Visual Colorimetric Sensor Research Articles

Double-Fluorescent Response Spiropyran-Functionalized Graphitic Carbon Nitride Solvent Decoder and Nanofiber Mat-Based Visual Colorimetric Sensor for Humidity Detection within Grain Heaps.

Spiropyran-functionalized graphitic carbon nitride (MCH@g-C3N4) in acidic conditions was prepared for the first time. MCH@g-C3N4 exhibited dual-wavelength fluorescence emission with two distant bands and distinct solvatochromic behavior owing to the different molecular structures of spiropyran. A three-dimensional organic solvent decoder was fabricated for solvent identification based on the emission wavelength and fluorescence quantum yield of MCH@g-C3N4 in different polar solvents. The ratiometric fluorescence sensing of the water content in organic solvent was realized using water to induce the structural change of spiropyran. A solid-phase MCH@g-C3N4/polyvinylpyrrolidone (PVP) nanofiber mat-based sensor was prepared via cospinning MCH@g-C3N4 with hydrophilic PVP. A fluorescent humidity sensor was fabricated using a commercial syringe equipped with a stainless-steel needle (catcher) to obtain the air inside the grain heap, which enabled the accurate location of the sampling spot. The space between the syringe and piston was used as an enclosed sensing space. Thus, a simple, accurate, and intuitive visual colorimetric method for the internal humidity of a grain heap was realized by analyzing the recorded images.

Sustainable and reusable probe-encapsulated porous poly(AMST-co-TRIM) monolithic sensor for the selective and ultra-sensitive detection of toxic cadmium(II) from industrial/environmental wastewater

This study focuses on a new type of fast responsive solid-state visual colorimetric sensor, custom engineered with dual-entwined porous polymer imbued with chromoionophoric 4-(sec-butyl)− 2-((5-mercapto-1,3,4-thiadiazol-2-yl)diazenyl)phenol (SMDP) probe for selective and ultra-sensitive colorimetric sensing of Cd(II). The polymer monolith, i.e., poly(aminostyrene-co-trimethylolpropanetrimethacrylate) denoted as poly(AMST-co-TRIM), is designed through a stoichiometric blending of monomer, crosslinker, and porogens leading to superior surface area, pore and adsorption properties for the voluminous incorporation of SMDP probe for target specific ion sensing. The porosity, surface and structural characteristics of the poly(AMST-co-TRIM)monolith and poly(AMST-co-TRIM)SMDP sensor are investigated using p-XRD, XPS, TG-DTA, FT-IR, BET/BJH, FE-SEM, HR-TEM, EDAX, and SAED techniques. The poly(AMST-co-TRIM)SMDP sensor reveals a frozen geometrical orientation of SMDP molecules to bind selectively with Cd(II), forming stable charge-transfer complexes by exhibiting transitional visible color shifts from light yellow to dark green (λmax 608 nm). The sensor imposes a linear response from 0–200 ppb, with quantification and detection limits of 0.95 and 0.28 ppb. The fabricated sensor material is cost-effective and versatile in its solid-state naked-eye sensing, with excellent reusability. The sensor performance has been verified using various environmentally contaminated water and commercial cigarette samples, with a recovery of ≥ 99.12% and an RSD of ≤ 1.95%, thus reflecting exceptional data reproducibility/reliability.

Fe single atoms encapsulated in N, P-codoped carbon nanosheets with enhanced peroxidase-like activity for colorimetric detection of methimazole

