Immobilization Kinetics of Cyclodextrins at Gold Surfaces - The

J. Phys. Chem. 1996, 100, 17893-17900

17893

Immobilization Kinetics of Cyclodextrins at Gold Surfaces Michael Weisser, Gabriele Nelles, Peter Wohlfart, Gerhard Wenz,† and Silvia Mittler-Neher* Max-Planck-Institut fu¨ r Polymerforschung, Ackermann Weg 10, 55128 Mainz, Germany ReceiVed: May 29, 1996; In Final Form: August 31, 1996X

In a previous paper we could demonstrate the immobilization of mono- and multithiolated cyclodextrins on gold surfaces and investigated the film properties. In this paper we follow the self-assembly processes in time by optical and physicochemical methods for those three monothiolated and a mixture of multithiolated cyclodextrin derivatives. The formation of cyclodextrin films on gold substrates was found to be a multistep process involving an activation energy.

Introduction Molecular guest-host systems have attracted enormous research interest in recent years.1,2 Inclusion of a guest without covalently binding into the host material can serve many purposes, such as solubility enhancement,3 protection against degradation by light4 or oxygen5 or removing undesired substances.6 Cyclodextrins (CDs) are cyclic oligosaccharides consisting of at least six glucopyranose units. The oligosaccharide ring forms a torus with the primary hydroxyl groups of the glucose residues lying on the narrow side and the secondary hydroxyl groups on the wider side. A specific guest complexation can take place in the hydrophobic cavity of the host torus. Some examples are already known wherein the synthesis of thiolated CDs and their adsorption on gold or silver are described.7-10 The film thicknesses and the location and orientation of the CD cavities in these films are investigated, e.g., as a function of the spacer length between the cavity and the thiol group binding to the gold.7 The adsorption kinetics of thiolated CDs, however, have not been investigated so far. Bain et al. and Grunze et al.11,12 showed that alkanethiols undergo a two-step process to form densely packed monolayers. In order to investigate the kinetics of the immobilization reaction of various thiolated CDs, in situ adsorption measurements with plasmon surface polariton (PSP) spectroscopy13 have been carried out as well as rinsing experiments. Cyclic voltammetry14 and mass spectrometry15 were used to investigate the coverage of gold electrodes qualitatively with the assembly time. The changes in wetting properties with increasing self-assembly time have been followed by contact angle measurements.16 The derivatives used in this study are mono(6-deoxy-6mercapto)-β-cyclodextrin (CD(0)), mono(6-deoxy-6-[(mercaptodecamethylene)thio])-β-cyclodextrin (CD(10)), mono(6-deoxy6-[[[(mercaptoethoxy)ethoxy]ethyl]thio])-β-cyclodextrin (CD(8)), and heptakis(2,3-O-dimethyl)oligo[6-deoxy-6-[(mercaptodecamethylene)thio]]-β-cyclodextrin (CDx(10)). The synthesis has been described previously.7 The structures of the four derivatives are depicted in Figure 1. Experimental Section Immobilization of the Cyclodextrins. The self-assembled monolayers were formed on gold films vacuum-evaporated at † Polymer Institut der Universita ¨ t Karlsruhe, Hertzstr. 16, 76187 Karlsruhe, Germany. * Corresponding author. [email protected]. fax: 0049 (0)6131-379100. X Abstract published in AdVance ACS Abstracts, October 15, 1996.

S0022-3654(96)01547-X CCC: $12.00

Figure 1. Structure of the thiolated CDs.

a pressure of 5 × 10-6 mbar onto cleaned LaSFN9 substrates. After preparation, the gold-coated substrates were allowed to cool under vacuum for approximately 30 min and were then immediately placed in solution of the appropriate selfassembling cyclodextrin derivative or stored under argon. All adsorptions were performed in ethanol, with a concentration of 1 × 10-4 M if not otherwise mentioned. Plasmon Surface Polariton Measurements. A diagram of the plasmon surface polariton spectroscopy setup is shown in Figure 2a. The Kretschmann configuration is used17 with a 50 nm gold film evaporated onto a substrate, which is then optically matched to the base of a 90° LaSFN9 glass prism (n ) 1.85 at λ ) 632.8 nm). Thus, the plasmon surface polaritons are excited at the metal/dielectric interface, upon total internal reflection of the laser beam (HeNe, λ ) 632.8 nm, power 5 mW) at the prism base. By varying the angles of incidence of the laser beam, we obtain a plot of reflected intensity as a function of the angle of incidence, similar to that shown in Figure 2b. The reflected intensity shows a sharp minimum at the resonance angle, θ1, which depends upon the precise architecture of the metal/dielectric interface and is defined by the matching condition for energy and momentum between the evanescent photons and the plasmon surface polaritons. Adsorption processes occurring at the gold interface were followed in real time, as shown in Figure 2c, by selecting an appropriate angle of incidence, θk, and monitoring the reflected intensity as a © 1996 American Chemical Society

