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Multiresponsive Hybrid Colloids Based on Gold Nanorods and Poly(NIPAM-co-allylacetic acid) Microgels: Temperature- and pH-Tunable Plasmon Resonance Matthias Karg,†,‡ Yan Lu,‡ Enrique Carbo´-Argibay,§ Isabel Pastoriza-Santos,§ Jorge Pe´rez-Juste,§ Luis M. Liz-Marza´n,*,§ and Thomas Hellweg*,‡ Stranski-Laboratorium, Institut fu¨r Chemie, TU Berlin, Strasse des 17.Juni 124, 10623 Berlin, Germany, Physikalische Chemie I, UniVersita¨t Bayreuth, UniVersita¨tsstrasse 30, 95440 Bayreuth, Germany, and Departamento de Quimica Fisica, and Unidad Asociada CSIC-UniVersidade de Vigo, 36310 Vigo, Spain ReceiVed October 17, 2008. ReVised Manuscript ReceiVed January 9, 2009 This work describes the control and manipulation of the optical properties of multiresponsive organic/inorganic hybrid colloids, which consist of thermo-responsive poly-(NIPAM-co-allylacetic acid) microgel cores and gold nanorods assembled on their surface. These composites are multifunctional, in the sense that they combine the interesting optical properties of the rod-shaped gold particles;exhibiting two well-differentiated plasmon modes;with the sensitivity of the copolymer microgel toward external stimuli, such as temperature or solution pH. It is shown that the collapse of the microgel core, induced by changes in either temperature or pH, enhances the electronic interactions between the gold nanorods on the gel surface, as a result of the subsequent increase of the packing density arising from the surface decrease of the collapsed microgel. Above a certain nanorod density, such interactions lead to remarkable red-shifts of the longitudinal plasmon resonance.
1. Introduction A growing number of publications related to thermoresponsive microgels made of cross-linked poly(N-isopropylacrylamide) (PNIPAM) demonstrates the extraordinary interest in these organic colloids. Because of their volume phase transition stemming from the lower critical solution temperature (LCST) behavior of PNIPAM, these particles are often classified as smart materials1-5 bearing potential with respect to a large number of applications.6-9 A powerful method to influence the responsive character of the microgel is copolymerization with organic comonomers such as acrylic acid,10-14 vinylacetic acid,15,16 and allylacetic acid (AAA).17 In these particular cases, the comonomers are acids and, hence, allow one to introduce charges into the microgel network. Such an increase in network charge does * To whom correspondence should be addressed. E-mail:
[email protected] (L.M.L.-M);
[email protected] (T.H.). † TU Berlin. ‡ Universita¨t Bayreuth. § Universidade de Vigo.
(1) Pelton, R. AdV. Colloid Interface Sci. 2000, 85, 1. (2) Nayak, S.; Lyon, L. A. Angew. Chem., Int. Ed. 2005, 44, 7686. (3) Kratz, K.; Hellweg, T.; Eimer, W. Polymer 2001, 42, 6531. (4) Crowther, H. M.; Saunders, B. R.; Mears, S. J.; Cosgrove, T.; Vincent, B.; King, S. M.; Yu, G.-E. Colloids Surf. A: Physicochem. Eng. Aspects 1999, 152, 327. (5) Senff, H.; Richtering, W. J. Chem. Phys. 1999, 111, 1705. (6) Guenet, J.-M. ThermoreVersibel gelation of polymers and biopolymers; Academic Press: San Diego, 1992. (7) Antonietti, M. Angew. Chem. 1988, 100, 1813. (8) Freitas, R. F. S.; Cussler, E. L. Chem. Eng. Sci. 1987, 42, 97. (9) Hoare, T. R.; Kohane, D. S. Polymer 2008, 49, 1993. (10) Snowden, M. J.; Chowdhry, B. Z.; Vincent, B.; Morris, G. E. J. Chem. Soc., Faraday Trans. 1996, 92, 5013. (11) Morris, G. E.; Vincent, B.; Snowden, M. J. J. Colloid Interface Sci. 1997, 190, 198. (12) Kim, J.-H.; Ballauff, M. Colloid Polym. Sci. 1999, 277, 1210. (13) Kratz, K.; Hellweg, T.; Eimer, W. Colloids Surf. A 2000, 170, 137. (14) Debord, J. D.; Lyon, L. A. Langmuir 2003, 19, 7662. (15) Ho¨fl, S.; Zitzler, L.; Hellweg, T.; Herminghaus, S.; Mugele, F. Polymer 2007, 48, 245. (16) Hoare, T.; Pelton, R. Macromolecules 2004, 37, 2544. (17) Karg, M.; Pastoriza-Santos, I.; Rodriguez-Gonza´lez, B.; von Klitzing, R.; Wellert, S.; Hellweg, T. Langmuir 2008, 24, 6300.
