Surface plasmon resonance-enhanced photoelectrochemical immunoassay with Cu-doped porous Bi2WO6 nanosheets

The development of rapid screening sensing platforms to improve pre-screening mechanisms in community healthcare is necessary to meet the significant need for portable testing in biomarker diagnostics. Here, we designed a portable smartphone-based photoelectrochemical (PEC) immunoassay for carcinoembryonic antigen (CEA) detection using Cu-doped ultrathin porous Bi2WO6 (CuBWO) nanosheets as the photoactive material. The CuBWO nanosheets exhibit a fast photocurrent response and excellent electrical transmission rate under UV light due to their surface plasmon resonance effect (SPR). The method uses glucose oxidase-labeled secondary antibody as a signal indicator for sandwich-type immune conjugation. In the presence of the target CEA, the electrons and holes generated at the surface of the photo-excited ultrathin porous CuBWO were rapidly consumed by the production of H2O2 from glucose oxidase oxidizing glucose, resulting in a weakened photocurrent signal. The photocurrent intensity increased logarithmically and linearly with increasing CEA concentration (0.02-50 ng mL-1), with a detection limit of 15.0 pg mL-1 (S/N = 3). The system provides a broader idea for inferring the electron-hole transport mechanism in ultrathin porous nanosheet layer materials and developing efficient PEC sensors.

Carbon quantum dots(CQDs)-sensitized CdS/CuInS2 heterojunction as a photoelectrochemical biosensing platform for highly sensitive detection of prostate-specific antigen

Sensitive and quantitative detection of prostate-specific antigen (PSA) has been determined to be indispensable for clinical diagnostics of prostate cancer, whereas such detection is quite challenging due to the extremely low concentration of biomarkers in human serum samples. In this study, a photoelectrochemical (PEC) sensor was effectively developed for the high-sensitivity analysis of prostate-specific antigen (PSA) using a signal amplification method utilizing sensitized carbon quantum dots (CQDs). In this experiment, cadmium sulfide quantum dots were employed as the substrate materials, and indium copper sulfide quantum dots were loaded on their surfaces. Moreover, the efficient matching of energy levels in these two materials contributed to the generation of photocurrents. The aforementioned heterojunction semiconductor QDs were thus combined with CQDs to produce CQDs on their surfaces. As a result of the presence of CQDs, the ability of heterojunction materials to absorb light was remarkably enhanced, increasing the photocurrent by over ten times. Consequently, in this study, CQDs were combined with PEC sensors, and the developed PEC biosensors exhibited excellent optical performance, sensitivity, repeatability, and stability. The results obtained from the analysis of actual samples were satisfactory and have promising application prospects.

Portable photoelectrochemical immunoassay with micro-electro-mechanical-system for alpha-fetoprotein in hepatocellular carcinoma

Early detection of cancer has a profound impact on patient survival and treatment outcomes considering high treatment success rates and reduced treatment complexity. Here, we developed a portable photoelectrochemical (PEC) immune platform for sensitive testing of alpha-fetoprotein (AFP) based on Pt nanocluster (Pt NCs) loaded defective-state g-C3N4 photon-electron transducers. The broad forbidden band structure of g-C3N4 was optimized by the nitrogen doping strategy and additional homogeneous porous structure was introduced to further enhance the photon utilization. In addition, the in-situ growth of Pt NCs provided efficient electron transfer catalytic sites for sacrificial agents, which were used to further improve the sensitivity of the sensor. Efficient photoelectric conversion under a hand-held flashlight was determined by the geometry of the transducer and the energy band design, and the portable design of the PEC sensor was realized. The developed sensing platform exhibited a wide linear response range (0.1-50 ng mL-1) and low limit of detection (0.043 ng mL-1) for AFP under optimum conditions. This work provides a new idea for designing portable PEC biosensing platforms to meet the current mainstream POC testing needs.

