Advances in Biosensing of Chemical Food Contaminants Based on the MOFs-Graphene Nanohybrids

Food safety remains one of the most critical subjects around the world and assessment of food quality is of great import- ance to human health, which decides whether the food could be distributed and consumed in the market. Water scarcity, chemical pollutants, pesticides and veterinary drugs overuse are significant elements that affect food safety and endanger human health. In today’s world, with the deepen- ing of globalization, food poisoning or foodborne diseases have affected both developed and developing countries. Therefore, the development of fast, robust, and sensitive approaches to identifying food contaminants is of great sig- nificance for controlling, preventing, and mitigating the effects of possible outbreaks. Though most of the classical laboratory techniques like chromatography and chemical techniques, for assessing food safety have shortcomings of being well-trained personnel, complex sample pretreatment, and time-consuming, particularly based on expensive appa- ratuses, restricting their practical applications in routine screening food contaminants [1]. In this context, novel sens- ing techniques possess the benefits of being accurate, sensi- tive, rapid, and easy to use for real-time detection of harmful substances. A sensor mentions a system or device which is typically contained in signal conversion and a recognition element to convert the response into another signal form like spectral, electrical, and optical responses. Hence, the detected data from the recognition unite is converted to a measurable sig- nal once it interacts with the target [2]. In a sensor, the key detecting properties, such as cost, stability, selectivity, and sensitivity, is mainly specified via the selection and design of sensing nanomaterials engaged in the sensor [3,4]. Nowadays, with the progress of material science, numerous materials like metals and metal oxides, magnetic and noble nanopar- ticles (NPs), quantum dots, carbon-based materials, and MOF (metal-organic framework), were focused on fabricat- ing new nanoprobes with acceptable detecting efficiency [5,6] . Especially, MOF, which is a novel type of crystalline porous material produced via self-assembly of organic ligands as linkers and metallic centers as nodes through coordination interactions, exhibiting enormous potential in the (bio)sensing areas [7]. The important driving forces toward the utilization of MOFs in bio-and chemo-sensing are their excellent characteristics (e.g., non-toxicity, bio- degradability, luminescence, and accessibility of active sur- plus elements) [8]. More importantly, the host-guest binding (together with large surface area and tunable porosity), because of the overlap between the electron acceptor and donor has deliberated an ideal option for sensor applica- tions. Additionally, the framework of crystalline porous MOFs provides adsorption properties (in principle) with high selectivity toward molecules [9]. However, most primi- tive MOFs have intrinsic limitations such as low structural stability and poor electrical conductivity, affecting their feas- ible efficiency [10]. To address these challenges, investigators have introduced conductive MOFs based on the organic ligands and metal ions with free bonding electrons [11,12]. MOFs can also get a good electrical conductivity through direct-carbonization method[13–16]. The high carbon content and porous structure of MOFs render them brilliant precur- sors for the direct carbonization of porous carbon materials. Metal units in MOF crystals could be evaporated at high temperatures or by acid treatment to remove transition met- als. However, the direct carbonization of dissociative MOFs decreases the amount of active sites and affects the conduct- ivity to some extent. The integration of MOF-derived carbon with graphene could inhibit the aggregation of carbon mate- rials at high temperatures, increase the active sites density, and solve the effect of direct carbonization of dissociated MOFs on their electrical conductivity.