A Cu(II)-MOF Based on a Propargyl Carbamate-Functionalized Isophthalate Ligand as Nitrite Electrochemical Sensor

Abstract

This paper investigates the electrochemical properties of a new Cu(II)-based metal-organic framework (MOF). Noted as Cu-YBDC, it is built upon a linker containing the propargyl carbamate functionality and immobilized on a glassy carbon electrode by drop-casting (GC/Cu-YBDC). Afterward, GC/Cu-YBDC was treated with HAuCl₄ and the direct electro-deposition of Au nanoparticles was carried at 0.05 V for 600 s (GC/Au/Cu-YBDC). The performance of both electrodes towards nitrite oxidation was tested and it was found that GC/Au/Cu-YBDC exhibited a better electrocatalytic behavior toward the oxidation of nitrite than GC/Cu-YBDC with enhanced catalytic currents and a reduced nitrite overpotential from 1.20 to 0.90 V. Additionally GC/Au/Cu-YBDC showed a low limit of detection (5.0 μM), an ultrafast response time (<2 s), and a wide linear range of up to 8 mM in neutral pH.

Keywords: electroactive metal-organic frameworks; electrocatalysis; electrochemical sensor; nitrite detection.

Figure 1

(A) A conventional sketch of the 1,3-H₂YBDC moiety, showing molecular connectivity; (B) a 3D drawing of the Cu-YBDC paddlewheel fragment and the location of the carbamate carbonyl oxygen atoms (highlighted in yellow) completing the Cu(II) coordination through C=O^…Cu bonds. Atomic coordinates are taken from ref. [26]. Color codes: Carbon (black); Hydrogen (white); Nitrogen (blue), Oxygen (red) and Copper (orange). The dashed bonds address the weak intermetallic interaction and the apical carbamate coordination mode.

Figure 2

SEM images of GC/Cu-YBDC; magnified: (A) 50 KX and (B) 150 KX.

Figure 3

Raw diffraction data for GC/Cu-YBDC (in red) and simulated powder diffraction data from the known structure of the Cu-YBDC MOF. Vertical bars indicate the angular location of the most intense diffraction peaks of the latter. The large hump in the 7–11° 2θ range and the triplet of peaks falling above 25° 2θ stem from the sample holder (amorphous plastics and crystalline substrate, respectively).

Figure 4

CVs of GC/Cu-YBDC at a scan rate of 50 mV/s in: A—0.1 M TRIS buffer; B—0.1 M NaCl; C—0.1 M PBS buffer electrolyte solution; the third scan is reported in all cases. The abscissa is Potential vs. SCE.

Figure 5

Nyquist plot obtained with A—GC, B—GC/Cu-YBDC, and C—GC/Au/Cu-YBDC electrodes recorded in 2 mM [Fe(CN)₆]^3−/4− and 0.1 M KCl solution. The experimental data were well fitted (solid lines) according to the inset equivalent circuit.

Figure 6

CV curves on GCE: A—bare, B—Cu-YBDC, and C—Au/Cu-YBDC in 0.1 M PBS (pH 7.2) in the absence (dashed line) and presence of 5.0 mM nitrite. Scan rate 50 mVs⁻¹.

Figure 7

CVs of: A—bare GC, B—GC/Au/Cu-YBDC and C—GC/Au electrodes in PBS 0.1 M (pH 7.2) containing 5.0 mM nitrite; 50 mVs⁻¹.

Figure 8

(A) Typical chronoamperometric response of GC/Au/Cu-YBDC to successive additions of nitrite in a mildly stirred 0.1 M PBS (pH 7.2) solution. Calibration plots of steady-state current versus nitrite concentration for three distinct ranges: (B) 20 μM to 160 μM; (C) 160 μM to 1.20 mM and (D) 1.20 mM to 8.0 mM.