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.

Photocurrent Polarity Switchable Sensing of Hyaluronidase Activity by Regulating Electrostatic Interactions between Two Semiconductors.

Photocurrent polarity switchable photoelectrochemical (PEC) sensing has superior accuracy and anti-interference ability to conventional PEC sensing. The development of a novel strategy for photocurrent polarity switchable sensing is of great interest. Herein, a novel strategy for photocurrent polarity switchable sensing is reported by regulating electrostatic interactions between two semiconductor photoactive materials. Hyaluronic acid (HA)-modified CuO nanosheets show a negatively charged surface, which prevents the attachment of CuO nanosheets to negatively charged CdS nanodendrite-modified photoelectrodes because of the strong electrostatic repulsion. In the presence of hyaluronidase (HAase), the specific hydrolysis of HA on the surface of CuO by HAase can yield a positively charged surface, so CuO can be attached to a CdS-modified photoelectrode via electrostatic attraction, leading to photocurrent polarity switching. The photocurrent polarity switchable detection of HAase activity is achieved with an ultralow detection limit of 2 × 10-3 U mL-1 and a wide linear detection range between 0.01 and 100 U mL-1. This work provides a new and effective photocurrent polarity switching strategy for PEC sensing and a simple and efficient method for detecting HAase activity.

Tailoring enzymatic loading capacity on 3D macroporous gold by catalytic hairpin assembly and hybridization chain reaction: Application for ultrasensitive self-powered microRNA detection.

It is important to develop effective strategies to construct enzymatic biofuel cell based self-powered biosensors. We report here the facile regulation of enzymatic loading capacity on the bioanode by utilizing a concatenated catalytic hairpin assembly (CHA)/hybridization chain reaction (HCR) and its application for self-powered microRNA-141 (miRNA-141) detection. To construct the bioanode, a concatenated CHA/HCR process triggered by miRNA-141 was conducted on the three-dimensional macroporous gold (3DMG) electrode to generate long double-stranded DNA nanowires for glucose oxidase immobilization. Quartz crystal microbalance study reveals that the enzymatic loading capacity on the bioanode increases at an increasing miRNA-141 concentration, leading to enhanced catalytic performance for glucose oxidation. The short-circuit currents of the assembled glucose/O2 biofuel cells increase at increasing miRNA-141 concentrations, enabling ultrasensitive detection of miRNA-141. The self-powered biosensor features a wide dynamic range for detecting miRNA-141 from 10-17 to 10-11 M, with an ultralow detection limit of 1.3 aM. This work provides a highly sensitive self-powered biosensing platform for miRNA detection.

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

Photosensitizers and enzyme mimics are extensively used in photoelectrochemical (PEC) sensing, but few materials can be used as both photosensitizers and enzyme mimics in PEC sensing. Herein, we report Au nanoclusters (AuNCs) as both photosensitizers and peroxidase mimics for sensitive PEC sensing of Hg2+. It is found that AuNCs can act as photosensitizers to improve the PEC activity of WO3 nanorods; so the WO3/AuNCs composite material can be used as an advanced photosensitive material for PEC detection. AuNCs can also catalyze precipitate formation on the photoelectrode because of their peroxidase mimetic activity, and the interface electron transfer is hindered by the formed precipitate. Thus, the photocurrent of the WO3/AuNCs-based photoelectrode is quenched. When Hg2+ is present, the AuNCs-catalyzed precipitate formation is inhibited by Hg2+ because of the binding of Hg2+ to AuNCs through Hg2+-Au+ interactions. The photocurrent of the WO3/AuNCs-based photoelectrode increases accordingly, enabling "signal on" PEC detection of Hg2+. A broad linear range for Hg2+ detection is achieved between 1.0 pM and 50 nM with a detection limit of 0.2 pM. We have developed an advanced photosensitive material and introduced a simple method for PEC detection of Hg2+.

A Novel Signaling Strategy for an Ultrasensitive Photoelectrochemical Immunoassay Based on Electro-Fenton Degradation of Liposomes on a Photoelectrode.

A signaling strategy can directly determine the analytical performance and application scope of photoelectrochemical (PEC) immunoassays, so it is of great significance to develop an effective signaling strategy. The electro-Fenton reaction has been extensively used to degrade organic pollutants, but it has not been applied to PEC immunoassays. Herein, we report a novel signaling strategy for a PEC immunoassay based on electro-Fenton degradation of liposomes (Lip) on a photoelectrode. Lip vesicles are coated on Au@TiO2 core-shell photoactive material, which can prevent ascorbic acid (AA) from scavenging photogenerated holes. In the presence of a target, the immunomagnetic bead labels are converted to Fe3+ for electro-Fenton reaction, and hydroxyl radicals generated by the electro-Fenton reaction can degrade the Lip vesicles on the photoelectrode. Because of the degradation of Lip vesicles, photogenerated holes can be scavenged more effectively by AA, leading to an increase in photocurrent. Based on the electro-Fenton-regulated interface electron transfer, the sensitive "signal on" PEC immunoassay of a carcinoembryonic antigen is achieved, which features a dynamic range from 0.05 to 5 × 104 pg mL-1 and a detection limit of 0.01 pg mL-1. Our work provides a novel and efficient PEC immunoassay platform by introducing the electro-Fenton reaction into PEC analysis.

