Surface functionalization of graphene oxide with tannic acid: Covalent vs non-covalent approaches

Graphene oxide (GO) is gaining a lot of interest in material science, biomedicine and biotechnology due to its outstanding physical properties, combined with its surface functionalization capacity, processability in aqueous media and biocompatibility. However, van der Waals forces among GO layers result in aggre-gation, yet its dispersion, large-scale production, and reinforcing efficiency remain challenging. Herein, simple and environmentally friendly methods via covalent and non-covalent routes have been developed to exfoliate and prepare surface-functionalized GO nanosheets with tannic acid (TA), a biological macro-molecule with antioxidant activity. Four esterification strategies were tested: direct, carbodiimide acti-vated, oxalyl chloride acylation and via an acid-functionalized GO intermediate. The resulting samples have been extensively characterized to get knowledge on the GO-TA interactions and the degree of graft-ing, as well as their surface topography, level of hydrophilicity, solubility/dispersibility, thermal and antibacterial properties. The covalent grafting of TA renders the GO surface more hydrophobic, resulting in improved dispersion in organic solvents. Besides, TA acts as a crosslinker between the GO nanosheets, leading to higher thermal resistance. A synergistic effect of both GO and TA on inhibiting bacterial growth has also been found. The esterification via carbodiimide leads to the highest grafting degree, the best thermal stability and the most effective antibacterial activity. This work not only highlights the great potential of TA for both exfoliation and surface functionalization of GO, but also extends its applications in biomedicine and for the development of green nanocomposites. (c) 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Easy, fast, and clean fluorescence analysis of tryptophan with clays and graphene/clay mixtures

Clays (C) such as sepiolite (SEP) or bentonite (BEN) and their mixtures with graphene (G) have been used as sorbents in dispersive solid phase extraction (dSPE). Tryptophan (TRP) analysis by fluorescence combined with a sample preparation step using G/C 30/70 w/w mixture provides a quantitative TRP retention, independently of the amino acid concentration with a desorption process feasible in 80 mM aqueous solution of the surfactant Brij L23. Under these conditions, the detection and quantification limits are 3.5 and 11.8 mu g L-1, respectively. Additionally, a novel, simple and inexpensive method has been developed to directly analyse TRP in real sam-ples, in which the presence of matrix interferents typically limits to obtain accurate results. For the first time, BEN has been used as an effective clean-up sorbent for the fluorimetric analysis of TRP in beer, leading to results without statistical differences versus those of a reference HPLC method free of interferences, with recoveries of 90 % and 100 %. The proposed method can be applied to accurately analyse TRP in complex matrices in a direct, easy, fast and sustainable way.Ministerio de Innovación, Ciencia y Universidade

Bioactive trans-resveratrol as dispersant of graphene in water. Molecular interactions

Currently, the preparation of high-quality graphene (G) dispersions is crucial due to the increasing demand for this nanomaterial in a wide range of industries. However, given the strong 7C-7C stacking tendency between G sheets, dispersant agents such as surfactants or polymers are required to attain stable and homogenous disper-sions in liquid media. In the present work, the effectiveness of resveratrol (RV), a fluorescent bioactive compound with antioxidant activity, as a dispersing agent for G in aqueous solutions was assessed. The interaction between G and RV was investigated via absorbance, fluorescence and Raman measurements. Dispersions were prepared via bath sonication, followed by probe ultrasonication and centrifugation, though the two last stages have little effect on the dispersion quality. The addition of G causes a quenching on RV fluorescence, and its magnitude raises with increasing G concentration, being the effect stronger up to 10 mg L-1. The change in the centrifugation speed and time have hardly influence on the RV fluorescence in the presence of G. The RV content remaining in the bulk solution after G dispersion, measured via UV-Vis absorption, decreases linearly with increasing G concentration. RV can effectively disperse all the G present in the sample up to 10 mg L-1; at higher concen-trations, the dispersing ability slightly decreases, and the G content is distributed between the bulk solution and the centrifuged residue. Besides, the fluorescence of RV dispersed in G is much higher in methanol than in an aqueous medium. TEM analysis confirms the good exfoliation of G upon ultrasonication in RV solutions and indicates that the layer thickness depends on the G/RV ratio. The results of this study could open new per-spectives for using natural products like resveratrol as biocompatible and efficient dispersing agents of G to be used in numerous applications, especially in biomedicine.Ministerio de Ciencia, Innovación y Universidade

Dispersive solid phase extraction/ fluorescence analysis of riboflavin using sepiolite as sorbent

