In Vitro Assessment of the Physiologically Relevant Oral Bioaccessibility of Metallic Elements in Edible Herbs Using the Unified Bioaccessibility Protocol.

In this work, the total content of seven metallic elements (Fe, Cu, Zn, Mg, Pb, Ni, and Co) in common edible herbs was determined and related to their bioaccessibility by an in vitro human digestion model. Specifically, the unified bioaccessibility protocol developed by the BioAccessibility Research Group of Europe (BARGE) was used to determine the release of each element during gastric and gastrointestinal digestion. The results show that Fe, Zn, and Mg are released during gastric digestion (34-57% Fe, 28-80% Zn, 79-95% Mg), but their overall bioaccessibility is reduced in the gastrointestinal tract (<30%). On the contrary, Cu is more bioaccessible during gastrointestinal digestion (38-60%). Pb, Ni, and Co exhibited similar bioaccessibility in both gastric and gastrointestinal fluids. Principle component analysis of the data shows that the classification of the nutritional value of herbs differs between the total and the gastrointestinal concentration, suggesting that the total concentration alone is not an adequate indicator for drawing secure conclusions concerning the nutritional benefits of edible plant species.

Analyte-mediated formation and growth of nanoparticles for the development of chemical sensors and biosensors.

The cornerstone of nanomaterial-based sensing systems is the synthesis of nanoparticles with appropriate surface functionalization that ensures their stability and determines their reactivity with organic or inorganic analytes. To accomplish these requirements, various compounds are used as additives or growth factors to regulate the properties of the synthesized nanoparticles and their reactivity with the target analytes. A different rationale is to use the target analytes as additives or growth agents to control the formation and properties of nanoparticles. The main difference is that the analyte recognition event occurs before or during the formation of nanoparticles and it is based on the reactivity of the analytes with the precursor materials of the nanoparticles (e.g., metal ions, reducing agents, and coatings). The transition from the ionic (or molecular) state of the precursor materials to ordered nanostructured assemblies is used for sensing and signal transduction for the qualitative detection and the quantitative determination of the target analytes, respectively. This review focuses on assays that are based on analyte-mediated regulation of nanoparticles' formation and differentiate them from standard nanoparticle-based assays which rely on pre-synthesized nanoparticles. Firstly, the principles of analyte-mediated nanomaterial sensors are described and then they are discussed with emphasis on the sensing strategies, the signal transduction mechanisms, and their applications. Finally, the main advantages, as well as the limitations of this approach, are discussed and compared with assays that rely on pre-synthesized nanoparticles in order to highlight the major advances accomplished with this type of nano-sensors and elucidate challenges and opportunities for further evolving new nano-sensing strategies.

Fabric phase sorpitive extraction and passive sampling of ultraviolet filters from natural waters using a zirconium metal organic framework-cotton composite.

In this work, we introduce the use of MOFs immobilized on cotton fabrics as a sorbent phase for the fabric phase sorptive extraction and passive sampling of non-polar organic compounds from water samples. A water-stable, Zr4+-based MOF (UiO-66(Zr)-NH2) was irreversibly immobilized on polydopamine decorated cotton through a step-wise synthetic procedure that maximized the amount of MOF immobilized on the fabric surface. In this manner it was possible to combine the permeability and the large contact surface area of the host cotton substrate with the high specific surface area and sorption capacity of the MOF. The MOF@cotton composite was used as a new sorbent phase for the fabric phase sorptive extraction of UV filters, as model organic compounds, not only under static (i.e. stirring assisted) but also in dynamic, flow-through extraction mode (i.e. as a solid phase extraction sorbent phase), producing satisfactory analytical results in terms of linearity of calibration curves (10-250 µg L-1), precision (<11%), detection limits <10 µg L-1 (using a single wavelength UV detector) and recoveries (86 - 119%) from various natural water samples. As a passive sampling sorbent phase, the MOF@cotton composite could linearly accumulate UV filters over time period of 35 days with sampling rates from 0.026 to 0.352 L d-1, which are comparable to other passive sampling sorbent phases.

Fundamentals and applications of stir bar sorptive dispersive microextraction: A tutorial review.

