Recent advances in photoelectrochemical platforms based on porous materials for environmental pollutant detection

Human health and ecology are seriously threatened by harmful environmental contaminants. It is essential to develop efficient and simple methods for their detection. Environmental pollutants can be detected using photoelectrochemical (PEC) detection technologies. The key ingredient in the PEC sensing system is the photoactive material. Due to the unique characteristics, such as a large surface area, enhanced exposure of active sites, and effective mass capture and diffusion, porous materials have been regarded as ideal sensing materials for the construction of PEC sensors. Extensive efforts have been devoted to the development and modification of PEC sensors based on porous materials. However, a review of the relationship between detection performance and the structure of porous materials is still lacking. In this work, we present an overview of PEC sensors based on porous materials. A number of typical porous materials are introduced separately, and their applications in PEC detection of different types of environmental pollutants are also discussed. More importantly, special attention has been paid to how the porous material's structure affects aspects like sensitivity, selectivity, and detection limits of the associated PEC sensor. In addition, future research perspectives in the area of PEC sensors based on porous materials are presented.

Nano-enabled sensing of per-/poly-fluoroalkyl substances (PFAS) from aqueous systems - A review

Per-/poly-fluoroalkyl substances (PFAS) are an emerging class of environmental contaminants used as an additive across various commodity and fire-retardant products, for their unique thermo-chemical stability, and to alter their surface properties towards selective liquid repellence. These properties also make PFAS highly persistent and mobile across various environmental compartments, leading to bioaccumulation, and causing acute ecotoxicity at all trophic levels particularly to human populations, thus increasing the need for monitoring at their repositories or usage sites. In this review, current nano-enabled methods towards PFAS sensing and its monitoring in wastewater are critically discussed and benchmarked against conventional detection methods. The discussion correlates the materials' properties to the sensitivity, responsiveness, and reproducibility of the sensing performance for nano-enabled sensors in currently explored electrochemical, spectrophotometric, colorimetric, optical, fluorometric, and biochemical with limits of detection of 1.02 × 10^-6 μg/L, 2.8 μg/L, 1 μg/L, 0.13 μg/L, 6.0 × 10^-5 μg/L, and 4.141 × 10^-7 μg/L respectively. The cost-effectiveness of sensing platforms plays an important role in the on-site analysis success and upscalability of nano-enabled sensors. Environmental monitoring of PFAS is a step closer to PFAS remediation. Electrochemical and biosensing methods have proven to be the most reliable tools for future PFAS sensing endeavors with very promising detection limits in an aqueous matrix, short detection times, and ease of fabrication.

A novel electrochemical sensor for the detection of fipronil and its toxic metabolite fipronil sulfone using TiO2-polytriazine imide submicrostructured composite as an efficient electrocatalyst

For the first time, a simple and sensitive electrochemical sensor based on a screen printed electrode (SPE) modified with titanium dioxide (TiO₂) and polytriazine imide submicrostructured composite (TiO₂-PTI) has been developed for the simultaneous detection of fipronil (FIP) and its toxic metabolite fipronil sulfone (FIP-S). The submicrostructured composite material based on TiO₂ and PTI was obtained by simple hydrothermal treatment of the Ti peroxocomplexes in the presence of pristine. This carbon nitride allotrope has better crystallinity and conductivity than its graphitic analog. It was found that the TiO₂-PTI submicrostructured composite enhanced the electrochemical sensing of the SPE electrode towards FIP and its metabolite FIP-S in 0.1 M Britton-Robinson buffer (pH 10) at the oxidation potentials of 0.82 V and 0.94 V, respectively. In addition, it showed good stability and reproducibility for the determination of both analytes. Under optimal conditions, the peak currents by square wave voltammetry were found to vary linearly with FIP and FIP-S concentrations in the range from 0.01 to 10 μM and from 10 to 50 μM, with a detection limit of 8.42 nM, 3.6 μg/kg for FIP and 9.72 nM, 4.04 μg/kg for FIP-S. This sensor was successfully used to detect FIP and FIP-S in eggs and water samples with good recoveries of 90%-106.6%.

