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J. Phys. Chem. C 2008, 112, 6462-6468
In-Situ Measurement of Colloidal Gold Adsorption on Functionalized Silica Surfaces Mikhail Mazurenka,† Suzanne M. Hamilton,† Patrick R. Unwin,‡ and Stuart R. Mackenzie*,† Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK, and Department of Chemistry, UniVersity of Warwick, Gibbet Hill Road, CoVentry CV4 7AL, UK ReceiVed: January 24, 2008; In Final Form: February 25, 2008
Evanescent wave cavity ring-down spectroscopy (EW-CRDS) has been applied to study, in situ, the deposition kinetics of commercially available gold colloids on functionalized silica surfaces from quiescent solution. Neither 5 nor 20 nm citrate-stabilized nanoparticles were observed to adsorb on clean silica surfaces. Adsorption on a poly-L-lysine-functionalized surface, however, occurs readily and irreversibly with the kinetics of adsorption differing markedly for the two particle sizes studied. 5 nm particles adsorb to form a highly disperse submonolayer of individual particles with atomic force microscope images showing no evidence of aggregation. The controlled growth of multilayer nanoparticle/polyelectrolyte films is demonstrated by alternately depositing colloidal particles and poly-L-lysine films. The deposition of multilayer nanoparticle films increases the sensitivity of the functionalized surface to changes in the solvent refractive index. The adsorption kinetics of the 20 nm colloid is more complex than that of the smaller colloid with adsorbed particles acting as nucleation sites for subsequent aggregation with the result that the interfacial absorbance continues to increase indefinitely with time.
I. Introduction The unique properties of metal nanoparticles (NPs) have attracted intense interest for more than a decade following the development of convenient methods for their synthesis.1-7 Recent advances in tuning the physical and chemical properties of NPs have spread the applicability of NPs to a wide range of scientificproblems,includingchemicalcatalysis,3,5,8-10 biosensing,11-13 and nanoscale electronic devices.14 Many applications of NPs require their controlled deposition from quiescent solution onto solid substrates in well-defined architectures while avoiding uncontrolled aggregation. Some applications, for example, require linking of adjacent NPs by conducting bridges either by attaching conducting molecules or by deposition onto surfaces modified by conductive polymers.14-16 In many cases, immobilization of individual nanoparticles itself significantly influences their optical and chemical properties,17 while other studies have shown that aggregation of nanoparticles can enhance the overall catalytic activity.18 Clearly, a better understanding of the surface chemistry of individual NPs and ensembles as well as the adsorption kinetics of NPs onto functionalized surfaces is required in order to better control the deposition process. The need for an improved description of the adsorption process has triggered extensive experimental and theoretical research into the irreversible adsorption of colloidal metals onto various native and functionalized surfaces. Theoretical descriptions include the application of random sequential adsorption (RSA)19-21 models originally developed for mesoscopic particles and the full numerical solution of the mass transport equations for nanoparticles and proteins.22 The experimental approaches which have been applied are many and varied but often combine surface imaging techniques, such as atomic force microscopy (AFM) or scanning tunneling microscopy (STM) to determine * Corresponding author. E-mail:
[email protected]. † University of Cambridge. ‡ University of Warwick.
