Aqueous and Thermal Oxidation of Porous Silicon Microparticles

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Aqueous and Thermal Oxidation of Porous Silicon Microparticles: Implications on Molecular Interactions Karyn L. Jarvis, Timothy J. Barnes, and Clive A. Prestidge* Ian Wark Research Institute, Australian Research Council Special Research Centre for Particle and Material Interfaces, UniVersity of South Australia, Mawson Lakes, SA 5095, Australia ReceiVed July 20, 2008. ReVised Manuscript ReceiVed October 7, 2008 Links between the mechanisms and kinetics of aqueous and dry thermal oxidation of porous silicon (pSi) microparticles have been investigated and the influence on molecular interaction established. ζ potential measurements have established the interplay between the dry oxidation state of pSi microparticles and their interfacial chemistry in aqueous solution, and Fourier transform infrared spectroscopy has demonstrated the effect of immersion time and oxidation temperature on surface chemistry. The influence of aqueous and thermal oxidation on molecular interactions and loading was investigated using methylene blue as a probe molecule. Aqueous immersion of pSi microparticles results in an initial increase in OySiH (y ) 1-3) species with increasing immersion times, reducing O2SiH concentration, while O3SiH concentration remained constant. Thermal oxidation from 473 to 1073 K causes the gradual transition from SiySiHx to OySiH and finally OySiOH species. Both aqueous and thermal oxidations had an effect on the ζ potentials of pSi microparticles. Methylene blue discoloration occurred due to its reduction by the SiSiHx-terminated surface thereby demonstrating the reactivity of such species. Aqueous and thermal oxidations modify pSi microparticle surface chemistry, which has therefore shown to influence molecular interactions. Understanding the aqueous oxidation of pSi is crucial when loading pSi from aqueous solution due to its impact on molecular interactions. These molecular interactions play an important role in the loading of pSi since they dictate the attraction of the molecule toward the surface and therefore ultimately the loading level.

Introduction 1

Since the discovery of porous silicon (pSi) over 50 years ago, it has been investigated for many applications, including; optical,2 biomedical,3,4 and pharmaceutical.5-10 Initial research focused on producing pSi for optoelectronics11 with oxidation utilized to improve the photostability of luminescent films,12 i.e., oxidation modifies the physical and chemical characteristics of pSi as well as the optical properties.13 More recently modified pSi has found use in drug delivery applications6 where the porous matrix is loaded with a drug which may then be released into the body as the matrix dissolves.7 Since the SiHx-terminated pSi surface is highly reactive toward many drug molecules, passivation of * To whom correspondence should be addressed. Phone:+61 8 8302 3569. Fax: +61 8 8302 3683. E-mail: address: [email protected]. (1) Uhlir, A. Bell Labs Tech. J. 1956, 35, 333–347. (2) Cullis, A. G.; Canham, L. T.; Calcott, P. D. J. J. Appl. Phys. 1997, 82, 909–965. (3) Canham, L. T.; Stewart, M. P.; Buriak, J. M.; Reeves, C. L.; Anderson, M.; Squire, E. K.; Allcock, P.; Snow, P. A. Phys. Status Solidi A 2000, 182, 521–525. (4) Coffer, J. L.; Whitehead, M. A.; Nagesha, D. K.; Mukherjee, P.; Akkaraju, G.; Totolici, M.; Saffie, R. S.; Canham, L. T. Phys. Status Solidi A 2005, 202, 1451–1455. (5) Salonen, J.; Laitinen, L.; Kaukonen, A. M.; Tuura, J.; Bjorkqvist, M.; Heikkila, T.; Vaha-Heikkila, K.; Hirvonen, J.; Lehto, V. P. J. Control. Release 2005, 108, 362–374. (6) Prestidge, C. A.; Barnes, T. J.; Lau, C.-H.; Barnett, C.; Loni, A.; Canham, L. Exp. Opin. Drug DeliV. 2007, 4, 101–110. (7) Vaccari, L.; Canton, D.; Zaffaroni, N.; Villa, R.; Tormen, M.; di Fabrizio, E. Microelectron. Eng. 2006, 83, 1598–1601. (8) Foraker, A. B.; Walczak, R. J.; Cohen, M. H.; Boiarski, T. A.; Grove, C. F.; Swaan, P. W. Pharm. Res. 2003, 20, 110–116. (9) Prestidge, C. A.; Barnes, T. J.; Mierczynska-Vasilev, A.; Skinner, W.; Peddie, F.; Barnett, C. Phys. Status Solidi A 2007, 204, 3361–3366. (10) Prestidge, C. A.; Barnes, T. J.; Mierczynska-Vasilev, A.; Kempson, I.; Peddie, F.; Barnett, C. Phys. Status Solidi A 2008, 205, 311–315. (11) Cullis, A. G.; Canham, L. T.; Williams, G. M.; Smith, P. W.; Dosser, O. D. J. Appl. Phys. 1994, 75, 493–501. (12) Bisi, O.; Ossicini, S.; Pavesi, L. Surf. Sci. Rep. 2000, 38, 1–126. (13) Pirasteh, P.; Charrier, J.; Soltani, A.; Haesaert, S.; Haji, L.; Godon, C.; Errien, N. Appl. Surf. Sci. 2006, 253, 1999–2002.

