Nanoparticles in Fundamental and Applied Research

Local probing of nanoscopic matter by spectroscopy and spectromicroscopy is reviewed. Targets are free nanoparticles in the gas phase, nanoscopic matter in liquid microdroplets, deposited nanosystems, and nanoscopic matter in biological surroundings. The experimental studies are primarily performed by tunable soft X-rays, in the infrared regime, as well as complimentary radiation sources, including free electron lasers and laboratory-based laser sources.
 
Properties of free nanoparticles prepared in a narrow beam are investigated by soft X-rays. This approach has the advantage that single particles without any contact to a substrate are probed and radiation damage and charging effects are efficiently suppressed. The emission of electrons or ions is probed as a function of photon energy. Especially photoemission studies reveal distinct information on the surface composition of heterogeneous nanoparticles, indicating segregation phenomena [1]. Characteristic asymmetries in photoelectron angular distributions have been probed yielding detailed information on photoelectron elastic scattering processes allowing for a quantification of the number of elastic scattering events the photoelectrons have undergone prior to leaving the sample [2]. The dynamics of photoemission from free nanoparticles leading to processes occurring in the femto- and atto-second regimes will be briefly mentioned. This requires the use of free electron lasers and ultra-short laser pulses [3].
 
Nanoscopic matter can also be formed in levitated supersaturated and supercooled microdroplets for investigating nucleation processes in metastable liquids. Structural properties of pre-nucleation clusters are identified by a combination of near-edge spectroscopy and molecular dynamics calculations [4]. In the role of excess charges on the nucleation of liquid microdroplets has been evaluated, since these influence massively the nucleation processes [5].
 
 
Finally, topical drug delivery into skin probed by label-free spectromicroscopy is reported. The role of drug formulations and polymeric nanocarriers as efficient drug transport vehicles is evaluated regarding their penetration into deeper skin layers [6, 7]. Selective and high spatial resolution detection of drugs and drug nanocarriers is accomplished by X-ray microscopy and complementary methods, such as atomic force microscopy-based spectroscopic approaches in the infrared regime [8, 9] and stimulated Raman  microscopy [10]. Recent results on the penetration of the anti-inflammatory drug dexamethasone are reported, where the drug is topically applied to human and murine skin samples ex vivo, reaching a spatial resolution below 10 nm.