Alkaline Phosphatase Tagged Antibodies on Gold Nanoparticles/TiO2 Nanotubes Electrode: A Plasmonic Strategy for Label-Free and Amplified Photoelectrochemical Immunoassay

Progress in Electrochemical Immunosensors with Alkaline Phosphatase as the Signal Label

Electrochemical immunosensors have shown great potential in clinical diagnosis, food safety, environmental protection, and other fields. The feasible and innovative combination of enzyme catalysis and other signal-amplified elements has yielded exciting progress in the development of electrochemical immunosensors. Alkaline phosphatase (ALP) is one of the most popularly used enzyme reporters in bioassays. It has been widely utilized to design electrochemical immunosensors owing to its significant advantages (e.g., high catalytic activity, high turnover number, and excellent substrate specificity). In this work, we summarized the achievements of electrochemical immunosensors with ALP as the signal reporter. We mainly focused on detection principles and signal amplification strategies and briefly discussed the challenges regarding how to further improve the performance of ALP-based immunoassays.

Label-free photoelectrochemical immunosensing of α-fetoprotein based on Eu-TiO2 nanocomposites sensitized with dye-encapsulated HMA

In this study, a sensitive photoelectrochemical immunosensor with dye-enhanced anodic photocurrent response was proposed for sensitive detection of α-fetoprotein (AFP). Specifically, europium-doped TiO₂ (Eu-TiO₂) was used as the photoelectrochemical functional material and coated onto indium tin oxide (ITO) electrode. Doxorubicin (DOX) as an excellent fluorescent dye was encapsulated in the hydrophobically modified alginate (HMA). Then the dye-loaded HMA was modified onto the surface of Eu-TiO₂ to further sensitize the photocurrent response. The results showed that the photoelectrical signal was enhanced and stabilized due to the effect of sensitization of DOX on Eu-TiO₂ material. The constructed PEC sensor revealed a good linear response to AFP antigen ranging from 0.5 to 100 ng/mL with a detection limit of 0.41 pg/mL. The clinical patient's serum test results obtained from the proposed PEC immunosensor were consistent with those obtained from the commercial electrochemilunescence assay. The proposed PEC sensing method could be a promising analytical tool for the detection of AFP in clinical analysis.

Advances in Electrochemical Biosensors Based on Nanomaterials for Protein Biomarker Detection in Saliva

The focus on precise medicine enhances the need for timely diagnosis and frequent monitoring of chronic diseases. Moreover, the recent pandemic of severe acute respiratory syndrome coronavirus 2 poses a great demand for rapid detection and surveillance of viral infections. The detection of protein biomarkers and antigens in the saliva allows rapid identification of diseases or disease changes in scenarios where and when the test response at the point of care is mandated. While traditional methods of protein testing fail to provide the desired fast results, electrochemical biosensors based on nanomaterials hold perfect characteristics for the detection of biomarkers in point-of-care settings. The recent advances in electrochemical sensors for salivary protein detection are critically reviewed in this work, with emphasis on the role of nanomaterials to boost the biosensor analytical performance and increase the reliability of the test in human saliva samples. Furthermore, this work identifies the critical factors for further modernization of the nanomaterial-based electrochemical sensors, envisaging the development and implementation of next-generation sample-in-answer-out systems.

Recent Advances of Nanostructured Materials for Photoelectrochemical Bioanalysis

Nowadays, the emerging photoelectrochemical (PEC) bioanalysis has drawn intensive interest due to its numerous merits. As one of its core elements, functional nanostructured materials play a crucial role during the construction of PEC biosensors, which can not only be employed as transducers but also act as signal probes. Although both chemical composition and morphology control of nanostructured materials contribute to the excellent analytical performance of PEC bioassay, surveys addressing nanostructures with different dimensionality have rarely been reported. In this review, according to classification based on dimensionality, zero-dimensional, one-dimensional, two-dimensional, and three-dimensional nanostructures used in PEC bioanalysis are evaluated, with an emphasis on the effect of morphology on the detection performances. Furthermore, using the illustration of recent works, related novel PEC biosensing patterns with promising applications are also discussed. Finally, the current challenges and some future perspectives in this field are addressed based on our opinions.

