Magnetic nanoparticle based solid-phase extraction of heavy metal ions: A review on recent advances

Magnetic Solid Phase Extraction as a Promising Technique for Fast Separation of Metallic Nanoparticles and Their Ionic Species: A Review of Recent Advances

The widespread use of silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) in a wide variety of industrial as well as medical sectors is indisputable. This leads to a new concern about their presence in various environmental compartments. Since their negative effect and potential toxicity impact have been confirmed, analytical chemists focus on the development of different procedures for their reliable detection, identification, characterization, and quantification, not only in homogenous and simple matrices but also in complex environmental matrices. However, nanoparticles and their ionic species can coexist and their toxicity may differ; therefore, novel analytical approaches are necessary to monitor not only the nanoparticles but also their ionic species. The aim of this article is to bring a review of recent works where magnetic solid-phase extraction (MSPE) procedures in connection with spectrometric methods were used for separation/preconcentration and quantification of (1) silver and gold ions in various environmental samples, (2) AgNPs and AuNPs in real water samples in the presence of various coexisting ions, and (3) both species (it means Ag ions and AgNPs; Au ions and AuNPs) in real water samples. The results presented herein show the great analytical potential of MSPE procedures in connection with spectrometric methods used in these fields and can be helpful in guiding analytical chemists who aim to work on this subject.

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.

Chitosan/thiol functionalized metal-organic framework composite for the simultaneous determination of lead and cadmium ions in food samples

In this work, a facile solid phase extraction (SPE) method was developed for the analysis of trace Pb²⁺ and Cd²⁺ by using chitosan/thiol modified metal-organic frameworks (CS/MOF-SH) composite as adsorbent followed by graphite furnace atomic absorption spectrometer (GF-AAS) detection. The potential influencing factors, such as solution pH, adsorbent dosage, and extraction time, were fully estimated. Under the optimized extraction conditions, the detection limits of Pb²⁺ and Cd²⁺ were 0.033 µg L^-1 and 0.008 µg L^-1, respectively. Compared to other studies, CS/MOF-SH not only possessed superior adsorption performance, but also had the advantages of ease of handling and recyclability. Encouragingly, the developed method was of high accuracy and could monitor trace Pb²⁺ and Cd²⁺ in various certified reference materials (rice, wheat and tea) with complicated matrices, demonstrating its practical potential for regular monitoring of trace heavy metal ions in real food samples.

Magnetic Cu: CuO-GO nanocomposite for efficient dispersive micro-solid phase extraction of polycyclic aromatic hydrocarbons from vegetable, fruit, and environmental water samples by liquid chromatographic determination

In this research, we presented a magnetic dispersive micro-solid phase extraction (MD-μ-SPE) method coupled with high performance liquid chromatography (HPLC) based on the use of magnetic Cu: CuO-Graphene Oxide (GO) nanocomposite (Fe₃O₄/Cu: CuO/GO-NC) for the separation and preconcentration of polycyclic aromatic hydrocarbons (PAHs), i.e. naphthalene (Nap), phenanthrene (Phe), anthracene (Ant), and pyrene (Pyr), in vegetable (onion, tomato, carrot, herb, watermelon, lettuce, eggplant, and chili pepper), fruit (apple, watermelon, and grape), wastewater, and water samples. The MD-μ-SPE of PAHs in matrix samples was carried out, and the impacts of pH, ionic strength, extraction time, temperature, eluent volume, and sorbent mass on the recovery of PAHs were investigated by using Placket-Burman design (PBD). In addition, by using the central composite design (CCD), the best combination of each important variable was measured. Sorbent mass of 14 mg, eluent volume of 200 μL, and 12 min extraction time at the central level of other factors were optimal conditions of pretreatment for the highest extraction recovery (ER%) of trace PAHs. Under the optimal conditions, the method proposed herein provided high enrichment factors ranged from 116.51 to 133.05, good linearity in the range of 10-3800 ng mL^-1 for Pyr, 3.0-3500 ng mL^-1 for Phe, 5.0-3200 ng mL^-1 for Nap, and 5.0-3000 ng mL^-1 for Ant with coefficient of determination (R²) values between 0.9889 and 0.9963, low limits of detection (LOD) and quantification (LOQ) in the range of 0.015-0.061 and 0.485-2.034 ng mL^-1, respectively, and also satisfactory spiked recoveries (between 95.1% and 106.8%) with the relative standard deviations (RSDs) values in the range of 1.73%-5.62%. The Fe₃O₄/Cu: CuO/GO-NC-based MD-μ-SPE followed by HPLC-UV corroborated promising results for the convenient and effective determination of PAHs in the samples of vegetables, fruits, and environmental water. The results of this study revealed that our developed method is easy, feasible, precise, highly effective, and convenient to operate for the trace analysis of PAHs in different real samples. The extraction recovery was about 90% of the initial recovery after the sorbent usage for three times; therefore, the Fe₃O₄/Cu: CuO/GO-NC can readily be regenerated.

