Metal nanoparticles such as silver and gold nanoparticles (Au NPs) exhibit unique plasmonic properties. This property has made them attractive for offering the suspension of the nanoparticles in optics and sensor technologies. The plasmonic properties depend on the nanoparticle morphology including the size and shape of nanoparticles. However it is difficult to obtain specific plasmonic properties from the metal nanoparticles due to difficulty in controlling the morphology. Therefore, researchers have been looking for methods to control the morphology and have identified the application of an AC electric field to metal nanoparticle suspensions as a promising solution.
Generally, the plasmonic properties of nanoparticles can also be tuned by controlling the clustering states of the nanoparticles. The clustering of nanoparticles forms “hotspots” between the adjacent nanostructured metals, leading to surface-enhanced Raman Scattering (SERS). External stimuli like temperature and applications of electric and magnetic fields have been used to control the clustering states of the metal nanoparticles. Among nano-sized particles, Au NPs exhibit high responsivity to AC electric field hence can be used to effectively control their clustering states.
Recently, Kanako Watanabe and colleagues at Tohoku University in Japan, investigated the plasmonic properties of nanostructured gold nanoparticles that were formed by the application of the AC electric field. They aimed at clarifying the relationship between the plasmonic properties and clustering states of Au NPs suspension as a key factor in controlling the plasmonic properties of metal nanoparticles. Their work is published in the journal, Soft Matter.
The research team commenced their experimental work by applying an external AC electric field of frequency 0.1 kHz-1 MHz and a field strength of 10 Vmm-1 – 50 Vmm-1 to Au NPs suspensions. The differences in the UV-vis spectra were observed for both the spectra with and without the applied AC electric field. Furthermore, the difference in the Raman intensity induced by the application of the electric field was used to determine the SERS effect of gold nanoparticle clusters.
The authors observed that the application of the AC electric field resulted in broader UV-vis spectra thus signifying the clustering of the gold nanoparticles. Consequently, they noted that clustering in suspension was dependent on the frequency of AC field. The AC field led to the aggregation of the Au NPs indicating the darkening of the red color. Furthermore, the sizes of the gold nanoparticle clusters formed by the electric filed highly depended on the frequency of the electric field.
Watanabe and colleagues’ findings showed that at a low field frequency of about 0.1 kHz in the case of 30 Vmm-1, rapid aggregation of Au NPs occurred thereby resulting in low Raman intensities. However, high field frequency above 1 kHz significantly enhanced the Raman intensities of target molecules due to the formation of the hotspots induced by the clustering of the nanoparticles. The strong relationship between the clustering states and the optical and sensing properties of the Au NPs showed that application of an external AC electric field is a promising alternative for controlling plasmonic properties of metal nanoparticles in suspension. Therefore, it will advance the development of highly sensitive sensors and optics materials.
Watanabe, K., Tanaka, E., Ishii, H., & Nagao, D. (2018). The plasmonic properties of gold nanoparticle clusters formed via applying an AC electric field. . Soft Matter, 14(17), 3372-3377.Go To Soft Matter