Design and construction of Z-scheme Bi2S3/nitrogen-doped graphene quantum dots: Boosted photoelectric conversion efficiency for high-performance photoelectrochemical aptasensing of sulfadimethoxine

Carbon-based photoelectrochemical sensors: recent developments and future prospects

Owing to the considerable potential of photoelectrochemical (PEC) sensors, they have gained significant attention in the analysis of biological, environmental, and food markers. However, the limited charge mass transfer efficiency and rapid recombination of electron hole pairs have become obstacles in the development of PEC sensors. In this case, considering the unique advantages of carbon-based materials, they can be used as photosensitizers, supporting materials and conductive substrates and coupled with semiconductors to prepare composite materials, solving the above problems. In addition, there are many types of carbon materials, which can have semiconductor properties and form heterojunctions after coupling with semiconductors, effectively promoting the separation of electron hole pairs. Herein, we aimed to provide a comprehensive analysis of reports on carbon-based PEC sensors by introducing their research and application status and discussing future development trends in this field. In particular, the types and performance improvement strategies of carbon-based electrodes and the working principles of carbon-based PEC sensors are explained. Furthermore, the applications of carbon-based photoelectric sensors in environmental monitoring, biomedicine, and food detection are highlighted. Finally, the current limitations in the research on carbon-based PEC sensors are emphasized and the need to enhance the sensitivity and selectivity through material modification, structural design, improved device performance, and other strategies are emphasized.

Dual amplification for PEC ultrasensitive aptasensing of biomarker HER-2 based on Z-scheme UiO-66/CdIn2S4 heterojunction and flower-like PtPdCu nanozyme

As an important prognostic indicator in breast cancer, human epithelial growth factor receptor-2 (HER-2) is of importance for assessing prognosis of breast cancer patients, whose accurate and facile analysis are imperative in clinical diagnosis and treatment. Herein, photoactive Z-scheme UiO-66/CdIn2S4 heterojunction was constructed by a hydrothermal method, whose optical property and photoactivity were critically investigated by a range of techniques, combined by elucidating the interfacial charge transfer mechanism. Meanwhile, PtPdCu nanoflowers (NFs) were fabricated by a simple aqueous wet-chemical method, whose peroxidase (POD)-mimicking catalytic activity was scrutinized by representative tetramethylbenzidine (TMB) oxidation in H2O2 system. Taken together, the UiO-66/CdIn2S4 based photoelectrochemical (PEC) aptasensor was established for quantitative analysis of HER-2, where the detection signals were further magnified through catalytic precipitation reaction towards 4-chloro-1-naphthol (4-CN) oxidation (assisted by the PtPdCu NFs nanozyme). The PEC aptasensor presented a broader linear range within 0.1 pg mL-1-0.1 μg mL-1 and a lower limit of detection of 0.07 pg mL-1. This work developed a new PEC aptasensor for ultrasensitive determination of HER-2, holding substantial promise for clinical diagnostics.

Photoelectrochemical quenching-recovery biosensor based on NSCQDs/Fe2O3@Bi2S3 for the detection of trypsin

BACKGROUND

The content of trypsin will change when pancreatic diseases occur, therefore developing a high-performance method for trypsin detection is of great significance for guiding patients on medication plans and improving their prognosis. Photoelectrochemical (PEC) analysis techniques have emerged as a solution to apply for bioassays.

RESULTS

Herein, the Fe2O3@Bi2S3 and Nitrogen and sulfur co-doped carbon quantum dots (NSCQDs) were successfully synthesized by a hydrothermal method. Subsequently, NSCQDs/Fe2O3@Bi2S3 with a photocurrent amplification effect covered on fluorine-doped tin oxide (FTO) electrode as the substrate material and apoferritin (APO) as a bio-recognition element to quench the photocurrent of the substrate material which can be excited with light. Due to the decomposition specifically between APO and trypsin, the photocurrent response increased. The linear range for trypsin detection showed satisfied results from 2 to 1000 ng mL-1 under optimal conditions, with a detection limit of 0.42 ng mL-1 and a recovery rate of 97.41 %-103.02 %, enabling efficient quantitative analysis of trypsin.

