Deposition of CTAB-Terminated Nanorods on Bacteria to Form Highly Conducting Hybrid Systems
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Interaction of nanoparticles with biological systems ranging from biomolecules to biological cells is of importance for a range of applications, such as high-resolution biomedical imaging,1 gene sequencing for molecular diagnostics,2 and sensitive electronic devices.3 In this report, we demonstrate that positively charged cetyltrimethylammonium bromide (CTAB), which is a stabilizing agent used to synthesize different metal nanoshapes4-7 (such as rods, spheres, cubes, prisms, stars, and hexagons), is an effective nanoparticle coating for self-assembling an electrically percolating monolayer of different nanoshapes on gram-positive bacterium, such as Bacillus cereus. The versatility of CTAB is especially realized for deposition of nanorods, where we observe 4 orders of magnitude larger conductivity compared to that of nanospheres at 3 times smaller area coverage. For the deposition on a “physical surface”, the rods do not form electrically percolating channels.8 Formation of such a percolating-conducting network on bacterium is attributed to high adhesion that overcomes steric interaction (responsible for liquid-crystalline order (see Figure 1a)), leading to random orientation (see Figure 2a). Furthermore, the strong adhesion (evidenced by conformal deposition of a rod causing bending) lowers the contact resistance, leading to 104 increases in conductivity at 13.5% area coverage compared to spheres with 41% area coverage (see Figure 3). This high conductivity is achieved well below the percolation threshold for random structure at 45% area coverage in two dimensions.9 With only �10% of bacterium surface covered, the microorganism may remain alive for a time longer than that for >40% coverage systems with nanosphere deposition.10 While CTAB alone is toxic to cells, CTAB-coated nanoparticles are nontoxic.11 Electronic coupling between nanorod monolayers with microorganisms can open the possibility of novel hybrid devices utilizing the machinery of the biological system.
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