Sensitive Colorimetric Detection Research Articles

Ultrafast sensitive colorimetric detection of cyanide based on dissolving AgI on Au-AgI heterodimeric nanoparticles via UV–vis spectroscopy and single nanoparticle imaging

The high toxicity of cyanide and its great variety of uses in industrial processes make it necessary to develop sensitive and selective sensors for its determination in food and environmental samples. However, the developed methods usually require long time and expensive instrument, hindering on-site detection. In this work, novel colorimetric assays for cyanide detection were achieved by using UV–vis spectroscopy and single nanoparticle (NP) imaging technology based on a mechanism wherein cyanide dissolves AgI on Au-AgI heterodimeric NPs by forming Ag(CN)2-. The reaction occurred in less than 1 s, leading to the change from Au-AgI NPs to Au NPs and thus blueshift of localized surface plasmon resonance. The solution color turned to purple, and then pink from blue, while single NPs under darkfield microscopy transformed from red to yellow and green. In UV–vis spectroscopy, the shift of peak wavelength (Δλ) is linear to cyanide concentration with an equation of Δλ=0.8784 Ccyanide-2.4910 over the range of 0–75 µM. Under darkfield microscopy, the linear regression equations were G/R=0.0677 Ccyanide+0.3717 (R2=0.992) with cyanide over the range of 0.2–4.0 μM and G/R=0.2945 Ccyanide+0.4835(R2=0.994) in the range of 4.0–15.0 μM, where G/R is the intensity ratio in green and red channel. The methods were applied for analysis of waters and foods, achieving recovery rates of 97.4–106.3% for UV–vis spectroscopy and 98.6–108.1% for single NP imaging. The techniques offer expeditious, highly discriminating, and on-site capable solutions for cyanide detection, with high practicality.

MOFs@POMs-derived bimetallic oxide Fe2(MoO4)3 nanoparticles for sensitive colorimetric detection of salicylic acid in aspirin.

Acolorimetric sensing method for salicylic acid (SA) was developed by designing and fabricating bimetallic oxide nanozymes. Firstly, by calcinating MIL-100(Fe)@PMo12 (MOFs@POMs) at different temperature, Fe2(MoO4)3-Ts (T = 400℃, 500℃, 600℃, 700℃) nanoparticles (NPs) were successfully prepared. Secondly, by evaluating the peroxidase-like activities, Fe2(MoO4)3-600 NPs shows the best peroxidase-like activity attributed to the Fenton-like effect and the synergistic coupling interaction between Mo and Fe. Finally, based on the specific complexation between SA and Fe3+, a sensitive colorimetric sensor for SA was established, which exhibits superior selectivity and interference with a detection limit of 0.11μM anda linear range of 10 to 100μM, the lowest LOD for SA to date, to the best of our knowledge.

Enhanced detection of copper residues on citrus surfaces: An innovative copper fenton-catalysed colorimetric approach using creatinine and molybdenum trioxide

Accurate quantification of residual copper from inorganic copper reagents on the surface of citrus fruits is crucial for preventing diseases caused by excessive heavy metal intake. In this study, we developed a rapid colorimetric strategy for copper detection that is enhanced under near-neutral pH conditions, using molybdenum trioxide (MoO3) and creatinine for copper-Fenton catalysis. The introduction of MoO3 enriches the redox reactions of Cu2+/Cu+ within the complex formed by copper ions and creatinine, leading to enhanced copper-Fenton catalytic activity at near-neutral pH. In the presence of H2O2, this system oxidises colourless 3,3′,5,5′-tetramethylbenzidine (TMB) to blue oxTMB, thereby affording a sensitive colorimetric detection platform for Cu2+. The designed method offers high sensitivity for Cu2+ detection, with a linear range of 0.011.0 μg/mL and a detection limit as low as 0.007 μg/mL. This method was effectively used to analyse copper reagent residues on citrus surfaces, achieving recovery rates of 92.0%115.0%. With its high sensitivity, selectivity, simplicity, and low cost, this colorimetric method shows great potential as an effective approach for determining copper ion levels, particularly for monitoring copper residues in agricultural products.

