Surface chemistry of porous silicon and implications for drug encapsulation and delivery applications

Advances in Colloid and Interface Science 175 (2012) 25–38

Contents lists available at SciVerse ScienceDirect

Advances in Colloid and Interface Science journal homepage: www.elsevier.com/locate/cis

Surface chemistry of porous silicon and implications for drug encapsulation and delivery applications Karyn L. Jarvis, Timothy J. Barnes, Clive A. Prestidge ⁎ Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA, 5095, Australia

a r t i c l e

i n f o

Available online 30 March 2012 Keywords: Porous silicon Surface modification Surface chemistry Drug loading Drug delivery

a b s t r a c t Porous silicon (pSi) has a number of unique properties that appoint it as a potential drug delivery vehicle; high loading capacity, controllable surface chemistry and structure, and controlled release properties. The native SiySiHx terminated pSi surface is highly reactive and prone to spontaneous oxidation. Surface modification is used to stabilize the pSi surface but also to produce surfaces with desired drug delivery behavior, typically via oxidation, hydrosilylation or thermal carbonization. A number of advanced characterization techniques have been used to analyze pSi surface chemistry, including X-ray photoelectron spectroscopy and time of flight secondary ion mass spectrometry. Surface modification not only stabilizes the pSi surface but determines its charge, wettability and dissolution properties. Manipulation of these parameters can impact drug encapsulation by altering drug–pSi interactions. pSi has shown to be a successful vehicle for the delivery of poorly soluble drugs and protein therapeutics. Surface modification influences drug pore penetration, crystallinity, loading level and dissolution rate. Surface modification of pSi shows great potential for drug delivery applications by controlling pSi–drug interactions. Controlling these interactions allows specific drug release behaviors to be engineered to aid in the delivery of previously challenging therapeutics. Within this review, different pSi modification techniques will be outlined followed by a summary of how pSi surface modification has been used to improve drug encapsulation and delivery. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porous silicon production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porous silicon surface chemistry and its modification . . . . . . . . . . . . . . . . . . . 3.1. Native porous silicon surface chemistry . . . . . . . . . . . . . . . . . . . . . . 3.2. Porous silicon oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Thermal oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Aqueous oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Thermal carbonization of porous silicon . . . . . . . . . . . . . . . . . . . . . . 3.4. Porous silicon hydrosilylation . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Impact of surface chemistry modification on factors affecting drug loading and encapsulation 4.1. Dissolution of porous silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Surface wettability of porous silicon . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Zeta potential of porous silicon . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Drug interactions, loading and delivery . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Rationale for using porous silicon for drug delivery . . . . . . . . . . . . . . . . . 5.2. Drug delivery from native porous silicon . . . . . . . . . . . . . . . . . . . . . . 5.3. Drug interactions with porous silicon . . . . . . . . . . . . . . . . . . . . . . . 5.4. Drug loading and release from surface modified porous silicon . . . . . . . . . . . . 5.5. Delivering chemotherapy drugs . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6. Protein loading, release and delivery . . . . . . . . . . . . . . . . . . . . . . . . 6. Conclusions and future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⁎ Corresponding author. Tel.: + 61 8 8302 3569; fax: + 61 8 8302 3683. E-mail address: [email protected] (C.A. Prestidge). 0001-8686/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2012.03.006

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

26 26 28 28 28 28 29 30 30 31 31 32 32 32 32 33 33 33 36 36 37 37

26

K.L. Jarvis et al. / Advances in Colloid and Interface Science 175 (2012) 25–38

1. Introduction Porous silicon (pSi) was inadvertently discovered over 50 years ago when attempting to produce electrochemically machined silicon wafers [1]. The large surface area and surface chemistry of pSi alter its surface reactivity and stability from that of conventional silicon [2]. pSi is most commonly produced by anodization which involves immersing silicon wafers into a HF/ethanol solution and exposing the wafers to a specific current for the desired length of time [3]. Since 1990 when pSi was found to emit visible photoluminescence at room temperature [4], significant research has been undertaken on developing pSi for use in light-emitting devices and optoelectronics [5]. The structure of pSi is highly dependent on fabrication parameters with the modification of surface area, porosity and pore size distribution possible via changes to current density, anodization time, temperature and HF concentration [6]. More recently pSi has been investigated for use in a number of biomedical applications, in particular; biosensing [7] and tissue engineering scaffolds [8]. Such interest in the biological applications of pSi is due to its biocompatible nature and particularly that cells adhere to the porous layer [9]. One of the most recent pSi applications investigated has been drug delivery [10,11]. Exploiting pSi for drug delivery conventionally involves loading the drug into the porous matrix which is then released into the body as the matrix begins to degrade [12]. Drug diffusion from the pores has also shown to occur from pSi, especially for surface modified pSi where its degradation is inhibited [13]. Drug delivery with pSi shows promise, particularly for the controlled delivery of poorly soluble drugs and proteins that cannot otherwise be delivered easily [11,14]. Native pSi has been used to successfully deliver a number of drugs [12,15], however native pSi possesses some undesirable characteristics that may mitigate its use in drug delivery. Freshly etched pSi is highly reactive due to its hydride terminated (SiySiHx, x + y = 4) surface. Such a reactive surface can result in undesirable chemical reactions with loaded drugs [16]. The hydride terminated pSi also reacts slowly with ambient air, altering both the structural and optoelectronic properties. It has been established that hydride terminated pSi is converted to native oxide with oxide growth dependent on atmospheric conditions [5]. The reactivity of the pSi surface however can be exploited to stabilize the surface [17] and introduce species onto the surface for a variety of applications, especially drug delivery [18,19]. By developing a number of surface modification methods, pSi can meet the needs of specific applications and can become increasingly useful in a wide range of fields. Thermal oxidation is frequently used for pSi surface modification. Transitions in pSi surface chemistry by thermal oxidation are often

characterized by infrared spectroscopy, in particular by observing changes to the SiHx stretching peaks. Backbond oxidation (OySiH) is observed at temperatures around 300 °C with complete oxidation at temperatures up to 600–700 °C [20]. Oxidation affects the structure of pSi by reducing the average pore diameter [21] and surface area [22] due to expansion of the silicon structure upon heating [23]. Hydrosilylation is another common method for pSi surface modification, where carbon chains are attached to the hydride terminated pSi surface [24]. Thermal or light induced hydrosilylation with alkenes or alkynes are the most commonly used. Hydrosilylation can also be mediated using either Lewis acid (EtAlCl2), late transition metals such as rhodium, palladium and platinum or organohalides [25,26]. Thermal carbonization is a more recent method which has been used to modify pSi surface chemistry via the thermal decomposition of acetylene [27,28]. Although several pSi review articles have previously been published discussing surface functionalization [29–32] and aspects of pSi drug delivery [11,33–35], minimal links have been made between the two topics. No previous review is available which bridges the gap between surface functionalization and drug encapsulation and delivery, outlining how manipulation of the surface chemistry can influence drug loading and release behavior. In this review we will start with a brief outline of pSi production. Different surface modification methods will be outlined followed by how such modification impacts the various properties of pSi such as hydrophobicity and dissolution. Lastly we will review in detail how pSi surface modification has been used to improve drug encapsulation and delivery. 2. Porous silicon production The production of pSi can be achieved using p or n-type silicon wafers. P-type wafers are boron doped while phosphorous is used to dope n-type wafers [36]. Doping is used to achieve specific electrical properties. Heavily doped p +-type wafers are the most commonly used to produce pSi. Although p +-type wafers produce pSi with lower surface areas than other types, they are more stable against oxidation [37]. Anodization is the most commonly used method to produce pSi. Several other less common etching methods such as stain etching [38], light stimulated [39], gas [40], vapor [41] and spark erosion etching [42] have also been investigated. Anodization occurs via the immersion of silicon wafers into an electrochemical cell containing a hydrofluoric acid/ethanol electrolyte. Ethanol is added to the electrolyte to increase surface wettability. Aqueous ethanol solutions

Si (anode)

Pt (cathode)

-

From pump

+ Pt grid (cathode)

+ Pt

Pt HF

HF

Si wafer Aluminium plate

HF Si

To pump

Lateral anodization cell

Single tank anodization cell

Double tank anodization cell

Fig. 1. Cross sectional view of lateral, single and double tank anodization cells. Adapted with permission from [6]. Copyright 2000 Elsevier.

K.L. Jarvis et al. / Advances in Colloid and Interface Science 175 (2012) 25–38

are able to penetrate pores while pure HF solutions cannot. The anodic current or potential can be manipulated, however a constant current is preferential as it provides better control over porosity, thickness and produces greater reproducibility. The silicon wafer acts as the anode while the cathode is conventionally made of platinum. The anodization cell needs to be made from an acid resistant polymer such as Teflon. Using a silicon wafer as an anode results in the porosification of the wafer surface exposed to HF. Several different types of anodization cells can be used, as shown in Fig. 1, however lateral anodization is the most simple. Both the anode and cathode are placed vertically in the electrolyte and the current applied. Lateral anodization produces non uniform porosity and thickness, due to a lateral potential drop which varies the local current density, therefore producing porosity and thickness gradients. Single tank cells [6] are commonly used with aluminum back side contact. A metal plate is used to protect the back of the wafer, therefore only the front side of the wafer is porosified. A third type of anodization cell is the double tank cell which uses electrolytic back side contact. Two half cells are fitted with platinum electrodes with silicon wafer separating the half cells. The electrolyte is recirculated using a pump which removes gas bubbles generated during etching and prevents any decreases in local HF concentration. The platinum electrodes are connected to a power supply and the current flows from one side to the other via the silicon wafer [6,36,43]. The morphology of the pSi produced is dependent on the anodization parameters such as HF concentration, current density, anodization time and temperature. Over the last 15 years investigations into pore formation mechanisms have been conducted with the first mechanism suggested in 1995 [44] and reiterated several times since [45–47]. Pore formation occurs when silicon atoms are dissolved producing SiF6−2 complexes, which requires F − from HF and electrons from the silicon wafers [48]. The growth of a pore is controlled by a number of parameters including wafer type, doping level, HF concentration, ethanol concentration, current density, temperature and time [49]. Pore formation occurs at low current densities when sufficient F − is present but is scarce at the interface. Due to the electric field distribution the chance for electrons from the silicon surface to reach the bottom of a valley is slightly increased in comparison to the top of the valley. Therefore more silicon is removed from the valley, resulting in pore growth. With a large current density, electropolishing occurs as the diffusion of F − to the interface is slower than electron transport. Whenever a F − ion reaches the surface, sites are awaiting dissolution. Therefore the tops of the valleys receive more ions than the bottom and the silicon peaks are reduced in height [48]. The pore formation mechanism is depicted in Fig. 2, where pore formation occurs as long as steps 1 and 2 are slower than steps 3–5

1

4

27

Fig. 3. SEM cross sections of a. cylindrical dead end pores and b. cylindrical interconnected pores in porous silicon wafers. Panel a. adapted with permission from [52]. Copyright 2002 American Chemical Society. Panel b. adapted with permission from [51]. Copyright 2001 Elsevier.

