Ultrasensitive detection of carcinogenic chromium(VI) species below the WHO limit using a LaCeO3/carbon black screen printed electrode in batch injection analysis

The widespread industrial use of chromium and its subsequent release into the environment as toxic and carcinogenic hexavalent chromium (Cr(VI)) species pose significant risks to human health and the environment. The World Health Organization (WHO) has established a limit of 50 ppb (960 nM) for Cr(VI) in water samples. Developing simple, selective, and separation-free methods for the direct detection of Cr(VI) species in the environment remains a challenging task. Herein, we present a highly crystalline lanthanum cerate/carbon black chemically modified screen-printed electrode (SPE/CB@LaCeO3) as an effective electrochemical system for the high-performance and selective electrochemical reduction of toxic Cr(VI) species in pH 2 KCl-HCl solution. The CB@LaCeO3 composite is characterized by its high-density electroactive sites and enhanced electrical conductivity, which facilitate the efficient diffusion-controlled reduction of Cr(VI) species at a low reduction potential of 0.55 V vs. Ag/AgCl. The modified electrode demonstrated stability and resistance to surface fouling during continuous voltammetry analysis of high Cr(VI) concentrations. A batch-injection analysis using a three-in-one screen-printed electrode, comprising carbon working, silver-ink reference, and CB@LaCeO3 modified carbon working electrodes, exhibited excellent concentration linearity within the ranges of 2-30 ppb and 10-35 ppm, with a low detection limit of 682 ppt (signal-to-noise ratio, 3). This method was not interfered by dissolved oxygen or other common chemicals present in environmental and water systems. The linear range and detection limit achieved in this study surpass those reported in several previous works involving precious metal and organic molecule-based chemically modified electrodes. The analytical method was validated with t-test analysis. To demonstrate the applicability of this new system, batch injection analysis was performed on a wide range of real samples, including water (tap, ground, well, and reverse osmosis), consumable products (coffee, tea and milk powders), and tannery effluent, using the standard addition method. This approach yielded accurate and sensitive detection of Cr(VI) species in the samples, with recovery values of approximately 100%.

Highly Efficient Adsorption of Cr(VI) by Mechanically Enhanced Chitosan Oligosaccharide Aerogel with Hierarchical Porosity

A three-dimensional porous aerogel was synthesized by combining soft chitosan oligosaccharide to hard multi-walled carbon nanotubes (MWCNTs) under mild conditions. Owing to the support of MWCNTs, mesopores can be detected on the inner surfaces of the aerogel. The cross-linking by tetraethylenepentamine forms a macropore structure after freeze drying, using water as the template. The hydrogel can bear 30% recoverable compression with the aid of hydrogen bonds between the amide groups. The aerogel exhibits excellent adsorption capabilities for Cr(VI) in water. The adsorption capacity can reach 367.6 mg/g within 100 min. Notably, the aerogel demonstrated an extremely high efficiency in removal for trace Cr(VI). Soluble Cr(VI) of 1 mg/L can be completely removed by the aerogel at a dosage of 0.2 g/L. This work provides a new strategy to synthesize adsorbents with hierarchical pores that exhibit high porosity and a fast mass transfer rate.

Colorimetric detection of Cr(VI) in water using tetramethyl benzidine (TMB) as an indicator

Hexavalent chromium (CrVI) poses a serious risk to both human and environment health. Hence, a simple, robust, and efficient analytical method must be developed to monitor the presence of Cr(VI) in the environment. The current investigation concentrated on the colorimetric detection of Cr(VI) using TMB as indicator in the presence of H2O2. The study found that Cr(VI) reacts with H2O2 to generate hydroxyl radicals which oxidize TMB in a concentration dependent manner. Under optimized conditions, the method obtained a good linearity range (0.025-0.5 mg/L, r2 = 0.9944) with LOD and LOQ of 0.009 mg/L and 0.029 mg/L, respectively. The technique was further improved by the addition of EDTA in the sample preparation protocol to reduce the false positive result by the presence of ions like Cu2+, Fe3+, etc. The study recorded improved Cr(VI) recoveries (81.73-111.40 %) at different fortification levels (0.1-0.5 mg/L). Under optimized conditions, the EDTA added method obtained a good linear response (r2 = 0.9952) with a detection limit of 0.023 mg/L which is less than the prescribed limits by WHO (0.05 mg/L) and US EPA (0.1 mg/L) for drinking water. The developed analytical method is very simple without use of any nanomaterial and the results with natural water samples show that it has the potential for real-time detection of Cr(VI) in the environment.

