Role of topological scale in the differential fouling of Pseudomonas aeruginosa and Staphylococcus aureus bacterial cells on wrinkled gold-coated polystyrene surfaces

Duy H. K. Nguyen a, Vy T. H. Pham a, Vi Khanh Truong a, Igor Sbarski b, James Wang b, Armandas Balčytis a, Saulius Juodkazis a, David E. Mainwaring a, Russell J. Crawford c and Elena P. Ivanova *a
aSchool of Science, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, VIC 3122, Australia. E-mail:
bSchool of Engineering, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, VIC 3122, Australia
cSchool of Science, College of Science, Engineering and Health, RMIT University, Melbourne, VIC 3001, Australia

Received 3rd November 2017 , Accepted 7th February 2018

First published on 7th February 2018

Wrinkled patterns, which possess an extensive surface area over a limited planar space, can provide surface features ranging across the nano- and microscale that have become an engineering material with the flexibility to be tuneable for a number of technologies. Here, we investigate the surface parameters that influence the attachment response of two model bacteria (P. aeruginosa and S. aureus) to wrinkled gold-coated polystyrene surfaces having topologies at the nano- and microscale. Together with flat gold films as the controls, surface feature heights spanned 2 orders of magnitude (15 nm, 200 nm, and 1 micron). The surface wrinkle topology was shown through confocal laser scanning microscopic, atomic force microscopic and scanning electron microscopic image analyses to consist of air–water interfacial areas unavailable for bacterial attachment, which were also shown to be stable by time-lapsed contact angle measurements. Imposition of the nanoscale wrinkles reduced P. aeruginosa attachment to 57% and S. aureus attachment to 20% of their flat equivalent surfaces whereas wrinkles at the microscale further reduced these attachments to 7.5% and 14.5%, respectively. The density of attachments indicated an inherent species specific selectivity that changed with feature dimension, attributable to the scale of the air–water interfaces in contact with the bacterial cell. Parameters influencing static bacterial attachment were the total projected surface areas minus the air–water interface areas and the scale of these respective air–water interfaces (area distribution) with respect to the cell morphology. The range of these controlling parameters may provide new design principles for the evolving suite of physical anti-biofouling materials not reliant on biocidal agents under development.


Surface wrinkling with features ranging from macro-, micro- and nanoscale has become one of the most fascinating morphological patterns for both artistic and scientific inspiration in many decades.1–3 Various examples can be found in the bulking of tree barks, the ridging of fruits and vegetable skins or the folding pattern of cortex during brain development.1–3 Wrinkled patterns, which possess an extensive surface area over a limited planar space, generally occur when a layer is forced to extend or shrink under certain conditions, resulting in a dramatic, yet potentially beneficial, change of material properties.4 This principle has become the foundation of a new focus for engineering materials with tuneable surface topologies providing a flexibility in manipulating surface physical, mechanical and biological properties for various applications.5–7

To create a wrinkled surface, compression is frequently required to trigger surface instability of a layered structure comprised of two or more materials with different surface stiffness.8,9 The deformation mismatch caused by the compressive strain then leads to surface undulation creating certain topographical wrinkled patterns. Such deformation can be induced by thermal expansion, oxidation, diffusion, electromagnetic field, ion irradiation or mass extension of cells.9,10 Studies have predominately focused on applying this principle to different bilayer systems, comprising of a polymeric substratum and a thin layer of metal. A heating process can then be used to induce wrinkling, whereby tuneable wrinkled features are generated by varying the thickness of the metal layer.11,12 This fast and facile fabrication approach has been applied to create different physical, optical and electrical effects in various devices.13–15

