3.1 Synthesis and composition of the particles.
Just as shown in Fig. 1, two kinds of m-SiO2 were synthesized by the simple one pot method based on the feed sequence and reactivity difference of the raw materials. The reactivity order of the raw materials was TEOS > APTES > TUSA. So as TUSA was feed together with the other raw materials, the slow reactivity of TUSA resulted in their location most likely in the out layer of m-SiO2. In order to fabricated particles with azobenzene groups dispersed in the inner layer of the particles, TUSA was added firstly and reacted for a period time and formed network before TEOS and APTES beginning to react. Thus, -NH2 groups in the final particles endowed the samples with higher IBU loading and azobenzene groups caused the sample photo-responsive ability. Also the azobenzene groups located in the core of m-SiO2 produced higher driving force for the IBU release.
FTIR confirmed the composition of the designed nanoparticles. In the spectra of TUSA, SiO2 and SiO2-TSUA shown in Fig. 2a, the broad peaks at 1,100cm− 1 assigning to the stretching vibrations of Si–O–Si were clearly observed[27]. The peak at 2900cm− 1 corresponding to the C-H stretching vibration of -CH3 in CTAB disappeared in the spectrum of SiO2-TSUA showing the successful removal of the surfactant[28, 29]. Compared to the spectra of TUSA, the characteristic bands at 1550cm− 1 and 3060cm− 1 corresponding to N = N and C-H of azobenzene groups in the spectrum of SiO2-TSUA indicated that the TSUA had grafted onto silica successfully[30]. Meanwhile the broad peak at around 3447cm− 1 in the spectrum of SiO2 corresponding to stretching vibration of O-H shifted to 3350cm− 1 in the spectra of SiO2-TSUA indicating that the amino groups were successfully incorporated in silica. The FTIR spectra of TUSA-SiO2 were similar with that of SiO2-TSUA confirming the fact that the samples contained amino and azobenzene groups at the same time.
Zeta potential can directly indicate the surface charge of the particles which reflects the amount of -NH2 groups on the surface of m-SiO2. The Zeta potential of TSUA-SiO2 was higher than the corresponding samples of SiO2-TSUA with the same feed ratio showing higher amount of -NH2 on the surface of TSUA-SiO2 (Fig. 2b). These data strongly confirmed more -NH2 groups dispersed on the out layer of TSUA-SiO2, which meant that the azobenzene groups dispersed in the inner core of TSUA-SiO2 and the reaction performed toward the designed direction.
Thermogravimetric analysis revealed the successful functionalization of m-SiO2 with amino and azobenzene groups. All materials exhibited small weight loss around 100℃ due to the loss of physically adsorbed water molecules. On the TGA curves of SiO2-TSUA(Fig. 2c) and TUSA-SiO2(Fig. 2d), the weight loss between 250–800℃ derived from the decomposition of the organic groups[31] which were about 25.45wt% and 28.99wt% confirming the two samples possessed almost the same amount of organic groups.
3.2 Morphology of the nanoparticles.
Morphology observation by SEM showed SiO2-TSUA was a mixture of more irregular rods and less spheres (Fig. 3a), while the morphology of TSUA-SiO2 was almost spheres (Fig. 3b) with average diameter of 99nm (Fig. 3e). TEM observation indicated SiO2-TSUA (Fig. 3c) and TSUA-SiO2 (Fig. 3d) both had radial ordered mesoscopic structure and visual channels. But TSUA-SiO2 showed observable edge between inner dark area and outer light gray area indicating the obvious core-shell structural characteristic.
The location of azobenzene and -NH2 groups was further studied by the energy disperse spectroscopy (EDS). In the spectra of SiO2-TSUA (Fig. 4a) and TSUA-SiO2 (Fig. 4b) the distribution of C and N elements was mapped out. As more azobenzene groups were dispersed in the out layer of m-SiO2, the C sphere of SiO2-TSUA had obvious and clear edge with background. But the C sphere of TSUA-SiO2 had light center and relative dark out layer confirming most azobenzene groups located at the inner of TSUA-SiO2. The N spheres of TSUA-SiO2 and SiO2-TSUA were similar. No obvious N edge was observed due to the difficult detection of N.
Figure 5 was the XPS spectra of SiO2-TSUA and TSUA-SiO2. The peaks located at 284.28, 285.40 and 288.42eV were assigned to C1s in C = C, C = O of azobezene groups, while the peak at 285.5 was C1s with sp2 hybrid orbital derived from azobezene and amino propyl groups[32, 33]. The binding energies of N1s in N = N and NH2 were 401.5 and 399.5eV respectively[32, 34]. The C = O peak integral area of SiO2-TSUA was 3.85% higher than that of TSUA-SiO2 which was 3.63% (Fig. 5c and 5d). The integral area of N1s in N = N was 51.8% obviously higher than NH2(48.62%) (Fig. 5e and 5f). Accordingly, the ratio of azobenzene to -NH2 was calculated to be 1:2.5 which was higher than the feed ratio. These data strongly confirmed that azobenzene groups located in the corner of TSUA-SiO2 and more likely dispersed at the gates of the pores in SiO2-TSUA.
