Ashkan
Dehsorkhi
and
Ian W.
Hamley
*
School of Chemistry, Pharmacy and Food Biosciences, University of Reading, Whiteknights, Reading, RG6 6AD, UK
First published on 31st January 2014
The peptide amphiphile C16-KTTKS templates silica polymerization, enabling the production of silica nanotape structures, imaged via electron microscopy (TEM and SEM). X-ray scattering shows that the nanotapes comprise stacked layers, as for the parent peptide amphiphile, but with a substantially increased layer spacing resulting from silica polymerization.
Peptide amphiphiles (PA) have gained a huge amount of attention over the past decade due to their ability to self-assemble into a range of novel nanostructures.10,11 Their self-assembling abilities are dictated by their amphiphilic nature due to the inclusion of a lipid chain attached to a biofunctional peptide epitope that can participate in secondary structures such as β-sheets.12 The unique interplay of intermolecular hydrogen bonding along with hydrophobic and electrostatic interactions leads to well defined self-assembled nanostructures.13,14 The ability to fine-tune these non-covalent interactions provides flexibility in PA morphologies and therefore makes them ideal and attractive as organic templates to construct novel silica nanostructures.
Previous work using surfactant like peptides (SLP) as templates to fabricate silica hollow nanotubes has been reported. The Guler group reported the formation of silica nanofibres by using an amyloid-like peptide, Ac-KFFAAK-Am as a template to mimic the biomineralisation process.15 SLPs comprising A6K and V6K were studied by Wang et al.16 A6K self-assembled into nanofibers whereas V6K formed sheets with a lamellar stacking arrangement. Various anions were introduced to observe the influence on the morphology of the self-assembled nanostructures. Silica nanostructures were synthesised by the addition of silicic acid to solutions of peptide/anion mixtures. The formation of flower-like, fibrillar and lamellar morphologies was observed. The silica templating of a short SLP, I3K from TEOS has been investigated by Zhang and co-workers.17 The amphiphilic peptide was reported to self-assemble into nanotubes and therefore the sol–gel condensation reaction of TEOS could be used to produce silica nanotubes. Their work also investigated the effects of TEOS concentration, pH and ageing on silica deposition. The Woolfson group conducted work on the silica templating of a designer peptide consisting of two α-helical peptides, which co-assemble into a heterodimer.18 The resulting fibres were reported to be 50–100 nm wide. Long hollow silica nanotubes were formed by the silicic acid method. Investigation into the removal of the organic peptide core using a variety of methods was carried out which included enzyme degradation.
Previously the Hartgerink group investigated the templating effect of PAs using the sol–gel condensation of TEOS to synthesis hollow silica nanotubes.19 Five designer PAs were prepared to study the catalytic activities of amino acids on silica polymerisation. All five PAs self-assembled into β-sheet forming nanofibres. The PAs which contained lysine or histidine residues were reported to produce well-defined silica nanotubes due to the catalytic effects on their side chains. In this study we investigate the use of a collagen stimulating PA, C16-KTTKS as a template to construct nanotape silica structures using the sol–gel condensation reaction of TEOS. The self-assembly of C16-KTTKS has been well characterised by us previously.20–22 To our knowledge this is the first report on the production of flat tape-like silica nanostructures using a PA as the organic template.
SAXS data were also collected on the bioSAXS beamline BM29 at the ESRF, Grenoble, France. A solution of 1 wt% C16-KTTKS was loaded into a PCR tube in an automated sample changer. SAXS data was collected using a Pilatus 1M detector. The sample-detector distance was 2.84 m. The X-ray wavelength was 0.99 Å.
SAXS/WAXS for the calcined powder was performed using a Bruker Nanostar instrument using CuKα radiation from an Incoatec microfocus source. The beam was collimated by a three slit system. The sample was mounted in a glass capillary (1 mm diameter). The sample-detector distance was 105 cm and a Vantec-2000 photon counting detector was used to collect SAXS patterns. The wavenumber q = 4πsinθ/λ (where 2θ is the scattering angle and λ = 1.54 Å is the wavelength) scale was calibrated using silver behenate. The powdered sample was placed on and wrapped with kapton tape which was then stuck onto a metal plate and placed in front of the beam.
