CoMnOx Nanoflower-Based Smartphone Sensing Platform and Virtual Reality Display for Colorimetric Detection of Ziram and Cu2.

Transition metal doping is an ideal strategy to construct multifunctional and efficient nanozymes for biosensing. In this work, a metal-doped CoMnOx nanozyme was designed and synthesized by hydrothermal reaction and high-temperature calcination. Based on its oxidase activity, an "on-off-on" smartphone sensing platform was established to detect ziram and Cu2+. The obtained flower-shaped CoMnOx could exhibit oxidase-, catalase-, and laccase-like activities. The oxidase activity mechanism of CoMnOx was deeply explored. O2 molecules adsorbed on the surface of CoMnOx were activated to produce a large amount of O2·-, and then, O2·- could extract acidic hydrogen from TMB to produce blue oxTMB. Meanwhile, TMB was oxidized directly to the blue product oxTMB via the high redox ability of Co species. According to the excellent oxidase-like activity of CoMnOx, a versatile colorimetric detection platform for ziram and Cu2+ was successfully constructed. The linear detection ranges for ziram and Cu2+ were 5~280 µM and 80~360 µM, and the detection limits were 1.475 µM and 3.906 µM, respectively. In addition, a portable smartphone platform for ziram and Cu2+ sensing was established for instant analysis, showing great application promise in the detection of real samples including environmental soil and water.

Introducing molecular imprinting onto nanozymes: toward selective catalytic analysis.

The discovery of enzyme-like catalytic characteristics in nanomaterials triggers the generation of nanozymes and their multifarious applications. As a class of artificial mimetic enzymes, nanozymes are widely recognized to have better stability and lower cost than natural bio-enzymes, but the lack of catalytic specificity hinders their wider use. To solve the problem, several potential strategies are explored, among which molecular imprinting attracts much attention because of its powerful capacity for creating specific binding cavities as biomimetic receptors. Attractively, introducing molecularly imprinted polymers (MIPs) onto nanozyme surfaces can make an impact on the latter's catalytic activity. As a result, in recent years, MIPs featuring universal fabrication, low cost, and good stability have been intensively integrated with nanozymes for biochemical detection. In this critical review, we first summarize the general fabrication of nanozyme@MIPs, followed by clarifying the potential effects of molecular imprinting on the catalytic performance of nanozymes in terms of selectivity and activity. Typical examples are emphatically discussed to highlight the latest progress of nanozyme@MIPs applied in catalytic analysis. In the end, personal viewpoints on the future directions of nanozyme@MIPs are presented, to provide a reference for studying the interactions between MIPs and nanozymes and attract more efforts to advance this promising area.

Target-triggered dual signal amplification based on HCR-enhanced nanozyme activity for the sensitive visual detection of Escherichia coli.

The detection of foodborne pathogens is crucial for food hygiene regulation and disease diagnosis. Colorimetry has become one of the main analytical methods in studying foodborne pathogens due to its advantages of visualization, low cost, simple operation, and no complex instrument. However, the low sensitivity limits its applications in early identification and on-site detection for trace analytes. In order to overcome such a limitation, herein we propose a joint strategy featuring dual signal amplification based on the hybridization chain reaction (HCR) and DNA-enhanced peroxidase-like activity of gold nanoparticles (AuNPs) for the sensitive visual detection of Escherichia coli. Target bacteria bound specifically to the aptamer domain in the capture hairpin probe, exposing the trigger domain for HCR and forming the extended double-stranded DNA (dsDNA) structures. The peroxidase-like catalytic capacity of AuNPs can be enhanced significantly by dsDNAs with the sticky ends of dsDNAs being adsorbed on AuNPs and the rigidity of dsDNAs causing the spatial regulation of AuNP concentration. The intensity of the enhancement was linearly related to the number of target bacteria. With the above strategy, the detection limit of our colorimetric method for Escherichia coli was down to 28 CFU mL-1 within a short analytical time (50 min). This study provides a new perspective for the sensitive and visual detection of early bacterial contamination in foods.

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.

MnOx In Situ Growth-Induced Luminescence and Oxidase-Like Feature Bimodulation of CePO4:Tb Nanorods: Toward Ascorbic Acid-Related Bioanalysis in a "One-Stone-Two-Birds" Manner.

