Triple-Indicator-Based Multidimensional Colorimetric Sensing Platform for Heavy Metal Ion Detections

Strategies for preparation of chitosan based water-soluble fluorescent probes to detect Cr3+ and Cu2+ ions

The low solubility of chitosan (CS) imposes adverse effects on its application. In this work, one of the aims is to improve the water solubility of CS. By introducing water-soluble side chains to CS, this aim was achieved. Besides, fluorescent moieties were incorporated into the side chains, the fluorescent copolymers were endowed with Cr3+ and Cu2+ ions recognition ability. Firstly, a reversible addition-fragmentation chain transfer polymerization (RAFT) reagent with naphthalimide units and CC groups was prepared. Water-soluble monomer methyl acrylic acid (MAA) was employed in the RAFT polymerization. Thus, water-soluble polymer with fluorescent unit and -C ≡ C on both ends of the polymer was obtained. They were introduced into CS, and the CS-based fluorescent copolymers were obtained eventually. The amount of MAA introduced could be tuned to obtain three side chains of different lengths. It was found that the more MAA was introduced, the better the solubility of CS-TP was. The detection limits (LOD) of Cr3+ and Cu2+ were 44.6 nM and 54.5 nM, respectively. The detection of Cr3+ and Cu2+ ions is further combined with a mobile APP to realize real-time, portable, and visual detection. And the application in the logic gate, a new detection platform, is prepared.

Paper-based sensors: affordable, versatile, and emerging analyte detection platforms

Paper-based sensors, often referred to as paper-based analytical devices (PADs), stand as a transformative technology in the field of analytical chemistry. They offer an affordable, versatile, and accessible solution for diverse analyte detection. These sensors harness the unique properties of paper substrates to provide a cost-effective and adaptable platform for rapid analyte detection, spanning chemical species, biomolecules, and pathogens. This review highlights the key attributes that make paper-based sensors an attractive choice for analyte detection. PADs demonstrate their versatility by accommodating a wide range of analytes, from ions and gases to proteins, nucleic acids, and more, with customizable designs for specific applications. Their user-friendly operation and minimal infrastructure requirements suit point-of-care diagnostics, environmental monitoring, food safety, and more. This review also explores various fabrication methods such as inkjet printing, wax printing, screen printing, dip coating, and photolithography. Incorporating nanomaterials and biorecognition elements promises even more sophisticated and sensitive applications.

Microfluidics in environmental analysis: advancements, challenges, and future prospects for rapid and efficient monitoring

Microfluidic devices have emerged as advantageous tools for detecting environmental contaminants due to their portability, ease of use, cost-effectiveness, and rapid response capabilities. These devices have wide-ranging applications in environmental monitoring of air, water, and soil matrices, and have also been applied to agricultural monitoring. Although several previous reviews have explored microfluidic devices' utility, this paper presents an up-to-date account of the latest advancements in this field for environmental monitoring, looking back at the past five years. In this review, we discuss devices for prominent contaminants such as heavy metals, pesticides, nutrients, microorganisms, per- and polyfluoroalkyl substances (PFAS), etc. We cover numerous detection methods (electrochemical, colorimetric, fluorescent, etc.) and critically assess the current state of microfluidic devices for environmental monitoring, highlighting both their successes and limitations. Moreover, we propose potential strategies to mitigate these limitations and offer valuable insights into future research and development directions.

Challenges and Opportunities of Using Fluorescent Metal Nanocluster-Based Colorimetric Assays in Medicine

Development of rapid colorimetric methods based on novel optical-active metal nanomaterials has provided methods for the detection of ions, biomarkers, cancers, etc. Fluorescent metal nanoclusters (FMNCs) have gained a lot of attention due to their unique physical, chemical, and optical properties providing numerous applications from rapid and sensitive detection to cellular imaging. However, because of very small color changes, their colorimetric applications for developing rapid tests based on the naked eye or simple UV-vis absorption spectrophotometry are still limited. FMNCs with peroxidase-like activity have significant potential in a wide variety of applications, especially for point-of-care diagnostics. In this review, the effect of using various capping agents and metals for the preparation of nanoclusters in their colorimetric sensing properties is explored, and the synthesis and detection mechanisms and the recent advances in their application for ultrasensitive chemical and biological analysis regarding human health are highlighted. Finally, the challenges that remain as well as the future perspectives are briefly discussed. Overcoming these limitations will allow us to expand the nanocluster's application for colorimetric diagnostic purposes in medical practice.

