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

Hydroxyl radical-mediated synthesis of carbonyl functionalized graphene quantum dots-like as enzyme mimics with tunable fluorescence emission

The synthesis of graphene quantum dots-like enriched with specific oxygenated groups (o-GQDs) exhibiting great catalytic performance offers a promising tool for diagnosis and biomedicine, but introducing specific oxygen groups remains a challenge. Here, we propose a mild synthetic protocol for producing regulated fluorescence emission (from blue to yellow) carbonyl functionalized GQDs with double catalytic function through Fe3O4-catalyzed hydroxyl radical (·OH) oxidation the precursors like graphene oxide, polyaniline (PANI) and polydopamine (PDA). The method can be carried out at room temperature than the traditional high-temperature oxidation in concentrated acid. Interestingly, o-GQDs exhibit excellent peroxidase (POD)- and ascorbate oxidase-like activity. XPS characterization showed a significant increase in carbonyl content in o-GQDs compared to the precursor, even a 14-fold increase in blue-emitting iron-doped GQDs (b-Fe-GQDs). The introduction of Fe3O4 during the synthesis process results in a minor degree of Fe doping, which enhances the catalytic activity of b-Fe-GQDs through coordination with N. Based on this feature, highly sensitive single-signal and ultra-selective dual-signal methods for alkaline phosphatase detection were developed. This low cost and safe synthesis strategy paves the way for practical usage of o-GQDs.

Intelligent onsite dual-modal assay based on oxidase-like fluorescence carbon dots-driven competitive effect for ethyl carbamate detection

Intelligent onsite accurate monitoring ethyl carbamate (EC, a group 2 A carcinogen) in environment is of great significance to safeguard environmental health and public safety. Herein, we reported an intelligent dual-modal point-of-care (POC) assay based on the bimetallic Mn and Ce co-doped oxidase-like fluorescence carbon dots (Ce&MnCDs) nanozyme-driven competitive effect. In brief, the oxidase-like activity of Ce&MnCDs was inhibited by thiocholine (TCh, originating from the hydrolysis of acetylcholinesterase (AChE) to acetylthiocholine (ATCh)), preventing the oxidation of o-phenylenediamine (OPD) to 2,3-diaminophenothiazine (DAP). However, with the aid of Br2 + NaOH, EC inactivated AChE to prevent TCh generation for re-launching the oxidase-like activity of Ce&MnCDs to trigger the oxidation of OPD into DAP, thereby outputting an EC concentration-dependent ratiometric fluorescence and colorimetric readouts by employing Ce&MnCDs and OPD as the optical signal reporters. Interestingly, these dual-modal optical signals could be transduced into the gray values that was linearly proportional to the residual levels of EC on a smartphone-based portable platform, with a detection limit down to 1.66 μg/mL, qualifying the requirements of analysis of EC residues in real samples. This opened up a new avenue for onsite assessment of the risk of residues of EC, safeguarding environmental health and public safety.

Ultra-trace Ag doped carbon quantum dots with peroxidase-like activity for the colorimetric detection of glucose

Carbon quantum dots (CQDs) have significant applications in nanozymes. However, previous studies have not elucidated the structure-activity relationship and enzyme mechanism. In this study, we employed a one-step microwave method to synthesize ultra-trace Ag-doped carbon quantum dots (Ag-CQDs). In the presence of hydrogen peroxide (H2O2), we used the oxidative coupling reaction of 3,3',5,5'-tetramethylbenzidine (TMB) to evaluate the intrinsic peroxidase-like activity, kinetics, and mechanism of Ag-CQDs. The trace amount of doped Ag (1.64 %) facilitated electron transfer from the CQDs interior to the surface. The electron transfer triggered the peroxide activity of CQDs, producing hydroxyl radical (·OH), which oxidized the colorless TMB to blue-colored TMB (oxTMB). By coupling with glucose oxidase (GOx), the Ag-CQDs/H2O2/TMB system has been used for colorimetric glucose determination. The system demonstrated a low detection limit (0.17 µM), wide linear range (0.5-5.5 µM), and satisfactory results when fruit juice was analyzed. This study reports a feasible method for the colorimetric detection of glucose by synthesizing ultra-trace Ag-doped carbon quantum dots with peroxidase-mimicking activity.

