Simultaneous determination of L-cysteine and L-tyrosine using Au-nanoparticles/poly-eriochrome black T film modified glassy carbon electrode

Decorated graphene oxide with gold nanoparticles as a sensitive modified carbon paste electrode for simultaneous determination of tyrosine and uric acid

It is presented here as a simple, selective, rapid, low-cost, with a wide linear range method to simultaneously determine tyrosine and uric acid using a modified carbon paste electrode decorated with graphene oxide and gold nanoparticles (GO/AuNPs/MCPE). In order to characterize and evaluate the morphology and constituents of the nanostructures, X-ray diffraction spectroscopy, Transmission electron microscopes, Dynamic light scattering, Zeta potential, electrochemical impedance spectroscopy, and Voltammetry were employed. The current response on the surface of the modified electrode had a dynamic linear range relationship in the concentrations of 0.14-340.00 µmol L-1 and 0.06-141.00 µmol L-1 for tyrosine and uric acid, respectively, and the method detection limit (MDL) was 0.0060 µmol L-1 and 0.0037 µmol L-1, respectively. This modified electrode provided high stability, sensitivity, and acceptable reproducibility for voltammetric measurements of tyrosine and uric acid simultaneously in a biological matrix.

Detection of Residual 2-Phenylphenol on Lemon Rind by Electrochemically Deposited Poly(hydroxybenzaldehyde) and Poly(hydroxybenzoic acid) Polymeric Stackings as Electrode Modifiers

This study explores the characteristics of electrodeposition of the three hydroxybenzaldehyde isomers and selected hydroxybenzoic acids (4-hydroxybenzoic acid, salicylic acid, 3,5-dihydroxybenzoic acid) from mesityl oxide solvent. Similar to recent advances of this solvent, used by electrochemical studies, the carbon-carbon double bond had significant influence on the formation of polymers from the outlined molecules. In case of most substrates the peak currents increased to a steady-state but electropolymerization of some substrates caused significant deactivation. Scanning electron microscopic and complementary voltammetric studies facilitated that the electrochemically formed polymers are present on the electrode surface in stackings. In viewpoint of analysis of 2-phenylphenol, the modifying deposit formed from 4-hydroxybenzaldehyde was the best with 5 µM detection limit obtained with differential pulse voltammetry. Furthermore, a new procedure was chosen for the involvement of a cavitand derivative into the organic layers with the purpose to improve the layer selectivity (subsequent electrochemical polymerization in an other solution). Further studies showed that in this way the sensitivities of as-modified electrodes were a little worse than without this step, thus indicating that application of this step is disadvantageous. Recovery studies of 2-phenylphenol were carried out on lemon rind without any treatment, and it was compared with the case when the outer yellow layer was removed by rasping. The inner tissues showed very high adsorption affinity towards 2-phenylphenol.

Electrochemical Amino Acid Sensing: A Review on Challenges and Achievements

The rapid growth of research in electrochemistry in the last decade has resulted in a significant advancement in exploiting electrochemical strategies for assessing biological substances. Among these, amino acids are of utmost interest due to their key role in human health. Indeed, an unbalanced amino acid level is the origin of several metabolic and genetic diseases, which has led to a great need for effective and reliable evaluation methods. This review is an effort to summarize and present both challenges and achievements in electrochemical amino acid sensing from the last decade (from 2010 onwards) to show where limitations and advantages stem from. In this review, we place special emphasis on five well-known electroactive amino acids, namely cysteine, tyrosine, tryptophan, methionine and histidine. The recent research and achievements in this area and significant performance metrics of the proposed electrochemical sensors, including the limit of detection, sensitivity, stability, linear dynamic range(s) and applicability in real sample analysis, are summarized and presented in separate sections. More than 400 recent scientific studies were included in this review to portray a rich set of ideas and exemplify the capabilities of the electrochemical strategies to detect these essential biomolecules at trace and even ultra-trace levels. Finally, we discuss, in the last section, the remaining issues and the opportunities to push the boundaries of our knowledge in amino acid electrochemistry even further.

