on the surface of a glassy carbon electrode. The effect was used for signal amplification in an electrochemical immunoassay for zearalenone.

Au@Pt nanodendrites enhanced multimodal enzyme-linked immunosorbent assay

Single modal enzyme-linked immunosorbent assay (ELISA) covering colorimetric, fluorescence, and chemiluminescence techniques has been widely reported in recent years, whereas the combination of multiple signal channels in one immunosensing platform still faces huge challenges. Multimodal imaging will provide more comprehensive and precise diagnostic information by the combination of two or more imaging modalities. Inspired by this concept, we established a new kind of multimodal enzyme-linked immunosorbent assay (M-ELISA) based on the unique properties of Au@Pt nanodendrites. As a proof-of-concept demonstration, cardiac troponin I (cTnI) has been chosen as a model biomarker. Owing to the excellent photothermal effect and intrinsic peroxidase-like activity of Au@Pt nanodendrites, the concentration of cTnI can be quantificationally reflected by photothermal immunoassay, colorimetric immunoassay and ratiometric fluorescence immunoassay simultaneously. This developed M-ELISA demonstrated the good consistency with the chemiluminescence immunoassay method in clinical diagnosis for real serum samples.

Photovoltage-triggered electrochromic tablet for visualized photoelectrochemical sensing

In this work, we designed an integrated photoelectrochemical (PEC) analytical system with photovoltage-regulated electrochromic tablet for visualized sensing. The electrochromic tablet consisted of electron-injector (EI) part and colorimetric system, which was functionalized with dye-sensitized titanium dioxide and nickel oxide nanosheet film as the recognition element and signal readout, respectively. Under irradiation of home-built white light, the EI part provided sufficient and stable photovoltage and injected the photoexcited electrons to nickel oxide nanosheet through the conductive inner side of indium tin oxide slide. Thus, the different color states of nickel oxide nanosheet were observed, resulting in a detectable signal by naked eyes for visual analysis. More importantly, the color change was identified to be dependent on the photovoltages generated from EI part. Using copper ions as a model analyte, the electrochromic tablet was successfully applied to visually detect copper ions based on its quenching effect on photovoltage due to the formation of exciton trapping. As a result, the copper ions could be simultaneously determined over a wide range through photovoltage record, average RGB value, and naked eyes. Furthermore, the device exhibited considerable stability and could be reused through simple washing steps. The smart tablet PEC system provides a new avenue to design practical PEC sensor in point-of-care diagnosis.

WS2 nanosheets-sensitized CdS quantum dots heterostructure for photoelectrochemical immunoassay of alpha-fetoprotein coupled with enzyme-mediated biocatalytic precipitation

A sensitive photoelectrochemical immunoassay for AFP was developed via signal enhancement of WS₂/CdS heterojunction and signal quenching of enzyme-mediated biocatalytic precipitation.

Quantum-dots-based photoelectrochemical bioanalysis highlighted with recent examples

Photoelectrochemical (PEC) bioanalysis is a newly developed methodology that provides an exquisite route for innovative biomolecular detection. Quantum dots (QDs) are semiconductor nanocrystals with unique photophysical properties that have attracted tremendous attentions among the analytical community. QDs-based PEC bioanalysis comprises an important research hotspot in the field of PEC bioanalysis due to its combined advantages and potentials. Currently, it has ignited increasing interests as demonstrated by increased research papers. This review aims to cover the most recent advances in this field. With the discussion of recent examples of QDs-PEC bioanalysis from the literatures, special emphasis will be placed on work reporting on fundamental advances in the signaling strategies of QDs-based PEC bioanalysis from 2013 to now. Future prospects in this field are also discussed.

A novel label-free electrochemical immunosensor based on the composite of LPCs-SnS2 and AuNPs for the detection of human chorionic gonadotropin

LPCs-SnS₂ with a 3D conduction architecture was prepared by a facile hydrothermal process.

Recent advances in the use of quantum dots for photoelectrochemical bioanalysis

Photoelectrochemical (PEC) bioanalysis is a newly developed technique for innovative biomolecular detection. Quantum dots (QDs) with unique photophysical properties are key components in realization of various exquisite PEC bioanalyses. Particularly, significant progress has been made in the QD-based PEC bioanalysis. In this work, we briefly summarize the most recent and important developments in the use of traditional and newly emerging QDs for novel PEC bioanalytical applications. The future prospects in this dynamic field are also highlighted.