Graphene-based optical nanosensors for detection of heavy metal ions

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.

Enhanced electrocatalytic activity in MOFs-derived 3D hollow NiCo-LDH nanocages decorated porous biochar for simultaneously ultra-sensitive electrochemical sensing of Cu2+ and Hg2

Layered double hydroxides (LDHs) have attracted significant attention due to their compositional and structural flexibility. However, it is challenging but meaningful to design and fabricate hierarchical mixed-dimensional LDHs with synergistic effects to increase the electrical conductivity of LDHs and promote the intrinsic activity. Herein, 3D hollow NiCo-LDH nanocages decorated porous biochar (3D NiCo-LDH/PBC) has been synthesized by using ZIF-67 as precursor, which was utilized for constructing electrochemical sensing platform to realize simultaneous determination of Cu2+ and Hg2+. The 3D NiCo-LDH/PBC possessed the characteristics of hollow material and three-dimensional porous material, revealing a larger surface area, more exposed active sites, and faster electron transfer, which is beneficial to enhancing its electrochemical performance. Consequently, the developed sensor displayed good performance for simultaneously detecting Cu2+ and Hg2+ with ultra-low limit of detection (LOD) of 0.03 μg L-1 and 0.03 μg L-1, respectively. The proposed sensor also demonstrated excellent stability, repeatability and reproducibility. Furthermore, the sensor can be successfully used for the electrochemical analysis of Cu2+ and Hg2+ in lake water sample with satisfactory recovery, which is of great feasibility for practical application.

Chromoionophoric molecular probe infused bimodal porous polymer rostrum as solid-state ocular sensor for the selective and expeditious optical sensing of ultra-trace toxic mercury ions

This study presents a distinctive solid-state naked-eye colorimetric sensing approach by encapsulating a chromoionophoric probe onto a hybrid macro-/meso-pore polymer scaffold for fast and selective sensing of ultra-trace Hg(II). The customized structural/surface properties of the poly(VPy-co-TM) monolith are attained by specific proportions of 2-vinylpyridine (VPy), trimethylolpropane trimethacrylate (TM), and pore-tuning solvents. The interconnected porous network of poly(VPy-co-TM), inherent superior surface area and porosity, is captivating for the homogeneous/voluminous incorporation of probe molecules, i.e., 7-((4-methoxyphenyl)diazenyl)quinoline-8-ol (MPDQ), for the target-specific colorimetric detection. The structural morphology, surface topography, and phase characteristics of the bare poly(VPy-co-TM) monolith and MPDQ@poly(VPy-co-TM) sensor are examined using HR-TEM-SAED (High-Resolution Transmission Electron Microscopy - Selected Area Electron Diffraction), FE-SEM-EDAX (Field Emission Scanning Electron Microscopy - Energy Dispersive X-ray Spectroscopy), XPS (X-ray Photoelectron Spectroscopy), p-XRD (Powder X-Ray Diffraction), FT-IR (Fourier Transform Infrared Spectroscopy), UV-Vis-DRS (Ultraviolet-Visible Diffuse Reflectance Spectroscopy), and BET/BJH (Brunauer-Emmett-Teller / Barrett-Joyner-Halenda) analysis. The distinctive properties of the sensor reveal a constrained geometrical orientation of the MPDQ probe onto the long-range continuous monolithic network of meso-/-macropore template, enabling selective interaction with Hg(II) with peculiar color transfiguration from pale yellow to deep brown. The sensor demonstrates a linear spectral-color alliance in the 0-200 ppb concentration range for Hg(II), with quantification and detection limits of 0.63 and 0.19 ppb. The sensor efficacy is verified using certified contaminated water and tobacco samples, with excellent reusability, reliability, and reproducibility of ≥ 99.23 % (RSD ≤1.89 %) and ≥ 99.19 % (RSD ≤1.94 %) of Hg(II), respectively.

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.

Food nanotechnology: opportunities and challenges

Food nanotechnology, which applies nanotechnology to food systems ranging from food production to food processing, packaging, and transportation, provides tremendous opportunities for conventional food science and industry innovation and improvement. Although great progress and rapid growth have been achieved in food nanotechnology research owing to the unique food features rendered by nanotechnology, at a fundamental level, food nanotechnology is still in its initial stages and the potential adverse effects of nanomaterials are still a controversial problem that attract public attention. Food-derived nanomaterials, compared to some inorganic nanoparticles and synthetic organic macromolecules, can be digested rapidly and produce similar digestion products to those produced normally, which become the mainstream and trend for food nanotechnology in practical applications, and are expected to be a vital tool for addressing the security problem and easing public concerns. These food-derived materials enable the favourable characteristics of nanostructures to be combined with the safety, biocompatibility, and bioactivity of natural food. Very recently, diverse food-derived nanomaterials have been explored and widely applied in multiple fields. Herein, we thoroughly summarize the fabrication and development of nanomaterials for use in food technology, as well as the recent advances in the improvement of food quality, revolutionizing food supply, and boosting food industries based on foodborne nanomaterials. The current challenges in food nanotechnology are also discussed. We hope this review can provide a detailed reference for experts and food manufacturers and inspire researchers to participate in the development of food nanotechnology for highly efficient food industry growth.

Sensitive and selective detection of heavy metal ions and organic pollutants with graphene-integrated sensing platforms

Graphene-based sensors have emerged as promising tools for environmental monitoring due to their exceptional properties such as high surface area, excellent electrical conductivity, and sensitivity to various analytes. This paper presents a review of recent advancements in the development and application of graphene-based sensors for the detection of heavy metal ions and organic pollutants. These sensors employ either graphene or its derivatives, often in combination with graphene hybrid nanocomposites, as the primary sensing material. The synthesis methods of graphene and sensing mechanisms of graphene-based sensors are discussed. Furthermore, performance metrics including the determination range and detection limits of these sensors are itemized. The potential challenges and future directions in the field of graphene-based sensors for environmental monitoring are also highlighted. Overall, this review provides valuable insights into the current state-of-the-art technologies and paves the way for the development of highly efficient and reliable sensors for environmental monitoring purposes.

