Towards high aspect ratio silica coated gold nanorods capable of smectic ordering for Surface Enhanced Raman spectroscopy

Abstract

Raman Spectroscopy is used to investigate the vibrational energy modes of molecules. However, the Raman signals are 1010 till 1015 times weaker than typical fluorescence spectroscopy. Surface Enhanced Raman Scattering (SERS) is a methodology that uses colloidal metal nanoparticles with plasmonic prop- erties to boost the Raman signal by orders of magnitude via electromagnetic enhancement. Silver particles would give the strongest enhancement. However, gold is used more often as it chemical more stable and there is an wider variety of available shapes. The shape of the nanoparticle also strongly influences the enhancement factor. The local enhancement will be stronger at strong curvatures due to the near field effecs of the plasmon resonance. Because of this, a rod shape is a good shape to use. These nanorods will have a strong enhancement at the tips. Further, if the local electromagnetic fields of nanoparticles overlap, they create so called hotspots, increasing the electromagnetic enhancement. By configuring the gold nanorods in an end-to-end orientation would, in theory, create the strongest enhancement, because of the linking of the hotspot. A novel way to do this is by creating a smectic- ordered liquid crystal. In a smectic ordered system, the tips of the rods are aligned as the rods are oriented the same way and layer by layer. When rods have an aspect ratio (length divided by width) of at least 4.1 to 1, then rods can form this smectic ordering through self-assembly. The gold nanorods were coated with mesoporous silica oxide. This improves thermal stability and reduces toxicity. Further, the silica layer will prevent aggregation of the gold nanorods. To do this, the silica layer needs a certain thickness. How thick depends on the aspect ratio of the gold nanorod. However, the thicker the silica layer, the lower the aspect ratio. It was calculated that using a gold rod with an aspect ratio of at least 6.9 gives enough room to create a thick enough silica layer to prevent the aggregation of the gold nano rods without silica layer becoming to thick that the aspect ratio of the total particle would be lower than 4.1. In this thesis, gold nanorods were achieved with various aspect ratios ranging from 2.2 to 5.6 via the Ye & Murray’s method. Using the Chang & Murphy’s method gold nanorods with an aspect ratio up to 8.5 could be synthesized. However, due to the ageing of NaBH4 or bad stock solutions of NaOH, it was impossible to create such a high aspect ratio at the end of the thesis. Controlling the thickness of the silica layer by adjusting the concentration of tetraethyl orthosilicate (TEOS) is difficult. Another way to do this is by adding 2-(Methoxy(polyethyleneoxy)propyl)trimethoxysilane (PEG-silane). By adding this chemical at different time intervals during the silica shell formation, the silica-coated gold nanorods could be created with various thicknesses. Further, it is shown that dissolving TEOS in methanol resulted in thinner and smoother silica layers. Further, it is shown that the concen- tration and the dimension of the gold nanorod strongly influence the formation of the silica layer. In the end, a silica-coated gold nanoparticle was created with an aspect ratio of 3.9.