Fabrication of a new three-dimensional Cu(II) metal-organic framework, {[Cu4(4,4'-bipy)3(OH)2(mal)3]·4H2O}n (1a; 4,4'-bipy = 4,4'-bipyridine, H2mal = malonic acid; P21/m), that undergoes an unprecedented redox-versatile ligand-substitution-induced single-crystal to single-crystal transformation, for smartphone-based detection of iodide was studied. The Cu-MOF 1a has been effortlessly synthesized by the microwave-heating technique. Phase formation of the Cu-MOF 1a depended on counter-anions. The transformations can be triggered by halides to corresponding coordination polymers through both non-redox and redox-associated pathways. The changes in the local structure and oxidation state of copper during the transformation were studied by ex situ and in situ synchrotron X-ray absorption spectroscopies. The selectivity of the halide-triggered transformation was investigated. A study on smartphone-based colorimetric detection of iodide was found to be linearly proportional to the iodide concentration in the range 10-1500 mg/L with a limit of detection of 5 mg/L and good precision relative standard deviation of 1.9% (n = 11), possibly to construct the iodide test kit.