Instant high-selectivity Cd-MOF chemosensor for naked-eye detection of Cu(II) confirmed using in situ microcalorimetry

A Cd(II) metal–organic framework, {NH2(CH3)2·Cd2.5(L)2(H2O)·(H2O)}n (1; H3L = tricarboxytriphenylamine), has been synthesized and features a 3D microporous framework with two different types of 1D channel. Addition of Cu2+ to crystals of 1 instantly leads to a clear color change from yellow to dark green crystalline samples (2), which are structurally identified as {NH2(CH3)22·Cd3Cu2(L)4(H2O)2·(H2O)2}n. In situ microcalorimetry shows that the response time of 1 to a Cu2+ concentration of 10−4 mol L−1 is as short as 9 s. Naturally, MOF 1 could potentially be used for the naked eye detection of Cu2+ ions in real-world applications. In addition, in situ microcalorimetry is successfully employed to real-time record a SC–SC guest transformation, where the guest water molecules can be selectively exchanged by methanol or N,N-dimethylacetamide (DMA) molecules (1·CH3OH and 1·DMA), and the molar enthalpies, ΔHθm, are quantitatively determined as being (−71.34 ± 0.52) and (−87.13 ± 0.86) kJ mol−1, respectively. This is the first report where the processes of SC–SC transformation and detection of Cu2+ are objectively presented.