3D printing-based lateral flow visual assay utilizing the dual-excitation property of nitrogen-doped carbon dots for the ratiometric determination and chemometric discrimination of flavonoids

A ratiometric fluorescence sensor has been established based on dual-excitation carbon dots (D-CDs) for the detection of flavonoids (morin is chosen as the typical detecting model for flavonoids). D-CDs were prepared using microwave radiation with o-phenylenediamine and melamine and exhibit controllable dual-excitation behavior through the regulation of their concentration. Remarkably, the short-wavelength excitation of D-CDs can be quenched by morin owing to the inner filter effect, while the long-wavelength excitation remains insensitive, serving as the reference signal. This contributes to the successful design of an excitation-based ratiometric sensor. Based on the distinct and differentiated variation of excitation intensity, morin can be determined from 0.156 to 110 µM with a low detection limit of 0.156 µM. In addition, an intelligent and visually lateral flow sensing device is developed for the determination  of morin content in real samples with satisfying recoveries, which indicates the potential application for human health monitoring.

Fluorescent multichannel sensor array based on three carbon dots derived from Tibetan medicine waste for the quantification and discrimination of multiple heavy metal ions in water

A fluorescent multichannel sensor array has been established based on three carbon dots derived from Tibetan medicine waste for rapid quantification and discrimination of six heavy metal ions. Due to the chelation between metal ions and carbon dots (CDs), this fluorescence "turn off" mode sensing array can quantify six metal ions as low as "μM" level. Moreover, the six heavy metal ions display varying quenching effects on these three CDs owing to diverse chelating abilities between each other, producing differential fluorescent signals for three sensing channels, which can be plotted as specific fingerprints and converted into intuitive identification profiles via principal component analysis (PCA) and hierarchical cluster analysis (HCA) technologies to accurately distinguish Cu2+, Fe3+, Mn2+, Ag+, Ce4+, and Ni2+ with the minimum differentiated concentration of 5 μM. Valuably, this sensing array unveils good sensitivity, exceptional selectivity, ideal stability, and excellent anti-interference ability for both mixed standards and actual samples. Our contribution provides a novel approach for simultaneous determination of multiple heavy metal ions in environmental samples, and it will inspire the development of other advanced optical sensing array for simultaneous quantification and discrimination of multiple targets.

Bifunctional MOF-Encapsulated Cobalt-Doped Carbon Dots Nanozyme-Powered Chemiluminescence/Fluorescence Dual-Mode Detection of Aflatoxin B1

A novel bifunctional MOF-encapsulated cobalt-doped carbon dots nanozyme (Co-CD/PMOF) with excellent peroxidase-mimic catalytic activity and fluorescence property was synthesized and employed to fabricate a chemiluminescence/fluorescence (CL/FL) dual-mode immunosensor for AFB1 detection. Co-CD/PMOF could catalyze the luminol/H2O2 system to generate robust and long-lasting CL signals due to the slow diffusion effect and continuous generation of •OH, O2•-, and 1O2 species. Differing from traditional flash-type CL emissions, this glow-type CL emission is helpful to fabricate a sensitive and accurate CL sensing platform. Then the CL/FL dual-mode detection of AFB1 was developed using antibody-functionalized Co-CD/PMOF as the signal-amplifying nanoprobe. The CL mode assay based on indirect competitive immune principle was carried out on a chemiluminescence optical fiber platform, where the AFB1-OVA-functionalized optical fiber probe was employed for biorecognition, separation, and signal conducting. The AFB1 detection range and LOD were 0.63-69.36 ng/mL and 0.217 ng/mL, respectively. Using AFB1 antibody-functionalized immunomagnetic beads for capturing and separation, the FL mode detection of AFB1 was established based on the sandwich immune principle. A linear range of 0.54-51.91 ng/mL and a LOD of 0.027 ng/mL were obtained. This work designed a sensitive, rapid, and reliable nanozyme-powered dual-mode assay strategy and provided technical support in the field of environmental monitoring and food safety.

