Visual detection of melamine based on the peroxidase-like activity enhancement of bare gold nanoparticles

A dual-mode fluorometric/colorimetric sensor for sulfadimethoxine detection based on Prussian blue nanoparticles and carbon dots

A dual-mode (colorimetric/fluorescence) nanoenzyme-linked immunosorbent assay (NLISA) was developed based on Au-Cu nanocubes generating Prussian blue nanoparticles (PBNPs). It is expected that this method can be used to detect the residues of sulfonamides in the field, and solve the problem of long analysis time and high cost of the traditional method. Sulfadimethoxine (SDM) was selected as the proof-of-concept target analyte. The Au-Cu nanocubes were linked to the aptamer by amide interaction, and the Au-Cu nanocubes, SDM and antibody were immobilized on a 96-well plate using the sandwich method. The assay generates PBNPs by oxidising the Cu shells on the Au-Cu nanocubes in the presence of hydrochloric acid, Fe3+ and K3[Fe (CN)6]. In this process, the copper shell undergoes oxidation to Cu2+ and subsequently Cu2 + further quenches the fluorescence of the carbon point. PBNPs exhibit peroxidase-like activity, oxidising 3,3',5,5'-tetramethylbenzidine (TMB) to OX-TMB in the presence of H2O2, which alters the colorimetric signal. The dual-mode signals are directly proportional to the sulfadimethoxine concentration within the range 10- 3~10- 7 mg/mL. The limit of detection (LOD) of the assay is 0.023 ng/mL and 0.071 ng/mL for the fluorescent signal and the colorimetric signal, respectively. Moreover, the assay was successfully applied to determine sulfadimethoxine in silver carp, shrimp, and lamb samples with satisfactory results.

A colorimetric and SERS dual-mode aptasensor for the detection of Shiga toxin type II based on Mn/Fe-MIL(53)@AuNSs

Shiga toxin type II (Stx2), the major virulence component of enterohemorrhagic Escherichia coli, is strongly associated with the life-threatening hemolytic uremic syndrome thus posing a substantial risk to food safety and human health. In this work, a dual-mode aptasensor with colorimetric and surface-enhanced Raman scattering was developed for Stx2 specific detection based on noble metal nanoparticles and Raman reporter loaded metal-organic framework (Mn/Fe-MIL(53)@AuNSs-MBA). The Mn/Fe-MIL(53)@AuNSs could catalyze the H2O2-mediated oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), thereby enabling visual detection. Meanwhile, the SERS signal from MBA can be enhanced by the decorated AuNSs. Under optimal conditions, a linear range of 0.05-500 ng/mL with limit of detection (LOD) of 26 pg/mL was achieved in colorimetric mode and a linear range of 5-1000 ng/mL with LOD of 0.82 ng/mL in SERS mode, in which the dual-mode results complement each other, widening the linear range, increasing the accuracy and reliability of the detection. The method was further applied to the detection of Stx2 in milk with average recovery of 101.1 %, demonstrating its superior potential for bacterial toxin monitoring.

PEG mediated NiMn2O4 nanomaterials as a nano catalyst for peroxidase mimetic activity and photocatalytic degradation

Artificial peroxidases have garnered a lot of attention owing to their tremendous superiority over their natural counterparts. Here, NiMn2O4 nanoparticles have been successfully prepared through PEG assisted hydrothermal method. The varied PEG concentrations significantly altered the morphology and particle size of the synthesizedmaterials. We demonstrate the improved peroxide-like assay of different NiMn2O4 nanoparticles for the first time. Among them, Ni4 nanoparticles exhibit good peroxidase-like activity by generating the oxidation of chromogenic substrate 3, 3', 5, 5'-tetramethylbenzidine (TMB) in the presence of H2O2 and a blue color charge transfer product with an absorption maximum is positioned at 652 nm. These observations led to the development of a method for assessingH2O2 that can be read visually and photometrically. The Ni4 nanoparticles show enhanced kinetics compared to the natural enzyme horse radish peroxidase (HRP) with a lower Km (0.168 mM) value. Additionally, this Ni4 nanosphere applies as a visible light photocatalyst for the degradation of methylene blue (MB) and rhodamine B (Rh B) dyes under visible-light irradiation. Under optimized conditions, the degradationrates of MB and Rh B are 68 and 80.7 %, respectively, after 210 min, and recyclable efficiency is about 99 % for Rh B photocatalytic degradation in the first test and 98 % for five cycles, and about 98 % for MB photocatalytic degradation in the first test and 97 % for five cycles.

