Increasing SERS performance of silver nanoparticles with nanometric coating of polydopamine: A novel approach for methylene blue detection

• PDA coatings with nanometric thickness were obtained on silver nanoparticles. • Enhancement of Methylene Blue signals exploiting PDA coated AgNPs was investigated. • Direct SERS detection of Methylene Blue in real water samples exploited without any pre-treatment. • Computational calculations were made to assess SERS data obtained. • The role of PDA in the enhancement of Methylene Blue SERS signals was validate comparing different dyes. Polydopamine (PDA) is a bioinspired polymer widely employed mainly because of its surface adhesive features. In this study silver nanospheres (AgNPs) were coated with a thin layer of PDA to exploit the affinity of its surface toward Methylene Blue (MB) and subsequently improve the Surface Enhanced Raman Spectroscopy (SERS) based MB detection. Using PDA thickness below 2 nm, we obtained a strong SERS signal enhancement for MB, with a tenfold increase compared to uncoated AgNPs, not observed for other investigated dyes. With the support of computational modelling the strong signal increase registerd for MB was explained on the basis of its completely planar structure giving optimal π-π stacking and electrostatic interactions with PDA surfaces. In pure water a limit of detection (LOD) of 0.025 µM concentration was found. Direct analysis on tap water samples spiked with MB gave a LOD of 0.1 µM concentration. The PDA coated SERS substrate here presented shows high performances in terms of signal enhancement, sensitivity for MB and reproducibility of the signals (RSD = 2.7 %). Moreover, this approach allows to perform fast analysis without any sample pre-treatment, with a potential for in-situ analysis using portable Raman instrumentation.