Sensitive photoelectrochemical determination of T4 polynucleotide kinase using AuNPs/SnS2/ZnIn2S4 photoactive material and enzymatic reaction-induced DNA structure switch strategy.

We report here Au nanoparticles (AuNPs)/SnS2/ZnIn2S4 as a high-performance active material for sensitive photoelectrochemical (PEC) determination of T4 polynucleotide kinase (T4 PNK) using an enzymatic reaction-induced DNA structure switch strategy. To construct the PEC biosensor, a double-stranded DNA probe consisting of a CdS quantum dots (QDs)-labeled single-stranded DNA (sDNA) and its complementary DNA (cDNA) is immobilized on the AuNPs/SnS2/ZnIn2S4 photoactive material. T4 PNK can catalyze the phosphorylation of 5'-OH-terminated sDNA in the double-stranded DNA probe when ATP is present, and λ-exonuclease can catalyze the degradation of the phosphorylated sDNA into small fragments. Then the cDNA forms a hairpin structure so that CdS QDs and AuNPs are in close contact, which can induce exciton-plasma interactions between CdS QDs and AuNPs. The exciton-plasma interactions significantly boost the photocurrent, enabling the "signal on" PEC determination of T4 PNK in the range of 10-4 - 1 U mL-1 with a detection limit of 6 × 10-5 U mL-1. The PEC biosensor can also be used to screen enzyme inhibitors.

Controllable sensitization of Zr-MOFs by using CdS and its application for photoelectrochemical detection of alkaline phosphatase.

CdS quantum dots (QDs) are attached onto zirconium-based metal-organic frameworks (Zr-MOFs) with DNA as a bridge to boost the photoelectrochemical (PEC) activity of Zr-MOFs, and the sensitization of Zr-MOFs by using CdS QDs is regulated by the alkaline phosphatase (ALP)-catalyzed hydrolysis of tripolyphosphate, enabling sensitive "signal-on" PEC detection of ALP activity.

A glucose/O2 biofuel cell integrated with an exonuclease-powered DNA walker for self-powered sensing of microRNA.

With the aid of an exonuclease-powered DNA walker, the amount of glucose oxidase immobilized on the bioanode can be facilely tailored by varying the concentration of microRNA-141, so a glucose/O2 biofuel cell is employed as a self-powered sensor for sensitive and selective detection of microRNA-141.

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 (SO32-) 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.

Photoelectrochemical immunoassay of interleukin-6 based on covalent reaction-triggered photocurrent polarity switching of ZnO@fullerenol.

We report here a novel photocurrent polarity switching strategy for a photoelectrochemical immunoassay driven by the covalent reaction between fullerenol (COH) and chloranilic acid (CA). The sensitive detection of interleukin-6 is achieved by using CA-encapsulated liposome as the label and COH-coated ZnO as the photoactive material, with a detection limit of 1.0 fg mL-1.

MWCNTs-CoP hybrids for dual-signal electrochemical immunosensing of carcinoembryonic antigen based on overall water splitting.

Great efforts have been made to search for highly active catalysts toward electrochemical water splitting, but double-signal immunosensors have not been reported based on bifunctional water splitting electrocatalysts. We report here a dual-signal electrochemical immunosensor for detecting carcinoembryonic antigen (CEA) using multi-wall carbon nanotubes (MWCNTs)-cobalt phosphide (CoP) as an electrocatalytic label. The preparation of MWCNTs-CoP involves the growth of Co3O4 nanoparticles on MWCNTs and low-temperature phosphatization of Co3O4 nanoparticles. The MWCNTs-CoP catalyst shows excellent electrocatalytic activities in a neutral medium toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), enabling MWCNTs-CoP as the electrocatalytic label for sensitive immunosensing. The linear range of the sandwich-type immunosensor for detecting CEA based on the HER signal is from 10-4-100 ng mL-1, whereas a linear range for detecting CEA based on the OER signal is achieved from 10-4 to 10 ng mL-1. The detection limits for detecting CEA using HER and OER signals are 10 and 12 fg mL-1, respectively. This work can provide a new double-signal immunosensing platform based on a bifunctional water splitting electrocatalyst.

NiCoO2@CeO2 Nanoboxes for Ultrasensitive Electrochemical Immunosensing Based on the Oxygen Evolution Reaction in a Neutral Medium: Application for Interleukin-6 Detection.

