Detection of the Sulfamethoxazole Antibiotic by Gold Nanoparticles: Binding Mechanism and SERS Spectra

Quantum chemical calculations using density functional theory are performed to evaluate the adsorption behavior and characteristics of surface-enhanced Raman spectroscopy (SERS) of sulfamethoxazole (SMZ), a powerful antibiotic, on gold nanoparticles. The gold nanostars are experimentally prepared and characterized by SEM and TEM techniques and X-ray and UV spectroscopies. Small gold clusters Au _n with n = 6 and 20 are used to mimic the gold nanostructured surfaces. By attaching gold atoms to the nitrogen atom of the methylisoxazole ring, the main binding mechanism is identified. The SMZ molecule is typically deprotonated in biological or aqueous environments due to its high acidity. The anion SMZ^- possesses a higher affinity for Au nanoparticles than its neutral counterpart SMZ, with binding energies ranging from -18 to -25 kcal/mol. Drug adsorption significantly adjusts the band gap and optical properties of the Au metals. Frontier orbital analysis reveals that the Au-SMZ bond is formed by forward donation from SMZ's HOMO to Au _n 's LUMO. In the SERS spectra of SMZ on gold surfaces, NC-N bending modes are directly orientated to the gold surface and likely offer the main contribution to the strongest band observed at ∼902 cm^-1, whereas CN bond vibrations of the methylisoxazole ring enjoy a significant amplification near 1375 cm^-1, and these vibrational parameters can be used as an index for detection.