the surface is often carried out to stabilize the surface,14 produce specific surface chemistries to manipulate drug dissolution, and to facilitate solid state stability. Common treatments of pSi for molecular loading and drug delivery are thermal oxidation or carbonization.5,15 However thermal oxidation of pSi results in a reduction in surface area,16 which is undesirable for maximizing the loading of drug molecules. Thermal oxidation is frequently utilized to passivate pSi wafers17,18 due to well established surface chemistry modification; however, equivalent passivation and characterization of pSi microparticles has not been reported. Transitions in pSi surface chemistry due to thermal oxidation can be monitored via IR spectroscopy, in particular by observing changes to the Si-Hx stretching peaks. The unoxidized pSi wafer surface is SiySiHx terminated,19 and an increase in oxidation temperature results in the conversion of these species to O2SiH and O3SiH.12 Hightemperature thermal oxidation completely removes SiHx species with Si-O bonds forming.20 Drug release from unoxidized,8 thermally oxidized, and carbonized pSi5,16,21 microparticles has been investigated with promising results. However, since unoxidized pSi undergoes oxidation in aqueous solution22 and given that many of the applications of pSi involve immersion in aqueous solution, it is (14) Lees, I. N.; Lin, H.; Canaria, C. A.; Gurtner, C.; Sailor, M. J.; Miskelly, G. M. Langmuir 2003, 19, 9812–9817. (15) Salonen, J.; Paski, J.; Vaha-Heikkila, K.; Heikkila, T.; Bjorkqvist, M.; Lehto, V. P. Phys. Status Solidi A 2005, 202, 1629–1633. (16) Limnell, T.; Riikonen, J.; Salonen, J.; Kaukonen, A. M.; Laitinen, L.; Hirvonen, J.; Lehto, V. P. Int. J. Pharm. 2007, 343, 141–147. (17) Mawhinney, D. B.; Glass, J. A.; Yates, J. T. J. Phys. Chem. B 1997, 101, 1202–1206. (18) Salonen, J.; Lehto, V. P.; Laine, E. Appl. Phys. Lett. 1997, 70, 637–639. (19) Nakajima, A.; Itakura, T.; Watanabe, S.; Nakayama, N. Appl. Phys. Lett. 1992, 61, 46–48. (20) Kumar, R.; Kitoh, Y. Appl. Phys. Lett. 1993, 63, 3032–3034. (21) Kaukonen, A. M.; Laitinen, L.; Salonen, J.; Tuura, J.; Heikkila, T.; Limnell, T.; Hirvonen, J.; Lehto, V.-P. Eur. J. Pharm. Biopharm. 2007, 66, 348–356. (22) Bateman, J. E.; Eagling, R. D.; Horrocks, B. R.; Houlton, A.; Worrall, D. R. Chem. Commum. 1997, 2275–2276.

