Rationally engineered nanosensors: A novel strategy for the detection of heavy metal ions in the environment

Causonis trifolia-based green synthesis of multifunctional silver nanoparticles for dual sensing of mercury and ferric ions, photocatalysis, and biomedical applications

Health and environmental concerns are often raised by the development of antibiotic resistance and water contamination from various aquatic contaminants, including antibiotic residues, dyes, and heavy metal ions. This paper outlines a facile, affordable, and eco-friendly way to address these issues by green synthesis of silver nanoparticles (CT@AgNPs) under sunlight irradiation using Causonis trifolia leaf extract (CTLE), known for its medicinal properties. The greenly synthesized CT@AgNPs exhibited antioxidant, antibacterial, and photocatalytic properties and were an effective nanoprobe for the selective detection of Fe3+ and Hg2+ in water. CT@AgNPs were thoroughly examined using several sophisticated analytical methods, including FTIR, UV-vis spectroscopy, Scanning electron microscopy (SEM), Powder X-ray diffraction (PXRD), Energy dispersive X-ray (EDX), and Zeta potential (ZP). FTIR demonstrated the effective functionalization of CT@AgNPs with the polar leaf extract of Causonis trifolia. The optical properties of CT@AgNPs in solution were monitored using UV-vis spectrophotometric analysis. The synthesis of spherical shaped CT@AgNPs with a face-centered cubic geometry and a 12.7 nm average crystallite size was assessed by SEM and XRD, respectively. CT@AgNPs showed a potent antibacterial activity against Gram-positive bacteria (L. monocytogenes and S. epidermidis) and Gram-negative bacterial strains (P. aeruginosa and B. bronchiseptica). CT@AgNPs showed high sensitivity for colorimetric detection of Hg2+ and Fe3+ with a limit of detection of 1.04 μM and 47.57 μM, respectively in spiked water samples, highlighting their potential for use in environmental monitoring applications. CT@AgNPs showed remarkable antioxidant ability, assessed by DPPH, TFC, and TPC assays. On exposure to sunlight, CT@AgNPs also showed good photocatalytic capability by degradation of methyl orange (79%) and crystal violet (77%) with rate constant values of 0.0157 min-1, and 0.0150 min-1, respectively. This work demonstrates the potential of green route-synthesized AgNPs as efficient and sustainable materials for biomedical and environmental applications.

Electrochemical sensing of Hg(II) ions based on ultramicrotome-crafted strip ultramicroelectrode

The presence of heavy metal ions in the biosphere constitutes a significant source of pollution, posing severe threats to both human health and the environment. Thus, the accurate detection of heavy metal ions assumes paramount importance. The present study involves the preparation of electrochemical sensors based on strip ultramicroelectrode, enabling the detection of Hg2+. The electrochemical performance of strip ultramicroelectrode is characterized firstly. The Au strip ultramicroelectrode demonstrates exceptional suitability for the detection of Hg2+. Moreover, the acetate buffer is confirmed as an advantageous detection medium for Hg2+ owing to its slight influence on the result. The electrochemical detection parameters of the strip ultramicroelectrode, such as the pH value of the acetate buffer, enrichment potential, and enrichment time, have been optimized. The prepared electrochemical sensors based on strip ultramicroelectrode are utilized to detect the Hg2+ in tap water, snow melt water and bottled water. The detection limitation exceeds the thresholds established by the drinking water quality standards of China, the World Health Organization, and the European Union. The sensor prepared based on the strip ultramicroelectrode has exceptional accuracy, reliability, and practical applicability.

Recent Developments in Enzyme-Free PANI-Based Electrochemical Nanosensors for Pollutant Detection in Aqueous Environments

Wastewater management has a direct impact on the supply of drinking water. New cutting-edge technologies are crucial to the ever-growing demand for tailored solutions for pollutant removal, but these pollutants first need to be detected. Traditional techniques are costly and are no longer competitive in the wastewater cleaning market. One sustainable and economically viable alternative is the fabrication of integrated nanosensors composed of conducting polymers. These include polyaniline doped with various types of nanomaterials such as nanocarbons (carbon nanotubes and graphene), metal oxide nanoparticles/nanostructures, and quantum dots. The synergistic properties of these components can endow sensing materials with enhanced surface reactivity, greater electrocatalytic activity, as well as tunable redox activity and electrical conductivity. This review covers key recent advances in the field of non-enzyme electrochemical conductive polymer nanosensors for pollutant detection in aqueous environments or simulated polluted samples. It provides an introduction to these sensors, their preparation, applications, the environmental and economic hurdles impeding the large-scale development of PANI-based nanomaterials in sensing applications, and future directions for research and real-world applications.