Current trends in sample preparation focus on the miniaturization of the process resulting in what is known as microextraction techniques. Among them stir bar sorptive dispersive microextraction (SBSDME) is one of the most recent techniques. It was presented a few years ago as a hybrid microextraction technique that combines the principles of stir bar sorptive extraction (SBSE) and dispersive solid phase extraction (DSPE). This novel technique introduces, into the sample solution, a bar-shaped magnet coated with a magnetic (nano)material, which is maintained on the surface by magnetism. Once stirring starts, the competitive action between magnetism and rotational force is exploited. At low rotational speed the (nano)material is retained on the magnet reminding SBSE, whereas at high sufficiently rotational speed the rotational force surpasses magnetism so that the magnetic (nano)material is dispersed into the sample solution as in DSPE. Upon halting rotation, the magnetic field prevails again and attracts the magnetic (nano)material containing the analytes back on the surface of the stirring bar. Finally, the extracted analytes are chemically or thermally desorbed before their measurement with the appropriate analytical instrument. When compared separately to SBSE and DSPE procedures, SBSDME affords, in a single approach, advantages such as lower extraction times than SBSE and easier extraction and post-extraction treatment than DSPE. The main purpose of this tutorial review is to describe the fundamentals and the experimental variables involved in this technique, as well as to compile and critically discuss the published papers concerning analytical methods based on the use of the SBSDME approach, analyzing its evolution and future prospects.

Biothiol modulated growth and aggregation of gold nanoparticles and their determination in biological fluids using digital photometry.

This work describes a novel and easy to use method for the determination of biologically important thiols that relies on their ability to inhibit the catalytic enlargement of AuNP seeds in the presence of ACl4- ions and trigger their aggregation. UV-vis spectroscopic monitoring of the plasmon resonance bands of the formed AuNPs showed that the spectral and color transitions depend both on the concentration and the structure of biothiols. The colorimetric changes induced by biothiols were quantified in the concentration range from 5 to 300 µM in the RGB color system with digital photometry using a commercially available flatbed scanner as detector. On the basis of these results, the applicability of the method was tested to the determination of glutathione in red blood cells and cysteine in blood plasma with satisfactory recoveries (88.7-96.5%), low detection limits (1.0 µM), good selectivity against major biomolecules under physiologically relevant conditions and satisfactory reproducibility (<8%). The method requires minimum technical expertise, is easy to use and is performed without scientific equipment, holding promise as a simple assay of biothiol testing even by non-experts.

Development of a sequential extraction and speciation procedure for assessing the mobility and fractionation of metal nanoparticles in soils.

This study describes the development of a sequential extraction procedure for the evaluation of metal nanoparticle mobility and bioaccessibility in soils. The procedure, that was developed using gold nanoparticles (AuNPs) as model species, relies on the fractionation of nanoparticles by sequentially dissolving soil matrix components (carbonates, metal oxides, organic matter and mineral phases) in order to release the entrapped nanoparticle species in the extract solution. By summing up the concentration of AuNPs recovered in each fraction it was found that 93.5% of the spiked AuNP concentration could be recovered which satisfactorily represents the nominal AuNP concentration in the soil. The efficiency of the procedure was found to depend on several procedural artifacts related to the separation of AuNPs from soil colloids and the reactivity of the extraction reagents with AuNPs and their precursor metal ions. Based on the results obtained a protocol for the speciation of the AuNPs and Au ions in the soil sample was also developed. The results of the study show that both AuNPs and Au ions are mainly associated with soil organic matter, which significantly reduces their mobility, while a small amount (<10%) is associated with metal oxides which are more mobile and potentially bioaccessible. The developed procedure provides a springboard for further development of sequential extraction procedures of metal nanoparticles in soils that could be used to assess both the exposure and release of metal nanoparticles and their precursor metal ions in the environment (as total extractable concentration) as well as provide evidence regarding their bioaccessibility and potential bioavailability by determining the concentration of nanoparticles in each specific soil fraction.

On-line extraction coupled to liquid chromatographic analysis of hydrophobic organic compounds from complex solid samples-Application to the analysis of UV filters in soils and sediments.