Activating the Neutral pH Photozymatic Activity of g-C3N4 Nanosheet through Post-Synthetic Incorporation of Pt

The catalytic activity of photozyme can be regulated through light irradiation time and intensity, but it still suffers from low activity in physiological neutral pH (typically, pH < 5). Herein,...

Intense Visible-Light Absorption in SrRuO3/C3N4 Heterostructures for the Highly Efficient Reduction of Hg(II)

Strontium ruthenium oxide (SrRuO₃) is recognized as a metallic itinerant ferromagnet and utilized as a conducting electrode in heterostructure oxides with unforeseen optical characteristics, including remarkably low-reflection and high-absorption visible-light spectrum compared to classical metals. By coupling mesoporous SrRuO₃ nanoparticles (NPs) with porous g-C₃N₄ nanosheets for the first time, we evidence remarkably promoted visible light absorption and superior photocatalytic performances for Hg(II) reduction under illumination with visible light. The photocatalytic performance of g-C₃N₄ increased upon boosting the SrRuO₃ percentage to 1.5%, and this (1.5% SrRuO₃/g-C₃N₄ heterostructure) is considered the optimum condition to obtain a high photocatalytic efficiency of about 100% within 50 min. It was promoted 3.68 and 5.75 times compared to SrRuO₃ and g-C₃N₄, respectively. Also, a Hg(II) reduction rate of 1.5% SrRuO₃/g-C₃N₄ was enhanced3.84- and 6.28-fold than those of pure SrRuO₃ NPs and g-C₃N₄, respectively. Such a high photocatalytic performance over SrRuO₃/g-C₃N₄ photocatalysts was explained by the characteristics of SrRuO₃ NPs incorporated on porous g-C₃N₄ layers, which demonstrate strong absorption of visible light with a narrow band gap, a large photocurrent density of ∼9.07 mA/cm², well-dispersed and small particle sizes, and cause facile diffusion of HCOOH and Hg(II) ions and electrons. The present work provides a dramatic novel approach to the challenge of constructing visible-light photosensitive photocatalysts for wastewater remediation.

Novel strategy of natural antioxidant nutrition quality evaluation in food: Oxidation resistance mechanism and synergistic effects investigation

Effective evaluation methods for assessing the nutritional quality of foods that eliminate free radicals (i.e., foods that are classified as antioxidants) have long attracted the attention of scientists and the populace. In this case, constructing a corresponding photoelectrochemical sensor that has the advantages of being intuitive, rapid, and capable of accurate assessment for global antioxidant capacity is of profound significance. In this study, a novel g-C₃N₄/NiS/TiO₂ photoelectric sensitive platform was constructed and afforded the possibility of a synergistic/antagonistic effect for estimating intrinsic antioxidant ingredients in food. Further investigation revealed that the internal influences of the compound structure, such as the redox potential and type of groups on the molecular benzene ring should be the main internal reasons for antioxidant synergistic behaviors. The photochemical strategy of concern is expected to provide benefits for on-site foods nutrition assays that should become a guide for health care diets.

Nanostructure-based photoelectrochemical sensing platforms for biomedical applications

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

Oxidized titanium carbide MXene-enabled photoelectrochemical sensor for quantifying synergistic interaction of ascorbic acid based antioxidants system

