Mesoporous Carbon Nitrides as Emerging Materials: Nanoarchitectonics and Biosensing Applications

Mesoporous carbon nitrides (MCNs) are versatile materials and find use in applications such as catalysis, gas capture, and remediation. The development of new forms of MCNs, such as C3N5, C3N6, and C3N7, has expanded their scope further. One of the least reviewed applications of these materials is in the field of sensing, though it has been demonstrated that their sensing abilities are on par with other comparable materials. Their excellent properties such as high surface area, tunable stoichiometry, adjustable electronic structure, and basicity, make them well‐suited for the adsorption and detection of a wide range of analytes. Herein, the new findings in the synthesis of MCNs and their exciting prospects for sensing are reviewed. The review is divided into two broad sections: 1) discussions on the synthesis of MCNs using hard/soft templating, sol–gel, and template‐free methods; and 2) their capabilities for photoelectrochemical, optical, and quartz microbalance‐based sensing. The findings from the recent literature are showcased and the covered topics are explained with comparative analyses. The current review is a timely presentation of the concerned topic and will serve as a useful piece of information for developing advanced sensors using MCN based materials.


Introduction
The continuous and rapid industrial development has dramatically increased the consumption of various natural and synthetic chemicals leading to the accumulation of toxic pollutants, greenhouse gases, heavy metal impurities, and pathogenic microorganisms in the environment. [1] The presence of such tivity for different chemical and biological molecules. [28] However, its sensing applications are severely impacted owing to its low water solubility, moderate electrical conductivity and selectivity, low specific surface area, and limit of detection as well as optical absorption in the blue-green range. Hence, there is a need to design and fabricate functionalized g-CN based materials with high specific surface area/porosity, bio-degradability, and light absorption capacity which could motivate researchers to conduct extensive work on the preparation of functionalized and hybrid g-CN. [29] To overcome these issues, researchers moved toward the preparation of porous CN structures containing higher surface area and a large number of active sites for capturing analytes, which is essential for efficient detection. [30] Ryoo et al. first reported nano casting as an approach for the preparation of highly ordered mesoporous carbon (CMK-1). [31] The discovery of CMK-1 opened a new avenue for preparing mesoporous materials such as silica, carbons, polymers, metals, and metal oxides using template materials. [31] The structural and physical properties of these mesoporous materials can be controlled by using various templates such as MCM-48, KIT-5, KIT-6, SBA-1, SBA-15, and SBA-16. [32] Vinu and co-workers were the first to replicate the mesoporous structure of SBA-15 into mesoporous carbon nitride (MCN). [33] Since then, extensive research is being carried out to explore the physical and structural properties of MCNs along with organic and inorganic materials present inside the porous channels for surface engineering and their functionalization and their applications in a wide range of applications in the field of selective sensing, catalysis, energy generation, adsorption, and gas storage. [33,34] Lately, studies have focused on tuning the nitrogen content in the MCN framework by choosing appropriate precursors and different synthesis routes to control the physical, electrical, and optical properties and the band structures as they can greatly impact their use in sensing applications. [35] While there are some published reviews on g-CN/polymeric CN, these publications have mostly covered applications such as energy conversion and storage, [36] catalysis, [37] and environmental remediation. [38] Our group comprehensively reviewed the field of MCN for different applications; however, it primarily focused on the synthesis and functionalization and specific catalytic, photocatalytic and energy storage and conversion applications. [30] Given the growing interest in the use of bare, hybrid and functionalized MCN as opposed to bulk g-CN in sensing applications and there is lack of a specific review completely focused on the area of MCN for sensing, this will be a timely review to summarize as well as propel the research in the right direction.
In this review, we will discuss various methods for the synthesis of MCNs, their structural properties and their application in sensing various analytes such as biomolecules, heavy metal atoms, and different chemical molecules such as volatile organic compounds (VOC). The principles and application of MCNs in the sensing of different analytes via photo/electrochemical (PEC), optical and quartz crystal microbalance (QCM) techniques are discussed in detail (Figure 1). This review will specifically focus on the advantages of using MCN over the bulk and their applicability in enhancing the selectivity and sensitivity for the detection of analytes. The review also discusses the effect of mesoporosity in various sensing techniques and emphasizes the benefits of the same. Finally, the review summarizes the field and provides the future direction for the research on MCN to make further progress in the field. We strongly believe that the critical discussions covered on various features of MCNs and sensing in this review will enhance the overall knowledge of nitrogenenriched mesoporous carbon nitride-based materials in addition to MCN.

