820 DOI: 10.1021/la1039249 Langmuir 2011, 27(2), 820–827 Published on Web 12/14/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Surface Plasmon Spectroscopy of Gold-Poly-N-isopropylacrylamide Core-Shell Particles Matthias Karg,* ,† Sarah Jaber, † Thomas Hellweg, ‡ and Paul Mulvaney † † School of Chemistry & Bio21 Institute, University of Melbourne, 30 Flemington Road, 3010 Victoria, Australia, and ‡ Physikalische und Biophysikalische Chemie, Universit € at Bielefeld, Universit € atsstrasse 25, 33615 Bielefeld, Germany Received September 30, 2010. Revised Manuscript Received November 22, 2010 Highly uniform, core-shell microgels consisting of single gold nanoparticle cores and cross-linked poly-N- isopropylacrylamide (PNIPAM) shells were prepared by a novel, versatile protocol. The synthetic pathway allows con- trol over the polymer shell thickness and its swelling behavior. The core-shell structure was investigated by electron microscopy and atomic force microscopy, whereas the swelling behavior of the shell was studied by means of dynamic light scattering and UV-vis spectroscopy. Furthermore, the latter method was used to investigate the optical properties of the hybrid particles. By modeling the scattering contribution from the PNIPAM shells, the absorption spectra of the gold nanoparticle cores could be recovered. This allows the particle concentration to be determined, and this in turn permits the calculation of the molar mass of the hybrid particles as well as the refractive index of the shells. Introduction A key goal in colloid science is understanding and controlling the interparticle spacing in dense colloidal systems. This spacing is extremely important in a number of nanocrystal systems because there can also be electronic coupling between the optical modes of the individual particles. It is of particular interest to fabricate systems where this spacing could be tuned or modulated. In the past, silica shells, polyelectrolytes, DNA, and polypeptides have all been employed as colloid stabilizers and spacers to determine the spacing between gold nanocrystals. However, these capping layers cannot be varied in situ. To achieve tunability of such a capping layer, we have exploited microgels as colloid coatings to control nanocrystal interactions. Microgels are colloidal gel particles with dimensions in the sub- micrometer size regime. If they can respond to external stimuli such as temperature or pH by shrinking or swelling, they are often referred to as “smart” or “intelligent” microgels. 1-10 Their interesting swelling behavior has led to applications in sensor design 7,11-13 and as separation media, 14 while more recently they have also been utilized in drug delivery 15-18 and catalysis. 19-21 In this paper, we present a simple and versatile protocol for the preparation of well-defined, core-shell hybrid microgels with gold cores. The synthesis is readily scaled up. The synthetic pathway allows the incorporation of a single gold nanoparticle core into a temperature sensitive shell of poly-N-isopropylacry- lamide (PNIPAM). PNIPAM was chosen because it is a well- known temperature responsive polymer with a volume phase transition temperature (VPTT) in the range of 32-33 °C. The thickness and hence distance of closest approach between neigh- boring nanocrystals can then be tuned through temperature. In addition, different cross-linker densities can also be used to tune the responsive character of the hybrids and the local refractive index environment of the cores. The difference in the network morphology of our particles is demonstrated schematically in Figure 1. Although gold-PNIPAM core-shell hybrid microgels have been prepared previously by precoating gold nanoparticles with polystyrene 22,23 or silica, 24 a simple approach without precoating is more practical. Without such a precoating of the gold cores, Kim et al. employed surface-initiated atom transfer radical *To whom correspondence should be addressed. E-mail: mkarg@ unimelb.edu.au. (1) Antonietti, M. Angew. Chem. 1988, 100, 1813–1817. (2) Crowther, H. M.; Saunders, B. R.; Mears, S. J.; Cosgrove, T.; Vincent, B; King, S. M.; Yu, G.-E. Colloids Surf., A 1999, 152, 327–333. (3) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247–256. (4) Scherzinger, C.; Lindner, P.; Keerl, M.; Richtering, W. Macromolecules 2010, 43, 6829–6833. (5) Hertle, Y.; Zeiser, M.; Hasen€ ohrl, C.; Busch, P.; Hellweg, T. Colloid Polym. Sci. 2010, 288, 1047–1059. (6) Karg, M.; Pastoriza-Santos, I.; Rodriguez-Gonz alez, B.; von Klitzing, R.; Wellert, S.; Hellweg, T. Langmuir 2008, 24, 6300–6306. (7) H€ ofl, S.; Zitzler, L.; Hellweg, T.; Herminghaus, S.; Mugele, F. Polymer 2007, 48, 245–254. (8) Hoare, T.; Pelton, R. Macromolecules 2004, 37, 2544–2550. (9) Pelton, R. Adv. Colloid Interface Sci. 2000, 85,1–33. (10) Kratz, K.; Hellweg, T.; Eimer, W. Colloids Surf., A 2000, 170, 137–149. (11) Guenet, J.-M. Thermoreversible gelation of polymers and biopolymers; Academic Press: San Diego, 1992. (12) Wiedemair, J.; Serpe, M. J.; Kim, J.; Masson, J. F.; Lyon, L. A.; Mizaikoff, B.; Kranz, C. Langmuir 2007, 23, 130–137. (13) FitzGerald, P. A.; Dupin, D.; Armes, S. P.; Wanless, E. J. Soft Matter 2007, 3, 580–586. (14) Freitas, R. F. S.; Cussler, E. L. Chem. Eng. Sci. 1987, 42, 97. (15) Chai, S.; Zhang, J.; Yang, T.; Yuan, J.; Cheng, S. Colloids Surf., A 2010, 356, 32–39. (16) Hoare, T. R.; Kohane, D. S. Polymer 2008, 49, 1993–2007. (17) Duracher, D.; Elaissari, A.; Mallet, F.; Pichot, C. Langmuir 2000, 16, 9002– 9008. (18) Bromberg, L.; Temchenko, M.; Hatton, T. A. Langmuir 2002, 18, 4944– 4952. (19) Wunder, S.; Polzer, F.; Lu, Y.; Mei, Y.; Ballauff, M. J. Phys. Chem. C 2010, 114, 8814–8820. (20) Lu, Y.; Wittemann, A.; Ballauff, M. Macromol. Rapid Commun. 2009, 30, 806–815. (21) Proch, S.; Mei, Y.; Villanueva, J. M. R.; Lu, Y.; Karpov, A.; Ballauff, M.; Kempe, R. Adv. Synth. Catal. 2008, 350, 493–500. (22) Contreras-C aceres, R.; Pacifico, J.; Pastoriza-Santos, I.; P erez-Juste, J.; Fern andez- Barbero, A.; Liz-Marz an, L. M. Adv. Funct. Mater. 2009, 19,1–7. (23) Contreras-C aceres, R.; Sanchez-Iglesias, A.; Karg, M.; P erez-Juste, I. P.-S. J.; Pacifico, J.; Hellweg, T.; Liz-Marz an, A. F.-B. L. M. Adv. Mater. 2009, 20, 1666–1670. (24) Karg, M.; Pastoriza-Santos, I.; Liz-Marz an, L. M.; Hellweg, T. Chem- PhysChem 2006, 7, 2298–2301.