10.1021/la802316p CCC: $40.75  2008 American Chemical Society Published on Web 11/16/2008

Oxidation of Porous Silicon Microparticles

imperative to understand the influence such immersion has on pSi surface chemistry. In contrast to the extensive studies of pSi oxidation in humid air,23-25 minimal research has been carried out on pSi oxidation in aqueous solution. Water immersion of pSi increases the Fourier transform infrared (FTIR) Si-O stretching and decreases Si-H stretching peak intensities.22 Aqueous oxidation of pSi is anticipated to occur via a similar mechanism to the oxidation of pSi in humid air. pSi exposed to air saturated with water vapor has shown that interaction initially occurs via the splitting of the Si-Si bond by water, which forms SiH and SiOH species. The formation of these species strains the structure, resulting in backbond oxidation by the attack of oxygen.26 A variety of proteins9,10,27,28 and drugs5,16,21,29 have been loaded into pSi; however, the specific interactions between the pSi and these molecules are not fully understood nor optimized. By use of probe molecules with relatively simple chemistry, the nature of these interactions can be reosolved at a fundamental level. Methylene blue has been used as a probe molecule to investigate interactions with a variety of surfaces including coal,30,31 titania,32 and activated carbon.33 Methylene blue is able to undergo a redox reaction34,35 in which it is reduced to the colorless leucomethylene blue. It has been established that the native SiHx-terminated surface is highly reactive and is able to act as a reducing agent,36,37 and our previous research has shown that the native pSi surface reduces the methylene blue molecule and subsequently oxidizes the surface.38 The conversion of methylene blue to leucomethylene blue can be used to indicate the reactivity of the pSi surface, hence the nature of molecular interactions can be quantitatively probed. The research described here is focused on understanding the impact of aqueous immersion and thermal oxidation in air on the surface speciation and surface potential of porous silicon microparticles. Aqueous and thermal oxidation of pSi microparticles has been analyzed with FTIR and surface potentials determined by ζ potential measurements. The influence of aqueous and thermal oxidation on methylene blue discoloration has been observed to quantify the molecular interactions with the pSi microparticles oxidized via different routes. (23) Mattei, G.; Valentini, V.; Yakovlev, V. A. Surf. Sci. 2002, 502(503), 58–62. (24) Salonen, J.; Lehto, V. P.; Laine, E. Appl. Surf. Sci. 1997, 120, 191–198. (25) Ogata, Y. H.; Tsuboi, T.; Sakka, T.; Naito, S. J. Porous Mater. 2000, 7, 63–66. (26) Kostishko, B. M.; Appolonov, S. V.; Salomatin, S. Y.; Kostishko, A. E. Tech. Phys. Lett. 2004, 30, 259–261. (27) Karlsson, L. M.; Tengvall, P.; Lundstrom, I.; Arwin, H. J. Colloid Interface Sci. 2003, 266, 40–47. (28) Zangooie, S.; Bjorklund, R.; Arwin, H. Thin Solid Films 1998, 313(314), 825–830. (29) Anglin, E. J.; Schwartz, M. P.; Ng, V. P.; Perelman, L. A.; Sailor, M. J. Langmuir 2004, 20, 11264–11269. (30) Mittal, A. K.; Venkobachar, C. Ind. Eng. Chem. Res. 1996, 35, 1472– 1474. (31) Qada, E. I.; Allen, E.S-S. J.; Walker, G. M. Chem. Eng. J. 2006, 124, 103–110. (32) Fetterolf, M. L.; Patel, H. V.; Jennings, J. M. J. Chem. Eng. Data 2003, 48, 831–835. (33) Vasanth Kumar, K.; Sivanesan, S. J. Hazard. Mater. 2006, 134, 237–244. (34) Bodoardo, S.; Borello, L.; Fiorilli, S.; Garrone, E.; Onida, B.; Arean, C. O.; Penazzi, N.; Palomino, G. T. Microporous Mesoporous Mater. 2005, 79, 275–281. (35) Pande, S.; Ghosh, S. K.; Nath, S.; Praharaj, S.; Jana, S.; Panigrahi, S.; Basu, S.; Pal, T. J. Colloid Interface Sci. 2006, 299, 421–427. (36) Low, S. P.; Williams, K. A.; Canham, L. T.; Voelcker, N. H. Biomaterials 2006, 27, 4538–4546. (37) Laaksonen, T.; Santos, H.; Vihola, H.; Salonen, J.; Riikonen, J.; Heikkil, T.; Peltonen, L.; Kumar, N.; Murzin, D. Y.; Lehto, V.-P.; Hirvonen, J. Chem. Res. Toxicol. 2007, 20, 1913–1918. (38) Jarvis, K. L.; Barnes, T. J.; Badalyan, A.; Pendleton, P.; Prestidge, C. A. J. Phys. Chem. C 2008, 112, 9717–9722.