Recent developments in voltammetric and amperometric sensors for cysteine detection

This review article aims to provide an overview of the recent advances in the voltammetric and amperometric sensing of cysteine (Cys). The introduction summarizes the important role of Cys as an essential amino acid, techniques for its sensing, and the utilization of electrochemical methods and chemically modified electrodes for its determination. The main section covers voltammetric and amperometric sensing of Cys based on glassy carbon electrodes, screen printed electrodes, and carbon paste electrodes, modified with various electrocatalytic materials. The conclusion section discusses the current challenges of Cys determination and the future perspectives.

TiO2 nanotube array-modified electrodes for L-cysteine biosensing: experimental and density-functional theory study

We report a non-enzymatic facile method for the detection of L-cysteine (L-Cyst) using free-standing TiO₂ nanotube (TNT) array-modified glassy carbon electrodes (GCEs). Self-organized, highly ordered, and vertically oriented TNT arrays were fabricated by anodization of titanium sheets in ethylene glycol-based electrolyte. Detailed electrochemical measurements were performed and it was found that modified GCE exhibited high current compared to the pristine counterpart. The high current of the modified electrode was attributed to the high surface area and enhanced electrocatalytic activities of the TNTs toward the L-Cyst oxidation. Under the optimum conditions, the modified electrode exhibited a high sensitivity of ∼1.68 µA mM^-1 cm^-2 with a low detection limit of ∼0.1 mM. The fabricated electrode was found to be sensitive to pH and electrolyte temperature. The real sample analysis of the proposed method showed a decent recovery toward L-Cyst addition in human blood serum. Furthermore, the density-funcational theory (DFT) analysis revealed that TNTs have greater affinity toward L-Cyst, having stronger binding distance after its adsorption. The higher negative E _ads values suggested a stable and chemisorption nature. The density of states results show that the E _gap of TNTs is significantly reduced after L-Cyst adsorption. The modified GCE showed excellent selectivity, enhanced stability, and fast response, which make TNTs a promising candidate for the enzyme-free detection of other biological analytes.

PEGylated NALC-functionalized gold nanoparticles for colorimetric discrimination of chiral tyrosine

In this work, acid and matrix-tolerant multifunctionalized gold nanoparticles (AuNPs) with an integrated chiral selector towards tyrosine (Tyr) and polyethylenglycol (PEG) chains were developed for visual chiral discrimination of Tyr in biological samples under acid conditions. In brief, AuNPs multifunctionalized with N-acetyl-l-cysteine (NALC) and PEG (PEG/NALC-AuNPs) were prepared via a simple strategy. In the presence of l-Tyr, the color of PEG/NALC-AuNP solution changed from red to gray, while no obvious color change was observed with the introduction of d-Tyr, which indicated that the introduction of PEG onto the surface of AuNPs has no effect on the chiral recognition between l-Tyr and NALC. A computer-aided molecular model was used to clarify the chiral recognition mechanism between NALC and Tyr enantiomers and to further guide the optimization of sensitivity. The resultant PEG/NALC-AuNP sensor presented a significantly improved stability under acid and alkali conditions compared with conventional NALC-AuNPs, resulting in a wider dynamic range (500 nM-100 μM) and a 50 times reduced detection limit by simply adjusting the pH of the sensor system under acid conditions (pH 2-2.5). More importantly, the PEG/NALC-AuNPs can realize the visual chiral discrimination of Tyr enantiomers in biological samples due to their significantly improved long-term stability and reduced interaction towards non-target species.