Synthesis of MCN
As mentioned earlier, eventhough g-CN has unique optical and electrical properties and high thermochemical and mechanical stability; it suffers from some drawbacks such as low surface area, low nitrogen density, less availability of surface for the functionalization, cost of preparation, and less sensitivity. [39] These drawbacks of the g-CN can be overcome as synthesis strategies can be employed to manipulate its physical/chemical properties. [40] Moreover, since surface area determines the number of adsorbed molecules, porosity can enhance sensing capability. Researchers therefore, came up with solution of introducing the pores into g-CN structure by using porous templates and/or by post treatment with different precursor materials. [41] However, generating a well-ordered structure and porosity is challenging and needs special synthesis techniques. Moreover, basicity of the material depends on the available number of electrons which can be tuned by manipulating carbon to nitrogen (C/N) ratio in the material. Tuning the C/N ratio is also easy in porous carbon nitride which make them suitable to functionalize with other heteroatoms due to the presence of lone pairs of electrons. [42,43] The commonly employed methods for this purpose are templating approaches, including hard and soft templating, wherein structure-directing agents are used to introduce porosity in CNs. [44] There are a few reports that the MCN without an ordered porous structure can be prepared either without the assistance of a template or via the sol-gel method. [45,46] All these different pathways for synthesizing MCN are illustrated in Figure 2. The MCNs synthesized via either of the above methods can readily be functionalized with species, including metals and metal oxides, to enhance their efficiency and application potential in various fields. The forthcoming subsections discuss various synthesis methods of MCNs and the evolution of their structural and functional features in detail. Table 1 summarizes various aspects, including synthesis param-eters, properties, and applications of MCN materials that are synthesized using various methods described in this review.

Hard Templating
Complete replication of ordered and porous structure of the mesoporous silica template into resultant MCN via hard templating route is a well-known and widely accepted method for the synthesis of MCN. [44b,c] The famous process, also known as nano casting, involves either inner filling or surface coating of a hard template such as silica with a C and N containing precursor, followed by carbonization and removal of the template to obtain the porous CN with a structure mimicking the template. The choice of precursors, concentration, wetting of the pores, and processing temperature greatly influence the physico-chemical properties of the corresponding CN in addition to the textural properties. Along with the replication of structural features, hard templating is a useful tool to generate considerable porosity in both micro and meso domains in the CN.
Vinu et al. are credited with the first report on the synthesis of MCN by employing SBA-15 as a hard template and carbon tetrachloride (CTC) and ethylenediamine as C and N precursors, respectively. [33] The resultant MCN not only replicated the ordered morphology of the silica but also displayed significant mesoporosity in terms of high surface area (505 m 2 g −1 ), uniform pore diameter (4 nm), and a large pore volume (0.55 cm 3 g −1 ). This discovery opened the field of MCN, which witnessed the development of various variants of MCN over the years created through interplay of the type of silica, C, and N precursors and experimental conditions. Several researchers have studied the effect of different silica templates on the synthesis of MCN. For example, Vinu et al. also studied the impact of the SBA-15 synthesized at different temperatures on the porous features of MCNs [47] (Figure 3). The synthesis procedure resembled their earlier study except that the silica template was synthesized at three different temperatures of 100, 130, and 150°C. Consequently, the resultant MCN was also different in their crystalline structure and porous features. While the SBA-15 template can be used for the synthesis of 2D  Synthesis strategies for mesoporous carbon nitride. Hard templating (prepared using nanocasting, replication of template), soft templating (prepared using self-assembly of templating materials), non-templating (pore generation by different thermos-chemical method) and sol-gel method (prepared by silica sol and gel development). porous MCN, 3D cube like MCN can be prepared by using different types of 3D silica templates such as KIT-6 [48] and FDU-12. [49] Guo and group synthesized ordered mesoporous carbon nitride (OMGCN) by direct thermal condensation of KIT-6 (silica template) and melamine (CN precursor) at 550°C with a surface area of 279 m 2 g −1 , a C/N ratio of 0.67 and good photocatalytic dye degradation ability. [48b] Xu et al. synthesized OMGCN using CTC and ethylenediamine as CN precursor and FDU-12 as silica template via the nanocasting method. The reported material showed significantly higher catalytic activity than the 1D MCN and bulk carbon nitride (BCN) owing to its higher specific surface area (702 m 2 g −1 ). [49b] Apart from their structural properties, the nitrogen content of the MCNs can also be controlled by the degree of polymerization and infiltration of nitrogen precursors. This was again demonstrated by Vinu et al. in 2007, where they reported the formation of a 3D cage-like MCN (MCN-2) using SBA-16 as a silica template and CTC and ethylenediamine (EDA) as carbon and nitrogen sources, respectively. [50] Here, they demonstrated that when a higher amount of nitrogen precursor, EDA was used, MCNs with  [52] Copyright 2017, Royal society of Chemistry) and b) C 3 N 5 by 5-amino 1-H triazole (Reproduced with permission. [53] Copyright 2018, Wiley). a higher nitrogen content can be prepared as a result of a higher degree of polymerization between the carbon and nitrogen precursors (CTC and EDA, respectively) which retains higher nitrogen content in the MCN. MCN-2 showed a greater surface area (810 m 2 g −1 ) and pore volume (0.81 cm 3 g −1 ) compared to MCN-1, due to the 3D cage-like structure of SBA-16. Although these materials exhibit high surface area and ordered porous structure, the nitrogen content of these materials is low owing to the low thermodynamic stability of N in the carbon matrix as these materials were prepared at the temperature ≈600°C. To obtain higher nitrogen content in the MCN materials, Vinu et al. utilized the nanoparticle strategy in which nanoparticles of mesoporous silica with a size less than 100 nm were used as the templates and prepared high nitrogen-containing MCN nanoparticles (MCN-3). It was believed that the small templates help to preserve the large amount of N in the CN matrix as the energy required to break the C─N bond in the small matrix is high. [51] The nitrogen content of MCN-3 (C/N = 2.3) was higher than the previously synthesized materials, MCN-1, and MCN-2 (C/N = 4.5) but the desirable C/N ratio of 0.75 (theoretically predicted value) was yet to be reached. To synthesize carbon nitride with C/N ratio of 0.75, Vinu and his group further came up with a twofold strategy in which they made use of a single precursor with a high N content for both the C and N molecules as the carbonization/polymerization of such a precursor on silica template at a lower temperature can help reach a higher nitrogen content within the CN framework. For confirmation of their hypothesis, they prepared two MCNs namely, MCN-4 and MCN-5 with C/N ratios of 0.64 and 0.92, re-spectively, which were much lower than the C/N ratio achieved in any previously synthesized MCNs. [34e,35] In this synthesis, a single CN precursor such as, guanidine (MCN-4) or 3-amino-1,2,4triazine (MCN-5) was used and the carbonization reaction was carried out at 400 and 450°C, respectively to get ordered mesoporous structures. The difference in the starting CN precursors and the reaction temperature resulted in different C/N ratios in MCN-4 and MCN-5. With the simple adjustment of the morphology of the templates, various MCN with different morphologies, including disc, [54] spherical, [55] 2D hexagonal, and the 3D cube can also be prepared.