Tailoring Acid Free-Paper based Analytical Devices (Af-PADs) via radiation assisted modification of cellulose paper

A novel green fabrication approach is being proposed based on radiation assisted modification of Whatman filter paper 1 (WFP) for development of Acid Free-Paper based Analytical Devices (Af-PADs). Af-PADs exude immense potential as handy tools for on-site detection of toxic pollutants such as, Cr(VI), boron, etc., which have established detection protocols involving acid mediated colorimetric reactions that necessitate external acid addition. The proposed Af-PAD fabrication protocol asserts its novelty through elimination of external acid addition step, making the detection process safer and simpler. To achieve this, poly(acrylic acid) (PAA) was grafted onto WFP via a single step, room temperature process of gamma radiation induced simultaneous irradiation grafting, introducing acidic -COOH groups in the paper thereon. Grafting parameters namely, absorbed dose and concentrations of monomer, homopolymer inhibitor and acid were optimized. The -COOH groups incorporated in PAA-grafted-WFP (PAA-g-WFP) provide localized acidic conditions for colorimetric reactions between pollutants and their sensing agents, anchored on the PAA-g-WFP. Af-PADs loaded with 1,5-diphenylcarbazide (DPC) have been ably demonstrated for visual detection and quantitative estimation of Cr(VI) in water samples using RGB image analysis, with LOD value of 1.2 mg.L^-1 and a measurement range comparable to that of commercially available PADs based Cr(VI) visual detection kits.

Sensitive and simple multi-ion detection using organic nanocrystal enrichment in paper analytical devices

Microfluidic paper analytical devices (μPADs) are among the most promising platforms for heavy metal ion analysis. On the other hand, achieving simple and highly sensitive analysis of μPADs is challenging. In this study, we developed a simple enrichment method for sensitive multi-ion detection utilizing water-insoluble organic nanocrystals accumulated on μPAD. By combining the enrichment method with multivariate data analysis, three metal ion concentrations in the ion mixtures were simultaneously quantified with high sensitivity owing to the sensitive responses of the organic nanocrystals. In this work, we successfully quantified Zn²⁺, Cu²⁺, and Ni²⁺ at 20 ng L^-1 in the mixed ion solution using only two dye indicators with a larger sensitivity improvement than those reported in previous studies. Interference studies revealed possibilities for a practical application in real sample analysis. This developed approach also can be used for other analytes.

Trends in Paper-Based Sensing Devices for Clinical and Environmental Monitoring

Environmental toxic pollutants and pathogens that enter the ecosystem are major global issues. Detection of these toxic chemicals/pollutants and the diagnosis of a disease is a first step in efficiently controlling their contamination and spread, respectively. Various analytical techniques are available to detect and determine toxic chemicals/pathogens, including liquid chromatography, HPLC, mass spectroscopy, and enzyme-linked immunosorbent assays. However, these sensing strategies have some drawbacks such as tedious sample pretreatment and preparation, the requirement for skilled technicians, and dependence on large laboratory-based instruments. Alternatively, biosensors, especially paper-based sensors, could be used extensively and are a cost-effective alternative to conventional laboratory testing. They can improve accessibility to testing to identify chemicals and pollutants, especially in developing countries. Due to its low cost, abundance, easy disposal (by incineration, for example) and biocompatible nature, paper is considered a versatile material for the development of environmentally friendly electrochemical/optical (bio) sensor devices. This review presents an overview of sensing platforms constructed from paper, pointing out the main merits and demerits of paper-based sensing systems, their fabrication techniques, and the different optical/electrochemical detection techniques that they exploit.

A novel and capable supported phenylazophenylenediamine-based nano-adsorbent for removal of the Pb, Cd, and Ni ions from aqueous solutions

Herein, we report the synthesis and characterization of chrysoidine (4-phenylazo-m-phenylenediamine) grafted on magnetic nanoparticles (Fe₃O₄@SiO₂@CPTMS@PhAzPhDA = FeSiPAPDA) as a novel and versatile adsorbent used for the satisfactory removal of Pb, Ni, and Cd ions from contaminated water via the formation of their complexes. The Freundlich, Langmuir, Temkin, and Redlich-Patterson isotherm models were studied to reveal the adsorption capability of the adsorbent and were found out that the Langmuir model is more compatible with the nano-adsorbent behavior. Moreover, according to the ICP tests as well as based on the Langmuir isotherm, the maximum adsorption capacity of the FeSiPAPDA-based adsorbent for the Pb ions (97.58) is more than that of Cd (78.59) and Ni ions (64.03).