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Experimental Section Materials. Porous silicon (BioSilicon) microparticles were used as received (pSiMedica Ltd., Malvern UK). Porous silicon layers were prepared from p+ silicon wafers (0.005-0.020 Ω cm) via electrochemical anodization using hydrofluoric acid/ethanol as the electrolyte. The current density was fixed to give an average porosity of 70%. The porous layer is detached from the underlying silicon substrate electrochemically, resulting in a free-standing pSi membrane with a typical thickness of 150 µm. Microparticles were produced from the membranes by a jet milling process with the particles subsequently classified to give an average particle diameter of 50 µm. An average pore diameter of 10.1 nm and a surface area of 325.7 m2/g were determined by N2 adsorption.38 Surface Chemical Treatment. Thermally oxidized pSi was produced by heating batches of approximately 50 mg in air at 40 K/minute to the required oxidation temperature, which was maintained for 1 h and then returned to ambient temperature. Aqueous oxidation of pSi microparticles was facilitated by prewetting with a small amount of methanol to aid dispersion and then immersed in water. Solutions were mixed for specific time periods to suspend particles and prevent settling. The microparticles were then filtered and dried in an oven at 373 K for 1 h. ζ Potential. A Malvern Zetasizer Nano (Malvern Co. UK) was used to measure the ζ potential of pSi particles in 10-3 M NaCl. The electrophoretic mobility of the pSi microparticles was obtained using the M3-PALS technique, which is a combination of laser doppler velocimetry and phase analysis light scattering. ζ potential was determined from electrophoretic mobility data using the Smoluchowski equation, since κa . 1, where κ is the Debye-Huckel parameter and a is the particle radius.39 Measurements were obtained by suspending 2.5 mg of pSi microparticles in 5 mL of 10-3 M NaCl at various pH values. Solutions of desired pH (2, 4, 6, and 10) were produced by the addition of HCl or NaOH to 10-3 M NaCl. FTIR Spectroscopy. A Nicolet Magna-IR 750 FTIR was used in transmission mode. pSi (3 mg) was ground with dry KBr (300 mg, Sigma-Aldrich FT-IR grade) and pressed into a 13-mm disk using 9 tonnes of pressure. The disk was placed in the sample chamber, which was purged prior to analysis with N2 to remove atmospheric CO2. Methylene Blue Discoloration. A dispersion of pSi particles was produced by immersing 5 mg in 2.46 mL of Milli-Q water. Methanol (40 µL) was added prior to Milli-Q to aid dispersion. Aqueous oxidation of pSi particles was carried out by mixing the dispersion for specific time periods (0.5, 1, 6, and 18 h) prior to methylene blue addition. An initial methylene blue concentration of 100 mg/L was produced by the addition of 2.5 mL of 200 mg/L methylene blue solution to the aqueous or thermally oxidized pSi dispersion. The methylene blue dispersions were mixed for 1 h; the pSi microparticles filtered off, and the solution immediately analyzed with a UV-vis spectrophotometer (Cary 5) at λ ) 665 nm. Measurements were taken at 5-10-min intervals until constant absorbance values were observed. The mass of methylene blue discolored to leucomethylene blue was calculated by measuring the initial and final concentrations of methylene blue and converting it to mass.

