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Control of Primary Particle Spacing in Gold Nanoparticle Clusters for Both High NIR Extinction and Full Reversibility Ehsan Moaseri, Robert J. Stover, Behzad Changalvaie, Alexis J Cepeda, Thomas M Truskett, Konstantin Sokolov, and Keith P. Johnston Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04453 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017
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Langmuir
Control of Primary Particle Spacing in Gold Nanoparticle Clusters for Both High NIR Extinction and Full Reversibility Ehsan Moaseri,† Robert J. Stover,‡ Behzad Changalvaie,† Alexis J. Cepeda,†, Thomas M. Truskett,†‡ Konstantin V. Sokolov,§ and Keith P. Johnston†‡ †
McKetta Department of Chemical Engineering, ‡Texas Materials Institute, University of Texas
at Austin, Austin, TX 78712, USA §
Department of Imaging Physics, MD Anderson Cancer Center, Houston, TX 77030, USA
Abstract Reversible NIR-active nanoparticle clusters with controlled size from 20 to 100 nm were assembled from 5nm gold nanoparticles (Au NP), with either citrate (CIT) or various binary ligands on the surface, by tuning the electrostatic repulsion and the hydrogen bonding via pH. The nanoclusters were bound together by vdW forces between the cores and the hydrogen bonds between the surface ligands and dissociated to primary nanoparticles over a period of 20 days at pH 5 and at pH 7. When high levels of citrate ligands were used on the primary particle surfaces, the large particle spacings in the nanoclusters led to only modest NIR extinction. However, a NIR extinction (E1000/525) ratio of up to ~0.4 was obtained for nanoclusters with binary ligand mixtures composed of citrate and either cysteine (CYS), glutathione (GSH) or thioctic acid zwitterion (TAZ) , while maintaining full reversibility to primary particles. The optimum ligand ratio for both an E1000/525 of ~0.4 and full reversibility decreased with increasing length of the secondary ligand (1.5/1 for CYS/CIT, 0.75/1 for GSH/CIT and 0.5/1 for TAZ/CIT) as a longer secondary ligand maintains a sufficient interparticle spacing required for dissociation more effectively. Interestingly, the zeta potential and the first order rate constant for nanocluster dissociation were similar for all three systems at the optimum ligand ratios. After incubation in 10 mM GSH solution (intracellular concentration), only the TAZ/CIT primary nanoparticles were resistant to protein opsonization in 100% fetal bovine serum, as the bidentate binding and zwitterion tips of TAZ resisted GSH exchange and protein opsonization, respectively.
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1. Introduction Gold nanoparticles (Au NPs) with high extinction in the near-infrared (NIR) window (700-1100 nm), where blood and tissue absorb weakly, 1 are of interest in biomedical imaging, in particular, photoacoustic imaging 1, 2, 3 4 and photo-thermal therapy. 5 Whereas NIR extinction is very weak for the surface plasmon resonance (SPR) of spherical Au NPs, it becomes strong for particles with asymmetry including nanorods 6, nanocages 7, nanoshells 8, and nanostars 9 and nanoroses.10 Furthermore, as two spherical Au NPs approach each other, the SPR shifts to the NIR as a consequence of hybridization of the SPRs of the individual nanoparticles, given interaction and mixing of the higher multipole oscillations.11, 12 Consequently, strong extinction has also been observed with nanoparticle clusters (nanoclusters) composed of closely-packed spherical primary Au NPs assembled in vivo 13 or in vitro.14, 15, 16, 17,18 From simulations of Maxwell’s equations, Romero et al. 19 showed the required spacing for a single dimer to reach the NIR window absorbance should be
