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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.

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Vascular endothelial growth factor 165 (VEGF165) is a crucial regulator of angiogenesis and works as a major protein biomarker of cancer metastasis. Therefore, its quantitative detection is pivotal in clinic. In this work, CuS/ZnIn2S4 flower-like heterojunctions had strong and stable photocurrents, which behaved as photoactive material to construct a photoelectrochemical (PEC) aptasensor for detecting VEGF165, combined by home-prepared (MnCo)Fe2O4 nanozyme-mediated signal amplification. The interfacial photo-induced electron transfer mechanism was chiefly discussed by UV-vis diffuse reflectance spectroscopy in details. Specifically, the (MnCo)Fe2O4 modified VEGF165 aptamer was released from the PEC aptasensing platform for its highly specific affinity to target VEGF165, which terminated the color precipitation reaction, ultimately recovering the PEC signals. The developed sensor displayed a wider linear range from 1 × 10-2 to 1 × 104 pg mL-1 with a smaller limit of detection (LOD) of 0.1 fg mL-1. This study provides some valuable insights for building other ultrasensitive aptasensors for clinical assays of cancer biomarkers in practice.

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Streptomycin (STR) is extensively employed for preventive and curative purposes in animals, which is accumulated in human body through food chain and induces serious health problems. Herein, highly photoactive type II heterojunctions of porous CdIn2S4/SnO2 microspheres were initially prepared, which can effectively inhibit the recombination of the charge-hole pairs. Besides, the peroxidase-mimicking catalytic property of the hollow PtCu nanocages (PtCu NCs) was carefully investigated by UV-vis spectroscopy, where catalytic oxidation of tetramethylbenzidine behaved as the benchmarked reaction. On such basis, a highly selective photoelectrochemical (PEC) aptasensor was established with the CdIn2S4/SnO2 heterojunctions for bioanalysis of streptomycin, coupled by the PtCu NCs nanozyme-catalyzed biocatalytic precipitation to achieve signal magnification. Specifically, the home-made nanozyme was applied for catalytic oxidation of 3,3'-diaminobenzidine to generate insulating precipitate in aqueous H2O2 system and thereby block the light harvesting on the photoanode, showing steeply declined PEC responses. The as-built aptasensor showed a broad linear range of 0.01-200 nM with a low limit of detection of 7.50 pM (S/N = 3) for such analysis, combined by exploring its practical detection in milk samples. This work shows excellent nanozyme-catalyzed signal amplification for fabrication of ultrasensitive PEC biosensors towards other antibiotics detection.

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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.

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Constructing of heterostructures can significantly improve the photoelectrical (PEC) response signal by promoting the migration and suppressing the recombination of photogenerated carries. A bifunctional PEC sensing platform was designed for simultaneous high-performance detection of mucin-1 (MUC1) and carcinoembryonic antigen (CEA), which was based on generated Z-scheme heterostructured Ag3PO4/Ag/TiO2 nanorod arrays (NAs) and enzyme-mediated catalytic precipitation by alkaline phosphatase (ALP) and Au/hollow Co3O4 polyhedron. The proposed aptasensor displayed linear ranges of 1.0-100 ng mL-1 and 0.1-50 ng mL-1 for MUC1 and CEA with limit of detections of 0.430 and 0.058 ng mL-1, respectively. This strategy offers potential applications for early diagnosis, monitoring progression, and even evaluating the prognosis of breast cancer in practice.

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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.