Recently it was reported that wrinkled surface patterns at the micron-scale level also exhibited antifouling characteristics.16–19 Initially a hierarchical wrinkled poly(dimethyl siloxane) (PDMS) surface was shown to exhibit antifouling activity lasting 18 months,16 which stimulated further studies focussed on anti-biofouling properties of various such surfaces. Freschauf et al. showed a reduction of Escherichia coli cells attachment to wrinkled surfaces of common plastic substrata; however, Alexander et al. reported an opposite behaviour, e.g., in static conditions increased attachment propensity of E. coli bacterial cells on wrinkled PDMS surfaces.17,18 A mixed response of other microorganisms including P. aeruginosa and S. aureus bacterial cells has been also reported.18 Despite several studies on the micro- and macro-sized wrinkled surfaces, the antibiofouling effect of nano-scale wrinkled surfaces remains inconsistent and poorly understood.16–19 Importantly, due to such inconsistencies reported on systems containing surfaces that possessed antifouling polymer functionality, we address here the underpinning effects of topology on antibiofouling using model inert surfaces. This study reports the first antibiofouling effect of nano-wrinkled surfaces and in doing so identifies a range of controlling parameters critical for anti-bacterial materials design. This paper details the attachment response of two pathogenic biofilm forming bacteria, P. aeruginosa ATCC 9721 and S. aureus CIP 65.8T, on gold coated thermally-induced wrinkled polystyrene surfaces. Two distinct micron and nano-scale biaxial disordered wrinkled patterns were fabricated on a planar polymer substratum by varying the gold coating thickness.

Experimental details

Fabrication of wrinkled surfaces

A transparent Pla-Plate® polystyrene sheet (Tamiya Corporation, Japan), with a thickness of 0.4 mm, was cut in squares (10 mm × 10 mm). Samples were washed with 70% ethanol and deionized water, dried with N2 gas before use. Gold sputtering was performed using MP-19020NCTR NeoCoater (JEOL, Japan) in order to yield the coating thicknesses of 2.2 nm and 11 nm. Samples were then heat-treated at 130 °C for 30 minutes in an UNB500 universal oven (Memmert GmbH+Co. KG, Germany) without mechanical constrains, which allowed a biaxial heat-induced wrinkling to occur. The samples were shrunk to 50% of their original dimensions in x and y directions (5 mm × 5 mm), while thickness increased in z direction to 2 mm. The wrinkled polystyrene surfaces coated with 2.2 nm (W2) and 11 nm (W11) gold layer were used throughout this study, together with as-received flat polystyrene (PS) and equivalent planar gold films tested prior to thermal treatment (F2) and (F11) as respective controls. For sterilization, all samples were washed with 70% ethanol three times and dried at 22 °C overnight in biological safety cabinet prior each experiment.

Scanning electron microscopy (SEM)

High-resolution scanning electron micrographs were obtained at 3 kV under 25[thin space (1/6-em)]000×, 50[thin space (1/6-em)]000× and 100[thin space (1/6-em)]000× magnification using a ZEISS SUPRA 40VP field-emission scanning electron microscope (SEM) (Carl Zeiss NTS GmbH, Oberkochen, BW, Germany). A quantitative morphological analysis was carried out using top-viewed images at 25[thin space (1/6-em)]000× and 50[thin space (1/6-em)]000× magnification using the Fiji distribution of ImageJ software.20 Wrinkle parameters were defined as wrinkle wavelength (λ) and wrinkle amplitude (A). Fast Fourier Transform (FFT) analysis was carried out on five converted binary SEM micrographs for each sample. Two samples of each surface type were then analysed. The wavelength of the wrinkle and preferential directionality of micron scale features was then estimated using radial profile angle and directionality plugin.21

Atomic force microscopy (AFM)

The surface topography was visualized using an Innova® atomic force microscope (Bruker, USA). Measurements were performed in air, at ca. 22 °C using a phosphorus doped silicon probe (MPP-31120-10, Bruker, USA) in tapping mode. AFM scans obtained from 5 μm × 5 μm surface areas were processed using Gwyddion software.22 The amplitude and wavelength were further analysed using five surface line profiles for each AFM scan. Three AFM micrographs were obtained for each of the surface types. Surface roughness (Sa) was also retrieved from AFM data.23

X-ray photoelectron spectroscopy (XPS)

Surface elemental analysis was performed using a Thermo Scientific™ K-alpha X-ray Photoelectron Spectrometer (ThermoFischer), equipped with a monochromatic X-ray source (Al Kα, = 1486.6 eV) operating at 150 W.24 Photoelectrons emitted at 90° to the surface from an area of 700 μm × 300 μm were analysed with 160 eV for survey spectra and then with 20 eV for region spectra. Survey spectra were recorded at 1.0 eV per step, while the region spectra were taken at 0.05 eV per step. The relative atomic concentration of elements determined using XPS was quantified based on the peak area in the selected high-resolution region, with the appropriate sensitivity factors for the instrument being used. High-resolution scans were performed across each of the carbon 1s and gold 4f peaks.

