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 and peroxyacetic acid from disinfectants, were selected and then tested by one simple adsorption process with two separate comparative carriers, i.e. activated and solid porous silica. It was demonstrated that a high storage capacity . / (9 6mlperfume mgcarrier) of 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. ( peroxide), and then the mixture was stirred for 12 h

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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.

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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. Subsequently, 4 ml of 250 300 350 400 450 500 the supernatant was withdrawn, followed by the addition of Wavelength (nm) H2SO4. Finally, titrimetric analysis was adopted to evaluate the release behaviour of effective ingredient () 15 having sterilized effects by adding KMnO4 titrant. When the 0.70 colour of the solution changed from colourless to a little pink 0.68 and kept constant for 30 s, the titration end point was reached. 12 0.66 The titrimetric principle is described by the following equation: 0.64 0.62 − + + + = 2+ + ↑+ . 2MnO4 5H2O2 6H 2Mn 5O2 8H2O (2) 9 0.60 Activated carbon 0.58 Perfume stored in carriers (mL/mg) carriers stored in Perfume 2.5. Characterization 0 5 10 15 20 25 30 6 Time (Day) The hollow structure of PHSNSs was observed by transmission electron microscopy (TEM) (JEM-2010). The BET surface PHSNSs area and the pore size distribution of samples were determined 3 Activated carbon

by a volumetric adsorption analyser (ASAP-2010). The Perfume stored in carriers (mL/mg) concentration of perfume in ethanol was measured by UV spectrometry (UV-2501). 0 0 5 10 15 20 25 30 35 40 45 Time (Day) 3. Results and discussion Figure 4. The UV absorbance spectrum of Indian pipal and the TEM image of the as-prepared PHSNSs was shown in figure 2, release profiles from PHSNSs and activated carbon. from which it could be clearly found that PHSNSs had hollow cavity with a diameter of 60–80 nm and a wall thickness of perfume and disinfectant. Thus, the two kinds of small volatile ≈15–20 nm. The shape and size of PHSNSs could be readily molecules could be ensured to diffuse inside pores so that they controlled by adopting different nanosized CaCO3 templates. could be efficiently accumulated, adsorbed, stored in PHSNSs Figure 3 gave the BJH pore size distribution of PHSNSs, and released from PHSNSs. indicating that most pores in the wall of PHSNSs had a The UV absorbance spectrum of Indian pipal in ethanol diameter around 1.8 nm, larger than the molecular sizes of and the release profiles of perfume from sample I and sample

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100

90

80

70 PHSNSs Activated carbon 60 Release percentage (%)

50

010203040 Time (day)

Figure 5. The accumulated release percentages of Indian pipal from PHSNSs and activated carbon.

II are depicted in figure 4. Despite the similar BET surface area, the perfume storage capacity of the PHSNS system / reached a high value of 9.6 mlperfume mgcarrier,whichwas almost 14 times as much as that of activated carbon (only / 0.7 mlperfume mgcarrier). The release profile from sample I could be roughly divided into three stages. The first stage was a fast release of perfume, an obvious decrease from / 9.6 to 6.3 mlperfume mgcarrier within 5 days. This may be attributed to perfume molecules adsorbed on the surface or the outer part of pore channels of PHSNSs. The second / stage showed a slow delivery from 6.3 to 5.5 mlperfume mgcarrier in the next 10 days, possibly due to the release of perfume molecules adsorbed inside pore channels. The final stage from Perfume stored in carriers Perfume stored in carriers Perfume stored in carriers 15 to 40 days emerged as a long release period, and hence the stored perfume amount of PHSNSs slightly reduced to / 5.07 mlperfume mgcarrier. This reason might be that the release of the perfume molecules stored inside the hollow core depended Figure 6. The perfume stored in PHSNSs versus time0.5 (a) and the . on the molecule diffusion rate and could last a comparatively perfume stored in activated carbon versus time0 5 (b) and time (c). long time, leading to a significantly delayed release. As a comparison, the release from sample II maintained a slow effectively high concentration in a long release period due to delivery process with a nearly constant rate, changing from 0.7 an obviously continuous release. / to 0.58 mlperfume mgcarrier during the period of 30 days, which The kinetics of the release of a drug from porous carrier would not meet the requirement of a sustained perfume release materials has frequently been described by using the Higuchi process in practical applications. model [19, 20]. According to this model, the release of a drug Figure 5 exhibits the relationship between the accumulated from an insoluble, porous carrier matrix could be described as release percentages of perfume from both carriers and time. a square root of a time-dependent process based on Fickian Depending on the material structure characteristics, different diffusion. Accordingly, the perfume release kinetics was perfume release behaviours were observed for a long period. similarly investigated according to the same principle. The This diagram clearly proved that PHSNS carriers displayed amount of perfume stored, Qp, per unit of exposed area at time much better sustained-release properties than their counterpart. t could then be described by the following simple relation: The amounts released from the two systems reached about 35 √ Q = k · t (3) and 3.5% in the initial 5 days, respectively. In the following p H long release period, the perfume release rate of PHSNSs where kH is the release rate constant for the Higuchi model. decreased greatly while that of activated carbon kept nearly Thus, for a purely diffusion sustained process, the amount stable at a low release level. This result implied that PHSNSs of perfume stored at time t exhibited a linear relationship could provide us with a better fragrance surrounding with a if plotted against the square root of time. The use of this long term when compared to activated carbon. The sustained model should be beneficial for the system under study as the perfume release from sample I ensured an effective perfume model is valid for releases of relatively small molecules that are concentration in the surroundings with a primary burst effect uniformly distributed throughout the matrix. However, from in a comparatively short time, and subsequently retained the the Higuchi square root of time plots presented in figure 6,

