Femtosecond Laser Synthesis of Carbon Dots from Amino Acids for Fluorescent Bioimaging and Sensing

Luminescent nanomaterials derived from amino acids offer a promising alternative to conventional π-conjugated fluorescent dyes for fluorescent imaging and sensing. We present a method to produce luminescent carbon dots (CD) from amino acids using high-intensity femtosecond laser pulses. Laser synthesis of CD was achieved through nonlinear absorption and femtosecond optical breakdown in an aqueous solution, which triggered polymerization and other chemical transformations of amino acids. Several types of proteinogenic amino acids with polar and nonpolar nonaromatic side chains were evaluated and yielded CD, which showed similar excitation-dependent and multicomponent visible fluorescence. At the same time, the side chain affected their synthesis rate and luminescence characteristics. Examination of CD emission revealed its similarities to nonconjugated luminescence in aggregated amino acids and peptides and thereby suggested polymerization-enhanced emission of the carbonyl moiety. Furthermore, we demonstrated pH-sensing behavior of amino acid-derived CD and their capacity to directly stain live cells for fluorescent bioimaging. Our findings underscore the potential of ultrashort laser pulse-mediated polymerization and chemical modification of amino acids as an efficient strategy to produce biocompatible nanomaterials with unconventional nonconjugated luminescence.