Exploring the fluorescence properties of tellurium-containing molecules and their advanced applications

This review article explores the fascinating realm of fluorescence using organochalcogen molecules, with a particular emphasis on tellurium (Te). The discussion encompasses the underlying mechanisms, structural motifs influencing fluorescence, and the applications of these intriguing phenomena. This review not only elucidates the current state of knowledge but also identifies avenues for future research, thereby serving as a valuable resource for researchers and enthusiasts in the field of fluorescence chemistry with a focus on Te-based molecules. By highlighting challenges and prospects, this review sparks a conversation on the transformative potential of Te-containing compounds across different fields, ranging from environmental solutions to healthcare and materials science applications. This review aims to provide a comprehensive understanding of the distinct fluorescence behaviors exhibited by Te-containing compounds, contributing valuable insights to the evolving landscape of chalcogen-based fluorescence research.

Helical Self-Assembly and Fe3+ Detection of V-Shaped AIE-Active Chiral Tetraphenylbutadiene-Based Polyamides

Chiral aggregation-induced emission (AIE) molecules have drawn attention for their helical self-assembly and special optical properties. The helical self-assembly of AIE-active chiral non-linear main-chain polymers can produce some desired optical features. In this work, a series of V-shaped chiral AIE-active polyamides P1-C3, P1-C6, P1-C12 and linear P2-C3, P2-C6, bearing n-propyl/hexyl/dodecyl side-chains, based on tetraphenylbutadiene (TPB), were prepared. All target main-chain polymers exhibit distinct AIE characteristics. The polymer P1-C6 with moderate length alkyl chains shows better AIE properties. The V-shaped main-chains and the chiral induction of (1R,2R)-(+)-1,2-cyclohexanediamine in each repeating unit promote the polymer chains display helical conformation, and multiple helical polymer chains induce nano-fibers helicity when the polymer chains aggregate and self-assemble in THF/H2 O mixtures. Simultaneously, the helical conformation polymer chains and helical nano-fibers cause P1-C6 produce strong circular dichroism (CD) signals with positive Cotton effect. Moreover, P1-C6 could also occur fluorescence quenching response to Fe3+ selectively with a low detection limit of 3.48 μmol/L.

Two-dimensional bismuth oxyselenide quantum dots as nanosensors for selective metal ion detection over a wide dynamic range: sensing mechanism and selectivity

Bismuth oxyselenide (Bi2O2Se) nanosheets, a new 2D non-van der Waals nanomaterial having unique semiconducting properties, could be favorable for various sensing applications. In the present report, a top-down chemical approach was adopted to synthesize ultrathin Bi2O2Se quantum dots (QDs) in an appropriate solution. The as-prepared 2D Bi2O2Se QDs with an average size of ∼3 nm, exhibiting strong visible fluorescence, were utilized for heavy-metal ion detection with high selectivity. The QDs show a high optical band gap and a reasonably high fluorescence quantum yield (∼4%) in the green region without any functionalization. A series of heavy metal ions were detected using these QDs. The as-prepared QDs exhibit selective detection of Fe3+ over a wide dynamic range with a high quenching ratio and a low detection limit (<0.5 μM). The mechanism of visible fluorescence and Fe3+ ion-induced quenching was investigated in detail based on a model involving adsorption and charge transfer. Density functional theory (DFT) first principles calculations show that fluorescence quenching occurred selectively due to the efficient trapping of electrons in the bandgap states created by the Fe atoms. This work presents a sustainable and scalable method to synthesize 2D Bi2O2Se QDs for heavy metal ion sensing over a wide dynamic range and these 2D QDs could find potential uses in gas sensors, biosensors and optoelectronics.

Aptamer-functionalized field-effect transistor biosensors for disease diagnosis and environmental monitoring

Nano-biosensors that are composed of recognition molecules and nanomaterials have been extensively utilized in disease diagnosis, health management, and environmental monitoring. As a type of nano-biosensors, molecular specificity field-effect transistor (FET) biosensors with signal amplification capability exhibit prominent advantages including fast response speed, ease of miniaturization, and integration, promising their high sensitivity for molecules detection and identification. With intrinsic characteristics of high stability and structural tunability, aptamer has become one of the most commonly applied biological recognition units in the FET sensing fields. This review summarizes the recent progress of FET biosensors based on aptamer functionalized nanomaterials in medical diagnosis and environmental monitoring. The structure, sensing principles, preparation methods, and functionalization strategies of aptamer modified FET biosensors were comprehensively summarized. The relationship between structure and sensing performance of FET biosensors was reviewed. Furthermore, the challenges and future perspectives of FET biosensors were also discussed, so as to provide support for the future development of efficient healthcare management and environmental monitoring devices.

A sensitive electrochemical sensor based on 3D porous melamine-doped rGO/MXene composite aerogel for the detection of heavy metal ions in the environment

Herein, we developed a unique screen-printed carbon electrode (SPCE) with three-dimensional melamine-doped graphene oxide/MXene composite aerogel (3D MGMA) modification, which is used for the simultaneous and sensitive detection of three metal ions (Zn²⁺, Cd²⁺, and Pb²⁺) in the environment. A self-assembly method was used to fabricate 3D MXene aerogels based on MXene, graphene oxide (GO), and melamine. Notably, the network-like 3D structure combining 2D MXene and rGO sheets can provide a high ratio of surface area and enriched functional clusters, which are beneficial for improving the electrical conductivity and promoting the uptake of heavy metal ions. In the linear range of 3-900 μg L^-1, the constructed innovative sensing platform can sensitively detect Zn²⁺, Cd²⁺, and Pb²⁺ simultaneously, with detection limits of 0.48 μg L^-1,0.45 μg L^-1 and 0.29 μg L^-1 respectively. This work reflects precision and reliability in the detection of three water samples (tap water, Minzhu lake and Yangtze River) and four cereal samples (sorghum, rice, wheat and corn), proposing a novel strategy for monitoring heavy metal ions in the natural environment.