Detection of heavy metals in soil using Au@SiO2 nanoparticles and surface microstructure combined with laser-induced breakdown spectroscopy

The detection of heavy metals in soil is of great scientific significance for food security and human health. However, traditional detection methods are complicated, time-consuming, and labor-intensive. Herein, we developed a novel method using Au@SiO2 nanoparticles (NPs) and surface microstructure combined with laser-induced breakdown spectroscopy (Au@SiO2 NPs-SMS-LIBS) for the rapid detection of lead (Pb), chromium (Cr), and copper (Cu) in soil samples. The surface microstructures and Au@SiO2 NPs were prepared to improve detection sensitivity and stability. The limits of detection (LODs) for Pb, Cr, and Cu were 0.36 mg/kg, 0.32 mg/kg, and 0.28 mg/kg, respectively, with relative standard deviations (RSDs) of 5.47-6.72 %. The mechanisms of spectral performance enhancement of LIBS detection were thoroughly investigated. Furthermore, the stacking combination model was developed to improve quantitative accuracy, with the correlation of the prediction set (Rp2) for Pb, Cr, and Cu being 0.9285, 0.8625, and 0.9160, respectively. This work offers a very promising solution to improve the sensitivity, stability, and accuracy of heavy metal detection. The developed method holds great application potential for large-scale soil assessments and real-time heavy metal pollution monitoring.

Dual-emission Si dots-based sensing array for identification of metal ions

Achieving rapid recognition and sensitive detection of multiple heavy metal ions simultaneously is of great significance for the monitoring of public health and environment. Herein, a fluorescence sensing array was constructed using Si dots with two emission centers for efficient discrimination of seven metal ions. Each metal ion had different binding capacities with -OH/-NH2 groups present on the surface of Si dots and thereby resulting in different changes in fluorescence intensity of the two emission peaks. The changes in fluorescence signals were transformed into unique "fingerprints" and "Euclidean distances" through linear discriminant analysis and hierarchical cluster analysis. The constructed sensing array provided multi-dimensional information to distinguish seven metal ions through a single material and a signal mode and show good linearity for individual metal ion and good performance in the discrimination of metal mixtures, effectively reducing the cost and response time and simplifying the experimental process. This system achieved accurate discrimination of metal ions in actual water samples and has broad application prospects in environmental monitoring.

Ultra-sensitive immunosensing of snake venom by functionalized Sm-Co doped antimony-tungstate

Snake venom has long-term physiological effects on survivor's life. An electrochemical immunosensor based on samarium-cobalt-doped antimony tungstate (Sb2WO4@Sm-Co) is developed via a solvothermal method to detect snake venom antigens (SVA). The fabricated nanospheres are functionalized with carboxyl groups to enhance the linkage of the 3-mercaptopropionic acid linker (3-MPA). This modification increases the conjugation of antivenom polyvalent antibody with the nanomaterial on a glassy carbon electrode (Sb2WO4@Sm-Co-COOH-MPA-Ab/GCE). The modified nanospheres are characterized by UV-VIS spectroscopy, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). The electrochemical performance of formulated immunosensor for antigen sensing is tested by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), differential pulse voltammetry (DPV), linear sweep voltammetry (LSV), and chronoamperometry. This developed immunosensor has a wide linear range of 5-30 ng/mL with LODs of 0.08 ng/mL and 0.1 ng/mL from DPV and LSV, respectively. The amperometric immunosensor increases the tested antibody's loading capacity and accelerates the electron transfer rate. The analytical parameters reveal that this immunosensor is ultrasensitive, stable, reproducible, and selective for measuring SVA and can have potential applications in diagnostic clinics. KEY POINTS: • The hierarchical Sb2WO4@Sm-Co-COOH NPs were synthesized through a one-step solvothermal method • Monitoring the effect of doping Sm and Co on the characteristics of Sb2WO4 • MPA-linked IgG antibodys-based immunosensor was synthesized with good dispersity and high surface functional groups for capturing SVAs.

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.