Paper-based colorimetric sensor using a single-atom nanozyme for the ultrasensitive detection of Cr(VI) in short-necked clams

Single-atom nanozymes (SAzymes) as a class of highly active nanozymes with the advantages of high atom utilization, high catalytic activity and stability have attracted great attention. In this work, Fe-N-C SAzymes with exceptional oxidase (OXD)-like activity were achieved utilizing polyvinylpyrrolidone (PVP) as a template. The Fe-N-C SAzymes with remarkable OXD-like activity could oxidize TMB to blue oxTMB, but 8-hydroxyquinoline (8-HQ) as a metal chelator is capable of discoloring oxTMB. Thus, the addition of 8-HQ decolorized the solution. However, upon the introduction of Cr(VI) ions, 8-HQ preferentially chelated with the Cr(VI) ions, reversing the inhibition of the color reaction and restoring the blue color. Based on this phenomenon, we constructed a novel paper-based analytical device (PAD) that exhibited a linear range of 5-1000 μM and an LOD of 1.2 μM. Importantly, the PAD used in this study shows the merits of simplicity, low preparation costs, and rapid reaction times. When combined with smartphone RGB analysis, it enables the simultaneous analysis of eight different Cr(VI) concentrations without the need for large-scale instrumentation. Moreover, the proposed PAD displays high selectivity, accuracy and utility in testing actual short-necked clam samples. This work not only provides a simple and cost-effective method to detect Cr(VI) but also makes a contribution to rapid food testing.

Coating Fe3O4 quantum dots with glutamic acid to achieve enhanced catalysis for facile and sensitive detection of chromium(VI) in water

A universal green synthesis approach for Glu-Fe3O4 quantum dots (QDs) with ultrasmall size (∼3.3 nm) and enhanced catalysis was demonstrated using glutamic acid (Glu) as a dopant. Abundant carboxylic groups and amino groups on the surfaces with a zeta potential of -6.28 mV resulted in greatly improved environmental stability. The Glu-Fe3O4 QDs exhibited oxidoreductase-like catalytic activity. The steady-state kinetic assays indicated that the QDs had a high binding affinity for the substrate TMB. A facile and sensitive colorimetric platform was developed using the Glu-Fe3O4 QDs for the detection of Cr(VI) in naturally sourced water and waste water, which showed a detection limit of 31.02 nM, much lower than the maximum Cr(VI) level permitted in drinking water by the World Health Organization (WHO) and the industrial water threshold. This universal green synthesis approach and the unique catalytic activity of Fe3O4 QDs open a new horizon in materials chemistry and the development of colorimetric biosensors for environmental protection and public health.

Bridging Component Strategy in Phosphomolybdate-Based Sensors for Electrochemical Determination of Trace Cr(VI)

Rapid and sensitive electrochemical determination of trace carcinogenic Cr(VI) pollutants remains an urgent and important task, which requires the development of active sensing materials. Herein, four cases of reduced phosphomolybdates with formulas of the (H2bib)3[Zn(H2PO4)]2{Mn[P4Mo6O31H7]2}·6H2O (1), (H2bib)2[Na(H2O)]2[Mn(H2O)]2{Mn[P4Mo6O31H6]2}·5H2O (2), (H2bib)3[Mo2(μ2-O)2(H2O)4]2{Ni[P4Mo6O31H2]2}·4H2O (3), and (H2bib)2{Ni[P4Mo6O31H9]2}·9H2O (4) (bib = 4,4'-bis(1-imidazolyl)-biphenyl) were hydrothermally synthesized under the guidance of a bridging component strategy, which function as effective electrochemical sensors to detect trace Cr(VI). The difference of hybrids 1-4 is in the inorganic moiety, in which the reduced phosphomolybdates {M[P4MoV6O31]2} (M{P4Mo6}2) exhibited different arrangements bridged by different cationic components ({Zn(H2PO4)} subunit for 1, [Mn2(H2O)2]4+ dimer for 2, and [MoV2(μ2-O)2(H2O)4]6+ for 3). As a result, hybrids 1 and 3 display noticeable Cr(VI) detection activity with low detection limits of 14.3 nM (1.48 ppb) for 1 and 6.61 nM (0.69 ppb) for 3 and high sensitivities of 97.3 and 95.3 μA·mM-1, respectively, which are much beyond the World Health Organization's detection threshold (0.05 ppm) and superior to those of the contrast samples (inorganic Mn{P4Mo6}2 salt and hybrid 4), even the most reported noble-metal catalysts. This work supplies a prospective pathway to build effective electrochemical sensors based on phosphomolybdates for environmental pollutant treatment.