[45]. In steps 1 and 2 Si\H is converted to Si\OH and then to Si\F in step 3. In step 3 HF and H2O attack the Si\Si backbonds, leaving hydrogen attached to silicon on the surface [50]. Two main pore morphologies are generally featured in pSi, as demonstrated in Fig. 3. The types of pores that are found in pSi are generally cylindrical. They can be interconnected and branched [51] or dead ended [52]. Other pore morphologies that may exist in pSi are closed pores which do not have an opening to the surface or through pores which extend all the way through a porous membrane [53]. A wide range of surface areas (b1 m 2/g for macroporous to 800 m 2/g for microporous) can be produced in pSi and are dependent on fabrication parameters [53]. Fabrication parameters influence surface

2

3

5

Fig. 2. Schematic representation of the chemistry of pore formation and hydrogen termination. Reprinted with permission from [45]. Copyright 2003 Elsievier.

28

K.L. Jarvis et al. / Advances in Colloid and Interface Science 175 (2012) 25–38

area in different ways, with an increase in the surface area observed by increasing the HF concentration, while decreases in the surface area result from increasing the current density and anodization time [6]. The silicon substrate also influences pore size with n-type wafers producing pore sizes of 25–40 nm, while the anodization of p-type wafers produced average pore diameter of 100 nm [49]. It has been determined that the porosity of pSi increases with increasing current density [2,54,55] and anodization and decreasing HF concentration [6]. As porosity increases, the average pore size generally increases [56]. Increasing current density tends to increase the pSi surface area, porosity, average pore size and pore size distribution [56–58]. Anodization at higher temperatures can either increase [59,60] or decrease [45,55] the porosity of pSi, depending on the other fabrication parameters. HF concentration has a significant effect on pSi structure. Increasing the HF concentration increases the pSi specific surface area and produces smaller average pore sizes, while lower HF concentrations produce broader pore size distributions [58,61]. Higher HF concentrations also decrease porosity with pSi formed at lower current densities showing the most significant decrease [58,61]. The effect of various HF concentrations on pSi formation was investigated by producing pSi electrochemically using 80% or 20% HF solutions and a current density of 30 mA/cm 2. The pSi produced in the concentrated solution was mesoporous and had pore diameters of 3–8 nm, and had a porosity of 60%. Dilute HF produced macroporous pSi with pore diameters of 0.5–3 μm and a porosity of 86% [61]. 3. Porous silicon surface chemistry and its modification

Fig. 4. FTIR spectrum of native and oxidized porous silicon. Adapted with permission from [62]. Copyright 1997 American Chemical Society.

indicating that silicon is present in its elemental form [66–68]. Slight oxidation is also usually observed [69] due to atmospheric exposure. The SiySiHx species, SiH, SiH2 and SiH3 can also feature in the spectrum and are reported to have shifts of 0.335, 0.67 and 1.00 eV respectively from the Si 0 peak [67].

3.1. Native porous silicon surface chemistry 3.2. Porous silicon oxidation The freshly etched pSi surface is SiySiHx (x + y = 4) terminated and reacts slowly with ambient air, affecting both its structural and optoelectronic properties. It has been established that hydride terminated pSi is converted to native oxide, where oxide growth is dependent on atmospheric conditions [5]. To prevent native oxide growth, surface modification has been investigated. Surface modification can also be used to add functionalities to the pSi surface to enable use in specific applications. Surface modification of pSi can be divided into two broad categories: oxidation and chemical functionalization. Oxidation occurs via the controlled exposure of pSi to various oxidizing agents to induce the formation of oxide species (OySiH, OySiOH and O\Si\O) on the surface. Functionalization is generally regarded as the attachment of carbon chains to the surface via various mechanisms, where both the Si\H and Si\Si bonds are reactive. The native pSi surface is terminated by Si3SiH, Si2SiH2 and SiSiH3 groups. Their relative concentrations are dependent on fabrication parameters. Fourier Transform Infrared spectroscopy (FTIR) has been used extensively to characterize the surface chemistry of native pSi with individual peaks for a number of surface species [62]. The FTIR spectrum of native pSi exhibits several features, including three peaks present together at approximately 2140, 2100 and 2090 cm − 1 (Fig. 4). These peaks are assigned to SiSiH3, Si2SiH2 and Si3SiH stretching respectively [63]. A peak at 1100 cm − 1 is due to bulk Si\O\Si stretching. Other peaks are featured at approximately 910, 640 and 617 cm − 1 and assigned to SiH2 scissor, SiHx deformation and bulk Si\Si stretching respectively [64]. Porosity has been shown to influence surface chemistry of pSi. As the porosity of the pSi increases, the concentration of SiySiHx species increases. Such behavior can be explained by the fact that pSi with higher porosities has a greater surface area and therefore a greater number of SiySiHx groups [65]. X-ray photoelectron spectroscopy (XPS) can also be utilized to investigate native pSi surface chemistry as SiySiHx species also contribute to the Si 2p spectrum. The XPS spectrum of a native pSi surface is dominated by the Si 0 peak at approximately 100 eV (Fig. 5),

3.2.1. Thermal oxidation Thermal oxidation is often studied using FTIR and demonstrates that various surface chemistries can be produced by controlling oxidation temperature. The surface of pSi undergoes significant changes when subjected to thermal oxidation with complete removal of SiySiHx species by oxidation at 300–400 °C [70]. The surface remains hydride terminated until approximately 500 °C [64,71] due to the formation of backbonded species (OySiH) with increasing oxidation temperature. Backbond oxidation occurs via the cleavage of Si\Si bonds resulting in the incorporation of oxygen within the silicon backbond. Thermal oxidation induces the appearance of new peaks in the FTIR spectrum at 2200 and 2250 cm − 1 (Fig. 4) and can be assigned to the backbond species O2SiH and O3SiH respectively [6,71]. Backbond species gradually diminish with increased oxidation

Si0

Si+ Si2+Si3+Si4+

Fig. 5. X-ray photoelectron spectrum of porous silicon exhibiting Si0, intermediate oxide and Si4+ peaks. Adapted with permission from [66]. Copyright 2007 Elsevier.

K.L. Jarvis et al. / Advances in Colloid and Interface Science 175 (2012) 25–38

a

b

pSi

silicon c

d pSi Si silicon

Fig. 6. 100× 100 μm ToF-SIMS images of unoxidized a. total ion−, b. O− and 600 °C oxidized c. total ion−, and d. O− pSi wafer fractured cross sections (scale bar= 10 μm) [85].

temperature and are completely removed by oxidation at 600 °C [62,64,71,72]. Oxidation at 600 °C and above removes all SiHx species from the surface and forms OySiOH and Si\O\Si species [20,64,73]. Such oxidation is supported by the observed increase in absorption at 3400 cm− 1 (Si\OH) and the disappearance of peaks at 2150–2300 cm− 1 due to backbonded species [20]. High temperature thermal oxidation replaces OySiH with Si\O bonds, thus increasing the intensities of the Si\O peaks at 460, 870 and 1100 cm− 1. The bulk Si\O\Si stretching peak at 1100 cm− 1 becomes broader during oxidation in addition to a shoulder forming at 1170 cm− 1 demonstrating the incorporation of oxygen as a suboxide species [64]. XPS is also a useful tool in the characterization of oxidized pSi due to its sensitivity to oxidation states. XPS has been widely used to characterize thermally oxidized pSi. The Si 2p spectrum of pSi generally has an elemental silicon (Si 0) peak and also a fully oxidized SiO2 (Si 4+) peak at approximately 104.4 eV [66]. With increasing oxidation time or temperature, a significant increase in SiO2 peak intensity and corresponding decrease in the Si 0 peak due to oxide formation is observed [67,68,74]. Intermediate oxide species (Si +\Si 3+) are also formed during oxidation (Fig. 5). The intermediate oxide species are denoted as Si +, Si 2+ and Si 3+ and correspond to the number of oxygen atoms bonded to the silicon atom. Elemental silicon (Si 0) is bonded to 4 other silicon atoms while Si +, Si 2+ and Si 3+ are bonded to 1, 2 and 3 oxygen atoms respectively. Fully oxidized SiO2 is bonded to 4 oxygen atoms [75]. The oxidation of pSi tends to be complete with the majority of the silicon existing in the Si 0 or Si 4+ states and lower concentrations of the intermediate

29

oxide species typically observed [76,77]. Early XPS characterization studies have mentioned, but did not attempt to fit these intermediate oxide species [68,69]. Later characterization studied fitted a broad peak between the Si 0 and Si 4+ peaks and labeled the peak as an “intermediate oxide phase” [78] or “siloxene like compound” [74] rather than fitting the individual components. The chemical shift of the XPS intermediate oxide species has been determined as 1, 1.8, 2.6 and 3.5 eV for Si +, Si 2+, Si 3+ and Si 4+ respectively [76]. These intermediate oxide species have been fitted to the Si 2p spectrum, however it is difficult to determine their precise positions and a range of values have been reported [66,79]. Time of flight secondary ion mass spectrometry (ToF-SIMS) is a surface characterization technique which has been utilized for the characterization of pSi [80–83]. There are two types of SIMS: dynamic and static. ToF-SIMS is a static SIMIS technique which is highly surface sensitive and provides information about the elemental and molecular species present within the top ~1 nm of a sample surface. Static SIMS should not be confused with dynamic SIMS which is much less surface sensitive and is better suited to bulk and elemental analysis [84]. Unoxidized thermally oxidized pSi wafers have been fractured and mounted with the fresh facture edge facing upwards to enable mapping of species across the porous layer. The negative ToF-SIMS images for unoxidized and 600 °C oxidized pSi wafers in Fig. 6 show a porous layer of approximately 30 μm and the underlying silicon substrate. The thickness of the porous layer is controlled by fabrication parameters, therefore a 30 μm thick porous layer is not representative of all pSi wafers. For unoxidized pSi, little oxygen is observed in the porous layer. Oxidation at 600 °C however produces a porous layer which has a high intensity of O − fragments. The distribution of this fragment is also uniform across the porous layer, indicating that thermal oxidation occurs uniformly [85]. ToF-SIMS shows promise in its ability to spatially image the distribution of chemical species within pSi, which is of particular use for surface modification and drug loading.