Xanthan gum-capped Chromia Nanoparticles (XG-CrNPs): A promising nanoprobe for the detection of heavy metal ions

The present study reports on the selective and sensitive detection of metals using xanthan gum-capped chromia nanoparticles (XG-CrNPs). The nanoparticles were synthesized by the chemical reduction method using sodium borohydride and xanthan gum as the reducing and capping agents, respectively. The synthesis of XG-CrNPs was confirmed by the appearance of the two absorption peaks at 272 nm and 371 nm in the UV-visible region. The nanoparticles have been extensively characterized by FTIR, TEM-EDX, XRD, and TGA analyses. The well-dispersed XG-CrNPs exhibited a quasi-spherical structure with an average particle size of 3 nm. A significantly low amount (2 μg/L) of XG-CrNPs was used for selective and sensitive detection of heavy metal ions. It showed excellent metal detecting properties by quenching its band gap signal which was extraordinarily conspicuous for Co(II), Hg(II), and Cd(II) in comparison to other metal ions like Ag(I), Ba(II), Mg(II), Mn(II), Ni(II), and Zn(II). The limit of detection of Co(II), Cd(II), and Hg(II) with this nanoprobe was found to be 2.167 μM, 1.065 μM, and 0.601 μM respectively. The nanoparticles manifested higher shelf-life and can be reused up to three consecutive cycles where most of its activity was conserved even after being used. Thus, it may find use in metal sensor devices for the detection of hazardous metals.

Alginic acid-functionalized silver nanoparticles: A rapid monitoring tool for detecting the technology-critical element tellurium

The increasing global demand for tellurium, driven by its critical role in alloys, photovoltaic devices, and electronics, has raised concerns about its environmental pollution and neurotoxicity. In response, the potential of alginic acid (AA), a renewable, low-cost, and sustainable biopolymer, was explored for the biosynthesis of ultra-small silver nanoparticles (AgNPs) and their application in the detection of tellurium (Te(IV)). The effect of key synthesis parameters on desired physicochemical properties and yield of AgNPs was established to ensure high specificity and sensitivity towards Te(IV). The purified AgNPs with AA surface ligands were utilized to demonstrate a ratiometric absorbance sensor that exhibits excellent linearity and nanomolar-level affinity. This approach achieved a high correlation coefficient of ∼ 0.982, with a low detection limit of about 22 nM. Further investigations into the effect of pH, ionic strength, and organic molecules were conducted to elucidate detection performance and molecular understanding. The detection mechanism relies on the coordination between Te(IV) ions and the carboxylate groups of AA, which initiates aggregation-induced plasmon coupling in adjacent AgNPs. The capability of this analytical method to monitor Te(IV) in real-world water samples features its rapidity, user-friendliness, and suitability for point-of-care monitoring, making it a promising alternative to more complex techniques.

Synergistic activation of P and orbital coupling effect for ultra-sensitive and selective electrochemical detection of Cd(II) over Fe-doped CoP

Despite significant advancements in the detection of cadmium (Cd(II)) based on nanomaterial adsorbability, limited research has been conducted on ultra-sensitive and selective detection mechanisms, resulting in a lack of guidance for designing efficient interface materials to detect Cd(II). Herein, reductive Fe doping on CoP facilitates an efficient Fe-Co-P electron transfer path, which renders P the electron-rich site and subsequently splits a new orbital peak that matches with that of Cd(II) for excellent electrochemical performance. The sensitivity of Cd(II) was remarkably up to 109.75 μA μM-1 on the Fe-CoP modified electrode with excellent stability and repeatability, surpassing previously reported findings. Meanwhile, the electrode exhibits exceptional selectivity towards Cd(II) ions compared to some bivalent heavy metal ions (HMIs). Moreover, X-ray absorption fine structure (XAFS) analysis reveals the interaction between P and Cd(II), which is further verified via density functional theory (DFT) calculation with the new hybrid peaks resulting from the splitting peak of P atoms coupled with the orbital energy level of Cd(II). Generally, doping engineering for specific active sites and regulation of orbital electrons not only provides valuable insights for the subsequent regulation of electronic configuration but also lays the foundation for customizing highly sensitive and selectivity sensors.

