Figure 6: Brightness comparisons of ST-QDs and BE-QDs with two-photon excitation.

(a) Measured brightness of three colours of conventional ST-QDs upon excitation at wavelengths between 700–1,000 nm. Inset shows fluorescence intensity (arbitrary units) versus excitation power (mW) plots for each QD with 780 nm excitation, with indicated log-log plot slopes (m). (b) Measured fluorescence brightness of three colours of BE-QDs upon excitation between 700 and 1,000 nm. Inset shows intensity-power plots. (c,d) Intravital multiphoton fluorescence images of a mouse mammary tumour showing fluorescence in blood vessels after intravenous injection of a mixture of green and red QDs in a 1:1 molar ratio. ST-QDs were injected into the mouse in c, n=3, and BE-QDs were injected into the mouse in d, n=3. Scale bar, 50 μm. Tumour cells expressing cyan fluorescent protein (CFP) provide contrast for interstitial tissue. (e) Measured brightness values of the red and green channels were divided for each in vivo experiment and plotted next to the corresponding ratio for in vitro values. Error bars denote standard error of measured brightness.