surface number densities, with spectroscopic methods. Experiments typically involve immersion of the substrate in solution for fixed times followed by subsequent spectroscopic/microscopic analysis. By contrast, techniques for in-situ monitoring of surface adsorption remain rare. Many of the uses of metal nanoparticles stem from their optical properties. In many cases, especially colloidal gold and silver, the spectrum in the visible region is dominated by a strong localized surface plasmon resonance (LSPR) peak. The maximum of extinction corresponding to the LSPR, λmax, depends on various factors, such as size, shape, the dielectric constant of the material concerned, the distance between adsorbed NPs on the surface, and the dielectric constant of the local environment (e.g., the solvent or the substrate).23-25 Optimization of these physical properties to suit particular applications is the subject of much research.4,7,26 The sensitivity of the LSPR to the local environment, for example, is the basis for the use of gold nanoparticles in biosensors, where the presence of an adsorbed biomolecule significantly perturbs the local refractive index which results in observable spectral shifts in the LSPR.25,27-29 In this paper we describe investigations of the adsorption kinetics of commercially available colloidal gold onto functionalized silica surface by means of evanescent wave cavity ring-down spectroscopy (EW-CRDS).30-35 This technique is ideally suited to time-resolved in-situ measurements at the solid-liquid interface and combines the high sensitivity, temporal and spatial resolution of cavity ring-down spectroscopy (CRDS)36-38 with the inherent interfacial sensitivity arising from the evanescent field.39 EW-CRDS has recently joined a limited range of in-situ techniques suitable for this type of application (including optical reflectometry40-42 and broadband timeresolved optical waveguide spectroscopy43) and has the advantage of simplicity and high sensitivity compared to other methods. The first application of EW-CRDS to gold nanoparticle deposition was reported recently by Fisk et al.,44 who measured the adsorption of home-synthesized gold nanoparticles to bare
10.1021/jp800706j CCC: $40.75 © 2008 American Chemical Society Published on Web 04/03/2008
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silica surfaces under continual flow conditions. Another variant of this technique, Brewster’s angle cavity ring-down spectroscopy, has recently been applied to the study of deposited gold nanoparticles by Gilb et al.45 In this study, we apply EW-CRDS to measure the adsorption of commercial gold nanoparticles from a quiescent solution onto functionalized silica surfaces. The use of commercial colloids was deliberate as these are the solutions used in the majority of biosensing applications. Despite the similarity of the experiments, we observed key differences to the results of Fisk et al., not the least of which is that we observe no surface adsorption whatsoever from fresh colloidal solution onto optically clean pure silica surfaces. Significantly, our studies embrace functionalized substrates which are of key technological importance for the creation of monolayer and multilayer nanoparticle ensembles.46,47 II. Experimental Section Apparatus. The cavity ring-down spectrometer used in this experiment was a modified version of the ring cavity spectrometer described in detail previously.31 Briefly, an optical cavity is formed between two highly reflective mirrors and the total internal reflection at the hypotenuse of a right angled fused silica prism. The latter constitutes the base of a liquid cell which contains the colloidal suspension. Light from an external laser source is injected into the cavity and the light decay within the cavity is measured by a photomultiplier tube. The ring-down time is sensitive to absorbance or scattering within the evanescent field which extends beyond the surface of the prism into the aqueous solution, a distance comparable with the wavelength of light. The laser used in these studies was a 50 mW diode laser operating at 405 nm (Power Technology IQ1H). The laser can be modulated using a TTL signal at up to 1 MHz. This rapid pulse capability increases the effective experiment repetition rate to 2 kHz, limited by the PCI bus data transfer rate from the NI PCI-5124 12-bit digitizer to the PC. Further, the “broadband” nature of the laser (∆λ ∼ 1 nm) means light is always coupled into a large number of the cavity modes, obviating the need to scan the laser wavelength or the cavity length and significantly simplifying the optical and electronic components of the experimental setup. To perform adsorption kinetic measurements of 5 and 20 nm colloids, the optical cavity was first aligned with Milli-Q water in the cell, giving a background ring-down time, τ0. The water was then removed from the cell, and 10 drops of poly-L-lysine (PLL) solution (1 mg/mL, Aldrich) were introduced to cover the whole silica surface inside the cell. The PLL solution was left to deposit for 20 min before being rinsed with Milli-Q water four times. At this point the gold colloid solution (2.5 mL of 5 or 20 nm gold colloid at various dilutions, Aldrich) was introduced into the cell. Cavity ring-down measurements of the interfacial absorbance were taken at 2 kHz. To improve the signal-to-noise ratio, 100 ring-down events were stored in the digitizer card memory and then transferred to the PC, where a decay constant was determined for each single transient and the average ring-down time calculated for this group. This averaging provides a data point every 50 ms. The whole experiment, including the fitting of the ring-down traces, was controlled by a purpose-built LabView program. Colloidal Solutions. The concentration of the commercial colloidal solutions (Sigma-Aldrich) was unknown; only the absorbance at the peak of the surface plasmon band is specified (A ) 0.75 at λ ) 525 nm in each case). Various dilutions of the stock solution were used, ranging from 10-2 to 1.0 of the
Figure 1. UV-vis spectra of 5 nm (upper) and 20 nm (lower) colloidal solutions. The spectrum is dominated by contributions from the surface plasmon resonance peak centered at 525 nm (dotted line) and the 5d6sp gold cluster transition (dashed line).
