21960
J. Phys. Chem. C 2010, 114, 21960–21968
Temperature-Induced Flocculation of Gold Particles with an Adsorbed Thermoresponsive Cationic Copolymer Ramo´n Pamies,†,‡ Kaizheng Zhu,† Sondre Volden,§ Anna-Lena Kjøniksen,†,| Go¨ran Karlsson,⊥ Wilhelm R. Glomm,§ and Bo Nystro¨m*,† Department of Chemistry, UniVersity of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway, Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, UniVersity of Murcia, Murcia, Spain, Department of Pharmaceutics, School of Pharmacy, UniVersity of Oslo, P.O. Box 1068, Blindern, N-0316 Oslo, Norway, Department of Physical and Analytical Chemistry, Uppsala UniVersity, Box 579, S-751 23 Uppsala, Sweden, and Ugelstad Laboratory, Department of Chemical Engineering, Norwegian UniVersity of Science and Technology (NTNU), N-7491 Trondheim, Norway ReceiVed: July 14, 2010; ReVised Manuscript ReceiVed: NoVember 8, 2010
In this article, we report a study of the adsorption of a charged thermoresponsive poly(N-isopropylacrylamide)block-poly(3-acrylamidopropyl)trimethylammonium chloride) [abbreviated as PNIPAAM24-b-PAMPTMA(+)18] block copolymer onto gold nanoparticles by means of dynamic light scattering (DLS), zeta-potential measurements, cryo-TEM, rheology small-angle light scattering (rheo-SALS), and UV-visible absorbance spectroscopy. At low surface coverage, the cationic copolymer was found to mediate flocculation of the particles at elevated temperatures, whereas at higher polymer concentration, the gold particles were stabilized by the adsorbed copolymer layer. The DLS results showed that the adsorbed polymer was compressed prior to flocculation at elevated temperatures and that the flocs coexisted with single particles. The latter finding was confirmed by cryo-TEM. Rheo-SALS measurements revealed that shear flow disrupted the large flocs formed at higher temperatures. UV-visible absorbance measurements demonstrated that the temperatureinduced flocculation process gives rise to a red shift of the localized surface plasmon resonance (LSPR) peak, together with the appearance of a second peak at higher wavelengths. Introduction As a virtually inert and biocompatible nanomaterial, gold nanoparticles (AuNPs) exhibit great potential in biotechnology and biomedicine, from protein and DNA detection to cancer therapy and drug delivery.1-6 In addition to their prospective use in numerous applications, gold nanoparticles have also attracted immense interest in recent years because of their unique optical and electronic features.7,8 Gold nanoparticles frequently display a strong absorption band in the UV-visible light region; the physical origin of this phenomenon is the collective oscillation of the conduction-band electrons induced by the interacting electromagnetic field, the so-called localized surface plasmon resonance (LSPR).8,9 The adsorption of a polymer onto the gold particles can affect the LSPR properties, as well as the stability of the colloidal suspension. There are, in principle, two basic mechanisms for the stabilization of colloidal particles, namely, charge stabilization and steric stabilization. The former can result from repulsive electrostatic interactions through the adsorption of a charged polymer, whereas the latter phenomenon is due to the adsorption of uncharged polymers onto the surface of the colloidal particles, leading to steric repulsion between them. The thickness of an adsorbed polymer layer generally depends on the choice of substrate, the chemical nature of the polymer, and especially the molecular weight of the adsorbed * To whom correspondence should be addressed. E-mail: bo.nystrom@ kjemi.uio.no. † University of Oslo. ‡ University of Murcia. § Norwegian University of Science and Technology (NTNU). | University of Oslo. ⊥ Uppsala University.
polymer. Under certain circumstances, however, it is known that high-molecular-weight polymers can adsorb on separate particles and draw them together, an act referred to as polymer bridging flocculation.10,11 This phenomenon has many vital industrial applications, such as preparation of paints, stabilization of drilling fluids, paper making, and purification of water, where the flocs are utilized to remove unwanted particles. Bridging flocculation of charged colloidal particles, mediated through an oppositely charged low-molecular-weight amphiphilic copolymer is less known. In the present work, we demonstrate a novel and special polymer bridging flocculation effect that is induced through Coulomb attractions at elevated temperatures. This type of flocculation has in the past been referred to as polyelectrolyte bridging.12-14a Amphiphilic copolymers containing a block of the temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAM) constitute interesting materials for adsorption onto gold nanoparticles because the affinity for adsorption can be tuned by temperature.14b It is well-known15 that high-molecular-weight PNIPAAM dissolved in water undergoes a coil-to-globule transition at a temperature exceeding its lower critical solution temperature (LCST ≈ 32 °C), although it has been reported16 that the value of the cloud point for low-molecular-weight PNIPAAM samples depends on both the length of the chains and the polymer concentration. In the present study, a charged diblock copolymer, containing PNIPAAM and a cationic block, was synthesized by atomtransfer radical polymerization (ATRP).17 The product obtained from the synthesis is poly(N-isopropylacrylamide)-block-poly(3acrylamidopropyl)trimethylammonium chloride) with the following composition: PNIPAAM24-b-PAMPTMA(+)18. The
10.1021/jp106520k 2010 American Chemical Society Published on Web 12/02/2010
Temperature-Induced Flocculation of Gold Particles
Figure 1. Illustration of the molecular weight distribution of the charged thermoresponsive PNIPAAM24-b-PAMPTMA(+)18 diblock copolymer measured in aqueous solution (0.01 M NaCl) with the aid of AFFFF. The inset shows the chemical structure of the copolymer.
