Complexation Chemistry for Tuning Release from Polymer Coatings

Oct 6, 2006 - However, the adsorbed amount of Medetomidine was lower ... of tuning the release of Medetomidine by altering the coordinating metal ion,...
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J. Phys. Chem. B 2006, 110, 21808-21815

Complexation Chemistry for Tuning Release from Polymer Coatings Camilla Fant,† Paul Handa,‡ and Magnus Nyde´ n*,‡ Cell and Molecular Biology/Biolpolymer Products AB, Go¨teborg UniVersity, Box 462, SE-405 30 Go¨teborg, Sweden, and Applied Surface Chemistry, Department of Chemical and Biological Engineering, Chalmers UniVersity of Technology, SE-412 96, Go¨teborg, Sweden ReceiVed: June 16, 2006; In Final Form: August 11, 2006

The strategy of metal ion complexation is employed to design a delivery system for an antifouling agent (AFA) in marine paints. A poly(1-vinylimidazole-co-methyl methacrylate) copolymer (PVM), together with Cu2+ or Zn2+ formed a PVM-M2+ complex. The AFA, Medetomidine, was then coordinated into the complex. The coordination strength was investigated in solution by 1H NMR and on solid surfaces by using the Quartz Crystal Microbalance with Dissipation monitoring technique (QCM-D) and Surface Plasmon Resonance (SPR). From the 1H NMR experiments strong interactions were observed between Cu2+ and the PVM-polymer and between Medetomidine and the PVM-Cu2+ complex. From the QCM-D and SPR measurements it was shown that Cu2+, compared to Zn2+, exhibited a larger affinity for the PVM-copolymer surface that resulted in higher degree of swelling of the polymer film. Large amounts of Medetomidine were adsorbed to the PVMCu2+ complex resulting in low desorption rates. However, the adsorbed amount of Medetomidine was lower to the Zn2+ doped polymer and a higher desorption rate was observed. These results indicate the possibility of tuning the release of Medetomidine by altering the coordinating metal ion, which may prove to be favorable in a paint formulation.

Introduction The complexation of metal ions to various organic compounds has proven to be a valuable and effective technique in a wide range of technical applications, such as separation of heavy metal ions in wastewater,1,2 design of chelating biosensor chips in quartz microbalance (QCM)3 and surface plasmon resonance (SPR) techniques,4,5 and total internal reflection fluorescence (TIRF)4 in the study of detection and immobilization of proteins. One of the most common and important separation and purification techniques for biomolecules, such as proteins and enzymes, is metal ion affinity chromatography (IMAC).6-10 This technique is based on materials with selective affinity for chelation to transition metal ions.8,9 With the use of the principle of hard and soft acids and bases (HSAB principle) a prediction of what type of ligand (Lewis base) can be chelated to certain metal ions (Lewis acid) is obtained and states that hard acids prefer hard bases and soft acids prefer soft bases.11,12 For the separation of biomolecules the histidine amino acid residue plays an important role in the chelation to transition metal ions. The histidine residue contains an imidazole moiety that is classified as a borderline base (together with pyridine) according to the HSAB principle and has the possibility to coordinate to Cu2+, Ni2+, Zn2+, and Co2+ with decreasing complexation constants in that specific order.9,13,14 Due to the favorable chelating properties of imidazole several synthetic polymers containing imidazole moieties have been designed for use in biomedical, pharmaceutical, and industrial applications, such as poly(N-vinylimidazole) and poly[(Nvinylimidazole)maleic acid].1,9,15,16 From conductometric and * Address correspondence to this author. Phone: +46-31-772973. Fax: +46-31-160062. E-mail: [email protected]. † Go ¨ teborg University. ‡ Chalmers University of Technology.