3.2.2. Aqueous oxidation Aqueous oxidation of pSi can be induced via either water immersion or exposure to humid atmospheres, with oxidation in humid air more rapid than in dry air [86]. The oxidation of pSi via water immersion has not been extensively studied due to its highly hydrophobic nature. Water can only oxidize pSi if it is in intimate contact, therefore immersion wetting of pSi is required for oxidation to occur. pSi oxidation has been observed in water and results in an increase in FTIR Si\O stretching intensity with immersion time. Such oxidation occurs in solution due to the water soluble nature of the SiySiHx surface groups which produces a hydrophilic surface [87]. A reduction in SiH peak intensity has also been observed with immersion time and complete removal results after approximately 100 h [88]. Oxidation of pSi in water vapor is studied more frequently and proceeds via the formation of OySiH and SiOH species, as with other oxidation methods. In the presence of water the Si\Si bond is broken, resulting in the formation of SiH and SiOH bonds (Fig. 7). The pSi structure then becomes strained, resulting in the backbond oxidation due to oxygen attack. O3SiH species dominate the surface due to greater stability over O2SiH and OSiH species [89,90].

Fig. 7. Aqueous oxidation steps of pSi. (a) Initial attack of Si\Si bonds by water, (b) backbond oxidation by oxygen, and (c) oxidation to O3SiH. Reprinted with permission from [90]. Copyright 2008 American Chemical Society.

30

K.L. Jarvis et al. / Advances in Colloid and Interface Science 175 (2012) 25–38

Oxidation in humid air initially results in the reduction in the concentrations of SiySiH species while OySiH [86,89,91,92] and Si(OH)x [92] species form. pSi surface chemistry is highly dependent on oxidation conditions with a surface still remaining SiySiHx terminated after exposure to humid air for 1200 h [86]. Exposure to humid air can however produce O3SiH [91] or SiOSi/Si(OH)x [92] terminated surfaces after only 260 and 170 h respectively. A humid atmosphere can also be combined with high temperatures to produce a significant oxide layer. Oxidation for 30 s in a humid atmosphere at 300–500 °C and 800 °C produces O3SiH and Si\OH terminated surfaces respectively [93]. Oxidation in humid air at 500 °C for 1 h removed all SiHx species from the surface while dry thermal oxidation at 500 °C leaves the surface O3SiH terminated [94]. Therefore thermal oxidation in humid air generates a significantly higher oxidation rate than dry thermal oxidation alone.

, hv, EtAlCl 2 Pt, Pd, Rh

3.3. Thermal carbonization of porous silicon Salonen et al. have developed thermal carbonization as an alternative method of modifying pSi surface chemistry [27,28,95–104]. Modification is achieved via the thermal decomposition of acetylene which produces a stable and chemically inert surface. Thermal carbonization shows advantages over other functionalization methods as it improves thermal and electrical conductivity, chemical stability and oxidative resistance [95]. Thermal carbonization is a more efficient stabilization process than thermal oxidation with thermal carbonization producing a pSi surface that is stable in humid atmospheres and harsh chemical environments [97]. In this process pSi is carbonized via an initial acetylene flush followed by thermal treatment [27,28]. The surface groups produced via thermal carbonization are dependent on temperature. Treatment at 520 °C produces a surface possessing Si\H, Si\C and C\H bonds. At 650 °C the Si\H bonds are removed with Si\C and C\H bonds remaining. Surfaces treated at 730 °C and above retain only Si\C bonds [96]. The hydrophilicity of thermal carbonized pSi can be modified according to treatment temperature, making it a desirable material for humidity sensing applications. pSi carbonized below 650 °C with continuous acetylene flow remained hydrophobic while treatment above 700 °C without acetylene flow produced hydrophilic pSi surfaces [99]. Thermal carbonization modifies pSi surface chemistry without significant reduction in surface area. Decreases of only 15–25% were observed [27,96]. 3.4. Porous silicon hydrosilylation Hydrosilylation of pSi is carried out using heat, light, metal or Lewis acid catalysts, via the reaction of surface SiySiHx groups with an unsaturated compound possessing C_C, C`C or C`O functionalities. These reactions break the Si\H bond and form a Si\C\C, Si\C_C or Si\C\O bond with alkenes, alkynes and aldehydes respectively. Functionalization has been used to increase pSi stability [27] and to attach desired species for use in applications such as; biosensors [52], gas and humidity sensors [105] and fouling prevention [106]. By developing selective surface modification methods pSi can meet the needs of specific applications and can become increasingly useful in a wide range of fields. Thermal hydrosilylation involves the immersion of hydride terminated pSi into dilute solutions of alkene or alkynes and refluxing for varying periods of time. Hydrosilylation attaches alkenes [105,107–112], alkynes [108] and aldehydes [110,111], as summarized in Fig. 8. Alkenes can also be attached with specific functional groups on the opposite end of the chain to produce a pSi surface terminated with carboxylic acid [112–114], alkene [110] and ester [110,115] functional groups. An alkyne has been used to produce a fulleropyrrolidine functionalized pSi surface [116] while an alkene has produced a spiropyran coated surface [117]. Although thermal hydrosilylation is conventionally carried out on hydride terminated

, hv, EtAlCl 2 Pt, Pd, Rh Fig. 8. Summary of hydrosilylation reactions with porous silicon.

pSi, functionalization is also successful on partially oxidized pSi surfaces forming both Si\C and Si\O\C bonds [118]. Initial electrochemical oxidation in the presence of sulfuric acid has resulted in partial oxidation with both SiySiHx and OySiH species present. Subsequent thermal hydrosilylation by alkenes [109,114,118,119] and aldehydes [118] then produced a mixture of oxidized and functionalized surface species. Light induced hydrosilylation has been carried out by immersing pSi in a neat alkene [120,121] or alkyne [121–123] at room temperature and illuminating with a tungsten or mercury lamp. Such hydrosilylation prevents pSi oxidation [120] and can be used to photopattern a surface by masking sections from light [121]. This process can also be applied to attach larger species to the surface such as cyclodextrins [124], providing they have a terminal alkene or alkyne group. A number of functional groups have been added to pSi surface via click chemistry. Initially the surfaces were functionalized using 1,6 heptadiyne [123] or 1,8 nonadiyne [125,126]. A Huisgen 1,3-dipolar cycloaddition was then carried out between the alkyne terminated pSi surface and the desired azide terminated molecule. Click chemistry has been used to attach PEG [123], alcohol l [125,126], methyl [125,126], bromine [125,126] and amine [125] functional groups to the pSi surface. Hydrosilylation in the presence of metals enables a wide variety of functionalities to be incorporated onto the pSi surface that are not possible by other functionalization methods. Late transition metal complexes containing rhodium and platinum are effective hydrosilylation catalysts. Rhodium catalyzed hydrosilylation is performed via the addition of Rh2(OAc)4 and ethyldiazoacetate. These two species initially react together to form a metallocarbene intermediate, which in turn reacts with the hydride terminated pSi surface [127]. Rhodium in the form of Wilkinson's catalyst (RhCl(PPh3)3) is also effective in the hydrosilylation of alkenes and alkynes and results in the attachment of carbon chains to the hydride terminate surface [128]. Platinum (Karstedt's catalyst) can be applied to catalyze hydrosilylation with alkenes, which also results in the addition of a carbon chain to the surface [127]. The use of platinum and rhodium catalysts however carries a risk of contamination which would be undesirable and potentially hazardous for biological applications. Hydrosilylation using Lewis acids can functionalize the pSi surface to introduce a variety of surface functionalities. Such hydrosilylation is carried out via the addition of the Lewis acid (EtAlCl2) to a solution containing pSi and an alkene or alkyne [25,26,122,128–130]. In comparison to other hydrosilylation reactions, this method is slower, requiring at least 12 h for the reaction to occur [26]. pSi surfaces

K.L. Jarvis et al. / Advances in Colloid and Interface Science 175 (2012) 25–38

4.1. Dissolution of porous silicon The biocompatibility of porous silicon is one of the characteristics of pSi that renders it most suitable for drug delivery and other biological applications. Its biocompatibility has been demonstrated by its ability to interface with cells [9,106,133–139], improved cell adhesion over crystalline silicon [9], non-toxicity [140] and reducing foreign body reactions over planar silicon, and porous and planar titanium [141,142]. pSi has also been shown to be biodegradable which results in dissolution in biological media [143,144] to nontoxic silicic acid [32,106]. When pSi is exposed to physiological conditions, the native SiHx terminated surface undergoes oxidation and subsequently dissolves into non-toxic orthosilicic acid (Si(OH)4). Orthosilicic acid is the soluble form of silicon and the major form of silicon in the human body which is required in both bone [139] and collagen [143] growth. Orthosilicic acid has also been shown to stimulate calcification and produce a negligible inflammatory tissue response [8]. High levels of orthosilicic acid can be toxic [139], however the urine excretion of orthosilicic acid from the human body is efficient at expelling all ingested silicon [32]. The dissolution of porous silicon in aqueous solution can be simplified to the following equations: Si þ 2H2 O→SiO2 þ 2H2

ð1Þ

SiO2 þ 2H2 O→SiðOHÞ4 :

ð2Þ

Hydrogen is generated in the oxidation step while the final product is orthosilicic acid [11]. Dissolution of pSi decreases the pH of the medium due to orthosilicic acid formation and the liberation of hydrogen from the pore walls [145]. The modification of pSi surface chemistry by thermal oxidation has a significant effect on the dissolution rate. The dissolution of pSi is attributed to the water soluble SiySiHx groups on the surface therefore removal of these species reduces dissolution. The dissolution of anodically oxidized pSi was investigated with oxidation potential identified as the limiting factor. Native pSi has shown 98% dissolution in simulated body fluid after 30 days while oxidation potentials of 1 and 4 V which partially oxidize the surface resulted in 80% and 25% dissolution respectively. Oxidation at 10 V which fully oxidizes the pSi surface prevented any dissolution [144]. Comparisons between the effect of thermal hydrosilylation and ozone oxidation on pSi dissolution have also been investigated. Hydrosilylated surfaces produce more stable surfaces than ozone oxidation which resists dissolution in the first 2 h. The formation of Si\C bonds during

