Neuro/brain study has attracted much attention during past few years, and many optical methods have been utilized in
order to obtain accurate and complete neural information inside the brain. Relying on simultaneous absorption of two
or more near-infrared photons by a fluorophore, multiphoton microscopy can achieve deep tissue penetration and
efficient light detection noninvasively, which makes it very suitable for thick-tissue and in vivo bioimaging.
Nanoparticles possess many unique optical and chemical properties, such as anti-photobleaching, large multiphoton
absorption cross-section, and high stability in biological environment, which facilitates their applications in long-term
multiphoton microscopy as contrast agents. In this paper, we will introduce several typical nanoparticles (e.g. organic
dye doped polymer nanoparticles and gold nanorods) with high multiphoton fluorescence efficiency. We further
applied them in two- and three-photon in vivo functional brain imaging of mice, such as brain-microglia imaging, 3D
architecture reconstruction of brain blood vessel, and blood velocity measurement.