original concentration in ultrapure water. Both the 5 and 20 nm colloids were produced by the well-known tannic acid/citrate method resulting in citrate-stabilized particles.48,49 Despite refrigerated storage, there was some evidence of aggregation after 2 months. This was observed by both a slight broadening of the LSPR band in the UV-vis spectra of the stock colloid solutions and the deposition of large aggregated particles, even on clean silica surfaces. Neither was observed with fresh samples which were used for the studies described here. Care was taken to ensure that even the maximum surface adsorption observed in these experiments did not significantly deplete the bulk concentration of the sample in the cell. This was tested following each experiment by removing the solution and replacing it with fresh colloidal solution. No change in the surface adsorption was observed. Between experiments, the fused silica surface of the prism was cleaned thoroughly by repeated plasma ashing (20 min in 100 W O2 plasma, Diener plasma asher), interspersed with rinsing in methanol. In each case care was taken to ensure that the original background ring-down time was achieved before proceeding. All atomic force microscope images were recorded using a DI AFM Nanoscope Dimension 3100 instrument operating in tapping mode. III. Results and Discussion UV-vis spectra of the 5 and 20 nm colloid solutions are shown in Figure 1. At the measurement wavelength of 405 nm, the extinction arises from absorption in the tail of the plasmon resonance band itself as well as from the 5d f 6sp interband transition of bulk gold.50 The surface plasmon band is considerably more prominent in the spectrum of the larger colloid. Experiments were originally performed on clean unfunctionalized silica surfaces which are known to be negatively charged under water due to the deprotonation of the terminal -Si-OH groups. No interfacial adsorption of nanoparticles was observed even over periods of several hours. This is no surprise as the citrate groups which stabilize the particles with respect to
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Figure 2. Typical adsorption kinetics for 0.1 dilution, 5 nm colloid solution onto PLL-functionalized silica. (inset) Tapping mode 1 µm × 1 µm AFM image of the surface recorded after 25 min of adsorption.
aggregation will give rise to repulsion from the charged surface. Some nanoparticle adsorption was observed on prisms which subsequent AFM imaging showed to be significantly scratched or contaminated. Following implementation of our plasma ashing cleaning protocol with new optical quality prisms, no adsorption on clean silica surfaces was observed. This is in marked contrast to the previous EW-CRDS study by Fisk et al., who observed extensive deposition on such surfaces for slightly larger self-synthesized colloids.44 The reasons for the disparity are unclear but must lie in one of several significant differences between the two experiments. First, Fisk et al. used a continually flowing sample of colloidal solution which increases the mass transport to the surface compared with our static solution. Second, Fisk et al. used self-prepared colloids which, although citrate-stabilized, may have had very different surface charge (and thus kinetic stability) than those used here. Fisk et al. report that their solutions developed a “pearlescent purple” appearance in the 24 h between preparation of their solutions and their use, which may reflect aggregation in solution. No such pearlescence was observed in our solutions. Third, the surface cleaning protocols were very different in the two studies (aqua regia vs plasma cleaning), and it is possible that Fisk et al. observed initial deposition onto scratches on the surface which acted as nucleation sites for further aggregation, resulting in the dense layers they report. A. Adsorption of 5 nm Particles on PLL-Functionalized Silica. As discussed earlier, there is considerable interest in the adsorption of nanoparticles onto PLL-functionalized surfaces. PLL is a positively charged polyelectrolyte which adsorbs readily onto the negative silica surface to yield a net positive surface. A typical adsorption kinetics curve, recorded by EWCRDS, for 5 nm colloidal particles on the PLL-functionalized prism is shown in Figure 2. Rapid initial adsorption for the first minute is followed by ever slower net adsorption as the coverage asymptotically approaches a maximum. The inset shows a 1 µm × 1 µm AFM image of the prism surface after 25 min of deposition. The image was recorded in tapping mode following removal of the colloidal solution and drying of the prism surface in a nitrogen atmosphere. Repeated flushing with water had no effect on the interfacial absorbance (see below), indicating that adsorption is irreversible, and thus this procedure is believed to have no significant effect on the nanoparticle films deposited. It is clear that the maximum coverage achieved is far short of a full monolayer. For concentrations used in this study after ∼4 h the interfacial absorbance remained constant within the
