Oligonucleotide-modulated photocurrent enhancement of a tetracationic porphyrin for label-free homogeneous photoelectrochemical biosensing

Development of a label-free photoelectrochemical immunosensor for novel astrovirus detection

Since 2017, an infectious goose gout disease characterized by urate precipitation in viscera, mainly caused by novel goose astrovirus (GoAstV) infection, has emerged in the main goose-producing region of China. The current challenge in managing goose gout disease is largely due to the absence of a rapid and efficient detection method for the GoAstV pathogen. Notably, the potential application of immunosensors in detecting GoAstV has not yet been explored. Herein, a label-free PEC immunosensor was fabricated by using purchased TiO2 as the photoactive material and antibody against GoAstV P2 proteins as the specific recognition element. First, we successfully expressed the capsid spike domain P2 protein of ORF2 from GoAstV CHSH01 by using the pET prokaryotic expression system. Meanwhile, the polyclonal antibody against GoAstV capsid P2 protein was produced by purified protein. To our knowledge, this is the first establishment and preliminary application of the label-free photoelectrochemical immunosensor method in the detection of AstV. The PEC immunosensor had a linear range of 1.83 fg mL-1 to 3.02 ng mL-1, and the limit of detection (LOD) was as low as 0.61 fg mL-1. This immunosensor exhibited high sensitivity, great specificity, and good stability in detecting GoAstV P2 proteins. To evaluate the practical application of the immunosensor in real-world sample detection, allantoic fluid from goose embryos was collected as test samples. The results indicated that of the eight positive samples, one false negative result was detected, while both negative samples were accurately detected, suggesting that the constructed PEC immunosensor had good applicability and practical application value, providing a platform for the qualitative detection of GoAstV.

Immobilization-free dual-aptamer-based photoelectrochemical platform for ultrasensitive exosome assay

Exosomes, involved in cancer-specific biological processes, are promising noninvasive biomarkers for early diagnosis of cancer. Herein, an immobilization-free dual-aptamer-based photoelectrochemical (PEC) biosensor was proposed for the enrichment and quantification of cancer exosome based on photoactive bismuch oxyiodide/gold/cadmium sulfide (BiOI/Au/CdS) composites, nucleic acid-based recognition and signal amplification. In this biosensor, the recognition of exosome by two aptamers would trigger the deoxyribonucleotidyl transferase (TdT) enzyme-aided polymerization, leading to the enrichment of alkaline phosphatase (ALP) on Fe3O4 surface. After magnetic separation, ALP could catalyze the generation of ascorbic acid (AA) as electron donor and initiate the following redox cycle reaction for further signal amplification. Furthermore, all the above processes were performed in solution, the recognition and signal amplification efficiency would be superior than the heterogeneous strategy owing to the avoidance of steric hindrance effect. As a result, the proposed PEC biosensor was capable of enriching and detecting of cancer exosomes with high sensitivity and selectivity. The linear range of the biosensor was from 1.0 × 102 particles·μL-1 to 1.0 × 106 particles·μL-1 and the detection limit was estimated to be 21 particles·μL-1. Therefore, the proposed PEC biosensor holds great promise in quantifying tumor exosome for nondestructive early clinical cancer diagnosis and various other bioassay applications.

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.

Organic-Inorganic Porphyrinoid Frameworks for Biomolecule Sensing

Porphyrinoids and their analogous compounds play an important role in biosensing applications on account of their unique and versatile catalytic, coordination, photophysical, and electrochemical properties. Their remarkable arrays of properties can be finely tuned by synthetically modifying the porphyrinoid ring and varying the various structural parameters such as peripheral functionalization, metal coordination, and covalent or physical conjugation with other organic or inorganic scaffolds such as nanoparticles, metal-organic frameworks, and polymers. Porphyrinoids and their organic-inorganic conjugates are not only used as responsive materials but also utilized for the immobilization and embedding of biomolecules for applications in wearable devices, fast sensing devices, and other functional materials. The present review delineates the impact of different porphyrinoid conjugates on their physicochemical properties and their specificity as biosensors in a range of applications. The newest porphyrinoid types and their synthesis, modification, and functionalization are presented along with their advantages and performance improvements.

