Self-Assembly of Palmitoyl Lipopeptides Used in Skin Care Products

Jul 3, 2013 - Here, we investigate the self-assembly of three lipopeptides (Scheme 1) used in commercial skin care products. Surprisingly, there has b...
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Self-Assembly of Palmitoyl Lipopeptides Used in Skin Care Products Valeria Castelletto and Ian W. Hamley* Department of Chemistry, University of Reading, Reading RG6 6AD, United Kingdom

Conor Whitehouse and Paul J. Matts Procter and Gamble Technical Centres Ltd., London Innovation Centre, Whitehall Lane, Egham, Surrey TW20 9NW, United Kingdom

Rosemarie Osborne and Ellen S. Baker The Procter & Gamble Company, Mason Innovation Center, 8700 Mason Montgomery Road, Mason, Ohio 45050-9462, United States ABSTRACT: The self-assembly of three cosmetically active peptide amphiphiles C16-GHK, C16-KT, and C16-KTTKS (C16 denotes a hexadecyl, palmitoyl chain) used in commercial skin care products is examined. A range of spectroscopic, microscopic, and X-ray scattering methods is used to probe the secondary structure, aggregate morphology, and the nanostructure. Peptide amphiphile (PA) C16-KTTKS forms flat tapes and extended fibrillar structures with high β-sheet content. In contrast, C16-KT and C16-GHK exhibit crystal-like aggregates with, in the case of the latter PA, lower β-sheet content. All three PA samples show spacings from bilayer structures in small-angle X-ray scattering profiles, and all three have similar critical aggregation concentrations, this being governed by the lipid chain length. However, only C16-KTTKS is stained by Congo red, a diagnostic dye used to detect amyloid formation, and this PA also shows a highly aligned cross-β X-ray diffraction pattern consistent with the high β-sheet content in the self-assembled aggregates. These findings may provide important insights relevant to the role of self-assembled aggregates on the reported collagen-stimulating properties of these PAs.

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corneum.2,6 The resulting lipopeptides may be classified as peptide amphiphiles (PAs), which combine bioactivity from the peptide headgroup with a strong amphiphilic character from the lipid tail. This molecular structure leads to the potential for selfassembly into a variety of aggregates such as cylindrical fibrils, sheets, vesicles, and micelles.7−10 The spontaneous selfassembly of the peptide amphiphiles in water creates extended aggregates where the lipid tails are packed into the core and the peptide headgroups are displayed at the exterior surface. This unique self-assembled structure may play a role in the demonstrated in vitro bioactivity of these materials, and may also be relevant to potential uses in tissue engineering and regenerative medicine.8,11−13 Here, we investigate the self-assembly of three lipopeptides (Scheme 1) used in commercial skin care products. Surprisingly, there has been very little research on the selfassembly of these materials. In 2010, we reported that C16-KTTKS (Scheme 1) selfassembles into extended fibrils, which have a cross-β tape-like

INTRODUCTION An increasingly diverse range of cosmetically active ingredients are being used in topically applied dermatological formulations to deliver acute and chronic skin care benefits including vitamins, α-hydroxy acids, ceramides, and peptides.1−4 Examples include skin care and cosmetic formulations that address the appearance of aged skin by reducing fine lines, wrinkles, and age spots, and improving skin tone and elasticity. Peptides offer considerable potential due to their demonstrated ability to produce specific biological responses from defined sequences under in vitro conditions.5 These peptides include a range of both naturally occurring and synthetic peptides. Peptides termed matrikines are involved in remodelling of the extracellular matrix,1,4 and peptides based on sequences from procollagen I and elastin are commonly used in skin care products. Other peptides have been designed to improve appearance of expression-induced wrinkles, in analogy to the botulinum neurotoxin in Botox. Peptides designed to inhibit degradation of the extracellular matrix (ECM) via matrix metalloproteases are also employed.4 The peptides used in skin care products are commonly lipidated through attachment of a palmitoyl chain, which is purported to improve permeability across the stratum © 2013 American Chemical Society

