## 3 Results and Discussion
### Fabrication of Aptamer-Functionalized DNA Hydrogel  
The ideal molar ratio of the L-DNA to X-DNA was 2:1, where the initial DNAs were entirely cross-linked and formed the most indurative network [38; 39]. Next, 10 \(\upmu\)L of X-DNA and 10 \(\upmu\)L of L-DNA with different concentrations were mixed with 1 \(\upmu\)L and 200 \(\upmu\)M of S-DNA to form aptamer-functionalized DNA hydrogels. As shown in Figure S4, the solutions with different concentrations of X and L-DNA have lost all their fluidity in a minute and seemed to be reaching a gel state, which turned out to be the transformation of the sol-to-gel. Besides, the frequency-scanning test was used to examine the mechanical properties of DNA hydrogels to further confirm the formation of DNA hydrogels. As shown in Figure 2A, the loss modulus (G\") was significantly smaller than the storage modulus (G\") in the whole process, which provided the obvious signal for the formation of gel. The result indicated that pure DNA hydrogels assuredly came into being, which were self-assembled from the DNA building blocks. The mechanical strength of the DNA hydrogel was enhanced along with the enhancing concentration of the DNA building units. The rheological properties of the DNA hydrogel with grafted S-DNA and without S-DNA that were shown in Figure 2B proved that there was no significant difference between the hydrogel with S-DNA and the hydrogel without S-DNA.  
In this work, the concentration of S-DNA was very low relative to that of X-DNA. The grafting degree of S-DNA into hydrogel was calculated in Figure S5 and Table S2. It demonstrated that only a few \"sticky ends\" of X-DNA had been consumed by S-DNA. Thus, the effect of S-DNA on the mechanical properties of the hydrogel could be ignored. The inner morphology of the hydrogel was characterized by SEM and provided here. The DNA hydrogel samples were freeze-dried as shown in  
Figure 2: (**A**) The rheological property of DNA hydrogels with different concentrations (400 \(\upmu\)M, 500 \(\upmu\)M, 600 \(\upmu\)M, 700 \(\upmu\)M, and 800 \(\upmu\)M for X-DNA, the molar ratio of X-DNA and L-DNA is fixed at 1:2). (**B**) The storage and loss modulus of DNA hydrogel (X: 400 \(\upmu\)M and L: 800 \(\upmu\)M) with S-DNA or without S-DNA. (**C**) The SEM image of DNA hydrogel after freeze-drying (X: 400 \(\upmu\)M and L: 800 \(\upmu\)M). (**D**) Fluorescent images of hydrogels (X: 400 \(\upmu\)M and L: 800 \(\upmu\)M) without S-DNA, with carboxyfluorescein (FAM)-labeled S-DNA, and with FAM-labeled S-DNA with scrambled ”sticky ends”.  
Figure 2C; the freeze-dried hydrogel owned the interconnected pore structures, and the pore size was about 10 \(\upmu\)m to 50 \(\upmu\)m, so that the protein could easily diffuse into or out of the hydrogel.  
In order to examine whether the S-DNA could be assembled into a hydrogel, the FAM (carboxyfluorescein)-labeled S-DNA and FAM-labeled S-DNA with a scrambled \"sticky end\" was mixed with X-DNA and L-DNA. The formed DNA hydrogels were washed in DNA buffer to eliminate non-cross-linked S-DNA, and observed by a fluorescent microscope. As shown in Figure 2D, the blank control group formed a hydrogel only by X-DNA and L-DNA without adding S-DNA, and no fluorescence was detected. The hydrogel with FAM-labeled S-DNA exhibited strong fluorescence intensity. However, if the hydrogel was assembled from FAM-labeled S-DNA with the scrambled \"sticky ends\", no fluorescence was detected, too. This could be explained by the S-DNA with the scrambled sticky ends not being complementary to that of the X-DNA. The results suggested that the S-DNA with a complementary \"sticky end\" to X-DNA could be successfully assembled into the hydrogel network through the base complementation pairing rule.