Colorimetric detection of heavy metal ions with various chromogenic materials: Strategies and applications

Enhanced near-infrared Ru (II) complex fluorescence sensor for sensitive sensing of Al3+ and cell imaging

A near-infrared (NIR) fluorescent sensor (λem = 790 nm), [Ru ((CH3O)2bipy)2 (BIMPY)]2+, was synthesized and thoroughly characterized, which can selectively recognize Al3+ ions in THF. The [Ru ((CH3O)2bipy)2 (BIMPY)]2+ has excellent sensitivity (LOD = 3.48 × 10-8 mol/L) towards Al3+ with a 2:1 (Ru complex/Al3+) complex ratio and opportune binding constant (K = 606.82 mol/L). The change mechanism of photophysical properties was determined by time-dependent density functional theory (TDDFT) method, which illustrates fluorescence enhancement sensing to Al3+ ions. The [Ru ((CH3O)2bipy)2 (BIMPY)]2+ was used as a field-deployable sensor, achieving on-site Al3+ monitoring via the RGB analysis. Furthermore, the [Ru ((CH3O)2bipy)2 (BIMPY)]2+ succeed in imaging Al3+ in living HepG2 cells.

A simple colorimetric and paper-based-smartphone sensing platform based on the enhanced peroxidase-like activity of Al doping Prussian blue for point-of-care detection of GSH

Glutathione (GSH) is a very important antioxidant and also participates in many important physiological processes, accurate determination of GSH in food and blood fluid is crucial for human health. Herein, we introduce a simple colorimetric and paper-based-smartphone dual signal output platform for point-of-care (POC) detection of GSH in garlic and human serum. The experiment revealed that the peroxidase-like activity of Aluminum-doped Prussian blue (AlPB) could be enhanced by the doping of Al element on Prussian blue. AlPB exhibited excellent peroxidase-like activity and efficiently catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2 to generate blue oxidized TMB, resulting in an absorbance and color-based-smartphone dual signal readouts. Enzyme kinetic studies also indicated that the AlPB showed high affinity towards TMB. Hence, a reliable colorimetric and color-based-smartphone dual signal readouts assay was constructed based on AlPB-mediated the peroxidase-like activity, it was used for highly sensitive colorimetric and POC detection of GSH in food and human serum within 7 min with a wide range of linear response from 1 to 20 μM, a low detection limit of 0.1748 μM. This work demonstrates a novel and versatile strategy to develop superior peroxidase mimics and holds great potential for rapid and portable detection of GSH in food, healthcare and clinical diagnosis, and also open promising avenues for more powerful paper-based POC tests.

Paper-based sap enrichment device combined with laser-induced breakdown spectroscopy for the minimally invasive detection of Cd(Ⅱ) and Pb(Ⅱ) in plants

Detecting heavy metals in plants is highly important for diagnosing plant health and understanding the stress mechanisms induced by heavy metals. However, the minimally invasive detection of heavy metals in plants remains a challenge. A novel paper-based sap enrichment device (PBSED), combined with laser-induced breakdown spectroscopy (LIBS) was proposed for the minimally invasive detection of Cd(Ⅱ) and Pb(Ⅱ) in plants. The PBSED included a stainless-steel capillary and heavy metal ion enrichment filter paper (HMIE-FP). The stainless-steel capillary was inserted into the plant stem, where plant sap was transported onto the paper substrate through capillary action. The heavy metal ions (HMIs) in the plants were enriched on the HMIE-FP, and LIBS was used to detect Cd(Ⅱ) and Pb(Ⅱ) on the HMIE-FP to determine the Cd(Ⅱ) and Pb(Ⅱ) concentration within the plant. COMSOL simulations were employed to analyse the flow dynamics of plant sap within the PBSED. To increase the heavy metal enrichment amount, the HMIE-FP was modified with AuAg bimetallic nanoparticles (AuAgBNPs). The PBSED-LIBS method was applied to detect Cd(Ⅱ) and Pb(Ⅱ) in cucumber plants, and the results were strongly correlated with the inductively coupled plasma mass spectrometry (ICP-MS) results (R² = 0.99 for Cd(Ⅱ) and 0.96 for Pb(Ⅱ)). The proposed PBSED-LIBS method demonstrated high sensitivity and minimal invasiveness; thus, it is suitable for rapid, in vivo detection of HMIs in plants. These findings provide valuable insights for the development of efficient, nondestructive tools for environmental applications.

Ultrasensitive adsorptive stripping voltammetry-free electrochemical sensor for Cu2 + based on its specific catalytic etching to cytosine-rich oligonucleotide templated silver nanoparticles

The development of reliable and highly sensitive copper ions (Cu2+) detection technologies is crucial for both environmental conservation and health surveillance. To address the challenges associated with conventional adsorptive stripping voltammetry, such as potential matrix interference, lengthy pre-electrolysis times, and limited detection sensitivity, we herein introduce an innovative electrochemical sensing approach for Cu2+. This method utilizes the unique catalytic etching capability of Cu2+ on cytosine-rich oligonucleotide (CRO)-templated silver nanoparticles (AgNPs). The thiolated CRO was assembled onto the Au electrode through the Au-S bond. Subsequently, the AgNPs were generated by in-situ chemical reduction of Ag+, which pre-absorbed on CRO via the cytosine-Ag+-cytosine (C-Ag+-C) structure. The results demonstrated that Cu2+ could markedly speed up the etching of AgNPs, which in turn reduced the solid-state electrochemical response of AgNPs. This reduction allowed for the detection of Cu2+ within a wide concentration range from 0.1 pM to 1.0 nM, with an impressively low detection limit of 0.03 pM. The practicality of this method has been validated through its successful application in analyzing Cu2+ levels of the actual water samples.

True Inhibition of CeO2 Nanozymes by Antioxidants: Mechanistic Insights and Colorimetric Detection of Ergothioneine

Some nanozyme-based detection methods for antioxidants often rely on the direct decolorization of colorimetric substrates by corresponding antioxidants, which weakens the role of nanozymes. Here, CeO2 nanozymes exhibiting inherent oxidase-like activity were prepared via a one-pot hydrothermal technique. Ergothioneine (ERG), a natural antioxidant, was found to induce the true inhibition of the oxidase-like activity. Unlike other antioxidants including ascorbic acid, gallic acid and glutathione, ERG could really inhibit the oxidase-like activity of CeO2 nanozymes, instead of directly reducing colorimetric substrates. Further kinetic analysis revealed that ERG-induced inhibition is reversible and noncompetitive. This discovery highlights the unique behavior of ERG and provides an important mechanistic understanding of nanozyme inhibition by antioxidants. Leveraging the inhibitory effect, a novel colorimetric sensing platform for ERG was established with a low limit of detection (0.21 μM), excellent reusability, good storage stability and high accuracy, making it a simple, sensitive and rapid method for assessing ERG levels in functional foods. This study not only establishes a framework for investigating the interactions between nanozymes and small molecules but also paves a new pathway for expanding the applications of nanozymes in food and agricultural biosensing.

Recent Advances (2019-2025) in Mercury Ion Detection

As a persistent bioaccumulative toxin, mercury ion (Hg2+) presents grave environmental and public health risks due to their extreme toxicity and neurological damage potential. The advancement of reliable Hg2+ detection methodologies become imperative for safeguarding ecological security and food supply chains. In recent years, different signal output modes have been explored for Hg2+ detection. This review systematically summarizes the latest detection techniques and strategies for Hg2+ based on electrochemical signal, fluorescence signal, SERS signal and colorimetric signal. Innovation designs of functional materials, small molecules, nanoprobes and strategies for Hg2+ detection have been introduced and their detection performances including sensitivity, selectivity and portability are compared with each other. The comparison results show that some methods can achieve ultrasensitive detection of Hg2+ at fM level. In addition, the challenges and perspective for Hg2+ detection have been discussed. The review is expected to provide insights for the development of highly sensitive and portable detection tools for Hg2+, and promote the transformation of Hg2+ detection technology from the laboratory to industrialization.