Biocompatible Folic-Acid-Strengthened Ag-Ir Quantum Dot Nanozyme for Cell and Plant Root Imaging of Cysteine/Stress and Multichannel Monitoring of Hg2+ and Dopamine

To boost the enzyme-like activity, biological compatibility, and antiaggregation effect of noble-metal-based nanozymes, folic-acid-strengthened Ag-Ir quantum dots (FA@Ag-Ir QDs) were developed. Not only did FA@Ag-Ir QDs exhibit excellent synergistic-enhancement peroxidase-like activity, high stability, and low toxicity, but they could also promote the lateral root propagation of Arabidopsis thaliana. Especially, ultratrace cysteine or Hg2+ could exclusively strengthen or deteriorate the inherent fluorescence property with an obvious "turn-on" or "turn-off" effect, and dopamine could alter the peroxidase-like activity with a clear hypochromic effect from blue to colorless. Under optimized conditions, FA@Ag-Ir QDs were successfully applied for the turn-on fluorescence imaging of cysteine or the stress response in cells and plant roots, the turn-off fluorescence monitoring of toxic Hg2+, or the visual detection of dopamine in aqueous, beverage, serum, or medical samples with low detection limits and satisfactory recoveries. The selective recognition mechanisms for FA@Ag-Ir QDs toward cysteine, Hg2+, and dopamine were illustrated. This work will offer insights into constructing some efficient nanozyme sensors for multichannel environmental analyses, especially for the prediagnosis of cysteine-related diseases or stress responses in organisms.

A portable colorimetric sensing platform for rapid and sensitive quantification of dichlorvos pesticide based on Fe-Mn bimetallic oxide nanozyme-participated highly efficient chromogenic catalysis

BACKGROUND

Dichlorvos (DDVP), as a highly effective insecticide, is widely used in agricultural production. However, DDVP residue in foodstuffs adversely affects human health. Conventional instrumental analysis can provide highly sensitive and accurate detection of DDVP, while the need of bulky and expensive equipment limits their application in resource-poor areas and on-site detection. Therefore, the development of easily portable sensing platforms for convenient, rapid and sensitive quantification of DDVP is very essential for ensuring food safety.

RESULT

A portable colorimetric sensing platform for rapid and sensitive quantification of DDVP is developed based on nanozyme-participated highly efficient chromogenic catalysis. The Fe-Mn bimetallic oxide (FeMnOx) nanozyme possesses excellently oxidase-like activity and can efficiently catalyze oxidation of 3, 3', 5, 5'-tetramethylbenzidine (TMB) into a blue oxide with a very low Michaelis constant (Km) of 0.0522 mM. The nanozyme-catalyzed chromogenic reaction can be mediated by DDVP via inhibiting the acetylcholinesterase (AChE) activity. Thus, trace DDVP concentration-dependent color evolution is achieved and DDVP can be sensitively detected by spectrophotometry. Furthermore, a smartphone-integrated 3D-printed miniature lightbox is fabricated as the colorimetric signal acquisition and processing device. Based on the FeMnOx nanozyme and smartphone-integrated lightbox system, the portable colorimetric sensing platform of DDVP is obtained and it has a wide linear range from 1 to 3000 ng mL-1 with a low limit of detection (LOD) of 0.267 ng mL-1 for DDVP quantification.

SIGNIFICANCE

This represents a new portable colorimetric sensing platform that can perform detection of DDVP in foodstuffs with simplicity, sensitivity, and low cost. The work not only offers an alternative to rapid and sensitive detection of DDVP, but also provides a new insight for the development of advanced sensors by the combination of nanozyme, 3D-printing and information technologies.

Multi-modal nanoprobe-enabled biosensing platforms: a critical review

Nanomaterials show great potential for applications in biosensing due to their unique physical, chemical, and biological properties. However, the single-modal signal sensing mechanism greatly limits the development of single-modal nanoprobes and their related sensors. Multi-modal nanoprobes can realize the output of fluorescence, colorimetric, electrochemical, and magnetic signals through composite nanomaterials, which can effectively compensate for the defects of single-modal nanoprobes. Following the multi-modal nanoprobes, multi-modal biosensors break through the performance limitation of the current single-modal signal and realize multi-modal signal reading. Herein, the current status and classification of multi-modal nanoprobes are provided. Moreover, the multi-modal signal sensing mechanisms and the working principle of multi-modal biosensing platforms are discussed in detail. We also focus on the applications in pharmaceutical detection, food and environmental fields. Finally, we highlight this field's challenges and development prospects to create potential enlightenment.