Quantitative and qualitative analyses of metal ions in food and water by using a multicolor sensor array and chemometrics

Rapid and accurate detection of toxic metal ions is the key to combating food contamination and environmental pollution. In sensor arrays, gold nanoparticles play a crucial role in monitoring metal ions based on surface plasmon resonance. However, identifying metal ions with unknown concentrations in a complex system through this assay is difficult because of its monotonous color change and weak anti-interference ability. To overcome these limitations, a sensitive, flexible, low-cost, and multicolor sensor array was designed herein. The applicability of the sensor array for the qualitative and quantitative analyses of metal ions in food and water was also verified. The developed sensor array could classify 14 metal ions (Cu²⁺, Fe²⁺, Fe³⁺, Mn²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Cr³⁺, Co²⁺, Ba²⁺, K⁺, Tl⁺, Pb²⁺, and Hg²⁺) of unknown concentration with an accuracy of 100%. In addition, partial least squares models were established to quantify Tl⁺, Pb²⁺, and Hg²⁺ in water and rice samples, with square correlation coefficients (R²) of 0.9991, 0.9742, and 0.9731, respectively. This method can be used for accurate quantitative and qualitative analyses of heavy metal ions in water and food.

A review on recent advancements on removal of harmful metal/metal ions using graphene oxide: Experimental and theoretical approaches

Graphene oxide is a two-dimensional carbon nanomaterial and has gained huge popularity over the last decade. Because, the graphene oxide can be dispersed in water easily and it is one of the most researched two-dimensional materials in the current time. The extraordinary properties shown by graphene oxide (GO) are due to its unique chemical structure; includes various hydrophilic functional groups containing oxygen such as carboxyl, hydroxyl, carbonyl and tiny sp² carbon domains surrounded by sp³ domains. These groups are very peculiar for various applications as they allow covalent functionalisation with a plethora of compounds. Large surface area, intrinsic fluorescence, excellent surface functionality, amphiphilicity, improved conductivity, high adsorption capacity and superior biocompatibility are some of the chemical properties have drawn research from various fields. Graphene oxide has various interactions such as coordination, chelation, hydrogen bonding, electrostatic interaction, hydrophobic effects, π-π interaction, acid base interaction etc., with various metal ions. This review is focused on the removal of metals and metal ions due to their interactions mentioned above. Further, potential of composites of graphene oxide in the removal of metal and metal ions is also discussed. Further, the current challenges in this field at industrial-scale are also discussed.

Carbon-polymer dot-based UV absorption and fluorescence performances for heavy metal ion detection

In previous reports, carbon dots (CDs) were customarily used as fluorescent probes to detect heavy metal ions. However, scientists neglected to take advantage of the excellent UV absorption properties of CDs to detect heavy metal ions. Herein, we synthesized nitrogen-containing carbon polymer dots (N-CPDs) for the determination of Co²⁺ ions in water samples by a one-step hydrothermal method using l-histidine and ethylene imine polymer as raw materials. The N-CPDs were characterized by ultraviolet-visible spectrum (UV-vis), infrared spectrum (FT-IR), X-ray photoelectron spectrum (XPS) and transmission electron microscopy (TEM) techniques. They possess superior full-band UV absorption performance and the surface is rich in multifunctional groups such as -COOH, -CN-, -OH, etc. When Co²⁺ was added to N-CPDs solution, the color of the solution rapidly changed from colorless to yellow-brown, which was visible to the naked eye. The UV absorption intensity of N-CPDs changed, and the fluorescence was instantly quenched, due to the formation of chelate between Co²⁺ and N-CPDs, and the FRET process occurred. The detection of Co²⁺ showed good linearity for both fluorescence and UV absorption spectroscopy modes in the range of 0-200 μM, and the limit of detection were 1.0023 μM and 0.75 μM, respectively. These two methods have the advantages of simple operation, remarkable selectivity and small sample size, which can be applied to the field detection of Co²⁺ in water samples. It is possible to develop the UV absorption properties of CDs to detect the ions.

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

Detection of heavy metal ions has drawn significant attention in environmental and food area due to their threats to the human health and ecosystem. Colorimetry is one of the most frequently-used methods for the detection of heavy metal ions owing to its simplicity, easy operation and rapid on-site detection. The development of chromogenic materials and their sensing mechanisms are the key research direction in the area of colorimetric method. Since each chromogenic material has their unique optical and chemical properties, they have totally different colorimetric sensing mechanisms. This review focuses on the chromogenic materials and their sensing strategies for the colorimetric detection of heavy metal ions. We divide the chromogenic materials into three types, including organic materials, inorganic materials, and other materials. As for each type of chromogenic material, we discuss their detailed sensing strategies, sensing performance, and real sample applications. Moreover, current challenges and perspectives related to the colorimetry of heavy metal ions are also discussed in this review. The aim of this review is to help readers to better understand the principles of colorimetric methods for heavy metal ions and push the development of rapid detection of heavy metal ions.