The mesoporosity and the amount of nitrogen in the MCNs are greatly dependent on starting precursors, templates, and reaction temperature. Vinu and the group also reported several instances of the effect of reaction temperature on the textural and physicochemical properties of MCNs. In their studies, they noted that the amount of nitrogen content in the MCN ring could be controlled by polymerization temperature. [56] To prove this, they prepared different MCNs with N/C ratios of 1.41 (C 3 N 4 ), 1.56 (C 3 N 5 ), 1.87 (C 3 N 6 ), 2.33 (C 3 N 7 ) at 550, 500, 300, and 250°C, respectively. Apart from temperature, the CN precursors also played an important role in determining the nitrogen content in the ring structure, the texture, and the chemical properties of MCN. [42] For example, different precursors like 5-amino-1H-tetrazole and aminoguanidine hydrochloride were used for the synthesis of MCN with high nitrogen contents, including C 3 N 6 (Figure 4a) and C 3 N 5 (Figure 4b). [52-53,56a] Not only mesoporosity, microporosity can also be introduced into the CN matrix by replacing www.advancedsciencenews.com www.advsensorres.com the templates of mesoporous silica with zeolites. For example, Sathish et al. reported the synthesis of microporous C 3 N 5.4 with a lot of porosity. [57] In this study, the MCN was synthesized using a nitrogen-rich precursor, aminoguanidine hydrochloride, to achieve a surface area of 130 m 2 g −1 . The presence of free -NH 2 groups, microporosity and abundant basic sites increased CO 2 adsorption (47.54 μmol m −2 ) and selective sensing for aliphatic hydrocarbons. Although silica templates play a crucial role in the preparation of MCNs in hard templating method, the requirement of toxic chemicals (HF or ammonium bifluoride) for the removal of templates makes it hard to scale up the overall process and thus there is a need for the development of a quicker and environmentally benign process.

Soft Templating
Soft templating is another feasible route for the synthesis of MCN and various organic moieties, such as amphiphilic polymers and surfactants, have been used for this purpose. These templates act as the structure-directing agents, and their careful selection is important in obtaining CN with desired porous features such as surface area, pore volume, and pore size. This method was first reported to prepare porous carbon film using the self-assembly of block copolymers (BCP) acting as a soft template. [58] This simple approach was extended for the fabrication of MCN materials. For example, Fen et al. synthesized MCNs, using Triton X-100 as a template, with a mesopore size of 10-40 nm. [59] One of the limitations of using soft templates is their low decomposition temperature, as they can decompose from the reaction mixture before the CN formation temperature is reached, which is usually ≈400°C. Wang et al. proposed a solution to this, wherein, they employed stepwise heating to control the decomposition/volatilization of the soft template. In this study, several surfactants and ionic liquids were used for the synthesis of MCN. [60] It was noted that ionic liquids (IL) are more stable at condensation temperature as compared to polymers and surfactants. By using ILs, it was possible to obtain a nanoporous structure without intermittent heating. However, the final carbon content of the synthesized MCN using IL was found to be higher, as it was suspected that the carbon from the soft template was also added to the MCN matrix. Similarly, Sheng and their group demonstrated the production of MCN spheres using melamine and cyanuric acid and an ionic liquid. [61] To further improve the porosity, MCN was prepared using P123 as a soft template and dicyano-diamide as CN precursor that resulted in good textural properties such as a high specific surface area of 299 m 2 g −1 with a pore volume of 0.128 cm 3 g −1 . Yan et al. reported a novel method for synthesizing worm like MCN, using melamine and pluronic P123 as CN precursor and template, respectively. [59b,62] In their study, the mixture of precursor and templates were passed through a series of reactions to obtain a mesoporous structure. The Brunauer, Emmett and Teller (BET) surface area of the resultant MCN increased from 30 to 90 m 2 g −1 after the calcination process, but it lacked an ordered structure. Mustapha and Sappani also used P123-5800 as a soft template for the preparation of MCNs. In this study, the melamine was polymerized with a mixture of P123-5800 and glutaraldehyde at 80°C. The calcination of the resultant material at 600°C gives the MCN with a surface area of 93 m 2 g −1 and 0.15 cm 3 g −1 pore volume. [64] The use of metals as dopants has also been explored to influence the mesoporosity on the MCN. Recently, Ahmed et al. synthesized zinc-doped MCN using a mixture of melamine, PEG-1500, and ZnCl 2 ·2H 2 O by heating it