Construction of Peroxidase-like Metal-Organic Frameworks in TiO2 Nanochannels: Robust Free-Standing Membranes for Diverse Target Sensing

Bacterial cellulose/gelatin-based pH-responsive functional film for food freshness monitoring

Food safety is related to public health and environmental safety. Therefore, it is necessary to develop accurate and effective detection methods to assess food quality and safety. In this study, a pH-responsive functional film (BC/GA/FITC/PCA) was generated for the real-time and visual monitoring of shrimp freshness. Bacterial cellulose /Gelatin (BC/GA) was used as a film-forming matrix, and fluorescein isothiocyanate (FITC) and red cabbage (PCA) were used as the response signals. The addition of FITC and PCA increased the shading capacity (< 30 %) and antioxidant properties (22.8 %) of the films. WCA (82.73 ± 0.95°), WVP (1.48 × 10-11 g·cm/cm2·s·Pa) and OTR (2.42 × 10-15 cm3·cm/cm2·s·Pa) indicated that the film possessed water resistance and oxygen barrier properties. When exposed to daylight, the film underwent a color transition from purple to green as the ammonia concentration increased. In addition, the blue-green fluorescence of the films gradually increased and the detection limit was low (170 ppb). In particular, the change in film color caused by shrimp spoilage corresponded to the TVBN value. This study work provides a new strategy for controlling and monitoring food safety and has a wide range of applications in the fields of food-active packaging and smart packaging.

Triple-Emission Single Sensing Element-Enabled Ratiometric Fluorescent Array Identification of Multiple Antibiotics

Given that intricate toxicological profiles exist among different antibiotics and pose serious threats to the environment and human health, synchronous analysis of multiple residues becomes crucial. Sensor arrays show potential to achieve the above purpose, but it is challenging to develop easy-to-use and high-sensitivity tools because the state-of-the-art arrays often require more than one recognition unit and are monosignal dependent. Here we exquisitely designed a fluorescent nanoprobe (2-aminoterephthalic acid-anchored CdTe quantum dots with Eu3+ coordination, CdTe-ATPA-Eu3+) featuring triple emissions at the same excitation as the only element to fabricate a luminescent sensor array with ratiometric calculations for identifying multiple antibiotics. By taking tetracycline, chlortetracycline, doxycycline, oxytetracycline, penicillin G, and sulfamethoxazole as models, the six species exhibited distinguishable motivation or/and quenching impacts on the three emissions of CdTe-ATPA-Eu3+, which were employed as indicators to perform the ratiometric logical operation and further combined with pattern recognition analysis for multitarget determination. Evidently, such a design exhibits two advances: (1) with the triple-emission probe as the sole receptor requiring neither internal nor external adjustments, the fabricated array acts as an extremely facile tool for multianalyte detection; (2) the ratiometric calculations offer excellent sensitivity and reliability for high-performance determination. Consequently, accurate identification and quantification of individual antibiotics and their combinations at various levels were verified in both laboratory and practical matrices. Our work provides a new tool for simultaneously detecting multiple antibiotics, and it will inspire the development of advanced sensor arrays for multitarget analysis.

Target-modulated mineralization of wood channels as enzyme-free electrochemical sensors for detecting amyloid-β species