SIGNIFICANCE

In this experiment, a PEC biosensor with simple operation, low detection limit, excellent selectivity and strong stability was successfully prepared, enabling quantitative analysis of trypsin in human serum samples through the quenching-recovery mechanism. It holds great significance for diagnosis and serves as a practical method for the detection of trypsin in the future.

Recent Progress in Photoelectrochemical Sensing of Pesticides in Food and Environmental Samples: Photoactive Materials and Signaling Mechanisms

Pesticides have become an integral part of modern agricultural practices, but their widespread use poses a significant threat to human health. As such, there is a pressing need to develop effective methods for detecting pesticides in food and environmental samples. Traditional chromatography methods and common rapid detection methods cannot satisfy accuracy, portability, long storage time, and solution stability at the same time. In recent years, photoelectrochemical (PEC) sensing technology has gained attention as a promising approach for detecting various pesticides due to its salient advantages, including high sensitivity, low cost, simple operation, fast response, and easy miniaturization, thus becoming a competitive candidate for real-time and on-site monitoring of pesticide levels. This review provides an overview of the recent advancements in PEC methods for pesticide detection and their applications in ensuring food and environmental safety, with a focus on the categories of photoactive materials, from single semiconductor to semiconductor-semiconductor heterojunction, and signaling mechanisms of PEC sensing platforms, including oxidation of pesticides, steric hindrance, generation/decrease in sacrificial agents, and introduction/release of photoactive materials. Additionally, this review will offer insights into future prospects and confrontations, thereby contributing novel perspectives to this evolving domain.

Ag2S QDs integration with MnO2 nanosheets for the sensitive detection of Cr (VI) via the redox reaction induced photoelectrochemical variation

The heavy metal Cr (VI) will remain, accumulate, and migrate after entering the environment or ecosystem, causing serious harm to the environment. Here, a photoelectrochemical sensor was developed for Cr (VI), utilizing the Ag₂S quantum dots (QDs) and MnO₂ nanosheets as photoactive components. By introducing Ag₂S QDs with a narrow gap, a staggered energy level match is created which effectively prevents the carrier recombination in MnO₂ nanosheets, resulting in an enhanced photocurrent response. In the presence of the electron donor, l-ascorbic acid (AA), the photocurrent of the Ag₂S QDs and MnO₂ nanosheets modified photoelectrode is further enhanced. As AA has the ability to convert Cr (VI) to Cr (Ⅲ), the photocurrent may decline due to the decrease in the electron donors when Cr (VI) is added. This phenomenon can be utilized for the sensitive detection of Cr (VI) over a wider linear range (100 pM-30 μM) with a lower detection limit of 6.46 pM (S/N = 3). This work using the strategy that the targets induced the variations of the electron donor shows the advantages of good sensitivity and nice selectivity. The sensor holds many advantages such as simple fabrication process, economical material expense, and consistent photocurrent signals. It also holds significant potential for environmental monitoring and serves as a practical photoelectric sensing approach for detecting Cr (VI).

Ultrasensitive PEC cytosensor for breast cancer cells detection and inhibitor screening based on plum-branched CdS/Bi2S3 heterostructures

Breast cancer is the most common malignant tumor in women, which seriously threatens the life and health of patients. Therefore, facile and sensitive detection of human breast cancer cells is crucial for cancer diagnosis. In this work, plum-branched CdS/Bi₂S₃ heterostructures (CdS/Bi₂S₃ HSs) were synthesized under hydrothermal condition, whose photoelectrochemical (PEC) property and biocompatibility were scrutinously investigated. In parallel, a signal amplification strategy was designed based on immune recognition between epidermal growth factor receptor (EGFR) overexpressed on membrane of breast cancer cells MDA-MB-231 and its aptamer. Integration of the above together, a highly sensitive PEC cytosensor was developed for analysis of target MDA-MB-231 cells, exhibiting a wider linear range of 1 × 10² ∼ 3 × 10⁵ cells mL^-1 with a limit of detection (LOD) down to 6 cells mL^-1 (S/N = 3). Further, the biosensor was explored for anticancer drug (e.g., dacomitinib) screening by monitoring the variations in the PEC signals of the expressed EGFR upon drug stimulation. The obtained CdS/Bi₂S₃ HSs are identified as promising and feasible photoactive material for determination of cancer cells and drug screening in clinic and related research.

