A signal-off sandwich photoelectrochemical immunosensor using TiO2 coupled with CdS as the photoactive matrix and copper (II) ion as inhibitor

Ultrasensitive self-powered biosensor with facile chemical signal amplification strategy using hydrogen peroxide-triggered silver oxidation reaction

The amplification strategies used for self-powered biosensor based on biofuel cell (BFC-SPB) need to be further developed. Because the currently developed strategies utilized the complicated hybridization of DNA or poorly readable current signal of capacitors for amplification, which limits the practical application in public health emergencies. Here, we present a facile chemical amplification strategy for BFC-SPB. The 5-min amplification was triggered by simply adding H2O2 solution dropwise to the sensing cathode after the formation of the immune sandwich. The Ag NP of immunoprobe were oxidized to Ag(I), which can be served as the electron acceptor of the cathode. The amount of immunoprobe was positively correlated with that of the antigen, resulting in corresponding and high concentration of Ag(I) after the amplification, which enhanced the ability of the cathode as the electron acceptor. Meanwhile the glucose oxidation reaction (GOR) was performed on the bioanode modified with glucose oxidase (GOx). After assembling the bioanode and sensing cathode, the open circuit voltage of the BFC-SPB, measured by digital multimeter, distinctly rised with the elevated concentration of the antigen. To demonstrate the proof of concept, immunoglobulin G (IgG), selecting as a model analyte, was sensitively detected using this method. Result indicated that the limit of detection was 4.4 fg mL-1 (0.03 amol mL-1) in the linear range of 1 pg mL-1-10 μg mL-1. This work initiates a brand-new way of chemical amplification strategy for BFC-SPB, and offers a promising platform for practical applications.

Synergistic effect of 2D covalent organic frameworks confined 0D carbon quantum dots film: Toward molecularly imprinted cathodic photoelectrochemical platform for detection of tetracycline

The development of high photoactive cathode materials combined with the formation of a stable interface are considered important factors for the selective and sensitive photoelectrochemical (PEC) detection of tetracycline (TC). Along these lines, in this work, a novel type II heterostructure composed of two-dimensional (2D) covalent organic frameworks confined to zero-dimensional (0D) carbon quantum dots (CDs/COFs) film was successfully synthesized using the rapid in-situ polymerization method at room temperature. The PEC signal of CDs/COFs was significantly amplified by improving the light absorption and electron transfer capabilities. Furthermore, a cathodic molecularly imprinted PEC sensor (MIP-PEC) for the detection of TC was constructed through fast in-situ Ultraviolet (UV) photopolymerization on the electrode. Finally, a "turn-off" PEC cathodic signal was achieved based on the selective recognition of the imprinted cavity and the mechanism of steric hindrance increase. Under optimal conditions, the proposed sensor demonstrated a wide linear relationship with TC in the concentration range of 5.00 × 10-12-1.00 × 10-5 M, with a detection limit as low as 6.00 × 10-13 M. Meanwhile, excellent stability, selectivity, reproducibility, and applicability in real river samples was recorded. Our work provides an effective and rapid in situ construction method for fabricating highly photoactive cathode heterojunctions and uniform stable selective MIP-PEC sensing interfaces, yielding accurate antibiotics detection in the environment.

Recent Progress in Photoelectrochemical Sensing of Pesticides in Food and Environmental Samples: Photoactive Materials and Signaling Mechanisms

Pesticides have become an integral part of modern agricultural practices, but their widespread use poses a significant threat to human health. As such, there is a pressing need to develop effective methods for detecting pesticides in food and environmental samples. Traditional chromatography methods and common rapid detection methods cannot satisfy accuracy, portability, long storage time, and solution stability at the same time. In recent years, photoelectrochemical (PEC) sensing technology has gained attention as a promising approach for detecting various pesticides due to its salient advantages, including high sensitivity, low cost, simple operation, fast response, and easy miniaturization, thus becoming a competitive candidate for real-time and on-site monitoring of pesticide levels. This review provides an overview of the recent advancements in PEC methods for pesticide detection and their applications in ensuring food and environmental safety, with a focus on the categories of photoactive materials, from single semiconductor to semiconductor-semiconductor heterojunction, and signaling mechanisms of PEC sensing platforms, including oxidation of pesticides, steric hindrance, generation/decrease in sacrificial agents, and introduction/release of photoactive materials. Additionally, this review will offer insights into future prospects and confrontations, thereby contributing novel perspectives to this evolving domain.

