Functional layered materials for chemical detection using surface-enhanced Raman spectroscopy

Surface-enhanced Raman scattering (SERS) is one of the useful techniques for identifying a specific fingerprint of molecules. Conventional SERS consists of colloid based and thin film-based types. The colloid-based SERS uses a nanoparticle metal colloid to enhance the Raman signal of the target molecules while the thin film SERS uses coating technique to make a cluster of metal nanoparticles on the substrate to enhance the Raman signal. The challenge in developing the thin film-based SERS technique besides the sensitivity is the improvement of the reproducibility and stability of the SERS substrate. In this work, a graphene/Ag-nanoparticle/polymer (G/AgNP/polymer) SERS substrate was successfully fabricated using chemical vapor deposition (CVD), electrodeposition, transferring, and etching techniques. CVD was used for growing a graphene layer on a copper foil substrate. Graphene functions as a protective layer of the SERS substrate and help enhance Raman signal via chemical enhancement. Silver nanoparticles as an enhancement material were coated on graphene by electrodeposition technique. The structure of the silver nanoparticles growth on the graphene/copper substrate can be tuned by varying the electrodeposition condition to obtain proper ‘hot spot’ for Raman amplification. The electrodeposition condition at the current density of 10 uA cm-2 for 3 min generates a rattan ball-like structure of the Ag nanoparticles with the gaps between metal nanoparticles between 30-100 nm. The as-prepared layers were then transferred onto two types of polymer substrates which are polyimide (PI) tape and polydimethylsiloxane (PDMS) substate. The copper layer was etched out using a ferric chloride solution (FeCl3). The developed SERS substrate could detect methyl parathion with the enhancement factor (EF) of the primary peak at 1344 cm-1 of 1.5x104 and 4.7x104 for the PMDS and PI SERS substrates, respectively. Compare between two types of substrates, PDMS SERS substrate shows better enhancement factor, while the uniformity of the prepared PI SERS substrate was better than that of the PDMS SERS substrate. The shelf life of both SERS substrates were longer than 48 days at room temperature, thanks to the protective graphene layer, making the developed SERS substrate practical for field use. The developed technique offers a facile fabrication with low cost and ease of SERS storage without additional packaging process. This polymer-based SERS substrate is a promising platform with potential for use as a screening technique in field tests.