Surface chemical modification to control molecular interactions with porous silicon

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Abstract

Interactions between porous silicon (pSi) particles and probe molecules were evaluated to determine the effect of pSi and probe molecule chemistry on adsorption. Methylene blue, ethyl violet and orange G dyes were chosen for investigation as they possess distinct functionalities and charges. Several distinct pSi surface species were produced via thermal oxidation at 200–800 °C and their effect on adsorption investigated. The adsorption mechanisms were elucidated from equilibrium adsorption and desorption isotherms. Methylene blue adsorption was attributed to electrostatic attraction where a gradual increase in adsorption with oxidation temperature was observed. Significant methylene blue desorption was observed at pH 3, confirming adsorption occurs via electrostatic attraction. Ethyl violet demonstrated an increase in plateau adsorption capacity and affinity with increased oxidation temperatures and adsorption was initially attributed to electrostatic attraction, however desorption of ethyl violet was not observed, thus indicating potential chemisorption. Orange G exhibited high affinity adsorption for SiySiHx terminated surfaces but no orange G desorption was detected, indicating a chemisorption adsorption mechanism. It has been successfully demonstrated that the surface modification of pSi enabled the manipulation of molecular interactions. By interacting probe molecules with similar functionalities to drug molecule with pSi, greater understanding of drug-pSi interactions can be ascertained which are of great importance. pSi surface chemistry can be tailored to enable control over molecular interactions and ultimately dictate loading, encapsulation and release behavior.

Graphical abstract

Effect of pH on methylene blue desorption from unoxidized (◊) and 800 °C (o) oxidized porous silicon particles.

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Highlights

► Dye adsorption onto unoxidized and thermally oxidized porous silicon. ► Porous silicon oxidation temperature controls dye mass adsorbed. ► Adsorption mechanism dependent on dye chemistry. ► Electrostatic attraction and chemisorption mechanisms observed. ► Desorption of dye adsorbed via electrostatic attraction but not chemisorption.

Introduction

In recent years porous silicon (pSi) has become a material of interest after its photoluminescence properties were revealed in 1990 [1] and has since been investigated for a wide variety of applications. Research was initially focused on optoelectronics [2], however in 1995 hydroxyapatite growth on pSi was demonstrated, suggesting the potential of pSi as a biomaterial [3]. pSi can be applied to a variety of biological applications due to it biocompatability [4], dissolution [5] and cell adhesion [6]. Another application of pSi with great potential is drug delivery, where the pores are filled and the drug is subsequently released as the pSi matrix degrades [7]. To develop pSi as an effective drug delivery system, it is necessary to understand how pSi interacts with a range of functionalities and charges. Molecular interactions play an important role in pSi loading since they dictate the attraction of the molecule towards the surface and therefore ultimately the loading level. A number of proteins [8], [9] and drugs [10], [11], [12], [13], [14], [15] have been loaded into pSi, however the specific interactions between the pSi and these molecules are not fully understood. By using probe molecules with simpler and well defined chemistry, such as dyes, the nature of these interactions can be resolved at a fundamental level and can be used to predict the interactions with specific drug functionalities.

The surface chemistry of pSi plays a crucial role in molecular interactions and subsequently controls adsorption. The native pSi surface is hydride terminated (SiySiHx, x + y = 4) [16] and highly reactive. A low SiOH concentration is expected on the native pSi surface due to atmospheric oxidation [17], [18] and aqueous immersion [16], [19] during the dye adsorption process. Thermal oxidation is commonly used to modify pSi surface chemistry [20], [21]. Thermal oxidation at 300 °C and above forms backbond oxidation species (OySiH) [20], [22]. Oxidation at 600 °C removes the majority of backbond species [23], [24] with complete removal at 750–800 °C [17], [25] where OySiOH species form, which are expected to enhance the adsorption of cationic dyes by electrostatic attraction.