Figure 1. Sketch of a cut through a microgel sphere covered with polyelectrolyte-coated gold nanorods (Au-rods@PSS@PAH). The temperature or pH-induced collapse of the responsive copolymer microgel leads to a decrease of the mutual space between the gold nanorods, resulting in an increase of electronic interactions and plasmon coupling.
not only affect the thermoresponsive behavior of the polymer, but also enhances its sensitivity toward changes of the solution pH and the ionic strength.13,17,18 Particular interest is currently being received by hybrid materials made of inorganic nanoparticles and thermoresponsive microgels, mostly dealing with core-shell structures19-24 or a random distribution of nanoparticles in the microgel network.25,20,26-29 In the latter case, the microgels act as a template for the formation of nanoparticles. Such composites benefit from the properties of both components, which are the stimuli-responsive character of the polymer part and, for example, interesting optical22,23 or magnetic27,30,31 properties stemming from the inorganic nanoparticles. The present work focuses on a modification of a system recently developed by us and by Kumacheva et al.,32,33 comprising hybrids (18) Hoare, T.; Pelton, R. J. Phys. Chem. B 2007, 111, 11895. (19) Karg, M.; Pastoriza-Santos, I.; Liz-Marzan, L. M.; Hellweg, T. Chem. Phys. Chem. 2006, 7, 2298. (20) Lu, Y.; Mei, Y.; Drechsler, M.; Ballauff, M. Angew. Chem. 2006, 118, 827. (21) Lu, Y.; Mei, Y.; Ballauff, M. J. Phys. Chem. B 2006, 110, 3930. (22) Kim, D. J.; Kang, S. M.; Kong, B.; Kim, W.-J.; Paik, H.-J.; Choi, I. S. Macromol. Chem. Phys. 2005, 206, 1941.
10.1021/la803458j CCC: $40.75 2009 American Chemical Society Published on Web 02/09/2009
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Table 1. Composition of Hybrid Samples a and b with Different Nanorod Loadingsa sample
VAu [µ L]
mCopolymer [mg]
λMax (15 °C)
λMax (42 °C)
λMax (60 °C)
∆λ [nm]
a b
20 30
0.10 0.10
916 916
939 955
947 971
31 55
a VAu is the added volume of Au-rod stock solution ([Au] ) 2.77 mM) to a dispersion of the poly-(NIPAM-co-AAA) microgel having a polymer mass mCopolymer. Both components were mixed in a way that the final volume was 1 mL. Also, a comparison of the longitudinal plasmon band positions of samples a and b as a function of temperature is given. The absolute red-shift between the swollen state at 15 °C and the collapsed state at 60 °C is given by ∆λ.