at 550°C. The material showed a surface area of 9.8 m 2 g −1 and a pore size of 18.8 nm. The mesoporosity in the material was ascribed to both soft templating and interaction between zinc and nitrogen. [44d] To lower the synthesis cost and environmental pollution and to increase industrial production, Liang et al. used a recyclable soft template, octamethyl polyhedral oligomeric silsesquioxane (mPOSS), for the synthesis of mesoporous CN. For this preparation, they dispersed mPOSS into dimethylformamide (DMF) along with melamine and calcined at 550°C. This synthesized material showed an improvement in the surface area (35.5 m 2 g −1 ) and pore diameter, along with an enhanced photocatalytic H 2 evolution, charge separation, and photo absorption. [44e] Overall, when compared to hard templating, soft templating is a relatively simpler approach to synthesize MCN. However, the associated disadvantages, such as, low-temperature decomposition of the template, formation of less ordered structures with diminished porosity, and leftover carbon after carbonization, are critical drawbacks that will need significant innovation input from the research community to obtain MCN with superior textural properties and high nitrogen contents. In the absence of any novel ideas and synthesis methods, it will be difficult to consider this method as an alternative to the hard templating approach.

Template Free Methods
Template free method is another way to introduce mesoporosity in CN that has gained significant recent interest. Templatefree methods can be classified into two separate approaches. In the first approach, MCN is directly synthesized from the precursors following a thermal polymerization, while in the second approach; mesopores are introduced in a secondary process after the synthesis of bulk CN. This was first reported by Min and Lu, who prepared MCN by pyrolysis of urea at 600°C for 4 h. The obtained material exhibits a specific surface area of 51 m 2 g −1 with a pore size of 20-40 nm, which was much lower as compared to those of MCN prepared from the hard templating routes. [65] Similarly, Wang et al. opted for a thermopolymerization approach in which urea was thermopolymerized at 550°C for 1 h with a heating rate of 10°C min −1 followed by thermal sintering at 500°C for 45 min (Figure 5a). [63] The SEM images of the prepared MCN samples confirm the successful synthesis of thermo-etched MCN with mesoporosity ( Figure 5b). The material also showed a surface area of 63 m 2 g −1 and a pore diameter in the range of 10-40 nm. Han and the group introduced an eco-friendly approach and prepared ultrathin mesoporous carbon nitride (UMCN) via thermal polycondensation of urea and ammonium carbonate at 550°C. The UMCN showed a type IV adsorption isotherm with a surface area of 85 m 2 g −1 and a pore diameter between the range of 10-30 nm and exhibited excellent performance in the degradation of organic pollutant bisphenol A and photocatalytic water splitting. [45b] Instead of generating mesopores during the synthesis process, Kumar et al. opted for a second approach and added a secondary step of ultrasonication to synthesize porous sheets like g-CN. In this process, bulk g-CN was dispersed in water: ethanol (2:1) mixture and then sonicated at optimum temperature for 5 h to generate a porous sheet. Due to their higher surface area and pore volume, the resultant porous g-CN showed higher photo-catalytic activity under visible light irradiation than bulk g-CN. [66] Similarly, Yang et al. introduced mesopores in bulk polymeric carbon nitride by using ascorbic acid. In this study, bulk polymeric carbon nitride was prepared from melamine and mixed with the ascorbic acid solution. After an ultrasonic treatment for 30 min, the mixture was transferred to an autoclave for the hydrothermal process at 180°C for 4 h. The resultant materials had type IV adsorption isotherm with an H3 hysteresis loop along with a surface area of 36 m 2 g −1 and a pore diameter of 3.8 nm and were utilized for photocatalytic H 2 generation. [45c] Using a secondary treatment approach, Zhou and his group developed the process for the synthesis of defective ultra-thin mesoporous graphitic carbon nitride (DUMCN) by a three-step heat treatment strategy. [45a] Here, melamine was first calcined at 550°C for 4 h to obtain BCN. This bulk carbon nitride was then heated again at 500°C for 3 h to achieve ultrathin mesoporous carbon nitride, following which a third heating step was done at 500°C for 2 h to get the final DUMCN material (Figure 5c,d). The N 2 adsorption isotherms are of type-IV with H-type hysteresis loops along with a surface area of 146 m 2 g −1 and 16.9 nm pore diameter. N-vacancies are gen-erated because of the high-temperature surface hydrogenation, which significantly enhanced the performance of MCN in visible light photocatalytic water splitting.
