Quantum-dots-based photoelectrochemical bioanalysis highlighted with recent examples

Modular probe integrating with quantum dots based versatile platform for sensitive and label-free biomarker detection

Multiplexed analysis of biomarkers in a single sample tube is essential for accurate diagnosis and therapy of diseases. However, the existing detection platforms suffer from many drawbacks, such as low specificity, limited applicable sceneries, and complicated operation. Hence, it is highly important to develop a versatile biomarker detection platform that can be used for disease diagnosis and pathophysiological research. In this study, we provide a versatile method for detecting biomarkers using dual-loop probes and quantum dots (QDs). This approach utilizes a dual-loop probe that consists of a recognition module for identifying specific targets, a template recognition module for initiating subsequent chain replacement cycles, and a signal module for facilitating the fixation of QDs on the 96-well plate. The lower limit of detection for miRNA-21 is determined to be at the aM level. Furthermore, this design may be easily expanded to simultaneously detect several targets, such as miRNA and C-reactive protein. The experimental results demonstrated the successful construction of the versatile biomarkers detection platform, and indicated that the sensitive and versatile platform has significant potential in the areas of bio-sensing, clinical diagnostics, and environmental sample analysis.

Photoelectrochemical Enzyme Biosensor for Malate Using Quantum Dots on Indium Tin Oxide/Plastics as a Sensing Surface

A photoelectrochemical biosensor for malate was developed using an indium tin oxide (ITO) layer deposited on a poly(ethylene terephthalate) plastic sheet as a transparent electrode material for the immobilization of malate dehydrogenase together with CdTe quantum dots. Different approaches were compared for the construction of the bioactive layer; the highest response was achieved by depositing malate dehydrogenase together with CdTe nanoparticles and covering it with a Nafion/water (1:1) mixture. The amperometric signal of this biosensor was recorded during irradiation with a near-UV LED in the flow-through mode. The limit of detection was 0.28 mmol/L, which is adequate for analyzing malic acid levels in drinks such as white wines and fruit juices. The results confirm that the cheap ITO layer deposited on the plastic sheet after cutting into rectangular electrodes allows for the economic production of photoelectrochemical (bio)sensors. The combination of NAD+-dependent malate dehydrogenase with quantum dots was also compatible with such an ITO surface.

Determination of chloramphenicol in food using nanomaterial-based electrochemical and optical sensors-A review

Chloramphenicol (CAP) is a widely used antibiotic for the treatment of sick animals owing to its potent action and low cost. However, the accumulation of CAP in the human body can cause irreversible aplastic anemia and hematopoietic toxicity. Accordingly, development of various analytical techniques for the rapid detection of CAP in animal products and the related processed foods is necessary. Among these analytical techniques, electrochemical and optical sensors offer many advantages for CAP detection, including high sensitivity, simple operation and fast analysis speed. In this review, we summarize recent application of carbon nanomaterials, metal nanoparticles, metal oxide nanoparticles and metal organic framework in the development of electrochemical and optical sensors for CAP detection (2010-2022). Based on the advantages and disadvantages of nanomaterials, electrochemical and optical sensors are summarized in this review. The preparation and synthesis of electrochemical and optical sensors and nanomaterials in the field of rapid detection are prospected.

Photoelectrochemical sensors based on paper and their emerging applications in point-of-care testing

Point-of-care testing (POCT) technology is urgently required owing to the prevalence of the Internet of Things and portable electronics. In light of the attractive properties of low background and high sensitivity caused by the complete separation of excitation source and detection signal, the paper-based photoelectrochemical (PEC) sensors, featured with fast in analysis, disposable and environmental-friendly have become one of the most promising strategies in POCT. Therefore, in this review, the latest advances and principal issues in the design and fabrication of portable paper-based PEC sensors for POCT are systematically discussed. Primarily, the flexible electronic devices that can be constructed by paper and the reasons why they can be used in PEC sensors are expounded. Afterwards, the photosensitive materials involved in paper-based PEC sensor and the signal amplification strategies are emphatically introduced. Subsequently, the application of paper-based PEC sensors in medical diagnosis, environmental monitoring and food safety are further discussed. Finally, the main opportunities and challenges of paper-based PEC sensing platforms for POCT are briefly summarized. It provides a distinct perspective for researchers to construct paper-based PEC sensors with portable and cost-effective, hoping to enlighten the fast development of POCT soon after, as well as benefit human society.

