Fluorescent dual-mode sensor for the determination of graphene oxide and catechin in environmental or food field.

The novel fluorescent sensor is proposed in this work based on the competitive interactions between the known bioactive compounds, riboflavin and catechin, which act as guests, and graphene oxide (GO) material that acts as host. Specifically, this proposal is based on an indicator displacement assay which allows the detection of GO and catechin (fluorescence quenching of riboflavin signal by GO and increase in fluorescence by catechin on the GO-riboflavin system). Three GO structures with different lateral dimensions and thickness were synthesized and tested, being able to be the three differentiated depending on the attenuation capacity of the fluorescent signal that each one possesses. The environmental analytical control of GO is more and more important, being this method sensitive and selective in the presence of other potential interferent substances. On the other hand, the other sensing capacity of the sensor also allows the determination of catechin in food samples based on the formation of riboflavin-GO complex. It is a rapid, simple and non-expensive procedure. Thus, the same 2D nanomaterial (GO) is seen to play a double role in this sensing strategy (analyte and analytical tool for the determination of another compound).

Graphene quantum dots an efficient nanomaterial for enhancing the photostability of trans-resveratrol in food samples.

Given the health-promoting properties associated with trans-resveratrol consumption and knowing its photochemical instability, as it rapidly isomerizes to the less biologically active conformer, the photoprotective role of a carbon-based nanomaterial was investigated. The resveratrol adsorption on graphene quantum dots (GQDs), synthesized from uric acid, produces a considerable inhibition in the trans- to cis-resveratrol conversion process under light-induced radiation, conferring photostability to the bioactive. The influence of different parameters affecting the adsorption efficiency and loading capacity of resveratrol on GQDs was explored. Several characterization techniques confirmed this interaction, even proving to be non-toxic at the concentration at which the maximum inhibition of isomerization occurs. This adsorption also implies an increase in the antioxidant capacity of the polyphenol. The photoprotective effect was evaluated in food samples, resulting in a considerable slowing down of isomerization. This fact confirms the potential of GQDs to be an effective vehicle of trans-resveratrol to supplement food systems.

Innovative and versatile nanoplasmonic approach for the full sensing of proteinogenic aminoacids in nutritional supplements.

A current nourishment issue is the development of smart and reliable analytical strategies to control in a simple way main bioactive compounds of nutritional supplements whose increasing use is deemed a trend nowadays. With this aim a quick and highly sensitive plasmonic sensor using simple citrate coated gold nanoparticles (AuNPs) as optical probe, was developed for both qualitative and quantitative global assessment of all the proteinogenic amino acids in nutritional supplements. AuNPs of five different sizes (from 19 to 74 nm) were synthesized, characterized and evaluated as optimal transductor element for the sensing approach. Critical physic-chemical conditions controlling aggregation (pH, incubation time, AuNPs amount and ionic strength) were investigated on the main five types of aas, structurally different attending to their R-side chain and with expected distinctive behaviour on aggregation mechanisms, which are also discussed. All proteinogenic amino acids induced AuNPs aggregation at low pH (2.5) causing a change in the colour solution from red to blue, as well as a redshift in the plasmon band from 518 nm (disperse NPs) to 650 nm (aggregated NPs). Based on this sensing approach two different strategies are allowed, a preliminary qualitative/semi-quantitative screening just by the naked eye (simple spot test) and a second quantitative confirmation procedure using the analytical signal (A650/A518). Reliability of quantitative approach was assessed by an exhaustive validation procedure, where matrix effects and potential interferences usually present in commercial samples and affecting the analytical signal were mainly focussed. The results for the analysis of complex nutritional samples were validated by means of a statistical comparison with those ones of the official reference Kjeldahl method (paired Student test-t) at a 95% confidence level. This is the first sensing approach able to provide the global estimation of proteinogenic aas amount based on their simply AuNPs aggregation induction, irrespectively of their R-side chain structure.

Cyclodextrin-modified graphene quantum dots as a novel additive for the selective separation of bioactive compounds by capillary electrophoresis.