chemical structure of the cationic diblock copolymer is displayed in the inset in Figure 1. This copolymer has a low molecular weight and a fairly narrow molecular weight distribution (see Figure 1). Because the polymer is charged, the tendency for self-assembly is suppressed, and higher temperatures can be reached before macroscopic phase separation takes place.18 Furthermore, the gold nanoparticles are negatively charged, and this promotes electrostatic interactions and adsorption of the oppositely charged copolymer. This work demonstrates that, by changing the temperature, contraction of the adsorbed layer can be observed, and at low concentrations, polymer bridging flocculation appears at elevated temperatures. To the best of our knowledge, we show for the first time that a low-molecularweight charged copolymer can generate a temperature-induced flocculation between gold particles through electrostatic interactions where, prior to particle aggregation, the brushes are compressed. The effect of polymer concentration on the flocculation was examined, and dynamic light scattering (DLS) measurements and cryogenic transmission electron microscopy (cryo-TEM) revealed the coexistence of single gold particles with more or less adsorbed polymer and flocs of particles. The effects of adsorbed polymer and particle flocculation on the optical properties of gold nanoparticles were investigated by means of UV-visible spectroscopy. Experimental Section Materials, Synthesis, and Solution Preparation. Aqueous suspensions of gold nanoparticles were purchased from Ted Pella, Inc., Redding, CA. The hydrodynamic radius of the gold particles measured by DLS was approximately 22 nm. These spherical particles had a narrow size distribution and were covered with a citrate layer, making them stable against aggregation over a long time (months). The particle concentration in all final solutions was 1 × 1011 particles/mL. The samples were prepared by mixing polymer solutions of various concentrations with the suspension of nanoparticles at 25 °C. All chemicals used for the synthesis of PNIPAAM24-bPAMPTMA(+)18 were purchased from Aldrich and Fluka. The cationic copolymer was synthesized by a simple one-pot twostep ATRP, which was carried out in a water/dimethylformamide (DMF) 50:50 (v/v) mixture solvent at 25 °C using ethyl 2-chloropropionate (ECP)/CuCl/CuCl2/Me6TREN {at a molar feed ratio of [NIPAAM]/[AMPTMA]/[ECP]/[CuCl]/[CuCl2]/
J. Phys. Chem. C, Vol. 114, No. 50, 2010 21961 [Me6TREN] ) 35/33/1/1/0.6/1.6, with [NIPAAM] ) 2 M} as the initiator/catalyst system. The preparation and purification procedure of the polymer was conducted under similar conditions as described in detail previously.16,18,19 The polymer was further purified by dialyzing it against distilled water for several days using a dialysis membrane of regenerated cellulose with a molecular-weight cutoff of 3500. The white solid product was finally isolated by lyophilization. Characterization of the Copolymer. The chemical structure and composition of the charged diblock copolymer were ascertained by its 1H nuclear magnetic resonance (NMR) spectrum with a Bruker AVANCE DPX 300 NMR spectrometer (Bruker Biospin, Fa¨llanden, Switzerland), operating at 300.13 MHz at 25.0 °C using heavy water (D2O) as the solvent. Asymmetric-Flow Field-Flow Fractionation (AFFFF). The AFFFF experiments20 were conducted on an AF2000 FOCUS system (Postnova Analytics, Landsberg, Germany) equipped with a refractive index (RI) detector (PN3140, Postnova) and a multiangle (seven detectors in the range of 35-145°) lightscattering detector (PN3070, λ ) 635 nm, Postnova). The PNIPAAM24-b-PAMPTMA(+)18 sample (2.0 wt % in 0.01 M NaCl) was measured using a 500-µm spacer, a regenerated cellulose membrane with a cutoff of 1000 (Z-MEM-AQU-425N, Postnova), and an injection volume of 20 µL. To minimize aggregate formation, the measurements were carried out at 10 °C. Processing of the measured data was achieved using the Postnova software (AF2000 Control, version 1.1.025). A weightaverage molecular weight of Mw ) 6080 for the sample in the dilute concentration regime was determined using this software with a Zimm-type fit and a refractive index increment (dn/dc) of 0.156 (determined using the RI detector at 32 °C). This diblock copolymer has a fairly narrow molecular weight distribution (see Figure 1) with a polydispersity index of Mw/ Mn ) 1.1. Zeta-Potential Experiments. Zeta-potential measurements were performed on a Malvern Zetasizer 2000 instrument. The experiments were carried out at several temperatures, and the equilibrium time at each temperature was ca. 1 min. Quartz Crystal Microbalance (QCM). Adsorption experiments were performed on a QCM D300 instrument from Q-sense, with gold-coated quartz crystals from Biolin Scientific AB. Adsorption of PNIPAAM24-b-PAMPTMA(+)18 was done at 25 °C, whereas flushing with solvent was performed at progressively higher temperatures up to 70 °C. Two different flushing protocols were applied: (i) While keeping the temperature of the measuring chamber constant, solvent was heated externally on an oil bath before being flushed through the measuring chamber. (ii) The temperature in the measuring chamber was raised to 44 °C (maximum temperature of the D300 microbalance) after adsorption of the polymer, the system was flushed with solvent equilibrated in the temperature loop of the instrument, and the temperature was again lowered to 25 °C. Cryogenic Transmission Electron Microscopy (CryoTEM). The cryo-TEM experiments were performed with a Zeiss EM 902A transmission electron microscope (Carl Zeiss NTS, Oberkochen, Germany). The instrument was operated at 80 kV in zero-loss bright-field mode. Digital images were recorded under low-dose conditions with a BioVision Pro-SM Slow Scan CCD camera (Proscan elektronische Systeme GmbH, Lagerlechfeld, Germany) and iTEM software (Soft Imaging System, GmbH, Mu¨nster, Germany). To enhance the image contrast, an underfocus of 2-3 µm was used. The experimental technique,
21962
J. Phys. Chem. C, Vol. 114, No. 50, 2010
Pamies et al.
