Rapid voltammetric monitoring of melatonin in the presence of tablet excipients

Development of indirect competitive ELISA and CLEIA for quantitative analysis of melatonin in health products

Melatonin (MT), as a hormone regulating the rhythm of sleep, is widely used in health products. However, illicit and excessive use of MT might cause undesirable effects. Therefore, it is essential to establish highly sensitive and specific rapid methods for MT analysis in health products. In this study, we established indirect competitive enzyme-linked immunosorbent assay (ic-ELISA) and indirect competitive chemiluminescent enzyme immunoassay (ic-CLEIA) for sensitive and selective detection of MT in health products. Under optimal conditions, half-maximum inhibitory concentration (IC50 value) and limit of detection (LOD, IC10 value) for MT by ic-ELISA were 0.25 ng mL-1 and 0.03 ng mL-1 respectively, while the IC50 and LOD of ic-CLEIA were lower at 0.17 ng mL-1 and 0.03 ng mL-1 respectively. Three MT-free Chinese patent medicines were spiked with MT and the recovery rates ranged from 71.89% to 117% (ic-ELISA) and 83.66% to 107.17% (ic-CLEIA). The level of MT in six MT-containing health products was assessed in parallel using the developed methods and HPLC. Both ic-ELISA and ic-CLEIA showed good consistencies (R2 = 0.999 and 0.993, respectively) with HPLC, indicating that the two methods developed were sensitive, fast, and reliable for application in MT analysis.

Advances on Hormones and Steroids Determination: A Review of Voltammetric Methods since 2000

This article presents advances in the electrochemical determination of hormones and steroids since 2000. A wide spectrum of techniques and working electrodes have been involved in the reported measurements in order to obtain the lowest possible limits of detection. The voltammetric and polarographic techniques, due to their sensitivity and easiness, could be used as alternatives to other, more complicated, analytical assays. Still, growing interest in designing a new construction of the working electrodes enables us to prepare new measurement procedures and obtain lower limits of detection. A brief description of the measured compounds has been presented, along with a comparison of the obtained results.

Detection of falsified clopidogrel in the presence of excipients using voltammetry

There has been a recent surge in the amount of substandard and falsified clopidogrel. Pharmacopeial based assays using high performance liquid chromatography and mass spectroscopy are widely used for the measurement of clopidogrel but are not accessible in low to middle income countries. Therefore, our study explored four different techniques (mid-infrared spectroscopy, thin layer chromatography, ultraviolet visible spectroscopy, and differential pulse voltammetry), which could be used in low to middle income countries. Differential pulse voltammetry showed the best performance for accurate and precise determination of clopidogrel in the presence of excipients. Clopidogrel tablets were fully crushed and sonicated in buffer for 30 seconds prior to differential pulse voltammetry measurements using a 3 mm glassy carbon electrode. Measurements were made without removing the excipients and the limit of detection was 0.08 mg ml^-1 and the sensitivity was 15.7 μA mg ml^-1. When conducting a blinded study, differential pulse voltammetry was able to identify varying types of substandard and falsified samples. Our findings highlight that voltammetry could be a vital analytical technique for the determination of substandard and falsified medicines in low- and middle-income countries.

Comparison of electrochemical performance of various boron-doped diamond electrodes: Dopamine sensing in biomimicking media used for cell cultivation

Chemically inert and biocompatible boron-doped diamond (BDD) has been successfully used in neuroscience for sensitive neurochemicals sensing and/or as a growth substrate for neurons. In this study, several types of BDD differing in (i) fabrication route, i.e. conventional microwave plasma enhanced chemical vapour deposition (MW-PECVD) reactor vs. MW-PECVD with linear antenna delivery system, (ii) morphology, i.e. planar vs. porous BDD, and (iii) surface treatment, i.e. H-terminated (H-BDDs) vs. O-terminated (O-BDDs), were characterized from a morphological, structural, and electrochemical point of view. Further, planar and porous BDD-based electrodes were tested for sensing of dopamine in common biomimicking environments of pH 7.4, namely phosphate buffer (PB) and HEPES buffered saline (HBS). In HBS, potential windows are narrowed due to electrooxidation of its buffering component (i.e. HEPES), however, dopamine sensing in HBS is possible. H-BDDs (both planar and porous) outperformed O-BDDs as they provided clearer dopamine signals with higher peak currents. As expected, due to its enlarged surface area and increased sp² content, the highest sensitivity and lowest detection limits of 8 × 10^-8 mol L^-1 and 6 × 10^-8 mol L^-1 in PB and HBS media, respectively, were achieved by square-wave voltammetry on porous H-BDD.

