In Situ Monitoring of Palladium-Catalyzed Chemical Reactions byNanogap-Enhanced Raman Scattering using Single Pd Cube Dimers

The central dilemma in label-free in situ surface-enhanced Raman scattering (SERS) for monitoring of heterogeneouslycatalyzed reactions is the need of plasmonically active nanostructures forsignal enhancement. Here, we show that the assembly of catalyticallyactive transition-metal nanoparticles into dimers boosts their intrinsi-cally insufficient plasmonic activity at the monomer level by severalorders of magnitude, thereby enabling the in situ SERS monitoring ofvarious important heterogeneously catalyzed reactions at the single-dimer level. Specifically, we demonstrate that Pd nanocubes (NCs),which alone are not sufficiently plasmonically active as monomers, canact as a monometallic yet bifunctional platform with both catalytic andsatisfactory plasmonic activity via controlled assembly into single dimerswith an similar to 1 nm gap. Computer simulations reveal that the highestenhancement factors (EFs) occur at the corners of the gap, which has important implications for the SERS-based detection ofcatalytic conversions: it is sufficient for molecules to come in contact with the"hot spot corners", and it is not required that theydiffuse deeply into the gap. For the widely employed Pd-catalyzed Suzuki-Miyaura cross-coupling reaction, we demonstrate thatsuch Pd NC dimers can be employed for in situ kinetic SERS monitoring, using a whole series of aryl halides as educts. Our genericapproach based on the controlled assembly into dimers can easily be extended to other transition-metal nanostructures.