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

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.

The immediate impact of this one year postdoctoral mobility study will be the training of Dr Neil Creamer in the use of muon spin rotation spectroscopy (experimental design, beam line equipment and its operation, data handling and interpretation] as he moves from his parent disciplines of microbiology (~20 years) and solid state chemistry (~10 years) into physics. This will be achieved via his embedment at ISIS and immersion in the muSR technique. During the one year he will also learn about pulsed low energy muons and gain sufficient expertise to be able to formulate bids for beam time access and help others from the Life Science disciplines to do so.
Dr Creamer will also carry out a 'reverse discipline hop' training Anna Williams, a physics graduate and first year PhD student in the EPSRC DTC 'Hydrogen Fuel Cells and their Applications' (biomanufactured metallic-NPs for fuel cell catalytic applications). He will train Anna to make the bio-NPs and carry out SQUID analysis on them, followed by specialist muon training on the beam line, with an introductory session in July 2011 (beam time allocated already) and a follow up slot planned for early 2012 (to be applied for). Dr Creamer will help Anna to formulate a joint bid for beam time so that after the one year PDRA mobility with his help she will be able to pursue and complete this part of her PhD. With demonstrated unexpected durable activity of bio-NPs in fuel cells Anna is keen to uncover the root of this activity and stability at the electronic level.
In a similar way, Prof G. Attard is a surface scientist with a strong interest in the exact nature of reactive metallic surfaces (and interfaces) with respect to catalytic applications, including chirality. Attard will participate actively in the muon beam slot and Dr Creamer will help him to explore ways in which the use of muons can expand the capability of his catalysis group at Cardiff.
Scientifically, we may not achieve all of the objectives within one year (and we will resist the contemporary pressures to disseminate until we are sure that our conclusions are correct) but we realistically expect to achieve a high impact via the demonstration of new methodology that could help to settle the ongoing controversy of the root of ferromagnetism: are all atoms involved or just those at the surface of a NP?; certainly NPs may show ferromagnetism but this question cannot be answered experimentally at present
We also report the controlled synthesis of bimetallic Pd/Au core shell NPs that largely has eluded chemical manufacturing and we hope to maintain this global lead by being the first to obtain magnetic information about them. Given that the superparamagnetic/ferromagnetic status of even single metallic NPs is still conjectural it is clear that a bimetallic will compound this problem and nothing at all is known about the magnetic domain at this core/shell interface or at the surface where the catalytic uniquness lies. We will not make inroads into the actual use of pulsed low energy muons to unveil these phenomena but hope to develop the experimental system by which this could be done- opening, then, a potential marriage of muSR and spintronics, which could have a high impact to physics and electronics academic groups
For many applications thin films are required and it is worth mentioning that the use of a biofilm, extensively developed by the PI, gives the means to manufacture thin films/arrays of metallic NPs and even pattern them using the tools of molecular biology. Such a level of sophistication is hard to achieve by chemical synthesis and, if proved possible in follow-up work, would take biomanufacturing towards layered devices and into the electronics communities and beyond into applications