Storage and Sustained Release of Volatile Substances from a Hollow Silica Matrix
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IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 18 (2007) 245705 (6pp) doi:10.1088/0957-4484/18/24/245705 Storage and sustained release of volatile substances from a hollow silica matrix Jiexin Wang1, Haomin Ding2,XiaTao1 and Jianfeng Chen1,2,3 1 Key Lab for Nanomaterials, Ministry of Education, Beijing 100029, People’s Republic of China 2 Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China E-mail: [email protected] Received 26 December 2006, in final form 26 March 2007 Published 25 May 2007 Online at stacks.iop.org/Nano/18/245705 Abstract Porous hollow silica nanospheres (PHSNSs) prepared by adopting a nanosized CaCO3 template were utilized for the first time as a novel carrier for the storage and sustained release of volatile substances. Two types of volatile substances, Indian pipal from perfumes and peroxyacetic acid from disinfectants, were selected and then tested by one simple adsorption process with two separate comparative carriers, i.e. activated carbon and solid porous silica. It was demonstrated that a high storage capacity . / (9 6mlperfume mgcarrier) of perfume could be achieved in a PHSNS matrix, which was almost 14 times as much as that of activated carbon. The perfume release profiles showed that PHSNSs exhibited sustained multi-stage release behaviour, while the constant release of activated carbon at a low level was discerned. Further, a Higuchi model study proved that the release process of perfume in both carriers followed a Fickian diffusion mechanism. For peroxyacetic acid as a disinfectant model, PHSNSs also displayed a much better delayed-delivery process than a solid porous silica system owing to the existence of unique hollow frameworks. Therefore, the aforementioned excellent sustained-release behaviours would make PHSNSs a promising carrier for storage and sustained delivery applications of volatile substances. 1. Introduction of our knowledge, it is reported for the first time that hollow silica spheres are employed as a novel carrier for storage and Recently, there has been continuous intense interest surround- delivery application of volatile substances in this paper. ing the diverse synthesis of hollow silica spheres, because Perfumes and disinfectants are two classes of representa- they can be potentially used as delivery vehicles for the con- tive volatile substances. Currently, to increase the performance trolled release of various substances such as drugs, cosmet- of consumer products, and, in particular to prolong the percep- ics, dyes, and inks, and the protection of biologically active tion of volatile molecules such as flavours or fragrances, the macromolecules, together with as fillers, and in catalysts and development of suitable delivery systems for functional per- waste removal [1–3]. However, there have just been a few fumery is becoming increasingly attractive due to their broad publications reported on the genuine application study of hol- applications in our daily life or the flavour and fragrance indus- low silica spheres till very recent years, which are mainly fo- try [12]. Various polymer microcapsules combined with micro- cused on the sustained release of some drugs, pesticides, and encapsulation technology have been widely employed as car- dyes, and the immobilization of proteins and enzymes [4–11]. riers for perfume slow release [13, 14]. The fragrance release Obviously, these delivery systems only aimed at involatile membrane is another new developed technology [15]. How- molecules, thereby limiting their application scopes. There- ever, these techniques involve complex procedures and high fore, it will be very valuable if a hollow silica matrix is devel- cost, hence leading to restriction of their wider applications and oped for the sustained release of a volatile system. To the best further commercialization. Apart from perfume, the sustained- 3 Author to whom any correspondence should be addressed. release disinfectants with a long antibacterial and sterilization 0957-4484/07/245705+06$30.00 1 © 2007 IOP Publishing Ltd Printed in the UK Nanotechnology 18 (2007) 245705 JWanget al effect also need to be urgently and effectively developed to meet the human demand for health. There are few related re- ports in this field. In this case, it is worth noting that hollow silica nanospheres as a good release carrier provide a reservoir for the efficient accumulation and the storage of small volatile molecules because of their unique hollow nature and high spe- cific surface area. In this paper, Indian pipal from perfumes and peroxyacetic acid from disinfectants were selected and then tested by one simple adsorption process. Activated carbon and solid porous silica were employed as comparative carriers, respectively. The experimental results indicated that porous hollow silica nanospheres (PHSNSs) exhibited more remarkable storage and sustained release performances for volatile substances than activated carbon and solid porous silica nanoparticles. Figure 1. A schematic drawing of the adsorption equipment for the perfume. 