Article pubs.acs.org/JPCC
Optical Tracking of Single Ag Clusters in Nanostructured Water Films Stefan Krause,*,† Martin Hartmann,† Ingolf Kahle,‡,§ Martin Neumann,† Mario Heidernaẗ sch,† Stefan Spange,‡ and Christian von Borczyskowski† †
Technische Universität Chemnitz, Faculty of Sciences, Institute of Physics and nanoMA (Centre for Nanostructured Materials and Analytics), 09107 Chemnitz, Germany ‡ Technische Universität Chemnitz, Faculty of Sciences, Institute of Chemistry, 09107 Chemnitz, Germany S Supporting Information *
ABSTRACT: The spatial diffusion and size distribution of monodisperse silver nanoclusters synthesized via Ag(I) carboxylate in zeolite Y cages are investigated in nanostructured water films on silicon dioxide (SiO2) and mica surfaces with optical and atomic force techniques. Subnanometer clusters escaping the zeolite Y cage show a strong and photostable fluorescence emission in the visible range and allow for optical singlecluster tracking. Heterogeneous diffusion dynamics reflect the transition from an ice-like to a liquid-like water film as a function of film thickness. The contributions of the different diffusion coefficients strongly correlate with the water film thickness and the chemical composition of the interface. The heterogeneity of the diffusion is caused by ad- and desorption of Ag clusters to silanol groups at the SiO2 interface which couple vibronically to the Ag clusters as can be seen from single cluster fluorescence spectra.
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INTRODUCTION Fluorescence-based techniques, especially single molecule or single particle techniques, have become standard methods in many biological and medical applications mainly due to their in vivo applicability and the specific labeling of cell domains.1,2 The steadily increasing field of fluorescence applications is accompanied by a rising demand for suitable fluorescent probes and markers. While organic dyes fulfill nontoxicity they often lack high photostability.3 The problem of photoinstability can in principle be solved by semiconducting quantum dots which are unfortunately strongly toxic. Already in the 1980s, another class of particles was shown to exhibit a pronounced fluorescence, namely, metal nanoclusters and especially Ag clusters.4,5 In experiments with Ag clusters formed by only a few atoms embedded in rare gas matrices, it turned out that the absorption and emission characteristics of such clusters strongly depend on the cluster size.4,6,7 Previous experiments on metal nanoclusters in liquids proved the strong dependence of the optical emission energy Eem on the number n of atoms forming these clusters according to Eem ∼ n1/3.8 This becomes especially important for a few atom clusters where a change by a single atom or even a change in the atomic arrangement can lead to intense modifications of the optical allowed transitions.4,6,9 In this case most or even all of the silver atoms are at the surface of the cluster and are therefore strongly influenced by environmental changes or surface attached ligands which are also capable of stabilizing the clusters.10,11 Thus, a lot of effort was undertaken to design fluorescent and stable noble metal nanodots and to hinder their agglomeration in highly concentrated solutions by steric interaction with polymers, peptides, or DNA shells.7,12 © 2013 American Chemical Society
In comparison to the difficult size selection inherent to these experiments, the use of a zeolite cage allows for a simple synthesis of nearly monodisperse clusters with a size less than 1 nm which is defined by the embedding host cage, its structure, and the chemical composition as has been shown experimentally13−17 and by simulation.18 The application of these clusters as fluorescent probes after extraction from the host cage is the main subject of this paper. In most cases optical investigations on few-atom Ag clusters aimed at the understanding of the photophysical properties. However, due to their small size and surface sensitivity, single quantum objects are also ideal probes to investigate structures and the related dynamics of nanomaterials such as pores,19 ultrathin films,20,21 or liquid crystal films.22,23 Up to now most of such experiments have been performed using single dye molecules. However, with respect to the above outlined experiments Ag nanoclusters offer a still unexplored rich potential to investigate both static and dynamic properties of various materials on the nanoscale and allow, e.g., for the observation of diffusion processes in ultrathin water films which were not accessible with organic probe molecules until now. Recently the structure of ultrathin water films has been investigated by various static techniques.24,25 Consequently, it is of special interest whether the structure influences dynamics such as spatial diffusion within films of a thickness less than 2 nm. In the present work we investigate via tracking of single Ag clusters with 0.9 nm diameter diffusion trajectories on SiO2 Received: August 1, 2013 Revised: October 2, 2013 Published: October 30, 2013 24822
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Figure 1. (a) AFM height image of immobile Ag nanoclusters on mica. (b) Histogram of cluster diameters and a Gaussian fit with a peak value of (0.90 ± 0.17) nm. (c) Phase image of the same area as in part (a). (d) 1 μm × 1 μm section showing agglomeration of some of the clusters.
