Colloids and Surfaces B: Biointerfaces 54 (2007) 165–172
Silica xerogel–hydrogen peroxide composites: Their morphology, stability, and antimicrobial activity ˙ Jacek Zegli´ nski a,∗ , Agnieszka Cabaj b , Michał Strankowski c , Justyna Czerniak a , J´ozef Tadeusz Haponiuk c b
a Department of Inorganic Chemistry, Medical University of Gda´ nsk, Al. Gen. Hallera 107, 80-416 Gda´nsk, Poland Marine Chemistry and Biochemistry Department, Institute of Oceanology Polish Academy of Sciences, Ul. Powsta´nc´ow Warszawy 55, 81-712 Sopot, Poland c Polymer Technology Department, Chemical Faculty, Gda´ nsk University of Technology, Ul. G. Narutowicza 11/12, 80-952 Gda´nsk, Poland
Received 21 August 2006; received in revised form 30 September 2006; accepted 4 October 2006 Available online 17 October 2006
Abstract Hydrogen peroxide was incorporated into silica xerogel matrix over the concentration range from 3.8 to 68.0 wt% via the sol–gel route. The obtained composites were characterized by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The release rates of H2 O2 from the composites into the aqueous phase were examined. In most cases, a 90% release was attained after ca. 10 min, and it was only slightly dependent on H2 O2 concentration and particle size. The antimicrobial activity of the composite containing 3.59% H2 O2 was evaluated against Escherichia coli and Micrococcus luteus. A comparative assay was carried out for aqueous solution of H2 O2 of the same concentration. The results demonstrated a potent microbicidal efficacy of the composite. Furthermore, diffusion range of the hydrogen peroxide from the solid composite into an agar medium matched that of the H2 O2 in aqueous solution. The stability tests with the xerogels containing 3.8, 26.4, and 68.0% of H2 O2 showed that after 63 days respective losses of the H2 O2 at 3 ◦ C were 8.8, 9.7, and 6.2%. Both the DSC results and the stability tests have shown that the molecular water present in the pores stabilizes the composite, probably through improving the binding of the H2 O2 molecules onto the silica surface. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen peroxide; Silica xerogel; DSC; Release rate; Antimicrobial activity
1. Introduction In today’s highly industrialized environment, pollution control is a crucial factor taken into consideration when new technologies are designed. Simple transfer of contaminants from one medium to another becomes no longer acceptable. For this reason, there is an urgent need to promote environmentally friendly chemicals. Hydrogen peroxide is such a “green” powerful and versatile oxidant whose degradation products are oxygen and water [1]. Unfortunately, hydrogen peroxide poses certain risks owing to its instability in the presence of catalytic amounts of heavy metals, alkalies, and even air-borne particulates. For this reason, efforts have been continued to design forms of the compound with improved safety during storage and han-
∗
Corresponding author. Tel.: +48 58 3493225; fax: +48 58 3493224. ˙ E-mail address:
[email protected] (J. Zegli´ nski).
0927-7765/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2006.10.013
dling. Three stable, solid peroxohydrates have been developed and marketed, namely sodium percarbonate (2Na2 CO3 ·3H2 O2 ), sodium perborate (NaBO3 ·xH2 O2 ; x = 1 or 4), used mainly as bleaching agents in washing powders [2], and an urea–hydrogen peroxide (1:1) adduct, used in hair bleaching, skin disinfection, teeth whitening, and as an efficient oxidant in organic synthesis [3,4]. Relatively stable monooleic cubic phases containing hydrogen peroxide have been reported as a gel for wounded skin disinfection [5]. A sol–gel technology that was developed during the last two decades offers new possibilities for incorporating active agents within silica matrix [6–9]. The most essential features of the technique are: (i) ultrahomogeneity—incorporated molecules can be separated at a nano-scale level; (ii) low processing temperature—also temperature-sensitive molecules can be processed;
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(iii) non-toxicity of silica gel matrix—final composites can be implemented as sustained-release drug delivery systems or biocompatible implantable materials. Recently, a composite has been developed in our laboratory by using the sol–gel technique, which contains hydrogen peroxide embedded in silica gel. Upon drying, the resulting xerogel contains up to 70% of H2 O2 . Because the H2 O2 molecule is the smallest one showing internal rotation about the O O bond, interactions between hydrogen peroxide and silica gel seem to be of fundamental importance for understanding adsorption phenomena. In our recent work, we highlighted some of the structural and energetic features of the system by means of Fourier transform infrared (FTIR) spectroscopy and quantumchemical calculations [10]. The title composites are a new solid form of hydrogen peroxide. We expect that the H2 O2 incorporated in the inert and non-toxic silica xerogel, will be an interesting alternative for teeth whitening systems based on the urea–hydrogen peroxide adduct. Besides, silica xerogel seems to be a better carrier of H2 O2 in wounded skin disinfection taking into account