Fabrication of 3D chiral nanoplasmonic structures is always challenging, while the principles for their chiroptical properties are still ambiguous.
We will present a combined experimental and theoretical study on 3D chiroplasmonic activity of silver nanospirals (AgNSs), fabricated on sapphire by low temperature glancing angle deposition. AgNSs exhibit bisignated CD spectra in the UV-visible range, in the form of not only individual AgNSs or an array. Compared to individual AgNSs, the array of AgNSs show CD with an intensity 3 order of magnitude higher. It is demonstrated the engineering of chiroplasmonic CD via adjusting spiral parameters, including spiral pitches, number of turns and handedness.
Finite element simulations were performed and are in good agreement with the experiments. A LC theory is also employed to explain the difference of chiroplasmonic CD in the UV and visible region.
Chirality does naturally exist, and the building blocks of life (e.g. DNA, proteins, peptides and sugars) are usually chiral. Chirality inherently imposes chemical/biological selectivity on functional molecules; hence the discrimination in molecular chirality from an enantiomer to the other mirror image (i.e. enantioselection) has fundamental and application significance. Enantiomers interact with left and right handed circularly polarized light in a different manner with respect to optical extinction; hence, electronic circular dichroism (ECD) has been widely used for enantioselection. However, enantiomers usually have remarkably low ECD intensity, mainly owing to the small electric transition dipole moment induced by molecular sizes compared to the ECD-active wavelength in the UV-visible-near IR region. To enhance ECD magnitude, recently it has being developed 3D chiral nanoplasmonic structures having a helical path, and the dimensions are comparable to the ECD wavelength. However, it is still ambiguous the origin of 3D chiroplasmonics, and there is a lack of studying the interaction of 3D chiroplasmoncs with enantiomers for the application of enantioselection.
Herein, we will present a one-step fabrication of 3D silver nanospirals (AgNSs) via low-substrate-temperature glancing angle deposition. AgNSs can be deposited on a wide range of substrates (including transparent and flexible substrates), in an area on the order of cm2. A set of spiral dimensions (such as spiral pitches, number of turns and handedness) have been easily engineered to tune the chiroptic properties, leading to studying the chiroplasmonic principles together with finite element simulation and the LC model. At the end, it will be demonstrated that 3D chiroplasmonics can differentiate molecular chirality of enantiomers with dramatic enhancement in the anisotropy g factor. This study opens a door to sensitively discriminate enantiomer chirality.
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