Aggregation-induced enhancement of peroxidase-mimetic activity of DNAzyme-gold nanoparticles for ultrasensitive detection of lead ions

Lead ion (Pb2+) detection is critically important in environmental protection and health management. In this work, we developed a simple signal-enhanced colorimetric sensor for the detection of Pb2+ based on the peroxidase-mimetic property of gold nanoparticles (AuNPs). When a certain concentration of Pb2+ was added to a solution of DNAzyme-modified AuNPs, aggregation was triggered, and the result was an enhancement of the peroxidase-mimetic activity of AuNPs. Then, the chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) by the catalyst of AuNPs was used for the sensitive UV-Vis and colorimetric detection of Pb2+. When a higher concentration of Pb2+ was added, the greater amount of aggregation of AuNPs resulted in the enhancement of the UV-Vis adsorption of the solution at 652 nm, with a deepening of the blue color of the solution. After optimization of the experimental conditions, a linear relationship between the absorbance of oxidized TMB at 652 nm and the logarithm of Pb2+ concentration was obtained, which had been divided into two parts (25 pM to 2.5 μM, and 2.5 μM to 250 μM). The detection limit was as low as 10 pM. The satisfactory specificity and rapid response of the sensor showed that it has promising application for the detection of Pb2+ in real samples.

CunO/Au heterostructure dendrimer anchored on Cu foam as dual functional catalytic nanozyme for glucose sensing by enzyme mimic cascade reaction

Multifunctional catalytic performance plays a crucial role in bio-applications through the diversity and durability of artificial nanozymes. An effective synergy with sufficient accessible active sites and high specific surface area is a challenge for composite catalysts, especially to avoid uncontrollable aggregation and structural instability. Here, we fabricated a Cu_nO/Au heterostructure dendrimer on copper foam (Cu_nO/Au HD/CF) as dual functional catalytic nanozyme to achieve enzyme mimic cascade reactions for efficient colorimetric analysis. A highly porous CF skeleton-based CuO nanowire array (CuO NWA) with a large specific surface area supported an efficient load capacity to assemble sufficient Cu_nO/Au HD by electrodeposition. The bimetallic Au-Cu nanozyme successfully achieved an oxidase-like and peroxidase-like cascade catalysis by a target-responsive sensing mechanism. Due to the confirmed catalytic performance of selectivity, anti-interference ability, and reproducibility, a Cu_nO/Au HD/CF-based quantitative analytical method was developed for glucose detection with a wide linear range and considerable detection limit of 8.4 μM. The robust nonenzymatic catalytic strategy for colorimetric detection not only confirmed the dual functional catalytic activity of Cu_nO/Au HD/CF, but also showed great potential for applications in clinical diagnostics and biochemical analysis.

Application of Peroxidase-Mimic Mn2BPMP Boosted by ADP to Enzyme Cascade Assay for Glucose and Cholesterol

The Mn2BPMP complex has an intrinsic peroxidase-like activity in the pH range of 5 to 8, especially a maximum activity at pH 7, while most peroxidase mimics operate at an acidic pH (mainly pH 4). Its peroxidase-like activity is high among small-molecule-based peroxidase mimics with a high reproducibility. In addition, we recently revealed that adenosine mono/diphosphate (AMP and ADP) significantly boosted the peroxidase-like activity of Mn2BPMP. These advantages imply that Mn2BPMP is suitable for biosensing as a substitute for horseradish peroxidase (HRP). Herein, we established a colorimetric one-pot assay system using the enzyme cascade reaction between analyte oxidase and ADP-boosted Mn2BPMP. The simple addition of ADP to the Mn2BPMP-based assay system caused a greater increase in absorbance for the same concentration of H2O2, which resulted in a higher sensitivity. It was applied to one-pot detection of glucose and cholesterol at 25 °C and pH 7.0 for a few minutes.

Enhanced peroxidase-like activity of bimetal (Fe/Co) MIL-101 for determination of tetracycline and hydrogen peroxide

The peroxidase-like activity of MIL-101(Fe/Co) is improved by adding tetracycline. On the basis of MIL-101(Fe/Co), fast colorimetric sensors of tetracycline and H2O2 have been successfully constructed.

Highly efficient peroxidase-like activity of a metal–oxide-incorporated CeO2–MIL(Fe) metal–organic framework and its application in the colorimetric detection of melamine and mercury ions via induced hydrogen and covalent bonds

The illegal addition of melamine to dairy products and the contamination of water with Hg2+ are serious threats to human health. This necessitates the search for new and efficient probe for detection of melamine in foods and Hg2+ in water samples.

