Heparin-stabilized gold nanoparticles embedded in graphene for the electrochemical determination of esculetin.

A conductive nanocomposite consisting of heparin-stabilized gold nanoparticles embedded in graphene was prepared and characterized to develop an electrochemical sensor for the determination of esculetin in tea and jam samples. The gold nanoparticles were characterized by spectroscopic and microscopic techniques. The different proportions of graphene in the nanocomposite were evaluated and characterized by electrochemical practices. The heterostructure material on the glassy carbon electrode with esculetin showed π-π stacking interactions with an adsorption-controlled process. The voltammetric profile of esculetin using the proposed nanomaterial presented oxidation and reduction peaks at +0.61 and +0.58 V vs. Ag/AgCl, respectively, facilitating the electron transfer with esculetin through the transfer of two moles of protons and two moles of electrons per mole of esculetin. Using optimized conditions and square wave voltammetry, the calibration curve was obtained with two linear ranges, from 0.1 to 20.5 µmol L-1, with a detection limit of 43.0 nmol L-1. The electrochemical sensor showed satisfactory results for repeatability and stability, although interferences were observed in the presence of high concentrations of ascorbic acid or quercetin. The sensor was successfully applied in the determination of esculetin in samples of mulberry jam, white mulberry leaf tea, and white mulberry powder tea, presenting adequate recovery ranges. This directive provides valuable insights for the development of novel electrochemical sensors using heparin-based conductive nanomaterials with improved sensitivity and sensibility.

Nanocomposite based on green synthesis of gold nanoparticles decorated with functionalized multi-walled carbon nanotubes for the electrochemical determination of hydroxychloroquine.

In this study, a selective and sensitive electrochemical approach for determining hydroxychloroquine (HCQ) was proposed. A novel nanocomposite based on gold nanoparticles synthesized by green synthesis in an extract of white pitaya (Hylocereus undatus) (AuNP-Ext) decorated with functionalized multi-walled carbon nanotubes (f-MWCNTs) was presented. AuNP-Ext was characterized by ultraviolet-visible spectroscopy and the f-MWCNTs/AuNP-Ext nanocomposite by transmission electron microscopy. The nanocomposite was used to modify a glassy carbon electrode (GCE). Using the f-MWCNT-AuNP-Ext/GCE sensor, an irreversible oxidation peak at +0.74 V vs. Ag/AgCl was verified by HCQ. The calibration plot was studied in two linear ranges, from 0.03 to 3.5 µmol/L and from 3.5 to 17.0 µmol/L, with a limit of detection of 0.0093 µmol/L and a limit of quantification of 0.031 µmol/L, regarding the first linear range. The proposed sensor was successfully applied to the determination of HCQ in pharmaceutical and clinical samples without any special purification, separation or pre-treatment steps. The accuracy was verified by UV-Vis spectrometry, and this revealed that the proposed method was accurate and precise, as evidenced by F- and t-tests.

Graphene and gold nanoparticle-based bionanocomposite for the voltammetric determination of bisphenol A in (micro)plastics.

The monitoring of endocrine disruptors in the environment is one of the main strategies in the investigation of potential risks associated with exposure to these chemicals. Bisphenol A is one of the most prevalent endocrine-disrupting compounds and is prone to leaching out from polycarbonate plastic in both freshwater and marine environments. Additionally, microplastics also can leach out bisphenol A during their fragmentation in the water environment. In the quest for a highly sensitive sensor to determine bisphenol A in different matrices, an innovative bionanocomposite material has been achieved. This material is composed of gold nanoparticles and graphene, and was synthesized using a green approach that utilized guava (Psidium guajava) extract for reduction, stabilization, and dispersion purposes. Transmission electron microscopy images revealed well-spread gold nanoparticles with an average diameter of 31 nm on laminated graphene sheets in the composite material. An electrochemical sensor was developed by depositing the bionanocomposite onto a glassy carbon surface, which displayed remarkable responsiveness towards bisphenol A. Experimental conditions such as the amount of graphene, extract: water ratio of bionanocomposite and pH of the supporting electrolyte were optimized to improve the electrochemical performance. The modified electrode displayed a marked improvement in current responses for the oxidation of bisphenol A as compared to the uncovered glassy carbon electrode. A calibration plot was established for bisphenol A in 0.1 mol L-1 Britton-Robinson buffer (pH 4.0), and the detection limit was determined to equal to 15.0 nmol L-1. Recovery data from 92 to 109% were obtained in (micro)plastics samples using the electrochemical sensor and were compared with UV-vis spectrometry, demonstrating its successful application with accurate responses.