Mycophenolic acid is a secondary metabolite of Penicillium brevicompactum and has been extensively studied for its antineoplastic, immunosuppressive, anti-inflammatory, antiviral, antipsoriasis, and antifungal activities. In the present study, a high-performance thin-layer chromatography method was developed for determining mycophenolic acid and a chitosan-linked prodrug using 60 F254 silica gel plates. The mobile phase used was chloroform–methanol–water (9.2 : 0.7 : 0.1, v/v/v), which provided good separation. The linearity was found to be in the range of 4–12 µg/spot with an R2 value of 0.9973. The percentage of RSD for intra-day and inter-day precision was found to be in the range of 0.7–1.1 and 0.9–2.0, respectively. The limit of detection and limit of quantification of the prodrug were determined to be 0.27 and 0.82 µg/spot, respectively. The recovery of the prodrug obtained was found to be 99.79%, suggesting that the method is simple, accurate, and reproducible. The proposed method proved to be sensitive, accurate, precise, economical, and less time-consuming, making it suitable for successful analysis in microsphere formulations.

To recognize and determine clopidogrel enantiomers, a sensor system was developed based on a glassy carbon electrode modified with mesoporous Carbopack X carbon black and cyclopentadiene derivatives (1S)-2-cyclopent-2,4-dien-1-yl-1,7,7-trimethylbicyclo[2.2.1]heptane, (1S,2S,4R)-2-cyclopent-1-en-1-yl-1-isopropyl-4-methylcyclohexane, and 9-[(1S,2S,5R)-2-isopropyl-5-methylcyclohexyl]-9H-fluorene. Because of the unique properties of Carbopack X, such as a large surface area and high conductivity, a mechanically stable and a sensitive sensor layer was obtained, which firmly retains chiral selector molecules on its surface. The morphological, electrochemical, and analytical properties of the obtained sensors were studied using scanning electron microscopy, electrochemical impedance spectroscopy, and cyclic and differential pulse voltammetry. Сlopidogrel enantiomers were determined in biological fluids, and linear dependences of oxidation peak currents on their concentration in solution were maintained for all sensors in the concentration range from 1 × 10–6 to 5 × 10–4 M. The sensors exhibited cross-sensitive properties, enabling them to be combined into a sensor array with high enantioselectivity and sensitivity towards clopidogrel enantiomers. Using the proposed sensor array system increases the probability of correctly identifying samples compared to individual sensors. In all cases, the concentration of R-clopidogrel in a mixture was determined correctly with a relative standard deviation not exceeded 9% and recovery ranged from 96 to 102%.

Flow injection amperometry provides an opportunity to avoid common drawbacks of classical potentiometry for solid-contact ion-selective electrodes. The current is generated by solid-contact in course of the polymer chain recharging in response to an excess of cations or anions in the polymer film. Cations cause cathodic peaks on the chronoamperogram, while anions result in anodic peaks. For polyaniline with negatively charged substituents, a response to cations is observed, whereas the presence of uncharged ones results in anion detection. Moreover, in the case of the protonated –SO3H group, this compensating current is an order of magnitude lower than that for the deprotonated one. For electroactive self-doped polyanilines, the maximal amperometric signal is achieved at the working potential within the electroactivity range. The proposed constant potential amperometric approach was validated by the successful analysis of human serum with a Na+-selective electrode based on polyaniline.

A voltammetric DNA sensor is developed for the determination of doxorubicin, the signal of which is the change in the current of the redox mediator Methylene green, recorded by cyclic voltammetry on a glassy carbon electrode modified by electrochemically reduced graphene oxide, electropolymerized Azure A, and DNA saturated with Methylene green. The influence of the nature of the polymer layer and the redox mediator used on the sensitivity of the determination of doxorubicin is revealed. With an optimal composition of the surface layer, the DNA sensor allows the determination of up to 0.01 pM of doxorubicin. The sensor was tested on a number of model (artificial urine, Ringer−Locke solution, bovine serum albumin solution) and biological samples (saliva, urine) containing doxorubicin.

The hydroxyl radical is the most aggressive and dangerous product of incomplete oxygen reduction in a body, making its determination and the effect of various antioxidants on the inhibition of OH radicals a pertinent task. We propose an impedance sensor for the determination of hydroxyl radicals, based on a graphite electrode modified with electrochemically deposited gold and a self-assembled monolayer of N-acetyl-L-cysteine (NAC). The use of NAC ensures the high selectivity of the sensor, and employing non-Faradaic impedance as a method of recording the analytical signal ensures the limit of detection for OH radicals at the level 0.01 nM with a linearity range of detectable concentrations from 0.08 to 8 nM. The developed sensor has been successfully used to assess the activity of certain antioxidants (ascorbic acid, glutathione, coenzyme Q10) towards OH radicals.

