Toward Dense Biotemplated Magnetic Nanoparticle Arrays: Probing the Particle-Template Interaction

We report on experiments to elucidate the underlying mechanism of the recently observed template-directed organization of gas phase deposited FePt nanoparticles into regular two-dimensional (2D) arrays on the bacterial surface protein layer (S-layer) of Bacillus sphaericus NCTC 9602. To this end, the size and charge state of the particles have been tuned prior to their deposition onto the S-layer, and the preferred particle deposition sites have been identified and correlated to the protein template lattice by means of statistical analysis of transmission electron microscopy images of the obtained hybrid structures. The experiments reveal that the match between the nanoparticle geometry and the regularly patterned template surface morphology is most important to achieve a high degree of nanoparticle ordering. Deposited nanoparticles were preferentially located at those sites where the S-layer surface exhibits hollows of appropriate size, so that the particle can reduce its surface free energy by maximizing its contact area with the exposed S-layer surface. Particular sites at the protein layer possessed a bimodal occupation frequency distribution. This can be explained by characteristic differences in the morphologies of the inner and outer faces of the S-layer sheets immobilized at the substrate surface. Experiments with nanoparticles of different charge states did not show significant variations in the particle distribution, indicating that the occurrence of a periodic surface charge modulation at the 2D protein crystal, which is often claimed to be the origin for the self-organization of particles deposited from solution, was not the relevant driving force under the chosen experimental conditions.