Mixed-ligand based water-stable Mn(ii)-MOF for quick, sensitive, and reusable IFE-PET-RET facilitated detection of formaldehyde and Cr(vi)-oxoanions in real-field samples like food and industrial water: experimental and theoretical insights

Abstract

We report the luminescence-based detection of Group-1 carcinogen formaldehyde (FA) and Cr(VI)-oxoanions with a mesoporous Mn(II)-MOF (1), featuring a uninodal 4-c net topology and linear 1D square channels forming a polymeric 2D network. The Mn-MOF i.e., [Mn(phen)(hia)(H₂O)]_∞ was solvothermally constructed using π-conjugated, chelating phenanthroline (phen) and µ³–η²:η¹ binding 5-hydroxyisophthalic acid (hia) ligands. The 2D rod-like crystallites of 1 demonstrated excellent phase purity, high thermal and photostability, and robustness under harsh conditions. The SCXRD and XPS studies established the structural framework and elemental composition, while the Hirshfeld surface analysis and NCI-RDG plot confirmed the presence of π–π stacking and weak interactions in 1. We explored the bright-blue emission of 1 for recyclable and fast-responsive (∼70 s) ‘turn-off’ detection of FA, with a low limit of detection (LOD) of 8.49 µM. Based on this, a 04-input-03-output molecular logic gate was proposed, which can be useful as a molecular switch for future applications. Furthermore, a unique experimental setup using the MOF film demonstrated ∼57% quenching upon exposure to FA vapor (an indoor VOC). Additionally, 1 exemplified itself as an efficient probe towards Cr(VI)-oxyanions, depicting LODs of 79 and 170 ppb, Stern–Volmer constants (K_SV) of 16.13 × 10⁴ and 12.73 × 10⁴ M⁻¹, and response times of ∼48 and ∼40 s for CrO₄²⁻ and Cr₂O₇²⁻, respectively. DFT calculations and specific wet-chemical investigations elucidated the FA detection to be triggered by photo-induced electron transfer (PET), while the Cr(VI)-sensing involved a combination of PET, the inner-filter effect (IFE), resonance energy transfer (RET), and electrostatic H-bonding interactions. The FA detection was validated using food samples (fish and meat) and wastewater specimens, achieving excellent recovery rates of ∼92–95%. Furthermore, the MOF's efficacy in recognizing the Cr(VI)-species in complex matrices (coal mine wastewater, sewage, and tap water) was investigated to yield high K_SV values (3.10–5.17 × 10⁴ and 2.16–7.03 × 10⁴ M⁻¹ for CrO₄²⁻ and Cr₂O₇²⁻), which demonstrated the probe's consistency and reliability.