Label-free photoelectrochemical immunosensing of α-fetoprotein based on Eu-TiO2 nanocomposites sensitized with dye-encapsulated HMA

In this study, a sensitive photoelectrochemical immunosensor with dye-enhanced anodic photocurrent response was proposed for sensitive detection of α-fetoprotein (AFP). Specifically, europium-doped TiO₂ (Eu-TiO₂) was used as the photoelectrochemical functional material and coated onto indium tin oxide (ITO) electrode. Doxorubicin (DOX) as an excellent fluorescent dye was encapsulated in the hydrophobically modified alginate (HMA). Then the dye-loaded HMA was modified onto the surface of Eu-TiO₂ to further sensitize the photocurrent response. The results showed that the photoelectrical signal was enhanced and stabilized due to the effect of sensitization of DOX on Eu-TiO₂ material. The constructed PEC sensor revealed a good linear response to AFP antigen ranging from 0.5 to 100 ng/mL with a detection limit of 0.41 pg/mL. The clinical patient's serum test results obtained from the proposed PEC immunosensor were consistent with those obtained from the commercial electrochemilunescence assay. The proposed PEC sensing method could be a promising analytical tool for the detection of AFP in clinical analysis.

Photogenerated Hole-Induced Chemical-Chemical Redox Cycling Strategy on a Direct Z-Scheme Bi2S3/Bi2MoO6 Heterostructure Photoelectrode: Toward an Ultrasensitive Photoelectrochemical Immunoassay

To achieve high sensitivity for biomolecule detection in photoelectrochemical (PEC) bioanalysis, the ideal photoelectrode and ingenious signaling mechanism play crucial roles. Herein, the feasibility of the photogenerated hole-induced chemical-chemical redox cycling amplification strategy on a Z-scheme heterostructure photoelectrode was validated, and the strategy toward enhanced multiple signal amplification for advanced PEC immunoassay application was developed. Specifically, a direct Z-scheme Bi₂S₃/Bi₂MoO₆ heterostructure was synthesized via a classic hydrothermal method and served as a photoelectrode for the signal response. Under the illumination, the PEC chemical-chemical redox cycling (PECCC) among 4-aminophenol generated by the enzymatic catalysis from a sandwich immunoassay, ferrocene as a mediator, and tris (2-carboxyethyl) phosphine as a reducing agent was run on the Z-scheme Bi₂S₃/Bi₂MoO₆ heterostructure photoelectrode. Exemplified by interleukin-6 (IL-6) as the target, the applicability of the strategy was studied in a PEC immunoassay. Thanks to the multiple signal amplification originating from the high efficiency of the PECCC redox cycling system, the enzymatic amplification, and the fine performance of the Z-scheme Bi₂S₃/Bi₂MoO₆ heterostructure photoelectrode, the assay for IL-6 exhibits a very low detection limit of 2.0 × 10^-14 g/mL with a linear range from 5.0 × 10^-14 to 1.0 × 10^-8 g/mL. This work first validates the feasibility of the PECCC redox cycling on the Z-scheme heterostructure photoelectrode and the good performance of the strategy in PEC bioanalysis. We envision that it would provide a new prospective for highly sensitive PEC bioanalysis on the basis of a Z-scheme heterostructure.