Vanadium-doped metal-organic framework@Znln2S4 core-shell heterojunction-attenuated photoelectrochemical immunoassay

Considering that cancer has become the second leading cause of death in humans, it is essential to develop an analytical approach that can sensitively detect tumor markers for early detection. We report an attenuated photoelectrochemical (PEC) immunoassay based on the organic-inorganic heterojunction 10MIL-88B(FeV)/ZnIn2S4 (10M88B(FeV)/ZIS) as a photoactive material for monitoring carcinoembryonic antigen (CEA). The 10M88B(FeV)/ZIS heterojunctions have excellent light-harvesting properties and high electrical conductivity, which are attributed to the advantages of both organic and inorganic semiconductors, namely, remarkable photogenerated carrier separation efficiency and long photogenerated carrier lifetime. Horseradish peroxidase (HRP) in the presence of H2O2 can catalyze 3,3'-diaminofenamide (DAB) producing brown precipitates (oxDAB), which is then loaded onto the 10M88B(FeV)/ZIS heterojunction to reduce the photocurrent and enable the quantitative detection of CEA. Under optimal conditions, the photocurrent values of the PEC biosensor are linearly related to the logarithm of the CEA concentrations, ranging from 0.01 ng mL-1 to 100 ng mL-1 with a detection limit (LOD) of 4.0 pg mL-1. Notably, the accuracy of the PEC biosensor is in agreement with that of the human CEA enzyme-linked immunosorbent assay (ELISA) kit.

Photocurrent-polarity switching between methylene blue-loaded liposome and iodine-doped BiOCl for in-situ amplified immunoassay

This work designed a liposome-mediated photocurrent polarity switching immunosensor depending on the reversed photocurrent of iodine-doped BiOCl (I-BOC) nanoflowers induced by the released methylene blue (MB) for the detection of prostate-specific antigen (PSA). Initially, MB-loaded liposomes as indicators were confined within the microplates to participate in the sandwiched immunoreaction and lysed under the treatment of Triton X-100 to release numerous MB. Owing to the host-guest recognition between β-cyclodextrin (β-CD) and MB, the released MB was immobilized on the β-CD-modified I-BOC/FTO electrode and triggered the photocurrent polarity reversal from cathodic photocurrent to anodic photocurrent. The sensing platform realized an accurate and sensitive assay of PSA due to the effective elimination of false-positive/negative signals in a linear range of 0.02-50 ng mL-1 with a limit of detection of 12 pg mL-1. Furthermore, this work not only conjugated liposome-assisted signal amplification strategy with the photocurrent polarity switching system but also provided a novel pathway for various protein determinations.

Self-assembled p-n Ag2O@Bi2O2S nanoflower heterojunctions for sensitive photoelectrochemical immunoassay

A new photoelectrochemical immunoassay based on self-assembled p-n Ag2O@Bi2O2S nanoflower heterojunction was designed and developed for quantitative monitoring of prostate-specific antigen (PSA) in biological fluids. Primarily, self-assembled p-n Ag2O@Bi2O2S nanoflower heterojunctions were served as the photoactive materials and coated onto the surface of electrodes. Subsequently, the glucose oxidase (GOx) was bound to the detection antibody (mAb2) labeled gold nanoparticles (Au NPs) and then were employed to accomplish a sandwich-like immunoreaction to generate H2O2 on a microplate incubated with monoclonal anti-PSA antibodies. In the presence of PSA, the product (H2O2) was catalyzed by the substrate, which was used as an electron sacrificial agent to improve signal conversion and capture of photogenerated electrons. Under optimum conditions, a wide linear range of 0.01-50 ng mL-1 and a low detection limit of 5.3 pg mL-1 were accomplished with the sensor, exhibiting an excellent photocurrent response. Moreover, the proposed sensor revealed satisfactory reproducibility, high selectivity, and acceptable accuracy for the real sample testing. Importantly, our work provides a novel strategy for high sensitivity detection of disease-associated biomarkers for the early diagnosis of cancers.

Ultrathin mesoporous BiOCl nanosheets-mediated liposomes for photoelectrochemical immunoassay with in-situ signal amplification

Designing new biochemical sensors and achieving selectivity and high-sensitivity analysis is one of main research directions for immunoassays. Herein, a liposome-amplification photoelectrochemical (PEC) immunoassay was developed using ultrathin mesoporous bismuth chloride oxide nanosheets (BiOCl MSCN) for the highly selective and sensitive detection of carcinoembryonic antigen (CEA). Based on good photocurrent response of BiOCl MSCN toward dopamine, a liposome-conjugated secondary antibody loaded with dopamine was added for specific recognition in the presence of CEA. After the lysis treatment, the liberated dopamine was injected into the three-electrode electrolytic cell to enhance the photocurrent of BiOCl MSCN. Under the optimized conditions, the constructed liposome-mediated PEC immunoassay showed high sensitivity against CEA, with a dynamic response in the linear range of 0.05 ng mL-1 to 100 ng mL-1 and a detection limit of 35 pg mL-1. The present study proposes a new approach to the liposome-mediated PEC immunoassay constructed on ultrathin mesoporous BiOCl nanosheets, which can be used to target further the study of the sensing mechanism.