Alzheimer's disease (AD) is an irreversible brain disorder, which has been found to be associated with neurotoxic amyloid-β oligomers (AβO). The early diagnosis of AD is still a great challenge. Herein, inspired by the hierarchical channel structure of natural wood, we design and demonstrate a low-cost and sensitive wood channel-based fluidic membrane for electrochemical sensing of AβO1-42. In this design, Zn/Cu-2-methylimidazole (Zn/Cu-Hmim) with artificial peroxidase (POD)-like activity was asymmetrically fabricated at one side of the wood channels by biomimetic mineralization and a subsequent ion exchange reaction. The strong affinity between Cu(II) and AβO1-42 enables Cu(II) species in Zn/Cu-Hmim to be extracted by AβO1-42, thus suppressing the POD-like performance via Zn/Cu-Hmim disassembly. Using Zn/Cu-Hmim to catalyze the oxidation reaction of 2,2'-diazo-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) by H2O2, the current-voltage (I-V) properties of wood channels are influenced by the generated oxidation product (ABTS•+), thus providing information useful for the quantitative analysis of AβO1-42. Importantly, the three aggregation states of Aβ1-42 (AβM1-42, AβO1-42, and AβF1-42) can also be identified, owing to the affinity difference and available reaction sites. The proposed wood membrane provides a novel, assessable, and scalable channel device to develop sensitive electrochemical sensors; moreover, the sustainable wood materials represent alternative candidates for developing channel-structured sensing platforms.

Improved Sensitivity and Selectivity of Glutathione Detection through Target-Driven Electron Donor Generation in Photoelectrochemical Electrodes

Glutathione (GSH) plays a vital role in many physiological processes, and its abnormal levels have been found to be associated with several diseases. In contrast to traditional methods using electron donor-containing electrolytes for photoelectrochemical (PEC) sensing, in this study, a target-driven electron donor generation in a PEC electrode was developed to detect GSH. Using well-aligned TiO2 nanotube arrays (TNTs) as the PEC substrate, mesoporous MIL-125(Ti) was grown in the TNTs through an in situ solvothermal method and subsequent two-step annealing treatment. The accommodation capacity of mesoporous MIL-125(Ti) allows a well loading of cystine and Pt nanoclusters (NCs). Taking advantage of the specific cleavage ability of disulfide bonds by GSH, cystine was converted to cysteine, which served as the electron donor for the PEC process. Benefiting from the confinement effect of mesoporous MIL-125(Ti), cysteine was effectively oxidized to cysteine sulfinic acid by the photogenerated holes. Importantly, the highly active Pt NCs decorated in the mesopores not only improved the charge transfer but also accelerated the above oxidation reaction. The synergistic effect of these factors enabled the efficient separation of the photogenerated electron-hole pairs, which induced a significant photocurrent increase and in turn led to the high-sensitivity detection of GSH. Consequently, the proposed PEC biosensor exhibited excellent performance in the detection of GSH in serum specimens. The target-driven electron donor generation designed in this study might open a new route for developing sensitive and selective PEC biosensors with application in complex biological environments.

A Colorimetric Sensor Enabled with Heterogeneous Nanozymes with Phosphatase-like Activity for the Residue Analysis of Methyl Parathion

In this study, a colorimetric sensor was developed for the detection of organophosphorus pesticides (OPs) using a heterogeneous nanozyme with phosphatase-like activity. Herein, this heterogeneous nanozyme (Au-pCeO2) was obtained by the modification of gold nanoparticles on porous cerium oxide nanorods, resulting in synergistic hydrolysis performance for OPs. Taking methyl parathion (MP) as the target pesticide, the catalytic performance and mechanism of Au-pCeO2 were investigated. Based on the phosphatase-like Au-pCeO2, a dual-mode colorimetric sensor for MP was put forward by the analysis of the hydrolysis product via a UV-visible spectrophotometer and a smartphone. Under optimum conditions, this dual-mode strategy can be used for the on-site analysis of MP with concentrations of 5 to 200 μM. Additionally, it can be applied for MP detection in pear and lettuce samples with recoveries ranging from 85.27% to 115.87% and relative standard deviations (RSDs) not exceeding 6.20%, which can provide a simple and convenient method for OP detection in agricultural products.

