Highly Sensitive Photoelectrochemical Immunoassay with Enhanced Amplification Using Horseradish Peroxidase Induced Biocatalytic Precipitation on a CdS Quantum Dots Multilayer Electrode

Organic photoelectrochemical transistor based on cascaded DNA network structure modulated ZnIn2S4/MXene Schottky junction for sensitive ATP detection

Organic photoelectrochemical transistor (OPECT) biosensor is now appearing in perspective of public, which characterized by amplified the grating electrode potential by ion transport. In this study, the DNA network formed by the hybridization chain reaction (HCR) detects the target adenosine triphosphate (ATP) by adjusting the surface potential of the new heterojunction of ZnIn2S4/MXene. The formation of DNA network amplifies the detection signal of ATP. Significantly, OPECT biosensor could further amplify the signal, which calculated the gain achieved 103, which is consistent with the gain signal of the previously reported OPECT biosensor. Furthermore, the OPECT biosensor achieved a highly sensitivity detection of the target ATP, which the linear detection range is 0.03 pM-30 nM, and the detection limit is 0.03 pM, and illustrated a high selectivity to ATP. The proposed OPECT biosensor achieved signal amplification by adjusting the surface potential of ZnIn2S4/MXene through cascade DNA network, which provides a new direction for the detection of biomolecules.

Development of a label-free photoelectrochemical immunosensor for novel astrovirus detection

Since 2017, an infectious goose gout disease characterized by urate precipitation in viscera, mainly caused by novel goose astrovirus (GoAstV) infection, has emerged in the main goose-producing region of China. The current challenge in managing goose gout disease is largely due to the absence of a rapid and efficient detection method for the GoAstV pathogen. Notably, the potential application of immunosensors in detecting GoAstV has not yet been explored. Herein, a label-free PEC immunosensor was fabricated by using purchased TiO2 as the photoactive material and antibody against GoAstV P2 proteins as the specific recognition element. First, we successfully expressed the capsid spike domain P2 protein of ORF2 from GoAstV CHSH01 by using the pET prokaryotic expression system. Meanwhile, the polyclonal antibody against GoAstV capsid P2 protein was produced by purified protein. To our knowledge, this is the first establishment and preliminary application of the label-free photoelectrochemical immunosensor method in the detection of AstV. The PEC immunosensor had a linear range of 1.83 fg mL-1 to 3.02 ng mL-1, and the limit of detection (LOD) was as low as 0.61 fg mL-1. This immunosensor exhibited high sensitivity, great specificity, and good stability in detecting GoAstV P2 proteins. To evaluate the practical application of the immunosensor in real-world sample detection, allantoic fluid from goose embryos was collected as test samples. The results indicated that of the eight positive samples, one false negative result was detected, while both negative samples were accurately detected, suggesting that the constructed PEC immunosensor had good applicability and practical application value, providing a platform for the qualitative detection of GoAstV.

A sensitive immunosensing platform based on the high cathodic photoelectrochemical activity of Zr-MOF and dual-signal amplification of peroxidase-mimetic Fe-MOF

Cathodic photoelectrochemical (PEC) analysis has received special concerns because of its outstanding anti-interference capability toward reductive substances in samples, so it is highly desirable to develop high-performance photocathodic materials for PEC analysis. Herein, a Zr-based metal-organic framework (Zr-MOF), MOF-525, is explored as a photoactive material in aqueous solution for the first time, which shows a narrow band-gap of 1.82 eV, excellent visible-light absorption, and high cathodic PEC activity. A sandwiched-type PEC immunosensor for detecting prostate-specific antigen (PSA) is fabricated by using MIL-101-NH2(Fe) label and MOF-525 photoactive material. MIL-101-NH2(Fe) as a typical Fe-MOF can serve as a peroxidase mimic to catalyze the production of precipitates on the photoelectrode. Both the produced precipitates and the MIL-101-NH2(Fe) labels can quench the photocathodic current, enabling "signal-off" immunosensing of PSA. The detection limit is 3 fg mL-1, and the linear range is between 10 fg mL-1 and 100 ng mL-1 for detecting PSA. The present study not only develops a high-performance Zr-MOF photoactive material for cathodic PEC analysis but also constructs a sensitive PEC immunosensing platform based on the dual-signal amplification of peroxidase-mimetic Fe-MOF.

Hydrogel-Based Biosensors for Bacterial Infections

Hydrogels are known to have the advantages such as good biodegradability, biocompatibility, and easy functionalization, making them ideal candidates for biosensors. Hydrogel-based biosensors that respond to bacteria-induced microenvironmental changes such as pH, enzymes, antigens, etc., or directly interact with bacterial surface receptors, can be applied for early diagnosis of bacterial infections, providing information for timely treatment while avoiding antibiotic abuse. Furthermore, hydrogel biosensors capable of both bacteria diagnosis and treatment will greatly facilitate the development of point-of-care monitoring of bacterial infections. In this review, the recent advancement of hydrogel-based biosensors for bacterial infection is summarized and discussed. First, the biosensors based on pH-sensitive hydrogels, bacterial-specific secretions-sensitive hydrogels, and hydrogels directly in contact with bacterial surfaces are presented. Next, hydrogel biosensors capable of detecting bacterial infection in the early stage followed by immediate on-demand treatment are discussed. Finally, the challenges and future development of hydrogel biosensors for bacterial infections are proposed.

