## 4 Discussion  
### Characterizations  
We first characterized citrate-stabilized AuNPs with TEM, demonstrating that synthesized nanoparticles were homogeneous and that their average diameter was 13.1 \(\pm\) 1.8 nm (Figure 2a and Figure S3). As-synthesized AuNPs appeared clustered in groups of two or three, with no clear spaces between individual nanoparticles within the groups. However, the images of DNA\(-\)AuNP conjugates showed that DNA-attached AuNPs have a clear and much more uniform spacing between neighboring particles, which was measured to have an average of 0.78 nm (Figure 2b). By comparing the morphologies of AuNP clustering in Figure 2a,b, we showed that DNA strands modified the surfaces of AuNPs. We further characterized DNA\(-\)AuNP conjugates using gel electrophoresis. Non-DNA-attached AuNPs aggregated in the well and failed to enter the gel. DNA-modified AuNPs did not aggregate and were able to migrate into the gel (as shown in Figure S4). The TEM images agreed with the gel electrophoresis results and demonstrated that we successfully decorated AuNPs with ssDNA. Next, we constructed DNA\(-\)AuNP hydrogel composites by combining DNA\(-\)AuNP conjugates and long spacers (Sl). We further confirmed the  
Figure 5: Current–voltage (IV) curves of dehydrated samples. (**a**) DNA\(-\)SWNT conjugates (left) vs. DNA\(-\)SWNT hydrogel (right); (**b**) DNA\(-\)MWNT conjugates (left) vs. DNA\(-\)MWNT hydrogel (right); (**c**) DNA\(-\)AuNP conjugates (left) vs. DNA\(-\)AuNP hydrogel (right). All measured across gold electrodes with 200 \(\upmu\)m spacings. Legend shows pulse number.  
formation of hydrogel composites by characterization of a hydrogel sample using TEM (Figure 2c). The TEM images showed multiple layers of AuNPs on top of each other that appeared to be held together during sample collapse from drying. Comparing that with Figure 2b, it is apparent that DNA spacers had linked AuNPs together in a 3D structure.  
Although differences in clustering and morphology between DNA-SWNT in conjugates versus hydrogel are not entirely distinctive using TEM characterization (Figure 2d,e), the differences were apparent in the MWNT samples (Figure 2f,g). DNA-MWNT conjugates appeared to gather on the lacy carbon film on the copper grids and did not appear to fill in most holes in the film. On the other hand, the dehydrated hydrogel made of DNA-MWNT conjugates with long spacers (SI) covered the entire lacy film (including holes in the film) with its own networks formed by the nanotubes. DNA spacers helped to connect MWNTs together into a web-like structure over a large area. When taking a closer look at individual MWNTs, we observed regions of coating over the nanotubes, with an average thickness of 1.51 nm. Figure 2h clearly shows that the middle area of the MWNT was not wrapped by DNA, while the areas close to MWNT junctions were coated. Figure 2h also shows a thicker coating around MWNT junctions, indicating the location of spacers. These images represent a direct observation of DNA acting as a \"smart\" glue to bind and connect MWNTs together. Overall, we successfully constructed pure DNA hydrogels and DNA hydrogel composites with nanomaterials.