Chiral plasmonic DNA nanostructures with switchable circular dichroism

Abstract

Circular dichroism spectra of naturally occurring molecules and also of synthetic chiral arrangements of plasmonic particles often exhibit characteristic bisignate shapes. Such spectra consist of peaks next to dips (or vice versa) and result from the superposition of signals originating from many individual chiral objects oriented randomly in solution. Here we show that by first aligning and then toggling the orientation of DNA-origami-scaffolded nanoparticle helices attached to a substrate, we are able to reversibly switch the optical response between two distinct circular dichroism spectra corresponding to either perpendicular or parallel helix orientation with respect to the light beam. The observed directional circular dichroism of our switchable plasmonic material is in good agreement with predictions based on dipole approximation theory. Such dynamic metamaterials introduce functionality into soft matter-based optical devices and may enable novel data storage schemes or signal modulators.

Figure 1. Surface-bound chiral plasmonic nanostructure.

(a) A rigid DNA origami bundle composed of 24 parallel double helices. Gold nanoparticles are arranged in a secondary left-handed helix on the DNA origami structure. Zoom in: 10 nm gold nanoparticle functionalized with thiolated ssDNA hybridized to the DNA origami 24HB. The origami structure is functionalized with biotin groups (green) on one end for the attachment to a BSA–biotin–neutravidin-coated surface (red, green and grey). (b) Transmission electron microscopy image of a nanohelix adsorbed non-specifically to a carbon-coated grid. Scale bar, 50 nm.

Figure 2. Dynamic material with switchable CD.

(a) Theoretical (grey curve) and experimental (black curve) CD spectra for L-NHs dispersed in solution. (b) Theoretical CD spectra of L-NHs oriented parallel (red curve) and perpendicular (blue curve) to the incident beam. (c) Experimental CD spectra (top) and scheme of orientation state (bottom) of the L-NHs. The light beam penetrates the quartz substrate perpendicular to its surface plane. Measurements shown were performed with the same ensemble of L-NHs and in the displayed order. L-NHs stood upright if dispersed in solution and were hence aligned parallel to the incoming light beam. The expected transverse CD_z was observed in this case. After removal of the buffer and drying of the surface, the L-NHs laid flat on the substrate and were thus oriented perpendicular to the incoming light beam. Here, the expected longitudinal CD_xy was measured. The cuvette was re-filled with buffer, which led to re-alignment of the L-NHs parallel to the light beam. Again the transverse CD_z was obtained. The sample was dried again and the longitudinal CD_xy reappeared.

Figure 3. CD splitting theory and comparison to experimental results.

(a) Scheme of L-NHs aligned orthogonal and parallel to a polarized light beam. The possible CD excitation modes are indicated with double arrows (red, CD_z; blue, CD_xy). (b) The simulated peak-dip CD signal of randomly oriented L-NHs can be composed by the superposition of weighted transverse CD_z and longitudinal CD_xy signals. The directional CD signal (CD_z or CD_xy) is much stronger than the signal originating from randomly dispersed helices as averaging over the orientation of the chiral objects typically strongly reduces the CD signal. (c) Simulated CD spectra for helices oriented at angles restricted to a cone with a given opening angle θ. (d) The CD_z (red curve) and the CD_xy (blue curve) were measured from the same ensemble of L-NHs by switching the orientation of the L-NHs (cf. Fig. 2). The black curve shows the calculated superposition of weighted CD_z and CD_xy. Importantly, this curve resembles the CD signal of nanohelices dispersed randomly in solution. Note that the peak shift between the inverted modes (indicated by the grey vertical lines) is larger than expected from theory (as indicated in b). This can be attributed to differences of the refractive indices of the particle-surrounding media in dry and wet samples.

Figure 4. Darkfield scattering spectroscopy of surface-bound nanohelices.

Aligned nanohelices orthogonal to a surface in solution (top row) and aligned nanohelices parallel to a surface dry on glass (bottom row) were studied. (a) Schemes of nanohelices aligned parallel (top) or orthogonal (bottom) to a light beam. (b) Theoretical calculations. Top: only transverse plasmons are excited if the nanohelices are aligned parallel to the light beam. Bottom: longitudinal (blue) and transverse (red) plasmons are excited if the nanohelices are aligned orthogonal to the light beam. The expected shift of the plasmon resonance is 20.5 nm. (c) Darkfield images of aligned nanohelices in the same area are shown in colour. An ozone-free Xe lamp is used as a light source. (d) The interpolated scattering spectra of a typical nanohelix as highlighted by the white circles in c. Top: the scattering spectrum of a surface-bound nanohelix in solution shows no clear polarization dependency. Only transverse plasmon excitation is obtained (peak indicated by the grey vertical line). Bottom: a dried nanohelix exhibits polarization dependent scattering spectra consistent with xy orientation of the nanohelix. Longitudinal (blue) and transverse (red) plasmon excitation is obtained. The observed shift of the plasmon resonance is 22±4 nm (peaks are indicated by the blue and red vertical lines, respectively).