Chiral chromatography and surface chirality of carbon nanoparticles

Chiral carbon nanoparticles (CNPs) represent a rapidly evolving area of research for optical and biomedical technologies. Similar to small molecules, applications of CNPs as well as fundamental relationships between their optical activity and structural asymmetry would greatly benefit from their enantioselective separations by chromatography. However, this technique remains in its infancy for chiral carbon and other nanoparticles. The possibility of effective separations using high performance liquid chromatography (HPLC) with chiral stationary phases remains an open question whose answer can also shed light on the components of multiscale chirality of the nanoparticles. Herein, we report a detailed methodology of HPLC for successful separation of chiral CNPs and establish a path for its future optimization. A mobile phase of water/acetonitrile was able to achieve chiral separation of CNPs derived from L- and D-cysteine denoted as L-CNPs and D-CNPs. Molecular dynamics simulations show that the teicoplanin-based stationary phase has a higher affinity for L-CNPs than for D-CNPs, in agreement with experiments. The experimental and computational findings jointly indicate that chiral centers of chiral CNPs are present at their surface, which is essential for the multiple applications of these chiral nanostructures and equally essential for interactions with biomolecules and circularly polarized photons.

Ti3C2 MXene-anchored photoelectrochemical detection of exosomes by in situ fabrication of CdS nanoparticles with enzyme-assisted hybridization chain reaction

Exosomes that carry large amounts of tumor-specific molecular information have been identified as a potential non-invasive biomarker for early warning of cancer. In this work, we reported an enzyme-assisted photoelectrochemical (PEC) biosensor for quantification of exosomes based on the in situ synthesis of Ti₃C₂ MXene/CdS composites with magnetic separation technology and hybridization chain reaction (HCR). First, exosomes were specifically bound between aptamer-labeled magnetic beads (CD63-MBs) and a cholesterol-labeled DNA anchor. The properly designed anchor ends acted as a trigger to enrich the alkaline phosphatase (ALP) through HCR. It catalyzed more sodium thiophosphate to generate the sulfideion (S²⁻), which combined with Cd²⁺ for in situ fabrication of CdS on Ti₃C₂ MXene resulting in elevated photocurrent. The Ti₃C₂ MXene-anchored PEC method was realized for the quantitative detection of exosomes, which exhibited the dynamic working range from 7.3 × 10⁵ particles per mL to 3.285 × 10⁸ particles per mL with a limit of detection of 7.875 × 10⁴ particles per mL. The strategy showed acceptable stability, high sensitivity, rapid response and excellent selectivity. Furthermore, we believe that the PEC biosensor has huge potential as a routine bioassay method for the precise quantification of exosomes from breast cancer in the future.

An enzyme-assisted photoelectrochemical (PEC) biosensor was established for quantification of exosomes based on the in situ synthesis of Ti₃C₂ MXene/CdS composites with magnetic separation technology and hybridization chain reaction (HCR).

Signal-on photoelectrochemical immunoassay for salivary cortisol based on silver nanoclusters-triggered ion-exchange reaction with CdS quantum dots

Nowadays, the epidemic, employment, and academic pressures are seriously affecting our physical and mental health. Herein, we designed a magneto-controlled photoelectrochemical immunosensor for noninvasive monitoring of salivary cortisol regarded as a pressure biomarker. A competitive immunoassay model was established by coupling bovine serum albumin-cortisol modified magnetic beads (MB-BSA-cortisol) with silver nanoclusters (Ag NCs)-labelled anti-cortisol antibody, and quantity analysis was operated by photoelectrochemical measurement of the CdS/Au electrode as an ion-exchange platform. Accompanying the formation of immune complexes, the carried Ag NCs were readily dissolved with nitric acid to produce abundant silver ions, which transferred to the electrode for ion-exchange reaction with CdS quantum dots to produce Ag₂S, a new electron–hole capture site, leading to a decrease in the photocurrent intensity. The photocurrent signal gradually recovered with the increase of concentration of target cortisol, acquiring the signal-on mode competitive immunosensing system, which is propitious to the detection of small molecules. Within optimal conditions, this sensor had a satisfactory linear relationship in the range of 0.0001–100 ng mL⁻¹ with favorable repeatability, specificity, and acceptable method accuracy. The detection limit was as low as 0.06 pg mL⁻¹. In addition, this strategy provided new thought for the test of other small-molecule analytes and immunosensor applied in the complex biological system.