Overall, it can be concluded that the template-free method for the synthesis of MCN is a great way to avoid the use of toxic templates and harsh etching agents and is hence environmentally friendly and cost-effective as compared to both hard and softtemplating strategies. However, it comes at the cost of inferior textural, crystalline and physico-chemical properties of the MCN and will need significant improvement in the process design to be able to compete with the MCN synthesized by templating approaches.

Sol-Gel Synthesis
Sol-gel synthesis is a well-established method generally used to synthesize metal oxide nanoparticles. For synthesizing MCN through sol-gel, silica and CN precursors are processed together, which involves precursors mixing, gel development, polymerisation and calcination of CN precursor followed by silica removal. This method uses silica as an in situ formed template as compared to hard templating, where the silica template is prepared beforehand and used for nanocasting. [59b] However, this method also requires removing the silica template similar to the hard- Figure 6. Synthesis of dual pore MCN by sol-gel method. Reproduced with permission. [69] Copyright 2023, Elsevier.
templating method and can thus be considered an extended case of hard-templating. Goettmann and Antonietti, in 2006, reported MCN synthesized using 12 nm silica nanoparticles and liquid cyanamide (CA) as C and N precursors. [55a] This material showed a large 12 nm pore diameter; however, it had a disordered structure. Kailasam et al. synthesized MCN via the sol-gel route using cyanamide and tetraethyl orthosilicate (TEOS) as CN and silica precursors, respectively. [67] In this route of synthesis, the solvent was evaporated to form a composite gel which, upon calcination (550°C) and subsequent removal of silica (ammonium bifluoride (NH 4 HF 2 ) treatment) produced MCN. The synthesized MCNs had a surface area in the range of 1-167 m 2 g −1 with a pore diameter between 2.4-3.6 nm. It was observed that weight loss in this process was higher compared to other methods; hence, they could get a monolith or a thin film of MCN. Instead of using alkoxy silane precursor, TEOS, Huang et al. synthesized MCN with Ludox-HS 40 and cyanamide. [68] After drying the homogenous mixture of two precursors at 70°C to form a transparent gel, subsequent calcination at 550°C for 4 h and subsequent treatment with NH 4 HF 2 to remove the silica template produced MCN with desired textural properties. A similar method was also used by Shiraishi et al. [46a] and Cui et al. [46b] for the synthesis of MCN. Rashidi and colleagues recently synthesized highnitrogen-containing MCN with tunable pores for ultra-high gas adsorption using a method similar to hard templating. [69] They began by polymerizing a carbon nitride precursor on SiO 2 under continuous reflux for 6 h at 90°C. The polymerized material was then dried overnight and carbonized at 600°C for 5 h before being washed with HF to completely remove the SiO 2 template (Figure 6). By varying the ratio of ethylenediamine to CTC, as well as the concentration of potassium hydroxide, they were able to tune the pore structure and surface area. The sample exhibited the specific surface area of 329.1 m 2 g −1 by using a 0.55 ratio of ethylenediamine to CTC. However, a huge reduction in the specific surface area was observed when the ratio was increased.
With the simple activation with KOH, which is used during synthesis; the specific surface area of the MCN can be increased to 2036.9 m 2 g −1 . Instead of silica precursors, Idris et al. made use of sodium carboxymethyl cellulose (CMC) to form the gel material for the synthesis of MCN and varied the concentrations of CMC solution with hexamethylenetetramine, followed by gel formation (at 80°C), polymerization, calcination (600°C for 12 h) and removal of the template (washed with 1 m HCl). Upon drying of washed samples, they were able to obtain MCN with a surface area in the range of 86-132 m 2 g −1 and a pore diameter of 3.5 nm. [46c] Apart from this, MCN can be synthesized successfully using several other precursors like thiourea, urea, cyanamide, and ammonium thiocyanate by similar methods employing different porous silica templates. [30] Due to the simpler method and use of less hazardous chemicals for template removal compared to other procedures, this sol-gel method can be used for the largescale production of MCN.
In general, sol-gel synthesis is an effective alternative to the hard templating process. This mode of synthesis allows control over pore structure and morphology; however, it suffers from longer synthesis time and comparably lower specific surface areas. It also provides less control over the combination of morphology and the desired ordered pore structure. In addition, it requires a tedious and unfriendly template removal procedure analogous to the hard templating process, even though some methods may require less hazardous chemicals. Despite the increasing advantages of sol-gel-based synthesis of MCN, the inferior textural properties of MCN have resulted in keeping the hard and soft templating process as the preferred method of synthesizing MCN. It is noted that MCN synthesized using hard/soft templating, template-free, and sol-gel approaches show superior textural properties for the adsorption of analytes; however, their application potential can be further enhanced by functionalizing their surfaces with moieties that can selectively modify the properties required for the selective and sensitive detection of analytes.