Fe-Co-Based Metal-Organic Frameworks as Peroxidase Mimics for Sensitive Colorimetric Detection and Efficient Degradation of Aflatoxin B1.

Building multifunctional platforms for integrating the detection and control of hazards has great significance in food safety and environment protection. Herein, bimetallic Fe-Co-based metal-organic frameworks (Fe-Co-MOFs) peroxidase mimics are prepared and applied to develop a bifunctional platform for the synergetic sensitive detection and controllable degradation of aflatoxin B1 (AFB1). On the one hand, Fe-Co-MOFs with excellent peroxidase-like activity are combined with target-induced catalyzed hairpin assembly (CHA) to construct a colorimetric aptasensor for the detection of AFB1. Specifically, the binding of aptamer with AFB1 releases the prelocked Trigger to initiate the CHA cycle between hairpin H2-modified Fe-Co-MOFs and hairpin H1-tethered magnetic nanoparticles to form complexes. After magnetic separation, the colorimetric signal of the supernatant in the presence of TMB and H2O2 is inversely proportional to the target contents. Under optimal conditions, this biosensor enables the analysis of AFB1 with a limit of detection of 6.44 pg/mL, and high selectivity and satisfactory recovery in real samples are obtained. On the other hand, Fe-Co-MOFs with remarkable Fenton-like catalytic degradation performance for organic contaminants are further used for the detoxification of AFB1 after colorimetric detection. The AFB1 is almost completely removed within 120 min. Overall, the introduction of CHA improves the sensing sensitivity; efficient postcolorimetric-detection degradation of AFB1 reduces the secondary contamination and risk to the experimental environment and operators. This strategy is expected to provide ideas for designing other multifunctional platforms to integrate the detection and degradation of various hazards.

Absorption-based colorimetric detection of nickel(II) ion by phase separation of thermoresponsive magnetic nanoparticles under microflow.

Therma-Max™ LSA Streptavidin is a thermoresponsive magnetic nanoparticle (TMNP). It can be introduced conveniently to molecular recognition groups by avidin-biotin interaction. In this study, we demonstrated the detection of nickel(II) ions by the magnetic separation of TMNP induced by their phase transition under microflow. The NTA-tagged TMNP solution mixed with a Ni2+ sample was introduced into a microchannel with a well structure. Moreover, the sample was heated to induce the thermally induced aggregation of TMNP. The Ni-capturing TMNP were trapped in the well by magnetic fields. The supernatant was removed from the outlet, and a dimethylglyoxime (DMG) solution was introduced into the device for colorimetric detection in the well. Because DMG has a higher stability constant with Ni2+, sensitive colorimetric detection of Ni2+ can be achieved in devices where the sample volume, e.g., optical pathlength, is short. To demonstrate the feasibility of the proposed method, a recovery test was conducted using a commercially available cosmetic sample. Therein, complete collection was achieved.

Near-Infrared Responsive Ag@Au Nanoplates with Exceptional Stability for Highly Sensitive Colorimetric and Photothermal Dual-Mode Lateral Flow Immunoassay.

