Stabilization of Metal Nanoparticle Films on Glass Surfaces Using

Oct 9, 2013 - ... previously used for modifying NPs in solution(26, 27) or as a coating on evaporated gold island films.(28, 29) The method includes a...
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Stabilization of Metal Nanoparticle Films on Glass Surfaces Using Ultrathin Silica Coating Yulia Chaikin, Ofer Kedem, Jennifer Raz, Alexander Vaskevich,* and Israel Rubinstein* Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 7610001, Israel S Supporting Information *

ABSTRACT: Metal nanoparticle (NP) films, prepared by adsorption of NPs from a colloidal solution onto a preconditioned solid substrate, usually form well-dispersed random NP monolayers on the surface. For certain metals (e.g., Au, Ag, Cu), the NP films display a characteristic localized surface plasmon resonance (LSPR) extinction band, conveniently measured using transmission or reflection ultraviolet−visible light (UV-vis) spectroscopy. The surface plasmon band wavelength, intensity, and shape are affected by (among other parameters) the NP spatial distribution on the surface and the effective refractive index (RI) of the surrounding medium. A major concern in the formation of such NP assemblies on surfaces is a commonly observed instability, i.e., a strong tendency of the NPs to undergo aggregation upon removal from the solution and drying, expressed as a drastic change in the LSPR band. Since various imaging modes and applications require dried NP films, preservation of the film initial (wet) morphology and optical properties upon drying are highly desirable. The latter is achieved in the present work by introducing a convenient and generally applicable method for preventing NP aggregation upon drying while preserving the original film morphology and optical response. Stabilization of Au and Ag NP monolayers toward drying is accomplished by coating the immobilized NPs with an ultrathin (3.0−3.5 nm) silica layer, deposited using a sol−gel reaction performed on an intermediate self-assembled aminosilane layer. The thin silica coating prevents NP aggregation and maintains the initial NP film morphology and LSPR response during several cycles of drying and immersion in water. It is shown that the silica-coated NP films retain their properties as effective LSPR transducers. leading to a drastic modification of the film morphology and optical properties.15−19 As various characterization techniques (such as electron microscopy) and applications (e.g., photovoltaic cells,20 LSPR based sensors21−25) require dry NP films, preserving the initial (wet) morphology of the NP monolayer upon drying while preventing NP aggregation is highly desirable for obtaining stable, homogeneously distributed, dry NP monolayers in a reproducible manner. In the present work, a simple and scalable scheme for achieving this goal is presented. The morphology and optical response of metal (Au, Ag) NP films, prepared by adsorption of prefabricated metal NPs onto surface-modified glass surfaces, were stabilized toward drying using an ultrathin sol−gel derived silica overlayer, previously used for modifying NPs in solution26,27 or as a coating on evaporated gold island films.28,29 The method includes a two-step procedure, wherein a self-assembled layer of 3-aminopropyl trimethoxysilane (APTS) is first adsorbed on the NP film, followed by deposition of an ultrathin (ca. 3.0−3.5 nm) silica layer from a pH-adjusted sodium silicate solution. It is shown that the

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he wide interest in metal nanoparticles (NPs) is rooted in their special properties, such as small dimensions, large surface-to-volume ratio, the possibility to chemically modify the surface, chemical inertness (in particular Au NPs), and high contrast in various imaging techniques, thus introducing numerous potential applications in nanotechnology and nanomedicine. In the case of metals such as gold, silver, and copper, the NPs display special optical properties, namely, an extinction band in the visible spectral range, attributed to coupling of the free electrons with the incident light through resonant excitation, known as localized surface plasmon resonance (LSPR).1,2 The wavelength, intensity, and shape of the LSPR band are sensitive to the size, shape, and composition of the metal NPs, the interparticle distance, and the dielectric properties of the surrounding medium.3−8 Various applications of metal NPs require their arrangement as thin films on substrates. A common scheme for the preparation of metal NP monolayers on solid supports includes adsorption of presynthesized NPs from a colloid solution onto a preconditioned substrate to which the NPs attach by chemical or electrostatic interactions.9−14 Adsorption from a colloid solution can offer monodisperse and homogeneously distributed NP monolayers on the surface. However, such immobilized NP layers are often unstable in that the NPs tend to aggregate upon removal from the solution and drying, © 2013 American Chemical Society

