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, andDepartment 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 depositionkinetics 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. Adsorptionon a poly-L-lysine-functionalized surface, however, occurs readily and irreversibly with the kinetics of adsorptiondiffering markedly for the two particle sizes studied. 5 nm particles adsorb to form a highly dispersesubmonolayer 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 depositingcolloidal particles and poly-L-lysine films. The deposition of multilayer nanoparticle films increases thesensitivity of the functionalized surface to changes in the solvent refractive index. The adsorption kinetics ofthe 20 nm colloid is more complex than that of the smaller colloid with adsorbed particles acting as nucleationsites for subsequent aggregation with the result that the interfacial absorbance continues to increase indefinitelywith time.

I. Introduction

The unique properties of metal nanoparticles (NPs) haveattracted intense interest for more than a decade following thedevelopment of convenient methods for their synthesis.1-7

Recent advances in tuning the physical and chemical propertiesof NPs have spread the applicability of NPs to a wide range ofscientificproblems,includingchemicalcatalysis,3,5,8-10biosensing,11-13

and nanoscale electronic devices.14

Many applications of NPs require their controlled depositionfrom quiescent solution onto solid substrates in well-definedarchitectures while avoiding uncontrolled aggregation. Someapplications, for example, require linking of adjacent NPs byconducting bridges either by attaching conducting moleculesor by deposition onto surfaces modified by conductivepolymers.14-16 In many cases, immobilization of individualnanoparticles itself significantly influences their optical andchemical properties,17 while other studies have shown thataggregation of nanoparticles can enhance the overall catalyticactivity.18 Clearly, a better understanding of the surfacechemistry of individual NPs and ensembles as well as theadsorption kinetics of NPs onto functionalized surfaces isrequired in order to better control the deposition process.

The need for an improved description of the adsorptionprocess has triggered extensive experimental and theoreticalresearch into the irreversible adsorption of colloidal metals ontovarious native and functionalized surfaces. Theoretical descrip-tions include the application of random sequential adsorption(RSA)19-21 models originally developed for mesoscopic particlesand the full numerical solution of the mass transport equationsfor nanoparticles and proteins.22 The experimental approacheswhich have been applied are many and varied but often combinesurface imaging techniques, such as atomic force microscopy(AFM) or scanning tunneling microscopy (STM) to determine

surface number densities, with spectroscopic methods. Experi-ments typically involve immersion of the substrate in solutionfor fixed times followed by subsequent spectroscopic/micro-scopic analysis. By contrast, techniques for in-situ monitoringof surface adsorption remain rare.

Many of the uses of metal nanoparticles stem from theiroptical properties. In many cases, especially colloidal gold andsilver, the spectrum in the visible region is dominated by a stronglocalized surface plasmon resonance (LSPR) peak. The maxi-mum of extinction corresponding to the LSPR,λmax, dependson various factors, such as size, shape, the dielectric constantof the material concerned, the distance between adsorbed NPson the surface, and the dielectric constant of the local environ-ment (e.g., the solvent or the substrate).23-25 Optimization ofthese physical properties to suit particular applications is thesubject of much research.4,7,26 The sensitivity of the LSPR tothe local environment, for example, is the basis for the use ofgold nanoparticles in biosensors, where the presence of anadsorbed biomolecule significantly perturbs the local refractiveindex which results in observable spectral shifts in theLSPR.25,27-29

In this paper we describe investigations of the adsorptionkinetics of commercially available colloidal gold onto func-tionalized silica surface by means of evanescent wave cavityring-down spectroscopy (EW-CRDS).30-35 This technique isideally suited to time-resolved in-situ measurements at thesolid-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 fromthe evanescent field.39 EW-CRDS has recently joined a limitedrange of in-situ techniques suitable for this type of application(including optical reflectometry40-42 and broadband time-resolved optical waveguide spectroscopy43) and has the advan-tage of simplicity and high sensitivity compared to othermethods. The first application of EW-CRDS to gold nanoparticledeposition was reported recently by Fisk et al.,44 who measuredthe adsorption of home-synthesized gold nanoparticles to bare

* Corresponding author. E-mail: srm49@cam.ac.uk.† University of Cambridge.‡ University of Warwick.