Excessive use of antithyroid drug methimazole (MMI) in pharmaceutical samples can cause hypothyroidism and symptoms of metabolic decline. Hence, it is urgent to develop rapid, low cost and accurate colorimetric method with peroxidase-like nanozymes for determination of MMI in medical, nutrition and pharmaceutical studies. Herein, Fe single atoms were facilely encapsulated into N, P-codoped carbon nanosheets (Fe SAs/NP-CSs) by a simple pyrolysis strategy, as certified by a series of characterizations. UV–vis absorption spectroscopy was employed to illustrate the high peroxidase-mimicking activity of the resultant Fe SAs/NP-CSs nanozyme through the typical catalysis of 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation. The catalytic mechanism was scrutionously investigated by the fluorescence spectroscopy and electron paramagnetic resonance (EPR) tests. Additionally, the introduced MMI had the ability to reduce the oxidation of TMB (termed oxTMB) as a peroxidase inhibitor, coupled by fading the blue color. By virtue of the above findings, a visual colorimetric sensor was established for dual detection of methimazole (MMI) with a linear scope of 5–50 mM and a LOD of 1.57 mM, coupled by assay of H2O2 at a linear range of 3–50 mM. According to the irreversible oxidation of the drug, its screening with acceptable results was achieved on the sensing platform even in commercial tablets The Fe SAs/NP-CSs nanozyme holds great potential for clinical diagnosis and drug analysis.

Smartphone‐based colorimetric determination of some physicochemical properties of polyaniline on flexible cellulose substrate

AbstractPolymer‐based materials are of high potential for application as active materials in contemporary electronic gadgets. The functionality of such materials is highly dependent on their oxidation state and requires fine control during their synthesis. Owing to that, we have designed a cost‐effective, simple, portable, system for calibration and the detection of the optical and electrical properties of polyaniline loaded on a paper substrate using a smartphone‐based colorimetric sensor. The vapor phase polymerization technique was adopted for preparing polyaniline. Colorimetric measurements of polyaniline‐loaded papers were achieved by estimating the hue, saturation and value (HSV) color parameter values using an open‐source smartphone application called “Color Grab (Color Detection)” during synthesis at different time intervals. The proposed sensor relies on the principle of Beer–Lambert law. The rear camera of the smartphone acts as a sensor and captures the light signal from the samples and provides information to the colorimetric app. The colorimetric results showed a promising correlation between the color parameter values with their electrical properties. The optical absorbance results of all polyaniline‐loaded papers are also in accordance with the H parameter values. The proposed sensor can be used as a portable smartphone‐based point‐of‐care testing kit to bring in‐situ control during synthesis.Highlights Conducting polymer on flexible cellulose substrate. Smartphone‐based visual colorimetric sensor. Biodegradable and environment‐friendly electronic material. Variation of electrical properties in terms of hue values. Cheap, portable sensor for point‐of‐care testing.

Highly selective and sensitive recognition of multi-ions in aqueous solution based on polymer-grafted nanoparticle as visual colorimetric sensor

A novel, selective and sensitive colorimetric sensor for naked-eye detection and adsorption of multi-ions in aqueous solution was synthesized using hybridization of organic–inorganic phase. The polymer-grafted nanoparticles (PGNPs) was synthesized via atom transfer radical polymerization (ATRP) of monomers on modified TiO2 NPs and applied under optimized conditions for naked-eye detection: sensor mass: 15 mg; response time: 30 s with limits of detection (LODs) as small as 10, 1, 0.5, and 1 ppb Hg (II), Cd (II), Cu (II), and UO2 (II) at pH = 8, 9, 6, and 7, respectively. The efficient selectivity of the naked eye sensor to multi-ions in the presence of various ions was affirmed wherein the color of the chemosensor in the presence of Hg (II), Cd (II), Cu (II), and UO2 (II) shifted from gray to violet, orange, green and yellow, respectively. The salient advantages of this method comprise expeditious, selectable, high reproducibility, with reasonable adsorption capacity (133 mg g-1) and inexpensive nature for rapid detection of heavy metal ions contamination in aqueous solution in an inexpensive manner. The adsorption mechanism was studied via adsorption kinetics and adsorption isotherm models and the accuracy of the chemosensor has been confirmed and supported by XRD, FT-IR, TGA, 1H-NMR, SEM, TEM, EDX mapping, DLS, BET, and EDS analysis.