Received: July 3, 2013 Accepted: September 23, 2013 Published: October 9, 2013 10022

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ultrathin silica coating perfectly stabilizes the NP film toward multiple drying/immersion cycles without any apparent NP aggregation, while enabling use of the NP film as an LSPR transducer in common sensing schemes.

previously reported procedures26,29 with certain modifications. After formation of the metal NP monolayer on glass, the wet slide was washed with MeOH (avoiding drying of the slide) and immersed for 2 h in a freshly prepared solution of 1% (w/ w) APTS in MeOH, washed with MeOH and then with H2O several times and left in H2O. Sodium silicate solution (∼1.5 wt % SiO2, pH ∼12) was prepared by dilution of 2 mL of the original solution (27 wt % SiO2) with triply distilled water to a final volume of 50 mL. The solution was then adjusted to pH ∼8.5−9 using the strongly acidic cation exchanger Amberlite IR-120. Deposition of ultrathin silica layers on Au NP films was carried out by dipping the slides into the sodium silicate solution for 2 h at ∼90 °C, as described previously.29 Immersion of Ag NP film into a silica growth solution at ∼90 °C resulted in fast disappearance of the Ag NP LSPR band, most likely due to silver oxidation. Therefore, ultrathin silica layers were deposited on Ag NP films at room temperature for 3 days, similar to the original protocol of Liz-Marzan et al.26 Polyelectrolyte (PE) Layer-by-Layer (LbL) Assembly. The LbL procedure was carried out using the positive polyelectrolytes PEI and PAH, and the negative polyelectrolyte PSS. The PEI concentration used was 1 mM in H2O, while the concentration of PAH and PSS used was 1.0 mM in 0.1 M NaCl in H2O. All of the polyelectrolyte (PE) solution concentrations were calculated with respect to the PE monomer. The slide to be coated was first immersed in the PEI solution for 15 min, rinsed with water, dipped into an aqueous solution of 0.1 M NaCl, and immersed in the PSS solution for 15 min. After completing the assembly of the first PE bilayer, the slide was washed in H2O, dried under N2, and measured. Subsequent construction of PE bilayers was carried out with PAH/PSS solutions using the same procedure. Stability of the Optical Response. Stability of the metal NP LSPR extinction band was assessed by drying and wetting of the NP-coated slide once or several times. All the spectra were measured in water, before and after drying. After obtaining the initial transmission spectrum of the as-adsorbed, wet NP film, it was dried under a N2 stream, immersed in water again and measured. In some cases, additional drying/ immersion cycles were applied. Characterization Methods. UV-vis Spectroscopy. Transmission UV-vis spectra were obtained with a Varian CARY 50 spectrophotometer. The wavelength resolution was 1 nm, and the average acquisition time was 0.2 s per point. Unless otherwise specified, spectra were measured in the solvent; dried samples were reimmersed in water for the measurement. Spectra were taken using a cuvette with the respective solvent as the baseline. Samples coated with a PE multilayer were dried and measured in air, using a slide holder that ensures reproducible positioning of the sample, with air as a baseline. Transmission Electron Microscopy (TEM). TEM imaging was performed with a Philips CM-120 transmission electron microscope operating at 120 kV, equipped with a chargecoupled device (CCD) camera (2k × 2k, Gatan Ultrascan 1000). High-Resolution Scanning Electron Microscopy (HRSEM). High-resolution scanning electron microscopy (HRSEM) imaging was carried out using an ULTRA 55 FEG ZEISS microscope. Measurements were performed at a working distance of 4 mm, using an Everhart−Thornley secondary electron (SE) detector at an applied voltage of 2 kV.