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A CdS/AuNPs/NiO Z-scheme heterojunction was prepared on a fluorine-doped tin oxide (FTO) electrode by hydrothermal synthesis of NiO on FTO, electrodeposition of AuNPs on NiO/FTO electrode and then cast-coating of CdS quantum dots. The CdS/AuNPs/NiO/FTO electrode gave a notably increased photocurrent versus NiO/FTO, CdS/FTO, AuNPs/NiO/FTO, CdS/AuNPs/FTO and CdS/NiO/FTO electrodes. The CdS/AuNPs/NiO/FTO electrode was further cast-coated with chitosan to immobilize d-mannose by Schiff base reaction, and concanavalin A (ConA) and then horseradish peroxidase (HRP) were captured on the electrode surface by lectin-sugar binding. 4-Chloro-1-naphthol (4-CN) was oxidized to form an insoluble precipitate catalyzed by HRP in the presence of H2O2, and the presence of precipitate on the photoelectrode inhibited the photocurrent in the presence of holes scavenger ascorbic acid. The relevant electrodes were characterized by electrochemistry, quartz crystal microbalance (QCM), UV-vis spectrophotometry, scanning electron microscopy/energy dispersive spectroscopy, and transmission electron microscopy. The QCM revealed that the collection efficiency (η) of the 4-CN-electrooxidation precipitate on the electrode can be as high as 91.8%. Under the optimal conditions, the decline of photocurrent responded linearly to the common logarithm of ConA concentration from 50 pM to 500 nM, with a limit of detection of 17 pM (S/N = 3). Satisfactory results were obtained in the detection of real soybean samples.

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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.

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MicroRNA (miRNA) is a new class of tumor biomarkers in human body for early diagnosis and therapy of cancers, whose detection has scientific significance and potential applications. Herein, a sensitive heterostructured BiVO4/CoPi photoelectrochemical (PEC) biosensor was established for sensing miRNA 141 with assistance of home-synthesized AuPt nanodendrites (NDs) as nanozyme. Specifically, the BiVO4/CoPi heterostructures displayed rough worm-like internetworks, as characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). In parallel, the PEC and UV-vis diffuse reflectance spectroscopy tests confirmed their excellent optical property, combined by discussing the interfacial electron transfer mechanism. Additionally, the AuPt NDs displayed superior peroxidase-like property in the presence of H2O2 as identified by benchmarked tetramethylbenzidine (TMB) oxidation, coupled by showing remarkable catalysis for 3-amino-9-ethylcarbazole (AEC) oxidation to form biocatalytic precipitation (BCP). Integrated by a cyclic enzyme strategy, the developed PEC biosensor exhibited a wider linear range of 5 fM ∼1 pM and a lower limit of detection (LOD) as low as 0.17 fM (S/N = 3). This work provides some valuable insights for sensitive analysis of tumor-associated miRNA in clinic.

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Enzyme-mediated signal amplification strategies have gained substantial attention in photoelectrochemical (PEC) biosensing, while natural enzyme on the photoelectrode inevitably obstructs the interfacial electron transfer, in turn deteriorating the photocurrent responses. Herein, Au nanoparticles and Cu2+-modified boron nitride nanosheets (AuNPs/Cu2+-BNNS) behaved as nanozyme to achieve remarkable magnification in the PEC signals from a novel signal-off aptasensor for ultra-sensitive assay of telomerase (TE) activity based on Ag2S/Ag nanoparticles decorated ZnIn2S4/C3N4 Z-scheme heterostructures (termed as Ag2S/Ag/ZnIn2S4/C3N4, synthesized by hydrothermal treatment). Specifically, telomerase primer sequences (TS) were extended by TE in the presence of deoxyribonucleoside triphosphates (dNTPs), which was directly bond with the thiol modified complementary DNA (cDNA), achieving efficient linkage with the nanozyme via Au-S bond. The immobilized nanoenzyme catalyzed the oxidation between 4-chloro-1-naphthol (4-CN) and H2O2 to generate insoluble precipitation on the photo-electrode. By virtue of the inhibited PEC signals with the TE-enabled TS extension, an aptasensor for assay of TE activity was developed, showing the wide linear range of 50-5×105 cell mL-1 and a low detection limit of 19 cell mL-1. This work provides some valuable guidelines for developing advanced nanozyme-based PEC bioanalysis of diverse cancer cells.