Construction of 3D Bi/ZnSnO3 hollow microspheres for label-free highly selective photoelectrochemical recognition of norepinephrine

Herein, we reported a label-free and reliable photoelectrochemical (PEC) platform for highly selective monitoring of norepinephrine (NE) based on metallic Bi nanoparticles anchored on hollow porous ZnSnO3 microspheres (3D Bi/ZnSnO3) via a simple solvothermal strategy. The designed 3D Bi/ZnSnO3 Schottky junction exhibited a unique photoanodic response toward NE among other catechol derivatives, such as epinephrine (EP) and dopamine (DA), and effectively shielded the interference from thirteen coexisting biomolecules like uric acid (UA) and ascorbic acid (AA). High selectivity and excellent sensitivity could be correlated to the unique chelating coordination interaction between NE and Zn2+ at surface sites as well as the efficient carrier separation of Bi/ZnSnO3, thereby developing a novel "signal-on" label-free and selective strategy for NE detection. The proposed Bi/ZnSnO3-based PEC sensor achieved remarkable NE biosensing with a low detection limit of 0.68 nmol L-1 and a wide response ranging from 0.002 to 350.0 μmol L-1. The applicability of this biosensor was realized for the selective analysis of NE in human serum, human urine and injection samples, laying the foundation for the label-free PEC monitoring of NE in biological fluids.

CdS/TiO2 Nanocomposite-Based Photoelectrochemical Sensor for a Sensitive Determination of Nitrite in Principle of Etching Reaction

The CdS/TiO₂ nanocomposite (NC) photoelectrochemical (PEC) sensor was constructed based on a new sensing strategy for nitrite assay. The CdS etching process caused by nitrite-in-acid solution was confirmed and applied to nitrite sensing. The CdS etching phenomenon occurring on the sensor led to an obvious reduction in the photocurrent response under visible-light irradiation, which responded to the nitrite concentration. The CdS/TiO₂ NC-based PEC sensor exhibited excellent performance on nitrite detection. The linear range for nitrite determination was from 1-100 and 100-500 μM, and the sensitivity of the PEC sensor was 2.91 and 0.186 μA μM^-1 cm^-2, respectively. The detection limit of the sensor was 0.56 μM (S/N = 3). In addition, the PEC sensor was also equipped with advantages such as good selectivity, excellent stability, low background, and recyclability. Satisfying results were obtained for the nitrite assay in real samples by such a PEC sensor. In summary, this work contributed a fresh idea to precisely determinate nitrite through PEC sensing.

Detection signal amplification strategies at nanomaterial-based photoelectrochemical biosensors

This review focusses on unique material modification and signal amplification strategies reported in developing photoelectrochemical biosensors with utmost sensitivity and selectivity.

Three-dimensional CdS nanosheet-enwrapped carbon fiber framework: Towards split-type CuO-mediated photoelectrochemical immunoassay

This work reports a customized methodology for the fabrication of 3D CdS nanosheet (NS)-enwrapped carbon fiber framework (CFF) and its utilization for sensitive split-type CuO-mediated PEC immunoassay. Specifically, the 3D CdS NS-CFF was fabricated via a solvothermal process, while the sandwich immunocomplexing was allowed in a 96 well plate with CuO nanoparticles (NPs) as the signaling labels. The subsequent release of the Cu²⁺ ions was directed to interact with the CdS NS, generating trapping sites and thus inhibiting its photocurrent generation. In such a protocol, the 3D CdS NS-CFF photoelectrode could not only guarantee its sufficient contact with the Cu²⁺-containing solution but also supply plenty CdS surface for the Cu²⁺ ions. Because of the target-dependent release of the Cu²⁺ ions and its proper coupling with the 3D CdS NS-CFF photoelectrode, a sensitive split-type PEC immunoassay was achieved for the detection of brain natriuretic peptide (BNP). This proposed system exhibited good stability and selectivity, and its applicability for real sample analysis was also demonstrated via comparison with the commercial BNP enzyme-linked immunosorbent assay (ELISA) kit. We expect this work could stimulate more interest in the design and utilization of 3D photoelectrodes for novel PEC bioanalysis.

Photoelectrochemical Determination of Cu2+ Using a WO3/CdS Heterojunction Photoanode

Copper ions are not only physiologically essential for life but also hazardous materials causing a series of neurodegenerative diseases. Photoelectrochemical (PEC) detection has attracted a large amount of focus as a potential strategy to develop Cu²⁺ ion sensors. However, relatively low photocurrent signals with poor antidisturbance ability and the limited concentration range have prevented its practical applications. Here, we designed a WO₃/CdS heterojunction photoanode for the PEC determination of Cu²⁺ in aqueous solution through a simple two-step chemical bath deposition method. The obtained WO₃/CdS photoanode had a nanoplate morphology and showed an enhanced photoresponsivity with a photocurrent density of 1.5 mA/cm² at 1.23 V versus RHE under illumination. Naturally, it exhibited a low detection limit (0.06 μM) and wider range (0.5 μM to 1 mM) for Cu²⁺ PEC detection first, suggesting that the WO₃/CdS heterojunction photoanode is a promising tool to monitor copper pollution in natural environments.