A signal "on-off-on"-type electrochemiluminescence aptamer sensor for detection of sulfadimethoxine based on Ru@Zn-oxalate MOF composites

An "on-off-on"-type electrochemiluminescence (ECL) aptamer sensor based on Ru@Zn-oxalate metal-organic framework (MOF) composites is constructed for sensitive detection of sulfadimethoxine (SDM). The prepared Ru@Zn-oxalate MOF composites with the three-dimensional structure provide good ECL performance for the "signal-on." The MOF structure with a large surface area enables the material to fix more Ru(bpy)₃²⁺. Moreover, the Zn-oxalate MOF with three-dimensional chromophore connectivity provides a medium which can accelerate excited-state energy transfer migration among Ru(bpy)₃²⁺ units, and greatly reduces the influence of solvent on chromophore, achieving a high-energy Ru emission efficiency. The aptamer chain modified with ferrocene at the end can hybridize with the capture chain DNA1 fixed on the surface of the modified electrode through base complementary pairing, which can significantly quench the ECL signal of Ru@Zn-oxalate MOF. SDM specifically binds to its aptamer to separate ferrocene from the electrode surface, resulting in a "signal-on" ECL signal. The use of the aptamer chain further improves the selectivity of the sensor. Thus, high-sensitivity detection of SDM specificity is realized through the specific affinity between SDM and its aptamer. This proposed ECL aptamer sensor has good analytical performance for SDM with low detection limit (27.3 fM) and wide detection range (100 fM-500 nM). The sensor also shows excellent stability, selectivity, and reproducibility, which proved its analytical performance. The relative standard deviation (RSD) of SDM detected by the sensor is between 2.39 and 5.32%, and the recovery is in the range 97.23 to 107.5%. The sensor shows satisfactory results in the analysis of actual seawater samples, which is expected to play a role in the exploration of marine environmental pollution.

Ionic liquid functionalized trapezoidal Zn-MOF nanosheets integrated with gold nanoparticles for photoelectrochemical immunosensing alpha-fetoprotein

An imidazolium based ionic liquid was successfully prepared and used as an organic ligand to coordinate with Zn²⁺ to prepare trapezoidal metal-organic frameworks (Zn-MOF) nanosheets. Then, gold nanoparticles (AuNPs) were integrated onto Zn-MOF nanosheets surface to produce AuNPs@Zn-MOF nanocomposites by in-situ reduction of chloroauric acid. AuNPs with size less than 5 nm were uniformly dispersed on the entire surface of Zn-MOF nanosheets. AuNPs can significantly promote the photocurrent response of Zn-MOF nanosheets and supply an efficient photoelectrochemical sensing platform for fabricating an immunosensor for alpha-fetoprotein (AFP). For AFP determination, the photocurrent response of the immunosensor was linearly related to the logarithm of AFP concentration in the range of 0.005-15.0 ng/mL. The detection limit was calculated to be 1.88 pg/mL. The PEC immunosensor can be facilely fabricated, and provided some superior analytical characteristics such as excellent selectivity, sensitivity, stability and reproducibility for AFP determination. Practicability of the photoelectrochemical immunosensor was demonstrated by using it in assaying AFP in clinical serum samples.

Heterostructured BiVO4/CoPi nanoarrays as high-efficiency photoanode and AuPt nanodendrites as nanozyme for sensitive sensing of miRNA 141

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

Disposable and cost-effective label-free electrochemical immunosensor for prolactin based on bismuth sulfide nanorods with polypyrrole