6462 J. Phys. Chem. C2008,112,6462-6468

10.1021/jp800706j CCC: $40.75 © 2008 American Chemical SocietyPublished on Web 04/03/2008

silica surfaces under continual flow conditions. Another variantof this technique, Brewster’s angle cavity ring-down spectros-copy, has recently been applied to the study of deposited goldnanoparticles by Gilb et al.45 In this study, we apply EW-CRDSto measure the adsorption of commercial gold nanoparticles froma quiescent solution onto functionalized silica surfaces. The useof commercial colloids was deliberate as these are the solutionsused in the majority of biosensing applications. Despite thesimilarity of the experiments, we observed key differences tothe results of Fisk et al., not the least of which is that we observeno surface adsorption whatsoever from fresh colloidal solutiononto optically clean pure silica surfaces. Significantly, ourstudies embrace functionalized substrates which are of keytechnological importance for the creation of monolayer andmultilayer nanoparticle ensembles.46,47

II. Experimental Section

Apparatus. The cavity ring-down spectrometer used in thisexperiment was a modified version of the ring cavity spectrom-eter described in detail previously.31 Briefly, an optical cavityis formed between two highly reflective mirrors and the totalinternal reflection at the hypotenuse of a right angled fused silicaprism. The latter constitutes the base of a liquid cell whichcontains the colloidal suspension. Light from an external lasersource is injected into the cavity and the light decay within thecavity is measured by a photomultiplier tube. The ring-downtime is sensitive to absorbance or scattering within the evanes-cent field which extends beyond the surface of the prism intothe aqueous solution, a distance comparable with the wavelengthof light.

The laser used in these studies was a 50 mW diode laseroperating at 405 nm (Power Technology IQ1H). The laser canbe modulated using a TTL signal at up to 1 MHz. This rapidpulse capability increases the effective experiment repetitionrate to 2 kHz, limited by the PCI bus data transfer rate fromthe NI PCI-5124 12-bit digitizer to the PC. Further, the“broadband” nature of the laser (∆λ ∼ 1 nm) means light isalways coupled into a large number of the cavity modes,obviating the need to scan the laser wavelength or the cavitylength and significantly simplifying the optical and electroniccomponents of the experimental setup.

To perform adsorption kinetic measurements of 5 and 20 nmcolloids, the optical cavity was first aligned with Milli-Q waterin the cell, giving a background ring-down time,τ0. The waterwas then removed from the cell, and 10 drops of poly-L-lysine(PLL) solution (1 mg/mL, Aldrich) were introduced to coverthe whole silica surface inside the cell. The PLL solution wasleft to deposit for 20 min before being rinsed with Milli-Q waterfour times. At this point the gold colloid solution (2.5 mL of 5or 20 nm gold colloid at various dilutions, Aldrich) wasintroduced into the cell. Cavity ring-down measurements of theinterfacial absorbance were taken at 2 kHz. To improve thesignal-to-noise ratio, 100 ring-down events were stored in thedigitizer card memory and then transferred to the PC, where adecay constant was determined for each single transient andthe average ring-down time calculated for this group. Thisaveraging provides a data point every 50 ms. The wholeexperiment, including the fitting of the ring-down traces, wascontrolled by a purpose-built LabView program.

Colloidal Solutions. The concentration of the commercialcolloidal solutions (Sigma-Aldrich) was unknown; only theabsorbance at the peak of the surface plasmon band is specified(A ) 0.75 atλ ) 525 nm in each case). Various dilutions ofthe stock solution were used, ranging from 10-2 to 1.0 of the

original concentration in ultrapure water. Both the 5 and 20nm colloids were produced by the well-known tannic acid/citratemethod resulting in citrate-stabilized particles.48,49 Despiterefrigerated storage, there was some evidence of aggregationafter 2 months. This was observed by both a slight broadeningof the LSPR band in the UV-vis spectra of the stock colloidsolutions and the deposition of large aggregated particles, evenon clean silica surfaces. Neither was observed with fresh sampleswhich were used for the studies described here.