instrumentation, and sample preparation have been described elsewhere,21 as has the performance of the measurements.22 Dynamic Light Scattering. Dynamic light scattering (DLS) experiments were carried out by means of an ALV/CGS-8F multidetector version compact goniometer system, with eight fiber-optical detection units, from ALV-GmbH., Langen, Germany. Further experimental details of this equipment can be found in a previous work.18 The DLS experiments were performed with a temperature gradient of 0.2 °C/min, as for the rheo-small-angle light scattering (rheo-SALS) measurements. The correlation function data were recorded continuously, with an accumulation time of 1 min. 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. For dilute polymer solutions at temperatures below the transition zone, the correlation functions could be described by a single stretched exponential
[ ( )]
g1(t) ) exp -
t τfe
β
(1)
where τfe 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
τf )
τfe 1 Γ β β
()
(1a)
where Γ(1/β) is the gamma function of β-1. At higher polymer concentrations (the gold particles are saturated and stabilized by polymer) the correlation functions can be analyzed with the aid of eq 1 over the whole considered temperature region. This relaxation mode was always found to be diffusive. For the gold nanoparticles with a small amount of adsorbed polymer, polymer bridging flocculation takes place at elevated temperatures, and two populations of species appear: single particles with more or less adsorbed polymer and flocs of particles. At this stage, the correlation function consists of two relaxation modes, one fast mode (single particles) and a slow relaxation mode (flocs). In this case, the correlation functions are described by the sum of a single exponential and a stretched exponential
g1(t) ) Af exp[-(t/τf)] + As exp[-(t/τse)γ]
(2)
with Af + As ) 1. The parameters Af and As are the amplitudes for the fast and slow relaxation times, respectively. The variables τf and τse are the relaxation times characterizing the fast and the slow process, respectively. The slow mean relaxation time is given by
and refractive index of the medium, respectively] of both the fast and slow relaxation times always revealed that both modes were diffusive (τ-1 ∝ q2). Because all of the relaxation modes are diffusive, the apparent hydrodynamic radii Rh of the single particles and flocs can be calculated through the Stokes-Einstein relationship from their respective relaxation times
Rh )
kBT 6πηD
(3)
where kB is the Boltzmann constant; T is the absolute temperature; η is the viscosity of the medium; and D is the mutual diffusion coefficient, with D ) 1/τq2. The Stokes-Einstein expression is strictly valid for spherical particles and for aggregates it can be considered as an approximation that yields an approximate size of the flocs. Rheo-Small-Angle Light Scattering. Combined rheological and small-angle light scattering (rheo-SALS) experiments during shear flow were performed simultaneously using a Paar-Physica MCR 300 rheometer, equipped with a specially designed parallel plate-plate configuration (the diameter of the plate is 43 mm, and the gap between the plates is 0.5 mm) in glass. The experimental details and a sketch of the setup have been given previously.20 The two-dimensional scattering patterns formed on the semitransparent screen were captured using a CCD camera (driver LuCam V. 3.8), positioned so that its plane was parallel to that of the screen. The images were acquired by the CCD camera with an exposure time of 200 ms. Subsequently, the images were analyzed using homemade software. The radius of gyration (Rg) was determined using the Guinier equation23
(
Rg2q2 I(q) ) I(0) exp 3
)
(4)
where I(0) is the scattered intensity as q approaches zero. This method was tested on some polystyrene (PS) latex particles of various sizes and was found to give Rg values close to the sizes given by the manufacturers. Ultraviolet-Visible Absorbance. The absorption spectra were recoded using a temperature-controlled Helios Gamma (Thermo Spectronic, Cambridge, U.K.) spectrophotometer, over wavelengths from 330 to 900 nm. The instrument is a singlebeam UV-visible spectrophotometer equipped with a temperature unit (Peltier plate) that gives a good temperature control over an extended temperature domain and time. The apparatus was computer-controlled through a homemade program. The results from the spectrophotometer are presented in terms of absorbance. Results and Discussion
τse 1 τs ) Γ γ γ
()
(2a)
The correlation functions were analyzed with a nonlinear fitting algorithm to obtain best-fit values of the parameters τfe and β in eq 1 and the parameters Af, τfe, τse, and γ appearing on the right-hand side of eq 2. Repeating controls of the q dependence [q ) (4πn/λ) sin(θ/2), with λ, θ, and n being the wavelength of the incident light in a vacuum, scattering angle,
Dynamic light scattering is a powerful technique24 for monitoring the size of mesoscopic particles and growing aggregates, as well as measuring hydrodynamic thickness of adsorbed polymer layers. The amount and layer thickness of polyions adsorbed onto oppositely charged gold nanoparticles depends on factors such as the nature of the polymer (e.g., molecular weight and hydrophobicity) and the charge densities of the polymer and particle surfaces. In addition, aspects such as the concentration and solubility of the polymer, the chemical
Temperature-Induced Flocculation of Gold Particles
Figure 2. (A) First-order correlation functions (at a scattering angle of 90°) versus time for a suspension of gold nanoparticles (Rh ≈ 22 nm) in the presence of a copolymer concentration of 0.05 wt % at the temperatures indicated. Every third point is shown. The solid curves were fitted with the aid of eqs 1 and 2 (see the text). (B) Temperature dependencies of the apparent hydrodynamic radii (see eq 3) of citratecovered gold nanoparticles in the presence of the copolymer concentrations indicated. During bridging flocculation, two relaxation modes are observed (cf. eq 2), and Rhf and Rhs represent the hydrodynamic radii for the fast and slow modes, respectively. (C) Temperature dependence of the zeta-potential for the copolymer without AuNPs at a concentration of 0.05 wt %.