Organic chemical Nano sensors: synthesis, properties, and applications

Nanosensors work on the "Nano" scale. "Nano" is a unit of measurement around 10- 9 m. A nanosensor is a device capable of carrying data and information about the behavior and characteristics of particles at the nanoscale level to the macroscopic level. Nanosensors can be used to detect chemical or mechanical information such as the presence of chemical species and nanoparticles or monitor physical parameters such as temperature on the nanoscale. Nanosensors are emerging as promising tools for applications in agriculture. They offer an enormous upgrade in selectivity, speed, and sensitivity compared to traditional chemical and biological methods. Nanosensors can be used for the determination of microbe and contaminants. With the advancement of science in the world and the advent of electronic equipment and the great changes that have taken place in recent decades, the need to build more accurate, smaller and more capable sensors was felt. Today, high-sensitivity sensors are used that are sensitive to small amounts of gas, heat, or radiation. Increasing the sensitivity, efficiency and accuracy of these sensors requires the discovery of new materials and tools. Nano sensors are nanometer-sized sensors that, due to their small size and nanometer size, have such high accuracy and responsiveness that they react even to the presence of several atoms of a gas. Nano sensors are inherently smaller and more sensitive than other sensors.

Graphene Synthesis Techniques and Environmental Applications

Graphene is fundamentally a two-dimensional material with extraordinary optical, thermal, mechanical, and electrical characteristics. It has a versatile surface chemistry and large surface area. It is a carbon nanomaterial, which comprises sp² hybridized carbon atoms placed in a hexagonal lattice with one-atom thickness, giving it a two-dimensional structure. A large number of synthesis techniques including epitaxial growth, liquid phase exfoliation, electrochemical exfoliation, mechanical exfoliation, and chemical vapor deposition are used for the synthesis of graphene. Graphene prepared using different techniques can have a number of benefits and deficiencies depending on its application. This study provides a summary of graphene preparation techniques and critically assesses the use of graphene, its derivates, and composites in environmental applications. These applications include the use of graphene as membrane material for the detoxication and purification of water, active material for gas sensing, heavy metal ions detection, and CO₂ conversion. Furthermore, a trend analysis of both synthesis techniques and environmental applications of graphene has been performed by extracting and analyzing Scopus data from the past ten years. Finally, conclusions and outlook are provided to address the residual challenges related to the synthesis of the material and its use for environmental applications.

Aerogel Assembled by Two Types of Carbon Nanoparticles for Efficient Removal of Heavy Metal Ions

Both sodium alginate and polyethyleneimine (PEI) have a good ability to adsorb heavy metal ions. PEI and sodium alginate were used as important precursors to synthesize positively charged carbon nanoparticles (p-CNDs) with hydroxyl and carboxyl, and negatively charged carbon nanoparticles (n-CNDs) with amino, respectively. The carbon nanoparticles (CNDs) aerogel with a large specific surface area and rich functional groups were constructed by self-assembled p-CNDs and n-CNDs via electrostatic attraction for adsorption of heavy metal ions in water. The results show that CNDs aerogel has good adsorption properties for Pb²⁺ (96%), Cu²⁺ (91%), Co²⁺ (86%), Ni²⁺ (82%), and Cd²⁺ (78%). Furthermore, the fluorescence emission intensity of CNDs aerogel will gradually decrease with the increase in the adsorption rate, indicating that it can detect the adsorption process synchronously. In addition, the cytotoxicity test reveals that CNDs have good biocompatibility and will not cause secondary damage to biological cells.

Novel NBN-Embedded Polymers and Their Application as Fluorescent Probes in Fe3+ and Cr3+ Detection

The isosteric replacement of C═C by B–N units in conjugated organic systems has recently attracted tremendous interest due to its desirable optical, electronic and sensory properties. Compared with BN-, NBN- and BNB-doped polycyclic aromatic hydrocarbons, NBN-embedded polymers are poised to expand the diversity and functionality of olefin polymers, but this new class of materials remain underexplored. Herein, a series of polymers with BNB-doped π-system as a pendant group were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization from NBN-containing vinyl monomers, which was prepared via intermolecular dehydration reaction between boronic acid and diamine moieties in one pot. Poly{2-(4-Vinylphenyl)-2,3-dihydro-1H-naphtho[1,8-de][1,3,2]diazaborinine} (P1), poly{N-(4-(1H-naphtho[1,8-de][1,3,2]diazaborinin-2(3H)-yl)phenyl)acrylamide} (P2) and poly{N-(4-(1H-benzo[d][1,3,2]diazaborol-2(3H)-yl)phenyl)acrylamide} (P3) were successfully synthesized. Their structure, photophysical properties and application in metal ion detection were investigated. Three polymers exhibit obvious solvatochromic fluorescence. As fluorescent sensors for the detection of Fe³⁺ and Cr³⁺, P1 and P2 show excellent selectivity and sensitivity. The limit of detection (LOD) achieved by Fe³⁺ is 7.30 nM, and the LOD achieved by Cr³⁺ is 14.69 nM, which indicates the great potential of these NBN-embedded polymers as metal fluorescence sensors.

Metal organic frameworks for electrochemical sensor applications: A review

Metal-organic frameworks (MOFs) are broadly known as porous coordination polymers, synthesized by metal-based nodes and organic linkers. MOFs are used in various fields like catalysis, energy storage, sensors, drug delivery etc., due to their versatile properties (tailorable pore size, high surface area, and exposed active sites). This review presents a detailed discussion of MOFs as an electrochemical sensor and their enhancement in the selectivity and sensitivity of the sensor. These sensors are used for the detection of heavy metal ions like Cd²⁺, Pb²⁺, Hg²⁺, and Cu²⁺ from groundwater. Various types of organic pollutants are also detected from the water bodies using MOFs. Furthermore, electrochemical sensing of antibiotics, phenolic compounds, and pesticides has been explored. In addition to this, there is also a detailed discussion of metal nano-particles and metal-oxide based composites which can sense various compounds like glucose, amino acids, uric acid etc. The review will be helpful for young researchers, and an inspiration to future research as challenges and future opportunities of MOF-based electrochemical sensors are also reported.

