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Electrochemistry
Lectrochemical sensors (ECSs) is a powerful method with great sensitivity and reliability for food evaluation. Metal-organic frameworks (MOFs) with surprisingly porous morphology provide uniform yet tunable features, a high specific surface, and established practical applications in various fields. MOF-based ECSs present novel routes for the fast and effective detection of food contaminants or nutrients.
- electrochemical food monitoring
- metal-organic frameworks
- food additives
- electrochemical sensors
1. Introduction
Food analysis is essential for quality observation and safety approval. Recently, it has seen a drastic growth in novel analytical methods utilized in food quality confirmation. Along with this, there has been a parallel prosperity in the functional materials area to improve selectivity, sensitivity, and stability. Owing to their unique characteristics, MOFs present a niche in the material field, enabling them to become an essential and highly favorable part of the food industry. To date, MOF has demonstrated its capabilities in various aspects of the food industry, including contamination removal, food packaging, improvement of food preservation, and monitoring of food products. Many researchers have recently concentrated on applying MOFs in sensor development for food evaluation, such as fluorescence, EC, colorimetric, and surface-enhanced Raman scattering (SERS) sensors [25,26]. Among them, MOF-based ECSs have attracted immense attention because the superiority of MOFs relies on two essential parameters: high surface area and extraordinarily tunable structures. They also displayed outstanding anchoring capacity for chemical functionalization by different functional groups like –NH2 and –COOH through in-situ or post-modification [27,28]. These characteristics make MOFs ideal candidates as a supporting platform for co-immobilizing catalysts and enzymes via interactions like hydrogen bonding, π–π stacking, and electrostatic interaction between MOF and biological ligands [29,30]. Improvement of key physio-chemical characteristics of MOFs can also be performed through combination or scaffolding with a variety of functional substances like metal oxide nanoparticles, metallic nanoparticles, quantum dots, polyoxometalates, and carbonaceous materials (carbon nanotubes, graphene (Gr), etc. [31,32]. These classes of MOF-based composites present exceptional physiochemical features that cannot be obtained with pristine MOFs. In recent years, a variety of MOF composites have been introduced with excellent stability in severe conditions (highly acidic or base), improved catalytic activity, and increased signal transduction in sensing investigations. In the context of evaluating food-based analytes using electrochemistry, MOF-based modifiers can play different roles, such as improving electrochemical performance by increasing the effective surface area of the electrode, resulting in more active sites for analyte adsorption, and facilitating electron transfer processes. Furthermore, analyte preconcentration can be carried out by MOFs by selectively adsorbing specific analytes from complex food matrices onto their porous structures. This preconcentration step assists in overcoming the challenges related to trace analyte contents, interference, and the complexity of sample environments.
Therefore, MOF materials are favorable in sensor development, and in the following section, recent advancements in MOF-based ECSs in sensor development for food analysis and related studies are reviewed.
A. Heavy Metal Ions
Heavy metals are elements spread in trace amounts in nature, a number of which, in low quantities, perform an essential role in human bodies; even so, they could give rise to perniciousness in higher concentrations [33]. Moreover, Pb and Cd are very poisonous pollutants that could have critical unfavorable health impacts, and food and water are the main sources of these toxic metals introduced to living organs. Therefore, to protect users, the quantity of heavy metals should be repeatedly and quickly determined at different steps of the food chain [34].
A voltammetric sensor for cadmium ions (Cd2+) detection in meat samples, based on amine-functionalized Zr(IV)-MOF (UiO-66-NH2) and MWCNT composite fabricated through a one-pot hydrothermal reaction was reported by Zou et al. [37]. The prepared sensor displayed outstanding EC activity, with an excellent linear range from 0.5 to 170 μg/L and a DL of 0.2 μg/L for Cd2+ detection with a recovery of 95.1–107.5%. The plentiful amino groups in UiO-66-NH2 simplified the preconcentration of Cd2+ on the electrode interface. Exploiting the high affinity of amino and carboxyl functional groups toward metal ions, Lin and colleagues also presented an EC constructed of a NH2-MIL-53(Al)/polypyrrole (PPy) hybrid structure coated on a gold electrode [38]. The nanocomposite-modified electrode displayed sensing activity toward Cu2+ and Pb2+ with a DL of 0.244 μg L−1 and 0.315 μg L−1, respectively, with great selectivity in the presence of other popular interferences. The adsorption volume of the chelating groups reduced in the order of Cu ≫ Pb ≫ Cd ≫ Zn ≫ Ni, Hg.
B. Food Additives
In recent years, food additives have become a severe threat to community health and food protection [44]. Food adulteration is explained as the insertion or extraction of any materials to/from food that impact the natural condition and constitution of the food substance. Sudan dyes have been recognized in flavoring powders, pepper sauces, piquant soups, colorful snacks, and even soda. Such illegal artificial dyes, including Sunset Yellow (SY), Sudan Red, and Ponceau 4 R, are non-expensive and accessible as colorants to intensify the natural color or improve the appearance, taste, and composition of products [45]. However, most of them have an aromatic ring in their chemical structure that is harmful to human health when extended. Contamination of natural milk with synthetic materials is also an important issue in different communities. For instance, melamine is a synthetic substance with a high quantity of nitrogen, which is frequently employed in melamine resin preparation. Recently, it has been illegally poured in milk to incorrectly display a higher amount of protein, which is typically determined based on nitrogen content with the Kjeldahl procedure, resulting in dangerous sickness and health risks [46]. Therefore, monitoring illegal additive chemicals at trace concentrations would be beneficial.