Recent advances in metal-organic framework (MOF)-based agricultural sensors for metal ions: a review

Metal ions have great significance for agricultural development, food safety, and human health. In turn, there exists an imperative need for the development of novel, sensitive, and reliable sensing techniques for various metal ions. Agricultural sensors for the diagnosis of both agricultural safety and nutritional health can establish quality and safety traceability systems of both agro-products and food to guarantee human health, even life safety. Metal-organic frameworks (MOFs) are utilized widely for the design of diversified sensors due to their distinctive structural characteristics and extraordinary optical and electrical properties. To serve agricultural sensors better, this review is dedicated to providing a brief overview of the synthesis of MOFs, the modification of MOFs, the fabrication of MOF-based film electrodes, the applications of MOF-based agricultural sensors for metal ions, which are centered on electrochemical sensors and optical sensors, and current challenges of MOF-based agricultural sensors. In addition, this review also provides potential future opportunities for the development and practical application of agricultural sensors.

Recent advances in the modification of electrodes for trace metal analysis: a review

This review paper summarizes the research published in the last five years on using different compounds and/or materials as modifiers for electrodes employed in trace heavy metal analysis. The main groups of modifiers are identified, and their single or combined application on the surface of the electrodes is discussed. Nanomaterials, film-forming substances, and polymers are among the most used compounds employed mainly in the modification of glassy carbon, screen-printed, and carbon paste electrodes. Composites composed of several compounds and/or materials have also found growing interest in the development of modified electrodes. Environmentally friendly substances and natural products (mainly biopolymers and plant extracts) have continued to be included in the modification of electrodes for trace heavy metal analysis. The main analytical performance parameters of the modified electrodes as well as possible interferences affecting the determination of the target analytes, are discussed. Finally, a critical evaluation of the main findings from these studies and an outlook discussing possible improvements in this area of research are presented.

Exploration of PVC@SiO2 nanostructure for adsorption of methylene blue via using quartz crystal microbalance technology

Methylene blue (MB) dye is considered a well-known dye in many industries and the low concentration of MB is considered very polluted for all environment if it discharged without any treatment. For that reason, many researchers used advanced technologies for removing MB such as the electrochemical methods that considered very simple and give rapid response. Considering these aspects, a novel quartz crystal microbalance nanosensors based on different concentrations of PVC@SiO2 were designed for real-time adsorption of MB dye in the aqueous streams at different pHs and different temperatures. The characterization results of PVC@SiO2 showed that the PVC@SiO2 have synthesized in spherical shape. The performance of the designed QCM-Based PVC@SiO2 nanosensors were examined by the QCM technique. The sensitivity of designed nanosensors was evaluated at constant concentration of MB (10 mg/L) at different pHs (2, 7 and 11) and temperatures (20 °C, 25 °C, and 30 °C). From the experimental, the best concentration of PVC@SiO2 was 3% for adsorbed 9.99 mg of cationic methylene blue at pH 11 and temperature 20 °C in only 5.6 min.

Hierarchical porous loofah-like carbon with sulfhydryl functionality for electrochemical detection of trace mercury in water

Mercury is a common contaminant found in natural waters, which is highly toxic to human health. Thus, the facile and reliable monitoring of mercury in waters is of great significance. In this study, we fabricated a novel loofah-like hierarchical porous carbon with sulfhydryl functionality (S-LHC), and applied it as an ultrasensitive sensor for the electrochemical detection of mercury in water. The S-LHC was prepared through the direct pyrolysis of a triazole-rich metal-organic framework (MOF), followed by chemical modification using thioglycolic acid. The highly conductive N-doped carbon framework of S-LHC facilitated the electron transfer in mercury electrochemical sensing. Meanwhile, the open hierarchical pore structure and abundant sulfhydryl groups allowed the fast diffusion and effective enrichment of mercury ions. Consequently, the S-LHC sensor exhibited an exceptionally high sensitivity for mercury ions, with the mercury detection limit (0.36 nM) orders of magnitude lower than the regulated values in drinking water (typically 10∼30 nM). The constructed sensor also afforded good anti-interference ability and excellent stability for long-term detection of mercury in a variety of complex real water samples. The present study provides not only a facile method for mercury detection, but also a new idea for the construction of highly sensitive electrochemical sensors.