affinity of the polymer to the surface, and the ionic strength of the surrounding medium play a dominant role. Figure 2A shows the correlation functions for citrate-stabilized nanoparticles in the presence of a low concentration (0.05 wt %) of the cationic copolymer at low and high temperature. At 25 °C, the decay can be described by a single stretched exponential (see eq 1). At 70 °C, the profile of the correlation function is changed, and two relaxation modes appear that can be described by eq 2. As discussed above, the fast mode represents the single gold particles with an adsorbed polymer layer, whereas the slow mode represents flocs of particles. Figure 2B illustrates the temperature dependence of the apparent hydrodynamic radius for a suspension of gold particles in the presence of different copolymer concentrations. The citratecovered gold particles (Rh ≈ 22 nm) are not affected by temperature, whereas a drastic alteration is observed for the particles with a low amount of adsorbed polymer. We first discuss the behavior of the unsaturated surfaces, where high values of Rh are observed at low temperatures. At this stage, our hypothesis is that the gold nanoparticles are sparsely covered by polyions, which are adsorbed onto the oppositely charged particle surfaces through attractive electrostatic interactions. As the temperature increases, the adsorbed polymer chains contract and/or polymer is desorbed (as discussed further below), causing a decrease in the value of Rh. At a temperature of about 60 or 72 °C (for a copolymer concentration of 0.05 or 0.075 wt %, respectively), the correlation function exhibits two relaxation modes. The two relaxation modes disclose the existence of two populations of species, namely, copolymer-covered single particles (Rhf) that coexist with the growth of flocs (Rhs). At this stage, a dramatic increase of the floc size occurs simultaneously as the polymer layers on the individual particles are compressed or desorption takes place. The flocculation feature can be rationalized in the following way: As a general rule, saturated surfaces repel each other because the crowding of polymer segments pushes them apart, and this effect is usually referred to as steric hindrance.25 The
J. Phys. Chem. C, Vol. 114, No. 50, 2010 21963 repulsion is of entropic origin, and its strength is determined by the osmotic pressure of polymer segments in the region where saturated layers overlap.25,26 When the adsorbed amount of polymer is below saturation and the adsorbent surfaces are sufficiently close, polymer bridging flocculation can occur. Usually, this happens when the polymer chains are long.27 Polymer bridging can also be established through hydrophobic interactions28 between two or more particles. Another possible mechanism if the polymer is charged and the colloid particles are oppositely charged is bridging mediated by attractive Coulombic interactions, which frequently is referred to as polyelectrolyte bridging interaction.14,29 The hypothesis for the flocculation displayed in Figure 2B is that polymer bridging is generated through attractive electrostatic interactions between the cationic copolymer chains and the available free sites on the negatively charged gold particles. This process is facilitated by the compression of the adsorbed copolymer species and/or desorption of polymer (making more sites available for adsorption) observed at elevated temperatures. In the presence of the lowest copolymer concentration, the hydrodynamic radius exhibits a plateau at temperatures above 65 °C. At this stage, most of the particles available for the formation of flocs have been consumed. The whole surface of the flocs is covered by polymer, and no free sites are available for further flocculation. At a higher polymer concentration (0.075 wt %), the bridging process occurs at higher temperatures because, at higher surface coverage, a more densely populated surface can slow the transition and a higher temperature is needed to start the flocculation process. In a previous adsorption and flocculation study30 of negatively charged silica particles in the presence of the cationic poly(diallyldimethylammonium chloride) (PDADMAC) polymer, flocculation was observed when the silica particles were unsaturated with polymer. It was argued that flocculation of silica particles in this system is governed by interaction of the cationic charged patches on one silica particle with negatively charged surface sites on another one. This mechanism is compatible with the results reported in this work. A conspicuous feature in Figure 2B is the monotonic decrease of Rh with increasing temperature, even at temperatures above the bridging flocculation stage where single particles coexist with flocs. At the highest studied temperature, the hydrodynamic layer thickness is only about 4 nm for single particles. In principle, there are two scenarios that can explain the low values of Rh at elevated temperatures, namely, desorption of polymer chains from the gold particles, and the other is a collapse of the adsorbed layer at higher temperatures. We first discuss the desorption hypothesis. In terms of thermodynamics, the electrostatic attraction between the gold particles and the oppositely charged copolymer generates a negative adsorption enthalpy (∆H < 0; exothermic process). As the temperature is raised, desorption of polymer can become a more favorable process than adsorption because enhanced hydrophobic stickiness at elevated temperatures can promote the formation of micelles and intermicellar structures in the bulk. This leads to augmented availability of sites on the gold particles for polyelectrolyte bridging flocculation. However, a finding that weakens this desorption hypothesis is that the charge density of the copolymer chains in the bulk rises strongly at higher temperatures (see Figure 2C) because the shrinking of the micelle-like clusters (caused by the enhanced hydrophobicity of PNIPAAM at elevated temperatures) forces the charged ammonium groups out to the surfaces of the copolymer moieties. Our conjecture is that this behavior will reduce the tendency for forming association complexes in the bulk, and it might also facilitate
21964
J. Phys. Chem. C, Vol. 114, No. 50, 2010
Figure 3. Temperature dependence of the (a) zeta-potential and (b) turbidity for mixtures of PNIPAAM24-b-PAMPTMA(+)18 and AuNPs at the polymer concentrations indicated.