An electrochemical sensor based on an Eriochrome Black T polymer and deep eutectic solvent for the simultaneous determination of omeprazole and lansoprazole

Here we report the fabrication of an electrochemical sensor for the simultaneous determination of the concentrations of omeprazole (OMZ) and lansoprazole (LAN) in biological fluids and pharmaceuticals in Britton-Robinson buffer solution (pH 6.0). The sensor was prepared by bulk modifying a carbon paste electrode with deep eutectic solvent (DES), followed by carrying out electropolymerization of Eriochrome Black T (EBT) at this electrode. The presence of DES increased the extent of the polymerization of EBT on the surface of the electrode, and this increase led to better OMZ and LAN electron transfer kinetics at the electrode surface. The modified electrode was evaluated and characterized by performing cyclic voltammetry, electrochemical impedance spectroscopy, scanning electronic microscopy, and Fourier-transform infrared spectroscopy. The best responses were obtained in drug-free Britton-Robinson buffer solution (pH 6.0) after 600 s of stirring at the open circuit potential. Linear relationships were obtained between the anodic peak currents and the concentrations of OMZ and LAN in the ranges of 0.010-0.276 and 0.010-0.300 μM, with detection limits of 0.006 and 0.009 μM, respectively. Satisfactory results were also obtained for the analysis of OMZ and LAN in real samples. The excellent sensitivity and easy regeneration and modification of this electrode with DES followed by the polymerization of EBT make this electrode relatively highly applicable for flow methods.

Electroanalytical Analysis of Guaifenesin on Poly(Acridine Orange) Modified Glassy Carbon Electrode and its Determination in Pharmaceuticals and Serum Samples

BACKGROUND

Electroanalytical methods are very functional to detect drugs in pharmaceuticals (tablets, syrups, suppositories, creams, and ointments) and biological samples.

OBJECTIVE

This study is aimed to make selective, sensitive, simple, fast, and low cost electrochemical analysis of expectorant drug guaifenesin in pharmaceuticals and serum samples.

METHODS

Differential pulse adsorptive stripping voltammetric method for determination of guaifenesin on a poly(acridine orange) modified glassy carbon electrode has been developed. Glassy carbon electrode was modified with electropolymerization of the acridine orange monomer for the sensitive determination of guaifenesin. Guaifenesin provided highly reproducible and welldefined irreversible oxidation peaks at +1.125 V and +1.128 V (vs. Ag/AgCl) in the selected supporting electrolyte and human serum samples, respectively.

RESULTS

Under optimized conditions, linear response of peak current on the concentration of guaifenesin has been obtained in the ranges of 2.00×10-7 to 1.00×10-4 M in Britton Robinson buffer solution at pH 7.0 and 4.00×10-7 to 1.00×10-4 M in serum samples. The precision of the method was detected by intraday and inter-day repeatability studies in the supporting electrolyte and serum samples media.

CONCLUSION

The analytical applicability of the proposed method exhibited satisfying determination results for guaifenesin from pharmaceutical dosage forms (syrup) and human serum samples without any pre-separation procedures.

Interactive Studies on Synthetic Nanopolymer decorated with Edible Biopolymer and its Selective Electrochemical determination of L-Tyrosine

Herein, an edible biopolymer amine Modified Gum Acacia (MGA), successfully encumbered with Electron Beam irradiated Polypyrrole Nanospheres (EB-PPy NSs), was investigated for the effective role in L-Tyrosine (Tyr) biosensing application. The morphology of EB-PPy NSs decorated MGA (EB-PPy/MGA) hybrid nanobiocomposite has been studied by Scanning electron microscopy and its affirmed interactions were characterized by X-ray diffraction, Raman, FT-IR spectroscopy, UV-Visible spectroscopy, Thermo Gravimetric Analysis and Vibrating Sample Magnetometer. The hybrid nanobiocomposite manifested diamagnetic behavior with reduced saturation magnetization (Ms = 1.412 × 10-4 emu/g) to produce more adhesive surface. Amine chains in EB-PPy NSs and hydroxyl groups of MGA contributed to effective immobilization, thus enabling suitable orientation for Tyr determination. The electrochemical analysis illustrated that the proposed nanobiocomposite based sensor exhibited an excellent electrocatalytic activity toward selective determination of Tyr in the linear range of 0.4 to 600 µM with a lower detection limit of 85 nM, low oxidation potential of 0.72 V and good selectivity. Finally, the reliability of the constructed EB-PPy/MGA for Tyr detection was demonstrated in real samples.