Recent advances in nanomaterials for prostate cancer detection and diagnosis

Despite the significant progress in the discovery of biomarkers and the exploitation of technologies for prostate cancer (PCa) detection and diagnosis, the initial screening of these PCa-related biomarkers using current...

Nanomaterials as Promising Theranostic Tools in Nanomedicine and Their Applications in Clinical Disease Diagnosis and Treatment

In recent decades, with the rapid development of nanotechnology, nanomaterials have been widely used in the medical field, showing great potential due to their unique physical and chemical properties including minimal size and functionalized surface characteristics. Nanomaterials such as metal nanoparticles and polymeric nanoparticles have been extensively studied in the diagnosis and treatment of diseases that seriously threaten human life and health, and are regarded to significantly improve the disadvantages of traditional diagnosis and treatment platforms, such as poor effectiveness, low sensitivity, weak security and low economy. In this review, we report and discuss the development and application of nanomaterials in the diagnosis and treatment of diseases based mainly on published research in the last five years. We first briefly introduce the improvement of several nanomaterials in imaging diagnosis and genomic sequencing. We then focus on the application of nanomaterials in the treatment of diseases, and select three diseases that people are most concerned about and that do the most harm: tumor, COVID-19 and cardiovascular diseases. First, we introduce the characteristics of nanoparticles according to the excellent effect of nanoparticles as delivery carriers of anti-tumor drugs. We then review the application of various nanoparticles in tumor therapy according to the classification of nanoparticles, and emphasize the importance of functionalization of nanomaterials. Second, COVID-19 has been the hottest issue in the health field in the past two years, and nanomaterials have also appeared in the relevant treatment. We enumerate the application of nanomaterials in various stages of viral pathogenesis according to the molecular mechanism of the complete pathway of viral infection, pathogenesis and transmission, and predict the application prospect of nanomaterials in the treatment of COVID-19. Third, aiming at the most important causes of human death, we focus on atherosclerosis, aneurysms and myocardial infarction, three of the most common and most harmful cardiovascular diseases, and prove that nanomaterials could be involved in a variety of therapeutic approaches and significantly improve the therapeutic effect in cardiovascular diseases. Therefore, we believe nanotechnology will become more widely involved in the diagnosis and treatment of diseases in the future, potentially helping to overcome bottlenecks under existing medical methods.

Smartphone-based photoelectrochemical biosensing system with graphitic carbon nitride/gold nanoparticles modified electrodes for matrix metalloproteinase-2 detection

Photoelectrochemical analysis has been widely used in the field of biosensing due to its high sensitivity and strong anti-interference ability. Herein, a portable and versatile smartphone-based photoelectrochemical biosensing platform was developed for the rapid and on-site biomedical analysis. In the system, light excitation and photocurrent measurements were accomplished by a miniaturized and integrated circuit board. Smartphone with a specifically designed application was utilized to wirelessly control the system via Bluetooth. For photoelectrochemical sensor, graphitic carbon nitride (g-C₃N₄) and gold nanoparticles loaded on indium tin oxide electrodes were utilized as photoactive materials and signal amplification elements, respectively. The gold nanoparticles were also used to immobilized matrix metalloproteinase-2 (MMP-2) specific cleavage peptide that modified with bovine serum albumin (BSA) on the terminal. In the presence of MMP-2, the peptide was specifically hydrolyzed and cleaved. Thus, parts of the peptide chain and BSA were detached from the electrode resulting in the decrease of steric hindrance and the increase of photoelectrochemical currents. The photocurrents changed linearly with the logarithm of MMP-2 concentrations ranging from 1 pg/mL to 100 ng/mL in both buffer and artificial serum with correlation coefficient of 0.9943 and 0.9698. The limit of detections were as low as 0.48 pg/mL in buffer and 0.55 pg/mL in artifical serum. It indicated that the biosensor has good linearity and high sensitivity, which also verified the effectiveness of the portable instrument. This system provides a pioneering solution for the development of miniaturized and portable photoelectrochemical analysis instruments used for the field monitoring of different analytes.