the electrostatic attraction between particles through polyelectrolyte bridging interaction. To scrutinize the two scenarios, zeta-potential experiments were performed (Figure 3a) on mixtures of gold particles and copolymer at various temperatures. If desorption were the dominant process, it is expected that the zeta-potential should approach the value of unmodified gold particles at elevated temperatures. However, the zeta-potential rises with increasing temperature. This finding lends support to the hypothesis of the evolution of a compressed adsorbed layer at high temperatures. This is probably a result of both hydrophobic interactions in the layer and strong lateral electrostatic repulsions of the chains that lead to thin layers in which the polyions adopt a flat pancakelike conformation at elevated temperatures. Strong temperature-induced hydrophobic compaction effects have been reported18 in aqueous solutions of copolymer complexes containing PNIPAAM. Turbidimetric experiments are used to reveal the formation of large-scale association complexes in solutions or suspensions. Figure 3b shows the temperature dependence of the turbidity for bare gold particles and for 0.1 wt % solutions of PNIPAAM24b-PAMPTMA(+)18 with and without gold particles. The turbidity for the bare gold particles is not affected by temperature, whereas a sharp transition in the turbidity was found in the presence of polymer, and the cloud point (determined as the temperature where an incipient increase in the turbidity was observed) was estimated to be 59 °C. The upturn of the turbidity starts at a lower temperature for the corresponding solution without gold particles because the bulk concentration of polymer is higher than for the sample containing gold particles, which adsorb some polymer. The cloud point for a 0.05 wt % solution of the copolymer without particles is ca. 61 °C (the data is not shown here).The cloud point for a 0.05 wt % aqueous solution of a PNIPAAM homopolymer of approximately the same molecular weight (Mw ) 5350) as the present copolymer is approximately 44 °C,16 and it has been shown that the value of
Pamies et al. the cloud point depends on both the length of the PNIPAAM chain and the concentration of the polymer in the low-molecularweight range. It is frequently demonstrated18 that the value of the cloud point is higher when charges are attached to the copolymer because the tendency to form intermicellar association complexes is reduced. To distinguish between the hypotheses of desorption and compression, temperature-dependent adsorption studies using the quartz crystal microbalance (QCM) technique were performed. Here, 0.05 wt % polymer solutions were applied to planar citrate-covered gold surfaces, and adsorption was monitored as a function of time. The effect of temperature was studied by means of two approaches as described in the Experimental Section. Increasing the temperature in situ induced a large, irreversible baseline drift that completely drowned out the signalto-noise level of the adsorbed layer. Hence, it was not possible to determine the effect of temperature on the adsorbed layer. The other approachsflushing the sample chamber with an externally heated solutionsdid not result in any significant changes in the adsorbed mass, which might support the compression hypothesis. Polymer adsorbed at 25 °C (∼246 ng/ cm2) did not appear to desorb significantly upon repeated flushing at progressively higher temperatures up to 70 °C (∼230-252 ng/cm2 remaining). It should be mentioned that the QCM experimental design deviated from the other experimental techniques in this study in two respects: (i) Whereas the other experiments conserved the mass balance and thus did not induce any concentration-dependent changes in the bulk-surface partition coefficients, the sample chamber was flushed with solvent in the QCM experiments, possibly influencing the surface-bulk equilibrium. (ii) Using the QCM, adsorption was performed on planar surfaces, whereas the other techniques study highly curved surfaces. Curvature is known to affect both the amount and conformation of adsorbed macromolecules. At high surface coverage, the gold particles are electrosterically stabilized, and flocculation is not expected to take place. By comparing the hydrodynamic thicknesses of the adsorbed layers at low and high surface coverage (Figure 2B) at low temperatures, it is obvious that the adsorbed layer is thinner at a higher polymer concentration. This feature can be rationalized in the following scenario. At higher polyelectrolyte concentration (0.1 wt %), charge reversal occurs, and the charge density in the adsorbed layer increases as more polyions are adsorbed. During the adsorption process, the adsorbed polyions will attempt to neutralize any opposite charge present on the surface, and the adsorption of many polyions can lead to an overcompensation of the surface charge and, hence, charge reversal.31-33 By employing an idealized model for the adsorption of weakly charged polyelectrolyte, it was predicted34 that the connectivity between the charges of the polyion leads to an overcharge of the colloidal particle, which can adsorb a chain of total charge up to 5/2 times its own charge. Adsorbed polyions are generally oriented parallel to the surface in a flat conformation because of a strong interaction between polymer segments and the surface, as well as larger electrostatic repulsion between polymer segments. This scenario is supported by both theoretical35 and simulation36-38 studies. In addition, a recent experimental study39 on the adsorption of cationic hydroxyethylcellulose derivatives onto gold particles revealed a more compressed adsorbed layer for the polymer with the highest charge density. In view of these findings, the observed decrease of the hydrodynamic layer thickness with increasing polyelectrolyte concentration at low temperatures (Figure 2B) is ascribed to the strong lateral
Temperature-Induced Flocculation of Gold Particles
J. Phys. Chem. C, Vol. 114, No. 50, 2010 21965
Figure 4. Cryo-TEM micrographs of a suspension of gold nanoparticles in the presence of 0.05 wt % PNIPAAM24-b-PAMPTMA(+)18. The sample was maintained at (a) 25, (b) 55, and 65 °C before vitrification. The length of each scale bar represents 100 nm. It should be noticed that the magnification at 65 °C is different.
Figure 5. Temperature dependencies of the scattered intensity and scattered intensity patterns for suspensions of gold nanoparticles in the presence of 0.05 or 0.1 wt % PNIPAAM24-b-PAMPTMA(+)18. Temperature scans at a rate of 0.2 °C/min from 25 to 54 °C were performed in the absence of shear flow. After reaching 54 °C, the sample was exposed to shear forces, and the influence of shear rate (on a logarithmic scale) on the scattered intensity was monitored over a shear rate range from 1 to 50 s-1. The panels on the right-hand side display the two-dimensional patterns of the scattered intensity under quiescent conditions and at a shear rate of 10 s-1 for a suspension of gold nanoparticles in the presence of (c1,c2) 0.05 or (c3,c4) 0.1 wt % polymer at 54 °C.
