## 4 Discussion
### Rheological Results  
We would like to construct hydrogels of reasonable mechanical strength where the crosslinked networks can effectively prevent diffusion of the nanomaterials and, thus, to achieve hydrogels with confined architecture for further applications. Having a higher storage modulus than loss modulus from oscillation frequency tests indicated that we indeed made hydrogels with solid-like properties. When using the same DNA tiles as crosslinkers, we observed that shorter spacers constructed hydrogels of higher mechanical strengths as they are able to build a denser network. The situation is reversed and longer spacers constructed more solid-like hydrogels when using DNA/nanomaterial conjugates as crosslinkers. This is because the CNTs and AuNPs we used are much larger in scale compared to DNA molecules; thus, having longer spacers helped to build more and more stable bridges between nanomaterial/crosslinker components. Therefore, we used SI for all electrical studies and structural characterization analysis.  
CNTs are well known for their mechanical reinforcement applications. Consistent with previous observations in other composites, we observed a huge increase in storage modulus with DNA-CNT hydrogel composites compared to pure DNA hydrogels. When using one type of crosslinker, DNA-SWNT conjugates constructed the most solid-like hydrogels, followed by using DNA-MWNT conjugates, DNA-AuNP conjugates, X-shaped DNA tiles, and, finally, Y-shaped DNA tiles. We believe that we observed a higher storage modulus from hydrogels constructed with DNA-SWNT conjugates than hydrogels constructed with DNA-MWNT conjugates because DNA wraps around SWNTs more tightly with more turns [51]. The TEM images (Figure 2) also showed that DNA coats on SWNTs much better than on MWNTs. As the average diameter of CNTs increases from SWNTs (0.78 nm) to MWNTs (\(\sim\)9 nm), (GT)\({}_{20}\) loses its strong binding around the nanotubes, which resulted in a decrease in mechanical performance of hydrogels made with these conjugates. This result shows the importance of crosslinked DNA as the binding material to connect nanomaterials and to build networks. Furthermore, hydrogels constructed with both types of DNA-CNT conjugate crosslinkers showed higher mechanical strength than hydrogels constructed with DNA-AuNP conjugates because the high aspect ratios of CNTs are inherently in favor of providing reinforcement to composites compared to sphere-shaped materials.  
We further investigated and rationally improved the mechanical properties of hybrid hydrogels by combining DNA/nanomaterial conjugates and DNA tiles as crosslinkers. Since CNTs and AuNPs are in much larger scales than DNA molecules, there are available spaces between individual nanotubes and nanoparticles in the hydrogels, where there is no DNA filling besides the DNA network associated with binding strands and spacers. These hydrogels have the capacity to integrate more materials that can fill in the spaces. Therefore, we used different compositions of DNA/nanomaterial conjugates and DNA tiles to construct hydrogels in which crosslinkers come in different scales. When we used DNA tiles to make up to 25% of crosslinkers and DNA-CNT conjugates for the rest, the resulted hydrogels were even more solid-like than when using 100% DNA-CNT conjugates as crosslinkers. This is because substituting some of the DNA-CNT conjugates with a much smaller type of crosslinkers helped to fill in the open spaces between CNTs and, thus, made a denser hydrogel. However, DNA molecules are significantly weaker than CNTs, so we observed a decrease in mechanical strength when substituting more DNA-CNT conjugates with DNA tiles. In summary, we demonstrated adjustment of the mechanical properties of hybrid hydrogels by combining different compositions of crosslinkers and achieved the most solid-like hydrogel when using DNA-CNT conjugates as 75% of crosslinkers and X-shaped DNA tiles for the remaining 25% of crosslinkers.