In Situ Assembly of Nanomaterials and Molecules for the Signal Enhancement of Electrochemical Biosensors

The physiochemical properties of nanomaterials have a close relationship with their status in solution. As a result of its better simplicity than that of pre-assembled aggregates, the in situ assembly of nanomaterials has been integrated into the design of electrochemical biosensors for the signal output and amplification. In this review, we highlight the significant progress in the in situ assembly of nanomaterials as the nanolabels for enhancing the performances of electrochemical biosensors. The works are discussed based on the difference in the interactions for the assembly of nanomaterials, including DNA hybridization, metal ion-ligand coordination, metal-thiol and boronate ester interactions, aptamer-target binding, electrostatic attraction, and streptavidin (SA)-biotin conjugate. We further expand the range of the assembly units from nanomaterials to small organic molecules and biomolecules, which endow the signal-amplified strategies with more potential applications.

Gold and Silver Nanoparticle-Based Colorimetric Sensors: New Trends and Applications

Gold and Silver nanoparticles (AuNPs and AgNPs) are perfect platforms for developing sensing colorimetric devices thanks to their high surface to volume ratio and distinctive optical properties, particularly sensitive to changes in the surrounding environment. These characteristics ensure high sensitivity in colorimetric devices. Au and Ag nanoparticles can be capped with suitable molecules that can act as specific analyte receptors, so highly selective sensors can be obtained. This review aims to highlight the principal strategies developed during the last decade concerning the preparation of Au and Ag nanoparticle-based colorimetric sensors, with particular attention to environmental and health monitoring applications.

Three-dimensional MoS2 nanoflowers supported Prussian blue and Au nanoparticles: A peroxidase-mimicking catalyst for the colorimetric detection of hydrogen peroxide and glucose

Well-dispersed Prussian blue (PB) and Au nanoparticles (Au NPs) loaded three dimensional MoS₂ nanoflowers (PB-Au@MoS₂ NFs) was synthesized by a simple and economical method. The structure, morphology and composition of the hybrid were characterized by XRD, SEM and EDS. Similar to the reported literature, MoS₂ nanoflowers showed peroxidase-like activity in catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). This peroxidase-mimicking activity could be enhanced with the introduction of PB and Au NPs. Herein, PB-Au@MoS₂ NFs could be used to establish a new platform for the determination of H₂O₂ and glucose by the chromogenic reaction. Wide linear ranges with 0-15 μM and 0-120 μM for H₂O₂ and glucose detection were finally obtained. The detection limits were as low as 0.25 μM and 3 μM (with signal to noise ratio of 3), respectively. The established platform was also used successfully for the determination of glucose in human serum and fruit juice samples with excellent sensitivity and stability.

Nanozymes in Point-of-Care Diagnosis: An Emerging Futuristic Approach for Biosensing

Nanomaterial-based artificial enzymes (or nanozymes) have attracted great attention in the past few years owing to their capability not only to mimic functionality but also to overcome the inherent drawbacks of the natural enzymes. Numerous advantages of nanozymes such as diverse enzyme-mimicking activities, low cost, high stability, robustness, unique surface chemistry, and ease of surface tunability and biocompatibility have allowed their integration in a wide range of biosensing applications. Several metal, metal oxide, metal-organic framework-based nanozymes have been exploited for the development of biosensing systems, which present the potential for point-of-care analysis. To highlight recent progress in the field, in this review, more than 260 research articles are discussed systematically with suitable recent examples, elucidating the role of nanozymes to reinforce, miniaturize, and improve the performance of point-of-care diagnostics addressing the ASSURED (affordable, sensitive, specific, user-friendly, rapid and robust, equipment-free and deliverable to the end user) criteria formulated by World Health Organization. The review reveals that many biosensing strategies such as electrochemical, colorimetric, fluorescent, and immunological sensors required to achieve the ASSURED standards can be implemented by using enzyme-mimicking activities of nanomaterials as signal producing components. However, basic system functionality is still lacking. Since the enzyme-mimicking properties of the nanomaterials are dictated by their size, shape, composition, surface charge, surface chemistry as well as external parameters such as pH or temperature, these factors play a crucial role in the design and function of nanozyme-based point-of-care diagnostics. Therefore, it requires a deliberate exertion to integrate various parameters for truly ASSURED solutions to be realized. This review also discusses possible limitations and research gaps to provide readers a brief scenario of the emerging role of nanozymes in state-of-the-art POC diagnosis system development for futuristic biosensing applications.