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Figure 2. (a) Schematic of the experimental setup used for plasmon surface polariton characterization of layer formation. The prism is brought into optical contact with the glass slide, on which the chemical reaction takes place, using an index match liquid. (b) Schematic reflectivity curves without and with a dielectric layer on top of the metal, respectively, with different minima positions θ1 and θ2. (c) Schematic reflectivity versus time curve obtained with the surface plasmon spectrometer. The formation of a layer can be followed in-situ by monitoring the reflected intensity at an angle θk.

function of time. Knowledge of the form of the resonance curve allowed this intensity to be interpreted as a shift in the angle of resonance. From a Fresnel fit to the resonance curve for bare gold surfaces, it is possible to obtain the dielectric constant and the thickness of the gold layer. Addition of a thin layer to the surface of the gold typically shifts the position of the resonance to a higher angle, θ2 (Figure 2c), and fitting to this second curve determines the optical thickness, ∆nd, of the layer. Although plasmon surface polariton measurements allow the determination of an average optical thickness of an adsorbed film, accurate conversion of this optical thickness to a geometrical thickness requires knowledge of the refractive index of the film, a parameter which depends on both the molecular composition of the film and the packing density. In practice, it is not possible to distinguish between a thin film with a high refractive index and a film twice as thick but with half the contrast in the

medium. The present data are evaluated using a concentrationdependent refractive index measurement (dn/dc) of native β-CD in water18 and calculate the surface concentration c in mol/cm2.19

c ) (dc/dn)∆nd

(1)

One has to be careful in interpreting these data, because the adsorption kinetics of thiolated CDs had been measured in ethanol, whereas the dn/dc measurement has been carried out with native β-CD in water due to the poor solubility of the thiolated CD derivatives in both water and ethanol and the poor solubility of native β-CD in ethanol. This is taken into account in the error bars depicted in all plots where surface concentrations are given. They have been calculated by the combination of the experimental error of the surface plasmon experiment and the errors in the slope of the dn/dc measurement with the Gaussian error function for multiple sources of errors.