Insertable, Scabbarded, and Nanoetched Silver Needle Sensor for Hazardous Element Depth Profiling by Laser-Induced Breakdown Spectroscopy

Sensing of hazardous metals is urgent in many areas (e.g., water pollution and meat products) as heavy metals threaten people's health. Laser-induced breakdown spectroscopy (LIBS), as a rapid, in situ, and multielemental analytical technique, has been widely utilized in rapid hazardous heavy metal sensing. However, loose and water-containing samples (e.g., meat, plant, and soil) are hard to analyze by LIBS directly, and heavy metal depth profiling for bulk samples remains suspenseful. Here, inspired by the Needle, the sword of Arya Stark in Game of Thrones, we propose an insertable, scabbarded, and nanoetched silver (NE-Ag) needle sensor for rapid hazardous element sensing and depth profiling. The NE-Ag needle sensor features a micro-nanostructure surface for inserting into the bulk sample and absorbing hazardous analytes. For accurate elemental depth profiling, we design a stainless-steel scabbard to wrap and protect the NE-Ag needle from pollution (unexpected contaminant absorption) during the needle insertion and extraction process. The results for cadmium (Cd) show that the relative standard deviation equals to 6.7% and the limit of detection reaches 0.8 mg/L (ppm). Furthermore, the correlations (Pearson correlation coefficient) for Cd and chromium (Cr) depth profiling results are no less than 0.96. Furthermore, the total testing time could be less than 1 h. All in all, the insertable and scabbarded NE-Ag needle senor has high potential in rapid hazardous heavy metal depth profiling in different industries.

A Comprehensive Review on Water Quality Monitoring Devices: Materials Advances, Current Status, and Future Perspective

Water quality monitoring has become more critical in recent years to ensure the availability of clean and safe water from natural aquifers and to understand the evolution of water contaminants across time and space. The conventional water monitoring techniques comprise of sample collection, preservation, preparation, tailed by laboratory testing and analysis with cumbersome wet chemical routes and expensive instrumentation. Despite the high accuracy of these methods, the high testing costs, laborious procedures, and maintenance associated with them don't make them lucrative for end end-users and field testing. As the participation of ultimate stakeholders, that is, common man for water quality and quantity can play a pivotal role in ensuring the sustainability of our aquifers, thus it is essential to develop and deploy portable and user-friendly technical systems for monitoring water sources in real-time or on-site. The present review emphasizes here on possible approaches including optical (absorbance, fluorescence, colorimetric, X-ray fluorescence, chemiluminescence), electrochemical (ASV, CSV, CV, EIS, and chronoamperometry), electrical, biological, and surface-sensing (SPR and SERS), as candidates for developing such platforms. The existing developments, their success, and bottlenecks are discussed in terms of various attributes of water to escalate the essentiality of water quality devices development meeting ASSURED criterion for societal usage. These platforms are also analyzed in terms of their market potential, advancements required from material science aspects, and possible integration with IoT solutions in alignment with Industry 4.0 for environmental application.

Adsorptive colorimetric determination of chromium(VI) ions at ultratrace levels using amine functionalized mesoporous silica

There is an urgent need for a rapid, affordable and sensitive analytical method for periodic monitoring of heavy metals in water bodies. Herein, we report for the first time a versatile method for ultratrace level metal detection based on colorimetric sensing. The method integrates preconcentration using a nanomaterial with a colorimetric assay performed directly on the metal-enriched nanomaterial surface. This method circumvents the need for tedious sample pre-processing steps and the complex development of colorimetric probes, thereby reducing the complexity of the analytical procedure. The efficacy of the proposed method was demonstrated for chromium(VI) ions detection in water samples. Amine functionalized mesoporous silica (AMS) obtained from a one-pot synthesis was utilized as a pre-concentration material. The structural and chemical analysis of AMS was conducted to confirm its physico-chemical properties. The pre-concentration conditions were optimized to maximise the colorimetric signal. AMS exhibited a discernible colour change from white to purple (visible to the naked eye) for trace Cr(VI) ions concentration as low as 0.5 μg L^-1. This method shows high selectivity for Cr(VI) ions with no colorimetric signal from other metal ions. We believe our method of analysis has a high scope for de-centralized monitoring of organic/inorganic pollutants in resource-constrained settings.

