Synthesis and Characterization of Gold Nanorods and Gold Nanorod Dimers  

By Kayli Vieira 

Introduction 

Gold nanoparticles have become a hot spot in recent research due to their unique electrochemical properties. When light hits the surface of a gold nanoparticle, a collective oscillation of electrons results in a strong electromagnetic field, enhancing absorption and scattering properties. This phenomenon, known as plasmon resonance, gives gold nanoparticles their uniqueness and versatility in different areas of research. For example, gold nanoparticles have been utilized for research in drug delivery, biosensing, spectroscopy, and catalysis. In Professor Wei-Shun Chang’s lab at UMass Dartmouth, gold nanoparticles are studied using spectroscopic techniques to understand their physical and chemical properties for application in catalysis, microscopy, and biosensing. For this project, gold nanorods will be used to study substrate–induced chirality through circular dichroism (CD) measurements on a hyperspectral microscope. Samples of both chiral and achiral gold nanorods are required for single particle measurement, which can be synthesized using simple wet-lab procedures.  

Results 

The first step of the project requires the synthesis of pure, homogenous, medium–sized gold nanorods (22×66 nm in size). To accomplish this, multiple syntheses were performed using the seed-mediated growth method first developed by the Murphy Group (see Figure 1 for procedure schematic). 

Figure 1. Schematic of Seed-Mediated Growth Method of Gold Nanorods (Adapted from the Murphy Group) 

After extensive background research and alterations of reagent concentrations, the procedure was used to synthesize medium–sized gold nanorods, optimized for single particle measurement. For characterization, samples were analyzed using UV-Vis spectroscopy, scanning electron microscopy, and hyperspectral microscopy.  

Figure 2. UV-Vis Absorption Graph of Medium Sized Gold Nanorods

As seen in Figure 2, the UV-Vis absorption graph yielded important information about the sample’s purity and general aspect ratio, evident by the transversal and longitudinal plasmon bands. To determine the average size of the nanorods produced, SEM (scanning electron microscope) images were taken and analyzed. After analyzing over 150 individual gold nanorods, the dimensions of the rods produced yielded 59.6 ∓ 7.0 nm X 20 ∓ 3.18 nm (image can be seen in Figure 3).  

Furthermore, after being spin cast on a glass substrate and imaged on a darkfield inverted hyperspectral microscope, individual spectra for a given nanorod were produced using analyses of Hyperspectral images through the MATLAB program (see Figures 4 and 5). 

Figures 3 and 4. SEM Image and Hyperspectral Image of Medium Gold Nanorod Dimers 

Figure 5. Individual Gold Nanorod Spectra 

After successful synthesis of singular medium–sized gold nanorods, synthesis of a chiral sample is required. Using the previously made gold nanorod samples and a method developed by Kar et. al, samples of gold nanorod dimers (two gold nanorods linked end-to-end) were assembled. Using the polyelectrolyte coating and thiol-linking procedure schematized in Figure 6, chiral samples of gold nanorod dimers were synthesized and stable for a very short time.

Figure 6. Schematic of Gold Nanorod Dimers developed by Kar et. al 

Unfortunately, results varied from those in the literature. After a successful first attempt, reproducing the same results was almost impossible. Countless experiments were done altering individual factors at every part of the procedure to determine the driving force of the reaction. The hydrochloric acid concentration was determined to be the main contributor as to whether dimers were produced, likely due to the decrease of surface charge on the rods resulting in the ability for linkage. Furthermore, it was discovered that the addition of DABCO resulted in the destabilization of dimers formation likely due to surface charge imbalances. Thus, successful samples produced were only stable for a few hours.  

Figure 7. UV-Vis Absorption Graph of Gold Nanorod Dimers Sample

Characterization of each gold nanorod dimer sample was preformed using the same methods mentioned above. In Figure 7, the UV-Vis absorption graph revealed important information as to whether or not dimers were formed in the reaction, evident by the shoulder created after the longitudinal plasmon band. Similar graphs can be seen in the resulting spectra generated using the hyperspectral microscope (see Figures 8 and 9). Furthermore, SEM images depicted in Figure 10 clearly showed the configuration of the rods as dimers, aligning with the results of the hyperspectral images and UV-Vis graph. 

Figures 8 and 9. SEM Image and Hyperspectral Image of Medium Gold Nanorod Dimers

Discussion and Future Work 

Homogeneous and pure medium-sized gold nanorods were successfully synthesized for future single-particle measurement. Additionally, gold nanorod dimers were assembled using the polyelectrolyte coating and thiol linking method previously developed in the literature. Hydrochloric acid was identified to be the key parameter of the dimerization reaction, with alterations needed for each concentration of rods used for assembly. Currently, the gold nanorod dimers synthesis is continuing to be optimized for maximum reproducibility, with future plans of single-particle measurement. Both products synthesized will be used for analysis in the future study of substrate-induced chirality.  

Works Cited

Synthesis of Solution-Stable End-to-End Linked Gold Nanorod Dimers via pH-Dependent Surface Reconfiguration Ashish Kar, Varsha Thambi, Diptiranjan Paital, Gayatri Joshi, and Saumyakanti Khatua Langmuir 2020 36 (33), 9894-9899 DOI: 10.1021/acs.langmuir.0c01516

Seeded High Yield Synthesis of Short Au Nanorods in Aqueous Solution Tapan K. Sau, and Catherine J. Murphy Langmuir 2004 20 (15), 6414-6420 DOI: 10.1021/la049463z