Lateral flow immunoassay (LFIA) has played a vital role in point-of-care (POC) testing on account of its simplicity, rapidity, and low cost. However, the low sensitivity and difficulty of quantitation limit its further development. Sensitive markers with new detection modes are being developed to dramatically improve LFIA's performance. Herein, a ligand-complex approach was proposed to uniformly coat a thin layer of Au onto Ag triangular nanoplates (Ag TNPs) without etching the Ag cores, which not only retain the unique optical properties from Ag TNPs but also acquire the surface stability and biocompatibility of gold. The localized surface plasmon resonance absorption of these Ag@Au TNPs could be finely adjusted from visible (550 nm) to the second near-infrared region (NIR-II) (1100 nm), and even longer, by simply adjusting the ratio between edge length and thickness. Utilizing the Ag@Au TNPs as new markers for LFIA, a highly sensitive colorimetric and photothermal dual-mode detection of the SARS-CoV-2 nucleocapsid protein was achieved with a very low background. The Ag@Au TNPs showed an exceedingly high photothermal conversion efficiency of 61.4% (ca. 2 times higher than that of Au nanorods), endowing the LFIA method with a low photothermal detection limit (40 pg/mL), which was 25-fold lower than that of the colorimetric results. The generality of the method was further verified by the sensitive and accurate analysis of cardiac troponin I (cTnI). This method is robust, reproducible, and highly specific and has been successfully applied to SARS-COV-2 detection in 35 clinical samples with satisfactory results, demonstrating its potential for POC applications.

Low-cost and highly sensitive colorimetric and visual detection of amikacin in milk using melamine functionalized gold nanoparticles.

This study developed a rapid, low-cost and highly sensitive analytical method using melamine functionalized gold nanoparticles (MA-Au NPs) as a colorimetric sensor to detect amikacin in milk. The MA-Au NPs were synthesized using a self-assembly process and analyzed using absorption spectroscopy, transmission electron microscopy (TEM), and Fourier transform infrared (FT-IR) spectroscopy. The MA-Au NPs exhibited a distinct surface plasmon resonance (SPR) absorption peak at 520 nm and demonstrated excellent spherical dispersion. The reaction parameters were optimized, with a Au NP concentration of 2.0 × 10-7 mol L-1, MA concentration of 1.0 × 10-6 mol L-1, pH of 4.0, and reaction time of 15 minutes. In the optimization experiment, the effects of unmodified Au NPs and MA-Au NPs as colorimetric sensors for amikacin determination were compared. As a specific amount of amikacin was added to the Au NP solution, the reaction solution showed a visible change from wine red to purple and then to blue gray; a red-shift in the absorption spectrum of the solutions was observed, which could also be analyzed using the A640/A520 value, and the color changes were recorded using smartphones for visual detection through RGB (red-green-blue) data image analysis, which made the detection easy and fast. As a result, two approaches were devised using visible absorption spectra and smartphones. A linear correlation was established between the A640/A520 value and the concentration of amikacin in the range of 4.0 to 16.0 × 10-8 mol L-1. The minimal limit of detection (LOD) for absorption spectrum analysis was 3.75 × 10-9 mol L-1, whereas for smartphone analysis, it was 6.0 × 10-8 mol L-1. The recoveries ranged from 103.09% to 104.51%, while the relative standard deviations (RSDs) ranged from 3.76% to 4.25%. The proposed method was used to detect actual milk samples. The results showed that the method is simple, selective, and practical.

A low iridium content greatly improves the peroxidase-like activity of noble metal nanozymes for sensitive colorimetric detection.

The enzyme-like activity of noble metal nanomaterials has been widely demonstrated. However, as an important noble metal, iridium (Ir) and its alloy nanomaterials have been less studied, particularly regarding the effect of Ir content on enzyme-like activity. Here, we demonstrated for the first time that a low Ir content can greatly improve the peroxidase-like activity of Pt-based nanozymes. When the weight percentage of Ir was 3.45% in trimetallic PtAuIr hollow nanorods (HNRs) and 2.86% in bimetallic PtIr HNRs, their specific activity increased by approximately 70% compared to their PtAu and Pt counterparts, respectively. However, a slightly higher percentage of Ir significantly diminished the enhancement effect on their specific activity. Density functional theory (DFT) calculations show that the rate-determining step (RDS) energy barrier of the nanozyme with low Ir content is lower than that of the nanozyme with slightly higher Ir content. Furthermore, we studied the kinetic properties of the PtAuIr nanozyme using TMB as the substrate. Its Michaelis-Menten constant (Km) and Vmax were 1.756 mM and 2.152 × 10-6 M s-1, respectively. Additionally, a colorimetric detection platform based on the PtAuIr nanozyme was established and applied to detect o-phenylenediamine (OPD), with a detection limit as low as 0.076 μM. This study highlights the important role of the Ir content in Pt-based nanozymes and demonstrates that PtAuIr nanozymes have potential applications in environmental detection.

