Inorganic-Ion-Stabilized Au Nanoparticles Supported on SiO2: Synthesis, Physicochemical Characterization, and Proof-of-Concept Raman Response Toward Methylene Blue

Silica nanostructures are attractive platforms for hybrid nanomaterials because of their chemical stability and versatile surface functionalization. Here, spherical SiO2 nanoparticles were synthesized by a modified Stöber method, functionalized with (3-aminopropyl)triethoxysilane (APTES), and decorated with gold nanoparticles (AuNPs) formed using an inorganic stabilization strategy involving OH− and Cl− ions, without conventional organic AuNP capping agents. Complementary characterization by Fourier-transform infrared spectroscopy (FTIR), ultraviolet–visible diffuse reflectance spectroscopy (UV–VIS DRS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) supported the sequential surface modification and formation of the SiO2–APTES–AuNPs hybrid material. TEM yielded average diameters of 100.81 ± 1.74 nm for SiO2 nanoparticles (n = 50) and 2.00 ± 0.31 nm for supported AuNPs (n = 80). Methylene blue (MB) was subsequently used for a proof-of-concept Raman evaluation. At 10−3 and 10−4 M MB, relative Raman signal-amplification ratios of approximately 7.5 and 3.5, respectively, were obtained from the integrated response near 1620 cm−1; these values are not conventional surface-enhanced Raman scattering (SERS) enhancement factors. At 10−6 M, a fluorescence-like background was observed without establishing metal-enhanced fluorescence. Density functional theory calculations using an Au32 cluster provided qualitative insight into local S···Au and π···Au interactions. Overall, these results establish an inorganic SiO2-AuNPs as a physicochemically characterized hybrid nanomaterial prepared through an inorganic AuNP-stabilization strategy and support its further investigation in Raman-based applications.