repulsion that builds up in the adsorbed layer at high charge density, giving rise to a flatter conformation.40 For the highest copolymer concentration (0.1 wt %), only one relaxation mode was detected at all temperatures, and Rh increased with increasing temperature, because a thicker layer developed through enhanced hydrophobic interactions and the formation of interchain association complexes on the particle surface at elevated temperatures. In this case, Rh passed through a maximum at ca. 62 °C and dropped at higher temperatures. This suggests that, at the highest polyelectrolyte concentration, the adsorbed polymer layer has hydrophobic microdomains with a sufficient number of hydrophobic chains that contract to avoid water exposure. In previous studies18,20 on aqueous bulk solutions of uncharged and charged copolymers containing PNIPAAM, a temperature-induced collapse of interchain complexes was observed at elevated temperatures. Figure 4 displays images obtained by cryo-TEM at different temperatures; the samples were equilibrated prior to vitrification. One should bear in mind that, based on the particle concentration in solution (1 × 1011 nanoparticles/mL), the expected number of reproduced particles in the images will be, on average, 1 particle per hole in the polymer film21 for a nonflocculated solution. Consequently, this means that after huge flocs had formed, they were found only in very few positions on the grid. The micrographs representing 25 and 55 °C show individual gold particles, but it is not possible to see the corona of adsorbed copolymer as was observed in the DLS measurements. The particles appear spherical and monodisperse, with an average
radius of 14 ( 3 nm. However, at 65 °C (Figure 3c), a dramatic change of the features appears. The results disclose that huge flocs with diameters of several hundreds of nanometers formed and that these flocs coexist with single particles. These results are compatible with the DLS results, where both single particles and large flocs were detected. This micrograph is consistent with the hypothesis proposed above about polyelectrolyte bridging flocculation at high temperatures. Rheo-SALS is a technique that can provide information about structural changes of flocs on a global dimensional scale under the influence of shear flow. The conjecture is that bridging flocculation can generate rather large, weakly bound complexes, and when these flocs are exposed to high shear rates, they are anticipated to break up under the power of intensive shear stresses. By this method, it is possible to reveal the formation of large association complexes. Figure 5 shows the influence of temperature and shear flow on the scattered intensity for a suspension of gold nanoparticles in the presence of 0.05 or 0.1 wt % copolymer. In view of the fact that the results above show that flocculation occurs in the presence of the low polymer concentration, it is expected that shear flow should affect the size of the flocs. When the temperature was raised (0.2 °C/min) to 54 °C (instrumental difficulties prevented us from reaching higher temperatures for the rheo-SALS measurements) for the 0.05 wt % sample in the absence of shear flow, the intensity was virtually constant. However, when a shear rate from 1 to 50 s-1 was applied at the above indicated temperature, it was evident that the scattered
21966
J. Phys. Chem. C, Vol. 114, No. 50, 2010
intensity fell off, which was attributed to shear-induced disruption of the flocs. This hypothesis is supported by the observed drop of Rg from 99 nm in the absence of shear forces to 66 nm at a shear rate of 10 s-1. The reason this effect is not as dramatic as observed with the other techniques can probably be traced to the fairly low temperature and shear rates. The corresponding SALS images show that the overall scattered intensity decreases with increasing shear rate, and the scattered patterns in the vorticity plane are practically isotropic at the considered shear rates; the black circle at the center of each pattern is the beam stop. In the presence of 0.1 wt % polymer, the scattered intensity was found to rise as the temperature increased to 54 °C under quiescent conditions. This trend is compatible with the increase of the size of the particles as observed from the DLS measurements (cf. Figure 2B) on this sample. In the region where the shear rate increased, the scattered intensity was virtually constant over the studied shear rate domain. The SALS image was only moderately affected by the shear rate in this case. For this sample, there was actually a slight increase in Rg from 29 nm under quiescent conditions to 38 nm at a shear rate of 10 s-1. This suggests that no large association clusters formed. The higher values of Rg at γ˙ ) 10 s-1 was probably caused by a continuing adsorption of polymer onto the AuNPs when the sample was kept at 54 °C for some time (the time between the two measurements was ca. 7 min). The localized surface plasmon resonance band is a strong and broad band observed in absorption in the UV-visible spectrum for gold nanoparticles larger than 2 nm. The width and position of the surface plasmon peak are usually strongly dependent on factors such as the size and shape of the nanoparticle, the state of aggregation, the dielectric features of the metal from which the nanoparticle is composed, and the dielectric properties of the local environment (e.g., suspension medium, citrate layer, and polymer coating) in which the nanoparticles are embedded.8 The effects of copolymer (0.1 wt %) adsorption onto the gold nanoparticles on the UV-visible absorbance spectra at various temperatures are depicted in Figure 6a, and the corresponding normalized spectra (the UV-visible absorbance of the polymer is taken into account by subtracting the values of the polymer in bulk) are displayed in Figure 6b. In Figure 6a, the height of the plasmon peak is displaced with temperature, but this effect is virtually removed when the data are corrected for the absorbance of the bulk polymer in the absorbance spectra of the gold nanoparticles. The plasmon peak is centered on 520 nm (this value is usually observed in suspensions of unmodified gold particles), and practically no temperature dependence can be traced. In a recent investigation41 on suspensions of unmodified gold nanoparticles with adsorbed PNIPAAM, a temperature increase was found to give rise to a red shift of the plasmon peak, which was ascribed to incipient aggregation of the particles. For the present gold particles in a 0.1 wt % copolymer solution, no temperature-induced flocculation was observed with the different experimental techniques, and this is probably why no shift of the plasmon peak was detected (Figure 6b). To examine the effect of interparticle aggregation on the plasmon peak, the simple case of salt-induced flocculation of bare gold particles is shown in Figure 6c. As the aggregation process proceeds, two peaks are observed in the spectra. The LSPR peaks (at long wavelengths) after 24 and 48 h are much broader and significantly red-shifted compared to the peaks of single gold particles, suggesting aggregation and/or changes in the shape43 of the gold nanoclusters. For the second peak appearing at short wavelengths, a small (from 520 to 535 nm)
Pamies et al.
Figure 6. (a) Absorbance spectra and (b) normalized spectra of gold nanoparticles in the presence of 0.1 wt % PNIPAAM24-b-PAMPTMA(+)18 at the temperatures indicated. The normalized spectra were constructed by subtracting the absorbance values of the polymer in bulk solution. (c) Absorbance spectra of a suspension of gold particles with a salt concentration of 10 mM NaCl at different stages during the aggregation process. (d) Absorbance spectra of gold nanoparticles in the presence of 0.05 wt % PNIPAAM24-b-PAMPTMA(+)18 at the temperatures indicated. The inset plot shows the position of the localized surface plasmon resonance peak for this system as a function of temperature.
