Hybrid nanoparticles composed of an efficient nonlinear optical core and a gold shell can enhance and tune the nonlinear optical emission thanks to the plasmonic effect. However the influence of an incomplete gold shell, i.e., isolated gold nano-islands, is still not well studied. Here LiNbO3 (LN) core nanoparticles of 45 nm were coated with various densities of gold nano-seeds (AuSeeds). As both LN and AuSeeds bear negative surface charge, a positively-charged polymer was first coated onto LN. The number of polymer chains per LN was evaluated at 1210 by XPS and confirmed by fluorescence titration. Then, the surface coverage percentage of AuSeeds onto LN was estimated to a maximum of 30% using ICP-AES. The addition of AuSeeds was also accompanied with surface charge reversal, the negative charge increasing with the higher amount of AuSeeds. Finally, the first hyperpolarizability decreased with the increase of AuSeeds density while depolarization values for Au-seeded LN were close to the one of bare LN, showing a predominance of the second harmonic volumic contribution.

In this paper, we report a simple, facile, and efficient method to obtain LiNbO3/Au and LiNbO3/Pt nanoparticles composed of LiNbO3 NPs (LN NPs) (30 nm) and ultrasmall nanoparticles (<5 nm, seeds) of Au and Pt, respectively. The synthesis of the nanocomposites followed a simple layer-by-layer method introducing branched poly­(ethyleneimine) (BPEI) as a linker. By varying the volume of metal seeds dispersion added to the LN coated with BPEI, the loading of metal on the surface of the LN is controlled. The morphology of the as-prepared composite is characterized by transmission electron microscopy (TEM), energy-dispersive spectrometry (EDS), and dynamic light scattering (DLS). Its applicability as peroxidase mimics is investigated and compared to the separated counterparts (LN NPs, LN@BPEI, AuSeeds, PtSeeds, and a simple mixture of LN NPs and metal seeds). The synthesized nanoparticles follow the Michaelis–Menten kinetic model with enhanced catalytic activity compared to the metal seeds alone. This intrinsic peroxidase activity makes them promising candidates for applications in biomedicine. In particular, the low Michaelis constant K m and high maximum velocity V max for the H2O2 are of great interest for H2O2 sensing and generation of reactive oxygen species for antimicrobial applications.