Care was taken to ensure that even the maximum surfaceadsorption observed in these experiments did not significantlydeplete the bulk concentration of the sample in the cell. Thiswas tested following each experiment by removing the solutionand replacing it with fresh colloidal solution. No change in thesurface adsorption was observed.

Between experiments, the fused silica surface of the prismwas cleaned thoroughly by repeated plasma ashing (20 min in100 W O2 plasma, Diener plasma asher), interspersed withrinsing in methanol. In each case care was taken to ensure thatthe original background ring-down time was achieved beforeproceeding. All atomic force microscope images were recordedusing a DI AFM Nanoscope Dimension 3100 instrumentoperating in tapping mode.

III. Results and Discussion

UV-vis spectra of the 5 and 20 nm colloid solutions areshown in Figure 1. At the measurement wavelength of 405 nm,the extinction arises from absorption in the tail of the plasmonresonance band itself as well as from the 5df 6sp interbandtransition of bulk gold.50 The surface plasmon band is consider-ably more prominent in the spectrum of the larger colloid.

Experiments were originally performed on clean unfunction-alized silica surfaces which are known to be negatively chargedunder water due to the deprotonation of the terminal-Si-OHgroups. No interfacial adsorption of nanoparticles was observedeven over periods of several hours. This is no surprise as thecitrate groups which stabilize the particles with respect to

Figure 1. UV-vis spectra of 5 nm (upper) and 20 nm (lower) colloidalsolutions. The spectrum is dominated by contributions from the surfaceplasmon resonance peak centered at 525 nm (dotted line) and the 5d-6sp gold cluster transition (dashed line).

Colloidal Gold Adsorption on Silica Surfaces J. Phys. Chem. C, Vol. 112, No. 16, 20086463

aggregation will give rise to repulsion from the charged surface.Some nanoparticle adsorption was observed on prisms whichsubsequent AFM imaging showed to be significantly scratchedor contaminated. Following implementation of our plasmaashing cleaning protocol with new optical quality prisms, noadsorption on clean silica surfaces was observed. This is inmarked contrast to the previous EW-CRDS study by Fisk etal., who observed extensive deposition on such surfaces forslightly larger self-synthesized colloids.44 The reasons for thedisparity are unclear but must lie in one of several significantdifferences between the two experiments. First, Fisk et al. useda continually flowing sample of colloidal solution whichincreases the mass transport to the surface compared with ourstatic solution. Second, Fisk et al. used self-prepared colloidswhich, although citrate-stabilized, may have had very differentsurface charge (and thus kinetic stability) than those used here.Fisk et al. report that their solutions developed a “pearlescentpurple” appearance in the 24 h between preparation of theirsolutions and their use, which may reflect aggregation insolution. No such pearlescence was observed in our solutions.Third, the surface cleaning protocols were very different in thetwo studies (aqua regia vs plasma cleaning), and it is possiblethat Fisk et al. observed initial deposition onto scratches on thesurface which acted as nucleation sites for further aggregation,resulting in the dense layers they report.

A. Adsorption of 5 nm Particles on PLL-FunctionalizedSilica. As discussed earlier, there is considerable interest in theadsorption of nanoparticles onto PLL-functionalized surfaces.PLL is a positively charged polyelectrolyte which adsorbsreadily onto the negative silica surface to yield a net positivesurface. A typical adsorption kinetics curve, recorded by EW-CRDS, for 5 nm colloidal particles on the PLL-functionalizedprism is shown in Figure 2. Rapid initial adsorption for the firstminute is followed by ever slower net adsorption as the coverageasymptotically approaches a maximum. The inset shows a 1µm × 1 µm AFM image of the prism surface after 25 min ofdeposition. The image was recorded in tapping mode followingremoval of the colloidal solution and drying of the prism surfacein a nitrogen atmosphere. Repeated flushing with water had noeffect on the interfacial absorbance (see below), indicating thatadsorption is irreversible, and thus this procedure is believedto have no significant effect on the nanoparticle films deposited.It is clear that the maximum coverage achieved is far short ofa full monolayer. For concentrations used in this study after∼4 h the interfacial absorbance remained constant within the