Recent Advances in the Application of Bionanosensors for the Analysis of Heavy Metals in Aquatic Environments

Heavy-metal ions (HMIs) as a pollutant, if not properly processed, used, and disposed of, will not only have an influence on the ecological environment but also pose significant health hazards to humans, making them a primary factor that endangers human health and harms the environment. Heavy metals come from a variety of sources, the most common of which are agriculture, industry, and sewerage. As a result, there is an urgent demand for portable, low-cost, and effective analytical tools. Bionanosensors have been rapidly developed in recent years due to their advantages of speed, mobility, and high sensitivity. To accomplish effective HMI pollution control, it is important not only to precisely pinpoint the source and content of pollution but also to perform real-time and speedy in situ detection of its composition. This study summarizes heavy-metal-ion (HMI) sensing research advances over the last five years (2019-2023), describing and analyzing major examples of electrochemical and optical bionanosensors for Hg2+, Cu2+, Pb2+, Cd2+, Cr6+, and Zn2+.

Hollow porphyrin-based porous organic polymer with dual enzyme-like activities for ultra-fast colorimetric detection of Cr (VI) in wastewater

A hollow porphyrin-based porous organic polymer (H-Fe-POP) was prepared for rapid and sensitive colorimetric determination of Cr(VI), which exhibited excellent dual enzyme-like activities, including oxidase-like and peroxidase-like activities. Due to the specific binding of 8-hydroxyquinoline (8-HQ) to Cr(VI), 3,3',5,5'-tetramethylbenzidine (TMB) was liberated, and TMB was oxidized to blue ox-TMB catalyzed by H-Fe-POP. Based on the excellent oxidase-like activity of H-Fe-POP, an ultra-fast colorimetric platform for the detection of Cr(VI) was constructed, allowing the quantification of Cr(VI) in the range 2-130 μM within 30 s with a detection limit of 0.23 μM. Importantly, the sensor can accurately determine Cr(VI) in industrial wastewater, indicating its high potential for environmental monitoring.

Facile one-pot synthesis of fluorescent aminoclay and the applications for fluorescence sensing Cr2 O7 2

Here, we developed a facile one-pot strategy for the fabrication of fluorescent aminoclay (F-AC) through in situ solvothermal treatment of 3-aminopropyltrimethoxysilane, MgCl2 , and sodium ascorbate at 180°C for 6 h. The obtained F-AC exhibited blue emission, good water solubility, and satisfactory photostability. It was observed that Cr2 O7 2- could selectively quench the fluorescence of F-AC through the inner filter effect and static quenching process. As a result, a novel fluorescent F-AC-based nanosensor was constructed with good linearity in the range 0.1-75 μM. The nanosensor was successfully applied in real water samples with satisfactory results. This work not only provides a novel nanosensor for Cr2 O7 2- , but also highlights the F-AC's promising applications in wider fields due to the versatility and simplicity of the preparation strategy.

A Comprehensive Review of the Current Progress of Chromium Removal Methods from Aqueous Solution

Chromium (Cr) exists in aqueous solution as trivalent (Cr³⁺) and hexavalent (Cr⁶⁺) forms. Cr³⁺ is an essential trace element while Cr⁶⁺ is a dangerous and carcinogenic element, which is of great concern globally due to its extensive applications in various industrial processes such as textiles, manufacturing of inks, dyes, paints, and pigments, electroplating, stainless steel, leather, tanning, and wood preservation, among others. Cr³⁺ in wastewater can be transformed into Cr⁶⁺ when it enters the environment. Therefore, research on Cr remediation from water has attracted much attention recently. A number of methods such as adsorption, electrochemical treatment, physico-chemical methods, biological removal, and membrane filtration have been devised for efficient Cr removal from water. This review comprehensively demonstrated the Cr removal technologies in the literature to date. The advantages and disadvantages of Cr removal methods were also described. Future research directions are suggested and provide the application of adsorbents for Cr removal from waters.