Nanostructured Shell-Layer Artificial Antibody with Fluorescence-Tagged Recognition Sites for the Trace Detection of Heavy Metal Ions by Self-Reporting Microsensor Arrays

Herein, a strategy for a metal ion-imprinted artificial antibody with recognition sites tagged by fluorescein was carried out to construct the selective sites with a sensitive optical response signal to the specific metal ion. The synthesized silica nanoparticles were modified by the derivative residue group of 3-aminopropyltriethoxysilane conjugated with a 4-chloro-7-nitro-1,2,3-benzoxadiazole (NBD-Cl) molecule through the hydrolysis and condensation reactions. The as-prepared silica nanoparticles were encapsulated by metal ion (Cu²⁺, Cd²⁺, Hg²⁺, and Pb²⁺)-imprinted polymers with nanostructured layers through the copolymerization of ethyl glycol dimethyl methacrylate (EGDMA) as a cross-linker, AIBN as an initiator, metal ions as template molecules, AA as a functional monomer, and acetonitrile as a solvent. The layers of molecular imprinted polymers (MIPs) with a core-shell structure removed template molecules by EDTA-2Na to retain the cavities and spatial sizes to match the imprinted metal ions. The microsensor arrays were achieved by the self-assembly technique of SiO₂@MIP nanoparticles on the etched silicon wafer with regular dot arrays. The nanostructured-shell layers with fluorescence-tagged recognition sites rebound metal ions by the driving force of concentration difference demonstrates the high selective recognition and sensitive detection to heavy metal ions through the decline of fluorescence intensity. The LOD concentration for four metal ions is down to 10^-9 mol·L^-1. The method will provide biomimetic synthesis, analyte screen, and detection of highly dangerous materials in the environment for theoretical foundation and technological support.

Paper-Based Analytical Devices for Colorimetric and Luminescent Detection of Mercury in Waters: An Overview

Lab-on-paper technologies, also known as paper-based analytical devices (PADs), have received increasing attention in the last years, and nowadays, their use has spread to virtually every application area, i.e., medical diagnostic, food safety, environmental monitoring, etc. Advantages inherent to on-field detection, which include avoiding sampling, sample preparation and conventional instrumentation in central labs, are undoubtedly driving many developments in this area. Heavy metals represent an important group of environmental pollutants that require strict controls due to the threat they pose to ecosystems and human health. In this overview, the development of PADs for Hg monitoring, which is considered the most toxic metal in the environment, is addressed. The main emphasis is placed on recognition elements (i.e., organic chromophores/fluorophores, plasmonic nanoparticles, inorganic quantum dots, carbon quantum dots, metal nanoclusters, etc.) employed to provide suitable selectivity and sensitivity. The performance of both microfluidic paper-based analytical devices and paper-based sensors using signal readout by colorimetry and luminescence will be discussed.

Enzyme Method-Based Microfluidic Chip for the Rapid Detection of Copper Ions

Metal ions in high concentrations can pollute the marine environment. Human activities and industrial pollution are the causes of Cu²⁺ contamination. Here, we report our discovery of an enzyme method-based microfluidic that can be used to rapidly detect Cu²⁺ in seawater. In this method, Cu²⁺ is reduced to Cu⁺ to inhibit horseradish peroxidase (HRP) activity, which then results in the color distortion of the reaction solution. The chip provides both naked eye and spectrophotometer modalities. Cu²⁺ concentrations have an ideal linear relationship, with absorbance values ranging from 3.91 nM to 256 μM. The proposed enzyme method-based microfluidic chip detects Cu²⁺ with a limit of detection (LOD) of 0.87 nM. Other common metal ions do not affect the operation of the chip. The successful detection of Cu²⁺ was achieved using three real seawater samples, verifying the ability of the chip in practical applications. Furthermore, the chip realizes the functions of two AND gates in series and has potential practical implementations in biochemical detection and biological computing.

Rapid heavy metal sensing platform: A case of triple signal amplification strategy for the sensitive detection of serum copper

Heavy metals are considered as hazardous substances to human because of their toxicity, persistence and bioaccumulation, and the level in serum is an important factor to evaluate the caused health risk, which depends on efficient and sensitive analytical methods. Here, a triple signal-amplified electrochemical sensing platform based on metal-dependent DNAzymes was fabricated for sensitive determination of heavy metals in serum (copper as a model target). Under the optimized conditions, the proposed method showed good sensitivity (limit of detection, 0.33 fM for Cu²⁺) with excellent selectivity and stability, which is ascribed to: (i) tetrahedral DNA nanostructures (TDNs) that was used as a promising scaffold to adjust the selective transformation between heterogeneous and homogeneous reactions, preventing the nonspecific binding of electrodes surface and DNA probes; (ii) the magnetic beads (MBs) used which led to signal amplification and decreased background owing to its excellent properties of extracting equivalent targets from the complex samples; (iii) two signal amplification strategy of catalytic hairpin assembly (CHA) and hybridization chain reaction (HCR). In addition, the proposed sensing platform displayed satisfactory accuracy through the validation with inductively coupled plasma-mass spectrometry (ICP-MS) and a spike-recovery analysis (recoveries, 87.92-111.61%; RSD, 4.89-8.85%), indicating the great potential for rapid and sensitive detection of Cu²⁺ or other metal ions.