Nanozyme-based multimode detection is a useful means to improve the accuracy and stability of analytical methods. However, both multifunctional nanozymes and related multimodal sensing strategies are still very scarce. Besides, they require complex processes to fabricate and operate. To fill this gap, here we propose a spontaneous interfacial in situ growth strategy to prepare a new bifunctional material (CePO4:Tb@MnOx) featuring good oxidase-like activity and green photoluminescence for the dual-mode colorimetric/luminescence determination of ascorbic acid (AA)-related biomarkers specifically. CePO4:Tb@MnOx was gained through the controllable redox reaction between KMnO4 and CePO4:Tb nanorods. It was interestingly found that MnOx in situ growth not only significantly enhanced the enzyme-like activity but also could reversibly regulate the luminescence of CePO4:Tb via a dual quenching mechanism. More interestingly, CePO4:Tb@MnOx exhibited a distinctive response toward AA against other reducing species. A double-coordination regulation mechanism was further verified to clarify the catalytic activity and luminescence switching behaviors in CePO4:Tb@MnOx. Based on these findings, a dual-mode colorimetric/luminescence approach was established for AA sensing in a "one-stone-two-birds" manner, providing excellent selectivity, sensitivity, and practicability. Furthermore, the determination of AA-related biomarkers, including acid phosphatase activity and organophosphorus residue, was also validated by the sensing principle. Our work not only deepens the understanding of the coordinated regulation of the luminescence and enzyme-like features in lanthanide-based materials but also offers a novel way to design and develop multifunctional nanozymes for advanced bioanalytical applications.

Multifunctional light-controllable nanozyme enabled bimodal fluorometric/colorimetric sensing of mercury ions at ambient pH.

Nanomaterials with enzyme-like catalytic features (nanozymes) find wide use in analytical sensing. Apart from catalytic characteristics, some other interesting functions coexist in the materials. How to combine these properties to design multifunctional nanozymes for new sensing strategy development is challenging. Besides, in nanozymes it is still a challenge to conveniently control the catalytic process, which also hinders their further applications in advanced biochemical analysis. To remove the above barriers, here we design a light-controllable multifunctional nanozyme, namely manganese-inserted cadmium telluride (Mn-CdTe) particles, that integrates oxidase-like activity with luminescence together, to achieve the fluorometric/colorimetric dual-mode detection of toxic mercury ions (Hg2+) at ambient pH. The Mn-CdTe exhibits a light-triggered oxidase-mimicking catalytic behavior to induce chromogenic reactions, thus enabling one to start or stop the catalytic progress easily via applying or withdrawing light irradiation. Meanwhile, the quantum dot material can exhibit bright photoluminescence, which provides the fluorometric channel to sense targets. When Hg2+ is introduced, it rapidly leans toward Mn-CdTe through electrostatic interaction and Te-Hg bonding and induces the aggregation of the latter. As a result, the luminescence of Mn-CdTe is dynamically quenched, and the masking of active sites in aggregated Mn-CdTe leads to the decrease of light-initiated oxidase-mimetic activity. According to this principle, a new fluorometric/colorimetric bimodal method was established for Hg2+ determination with excellent performance. A 3D-printed portable platform combining paper-based test strips and an App-equipped smartphone was further fabricated, making it possible to achieve in-field sensing of the analyte in various matrices.

Joint concanavalin A-aptamer enabled dual recognition for anti-interference visual detection of Salmonella typhimurium in complex food matrices.

Given that food poisoning and infectious diseases caused by Salmonella typhimurium (S. typhimurium) draw intensive public health concerns, developing rapid, accurate, and cost-effective approaches to detect the pathogen is of crucial importance. Herein, we proposed a concanavalin A (Con A)-aptamer joint strategy to realize dual recognition for the strongly specific, visual, and highly sensitive determination of S. typhimurium. Compared with currently used single identification strategies, Con A and aptamer could recognize different sites of S. typhimurium to enhance the utilization rate of these sites for better sensing. The developed assay offered specific detection of S. typhimurium against other bacteria in a remarkably wide concentration range of 7.0 × 101 âˆ¼ 7.0 × 109 CFU/mL, along with a detection limit as low as 23 CFU/mL. Real sample analyses of milk and pork demonstrated the excellent reliability and practicability of our assay, providing great potential for food safety analysis.

Alkali-Etched Imprinted Mn-Based Prussian Blue Analogues with Superior Oxidase-Mimetic Activity and Precise Recognition for Tetracycline Colorimetric Sensing.

