Fig. 4: Design and structural and optical characterization of the spherical helix NP cluster.

a Schematic of the spherical helix NP cluster. The spherical helix valence mode is realized for the meshframe by programming two alternating sets of sticky ends (green and purple, with five strands for each color) at desired vertices. Two types of AuNPs with identical 10 nm core size but distinct DNA shells (green and purple) are directed through the hybridization to binding sites and assembled into a spherical helix cluster. b (Left) Representative negative-stained TEM image of spherical helix clusters; scale bar, 200 nm (inset: zoomed-in image of a spherical helix cluster; side length, 150 nm). (Right) Population analysis of assembled spherical helix clusters (from 611 measured clusters). c Designed NP positions of the spherical helix cluster (numbered from 1 to 13) on a sphere-like meshframe (left). Seven particles (Particles 4–10) are on the equatorial plane of the sphere-like structure. Particle 1 and Particle 13 denote the summits of Upper and Lower hemispheres, respectively. Projections of designed spherical helix cluster from the Upper (upper right) and Lower (lower right) summits. The grey sphere provides visual guidance. The blue line with arrow defines particles on the equatorial plane (Particles 4–10). d Spherical helix cluster reconstructed from cryo-TEM tomography. 3D view of the reconstructed structure (left) and projections from the Upper (upper right) and Lower (lower right) summits. e Center-to-center distance between adjacent NPs, D, and center-to-center distance between sphere-like meshframe and NPs, R, were measured. Designed distances (bottom histogram, shadow columns): D = 20.2 nm and 22.8 nm, and R = 32.4 nm. Average distances obtained from three independently reconstructed clusters (bottom histogram, solid columns): D = 24.0 ± 4.9 nm and R = 34.5 ± 3.8 nm. Error bars indicate standard deviation of experimental data. f Computed CD spectrum (blue curve) and absorption spectrum (green curve) for spherical helix clusters. g Experimental CD spectrum (blue dotted curve), Lorentzian fit (blue solid curve) and absorption spectrum (green curve) for spherical helix clusters as described in the text.