Cheap and portable paper chip with terrific oxidase-like activity and SERS enhancement performance for SERS-colorimetric bimodal detection of intracellular glutathione

Glutathione (GSH) acts a crucial role in the normal operation of manifold life activities and is closely bound up with many human diseases. Here, a SERS-colorimetric bimodal paper-based biosensor based on Mn-doped CDs/silver nanoparticles (Mn-CDs/AgNPs) has been fabricated for high-efficiency quantification of intracellular GSH. The Mn-CDs/AgNPs with fine oxidase-like characteristic and SERS enhancement ability has been assembled onto the Whatman filter paper (WFP) to cleverly fabricate paper chip (Mn-CDs/AgNPs@WFP) which can trigger the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue TMBox and simultaneously enhance the SERS signal of TMBox. However, the introduction of GSH inhibits the oxidation of TMB, leading to color fading of paper chip and diminishment of SERS signal. Considering this, the bimodal paper-based sensing platform can be exploited for SERS-colorimetric detection of GSH, manifesting excellent selectivity, reliable stability, and satisfactory precision. The detection limits of SERS and colorimetric detection modes are as low as 0.41 μM and 0.53 μM, respectively. Furthermore, this proposed bimodal biosensor has been successfully utilized for the determination of intracellular GSH and validated by commercial GSH assay kit, which provides a mighty and convenient tool for intracellular GSH detection and can boost future effort about exploitation of other multimode paper-based biosensors as well as promote their appliances in disease diagnosis.

Zirconium Phosphate-Incorporated Polyaniline-Graphene Oxide Composite Modified Electrodes for Effective and Selective Detection of Nitrite

Nitrite contamination in food, water, and environmental samples poses a substantial health hazard, owing to its capacity for transformation into carcinogenic compounds. Given the profound ecological and physiological implications, precise and highly sensitive surveillance of nitrite has emerged as an imperative area of concern, addressing the substantial detrimental impact that it can have on both terrestrial and aquatic ecosystems. The novel electroactive polyaniline-graphene oxide composite, incorporating hexagonal zirconium phosphate discs (PGZrP), was systematically engineered as a foundation for an advanced electrochemical sensor, enabling precise nitrite detection in diverse aqueous and biological matrices. At a specific potential peak of +0.85 V, observed within a pH 7.0 phosphate buffer solution, the PGZrP-modified glassy carbon electrode (GCE) exhibited exceptional electrocatalytic proficiency in the sensing nitrite ions (NO2-), surpassing the performance of alternative electrode configurations, including the zirconium phosphate-modified GCE (ZrP/GCE), graphene oxide-modified GCE (GO/GCE), polyaniline-graphene oxide-modified GCE (PG/GCE), and the unmodified bare glassy carbon electrode. The constructed sensor demonstrated an impressive limit of detection at 80 nM along with a broad and linear detection range spanning from 124 nM to 40 mM. The synergistic effect created by the close contact between ZrP and PG, which resulted in a well-enhanced electrochemical sensing capability, was responsible for this exceptional activity. The developed sensor exhibited an enhanced electrochemical performance characterized by an extended operational range, a heightened detection threshold, and exceptional sensitivity. The PGZrP/GCE sensor, as fabricated, consistently demonstrated commendable operational stability, robust reproducibility, and remarkable repeatability in its capacity for nitrite detection. Furthermore, its successful application in the precise quantification of nitrite levels within environmental water samples and blood specimens showcased an impressive recovery rate, establishing it as a promising tool for diverse analytical applications. These findings indicate the promising potential of the PGZrP composite for integration into electrochemical devices designed to deliver rapid response times, heightened sensitivity, and sustained stability, thereby placing it as a potential candidate for the production of cutting-edge sensors, particularly those employed for the precise recognition of nitrite in aquatic and biological specimens.