A gold nanoparticle-based colorimetric strategy for DNA detection: principles and novel approaches

The development of nanotechnology has led to the rapid growth of many different fields, including sensors. Bulky and complex sensor systems are gradually being replaced by streamlined sensor devices with advantages in size, simplicity, cost-effectiveness, and fast response, allowing qualitative detection of target analyte on-site application for clinical diagnosis. Significantly, since the COVID-19 pandemic, research on developing test kits for detecting biological molecules has grown rapidly, with an increasing number of publications. The number of studies developing colorimetric sensors based on gold nanoparticles (AuNPs) has increased continuously over the years, demonstrating the potential application of this material. The surface plasmon resonance (SPR) effect and high biocompatibility of AuNPs make them different from many other metal nanomaterials. In addition, the peroxidase activity properties of AuNPs have also received much attention in colorimetric sensors. In this review, the colorimetric sensors developed based on the AuNP material platform for DNA detection will be discussed in detail. Among them, the commonly used synthesis methods of AuNPs based on their applications and the primary mechanism of AuNP-based colorimetric sensors for DNA detection will be discussed. In addition, AuNP-based colorimetric applications in POCT for pathogenic bacteria and viruses are also mentioned in this review to provide a broader perspective on the potential and developmental direction of AuNP-based colorimetric sensors. Another aspect this review provides is development strategies that allow simple readout using the naked eye, a spectrophotometer, or a smartphone camera, which present many opportunities for integration into other electronic devices.

An azacrown ether-based near-infrared fluorescent probe for the detection of Pb2+ and its applications in food, environmental water, plant and animal samples

BACKGROUND

Lead ion (Pb2+), as a kind of heavy metal ion, is particularly harmful to human health and ecosystems due to its high toxicity and easy bioaccumulation. Fluorescent probes capable of selective and sensitive detection of Pb2+ are crucial for enabling rapid and on-site monitoring and regulation, thereby mitigating its adverse health and environmental impacts. Additionally, the development of fluorescence probes for the detection of Pb2+ in plant systems is rarely reported. Accordingly, the development of near-infrared (NIR) emission fluorescence probe for the detection of Pb2+ in food, environment and in vivo is of great significance.

RESULTS

In this work, an azacrown ether-based NIR fluorescence probe LCE1 was reported for the detection of Pb2+. Probe LCE1 can generate 1:1 complex with Pb2+, resulting in the inhibition of ICT effect to reduce the fluorescence signal. LCE1 exhibited many advantages, including NIR emission (λem = 670 nm), high selectivity and sensitivity (LOD = 0.34 μM) and fast response (30 s). The quantitative determination of Pb2+ in real food and water samples has been achieved with good recovery using LCE1 as the probe. Concurrently, the on-site and rapid determination of Pb2+ in water sample was realized by smartphone-assisted LCE1-based test strip technology. Notably, the fluorescence imaging of Pb2+ in cells and animals has been successfully implemented using the probe LCE1. Most importantly, the fluorescence imaging of Pb2+ in Pb-hyperaccumulator plant samples has been successfully demonstrated.

SIGNIFICANCE

LCE1 could provide new methods for understanding the physiopathological roles of Pb2+, evaluating food safety and selecting plants used to remediate soil contaminated by heavy metals.

Colorimetric and Fluorometric N-Acylhydrazone-based Chemosensors for Detection of Single to Multiple Metal Ions: Design Strategies and Analytical Applications

The development of optical sensors for metal ions has gained significant attention due to their broad applications in biology, the environment, and medicine. Colorimetric and fluorometric detection methods are particularly valued for their simplicity, cost-effectiveness, high detection limits, and analytical power. Among various chemical probes, the hydrazone functional group stands out for its extensive study and utility, owing to its ease of synthesis and adaptability. This review provides a comprehensive overview of N-acylhydrazone-based probes, serving as highly effective colorimetric and fluorometric chemosensors for a diverse range of metal ions. Probes are categorized into single-ion, dual-ion, and multi-ion chemosensors, each further classified based on the detected metal(s). Additionally, the review discusses detection modes, detection limits, association constants, and spectroscopic measurements.

DNA Sensors for the Detection of Mercury Ions

Ecosystem pollution by mercury ions (Hg2+) is a major health concern, yet classical analytical methods for mercury analysis are limited. This paper reviews the advances in Hg2+ detection using DNA as recognition elements in the sensors. DNA as a recognition molecule is inexpensive, simple, and appropriate for real-time detection of Hg2+. This paper discusses the DNA-based sensors that were used for the detection of Hg2+. These can be carried out by electrochemistry, field effect transistors (FET), Raman spectroscopy, colorimetry, and fluorescence resonance energy transfer (FRET). The detection principles and the advantages of DNA in these sensors are also revealed. Finally, the paper provides an overview of prospects and potential challenges in the field.

Eco-friendly colorimetric detection of lead and mercury using l-cysteine-functionalized gold nanoparticles: a step towards greening the exposome

Heavy metal toxicity, particularly from lead (Pb) and mercury (Hg), poses a significant threat to the biological system and the exposome, even at trace levels. In alignment with the 'Greening the Exposome' initiative, this study presents an eco-friendly and rapid colorimetric technique for detecting Pb and Hg in water. This method utilizes biogenic l-cysteine functionalized gold nanoparticles (AuNPs) of varying sizes as a colorimetric sensor, offering selective detection of these heavy metals. The utilization of l-cysteine, a naturally occurring amino acid, enhances the sustainability of the assay. The l-cysteine functionalized AuNPs demonstrate selectivity towards Pb and Hg through visually observable color changes, directly correlated with shifts in the surface plasmon resonance (SPR). The characteristic SPR peak of AuNPs, initially observed around 525 nm, undergoes a red shift upon aggregation in the presence of Pb and Hg, resulting in peaks at approximately 725 nm for Pb and 700 nm for Hg. The calibration curve shows a linearity range of 100-500 ppb for determination of Pb and Hg with a limit of detection of 290 ppb and 140.35 ppb. This simple, cost-effective, and environmentally conscious approach offers a promising tool for monitoring heavy metal contamination in aqueous environments, contributing to a better understanding and management of the chemical exposome.

Evaluation of Chlorella vulgaris as sensitive, cost-effective, and environmentally sustainable biosensor tools for heavy metal monitoring in aquatic ecosystems

This study evaluates the effectiveness of single-cell microalgae as sensitive, cost-effective, and environmentally sustainable biosensors for detecting heavy metal contamination in aquatic ecosystems. A preliminary investigation compared the sensitivity of Nostoc commune and Chlorella vulgaris to chromium (Cr), cadmium (Cd), and mercury (Hg). C. vulgaris exhibited greater sensitivity, with Kautsky fluorescence increases of 67.82% (Cr VI, 200 µmol), 67.1% (Cd II, 100 µmol), and 35.27% (Hg, 1 µmol), whereas N. commune showed 124.70% (Cr VI, 200 µmol), 118.04% (Cd II, 200 µmol), and 61.96% (Hg, 1 µmol). Given its higher sensitivity, C. vulgaris was selected for biosensor development. The biosensor was optimized for pH (5-9), metal concentrations (Cr⁶⁺ and Cd2⁺ at 1-100 µmol, Hg2⁺ at 1-20 µmol), and algal density, while also evaluating immobilization effects on storage stability and sensitivity. Results showed a time-dependent increase in fluorescence with rising metal concentrations, demonstrating the biosensor's efficacy in detecting heavy metals. The calculated LC50 values were 67.32 µmol (Cd2⁺), 79.2 µmol (Cr⁶⁺), and 7.2 µmol (Hg2⁺), indicating the highest sensitivity to mercury. Immobilization enhanced biosensor stability, but sensitivity declined over extended storage, particularly at higher metal concentrations. Naked eye assessments confirmed superior sensitivity to mercury, reinforcing C. vulgaris as a promising biosensor for trace metal detection.

