Using bacterial cell growth to template catalytic asymmetryw

The use of biological structures to template inorganic materials has become an increasingly widespread strategy to develop otherwise synthetically inaccessible hierarchical structures under mild chemical conditions, for use as catalytic and device materials. For example, proteins, peptides, DNA, viruses, cells and multicellular structures have been employed as templates to develop inorganic particles, wires, and optical devices. While most of these approaches have treated the biological structure as a static template, the development of strategies to recruit biological processes (e.g., cell growth and differentiation) to direct active templating/positioning would greatly expand the pool of synthetic outcomes and potential applications for a given biotemplate. Here, we report an approach to (1) metalize the bacterial cell envelope and (2) segregate metallic regions by exploiting the polarity of the cell envelope undergoing growth and division. This strategy is applied to the synthesis of asymmetric catalytic micro-particles that are capable of ‘bacterial-like’ swimming behavior in the presence of chemical fuel. Fig. 1A describes the approach used to metalize the bacterial cell envelope. Similar to previous work, positively charged gold nanoparticles (AuNPs) deposit on the bacterial cell envelope via electrostatic interactions with negatively charged moieties [predominantly carboxyl and phosphate groups] located on the cell envelope. In a typical experiment, E. coli cells were incubated with AuNP seeds (1.4 nm diameters, see ESIw for experimental details) at a ratio B10 per bacterial cell for B20 minutes at room temperature. AuNPs bound to the cell envelope catalyze reduction of aqueous metal ions (e.g., Au, Ag, Pt) in the presence of a reducing agent, leading to particle growth that can be visualized as opaque areas on cells using optical microscopy (Fig. 1B). Thus, autometallographic development of AuNPs indicates their location on the cell surface. Development of AuNPs using Au and Pt immediately following nanoparticle incubation with stationary phase cells showed AuNP binding occurs homogenously over the whole cell envelope across the population (499% of cells) to produce hollowmetallic shells (S1, ESIw). X-ray diffraction (XRD) of cell-templated platinum shells showed sharp peaks attributed to face-centered cubic (fcc) platinum particles of ca. 5 nm in size (S2, ESIw). Cell-templated gold shells displayed resistivities nearing that of bulk gold, and the length of the wire could be modified using antibiotic treatment (S3, ESIw). The use of whole cells as templates for inorganic materials produces, in general, structures with relatively larger feature sizes as compared to other studies that have used cell constituents (e.g., DNA, peptides, microtubules, etc.) as templates. Yet, employing viable microorganisms to fabricate/template device materials may offer a number of advantages, for instance, to organize a desired functionality on a bio-template by exploiting cell processes (e.g., growth), engineering cell behaviors to achieve desired morphologies as well as to enable the self-assembly of functionalized biotemplates using innate or engineered environmental responses (e.g., chemotaxis of motile cells). These strategies require that aspects of the synthesis must be compatible with cell viability. Therefore, we investigated the effects of AuNPs bound to the cell envelope on cell behavior. Fig. 1C shows AuNP Fig. 1 Biocompatible catalyst seeding and subsequent metallization of bacterial cells. (A) AuNPs bind to the bacterial cell envelope via electrostatic interactions. AuNPs catalyze metallization of the cell envelope via reduction of solution ions of Au or Pt. (B; scale bars, 1 mm) Optical microscopy image of an AuNP targeted E. coli before (upper left panel) and after (lower left panel) metallization with gold. (Upper right panel) SEM of a platinum metallized bacterium (lower right panel; EDS map of Pt). (C) E. coli seeded with AuNPs (labeled, ‘S’) and recovered via centrifugation (left inset) display no significant difference in growth rates and chemotaxis properties (right inset) compared to controls (labeled ‘C’). (D) Zeta potential of an E. coli population, maintained in growth media, following incubation of AuNPs (blue). Red line shows first 4 points of the ‘seeded’ exponential growth curve shown in C.