Ratiometric colorimetric detection of nitrite using CoMnO3 nanofibers as an oxidase-like enzyme to induce diazotization reaction

Nitrite is a typical food additive and preservative used in the food industry, which has attracted considerable attention due to its severe adverse effects on human health. Herein, a sensitive and highly selective ratiometric colorimetric sensing platform for the detection of nitrite was created based on a polymetallic oxide nanozyme, CoMnO3 nanofibers (CMO) catalysis integrated with the particular diazotization reaction. The nanozyme has superior oxidase-like activity (Km was 0.105 mM and Vmax was 63.7 × 10-8 M S-1) and could catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to oxidized TMB (oxTMB), as CMO could achieve the conversion of oxygen in the solution to superoxide anion (O2˙-). In addition, it is interesting to note that oxTMB can be diazotized in the presence of nitrite under acidic conditions, causing a shift in the ratio of nitrite concentration to the absorbance peaks at 450 and 652 nm (A450/A652). The ratio of A450/A652 exhibited a positive linear relationship with the concentration of nitrite within the concentration range of 0.2-200 μM, with a detection limit of 0.094 μM. Simultaneously, this method was also successful in quantifying the nitrite produced by brined and pickled foods and the dynamic tracking of the nitrite levels in various types of dishes. The analysis method not only offers dual-signal ratio sensing with high sensitivity but also holds the benefit of outstanding selectivity for the use of the particular reaction, which has a wide range of application prospects in food safety management.

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.

A molecularly imprinted ratiometric fluorescence sensor based on blue/red carbon quantum dots for the visual determination of thiamethoxam

Neonicotinoids such as thiamethoxam (TMX) were widely used in agricultural production and tended to accumulate in the environment, potentially harming human and ecosystem health. To enable widespread monitoring of TMX residues, it was essential to design a reliable and sensitive detection method. Here, we developed a novel smartphone-enablled molecularly imprinted ratiometric fluorescence sensing system for selective on-site detection of TMX. It was based on blue-emission carbon dots (CDs) wrapped with a molecularly imprinted layer (B-CDs@MIPs), which provided the response signal, while red-emission CDs (R-CDs) served as an internal reference. The fluorescence signal ratio of the sensor increased with the TMX concentration, resulting in an obvious fluorescence color change from red to blue. The sensor exhibited a satisfactory limit of detection (LOD) of 13.5 nM in fluorescence analysis while LOD of 70.1 nM in visual determination. In addition, the sensing system was validated using food and environment samples, exhibiting recoveries from 91.40% to 105.7%, indicating excellent reliability for TMX detection in actual samples. Thus, the sensing system developed in this study offered promising prospects for visual detection of pesticide residues in complex environmental samples.

Diazo-reaction based dual-mode colorimetric-electrochemical sensing of nitrite in pickled food

Development of an effective and convenient sensor for sensitive detection of nitrites is of great concern since excessive amounts of nitrites can be harmful to both human health and the environment. In this work, Cu-MOF modified exfoliated graphite paper (EGP) was employed as a signal reporter to enable the visual and electrochemical dual-mode sensing of nitrites. Cu-MOFs were in situ synthesized on EGP, which exhibited an excellent oxidase enzyme-like activity to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into its oxidation product (oxTMB). The multi-layer structure and the superior electrical conductivity of EGP not only facilitated the loading of the Cu-MOF nanozyme for colorimetric sensing but also enabled its use as an underlying backbone to support electroanalysis. Based on the recognition of nitrite through a highly specific diazo reaction between nitrite and oxTMB, the addition of nitrite caused the colorimetric sensing solution to change color from blue to green, which allowed for the colorimetric sensing of nitrite with a limit of detection (LOD) of 8.5 × 10-6 mol L-1. Meanwhile, the Cu-MOF/EGP electrochemical platform was employed for ratiometric detection of nitrite based on the electrochemical oxidation of nitrite and TMB. Compared with the colorimetric mode, the electrochemical mode possessed higher sensitivity with a LOD of 5.4 × 10-7 mol L-1, indicating the high sensitivity and accuracy of the proposed dual-mode sensing strategy. Furthermore, the determination of nitrite in different pickled food samples is demonstrated.

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.

Cobalt-Nitrogen Co-Doped Carbon as Highly Efficient Oxidase Mimics for Colorimetric Assay of Nitrite

Transition metal-N-doped carbon has been demonstrated to mimic natural enzyme activity; in this study, cobalt-nitrogen co-doped carbon (Co-N-C) nanomaterial was developed, and it could be an oxidase mimic. Firstly, Co-N-C with oxidase-like activity boosts the chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) to produce the oxidized TMB (oxTMB). And the aromatic primary amino group of oxTMB reacts with nitrite (NO₂^-) to form diazo groups. Based on this background, we developed a cascade system of a Co-N-C-catalyzed oxidation reaction and a diazotization reaction for nitrite determination. The low detection limit (0.039 μM) indicates that Co-N-C is superior compared with the vast majority of previously reported nitrite assays. This study not only provides a novel nanozyme with sufficiently dispersed active sites, but it also further applies it to the determination of nitrite, which is expected to expand the application of nanozymes in colorimetric analysis.

