Magnetic Nanomaterials—From Unexpected Luminescence to Unexcpected Magnetism

Magnetic materials find use in a variety of applications, and still offer new surprises in terms of unexpected physical behaviours. Nowhere is this more prevalent than in nanomaterials. In this talk, I will present recent work on two areas of nanomagnetism, where both led to unexpcted combinations of magnetic and photophysical properties.<br/>In the first part of this talk, I will discuss our ongoing work on the design of new nanoparticulate metal-oxide nanomaterials for use as contrast agents in magnetic resonance imaging. Our focus has been in the design of contrast agents for high-field magnetic resonance imaging (MRI). We investigate several parameters, such as low-molecular weight capping ligands, magnetochemical series across mixed metal ferrites, and the effect of shape in new Mn-based agents. Multimodal probes are an asset for simplified, improved medical imaging. In particular, fluorescence and MRI are sought-after combined capabilities. Here, we show that pyrrolidin-2-one-capped manganese oxide nanoparticles (MnO^pyrr NPs) combine MRI with fluorescence microscopy to function as efficient bifunctional bionanoprobes. We employ a one-pot synthesis for ca. 10 nm MnO NPs, wherein manganese(II) 2,4-pentadionate is thermally decomposed using pyrrolidin-2-one as a solvent and capping ligand. The MnO^pyrr NPs are soluble in water without any further postsynthetic modifications. The <i>r</i>₁ relaxivity and <i>r</i>₂/<i>r</i>₁ ratio indicate that these NPs are potential <i>T</i>₁ MRI contrast agents at clinical (3 T) and ultrahigh (9.4 T) magnetic fields. Serendipitously, the as-prepared NPs are photoluminescent. The unexpected luminescence is ascribed to the modification of the pyrrolidin-2-one during the thermal treatment. MnOpyrr NPs are successfully used to enable fluorescence microscopy of HeLa cells, demonstrating bifunctional imaging capabilities. A low cytotoxic response in two distinct cell types (HeLa, HepG2) supports the suitability of MnO^pyrr NPs for biological imaging applications.<br/>In the second part of this talk, I will discuss our study of the unconventional magnetism observed in coinage metal nanomaterials. Room-temperature ferromagnetic behaviour has been reported in nanoscale materials expected to be diamagnetic, including gold. However, it is yet unclear which factors (size, shape, surface coating) predominantly influence the magnitude of the magnetic response. Our own work shows one take great care in studying, and assigning, the source of magnetism in these materials. We studied the magnetic and electronic properties of similarly-sized gold nanoparticles (Au NPs) coated with four different n-alkanethiols, as well as hydroxyl- and carboxyl-functionalized alkanethiols using superconducting quantum interference device (SQUID) magnetometry and ultraviolet photoelectron spectroscopy (UPS). We find room-temperature behaviour (hysteresis in magnetization vs. field strength loops) in all samples, as well as large effective magnetic anisotropy. Importantly, we find the nanoparticles coated with polar chain endgroups (–OH and –COOH) show markedly higher magnetization; this increased magnetization correlates with a higher work function. This work establishes chemical handles to enhance magnetism in nanoscale gold particles.