Photothermal properties and applications of gold nanorods

gold nanorod;photothermal;thermal transport;ultrafast laser;plasmonics

Huang, Jingyu

关键词: gold nanorod;    photothermal;    thermal transport;    ultrafast laser;    plasmonics;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/50432/Jingyu_Huang.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Gold nanorods (GNRs) with their special optical and photothermal properties have attracted attention recently for their potential applications in fields such as photothermal therapy, drug delivery, biomedical imaging and chemical sensing. These anisotropic gold nanocrystals have tunable surface plasmonic resonance (SPR) bands ranging from the visible to the near infrared region. Their small dimensions of less than 100 nm and inert reactivity as well as the ease of surface functionalization facilitate their interaction with biological substances for various purposes. Before these GNR-based techniques can be applied in clinical use, there is still a large number of fundamental problems that need to be addressed to better align material properties with the desired application metrics. This thesis focuses on the fundamental photothermal and optical properties of GNRs when interacting with laser beams, which includes developing photothermal molecular release systems for drug delivery; investigating the nanoscale thermal transport properties at the solid-liquid interfaces; and studying the light emission induced by hot electron-hole pairs in GNRs.In the first chapter, the colloidal synthetic strategy of GNRs, developed by the Murphy research group, and their optical and photothermal properties are introduced. The as-synthesized GNRs bear a bilayer of surfactant molecules cetyltrimethylammonium bromide (CTAB) on the surface and exhibit two SPR peaks ranging from 500 to 900 nm depending on the aspect ratio of the GNRs. The surface of the GNRs can be modified by different ligands or polyelectrolyte layers in the aqueous environment. GNRs can also be immobilized on different substrates for further studies and applications. In the second chapter, a light-controlled molecular release system was developed. Model drug molecules (dyes) were loaded within a variable number of polyelectrolyte multilayers wrapped around GNRs. A NIR continuous-wave (cw) laser was used to heat the global sample solution to release the dye molecules, which was detected using fluorescent signals from the dyes. The photo-induced release rate depended on the quantity and type of polyelectrolyte trapping layers and could be tuned by a factor of 100. Comparison of the photo-triggered molecular release rate to a pure thermal experiment provides an estimate of the effective temperature of the GNR solution upon irradiation.In the third chapter, since knowledge of the spatiotemporal temperature distribution around a laser heated nanoparticle is essential for the study of photothermal therapeutics and the interface is playing an important role in controlling nanoscale heat transfer, a transient absorption method was used to investigate the thermal conductivity and heat capacity of surfactant and polyelectrolyte coatings of GNRs in aqueous solution, following femtosecond pumping of the longitudinal localized surface plasmons. Surfactant and polyelectrolyte layer thicknesses are measured by dynamic light scattering (DLS). The cooling dynamics of GNRs are best measured by tuning the pump-probe laser wavelength to the absorption peak of the sample solutions. The heat capacity and thermal conductivity were calculated numerically based on a cylindrical heat transfer model. It was found that the thermal properties have significant dependence on: (1) the external surfactant critical micelle concentration for CTAB-capped GNRs; and (2) the charge of the terminal capping layer for polyelectrolyte-capped GNRs.In the fourth chapter, light emission from plasmonic GNRs in aqueous suspensions was studied with cw and pulsed-laser excitation, which could be widely applied in biomedical imaging. Resonant secondary light emission contributes significantly to the background commonly observed in surface-enhanced Raman scattering (SERS) and to the light emission generated by pulsed-laser excitation of metallic nanostructures that is often attributed to two-photon luminescence. We proposed an electronic Raman scattering mechanism to describe the origin of the light emission quantitatively by comparing intensity of anti-Stokes emission excited by 785 nm cw and subpicosecond laser pulses. The result indicated that anti-Stokes emission is consistent with electronic Raman scattering by a high-temperature distribution of electronic excitations predicted by a two-temperature model.In the last chapter, femtosecond pulsed laser induced photothermal release of molecules was explored with surface-enhanced Raman scattering techniques. Distance-dependent SERS enhancement factors were first studied by immobilizing Raman reporters on different spacer layers. Laser pulse-triggered molecular release was monitored by SERS signals, since the SERS signal decreased significantly with the distance from the GNR surface. It was found the SERS signal decreases with the average power of laser pulses and the irradiation time. However, it is observed that laser pulses could also induce severe photobleaching of the Raman reporters, which should be considered in drug delivery to prevent the damaging of drug molecules.