Advances in the Immunoassays for Detection of Bacillus thuringiensis Crystalline Toxins

Insect-resistant genetically modified organisms have been globally commercialized for the last 2 decades. Among them, transgenic crops based on Bacillus thuringiensis crystalline (Cry) toxins are extensively used for commercial agricultural applications. However, less emphasis is laid on quantifying Cry toxins because there might be unforeseen health and environmental concerns. Immunoassays, being the preferred method for detection of Cry toxins, are reviewed in this study. Owing to limitations of traditional colorimetric enzyme-linked immunosorbent assay, the trend of detection strategies shifts to modified immunoassays based on nanomaterials, which provide ultrasensitive detection capacity. This review assessed and compared the properties of the recent advances in immunoassays, including colorimetric, fluorescence, chemiluminescence, surface-enhanced Raman scattering, surface plasmon resonance, and electrochemical approaches. Thus, the ultimate aim of this study is to identify research gaps and infer future prospects of current approaches for the development of novel immunosensors to monitor Cry toxins in food and the environment.

Fluorescent nanoparticles as tools in ecology and physiology

Fluorescent nanoparticles (FNPs) have been widely used in chemistry and medicine for decades, but their employment in biology is relatively recent. Past reviews on FNPs have focused on chemical, physical or medical uses, making the extrapolation to biological applications difficult. In biology, FNPs have largely been used for biosensing and molecular tracking. However, concerns over toxicity in early types of FNPs, such as cadmium-containing quantum dots (QDs), may have prevented wide adoption. Recent developments, especially in non-Cd-containing FNPs, have alleviated toxicity problems, facilitating the use of FNPs for addressing ecological, physiological and molecule-level processes in biological research. Standardised protocols from synthesis to application and interdisciplinary approaches are critical for establishing FNPs in the biologists' tool kit. Here, we present an introduction to FNPs, summarise their use in biological applications, and discuss technical issues such as data reliability and biocompatibility. We assess whether biological research can benefit from FNPs and suggest ways in which FNPs can be applied to answer questions in biology. We conclude that FNPs have a great potential for studying various biological processes, especially tracking, sensing and imaging in physiology and ecology.

Nanostructure-based photoelectrochemical sensing platforms for biomedical applications

As a newly developed and powerful analytical method, the use of photoelectrochemical (PEC) biosensors opens up new opportunities to provide wide applications in the early diagnosis of diseases, environmental monitoring and food safety detection. The properties of diverse photoactive materials are one of the essential factors, which can greatly impact the PEC performance. The continuous development of nanotechnology has injected new vitality into the field of PEC biosensors. In many studies, much effort on PEC sensing with semiconductor materials is highlighted. Thus, we propose a systematic introduction to the recent progress in nanostructure-based PEC biosensors to exploit more promising materials and advanced PEC technologies. This review briefly evaluates the several advanced photoactive nanomaterials in the PEC field with an emphasis on the charge separation and transfer mechanism over the past few years. In addition, we introduce the application and research progress of PEC sensors from the perspective of basic principles, and give a brief overview of the main advances in the versatile sensing pattern of nanostructure-based PEC platforms. This last section covers the aspects of future prospects and challenges in the nanostructure-based PEC analysis field.

In Situ Generation of Regularly Ordered 2D Ultrathin Covalent Organic Framework Films for Highly Sensitive Photoelectrochemical Bioanalysis