viscometric measurements it has been shown that chelation of poly(N-vinylimidazole) to Cu2+, Co2+, and Cd2+ results in four imidazole units per metal ion.2 However, with Ni2+, Zn2+, and Co2+ it is possible to obtain crystalline complexes with six imidazole molecules per metal ion.13 The coordination occurs via the more basic nitrogen of imidazole (position 3 in Figure 1a,b).1,13 The purpose of this work is to present the possibility for a new application for metals chelated to an imidazole based polymer, namely as an additive in a marine self-polishing paint for controlled release purposes of imidazole-containing AFAs (antifouling agent(s)). With this concept the diffusion rate of AFAs through the paint film is minimized, thereby preventing premature depletion of the AFA concentration. The highly selective R2-adrenoceptor agonist Medetomidine or (S,R)-4(5)[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole (see Figure 1a) has been chosen as AFA due to its inhibitory properties toward fouling of the barnacle (Balanus improvisus) already at low (1 to 10 nM) biocide concentrations.17-19 Metal-ligand complexes of poly(1-vinylimidazole-co-methyl methacrylate) (PVM, see Figure 1b) and Cu2+ and Zn2+ have been synthesized (PVM-M2+ complex, see Scheme 1). Medetomidine is then chelated into the metal-ligand complex producing a PVM-M2+-Medetomidine complex (see Scheme 1). With NMR, qualitative information of the differences in interaction between the complexes has been obtained. More detailed investigations were performed on solid surfaces by using the Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) and Surface Plasmon Resonance (SPR) technique revealing differences in adsorbed amounts (dry and wet mass), film swelling, amount of bound water, and desorption rates depending on polymer film thickness and metal ion.

10.1021/jp0637532 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/06/2006

Tuning Release from Polymer Coatings

Figure 1. Structures of (a) Medetomidine and (b) poly(1-vinylimidazole-co-methyl methacrylate), PVM-copolymer.

SCHEME 1: Formation of the PVM-M2+-Medetomidine Complexa

J. Phys. Chem. B, Vol. 110, No. 43, 2006 21809 Preparation of Samples for 1H NMR. The general procedure for the 1H NMR samples follows: To the pulverized PVMpolymer (1.0 g) a M2+ solution was added (10 mL of CuSO4 or ZnSO4 both with the concentration of 0.15 M in MilliQ). The mixture was shaken for 2.5 h, filtered, and washed with MilliQ-water. The resulting powder was dried under vacuum overnight. Dimethyl sulfoxide-d6 (DMSO-d6) was added to the PVM-M2+ complex and the mixture was heated to 70 °C for 4 h. Five samples were prepared (all samples used tetramethylsilane (TMS) as zero point reference): (1) DMSO-d6, (2) PVMpolymer (1.3 wt %) in DMSO-d6, (3) PVM-M2+ mixture (0.6 wt % PVM-M2+ mixture) in DMSO-d6, (4) Medetomidine (0.2 wt %) in DMSO-d6, and (5) PVM-M2+-Medetomidine mixture (0.2 wt % Medetomidine, 0.6 wt % PVM-M2+ mixture) in DMSO-d6. All samples were placed on a shaker overnight before being measured. The samples were analyzed on a 400 MHz Varian Unity spectrometer and on a 600 MHz Varian Inova spectrometer. Quartz Crystal Microbalance with Dissipation Monitoring Technique (QCM-D). The quartz crystal microbalance technique used in the present work is an extension of the traditional QCM technique and is called QCM-D.23 The new feature in this setup is that the energy dissipation, D (or Q-factor ∝ 1/D), is measured in addition to the frequency (mass) change. The merit of the traditional QCM technique lies primarily in the simplicity and sensitivity (below ng‚cm-2) by which an adsorbed mass, ∆mQCM, can be deduced from measured changes in the resonant frequency, ∆f, by using the Sauerbrey relation (see eq 1):24

∆mQCM ) Ffilmδfilm )

a

Medetomidine is chelated into a metal-ligand complex (PVMcomplex) creating a PVM-M2+-Medetomidine complex. M2+ can be either Zn2+ or Cu2+.

M2+

Materials and Methods Synthesis of the Poly(1-vinylimidazole-co-methyl methacrylate) Copolymer (PVM) and Medetomidine, (S,R)-4(5)[1-(2,3-Dimethylphenyl)ethyl]-1H-imidazole. The poly(1vinylimidazole-co-methyl methacrylate) copolymer (PVM) was prepared from a 1:1 (molar) ratio of the monomers 1-vinylimidazole (VI) and methyl methacrylate (MMA) as described by Wu et al.20 The molar fractions of VI and MMA were determined by IR spectroscopy measurements of the carbonyl peak vibration (1722 cm-1) and the imidazole ring torsion stretching (665 cm-1). The synthesis resulted in a 47:53 copolymer ratio of the monomers VI and MMA, respectively. This can be compared to the copolymer composition, 19:81, as determined by Wu et al. for the same monomer ratio (1:1 molar ratio).20 Medetomidine was prepared from a five-step synthesis.21,22 The compound was synthesized in an overall yield of 36% and with a purity of g99% according to 1H NMR. 1H NMR (DMS0-d ): δ 1.45 (d, 3 H), 2.21 (s, 3 H), 2.24 (s, 6 3 H), 4.30 (q, 1 H), 6.68 (d, 1 H), 6.92-6.97 (m, 3H), 7.48 (d, 1 H).