Hexavalent Chromium Removal from Water and Wastewaters by Electrochemical Processes: Review

Hexavalent chromium (Cr(VI)) is a toxic, mutagenic, teratogenic, and carcinogenic species. Its origin is in industrial activities. Therefore, its effective control is realized on a source basis. Although chemical methods proved effective in removing Cr(VI) from wastewaters, more economic solutions with a minimum sludge production have been sought. Among them, the use of electrochemical processes has emerged as a viable solution to the problem. Much research was conducted in this area. The aim of this review paper is to make a critical evaluation of the literature on Cr(VI) removal by electrochemical methods, particularly electrocoagulation with sacrificial electrodes, and to assess the present data as well as to point out the areas that need further elaboration. Following the review of the theoretical concepts of electrochemical processes, the literature on the electrochemical removal of Cr(VI) was evaluated on the basis of important elements of the system. Among them are initial pH, initial Cr(VI) concentration, current density, type and concentration of supporting electrolyte, and the material of electrodes and their operating characteristics and process kinetics. Dimensionally stable electrodes that realize the reduction process without producing any sludge were evaluated separately. Applications of electrochemical methods to a wide spectrum of industrial effluents were also assessed.

Two new bis(pyridine)-bis(amide)-based copper(II) coordination compounds for the electrochemical detection of trace Cr(VI) and efficient electrocatalytic oxygen evolution

Two new metal-organic compounds (MOCs) [Cu(L)0.5(3-nba)2] (1) and [Cu(L)(2,5-tdc)] (2) have been hydrothermally synthesized by employing the ligand N,N′-di(3-pyridyl)adipoamide (L) and two carboxylic acids (3-Hnba = 3-nitrobenzoic acid, 2,5-H2tdc = 2,5-thiophenedicarboxylic acid) as ligands. Compound 1 displays a metal-organic chain-like structure formed by the {Cu2(3-nba)4} double-paddle wheel units and the µ 2-bridging L ligands. The adjacent polymeric chains form a supramolecular layered structure through hydrogen bonding. Compound 2 shows a 3D metal-organic polymeric framework derived from Cu-L layers and µ 2-bridging 2,5-tdc ligands, which presents a 3,5-connected {4.62}{4.66.83} topology. The electrochemical and electrocatalytic behavior of the two compounds has been studied in detail. Carbon paste working electrodes modified with compounds 1 and 2 can be used as highly selective sensors for detecting traces Cr(VI). Both electrodes show also electrocatalytic performance in oxygen evolution reactions (OERs).

Sensitive and selective detection of chromium (VI) based on two-dimensional luminescence metal organic framework nanosheets via the mechanism integrating chemical oxidation-reduction and inner filter effect

Two-dimensional luminescence metal organic framework nanosheets (LMOF) named NH₂-CuMOFs were synthesized using Cu (II) nodes coordinated with negatively charged 2-aminoterephthalic acid (NH₂-BDC) via a bottom-up strategy, which were first used as the fluorescent probes for the detection of chromium Cr (VI). The nanosheets possess stable fluorescence with the maximum emission wavelength of 436 nm at excitation of 338 nm that can be effectively quenched by hexavalent chromium Cr (VI). The NH₂-CuMOFs nanosheets show superior advantage over the linker of NH₂-BDC for the excellent selectivity to Cr (Ⅵ) without the interferences of other metal ions. The mechanism investigation suggested that the sensitive detection of Cr (VI) was attributed to the chemical oxidation-reduction (redox) reaction and internal filtration effect (IFE) between Cr (VI) and NH₂-CuMOFs nanosheets. Based on this mechanism, the quantitation of Cr (VI) was realized in the linear range of 0.1-20 μM with a detection limit of 18 nM. Moreover, the detection of Cr (VI) in real samples was also conducted with good recovery. This work provides an optical sensing nanoplatform for heavy metal ions based on two-dimensional LMOFs via a novel mechanism integrating chemical redox reaction and IFE, which may promise broad application prospect for two-dimensional luminescence nanosheets.

Evaluation of Cr(VI) adsorption on glutaraldehyde crosslinked chitosan beads using cyclic voltammetry employing gold electrode

Hexavalent chromium is one of the toxic heavy metals found in wastewaters from industries like electroplating, leather tanning, and steel manufacturing. The Cr(VI) removal and quantitative detection are among the major concerns from an environmental toxicity point of view. In the present work, we report an effective electrochemical technique for Cr(VI) monitoring based on the utilization of gold electrode. The technique developed is user-friendly, non-destructive, and provides real-time monitoring of Cr(VI) from wastewaters in contrast to other spectroscopic and optical techniques. The technique developed was applied in monitoring Cr(VI) removal from synthetic Cr(VI) samples using chitosan cross-linked glutaraldehyde (C + G) beads. The parameter optimization for Cr(VI) removal using C + G beads was done. The experimental data of Cr(VI) adsorption on C + G beads obtained using cyclic voltammetry were further used for kinetic and thermodynamic studies. Kinetic and thermodynamic studies found that Cr(VI) adsorption follows pseudo-second-order kinetics and Modified Langmuir isotherm. The maximum adsorption capacity of C + G beads for Cr(VI) was found to be 28.65 mg g−1. These results obtained indicate that the cyclic voltammetric technique using gold electrode can be effectively applied for Cr(VI) analysis from wastewaters.