red shift compared to the original peak is registered, and gradually, the peak height decreases. Similar features have been reported43,44 in the literature for aggregation of gold nanoparticles. The trend in Figure 6c is consistent with the theoretical calculations by Jensen et al45 for aggregates of spherical particles. In this approach, the peak around 600 nm arises from dipole plasmon resonance, whereas the shorter-wavelength peaks come from quadrupole resonance when the aggregate size is increased. The authors found that, when two nanoparticles were sufficiently close to each other, the surface plasmon resonance peak was shifted toward longer wavelengths. The closer the two particles, the more red-shifted the peak. This effect was accompanied by the appearance of a second peak, the quadrupole resonance peak, which was located at a slightly longer wavelength than the peak for single particles, and its position and amplitude did not change much with the distance between the two particles. When a third particle was added to the cluster, the dipole plasmon resonance peak was shifted even farther toward longer wavelengths, and at the same time, its amplitude increased, indicating that larger aggregates are more red-shifted and display higher peak amplitudes than small aggregates. Larger aggregates were observed to exhibit a broader peak. The quadrupole resonance peak was located at approximately the same wavelength, but it decreased somewhat in height as the aggregate grew. For the gold particles with 0.05 wt % copolymer, a dramatic change in behavior occurred with increasing temperature (cf. Figure 6d). As the temperature was raised, the LSPR peak was gradually shifted toward longer wavelengths. The position of this peak is displayed as a function of temperature in the inset of Figure 6d. In this process, the height of the second peak (which was located close to the peak observed at 25 °C) decreased as the temperature increased. This is consistent with the picture that large Au-containing clusters are formed in this system, which is compatible with the charged-induced flocculation that was suggested above for this sample. The dipole plasmon peak is more pronounced and has a higher peak amplitude for the flocculated sample at elevated temper-
Temperature-Induced Flocculation of Gold Particles atures than was observed for the salt-induced aggregation of the citrate-covered gold particles. Theoretical studies42,45 have shown that the profile of this peak depends on factors such as the size and shape of the clusters, as well as the number of particles within the cluster and the interparticle separation of the coupled particles. For the salt-induced aggregation (after 24 h) of unmodified gold particles, DLS measurements revealed that the value of the stretched exponential γ (eq 2) is close to 1. This suggests a narrow distribution of cluster sizes. Dipole plasmon resonance peaks have previously been observed46,47 for aggregates of gold particles. In the case of the polyelectrolyte bridging flocculated particles, different numbers of particles are connected through bridges, and the morphology of the flocs of particles is expected to be different from the salt-induced aggregates of citrate-covered gold particles. In light of the theoretical approaches, the number of particles and the distance between the particles within the cluster, as well as the shape and size of the clusters, are factors that are expected to influence the height and the magnitude of the red shift of the dipole plasmon resonance peak. The difference in surface plasmon resonance characteristics between salt-induced aggregates and polymer-mediated flocs might reflect differences in factors such as interparticle spacing within the clusters and shape and size of the clusters. Actually, the average size of the flocculated species at 75 °C is Rhs ) 443 nm, which is much smaller than the Rhs ) 740 nm observed for the salt-induced aggregates after 24 h. Furthermore, the DLS experiments disclosed a value of the stretched exponent γ (eq 2) of 0.6 at 75 °C. This indicates that bridging flocculation leads to a much broader size distribution than observed for salt-induced aggregation, and this effect can also influence the plasmon resonance features.. The picture that emerges is that the flocs consist of separated particles, and the shape of the clusters can change during temperature-induced flocculation. These results indicate that UV-visible absorbance measurements can be used to monitor the appearance and progress of the flocculation process. To provide more evidence for the hypothesis that electrostatic attractions are responsible for the polymer bridging flocculation, it is a common strategy to alter the ionic strength of the system by adding salt and, thereby, screening the Coulomb interactions. However, upon addition of salt, an intricate situation evolves because the charges on both the gold particles and the polymer will be screened, and this can lead to aggregation of the particles if they are not fully covered with polymer. Furthermore competition for the surface sites between the polymer segments and small salt ions may result in desorption of the polymer from the surfaces. To circumvent these problems, we carried out UV-visible absorbance experiments on the same concentration of gold particles as reported here, but in the presence of 0.05 wt % concentration of an uncharged PNIPAAM homopolymer of approximately the same molecular weight (Mw ) 5350) as for the considered charged copolymer. The UV-visible absorbance spectra at various temperatures are depicted in Figure 7a, and the corresponding normalized spectra are displayed in Figure 7b. Again, the surface plasmon peak is centered on 520 nm, and no temperature-induced shift of the peak is detected, which indicates that no pronounced flocculation occurs at elevated temperatures. This lends support to our conjecture that electrostatic attractions are accountable for the observed flocculation phenomenon (cf. Figure 6d) reported in this work. Conclusions The experimental results from DLS, zeta-potential measurements, cryo-TEM, rheo-SALS, and UV-visible absorbance
J. Phys. Chem. C, Vol. 114, No. 50, 2010 21967
Figure 7. (a) Absorbance spectra and (b) normalized spectra of gold nanoparticles in the presence of 0.05 wt % PNIPAAM (Mw ) 5350) at the temperatures indicated.
reported in this work demonstrate that a temperature-induced electrostatic flocculation of gold nanoparticles can be accomplished in the presence of a low concentration of a lowmolecular-weight oppositely charged copolymer containing PNIPAAM. The results from DLS demonstrate that the adsorbed polymer contracts prior to flocculation and single particles and flocs coexist during the flocculation process. The conjecture is that the flocculation taking place at elevated temperatures is generated through a combination of enhanced charge density of the polymer and weakening of the steric hindrance. RheoSALS is a powerful technique for monitoring the influence of shear flow on the evolution and disruption of huge flocs; the flocculation process can be probed by observing the red shift of the LSPR peak as the temperature increases. At a higher polymer concentration, the gold particles are better stabilized, both sterically and electrostatically, and no interparticle aggregation is detected. Acknowledgment. We gratefully acknowledge support from the Norwegian Research Council for the project (177665/V30). R.P. acknowledges a postdoctoral fellowship from Fundacio´n Se´neca-CARM. B.N. thanks Prof. K. Schille´n for valuable discussions concerning the article. References and Notes (1) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078–1081. (2) Cao, Y. W. C.; Jin, R. C.; Mirkin, C. A. Science 2002, 297, 1536– 1540. (3) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293–346. (4) Stoeva, S. I.; Lee, J. S.; Thaxton, C. S.; Mirkin, C. A. Angew. Chem., Int. Ed. 2006, 45, 3303–3306. (5) Jain, P. K.; Huang, X.; El-Sayed, M. A. Acc. Chem. Res. 2008, 41, 1578–1586. (6) Melancon, M. P.; Lu, W.; Yang, Z.; Zhang, R.; Cheng, Z.; Elliot, A. M. Mol. Cancer Ther. 2008, 7, 1730–1739. (7) Moores, A.; Goettmann, F. New J. Chem. 2006, 30, 1121–1132. (8) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Sringer-Verlag: Berlin, 1995. (9) Noguez, C. J. Phys. Chem. C 2007, 111, 3806–3819. (10) Spalla, O.; Cabane, B. Colloid Polym. Sci. 1993, 271, 357–371.