made of polyelectrolyte-coated gold nanorods (Aurods@PSS@PAH), statistically adsorbed on poly(N-isopropyl acrylamide)-co-poly(allylacetic acid) (poly(NIPAM-co-AAA) microgels. Figure 1 shows a scheme of the cross-section of such a hybrid microgel, with the microgel surface covered by oppositely charged gold nanorods due to electrostatic attraction.34 In the swollen state, the distance between the nanorods is much higher than in the collapsed state, when the volume of the microgel core is dramatically decreased. In contrast to previous studies on related thermo-responsive hybrids33,32,35 the present work was aimed at preparing particles with a high surface coverage. To follow the changes of the optical properties of the new system, UV-vis spectroscopy was employed, and absorbance spectra were measured at different temperatures along the transition between the swollen and the collapsed state of the polymer core. Additionally, the influence of pH changes on the optical properties of the composite colloids was monitored by means of titration combined with UV-vis spectroscopy. Both the TEM and the SEM examination of the hybrid structure were used and demonstrated nearly full surface coverage of the particles in the collapsed state (dehydrated in vacuum), which can only be reached using charged copolymer microgels. The thermoresponsive character of the pure copolymer microgel was studied by dynamic light scattering (DLS) (see also ref 17), which is an established method to follow the volume phase transition of this type of polymer-based smart colloids.4,36
2. Experimental Section Materials. N-isopropylacrylamide (NIPAM; Aldrich) was purified by recrystallization from hexane. The cross-linker agent N,Nmethylenebisacrylamide (BIS; Fluka), the comonomer AAA (Fluka) and the radical initiator potassium peroxodisulfate (KPS; Fluka) were used as received. Tetrachloroauric acid (Aldrich), silver nitrate (Aldrich), sodium borohydride (Aldrich), sodium chloride (Aldrich), ascorbic acid (Aldrich), and cetyltrimethyl ammonium bromide (CTAB; Aldrich) were used without further purification. The polyelectrolytes poly(styrene sulfonate) (PSS, Mw ) 14.9 kDa; Polymer Standards Service) and poly(allylamine hydrochloride) (PAH, Mw ) 15 kDa; Aldrich) were used as received. Milli-Q (Millipore) water was used for all solutions. Synthesis of the Copolymer Microgel. Copolymerization of NIPAM and AAA was done as described previously,17 through a (23) Suzuki, D.; Kawaguchi, H. Langmuir 2006, 22, 3818. (24) Contreras-Ca´ceres, R.; Sa´nchez-Iglesia, A.; Karg, M.; Pastoriza-Santos, I.; Pe´rez-Juste, J.; Pacifico, J.; Hellweg, T.; Ferna´ndez-Barbero, A.; Liz-Marza´n, L. M. AdV. Mater. 2008, i20, 1666. (25) Jones, C. D.; Serpe, M. J.; Schroeder, L.; Lyon, L. A. J. Am. Chem. Soc. 2003, 125, 5292. (26) Pich, A.; Karak, A.; Lu, Y.; Ghosh, A. K.; Adler, H.-J. P. Macromol. Rapid Commun. 2006, 27, 344. (27) Wong, J. E.; Gaharwar, A. K.; Mueller-Schulte, D.; Bahadur, D.; Richtering, W. J. Magnet. Magnet. Mater. 2007, 311, 219. (28) Rubio-Retama, J.; Zafeiropoulos, N. E.; Serafinelli, C.; Rojas-Reyna, R.; Voit, B.; Lopez-Cabarcos, E.; Stamm, M. Langmuir 2007, 23, 10280. (29) Pich, A. Z.; Adler, H.-J. P. Polym. Int. 2007, 56, 291. (30) Bhattacharya, S.; Eckert, F.; Boyko, V.; Pich, A. Small 2007, 3, 650. (31) Schmidt, A. M. Colloid Polym. Sci. 2007, 285, 953. (32) Karg, M.; Pastoriza-Santos, I.; Perez-Juste, J.; Hellweg, T.; Liz-Marzan, L. M. Small 2007, 3, 1222. (33) Das, M.; Sanson, N.; Fava, D.; Kumacheva, E. Langmuir 2007, 23, 196. (34) Decher, G. Science 1997, 277, 1232. (35) Das, M.; Mardoukhovski, L.; Kumacheva, E. AdV. Mater. 2008, 20, 2371. (36) Xia, X.; Hu, Z. Langmuir 2004, 20, 2094.
Figure 2. Hydrodynamic radius, Rh, and inverse swelling ratio R-1 for the poly-(NIPAM-co-AAA) microgels at pH 8 (filled symbols) and pH 10 (open symbols) (measurements were done before mixing with the nanorods). The solid lines are guides to the eye. The shown data are results from temperature-dependent DLS measurements. Further information about the volume phase transition of the copolymer microgel can be found in ref 17.