As a typical antibiotic pollutant, tetracycline (TC) is producing increasing threats to the ecosystem and human health, and exploring convenient means for monitoring of TC is needed. Here, we proposed alkali-etched imprinted Mn-based Prussian blue analogues featuring superior oxidase-mimetic activity and precise recognition for the colorimetric sensing of TC. Simply etching Mn-based Prussian blue analogues (Mn-PBAs) with NaOH could expose the sites and surfaces to significantly improve their catalytic activity. Density functional theory calculations were employed to screen the molecularly imprinted polymer (MIP) layer for target identification. Consequently, the designed Mn-PBANaOH@MIP possessed the rich channels for substrates to get in touch with the active Mn-PBANaOH core, showing an excellent catalytic capacity to trigger the chromogenic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) without the use of H2O2. If TC was introduced, it would be recognized selectively by the MIP shell and masked the channels for TMB access, resulting in the obstruction of the chromogenic reaction. According to this mechanism, selective optical detection of TC was achieved, and performance stability, reusability, and reliability as well as practicability were also verified, promising potential for TC monitoring in complex matrices. Our work not only presents an effective way to enhance the enzyme-like activity of Prussian blue analogues but also provides a facile approach for TC sensing. Additionally, the work will inspire the exploration of molecularly imprinted nanozymes for various applications.

Amorphous Fe-Containing Phosphotungstates Featuring Efficient Peroxidase-like Activity at Neutral pH: Toward Portable Swabs for Pesticide Detection with Tandem Catalytic Amplification.

Peroxidase-mimetic materials are intensively applied to establish multienzyme systems because of their attractive merits. However, almost all of the nanozymes explored exhibit catalytic capacity only under acidic conditions. The pH mismatch between peroxidase mimics in acidic environments and bioenzymes under neutral conditions significantly restricts the development of enzyme-nanozyme catalytic systems especially for biochemical sensing. To solve this problem, here amorphous Fe-containing phosphotungstates (Fe-PTs) featuring high peroxidase activity at neutral pH were explored to fabricate portable multienzyme biosensors for pesticide detection. The strong attraction of negatively charged Fe-PTs to positively charged substrates as well as the accelerated regeneration of Fe2+ by the Fe/W bimetallic redox couples was demonstrated to play important roles in endowing the material with peroxidase-like activity in physiological environments. Consequently, integrating the developed Fe-PTs with acetylcholinesterase and choline oxidase led to an enzyme-nanozyme tandem platform with good catalytic efficiency at neutral pH for organophosphorus pesticide response. Furthermore, they were immobilized onto common medical swabs to fabricate portable sensors for paraoxon detection conveniently based on smartphone sensing, showing excellent sensitivity, good anti-interference capacity, and low detection limit (0.28 ng/mL). Our contribution expands the horizon of acquiring peroxidase activity at neutral pH, and it will also open avenues to construct portable and effective biosensors for pesticides and other analytes.

Bifunctional Mn-Doped N-Rich Carbon Dots with Tunable Photoluminescence and Oxidase-Mimetic Activity Enabling Bimodal Ratiometric Colorimetric/Fluorometric Detection of Nitrite.

Multimodal detection is a promising paradigm because of its advantages of expanding usage scenarios and improving reliability. However, it is very challenging to design reasonable strategies to achieve the multimodal sensing of targets. Herein, we developed an unprecedented bimodal ratiometric colorimetric/fluorometric method by exploring a novel bifunctional artificial oxidase mimic, Mn-doped N-rich carbon dots (Mn-CDs), to achieve the high-performance determination of nitrite in complicated matrices. The Mn-CDs exhibited both tunable photoluminescence and high oxidase-like activity, effectively catalyzing the colorless 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to generate blue TMB+. When nitrite was introduced, the TMB+ species generated would specifically react with nitrite to produce diazotized TMB+, resulting in a color change from blue to green and finally to yellow. Simultaneously, the fluorescence of Mn-CDs was quenched by the diazotized TMB+ product via the inner filter effect. Hence, the existence of nitrite could lead to the simultaneous variations of visual color and photoluminescence, providing the principal basis for the bimodal ratiometric colorimetric/fluorometric quantification of the target. With the method, excellent sensitivity, selectivity, reliability, and practicability for nitrite detection were verified. Our work proposes a new bimodal strategy for nitrite measurement using bifunctional CDs-based enzyme mimics, which will inspire future effort on the exploration of promising multifunctional nanozymes and their advanced applications in biochemical sensing.