Porous Au@Pt nanoparticles with superior peroxidase-like activity for colorimetric detection of spike protein of SARS-CoV-2

The development of colorimetric assays for rapid and accurate diagnosis of coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is of practical importance for point-of-care (POC) testing. Here we report the colorimetric detection of spike (S1) protein of SARS-CoV-2 based on excellent peroxidase-like activity of Au@Pt nanoparticles, with merits of rapidness, easy operation, and high sensitivity. The Au@Pt NPs were fabricated by a facile seed-mediated growth approach, in which spherical Au NPs were premade as seeds, followed by the Pt growth on Au seeds, producing uniform, monodispersed and porous Au@Pt core-shell NPs. The as-obtained Au@Pt NPs showed a remarkable enhancement in the peroxidase-mimic catalysis, which well abided by the typical Michaelis-Menten theory. The enhanced catalysis of Au@Pt NPs was ascribed to the porous nanostructure and formed electron-rich Pt shells, which enabled the catalytic pathway to switch from hydroxyl radical generation to electron transfer process. On a basis of these findings, a colorimetric assay of spike (S1) protein of SARS-CoV-2 was established, with a linear detection range of 10-100 ng mL^-1 of protein concentration and a low limit of detection (LOD) of 11 ng mL^-1. The work presents a novel strategy for diagnosis of COVID-19 based on metallic nanozyme-catalysis.

Fe(III)-mediated reversible catalytic activity of MoS2 nanozymes for bisphosphonate drug sensing

Peroxidase-like activity of MoS₂ quantum dots (QDs) can be reversibly regulated by means of Fe³⁺/alendronate sodium (ALDS)-induced aggregation/disaggregation of the QDs in solution. Specifically, Fe³⁺ can selectively aggregate the MoS₂ QDs and thus greatly enhance their peroxidase-like activity, while such enhancement can be inhibited in the presence of ALDS owing to the competitive coordination of ALDS with Fe³⁺. By regulating the enzyme-like activity of MoS₂ QDs, different colorimetric signal of a typical substrate of horseradish peroxidase, 3,3΄,5,5΄-tetramethylbenzidine, can be measured in the presence of H₂O₂. Based on this mechanism, we develop a colorimetric approach for the determination of ALDS and further applied in quality control of pharmaceutical products, utilizing either smartphone or UV-vis spectrometer as a readout. This detection method is rapid and selective, where derivatization of ALDS before detection is not needed. Such a smartphone-based colorimetric detection platform is promising to be applied in point-of-care testing at home, small clinics, or underdeveloped regions.

Photothermometric analysis of bismuth ions using aggregation-induced nanozyme system with a target-triggered surface cleaning effect

The development of nanozyme-based photothermometric sensing for point-of-care testing (POCT) heavy metal ions is of great significance for disease diagnosis and health management. Considering the low catalytic activity of most nanozymes at physiological pH, we found bismuth ions (Bi³⁺) could effectively enhance the peroxidase (POX)-like activity of cetyltrimethylammonium bromide and citrate-capped octahedral gold nanoparticle (CTAB/Cit-AuNP) nanozymes. It is mainly based on Bi³⁺ ions being able to trigger the surface cleaning effect of CTAB/Cit-AuNPs. Because the more active Bi³⁺ ions could effectively bind with citrate on the gold surface and competitively destroy the electrostatic interaction between citrate and CTAB, resulting in the removal of CTAB ligands from the gold surface. Without the ligand protection, CTAB/Cit-AuNPs aggregated immediately, and further resulted in a significant activation of the POX-like activity of AuNP nanozymes. Based on this principle, we introduced the enzyme substrate 3,3',5,5'-tetramethylbenzidine (TMB) into this aggregation-induced nanozyme system, and rationally designed a photothermometric platform to quickly and sensitively detect Bi³⁺ ions by using the good photothermal effect of the oxidation product of TMB (oxTMB). The developed photothermometric method only using a common thermometer has a limit of detection (LOD) as low as 45.7 nM for POCT analysis of Bi³⁺ ions. This study not only provides a more accurate understanding of the aggregation-induced nanozymes based on the surface cleaning principle, but also shows the potential applications of aggregation-induced nanozymes in the POCT field.