Microfluidic Devices for Heavy Metal Ions Detection: A Review

The contamination of air, water and soil by heavy metal ions is one of the most serious problems plaguing the environment. These metal ions are characterized by a low biodegradability and high chemical stability and can affect humans and animals, causing severe diseases. In addition to the typical analysis methods, i.e., liquid chromatography (LC) or spectrometric methods (i.e., atomic absorption spectroscopy, AAS), there is a need for the development of inexpensive, easy-to-use, sensitive and portable devices for the detection of heavy metal ions at the point of interest. To this direction, microfluidic and lab-on-chip (LOC) devices fabricated with novel materials and scalable microfabrication methods have been proposed as a promising approach to realize such systems. This review focuses on the recent advances of such devices used for the detection of the most important toxic metal ions, namely, lead (Pb), mercury (Hg), arsenic (As), cadmium (Cd) and chromium (Cr) ions. Particular emphasis is given to the materials, the fabrication methods and the detection methods proposed for the realization of such devices in order to provide a complete overview of the existing technology advances as well as the limitations and the challenges that should be addressed in order to improve the commercial uptake of microfluidic and LOC devices in environmental monitoring applications.

Fluorescence off-on nanosensor based on MoS2 nanosheets and oligonucleotides for the alternative detection of mercury(II) ions or silver(I) ions

As traditional methods for detection of heavy metal pollution in water involve complex procedures and require expensive equipment, there is a great deal of interest in the development of rapid and simple methods for determining heavy metal ions in water. Here, a nanobiosensor based on molybdenum disulphide (MoS₂) nanosheets and fluorophore (FAM) labeled oligonucleotides was proposed, and fluorescence spectroscopy was adopted for detection of Hg²⁺ or Ag⁺ ions in aqueous solution. The principle underlying detection by the sensor involves the formation of T-Hg²⁺-T or C-Ag⁺-C mismatches by single-stranded DNA (ssDNA) rich in thymine (T) or cytosine (C), thereby forming stable double-stranded DNA (dsDNA) structures. By exploiting the different adsorption capacity of MoS₂ nanosheets for ssDNA and dsDNA, when oligonucleotides were in a single chain state, MoS₂ nanosheets possessed a strong adsorption capacity for ssDNA, resulting in fluorescence quenching of FAM. After the addition of Hg²⁺ or Ag⁺, ssDNA formed double chains structure, the fluorescence recovered due to the weak adsorption capacity of MoS₂ nanosheets for dsDNA. Along this line, an "off-on" mode fluorescence nanobiosensor was designed to alternatively detect these two heavy metal ions in water. The sensor showed high sensitivity and excellent selectivity for both Hg²⁺ and Ag⁺ ions, with minimum detection limits of 6.8 nM and 8.9 nM, respectively.

Ternary nanocomposite-based smart sensor: Reduced graphene oxide/polydopamine/alanine nanocomposite for simultaneous electrochemical detection of Cd2+, Pb2+, Fe2+, and Cu2+ ions

Heavy metal ion (HMI) sensors are the most sought commercial devices for environmental monitoring and food analysis research due to serious health concerns associated with HMI overdosage. Herein, we developed an effective electrochemical sensor for simultaneous detection of four HMI (Cd²⁺, Pb²⁺, Fe²⁺, and Cu²⁺) using a ternary nanocomposite of reduced graphene oxide functionalized with polydopamine and alanine (ALA/pDA/rGO). Comprehensive spectroscopic and microscopic characterizations were performed to ensure the formation of the ternary nanocomposite. The developed nanocomposite on glassy carbon electrode (GCE) yields >2-fold higher current than GO/GCE electrode with excellent electrochemical stability and charge transfer rate. Using DPV, various chemical and electrochemical parameters, such as supporting electrolyte, buffer pH, metal deposition time, and potential, were optimized to achieve highly sensitive detection of targeted HMI. For Cd²⁺, Pb²⁺, Fe²⁺, and Cu²⁺ sensing devised sensor exhibited detection limits of 1.46, 2.86, 50.23, and 17.95 ppb and sensitivity of 0.0929, 0.0744, 0.0051, and 0.0394 μA/ppb, respectively, with <6% interference. The sensor worked similarly well for real water samples with HMI. This study demonstrates a novel strategy for concurrently detecting and quantifying multiple HMI in water and soil using a smart ternary nanocomposite-based electrochemical sensor, which can also detect HMI in food samples.