Recent Trends in Self-Powered Photoelectrochemical Sensors: From the Perspective of Signal Output

Revolutionary developments in analytical chemistry have led to the rapid development of self-powered photoelectrochemical (PEC) sensors. Different from conventional PEC sensors, self-powered PEC sensors do not require an external power source or complex devices for the sensitive detection of targets. As a result, these sensors have enormous application potential for the development of novel portable sensors. An increasing body of work is making excellent progress toward the implementation of self-powered PEC sensors for detection, but there have been no reviews to date. The present review first introduces the state of the art in the development of self-powered PEC sensors. Then, different types of self-powered PEC sensors are summarized and discussed in detail, including their current, power, and potential. Additionally, single- and dual-photoelectrode systems are classified and systematically compared. Finally, the current developments and major challenges that need to be addressed are also summarized. This review provides valuable insights into the current state of self-powered PEC sensors to promote further progress in this field.

Inspired by game theory: Multi-signal output photoelectrochemical point-of-care immunoassay based on target-triggered organic electronic barriers

This work presents an integrated photoelectrochemical, impedance and colorimetric biosensing platform for flexible detection of cancer markers based on the targeted response by combining liposome amplification strategies and target-induced non-in situ formation of electronic barriers as the signal transduction modality on carbon-modified CdS photoanodes. Inspired by game theory, the carbon layer modified CdS hyperbranched structure with low impedance and high photocurrent response was firstly obtained by surface modification of CdS nanomaterials. Through a liposome-mediated enzymatic reaction amplification strategy, a large number of organic electron barriers were formed by a biocatalytic precipitation (BCP) reaction triggered by horseradish peroxidase released from cleaved liposomes after the introduction of the target molecule, thereby increasing the impedance characteristics of the photoanode as well as attenuating the photocurrent. The BCP reaction in the microplate was accompanied by a significant color change, which opened up a new window for point-of-care testing. Taking carcinoembryonic antigen (CEA) as a proof of concept, the multi-signal output sensing platform showed a satisfactory sensitive response to CEA with an optimal linear range of 20 pg mL^-1-100 ng mL^-1. The detection limit was as low as 8.4 pg mL^-1. Meanwhile, with the assistance of a portable smartphone and a miniature electrochemical workstation, the electrical signal obtained was synchronized with the colorimetric signal to correct the actual target concentration in the sample, further reducing the occurrence of false reports. Importantly, this protocol provides a new idea for the sensitive detection of cancer markers and the construction of a multi-signal output platform.

Hybridization Chain Reaction-Enhanced Biocatalytic Precipitation on Flower-like Bi2S3: Toward Organic Photoelectrochemical Transistor Aptasensing with High Transconductance

Organic photoelectrochemical transistor (OPECT) bioanalysis has recently emerged as a promising avenue for biomolecular sensing, providing insight into the next-generation of photoelectrochemical biosensing and organic bioelectronics. Herein, this work validates the direct enzymatic biocatalytic precipitation (BCP) modulation on a flower-like Bi₂S₃ photosensitive gate for high-efficacy OPECT operation with high transconductance (g_m), which is exemplified by a prostate-specific antigen (PSA)-dependent hybridization chain reaction (HCR) and subsequent alkaline phosphatase (ALP)-enabled BCP reaction toward PSA aptasensing. It has been shown that light illumination could ideally achieve the maximized g_m at zero gate bias, and BCP could efficiently modulate the device's interfacial capacitance and charge-transfer resistance, resulting in a significantly changed channel current (I_DS). The as-developed OPECT aptasensor realizes good analysis performance for PSA with a detection limit of 10 fg mL^-1. This work features direct BCP modulation of organic transistors and is expected to stimulate further interest in exploring advanced BCP-interfaced bioelectronics with unknown possibilities.

Highly sensitive photoelectrochemical immunosensor for detecting cancer marker CA19-9 based on a new SnSe quantum dot

A sandwich-type photoelectrochemical (PEC) immunosensor was constructed on a screen-printed electrode (SPE) using gold-coated tin selenide quantum dots (Au-SnSe QDs) to determine the carbohydrate antigen 19 9 (CA19-9). Water-soluble Au-SnSe QDs were prepared by coating low-cost SnSe QDs, prepared by reacting tin(II) 2-ethyl hexanoate with selenium ions (HNaSe) without needing to add an external capping agent (SnSe QDs). SnSe-based QDs were characterized using high-resolution transmission electron microscopy (HR-TEM) and dynamic light scattering (DLS). DSP (dithio-bis (succinimidyl propionate)) as a linker was attached on Au@SnSe QDs and conjugated with CA19-9 monoclonal antibodies (Ab₂-DSP-Au@SnSE QD). For capture probe assembling, an Au nano-layer was electrochemically deposited on a SPE by HAuCl₄ reduction using 12 cycles of cyclic voltammetry (0 to - 1.4 V) at the scan rate of 50 mV s^-1, then covered by self-assembly of DSP and covalent conjugation of CA19-9 Ab₁. Our developed PEC immunosensor showed a significant photoelectrochemical response, recorded using chronoamperometry (0.3 V), for the presence of CA19-9 antigen in serum samples under light irradiation, with a detection limit (LOD) of 0.0011 U mL^-1 and a dynamic range of 0.005-100 U mL^-1. The recovery of CA19-9 determination from serum samples was 101 to 113%.

