Selective sensing of submicromolar iron(III) with 3,3',5,5'-tetramethylbenzidine as a chromogenic probe

Peroxidase activity of a Cu-Fe bimetallic hydrogel and applications for colorimetric detection of ascorbic acid

A Cu-Fe bimetallic hydrogel (2-QF-CuFe-G) was constructed through a simple method. The 2-QF-CuFe-G metallohydrogel possesses excellent peroxidase-like activity to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2. The catalytic mechanism was confirmed by the addition of •OH radical scavenger isopropyl alcohol (IPA), tert-butyl alcohol (TBA) and ˙OH trapping agent terephthalic acid (TA). Remarkably, the resultant blue ox-TMB system can be used to selectively and sensitively detect ascorbic acid (AA) with an LOD of 0.93 μM in the range of 4-36 μM through the colorimetric method. Moreover, the assay based on the 2-QF-CuFe-G metallohydrogel can be successfully applied to detect AA in fresh fruits.

Headspace single drop microextraction based visual colorimetry for highly sensitive, selective and matrix interference-resistant determination of sulfur dioxide in food samples

Excessive intake of SO₂, a widely-used food additive, is able to cause respiratory, cardiovascular and neurological disease. For effectively monitoring SO₂ level, we have developed a headspace single drop microextraction based visual colorimetry for highly sensitive and selective sensing of SO₂ with TMB (3,3',5,5'-tetramethylbenzidine) as a classic chromogenic reagent. A combination of single drop and headspace microextraction integrated merits of high extraction efficiency, low consumption of reagents and excellent matrix interference-resistant ability. The colorimetric principle was based on oxidation of TMB, and SO₂ could compete with TMB to preferentially react with ·OH, resulting in the fading of color blue that could be easily read out by naked eye. LOD was calculated to be 0.53 μM and 5 μM by UV-vis and naked eye, respectively. The method was successfully utilized to analysis of food samples, and the experimental device was miniaturized and easy to construct, thus showing a promising potential in field analysis.

Colorimetric/photothermal dual-mode sensing detection of ascorbic acid based on a Ag[i] ion/3,3',5,5'-tetramethylbenzidine (TMB) system

In this work, a novel strategy of colorimetric and photothermal dual-mode sensing determination of ascorbic acid (AA) based on a Ag⁺/3,3',5,5'-tetramethylbenzidine (TMB) system was developed. In this sensing system, Ag⁺ could oxidize TMB with a distinct color change from colorless to blue color, strong absorbance at 652 nm and a photothermal effect under 808 nm laser irradiation due to the formation of oxidized TMB (oxTMB). When AA was present, oxTMB was reduced accompanied by a change from blue to colorless, and a decrease in absorption peak intensity and the photothermal effect. AA concentration showed a negative linear correlation with the value of both the absorbance intensity at 652 nm and temperature in the range of 0.2-10 μM (A = -0.03C + 0.343 (R ², 0.9887; LOD, 50 nM); ΔT = -0.57C + 8.453 (R ², 0.997; LOD, 7.8 nM)). Based on this, a sensing approach for detection of AA was proposed with dual-mode and without the complicated synthesis of nanomaterials. The photothermal effect and colorimetric signal provided a dual-mode detection strategy for AA, overcoming the limitations of any single mode. This colorimetric and photothermal dual-mode detection has great potential in the detection of AA in clinical pharmaceuticals and the construction of portable and highly sensitive sensors.

Colorimetric Systems for the Detection of Bacterial Contamination: Strategy and Applications

Bacterial contamination is a public health concern worldwide causing enormous social and economic losses. For early diagnosis and adequate management to prevent or treat pathogen-related illnesses, extensive effort has been put into the development of pathogenic bacterial detection systems. Colorimetric sensing systems have attracted increasing attention due to their simple and single-site operation, rapid signal readout with the naked eye, ability to operate without external instruments, portability, compact design, and low cost. In this article, recent trends and advances in colorimetric systems for the detection and monitoring of bacterial contamination are reviewed. This article focuses on pathogen detection strategies and technologies based on reaction factors that affect the color change for visual readout. Reactions used in each strategy are introduced by dividing them into the following five categories: external pH change-induced pH indicator reactions, intracellular enzyme-catalyzed chromogenic reactions, enzyme-like nanoparticle (NP)-catalyzed substrate reactions, NP aggregation-based reactions, and NP accumulation-based reactions. Some recently developed colorimetric systems are introduced, and their challenges and strategies to improve the sensing performance are discussed.

Fe–Ni metal–organic frameworks with prominent peroxidase-like activity for the colorimetric detection of Sn2+ ions

The synthesized Fe–Ni-MOF could oxidize TMB to produce oxTMB with blue color. The presence of Sn²⁺ ions could make the oxTMB color lighter, hence colorimetric detection of Sn²⁺ ions is realized.

A sensitive “ON–OFF” fluorescent probe based on carbon dots for Fe2+ detection and cell imaging

A sensitive fluorescent probe based on carbon dots has been synthesized by a one-pot hydrothermal method for the rapid detection of intracellular Fe²⁺.