Immobilization Kinetics of Cyclodextrins at Gold Surfaces Contact Angle Measurement. Advancing and receding contact angles of water on the films were measured using a contact angle microscope (Kru¨ss G-1) under ambient conditions, while the volume of the drop is increased or decreased at the minimum rate required for movement of the water/air/solid triple point. Cyclic Voltammetry. Cyclic voltammetry was performed using the modified gold surfaces as the working electrode. Specifically, a 1.2 × 1.6 cm2 gold working electrode (evaporated onto LaSFN9 substrates using a mask), Ag/AgCl reference electrode (Bio Analytical Systems, Inc.), and a platinum wire auxiliary electrode were used. The cyclic voltammograms were measured in 0.1 M HNO3 and 0.001 M K3[Fe(CN)6], at a sweep rate of 100 mV/s, using a Princeton Applied Research 270 potentiostat. Time-of-Flight Mass Spectrometry. The measurements were performed using a linear time-of-flight (TOF) mass spectrometer in high vacuum at a pressure of about 1 × 10-6 mbar. The self-assembled monolayers were deposited on 50 nm thick gold films evaporated on glass slides that have been covered first with a 2 nm chromium film in order to increase the mechanical stability. Atomic and molecular ions from the sample were released by spontaneous desorption, a secondary ion process in which the sample is not bombarded by particles from an external source.15 Primary ions of adsorbates are field desorbed from the edges of an acceleration grid located in front of the sample. These ions are accelerated towards the sample gaining kiloelectronvolt energies. They finally sputter secondary ions from the sample that are analyzed with the TOF mass spectrometer. Spectra of negative secondary ions are recorded using the simultanously emitted secondary electrons from the sample surface as trigger particles. An acceleration voltage of 9.5 kV was applied to the sample. The recording time of a spectrum was 20-40 min. Within one spectrum mass peaks of interest were integrated and normalized with the number of start events with one or more corresponding stop events. This leads to a relative ion yield that allows to compare the intensities of equivalent peaks in different spectra. Atomic Force Microscopy. The samples were imaged with a Nanoscope III (Digital Instruments, Santa Barbara, CA) in contact, constant force mode. We used silicon nitride cantilevers of 100 µm length (spring constant 0.09 N/m) with integrated sharpened tips (Olympus, Tokyo, Japan). Images were taken in ethanol at almost the smallest force possible (typically 0.1 nN). The gold(111), onto which the cyclodextrins were selfassembled, was prepared by evaporating gold at a slow rate (0.2 nm/s) onto freshly cleaved mica at a pressure of 10-6 mbar. After evaporation the gold substrates were annealed at 400 °C under vaccum and cooled in methanol. Gold samples were placed in 0.1 mM ethanolic cyclodextrin solution for 8 h. Then they were taken out, briefly rinsed, dried with nitrogen, and placed onto the scanner of the atomic force microscope. Results Plasmon Surface Polariton Spectroscopy. The adsorption processes of all CD derivatives have been followed in situ by plasmon surface polariton spectroscopy. In this case all molecules which are adsorbed at the surface are measured. Figure 3 shows the adsorption kinetics of the thiolated CD derivatives from a 10-4 M solution in ethanol. The CD derivatives show different adsorption behavior. The spacerless CD(0) exhibits a quite different assembly kinetic than the other three derivatives. It seems to start and continue the concentration increase with a single-exponential time behavior, whereas

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Figure 3. In-situ surface plasmon adsorption kinetic measurements for CD(0), CD(8), CD(10), and CDx(10).

Figure 4. Two models for the arrangement of the CD tori on a surface.7 (a) Hexagonal closed packed model: Θmono) 8.11 × 10-11 mol/cm2. (b) Brick packing model: Θmono) 1.90 × 10-10 mol/cm2.

CD(8), CD(10), and CDx(10) exhibit a fast surface concentration increase at the beginning, which then changes in an ongoing slower concentration increase. But all four derivatives pass the surface concentration values of a closely aligned monolayer of CD tori in hexagonal closed packing parallel to the surface (8.11 × 10-11 mol/cm2) or the closed packing of perpendicular aligned CDs (1.90 × 10-10 mol/cm2) (Figure 4). Here we obviously observe not only the formation of a covalently bound monolayer but also molecules at the surface which are physisorbed on top of the monolayer. It is very typical for the CD self-assembly kinetics that the signal keeps increasing with time. The films become thicker and thicker. An additional adlayer of physisorbed CD-molecules must be the reason for this thickness increase. In order to observe only the covalently bound molecules, insitu rinsing experiments were performed. Figure 5 shows the result of such rinsing processes for CD(0) and CD(10): the plasmon signal and therefore the surface concentration decrease rapidly by changing to and rinsing with the bare solvent. The physisorbed molecules are rinsed off. The adsorption time of the molecules was varied systematically, and the resulting rest surface concentration after rinsing was measured. The concentration versus time curve for all CD derivatives is depicted in Figure 6. For all four derivatives a surface concentration plateau can be found, which we assume to be the surface concentration of the covalently bound monolayer. The adsorption time at which the plateau begins is the monolayer formation

17896 J. Phys. Chem., Vol. 100, No. 45, 1996

Weisser et al. TABLE 1: Fit Parameters Achieved by the Elovich Equationa fitted time interval, min 1010 Θmono, mol/cm2 τ ) 1/kc, s R, kJ/mol

CD(0)

CD(8)

0.15-10

0.15-60

CD(10) 0.15-60

CDx(10) 0.15-60

0.66 ( 0.09 2.29 ( 0.25 2.59 ( 0.16 1.55 ( 0.13 68 ( 36 246 ( 92 83 ( 25 80 ( 32 0.00 ( 1.03 1.78 ( 0.39 1.39 ( 0.36 1.14 ( 0.54

a Θ is the surface coverage, τ the adsorption time constant, and R the maximum activation energy for the chemisorption kinetics of the thiolated CD. The time intervals are given in which the fits were performed.