Recent progress in smartphone-based techniques for food safety and the detection of heavy metal ions in environmental water

Emerging smartphone-based point-of-care tests (POCTs) are cost-effective, precise, and easy to implement in resource-limited areas. Thus, they are considered a potential alternative to conventional diagnostic testing. This review explores food safety and the detection of metal ions in environmental water based on unprecedented smartphone technology. Specifically, we provide an overview of various methods used for target analyte detection (antibiotics, enzymes, mycotoxins, pathogens, pesticides, small molecules, and metal ions), such as colorimetric, fluorescence, microscopic imaging, and electrochemical methods. This paper performs a comprehensive review of smartphone-based POCTs developed in the last three years (2018-2020) and evaluates their relative advantages and limitations. Moreover, we discuss the imperative role of new technology in the progress of POCTs. Sensor materials (metal nanoparticles, carbon dots, quantum dots, organic substrates, etc.) and detection techniques (paper-based, later flow assay, microfluidic platform, etc.) involved in POCTs based on smartphones, and the challenges faced by these techniques, are addressed.

Trends in sensor development toward next-generation point-of-care testing for mercury

Mercury is one of the most common heavy metals and a major environmental pollutant that affects ecosystems. Since mercury and its compounds are toxic to humans, even at low concentrations, it is very important to monitor mercury contamination in water and foods. Although conventional mercury detection methods, including inductively coupled plasma mass spectrometry, atomic absorption spectroscopy, and gas chromatography-mass spectrometry, exhibit excellent sensitivity and accuracy, they require operation by an expert in a sophisticated and fully controlled laboratory environment. To overcome these limitations and realize point-of-care testing, many novel methods for direct sample analysis in the field have recently been developed by improving the speed and simplicity of detection. Commonly, these unconventional sensors rely on colorimetric, fluorescence, or electrochemical mechanisms to transduce signals from mercury. In the case of colorimetric and fluorescent sensors, benchtop methods have gradually evolved through technology convergence to give standalone platforms, such as paper-based assays and lab-on-a-chip systems, and portable measurement devices, such as smartphones. Electrochemical sensors that use screen-printed electrodes with carbon or metal nanomaterials or hybrid materials to improve sensitivity and stability also provide promising detection platforms. This review summarizes the current state of sensor platforms for the on-field detection of mercury with a focus on key features and recent developments. Furthermore, trends for next-generation mercury sensors are suggested based on a paradigm shift to the active integration of cutting-edge technologies, such as drones, systems based on artificial intelligence, machine learning, and three-dimensional printing, and high-quality smartphones.

Coordination array for accurate colorimetric sensing of multiple heavy metal ions

Heavy metals detection is of great significance to the environment and human health, and most of the detection methods require expensive instruments and laborious operation. Herein, we present a coordination array for accurate and rapid colorimetric heavy metal ions sensing. The array was composed of six chelates and has cross response to Hg²⁺, Cd²⁺, Pb²⁺ and Cu²⁺. The results could be observed by naked eyes or detected by plate reader combined with pattern analysis. Linear discrimination analysis was applied for the pattern analysis and the four heavy metal ions (Pb²⁺, Cd²⁺, Hg²⁺ and Cu²⁺) generated a clustering map at 1 μM. The coordination array demonstrates a great potential for sensing heavy metal ions simultaneously.

