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

Abstract

A Cd(II) metal–organic framework, {NH₂(CH₃)₂·Cd_2.5(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, ΔH^θ_m, 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.