Bioinspired Synthesis of ZnO@polydopamine/Au for Label-Free Photoelectrochemical Immunoassay of Amyloid-β Protein

Amyloid-β protein (Aβ) is an important biomarker and plays a key role in the early stage of Alzheimer's disease (AD). Here, an ultrasensitive photoelectrochemical (PEC) sensor based on ZnO@polydopamine/Au nanocomposites was constructed for quantitative detection of Aβ. In this sensing system, the ZnO nanorod array decorated with PDA films and gold nanoparticles (Au NPs) have excellent visible-light activity. The PDA film was used as a sensitizer for charge separation, and it also was used for antibody binding. Moreover, Au NPs were loaded on the surface of PDA film by in situ deposition, which further improved the charge transfer efficiency and the PEC activity in visible light due to the localized surface plasmon resonance effect of Au NPs. Therefore, in ZnO@polydopamine/Au nanocomposites, a significantly enhanced photocurrent response was obtained on this photoelectrode, which provides a good and reliable signal for early detection of AD. Under the optimized conditions, the PEC immunosensor displayed a wide linear range from 1 pg/mL to 100 ng/mL and a low detection limit of 0.26 pg/mL. In addition, this PEC immunosensor also presented good selectivity, stability, and reproducibility. This work may provide a promising point-of-care testing method toward advanced PEC immunoassays for AD biomarkers.

Optical/electrochemical methods for detecting mitochondrial energy metabolism

This review highlights the biological importance of mitochondrial energy metabolism and the applications of multiple optical/electrochemical approaches to determine energy metabolites. Mitochondria, the main sites of oxidative phosphorylation and adenosine triphosphate (ATP) biosynthesis, provide the majority of energy required by aerobic cells for maintaining their physiological activity. They also participate in cell growth, differentiation, information transmission, and apoptosis. Multiple mitochondrial diseases, caused by internal or external factors, including oxidative stress, intense fluctuations of the ionic concentration, abnormal oxidative phosphorylation, changes in electron transport chain complex enzymes and mutations in mitochondrial DNA, can occur during mitochondrial energy metabolism. Therefore, developing accurate, sensitive, and specific methods for the in vivo and in vitro detection of mitochondrial energy metabolites is of great importance. In this review, we summarise the mitochondrial structure, functions, and crucial energy metabolic signalling pathways. The mechanism and applications of different optical/electrochemical methods are thoroughly reviewed. Finally, future research directions and challenges are proposed.

Alkaline Phosphatase-Triggered Etching of Au@FeOOH Nanoparticles for Enzyme Level Assay under Dark-Field Microscopy

In clinical diagnosis, the level of biological enzymes in serum has been generally regarded as markers of human diseases. In this work, a kind of simple and sensitive plasmonic probe (indicated as Au@FeOOH) has been synthesized with the guidance of plasmonic imaging and subsequently developed for the alkaline phosphatase (ALP) level detection under dark-field microscopy (DFM). As a kind of hydrolysis enzyme, ALP can promote the hydrolysis of l-ascorbic acid 2-phosphate to ascorbic acid (AA). AA further acts as a strong reduction reagent for the decomposition of the FeOOH shell, which results in a blue shift of localized surface plasmon resonance spectra and an obvious color change under DFM. RGB analyses show that using a ΔR/G value instead of scattering wavelength or R/G value as the analytical signal, the deviation attributed to the size distribution of the initial Au NPs is greatly suppressed, and a linear range from 0.2 to 6.0 U/L (R² = 0.99) and a limit of detection of 0.06 U/L are acquired with various concentrations of ALP during the detection. Besides, this approach exhibits excellent selectivity in complex biological serum samples, which is expected to be applied for the early diagnosis of clinical diseases by monitoring various biomarkers in the future.

Mesoporous TiO2-based architectures as promising sensing materials towards next-generation biosensing applications

In the past two decades, mesoporous TiO2 has emerged as a promising material for biosensing applications. In particular, mesoporous TiO2 materials with uniform, well-organized pores and high surface areas typically exhibit superior biosensing performance, which includes high sensitivity, broad linear response, low detection limit, good reproducibility, and high specificity. Therefore, the development of biosensors based on mesoporous TiO2 has significantly intensified in recent years. In this review, the expansion and advancement of mesoporous TiO2-based biosensors for glucose detection, hydrogen peroxide detection, alpha-fetoprotein detection, immobilization of enzymes, proteins, and bacteria, cholesterol detection, pancreatic cancer detection, detection of DNA damage, kanamycin detection, hypoxanthine detection, and dichlorvos detection are summarized. Finally, the future perspective and research outlook on the utilization of mesoporous TiO2-based biosensors for the practical diagnosis of diseases and detection of hazardous substances are also given.