Mazurenka et al. (small) long-term drift of the spectrometer. From multiple images across the surface of several prisms, we estimate the maximum coverage of nanoparticles deposited from 0.1× stock concentration solution to be 55 ( 14 µm-2 or an effective coverage of ca. (1.2 ( 0.3) × 10-3 of a monolayer. With knowledge of both the number density and the corresponding interfacial absorbance it is possible to derive the extinction at 405 nm arising from individual nanoparticles on the surface. For the 5 nm colloid we estimate an extinction 8 3 -1 cm-1. coefficient of 405 5nm ) (3.76 ( 0.92) × 10 dm mol Under the assumption that the extinction coefficient is unchanged upon adsorption (and our 405 nm wavelength is some way from the peak of the surface plasmon band), this value can be used to estimate the concentration of the original colloidal solution for which the bulk absorbance is known from the conventional UV-vis spectrum. Using this method, we calculate the concentration of the original undiluted 5 nm colloidal sample, as purchased, to be 3.6 ( 0.9 nM. B. Adsorption Kinetics of 5 nm Particles. Diffusioncontrolled adsorption of colloidal particles from stagnant solution has been discussed theoretically in the context of several models including a random sequential adsorption (RSA) model,51 a ballistic model,20 and Brownian dynamics.21 All models assume that adsorption is irreversible, and the validity of this assumption was confirmed in our experiment by removing the colloidal solution from the cell after the NPs had been adsorbed for an hour and refilling the cell with pure Milli-Q water. The resulting absorbance changes were within the long-term spectrometer signal drift, indicating that desorption can indeed be neglected. The ballistic model of desorption includes gravitational effects on the colloid particle and is applicable when the effective radius of the colloid particle, R*, is larger than 1.8.46 In the case of our nanoparticles R* ) 9.2 × 10-3 and 3.7 × 10-2 for the 5 and 20 nm colloids, respectively. In both cases R* , 1.8, and thus the ballistic model can be discarded from the following data analysis. Similarly, the RSA model, developed to describe mesoscopic sized particles, fails for small particles such as these. Brownian dynamics apply best at near-monolayer coverages where jamming becomes important. As shown above this is not the situation here. Adamczyk has developed an approach to describing the kinetics of diffusion-limited irreversible adsorption of colloids from stagnant solutions which involves solving, in a quasistationary manner, the mass transfer equation, whereby blocking effects are incorporated into the activity coefficient.22 Within this model the mass transfer equation can be solved analytically for two limiting cases:
t π < Θ 2 tch 4 mx
(1)
t >1 tch
(2)
and
where t is the adsorption time and Θmx is the jamming (or saturation) coverage. tch is a characteristic time, defined as
tch )
1 πa nbD∞ 2
(3)
in which nb is the bulk colloid concentration, D∞ is the bulk diffusion coefficient, and a is the particle radius.
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J. Phys. Chem. C, Vol. 112, No. 16, 2008 6465
Figure 3. Adsorption kinetics curves for (a) 0.01, (b) 0.02, (c) 0.05, and (d) 0.1 dilution 5 nm colloidal solutions, plotted against t1/2, illustrating the extent of diffusion-controlled adsorption.
Early stage adsorption kinetics, satisfying expression 1, leads to the familiar form of diffusion-controlled kinetics in which the coverage as a function of time is given by
Θ)
2 xπ
x
t tch
(4)
At the other extreme, as the maximum coverage is approached asymptotically, i.e., t > tch, the coverage behaves as
K1
Θ ) Θmx -
xt/tch
n -1
(5)
where
x
K1 ) Θmx n - 1
Θmx
c(n - 1)ka
(6)
and for spherical particles n ) 3, c ) 2.3. The predicted time dependence is thus 1 - t -1/2. For intermediate times, the mass transfer equation cannot be solved analytically and requires numerical solution.22 We have tested our data for consistency with the above kinetics by replotting the interfacial absorbance for several different colloidal concentrations as a function of t1/2, as shown in Figure 3. Particularly in the adsorption from the more dilute solutions, a clear linear region is visible, indicating the extent to which the system behaves in a diffusion-controlled manner. The linearity of the data for 0.01 dilution solution for t < 25 min is sufficiently good to permit a determination of the diffusion constant via22
x
Θ ) 2πa2
Dbt n π b
(7)
and noting that
Abs )
405 5nmΘ πa2NA
(8)
-11 Gratifyingly, the value obtained, Dexp b ) (8.01 ( 1.96) × 10 2 -1 m s , is in excellent agreement with that calculated using the Stokes-Einstein equation, 8.58 × 10-11 m2 s-1.