Microscale quantitation of heavy metals by back-scattering interferometry in conjunction with photothermal effect using a single laser beam

A microanalytical technique based on the photothermal effect in conjunction with back-scattering interferometry (BSI) using a single laser beam was developed for quantitative detection of heavy metals. After the chromogenic reaction of an analyte in a capillary tube, the photothermal effect induced by irradiation with the same laser beam leads to a change of the refractive index of the solution, which can be "quantified" using the BSI technique. For prove-of-concept, Cu(II) was chosen as the trial analyte, for which the solution changes to purplish through reacting with the chromogenic reagent; a single laser beam of 532 nm was adapted for both inducing the photothermal effect and realizing BSI detection. With as little as 1.0 μL solution, a limit of detection (LOD) of 0.10 mg/L for Cu(II) was achieved. In addition, the versatility of the technique was demonstrated by detecting other two heavy metal ions, Fe(II) and Cr(VI), with limits of detection of 0.06 mg/L and 0.04 mg/L, respectively. The demonstrated detection sensitivity, application versatility, and instrumentation simplicity of this new technique promises it as a practical tool for environmental monitoring and beyond.

Chitosan-based antibacterial AIE luminogens for bioimaging and dual-mode detecting of nitrite in food samples

The polysaccharides are abundant in nature and are typically considered harmless. They can be chemically modified to exhibit a diverse range of fluorescent behaviors. Moreover, these characteristics render polysaccharides particularly promising future for the development of environmentally friendly materials, such as chemical sensors. In this study, a chitosan-based dual-mode sensor (CS-DAS) with aggregation-induced emission (AIE) properties was designed for both fluorometric and colorimetric detection of nitrite. The unique AIE property of CS-DAS enables enhanced fluorescence in aggregated states, overcoming conventional quenching limitations. CS-DAS also showed high selectivity and sensitivity for nitrite detection. By fluorescence and colorimetry, the limits of detection were calculated to be 0.021 μM (1.45 mg/kg) and 0.027 μM (1.86 mg/kg), respectively. This sensor was successfully utilized for the detection of nitrite in sausage samples. Additionally, it exhibited significant antibacterial activity against typical Gram-positive and Gram-negative bacteria. Moreover, CS-DAS showed low cytotoxicity, demonstrating its potential as an excellent fluorescent probe for cell imaging applications.

Employing Mesoporous Nitrogen Containing Carbon for Simultaneous Electrochemical Detection of Heavy Metal Ions

Heavy metal ions are major contributors to water pollution, posing significant threats to both ecological balance and human health due to their carcinogenic properties. The increasing need for heavy metal detection highlights the advantages of electrochemical methods, which offer high sensitivity and efficiency. Herein mesoporous nitrogen containing carbon (MNC) was utilized for the simultaneous determination of heavy metals using square wave voltammetry technique in the established conditions of a buffer pH of 5.0. MNC demonstrated low detection limits (1, 10 and 50 μM), wide linear ranges (1 μM-6 mM, 10 μM-7 mM and 50 μM-17 mM), and high sensitivities (2.5 μA μM-1 cm-2, 1.03 μA μM-1 cm-2 and 5.14 mA mM-1 cm-2) for, Pb2+, Cd2+ and Cu2+, respectively. Moreover, the reproducibility, and selectivity of the sensor was investigated in the presence of K+, Mg2+, Zn2+, Ni2+, and Fe3+ which are the possible interferents present in water.

A portable and 3D printing microplasma-based device coupling with visual colorimetry for field speciation analysis of SO32-/S2- in environmental water sample

In this work, a portable and 3D printing sulfur speciation analysis device was constructed, which effectively integrated with a vapor generation system, a microplasma chamber and a colorimetric unit. Point discharge microplasma was used for highly efficient oxidation of gaseous H2S to SO2, thus the simple and time-saving nonchromatographic speciation analysis of S2- and SO32- was achieved by simply adjusting the plasma "on" or "off". In this process, S2- were converted to volatile H2S by acidification reaction and then oxidized to SO2 by microplasma, prior to a specific discoloring reaction with yellow fluorescein derivative, which effectively alleviated the interference from sample matrix and further improved the analytical sensitivity. The absorbance value of fluorescein derivative at 482 nm was used for quantitative analysis of S2- and SO32-. Under optimal conditions, the detection limit was calculated to be 6.22 μmol L-1 for both analytes in the concentration range of 30-210 μmol L-1, while 60 μmol L-1 analytes were recognizable by the naked eye. The whole 3D-printed device was miniaturized, portable and easy to operate, with fast response time (<1 min), which was only the size of an adult's palm and equipped with one 3.7 V lithium battery for power. This method has been successfully utilized to field analysis of toxic S2- and SO32- in real environmental water samples.

Colorimetric and smartphone-visual detection of biothiols in human serum and red wine based on POD-like activity of Fe-CDs

An efficient mimic peroxidase platform of Fe-CDs/GOx based on the hybrid cascade system to produce in-situ H2O2 for the visualized detection of glucose has been also developed in our group. Herein, the [Fe-CDs + H2O2 + TMB] system was further performed to detect the biothiols (GSH, Cys, and Hcy) as representatives of the -SH group. The result exhibits that the higher concentration of biothiols can cause the absorbance signal at 652 nm to decrease, and the deep-blue color of oxTMB turns green until it is faded in 20 min by the naked eye. The colorimetric method showed the LOD are 0.54, 0.29, and 1.41 μM for GSH, Cys, and Hcy respectively, without interference in the presence of other amino acids. The smartphone-based and paper-based determination display good respect for biothiols in human serum or GSH in red wine. These simple and convenient strategies provide potential applications in the fields of clinical diagnosis and foods.

Magnetic Field-Accelerated Nonthermal Plasma Digestion for Field Pretreatment and Determination of Heavy Metals in Biological Samples

Field analysis of heavy metals in biological samples is essential for assessing their potential threats to human health. The development of portable pretreatment and detection devices is crucial to address this challenge. Herein, a magnetic field-accelerated nonthermal plasma digestion device using dielectric barrier discharge (DBD) is designed for the rapid and environmentally friendly pretreatment of biological samples and subsequently combined with point discharge-optical emission spectrometry (PD-OES) for sensitive determination of heavy metals. With the assistance of a magnetic field, the DBD plasma digestion of a batch of six samples using a H2O + H2O2 + HNO3 system can be completed within 25-40 min, achieving the digestion efficiencies of 92-99%. The application of magnetic field enhances the electron density, excitation temperature, discharge current, and power of the DBD plasma, thereby reducing the digestion time by 40-69%. The determination of heavy metals in the digestion solution is directly performed using a miniature injection-detector based on hydride generation-PD-OES. Under the optimized conditions, the detection limits for Pb, Sb, Sn, and Bi are 2.4-8.2 μg L-1 (0.048-0.16 mg kg-1), with precisions of <5%. The accuracy and practicability of the present device are verified by measuring several certified reference materials, including GBW10023 (laver) and GBW10044 (rice), and real rice, laver, fish, milk, and blood samples. With its advantages of environmental sustainability, short digestion time, compact size, and lightweight design, the present device provides a simple and efficient tool for field analysis of toxic elements in biological samples.

A new quinoline-based fluorescent-colorimetric chemosensor for sensitive and selective "on-off" detection of Pb2+ ions

For the first time, a novel mono Schiff base of a quinoline-based reversible fluorescent-colorimetric receptor (L) has been designed for the detection of Pb2+ ions in a semi-aqueous medium. The receptor L exhibited a very good selective fluorescent-colorimetric quick on-off response when exposed to Pb2+. The colorimetric assay showed a sensitivity of 1.2 × 10-6 M, while the fluorescent assay exhibited a sensitivity of 9.9 × 10-7 M for Pb2+. A 1 : 1 stoichiometric complexation between L and Pb2+ was determined using ESI-MS spectra and Job's plot measurements. Pb2+ was successfully measured using receptor L in both real samples and in an aqueous solution of the protein bovine serum albumin, as well as in the construction of an INHIBIT-type molecular logic gate.