Specific ultralow level chemo-recognition using Graphene-fluorophore supramolecular assembly: Fine-tuning the fluorophore framework

The non-covalent interactions between graphene and aromatic fluorophores have generated highly sensitive fluorimetric turn-on sensors for various significant analytes. Herein, the supramolecular interaction between reduced graphene oxide and 7-Hydroxy-4-Methyl-8-Amino Coumarin is made use of for tracing Cu²⁺ at sub-zeptomole level with excellent selectivity among a collection of nineteen metal ions. The system enables quantification of the analyte in a commendably wide range, from micromolar to zeptomolar, a feature that almost all-optical sensors lack. Handy solid-state sensor strip fabricated using the above-mentioned supramolecular combination enabled visual recognition of Cu²⁺ions at the molecular level. Based on the chemo recognition ability of the fluorophore, multiple Boolean logic devices operating at the molecular level are proposed. By screening pertinent coumarin derivatives, it is demonstrated that the selectivity and sensitivity of the sensors of this sort are decided by the number of π- interaction centers of the fluorophores and the strength by which they interact with graphene, respectively, which will enable identification and modification of proper fluorophores for ultra-trace detection of contaminants of environmental relevance from aqueous solutions.

A novel 3D Zn-coordination polymer based on a multiresponsive fluorescent sensor demonstrating outstanding sensitivities and selectivities for the efficient detection of multiple analytes

A novel and unusual 3D luminescent coordination polymer (CP) [Zn₂(3-bpah)(bpta)(H₂O)]·3H₂O (1), where 3-bpah denotes N,N'-bis(3-pyridinecarboxamide)-1,2-cyclohexane and H₄bpta denotes 2,2',4,4'-biphenyltetracarboxylic acid, was successfully synthesized via hydrothermal methods from Zn(II) ions and 3-bpah and bpta ligands. The structure of this CP was investigated via powder X-ray diffraction (PXRD) analysis along with single crystal X-ray diffraction. Notably, 1 exhibits remarkable fluorescence behavior and stability over a wide pH range and in various pure organic solvents. More importantly, 1 can become an outstanding candidate for the selective and sensitive sensing of Fe³⁺, Mg²⁺, Cr₂O₇^2-, MnO₄^-, nitrobenzene (NB) and nitromethane (NM), at an extremely low detection limit. The changes in the fluorescence intensity exhibited by these six analytes in the presence of 1 over a wide pH range indicate that this polymer can be an excellent luminescent sensor. To the best of our knowledge, 1 is a rare example of a CP-based multiresponsive fluorescent sensor for metal cations, anions, and toxic organic solvents.

Development of QDs-based nanosensors for heavy metal detection: A review on transducer principles and in-situ detection

Heavy metal pollution has severe threats to the ecological environment and human health. Thus, it is urgent to achieve the rapid, selective, sensitive and portable detection of heavy metal ions. To overcome the defects of traditional methods such as time-consuming, low sensitivity, high cost and complicated operation, QDs (Quantum dots)-based nanomaterials have been used in sensors to significantly improve the sensing performance. Due to their excellent physicochemical properties, high specific surface area, high adsorption and reactive capacity, nanomaterials could act as potential probes or offer enhanced sensitivity and create a promising nanosensors platform. In this review, the rapidly advancing types of QDs for heavy metal ions detection are first summarized. Modified with ligands, nanomaterials, or biomaterials, QDs are assembled on sensors by the interaction of electrostatic adsorption, chemical bonding, steric hindrance, and base-pairing. The stability of QDs-based nanosensors is improved by doping the elements to QDs, providing the reference substance, optimizing the assemble strategies and so on. Then, according to transducer principles, the two most typical sensor categories based on QDs: optical and electrochemical sensors are highlighted to be discussed. In the meanwhile, portable devices combining with QDs to adapt the practical detection in complex situations are summarized. The deficiencies and future challenges of QDs in toxicity, specificity, portability, multi-metal co-detection and degradation during the detection are also pointed out. In the end, the development trends of QDs-based nanosensors for heavy metal ions detection are discussed. This review presents an overall understanding, recent advances, current challenges and future outlook of QDs-based nanosensors for heavy metal detection.

Synthesis of Highly Near-Infrared Fluorescent Graphene Quantum Dots Using Biomass-Derived Materials for In Vitro Cell Imaging and Metal Ion Detection

Graphene quantum dots (GQDs) are a subset of fluorescent nanomaterials that have gained recent interest due to their photoluminescence properties and low toxicity and biocompatibility features for bioanalysis and bioimaging. However, it is still a challenge to prepare highly near-infrared (NIR) fluorescent GQDs using a facile pathway. In this study, NIR GQDs were synthesized from the biomass-derived organic molecule cis-cyclobutane-1,2-dicarboxylic acid via one-step pyrolysis. The resulting GQDs were then characterized by various analytical methods such as UV-Vis absorption spectroscopy, fluorescence spectroscopy, dynamic light scattering, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Moreover, the photostability and stability over a wide pH range were also investigated, which indicated the excellent stability of the prepared GQDs. Most importantly, two peaks were found in the fluorescence emission spectra of the GQDs, one of which was located in the NIR region of about 860 nm. Finally, the GQDs were applied for cell imaging with human breast cancer cell line, MCF-7, and cytotoxicity analysis with mouse macrophage cell line, RAW 246.7. The results showed that the GQDs entered the cells through endocytosis on the fluorescence images and were not toxic to the cells up to a concentration of 200 μg/mL. Thus, the developed GQDs could be a potential effective fluorescent bioimaging agent. Finally, the GQDs depicted fluorescence quenching when treated with mercury metal ions, indicating that the GQDs could be used for mercury detection in biological samples as well.

