Second-Harmonic Generation of Halloysite Nanotubes for Bioimaging

This Letter reports the first characterization of nonfluorescent halloysite nanotubes (HNTs) by multiphoton microscopy via their endogenously generated second-harmonic (SH) resonance. The unveiled discovery that HNTs generate a SH signal in the nanoscale regime is supported by studies of the signal’s propagation, polarization, and coherence properties and, finally, is directed toward the understanding of the origin of signal’s generation from the nanoparticle’s aluminosilicate skeleton. The study is significant, since it sheds light on halloysite nanotube technological applications in various domains, including alternative bioimaging, biosensing, and noninvasive biomedicine, as well as possibly nonlinear spectroscopy, optoelectronics, and many others.