(small) long-term drift of the spectrometer. From multipleimages across the surface of several prisms, we estimate themaximum coverage of nanoparticles deposited from 0.1× stockconcentration solution to be 55( 14 µm-2 or an effectivecoverage of ca. (1.2 ( 0.3) × 10-3 of a monolayer.

With knowledge of both the number density and the corre-sponding interfacial absorbance it is possible to derive theextinction at 405 nm arising from individual nanoparticles onthe surface. For the 5 nm colloid we estimate an extinctioncoefficient of ε5nm

405 ) (3.76 ( 0.92) × 108 dm3 mol-1 cm-1.Under the assumption that the extinction coefficient is un-changed upon adsorption (and our 405 nm wavelength is someway from the peak of the surface plasmon band), this valuecan be used to estimate the concentration of the original colloidalsolution for which the bulk absorbance is known from theconventional UV-vis spectrum. Using this method, we calculatethe concentration of the original undiluted 5 nm colloidalsample, as purchased, to be 3.6( 0.9 nM.

B. Adsorption Kinetics of 5 nm Particles. Diffusion-controlled adsorption of colloidal particles from stagnant solutionhas been discussed theoretically in the context of several modelsincluding a random sequential adsorption (RSA) model,51 aballistic model,20 and Brownian dynamics.21 All models assumethat adsorption is irreversible, and the validity of this assumptionwas confirmed in our experiment by removing the colloidalsolution from the cell after the NPs had been adsorbed for anhour and refilling the cell with pure Milli-Q water. The resultingabsorbance changes were within the long-term spectrometersignal drift, indicating that desorption can indeed be neglected.

The ballistic model of desorption includes gravitational effectson the colloid particle and is applicable when the effective radiusof the colloid particle,R*, is larger than 1.8.46 In the case ofour nanoparticlesR* ) 9.2 × 10-3 and 3.7× 10-2 for the 5and 20 nm colloids, respectively. In both casesR* , 1.8, andthus the ballistic model can be discarded from the followingdata analysis. Similarly, the RSA model, developed to describemesoscopic sized particles, fails for small particles such as these.Brownian dynamics apply best at near-monolayer coverageswhere jamming becomes important. As shown above this is notthe situation here.

Adamczyk has developed an approach to describing thekinetics of diffusion-limited irreversible adsorption of colloidsfrom stagnant solutions which involves solving, in a quasi-stationary manner, the mass transfer equation, whereby blockingeffects are incorporated into the activity coefficient.22 Withinthis model the mass transfer equation can be solved analyticallyfor two limiting cases:

and

where t is the adsorption time andΘmx is the jamming (orsaturation) coverage.tch is a characteristic time, defined as

in which nb is the bulk colloid concentration,D∞ is the bulkdiffusion coefficient, anda is the particle radius.

Figure 2. Typical adsorption kinetics for 0.1 dilution, 5 nm colloidsolution onto PLL-functionalized silica. (inset) Tapping mode 1µm ×1 µm AFM image of the surface recorded after 25 min of adsorption.

ttch

< π4

Θmx2 (1)

ttch

> 1 (2)

tch ) 1

πa2nbD∞

(3)

6464 J. Phys. Chem. C, Vol. 112, No. 16, 2008 Mazurenka et al.

Early stage adsorption kinetics, satisfying expression 1, leadsto the familiar form of diffusion-controlled kinetics in whichthe coverage as a function of time is given by

At the other extreme, as the maximum coverage is approachedasymptotically, i.e.,t > tch, the coverage behaves as

where

and for spherical particlesn ) 3, c ) 2.3. The predicted timedependence is thus 1- t -1/2. For intermediate times, the masstransfer equation cannot be solved analytically and requiresnumerical solution.22