Received: May 9, 2013 Revised: July 1, 2013 Published: July 3, 2013 9149

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wrinkle filling, skin tightening, barrier properties, and other physical benefits in skin care products. Currently, the use of cosmetically active peptides in skin care products at the concentrations where self-assembly occurs is likely to be restricted due to cost considerations. In doubleblind, placebo-controlled human studies, topical C16-KTTKS in skin cream formulations was shown to improve the appearance of wrinkled skin at concentration as low as 3 ppm.22 In contrast, the use of C16-KTTKS and many other cosmetically active peptides in their self-assembled form would require significantly higher concentrations (∼100 ppm and greater) and would be vastly more expensive. Fortunately, chemical synthesis and biofermentation production methods have advanced considerably in recent years, and this cost barrier may soon become less problematic making self-assembled peptide materials a realistic prospect for commercial skin care products. Here, we compare for the first time the self-assembly of the three PAs C16-KTTKS, C16-KT, and C16-GHK, the aggregation properties of the latter two not having been reported before to the best of our knowledge. The critical aggregation concentration for the three compounds is obtained from concentration-dependent fluorescence measurements. The secondary structure is elucidated using FTIR and CD spectroscopic methods and X-ray diffraction. The selfassembled structure is probed via small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). This powerful combination of techniques provides unprecedented insights into the aggregation process, which is expected to have a significant impact on in vitro collagen stimulation.

Scheme 1. Molecular Structures of the Three Peptide Amphiphiles: (a) C16-GHK, (b) C16-KT, and (c) C16-KTTKS

structure.14 The KTTKS headgroup was discovered as a sequence from the procollagen I peptide able to stimulate collagen and fibronectin production in fibroblasts.15 The shorter peptide C16-KT (Scheme 1) has also been found to have promise in skin care products,16,17 and its self-assembly is examined here for the first time. The lipopeptide C16-GHK (Scheme 1) contains a tripeptide sequence (a fragment of dermal collagen) having copper-binding ability and reported to boost collagen synthesis in fibroblasts.18 This leads to antioxidant properties because copper is a cofactor for antioxidant enzymes such as superoxide dismutase2 or lysyl oxidase involved in collagen synthesis19 or the downregulation of MMPs.2 Copper also enhances ECM synthesis and repair in a possible self-healing matrikine mechanism.1,20 More recently, we have demonstrated that the collagenstimulating behavior of the C16-KTTKS lipopeptide and its selfassembling behavior are inter-related.21 Our studies showed a significant increase in collagen production for dermal fibroblasts in the presence of self-assembled lipopeptide, whereas collagen production for solution concentrations below the critical aggregation concentration (cac) was dose-dependent. These results suggest that self-assembled lipopeptide materials can be used to boost cell growth and tissue regeneration in applications such as tissue engineering or invasive cosmetic procedures, where the peptide materials directly contact viable cells in the ECM. There are also several potential physical and chemical benefits that may be envisioned for self-assembled lipopeptides in topical skin care formulations. The selfassembled aggregates themselves are too large to permeate the physical barrier of the stratum corneum; however, the presence of self-assembled aggregates in a skin care or similar personal care formulation could potentially provide a reservoir of cosmetically active peptide that is slowly released through the dynamic equilibrium of the self-assembly process (monomer ↔ self-assembled structure), thereby maintaining the maximum steady-state concentration of free monomer for absorption through the stratum corneum, which could provide a sustained delivery benefit not currently possible with the monomeric peptide amphiphile. Additionally, the selfassembled gels formed by peptide amphiphiles may provide

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EXPERIMENTAL SECTION

Materials. Palmitoyl-lysine-threonine-threonine-lysine-serineCOOH (C16-KTTKS; MW 802.07 Da, purity = 83.8%), palmitoyllysine-threonine-COOH (C16-KT; MW 485.71 Da, purity = 96.6%), and palmitoyl-glycine-histidine-lysine-COOH (C16-GHK; MW 566.79, purity = 95.4%) were obtained as a pure freeze-dried peptide powders from Sederma (Le Perray-en-Yvelines, France). Purities, molecular weights, and log P values (measuring hydrophobicity) for the three PAs are listed in Table 1.