Prolactin (PRL) is produced by the pituitary gland and plays a vital role in the production of milk after a baby is born. PRL levels are normally elevated in pregnant and nursing women, and high levels of PRL in the human body cause hyperprolactinemia, infertility, galactorrhea, infrequent or irregular periods, amenorrhea, breast pain, and loss of libido. Accordingly, herein, a novel label-free immunosensor using a bismuth sulfide/polypyrrole (Bi2S3/PPy)-modified screen-printed electrode (SPE) for the fast and facile detection of the peptide hormone PRL. Bi2S3 nanorods were synthesized via a facile hydrothermal technique, and PPy was prepared by chemical polymerization method. Subsequently, the Bi2S3/PPy/ SPE was modified with 3-mercaptopropionic acid (MPA) and EDC/NHS. Owing to the cross-linking effect of EDC/NHS, antibody-PRL (anti-PRL) was firmly stabilized on the modified SPE surface. These layer-by-layer modifications enhanced the conducting properties, anti-PRL loading capacity, and sensitivity of the developed immunosensor. Under optimized conditions, the PRL immunosensor demonstrated a broad linear range of approximately 1-250 ng/mL, a low detection limit of approximately 0.130 ng/mL (3 × SD/b), good specificity, reproducibility, and stability. PRL was successfully evaluated in human and mouse serum samples, and the corresponding outcomes were compared with those of the electrochemical and ELISA methods.

Facile and green synthesis of carbon nanodots from environmental pollutants for cell imaging and Fe3+ detection

An economical and green approach has been provided to turn environmental pollutants into carbon nanodots for their potential applications in both bioimaging and Fe3+ detection.

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.

Recent applications of quantum dots in optical and electrochemical aptasensing detection of Lysozyme

Lysozyme (Lyz) is a naturally occurring enzyme that operates against Gram-positive bacteria and leads to cell death. This antimicrobial enzyme forms the part of the innate defense system of nearly all animals and exists in their somatic discharges such as milk, tears, saliva and urine. Increased Lyz level in serum is an important indication of several severe diseases and so, precise diagnosis of Lyz is an urgent need in biosensing assays. Up to know, various traditional and modern techniques have been introduced for Lyz determination. Although the traditional methods suffer from some significant limitations such as time-consuming, arduous, biochemical screening, bacterial colony isolation, selective enrichment and requiring sophisticated instrumentation or isotope labeling, some new modern approaches like aptamer-based biosensors (aptasensors) and quantum dot (QD) nanomaterials are the main goal in Lyz detection. Electrochemical and optical sensors have been highlighted because of their adaptability and capability to decrease the drawbacks of common methods. Using an aptamer-based biosensor, sensor selectivity is enhanced due to the specific recognition of the analyte. Thereby, in this review article, the recent advances and achievements in electrochemical and optical aptasensing detection of Lyz based on different QD nanomaterials and detection methods have been discussed in detail.

Ultra-sensitive photoelectrochemical aptamer biosensor for detecting E. coli O157:H7 based on nonmetallic plasmonic two-dimensional hydrated defective tungsten oxide nanosheets coupling with nitrogen-doped graphene quantum dots (dWO3•H2O@N-GQDs)

Light absorption and interfacial engineering of photoactive materials play vital roles in photoexcited electron generation and electron transport, and ultimately boost the performance of photoelectrochemical (PEC) biosensing. In this work, a novel high-performance photoelectrochemical (PEC) biosensing platform was fabricated based on nonmetallic plasmonic tungsten oxide hydrate nanosheets (WO₃•H₂O) coupling with nitrogen doped graphene quantum dots (N-GQDs) by a facile one-step hydrothermal approach. The localized surface plasmon resonance (LSPR) properties were achieved by oxygen vacancy engineered WO₃·H₂O (dWO₃•H₂O), which could greatly extend the light absorption from visible light to near-infrared light. Moreover, by coupling with N-GQDs, the as-fabricated heterojunction (dWO₃•H₂O@N-GQD) provided a much enhanced photoelectric response due to the efficient charge transfer. By conjugation with E.coli O157:H7 aptamer, a novel PEC aptasensor based on dWO₃•H₂O@N-GQD heterojunction was fabricated with a high sensitivity for detection of E.coli O157:H7. The limit of detection (LOD) of this PEC aptasensor is 0.05 CFU/mL with a linear detection range from 0.1 to 10⁴ CFU/mL. Moreover, high reproducibility and good accuracy could also be achieved for analysis in milk samples. This work could provide a promising platform for the development of PEC bioanalysis and offer an insight into the non-metallic plasmonic materials based heterojunctions for high-performances PEC biosensing.

