Enzyme-Initiated Quinone-Chitosan Conjugation Chemistry: Toward A General in Situ Strategy for High-Throughput Photoelectrochemical Enzymatic Bioanalysis

γ-Glutamyltranspeptidase (GGT) Sensitive Fluorescence Probes for Cancer Diagnosis; Brief Review

Millions of deaths occur each year due to the late diagnosis of abnormal cellular growth within the body. However, the devastating impact of this can be significantly reduced if cancer metastasis is detected early through the use of enzymatic biomarkers. Among several biomarkers, γ-glutamyltranspeptidase (GGT) stands out as a member of the aminopeptidase family. It is primarily found on the surface of cancer cells such as glioma, ovarian, lung, and prostate cancer, without being overexpressed in normal cells or tissues. Recent years have witnessed significant progress in the field of cancer monitoring and imaging. Fluorescence sensing techniques have been employed, utilizing organic small molecular probes with enzyme-specific recognition sites. These probes emit a fluorescent signal upon interacting with GGT, enabling the imaging, identification, and differentiation of normal and cancerous cells, tissues, and organs. This review article presents a concise overview of recent progress in fluorescent probes developed for the selective detection of GGT, focusing on their applications in cancer imaging. It highlights the observed alterations in the fluorescence and absorption spectra of the probes before and after interaction with GGT. Additionally, the study investigates the changes in the probe molecule's structure following enzyme treatment, evaluates the sensor's detection limit, and consolidated imaging studies conducted using confocal fluorescence analysis. This comprehensive survey is expected to contribute to the advancement of sensing techniques for biomarker detection and cancer imaging, providing valuable insights for refining methodologies and inspiring future developments in this field.

Thioredoxin Reductase-Mediated Reaction Evokes In Situ Surface Polarization Effect on BiOIO3: Toward a New Sensing Strategy for Cathodic Photoelectrochemistry

We have witnessed the fast progress of cathodic photoelectrochemistry over the past decades, though its signal transduction tactic still lacks diversity. Exploring new sensing strategies for cathodic photoelectrochemistry is extremely demanding yet hugely challenging. This article puts forward a unique idea to incorporate an enzymatic reaction-invoked surface polarization effect (SPE) on the surface of BiOIO3 to implement an innovative cathodic photoelectrochemical (PEC) bioanalysis. Specifically, the thioredoxin reductase (TrxR)-mediated reaction produced the polar glutathione (GSH), which spontaneously coordinated to the surface of BiOIO3 and induced SPE by forming a polarized electric field, resulting in improved electron (e-) and hole (h+) pair separation efficiency and an enhanced photocurrent output. Correlating this phenomenon with the detection of TrxR exhibited a high performance in terms of sensitivity and selectivity, achieving a linear range of 0.007-0.5 μM and a low detection limit of 2.0 nM (S/N = 3). This study brings refreshing inspiration for the cathodic PEC signal transduction tactic through enzyme-mediated in situ reaction to introduce SPE, which enriches the diversity of available signaling molecules. Moreover, this study unveils the potential of in situ generated SPE for extended and futuristic applications.

Polarity Conversion of the Ag2S/AgInS2 Heterojunction by Radical-Induced Positive Feedback Polydopamine Adhesion for Signal-Switchable Photoelectrochemical Biosensing

Efficient tuning of the polarity of photoactive nanomaterials is of great importance in improving the performance of photoelectrochemical (PEC) sensing platforms. Herein, polarity of the Ag2S/AgInS2 heterojunction is converted by radical-induced positive feedback polydopamine (PDA) adhesion, which is further employed to develop a signal-switchable PEC biosensor. In the nanocomposites, Ag2S and AgInS2 achieve electron-hole separation, exhibiting a strong anodic PEC response. Under the irradiation of light, the Ag2S/AgInS2 heterojunction is able to produce superoxide radical and hydroxyl radical intermediate species, leading to the polymerization of dopamine (DA) and the subsequent adhesion of PDA onto the Ag2S/AgInS2 heterojunction (Ag2S/AgInS2@PDA). By constructing a new electron-transfer pathway with PDA, the polarity of the Ag2S/AgInS2 heterojunction is converted, and the PEC response changes from anodic to cathodic photocurrents. In addition, since the photoreduction activity of PDA is stronger than that of the Ag2S/AgInS2 heterojunction, more superoxide radical can be produced by Ag2S/AgInS2@PDA once PDA is generated, thereby promoting the generation of PDA. Consequently, a positive feedback mechanism is established to enhance the polarity conversion of the Ag2S/AgInS2 heterojunction and amplify the responding to DA. As a result, the bioanalytical method is capable of sensitively quantifying DA in 10 orders of magnitude with an ultralow limit of detection. Moreover, the applicability of this biosensor in real samples is identified by measuring DA in fetal bovine serum and compared with a commercial ELISA method. Overall, this work offers an alternative perspective for adjusting photogenerated carriers of nanomaterials and designing high-performance PEC biosensors.