CQCM ∆f n

(1)

where Ffilm and δfilm are the effective density and the film thickness, respectively, CQCM ()17.7 ng‚cm-2‚Hz-1) is the mass-sensitivity constant, and n ()1, 3, ...) is the overtone number. This relationship holds, however, only under certain conditions:24-26 The adsorbed film must be sufficiently thin and not too “soft”. Moreover, if a viscoelastic film is deposited on the sensor surface, it will slightly and periodically deform during the sensor’s shear oscillation. This process induces internal friction in the adsorbed film, leading to dissipation of additional energy, which results in an underestimation of the adsorbed mass. Thus, in such situations a proper treatment of the QCM response requires measurements of changes in both energy dissipation and frequency, combined with a model describing both the elastic and inelastic (loss) modulus of the film.25,26 However, since the conclusions drawn from the present work essentially require information about relative changes in adsorbed mass, we have chosen to present the data as relative changes in resonance frequency and dissipation. Regarding quantitative estimations based on the QCM-D response, it is also important to be aware that the detection principle in certain aspects differs from most other surface sensitive techniques. In contrast to common optical techniques, such as ellipsometry or surface plasmon resonance (SPR), the mechanically coupled mass detected via ∆f is sensitive not only to the adsorption of solutes but also to solvent associated with the film.27,28 The QCM-D technique measures ∆D and ∆f, by periodically switching on and off the driving power to the thickness-shearmode oscillation of the sensor crystal and recording the decay of the damped oscillation. The time constant of the decay is inversely proportional to D, and f is the period of the decaying signal. This measurement is fully automated and measurement points are taken at a rate of about 1 Hz. The system also allows

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subsequent measurements of up to four harmonics (for a 5 MHz crystal). In the present work we used polished, AT-cut crystals with a fundamental resonant frequency of about 5 MHz operating at its third harmonic (i.e. at 15 MHz). All measurements were done in a static solution, i.e., in batch mode, in a cell designed to provide a fast nonperturbing exchange of a stagnant liquid (Q-Sense AB, Sweden). The measurement chamber was temperature stabilized to 22 ( 0.1 °C and each measurement was performed at least three times. The swelling ratio was calculated by using the expression

swelling ratio )

Ws - Wo × 100 Wo

(2)

where Wo and Ws denote the weights of the polymer film before and after swelling, respectively. Surface Plasmon Resonance (SPR). The surface plasmon resonance (SPR) technique relies on a strongly localized nonradiative optical wave that propagates along a metal surface (often gold) and is, as such, very sensitive to changes in refractive index (n) of the medium in close proximity (≈200 nm) to the sensor surface. SPR is obtained by using monochromatic and plane-polarized light that under total internal reflection conditions is directed through a quartz prism at the interface between the prism and a thin layer of gold (≈50 nm). At the a certain angle, Θ, of the incident light, a sharp minimum in the intensity of the reflected light is observed resulting in SPR. If the refractive index changes in the region of the medium outside but close (≈200 nm) to the gold surface, e.g., from protein adsorption, there is a proportional change in the angle, ∆Θ, at which SPR is observed. Accordingly, SPR measurements allow real-time measurements of the amount of adsorbed molecules, ∆mSPR, via the relationship:29

neff - nbuffer CSPR∆RU ) dn/dC β

∆mSPR ) d

(3)

where d is the effective thickness of the adsorbed molecule, neff is the effective refractive index of the adsorbed molecule, nbuffer is the effective refractive index of the buffer, dn/dC is the incremental change in refractive index with concentration, and CSPR is the proportionality constant, which varies with different values for dn/dC (for instance, CSPR is equal to 6.5 × 10-2 ng‚cm-2 for protein adsorption on flat surfaces).30-32 ∆RU is the change in response units (a dimensionless quantity that corresponds to the changes in Θ) and β is a factor correcting for the decrease in SPR signal when the event of adsorption occurs at a given distance from the sensor chip. In this study dn/dC for the metal ions and Medetomdine have been experimentally determined to quantitatively determine the mass uptake, ∆mSPR. The proportionality between ∆n and ∆m has been shown to correlate well with the molecular weight of different proteins. This is due to the fact that, upon adsorption, protein molecules replace the previously accumulated water in the interfacial region. Since SPR measures ∆n at the interfacial region, the obtained signal is due to the differences in n between the accumulated protein molecules and the displaced water. Hence, the water present in the adlayer is not included in the mass determination. The SPR measurements were performed on a BIAcore 2000 system in a flow cell providing a laminar flow (BIAcore AB), using a flow rate of 3 µL/min at 22 °C with each measurement being performed at least three times. The refractive index increment, dn/dC, was determined by using a differential refractometer, PN3/20dndC (Radiometer Analytical). The instru-