conventional emulsion polymerization (sometimes also called precipitation polymerization) as reported by Pelton and Chibante.37 A solution of 3.961 g of NIPAM, 0.270 g of BIS, and 0.350 g of AAA dissolved in 350 mL of Milli-Q water was prepared in a three-neck flask equipped with a reflux condenser and a strong stirring device. The solution was kept under nitrogen atmosphere to remove oxygen. After heating to 70 °C, the mixture was allowed to equilibrate for 30 min, and the polymerization was started by the rapid addition of a solution of 1.0 mg of KPS dissolved in water (1 mL). The initially colorless solution became turbid within 10 min, and the reaction was allowed to proceed for 4 h at 70 °C resulting in an opaque mixture. After cooling to room temperature, the dispersion was stirred overnight. Cleaning of the gel particles was done by intensive dialysis lasting 14 days with daily water exchange. This way of purification was sufficient for our experiments, even if longer polymer chains above the molecular weight cutoff of the dialysis membrane, if present, remained in the dispersion. Finally, the microgel was freeze-dried. Synthesis of the Gold Nanorods. The synthesis as well as the polyelectrolyte-coating procedure34,38 were carried out as described elsewhere.39 The dimensions of the gold nanorods (57 nm in length and 15 nm in diameter) used within this investigation were the same as those used in our previous study.32 Because of the polyelectrolytecoating with a PSS/PAH bilayer, the gold nanorods possessed a high, positive surface charge (ζ-potential of +36 mV). Preparation of the Hybrid System. The nanorod assembly on the polymer colloids was carried out as reported previously.32 A constant amount of aqueous solution of the polyelectrolyte-coated gold nanorods was added to aqueous microgel dispersions under sonication, leading to a final volume of 1 mL (see Table 1 for further information). The pH of the solution was adjusted to be close to 8. Two different surface coverages were achieved by variations of the copolymer-to-nanorod mixing ratio, and both obtained coverages are in the regime of high surface coverage as defined in previous studies.33,32 The sample with a higher coverage reaches the highest possible coverage, and excess nonadsorbed rods were already (37) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247. (38) Caruso, F.; Schu¨ler, C.; Kurth, D. G. Chem. Mater. 1999, 11, 3394. (39) Pastoriza-Santos, I.; Pe´rez-Juste, J.; Liz-Marza´n, L. M. Chem. Mater. 2006, 18, 2465.
Hybrid Colloids of Au Nanorods and Poly(MIPAM-co-AAA)
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Figure 3. Gold nanorod-loaded poly-(NIPAM-co-AAA) microgel with high surface coverage. Left: TEM image of a single hybrid particle. Middle: SEM image. Right: Tilted SEM image of hybrid particles.
Zeta potential measurements were carried out with a Malvern Zetasizer 2000 using highly dilute colloidal dispersions and were measured at 25 °C and at a solution pH of 8.
3. Results and Discussion
Figure 4. UV-vis spectra of hybrid samples a and b, having different loadings of nanorods (see Table 1 for sample information). For each composite material, spectra are shown at 15 °C (swollen state), at 42 °C, and at 60 °C (collapsed state). The dispersions have a pH of 8. ∆λT is the plasmon shift between the plasmon band position at 15 and 60 °C for the respective sample. (The dashed lines indicate the spectrum of the used nanorods. The nanorods do not change their plasmon band positions when the temperature is changed.)
observable on the TEM images obtained for this sample. A further increase of the amount of nanorods invariably lead to the formation of a brownish residue after centrifugation of the hybrids, indicating the presence of an excess of rods. The pH of the hybrid dispersions was controlled by small additions of ammonia and monitored a second time 24 h after the pH adjustment. The influence of ionic strength was neglected in the present study. Particle Characterization. The hybrid particles were imaged using transmission electron microscopy (TEM) as well as scanning electron microscopy (SEM). For the TEM observations, we employed a JEOL JEM 1010 microscope working with an acceleration voltage of 100 kV, while a JEOL JSM 6700F microscope with an acceleration voltage of 5 kV was used for the SEM investigations. The specimens for both techniques were prepared on carbon-coated copper grids as substrates. Typically a drop (7 µL) of a highly dilute dispersion of the nanorodcovered copolymer microgels in water was dried on the grid in air. The thermoresponsive behavior of the pure poly-(NIPAM-coAAA) microgel at different pH values was studied using DLS.17 Time intensity autocorrelation functions g2(τ) were recorded at a constant scattering angle of 75° using an ALV goniometer setup. As a light source we used a frequency-doubled Nd:YAG laser with a wavelength of 532 nm and an output power of 150 mW (Compass 150, Coherent, USA). The sample temperature was controlled by a Haake thermostat working with a precision of ( 0.1 K and a toluene matching bath. An ALV-5000/E multiple τ digital correlator was employed to generate g2(τ) from the measured intensity fluctuations. The final data analysis was carried out by means of inverse Laplace transformations (CONTIN)40,41 of g2(τ). Ultraviolet-visible (UV-vis) spectra were recorded employing an Agilent 8453 spectrophotometer with a temperature-controlled sample holder. The temperature was set through a thermostat giving a precision of ( 0.5 K at the sample position. (40) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213. (41) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 229.