PB@PDA nanocomposites as nanolabels and signal reporters for separate-type cathodic photoelectrochemical immunosensors in the detection of carcinoembryonic antigens

Photoelectrochemical (PEC) immunoassays exhibiting high sensitivity and decent operability have considerable potential in areas such as cancer diagnostics. In particular, cathodic PEC configurations can prevent interference from reductive substances, which can occur in biological samples; however, challenges remain in terms of sensitivity and operability. In this study, separate-type PEC immunoassays were developed for carcinoembryonic antigen (CEA) by combining microplate-based immune recognition and off-on cathodic PEC detection. Polydopamine (PDA)-coated Prussian blue (PB) nanoparticles (PB@PDA NPs) were used as signal tags to label the detection antibody. The PB NPs and PDA captured on the microplates both disassembled under strongly alkaline conditions to generate redox-active electron acceptors. The disassembled products were quantitatively transferred to PEC detection cells and synergistically enhanced the PEC current with microstructured BiOI, which operated as a cathodic semiconductor electrode. As proof of principle, carcinoembryonic antigen (CEA) was applied to elucidate the potential application of PEC immunoassay in clinical diagnosis, and the obtained linear range of the sensor was 0.001-100 ng mL^-1 with the detection limit of 54.9 fg mL^-1 (S/N = 3). The proposed separate-type off-on PEC strategy showed high sensitivity and decent operability for CEA detection, indicating its potential for the identification of other tumor markers.

Dual Functional Conjugated Acetylenic Polymers: High-Efficacy Modulation for Organic Photoelectrochemical Transistors and Structural Evolution for Bioelectronic Detection

Conjugated acetylenic polymers (CAPs) have emerged as a unique class of metal-free semiconductors with tunable electrical and optical properties yet their full potential remains largely unexplored. Organic bioelectronics is envisioned to create more opportunities for innovative biomedical applications. Herein, we report a poly(1,4-diethynylbenzene) (pDEB)/NiO gated enhancement-mode poly(ethylene dioxythiophene)-poly(styrene sulfonate) organic photoelectrochemical transistor (OPECT) and its structural evolution toward bioelectronic detection. pDEB was synthesized via copper-mediated Glaser polycondensation of DEB monomers on the NiO/FTO substrate, and the as-synthesized pDEB/NiO/FTO can efficiently modulate the enhancement-mode device with a high current gain. Linking with a sandwich immunoassay, the labeled alkaline phosphatase can catalyze sodium thiophosphate to generate H₂S, which will react with the diacetylene group in pDEB through the Michael addition reaction, resulting in an altered molecular structure and thus the transistor response. Exemplified by HIgG as the model target, the developed biosensor achieves highly sensitive detection with a linear range of 70 fg mL^-1-10 ng mL^-1 and a low detection limit of 28.5 fg mL^-1. This work features the dual functional CAP-gated OPECT, providing not only a novel gating module but also a structurally new rationale for bioelectronic detection.

