Photocurrent switchable dual-target bioassay: Signal distinction and interface reconfiguration via pH-responsive triplex DNA programming

Most multiplexed photoelectrochemical (PEC) sensors require additional instrumentation and cumbersome electrode modification and surface partitioning, which limits their portability and instrument miniaturization. Herein, a pH-responsive programmable triple DNA nanomachine was developed for constructing a reconfigurable multiplex PEC sensing platform. By programming the base sequence, T-A·T-riched triple DNA was designed to construct integrated nano-controlled release machine (INCRM) for simultaneous recognition of multiple targets. The INCRM enables to recognize two targets in one step, and sequentially separate the signal labels by pH adjustment. The detached signal label catalyzes glucose to produce gluconic acid, causing the C-riched DNA fold into a triple structure on the electrode surface. As a result, one target can be detected relying on the enhanced photocurrent due to accelerated electron transfer between the CdS QD labeled at the end of C-riched DNA and the electrode. The triplex DNA dissociation in pH 7.4 buffer reconfigures the electrode interface, which can be continued to detect another target. The feasibility of the multiplexed sensor is verified by the detection of extensively coexisting antibiotics enrofloxacin (ENR) and ciprofloxacin (CIP). Under the optimal conditions, wide linear range (10 fg/mL ∼ 1 μg/mL) and low detection limit (3.27 fg/mL and 9.60 fg/mL) were obtained. The pH-regulated programmable triplex DNA nanomachine-based sensing platform overcomes the technical difficulties of conventional multiplexed PEC assay, which may open the way for miniaturization of multiplexed PEC sensors.

An in situ ratiometric fluorescence immunosensor via Mn2+-triggered aggregation-induced emission transformation of levodopa fluorescent copolymer nanoparticles

Herein, a direct in situ alkaline phosphatase (ALP)-labeled luminescent nanoimmunoassay platform was constructed using Mn2+-triggered aggregation-induced emission transformation of levodopa fluorescent copolymer (LFC) nanoparticles. Using cardiac troponin I (cTn I) as the model antigen, the proposed nanoimmunosensor has been applied to detect cTn I in clinical samples with satisfactory results.

Digital multimeter-based portable photoelectrochemical immunoassay with enzyme-catalyzed precipitation for screening carbohydrate antigen 125

The degree of the carbohydrate antigen 125 (CA-125) level in serum is positively correlated with the severity of ovarian cancer. In this study, a facile photoelectrochemical (PEC) immunoassay was devised for sensitive detection of CA-125 employing enzyme-catalyzed precipitation to weaken the photocurrent of hollow porous In2O3 nanotubes incorporating CdS nanoparticles. Upon the addition of the target analyte, horseradish peroxidase (HRP) enriches as a result of the formation of the sandwich immunocomplex, which can catalyze the conversion of 4-chloro1-naphthol (4-CN) to benzo-4-chlorohexadienone (4-CD) employing H2O2 as a cofactor. The as-produced insoluble precipitate acts as an obstacle to hinder the absorption of visible light by photoactive materials, thereby resulting in a decrease in photocurrent. Moreover, the weakened signal can be easily read out by a digital multimeter (DMM), advancing the convenience of the detection system. The preliminary analysis data indicate that the PEC immunoassay shows an efficient response to CA-125 levels ranging from 0.1 to 100 U mL-1 with a limit of detection (LOD) as low as 0.046 U mL-1 (S/N = 3). Most importantly, the proposed portable method has shown satisfactory performance in terms of selectivity, reproducibility, stability, and analysis in complex biological matrices.

A self-driven, microfluidic, integrated-circuit biosensing chip for detecting four cardiovascular disease biomarkers

Cardiovascular diseases (CVDs) claimed the lives of nearly 21 million people worldwide in 2021, accounting for 30% of global deaths. However, one in five CVD patients is unaware that they have the disease, emphasizing the need for accurate biomarker monitoring. Herein we developed an integrated microfluidic system (IMS) for rapid quantification of four CVD biomarkers, including N-terminal pro B-type natriuretic peptide (NT-proBNP), fibrinogen, cardiac troponin I (cTnI), and C-reactive protein (CRP)- via aptamer-coated interdigitated electrodes (IDE) with integrated circuits (IC) and a self-driven IMS for sample treatment. The device was composed of plasma filtration, metering, and fluidic delay modules, and the former could extract 45% of plasma from a 20-μL blood sample; the metering module could quantify 5 μL of plasma within 90 s. Subsequently, the plasma was transported to a detection chamber, where IC-based IDE sensors made measurements within 5 min. The entire 15-min process allowed us to evaluate biomarkers across a wide dynamic range: NT-proBNP (0.1-10,000 pg/mL), fibrinogen (50-1,000 mg/dL), cTnI (0.1-10,000 pg/mL), and CRP (0.5-9 mg/L). Given that spiked blood samples were measured with reasonable accuracy (>80%), the IMS could see utility in CVD risk assessment and personalized medicine.

