Multimodal optical microscopy in combination with gold nanorods for cancer cell imaging.

The multimodal optical imaging technique, which utilizes nonlinear and linear optical processes, plays an important role in biological and biomedical research. As second-order nonlinear phenomenon, the two-photon luminescence (TPL) results from the nonlinear excitation of fluorescent molecules, while the second harmonic generation (SHG) depends on the second order nonlinear polarization, orientation, and noncentrosymmetric properties of molecules. In contrast, the linear resonance light scattering (RLS) occurs when the molecules are excited by a light beam with a wavelength close to their absorption bands. Since SHG, TPL, and RLS involve different kinds of optical processes, they might be used in parallel to provide complementary information about the structure and function of cells and tissues. Herein, we develop for the first time a multimodal optical microscopy with the capability of simultaneous SHG, TPL, and RLS imaging. We analyze theoretically and demonstrate experimentally the near-infrared irradiation-induced SHG, TPL, and RLS from the gold nanorods with nanometer spatial resolution. With the gold nanorods as the contrast agents, we further demonstrate the simultaneous SHG, TPL, and RLS imaging of A431 human epithelial skin cancer cells. This multimodal optical microscopy might provide a reliable and complementary approach for biological and biomedical research.