TABLE 1: Effective Polymer Thickness in Air (tf(air)), Water (tf(water)) and Adsorbed Mass after 60 min of Exposure of the Polymer Films to Cu2+ or Zn2+ Solutions Determined by Both QCM-D and SPRa tf(air) tf(water) swelling ratio ∆mQCM-D Cu ∆mSPR Cu ∆mQCM-D Zn ∆mSPR Zn

PVM0.5%

PVM1.0%

21 ( 2 nm 43 ( 4 nm 105% 537 ( 21 ng/cm2 212 ( 7.4 ng/cm2 191 ( 59 ng/cm2 124 ( 11 ng/cm2

42( 2 nm 100 ( 10 nm 138% 1874 ( 307 ng/cm2 418 ( 47 ng/cm2

a PVM0.5% and PVM1.0% are the polymer films prepared by the spincoating technique from 0.5 and 1.0 wt % solutions.

ment was calibrated against KCl (7.419 g/1000 g solution) and measurements were performed at 25 °C, using a wavelength of 620 nm resulting in dn/dC values of 0.1640 ( 0.0035, 0.1702 ( 0.002, and 0.2550 ( 0.003 mg/g for CuSO4, ZnSO4, and Medetomidine, respectively. Preparation of Solutions for QCM-D and SPR Measurements. The 0.15 M M2+-stock solutions were prepared by dissolving CuSO4 or ZnSO4 in MilliQ water. Prior to use, a 7.5 mM solution in MilliQ, of either M2+, was prepared. Medetomidine was prepared as a 0.67 mM solution of artificial seawater (pH 8). PVM-polymer was dissolved in ethanol (96%) to give 1.0 and 0.5 wt % solutions. Artificial seawater was prepared by mixing NaCl (24.615 g/L), KCl (0.783 g/L), Na2SO4 (4.105 g/L), MgCl2‚6H2O (11.06 g/L), CaCl2‚2H2O (1.558 g/L), and NaN3 (0.5 g/L) and thereafter adjusting to pH 8 with a NaOH solution (0.2 M). Preparation of Surfaces for QCM-D and SPR Measurements. AT-cut piezoelectric quartz crystals with gold electrodes on its two faces and with a fundamental frequency of 5 MHz were used (Q-Sense AB) for the QCM-D measurements, and for the SPR measurements gold sensor chips (SIAkit, Biacore AB) were used. The QCM-D sensor crystals and the BIAcore sensor chips were cleaned between each measurement by first being immersed in ethanol (96%) for 24 h followed by cleaning in an UV/ozone chamber for 10 min. Thereafter the gold surfaces were exposed to a 1:1:5 mixture of H2O2 (30%), NH3 (25%), and Milli-Q water (Millipore) for 10 min at 75-80 °C and left for 10 min in an UV/ozone chamber. A thin film of PVM-copolymer was formed on the sensor surface, using the spin-coating technique. The polymer was added from an ethanol solution (either 1.0 or 0.5 wt % of polymer) and the sensor surfaces were spun at ≈2000 rpm for 60 s, resulting in the polymer thicknesses shown in Table 1. The film thickness was determined by measuring the resonance frequency in air before and after spin-coating. The mass of the polymer film was calculated by using the Sauerbrey relation (see eq 1, above) and the film thickness could be roughly estimated by using a polymer density of 1 g/mL. Results and Discussion NMR Studies of Complexation Behavior in Solution. From simple 1D 1H NMR experiments, qualitative information concerning the interaction between Cu2+ or Zn2+ and the PVMpolymer as well as the interaction between Medetomidine and metal doped polymers can be obtained from the shape and position of signals in the spectrum. In Figure 2 the 1H NMR spectra for PVM, PVM-Cu2+ complex, Medetomidine, and the PVM-Cu2+-Medetomidine complex in DMSO-d6 are displayed (the signal from the solvent, DMSO-d6, appears at 2.50 ppm as a singlet and water appears

Tuning Release from Polymer Coatings

J. Phys. Chem. B, Vol. 110, No. 43, 2006 21811

Figure 2. 1H NMR spectra of (a) PVM, (b) PVM-Cu2+ complex, (c) Medetomidine and (d) PVM-Cu2+-Medetomidine complex.