that urea may be irritant to sensitive skin in topical applications [11]. We also expect that the composite with the low H2 O2 concentration (up to 4%) could be applicable in a sticking plaster disinfectant. The silica xerogel with high load of the H2 O2 could act as an active sorbent for the neutralization of locally contaminated areas. The other potential application of the xerogel could be for small portable drinking water disinfectant systems used under field conditions. In that case, there should be a need for removing of the excessive peroxide from the treated solution. This is, however, a prospect for another research. The purpose of the present study was to work out a method for preparation of silica xerogel–hydrogen peroxide composites over a broad spectrum of H2 O2 concentrations. The composites were characterized by means of scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). An effort was undertaken to learn whether the co-adsorbed water molecules could stabilize the system (as it was shown in our theoretical calculations). In addition, release rates of H2 O2 from the composites into the aqueous phase were examined as well as the antimicrobial activity of the xerogels. The stability tests were performed at different temperatures, H2 O2 concentrations, and water-to-composite ratios. 2. Experimental 2.1. Materials Sodium metasilicate solution (water glass); R-145, SiO2 / Na2 O mole ratio 2.5 (Enterprise WAMA, Lebork, Poland); hydrogen peroxide (30 wt%) (POCh, Gliwice, Poland); orthophosphoric acid (85 wt%) (POCh, Gliwice, Poland); Amberlite® IR 120 (Fluka AG, Germany). Escherichia coli ATCC 8739 strain was provided by Polish Collection of Microorganisms, Polish Academy of Sciences, Wrocław, Poland. Micrococcus luteus strain was provided by the Marine Chemistry and Biochemistry Department, IO PAS, Sopot,
Poland. Tests with E. coli were carried out in the Nutrient Agar medium [12], whereas with M. luteus the ZoBell medium was applied [13]. Microbicidal tests were performed using autoclaved deionized water from Milli-Q water purification system (Millipore Corp.). 2.2. Preparation of the silica xerogel–hydrogen peroxide composites The cation-exchange method was used to prepare the silicic acid sol. Water glass, diluted 1:4 with distilled water was passed through a glass column packed with a swollen Amberlite, cationexchange resin, to afford a solution of silicic acids (sol). The Si content of the sol, expressed as SiO2 was 5.0 wt%. Details of this procedure are described elsewhere [14]. The freshly prepared sol of silicic acids was mixed with 30% H2 O2 to give six solutions containing 0.5, 1, 2.5, 5, 10, and 20% of H2 O2 . To immobilize trace amounts of metal impurities which catalyze H2 O2 decomposition, the same amount of a stabilizer (0.03 wt% of orthophosphoric acid) was added to all the solutions. Fifty grams of each solution was then poured out onto the glass Petri dishes (90 mm in inner diameter and 15 mm in height). In the next step, the samples were dried at 70 ◦ C up to 90% of a total mass loss to give xerogels containing from 3.8 to 68.0% of H2 O2 . In addition, for the system with 5% of initial H2 O2 concentration in the sol, composites were obtained of various mass losses. The hydrogen peroxide concentration in all the composites was determined by KMnO4 titration. To investigate the morphology of the samples, the intact xerogel monoliths were used. For the study of the release rate, both crushed monoliths (mean particle diameter of ca. 2 mm) and finely powdered xerogel samples (mean particle diameter of 13 m) were used. For the differential scanning calorimetry measurements, antimicrobial assay and stability assessment, only powdered xerogel samples were taken. The particles size of powdered samples was determined by manual choosing of 150 particles from the optical microscope images (microscope—Motic B1-220A, Wetzlar, Germany, coupled with digital camera—Panasonic GP-KR 222E, Matsushita Communication Industrial, Japan) and measuring their area and diameter using UTHSCSA Image Tool software, Version 3.00. 2.3. Modelling approach The geometry optimization of the H-bonded clusters of the water–four-fold siloxane ring and hydrogen peroxide–four-fold siloxane ring was performed using the Dgauss program [15] implemented within CAChe Software, Version 7.5.0.85. As the computational method, density functional theory (DFT) and exchange-correlation energy functional, B88-LYP, developed by Lee et al. [16] were applied, with a double zeta, DZVP basis set [17]. 2.4. Morphology of the composites For the scanning electron microscopy, a Tesla BS-300 instrument (Tesla Brno, Czech Republic) was employed. For the samples preparation, a piece of silica xerogel–hydrogen perox-
