Environmental biogeochemistry studied by second-harmonic generation: A look at the agricultural antibiotic oxytetracycline

The binding behavior of the veterinary antibiotic oxytetracycline (OTC) to tailor-made environmental interfaces was investigated using second-harmonic generation (SHG). Specifically, four environmentally relevant chemical moieties were covalently tethered to fused quartz surfaces using silane and amide coupling chemistry to survey how amides, polyfunctional binding sites, and phenyl rings that are common in humic acids control OTC binding at pH 8. The model geochemical surfaces prepared for this study were the following: methylamide-terminated fused quartz as well as amide-linked carboxylic acid-, benzoic acid-, and benzyl-terminated fused quartz. We find that amide-linked benzoic acid-terminated silanes display the highest equilibrium binding constant (1.1 x 10(6) M-1) among the systems surveyed. A straightforward predictive method of using contact angle measurements and the corresponding interfacial energy densities to predict OTC mobility across humic acid-containing mineral/water interfaces is discussed in the context of the emerging bacterial antibiotic resistance development threat.