The resulting silica templated structures of C16-KTTKS were examined by a combination of microscopy techniques. TEM images of the silica nanotape structures are shown in Fig. 1a and b. Flat sheet like structures were observed, which have strikingly similar structures to those of the PA C16-KTTKS itself.20 We also observed the presences of well-defined stripes running parallel to the surface of the silica nanotapes as presented in Fig. 1a. The stripe spacing from the TEM image was determined to be 4 nm. Stripes have been noted previously for the PA itself.23 The spacing is lower than the usual bilayer spacing (5.2 nm), and has been ascribed to the existence of a population of dehydrated bilayer structures.24,25 The striped appearance is attributed to the nanostructures being viewed side-on. The previously reported 4 nm stripe spacing is in good agreement with that obtained from the TEM image in Fig. 1a for the calcined sample. A SEM image at high magnification of the calcined material shown in Fig. 1c reveals fibrillar structures as does the cryo-TEM image shown in ESI Fig. S1a.† An energy-dispersive X-ray spectrum was obtained via SEM shown in ESI Fig. S2† for the calcined sample. The spectrum indicates that silica is present along with carbon and oxygen, which reveals that calcination was not fully completed. Nonetheless, calcination produced nanostructures with a predominant silica content.
Fig. 1 (a and b) TEM images of the calcined C16-KTTKS-templated silica sample (c) SEM image of the silica nanostructure at high magnification. |
To further elucidate the silica nanotape structure, SAXS was employed to complement the microscopy images. SAXS intensity profiles for 1 wt% C16-KTTKS, precursor solution and calcined powder are shown in Fig. 2. The SAXS profile for a 1 wt% solution of C16-KTTKS dissolved in water contains peaks with positions in the ratio 1:2, which is indicative of layered structure due to the bilayer arrangement of the nanotape as observed previously.20 A solution containing 1 wt% C16-KTTKS dissolved in ethanol was mixed with TEOS and left to age for a week for silica deposition. SAXS was measured for the precursor solution, which exhibits a peak with a d-spacing = 5.2 nm, the same as that for 1 wt% C16-KTTKS in water.20,21,26 The second order lamellar reflection is not present, indicating a reduction in long-range order. SAXS measurements for the calcined silica powder are also shown in Fig. 2, the profile contains Bragg reflections associated with a layered structure with a periodicity of 11.2 nm. This indicates a substantial increase in the bilayer spacing, which we ascribe to swelling of the bilayers by silica upon drying. The precursor TEOS is believed to associate with the peptide headgroup due to electrostatic interactions, however there is no change in layer spacing for the precursor TEOS solution. The large increase in layer spacing for the calcined sample shows that silica templating does not occur via the direct liquid crystal templating route,27 but that it involves changes in the layer spacing due to silica polymerization upon calcination. Allowing the precursor solution to age for a week has been shown to provide sufficient time for silica polymerisation to occur.
Fig. 2 SAXS data comparing 1 wt% solution of C16-KTTKS with C16-KTTKS precursor solution and calcinated powder. |
X-ray diffraction measurements were performed on calcined powders to observe any additional structural features in the wide angle region. A representative intensity profile is shown in Fig. 3. This shows a series of Bragg reflections which do not appear to be higher order peaks from the layered structure with 11.2 nm observed by SAXS. The absence of the meridional (4.7–4.8 Å) β-sheet peak as previously observed20 indicates that β-sheet ordering is disrupted upon calcination. This is also supported by the near absence of the 11 Å β-sheet spacing peak,23 which is only present as a weak shoulder. We ascribe the 14.96 Å and 7.57 Å reflections as second and third order peaks from a 3 nm periodicity dehydrated lamellar structure noted previously24 which may coexist with the TEOS-swollen 11.2 nm layer structure.
The primary amine group of lysine is known to exert a catalytic effect on TEOS hydrolysis.17,19,28 The PA we investigated, C16-KTTKS contains two basic lysine residues, which can catalyse the hydrolysis of TEOS. A mechanism for silica templating has been proposed by Hartgerink that the hydrolysed TEOS forms negative ions by deprotonation, which can in turn adhere to the positively charged primary amine group of lysine residues via electrostatic interactions.19 As C16-KTTKS self-assembles into a nanotape construct, the peptide epitope resides on the tape surface with the hydrophobic alkyl chains forming the core. We propose that silica deposition therefore occurs specifically on the surface of the nanotape structure, where the lysine residues are located.
As mentioned in the introduction, other authors have reported constructing silica nanotubes using amphiphilic peptides and PAs as templates but none have reported silica nanotapes. We report for the first time that a commercially available collagen stimulating PA, which can self-assemble into bilayers comprising a nanotape architecture can be templated using the sol–gel condensation reaction of TEOS to mimic silica biomineralisation.
In summary we have shown that flat sheet-like silica nanotapes with striations on some of the nanotape surfaces can be fabricated using a PA as a template. The silica nanotape stuructures were characterised using microscopy and scattering techniques, which reveal a similar structure as C16-KTTKS but with an increase in bilayer periodicity upon silica polymerization. XRD indicates that the inorganic polymerization overwhelms the initial β-sheet template features. Lysine residues present on the outer surface are believed to be responsible for catalysing the sol–gel condensation reaction of TEOS and subsequently template-directed synthesis of silica nanotapes.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3sm52324a |
This journal is © The Royal Society of Chemistry 2014 |