Oxygen Vacancy Induced Atom-Level Interface in Z-Scheme SnO2/SnNb2O6 Heterojunctions for Robust Solar-Driven CO2 Conversion

The modulation of Z-scheme charge transfer is essential for efficient heterostructure toward photocatalytic CO₂ reduction. However, constructing a compact hetero-interface favoring the Z-scheme charge transfer remains a great challenge. In this work, an interfacial Nb-O-Sn bond and built-in electric field-modulated Z-scheme O_v-SnO₂/SnNb₂O₆ heterojunction was prepared for efficient photocatalytic CO₂ conversion. Systematic investigations reveal that an atomic-level interface is constructed in the O_v-SnO₂/SnNb₂O₆ heterojunction. Under simulated sunlight irradiation, the obtained O_v-SnO₂/SnNb₂O₆ photocatalyst exhibits a high CO evolution rate of 147.4 μmol h^-1 g^-1 from CO₂ reduction, which is around 3-fold and 3.3-fold of SnO₂/SnNb₂O₆ composite and pristine SnNb₂O₆, respectively, and favorable cyclability by retaining 95.8% rate retention after five consecutive tests. As determined by electron paramagnetic resonance spectra, in situ Fourier transform infrared spectra, and density functional theory calculations, Nb-O-Sn bonds and built-in electric field induced by the addition of oxygen vacancies jointly accelerate the Z-scheme charge transfer for enhanced photocatalytic performance. This work provides a promising route for consciously modulating Z-scheme charge transfer by atomic-level interface engineering to boost photocatalytic performance.

Self-Cleaning Recyclable Multiplexed Photoelectrochemical Sensing Strategy Based on Exonuclease III-Assisted Signal Discrimination

For discriminating the signals of multi-targets, multiplexed photoelectrochemical (PEC) detection is generally accomplished by modulating the light source or voltage, which prospect is usually limited by expensive instrumentation, tedious operational steps, and time-consuming material screening. To realize multiplexed determination on single photoelectric interface using the routine technique, a non-instrument-assisted strategy for signal discrimination needs to be explored. Herein, we propose an exonuclease III-mediated multiple PEC signals resolution strategy and construct a self-cleaning recyclable multiplexed PEC sensor using a porphyrin-bipyridine-based covalent organic framework (Por-Bpy COF) photocathode. Specifically, following the dual-target recognition event, exonuclease III cleaves the DNA strand attached to the magnetic bead so that the two signal labels can be separated. Once the signal label binds to the DNA on the electrode surface (E-DNA), exonuclease III turns to excise the DNA strand of the signal label and consequently the E-DNA can repeatedly bind different signal labels. As a result, distinguishable photocurrent signals of different targets can be generated on a single photoelectric interface. The feasibility of this multiplexed sensor is verified by detecting two coexisting mycotoxins aflatoxin B1 and zearalenone. On account of eliminating the instrumentation constraints and simplifying the experimental procedures, the proposed sensing strategy may provide a brand-new idea for the exploration of portable multiplexed PEC sensing devices.

Light-Addressable Electrochemical Sensors toward Spatially Resolved Biosensing and Imaging Applications

The light-addressable electrochemical sensor (LAES) is a recently emerged bioanalysis technique combining electrochemistry with the photoelectric effect in a semiconductor. In an LAES, a semiconductor substrate is illuminated locally to generate charge carriers in a well-defined area, thereby confining the electrochemical process to a target site. Benefiting from the unique light addressability, an LAES can not only detect multiple analytes in parallel within a single sensor plate but also act as a bio(chemical) imaging sensor to visualize the two-dimensional distribution of specific analytes. An LAES usually has three working modes: a potentiometric mode using light-addressable potentiometric sensors (LAPS) and an impedance mode using scanning photoinduced impedance microscopy (SPIM), while an amperometric mode refers to light-addressable electrochemistry (LAE) and photoelectrochemical (PEC) sensing. In this review, we describe the detection principles of each mode of LAESs and the concept of light addressability. In addition, we highlight the recent progress and advance of LAESs in spatial resolution, sensor system design, multiplexed detection, and bio(chemical) imaging applications. An outlook on current research challenges and future prospects is also presented.