Photoelectrochemical enzymatic sensor for glucose based on Au@C/TiO2 nanorod arrays

In this work, a photoelectrochemical (PEC) glucose biosensor was synthesized on the basis of a type of Au@C/TiO₂ composite by using an unsophisticated secondary hydrothermal strategy. The compounded Au@C/TiO₂ material was characterized by XRD, SEM, TEM, UV-vis, and XPS to identify the composition and the purity of the phase. Glucose oxidase (GODx) was immobilized on the Au@C/TiO₂. The PEC biosensor presented satisfactory stability and outstanding reproducibility. The range and the sensitivity of the linear measurement were 0.1–1.6 mM and 29.76 μA mM⁻¹ cm⁻², respectively, and the detection limit was low (0.049 mM). The GODx/Au@C/TiO₂/FTO biosensor presented excellent efficiency in detecting glucose, suggesting the great potential application of this synthesized material in PEC biosensors.

In this work, a photoelectrochemical (PEC) glucose biosensor was synthesized on the basis of a type of Au@C/TiO₂ composite by using an unsophisticated secondary hydrothermal strategy.

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.

Modification-free amperometric biosensor for the detection of wild-type p53 protein based on the in situ formation of silver nanoparticle networks for signal amplification

Sensitive and accurate quantification of wild-type p53 protein is of great importance for biological research and clinical diagnosis. Herein, a modification-free amperometric biosensor was proposed for sensitive detection of wild-type p53 protein by the signal amplification of silver nanoparticles (AgNPs) networks formed in situ on electrode surface. Double-stranded DNA (dsDNA) probe containing two consensus sites was immobilized on gold electrode surface to capture wild-type p53 protein. The cysteine thiol and amine groups on the exterior of the protein allowed for the attachment of bare AgNPs through the AgS or AgN interactions. Meanwhile, benzene-1,4-dithiol (BDT) molecules in solution triggered the assembly of more AgNPs on electrode surface through the AgS interactions, thus leading to the in situ formation of AgNPs networks for signal amplification. The target at the concentration as low as 0.1 pM can be readily determined. This method was further applied to determine wild-type p53 protein in spiked human serum and cell lysates with satisfactory results. Moreover, the biosensor is regenerative and does not require the modification of AgNPs with recognition element for signal readout. The modification-free strategy can potentially be applied to develop novel biosensors for detection of other biological macromolecules.

Boosting the biocatalytic precipitation with enzyme-loaded liposomes: Toward a general platform for amplified photoelectrochemical immunoassay

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.

Affinity-Based Detection of Biomolecules Using Photo-Electrochemical Readout

Detection and quantification of biologically-relevant analytes using handheld platforms are important for point-of-care diagnostics, real-time health monitoring, and treatment monitoring. Among the various signal transduction methods used in portable biosensors, photoelectrochemcial (PEC) readout has emerged as a promising approach due to its low limit-of-detection and high sensitivity. For this readout method to be applicable to analyzing native samples, performance requirements beyond sensitivity such as specificity, stability, and ease of operation are critical. These performance requirements are governed by the properties of the photoactive materials and signal transduction mechanisms that are used in PEC biosensing. In this review, we categorize PEC biosensors into five areas based on their signal transduction strategy: (a) introduction of photoactive species, (b) generation of electron/hole donors, (c) use of steric hinderance, (d) in situ induction of light, and (e) resonance energy transfer. We discuss the combination of strengths and weaknesses that these signal transduction systems and their material building blocks offer by reviewing the recent progress in this area. Developing the appropriate PEC biosensor starts with defining the application case followed by choosing the materials and signal transduction strategies that meet the application-based specifications.