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EXPERIMENTAL SECTION Chemicals and Materials. Ethanol (Abs. AR, Gadot), methanol (Abs. AR, Mallinckrodt), sulfuric acid (95%−98%, BioLab), hydrogen peroxide (30%, Frutarom), ammonium hydroxide (25%, chemically pure, Frutarom), 3-aminopropyl trimethoxysilane (APTS) (Aldrich), N-[3-(trimethoxysilyl)propyl]ethylenediamine (TMSPEDA) (Aldrich), sodium silicate solution (27 wt %, Sigma−Aldrich), ion-exchanger Amberlite IR-120 (Merck), polystyrene sulfonate, sodium salt (PSS) (70 kDa, Polysciences, Inc.), polyallylamine hydrochloride (PAH) (56 kDa, Sigma−Aldrich), and branched polyethyleneimine (PEI) (750 kDa, 50 wt % solution in water, Aldrich), were used as received. Water was purified by ionexchange followed by double distillation (denoted triply distilled water). Samples were dried under a stream of household nitrogen (from liquid N2) or under ambient conditions (air). Glass Cleaning Procedure. Microscope glass cover slides (No. 3, Schott AG borosilicate glass D263T, supplied by Menzel-Gläser) were cut to 22 mm × 9 mm, using a diamond pen and cleaned using the following procedure: immersion in freshly prepared hot piranha solution (1:3 H2O2:H2SO4) for 1 h, followed by rinsing with triply distilled water, then immersion in “RCA” solution (1:1:5 H2O2:NH4OH:H2O) for 1 h at 70 °C, followed by rinsing with triply distilled water, and rinsing with methanol or ethanol (depending on the silanization procedure, see below) three times, 5 min each, in an ultrasonic bath (Cole-Parmer, Model 8890). [Caution: Piranha solution reacts violently with organic materials and should be handled with extreme care.] Silanization of Glass Substrates for NP Monolayer Assembly. After cleaning, the glass slides were silanized using one of the silanes TMSPEDA or APTS, as follows: The substrates were immersed in 5% (v/v) solution of TMSPEDA in ethanol for 30 min or in 1% (v/v) APTS in MeOH overnight. Then, the TMSPEDA or APTS coated slides were sonicated three times in EtOH or MeOH, respectively, 5 min each, rinsed with water, dried under a N2 stream, and heated in an oven at 120 °C for 30 min. Synthesis of Metal NPs. Water-soluble, citrate-stabilized Au NPs were prepared according to the method of Turkevich,30 using sodium citrate as the reducing and capping agent. Watersoluble, aminomethylene phosphonic acid (AMP) capped Ag NPs were prepared according to our recently published procedure.31 Metal NP Monolayer Formation. A monolayer of citratestabilized Au NPs on glass surface was prepared by placing 100 μL of the colloid solution on an APTS- or TMSPEDA-modified glass slides for 4 h; the slide was then washed in H2O. To form a monolayer of AMP-capped Ag NPs, glass slides silanized with TMSPEDA were covered with a drop containing 50 μL colloid solution and 20 μL 10 mM NaClO4 for 2 h, followed by washing in H2O. Immobilization of Ag NPs on APTS-modified slides was studied by us previously.31 Note that no differences were observed between NP films (gold or silver) prepared using either of the aminosilane linkers (APTS or TMSPEDA). Coating of Metal NP Films with Silica. An ultrathin silica layer on metal NP monolayers was deposited according to 10023

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Image Analysis. HRSEM images were analyzed using ImageJ image analysis software (Wayne Rasband, NIH, USA). Prior to analysis, particle aggregates in the images were manually separated by thin lines (see Figure S1 in the Supporting Information), to allow the individual particles to be recognized by the software. Particles partially located on edges were not processed. The resulting particle center-of-mass location tables were processed by a custom-built MATLAB script. Two parameters were determined for each particle: (i) the distance to the nearest neighbor, and (ii) the standard deviation (SD) in the distances from the particle to its four nearest neighbors. TEM images were analyzed using ImageJ software to determine the average particle dimensions.