Addressable Label-Free Photoelectric Sensor Array with Self-Calibration for Detection of Neuron Specific Enolase

An addressable label-free photoelectric immunosensor array was designed for detection of neuron specific enolase (NSE) based on TiO₂/CdS as substrate materials. In this work, the hydrothermal synthesized TiO₂ nanorod film is evenly grown on the surface of the fluorine-doped tin oxide (FTO), and then CdS with a narrow band gap is added for sensitization through successive ionic layer adsorption reactions. The obtained TiO₂/CdS composite materials with matched energy band structures promote the rapid electron transfer and effectively reduce the recombination of electron hole pairs, which greatly enhance the visible light absorption and increased photocurrent intensity. In order to construct a suitable sensor array, the sensitized FTO electrode is divided into multiple regions of equal size by insulating stickers, and then the addressable and continuous detection of multiple samples can be achieved. Because multiple detection regions are prepared and tested under the same conditions, the difference effectively reduces, and the sensor can realize self-calibration and obtain more accurate results. Under optimal conditions, this sensor array can detect NSE in the linear range of 0.01-100 ng mL^-1 with a detection limit of 2.49 pg mL^-1 (S/N = 3). The sensor array has good selectivity, stability, and reproducibility, making it a viable approach for real sample detection.

Exploiting Photoelectric Activities and Piezoelectric Properties of NaNbO3 Semiconductors for Point-of-Care Immunoassay

Point-of-care testing (POCT) technology has made major breakthroughs in community medicine and physician office situations, in tandem with the more ubiquitous and intensive usage of highly integrated quick detection equipment for illness diagnosis, personal care, and mobile healthcare. Although the photoelectrochemical (PEC)-based POCT platform offers the benefits of cheap cost and good user engagement, its commercialization is still limited by the photodetection components' downsizing and mobility, among other factors. In this work, a novel highly integrated PEC biosensor aided by piezophototronics to enhance the efficiency of PEC testing was reported for flexible detection of cancer-associated antigens in biological fluids (prostate-specific antigen, PSA, used as an example). Multiple signal enhancement strategies, including a magnetic bead-linked enzyme-linked immune system catalyzing the production of ascorbic acid from the substrate and a piezoelectric-assisted enhancement strategy, were used for sensitive detection of the analyte to be tested in human body fluids. Unlike the electron transfer mechanism in heterojunctions, piezoelectric semiconductors promote the transfer of electrons and holes by generating piezoelectric potentials in the ultrasonic field, thus contributing to the performance of the PEC testbed. Under optimized conditions, the test platform achieves good correspondence for PSA at 0.02-40 ng mL-1. Impressively, the test devices are comparable to or even superior to gold standard ELISA kits in terms of cost approval and batch testing. This research demonstrates the potential of piezoelectric semiconductors for POC applications in revolutionary PECs and offers innovative thoughts for the development of new PEC bioanalytical components.

Enhanced photoactivity of ZnPc@WS2 heterojunction by CuBi2O4 and its application for photoelectrochemical detection of 5-formyl-2'-deoxycytidine

The endogenous epigenetic marker 5-formylcytosine (5 fC) is introduced by 5-methylcytosine (5 mC) oxidation under action of enzyme oxidation, and plays an important role in many life activities. Since the content of 5 fC in mammalian tissues and cells is very low, it is necessary to exploit a sensitive and specific detection method to further understand the function of 5 fC. In this work, a sensitively and selectively photoelectrochemical (PEC) biosensor was developed for 5-formyl-2'-deoxycytidine (5fdC) detection. CuBi₂O₄/ZnPc@WS₂ was used as photoactive material, where the formed ternary heterojunction structure greatly enhanced the PEC response and increased the detection sensitivity. Positively charged polyethyleneimine (PEI) was employed as 5fdC recognition and capture unit, where the amine group on PEI specifically reacted with aldehyde group of 5fdC to form stable amide bond. 4-Carboxyphenylboronic acid (4-CPBA) was adopted as crosslinker for 5fdC and amino functionalized CuBi₂O₄ based on the covalent interaction between 1,3-diol bond on 5fdC and boric acid structure on 4-CPBA, and the covalent interaction between -COOH on 4-CPBA and -NH₂ on amino functionalized CuBi₂O₄. On the basis of the positive synergistic effect of ZnPc and CuBi₂O₄ on improving the photoelectric performance of WS₂, the separation of photo-generated electron-hole pairs in semiconductors were promoted, and the examination range was expanded from 0.1 to 500 nM, and the detection limit was 0.0483 nM (3σ). Based on the unique covalent reaction between -NH₂ and -CHO, the PEC biosensor has excellent detection sensitivity, and can even separate 5fdC from 5-methylcytosine deoxyribonucleoside and 5-hydroxymethylcytosine deoxyribonucleoside. The effect of antibiotics and heavy metals on the 5fdC content in wheat tissue genome has also been further investigated using this sensor.

