A self-powered photoelectrochemical aptasensor using 3D-carbon nitride and carbon-based metal-organic frameworks for high-sensitivity detection of tetracycline in milk and water

Antibiotic residues have become a significant challenge in food safety, threatening both ecosystem integrity and human health. To combat this problem, we developed an innovative photo-powered, self-powered aptasensor that employs a novel carbon-doped three-dimensional graphitic carbon nitride (3D-CN) combined with a metal-organic framework composed of N-doped copper(I) oxide-carbon (Cu2O@C) skeletons. The 3D-CN serves as the photoanode, offering stable photocurrent production due to its three-dimensional open framework structure. The N-doped Cu2O@C acts as the photocathode, providing oxidation protection for the metal core and enhancing light absorption due to its metal-organic framework structure. A key feature of our work is exploiting the Fermi level difference between the n-type photoanode and p-type photocathode, which facilitates faster migration of photogenerated electrons toward the photocathode, thereby enhancing the sensor's self-powered effect. Experimental results reveal that upon aptamer loading, the sensor can linearly detect tetracycline (TC) within a range of 0.5 pmol/L to 300 nmol/L, with a detection limit as low as 0.13 pmol/L. It also demonstrates excellent selectivity, stability, and reproducibility, making it applicable to real samples such as milk and river water. Consequently, our research provides a highly efficient and sensitive method for monitoring TC in food, with significant practical implications and profound impacts on food safety.

Detection of Tyrosinase Activity and Inhibitor Validation Based on N-GQDs Fluorescence Sensor

Tyrosinase inhibitors have the ability to resist melanin formation and can be used for clinical and cosmetic, so it is becoming extremely crucial to search a rapid and effective method for detecting t the activity of tyrosinase. In this study, a sensing probe based on Nitrogen-doped graphene quantum dots (N-GQDs) were prepared with carbamide and citric acid. Tyrosinase can oxidize dopamine to dopamine quinone, which can quench the fluorescence of N-GQDs based on the principle of fluorescence resonance energy transfer (FRET) process, and then the detection of tyrosinase activity can be achieved. The result demonstrated that the fluorescence intensity of N-GQDs was a linear correlation with the activity of tyrosinase. Wide detection linear ranges between 0.05 and 5 U/mL and high selectivity. The detection range of tyrosinase was 0.05 to 5 U/mL and LOD of 0.005 U/mL. According to the above, the fluorescence method established in this work could be successfully used for the trace analysis of tyrosinase and it was verified that KA is an inhibitor of tyrosinase.

A dual-photoelectrode fuel cell-driven self-powered aptasensor based on the 1D/2D In2S3/MoS2@Fe-CNTs heterojunction for the ultrasensitive detection of Staphylococcus aureus

A novel dual-electrode photo-fuel cell (PFC)-driven self-powered aptasensor was manufactured for the sensitive and selective detection of Staphylococcus aureus (S. aureus) using the one-dimensional (1D)/2D Schottky heterojunction comprising bimetallic indium/molybdenum sulfide nanosheets and iron-doped carbon nanotube (Fe-CNT) (denoted as In₂S₃/MoS₂@Fe-CNTs) as the photocathode. Given the generation of a robust interface at In₂S₃/MoS₂ and Fe-CNTs, the charge separation and transfer ability of photoexcited electron-hole pairs were enforced, thus improving the output voltage of the assembled PFC. In addition, the numerous active sites of the 1D/2D In₂S₃/MoS₂@Fe-CNTs Schottky heterojunction enabled the immobilization of large amounts of aptamer. Accordingly, the proposed PFC-driven self-powered aptasensor exhibited a wide linear range in 10-1 × 10⁷ CFU mL^-1 with a detection limit of 1.2 CFU mL^-1 toward S. aureus. High selectivity, excellent reproducibility, good stability, and acceptable regenerability, as well as great potential practicality, were also achieved for the detection of S. aureus using the developed PFC-driven self-powered aptasensor. This work not only provides a new photoactive material based on a robust 1D/2D Schottky heterojunction, but also constructs a novel PFC-based self-powered aptasensing strategy based on dual-photoelectrodes and with satisfactory performance for the detection of foodborne pathogens in diverse environments.

