Fig. 2.

SHG nanoprobes signal properties. (A) Displayed is the normalized SHG signal spectrum of BaTiO₃ nanocrystals of the size of around 30 nm (Inset, SEM picture of BaTiO₃ nanocrystals) immobilized in 20% polyacrylamide gel (signal ranging from 380 to 710 nm) generated by tuning the excitation wavelength to 820 nm. (B) BaTiO₃ nanocrystal immobilized in 20% polyacrylamide gel and illuminated with low-intensity levels of 820 nm light 300 times with a scanning speed of 20 frames/s. The SHG signal intensity of BaTiO₃ is constant and does not display blinking. (C) BaTiO₃ nanocrystals were immobilized in 20% polyacrylamide gel and rapidly illuminated with gradually increasing 820-nm light intensity. The SHG signal intensity of BaTiO₃ nanocrystals increases quadratically. The dots show independent experimental results, whereas the line is a quadratic fit for BaTiO₃ (n = 4). Bar shows direct comparison of the obtained intensity range of QDs (see E) when rapidly illuminated under comparable conditions with increasing (up to 40-fold) 820-nm light intensity. (D) Water-soluble CdSe/ZnS QD immobilized in 20% polyacrylamide gel and illuminated with low-intensity levels of 820-nm light 300 times with a scanning speed of 20 frames/s. In contrast to BaTiO₃, the QD signal fluctuates displaying subblinking as well as major blinking. (E) QDs were immobilized in 20% polyacrylamide gel and rapidly illuminated with gradually increasing 820-nm light intensity. Signal saturation of the QD fluorescence occurs at very low-power levels. The dots show independent experimental results, whereas the line is a polynomial fit for QDs (n = 4).