Two new polyoxometalate-based metal-organic complexes for the detection of trace Cr(VI) and their capacitor performance

Two new Keggin-type polyoxometalate (POM)-based metal-organic complexes (MOCs) H3[Cu2(4-dpye)2(PMo12O40)] (1) and H[Cu2(4-Hdpye)2(PMo12O40)(H2O)4]·2H2O (2) were constructed with a new N,N'-bis (4-pyrimidinecarboxamido)-1,2-ethane (4-H2dpye) ligand by the hydrothermal/solvothermal method. Complex 1 was a 2D layered structure constructed from 1D metal-organic chains [Cu(4-dpye)]n and Keggin-type [PMo12O40]3- polyoxoanions. Complex 2 displays a 3D supramolecular framework formed by discrete [PMo12O40]3- polyoxoanions and binuclear metal-organic loops [Cu2(4-Hdpye)2]. The electrocatalytic behaviors of carbon paste electrodes modified by complexes 1 and 2 (1-CPE and 2-CPE) were investigated. The 1-CPE and 2-CPE were used as electrochemical sensors to detect trace Cr(vi), and the low limits of detection (LOD) are 1.27 × 10-7 M for 1 and 1.71 × 10-7 M for 2, which are lower than the maximum allowable concentration of Cr(vi) in drinking water specified by the World Health Organization (WHO). In addition, the performances of complexes 1 and 2 modified carbon cloth electrodes (1-CC and 2-CC) as supercapacitor materials have also been studied. The influence of the structure on electrocatalytic and capacitor performances is discussed.

Single-atom nanozyme enabled fast and highly sensitive colorimetric detection of Cr(VI)

As a high biologically toxic heavy metal ion, Cr(VI) will cause environmental pollution and endanger human health. Therefore, the development of fast, simple and visible detection methods for Cr(VI) is extremely important to control its harm. Toward this end, we report the establishment of a colorimetric sensing method for Cr(VI) based on single-atom nanozymes for enhanced detection performance. Firstly, we prepared SA-Fe/NG as peroxidase mimetic by anchoring Fe single-atom onto a single-layer of two-dimensional nitrogen-doped graphene. The SA-Fe/NG showed superiorly high oxidation catalytic activity due to its 100% atomic utilization and existing Fe-N-C structure. Furthermore, with 3,3',5,5'-tetramethylbenzidine (TMB) as a colorimetric sensing probe, and 8-hydroxyquinoline (8-HQ) as an inhibitor for the oxidation of TMB, the detection of Cr(VI) was realized through specific interaction between Cr(VI) and 8-HQ, which led to the recovery of oxTMB in blue color. Our established method showed superior sensitivity with a detection limit of 3 nM and a linear range of 30 nM to 3 μM. It also exhibited high selectivity for a series of metal cations, and has been successfully applied to the detection of Cr(VI) in tap water and tuna samples.

Electrochemical reduction of Cr(VI) in water: lessons learned from fundamental studies and applications

Converting toxic Cr(vi) to benign Cr(iii) would offer a solution to decontaminate drinking water. Electrochemical methods are ideally suited to carry out this reduction without added external reductants. Achieving this transformation at low overpotentials requires mediating the transfer of protons and electrons to Cr(vi). In this review thermodynamic parameters will be discussed to understand Cr(vi) speciation in water and identify reduction pathways. The electrochemical reduction of Cr(vi) at bare electrodes is reviewed and mechanistic considerations are discussed. Works on modified electrodes are compared to identify key parameters influencing the reduction. An overview of current applications to Cr(vi) reduction is briefly discussed to link fundamental studies to applications.