Quantum dots encoded white-emitting polymeric superparticles for simultaneous detection of multiple heavy metal ions

Simultaneous detection of multiple heavy metal ions (HMI) is of great importance for the environmental monitoring, and the analytical tools based on multiband emissive fluorescent probes have been regarded as one of the most promising candidate for multiple HMI detection. Herein, the rod-coil amphiphilic block copolymer (BCP) with intrinsic blue fluorescence emission has been synthesized and subsequently employed to encapsulate two types of hydrophobic quantum dots (QD) with green and red fluorescence emission via the three dimensionally confined emulsion self-assembly, leading to the generation of white-emitting superparticles showing good colloidal stability and stable aqueous phase fluorescence. Furthermore, it was found that the fluorescence emission intensity of obtained superparticles can be selectively quenched by Ag⁺, Hg²⁺, Cu²⁺ and Fe³⁺ ions via different mechanisms, and the four metal ions can be further discriminated according to their distinct combinational quenching effects onto three fluorescent bands of white-emitting superparticles. In addition, an analytical model was built to enable the simultaneous detection of Cu²⁺, Hg²⁺ and Fe³⁺ in the real sample. Basically, the current work opens the new way to fabricate fluorescent probes with multiple emission bands, which can be easily adapted to prepare more complicated QD encoded fluorescent probes for high throughput detection.

Highly sensitive determination of heavy metals in water prior to and after remediation using Citrofortunella Microcarpa

A highly sensitive bismuth/silver nanoparticles/Nafion-modified screen-printed graphene electrode was fabricated and was utilized for the detection of trace lead (Pb) concentrations in river water samples prior to and after remediation using calamansi (Citrofortunella Microcarpa) rinds in different forms viz., ground sun-dried, dry-ashed, food-grade pectin, fractionated pectin, and alcohol insoluble solids-extracted pectin. All these forms of pectin remediated Pb in the water samples. Hence, this novel method of using calamansi rinds in different forms is an effective method for the removal of lead in water. The electrode was characterized using scanning electron microscopy and energy dispersive x-ray spectrometry which confirmed the presence of the modifiers on the electrode surface. The limit of detection of 267.6 ppt and the strong linear relationship between the Pb concentration and the anodic current response (R2 = 0.999) were obtained under optimized experimental conditions and parameters.

Naphthalimide-decorated imino-phenol: supramolecular gelation and selective sensing of Fe3+ and Cu2+ ions under different experimental conditions

Compound 1 forms gels in DMF–H₂O (1 : 1, v/v) and DMSO–H₂O (1 : 1, v/v). While it was insensitive to any metal ion in DMF–H₂O, the gel state was responsive to Fe³⁺ over the other metal ions studied. In CH₃CN or aqueous CH₃CN compound 1 senses Cu²⁺ ion.

Use of an ether-derived 3-hydroxy-4-pyridinone chelator as a new chromogenic reagent in the development of a microfluidic paper-based analytical device for Fe(III) determination in natural waters

This article reports on the development and validation of a disposable microfluidic paper-based analytical device (μPAD) for on-hand, in-situ, and cheap Fe(III) determination in natural waters complying with World Health Organization guidelines. The developed μPAD used 3-hydroxy-4-pyridinone (3,4-HPO) as a colour reagent due to its considerably lower toxicity than traditionally used iron analytical reagents. It was selected among a group of hydrophilic 3,4-HPO chelators containing ether-derived chains in their structure which were prepared using green methods. The relatively high water solubility of these chelators improved the detection limit and applicability as μPAD reagents. Under optimal conditions, the μPAD is characterised by a quantification range between 0.25 and 2.0 mg/L, a detection limit of 55 μg/L and 15 min of analysis time. The signal stability extends up to 4 h and the device is stable for at least one month. The reagent consumption is below 0.2 mg per analysis and the μPAD method was validated by analysis certified reference materials and by comparison with atomic absorption results (RD < 10%). The newly developed μPAD was successfully applied to the determination of iron in river, well and tap waters with no need of any prior sample pre-treatment.

Tripeptide Derivative-Modified Glassy Carbon Electrode: A Novel Electrochemical Sensor for Sensitive and Selective Detection of Cd2+ Ions

A N-[(Benzyloxy)carbonyl]-l-alanyl-l-prolyl-l-leucine-N-cyclohexylcyclohexanamine (Cbz-APL) tripeptide-coated glassy carbon electrode (GCE)-based sensor was used for sensitive and selective recognition of cadmium ions in environmental water. Detailed cyclic voltammetric and electrochemical impedance spectroscopic studies were performed to investigate the charge transfer and sensing activity of the developed electrochemical sensor. Square wave anodic stripping voltammetry (SWASV) was employed to further investigate the sensitivity, selectivity, validity, and applicability of the developed sensor. A sharp electrochemical signal of oxidized Cd at -0.84 V versus Ag/AgCl provides evidence for the higher sensing ability of Cbz-APL/GCE than bare GCE at -0.79 V. Moreover, on Cbz-APL/GCE, extraordinary low detection limits of 4.34 fM and linearity range of 15 nM to 0.1 pM with coefficients of correlation higher than 0.99 for Cd2+ were achieved. Besides, the influence of inorganic and organic interferents on the targeted analyte signals was examined, and high selectivity of Cbz-APL/GCE for Cd2+ ions was observed. Lastly, the validity and applicability of the developed electrochemical sensor for the detection of Cd2+ ions were checked in real water samples, and 100% recovery was obtained.