Bacterial strains and sample preparation

Pseudomonas aeruginosa ATCC 9721 and Staphylococcus aureus CIP 65.8T were obtained from the American Type Culture Collection (ATCC, USA) and Culture Collection of the Institute Pasteur (CIP, France), respectively. Bacterial stocks were prepared in 20% glycerol nutrient broth (Oxoid) and stored at −80 °C until needed. Bacterial cultures were refreshed from stocks on nutrient agar (Oxoid) for 24 h at 37 °C and the cells were collected prior to each experiment. A fresh bacterial suspension with OD600 = 0.1 was prepared for each of the strains in 20 mL of nutrient broth. Sterilized samples were immersed in 1 mL bacterial suspension at 25 °C. After 18 h of incubation, the samples were retrieved from the suspension, washed gently three times with 10 mM phosphate buffer saline (PBS) and used for visualization.

Confocal laser scanning microscopy (CLSM)

Visualization and quantification of viable bacteria were performed using Olympus Fluoview 1000 IX81 (Japan) confocal laser scanning microscope, operated using 60× water-immersion objective (UPlanSApo 60×/1.20 W, Olympus, Japan, NA = 1.2) combined with 3× optical zoom. Bacterial cells were stained with a LIVE/DEAD BacLight Bacterial Viability Kit (ThermoFisher Scientific); live cells were stained green with SYTO 9 and damaged cells were stained red with propidium iodide as described elsewhere.25 Bacterial cells were counted using five microscope fields at 180× magnification for each sample. The total number of attached cells was evaluated as a sum of live and damaged cells. The results were derived from the average of at least three independent experiments, containing two replicates for each experiment.

Surface wettability

Surface wettability was examined using the sessile drop method by the measurements of the static water contact angles on the samples. The contact angle measurements were carried out in air using an FTA1000c equipped with a nanodispenser (First Ten Ångstroms, Inc., Portsmouth, VA, USA) with droplets of approximately 8.0 μL. The results were acquired over five measurements for each sample type. A time lapsed imaging of water droplet was conducted to monitor the transition of water, which replaced air-trapped in wrinkled pattern, as detailed in the ESI.

Composition of sample interface

The presence of any air entrapped within the respective wrinkled surfaces structures which may influence bacterial attachment was estimated by CLSM and SEM. Here, W11 samples were immersed in fluorescein solution, 0.1% (w/v) fluorescein solubilized in MilliQ water prior to CLSM examination, where it was considered that the fluorescent solution would not penetrate entrapped air regions.26 Here black areas within the CLSM image represent both the air-solution interface and the upper regions of the Au topography. The extent of the air–water interface was estimated by the difference between these values for W11 samples. Due to the limitation of the CLSM resolution, the extent of trapped air in the W2 sample was estimated from SEM images using the relationship between the W11 CLSM image and its corresponding SEM. Three hour time-lapse droplet imaging was then carried out to observe the stability of trapped air.

Statistical analysis

Quantification of attached cells, available surface areas and water contact angles were statistically analysed using two-tailed t-tests for the significance of the difference between the means of two independent samples (Microsoft Excel 2016). Statistically significant differences were considered if p-values were less than 0.05 (*p < 0.05).

Results and discussion

Characterisation of wrinkled surfaces

The two wrinkled patterns, W2 and W11, fabricated using heat-shrinkable gold-coated polystyrene substrata are shown in Fig. 1. Further discussion of wrinkled pattern formation was provided in ESI, together with glass transition temperature and chemical bondings derived from Tan Delta peak and Fourier transform infared spectroscopic spectra, respectively (Table S1 and Fig. S1). Surface physico-chemical characteristics were assessed using SEM, AFM and XPS. XPS high-resolution spectra displayed characteristic π–π* and C–C peaks, resulting from the aromatic ring of the polystyrene (Fig. S2), whereas, C[double bond, length as m-dash]O and C–O peaks may be attributed to adventitious species adsorbed when Au surfaces are exposed to air. The results of XPS elemental analysis of Au, C, O and N are given in Table S2. Elemental analysis showed that with the increasing atomic percentage of Au (W11), the atomic % of C, O and N decreased consistent with the profile depth sensed and adventitious adsorption on the Au films as above.
image file: c7nr08178b-f1.tif
Fig. 1 Surface morphology of the wrinkled polystyrene surfaces covered with a 2.2 nm (W2) and 11 nm (W11) gold coating visualized with (a) SEM and (b) AFM. The typical undulation of the nanowrinkles and microwrinkles, as detected from the micrographs, are reflected in surface buckling at different film thickness. AFM micrographs taken over a scanning area of 5 μm × 5 μm, with (c) the corresponding surface line profile representing the roughness. Colour scale bars in AFM micrographs are in nm. Scale bars in SEM micrographs are 500 nm.

