Determination of trace heavy metal ions by anodic stripping voltammetry using nanofibrillated cellulose modified electrode

A Prompt Study on Recent Advances in the Development Of Colorimetric and Fluorescent Chemosensors for "Nanomolar Detection" of Biologically Important Analytes

Fluorescent and colorimetric chemosensors for selective detection of various biologically important analytes have been widely applied in different areas such as biology, physiology, pharmacology, and environmental sciences. The research area based on fluorescent chemosensors has been in existence for about 150 years with the development of large number of fluorescent chemosensors for selective detection of cations as metal ions, anions, reactive species, neutral molecules and different gases etc. Despite the progress made in this field, several problems and challenges still exist. The most important part of sensing is limit of detection (LOD) which is the lowest concentration that can be measured (detected) with statistical significance by means of a given analytical procedure. Although there are so many reports available for detection of millimolar to micromolar range but the development of chemosensors for the detection of analytes in nanomolar range is still a challenging task. Therefore, in our current review we have focused the history and a general overview of the development in the research of fluorescent sensors for selective detection of various analytes at nanomolar level only. The basic principles involved in the design of chemosensors for specific analytes, binding mode, photophysical properties and various directions are also covered here. Summary of physiochemical properties, mechanistic view and type of different chemosensors has been demonstrated concisely in the tabular forms.

Green synthesis of copper oxide nanoparticles using Ficus elastica extract for the electrochemical simultaneous detection of Cd2+, Pb2+, and Hg2

In this paper, for the first time, we report the use of a new carbon paste electrode based on a low-cost pencil graphite powder modified with polyaniline (PANI) and green synthesized copper oxide nanoparticles using Ficus elastica extract as a sensor for Cd²⁺, Pb²⁺, and Hg²⁺. The elaborated electrode was characterized by FT-IR spectroscopy, field-emission gun scanning electron microscopy (FEG-SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and simultaneous thermal analysis (TGA/DSC). The electrochemical behavior of the sensor was evaluated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy techniques. According to CV, as well as square wave voltammetry (SWV) results, it was found that the CuONPs/PANI-CPE sensor was able to determine very low concentrations of Cd²⁺, Pb²⁺, and Hg²⁺ in HCl (0.01 M) either in single metal or in multi-metal solutions with a high sensitivity. Furthermore, Cd²⁺, Pb²⁺, and Hg²⁺ simultaneous detection on CuONPs/PANI-CPE achieved very low limits of detection (0.11, 0.16, and 0.07 μg L^-1, respectively). Besides, the designed sensor displayed a good selectivity, reproducibility, and stability. Moreover, CuONPs/PANI-CPE enabled us to determine with high accuracy Cd²⁺, Pb²⁺, and Hg²⁺ traces in environmental matrices.

Real scenario of metal ion sensor: is conjugated polymer helpful to detect hazardous metal ion

Metal ions from natural and anthropogenic sources cause pollution to society and the environment is major concern in the present scenario. The deposition and contamination of metal ions in soil and water affect the biogeochemical cycles. Thus, it threatens the everyday life of living and non-living organisms. Reviews on the detection of metal ions through several techniques (Analytical methods, electrochemical techniques, and sensors) and materials (Nanoparticles, carbon dots (quantum dots), polymers, chiral molecules, metal-organic framework, carbon nanotubes, etc.) are addressed separately in the present literature. This review reveals the advantages and disadvantages of the techniques and materials for metal ion sensing with crucial factors. Furthermore, it focus on the capability of conjugated polymers (CPs) as metal ion sensors able to detect/sense hazardous metal ions from environmental samples. Six different routes can synthesize this type of CPs to get specific properties and better metal ion detecting capability in vast research areas. The metal ion detection by CP is time-independent, simple, and low cost compared to other materials/techniques. This review outlines recent literature on the conjugated polymer for cation, anion, and dual ion sensors. Over the last half decades published articles on the conjugated polymer are discussed and compared.