100

% Silicon dissolved

4. Impact of surface chemistry modification on factors affecting drug loading and encapsulation

hydrosilylation prevents dissolution by protecting the pSi against hydrolytic attack [146]. Surface modification via PEGylation also influences dissolution where the dissolution rate decreased for larger molecular weight PEG's [147]. In vitro pSi dissolution is significantly reduced by thermal oxidation, with only approximately 3% of thermally oxidized pSi particles with a particle diameter of ~ 50 μm dissolved after 7 days at pH 6 (Fig. 9a). Passivation of the pSi surface by thermal oxidation appears to significantly impair dissolution [148]. Dissolution of unoxidized and oxidized pSi is accelerated in pH 9 buffer (Fig. 9b). The most significant pH effects are observed for thermally oxidized pSi particles. At pH 6, only slight dissolution of 400 °C oxidized pSi was observed, while at pH 9 complete dissolution occurred after 7 days. Enhanced dissolution of 800 °C oxidized pSi particles was also observed at pH 9 with 35% dissolution in 7 days [85]. Particle size appears to accelerate the dissolution of both native and thermally oxidized pSi particles. Native pSi nanoparticles with a particle diameter of 126 nm almost completely dissolve in 4 h in phosphate buffered saline [149] while 164 nm diameter 200 °C thermally oxidized nanoparticles exhibit 80% dissolution at pH 6.5 after 6 days [150]. The dissolution of unoxidized and thermally oxidized pSi particles is both particle size and pH dependent, which will have an impact on the use of pSi as a drug delivery vehicle. Particle size and surface chemistry can be used to tailor pSi dissolution rates. In low pH environments (e.g. the stomach), pSi has reduced dissolution, which protects the loaded molecule against degradation and release, while at higher pH (i.e. intestines), pSi dissolution will occur thereby releasing the loaded molecule. The in vivo dissolution of native and surface modified pSi has been demonstrated via intraocular injection. Native pSi particles had a vitreous half-life of 1 week and a maximum residence time of

a unoxidized

80

60

40

20

400°C & 800°C 0 0

1

2

3

4

5

6

7

5

6

7

Time (days) 100

b % Silicon dissolved

functionalized by Lewis acid mediated hydrosilylation show strong resistance to atmospheric oxidation [26], KOH [25] and simulated human blood plasma [131]. Organohalides functionalize pSi by the cleavage of Si\H bonds to form Si\C bonds, which significantly improves the stability of the surface. Functionalization is achieved via the reductive electrolysis of organohalides. pSi is immersed in a solution containing the organohalide (RX, X_I or Br) with a cathodic current passed through the solution. The reactions occur via either a direct reaction between the silicon radical and the alkyl radical or the reduction of the silicon radical to an anion followed by nucleophilic attack of the organohalide [132]. In general, hydrosilylation only functionalizes 20–80% of the Si\H bonds, making the surface more stable however the remaining Si\H bonds are still vulnerable to attack. Organohalide functionalization has been followed by methylation using CH3I to react with any remaining Si\H species to form a completely Si\C terminated surface [17].

31

unoxidized

80

400°C 60

40

20

800°C 0 0

1

2

3

4

Time (days) Fig. 9. Dissolution profiles of unoxidized, 400 °C and 800 °C thermally oxidized porous silicon particles in a. pH 6 and 10−3 M NaCl and b. pH 9 sodium tetraborate buffer [85].

32

K.L. Jarvis et al. / Advances in Colloid and Interface Science 175 (2012) 25–38

4 weeks. Thermal oxidation at 220 °C increased the half-life to 5 weeks and the maximum residence time to 12–18 weeks. Thermal hydrosilylation with 1-dodecene extended the half-life further to 16 weeks. High temperature thermal oxidation (800 °C) produces pSi particles with a half-life of 8 weeks and the greatest maximum residence time of 10–12 months [151]. Surface modification can be used to easily tune the dissolution of a pSi sample in solution. Faster pSi dissolution can be used to produce sustained drug release by releasing the drug from the pores during dissolution while slower pSi dissolution can be used to enhance drug release via diffusion from the pores while the pSi remains intact. 4.2. Surface wettability of porous silicon Modification of pSi surface chemistry can be used to influence its wettability. The pSi surface is highly hydrophobic [88,152] due to its SiySiHx termination, making it difficult for use in aqueous applications with contact angles of up to 130° observed. The wettability of pSi impacts upon its use in drug delivery since aqueous immersion is generally required. Successful immersion of pSi particles in aqueous solutions can be achieved by either pre-wetting or oxidation. Prewetting with a solvent reduces the surface tension and allows water molecules to penetrate the porous structure [88]. This then induces aqueous oxidation causing the pSi to become more hydrophilic. Thermal oxidation can be applied to increase wettability, with water adsorption onto oxidized pSi increasing with oxidation temperature [152]. Oxidized pSi is hydrophilic as the SiO2 — air interfacial energy is larger than the sum of the water — air and SiO2 — water interfacial energies. Contact angle measurements have also shown that a significant change in wettability can be produced by increasing the oxidation temperature, shown in Fig. 10. Unoxidized pSi wafers are hydrophobic and generally have a contact angle of ~ 115° [85,116,153]. Oxidation at 200 °C slightly reduces the contact angle while oxidation at 400 °C decreases the contact angle to 19° which was attributed to the removal of SiySiHx species from the surface and the formation of more hydrophilic SiOH species [85]. Thermal carbonization has also been used to modify pSi wettability, where treatment temperatures of 600–820 °C with no acetylene flushed produced hydrophilic pSi surfaces with contact angles of 20– 35° [99]. Thermal carbonization at lower temperatures with a continuous acetylene flush [28] produced particles that were hydrophobic due to retained hydride groups on the surface. These particles proved difficult to use in drug dissolution studies due to their minimal wetting properties [18]. Modification of pSi wettability will have important implications on drug encapsulation and loading by influencing surface energetics and molecular interactions. Increased wettability of pSi surfaces enables drug loading to be

undertaken in an aqueous medium, which is advantageous for biological applications. 4.3. Zeta potential of porous silicon Zeta potential is a useful method for monitoring particle surface chemistry at solid–liquid interfaces. Surface modification by thermal oxidation will have an impact on the zeta potential of pSi particles. Zeta potential indicates the overall charge of the pSi surface and will therefore influence drug loading. The zeta potentials of unoxidized and thermally oxidized pSi particles were monitored over a range of pH values (Fig. 11). The unoxidized pSi surface is hydride terminated and is therefore expected to have no zeta potential due to the absence of charge determining SiOH species. Unoxidized pSi surfaces however have a negative zeta potential at pH > 4, indicating the presence of SiO − species. Atmospheric oxidation [20,68] and aqueous oxidation are accountable for the majority of Si\OH species since water accelerates the pSi oxidation process [6,89,154]. Differences in the zeta potentials of unoxidized and oxidized pSi indicated the formation of SiOH species by oxidation. SiOH dissociates in water to either Si\O − or Si\OH2+ depending on the pH of the solution. For oxidized pSi at pH b 4 the surface had a positive zeta potential, while at pH > 4 the pSi surface had a negative zeta potential [90] due to the deprotonation of Si\OH [155]. Zeta potential can influence drug loading, especially from solution. Thermal oxidation produces a more negatively charged pSi surface and can therefore be used to load positively charged drugs via electrostatic attraction [149]. The negative charge of unoxidized and oxidized pSi surfaces will however hinder the loading of negatively charged drugs. Surface modification however can be utilized to overcome zeta potential issue by either altering the zeta potential of the pSi surface with positively charged functionalities [147,156] or to attach specific functionalities for covalent drug binding [157,158]. 5. Drug interactions, loading and delivery 5.1. Rationale for using porous silicon for drug delivery For pSi to be utilized in drug delivery, the porous network is loaded with a drug to be released into the body by pSi dissolution [12] or pore diffusion. The loading of molecules into pSi can be carried out via a number of methods including physical adsorption, solvent evaporation, covalent attachment or drug entrapment by oxidation [33]. The major focus of pSi in respect to drug delivery has been on controlled drug release and increasing the oral bioavailability of 10

120 0 2

80

ζ (mV)

Contact angle (°)

100

60

4

6

8

-10

-20

unox

200°C

40

400°C -30

20

600°C 800°C

0 0

200

400

600

800

-40

pH

Oxidation temperature (°C) Fig. 10. Static contact angle as a function of oxidation temperature for porous silicon wafers [85].

Fig. 11. Zeta potentials of porous silicon particles as a function of pH at oxidation temperatures of 200–800 °C. Reprinted with permission from [90]. Copyright 2008 American Chemical Society.

K.L. Jarvis et al. / Advances in Colloid and Interface Science 175 (2012) 25–38

poorly soluble drugs. The manufacture of implantables such as microneedles [159–161] has also been investigated. These microneedles have been manufactured with pSi tips which can be loaded with drugs and then delivered transdermally. Implantation of pSi particles into the eye has been investigated for dissolution [151] and biocompatibility [148] and demonstrates potential as a future intraocular drug delivery vehicle. Poor drug oral bioavailability is due to poor dissolution and gastrointestinal permeation [162]. pSi can enhance bioavailability of poorly soluble drugs by improving the solubility of the drug and has also shown to enhance in vitro drug permeation [15,162]. pSi has the potential to increase the solubility of large hydrophobic drugs by pore confinement. When the pores of pSi are only a few times larger than that of the drug molecule; the drug is unable to revert back to its crystalline phase due to the confined space. The drug therefore remains in a non-crystalline, amorphous form which is known to display higher dissolution rates than drugs in a crystalline phase [10]. Depending on pSi surface chemistry and pore size either increased dissolution rate or sustained release can be achieved [34]. Surface modification can be used to inhibit pSi dissolution which in turn enhances drug release via pore diffusion, especially for poorly soluble drugs. The modification of pSi surface chemistry has important implications on loading capacity by controlling the interactions of molecules with the pSi surface. Drug release from pSi can be controlled by varying the surface chemistry and/or pore sizes [18] to produce favorable surfaces for drug adsorption [10].

5.2. Drug delivery from native porous silicon Protein [15,83,163] and small molecule drug loading [12] and release from native pSi have been investigated and demonstrates the potential of pSi as a drug delivery vehicle. The delivery of proteins using pSi was successfully demonstrated by Foraker et al. using insulin loaded pSi particles. Insulin and a permeation enhancer were loaded via capillary action into pSi particles to investigate the permeation across a Caco-2 cell layer. A sodium laurate permeation enhancer increased transportation across the Caco-2 cell layer nearly tenfold with the transportation of insulin also enhanced via loading into pSi particles [15]. The anti-cancer drug doxorubicin has been loaded into a pSi matrix by Vaccari et al. and the cytotoxicity of doxorubicin loaded pSi investigated via exposure to human colon adenocarcinoma cells. Doxorubicin loaded pSi demonstrated a controlled release profile (Fig. 12) which attained its maximum

Fig. 12. Doxorubicin release profile (●) and cytotoxicity (■) of doxorubicin loaded porous silicon. Reprinted with permission from [12]. Copyright 2006 Elsevier.