In situ images of Cd2+ in rice reveal Cd2+ protective mechanism using DNAzyme fluorescent probe

As a common pollutant, cadmium (Cd) poses a serious threat to the growth and development of plants. Currently, there is no effective method to elucidate the protective mechanism of Cd2+ in plant cells. For the first time, we designed a Cd2+ fluorescent probe to observe the adsorption and sequestration of Cd2+ in rice cell walls and vacuoles. Specifically, Cd2+ is blocked by the Casparian strip and electrostatically attracted to hemicellulose, which is abundantly adsorbed and fixed to the cell walls of the endodermis. For Cd2+ that successfully entered the endodermis, one part entered the cells and was compartmentalised and fixed in the vacuoles, while the other part entered the vascular bundles and precipitated in the cell walls of the sclerenchyma through the ion exchange effect. Furthermore, with prolonged exposure to Cd2+, compartmentalised bodies that were strongly labelled by fluorescence gradually appeared in the vacuoles, which were assumed to be a new heavy metal protective mechanism activated by plants in response to continuous Cd2+ exposure. In conclusion, this study provides an innovative and effective method for the detection of adsorption, transportation, and accumulation of Cd2+ in plant tissues, which can be employed for the rapid identification of crops with low Cd accumulation.

Emerging insights into the application of metal-organic framework (MOF)-based materials for electrochemical heavy metal ion detection

Heavy metal ions are one of the main sources of water pollution, which has become a major global problem. Given the growing need for heavy metal ion detection, electrochemical sensor stands out for its high sensitivity and efficiency. Metal-organic frameworks (MOFs) have garnered much interest as electrode modifiers for electrochemical detection of heavy metal ions owing to their significant specific surface area, tailored pore size, and catalytic activity. This review summarizes the progress of MOF-based materials, including pristine MOFs and MOF composites, in the electrochemical detection of various heavy metal ions. The synthetic methods of pristine MOFs, the detection mechanisms of heavy metal ions and the modification strategies of MOFs are introduced. Besides, the diverse applications of MOF-based materials in detecting both single and multiple heavy metal ions are presented. Furthermore, we present the current challenges and prospects for MOF-based materials in electrochemical heavy metal ion detection.

Trace level detection of Pb2+ ion using organic ligand as fluorescent-on probes in aqueous media

In this study, an optical sensor, JA/(2,6-di((E)-benzylidene)cyclohexan-1-one), was synthesized and characterized using 1H NMR and FT-IR spectroscopy. The sensor exhibited high efficiency and selectivity in detecting Pb2+ ions, even in the presence of potential interfering ions such as Mn2+, Cu2+, Co2+, Cr3+, Ni2+, Ce3+, Hg2+, and Cd2+ in aqueous solutions. The interaction of JA with Pb2+ resulted in a significant enhancement of fluorescence intensity, suggesting the formation of a stable complex. A 2:1 binding ratio between JA and Pb2+ was confirmed through fluorometric analysis, absorption spectra, and theoretical calculations (DFT). The association constant for the complex was calculated to be 3 × 106 M-2. The sensor demonstrated high sensitivity towards Pb2+ with a detection limit of 5 × 10-7 M. Additionally, JA was successfully reused by applying EDTA to release Pb2+ from the sensor. Real sample analysis under optimized conditions of pH, time, and concentration of JA and Pb2+ further validated the practical applicability of the sensor.

Pattern recognition of multiple metal ions using fluorescent Perylenyl nanoaggregates and application in food traceability

Metal ions (MIs) identification is essential for the safety assessment, traceability and authentication of food. Most current approaches for detecting MIs are difficult to reconcile the simplicity, sensitivity and stability simultaneously. In this work, we proposed a novel strategy for discriminating MIs based on fluorescent supramolecular nanoaggregates (SNAs). In the presence of MI, perylene diimide derivatives (PDI)-based SNAs could be formed through the multiple non-covalent interactions between them including electrostatic, coordination and π-π interactions. With the assistance of discriminant analysis (LDA), different MIs (Hg2+, Ag+, Cd2+, Fe3+, Cr3+, Fe2+, Zn2+, Cu2+ and Pb2+) were successfully identified at three different concentration levels. It featured good quantitative sensing abilities in buffer solutions and practical samples. Furthermore, a water-quality evaluation model was successfully constructed for the distinction of different sources of drinking water, and the fluorescence array sensor technology was applied for the first time to the geographical traceability of apples.

Cerium oxide mimetic enzyme based colorimetric detection of potential oesophageal cancer biomarkers

Oesophageal cancer (OC) is a prevalent malignant tumor that poses a significant threat to individuals. Current mainstream detection method is endoscopy, which requires professional operators and expensive instruments. Therefore, it is crucial to develop a rapid, easy-to-operate, and low-cost detection method. In this study, an RNA colorimetric biosensor was successfully constructed using cerium oxide mimetic enzyme. The sensor is constructed on 96-well plates, which are immobilized with DNA-RNA-DNA complexes in microtiter wells when target RNA is present. This immobilization is based on the principle of base complementary pairing. The CeO2 immobilized has the unique advantage of catalyzing the bluing of 3,3',5,5'-tetramethylbenzidine (TMB) directly without the need any additional oxidant in microtiter wells. This property allows for the detection of RNA and enables the visualization of multiple sample assays. Furthermore, the RNA colorimetric sensor demonstrates good selectivity, immunity to interference, and high stability. Under optimal conditions, the sensor exhibited linearity in the range of 10-13 to 10-9 M with a detection limit of 33.26 fM. Therefore, this study presents a new detection method for oesophageal cancer screening.

Anion-binding-induced selective aggregation and self-degradation: Influence of positional isomerism on the anion selectivity and mode of binding of an imine-based receptor

Imine based positional isomers (8E)-N-(4-((E)-(perfluorophenylimino)methyl)benzylidene)-2,3,4,5,6-pentafluorobenzenamine, L and (10E)-N-(3-(E-Perfluorophenylimino)methyl)benzylidene)-2,3,4,5,6-pentafluorobenzenamine, L1 have been designed, and synthesized by functionalizing two electron deficient aromatic moieties at the para-para'/ortho-ortho' positions in the phenyl core of the L and L1 respectively. The responses of L and L1 towards various anionic species are examined. The positional isomers L and L1 differs not only by showing distinguishable color change upon addition of anions but also differentiates themselves by the way of self-assembling together upon binding with cyanide anion. The naked-eye colorimetric experiments, UV-Vis, Nuclear Magnetic Resonance, and Infra-Red spectroscopic analyses reveal that the isomer L binds fluoride anion through 2:1 stoichiometry ratio. Unlike fluoride complex, the isomer L form aggregates while binding with cyanide ion. On the other hand, isomer L1 does not show any instant color change upon additions of any anion. Interestingly, after thirty minutes, only the color of the cyanide complex is turned into dark brown. While analyzing the spectroscopic results of cyanide complex of L1, it is found that the cyanide complex begins to decompose and finally it is completely decomposed within 30 min. This unprecedented phenomenon about the colorimetric sensing of cyanide and destruction of cyanide complex with respect to time has not been reported in the literature yet. To the best of our knowledge this is the first example of study of sensing controlling the selectivity, mode of binding, self-aggregating and degradation properties of anionic complexes under the influence of positional isomeric effects. This present investigation provides simple and effective strategy to construct the sensor molecules with tunable binding properties in terms of easy to prepare as well as easy to use as a colorimetric sensor. _____________________________________________________________________________________________________.

