Adsorption of Cationic Hydroxyethylcellulose Derivatives onto Planar

Sep 14, 2010 - P.O. Box 1068, Blindern, N-0316 Oslo, Norway. Received July 7, 2010. Revised Manuscript Received August 27, 2010. The adsorption of two...
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Adsorption of Cationic Hydroxyethylcellulose Derivatives onto Planar and Curved Gold Surfaces Ramon Pamies,†,‡ Sondre Volden,§ Anna-Lena Kjøniksen,†, Kaizheng Zhu,† Wilhelm R. Glomm,§ and Bo Nystr€om*,†

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† Department of Chemistry, University of Oslo, P.O. Box 1033, N-0315 Oslo, Norway, ‡Department of Physical Chemistry, Faculty of Chemistry, University of Murcia, 30071 Murcia, Spain, §Ugelstad Laboratory, Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU ), N-7491 Trondheim, Norway, and Department of Pharmaceutics, School of Pharmacy, University of Oslo, P.O. Box 1068, Blindern, N-0316 Oslo, Norway

Received July 7, 2010. Revised Manuscript Received August 27, 2010 The adsorption of two positively charged hydroxyethylcellulose derivatives with 7 and 60 mol % positively charged groups and a cationic, hydrophobically modified hydroxyethylcellulose containing 1 mol % hydrophobic groups and 7 mol % charged groups onto flat and spherical citrate-coated gold surfaces of different sizes has been investigated. The planar surfaces were studied by means of the quartz crystal microbalance with dissipation monitoring, whereas nanoparticle suspensions were examined using dynamic light scattering and UV-vis spectroscopy. Two different driving forces for adsorption have been evaluated: the electrostatic interaction between the positive charges on the polymers and the negatively charged gold surfaces and the affinity of the polymers for gold due to hydrophobic interactions. The comparison between the data obtained from curved and planar surfaces suggests a strong correlation between surface curvature and adlayer conformation in the formation of the hybrid polymer-gold nanoparticles. The influence of particle size on the amount of adsorbed polymer has been evaluated for the different polymers. The impact of the ionic strength on polymer adsorption has been explored, and the adsorbed polymer layer has been found to protect the gold nanoparticles from aggregation when salt is added to the solution. The addition of salt to a mixture of gold particles and a charged polymer can induce a thicker adsorbed layer at low salinity, and desorption was found at high levels of salt addition.

Introduction and Basic Considerations Advances in nanomedicine have promoted the development of nanoparticles, such as quantum dots, carbon nanotubes, and metallic nanostructures, for many innovative applications regarding in vitro and in vivo diagnostics, imaging, immunolabeling, gene delivery, and cancer therapy because of their biometric features, high surface area to volume ratio, and the possibility of modulating their surface properties with biomolecules.1-3 Nowadays, nanoparticles (NPs) are broadly divided into two types, namely, inorganic and polymeric NPs. To tailor the morphology and properties of AuNP’s, there are two different strategies: either the modification of the surface of the nanoparticle3,4 or the use of suitable additives that can be physically adsorbed onto the surface.5,6 Polymer coating of colloid particles is one method of modifying surface properties, with the polymer type determining the final surface characteristics of the particles. A major area of polymer coatings uses polyelectrolytes to modify surfaces and colloids, exploiting electrostatic attraction for their deposition.7,8 The adsorption of polyions onto oppositely charged curved and (1) Liu, Y.; Wu, D. C.; Zhang, W. D.; Jiang, X.; He, C. B.; Chung, T. S.; Goh, S. H.; Leong, K. W. Angew. Chem. 2005, 44, 4782. (2) Rosi, N. L.; Gilijohann, D. A.; Thaxon, C. S.; Lytton-Jean, A. K. R.; Han, M. S.; Mirkin, C. A. Science 2006, 312, 1027. (3) Chak, C.-P.; Xuan, S.; Mendes, P. M.; Yu, J. C.; Cheng, C. H. K.; Leung, K.C.-F. ACS Nano 2009, 3, 2129. (4) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664. (5) Li, D.; Li, G.; Guo, W.; Li, P.; Wang, E.; Wang, J. Biomaterials 2008, 29, 2776. (6) Caruso, F.; Caruso, R. A.; M€ohwald, H. Science 1998, 282, 1111. (7) Mahtab, R.; Rogers, J. P.; Murphy, C. J. Am. Chem. Soc. 1995, 117, 9099. (8) Gittins, D. I.; Caruso, F. J. Phys. Chem. B 2001, 105, 6846. (9) Cohen Stuart, M. A. J. Phys. (Paris) 1988, 49, 1001.

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planar surfaces has attracted a great deal of interest and stimulated experimental,9-13 theoretical,14-18 and simulation19-23 studies. These investigations have revealed that many parameters affect the adsorption of polyions onto oppositely charged colloidal surfaces. Generally, the strength of the electrostatic interaction has been found to depend on polyelectrolyte characteristics such as the charge density of the polyion, chain length, and chain flexibility as well as colloidal features such as the colloidal surface charge density, size, and shape. In addition, extensive system variables (i.e., temperature, pH, and ionic strength of the medium) can play important roles. It is usually observed that with increasing polyelectrolyte concentration, adsorption increases until saturation occurs. In this process, the surface charge decreases, changes sign, and continue to increase in value. Theoretical considerations16 suggest that the connectivity between the charges of the polyion leads to an overcharge or charge inversion of the colloidal surface, (10) Lowack, K.; Helm, C. A. Macromolecules 1998, 31, 823. (11) Kayitmazer, A. B.; Shaw, D.; Dubin, P. L. Macromolecules 2005, 38, 5198. (12) Samoshina, Y; Nylander, T.; Shubin, V.; Bauer, R.; Eskilsson, K. Langmuir 2005, 21, 5872. (13) Vaccaro, A.; Hierrezuelo, J.; Skarba, M.; Galletto, P.; Kleimann, J.; Borkovec, M. Langmuir 2009, 25, 4864. (14) Von Goeler, F.; Muthukumar, M. J. Chem. Phys. 1994, 100, 7796. (15) Netz, R. R.; Joanny, J.-F. Macromolecules 1999, 32, 9026. (16) Gurovitch, E.; Sens, P. Phys. Rev. Lett. 1999, 82, 339. (17) Dobrynin, A. V.; Deshkovski, A.; Rubinstein, M. Macromolecules 2001, 34, 3421. (18) Li, Z.; Wu, J. Phys. Rev. Lett. 2006, 96, 048302–1. (19) van de Steeg, H. G. M.; Cohen Stuart, M. A.; de Kaizer, A.; Bijsterbosch, B. H. Langmuir 1992, 8, 2538. (20) Linse, P. Macromolecules 1996, 29, 326. (21) Chodanowski, P.; Stoll, S. J. Chem. Phys. 2001, 115, 4951. (22) Laguecir, A.; Stoll, S. Polymer 2005, 46, 1359. (23) Carrillo, J.-M. Y.; Dobrynin, A. V. Langmuir 2007, 23, 2472.