4 Nanotechnology 18 (2007) 245705 JWanget al it could be clearly seen that PHSNS material revealed a two- 48 step release (figure 6(a)), which was different from a good PHSNSs linear relationship of activated carbon (figure 6(b)). This result 40 Solid porous silica indicated that the perfume release from activated carbon was completely consistent with a Fickian diffusion mechanism. 32 However, because of the unique hollow structure, perfume solution (ml) molecules accumulated in the hollow core could diffuse into 4 24 the air only through the whole channels in the shell. Further, the diffusion rate apparently decreased with the decline of diffusion driving force induced by the great reduction of the 16 perfume molecules inside the inner confined space. Figure 6(c) shows the linear profile of the stored perfume amount in Volume of KMnO 8 carriers versus time. This is probably because activated carbon possesses a uniform pore structure in the whole carrier, thereby 0 0 750 1500 2250 3000 3750 resulting in a nearly constant perfume release rate. Time (minute) To further study the characteristics of the perfume stored in the two carriers, the concept of perfume storage period was Figure 7. The release evaluation of peroxyacetic acid from PHSNSs introduced and then represented the half-time of the perfume and solid porous silica. release process. It was presumed that the perfume release rate, K , was in direct proportion to the perfume storage amount per weight of carriers, M, which could be described by the behaviour than activated carbon despite the similar perfume following equation: release period of both carriers. Currently, disinfectant is another kind of typical volatile dM − = KM (4) substance closely related to human life. The development dt of a sustained release type disinfectant would reduce the or usage amount and frequency of disinfectant greatly, thereby InM =−Kt + a. (5) decreasing the damage for humans. Among them, peroxyacetic acid is commonly used as a disinfectant in our life owing From equation (5), the half-time of perfume release could be to its excellent antibacterial effects. So it was selected as a expressed as disinfectant model in our study. In2 Since KMnO4 titrant had a colour-changeable reaction t1/2 = . (6) K with the main effective ingredient, hydrogen peroxide, in the According to figure 5, the perfume release rate, K , titrimetric process, the consumption amount of KMnO4 would was roughly determined by the slope of release percentage be a directly corresponding reflection of hydrogen peroxide versus t (t > 5), and t1/2 could then be calculated (H2O2). Consequently, the relation between the KMnO4 usage to be approximately 110 days (PHSNSs) and 105 days amount (volume) and time revealed the disinfectant sustained (activated carbon), respectively. This result indicated that release process. Figure 7 displays the typical cumulative both systems had a long release period, but PHSNSs only peroxyacetic acid delivery from sample III (in PHSNSs) and possessed almost the same half-time of perfume release to sample IV (in solid porous silica nanoparticles). There was an that of activated carbon in spite of very high perfume storage interesting result in our experiment that KMnO4 usage volume capacity (14 times as much as that of activated carbon as first increased and then decreased with time in the two profiles mentioned above). The probable reason was that although the (two maximum values). Although the reason is unclear, it two kinds of carriers possessed enormous porous networks, is reasonable to suggest the occurrence of the following two activated carbon was a kind of excellent adsorbent with much reactions (equations (7)and(8)) in the peroxyacetic acid stronger adsorption ability for gas molecules than porous aqueous solution. silica. So the affinity of activated carbon for perfume + ⇔ + molecules would be greatly stronger than that of silica. This CH3COOOH H2O CH3COOH H2O2 (7) would effectively restrain the desorption of perfume molecules H O ⇒ H O + 1 O ↑ K θ = 1.067. (8) from activated carbon, thereby decreasing the release rate of 2 2 2 2 2 perfume molecules from activated carbon significantly. As a In the initial reaction period, equation (7)isamain result, the release rate was very slow and the release process reaction and equation (8) is a side one, resulting in the increase was greatly prolonged (figures 4 and 5). Furthermore, it of the hydrogen peroxide amount. However, with the rise of could also be concluded that the hollow cavity of PHSNSs hydrogen peroxide, the reaction rate of equation (8) would played a very crucial role in perfume storage mainly due boost greatly. In contrast, the reaction rate of equation (7) to similar BET surface area, significant perfume storage decreased with the large consumption of peroxyacetic acid. capacity difference and the different ease of capture of perfume When there was a balance between the two reactions, the molecules from the prominently different affinity or adsorption amount of hydrogen peroxide reached a maximum value, ability of activated carbon and hollow silica for the perfume showninfigure7. Later, equation (8) played a dominant molecules. Obviously, PHSNSs offered considerably more role, which would cause an obvious decline of the amount perfume storage capacity and much better sustained-release of hydrogen peroxide because of its decomposition. From