A dual-ligand lanthanide-based metal-organic framework for highly selective and sensitive colorimetric detection of Fe2.

Accumulation of heavy metals in humans and mammals causes health problems due to their abundance as transition metal ions. Iron (Fe2+) serves significantly in numerous biological processes as a heavy metal ion. In this study, we have designed and prepared a metal-organic framework (MOF) utilizing a one-step solvothermal process, incorporating a dual-ligand combination of terephthalic acid (H2BDC) and α,α',α''-tert-pyridine (TPY) with Eu3+ as the metal node. For this MOF, we termed it Eu-BDC/TPY. Eu-BDC/TPY has superior selectivity over other metal cations. It provides an accurate, sensitive, broad linear range colorimetric method for detecting Fe2+ in a concentration range of 1-50 μM with a modest limit of detection (0.33 μM). Eu-BDC/TPY detects the absence of Fe2+ quickly (within 5 seconds), which is very valuable in practical applications. In addition, the results can be used to create a digital image colorimetric card (DIC) using colorimetric software, enabling instantaneous detection of Fe2+ concentration using a smartphone.

Eu MOF-enhanced FeNCD nanozymes for fluorescence and highly sensitive colorimetric detection of tetracycline.

The constrained enzymatic activity and aggregation challenges encountered by small-sized nanozymes pose obstacles to their practical utility, necessitating a strategy to mitigate aggregation and boost enzymatic catalytic efficiency. In this work, a negatively charged Eu MOF was utilized as the encapsulation matrix, encapsulating the small-sized nanozymes FeNCDs into the Eu MOF to synthesize an FeNCDs@Eu MOF. The dispersibility of the encapsulated FeNCDs was increased, and owing to the negative charge of the FeNCDs@Eu MOF, electrostatic pre-concentration of the positively charged target molecule tetracycline (TC) was facilitated, thereby amplifying the enzymatic catalytic efficiency of the FeNCDs. The response of the FeNCDs to TC increased by nearly 6 times upon encapsulation. The TC detection limit (LOD) of the FeNCDs@Eu MOF-based sensor is as low as 11.63 nM. The incorporation of fluorescence detection expanded the linear range of the sensor, rendering it more suitable for practical sample detection.

A two-dimensional coordination polymer with high laccase-like activity for sensitive colorimetric detection of thiram.

In contrast to natural enzymes, nanozymes show promising advantages of low cost and high stability for analytical applications. The simple mix of L-phenylalanine (F) and Cu2+ produces two-dimensional nanosheets of a coordination polymer with a high surface area ratio and rich exposed active sites as a novel catalyst. As the mimetic of natural laccase, this nanozyme (F-Cu) can catalyze the oxidative coupling reaction of 2,4-dichlorophenol (2,4-DP) and 4-aminoantipyrine (4-AP) to produce a distinct red product, thus establishing an intuitive and simple method for the detection of thiram. In the range of 0-7.5 μM, the absorbance intensity was linearly related to the concentration of thiram, and the detection limit was 0.0845 μM. The F-Cu nanozyme was successfully applied to the colorimetric detection of thiram in real samples.