Advances in Heavy Metal Sensing: Utilizing Immobilized Chromogenic Reagents, Nanomaterials Perovskite and Nanonzymes

Heavy metal pollution is a major environmental and health problem due to the toxicity and persistence of metals such as lead, mercury, cadmium, and arsenic in water, soil, and air. Advances in sensor technology have significantly improved the detection and quantification of heavy metals, providing real-time monitoring and mitigation tools. This review explores recent developments in heavy metal detection, focusing on innovative uses of immobilized chromogenic reagents, nanomaterials, perovskites, and nanozymes. Immobilized chromogenic reagents, with their high specificity and visual detection capabilities, provide cost effective solutions for heavy metal detection. Techniques to improve their stability and sensitivity, including surface modifications and hybrid materials, are discussed. Nanomaterials, including quantum dots, metal-organic frameworks, and carbon-based nanostructures, have emerged as versatile platforms due to their unique physicochemical properties. These materials enable highly sensitive and selective sensing mechanisms, such as fluorescence quenching and electrochemical sensing. Perovskites, a class of materials known for their tunable optoelectronic properties, have shown great promise in the optical and electrochemical detection of heavy metals. Despite challenges related to stability and environmental safety, their potential for low-cost and scalable applications is remarkable. Nanozymes, synthetic enzyme mimics, offer robust and catalytic sensing capabilities, particularly in colorimetric and electrochemical analyses. Their superior stability and reusability compared to natural enzymes make them ideal candidates for environmental monitoring. This review provides a comparative analysis of these techniques, highlighting their strengths, limitations, and real-world applicability. Emerging trends include hybrid systems that combine the benefits of multiple approaches. The discussion concludes by addressing current challenges and providing perspectives on future directions for advancing heavy metal detection technologies to improve environmental health and safety. Integrating chromogenic reagents with perovskite materials represents a promising direction for developing robust, sensitive, and easy-to-use sensors for health and environmental safety monitoring.

A Biodegradable, Porous Flier Inspired by a Parachute-Like Tragopogon Fruit for Environmental Preservation

New devices inspired by flying seeds, or more technically by fruits with dispersal units, could have a significant impact for environmental monitoring and aerial seeding. Among the various types of dispersal units (e.g., winged, gliding), parachuted or plumed units offer the lowest vertical descent speed (i.e., 0.3-0.7 m s-1), making them an appealing solution for wind-driven distribution over large areas. Here, a biodegradable and porous parachute flier based on cellulose acetate, inspired by a Tragopogon pratensis fruit is presented. A micrometric-thick pappus is 3D printed and integrated with a porous colorimetric indicator or a porous beak, with micrometric pores, fabricated through injection molding and leaching techniques. Thanks to the bioinspired design and the lightweight porous structure, the artificial Tragopogon mimics the aerodynamics and descent speed of the natural species. Its feasibility is demonstrated in aerial seeding by integrating the beak with a mustard seed (as a model), and in environmental monitoring by coupling it with colorimetric indicators for rain pH and nitrate levels in soils. The proposed flier represents a significant advancement as it is the first parachute-like biodegradable solution, seamlessly integrated into natural ecosystems, thus contributing to moving a step forward in artificial solutions with zero impact.

Fluorescent carbon quantum dots for heavy metal sensing

Many heavy metals pose significant threats to the environment and human health. Traditional methods for detecting heavy metals are often limited by complex procedures, high costs, and challenges in field monitoring. Carbon quantum dots (CQDs), a novel class of fluorescent carbon nanomaterials, have garnered significant interest due to their excellent biocompatibility, low cost, and minimal toxicity. This paper reviews the primary synthesis methods, luminescence mechanisms, and fluorescence quenching mechanisms of CQDs, as well as their recent applications in detecting heavy metals. In heavy metal sensing applications, the simplest hydrothermal method is commonly employed for the one-step synthesis and surface modification of CQDs. Various green reagents and biomass materials, such as citric acid, glutathione, orange peel, and bagasse, can be used for CQDs' preparation. Quantum confinement effects and surface defects give CQDs their distinctive luminescence properties, enabling the detection of heavy metals through fluorescence quenching or enhancement. Additionally, CQDs can be applied in biological imaging and smart detection, and when combined with adsorption materials, they can offer multifunctional capabilities. This review also discusses the future development prospects of CQDs.

Recent Advancements in Electrochemical Sensors Based on MOFs and Their Derivatives

Metal-organic frameworks (MOFs) are composed of metal nodes and organic linkers that can self-assemble into an infinite network. The high porosity and large surface area of MOFs facilitate the effective enrichment and mass transfer of analytes, which can enhance the signal response and improve the sensitivity of electrochemical sensors. Additionally, MOFs and their derivatives possess the properties of unsaturated metal sites and tunable structures, collectively demonstrating their potential for electrochemical sensing. This paper summarizes the preparation methods, structural properties, and applications of MOFs and their derivatives in electrochemical sensing, emphasizing sensors' selectivity and sensitivity from the perspectives of direct and indirect detection. Additionally, it also explores future directions and prospects for MOFs in electrochemical sensing, with the aim of overcoming current limitations through innovative approaches.

A fast and sensitive colorimetric sensor for residual chlorine detection made with oxidized cellulose

Residual chlorine from widespread disinfection processes forms byproducts in water that are harmful to humans and ecosystems. Portable sensors are essential tools for the on-site monitoring of residual chlorine in environmental samples. Here, an inexpensive colorimetric sensor was developed by grafting via amidation the chromogen orthotolidine (OTO) to the surface of a TEMPO-oxidized cellulose filter paper (O-TOFP). A thorough characterization of the sensor strip demonstrated that it was highly stable and that it could be stored for a long period before usage. O-TOFP had a fast response time of 30 s, was highly selective for residual chlorine ions (ClO-) with an accuracy of at least 95 %, and exhibited an excellent limit of detection of only 0.045 mg/L when combined with smartphone image acquisition. With its many positive features, the easy-to-use and robust O-TOFP sensor described here could become a useful tool for the determination of residual chlorine in different water samples.

Smartphone-assisted colorimetric sensor arrays based on nanozymes for high throughput identification of heavy metal ions in salmon

The rapid, precise, and high-throughput identification of multiple heavy metals ions holds immense importance in ensuring food safety and promoting public health. This study presents a novel smartphone-assisted colorimetric sensor array for the rapid and precise detection of multiple heavy metals ions. The sensor array is based on three signal recognition elements (AuPt@Fe-N-C, AuPt@N-C, and Fe-N-C) and the presence of different heavy metal ions affects the nanozymes-chromogenic substrate (TMB) catalytic color production, enabling the differentiation and quantification of various heavy metal ions. Combined with a smartphone-based RGB mode, the colorimetric sensor array can successfully identify five different heavy metal ions (Hg2+, Pb2+, Co2+, Cr6+, and Fe3+) as low as 0.5 μM and different ratios of binary and ternary mixed heavy metal ions in just 5 min. The sensor array successfully tested seawater and salmon samples with a total heavy metal content of 10 μM in the South China Sea (Haikou and Wenchang). Overall, this study highlights the potential of smartphone-assisted colorimetric sensor arrays for the rapid and precise detection of multiple heavy metal ions, which could significantly contribute to food safety and public health monitoring.

Lateral flow chromatography strip system for rapid fluorescence determination of phycocyanin in water samples

Phycocyanin (PC) is of great significance to biomedicine and water environmental safety. Hence, it is indispensable to develop facile and rapid method for PC determination. In this investigation, a system containing lateral flow chromatography (LFC) strip (which was deposited with molecularly imprinted polymer (MIP) capped CdTe quantum dots (QDs) based mesoporous structured coated silica nanoparticles, SiO2@QDs@ms-MIP NPs) and miniaturized fluorimeter was first fabricated. In detail, a two-step strategy was utilized for preparation of SiO2@QDs@ms-MIP NPs, which consisted of modification of CdTe QDs onto the silica NPs first, and synthesis of mesoporous imprinting shell by using PC as template molecule and cetyltrimethylammonium bromide (CTAB) as surfactant. After that, novel fluorescence NPs possessing specific recognition and sensitivity toward PC in seawater and lake water were acquired. The resulting fluorescent sensing system exhibited outstanding performances, which included excellent sensitivity (4.5 nmol/L), satisfactory specificity (imprinting factor, 2.31), appropriate linearity range (0.01-5 μmol/L), good recovery (96.0-101.7 %), excellent stability (relative standard deviation, RSD<1.1 %), wonderful reproducibility (RSD<1.1 %), and excellent anti-interference ability. The results of the fluorescent sensing system were superior to those of the commonly used ultraviolet (UV) method. The proposed strategy showed great potential for fast (<10 min) and convenient fluorescence detection of PC in real samples.