Contribution of Nanomaterials to the Development of Electrochemical Aptasensors for the Detection of Antimicrobial Residues in Food Products

The detection of antimicrobial residues in food products of animal origin is of utmost importance. Indeed antimicrobial residues could be present in animal derived food products because of animal treatments for curative purposes or from illegal use. The usual screening methods to detect antimicrobial residues in food are microbiological, immunological or physico-chemical methods. The development of biosensors to propose sensitive, cheap and quick alternatives to classical methods is constantly increasing. Aptasensors are one of the major trends proposed in the literature, in parallel with the development of immunosensors based on antibodies. The characteristics of electrochemical sensors (i.e., low cost, miniaturization, and portable instrumentation) make them very good candidates to develop screening methods for antimicrobial residues in food products. This review will focus on the recent advances in the development of electrochemical aptasensors for the detection of antimicrobial residues in food products. The contribution of nanomaterials to improve the performance characteristics of electrochemical aptasensors (e.g., Sensitivity, easiness, stability) in the last ten years, as well as signal amplification techniques will be highlighted.

A signal-on photoelectrochemical aptasensor for chloramphenicol assay based on 3D self-supporting AgI/Ag/BiOI Z-scheme heterojunction arrays

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

Visible light-activated self-powered photoelectrochemical aptasensor for ultrasensitive chloramphenicol detection based on DFT-proved Z-scheme Ag2CrO4/g-C3N4/graphene oxide

A visible light self-powered photoelectrochemical (PEC) aptasensor based on silver chromate particles, graphitic carbon nitride nanosheets and graphene oxide sheets (Ag₂CrO₄/g-C₃N₄/GO) for the ultrasensitive detection of chloramphenicol (CAP) was reported in this work. g-C₃N₄ was considered to be the fundamental photoelectric material because of its great oxidation ability of photogenerated hole as well as excellent biocompatibility and low toxicity. However, the narrow light absorption range and rapid carrier recombination rate limit the application of pure g-C₃N₄. Herein, Ag₂CrO₄ and GO as photosensitizer were introduced to improve the photoelectric properties of g-C₃N₄. The photocurrent of the developed ternary composite was about 3 times higher than that of pristine g-C₃N₄, which proves it can be used as a suitable photoelectric active material. Moreover, the mechanism of Z-scheme electron transfer path was proved by density functional theory (DFT) calculation. The fabricated PEC aptasensor exhibited high sensitivity toward CAP with a wide liner response of 0.5 pM to 50 nM and a detection limit of 0.29 pM. The specific recognition mechanism and excellent sensing performance indicated this aptasensor could serve as a useful tool for selective and ultrasensitive CAP detection in practical analysis.

A sensitive and stable visible-light-driven photoelectrochemical aptasensor for determination of oxytetracycline in tomato samples

Sensitive detection of oxytetracycline (OTC) has attracted increasing attention worldwide due to the relationship between food safety and human health problems. In this work, a visible-light-driven photoelectrochemical (PEC) OTC aptasensor was constructed using Bi₄VO₈Cl/nitrogen-doped graphene quantum dots (Bi₄VO₈Cl/N-GQDs) nanohybrids as photoactive material and OTC aptamer as identification element. Owing to the well matched heterojunction of Bi₄VO₈Cl and nitrogen-doped graphene quantum dots (N-GQDs), the photogenerated electron-hole pairs could be separated effectively, so that the photocurrent intensity of as-prepared Bi₄VO₈Cl/N-GQDs nanohybrids was about 7 times higher than pure Bi₄VO₈Cl and had higher stability. The constructed "signal-off" PEC aptasensor realized OTC detection in tomato samples with excellent sensitivity, specificity and repeatability. The photocurrent decreased with the increase of OTC concentration in a range from 0.1 nM to 150 nM, and the detection limit was 0.03 nM (S/N = 3). The national standard method was used to compare with our method and the results were consistent.