Figure 3. 1H NMR spectra of (a) PVM, (b) PVM-Zn2+ complex, (c) Medetomidine and (d) PVM-Zn2+-Medetomidine complex.

at 3.33 ppm, also as a singlet). In the PVM spectrum (see Figure 2a) the signals from the imidazole ring appear between 6.62 and 7.95 ppm corresponding to positions 2, 4, and 5 in Figure 2. In the region of 3-4 ppm signals from the methoxy and methylene group bound to the carbonyl group appear, and in the 0-2 ppm region, signals from the methylene and methine groups of the polymers hydrophobic backbone are seen. Upon addition of Cu2+ to PVM the imidazole signals are completely reduced, while remaining signals are not as severely affected, indicating coordination of imidazole to the paramagnetic Cu2+ (see Figure 2b). With the addition of Medetomidine, the signals belonging to the imidazole moiety of Medetomidine also become reduced (positions 2 and 5 in Figure 2), indicating that Cu2+ coordinates to the imidazole part in Medetomidine,

as well as to the PVM-polymer (Figures 2d). However, the benzylic protons of Medetomidine now appear in the region of 7 ppm. These are absent in the PVM-Cu2+ complex spectrum. When using Zn2+ instead of Cu2+, the results are qualitatively the same (see Figure 3). However, the reduction in signal intensities is not as pronounced. These results coincide with previous NMR studies of chelating paramagnetic species to imidazole containing compounds, such as proteins and peptides, for instance in the study of histidyl residuals on Ribonuclease A coordinating to Cu2+ where the interaction was observed through the broadening of histidyl signals.33 The same effect is essentially displayed in the 1H NMR spectra of the PVM-Cu2+ and PVM-Cu2+-Medetomidine complex (see Figure 2b,d) where the loss of signal intensity arises

21812 J. Phys. Chem. B, Vol. 110, No. 43, 2006 from an increase in 1H NMR transverse relaxation rates when a paramagnetic relaxation agent, such as Cu2+, Zn2+, or chromium(III) acetylacetonate (Cr(acac)3) (see Cookson et al.34) was used. This effect was also reported by Wu et al.20 where the reduction, or even the disappearance, of 13C signals (in a CP/MAS NMR experiment) from the imidazole group in a PVM-Cu2+ complex was interpreted as a coordination of Cu2+ to the imidazole ring. The very large increase in 1H transverse relaxation rate is not observed for Zn2+ due to the diamagnetic properties of this particular ion (the Zn2+ has a d10 electronic configuration). However, the signals are broader with Zn2+ present, which can be accounted for by the decrease in Medetomidine mobility upon coordination to the PVM-Zn2+ complex. QCM-D and SPR Studies of Swelling and Complexation at Model Surfaces. To develop a better understanding of the surface behavior of the polymer as well as the interaction between the two different metal doped polymers and Medetomidine, QCM-D and SPR were used. Swelling of the polymer films (PVM0,5% and PVM1,0%) was quantified by measuring the frequency in air and water before and after spin coating and the adsorbed amount is presented in Table 1. Table 1 shows that the polymer thickness in air depends linearly on the polymer concentration in solution resulting in film thicknesses of 21 and 42 nm for PVM0.5% and PVM1.0%, respectively. Upon addition of water, the effective polymer thickness increases more than 100% for both polymer films and the swelling ratios are 105% and 138%, for PVM0.5% and PVM1.0%, respectively (determined by eq 2). The swelling beahavior of similar polymers, poly(N-vinylimidazole) and poly[(N-vinylimidazole)maleic acid], has previously been investigated in solution where hydrogels were formed with large swelling ratios.9,16 In the swollen state, it is most likely that a more permeable matrix is formed, thereby resulting in efficient adsorption of both solvent and small solutes that interact favorably with the polymer. The binding of metal ions to the polymer films was investigated by measuring polymer thickness, adsorbed amount, and the difference between optical (dry) mass and acoustic (wet) mass, as well as differences in binding level after 60 min exposure to Cu2+ or Zn2+ solutions. Panels a and b of Figure 4 display ∆mQCM-D and ∆D versus time upon exposure of 21 and 42 nm thick polymer films to water solutions of either Cu2+ or Zn2+. After 60 min of exposure, the surfaces were rinsed with pure water. The changes in mass upon addition of Cu2+ (∆mQCM-D ) were 537 and 1874 Cu ng/cm2 for the 21 and 42 nm films, respectively. The increase in adsorbed mass upon addition of Zn2+ was 191 ng/cm2 for the 21 nm film and 418 ng/cm2 for the thicker 42 nm film. Upon adsorption of either Cu2+ or Zn2+ to the 21 nm film the change in dissipation was between 1 × 10-6 and 1.5 × 10-6, which was also observed when adsorbing Zn2+ to the 42 nm film. However, the adsorption of Cu2+ to the 42 nm film resulted in a considerably larger change in dissipation (∆D ) 6.8 × 10-6). The swelling can be understood from an electrostatic argument. Ion adsorption in the film creates strong repulsive coulomb interactions and a subsequent swelling of the film. As shown in Figure 4, the adsorbed amount of Zn2+ depends linearly on the film thickness. However, this is not the case for Cu2+. The results presented here are in contrast to the results shown by Pekel et al.16 and Salih et al.9 where addition