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Severe challenges are still remained for development of highly sensitive, selective and stable photoelectrochemical (PEC) sensing technology, albeit with its broad application for chloramphenicol (CAP) detection. Herein, a novel "signal-on" PEC aptasensor was fabricated based on a 3D self-supporting Z-scheme AgI/Ag/BiOI heterojunction arrays subtly integrated with in-situ formed biocatalytic precipitation (BCP) for highly sensitive and selective determination of CAP. Impressively, the HRP modified CAP aptamer (HRP-CAP aptamer) was released from the electrode by its strong affinity to the introduced CAP, and gradually terminated the BCP reaction, in turn recovering the photocurrent. By virtues of the 3D self-supporting AgI/Ag/BiOI Z-scheme heterojunction arrays and BCP signal amplification strategy, the resultant PEC sensor exhibited a wide linear range of 2-250 nM with a limit of detection (LOD) as low as 0.226 nM (S/N = 3). This work opens a new avenue for design of PEC aptasensing strategy and exhibits the marvelous potential in bioanalysis of environmental samples.

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

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Three-dimensional (3D) nanostructured materials have recently attracted intensive research interest in various fields. For the photoelectrochemical (PEC) bioanalysis, unique 3D nanostructured semiconductors are also of special appeal because they allow direct mass transport and offer large surface areas for biomolecular immobilization. Currently, two-dimensional and 3D metal-sulfide semiconductor materials have drawn tremendous interest because of their different chemophysical features. This work presents the innovative fabrication of 3D ZnInS nanoflakes (NFs)@carbon fiber (CF) frameworks and its validation as a unique 3D platform for the biocatalytic precipitation (BCP)-based PEC immunoassay. Experimental results revealed that the as-developed 3D framework photoelectrode was advantageous for the accommodation of biomolecules and in situ deposition of BCP. The corresponding BCP growth process on the photoelectrode was also systematically characterized. Exemplified by lipoprotein-associated phosPhohPaseA2 (LpPLA2), the as-derived PEC immunoassay exhibits good analytical performance. This study features the first fabrication of 3D ZnInS NFs@CF frameworks for advanced BCP-based PEC bioanalysis and is expected to inspire more interest in the development of 3D nanoframework photoelectrodes for future PEC bioanalysis development.

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The photoelectrochemistry (PEC) performance of TiO2 is somewhat limited by its wide band gap and low quantum efficiency, and the innovation of its composite materials provides a promising solution for an improved performance. Herein, a composite of a Au nanorod@TiO2 core-shell nanostructure (AuNR@TiO2) and a melanin-like l-DOPA polymer (PD) is designed and prepared, where the outer layer PD tethered by TiO2-hydroxyl complexation and the AuNR core can intensify the long-wavelength light harvesting, and the AuNR@TiO2 core-shell structure can strengthen the hot-electron transfer to TiO2. The photocurrent of PD/AuNR@TiO2 is 8.4-fold improved versus that of commercial TiO2, and the maximum incident photon-to-electron conversion efficiency reaches 65% in the UV-visible-near-infrared region. In addition, the novel PD/AuNR@TiO2 photocatalyst possesses the advantages of good biocompatibility and stability, which can act as a versatile PEC biosensing platform for providing a biocompatible environment and improving detection sensitivity. Herein, a PEC enzymatic biosensor of glucose is developed on the basis of the immobilization of dual enzyme [glucose oxidase (GOx) and horseradish peroxidase (HRP)] in PD and the signaling strategy of biocatalytic precipitation. In phosphate buffer containing glucose and 4-chloro-1-naphthol, the HRP-catalyzed oxidation of 4-chloro-1-naphthol by GOx-generated H2O2 can form a precipitate on the electrode, by which the decrement of photocurrent intensity is proportional to the common logarithm of glucose concentration. The linear detection range is from 0.05 µM to 10.0 mM glucose, with a limit of detection of 0.01 µM (S/N = 3). Glucose in some human serum samples is analyzed with satisfactory results.