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ide composite monolith was deposited onto the aluminum target. The monoliths were sputtered with a gold layer to improve conductivity. An operational voltage was 15 kV. 2.5. Differential scanning calorimetry (DSC) DSC measurements were carried out on a PerkinElmer, DSC-6 instrument. The powdered composite samples (38 ± 4.9 mg) were placed inside aluminum pans and scanned from 20 to 200 ◦ C at a heating rate 10 ◦ C min−1 in a nitrogen atmosphere. 2.6. Release rate of H2 O2 from the composites Each sample (0.3 g) was placed in a 20-mL twisted test tube (13 mm in inner diameter; 160 mm in height). Seven identical samples were prepared to determine the amount of the H2 O2 released at various time intervals. Ten milliliters of distilled water was added to each sample. The first sample was assayed after 30 s without shaking, whereas the next six tubes were placed horizontally in a thermostated water bath (Julabo Exatherm U3, Germany) coupled with a laboratory shaker (Elpin+ 358S, Poland). The test temperature was 25 ◦ C, the shaking speed was 100 c.p.m. at amplitude 1. At predetermined time intervals the supernatants were filtered (Filtrak 3W, Germany) and the amount of the released H2 O2 was determined by KMnO4 titration. 2.7. Antimicrobial acitivity test The purpose of the test was to evaluate antibacterial effect of the silica xerogel–hydrogen peroxide composite. Bacterial strains used in the test were both Gram-negative (E. coli that is a common pathogen appearing in water, soil, food, sewage, and plants) and Gram-positive (M. luteus occurring mainly in soil, water, dust, as well as on human skin). Both strains show catalase activity, an enzyme ensuring natural protection from oxidative damage. This defence mechanism can be, however, defeated by a higher concentration of the oxidant. As the material tested here is insoluble in water, the cut plug method was applied which is routinely used for the determination of antimicrobial activity of polymers [18]. The test was performed in sterile plastic Petri dishes containing 15 mL of solid agar medium. The whole surface of each plate was inoculated with 100 L of 1day-old bacterial culture of optical density ∼0.5 at λ = 600 nm. This density equals approximately 107 CFU mL−1 (107 of bacterial cells in 1 mL). A well of 4 mm in diameter was made in each plate, and 20 mg of the powdered composite containing 3.59% of H2 O2 (the sample of the composite initially containing 3.8% of H2 O2 —after 2 weeks of storage) was added into each well. The control sample contained equivalent amount of silica xerogel without hydrogen peroxide. The plates were then incubated for 24 h at 37 ◦ C and the diameters of the growth inhibition zone were measured. A comparative assay was carried out using aqueous solution of H2 O2 . In this case, 20 L of a 3.59% solution was poured into the wells in the agar plates with bacteria.
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2.8. Storage-stability of the composites All the examined composites were stored for 63 days in a closed, plastic Eppendorf test tubes in the dark. The samples of the composite containing 3.8% of H2 O2 (dried up to a 90% mass loss) were stored at room temperature, 3, and −25 ◦ C, whilst the composites obtained from sols with 5 and 20% of initial H2 O2 concentration differing in mass losses degrees, were stored at the same temperature (3 ◦ C). 3. Results and discussion 3.1. Loading efficiency When the silicic acid sol with dissolved hydrogen peroxide is dried under mild conditions, water and part of H2 O2 gradually escape from the system. Simultaneously Si OH functions of the silicic acid condense to form Si O Si bridges, being converted initially to a hydrogel, followed by further dehydration and densification of a three-dimensional silica network resulting in a xerogel. The evaporation process is accompanied by formation of a silica framework, both resulting in gradual increase in H2 O2 concentration. A maximum loading efficiency (LE) of H2 O2 could be achieved at evaporation of about 90% of the total sol mass. Schematic representation of the preparation steps of the silica xerogel–hydrogen peroxide composites as well as the LE values are shown in Fig. 1. LE was calculated according to the following equation: LE (%) =
Qxerogel × 100, Qsol
where Qxerogel is the mass of hydrogen peroxide loaded in the xerogel and Qsol is the initial mass of hydrogen peroxide in the sol (prior to drying). The highest LE was obtained at low H2 O2 concentrations in the sol (LE = 82 for 0.5% H2 O2 in the sol), whilst the lowest LE was found in the sol with the highest excess of hydrogen peroxide (LE = 33.5 for 20% H2 O2 in the sol). However, such a notable excess resulted in a very high H2 O2 concentration (up to 67%) in the xerogel. The results clearly show a stronger adsorption of the H2 O2 molecules on the silica surface in comparison to that of the water molecules, thus indicating that hydrogen peroxide wins the competition in access to the sorbent. First explanation for this phenomenon can be that the H2 O2 molecule possesses a second oxygen atom that is a H-bond sensitive nucleophilic centre. Hence, hydrogen peroxide can generate a larger quantity of hydrogen bonds than does water. Since the water molecule is not a free rotor, but rather tightly involved in hydrogen bonding, and hydrogen peroxide shows a hindered rotation about the O O bond [19], H2 O2 molecules can better adjust to the adsorption centres of silica gel than do the H2 O molecules. This implies constituting of less strained and shorter, and consequently, stronger hydrogen bonds. This can clearly be seen in proposed DFT-optimized model structures shown in Fig. 2.
