Vanesa Romero

In Situ Building of a Nanoprobe Based on Fluorescent Carbon Dots for Methylmercury Detection

A new fluorescent assay based on in situ ultrasound-assisted synthesis of carbon dots (CDs) as optical nanoprobes for the detection of methylmercury has been developed. Application of high-intensity sonication allows simultaneous performance of the synthesis of fluorescent CDs within the analytical time scale and the selective recognition of the target analyte. Microvolume fluorospectrometry is applied for measurement of the fluorescence quenching caused by methylmercury. The assay uses low amounts of organic precursors (fructose, poly(ethylene glycol), and ethanol) and can be accomplished within 1 min. A detection limit of 5.9 nM methylmercury and a repeatability expressed as a relative standard deviation of 2.2% (N = 7) were obtained. CDs displayed a narrow size distribution with an average size of 2.5 nm as determined by electron transmission microscopy. To study the quenching mechanism, fluorescence, atomic absorption spectrometry, and Fourier transform infrared spectrometry were applied. Hydrophobicity of methylmercury and its ability to facilitate a nonradiative electron/hole recombination are suggested as the basis of the recognition event. A simple and green assay is achieved for quick detection of methylmercury without the use of tedious sample preparation procedures or complex and expensive instrumentation.

Green chemistry in analytical atomic spectrometry: a review

As a result of the greater consciousness within the analytical community on the impact of chemicals on human health and environment, green issues are increasingly taken into account when choosing an established analytical method or developing a new one. Apart from typical analytical characteristics (e.g., sensitivity, limit of detection, repeatability, etc.), other features such as the amount of sample/reagents, operation time, use of energy-effective apparatus, waste production, etc. should be highlighted in order to meet the principles of Green Chemistry. Although conventional approaches for trace element analysis by atomic spectrometry usually involve well-established sample pre-treatments based on ‘wet chemistry’, and high consumption of gases, reagents, etc. is inherent to many techniques in this group, there are still many avenues where green issues can be implemented. For greening atomic spectrometry, green chemistry principles should be applied to every step of the analytical process, i.e., from sampling and sample pre-treatment to data processing. In this review, main pathways for greening atomic spectrometry such as downsizing of instrumentation, use of portable instruments, solid sampling, application of clean energies (ultrasound, microwaves, etc.) for sample pre-treatment, development of on-site, on-line and at-line approaches vs. typical off-line methods, application of modern extraction techniques (e.g., solid- and liquid-phase microextraction), green solvents and derivatization agents and use of chemometric tools for method optimization, signal processing, etc. are discussed in a critical way.

Quantum dot-based headspace single-drop microextraction technique for optical sensing of volatile species.

Core-shell CdSe/ZnS quantum dots (QDs) dispersed in a droplet of organic solvent have been applied for the first time as luminescent probes for the selective detection of volatile species. Luminescence quenching caused by volatile species was examined after their trapping onto a drop using the headspace single-drop microextraction (HS-SDME) approach along with microvolume fluorospectrometry. The novel method is characterized by low reagent and sample consumption, especially regarding QDs, a reduction about 500-fold for each analysis being attained in comparison with luminescent probing in aqueous phase using conventional luminescence spectrometers with 1 cm quartz cells for measurement. To assess QDs as luminescent probes along with HS-SDME, 14 volatile species were tried. Strong luminescence quenching (i.e., I(0)/I > 2.5) was observed for species such as CH(3)Hg(+) and Se(IV) after hydridation with NaBH(4). Moderate luminescent quenching (I(0)/I ≈ 2) was observed for species such as Hg(II) after its conversion into Hg(0), H(2)S, and methylcyclopentadienyl-manganese tricarbonyl (MMT). Small luminescence quenching effects (i.e., 1< I(0)/I <2) were caused by other hydride forming species such as As(III), Sb(III), Te(IV), and Bi(III), as well as SnBu(4), volatile amines, and endosulfan. Detection limits of 6.3 × 10(-9) and 1.6 × 10(-7) M were obtained for Se(IV) and CH(3)Hg(+), respectively. Repeatability expressed as relative standard deviation (N = 7) was about 5%. QD-HS-SDME-μvolume-fluorospectrometry allows one to carry out matrix separation, preconcentration, and confinement of QDs, hence achieving a selective, sensitive, fast, environmentally friendly, and miniaturized luminescence assay.