A Ratiometric Fluorescence Probe for Visualized Detection of Heavy Metal Cadmium and Application in Water Samples and Living Cells

Heavy metal cadmium (II) residuals have inflicted severe damage to human health and ecosystems. It has become imperative to devise straightforward and highly selective sensing methods for the detection of Cd2+. In this work, a ratiometric benzothiazole-based fluorescence probe (BQFA) was effortlessly synthesized and characterized using standard optical techniques for the visual detection of Cd2+ with a change in color from blue to green, exhibiting a significant Stokes shift. Moreover, the binding ratio of BQFA to Cd2+ was established as 1:1 by the Job's plot and was further confirmed by FT-IR and 1HNMR titrations. The ratiometric fluorescence response via the ICT mechanism was confirmed by DFT calculations. Furthermore, the limit of detection for detecting Cd2+ was determined to be 68 nM. Furthermore, it is noteworthy that BQFA showed good performance in real water samples, paper strips, smartphone colorimetric identification, and cell imaging.

Optical detection strategies for Ni(II) ion using metal-organic chemosensors: from molecular design to environmental applications

Nickel is an important element utilized in various industrial/metallurgical processes, such as surgical and dental prostheses, Ni-Cd batteries, paint pigments, electroplating, ceramics, computer magnetic tapes, catalysis, and alloy manufacturing. However, its extensive use and associated waste production have led to increased nickel pollution in soils and water bodies, which adversely affects human health, animals and plants. This issue has prompted researchers to develop various optical probes, hereafter luminescent/colorimetric sensors, for the facile, sensitive and selective detection of nickel, particularly in biological and environmental contexts. In recent years, numerous functionalized chemosensors have been reported for imaging Ni2+, both in vivo and in vitro. In this context, metal-based receptors offer clear advantages over conventional organic sensors (viz., organic ligands, polymers, and membranes) in terms of cost, durability, stability, water solubility, recyclability, chemical flexibility and scope. This review highlights recent advancements in the design and fabrication of hybrid receptors (i.e., metal complexes and MOFs) for the specific detection of Ni2+ ions in complex environmental and biological mixtures.

Nanoarchitectonics with cetrimonium bromide on metal nanoparticles for linker-free detection of toxic metal ions and catalytic degradation of 4-nitrophenol

Heavy metal ions and organic pollutants, such as 4-nitrophenol (4-NP), pose significant environmental and human health threats. Addressing these challenges necessitates using advanced nanoparticle-based systems capable of efficient detection and degradation. However, conventional approaches utilizing strong capping agents like cetrimonium bromide (CTAB) on nanoparticles lead to limitations due to the rigid nature of CTAB. This restricts its utility in heavy metal detection and 4-NP degradation, requiring additional surface modifications using linker molecules, thereby increasing process complexity and cost. To overcome these limitations, there is a critical need for the development of an easy-to-use, dual-functional, linker-free nanosystem capable of simultaneous detection of heavy metals and efficient degradation of 4-NP. For enabling linker-free/ligand-free detection of heavy metal ions and catalytic degradation of 4-NP, CTAB was engineered as a versatile capping agent on gold and silver nanoparticles. Various factors, including nanoparticle characteristics such as shape, size, metal composition, centrifugation, and NaOH amount, were investigated for their impact on the performance of CTAB-capped nanoparticles in heavy metal detection and 4-NP degradation. CTAB-Au nanospheres demonstrated limited heavy metal ion detection capability but exhibited remarkable efficiency in degrading 94.37% of 4-NP within 1 min. In contrast, silver nanospheres effectively detected Hg2+, Cu2+, and Fe3+ at concentrations as low as 1 ppm and degraded 90.78% of 4-NP within 30 min. Moreover, anisotropic gold nanorods (CTAB-AuNR1 and CTAB-AuNR2) showed promising sensing capabilities towards Cu2+, Cr3+, and Hg2+ at 0.5 OD, while efficiently degrading 4-NP within 5 min at 1 OD. This study emphasizes the importance of tailoring parameters of CTAB-capped nanoparticles for specific sensing and catalytic applications, offering potential solutions for environmental remediation and human health protection.

Rapid and on-site detection of bisulfite via a NIR fluorescent probe: A case study on the emission wavelength of probes with different quinolinium as electron-withdrawing groups

The emission of venenous sulfur dioxide (SO2) and its derivatives from industrial applications such as coking, transportation and food processing has caused great concern about public health and environmental quality. Probes that enable sensitivity and specificity to detect SO2 derivatives play a crucial role in its regulations and finally mitigating its environmental and health impacts, but fluorescent probes that can accurately, rapidly and on-site detect SO2 derivatives in foodstuffs and environmental systems rarely reported. Herein, a near-infrared (NIR) fluorescent probe (ZTX) for the ratiometric response of bisulfite (HSO3-) was designed and synthesized by regulating the structure of high-performance HSO3- fluorescent probe SL previously reported by us based on structural analyses, theoretical calculations and related literature reports. The Michael addition reaction between the electronic-deficient C=C bond and HSO3- destroys ZTX's π-conjugation system and blocks its intramolecular charge transfer (ICT) process, resulting in a significant fading of the fuchsia solution and the bluish-purple fluorescence turned light blue fluorescence. Fluorescent imaging of HSO3- in live animals utilizing ZTX has been demonstrated. The quantitative analysis of HSO3- in food samples using ZTXvia a smartphone has been also successfully implemented. Simultaneously, the ZTX-based test strips were utilized to quantificationally determine HSO3- in environmental water samples by a smartphone. Consequently, probe ZTX could provide a new method to understand the physiopathological roles of HSO3-, evaluate food safety and monitor environment, and is promising for broad applications.

Facile Preparation of Tannic Acid-Gold Nanoparticles for Catalytic and Selective Detection of Mercury(II) and Iron(II) Ions in the Environmental Water Samples and Commercial Iron Supplement

Tannic acid (TA), a plant-derived polyphenol rich in hydroxyl groups, serves as both a reducing agent and stabilizer for synthesizing gold nanoparticles (TA-AuNPs). This study presents a groundbreaking method that utilizes TA to fabricate TA-AuNPs and develop two distinct colorimetric detection systems for mercury (Hg2+) and iron (Fe2+) ions. The first detection system leverages the interaction between TA-AuNPs and Hg2+ to enhance the peroxidase-like activity of TA-AuNPs, facilitating the production of hydroxyl radicals upon reaction with hydrogen peroxide, which subsequently oxidizes 3,3',5,5'-tetramethylbenzidine (TMB) into a blue-colored product (ox-TMB). The second system capitalizes on TA-AuNPs to catalyze the Fenton reaction between Fe2+ and hydrogen peroxide in the presence of 2, 6-pyridinedicarboxylic acid, boosting the generation of hydroxyl radicals that oxidize TMB into a blue-colored ox-TMB. Absorbance measurements at 650 nm display a linear relationship with Hg2+ concentrations ranging from 0.40 to 0.60 μM (R2 = 0.99) and Fe2+ concentrations from 0.25 to 2.0 μM (R2 = 0.98). The established detection limits for Hg2+ and Fe2+ are 18 nM and 96 nM, respectively. Applications to real-world samples achieved an excellent spiked recovery, spanning 101.6% to 108.0% for Hg2+ and 90.0% to 112.5% for Fe2+, demonstrating the method's superior simplicity, speed, and cost-effectiveness for environmental monitoring of these ions compared to existing techniques.

Single-atom nanozymes: Emerging talent for sensitive detection of heavy metals

In recent years, the increasingly severe pollution of heavy metals has posed a significant threat to the environment and human safety. Heavy metal ions are highly non-biodegradable, with a tendency to accumulate through biomagnification. Consequently, accurate detection of heavy metal ions is of paramount importance. As a new type of synthetic nanomaterials, single-atom nanozymes (SANs) boast exceptional enzyme-like properties, setting them apart from natural enzymes. This unique feature affords SANs with a multitude of advantages such as dispersed active sites, low cost and variety of synthetic methods over natural enzymes, making them an enticing prospect for various applications in industrial, medical and biological fields. In this paper, we systematically summarize the synthetic methods and catalytic mechanisms of SANs. We also briefly review the analytical methods for heavy metal ions and present an overall overview of the research progress in recent years on the application of SANs in the detection of environmental heavy metal ions. Eventually, we propose the existing challenges and provide a vision for the future.