The use of a natural clay, sepiolite (Sep), and graphene (G)/Sep mixtures as sorbents for the solid phase extraction in dispersive mode (dSPE) with fluorescence detection of a biologically interesting molecule, riboflavin, is reported. The retention of riboflavin by Sep is quantitative with different volumes of the sample and 10?50?mg of the clay. The desorption process has been performed using aqueous solutions of surfactants of different nature: anionic sodium dodecylsulphate (SDS), cationic hexadecyltrimethylammonium chloride (CTAC) and non-ionic polyoxyethylene-23-lauryl eter (Brij L23). The non-ionic surfactant provides the highest riboflavin recoveries; upon optimization of the process, that takes less than half an hour, an extraction yield of 93% was attained using 40?mM Brij L23. The developed method has been applied to the analysis of beer, soya drink and infant milk samples with excellent accuracy (recoveries of 100%) and very good precision, avoiding the interferences of the matrix components that hinder the direct fluorescence analysis of riboflavin.Ministerio de Economía y Competitivida

Study of graphene dispersions in sodium dodecylsulfate by steady-state fluorescence of pyrene

Hypothesis: Aqueous solutions of ionic surfactants allow the exfoliation of graphene, that can be explained considering the adsorption model of ionic surfactants to hydrophobic surfaces. For many years, pyrene has been used as a fluorescent probe because its sensitivity to the micro-environment. The study of pyrene fluorescence in the presence of different graphene dispersions in an ionic surfactant, would improve the knowledge of the graphene-surfactant interactions.Experiments: Different dispersions of graphene in sodium dodecylsulfate were prepared at different weight ratios 0.5, 1 and 2%. The dispersions have been studied by Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. The influence of the dispersions on the pyrene fluorescence has been investigated.Findings: The graphene sheets modified by the surfactant quench the fluorescence of pyrene, which depends on the amount of graphene, the concentration of surfactant and the weight ratio. For surfactant concentrations below the critical micelle concentration, the quenching effect is higher as the weight ratio increases. Once this concentration is reached, the fluorescence increases slightly and then levels off. This behavior has been explained by the adsorption model. For a constant surfactant concentration, twostraight lines can be observed in the Stern-Volmer plots whose cut-off point is approximately 20 mg L-1 of graphene.Ministerio de Economía y CompetitividadUniversidad de Alcal

Fluorescence Study of Riboflavin Interactions with Graphene Dispersed in Bioactive Tannic Acid

The potential of tannic acid (TA) as a dispersing agent for graphene (G) in aqueous solutions and its interaction with riboflavin have been studied under different experimental conditions. TA induces quenching of riboflavin fluorescence, and the effect is stronger with increasing TA concentration, due to pi-pi interactions through the aromatic rings, and hydrogen bonding interactions between the hydroxyl moieties of both compounds. The influence of TA concentration, the pH, and the G/TA weight ratio on the quenching magnitude, have been studied. At a pH of 4.1, G dispersed in TA hardly influences the riboflavin fluorescence, while at a pH of 7.1, the nanomaterial interacts with riboflavin, causing an additional quenching to that produced by TA. When TA concentration is kept constant, quenching of G on riboflavin fluorescence depends on both the G/TA weight ratio and the TA concentration. The fluorescence attenuation is stronger for dispersions with the lowest G/TA ratios, since TA is the main contributor to the quenching effect. Data obey the Stern-Volmer relationship up to TA 2.0 g L-1 and G 20 mg L-1. Results demonstrate that TA is an effective dispersant for graphene-based nanomaterials in liquid medium and a green alternative to conventional surfactants and synthetic polymers for the determination of biomolecules.Ministerio de Ciencia, Innovación y Universidade

Graphene/sepiolite mixtures as dispersive solid-phase extraction sorbents for the anaysis of polycyclic aromatic hydrocarbons in wastewater using surfactant aqueous solutions for desorption

tDifferent graphene/sepiolite (G/Sep) solid mixtures have been prepared and tested as nanometric sor-bents for the analysis of polycyclic aromatic hydrocarbons (PAHs) using dispersive solid phase extraction(dSPE) and aqueous solutions of surfactants as environmentally friendly agents for desorption. Quantifi-cation of the PAHs was carried out by reversed-phase liquid chromatography (RP-HPLC) with fluorescencedetection. The adsorption of four PAHs with increasing number of benzene rings into a G/Sep mixture(2/98, w/w) was investigated. A 100% retention was attained for phenanthrene (Phe), pyrene (Pyr) andbenzo(a)pyrene (BaP), while for naphthalene (Nap) the maximum retention was close to 75%. The G/Sepmixtures can be used to remove the PAHs from wastewater. The desorption step was carried out usingan aqueous surfactant solution: 100 mM non-ionic polyoxiethylen-23-lauryl eter (Brij L23). Consideringthe whole extraction process, the highest PAH recoveries (50, 92, 83 and 76% for Nap, Phe, Pyr and BaP,respectively) were obtained using 100 mM Brij L23. The developed method shows very high sensitivity,robustness and precision, as well as low limits of detection and quantification, and has been successfullyapplied to the analysis of PAHs in wastewater samples.Ministerio de Economía y Competitivida