Highly reliable separation and determination of various biologically active compounds were achieved using capillary electrophoresis (CE) based on ß-cyclodextrin-functionalized graphene quantum dots (ßcd-GQDs) as the background electrolyte additive. ßcd-GQDs improve the separation efficiency between peaks of all analytes. No addition of surfactants or organic solvents was needed in the running buffer containing ßcd-GQDs. Up to eight consecutive runs were acquired with high precision for the separation of resveratrol, pyridoxine, riboflavin, catechin, ascorbic acid, quercetin, curcumin, and even of several of their structural analogs. Baseline separation was achieved within just 13 min as a result of the effective mobility of the analytes along the capillary owing to the differential interaction with the additive. The proposed analytical method displayed a good resolution of peaks for all species selecting two absorption wavelengths in the diode array detector. Detection limits lower than 0.28 µg mL-1 were found for all compounds and precision values were in the range of 2.1-4.0% in terms of the peak area of the analytes. The usefulness of the GQD-assisted selectivity-enhanced CE method was verified by the analysis of food and dietary supplements. The applicability to such complex matrices and the easy and low-cost GQD preparation open the door for routine analyses of food and natural products. The concept of using such a dual approach (macromolecules and nanotechnology) has been explored to tackle the separation of various bioactive compounds in nutritional supplements and food. Schematic illustration of the electrophoretic separation of the bioactive molecules in the capillary which is filled with the running solution without (top) and with ßcd-GQDs (bottom). The fused silica capillary with negatively ionizable silanol groups at the wall. The voltage is applied at positive polarity at the outlet. R, riboflavin; r, resveratrol; P, pyridoxine; C, catechin; c, curcumin; A, ascorbic acid; Q, quercetin.

Discrimination between nanocurcumin and free curcumin using graphene quantum dots as a selective fluorescence probe.

Accurate-controlled sized graphene quantum dots (GQDs) have been used as an analytical nanoprobe for detecting curcumin as a function of the photoluminescent quenching upon increasing concentrations of the analyte. Regarding the importance of curcumin nanoparticles in nutraceutical food, the analytical method described herein was also proven for the discrimination of curcumin remaining in free solution from that encapsulated into water-soluble nanomicelles of ca. 11 nm. This recognition is based on the displacement of GQD emission when interacting with both curcumin species. Maximum emission wavelength of GQDs suffers a gradual quenching as well as a red-shifting upon increasing concentrations of free curcumin (from 458 to 490 nm, exciting at 356 nm). On the other hand, in the presence of nanocurcumin, GQD photoluminescent response only displays a quenching effect (458/356 nm). The sensitivity of the described method in terms of detection limits was 0.3 and 0.1 µg mL-1 for curcumin and nanocurcumin, respectively. The applicability of the photoluminescent probe for the quantification and discrimination between both curcumin environments was demonstrated in nutraceutical formulations namely functional food capsules and fortified beverages such as ginger tea. Graphical abstract.

A screen-printed electrode modified with silver nanoparticles and carbon nanofibers in a nafion matrix for ionic liquid-based dispersive liquid-liquid microextraction and voltammetric assay of heterocyclic amine 8-MeIQx in food.

An electrochemical method is described for the determination of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (8-MeIQx) which is a heterocyclic aromatic amine formed in cooked food samples. The method uses a screen-printed carbon nanofiber electrode that is modified with silver nanoparticles (AgNPs) in a Nafion matrix. The surface of the modified electrode was characterized by UV-vis spectrometry, dynamic light scattering, scanning electron microscopy and Raman spectroscopy. The average size of the AgNPs is 14 nm. The modified electrode exhibits good properties in terms of reversibility, fast kinetics of electron transfer, and large electroactive area toward the reduction of 8-MeIQx. Differential pulse voltammetry is the most suitable electrochemical technique for quantification of 8-MeIQx, best at a voltage of -0.21 V (versus Ag reference electrode). The first derivative serves as the analytical signal that increases linearly in the 0.015-40 mg L-1 8-MeIQx concentration range, with a 5 µg L-1 detection limit. A dispersive liquid-liquid microextraction procedure assisted via ionic liquid was developed to isolate the analyte from real samples. The whole extraction-preconcentration and voltammetric method allows to determine 30 and 70 µg L-1 in (spiked) bouillon cube, meat broth, beer and wine, with recoveries in the 93.6-110.4% range. Graphical abstractSchematic presentation for the analysis of aromatic amine 8-MeIQx, resultant compound from cooking meat. Extracted sample solution was placed onto modified electrode surface thus obtaining voltammetric analytical signal. So, quantification atrelevant levels can be performed.

Determination of vanillin by using gold nanoparticle-modified screen-printed carbon electrode modified with graphene quantum dots and Nafion.