This work investigates the applicability of on-line extraction (OLE) directly coupled to liquid chromatography (LC) for the dynamic extraction of hydrophobic organic compounds from complex solid samples. The method operates within the existing apparatus of the LC by replacing the valve loop with an extraction vessel comprised of an empty guard column loaded with the sample. In this manner, extraction is accomplished by the mobile phase as it flows through the extraction vessel under high pressure without the need for additional apparatus other than those comprising the LC system or additional sample pretreatment steps. The experimental parameters affecting the extraction efficiency of the method were investigated and discussed in relation to both the extraction performance and the chromatographic efficiency. The method was optimized and applied for the first time to the extraction of hydrophobic UV filters from spiked soil and sediment samples yielding recoveries between 59 and 117% and reproducibility from 4.8 to 14.3%, which are comparable to those reported from more advanced sample preparation methods.

Single-point calibration and standard addition assays on calibrant-loaded paper-based analytical devices.

This article describes a new, simplified approach for performing quantitative colorimetric assays on paper-based analytical devices that uses calibrant-loaded paper devices to perform external calibration and standard addition calibration using one calibration point. Calibrant-loaded devices consist of sensing areas pre-loaded with a colored product which is produced from the reaction of a standard solution of the analyte with the appropriate colorimetric reagents. When the sample is added into the calibrant-loaded sensing zone the analytical signal (i.e. color intensity) increases proportionally to the concentration of the analyte in the tested sample. The total measured signal corresponds to the sum of the concentration of the analyte in the sample and the standard solution pre-stored in the device and is used to calculate the concentration of the analyte in the sample based on the principles of linear calibration. The applicability of this approach was benchmarked in three colorimetric assays (i.e., for the determination of iron, nickel, and amino acids) that use different reaction chemistries. This work demonstrates a simplified calibration-free approach for assays performed on paper-based analytical devices that requires minimum experimental and computational effort, it can be used for either external or standard addition calibration and can be used to identify and eliminate the presence of interferences in a sample.

Determination of silver nanoparticles by atomic absorption spectrometry after dispersive suspended microextraction followed by oxidative dissolution back-extraction.

This study presents a liquid phase microextraction method for the quantification of silver nanoparticles (AgNPs) in environmental samples by means of directly suspended microextraction and atomic absorption spectrometry. The method is based on the dispersion of octanol into fine droplets where AgNPs are partitioned and extracted from the bulk aqueous phase. Then octanol is re-dispersed again in a strongly acidic and oxidizing aqueous medium in order to selectively decompose AgNPs to silver ions. The aqueous extract is then analyzed by atomic absorption spectrometry. Under the working conditions, the method offers high selectivity for AgNPs since most nanoparticle species cannot be extracted into the octanol phase, while the selective back-extraction procedure ensures that only AgNPs are dissolved to the final extract medium. The method works equally well for AgNPs of different size and coatings, thus enabling the determination of the total concentration AgNPs in real samples. Method application in different water samples was used to evaluate the utility of the method in the analysis of real samples of variable complexity with satisfactory results in terms of detection limit (less than 100 femto-mole of AgNPs), recoveries (90-104%) and reproducibility (lower than 7%).

Paper-based devices for biothiols sensing using the photochemical reduction of silver halides.

This study describes the development of paper-based devices for the determination of biothiols. The devices are inexpensive (composed of paper and silver halide particles), and the analytical protocol is easily executable with minimum technical expertise and without the need of specialized equipment; the user has to add a test sample, illuminate the device with a UV lamp, and read the color change of the sensing area using a simple imaging device (i.e., cell-phone camera) or a bare eye. The detection mechanism of the assay is based on the biothiols-mediated photoreduction of nanometer-sized silver chloride particles deposited on the surface of paper; photoreduced silver chloride particles have a grayish coloration that depends on the concentration of biothiols in the tested solution. This is the first time that the UV-mediated photoreduction of solid silver halides particles is used for analytical purposes. The performance of the devices has been tested on the detection of total biothiols content of artificial body fluids and protein-free human blood plasma samples, and the results were satisfactory in terms of sensitivity, selectivity, recoveries and reproducibility.

Expanding the application of stir bar sorptive-dispersive microextraction approach to solid matrices: Determination of ultraviolet filters in coastal sand samples.

