Triple modifications of Ni(II) metal-organic frameworks as electrodes for enhanced electrochemical detection of dopamine

Metal-organic frameworks (MOFs) have been recognized as promising modified electrodes for electrochemical detections due to their porous nature with larger surface area and more exposed active sites facilitating adsorption of analytes. However, the inherent poor conductivity, unsatisfied sensitivity and limited selectivity of pristine MOFs hampered their electrochemical applications. Herein triple modifications, i.e. incorporation of second redox-active metal ions, confined pyrolysis and carbon nanotubes (CNTs) addition were implemented simultaneously to boost electrochemical detections towards dopamine (DA) of Ni(II)-MOFs assembled by three similar ligands (Ln, n = 1,2,3) to glutaric acid (L0). Comprehensive characterizations including XRD, FT-IR, XPS, SEM, N2 adsorption, and inductively coupled plasma (ICP) unveiled the structures and morphologies of MOFs and their composites. Cyclic voltammetry (CV) analysis revealed that Fe3+ doping accelerated electrochemical oxidation of DA through charge transport in the redox process of Fe3+ to Fe2+, resulting enhanced current responses. N2 adsorption and electrochemical impedance spectroscopy (EIS) measurement confirmed the improved surface areas and conductivity by subsequent confined pyrolysis, which are ascribed mainly to the created metaloxides by pyrolysis. The final generated Ni/Fe8:2-Ln-P-CNTs by triple modifications exhibited anticipated 5 to 10-fold amplified current responses due to the interesting synergistic effect of confined calcination and conductive CNTs. Ni/Fe8:2-L1-P-CNTs with lamellar morphology showed the best comprehensive detection of DA with the largest peak current of 259.3 mu A and the lower detection limit of 0.08 mu M (S/N = 3), the latter of which is 121 times lower than that of Ni/Fe8:2-L1, coupled with strong stability, good anti-interference and repeatability. This work highlighted the potential of combined metal ions doping, confined calcination and CNTs addition as a facile strategy for amplification of electrochemical detections.