Trap remediation of CuBi2O4 nanopolyhedra via surface self-coordination by H2O2: an innovative signaling mode for cathodic photoelectrochemical bioassay

This work conveys a new philosophy of surface self-coordination mediated trap remediation for innovative cathodic photoelectrochemical (PEC) signal transduction. Initially, the surface trap states of CuBi₂O₄ nanopolyhedra resulting from dangling bonds can function as charge carrier recombination centers, which suppress the carrier separation efficiency and result in a low photocurrent output. Particularly, hydrogen peroxide (H₂O₂) spontaneously interacts with the uncoordinated Cu(II) on the surface of CuBi₂O₄, enabling efficient elimination of dangling bonds and remedy of trap states, thereby outputting intensified photocurrent readout. Exemplified by Flap endonuclease 1 (FEN1) as a model target, a tetrahedron DNA (THD)-based strand displacement amplification (SDA) was introduced to manipulate the formation of hemin impregnated G-quadruplex (G-quadruplex/hemin) DNAzyme and the resultant catalytic reduction for H₂O₂. In addition, a highly efficient and ultra-sensitive PEC sensing platform was achieved for FEN1 detection with a wide linear range from 1.0 fM to 100.0 pM and a detection limit of 0.3 fM (S/N = 3). This work not only establishes a new idea of cathodic PEC signal transduction, but also offers an efficient biosensing platform for FEN1.

Integrating Ti3C2/MgIn2S4 heterojunction with a controlled release strategy for split-type photoelectrochemical sensing of miRNA-21

The harsh operating conditions and time-consuming fabrication process of the photoelectrode modification process have limited the potential applications of photoelectrochemical (PEC) sensors. To overcome these drawbacks, this study introduced a unique split-type PEC biosensor for microRNA-21 (miRNA-21) detection. Specifically, a Ti₃C₂/MgIn₂S₄ heterojunction was adopted as the photosensitive material, and a target-controlled glucose release system, comprising a multifunctional porphyrin-based metal-organic framework (PCN-224), was used for signal amplification. The Ti₃C₂/MgIn₂S₄ heterojunction effectively separated the photogenerated electrons and holes, and improved the photoelectric conversion efficiency, offering a strong initial photocurrent signal during PEC biosensing. Meanwhile, the porous PCN-224 acted as a nimble nanocontainer that encapsulated glucose using a capture probe (CP). In the presence of miRNA-21, the CP formed a CP-miRNA-21 complex and then detached from PCN-224, controllably releasing the trapped glucose. The oxidization of glucose by glucose oxidase resulted in hydrogen peroxide generation, which acted as a scavenger for the holes generated on the surface of Ti₃C₂/MgIn₂S₄, and significantly enhanced the photocurrent response under visible light irradiation. Finally, the sensor exhibited good performance for miRNA-21 detection with a low detection limit (0.17 fM) and wide linearity range (0.5 fM-1.0 nM). Thus, the proposed Ti₃C₂/MgIn₂S₄-based split-type PEC sensor is a promising tool for sensitive and accurate detection of miRNA-21 and provides an innovative basis for the preparation of other high-performance sensors.

Photoelectrochemical sandwich immunoassay of brain glycogen phosphorylase based on methyl orange-sensitized TiO2 nanorods

The photoelectrochemical immunoassay of glycogen phosphorylase BB (GPBB) was studied. A methyl orange/TiO₂ nanorod heterojunction was constructed on a fluorine-doped tin oxide electrode by hydrothermal synthesis, calcination, and chemical adsorption. A sandwich immune structure consisting of GPBB as the first antibody, GPBB, and a CdS@mesoporous silica-ascorbic acid (AA)-GPBB as secondary antibody composite was constructed on each of the selected well surfaces of a 96-well microplate. By adding mercaptoethylamine to structurally destroy the secondary antibody composite and release the electron donor AA, the amplification of photocurrent, and thus the "off-on" photoelectrochemical biosensing of GPBB were realized. The use of the 96-well microplate provides good reproducibility of the assembled immune structures and eliminates the possible effect of the photogenerated hole-induced protein oxidation on the photocurrent. The relevant electrodes and materials were characterized by electrochemistry, UV-vis diffuse reflectance spectra, Fourier transform infrared spectroscopy, X-ray diffractometer, scanning electron microscopy/energy dispersive spectroscopy, transmission electron microscopy and BET method. Under the optimal conditions, the photocurrent was linear with the logarithm of GPBB concentration from 0.005 to 200 ng mL^-1 and with a limit of detection of 1.7 pg mL^-1 (S/N = 3). Satisfactory results were obtained in the analysis of real serum samples. A sandwich immune structure consisting of GPBB first antibody, GPBB, and a CdS@mesoporous silica-ascorbic acid (AA)-GPBB secondary antibody composite was constructed on each of the selected well surfaces of a 96-well microplate. By adding mercaptoethylamine to structurally destroy the secondary antibody composite and release the electron donor AA, the amplification of photocurrent, and thus the "off-on" photoelectrochemical biosensing of GPBB were realized.