Table 1 sample C16-KTTKSCOOH C16-KTCOOH C16-GHKCOOH

Mi log P

Mw

purity

802.07

83.8%

acetate counterions

3.371

485.71

96.6%

2.734

566.79

95.4%

no counterions, it is an internal salt acetate counterions

−0.4

salt/counterions

Solution Preparation. PA solutions were dissolved in Milli-Q water to the desired concentration and mixed using sonication in an ultrasonic bath at ∼50 °C for 5−10 min. Thioflavin (ThT) Fluorescence Spectroscopy. Spectra were recorded with a Varian Cary Eclipse fluorescence spectrometer with samples in 4 mm inner width quartz cuvettes. The spectra were recorded from 460 to 600 nm using an excitation wavelength λex = 440 nm. ThT assays were performed using a 5.0 × 10−3 wt % ThT solution. Circular Dichroism (CD). Spectra were recorded using a Chirascan spectropolarimeter (Applied Photophysics, UK). CD was performed on PA solutions in water and placed in coverslip cuvettes (0.1 mm thick). Spectra are presented with absorbance A < 2 at any measured point with a 0.5 nm step, 1 nm bandwidth, and 1 s collection time per step at 20 °C. 9150

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Fourier Transform Infrared (FTIR) Spectroscopy. Spectra were measured on a Nicolet Nexus spectrometer with DTGS detector. FTIR data were measured for PA solutions in D2O. Samples were sandwiched between two CaF2 plate windows (spacer 0.025 mm). Spectra were scanned 128 times over the range of 4000−900 cm−1. Data were corrected by baseline subtraction. Small-Angle X-ray Scattering (SAXS). SAXS was performed using a Bruker Nanostar instrument using Cu Kα 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. Polarized Optical Microscopy (POM) and Congo Red Assay. For POM experiments, a drop of PA solution was placed onto a glass microscope slide and then placed under a coverslip. Samples were observed with the microscope through crossed polarizers, using an Olympus BX41 polarized microscope. Congo red assays were performed by staining the PA solution with a freshly prepared and filtered 1 wt % Congo red solution in water. Transmission Electron Microscopy (TEM). Imaging was performed using a Philips CM20 TEM microscope operated at 200 kV. Droplets of PA solutions solution were placed on Cu grids coated with a carbon film (Agar Scientific, UK), stained with phosphotungstic acid (2 wt %) (Sigma-Aldrich, UK), and dried. X-ray Diffraction (XRD). X-ray diffraction was performed on stalks prepared by suspending drops of PA solutions between the ends of wax-coated capillaries, and allowing them to dry. The stalk was mounted vertically onto the four axis goniometer of a RAXIS IV++ Xray diffractometer (Rigaku) equipped with a rotating anode generator. The XRD data were collected using a Saturn 992 CCD camera.

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RESULTS We first determined the cac of the three palmitoyl peptides shown in Scheme 1 via Thioflavin T fluorescence. Thioflavin T is a dye that binds amyloid fibrils, and this assay is therefore sensitive to aggregation into β-sheet fibrils.23−25 Figure 1 shows measured fluorescence intensities versus concentration along with representative spectra inset. By linear extrapolations as shown, the cac values were found to be 0.024 wt % for C16GHK, 0.024 wt % for C16-KT, and 0.02 wt % for C16-KTTKS. Given the uncertainty in the extrapolations, these three values can be considered to be the same within experimental error. Using pyrene fluorescence, we have determined the cac for the TFA salt of C16-KTTKS to be 0.03 wt %. All of the aggregation curves in Figure 1 show a much broader transition than we have previously observed for peptide amphiphiles such as C16-βAH (C16-carnosine, βA denotes β-alanine)26 or C16-KTTKS.27 We ascribe this to the higher purity of the peptide amphiphiles previously investigated (which had typical purity levels of 97% +). Consistent with this, we observed a broader transition with a cac = 0.0038 wt % for C18-KTTKS.28 To probe the secondary structure of the peptide, and to provide information on the ordering of the lipid chains, we measured FTIR spectra above the cac. Figure 2 shows spectra in the amide I and II regions (Figure 2a−c) sensitive to secondary structure and the region (Figure 2d−f) with peaks corresponding to CH2 group deformations, respectively. For all samples, the principal peak intensities increase with peptide concentration. PA C16-GHK shows a series of peaks indicating a well-defined secondary structure. In particular, the amide I peaks at 1638 and 1675 cm−1 correspond to β-sheets, although there is a peak at 1658 cm−1 indicating some apparent turn/ bend content.29 The amide II C−N stretch and N−H bend deformations at 1577 and 1551 cm−1 are prominent. C16-KT shows a strong β-sheet peak at 1631 cm−1. For C16-KTTKS, this is shifted to 1608 cm−1, and the shoulder at 1646 cm−1 is