Polyoxometalate-based crystalline materials as a highly sensitive electrochemical sensor for detecting trace Cr(vi)

It is crucial to find a convenient and sensitive method for quantitative determination of heavy metal chromium(vi) ions. Developing crystalline materials coupled with polyoxometalates as an electrochemical sensor is a promising approach to address the above issues. Here we reported two reductive polyoxometalate-based crystalline compounds with the formula of (H2bpp)2[Na4Fe(H2O)7][Fe(P4Mo6O31H6)2]·2H2O (1) and (H2bpp)6(bpp)2[Fe(P4Mo6O31H8)2]2·13H2O (2) (bpp = 1,3-bi(4-pyridyl)propane). Structural analysis indicated that both two compounds were composed of inorganic polyanionic clusters and organic protonated bpp cations. The difference lies in the arrangement mode of the inorganic moiety: crystal 1 shows a unique three-dimensional (3-D) inorganic porous skeleton, while crystal 2 consists of isolated 0-D polyanionic clusters. When used as electrochemical sensors in the determination of trace Cr(vi), crystal 1 shows a broad linearity range (2-2610 μM) with a low limit of detection (LOD) of 0.174 μM (9 ppb), which is superior to that of compound 2 (a LOD of 0.33 μM) and meets the standard of Cr(vi) in drinking water set by the WHO (less than 0.962 μM or 50 ppb). Importantly, crystal 1 showed benign selectivity to Cr(vi) in the presence of various heavy metal ions and good reproducibility in a real water sample, which prove its strong anti-interference ability. In addition, experimental results showed that the spatial arrangement of polyanionic clusters could affect the final electrochemical behavior of crystalline materials. This work provides some insights into the design of cost-effective POM-based electrochemical sensors at the molecular level.

Molecular spectra of a D-π-A typed polydentate ligand chromophore and its simultaneous response to trace Cu2+ and Co2

A D-π-A conjugated polydentate ligand chromophore, N-8'-quinolyl-2,4,6- trihydroxyl benzamide (NQTB), was identified and synthesized using tri-hydroxyl phenol as donated-electron group, N-heterocycle quinoline as accepted-electron one and CN bond as bridged one. It was expected to chelate some heavy metal ions with prominent colorimetric or spectral changes. After its UV-vis absorption spectrum was investigated in detail, it was noted that NQTB possessed excellent spectral recognition ability to Cu²⁺ and Co²⁺ from other coexisting ions in aqueous. Under the optimized conditions, NQTB could simultaneously discriminate trace Cu²⁺ and Co²⁺ in environmental aqueous samples with low detection limits (1.9 × 10^-8 mol/L and 5.7 × 10^-8 mol/L) and satisfying analytical precisions (R.S.D. ≤3.3% and ≤2.6%) respectively. The sensing mechanism was confirmed to form some stable 5-membered-co-6-membered condensed rings between Cu²⁺/Co²⁺ and O/N atoms in NQTB.

Spatial varying profiling of air PM constituents using paper-based microfluidics

Accurate and quantitative profiling of air particulate matter (PM) compositions is essential for assessing local pollution information. The method combining mobile aerial sampling using unmanned aerial vehicles (UAVs) and prompt analysis excels in this regard as it allows spatiotemporal mapping of air pollution, especially in the vertical direction. However, applications of the method are still scarce as it is limited by a lack of sampling reliability due to insufficient aerial sampling time and a lack of accurate, portable quantification techniques. In this work, by integrating mobile aerial sampling with in-flight tethered charging and smartphone-based colorimetric analysis in a cost-effective paper microfluidic device, we present a method for quantitative, reliable profiling of spatiotemporal variation in air PM compositions. The method extends aerial sampling time to 12-15 flight hours per deployment, thereby significantly improving sampling reliability while maintaining the maneuverability of the UAVs. Also, smartphone-based colorimetric analysis combined with paper-based microfluidics enables portable, economically efficient analysis and is well-suited for using in low-resource settings. We demonstrated the utility of the method by carrying out a spatiotemporal variation study of air PM trace metal components (Fe, Ni, and Mn) at 4 geographical locations in Fuzhou, China, for a period of 21 days, and the results were in good agreement with results obtained from using a commercial instrument. Beside air PM composition study, this method is universally applicable and holds great potential to be extended to multipollutant analysis, such as prompt detection of airborne viruses, bacteria, and others.