Surface polarization of BiOI to boost the photoelectrochemical signal transduction for high performance bioassays

The surface hydroxylation induced polarization (SHIP) is disclosed as an effective tactic to promote the cathodic photoelectrochemical (PEC) communication of bismuth oxyiodide with doxorubicin (Dox) by as large as three...

Intercalated doxorubicin acting as stimulator of PbS photocathode for probing DNA-protein interactions

Label-free and turn-on DNA-binding protein detection based on the doxorubicin (Dox)-intercalated DNA as a signal stimulator in cathodic photoelectrochemistry is reported. The double-stranded DNA (dsDNA) acted as the matrix accommodating the intercalative Dox and allowed its effective photoelectrochemical (PEC) communication with the PbS quantum dots (QDs) for realizing cathodic photocurrent readout. In the presence of the target of the vascular endothelial growth factor (VEGF), the dsDNA was prevented from being digested by the exonuclease III (Exo III), allowing the anchor of Dox to perform as activation stimuli of the photocurrent. The VEGF can be detected in the linear range from 1.5 pM to 100 nM, with an impressively low detection limit of 0.49 pM. This study hints the prospect of DNA intercalated architectures as innovative signaling transduction elements for wide and versatile cathodic PEC bioassays. Effective signaling molecules that are conducive to probe-related cathodic PEC bioassays using DNA as the recognition or signification elements are scarce but very demanding. Herein, the doxorubicin intercalated in duplex DNA functions as an efficient signal stimulator of PbS-consisted photocathode, and thus hints the versatility of the strategy for various targets through cathodic photoelectrochemistry.

An electron donor-acceptor organic photoactive composite with Schottky heterojunction induced photoelectrochemical immunoassay

A signal enhancement photoelectrochemical (PEC) immunoassay system induced by the composite (PTCs@Au) of electron donor-acceptor with Schottky heterojunction was designed. Carcinoembryonic antigen (CEA) was selected as a model target. Initially, the capture anibody (Ab1) was linked to gold nanoparticles electrodeposited on glassy carbon electrode and sealed by bovine serum albumin. Meanwhile, the organic semiconductor (PTCs) with the structure of electron donor-acceptor was synthetized from perylene tetracarboxylic dianhydride (acceptor) and dopamine (donor) via amidation reaction. Then PTCs@Au composite with Schottky heterojunction was formed through gold nanoparticles in situ reduction and functionalization with PTCs. Next, the detection antibody was labeled by PTCs@Au composite (Ab2-PTCs@Au) as an immuno-probe. The PTCs@Au was introduced via sandwich immune reaction leading to enhancement PEC signal without additional electron donor nor acceptor for achieving quantitative detection of CEA under external light. The proposed immunoelectrode showed dynamic ranges of 0.5 fg mL^-1 to 10 pg mL^-1 and 10 pg mL^-1 to 1 μg mL^-1 with the detection limit of 0.17 fg mL^-1. In addition, this PEC strategy with acceptable selectivity and stability can be potentially applied to detect other targets by choosing appropriate target recognition unit.

A new photoelectrochemical biosensor based on FeOOH and exonuclease III-aided dual recycling signal amplification for HPV-16 detection

Herein, based on iron oxyhydroxide (FeOOH) as the photoactive material and exonuclease III (exo III)-aided dual recycling signal amplification, a new photoelectrochemical (PEC) biosensor was successfully developed for human papillomavirus-16 (HPV-16) detection with a wide linear range from 0.5 fM to 1 nM and a low detection limit of 0.17 fM.

