# pH-Responsive Hollow Polymeric Microspheres from Irradiated Cyclic Ether Aqueous Solution
## 3 Results and Discussions
### pH-Trigged Molecular Release  
By using a combination method of CLSM and labeling of dyes, we could investigate the hydrophobicity of the hollow microspheres. As shown in Figure 2b, the hydrophilic dye was only confined to the shells, indicating that the prepared hollow microspheres possessed hydrophilic shells and hydrophobic cores. Moreover, when the pH was lower than 5.0, the hydrophobic dye could be easily encapsulated into the hollow cores of the microspheres. Notably, different from other tedious methods for matter encapsulation, immersion was a much easier way and was expected to be suitable for various hydrophobic matters. However, the situation was very different when the solution pH increased from 5.0 to about 8.0. The CLSM image showed that the hollow microspheres shrank and less hydrophobic dye was encapsulated (Figure 2c). These results may be attributed to the pH stimuli-responsiveness of the hollow microspheres. Therefore, the relationship between the morphology of the obtained hollow microspheres and the solution pH was investigated in detail in a wide pH range. As shown in Figure 2d, when the pH was lower than 5.0, the hollow microspheres remained swollen. Herein, the hydrophobic dye could be encapsulated and taken away from the aqueous solution. However, the sharp shrinkage of the outer and inner diameters occurred immediately when the pH was higher than 5.0. In addition, the core fraction of the hollow microspheres also decreased simultaneously (Figure 2e). In fact, the core fraction decreased extremely from about 70% to nearly 0%, when the pH increased from about 1.0 to 8.0. Therefore, the encapsulated hydrophobic dye could be rapidly squeezed out of the hollow cores and released in aqueous solution by simply adjusting the solution pH. Moreover, no further shrinkage was observed at pH above 8.0. This pH stimuli-responsiveness may be ascribed to the alteration of ionization degree of the prepared amphiphilic polymers bearing COOH groups (pKa \(\sim\)4.80) [21]. When the pH was less than 5.0, only a few COOH groups were deprotonated and the hydrogen bonds among the obtained amphiphilic polymers promoted the formation of compact aggregates. With the increase of pH, the obtained polymers existed in the more stretched conformation due to the deprotonation of the COOH groups, leading to the formation of the incompact aggregated structures with disappearing hollow cores [25]. The as-prepared pH-responsive hollow microspheres are possibly applicable as a drug delivery in the target tissues or cells, e.g., intestine (pH 8.4) or endosome (pH 6.0-6.5) [7]. Fascinatingly, when the labeled hollow microspheres were subjected to 1 mol L\({}^{-1}\) KOH aqueous solution, they decomposed in seconds, leaving the hydrophobic dye completely released in water (the inset of Figure 3a). Moreover, TEM (Figure 3a) and DLS analysis showed that the hollow microspheres were disassembled to well-defined core-shell aggregates with a \(<\) D\({}_{\text{h}}\) \(>\) of 133.4 nm (Figure S6). This behavior might be attributed to the intensive weakening of interactions among amphiphilic polymers by adding strong alkane. Moreover, after KOH had been removed via a dialysis process, the core-shell aggregates could be reassembled back to hollow microspheres with an average diameter of ca. 1.9 \(\upmu\)m by simply irradiating the aqueous solution (Figure 3b and Figure S7). It indicated the self-organization of the hydrophobic segments of the obtained core-shell micelles under irradiation so as to minimize their contact with surrounding water. Notably, this reversible disassembly/assembly behavior between dispersing core-shell micelles and hollow microspheres could be repeated for several times. Therefore, the reversible encapsulation/release of the hydrophobic matters can be expected for the prepared hollow microspheres.