Figure 5. In-situ surface plasmon adsorption kinetics with a rinsing process.

Figure 6. Surface concentration after the rinsing processes, measured with the surface plasmon spectroscopy like shown in Figure 5 for ()) CD(0), (O) CD(8), (4) CD(10), and (3) CDx(10). The lines are fits to an Elovich kinetic (eq 3).

time and differs for all derivatives. For CD(0) it appears after 5 min, for CD(8) after 60 min and after 20 min for CD(10). The oligo-thiolated derivative CDx(10) reaches the plateau after 10 min. The lines in Figure 6 are fits to a by Elovich introduced kinetic.20-22 The fit was broken off at the end of the plateau, because after the formation of the monolayer the rinsing process is not totally efficient anymore. The Elovich rate equation (eq 2) takes into account that a molecule needs to overcome an energy barrier E before binding. This energy E increases with increasing surface coverage Θ, due to more and more unoriented molecules present on the surface (E(Θ) ) RΘ).

dΘ 1 -RΘ ) (1 - Θ) exp dt τ RT

(

)

(2)

Integration of eq 2 leads the Elovich adsorption kinetic equation (eq 3).

[

exp(-R/RT)

( τt )

Θ(t) ) Θmono 1 - exp -

]

(3)

τ is the time constant of the adsorption process, R the maximum activation energy the molecule has to overcome for binding, R the gas constant, T the absolute temperature, and Θmono the surface concentration of a complete monolayer. The fitted monolayer concentration Θmono converted into the surface concentration c, the activation energy R, the time constant τ, and the time interval for the fitting routine are listed

in Table 1. Like in the in-situ kinetic measurements depicted in Figure 3 the CD(0) derivate exhibits another assembly behavior than the other derivatives. It is the only derivative which does not show an activation energy, within the experimental error. It can be descibed by a Langmuir adsorption kinetic, which is the R f 0 limit for the Elovich kinetic. The highest activation energy exhibits CD(8) with 1.8 kJ/mol, compared to the smaller value of 1.4 kJ/mol for CD(10) and 1.1 kJ/mol for CDx(10). The activation energy is obviously dependent on the length and kind of the spacer chain between the binding thiol group and the CD torus. The order for the time contants τ and the activation energies R are the same: τ[CD(0)] < τ[CDx(10)] < τ[CD(10)] < τ[CD(8)]. Cyclic Voltammetry. In an usual cyclic voltammetry investigation of a reversible redox pair at an unmodified electrode surface, the redox currents are proportional to the electrode area. Thus, for modified electrode surfaces with decreased electrode area available for redox processes, the redox currents are expected to decrease. Furthermore, as the redox currents are confined to smaller and smaller electrode areas, the current density decreases and regular Nernstian redox behavior is no longer observed. Therefore, the cyclic voltammetry can be a useful probe of films immobilized on electrode surfaces and has already been used effectively in the investigation of several modified electrode systems, including alkanethiol films immobilized on gold.23-25 In this work cyclic voltammetry was used as a qualitative tool to probe the density or permeability of CD films on gold elctrodes. The oxidation and reduction processes of ferro-/ ferricyanide at the modified gold surfaces were investigated with respect to the adsorption time of CD(10) and compared to the same redox processes at the bare gold surface. Cyclic voltammograms of ferro-/ferricyanide are shown in Figure 7 for gold working electrodes modified with CD(10). The substrates were investigated after different adsorption times and thoroughly rinsing to remove physisorbed molecules. At the modified electrodes, the oxidative and reductive peak currents, ipox and ipr, are lower and the peak potential separation, ∆Ep, is larger than those measured at the bare gold electrode. These effects increase with increasing adsorption time of CD(10). One minus the ratio of the oxidative peak currents of Fe(CN)63-/4- measured at the various modified gold electrodes, ipox(CD), to the oxidative peak current of Fe(CN)63-/4- measured at the bare gold electrode, ipox(Au), yields the hindrance B of the electrode surface (eq 4). B is a qualitative value for the layer density.