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RESULTS AND DISCUSSION The present work aims at introducing a simple and widely applicable scheme for producing metal NP films on glass surfaces that remain unchanged in terms of NP lateral distribution and LSPR spectroscopy upon drying of the films, i.e., avoiding any drying-induced NP aggregation, by means of an ultrathin silica coating on the metal NPs. TEM image of the citrate-stabilized Au NPs used in this work is presented in Figure S2a in the Supporting Information. Quantitative determination of the average NP size and size distribution was carried out on TEM images of ∼250 particles. The NPs are approximately spherical in shape with an average diameter of 15.4 ± 2.2 nm (see Figure S2b in the Supporting Information). Citrate-stabilized Au NP films were immobilized on APTSsilanized glass substrates. Figure 1a shows transmission spectra of the NPs in solution and assembled on the glass surface, before and after drying. Note that all the spectra in Figure 1a (including those of samples after drying) were taken in water to eliminate the effect of the refractive index (RI) of the medium. The spectra of the two NP films on glass before drying (Figure 1a) show characteristic LSPR bands of the Au NP layers on the surface. Both spectra are red-shifted, with respect to the spectrum of the NPs in solution, reflecting the change of the effective refractive index (RI) of the NPs’ surrounding medium upon binding to the surface, as well as a small number of NP aggregates. The drying of NP films prepared by adsorption from colloidal solutions frequently results in NP aggregation on the surface and large changes in the spectrum, a process usually considered detrimental to the system’s reproducibility and applicability. To assess the effect of the drying method on the aggregation process, two drying schemes were tested: drying in air (22.5 ± 1.0 °C, 50% ± 5% relative humidity) and drying under a N2 stream. Evidently, both lead to dramatic changes of the LSPR spectra (Figure 1a). The spectrum after drying in air shows a decrease of the peak at 534 nm, corresponding to isolated NPs, and the appearance of a red-shifted band, attributed to NP aggregates. The spectrum after drying under a N2 stream exhibits the same qualitative changes, somewhat less pronounced, indicating that drying under N2 also promotes aggregation, but to a lesser extent. (As noted above, dried samples were reimmersed in water for the spectral measurements.) Figures 1b and 1c present, respectively, transmission spectra of Au NP films before and after APTS adsorption (Figure 1b) and after silica deposition (Figure 1c). An increase in the LSPR band intensity and a red-shift of the band maximum are seen after each step of the coating procedure, as expected from the

Figure 1. (a−c) Transmission spectra of citrate-stabilized Au NP films on glass substrates showing the effect of drying on Au NPs, Au NPs/ APTS, and Au NPs/APTS/SiO2, respectively. Panel (a) shows data for Au NPs in solution (purple line); two samples of Au NPs immobilized on silanized glass (black and orange lines); also shown are data for the same samples after drying in air (pink line) or under a N2 stream (green line). Panel (b) shows data for Au NPs (black line), Au NPs/ APTS (blue line), and Au NPs/APTS after drying under a N2 stream (green line). Panel (c) shows data for Au NPs (black line), Au NPs/ APTS (blue line), Au NPs/APTS/SiO2 (red line), and Au NPs/ APTS/SiO2 after drying under a N2 stream (green line); note that the red and green spectra practically overlap (see inset). All of the spectra were measured in water.

increase in the effective RI of the adjacent medium. Drying under N2 after APTS adsorption on the Au NPs (Figure 1b) shows a red-shifted LSPR peak with a tail in the red part of the spectrum, indicating the existence of NP aggregates, although to a much lesser extent than in the case of uncoated Au NPs (Figure 1a). Notably, the spectra of the silica-coated Au NPs (Figure 1c) measured before and after drying are essentially identical (Figure 1c, inset), implying that the ultrathin silica overlayer stabilizes the NP film and effectively prevents dryinginduced NP aggregation. The HRSEM images corresponding to the different stages in the SiO2 coating procedure, shown after drying (see Figures 2a, 2b, and 2c) are in complete agreement with the spectroscopic data. The HRSEM image of the dried uncoated Au NPs (Figure 10024

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and less aggregated NP film is expected to display smaller SDs, while a NP film with extensive aggregation is expected to exhibit large SDs due to the considerable dispersion of interparticle distances. This trend is clearly seen in the histograms presented in Figure S3 in the Supporting Information. As shown above, drying of uncoated Au NP films under different conditions (in air or under a N2 stream) produces, in both cases, spectra which are exceedingly different from the spectrum of the adsorbed (not dried) NP film, and somewhat different for the two drying schemes. The same drying methods were also applied to SiO2-coated NP films (see Figure S4 in the Supporting Information). When the Au NP films are protected with the ultrathin SiO2 layer, the LSPR spectra remain perfectly unchanged after drying, irrespective of the drying procedure used. TEM image of an Au NP after silica deposition shows a homogeneous layer of SiO2 engulfing the NP, with a relatively rough outer surface (Figure 3). The thickness of the silica layer,

Figure 2. SEM images (left panels) and histograms of the distance to the nearest neighbor for each Au NP (right panels) for (a, d) uncoated Au NPs, (b, e) Au NPs/APTS, and (c, f) Au NPs/APTS/SiO2 films. Scale bars = 100 nm. All samples were dried under a N2 stream. The distance to the nearest neighbor was measured as the distance between the particles’ centers of mass.

