ORIGINAL PAPER Gold nanorod separation and characterization by asymmetric-flow field flow fractionation with UV–Vis detection Julien Gigault & Tae Joon Cho & Robert I. MacCuspie & Vincent A. Hackley Received: 10 August 2012 / Revised: 26 October 2012 / Accepted: 2 November 2012 # Springer-Verlag Berlin Heidelberg (outside the USA) 2012 Abstract The application of asymmetric-flow field flow fractionation (A4F) for low aspect ratio gold nanorod (GNR) fractionation and characterization was comprehen- sively investigated. We report on two novel aspects of this application. The first addresses the analytical challenge in- volved in the fractionation of positively charged nanopar- ticles by A4F, due to the interaction that exists between the negatively charged native membrane and the analyte. We show that the mobile phase composition is a critical param- eter for controlling fractionation and mitigating the membrane-analyte interaction. A mixture of ammonium nitrate and cetyl trimethyl ammonium bromide at different molar ratios enables separation of GNRs with high recovery. The second aspect is the demonstration of shape-based separation of GNRs in A4F normal mode elution (i.e., Brownian mode). We show that the elution of GNRs is due both to aspect ratio and a steric-entropic contribution for GNRs with the same diameter. This latter effect can be explained by their orientation vector inside the A4F channel. Our experimental results demonstrate the relevance of the theory described by Beckett and Giddings for non-spherical fractionation (Beckett and Giddings, J Colloid and Interface Sci 186(1):53–59, 1997). However, it is shown that this theory has its limit in the case of complex GNR mixtures, and that shape (i.e., aspect ratio) is the principal material parameter controlling elution of GNRs in A4F; the apparent translational diffusion coefficient of GNRs increases with aspect ratio. Finally, the performance of the methodology developed in this work is evaluated by the fractionation and characterization of individual components from a mixture of GNR aspect ratios. Keywords Field flow fractionation . Gold . Nanoparticle . Nanorod . Shape separation . Hyphenated technique . Elution mechanism . Positive charge Introduction Metallic nanorods have attracted increasing attention due to the ease of preparation, the large number of synthetic meth- ods available, the high uniformity achievable, and control over the aspect ratio, which in turn is primarily responsible for controlling the optical properties [2, 3]. Gold is widely recognized as the most important noble metal nanomaterial due to its unique optical response and its potential use in catalytic, biosensing, nanomedicine and electronic applica- tions [2, 4–9]. The development of well-controlled shapes and novel structures of gold nanoparticles has therefore developed into an emerging research topic in its own right. More specifically, gold nanorods (GNRs) have many fasci- nating properties and have been used in sensors, for infor- mation storage, and in a number of biomedical applications such as photothermal therapy [10–15]. While substantial research has been carried out to devel- op and utilize single GNRs and GNR ensembles as sensors, many important unexplored aspects and challenges remain before GNRs can be effectively used in practice. One of these challenges is the need to achieve sensitive and relevant in situ characterization of GNR shape and size distributions. It is in this context that advanced separation techniques coupled to multiple detection modalities can play a signifi- cant role. Chromatographic techniques, such as size Electronic supplementary material The online version of this article (doi:10.1007/s00216-012-6547-9) contains supplementary material, which is available to authorized users. J. Gigault : T. J. Cho : R. I. MacCuspie : V. A. Hackley (*) Materials Measurement Science Division, National Institute of Standards and Technology, 100 Bureau Drive, Stop 8520, Gaithersburg, MD 20899-8520, USA e-mail: vince.hackley@nist.gov Anal Bioanal Chem DOI 10.1007/s00216-012-6547-9