Smartphone-based photoelectrochemical immunoassay for carcinoembryonic antigen based on BiOCl/CuBi2O4 heterojunction

Photoelectrochemical (PEC) immunoassay has been widely developed for biomarker detection, but most include heavy and expensive instruments that are not suited for portable and on-site detection. In this work, the PEC immunoassay platform for mobile phones was reported for flexible, rapid, low-cost detection of carcinoembryonic antigen (CEA). The PEC detection platform was successfully composed of disposable screen-printed carbon electrodes, a micro-electrochemical workstation, a flashlight (the excitation light source), and a smartphone with a companion software with a micro-electrochemical workstation for rapid and on-site detection of target biomarkers. In this portable smartphone-based PEC system, the S-scheme heterojunction BiOCl/CuBi2O4 was effectively excited due to the efficient electron transfer rate and excellent photocurrent response under visible light. Specifically, the sandwich-type immunoreaction for capturing target biomarkers introduced alkaline phosphatase (ALP) labeled gold nanoparticles (Au NPs). The addition of CEA increased the ascorbic acid (AA) content and enhanced the photocurrent. The proposed immunoassay presented a good linear with the logarithm of CEA concentrations range within 0.01-40 ng mL-1, and the detection limit of 3.5 pg mL-1 (S/N = 3). Therefore, the portable detection platform offered an implementable approach to the development of miniaturized and portable photoelectrochemical detectors and on-site detection technology.

High-entropy effect with hollow (ZnCdFeMnCu)xS nanocubes for photoelectrochemical immunoassay

High entropy (HE) compounds with chemically disordered multi-cation structures have become a hot research topic because of their fascinating "cocktail effect". However, high entropy effect with the efficient photoelectric response has not been reported for photoelectrochemical (PEC) immunoassays. Herein, an innovative PEC immunoassay for the sensitive detection of prostate-specific antigen (PSA) was ingeniously constructed using hollow nanocubic (ZnCdFeMnCu)xS photoactive matrices with high entropic effect via the cation exchange. Initially, a sandwich-type immunoreaction has behaved using dopamine-loaded liposome labeled with anti-PSA secondary antibodies. In the presence of PSA, addition of Triton X-100 caused the liposomal cleavage to release dopamine, which was then detected as a reduced photocurrent on (ZnCdFeMnCu)xS-based photoelectrode. Under optimal condition, the PEC immunoassay showed good photocurrent responses toward target PSA with the dynamic linear range of 0.1-50 ng mL-1 with a limit of detection of 34.1 pg mL-1. Significantly, this system can provide a new platform for the development of PEC immunoassays by coupling with high-entropy photoactive materials.

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.

Enzyme-catalyzed high-performing reaction with in-situ amplified photocurrent on carbon-functionalized inorganic photoanode for immunosensing

An enzyme-catalyzed high-performing reaction with in-situ amplified photocurrent was innovatively designed for the quantitative screening of carcinoembryonic antigen (CEA) in biological fluids by coupling with carbon-functionalized inorganic photoanode. A split-type photoelectrochemical (PEC) immunoassay was initially executed with horseradish peroxidase (HRP)-labeled secondary antibody on the capture antibody-coated microtiter. Then, the photocurrent of carbon-functionalized inorganic photoanode were improved through enzymatic insoluble product. Experimental results revealed that introduction of the outer carbon layer on the inorganic photoactive materials caused the amplifying photocurrent because of the improving light harvesting and separation of photo-generated e^-/h⁺ pairs. Under optimum conditions, the split-type photoelectrochemical immunosensing platform displayed good photocurrent responses within the dynamic range of 0.01 - 80 ng mL^-1 CEA, and allowed the detection of CEA as low as a concentration of 3.6 pg mL^-1 at the 3S_blank level. The strong attachment of antibodies onto nano label and high-performing photoanode resulted in a good repeatability and intermediate precision down to 9.83%. No significant differences at the 0.05 significance level were encountered in the analysis of six human serum specimens between the developed PEC immunoassay and the commercially available CEA ELISA kits.