Developing new photoactive materials and electrode preparation technology with high stability, repeatability, easy fabrication, and a low electron-hole recombination rate is promising for ideal photoelectrochemical (PEC) biosensors, but it remains a great challenge. Here, a porous and crystalline oriented two-dimensional (2D) ultrathin covalent organic framework film (D-TA COF film) was formed in situ on indium-doped tin oxide (ITO) substrates under very mild conditions. The structure and morphology of D-TA COF film were characterized by means of Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and powder X-ray diffraction. Compared with the randomly oriented D-TA COF powder drop-coated on ITO, the photocurrent of the D-TA COF film grown on the ITO surface in situ achieved as high as ∼333-fold increase. This photocurrent can be further amplified by O₂ (acting as electron acceptors). Benefiting from the fabrication in situ, D-TA COF film also exhibited tough adhesion, assuring the film was difficult to separate from the electrode. Accordingly, D-TA COF film was applied as the photoactive material to build a PEC biosensor for H₂O₂ detection based on coupling with large amounts of catalase (CAT) through simple adsorption. The introduced CAT catalyzed the decomposition of H₂O₂ to O₂, leading to an enhancement of the photocurrent response. As a result, a "signal-on" PEC biosensor was fabricated with good sensitivity, rapid response, and high stability, and it can also detect H₂O₂ released from living cells. Taking into account these advantages, the D-TA COF film is expected to be an ideal photoactive material to construct various PEC biosensors, which as far as we know have not been reported.

An CuInS2 photocathode for the sensitive photoelectrochemical determination of microRNA-21 based on DNA-protein interaction and exonuclease III assisted target recycling amplification

A photocathode is described for the determination of microRNA-21 by using CuInS₂ as an active photocathode material. Exonuclease III assisted target recycling amplification was employed to enhance the detection sensitivity. The TATA-binding protein (TBP) was applied to enhance steric hindrance which decreases the photoelectrochemical intensity. This strategy is designed by combining the anti-interference photocathode material, enzyme assisted target recycling amplification and TBP induced signal off, showing remarkable amplification efficiency. Under the optimized conditions, the detection limit for microRNA-21 is as low as 0.47 fM, and a linear range was got from 1.0 × 10^-15 M to 1.0 × 10^-6 M. Graphical abstract Schematic representation of sensitive photoelectrochemical detection of microRNA-21.CuInS₂ is used as an active photocathode material. Combined Exonuclease III assisted target recycling amplification and TATA-binding protein decreased of photoelectrochemical intensity, the detection limit was 0.47 fM with good selectivity. (miR-21: microRNA-21; CS: chitosan).

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.

Cathodic "signal-on" photoelectrochemical aptasensor for chloramphenicol detection using hierarchical porous flower-like Bi-BiOI@C composite

A novel p-type semiconductor-based cathodic "signal-on" photoelectrochemical (PEC) aptasensor was proposed for highly sensitive and selective detection of chloramphenicol (CAP). The photocathode was fabricated with hierarchical porous flower-like Bi-BiOI@C composite synthesized via a one-pot solvothermal method using glucose as both green reductant and carbon precursor. Due to the surface plasmon resonance (SPR) effect of Bi and high-conductivity of carbon, the composite exhibited an enhanced cathodic photocurrent as compared with pure BiOI or Bi-BiOI. When CAP-binding aptamer was immobilized as recognition element on Bi-BiOI@C modified electrode, a cathodic PEC aptasensor showing specific "signal-on" response to CAP was constructed. Some influencing factors such as coating amount of Bi-BiOI@C suspension, applied bias potential, and aptamer concentration were studied. Under the optimum conditions, the cathodic photocurrent of the constructed PEC aptasensor increased linearly with CAP concentration from 2 to 250 nM, with a detection limit (3S/N) of 0.79 nΜ. The proposed sensor was successfully applied to the determination of CAP in pharmaceutical tablet, eye drop and lake water samples.

Gold Nanoparticle Couples with Entropy-Driven Toehold-Mediated DNA Strand Displacement Reaction on Magnetic Beads: Toward Ultrasensitive Energy-Transfer-Based Photoelectrochemical Detection of miRNA-141 in Real Blood Sample

Highly stable circulating microRNAs (miRNAs) are currently recognized as a novel potential biomarker for clinical cancer diagnosis in the early stage. However, limited by its low concentration, high sequence similarity, as well as the numerous interferences in body fluids, detection of miRNA in whole blood with sufficient selectivity and sensitivity is still challenging. Herein, we reported the integration of entropy-driven toehold-mediated DNA strand displacement (ETSD) reaction with magnetic beads (MB) toward the energy-transfer-based photoelectrochemical (PEC) detection of the prostate carcinoma (PCa) biomarker miRNA-141 in a real blood sample. In this protocol, the ETSD reaction was divided into two steps, and cooperated with magnetic separation, target extraction and amplification could be realized in a single test and ultrasensitive detection of miRNA-141 could be achieved in undiluted whole blood sample. This work proposed a new solution for sensitive biomolecular detection in a complex biological milieu and exhibited great promise for future clinical cancer diagnosis.