Ion-Exchange Reaction-Mediated Hierarchical Dual Z-Scheme Heterojunction for Split-Type Photoelectrochemical Immunoassays

Photoelectrochemical (PEC) immunoassays with ultrasensitive detection abilities are highly desirable for in vitro PEC diagnosis and biological detection. In this paper, dual Z-scheme PEC immunoassays with hierarchical nanostructures (TiO₂@NH₂-MIL-125@CdS) are synthesized through epitaxial growth of MOF-on-MOF and further in situ derivatization. The dual Z-scheme configuration not only extends the light absorption range but also increases the redox ability due to the interface structure nanoengineering, which synergistically suppresses bulk carrier recombination and promotes the charge transfer efficiency at the electron level. Furthermore, a smart MOF-derived labeling probe (CuO@ZnO nanocube) is designed to develop a split-type PEC biosensor by using prostate-specific antigen (PSA) as a target biomarker. In the presence of PSA, the Ab₂-labeled CuO@ZnO would specifically bond to the dual Z-scheme electrode. Then, the MOF-derived CuO@ZnO is dissolved by hydrochloric acid to release Cu²⁺, which could replace Cd²⁺ via an ion-exchange reaction, thus leading to the decrease of the photocurrent due to the destruction of the dual Z-scheme configuration. In typical applications, the split-type PEC immunoassay exhibits an excellent detection performance for PSA with a LOD as low as 0.025 pg·mL^-1.

An "off-on" electrochemiluminescence aptasensor for determination of lincomycin based on CdS QDs/carboxylated g-C3N4

A novel electrochemiluminescence (ECL) aptasensor for the determination of lincomycin (LIN) was developed based on CdS QDs/carboxylated g-C₃N₄ (CdS QDs/C-g-C₃N₄). CdS QDs/C-g-C₃N₄ served as the substrate of the aptasensor, and then CdS QDs/C-g-C₃N₄-modified electrode was incubated with aptamer DNA (Apt-DNA). When the non-specific sites of the electrode surface was blocked by 6-mercaptohexanol, the ferrocene-labeled probe (Fer-DNA) was assembled onto the electrode surface through base complementation with Apt-DNA. In the absence of LIN, the ECL signal was quenched effectively by Fer-DNA and a decreased ECL emission (off state) was acquired. On the contrary, LIN was specifically bond with Apt-DNA, and Fer-DNA was detached from the aptasensor surface because of the deformation of Apt-DNA, resulting in an effectively enhanced ECL signal (on state). The constructed ECL aptasensor exhibited a wide detection range for LIN determination (0.05 ng mL^-1-100 μg mL^-1) with a low detection limit (0.02 ng mL^-1). Importantly, the proposed ECL aptasensor showed outstanding accuracy and specificity for LIN determination, and also provided a potential strategy for other antibiotic determinations.

Ultrasensitive photoelectrochemical immunosensor based on Dual-Photosensitive electrodes

In the study, a photoelectrochemical (PEC) immunosensor based on dual-photosensitive electrodes was developed for cardiac troponin I (cTnI) detection. The sensing photocathode with biometric functions was prepared by CuInS₂ and narrow band gap semiconductor In₂S₃ as the counter electrode. In this way, the separation of photoanode and biometric events was realized, and the ability of stability of the immunosensor could be effectively improved. Moreover, the attraction to the photogenerated electrons (e^-) from photoanode would be increased by the abundant holes (h⁺) of photocathode, under the radiation of light. This tremendously improves the photoelectric response, which further improves the sensitivity of the immunosensor. The controllable-synthesis uncomplicated photoelectric material not only accords with the principle of simplicity of electrode modification but also makes the immunosensor more conducive to the practical application. Additionally, even in the case of zero bias voltage, the constructed PEC immunosensor can operate with high efficiency, namely, self-powered. The immunosensor could provide the quantitative readout photocurrent to a concentration of cTnI in the range of 0.10 pg/mL to 1.00 μg/mL and the detection limit was 0.0113 pg/mL under the optimal experimental conditions. With favorable performance in terms of anti-interference, stability, specificity and reproducibility, this immunosensor will provide new prospects for general PEC bioanalysis development.