Bio-inspired nanozyme with ultra-thin Fe-Bi2O2S nanosheets for in-situ amplified photoelectrochemical immunoassay of cancer-related protein

A Fe-loaded Bi₂O₂S nanosheet photoanode serving as photoelectric biomonitoring platform for the detection of prostate-specific antigen (PSA) using biologically inspired prussian nanoparticle (PB)-catalyzed biocatalytic precipitation strategy was developed. Primarily, the signal probe PB-mAb₂ obtained by electrostatic adsorption was immobilized on a microplate in the presence of target PSA, and 4-chloro-1-naphthol (4-CN) was oxidized to benzo-4-chloro-hexadienone (4-CD) with the assistance of exogenous hydrogen peroxide, which was generated by a large number of hydroxyl radicals catalyzed by PB. The generated 4-CD showed strongly low conductivity characteristics to burst the photocurrent of highly photoactive Fe-Bi₂O₂S photoanode. The split incubation reaction could be suitable for high volume and low-cost rapid detection. A dynamic response range of 0.1-100 ng mL^-1 with a limit of detection of 34.2 pg mL^-1 was achieved with the sensor based on a photoelectric sensing platform and a biomimetic catalytic precipitation reaction. Equally important, the sensor also showed good potential in the detection of real samples compared to commercially available ELISA kits. In conclusion, this work provides a fresh scheme for the development of sensitive biosensors through a bio-inspired catalytic strategy of versatility and a photoanode coupling with high photoelectric activity.

Paper-based photoelectrochemical immunoassay for ultrasensitive screening of carcinoembryonic antigen on hollow CdS/CdMoO4-functionalized photoanode

Lab-based testing systems utilizing photoelectrochemical (PEC) biosensing methodologies for the ultrasensitive carcinoembryonic antigen (CEA) have been developed, although the majority have shown complicated operating procedures and dependence on precise apparatus. Herein, a portable photoelectrochemical split diagnostic platform based on a hollow CdS/CdMoO₄ (h-CdS@CdMoO₄) shell-shell structured photoanode system was developed for ultrasensitive detection of CEA. Using a small LED flashlight as the excitation light source and a digital multimeter (DMM) as the signal readout device, real-time CEA on a paper-based printed screen electrode developed in-house was quickly detected. The composite h-CdS@CdMoO₄ featured a special hollow shell-shell heterojunction structure that optimizes photon usage in the bulk phase on the one hand, and facilitates directed separation of the electrons and holes therein on the other. A split-sandwich immunoassay and detection antibodies for modified glucose oxidase were introduced into the paper-based photoanode test system, and the signals were displayed with a DMM to realize a point-of-care test for CEA. Under optimized conditions, the constructed portable PEC sensing system was sensitive to the target CEA from 0.02 to 50.0 ng mL^-1 with a detection limit of 11.3 pg mL^-1. Interferent experiments and stability test evaluations demonstrate the specificity and robustness of the constructed paper-based portable PEC sensor. The portable, paper-based PEC immunoassay system developed offers a fresh way of exploring affordable, approachable sensors to satisfy both the relevant community medical testing demands and hospital objectives for quick testing.

Schottky-functionalized Z-scheme heterojunction: Improved photoelectric conversion efficiency and immunosensing

Designing photovoltaic materials with good photoelectric activity is the crucial to boost the sensitivity of photoelectrochemical (PEC) biosensors. To meet this concern, a Schottky-functionalized direct Z-scheme heterojunction photovoltaic material was proposed by electrodeposition of gold nanoparticles on two kinds of bismuth oxyhalide composites surface (bismuth oxybromide and bismuth oxyiodide with different but matched band gaps) (depAu/BiOI/BiOBr). Specifically, synergistic effect was achieved through the direct Z-scheme heterojunction formed by BiOBr and BiOI as well as the gold Schottky junction, resulting in the enhanced light harvest and photoelectric conversion efficiency. Meanwhile, combined with sandwich immunotechnology, a "signal-off" PEC biosensor was fabricated for highly sensitive detection of carcinoembryonic antigen (CEA). In which, using depAu/BiOI/BiOBr modified glassy carbon electrodes both as the photoactive sensing interface and capture antibody loading matrix, polyethyleneimine copper complex encapsulated gold nanoclusters labeled detection antibody (Ab₂-Au@PEI-Cu) as the quencher, the photocurrent decreased with the increasing target CEA introduced by sandwich immune reaction. The proposed smart PEC immunoassay platform exhibited a wide detection range (1.0 fg/mL-2.0 ng/mL) and a detection limit as low as 0.11 fg/mL with favorable selectivity and stability. In addition, this PEC sensing strategy can be easily extended for other tumor marker analysis, which offers a new perspective of using multiple bismuth oxyhalide as photoactive materials for early diseases diagnosis.

