Leonora
Velleman
a,
Cameron James
Shearer
a,
Amanda Vera
Ellis
a,
Dusan
Losic
b,
Nicolas Hans
Voelcker
a and
Joseph George
Shapter
*a
aCentre for NanoScale Science and Technology, Flinders University, School of Chemical and Physical Sciences, 5040, Australia. E-mail: Joe.shapter@flinders.edu.au
bIan Wark Research Institute, University of South Australia, Mawson Lakes Campus, 5095, Australia
First published on 7th July 2010
This study presents a simple approach to perform selective mass transport through freestanding porous silicon (pSi) membranes. pSi membranes were fabricated by the electrochemical etching of silicon to produce membranes with controlled structure and pore sizes close to molecular dimensions (∼12 nm in diameter). While these membranes are capable of size-exclusion based separations, chemically specific filtration remains a great challenge especially in the biomedical field. Herein, we investigate the transport properties of chemically functionalized pSi membranes. The membranes were functionalized using silanes (heptadecafluoro-1,1,2,2-tetrahydrodecyl)dimethylchlorosilane (PFDS) and N-(triethoxysilylpropyl)-o-polyethylene oxide urethane (PEGS) to give membranes hydrophobic (PFDS) and hydrophilic (PEGS) properties. The transport of probe dyes tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate (Rubpy) and Rose Bengal (RB) through these functionalized membranes was examined to determine the effect surface functionalization has on the selectivity and separation ability of pSi membranes. This study provides the basis for further investigation into more sophisticated surface functionalization and coupled with the biocompatibility of pSi will lead to new advances in membrane based bio-separations.
Emerging applications for pSi membranes are based on the ability to functionalize the surfaces of the membrane with organic monolayers to provide rich surface chemistries and a platform capable of incorporating other functional groups and materials, thus providing a membrane where the surface chemistry can be tailored and optimized for a specific purpose. Surface functionalization of pSi has given access to a broad range of novel organic–inorganic hybrid materials relevant to a variety of applications.1,3,4,7 The modification of inorganic materials by the adsorption of organic molecules is a common technique for tailoring interface properties.8,9 The attachment of a variety of functional molecules onto pSi membranes is possible via attachment to the silanol (Si–OH) groups present on the inner and outer surfaces after oxidation. Silanes can attach to the silanol groups to form highly ordered self-assembled monolayers (SAMs). In addition to the ease of preparation of these SAMs, they possess advantageous characteristics such as high structural stability due to covalent attachment between the silane molecules and silicon surface.10–12
There is much interest in increasing the efficacy of surface chemistry driven molecular transport and separation.8 However, the detailed mechanism behind transport properties through functionalized membranes is not sufficiently understood. This work is a preliminary investigation into the effect on the transport properties of pSi membranes coated with various SAMs. The pores in pSi can be made controllably with molecular dimensions and hence provide an opportunity to develop readily tunable, molecularly selective membranes.
The silanes (heptadecafluoro-1,1,2,2-tetrahydrodecyl)dimethylchlorosilane (PFDS) and N-(triethoxysilylpropyl)-o-polyethylene oxide urethane (PEGS) were chosen to functionalize pSi membranes due to their hydrophobic and hydrophilic properties respectively.13 Through examining the transport of probe dyes through these functionalized membranes the transport mechanism of molecules through SAM modified pores can be probed. Perfluorinated SAMs are highly hydrophobic and provide optimal properties such as high resistance to thermal decomposition and stability towards various chemicals.14 Furthermore, PFDS and PEGS are used due to their ability to form low free energy surfaces14–17 and their non-fouling properties.18–21
To our knowledge, there has not been any previous research performed regarding the transport of molecules through pSi membranes primarily due to the fragility of the membranes. In this work, we describe a simple procedure to provide support to the pSi membranes in order to directly observe the transport properties of these membranes. Furthermore, we present the ability for these freestanding pSi membranes to perform molecular separations and investigate the selectivity of silane-functionalized membranes towards the transport of organic dyes with different hydrophilicity.
