702 2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim DOI: 10.1002/smll. 200500021 small 2005, 1, No. 7, 702–706 communications of positioning on the nanoscale,[7, 8] by using a particle that can either interface with lithographically defined structures, or undergo further self-assembly into extended structures by itself.[9, 10] As a test case for proving the efficacy of this approach, we built 3D conductive molecular networks using cowpea mosaic virus (CPMV) as a scaffold. CPMV is an icosahedral particle made of 60 copies of a protein subunit, with a spherically averaged diameter of 30 nm (Figure 1a). Its structure has been determined by X-ray crystallography to a resolution of 2.8.[11] CPMV has been genetically engineered, resulting in the production of a large number of mutants.[12, 13] Its icosahedral symmetry allows the generation of specific patterns of functional residues, which provide a means to assemble complex structures with high spatial specificity on the nanometer scale. In its natural state, CPMV contains no thiol-containing cysteine residues on the capsid exterior. We have genetically engineered CPMV to present cysteine residues at selected positions, which allow us to anchor gold nanoparticles that are subsequently interconnected by molecular wires to create a 3D conducting network on the nanoscale. The properties of the network can be changed by altering the position and hence the pattern of cysteine residues on the capsid surface. The current work utilizes two different CPMV mutants, designated EF and DM. The EF mutant has a single cysteine (Figure 1b) inserted into the protein subunit as a GGCGG loop,[14] while the DM mutant (Figure 1c) has two cysteines inserted per subunit, replacing alanine and glutamic acid residues at positions 235 and 2319, respectively. Gold nanoparticles were bound to these engineered cysteine residues to produce patterns in three dimensions with specific interparticle distances (Figure 1d and e).[15, 16] Viruses decorated with gold nanoparticles (EF with 5 nm particles and DM with 2 nm particles) were exposed to molecules with thiol end groups to produce a conductive network on the virus (Figure 1 f and g), which we refer to as a viral nanoblock (VNB). The two molecules chosen for this study, 1, 4-C6H4 [trans-(4-AcSC6H4C CPt-(PBu3) 2C C] 2 (di-Pt) and oligophenylenevinylene (OPV), for which detailed I/V characterization is available,[17–20] are shown in Table 1. Due to the interparticle spacing, the network on the EF mutant (EF-VNB) was formed using both OPV and di-Pt molecules, while the network on the DM mutant (DM-VNB) was formed from OPV alone. Molecular attachment was confirmed with fluorescence spectroscopy, using the emission of OPV at 457 nm (see Supporting Information, Figure 1). Although it is possible for the gold–sulfur bond to dissociate or exchange with other thiol compounds, we find that the gold nanoparticles are well-attached to the viral scaffold, and do not detach from the virus in the presence of the molecules. Any weakly bound gold nanoparticles are more likely to be removed by the electric field during electrophoresis for purification or electroelution for recovery.[16] Furthermore, while the location of the cysteines on the capsid determines the positions of the gold nanoparticles, the bound particles are likely stabilized by interactions with the surrounding amino acids of the capsid, as colloidal gold has also been used as a nonspecific protein label for electron microsco-