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Fig. 1. Schematic representation of the preparation steps of the silica xerogel–hydrogen peroxide composites. The H2 O2 concentration (wt%) in each composite is the mean ± M.D. (mean deviation) of two independent experiments. Loading efficiency (%) of the H2 O2 encapsulated in the xerogels in parentheses.
3.2. SEM study The morphologies of the composites containing 8 and 27% of H2 O2 were observed by scanning electron microscopy as shown in Fig. 3. Both composites were obtained by drying respective sols up to a 90% mass loss (according to the scheme in Fig. 1). On a dark surface representing a homogenous phase of the silica xerogel and hydrogen peroxide there are white spots revealing the phosphate stabilizer and residual sodium chloride released during ion exchange. There are distinct differences in surfaces of both composites. The surface of the monolith containing 8% of H2 O2 consists of plain irregularly shaped plates separated by cracks. On the other hand, the surface of the monolith containing 27% of H2 O2 is almost smooth at a 500-fold magnification (photo d in Fig. 3), whereas complexity of its structure becomes evident at a 10,000-fold magnification only (photo f). The difference in the morphology of the two surfaces can be attributed to different hydrogen peroxide/water ratios, since both the water and H2 O2 molecules saturate the xerogels strengthening their frameworks through hydrogen bonds, thus preventing erosion. The composite with a cracked surface was obtained from a sol of lower contents of both components. 3.3. DSC Thermograms of intact silica xerogel, and silica xerogel– H2 O2 composites that differ in dehydration degree are shown in Fig. 4. The samples were dried up to a total mass loss ranging from 87.0 to 92.0%.
As just mentioned, the final concentration of H2 O2 in the composite strongly depends on the degree of dehydration of the xerogel and is the highest at about 90% of a total mass loss (30.3% H2 O2 at a 89.8% mass loss—curve c). Upon thermal treatment of samples, there is a strong endothermic event emerging both in xerogel samples and in the composite samples. The input of heat results in breaking hydrogen bonds between silica gel hydroxyls, Si OH, and the molecules of water and hydrogen peroxide. Most probably, upon raising the temperature, weakly adsorbed water molecules would be released first from the silica surface. With the silica xerogel, an endothermic peak (curve a) emerges at 106 ◦ C and total desorption of physically adsorbed water occurs at 170 ◦ C. On the other hand, with the composite, thermal degradation of the H2 O2 occurs at a critical temperature according to the equation [20]: H2 O2 = H2 O + 21 O2 + 23.44 kcal Respective exothermic heat effects are seen in the thermograms (b–e) over the range 145–151 ◦ C. At the same time, desorption of the water resulting from thermal degradation of H2 O2 emerges as small endothermic peaks over the range 153–157 ◦ C. The thermograms of the composite do not display separate peaks for thermal effects due to desorption of water and hydrogen peroxide. This is indicative of a complex desorption mechanism. This notwithstanding, a closer inspection of the shapes of the curves allows to discern some fragments indicating preferred desorption of either water or hydrogen peroxide. For instance, in curve (c), the left-hand hump of the main endothermic effect peaked at 105 ◦ C is mostly due to water desorption, whereas
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H-bonded clusters of the H2 O2 , H2 O, and Si(OH)4 molecules [10]. 3.4. Release rate of H2 O2 from silica xerogel
Fig. 2. A three-dimensional model of silica gel adsorption centre, siloxane ring, consisting of four condensed Si(OH)4 units with adsorbed H2 O molecule—upper structure, and H2 O2 molecule—lower structure. The structures were optimized by using the B88LYP DFT functional and DZVP basis set. ˚ units and H-bond angles Hydrogen bond lengths are expressed in angstrom (A) (O–H· · ·O)—in degrees.