Responsive methylene blue release from lanthanide coordination polymer for label-free, immobilization-free and sensitive electrochemical alkaline phosphatase activity assay

Alkaline phosphatase (ALP) is an important enzyme related to many clinical diseases and also widely used as a labeling enzyme for immunoassay. Herein, a new electrochemical sensing strategy for ALP activity was proposed, which was based on the ALP-triggered methylene blue (MB) release from a lanthanide coordination polymer and successive penetration through a self-assembled dodecanethiol monolayer for electrochemical response. The supramolecular lanthanide coordination polymer was constructed by using guanine monophosphate (GMP) and Tb³⁺ as the ligand and the metal ion, respectively, and the encapsulated MB as the signal molecule. ALP catalyzed the cleavage of the phosphate group from the GMP ligand and disrupted the coordination polymer network to release abundant MB molecules for electrochemical responses related to ALP activity. The obtained lanthanide coordination polymers were well characterized by various techniques. The fabricated electrochemical sensor for ALP activity assay shows distinct advantages such as being one-step, label-free, immobilization-free and highly sensitive. The detection limit toward ALP activity was down to 0.5 U L^-1. With the aid of a MB enrichment process on the modified electrode before measurement, the detection limit could be further improved to 0.1 U L^-1. Moreover, the assay method could be applied for ALP detection in complex matrixes such as human serum and also for efficient inhibitor evaluation. Thus, the current study provides a new pathway to the fabrication of a coordination polymer-based electrochemical sensing platform for applications in disease diagnosis and drug discovery.

Photoelectrochemical detection of breast cancer biomarker based on hexagonal carbon nitride tubes

Photoelectrochemical (PEC) sensor for sensitive detection of breast cancer biomarker human epidermal growth factor receptor 2 (HER2) utilizing hexagonal carbon nitride tubes (HCNT) as photoactive material is reported. The detection is based on suppression of the PEC current intensity of the sensor. HCNT were synthesized via a facile hydrothermal method with large specific surface area and low electron-hole recombination. Au nanoparticles (AuNPs) were deposited onto the surface of the HCNT, which enhanced the photocurrent intensity of the HCNT by one time. For HER2 detection, peptide specific to HER2 was immobilized on the AuNPs surface for capturing HER2 molecules. The following binding of HER2 with HER2 aptamer and the reaction of phosphate groups on aptamer with molybdate can form molybdophosphate precipitate, which sticks to the surface of HCNT and impedes electron transport. Thus, photocurrent intensity of the sensor was suppressed. Under optimal conditions, the linear relationship between the PEC intensity and the logarithm of HER2 concentration was from 0.5 to 1 ng mL^-1 with low limit of detection (LOD) of 0.08 pg mL^-1. Furthermore, the PEC sensor also displayed capability for detecting HER2 in human serum samples. This PEC sensor signal detection strategy can be easily adapted to other PEC sensors involving DNA and find wide applications. Graphical abstract.

Electrochemical modulation of plasmon-induced charge separation behaviour at Au-TiO2 photocathodes

Plasmon-induced charge separation (PICS) at the interface between a plasmonic nanoparticle and a semiconductor becomes less efficient as the plasmon resonance wavelength increases, because the energy of a photon may not be sufficiently higher than the interfacial Schottky barrier height. In this study, we developed PICS photocathodes by coating Au nanoparticles of different sizes on an ITO electrode with a thin TiO2 layer, and applied negative potentials to those photocathodes so as to suppress back electron transfer and improve the PICS photocurrent responses. The photocurrent enhancement factor was increased as the particle size was decreased, and enhancement of about two orders of magnitude was observed for small Au nanoparticles when bias voltage of 0.5 V was applied. In some cases the photocurrent enhancement was accompanied by a slight redshift of the photocurrent peak, which was caused by a lowered barrier. This technique would be useful for tuning the photocurrents when it is applied to devices such as electrochemical LSPR sensors and photodetectors.