Quantum Dots Based Fluorescent Probe for the Selective Detection of Heavy Metal Ions

Heavy metal ions are one of the primary causes of environmental pollution. A marshal effect of heavy metal ions is a paramount ultimatum to humans, aquatic animals and other organisms present in nature. Multitude arrays of materials have been proclaimed for sensing of heavy metal ions and also many methodologies are applied for heavy metal ion sensing. Due to their toxicity and non-biodegradability, it is required to be perceived immediately prior to its manifestation of harmful effects. Quantum Dots (QDs) are zero-dimensional nanomaterial particles and owing to their distinctive optical and electronic properties, they are utilized as nanosensors. QDs have enriched fluorescence properties which includes broad excitation spectrum, narrow emission spectrum and photostability. QDs offer eclectic and sensitive detection of heavy metal ions due to presence of discrete capping agents and different functional groups present on the surface of the QDs. These capping layers and functional groups attune the sensing capability of the QDs, which leverages the interactions of QDs with various analytes by different mechanisms. This review, comprising of papers from 2011 to 2020,focuses on heavy metal ions sensing potential of various quantum dots and its applicability as a nanosensor for on field heavy metal ions detection in water. Quantum Dots (QDs) based Heavy Metal Detection.

A Review on Biosensors and Nanosensors Application in Agroecosystems

Previous decades have witnessed a lot of challenges that have provoked a dire need of ensuring global food security. The process of augmenting food production has made the agricultural ecosystems to face a lot of challenges like the persistence of residual particles of different pesticides, accretion of heavy metals, and contamination with toxic elemental particles which have negatively influenced the agricultural environment. The entry of such toxic elements into the human body via agricultural products engenders numerous health effects such as nerve and bone marrow disorders, metabolic disorders, infertility, disruption of biological functions at the cellular level, and respiratory and immunological diseases. The exigency for monitoring the agroecosystems can be appreciated by contemplating the reported 220,000 annual deaths due to toxic effects of residual pesticidal particles. The present practices employed for monitoring agroecosystems rely on techniques like gas chromatography, high-performance liquid chromatography, mass spectroscopy, etc. which have multiple constraints, being expensive, tedious with cumbersome protocol, demanding sophisticated appliances along with skilled personnel. The past couple of decades have witnessed a great expansion of the science of nanotechnology and this development has largely facilitated the development of modest, quick, and economically viable bio and nanosensors for detecting different entities contaminating the natural agroecosystems with an advantage of being innocuous to human health. The growth of nanotechnology has offered rapid development of bio and nanosensors for the detection of several composites which range from several metal ions, proteins, pesticides, to the detection of complete microorganisms. Therefore, the present review focuses on different bio and nanosensors employed for monitoring agricultural ecosystems and also trying to highlight the factor affecting their implementation from proof-of-concept to the commercialization stage.

GaOOH-modified metal-organic frameworks UiO-66-NH2: Selective and sensitive sensing four heavy-metal ions in real wastewater by electrochemical method

Heavy metal pollution in the environment poses a serious threat to the ecosystem and human health, which has attracted widespread attention. In this study, an octahedral structure composite composed of UiO-66-NH₂ MOFs and semiconductor GaOOH materials has been prepared and used as electrode materials successfully. These composites can be used for the real-time and online determination of Cd²⁺, Cu²⁺, Hg²⁺, and Pb²⁺ in real water samples simultaneously or alone via an electrochemical method. Zr-MOF has a large and unique surface area that is beneficial to the adsorption and preconcentration of heavy metal ions. The experiment parameters such as pH, deposition potential, and deposition time were optimized. Under the optimized conditions, the electrochemical performances and practical applications of Zr-MOF composites modified electrode have been investigated, which shows excellent wider linear range and lower detection limit (LOD). The results demonstrated excellent selectivity, reproducibility, stability and applicability for the detection of four metal ions. These superior features stem from the synergistic reaction mechanism of UiO-66-NH₂ and GaOOH. In addition, it has been established a new detection strategy for heavy metal ions through the form of metal-organic framework (MOF) composite in this work. It may provide a novel platform for the quantitative determination of heavy metal ions in various environmental samples.

Advances in aptamer screening and aptasensors' detection of heavy metal ions

Heavy metal pollution has become more and more serious with industrial development and resource exploitation. Because heavy metal ions are difficult to be biodegraded, they accumulate in the human body and cause serious threat to human health. However, the conventional methods to detect heavy metal ions are more strictly to the requirements by detection equipment, sample pretreatment, experimental environment, etc. Aptasensor has the advantages of strong specificity, high sensitivity and simple preparation to detect small molecules, which provides a new direction platform in the detection of heavy metal ions. This paper reviews the selection of aptamers as target for heavy metal ions since the 21th century and aptasensors application for detection of heavy metal ions that were reported in the past five years. Firstly, the selection methods for aptamers with high specificity and high affinity are introduced. Construction methods and research progress on sensor based aptamers as recognition element are also introduced systematically. Finally, the challenges and future opportunities of aptasensors in detecting heavy metal ions are discussed.

Carbon Nanomaterials: Synthesis, Functionalization and Sensing Applications

Recent advances in nanomaterial design and synthesis has resulted in robust sensing systems that display superior analytical performance. The use of nanomaterials within sensors has accelerated new routes and opportunities for the detection of analytes or target molecules. Among others, carbon-based sensors have reported biocompatibility, better sensitivity, better selectivity and lower limits of detection to reveal a wide range of organic and inorganic molecules. Carbon nanomaterials are among the most extensively studied materials because of their unique properties spanning from the high specific surface area, high carrier mobility, high electrical conductivity, flexibility, and optical transparency fostering their use in sensing applications. In this paper, a comprehensive review has been made to cover recent developments in the field of carbon-based nanomaterials for sensing applications. The review describes nanomaterials like fullerenes, carbon onions, carbon quantum dots, nanodiamonds, carbon nanotubes, and graphene. Synthesis of these nanostructures has been discussed along with their functionalization methods. The recent application of all these nanomaterials in sensing applications has been highlighted for the principal applicative field and the future prospects and possibilities have been outlined.