The surface topologies of the wrinkled surfaces (W2, W11, and control surfaces) were visualized using AFM and SEM (Fig. 1 and 2, ESI Fig. S3). The cross-sectional surface profiles showed significant differences in height between the planar control surfaces (PS, F2, F11) and the wrinkled W2 and W11 surfaces that extended over 2 orders of magnitude (15 nm, 200 nm and 1 μm), respectively. The surface roughness Sa also showed a consistent rise from planar controls to wrinkled surfaces due to the increase in coating thickness and the formation of nano- and microwrinkles. Therefore, the wrinkled surfaces appeared to be more hydrophobic than planar surfaces (Table 1, ESI Table S3).

image file: c7nr08178b-f2.tif
Fig. 2 Wavelength and wrinkle orientation analysis on wrinkled surfaces covered with a 2.2 nm (W2) and 11 nm (W11) gold films. (a) Binary converted SEM micrographs with corresponding FFT micrographs (inset) employed to determine the wavelength of the wrinkles. (b) Directionality histogram (Fourier component analysis) shows the lack of preferential orientation of wrinkles, indicating a random self-organized wrinkle formation (see Wrinkle pattern characteristics, ESI). Scale bars are 2 μm.
Table 1 Characterization of wrinkled surfaces covered with a 2.2 nm (W2) and 11 nm (W11) gold film
Parameter Positiona W2 W11
a The topology comprised small wrinkles organized on top of the larger wrinkles.
Amplitude (nm) Upper 45 ± 14 317 ± 52
Lower 178 ± 47 1113 ± 132
Wavelength (nm) Upper 91 ± 15 318 ± 116
Lower 641 ± 89 3295 ± 1044
Water contact angle, θ (°) 124 ± 6 130 ± 4
Surface roughness, Sa (nm) 41 ± 1 258 ± 33

Amplitude, wavelength and directionality of wrinkled patterns were further studied using SEM images converted into binary data which were further analysed through the FFT plugin (Fig. 2). Both types of wrinkled surfaces appeared as biaxial, disordered and hierarchically structured system, similar to those reported previously.11,27 For example, the amplitude of two-tier wrinkles on W11 surfaces was found to be 6.5–7.1 times greater compared to those in W2. The W11 surfaces had a wavelength of 318 nm ± 116 nm, comparable to that of 270 nm ± 50 nm of reported wrinkled surfaces fabricated using 12 nm gold layer.12 Overall, the W11 samples wavelength was found to be between 3.5–5.1 times greater than that of W2 (Table 1).

Notably, the length scale of the wrinkles is strongly dependent on the thickness of the metal film, which reflected the overall mechanical properties of the coated film – substratum system such as the compressive strains produced. The experimental characteristics were then compared with theoretical estimations based on elastic buckling theory,8,28,29 where wrinkled surfaces arising from the interface of soft matter with hard skin can be described in terms of the critical strain εc, the spatial periodicity of wrinkles along the planar surface (characteristic wavelength λ) and their amplitude A through the following expressions.8,28,29

image file: c7nr08178b-t1.tif(1)
image file: c7nr08178b-t2.tif(2)
image file: c7nr08178b-t3.tif(3)
where ν is the Poisson ratio, E is the Young's modulus, h is the thickness of the film and ε is the compressive strain, and subscripts c, s and f denote critical, substrate and film, respectively.