Malachite green is beneficial in cases of parasite and microbial contagion in fish. However, as a chemical substance, malachite green would occupy their organs for a long time, resulting in ramifications. Recently, malachite green has been illegitimately employed in the aquaculture industry. To sensitively detect malachite green, Zhou and coworkers developed a new ECS utilizing Ag/Cu-MOFs prepared in a single-step solvothermal procedure [47]. MOF identification was carried out by XPS measurements. Besides photoelectron peaks related to C, O, and Cu, doublet peaks attributed to Ag 3d3/2 and Ag 3d5/2 were observed. The Ag/Cu proportion was calculated to be 7.65%, which was in agreement with the EDS experiment. The higher values of binding energy compared to common chemical states of Ag are attributed to the four electronegative oxygen atoms of carboxylic ligands, which lead to larger inner electron binding energies. This result confirmed that the mixed-node MOFs (Ag/Cu-MOFs) were successfully prepared by the one-step synthesis. According to CV studies, malachite green displayed well-defined oxidation peaks around 0.57 V without a cathodic peak on the reverse sweep, indicating an irreversible process. Investigation of the relation between scan rate and the oxidation peak potential revealed a peak shift to a less positive value with the increase in scan rate, with Na estimated to be 2. This is explained as follows: The electrochemical oxidation of malachite green occurs with the ejection of an integral unit of the central carbon connected to a phenyl group, followed by the intramolecular coupling of two phenyl fragments. This process is an electrochemically irreversible, two-electron transfer process. In comparison with the bare electrode, the modified one efficaciously resulted in more sensitive detection with a low DL of 2.2 nM in a linearity range of 10–140 nM and an acceptable anti-interference potential. In another study, a highly sensitive, conductive, and selective sensor has been introduced by Shabani-Nooshabadi and colleagues for simultaneous measurement of tartrazine, patent blue V, acid violet 7, and ponceau 4RA in food products like jellies, condiments, soft drinks, and candies [48]. A modified carbon paste electrode was prepared using copper-based MOF (Cu-BTC MOF) and 1-ethyl-3-methylimidazolium chloride as an ionic liquid (IL) to improve the surface area and conductivity and, consequently, electron transfer rate enhancement. The excellent analytical performance of the suggested system was approved for patent blue V by providing a low DL of 0.07 µM, a broad linearity range between 0.08 and 900 µM and reasonable recovery.
C. Foodborne Pathogens
One valuable activity to overcome bacterial foodborne disease is improvement in pathogen detection prior to human infection. Since the environments in which these pathogens are present vary from food packaging instruments to restaurants and home kitchens, detection techniques for contagions must be low-cost, easy to use, and rapid while being stable, sensitive, and selective. Among the various available methods, EC sensing is uniquely suitable for point-of-contaminant detection [57]. In the field of mycotoxins sensing, most utilizations of these novel materials are based on their integration with specific target detection elements, including aptamers, antibodies, and molecularly imprinted polymers (MIP). For instance, He and his coworker presented an ultrasensitive EC aptasensor utilizing CoSe2/Au nanorods, 3D-structured DNA-PtNi@Co-MOF, and nicking endonuclease to determine the zearalenone (ZEN) [58]. The Co-MOF is prepared to improve the aptasensor sensitivity, and the PtNi nanostructure is used for both immobilization DNA and thionine (Thi) via Pt-N or Pt-S connections to make a three-dimensional network and to provide synergetic catalytic influence with the Co-MOF towards signal tags such as Thi. The CoSe2/Au is also employed to provide total activity of the nicking enzyme and DNA-PtNi@CoMOF networks in signal increase. In the presence of ZEN, the aptamer forms a complex with ZEN via a nicking enzyme, leading to DNA H1 production. Consequently, a significant Thi signal could be discerned because of the incubation of Thi-labeled DNA H2–PtNi@Co-MOF and DNA-PtNi@Co-MOF networks onto the aptasensor. In comparison with traditional hybridization chain reactions, the DNAPtNi@Co-MOF structure can couple with abundant signal tags (Thi), as well as present multiple signal boosters (PtNi@Co-MOF). Additionally, the aptasensor displayed an appreciative response to ZEN evaluation in maize samples, establishing the applicable capacity for mycotoxins detection.
D. Hydrogen Peroxide
Hydrogen peroxide (H2O2) is a vital oxidant with broad applications in different fields such as food processing, paper bleaching, sewage treatment, biotechnology, and the chemical industry. Some food industries employ H2O2 for sanitation, which undoubtedly results in the remaining H2O2 in their products. Investigations have proven that extra H2O2 is cytotoxic for living organs, leading to serious diseases, from metabolic disorders like diabetes to abnormalities in tissue like cancer. According to the literature, Cu-MOFs have been considered encouraging substances for EC sensing platforms toward H2O2 determination for different reasons, including that, firstly, these Cu-contained frameworks obtain good catalytic performance for H2O2 because of the well-matched potential between Cu+/Cu2+ redox peaks and H2O2 reduction. Additionally, the flower-like morphology of Cu-MOF provides an abundant active site, which facilitates H2O2 enrichment in the catalytic centers.
This entry is adapted from the peer-reviewed paper 10.3390/chemosensors11070357
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