Fluorescence sensing of picomolar ammonia by covalent organic framework

Fluorescence sensing has exhibited its power in analytical sciences due to its sensitivity, selectivity and convenience. However, it is still challenging to further improve the sensitivity to picomolar (pM) and sub-pM level as LPRS- and SERS-enhanced electrochemical, ELISA and aptamer-based highly sensitive methods. The main obstacle lies that even if strongly absorbing fluorophores with 100% photoluminescence quantum yield, the signals it generates at pM level still cannot exceed the background noise by solvent and particle Rayleigh and Raman scattering and detector dark current and a satisfactory signal to noise ratio cannot be achieved. To overcome this issue, non-linear amplification strategies were undertaken such as enzyme catalyzed processes such as ELISA and aptamer-based fluorescence sensing. However, due to the rigorous physiological conditions enzyme required, it is highly desired to develop artificial catalytic amplification method to non-linearly enhance the weak fluorescence signals generated by the fluorophores, which responds to pM level analytes. Here, we developed a TAPA-BTD-COF, which complexes with NH3 by its C=N-H bond, and the as-formed strong interaction strengthen the C=N imine bond and fix the trans-configuration. Given the infinite extending two-dimensional network by strong covalent bond, the fixation of a single imine bond to trans-configuration leads to a domino effect rendering the C=N bond in the two-dimensional plane to be locked one by one to trans-configuration. Since trans-configuration bears stronger fluorescence than cis-configuration because of more rigidified and conjugated structures. This domino amplification effect by a single NH3 molecule drives about 10 million TAPA-BTD-COF C=N-H bond be locked due to the whole rigid two-dimensional COF structure not allowing the flexible configuration isomerization and blocked the non-radiative channel realizing unprecedented pM concentration fluorescence sensing by common chemical sensors without pursuing to biologically enzyme catalyzed amplification process such as ELISA or aptamer.