Photoelectrochemical immunoassay for thyroglobulin on nanogold-functionalized BiVO4 photoanode coupling with enzymatic biocatalytic precipitation

Methods derived from photoelectrochemical (PEC) have been constructed for immunoassays, but most involve the split-type immunoreaction modes, and thus easily cause unpredictable intermediate precision. Herein, we innovatively designed an integrated PEC immunosensing platform for the quantitative monitoring of thyroglobulin (TG) on the gold nanoparticles (AuNPs)-functionalized BiVO₄ photoanode coupling with enzymatic biocatalytic precipitation (EBCP). This sensing system could simultaneously implement the immunoreaction and photocurrent measurement. Anti-TG capture antibodies were modified onto AuNPs-decorated BiVO₄ photoelectrode. A sandwich-type immunoreaction was carried out in the presence of target TG using horseradish peroxidase (HRP)-conjugated anti-TG detection antibody. The carried HRP molecules catalyzed 4-chloro-1-naphthol (4-CN) to generate an insoluble benzo-4-chlorohexadienone product on the photoanode in the presence of peroxide hydrogen, thereby decreasing the photocurrent. Under optimal conditions, the PEC immunosensors gave good photocurrent responses toward target TG within the dynamic range of 0.01-10 ng mL^-1 at a detection limit of 7.6 pg mL^-1. Good repeatability and precision, high specificity and acceptable storage stability were acquired during the measurement. No significant differences were encountered for screening 15 human serum specimens between the developed PEC immunoassay and commercially available enzyme-linked immunosorbent assay (ELISA) method for the detection of target TG. Significantly, PEC immunosensing system offers promise for simple and cost-effective analysis of disease-related biomarkers.

In2O3/CdIn2S4 heterojunction-based photoelectrochemical immunoassay of carcinoembryonic antigen with enzymatic biocatalytic precipitation for signal amplification

This work reported a split-type photoelectrochemical (PEC) immunoassay for the detection of carcinoembryonic antigen (CEA) based on target-induced biocatalytic precipitation (BCP) by using In₂O₃/CdIn₂S₄ heterojunctions as the photosensitizers. The synthesized In₂O₃/CdIn₂S₄ heterojunctions improved the efficiency of charge separation and shortened the electron convey path to enhance the photocurrent, thus exhibiting high conductivity and low complexation rates of photogenerated electrons and holes. In the presence of CEA, horseradish peroxidase (HRP) catalyzed 4-chloro-1-naphthol (4-CN) to produce benzo-4-chloro-hexadienone (4-CD) through H₂O₂. Then, 4-CD was deposited onto the surface of In₂O₃/CdIn₂S₄ to reduce the photocurrent and realized the signal amplification. The PEC immunoassay revealed an excellent photocurrent toward target CEA within a wide range of 0.01-50 ng mL^-1 at a low limit of detection of 2.8 pg mL^-1 under the optimum conditions. Multiple switching light excitation tests demonstrated the good reliability and stability of the fabricated PEC biosensor. The accuracy was acceptable in comparison with human CEA enzyme-linked immunosorbent assay (ELISA) kit.

A sandwiched photoelectrochemical biosensing platform for detecting Cytokeratin-19 fragments based on Ag2S-sensitized BiOI/Bi2S3 heterostructure amplified by sulfur and nitrogen co-doped carbon quantum dots

A sandwiched photoelectrochemical (PEC) immunosensor based on BiOI/Bi₂S₃/Ag₂S was designed for the quantitative detection of cytokeratin-19 fragments (CYFRA21-1) in serum. In this work, due to the intervention of the narrow band gap Bi₂S₃, the absorption of the light source by the BiOI/Bi₂S₃ heterostructure has been significantly enhanced. Meanwhile, the matched band structure of BiOI, Bi₂S₃ and Ag₂S promoted the rapid transfer of electrons between the conduction bands and effectively inhibited the recombination of electron-hole pairs, thus enhanced the photoelectric signals. Sulfur and nitrogen co-doped carbon quantum dots (S,N-CQDs) with up-conversion luminescence properties provided more light energy for the base materials. On the other hand, S,N-CQDs were combined with Ab₂ through polydopamine (PDA), as secondary antibody labels, further enhanced the sensitivity of the sensor. Herein, the linear range of the sensor was from 0.001 to 100 ng mL^-1 and the detection limit was 1.72 pg mL^-1. In addition, the sensor provides a feasible way for the detection of tumor markers due to its excellent selectivity, repeatability and good stability.