Fant et al.

Figure 4. (a) ∆mQCM-D versus time and (b) ∆D versus time for addition of Cu2+ and Zn2+ in water to 21 and 42 nm thick PVM surfaces. The adsorbed amount of Zn2+ is 191 ng/cm2 for the 21 nm thick surface (open squares) and 418 ng/cm2 for the 42 nm thick surface (open diamonds), i.e., almost linearly dependent on the polymer thickness. The adsorbed amount of Cu2+, however, is 537 ng/cm2 for the 21 nm thick surface (solid squares) and 1874 ng/cm2 for the 42 nm thick surface (solid diamonds), i.e., not linear with respect to polymer thickness.

of Cu2+ and Co2+ to poly(N-vinylimidazole) and poly[(Nvinylimidazole)maleic acid] resulted in shrinking of the hydrogels. Compared to Zn2+, Cu2+ clearly adsorbs more regardless of polymer film thickness. However, the slightly larger increase in dissipation upon addition of Cu2+ to 21 nm film (see Figure 4b) and the significantly larger increase in dissipation with Cu2+ to 42 nm film indicates that the solvent induced swelling of the polymer film, in addition to metal ion adsorption, contributes significantly to the changes in frequency. To quantify these two processes, the binding of metal ions was further investigated with the SPR technique. The adsorbed amount of ions as measured by QCM-D and SPR are shown in Figure 5 where ∆mQCM-D and ∆mSPR versus time when adding Cu2+ or Zn2+ to the 21 nm thick film are plotted. After 60 min of exposure to metal ion solutions, the surfaces were rinsed with pure water. The main difference between adsorbed amount measured by SPR, ∆mSPR, and QCMD, ∆mQCM-D, is that ∆mQCM-D also includes water in the swelled film. SPR, however, senses only the change in refractive index occurring when metal ions and counterions replace water upon complexation to the imidazole moiety at the surface. SPR therefore measures the dry mass of adsorbed metal ions and when used in combination with QCM-D data the amount of water in the film may be calculated. After 60 min of exposure,