Molecularly imprinted sensor based on cascade enzyme system supported by metal-organic framework (UiO-66-NH2) for sensitive colorimetric detection of cholesterol

Cholesterol is a crucial cellular molecule in the human body, and a neutral lipid necessary for maintaining the integrity and fluidity of cell membranes. Additionally, it acts as a precursor for the synthesis of essential substances for living organisms, including steroid hormones, bile acids and vitamins. Nonetheless, cholesterol is not always advantageous, and both excess and deficiency of cholesterol can result in serious diseases. Therefore, maintaining cholesterol at a normal level is of paramount importance for the maintenance of normal cellular activity. In this study, an amino-functionalized metal-organic framework (UiO-66-NH2, abbreviated as U6NH2) was used as a carrier, on the surface of which gold nanoparticles (AuNPs) was embedded and cholesterol oxidase (ChOx) was immobilized, and then cholesterol was employed as a template molecule, followed by the synthesis of molecularly imprinted polymers (MIPs). Remarkably, the obtained U6NH2@AuNPs-ChOx@MIPs can be used to facilitate efficacious and selective detection of blood cholesterol through a facile colorimetric method over a range of 2.9 mM - 6.7 mM with a limit of detection (LOD) = 2.4 mM, while the normal level of blood cholesterol should be less than 5.18 mM. Moreover, the sensing system demonstrates excellent stability and satisfactory selectivity. The constructed cholesterol selective detection system reported in this paper is anticipated to have a crucial impact on bioanalysis.

Flower-like L-Cys-FeNiNPs nanozyme aptasensor for sensitive colorimetric detection of aflatoxin B1

It is vital to create a sensitive and trustworthy sensor for aflatoxin B1 (AFB1) considering the potential hazards this substance poses to the safety of food. Herein, a colorimetric sensor, namely, aptamer-modified flower-like L-cysteine-functionalized FeNi bimetallic nanoparticles (L-Cys-FeNiNPs) with peroxidase-mimicking enzymatic activity, was established for the detection of aflatoxin B1. Interestingly, based on the synergies between FeNi bimetals and the protection and regulation of morphology by L-Cys on FeNiNPs, we successfully synthesized flower-like L-Cys-FeNiNPs, which were employed as outstanding peroxidase-mimic enzymes to oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H2O2. Kinetic analysis showed that the flower-like L-Cys-FeNiNPs have an ultralow Michaelis constant (Km) and an ultrahigh affinity for H2O2. Furthermore, based on the peroxidase-mimicking activity of flower-like L-Cys-FeNiNPs, a new colorimetric aptamer sensing strategy was employed for the easy and specific detection of AFB1 with a wide response range (0.12–2 μg/mL) and a low detection limit (36.57 ng/mL). Finally, to verify the practicality of this aptamer detection strategy, we successfully applied it to the detection of AFB1 in actual millet and maize samples.

A versatile CuCo@PDA nanozyme-based aptamer-mediated lateral flow assay for highly sensitive, on-site and dual-readout detection of Aflatoxin B1

Mycotoxin contaminations in food and environment seriously harms human health. Constructing sensitive and point-of-test early-warning tools for mycotoxin determination is in high demand. In this study, a CuCo@PDA nanozyme-based aptamer-mediated lateral flow assay (Apt-LFA) has been elaborately designed for on-site and sensitive determination of mycotoxin Aflatoxin B1 (AFB1). Benefiting from the rich functional groups and excellent peroxidase-like activity, the CuCo@PDA with original dark color can be conjugated with the specific recognition probe (i.e., aptamer), generating colorimetric signal on the test lines of Apt-LFA via a competitive sensing strategy. The signal can further be amplified in-situ by catalytic chromogenic reaction. Therefore, a visual and dual-readout detection of AFB1 has been realized. The developed Apt-LFA provides a flexible detection mode for qualitative and quantitative analysis of AFB1 by naked-eyes observation or smartphone readout. The smartphone-based LFA platform shows a reliable and ultrasensitive determination of AFB1 with the limit of detection (LOD) of 2.2 pg/mL. The recoveries in the real samples are in the range of 95.11–113.77% with coefficients of variations less than 9.84%. This study provides a new approach to realize point-of-test and sensitive detection of mycotoxins in food and environment using nanozyme-based Apt-LFAs.