Enzymatic Catalysis in Size and Volume Dual-Confined Space of Integrated Nanochannel-Electrodes Chip for Enhanced Impedance Detection of Salmonella

Nanochannel-based confinement effect is a fascinating signal transduction strategy for high-performance sensing, but only size confinement is focused on while other confinement effects are unexplored. Here, a highly integrated nanochannel-electrodes chip (INEC) is created and a size/volume-dual-confinement enzyme catalysis model for rapid and sensitive bacteria detection is developed. The INEC, by directly sandwiching a nanochannel chip (60 µm in thickness) in nanoporous gold layers, creates a micro-droplet-based confinement electrochemical cell (CEC). The size confinement of nanochannel promotes the urease catalysis efficiency to generate more ions, while the volume confinement of CEC significantly enriches ions by restricting diffusion. As a result, the INEC-based dual-confinement effects benefit a synergetic enhancement of the catalytic signal. A 11-times ion-strength-based impedance response is obtained within just 1 min when compared to the relevant open system. Combining this novel nanoconfinement effects with nanofiltration of INEC, a separation/signal amplification-integrated sensing strategy is further developed for Salmonella typhimurium detection. The biosensor realizes facile, rapid (<20 min), and specific signal readout with a detection limit of 9 CFU mL-1 in culturing solution, superior to most reports. This work may create a new paradigm for studying nanoconfined processes and contribute a new signal transduction technique for trace analysis application.

Copper Immobilized over 2D Hexagonal SBA-15 for Electrochemical and Colorimetric Sulfite Sensing

Sulfite (SO₃^2-) is considered a highly toxic anion for living organisms. Herein, we report the synthesis of copper immobilized over a 2D hexagonally ordered mesoporous silica material CuMS as an electrochemical and colorimetric dual-technique-based sensing platform for sulfite detection. The immobilization of copper on silica was achieved through the bis[3-(triethoxysilyl)propyl]tetrasulfide (TEPTS) ligand. Morphological and physical properties of the material were confirmed by several characterization techniques, including scanning electron microscopy, transmission electron microscopy, X-ray diffraction, N₂ sorption, and X-ray photoelectron spectroscopy. The CuMS material retained mesoporosity with a narrow pore size distribution (D ≈ 5.4 nm) and a high Brunauer-Emmett-Teller surface area of 682 m² g^-1 after the immobilization of copper. The prepared catalyst shows promising electrocatalytic activity toward sulfite oxidation. A linear variation in the peak current was obtained for SO₃^2- oxidation in the 0.2-15 mM range with a high sensitivity of 62.08 μA cm^-2, under optimum experimental conditions. The limit of detection (LOD) was found to be 1.14 nM. CuMS also shows excellent activity toward colorimetric detection of sulfite anions with an LOD of 0.4 nM. The proposed sensor shows high selectivity toward the sulfite anion, even in the presence of common interferents. The detection of sulfite in white wine with excellent recovery demonstrates the practical applicability of this sensor.

Target Recognition-Triggered Peroxidase-Mimicking Activity Depression in Homochiral Nanochannels for Identifying Cystine Enantiomers

Enantioselective identification of chiral molecules is of paramount importance in medical science, biochemistry, and pharmaceutics owing to the configuration-dependent activities of enantiomers. However, the identical physicochemical properties of enantiomers remain challenging in chiral sensing. In this study, inspired by the peroxidase-mimicking activity of Fe(III)-based nanomaterials, an enantioselective artificial architecture is constructed on TiO₂ nanochannels. Homochiral Ti-based metal-organic frameworks (MOFs) use a 2,2'-bipyridine-5,5'-dicarboxylic acid ligand as the artificial enzyme skeleton, Fe(III) as peroxidase-mimicking centers, and l-tartaric acid (TA) as a chiral recognition selector. Using l-/d-cystine as model enantiomers, the chiral moieties of l-TA on Ti-MOFs allow stereoselective recognition of guest molecules through hydrogen bonds formed between chiral cystine and the host. In a tris(2-carboxyethyl)phosphine hydrochloride-containing environment, the disulfide bonds in cystine molecules are further cleaved, and the HS-tails react with Fe(III) active sites, causing the loss of peroxidase-like performance of nanochannels. Benefitting from the nanochannel architecture's current-potential (I-V) properties, the selective recognition of cystine enantiomers is directly monitored through the peroxidase-like activity change-induced ionic current signatures. This study provides a new and universal strategy for distinguishing disulfide- and thiol-containing chiral molecules.