We have tested our data for consistency with the abovekinetics by replotting the interfacial absorbance for severaldifferent colloidal concentrations as a function oft1/2, as shownin Figure 3. Particularly in the adsorption from the more dilutesolutions, a clear linear region is visible, indicating the extentto which the system behaves in a diffusion-controlled manner.The linearity of the data for 0.01 dilution solution fort < 25min is sufficiently good to permit a determination of thediffusion constant via22

and noting that

Gratifyingly, the value obtained,Dbexp ) (8.01( 1.96)× 10-11

m2 s-1, is in excellent agreement with that calculated using theStokes-Einstein equation, 8.58× 10-11 m2 s-1.

We have fitted our data to the predicted functional forms inboth early and long time regimes, and the results for the 0.01and 0.1 dilutions are shown in Figure 4. The early time kineticshave been fitted to Abs) A + B xt-t0 to reflect the fact thatfor the first few seconds the solution in the cell experiencesturbulent flow with the consequence thatt ) 0 is ill-defined.Convergence of the fit parameters was ensured by fitting tovarious subsets of the data until no noticeable change in the fitparameters was observed. Likewise, the long time adsorptionkinetics were fitted to the function Abs) A - B/xt with asimilar procedure. The results demonstrate good agreementbetween the data and the predicted functional forms of the timedependence in the limits of both short and long deposition times.Despite strenuous attempts to make our deposition protocol asreproducible as possible, there is still typically a(17% statisticalvariation in the final coverage which we attribute to variationsin the PLL layer and/or the way the colloid is introduced to thecell.

Despite this qualitative agreement, there is some discrepancybetween the experimental data and the predicted time scales.Using the diffusion coefficient and stock concentrations deter-mined above, the characteristic timetch can be calculated (eq3) to range from 6.5× 1010 s (0.01 dilution) to 6.5× 109 s(0.1 dilution). These times are substantially longer than theduration of any experiment performed in this study. In otherwords, we observe jamming, 1- t -1/2, kinetics at much earliertimes (103-104 s) than predicted (Figure 4). This discrepancymay result from the neglect of particle-surface and particle-particle interactions in the model and the unknown behavior ofthe PLL film when charged particles adsorb. It is possible that,upon adsorption, a nanoparticle strips the PLL film from thesurface around it thereby removing the electrostatic attractiondriving surface adsorption. This would substantially decreasethe number of available sites and saturation coverage would bereached much sooner and be much smaller than the jammingcoverage predicted by theory. What is clear from the AFMimage in Figure 2 is that an adsorbed nanoparticle preventssimilarly charged particles from adsorbing within relatively longdistances (ca. 135 nm) around it. This is presumably due toelectrostatic effects arising from either disruption of the PLLfilm or inefficient screening of the nanoparticle charge onceadsorbed, resulting in a buildup of surface charge.

C. Adsorption Kinetics of 20 nm Particles.Similar experi-ments were carried out with 20 nm nanoparticle suspensions,and a typical interfacial absorbance time curve is shown inFigure 5. Clearly, the adsorption kinetics are qualitativelydifferent to those observed for the 5 nm colloid. Two distinctkinetic regimes are observed: At early times, up to 10 min,individual 20 nm particles adsorb at the surface in much thesame way as the 5 nm particles. The adsorbed nanoparticlesthen act as sites for subsequent aggregation, ultimately leadingto substantial structures comprising many hundreds of nano-particles. As a result, the measured interfacial absorbance neverplateaus but rather continues increasing approximately linearlywith time.

These interpretations are confirmed by the AFM imagesshown in Figure 5. After 1 min it is clear that only individualparticles are adsorbed. Other particles from solution aggregatearound these until large agglomerations are present. From AFMimages taken in the early stages (after 1 min of deposition), thesurface concentration of 20 nm nanoparticles was measured tobe 2.55( 0.64µm-2, from which we calculate the extinctioncoefficient to beε20nm

405 ) (2.6 ( 0.7) × 109 dm3 mol-1 cm-1.We thus estimate the bulk concentration of the stock solution

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 againstt1/2,illustrating the extent of diffusion-controlled adsorption.