2a) shows areas with evenly distributed, isolated Au NPs, as well as a large fraction of NP aggregates. The APTS coating eliminates some of the large aggregates (Figure 2b), still exhibiting mixed isolated and aggregated NPs. The image of the SiO2-coated NP film (Figure 2c) shows that almost all the NPs are isolated, with a minimal degree of aggregation, preserving well the morphology of the NP film before drying. To quantify the imaging results, the SEM images were analyzed to determine interparticle distances (defined as the distance between the particles’ centers of mass). Figures 2d, 2e, and 2f present histograms of the distance to the nearest neighbor for all the NPs in the three NP films. The histogram for the uncoated NP film (Figure 2d) shows a wide distribution of interparticle distances, including a large fraction of small distances corresponding to touching (i.e., aggregated) NPs. The wide distribution indicates a highly nonuniform system. The histogram for the APTS-coated NP film (Figure 2e) is better defined, showing two maxima: one at ca. 14 nm (i.e., approximately twice the average NP radius, corresponding to touching (aggregated) NPs) and a second at ca. 18 nm (attributed to the distribution of isolated NPs). The SiO2 coating (Figure 2f) eliminates most of the aggregates, sharply reducing the population of touching particles. The distribution of isolated NPs is narrower than that in Figure 2e, with the SiO2 coating reducing the tail at long interparticle distances, thus indicating a more uniform NP distribution. A different statistical presentation showing the same trend is achieved by analyzing the standard deviation (SD) of the distances from each particle to its four nearest neighbors (distances between particles’ centers of mass). A more uniform

Figure 3. Representative TEM image of a SiO2-coated citratestabilized Au NP.

determined from the TEM imaging, is ca. 3.0−3.5 nm. Variation of the deposition time may provide convenient control over the silica layer thickness.26 To demonstrate the versatility of the use of ultrathin SiO2 coatings for preventing drying-induced effects in metal NP films, the SiO2 coating scheme was applied to phosphonatestabilized, AMP-capped Ag NPs, assembled on TMSPEDAmodified glass substrates as described previously.31 TEM imaging and size distribution of the Ag NPs used in the present study are shown in Figure S5 in the Supporting Information. The mean diameter of the Ag NPs is ca. 14.5 nm. Figure 4 shows, respectively, transmission spectra of uncoated (Figure 4a) and SiO2-coated (Figure 4b) Ag NP films, before and after drying. The spectrum of the uncoated Ag NP film after drying (Figure 4a) shows a tail at longer wavelengths, indicating a certain degree of NP aggregation, whereas upon drying the silica-coated Ag NP film (Figure 4b), the spectrum remains essentially unchanged with no tail formation, i.e., no drying-induced NP aggregation. The respective HRSEM images of the dried films (Figures 4c and 4d) support the spectroscopic results: in the case of the uncoated Ag NPs (Figure 4c) a number of aggregates of 10025

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Figure 4. (a, b) Transmission spectra of an AMP-capped Ag NP film (a) and a SiO2 coated AMP-capped Ag NP film (b) on glass substrates, as deposited (not dried) (black lines) and after drying under a N2 stream (red lines). All spectra were measured in water. (c, d) Corresponding SEM images of the dried Ag NP films (scale bars = 100 nm). (e, f) Corresponding histograms of the distance to the nearest neighbor for each Ag NP.

different sizes are evident in the image, whereas the SiO2-coated Ag NP film (Figure 4d) shows well-separated NPs with no apparent aggregates. Histograms of the distance to the nearest neighbor for all the NPs in the two types of dried Ag NP films, extracted from Figures 4c and 4d, are presented in Figures 4e and 4f. The histogram of the uncoated NP film (Figure 4e) shows a bimodal distribution with a peak at short distances corresponding to touching (aggregated) NPs and a main peak at longer distances attributed to isolated NPs, whereas in the case of the SiO2-coated NP film (Figure 4f), the aggregation peak is essentially absent within the noise level. Stability of the optical response of metal NP films toward multiple drying cycles was evaluated for citrate-stabilized Au NP films with and without the ultrathin silica coating. Uncoated Au NP films (Figure 5a) exhibit substantial spectral changes following each cycle of drying and reimmersion in water. Conversely, after coating of an Au NP film with the silica layer (Figure 5b), the LSPR spectrum shows excellent stability toward multiple drying/reimmersion cycles, remaining perfectly unchanged within the experimental error.