A halide perovskite/lead sulfide heterostructure with enhanced photoelectrochemical performance for the sensing of alkaline phosphatase (ALP)

For the construction of halide perovskite-based photoelectrochemical (PEC) biosensors in aqueous solution, the balance between retaining the excellent photoelectric performances of the halide perovskite and the protective modification of the halide perovskite has always been a challenging problem. In this work, a simple and sensitive photoelectrochemical biosensor based on inorganic halide perovskite CsPbBr₃ as the photoactive material for the detection of alkaline phosphatase (ALP) was reported. The substrate sodium thiophosphate (Na₃SPO₃) can be catalyzed by ALP to produce hydrogen sulfide (H₂S), which can react with the lead site on the surface of the CsPbBr₃ film to form lead sulfide (PbS), resulting in a stable heterostructure and enhanced photocurrent intensity. The possible mechanism of enhanced photocurrent response of CsPbBr₃/PbS heterojunctions was studied in detail. This work paves a new way for applying halide perovskites in different biosensor designs.

MnO2 Nanozyme-Mediated CRISPR-Cas12a System for the Detection of SARS-CoV-2

The CRISPR-Cas system was developed into a molecular diagnostic tool with high sensitivity, low cost, and high specificity in recent years. Colorimetric assays based on nanozymes offer an attractive point-of-care testing method for their low cost of use and user-friendly operation. Here, a MnO2 nanozyme-mediated CRISPR-Cas12a system was instituted to detect SARS-CoV-2. MnO2 nanorods linked to magnetic beads via a single-stranded DNA (ssDNA) linker used as an oxidase-like nanozyme inducing the color change of 3,3',5,5'-tetramethylbenzidine, which can be distinguished by the naked eye. The detection buffer color will change when the Cas12a is activated by SARS-CoV-2 and indiscriminately cleave the linker ssDNA. The detection limit was 10 copies per microliter and showed no cross-reaction with other coronaviruses. The nanozyme-mediated CRISPR-Cas12a system shows high selectivity and facile operation, with great potential for molecular diagnosis in point-of-care testing applications.

Ag nanozyme strengthened by folic acid: Superior peroxidase-mimicking activity and application for visual monitoring of dopamine

Dopamine (DA) is an important neurotransmitter; however, any excess or deficiency of DA will cause several diseases in humans. To monitor DA efficiently and conveniently, a Ag nanozyme strengthened by bioactive folic acid (FA@AgNPs) was developed by homogeneous redox assembly. After the microstructure and performance were characterized in detail, it was noted that the proposed FA@AgNPs possessed superior peroxidase-like activity due to the ultra-small Ag nanoparticles and multiple amino, hydroxyl, and aromatic rings in FA. FA@AgNPs accelerated the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with a low Michaelis constant (K_m) and high maximal reaction rate (V_max). Importantly, the characteristic absorbance intensity of FA@AgNPs-TMB-H₂O₂ at 652 nm (A₆₅₂) was exclusively deteriorated in the presence of trace DA, accompanied by a visual color change from blue to colorless. Under the optimized conditions (pH 4.0, 300 μL 1.5 mM TMB, 300 μL 1.0 M H₂O₂ and incubated for 30 min at room temperature), there expressed an excellent linear relationship between lgA₀/A₆₅₂ and c_DA from 1.0 ×10^-8 to 6.67×10^-6 mol/L with a low limit detection of 7.1×10^-10 mol/L (S/N=3). When applied for monitoring of DA in real fruit juice and pharmaceutical samples, the recovery was between 96.6% and 104.9%, with RSD less than 2.2%. The enhanced peroxidase-like activity of the FA@AgNP system and its selective recognition mechanism for DA are also proposed.