33

after 4 h. Significant cell death was observed with cell numbers decreasing from approximately 2000 to 600 in 6 h [12]. 5.3. Drug interactions with porous silicon The native pSi surface is hydride terminated and highly reactive, which has shown to result in chemical interactions with a number of drugs. The hydride termination acts as a reducing agent, thus potential redox reactions with drugs need to be considered. Wu et al. [16] have suggested that anthracycline drugs such as doxorubicin and daunorubicin likely undergo a redox reaction with hydride terminated pSi. To investigate this hypothesis, 3 sets of carboxylic acid terminated pSi surfaces were produced prior to daunorubicin attachment. Hydrosilylation with neat undecylenic acid was carried out which produced a surface with a significant quantity of hydride groups remaining. Mild oxidation at 150 °C following hydrosilylation was also conducted to remove any remaining hydride groups. pSi surfaces were also produced via high temperature oxidation at 800 °C followed by silanization to produce a carboxylic acid terminated SiO2 substrate. Daunorubicin was attached to these three types of surface and then placed into phosphate buffered saline for 24 h to initiate release to determine the structure of the released drug, which is summarized in Fig. 13. Hydrosilylated pSi did not release unmodified daunorubicin, only the degraded form. For hydrosilylated pSi oxidized at 150 °C, less degraded daunorubicin was observed in addition to unmodified daunorubicin. Silanized 800 °C oxidized pSi released only unmodified daunorubicin. This study demonstrates the importance of pSi surface modification on drug interactions, release and delivery which has been shown to play a role in the structure and therefore activity of released drugs. Similar behavior has also been observed for protein loading onto unoxidized and thermally oxidized pSi. The activity of lysozyme and papain proteins was significantly reduced upon loading into unoxidized pSi, in contrast for pSi thermally oxidized at both 400 °C and 800 °C the activities of the proteins were retained. Thermal oxidation retained protein activity due to the removal of reactive surface hydride groups and conversion to a hydrophilic surface [81]. 5.4. Drug loading and release from surface modified porous silicon Surface modified pSi is more frequently utilized than native pSi for drug delivery due to its enhanced stability. Several different surface modification methods have been investigated for the release of a number of different drugs, as highlighted in Table 1. Thermal oxidation is a simple stabilization method that also changes the pSi surface from hydrophobic to hydrophilic, which is also advantageous for drug delivery under physiological conditions [150]. Stabilization via thermal oxidation has been used for the loading and release of griseofulvin [150], indomethacin [14] and intraconazole [164]. Thermal carbonization [164] and thermal hydrocarbonization [165,166] have also been utilized. Apart from simple stabilization, modification of pSi surface chemistry can also be utilized to introduce desired functionalities to the surface. Hydrosilylation has been used to introduce carboxylic acid functionalities to the pSi surface to allow for the covalent attachment of anticancer drugs e.g. doxorubicin [16,157] and daunorubicin [16,158]. Thermally responsive poly(N-isopropylacrylamide) has also been grafted on pSi to manipulate camptothecin release via temperature modification [167]. Surface modification has also been used to conjugate a E-selectin thioaptamer ligand by initial oxidation and 3-aminopropyltriethoxysilane functionalization [168]. Modification of pSi surface chemistry has been investigated as a method to manipulate the loading and release of a number of drugs [10,13,18,19,157,162,169–172]. Extensive work has been carried out by Salonen et al. on the modification of pSi by thermal annealing, oxidation and carbonization for drug loading and release. Poorly

34

K.L. Jarvis et al. / Advances in Colloid and Interface Science 175 (2012) 25–38

Si O

Less degraded daunorubicin Unmodified danunoribicin

Si COOH O Si

150°C

Si

H

Si COOH Si

Degraded daunorubicin No unmodified danunoribicin

H

undecanoic acid Si

H

Si

H

Si

H

800°C silanization O O

Si COOH O

No degraded daunorubicin Only unmodified danunoribicin

Fig. 13. Summary of danuorubicin release from hydrosilylated, hydrosilylated/oxidized and oxidized/silanized porous silicon particles.

soluble drugs ibuprofen, griseofulvin and furosemide and soluble drugs antipyrine and ranitidine have been investigated. These drugs represent a wide range of drug properties; solubility, acid/base character and lipophilicity. Ibuprofen, antipyrine and ranitidine were loaded into unoxidized, thermally oxidized and thermally carbonized pSi to investigate the effect of loading on drug crystallinity. Ibuprofen could easily penetrate the pores of unoxidized and thermally carbonized pSi but not thermally oxidized pSi. In contrast antipyrine could not be loaded into unoxidized particles. Ibuprofen was initially found in a crystalline form in the pores however upon drying at 65 °C, the ibuprofen on the surface did not melt while the drug in the pores did melt. Such behavior suggests that the ibuprofen in the pores is already melted while it remains in a crystalline form on the surface. Antipyrine was found in a crystalline form on thermally carbonized pSi but not thermally oxidized pSi. Loaded ranitidine was not found in the pores in its crystalline form [169]. Unoxidized, thermally oxidized and carbonized pSi have also been used to determine the effect of pSi surface chemistry on drug dissolution. Loading of drugs into mesoporous materials has the potentials to improve the permeability of large hydrophilic drugs and increasing the dissolution of poorly soluble drugs by decreasing crystallinity. When pores are only slightly larger than a drug, size constraints retain the drug in its noncrystalline, amorphous form. In general, more drug could be loaded into thermally carbonized than thermally oxidized pSi. For the highly soluble drug antipyrine, loading into thermally carbonized pSi decreased dissolution. Ibuprofen, which has low aqueous solubility at acidic pH, demonstrated significantly faster dissolution upon pSi loading at pH 5.5. Similar dissolution behavior was observed for

poorly soluble griseofulvin where greater dissolution was observed upon pSi loading than unloaded griseofulvin at all pH values. Ranitidine behaved in a similar manner to antipyrine with pSi loading reducing dissolution; however the difference in dissolution between loaded and unloaded rantidine was greater. Drug dissolution was affected as surface modification induced delayed release for drugs with high dissolution rates and improved dissolution for poorly dissolving drugs, as shown in Fig. 14 for ranitidine and furosemide. Such behavior suggests that it may be necessary in the future to treat the surface of the pSi to suit a specific drug [10]. Native, thermally oxidized, electrochemically methylated (Si\CH3 terminated) and undecylenic acid (Si\C10H20COOH terminated) functionalized pSi have been loaded with the antibiotic vancomycin via spin coating. All four pSi samples demonstrated burst release, resulting in virtually complete release within the first 15 min for thermally oxidized and methylated pSi. Native and undecylenic acid functionalized pSi exhibited a more sustained release rate due to their amphiphilic nature which increased the binding affinity of vancomycin. The pH triggered release of vancomycin was also investigated so that oral delivery would result in release within the intestines, where the pH is higher, as vancomycin is degraded by stomach acids. Vancomycin loaded native pSi was coated with a BSA capping layer, as BSA has a higher solubility at pH 7.4 than pH 4. At pH 4 minimal vancomycin was released whereas once the pH was adjusted to pH 7.4, 90% release was observed in the first 90 min [19]. The steroid dexamethasone has been loaded into native pSi and pSi functionalized via hydrosilylation with dodecene. Dexamethasone loading into functionalized pSi was initially unsuccessful due to steric crowding of

K.L. Jarvis et al. / Advances in Colloid and Interface Science 175 (2012) 25–38

35

Table 1 Summary of drug delivery studies using surface modified porous silicon. No.

Surface modification method

Drug

Result

Ref.

1

Thermal carbonization Oxidation @ 300 °C

Controlled release for highly soluble drugs Improved dissolution for poorly soluble drugs

[10]

2

Thermal carbonization

Reduced drug crystallinity

[169]

3

Oxidation @ 800 °C Electrochemical methylation Hydrosilylation Bovine serum albumin capping Hydrosilylation Thermal carbonization Oxidation @ 300 °C pNIPAM grafting Thermal carbonization Thermal hydrocarbonization Oxidation @ 300 °C Thermal carbonization Oxidation @ 300 °C Oxidation @ 600 °C

Ibuprofen Griseofulvin Furosemide Antipyrine Ranitidine Ibuprofen Antipyrine Ranitidine Vancomycin

Controlled release pH triggered release

[19]

Dexamethasone Furosemide Griseofulvin Camptothecin Ibuprofen

Controlled release Reduction in pH dependent solubility Increased Caco-2 cell layer permeation Temperature controlled release Increased dissolution from smaller pores

[13] [162] [150] [167] [18]

Ibuprofen

Reduced drug crystallinity for thermal carbonization

[171]

Indomethacin

[14]

Intraconazole

Reduced drug crystallinity Higher in-vivo blood concentration Faster release from thermally carbonized

Cisplatin Cisplatin Doxorubicin Danorubicin Danorubicin

Increased release from higher loading conc. Increased toxicity to cancer cells Controlled release for covalent attachment Controlled release for covalent attachment Reduction in daunorubicin degradation

[172] [173] [157] [158] [16]

Lysozyme Papain Melanotan II Ghrelin antagonist Peptide YY3-36

Improved protein activity

[81]

Sustained release Sustained release Sustained release

[165] [166] [174]

