LIU ET AL. VOL. 7 ’ NO. 7 ’ 5901–5910 ’ 2013 www.acsnano.org 5901 June 20, 2013 C 2013 American Chemical Society In Situ Plasmonic Counter for Polymerization of Chains of Gold Nanorods in Solution Kun Liu, †,‡,r, * Aftab Ahmed, §,r Siyon Chung, ‡ Kota Sugikawa, ‡ Gaoxiang Wu, ‡ Zhihong Nie, ^ Reuven Gordon, §, * and Eugenia Kumacheva ‡, ) ,#, * † State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China, ‡ Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada, § Department of Electrical and Computer Engineering, University of Victoria, Victoria, BC V8W 3P6, Canada, ^ Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States, ) Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada, and # The Institute of Biomaterials and Biomedical Engineering, University of Toronto, 4 Taddle Creek Road, Toronto, Ontario M5S 3G9, Canada. r These authors contributed equally. S elf-assembly of nanoparticles attracts great interest for several reasons. First, in the course of self-assembly, nanopar- ticles act as artificial molecules, thereby offer- ing the ability to create simple and complex model systems for their molecular analogues undergoing a chemical reaction. 1À12 For example, self-assembly of nanoparticles into small clusters, chiral structures, or polymer- like chains can be exploited to study reaction mechanisms, kinetics, and thermodynamics, as well as the formation of isomers. 13À20 This new direction of research necessitates the development of new, efficient tools for quan- titative characterization of self-assembly of nanoparticles in solution. Plasmonic nanostructures offer a very beneficial property that can be used for their structural characterization, that is, the coupling of the plasmonic properties of neighboring nanoparticles. 21À25 For poly- mer chains formed from plasmonic nano- particles, the red shift in the localized surface plasmon resonance band depends on chain degree of polymerization 26,27 and offers a conceptually new strategy for poly- mer characterization, in comparison with con- ventional Gel Permeation Chromatography, Nuclear Magnetic Resonance, or viscosity techniques used for molecular polymers. 28 We stress the importance of utilization of ensemble-averaged plasmonic properties that describe the entire “reaction” system, while single particle spectroscopy provides information about each particular nano- structure but cannot be used for such a * Address correspondence to kliu@jlu.edu.cn, rgordon@uvic.ca, ekumache@chem.utoronto.ca. Received for review March 13, 2013 and accepted June 20, 2013. Published online 10.1021/nn402363p ABSTRACT Self-assembly of gold nanorods (NRs) in linear, polymer-like chains offers the ability to test and validate theoretical models of molecular polymerization. Practically, NR chains show multiple promising applications in sensing of chemical and biological species. Both areas of research can strongly benefit from the development of a quantitative tool for characterization of the structure of NR chains in the course of self-assembly, based on the change in ensemble-averaged optical properties of plasmonic polymers; however, quantitative correlation between the extinction spectra and the structural characteristics of NR chains has not been reported. Here, we report such a tool by a quantitatively correlating the red shift of the longitudinal surface plasmon band of gold NRs and the average aggregation number of NR chains. The generality of the method is demonstrated for NRs with different aspect ratios, for varying inter-rod distances in the chains, and for varying initial concentrations of NRs in solution. We modeled the extinction spectra of the NR chains by combining the theory of step-growth polymerization with finite-difference time-domain simulations and a resistor-inductor-capacitor model, and obtained agreement between the theoretical and experimental results. In addition to capturing quantitatively the ensemble physics of the polymerization, the proposed 'plasmonic counter' approach provides a real-time cost- and labor-efficient method for the characterization of self-assembly of plasmonic polymers. KEYWORDS: self-assembly . plasmonic polymers . gold nanorods . aggregation number . extinction spectra . simulations ARTICLE