Fluorescent nanosized PAMAM dendrimers: One-step formation of a bright blue fluorophore on terminal groups and its optical properties

Background and Objectives: Polyamidoamine dendrimers (PAMAM) are nanoscale monodisperse compounds with a multifunctional terminal surface. Structural features of PAMAM, such as a nanosize of high homogeneity, highly developed terminal surface and cavities in the structure open up wide possibilities for their application. The most promising use of PAMAM is for biomedical purposes, in particular for the targeted drug delivery (for example, anticancer drugs). The interaction of PAMAM with target cells can be assessed using fluorescent imaging. This suggests the preliminary modification of PAMAM with various fluorescent molecules or the development of approaches to increase the intrinsic fluorescence of PAMAM. Materials and Methods: In this paper, we will consider a one-step modification of PAMAM based on the double cyclization reaction of PAMAM terminal groups and citric acid. Two approaches are chosen for modification: hydrothermal and boiling methods. The methods of optical spectroscopy and dynamic light scattering will be used as the main research tools. The methods used make it possible to determine the efficiency of fluorophore formation under given conditions. Results: In this work, we have proposed and implemented a one-step modification of PAMAM with a bright blue fluorophore (1,2,3,5-tetrahydro-5-oxo-imidazo[1,2-a] pyridine-7-carboxylic acid, IPCA), which is formed by a double cyclization reaction between citric acid and terminal ethylenediamine fragments of PAMAM. It has been shown that as a result of modification the hydrodynamic diameter of PAMAM does not change, the fluorescence intensity increases significantly (the quantum yield increases from < 1 to 28$\%$), $\zeta$-potential changes from 42 $\pm$ 5 to - 24 $\pm$ 4 mV. Conclusion: Reaction of PAMAM and citric acid leads to the appearance of bright-blue fluorescence, which is significantly higher than the intrinsic fluorescence of PAMAM. A combination of bright fluorescence and a multifunctional terminal surface make it possible to further use the obtained structures for biovisualization.