sensors Review Graphitic Carbon Nitride: A Highly Electroactive Nanomaterial for Environmental and Clinical Sensing Azeez O. Idris * , Ekemena O. Oseghe, Titus A. M. Msagati , Alex T. Kuvarega, Usisipho Feleni and Bhekie Mamba Institute for Nanotechnology and Water Sustainability (iNanoWS), Florida Campus, College of Science, Engineering and Technology, University of South Africa, Johannesburg 1709, South Africa; [email protected] (E.O.O.); [email protected] (T.A.M.M.); [email protected] (A.T.K.); [email protected] (U.F.); [email protected] (B.M.) * Correspondence: [email protected] Received: 12 August 2020; Accepted: 23 September 2020; Published: 10 October 2020 Abstract: Graphitic carbon nitride (g-C3N4) is a two-dimensional conjugated polymer that has attracted the interest of researchers and industrial communities owing to its outstanding analytical merits such as low-cost synthesis, high stability, unique electronic properties, catalytic ability, high quantum yield, nontoxicity, metal-free, low bandgap energy, and electron-rich properties. Notably, graphitic carbon nitride (g-C3N4) is the most stable allotrope of carbon nitrides. It has been explored in various analytical fields due to its excellent biocompatibility properties, including ease of surface functionalization and hydrogen-bonding. Graphitic carbon nitride (g-C3N4) acts as a nanomediator and serves as an immobilization layer to detect various biomolecules. Numerous reports have been presented in the literature on applying graphitic carbon nitride (g-C3N4) for the construction of electrochemical sensors and biosensors. Different electrochemical techniques such as cyclic voltammetry, electrochemiluminescence, electrochemical impedance spectroscopy, square wave anodic stripping voltammetry, and amperometry techniques have been extensively used for the detection of biologic molecules and heavy metals, with high sensitivity and good selectivity. For this reason, the leading drive of this review is to stress the importance of employing graphitic carbon nitride (g-C3N4) for the fabrication of electrochemical sensors and biosensors. Keywords: graphitic carbon nitride (g-C3N4); heavy metals; biosensors; electrochemical sensors; nanoparticles 1. Introduction Graphitic carbon nitride (g-C3N4) is a two-dimensional conjugated polymer consisting of carbon and nitrogen. It is obtained from different carbon materials analogues by replacing carbon atoms with nitrogen atoms [1]. Also, graphitic carbon nitride (g-C3N4) is one of the oldest documented polymers in the literature with the general formula (C3N3H)n; the history of this material can be dated as far back as 1834 [2]. According to the literature, the carbon nitride (CN) material was first studied in 1834 when Berzelius obtained a linear CN polymer and named it “melon” [2]. This breakthrough led to the discovery of graphitic carbon nitride (g-C3N4), prepared by thermal decomposition of mercuric thiocyanate by Franklin in 1922 [3]. In 1989, numerous studies indicated that if carbon could replace silicon in the structure of silicon nitride (β-Si3N4), a very hard carbon nitride (β-C3N4) may be obtained [3]. Furthermore, in 1966, Teter and Hemley predicted various phases of several allotropic forms of carbon nitride, which include α-C3N4, β-C3N4, cubic-C3N4, pseudo-C3N4, and graphitic-C3N4 [3]. It was reported that all the phases of the CN materials are super-hard except g-C3N4, and it was Sensors 2020, 20, 5743; doi:10.3390/s20205743 www.mdpi.com/journal/sensors SensorsSensors 20202020, 20, ,20 x, FOR 5743 PEER REVIEW 2 of2 of29 28 to be the most stable allotrope under ambient conditions [3]. This report revealed the ease of modifyingdocumented the structure to be the mostand the stable morphology allotrope under of graphi ambienttic carbon conditions nitride. [3 ].As This a result, report the revealed chemistry the easeof differentof modifying morphologies, the structure structures, and the and morphology graphitic ofcarbon graphitic nitride carbon (g-C3N4) nitride. applications As a result, has the attracted chemistry considerableof different morphologies,interest from structures,scientists and and industrial graphitic carboncommunities nitride (g-Cin employing3N4) applications this material has attracted for variousconsiderable analytical interest applications. from scientists and industrial communities in employing this material for various analytical applications. The carbon and hydrogen atoms in graphitic carbon nitride (g-C3N4) are bonded together by sp2 The carbon and hydrogen atoms in graphitic carbon nitride (g-C N ) are bonded together by hybridization. In graphitic carbon nitride (g-C3N4), both carbon and3 nitrogen4 atoms are sp2 2 2 hybridizedsp hybridization. and are connected In graphitic