A novel split-type photoelectrochemical immunosensor based on chemical redox cycling amplification for sensitive detection of cardiac troponin I

Photoelectrochemical (PEC) immunoassay is a burgeoning and promising bioanalytical method. However, the practical application of PEC still exist some challenges such as the inevitable damage of biomolecules caused by the PEC system and the unsatisfactory sensitivity for biomarkers with low abundance in real sample. To solve the problems, we integrated the cosensitized structure of Ag2S/ZnO nanocomposities as photoelectrode with photogenerated hole-induced chemical redox cycling amplification (CRCA) strategy to develop a split-type PEC immunosensor for cardiac troponin I (cTnI) with high sensitivity. Initially, the immunoreaction was carried out on the 96-well plates in which alkaline phosphatase (ALP) could catalyze ascorbic acid 2-phosphate (AAP) to generate the signal-reporting species ascorbic acid (AA). Subsequently, the AA participated and the tris (2-carboxyethyl) phosphine (TCEP) mediated chemical redox cycling reaction took place on the photoelectrode, thus leading to signal amplification. Under the optimized conditions, the immunosensor demonstrated a detection limit (LOD) of 3.0 × 10^-15 g mL^-1 with a detection range of 1.0 × 10^-14 g mL^-1 to 1.0 × 10^-9 g mL^-1 for cTnI. Impressively, the proposed method could determine the cTnI in human serum samples with high sensitivity and satisfactory accuracy. Considering the virtues of the photoelectrode and the chemical redox cycling strategy, the method would hold great potential for highly sensitive biosensing and bioanalysis.

Signal-on photoelectrochemical immunoassay mediated by the etching reaction of oxygen/phosphorus co-doped g-C3N4/AgBr/MnO2 nanohybrids

We designed a signal-on photoelectrochemical (PEC) immunoassay for the sensitive monitoring of prostate-specific antigen (PSA) based on the etching reaction of hydrogen peroxide (H₂O₂) toward oxygen/phosphorus co-doped graphitic C₃N₄/AgBr/MnO₂ nanosheets (OP-g-C₃N₄/AgBr/MnO₂). Initially, glucose oxidase (GO_X)-labeled detection antibodies were introduced into the capture antibody-coated microplate with a sandwich-type immunoreaction in the presence of PSA. Then, the as-generated H₂O₂ from the decomposition of glucose by GO_X etched the manganese dioxide (MnO₂) nanosheets into manganese ions (Mn²⁺), thereby causing the exposure of the underlying OP-g-C₃N₄/AgBr. Meanwhile, H₂O₂ could be also used as an electron scavenger, and restrain the recombination of the electron-hole pairs of OP-g-C₃N₄/AgBr. Two advantages of H₂O₂ enhanced the photocurrent synergistically. Under optimum conditions, the PEC immunoassay showed high sensitivity toward target PSA within a dynamic working range of 0.05-50 ng mL^-1 with a limit of detection of 17 pg mL^-1. In addition, our system possessed high specificity, favorable selectivity, and good stability. Relative to commercialized PSA ELISA kits, the accuracy of our strategy was acceptable. More importantly, our strategy can be easily extended to screen other biomarkers by controlling the corresponding antibodies.

Double ion-exchange reaction-based photoelectrochemical immunoassay for sensitive detection of prostate-specific antigen

This work developed a double ion-exchange reaction-based photoelectrochemical (PEC) immunoassay with the split-type detection mode for sensitive detection of prostate-specific antigen (PSA, used as a model). The nanocomposite of cadmium sulfide and nickel sulfide (CdS@NiS nanocomposite), as the photoactive material, was rapidly synthesized by two-step hydrothermal treatment. In the presence of target PSA, the cupric oxide nanoparticle (CuO NP) labeled detection antibody was introduced into the detection system by sandwich immunoreaction and the copper (Cu²⁺) ions was released from CuO nanoparticles by acid to participate in double ion-exchange reaction. The double ion-exchange reaction on the photoelectric sensing interface between Cu²⁺ and CdS@NiS nanocomposites formed the weak photoactive material Cu_xS (x = 1, 2) to reduce the photocurrent. Under optimal conditions, the double ion-exchange reaction-based PEC immunoassay exhibited good photocurrent responses toward target PSA within the dynamic working range from 0.01 ng mL^-1 to 50 ng mL^-1 at a low limit of detection (LOD) of 2.9 pg mL^-1. Besides, our work could achieve good reproducibility and high specificity under the split-type detection mode. Compared with human PSA ELISA kit, the accuracy obtained by our strategy was satisfactory. Importantly, this Cu²⁺-activated double ion-exchange reaction-based PEC immunoassay provides a promising platform for the detection of biomarkers.

