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



Cu2+-based metal-organic framework (Cu−TCA) (H3TCA = tricarboxytriphenyl amine) having triphenylamine emitters was assembled and structurally characterized. Cu−TCA features a three-dimensional porous structure consolidated by the well-established Cu2(O2CR)4 paddlewheel units with volume of the cavities approximately 4000 nm3. Having paramagnetic Cu2+ 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 mM, 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 H2O2, NO3, NO2, ONOO, ClO and 1O2, 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.