In situ electro-organic synthesis of hydroquinone using anisole on MWCNT/Nafion modified electrode surface and its heterogeneous electrocatalytic reduction of toxic Cr(vi) species

Owing to its electro-inactive character, anisole (phenylmethyl ether, PhOCH3) and its related derivatives have been used as electrolytes in electrochemistry. Herein, we report a simple one-step electro-organic conversion of PhOCH3 to hydroquinone (HQ) on a pristine-MWCNT-Nafion modified electrode glassy carbon electrode surface, GCE/Nf-MWCNT@HQ, in pH 2 KCl-HCl solution within 15 min of working time. The chemically modified electrode showed a highly redox-active and well-defined signal at an apparent standard electrode potential, E o' = 0.45 V vs. Ag/AgCl (A2/C2) with a surface excess value, Γ HQ = 2.1 × 10-9 mol cm-2. The formation of surface-confined HQ is confirmed by collective physicochemical and spectroscopic characterizations using TEM, UV-Vis, Raman, FTIR, NMR and GC-MS techniques and with several control experiments. Consent about the mechanism, the 2.1% of intrinsic iron present in the pristine-MWCNT is involved for specific complexation with oxygen donor organic molecule (PhOCH3) and hydroxylation in presence of H2O2 (nucleophilic attack) for HQ-product formation. The GCE/Nf-MWCNT@HQ showed an excellent heterogeneous-electrocatalytic reduction of Cr(vi) species in acidic solution with a linear calibration plot in a range, 5-500 ppm at an applied potential, 0.4 V vs. Ag/AgCl with a detection limit, 230 ppb (S/N = 3; amperometric i-t). As a proof of concept, selective detection of toxic Cr(vi) content in the tannery-waste water has been demonstrated with a recovery value ∼100%.

Cathodically Pretreated AuNPs-BDD Electrode for Detection of Hexavalent Chromium

Hexavalent chromium (Cr (VI)) has strong oxidizing properties and can result in strong carcinogenic effects on human bodies. Therefore, it is necessary to detect hexavalent chromium sensitively and accurately. This article proposes the gold nanoparticles (AuNPs)–boron-doped diamond (BDD) electrode for the direct determination of chromium with a green and simple detection process by cathodic stripping voltammetry. Gold nanoparticles are used to enhance the detection performance toward Cr (VI). The effect of different pretreatment methods on electrode modification has been studied, and the detection parameters have been optimized. With the optimized conditions, the AuNPs–BDD electrode presents a good linear behavior in a Cr (VI) concentration range of 10 to 1000 μg/L. A low limit of detection of 1.19 μg/L is achieved. The detection process is simple and environmentally friendly. The sensor has been tested for the detection of Cr (VI) in a real water sample with satisfactory results, which indicates potential application of the AuNPs–BDD electrode for the sensitive and onsite detection of Cr (VI).

Toward the Development of Ultrasensitive Detectors for Environmental Applications: A Kinetic Study of Cr(III) Monitoring in Water Using EDTA and SERS Techniques

We report for the first time kinetic studies on chromium(III) detection in aqueous solution using citrate-capped silver nanoparticles (AgNPs) and the surface-enhanced Raman spectroscopy (SERS) technique. Moreover, we have shown an important effect of adding ethylenediaminetetraacetic acid (EDTA) on the enhancement and the stability of the Raman signal. The origin of the SERS signal was attributed to the coordination of Cr(III) by citrate/EDTA molecules and the formation of hot spots on aggregated AgNPs. Depending on the mixing method of Cr(III) and EDTA with AgNPs, the temporal SERS spectral features reveal a Prout-Tompkins or a Langmuir kinetic detection model. The UV-visible data, the temporal response of the Raman signal, and the scanning electron microscopy analysis have allowed us to elucidate the mechanism of Cr(III) detection. We observed that mixing simultaneously Cr(III), AgNPs, and EDTA leads to the most stable and intense time-dependent SERS signal. The obtained results should open the way to perform kinetic studies on different host-guest interactions in solution using the SERS technique.

Biologically inspired catalyst for electrochemical reduction of hazardous hexavalent chromium

An indirect electrochemical detoxification and detection platform has been demonstrated for toxic hexavalent chromium (Cr(vi)) based on the biologically important N-4 macrocycle. The research work describes a simple, green, low-cost and potential way for the synthesis of a new N-4 macrocyclic molecule and the molecule is characterized by various analytical and spectroscopic techniques like elemental analysis, TGA, FT-IR, UV-visible, mass spectrometry and NMR spectroscopies, and cyclic voltammetry. The synthesized molecule was explored for the electrochemical reduction of Cr(vi) using both voltammetric and amperometric methods. Amperometric studies exhibited 50 to 2500 nM linear range and the detection limit and quantification limit are 18 and 50 nM, respectively. The common coexisting metal ions did not interfere with Cr(vi) even in the presence of 40-fold excess interfering ions. The real sample analysis was carried out with the fabricated sensor and successfully quantified a recovery result (98-104%) of Cr(vi) in water. This proposed sensor is helpful in the detection of chromium ions in drinking water and is capable of detecting Cr(vi) in the limits set by the World Health Organization (WHO). In addition, this sensor satisfactorily demonstrated considerable stability and reproducibility.

