The fabrication technique involved in development of helically plasmonic nanoparticles with the use of glancing angle deposition is of high interest due to its technology advantage over others. Also, chiroptical responses characterized by circular dichroism (CD) have shown that plasmonic spirals with nanometric-sized helical pitch can be effectively noticed in a certain ultraviolet-visible region.
Researchers led by Professor Zhifeng Huang from Hong Kong Baptist University performed a glancing angle deposition process that produces plasmonic nanospirals to generate silver nanoparticles with a helical pitch less than wire diameter while operating at a fast substrate rotation and at a substrate temperature of 0 °C in order to study chiral plasmonics.
The hidden chiroptical activity of the silver nanoparticle arrays with helical pitch range of 3-70 nm showed that the localized surface plasmon resonance (LSPR), plasmonic CD and anisotropy factor tend not to shift with an increase in helical pitch. However, the logarithmic increase of plasmonic CD and anisotropic g factor mode was similarly to that of the nominal helical pitch.
The increase in nominal helical pitch favored its coupling with LSPR wavelength which provides an explanation on the helical pitch-induced CD amplification.
The water effect on chiroptical activities of silver nanoparticles in a helical pitch range of 3-70 nm had a hydrophilic array with water contact angles less than 90°. This led to a red-shift of amplified LSPR and CD peaks by approximately 40 nm.
A reversible water effect on chiroptical activity was noticed during the multiple alternating wetting-drying processes, leading to an observed red shifting of the plasmonic amplification blue shifting of the plasmonic reduction.
The water effect of silver nanospirals with a helical pitch of 215 nm gave a hydrophobic array and water contact angle of 120°. Despite that its chiroptical activity has a reversible effect on water in terms of the CD shift, the reversibility observed was much lesser than that of the silver nanoparticles. The chiroptical activity of silver nanospirals however, decreased during multiple alternating wetting-drying processes, showing a high irreversibility range in the chiroptical water effect.
The low porosity of the hydrophilic silver nanoparticle arrays contributed to reversible water effect on the chiroptical activity while the hydrophobic silver nanospirals with excessive array porosity contributed to the irreversible water effect.
This study developed an economical and easy method of fabricating plasmonic helical pitch with sub-10-nm molecularly comparable sizes that would be of relevance to the chirality-related biomedical applications.
Liu, J1, Yang, L1, Huang, Z2,3,4. Chiroptically Active Plasmonic Nanoparticles Having Hidden Helicity and Reversible Aqueous Solvent Effect on Chiroptical Activity, Small 12 (2016) 5902-5909.Show Affiliations
- Department of Physics, Hong Kong Baptist University (HKBU), Kowloon Tong, Kowloon, Hong Kong SAR, China
- Department of Physics, Hong Kong Baptist University (HKBU), Kowloon Tong, Kowloon, Hong Kong SAR, China. [email protected]
- Institute of Advanced Materials, Partner State Key Laboratory of Environmental and Biological Analysis, HKBU, Kowloon Tong, Kowloon, Hong Kong SAR, China. [email protected]
- HKBU Institute of Research and Continuing Education, 9F, the Industrialization Complex of Shenzhen Virtual University Park, No. 2 Yuexing 3rd Road, South Zone, Hi-tech Industrial Park Nanshan District, Shenzhen, 518057, Guangdong, China. [email protected]
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