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Nonlinear optical nanocrystals have been recently introduced as a promising alternative to fluorescent probes for multiphoton microscopy. We present for the first time a complete survey of the properties of five nanomaterials (KNbO3, LiNbO3, BaTiO3, KTP, and ZnO), describing their preparation and stabilization and providing quantitative estimations of their nonlinear optical response. In the light of their prospective use as biological and clinical markers, we assess their biocompatibility on human healthy and cancerous cell lines. Finally, we demonstrate the great potential for cell imaging of these inherently nonlinear probes in terms of optical contrast, wavelength flexibility, and signal photostability.

Hyper Rayleigh scattering (HRS) and second harmonic (SH) microscopy were used to study the second order nonlinear optical response of ZnO and BaTiO3nanocrystals, which have been recently proposed as new markers for bioimaging. HRS, combined with dynamic light scattering, was first used to retrieve hyperpolarizabilities and nonlinear coefficients of both type of nanocrystals. The results indicate that the main contribution to the SH signal is related to the bulk noncentrosymmetric structure of the particles. Successively, we carried out the inspection of individual nanocrystals by SH microscopy, interpreting their response using the nonlinear coefficients deduced from HRS.

A detailed experimental investigation of the formation mechanisms of acentric iron iodate [Fe(IO3)3] nanocrystals in AOT-based reverse micelles according to the water-in-oil (w/o) microemulsion (ME) composition and temperature was performed. A low chemical reaction rate was first demonstrated by means of UV−vis absorption spectroscopy. Attainment of iron iodate crystalline nanorods was then attributed to an oriented aggregation mechanism of 10−20-nm amorphous primary nanoparticles. As for the intermicellar exchange of reactants, the aggregation mechanism was also found to be determined by the ME composition because clear effects of the surfactant and reactant concentrations, the nature of the oil phase, and the water droplet size were observed. Moreover, a combination of dynamic light scattering, X-ray diffraction, and transmission electron microscopy experiments revealed that hyper-Rayleigh scattering is a fast, valuable, and nondestructive alternative to probe, in real time, the crystallization and formation dynamics of acentric nanoparticles in microemulsions. This experimental approach can be extended to studies dealing with the formation of metal nanoparticles for which scattering of second-harmonic light is readily observed.