Signal-switchable lab-on-paper photoelectrochemical aptasensing system integrated triple-helix molecular switch with charge separation and recombination regime of type-II CdTe@CdSe core-shell quantum dots

Herein, a new "on-off-on" signal switch system combined triple helix molecular switch with efficient charge separation and transfer between different sensitization units was designed for the ultrasensitive photoelectrochemical (PEC) determination of prostate-specific antigen (PSA). Concretely, the initial "signal-on" state was obtained via the cascaded sensitization structure consisting of type-II CdTe@CdSe core-shell quantum dots (QDs), CdS QDs, and ZnO nanotubes, which were assembled on Au nanoparticles modified paper fibers with the aid of signal transduction probe (STP). Thereinto, the type-II CdTe@CdSe QDs with hole-localizing core and electron-localizing shell could enable the ultrafast charge transfer and retard the charge recombination, magnifying the initial photocurrent response and preserving the high efficiency of signal-switchable PEC aptasensing system. Subsequently, the PSA aptamer (PSA-Apt) modified with gold nanoparticles (GNPs) was introduced by the hybridization of PSA-Apt with STP and the hairpin configuration of STP changed from closed to open state, forming a triple-helix structure. Hence, the CdTe@CdSe QDs labeled on the terminal of STP moved away from the electrode surface while the GNPs kept attached close to it. The proposed aptasensor turned to "signal-off" state because of the dual inhibition of vanished cosensitization effect and signal quenching effect of GNPs. Upon the target recognition, the triple-helix structure was perturbed with the formation of DNA-protein complex and the recovery of STP hairpin structure, resulting in the second "switch-on" state. Based on the target-induced photocurrent enhancement, the proposed PEC aptasensor was utilized for the determination of PSA with high sensitivity, persuasive selectivity, and excellent stability.

A Cu2+-doped two-dimensional material-based heterojunction photoelectrode: application for highly sensitive photoelectrochemical detection of hydrogen sulfide

In this work, on the basis of a Cu2+-doped two-dimensional material-based heterojunction photoelectrode, a novel anodic photoelectrochemical (PEC) sensing platform was constructed for highly sensitive detection of endogenous H2S. Briefly, with g-C3N4 and TiO2 as representative materials, the sensor was fabricated by modifying g-C3N4/TiO2 nanorod arrays (NAs) onto the surface of fluorine-doped tin oxide (FTO) and then doping Cu2+ as a Cu x S (x = 1, 2) precursor. After the binding of S2- with surface-attached Cu2+, the signal was quenched owing to the in situ generation of Cu x S which offers trapping sites to hinder generation of photocurrent signals. Since the photocurrent inhibition was intimately associated with the concentration of S2-, a highly sensitive PEC biosensor was fabricated for H2S detection. More importantly, the proposed sensing platform showed the enormous potential of g-C3N4/TiO2 NAs for further development of PEC bioanalysis, which may serve as a common basis for other semiconductor applications and stimulates the exploration of numerous high-performance nanocomposites.

One-Pot Synthesis of N-Graphene Quantum Dot-Functionalized I-BiOCl Z-Scheme Cathodic Materials for "Signal-Off" Photoelectrochemical Sensing of Chlorpyrifos

A Z-scheme I-BiOCl/N-GQD (i.e., nitrogen-doped graphene quantum dot) heterojunction was prepared by a one-pot precipitation method at room temperature. The doped iodine decreased the band gap of BiOCl, the introduced N-GQDs enhanced light harvesting and prolonged the photogenerated electron lifetime, and the resultant Z-scheme heterojunction promoted the spatial separation of interfacial charges. Thus, the composite showed high photoelectrochemical activity and a big cathodic photocurrent signal. On the basis of the coordination of chlorpyrifos with surface Bi(III) of the composite, a cathodic photoelectrochemical sensor was constructed for the selective detection of chlorpyrifos. In this case, chlorpyrifos decreased the lifetime of photogenerated electrons, so the photocurrent became small. Furthermore, the photocurrent changed and the logarithm of chlorpyrifos concentration presented a linear relationship. The linear range was 0.3-80 ng mL^-1, and the limit of detection was estimated to be 0.01 ng mL^-1 (defined as S/N = 3). The present strategy can also be used for the design and fabrication of other PEC sensors suitable for different analytes.

