Modification and Application of Gold Nanorods in Surface Enhanced Raman Scattering Based Assays
Elbayomy,Shereen A
Surface enhanced Raman scattering (SERS) is an ultrasensitive vibrational spectroscopic technique to detect molecules on or near the surface of plasmonic nanoparticles. More recently, this technique has been used to design novel nanoprobes named “SERS labels” that combine metallic nanoparticles and specific organic Raman reporter molecules. Such SERS labels can be conjugated to capture probes of biological molecules to be used to indirectly sense the target biological molecules by using laser Raman spectroscopy. Various metal nanoparticles act as a Raman signal amplifier for engineering of nanoprobes. In general, their size, geometry, chemical composition, and surface chemistry can influence the Raman enhancement ability. Recently, interest in gold nanorods (GNRs) has increased, as they possess unique optical and electronic properties. Many of the targeted applications for GNRs require their surface modification, but it can often be a challenge due to their cetyltrimethylammonium bromide (CTAB) coating, which is a stabilizing agent used during GNR synthesis. The work presented in Chapter 2 of this thesis explored spectroscopic and electronic microscopy characterization of GNRs after CTAB replacement with a mixed thiolate layer of a Raman reporter such as 4-nitrobenzenethiol (tNB) and 2-(2-{2-[2-(2-[2-(11-mercapto-undecyloxy)-ethoxy]-ethoxy)–ethoxy]-ethoxy}–ethoxy)-ethoxy–acetic acid (HSC11(EO)6~COOH). This HSC11(EO)6~COOH linker provides; steric stability through hydrophobic alkyl chain; water solubility due to presence of an ethoxy moiety that improves ligand exchange in aqueous solution; and anchor points such as carboxylic acid or amino groups for further conjugation with biological molecules. Chapter 3 presents the SERS response of the −SC11(EO)6~COO-/tNB modified GNRs of four different aspect ratios and 30 nm diameter spherical gold nanoparticles, the characteristic Raman spectrum of 4-nitrobenzenethiol was measured for five gold nanoparticle solutions. The capability of using the −SC11(EO)6~COO-/tNB modified GNRs of aspect ratio 2.4, which were covalently linked to immunoglobulin G (IgG) through terminal-carboxylic acid group of thiolate linker were explored with a chip-based SERS immunoassay in Chapter 4. The sensitivity of SERS based sandwich immunoassay utilizing gold nanorods of aspect ratio 2.4 for goat IgG detection was translated to a limit of detection (LOD) of 15 fM. The detection and quantification of small metabolite molecules is being targeted as a promising diagnostic method in disease assessment. Chapter 5 in this thesis presents an indirect competitive SERS based assay for the analysis of the thyroid hormone thyroxine in its free form (fT4). In this assay, we used fT4 conjugated SERS labels of GNRs to compete with fT4 standard solutions for monoclonal antibody binding sites. Lower levelss of free thyroxine than threshold, that accompanies hypothyroidism disease, can be detected and the results were correlated well with the results from a commercial enzyme-linked immunosorbent assay (ELISA) kit.