Single-Atomic Iron Doped Carbon Dots with Both Photoluminescence and Oxidase-Like Activity.

Multifunctional nanozymes can benefit biochemical analysis via expanding sensing modes and enhancing analytical performance, but designing multifunctional nanozymes to realize the desired sensing of targets is challenging. In this work, single-atomic iron doped carbon dots (SA Fe-CDs) are designed and synthesized via a facile in situ pyrolysis process. The small-sized CDs not only maintain their tunable fluorescence, but also serve as a support for loading dispersed active sites. Monoatomic Fe offers SA Fe-CDs exceptional oxidase-mimetic activity to catalyze 3,3',5,5'-tetramethylbenzidine (TMB) oxidation with fast response (Vmax  = 10.4 nM s-1 ) and strong affinity (Km  = 168 µM). Meanwhile, their photoluminescence is quenched by the oxidation product of TMB due to inner filter effect. Phosphate ions (Pi) can suppress the oxidase-mimicking activity and restore the photoluminescence of SA Fe-CDs by interacting with Fe active sites. Based on this principle, a dual-mode colorimetric and fluorescence assay of Pi with high sensitivity, selectivity, and rapid response is established. This work paves a path to develop multifunctional enzyme-like catalysts, and offers a simple but efficient dual-mode method for phosphate monitoring, which will inspire the exploration of multi-mode sensing strategies based on nanozyme catalysis.

Smartphone-assisted bioenzyme-nanozyme-chromogen all-in-one test strip with enhanced cascade signal amplification for convenient paraoxon sensing.

Monitoring of pesticide residues in food and environmental matrices is undoubtedly crucial to guarantee food safety and ecological health, yet how to realize their sensitive and convenient detection is still challenging. Herein, we propose an all-in-one test strip that elaborately integrates bioenzyme, nanozyme and chromogen together, and achieve the highly sensitive and convenient sensing of pesticide residues assisted by a smartphone. A sequential self-assembly strategy was first explored to acquire an integrative bioenzyme-nanozyme-chromogen assembly, and then the assembly was confined in a biocompatible hydrogel to construct the test strip. Thanks to both the proximity and confinement effects, a ∼1.2-fold improvement of the cascade catalytic efficiency was gained to benefit high-sensitivity detection. More importantly, since all the sensing elements, including target recognition units and signal amplification modules, were rationally integrated in the test strip, detection operation was significantly simplified, making it possible for in-field rapid analysis. Besides, the microenvironment provided by the alginate hydrogel carrier endowed the test strip with an excellent sensing stability. By taking paraoxon as a typical pesticide, high-performance detection of the target was accomplished via the smartphone-assisted all-in-one test strip. Moreover, the test strip was successfully applied for paraoxon detection in various real samples and exhibited good correlations with commercial kits, demonstrating its great prospect for practical applications. Our work not only offers a new tool for the high-sensitivity and convenient monitoring of pesticide residues, but will also inspire the development of efficient multi-enzyme sensing platforms.

Nanozymes with Multiple Activities: Prospects in Analytical Sensing.

Given the superiorities in catalytic stability, production cost and performance tunability over natural bio-enzymes, artificial nanomaterials featuring enzyme-like characteristics (nanozymes) have drawn extensive attention from the academic community in the past decade. With these merits, they are intensively tested for sensing, biomedicine and environmental engineering. Especially in the analytical sensing field, enzyme mimics have found wide use for biochemical detection, environmental monitoring and food analysis. More fascinatingly, rational design enables one fabrication of enzyme-like materials with versatile activities, which show great promise for further advancement of the nanozyme-involved biochemical sensing field. To understand the progress in such an exciting field, here we offer a review of nanozymes with multiple catalytic activities and their analytical application prospects. The main types of enzyme-mimetic activities are first introduced, followed by a summary of current strategies that can be employed to design multi-activity nanozymes. In particular, typical materials with at least two enzyme-like activities are reviewed. Finally, opportunities for multi-activity nanozymes applied in the sensing field are discussed, and potential challenges are also presented, to better guide the development of analytical methods and sensors using nanozymes with different catalytic features.

Biomimic Nanozymes with Tunable Peroxidase-like Activity Based on the Confinement Effect of Metal-Organic Frameworks (MOFs) for Biosensing.