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which can adsorb a polyion chain with a total charge of up to 15/6 times its own charge. In another theoretical approach,18 it was shown that the adsorption of polyions near an oppositely charged surface is mainly governed by the direct Coulomb attraction from the surface and the excluded volume effects. It was also demonstrated that charge inversion becomes more momentous when the surface charge density or polyion chain length increases. There is consensus about the view that electrostatic attraction plays an important role in polyion adsorption at an oppositely charged surface. The simplest theoretical approach is called the ionexchange model,24 which is based on the idea that the adsorbed polyions successfully compete with small monovalent ions and neutralize the surface because of entropic effects. Although this model can qualitatively explain several properties, some phenomena such as charge overcompensation cannot be interpreted in the framework of the simple ion-exchange model. In this case, it was argued24 that it is not the net surface charge that controls the polyion adsorption but rather the possibility of forming ion pairs between polymer chains and surface groups. Ion pairs are considered to be contacting molecular groups of opposite charge; the electrostatic binding energy is deemed to be weak, ∼kBT. Therefore, the polyion sticks to the surface because there are many of these bonds, not because these bonds are strong. In many cases, the adsorbed polyion layer is thicker than the layer adsorbed by the corresponding uncharged polymer. Theoretical models for polyion adsorption to an oppositely charged surface suggest that at low surface charge densities the thickness of the adsorbed layer is determined by the balance between electrostatic attraction to the charged surface and chain entropy. For higher surface charge densities, the thickness of adsorbed polyion chains is governed by the balance of the energy gain due to electrostatic attraction and the confinement entropy lost because of chain localization. The thickness of the adsorbed layer is expected to decrease with increasing surface charge density. The strong lateral repulsion that builds up when the polymer charge density increases frequently leads to a rather flat polyelectrolyte layer on an oppositely charged surface. The adsorption of a hydrophobically modified polyelectrolyte may lead to a more intricate situation because in this case molecular simulation studies23 show that the conformation of the polymer chains may change and this can lead to a redistribution of the number of trains, loops, and tails in the adsorbed layer. In addition, the enhanced hydrophobic interactions may generate a multilayer of the adsorbed polymer. The addition of salt to a suspension of particles containing a hydrophobically modified polyion may lead to an alteration of the adsorption process because of hydrophobic interactions between the surface and polyions.19 Earlier studies25,26 on the adsorption of cellulose derivatives show that both the adsorbed amount and the conformation of the adsorbed polymers are sensitive to temperature changes and the hydrophobicity of the surface and the polymer. In previous work,27-29 the study of the adsorption of uncharged cellulose derivatives onto flat and spherical gold surfaces demonstrated (24) Cohen Stuart, M. Polyelectrolytes on Solid Surfaces. In Short and Long Chains at Interfaces; Proceedings of the XXXth Rencontres de Moriond; Daillant, J., Guenoun, P., Marques, C., Muller, P., Van, J. T. T., Eds.; Editions Ffronttieres; Gif-sur-Yvette, France, 1996; pp 1-12. (25) Malmsten, M.; Claesson, P. M. Langmuir 1991, 7, 1441. (26) Pezron, I.; Pezron, E.; Claesson, P. M.; Malmsten, M. Langmuir 1991, 7, 2248. (27) Malmsten, M.; Claesson, P. M.; Pezron, E.; Pezron, I. Langmuir 1990, 6, 1572–1578. (28) Malmsten, M.; Tiberg, F. Langmuir 1993, 9, 1098–1103. (29) Amirkhani, M.; Volden, S.; Zhu, K.; Glomm, W. R.; Nystr€om, B. J. Colloid Interface Sci. 2008, 328, 20.

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that hydrophobic interactions between the polymer and the surface constitute the main driving force for adsorption. In this article, we address how the attachment of positive charges onto the polymeric chain, together with incorporated hydrophobic groups, affects the adsorption of polymers onto planar and curved gold surfaces. The resulting complex is an inorganic nanoparticle with a surrounding biopolymer corona, which might have a combination of the properties of polymeric and inorganic nanoparticles mentioned above. Three different positively charged derivatives of hydroxyethylcellulose (HEC) have been synthesized. HEC is a nonionic hydrophilic and biodegradable biopolymer, which is susceptible to modification by the inclusion of new groups.30 The polymer has been modified by incorporating positive charges of 7 and 60 mol %, yielding two cationic polyelectrolytes with different charge densities. In addition, a hydrophobically modified analogue containing 7 mol % positive charges and 1 mol % hydrophobic groups has been synthesized. Thus, the competition between electrostatic effects and hydrophobic interactions in the adsorption of these biopolymers onto gold surfaces can be scrutinized. UV-visible spectroscopy, dynamic light scattering (DLS), and quartz crystal microbalance with dissipation monitoring (QCM-D) are the techniques employed in this study to monitor the adsorption of the different polymers onto gold surfaces. The aim of this work is to elucidate the intricate interplay between hydrophobic and electrostatic interactions on the adsorption of polymers onto gold surfaces. To the best of our knowledge, the combination of the varying charge density and hydrophobicity of a polysaccharide with respect to adsorption onto gold surfaces has not previously been addressed.