Nanocellulose-based sensing platforms for heavy metal ions detection: A comprehensive review

Increase in industrial activities has been arising a severe concern about water pollution caused by heavy metal ions (HMIs), such us lead (Pb²⁺), cadmium (Cd²⁺) or mercury (Hg²⁺). The presence of substantial amounts of these ions in the human body is harmful and can cause serious diseases. Hence, the detection of HMIs in water is of great importance. As technological advances have developed, some conventional methods have become obsolete due to some methodological disadvantages, giving way to a second generation that uses novel sensors. Recently, nanocellulose, as a biocompatible material, has drawn a remarkable attraction for developing sensors owing to its extraordinary physical and chemical properties. This review pays a special attention to the different dimensional nanocellulose-based sensors devised for HMIs recognition. What is more, different sensing techniques (optical and electrochemical), sensing mechanisms and the roles of nanocellulose in such sensors are discussed.

Nanocellulose as a promising substrate for advanced sensors and their applications

Nanocellulose has broad and promising applications owing to its low density, large specific surface area, high mechanical strength, modifiability, renewability. Recently, nanocellulose has been widely used to fabricate flexible, durable and environmental-friendly sensor substrates. In this contribution, the construction and characteristics of nanocellulose-based sensors are comprehensively reviewed. Various nanocellulose-based sensors are summarized and divided into colorimetric, fluorescent, electronic, electrochemical and SERS types according to the sensing mechanism. This review also introduces the applications of nanocellulose-based sensors in the fields of biomedicine, environmental monitoring, food safety, and wearable devices.

A Review on Conducting Polymers for Colorimetric and Fluorescent Detection of Noble Metal Ions (Ag+, Pd2+, Pt2+/4+, and Au3+)

Conducting polymers (CPs) are conductive materials composed of organic polymers. CPs have excellent properties such as easy synthesis and effortless fabrication, tunable electrical property, high environmental stability, high mechanical and optical properties. These unique properties have attracted researchers to discover a wide variety of uses, such as batteries, solar cells, sensors, supercapacitors, electrochromic devices, and biochemical applications. Although CPs have many limitations in their pristine form, hybridization with other materials overcomes these limitations. Here in this review article, we discuss different CPs based chemosensors for colorimetric and fluorimetric detection and determination of noble metal ions (Ag+, Pd2+, Pt2+/4+, and Au3+) in different environmental, agricultural, and biological samples. Further, the sensing performances of these chemosensors have been compared and discussed. We hope this article will help the readers with the future design of CPs based optical sensor (colorimetric and fluorescent) for detecting noble metal cations.

Characterization of Neodymium Speciation in the Presence of Fulvic Acid by Ion Exchange Technique and Single Particle ICP-MS

It has been well known that the free ion concentration of metals plays a vital role in metal bioavailability. However, measurement of this fraction is still not easy over years of development. Nowadays, rare earth elements (REEs) are drawing more attentions as an emerging contaminant due to their wide applications in our daily life. To analyze the free ion concentration of neodymium (Nd), we adopted ion-exchange technique (IET) to investigate the changes on Nd free ion concentration in the presence of fulvic acid (FA). With the dynamic mode of IET analysis, the concentrations of Nd free ion were in the range of 0.85-36.8 × 10^-8 M at the total Nd concentration of 5 × 10^-7 M when FA varied from 0.4 to 10 M. However, these concentrations were 3-58 times higher than the one calculated by WHAM 7.0, which may be due to the particulate Nd spontaneously formed in solution. With single particle ICP-MS analysis, we found 0.25%-2.36% of Nd was in the form of colloids when the total Nd concentrations varied from 8.5 × 10^-9 to 4.7 × 10^-7 M, with the average particle sizes in the range of 26.5-39.2 nm. The presence of FA significantly decreased the number of Nd colloids, but increased the average particle size. Under the TEM, we found that Nd colloids were amorphous, with the size less than 200 nm. The present study provided a relatively new perspective on REE speciation in water. The natural organic matters not only affect the free ion concentration of Nd, but also influenced the size and numbers of Nd colloids in solution.