Results and Discussion Aqueous and Thermal Oxidation of pSi Particles. Modification of pSi microparticle surface chemistry by aqueous oxidation has been monitored by the SiySiHx and OySiH stretching peaks, which are positioned in the range of 2000-2300 cm-1, as presented in Figure 1. Silicon hydride stretching peaks are observed for native water-immersed and 473 K oxidized pSi at 2130, 2100, and 2080 cm-1 and are assigned to SiSi-H3, Si2Si-H2, and Si3Si-H species, respectively.40 pSi exposed to water for 30 min (Figure 1) also shows FTIR absorbance peaks (39) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: Oxford, 2001. (40) Harper, J.; Sailor, M. J. Langmuir 1997, 13, 4652–4658.

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Figure 1. FTIR spectra of unoxidized, water oxidized, and thermally oxidized porous silicon particles, Si-Hx (x ) 1-3) stretching region.

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Figure 3. Absorbance of Si3SiH, O3SiH, and O2SiH stretching peaks as a function of immersion time in water.

Figure 4. Thermal oxidation stages of pSi surface. Figure 2. Aqueous oxidation steps of pSi. (a) Initial attack of Si-Si bond by water, (b) backbond oxidation by oxygen, and (c) oxidation to O3SiH.

at 2187 and 2239 cm-1, which correspond to O3Si-H and O2Si-H stretching, respectively,18 with SiySiHx species still present on the pSi surface. OySiH species form when oxygen atoms are inserted between the silicon atoms of the backbone and is referred to as backbond oxidation. Similarly to aqueous oxidation, thermal oxidation at 573 K results in the formation of O3Si-H and O2Si-H stretching peaks, as shown in Figure 1. However unlike aqueous oxidation, thermal oxidation results in the removal of SiySiHx peaks with oxidation at 573 K. Similar spectral changes have been observed for OySiH formation on pSi wafers with an increase in oxidation temperature,17 indicating equivalent oxidation mechanisms for wafer and microparticle oxidation. Complete removal of OySiH species is observed subsequent to oxidation at 873 K, which is not observed in aqueous oxidation. Although the oxidation mechanism of pSi particles in aqueous solution has not been definitively established, it is anticipated that both dissolved oxygen and water are involved. The mechanism of aqueous oxidation occurs via the initial attack of the Si-Si bond-forming SiH and SiOH species. Such attack leaves the structure strained so it is more susceptible to oxygen attack and results in back-bond oxidation.25 This mechanism is depicted in Figure 2 and accounts for the backbonded peaks in the FTIR spectrum. Hydrogen gas is also liberated in aqueous oxidation and is anticipated to be due to the direct reaction of water with the SiySiHx-terminated surface, forming SiOH species. The aqueous oxidation of pSi microparticles is attributed to dissolution41,42 with oxidation anticipated to be the first stage. Aqueous immersion alters the surface chemistry via dissolution, which produces OySiH and SiOH species on the surface. Aqueous dissolution is expected to proceed in a similar manner to pore formation,43 i.e., resulting in the regeneration of surface SiHx species, which is confirmed by the observed SiySiHx stretching absorbances. Dissolution experiments were carried out in Tris buffer at pH 7.4 with the accumulated silicon released determined (41) Anderson, S. H. C.; Elliott, H.; Wallis, D. J.; Canham, L. T.; Powell, J. J. Phys. Status Solidi A 2003, 197, 331–335. (42) Pastor, E.; Matveeva, E.; Parkhutik, V.; Curiel-Esparza, J.; Millan, M. C. Phys. Status Solidi C 2007, 4, 2136–2140. (43) Blackwood, D. J.; Zhang, Y. Electrochim. Acta 2003, 48, 623–630.