Experimental Section Materials and Methods. The hydrophobically modified cationic HEC derivative with 7 mol % charges (HM-HEC(þ)7) was prepared by reacting a hydrophobically modified HEC (HMHEC) with 2,3-epoxypropyltrimethylammonium (Fluka) following a slightly modified procedure described elsewhere.31,32 The charged HEC samples with 7 mol % (HEC(þ)7) and 60 mol % (HEC(þ)60) charges were prepared directly from HEC. First, 10.0 g of the HEC derivative (Natrosol 250 GR; lot. no. A-0832 obtained from Hercules, Aqualon Division) was added to an alkaline solution containing 1.5 g of sodium hydroxide (from Sigma-Aldrich) in 400 mL water and mixed for 24 h at room temperature. An aqueous solution of the reactant (4.7 g of 2,3epoxypropyltrimethylammonium (Sigma-Aldrich) dissolved in 40 mL of water) was then added, and the reaction bath was heated and kept at 50 °C for 24 h under nitrogen. The solution was then neutralized by the addition of 6 N HCl until a pH of 6 was reached. The polymer was further purified through dialysis (with a regenerated cellulose membrane with a molecular-weight cutoff of 8000) against Millipore water, and after being freeze dried, the positively charged HEC derivatives were recovered. The degree of substitution corresponding to the incorporation of the ammonium group per repeating anhydroglucose unit was calculated from NMR spectra using the ratio of the integration of the peak value at d = 3.2 ppm assigned to the proton -CH3 that is linked to the ammonium group (Nþ) to that of the anomeric protons’ signal of the HEC. The degrees of cationic substitution were found to be approximately 7 and 60 mol % for samples HEC(þ)7 and HEC(þ)60, respectively. The chemical structures of the synthesized polymers are depicted in Figure 1. (30) Beheshti, N.; Zhu, K.; Kjøniksen, A.-L.; Nystr€om, B. Colloids Surf. A 2008, 328, 79. (31) Blomberg, E.; Kumpulainen, A.; David, C.; Ariel, C. Langmuir 2004, 20, 10449. (32) Beheshti, N.; Zhu, K.; Kjøniksen, A.-L.; Nystr€om, B. J. Non-Cryst. Sol. 2007, 353, 3906.

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Article In the dilute concentration regime probed in this study, the scattering field obeys Gaussian statistics and the measured correlation function g2(q, t), where q = (4πno/λ)sin(θ/2), with λ, θ, and no being the wavelength of the incident light in a vacuum, the scattering angle, and the refractive index of the medium, respectively, can be related to the theoretically amenable first-order electric field correlation function g1(q, t) by the Siegert relationship g2(q, t) = 1 þ B|g1(q, t)|2, where B is usually treated as an empirical factor. In this case, the correlation functions were found to be described by a stretched exponential with a much better goodness-of-fit procedure (random distribution and small values of the residuals) in comparison to a single-exponential fit (Supporting Information). g 1 ðtÞ ¼ exp½ - ðt=τe Þβ 

Figure 1. Structure of the positively charged HEC derivatives, where HEC(þ)7 and HEC(þ)60 contain 7 and 60 mol % R groups, respectively. For these samples, R0 = H. HM-HEC(þ) contains 7 mol % R groups and 1 mol % R0 . Table 1. Characteristics of the Different Gold Nanoparticle Suspensions radii (nm), manufacturer

concentration of nanoparticles (particles/mL), manufacturer

Rh,app (nm) measured by DLS

10 15 20

7  1011 2  1011 9  1010

15 19 25

Three different types of aqueous suspensions of AuNP were purchased from Ted Pella, Inc. (Redding, CA) and stored at 4 °C in the refrigerator. In Table 1, the sizes (radii) of the gold particles and the concentrations of the suspensions given by the manufacturer are displayed, together with the values of the hydrodynamic radius determined by DLS. The employed samples were simply prepared by mixing the polymer solutions with the AuNP suspensions at room temperature. The concentrations, which appear in the text, correspond to the values in the final solution for both particles and polymer. Zeta-Potential Experiments. Zeta-potential measurements were carried out on a Malvern Nano-ZS zetasizer. All experiments were performed at 25 °C with 5 series, each consisting of 10 repeating runs, being recorded for each sample. Zeta potentials were calculated from the polymer/particles’ electrophoretic mobility using the Smoluchowski approximation. Dynamic Light Scattering. The dynamic light scattering (DLS) experiments were conducted with the aid of an ALV/ CGS-8F multidetector compact goniometer system, with eight off-fiber optical detection units, from ALV-GmbH, Langen, Germany. The beam from a Uniphase cylindrical 22 mW He-Ne laser, operating at a wavelength of 632.8 nm with vertically polarized light, was focused onto the sample cell (10 mm NMR tubes) through a temperature-controlled cylindrical quartz container (with two plane-parallel windows) that is filled with a refractive index matching liquid (cis-decalin). The polymer and AuNP and polymer solutions were filtered separately through a 5 μm filter (Millipore), mixed in the required proportions, and injected into precleaned NMR tubes. All measurements were performed at 25 °C, and correlation functions were recorded every minute. At least 10 parallels were carried out, and 4 to 5 of them were used to calculate the final values with the corresponding standard deviation. In the DLS measurements, the intensity correlation function was measured at eight scattering angles simultaneously in the range of 22-141° with four ALV5000/E multiple-τ digital correlators. Langmuir 2010, 26(20), 15925–15932