CuO Nanozymes as Multifunctional Signal Labels for Efficiently Quenching the Photocurrent of ZnO/Au/AgSbS2 Hybrids and Initiating a Strong Fluorescent Signal in a Dual-Mode Microfluidic Sensing Platform

A novel dual-mode microfluidic sensing platform based on CuO nanozymes as a photoelectrochemical (PEC)-fluorescent (FL) multifunctional signal label was developed for ultrasensitive neuron specific enolase (NSE) detection. Herein, ZnO/Au/AgSbS₂ hybrids, possessing excellent PEC properties, were first exploited as a sensing matrix to provide a stable photocurrent. The controlled synthesis of photoactive ZnO nanoflowers (NFs) was successfully conducted using a microfluidic reactor in the scale of seconds. Furthermore, the photocurrent of ZnO NFs decorated by Au and AgSbS₂ nanoparticles significantly improved, owing to the local surface plasma resonance effect of Au and matching band structure between ZnO and AgSbS₂. A strategy of catalytic oxidation ascorbic acid (AA) by CuO nanozymes was proposed to quench the PEC signals and initiate FL signals. CuO nanoparticles growing on conductive carbon spheres (CuO@CSs) as secondary antibodies' labels could efficiently catalyze the oxidation of AA to achieve a PEC "signal-off" state. Then, the produced dehydroascorbic acid reacting with o-phenylenediamine opportunely generated a strong FL signal. Importantly, wide linear ranges of 0.0001-150 ng/mL for the PEC technique and 0.001-150 ng/mL for the FL method with a low detection limit of 0.028 and 0.25 pg/mL, respectively, could guarantee the sensitive detection of NSE.

Optical fiber tip integrated photoelectrochemical sensors

In this work, we design and fabricate a compact photoelectrochemical (PEC) sensor by integrating a graphene-MoS₂ heterostructure on an optical fiber tip. The graphene serves as a transparent carrier transport layer, and the MoS₂ presents a photoelectrical transducer that generates photocarriers and interacts with ascorbic acid (AA) in solution. This device is used to demonstrate a self-powered detection of AA with a concentration range between 1 mM and 50 mM, and a time response of ∼ 6 ms. The device downsizes traditional PEC systems to the micrometer scale, benefiting the real-time monitoring of biochemical changes in small areas and opening the pathway for miniaturized PEC sensing applications.

A microfluidic cloth-based photoelectrochemical analytical device for the detection of glucose in saliva

Photoelectrochemical (PEC) detection is a widely used detection method that uses light to stimulate and photocurrent signals to detect the target. Due to the disengagement of the excitation unit and the detection unit, the PEC background signal is reduced, and the detection sensitivity is improved. In this work, we report the first demonstration of PEC detection for microfluidic cloth-based analytical devices (μCADs). Using PEC μCADs integrated with cadmium sulfide quantum dots (CdS QDs) and multiwalled carbon nanotubes (MWCNTs), the nonenzymatic, sensitive and rapid measurement of glucose in saliva has been achieved. For the cloth-based device, the PEC reaction zone and cloth-based electrodes can be fabricated by inexpensive wax-based and carbon ink-based screen-printing, respectively. By the layer-by-layer method, the as-prepared poly (dimethyl diadly ammonium chloride-functionalized) MWCNTs (PDDA-MWCNTs) and CdS QDs are successively adsorbed onto the working electrode surface of the cloth-based device. In the presence of an excitation source and glucose, the CdS QDs generate a strong oxidizing electron hole that can then continuously oxidize glucose to produce an electrical signal for glucose detection. Under optimized conditions, a linear dependence is obtained between the PEC signal and glucose concentrations in the range of 0.05-1000 μM with a detection limit of 15.99 nM. In the detection range, the cloth-based device also shows acceptable selectivity, reproducibility, and long-term stability. Moreover, the method has been implemented for the detection of glucose in real saliva samples, suggesting good potential for biochemical applications.

Disposable and cost-effective label-free electrochemical immunosensor for prolactin based on bismuth sulfide nanorods with polypyrrole

Prolactin (PRL) is produced by the pituitary gland and plays a vital role in the production of milk after a baby is born. PRL levels are normally elevated in pregnant and nursing women, and high levels of PRL in the human body cause hyperprolactinemia, infertility, galactorrhea, infrequent or irregular periods, amenorrhea, breast pain, and loss of libido. Accordingly, herein, a novel label-free immunosensor using a bismuth sulfide/polypyrrole (Bi2S3/PPy)-modified screen-printed electrode (SPE) for the fast and facile detection of the peptide hormone PRL. Bi2S3 nanorods were synthesized via a facile hydrothermal technique, and PPy was prepared by chemical polymerization method. Subsequently, the Bi2S3/PPy/ SPE was modified with 3-mercaptopropionic acid (MPA) and EDC/NHS. Owing to the cross-linking effect of EDC/NHS, antibody-PRL (anti-PRL) was firmly stabilized on the modified SPE surface. These layer-by-layer modifications enhanced the conducting properties, anti-PRL loading capacity, and sensitivity of the developed immunosensor. Under optimized conditions, the PRL immunosensor demonstrated a broad linear range of approximately 1-250 ng/mL, a low detection limit of approximately 0.130 ng/mL (3 × SD/b), good specificity, reproducibility, and stability. PRL was successfully evaluated in human and mouse serum samples, and the corresponding outcomes were compared with those of the electrochemical and ELISA methods.