grown SiO2 layer (Center for Micro Technologies, Chemnitz, Germany). To achieve high silanol group densities and a high degree of hydroxylation of the SiO2 surface the substrates were treated for one hour with a (2:1) mixture of H2SO4 and H2O2 at 80 °C in an ultrasonic bath.26 Strong annealing of the substrates up to 1100 °C for several minutes removed most of the silanol groups and dehydroxylated the surface.27 These procedures change the wettability of the substrate (see Figure S3 in the Supporting Information). Single cluster spectra were taken with a home-built confocal laser scanning microscope28 using the 488 nm excitation line of an Ar/Kr laser (Coherent, Santa Clara, California). The light was focused into the back focal plane of an air immersion objective (NA = 0.9, 100×, Zeiss, Göttingen, Germany). The emitted light was separated from the excitation light by a 500 nm long pass filter (Omega Optical, Brattleboro, Vermont) and spectrally dispersed with a monochromator (205 mm optical pathway) equipped with a 300 grooves/mm grating. The spectra were imaged onto a liquid nitrogen cooled CCD camera (Princeton Instruments, Acton SpectraPro-275, Trenton, New Jersey). For optical single cluster tracking experiments29 we used the 514 nm line of an Ar/Kr laser which was stabilized by an electro optical modulator (Conoptics, Danbury, Connecticut) to reduce frequency jittering and circularly polarized by the combination of a lambda half and lambda quarter wave plate. The fluorescence light was separated from the excitation by a 530 nm long pass filter (Omega Optical) and then imaged onto a Peltier cooled CCD camera (Pentamax, Princeton Instruments). For adjustment of the humidity the samples were placed within a perspex box equipped with a humidity sensor
surfaces under ambient conditions. We report how diffusion depends on surface properties such as the wettability with ultrathin water films. Specifically, we investigated whether the diffusion of Ag nanoclusters is sensitive to the thickness of the water film and the related transition from “ice-like” to “liquid-like” water films which consist of about 2−3 monolayers at the SiO2 surface.24,25
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EXPERIMENTAL SECTION The synthesis of Ag clusters in zeolites is described in the Supporting Information. Tapping mode atomic force microscopy (AFM) measurements were performed with a Nanowizard II (JPK instruments, Berlin, Germany). The AFM was assembled with Pointprobe NCH-W silicon tips (Nanoworld, Neuchâtel, Switzerland) with a standard resonance frequency of approximately 320 kHz. Ensemble fluorescence and excitation spectra were taken with a commercial UV/vis spectrometer (Varian Cary Eclipse, Agilent Technology, Santa Clara, California). An amount of 1 mg of the Ag(I) complex encapsulated in the zeolite Y was mixed with 4 mL of ultraclean Millipore water ( 0.15 μm2/s). Nevertheless, there may be much faster components escaping detection at the given integration time of 300 ms. All Aj and Dj parameters are shown in Figure 4 (a) and (b) for the total range of humidities on an as-grown SiO2 surface.