Chemical sensing with Au and Ag nanoparticles

Noble metal nanoparticles (NPs) are ideal scaffolds for the fabrication of sensing devices because of their high surface-to-volume ratio combined with their unique optical and electrical properties which are extremely sensitive to changes in the environment. Such characteristics guarantee high sensitivity in sensing processes. Metal NPs can be decorated with ad hoc molecular building blocks which can act as receptors of specific analytes. By pursuing this strategy, and by taking full advantage of the specificity of supramolecular recognition events, highly selective sensing devices can be fabricated. Besides, noble metal NPs can also be a pivotal element for the fabrication of chemical nose/tongue sensors to target complex mixtures of analytes. This review highlights the most enlightening strategies developed during the last decade, towards the fabrication of chemical sensors with either optical or electrical readout combining high sensitivity and selectivity, along with fast response and full reversibility, with special attention to approaches that enable efficient environmental and health monitoring.

Norfloxacin detection based on the peroxidase-like activity enhancement of gold nanoclusters

The use of nanomaterials as mimic enzymes provides a promising way to implement bio-molecule detection in living systems. However, to achieve highly efficient catalytic processes with gold nanocluster-based nanozymes is still challenging. In this study, a facile reduction method was utilized to synthesize gold nanoclusters with 1-methyl-D-tryptophan as the reducing and capping agent. The obtained gold nanoclusters exhibited a peroxidase-mimicking property in the redox reaction of 3,3',5,5'-tetramethylbenzidine to blue oxidized 3,3',5,5'-tetramethylbenzidine in the presence of H₂O₂. The addition of norfloxacin endowed the nanozymes with a 10-fold enhancement in catalytic efficiency due to the surface charge-controlled electron transfer modulation. The colorimetric sensing system presented a high selectivity toward norfloxacin. The good linear relationship of norfloxacin monitoring was gained in the range of 1.25~8.0 μM (R² = 0.996), with a detection limit of 0.2 μM. The practical application of the proposed protocol for the measurement of norfloxacin in capsules was realized. This demonstrates that on account of their significant catalytic efficiency enhancement, the gold nanocluster-based nanozymes hold great promise in realizing the selective detection of drugs. Graphical Abstract.

Protein-mediated wool-ball-like copper sulfide as a multifunctional nanozyme for dual fluorescence "turn-on" sensors of cysteine and silver ions

Developing a multifunctional nanozyme based biosensor with a convenient approach and high reliability is of vital interest for multiplex detection. In this study, wool-ball-like copper sulfide (WBLCS) was obtained facilely using an amphiphilic protein. The acidic amino acid residues and the amphiphilic properties of protein molecules play cooperative roles in the fabrication of hierarchical nanostructures. Unlike copper sulfide with irregular morphologies, the single component WBLCS acts as a multifunctional nanozyme possessing both superior cysteine oxidase- and peroxidase-mimicking activity. Fascinatingly, the addition of silver ions can significantly enhance the performance of the cascade system at a very low fluorescence substrate concentration. Based on this, dual fluorescence "turn on" sensors of cysteine and silver ions with extremely high sensitivity and selectivity are developed. This is the first report to explore multiple fluorescence "turn on" sensing systems based on one single nanozyme. Hence, the present finding has significant implication towards the design of superstructured nanozymes combining different multi-functionalities at the nanoscale for sensing multiplex target molecules sensitively and selectively in practice.

Conjugation of antibodies and aptamers on nanozymes for developing biosensors

Nanozymes are engineered nanomaterials with enzyme-like activities. Over the past decade, impressive progresses on nanozymes in biosensing have been made due to their unique advantages of high stability, low cost, and easy modification compared to natural enzymes. For many biosensors, it is critical to conjugate nanozymes to affinity ligands such as antibodies and aptamers. Since different nanomaterials have different surface properties, conjugation methods need to be compatible with these properties. In addition, the effect of biomolecules on nanozyme activity needs to be considered. In this review, we first categorized nanozyme-based biosensors into four parts, respectively describing noncovalent and covalent modifications with antibodies and aptamers. Meanwhile, recent advances in antibody and aptamer labeled nanozyme biosensors are summarized, and the methods of their conjugation are further illustrated. Finally, conclusions and future perspectives for the development and application of nanozyme bioconjugates are discussed.

