Polydopamine-coated flat glass surfaces for nanoplastics uptake and Raman-based detection: a case study with polystyrene

Pollution caused by nanoplastics presents great challenges for researchers because of the lack of sensitivity of traditional analytical methods, with Raman and surface-enhanced Raman spectroscopy (SERS) becoming strategic for their detection. The main drawbacks lie in the poor signals of traditional Raman spectroscopy, requiring high concentrations of analytes, and the non-homogeneous distribution often limiting reliable detection when exploiting SERS on dried samples. Herein, we propose a simple strategy based on a coating layer of polydopamine (PDA) on simple glass substrates to exploit the adhesive properties of the biopolymer for the grafting and thus evenly pre-concentration of polystyrene nanoplastics (PS-NPs), further analysed using Raman and SERS. An in-depth analysis on the role of pH in PDA adhesive properties demonstrates the importance of electrostatic interactions toward different kinds of PS-NPs, presenting different Z-potential values. Moreover, PS-NPs of different sizes were analysed, ranging from 1 μm down to 15 nm. Raman detection of 100 nm and 1 μm PS-NPs was achieved, demonstrating that the PDA coating layer enables NPs pre-concentration and their subsequent detection by Raman spectroscopy. The versatility of the PDA substrate was also proven by grafting gold nanostars, creating a SERS substrate capable of detecting PS-NPs down to 15 nm.