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Cathode photoelectrochemical immunoassay usually shows better anti-interference capacity toward real samples than anode photoelectrochemical immunoassay. However, its poor photocurrent response has greatly restricted the detection sensitivity. Herein, a promising ultrasensitive cathode photoelectrochemical immunoassay was developed based on TiO2 photoanode-enhanced 3D Cu2O nanowire array (NWA) photocathode, and coupled with signal amplification by horseradish peroxidase (HRP)-induced biocatalytic precipitation (BCP). Carcinoembryonic antigen (CEA, Ag) was used as a detection model, TiO2 nanoparticle-modified indium tin oxide (ITO) electrode served as the photoanode, and Cu2O NWAs grown in situ on Cu mesh was both the photocathode and photoelectrochemical matrix to immobilize the capture CEA antibodies (Ab1). The signal CEA antibodies (Ab2) were labeled with HRP to form Ab2-HRP bioconjugates, and employed as signal amplifiers when the specific immunoreaction occurred. The developed photoanode-enhanced cathode photoelectrochemical immunoassay has good anti-interference capability, outstanding photocurrent response, and high sensitivity for target Ag detection, which was attributed to the synergistic effects of the 3D nanostructure of Cu2O NWA photocathode, the introduction of TiO2 photoanode as counter electrode, and the signal amplification of Ab2-HRP bioconjugate-induced BCP. The developed cathode photoelectrochemical immunoassay showed a low limit of detection (0.037 pg mL-1) with a wide linear range (from 0.1 pg mL-1 to 50 ng mL-1) for CEA detection.

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Methods based on photoelectrochemistry have been developed for immunoassay, but most involve in a low sensitivity and a relatively narrow detectable range. Herein a new bio-bar-code-based split-type photoelectrochemical (PEC) immunoassay was designed for sensitive detection of prostate-specific antigen (PSA), coupling rolling circle amplification (RCA) with enzymatic biocatalytic precipitation. The bio-bar-code-based immunoreaction was carried out on monoclonal anti-PSA antibody (mAb1)-coated microplate using primer DNA and polyclonal anti-PSA antibody-conjugated gold nanoparticle (pDNA-AuNP-pAb2) with a sandwich-type assay format. Accompanying the immunocomplex, the labeled primer DNA on gold nanoparticle readily triggered RCA reaction in the presence of padlock probe/dNTPs/ligase/polymerase. The RCA product with a long single-stranded DNA could cause the formation of numerous hemin/G-quadruplex-based DNAzyme concatamers. With the assistance of nicking endonuclease, DNAzyme concatamers were dissociated from gold nanoparticle, which catalyzed the precipitation of 4-chloro-1-naphthol in the presence of H2O2 onto CdS nanorods-coated electrode (as the photoanode for the generated holes). The formed insoluble precipitate inhibited the electron transfer from the solution to CdS nanorods-modified electrode by using ascorbic acid as the electron donor. Under the optimum conditions, the photocurrent of the modified electrode decreased with the increasing of PSA concentration. A detectable concentration for target PSA with this system could be achieved as low as 1.8pgmL-1. In addition, our strategy also showed good reproducibility, high specificity and accuracy matched well with commercial PSA ELISA kits for real sample analysis. These remarkable properties revealed that the developed PEC immunoassay has great potential as a useful tool for the detection of PSA in practical application.

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In this study, quartz crystal microbalance (QCM) and surface plasmon resonance (SPR) sensors were combined with template enhanced hybridization processes (TEHP), rolling circle amplification (RCA) and biocatalytic precipitation (BCP) for ultrasensitive detection of DNA and protein. The DNA complementary to the aptamer was released by the specific binding of the aptamer to the target protein and then hybridized with the capture probe and the assistant DNA to form a ternary "Y" junction structure. The initiation chain was generated by the template-enhanced hybridization process which leaded to the rolling circle amplification reaction, and a large number of repeating unit sequences were formed. Hybridized with the enzyme-labeled probes, the biocatalytic precipitation reaction was further carried out, resulting in a large amount of insoluble precipitates and amplifying the detection signal. Under the optimum conditions, detection limits as low as 43 aM for target DNA and 53 aM for lysozyme were achieved. In addition, this method also showed good selectivity and sensitivity in human serum.