Nanoporous Semiconductor Electrode Captures the Quantum Dots: Toward Ultrasensitive Signal-On Liposomal Photoelectrochemical Immunoassay

Liposomal photoelectrochemical (PEC) bioanalysis has recently emerged and exhibited great potential in sensitive biomolecular detection. Exploration of the facile and efficient route for advanced liposomal PEC bioanalysis is highly appealing. In this work, we report the split-type liposomal PEC immunoassay system consisting of sandwich immunorecognition, CdS quantum dots (QDs)-loaded liposomes (QDLL), and the release and subsequent capture of the QDs by a separated TiO₂ nanotubes (NTs) electrode. The system elegantly operated upon the protein binding and lysis treatment of CdS QDLL labels within the 96-well plate, and then the CdS QDs-enabled sensitization of TiO₂ NTs electrode. Exemplified by cardiac markers troponin I (cTnI) as target, the proposed system achieved efficient activation of TiO₂ NTs electrode and thus the signal generation toward the split-type PEC immunoassay. This work features the first use of QDs for liposomal PEC bioanalysis and is expected to inspire more interests in the design and implementation of numerous QDs-involved liposomal PEC bioanalysis.

A 3D printing-based portable photoelectrochemical sensing device using a digital multimeter

An enzyme-free photoelectrochemical sensing method based on a 3D-printing device was developed for CEA detection coupling glucose-encapsulated liposomes with digital multimeter readout.

Amplified photoelectrochemical immunoassay for the tumor marker carbohydrate antigen 724 based on dye sensitization of the semiconductor composite C3N4-MoS2

The authors describe an amplified photoelectrochemical immunoassay for the tumor marker carbohydrate antigen 724 (CA724). The method employs a C₃N₄-MoS₂ semiconductor as the photoelectric conversion layer. The nanocomposite was characterized by transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray powder diffraction, and UV-vis diffuse reflectometry. The dye eosin Y was encapsulated into CaCO₃ nanospheres which then were used as labels for antibody against CA724. In addition, Fe₃O₄ nanospheres were employed as magnetic platform for constructing photoelectrochemical sandwich immunoassay. The CaCO₃ nanospheres can be dissolved with aid of ethylene diamine tetraacetic acid (EDTA) and the carried eosin Y in CaCO₃ is released. The released dyes sensitizes the C₃N₄-MoS₂ semiconductor, which induces photocurrent amplification. Under optimal conditions and at a typical working voltage of 0 V (vs. SCE), the photocurrent increases linearly in the range of 0.05 mU mL^-1 to 500 mU mL^-1 of CA724, with a 0.02 mU mL^-1 detection limit. Graphical abstract The C₃N₄-MoS₂ complex, with high efficiency of electron transport, was synthesized to construct a photoelectrochemical analytical platform. A sandwich-type immunoassay was established on the surface of magnetic beads. Carbohydrate antigen 724 in sample was detected sensitively by using sensitization of released eosin Y as signal amplifiery.

3D Semiconducting Polymer/Graphene Networks: Toward Sensitive Photocathodic Enzymatic Bioanalysis

This work reports the development of three-dimensional (3D) semiconducting polymer/graphene (SP/G) networks toward sensitive photocathodic enzymatic bioanalysis. Specifically, the porous 3D graphene was first synthesized via the hydrothermal and freeze-dry processes and then mixed with semiconducting polymer to obtain the designed hierarchical structure with unique porosity and large surface area. Afterward, the as-prepared hybrid was immobilized onto the indium tin oxide (ITO) for further characterizations. Exemplified by sarcosine oxidase (SOx) as a model biocatalyst, an innovative 3D SP/G-based photocathodic bioanalysis capable of sensitive and specific sarcosine detection was achieved. The suppression of cathodic photocurrent was observed in the as-developed photocathodic enzymatic biosystem due to the competition of oxygen consumption between the enzyme-biocatalyst process and O₂-dependent photocathodic electrode. This work not only presented a unique protocol for 3D SP/G-based photocathodic enzymatic bioanalysis but also provided a new horizon for the design, development, and utilization of numerous 3D platforms in the broad field of general photoelectrochemical (PEC) bioanalysis.

