Facile synthesis of size and wavelength tunable hollow gold nanostructures for the development of a LSPR based label-free fiber-optic biosensor†
Hollow bimetallic nanostructures have recently emerged as attractive plasmonic materials due to the ease of optical tunability by changing their size/composition. Currently available methods, in addition to being tedious and time-consuming, result in polydispersed nanostructures, particularly due to polydispersed templates. In this study, optically tunable hollow gold nanostructures (HGNS) were synthesized by galvanic replacement reaction between silver nanospheres (AgNS) templates and gold salt. Monodispersed AgNS were created using a gold seed-mediated heteroepitaxial growth. Since it is easier to ensure monodispersed gold nanosphere seeds, the resulting AgNS showed a tight control on size. Hollow gold nanostructures 43–70 nm in size with extinction maxima ranging between 450–590 nm were produced by varying the gold to silver molar ratio. The nanostructures were observed to be monodispersed and uniform (SD ≤ 11%) in all the batches. Furthermore, the synthesized HGNS were immobilized on dendrimer-functionalized U-shaped fiber-optic probes to develop a localized surface plasmon resonance (LSPR) based sensor. Refractive index sensitivity of the HGNS based sensors was found to be 1.5-fold higher than solid gold nanosphere (GNS)-based fiber-optic sensors. These HGNS-based fiber-optic probes were subsequently used to develop an immunosensor with improved sensitivity by using human immunoglobulin-G (HIgG) as receptor molecules and goat-anti-HIgG as a target analyte.