Luminescent Metal‐Organic Frameworks for Selectively Sensing Nitric Oxide in an Aqueous Solution and in Living Cells

Cu 2+ ‐based metal‐organic framework (Cu− TCA ) (H 3 TCA = tricarboxytriphenyl amine) having triphenylamine emitters was assembled and structurally characterized. Cu− TCA features a three‐dimensional porous structure consolidated by the well‐established Cu 2 (O 2 CR) 4 paddlewheel units with volume of the cavities approximately 4000 nm 3 . Having paramagnetic Cu 2+ ions to quench the luminescence of triphenylamine, Cu− TCA only exhibited very weak emission at 430 nm; upon the addition of NO up to 0.1 m M , the luminescence was recovered directly and provided about 700‐fold fluorescent enhancement. The luminescence detection exhibited high selectivity – other reactive species present in biological systems, including H 2 O 2 , NO 3 − , NO 2 − , ONOO − , ClO − and 1 O 2 , did not interfere with the NO detection. The brightness of the emission of Cu− TCA also led to its successful application in the biological imaging of NO in living cells. As a comparison, lanthanide metal‐organic framework Eu− TCA having triphenylamine emitters and characteristic europium emitters was also assembled. Eu− TCA exhibited ratiometric fluorescent responses towards NO with the europium luminescence maintained as the internal standard and the triphenylamine emission exhibited more than 1000‐fold enhancement.