Ultrasensitive Electrochemical Biosensors Based on Allosteric Transcription Factors (aTFs) for Pb2+ Detection

Exposure to Pb2+ in the environment, especially in water, poses a significant threat to human health and urgently necessitates the development of highly sensitive Pb2+ detection methods. In this study, we have integrated the high sensitivity of electrochemical techniques with allosteric transcription factors (aTFs) to develop an innovative electrochemical biosensing platform. This biosensors leverage the specific binding and dissociation of DNA to the aTFs (PbrR) on electrode surfaces to detect Pb2+. Under the optimal conditions, the platform has a broad linear detection range from 1 pM to 10 nM and an exceptionally low detection threshold of 1 pM, coupled with excellent selectivity for Pb2+. Notably, the biosensor demonstrates regenerative capabilities, enabling up to five effective Pb2+ measurements. After one week of storage at 4 °C, effective lead ion detection was still possible, demonstrating the biosensor's excellent stability, this can effectively save the cost of detection. The biosensor also achieves a recovery rate of 93.3% to 106.6% in real water samples. The biosensor shows its potential as a robust tool for the ultrasensitive detection of Pb2+ in environmental monitoring. Moreover, this research provides new insights into the future applications of aTFs in electrochemical sensing.

In Situ Formed 3,3',5,5'-Tetramethylbenzidine-Cu(I/II) Charge-Transfer Complex Intermediates Promoting Colorimetric Assay of Cr6

Developing a synthesis-free, multifunctional, and effective colorimetric assay system based on 3,3',5,5'-tetramethylbenzidine (TMB) oxidation is attractive yet challenging. Herein, we established a synergetic Cu2+/Cr6+-promoting strategy for TMB-based colorimetric detection of Cr6+. By introducing Cu2+, critical TMB·+···Cu(I/II)···TMB charge transfer complex (TMC) intermediates were in situ formed to reduce the activation energy of TMB oxidation, thereby accelerating Cr6+-mediated TMB oxidation. TMC intermediates also played a pivotal role in H2O2-participated TMB oxidation, clarifying the secondary responsibility of reactive oxygen species frequently caused by Fenton-like reactions. Leveraging the synergetic capacity between Cu2+ and Cr6+ for TMB oxidation, we demonstrated sensitive and specific colorimetric detections for Cr6+ with a limit of detection of 0.006 μM. With its convenient operation and rapid responsiveness, this strategy successfully enabled the practical detection of Cr6+ in real water samples. This work not only enhances the understanding of the internal acceleration mechanism in colorimetric sensing but also provides valuable opportunities to advance synthesis-free detection platforms.

Xanthine oxidase immobilized cellulose membrane-based colorimetric biosensor for screening and detecting the bioactivity of xanthine oxidase inhibitors

Xanthine oxidase (XO) is a typical target for hyperuricemia and gout, for which there are only three commercial xanthine oxidase inhibitors (XOIs): febuxostat, topiroxostat and allopurinol. However, these inhibitors have problems such as low bioactivity and several side effects. Therefore, the development of novel XOIs with high bioactivity for the treatment of hyperuricemia and gout is urgently needed. In this work we constructed a XO immobilized cellulose membrane colorimetric biosensor (XNCM) by the TEMPO oxidation, amide bond coupling and nitro blue tetrazolium chloride (NBT) loading method. As expected, the XNCM was able to detect xanthine, with high selectivity and sensitivity by colorimetric method with a distinctive color change from yellow to purple, which can be easily observed by the naked-eye in just 8 min without any complex instrumentation. In addition, the XNCM sensor performed screening of 21 different compounds and have been successfully pre-screened out XOIs with biological activity. Most importantly, the XNCM was able to quantitatively detect the IC50 values of two commercial inhibitors (febuxostat and allopurinol). All the results confirmed that the XNCM is a simple and effective tool which can be used for the accelerated screening of XOIs and has the potential to uncover additional XOIs.

Recent advances and trends in innovative biosensor-based devices for heavy metal ion detection in food

Low-cost, reliable, and efficient biosensors are crucial in detecting residual heavy metal ions (HMIs) in food products. At present, based on distance-induced localized surface plasmon resonance of noble metal nanoparticles, enzyme-mimetic reaction of nanozymes, and chelation reaction of metal chelators, the constructed optical sensors have attracted wide attention in HMIs detection. Besides, based on the enrichment and signal amplification strategy of nanomaterials on HMIs and the construction of electrochemical aptamer sensing platforms, the developed electrochemical biosensors have overcome the plague of low sensitivity, poor selectivity, and the inability of multiplexed detection in the optical strategy. Moreover, along with an in-depth discussion of these different types of biosensors, a detailed overview of the design and application of innovative devices based on these sensing principles was provided, including microfluidic systems, hydrogel-based platforms, and test strip technologies. Finally, the challenges that hinder commercial application have also been mentioned. Overall, this review aims to establish a theoretical foundation for developing accurate and reliable sensing technologies and devices for HMIs, thereby promoting the widespread application of biosensors in the detection of HMIs in food.

A fluorescent sensor array based on a single cucurbit[5]uril-truxene probe for simultaneous identification of five heavy metal ions

There is a need to develop simple and effective strategies for the rapid detection of heavy metal ions (HMIs) in order to protect the environment and human health. A simple fluorescent sensor array based on a single cucurbit[5]uril-truxene probe was proposed to simultaneously identify five HMIs (Pb2+, Cu2+, Ag+, Fe2+ and Fe3+). This probe was synthesized using monohydroxyl cucurbit[5]uril and monobromohexyl truxene by a substitution reaction between them. It could be observed that the fluorescence response of this synthesized probe to HMIs was closely related to the pH of the aqueous solution, exhibiting different fluorescence intensities at pH 3.0, 7.0, and 9.0. Based on this phenomenon, a fluorescent sensor array based on a single cucurbit[5]uril-truxene probe was then constructed by simply altering the pH in the sensor element. These unique fluorescence responses were analyzed using linear discriminant analysis (LDA) to identify metal ions. A concentration limit classification of 0.1 μM was applied to the above five HMIs. Moreover, the quantification of metal ions was implemented even at low concentrations of 48-121 nM. This array showed good results in the recognition of metal ions in real water samples (lake water and tap water samples), which shows its broad application prospects in many fields, including monitoring of the environmental water quality and so on.

A Review on Small Organic Colorimetric and Fluorescent Hosts for the Detection of Cobalt and Nickel Ion

In recent years, there has been a notable increase in efforts to advance efficient hosts for detecting cobalt and nickel ions, driven by their extensive industrial applications and environmental significance. This review meticulously examines the progress made in small organic colorimetric and fluorescent hosts tailored specifically for the sensitive and selective detection of cobalt and nickel ions. It delves into a diverse range of molecular architectures, including organic ligands, elucidating their unique attributes such as sensitivity, selectivity, and response time. Moreover, the review precisely explores the underlying principles governing the colorimetric and fluorescent mechanisms employed by these hosts, shedding light on the intricate interactions between the sensing moieties and the target metal ions. Furthermore, it critically evaluates the practical applicability of these hosts, considering crucial factors such as detection limits, recyclability, and compatibility with complex sample matrices. Additionally, exploration extends to potential challenges and prospects in the field, emphasizing the imperative for ongoing innovation to address emerging environmental and analytical demands. Eventually, through this comprehensive examination, the review seeks to contribute to the ongoing endeavor to develop robust and efficient tools for monitoring and detecting cobalt and nickel metal ions in diverse analytical scenarios.