This work describes a new method for the determination of organic compounds in solid samples based on the equilibrium desorption of the analytes in an aqueous phase followed by stir bar sorptive-dispersive microextraction (SBSDME). Sand samples, contaminated with UV filters due to bathing and recreational activities, were dispersed in an aqueous medium by using a coated stir bar with CoFe2O4@oleic acid magnetic nanoparticles. The UV filters were physically desorbed from the surface of the sand particles and rapidly adsorbed on the hydrophobic coating of the nanoparticles, which were retrieved by means of their magnetism after stopping the stirring. In this manner, both preconcentration of the analytes and clean-up were simultaneously accomplished without the requirement of any additional sample preparation steps. After extraction, the analytes were desorbed in ethanol and then analyzed by gas chromatography-mass spectrometry (GC-MS). The main variables involved in the extraction process (i.e., sorbent amount, extraction time, pH and ionic strength) were studied to provide the best extraction efficiencies and maximum enrichment factors. Under the selected conditions, the figures of merit of the proposed method were evaluated providing limits of detection in the low ng g-1 range, enrichment factors between 23 and 80, and relative standard deviations (RSD%) below 14% for all the target analytes. Matrix effects were observed in real costal sand samples and thus standard addition calibration was employed for quantification. The method was successfully applied to the analysis of UV filters in several coastal sand samples of different origin. This work expands the analytical potential of the novel SBSDME approach to the determination of organic compounds in solid matrices.

Generic Assay of Sulfur-Containing Compounds Based on Kinetics Inhibition of Gold Nanoparticle Photochemical Growth.

This work describes a new, equipment-free, generic method for the determination of sulfur-containing compounds that is based on their ability to slow down the photoreduction kinetics of gold ions to gold nanoparticles. The method involves tracking the time required for a red coloration to appear in the tested sample, indicative of the formation of gold nanoparticles, and compare the measured time relative to a control sample in the absence of the target analyte. The method is applicable with minimal and simple steps requiring only two solutions (i.e., a buffer and a gold solution), a source of light (UV or visible), and a timer. The method responds to a large variety of sulfur-containing compounds including thiols, thioesters, disulfides, thiophosphates, metal-sulfur bonds, and inorganic sulfur and was therefore applied to the determination of a variety of compounds such as dithiocarbamate and organophosphorous pesticides, biothiols, pharmaceutically active compounds, and sulfides in different samples such as natural waters and wastewater, biological fluids, and prescription drugs. The analytical figures of merit of the method include satisfactory sensitivity (quantitation limits at the low µM levels), good recoveries (from 93 to 109%), and satisfactory reproducibility (from 4.8 to 9.8%). The method is easily adoptable to both laboratory settings and nonlaboratory conditions for quantitative and semiquantitative analysis, respectively, is user-friendly even for the minimally trained user, and can be performed with limited resources at low cost.

Paper-Based Microfluidic Device with Integrated Sputtered Electrodes for Stripping Voltammetric Determination of DNA via Quantum Dot Labeling.

This work reports a microfabricated electrochemical paper-based analytical device (ePAD) for the voltammetric determination of DNA. The device is patterned by wax-printing on paper and features a circular assay zone connected to an inlet zone and a sink via grooved microfluidic channels for accelerated flow rate. An electrochemical cell with integrated electrodes is formed on the reverse side of the paper by sputtering of thin metal films (Sn, Pt and Ag as the working, counter and reference electrode, respectively). Proof-of-principle of the ePAD for biosensing is demonstrated for a DNA assay involving attachment of capture DNA, hybridization with biotinylated target oligonucleotide and labeling with streptavidin-conjugated CdSe/ZnS quantum dots (QDs). After the acidic dissolution of the QDs, the released Cd(II) is quantified by anodic stripping voltammetry (ASV) at the Sn-film working electrode. Thanks to the synergistic effects of QDs amplification, the inherent sensitivity of ASV and the excellent detection capabilities of the Sn-film working electrode for Cd(II), the target DNA can be detected at levels as low as 0.11 pmol L-1 using sample volumes as low as 1 µL. The developed microfluidic ePAD costs only 0.11$ and presents favorable fabrication and operational features that make it an excellent candidate biosensor for simple and ultrasensitive point-of-need testing.

