Letter pubs.acs.org/Langmuir
pH Control of the Electrostatic Binding of Gold and Iron Oxide Nanoparticles to Tobacco Mosaic Virus Abid A. Khan,†,⊥ Eoin K. Fox,‡ Marcin Ł. Górzny,† Elizaveta Nikulina,† Dermot F. Brougham,‡ Christina Wege,§ and Alexander M. Bittner*,†,∥ †
CIC nanoGUNE Consolider, Avenida Tolosa 76, 20018 Donostia-San Sebastián, Spain School of Chemical Sciences, National Institute for Cellular Biotechnology, Dublin City University, Dublin 9, Ireland § Institute of Biology, University of Stuttgart, Pfaffenwaldring57, 70569 Stuttgart, Germany ∥ Ikerbasque, Basque Foundation for Science, 48011 Bilbao, Spain ⊥ Biosciences Department, COMSATS Institute of Information Technology, Park Road, Chak Shehzad, 44000 Islamabad, Pakistan ‡
S Supporting Information *
ABSTRACT: We report the binding of nanoparticles (NPs) to wild type (unmodified) tobacco mosaic virus (TMV). The viruses are simply mixed with citrate-coated, negatively charged gold and iron oxide nanoparticles (IONPs) in acidic solution. This results in TMV decorated along its whole length by the respective particles. Such a decoration usually requires chemical modification or mutation of TMV (e.g., cysteine residues), but here we simply reduce TMV’s natural negative charge by protonation. The particles are protonated to a much smaller extent. This charge-based mechanism does not operate for neutral particles.
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INTRODUCTION Design and synthesis of materials or devices at the nanoscale ask for excellent control over size, shape, and composition. Naturally occurring biomaterials offer here the advantage of being always well controlled in size, shape, and composition. A number of organic biotemplates such as DNA, proteins, lipids, and polysaccharides had been exploited for the synthesis of hybrid organic−inorganic structures at the nanoscale.1−4 It is very important that the selected biotemplate is physically sufficiently stable to withstand typical synthesis conditions. Tobacco mosaic virus (TMV) qualifies as an excellent candidate for this purpose.5−9 TMV is a rod-shaped plant virus. The naturally occurring wild type has a low pI (3.5), due to the negative charges of the incorporated RNA, and of the coat proteins. TMV is 300 nm long (or arranges in multiples of 300 nm), with a diameter of 18 nm. The exterior TMV surface harbors amine, hydroxyl, and carboxylate groups.10,11 TMV is used for binding, deposition, and assembly of a variety of inorganic and organic materials, for example, polymers, silica, iron oxide, nickel, and gold.8,9,12−18 Of all nanoparticles (NPs), gold is most widely used in (bio)chemistry. This preference is based on simple syntheses and on control over size, shape, and bioconjugation.19−22 The size is usually controlled by variations in the concentrations of adsorbents/coatings, such as citrate.23,24 The coating can be easily replaced for functionalization with DNA, peptides, or © 2013 American Chemical Society
antibodies, for example, for biosensors, immunoassays, optical imaging, and for photothermolysis of cancer cells.19,24−26 Iron oxide nanoparticles (IONPs), too, are staples in nanoscale science and technology. Also their synthesis has been known for a long time and is extremely well developed. Their nontoxic nature and inherent magnetism opens a unique therapeutic option with hyperthermia, based on local heating in oscillating magnetic fields.27 We here show how and why citrate-coated gold NPs and IONPs bind to wild type TMV, produced as in.28 Gold binding to TMV has already been reported;29−31 however, our method is simple and fast, and it does not require TMV mutation. We merely require pH values close to the pI, that is, 2.9−3.4. The mechanism can be explained with electrostatic arguments, on the basis of DLVO theory;32 however, the binding itself is irreversible. Our results should be very generally applicable, and offer one of the simplest routes to controlled nanoarchitectures.33
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EXPERIMENTAL METHODS
Water was produced with a Millipore Gradient A10; 18 MΩ cm,