Dopamine-modified magnetic graphene oxide as a recoverable sorbent for the preconcentration of metal ions by an effervescence-assisted dispersive micro solid-phase extraction procedure

Nanomagnetic graphene oxide modified with dopamine (GO-Fe3O4-DA) was synthesized via a very simple procedure. Using GO-Fe3O4-DA as the new adsorbent, the effervescence-assisted dispersive micro solid-phase extraction procedure was exploited for the preconcentrative extraction of Cu(ii), Pb(ii) and Ni(ii) ions. Structural characteristics of the adsorbent were studied via FT-IR, FE-SEM, EDX and XRD analyses. The rapid dispersion and high adsorption capability of GO-Fe3O4-DA, along with the rapid separation of the adsorbent from the aqueous phase by a magnet, led to a decrease in the extraction time of the target metal ions. In effect, high extraction percentages were attained in a very short time period. In this work, the relative standard deviations (RSD; n = 3) calculated for the proposed method were 1.09, 1.25 and 1.03% for the Pb(ii), Cu(ii) and Ni(ii) ions, respectively, the calibration curve was dynamically linear in the range of 0.25 to 50 μg L-1, and the limits of detection were obtained as 0.5, 0.1, and 0.7 μg L-1. The procedure was also implemented on real sausage (herbal and meaty) samples and a water sample, vouchsafing the success of the proposed method in tackling real samples with a complicated matrix.

Recent Advances in the Extraction of Polycyclic Aromatic Hydrocarbons from Environmental Samples

Polycyclic aromatic hydrocarbons (PAHs) comprise a group of chemical compounds consisting of two or more fused benzene rings. PAHs exhibit hydrophobicity and low water solubility, while some of their members are toxic substances resistant to degradation. Due to their low levels in environmental matrices, a preconcentration step is usually required for their determination. Nowadays, there is a wide variety of sample preparation techniques, including micro-extraction techniques (e.g., solid-phase microextraction and liquid phase microextraction) and miniaturized extraction techniques (e.g., dispersive solid-phase extraction, magnetic solid-phase extraction, stir bar sorptive extraction, fabric phase sorptive extraction etc.). Compared to the conventional sample preparation techniques, these novel techniques show some benefits, including reduced organic solvent consumption, while they are time and cost efficient. A plethora of adsorbents, such as metal-organic frameworks, carbon-based materials and molecularly imprinted polymers, have been successfully coupled with a wide variety of extraction techniques. This review focuses on the recent advances in the extraction techniques of PAHs from environmental matrices, utilizing novel sample preparation approaches and adsorbents.

Recent Advances in Nanomaterials for Analysis of Trace Heavy Metals

In an effort to achieve high sensitivity analysis methods for ultra-trace levels of heavy metals, numerous new nanomaterials are explored for the application in preconcentration processes and sensing systems. Nanomaterial-based methods have proven to be effective for selective analysis and speciation of heavy metals in combination with spectrometric techniques. This review outlined the different types of nanomaterials applied in the field of heavy metal analysis, and concentrated on the latest developments in various new materials. In particular, the functionalization of traditional materials and the exploitation of bio-functional materials could increase the specificity to target metals. The hybridization of multiple materials could improve material properties, to build novel sensor system or achieve detection-removal integration. Finally, we discussed the future perspectives of nanomaterials in the heavy metal preconcentration and sensor design, as well as their respective advantages and challenges. Despite impressive progress and widespread attention, the development of new nanomaterials and nanotechnology is still hampered by numerous challenges, particularly in the specificity to the target and the anti-interference performance in complex matrices.