The development of highly active electrocatalytic labels is important for constructing sensitive electrochemical immunosensors. Great progress has been made in developing non-noble-metal nanocatalysts toward the oxygen evolution reaction (OER) in the past decade, but non-noble-metal OER nanocatalysts have not been explored as electrocatalytic labels for immunosensing. Herein, we report NiCoO2@CeO2 nanoboxes (NBs) as novel electrocatalytic labels for ultrasensitive immunosensing based on the excellent OER activity of NiCoO2@CeO2 NBs in a neutral solution. The synthesis of NiCoO2@CeO2 NBs involves Ni2+ exchange and heat treatment of ZIF-67 nanocubes to produce NiCoO2 NBs, followed by the growth of CeO2 nanoparticles on the surface of NiCoO2 NBs. The NiCoO2@CeO2 NBs offer superior OER activity to NiCoO2 NBs because of the synergetic effect between NiCoO2 NBs and CeO2 nanoparticles. The formation of ester-like bridging between CeO2 and the carboxylic groups of antibody enables direct immobilization of the antibody on the NiCoO2@CeO2 surface. A sandwich-type electrochemical immunosensor using NiCoO2@CeO2 NBs as electrocatalytic labels features a broad linear range for interleukin-6 detection from 2.5 × 10-5 to 10 ng mL-1, with a low detection limit of 7 fg mL-1. Our work lays the foundation for developing electrochemical immunosensors and aptasensors based on non-noble-metal OER electrocatalysts.

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.

Three-dimensional macroporous gold electrodes superior to conventional gold disk electrodes in the construction of an electrochemical immunobiosensor for Staphylococcus aureus detection.

Herein, a three-dimensional macroporous gold (3DMG) electrode is demonstrated to be a better choice than a conventional gold disk electrode in the construction of an electrochemical immunobiosensor for Staphylococcus aureus (S. aureus) detection. The 3DMG electrode was prepared on a gold disk electrode by one-step electrodeposition using hydrogen bubbles as dynamic templates. The 3DMG electrode has a high electrochemically active surface area with pore sizes ranging from 20 to 50 µm, and these unique features are conducive to the immobilization of primary antibodies and the capture of S. aureus. Secondary antibodies (Ab2) and alkaline phosphatase (ALP) were immobilized on mesoporous silica nanospheres (MSNs), and the resulting ALP-MSNs-Ab2 composites were utilized as signal tags to construct a sandwich-type electrochemical immunobiosensor. S. aureus was measured based on alkaline phosphatase-catalyzed silver deposition and differential pulse voltammetric detection. The linear range is from 5 to 109 CFU mL-1, and the detection limit is 2 CFU mL-1 for S. aureus detection. Due to the signal amplification of the 3DMG electrode, the sensitivity of the immunobiosensor constructed on the 3DMG electrode is 9 times that of an immunobiosensor constructed on a gold disc electrode. The proposed biosensor was successfully applied for detecting S. aureus in milk samples.

An immunosensor for sensitive photoelectrochemical detection of Staphylococcus aureus using ZnS-Ag2S/polydopamine as photoelectric material and Cu2O as peroxidase mimic tag.

We report here sensitive photoelectrochemical immunosensing of Staphylococcus aureus (S. aureus) using ZnS-Ag2S/polydopamine (PDA) as a novel photoelectric material and Cu2O as the peroxidase mimic tag. ZnS-Ag2S heterojunctions were prepared on indium tin oxide (ITO) via electrodeposition of ZnS nanoparticles, followed by silver ion exchange. To prepare a PDA/ZnS-Ag2S/ITO, the ZnS-Ag2S/ITO electrode was coated with PDA by self-polymerization of dopamine. The photocurrent of the PDA/ZnS-Ag2S/ITO is 1.55 times that of the ZnS-Ag2S/ITO and 7.87 times that of the ZnS/ITO, indicating a high-performance photoelectric material. A sandwiched-type photoelectrochemical immunosensor was constructed by using PDA/ZnS-Ag2S/ITO as the photoelectrode and Cu2O nanocubes as the labels. Cu2O nanocubes can serve as peroxidase mimic to generate catalytic precipitates on the immunoelectrodes, and both the Cu2O nanocubes and the generated precipitates can decrease the photocurrents of the immunoelectrodes, so a photoelectrochemical immunosensor for detecting S. aureus was constructed, showing a linear range between 10 and 107 CFU mL-1 and a low detection limit of 2 CFU mL-1. Owing to the signal amplification of Cu2O labeling, the sensitivity of the Cu2O-labeled immunosensor is 4 times that of a label-free immunosensor for detecting S. aureus, and the detection limit (2 CFU mL-1) is lower than that of a label-free immunosensor (10 CFU mL-1). This work not only provides a new and efficient photoelectric material but also demonstrated an efficient signal-amplification strategy for photoelectrochemical biosensing.