3D NiO nanoflakes/carbon fiber meshwork: Facile preparation and utilization as general platform for photocathodic bioanalysis

Herein, we describe a customized approach for facile preparation of three-dimensional (3D) NiO nanoflakes (NFs)/carbon fiber meshwork (CFM) and its validation as a common photocathode matrix for photoelectrochemical (PEC) bioanalysis, which to our knowledge has not been reported. Specifically, 3D NiO NFs/CFM was fabricated by a sequential liquid phase deposition and annealing process, which was then characterized by scanning electron microscopy, X-ray photoelectron spectrum, UV-vis absorption spectra and N₂ adsorption-desorption measurement. Sensitized by BiOI and incorporated with an alkaline phosphatase (ALP)/tyrosinase (TYR) bi-enzyme cascade system, a sensitive split-type cathodic PEC bioanalysis for the determination of ALP was achieved. This method can detect ALP concentrations down to 3 × 10^-5 U L^-1 with a linear response range of 0.001-10 U L^-1. Moreover, this proposed system exhibited good selectivity, stability and excellent performance for real sample analysis. This research features the facile preparation of 3D NiO NFs/CFM that could acts as a universal matrix for photocathodic analysis, and is envisioned to stimulate more effort for advanced 3D photocathode for PEC bioanalysis and beyond.

Integration of fluorescent polydopamine nanoparticles on protamine for simple and sensitive trypsin assay

As an important protease, trypsin (TRY) has been identified as a key indicator of various diseases. A simple and sensitive strategy for TRY detection by using an environment-friendly biosafe probe is significant. Herein, we introduced negatively charged fluorescent polydopamine nanoparticles (PDNPs) with 4.8 nm diameter obtained through a controllable method as an effective probe for TRY. PDNPs exhibited excellent fluorescence property but integrated with protamine (Pro) to form an aggregation-caused quenching system via a static quenching mechanism. The quenching mechanism of Pro to PDNPs revealed the significant effect of the surface charge, functional groups, and appropriate size of PDNPs on quenching process. Given the specific hydrolysis of Pro by TRY, PDNPs were released from the quenching integration of PDNPs and Pro (PDNPs-Pro) and recovered their fluorescence. Thus, a fluorescence sensor for TRY with a linear range of 0.01 and 0.1 μg/mL and a detection limit of 6.7 ng/mL was developed without the disturbing from other proteases. Compared with other TRY assays, the biosensor based on PDNPs-Pro has the advantages of simple operation, environmental friendliness, and high sensitivity. This specific controlled-synthesis PDNPs would open up a new window for the extended application of fluorescent nanomaterials in biomedicine based on fluorescence changes induced by biological interaction.

An enhanced photoelectrochemical sensor for aflatoxin B1 detection based on organic-inorganic heterojunction nanomaterial: poly(5-formylindole)/NiO

A new strategy for enhancing the photoelectric activity of poly(5-formylindole) (P5FIn) was developed by introducing the inorganic semiconductor material (NiO) to form organic-inorganic heterojunctions. P5FIn/NiO heterojunctions were firstly prepared by combining hydrothermal synthesis and electrochemical polymerization. Due to the synergistic effect between P5FIn and NiO, the photoelectrochemical (PEC) performance of this heterojunction was significantly enhanced compared to pure P5FIn and NiO. The reason for the enhanced PEC performance is mainly attributed to the increased visible light utilization and the bandgap matching effect of the P5FIn/NiO heterojunctions. Based on the prepared P5FIn/NiO heterojunctions, a novel PEC sensor for aflatoxin B1 (AFB1) detection was also constructed with a wide linear range of 0.005-50 ng mL^-1 and a limit of detection (LOD) of 0.0015 ng mL^-1. Moreover, this constructed PEC sensor also had good stability, reproducibility, selectivity, and satisfactory actual sample detection ability. This strategy may inspire more design and application of high-performance photoelectric active material based on inorganic semiconductor and organic conducting polymer heterojunctions. Graphical abstract.

Multimode detection of β-glycosidase and pathogenic bacteria via cation exchange assisted signal amplification

