Preparation and optical properties of bio-dots. Preparation of the bio-dots is schematically shown in Fig. 1a.
Double-strand DNA was water-dissovled in a sealed container at temperature of 80uC or above for 12 hours or
longer and become single strand DNA firstly, followed by self-assembly and converting to bio-dots. Under
illumination at 365 nm, the bio-dots have a bright blue photoluminescence (PL) while no PL is observed for
DNA (Fig. 1b), which rules out the possibility of PL from DNA. With respect to the adsorption, DNA has a
maximum absorption peak at 260 nm, which is attributed to the presence of delocalise π electron from
heterocyclic rings of purines and pyrimidines in DNA 21 . The bio-dots have two absorption peaks, of which
one is located as same as DNA while another one is in the range of 300 to 400 nm (Fig. 1c), indicating that
they well retain the heterocyclic rings of DNA but with a different molecular structure. Fig. 1c displays bio-dots’
emission spectra under different excitation wavelengths (l ex ) ranging from 350 to 410 nm. As the excitation wavelength increases, the emission peak position of the bio-dots
displays a red-shift behavior and the emission peak intensity
decreases, exhibiting excitation-dependent PL emission.