Functionalization of MCN
MCNs exhibit ring-like frameworks and excess nitrogen, which offer unique electronic and optical properties and have been used for various sensing applications. In addition to the inherent properties of MCN, in situ or post-synthesis modification of MCN can also enhance its properties for sensing applications by modifying the electronic structure led by enhancement of surface charge, improvement in charge transfer properties or reduction of charge carrier recombination. For example, several non-metal atoms (S, P, and O) have been shown to influence the band gap by influencing the position of conduction and valence band and thereby influence the fluorescence properties of both g-CN and MCN that has been utilized for devising sensing assays of these ions. [70] Similarly, forming heterojunctions with metal oxides has been shown to influence the band structure and drastically influence the electronic and optical properties of g-CN and MCN.
The structural and functional properties of such functionalized MCN are highly dependent on the types of material (metal, metal oxide, nitrogen, and non-metal) loaded into the CN framework. To analyze the effect of doping on MCNs, Azimi et al. used the boron doping approach to alter the acid-base character of MCN. [71] They opted for the hard templating method wherein ethylenediamine, and CTC were polymerized inside SBA-15 and carbonized at 600°C. For boron-doped MCN (BMCN) preparation, MCN was mixed with a boric acid solution, followed by an ultrasound treatment for 30 min. The resultant material was then calcined at 600°C for 3 h to produce B-doped MCN with a high surface area (431 m 2 g −1 ). Because of the Lewis acidic and electron-deficient character of boron, the CN framework of BMCN showed an acid-base contrast which was used for increasing the adsorption of analytes. Similar to the boron-doping study, Hu et al. showed the effect of phosphorous doping on visible light absorption of MCN. [72] In this study, a mixture of melamine, SBA-15 and melamine polyphosphate was polymerized at high temperatures. Phosphorous doping helped in extending the visible light adsorption range to 460 nm. Koohard and group added magnetic properties to the MCN by creating a composite of CuFe 2 O 4 with mesoporous carbon nitride. [73] In this study, MCN and CuFe 2 O 4 were prepared separately, and then mixed materials were treated with NaOH. To create a better composite material, the mixture was transferred to an autoclave for thermal hydrolysis to obtain magnetic CuFe 2 O 4 loaded MCN with a surface area of 81 m 2 g −1 . The major advantage of this material is its stability and the fact that this could easily be separated from the reaction by using magnetic field. Such magnetic MCN could be useful for precise attachment of analyte using a magnetic field. Huang et al. used SnO 2 doping to reduce the electron-hole recombination rate of MCN. [75] The SnO 2 doped MCN (mpg-C 3 N 4 /SnO 2 ) was prepared by a two-step process via the non-templating method. The steps included, (i) synthesis of MCN by thermal condensation of melamine and ammonium nitrate and (ii) doping of SnO 2 on presynthesized MCN via in situ growth method. The morphology of this synthesized material showed a thin nanosheet-like structure. The BET surface area of mpg-C 3 N 4 /SnO 2 suggests that with an increase in the concentration of SnO 2 doping (5% to 30%), the surface area decreased gradually (37 to 29 m 2 g −1 ) whereas the pore diameter slightly increased (11.3-14.3 nm). Metals and metal oxides are not the only functionalities attached to MCN, composites of MCN with nanotubes and 2D materials have also been reported to improve its properties. For example, Chen et al. synthesized metal-free carbon nanotube-MCN (mpg-CN-CNT) composites with a specific surface area of 195 m 2 g −1 and a pore diameter of 9.6 nm via polymerization of melamine in the presence of CNT. [76] This composite showed a higher visible light activity of mpg-CN-CNT by improving the absorption of incident visible light, increased transfer of photogenerated electrons and availability of specific channels for oxygen photoreaction and adsorption. On the other hand, Vinu and the group also showed that 2D molybdenum disulfide (MoS 2 ) could be mixed with 3D MCN (CN/MoS 2 ) [77] to form a highly functional composite material. A single step hard templating method was used for the synthesis of the composite where polymerization and pyrolysis of 5-amino-1H-tetrazole (5-ATTZ), dithio-oxamide (DTO), and phosphomolybdic acid hydrate (PMA) resulted in the synthesis of CN/MoS 2 using KIT-6 (silica template). The pyrolysis temperature influenced the textural properties of the composite structure as the BET surface area and pore diameter of the composite increased with an increase in temperature to 700°C after which, surface area and pore diameter showed a decreasing trend due to the collapse of the ordered structure. In a recent study, Wang and colleagues demonstrated the functionalization of MCN with copper phosphide (Cu 3 P) for the efficient oxidation of aqueous contaminants (Figure 7). [74] The authors utilized the Fenton activity of copper to generate free radicals in the presence of hydrogen peroxide (H 2 O 2 ). The rationale behind selecting Cu 3 P was its ability to efficiently oxidize H 2 O 2 and generate free radicals, utilizing both the Cu cations and P anions. Furthermore, MCN was utilized to enhance the dispersibility, electron transfer, and catalytic performance of Cu 3 P. Based on nitrogen sorption analysis, an increase in pore volume was observed in Cu 3 P-functionalized MCN, indicating that Cu 3 P was anchored on the surface, promoting heterogeneous Fenton-like reactions and generating more free radicals for the oxidation of aqueous pollutants.