Tuning Release from Polymer Coatings

Figure 5. ∆m versus time for Cu2+ and Zn2+ addition to the 21 nm thick film measured by QCM-D and SPR. With SPR the adsorbed amount reaches 212 ng/cm2 after 60 min of exposure of Cu2+ (open squares) and 124 ng/cm2 upon exposure to Zn2+ (open triangles). The corresponding QCM-D results are 537 ng/cm2 for Cu2+ (solid squares) and 191 ng/cm2 for Zn2+ (solid triangles). SPR 2 ∆mSPR reaches 212 (∆mSPR Cu ) and 124 ng/cm (∆mZn ), for 2+ 2+ addition of Cu and Zn , respectively. The corresponding changes in ∆mQCM-D are 537 ng/cm2 for Cu2+ and 191 ng/cm2 for Zn2+. By using this strategy, the number of water molecules which are coadsorbed with each CuSO4 is calculated to as approximately 18 and for ZnSO4 the number is approximately 5. The calculated values can be compared to the coordination number of Zn2+ and Cu2+ in aqueous solutions; however, this direct comparison may be misleading. It is necessary to consider the water that is released from a coordination site on the metal ion upon adsorption. In addition, the metal ion adsorption will, as mentioned above, generate electrostatic repulsion between positively charged polymer segments and thereby space, which water will occupy. When comparing the ratio between ∆ mQCM-D and ∆mQCM-D with the ratio between ∆mSPR Cu Zn Cu and ∆ SPR 2+ 2+ mZn for Cu and Zn adsorbed to 21 nm thick film the value is almost double for the QCM-D measurements (∆ SPR /∆mQCM-D equals 2.8 whereas ∆mSPR mQCM-D Cu Zn Cu /∆mZn is equal to 1.7). This provides further evidence for the larger swelling of the polymer film induced by Cu2+ adsorption, since the difference in ratios relates to the water content in the polymer film. After generating the metal doped polymer films, the interaction with Medetomidine was investigated in artificial seawater. As control measurements metal free films were also exposed to Medetomidine solutions. Figure 6 shows ∆mQCM-D versus time upon exposure of the different surfaces to Medetomidine. After approximately 20 min of exposure, the Medetomidine solution was replaced by pure artificial seawater and the reversibility of the Medetomidine interaction was monitored. The changes in ∆mQCM-D upon binding of Medetomidine to the Zn2+ doped 21 and 42 nm films are 174 and 302 ng/cm2, respectively. For the Cu2+ doped surfaces the adsorbed amounts are slightly higher, 400 ng/cm2 for the 42 nm film and 198 ng/ cm2 for the 21 nm film. Adsorption of Medetomidine to the metal free films induces a change in mass of 100 and 50 ng/ cm2 for the 42 and 21 nm films, respectively. After 20 min of exposure to Medetomidine, an almost linear adsorption with regard to film thickness is observed for all surfaces. As noted from the changes in dissipation, Medetomidine did not induce any significant swelling of the films (data not shown).

J. Phys. Chem. B, Vol. 110, No. 43, 2006 21813

Figure 6. Plot of ∆mQCM-D versus time for the Medetomidine addition to Cu2+ doped 42 nm thick film (before Cu2+ adsorption, solid circles), Cu2+ doped 21 nm thick film (before Cu2+ adsorption, open circles), Zn2+ doped 42 nm thick film (before Zn2+ adsorption, solid squares), and Zn2+ doped 21 nm thick film (before Zn2+ adsorption, open squares). Also shown are control measurements of the addition of Medetomidine to 42 (solid triangles) and 21 nm thick films (open triangles).

Figure 7. Plot of ∆mQCM-D and ∆mSPR versus time for the adsorption of Medetomidine to Cu2+ doped, Zn2+ doped, and metal free 21 nm thick films. SPR measurements result in ∆mSPR reaching 75 ng/cm2 upon adsorption of Medetomidine to the Cu2+ doped film (solid circles), 48 ng/cm2 for Medetomidine to the Zn2+ doped film (solid squares), and 33 ng/cm2 for Medetomidine to the metal free film (dashed line without markers). The corresponding measurements by QCM-D result in 198 ng/cm2 for the Cu2+ doped film (open circles), 174 ng/cm2 for the Zn2+ doped film (open squares), and 51 ng/cm2 for the metal free film (solid line without markers).

Upon addition of pure artificial seawater desorption occurs for all polymer films (∆mQCM-D decreases). The desorption rate is significantly higher for Medetomidine adsorbed to the Zn2+ doped and the metal free films compared to Cu2+ doped films. To investigate the desorption rate under controlled flow conditions, the adsorption/desorption processes for Medetomidine on polymer films were further studied with use of SPR. In Figure 7 the changes in adsorbed amount measured by QCM-D and SPR are plotted versus time for the adsorption of Medetomidine to metal doped 21 nm and metal free 21 nm

21814 J. Phys. Chem. B, Vol. 110, No. 43, 2006

Fant et al. toward imidazole14 and a more stable PVM-M2+-Medetomidine complex. Nevertheless, the desorption for Medetomdine from both PVM-Cu2+-Medetomidine and PVM-Zn2+-Medetomidine is considerably slower compared to the Medetomidine on the metal free film. Conclusions

Figure 8. Desorption rate of Medetomidine after 20 min exposure of Medetomidine, followed by rinsing, to the Cu2+ doped (solid triangles, dashed line), Zn2+ doped (open triangles, dashed line), and metal free 21 nm films (dashed line without markers) measured by SPR.