Visual artificial tongue for identification of various metal ions in mixtures and real water samples: a colorimetric sensor array using off-the-shelf dyes

A three-unit colorimetric sensor array in aim of detecting heavy metal ions has been constructed with two off-the-shelf dyes. Multivariate data analysis is performed using LDA and HCA to recognize colour change patterns based on both absorption spectra and RGB values from image scans. The sensor array is able to differentiate 15 metal ions not only in separate solutions, but also mixtures of 3, 5, and 7 different metal ions and real water samples.

Heavy Metal/Toxins Detection Using Electronic Tongues

The growing concern for sustainability and environmental preservation has increased the demand for reliable, fast response, and low-cost devices to monitor the existence of heavy metals and toxins in water resources. An electronic tongue (e-tongue) is a multisensory array mostly based on electroanalytical methods and multivariate statistical techniques to facilitate information visualization in a qualitative and/or quantitative way. E-tongues are promising analytical devices having simple operation, fast response, low cost, easy integration with other systems (microfluidic, optical, etc) to enable miniaturization and provide a high sensitivity for measurements in complex liquid media, providing an interesting alternative to address many of the existing environmental monitoring challenges, specifically relevant emerging pollutants such as heavy metals and toxins.

Multifunctional supramolecular self-assembly system for colorimetric detection of Hg2+, Fe3+, Cu2+ and continuous sensing of volatile acids and organic amine gases

A novel multifunctional gelator (1) based on an azobenzene derivative was designed and characterized. This compound could gelate some solvents including hexane, petroleum ether, DMSO, acetonitrile and ethanol through a heating-cooling procedure. The self-assembly process in different solvents was studied by means of UV-vis absorption and Fourier transform infrared (FTIR) spectra, field emission scanning electron microscopy (FESEM), rheological measurements, X-ray powder diffraction and water contact angle experiments. Interestingly, compound 1 had a high-contrast colorimetric detection ability towards Hg2+, Cu2+, Fe3+ and volatile acids and further organic amine gases in solution through its color change. At the same time, organogel 1 in acetonitrile also exhibited detection performance through a color or gel state change. In the response process, the self-assembly structures were changed from a nanofiber into a microsphere under induction by analytes. More significantly, film 1 could continuously detect volatile acids and organic amine gases. The number of cycles of film 1 for the detection of volatile acids and organic amine gases was at least seven times. The limit of detection (LOD) of film 1 towards TFA was calculated to be 0.0848 ppb. The sensing mechanisms were studied using 1HNMR, FESEM, UV-vis absorption spectra and HRMS. The intramolecular cyclization occurred on molecule 1 and a H2S molecule was lost during the detection process of Hg2+. It was proposed that the -N[double bond, length as m-dash]N- bonding could be coordinated by Fe3+ and Cu2+ and this further induced the absorption spectra and color change. For a volatile acid, it was possible that the volatile acid was combined with the N,N-dimethyl amine group of molecule 1. This research opens up a novel pathway to the fabrication of supramolecular self-assembly gels to detect polymetallic ions and trace volatile acids in the environment.

Chromium Monitoring in Water by Colorimetry Using Optimised 1,5-Diphenylcarbazide Method

Chromium contamination of drinking water has become a global problem due to its extensive use in industry. The most commonly used methods for chromium detection in water are laboratory-based methods, such as atomic absorption spectroscopy and mass spectroscopy. Although these methods are highly selective and sensitive, they require expensive maintenance and highly trained staff. Therefore, there is a growing demand for cost effective and portable detection methods that would meet the demand for mass monitoring. Microfluidic detection systems based on optical detection have great potential for onsite monitoring applications. Furthermore, their small size enables rapid sample throughput and minimises both reagent consumption and waste generation. In contrast to standard laboratory methods, there is also no requirement for sample transport and storage. The aim of this study is to optimise a colorimetric method based on 1,5-diphenylcarbazide dye for incorporation into a microfluidic detection system. Rapid colour development was observed after the addition of the dye and samples were measured at 543 nm. Beer's law was obeyed in the range between 0.03-3 mg·L^-1. The detection limit and quantitation limit were found to be 0.023 and 0.076 mg·L^-1, respectively.