It is clear from the above examples that MCN materials can be functionalized with different groups such as metals, metal oxides, 2D layered materials, and nanotubes. The porous structure of the MCN allows the integration of secondary materials and functionalities that further improve the properties of MCN. These properties are utilized for creating better biosensors using electrochemical, photo-electrochemical, optical or gravimetric methods and are discussed in the next section.

Application of MCN in Sensing
As discussed in the sections above, MCNs, owing to their unique mesoporous structure and interesting physico-chemical properties like metal-free carbonaceous material, semiconducting nature, tuneable carbon-to-nitrogen ratio, and medium band gap, are suitable for applications in several fields including sensing, adsorption, energy storage, and catalysis. The application areas of MCNs could be widened by designing their modified counterparts. Owing to the synergistic properties of graphitic carbon nitride and the mesoporous structure, MCNs show a greater number of active sites and an easier -* electronic transition, which along with lower synthesis cost, greater biocompatibility, stability, and unique optical and electronic properties, make it more www.advancedsciencenews.com www.advsensorres.com Figure 7. Synthesis and modification of MCN with copper phosphide (Cu 3 P). Adapted with permission. [74] Copyright 2023, Elsevier.
appealing for sensing application. [81] The sections below discuss some prominent advancements made in research on MCNs in sensing ( Table 2).

Electrochemical and Photoelectrochemical (PEC) Biosensors
The development of biological and chemical sensors with high selectivity, sensitivity, portability, cost-effectiveness, and ease of operation is rapidly evolving. [82] As discussed earlier, MCNs have gained significant recognition, especially in analytical chemistry, due to their unique properties and have been used for the detection of heavy metal ions, small molecules, and various biomolecules with high sensitivity. Carbon nitride is an electrochemically active material. When any analyte molecule comes in close vicinity of carbon nitride layers, it could modify its electrochemical potential and thus provide an excellent opportunity for electrochemical sensing. The excellent charge/electron transfer properties of carbon nitride at the electrode/electrolyte interface helps in electrochemical sensing. In addition, the high surface area of MCN offers several active sites that can act as anchor points for interaction with the analytes. Doping with heteroatoms or creating a composite with metals further increases the electrochemical properties of the MCN, which have been especially used for the detection of analytes.
Zhang et al. demonstrated the effect of different amounts of Nbonding on ordered MCN for electrochemical sensing of hydrogen peroxide (H 2 O 2 ), nicotinamide adenine dinucleotide (NAD), and nitrobenzene. [84] It was found that the MCN pyrolyzed at 800°C possesses the highest amount of pyridinic nitrogen, which enhanced the electrochemical detection of these three analytes at neutral pH. In this study, a linear range of 4-40, 0.5-1000, and 2-2200 μm with a limit of detection (LOD) of 1.52, 0.18, and 0.82 μm for H 2 O 2 , nitrobenzene and NAD, respectively was reported. Zhang et al. detected heavy metal ions cadmium (Cd 2+ ) and lead (Pb 2+ ) via stripping voltammetry by using a glassy carbon electrode (GCE) modified with self-doped polyaniline nanofibers (SPAN), MCN, and bismuth. [85] Modification of GCE with SPAN, MCN, and bismuth nanoparticles was done to increase electron transfer and stability for the detection of heavy metal ions. It was possible to detect Cd 2+ and Pb 2+ with a LOD of 0.7 and 0.2 nm, respectively. Based on the Pb 2+ dependent RNA cleaving by DNAzymes, Zeng et al. introduced a selective detection of Pb 2+ via DNA/methylene blue (MB)/gold nanoparticle (AuNPs)/MCN fabricated GCE. [86] In this study, MCN and AuNPs were used for enhancement of electrical conductivity, whereas MB was immobilized for signal amplification, and the DNAzymes were used to improve selectivity. Sensing via modified GCE showed a wide linear response range of 10 −3 to 10 −14 m with a LOD of 10 −14 m for Pb 2+ .