A rapid strategy for the β-glycosidase (β-Gal) and Escherichia coli (E. coli) sensing is presented, which is based on selective recognition reactions of QDs using visualization/fluorescence (FL)/atomic fluorescence spectrometry (AFS)/inductively coupled plasma mass spectrometry (ICP-MS) multimode assay. CdTe QDs can selectively recognize Ag⁺ and Ag NPs with a cation exchange reaction (CER) where Ag⁺ triggers the release of Cd²⁺ and quenches the fluorescence signal of QDs. Taking advantage of the fact that β-Gal can hydrolyze 4-Aminophenyl β-D-galactopyranoside (PAPG) to produce p-aminophenol (PAP), which has the ability to reduce Ag⁺ to form Ag NPs. The β-Gal can be easily detected by visualization or FL in a turn-on manner. Furthermore, combining with the selective separation of Cd²⁺ by filter membrane, AFS and ICP-MS with higher sensitivity were used for the determination of the enzyme. Under optimized conditions, the system limits of detections (LODs) were 0.01 U/L, 0.03 mU/L, and 0.02 mU/L using FL, AFS, and ICP-MS as the detector, respectively. The relative standard deviations (RSDs, n = 7) for 0.1 U/L β-Gal were 2.2, 2.0, and 1.3% using FL/AFS/ICP-MS as the detector, respectively. And 0.1 U/L of β-Gal can be discriminated from the blank solution with the naked eye. In addition, given that the β-Gal can serve as an indicator of E. coli, we have successfully applied this strategy for the detection of E. coli with a LOD of 25 CFU/mL. Application of the method was demonstrated by analyzing human urine samples and milk samples for ultra-trace detection of E. coli. Graphical abstract The CVG-AFS/ICP-MS/visual/FL multimode β-Gal and E.coli detection via CER.

Plasmonic Enhanced Gold Nanoclusters-Based Photoelectrochemical Biosensor for Sensitive Alkaline Phosphatase Activity Analysis

Low-toxicity gold nanoclusters-decorated Ag@SiO₂ (Au NCs-Ag@SiO₂) nanocomposites modified plasmonic photoelectrodes were first fabricated to improve the photoelectric properties of Au NCs and practical application in biological detection. Through adjusting distance between Au NCs and plasmonic silver nanoparticles (Ag NPs), the photocurrent intensity of Au NCs enhanced by 3.8 times attributed to strong competition between enhancement functions of hot electron transfer, local electric field, light scattering effects, and quenching functions of nonradiative energy transfer. Further comparison between experimental results and theoretical simulations were conducted to gain a deeper understanding toward the photoelectric enhancement mechanism. Moreover, Au NCs-Ag@SiO₂ nanocomposites was successfully applied to the construction of photoelectrochemical (PEC) biosensors for sensitively detecting alkaline phosphatase activity. This proposed PEC biosensor showed a wide linear range from 0.04 to 400 U·L^-1, and a low detection limit of 0.022 U·L^-1.

Tyrosinase-mediated dopamine polymerization modified magnetic alginate beads for dual-enzymes encapsulation: Preparation, performance and application

In this study, a simple and efficient method to obtain entrapment of mixtures of double enzymes is developed. As a proof of principle, double enzymes (tyrosinase (TYR) and β-glucosidase (β-Glu)) were co-immobilized in magnetic alginate-polydopamine (PDA) beads using in situ TYR-mediated dopamine polymerization and internal setting strategy-mediated magnetic alginate-PDA gelation. The leakage of enzymes from the magnetic alginate beads was significantly reduced by exploiting the double network cross-linking of alginate and PDA, which was induced by the d-(+)-Gluconic acid δ-lactone (GDL) and TYR, respectively. The physicochemical properties of the prepared magnetic alginate beads were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. After that, the enzymatic reaction conditions and the performance of the entrapped TYR and β-Glu, such as enzyme kinetics and inhibition kinetics, were investigated. The Michaelis-Menten constants (K_m) of the entrapped TYR and β-Glu were determined as 2.72 and 3.45 mM, respectively. The half-maximal inhibitory concentrations (IC₅₀) of kojic acid and castanospermine for the entrapped TYR and β-Glu were determined as 13.04 and 56.23 μM, respectively. Finally, the entrapped double enzymes magnetic alginate beads were successfully applied to evaluate the inhibitory potency of six kinds of tea polyphenols extracts. Black tea and white tea showed high inhibition activity against TYR were (36.14 ± 1.43)% and (36.76 ± 2.35)%, respectively, while the black tea and dark tea showed high inhibition activity against β-Glu were (37.89 ± 6.70)% and (21.28 ± 4.68)%, respectively.