1 - ipox(CD)/ipox(Au) ) B

(4)

The hindrance B versus the assembly time is plotted in the inset of Figure 7. The time scale found here differs from the in-situ plasmon surface polariton measurements but is in the same range

Immobilization Kinetics of Cyclodextrins at Gold Surfaces

J. Phys. Chem., Vol. 100, No. 45, 1996 17897

Figure 7. Cyclic voltammograms of CD(10) films with four different assembly times. Inset: calculated hindrance B (eq 4) versus selfassembly time.

than the plasmon rinsing experiments. Both kinetic curves depict qualitatively the same shape. This is reasonable if one has in mind that before investigation the modified electrodes were rinsed thoroughly to carry only chemisorbed material. Because of their qualitative nature, these data are not fitted to any theoretical assembly kinetic. Contact Angle Measurements. Contact angles of water on a surface are a qualitative measure of the hydophilicity or hydrophobicity of that surface. Small angles indicate a hydrophilic surface, whereas large angles indicate a hydrophobic surface. The difference between the advancing and the receding contact angle, the hysteresis, is a measure for the surface roughness and inhomogeneities: it increases with increasing inhomogeneity. Advancing and receding contact angles of water on substrate surfaces after different assembly times were measured. The results are depicted in Figure 8a-d. The contact angle of the surface first decreases with increasing adsorption time, reaches a minimum, and increases again. The hysteresis shows a minimum at the same adsorption time where the contact angles have their minima. Mass Spectrometry. To investigate the chemical composition of a monolayer on a molecular level, mass analysis is very useful. In the experiment the sample surface is bombarded with kiloelectronvolt ions and atoms and molecules are released into the gas phase, partially as ions. Negatively charged secondary ions are analyzed with a linear time-of-flight mass spectrometer. Substrates which had been given different self-assembly times and a thorough rinse were measured directly after the film formation. In Figure 9 the intensities of the negatively charged gold (Au-) and CD(8) molecular peaks are shown with respect to the self-assembly time. The intensity of the gold peak decreases with increasing adsorption time, whereas the CD(8) molecular peak intensity increases. The desorption probability for gold decreases due to the increasing surface coverage, wheras at the same time the probability for the CD(8) molecular peak increases. Again, we have renounced to fit the data, because of the statistical character of the peak intensity of specific ions and the dependence on a variety of factors, like desorption and ionization probabilities. AFM Measurements. An image of the gold surface covered with CDx(10) is shown in Figure 10. The brainlike structure is the gold(111) film on the mica substrate. The surface looks rougher than bare gold(111) or gold(111) covered with a wellordered monolayer of e.g. alkanethiols.25,27 Striking on this picture are the 10-50 nm big islands with a hight of typically

Figure 8. Contact angle data taken with increasing self-assembly time for CD(0), CD(8), CD(10), and CDx(10).

Figure 9. Peak intensity of Au- and a CD(8)- ion versus self-assembly time. The lines are a guide to the eyes.

15 nm and a surface coverage of 2-3%. These islands were not observed on bare gold surfaces, but are found on all samples which were immersed for a few hours in the solutions of CDx(10) and CD(8). We attribute these islands to lumps of CD as the adlayer of physisorbed molecules. Discussion The adsorption kinetic was investigated in real time by plasmon surface polariton experiments. The assembly process was also investigated in a sample-to-sample manner with cyclic voltammetry, mass spectrometry and contact angle measurements. AFM images were taken from “thick” samples. There is a distinct difference in the assembly kinetics measured in experiments which are carried out in situ or without

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Figure 10. AFM image on a 8 h assembled film of CDx(10).

