MOF@PEDOT Composite Films for Impedimetric Pesticide Sensors

Abstract

Due to its deleterious effects on health, development of new methods for detection and removal of pesticide residues in primary and derived agricultural products is a research topic of great importance. Among them, imazalil (IMZ) is a widely used post-harvest fungicide with good performances in general, and is particularly applied to prevent green mold in citrus fruits. In this work, a composite film for the impedimetric sensing of IMZ built from metal-organic framework nanocrystallites homogeneously distributed on a conductive poly(3,4-ethylene dioxythiophene) (PEDOT) layer is presented. The as-synthetized thin films are produced via spin-coating over poly(ethylene terephtalate (PET) substrate following a straightforward, cost-effective, single-step procedure. By means of impedance spectroscopy, electric transport properties of the films are studied, and high sensitivity towards IMZ concentration in the range of 15 ppb to 1 ppm is demonstrated (featuring 1.6 and 4.2 ppb limit of detection, when using signal modulus and phase, respectively). The sensing platform hereby presented could be used for the construction of portable, miniaturized, and ultrasensitive devices, suitable for pesticide detection in food, wastewater effluents, or the assessment of drinking-water quality.

Keywords: PEDOT; conductive polymers; imazalil; impedance spectroscopy; metal–organic frameworks; pesticide sensors.

Scheme 1

A) Imazalil molecular structure. B) Electrostatic potential map.

Figure 1

A) Adsorption equilibrium isotherm for IMZ on native MOF. B) Diffraction patterns for native and amino‐substituted MOF in comparison with the theoretical diffraction pattern for the native MOF. C) Sheet resistance of the PEDOT and MOF‐NH₂@PEDOT thin films on PET substrates. D) SEM micrographs of the MOF‐NH₂@PEDOT composite films and the bare PEDOT thin film (inset), both deposited on PET foils (scale bar is 2 µm).

Figure 2

A,B) Experimental setup for the impedance experiments applying a 20 mV AC sinusoidal signal from 0.1 Hz to 10 kHz of a MOF‐NH₂@PEDOT composite deposited on a PET substrate. The module and phase of the electrical impedance in air, DIW, and 0.1 m KCl are shown for C,D) the PEDOT and E,F) MOF‐NH₂@PEDOT composite films.

Scheme 2

Conduction mechanisms inside the PEDOT film upon the addition of 0.1 m KCl.

Figure 3

Experimental data (circles) and corresponding fitting (continuous lines) of the impedance module and phase response for the impedimetric sensors upon the addition of KCl 0.1 m and 1 ppm IMZ. Samples: A,B) PEDOT, C,D) MOF‐NH₂@PEDOT. The equivalent electrical circuit used for modeling the impedance module and phase is described in the Supporting Information.

Figure 4

Experimental data obtained from the A) impedance module and B) phase of an MOF‐NH₂@PEDOT films after the addition of IMZ at different concentrations (0.015, 0.03, 0.06, 0.12, 0.25, 0.5, and 1 ppm). In the insets, the calibration plots are obtained from the difference of each parameter with its blank obtained for three different samples. In (C), the relative change of the impedance modulus for the MOF‐NH₂@PEDOT film is compared (air, DIW, 0.1 m KCl and 1 ppm IMZ in 0.1 m KCl). Error bars represent the standard deviation of three independent measurements.