Fabrication and characterization of glycine-loaded PEG nanoparticles for drug delivering: A comprehensive SERS study

Highlights: • A biocompatible suspension comprising glycine (Gly) loaded into PEG/NPs was prepared. • SERS was used to assess information from the Gly molecule while loaded into PEG/NPs. • A mechanism of loading of Gly into PEG nanoparticles is suggested. This study reports on the successful preparation and characterization of a biocompatible colloidal suspension comprising glycine (Gly) loaded into polyethylene glycol (PEG) nanoparticles (Gly@PEG/NPs) and suspended within PBS. The interaction between the Gly molecule and the PEG nanoparticles is herein investigated using normal Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS), the latter employing silver substrate. Additionally, scanning and transmission electron microscopy (TEM), dynamic light scattering, zeta-potential, and Fourier transform infrared spectroscopy (FTIR) were employed to support our analyses. Morphological analyzes revealed that Gly@PEG/NPs have non-spherical shape with average diameter (polydispersion index) of 54.3 nm (0.16). Moreover, high-resolution TEM images demonstrated the coexistence of crystalline and amorphous phases in the Gly@PEG/NPs structure. The FTIR, normal Raman and SERS spectra strongly suggest that the protonated amino groups (${\text{NH}}_3^{+}$) of the Gly molecules bind to the oxygen atoms of the PEG molecule by means of hydrogen bonds, leaving the carboxylate groups (${\text{COO}}^{-}$) facing outwards to the bulk aqueous-like medium. Finally, MTT assay demonstrated that Gly@PEG/NPs are very much biocompatible at Gly concentrations of up to 1.0 mol/L while free Gly shows toxicity in the same range.