2. Experimental details 2.1. Chemicals 2.3. Studies of the perfume adsorption and release We prepared calcium carbonate nanoparticles as templates with Indian pipal and dehydrated alcohol were mixed in the flask an average diameter of about 60 nm by a unique high grav- with a volume ratio of 1:10. The perfume catch pipe packed ity reactive precipitation technology [16]. Sodium silicate with hollow silica matrix was then fixed at the top of the (Na2SiO3·9H2O), hydrochloric acid, hexadecyltrimethylam- flask. Subsequently, the perfume mixture was heated at monium bromide (C16TMABr), and ethanol were purchased about 333 K and maintained for 3 days to realize adsorption from Beijing Chemical Factory, China. All chemicals were saturation of the hollow silica for the perfume. The excess reagent grade and used as purchased without further purifica- and unabsorbed perfume was adsorbed with alcohol. Finally, tion. Perfume (Indian pipal) was purchased from Nanchang the perfume-adsorbed PHSNSs were collected and designated Bai-ShiTe Flavour & Fragrance Co., Ltd, China; its main com- as sample I. For comparison, activated carbon with a BET − ponent is hydroxycitronellal (7-hydroxy-3,7-dimethyl octanal; surface area of 634 m2 g 1 was selected, and then treated by molecular weight (M) about 172 g mol−1; molecular dimen- the same adsorption procedure. The as-prepared powder was sion ≈0.9nm).Peroxyacetic acid was also commercially pur- named sample II. The adsorption equipment is schematically chased. As comparative carriers, activated carbon was pur- illustrated in figure 1. chased from the indicated suppliers while solid porous silica To assess their perfume release properties, the two nanoparticles were fabricated in our laboratory as reported pre- samples were divided into several portions (0.3 g per portion) viously [17]. Deionized water was used throughout the study. respectively. One portion was taken each time at different time intervals and immersed in 30 ml of alcohol to dissolve perfume, and then the above suspension was centrifuged. Filtrates 2.2. Preparation of PHSNSs (4.0 ml) were afterwards extracted and diluted to 100 ml, and finally analysed by UV–vis spectroscopy at a wavelength of PHSNSs were synthesized as our group previously reported 272 nm determined by a full-wavelength scan, as shown in after a little modification [18]. A typical procedure was figure 4. The perfume adsorption amount of carriers was performed as follows: nanosized CaCO3 aqueous suspension calculated by the following equation: (8 wt%) was heated and kept at 353 K under vigorous stirring. C TMABr (3 wt%) was then put into the above CV 16 M = (1) suspension. Subsequently, sodium silicate solution (2 wt% W SiO ) was added dropwise into the suspension to form core– 2 where M (ml/mg) is the amount of the perfume adsorbed in shell composites with the weight ratio of SiO /CaCO = 1/5, 2 3 unit weight of the carrier, C (ml/ml) is the concentration of while the system was maintained at pH 9.0 by simultaneously perfume dissolved in the alcohol, V (ml) is the volume (30 ml) adding HCl dilute solution. After the addition was completed, of alcohol and W (mg) is the weight (300 mg) of the perfume- the slurry was further stirred at the same conditions as above adsorbed carriers, respectively. for 3 h and subsequently filtered, rinsed with deionized water and dried, followed by calcination in air at 973 K for 5 h. 2.4. The disinfectant storage and release evaluation The as-prepared composite was afterwards put into HCl dilute solution maintaining pH < 1for12htoremovetheCaCO3 The disinfectant loading experiment was conducted as follows. templates completely. Finally, the resulting gel was filtered, 4.0 g of PHSNSs was added to 80 ml of commercial rinsed with deionized water and ethanol in sequence, and dried peroxyacetic acid aqueous solution (16 wt%). The vial to obtain hollow silica powder. PHSNSs with a BET surface was sealed to prevent the evaporation of effective ingredient area of 663 m2 g−1 could be thus prepared. (hydrogen peroxide), and then the mixture was stirred for 12 h 2 Nanotechnology 18 (2007) 245705 JWanget al 0.040 0.035 0.030 0.025 0.020 0.015 0.010 0.005 Pore Volume dV/dD(cm g-A) 0.000 10 20 30 40 50 60 Pore Diameter (A) Figure 3. The pore size distribution in the wall of PHSNSs. 5 Figure 2. TEM image of PHSNSs. 4 to reach adsorption equilibrium. Finally, the suspension was 272 nm filtered until no liquid dropped. The as-obtained PHSNSs 3 were called sample III. The same process was also carried out for solid porous silica nanoparticles with a BET value of 2 −1 2 286 m g . The as-prepared sample was sample IV. Absorbance The release evaluation experiments were then performed below: 0.2 g of the above powder in the open air was taken 1 each time at different time intervals, and put into 25 ml of water and immersed for 1 h to dissolve adsorbed peroxyacetic 0 acid and sedimentate the powder.