Biomimic nanozymes coassembled by peptides or proteins and small active molecules provide an effective strategy to design attractive nanozymes. Although some promising nanozymes have been reported, rational regulation for higher catalytic activity of biomimic nanozymes remains challenging. Hence, we proposed a novel biomimic nanozyme by encapsulating the coassembly of hemin/bovine serum albumin (BSA) in zeolite imidazolate frameworks (ZIF-8) to achieve controllable tailoring of peroxidase-like activity via the confinement effect. The assembly of Hemin@BSA was inspired by the structure of horseradish peroxidase (HRP), in which hemin served as the active cofactor surrounded by BSA as a blocking pocket to construct a favorable hydrophobic space for substrate enrichment. Benefiting from the confinement effect, ZIF-8 with a porous intracavity was identified as the ideal outer layer for Hemin@BSA to accelerate substrate transport and achieve internal circulation of peroxidase-like catalysis, significantly enhancing its peroxidase-like activity. Especially, the precise encapsulation of Hemin@BSA in ZIF-8 could also prevent it from decomposition in harsh environments by rapid crystallization around Hemin@BSA to form a protective shell. Based on the improved peroxidase-like activity of Hemin@BSA@ZIF-8, several applications were successfully performed for the sensitive detection of small molecules including H2O2, glucose, and bisphenol A (BPA). Satisfactory results highlight that using a ZIF-8 outer layer to encapsulate Hemin@BSA offers a very effective and successful strategy to improve the peroxidase-like activity and the stability of biomimic nanozymes, broadening the potential application of biocatalytic metal-organic frameworks (MOFs).

Dual-mode fluorescence and colorimetric detection of pesticides realized by integrating stimulus-responsive luminescence with oxidase-mimetic activity into cerium-based coordination polymer nanoparticles.

The great threat of pesticide residues to the environment and human health has drawn widespread interest to explore approaches for pesticide monitoring. Compared to commonly developed single-signal pesticide assays, multi-mode detection with inherent self-validation and self-correction is expected to offer more reliable and anti-interference results. However, how to realize multi-mode analysis of pesticides still remains challenging. Herein, we propose a dual-mode fluorescence and colorimetric method for pesticide determination by integrating stimulus-responsive luminescence with oxidase-mimetic activity into cerium-based coordination polymer nanoparticles (CPNs(Ⅳ)). The CPNs(Ⅳ) exhibit good oxidase-like activity of catalyzing the colorless 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to its blue oxide, offering a visible color signal; by employing acid phosphatase (ACP) to hydrolyze ascorbic acid 2-phosphate (AAP), the generated ascorbic acid (AA) can chemically reduce the CPNs(Ⅳ) to CPNs(Ⅲ), which exhibit a remarkable fluorescence signal but lose the oxidase-mimicking ability to trigger the TMB chromogenic reaction; when pesticides exist, the enzymatic activity of ACP is restrained and the hydrolysis of AAP to AA is blocked, leading to the recovery of the catalytic TMB chromogenic reaction but the suppression of the fluorescence signal of CPNs(Ⅲ). According to this principle, by taking malathion as a pesticide model, dual-mode 'off-on-off' fluorescence and 'on-off-on' colorimetric detection of the pesticide with good sensitivity was realized. Excellent interference-tolerance and reliability were verified by applying it to analyze the target in real sample matrices. With good performance and practicability, the proposed dual-mode approach shows great potential in the facile and reliable monitoring of pesticide residues.

Nanozyme catalysis-assisted ratiometric multicolor sensing of heparin based on target-specific electrostatic-induced aggregation.

Monitoring the level of heparin in clinical matrices is significant because of its pivotal role in preventing thrombosis. Compared to traditional single-signal sensors, multi-signal ratiometric detection can provide anti-interference results especially in complicated environments. However, fabricating an easy-to-operation, low-cost and robust sensor for the ratiometric detection of heparin still remains challenging. Here we propose a novel nanosensor for the ratiometric multicolor sensing of heparin with high performance. The sensor is based on the specific electrostatic interaction between the target and a positively charged species generated from nanozyme catalysis. FeMoO4 nanorods are explored as an oxidase mimic for the first time, showing a high activity at neutral pH to catalyze the colorless 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to blue TMBox with an absorbance at 652 nm. Heparin can induce the rapid aggregation of the produced TMBox intermediate with rich positive charges due to their strong electrostatic interaction, leading to the formation of a purple Heparin-TMBox complex providing a signal at 565 nm. With the increase of heparin, the color changes from blue to indigo and further purple, enabling the multicolor sensing of the target. As a result, ultrasensitive determination of heparin was obtained with a very low detection limit. The fabricated nanosensor could differentiate heparin from complex species with no interferences, and it provided reliable analytical results for heparin in both serum and plasma. With robust performance, low cost and facile fabrication, the sensor holds great potential in monitoring heparin for clinical applications.