Photoelectrochemical immunoassay platform based on MoS2 nanosheets integrated with gold nanostars for neuron-specific enolase assay

MoS₂ nanosheets were prepared by exfoliating MoS₂ bulk crystals with ultrasonication in N-methylpyrrolidone and were integrated with gold nanostars (AuNS) to fabricate an AuNS/MoS₂ nanocomposite. All nanomaterials were characterized by transmission electron microscope, scanning electron microscope, ultraviolet-visible spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. AuNS/MoS₂ nanocomposites were coated onto a glassy carbon electrode (GCE) surface to construct a nanointerface for immobilizing neuron-specific enolase antibody (anti-NSE) thus forming a photoelectrochemical immunoassay system. AuNS can significantly promote the photoelectric conversion of MoS₂ nanosheets improving the performance for a photoelectrochemical assay. Being illuminated with white light LED and controlling the potential at 0.05 V (vs. SCE), the photocurrent generated from anti-NSE(BSA)/AuNS/MoS₂/GCE using 0.15 mol L^-1 ascorbic acid as electron donor can be recorded with amperometry and used as an output signal for NSE quantitative assay. Under optimized experimental conditions, the photocurrent variation for the affinity-binding NSE is proportional to the logarithm of NSE concentration in the range 5.0 pg mL^-1   to 1.5 ng mL^-1 with a detection limit of 3.5 pg mL^-1 (S/N = 3). The practicability of the PEC immunoassay system was evaluated by determining NSE in clinical serum samples. The recoveries ranged from 93.0 to 103% for the determination of NSE in serum samples with a standard addition method. The PEC immunoassay system possesses good accuracy for determining NSE in real samples. Graphical abstract.

A photoelectrochemical sandwich immunoassay for protein S100β, a biomarker for Alzheimer's disease, using an ITO electrode modified with a reduced graphene oxide-gold conjugate and CdS-labeled secondary antibody

A sandwich-type photoelectrochemical immunoassay is described for the protein S100ß which is an Alzheimer's disease biomarker found in the astrocytes of the brain. Antibody against S100ß (anti-S100ß) was labeled with CdS quantum dots and then acted as a secondary antibody. The labeled antibody was characterized by FTIR, ultraviolet-visible and fluorescence spectroscopy. An indium-tin oxide (ITO) electrode was modified with a nanocomposite prepared from reduced graphene oxide and gold nanoparticles. Then, a sol-gel film containing isocyanate functional groups (-N=C=O) was cast on the surface of the electrode. The NCO group reacts with amino groups of the labeled antibody to covalently bind them to the surface. The S100β was bound by the primary immobilized antibody on the rGO-Au/ITO electrode and then sandwiched with the labeled secondary antibody. Cyclic voltammetry and electrochemical impedance spectroscopy were applied to confirm the stepwise changes in the electrochemical properties of the electrode surface. The photoelectrochemical immunoassay, typically operated at a potential of +0.2 V (vs. Ag|AgCl_sat) gives a signal that is related to the logarithm of the S100β concentration in the range from 0.25 to 10 ng·mL^-1 with a lower detection limit of 0.15 pg·mL^-1. The method was successfully applied to the determination of S100β in human serum samples. Graphical abstract Schematic presentation of an immunosensor which is based on an indium tin oxide modified with reduced graphene oxide decorated with gold nanocomposite and antibody. The immunosensor was applied for the determination of S100β biomarker by using in the labeled antibody.

Liposome-amplified photoelectrochemical immunoassay for highly sensitive monitoring of disease biomarkers based on a split-type strategy