A label-free aptamer-based cytosensor for specific cervical cancer HeLa cell recognition through a g-C3N4-AgI/ITO photoelectrode

Facile and efficient detection of cancer cells in the early phases of the disease is one of the main challenges in cancer diagnostics. It has been found that photoactive materials and bio-recognition elements are two key factors in the development of promising photoelectrochemical (PEC) biosensors for cancer cell detection, which can play significant roles for realizing early cancer diagnostics with high sensitivity and selectivity. In this study, we designed a novel label-free PEC aptamer-based cytosensor for the specific detection of cancer cells such as HeLa cells by using water-dispersible g-C₃N₄-AgI nanocomposites as visible light-sensitive materials and anti-CEM/PTK7 aptamer as the bio-recognition element. It was observed that when a suitable amount of AgI nanoparticles was doped in two-dimensional graphite-like carbon nitride nano-sheets (g-C₃N₄ NSs), the visible light photocurrent response could be significantly improved. The PEC response of the as-prepared biosensor based on the g-C₃N₄-AgI/ITO photoelectrode was linearly proportional to the relevant cancer cells such as HeLa cells at concentrations ranging from 10 to 10⁶ cells per mL with a limit of detection of 5 cells per mL. In addition, the g-C₃N₄-AgI/ITO photoelectrode and the fabricated cytosensor exhibited long-term stability, good reproducibility, excellent selectivity, and high sensitivity, demonstrating the successful conjugation of g-C₃N₄-AgI NSs with the aptamer and target cancer cells in the high performance PEC cytosensor.

DNA-fueled target recycling-induced two-leg DNA walker for amplified electrochemical detection of nucleic acid

Taking advantage of the homogeneous and heterogeneous electrochemical biosensors, a simple, sensitive, and selective electrochemical biosensor is constructed by combining entropy-driven amplification (EDA) with DNA walker. This electrochemical biosensor realizes the biorecognition and EDA operation in homogeneous solution, which is beneficial to improve the recognition and amplification efficiency. A two-leg DNA walker generated by EDA can walk on the surface of gold electrode for cleaving the immobilized substrate DNA and releasing the electroactive labels, giving rise to a significant decrease of the electrochemical signal. The immobilization of the electroactive labels ensures the reproducibility and reliability of the biosensor. The present cascade amplification assay can be applied to detect target DNA with a detection limit of 0.29 fM, and base mutations can be easily distinguished. Moreover, the proposed electrochemical biosensor shows a satisfactory performance for the detection of target DNA in human serum. Thus, the novel electrochemical biosensor holds promising potential for a future application in disease diagnosis.

Catechol-chitosan redox capacitor for added amplification in electrochemical immunoanalysis

Antibodies are common recognition elements for molecular detection but often the signals generated by their stoichiometric binding must be amplified to enhance sensitivity. Here, we report that an electrode coated with a catechol-chitosan redox capacitor can amplify the electrochemical signal generated from an alkaline phosphatase (AP) linked immunoassay. Specifically, the AP product p-aminophenol (PAP) undergoes redox-cycling in the redox capacitor to generate amplified oxidation currents. We estimate an 8-fold amplification associated with this redox-cycling in the capacitor (compared to detection by a bare electrode). Importantly, this capacitor-based amplification is generic and can be coupled to existing amplification approaches based on enzyme-linked catalysis or magnetic nanoparticle-based collection/concentration. Thus, the capacitor should enhance sensitivities in conventional immunoassays and also provide chemical to electrical signal transduction for emerging applications in molecular communication.

Bismuth Oxyiodide Couples with Glucose Oxidase: A Special Synergized Dual-Catalysis Mechanism for Photoelectrochemical Enzymatic Bioanalysis

On the basis of a special synergized dual-catalysis mechanism, this work reports the preparation of a BiOI-based heterojunction and its use for cathodic photoelectrochemical (PEC) oxidase biosensing, which, unexpectedly, revealed that hydrogen peroxide (H₂O₂) had a greater impact than dioxygen (O₂). Specifically, the BiOI layer was in situ formed on the substrate through an impregnating hydroxylation method for the following coupling with the model enzyme of glucose oxidases (GOx). The constructed cathodic PEC enzyme sensor exhibited a good analytical performance of rapid response, high stability, and good selectivity. Especially, glucose-induced H₂O₂-controlled enhancement of the photocurrent was recorded rather than the commonly observed O₂-dependent suppression of the signal. This interesting phenomenon was attributed to a special synergized dual-catalysis mechanism. Briefly, this study is expected to provide a new BiOI-based photocathode for general PEC bioanalysis development and to inspire more interest in the design and construction of a novel heterojunction for advanced photocathodic bioanalysis. More importantly, the mechanism revealed here would offer a totally different perspective for the use of a biomimetic catalyst in the design of future PEC enzymatic sensing and the understanding of relevant signaling routes as well as the implementation of innovative PEC devices.

