Electrocatalytic oxidation of ascorbic acid by norepinephrine embedded in lipid cast film at glassy carbon electrode

Electrochemical determination of ascorbic acid using palladium supported on N-doped graphene quantum dot modified electrode

To precise screening concentration of ascorbic acid (AA), a novel electrochemical sensor was prepared using palladium nanoparticles decorated on nitrogen-doped graphene quantum dot modified glassy carbon electrode (PdNPs@N-GQD/GCE). For this purpose, nitrogen doped GQD nanoparticles (N-GQD) were synthesized from a citric acid condensation reaction in the presence of ethylenediamine and subsequently modified by palladium nanoparticles (PdNPs). The electrochemical behavior of AA was investigated, in which the oxidation peak appeared at 0 V related to the AA oxidation. Considering the synergistic effect of Pd nanoparticles as an active electrocatalyst, and N-GQD as an electron transfer accelerator and electrocatalytic activity improving agent, PdNPs@N-GQD hybrid materials showed excellent activity in the direct oxidation of AA. In the optimal conditions, the voltammetric response was linear in the range from 30 to 700 nM and the detection limit was calculated to be 23 nM. The validity and the efficiency of the proposed sensor were successfully tested and confirmed by measuring AA in real samples of chewing tablets, and fruit juice.

A Comparison of the Sensing Behavior for Pt-Mo/C-, Pt-Zr/C-, Pt-Fe-Ir/C-, and Pt/C-Modified Glassy Carbon Electrodes for the Oxidation of Ascorbic Acid and Dopamine

This study compares the sensing performance for platinum-molybdenum-, platinum-zirconium-, platinum-iron-iridium-, and platinum-modified electrodes in terms of the amperometric detection of ascorbic acid (AA) and dopamine (DA). The Pt, Pt-Mo, Pt-Zr, and Pt-Fe-Ir electrocatalysts are fabricated by chemical reduction on a carbon black support (XC-72) and are further modified on a glassy carbon electrode (GCE) as sensing electrodes. The Pt-Mo/C/GCE exhibits better electrocatalytic activity toward AA and DA than the Pt/C/GCE, Pt-Zr/C/GCE, and Pt-Fe-Ir/C/GCE. The Pt-Mo/C/GCE exhibits a sensitivity of 31.29 µA mM−1 to AA at 0.3 V vs. Ag/AgCl and a sensitivity of 72.24 µA mM−1 to DA at 0.6 V vs. Ag/AgCl and is reproducible and stable. This electrode has a respective limit of detection of 7.69 and 6.14 µM for AA and DA. Sucrose, citric acid, tartaric acid, and uric acid do not interfere with AA and DA detection. The diffusion coefficient and kinetic parameters, such as the catalytic rate constant and the heterogeneous rate constant for AA and DA, are determined using electrochemical approaches.

Gold-Platinum Core-Shell Nanoparticles with Thiolated Polyaniline and Multi-Walled Carbon Nanotubes for the Simultaneous Voltammetric Determination of Six Drug Molecules

In this proof-of-concept study, a novel nanocomposite of the thiolated polyaniline (tPANI), multi-walled carbon nanotubes (MWCNTs) and gold–platinum core-shell nanoparticles (Au@Pt) (tPANI-Au@Pt-MWCNT) was synthesized and utilized to modify a glassy carbon electrode (GCE) for simultaneous voltammetric determination of six over-the-counter (OTC) drug molecules: ascorbic acid (AA), levodopa (LD), acetaminophen (AC), diclofenac (DI), acetylsalicylic acid (AS) and caffeine (CA). The nanocomposite (tPANI-Au@Pt-MWCNT) was characterized with transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). Using the sensor (GCE-tPANI-Au@Pt-MWCNT) in connection with differential pulse voltammetry (DPV), the calibration plots were determined to be linear up to 570.0, 60.0, 60.0, 115.0, 375.0 and 520.0 µM with limit of detection (LOD) of 1.5, 0.25, 0.15, 0.2, 2.0, and 5.0 µM for AA, LD, AC, DI, AS and CA, respectively. The nanocomposite-modified sensor was successfully used for the determination of these redox-active compounds in commercially available OTC products such as energy drinks, cream and tablets with good recovery yields ranging from 95.48 ± 0.53 to 104.1 ± 1.63%. We envisage that the electrochemical sensor provides a promising platform for future applications towards the detection of redox-active drug molecules in pharmaceutical quality control studies and forensic investigations.

Catechol-based biomimetic functional materials

Catechols are found in nature taking part in a remarkably broad scope of biochemical processes and functions. Though not exclusively, such versatility may be traced back to several properties uniquely found together in the o-dihydroxyaryl chemical function; namely, its ability to establish reversible equilibria at moderate redox potentials and pHs and to irreversibly cross-link through complex oxidation mechanisms; its excellent chelating properties, greatly exemplified by, but by no means exclusive, to the binding of Fe(3+); and the diverse modes of interaction of the vicinal hydroxyl groups with all kinds of surfaces of remarkably different chemical and physical nature. Thanks to this diversity, catechols can be found either as simple molecular systems, forming part of supramolacular structures, coordinated to different metal ions or as macromolecules mostly arising from polymerization mechanisms through covalent bonds. Such versatility has allowed catechols to participate in several natural processes and functions that range from the adhesive properties of marine organisms to the storage of some transition metal ions. As a result of such an astonishing range of functionalities, catechol-based systems have in recent years been subject to intense research, aimed at mimicking these natural systems in order to develop new functional materials and coatings. A comprehensive review of these studies is discussed in this paper.

Electrocatalytic Determination of L-Ascorbic Acid by Modified Glassy Carbon with Ni(Me2(CH3CO)2[14]tetraenoN4) Complex

A novel modified glassy carbon electrode containing Ni(Me2(CH3CO)2[14]tetraenoN4) complex was used as an electrocatalytic sensor for the determination of L-ascorbic acid in pH = 6.6. The peak potential shifted to negative by 205 mV compared with that for a bare electrode in cyclic voltammograms. The calibration curve was linear up to 6.2 x 10(-3) M with a detection limit 3.1 x 10(-7) M and an RSD% better than 2.47%. This newly modified electrode was applied to commercial pharmaceutical tablets, injections and foods. The obtained results were identical to those obtained by the classical 2,6-dichlorophenolindophenol method.

Studies of sol-gel ceramic film incorporating methylene blue on glassy carbon: an electrocatalytic system for the simultaneous determination of ascorbic and uric acids

In this work, we investigated the immobilization of methylene blue in a methyltrimethoxysilane sol-gel ceramic film on a glassy carbon electrode. Up to a certain saturation level, under our conditions, it was found that the methylene blue was tightly held and did not leach out into aqueous solutions, even with continuous immersion for up to 1 month. The electrochemical behavior of the immobilized methylene blue was then studied. pH variation revealed that there were two distinct redox couples whose existences were pH-dependent. The methylene blue/sol-gel film was also examined as an electrocatalytic system for ascorbic and uric acid oxidations. It was revealed that this system was highly sensitive for ascorbic and uric acid sensing (practical determination limits of 5.00 nM and 1.00 nM for ascorbic acid and uric acid, respectively) and also allowed simultaneous determination of these biomolecules. The simultaneous determination of these two analytes in a human urine sample was demonstrated. The stability of the methylene blue/sol-gel film/glassy carbon electrode sensing system was good, with up to at least a month of continual operation.