Confinement of Gold Nanoparticles in a Supramolecular Cross-Linked Polymer Network as a Robust Nanofilm for Efficient SERS Detection of Antibiotics

The self-organization of gold nanoparticles (AuNPs) into a closely packed film is of great interest for both basic research and practical applications in surface-enhanced Raman spectroscopy (SERS). Here, we report an approach to facilitate the self-assembly of AuNPs at the oil-water interface, driven by cross-linking of two block copolymers. The diblock copolymer poly(ethylene glycol)-block-poly(acrylic acid) (PEG-b-PAA) in the aqueous phase and the diblock copolymer poly(ethylene glycol)-block-poly(4-vinylpyridine) (PEG-b-P4VP) in the oil phase form a cross-linked polymer network (CPN) at the water-oil interface. Meanwhile, due to the electrostatic adsorption of P4VP chains on the citrate-capped AuNPs, the AuNPs in the water are also drawn to the water-oil interface and confined within the physically cross-linked network formed by PEG-b-PAA and PEG-b-P4VP. This technique efficiently produces a two-dimensional (2D) AuNP-polymer composite film suitable for SERS sensing applications. The physically cross-linked network acts as a robust framework that significantly enhances the flexibility of the film through hydrogen-bonding interactions between PEG-b-PAA and PEG-b-P4VP. In addition, compared to AuNP films prepared by traditional methods, this 2D cross-linked polymer network film imbued with closely packed AuNPs (i.e., AuNPs@CPN) exhibits significantly improved mechanical strength, making it less prone to fracture. Thus, it can exist as a free-standing film and be transferable to various solid substrates, which is very convenient for practical use.