We have fitted our data to the predicted functional forms in both early and long time regimes, and the results for the 0.01 and 0.1 dilutions are shown in Figure 4. The early time kinetics have been fitted to Abs ) A + B xt-t0 to reflect the fact that for the first few seconds the solution in the cell experiences turbulent flow with the consequence that t ) 0 is ill-defined. Convergence of the fit parameters was ensured by fitting to various subsets of the data until no noticeable change in the fit parameters was observed. Likewise, the long time adsorption kinetics were fitted to the function Abs ) A - B/xt with a similar procedure. The results demonstrate good agreement between the data and the predicted functional forms of the time dependence in the limits of both short and long deposition times. Despite strenuous attempts to make our deposition protocol as reproducible as possible, there is still typically a (17% statistical variation in the final coverage which we attribute to variations in the PLL layer and/or the way the colloid is introduced to the cell. Despite this qualitative agreement, there is some discrepancy between the experimental data and the predicted time scales. Using the diffusion coefficient and stock concentrations determined above, the characteristic time tch can be calculated (eq 3) to range from 6.5 × 1010 s (0.01 dilution) to 6.5 × 109 s (0.1 dilution). These times are substantially longer than the duration of any experiment performed in this study. In other words, we observe jamming, 1 - t -1/2, kinetics at much earlier times (103-104 s) than predicted (Figure 4). This discrepancy may result from the neglect of particle-surface and particleparticle interactions in the model and the unknown behavior of the PLL film when charged particles adsorb. It is possible that, upon adsorption, a nanoparticle strips the PLL film from the surface around it thereby removing the electrostatic attraction driving surface adsorption. This would substantially decrease the number of available sites and saturation coverage would be reached much sooner and be much smaller than the jamming coverage predicted by theory. What is clear from the AFM image in Figure 2 is that an adsorbed nanoparticle prevents similarly charged particles from adsorbing within relatively long distances (ca. 135 nm) around it. This is presumably due to electrostatic effects arising from either disruption of the PLL film or inefficient screening of the nanoparticle charge once adsorbed, resulting in a buildup of surface charge. C. Adsorption Kinetics of 20 nm Particles. Similar experiments were carried out with 20 nm nanoparticle suspensions, and a typical interfacial absorbance time curve is shown in Figure 5. Clearly, the adsorption kinetics are qualitatively different to those observed for the 5 nm colloid. Two distinct kinetic regimes are observed: At early times, up to 10 min, individual 20 nm particles adsorb at the surface in much the same way as the 5 nm particles. The adsorbed nanoparticles then act as sites for subsequent aggregation, ultimately leading to substantial structures comprising many hundreds of nanoparticles. As a result, the measured interfacial absorbance never plateaus but rather continues increasing approximately linearly with time. These interpretations are confirmed by the AFM images shown in Figure 5. After 1 min it is clear that only individual particles are adsorbed. Other particles from solution aggregate around these until large agglomerations are present. From AFM images taken in the early stages (after 1 min of deposition), the surface concentration of 20 nm nanoparticles was measured to be 2.55 ( 0.64 µm-2, from which we calculate the extinction 9 3 -1 cm-1. coefficient to be 405 20nm ) (2.6 ( 0.7) × 10 dm mol We thus estimate the bulk concentration of the stock solution
6466 J. Phys. Chem. C, Vol. 112, No. 16, 2008
Mazurenka et al.
Figure 4. Examples of the fits (red lines) of experimental data to the models outlined in the paper for the 0.01 (upper) and 0.1 (lower) dilution 5 nm colloidal solutions, in the limits of short (left) and long (right) times.