Magnetic Solid-Phase Extraction of Organic Compounds Based on Graphene Oxide Nanocomposites

Graphene oxide (GO) is a chemical compound with a form similar to graphene that consists of one-atom-thick two-dimensional layers of sp²-bonded carbon. Graphene oxide exhibits high hydrophilicity and dispersibility. Thus, it is difficult to be separated from aqueous solutions. Therefore, functionalization with magnetic nanoparticles is performed in order to prepare a magnetic GO nanocomposite that combines the sufficient adsorption capacity of graphene oxide and the convenience of magnetic separation. Moreover, the magnetic material can be further functionalized with different groups to prevent aggregation and extends its potential application. Until today, a plethora of magnetic GO hybrid materials have been synthesized and successfully employed for the magnetic solid-phase extraction of organic compounds from environmental, agricultural, biological, and food samples. The developed GO nanocomposites exhibit satisfactory stability in aqueous solutions, as well as sufficient surface area. Thus, they are considered as an alternative to conventional sorbents by enriching the analytical toolbox for the analysis of trace organic compounds.

Trends in sensitive detection and rapid removal of sulfonamides: A review

Sulfonamides in environmental water, food, and feed are a major concern for both aquatic ecosystems and public health, because they may lead to the health risk of drug resistance. Thus, numerous sensitive detection and rapid removal methodologies have been established. This review summarizes the sample preparation techniques and instrumental methods used for sensitive detection of sulfonamides. Additionally, adsorption and photocatalysis for the rapid removal of sulfonamides are also discussed. This review provides a comprehensive perspective on future sulfonamide analyses that have good performance, and on the basic methods for the rapid removal of sulfonamides.

Preparation of levofloxacin-imprinted nanoparticles using designed deep eutectic solvents for the selective removal of levofloxacin pollutants from environmental waste water

Designed deep eutectic solvents was applied to preparation of levofloxacin-imprinted nanoparticles as functional monomer. The nanoparticles using for the selective removal of levofloxacin pollutants from a natural environmental system.

Smartphone-based image analysis for evaluation of magnetic textile solid phase extraction of colored compounds

Novel modern easily feasible methods for direct evaluation of a new, simple preconcentration analytical procedure have been developed. Two types of smartphone image analysis applications (ON Color Measure and Color Lab) were evaluated to obtain RGB and HSV color spaces data for the quantification of Magnetic textile solid phase extraction of colored compounds (e.g. water-soluble organic dyes). Both direct measurement of color spaces values via the smartphone camera and image analysis of the photograph can be used successfully. The obtained data were similar to those obtained by previously examined professional ImageJ software. The saturation (S) values of the HSV color space are directly proportional to the concentrations of the analyzed dye.

A composite consisting of a deep eutectic solvent and dispersed magnetic metal-organic framework (type UiO-66-NH2) for solid-phase extraction of RNA

A cactus-shaped magnetic composite was prepared for solid-phase extraction of RNA. It is composed of the metal organic framework UiO-66-NH₂ that was modified with Fe₃O₄ nanoparticles. The composite was then dispersed in a lactic acid-based deep eutectic solvent (DES, Fe₃O₄-COOH@UiO-66-NH₂@DES). The structures of the sorbents were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectrometry, vibrating sample magnetometry and thermogravimetric analysis. The extraction performance of sorbents was optimized and the maximum extraction capacity reached 246 mg·g^-1. Extraction is shown to mainly rely on chelation interaction, electrostatic interaction, hydrophobic interaction and hydrogen bonding interaction. The sorbent can selectively extract RNA over DNA, bovine hemoglobin and amino acids. Regeneration studies indicated that the sorbent can be re-used (after regenreation with DES) several times without obvious change of the extraction capacity. The successful extraction of RNA from yeast testified the practical application of the sorbent. Graphical abstractSchematic representation of the fabrication Fe₃O₄-COOH@UiO-66-NH₂@DES, and its application in the magnetic solid phase extraction of RNA.

Effect of green GO/Au nanocomposite on in-vitro amplification of human DNA

Recently nanomaterials have attracted interest for increasing efficiency of polymerase chain reaction (PCR) systems. Here, the authors report on the usefulness of green graphene oxide/gold (GO/Au) nanocomposites for enhancement of PCR reactions. In this study, green GO/Au nanocomposite was prepared with Matricaria chamomilla extract as reducing/capping agent for site-directed nucleation of Au^o atoms on surface of GO sheets. The as-prepared green GO/Au nanocomposites were then characterised with UV-VIS spectrophotometer and scanning electron microscopy. Later, the effect of these nanocomposites was studied on end-point and real-time PCR employed for amplification of human glyceraldehyde-3-phosphate dehydrogenase gene. The results indicated that GO/Au nanocomposite can improve both end-point and real-time PCR methods at the optimum concentrations, possibly through interaction between GO/Au nanocomposite and the materials in PCR reaction, and through providing increased thermal convection by the GO surface as well as the Au nanostructures. In conclusion, it can be suggested that green GO/Au nanocomposite is a biocompatible and eco-friendly candidate as enhancer of in-vitro molecular amplification strategies.