A critical review on the use of potentiometric based biosensors for biomarkers detection

Potentiometric-based biosensors have the potential to advance the detection of several biological compounds and help in early diagnosis of various diseases. They belong to the portable analytical class of biosensors for monitoring biomarkers in the human body. They contain ion-sensitive membranes sensors can be used to determine potassium, sodium, and chloride ions activity while being used as a biomarker to gauge human health. The potentiometric based ion-sensitive membrane systems can be coupled with various techniques to create a sensitive tool for the fast and early detection of cancer biomarkers and other critical biological compounds. This paper discusses the application of potentiometric-based biosensors and classifies them into four major categories: photoelectrochemical potentiometric biomarkers, potentiometric biosensors amplified with molecular imprinted polymer systems, wearable potentiometric biomarkers and light-addressable potentiometric biosensors. This review demonstrated the development of several innovative biosensor-based techniques that could potentially provide reliable tools to test biomarkers. Some challenges however remain, but these can be removed by coupling techniques to maximize the testing sensitivity.

A Multiplexed Self-Powered Dual-Photoelectrode Biosensor for Detecting Dual Analytes Based on an Electron-Transfer-Regulated Conversion Strategy

Due to the inherent mechanism limitation of photocatalytic fuel cell based self-powered biosensors (PFC-SPBs), it was difficult to distinguish the power density of various photoactive materials or recognition events in one detection process, which made it lack multitarget quantitative capacity. In order to solve this problem, we proposed an electron-transfer-regulated conversion strategy for the construction of multiplexed PFC-SPBs. Herein, the n-type CdS/Fe₂O₃ nanorod array (NR) heterojunction and p-type CuBr semiconductor were used as photoactive materials to prepare the photoanode and photocathode. Based on the appropriate Fermi level differentiation between these two photoelectrodes, a self-powered sensing platform driven by visible light without external energy supply was achieved. In this design, two kinds of common and easily coexisting mycotoxins OTA and AFB1 acted as model analytes. The coupling of "signal off" and "signal on" was realized by controlling the electronic transmission on the interface between the photoanode and photocathode, so as to achieve the simultaneous detection of two mycotoxins. This work established a proof-of-concept for the integration of a dual-photoelectrode with dual-assay that could provide the innovative inspiration for the formation of a general multiplexed self-powered sensing platform.

Facile one-step hydrothermal synthesis of single-crystalline SnNb2O6 nanosheets with greatly extended visible-light response for enhanced photocatalytic performance and mechanism insight

The conventional preparation of SnNb₂O₆ invariably involves complex and laborious steps, which unavoidably introduces defect into the host lattice and also increases the reaction period and costs, resulting in undesired recombination of photo-generated electron-hole pairs. For the first time in this work, we manage to synthesize single-crystalline two-dimensional (2D) SnNb₂O₆ nanosheets with ultrathin structure through a facile one-step hydrothermal method. Comparative studies were explored to analyze the structure and phase evolution during the preparation course. The synthesized 2D structure demonstrated a narrower band gap of 2.09 eV and specific surface area of 76.1 m² g^-1, which exhibited significantly extended visible-light-responsive range and larger surface area by contrast with the state-of-the-art reports, resulting in excellent visible-light-driven photoactivity towards H₂ production and water purification as well. Additionally, further enhanced photocatalytic performance was achieved by the incorporation of Pt as co-catalyst to indirectly indicate the advantage of the SnNb₂O₆ nanosheets in this method over other reported counterparts. It was found that, a very small amount of Pt loaded on the surface of SnNb₂O₆ nanosheets would contribute to remarkably higher activity than pure SnNb₂O₆ nanosheets and exhibit superior stability as well. Moreover, a deep insight into the underlying photocatalytic mechanism was proposed. This work sheds light on a new facile way to fabricate high-performance photocatalytic materials and provided new opportunities for solar-energy conversion.

Light-controlled imaging of biocatalytic reactions via scanning photoelectrochemical microscopy for multiplexed sensing

A light-controlled multiplexing platform has been developed on the basis of a quantum dot-sensitized inverse opal TiO₂ electrode with integrated biocatalytic reactions. Spatially resolved illumination enables multiplexed sensing and imaging of enzymatic oxidation reactions at relatively negative applied potentials.