A Blue/NIR ratiometric fluorescent probe for intracellular detection of Tyrosinase and the inhibitor screening

A novel ratiometric fluorescent tyrosinase assay is developed based on hybrid nano-assembly of gold nanocluster and tyrosine-containing peptides. The AuNCs@YCY nano-probe (AYNP) is fabricated through the hydrophobic interactions and π-π stacking between the tyrosine residues of the Tyr-Cys-Tyr tripeptide (YCY) and the ligands on the surfaces of AuNCs under the near-isoelectric pH value. The resulted AYNP shows distinct fluorescence responses, spontaneous turn-on of the blue emission and turn-off of the near-infrared emission, with a single wavelength excitation. It is demonstrated that the enhancement and quenching are due to the production of pheomelanin and dopaquinone structures, respectively, induced by tyrosinase oxidation. The internal referencing system provides the tyrosinase assay with superior sensitivity and a detection limit as low as 6.3 U L^-1 could be achieved. The experimental results also demonstrate the excellent selectivity, good photo-stability, and both in vitro and cellular applications of AYNP. This assay technique is low-cost, easy to prepare, and shows excellent potential as a novel melanoma clinical diagnostic platform and a tyrosinase inhibitor screening tool.

Synergy of photocatalysis and fuel cells: A chronological review on efficient designs, potential materials and emerging applications

The rising demand of energy and lack of clean water are two major concerns of modern world. Renewable energy sources are the only way out in order to provide energy in a sustainable manner for the ever-increasing demands of the society. A renewable energy source which can also provide clean water will be of immense interest and that is where Photocatalytic Fuel Cells (PFCs) exactly fit in. PFCs hold the ability to produce electric power with simultaneous photocatalytic degradation of pollutants on exposure to light. Different strategies, including conventional Photoelectrochemical cell design, have been technically upgraded to exploit the advantage of PFCs and to widen their applicability. Parallel to the research on design, researchers have put an immense effort into developing materials/composites for electrodes and their unique properties. The efficient strategies and potential materials have opened up a new horizon of applications for PFCs. Recent research reports reveal this persistently broadening arena which includes hydrogen and hydrogen peroxide generation, carbon dioxide and heavy metal reduction and even sensor applications. The review reported here consolidates all the aspects of various design strategies, materials and applications of PFCs. The review provides an overall understanding of PFC systems, which possess the potential to be a marvellous renewable source of energy with a handful of simultaneous applications. The review is a read to the scientific community and early researchers interested in working on PFC systems.

Zinc-Air Battery-Based Self-Powered Sensor with High Output Power for Ultrasensitive MicroRNA let-7a Detection in Cancer Cells

Self-powered sensors do not require a power supply and are easy to miniaturize, which have potential for constructing wearable, portable, and real-time detection devices. However, it is challenging for the detection of low abundant targets due to the low output power density of fuel cells and much interference of complex biological environment. Herein, a new kind of photocatalytic zinc-air battery-based self-powered electrochemical sensor (ZAB-SPES) was constructed for the detection of microRNA let-7a (miRNA let-7a) by combining magnetic nanobeads (MBs) with a metal-organic framework loaded with glucose oxidase (MOFs@GOX). Poly(1,4-di(2-thienyl))benzene (PDTB) was used as the photocathode material, and the proposed ZAB-SPES had a high power density of 22.8 μW/cm², which was 2-3-fold of commonly used photofuel cells. MBs can capture and separate miRNA from complex samples quickly with a high separation efficiency of 99% within 60 s. The competitive reaction of oxygen reduction reaction between PDTB and MOFs@GOX would change the output power density of the ZAB-SPES. Based on the relationship between output power density and target concentration, the ZAB-SPES realized ultrasensitive detection of miRNA let-7a with a detection limit down to 1.38 fM. Furthermore, the successful detection of miRNA let-7a in A549 cancer cells indicated the great prospects of ZAB-SPES in clinical analysis and early diagnosis of cancers.