the right-hand hump, peaked around 123 ◦ C, is due to desorption of hydrogen peroxide. Furthermore, the thermograms show that the content of the weakly adsorbed water declines in the following sequence: (b) > (c) > (d) > (e), thus being compatible with the dehydration degree of the composites (cf. caption to Fig. 4). The right-hand hump of the main peak in curve (b) is the most shifted towards upper temperature thus suggesting the strongest binding of the H2 O2 molecules with the silica surface. However, the highest temperature of the exothermic decomposition of hydrogen peroxide was recorded at 151 ◦ C (curve c) thus revealing the most thermally stable composite. Taking all this into consideration, it can be assumed that some excess of water enclosed in silica xerogel–H2 O2 composites can stabilize H2 O2 molecules adsorbed on silica surface, this manifesting itself in a higher thermal energy needed for their detachment. This finding is in a good agreement with our theoretical calculations on
Fig. 5 shows relative release rates (i.e. the ratio of the amount released to the total amount of H2 O2 over time), in different concentrations, from silica xerogel matrix. A very high release rate of hydrogen peroxide to water solution was achieved already under mild conditions (test temperature: 25 ◦ C, shaking speed: 100 c.p.m. at amplitude 1), and was only slightly dependent on H2 O2 concentration and particle size. In most cases, a 90% release was attained after ca. 10 min. Only for the powdered sample containing 19.2% of H2 O2 the rate of release was distinctly higher (90% release after ca. 5 min). It has been found that the release of incorporated species from silica xerogel is governed by combined effects of diffusion and matrix erosion, and in most cases it is inversely proportional to particle size [6]. On the basis of the experimental results, it can be assumed that liberation of hydrogen peroxide occurs through undisturbed penetration of water into pores of the silica xerogel followed by destruction of the matrix. Hydrogen peroxide can be then easily leached from silica surface by excessive water. In this case, particle-size effect does not affect appreciably the rate of H2 O2 release. Bearing in mind the above results, the quick release of hydrogen peroxide allows for a nearly immediate transfer of most of the H2 O2 from the silica matrix to the solution. This can be useful in applications where accurate concentration of the peroxide is needed, or where its long-term exposition is not desirable. It seems that a sustained release of H2 O2 can be profitable in teeth whitening and a long-term disinfecting applications. From the results of Ahola et al., it is clear that the release rate of active agents from the silica xerogel can be slowed down by a covalent binding of the polyethylene glycol to the silanol groups of the xerogel [7]. Leonard et al. produced evidence for prolongation of H2 O2 release from urea–hydrogen peroxide adduct by addition of a carboxypolymethylene polymer. Another advantage of using this additive results from its acting as an agent for improving the tissue adherence [21]. Because in topical applications side effects (mainly tissue irritation) are likely to occur and depend upon peroxide concentration and the contact time [22], thus, there is a need for precise determination of these two parameters in respect not only of an optimal biocidal efficiency. There is also a need for considering the influence of these parameters on the healing process. 3.5. Antibacterial effects In Table 1, the diameters of the growth inhibition zones (GIZ) produced by H2 O2 solution and the composite are compared for the two strains. Test results for E. coli incubated with the powdered composite and without the composite are presented in Fig. 6. The mean GIZ values for E. coli and M. luteus were slightly lower for the composite, making up, respectively, 94.8 and 97.5% of those produced by H2 O2 solution. The control sam-
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Fig. 3. SEM pictures of the surface morphology of the monolithic silica xerogel–hydrogen peroxide composites containing 8% H2 O2 (photos a–c) and 27% H2 O2 (photos d–f) with magnifications: ×500 (a and d), ×1000 (b and e), and ×10,000 (c and f).