Sensitive photoelectrochemical immunoassay of Staphylococcus aureus based on one-pot electrodeposited ZnS/CdS heterojunction nanoparticles.

We report here a facile synthesis of ZnS/CdS heterojunction nanoparticles on an indium-tin oxide (ITO) electrode and their application in the ultrasensitive photoelectrochemical detection of Staphylococcus aureus (S. aureus). The ZnS/CdS/ITO electrode was prepared using one-pot electrodeposition in an acidic solution containing ZnCl2, CdCl2 and Na2S2O3. The optimal ZnS/CdS heterojunction nanoparticles with a Zn/Cd atomic ratio of 1 : 1 showed a high photoelectrochemical response to l-cysteine. l-Cysteine-encapsulated liposome (cysteine@liposome) immunonanocapsules were prepared and used as the labels for photoelectrochemical detection of S. aureus. By coupling cysteine@liposome immunonanocapsule labeling with immunomagnetic separation/enrichment and photoelectrochemical analysis using the ZnS/CdS/ITO electrode, sensitive photoelectrochemical detection of S. aureus was achieved. Under optimal conditions, the linear range for photoelectrochemical detection of S. aureus was from 1 to 4000 CFU mL-1. The proposed method was successfully used for photoelectrochemical detection of S. aureus in milk and juice samples.

Ruthenium Ion-Complexed Carbon Nitride Nanosheets with Peroxidase-like Activity as a Ratiometric Fluorescence Probe for the Detection of Hydrogen Peroxide and Glucose.

Detection of hydrogen peroxide is of great significance for clinical diagnosis and biomedical research. Ratiometric detection represents an effective method that is generally based on horseradish peroxidase. In the present study, ruthenium ion-complexed carbon nitride (Ru-C3N4) nanosheets are found to serve as a peroxidase mimic and can catalyze the conversion of o-phenylenediamine to fluorescent 2,3-diaminophenazine in the presence of H2O2. The produced 2,3-diaminophenazine also results in the apparent quenching of the Ru-C3N4 photoluminescence due to the inner filter effect. These unique characteristics can be exploited for the construction of an effective, peroxidase-free ratiometric fluorescence framework for the detection of H2O2 and glucose, which has also been used in the successful detection of glucose in human serum. Results from this study not only demonstrate a new peroxidase mimic but also provide a novel ratiometric fluorescence platform for the detection of H2O2 and metabolites involving reactions of H2O2 generation in the absence of horseradish peroxidase.

Ultrasensitive immunoassay of Staphylococcus aureus based on colorimetric and fluorescent responses of 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole to l-cysteine.

We report here a sensitive method for immunoassay of Staphylococcus aureus (S. aureus) based on colorimetric and fluorescent responses of 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) to l-cysteine (Cys). S. aureus cells were separated from samples by immunomagnetic nanoparticles and labeled with Cys-encapsulated liposome (Cys@liposome) immuno-nanocapsules. After mangnetic separation, the Cys@liposome immuno-nanocapsules linked to bacterial cells were destructed by Tween-20, and then the released Cys molecules reacted with NBD-Cl to generate colorimetric and fluorescent signals. Due to the amplification effect of liposome nanocapsules, the high capture efficiency of immunomagnetic nanoparticles, and the high responses of NBD-Cl to Cys, the detection limits for colorimetric and fluorescent detection of S. aureus were as low as 10 and 1 CFU mL-1, respectively. The proposed method was successfully used to detect S. aureus in milk samples. This work has provided a novel NBD-Cl based colorimetric and fluorescent platform for immunoassay.

Improving Photovoltaic and Enzymatic Sensing Performance by Coupling a Core-Shell Au Nanorod@TiO2 Heterostructure with the Bioinspired l-DOPA Polymer.

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.

Double Perovskite LaFex Ni1-x O3 Nanorods Enable Efficient Oxygen Evolution Electrocatalysis.

Perovskite-based electrocatalysts are one of the most promising materials for oxygen evolution reaction (OER), but their activity and durability are still far from desirable. Herein, we demonstrate that the double perovskite LaFex Ni1-x O3 (LFNO) nanorods (NRs) can be adopted as highly active and stable OER electrocatalysts. The optimized LFNO-II NRs with Ni/Fe ratio of 8:2 achieve a low overpotential of 302 mV at 10 mA cm-2 and a small Tafel slope of 50 mV dec-1 , outperforming those of the commercial Ir/C. The LFNO-II NRs also show high OER stability with slight current decrease after 20 h. The enhanced activity is explained by the improved surface area, tailored electronic structure as well as strong hybridization between O and Ni.