21968
J. Phys. Chem. C, Vol. 114, No. 50, 2010
(11) Swenson, J.; Smalley, M. V.; Hatharasinghe, H. L. M. Phys. ReV. Lett. 1998, 81, 5840–5843. (12) Podgonik, R.; Åkesson, T.; Jo¨nsson, B. J. Chem. Phys. 1995, 102, 9423–9434. (13) Furusawa, K.; Ueda, M.; Nashima, T. Colloids Surf. A 1999, 153, 575–581. (14) (a) Podgornik, R.; Licˇer, M. Curr. Opin. Colloid Interface Sci. 2006, 11, 273–279. (b) Chakraborty, S.; Bishnoi, S. W.; Pe´rez-Luna, V. H. J. Phys. Chem. C 2010, 114, 5947–5955. (15) Zhang, G.; Winnik, F. M.; Wu, C. Phys. ReV. Lett. 2003, 90, 35506-1–035506-4. (16) (a) Pamies, R.; Zhu, K.; Kjøniksen, A.-L.; Nystro¨m, B. Polym. Bull. 2009, 62, 487–502. (b) Xia, Y.; Burke, N. A. D.; Sto¨ver, H. D. H. Macromolecules 2006, 39, 2275. (17) Matyjaszewski, K.; Xia, J. Chem. ReV. 2001, 101, 2921–2990. (18) Kjøniksen, A.-L.; Zhu, K.; Pamies, R.; Nystro¨m, B. J. Phys. Chem. B 2008, 112, 3294–3299. (19) Dedinaite, A.; Thormann, E.; Olanya, G.; Claesson, P. M.; Nystro¨m, B.; Kjøniksen, A.-L.; Zhu, K. Soft Matter 2010, 6, 2489–2498. (20) Zhu, K.; Jin, H.; Kjøniksen, A.-L.; Nystro¨m, B. J. Phys. Chem. B 2007, 111, 10862–10870. (21) Almgren, M.; Edwards, K.; Karlsson, G. Colloid Surf. A 2000, 174, 3–21. (22) Kjøniksen, A.-L.; Zhu, K.; Karlsson, G.; Nystro¨m, B. Colloid Surf. A 2008, 333, 32–45. (23) Guinier, A. Ann. Phys. 1939, 12, 161–>237. Guinier, A. ; Fournet, G.; Walker, C. B. ; Yudowitch, K. L. Small Angle Scattering of X-rays; Wiley: New York, 1955. (24) Gregory, J. AdV. Colloid Interface Sci. 2009, 147-148, 109–123. (25) Naper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983. (26) de Gennes, P. G. AdV. Colloid Interface Sci. 1987, 27, 189–209. (27) Scheutjens, J. M. H. M.; Fleer, G. J. Macromolecules 1985, 18, 1882–1900. (28) Paris, E.; Cohen Stuart, M. A. Macromolecules 1999, 32, 462– 470.
Pamies et al. (29) Abraham, T.; Christendat, D.; Xu, Z.; Masliyah, J.; Goby, J. F.; Je´roˆme, R. AIChE J. 2004, 50, 2613–2626. (30) Buchhammer, H.-M.; Petzold, G.; Lunkwitz, K. Langmuir 1999, 15, 4306–4310. (31) Stoll, S.; Chodanowski, P. Macromolecules 2002, 35, 9556–9562. (32) Samoshina, Y.; Nylander, T.; Shubin, V.; Bauer, R.; Eskilsson, K. Langmuir 2005, 21, 5872–5881. (33) Li, Z.; Wu, J. Phys. ReV. Lett. 2006, 96, 048302-1s048302-4. (34) Gurovitch, E.; Sens, P. Phys. ReV. Lett. 1999, 82, 339–342. (35) Dobrynin, A. V.; Deshkovski, A.; Rubinstein, M. Phys. ReV. Lett. 2000, 84, 3101–3104. Dobrynin, A. V.; Deshkovski, A.; Rubinstein, M. Macromolecules 2001, 29, 3421–3436. (36) Linse, P. Macromolecules 1996, 29, 326–336. (37) Kong, C. Y.; Muthukumar, M. J. Chem. Phys. 1998, 109, 1522– 1527. (38) Carrillo, J.-M. Y.; Dobrynin, A. V. Langmuir 2007, 23, 2472–2482. (39) Pamies, R.; Volden, S.; Kjøniksen, A.-L.; Zhu, K.; Glomm, W. R.; Nystro¨m, B. Langmuir 2010, 26, 15925–15932. (40) Cohen Stuart, M. A. J. Phys. (Paris) 1988, 49, 1001–1008. (41) Nash, M. A.; Lai, J. J.; Hoffman, A. S.; Yager, P.; Stayton, P. S. Nano Lett. 2010, 10, 85–91. (42) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668–677. (43) Fujiwara, H.; Yanagida, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 2589–2591. (44) Li, D.; Huang, Y.; Li, J. J. Colloid Interface Sci. 2005, 283, 440– 445. (45) Jensen, T.; Kelly, L.; Lazarides, A.; Schatz, G. C. J. Cluster Sci. 1999, 10, 295–347. (46) Khlebtsov, N. G.; Dykman, L. A.; Krasnov, Y. M.; Melnikov, A. G. Colloid J. 2000, 6, 765–779. (47) Kim, T.; Lee, C.-H.; Joo, S.-W.; Lee, K. J. Colloid Interface Sci. 2008, 318, 238–243.
JP106520K