ð1Þ

In the analysis of the correlation functions by means of eq 1, a nonlinear fitting algorithm was employed to obtain best-fit values of parameters τe and β. Parameter τe is some effective relaxation time, and β (0 < β e 1) is a measure of the width of the distribution of relaxation times. The mean relaxation time is given by   τe 1 τ ¼ ð2Þ Γ β β where Γ is the gamma function. If the relaxation mode is diffusive, then it yields the mutual diffusion coefficient D (τe-1 = Dq2), which allows us to calculate the apparent hydrodynamic radii (Rh,app) using the Stokes-Einstein equation D ¼

kT 6πη0 Rh, app

ð3Þ

where k is the Boltzmann constant, η0 is the viscosity of the solvent, and T is the absolute temperature. All of the relaxation times calculated in this work were found to be diffusive. Quartz Crystal Microbalance. The Au-coated QCM crystals were AT-cut quartz crystals (Biolin Scientific AB) with a fundamental frequency ( f0) of 5 MHz and an active sensor area of 20 mm2. The gold crystals were cleaned in piranha solution consisting of 3:1 H2SO4 (98%)/H2O2 (30%) for approximately 30 min, followed by rinsing with Milli-Q water and drying under a stream of N2 gas. All crystals were used immediately after preparation. Mass adsorption data were acquired at 25 °C on a D300 supplied by Q-sense, and the adsorption density (surface coverage) for each species was calculated using the Sauerbrey equation on data extracted from the third harmonic of the resonance.33 However, the surface coverage thus calculated is considered to be an approximation because there will be additional mass registered from water trapped within or associated with the adsorbed layer.34,35 The Q-sense D300 continuously measures the dissipation change following adsorption, ΔD = D - D0, throughout the adsorption process. This in turn allows for plotting these dissipation changes as a function of changes in frequency (ΔD - Δ( f/n) plot), which yields information regarding layer formation and film rigidity. In this study, frequency and dissipation responses were recorded at around 5, 15, 25, and 35 MHz, corresponding to the fundamental frequency and harmonics n = 3, 5, and 7, respectively. For clarity, only the normalized frequency shifts, Δf = Δfn/n, and the dissipation shifts, ΔD, for the third harmonic are represented. The experiments were conducted in a direct deposition protocol wherein a polymer solution of a certain concentration was (33) Sauerbrey, G. Z. Phys. 1959, 155, 206. (34) Hook, F.; Kasemo, B.; Nylander, T.; Fant, C.; Sott, K.; Elwing, H. Anal. Chem. 2001, 73, 5796. (35) Hook, F.; Voros, J.; Rodahl, M.; Kurrat, R.; Boni, P.; Ramsden, J. J.; Textor, M.; Spencer, N. D.; Tengvall, P.; Gold, J.; Kasemo, B. Colloids Surf., B 2002, 24, 155.

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Table 2. Values (mV) of the Zeta Potential for 0.1 wt % Solutions of the Indicated Polyelectrolytes (with and without 10 mM NaCl) in the Bulk and in the Presence of Gold Nanoparticles (Rh,app = 19 nm)

HEC(þ)7 HEC(þ)60 HM-HEC(þ)7

water

NaCl 10 mM

AuNP

4.3 ( 0.5 14 ( 2 7.0 ( 0.5

-1.3 ( 0.2 9.4 ( 0.6

3.3 ( 0.1 12.2 ( 0.3 3.8 ( 0.2

deposited onto a freshly prepared gold slide for each measurement. Polymer solutions of 0.0125, 0.025, 0.05, and 0.1 wt % concentrations were prepared both in water and in 10 mM NaCl, and they were added to the adsorption chamber after a stable baseline was found. A citrate layer covering the crystal was prepared by adding a solution of sodium citrate (10 mM) to the measurement chamber. Ultraviolet-Visible Absorbance. The absorption spectra of bare gold nanoparticles and particles with adsorbed polymer were collected using a temperature-controlled Helios Gamma (Thermo Spectronic, Cambridge, U.K.) spectrophotometer at wavelengths in the range from 330 to 900 nm. The apparatus is a single-beam UV-visible spectrophotometer equipped with a temperature unit (Peltier plate) that gives good temperature control over an extended temperature interval and time. The instrument can scan a wavelength range from 190 to 1100 nm and is computer controlled through a homemade program. The results from the spectrophotometer will be presented in terms of absorbance, and all measurements were performed at 25 °C.

Results and Discussion The charge densities of polyelectrolytes in the bulk and the charges on gold nanoparticles with adsorbed polyions have been determined with the aid of a zetasizer at 25 °C. For polyelectrolytes in the bulk, HEC(þ)60 has the highest ζ potential and HEC(þ)7 exhibits a lower value than does the hydrophobically modified analogue, probably because of interchain associations through hydrophobic interactions. When salt is added (10 mM), the charge density of the HEC(þ)60 sample is reduced, and for the HEC(þ)7 sample, charge reversal occurs. At this level of salt addition, it is not possible to dissolve 0.1 wt % of the HM-HEC(þ)7 polymer. When HEC(þ)7 or HM-HEC(þ)7 is adsorbed onto gold particles, the charge density is practically the same but a much higher zeta potential is observed for the particles with the adsorbed HEC(þ)60 sample. The values of the zeta potential of the polymer/ AuNP constructs are significantly lower than the corresponding values for the polymer in the bulk in the absence of particles because of the partial neutralization when the polyions are adsorbed onto the oppositely charged gold surfaces. Adsorption of Polymers onto Spherical and Planar Gold Surfaces. The adsorption onto the gold surfaces of the three positively charged HEC derivatives can be ascribed to two main driving forces, namely, electrostatic and hydrophobic interactions. In the case of HEC(þ)7 and HEC(þ)60, the main force is the electrostatic interactions between the positive charges along the polymeric backbone and the negative charges of the citratecovered AuNP. However, for HM-HEC(þ), the presence of both charges and hydrophobic groups favors adsorption. The correlation functions obtained from the suspensions of gold nanoparticles (Rh,app = 19 nm) in the presence of 0.1 wt % polymer at a temperature of 25 °C are displayed in Figure 2a. When the correlation functions are fitted by means of eq 1, the hydrodynamic radii can be calculated through the StokesEinstein relationship as described above. We have determined Rh,app of the polymer-AuNP complexes at different polymer concentrations, and the apparent hydrodynamic layer thickness δh,app is calculated by subtracting Rh,app of bare AuNP from Rh,app 15928 DOI: 10.1021/la102716m

