A hybrid technique based on a catalytic reaction carried out on the surface of a paper-based sorbent is proposed. It is shown that Mn^II exhibits its catalytic action in the oxidation of 3,3′,5,5′-tetramethylbenzidine with periodate in aqueous solution as well as on filter-paper with or without attached diethylenetriaminetetraacetate (DETATA) groups. Optimum conditions differ for the reaction in solution and on filter-paper. For equal catalyst concentrations, higher initial reaction rates are attainable on the filter-papers. Preconcentration of manganese on the DETATA sorbent combined with the subsequent catalytic reaction improves the selectivity, reduces the limit of determination down to 5 × 10^{– 6} mg l^{– 1} (as compared with 6 × 10^{– 5} mg l^{– 1} in solution) and expands the linear range by an order of magnitude (to 5 × 10^{– 6}–2.5 × 10^{– 3} mg l^{– 1}). The precision of manganese determination on the sorbent is high (the RSDs are ≤5% for ≥6 × 10^{– 4} µg Mn). Samples of tap and river water were analyzed by use of the proposed sorption–catalytic technique. A rapid procedure for the determination of manganese with visual detection was also developed.

Array-based optical sensing is an efficient technique for the determination and discrimination of small organic molecules. This study is aimed at the development of a simple and rapid strategy for obtaining an optical response from a wide range of low-molecular-weight organic compounds. We have suggested a colorimetric and fluorimetric sensing platform based on the combination of two response mechanisms using carbocyanine dyes: aggregation and oxidation. In the first one, the analyte forms ternary aggregates with an oppositely charged surfactant wherein the dye is solubilized in the hydrophobic domains of the surfactant accompanied with fluorescent enhancement. The second mechanism is based on the effect of the analyte on the catalytic reaction rate of dye oxidation with H₂O₂ in the presence of a metal ion (Cu²⁺, Pd²⁺), which entails fluorescence waning and color change. The reaction mixture in a 96-well plate is photographed in visible light (colorimetry) and the near-IR region under red light excitation (fluorimetry). In this proof-of-concept study, we demonstrated the feasibility of discrimination of nine medicinal compounds using principal component analysis: four cephalosporins (ceftriaxone, cefazolin, ceftazidime, cefotaxime), three phenothiazines (promethazine, promazine, chlorpromazine), and two penicillins (benzylpenicillin, ampicillin) in an aqueous solution and in the presence of turkey meat extract. The suggested platform allows simple and rapid recognition of analytes of various nature without using spectral equipment, except for a photo camera.

The work aims at developing fluorimetric methods used for bioassay to expand the range of the determined low-molecular-weight organic analytes and cut down sample preparation operations using available fluorophores, reagents, and identical fluorimetric systems for qualitative and quantitative analysis. We proposed using the reaction of carbocyanine fluorophore oxidation by hydrogen peroxide, catalyzed by copper(II), changing fluorescence intensity in the near-IR region (700 nm). Several organic compounds of different nature accelerate or slow down the indicator reaction, to varying degrees, and at different times of the process. The model analytes were eight sulfonamides, which can be distinguished qualitatively using the kinetic factor in data processing by principal component analysis. We demonstrated on an example of phthalylsulfathiazole that the signal could be obtained not only in an aqueous solution but also in the presence of a turkey muscle homogenate at a level of 0.08–0.5 mM (RSD = 9%) without separation. The prospects for the development of such fluorescence platforms are discussed.