Fe3+-sensing by 3,3',5,5'-tetramethylbenzidine-functionalized upconversion nanoparticles

Detection of Fe³⁺ ion is essential for human health because it is an important element of hemoglobin, which carries oxygen to cells in the body. Here, a 3,3',5,5'-tetramethylbenzidine (TMB) functionalized NaYF₄: Yb³⁺, Er³⁺@NaYF₄ composite upconversion probe was developed, and demonstrated Fe³⁺ sensing ability with high sensitivity and selectivity. The red emission of upconversion nanoparticles (UCNPs) has a higher penetration depth in tissue than green light and works within the biological window. The obtained hydrophobic NaYF₄: Yb³⁺, Er³⁺@NaYF₄ nanoparticles were treated with HCl to achieve hydrophilic ligand-free nanoparticles with non-saturated metal ions on their surface. Then, a Fe³⁺ responsive TMB-UNCPs composite luminescence probe was formed through linking TMB onto the ligand-free UCNPs by a coordination bond between the NH₂ groups in TMB and the metal ions on the UCNPs. Due to the efficient fluorescence resonance energy transfer from UCNPs to Fe³⁺-TMB, the obtained probe shows high sensitivity for detecting Fe³⁺ in the range of 0-100 μM with a detection limit of 0.217 μM. And the color change of the detection system can also be easily recognized by the naked eye. The 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) experiments and the bioimaging experiments show promising prospects in tissue imaging.

N-doped carbon dots/H2O2 chemiluminescence system for selective detection of Fe2+ ion in environmental samples

The nitrogen doped carbon dots (N-CDs) produces strong chemiluminescence (CL)-emission due to hydroxyl radical (^•OH) induced electron-hole transition in N-CDs. The Fe²⁺ has the ability to generate ^•OH from available hydrogen peroxide (H₂O₂). Therefore, a pre-mixed N-CDs/H₂O₂ solution was utilized for selective quantification of Fe²⁺ in solution via CL-emission. A linear increase in the CL-emission intensity was observed within increase in Fe²⁺ concentration. The N-CDs/H₂O₂ system enabled the detection of Fe²⁺ up to lower concentration of 0.2 × 10^-9 M with a linear dynamic range of 1.0 × 10^-9-1.0 × 10^-6 M. Significantly, no CL-emission was observed when other divalent cations, Al³⁺, Fe³⁺, or Cr³⁺ were injected to this system. Moreover, no interference was observed when a mixed solution of Fe²⁺ and other cations were introduced to N-CDs/H₂O₂. The practical evaluation of N-CDs/H₂O₂ system was demonstrated for detection of Fe²⁺ in tap, lotus pond, and canal water samples. The easy detection, high sensitivity, and selectivity make this method a significant tool for analysis of Fe²⁺ in solution.

Simple preparation and highly selective detection of silver ions using an electrochemical sensor based on sulfur-doped graphene and a 3,3′,5,5′-tetramethylbenzidine composite modified electrode

An electrochemical sensor based on S-Gr-TMB/GCE was prepared to solve the nonselectivity problem of TMB as a spectral probe.

Photo-mediated optimized synthesis of silver nanoparticles for the selective detection of Iron(III), antibacterial and antioxidant activity

The AgNPs synthesized by green method have shown great potential in several applications such as biosensing, biomedical, catalysis, electronic etc. The present study deals with the selective colorimetric detection of Fe³⁺ using photoinduced green synthesized AgNPs. For the synthesis purpose, an aqueous extract of Croton bonplandianum (AEC) was used as a reducing and stabilizing agent. The biosynthesis was confirmed by UV-visible spectroscopy where an SPR band at λ_max 436nm after 40s and 428nm after 30min corresponded to the existence of AgNPs. The optimum conditions for biosynthesis of AgNPs were 30min sunlight exposure time, 5.0% (v/v) AEC inoculum dose and 4mM AgNO₃ concentration. The stability of synthesized AgNPs was monitored up to 9months. The size and shape of AgNPs with average size 19.4nm were determined by Field Emission Scanning Electron Microscope (FE-SEM) and High-Resolution Transmission Electron Microscope (HR-TEM). The crystallinity was determined by High-Resolution X-ray Diffractometer (HR-XRD) and Selected Area Electron Diffraction (SAED) pattern. The chemical and elemental compositions were determined by Fourier Transformed Infrared Spectroscopy (FTIR) and Energy Dispersive X-ray Spectroscopy (EDX) respectively. The Atomic Force Microscopy (AFM) images represented the lateral and 3D topological characteristics of AgNPs. The XPS analysis confirmed the presence of two individual peaks which attributed to the Ag 3d3/2 and Ag 3d5/2 binding energies corresponding to the presence of metallic silver. The biosynthesized AgNPs showed potent antibacterial activity against both gram-positive and gram-negative bacterial strains as well as antioxidant activity. On the basis of results and facts, a probable mechanism was also proposed to explore the possible route of AgNPs synthesis, colorimetric detection of Fe³⁺, antibacterial and antioxidant activity.