The synthesis of poly-(NIPAM-co-AAA) microgels has been reported recently.17 This copolymer microgel is sensitive to temperature, pH, and ionic strength, since it has charged functionalities stemming from the comonomer AAA. The AAA content of the microgels used in the present article was 1.53 mol%.17 This small AAA content is nevertheless sufficient to achieve a rather high sensitivity of the particles with respect to pH, and already a small change in pH (from 8 to 10) can induce a drastic answer of the particles in terms of swelling ratio and transition temperature, as will be discussed more extensively in what follows. The swelling behavior of the pure poly(NIPAM-coAAA) microgel in aqueous dispersion at two different pH values was already studied by DLS in our previous work.17 Figure 2 summarizes the results from DLS experiments on the pure poly(NIPAM-co-AAA) microgel in aqueous dispersion (prior to nanorod assembly). In this figure, the hydrodynamic radius is given as a function of temperature. In addition, the inverse swelling ratio R-1 is also shown. The swelling ratio R is defined as the ratio between the particle volume in the collapsed Vcollapsed and the swollen state Vswollen:
R)
( )
Vcollapsed Rh(T) ) Vswollen R0
3
(1)
Rh(T) is the hydrodynamic radius at a certain temperature, and R0 is the hydrodynamic radius in the fully swollen state. For R0, the radius at T ) 15 °C is used in the present work. In case of objects with a spherical shape, the hydrodynamic radii in the respective states can be used instead of the volumes. Figure 2 reflects the volume phase transition of the used copolymer in terms of the decrease in hydrodynamic dimensions as a function of temperature. The hydrodynamic diameter at 15 °C is around 700 nm, while at 60 °C a value of 270 nm was computed from the intensity correlation functions. In the swollen state, the dimensions of the copolymer are much bigger than those for pure poly-NIPAM microgels.3 Therefore, the studied copolymer particles offer two main advantages for the preparation of the nanorod microgel hybrids. They have a higher surface charge density than the NIPAM-based homopolymer microgels, and the particle dimensions are increased compared to those of the homopolymer system, offering a larger particle surface area for the nanorod assembly. In order to obtain the surface charge of the copolymer microgels, we measured the ζ potential and found a value of -19 mV (pH 8; the Au nanorods were adsorbed under similar conditions). In case of soft, rather large polymer colloids, the
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Figure 5. The effect of pH on the optical properties of the hybrid sample a. Shown are UV-vis spectra at different temperatures, for dispersions with pH 8 and pH 10. ∆λpH is the shift in the longitudinal plasmon resonance between the hybrid colloid at pH 8 and pH 10. See Table 1 for additional information.
Figure 6. Plasmon band positions λMax for the hybrid sample b (higher nanorod coverage) as a function of pH and temperature. The filled symbols show values for hybrids at pH 8, while the open symbols were measured for dispersions at pH 10. ∆λpH is the plasmon shift between both pH values. Table 2. Effect of pH on the Swelling State of the Two Hybrid Samples Expressed in Terms of the Plasmon Band Shift Obtained for the Gold Nanorodsa temperature ∆λ (sample a) ∆λ (sample b) R (pH 8) R (pH 10) 15 °C 42 °C 60 °C
4 nm 15 mm 17 nm
4 nm 27 nm 41 nm
1.00 0.48 0.07
1.00 0.29 0.06
a ∆λ gives the difference of the plasmon band position between pH 8 and pH 10. The swelling ratios R as a result of DLS measurements are given for pH 8 and pH 10.
interpretation of the ζ potential is not an easy task.42 However, even if this value can not be taken as an absolute measure of the surface charge, it clearly shows an increase of charge compared to a pure poly-NIPAM microgel prepared with a similar amount of ionic radical initiator (-3 mV).17 The increase of the effective surface charge obtained by copolymerization is a necessary prerequisite to achieve higher surface coverages with gold nanorods compared to our previous study.32 Both the increased charge and the enlarged surface area should guarantee a higher number of adsorbed nanorods on the swollen microgels, still having a considerable interparticle distance between the rods, which is reflected by the plasmon band position at 15 °C (see Figure 4). The surface coverage of the hybrid microgels was studied by TEM and SEM, and Figure 3 reveals the achieved almost complete occupation of the negatively charged copolymer microgel surface (42) Ohshima, H. Colloid Polym. Sci. 2007, 285, 1411.