Figure 2. (a) Correlation functions for the Au nanoparticles in the presence of 0.1 wt % of the indicated polymers. (b) Effects of polymer adsorption on the hydrodynamic layer thickness (measured with DLS) of Au nanoparticles (Rh,app = 19 nm for the bare particles) at different concentrations for the systems indicated. (c) Mass of samples adsorbed onto flat gold surfaces as measured by QCM-D in water.

of polymer-covered AuNP. To elucidate the effect of charged groups on the adsorption behavior, the adsorption features of uncharged HEC and HM-HEC have also been measured. Whereas δh,app for uncharged HM-HEC increases only slightly with polymer concentration, uncharged HEC and all of the charged cellulose derivatives exhibit a pronounced upturn in the hydrodynamic layer thickness with increasing polymer concentration (Figure 2b). This type of behavior has been reported for HEC29 and polyelectrolytes of various natures. Unmodified HEC, even though it lacks hydrophobic groups, still adsorbs onto the gold particles. Apparently, the polysaccharide ring interacts strongly enough (presumably van der Waals forces) with the gold surfaces to induce adsorption. In this study, the gold surfaces are covered with a citrate layer, and it is known36 that citrate ions have a strong affinity for gold surfaces and are frequently employed to stabilize gold particles by conferring negative charges on the surfaces and thereby repulsive forces between the particles. Actually, a thicker surface coverage of nonionic HEC on gold surfaces with a citrate layer has been reported;29 this indicates that precoating of the gold surfaces with citrate leads to an overall higher surface coverage for HEC than on the pure gold substrate. However, both HM-HEC and all of the charged polymers adsorb into thicker polymer layers than does unmodified HEC, indicating that both enhanced hydrophobicity and the incorporation of cationic charges favor adsorption onto the negatively charged AuNP. When the charged HEC analogue (HEC(þ)7) is adsorbed on the gold particles, a tremendous effect on the hydrodynamic layer thickness is registered, giving higher values of δh,app than does either of the other polymers. In spite of this, the amount of adsorbed polymer, measured with the aid of QCM-D, is fairly low (cf. Figure 2c). In addition, the adsorbed amount of polymer is the same for HEC(þ)7 and HEC(þ)60 (within experimental error), even though δh,app is clearly higher for HEC(þ)7. The high values of δh,app for HEC(þ)7, combined with the low adsorbed mass, seem to indicate that the polyions are associated with the surface via few attachment points. Long loops and tails dangle in the proximity of the oppositely charged surfaces. (36) Turkevich, J.; Stevenson, P. C.; Hillier, J. J. Phys. Chem. 1953, 57, 670.

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Figure 3. Changes in dissipation (ΔD) as a function of the change in frequency for the third harmonic of the resonance (-Δf/n) for the indicated charged polymers during the adsorption process. The fitted gray lines indicate the three different adsorption domains. (See the text for details.)

By using a mean-field lattice theory20 for flexible polyelectrolytes in solution, it has been shown that a low charge density of the polymer leads to a smaller amount of adsorbed polymer on the oppositely charged surface but a thicker adsorbed layer, accompanied by longer tails, fewer longer loops, and a smaller number of polyion segments in direct contact with the surface. For the HEC analogue with the highest charge density (HEC(þ)60), the values of δh,app are significantly lower than the corresponding ones for HEC(þ)7. In this case, the adsorption of polyions with a high charge density on the oppositely charged gold surface can lead to an overcompensation of the surface charge and hence charge reversal.16 As a consequence, the strong lateral repulsion that builds up in the adsorbed layer at high charge density will give rise to a flatter conformation.9 The values of δh,app are considerably larger in the presence of the uncharged HM-HEC than for the HEC analogue (Figure 2b), and this difference is ascribed to enhanced hydrophobic interactions between the polymer and the substrate. It has previously been reported19 that a larger amount of polymer is adsorbed on gold with HM-HEC than for HEC. However, the increase in the hydrodynamic layer thickness with increasing polymer concentration is moderate, and this is probably due to a weak expansion of the adsorbed layer as a consequence of hydrophobic interactions within the layer. For the charged analogue (HM-HEC(þ)7), only a slight increase in δh,app compared to its uncharged analogue (HM-HEC) is observed at low polymer concentrations. However, a much thicker layer evolves as the polymer concentration increases. This effect is attributed to the intricate interplay between hydrophobic interactions and attractive electrostatic interactions. This clearly discloses that the Coulombic forces have a strong impact on δh,app as well as on the amount of adsorbed polymer. From Figure 2c, it is evident that the amount of HMHEC(þ)7 adsorbed onto gold surfaces is large, and a strong concentration effect is found. But even though the adsorbed amount of HM-HEC(þ)7 is higher than for the charged polymers without hydrophobic groups, the hydrodynamic layer thickness is smaller (Figure 2b). This indicates that the hydrophobic groups cause the adsorbed polymer layer to contract. In Figure 3, the change in dissipation, ΔD, as a function of the variation of normalized frequency for the third harmonic of the resonance, -Δf/n, is displayed for HEC(þ)7, HEC(þ)60, and HM-HEC(þ)7 on citrate-capped planar gold surfaces. A close inspection of the data for HEC(þ)7 and HEC(þ)60 adsorbed Langmuir 2010, 26(20), 15925–15932

