SERS detection of negatively charged molecules by interface-induced self-assembly of PDDA modified Ag nanoparticles

Some noble metal colloids with negative charges were mostly used as surface-enhanced Raman scattering (SERS) substrates, that the Raman signals of negatively charged molecules are hard to be detected because of the electrostatic repulsion among them and the negatively charged metal nanoparticles . Herein, Ag nanoparticles (Ag NPs) with positive charges and its Ag nanofilm were prepared by poly dimethyl diallyl ammonium chloride (PDDA) to realize the SERS detection of negatively charged molecules directly. The zeta potential and its particle sizes were characterized to select the optimum condition of PDDA. In addition, the morphology of positive Ag NPs and Ag nanofilm was characterized by UV–Vis spectroscopy and transmission electron microscopy . The SERS effect of negatively charged thiamphenicol on positively charged Ag nanofilm was significantly enhanced at the optimum condition and its sensitivity was significantly improved compared with normal Ag nanofilm. The proposed novel positively charged Ag nanofilm can be further applied to detect other negatively charged molecules by SERS.