Spectroscopic Analysis of Melatonin in the Terahertz Frequency Range

There is a need for fast and reliable quality and authenticity control tools of pharmaceutical ingredients. Among others, hormone containing drugs and foods are subject to scrutiny. In this study, terahertz (THz) spectroscopy and THz imaging are applied for the first time to analyze melatonin and its pharmaceutical product Circadin. Melatonin is a hormone found naturally in the human body, which is responsible for the regulation of sleep-wake cycles. In the THz frequency region between 1.5 THz and 4.5 THz, characteristic melatonin spectral features at 3.21 THz, and a weaker one at 4.20 THz, are observed allowing for a quantitative analysis within the final products. Spectroscopic THz imaging of different concentrations of Circadin and melatonin as an active pharmaceutical ingredient in prepared pellets is also performed, which permits spatial recognition of these different substances. These results indicate that THz spectroscopy and imaging can be an indispensable tool, complementing Raman and Fourier transform infrared spectroscopies, in order to provide quality control of dietary supplements and other pharmaceutical products.

New insights on the spectrophotometric determination of melatonin pKa values and melatonin-βCD inclusion complex formation constant

Using UV-Vis spectrophotometry a stability study of melatonin at different pH values was done in aqueous media, finding that at acidic pH melatonin is unstable when interacting with the environment, however it becomes stable protecting it from light and oxygen. From the UV-Vis spectra and SQUAD software, melatonin pK_a values, in a completely protected aqueous medium, were estimated as 5.777±0.011 and 10.201±0.024. Using the same techniques, the melatonin and β-cyclodextrin inclusion complex formation constants were assessed at pH3, 7 and 11.5, giving the values of log β=(3.07±0.06), (2.94±0.01) and (3.07±0.06) M^-1, respectively. From the global acidity formation constants and the complexes' formation constants, the molar fractions were determined for each species of MT and MT-βCD, to build the molar fraction-[βCD]-pH 3D diagram and the molar fraction-pH 2D diagrams, where it was possible to observe the predominance of the MT species with and without βCD. A voltammetric study at pH3, allowed obtaining a value of log β=(3.15±0.01) M^-1, which corroborates that obtained through UV-Vis spectrophotometry, supporting strongly the rationale behind using simple, straightforward techniques.

Facile and novel synthesis of palladium nanoparticles supported on a carbon aerogel for ultrasensitive electrochemical sensing of biomolecules

Highly stable palladium nanoparticles (Pd NPs) supported on a porous carbon aerogel (Pd/CA) prepared by a facile microwave reduction route is reported. The as-prepared Pd/CA composites were characterized by various techniques, viz. XRD, Raman, SEM-EDX, FE-TEM, BET, and TGA. The Pd NPs were found to disperse uniformly in the porous carbon matrix, which possesses a large surface area (851.8 m² g^-1) and pore volume (3.021 cm³ g^-1). The Pd/CA composite was found to possess extraordinary electrocatalytic activity and excellent selectivity for simultaneous detection of dopamine (DA) and melatonin (ML). The Pd/CA-modified electrode exhibited a wide linear response range for electrochemical sensing of DA (0.01-100 μM) and ML (0.02-500 μM) with a detection limit of 0.0026 and 0.0071 μM, respectively. In addition, the electrochemical sensor reported herein was successfully applied for the detection of DA and ML in human serum and urine samples, revealing perspective practical applications.

Nanomolar simultaneous determination of tryptophan and melatonin by a new ionic liquid carbon paste electrode modified with SnO2-Co3O4@rGO nanocomposite

This work describes the development of a new sensor for simultaneous determination of tryptophan and melatonin. The proposed sensor was an ionic liquid carbon paste electrode modified with reduced graphene oxides decorated with SnO₂-Co₃O₄ nanoparticles. The voltammetric oxidation of the analytes by the proposed sensor confirmed that the electrooxidation process undergoes a two-electron/one-proton reaction for melatonin and a two-electron/two-proton reaction for tryptophan in diffusion-controlled processes. Moreover, based on the excellent electrochemical properties of the modified electrode, a sensitive voltammetric method was used for individual and simultaneous determination of melatonin and tryptophan in the aqueous solutions. Under the optimized experimental conditions, a linear response obtained in the range of 0.02 to 6.00μmolL^-1 with detection limits of 4.1 and 3.2nmolL^-1 for melatonin and tryptophan, respectively. The prepared sensor possessed accurate and rapid response toward melatonin and tryptophan with a good sensitivity, selectivity, stability, and repeatability. Finally, the applicability of the proposed sensor was verified by evaluation of melatonin and tryptophan in various real samples including human serum and tablet samples.

Simultaneous and sensitive determination of melatonin and dopamine with Fe3O4 nanoparticle-decorated reduced graphene oxide modified electrode

An electrochemical sensor based on a new nanocomposite of graphene/Fe₃O₄ has been fabricated for simultaneous determination of melatonin and dopamine.

A new electrochemical sensor based on a nitrogen-doped graphene/CuCo2O4 nanocomposite for simultaneous determination of dopamine, melatonin and tryptophan

An electrochemical sensor was fabricated for the simultaneous determination of dopamine, melatonin and tryptophan in biological and pharmaceutical samples.