spectrophotometrically using a molybdenum blue assay.44 Unoxidized pSi dissolves rapidly with ∼70% dissolution in the first 10 min of immersion. Dissolution continues over time with ∼90% dissolved in 18 h.45 The immersion of pSi particles in water for varying time periods shows an initial increase in the concentration of SiySiHx and OySiH species (Figure 3). The increase in Si3SiH concentration with immersion time can be attributed to the dissolution of the pSi microparticles, which is anticipated to expose additional Si3SiH species. After 6 h of immersion, the Si3SiH concentration has decreased due to a reduction in surface area produced by dissolution. Formation of OySiH species after immersion can be attributed to the rapid dissolution that occurs in the first 10 min. The concentration of O2SiH remains constant for immersion times of 0.5 to 6 h, and after 6 h of immersion there is a reduction in the O2SiH concentration and a plateau in O3SiH concentration was achieved after 6 h of immersion. Aqueous oxidation of pSi forms predominately O3SiH species as a result of their structure being more stable than O2SiH.25 Backbond oxidation is also produced during thermal oxidation; however, it proceeds via a different mechanism to aqueous oxidation, as highlighted in Figure 4. Thermal oxidation occurs in a similar way to aqueous oxidation with initial oxidation occurring via the attack of the Si-Si bond and results in backbond oxidation. In contrast the Si-Si bond is broken in aqueous oxidation form SiH and SiOH species. Thermal oxidation results in an increase in O2SiH and O3SiH concentration, shown in Figure 5. As the temperature increases, oxidation of the Si-Si backbone occurs due to the incorporation of oxygen atoms, resulting in a decrease in SiySiHx stretching groups, which is not observed in aqueous oxidation. The thermal oxidation of pSi progresses via backbond oxidation as opposed to the direct formation of Si-OH, i.e., oxygen preferentially attacks Si-Si backbonds rather than breaking Si-H surface bonds.19 Similar to aqueous oxidation, backbond oxidation is progressive and continues to form OSiH followed by O2SiH and then O3SiH. Thermal oxidation results in the removal of OySiH species, which can be observed in Figure 5 where OySiH peak absorbances reach their maximum at 673 K. A further increase (44) Duce, F. A.; Yamamura, S. S. Talanta 1970, 17, 143–149. (45) Wang, F. PhD Thesis, University of South Australia, Adelaide, in preparation.

Oxidation of Porous Silicon Microparticles

Figure 5. Absorbances of Si-Hx stretching peaks as a function of oxidation temperature (maxima position: SiH, 2080 cm-1; SiH2, 2100 cm-1; SiH3, 2130 cm-1; O3SiH, 2260 cm-1; O2SiH, 2210 cm-1).