Highly efficient and simultaneous magnetic solid phase extraction of heavy metal ions from water samples with l-Cysteine modified magnetic polyamidoamine dendrimers prior to high performance liquid chromatography

Due to the strong metal-sulfur interaction between mercapto groups and metal ions, which can be used to functionalize polyamidoamine dendrimer decorated Fe₃O₄ nanoparticles for high enrichment of trace heavy metal ions from waters. Based on this concept, polyamidoamine dendrimer modified Fe₃O₄ nanomaterials were functionalized with l-Cysteine and a new magnetic solid phase extraction for rapid adsorption and separation of Hg²⁺, Pb²⁺, Co²⁺ and Cd²⁺ from waters was established. The factors affecting extraction efficiency have been optimized. Upon the optimal parameters, the established method provided good linear ranges of 0.1-200 μg L^-1 for Hg²⁺ and 0.05-200 μg L^-1 for Pb²⁺, Co²⁺ and Cd²⁺, and high sensitivity with limits of detection (LOD) of 0.018 μg L^-1, 0.014 μg L^-1, 0.013 μg L^-1 and 0.025 μg L^-1 for Cd²⁺, Pb²⁺, Co²⁺ and Hg²⁺, respectively. Real water samples were utilized to validate the proposed method, and achieved results revealed that the proposed method was sensitive, effective, stable and suitable for monitoring Pb²⁺, Cd²⁺, Co²⁺and Hg²⁺ in environmental waters. This work provided a novel strategy for the simultaneous analysis of target cations in waters, and a new direction for developing decoration method of nanomaterials according to specific purpose.

Multifunctional nanomaterials and nanocomposites for sensing and monitoring of environmentally hazardous heavy metal contaminants

The applications of conventional sensors are limited by the long response time, high cost, large detection limit, low sensitivity, complicated usage and low selectivity. These sensors are nowadays replaced by Nanocomposite-based modalities and nanomaterials which are known for their high selectivity and physical and chemical properties. These nanosensors effectively detect heavy metal contaminants in the environment as the discharge of heavy metals into natural water as a result of human activity has become a global epidemic. Exposure to these toxic metals might induce many health-related complications, including kidney failure, brain injury, immune disorders, muscle paleness, cardiac damage, nervous system impairment and limb paralysis. Therefore, designing and developing novel sensing systems for the detection and recognition of these harmful metals in various environmental matrices, particularly water, is of extremely important. Emerging nanotechnological approaches in the past two decades have played a key role in overcoming environmentally-related problems. Nanomaterial-based fabrication of chemical nanosensors has widely been applied as a powerful analytical tool for sensing heavy metals. Portability, high sensitivity, on-site detection capability, better device performance and selectivity are all advantages of these nanosensors. The detection and selectivity have been improved using molecular recognition probes for selective binding on different nanostructures. This study aims to evaluate the sensing properties of various nanomaterials such as metal-organic frameworks, fluorescent materials, metal-based nanoparticles, carbon-based nanomaterials and quantum dots and graphene-based nanomaterials and quantum dots for heavy metal ions recognition. All these nano-architectures are frequently served as effective fluorescence probes to directly (or by modification with some large or small biomolecules) sense heavy metal ions for improved selectivity. However, efforts are still needed for the simultaneous designing of multiple metal ion-based detection systems, exclusively in colorimetric or optical fluorescence nanosensors for heavy metal cations.

A highly selective silver ion optical chemosensor based on isoxazolyl-azo pyrimidine: synthesis, spectroscopy, DFT calculations and applications

In this work, an isoxazolyl-azo pyrimidine optical chemosensor (PICS) was efficiently synthesized and applied for naked-eye detection of Ag⁺ ions in solution. The chemical formula of the PICS was recognized by UV-vis, FTIR and NMR analyses. The detection ability of PICS toward various ions was assessed. The results revealed the excellent selectivity and sensitivity of the chemosensor PICS to Ag⁺ ions in aqueous DMSO solutions. The PICS displayed an obvious color change from yellow to dark red in the presence of silver ions. The PICS could efficiently detect Ag⁺ ions over a wide pH range of 6-11, which makes it suitable for detection of Ag⁺ under physiological conditions. PICS also binds Ag⁺ ions to form a 1 : 1 stoichiometry complex (PICS-Ag⁺), resulting in a bathochromic shift in the absorption maximum from 372 to 410 nm. The detection limit of the probe PICS towards Ag⁺ was calculated to be 1.78 μM. Furthermore, the probe PICS shows excellent detection performance in the solid state, and PICS-based test strips were fabricated and applied as efficient Ag⁺ test kits for detection of silver ions in water samples. In addition, the sensing mechanism of PICS-Ag⁺ was completely evaluated using the density functional theory (DFT) calculations. Results indicated that the calculated energy gap between the HOMO and LUMO (3.41 eV) of PICS-Ag is lower than that of the free PICS (3.57 eV). This suggests that a red shift occurred upon addition of the Ag⁺ ion to PICS.