Recent Advances in Electrochemical Monitoring of Chromium

The extensive use of chromium by several industries conducts to the discharge of an immense quantity of its various forms in the environment which affects drastically the ecological and biological lives especially in the case of hexavalent chromium. Electrochemical sensors and biosensors are useful devices for chromium determination. In the last five years, several sensors based on the modification of electrode surface by different nanomaterials (fluorine tin oxide, titanium dioxide, carbon nanomaterials, metallic nanoparticles and nanocomposite) and biosensors with different biorecognition elements (microbial fuel cell, bacteria, enzyme, DNA) were employed for chromium monitoring. Herein, recent advances related to the use of electrochemical approaches for measurement of trivalent and hexavalent chromium from 2015 to 2020 are reported. A discussion of both chromium species detections and speciation studies is provided.

Facile synthesis of carboxymethyl cellulose sulfur quantum dots for live cell imaging and sensitive detection of Cr(VI) and ascorbic acid

Nowadays the synthesis of stable fluorescent sulfur quantum dots (SQDs) remains a big challenge. Herein, the utilization of carboxymethyl cellulose (CMC) to synthesis of SQDs is reported. Benefiting from the unique composition and structure of CMC macromolecule, the resulted CMC-SQDs simultaneously show high aqueous dispersibility and stability, tunable emission, stable fluorescence and low cytotoxicity, which make them promising for working as a fluorescent probe. Fluorescence detection experiments suggested that the CMC-SQDs could serve as a fluorescence on-off-on switch to sensitive and selective detection of Cr(VI) and ascorbic acid (AA) based on the inner filter effect (IFE). The limit of detection towards Cr(VI) and AA can reach 0.024 and 0.18 μM with linear range of 0.5-225 and 1-300 μM, respectively, which compares favorably to other reported fluorescent probes. In addition, the employment of fluorescent CMC-SQDs for practical detection of Cr(VI) and AA was also studied.

An overview of palladium supported on carbon-based materials: Synthesis, characterization, and its catalytic activity for reduction of hexavalent chromium

Palladium plays a pivotal role in most of the industrial heterogeneous catalysts, because of its unique properties such as well-defined structure, great intrinsic carrier, outstanding electronic, mechanical and thermal stability. The combination of palladium and various porous carbons (PCs) can widen the use of heterogeneous catalysts. This review highlights the advantages and limitations of carbon supported palladium-based heterogeneous catalyst in reduction of toxic hexavalent chromium (Cr^(VI)). In addition, we address recent progress on synthesis routes for mono and bimetallic palladium nanoparticles supported by various carbon composites including graphene-based materials, carbon nanotubes, mesoporous carbons, and activated carbons. The related reaction mechanisms for the Cr^(VI) reduction are also suggested. Finally, the challenge and perspective are proposed.

Zr-Based MOFs as new photocatalysts for the rapid reduction of Cr(vi) in water

The hydroxyl modified Zr-based MOFs is used for photocatalytic reduction of hexavalent chromium existing in wastewater.

Facile synthesis of hierarchically nanostructured bismuth vanadate: An efficient photocatalyst for degradation and detection of hexavalent chromium

Heterostructured nanomaterials can paid more significant attention in environmental safety for the detection and degradation/removal of hazardous toxic chemicals over a decay. Here, we report the preparation of hierarchically nanostructured shuriken like bismuth vanadate (BiVO₄) as a bifunctional catalyst for photocatalytic degradation and electrochemical detection of highly toxic hexavalent chromium (Cr(VI)) using the green deep eutectic solvent reline, which allows morphology control in one of the less energy-intensive routes. The SEM results showed a good dispersion of BiVO₄ catalyst and the HR-TEM revealed an average particle size of ca. 5-10 nm. As a result, the BiVO₄ exhibited good photocatalytic activity under UV-light about 95% reduction of Cr(VI) to Cr(III) was observed in 160 min. The recyclability of BiVO₄ catalyst exhibited an appreciable reusability and stability of the catalyst towards the photocatalytic reduction of Cr(VI). Also, the BiVO₄-modified screen printed carbon electrode (BiVO₄/SPCE) displayed an excellent electrochemical performance towards the electrochemical detection of Cr(VI). Besides, the BiVO₄/SPCE demonstrated tremendous electrocatalytic activity, lower linear range (0.01-264.5 μM), detection limit (0.0035 μM) and good storage stability towards the detection of Cr(VI). Importantly, the BiVO₄ modified electrode was also found to be a good recovery in water samples for practical applications.

