Applying muon spin rotation to understand the magnetic behaviour of metallic bionanoparticles

点击翻译按钮获取中文摘要

Nanoparticles (NPs) can have properties at odds with those of bulk material. Palladium and gold-NPs are excellent catalysts. Biomanufactured Pd and Au-NPs supported on bacteria have high catalytic potential in 'green chemistry' (e.g. selective hydrogenations/oxidations/production of platform chemicals) and in clean energy (fuel cell catalysts). 'Bio-Pd' and 'Bio-Au'-NPs are 5nm (Pd) to 20-60 nm (Au, Pd) supported (preventing coalescence) on the surface the bacteria that made them. Recently core-shell Pd/Au-NPs have been biomanufactured. These outperform commercially available catalysts. The free Pd atom is nonmagnetic and in bulk no spontaneous ferromagnetic order is observed. However, Pd-NPs formed by gas evaporation demonstrated ferromagnetism in a NP-population with mean diameter 5.9 nm which has been attributed to non-typical metal-metal bonding due to constraints of particle size. Ferromagnetic nano-Au is also claimed in the literature. These published conclusions are controversial.We found that Bio-Pd-NPs are magnetically active. The magnetic moment/NP size/catalytic activity are related. Biomanufacturing is NP-size-controllable and commercially scalable. Our 'position' paper (Biotech Letts) evaluated the potential of muon spin rotation (muSR) as a tool for bionanoparticle characterisation. The muon, an unstable lepton, has a magnetic moment ~3x that of the proton and is a sensitive microscopic magnetometer. Positive muons thermalise at an interstitial location and probe local magnetic fields in the regions between the atoms. The ISIS synchrotron produces a beam of positive muons with a unique momentum (29.8 MeV/c), 100% spin-polarised. Muons stop within the sample and decay, giving positrons, emitted preferentially in the direction of the muon spin, enabling the time evolution of the muon polarisation (or decay asymmetry) to be followed via the time dependence of the positron distribution. Hence one can measure the time dependent depolarisation of the muon signal and characterise the distribution and dynamics of internal fields in the sample. In insulating materials (here the residual bacteria) the implanted muon may bind an electron to form muonium, akin to H-dot. This reactive species may react with organic systems to form radicals; the muon-electron hyperfine coupling can complicate the signal measured. We precluded this.muSR has been previously applied to study heavily dislocated hydrogen-containing bulk Pd and also to ligand-capped Pd-NPs in the critical size range within which Pd is expected to demonstrate super-paramagnetic/ferromagnetic behaviour. Our pilot study was the first application of muSR as a probe for such catalytic bionanoparticles within an EPSRC project to develop these for catalysis. This one year PDRA mobility will train Dr N.Creamer in the use of muSR by embedding him into ISIS, enabling him to complete the Pd-NP study, extending this also to the study of Bio-Au and Bio-Pd/Au-NPs. This will utilise a controlled ligand-stripping method developed in the parent grant. By removing the thin layer of organic residuum capping the NPs just before the point of muSR analysis we increase the chance of acquiring magnetic data before the NPs coalesce. We aim to address a fundamental problem of magnetism: is it attributable to surface atoms, bulk atoms or both? We also aim to establish this study of the hard/matter/soft matter interface (bionanoparticles have not been muSR-probed before) and also provide the first muSR magnetic testing of Pd/Au core-shell bimetallics to inform their unique chemical activities. The outcome will be a novel biomanufacturing tool to lay the foundation to study intra-particle interfaces and surfaces via their magnetic domains, enabled by fusing life sciences, chemistry and hard physics disciplines. Dr Creamer, skilled in the former two but needing training in the third, has substantial teaching experience and is ideal to champion this new subdiscipline.