Fig. 1 SEM images of (a) top down and (b) side view of the pSi membranes and (c) a pore size distribution graph (as determined by SEM analysis). |
To improve the level of molecular separation of the pSi membranes, surface functionalization was completed to make the surface either hydrophobic or hydrophilic. This was achieved by first oxidising the surface thermally and then depositing silane layers utilising simple silane surface chemistry (Fig. 2). Surface functionalization of the pSi membranes was confirmed by contact angle measurements. The thermally oxidized pSi membrane showed a hydrophilic contact angle of approximately 20° (Table 1). This value is due to the surface chemistry consisting of silicon to oxygen bonds (e.g. –SiO, Si–OH and –Si–O–Si–). When functionalized with the hydrophobic PFDS silane the contact angle was found to be 135°. The high value is attributed to two factors: the highly hydrophobic nature of the PFDS surface coating and the roughness of the porous structure which has been shown to increase contact angle values.23,24Table 1 also shows a contact angle of approximately 13° for the hydrophilic PEGS modified pSi surface. Values of between 10 and 15° have previously been reported for a similar PEGylated silane on a PDMS surface.25 These contact angle values indicate that the surface functionalization was successful and that both hydrophobic and hydrophilic pSi surfaces have been created.
Fig. 2 Schematic of the chemical modification of pSi membranes with (a) a hydrophobic fluorinated silane (PFDS) and (b) a hydrophilic silane (PEGS). |
Surface modification | Contact angle/° |
---|---|
Unfunctionalized | 19.5 ± 4.5 |
PFDS | 135 ± 13 |
PEGS | 12.9 ± 1.8 |
Fig. 3 Transport of Rubpy (hydrophobic properties) and RB (hydrophilic properties) through pSi membranes where (a) is an unfunctionalized membrane, (b) is a hydrophobic (PFDS) functionalized membrane and (c) is a hydrophilic (PEGS) functionalized membrane. |
A summary of the permeation data of these dyes through functionalized and unfunctionalized pSi membranes is presented in Table 2. The pSi membrane permeability coefficients were determined from Fick's first law of diffusion (eqn (1)) for each of the dyes by documenting the flux of the dyes through the various surface functionalized pSi membranes at several feed concentrations: 500 µm, 1 mM and 1.5 mM (data for 500 µm and 1.5 mM not shown).
(1) |
Surface modification | Flux of permeate molecule/mol cm−2 h−1 | Flux ratio of Rubpy:RB | Membrane permeability, P/cm2 h−1 | ||
---|---|---|---|---|---|
Rubpy | RB | Rubpy | RB | ||
Unfunctionalized | 3.08 ± 0.17 × 10−7 | 1.46 ± 0.10 × 10−7 | 2.11 | 2.72 ± 0.32 × 10−9 | 1.35 ± 0.22 × 10−9 |
PFDS | 2.76 ± 0.12 × 10−7 | 3.87 ± 0.16 × 10−8 | 7.13 | 2.88 ± 0.07 × 10−9 | 4.73 ± 0.74 × 10−10 |
PEGS | 1.24 ± 0.01 × 10−7 | 1.92 ± 0.03 × 10−7 | 0.65 | 1.12 ± 0.06 × 10−9 | 1.39 ± 0.21 × 10−9 |
The permeability can be expressed as,
P = DK, | (2) |
c1 = KC1 | (3) |
For an unfunctionalized pSi membrane we find that the flux of RB is 2.11 times smaller than Rubpy (Table 2 and Fig. 3(a)). The observed difference in transport is due to the variation in the diffusion coefficients of the dyes which are attributed to factors such as the size, charge, shape and solubilities of the molecules in the solvent. The transport data obtained from the functionalized membranes are compared to the unfunctionalized membrane transport data in order to determine the impact of functionalization on the transport properties of the membrane.