E f and νf have been reported as 69.1 GPa and 0.44 respectively for Au thickness of 18 nm, while for a PS substratum Es and νs are reported as 0.4 GPa and 0.38 at 130 °C corresponding to the temperature at which fabrication was conducted and wrinkles generated.30–32 The calculated wavelength λ of W2 and W11 surfaces was 50 nm and 272 nm, respectively, which is close to the experimental data at 91 nm ± 15 nm for W2 surfaces and 318 nm ± 116 nm for W11 surfaces. Similarly, the theoretical estimation of critical strain εc was 0.016; and hence, the corresponding amplitude A (at constant compressive strain ε = 0.75) can be estimated as 13 nm for W2 and 74 nm for W11. Discrepancy between experimental data and theoretical values may be due to: (i) expressions 1–3 designed for a completely elastic substratum rather than the heat-shrinkable PS at temperatures greater than Tg; (ii) the model applied to linear buckling of a single wrinkle scale instead of multi-tier wrinkled surfaces, and (iii) numerical data of ν and E reported for both the Au film and PS substratum represent an estimation since data on the materials at the same thickness was unavailable.

Detection of trapped air and interfacial area available for bio-fouling

Control surface (PS) appeared in the CLSM to be homogeneously green fluorescent when immersed in fluorescein solution showing complete permeability of the solution at the surface, whereas with the W11 surface a heterogeneous pattern of green and black areas was clearly distinguished (Fig. 3) indicating a lack of ingress into the topology. Based on the CLSM areas and SEM areas, the projected available surface area was estimated from these binary images. The projected interfacial area of W2 and W11 available for microbial adhesion was found to be 82.3% and 70.5%, respectively, compared to that of planar control surfaces. The stability of air entrapped within W11 wrinkled surfaces were further confirmed by time lapsed imaging of water droplets for 3 h (ESI Fig. S4).
image file: c7nr08178b-f3.tif
Fig. 3 Detection of air entrapment within the wrinkled surfaces. (a) CLSM micrographs of planar polystyrene and wrinkled 11 nm Au (W11). The uniformity of fluorescein (green) signal indicated the permeability of this solution onto the different surfaces. The area of the surface containing entrapped air appeared as black due to the inability of the fluorescein solution to enter the surface due to the air barriers. (b) Time-lapse imaging over 3 h confirmed the stability of the air entrapped within W11 surfaces. (c) Histogram of green signal inside the red-squared region showed the consistency of the fluorescein signal, that the trapped air had not been replaced by liquid at least for 3 h. Scale bars are 5 μm.

Bacterial attachment on wrinkled surfaces

The dependency between the surface areas available for attachment and bacterial cell morpho-types has been studied experimentally, e.g. through attachment point mechanisms of diatoms on microtextured polycarbonate surfaces, air-manipulated S. aureus on lotus-like titanium surfaces and limited surface accessibility due to trapped interfacial air pockets for Amphora coffeaeformis settlement.33–35 Such studies generally consider that strong cell attachment only occurs if the surface provides sufficient anchoring points.

This study of wrinkled topography is consistent with these overall observations where pockets of entrapped air provide limited surface areas available as anchoring points, thereby restricting both P. aeruginosa and S. aureus cells attachment. Fig. 4 provides CLSM and SEM images of both types of wrinkled surfaces together with their corresponding flat controls in the presence of P. aeruginosa and S. aureus, where both live and damaged cells can be seen at each surface. Notably under static conditions, both the bacterial species and surface topology impose significant influences on the extent of overall cell attachment. Quantification of this attachment (Fig. 5 and Table S4) details both these controlling parameters. The planar control surfaces showed on average an overall increase in bacterial attachment to gold compared to the highly hydrophobic polystyrene surface (Fig. 5a) which was also sensitive to species. On planar gold surfaces (Fig. 5b), the surface density of both P. aeruginosa and S. aureus under the same conditions increased with film thickness, consistent with the increased surface roughness seen in Fig. S-3 and Table S-3, ESI.

image file: c7nr08178b-f4.tif
Fig. 4 CLSM and SEM micrographs of P. aeruginosa and S. aureus bacterial cells attached to planar and wrinkled surfaces after an 18 h incubation period. Live cells are stained green; damaged cells are stained red. Scale bars in confocal and SEM micrographs 10 μm and 1 μm, respectively.

image file: c7nr08178b-f5.tif
Fig. 5 Attachment of P. aeruginosa and S. aureus bacterial cells on planar and wrinkled surfaces after an 18 h incubation period. (a) Comparison between planar control surfaces polystyrene substratum and a gold film on polystyrene. (b) Correlation of available surface area and attached bacterial cells on flat gold surfaces (F2 & F11) and wrinkled surfaces (W2 & W11).