Quantum Dots Confined in an Organic Drop as Luminescent Probes for Detection of Selenium by Microfluorospectrometry after Hydridation: Study of the Quenching Mechanism and Analytical Performance

Following a preliminary work (Costas-Mora, I.; Romero, V.; Pena-Pereira, F.; Lavilla, I.; Bendicho, C. Anal. Chem.2011, 83, 2388-2393), a quenching mechanism has been established for the selective detection of Se (as selenium hydride) by microfluorospectrometry using CdSe quantum dots (QDs) as luminescent probes stabilized with hexadecylamine and confined in an organic droplet. For this purpose, luminescence, luminescence lifetime, UV-vis absorption, total reflection X-ray fluorescence, transmission electron microscopy, and atomic force microscopy measurements were performed. The presence of stabilizing agents of QDs in the droplet was found to cause a critical effect on both extraction efficiency of selenium hydride in the drop and luminescence quenching. A self-quenching mechanism due to the aggregation of QDs is suggested. Aggregation is thought to occur as a result of the binding between selenide trapped into the organic drop as selenium hydride and Cd(2+) present in the surface of QDs, which in turn, may cause the loss of stabilizing hexadecylamine groups. After full optimization of main variables influencing the luminescent response, the analytical performance was established. A detection limit as low as 0.08 μg L(-1) Se(IV) and a repeatability expressed as relative standard deviation of 4.6% were obtained. The method was validated against CRM NWTM-27.2 lake water, and a recovery study was performed with synthetic seawater. The use of CdSe as luminescent probes in an organic drop may constitute an extremely selective, sensitive, and miniaturized assay for in situ detection of Se(IV) in water.

Natural deep eutectic solvents in combination with ultrasonic energy as a green approach for solubilisation of proteins: application to gluten determination by immunoassay.

In this work, a fast and miniaturised procedure based on the use of a natural deep eutectic solvent (NADES) in combination with ultrasound-assisted extraction (UAE) has been proposed for gluten determination by a commercial enzyme-linked immunosorbent assay (ELISA). Fourteen NADESs were prepared by combining two natural primary metabolites and water. Studies on NADES viscosity and gluten solubilisation in NADESs and ethanol-water solutions (for comparison purposes) were carried out. Different strategies for speeding-up gluten solubilisation in NADESs were evaluated: dilution, temperature and sonication by a cup-horn sonoreactor. Diluted fructose-citric acid NADES and sonication were finally selected for gluten solubilisation. Solubilised proteins were characterized by electrophoresis and molecular fluorescence. The proposed procedure was also assessed in real samples, especially ultrasound time. Kit solvents (including reducing agents) were replaced by NADES, and hence, a reassessing of immunoassay system was necessary. Samples with and without gluten as well as recovery tests were used for this purpose. Recoveries were in the range of 79-106% and the repeatability, expressed as relative standard deviation, was better than 15%.

Solid-state chemiluminescence assay for ultrasensitive detection of antimony using on-vial immobilization of CdSe quantum dots combined with liquid-liquid-liquid microextraction

On-vial immobilized CdSe quantum dots (QDs) are applied for the first time as chemiluminescent probes for the detection of trace metal ions. Among 17 metal ions tested, inhibition of the chemiluminescence when CdSe QDs are oxidized by H2O2 was observed for Sb, Se and Cu. Liquid-liquid-liquid microextraction was implemented in order to improve the selectivity and sensitivity of the chemiluminescent assay. Factors influencing both the CdSe QDs/H2O2 chemiluminescent system and microextraction process were optimized for ultrasensitive detection of Sb(III) and total Sb. In order to investigate the mechanism by which Sb ions inhibit the chemiluminescence of the CdSe QDs/H2O2 system, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), UV-vis absorption and fluorescence measurements were performed. The selection of the appropriate CdSe QDs capping ligand was found to be a critical issue. Immobilization of QDs caused the chemiluminescence signal to be enhanced by a factor of 100 as compared to experiments carried out with QDs dispersed in the bulk aqueous phase. Under optimized conditions, the detection limit was 6 ng L(-1) Sb and the repeatability expressed as relative standard deviation (N=7) was about 1.3%. An enrichment factor of 95 was achieved within only 3 min of microextraction. Several water samples including drinking, spring, and river waters were analyzed. The proposed method was validated against CRM NWTM-27.2 fortified lake water, and a recovery study was performed with different types of water samples. Sb recoveries ranged from 94 to 105%. A fast, miniaturized and relatively inexpensive assay for selective and sensitive detection of Sb(III) and total Sb in waters is accomplished.

In situ ultrasound-assisted synthesis of Fe3O4 nanoparticles with simultaneous ion co-precipitation for multielemental analysis of natural waters by total reflection X-ray fluorescence spectrometry