4 5 6 7 8

9 10 11 12 13 14 15 16

17 18 19 20

Thermal carbonization Oxidation @ 300 °C Calcium phosphate growth Hydrosilylation Hydrosilylation Hydrosilylation Hydrosilylation Oxidation @ 150 °C Oxidation @ 800 °C + silanization Oxidation @ 400 °C Oxidation @ 800 °C Thermal hydrocarbonization Thermal hydrocarbonization Hydrosilylation Oxidation @ 300 °C Thermal hydrocarbonization

the drug and the added functional groups. To eliminate this effect, pore expansion by DMSO oxidation and subsequent HF was conducted prior to functionalization. Linear release was observed for both materials in the first 2 h, with complete dexamethasone release from native pSi in this time due to pSi dissolution in solution. Functionalization of pSi produced controlled release with continuous release observed over 3 days [13]. The release and in vitro permeation of furosemide across a Caco-2 cell layer as a function of pH has been studied. Furosemide was loaded into thermally carbonized pSi particles to produce a highly wettable and stable surface. Loading of furosemide resulted in the formation of non-crystalline analogues. The solubility of furosemide is normally pH dependent, however this was significantly reduced by loading into thermally carbonized pSi particles with 80% of the loaded furosemide released after 80 min. Furosemide dissolution was more rapid upon pSi loading and also increased the Caco-2-cell layer permeation at all pH values with the highest permeation observed at pH 6.8 [162]. Thermally oxidized pSi was also reported to improve the permeation of griseofulvin across a Caco-2 cell layer. In comparison to bulk griseofulvin, loading into thermally oxidized pSi increases drug permeation two fold at pH 5.5 and four fold at pH 7.4 [150]. The modification of pSi surface chemistry has shown to have a significant effect on drug loading and release, however pore size also plays a role. Prior to thermal oxidation and carbonization, pSi has been annealed to increase the pore size from 8–15 nm to 40–47 nm. Treatment at 820 °C under N2 increased the pore size by pore coalescence. Ibuprofen was loaded in the pSi with loading levels of approximately 33% w/w for all pSi samples except the annealed thermally carbonized pSi which was slightly higher at 41%. Both

[164]

surface chemistry and pore size influenced ibuprofen loading and release. The highest dissolution was from small pore thermally carbonized pSi. The increased dissolution was attributed to pore confinement of ibuprofen, where it resided in its amorphous rather than crystalline form which has a faster dissolution rate. In contrast, annealed thermally oxidized pSi had faster release than its smaller pore version, indicating that pore size also affects drug dissolution in addition to surface chemistry [18]. Native pSi has also been annealed at temperatures of 550–800 °C to produce a range of pore diameters and volume. The surface chemistry was then modified via thermal carbonization or thermal oxidation. The samples with the largest pore volumes, irrespective of surface chemistry resulted in the largest ibuprofen loading. A relationship between crystallinity and pore size was once again observed, where the greatest ibuprofen crystallinity resulted from the largest pore diameters. Surface chemistry however also had an effect on ibuprofen crystallinity which was attributed to surface charge. Thermally carbonized pSi had the strongest negative charge which resulted in the strongest interaction with ibuprofen and therefore the lowest crystallinity. Unoxidized pSi had the weakest negative charged which resulted in the greatest crystallinity due to the weakest ibuprofen–pSi interaction [171]. Thermally oxidized pSi has been used to produce a stable pSi surface for indomethacin loading and release. The loading of indomethacin into pSi thermally oxidized at 600 °C resulted in the molecule remaining in its amorphous form due to pore confinement. Upon oral administration pSi loaded indomethacin demonstrated a higher in vivo blood concentration and a shorter time to reached maximum concentration in comparison to unloaded indomethacin and a commercial available product, as shown in Fig. 15. A linear

36

K.L. Jarvis et al. / Advances in Colloid and Interface Science 175 (2012) 25–38

Intraconazole release was slightly faster from thermally carbonized pSi with 95% release in 10 min while 20 min was required for the same level of release from thermally oxidized pSi [164]. 5.5. Delivering chemotherapy drugs

Fig. 14. Drug dissolution profiles of a. rantidine and b. furosemide unloaded (○) and loaded (●) into thermally carbonized porous silicon particles. Reprinted with permission from [10]. Copyright 2005 Elsevier.

correlation between in vitro indomethacin dissolution and in vivo indomethacin absorption for both unloaded and pSi loaded indomethacin was observed, demonstrating that in vivo absorption can be predicted from in vitro dissolution [14]. Loading and release of intraconazole for thermally carbonized and thermally oxidized pSi has been investigated. Intraconazole is hydrophobic and poorly soluble and was loaded in its amorphous form. In comparison to intraconazole alone, loading into pSi resulted in significantly faster release as the drug was encapsulated in its amorphous form.

Fig. 15. Rat plasma concentration profiles of indomethacin (Δ), commercially available indomethacin (□) and pSi loaded indomethacin (◊). Reprinted with permission from [14]. Copyright 2010 American Chemical Society.

Chemotherapy drugs cisplatin and doxorubicin have also been loaded into surface modified pSi. The loading of cisplatin into pSi has been carried out by applying a cathodic bias and a cisplatin/simulated body fluid electrolyte. A cisplatin loading concentration of 3 mM results in a significant increase in the amount of cisplatin loaded into and released from the structure in comparison to 1 mM cisplatin. The amount of cisplatin released reached equilibrium after 10 h, with this concentration maintained for up to 70 h [172]. Cisplatin has been loaded into dodecene and undecylenic acid hydrosilylated pSi microparticles via trapping with metallic platinum. Dodecene modified pSi did not demonstrate significant cisplatin loading due to its hydrophobic surface. Undecylenic acid modification and cisplatin loading resulted in high toxicity towards human ovarian cancer cells with a significantly higher toxicity than free cisplatin [173]. Doxorubicin release from undecylenic acid functionalized pSi has also been investigated. Doxorubicin was covalently attached to the functionalized pSi which only enabled drug release once the covalent bond is broken or the pSi is oxidized/degraded. The release of physically adsorbed and covalently attached doxorubicin was compared. Release profiles demonstrated significant burst release in the first 2 h and complete release within 24 h for physically adsorbed doxorubicin. Covalently attached doxorubicin however did not exhibit initial burst release, but continuous slow release over 5 days. This study demonstrated that by minimizing physical drug adsorption into pSi, longer term drug delivery can be achieved [157]. Covalent attachment of daunorubicin has been achieved via initial functionalization with undecylenic acid. Daunorubicin was covalently attached to the functionalized pSi surface via coupling between the carboxylic acid group on the pSi surface and the pendant amino group of daunorubicin which resulted in sustained release with 88% released over 30 days [158]. 5.6. Protein loading, release and delivery The encapsulation and delivery of proteins via pSi is an emerging field as the use of protein therapeutics has increased recently. Introducing proteins into the body is a challenge as oral delivery results in enzymatic degradation and proteins have poor intestinal penetration which result in low oral bioavailability, therefore proteins are conventionally delivered by injection. Successful oral delivery of proteins also requires preservation of their structure, which is linked to bioactivity. Incorporation of proteins into a carrier system such as pSi can be used to enhance delivery and maintain bioactivity. Lysozyme, papain and human serum albumin (HSA) were loaded into unoxidized and thermally oxidized pSi particles. Significantly greater masses of all 3 proteins could be loaded into unoxidized than thermally oxidized pSi. Reduction is protein loading upon thermally oxidation was attributed to the hydrophilicity of the surface and reductions in pore penetration of the proteins. Loading of the proteins into unoxidized pSi resulted in changes in the structure of all proteins, due to the hydrophobicity and hydride termination of the pSi surface. Thermal oxidation at 800 °C retained the protein structure of lysozyme and papain but not HSA. Lysozyme and papain structures were retained due to increased hydrophilicity and removal of reactive hydride groups. HSA structure was not retained due to its globular nature, which resulted in structural changes upon loading irrespective of surface properties [81]. Peptides ghrelin antagonist (GhA) [166] and melanotan II (MTII) [165] have been loaded and released from thermally hydrocarbonized pSi particles and delivered orally. GhA inhibits food intake, with both GhA alone and pSi loaded

K.L. Jarvis et al. / Advances in Colloid and Interface Science 175 (2012) 25–38

GhA inhibiting food intake in mice over the first hour. Inhibition of food intake by GhA alone diminished after 4 h while pSi loaded GhA maintained food inhibition for up to 18 h after administration. MTII increases heart rate and reduces fluid intake. MTII loaded pSi maintained an increased heart rate in mice for 14 h while MTII alone increased the heart rate for only 7 h. MTII also reduced water intake in mice, with water intake lower for MTII loaded pSi than MTII alone for 150–300 min after administration. Peptide YY3-36, which has been shown to reduce food intake, has been loaded into thermally oxidized, thermally hydrocarbonized, and undecylenic acid functionalized thermally hydrocarbonized pSi. Loading into all three samples had similar loading levels of 12. 2–16% w/w. Peptide YY3-36 demonstrated sustained but incomplete release with maximum release from thermally oxidized pSi at 27% released after 14 days. In vivo studies showed that unloaded YY3-36 was cleared from the blood plasma within 12 h while for the three loaded pSi samples YY3-36 could be detected for up to 96 h [174]. 6. Conclusions and future directions pSi is a functional material for drug delivery due to its porous structure, biocompatibility and controllable dissolution behvaiour. The ease in which pSi surface chemistry can be modified further increases its usefulness as a drug delivery vehicle. Surface modification via oxidation, carbonization and hydrosilylation not only stabilizes the pSi surface but also introduces specific functional groups for the loading of drug molecules. Altering pSi surface chemistry manipulates its dissolution, wettability and surface charge, which in turn controls drug–pSi interactions and therefore also drug loading and release. Drug loading and release from surface modified pSi has demonstrated many advantages over both native pSi and unloaded drugs. Potential pSi–drug interactions have been alleviated via surface modification due to the removal of highly reactive surface hydride groups. Loading drugs into surface modified pSi increased loading levels in comparison to native pSi. Surface modification also decreased drug crystallinity, thus increasing its dissolution. Drug dissolution behavior has been controlled using surface modified pSi which increased the dissolution of poorly soluble drugs and slowed the dissolution of highly soluble drugs. As we develop pSi further for use in drug encapsulation and delivery we will only see an increase in the variety of functionalities that can be attached to the pSi surface. The use of characterization techniques such as ToF-SIMS will increase and will examine not only surface functional groups but also the distribution of loaded drugs throughout the porous network. It is expected that further in vivo studies of drug loaded surface modified pSi will be investigated, especially for challenging deliverables such as proteins. In the future we should begin to see the culmination of the past research as commercially available surface modified pSi pharmaceutical products. Surface modification has demonstrated great promise in increasing the usefulness and functionality of pSi, by tailoring the surface properties to produce effective drug delivery systems. References [1] Uhlir A. Bell Labs Tech J 1956;35:333. [2] Sharma SN, Sharma RK, Bhagavannarayana G, Samanta SB, Sood KN, Lakshmikumar ST. Mater Lett 2006;60:1166. [3] Harraz FA, Sakka T, Ogata YH. Electrochim Acta 2002;47:1249. [4] Canham LT. Appl Phys Lett 1990;57:1046. [5] Cullis AG, Canham LT, Calcott PDJ. J Appl Phys 1997;82:909. [6] Bisi O, Ossicini S, Pavesi L. Surf Sci Rep 2000;38:1. [7] Worsfold O, Voelcker NH, Nishiya T. Langmuir 2006;22:7078. [8] Coffer JL, Whitehead MA, Nagesha DK, Mukherjee P, Akkaraju G, Totolici M, et al. Phys Status Solidi A 2005;202:1451. [9] Sapelkin AV, Bayliss SC, Unal B, Charalambou A. Biomaterials 2006;27:842. [10] Salonen J, Laitinen L, Kaukonen AM, Tuura J, Bjorkqvist M, Heikkila T, et al. J Control Release 2005;108:362.