Nanomaterial-based fluorescent sensors for the detection of lead ions

Lead (Pb) poisoning has been a scourge to the human to pose sighnificant health risks (e.g., organ disorders, carcinogenicity, and genotoxicity) as observed from many different parts of the world, especially in developing countries. The demand for accurate sensors for its detection, especially in environmental media (soil, water, food, etc.) has hence been growing steadily over the years. The potential utility of fluorescent nanosensors as an important analytical tool is recognized due to their astonishing characteristics (e.g., high sensitivity/selectivity, enhanced detection performance, low cost, portability, and rapid on-site detection ability). This review is organized to offer insight into the recent developments in fluorescent nanosensing technology for the detection of lead ions (Pb²⁺). To this end, different types of nanomaterials explored for such applications have been classified and evaluated with respect to performance, especially in terms of sensitivity. This review will help researchers gain a better knowledge on the status and importance of optical nanosensors so as to remediate the contamination of lead and associated problems. The technical challenges and prospects in the development of nanosensing systems for Pb²⁺ are also discussed.

Fabrication of target specific solid-state optical sensors using chromoionophoric probe-integrated porous monolithic polymer and silica templates for cobalt ions

The article demonstrates the design of two solid-state sensors for the capturing of industrially relevant ultra-trace Co(II) ions using porous monolithic silica and polymer templates. The mesoporous silica reveals high surface area and voluminous pore dimensions that ensures homogeneous anchoring of 4-((5-(allylthio)-1,3,4-thiadiazol-2-yl)diazenyl)benzene-1,3-diol, as the chromoionophore. We report a first of its kind solid-state macro-/meso-porous polymer monolithic optical sensor from a monomeric chromoionophore, i.e., 2-(4-butylphenyl)diazenyl)-2-hydroxybenzylidene)hydrazine-1-carbothioamide. The monolithic solid-state sensors are characterized using HR-TEM-SAED, FE-SEM-EDAX, p-XRD, XPS, ²⁹Si/¹³C CPMAS NMR, FT-IR, TGA, and BET/BJH analysis. The electron microscopic images reveal a highly ordered hexagonal mesoporous network of honeycomb pattern for silica monolith, and a long-range macroporous framework with mesoporous channels for polymer monolith. The sensors offer exclusive ion-selectivity and sensitivity for trace cobalt ions, through a concentration proportionate visual color transition, with a response kinetics of ≤ 5 min. The optimization of ion-sensing performance reveals an excellent detection limit of 0.29 and 0.15 ppb for Co(II), using silica- and polymer-based monolithic sensors, respectively. The proposed sensors are tested with industrial wastewater and spent Li-ion batteries, which reveals a superior cobalt ion capturing efficiency of ≥ 99.2% (RSD: ≤ 2.07%).

Challenges and potential solutions for nanosensors intended for use with foods

Nanotechnology-adapted detection technologies could improve the safety and quality of foods, provide new methods to combat fraud and be useful tools in our arsenal against bioterrorism. Yet despite hundreds of published studies on nanosensors each year targeted to the food and agriculture space, there are few nanosensors on the market in this area and almost no nanotechnology-enabled methods employed by public health agencies for food analysis. This Review shows that the field is currently being held back by technical, regulatory, political, legal, economic, environmental health and safety, and ethical challenges. We explore these challenges in detail and provide suggestions about how they may be surmounted. Strategies that may have particular effectiveness include improving funding opportunities and publication venues for nanosensor validation, social science and patent landscape studies; prioritizing research and development of nanosensors that are specifically designed for rapid analysis in non-laboratory settings; and incorporating platform cost and adaptability into early design decisions.

Recent progresses in luminescent metal-organic frameworks (LMOFs) as sensors for the detection of anions and cations in aqueous solution

The discharge of excessive metal ions and anions into water bodies leads to the serious pollution of water and environment, which in turn has a certain impact on industry, agriculture, and human life. Because of the unique advantages of luminescent metal-organic frameworks (LMOFs), they have been successfully explored as various fluorescent probes to quickly and effectively detect these pollutants. This perspective not only introduces the design strategy and classification of LMOFs, especially the construction methods of water-stable LMOFs, but also reports the latest progresses in some LMOFs between 2016 and 2020 as well as expounds the mechanisms of LMOFs for detecting anions and cations. Moreover, the luminescence properties of LMOFs are related to the selection of metal ions, the structure of organic ligands, the pore size, and the interaction of guest molecules. Finally, the further development of LMOFs is summarized and prospected in this field.

UiO series of metal-organic frameworks composites as advanced sorbents for the removal of heavy metal ions: Synthesis, applications and adsorption mechanism

Heavy metal pollution has threatened the ecological environment and human health, therefore, effective removal of these toxic pollutants from various complex substrates is of great significance. So far, adsorption is still one of the most effective approaches. Metal-organic frameworks (MOFs), which are porous crystalline materials consisting of metal ions or metal clusters and organic ligands through coordination bonds. Due to their high surface area, porosity, as well as good chemical/thermal stability, the materials have recently attracted great attention in environmental analytical chemistry. This review mainly focused on the recent studies about the applications of UiO series MOFs and their composites as the emerging MOFs, which have been used effectively for the adsorption and removal of diverse heavy metal ions from a variety of environmental samples as novel adsorption materials. Moreover, an elaboration about UiO-MOFs and its composites including the synthetic methods and the applications of these materials in the removal of heavy metal ions were presented in detail. In addition, the adsorption characteristics and mechanism of UiO-MOFs as solid sorbents for heavy metal ions were discussed, including adsorption isotherms equation, adsorption thermodynamics, and kinetics. To this end, the developing trends of MOF-based composites for the removal of heavy metal ions had also prospected. This review will provide a new idea for the study of the adsorption mechanism of heavy metal ions on sorbents and the development of high-performance media for the efficient removal of pollutants in wastewater.

