Structurally tunable small signaling molecules have been specifically designed to probe-free formaldehyde concentrations as low as 0.1 ppm, establishing the utility of probing the exposure limit for safe human consumption according to the guidelines suggested by WHO. The substrate, 2-aminobenzoylhydrazide, fluorescent core skeleton was designed for the construction of diverse quinazoline compounds with combinatorial substituent-pending potentials via rapid condensation reactions with aldehyde groups. After identifying the fluorescent species by (HNMR)-H-1 and X-ray crystallography, we demonstrate that the fluorescent emission is substituent dependent and that, by simple structural variation, the fluorescence can be tunable through controlling internal charge transfer (ICT) within a dye platform. Finally, by introducing more electron-rich ferrocene into the substrate to further undermine the ICT process, we demonstrate highly sensitive formaldehyde sensing through dual signal output, including fluorescence and redox potential.