One-Step Synthesis of CuO-Cu2O Heterojunction by Flame Spray Pyrolysis for Cathodic Photoelectrochemical Sensing of l-Cysteine

CuO-Cu₂O heterojunction was synthesized via a one-step flame spray pyrolysis (FSP) process and employed as photoactive material in construction of a photoelectrochemical (PEC) sensing device. The surface analysis showed that CuO-Cu₂O nanocomposites in the size less than 10 nm were formed and uniformly distributed on the electrode surface. Under visible light irradiation, the CuO-Cu₂O-coated electrode exhibited admirable cathodic photocurrent response, owing to the favorable property of the CuO-Cu₂O heterojunction such as strong absorption in the visible region and effective separation of photogenerated electron-hole pairs. On the basis of the interaction of l-cysteine (l-Cys) with Cu-containing compounds via the formation of Cu-S bond, the CuO-Cu₂O was proposed as a PEC sensor for l-Cys detection. A declined photocurrent response of CuO-Cu₂O to addition of l-Cys was observed. Influence factors including CuO-Cu₂O concentration, coating amount of CuO-Cu₂O, and applied bias potential on the PEC response toward l-Cys were optimized. Under optimum conditions, the photocurrent of the proposed sensor was linearly declined with increasing the concentration of l-Cys from 0.2 to 10 μM, with a detection limit (3S/N) of 0.05 μM. Moreover, this PEC sensor displayed high selectivity, reproducibility, and stability. The potential applicability of the proposed PEC sensor was assessed in human urine samples.

Development of a novel and cost-effective redox sensor for voltammetric determination of pantoprazole sodium during pharmacokinetic studies

A pencil graphite electrode modified with poly (bromocresol green (BCG)) was prepared by electro-polymerization process for the determination of pantoprazole sodium. The surface morphology and structure of poly (BCG) film were characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. The determination of pantoprazole sodium in Britton-Robinson buffer (pH 7.0) was carried out by square wave adsorptive stripping voltammetric technique. Under optimum conditions, the linear response of the peak with concentration of the cited drug was in the range of 6.6-360 × 10^-8M with limit of detection of 2.2 × 10^-8M. Moreover, the poly (BCG)-modified electrode has been successfully applied to determine pantoprazole sodium in tablets, vials and during pharmacokinetic studies.

Development of Fluorescent FRET Probes for "Off-On" Detection of L-Cysteine Based on Gold Nanoparticles and Porous Silicon Nanoparticles in Ethanol Solution

A new type of fluorescence “off-on” probe was designed for L-Cysteine (L-Cys) based on the fluorescence resonance energy transfer (FRET) between negatively charged amino-capped porous silicon nanoparticles (SiNPs) and positively charged citrate-stabilized Au nanoparticles (AuNPs). In this proposed FRET immunosensor, novel water-soluble amino-conjugated porous SiNPs in ethanol with excellent photoluminescence properties act as the energy donor. Excellent quenching efficiency between SiNPs-ethanol and citrate-stabilized AuNPs by electrostatic interaction via FRET provides an ideal “off-state” (turn-off). The addition of L-Cys leads to releasing the adsorbed AuNPs from the surface of SiNPs and hence the fluorescence emission of SiNPs-ethanol is restored (turn-on), which means the coordination ability of the thiols with AuNPs is stronger than that of the electrostatic interaction. The fluorescence intensity of SiNPs-AuNPs in ethanol is sensitive to L-Cys, and such a restored fluorescence is proportional to the concentration of L-Cys. The method will broadly benefit the development of a new thiol biosensor based on nanostructured porous materials, and the proposed procedure is also expected to develop a variety of functional nanoparticles to form other novel kinds of nanosensors.

Simple synthesis of ZnO nanoparticles on N-doped reduced graphene oxide for the electrocatalytic sensing of l-cysteine

A new kind of ZnO nanoparticle/N-doped reduced graphene oxide nanocomposite (ZnONPs/N-rGO) was synthesized through a low temperature, low-cost and one step hydrothermal process.