Simplifying the complexity: Single enzyme (choline oxidase) inhibition-based biosensor with dual-readout method for organophosphorus pesticide detection

Organophosphorus pesticides (OPs) are widely used in agricultural production, but their residues could cause pollution to the environment and living organisms. In this paper, a simple dual-readout method for OPs detection was proposed based on ChOx single enzyme inhibition. Firstly, ChOx can catalyze the production of H₂O₂ from choline chloride (Ch-Cl). Bifunctional iron-doped carbon dots (Fe-CDs) with good peroxidase-like activity and superior fluorescence properties can catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxidized TMB (oxTMB) by H₂O₂ formed, and oxTMB could quench the fluorescence of Fe-CDs. In light of the fact that OPs exhibited activity in inhibiting ChOx, less H₂O₂ and the decreasing oxTMB led to a result that the fluorescence of the system recovered and the solution became lighter in blue color. Moreover, the process of ChOx inhibition by OPs was analyzed by molecular docking technique and it was found that OPs interact with key amino acid residues catalyzed by ChOx (Asn510, His466, Ser101, His351, Phe357, Trp331, Glu312). Finally, a dual-mode (colorimetry and fluorescence) sensor was created for the detection of OPs with the detection limit of 6 ng/L, and was successfully used in the quantitative determination of OPs in actual samples with satisfactory results.

"Three-in-one" platform based on Fe-CDs nanozyme for dual-mode/dual-target detection and NIR-assisted bacterial killing

As nanozymes, carbon dots (CDs) have attracted increasing attention due to their remarkable properties. Besides general enzyme activity, their photoluminescence and photothermal properties have been explored rarely, whereas their synergistic effects might produce CDs-based nanozymes of high performance. Here, iron-doped CDs (Fe-CDs) with tunable fluorescence and enhanced peroxidase-like activity were designed to develop a novel "three-in-one" multifunctional platform to provide dual-mode/dual-target detection and near infrared (NIR)-assisted antibacterial ability. This proposed strategy for a H₂O₂ test exhibited a wide linear relationship with a low limit of detection (LOD) of 0.16 μM (colorimetric) and 0.14 μM (ratiometric fluorescent). Furthermore, due to the nature of cholesterol being oxidized to H₂O₂ by cholesterol oxidase, sensitive and selective detection of cholesterol was realized, and the LOD was 0.42 μM (colorimetric) and 0.27 μM (ratiometric fluorescent), surpassing that reported previously. This result suggested that Fe-CDs could be used for dual-mode quantification of large family of H₂O₂-producing metabolites, thereby paving the way for developing multi-mode sensing strategies based on nanozymes. Moreover, this platform showed synergistic effects for antibacterial application, indicating great prospects for bacterial killing as well as wound disinfection and healing. Hence, this platform could contribute to the construction of multifunctional CDs with high performance.

Construction of fluorescence and colorimetric tandem dual-mode sensor by modulating fluorescence and oxidase-like activity via valence switching of cerium-based coordination polymer nanoparticles for sarcosine detection

A fluorescence and colorimetric tandem dual-mode sensor was established by modulating fluorescence and oxidase-like activity via valence switching of cerium-based coordination polymer nanoparticles (Ce-CPNs) for the detection of sarcosine (Sar) which is considered as a potential biomarker for the diagnosis of prostate cancer (PCa). In the present research, sarcosine oxidase (SOX) specifically catalyzes the oxidation of Sar to yield H₂O₂ which can rapidly oxidize Ce(III)-CPNs to generate Ce(IV)-CPNs in appropriate alkaline solution. The generated Ce(IV)-CPNs create a markedly weakened fluorescent signal at 350 nm, while they can induce oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to generate blue TMBox through emerging good oxidase-like activity. The sensing platform can realize accurate, stable, and high-throughput detection of Sar because of the tandem dual signal output mechanism. More attractively, the chromogenic hydrogel sensing device using smartphone photographing has achieved perfect results for the on-site sensing of Sar in urine specimens without large experimental equipments, demonstrating its considerable clinical application potential in the early diagnosis of PCa.