Graphene-Based Sensors for the Detection of Bioactive Compounds: A Review

Over the last years, different nanomaterials have been investigated to design highly selective and sensitive sensors, reaching nano/picomolar concentrations of biomolecules, which is crucial for medical sciences and the healthcare industry in order to assess physiological and metabolic parameters. The discovery of graphene (G) has unexpectedly impulsed research on developing cost-effective electrode materials owed to its unique physical and chemical properties, including high specific surface area, elevated carrier mobility, exceptional electrical and thermal conductivity, strong stiffness and strength combined with flexibility and optical transparency. G and its derivatives, including graphene oxide (GO) and reduced graphene oxide (rGO), are becoming an important class of nanomaterials in the area of optical and electrochemical sensors. The presence of oxygenated functional groups makes GO nanosheets amphiphilic, facilitating chemical functionalization. G-based nanomaterials can be easily combined with different types of inorganic nanoparticles, including metals and metal oxides, quantum dots, organic polymers, and biomolecules, to yield a wide range of nanocomposites with enhanced sensitivity for sensor applications. This review provides an overview of recent research on G-based nanocomposites for the detection of bioactive compounds, providing insights on the unique advantages offered by G and its derivatives. Their synthesis process, functionalization routes, and main properties are summarized, and the main challenges are also discussed. The antioxidants selected for this review are melatonin, gallic acid, tannic acid, resveratrol, oleuropein, hydroxytyrosol, tocopherol, ascorbic acid, and curcumin. They were chosen owed to their beneficial properties for human health, including antibiotic, antiviral, cardiovascular protector, anticancer, anti-inflammatory, cytoprotective, neuroprotective, antiageing, antidegenerative, and antiallergic capacity. The sensitivity and selectivity of G-based electrochemical and fluorescent sensors are also examined. Finally, the future outlook for the development of G-based sensors for this type of biocompounds is outlined

Graphene Oxides Derivatives Prepared by an Electrochemical Approach: Correlation between Structure and Properties

Graphene oxide (GO) can be defined as a single monolayer of graphite with oxygen-containing functionalities such as epoxides, alcohols, and carboxylic acids. It is an interesting alternative to graphene for many applications due to its exceptional properties and feasibility of functionalization. In this study, electrochemically exfoliated graphene oxides (EGOs) with different amounts of surface groups, hence level of oxidation, were prepared by an electrochemical two-stage approach using graphite as raw material. A complete characterization of the EGOs was carried out in order to correlate their surface topography, interlayer spacing, defect content, and specific surface area (SSA) with their electrical, thermal, and mechanical properties. It has been found that the SSA has a direct relationship with the d-spacing. The EGOs electrical resistance decreases with increasing SSA while rises with increasing the D/G band intensity ratio in the Raman spectra, hence the defect content. Their thermal stability under both nitrogen and dry air atmospheres depends on both their oxidation level and defect content. Their macroscopic mechanical properties, namely the Young's modulus and tensile strength, are influenced by the defect content, while no correlation was found with their SSA or interlayer spacing. Young moduli values as high as 54 GPa have been measured, which corroborates that the developed method preserves the integrity of the graphene flakes. Understanding the structure-property relationships in these materials is useful for the design of modified GOs with controllable morphologies and properties for a wide range of applications in electrical/electronic devices

Impact of recovery correction or subjecting calibrators to sample preparation on measurement uncertainty: PAH determinations in waters

The decision on the fitness of a measurement for its intended use and the interpretation of an analytical result requires the assessment of the measurement uncertainty. Frequently, the determination of analytes in complex matrices involves demanding sample preparations in which analyte losses are observed. These losses should be considered when reporting the results, which can be corrected for low recovery by taking the mean recovery observed in the analysis of reference items (e.g. spiked samples) or, alternatively, by subjecting calibrators to the same pre-treatment performed on the samples. In these cases, neat (NC) or adjusted (AC) calibrators are used, respectively. The way analyte losses are handled impacts on the measurement uncertainty. The top-down evaluation of the measurement uncertainty involves combining precision, trueness and additional uncertainty components. The trueness component is quantified by pooling various analyte recovery determinations. This work assesses and compares the uncertainty of polycyclic aromatic hydrocarbons (PAHs) measurements in water based on HPLC-FD calibrations with NC or AC. The trueness component is estimated by pooling mean recoveries observed from the analysis of different spiked samples to which mean recovery uncertainty and degrees of freedom are used to estimate a weighted mean recovery and respective uncertainty. The performance of measurements based on NC and AC are associated with equivalent uncertainty except when large analyte losses are observed, namely in the determination of Naphtalene. In this case, the processing of AC reduces the expanded relative uncertainty from 9.9% to 3.5%. The evaluated expanded uncertainty ranged from 3.5% to 12% of the measured value.Ministerio de Economía y CompetitividadBanco SantanderFundação para a Ciência e Tecnologí