Visual colorimetric sensor for nitroguanidine detection based on hydrogen bonding−induced aggregation of uric acid−functionalized gold nanoparticles

A colorimetric assay is proposed for the quantification of nitroguanidine (NQ), based on triggering the aggregation of uric acid−modified gold nanoparticles (AuNPs@UA) by intermolecular hydrogen bonding interaction between uric acid (UA) and NQ. The red-to-purplish blue (lavender) color change of AuNPs@UA with increasing NQ concentrations could be perceived with the naked eye or detected by UV–vis spectrophotometry. The absorbance versus concentration correlation gave a linear calibration curve in the range of 0.6–3.2 mg L−1 NQ, with a correlation coefficient of 0.9995. The detection limit of the developed method was 0.063 mg L−1, lower than those of noble metal aggregation methods in the literature. The synthesized and modified AuNPs were characterized using UV–vis spectrophotometry, scanning transmission electron microscopy (STEM), dynamic light scattering (DLS), and Fourier transform infrared spectroscopy (FTIR). Some critical parameters such as modification conditions of AuNPs, UA concentration, solvent environment, pH, and reaction time were optimized for the proposed method. The non-interference of common explosives (i.e., nitroaromatic, nitramine, nitrate ester, insensitive and inorganic explosives), common soil and groundwater ions (Na+, K+, Ca2+, Mg2+, Cu2+, Fe2+, Fe3+, Cl−, NO3−, SO42−, CO32−, PO43−) and possible interfering compounds (used as camouflage agents for explosives; D-(+)-glucose, sweeteners, acetylsalicylic acid (aspirin), household powder detergents, and paracetamol) on the proposed method was demonstrated, proving that the procedure was fairly selective for NQ, due to special hydrogen bonding interactions between UA−functionalized AuNPs and NQ. Finally, the proposed spectrophotometric method was applied to NQ−contaminated soil, and the obtained results were statistically compared with those of the liquid chromatography-tandem mass spectrometric (LC-MS/MS) method in the literature.

Nitrogen-Doped Carbon Nanoflowers Decorated with PtNi Nanoparticles for Colorimetric Detection of Total Antioxidant Capacity

A total antioxidant capacity (TAC) assay plays an essential role in evaluating antioxidant behaviors. It mainly involves examining the dosages of antioxidants such as ascorbic acid (AA), while TAC can be measured by nanozyme with peroxidase-like activity. Herein, nitrogen-doped carbon nanoflowers decorated with uniform PtNi nanoparticles (PtNi/NCFs) were fabricated by a simple pyrolysis strategy. The obtained PtNi/NCFs showed high peroxidase-mimicking activity to catalyze 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation by generating rich hydroxyl radicals (·OH) in the H2O2 system. By virtue of the PtNi/NCFs nanozyme, a visual colorimetric sensor was developed for assays of H2O2 and AA. The limits of detection were as low as 45.3 and 0.94 μM for determination of H2O2 and AA with linear ranges of 100–1000 and 1.0–20.0 μM, respectively. Moreover, such sensor was also successfully employed to detect AA and H2O2 in real tablets and juices. The PtNi/NCFs nanozyme holds great potential for practical applications.

Smartphone bioelectronic drug with visual colorimetric sensor and bulk nanoencapsulation optogenetic bacteria for chronic kidney disease theragnostics

Point-of-care testing (POCT) has been widely employed for chronic disease diagnosis due to its portable testing modalities and fast medical decisions. However, conventional POCT techniques lack the means of rapid and precise treatment, deviating from the requirement of an integrated theragnostics system for home diagnosis and personalized therapy. Here, we present a smartphone-controlled, system-level bioelectronic drug with programmable point-of-care theragnostics (POCTH) for chronic kidney disease monitoring. The system features a urine-based colorimetric sensor for pathological quantification and engineered bacteria that can generate targeted immune regulators (IL-10) for disease modulation, allowing real-time, at-home chronic disease diagnosis and treatment. Customized smartphone applications have been developed for image capture, colorimetric analysis, and wireless transmission. Besides, a flexible optoelectronic device has also been fabricated for conformal integration with rats and wireless modulation of optogenetic bacteria. The system leads to sophisticated applications in physiological monitoring and personalized diagnosis of chronic diseases.