Visible light-driven photoelectrochemical ampicillin aptasensor based on an artificial Z-scheme constructed from Ru(bpy)32+-sensitized BiOI microspheres

Dye sensitization is an alternative strategy to improve photoelectric activity of semiconductors and, particularly, to enhance the activity towards visible light domain. Herein, an artificial Z-scheme bipyridine ruthenium (Ru(bpy)32+) sensitizing narrow-gap bismuth oxy-iodide (BiOI) microspheres was constructed by a simple electrostatic interaction strategy for the first time. The electrochemical impedance spectroscopy (EIS) and photoluminescence (PL) analysis showed that this design of such Z-scheme structure was helpful to enhance the interfacial charge transfer and improve the photoelectric conversion efficiency. In addition, due to the sensitization of Ru(bpy)32+, the band gap was narrowed from 1.8 eV of BiOI microspheres to 1.3 eV of BiOI/Ru(bpy)32+ microspheres, leading to improve the utilization of visible light. So that, the photocurrent of the resulted BiOI/Ru(bpy)32+ was 13.0 times that of pure BiOI microspheres. In view of the outstanding photoelectrochemical (PEC) performance of BiOI/Ru(bpy)32+ and the high specificity of the aptamer, the PEC aptasensor for ampicillin (AMP) merits the excellent detection performance including a broad linear ranging from 1 × 10-7 nM to 100 nM as well as a low detection limit of 3.3 × 10-8 nM (S/N = 3). This work not only provides a novel way to construct and design highly efficient photoactive materials for PEC detection, but also broadens the application of Z-scheme in the field of sensing.

Nanobiosensing with graphene and carbon quantum dots: Recent advances

Graphene and carbon quantum dots (GQDs and CQDs) are relatively new nanomaterials that have demonstrated impact in multiple different fields thanks to their unique quantum properties and excellent biocompatibility. Biosensing, analyte detection and monitoring wherein a key feature is coupled molecular recognition and signal transduction, is one such field that is being greatly advanced by the use of GQDs and CQDs. In this review, recent progress on the development of biotransducers and biosensors enabled by the creative use of GQDs and CQDs is reviewed, with special emphasis on how these materials specifically interface with biomolecules to improve overall analyte detection. This review also introduces nano-enabled biotransducers and different biosensing configurations and strategies, as well as highlights key properties of GQDs and CQDs that are pertinent to functional biotransducer design. Following relevant introductory material, the literature is surveyed with emphasis on work performed over the last 5 years. General comments and suggestions to advance the direction and potential of the field are included throughout the review. The strategic purpose is to inspire and guide future investigations into biosensor design for quality and safety, as well as serve as a primer for developing GQD- and CQD-based biosensors.

Ultrasensitive Detection of Tetracycline Using Boron and Nitrogen Co-Doped Graphene Quantum Dots from Natural Carbon Source as the Paper-Based Nanosensing Probe in Difference Matrices

Herein, the boron and nitrogen co-doped 0-dimensional graphene quantum dots (B,N-GQDs) with high quantum yield (QY) were synthesized via microwave-assisted hydrothermal method at 170 °C for 20 min using fresh passion fruit juice and boric acid as the starting materials. The 3–6 layers of B,N-GQDs with mean particle size of 9 ± 1 nm were then used for ultra-sensitive and selective detection of tetracycline in aqueous and biological media. The hybridization of boron and nitrogen atoms into the GQD structures increases the intensity of electronegative, resulting in the enhancement of QY to 50 ± 1%. The B,N-GQDs show their excellent analytical performance on tetracycline determination after 2 min of reaction under an optimal condition at pH 5. The linear range of 0.04–70 µM and with limits of detection (LOD) of 1 nM in phosphate buffer saline (PBS), 1.9 nM in urine and 2.2 nM in human serum are obtained. Moreover, the high selectivity of tetracycline by B,N-GQDs over the other 23 interferences is observed. The π-π interaction and electron donor-acceptor principle play pivotal roles in enhancing the ultra-sensitivity and selectivity of B,N-GQDs toward TC detection. Moreover, the B, N-GQD based paper nanosensor exhibits an excellent analytical performance on visual detection of 0.1–30 µM TC in human serum. Results of this study clearly indicate the feasibility of synthesis of B,N-GQDs derived from passion fruit juice for ultrasensitive tetracycline detection, which can open an avenue to use natural products for the preparation of environmentally benign and biocompatible carbon nanomaterials for highly sensitive detection of drugs, antibiotics, organic compounds and biomarkers.