Design of a photoelectrochemical lab-on-a-chip immunosensor based on enzymatic production of quantum dots in situ

We present a new photoelectrochemical immunoassay based on a microfluidic device. Its operation employs enzymatic generation of CdS quantum dots.

Au nanoclusters-decorated WO3 nanorods for ultrasensitive photoelectrochemical sensing of Hg2+

Ultrasensitive photoelectrochemical sensing of Hg2+ is achieved using Au nanocluster-decorated WO3 nanorods as photoactive materials.

A visible and near-infrared light dual responsive "signal-off" and "signal-on" photoelectrochemical aptasensor for prostate-specific antigen

A visible and near-infrared light dual responsive "signal-off" and "signal-on" photoelectrochemical aptasensor was constructed for determining prostate-specific antigen (PSA) based on MoS₂ nanoflowers and gold nanobipyramids. The dual responsive photoelectrochemical aptasensor can provide accurate results for PSA determination. For the photoelectrochemical aptasensor fabrication, amino-group functionalized aptamers were immobilized on a MoS₂ nanoflowers modified glassy carbon electrode surface for the specific recognition, and thus to achieve a "signal-off" aptasensor for PSA under visible light illumination. Subsequently, gold nanobipyramids integrated with thiol-functional aptamer were introduced to the "signal-off" aptasensing interface after PSA recognition. Under excitation with near-infrared light at 808 nm, the photocurrent response can be amplified significantly due to the excellent conductivity and local surface plasmon resonance effect of gold nanobipyramids, thus to producing a "signal-on" model for determining PSA. Under the optimized conditions, the dual-responsive photoelectrochemical aptasensor shows a linear response to the logarithm of PSA concentration in the range of 0.005-100 ng/mL. The detection limits for PSA determination with a "signal-off" or a "signal-on" mode are 1.75 pg mL^-1 and 0.39 pg mL^-1, respectively. The dual-responsive photoelectrochemical aptasensor was also employed for determining PSA in clinical serum samples with satisfactory selectivity and excellent accuracy.

Tailoring the Photoelectrochemical Activity of Hexametaphosphate-Capped CdS Quantum Dots by Ca2+-Triggered Surface Charge Regulation: A New Signaling Strategy for Sensitive Immunoassay

The development of efficient signaling strategies is highly important for photoelectrochemical (PEC) immunoassay. We report here a new and efficient strategy for sensitive PEC immunoassay by tailoring the electrostatic interaction between the photoactive material and the electron donor. The photoelectric conversion of hexametaphosphate (HMP)-capped CdS quantum dots (QDs) in Na₂SO₃ solution is significantly boosted after Ca²⁺ incubation. The negative surface charges on CdS@HMP QDs decrease because of the complexation reaction between HMP and Ca²⁺, and the electrostatic repulsion between CdS@HMP QDs and electron donor (SO₃^2-) becomes weak accordingly, leading to an improved electron-hole separation efficiency. Inspired by the PEC response of CdS@HMP QDs to Ca²⁺, a novel "signal-on" PEC immunoassay platform is established by employing CaCO₃ nanoparticles as labels. By regulating the surface charge of CdS@HMP QDs with in situ-generated Ca²⁺ from CaCO₃ labels, sensitive detection of the carcinoembryonic antigen (CEA) is achieved. The linear detection range is 0.005-50 ng mL^-1 and the detection limit is 1 pg mL^-1 for CEA detection. Our work not only provides a facile route to tailor the photoelectric conversion but also lays the foundation for sensitive PEC immunoassay by simply regulating the surface charge of photoactive materials.

Cathode-Anode Spatial Division Photoelectrochemical Platform Based on a One-Step DNA Walker for Monitoring of miRNA-21

Photoelectrochemical (PEC) biosensors carried out the whole reaction process in the same solution, which would limit the sensitivity and selectivity of detection in the sensing system. Herein, we reported a promising new cathode-anode spatial division PEC platform based on the two-electrode synergistic enhancement strategy. With the photoanode and photocathode integrated in the same current circuit, the platform exhibited an increased photocurrent response, as well as an improved anti-interference ability led by separating the two electrodes spatially. In this proposal, red light-driven AgInS₂ nanoparticles (NPs) served as the photoanode to build biometric steps and amplify the signal, whereas p-type PbS quantum dots were selected as the photocathode to increase the signal. With the participation of alkaline phosphatase (ALP) labeled on Au NPs-DNA, ascorbic acid 2-phosphate was catalyzed to produce ascorbic acid as an electron donor, resulting in the enhancement of the PEC signal. Interestingly, in the presence of miRNA-21 and T7 Exo, the one-step DNA walker amplification can be triggered to reduce the PEC signal by releasing ALP-Au NP-DNA. The constructed PEC biosensor exhibited a detection limit of as low as 3.4 fM for miRNA-21, which was expected to be applied to early clinical diagnosis. Also, we believe that the proposed cathode-anode spatial division PEC platform can open up a new view for the establishment of other types of PEC biosensors.