Fluorescent multi-component polymer sensors for the sensitive and selective detection of Hg2+/Hg+ ions via dual mode fluorescence and colorimetry

Fluorescent multi-component polymers, which are sensitive and selective to Hg2+/Hg+ through fluorescence and colorimetry, were synthesized by the Heck coupling reaction.

Simultaneous quantitative analysis of K+ and Tl+ in serum and drinking water based on UV-Vis spectra and chemometrics

The simultaneous detection of K⁺ and Tl⁺ can serve as a toxicological diagnostic tool for thallium poisoning. Colorimetric-reaction-based nanoprobes have emerged as promising sensors for the rapid and ultrasensitive detection of molecular species in simple systems. However, the development of viable screening tools for multicomponent analysis in complex systems remains challenging owing to interference from coexisting materials in the media. Herein, a simple chemical sensor array based on the peroxidase-like activity of gold nanoparticles modified with single-stranded DNA (AuNPs-ssDNA) and chemometrics was developed for the simultaneous detection of K⁺ and Tl⁺ in aqueous solutions and serum. The use of a K⁺ adapter conferred high selectivity to the developed method. Optimized AuNPs-ssDNAs were used to construct a sensor array, which together with chemometrics provided fingerprints that can facilitate the simultaneous analysis of multiple components. The developed colorimetric reaction in combination with the chemometrics assay was directly used as a biosensor array, which exhibited detection limits of 107.33 nM for K⁺ and 19.26 nM for Tl⁺. The developed method could potentially serve as a diagnostic technique for investigating thallium poisoning and toxicology.

Simultaneous determination of heavy metals by an electrochemical method based on a nanocomposite consisting of fluorinated graphene and gold nanocage

Fluorinated graphene/gold nanocage (FGP/AuNC) nanocomposite was developed for simultaneous determination of heavy metals using square wave anodic stripping voltammetry. Under optimized conditions, with a buffer pH of 5.0, a deposition potential of - 1.25 V, and a deposition time of 140 s, the method can obtain the best results. The FGP/AuNC electrode exhibits low limits of detection (0.08, 0.09, 0.05, 0.19, 0.01 μg L^-1), wide linear ranges (6-7000, 4-6000, 6-5000, 4-4000, 6-5000 μg L^-1), and well-separated stripping peaks (at - 1.10, - 0.77, - 0.50, - 0.01, 0.31 V vs Ag/AgCl) towards Zn²⁺, Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺, respectively. Furthermore, the FGP/AuNC electrode is also used for simultaneous determination of Zn²⁺, Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ in real samples (peanut, rape bolt, and tea). Highly consistent results are found between the electrochemical method and atomic fluorescence spectrometry/inductively coupled plasma-mass spectrometry. The method has been successfully applied to the determination of heavy metal ions in agricultural food. Graphical abstract Schematic representation of simultaneous determination of heavy metal ions by electrochemical method. The FGP/AuNC (fluorinated graphene/gold nanocage) electrode is used to simultaneous determination of Zn²⁺, Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ by square wave anode stripping voltammetry.

Graphene and Graphene Oxide Applications for SERS Sensing and Imaging

Surface Enhanced Raman Spectroscopy (SERS) has a long history as an ultrasensitive platform for the detection of biological species from small aromatic molecules to complex biological systems as circulating tumor cells. Thanks to unique properties of graphene, the range of SERS applications has largely expanded. Graphene is efficient fluorescence quencher improving quality of Raman spectra. It contributes also to the SERS enhancement factor through the chemical mechanism. In turn, the chemical flexibility of Reduced Graphene Oxide (RGO) enables tunable adsorption of molecules or cells on SERS active surfaces. Graphene oxide composites with SERS active nanoparticles have been also applied for Raman imaging of cells. This review presents a survey of SERS assays employing graphene or RGO emphasizing the improvement of SERS enhancement brought by graphene or RGO. The structure and physical properties of graphene and RGO will be discussed too.

Recovery of nickel ions from wastewater by precipitation approach using silica xerogel

A facile route was developed to recover nickel ions from a synthetic wastewater. It involved the use of silica xerogel containing amine in the nickel sulphate solution resulting in the formation of a greenish precipitate. It was found that this precipitate was mostly amorphous Ni(OH)₂ spherical aggregate composed of nanosheets. The pH level of the solution was monitored, and it was maintained in the range of 10-10.5 due to the steady release of amine from the xerogel into the waste solution. The prepared silica xerogel would provide a stable environment for the chemical precipitation of metal ions in wastewater during the whole precipitation process. The silica xerogel was collected and reused for two more cycles of recovery. The nickel removal efficiencies (99.34˜99.65%) kept unchanged and higher than those reported earlier. The collected precipitate that contained nickel hydroxide with some residual silica could be utilized as glass colorant.

Development of Graphene Quantum Dots-Based Optical Sensor for Toxic Metal Ion Detection

About 71% of the Earth's surface is covered with water. Human beings, animals, and plants need water in order to survive. Therefore, it is one of the most important substances that exist on Earth. However, most of the water resources nowadays are insufficiently clean, since they are contaminated with toxic metal ions due to the improper disposal of pollutants into water through industrial and agricultural activities. These toxic metal ions need to be detected as fast as possible so that the situation will not become more critical and cause more harm in the future. Since then, numerous sensing methods have been proposed, including chemical and optical sensors that aim to detect these toxic metal ions. All of the researchers compete with each other to build sensors with the lowest limit of detection and high sensitivity and selectivity. Graphene quantum dots (GQDs) have emerged as a highly potential sensing material to incorporate with the developed sensors due to the advantages of GQDs. Several recent studies showed that GQDs, functionalized GQDs, and their composites were able to enhance the optical detection of metal ions. The aim of this paper is to review the existing, latest, and updated studies on optical sensing applications of GQDs-based materials toward toxic metal ions and future developments of an excellent GQDs-based SPR sensor as an alternative toxic metal ion sensor.