Magnetometry and electron paramagnetic resonance studies of phosphine- and thiol-capped gold nanoparticles

Abstract

In the last years, the number of studies performed by wholly independent research groups that confirm the permanent magnetism, first observed in our research lab, for thiol-capped Au nanoparticles ͑NPs͒ has rapidly increased. Throughout the years, the initial magnetometry studies have been completed with element-specific magnetization measurements based on, for example, the x-ray magnetic circular dichroism technique that have allowed the identification of gold as the magnetic moment carrier. In the research work here presented, we have focused our efforts in the evaluation of the magnetic behavior and iron impurities content in the synthesized samples by means of superconducting quantum interference device magnetometry and electron paramagnetic resonance spectrometry, respectively. As a result, hysteresis cycles typical of a ferromagnetic material have been measured from nominally iron-free gold NPs protected with thiol, phosphine, and chlorine ligands. It is also observed that for samples containing both, capped gold NPs and highly diluted iron concentrations, the magnetic behavior of the NPs is not affected by the presence of paramagnetic iron impurities. The hysteresis cycles reported for phosphine-chlorine-capped gold NPs confirm that the magnetic behavior is not exclusively for the metal-thiol system.

Magnetic Ordering in Gold Nanoclusters

“…46 In most cases, on the other hand, analogous samples were found to be EPR-silent or showed very weak and hardly interpretable signals. 22,48 Our study takes advantage of using a cluster, Au 25 (SC2Ph) 18 0 , whose structure in the neutral state has been refined very recently 49 and whose magnetic properties in solution are well-understood. The interactions between Au 25 (SC2Ph) 18 0 clusters in the solid state were studied by using a combination of experimental and theoretical analyses.…”

Magnetism in gold nanoparticles

“…68,69 The magnetic moment can be saturated 19,26 or not. 51,70 The samples are ESR silent 71,72 or exhibit a resonance line. 67 The saturation moment is very weak 57 or very strong.…”

Unexpected magnetism in gold nanostructures: making gold even more attractive

“…In addition to polymer-encapsulated [19,54,55,60,61,78] and alkanethiol-coated nanoparticles [24,40,[44][45][46][47][48][49][51][52][53], magnetism has now been observed in Au nanoparticles coated with ligands such as thiolated sugars [41,42] and biomolecules [48,51], thiolated azobenzenes [58,59], phosphine [53], and oleic acid and/or oleic amine [43,57]. The behaviors range from permanent, ferromagneticlike at room temperature, to superparamagnetic, to paramagnetic.…”