Figure 5. Top left: typical adsorption kinetics for 20 nm NPs. 5 µm × 5 µm AFM images at (a) 1 min, (b) 25 min, and (c) 2 h show aggregation of colloidal particles accounting for the continual rise in the interfacial absorbance.
to be 214 ( 54 pM. As expected, this is considerably lower than the concentration of the 5 nm colloid, reflecting the stronger surface plasmon resonance band. D. In-Situ Observation of the Growth of Multilayer Nanoparticle/Polyeletrolyte Films. There has been much interest in the growth and properties of multilayer films of metal nanoparticles and polyelectrolytes.47,52 Accordingly, we have studied in situ the growth of such films by repeatedly depositing alternate films of the 5 nm gold colloid and PLL. Each “layer” was deposited using the protocol described previously. The results are shown in Figure 6 and show clear steps in the interfacial absorbance upon the deposition of each layer of nanoparticles. These steps are only observed following the deposition of a PLL layer.
Figure 6. In-situ observation of the growth of multiple 5 nm colloidal gold/PLL films on a silica surface. Before the addition of either PLL solution or colloidal gold solution the surface was rinsed thoroughly with water.
The adsorption rate appears to be the same for each nanoparticle “layer” deposited, but the absorbance arising from each layer is slightly smaller (ca. 10%) than for the previous one. There are a number of ways in which these results can be interpreted. Either each new PLL layer covers the deposited nanoparticles, thereby effectively neutralizing their jamming effect on subsequent deposition, or else the positively charged surface between nanoparticles is renewed with each PLL layer, or both. The smaller adsorption of each successive layer suggests that the jamming effect is not perfectly negated, and the effective area is slightly reduced each time. Interestingly, the deposition of PLL onto an existing submonolayer of nanoparticles does not significantly affect the interfacial absorbance of that layer. Given the sensitivity of the surface plasmon resonance peak to local refractive index, this is perhaps somewhat surprising, although we note that in the case of the 5 nm colloid the SPR peak is not pronounced in the UV-vis spectrum, and our
Colloidal Gold Adsorption on Silica Surfaces
Figure 7. Variation in interfacial absorbance of (i) a PLL-functionalized silica surface (filled triangles), (ii) a poly-L-lysine/nanoparticle film (PLL-NP, filled squares), (iii) a PLL-NP-PLL layer (open squares), (iv) a PLL-NP-PLL-NP double layer (filled cirles), and (v) a PLL-NP-PLL-NP-PLL double layer (open circles) with changing solvent refractive index. Refractive indicies: nH2O ) 1.343, nIPA ) 1.388 at 405 nm.
detection wavelength of 405 nm is, in any case, some way from λmax,SPR ) 525 nm (see Figure 1). Much of the interest in supported NP films arises from their use as sensors in which the response of the localized surface plasmon resonance (LSPR) to changes in the external refractive index is a key factor. This sensitivity is usually quantified as the spectral shift in LSPR maximum per unit change of the refractive index, dλmax/dn, and is measured in 1/refractive index units (RIU-1). Alternatively, where measurements are performed at a single wavelength and information about the LSPR shift is not available, the variation of the total extinction with refractive index, dA/dn, can be used as a measure of the sensitivity.44 Van Duyne and co-workers have previously shown that the LSPR shows greater sensitivity to the solvent than to the substrate upon which the NPs are isolated,17 and it is possible to measure dA/dn arising from a change in the refractive index of the solvent. Following the method of Fisk et al., we have measured the change in interfacial absorbance at 405 nm for both submonolayer and multilayer films as a function of the RI of the solvent and compared it with the corresponding change observed in the bulk solution UV-vis spectrum. 5 nm colloidal films were deposited on PLL-functionalized silica, as before, using 0.1 dilution solution, and the interfacial absorbance measured as solution refractive index was changed systematically between 1.343 and 1.388 by using different H2O/isopropyl alcohol mixtures. The results are shown in Figure 7 for 0, 1, and 2 nanoparticle layers. As can be seen from Figure 6, depositing a covering layer of PLL on top of the particles does not significantly affect the absorbance while a “double layer” of colloid does, as expected, exhibit approximately twice the change in interfacial absorbance of a single layer. Similarly, the sensitivity of the interface to solvent refractive index change is also proportional to the number of colloidal particles adsorbed: A single 5 nm colloidal film exhibits a sensitivity to the solvent refractive index of dA/ dn ) (5.8 ( 0.2) × 10-2. This increases to (10.5 ( 0.2) × 10-2 when a second layer of nanoparticles is deposited, indicating no significant change in the RI sensitivity of the first layer despite the additional PLL layer. When normalized for effective path length and particle number density, these results indicate that immobilization of the nanoparticles has no significant effect on the RI sensitivity of individual particles at the wavelength used here. The fact that in the absence of a deposited colloid film the absorbance is insensitive to changes in refractive index confirms that total internal reflection remains complete despite the change in refractive index.