Low-cost colorimetric assay of biothiols based on the photochemical reduction of silver halides and consumer electronic imaging devices.

This work describes a new approach for the determination of free biothiols in biological fluids that exploits some of the basic principles of early photographic chemistry - that was based on silver-halide recording materials - and uses broadly-available imaging devices (i.e. flatbed scanners) as detectors. Specifically, the proposed approach relies on the ability of biothiols to bind to silver ions and dissociate the silver halide crystals thus changing the photosensitivity of silver halide crystal suspension. The changes induced by biothiols on the light intensity transmitted through the silver halide suspension, after photochemical reduction, were measured with a simplified photometric approach that employs a flatbed scanner operating in transmittance mode. The overall analytical procedure for the determination of biothiols was easily executable, fast and could be applied with inexpensive and commercially available materials and reagents. What is more, physiologically relevant biothiol levels could be inspected even by the unattended eye. The developed assay was successfully applied to the determination of biothiols in urine and blood plasma samples with detection limits as low as 10µM, satisfactory recoveries (92-97%), good reproducibility (6.7-8.8%) and high selectivity against other major components of biological fluids. The utility of the method to the determination of reduced/oxidized thiol ratio's as well as its application under natural light illumination, without external energy sources, was also demonstrated and is discussed with regard to point-of need applications in facility-limited settings.

Determination of gold nanoparticles in environmental water samples by second-order optical scattering using dithiotreitol-functionalized CdS quantum dots after cloud point extraction.

This work presents a new method for the sensitive and selective determination of gold nanoparticles in water samples. The method combines a sample preparation and enrichment step based on cloud point extraction with a new detection motif that relies on the optical incoherent light scattering of a nano-hybrid assembly that is formed by hydrogen bond interactions between gold nanoparticles and dithiotreitol-functionalized CdS quantum dots. The experimental parameters affecting the extraction and detection of gold nanoparticles were optimized and evaluated to the analysis of gold nanoparticles of variable size and surface coating. The selectivity of the method against gold ions and other nanoparticle species was also evaluated under different conditions reminiscent to those usually found in natural water samples. The developed method was applied to the analysis of gold nanoparticles in natural waters and wastewater with satisfactory results in terms of sensitivity (detection limit at the low pmolL-1 levels), recoveries (>80%) and reproducibility (<9%). Compared to other methods employing molecular spectrometry for metal nanoparticle analysis, the developed method offers improved sensitivity and it is easy-to-operate thus providing an additional tool for the monitoring and the assessment of nanoparticles toxicity and hazards in the environment.

Intrinsic peroxidase-like activity of rhodium nanoparticles, and their application to the colorimetric determination of hydrogen peroxide and glucose.

The intrinsic peroxidase-like activity of rhodium nanoparticles (RhNPs) and their use as catalytic labels for sensitive colorimetric assays is presented. RhNPs catalyze the oxidation of the peroxidase substrate 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H2O2 to produce a blue reaction product with a maximum absorbance at 652 nm. Kinetic studies show catalysis to follow Michaelis-Menten kinetics and a "ping-pong" mechanism. The calculated kinetic parameters indicate high affinity of RhNPs for both the substrate TMB and H2O2. In fact, they are better than other peroxidase mimicking nanomaterials and even the natural enzyme horseradish peroxidase. On the other hand, RhNPs exhibit no reactivity towards saccharides, thiols, amino acids and ascorbic acid. Based on these findings, a sensitive and selective colorimetric method was worked out for the determination of H2O2 in real samples with a linear response in the 1-100 µM concentration range. By employing glucose oxidase, the glucose assay has a linear range that covers the 5 to 125 µM glucose concentration range. The detection limits are <0.75 µM for both species. The methods were applied to the determination of H2O2 in spiked pharmaceutical formulations, and of glucose in soft drinks and blood plasma. Figures of merit include (a) good accuracy (with errors of <6%), (b) high recoveries (96.5-103.7%), and (c) satisfactory reproducibility (<6.3%). Graphical abstract Rhodium nanoparticles catalyze the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H2O2 to produce a blue reaction product. The effect is exploited in photometric assays for hydrogen peroxide and glucose.