Signal-on cathodic photoelectrochemical aptasensing of insulin: Plasmonic Au activated amorphous MoSx photocathode coupled with target-induced sensitization effect

Cathodic photoelectrochemical (PEC) bioassay is more resistant to reductive interferents, and development of high-performance photocathode is imperatively required in precise monitoring target in complex matrices. In this work, a plasmonic Au activated amorphous MoS_x photocathode (a-MoS_x/Au) was fabricated by sequential electrodeposition. Coupled with a sensitization amplification strategy induced by target-aptamer recognition, an ultrasensitive and high-affinitive signal-on cathodic PEC aptasensor for insulin detection was developed. Under optimum conditions, the sensor exhibits a wide linear range (0.1 pg/mL~100 ng/mL) and an ultralow detection limit (28 fg/mL) even lower than most sensors reported so far. Plasmonic Au activation and target-induced sensitization effect are responsible for high-performance PEC aptasensing of insulin at a-MoS_x photocathode.

The versatile biomedical applications of bismuth-based nanoparticles and composites: therapeutic, diagnostic, biosensing, and regenerative properties

Studies of nanosized forms of bismuth (Bi)-containing materials have recently expanded from optical, chemical, electronic, and engineering fields towards biomedicine, as a result of their safety, cost-effective fabrication processes, large surface area, high stability, and high versatility in terms of shape, size, and porosity. Bi, as a nontoxic and inexpensive diamagnetic heavy metal, has been used for the fabrication of various nanoparticles (NPs) with unique structural, physicochemical, and compositional features to combine various properties, such as a favourably high X-ray attenuation coefficient and near-infrared (NIR) absorbance, excellent light-to-heat conversion efficiency, and a long circulation half-life. These features have rendered bismuth-containing nanoparticles (BiNPs) with desirable performance for combined cancer therapy, photothermal and radiation therapy (RT), multimodal imaging, theranostics, drug delivery, biosensing, and tissue engineering. Bismuth oxyhalides (BiO_x, where X is Cl, Br or I) and bismuth chalcogenides, including bismuth oxide, bismuth sulfide, bismuth selenide, and bismuth telluride, have been heavily investigated for therapeutic purposes. The pharmacokinetics of these BiNPs can be easily improved via the facile modification of their surfaces with biocompatible polymers and proteins, resulting in enhanced colloidal stability, extended blood circulation, and reduced toxicity. Desirable antibacterial effects, bone regeneration potential, and tumor growth suppression under NIR laser radiation are the main biomedical research areas involving BiNPs that have opened up a new paradigm for their future clinical translation. This review emphasizes the synthesis and state-of-the-art progress related to the biomedical applications of BiNPs with different structures, sizes, and compositions. Furthermore, a comprehensive discussion focusing on challenges and future opportunities is presented.

A procalcitonin photoelectrochemical immunosensor: NCQDs and Sb2S3 co-sensitized hydrangea-shaped WO3 as a matrix through a layer-by-layer assembly

Electron-transfer mechanism of a PEC immunosensor based on WO₃/NCQDs/Sb₂S₃ composites in PBS electrolytes containing AA.

Ultrasensitive and visible light-responsive photoelectrochemical aptasensor for edifenphos based on Zinc phthalocyanine sensitized MoS2 nanosheets

Developing a simple, rapid detection method for the analysis of edifenphos (EDI) is crucial due to its residue is harmful to acetylcholinesterase on the human cellular system, and cause a lot of complications. Herein, we synthesized visible light-responsive MoS₂ nanosheets decorated with Zinc phthalocyanine (ZnPc) nanoparticles (ZnPc/n-MoS₂). Due to the sensitization of ZnPc nanoparticles, the resulting ZnPc/n-MoS₂ exhibited narrower energy bandgap and efficient charge transfer. Especially, the carrier lifetime of ZnPc/n-MoS₂ is 2 more times longer than n-MoS₂, and the photocurrent intensity of ZnPc/n-MoS₂ is 24 times of n-MoS₂ and 22 times of ZnPc nanoparticles under visible light irradiation. Further, a visible light-responsive ultrasensitive photoelectrochemical (PEC) aptasensor for selectivity recognition of EDI was triumphantly established by using EDI aptamer as a biorecognition element, which exhibited a wide linear ranking from 5 ng L^-1 to 10 μg L^-1 (R² = 0.996) and a low detection limit of 1.667 ng L^-1 (S/N = 3). The splendid performance of the ZnPc/n-MoS₂ nanosheet ultrasensitive sensing platform can be applied to detect the concentration of EDI in food, biomedical and environmental analysis.

