Irina Ermolina 1 * Joel Milner 2 Hywel Morgan 1 1 School of Electronics and Computer Science, University of Southampton, Southampton, UK 2 Institute of Biological and Life Sciences, University of Glasgow, Glasgow, UK Received December 16, 2005 Revised April 21, 2006 Accepted April 24, 2006 Research Article Dielectrophoretic investigation of plant virus particles: Cow Pea Mosaic Virus and Tobacco Mosaic Virus This paper reports experimental results on the dielectrophoretic (DEP) behaviour on two nonenveloped plant viruses of different geometrical shapes, namely Cow Pea Mosaic Virus (CPMV) and Tobacco Mosaic Virus (TMV). The DEP properties of car- boxy-modified latex beads of the same size are also reported. The DEP properties of single particles were obtained from measurement of the frequency at which the DEP force on a particle goes to zero (the crossover frequency). The DEP behaviour of par- ticle ensembles was also measured using image processing. The dielectric properties of the particles were evaluated from the DEP data. The surface conductance was found to be 0.3nS for CPMV, 0.38nS for TMV, and 0.52nS for 27nm diameter car- boxy–latex beads. Data analysis has shown that the optimal condition for separation of TMV and CPMV is a low-conductivity suspending medium – below 1 mS/m. Keywords: AC electrokinetics / Dielectrophoresis / Latex beads / Surface con- ductance / Virus particles DOI 10.1002/elps.200500928 1 Introduction Dielectrophoresis is a class of AC electrokinetic forces that has been widely used for the separation, manipula- tion and characterisation of bioparticles [1–6]. The origin of the force lies in differences between the dielectric properties of the particle and the surrounding suspending medium. Dielectrophoretic (DEP) is an effective method for particle manipulation within microdevices, as the for- ces are generated and controlled in situ, using micro- electrodes. Characterisation of the dielectric properties of particles is a prerequisite to the efficient design and function of these separation systems. Dielectric spec- troscopy has been used to characterise cells [7–9], DNA [10, 11] and proteins [12–14]. However, little information exists on the dielectric properties of viruses [15, 16]. The dielectric properties of bioparticles can be obtained from an analysis of their AC electrokinetic properties, using dielectrophoresis [2, 3] or electrorotation (ROT) [4, 17]. Although ROT has been successfully used to analyse bacteria and cells, it has limited use for characterising submicron particles owing to the limits of optical resolu- tion [18]. DEP has been widely used to manipulate and collect submicron particles, particularly latex beads and also to a lesser extent virus particles [19–21]. The DEP and dielectric properties of submicron latex beads have been extensively analysed [5, 19], but there is much less data concerning the properties of biological nano- particles. In this paper, the DEP properties of two virions, and latex particles of similar size is presented. The two viruses were of different geometrical shape, and were Cow Pea Mosaic Virus (CPMV) and Tobacco Mosaic Virus (TMV). The DEP properties of the particles were determined from meas- urement of the DEP crossover frequency [5, 19]. Analysis of the data was performed to obtain the dielectric pa- rameters of the viruses. In previous work [19] the DEP crossover frequency of single beads of sizes greater than 100 nm diameter was measured by observation of the behaviour of single particles using fluorescence micros- copy. In this paper, the DEP crossover frequency of the particles was determined by analysing the behaviour of an ensemble, using image processing. Correspondence: Professor Hywel Morgan, School of Electronics and Computer Science, The University of Southampton, Highfield, Southampton, SO17 1BJ England, UK E-mail: hm@ecs.soton.ac.uk Fax: 144-(0)-2380-593029 Abbreviations: CM, Clausius–Mossotti; DEP , dielectrophoretic; CPMV , Cowpea Mosaic Virus; TMV , Tobacco Mosaic Virus Electrophoresis 2006, 27, 3939–3948 3939 * Current address: School of Pharmacy, De Montfort University, Leicester, UK.  2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com