Highly sensitive photoelectrochemical detection of bleomycin based on Au/WS2 nanorod array as signal matrix and Ag/ZnMOF nanozyme as multifunctional amplifier

An ultrasensitive photoelectrochemical (PEC) biosensor was constructed based on gold nanoparticles (Au NPs)/tungsten sulfide nanorod array (WS₂ NA) photoelectrode as the PEC matrix and silver nanoparticles/flake-like zinc metal-organic framework (Ag/ZnMOF) nanozyme with the peroxidase mimetic enzyme property for sensitive detection of bleomycin (BLM). In particular, Au/WS₂ and Ag/ZnMOF were linked by thiolate DNA₁ and DNA₂ strand, respectively, and the Au/WS₂-Ag/ZnMOF probe was prepared via hybridization reaction between the two DNAs. The introduction of Ag/ZnMOF in the probe offers two functions: i) the steric hindrance effect can effectively impede electron transport and reduce the photocurrent; ii) Ag/ZnMOF nanozyme can also be used as mimic peroxidase to effectively catalyze 3,3-diaminobenzidine (DAB) to produce the relevant precipitation, which will further reduce photocurrent and eliminate false positive signals. When BLM exists, BLM with Fe²⁺ as irreversible cofactor can specifically recognize and cleave of the 5'-GC-3' active site of DNA₂, resulting in reduced precipitation deposited on the electrode and recovery of PEC signal. The highly sensitive PEC biosensor exhibits a the linear strategy from 0.5 nM to 500 nM with a detection limit down to 0.18 nM. Further, the unique strategy was conducted in biological samples for BLM detection with satisfactory consequence, offering available and efficient pathway for disease diagnosis.

Immobilization-free, split-mode cathodic photoelectrochemical strategy combined with cascaded amplification for versatile biosensing

We propose herein an immobilization-free, split-mode cathodic photoelectrochemical (PEC) strategy coupled with a cascaded amplification for versatile biosensing. Taking DNA and microRNA (miRNA) as the model targets, the hybridization between the targets and the hairpin probe triggers the digestion of the probe DNA by T7 exonuclease (T7 Exo), thus to generate G-quadruplex (G4) forming sequences, and then the released targets (DNA or miRNA) initiate the subsequent cycling processes and generate a large amount of G4 forming sequences. Subsequently, the formed G4 sequences associate with hemin to form the G4/hemin DNAzyme, which catalytically produces 1,4-bezoquinone (BQ) for conjugating onto the surface of the chitosan (CS) deposited BiOI/ITO photocathode via the quinone-chitosan conjugation chemistry (QCCC). Under photo excitation, the covalently attached quinones can act as electron acceptors of bismuth oxyiodine (BiOI), promoting the photocurrent generation and thus allowing the elegant and "signal-on" mode for probing targets of interest. Highly sensitive and selective PEC bioassays are readily realized, with the detection limits down to 2.2 fM (for DNA) and 0.2 fM (for miRNA). Since no labeling and no electrode modification processes are needed, this split-mode PEC biosensing strategy is amenable to convenient, time/labor saving, and high-throughput detections. More significantly, it provides a novel concept to design immobilization-free and label-free cathodic PEC biosensing systems, and showcases promise in general and versatile bioanalysis research.

In situ chemical redox and functionalization of graphene oxide: toward new cathodic photoelectrochemical bioanalysis

This report outlines the first exploration of graphene oxide (GO) itself as a light harvesting material with an innovative in situ chemical redox and functionalization (CRF) strategy for versatile and high-throughput cathodic photoelectrochemical (PEC) bioanalysis.

Fluorescence sensing of tyrosinase activity based on amine rich carbon dots through direct interaction in a homogeneous system: detection mechanism and application

As a vital, copper-containing oxidase, tyrosinase (TYR) is useful as a biomarker for the screening of skin diseases. In this paper, a convenient and sensitive homogeneous fluorescence detection platform for the assay of TYR activity without any modified steps is described. Inspired by the fact that carbon dots (CDs) with excellent properties can be obtained through some surface modification, amine rich carbon dots (N-CDs) using a nitrogen doping process were developed as the fluorescent probe for this assay. The effect and the response mechanism of the degree of nitrogen doping in relation to the response of different CDs to the sensing of TYR activity using dopamine (DA) as a substrate were investigated. The DA was oxidized to o-dopaquinone with the catalyzation of TYR and quenched the fluorescence of the N-CDs by direct interaction. By using a set concentration of DA and other optimized reaction conditions, the fluorescence intensity of the N-CDs was directly applied to monitor the TYR activity. This assay for TYR activity showed a broad linear range from 0.05 to 6.0 U mL-1 with a detection limit of 0.039 U mL-1. The satisfactory recovery of the sensor for TYR activity in diluted human serum illustrated a potential clinical application.

