## 3 Results and Discussion  
### Suitable Synthesis Conditions for Fragrance Silica Microcapsules  
Silica microcapsules can be synthesized using several techniques. High temperature, sonication, and high or low pH can be employed to rapidly synthesize microcapsules by increasing the TEOS hydrolysis rate [33; 36; 37; 38]. Before synthesizing fragrance containing silica microcapsules for our fragrance delivery system, we first examined the feasibility of each synthetic method. After emulsifying the fragrance compound and TEOS mixture at 11,000 rpm at 25 \({}^{\circ}\)C, samples of the emulsion were subjected to the following reactions: (i) a 7-d reaction at room temperature, (ii) a 2-d reaction at 50 \({}^{\circ}\)C followed by a 7-d reaction at room temperature, and (iii) 1 h of sonication followed by a 7-d reaction. The silica microcapsules formed under each set of reaction conditions had the following characteristics (Figure 1G): (i) particle size of 1.18 \(\upmu\)m and an encapsulation efficiency of 99.61 \(\pm\) 0.003%, (ii) particle size of 0.87 \(\upmu\)m and an encapsulation efficiency of 84.19 \(\pm\) 0.464%, and (iii) particle size of 0.43 \(\upmu\)m and an encapsulation efficiency of 99.13 \(\pm\) 0.004%. The silica microcapsule (0.2 wt%) was added to a 15% SDS solution to assess the fragrance retention. The fragrance in the capsules synthesized at room temperature was retained for 1000 h (approximately 40 d), whereas the fragrance in capsules synthesized at 50 \({}^{\circ}\)C was completely released in 4 h and that in the capsules synthesized by sonolysis was completely released within 2 d (Figure 1H). The fragrance retention of the silica microcapsules was also analyzed by measuring the concentration of hexyl cinnamaldehyde that leaked into the aqueous phase of the SDS solution from the silica microcapsules at room temperature by measuring the absorbance at 285 nm. The fragrance compound encapsulated in silica microcapsules, with silica coatings to reduce porosity, was added to a 15% SDS solution to mimic laundry detergent and fabric softener products. The leakage of the fragrance oil from the microcapsules was evaluated at various times and temperatures. The fragrance silica microcapsules synthesized at room temperature were found to be the most suitable (Figure 1H). These microcapsules had dense less porous microcapsule shell structures because the reaction occurred slowly at the interface of the fragrance compound and the TEOS mixture [31,32]. However, even at room temperature, the fragrance oil leaked within 2 h at a higher silication rate due to the high temperature. Although the fragrance capsules synthesized at 50 \({}^{\circ}\)C and those at room temperature showed minor size differences, the rapid silica reaction presumably formed more porous structures at 50 \({}^{\circ}\)C (Figure 1A,B,D,E). Meanwhile, after ultrasonic homogenization into a fragment nanoemulsion, the emulsion template reaction occurred, and the synthesized nanocapsules were narrowly dispersed to 400 nm in size (Figure 1C). However, these nanocapsules released the fragrance compound approximately 14 times faster than the microcapsules synthesized at room temperature, and the rate of leakage appeared to increase with increasing surface area (Figure 1E). The fragrance retention time of the microcapsules synthesized at room temperature (approximately 40 d) was too short for use in detergents; hence, the microcapsules required further treatment. pH is also important for the formation of silica. The reaction rate is accelerated by increasing the pH; however, this also increases porosity to the extent that the structure of the microcapsules cannot be maintained. The reaction rate can be controlled by lowering the pH level; however, when the pH level decreases to <4, the hydrolysis rate exceeds the condensation rate [39]. Thus, the concentration of hydrolyzed TEOS is increased, and the rate of migration of hydrophilic TEOS to the exterior water phase is higher than the rate of incorporation at the interface.