Intrinsic peroxidase-like activity of 4-amino hippuric acid reduced/stabilized gold nanoparticles and its application in the selective determination of mercury and iron in ground water

Herein, we report a method for the synthesis of 4-aminohippuric acid (4-AHA) reduced/stabilized gold nanoparticles and their peroxidase mimicking properties for the colorimetric detection of Fe³⁺ and Hg²⁺. The synthesis of nanoparticles was evidenced by appearance of bright red color and an absorption peak at 518 nm. Transmission electron microscopic (TEM) characterization revealed the nanoparticles to be spherical with average size of about 5.9 ± 1.7 nm. X-ray diffraction (XRD) analysis established highly crystalline nature of the nanoparticles. The synthesized nanoparticles have shown very good peroxidase mimicking property; exhibiting the catalytic oxidation of the chromogen 3,3',5,5'-tetramethyl benzidine (TMB) to a blue color product, in the presence of hydrogen peroxide. The peroxidase mimicking activity of the nanoparticles was found to be selectivity enhanced in the presence of Fe³⁺ and Hg²⁺ while there was no change in the activity in the presence of other concomitant ions. The mechanism studies revealed that the synthesized gold nanoparticles assisted in electron transfer during the catalytic process however the stimulation of peroxidase-like activity in the presence of Fe³⁺ and Hg²⁺ is owed to both generation of hydroxyl radical and accelerated electron transfer. The assay was made selective for iron by the addition of cysteine in acetate buffer; whereas the selective detection of mercury was achieved by carrying out the assay in citrate buffer. The linear ranges for the determination of Fe³⁺ and Hg²⁺ in deionized water were found to be: 5-50 ppb and 5-200 ppb respectively. The limits of quantification (LOQ) for Fe³⁺ and Hg²⁺ were 4.0 and 2.5 ppb respectively. The assay was applied for the determination of Fe³⁺ and Hg²⁺ in drinking and ground water samples. The method holds potential for the on-field screening of these metal ions in real environmental samples.

A colorimetric biosensor for simultaneous ochratoxin A and aflatoxins B1 detection in agricultural products

Colorimetric biosensors have been widely applied to mycotoxins testing. However, the colorimetric assay previously reported used a single color to detect one mycotoxin, and there were few reports on the simultaneous detection of multiple mycotoxins. In this work, a colorimetric biosensor for dual mycotoxins detection was developed. A Fe₃O₄/GO based platform for aflatoxins B1 (AFB1) detection and a Fe₃O₄@Au based platform for ochratoxin A (OTA) detection were fabricated. The quantification of OTA and AFB1 was respectively achieved by the release of thymolphthalein under alkaline conditions and 3,3',5,5'-tetramethylbenzidine was catalyzed by Au NPs under acidic conditions. Because of different conditions, two sensing methods didn't interfere with each other but could provide a higher detection efficiency. The detection range of AFB1 is 5-250 ng·ml^-1 and that of OTA is 0.5-80 ng·ml^-1. This biosensor has been successfully applied in real sample detection, which has a broad application prospect in fields of food safety.

Controllable and robust dual-emissive quantum dot nanohybrids as inner filter-based ratiometric probes for visualizable melamine detection

The ratiometric fluorescence technique is of great interest due to its visualization characteristics. The construction of a reliable fluorescent ratiometric nanoprobe for high-sensitivity visual quantification is highly sought after but it is limited by poor stability and controllability. Herein, we report a robust dual-emissive quantum dot nanohybrid with precise color tunability and demonstrate its potential as a two-signal-change ratiometric probe for visual detection. A novel assembly strategy was developed for spatially implanting hydrophobic green and red quantum dots (QDs) into a silica scaffold to form a dual-emissive hierarchical fluorescent silica nanohybrid. The fluorescence intensity ratio and color of the nanohybrid were precisely tailored by altering the amounts of green and red QDs. Particularly, after the alkylsilane-mediated phase transfer and exterior silica shell growth, the nanohybrid exhibited the well-preserved fluorescence features of the original QDs and robust optical/colloid stability. An inner filter-based ratiometric nanoprobe for the visual determination of melamine was ultimately devised by combining the spectra-overlapped two-colored fluorescent nanohybrid with analyte-specific gold nanoparticles. Furthermore, based on the reversible fluorescence signal changes in two-colored QDs induced by melamine, a logic gate strategy for melamine monitoring was constructed. The newly developed fluorescent ratiometric nanoprobe shows great prospects for the visual and quantitative determination of analytes in a complex biological matrix.