rinsing (in-situ plasmon surface polariton spectroscopy) and experiments with samples on which the thiolated CDs are adsorbed first and measured after thoroughly rinsing (plasmon surface polariton spectroscopy with rinsing, cyclic voltammetry, mass spectrometry, and contact angle measurements). The experiments which are carried out without rinsing take into account all adsorbed molecules, whether they are physisorbed or chemisorbed, whereas the experiments with carefully rinsed samples detect the left chemisorbed species. The first very fast exponential increase in the surface concentration seen by the in-situ plasmon surface polariton measurements (Figure 3) is due to a Langmuir adsorption of molecules physisorbed and partly chemisorbed onto the gold surface. Due to the geometry of the CD derivatives a spontaneous chemisorption is difficult. The thiol group must come into contact with the gold surface before the covalent binding can be formed. This takes time. This kineticsthe chemisorption kineticsis measured by the methods involving a rinse. All these methods show that the formation of a covalently bound densely packed monolayer takes minutes to hours and depends strongly on the chemical structure of the adsorbate. The formation of the covalently bound film increases not only the film thickness because of the possibility of a stretched orientation of the molecules after binding, but enhances the packing density of the film. This is seen by the analysis of the plasmon surface polariton rinsing data with an Elovich kinetic for the three derivatives carring an anchor chain. The derivative which carries the thiol group directly at the torus shows a Langmuir behavior: the activation energy R (Table 1) is zero within the experimental errors. There is no orientation and elongation of a chain. The monolayer formation time differs for all derivatives (Table 1), but the most extreme case is CD(8) with a 3 times longer monolayer formation time as the other derivatives (Table 1). This compound also exhibits the highest activation energy. The anchor group of CD(8) carries two ether groups leading to a small dipolemoment28 and has therefore stronger interactions

with the gold surface than an alkyl chain. For the adsorption of incoming molecules more energy and time is necessary, because it first has to push away already present but unoriented, probably flat lying molecules. The contact angle measurements confirm the PSP measurements. The time ranges where the minima appear are interpreted as the end of the chemisorption on the gold surface: the monolayer formation time. The hydrophobic gold is shielded more and more by the hydrophilic CD tori. The closed packed monolayer has formed, the surface is hydophilic and as smooth as possible. The hysteresis shows a minimum as well. The following increase in the contact angles and the hysteresis is due to the forming adlayer which cannot been rinsed off effectively from the chemisorbed monolayer. The same rinsing problem was found in the plasmon surface polariton rinsing experiments, where the surface concentration started to grow again after reaching the “plateau”- monolayer concentration (Figure 6). The overall higher contact angles of the CDx(10) derivative is due to the methylation of the secondary hydroxyl groups. The monolayer formation times found by these wetting experiments are in reasonable agreement with the optical data: [t(rinsing plasmon surface polariton): t(contact angle measurement)] for CD(0) [5 min:10 min] and CDx(10) [10 min:7 min]. A larger difference is found for CD(10) [ 20 min:3 min] and a slight discrepancy for CD(8) [60 min:20 min] (Figure 6 and Table 1). The monolayer formation time depends on the preparation of the gold substrate. If the gold surface is very clean the adsorbing molecules do not have to remove a contamination layer with the binding reaction (self-cleaning process of thiols29), which usually delays the film formation process.30 Therefore, we do not claim that the monolayer formation times which are found here enable us to prepare monolayers on all gold surfaces. They hold for the gold substrate preparation scheme and quality achieved with our evaporation system and used throughout this study.

