Organic coating of 1–2-nm-size silicon nanoparticles: Effect on particle properties

Abstract

Photoluminescent silicon nanoparticles 1–2 nm in size were synthesized by a wet chemical procedure and derivatized with propylamine (NH₂SiNP). Surface NH₂ groups were used as linkers for additional poly(ethylene glycol) (PEG) and folic acid (Fo) attachment (PEG-NHSiNP and Fo-NHSiNP, respectively) to enable efficient targeting of the particles to tumors and inflammatory sites. The particles were characterized by transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, ζ potential, dynamic light scattering, and time-resolved anisotropy. The photophysical properties and photosensitizing capacity of the particles and their interaction with proteins was dependent on the nature of the attached molecules. While PEG attachment did not alter the photophysical behavior of NH₂SiNP, the attachment of Fo diminished particle photoluminescence. Particles retained the capacity for ¹O₂ generation; however, efficient ¹O₂ quenching by the attached surface groups may be a drawback when using these particles as ¹O₂ photosensitizers. In addition, Fo attachment provided particles with the capacity to generate the superoxide anion radical (O₂⁻). The particles were able to bind tryptophan residues of bovine serum albumin (BSA) within quenching distances. NH₂SiNP and PEG–NHSiNP ground state complexes with BSA showed binding constants of (3.1 ± 0.3) × 10⁴ and (1.3 ± 0.4) × 10³ M⁻¹, respectively. The lower value observed for PEG-NHSiNP complexes indicates that surface PEGylation leads to a reduction in protein adsorption, which is required to prevent opsonization. An increase in particle luminescence upon BSA binding was attributed to the hydrophobic environment generated by the protein. NH₂SiNP-BSA complexes were also capable of resonance energy transfer.