Fast and sensitive smartphone colorimetric detection of whole blood samples on a paper-based analytical device

BackgroundMethods based on paper-based analytical devices (PAD) and smartphone photographic colorimetric detection have become representative instrument-independent point-of-care testing (POCT) platforms due to their low cost and simplicity. However, the detection of target components from whole blood sample still presents challenges in terms of field preparation of small amounts of blood sample and detection sensitivity. This paper presents a rapid online processing method for whole blood samples on PAD based on plasma separation membrane (PSM), and combined with electrokinetic stacking and selective chromatic reaction. Real-time smartphone-based colorimetric detection of free hemoglobin (FHb) and human serum albumin (HSA) was successfully demonstrated. ResultsWith the proposed method, both detections for low and high concentration analytes could be implemented. The limits of detection of 16.6 mg L-1 for FHb and 0.67 g L-1 for HSA were obtained, respectively, with RSD below 8 %. The reliability of the method was verified by the recovery test and desktop spectrophotometric method. The detection results for real blood samples were in agreement with that by clinical methods. Significance and noveltyThe PAD method is inexpensive, simple and fast, and detection of a whole blood sample of 5 μL can be finished in 5 min. This work shows that POCT of biomarkers from whole blood with PAD is possible without using any desktop facilities.

Molecular Self‐Assembly of Au Nanoparticles on a Porous Cellulose Membrane Template for Highly Sensitive Colorimetric Detection of Glucose

AbstractA sensitive glucose colorimetric detection system (Au@QCM) based on quaternized cellulose membrane (QCM) and enzyme‐mimicking Au nanoparticles (AuNPs) was successfully developed via a simple molecular self‐assembly strategy. The positively charged QCM functioned as a template to guide the negatively charged citric acid‐reduced AuNPs uniformly distributed on the surfaces along the pore walls of its porous structure, and thus gave AuNPs an ordered interconnected three‐dimensional network. This electrostatic interaction could effectively prevent the aggregation of the AuNPs and greatly increase their exposed surface area, thus endow the Au@QCM with highly active catalytic properties towards glucose oxidation with glucose oxidase (GOx). The experimental studies showed that the color change of redox indicator 3,3′,5,5′‐tetramethylbenzidine (TMB) can be clearly observed with naked eyes and the absorbance of the oxidized TMB (at 653 nm) was directly proportional to the concentration of glucose. Au@QCM could achieve a sensitive glucose detection with a limit of detection of 0.55 μM and linear response in the range of 0.01–0.5 mM. Besides, it also had good applicability in real samples detection and excellent selectivity to glucose. This novel detection system significantly improved detection performance compared to previous work based on cellulose‐based strips loaded with GOx and horseradish peroxides.

Novel benzohydrazide-based Schiff base for highly selective and sensitive colorimetric detection of Hg2+ ions; DFT analysis and test strips applications

Metal ions play a crucial role in serving the proper functioning of living beings. Whereas Hg2+ ions are non-essential and toxic even at a minute level for living organisms. Thus, a simple colorimetric approach for Hg2+ ion detection is used. In this work, the synthesis of novel benzohydrazide-based Schiff base ligand (PN) as a sensor and exploring its colorimetric recognition ability. The ligand PN has been designed and synthesized via an economical and simple one-step condensation process. The as-synthesized PN ligand were successfully characterized by various spectroscopic techniques like 1H NMR, 13C NMR, FT-IR, HRMS and ICP-OES. ligand PN manifest outstanding selectivity for Hg2+ ions leaving the rest of the examined metal ions in mixed aqueous medium. A colour change of colourless to crimson yellow is prominent under UV light. The Job plot has shown formation of complex of 1:1 stoichiometric ratio of PN and Hg2+ ions; which is produced by conducting UV–vis titrations using different molar fractions of Hg2+ ions. Moreover, theoretical DFT calculations are validated by experimental data. Furthermore, pre-stained paper strips are furnished and it also showed excellent instantaneous Hg2+ naked eye sensing with a limit of detection as low as 3.08 × 10−7 M.