The formation of mesopores in the amino/azobenzene modified m-SiO2 was directly reflected by the nitrogen adsorption-desorption isotherms (Fig. 6a) and corresponding pore size distributions (Fig. 6b). SiO2-TSUA and TSUA-SiO2 exhibit a typical IV isotherm with a H4-type hysteresis loop, indicating the presence of mesopores[35] Stepwise adsorption of nitrogen and capillary condensation at a relative pressure (P/P0) of 0.45 to 0.95 and 0.3 to 0.95 respectively was exhibited. The surface areas, pore diameters and pore volumes of SiO2, SiO2-TSUA and TSUA-SiO2 were 1030 m2/g, 854.426 m2/g, 708.944 m2/g, 3.061 nm, 2.455 nm, 2.453 nm, and 0.493 cc/g, 0.66 cc/g, 0.771 cc/g in order. SiO2-TSUA had higher surface area, lower pore volume, almost the same pore diameter compared with TSUA-SiO2. The stack accumulation of azobenzene groups in the inner core of TSUA-SiO2 resulted in lower surface area and larger pore volume.
These characterizations confirmed two kinds of m-SiO2 with azobenzene and -NH2 groups accurately dispersed in the inner and out layer were successfully fabricated. The position of azobenzene on m-SiO2 can be controlled just by adjustment the feeding sequence based on the reaction difference of the raw materials.
3.3 Photoresponsive ability of the particles
UV-Vis spectroscopic measurements shown in Fig. 7 indicate the successful immobilization of TSUA on the particle surface. Upon UV light irradiation, the absorption band centered at 358 nm declines remarkably, and concurrently the band centered at 450 nm increases a little which ascribed to π-π* transition of the (E)-azobenzene group and n-π* transitions of (Z)-azobenzene group form respectively. The change of the absorption bands induced by UV irradiation indicates the trans-to-cis photoisomerization of the azobenzene. When the nanoparticle was exposed to visible light, the π-π* absorption increases and the n-π* absorption decrease, which implied the process of back-conversion from cis to trans form. All these changes exhibited reversible trans–cis isomerization and photoresponsive ability of TSUA-SiO2[36, 37]. SiO2-TSUA showed the curves similar to TSUA-SiO2.
3.4 Loading and release of IBU by the particles
According to the TGA data (Fig. 2b), the loading amount of IBU by SiO2-TSUA and TSUA-SiO2 were 16.12% and 19.0% respectively after comparison the weight loss of particles before and after loading IBU. TSUA-SiO2 had larger loading ability which may derive from the stack accumulation of azobenzene groups in the core and leave more volume in the out part of the pores. The decrease of Zeta potential of the particles after loading of IBU (Fig. 2c) also showed the successful loading of IBU and the larger loading ability of TSUA-SiO2 as the negative charge of IBU neutralized part of the positive charges of the m-SiO2.
The release of IBU from SiO2-TSUA was shown in Fig. 8a. The release amount of IBU from SiO2-TSUA under UV light was higher than that under visible light. Just as shown in Fig. 1, as the loading of IBU was performed under visible light, the azobenzene groups in m-SiO2, which existed as trans form, experienced conformation transformation under UV light irradiation process. This process caused free volume decrease in the pore and driving force formation for IBU release. But the IBU release amount by SiO2-TSUA was very low and less than 55%. While the IBU release percent by TSUA-SiO2 under UV reached up to ~ 93% in 250 min, almost 2 times as much as the m-SiO2 with the azobenzene groups in the out layer of particles with the same composition. Also the release rate difference has increased significantly after 160 min under different release light and rapid release rate under UV light were observed. The reason causing the obvious release behavior difference of the two kinds of particles was shown in Fig. 9. As azobenzene located in the inner layer of the pores, the organic groups accumulated tightly on the surface of the pores. Under UV radiation, conformation change decreased the free volume in the pores and produced large pushing force which decrease the attraction between -NH2 with the drug molecules. Thus the drug molecules released out rapidly. While as azobenzene was in the out layer of the pores, the organic groups arranged on the surface of the pores loosely as more space was existed especially at the gate of the pores. The volume change of azobenzene groups had no obvious influence on the interaction between -NH2 and drug molecules. So less drug molecules was released out. Therefore the higher IBU release percent by TSUA-SiO2 was due to the conformation change of stack azobenzene groups which produced large pushing forces to the drug molecules.