Defective Metal–Organic Framework Nanocrystals as Signal Amplifiers for Electrochemical Dopamine Sensing

Abstract

The electrochemical sensing system has been recognized as a promising and facile approach to detect dopamine (DA), a crucial neurotransmitter in the human body. However, the rational design of electrode materials is important in order to selectively detect DA in the presence of coexisting interferents, such as ascorbic acid (AA) and uric acid (UA). In this study, a water-stable and nanoporous zirconium-based metal−organic framework (MOF), UiO-66, is synthesized with a tunable degree of missing-linker defects, and the corresponding crystallinity, morphology, porosity, and degree of defects are characterized. Although all the UiO-66 synthesized here is electrically insulating and electrochemically inactive, the thin film of defective UiO-66 deposited on the electrode surface can significantly amplify the electrochemical sensing signal for DA. The effect of the degree of defects on the resulting sensing response for DA is examined, and the origin of such a signal amplification effect, which is relevant to the hopping-based electrochemical process of the irreversibly adsorbed DA in the defective MOF, is investigated. By serving the defective UiO-66 as a signal amplifier casted on top of another electrically conductive active material capable for selective DA detection, the modified electrode for sensing DA with enhanced performances can be developed. As a proof-of-concept demonstration, the defective UiO-66 thin film was coated on the graphene oxide (GO) modified electrode. With the use of such a bi-layer thin film for DA sensing, a much higher sensitivity (6.4-fold), a much smaller limit of detection (0.23-fold), and a better selectivity toward DA against AA and UA (2.6-fold and 1.2-fold, respectively) can be achieved compared to those of the GO thin film. Experimental results here shed light on the use of such porous MOF coatings to effectively enhance the sensing performances of other developed electrochemical sensing systems for DA.

Sulfonate-Grafted Metal–Organic Framework─A Porous Alternative to Nafion for Electrochemical Sensors

“…By casting the MOF-808 thin film on the surface of GO, the sensing signal from DA can be significantly amplified but the current responses for the oxidation of AA and UA can be amplified as well (Figure b). Such amplifying effects should be attributed to the adsorption of analytes in the Zr-MOF, which has been investigated in another Zr-MOF in our very recent work . On the other hand, the Nafion thin film can selectively amplify the electrochemical response for DA oxidation owing to its negatively charged sulfonate groups that can attract DA (Figure c) .…”

“…All electrodes were then subjected to differential pulse voltammetric (DPV) experiments in 0.5 M KCl aqueous solutions, where the structural integrity of Zr-MOFs can be well preserved, 27 to probe their sensing behavior and selectivity toward the oxidation of DA against those of AA and UA. It is worth mentioning that the use of DPV technique is necessary to separate the oxidative responses from DA, UA, and AA; 27 simply utilizing linear sweep voltammetry would result in the overlapped peaks from these analytes (Figure S8). As shown in Figure 3a, the DPV curve of GO shows three well separated peaks located at +0.15 + 0.32, and +0.47 V (vs. Ag/AgCl/NaCl (3 M)) after adding all the three analytes, which correspond to the oxidation of AA, DA, and UA, respectively.…”

“…Such amplifying effects should be attributed to the adsorption of analytes in the Zr-MOF, which has been investigated in another Zr-MOF in our very recent work. 27 On the other hand, the Nafion thin film can selectively amplify the electrochemical response for DA oxidation owing to its negatively charged sulfonate groups that can attract DA (Figure 3c). 36 However, the peak current for DA after background subtraction achieved by Nafion/GO (84 μA/cm 2 ) is much smaller than that achieved by MOF-808/GO (192 μA/cm 2 ), presumably due to the mass-transfer limitation of DA in the less porous Nafion.…”

“…Among various electrochemical sensing purposes, the electrochemical detection of dopamine (DA) is particularly of interest, since DA is a crucial neurochemical in the human body and an electrochemical method can provide a rapid detection and ease of miniaturization. − Various recent studies have thus utilized water-stable MOF-based materials, including some Zr-MOFs, in the electrochemical detection of DA. − However, one critical issue for the electrochemical determination of DA is the coexistence of ascorbic acid (AA) and uric acid (UA) in physiological samples, which can be electrochemically oxidized at the potentials similar to that for DA oxidation and thus cause the selectivity issue of the sensor . Approaches such as the use of electrocatalysts that can selectively oxidize DA and the modification of the electrode surface to suppress the signals from UA and AA are thus required for the development of electrochemical DA sensors. , …”

“…Electrodes modified with GO, which is active for the electrochemical detection of DA, were then prepared, 27 and the MOF thin film was further deposited onto the surface of GO by drop-casting technique. Bilayered thin films can thus be prepared on the fluorine-doped tin oxide (FTO)-based electrodes (Figure 3e).…”