In this work, the mutual arrangement of polyelectrolytes of multilayer polyelectrolyte microcapsules (with layers—[PAH/PSS]₃PAH) by determination of the dissociation level of polyallylamine (PAH) from the surface of a polyelectrolyte microcapsules (PMC) of various types was studied: PMC with a dissolved CaCO₃ core after preparation, PMC with an undissolved CaCO₃ core and PMC with an encapsulated protein. It was concluded that the polyelectrolyte layers are mixed in the entire shell of the capsules with a dissolved CaCO₃ core. In the case of the PMC with an undissolved CaCO₃ core, such mixing of polyelectrolyte layers does not occur. That fact allows us to conclude that the mixing of polyelectrolytes layers mixing at the stage of dissolution of CaCO₃ core. The PMC with encapsulated protein has partial mixing of polyelectrolytes layers. That phenomenon may be due to the fact that seven-layered protein-containing microcapsules already have a dense and well-formed shell. The obtained data correlate with the data on the study of the surface charge of microcapsules.

Nowadays, the enzymological method used in clinical diagnostics has several problems: a short lifetime of the enzyme sensor in solution, its susceptibility in the presence of proteolytic enzymes, and the impossibility of multiple uses. We proposed the modification of the standard method of urea detection, which will solve the problems mentioned above. The main idea of the proposed approach was encapsulation of urease into microcapsules obtained by the method of alternate adsorption of oppositely charged polyelectrolytes. The determination was conducted with the help of unfixed (microdiagnosticum) and fixed microcapsules on a solid surface (diagnostic plate). Multiple uses of microdiagnosticum and diagnostic plate have been shown. The results obtained exhibited deviations and did not exceed the value of 9.6%. We determined the urea concentration in the serum blood by the standard Berthelot method using a diagnostic plate and a microdiagnosticum for detection in the urine. The results obtained differed from the results obtained using a free enzyme by no more than 4.26% for diagnostic plates and 1.67% for microdiagnosticum. The diagnostic plate exhibited detectable concentrations in the range of 5–50 mmol L⁻¹ and linearity up to 40 mmol L⁻¹; moreover, deviation from linearity did not exceed 10%, and the coefficient of variation did not exceed 7.17. Microdiagnosticum exhibited detectable concentrations in the range of 5–55 mmol L⁻¹ and linearity up to 50 mmol L⁻¹; also, deviation from linearity did not exceed 7%, and the coefficient of variation did not exceed 2.85.

Encapsulation of enzymes (lactate dehydrogenase and urease) in polyelectrolyte shells was assessed with a view to designing enzymic microdiagnostics for low-molecular compounds in native biological fluids. Polyelectrolyte microcapsules were prepared with two polyanions [poly(styrenesulfonate) PSS and dextran sulfate DS] and two polycations [poly(allylamine) PAA and poly(diallyldimethylammonium) PDADMA]; calcium carbonate microspherulites with embedded enzymes served as “cores.” It was demonstrated that the main problem in making such a biosensor is to select a pair of oppositely charged polyelectrolytes that would be optimal for enzyme functioning. The best pairs were PAA/DS and PAA/PSS for lactate dehydrogenase, and PSS/PAA and PSS/PDADMA for urease. We designed and prepared enzyme-containing microcapsules differing in polyelectrolyte composition and number of layers, and investigated their properties.

The degradation of polyelectrolyte microcapsules formed on protein-free CaCO₃ particles consisting of polyallylamine (PAH) and polystyrene sulfonate (PSS) and the resulting yield of protein in the presence of various salts of different concentrations, as well as at two pH values, was studied by fluorescence spectroscopy; the protein was incorporated into prepared microcapsules by adsorption. It was found that a high concentration of sodium chloride (2 M) leads to considerable dissociation of PAH, which is apparently due to the loosening of polyelectrolytes under the action of ionic strength. At the same time, 0.2 M sodium chloride and ammonium sulfate of the same ionic strength (0.1 M) exert less influence on the amount of dissociated polymer. In the case of ammonium sulfate (0.1 M), the effect is due to the competitive binding of sulfate anions to the amino groups of the polyelectrolyte. However, unlike microcapsules formed on CaCO₃ particles containing protein, the dissociation of polyelectrolyte from microcapsules formed on protein-free particles increased with increasing temperature. Apparently, a similar effect is associated with the absence of a distinct shell, which was observed on microcapsules formed on protein-containing CaCO₃ particles. The high level of the presence of Fluorescein isothiocyanate (FITC)-labeled Bovine Serum Albumin (BSA) in the supernatant is explained by the large amount of electrostatically bound protein and the absence of a shell that prevents the release of the protein from the microcapsules. In 2 M NaCl, during the observation period, the amount of the released protein did not exceed 70% of the total protein content in the capsules, in control samples, this value does not exceed 8%, which indicates the predominantly electrostatic nature of protein retention in capsules formed on protein-free CaCO₃ particles. The increase in protein yield and peeling of PAH with increasing pH is explained by the proximity of pH 7 to the point of charge exchange of the amino group of polyelectrolyte, as a result of the dissociation of the microcapsule.

This work investigated changes in the biochemical parameters of multilayer membrane structures, emerging at their modification with multiwalled carbon nanotubes (MWCNTs). The structures were represented by polyelectrolyte microcapsules (PMCs) containing glucose oxidase (GOx). PMCs were made using sodium polystyrene sulfonate (polyanion) and poly(allylamine hydrochloride) (polycation). Three compositions were considered: with MWCNTs incorporated between polyelectrolyte layers; with MWCNTs inserted into the hollow of the microcapsule; and with MWCNTs incorporated simultaneously into the hollow and between polyelectrolyte layers. The impedance spectra showed modifications using MWCNTs to cause a significant decrease in the PMC active resistance from 2560 to 25 kOhm. The cyclic current–voltage curves featured a current rise at modifications of multilayer MWCNT structures. A PMC-based composition was the basis of a receptor element of an amperometric biosensor. The sensitivity of glucose detection by the biosensor was 0.30 and 0.05 μA/mM for PMCs/MWCNTs/GOx and PMCs/GOx compositions, respectively. The biosensor was insensitive to the presence of ethanol or citric acid in the sample. Polyelectrolyte microcapsules based on a multilayer membrane incorporating the enzyme and MWCNTs can be efficient in developing biosensors and microbial fuel cells.