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.

Ratiometric Colorimetric Detection of Nitrite Realized by Stringing Nanozyme Catalysis and Diazotization Together.

Due to the great threat posed by excessive nitrite in food and drinking water to human health, it calls for developing reliable, convenient, and low-cost methods for nitrite detection. Herein, we string nanozyme catalysis and diazotization together and develop a ratiometric colorimetric approach for sensing nitrite in food. First, hollow MnFeO (a mixture of Mn and Fe oxides with different oxidation states) derived from a Mn-Fe Prussian blue analogue is explored as an oxidase mimic with high efficiency in catalyzing the colorless 3,3',5,5'-tetramethylbenzidine (TMB) oxidation to blue TMBox, presenting a notable signal at 652 nm. Then, nitrite is able to trigger the diazotization of the product TMBox, not only decreasing the signal at 652 nm but also producing a new signal at 445 nm. Thus, the analyte-induced reverse changes of the two signals enable us to establish a ratiometric colorimetric assay for nitrite analysis. According to the above strategy, facile determination of nitrite in the range of 3.3-133.3 µM with good specificity was realized, providing a detection limit down to 0.2 µM. Compared with conventional single-signal analysis, our dual-signal ratiometric colorimetric mode was demonstrated to offer higher sensitivity, a lower detection limit, and better anti-interference ability against external detection environments. Practical applications of the approach in examining nitrite in food matrices were also verified.

Molecularly imprinted polypyrrole nanotubes based electrochemical sensor for glyphosate detection.

An electrochemical sensor based on molecularly imprinted polypyrrole nanotubes (MIPNs) has been developed for the detection of glyphosate (Gly) with high sensitivity and specificity. Herein, the MIPNs are prepared by imprinting Gly sites on the surface of polypyrrole (PPy) nanotubes. The synthesized MIPNs have high electrical conductivity and exhibit rapid adsorption rate, enhanced affinity and specificity to Gly. An electrochemical sensor for Gly detection is fabricated by assembling MIPNs-modified screen-printed electrodes with a 3D-printed electrode holder, which is highly portable and suitable for real-time detection. The results demonstrate that the MIPNs-based electrochemical sensor for Gly exhibits a wide detection range of 2.5-350 ng/mL with a limit of detection (LOD) of 1.94 ng/mL. Besides, the Gly sensor possessed good stability, reproducibility, and excellent selectivity against other interferents. The practicability of the sensor is verified by detecting Gly in orange juice and rice beverages, indicating that the sensor is suitable for monitoring pesticides in actual food and environmental samples.

Target-induced synergetic modulation of electrochemical tag concentration and electrode surface passivation for one-step sampling filtration-free detection of acid phosphatase activity.

As a biomarker of several diseases, the activity of acid phosphatase (ACP) is generally used to assistantly diagnose these diseases. Thus, developing reliable ACP activity analytical methods becomes quite significant. Herein, we recommend a one-step sampling filtration-free electrochemical method for ACP activity determination based on the target-induced synergetic modulation of tag concentration and surface passivation. Mn3O4 microspheres with favorable oxidase-mimicking activity are synthesized to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to its product TMBox, resulting in a remarkable re-reduction signal of TMBox to TMB recorded by an integrated electrochemical system consisting of screen-printed electrode (SPE) and 3D-printed holder. When hexametaphosphate ions (HMPi) with rich negative charges are employed to interact positively charged TMBox, the formed flocculent precipitate TMBox-HMPi automatically sedimentates onto SPE surface, and both the decreased concentration of free TMBox in solution and the increased electrode surface passivation triggered by TMBox-HMPi sedimentation synergistically reduce the re-reduction signal of TMBox. When ACP is present, it hydrolyzes the HMPi substrate, greatly relieving the formation of the TMBox-HMPi precipitate and its sedimentation onto SPE surface. As a result, the electrochemical re-reduction signal of TMBox becomes remarkable again. With the strategy of using one stimulus to generate two-fold signal change, highly sensitive ACP activity detection was realized, with a wide linear range from 0.05 to 50 U/L and a limit of detection down to 0.024 U/L. Reliable monitoring of ACP activity in clinical serum was also demonstrated.