Separating photoanode from recognition events: toward a general strategy for a self-powered photoelectrochemical immunoassay with both high sensitivity and anti-interference capabilities

A general, efficient strategy for a self-powered PEC immunoassay, with both high sensitivity and anti-interference properties, by separating the photoanode from recognition events.

A ratiometric photoelectrochemical immunosensor based on g-C3N4@TiO2 NTs amplified by signal antibodies–Co3O4 nanoparticle conjugates

Herein, we report a ratiometric photoelectrochemical (PEC) immunosensor coupled with secondary antibodies–Co₃O₄ nanoparticle conjugates (Ab₂–Co₃O₄ NPs) for signal amplification.

Biofunctionalized mesoporous silica nanospheres for the ultrasensitive chemiluminescence immunoassay of tumor markers

Mesoporous silica nanospheres (SiO₂) are synthesized and biofunctionalized for the development of an ultrasensitive chemiluminescent (CL) immunosensor for tumor markers.

Gold Nanoparticles Decorated Hematite Photoelectrode for Sensitive and Selective Photoelectrochemical Aptasensing of Lysozyme

Photoelectrochemical aptasensor (PECAS) is a new and promising detection platform with both high sensitivity and good selectivity. Exploration of new photoelectrode materials and establishment of effective charge transfer channel between photoelectrode and aptamer are the main challenges in this field. In this work, an efficient PECAS based on Au nanoparticles (NPs) decorated Fe₂O₃ nanorod photoelectrode is rationally designed, fabricated, and exhibited excellent sensitivity and selectivity for detection of lysozyme (Lys) with an ultralow detection limit of 3 pM and wide detection range from 10 pM to 100 nM. The Au NPs not only act as anchor to establish an efficient charge transfer channel between the photoelectrode and the aptamer, but also help to enhance the PEC performance through adjusting the carrier density of Fe₂O₃. The rationally designed photoelectrode opens up a distinctive avenue for promoting the PECAS to be a versatile analysis method.

Near-Infrared-to-Ultraviolet Light-Mediated Photoelectrochemical Aptasensing Platform for Cancer Biomarker Based on Core-Shell NaYF4:Yb,Tm@TiO2 Upconversion Microrods

Titanium dioxide (TiO₂; as a potential photosensitizer) has good photocurrent performance and chemical stability but often exhibits low utilization efficiency under ultraviolet (UV) region excitation. Herein, we devised a near-infrared light-to-UV light-mediated photoelectrochemical (PEC) aptasensing platform for the sensitive detection of carcinoembryonic antigen (CEA) based on core-shell NaYF₄:Yb,Tm@TiO₂ upconversion microrods by coupling with target-triggered rolling circle amplification (RCA). The upconversion microrods synthesized through the hydrothermal reaction could act as a photosensing platform to convert the near-infrared (near-IR) excitation into UV emission for generation of photoinduced electrons. The target analyte was determined on a functional magnetic bead by using the corresponding aptamers with a sandwich-type assay format. Upon target CEA introduction, a complex was first formed between capture aptamer-1-conjugated magnetic bead (Apt1-MB) and aptamer-2-primer DNA (Apt2-pDNA). Thereafter, the carried primer DNA by the aptamer-2 paired with linear padlock DNA to trigger the RCA reaction. The guanine (G)-rich product by RCA reaction was cleaved by exonuclease I and exonuclease III (Exos I/III), thereby resulting in the formation of numerous individual guanine bases to enhance the photocurrent of core-shell NaYF₄:Yb,Tm@TiO₂ upconversion microrods under near-IR illumination (980 nm). Under optimal conditions, the near-IR light-mediated PEC aptasensing system could exhibit good photoelectrochemical response toward target CEA and allowed for the detection of target CEA as low as 3.6 pg mL^-1. High reproducibility and good accuracy were achieved for analysis of human serum specimens. Importantly, the near-IR-activated PEC aptasensing scheme provides a promising platform for ultrasensitive detection of other biomolecules.