Colorimetric determination of F-, Br- and I- ions by Ehrlich's bio-reagent oxidation over enzyme mimic like gold nanoparticles: Peroxidase-like activity and multivariate optimization

Citrate and polyvinyl alcohol capped gold nanoparticles (PVA-GNPs) were synthesized via chemical reduction technique and fully characterized by DLS, SEM, EDS, XRD, UV-Vis and FT-IR analysis. A simple and practical colorimetric sensor based on red-ox reaction of p-dimethylaminobenzaldehyde (DABA) as ehrlich's bio-reagent and Au(III) with H₂O₂ on PVA-GNPs mimic catalyst with enzyme-like activity, has been fabricated for determination of F^-, Br^- and I^- halide anions. Prepared PVA-GNPs, can simultaneously catalyze the disintegration of H₂O₂, that used to reduce Au(III) ions into co-doped Au-NPs and oxidation of p-dimethylaminobenzaldehyde ehrlich's bio-reagent while in the presence of halide ions Au-X complex can be formed and improved sensor selectivity. Halide ions (F^-, Br^- and I^-) effectively diminishes the catalytic activity of GNPs to disintegrate oxygenated water by the interaction among Au⁺ and Au⁰ and suppressing oxidation of p-dimethylaminobenzaldehyde ehrlich's bio-reagent. In this system which contains PVA-GNPs, H₂O₂, p-dimethylaminobenzaldehyde ehrlich's bio-reagent, and Au(III), increasing the halide ions (F^-, Br^- and I^-) concentration show color changes from deep green to red. In view of this rule, in this work, a novel colorimetric technique for sensitive determination of F^-, Br^- and I^- was developed. This method has the detection limits of 2.60 × 10^-6 M, 6.64 × 10^-8 M and 9.93 × 10^-9 M and linear ranges between 1.98 × 10^-5-1.22 × 10^-3 M, 1.99 × 10^-6-2.0 × 10^-4 M and 1.07 × 10^-7- 2.86 × 10^-5 M for F^-, Br^- and I^-, respectively. Assays are highly selective over other ions. They effectively applied to detection of halide ions in real water samples.

Nano-Affi: a solution-phase, label-free, colorimetric aptamer affinity assay based on binding-inhibited aggregation of gold nanoparticles

The ideal way to assess aptamer affinity is when both aptamer and target are in a native state, without the unpredictable interference associated with labelling and surface immobilization.

AuNPs@PMo12nanozyme: highly oxidase mimetic activity for sensitive and specific colorimetric detection of acetaminophen

The design of a highly specific and sensitive approach for the quantitative and qualitative determination of acetaminophen (AP) is crucial from a human health point of view. In this study, AuNPs@PMo₁₂, as a nanozyme, has been developed for the highly sensitive and selective detection of AP with 3,3′,5,5′-tetramethylbenzidine (TMB) within a few seconds without adding oxidizing reagents (e.g. H₂O₂). Synthesized nanosensors are able to oxidize TMB to yellow-brown oxidized TMB (oxTMB). The maximum peak wavelength of oxTMB was observed at 450 nm. The addition of AP and then increasing its concentration led to the production of different products in blue color. In experimental measurements, the limit of detection was obtained as 14.52 mg L⁻¹. The quantitative determination of AP concentrations can be carried out using UV-vis spectroscopy. The design of nanosensors is cost-effective and application of them in H₂O₂-free and enzyme-free conditions provides a rapid sensing approach for practical use in disease monitoring and diagnosis.

The design of a highly specific and sensitive approach for the quantitative and qualitative determination of acetaminophen (AP) is crucial from a human health point of view.

Aggregation-induced phosphorescence quenching method for the detection of picric acid based on melamine-passivated Mn-doped ZnS quantum dots

Melamine (MA)-passivated Mn-doped ZnS quantum dots (QDs) were synthesized by a hydrothermal method. The MA-passivated QDs can form a charge-transfer complex with picric acid (PA) at 80 °C, thereby quenching the phosphorescence of the QDs. A sensitive method for detecting PA was established based on this principle of phosphorescence quenching. When the PA concentration ranged from 2.0 to 180 ng mL^-1, the concentration was linearly related to the quenching intensity of the QDs, with a detection limit of 1.4 ng mL^-1. When applied to detect PA in environmental water samples, the proposed method showed superior analytical performance compared with other PA analysis methods. Furthermore, we verified the static quenching mechanism by density functional theory. MA on the surface of QDs and PA formed a stable structure with a binding energy of 12.43 eV.