The performance of LSPR transducers in sensing applications may be simulated using polyelectrolyte (PE) layer-bylayer (LbL) assembly.32,33 The PE LbL system used for this purpose here consisted of PEI/PSS in the first bilayer and PAH/PSS in all subsequent bilayers.32 The thickness of each PE bilayer (except the first bilayer adjacent to the Au NPs, which is different in composition and thickness) was previously determined to be ca. 2.1 nm.32 PE multilayers form conformal coatings on LSPR transducers, thus furnishing a convenient model for simulating the binding of full layers of analytes to immobilized receptor layers.32,33 The results presented in Figure 6 (measurements in this case were carried out in air)

Figure 6. Transmission spectra of a SiO2 coated citrate-stabilized Au NP film covered with an increasing number of polyelectrolyte bilayers (in the direction of the arrow). The spectra correspond to Au NPs/ APTS/SiO2 (black line), Au NPs/APTS/SiO2/PEI/PSS (red line), and Au NPs/APTS/SiO2/PEI/PSS/(PAH/PSS)x (x = 1, 2, 3, 4) (blue, green, purple, and brown lines, respectively). All measurements were carried out in air.

Figure 5. Transmission spectra of (a) a citrate-stabilized Au NP film and (b) a SiO2-coated citrate-stabilized Au NP film on glass substrates, after three consecutive cycles of drying under a N2 stream and immersion in water (black, red, and blue lines, respectively). Note that, in panel (b), the three lines practically overlap. All measurements were carried out in water.

show the expected behavior of a typical LSPR sensor, i.e., addition of PE bilayers leads to a red shift of the LSPR wavelength and an increase of the extinction intensity, with the effect diminishing as more bilayers are added, the latter reflecting the plasmon decay length.32 Hence, SiO2-coated 10026

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metal NP films behave as regular LSPR transducers, with the added value of being perfectly stable toward drying.

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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CONCLUSIONS The well-known tendency of adsorbed metal nanoparticle (NP) films on surfaces to undergo NP aggregation upon drying of the film, exhibiting dramatic changes of the morphology and optical response, may present serious limitations in cases where drying of the film is required for application or analysis. Such changes can prevent, for example, reliable analysis of the films using techniques such as electron microscopy, requiring sample drying before the analysis. The problem is solved in the present work by coating the (not dried) metal NP films on glass substrates with an ultrathin (ca. 3.0−3.5 nm) sol−gel-derived silica layer. The silica layer on the metal NPs stabilizes the film, preventing any NP aggregation, and perfectly preserves the film morphology and LSPR response during multiple drying/ reimmersion cycles. The method is simple and scalable; its general applicability was demonstrated by using different metal NPs (citrate-stabilized Au NPs and AMP-capped Ag NPs), as well as different drying schemes. Although the effect of changing the NP size was not tested here, we do not expect a significant size effect on the silica stabilization. Silica-coated metal NP films can be used as regular LSPR transducers in sensing applications, as demonstrated here using a simulated sensing scenario (polyelectrolyte layer-by-layer (PE LbL) assembly on a silica-coated Au NP film). It should be noted, however, that application of such transducers for sensing implies a certain compromise in sensor performance, as the silica coating reduces the transducer sensitivity, to some extent, in relation to the plasmon decay length of the specific transducer used. This sensitivity decrease may be minimized by tuning the decay length of the NP film used in a specific application.32 On the other hand, the perfect stability attained with the silica coating may well compensate for the small loss of sensitivity. It enables drying of the transducer when necessary as detailed above, and no less important, attributing changes in the LSPR band exclusively to analyte binding rather than to possible morphological changes of the transducer.

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Technical Note

AUTHOR INFORMATION

Corresponding Author

*Fax: +972-8-9344137 E-mail: alexander.vaskevich@weizmann. ac.il (A.V.), [email protected] (I.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support of this work by the Israel Science Foundation (Grant No. 1251/11) and the Grand Center for Sensors & Security (Weizmann Institute), is gratefully acknowledged. I.R. is Incumbent of the Agnes Spencer Professorial Chair of Physical Chemistry. The electron microscopy studies were conducted at the Irving and Cherna Moskowitz Center for Nano and BioNano Imaging, Weizmann Institute of Science. This research is made possible in part by the historic generosity of the Harold Perlman family. 10027

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