GaN-Based Substrates as Surface-Enhanced Raman Scattering Sensors for Penicillin G: Does the Size of the Analyte Molecule Matter?

Silver was deposited onto surfaces of GaN that were differently nanostructured (by formation of individual nanopillars, bunches of nanopillars, or hexagonal etch micropits). The different levels of surface-enhanced Raman scattering (SERS) activity of the nanomaterials obtained were compared by using them for the SERS detection of penicillin G in milk. The lowest limit of detection (LOD) of penicillin G was achieved for silver-covered GaN surfaces nanostructured with bunches of nanopillars (LOD = 1.1 × 10–9 M). The penicillin G LODs of other silver-covered GaN surfaces were higher, equal to 4.1 × 10–9 M for individual nanopillars, 7.3 × 10–8 M for hexagonal etch micropits, and 1.0 × 10–7 M for a flat GaN surface. SERS measurements carried out for analytes having significantly different sizes of molecules showed that the ratio of intensities of the SERS spectra measured on a layer of silver clusters deposited on a highly developed GaN surface or on a smooth GaN surface depends on the size of the analyte molecule and on the metal-analyte affinity. The larger the molecule, the smaller this ratio (for example, it was 9.5 for pyridine, 3.5 for penicillin G, and 1.5 for hemoglobin). These results show that some "SERS hot spots" cannot be reached by large molecules.