Colorimetric sensing for ascorbic acid based on peroxidase-like of GoldMag nanocomposites

A new scheme for sensitive and rapid colorimetric detection of ascorbic acid (AA) has been developed by the GoldMag-ABTS free radical scavenging system. The well-dispersed Gold and magnetic particles (GoldMag) was successfully prepared by self-assembly method and characterized by Fourier transform infrared (FTIR), X-Ray Photoelectron Spectroscopic (XPS) techniques. Nanocomposites combine the advantages of superparamagnetic, biocompatibility and high catalytic activity of Fe₃O₄ and gold nanoparticles (AuNPs) and exhibit enhanced the intrinsic peroxidase-like activity, which can be used to catalyze the oxidation of the peroxidase substrates 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) produce a green-colored product in presence of hydrogen peroxide. Ascorbic acid as an effective antioxidant have scavenging effects on ABTS radicals and induce the reduction of green ABTS.⁺ to colorless ABTS^2-, resulting in a significant green color fading. On this basis, a rapid, sensitive and selective colorimetric assay for ascorbic acid has been developed. Under optimal conditions, ascorbic acid has a linear response range from 0.01 mmol/L to 1 mmol/L with a detection limit of 0.12 μmol/L and a short assay time of the 30 s. Furthermore, the colorimetric system showed good sensitivity, stability, selectivity, and repeatability. It also successfully applied to the determination of ascorbic acid in real samples.

Colorimetric method for glucose detection with enhanced signal intensity using ZnFe2O4-carbon nanotube-glucose oxidase composite material

In this paper, a composite material comprised of ZnFe2O4 nanomaterial, carbon nanotubes (CNT) and glucose oxidase (GOD) was synthesized and used for glucose detection. ZnFe2O4-CNT was formed by a one-step solvothermal approach using acid-treated CNT as precursor, then GOD was linked to it by coupling reaction between -NH2 and -COOH. After addition of glucose, which is oxidized by GOD, the intermediate product (H2O2) further oxidizes the 3,3',5,5'-tetramethylbenzidine (TMB) substrate and forms a blue product. This process was accelerated in the presence of peroxidase-mimic ZnFe2O4 nanomaterial and the detected signal intensity was correspondingly enhanced. The linear detection range of glucose was 0.8 to 250 μM, with a limit of detection of 0.58 μM. This may originate from (1) the limited diffusion of intermediate species, which resulted in enhanced local concentrations of reaction compounds; (2) enhanced electron transmission among CNT, GOD and ZnFe2O4; (3) the synergistic enhancement of catalytic activity of ZnFe2O4 compared with other metal oxides; (4) the high loading capacity of ZnFe2O4-CNT for GOD molecules, because of its high surface-to-volume ratio. Meanwhile, this method has reasonable selectivity, stability and reusability and can be used for real serum detection, which may be useful for the development of sensitive biosensors.

Catalytic Mechanisms of Nanozymes and Their Applications in Biomedicine

The research on nanozymes has increased dramatically in recent years and a new interdiscipline, nanozymology, has emerged. A variety of nanomaterials have been designed to mimic the characteristics of natural enzymes, which connects an important bridge between nanotechnology and biological science. Unlike natural enzymes, the nanoscale properties of nanozymes endow them with the potential to regulate their enzymatic-like activity from different perspectives. The mechanisms behind those methods are intriguing. In this Review, we introduce these mechanisms from the aspects of surface chemistry, surface modification, molecular imprinting, and hybridization and then focus attention on some specific catalytic mechanisms of several representative nanozymes. The applications of nanozymes ranging from bioassay, imaging, to disease therapy are also discussed in detail to prove the fact that the inherent physicochemical properties of nanomaterials not only make nanozymes the analogues of biological enzymes, but also endow them with incomparable advantages and broad prospects in biomedical fields. Finally, four characteristics and some challenges of nanozymes are summarized.

Peroxidase-Like Activity of Smart Nanomaterials and Their Advanced Application in Colorimetric Glucose Biosensors

Diabetes is a dominating health issue with 425 million people suffering from the disease worldwide and 4 million deaths each year. To avoid further complications, the diabetic patient blood glucose level should be strictly monitored despite there being no cure for diabetes. Colorimetric biosensing has attracted significant attention because of its low cost, simplicity, and practicality. Recently, some nanomaterials have been found that possess unexpected peroxidase-like activity, and great advances have been made in fabricating colorimetric glucose biosensors based on the peroxidase-like activity of these nanomaterials using glucose oxidase. Compared with natural horseradish peroxidase, the nanomaterials exhibit flexibility in structure design and composition, and have easy separation and storage, high stability, simple preparation, and tunable catalytic activity. To highlight the significant progress in the field of nanomaterial-based peroxidase-like activity, this work discusses the various smart nanomaterials that mimic horseradish peroxidase and its mechanism and development history, and the applications in colorimetric glucose biosensors. Different approaches for tunable peroxidase-like activity of nanomaterials are summarized, such as size, morphology, and shape; surface modification and coating; and metal doping and alloy. Finally, the conclusion and challenges facing peroxidase-like activity of nanomaterials and future directions are discussed.