Novel Aggregation-Enhanced PEC Photosensitizer Based on Electrostatic Linkage of Ionic Liquid with Protoporphyrin IX for Ultrasensitive Detection of Molt-4 Cells

Nowadays, aggregation quenching of most organic photosensitizers in aqueous media seriously restricts analytical and biomedical applications of photoelectrochemical (PEC) sensors. In this work, an aggregation-enhanced PEC photosensitizer was prepared by electrostatically bonding protoporphyrin IX (PPIX) with an ionic liquid of 1-butyl-3-methylimidazole tetrafluoroborate ([BMIm][BF₄]), termed as PPIX-[BMIm] for clarity. The resultant PPIX-[BMIm] showed weak photocurrent in pure dimethyl sulfoxide (DMSO, good solvent), while the PEC signals displayed a 44.1-fold enhancement in a water (poor solvent)/DMSO binary solvent with a water fraction (f_w) of 90%. Such PEC-enhanced mechanism was critically studied by electrochemistry and density functional theory (DFT) calculation in some detail. Afterward, a label-free PEC cytosensor was built for ultrasensitive bioassay of acute lymphoblastic leukemia (molt-4) cells by electrodepositing Au nanoparticles (Au NPs) on the PPIX-[BMIm] aggregates and sequential assembly of protein tyrosine kinase (PTK) aptamer DNA (aptDNA). The resultant cytosensor showed a wide linear range (300 to 3 × 10⁵ cells mL^-1) with a limit of detection (LOD) as low as 63 cells mL^-1. The aggregation-enhanced PEC performance offers a valuable and practical pathway for synthesis of advanced organic photosensitizer to explore its PEC applications in early diagnosis of tumors.

Homogeneous photoelectrochemical biosensor for microRNA based on target-responsive hydrogel coupled with exonuclease III and nicking endonuclease Nb.BbvCI assistant cascaded amplification strategy

MicroRNAs can serve as biomarkers for many cancers, so it is significant to develop simple and sensitive strategies for microRNAs detection. Photoelectrochemical (PEC) detection has the advantages of simple equipment and high sensitivity. But in conventional PEC DNA sensors, tedious immobilization procedures of photoactive materials and capture probes on electrode surfaces are inevitable. To overcome those limitations, a homogeneous PEC biosensor based on target-responsive hydrogels has been developed (miRNA-155 has been chosen as a model target). PEC signal molecules (TiO₂ nanoparticles, TiO₂ NPs) were embedded in DNA hydrogels formed by hyaluronic acid sodium salt, amine-modified DNA double strands, and polyethylenimine rich in amine groups. In the presence of the target, DNA double strands in hydrogel were nicked by endonuclease and TiO₂ NPs were released to the supernate and a high PEC response was obtained when collecting the supernate for PEC test, while almost no TiO₂ NPs released in the absence of the target. Thanks to the exonuclease III and nicking endonuclease Nb.BbvCI-assisted cascaded amplification strategy, the proposed biosensor exhibits high sensitivity toward miRNA-155 with a low detection limit of 0.41 fM and a wide linear range from 1.0 fM to 100 pM. Since this method circumvents tedious electrode modification procedures, the proposed technique exhibits the advantages of simplicity and good reproducibility. Moreover, the prepared hydrogels have outstanding storage stability, so that they can be prepared in advance and shorten detection time. This biosensing platform provides a versatile strategy for the construction of homogeneous PEC biosensors for the detection of diverse targets. Photoelectrochemical detection techniques have been coupled with controlled release system to develop an immobilization-free microRNA biosensor. High sensitivity has been realized based on cascaded signal amplification strategy, and the proposed biosensor has been applied to detect the target in real sample with satisfied results. Since no tedious electrode modifications, the proposed homogeneous PEC sensor exhibits high reproducibility and good stability.