Immobilization Kinetics of Cyclodextrins at Gold Surfaces The kinetics which are found by Grunze et al.12 for the formation of a well-ordered alkanethiol film are much longer (days), because the alkanethiols form crystalline-like structures with very densely packed alkyl chains. Those have to align straight side by side each other, stabilized by interactions of the alkyl chains. This needs much more time to realize because of steric hindrance. One of the important differences between our CD systems and the alkanethiols are those missing interactions in between the alkyl chains. The thiolated CDs do not form a crystalline structure, the CD tori are packed without a main orientation, and the anchor groups are diluted (within the solvent) underneath the CD tori without stabilizing interactions between each other. The CD(0) and CDx(10) derivatives show a smaller surface coverage than the other derivatives (Table 1). We attribute this to an orientation and packing of the CD tori parallel to the gold surface, like depicted in Figure 4a. The surface coverage of CDx(10) is slightly to high for the hexagonal closed packing. This might be due to the possibility that not all the possible bonds of an individual CDx(10) molecule to the gold were formed. The molecule is then able to align with its main axis parallel to the substrate, which needs less space and stick out of the “monolayer” with the left binding sites. The applicability of the dn/dc measurement is critical here, therefore one has to take a large error into account. Cyclic voltammetry of Nelles et al. and Rojas et al.7,8 showed a high permeability for the redox couple in solution on with multithiolated CD modified gold electrodes. This shows that the layer density is not “perfect”.7,8 The monosubstituted long-chain derivatives show a surface concentration similar to the one depicted in Figure 4b). The surface coverage kinetics measured with the cyclic voltammetry (inset of Figure 7) and with mass spectrometry (Figure 9) follow qualitatively the kinetics measured with plasmon surface polariton investigations (with rinsing). In these qualitative measurements the monolayer formation times can be estimated. The hindrance B of the oxidation due to the CD(10) film reaches a “plateau” between 6 and 10 min, which lies in between the monolayer formation times found by the plasmon surface polariton and wetting investigations. In the mass spectrometry plot (Figure 9) a weak “plateau” can be found for the CD(8) molecular ion increase and for the Au ion decrease between 40 and 60 min. This time is in between the monolayer formation times measured with the other two methods as well. We propose that the film formation of CDs is a three-step process. First the molecules physisorb unoriented on the gold surface, and then the covalent sulfur-gold bond is formed and the molecules can orient away from the surface without leaving. Because of the missing anchor group, CD(0) has no orientation step of a spacer and therefore no activation energy. The small difference in the activation energy between the monothiolated CD(10) and multithiolated CDx(10) is not clear. There might be two counter acting effects: the 2 times bigger necessary space for the multifunctionalized CDx(10) may be compensated by the 2-4-fold higher binding probability. The third part in the film formation kinetics is a grow of a CD adlayer physisorbed on top of the chemisorbed CD monolayer. In the AFM pictures lumbs of typical diameters of 40 nm are found. It is not clear so far whether the lumbs are formed in solution and subsequently deposited on the substrate or whether the lumbs are formed on the surface with time. In Figure 11 the in-situ self-assembly kinetic from Figure 3 is plotted in a double-logarithmic fashion, and the long-time behavior, which we attribute to the adlayer or lump formation,

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Figure 11. In-situ surface plasmon kinetic data from Figure 3 plotted in a double logarithmic fashion. The lines are fits to y ) axb.

TABLE 2: Fit Parameters of the Kinetics of the Formation of the Physisorbed Adlayera Θ(t) ) axb

time domain of fit, min

a

b

CD(0) CD(8) CD(10) CDx(10)

10-840 60-840 60-840 60-840

0.63 ( 0.05 1.91 ( 0.21 2.21 ( 0.24 1.51 ( 0.19

0.32 ( 0.03 0.10 ( 0.01 0.10 ( 0.01 0.10 ( 0.01

a

b ) 0.5 would represent a diffusion-controlled process.

is fitted to y ) axb in order to find the power of the physisorption process with time. A power b of 0.5 would point to a diffussion process.31 It is found (Table 2) that the three derivatives carrying a spacer chain show a physisorption with an exponent of 0.1, whereas the CD(0) derivative exhibits an exponent of 0.3. This is obviously no diffusion process, but it shows a distinct difference between the compounds having a spacer chain, being a surfactant, and the compound having none. Insitu plasmon surface polariton kinetic measurements with CD(10) and solution concentrations ranging from 5 × 10-4 to 1 × 10-7 M were performed (not shown here). These measurements show a strong relationship of the adsorption time with the concentration. At the lowest concentration of 1 × 10-7 M the adsorbed layer does not even show a monolayer concentration after 14 h. The growth mechanisms of the adlayer are not understood so far. Conclusion The CD derivatives used in this study behave quite differently than the well-known alkane thiols. They can form densely packed monolyers (CD(10)), but also physisorbed undesired adlayers. The adsorption kinetics we find can be described by a three-step process: a physisorption process, a binding and orientation step, and an adlayer formation. The chemisorption process can be described by an Elovich kinetic. The length, structure, and number of the anchor groups have a strong influence on the assemble kinetics. Our interpretation is selfconsistent with all the data but might not be the only possible interpretation. Acknowledgment. The authers thank Manfred Jaschke and Hans-Ju¨rgen Butt from the Max-Planck-Institut fu¨r Biophysik in Frankfurt/Main for the AFM measurements. They also acknowledge the stimulating discussions with W. Knoll. Financial support came from Boehringer Mannheim GmbH and the Bundesministerium fu¨r Bildung, Forschung und Technologie (project: “Nachwachsende Rohstoffe” BE022/0319055A).

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