Enhanced mimic peroxidase activity of carbon nanozyme by simultaneous phosphorus, oxygen dual-heteroatom doping and nanosheet structure construction

The carbon-based nanomaterials with peroxidase-like property show huge potential as candidates for natural enzymes. The design of carbon nanozymes with high activity can expand their potential application. Herein, the carbon nanozymes with enhanced catalytic property by simultaneous phosphorus, oxygen dual-doping and nanosheet structure construction are fabricated through one-step pyrolysis process. The dual-doped phosphorus and oxygen element is conducive to accelerate electron transfer and form active site with moderate free energy for activation of H2O2 to ·OH. Besides, the nanosheet structure with enlarged surface area is beneficial to expose more catalytic sites. Benefiting from its great peroxides-like activity, the phosphorus, oxygen dual-doped carbon nanosheet materials (POCNS) are used in colorimetric determination of H2O2 and glucose with cascade reaction of glucose oxidase and POCNS. And the sensitive colorimetric detection assay for organophosphorus pesticide of chlorpyrifos was constructed with regulation of peroxidase-like activity by aptamer. The system of POCNS and H2O2 further displays great antibacterial property in inactivation of Escherichia coli. This work provides an avenue to construct high-efficient carbon nanozymes and develop their potential application in environmental and biological field.

Highly sensitive colorimetric detection of the toxic metal cadmium in rice using the 3-(1H-Tetrazol-5-yl) aniline@AuNPs sensor

A very poisonous heavy metal Cd2+ not only contaminates the living environment but also constitutes a serious menace to human health through water intake and food chains. Thus, monitoring and controlling the content of Cd2+ with rapidity, sensitivity, and simplicity in tap water, soil, and rice samples is important especially. In the work, a high-efficiency sensor based on gold nanoparticles (AuNPs) embellished and refined with 3-(1H-Tetrazol-5-yl) aniline (TAA) for the colorimetric detection of Cd2+ is rationally put forward. The interaction between Cd2+ and TAA@AuNPs leads to the agglomeration of the AuNPs solutions, resulting in the color of the system changing from wine-red to gray-blue. The limit of detection (LOD) of the sensor is 1.0 μM when observe with the unaided eye, and UV–vis spectroscopy also has a LOD of 31 nM with an excellent linear relationship (R2 = 0.99049) in the concentration range of 0.0–8.0 μM for Cd2+, indicating that AuNPs modified with TAA can be effectively used for the quantitative determination of Cd2+. Furthermore, the colorimetric sensor has been tested on tap water, soil, and rice samples, and it shows promising potential for the practical monitoring of harmful Cd2+ in environmental systems.

Fe-Ni dual-single atoms nanozyme with high peroxidase-like activity for sensitive colorimetric and fluorometric dual-mode detection of cholesterol

Cholesterol is widely existed in nerve myelin sheath and various membrane structures, whose abnormal level would deteriorate human cells or even cause diseases. Herein, Fe-Ni dual-single-atom nanozyme was efficiently incorporated into N-doped carbon nanosheets (FeNi DSAs/N-CSs) by a simple calcination method. Its nanozyme activity and catalytic mechanism were investigated in details. The FeNi DSAs/N-CSs nanozyme showed superior peroxidase-like property, which was applied for the dual-mode determination of hydrogen peroxide (H2O2) and cholesterol. The colorimetric/fluorometric assays of H2O2 displayed the linear ranges of 1–50 mM and 5–40 mM with low limits of detection of 0.45 mM and 3.33 mM, respectively. In parallel, there exhibited the linear ranges of 0.5–5.0 mM and 0.25–5.0 mM for the colorimetric/fluorometric analysis of cholesterol, coupled with the limits of detection down to 0.19 mM and 0.044 mM, respectively. This work provided a rapid, cost-effectiveness and simple colorimetric/fluorometric method for sensitive dual-mode detection of cholesterol in human serum samples.