Magnetic solid-phase extraction of pyrethroid insecticides from tea infusions using ionic liquid-modified magnetic zeolitic imidazolate framework-8 as an adsorbent

A simple, sensitive, and reliable magnetic solid-phase extraction (SPE) method coupled with GC-MS/MS for the effective analysis of four pyrethroids from tea infusions was developed. A magnetic adsorbent, named ionic liquid-modified magnetic zeolitic imidazolate framework-8 (Fe3O4/ZIF-8/IL), was prepared by immobilizing an ionic liquid (IL) on the surface of Fe3O4/ZIF-8. The textures of Fe3O4/ZIF-8/IL were confirmed by material characterization, and the results suggested that the adsorbent possessed high magnetism (59.0 emu g-1), an adequate Brunauer-Emmett-Teller (BET) surface area (104 m2 g-1), and a large pore volume (0.68 cm3 g-1). To confirm the extraction performance of the prepared Fe3O4/ZIF-8/IL, several experimental conditions affecting the extraction efficiency were investigated. Under the optimum conditions, the limits of determination (LODs) for the four pyrethroids were in the range of 0.0065-0.1017 μg L-1 (S/N = 3 : 1) with an intra-day relative standard deviation (RSD) of ≤9.70% and inter-day RSD of ≤11.95%. The linear ranges were 0.5-50 μg L-1 for bifenthrin and 0.5-500 μg L-1 for permethrin, cypermethrin, and flucythrinate, with determination coefficients higher than 0.999. Finally, the proposed technique was successfully applied for the determination of pyrethroids in real tea infusions. This work could be extended to other IL-modified metal-organic frameworks (MOFs) and to the development of different sample pretreatment techniques.

Magnetic Particles-Based Analytical Platforms for Food Safety Monitoring

Magnetic nanoparticles (MNPs) have attracted growing interest as versatile materials for the development of analytical detection and separation platforms for food safety monitoring. This review discusses recent advances in the synthesis, functionalization and applications of MNPs in bioanalysis. A special emphasis is given to the use of MNPs as an immobilization support for biomolecules and as a target capture and pre-concentration to increase selectivity and sensitivity of analytical platforms for the monitoring of food contaminants. General principles and examples of MNP-based platforms for separation, amplification and detection of analytes of interest in food, including organic and inorganic constituents are discussed.

Design of magnetic nanoparticles with high magnetic separation efficiencies and durability for Cu2+ adsorption

Fe3O4@SiO2 core-shell structured nanoparticles are promising candidates for adsorption of heavy metal ions from waste water due to their high adsorption capacities and feasible recycling process. However, their practical applications have been hindered by the trade-off between the magnetic separation efficiency and durability. In this study, we demonstrate the preparation of amino-functionalized Fe3O4@SiO2 absorbents with both high magnetic separation efficiencies and durability in strong acidic environment. Our key strategy was to use a thin but highly protective silica layer by manipulating the sol-gel chemistry. With the protection of a thin but poorly permeable silica shell, the durability of the Fe3O4 in strong acid solution was significantly enhanced while its magnetic separation efficiency was maintained. We also demonstrated the adsorption of Cu2+ from aqueous solution corresponded well with the pseudo-second-order kinetic model and the Langmuir adsorption isotherm. The adsorption capacity can be further improved by coating an additional layer of mesoporous SiO2 outside Fe3O4@SiO2. The design of highly durable magnetic absorbents without sacrificing the magnetic separation efficiency greatly facilitates the practical applications of magnetic nanoparticles in adsorption of heavy metal ions.