37

[11] Prestidge CA, Barnes TJ, Lau C-H, Barnett C, Loni A, Canham L. Expert Opin Drug Deliv 2007;4:101. [12] Vaccari L, Canton D, Zaffaroni N, Villa R, Tormen M, di Fabrizio E. Microelectron Eng 2006;83:1598. [13] Anglin EJ, Schwartz MP, Ng VP, Perelman LA, Sailor MJ. Langmuir 2004;20:11264. [14] Wang F, Hui H, Barnes TJ, Barnett C, Prestidge CA. Mol Pharm 2010;7:227. [15] Foraker AB, Walczak RJ, Cohen MH, Boiarski TA, Grove CF, Swaan PW. Pharm Res 2003;20:110. [16] Wu EC, Andrew JS, Buyanin A, Kinsella JM, Sailor MJ. Chem Commun (Camb) 2011;47:5699. [17] Lees IN, Lin H, Canaria CA, Gurtner C, Sailor MJ, Miskelly GM. Langmuir 2003;19: 9812. [18] Limnell T, Riikonen J, Salonen J, Kaukonen AM, Laitinen L, Hirvonen J, et al. Int J Pharm 2007;343:141. [19] Perelman LA, Pacholski C, Li YY, VanNieuwenhze MS, Sailor MJ. Nanomedicine (London) 2008;3:31. [20] Salonen J, Lehto VP, Laine E. Appl Phys Lett 1997;70:637. [21] Pap AE, Kordas K, Toth G, Levoska J, Uusimaki A, Vahakangas J, et al. Appl Phys Lett 2005;86:041501. [22] Herino R, Perio A, Barla K, Bomchil G. Mater Lett 1984;2:519. [23] Hou XY, Shi G, Wang W, Zhang FL, Hao PH, Huang DM, et al. Appl Phys Lett 1993;62:1097. [24] Buriak JM. Adv Mater 1999;11:265. [25] Buriak JM, Allen MJ. J Am Chem Soc 1998;120:1339. [26] Buriak JM, Stewart MP, Geders TW, Allen MJ, Choi HC, Smith J, et al. J Am Chem Soc 1999;121:11491. [27] Salonen J, Lehto VP, Björkqvist M, Laine E, Niinistö L. Phys Status Solidi A 2000;182:123. [28] Salonen J, Bjorkqvist M, Laine E, Niinisto L. Appl Surf Sci 2004;225:389. [29] Stewart MP, Buriak JM. Adv Mater 2000;12:859. [30] Sailor MJ, Lee EJ. Adv Mater 1997;9:783. [31] Song JH, Sailor MJ. Comment Inorg Chem 1999;21:69. [32] Salonen J, Lehto V-P. Chem Eng J 2008;137:162 (Amsterdam, Neth.). [33] Anglin EJ, Cheng L, Freeman WR, Sailor MJ. Adv Drug Deliv Rev 2008;60:1266. [34] Salonen J, Kaukonen AM, Hirvonen J, Lehto V-P. J Pharm Sci 2008;97:632. [35] Santos HA, Bimbo LM, Lehto V-P, Airaksinen AJ, Salonen J, Hirvonen J. Curr Drug Discov Technol 2011;8:228. [36] Halimaoui A. In: Canham LT, editor. Properties of porous silicon. INSPEC, The institution of Electrical Engineers; 1997. Chapter 1.2. [37] Salonen J, Bjorkqvist M, Laine E, Niinisto L. Phys Status Solidi A 2000;182:249. [38] Vazsonyi E, Szilagyi E, Petrik P, Horvath ZE, Lohner T, Fried M, et al. Thin Solid Films 2001;388:295. [39] Andersen OK, Frello T, Veje E. J Appl Phys 1995;78:6189. [40] Boughaba S, Wang K. Thin Solid Films 2006;497:83. [41] Saadoun M, Mliki N, Kaabi H, Daoudi K, Bessais B, Ezzaouia H, et al. Thin Solid Films 2002;405:29. [42] Hummel RE, Sung-Sik C. Appl Phys Lett 1992;61:1965. [43] Halimaoui A. In: Vial JC, Derrien J, editors. Porous silicon science and technology. Spinger-Verlag; 1995. Chapter 3. [44] Allongue P, Kieling V, Gerischer H. Electrochim Acta 1995;40:1353. [45] Blackwood DJ, Zhang Y. Electrochim Acta 2003;48:623. [46] Lehmann V, Gosele U. Appl Phys Lett 1991;58:856. [47] Smith RL, Collins SD. J Appl Phys 1992;71:R1. [48] Theiss W. Surf Sci Rep 1997;29:91. [49] Jakubowicz J. Superlattice Microstruct 2007;41:205. [50] Allongue P. In: Canham LT, editor. Properties of porous silicon. INSPEC, The Institution of Electrical Engineers; 1997. Chapter 1.1. [51] Chayen NE, Saridakis E, El-Bahar R, Nemirovsky Y. J Mol Biol 2001;312:591. [52] Wojtyk JTC, Morin KA, Boukherroub R, Wayner DDM. Langmuir 2002;18:6081. [53] Canham LT. In: Canham LT, editor. Properties of porous silicon. INSPEC, The Institution of Electrical Engineers; 1997. Chapter 2.1. [54] Hajji M, Khardani M, Khedher N, Rahmouni H, BessaIs B, Ezzaouia H, et al. Thin Solid Films 2006;511–512:235. [55] Setzu S, Lerondel G. J Appl Phys 1998;84:3129. [56] Herino R. In: Canham LT, editor. Properties of porous silicon. INSPEC, The Institution of Electrical Engineers; 1997. Chapter 2.2. [57] Bomchil G, Herino R, Barla K, Pfister J. Electrochem Soc 1983;130:1611. [58] Herino R, Bomchil G, Barla K, Bertrand C. J Electrochem Soc 1987;134:1994. [59] Balagurov LA, Loginov BA, Petrova EA, Sapelkin A, Unal B, Yarkin DG. Electrochim Acta 2006;51:2938. [60] Kumar P, Huber P. J Nanomater 2007;2007:4. [61] Gorbanyuk TI, Evtukh AA, Litovchenko VG, Solnsev VS, Pakhlov EM. Thin Solid Films 2006;495:134. [62] Mawhinney DB, Glass JA, Yates JT. J Phys Chem B 1997;101:1202. [63] Nakajima A, Itakura T, Watanabe S, Nakayama N. Appl Phys Lett 1992;61:46. [64] Kumar R, Kitoh Y. Appl Phys Lett 1993;63:3032. [65] Lysenko V, Vitiello J, Remaki B, Barbier D, Skryshevsky V. Appl Surf Sci 2004;230: 425. [66] Errien N, Vellutini L, Louarn G, Froyer G. Appl Surf Sci 2007;253:7265. [67] Zanoni R, Righini G, Mattogno G, Schirone L, Sotgiu G, Rallo F. J Lumin 1998;80: 159. [68] Torchinskaya TV, Korsunskaya NE, Khomenkova LY, Dhumaev BR, Prokes SM. Thin Solid Films 2001;381:88. [69] Murakoshi K, Uosaki K. Appl Phys Lett 1993;62:1676. [70] Gardelis S, Hamilton B. J Appl Phys 1994;76:5327. [71] Zoubir NH, Vergnat M. Appl Phys Lett 1994;65:82.

