Self-Calibrating Copper-Based Metal Organic Frameworks for Ratiometric Electrochemical Sensing of Creatinine

Abstract

Nanostructures that can undergo redox reactions have significant potential for electrochemical analysis. They can act as signal amplifiers to determine concentrations of targets in biological samples. In this work, a rational combination of electroactive copper-based metal-organic frameworks (Cu-MOFs) and ferrocene carboxylic acid (Fc) was designed for ratiometric electrochemical detection of creatinine. The synthesized Fc@Cu-MOFs exhibited two redox signals from the Cu²⁺/Cu⁺ and Fe³⁺/Fe²⁺ systems in the skeleton of Cu-MOFs and Fc, respectively. In the presence of chloride (Cl⁻), the oxidation current of Cu⁺ increased due to formation of solid-state cuprous chloride (CuCl). Adding strong Cu⁺ chelators, e. g. creatinine, coordinated with Cu⁺ and caused the current output of solid-state CuCl to decrease significantly. The anodic current of Fc did not appreciably change, serving as an internal reference signal. The ratiometric responses (I_Cu(I)/I_Fc) changed with increasing concentrations of creatinine from 0.017–130 μM with a detection limit (S/N=3) of 0.005 μM. The main advantages of Fc@Cu-MOFs are the low LOD, high selectivity, and reliability, making it a suitable platform for determining creatinine in human serum and urine samples. The as-fabricated sensor is a reliable approach for determining (bio) molecules that can form stable complexes with Cu⁺ ions.