Antioxidants can protect organization from damage by scavenging of free radicals. When two kinds of antioxidants are consumed together, the total antioxidant capacity might be enhanced via synergistic interactions. Herein, we develop a simple, direct, and effective strategy to quantify the synergistic interaction between ascorbic acid (AA) and other different antioxidants by photoelectrochemical (PEC) technology. MXene Ti₃C₂-TiO₂ composites fabricated via hydrogen peroxide oxidation were applied as sensing material for the antioxidants interaction study. Under excitation of 470 nm wavelength, the photogenerated electrons transfer from the conduction band of TiO₂ nanoparticles to the Ti₃C₂ layers, and the holes in TiO₂ can oxidize antioxidants, leading to an enhanced photocurrent as the detection signal. This PEC sensor exhibits a good linear range to AA concentrations from 12.48 to 521.33 μM as well as obvious antioxidants capability synergism. In particular, the photocurrents of AA + gallic acid (GA) and AA + chlorogenic acid (CHA) mixtures at 476.19 μM increase 1.95 and 2.35 times respectively comparing with the sum of photocurrents of AA and GA or CHA. It is found that the synergistic effect is mainly depending on the fact that AA with the low redox potential (0.246 V vs NHE) can reduce other antioxidants radical to promote regeneration, improving the overall antioxidant performance. Moreover, it is proved that the greater redox potential of antioxidants, the more obvious the synergistic effect. In addition, the sensor was used to real sample assay, which provides available information towards food nutrition analysis, health products design and quality inspection.

Single-Atom-Based Heterojunction Coupling with Ion-Exchange Reaction for Sensitive Photoelectrochemical Immunoassay

Benefiting from the maximum atom-utilization efficiency and distinct structural features, single-atom catalysts open a new avenue for the design of more functional catalysts, whereas their bioapplications are still in their infancy. Due to the advantages, platinum single atoms supported by cadmium sulfide nanorods (Pt SAs-CdS) are synthesized to build an ultrasensitive photoelectrochemical (PEC) biosensing platform. With the decoration of Pt SAs, the PEC signal of CdS is significantly boosted. Furthermore, theory calculations indicate the positively charged Pt SAs could change the charge distribution and increase the excited carrier density of CdS. Meanwhile, it also suggests that Cu²⁺ can severely hinder the photoexcitation and electron-hole separation of CdS. As a proof of concept, prostate-specific antigen is chosen as the target analyte to demonstrate the superiority of the Pt SAs-CdS-based PEC sensing system. As a result, the PEC biosensor based on Pt SAs-CdS exhibits outstanding detection sensitivity and promising applicability.

Generation of Hydrogen Gas Using CuCr2O4-g-C3N4 Nanocomposites under Illumination by Visible Light

In this research, nanocomposites made of CuCr₂O₄-g-C₃N₄ accommodating distinct contents of CuCr₂O₄ (1-4 wt %) nanoparticles (NPs) were endorsed for hydrogen gas production after illumination by visible light in the presence of aqueous glycerol solution. The ultrasonication-mixture method was applied to assure the homogeneous distribution of CuCr₂O₄ NPs over synthesized mesoporous g-C₃N₄. Such nanocomposites possess suppressed recombination between the photoinduced charges. High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy examinations affirmed the formation of CuCr₂O₄-g-C₃N₄ heterojunctions. The separation between the induced charges and the photocatalytic performance with the CuCr₂O₄ NP amount were investigated. The CuCr₂O₄-g-C₃N₄ heterojunction of 3 wt % CuCr₂O₄ content was documented as the optimal heterojunction. Upgraded hydrogen gas generation was attained over the optimal heterojunction with the extent of ten and thirty times as those registered for pure CuCr₂O₄ and g-C₃N₄ specimens, respectively, under illumination by visible light. The photocatalytic performance acquired by the diverse synthesized specimens was assessed not only by their effectiveness to absorb light in the visible region but also by their potential to separate the photoinduced charges.

Long-Lasting and Intense Chemiluminescence of Luminol Triggered by Oxidized g-C3N4 Nanosheets

Most of the known chemiluminescence (CL) systems are flash-type, whereas a CL system with long-lasting and strong emission is very favorable for accurate CL quantitative analysis and imaging assays. In this work, we found that the oxidized g-C₃N₄ (g-CN_OX) could trigger luminol-H₂O₂ to produce a long-lasting and intense CL emission. The CL emission lasted for over 10 min and could be observed by the naked eye in a dark room. By means of a CL spectrum, X-ray photoelectron spectra, and electron spin resonance spectra, the possible mechanism of this CL reaction was proposed. This strong and long-duration CL emission was attributed to the high catalytic activity of g-CN_OX nanosheets and continuous generation of reactive oxygen species from H₂O₂ on g-CN_OX surface. Taking full advantage of the long-lasting CL property of this system, we proposed one "non-in-situ mixing" mode of CL measurement. Compared with the traditional "in-situ mixing" CL measurement mode, this measurement mode was convenient to operate and had good reproducibility. This work not only provides a long-lasting CL reaction but also deepens the understanding of the structure and properties of g-C₃N₄ material.