Figure 3. Complementary cumulative probabilities of the diffusivities ddiff of Ag nanoclusters on SiO2 for four different humidities and differently treated substrates (the humidity-dependent experiments have been performed on as-grown SiO2 surfaces). The diffusivities can be roughly separated into three principle regimes as separated by the vertical dashed lines. The first one (left) includes only very small diffusivities which in fact represent immobile clusters, while the second and the third regime correspond to slow and fast diffusing clusters, respectively. Each data set is fitted by the sum of three exponential functions (except the curve for 0% relative humidity measured in vacuum). Thin lines correspond to fits according to eq 3. The dotted vertical line indicates that the most pronounced dependence of diffusivities on humidity is observed at about ddiff = 0.1 μm2/s. The inset shows three example trajectories (immobile, slow, and fast) of three different single Ag nanoclusters, as they are typical for the three diffusion regimes.
ddiff =
Δr 2 4·τ
(2)
with the squared displacement Δr2 between two consecutive positions along a single cluster trajectory and τ = 300 ms as the integration time of the experiment. All humidity-dependent distributions reveal a major part of very small (immobile) diffusivities below 0.01 μm2/s, at a relative humidity of 0%, realized in an evacuated cryostat removing all possibly remaining water films. In the range ddiff > 0.01 μm2/s the distributions depend strongly on humidity and the substrate treatment. The probability for diffusivities varies with humidity and with the density of silanol (OH−) groups on the substrate, which was increased by a treatment with Piranha and decreased by annealing (see Experimental Section). The percentage of mobile clusters increases with the thickness of the water layer on the SiO2 substrate which is directly related to the environmental humidity.24,25 In addition, increase of the silanol density improves the wettability (see Figure S3 in the Supporting Information) with H2O films.49,50 The CCP can be fitted by three exponential functions31 according to ⎛ ddiff ⎞⎟ ⎜ A · exp − ∑ j ⎜ 2Dj ⎟⎠ ⎝ j=1
Figure 4. Fitting parameters Aj and Dj of the three diffusion components immobile D1 (Δ), slow D2 (○), and fast D3 (□) as a function of the relative humidity. Solid lines serve as a guide for the eye. The error bars result from the variation of the relative humidity during measurement and the accuracy of the fit, respectively. (a) Amplitudes Aj. (b) Diffusion coefficients Dj. (c) Probability C0.1 of the CCP for the diffusivity of 0.1 μm2/s. Tentative behavior is visualized by solid lines. The solid blue line in (c) represents the measured surface potential ΔΦ given by Verdaguer et al.25 The water layer thickness scale on top of the figure refers to data from Verdaguer et al.25
The influence of the substrate treatment will be discussed later. Though the experimental error is quite large, all curves for Aj and Dj have in common that they show a “turnover” in increment at about 60−65% when proceeding from low to high humidities. At both ends of the humidity scale the related parameters remain nearly constant. This behavior is indicated by solid lines. It is obvious that the amplitudes assigned to the immobile clusters, which we related to the large component A1, decrease with increasing relative humidity, and this “feeds” the amount of mobile clusters (slow and fast) as can be seen by the increase of the respective amplitudes A2 and A3. The strongest changes of the amplitudes take place in the regime close to diffusivities of ddiff = 0.1 μm2/s (dotted vertical line Figure 3) which is the range of crossover from slow to fast diffusion. This becomes clearly apparent in Figure 4(c) where
3
C(ddiff , t ) =
(3)
with Aj and Dj representing the amplitude and the diffusion coefficient of the jth exponential function. Fitting parameters for the six example CCPs in Figure 3 are summarized in Table S1 in the Supporting Information. The 24826