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6467 E. Conclusions. The adsorption kinetics of commercial gold colloid from quiescent solution onto PLL-functionalized silica surfaces has been studied using a combination of evanescent wave cavity ring-down spectroscopy and atomic force microscopy. The adsorption kinetics for 5 and 20 nm particles differ qualitatively with individual 5 nm particles adsorbing until a maximum coverage is reached and 20 nm particles undergoing significant aggregation at the interface. The reason for the difference in the kinetics of the two colloids is unclear but must stem from differences in the kinetic stabilities of the two once adsorbed. This may arise from different surface charges or charge distributions for the two colloids originating in the original synthesis and/or from differences in their interaction with the PLL supporting layer. If, for example, the larger colloid were more effective in stripping PLL from the surface around it, it would end up more effectively shielded enhancing the probability of aggregation. The exact surface properties of these commercial colloids, including the surface ligand density are, unfortunately, unknown. The deposition of the 5 nm colloid follows diffusioncontrolled (t1/2-dependent) kinetics in the early stages which gradually changes to a 1 - t -1/2 dependence as blocking becomes more significant. The maximum coverages achieved are typically ca. 10-3 of a monolayer. Additional films of colloid can be deposited following further treatment of the surface with an additional polyelectrolyte layer. We have observed in situ the growth of multilayer films of gold colloid and PLL laid down with a fine degree of control. The extinction coefficients for adsorbed 5 and 20 nm particles are estimated to be (3.76 ( 0.92) × 108 and (2.6 ( 0.7) × 109 dm3 mol-1 cm-1, respectively from which bulk solution concentrations of 3.6 ( 0.9 nM and 214 ( 54 pM for the 5 and 20 nm colloids can be determined. The studies herein represent a further demonstration of the sensitivity of EW-CRDS to surface processes, such as adsorption. The silica prism, which is core to this variant of the technique, can be readily modified, as shown herein, and we expect that this will allow a myriad of surface processes to be followed in real time. Acknowledgment. The authors are grateful to the Engineering and Physical Sciences Research Council (EPSRC) which funded this work. S.R.M. is further grateful to EPSRC for his Advanced Research Fellowship. References and Notes (1) Schmid, G. Chem. ReV. 1992, 92, 1709. (2) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (3) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (4) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (5) Aiken, J. D.; Finke, R. G. J. Mol. Catal. A: Chem. 1999, 145, 1. (6) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545. (7) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306. (8) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (9) Boyen, H. G.; Ka¨stle, G.; Weigl, F.; Koslowski, B.; Dietrich, C.; Ziemann, P.; Spatz, J. P.; Riethmu¨ller, S.; Hartmann, C.; Mo¨ller, M.; Schmid, G.; Garnier, M. G.; Oelhafen, P. Science 2002, 297, 1533. (10) Zidki, T.; Cohen, H.; Meyerstein, D. Phys. Chem. Chem. Phys. 2006, 8, 3552. (11) Parak, W. J.; Gerion, D.; Pellegrino, T.; Zanchet, D.; Micheel, C.; Williams, S. C.; Boudreau, R.; Le Gros, M. A.; Larabell, C. A.; Alivisatos, A. P. Nanotechnology 2003, 14, R15. (12) Alivisatos, P. Nat. Biotechnol. 2004, 22, 47. (13) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547. (14) McConnell, W. P.; Novak, J. P.; Brousseau, L. C.; Fuierer, R. R.; Tenent, R. C.; Feldheim, D. L. J. Phys. Chem. B 2000, 104, 8925.
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