A voltammetric analytical assay for the selective quantification of vanillin is described. It is based on the use of a gold nanoparticle-modified screen-printed carbon electrode (SPCE) modified with graphene quantum dots (GQD) in a Nafion matrix. The GQD were synthesized by an acidic thermal method and characterized by UV-Vis, photoluminescence, and FTIR spectroscopy. The modified SPCE displays a strongly enhanced response to vanillin. Linear sweep voltammetry (LSV) and differential pulse voltammetry (DPV) were applied to optimize the methods. The analytical assay has linear responses in the 13 to 660 µM and 0.66 to 33 µM vanillin concentration ranges. The detection limits are 3.9 µM and 0.32 µM when using LSV and DPV, respectively. The analytical assay is selective and stable. It was applied to the determination of vanillin in several food samples with satisfactory results. Recoveries from spiked samples ranged between 92.1 and 113.0%. Graphical abstract The selective and sensitive quantification of vanillin is carried out by the use of a gold nanoparticle-modified screen-printed carbon electrode modified with graphene quantum dots in a Nafion matrix.

Analysis of penicillamine using Cu-modified graphene quantum dots synthesized from uric acid as single precursor.

A simple methodology was developed to quantify penicillamine (PA) in pharmaceutical samples, using the selective interaction of the drug with Cu-modified graphene quantum dots (Cu-GQDs). The proposed strategy combines the advantages of carbon dots (over other nanoparticles) with the high affinity of PA for the proposed Cu-GQDs, resulting in a significant and selective quenching effect. Under the optimum conditions for the interaction, a linear response (in the 0.10-7.50 µmol/L PA concentration range) was observed. The highly fluorescent GQDs used were synthesized using uric acid as single precursor and then characterized by high resolution transmission electron microscopy, Raman spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, fluorescence, and absorption spectroscopy. The proposed methodology could also be extended to other compounds, further expanding the applicability of GQDs.

ß-Cyclodextrin coated CdSe/ZnS quantum dots for vanillin sensoring in food samples.

An optical sensor for vanillin in food samples using CdSe/ZnS quantum dots (QDs) modified with ß-cyclodextrin (ß-CD) was developed. This vanillin-sensor is based on the selective host-guest interaction between vanillin and ß-cyclodextrin. The procedure for the synthesis of ß-cyclodextrin-CdSe/ZnS (ß-CD-CdSe/ZnS-QDs) complex was optimized, and its fluorescent characteristics are reported. It was found that the interaction between vanillin and ß-CD-CdSe/ZnS-QDs complex produced the quenching of the original fluorescence of ß-CD-CdSe/ZnS-QDs according to the Stern-Volmer equation. The mechanism of the interaction is discussed. The analytical potential of this sensoring system was demonstrated by the determination of vanillin in synthetic and food samples. The method was selective for vanillin, with a limit of detection of 0.99 µg mL(-1), and a reproducibility of 4.1% in terms of relative standard deviation (1.2% under repeatability conditions). Recovery values were in the 90-105% range for food samples.

Microwave-assisted synthesis of water soluble thiol capped CdSe/ZnS quantum dots and its interaction with sulfonylurea herbicides.

A simple and fast procedure for water solubilization of CdSe/ZnS quantum dots (QDs) using microwave irradiation (MW) has been optimized. The CdSe/ZnS QDs were synthesized in organic media and water solubilization was achieved by replacing the initial hydrophobic ligands (TOPO and TOP) with hydrophilic heterobifunctional thiol ligands, such as L-cysteine (L-Cys), 3-mercaptopropionic acid (3-MPA) and cysteamine (CTAM). The use of MW irradiation allowed carrying out the modification of the surface thiol of QDs in a simple and fast way (only 40 s was required). Different optimization studies based on activation-time, irradiation-time, concentration of ligands, pH and lifetime fluorescent properties were carried out in order to obtain the best results for the solubilization of QDs. By the proposed method, the resulting water-soluble QDs exhibit a strong fluorescence emission at about 590 nm, with a high and reproducible photostability and acceptable yields. With the aim of contributing to exploiting the advantages of synthetized QDs from an analytical point of view, the different behavior with sulfonylurea herbicides (SUHs) were studied.

Use of Cdse/ZnS quantum dots for sensitive detection and quantification of paraquat in water samples.

Based on the highly sensitive fluorescence change of water-soluble CdSe/ZnS core-shell quantum dots (QD) by paraquat herbicide, a simple, rapid and reproducible methodology was developed to selectively determine paraquat (PQ) in water samples. The methodology enabled the use of simple pretreatment procedure based on the simple water solubilization of CdSe/ZnS QDs with hydrophilic heterobifunctional thiol ligands, such as 3-mercaptopropionic acid (3-MPA), using microwave irradiation. The resulting water-soluble QDs exhibit a strong fluorescence emission at 596 nm with a high and reproducible photostability. The proposed analytical method thus satisfies the need for a simple, sensible and rapid methodology to determine residues of paraquat in water samples, as required by the increasingly strict regulations for health protection introduced in recent years. The sensitivity of the method, expressed as detection limits, was as low as 3.0 ng L(-1). The lineal range was between 10-5×10(3) ng L(-1). RSD values in the range of 71-102% were obtained. The analytical applicability of proposed method was demonstrated by analyzing water samples from different procedence.