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A new photoelectrochemical (PEC) immunosensor based on Mn-doped CdS quantum dots (CdS:Mn QDs) on g-C3N4 nanosheets was developed for the sensitive detection of prostate specific antibody (PSA) in biological fluids. The signal derived from the as-synthesized Cd:Mn QDs-functionalized g-C3N4 nanohybrids via a hydrothermal method and was amplified through DNAzyme concatamers on gold nanoparticles accompanying enzymatic biocatalytic precipitation. Experimental results by UV-vis absorption spectra and photoluminescence revealed that CdS:Mn QDs/g-C3N4 nanohybrids exhibited higher photocurrent than those of CdS:Mn QDs and g-C3N4 alone. Upon addition of target PSA, a sandwich-type immunoreaction was carried out between capture antibodies and the labeled detection antibodies. Accompanying introduction of gold nanoparticles, the labeled initiator strands on the AuNPs triggered hybridization chain reaction and the formation of DNAzyme concatamers in the presence of hemin. The formed DNAzyme catalyzed 4-chloro-1-naphthol (4-CN) to produce an insoluble/insulating precipitate on the Mn:CdS QDs/g-C3N4, and blocked the light harvesting of Mn:CdS QDs/g-C3N4, thus resulting in the decreasing photocurrent. Under optimal conditions, the immunosensor exhibited good photocurrent responses for determination of target PSA, and allowed detection of PSA at a concentration as low as 3.8pgmL-1. The specificity, reproducibility and precision of this system were acceptable. Significantly, this methodology was further evaluated for analyzing human serum samples, giving well-matched results with referenced PSA enzyme-linked immunosorbent assay (ELISA) method.

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This work reports on a new impedimetric immunosensing strategy for sensitive detection of prostate-specific antigen (PSA) in biological fluids. The assay was carried out on monoclonal anti-PSA capture antibody-modified glassy carbon electrode with a sandwich-type detection format. Gold nanoparticles-decorated g-C3N4 nanosheets (AuNP/g-C3N4), synthesized by the wet-chemistry method, were utilized for the labeling of polyclonal anti-PSA detection antibody and horseradish peroxidase (HRP). Upon target PSA introduction, the sandwiched immunocomplex could be formed between capture antibody and detection antibody. Followed by the AuNP/g-C3N4, the labeled HRP could catalyze 4-choloro-1-naphthol into benzo-4-chlorohexadienone. The as-generated insoluble product was coated on the electrode surface, thus increasing the Faradaic impedance of Fe(CN)6(4-/3)(-) indicator between the solution and the base electrode. Under the optimal conditions, the impedance increased with the increasing target PSA in the sample, and exhibited a wide linear range from 10pgmL(-1) and 30ngmL(-1) with a detection limit of 5.2pgmL(-1). A repeatability and intermediate precision of <14% was accomplished. The specificity and method accuracy in comparison with commercial PSA ELISA kit for analysis of human serum specimens were relatively satisfactory.

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Branched zinc oxide nanorods (BZR) arrays, an array with high charge carries collection efficiency and specific surface area, are grown on the reduced graphene oxide-paper working electrode for the first time to construct a paper-based electrochemical (EC) immunosensor. Typically, the BZR are fabricated via a simple hydrothermal process, which can provide abundant sites for antibodies loading. By combining the large surface area of porous zinc oxide (PZS) and good biocompatibility of gold nanoparticles (AuNPs), PZS-AuNPs (PZS@Au) nanocomposites are designed to label horseradish peroxide (HRP) and antigens. After a competitive reaction between antigens and PZS@Au nanocomposites labeled antigens, the signal labels are introduced into the immunosensor, in which, HRP participate in biocatalytic precipitation process. The produced precipitate reduces the electrode surface area and hinders the electron transfer. With the increase of concentration of antigens, the signal labels introduced into the sensor decrease, thus, a signal-on immunoassay for α-fetoprotein detection is constructed. The proposed paper-based EC immunosensor combines enzymatic biocatalytic precipitation reaction and competitive immunoassay mode for the first time, and possesses a wide linear range from 0.2 pg mL(-1) to 500 ng mL(-1) with a detection limit of 0.08 pg mL(-1). In addition, the proposed method is simple, sensitive and specific and can be a promising platform for other protein detection.

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