## 4 Conclusions  
In conclusion, the aptamer-functionalized DNA hydrogel was simply self-assembled from X-shaped DNA, L-shaped DNA, and S-DNA, avoiding a tedious DNA\(-\)polymer hybrid modification process. This hydrogel not only had a high protein-capture capacity, it also had a high retention rate in the absence of triggering CSs. The big drawback (i.e., initial burst release) of traditional hydrogel as a protein drug carrier could be resolved by using this aptamer-functionalized DNA hydrogel. Meanwhile, CSs can be used for controlling the wise-stage release of thrombin in the desired time from the hydrogels with designed concentration and dosage. The hydrogel was also workable in complex matrixes such as human serum. Furthermore, in vitro studies showed that RAW 264.7 cells cultured on the DNA hydrogel exhibited good proliferation and viability. This hydrogel will become a potential platform for the controlled release of protein drugs, which can regulate the release rate at a predetermined time point with excellent biocompatibility, and will facilitate the treatment of a mass of human diseases.  
The following are available online at [[http://www.mdpi.com/2076-3417/8/10/1941/s1](http://www.mdpi.com/2076-3417/8/10/1941/s1)]([http://www.mdpi.com/2076-3417/8/10/1941/s1](http://www.mdpi.com/2076-3417/8/10/1941/s1)). Figure S1: The detection mechanism of graphene oxide-based fluorescent aptasensor for thrombin protein. Figure S2: The recovered fluorescence intensity influenced by adding TB with concentrations of (2, 4, 6, 8 and 10 nM) and the linear relationship between TB concentration (nmol L\({}^{-1}\)) and relative fluorescence value. Figure S3: 3% agarose gel electrophoresis analysis. Lane 1, 2, 3 and 4 were the single-stranded DNAs, X1, X2, X3 and X4, respectively. Lane 5 was the X-DNA; lane 6 and 7 were the single-stranded DNAs, L1 and L2. Lane 8 was the L-DNA. Table S1: The sequences and the strand length of all oligonucleotides. Figure S4: The DNA hydrogel with different concentrations of X-DNA and L-DNA. (a) X: 400 uM, L: 800 uM; (b) X: 500 uM, L: 1000 uM; (c) X: 600 uM, L: 1200 uM; (d) X: 700 uM, L: 1400 uM; (e) X: 800 uM, L: 1600 uM, respectively. The concentration of S-DNA was fixed at 200 uM. Figure S5: Determination of the grafting amount of S-DNA by fluorescence spectroscopy. The initial amount of added S-DNA was respectively at (A) 0.2 nmol, (B) 0.4 nmol, (C) 0.6 nmol, (D) 0.8 nmol, (E) 1.0 nmol. Their fluorescence values were detected at 522 nm and the amount of grafted S-DNA was calculated through the difference of fluorescence intensity between the supernatant and blank solution. Table S2: The grafting degree of S-DNA. Figure S6: Comparison of the interaction force between the aptamer-TB complex and aptamer-CS duplex. Figure S7: The percentage of CS15 diffused into hydrogel. The initial amount of added CSs was at (A) CSs-S-DNA = 1/5, (B) CSs-S-DNA = 2/5, (C) CSs-S-DNA = 3/5, (D) CSs-S-DNA = 4/5, (E) CSs-S-DNA = 1. The amount of S-DNA was kept at 0.2 nmol. Their fluorescence values were detected at 522 nm and the percentage of CSs diffused into hydrogel was calculated through the difference of fluorescence intensities between the supernatant and blank solution. Table S3: The percentage of CS15 that diffused into hydrogel.  
[PERSON] conceived and designed the experiments; [PERSON] and [PERSON] performed the experiments and wrote the paper, [PERSON] and [PERSON] measured the characterizations and analyzed the data.  
This work was financially supported by the National Natural Science Foundation of China (Grant No. 21763009, 21404028), and Graduate Students Innovation Research Project of Hainan Province (Hys2018-59, Hys2018-60)[PERSON]  
The authors declare no conflict of interest.