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onto the gold substrate reveals three transition zones in the dissipation versus normalized (ΔD - Δ( f/n)) curves, whereas a continuous curve with no break points is observed for HMHEC(þ)7. The transitions indicate that the nature of the adsorption process changes (i.e., there are different kinetic regimes).34,35 The three domains for HEC(þ)7 and HEC(þ)60 correspond to a rather rigid layer at low surface coverage, which then becomes more viscoelastic before tapering off at higher adsorbed amounts. From this, it would seem that both HEC(þ)7 and HEC(þ)60 start out rather flat on the surface before stretching out into the bulk solution as more polymer approaches the surface. At the second break point, the polymers are so tightly packed that lateral movement is restricted, causing a reduction in the dampening of the system. The ΔD - Δ( f/n) graph for HEC(þ)7 has break points at around 65 and 110 Hz, and the corresponding ones for HEC(þ)60 are located at around 35 and 70 Hz, respectively. These differences in location of the break points can provide additional information about the adsorption process. Both transitions for HEC(þ)60 occur at lower frequencies than do the corresponding ones for HEC(þ)7, which implies that less polymer is adsorbed when the transition starts. This can be rationalized by the fact that the more strongly charged HEC(þ)60 will have a higher propensity to repel intermolecularly, both between polymers on the surface and also between surface-bonded and approaching polymers, invoking a stronger driving force to extend into the solution. Hydrophobically modified polyelectrolyte HM-HEC(þ)7 yields a ΔD - Δ( f/n) plot that displays a smoother crossover to a gradually more viscoelastic layer. In view of the large amount of adsorbed mass, pronounced hydrophobic interactions, and the feature of the ΔD - Δ( f/n) plot, the HM-HEC(þ)7 polyions appear to adsorb not in a layerwise manner but rather as small aggregates or in a nonordered multilayer formation. Adsorption onto the AuNP is expected to be different than onto flat surfaces not only because of the geometry but also because the area of the flat gold surfaces used in this study is around 103 times higher than the total surface area of the spherical gold nanoparticles. Obviously, the conformational aspects of the polymers are decisive in this kind of system. It is important to note that the size of these hybrid nanoparticles cannot always be related to the adsorbed mass of polymer onto the gold spheres. The presence of hydrophobic groups in the polymer provokes a lower solubility in water, and a more compact layer is formed than in the case of nonhydrophobically modified polymers.37 This contraction of the adsorbed layer causes δh,app values for HM-HEC(þ)7 to be smaller than what is observed for the polymers without hydrophobic groups (Figure 2b), even though the adsorbed mass is higher. Adsorption of Polymers onto Gold Particles and the Appearance of Surface Plasmon Peaks. AuNP’s are known38 to have an intense surface plasmon resonance absorption located between 500 and 550 nm. Changes in the size, shape, and dielectric environment provide an extraordinary degree of freedom for manipulating the localized surface plasmon oscillation frequency and lifetime for isolated metal particles.39,40 The surface plasmon peak absorption for our AuNP with an Rh,app of 19 nm occurs at a wavelength of 525 nm as can be seen in Figure 4, where the effect of the addition of HEC(þ)7, HEC(þ)60, and HMHEC(þ) onto AuNP is studied using different concentrations of the polymers. (37) Cosgrove, T.; Crowley, T. L.; Ryan, K.; Webster, J. R. P. Colloids Surf. 1990, 51, 255. (38) Hutter, E.; Fendler, J. H. Adv. Mater. 2004, 16, 1685. (39) Lal, S.; Link, S.; Halas, N. J. Nat. Photonics 2007, 1, 641. (40) Hauck, T. S.; Ghazani, A. A.; Chan, W. C. W. Small 2008, 4, 153.

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Figure 4. Surface plasmon resonance peaks for AuNP (Rh,app = 19 nm) in the presence and absence of (a) HEC(þ)7, (b) HEC(þ)60, and (c) HM-HEC(þ)7.

It has been reported that this value might be affected by the adsorption of polyelectrolytes onto AuNP’s because of the change in the refractive index, according to Mie-Drude theory.40,41 Although we do not register any significant change in the position of the maximum of the plasmon peak as the polymer concentration increases, a slight increase in absorbance was found for all systems with increasing polymer concentration. It is interesting that neither the nature nor the concentration of the polymer significantly affects the profile and maximum of the absorption spectra of the gold particles. When small amounts of polyelectrolytes are added to AuNP suspensions, the colloids will be more or less coated, depending on the charge density and polymer concentration, and as a result, the particles are stabilized through steric hindrance and/or repulsive electrostatic forces.42,43 When the interparticle distance becomes comparable to the wavelength of the interacting light, far-field radiative coupling44,45 strongly modifies the surface plasmon resonance (SPR) energy and width (i.e., if there is any kind of aggregation, a shift in the SPR absorption to longer-wavelength values is expected). When we add an HEC(þ)7, HEC(þ)60, or HM-HEC(þ)7 solution to the AuNP dispersion, no significant change in the plasmon peaks is found for any of the polyelectrolytes and concentrations studied (i.e., there is an overlap between the plasmon peaks of the polymeric-inorganic nanoparticles and pure AuNP’s). The peak height for all systems seems to rise somewhat as the polyelectrolyte concentration increases; this is ascribed to the contribution from a slight change in the absorbance in the bulk with increasing polymer concentration. These results suggest that the optical properties of the particle surface are virtually not affected by the adsorbed polyions, and this may indicate that the adsorbed layers are porous and the surface plasmon resonance features are not significantly affected. However, it has been found that (41) Templeton, A. C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P. J. Phys. Chem. B 2000, 104, 564. (42) Pugh, T. L.; Heller, W. J. Polym. Sci. 1960, 47, 203. (43) Heller, W.; Pugh, T. L. J. Polym. Sci. 1960, 47, 219. (44) Lamprecht, B.; Schider, G.; Lechner, R. T.; Ditlbacher, H.; Krenn, J. R.; Leitner, A.; Aussenegg, F. R. Phys. Rev. Lett. 2000, 84, 4721. (45) Haynes, C. L.; McFarland, A. D.; Zhao, L.; Van Duyne, R. P.; Schatz, G. C.; Gunnarsson, L.; Prikulis, J.; Kasemo, B.; M€all, M. J. J. Phys Chem. B 2003, 107, 7337. (46) Vasilev, K.; Casanal, A.; Challougui, H.; Griesser, H. J. J. Phys. Chem. 2009, 113, 7059.