Magnetic carbon nanotube modified with polymeric deep eutectic solvent for the solid phase extraction of bovine serum albumin

This article described the fabrication of novel magnetic carbon nanotube modified with polymeric deep eutectic solvent (M-CNT@PDES) and its application as extractant for the magnetic solid phase extraction (MSPE) of bovine serum albumin (BSA). The physicochemical properties and morphology of M-CNT@PDES were characterized by X-ray diffraction (XRD), vibrating sample magnetometer (VSM), thermo-gravimetric analysis (TGA), zeta potentials, fourier transform infrared spectrometry (FT-IR) and transmission electron microscope (TEM). Afterwards, several parameters such as pH value, initial concentration of BSA, extraction time, ionic strength and extraction temperature were optimized. The results indicated that the modification of PDES significantly improved the extraction performance for BSA, and the maximum extraction capacity was 225.15 mg/g under the optimized conditions. In addition, 0.20 mol/L NaCl-PBS solution was chosen as the appropriate eluent, and favourable elution rate (81.22%) was obtained. Circular dichroism spectroscopy (CD) indicated that the secondary structure of BSA has not changed during extraction and elution. The regenerative experiment and application in real calf serum confirmed the outstanding durability and practical application ability of M-CNT@PDES. All of above verified that the proposed M-CNT@PDES coupled with MSPE method has great application potential for the pre-concentration of biomolecules.

Magnetic Solid-Phase Extraction of Dichlorodiphenyltrichloroethane and Its Metabolites from Environmental Water Samples Using Ionic Liquid Modified Magnetic Multiwalled Carbon Nanotube/Zeolitic Imidazolate Framework-8 as Sorbent

As persistent organic pollutants, dichlorodiphenyltrichloroethanes (DDTs) and their metabolites pose considerable risks to human health and the environment. Therefore, monitoring DDTs in the environment is essential. Here, we developed a green, simple, and effective magnetic solid-phase extraction (MSPE) method coupled with gas chromatography tandem triple-quadrupole mass spectrometry to determine the DDT content of environmental water samples. A magnetic ionic liquid (IL) adsorbent was developed based on a modified magnetic multiwalled carbon nanotube/zeolitic imidazolate framework-8 (MM/ZIF-8/IL), synthesized by immobilizing the IL on the surface of MM/ZIF-8. We confirmed successful synthesis of MM/ZIF-8/IL by material characterization, and our results suggested that the MM/ZIF-8/IL had a high Brunauer-Emmett-Teller surface area (159.9 m2 g-1), good thermostability (<800 °C), and a high degree of superparamagnetism (52.9 emu g-1). Several experimental conditions affecting the MSPE efficiency were optimized. Under the best conditions, good detection linearity was achieved (0.5-500 µg L-1) with determination coefficients ranging from 0.9927 to 0.9971. The lower limits of detection (0.0016-0.0072 µg L-1) also had good precision, having an intraday relative standard deviation (RSD) ≤ 6.5% and an interday RSD ≤ 8.9%. Finally, we used the as-developed method to determine DDT levels in environmental water samples.

Magnetic nanostructures for preconcentration, speciation and determination of chromium ions: A review

Magnetic nanoparticles based solid-phase extraction is a new analytical technique based on the use of magnetic sorbents for the preconcentration and quantification of different inorganic and organic species. The present review concentrates on recent developments that have been built in magnetic nanostructures-based solid phase extraction, speciation and quantification of chromium ions. Besides, a description of the preparation, characterization as well as applications of various types of magnetic nanostructures, either with an inorganic or organic coating of the magnetic core, is presented. In addition, the most important analytical characteristics such as preconcentration factor, linear range, and limits of detection were carefully reported and compared. On the other hand, the removal of the chromium ions by magnetic solid phase extraction was not discussed in the review.

A magnetized fungal solid-phase extractor for the preconcentrations of uranium(VI) and thorium(IV) before their quantitation by ICP-OES

The fungus Bovista plumbea immobilized on γ-Fe₂O₃ nanoparticles is shown to be a novel sorbent for magnetic solid-phase extractions of U(VI) and Th(IV). The biosorbent was characterized by FT-IR, SEM, and EDX. The effects of pH value, flow rate and volume of sample, amounts of biomass and support material, eluent type, foreign ions and repeated use of the sorbent on extraction efficiency were investigated. The sorption capacities are 41 and 44 mg g^-1, respectively, for U(VI) and Th(IV). The results indicated that B. plumbea immobilized onto γ-Fe₂O₃ nanoparticles can be utilized as a novel material for the preconcentrations of U(VI) and Th(IV) in certified materials and in spiked tap, river and lake waters. Graphical abstract Schematic presentation of a method for preconcentrations of Th(IV) and U(VI) ions using γ-Fe₂O₃ nanoparticles loaded with the fungus Bovista plumbea.