in oxidation temperatures removes OySiH species, which can be attributed to the progression of oxidation where OySiH species are replaced by OySiOH, shown in Figure 4. Oxidation at 873 K removes all SiHx species (oxidized or otherwise) from the pSi surface, which is well established for high temperature thermal oxidation of pSi wafers.20,46,47 The removal of OySiH species is not observed in aqueous oxidation; however, equivalent behavior is observed for the oxidation of pSi wafers.46 Backbond oxidation in aqueous solution (Figure 2) proceeds via a similar chemical process to thermal oxidation, i.e., initially the concentration of both backbond oxidation species increase as oxidation progresses followed by a decrease in O2SiH concentration where it is further oxidized to O3SiH. Unlike aqueous oxidation, SiySiHx and O3SiH species are removed during thermal oxidation but remain during aqueous oxidation as they are restored during dissolution. It can also be concluded that thermal oxidation of pSi microparticles proceeds in a virtually identical manner to pSi wafers. The higher external surface area of pSi microparticles does not cause expedited oxidation with SiySiHx removal and OySiH formation occurring at comparable temperatures. It may be envisaged that the kinetics differ, but this is beyond the scope of the current study. The modification of pSi microparticles surface chemistry by aqueous and thermal oxidation has been monitored using ζ potential over a range of pH values at fixed ionic strength. The ζ potential originates from the dissociation of charge determining surface silanol (OySiOH, y ) 0-3) groups. The SiySiH (y ) 0-3) species do not undergo dissociation and therefore do not play a role in ζ potential. Differences in the ζ potentials of unoxidized and oxidized pSi are significant and indicate the increased formation of charge determining SiOH species with oxidation. SiOH dissociates/protonates in water to either Si-Oor Si-OH2+ depending on the pH of the solution. For waterimmersed pSi when the pH is less than 2-3.5, depending on the immersion time, the surface was positively charged. Above these pH values the pSi surface is negatively charged due to the deprotonation of SiOH.48 The ζ potentials for Si microparticles as a function of pH for various immersion times are presented in Figure 6 and demonstrate that immersion time has a minor effect on surface potential. Immersion of pSi microparticles for up to 2 h results in similar ζ potentials with isoelectric points of pH ≈ 3.5. Increasing immersion time does not initially have an effect on ζ potential since oxidation occurs via the formation of charge-determining (46) Zoubir, N. H.; Vergnat, M. Appl. Phys. Lett. 1994, 65, 82–84. (47) Takazawa, A.; Tamura, T.; Yamada, M. J. Appl. Phys. 1994, 75, 2489– 2495. (48) Binner, J.; Zhang, Y. J. Mater. Sci. Lett. 2001, 20, 123–126.

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Figure 6. ζ potentials of porous silicon microparticles as a function of pH for various immersion times.

Figure 7. ζ potentials of porous silicon microparticles as a function of pH for different oxidation temperatures.

SiOH species. For increased immersion times, dissolution of the microparticle occurs via the cleavage of Si-Si bonds, regenerating the SiOH species. Therefore upon dissolution pSi microparticle surface maintains a constant surface potential. Dissolution is likely to have an effect on the ζ potential of pSi microparticles that have been immersed for more than 4 h, where the isoelectric point has shifted to pH ≈ 2. It is suggested that these changes in ζ potentials with immersion time result from an increase in surface SiOH species, making the surface more negatively charged and thus lowering the isoelectric point. The native pSi surface is hydride terminated and is therefore expected to have no surface potential due to the absence of chargedetermining SiOH species. However, ζ potentials indicate the unoxidized pSi surface is negatively charged at pH > 4 (Figure 7), indicating the presence of SiOH species. A small number of SiOH groups would be present on the pSi surface due to atmospheric oxidation, which is well documented,18,49 and may also be introduced in the jet milling of free-standing pSi membranes into microparticles. As shown in Figure 6, aqueous oxidation results in more negative ζ potential values due to the formation of SiOH species as water results in the oxidation of the pSi surface. Water accelerates the pSi oxidation process12,25,50 due to silicon dissolution. The reaction of silicon with water is a nucleophilic substitution reaction with OH-, which results in the formation of SiOH on the surface and the removal of a hydride ion (H-).51 (49) Torchinskaya, T. V.; Korsunskaya, N. E.; Khomenkova, L. Y.; Dhumaev, B. R.; Prokes, S. M. Thin Solid Films 2001, 381, 88–93. (50) Gupta, P.; Dillon, A. C.; Bracker, A. S.; George, S. M. Surf. Sci. 1991, 245, 360–372. (51) Baum, T.; Schiffrin, D. J. J. Chem. Soc., Faraday Trans. 1998, 94, 691– 694.

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Figure 8. Oxidized and reduced structures of methylene blue.

Figure 9. Mass of methylene blue recolored per mass of pSi as a function of thermal and aqueous oxidation.