Practical immune-barometer sensor for trivalent chromium ion detection using gold core platinum shell nanoparticle probes

The technology progress of biosensors has markedly improved healthcare, disease diagnosis, environment monitoring, and food safety control over the past few decades. However, development of sensitive, robust, low-cost and portable assays for on-site bioanalysis is still a great challenge. In this study, we described a portable, feasible and miniaturized immune-barometer sensor (IBS), which can be used to sensitively measure the changes in a pressure signal, and we applied this IBS in the detection of Cr(iii). In this system, a competitive immunoassay was incorporated as a signaling technique for Cr(iii) detection. To generate a signal of pressure changes (ΔP), Au@PtNPs (gold core platinum shell nanoparticles) were prepared for decomposing H₂O₂ to generate O₂ in a sealed chamber. The expansion of gas volume was accurately detected using a sensitive barometer in the sealed reaction chamber. The ΔP correlated well with Cr(iii) concentrations ranging from 0.39 to 25 ng mL^-1. The limit of detection (LOD) of the IBS was estimated to be as low as 0.35 ng mL^-1. Furthermore, the IBS has high specificity and high recovery for Cr(iii) detection in tap water samples (97.5%-108.7%) and in the Pearl River water samples (95.6%-110.2%). Compared with the traditional enzyme-linked immunosorbent assay (ELISA), the IBS was observed to be more sensitive, of low-cost and portable for the on-site detection of Cr(iii). Therefore, the IBS is a promising potential method for the detection of heavy metals in aqueous solutions and many other fields.

Highly selective and sensitive probes for the detection of Cr(vi) in aqueous solutions using diglycolic acid-functionalized Au nanoparticles

In this study, a variety of diglycolic acid-functionalized gold nanoparticle (Au NP) probes are reported, which are highly sensitive for the detection of chromium ions, Cr(vi) ions, at low concentrations in aqueous solutions based on the application of surface plasmon resonance (SPR) theory. Due to its outstanding affinity for Cr(vi) ions, the capped diglycolic acid would induce the aggregation of the NP probes upon encountering them; this was evidenced by the obvious red-shifting of the SPR peak and the enlarged size of the NPs. For the same reason, the selectivity of the probe for Cr(vi) against other heavy metal ions was found to be remarkable. Under optimized conditions, the probe showed the limit of detection (LOD) of 0.32 ppb for Cr(vi) and a linear detection scale ranging from 0.32 ppb to 0.1 ppm. To the best of our knowledge, this is probably the lowest LOD reported for Cr(vi) detection among those of the methods based on SPR.

In this study, we proposed a diglycolic acid-functionalized gold nanoparticle (Au NP) probe, which are highly sensitive for the detection of chromium ions based on the application of surface plasmon resonance (SPR) theory.

One-pot synthesis of N-doped carbon dots by pyrolyzing the gel composed of ethanolamine and 1-carboxyethyl-3-methylimidazolium chloride and their selective fluorescence sensing for Cr(vi) ions

N-Doped carbon dots were synthesized by a one-pot pyrolysis method and used as a fluorescent sensor towards Cr(vi) ions.

An efficient “off–on” carbon nanoparticle-based fluorescent sensor for recognition of chromium(vi) and ascorbic acid based on the inner filter effect

A simple approach based on calcination treatment of diethylenetriaminepentaacetic acid (DTPA) was developed to prepare water-soluble nitrogen doped carbon nanoparticles (N-CNPs) with a high quantum yield of approximately 53.7%.

Au-Pd Bimetallic Nanoparticle Electrodes for Direct Electroreduction of Hexavalent Chromium Complexes

This paper reports a simple, low-cost, and effective electrochemical technique for sensing and reducing CrVI based on a Au-Pd bimetallic nanoparticle (BNP)-decorated indium tin oxide (ITO) conducting glass electrode. It was observed that the Au-Pd BNP-decorated ITO electrode could significantly boost the electrochemical reduction of CrVI when compared with either Au nanoparticle- or Pd nanoparticle-decorated ITO electrodes. These BNP-decorated electrodes exhibited a wide linear concentration range of 0.001–100 μM, a very low detection limit (signal-to-noise ratio = 3) of 0.3 nM, and a high sensitivity of 1.701 μA μM–1. From electrochemical impedance spectroscopy, it was revealed that this significant improvement was mainly due to the reduction in the charge-transfer resistance, which leads to faster free exchange of the reaction intermediates. The proposed Au-Pd BNP electrode also demonstrated excellent stability, selectivity, repeatability, and reproducibility.