Phenolic Tyrosinase Substrate with a Formal Potential Lower than That of Phenol to Obtain a Sensitive Electrochemical Immunosensor

The high and selective catalytic activities of tyrosinase (Tyr) have frequently led to its application in sensitive biosensors. However, in affinity-based biosensors, the use of Tyr as a catalytic label is less common compared to horseradish peroxidase and alkaline phosphatase owing to the fact that phenolic Tyr substrates have yet to be investigated in detail. Herein, four phenolic compounds that have lower formal potentials than phenol were examined for their applicability as Tyr substrates, and three reducing agents were examined as potential strong reducing agents for electrochemical-chemical (EC) redox cycling involving an electrode, a Tyr product, and a reducing agent. The combination of 4-methoxyphenol (MP) and ammonia-borane (AB) allows for (i) a high electrochemical signal level owing to rapid EC redox cycling and (ii) a low electrochemical background level owing to the slow oxidation of AB at a low applied potential and no reaction between MP and AB. When this combination was applied to an electrochemical immunosensor for parathyroid hormone (PTH) detection, a detection limit of 2 pg/mL was obtained. This detection limit is significantly lower than that obtained when a combination of phenol and AB was employed (300 pg/mL). It was also found that the developed immunosensor works well in PTH detection in clinical serum samples. This new phenolic substrate could therefore pave the way for Tyr to be more commonly used as a catalytic label in affinity-based biosensors.

Self-assembled peptides-modified flexible field-effect transistors for tyrosinase detection

Flexible biosensors have received intensive attention for real-time, non-invasive monitoring of cancer biomarkers. Highly sensitive tyrosinase biosensors, which are important for melanoma screening, remained a hurdle. Herein, high-performance tyrosinase-sensing field-effect transistor-based biosensors (bio-FETs) have been successfully achieved by self-assembling nanostructured tetrapeptide tryptophan-valine-phenylalanine-tyrosine (WVFY) on n-type metal oxide transistors. In the presence of target tyrosinase, the phenolic hydroxyl groups in WVFY are rapidly converted to benzoquinone with the consumption of protons, which could be detected potentiometrically by bio-FETs. As a result, the WVFY-modified bio-FETs exhibited an ultra-low detection limit of 1.9 fM and an optimal detection range of 10 fM to 1 nM toward tyrosinase sensing. Furthermore, flexible devices fabricated on ∼2.9-μm-thick polyimide (PI) substrates illustrated robust mechanical flexibility, which could be attached to human skin conformally. These achievements hold promise for wearable melanoma screening and provide designing guidelines for detecting other important cancer biomarkers with bio-FETs.

A membraneless self-powered photoelectrochemical biosensor based on Bi2S3/BiPO4 heterojunction photoanode coupling with redox cycling signal amplification strategy

The photoelectrochemical (PEC) self-powered system has attracted great attention in disease detection. The determination of a simple and efficient approach for disease-related biomarkers is highly interesting and appealing. Herein, an ingenious visible light-induced membraneless self-powered PEC biosensing platform was constructed, integrating a signal amplification strategy for ultrasensitive split-type PEC bioanalysis. The system was comprised of a Bi₂S₃/BiPO₄ heterojunction photoanode and a platinum (Pt) cathode in a one compartment chamber. An alkaline phosphatase (ALP)-loaded sandwich immunoassay was used to generate the signal reporter ascorbic acid (AA) in a 96-well plate, and myoglobin (Myo) was used as a model protein. In the presence of AA, ferrocene (Fc), and Tris (2-carboxyethyl) phosphine (TCEP), the chemical-chemical redox cycling scheme was operated upon the initial oxidation of Fc by the holes in the Bi₂S₃/BiPO₄ photoelectrode, and Fc was regenerated from Fc⁺ by AA. Subsequently, AA was regenerated by TCEP after its oxidation, and cycling was triggered. As a result, the proposed self-powered PEC sensing exhibited excellent performance with a wide linear range from 5.0 × 10^-13 to 1.0 × 10^-7 g/mL, and a low detection limit of 2.0 × 10^-13 g/mL for Myo. This work provided a new design of a redox cycling strategy in the self-powered PEC biosensor, and showed an effective approach for the clinical diagnosis.

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.