Fig. 3(b) displays the transport of Rubpy and RB through the hydrophobic PFDS modified membrane in which the transport of Rubpy through the PFDS modified membrane remains relatively unchanged in comparison to the transport through the unfunctionalized membrane (Fig. 3(a)), resulting in similar permeability coefficients (Table 2). Therefore the diffusion of the hydrophobic dye through the hydrophobic functionalized membrane is not hindered with the addition of the adsorbed monolayer and is allowed to pass freely through the membrane. Modification with silanes would be expected to slightly decrease the membrane pore size and consequently decrease the transport. However, it is also important to consider the partitioning of organic solutes into the organic SAM formed within the pores. The ability for molecules to partition into molecular layers such as SAMs and lipid bilayers and the impact this has on transport properties have been explored and reported in the recent literature.28–32 In our case, it is likely that the hydrophobic dye can partition into the hydrophobic silane layer and thus diffusion across the membrane can occur through both the silane layer and the solvent in the centre of the pore resulting in similar Rubpy transport rates through PFDS-modified and unfunctionalized membranes. From Fig. 3(b) it can be seen that the transport of RB through the PFDS modified membrane is significantly reduced in comparison to the transport through the unfunctionalized membrane (Fig. 3(a)). Table 2 shows that the permeability coefficient of RB into the PFDS–pSi membrane is approximately 2.8 times smaller than the permeability coefficient of the dye into the unfunctionalized membrane. Therefore the partition coefficient for RB transport in the PFDS–pSi membrane has decreased (eqn (2)). The PFDS–pSi membrane exhibits a hydrophobic environment in which the hydrophilic dye would not enter easily. Furthermore, it is unlikely that the RB will partition into the hydrophobic organic monolayer and hence the reduction of the transport rate can also be attributed to a reduction in the pore diameter from the adsorbed monolayer. These contributing factors lead to an overall decrease in the transport of RB through the PFDS–pSi membrane. Thus the PFDS–pSi membrane facilitates the transport of hydrophobic species through the membrane while hindering the transport of hydrophilic species.
The transport rates of Rubpy and RB through the hydrophilic PEGS modified membrane is presented in Fig. 3(c). In this case the transport of RB through the membrane is faster than Rubpy resulting in a flux ratio of 0.65 (Table 2). The permeability coefficients (Table 2) of RB through the PEG–pSi membrane and the unfunctionalized membrane are similar therefore the ability for RB to partition into the PEG-functionalized membrane is unaffected. Furthermore, the permeability of Rubpy through the membrane has decreased considerably which is due to a reduction in the partition coefficient explained by the poor solubility of Rubpy into the hydrophilic environment formed within the pores of the membrane due to adsorption of the hydrophilic PEGS. Therefore, complementary to the transport results obtained from the PFDS–pSi membranes, the transport of hydrophilic species through the PEGS modified membrane is facilitated while the transport of the hydrophobic species is hindered due to the hydrophilic environment exhibited by the PEGS modified membrane.
From Fig. 3 it is apparent that the PFDS-functionalized membrane favours the transport of the hydrophobic dye over the hydrophilic dye (Fig. 3(b)) and the PEGS-functionalized membrane favours the transport of the hydrophilic dye over the hydrophobic dye (Fig. 3(c)). After PFDS modification it is seen that the flux ratio of Rubpy:RB has increased by a factor of 3.38 (from 2.11 to 7.13) while after PEGS modification the flux ratio of Rubpy:RB has decreased by a factor of 3.25 (from 2.11 to 0.65) (Table 2). Silane modification has therefore improved the selectivity properties of pSi membranes considerably. Thus chemical sensitivity has been imparted to the membrane through the adsorption of silanes onto the inner and outer surfaces of the membrane resulting in an enhancement in the degree of separation between hydrophilic and hydrophobic molecules. These results confirm that surface modifications can be tailored to favor the transport of molecules with a specific chemical nature.
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