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Figure 5. Total hydrodynamic volume of adsorbed polymer as a function of the total hydrodynamic area of the gold surfaces. The inset depicts the hydrodynamic layer thickness as a function of the size of the bare AuNP. The polymer concentration is 0.1 wt %.

particles that are imbedded in a polymer matrix46 exhibit changes in the optical properties. Adsorption onto Gold Particles of Different Sizes. It has been claimed that the size and curvature of the gold nanoparticles play a crucial role in studies of the adsorption of biomolecules onto the surface.47-51 We have performed a series of DLS experiments to measure Rh,app of polymer-AuNP hybrids in the presence of HEC, HEC(þ)7, HEC(þ)60, and HM-HEC(þ)7 using AuNP’s of three different sizes. For all of the studied polymers, a minimum in the adsorbed layer thickness was found when the size of the AuNP’s was increased (inset in Figure 5). The number of gold particles in the samples varies with their size (Table 1). Unfortunately, it is not possible to obtain good data on samples containing the same number of AuNP’s because the degree of dilution affects the thickness of the citrate layer, thereby making the AuNP suspensions unstable. To take into account the different numbers of AuNP’s in the samples, we have plotted the total hydrodynamic volume of the adsorbed polymer (Vh,T = 4/3π(Rhp3 - RhAu3)NAu, where RhAu is Rh of the bare AuNP, Rhp is Rh,app of the AuNP with adsorbed polymer, and NAu is the number of AuNP per milliliter) as a function of the total available hydrodynamic surface area of the AuNP’s (Ah,T = 4π(Rhp2 - RhAu2)NAu) in Figure 5. The system seems to follow a scaling law where Vh,T ≈ Ah,Tν. If the adsorbed volume per unit area is constant, a value of ν = 1 is expected to be obtained. However, for the considered systems, ν is 1.3-1.4, indicating that the total adsorbed volume per unit area increases as the available surface area is raised. Accordingly, there is a larger adsorbed volume per surface area when there is more area available for the polymer to adsorb onto. It can be observed from the data that this behavior is general for all of the polymers in this work. Because it seems unlikely that a larger available surface area should cause more polymer to adsorb per unit area, this indicates that the adsorbed polymer layers swell when the total available surface increases. When Ah,T is increased, the total amount of polymer that is adsorbed onto AuNP’s increases as well. This may cause (47) Baker, J. A.; Pearson, R.; Berg, J. C. Langmuir 1989, 5, 339–342. (48) Li, J.-T.; Caldwell, K. D.; Rapoport, N. Langmuir 1994, 10, 4475–4482. (49) Jiang, X.; Jiang, J.; Jin, Y.; Wang, E.; Dong, S. Biomacromolecules 2005, 6, 46. (50) Ding, Y.; Chen, Z.; Xie, J.; Guo, R. J. Colloid Interface Sci. 2008, 327, 243. (51) Nystr€om, B.; Kjøniksen, A.-L.; Beheshti, N.; Maleki, A.; Zhu, K.; Knudsen, K. D.; Pamies, R.; Hernandez Cifre, J. G.; Garcı´ a de la Torre, J. Adv. Colloid Interface Sci. 2010, 158, 108–118.

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Figure 7. Surface plasmon peaks of gold nanoparticles (Rh,app = Figure 6. (a) Effect of polymer concentration on the value of the hydrodynamic radius (from DLS) for suspensions of gold particles (Rh,app = 19 nm) in the presence of HEC(þ)7 or HEC(þ)60 with a fixed NaCl concentration of 10 mM. (b) Amount of adsorbed polymer on planar gold surfaces at a salt concentration of 10 mM as determined by means of QCM-D. (c) Effect of salt addition on Rh,app for a suspension of gold particles in the presence of HEC(þ)7 or HEC(þ)60 at a constant concentration of 0.1 wt %.

the bulk concentration of nonadsorbed polymer to decrease. As a result, the chemical potential in the bulk is lowered and the adsorbed polymer layers swell to minimize the differences in chemical potential between the adsorbed polymer layer and the bulk solution. Effect of Salt Addition on Polyelectrolyte Adsorption on Gold Particles. To gain better insight into the impact of electrostatic interactions on the adsorption of polyelectrolytes on oppositely charged gold particles, the ionic strength has been changed by the addition of NaCl. It is well known that the addition of salt will screen the electrostatic interactions and thereby change the adsorption properties. Figure 6a shows the effect of polymer concentration for HEC(þ)7 and HEC(þ)60 on the hydrodynamic radius at a fixed salt concentration of 10 mM. The behavior of both polymers in aqueous solutions in the presence of NaCl was studied in previous work,51 where various physical parameters are given. At low polymer concentrations, Rh,app is independent of the number of charges attached to the polymer. For both polymers, the amount of adsorbed polymer increases with increasing polymer concentration, but whereas Rh,app for HEC(þ)7 continues to rise over the whole concentration region, Rh,app of the HEC(þ)60 sample flattens out when the concentration exceeds 0.05 wt %. However, Figure 6b shows that the adsorbed mass increases with increasing polymer concentration and is practically the same for both polymers over the whole concentration region. A comparison with the results in Figure 2c reveals that the adsorbed mass decreases when salt is added to the system because of the partial screening of the electrostatic attractive forces between the polymer and the AuNP’s. For the polymer with a small number of charges, most of the charges are screened and the formation of loops and tails of the polymer19,20 protruding out of the particle surfaces is promoted. For the HEC(þ)60 sample, most of the charges are screened at low polymer concentrations, and it behaves similarly to HEC(þ)7. At higher concentrations, the low salinity is not high enough to screen out the charges, and the progressive adsorption of charged chains onto the particles will lead to the formation of an adsorbed layer with a strong lateral repulsion between the polyions when the charge density increases further. In this case, the polyions tend Langmuir 2010, 26(20), 15925–15932