In this study, a molecularly imprinted co-polymer-based electrochemical sensor has been fabricated that shows sensitive and selective detection of paracetamol. Molecularly imprinted co-polymer synthesis was achieved using methacrylic acid and styrene as functional monomers, azobisisobutyronitrile as a free radical initiator, ethylene glycol dimethacrylate as a cross-linker, and dimethyl sulfoxide as a porogenic solvent. The molecularly imprinted co-polymer was synthesized using a thermal free radical bulk polymerization technique. The molecularly imprinted co-polymer is characterized by Fourier-transform infrared spectroscopy. The LCR meter was used to measure electrical parameters such as inductance and resistance. The sensor response was linear and concentration-dependent. The lower limit of detection of the fabricated sensor in terms of inductance and resistance was 12 and 5.33 mg/L, respectively. Furthermore, in the presence of competing substances such as ascorbic acid and uric acid, the fabricated sensor proved a selective response. The fabricated sensor also shows reproducibility and reversible response.

In the present study, a new approach that uses ultrasound-assisted extraction clean-up combined with dispersive liquid-liquid microextraction has been developed for the preconcentration of trace amounts of clodinafop-propargyl herbicide in wheat samples. The clodinafop-propargyl was then determined using gas chromatography with flame ionization detection. In this technique, wheat samples were extracted by ultrasound-assisted extraction followed by a dispersive liquid-liquid microextraction (DLLME) step for further purification and enrichment of clodinafop-propargyl. By using a Pasteur pipette as an extraction device, a low-density extraction solvent like water is possible to use in the DLLME method. The effects of various parameters on the extraction efficiency of the proposed technique were investigated and optimized. Good linearity was obtained from 20 to 1000 µg/kg. The limit of detection was 10 µg/kg (S/N = 3). The relative recoveries at the three spiked levels ranged from 88 to 96% with an RSD less than 13% (n = 3). The technique has been successfully applied to the determination of clodinafop-propargyl herbicide in wheat samples.

Microwave plasma atomic emission spectrometry is used to determine the composition of lanthanum sulfide and europium-doped gadolinium oxide crystals, and also of elements in the melt (tin, boron, and lithium). Calibration dependencies for rare-earth elements were found to be nonlinear and did not meet the required accuracy. The internal standard method was used to minimize errors and linearize calibration dependences. Molecular species N2, $${\text{N}}_{2}^{ + }$$ , and OH did not affect plasma conditions or interelement influences. Internal standards were selected based on the proximity of their first ionization potential to that of the analytes; Ba, Al, Ga, and In were considered. Using these elements as internal standards enabled the linearization of the calibration dependences, achieving an recovery of 95–105%. The determined total mass of elements was 97–103% of the sample weight, with accuracy confirmed by the standard addition method.

In this work, a melamine-based covalent organic framework (COF) was synthesized. Then its performance as a potential adsorbent for the removal of tetracycline (TC) was scrutinized by density functional theory computations. The theoretical results showed that TC interaction with COF is experimentally possible, exothermic, spontaneous, and thermodynamically favorable. The adsorption mechanism was checked out by natural bond orbital computations, the results of which demonstrated that TC interaction with COF is a physisorption process. The influence of temperature and the presence of the solvent was also investigated computationally, and the results showed that these parameters do not affect the interactions. Afterward, all effective operational parameters, including the amount of COF, agitation time, and pH, were optimized by the one-factor-at-a-time method to achieve the highest efficiency. Under optimum conditions, the experimental data were fitted to Langmuir, Freundlich, and Sips models. The TC adsorption behavior was well-described by the Sips isotherm with a maximum adsorption capacity of 168.98 mg/g. The desorption process was also discussed, and a mixture of methanol and acetic acid was selected as the most suitable desorption solvent.

Substitution reactions in carbocyanine dyes are used to determine nucleophilic compounds. The interaction of a chlorine-containing carbocyanine dye with a number of medicinal substances is studied. It is shown that, in the reaction with isoniazid in the presence of a surfactant, this dye selectively changes color from yellow-green to violet. The formation of a product of the substitution of chlorine for isoniazid is proven by chromatography–mass spectrometry. The reaction proceeds in 20 min in the presence of 1 mM cetyltrimethylammonium bromide. The limit of detection for isoniazid in water by the photometric method is 10 μg/mL and in diluted artificial urine using fluorimetry, 0.3 μg/mL. The procedure does not use full-spectrum equipment, which simplifies the determination.