Carbon Dots‐Decorated g‐C3N4 as Peroxidase Nanozyme for Colorimetric Detection of Cr(VI) in Aqueous Medium

AbstractCarbon dots (CD) generated from biomass were fabricated on g‐C3N4 to construct a metal‐free nanozyme with inherent peroxidase mimic functions. The formation of CD/g‐C3N4 was confirmed by different characterization techniques. The CD/g‐C3N4 displays distinct peroxidase mimic activities, as evidenced by the catalyzed oxidation of TMB (3,3’,5,5’, ‐tetramethylbenzidine) in the presence of H2O2, as well as color alteration from transparent to blue. The mechanistic analysis of peroxidase activity indicated that H2O2 was catalyzed by CD/g‐C3N4 to produce reactive oxygen species. In an acidic solution, the presence of Cr (VI) accelerates the breakdown of H2O2 to ⋅OH radicals thereby increasing the rate of TMB oxidation. Hence, a visual colorimetric sensor was developed for the purpose of detecting Cr(VI) with limits of detection (LOD) 0.31 μM, based on the accelerated electron transfer between TMB and H2O2 with the assistance of CD/g‐C3N4.

Highly sensitive visual colorimetric sensor for trichlorfon detection based on the inhibition of metallization of gold nanorods

In this study, a highly sensitive visual colorimetric sensor was designed for the detection of trichlorfon based on inhibiting ascorbate oxidase (AAO)-induced metallization of gold nanorods (Au NRs). Ascorbic acid (AA) can reduce silver ion (Ag+) to metal silver (Ag) that will be deposited on the surface of Au NRs, which results in the blue shift of longitudinal localized surface plasmon resonance (LSPR) peak of Au NRs, accompanying by perceptible color changes from red to cyan to red to yellow. In the presence of trichlorfon, the activity of AAO will be inhibited, resulting in less AA is hydrolyzed to dehydroascorbic acid (DHA), and therefore more Ag+ is reduced to Ag by AA. Under the optimized conditions, detection of trichlorfon has a wide linear range of 27.8–11111.1 μg/L with a limit of detection as low as 132.6 ng/L. Moreover, the sensor has a good sample spiked recovery (84.7–96.8%) for the determination of trichlorfon in lake water samples. The proposed method can achieve rapid analysis (about 10 min) of trichlorfon with simple operation when there are no other types of organophosphorus pesticides in the real samples.

A novel enhanced enrichment glucose oxidase@ZIF-8 biomimetic strategy with 3-mercaptophenylboronic acid for highly efficient catalysis of glucose

Herein, a glucose oxidase@ZIF-8 composite (3-MPBA/GOx@ZIF-8) with enhanced enrichment was enabled the rapid encapsulation of glucose oxidase (GOx) into microporous zeolitic imidazolate framework-8 (ZIF-8) for the first time. The 3-MPBA/GOx@ZIF-8 not only has improved affinity and catalytic efficiency to the substrate but also can shorten the formation time. The optimum loading amount of GOx on ZIF-8 was determined to be 470 mg/g. The as-prepared 3-MPBA/GOx@ZIF-8 composite maintained the native conformation of the enzyme and showed excellent bioactivity, even in chemical agents or at high temperature. Furthermore, the 3-MPBA/GOx@ZIF-8 showed satisfactory reusability, preserving almost 80.8 % activity after 7 cycles. The Michaelis constant Km and specificity constant kcat/Km of the 3-MPBA/GOx@ZIF-8 were 0.03 ± 0.02 mM and 63.87 ± 1.96 s−1 mM−1, respectively, which were superior to corresponding values of free GOx. Therefore, the 3-MPBA/GOx@ZIF-8 displayed high catalytic efficiency, high loading efficiency and enhanced stability. Moreover, a new type of visual colorimetric sensor for screening of the diabetes was realized through the 3-MPBA/GOx@ZIF-8, which provided a new strategy for the analysis field of glucose.