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the expected diffusion within a thin film. Thus, we conclude that stack-and-slide processes are the reason for the strong decrease of the diffusion coefficients as has already been observed for organic dye molecules.29 A way to clarify the influence of chemical surface properties on diffusion and electronic coupling to the surface is the variation of the silanol group density on the SiO2 surface. Figure 3 and Table S1 (Supporting Information) show at the same humidity of 70% besides the distributions of ddiff as a function of humidity also the CCPs of ddiff for Ag clusters diffusing on two different types of substrates, namely, those with very high and very low silanol density as compared to the as-grown surfaces. It is immediately evident that the silanol concentration has a strong impact on the overall diffusion, namely, increasing the number of diffusing clusters upon an increase of silanol density, respectively. The value of, e.g., C0.1 is 0.039 for substrates with a low and 0.117 for a high silanol density. These findings prove that the silanol density influences the wetting (see Figure S3 in the Supporting Information) of the surface as has already been shown experimentally49 and by simulations.50 As the diffusion processes take place in thin water layers on the SiO2 surface, wetting is the dominant contribution which controls the overall amount of mobile clusters. It is obvious that the relative contribution of fast diffusing clusters is much stronger in the case of a high silanol density. We have included the ratio A2/A3 of the amplitudes of the two diffusion constants D2 and D3 in Table S1 (Supporting Information). The comparison shows that the humidity and thus the thickness of the water film in the case of the as-grown SiO2 do not influence the relative contribution of the diffusion coefficients which remain constant at A2/A3 ≈ 2.1 since both components increase in parallel with humidity. However, A2/A3 increases (decreases) to 3.5 (1.4) upon a decrease (increase) of silanol density. Obviously, a high silanol density favors the fast diffusion more effectively than a slow one, probably because the SiO2 intrinsic hydrogen-bonded network of silanols is established.55 Attachment becomes obviously more effective in the case of isolated silanols. We observed a reduced electron−vibron coupling for mobile clusters as compared to immobile ones (see Figure S6 in the Supporting Information). We like to point out that a similar combination of stick-andslide diffusion processes has been reported recently.56
the related probabilities C0.1 are plotted as a function of the relative humidity. Approximately twice the number of clusters shows a diffusivity above ddiff = 0.1 μm2/s at high humidities in comparison to low humidities. Also the probabilities C0.1 show a “turnover” close to 60% humidity. Surprisingly, D3 decreases, while D2 increases with humidity. Without going into detail this might be explained by a rapid exchange between fast and slow diffusion (see example trajectories for such processes in the inset of Figure 3) on the time scale of the experiment.31 Finally, we would like to discuss the diffusion processes qualitatively. Water films on SiO2 have been investigated as a function of humidity by X-ray spectroscopy,25 Kelvin probe microscopy,25 and attenuated total reflection infrared (ATR-IR) techniques.24 Both investigations24,25 predict a transition from an “ice-like” structure at humidities below 17% and 30%, respectivelycorresponding to two and three molecular water layers, respectivelyto a liquid-like structure for humidities above 80% (>5 monolayers) (see also the thickness scale on top of Figure 4). According to the published data, a transition between these two regimes occurs between 55% and 60%. This finding is strikingly similar to our results for diffusion (see Figure 4). From this comparison we conclude that the structural inhomogeneity of thin H2O films influences in a distinct way the identified diffusion processes of Ag clusters. To push the comparison still further, we have in Figure 4(c) included the variation of the surface potential.25 The change in diffusion is in qualitative agreement with the potential down to the range from whereon only the “ice-like” structure of about 2−3 ML thickness remains. We like to note that 3 ML of H2O correspond to the average diameter of the Ag clusters. We do not claim that the surface potential directly controls the diffusion process, but it merely reflects the crossover of the structure of the film. Thus we believe that the structure formation in ultrathin