Photocatalytic Fuel Cell-Assisted Molecularly Imprinted Self-Powered Sensor: A Flexible and Sensitive Tool for Detecting Aflatoxin B1

The self-powered electrochemical sensor has gained big achievements in energy and devices, but it is challenging in analytical application owing to its low energy conversion efficiency and limited selectivity caused by the plentiful interference in actual samples. Herein, a new self-powered biosensor was constructed by the integration of a photocatalytic fuel cell (PFC) with a molecular imprinting polymer (MIP) to achieve sensitive and specific detection of aflatoxin B1 (AFB₁). Compared with other fuel cells, the PFC owns the advantages of low cost, high energy, good stability, and friendly environment by using light as the excitation source. MoS₂-Ti₃C₂T_x MXene (MoS₂-MX) served as the photoanode material for the first time by forming a heterojunction structure, which can enhance the photocurrent by about 3-fold and greatly improve the photoelectric conversion efficiency. Aiming at the poor selectivity of the self-powered sensor, the MIP was introduced to achieve the specific capture and separation of targets without sample pretreatment. Using the MIP and PFC as recognition and signal conversion elements, respectively, the proposed self-powered biosensor showed a wide dynamic range of 0.01-1000 ng/mL with a detection limit of 0.73 pg/mL, which opened opportunities to design more novel self-powered biosensors and promoted its application in food safety and environmental monitoring.

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.

A portable signal-on self-powered aptasensor for ultrasensitive detection of sulfadimethoxine based on dual amplification of a capacitor and biphotoelectrodes

A one-compartment photofuel cell with two photoelectrodes was combined with a capacitor to develop a portable self-powered sensor for sulfadimethoxine (SDM) detection. The developed sensor was applied to the assay of SDM in veterinary drug samples with desirable accuracy and precision.

Increase of tyrosinase activity at the wound site in zebrafish imaged by a new fluorescent probe

Tyrosinase plays a pivotal role in the hyperpigmentation of wounds. Here, we develop a new fluorescent probe and with it, we reveal an increase of tyrosinase activity at the wound site in zebrafish.

Chemical-chemical redox cycling amplification strategy in a self-powered photoelectrochemical system: a proof of concept for signal amplified photocathodic immunoassay

A chemical-chemical redox cycling amplification strategy was introduced into a photocathodic immunosensing system. To prove the applicability of the method, a novel self-powered photochemical system by integrating the photoanode and photocathode was designed for protein analysis.

Novel Bi2+xWO6 p-n Homojunction Nanostructure: Preparation, Characterization, and Application for a Self-Powered Cathodic Photoelectrochemical Immunosensor

Synthesizing novel cathodic photoactive materials with high photoelectrochemical (PEC) performance is urgently important for the development of photocathodic sensors. Herein, a novel photocathode material, Bi self-doped Bi₂WO₆ (i.e., Bi_2+xWO₆) p-n homojunction, is prepared via a simple ethylene glycol-assisted solvothermal reduction for the first time. Compared with pristine Bi₂WO₆, Bi_2+xWO₆ possesses a narrower band gap and higher light harvesting ability. Among the synthesized materials, Bi_2.1WO₆ exhibits the highest photocurrent response, which is 23 times that of pure Bi₂WO₆ because of the synergistic effect of doped Bi and the p-n homojunction. The open circuit potential, "V-shaped" Mott-Schottky plot, linear sweep voltammetry curve, and transient photocurrent demonstrate the p-n homojunction characteristics of the material well. By using the Bi_2+xWO₆ p-n homojunction as the photocathode for sensing and the plasmonic WO₃/Au composite as the photoanode for signal amplification, a new self-powered membraneless PEC immunosensor is established for a highly sensitive detection of human epididymal protein 4. This study offers a new idea for designing novel photocatalysts with satisfactory performance, and the Bi_2+xWO₆ p-n homojunction is expected to act as a promising PEC platform for developing various self-powered biosensors.

A self-powered aptasensor using the capacitor-amplified signal of a photofuel cell and a portable digital multimeter readout

A self-powered aptasensor for streptomycin detection was constructed with a photofuel cell combined with a capacitor and a digital multimeter. The sensitivity of the proposed sensor was 8.7 times of that without using a capacitor amplifier circuit.