Determination of phenolic compounds using spectral and color transitions of rhodium nanoparticles.

This work reports a new approach for the determination of phenolic compounds based on their interaction with citrate-capped rhodium nanoparticles. Phenolic compounds (i.e., catechins, gallates, cinnamates, and dihydroxybenzoic acids) were found to cause changes in the size and localized surface plasmon resonance of rhodium nanoparticles, and therefore, give rise to analyte-specific spectral and color transitions in the rhodium nanoparticle suspensions. Upon reaction with phenolic compounds (mainly dithydroxybenzoate derivatives, and trihydroxybenzoate derivatives), new absorbance peaks at 350 nm and 450 nm were observed. Upon reaction with trihydroxybenzoate derivatives, however, an additional absorbance peak at 580 nm was observed facilitating the speciation of phenolic compounds in the sample. Both absorbance peaks at 450 nm and 580 nm increased with increasing concentration of phenolic compounds over a linear range of 0-500 µM. Detection limits at the mid-micromolar levels were achieved, depending on the phenolic compound involved, and with satisfactory reproducibility (<7.3%). On the basis of these findings, two rhodium nanoparticles-based assays for the determination of the total phenolic content and total catechin content were developed and applied in tea samples. The obtained results correlated favorably with commonly used methods (i.e., Folin-Ciocalteu and aluminum complexation assay). Not the least, the finding that rhodium nanoparticles can react with analytes and exhibit unique localized surface plasmon resonance bands in the visible region, can open new opportunities for developing new optical and sensing analytical applications.

Stir bar sorptive-dispersive microextraction mediated by magnetic nanoparticles-nylon 6 composite for the extraction of hydrophilic organic compounds in aqueous media.

A new and sensitive analytical method based on the recently developed approach termed stir bar-sorptive dispersive microextraction (SBSDME) using a magnetic CoFe2O4@SiO2-nylon 6 composite as sorbent material is presented for the extraction of hydrophilic organic compounds. The simultaneous determination of four hydrophilic UV filters in environmental water samples has been chosen as a model analytical application due to the increasing awareness regarding the occurrence of sunscreen residuals in natural waters. The developed SBSDME approach combines the principles and benefits of stir bar sorptive extraction (SBSE) and dispersive solid phase extraction (DSPE) but allows for lower extraction time and easier post-extraction treatment. Moreover, most importantly, it enables the use of new magnetic materials that affords higher versatility and can be tailored to the needs of the analysis. The main experimental parameters involved in the SBSDME process (i.e. composite amount, extraction time, pH, ionic strength, desorption solvent and desorption time) were evaluated to provide the best enrichment factors. Under the optimized conditions, the method was successfully validated showing good linearity, enrichment factors between 105 and 145 depending on the analyte, limits of detection and quantification in the low ng mL(-1) range (1.6-2.9 ng mL(-1) and 5.4-9.6 ng mL(-1), respectively) and good intra- and inter-day repeatability (RSD < 13%). The developed method was applied to the analysis of water samples of different origin (sea, river and swimming pool). Relative recovery values ranged between 90 and 115%, thus showing that the matrices under consideration do not affect the extraction process.

In-situ suspended aggregate microextraction of gold nanoparticles from water samples and determination by electrothermal atomic absorption spectrometry.

This work describes a new method for the extraction and determination of gold nanoparticles in environmental samples by means of in-situ suspended aggregate microextraction and electrothermal atomic absorption spectrometry. The method relies on the in-situ formation of a supramolecular aggregate phase through ion-association between a cationic surfactant and a benzene sulfonic acid derivative. Gold nanoparticles are physically entrapped into the aggregate phase which is separated from the bulk aqueous solution by vacuum filtration on the surface of a cellulose filter in the form of a thin film. The film is removed from the filter surface and is dissociated into an acidified methanolic solution which is used for analysis. Under the optimized experimental conditions, gold nanoparticles can be efficiently extracted from water samples with recovery rates between 81.0-93.3%, precision 5.4-12.0% and detection limits as low as 75femtomolL(-1) using only 20mL of sample volume. The satisfactory analytical features of the method along with the simplicity indicate the efficiency of this new approach to adequately collect and extract gold nanoparticle species from water samples.