Voltammetric Determination of Hg2+, Zn2+, and Pb2+ Ions Using a PEDOT/NTA-Modified Electrode

A novel electrochemical sensor for determining trace levels of Hg²⁺, Pb²⁺, and Zn²⁺ ions in water using square wave voltammetry (SWV) is reported. The sensor is based on a platinum electrode (Pt) modified by poly(3,4-ethylenedioxythiophene) and N _α,N _α-bis-(carboxymethyl)-l-lysine hydrate (NTA lysine) PEDOT/NTA. The modified electrode surface (PEDOT/NTA) was prepared via the introduction of the lysine-NTA group to a PEDOT/N-hydroxyphthalimide NHP electrode. The (PEDOT/NTA) was characterized via cyclic voltammetry (CV), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). The effects of scan rates on the electrochemical properties of the polymer electrode were also investigated. The electrochemical results were used to estimate the coverage of the electrode polymer surface and its electrostability in background electrolyte solutions. Several analytical parameters, such as polymer film thickness, metal deposition time, and pH of the electrolyte, were examined. Linear responses to Hg²⁺, Pb²⁺, and Zn²⁺ ions in the concentration range of 5-100 μg L^-1 were obtained. The limits of detection (LODs) for the determination of Hg²⁺, Pb²⁺, and Zn²⁺ ions were 1.73, 2.33, and 1.99 μg L^-1, respectively. These promising results revealed that modified PEDOT/NTA films might well represent an important addition to existing electrochemical sensor technologies.

A new and efficient diaminopyrimidine-based colorimetric and fluorescence chemosensor for the highly selective and sensitive detection of Cu2+ in aqueous media and living cells

In this paper, a new and effective diaminopyrimidine-based chemosensor (DAPCS) was developed for the highly selective and ultra-sensitive detection of Cu²⁺ ion in aqueous media and living cell. Characterization and structure determining of DAPCS was determined by UV-Vis, FTIR and NMR analyses. It is observed that DAPCS and Cu (II) forms a ligand to metal charge transfer (LMCT) complex which produces distinguishable red color. The results also indicate that the DAPCS easily interacts with Cu²⁺ ion to form a 1:1 stoichiometry complex (DAPCS -Cu²⁺), resulting in a bathochromic shift in absorption maximum (429 nm to 449 nm) and remarkable quenching fluorescence intensity at the wavelength of 501 nm in DMSO-H₂O solution. Furthermore, the detection limit of DAPCS towards Cu²⁺ was calculated to be 3.19 µM. Meanwhile, DAPCS was applied as fluorescent probe for detection of Cu²⁺ ions with the detection limit of 0.014 µM. The optimal pH range of probe DAPCS for quantitative analysis of Cu²⁺ ions was 9-11, which renders it suitable for detection of Cu²⁺ under physiological conditions. Additionally, the DAPCS could be applied to detect Cu²⁺ in real water samples and in HeLa cells, indicating the practical uses of DAPCS in real analyses.

CeO2 nanostructured electrochemical sensor for the simultaneous recognition of diethylstilbestrol and 17β-estradiol hormones

A new highly sensitive, selective, and inexpensive electrochemical method has been developed for simultaneously detecting diethylstilbestrol (DES) and 17β-estradiol (E2) in environmental samples (groundwater and lake water) using a graphite sensor modified by cerium oxide nanoparticles (CPE-CeO₂ NPs). The developed sensor and the materials used in its preparation were characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The ab initio simulation was used to evaluate the adsorption energies between both DES and E2 with the surface of the sensor. The peak current of oxidation of both hormones showed two regions of linearity. The region of greatest sensitivity was observed for the linear range of 10 nM-100 nM. The detection and quantification limits for this concentration range were 0.8/2.6 nM and 1.3/4.3 nM for DES and E2, respectively. The analytical performance of the developed method showed high sensitivity, precision, repeatability, reproducibility, and selectivity. The CPE-CeO₂ NPs sensor was successfully applied to simultaneously detect DES and E2 in real samples with recovery levels above 98%.