Ultrasensitive Photoelectrochemical Biosensing Platform for Detecting N-Terminal Pro-brain Natriuretic Peptide Based on SnO2/SnS2/mpg-C3N4 Amplified by PbS/SiO2

A sandwich-type photoelectrochemical (PEC) immunosensor for detecting N-terminal pro-brain natriuretic peptide (NT-proBNP) was constructed on the basis of SnO₂/SnS₂/mpg-C₃N₄ nanocomposites and PbS/SiO₂, with the former as a photoactive matrix and the latter as an efficient quencher. SnO₂/SnS₂/mpg-C₃N₄ was synthesized via in situ growth of SnO₂ and SnS₂ on mesoporous graphene like C₃N₄ nanocomposites (mpg-C₃N₄). Specifically, SnO₂/SnS₂/mpg-C₃N₄ exhibited intense PEC signal responses, which are tens of times stronger than its each single component. Because of its superior performance, SnO₂/SnS₂/mpg-C₃N₄ was applied as a photoactive matrix and signal indicator for fabricating PEC immunosensor. Interestingly, the excellent PEC signals from SnO₂/SnS₂/mpg-C₃N₄ could be reduced severely with the addition of PbS/SiO₂. Hence, the secondary antibody bioconjugates (PbS/SiO₂-Ab₂) were prepared as an efficient quencher. The mechanism of the quench reaction was further discussed in detail. On the basis of the interaction between the matrix and the quencher, the NT-proBNP immunosensor was fabricated and a wide linear range of 0.1 pg·mL^-1 to 50 ng·mL^-1 was obtained with a low detection limit of 0.05 pg·mL^-1. Additionally, the PEC immunosensor manifested good stability, reproducibility, and selectivity, which could underlie robust platforms for detecting multitudinous biomarkers or other targets of interest.

Tremella-like ZnIn2S4/graphene composite based photoelectrochemical sensor for sensitive detection of dopamine

Tremella-like ZnIn₂S₄ (ZISt) and flower-like microsphere ZnIn₂S₄ (ZISm) were synthesized via a straightforward hydrothermal method. It was found that the ZISt was superior to ZISm for photoelectrochemical (PEC) sensing because of its large surface area and high photocatalytic activity. A composite of ZISt and graphene (GR) was prepared and used for the PEC sensing of dopamine (DA). Here DA acted as an electron donor to scavenge the hole and inhibit the charge recombination. The GR enhanced visible light absorption and accelerated electron transfer, amplifying the photocurrent signal. The strong chelating coordination interaction between DA and Zn(II) in ZISt guaranteed the selective adsorption of target analyte. Thus the resulting ZISt/GR photoelectrode showed sensitive and selective PEC response to DA. Under the optimized conditions, the linear response range was from 0.01 to 20 μM, and the detection limit was down to 0.001 μM. Additionally, the sensor had good stability and reproducibility, and it could be used for the detection of DA in real samples.

Spatial-Resolved Photoelectrochemical Biosensing Array Based on a CdS@g-C3N4 Heterojunction: A Universal Immunosensing Platform for Accurate Detection