38

K.L. Jarvis et al. / Advances in Colloid and Interface Science 175 (2012) 25–38

[72] Harper J, Sailor MJ. Langmuir 1997;13:4652. [73] Takazawa A, Tamura T, Yamada M. J Appl Phys 1994;75:2489. [74] Domashevskaya EP, Kashkarov VM, Manukovskii EY, Shchukarev AV, Terekhov VA. J Electron Spectrosc Relat Phenom 1998;88–91:969. [75] Yang DQ, Meunier M, Sacher E. J Appl Phys 2006;99. [76] Munder H. In: Vial JC, Derrien J, editors. Porous silicon science and technology. Springer-Verlag; 1995. Chapter 16. [77] Munder H, Berger MG, Frohnhoff S, Thonissen M, Luth H, Jeske M, et al. J Lumin 1993;57:223. [78] Terekhov VA, Kashkarov VM, Manukovskii, Schukarev AV, Domashevskaya EP. J Electron Spectrosc Relat Phenom 2001;114–116:895. [79] Leisenberger F, Duschek R, Czaputa R, Netzer FP, Beamson G, Matthew JAD. Appl Surf Sci 1997;108:273. [80] Kempson IM, Barnes TJ, Prestidge CA. J Am Soc Mass Spectrom 2010;21:254. [81] Jarvis KL, Barnes TJ, Prestidge CA. Langmuir 2010;26:14316. [82] Barnes TJ, Kempson IM, Prestidge CA. Int J Pharm 2011;417:61–9. [83] Prestidge CA, Barnes TJ, Mierczynska-Vasilev A, Kempson I, Peddie F, Barnett C. Phys Status Solidi A 2008;205:311. [84] Benninghoven A, Rudenauer FG, Werner HW. In: Elving PJ, Winefordner JD, editors. John Wiley & Sons; 1987. Chapter 5. [85] Jarvis KL, PhD Thesis, 2010, University of South Australia, Adelaide. [86] Salonen J, Lehto VP, Laine E. Appl Surf Sci 1997;120:191. [87] Bjorklund RB, Zangooie S, Arwin H. Langmuir 1997;13:1440. [88] Bateman JE, Eagling RD, Horrocks BR, Houlton A, Worrall DR. Chem Commun (Camb) 1997;2275. [89] Ogata YH, Tsuboi T, Sakka T, Naito S. J Porous Mater 2000;7:63. [90] Jarvis KL, Barnes TJ, Prestidge CA. Langmuir 2008;24:14222. [91] Ogata Y, Niki H, Sakka T, Iwasaki M. J Electrochem Soc 1995;142:1595. [92] Mattei G, Valentini V, Yakovlev VA. Surf Sci 2002;502–503:58. [93] Chen H, Hou X, Li G, Zhang F, Yu M, Wang X. J Appl Phys 1996;79:3282. [94] Park J-Y, Lee J-H. Mater Chem Phys 2003;82:134. [95] Salonen J, Laine E, Niinisto L. J Appl Phys 2002;91:456. [96] Bjorkqvist M, Salonen J, Laine E. Appl Surf Sci 2004;222:269. [97] Bjorkqvist M, Salonen J, Paski J, Laine E. Sens Actuators A 2004;112:244. [98] Salonen J, Bjorkqvist M, Paski J. Sens Actuators A 2004;116:438. [99] Paski J, Björkqvist M, Salonen J, Lehto V-P. Phys Status Solidi C 2005;2:3379. [100] Björkqvist M, Paski J, Salonen J, Lehto VP. Phys Status Solidi A 2005;202:1653. [101] Salonen J, Tuura J, Bjorkqvist M, Lehto VP. Sens Actuators B 2006;114:423. [102] Bjorkqvist M, Paski J, Salonen J, Lehto V-P. IEEE Sensors J 2006;6:542. [103] Torres-Costa V, Salonen J, Lehto VP, Martín-Palma RJ, Martínez-Duart JM. Microporous Mesoporous Mater 2008;111:636. [104] Torres-Costa V, Martin-Palma RJ, Martinez-Duart JM, Salonen J, Lehto VP. J Appl Phys 2008;103:083124. [105] Geobaldo F, Rivolo P, Ugliengo P, Garrone E. Sens Actuators B 2004;100:29. [106] Low SP, Williams KA, Canham LT, Voelcker NH. Biomaterials 2006;27:4538. [107] Salonen J, Laine E, Niinistö L. Phys Status Solidi A 2003;197:246. [108] Bateman JE, Eagling RD, Worrall DR, Horrocks BR, Houlton A. Angew Chem Int Ed 1998;37:2683. [109] Fan HJ, Kuok MH, Ng SC, Lim HS, Liu NN, Boukherroub R, et al. J Appl Phys 2003;94:1243. [110] Boukherroub R, Morin S, Wayner DDM, Lockwood DJ. Phys Status Solidi A 2000;182:117. [111] Boukherroub R, Morin S, Wayner DDM, Bensebaa F, Sproule GI, Baribeau JM, et al. Chem Mater 2001;13:2002. [112] Alekseev SA, Lysenko V, Zaitsev VN, Barbier D. J Phys Chem C 2007;111:15217. [113] Tay L, Rowell NL, Lockwood DJ, Boukherroub R. J Vac Sci Technol A 2006;24:747. [114] Gelloz B, Sano H, Boukherroub R, Wayner DDM, Lockwood DJ, Koshida N. Phys Status Solidi C 2005;2:3273. [115] Boukherroub R, Wayner DDM, Sproule GI, Lockwood DJ, Canham LT. Nano Lett 2001;1:713. [116] Dattilo D, Armelao L, Maggini M, Fois G, Mistura G. Langmuir 2006;22:8764. [117] Dattilo D, Armelao L, Fois G, Mistura G, Maggini M. Langmuir 2007;23:12945. [118] Gelloz B, Sano H, Boukherroub R, Wayner DDM, Lockwood DJ, Koshida N. Appl Phys Lett 2003;83:2342. [119] Boukherroub R, Wayner DDM, Lockwood DJ. Appl Phys Lett 2002;81:601. [120] Björkqvist M, Salonen J, Laine E, Niinistö L. Phys Status Solidi A 2003;197:374. [121] Stewart MP, Buriak JM. Angew Chem Int Ed 1998;37:3257. [122] Stewart MP, Robins EG, Geders TW, Allen MJ, Cheul Choi H, Buriak JM. Phys Status Solidi A 2000;182:109. [123] Britcher L, Barnes TJ, Griesser HJ, Prestidge CA. Langmuir 2008;24:7625. [124] Vrkoslav V, Jelinek I, Trojan T, Jindrich J, Dian J. Phys B 2007;38:200. [125] Ciampi S, Bocking T, Kilian KA, Harper JB, Gooding JJ. Langmuir 2008;24:5888. [126] Guan B, Ciampi S, Le Saux G, Gaus K, Reece PJ, Gooding JJ. Langmuir 2010;27: 328–34. [127] Saghatelian A, Buriak J, Lin VSY, Reza Ghadiri M. Tetrahedron 2001;57:5131. [128] Holland JM, Stewart MP, Allen MJ, Buriak JM. J Solid State Chem 1999;147:251.

[129] Romano E, Narducci D. Surf Sci 2007;601:2836. [130] Buriak JM, Allen MJ. J Lumin 1998;80:29. [131] Canham LT, Reeves CL, Newey JP, Houlton MR, Cox TI, Buriak JM, et al. Adv Mater 1999;11:1505 (Weinheim, Ger.). [132] Gurtner C, Wun AW, Sailor MJ. Angew Chem Int Ed 1999;38:1966. [133] Bayliss SC, Buckberry LD, Harris PJ, Tobin M. J Porous Mater 2000;7:191. [134] Mayne AH, Bayliss SC, Barr P, Tobin M, Buckberry LD. Phys Status Solidi A 2000;182:505. [135] Bayliss SC, Buckberry LD, Fletcher I, Tobin MJ. Sens Actuators A 1999;74:139. [136] Khung YL, Barritt G, Voelcker NH. Exp Cell Res 2008;314:789. [137] Bayliss SC, Heald R, Fletcher DI, Buckberry LD. Adv Mater 1999;11:318 (Weinheim, Ger.). [138] Chin V, Collins BE, Sailor MJ, Bhatia SN. Adv Mater 2001;13:1877 (Weinheim, Ger.). [139] Alvarez SD, Derfus AM, Schwartz MP, Bhatia SN, Sailor MJ. Biomaterials 2009;30: 26. [140] Pramatarova L, Pecheva E, Dimova-Malinovska D, Pramatarova R, Bismayer U, Petrov T, et al. Vacuum 2004;76:135. [141] Rosengren A, Wallman L, Bengtsson M, Laurell T, Danielsen N, Bjursten LM. Phys Status Solidi A 2000;182:527. [142] Rosengren A, Wallman L, Danielsen N, Laurell T, Bjursten LM. IEEE Trans Biomed Eng 2002;49:392. [143] Anderson SHC, Elliott H, Wallis DJ, Canham LT, Powell JJ. Phys Status Solidi A 2003;197:331. [144] Pastor E, Matveeva E, Parkhutik V, Curiel-Esparza J, Millan MC. Phys Status Solidi C 2007;4:2136. [145] Tutov EA, Pavlenko MN, Protasova IV, Kashkarov VM. Tech Phys Lett 2002;28: 729. [146] Sweetman MJ, Harding FJ, Graney SD, Voelcker NH. Appl Surf Sci 2011;257:6768. [147] Godin B, Gu J, Serda RE, Bhavane R, Tasciotti E, Chiappini C, et al. J Biomed Mater Res A 2010;94A:1236. [148] Low SP, Voelcker NH, Canham LT, Williams KA. Biomaterials 2009;30:2873. [149] Park J-h GuL, Von Maltzahn G, Ruoslahti E, Bhatia SN, Sailor MJ. Nat Mater 2009;8:331. [150] Bimbo LM, Mäkilä E, Laaksonen T, Lehto V-P, Salonen J, Hirvonen J, et al. Biomaterials 2011;32:2625. [151] Cheng L, Anglin E, Cunin F, Kim D, Sailor MJ, Falkenstein I, et al. Br J Ophthalmol 2008;92:705. [152] Zangooie S, Bjorklund R, Arwin H. J Electrochem Soc 1997;144:4027. [153] Stefano LD, Rea I, Giardina P, Armenante A, Rendina I. Adv Mater 2008;20:1529 (Weinheim, Ger.). [154] Gupta P, Dillon AC, Bracker AS, George SM. Surf Sci 1991;245:360. [155] Binner J, Zhang Y. J Mater Sci Lett 2001;20:123. [156] Tasciotti E, Liu X, Bhavane R, Plant K, Leonard AD, Price BK, et al. Nat Nanotechnol 2008;3:151. [157] Wu EC, Park J, Park J, Segal E, Cunin F, Sailor MJ. ACS Nano 2008;2:2401. [158] Wu EC, Andrew JS, Cheng L, Freeman WR, Pearson L, Sailor MJ. Biomaterials 2011;32:1957. [159] Rajaraman S, Henderson HT. Sens Actuators B 2005;105:443. [160] Jing J, Tay FEH, Miao J, Iliescu C. J Micromech Microeng 2006;16:958. [161] Jing J, Tay FEH, Miao J. J Phys Conf Ser 2006;34:1127. [162] Kaukonen AM, Laitinen L, Salonen J, Tuura J, Heikkila T, Limnell T, et al. Eur J Pharm Biopharm 2007;66:348. [163] Prestidge CA, Barnes TJ, Mierczynska-Vasilev A, Skinner W, Peddie F, Barnett C. Phys Status Solidi A 2007;204:3361. [164] Kinnari P, Mäkilä E, Heikkilä T, Salonen J, Hirvonen J, Santos HA. Int J Pharm 2011;414:148. [165] Kilpeläinen M, Mönkäre J, Vlasova MA, Riikonen J, Lehto V-P, Salonen J, et al. Eur J Pharm Biopharm 2011;77:20. [166] Kilpeläinen M, Riikonen J, Vlasova MA, Huotari A, Lehto VP, Salonen J, et al. J Control Release 2009;137:166. [167] Vasani RB, McInnes SJP, Cole MA, Jani AMM, Ellis AV, Voelcker NH. Langmuir 2011;27:7843. [168] Mann AP, Tanaka T, Somasunderam A, Liu X, Gorenstein DG, Ferrari M. Adv Mater 2011;23:H278 (Weinheim, Ger.). [169] Salonen J, Paski J, Vaha-Heikkila K, Heikkila T, Bjorkqvist M, Lehto VP. Phys Status Solidi A 2005;202:1629. [170] Lehto VP, Vaehae-Heikkilae K, Paski J, Salonen J. J Therm Anal Calorim 2005;80: 393. [171] Riikonen J, Mäkilä E, Salonen J, Lehto V-P. Langmuir 2009;25:6137. [172] Li X, St. John J, Coffer JL, Chen Y, Pinizzotto RF, Newey J, et al. Biomed Microdevices 2000;2:265. [173] Park JS, Kinsella JM, Jandial DD, Howell SB, Sailor MJ. Small 2011;7:2061. [174] Kovalainen M, Mönkäre J, Mäkilä E, Salonen J, Lehto V-P, Herzig K-H, et al. Pharm Res 2012;29:837.