by σ carbon bonds nitrideforming (g-C a hexagonal3N4), both struct carbonure. and This nitrogen structure atoms is called are spa σ triazinehybridized ring, and each are connected of these rings by isbonds linked forming to a small a hexagonal unit connected structure. by a This C–N structure bond. There is called is a a triazine ring, and each of these rings is linked to a small unit connected by a C–N bond. There is a consensus that graphitic carbon may have two chemical structures, namely g-C3N4 with a triazine consensus that graphitic carbon may have two chemical structures, namely g-C N with a triazine ring ring (C3 N3), which belongs to the R3 m space group, and the other structure3 consists4 of the tri-s- (C N ), which belongs to the R3 m space group, and the other structure consists of the tri-s-triazine triazine3 3ring (C6N7) [4]. In graphitic carbon nitride (g-C3N4,) each triazine ring is connected by the nitrogenring (C 6atomN7)[ at4]. the In end, graphitic forming carbon a noti nitrideceably (g-C expanded3N4,) each planar triazine grid ringstructure. is connected by the nitrogen atomGraphitic at the end, carbon forming nitride a noticeably (g-C3N4) expanded has recently planar fascinated grid structure. researchers' interest due to its outstandingGraphitic properties, carbon nitrideincluding (g-C low-cost,3N4) has recently large surf fascinatedace area, researchers’ earth-abundant, interest fast due electron to its outstanding transfer π π π-properties,π conjugation including structure, low-cost, metal-free, large surface excellent area, earth-abundant, visible-light-driven fast electron polymeric transfer semiconductor,- conjugation biocompatibility,structure, metal-free, and catalytic excellent properties visible-light-driven [5,6]. Figure 1 depicts polymeric graphitic semiconductor, carbon nitride's biocompatibility, key features and catalytic properties [5,6]. Figure1 depicts graphitic carbon nitride’s key features that spurred that spurred scientists towards employing graphitic carbon nitride (g-C3N4) for various analytical fields.scientists towards employing graphitic carbon nitride (g-C3N4) for various analytical fields. It Itis is important important toto emphasizeemphasize that that the the excellent excellen catalytict catalytic and biocompatibilityand biocompati propertiesbility properties of graphitic of carbon nitride (g-C N ) have inspired various scientists to utilize this material to construct multiple graphitic carbon nitride3 4(g-C3N4) have inspired various scientists to utilize this material to construct multiplesensors sensors and bio-(sensors). and bio-(sensors). Thus, Thus, this reviewthis review aims aims to provide to provide a comprehensive a comprehensive assessment assessment of of the synthesis and application of graphitic carbon nitride (g-C N ) to construct sensors and biosensors. the synthesis and application of graphitic carbon nitride (g-C3 3N44) to construct sensors and biosensors. FigureFigure 1. Critical 1. Critical properties properties of graphitic of graphitic carbon carbon nitride nitride (g-C3N (g-C4) that3N promote4) that promote its application its application for sensor for fabrication.sensor fabrication. 2. Preparation of Graphitic Carbon Nitride (g-C3N4) 2. Preparation of Graphitic Carbon Nitride (g-C3N4) Several synthesis routes have been reportedly used to prepare graphitic carbon nitride (g-C3N4), Several synthesis routes have been reportedly used to prepare graphitic carbon nitride (g-C3N4), taking cognizance of the relationship between the structure of the material and morphologic features. taking cognizance of the relationship between the structure of the material and morphologic features. In summary, top-down and bottom-up approaches are the synthesis routes that are generally employed In summary, top-down and bottom-up approaches are the synthesis routes that are generally to synthesize various morphologies of graphitic carbon nitride (g-C3N4)[6]. The former involves liquid employed to synthesize various morphologies of graphitic carbon nitride (g-C3N4) [6]. The former and thermal exfoliation methods, while the latter involves a solvothermal approach, supramolecular involves liquid and thermal exfoliation methods, while the latter involves a solvothermal approach, aggregation, and soft and hard templates [6]. The top-down synthesis route involves the sequential supramolecular aggregation, and soft and hard templates [6]. The top-down synthesis route involves the sequential breaking down of larger blocks of graphitic carbon nitride (g-C3N4) into smaller units, Sensors 2020, 20, 5743 3 of 28 breaking down of larger blocks of graphitic carbon nitride (g-C3N4) into smaller units, g-C3N4 nanosheets multilayer, and single-layer structures. Similarly,