Enhancing the Photoelectrochemical Response of DNA Biosensors Using Wrinkled Interfaces

Photoelectrochemical (PEC) biosensors, with optical biasing and electrochemical readout, are expected to enhance the limit-of-detection of electrochemical biosensors by lowering their background signals. However, when PEC transducers are functionalized with biorecognition layers, their current significantly decreases, which reduces their signal-to-noise ratio and dynamic range. Here, we develop and investigate a wrinkled conductive scaffold for loading photoactive quantum dots into an electrode. The wrinkled photoelectrodes demonstrate an order of magnitude enhancement in the magnitude of the transduced PEC current compared to their planar counterparts. We engineer PEC biosensors by functionalizing the wrinkled photoelectrodes with nucleic acid capture probes. We challenge the sensitivity of the wrinkled and planar biosensors with various concentrations of DNA target and observe a 200 times enhancement in the limit-of-detection for wrinkled versus planar electrodes. In addition to enhanced sensitivity, the wrinkled PEC biosensors are capable of distinguishing between fully complementary and targets with a single base-pair mismatch, demonstrating the suitability of these biosensors for use in clinical diagnostics.

Sensors Based on Bio and Biomimetic Receptors in Medical Diagnostic, Environment, and Food Analysis

Analytical chemistry is now developing mainly in two areas: automation and the creation of complexes that allow, on the one hand, for simultaneously analyzing a large number of samples without the participation of an operator, and on the other, the development of portable miniature devices for personalized medicine and the monitoring of a human habitat. The sensor devices, the great majority of which are biosensors and chemical sensors, perform the role of the latter. That last line is considered in the proposed review. Attention is paid to transducers, receptors, techniques of immobilization of the receptor layer on the transducer surface, processes of signal generation and detection, and methods for increasing sensitivity and accuracy. The features of sensors based on synthetic receptors and additional components (aptamers, molecular imprinted polymers, biomimetics) are discussed. Examples of bio- and chemical sensors' application are given. Miniaturization paths, new power supply means, and wearable and printed sensors are described. Progress in this area opens a revolutionary era in the development of methods of on-site and in-situ monitoring, that is, paving the way from the "test-tube to the smartphone".

Hierarchical CuInS2-based heterostructure: Application for photocathodic bioanalysis of sarcosine

In this study, on the basis of hierarchical CuInS₂-based heterostructure, a novel cathodic photoelectrochemical (PEC) enzymatic bioanalysis of the sarcosine detection was reported. Specifically, heterostructured CuInS₂/NiO/ITO photocathode was prepared and sarcosine oxidases (SOx) were integrated for the construction of the enzymatic biosensor. In the bioanalysis, the O₂-dependent suppression of the cathodic photocurrent can be observed due to the competition between the as-fabricated O₂-sensitive photocathode and the SOx-catalytic event toward O₂ reduction. Based on the sarcosine-controlled O₂ concentration, a novel photocathodic enzymatic biosensor could be realized for the sensitive and specific sarcosine detection. This work manifested the great potential of CuInS₂-based heterostructure as a novel platform for future PEC bioanalytical development and also a PEC method for sarcosine detection, which could be easily extended to numerous other enzymatic systems and to our knowledge has not been reported. This work is expected to stimulate more interest in the design and implementation of numerous CuInS₂-based heterostructured photocathodic enzymatic sensing.

On the Complex and Reversible Pathways of CdSe Quantum Dots Driven by Pyroelectric-Dielectrophoresis

Electrophoresis (EP) and dielectrophoresis (DEP) are the two well-established methodologies to manipulate nanoparticles (NPs). Recently, DEP by a virtual electrode platform was demonstrated on ferroelectric substrates, where the driving force is due to the strong electric field generated by the pyroelectric effect, thus opening new scenarios for manipulating the matter. Such an innovative approach named pyroelectric-DEP has several advantages over traditional EP and DEP. However, a detailed study on this novel approach is required for understanding the complex pathways traced by NPs under the action of the pyroelectric-driven forces and thus for explaining the final patterns. Here, we investigate experimentally the dynamic behavior of CdSe NPs through time-lapse fluorescence microscopy imaging. Complete visualization and measurement of the directed-assembling process of NPs immersed in polydimethylsiloxane fluid is reported, which shows some unpredicted results with respect to the previous works, thus opening the route for designing in principle a reversible and switchable device allowing two different and reversible final NP-patterned states. The observed phenomena are fully analyzed by experimental and simulated analysis, and the movements of NPs is performed to elucidate in depth the involved processes. The investigation furnishes an interesting result that the complex behavior of the NPs can be fully comprehended and explained by considering the superposition of both EP and DEP forces.

