UC San Diego

Metallic nanostructures can confine light to nanoscale dimensions through surface plasmon resonance, leading to extraordinary effects such as enhancements in light emission and scattering. This thesis explores the use of these enhanced light-matter interactions for applications in optical biosensing. First, we demonstrate plasmon-enhanced multiphoton absorption in covellite-phase copper sulfide (CuS) nanocrystals. The near-infrared plasmon resonance wavelength of these semiconductor nanocrystals is tuned towards the two-photon absorption edge using selenium doping. This tuning results in a 300% enhancement in the two-photon action cross section, resulting in a “brightness” that is an order of magnitude greater than conventional fluorophores. The resulting luminescence demonstrates the efficacy of CuS nanocrystals as a solid-state dye for biological imaging. CuS nanocrystals also offer the first “all-in-one” platform for studying plasmon-exciton coupling without the need for a physicochemical interface between the plasmonic and excitonic materials. Second, the use of lithographically-patterned gold nanobulbs for detection of complex bioanalytes using surface-enhanced Raman scattering (SERS) is reported. Highly uniform SERS enhancement factors in these gold nanobulbs permit the use of 2D-correlation spectroscopy for the identification of molecular rearrangement and complementary binding in single-stranded DNA. Third, we demonstrate the use of metal nanoislands on graphene for strain sensing using SERS. These metal nanoisland-graphene composite films contain gaps between the nanoislands which behave as hot spots for SERS when functionalized with benzenethiolate. Mechanical strain increases the sizes of the gaps; this increase attenuates the electric field and the resultant SERS signal. This compounded, SERS-enhanced “piezoplasmonic" effect can be quantified using a plasmonic gauge factor, and is among the most sensitive mechanical sensors of any type. These sensors can detect tensile deformations of less than 0.04% with a resolution of less than 0.002%. Since the nanoisland-graphene composites are thin films that are both conductive and optically active, they permit simultaneous electrical stimulation of myoblast cells and optical detection of the strains produced by the cellular contractions. Last, we demonstrate a method to fabricate metal nanoisland-graphene composites with a temperature coefficient of resistance (TCR) that is close to zero. A thin film with this property can be used as a piezoresistive sensor that is stable against temperature fluctuations of the type encountered in the real world—e.g., in a wearable sensor. The stability of a sensor fabricated with this method is demonstrated by subjecting a wearable pulse sensor to simulated solar irradiation.