Improving the longevity of plasmonic SERS-active substrates using functionalized graphene for trace TNT detection

Abstract

Silver (Ag) nanostructures have, by far, been the most effective SERS substrates investigated over the last two decades. However, the oxidation-induced instability limits the practical applications of many Ag-based SERS substrates. In this work, we tackle this problem by creating Ag nanostructures (AgNSs) using femtosecond laser ablation and then covering them with octadecylamine functionalized graphene (ODA-Gr) and reduced graphene oxide (r-GO). The laser-induced periodic surface structures (LIPSS) created on silver surfaces, which strengthen localized plasmonic fields, maximized the SERS activity. While r-GO and pristine graphene have been explored for surface modification of SERS substrates, integration of amine-functionalized graphene offers an unexplored route to synergistically maximize the chemical and electromagnetic enhancements. By introducing octadecylamine groups, we increase both TNT adsorption affinity and protect the Ag nanostructures from oxidation, resulting in unprecedented substrate longevity and detection sensitivity. Our findings show that the ODA-Gr-coated AgNSs outperformed the AgNSs (100 nM) and AgNSs/r-GO (10 nM) in terms of sensitivity, reaching a detection limit of 1 nM for trinitrotoluene (TNT). In addition, the ODA-Gr coating dramatically extended the lifespan of the substrate, maintaining ∼54 % of its original SERS intensity after 120 days, as opposed to ∼32 % for r-GO-coated AgNSs and ∼8 % for bare AgNSs under open-air conditions. Long-term stability and improved adsorption efficiency are facilitated by the combined chemical and electromagnetic enhancement mechanisms in the case of AgNSs/ODA-Gr. These results demonstrate the potential of amine-functionalized graphene-coated AgNSs as a reliable and sensitive SERS platform for explosives detection.