Fe Foil-Guided Fabrication of Uniform Ag@AgX Nanowires for Sensitive Detection of Leukemia DNA

Herein, we report a novel Fe foil-guided, in situ etching strategy for the preparation of highly uniform Ag@AgX (X = Cl, Br) nanowires (NWs) and applied the photoelectric-responsive materials for sensitive photoelectrochemical (PEC) detection of leukemia DNA. The Ag@AgX NW formation process was discussed from the redox potential and K_sp value. The fabricated PEC platform for sensing leukemia DNA showed good assay performance with a wide linear range (0.1 pM to 50 nM) and low detection limit of 0.033 pM. We envision that our Fe foil-guided synthetic method could be applied to synthesize more photoactive materials for sensitive PEC detections.

Laser-Cut Polymer Tape Templates for Scalable Filtration Fabrication of User-Designed and Carbon-Nanomaterial-Based Electrochemical Sensors

We report here a simple filtration method for the scalable fabrication of user-designed and carbon-nanomaterial-based electrode arrays using laser-cut poly(vinyl chloride) (PVC) tape templates. This method can produce electrode arrays with high uniformity and low resistance from the dilute dispersions of single-walled carbon nanotubes (SWNTs) and graphene nanoplatelets (GNPs). For these two carbon arrays, the SWNT array is demonstrated to possess several interesting properties, e.g., good mechanical properties, excellent flexibility, and favorable electrochemical behavior. Moreover, its porous structure enables the construction of a paperlike solid-state electrochemical sensor using Nafion electrolytes, which is suitable for the on-site monitoring of trace phenol pollutants in electrolyte-free water. Besides, an electrochemically addressable 36-zone sensor was constructed by this method. With the aid of an inexpensive 3D printer, the addressable sensor can achieve the semiautomatic and high-throughput evaluation of antioxidant capacity on a series of vegetables and fruits using a single-channel electrochemical analyzer.

Paper-Based Origami Photoelectrochemical Sensing Platform with TiO2/Bi4NbO8Cl/Co-Pi Cascade Structure Enabling of Bidirectional Modulation of Charge Carrier Separation

A bidirectional modulation of photoinduced charge carrier separation strategy based on TiO₂/Bi₄NbO₈Cl/Co-Pi was proposed in microfluidic paper based photoelectrochemical analytical device (μ-POAD). Perovskite Bi₄NbO₈Cl with high charge carrier mobility was employed as visible light absorber, sandwiching between electron transporting material (ETM) and hole transporting material (HTM). Paper based TiO₂ nanosheet arrays (PTNAs) serve as the ETM to provide a direct pathway for electron transport and Co-Pi works as the HTM to extract holes. Driven by a built-in electric field, the generated electrons of Bi₄NbO₈Cl are extracted by PTNAs, while holes are drawn toward Co-Pi, achieving efficient carrier separation. Remarkably, it is the first time that the HTM was introduced into μ-POAD to efficiently output holes and enhance the sensitivity. With the aid of ETM and HTM, 2.59 and 14.6 times higher photocurrent density was obtained compared with PTNAs/Bi₄NbO₈Cl and Bi₄NbO₈Cl photoelectrode, respectively. Benefiting from this dramatic photocurrent signal, ultrasensitive detection of β human chorionic gonadotrophin is realized with the linear range of 0.01-3000 IU L^-1 and detection limitation of 0.005 IU L^-1. This work demonstrates the importance of efficient carrier separation to the sensitivity of μ-POAD and paves the way for developing a high-performance analytical device.