Design of ultrathin nanosheet subunits ZnIn2S4 hollow nanocages with enhanced photoelectric conversion for ultrasensitive photoelectrochemical sensing

Herein, a high-efficiency photoactive material, hollow ZnIn₂S₄ nanocages (ZIS-HNCs) composed of ultrathin nanosheets were creatively synthesized via a metal-organic framework (MOF) derived solvothermal method. It had been specified the underlying mechanism of the ZIS-HNCs evolution under the MOF templated surface. Subsequently, the obtained ZIS-HNCs combined with annealing TiO₂ modified electrode (ZIS-HNCs@TiO₂), and the ZIS-HNCs@TiO₂ exhibited intense transient photocurrent. The enhanced photocurrent signal benefited from the multiple light capture effect of ZIS-HNCs, ultrathin nanosheet subunits of ZIS-HNCs, and typical type Ⅱ heterojunction, which could effectively retard the photoexcited electron-hole pairs recombination, and accelerated charge separation and transfer. Taking antibiotic lincomycin (Lin) as a model, a signal-off photoelectrochemical (PEC) aptasensor based on the ZIS-HNCs@TiO₂ was established and manifested a high sensitive detection for Lin with a linear response range from 0.0001 to 0.1 nM as well as an ultralow detection limit of 0.084 pM. Additionally, the proposed PEC aptasensor showed acceptable stability and remarkable selectivity. Therefore, this study provides a promising strategy to design nanomaterials with superior photoelectric activity for PEC sensing applications.

Porphyrins as Colorimetric and Photometric Biosensors in Modern Bioanalytical Systems

Advances in porphyrin chemistry have provided novel materials and exciting technologies for bioanalysis such as colorimetric sensor array (CSA), photo-electrochemical (PEC) biosensing, and nanocomposites as peroxidase mimetics for glucose detection. This review highlights selected recent advances in the construction of supramolecular assemblies based on the porphyrin macrocycle that provide recognition of various biologically important entities through the unique porphyrin properties associated with colorimetry, spectrophotometry, and photo-electrochemistry.

A label-free photoelectrochemical aptasensor for facile and ultrasensitive mercury ion assay based on a solution-phase photoactive probe and exonuclease III-assisted amplification

In typical photoelectrochemical (PEC) biosensing assays, electrodes are generally modified with photoactive probes and/or target recognition probes, which makes the processes complicated, time-consuming, and difficult to achieve excellent reproducibility. Hence, to overcome such shortcomings, we propose here an immobilization-free and label-free PEC aptasensor using solution-phase methylene blue (MB) as the PEC signal probe. Based on the unique T-Hg²⁺-T base pairs, and the diffusivity difference between free MB molecules and the MB/G-quadruplex composite towards the ITO electrode surface with negative charge, the "signal-off" approach for Hg²⁺ detection is developed. In the presence of target Hg²⁺, via the T-Hg²⁺-T bond formation, the two sticky ends of the hairpin DNA probe form a rigid duplex stem, which triggers the exonuclease III-facilitated target cycling amplification, and the formation of multiple G-quadruplexes. Upon the intercalation of MB in G-quadruplexes, significantly decreased photocurrent is obtained owing to the increased electrostatic repulsion between the MB/G-quadruplex composite and the ITO electrode. Therefore, highly sensitive and ultrasensitive Hg²⁺ determination is achieved, with a low detection limit of 1.2 pM, well below the maximum allowable Hg²⁺ level in drinking water defined by the WHO, China's Ministry of Health, and the US EPA. Due to the avoidance of sophisticated electrode modification and recognition probe immobilization processes, as well as an expensive labeling procedure, the PEC aptasensor proposed here demonstrates the advantages of simplicity, good reproducibility, rapidness and low cost.

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.