Investigation of the inhibited biotoxicity of heavy metals towards 5- formylcytosine in rice by hydrochar based on photoelectrochemical biosensor

A photoelectrochemical (PEC) biosensor was constructed for 5-formylcytosine (5fC) nucleotide detection based on Ag₂S@WS₂ photoactive material and FeVO₄ catalytic signal quenching. After Ag₂S@WS₂ was modified onto the ITO substrate surface, 5fC recognition reagent of Au@4-amino3hydrazino5mercapto-1,2,4-triazol (Au@AHMT) was further modified through electrostatic adsorption. Afterwards, based on the specific chemical reaction between -NH₂ and -CHO, 5fC can be selectively recognized and captured. Subsequently, the nanoenzyme of FeVO₄ was recognized based on the specific reaction between the phosphate group of 5fC nucleotide and Fe³⁺. Under the catalysis of FeVO₄, the 4-chloro-1-naphthol in the detection solution can be oxidized to generate a precipitate, which will be retained on the electrode surface to inhibit the PEC signal. The developed method presented a widely dynamic range from 0.1 to 400 nM. The detection limit was 0.062 nM (3σ). This method also showed good detection selectivity, reproducibility and stability. The applicability was verified by investigating 5fC content change in genomic DNA of rice tissues after incubated with heavy metals. Moreover, the inhibited influence of hydrochar towards heavy metals was also assessed.

Proton-Regulated Catalytic Activity of Nanozymes for Dual-Modal Bioassay of Urease Activity

Benefiting from the merits of high stability and superior activity, nanozymes are recognized as promising alternatives to natural enzymes. Despite the great leaps in the field of therapy and colorimetric sensing, the development of highly sensitive nanozyme-involved photoelectrochemical (PEC) biosensors is still in its infancy. Specifically, the investigation of multifunctional nanozymes facilitating different catalytic reactions remains largely unexplored due to the difficulty in synergistically amplifying the PEC signals. In this work, mesoporous trimetallic AuPtPd nanospheres were synthesized with both efficient oxidase and peroxidase-like activities, which can synergistically catalyze the oxidation of 4-chloro-1-naphthol to produce benzo-4-chlorohexadienone precipitation on the surface of photoactive materials, and thus lead to the decreased photocurrent as well as increased charge-transfer resistance. Inspired by the proton-dependent catalytic activity of nanozymes, a self-regulated dual-modal PEC and electrochemical bioassay of urease activity was innovatively established by in situ regulating the activity of AuPtPd nanozymes through urease-mediated proton-consuming enzymatic reactions, which can remarkably improve the accuracy of the assay. Meanwhile, the determination of urease activity in spiked human saliva samples was successfully realized, indicating the reliability of the biosensor and its application prospects in clinical diagnosis.

The discovery of lactoferrin dual aptamers through surface plasmon resonance imaging combined with a bioinformation analysis

An analytical method for screening aptamers for different recognition sites in lactoferrin (Lac) molecules has been developed based on Surface Plasmon Resonance imaging (SPRi), combined with the cluster classification calculation of a quasi-aptamer library strategy and molecular docking simulation analysis. Using the software simulation, a homology analysis was performed on the selected quasi-aptamer sequences, which could be divided into 8 different families. Based on the principle of biomolecular recognition, a label-free, high-throughput dual immune site screening method was established, in which the nucleic acid aptamers of recognizing ability for lactoferrin molecules were fixed onto the surface of the SPRi sensor chip and could bind to the lactoferrin molecules. Then, the aptamer candidates to be paired were introduced, and the recognition event of the second immune site was judged by observing the binding signal of SPRi. The paired SPRi signal was generated only when the site identified by the second nucleic acid molecule was different from the first immune site. Based on this principle, a pair of Lac nucleic acid aptamers (Lac-8 and Lac-25) was finally screened and confirmed using computerized simulation, and has been employed to assay Lac in milk by ELONA (Enzyme-Linked Oligonucleotide Assay).

Nanozyme-Activated Synergistic Amplification for Ultrasensitive Photoelectrochemical Immunoassay

At present, enzyme-mediated signal amplification strategies have been widely applied in photoelectrochemical (PEC) biosensing systems, while the introduction of natural enzymes onto the surface of photoelectrodes inevitably obstructs the electron transfer due to their insulating properties as proteins, leading to severe damage to photocurrent. In this work, the PdPt bimetallic nanozymes with the efficient peroxidase-like activity were used as alternatives to natural enzymes and amplified PEC biosensing signals via their efficient enzymatic reaction and remarkable enhancement in photocurrent. As a result, photoactive CdS nanorods modified with PdPt bimetallic nanozymes showed a boosted PEC performance compared with the pristine CdS nanorods due to the localized surface plasmon resonance effect and Schottky junction. On the basis of the as-prepared CdS/PdPt photoelectrode, a sensitive split-type glucose oxidase-mediated PEC immunoassay for carcinoembryonic antigen (CEA) detection was successfully constructed. Along with the sandwich immunocomplexing, the subsequently produced hydrogen peroxide (H₂O₂) can oxidize 4-chloro-1-naphthol into insoluble precipitates to inhibit photocurrent and simultaneously trigger the bio-etching of CdS to further restrain photocurrent signals due to the excellent peroxidase-mimicking activity of PdPt nanozymes. Owing to the synergistic signal amplification fulfilled by PdPt nanozymes, an ultrasensitive immunoassay of CEA was realized with a wider linear range from 1 to 5000 pg/mL and a low detection limit of 0.21 pg/mL, opening a new avenue for building ultrasensitive PEC biosensors with nanozymes.