in the collapsed state (nearly dehydrated) with positively charged gold nanorods (57 × 15 nm). The particle diameter of the microgels studied by electron microscopy (ca. 350 nm) is typically by about a factor two smaller compared to the swollen state (good solvent conditions) in aqueous dispersion. This is due to the partial loss of water when the samples are exposed to vacuum in the microscopes. Hence, despite the fact that the total number of nanorods per microgel is constant, a much lower nanorod coverage is expected in the swollen state in water due to the enormous increase of the surface, and the electron microscopy images shown in Figure 3 are more representative for the situation in the collapsed state. However, this will be studied by cryogenic TEM (cryo-TEM) in the future. In the present work we focused on two different nanorod coverages in the regime of high nanorod loads (compared to our previous work). Further details about the sample composition can be found in Table 1. The optical properties of the hybrid microgels were studied by UV-vis spectroscopy. In our previous investigations32 we have shown that the thermoresponsive optical properties strongly depend on the degree of surface coverage, which can be easily controlled by the nanorod-to-microgel mixing ratio. Temperature Sensitivity. While the dashed line in Figure 4 shows the UV-vis spectrum of the free polyelectrolyte-coated gold nanorods (Au-rods@PSS@PAH) with two well-separated and pronounced plasmon bands at 511 and 857 nm, the solid lines in this figure display the spectra for the two hybrid microgels with different surface coverages, respectively. The solution pH was 8 in both cases, and, despite maintaining the characteristic features of the Au rod spectrum, a clear red-shift of the longitudinal plasmon band from 857 to approximately 915 nm occurs, which is most likely due to two combined effects: the increase of the local refractive index when the Au particles are adsorbed on the microgel and plasmon coupling when the nanorods approach each other.31 Since the hybrid microgels studied here both have rather high surface coverage, it is believed that plasmon coupling is the predominant effect responsible for the observable temperature-dependent shift. Furthermore, the higher nanorod content in sample b as compared to that in sample a is also reflected in a longitudinal plasmon band that is slightly red-shifted for b in the swollen state (917 nm vs 915 nm). A temperature-induced red-shift can be observed in both cases. Each diagram of Figure 4 shows spectra at 15 °C (swollen microgel core), at 60 °C (collapsed microgel core), and also at 42 °C. For sample a, a red-shift of 31 nm is induced by the microgel collapse. Already this value is higher than the largest shift we achieved in our previous study.32 For sample b, having
Hybrid Colloids of Au Nanorods and Poly(MIPAM-co-AAA)
the higher nanorod coverage, an even larger shift of the longitudinal plasmon band position of 55 nm was observed, induced by the core collapse. At high coverage, the predominant effect is the interparticle coupling, in fact the differences in the optical response observed during the temperature-induced microgel collapse for samples a and b arise from different plasmon coupling intensities.32,33,43,35,44-47 Additionally, Figure 4 nicely shows two other effects that occur during the volume phase transition of the copolymer core particle. First, the absorbance increases when the core size decreases, and, second, the plasmon band gets broadened. This is again in good agreement with previous results.32 It should be pointed out that, because of the reversible thermoresponsive properties of the core microgel, the temperatureinduced plasmon shifts are fully reversible. The reproducibility was checked using two independent preparations for each sample. The only problem that occurred is related to the long-time colloidal stability of the dispersions at high temperature. When the samples were kept at high temperature for longer times, precipitation started, and the obtained solid was difficult to redisperse. pH Sensitivity. The influence of changes in solution pH on the dimensions of the copolymer microgels without nanorod coating was studied by measuring DLS. In Figure 2, the swelling behavior is plotted for these two pH values, with the general observation that pH changes induce drastic changes in the copolymer microgel size, as previously reported.17 The optical properties of the respective hybrids were again studied by UV-vis spectroscopy of dispersions of the composite colloids with pH values adjusted to 8 and 10. Figure 5 shows spectra for sample a at pH 8 and 10 at three different temperatures (15, 42, and 60 °C). While the difference in the plasmon band position is low at 15 °C, much stronger effects can be observed at 42 and 60 °C. Also, here a rather strong red-shift of 17 nm as well as an increase in absorbance and a broadening of the band was found when the pH was lowered from 10 to 8. This influence is much more pronounced if the nanorod coverage of the hybrids is increased. In Figure 6, the maximum positions of the longitudinal plasmon band for sample b are given as a function of pH and temperature. At 60 °C, a remarkable red-shift of 41 nm was generated already as a consequence of rather small changes in the solvent pH. At 60 °C the difference in the swelling ratio at both pH values is smaller (0.057 vs 0.071), but considering that the microgel is close to being completely collapsed and therefore the gold nanorods are already in close proximity, a small variation in size should induce large plasmon coupling and thus give rise to a large red-shift of the plasmon band. In both cases, the red-shift in the surface plasmon band position is accompanied by band broadening as expected from plasmon (43) Gorelikov, I.; Field, L. M.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 15938. (44) Gluodenis, M.; Foss, C. A. J. Phys. Chem. B 2002, 106, 9484. (45) Thomas, K. G.; Barazzouk, S.; Ipe, B. I.; Joseph, S. T. S.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 13066. (46) Sudeep, P. K.; Joseph, S. T. S.; Thomas, K. G. J. Am. Chem. Soc. 2005, 127, 6516. (47) Vial, S.; Pastoriza-Santos, I.; Pe´rez-Juste, J.; Liz-Marza´n, L. M. Langmuir 2007, 23, 4606.
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coupling.43 The observed shifts are in good agreement with the determined swelling ratios R as found for the pure copolymer microgel (see Table 2 for further information).
4. Conclusions Charged copolymer microgels can be covered with polyelectrolyte-coated gold nanorods achieving a rather dense and homogeneous nanoparticle distribution on their surface. The maximum reachable surface coverage strongly depends on the number of charges in the network and can be controlled up to a certain limit by variations in the nanorod-to-microgel ratio.32 In order to reach a high particle density on the microgel surface, a poly-(NIPAM-co-AAA) microgel with an effective comonomer content of 1.5 mol% was used.17 Because of the incorporation of the comonomer AAA, which can be charged by dissociation of the COOH groups, a high negative charge of the microgel particles was achieved on the microgel particles, and, as a result, a high surface coverage of the final hybrid particles was obtained, as confirmed by TEM and SEM. Within this study we focused on the optical properties of hybrid particles and their changes upon collapse of the multiresponsive microgel cores. It is shown that the temperature-induced decrease of the microgel volume leads to a red-shift of the longitudinal plasmon band of the gold nanorods that can be as large as 55 nm. Compared to hybrids using pure PNIPAM microgels,32 this shift is almost a factor of 2 higher. In contrast to pure PNIPAM microgels, the used copolymer is not only sensitive to temperature, but also to changes in solution pH.17 In this work we have demonstrated that our composite microgel systems are highly sensitive toward pH changes, with a change between pH 8 and 10 leading to a drastic, fully reversible red-shift of 41 nm for the highest surface coverage studied. These strong plasmon shifts can be explained by plasmon coupling between neighboring gold nanorods, which become more pronounced when the polymer core collapses. The collapse leads to a decrease in the microgel surface area and hence, to a decrease in the distance between the adsorbed gold nanorods. This kind of hybrid materials can thus be proposed as suitable candidates for the design of novel sensors. This application would not only benefit from the thermoresponsive character of the used copolymer microgel but also from their strong response toward pH changes. An additional response toward ionic strength can also be expected for these hybrid materials. However, this was not in the focus of the present work and will be studied in the near future. Acknowledgment. This work has been supported by the Spanish Ministerio de Ciencia e Innovacio´n, through Grant Nos. MAT2007-62696 and NAN2004-09133-C03-03, Xunta de Galicia (PGIDIT06TMT31402PR), and by the Deutsche Forschungsgemeinschaft within the framework of the priority program SPP 1259. In addition, partial support of COST Action D43 is acknowledged. The authors would like to acknowledge Benito Rodriguez-Gonza´lez and Jacinto Pe´rez-Borrajo for help with the TEM and SEM experiments. LA803458J