Θ ) 2

xπ x ttch

(4)

Θ ) Θmx -K1

n-1xt/tch

(5)

K1 ) Θmxn - 1x Θmx

c(n - 1)ka

(6)

Θ ) 2πa2xDbt

πnb (7)

Abs )ε5nm

405Θ

πa2NA

(8)

Colloidal Gold Adsorption on Silica Surfaces J. Phys. Chem. C, Vol. 112, No. 16, 20086465

to be 214( 54 pM. As expected, this is considerably lowerthan the concentration of the 5 nm colloid, reflecting the strongersurface plasmon resonance band.

D. In-Situ Observation of the Growth of MultilayerNanoparticle/Polyeletrolyte Films. There has been muchinterest in the growth and properties of multilayer films of metalnanoparticles and polyelectrolytes.47,52 Accordingly, we havestudied in situ the growth of such films by repeatedly depositingalternate films of the 5 nm gold colloid and PLL. Each “layer”was deposited using the protocol described previously. Theresults are shown in Figure 6 and show clear steps in theinterfacial absorbance upon the deposition of each layer ofnanoparticles. These steps are only observed following thedeposition of a PLL layer.

The adsorption rate appears to be the same for eachnanoparticle “layer” deposited, but the absorbance arising fromeach layer is slightly smaller (ca. 10%) than for the previousone. There are a number of ways in which these results can beinterpreted. Either each new PLL layer covers the depositednanoparticles, thereby effectively neutralizing their jammingeffect on subsequent deposition, or else the positively chargedsurface between nanoparticles is renewed with each PLL layer,or both. The smaller adsorption of each successive layer suggeststhat the jamming effect is not perfectly negated, and the effectivearea is slightly reduced each time. Interestingly, the depositionof PLL onto an existing submonolayer of nanoparticles doesnot significantly affect the interfacial absorbance of that layer.Given the sensitivity of the surface plasmon resonance peak tolocal refractive index, this is perhaps somewhat surprising,although we note that in the case of the 5 nm colloid the SPRpeak is not pronounced in the UV-vis spectrum, and our

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 5nm colloidal solutions, in the limits of short (left) and long (right) times.

Figure 5. Top left: typical adsorption kinetics for 20 nm NPs. 5µm× 5 µm AFM images at (a) 1 min, (b) 25 min, and (c) 2 h showaggregation of colloidal particles accounting for the continual rise inthe interfacial absorbance.

Figure 6. In-situ observation of the growth of multiple 5 nm colloidalgold/PLL films on a silica surface. Before the addition of either PLLsolution or colloidal gold solution the surface was rinsed thoroughlywith water.

6466 J. Phys. Chem. C, Vol. 112, No. 16, 2008 Mazurenka et al.

detection wavelength of 405 nm is, in any case, some way fromλmax,SPR) 525 nm (see Figure 1).

Much of the interest in supported NP films arises from theiruse as sensors in which the response of the localized surfaceplasmon resonance (LSPR) to changes in the external refractiveindex is a key factor. This sensitivity is usually quantified asthe spectral shift in LSPR maximum per unit change of therefractive index, dλmax/dn, and is measured in 1/refractive indexunits (RIU-1). Alternatively, where measurements are performedat a single wavelength and information about the LSPR shift isnot available, the variation of the total extinction with refractiveindex, dA/dn, can be used as a measure of the sensitivity.44 VanDuyne and co-workers have previously shown that the LSPRshows greater sensitivity to the solvent than to the substrateupon which the NPs are isolated,17 and it is possible to measuredA/dn arising from a change in the refractive index of thesolvent.