ple containing silica xerogel without hydrogen peroxide did not exhibit bacterial growth inhibition. The results show that the diffusion range of the hydrogen peroxide from the solid composite to the agar medium is comparable to that of the H2 O2 solution. Therefore, it can be expected that after a slight moistening, the composite will be biologically active to nearly the same extent as is the H2 O2 solution. 3.6. Evaluation of storage-stability of the composite Reduction of the hydrogen peroxide concentration in the composite runs via two parallel routes: (i) by a chemical decomposition of H2 O2 and (ii) by its desorption from the silica framework. Both these processes can be inhibited by temper-
Fig. 4. DSC curves of: (a) silica xerogel and (b–e) silica xerogel–H2 O2 composites obtained from the sol containing 5% of H2 O2 . The H2 O2 concentration in the composites and the degree of their mass loss (both in wt%) are, respectively, 21.0 and 87.0 (b); 30.3 and 89.8 (c); 29.6 and 91.0 (d); 25.3 and 92.0 (e).
Table 1 Comparison of the diameters of growth inhibition zones produced by 20 mg of the H2 O2 solution and the powdered silica xerogel–H2 O2 composite by E. coli and M. luteus as determined by the cut plug method after 24 h on the agar nutrient at 37 ◦ C Strain
Escherichia coli Micrococcus luteus a b
3.59% aqueous H2 O2 solution
3.59% H2 O2 –silica composite
Mean diametera
S.D.b
Mean diametera
S.D.b
27.0 31.6
2.0 1.7
25.6 30.8
2.2 1.3
Diameter of the growth inhibition zone (mm). Standard deviation (n = 5).
Fig. 5. Release rates of H2 O2 from the silica xerogel matrix with varying concentrations of hydrogen peroxide. a Mean particle diameter 13 m; b mean particle diameter ca. 2 mm.
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Fig. 6. Growth of E. coli after 24 h: (A) incubated with silica xerogel–H2 O2 composite powder; (B) incubated without the composite.
ature lowering. As can be seen in Fig. 7A, the slowest loss of hydrogen peroxide was observed for the samples stored at 3 and −25 ◦ C. In both cases, more than 90% of H2 O2 was retained after 63 days of storage. The results presented in Fig. 7B show that the preparation conditions strongly affect stability of the com-
posite. In the least dehydrated system (a) containing relatively high content of bulk water, the H2 O2 content even increases during the first 3 weeks of storage, this being in agreement with the DSC results (the weakly H-bonded H2 O molecules left the silica matrix first, thus increasing the relative amount of the hydrogen peroxide in the system). The H2 O2 content in this composite after 63 days is also the highest as compared to that of the more dried samples. It can thus be assumed that the water molecules can stabilize the hydrogen peroxide firstly through the reduction of its desorption rate from the silica matrix by an effective strengthening of the hydrogen bonding, and secondly by neutralizing catalytic sites of transition metals thus inhibiting chemical decomposition of the hydrogen peroxide. The concentration of H2 O2 in the composites does not influence apparently their stability. In the xerogels with 3.8, 26.4, and 68% of H2 O2 , after 63 days at 3 ◦ C, respectively, 91.2, 90.3, and 93.8% of the hydrogen peroxide still remained undecomposed. 4. Conclusions A sol–gel method was successfully applied for incorporation of hydrogen peroxide into a silica xerogel matrix over a concentration range of 3.8–68.0 wt%. Both the DSC results and the storage-stability tests show that the presence of molecular water in the pores stabilize H2 O2 molecules adsorbed on the silica surface. A very high release rate of hydrogen peroxide from the composite to aqueous solution was achieved. In most cases, a 90% release was attained after ca. 10 min and it was only slightly dependent on H2 O2 concentration and particle size. Also, the undisturbed diffusion of hydrogen peroxide from the solid composite to the agar medium was shown in the antimicrobial assay. Taking into account the good stability, the potent microbicidal efficacy, good availability of the embedded H2 O2 , as well as the broad range of its concentrations, various applications of the silica xerogel–hydrogen peroxide composites can be expected.
Fig. 7. Relative loss of hydrogen peroxide from silica xerogel–H2 O2 composites. (A) The composite containing 3.8% H2 O2 obtained from the sol dried up to a 90% mass loss—stored at room temperature, 3, and −25 ◦ C. (B) The composite differing in degrees of drying—all samples stored at 3 ◦ C. The H2 O2 concentration in the composite and degree of the mass loss (both in wt%) are, respectively, 25.8 and 87.0 (a); 26.4 and 90.0 (b); 24.6 and 92.0 (c); 21.7 and 93.0 (d).
Acknowledgment The authors are especially grateful to Professor Ryszard Pi˛eko´s for the essential contribution and for his help in translation of the manuscript into English.
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