films. The adsorbed amount measured with QCM-D was 198 ng/cm2 for the Cu2+ doped film, 174 ng/cm2 for the Zn2+ doped film, and 51 ng/cm2 for the metal free film. The changes in adsorbed amount measured with SPR resulted in 75 ng/cm2 for the Cu2+ doped, 48 ng/cm2 for the Zn2+ doped, and 33 ng/cm2 for the metal free film. The ratio between ∆mQCM-D and ∆mSPR is significantly larger for Medetomidine adsorption to the Zn2+ doped film compared to the ratios obtained for the Cu2+ doped and metal free films. The lowest ratio was obtained for the Medetomidine adsorption to the metal free polymer film. Thus, taken together these results indicate that the amount of adsorbed water is significantly larger for the formation of PVM-M2+Medetomidine complexes compared to the Medetomidine adsorption to metal free film. By using the results from SPR measurements presented in Figures 5 and 7, the number of Medetomidines that bind for each CuSO4 or ZnSO4 can be determined. For both metal doped films, the ratio between metal ion and Medetomidine is 10:3. However, Zn2+ binds less efficiently to the film and Medetomidine has a significantly higher desorption rate from Zn2+ doped films. In Figure 8 the desorption of Medetomidine from Cu2+ and Zn2+ doped and the metal free 21 nm films following rinsing (under controlled flow conditions) is presented. Prior to rinsing, the films had been exposed to Medetomidine for 20 min. After approximately 30 s of rinsing, the mass of the Cu2+ doped film decreases by 30%. The corresponding value for the Zn2+ doped film is 50% and that for the metal free film is 80%. Thus, it is clear that the PVM-Cu2+-Medetomidine complex is more stable than PVM-Zn2+-Medetomidine. This is not unforeseen when considering that the metal ions charge-to-size ratio, differences in electronegativity, and acidity all contribute to the stability constants of complexation toward imidazole containing polymers.1 Following this reasoning and comparing Cu2+ with Zn2+, Cu2+ is slightly smaller in size but more electronegative,35 thereby generating larger stability constants for the complexation

The complexation behavior of two Lewis acids, Cu2+ and Zn2+, has been investigated by QCM-D and SPR on PVM films. The AFA, Medetomidine, was then coordinated to the doped polymers. From the 1H NMR investigations of solution behavior, it was observed that through losses in signal intensity Cu2+ interacted substantially with both PVM and Medetomidine. This effect was also observed with Zn2+ but due to the diamagnetic properties of Zn2+ a direct comparison of interaction strength was not possible. The QCM-D measurements confirmed the observations obtained by NMR, where the addition of Cu2+ induced a significantly larger frequency shift (large mass-uptake) compared to the addition of Zn2+. However, the adsorbed amount of Zn2+ depended linearly on the polymer film thickness, which was not the case for Cu2+. The adsorption of Cu2+ generated large changes in dissipation for thick PVM films indicating film swelling. This process is most likely a result of an increased charge density on the polymer causing electrostatic repulsion between polymer segments. Zn2+, however, independent of polymer film thickness, did not induce the same degree of swelling. From SPR measurements the dry mass was determined and by using the difference in adsorbed amounts obtained by QCMD-D and SPR (∆mQCM-D and ∆mSPR) the amount of adsorbed water was also determined, which was considerably higher for Cu2+ than for Zn2+. Upon addition of Medetomidine to the metal doped films small differences in adsorbed amounts were observed between Cu2+ and Zn2+, observed by both QCM-D and SPR. However, from the SPR desorption measurements it could be seen that the PVM-Cu2+-Medetomidine complex was more stable resulting in 30% desorption when rinsing with artificial seawater. The PVM-Zn2+-Medetomidine complex was less stable, exhibiting a 50% desorption upon rinsing. The detailed mechanism by which Medetomidine acts with regards to preventing the settlement of the barnacle is still not known. However, there are indications that it is Medetomidine at the surface, and not in solution, that is active, which suggests that the use of a Cu2+ containing binder may be favorable.36 The use of the PVM-Cu2+-Medetomidine complex in a paint coating will result in retention of Medetomdine in the coating. However, if Zn2+ is used instead of Cu2+, Medetomidine is likely to be depleted from the coating at a higher rate. Choosing one of these metal ions, or a combination of them, may present one possibility to design a coating system where the Medetomidine will be released with an optimal rate. This study clearly demonstrates how combining the optical technique, SPR, with the acoustic technique, QCM-D, results in a powerful and versatile way of investigating polymers at surfaces. This combination of techniques provides a simple, quick, and accurate tool for measuring swelling of polymers upon exposure to solvents and/or a substrate as well as for the quantification of the number associated water molecules. The possibility of easily gaining information of polymer behavior upon interaction with other molecules (whether it is a solvent or an actual substrate molecule) may become a valuable and

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