Figure 1. Dependence of I485/I0 on the sample concentration for (a) C16-GHK, (b) C16-KT, and (c) C16-KTTKS. I485 is the fluorescence emission intensity at 485 nm for samples containing PA and ThT, while I0 is the fluorescence emission intensity at 485 nm for samples containing only ThT. The insets in (a)−(c) show representative examples of the emission fluorescence spectra.

associated with disordered structure. The location of the βsheet peak for this latter PA is consistent with previous measurements.14,30 It is shifted to lower frequency than usual for β-sheets, possibly indicating weaker hydrogen bonding or the influence of lysine side chains (this PA has two lysine side chains, the others only have one).14,29,31 All three samples show peaks at 2913−2921 and 2847−2850 cm−1 (Figure 2b,d,f). These modes correspond to antisymmetric and symmetric CH2 stretching deformations, respectively,32,33 and indicate ordering of the C16 lipid chains. There are no qualitative changes in the 9151

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Figure 2. FTIR spectra measured for (a,b) 0.1−1 wt % C16-GHK, (c,d) 0.1−1 wt % C16-KT, and (e,f) 0.1−1 wt % C16-KTTKS.

diffraction were employed along with optical and electron microscopy. Figure 4 shows SAXS intensity profiles measured

position of the peaks with concentration, as expected because the PAs are aggregated above the cac. Circular dichroism (CD) spectroscopy also provides information on peptide secondary structure. Figure 3 shows

Figure 4. SAXS spectra measured for 1 wt % solutions containing C16GHK, C16-KT, or C16-KTTKS. Figure 3. CD spectra measured for solutions containing 0.1 wt % of C16-GHK, C16-KT, and C16-KTTKS.

for 1 wt % samples of the three PAs. The data show first-order reflections with positions that correlate to the length of the peptide unit: C16-KTTKS shows a first-order peak corresponding to d = 51.6 Å, in good agreement with the value 52.5 Å reported previously.14,30 This spacing corresponds to an interdigitated bilayer of the peptide amphiphile. For C16GHK the spacing is 40.5 Å. We assign this also to an interdigitated bilayer structure because the estimated length of the PA is 16 × 1.2 + 3 × 3.2 Å = 29 Å (we have used the repeat distance 3.4 Å for residues in a parallel β-sheet38). For C16-KT, SAXS provides d = 25.8 Å. This is very close to the estimated length of the molecule and suggests a layer structure one molecule thick (likely comprising alternating “up” and “down” molecules so that the lipid chains are not exposed to water). Xray diffraction complemented the SAXS results and extended the structural measurements to the length scale of the cross-β

spectra measured for the three PAs. The spectrum for C16GHK contains a broad maximum centered at 228 nm. We have observed similar spectra for histidine-containing Fmoc-βAH34 [Fmoc: N-(fluorenyl-9-methoxycarbonyl)] and C16-βAH,26 and the CD spectrum for random coil fully protonated poly-Lhistidine is characterized by a maximum at 222 nm.35−37 C16KT exhibits a β-sheet like CD spectrum, with a minimum at 217 nm. In contrast, C16-KTTKS exhibits a spectrum with two minima at 202 and 218 nm, as reported previously.14 The latter corresponds to β-sheet structure, while the former is as yet unassigned. Turning to the nature of the self-assembled structures formed by the three PAs above their cac’s, SAXS and X-ray 9152