Facile construction of magnetic core-shell covalent organic frameworks as efficient solid-phase extraction adsorbents for highly sensitive determination of sulfonamide residues against complex food sample matrices

Integration of advanced sample pretreatment techniques, with the involvement of functional nano/micro-materials as adsorbents, is of great importance and value for food-safety precise inspection. For now, the major demands for functional adsorbents are ease of fabrication, fast adsorption and separation performance, low toxicity, robustness, and reusability. In the present work, core-shell structured magnetic covalent organic frameworks (COFs) that employed Fe3O4 microspheres as the magnetic core and TpBD COFs as the adsorption shell have been successfully constructed as efficient solid phase extraction (SPE) adsorbents for complex food sample analysis. In favor of the combination of magnetic separation and effective preconcentration, the proposed magnetic COF-SPE method gave a rapid detection performance of the simultaneous detection of ten sulfonamide residues as well as high sensitivity, with detection limits in the range of 0.28-1.45 μg L-1 under the optimized experimental conditions. The Fe3O4@TpBD core-shell adsorbents also demonstrated good stability, robust SPE preconcentration ability, excellent determination recovery, and good reusability. The applicability of the developed SPE method was well demonstrated by real sample analysis, with the recoveries ranging from 82-94%. Through this example, it was believed that the new emerging porous nano/micro-materials, like COFs, metal-organic networks, or hybrid structures, would play more and more important roles as functional materials in food-safety inspection, especially for highly efficient determination of targets against complicated food sample matrices.

Progress in rapid optical assays for heavy metal ions based on the use of nanoparticles and receptor molecules

This review (with 230 refs.) covers recent progress in rapid optical assays for heavy metals (primarily lead and mercury as the most relevant) based on the use of nanoparticles and receptor molecules. An introduction surveys the importance, regulatory demands (such as maximum permissible concentrations) and potential and limitations of various existing methods. This is followed by a general discussion on the use of nanoparticles in optical assays of heavy metals (including properties, basic mechanisms of signal generation). The next sections cover methods for the functionalization of nanoparticles with (a) sulfur-containing compounds (used for modification of nanoparticles or added to the reaction medium), (b) nitrogen-containing compounds (such as amino acids, polypeptides, and heterocyclic molecules), and (c) oxygen-containing species (such as hydroxy and carbonyl compounds). This is continued by a specific description of specific assays based on the use of aptamers as receptors, on the use of deoxyribozymes as synthetic reaction catalysts, of G-quadruplex aptamers, of aptamers in logic gate-type of assays of linear (unstructured) aptamers ("hairpins"), and on the use of aptamers in lateral flow assays. A next section covers assays based on the employment of antibodies as receptors (used in the immunoassay development). The properties of various nanoparticles and their applicability in optical assays are also discussed in some detail. Final sections discuss the selectivity of assays, potential interferences by other cations, methods for their elimination, and also matrix effects and approaches for sample pretreatment. A concluding section discusses current challenges and future trends. Analysis based on enzyme inhibition assay is not treated here but enzyme-like action of some receptor molecules such as DNAzymes is discussed. Graphical abstract Schematic presentation of main principles of application of various nanoparticles with receptor molecules (S-, N-, O-containing, heterocyclic compounds, proteins, antibody, aptamers) for heavy metals ions detection. The included methods cover optical assays with description of mechanisms of interactions and signal generation.

Fluorometric determination of pesticides and organophosphates using nanoceria as a phosphatase mimic and an inner filter effect on carbon nanodots

Nanoceria with a remarkable phosphatase mimicking activity was synthesized and used to catalyze the hydrolysis of phosphate esters in pH 10 solution. The catalytic effect of nanoceria was firstly investigated by selecting p-nitrophenyl phosphate as a model substrate. The pH value, incubation temperature, reaction time, and concentration of nanoceria were optimized. The catalytic effect was then confirmed by using methyl-paraoxon as a substrate. The p-nitrophenol anion released by the enzyme mimic is yellow and exerts an inner filter effect on the fluorescence of the carbon dots (with excitation/emission maxima at 400/520 nm). Response to methyl-paraoxon is linear in the 1.125-26.25 μmol L^-1 concentration range. The method was applied to the determination of pesticides in spiked Panax quinquefolius and water samples. Recoveries ranged from 85 to 103% (n = 3). The technique is rapid, reliable, and can be used for on-site detection of pesticides and organophosphates. Graphical abstract Schematic presentation of a fluorometric technique for the detection of organophosphate compound and pesticide using nanoceria as a phosphatase mimic and an inner filter effect on the blue fluorescence of carbon dots (with excitation/emission maxima at 400/520 nm).