Magnetic Decoration of Escherichia coli Loaded with Palladium Nanoparticles

Palladium (Pd) is a scarce metal, which is able to catalyze a variety of important chemical reactions. In the bulk form, Pd is anomalous paramagnetic, but in the form of nanoparticles, it has been reported as ferromagnetic. Detecting the magnetism of Pd nanoparticles is, however, difficult due to their reduced dimension. In addition, the ferromagnetic nature is a size-dependent property and, for Pd, the strength of magnetism decreases with slight increments in size. A “green” methodology for the preparation of small Pd nanoparticles was followed in this study. It is based on the ability of bacteria to take up Pd2+ ions from its surrounding solution and to enzymatically reduce it to metallic Pd0 nanoparticles. The efficiency of the production can be determined by transmission electron microscopy and, as a new technique, magnetic force microscopy. Notwithstanding, the study of nanoparticles of the size of just a few nanometers with these techniques is still difficult. Here we present a methodology for the enhancement of the magnetic signal of biologically produced Pd-based nanoparticles through the decoration of Escherichia coli bacteria cross-sections with Fe nanoparticles. This methodology allows the visualization of bacteria that are loaded with magnetic nanoparticles even when conventional transmission electron microscopy has difficulties to resolve them inside the microorganisms.