In this work, in situ ultrasound-assisted magnetite synthesis with simultaneous ion co-precipitation (UAMS-SIC) was applied to magnetic solid-phase extraction (MSPE) of Cu, Zn, Ge, As, Se, Re, Au, Hg, Tl, Bi and Pb. The magnetic solid phase (MSP) enriched with metal ions was directly analyzed by total reflection X-ray fluorescence (TXRF). Magnetite nanoparticles (Fe3O4 NPs) were synthesized by sonochemical treatment of a solution containing Fe(II) and Fe(III) in the presence of target analytes so that these are trapped by occlusion or surface adsorption during the formation of Fe3O4 NPs. In situ synthesized Fe3O4 NPs were easily separated from the aqueous matrix by applying an external magnetic field, and hence, no filtration or centrifugation steps were necessary. A 10 μL aliquot of MSP was deposited onto a sample quartz glass siliconized carrier and analyzed by TXRF, thus avoiding the elution step. The effect of the Fe(II):Fe(III) molar ratio, total mass of Fe in the MSP, ultrasound amplitude, time and interfering ions on the extraction efficiency of the metal ions was investigated. Detection limits ranged from 0.3 to 26 μg L−1 depending on the metal ion. Enrichment factors (EFs) in the range 8–180 were achieved. A recovery study carried out on spiked samples showed recoveries in the range 89–109% with relative standard deviations below 10% (N = 5). An effective, fast and sensitive procedure that can be used for field analysis is accomplished. A comprehensive characterization of in situ synthesized Fe3O4 NPs was carried out by atomic force microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Finally, a co-precipitation mechanism for trace metal ions with this novel synthesis of nanosized magnetite is approached using the Berthelot–Nernst and the Doerner–Hoskins laws.

Silver nanoparticle-assisted preconcentration of selenium and mercury on quartz reflectors for total reflection X-ray fluorescence analysis

In this work, a new analytical approach based on the trapping of Se and Hg vapours using silver nanoparticles (Ag NPs) immobilized on quartz reflectors, prior to their determination by total reflection X-ray fluorescence (TXRF), is proposed. Ag NPs were synthesized by reduction of the metal precursor with a reducing agent in aqueous solution. Silanization with 3-mercaptopropyltrimethoxysilane (MPTMS) was used in order to modify the quartz reflectors and immobilize Ag NPs by interaction between the organosilane compound and Ag NPs. A comprehensive characterization of synthesized Ag NPs was carried out by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). SeH2 and Hg(0) vapours were generated by means of a continuous flow vapour generation system and trapped onto the quartz reflector containing immobilized Ag NPs. Different parameters involved in both the Ag NP synthesis and the continuous flow system for vapour generation and preconcentration were optimized in order to obtain the best results. Detection limits were 0.18 and 0.55 μg L−1 for selenium and mercury, respectively. Enrichment factors of 265 and 175 were obtained for Se and Hg, respectively. The new method was successfully validated against three biological certified reference materials and applied to several seafood samples. Recoveries carried out on three certified reference materials were in the range 94–106% with a relative standard deviation of 5% (N = 5).

Graphene membranes as novel preconcentration platforms for chromium speciation by total reflection X-ray fluorescence

Fabrication of unmodified graphene membranes for their application as selective sorptive platforms of hexavalent chromium [Cr(VI)] is described for the first time. Multilayer graphene membranes are synthesized by drop-casting of graphene oxide (GO) onto a glass substrate followed by mild thermal reduction. As-prepared membranes are formatted to fit the measurement area of total reflection X-ray fluorescence (TXRF). Structural and morphological characterization by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) shows that graphene membranes are 122 nm height and contain non-reduced functional groups resulting in lattice defects. Adsorption isotherm models and characterization by time-of-flight secondary ion mass spectrometry (TOF-SIMS) indicate that adsorption sites on graphene membranes are uniformly distributed and bind Cr(VI) as a monolayer, both by electrostatic interaction and chemisorption. Graphene membranes display high flexibility and become conical-shaped when immersed into stirred liquid samples. When combining graphene membrane preconcentration and TXRF, a detection limit of 0.08 μg L−1 Cr(VI) is obtained. Repeatability expressed as relative standard deviation is 3% (N = 5). Two certified reference materials, i.e. CASS-4 seawater and NWTM-27.2 lake water, are used for testing accuracy. The proposed method is simple, solvent-free and sensitive, being suitable for Cr speciation in water including high salinity samples.

In situ photochemical synthesis of fluorescent carbon dots for optical sensing of hydrogen peroxide and antioxidants

A new synthesis approach for obtaining fluorescent carbon dots (CDs) based on UV irradiation of carbohydrates was developed. The photochemical synthesis pathway allows the formation of water soluble CDs of analytical usefulness within one min. CDs obtained by photochemical treatment from the sucrose/NaOH/poly(ethylene glycol) system are monodisperse with an average size of 8 nm as determined by transmission electron microscopy. A dramatic increase in the CDs fluorescence (turn on) is observed when H2O2 is added. The decrease in CDs size occurring by the action of highly oxidant OH radicals gives rise to confinement of emissive energy traps and, in turn, to fluorescence enhancement. Antioxidants such as ascorbic acid and glutathione inhibit the photochemical reaction giving rise to a decrease in fluorescence of the CDs/H2O2 system (turn on-off). The detection limit was 5 µM H2O2 and the repeatability expressed as the relative standard deviation was 3.8% (N=7). The photochemical synthesis of CDs allows building a green, low-cost, safe and fast assay for the detection of H2O2 and antioxidants. An application of the novel fluorescent nanoprobe to H2O2 detection in contact lens cleaning solutions is performed.