One-pot hydrothermal synthesis of metal-doped carbon dot nanozymes using protein cages as precursors

Metal-doped carbon dots represent a new class of promising nanomaterials with enzyme-like activity, whose properties such as fluorescence properties and enzyme-like activity are determined by the precursors and the conditions used to prepare them. Nowadays, the synthesis of carbon dots using naturally occurring precursors has attracted increasing attention. Here, using metal-loaded horse spleen ferritin as a precursor, we report a facile one-pot hydrothermal strategy to synthesise metal-doped fluorescent carbon dots with enzyme-like activity. The as-prepared metal-doped carbon dots exhibit high water solubility, uniform size distribution, and good fluorescence. In particular, the Fe-doped carbon dots exhibit prominent oxidoreductase catalytic activities, including peroxidase-like, oxidase-like, catalase-like, and superoxide dismutase-like activities. This study provides a green synthetic strategy for developing metal-doped carbon dots with enzymatic catalytic activity.

Antibacterial Carbon Dots-Based Composites

The emergence and global spread of bacterial resistance to conventionally used antibiotics have highlighted the urgent need for new antimicrobial agents that might replace antibiotics. Currently, nanomaterials hold considerable promise as antimicrobial agents in anti-inflammatory therapy. Due to their distinctive functional physicochemical characteristics and exceptional biocompatibility, carbon dots (CDs)-based composites have attracted a lot of attention in the context of these antimicrobial nanomaterials. Here, a thorough assessment of current developments in the field of antimicrobial CDs-based composites is provided, starting with a brief explanation of the general synthesis procedures, categorization, and physicochemical characteristics of CDs-based composites. The many processes driving the antibacterial action of these composites are then thoroughly described, including physical destruction, oxidative stress, and the incorporation of antimicrobial agents. Finally, the obstacles that CDs-based composites now suffer in combating infectious diseases are outlined and investigated, along with the potential applications of antimicrobial CDs-based composites.

Plasmon-Mediated Oxidase-like Activity on Ag@ZnS Heterostructured Hollow Nanowires for Rapid Visual Detection of Nitrite

Rational design of fast and sensitive determination of nitrite (NO₂^-) from a complicated actual sample overtakes a crucial role in constructing a high-efficiency sensing platform. Herein, a visual NO₂^- sensing platform with outstanding selectivity, sensitivity, and stability based on a surface plasmon resonance (SPR)-enhanced oxidase-like activity has been proposed. Benefiting from the intrinsic photocatalytic activity and limited light penetration of ZnS, the oxidase-like activity based on ZnS decorated on Ag nanowires (Ag@ZnS) is determined. It is demonstrated that the electrons are generated efficiently on the surface of ZnS and then transferred into the hot electrons of Ag with the help of localized SPR excitation, thus greatly oxidating the colorless 3,3',5,5'-tetramethylbenzidine (TMB) to produce dark blue oxidized TMB (oxTMB). When nitrite is added into the reaction system, the oxTMB will selectively react with NO₂^- to generate diazotized oxTMB, leading to a visual color change from dark blue to light green and subsequently to dark yellow. Owing to the specific recognition between nitrite and oxTMB, the recovery of catalytic activity induced an enhanced colorimetric test with a wider linear range for NO₂^- determination, an ultralow detection limit of 0.1 μM, excellent selectivity, and practicability for application in real samples. This plasmon-enhanced oxidase-like activity not only provides a smart approach to realize a colorimetric assay with high sensitivity and simplicity but also modulates oxidase-like activities.

Determination Methods of the Risk Factors in Food Based on Nanozymes: A Review

Food safety issues caused by foodborne pathogens, chemical pollutants, and heavy metals have aroused widespread concern because they are closely related to human health. Nanozyme-based biosensors have excellent characteristics such as high sensitivity, selectivity, and cost-effectiveness and have been used to detect the risk factors in foods. In this work, the common detection methods for pathogenic microorganisms, toxins, heavy metals, pesticide residues, veterinary drugs, and illegal additives are firstly reviewed. Then, the principles and applications of immunosensors based on various nanozymes are reviewed and explained. Applying nanozymes to the detection of pathogenic bacteria holds great potential for real-time evaluation and detection protocols for food risk factors.