Polydopamine nanospheres with multiple quenching effect on TiO2/CdS:Mn for highly sensitive photoelectrochemical assay of tumor markers

A photoelectrochemical (PEC) immunosensing strategy based on the multiple quenching of polydopamine nanoparticles (PDA NPs) to Mn²⁺-doped CdS-modified TiO₂ nanoparticles (TiO₂/CdS:Mn) was designed for the highly sensitive detection of carcinoembryonic antigen (CEA). The uniform PDA NPs possessed good dispersibility, good biocompatibility, and abundant functional groups for biomolecule assembly. They also had unique photophysical properties, with light absorption spanning the visible to infrared light range. When the immune-recognition brought the PDA NPs close to the TiO₂/CdS:Mn interface, the PDA NPs competed with TiO₂/CdS:Mn to absorb light, consumed photoelectrons generated in the TiO₂/CdS:Mn, and hindered the access of electron donors to photoactive materials. The contribution from these aspects thus led to a significant decrease in photocurrent. Benefiting from the multiple quenching mechanism, the PEC immunosensor showed high sensitivity for CEA detection. Under optimal conditions, a low detection limit of 0.02 pg/mL and a wide linear relationship from 0.1 pg/mL to 100 ng/mL were obtained. The immunoassay showed good reproducibility and stability, and good selectivity and high accuracy in serum sample analysis. In this regard, PEC immunosensors may have great application potential for screening tumor markers and the prevention and monitoring of serious diseases.

Applications of DNA-nanozyme-based sensors

Since the discovery of the enzyme-like activities of nanomaterials, the study of nanozymes has become one of the most popular research frontiers of diverse areas including biosensors. DNA also plays a very important role in the construction of biosensors. Thus, the idea of combined applications of nanozymes with DNA (DNA-nanozyme) is very attractive for the development of nanozyme-based biosensors, which has attracted considerable interest of researchers. To date, many sensors based on DNA-functionalized or templated nanozymes have been reported for the detection of various targets and highly accelerated the development of nanozyme-based sensors. In this review, we summarize the main applications and advances of DNA-nanozyme-based sensors. Additionally, perspectives and challenges are also discussed at the end of the review.

CdS Quantum-Dots-Decorated V2O5 Nanosheets as Chemically Etchable Active Materials for Sensitive Photoelectrochemical Immunoassay of Carcinoembryonic Antigen

We report here CdS quantum-dots (QDs)-decorated V₂O₅ nanosheets as high-performance and chemically etchable photoelectric active materials for constructing a photoelectrochemical (PEC) immunoassay platform. CdS QDs-decorated V₂O₅ nanosheets as new photoelectric materials can show superior photocurrent to V₂O₅ nanosheets and CdS QDs under visible-light irradiation because of the promoted photogenerated electron-hole separation and the increased visible-light absorption. V₂O₅ nanosheets can be etched by ascorbic acid (AA) because of the reduction of V₂O₅ to V⁴⁺, and the photocurrent of CdS/V₂O₅-nanocomposite-modified indium tin oxide electrode decreases significantly after being etched by AA. Inspired by this phenomenon, a PEC immunoassay platform is constructed for carcinoembryonic antigen (CEA) detection by using CdS/V₂O₅ nanocomposite as the photoelectric material and AA-encapsulated liposome immunonanocapsules as labels. The linear detection range for detecting CEA is from 0.5 pg mL^-1 to 1 ng mL^-1, with a limit of detection of 0.1 pg mL^-1. The proposed method also shows good selectivity, excellent reproducibility, and satisfactory recovery in detection of CEA in human serum samples. We believe that this work will lay the foundation for the future development of V₂O₅-based materials for PEC analysis, and also provide a reasonable design and implementation for the development of PEC immunoassay.

CdS Quantum Dots Modified Photoelectrochemical Biosensor for TATA-Binding Protein Probing

The photoelectrochemical (PEC) biosensor, in which light is utilized to excite the photoactive species and current is employed as the detection signal, is a newly appeared yet dynamically developing technique for biological analysis. Based on the assay of DNA binding proteins upon visible light irradiation, a PEC biosensor is constructed for successfully probing a DNA-protein interaction.

A procalcitonin photoelectrochemical immunosensor: NCQDs and Sb2S3 co-sensitized hydrangea-shaped WO3 as a matrix through a layer-by-layer assembly

Electron-transfer mechanism of a PEC immunosensor based on WO₃/NCQDs/Sb₂S₃ composites in PBS electrolytes containing AA.