Picomolar-Level Melamine Detection via ATP Regulated CeO2 Nanorods Tunable Peroxidase-Like Nanozyme-Activity-Based Colorimetric Sensor: Logic Gate Implementation and Real Sample Analysis

The capability of functional logic operations is highly intriguing, but far from being realized owing to limited recognition element (RE) and complex readout signals, which limit their applications. In this contribution, for a visual colorimetric sensor for melamine (MEL) we described the construction of two- and three-input AND logic gate by exploiting the intrinsic peroxidase (POD)-like activity of CeO2 nanorods (NRs) (~23.04% Ce3+ fraction and aspect ratio (RTEM) of 3.85 ± 0.18) as RE at acidic pH (4.5). Further ATP piloted catalytic tuning of POD-like activity in CeO2 NRs employed for a functional logic gate-controlled MEL sensing at neutral pH (7.4). AND logic circuit operated MEL sensing record colorimetric response time of 15 min to produce blue color proportionate to MEL concentration. The fabricated nanozyme (CeO2)-based logic gate sensor probe for MEL at pH 4.5 showed a linear response from 0.004 nM to 1.56 nM with a limit of detection (LOD) of 4 pM; while translation from acidic to neutral pH (at 7.4) sensor exhibited linear response ranging from 0.2 nM to 3.12 nM with a LOD value of 17 pM. Through CeO2 POD-like nanozyme behavior under acidic and neutral pH, the fabricated logic gate sensor showed high affinity for MEL, generating prominent visual output with picomolar sensitivity, good reproducibility, and stability with relative standard deviation (RSD) <1% and 2%, respectively. A feasibility study in real samples (raw milk and milk powder) showed good recoveries with negligible matrix effect, an anti-interference experiment revealed sensor selectivity, highlighting robust sensor practical utility. With the merits of high sensitivity, specificity, low cost, and simplified sample processing, the developed logic-controlled colorimetric MEL sensing platform with appropriate modifications can be recognized as a potent methodology for on-site analysis of various food adulterants and related applications.

Development of a Simple Fe(II) Ion Colorimetric Sensor from the Immobilization of 1,10-Phenanthroline In Alginate/Pectin Film

An optical analytical sensor was proposed based on the complexation reactions of 1,10-phenanthroline derivative in aqueous solutions. This study aims to synthesize a metal ion sensor for detecting Fe(ll) ion from the immobilization of 1,10-phenanthroline compound in alginate/pectin film. This was carried out by characterizing the films using the Fourier-transform infrared spectrometry (FT-IR) and scanning electron microscope (SEM). The determination of the optimal condition for Fe(II) ion detection and validation of the parameters was conducted by measuring the absorbance of the films using a UV-Vis spectrophotometer. After the addition of Fe(II) ion, the color of the alginate/pectin-phenanthroline film changed from transparent yellow to orange-red, showing its potential as a visual colorimetric sensor for iron(II) ion. It was found that the optimum condition for Fe(II) ion sensing was at 513 nm after 2 min of detection at pH 2. The alginate/pectin-phenanthroline film had good linearity, precision, selectivity, and accuracy with a detection limit as low as 0.446 mg L–1, which was remarkable.

Fast Orange Peel-Mediated Synthesis of Silver Nanoparticles and Use as Visual Colorimetric Sensor in the Selective Detection of Mercury(II) Ions

This paper presents the green synthesis of silver nanoparticles (AgNPs) using orange (Citrus sinensis) peel extract. The effects of different factors such as silver ion concentration, reaction time, temperature, pH, and extract quantity on the AgNPs synthesis were studied, enabling us to determine optimum synthesis conditions. We characterized the AgNPs with different techniques: UV–Visible Spectroscopy, Field Emission Scanning Electron Microscopy (FESEM), FTIR, and XRD. The AgNPs showed yellowish brown to golden brown colors and a surface plasmon resonance (SPR) absorption band around a wavelength of 420 nm. FESEM images showed polydisperse AgNPs having an average size of about 55 nm. The XRD profile of the synthesized nanoparticles showed peaks that are characteristic of silver while the FTIR spectrum highlighted the functional groups associated with reducing silver ions and stabilizing the AgNPs. With the selected optimum conditions, the AgNPs were formed in less than 1 min. This is the fastest reaction time so far reported and is significantly shorter than in earlier reports. The AgNPs colloid solution was applied as nanosensor in the visual colorimetric detection of mercury(II) ions in water. The golden brown AgNPs colloid solution turned colorless and the characteristic SPR absorption band disappeared when mercury(II) ions were added to the solution. While the AgNPs show good sensitivity and selectivity for the colorimetric detection of mercury(II) ions with a detection limit of 1.24 × 10−6 mol/L (0.25 ppm), we also demonstrated the suitability of the method for detecting mercury(II) ions in drinking water.