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.

Integration of Enzymes in Polyaniline-Sensitized 3D Inverse Opal TiO2 Architectures for Light-Driven Biocatalysis and Light-to-Current Conversion

Inspired by natural photosynthesis, coupling of artificial light-sensitive entities with biocatalysts in a biohybrid format can result in advanced photobioelectronic systems. Herein, we report on the integration of sulfonated polyanilines (PMSA1) and PQQ-dependent glucose dehydrogenase (PQQ-GDH) into inverse opal TiO₂ (IO-TiO₂) electrodes. While PMSA1 introduces sensitivity for visible light into the biohybrid architecture and ensures the efficient wiring between the IO-TiO₂ electrode and the biocatalytic entity, PQQ-GDH provides the catalytic activity for the glucose oxidation and therefore feeds the light-driven reaction with electrons for an enhanced light-to-current conversion. Here, the IO-TiO₂ electrodes with pores of around 650 nm provide a suitable interface and morphology needed for the stable and functional assembly of polymer and enzyme. The IO-TiO₂ electrodes have been prepared by a template approach applying spin coating, allowing an easy scalability of the electrode height and surface area. The successful integration of the polymer and the enzyme is confirmed by the generation of an anodic photocurrent, showing an enhanced magnitude with increasing glucose concentrations. Compared to flat and nanostructured TiO₂ electrodes, the three-layered IO-TiO₂ electrodes give access to a 24-fold and 29-fold higher glucose-dependent photocurrent due to the higher polymer and enzyme loading in IO films. The three-dimensional IO-TiO₂|PMSA1|PQQ-GDH architecture reaches maximum photocurrent densities of 44.7 ± 6.5 μA cm^-2 at low potentials in the presence of glucose (for a three TiO₂ layer arrangement). The onset potential for the light-driven substrate oxidation is found to be at -0.315 V vs Ag/AgCl (1 M KCl) under illumination with 100 mW cm^-2, which is more negative than the redox potential of the enzyme. The results demonstrate the advantageous properties of IO-TiO₂|PMSA1|PQQ-GDH biohybrid architectures for the light-driven glucose conversion with improved performance.

One-Step Synthesis of CuO-Cu2O Heterojunction by Flame Spray Pyrolysis for Cathodic Photoelectrochemical Sensing of l-Cysteine

CuO-Cu₂O heterojunction was synthesized via a one-step flame spray pyrolysis (FSP) process and employed as photoactive material in construction of a photoelectrochemical (PEC) sensing device. The surface analysis showed that CuO-Cu₂O nanocomposites in the size less than 10 nm were formed and uniformly distributed on the electrode surface. Under visible light irradiation, the CuO-Cu₂O-coated electrode exhibited admirable cathodic photocurrent response, owing to the favorable property of the CuO-Cu₂O heterojunction such as strong absorption in the visible region and effective separation of photogenerated electron-hole pairs. On the basis of the interaction of l-cysteine (l-Cys) with Cu-containing compounds via the formation of Cu-S bond, the CuO-Cu₂O was proposed as a PEC sensor for l-Cys detection. A declined photocurrent response of CuO-Cu₂O to addition of l-Cys was observed. Influence factors including CuO-Cu₂O concentration, coating amount of CuO-Cu₂O, and applied bias potential on the PEC response toward l-Cys were optimized. Under optimum conditions, the photocurrent of the proposed sensor was linearly declined with increasing the concentration of l-Cys from 0.2 to 10 μM, with a detection limit (3S/N) of 0.05 μM. Moreover, this PEC sensor displayed high selectivity, reproducibility, and stability. The potential applicability of the proposed PEC sensor was assessed in human urine samples.