Thermally oxidized pSi particles (Figure 7) have similar ζ potentials to pSi that has been immersed in water, indicating that these oxidation methods produce similar surface chemistries which is in contrast to the FTIR spectra. pSi oxidized at 473, 673, 873, and 1073 K have similar ζ potentials in solution, despite the FTIR spectra in Figure 1 showing distinctly different surface chemistries in the dry state. It is anticipated that aqueous immersion has an effect on the surface potential of thermally oxidized pSi. FTIR spectra confirm that pSi oxidized at e773 K retain SiHx species, and it is suspected that these remaining groups undergo oxidation in aqueous solution, eventually resulting in sufficient concentrations of surface silanol groups to establish similar ζ potentials to pSi microparticles that have been oxidized at 873 K and above. Molecular Interactions. Upon contact with the SiySiHxterminated surface in aqueous solution the methylene blue molecule is reduced to colorless leucomethylene blue. Removal of the pSi particles allowed leucomethylene to be converted back to methylene blue by oxidation (Figure 8). Therefore the discoloration and recoloration processes can be used to indicate the reactivity of the pSi surface and the effect of both thermal and aqueous oxidation on molecular loading. pSi particles either unoxidized or thermally oxidized at 473 K are able to discolor approximately 0.03 mg(MB)/mg(pSi) (Figure 9) due to the presence of SiySiHx surface species. Discoloration is not observed for 673 K oxidized pSi, which has an OySiH-terminated surface; hence it can be established that only SiySiHx species are capable of reducing the methylene blue molecule. No discoloration was observed for pSi microparticles that have been oxidized at 873 and 1073 K, which is due to the absence of SiHx species. Aqueous oxidation of pSi also discolors approximately 0.03 mg(MB)/

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mg(pSi), demonstrating aqueous immersion does not have an effect on the mass of methylene blue discolored. This is due to dissolution, which regenerates the SiySiHx-terminated surface. On the basis of the observed increase in Si3SiH concentration with increasing immersion times (Figure 3), an increase in the mass of methylene blue discolored would be expected. Since the mass of methylene blue discolored remains unchanged, it is postulated that the increase in discoloration due to higher Si3SiH concentrations is canceled out by a decrease in surface area due to pSi dissolution. Methylene blue discoloration has shown that thermal oxidation reduces pSi reactivity via the conversion of SiySiHx to OySiH and OySiOH species while aqueous oxidation does not have a significant impact on reactivity. Investigation of the thermal and aqueous oxidation of pSi microparticles has highlighted differences in the oxidation mechanism between these two processes. These mechanisms are expected to be identical for all pSi types due to similar surface chemistries, regardless of porosity. Although surface chemistry is strongly influenced by oxidation time and temperature, aqueous and thermally oxidized pSi microparticles do not show major changes in ζ potential. When loading material onto pSi from aqueous solution the oxidation process needs to be considered, despite aqueous immersion having minimal influence on molecular interactions between the adsorbate and the pSi surface. Dissolution will have an impact on the quantity of the molecule loaded with thermal oxidation having the most significant impact on molecular interactions and loading capacity.

Conclusions Differences and similarities in the mechanisms and surface chemistries produced via the aqueous and thermal oxidation of pSi microparticles have been determined. Thermal and aqueous oxidation commences via the breaking of Si-Si bonds. In thermal oxidation the bond is broken due to backbond oxidation however in aqueous oxidation SiH and SiOH species form. Thermal oxidation has the capability of removing SiySiHx and O3SiH species, but they are retained in aqueous oxidation. Surface chemistry has shown to have a significant effect on molecular interactions with thermal oxidation significantly reducing pSi reactivity. It has been shown that oxidation modifies the surface chemistry and ζ potential of the pSi microparticle. Modification of these parameters is important as they dictate the affinity of a molecule toward the pSi surface and therefore controls loading capacity. To develop pSi as a therapeutic delivery system, such fundamental knowledge is imperative to establish successful modification and loading protocols. Acknowledgment. Frank Wang is thanked for providing the dissolution data. pSiMedica Limited (a pSivida group Company) is gratefully acknowledged for supplying pSi samples. Funding from pSivida and the Australian Research Council’s Linkage grant scheme (Project No. LP040805) is also acknowledged. LA802316P