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

纳米颗粒（NPs）可以具有与本体材料的性质不一致的性质。钯和金纳米颗粒是优良的催化剂。负载在细菌上的生物制造的Pd和Au-NP在“绿色化学”（例如平台化学品的选择性氢化/氧化/生产）和清洁能源（燃料电池催化剂）中具有高催化潜力。“Bio-Pd”和“Bio-Au”-NPs是在制造它们的细菌的表面上支撑（防止聚结）的5 nm（Pd）至20-60 nm（Au，Pd）。最近，核-壳Pd/Au-NPs已经被生物制造。这些催化剂的性能优于市售催化剂。自由的Pd原子是无规的，并且在体中没有观察到自发的铁磁有序。然而，通过气体蒸发形成的Pd-NPs在平均直径为5.9 nm的NP-群体中表现出铁磁性，这归因于由于颗粒尺寸的限制而导致的非典型的金属-金属键合。铁磁性纳米Au也在文献中被要求保护。这些已发表的结论是有争议的。我们发现，生物钯纳米粒子是磁活性。磁矩/NP尺寸/催化活性相关。生物制造是NP规模可控和商业可扩展的。我们的“立场”论文（Biotech Linguistic）评估了μ子自旋旋转（muSR）作为生物纳米粒子表征工具的潜力。μ子是一种不稳定的轻子，它的磁矩是质子的3倍，是一种灵敏的微观磁力计。正μ子在间隙位置热化，并探测原子之间区域的局部磁场。ISIS同步加速器产生一束具有独特动量（29.8 MeV/c）的正μ子，100%自旋极化。μ子在样品内停止并衰变，给出正电子，优先在μ子自旋的方向上发射，使得μ子极化（或衰变不对称性）的时间演化能够通过正电子分布的时间依赖性来跟踪。因此，人们可以测量μ子信号的时间依赖性去极化，并测量样品中内部场的分布和动力学。在绝缘材料（这里是残留的细菌）中，植入的μ介子可以结合电子形成μ 鎓，类似于H点。这种活性物质可以与有机体系反应形成自由基; μ子-电子超精细耦合可以使测量的信号复杂化。我们排除了这一点。muSR先前已被应用于研究严重错位的含氢散装钯，也配体封端的钯纳米粒子在临界尺寸范围内，其中钯预计将表现出超顺磁性/铁磁性行为。我们的试点研究是第一次应用muSR作为探针的这种催化生物纳米粒子在EPSRC项目开发这些催化。这一年的PDRA流动性将通过将N.Creamer博士嵌入ISIS来培训他使用muSR，使他能够完成Pd-NP研究，并将其扩展到Bio-Au和Bio-Pd/Au-NP的研究。这将利用在母基金中开发的受控配体剥离方法。通过在muSR分析点之前去除覆盖NP的有机残留物的薄层，我们增加了在NP聚结之前获得磁性数据的机会。我们的目标是解决磁性的一个基本问题：它是归因于表面原子，体原子或两者兼而有之？我们还旨在建立硬/物质/软物质界面（生物纳米粒子之前尚未进行muSR探测）的研究，并提供Pd/Au核壳双金属的第一个muSR磁性测试，以告知其独特的化学活性。其结果将是一种新型的生物制造工具，为通过其磁畴研究颗粒内界面和表面奠定基础，通过融合生命科学，化学和硬物理学科实现。克里默博士精通前两门学科，但在第三门学科上需要培训，他有丰富的教学经验，是支持这一新学科的理想人选。

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.