Prof. Sebastian Schlücker
Faculty of Chemistry, University Duisburg-Essen and Center for Nanointegration Duisburg-Essen (CENIDE)

Gold nanoparticles (AuNP) are used in various areas, ranging from heterogeneous catalysis in chemistry to point-of-care testing in daily life (pregnancy test, Corona test etc.). Upon interaction with light, AuNP exhibit fascinating optical properties such as their reddish color. While this phenomenon was already explained theoretically by G. Mie in 1908, several fascinating properties of assemblies of AuNP still remain largely unexplored. This talk intends to give an overview of the multidisciplinary research activities in our group; during this journey both fundamentals and applications are discussed.

Gold Nanoparticle Dimers: A dimer of AuNP is the smallest system in which plasmon coupling/hybridization occurs – it is the simplest “plasmonic molecule“, conceptually very similar to molecular hydrogen. Although theory has been successful in describing the optical properties using classical electrodynamics, experimentalists have long failed to actually synthesize those computed “ideal dimers of AuNP“. We recently solved this experimental challenge and demonstrated that highly uniform optical properties result when assembling two highly spherical AuNP at very small nanometer gaps with Ångström control using molecular linkers. At very close gap distances quantum effects such as tunneling occur and we highlight that this classic-to-quantum transition depends on particle size.

Catalysis: One particular application of several noble metal NPs such as gold, palladium and platinum in heterogenous catalysis will be discussed: their catalytic activity is conventionally tested by monitoring the famous and very widely employed NaBH₄ reduction of 4-nitrophenol to 4-aminophenol using UV/vis absorption spectroscopy. Caveats of using different reaction conditions, in particular pH, for infering catalytic activity based on kinetic data, are highlighted.

Robotics: The synthesis of anisotropic AuNP such as nanorods requires training of personal and often yields diverging results when different people carried out the synthesis using the same protocol. Initial results on the automation of AuNP synthesis using a simple home-built robot are presented.

Point-of-Care Testing (POCT): The Corona pandemic has shown us, among many other things, that rapid tests based on the detection by the naked eye are not very sensitive. Our generic approach to a significantly more sensitive and quantitative detection of target molecules is based on molecularly functionalized AuNP; the latter  can be detected using surface-enhanced Raman scattering (SERS), a highly sensitive variant of normal Raman scattering with single-molecule sensitivity. Two central drawbacks have limited the use of SERS-based lateral-flow assays (rapid tests) in real-world POCT outside academic laboratories so far: the extrordinary time of many hours required for raster scanning the entire test line and the enormous prize for research-grade confocal Raman microscopes. We therefore designed and custom-built a significantly cheaper and compact instrument (SERS reader) for performing the corresponding assays within only few seconds rather than several hours to even days.

The talk closes with a short discussion of remaining challenges and opportunities in the entire field of plasmon-enhanced Raman spectroscopy and, more specifically, those ones which we would like to approach in future.