Mechanisms of Enzymatic Transduction in Nanochannel Biosensors

The immobilization of enzymes in solid-state nanochannels is a new avenue for the design of biosensors with outstanding selectivity and sensitivity. This work reports the first theoretical model of an enzymatic nanochannel biosensor. The model is applied to the system previously experimentally studied by Lin, et al. (Anal. Chem. 2014, 86, 10546): a hourglass nanochannel modified by glucose oxidase for the detection of glucose. Our predictions are in good agreement with experimental observations as a function of the applied potential, pH and glucose concentration. The sensing mechanism results from the combination of three processes: i) the establishment of a steady-state proton concentration gradient due to a reaction-diffusion mechanism, ii) the effect of that gradient on the charge of the adsorbed enzymes and native surface groups, and iii) the effect of the resulting surface charge on the ionic current. Strategies to improve the sensor performance based on this mechanism are identified and discussed.

Fabrication of Redox-Controllable Bioinspired Nanochannels for Precisely Regulating Protein Transport

Redox regulation is an inherent feature of nature and plays a crucial role in the transport of ions/small molecules. However, whether redox status affects the biomolecule transport remains largely unknown. To explore the effects of redox status on biomolecule transport, herein, we constructed a glutathione/glutathione disulfide (GSH/GSSG)-driven and pillar[5]arene (P5)-modified artificial nanochannel for protein transport. The results indicate that hemoglobin (Hb) protein is selectively and effectively transported across the GSH-driven P5-modified nanochannel, which suggests that the redox status of the nanochannel could affect the process of protein transport. Therefore, this redox-driven nanochannel could provide a potential application for biomolecule detection and redox-controllable biomolecular drug release.

Nanoarchitectonics of a MOF-in-Nanochannel (HKUST-1/TiO2) Membrane for Multitarget Selective Enrichment and Staged Recovery

Enrichment and separation of specific endogenous molecules are essential for disease diagnosis and the pharmaceutical industry. Although many solid sorbents have been developed for target molecule enrichment, simultaneous separation of multitargets is still a challenge for adsorbents. In this study, we develop a multitarget selective sorbent based on a nanochannel membrane prepared by the anodization of a Ti-Cu alloy. The in situ growth of a metal-organic framework (MOF, herein using Cu-based HKUST-1) in the nanochannels enables the resulting MOF-in-nanochannel membrane to act as a nanofilter. Benefitting from the size-exclusion effect of MOFs and the distinct surface characteristics of each component in the HKUST-1/TiO₂ nanochannels, the as-proposed membranes can be simply operated as a filter and exhibit satisfactory selectivities and enrichment capacities in the separation of aromatic amino acids, histidine-rich proteins, and phosphoproteins. More importantly, the adsorbed multitargets can be further controllably released from the membrane in a sequence via a staged recovery process. The use of this system is envisioned to provide an innovative and potential design for efficient sorption media for the selective enrichment and staged separation of specific biomolecules.

Porous Fe3O4@COF-Immobilized gold nanoparticles with excellent catalytic performance for sensitive electrochemical detection of ATP

In this work, a "signal-off" electrochemical biosensor was established for sensitive detection of adenosine triphosphate (ATP) based on Fe₃O₄@covalent organic framework-immobilized gold nanoparticles (Fe₃O₄@COF-Au NPs) porous composite material as a nanocarrier. The proposed Fe₃O₄@COF-Au NPs could effectively confine Au NPs in the uniform channels of the Fe₃O₄@COF, which successfully avoided Au NPs aggregation to a certain extent and provided a comparatively independent and stable micro-environment via its hydrophobic porous nanochannels, thereby owning excellent electro-catalytic performance for the reduction of 4-nitrophenol. Moreover, the Fe₃O₄@COF-Au NPs nanomaterials were served as functional platform for immobilizing DNA substrate (S0), which was used to bind with the conversion product (S1) of the target ATP for subsequent branched hybridization chain reaction (b-HCR) to form dendritic DNA strands to hinder electron transfer between Fe₃O₄@COF-Au NPs and 4-nitrophenol, finally achieving sensitive detection of ATP with a wide linear range of 5 pM-50 μM and a low detection limit of 1.6 pM. Such strategy provides a multifunctional immobilized platform for the sensitive detection of ATP and a versatile strategy for monitoring other biomolecules.