Organic-Inorganic Composite Nanorods as an Excellent Mimicking Peroxidases for Colorimetric Detection and Evaluation of Antioxidant.

N,N'-Dicarboxy methyl perylene diimide-coated CeO2 nanorods (PDI/CeO2 NR) were synthesized via an ultrasonic-assisted method. PDI/CeO2 exhibits the superb mimic peroxidase functions confirmed by the catalyzed oxidation of TMB by hydrogen peroxide in less than 60 s along with color transformation from colorless to blue. The catalytic mechanism was confirmed to be an electronic transfer mechanism by fluorescence experiment. Thus, a visual colorimetric sensor was constructed for hydrogen peroxide detection with a low detection limit (LOD = 2.23 μM). Comparing the inhibition effects of l-cysteine (Cys), ascorbic acid (AA) and glutathione (GSH) on the catalytic oxidation of TMB, it can be found that they possessed different types of inhibition on the oxidation of TMB by H2O2 using PDI/CeO2, and AA has better antioxidant effect, followed by Cys and GSH. On the basis of the excellent antioxidant effect of AA, a low-cost colorimetric sensor was also used to detect AA, and a lower LOD value (0.68 μM) was obtained in the linear range of 5.0-30 μM.

Chemically modified mesoporous wood: a versatile sensor for visual colorimetric detection of trinitrotoluene in water, air, and soil by smartphone camera.

There is great interest in detection of the level of 2,4,6-trinitrotoluene (TNT) explosive due to its importance in public security and environmental protection fields. The conventional chemical sensors do not simultaneously realize simple, rapid, sensitive, selective, and direct detection of TNT in different medium without sample pretreatment. Here we present a modified wood-based chemical sensor for visual colorimetric detection of TNT in water, air, and soil. The natural wood undergoes a delignified process, which is further functionalized by 3-aminopropyltriethoxysilane (APTES). When TNT solutions are introduced, the wood-based sensor shows a colorimetric transition from light yellow to brown for naked-eye readout because of the generation of Meisenheimer complex between APTES and TNT. The photographs are collected by smartphone camera, and the RGB components are extracted to calculate the adjusted intensity for qualitative detection of TNT. This visual colorimetric sensor for TNT solution displays a linearity in the range of 0.01-5 mM with a limit of detection of 3 μM. In addition, by taking advantage of its inherent mesostructure, the wood-based sensor can be employed for visual detection of TNT vapor as well. Furthermore, it is also able to directly detect TNT in wet soil samples based on capillary action, in which TNT carried by water transports upward along the wood microchannel, triggering the generation of Meisenheimer complex. Graphical Abstract.

Visual Sensor for Sterilization of Polymer Fixtures Using Embedded Mesoporous Silicon Photonic Crystals.

A porous photonic crystal is integrated with a plastic medical fixture (IV connector hub) to provide a visual colorimetric sensor to indicate the presence or absence of alcohol used to sterilize the fixture. The photonic crystal is prepared in porous silicon (pSi) by electrochemical anodization of single crystal silicon, and the porosity and the stop band of the material is engineered such that the integrated device visibly changes color (green to red or blue to green) when infiltrated with alcohol. Two types of self-reporting devices are prepared and their performance compared: the first type involves heat-assisted fusion of a freestanding pSi photonic crystal to the connector end of a preformed polycarbonate hub, forming a composite where the unfilled portion of the pSi film acts as the sensor; the second involves generation of an all-polymer replica of the pSi photonic crystal by complete thermal infiltration of the pSi film and subsequent chemical dissolution of the pSi portion. Both types of sensors visibly change color when wetted with alcohol, and the color reverts to the original upon evaporation of the liquid. The sensor performance is verified using E. coli-infected samples.

