# Sequential drug release via chemical diffusion and physical barriers enabled by hollow multishelled structures

Sep 7, 2020

### MIT molecule absorption in HoMS

Fourier transform infrared (FTIR) spectra (Fig. 2e) were collected after different MIT loading periods to study the interaction between MIT and TiO2–HoMS. The peaks in the FTIR spectrum of MIT-3s-TiO2–HoMS at 714, 1418, 1618, and 1640 cm−1 could be ascribed to C–S (stretching), CH3 (bending), C=C, and C=O (stretching) in MIT26, respectively (Fig. 2e). Interestingly, a redshift is observed for the C=O stretching band with extension of the drug-loading period, indicating different adsorption modes during MIT loading (Fig. 2f).

$$frac{1}{{q_e}} = frac{1}{{Q_0}} + frac{1}{{Q_0K_Lc_e}},$$

(1)

where ce = equilibrium concentration of adsorbate (mg L−1), qe = amount of MIT adsorbed per gram of carrier at equilibrium (mg g−1), Q0 = maximum monolayer coverage capacity (mg g−1), and KL = Langmuir isotherm constant (L mg−1):

$$lg,q_e = lg,K_f + frac{1}{n}lg,c_e,$$

(2)

where Kf = Freundlich isotherm constant (mg g−1), n = adsorption intensity, ce = equilibrium concentration of adsorbate (mg L−1), and qe = amount of MIT adsorbed per gram of carrier at equilibrium (mg g−1). The R2 values of the Langmuir model and the Freundlich model for MIT adsorption on TiO2–HoMS are 0.3685 and 0.9931, respectively (Supplementary Fig. 4). Therefore, the adsorption of MIT on TiO2–HoMS can be considered as multimolecular layer adsorption with Freundlich model.

### Sequential drug release

Taking the release of MIT, a model antibacterial compound, for characterization purposes, the mass release performance was tracked by UV–Vis spectrophotometer (Supplementary Fig. 5). The MIT release process shows roughly three stages (Fig. 4a). After the MIT-carrier composite entered the solution, a quick increase in drug concentration was observed at the first stage (~4 h), which is called burst release. Afterwards, the mass release speed slowed, and the MIT concentration remained stable at the second stage, which is named sustained release. 2s- and 3s-TiO2–HoMS and SBA-15 all have the capacity to maintain the concentration in the second stage (Fig. 4a). Notably, when carrying the same drug amount, 3s-TiO2–HoMS was found to be the most effective in inhibiting the growth of Escherichia coli (E. coli) when we continually introduced bacteria into the system and could inhibit bacterial growth even after 432 h (Fig. 4b). The presented fluorescence images32 show that on the 10th day, the bacterial viability was 66%, 58%, 33%, 0%, and 11% for MIT, MIT-loaded TiO2 hollow spheres, MIT-2s-TiO2–HoMS, MIT-3s-TiO2–HoMS, and MIT-SBA-15, respectively (Fig. 4c and Supplementary Fig. 6). Moreover, all the TiO2 carriers show good stability, which did not present any morphological changes even after 30 days (Supplementary Fig. 7).

Interestingly, this stimulus-responsive release, the third stage of sequential release, is a unique feature of the HoMS system. After the concentration of MIT stabilized at ~60 ppm (slightly higher than the minimum inhibitory concentration (MIC)) with the same total drug amount for various samples (Supplementary Fig. 8), bacteria were added to the solution to investigate the responsive release performance. 2s- and 3s-TiO2–HoMS and SBA-15 all presented responsive release performance, i.e., after the rapid decrease in the concentration of MIT, equilibrium was gradually recovered. 3s-TiO2–HoMS showed the best recycling performance among all the samples and maintained the concentration over the MIC after 14 rounds of addition of bacteria. Impressively, the recovery process was different for various samples. 3s-HoMS showed a smaller drop (9.8%) than 2s-HoMS (17.3%) with the same amount of bacteria added (Fig. 4f), indicating that the MIT molecules reserved in 3s-HoMS are easier to release under the stimulating conditions. In comparison, the MIT-loaded TiO2 hollow spheres and pure MIT do not show the ability of responsive release (Fig. 4e). MIT-loaded SBA-15 has excellent sustained-release performance; however, its responsive release is not as good as that of 3s-TiO2–HoMS. SBA-15 shows the largest drop of 18.1% after adding bacteria (Fig. 4f), and it cannot reach the initial concentration during the recovery stage, even after 20 h.

In addition to responding to bacteria, MIT–HoMS can also respond to pH changes (Supplementary Fig. 9a). A low pH value can change the zeta potential of TiO2 from −19.96 to 0.42 mV, weaken TiO2–MIT interactions and further affect MIT–MIT interactions; thus, more MIT molecules can be easily released (Supplementary Fig. 9b).