Template-Free Synthesized Gold Nanobowls Composed with Graphene Oxide for Ultrasensitive SERS Platforms

Engineering of plasmonic properties of gold nanostructures expands the field of their applications from photocatalysis and photothermal effects to ultrasensitive surface-enhanced Raman spectroscopy (SERS). The known methods of preparation of gold nanobowls involve the deposition of gold layer on polymers or silicon nanotemplates and the removal of the top layer of gold together with the template. Such gold nanobowls are characterized by very broad plasmonic bands due to the plasmon hybridization. The sharp edges on the top of nanobowls are potential sources of the strong electromagnetic field beneficial for SERS. We present a novel template-free synthesis of gold nanobowls (AuNBs). The AuNB layers are deposited on graphene oxide (GO) layers. We compare AuNBs with gold nanospheres (AuNSs) and gold nanourchins (AuNUs) having similar size. The gold nanoparticles are combined with pristine GO or graphene oxide conditioned in ammonia (GONH₃) or graphene oxide conditioned in sodium hydroxide (GONaOH). The SERS properties of the hybrid supports were studied using rhodamine 6G (R6G) as the SERS probe. The 633 nm laser line was used, which falls out of the molecular resonance with R6G. The results indicate that AuNBs show largely higher enhancement factors when compared to AuNUs and AuNSs. Furthermore, the GO materials are able to modify the SERS enhancement by 1 order of magnitude. We explain the influence of the GO material by three factors: (1) enabling or disabling the charge transfer between gold and R6G, which is crucial for the chemical part of SERS enhancement; (2) causing the aggregation of gold nanoparticles and formation of hot spots; (3) dipole contribution to the electromagnetic enhancement through the abundance of polar groups on the surface.