An innovative autonomous robotic system for on-site detection of heavy metal pollution plumes in surface water

Smart monitoring has been studied and developed in recent years to create faster, cheaper, and more user-friendly on-site methods. The present study describes an innovative technology for investigative monitoring of heavy metal pollution (Cu and Pb) in surface water. It is composed of an autonomous surface vehicle capable of semiautonomous driving and equipped with a microfluidic device for detection of heavy metals. Detection is based on the method of square wave anodic stripping voltammetry using carbon-based screen-printed electrodes (SPEs). The focus of this work was to validate the ability of the integrated system to perform on-site detection of heavy metal pollution plumes in river catchments. This scenario was simulated in laboratory experiments. The main performance characteristics of the system, which was evaluated based on ISO 15839 were measurement bias (Pb 75%, Cu 65%), reproducibility (in terms of relative standard deviation: Pb 11-18%, Cu 6-10%) and the limit of detection (4 µg/L for Pb and 7 µg/L for Cu). The lowest detectable change (LDC), which is an important performance characteristic for this application, was estimated to be 4-5 µg/L for Pb and 6-7 µg/L for Cu. The life span of an SPE averaged 39 measurements per day, which is considered sufficient for intended monitoring campaigns. This work demonstrated the suitability of the integrated system for on-site detection of Pb and Cu emissions from large and medium urban areas discharging into small water bodies.

Chromone functionalized pyridine chemosensor for cupric ions detection

A new Schiff base 2-ethoxy-3-{[(6-{[(2-ethoxy-4-hydroxy-2H-chromen-3-yl)methylidene]amino}pyridine-2-yl)imino]methyl}-2H-chromen-4-ol (CD) was synthesized as a result of the condensation of 2,6-diaminopyridine and 3-formyl chromone in 1:2 M ratio and used for cupric ions detection and characterized through FTIR, HRMS and ¹H NMR spectral techniques. The sensing capability of Schiff base for cupric ions as compared to other transition metal ions was examined by absorbance and emission studies. A considerable decrease in emission intensity appeared in Schiff base in the case of cupric ions while irrelevant changes were examined for the rest of the ions. The binding stoichiometry was obtained as 1:2 for CD: Cu²⁺ complex intended from the job's plot which was confirmed through HRMS spectral technique. DFT calculations were carried for the confirmation of structural relationships and absorption-emission data. The Regression coefficient, Limit of detection, and Association constant were obtained as 98.7%, 1.2 × 10^-6 M, and 3.26 × 10⁴ M^-1 respectively using Benesi-Hildebrand (B-H) equation. The sensing power of Schiff base CD to recognize cupric ions was unaltered by the addition of the rest of metal ions, which was authenticated through interference studies. Schiff base CD and its complex with cupric ions were found stable over an extensive time period as revealed by time-reliant studies. The data collected by pH studies revealed that the preferred pH range for detecting cupric ions by Schiff base CD was 6 to 11. The Schiff base was finally utilized for sensing cupric ions in a variety of spiked samples of water like canal water, tap water, groundwater, distilled water.

A review on metal nanozyme-based sensing of heavy metal ions: Challenges and future perspectives

Large scale mining, manufacturing industries, exploitation of underground water, depletion of groundwater level, and uncontrolled discharge of industrial wastes have caused severe heavy metal ion pollution to the environment throughout the world. Therefore, the rapid detection of such toxic metal ions is inevitable. However, conventional methods require sophisticated instruments and skilled manpower and are difficult to operate in on-field conditions. Recently, metal nanozyme-based assays have been found to have the potential as an alternative to conventional methods due to their portability, simplicity, and high sensitivity to detect metal ion concentration to as low as parts per trillion (ppt). Metal nanozyme-based systems for heavy metal ions enable rapid and cheap screening on the spot with a very simple instrument such as a UV-vis absorption spectrophotometer and therefore, are convenient for use in field operations, especially in remote parts of the world. The sensing mechanism of a nanozyme-based sensor is highly dependent on its surface properties and specific interactions with particular metal ion species. Such method often encounters selectivity issues, unlike natural enzyme-based assays. Therefore, in this review, we mainly focus our discussion on different types of target recognition and inhibition/enhancement mechanisms, and their responses toward the catalytic activity in the sensing of target metal ions, design strategies, challenges, and future perspectives.

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.