water films controls the diffusion of Ag clusters in an analogous way as has been reported for dye molecules in ultrathin viscous liquids20,21 or liquid crystal films.22,23 Besides the humidity dependency, the diffusion reveals strong heterogeneities reaching from about 50% immobile clusters over slow diffusing to fast diffusing clusters (see also inset in Figure 3). This fact can be explained on one hand by the surface roughness of the SiO2 substrate which is in the order of 0.5−1 nm and thus almost equal to the diameter of the diffusing clusters and on the other hand by chemical inhomogeneities depending strongly on the preparation process. Of special importance are silanols (OH groups, see Figure S2 in the Supporting Information)27 which form islands or clusters on a nanometer scale.51,52 The observed coupling to the interface (vibronic sideband Figure S6 in the Supporting Information) gives rise to a mobility controlled by ad- and desorption processes, e.g., to silanol groups. Related attachment processes have been reported for dye molecules on fused silica.53 Further indications for a strong influence of the surface on diffusion dynamics are the small diffusion coefficients between D2 ≈ 0.03 μm2/s and D3 ≈ 0.4 μm2/s as compared to the expected diffusion coefficient of D = 100 μm2/s for a spherical particle in H2O according to a Debye model. Even if one takes the solid−liquid and liquid−air interfaces into account54 (with a film thickness comparable to the particle diameter)22 the corresponding diffusion (for particles of 1 nm diameter) will only be slowed down by a factor of 0.5. However, the presently observed diffusion is at least 3 orders of magnitude slower than
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CONCLUSIONS Ag nanoclusters with an (AFM detected) narrow size distribution centered at 0.9 nm were synthesized within the cages of Y zeolite which served as a size selector. Further size selection was achieved with optical excitation at wavelengths of 488 and 514 nm, respectively, and results in spectra of single clusters with a pronounced fluorescence close to 545 nm. We observed diffusion processes of the Ag clusters on a wetted SiO2 surface. We conclude that diffusion takes place in at most 2 nm thick and nanostructured water films on the surface as the number of diffusing clusters increases with increasing humidity of the surrounding atmosphere. The diffusion dynamics reflect as a function of humidity the earlier identified crossover24,25 from “ice-like” to “liquid-like” water. In addition, diffusion can be effectively modified by changing the concentration of silanol groups which is accompanied by wetting and dewetting processes. Analysis of the diffusion data according to the method of complementary cumulative distributions30−32 reveals that apart from immobile clusters at 24827
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(6) Harb, M.; Rabilloud, F.; Simon, D.; Rydlo, A.; Lecoultre, S.; Conus, F.; Rodrigues, V.; Félix, C. Optical Absorption of Small Silver Clusters: Agn, (n = 4−22). J. Chem. Phys. 2008, 129, 194108. (7) Shang, L.; Dong, S.; Nienhaus, G. U. Ultra-Small Fluorescent Metal Nanoclusters: Synthesis and Biological Applications. Nano Today 2011, 6, 401−418. (8) Zheng, J.; Zhang, C.; Dickson, R. M. Highly Fluorescent, WaterSoluble, Size-Tunable Gold Quantum Dots. Phys. Rev. Lett. 2004, 93, 077402. (9) Bonaĉić-Koutecký, V.; Veyret, V.; Mitrić, R. Ab Initio Study of the Absorption Spectra of Agn (n = 5−8) Clusters. J. Chem. Phys. 2001, 115, 10450. (10) Zheng, J.; Petty, J. T.; Dickson, R. M. High Quantum Yield Blue Emission from Water-Soluble Au8 Nanodots. J. Am. Chem. Soc. 2003, 125, 7780−7781. (11) Oemrawsingh, S. S. R.; Markešević, N.; Gwinn, E. G.; Eliel, E. R.; Bouwmeester, D. Spectral Properties of Individual DNA-Hosted Silver Nanoclusters at Low Temperatures. J. Phys. Chem. C 2012, 116, 25568−25575. (12) Choi, S.; Dickson, R. M.; Yu, J. Developing Luminescent Silver Nanodots for Biological Applications. Chem. Soc. Rev. 2012, 41, 1867− 1891. (13) Wöhrle, D.; Schulz-Ekloff, G. Molecular Sieve Encapsulated Organic Dyes and Metal Chelates. Adv. Mater. 