Adsorption is frequently utilized to remove dyes from industrial effluent, however dye adsorption is also useful in characterizing adsorbent materials by providing information regarding the interactions of specific dye functional groups with various surface species. A variety of adsorbents have been characterized via dye adsorption including; sulfonated coal [26], maize waste [27], xerogels [28], diatomite [29], activated carbon [30] and silica [31]. The functional groups of both the dye and adsorbent dictate their interactions. Dye adsorption frequently results from electrostatic attraction between the charged functional groups of the dye and adsorbent after dissociation in solution. The contrasting adsorption behavior of anionic and cationic dyes was highlighted using a negatively charged maize waste adsorbent. Cationic dye adsorption was rapid and complete due to its electrostatic attraction while in contrast anionic dye adsorption was slow and incomplete [27]. pH has a significant effect on adsorption by controlling the protonation and deprotonation of the dye and adsorbent, dictating electrostatic repulsion or attraction. Adsorption generally decreases with increasing pH for anionic dyes while it increases with increasing pH for cationic dyes since adsorbents typically become more negatively charged as the pH increases [28], [32], [33], [34], [35]. Surface modification has shown to be effective at altering dye adsorption properties e.g. Silica, which is normally negatively charged, was silane modified to produce a positively charged surface. The positively charged surface adsorbed anionic dyes with the dye adsorption increased by increasing silane concentration [31]. Electrostatic attraction is the dominant adsorption mechanism for dyes, however the adsorption of a large anionic dye onto a negatively charged clay adsorbent has been attributed to van der Waals forces [36].

Dye desorption is a useful method to gain additional information regarding dye adsorption mechanisms. Successful desorption via the addition of pH controlled solutions to dye loaded adsorbents indicates physical adsorption. Such behavior has been observed for positively charged chitosan [37], bentonite [38] and silica [39] where the desorption of anionic dyes was achieved upon exposure to high pH aqueous solutions. Desorption occurred by deprotonation of the positively charged surfaces under basic conditions thus reducing the electrostatic interactions between the surface and dye anions [39]. Low desorption has been observed from a number of different adsorbents [40], [41], [42], [43] which was attributed to chemisorption as the dominant adsorption mechanism. Increases in dye adsorption capacity at higher temperatures [43], [44] and pH for an anionic dye [42] can also indicate chemisorption. A number of models can be applied to dye adsorption data to reveal further information regarding adsorption mechanisms. Dye adsorption data has been fitted to the pseudo-second-order kinetics model and when high correlation coefficients to the model were observed, chemisorption was inferred as the adsorption mechanism [44], [45], [46], [47], [48], [49].

This research investigates the effect of different probe molecule functional groups and pSi surface speciations on molecular interactions. Equilibrium adsorption isotherms were obtained to determine the effect of surface chemistry on adsorption affinity and capacity. The Langmuir model was fitted to the adsorption isotherms to gain further insight into the molecular interactions. Additional information regarding the adsorption mechanisms was obtained by dye desorption experiments.

Section snippets

Adsorbent

Porous silicon particles were produced by pSiMedica Ltd. and used as received. pSi wafers were produced from p+ silicon wafers (0.005–0.020 Ω cm) via electrochemical anodization with a hydrofluoric acid/ethanol electrolyte. A fixed current density produced an average porosity of 70%. Electrochemical detachment of the porous layer from the underlying silicon substrate resulted in a pSi membrane with a typical thickness of 150 μm. Particles were produced from the pSi membrane by jet milling to

Adsorption isotherms of probe molecules onto pSi particles

Adsorption isotherms for methylene blue, ethyl violet and orange G onto unoxidized and thermally oxidized pSi particles are shown in Fig. 2. These probe molecules were chosen to investigate the effect of charge on molecular interactions as they possess functionalities found commonly in drug molecules i.e. azo, sulfonate etc. Characterization of adsorbents is frequently carried out via the adsorption of methylene blue [30], [33], [51], [52] which is a well established probe molecule for

Conclusions

Adsorption of dye molecules onto thermally oxidized pSi particles have been studied to investigate the interactions between specific functional groups. To be able to predict and control how pSi will behave as a drug delivery vehicle, it is imperative to understand how its surface chemistry impacts molecular interactions. Adsorption of methylene blue, ethyl violet and orange G was dependent on pSi surface chemistry however the dye functionalities dictated the adsorption mechanism. pSi behaves in

Acknowledgments

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 is also acknowledged (Project No. LP040805).

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