Silicon nanoparticles (Si-NPs) with initial sizes of the order of 100 nm were prepared by femtosecond laserablation-fragmentation of microcrystalline silicon in water followed by drying and resuspending in wateror in aqueous solutions of dextran. The prepared aqueous suspensions of Si-NPs without dextran and withdextran coating were investigated by means of the scanning electron microscopy, dynamic light scatteringand optical absorption spectroscopy in the spectral range from 250 to 800 nm. The optical absorption ofuncoated Si-NPs in an aqueous medium with different pH-level varied from 4 to 8 was found to decreasewith time because of a process of the dissolution of those NPs in water. The dissolution rate dependednonmonotonically on the solution acidity (pH level) and the corresponding times were in the range from 50to 180 hours. The addition of dextran into the solution was found to significantly decrease the dissolutionrate of Si-NPs to 300 hours because of the coating of NPs with a polymer shell. The obtained results canbe useful to develop new biomedical technologies involving Si-NPs as stabilized theranostics (therapy anddiagnostics) agents. Key words: nanoparticles, dextran, biopolymer, optics, absorbance.