1994, 6, 875−880. (14) Warnken, M.; Lázá, K.; Wark, M. Redox Behaviour of SnO2 Nanoparticles Encapsulated in the Pores of Zeolites towards Reductive Gas Atmospheres Studied by in Situ Diffuse Reflectance UV/Vis and Mössbauer Spectroscopy. Phys. Chem. Chem. Phys. 2001, 3, 1870− 1876. (15) Mayoral, A.; Carey, T.; Anderson, P. A.; Lubk, A.; Diaz, I. Atomic Resolution Analysis of Silver Ion-Exchanged Zeolite A. Angew. Chem., Int. Ed. 2011, 50, 11230−11233. (16) Mayoral, A.; Carey, T.; Anderson, P. A.; Diaz, I. Atomic Resolution Analysis of Porous Solids: A Detailed Study of SilverionExchanged Zeolite A. Microporous Mesoporous Mater. 2013, 166, 117− 122. (17) Miyanaga, T.; Suzuki, Y.; Matsumoto, N.; Narita, S.; Ainai, T.; Hoshino, H. Formation of Ag Clusters in Zeolite X Studied by in Situ EXAFS and Infrared Spectroscopy. Microporous Mesoporous Mater. 2013, 168, 213−220. (18) Yumura, T.; Nanba, T.; Torigoe, H.; Kuroda, Y.; Kobayashi, H. Behavior of Ag3 Clusters Inside a Nanometer-Sized Space of ZSM-5 Zeolite. Inorg. Chem. 2011, 50, 6533−6542. (19) Hellriegel, C.; Kirstein, J.; Bräuchle, C.; Latour, V.; Pigot, T.; Olivier, R.; Lacombe, S.; Brown, R.; Guieu, V. Payrastre et al. Diffusion of Single Streptocyanine Molecules in the Nanoporous Network of Sol−Gel Glasses. J. Phys. Chem. B 2004, 108, 14699−14709. (20) Schuster, J.; Cichos, F.; von Borczyskowski, C. Diffusion in Ultrathin Liquid Films. Eur. Polym. J. 2004, 40, 993−999. (21) Täuber, D.; Trenkmann, I.; von Borczyskowski, C. Influence of van der Waals Interactions on Morphology and Dynamics in Ultrathin Liquid Films at Silicon Oxide Interfaces. Langmuir 2013, 29, 3583− 3593. (22) Schulz, B.; Täuber, D.; Friedriszik, F.; Graaf, H.; Schuster, J.; von Borczyskowski, C. Optical Detection of Heterogeneous Single Molecule Diffusion in Thin Liquid Crystal Films. Phys. Chem. Chem. Phys. 2010, 12, 11555−11564. (23) Schulz, B.; Täuber, D.; Schuster, J.; Baumgärtel, T.; von Borczyskowski, C. Influence of Mesoscopic Structures on Single Molecule Dynamics in Thin Smectic Liquid Crystal Films. Soft Matter 2011, 7, 7431−7440. (24) Asay, D. B.; Kim, S. H. Evolution of the Adsorbed Water Layer Structure on Silicon Oxide at Room Temperature. J. Phys. Chem. B 2005, 109, 16760−16763. (25) Verdaguer, A.; Weis, C.; Oncins, G.; Ketteler, G.; Bluhm, H.; Salmeron, M. Growth and Structure of Water on SiO2 Films on Si Investigated by Kelvin Probe Microscopy and in Situ X-ray Spectroscopies. Langmuir 2007, 23, 9699−9703.
least two diffusion constants are necessary to describe the diffusion quantitatively. Namely, we could identify immobile, slow, and fast diffusing clusters. Though diffusion coefficients differ by 1 order of magnitude, diffusion is still much slower than expected for diffusion in bulk water. We ascribe this effect to a coupling of the Ag clusters to the interface, which is supported by the observation of vibrational side bands of the fluorescence of single Ag clusters, probably due to electronic coupling with vibrations of surface-related silanols. Optical single cluster detection thus demonstrates that it is feasible to detect only a few (size selected) clusters and that agglomeration into large Ag particles is considerably suppressed at extremely low concentrations. This introduces a new approach to investigate small Ag clusters and their interaction with the environment and utilize these clusters as sensitive tracers. Moreover, we have shown that diffusion of nanoclusters reveals the underlying nanostructure of thin (water) films on chemically modified SiO2 substrates.
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ASSOCIATED CONTENT
* Supporting Information S
Materials and synthesis, modification of silanol density, absorption, fluorescence, and excitation data for bulk samples, discussion of electron−vibron coupling, emission spectra for single mobile and immobile Ag nanoclusters, diffusion properties and simulated transition rates for immobile, slow, and fast clusters, and example video clip of diffusing clusters. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address §
Schwan-STABILO Cosmetics GmbH & Co. KG, 90562, Heroldsberg, Germany. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the DFG Research Group FOR 877 (“From local constraints to macroscopic transport”) for financial support. AFM measurements have been performed in the laboratory of R. Magerle, TU Chemnitz. We thank Cornelius Krasselt, TU Chemnitz, for the supply of surface wetting data.
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