Ultrathin PtNi nanozyme based self-powered photoelectrochemical aptasensor for ultrasensitive chloramphenicol detection

Nanozymes have gained increasing attention in the field of biosensing. Rationally designed nanozymes with excellent catalytic activity are accessible to substitute natural enzymes. Herein, a novel self-powered photoelectrochemical (PEC) aptasensor was constructed for ultrasensitive detection of chloramphenicol (CAP) based on ultrathin PtNi nanowires (NWs) as nanozyme and benzene-ring doped g-C₃N₄ (BR-CN) as the photoactive material. The prepared 1-nm-thick PtNi nanozyme acted as a peroxidase, possessing higher catalytic activity than natural horseradish peroxidase (HRP) and other Pt-based mimic enzymes. Through the biotin-streptavidin specific interaction, streptavidin modified PtNi nanozyme was introduced into the dual-stranded DNA (dsDNA) formed by complementary DNA and biotinylated CAP aptamer. The PtNi nanozyme catalyzed 4-chloro-1-naphthol (4-CN) oxidation to generate insoluble precipitation on the electrode surface, resulting in an obvious photocurrent reduction. In the presence of CAP, the CAP aptamer was released from the electrode due to strong affinity with CAP, causing the decrease of catalytic precipitation and consequently the generation of a high photocurrent signal. On the basis of PtNi nanozyme signal amplification, the developed self-powered PEC aptasensor showed a wide linear range of 0.1 pM-100 nM with an ultralow detection limit of 26 fM for the determination of CAP. This work provides a feasible strategy for the design of high-activity nanozyme and self-powered PEC biosensor to achieve the ultrasensitive detection of target analyte.

An OFF-ON detection method for copper(ii) ions using a AgAu-NG nanocomposite modified electrode

An OFF-ON detection method for Cu²⁺ was developed at the AgAu bimetallic nanoparticle decorated nitrogen-doped graphene (AgAu-NG) nanocomposite modified electrode. The measurement was based on the copper-catalyzed oxidation of cysteamine (Cys) to regulate the oxidation peak current of Ag. In the absence of Cu²⁺, Cys can bind to the surface of AgAu-NG via the Ag-S or Au-S bond, thus leading to an obvious decrease of the oxidation peak current of Ag. However, in the presence of Cu²⁺, Cu²⁺ can greatly catalyze the oxidation of Cys by dissolved O₂ to form cystamine, which would fall off the surface of AgAu-NG nanocomposites, leading to the partial recovery of the oxidation peak current of Ag. With the increase in the concentration of Cu²⁺, the oxidation peak current of Ag in the presence of Cys increases accordingly. So, the concentration of Cu²⁺ can be measured. By using the optimum conditions, this method can detect Cu²⁺ concentrations down to 0.3 nM (S/N = 3) with a linear response range of 1 nM-1 mM. Furthermore, this method was applied to determine Cu²⁺ concentrations in river water samples and showed excellent analytical performance.

Recent developments of photoelectrochemical biosensors for food analysis

Recent developments of photoelectrochemical biosensors for food analysis are summarized and the future prospects in this field are discussed.

Target-induced photocurrent-polarity switching: a highly selective and sensitive photoelectrochemical sensing platform

Based on target-induced photocurrent-polarity switching, a highly selective and sensitive photoelectrochemical sensing platform was developed.

A sensitive photoelectrochemical methyltransferase activity assay based on a novel “Z-scheme” CdSe QD/afGQD heterojunction and multiple signal amplification strategies

Based on a “Z-scheme” CdSe QD/afGQD heterojunction and multiple signal amplification strategies, a sensitive photoelectrochemical methyltransferase activity assay was carried out.