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structure exhibited by β-sheet assemblies.39 Figure 5 shows XRD intensity profiles obtained by radial integration of the 2D

Figure 6. POM images obtained for 1 wt % lipopeptide solutions (a) C16-GHK, (b) C16-KT, and (c) C16-KTTKS. Image (d) corresponds to a 1 wt % C16-KTTKS sample stained with 0.2 wt % Congo red. Samples in (a) and (b) could not be stained with Congo red, probably due to a crystal-like structure of the textures.

Figure 5. XRD profiles measured for stalks prepared from 4 wt % lipopeptide solutions (a) C16-GHK, (b) C16-KT, and (c) C16-KTTKS. The insets in (a),(b) show the corresponding 2D XRD pattern.

fiber diffraction patterns shown inset. The long-spacing for C16GHK (Figure 5a) falls close to the beamstop and could not be reliably determined. Nonetheless, the pattern shows cross-β features with a 4.8 Å strand spacing. The diffraction pattern is not well oriented. The pattern for C16-KT (Figure 5b) shows stronger peaks, and some orientation is observed. A prominent equatorial reflection corresponds to d = 27.8 Å, close to the spacing observed by in situ SAXS along with a 4.7 Å β-strand spacing. The XRD data for C16-KTTKS (Figure 5c) show a greater number of sharp reflections including a second-order reflection from the bilayer structure (labeled 2). The pattern is highly oriented. The reflections for this sample have been analyzed in detail elsewhere,30 and many of the assignments of intermediate peaks can be used for the XRD data for the other two samples. Polarized optical microscopy (POM) was used to image the texture of the samples (Figure 6). Both C16-GHK and C16-KT form small aggregates resembling microcrystals (Figure 6a,b), whereas C16-KTTKS self-assembles into extended fibrillar structures, consistent with prior observations.14 This sample also shows staining with Congo red at a 1 wt % concentration, leading to green birefringence in the POM image. This is diagnostic of amyloid β-sheet formation.25 In contrast, C16GHK and C16-KT did not exhibit uptake of Congo red, possibly due to the short-range pseudocrystalline morphology. These conclusions concerning morphology were confirmed by TEM. Figure 7 shows representative images. C16-GHK and C16-KT show pseudocrystalline structures with plate-/tape-like morphology (consistent with the layered molecular organization revealed by SAXS and XRD). In contrast, C16-KTTKS shows

Figure 7. TEM images obtained from samples dried from (a,b) 0.1 wt % C16-GHK, (c,d) 0.1 wt % C16-KT, and (e,f) 1 wt % C16-KTTKS solutions.

noncrystalline extended tape-like structures, along with more flexible thin fibrils, features that closely resemble those reported in prior work.14,30 The tapes are several hundred nanometers wide but much thinner. 9153

