More detailed information(such as specific information about the reagents) will be published separately
Metamolecules, which constitue metafluid, have to has symmetry stucture and high magnetic response. So, we choose the final candidates of projects; tetrahedron and octahedron which are the symmetric geometry with lowest complexity (which means relatively easy to fabricate compared with higher symmetry structures. To compare the magnetic response between two clusters we performed numerical simulation.
Fig 1.(a) and (b) represent electric response(blue line) and magnetic response(red line). (c),(d) represent magnetic field distribution (color) of tetrahedron and octahedron at their magnetic resonance wavelength(668[nm] and 720[nm], respectively).
At graph (a),(b) in figure 1, electric means radiated power of sum of electric dipole and quadrupole, magnetic means radiated power of magnetic dipole. The methods of calculating electric and magnetic response are shown in 'Materials & Methods - simulation'.
In both bottom pictures in Figure 1 the magnetic responses under impinging light were mapped with rainbow color. Red color indicate strong magnetic response whereas blue color indicate weak magnetic response (same scale bar in both figure). The strong magnetic response can be identified in the center of the both structure. However, the octahedral structure shows stronger magnetic response in relatively large area than tetarhedron stucture. It indicate that there is high induced magnetic response. The result of comparison in top two figures(fig 1(a), (b)) also show that octahedron structure can induce more stronger magnetic response under incoming light which means octahedron structure is more appropriate to induce negative refraction in liquid phase. Thus, we chose an octahedron structure to construct metafluid.
More detailed information(such as specific information about the reagents) will be published separately
Fig 3. Seed AuNPs synthesis results. Experiment was conducted by varying sodium citrate(SC) concentration to achieve desired quality of AuNPs. 2.52mM SC was set as a reference condition. Old beaker indicate originally used one for other purpose which treated with aqua regia. New beaker was also treated with aqua regia before use.
Fig 4. Seed AuNPs synthesis results with TEM analysis. Size distribution was further analyzed by TEM image process (using Image J). Average size of 2.02mM old beaker(bottom, left) is 18.76nm and 2.02mM new beaker - 1(bottom, right) is 16.64nm.
Fig 5. DNA functionalization process snap shot. Written concentrations indicate the final concentration of monovalent cation. (Top) Functionalization of AuNPs using monothiolated DNA strands. At relatively high salt concentration, color of AuNPs turned to darkish color which is the evidence of low stability. (Middle) After 1 day incubation of above samples. During incubation, as thiolated DNA strands bind to AuNPs, the reddish color recovered. (Bottom) Functionalization of AuNPs with dithiolated using dithiolated DNA strands. Clear reddish color have maintained at all salt concentration.
Fig 8. Gel intensity analysis. Relative intensity indicates the intensity ratio of gel pocket against the each lane.
Fig 9. 1.25% agarose gel purification of assembled AuNPs cluster. The depicted lanes contain the following samples: 1kb DNA Ladder, M13mp18 scaffold strands, barrel-like DNA origami structures, assembled AuNPs cluster by barrel-like DNA origami and unbounded AuNPs. In the lane 4, the faint dark band in the middle is assembled cluster whereas the leading dark band is unbounded AuNPs. The cluster band was extracted and purified for TEM analysis
Fig 11. Quantitative analysis of the assembled clusters for yield calculation. Tri: trimer, Tetra: tetramer, Penta: pentamer, Hexa: hexamer(octahedron). Totally, 127 clusters are counted for analysis
Fig 12. (a)Assembled AuNPs further growth process and (b)results. a) During growth process, AuNPs does not aggregated in buffer condition even after washed by centrifugation. b)TEM image of the assembled AuNPs after further growth steps. To verify grown state more clearly, AuNP clusters in tetramer geometry (nano-ring consist of four particles) were used. c) Further growth result of non-preassembled AuNPs which does not show emerging between AuNPs
By utilizing DNA nanotechnology, our team successfully arrange seed AuNPs in specific structure and further growth it to achieve target size of AuNP clusters. Our team chose octahedron structure which shows strong magnetic response against the impinging light using the Finite Element Method(FEM) simulation. We assembled octahedron clusters with ~50% yield and verified the feasibility of seeding growth method using clusters with 3 to 4 AuNPs. Even though the sphericity of the assembled particles are not so high and some clusters are merged, we confirmed that assembled AuNP clusters can be successfully grown in salt and buffer condition.
Eventhough our works show the possibility that our approaches can be used to realize negative refraction in liquid phase, there are many issues must be solved for realization. First of all, octahedral AuNP clusters formation yield should be further increased. Even though additional purification such as density gradient centrifugation can be to selectively disperse octahedral clusters in suspension with ~50% of formation yield, this approach unavoidable accompanies loss of the assembled cluster. Thus, high yield is essential for practical application. Second, further growth processes should be optimized further. In our research, due to the low stability of AuNPs in salt condition, growth conditions are highly restricted which lead to polygonal AuNPs geomerty. Becasue the high sphericity is essential for homogeneous optical properties, AuNPs growth process should be further optimized.
Despite the mentioned issues, our results reveal that seeding growth method utilizing DNA nanotechnology can pave the to negative refractive index in liquid phase which provide a practical platform for the versatile translation of metamaterials into various real-world applications.