Functionalization of Carbon Nanoparticles and Defunctionalization-Toward Structural and Mechanistic Elucidation of Carbon Quantum Dots

Small carbon nanoparticles were functionalized by low-molecular weight amino compound 3-ethoxypropylamine (EPA) for ultracompact carbon dots of bright fluorescence emissions. The results from solution-phase NMR characterization support the expected dot structure of carbon nanoparticles surface-attached with EPA species. The optical properties of the EPA-carbon dots share many features with those found in carbon dots of other surface functionalities, but there is an extra absorption shoulder in the blue spectral region, into which the excitation results in much enhanced green fluorescence emissions. The EPA-carbon dots were defunctionalized by gradually removing the EPA species from the carbon nanoparticles, along with spectroscopic characterizations on the samples of different degrees of defunctionalization. The outcomes further confirm the critical role of surface functionalization in the superior optical properties of carbon dots. Results from systematic measurements of fluorescence quantum yields and decays at different excitation wavelengths for the EPA-carbon dots and their gradually defunctionalized samples are presented, and their mechanistic implications are discussed.