A nitrogen-doped nanographene molecule tetrabenzo[b,de,gh,j]-[1,10]phenanthroline (TB(phen)) was generated for selective transition metal cation sensing or as a proton fluorophore. In particular, TB(phen) fluorescence in solution could be enhanced with the peak wavelength changing depending on selective metal (e.g., Zn²⁺, Zr⁴⁺, Cd²⁺) or proton addition, so that the mixture could also be observed to undergo a significant bright emission color change under 365 nm, indicating its potential as an effective metal cation and proton modification material in stimuli-responsive organic optoelectronic materials.