Mixed metal conductive MOFs constructed from Trypan blue linked metal nodes: characteristic features and electrochemical performance

Abstract

Multifunctional ligands possess multiple coordination sites, enabling them to simultaneously connect with several metal nodes. In this study, we report the use of Trypan blue (Try) as a multifunctional organic linker to fabricate new three-dimensional (3D) 3d–3d and 3d–4f bimetallic metal–organic frameworks (MOFs). The multiple binding sites, including amino, hydroxyl, sulfonate, and azo groups, facilitated the coordination of metals (M = Cu, Co, Zn, Er, and Yb) with varying preferences and geometries. Depending on the metal interactions, these MOFs (Cu–Co–Try (P1), Cu–Zn–Try (P2), Cu–Er–Try (P3), Cu–Yb–Try (P4)) adopted empirical formulae such as Cu₂M₁C₂O_1.2N₁/Cu₁M₂C₂O₁N₁. They exhibit polymorphic crystal forms, predominantly featuring monoclinic symmetry in P2₁/c, P2₁/n, hexagonal symmetry in P3221, and triclinic symmetry in P space groups. Electronic and vibrational spectroscopy results indicate that Trypan blue remained intact throughout the synthesis. The paste of these MOFs can serve as a stand-alone electroactive working electrode material without the need for carbon additives. They exhibited high peak current responses of 719.4 μA (P1), 637.5 μA (P2), 79.7 μA (P3), and 173.7 μA (P4) at a scan rate of 0.01 V s⁻¹. The charge mobility values for the MOFs were determined as 1.08 × 10⁻⁷ (E), 1.25 × 10⁻³ (P1), 1.42 × 10⁻³ (P2), 1.58 × 10⁻⁴ (P3), and 1.51 × 10⁻⁴ (P4) cm² V⁻¹ s⁻¹, suggesting that electron transfer occurs via hopping mechanisms. Additionally, the effective utilization of Trypan blue in MOF synthesis, without generating any waste, may reduce environmental impact and enhance economic benefits.