5 Nanotechnology 18 (2007) 245705 JWanget al figure 7, it could be evidently found that the maximum value Acknowledgment and its appearance from sample III were respectively larger and later than sample IV. Moreover, the latter possessed a faster This work was financially supported by the National Natural release rate than the former by the slopes of the two curves. Science Foundation of China (grant Nos 20325621 and Most of all the disinfectant molecules stored in sample IV were 50642042). rapidly released in 2200 min, while only about 50% of the disinfectant was delivered into air from sample III in the same References period. Until 3600 min, the residue amount in sample III was still approximately 15%. It was apparent that the existence of [1] Cochran J K 1998 Curr. Opin. Solid State Mater. Sci. 3 474 the hollow inner core in the PHSNSs provided more storage [2] Caruso F 2000 Chem. Eur. J. 6 413 space than the solid counterpart, and benefited delaying the [3] Huang H and Remsen E E 1999 J. Am. Chem. Soc. 121 3805 disinfectant release process more effectively owing to the [4] Chen J F, Ding H M, Wang J X and Shao L 2004 Biomaterials 25 723 strong constraint on molecule diffusion from hollow structures. [5] Zhu Y F, Shi J L, Shen W H, Dong X P, Feng J W, Ruan M L and Li Y S 2005 Angew. Chem. Ind. Edn 44 5083 4. Conclusion [6] Zhu Y F, Shi J L, Shen W H, Chen H R, Dong X P and Ruan M L 2005 Nanotechnology 16 2633 In summary, we have successfully demonstrated the excellent [7] Zhu Y F, Shi J L, Li Y S, Chen H R, Shen W H and Dong X P 2005 J. Mater. Res. 20 54 properties of porous hollow silica spheres in volatile substance [8] Wen L X, Li Z Z, Zou H K, Liu A Q and Chen J F 2005 Pest storage and sustained release fields. The Indian pipal Manag. Sci. 61 583 perfume storage capacity of the PHSNS system reached [9] Botterhuis N E, Sun Q Y, Magusin P C M M, van Santen R A / and Sommerdijk N A J M 2006 Chem. Eur. J. 12 1448 9.6 mlperfume mgcarrier, which was nearly 14 times higher than that of the activated carbon. In addition, the PHSNS [10] Shiomi T, Tsunoda T, Kawai A, Chiku H, Mizukami F and Sakaguchi K 2005 Chem. Commun. 5 5325 carrier also displayed a significant sustained-release behaviour [11] Sharma R K, Das S and Maitra A 2005 J. Colloid Interface Sci. while the counterpart emerged a constant release process 284 358 with a low release level. Furthermore, a Higuchi model [12] Rogers K 1999 Cosmet. Toiletries 114 53 study demonstrated that the release process of perfume from [13] Hong K and Park S 1999 React. Funct. Polym. 24 193 both carriers followed Fick’s law basically. Compared [14] Peppas N A and Lisa B P 1996 J. Control. Release 40 245 [15] Hiroshi N, Isao K, Kumao U, Jyunya O, Toshiaki K and with the solid porous silica system, PHSNSs could deliver Yoshio M 2003 Radiat. Phys. Chem. 67 131 peroxyacetic acid as a disinfectant model more durably [16] Chen J F, Wang Y H, Guo F, Wang X M and Zheng C 2000 owing to their unique hollow framework. Therefore, Ind. Eng. Chem. Res. 39 948 these results proved that the as-prepared PHSNS matrix [17] Chen J F and Shao L 2003 China Particuol. 1 64 was a capacious and promising sustained perfume and [18] Chen J F, Wang J X, Liu R J, Shao L and Wen L X 2004 Inorg. Chem. Commun. 7 447 disinfectant release material, and would have potential for [19] Higuchi T 1961 J. Pharm. Sci. 50 874 wider applications in other volatile substance delivery fields. [20] Higuchi T 1963 J. Pharm. Sci. 52 1145

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