An CuInS2 photocathode for the sensitive photoelectrochemical determination of microRNA-21 based on DNA-protein interaction and exonuclease III assisted target recycling amplification

A photocathode is described for the determination of microRNA-21 by using CuInS₂ as an active photocathode material. Exonuclease III assisted target recycling amplification was employed to enhance the detection sensitivity. The TATA-binding protein (TBP) was applied to enhance steric hindrance which decreases the photoelectrochemical intensity. This strategy is designed by combining the anti-interference photocathode material, enzyme assisted target recycling amplification and TBP induced signal off, showing remarkable amplification efficiency. Under the optimized conditions, the detection limit for microRNA-21 is as low as 0.47 fM, and a linear range was got from 1.0 × 10^-15 M to 1.0 × 10^-6 M. Graphical abstract Schematic representation of sensitive photoelectrochemical detection of microRNA-21.CuInS₂ is used as an active photocathode material. Combined Exonuclease III assisted target recycling amplification and TATA-binding protein decreased of photoelectrochemical intensity, the detection limit was 0.47 fM with good selectivity. (miR-21: microRNA-21; CS: chitosan).

Ultrathin PtNi nanozyme based self-powered photoelectrochemical aptasensor for ultrasensitive chloramphenicol detection

Nanozymes have gained increasing attention in the field of biosensing. Rationally designed nanozymes with excellent catalytic activity are accessible to substitute natural enzymes. Herein, a novel self-powered photoelectrochemical (PEC) aptasensor was constructed for ultrasensitive detection of chloramphenicol (CAP) based on ultrathin PtNi nanowires (NWs) as nanozyme and benzene-ring doped g-C₃N₄ (BR-CN) as the photoactive material. The prepared 1-nm-thick PtNi nanozyme acted as a peroxidase, possessing higher catalytic activity than natural horseradish peroxidase (HRP) and other Pt-based mimic enzymes. Through the biotin-streptavidin specific interaction, streptavidin modified PtNi nanozyme was introduced into the dual-stranded DNA (dsDNA) formed by complementary DNA and biotinylated CAP aptamer. The PtNi nanozyme catalyzed 4-chloro-1-naphthol (4-CN) oxidation to generate insoluble precipitation on the electrode surface, resulting in an obvious photocurrent reduction. In the presence of CAP, the CAP aptamer was released from the electrode due to strong affinity with CAP, causing the decrease of catalytic precipitation and consequently the generation of a high photocurrent signal. On the basis of PtNi nanozyme signal amplification, the developed self-powered PEC aptasensor showed a wide linear range of 0.1 pM-100 nM with an ultralow detection limit of 26 fM for the determination of CAP. This work provides a feasible strategy for the design of high-activity nanozyme and self-powered PEC biosensor to achieve the ultrasensitive detection of target analyte.

Enhanced photoelectrochemical sensing based on novel synthesized Bi2S3@Bi2O3 nanosheet heterostructure for ultrasensitive determination of L-cysteine

The design of a low-cost and highly efficient photoactive heterojunction material for sensing is still a challenging issue. On the basis of the formation of sheet-like Bi₂O₃ via coating Bi₂S₃, a novel Bi₂O₃@Bi₂S₃ heterostructure is controllably synthesized via a facile water bath approach. The prepared Bi₂O₃@Bi₂S₃ nanosheets show a superior photoelectrochemical (PEC) performance for the detection of L-cysteine (L-Cys), and the photocurrent signal is three and four times higher than those of Bi₂S₃ and Bi₂O₃ under visible irradiation, respectively. Also, the heterostructure presents an outstanding linear range for the detection of L-Cys: 0.1-10,000 μM. In addition, the mechanism of improved PEC response of Bi₂O₃@Bi₂S₃ nanosheets is investigated according to the estimated energy band positions. Thus, the integration of the novel heterostructure and the photoelectrochemical technique demonstrates a rapid photocurrent response, showing a great effect on the performance of the sensing system and a new PEC method for highly selective and sensitive chemical detection. Graphical abstract.

Design and construction of Z-scheme Bi2S3/nitrogen-doped graphene quantum dots: Boosted photoelectric conversion efficiency for high-performance photoelectrochemical aptasensing of sulfadimethoxine

Rational design and fabrication of Z-scheme visible-light-driven photoactive materials have drawn much attention owing to their great potential in handling environment and energy crisis. In this work, Z-scheme Bi₂S₃/nitrogen-doped graphene quantum dots (NGQDs) with superior photoelectric conversion efficiency were designed and fabricated, which demonstrated enhanced photoactivity compared with Bi₂S₃ owing to the improved separation efficiency of photogenerated electron and hole pairs. The emphasis was put on designing Z-scheme Bi₂S₃/NGQDs, and then the mechanism of Z-scheme charge transfer mode was verified by the electron spin resonance (ESR) technique. On this basis, the proposed sensor exhibited a wide linear range of 0.1-120 nM and a detection limit of 0.03 nM (S/N = 3) for SDM, with high sensitivity (0.075 μA nM ^-1), good selectivity and stability. Moreover, the proposed PEC aptasensor using Bi₂S₃/NGQDs as the photoelectrode achieved sensitive and selective determination of sulfadimethoxine in milk samples. This work could provide some ideas for designing other Z-scheme photoactive species and insights into the charge transfer mechanism of Z-scheme. Furthermore, the promising applicability of PEC aptasensor using photoactive species could be extended to other accurate monitoring for contaminants.