Two-photon excitation of ethidium bromide labeled DNA

We examined the steady state and time-resolved emission of DNA stained with ethidium bromide (EB) when excited with 90 fs pulses from a mode-locked titanium sapphire laser. Over the wavelength range from 840 to 880 nm EB-DNA was found to display two-photon excitation, with a cross-section near 7 x 10(-50) cm4s/photon. Frequency-domain intensity decay measurements revealed similar multi-exponential intensity decays for one- and two-photon excitation. Time-resolved anisotropy decay measurements revealed similar correlation times, but different amplitudes as has been observed previously for two- versus one-photon excitation. These results indicate that two-photon excitation of EB-DNA can be accomplished with the fundamental output of a Ti:sapphire laser without obvious heating or perturbation of the DNA.