Highly fluorescent N doped C-dots as sensor for selective detection of Hg2+ in beverages

In this work, nitrogen doped carbon dots (NCDs) were synthesized through one step hydrothermal reaction using citric acid and ethylenediamine as precursors. The prepared NCDs exhibit high quantum yield of 67.4%, good stability, excellent selectivity and sensitivity. It was found that the NCDs have potential as a fluorescence sensor for the detection of Hg²⁺. Under optimal conditions, good linearity between the change in NCDs fluorescence intensity and Hg²⁺ concentration was obtained in the range of 0.3 to 2.0 μM with a detection limit at 0.24 μM. The possible detection mechanism was static quenching of NCDs by Hg²⁺. The established method was successfully applied to the determination of Hg²⁺ in beverage samples. The results indicated that the NCDs-based sensor has potential for detection of Hg²⁺ in real beverage sample.

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.

Surface Characterization of New Azulene-Based CMEs for Sensing

Films of 2-(azulen-1-yldiazenyl)-5-phenyl-1,3,4-thiadiazole (T) were successfully deposited on glassy carbon surfaces to prepare chemically modified electrodes (CMEs). Their surface characterization was analyzed by electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). This complexing monomer has been deposited through direct electropolymerization in conditions established during the electrochemical characterization of T performed by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and rotating disk electrode voltammetry (RDE). These methods put in evidence the high degree of asymmetry of oxidation and reduction curves, which is due to the irreversible processes occurring at opposite potentials. The film formation was confirmed by ferrocene redox assay probe. The properties of the electrodes modified with T (T-CMEs) were investigated for sensing heavy metal (HM) ions in water solutions, with promising results for Pb(II) among Cd(II), Cu(II), and Hg(II) ions.

Direct growth of two-dimensional phthalocyanine-based COF on Cu-MOF to construct a photoelectrochemical-electrochemical dual-mode biosensing platform for high-efficiency determination of Cr(III)

A photoelectrochemical (PEC)-electrochemical (EC) dual-mode biosensing strategy based on COF@MOF heterostructure was developed for efficiently analyzing Cr(III) ions. A two-dimensional phthalocyanine-based COF (CoPc-PT-COF) was in situ grown on a Cu-based MOF (Cu-MOF) substrate via covalent binding between carboxyl groups in Cu-MOF and amino groups in CoPc-PT-COF (denoted as CoPc-PT-COF@Cu-MOF). The coexistence of both phthalocyanine and bipyridine in CoPc-PT-COF@Cu-MOF affords the outperformed electro- and photo-activities, thus serving as a photoelectric beacon with favorable energy-band configuration and amplified electrochemical response. Due to the high porosity and rich functionality of the obtained heterostructure, the DNA strands can be tightly anchored over CoPc-PT-COF@Cu-MOF via diverse interactions. Thanks to the specific recognition between DNA strands and Cr³⁺ ions, the CoPc-PT-COF@Cu-MOF-based biosensor can be used to determine Cr³⁺ ions in an aqueous environment by PEC-EC mode. The gained biosensor shows an extremely low limit of detection (LOD) of 14.5 fM (for PEC) and 22.9 fM (for EC) within the Cr³⁺ concentration range from 0.1 pM to 100 nM, along with high selectivity, good reproducibility and stability. Moreover, this novel biosensor exhibits acceptable applicability for analyzing the trace Cr³⁺ from diverse samples (e.g., river and tap water). As a result, this work provides new insights into the construction of a high-efficiency PEC-EC dual-mode biosensor for detecting heavy metal ions from complex environments.

A novel copper(i) metal–organic framework as a highly efficient and ultrasensitive electrochemical platform for detection of Hg(ii) ions in aqueous solution

A novel copper(i) metal–organic framework was constructed and used to modify a glassy carbon electrode, and exhibits excellent electrochemical sensing of Hg(ii) ions.

A dithizone-anchored silica gel surface, {SiO2}@DZ for the selective sample cleanup of Gd(iii) amidst Fe(iii), Th(iv), and Ce(iv) employing ion pair complexation

Sorption–desorption equilibration, {extractor-HOMO}:{H₃O}⁺ + {metal-species}ⁿ⁺ ⇄ {extractor-HOMO}:{metal-species}ⁿ⁺ + {H₃O}⁺, an eventual ion-pair complexation controlled by {H₃O}⁺.