Imposition of the wrinkle nanotopology (W2) reduced the inherent density of rod-shaped P. aeruginosa and spherical S. aureus by 57% and 20% respectively while the microscale topology of W11 reduced attachment of both species still further where they now represented 7.5% and 14.5% respectively of their planar counterpart (F11). There was no statistically significant difference in S. aureus attachment between either two planar surfaces (F2 & F11) or two wrinkled surfaces (W2 & W11). The responses of the respective nano- and microtopologies are consistent with the projected surface areas available for attachment as well as indicating a clear reversal in attachment selectivity between species (Fig. 5). This further suggests that attachment of the smaller spherical S. aureus cell morphology (Fig. 4–6) is less sensitive to the scale of the air–water interfaces presented statically to the cell membrane compared to rod-shaped P. aeruginosa. As seen in Table S4, both topologies (W2, W11) yielded only ∼17% damaged P. aeruginosa cells and 5% damaged S. aureus cells, indicating that these wrinkled surfaces were antifouling rather than bactericidal. Previously the decrease of P. aeruginosa cell attachment has been achieved on uniaxial wrinkled surfaces under continuously dynamic (deforming shear) conditions,18 here we now show the extent and selectivity of anti-biofouling under static conditions.

image file: c7nr08178b-f6.tif
Fig. 6 Mechanisms underlying the influence of biaxial wrinkled surfaces on P. aeruginosa and S. aureus attachment. Wrinkled surfaces, including multiscale structures (shown by surface line profiles) and trapped air (coloured blue), provided unfavourable conditions for the attachment of bacterial cells after an 18 h incubation period, compared to their attachment on the control flat surfaces. The attachment of rod-like P. aeruginosa cells was influenced by the presence of both nanowrinkles (W2) and microwrinkles (W11) due to the limited surface area available for attachment. The attachment of spherical S. aureus cells, however, was less influenced by the micron-scale pattern due to the cell morphology.


In this study, we investigated the parameters that influence the attachment responses of Gram-negative rod-shaped P. aeruginosa and spherical Gram-positive S. aureus bacterial cells using two distinct topologies consisting of nanoscale and microscale biaxial disordered wrinkle patterns fabricated as thermally treated thin gold films on a polymer substrate. Varying the Au film thickness allowed systematic variation in surface feature height across a range that spanned two orders of magnitude, i.e., planar Au (Fav) film gave 15 nm, 2.2 nm Au (W2) gave 200 nm and 11.0 nm Au (W11) gave 1 μm high features, as determined by AFM. Time lapsed contact angle measurements indicated stable air entrapment within these surface features which was quantified by CLSM fluorescence and SEM imaging.

The nature of the surface participating in bacterial cell attachment was analysed and shown to consist of a corresponding range of projected areas available for adhesion, producing two influences on this adhesion: (1) total projected surface areas minus the air–water interface areas, and (2) the scale of these respective air–water interfaces (area distribution). The density of attachment to the planar Au surfaces indicated an inherent species specific selectivity that changed with the dimension of these surface features.

Imposition of the nanoscale wrinkles reduced P. aeruginosa attachment to 57% and S. aureus attachment to 20% of their flat equivalent surfaces whereas wrinkles at the microscale further reduced these attachments to 7.5% and 14.5%, respectively. The relative species attachment selectivity of W2 and W11, that indicated a lower sensitivity for the smaller spherical S. aureus cells, suggests that the size distribution of the air–water interfaces not available for attachment (Fig. 4 & 5) also plays a role in the underlying attachment efficiency, as illustrated figuratively in Fig. 6. The range of these controlling mechanisms may provide design principles for the evolving suite of physical anti-biofouling materials not reliant on biocidal agents under development.

Conflicts of interest

There are no conflicts to declare.


Authors gratefully acknowledge the RMIT Microscopy and Microanalysis Facility (RMMF) for providing access to the characterization instruments. DHKN is the recipient of Swinburne University Postgraduate Research Award (SUPRA). This research was undertaken on the FTIR beamline at the Australian Synchrotron, part of ANSTO.


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Electronic supplementary information (ESI) available: Supplementary experimental details; supplementary results and discussion; supplmeentary tables; and supplementary figures. See DOI: 10.1039/c7nr08178b

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