Liposomes are an excellent candidate component for biosensors to transduce and amplify detection signals due to their outstanding ability in encapsulating signal marker compounds. However, the use of liposomes for photoelectrochemical (PEC) signal transduction has not yet been achieved due the lack of appropriate sensing strategy. Herein, we report on a novel liposomes-amplified PEC immunoassay (LAPIA) method for sensitive HIV-p24 antigen (p24) detection based on a split-type strategy. Initially, liposomes were encapsulated with alkaline phosphatase (ALP) in their hydrophilic chamber and conjugated with secondary antibody on the surface to form the ALP-encapsulated liposomes (ALP-Ls) based PEC signal label. Sandwiched immunoassay based on the ALP-Ls label was then carried out in microwell plate. Upon addition of tween 20, the ALP molecules were released and catalyzed the hydrolysis of ascorbic acid 2-phosphate (AA-p) to produce ascorbic acid (AA). The latter then donated electron to the graphene/g-C₃N₄ nanohybrids based photoelectrode, arousing an increased photocurrent signal. The separation of immunoreaction step and PEC signal excitation (i.e. split-type) not only enabled the realization of liposomes based amplification strategy, but also could eliminate the PEC-caused biomolecules damage. The developed PEC method possessed a wide calibration range from 1.0pgmL^-1 to 50ngmL^-1 and a low detection limit of 0.63pgmL^-1. Its practicability was demonstrated by assaying human serum samples. Moreover, the universality of the liposomes-amplified PEC sensing strategy was also demonstrated by developing it into a sensitive microRNA detection method.

Bifunctional polydopamine thin film coated zinc oxide nanorods for label-free photoelectrochemical immunoassay

Photoelectrochemical (PEC) detection is a promising method for label-free immunoassay by reporting the specific biological recognition events with electrical signals. However, it is challenging to rationally incorporate immunosensing components with a photocurrent conversion interface, which generally necessitates multistep fabrication and careful tailoring of various components such as photoactive material and biological probe. For high detection reliability and reproducibility, it is highly desirable to rationally construct an efficient PEC interface with architecture as simple as possible. In this work, a novel yet simple PEC immunosensor based on bio-inspired polydopamine (PDA) thin film-coated zinc oxide (ZnO) nanorods was reported. In this PEC immunosensor, the PDA thin film serves simultaneously as a unique sensitizer for charge separation as well as a functional layer for probe antibody attachment. The photocurrent on this electrode under illumination decreases upon the immunoreaction on the surface, possibly due to the blocking effect of formed immunocomplexes on the access of reducing reagent to the photoelectrode, thus offering a simple and reliable platform for PEC label-free immunoassay. By using an antibody-antigen pair as a model, successful label-free immunoassay was achieved with a detection limit of 10pgmL^-1 and a dynamic range from 100pgmL^-1 to 500ngmL^-1. This work demonstrates intriguing electro-optical property and bioconjugation activity of PDA film and may pave the way toward advanced PEC immunoassays.

Microfluidic paper-based analytical device for photoelectrochemical immunoassay with multiplex signal amplification using multibranched hybridization chain reaction and PdAu enzyme mimetics

Combining multibranched hybridization chain reaction (mHCR), the photoelectrochemical (PEC) immunosensor was fabricated with a microfluidic paper-based analytical devices using different sizes of CdTe quantum dots (QDs) sensitized flower-like 3D ZnO superstructures as photoactive materials. Firstly, 4-aminothiophenol (PATP) functioned ZnO was anchored on gold-paper working electrode. With the aid of PATP, large-sized CdTe-COOH QDs (QDs1) were conjugated onto the ZnO surface because of the formation of a strong bond (Zn-S) between the thiol of PATP molecule and the ZnO, and the remaining amino group formed an amide bond with carboxylic acid group capping CdTe. Then the small-sized CdTe-NH2 QDs (QDs2) were modified on the QDs1 by forming amide bond, which leaded to a very strong photocurrent response because of the formation of cosensitized structure. The designed mHCR produced long products with multiple branched arms, which could attached multiple PdAu nanoparticles and catalyze the oxidation of hydroquinone (HQ) using H2O2 as anoxidant. Double strands DNA with multiple branched arms (mdsDNA) was formed by mHCR. In the presence of carcinoembryonic antigen (CEA), PdAu-mdsDNA conjugates-labeled CEA antibody was captured. The concentrations of CEA were measured through the decrease in photocurrent intensity resulting from the increase in steric hindrance of the immunocomplex and the polymeric oxidation product of HQ. In addition, the oxidation product of HQ deposited on the as-obtained electrode, which could efficiently inhibit the photoinduced electron transfer. Under optimal conditions, the PEC immunosensor exhibited excellent analytical performance: the detection range of CEA was from 0.001 to 90 ng mL(-1) with low detection limit of 0.33 pg mL(-1). The as-obtained immunosensor exhibited excellent precision, prominent specificity, acceptable stability and reproducibility, and could be used for the detection of CEA in real samples. The proposed assay opens a promising platform of clinical immunoassay for other biomolecules.