A new photoelectrochemical immunosensor for ultrasensitive assay of prion protein based on hemin-induced photocurrent direction switching

As a significant biomarker of prion diseases, ultrasensitive assay of infectious isoform of prion (PrP^Sc) is highly desirable for early diagnostics of prion diseases. Herein, taking normal cellular form of prion (PrP^C) as a model owing to a high risk of pathogenicity of PrP^Sc, a new photoelectrochemical immunosensor has been developed based on hemin-induced switching of photocurrent direction. In the presence of PrP^C, nitrogen-doped porous carbon-hemin polyhedra labeled with secondary antibody were introduced onto the CdS-chitosan (CS) nanoparticles-modified indium-tin oxide (ITO) electrode via the antigen-antibody specific recognition. Because of the matched energy level between CdS and hemin, the high-efficiency switch of photocurrent direction of the ITO/CdS-CS photoelectrode from anodic to cathodic photocurrent was observed even at very low concentration (0.4 aM) of PrP^C. Through changing the specific antibody, this method can be easily expanded to PrP^Sc assay. Such low detectable limit is very useful in the early diagnosis and screening of prion diseases. The developed method has also promising applications in bioanalysis, disease diagnostics, and clinical biomedicine.

An enzyme cascade-based electrochemical immunoassay using a polydopamine-carbon nanotube nanocomposite for signal amplification

By coupling tyrosinase (Tyr) and β-galactosidase (Gal) into one redox-cycling scheme, an enzyme cascade-based electrochemical immunosensor with boosted selectivity and sensitivity was constructed using polydopamine-functionalized multiwalled carbon nanotube (MWCNTs-PDA) nanohybrid modified electrodes. The MWCNTs-PDA nanohybrid presented a 5 times enhanced capability for antibody conjugation, which was responsible for signal amplification. In the proposed enzyme cascade scheme, Gal was captured on the immunosensor surface by a sandwiched immunoreaction, which catalyzed phenyl β-d-galactopyranoside (P-GP) into phenol based on a hydrolysis reaction. The resulting phenol was used as a substrate of Tyr, which was catalyzed to catechol and subsequently to o-quinone. The o-quinone was then electrochemically reduced to catechol, forming a redox cycle between catechol and o-quinone. The enzyme cascade-based immunoassay not only significantly amplified the electrochemical signal, but also led to a high selectivity. Taking the detection of CEA as an example, the enzyme cascade-based electrochemical immunosensor showed a detectable range of 10 pg mL^-1 to 10 ng mL^-1 and a low detection limit of 8.39 pg mL^-1 (S/N = 3), which was superior/comparable to those using other methodologies in previous reports. The selectivity of the enzyme cascade-based immunosensor was 44-80% higher than that of a single enzyme-based immunosensor. This work shows great potential of the coupling enzyme cascade in immunosensing for clinical diagnosis with boosted selectivity and sensitivity.

Photogenerated Hole-Induced Chemical Redox Cycling on Bi2S3/Bi2Sn2O7 Heterojunction: Toward General Amplified Split-Type Photoelectrochemical Immunoassay

This work reports the elegant bridging of enzymatic generation of electron donor with photogenerated hole-induced chemical redox cycling amplification (RCA) for innovative photoelectrochemical (PEC) immunoassay, by the aid of a heterojunction photoelectrode with split-type strategy. Specifically, the system was exemplified by the alkaline phosphatase (ALP) catalytic generation of ascorbic acid (AA), the redox cycling of AA by tris (2-carboxyethyl) phosphine (TCEP) as reductant, and the use of a novel Bi₂S₃/Bi₂Sn₂O₇ heterojunction and myoglobin (Myo) as the photoelectrode and the target, respectively. After the immunoreaction and ALP-induced production of AA, the subsequent oxidation of AA by the photogenerated holes of the Bi₂S₃/Bi₂Sn₂O₇ heterojunction could be cycled via the regeneration of AA by TCEP from the oxidized product of dehydroascorbic acid, leading to easy signal amplification for the sensitive immunoassay of Myo in real samples. It is believed that this work provided a basis for further design and development of general RCA-based PEC immunoassays.