Plasmonic Photocatalysis Driven by Indirect Gold Excitation Via Upconversion Nanoparticle Emission Monitored In Situ by Surface- Enhanced Raman Spectroscopy

Lanthanide-doped upconversion nanoparticles (UCNPs) absorb low-energy photons, in the near-infrared region (NIR), and emit high-energy photons, in the visible or ultraviolet region (UV/Vis). The higher energy radiation emissions can be used to promote plasmonic photocatalytic reactions by coupling UCNPs with plasmonic nanoparticles. Thus, in this work, highly crystalline beta- NaYF4:Yb3+,Er3+@NaYF4:Nd3+@NaYF4 UCNPs capped with an amino-modified nanosized silica shell were coupled with Au nanospheres (13 +/- 2 nm). The plasmonic-driven dimerization of 4-aminothiophenol (4-ATP) to 4,4 '-dimercap-toazobenzene (DMAB) was carried out solely by using the UCNP visible emissions to excite the Au nanoparticle localized surface plasmon resonance band. The reaction was monitored in situ through surface-enhanced Raman spectroscopy (SERS) and optimal conditions were achieved in order to eliminate secondary influences during spectra acquisition. The SERS substrates containing either UCNPs@SiO2-NH2/Au or SiO2-NH2/Au nanoparticles were prepared by the drop-cast method in silicon substrates. In the studied reaction conditions, plasmonic photocatalytic activity was observed only in the substrates containing UCNPs and Au nanoparticles. The plasmonicdriven reaction was achieved by UCNPs excitation with a 980 nm laser (25.7 mW), aligned in a Raman spectrometer, only after 5 s of exposition. Interactive document mapping (IDMAP), which is an unsupervised pattern recognition method, was applied to analyze the numerous SERS spectra acquired during the plasmonic photocatalytic measurements. The high values found for the silhouette coefficient (0.88 for the Si_SiO2-NH2/Au and 0.76 for the Si_NYF@SiO2-NH/Au substrate) indicate that strong discrimination among the Raman spectra was achieved. Thus, through IDMAP, it was observed that the SERS spectra obtained for the substrates containing either UCNPs@SiO2-NH2/Au or SiO2-NH2/Au corresponded to spectral features, respectively, from the DMAB product and the 4-ATP reagent, evidencing that, in the optimized reaction conditions, the dimerization occurred only through the interaction of the UCNPs with the Au nanospheres.