Preparation of Ag-cellulose nanocomposite for the selective detection and quantification of mercury at nanomolar level and the evaluation of its photocatalytic performance

Mercury is a toxic heavy metal that reaches to the water bodies mainly by coal burning, mining and petrol refining. The study was focused to investigate the application of Ag-cellulose nanocomposite to detect and quantify mercury colorimetrically. The Ag-cellulose nanocomposite was characterized by X-ray diffraction, Transmission electron microscopy, Fourier transform infrared spectroscopy, UV-visible spectroscopy, particle size analyzer and zetasizer. The study identified that the presence of other metal ions did not interfere with the detection of Hg²⁺ ion by the probe. The prepared Ag-cellulose nanocomposite-phenylalanine conjugate incorporated paper strip showed an excellent result in Hg²⁺ detection. The Ag-cellulose nanocomposite was used to quantify the unknown concentration of mercury on real sample (environmental sample) and it was found to be highly accurate by confirming with atomic absorption spectrophotometric analysis. The Ag-cellulose nanocomposite showed effective detection at 45 °C, pH 9 and 0.1% of salinity. The Ag-cellulose nanocomposite showed efficient photocatalytic performance under visible light irradiation. The half-life period of MB by Ag-cellulose nanocomposite under visible light was determined to be 90 min. The study suggests the application of prepared probe in photocatalysis and the detection of Hg²⁺ from various environmental samples.

Determination of Trace Lead and Cadmium in Decorative Material Using Disposable Screen-Printed Electrode Electrically Modified with Reduced Graphene Oxide/L-Cysteine/Bi-Film

* Correspondence: wanghui_lunwen@163 [...].

Early warning sensors for monitoring mercury in water

Poly-4-vinylpyridine grafted poly(vinylidene difluoride) (P4VP-g-PVDF) nanoporous polymer electrodes were found to be sensitive for Hg(II) analysis. The fabrication and characterization of functionalized nanoporous membrane-electrodes by FESEM and FTIR are presented. Functionalized nanopore charge state versus a large range of pH (1-10) was investigated by registering the streaming potential. This isoelectric point is achieved at the pKa of P4VP (pH = 5). Mercury adsorption at solid-liquid interface obeys a Langmuir law. A protocol for accurate Hg(II) analysis at ppb level was established. Calibration curves were performed and different real water samples (mineral water, ground water, surface water) were spiked and analyzed. The resulting sensor is intended to be integrated into existing systems or used standalone as portable devices. A first generation prototype exhibiting its own integrated potentiostat, its software and set of membrane-electrode pads is presented.

Tracking metal ions with polypyrrole thin films adhesively bonded to diazonium-modified flexible ITO electrodes

Adhesively bonded polypyrrole thin films doped with benzene sulfonic acid (BSA) were electrodeposited on aminobenzenediazonium-modified flexible ITO electrodes and further employed for the detection of Pb²⁺, Cu²⁺, and Cd²⁺ metal ions in aqueous medium. The aminophenyl (AP) adhesive layer was grafted to ITO by electroreduction of the in situ generated parent diazonium compound. Polypyrrole (PPy) thin films exhibited remarkable adhesion to aminophenyl (ITO-AP). The strongly adherent polypyrrole films exhibited excellent electroactivity in the doped state with BSA which itself served to chelate the metal ions in aqueous medium. The surface of the resulting, modified flexible electrode was characterized by XPS, SEM, and electrochemical methods. The ITO-AP-PPy electrodes were then used for the simultaneous detection of Cu²⁺, Cd²⁺, and Pb²⁺ by differential pulse voltammetry (DPV). The detection limits were 11.1, 8.95, and 0.99 nM for Cu²⁺, Cd²⁺, and Pb²⁺, respectively. In addition, the modified electrodes displayed a good reproducibility, making them suitable for the determination of heavy metals in real wastewater samples.

Comparison of backing materials of screen printed electrochemical sensors for direct determination of the sub-nanomolar concentration of lead in seawater

An anodic stripping voltammetric method is reported in this study for the determination of sub-nanomolar Pb concentration using disposable sensors, each consisting of three (counter, working and reference) screen-printed electrodes. Sensor performance was optimized for the determination of Pb through several surface modifications, by using single-walled carbon nanotubes, electro-reduced graphene oxide and gold nanoparticles. A scanning electron microscopy study of the deposition of electrogenerated gold nanoparticles of various sizes on the working electrode surface showed that spherical nanoparticles of around 100 nm provided the best results. The modification of working electrodes with graphene and gold nanoparticles permitted the determination of Pb²⁺ in seawater (Detection Limit: 3.21·10^-10 M) without modifying the pH of the sample. The electrode systems were printed on both rigid and textile backing materials, to observe the influence of those materials on the final performance of the sensor.