The detection of biomarkers with high sensitivity and accuracy in real biosamples remains challenging. Herein, a universal spatial-resolved photoelectrochemical (PEC) ratiometry for biodetection of prostate-specific antigen (PSA) as model biomarker was designed for the first time based on a dual-electrode array modified by CdS@g-C₃N₄ heterojunction coupled with CuS quantum dots (QDs) as signal amplification tags. Specifically, a new kind of photoactive material, the CdS@g-C₃N₄ p-n heterojunction with high photoelectric conversion efficiency and good chemical stability, was synthesized and immobilized on two spatial-resolved electrodes (WE1 and WE2). After immobilizing gold nanoparticles and capturing PSA antibodies on the electrodes, WE1 incubated with various concentrations of PSA was taken as a working electrode, whereas WE2 with a fixed concentration of PSA was used as an internal reference electrode. Next, signal antibodies of PSA-labeled CuS QDs as PEC signal quenchers were immobilized on the electrodes to form a sandwich-type immunocomplex. With the aid of a multiplexed disjunctor, the PEC responses of the dual electrodes were recorded, and the PSA was quantified via the ratio values of photocurrent intensities from WE1 and WE2. Combining the fine PEC performance of the CdS@g-C₃N₄ heterojunction with the superior quenching effect of CuS QDs in the spatial-resolved platform, the ratiometric system exhibits a linear range from 1.0 × 10^-11 to 5.0 × 10^-8 g mL^-1 with a limit of detection of 4.0 pg mL^-1. The results demonstrated herein may provide a new pattern for biomarker detection with high accuracy and good specificity as well as satisfactory applicability in real biosamples.

An in situ electron donor consumption strategy for photoelectrochemical biosensing of proteins based on ternary Bi2S3/Ag2S/TiO2 NT arrays

An ascorbic acid oxidase–ascorbic acid bioevent-based electron donor consumption mode is introduced into the PEC bioassay for the first time.

Design of a Dual Channel Self-Reference Photoelectrochemical Biosensor

Photoelectrochemical (PEC) biosensors are usually based on the single photocurrent change caused by biorecognition events between analytes and probes. However, the photocurrent may be influenced by other factors besides target analytes and bring a false result. To improve the accuracy and reliability of PEC detection, here we proposed the design of a dual channel self-reference PEC biosensors. CdTe and CdTe-graphene oxide (GO) were chosen as the two PEC active material and modified onto two adjacent areas on the ITO electrode. Then they were functionalized with Aflatoxin B1 (AFB1) aptamer through covalent binding or physical adsorption, respectively. The cathodic current from CdTe-GO and anodic current from CdTe can be well distinguished by adjusting the bias voltage. With the simultaneous application of "signal on" and "signal off" model, dual concentration information may be obtained in one detection and serve as a reference for each other. By comparing these two results, this sensor can clearly distinguish whether the signal change was caused by AFB1 or other interference factors. Compared to traditional PEC biosensors, this design can provide a better accuracy and reliability, which is promising in the future development of PEC detection.

Facile fabrication of an aptasensor for thrombin based on graphitic carbon nitride/TiO2 with high visible-light photoelectrochemical activity

A novel aptasensor for thrombin with high visible-light activity was facilely fabricated based on graphitic carbon nitride/TiO2 (g-C3N4/TiO2) photoelectrochemical (PEC) composite. Crystallization of TiO2 nanoparticles (NPs) and their strong interaction with g-C3N4 sheet were confirmed by high-resolution transmission electron microscope (HR-TEM), both of which contributed to the high photocurrent intensity under visible-light irradiation. Carboxyl functionalized thrombin aptamers were first successfully bound to the g-C3N4/TiO2 modified electrode as proven by photoelectrochemical test and electrochemical impedance spectroscopy (EIS) analysis. Ascorbic acid was utilized as the electron donor for scavenging photo-generated holes and inhibiting light driven electron-hole pair recombination. The specific recognition between thrombin aptamer and thrombin led to the linear decrease of photocurrent with the increase of logarithm of thrombin concentration in the range of 5.0×10(-13)molL(-1) to 5.0×10(-9)molL(-1) with a detection limit of 1.2×10(-13)molL(-1). This proposed low-cost, convenient and sensitive aptasensor showed promising applications in biosensor and photoelectrochemical analysis.

Electrochemiluminescence sensor for dopamine with a dual molecular recognition strategy based on graphite-like carbon nitride nanosheets/3,4,9,10-perylenetetracarboxylic acid hybrids

Preparation of an ECL sensor and graphite-like carbon nitride nanosheets/3,4,9,10-perylenetetracarboxylic acid hybrids.