Graphitic C3N4 nanosheet and hemin/G-quadruplex DNAzyme-based label-free chemiluminescence aptasensing for biomarkers

Here we first reported that graphitic carbon nitride nanosheet (g-C₃N₄ NS) could effectively quench the chemiluminescence (CL) of luminol-hydrogen peroxide (H₂O₂) system. According to the new discovery, a label-free and homogeneous CL aptasensing platform was designed for sensitive detecting of biomarkers. In the absence of target, DNA probe containing hemin/G-quadruplex DNAzyme structure was adsorbed on the surface of g-C₃N₄ NS, causing the CL quenching of luminol through an electron transfer process. However, in the presence of the target, a DNA-DNA duplex was formed due to DNA hybridization reaction and target recognition effect, which could not be adsorbed onto the g-C₃N₄ NS surface because of its weak affinity. Thus, the electron transfer was blocked and the CL emission of luminol could be enhanced. The proposed CL aptasensor could detect carcinoembryonic antigen (CEA) with a detection limit of 63.0 pg/mL and it can also be used as a general detecting strategy for adenosinetriphosphate (ATP) detection. This aptasensing platform exhibited high sensitivity toward biomarkers and the probe need not be labeled, showing great promise for disease diagnosis.

Negative ion laser desorption/ionization time-of-flight mass spectrometric analysis of small molecules by using nanostructured substrate as matrices

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is an excellent analytical technique for rapid and sensitive analysis of macromolecules such as polymers and proteins. However, the main drawback of MALDI-TOF MS is its difficulty to detect small molecules with mass below 700 Da because of the intensive interference from MALDI matrix in the low mass region. In recent years there has been considerable interest in developing matrix-free laser desorption/ionization by using nanostructured substrates to substitute the conventional organic matrices, which is often referred as surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS). Despite these attractive features, most of the current SALDI-TOF MS for the analysis of small molecules employ positive ion mode, which is subjected to produce multiple alkali metal adducts, and thus increases the complexity of the analysis. Different from the complicated adducts produced in positive ion mode, mass spectra obtained in negative ion mode are featured by deprotonated ion peaks without matrix interference, which simplifies the interpretation of mass spectra and detection of unknown. In this review, we critically survey recent advances in nanostructured substrates for negative ion LDI-TOF MS analysis of small molecules in the last 5 years. Special emphasis is placed on the preparation of the nanostructured substrates and the results achieved in negative ion SALDI-MS. In addition, a variety of promising applications including environmental, biological, and clinical analysis are introduced. The ionization mechanism of negative ionization is briefly discussed.

Glutathione-driven Cu(i)-O2 chemistry: a new light-up fluorescent assay for intracellular glutathione

Besides its widely known role as an endogenous antioxidant in scavenging free radicals, glutathione (GSH) can also play the role of prooxidant and promote CuO-induced formation of hydroxyl radicals to light up a fluorescent signal through Cu(i)-O2 chemistry without requiring additional H2O2. This approach is independent of the mechanisms of enzyme mimics, such as the well-known oxidase and peroxidase mimetics, providing a new method to simply and effectively analyze intracellular GSH.

A ratiometric photoelectrochemical immunosensor based on g-C3N4@TiO2 NTs amplified by signal antibodies–Co3O4 nanoparticle conjugates

Herein, we report a ratiometric photoelectrochemical (PEC) immunosensor coupled with secondary antibodies–Co₃O₄ nanoparticle conjugates (Ab₂–Co₃O₄ NPs) for signal amplification.