19 nm) with a level of salt (NaCl) addition of 10 mM for (a) bare particles without polymer, (b) particles with 0.1 wt % HEC(þ)7, and (c) particles with 0.1 wt % HEC(þ)60.

to adsorb at the particle surfaces in a flat conformation, forming numerous contacts with the surface. Figure 6c illustrates the effect of salt addition at a constant polymer concentration of 0.1 wt % for HEC(þ)7 and HEC(þ)60. A steady drop in the value of the hydrodynamic radius with increasing salinity is found for the HEC(þ)7 polymer, and this behavior is ascribed to screening of the electrostatic attraction and competition for surface sites between the polyion segments and small salt ions. This results in desorption of the polyions from the surfaces. In the case of the HEC(þ)60 sample, Rh,app passes through a sharp maximum before Rh,app falls off strongly with increasing salt concentration. At moderate levels of salt addition, the screening of electrostatic repulsive forces between the polyions in the adsorbed layer will lead to the evolution of a thicker layer with loops and tails, analogous to what is observed for HEC(þ)7 in the absence of salt (Rh,app at the maximum is close to Rh,app for HEC(þ)7 in the absence of salt). At higher salt concentrations, an increasing number of segments are displaced from the surface by salt cations. This gives rise to desorption and a thinner adsorbed layer. These effects of salt addition on the adsorption/desorption of polymer chains are consistent with the theoretical advances17,19,52 and simulation20-22 studies of polyelectrolytes at oppositely charged surfaces. Gold nanoparticles are stabilized by a citrate layer, and this layer can easily be charge neutralized by the addition of salt, after which the flocculation of particles occurs in the course of time. Salt-induced aggregation of stabilized gold nanoparticles can easily be accomplished by the addition of salt, even at a low salinity (10 mM). The time evolution of the surface plasmon peak in the presence of NaCl (10 mM) of bare gold particles in the absence of polymer is depicted in Figure 7a. The most conspicuous features are the appearance of a secondary peak and the reduction of the peak amplitudes after a long period of time. A close inspection of the data reveals a modest red shift of the peak at low wavelengths (located at around 525 nm) and a more pronounced red shift of the peak appearing at longer wavelengths in the course of time. Similar features have been reported previously53 for the salt-induced aggregation of citrate-covered silver particles in aqueous solutions of cellulose derivatives. In the (52) B€ohmer, M. R.; Evers, O. A.; Scheutjens, J. M. H. M. Macromolecules 1990, 23, 2288. (53) Trinh, L. T. T.; Kjøniksen, A.-L.; Zhu, K; Knudsen, K. D.; Volden, S.; Glomm, W. R.; Nystr€om, B. Colloid Polym. Sci. 2009, 287, 1391.

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presence of a 0.1 wt % solution of HEC(þ)7 or HEC(þ)60 (Figure 7b,c), no salt-induced flocculation of the gold particles is detected and the absorbance curves recorded at different times condense onto each other. This suggests that the particles are stabilized and no aggregation occurs. This can probably be attributed to both steric and electrostatic stabilization, especially for the polymer with the highest charge density.

Conclusions Positively charged hydroxyethylcellulose derivatives with and without hydrophobic modification have been found to be useful for the formation of hybrid polymeric-inorganic nanoparticles when solutions of these polymers are mixed with citrate-coated gold nanoparticles. The thickness of the adsorbed layer for these systems depends on the nature and concentration of the polymer and on the ionic strength of the solution. Much higher values of δh,app for the HEC sample with moderate charge density (HEC(þ)7) are observed than for the corresponding concentrations of the uncharged analogue. For the HEC(þ)60 sample, lower values of δh,app are found. A moderate charge density of the polymer results in a stronger polymer/surface attraction and a concomitant increased adsorption and thicker adsorbed polymer layer, but this layer is compressed by the strong lateral repulsion that builds up when the polymer charge density increases further. The amount of polymer adsorbed onto planar citrate-covered gold surfaces was found to be virtually independent of the charge density. For the hydrophobically modified counterpart (HM-HEC(þ)7), the adsorbed mass of polymer is higher but the values of δh,app are significantly lower than those for HEC(þ)7. This can be ascribed to the compression of the adsorbed layer in the case of the hydrophobically modified polymer.

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In the adsorption of HEC and HEC(þ) derivates onto gold nanoparticles of different sizes, a power law with an exponent value of 1.3-1.4 has been found in the representation of the total adsorbed hydrodynamic volume, Vh,T, versus the total adsorbed hydrodynamic area, Ah,T, indicating a swelling of the adsorbed polymer layer as the available AuNP surface area is increased. When nonhydrophobically modified polysaccharides are mixed with AuNP in 10 mM NaCl, the well-known salt-induced aggregation of the nanoparticles is avoided because of steric stabilization of the particles. The profile of the surface plasmon peak supports the idea that no flocculation of particles takes place upon addition of salt to a suspension of particles covered by polymer. A competition between the suppression of the electrostatic attraction between the polymer and the gold surface and the screening of the intrapolymer repulsions has been found to be dependent on the concentration of polymer in solution and the ionic strength. The addition of salt to mixtures of gold particles and charged HEC can lead to a thicker adsorbed layer at low salinity and the desorption of polymer at higher levels of salt addition. Acknowledgment. We gratefully acknowledge the Research Council of Norway (NFR) for financial support within the FRINAT program, project nos. 177556/V30 and 177665/V30. R.P. acknowledges a postdoctoral fellowship from Fundacion Seneca-CARM and from the University of Murcia. Supporting Information Available: Plot showing the correlation function obtained for AuNP-HEC(þ)60 at a concentration of 0.1 wt %. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(20), 15925–15932