Programable sewage-cleaning technology: Regenerating chitosan biofilms with anti-bacterial capacity via self-purification of water pollutants

In this paper, a novel programable sewage-cleaning technology for the regeneration of antibacterial nanocomposites via the removal of wastewater pollutants is presented. Montmorillonite (MMT) was encapsulated in poly(vinyl alcohol) (PVA)-enhanced chitosan (CTS) hydrogels to form MMT-loaded nanocomposite biofilms (PCM). The PCM nanocomposite biofilms exhibited increased breaking strength and elongation at break, by factors of approximately 1.38 and 1.40, respectively, compared with those of the pure PVA/CTS biofilms. The maximum adsorption capacity of the PCM nanocomposite biofilms toward tetracycline and Ag(I) is 275.0 and 567.0 mg/g, respectively. The adsorbed nanocomposite biofilms exhibited excellent antibacterial properties against Staphylococcus aureus and Escherichia coli. Meanwhile, the nanocomposite also showed an effective adsorption capacity toward other toxic components, where the highest adsorption capacity is 2748.0 mg/g (for methyl blue). The simulation results indicated that the adsorption behaviors of the malachite green, neutral red, methyl blue, tetracycline, Cu(II), Zn(II), and Ag(I) by the PCM nanocomposite biofilms followed pseudo-second-order kinetic and Freundlich isotherm models. Furthermore, the PCM nanocomposite biofilms are stable in PBS solution but degradable in lysozyme-containing PBS solution, suggesting their potential application in the wastewater treatment as well as antibacterial fields.

Current and emerging nanotechnology for sustainable development of agriculture: Implementation design strategy and application

Recently, agriculture systems have faced numerous challenges involving sustainable nutrient use efficiency and feeding, environmental pollution especially heavy metals (HMs), infection of harmful microorganisms, and maintenance of crop production quality during postharvesting and packaging. Nanotechnology and nanomaterials have emerged as powerful tools in agriculture applications that provide alternatives or support traditional methods. This review aims to address and highlight the current overarching issue and various implementation strategies of nanotechnology for sustainable agriculture development. In particular, the current progress of different nano-fertilizers (NFs) systems was analyzed to show their advances in enhancing the uptake and translocations in plants and improving nutrient bioavailability in soil. Also, the design strategy and application of nanotechnology for rapid detection of HMs and pathogenic diseases in plant crops were emphasized. The engineered nanomaterials have great potential for biosensors with high sensitivity and selectivity, high signal throughput, and reproducibility through various detection approaches such as Raman, colorimetric, biological, chemical, and electrical sensors. We obtain that the development of microfluidic and lab-on-a-chip (LoC) technologies offers the opportunity to create on-site portable and smart biodevices and chips for real-time monitoring of plant diseases. The last part of this work is a brief introduction to trends in nanotechnology for harvesting and packaging to provide insights into the overall applications of nanotechnology for crop production quality. This review provides the current advent of nanotechnology in agriculture, which is essential for further studies examining novel applications for sustainable agriculture.

Arsenic field test kits based on solid-phase fluorescence filter effect induced by silver nanoparticle formation

Millions of people worldwide are affected by naturally occurring arsenic in groundwater. The development of a low-cost, highly sensitive, portable assay for rapid field detection of arsenic in water is important to identify areas for safe wells and to help prioritize testing. Herein, a novel paper-based fluorescence assay was developed for the on-site analysis of arsenic, which was constructed by the solid-phase fluorescence filter effect (SPFFE) of AsH3-induced the generation of silver nanoparticles (AgNPs) toward carbon dots. The proposed SPFFE-based assay achieves a low arsenic detection limit of 0.36 μg/L due to the efficient reduction of Ag+ by AsH3 and the high molar extinction coefficient of AgNPs. In conjunction with a smartphone and an integrated sample processing and sensing platform, field-sensitive detection of arsenic could be achieved. The accuracy of the portable assay was validated by successfully analyzing surface and groundwater samples, with no significant difference from the results obtained through mass spectrometry. Compared to other methods for arsenic analysis, this developed system offers excellent sensitivity, portability, and low cost. It holds promising potential for on-site analysis of arsenic in groundwater to identify safe well locations and quickly obtain output from the global map of groundwater arsenic.

Fluorescent multichannel sensor array based on three carbon dots derived from Tibetan medicine waste for the quantification and discrimination of multiple heavy metal ions in water

A fluorescent multichannel sensor array has been established based on three carbon dots derived from Tibetan medicine waste for rapid quantification and discrimination of six heavy metal ions. Due to the chelation between metal ions and carbon dots (CDs), this fluorescence "turn off" mode sensing array can quantify six metal ions as low as "μM" level. Moreover, the six heavy metal ions display varying quenching effects on these three CDs owing to diverse chelating abilities between each other, producing differential fluorescent signals for three sensing channels, which can be plotted as specific fingerprints and converted into intuitive identification profiles via principal component analysis (PCA) and hierarchical cluster analysis (HCA) technologies to accurately distinguish Cu2+, Fe3+, Mn2+, Ag+, Ce4+, and Ni2+ with the minimum differentiated concentration of 5 μM. Valuably, this sensing array unveils good sensitivity, exceptional selectivity, ideal stability, and excellent anti-interference ability for both mixed standards and actual samples. Our contribution provides a novel approach for simultaneous determination of multiple heavy metal ions in environmental samples, and it will inspire the development of other advanced optical sensing array for simultaneous quantification and discrimination of multiple targets.

Hydrothermal synthesis of B, S, and N-doped carbon quantum dots for colorimetric sensing of heavy metal ions

In this study, glucose was used as the carbon source to synthesize carbon quantum dots (CQDs) and also aimed to synthesize CQDs doped with heteroatoms such as sulphur, nitrogen, and boron to enhance their functionality. The obtained material has been characterized by several techniques. According to FL analysis, the highest peaks for CQD, N-CQD, B-CQD, and S-CQD were determined as 432 nm (ex 350), 425 (ex 350), 430 nm (ex 340 nm), and 436 nm (ex 340 nm), respectively. FTIR spectra showed different characteristic peaks for CQD, and the FTIR results show that CQDs have a unique structure. According to TEM analysis, the morphology of all CQDs was found to be spherical and monodisperse with average sizes in the range of 5-7 nm. The characterization results of CQDs show that the addition of heteroatoms changes the properties of CQDs. The synthesized CQDs were also tested as colorimetric sensors for the detection of heavy metals. It was observed that CQDs detected Fe3+ metal ions, B-CQD and S-CQD detected Fe3+ and Ag+ metal ions, and N-CQDs detected Ca2+ metal ions. Sensor studies were performed for all CQDs and linear plots were obtained against metal concentrations in the range of 0.06-1.23 μM. LOD values for CQD, N-CQD, S-CQD, and B-CQD were calculated as 0.187 μM (Fe3+), 0.391 μM (Ca2+), 0.224 μM (Fe3+)-0.442 μM (Ag+), and 0.182 μM (Fe3+)-0.174 μM (Ag+), respectively. The results show that the addition of B, N, and S atoms to CQDs plays a role in the improvement and modification of colorimetric sensor properties and has the potential to be used in sensor applications for the detection of heavy metals in areas such as the environment and health.

Molecular engineering of a new method for effective removal of cadmium from water

Cadmium (Cd) is a widespread and highly toxic environmental pollutant, seriously threatening animal and plant growth. Therefore, monitoring and employing robust tools to enrich and remove Cd from the environment is a major challenge. In this work, by conjugating a fluorescent indicator (CCP) with a functionalized glass slide, a special composite material (CCPB) was constructed to enrich, remove, and monitor Cd2+ in water rapidly. Then Cd2+ could be effectively eluted by immersing the Cd-enriched CCPB in an ethylenediaminetetraacetic acid (EDTA) solution. With this, the CCPB was continuously reused. Its recovery of Cd2+was above and below 100 % after multiple uses by flame atomic absorption spectrometry (FAAS), which was excellent for practical use in enriching and removing Cd2+ in real aqueous samples. Therefore, CCPB is an ideal material for monitoring, enriching, and removing Cd2+ in wastewater, providing a robust tool for future practical applications of Cd enrichment and removal in the environment.