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DISCUSSION All three PAs show a similar cac in wt %,; however, converting to molar concentrations, the cac values will be in the order KT > GHK > KTTKS. The similarity in cac values suggests that this is dominated by the lipid chain length (consistent with this, we have reported a lower cac for C18-KTTKS and a higher one for C14-KTTKS as compared to C16-KTTKS28). FTIR spectra show β-sheet structures and lipid chain aggregation for all three PAs. C16-GHK appears to have a lower β-sheet content than C16-KT, which in turn displays less order than C16-KTTKS. Overall, the lipid chain order shown in all three peptides supports the assertion that lipid chain aggregation is the main driving force for self-assembly. CD spectroscopy also shows ordering of the three PAs, although the spectrum from C16GHK is dominated by the signal from the histidine chromophore. SAXS data are consistent with expected bilayer packings of the peptide amphiphiles with spacings proportional to the length of the peptide amphiphiles stacked in bilayer structures. XRD data show cross-β structure for all three peptide amphiphiles. Orientation in the XRD data for C16-KTTKS suggests a higher degree of order within the sample and longer fibers, which create some nematic alignment in the selfassembled gels at these high concentrations. C16-KT and C16GHK do not show bulk orientation, which suggests that the aggregates are still relatively small, most likely in the form of microcrystals, consistent with polarized optical microscopy images. Congo red staining shows the presence of organized cross-β structure, which is diagnostic of β-sheet aggregates; only C16KTTKS exhibits staining at 1 wt %, which is well above the cac. This suggests that the C16-KT and C16-GHK aggregates are not as highly ordered as C16-KTTKS and therefore do not bind the congo red dye effectively. TEM images for C16-KT and C16-GHK show the formation of small flat crystalline aggregates at 0.1 wt %. C16-KTTKS in comparison forms a mixture of large flat sheets and long fibrous aggregates. The structures observed in these images are consistent with the conclusions from XRD, CD, and FTIR data. The morphology of the self-assembled aggregates formed by the peptide amphiphiles is consistent with predictions from modeling simulations previously reported by Stupp and coworkers on peptide amphiphile self-assembly.40 From their simulations, it would be expected that flat stacks of β-sheets will form where total hydrogen-bonding energy from the peptide headgroups is similar to the total hydrophobic interaction energy from alkyl tail association. As the hydrogen-bonding contributions from the peptide headgroups increase in magnitude, long fibrous aggregates are predicted. This is consistent with our observations where the di- and tripeptide headgroups of C16-KT and C16-GHK do not form highly organized β-sheets as shown by FTIR, and these PAs are also observed to form flat crystallites, whereas C16-KTTKS has a high β-sheet content and forms highly organized fibrous aggregates due to increased hydrogen-bonding contributions. The fibrous aggregates formed by C16-KTTKS are also typical of the structures observed for peptide self-assembly where twisted β-sheet tapes form that twist around one another to form highly organized fibrils and fibers, suggesting that the βsheet forming headgroups play a more dominant role in the supramolecular structure than was observed for the two other PAs.41 Overall, lipid chain aggregation has been shown to be

the dominant factor driving self-assembly, although the peptide headgroup does have some influence upon self-assembly and a significant influence upon the morphology and stability of the self-assembled aggregates. It is interesting to speculate whether the morphology of the self-assembled aggregates may also have some importance when considering the potential collagen stimulating effect of these self-assembled PAs in the in vitro cell culture experiments. We have previously demonstrated that the presence of selfassembled C16-KTTKS aggregates can significantly increase the collagen production of human fibroblasts. However, in this Article, the self-assembled aggregates formed by C16-KT and C16-GHK have been shown to be smaller and less wellorganized than the C16-KTTKS aggregates and do not form the long fibrous aggregates that have been postulated to mimic the collagen and fibronectin fibers of the ECM. Comparison of the in vitro collagen stimulating effect of these three lipopeptide in their self-assembled form may deliver some important insights on the role of higher-order structure in collagen and fibronectin biosynthesis, which has immediate relevance for tissue engineering and biomedical applications.21 From a skin care perspective, although we have shown that C16-KTTKS forms the most stable and well-organized self-assembled structures, the monomeric C16-KTTKS would be expected to have the lowest permeability through the stratum corneum based on the molecular weight; therefore, monomeric C16-KT would be the preferred molecule of the three PAs for current skin care products where the monomeric peptide is used. Importantly, this insight suggests that there is an opportunity to design new PAs that combine efficient self-assembling behavior with maximum permeability through the stratum corneum for future skin care applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the supply of all peptide powder samples used in this study from Sederma, France, subsidiary of Croda International Plc, UK. We acknowledge use of facilties in the Centre for Advanced Microscopy and Chemical Analysis Facility at the University of Reading, and M.r A. Dehsorkhi for SAXS measurements.

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REFERENCES

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dx.doi.org/10.1021/la401771j | Langmuir 2013, 29, 9149−9155