Determination of neonicotinoid insecticides in environmental samples by micellar electrokinetic chromatography using solid-phase treatments.

A sensitive and reliable method based on MEKC has been developed and validated for trace determination of neonicotinoid insecticides (thiamethoxam, acetamiprid, and imidacloprid) and the metabolite 6-chloronicotinic acid in water and soil matrices. Optimum separation of the neonicotinoid insecticides was obtained on a 58 cm long capillary (75 µm id) using as the running electrolyte 40 mM SDS, 5 mM borate (pH 10.4), and 5% (v/v) methanol at a temperature of 25°C, a voltage of 25 kV and with hydrodynamic injection (10 s). The analysis time was less than 7 min. Prior to MEKC determination, the samples were purified and enriched by carrying out extraction-preconcentration steps. For aqueous samples, off-line SPE with a sorptive material such as Strata-X (polymeric hydrophobic sorbent) and octadecylsilane (C18) was carried out to clean up and preconcentrate the insecticides. However, for soil samples, matrix solid-phase dispersion (MSPD) was applied with C18 used as the dispersant. Good linearity, accuracy, and precision were obtained and the detection limits were in the range between 0.01 and 0.07 µg mL⁻¹ for river water and 0.17 and 0.37 µg g⁻¹ for soil samples. Recovery levels reached greater than 92% for all of the assayed neonicotinoids in river water samples with Strata-X. In soil matrices, the best recoveries (63-99%) were obtained with MSPD.

Determination of morphine, codeine, and paclitaxel in human serum and plasma by micellar electrokinetic chromatography.

A micellar electrokinetic chromatography method is proposed for the determination of morphine, codeine, and paclitaxel at clinical relevant levels in human serum and plasma, which are employed in the treatment of patients with cancer. Optimal conditions for the separation were investigated. A background electrolyte solutions consisting of 20 mM borate buffer adjusted to pH 8.5, sodium dodecyl sulphate 60 mM and 15% methanol, hydrodynamic injection, and 25 kV as separation voltage were used. Detection wavelength was 212 nm for morphine and codeine and 200 nm for paclitaxel. Aspects such as stability of the solutions, linearity, accuracy, precision, and robust and ruggedness were examined in order to validate the proposed method. Detection limits obtained for all the studied compounds ranged between 26 and 52 ng/mL. Before micellar electrokinetic chromatography determination, the samples were purified and enriched by means of an extraction-preconcentration step with a preconditioned C(18) cartridge. This method was applied to the analysis of serum and plasma samples from different cancer patients undergoing treatment with paclitaxel or/and codeine.

Analytical characterization of PEG polymers by MEKC.

Characterization of PEGs with average molecular masses of up to 2000 has been achieved using MEKC with UV detection. A rapid derivatization procedure with phenyl isocyanate using microwave radiation, in order to introduce chromophore groups in PEGs, has been developed involving a reaction time of 60 s. Different optimized conditions in accordance with the molecular weight have been studied to obtain the oligomer separation. The weight-average molecular mass the number-average molecular mass and the degree of polydispersity (molecular mass distribution) were calculated for the different PEGs obtaining similar results with those certified for standards. A good precision was obtained for characterizing the different oligomers. Ethylene glycol was used as the internal standard for the analysis of low-molecular-weight PEGs. The developed method was satisfactorily applied to the characterization of these polymers in several real samples, such as lubricant eye drops, toothpaste, tap water and eye make-up remover.

Development of a novel biotoxicity screening assay for analytical use.

A new biotoxicity assay has been developed and employed as an analytical screening tool. The proposed biotoxicity assay is simple and represents the first example of the use of Lactobacillus plantarum as a test micro-organism. The applicability of this method was demonstrated by evaluating the pollution of water due to the presence of several toxicants. Traditional parameters such as effective concentration (EC(50)) and lowest observed effect concentration (LOEC) provided by all the biotoxicity tests were employed in a screening method based on a binary toxic/non-toxic response. The threshold of the response was characterized by defining a mortality index (I(m)), which in this case was obtained by monitoring the consumption of glucose in the culture medium (Somogyi-Nelson method) or by turbidimetric measurements. Cut-off values established by the current legislation (or other preset values) were used as criteria to classify samples as positive or negative. Bacterial growth was inhibited by the presence of heavy metals such as mercury, cadmium, lead, copper and zinc. This methodology was applied to study the toxicity of heavy metals in different water samples.