Abstract
The circularly polarized reflection of nature is due to their distinct azimuthally twisted or helical character in the nanostructure of the surface films. Although many chiral inorganic powders have been successfully synthesised, the artificial synthesis of chiral inorganic films is rare. Herein, we reported a facile synthetic route for the growth of monolayered chiral film on the quaternary ammonium-modified silicon substrate. The films grew on the substrate surface because of the strong electrostatic interaction between positively charged quaternary ammonium groups and negatively charged phosphate groups of DNA, with subsequent growth to right-handed, vertically aligned, impeller-like helical architectures with left-handed two-dimensional square p4mm-structured DNA chiral packing. The DNA–silica composite films exhibited strong optical activity at 295 nm and in the range of 400–800 nm, corresponding to DNA chiral packing (absorption) and to the helical blade in the impeller (scattering), respectively. Upon removal of DNA templates, the pure inorganic impeller-like helical morphology was maintained; consequently, the scattering-based optical response was blue-shifted approximately 200 nm as a result of a decrease in the effective average refractive index. The hierarchical structures were reflected from the surfaces by cross-polarised light, which confirmed that the films were strongly birefringent, with long-range anisotropy.
Highlights
The silicon substrate was pretreated with H2SO4/H2O2 to increase the abundance of silanol groups24. (ii) The chemical modification of a positively charged quaternary ammonium functionalised surface via the silanisation of silanol-based surfaces using quaternary aminosilane (N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride, TMAPS) to terminate the substrate surface was employed (Figure 1B)24. (iii) The DNA layer was introduced onto the substrate surface via strong electrostatic interactions between the positively charged quaternary ammonium groups on the substrate surface and the negatively charged phosphate groups of DNA (Figure 1C). (iv) The synthesis gel mixture composed of DNA, MgCl2, TMAPS, TEOS and H2O was subsequently introduced to induce growth of the chiral DNA–silica films (CDSFs) on the substrate
TMAPS acts as both a condensing agent and a co-structure-directing agent (CSDA)[13] and results in the self-assembly of the DNA–silica composite with co-condensation of the silane group of TMAPS and the silica source of tetraethoxysilane (TEOS), and Mg21 acts as an inducer of DNA chiral packing[21] of the chiral morphological particles in the DNA–silica composite (Figure 1D). (v) We calcined the CDSFs to pure inorganic chiral silica films (CSFs) by removing the DNA templates (Figure 1E)
The impeller-like helical DNA–silica complex (IHDSC) with blades arranged in an anticlockwise manner in the side view is defined as right-handed, and the opposite case is defined as left-handed
Summary
The strategy that we used to synthesise chiral silica films relied on controlling the growth of helical DNA–silica impeller[21,22] on the substrate surface by facilitating the interaction between the DNA and the substrate and their subsequent self-assembly with a silica source[23]. (iv) The synthesis gel mixture composed of DNA, MgCl2, TMAPS, TEOS and H2O was subsequently introduced to induce growth of the chiral DNA–silica films (CDSFs) on the substrate In this process, TMAPS acts as both a condensing agent and a co-structure-directing agent (CSDA)[13] and results in the self-assembly of the DNA–silica composite with co-condensation of the silane group of TMAPS and the silica source of tetraethoxysilane (TEOS), and Mg21 acts as an inducer of DNA chiral packing[21] of the chiral morphological particles in the DNA–silica composite (Figure 1D).
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