Spatial organization and optical properties of layer-by-layer assembled upconversion and gold nanoparticles in thin films

Multilayers of NaYF4:Yb,Tm@.NaYF4 upconversion nanoparticles (UCNPs), up to 10 UCNP/polyelectrolyte bilayers, were prepared using the layer-by-layer (LBL) method. The assembled thin films of UCNPs were found to display good stability of upconversion luminescence (UCL) upon 980 nm near-infrared (NIR) excitation. Moreover, LBL assembled films comprising four UCNP/polyelectrolyte bilayers and four gold nanoparticles (AuNP)/polyelectrolyte bilayers were prepared while adjusting their deposition order to give rise to three different spatial organizations of the two types of nanoparticles. The UV-vis-NIR extinction spectra and UCL emission spectra revealed that the spatial organization determined by the sequence of deposition influences the interaction between UCNPs and AuNPs and thus their optical properties. The alternating deposition of UCNP and AuNP layers appears to be the best way to preserve their "individual" properties, as a result of reducing both the agglomeration of AuNPs and the scattering of the 980 nm excitation light that results in an apparent quenching effect on the UCL of UCNPs. By contrast, with the same numbers of UCNP and AuNP layers, the successive deposition of UCNPs followed by the successive addition of AuNP result in more agglomeration of AuNPs leading to a broad surface plasmon resonance (SPR) band in the NIR region and also a significant quenching of the UCL intensity of UCNPs. This study demonstrates an easy and effective way to prepare NIR-responsive and plasmonic hybrid thin films with the possibility to tune their optical properties through spatially controlled organization of the nanoparticles.