New Titanium Dioxide-Based Heterojunction Nanohybrid for Highly Selective Photoelectrochemical-Electrochemical Dual-Mode Sensors

A new titanium dioxide (TiO₂)-based heterojunction nanohybrid (HJNH) composed of TiO₂, graphene (G), poly[3-aminophenylboronic acid] (PAPBA), and gold nanoparticles (Au NPs) was synthesized and designated as TiO₂(G) NW@PAPBA-Au HJNH. The TiO₂(G) NW@PAPBA-Au HJNH possesses dual-mode signal photoelectrochemical (PEC) and electrochemical transduction capabilities to sense glucose and glycated hemoglobin (HbA1c) independently. The synthesis of the HJNH material involved two sequential stages: (i) simple electrospinning synthesis of G-embedded TiO₂ nanowires [TiO₂(G) NWs] and (ii) one-step synthesis of Au NP-dispersed PAPBA nanocomposite (NC) in the presence of TiO₂(G) NWs. The as-synthesized TiO₂(G) NW@PAPBA-Au HJNH was characterized by field emission scanning electron microscopy, field emission transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared, thermogravimetric analysis, and UV-visible diffuse reflectance spectroscopy. A PEC platform was developed with TiO₂(G) NW@PAPBA-Au HJNH for the selective detection of glucose without any enzyme auxiliary. The PEC glucose sensor presents an acceptable linear range (from 0.5 to 28 mM), good sensitivity (549.58 μA mM^-1 cm^-2), and low detection limit (0.11 mM), which are suited for diabetes glucose monitoring. Besides, the boronic acid groups in PAPBA were utilized as a host to capture HbA1c. We fabricated the electrochemical HbA1c sensor based on monitoring the electrocatalytic reduction current of hydrogen peroxide produced by HbA1c tethered to the sensor probe. The amperometric electrochemical sensor for HbA1c exhibited linear responses to HbA1c levels from 2.0 to 10% (with a detection limit of 0.17%). Notably, the performances of the fabricated glucose and HbA1c sensors are superior in the dual-signal transduction modes as compared to the literature, suggesting the significance of the newly designed bifunctional TiO₂(G) NW@PAPBA-Au HJNH.

Oxidized Quasi-Carbon Nitride Quantum Dots Inhibit Ice Growth

Antifreeze proteins (AFPs), a type of high-efficiency but expensive and often unstable biological antifreeze, have stimulated substantial interest in the search for synthetic mimics. However, only a few reported AFP mimics display thermal hysteresis, and general criteria for the design of AFP mimics remain unknown. Herein, oxidized quasi-carbon nitride quantum dots (OQCNs) are synthesized through an up-scalable bottom-up approach. They exhibit thermal-hysteresis activity, an ice-crystal shaping effect, and activity on ice-recrystallization inhibition. In the cryopreservation of sheep red blood cells, OQCNs improve cell recovery to more than twice that obtained by using a commercial cryoprotectant (hydroxyethyl starch) without the addition of any organic solvents. It is shown experimentally that OQCNs preferably bind onto the ice-crystal surface, which leads to the inhibition of ice-crystal growth due to the Kelvin effect. Further analysis reveals that the match of the distance between two neighboring tertiary N atoms on OQCNs with the repeated spacing of O atoms along the c-axis on the primary prism plane of ice lattice is critical for OQCNs to bind preferentially on ice crystals. Here, the application of graphitic carbon nitride derivatives for cryopreservation is reported for the first time.

New Insights toward Efficient Charge-Separation Mechanism for High-Performance Photoelectrochemical Aptasensing: Enhanced Charge-Carrier Lifetime via Coupling Ultrathin MoS2 Nanoplates with Nitrogen-Doped Graphene Quantum Dots

Deeply understanding the internal mechanism of the photoelectrohemical (PEC) process is conducive to fabricate high-performance PEC biosensors. In this work, we proposed a new insight toward an efficient charge-separation mechanism in high-performance PEC biosensors. Specifically, we disclosed that the lifetimes of photogenerated charge carriers of ultrathin MoS₂ nanosheets could be prolonged by approximately millisecond time scales after a proper mole ratio of NGQDs were coupled, leading to the promoted charge separation and a giant photocurrent signal magnification. Benefiting from the dramatic signal amplification and the introduction of acetamiprid aptamer, subfemtomolar level detection of acetamiprid is achieved, which makes our strategy among the most sensitive electronic approaches for PEC-based monitoring of targets. This study was beneficial to further understand the charge-separation mechanism in PEC biosensing, which paved the way for the development of more efficient PEC biosensors.