FePt-Au ternary metallic nanoparticles with the enhanced peroxidase-like activity for ultrafast colorimetric detection of H2O2

In this paper, FePt-Au ternary metallic hybrid nanaoparticles (FePt-Au HNPs) were prepared with FePt nanocubes as seeds and then the seeds were combined with Au(I) precursor through a facile hydrothermal approach. And the FePt-Au HNPs were characterized by a series of technical methods such as transmission electron microscopy (TEM), X-ray diffraction pattern (XRD) and UV–vis absorption spectra. Moreover, the as-prepared FePt-Au HNPs possessed the excellent peroxidase-like activity which could rapidly catalyze the oxidation reaction of substrate 3,3′,5,5′ −tetramethylbenzidine (TMB) to obtain a typical blue product which could be observed apparently by the naked eye only within 30 s. Notably, the color response is instant, due to the fast electron transfer between the substrate and H2O2 with the aid of FePt-Au HNPs. Based on the catalytic mechanism of fast electron transfer and the intrinsic peroxidase-like activity of FePt-Au HNPs, a visual colorimetric sensor for ultrafast detecting H2O2 was constructed with a wide linear range of 20–700 μM as well as a relative lower limit of detection (LOD) of 12.33 μM. Furthermore, the ultrafast sensor based on FePt-Au HNPs as peroxidase mimics was also successfully applied to detect H2O2 in milk samples.

3,3′,5,5′-tetramethylbenzidine-based quadruple-channel visual colorimetric sensor array for highly sensitive discrimination of serum antioxidants

A quadruple-channel visual colorimetric sensor array has been developed to discriminate serum antioxidants. The redox reaction between 3,3′,5,5′-tetramethylbenzidine (TMB) and sodium hypochlorite (NaClO) can successively generate colorful products with strong absorption at different wavelengths. The presence of five antioxidants containing glutathione, cysteine, lipoic acid, melatonin, and uric acid disrupts the redox reaction between TMB and NaClO, giving birth to distinct colorimetric response patterns. By taking advantage of principal component analysis, the five antioxidants are identified with an accuracy of 100%. The sensor array is capable of visually discriminating different concentrations of the same antioxidant and binary mixtures of two antioxidants as well as working well in real fetal bovine serum. The distinguishable lowest concentration of antioxidants using this sensor array is 1μM, which is about one or three orders of magnitude lower than that of the recently reported colorimetric sensor array.

Facile visual colorimetric sensor based on iron carbide nanoparticles encapsulated in porous nitrogen-rich graphene

Herein, via one-step pyrolysis of glucose, dicyandiamide (DCDA) and Fe containing metal–organic framework (Fe-MOF), small Fe3C nanoparticles were in-situ decorated in 3D porous network of N-rich graphene (NGr). The Fe-MOF served as size regulating precursor, layered g-C3N4 (derived from pyrolysis of DCDA) acted as not only a template to guide the growth of small Fe3C nanoparticles, but also the carbon source for 3D porous NGr network. The intrinsic peroxidase-like catalytic activity of Fe3C/NGr was unpredictably discovered, by taking the oxidation reaction of 3,3′,5,5′-tetramethylbenzidine (TMB) with H2O2 as a protocol. Fast and distinguished color change, improved stability in exposure to extreme H2O2 concentration and high temperature were obtained. The Michaelis–Menten kinetics was investigated. The detection of glucose was accomplished over a wide concentration range of 2.0~500.0μM with a detection limit lower than most of other similar systems. The reliability of the present sensor was further evaluated by practical monitoring glucose in diluted serum samples. Low cost and simple preparation, fast and distinguished color change, high tolerance to extreme H2O2 concentration and high temperature, endow Fe3C/NGr as one of the promising materials for fast visual colorimetry.