Following the method of Fisk et al., we have measured thechange in interfacial absorbance at 405 nm for both submono-layer and multilayer films as a function of the RI of the solventand compared it with the corresponding change observed in thebulk solution UV-vis spectrum. 5 nm colloidal films weredeposited on PLL-functionalized silica, as before, using 0.1dilution solution, and the interfacial absorbance measured assolution refractive index was changed systematically between1.343 and 1.388 by using different H2O/isopropyl alcoholmixtures. The results are shown in Figure 7 for 0, 1, and 2nanoparticle layers.

As can be seen from Figure 6, depositing a covering layer ofPLL on top of the particles does not significantly affect theabsorbance while a “double layer” of colloid does, as expected,exhibit approximately twice the change in interfacial absorbanceof a single layer. Similarly, the sensitivity of the interface tosolvent refractive index change is also proportional to thenumber of colloidal particles adsorbed: A single 5 nm colloidalfilm 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 firstlayer despite the additional PLL layer. When normalized foreffective path length and particle number density, these resultsindicate that immobilization of the nanoparticles has nosignificant effect on the RI sensitivity of individual particles atthe wavelength used here. The fact that in the absence of adeposited colloid film the absorbance is insensitive to changesin refractive index confirms that total internal reflection remainscomplete despite the change in refractive index.

E. Conclusions.The adsorption kinetics of commercial goldcolloid from quiescent solution onto PLL-functionalized silicasurfaces has been studied using a combination of evanescentwave cavity ring-down spectroscopy and atomic force micros-copy. The adsorption kinetics for 5 and 20 nm particles differqualitatively with individual 5 nm particles adsorbing until amaximum coverage is reached and 20 nm particles undergoingsignificant aggregation at the interface. The reason for thedifference in the kinetics of the two colloids is unclear but muststem from differences in the kinetic stabilities of the two onceadsorbed. This may arise from different surface charges orcharge distributions for the two colloids originating in theoriginal synthesis and/or from differences in their interactionwith the PLL supporting layer. If, for example, the larger colloidwere more effective in stripping PLL from the surface aroundit, it would end up more effectively shielded enhancing theprobability of aggregation. The exact surface properties of thesecommercial colloids, including the surface ligand density are,unfortunately, unknown.

The deposition of the 5 nm colloid follows diffusion-controlled (t1/2-dependent) kinetics in the early stages whichgradually changes to a 1- t -1/2 dependence as blockingbecomes more significant. The maximum coverages achievedare typically ca. 10-3 of a monolayer.

Additional films of colloid can be deposited following furthertreatment of the surface with an additional polyelectrolyte layer.We have observed in situ the growth of multilayer films of goldcolloid and PLL laid down with a fine degree of control.

The extinction coefficients for adsorbed 5 and 20 nm particlesare estimated to be (3.76( 0.92)× 108 and (2.6( 0.7)× 109

dm3 mol-1 cm-1, respectively from which bulk solutionconcentrations of 3.6( 0.9 nM and 214( 54 pM for the 5and 20 nm colloids can be determined.

The studies herein represent a further demonstration of thesensitivity of EW-CRDS to surface processes, such as adsorp-tion. The silica prism, which is core to this variant of thetechnique, can be readily modified, as shown herein, and weexpect that this will allow a myriad of surface processes to befollowed in real time.

Acknowledgment. The authors are grateful to the Engineer-ing and Physical Sciences Research Council (EPSRC) whichfunded this work. S.R.M. is further grateful to EPSRC for hisAdvanced Research Fellowship.

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Figure 7. Variation in interfacial absorbance of (i) a PLL-function-alized silica surface (filled triangles), (ii) a poly-L-lysine/nanoparticlefilm (PLL-NP, filled squares), (iii) a PLL-NP-PLL layer (opensquares), (iv) a PLL-NP-PLL-NP double layer (filled cirles), and(v) a PLL-NP-PLL-NP-PLL double layer (open circles) withchanging solvent refractive index. Refractive indicies:nH2O ) 1.343,nIPA ) 1.388 at 405 nm.

Colloidal Gold Adsorption on Silica Surfaces J. Phys. Chem. C, Vol. 112, No. 16, 20086467

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