Divergence in Cu(II)- and Zn(II)-Based Coordination Polymers: The Sensing of Melatonin and Tryptophan, and Their...

Accurate and selective recognition of ions and molecules is crucial in medical and diagnostic research. Cu(II)- and Zn(II)-based coordination polymers (CPs) have been designed in this work to detect trace levels of melatonin and tryptophan and evaluate their anticancer activity. The [Cu₂(4-bph)₂(adc)₄]_n (CP1) (4-bph = (1E,2E)-1,2-bis(pyridin-4-ylmethylene) hydrazine; Hadc = 1-adamantanecarboxylic acid) structure shows that 4-bph serves as a bridging pyridyl-N ligand and adc^- is a chelating and binuclear bridging ligand, forming an eight-membered Cu(μ-COO)₂Cu motif. In Zn(II)-CP, 4-bph is a bridging ligand, while adc^- is monodentate, yielding [Zn(4-bph)(adc)₂]_n (CP2). In CP1, π-π stacking (∼3.875 Å) and hydrogen bonding generate a 3D supramolecular network while CP2 forms pyridyl-N bridging zigzag 1D CP. The BET analysis measures higher pore volume of CP1 (0.06 cm³ g^-1) than CP2 (0.018 cm³ g^-1). The CP1 is weakly emissive, and upon irradiation at 312 nm, it emits at 392 nm which has been enhanced by the addition of tryptophan (Trp) (LOD, 44.65 nM), in the presence of 19 other amino acids. The CP1 senses melatonin (MEL) (LOD, 38 nM) also in the presence of various proteins, enzymes, and neuroactive metal ions. Blood serum is used for the measurement of melatonin in blood serum (pH 7.4) and also tryptophan measurement in milk. The CP2 is inactive toward sensing performance. DFT computation using crystallographic parameters reveals a stronger binding of CP1 with Trp (-221 kcal mol^-1) than that of CP2 (-38.22 kcal mol^-1). Anticancer assays show that CP1 is more potent than CP2 against MCF-7 breast cancer cells, IC₅₀ values are 196.8 ± 2.31 nM (CP1) and 258.2 ± 2.08 nM (CP2). Both CPs exhibit minimal toxicity toward normal PBMCs at these doses. Theoretical evaluation has also been used to explain the luminescence and selective sensing behavior to Trp and MEL.