Polypyrrole deposited on the core–shell structured nitrogen-doped porous carbon@Ag-MOF for signal amplification detection of chloride ions

An electrochemical platform for signal amplification probing chloride ions (Cl⁻) is constructed by the composite integrating core–shell structured nitrogen-doped porous carbon@Ag-based metal–organic frameworks (NC@Ag-MOF) with polypyrrole (PPy). It is based on the signal of solid-state AgCl derived from Ag-MOF, since both NC and PPy have good electrical conductivity and promote the electron transport capacity of solid-state AgCl. NC@Ag-MOF was firstly synthesized with NC as the scaffold and then, PPy was anchored on NC@Ag-MOF by chemical polymerization. The composite NC@Ag-MOF-PPy was utilized to modify the electrode, which exhibited a higher peak current and lower peak potential during Ag oxidation compared with those of Ag-MOF and NC@Ag-MOF-modified electrodes. More importantly, in the coexistence of chloride (Cl⁻) ions in solution, the NC@Ag-MOF-PPy-modified electrode displayed a fairly stable and sharp peak of solid-state AgCl with the peak potentials gradually approaching zero, which might effectively overcome the background interference caused by electroactive substances. The oxidation peak currents of solid-state AgCl increased linearly with the concentration of  Cl⁻ ions in a broad range of 0.15 µM–40 mM and 40–250 mM, with detection limits of 0.10 µM and 40 mM, respectively. The practical applicability for Cl⁻ ions determination was demonstrated using human serum and urine samples. The results suggest that NC@Ag-MOF-PPy composite could be a promising candidate for the construction of the electrochemical sensor.