Biocatalytic Metal-Organic Framework: Promising Materials for Biosensing

The high-efficient and specific catalysis of enzyme allow it to recognize a myriad of substrate that impels the biosensing. Nevertheless, the fragility of natural enzymes severely restricts their practical applications. Metal-organic frameworks (MOFs) with porous network and attractive functions have been intelligently employed as supports to encase enzymes for protecting them against hash environments. More importantly, the customizable construction and composition affords the intrinsic enzyme-like activity of some MOFs (known as nanozymes), which provides an alternative guideline to construct robust enzymes mimics. Herein, this review will introduce the concept of these biocatalytic MOFs, with the special emphasis on how the biocatalytic processes operated in these MOFs materials can reverse the plight of native enzymes-based biosensing. In addition, the present challenges and future outlooks in this research field are briefly put forward.

Interfacially Super-Assembled Tyramine-Modified Mesoporous Silica-Alumina Oxide Heterochannels for Label-Free Tyrosinase Detection

Tyrosinase (TYR) is a multifunctional copper-containing enzyme that plays a critical role in the biosynthetic pathway of melanin. Thus, the detection of TYR activity possesses vast importance from clinical diagnosis to the food industry. However, most TYR detection methods are expensive, complicated, and time-consuming. Herein, a functional nanofluidic heterochannel composed of an ultrathin tyramine-modified mesoporous silica layer (Tyr-MS) and alumina oxide (AAO) arrays is constructed by an interfacial super-assembly method. The heterochannel with plenty of enzyme catalytic sites for TYR provides the response of the ion current signal against TYR concentrations. Introducing enzymatic reaction paves the way for the heterochannel to achieve label-free, selective, specific detection of TYR. Notably, a highly sensitive detection of TYR with a limit of 2 U mL^-1 was obtained by optimizing the modified conditions. Detailed investigations and theoretical calculations further reveal the mechanism for the detection performance. This work provides a simple, low-cost, quick response, and label-free platform based on functional nanofluidic devices for enzyme-sensing technologies.

Nanozyme-Participated Biosensing of Pesticides and Cholinesterases: A Critical Review

To improve the output and quality of agricultural products, pesticides are globally utilized as an efficient tool to protect crops from insects. However, given that most pesticides used are difficult to decompose, they inevitably remain in agricultural products and are further enriched into food chains and ecosystems, posing great threats to human health and the environment. Thus, developing efficient methods and tools to monitor pesticide residues and related biomarkers (acetylcholinesterase and butylcholinesterase) became quite significant. With the advantages of excellent stability, tailorable catalytic performance, low cost, and easy mass production, nanomaterials with enzyme-like properties (nanozymes) are extensively utilized in fields ranging from biomedicine to environmental remediation. Especially, with the catalytic nature to offer amplified signals for highly sensitive detection, nanozymes were finding potential applications in the sensing of various analytes, including pesticides and their biomarkers. To highlight the progress in this field, here the sensing principles of pesticides and cholinesterases based on nanozyme catalysis are definitively summarized, and emerging detection methods and technologies with the participation of nanozymes are critically discussed. Importantly, typical examples are introduced to reveal the promising use of nanozymes. Also, some challenges in the field and future trends are proposed, with the hope of inspiring more efforts to advance nanozyme-involved sensors for pesticides and cholinesterases.