To show the versatility of MCN in the bio-sensing field, Zhou et al. studied the detection of avian leukosis virus-subgroup J (ALVs-J) via GCE modified with a sandwich structure of thionine, MCN, and primary antibodies (Th-mpg-C 3 N 4 -Ab 2 ). [83] Schematic representation of Th-mpg-C 3 N 4 -Ab 2 based sensor preparation and fabrication is shown in Figure 8a. Thionine (Th) was used as an electron transfer mediator, whereas MCN was utilized to enhance sensitivity and to provide a platform for primary antibody binding on GCE. Based on this study, they were able to detect ALVs-J viral titer in a linear range of 10 2.08 -10 4.0 TCID 50 per mL (Figure 8b,c) with LOD of 120 TCID 50 per mL (TCID 50 : 50% tissue culture infective dose). Subsequently, MCN functionalized with biomaterials has also shown significant application with electrochemical-based sensing having remarkable selectivity and sensitivity. Ono et al. identified the selectivity of silver ion (Ag + ) toward cytosine to form a stable C-Ag + -C complex in the presence of DNA. [87] Based on this study, Tang et al. showed successful detection of silver (I) ions by using unlabeled DNA mobilized MCN via electrochemical impedance spectroscopy (EIS). [88] They showed that the hairpin-like DNA structure of gold NPs/MCN/Llys-modified GCE in the presence of Ag + changed to a duplex-like structure which led to resistance in interfacial charge-transfer of the electrode to [Fe(CN) 6 ] 4-/3− electrolyte (redox couple). A decline in charge transfer resistance was measured, which corresponded  Reproduced with permission. [83] Copyright 2016, Elsevier. B) Differential pulse voltammetry (DPV) response of immune sensor for ALVs-A and C) calibration curve (concentration of a-h range includes 10 4.0 , 10 3.78 , 10 3.48 , 10 3.18 , 10 2.90 , 10 2.60 , 10 2.30 , 10 2.08 TCID 50 /mL. Reproduced with permission. [83] Copyright 2016, Elsevier. to Ag + concentration. Electrochemical impedance spectroscopybased sensors can accurately detect Ag + in the linear range of 10 −10 to 10 −5 m with a LOD of 5 × 10 −11 m. Zhou et al. showed the chronoamperometric detection of phenol and catechol by using GCE/MCN/Tyrosine biosensor. [89] GCE/MCN/Tyr, due to its selectivity, enzyme immobilization ability and easy charge transfer, can selectively detect phenol and catechol in the linear range of 50 nm-9.5 μm and 50 nm-12.5 μm, respectively. Chronoamperometric detection by this method showed the LOD of 10 and 10.24 nm for phenol and catechol, respectively. Due to their fast response, strong operability, high selectivity and sensitivity, enzyme-based DNA sensors are getting special attention in various disciplines. After successfully detecting Ag+, phenol and catechol, the same group detected manganese peroxidase genes using an electrochemical DNA sensor. [90] Electrochemical DNA sensors made up of streptavidin (SA)-horseradish peroxidase (HRP) conjugated on gold nanocluster (GNCs), MCN and a biotinylated detection probe modified electrode. In this study, the SA-HRP scaffold with GNCs served for triple signal amplification for biosensing. In contrast, GNCs and MCN on the surface make the probe an efficient conductor for accelerating electron transfer. Through this approach, nucleic acid was successfully detected in a linear range of 10 −17 to 10 −9 m with LOD of 8.0*10 −18 m.
Like enzymes, biomarkers are also important diagnostic tools for diseases like cancer. Ma et al. showed a single-step synthesis procedure for producing a luminol-H 2 O 2 -based electro chemiluminescent (ECL) sensor using MCN and Au NPs for the detection of prostate-specific antigen. [91] A combination of MCN and Au NPs was selected because of the high specific surface area and porous nature. MCN provided the anchoring sites for Au NPs and showed a synergistic catalytic effect on the luminol-H 2 O 2 system. In addition, MCN-Au NPs can be associated with a primary antibody via an Au-NH 2 bond to increase the selectivity of the ECL sensor. In this study, by using MCN-Au NPs-based ECL biosensor, PSA detection was achieved in a linear range of 0.001-15 ng mL −1 with LOD of 0.927 pg mL −1 .
Humidity sensors are another major type of sensor that is widely employed in areas like health, meteorology, and the protection of precise equipment. [92] Various technologies like the Fabry-Perot interferometer, quartz crystal microbalance and surface acoustic wave have also had a major impact on the advancement of humidity sensors. [93] Various sensing materials with high sensitivity, swift response/recovery, and long stability have been developed, which are crucial for humidity sensors. [94] The unique physicochemical properties and easy structural modification contributed to making carbon nitride-based RH sensing using EC technique. Carbon nitride is considered as an optimal material for RH sensing due to its high active site density, large specific area, semi-conducting nature, and high sensitivity to water adsorption. [95] Metal NPs are usually combined with MCN to enhance the efficacy of humidity sensors. Malik et al. reported on the indium nanoparticles loaded MCN nanocomposite for the detection of humidity with high accuracy in 11-98% RH range in an indoor climate. This was achieved by the preparation of In-SnO 2 /meso-g-C 3 N 4 nanocomposites. Indium was opted for its high conductivity, stability, and capacity to accelerate the chemical reaction in the sensors. This also demonstrates the high response with ≈5 orders of magnitude change in the impedance when switched between 11% RH and 98% RH at 25°C. [96] The humidity sensing capacity of the synthesized material was measured by evaluating the change in impedance with respect to the humidity changes in the 11-98% RH range. The factor that resulted in the doubled response time could be the mesochannels of MCN that enhance the sensing response. This also aided in improving the linearity of the sensor over the complete %RH range. The response and recovery time at room temperature was as low as 3.5 and 1.5 s, respectively and shows high potential for monitoring indoor climatic conditions. Similarly, Tomer et al. demonstrated that silver NPs modified MCN could be used for creating a reversible RH sensor. [97] The c-mpg-CN synthesized using KIT-6 as a hard template showed ≈4 orders of magnitude change in impedance in the 11-98% RH range, and its sensitivity was found to be 2.3 times higher than that of g-CN. The presence of Ag nanoparticles in c-mpg-CN also not only enhances the response but also improves the linearity of the nanocomposite sensor. Ag@mpg-CN material showed 3 and 1.4 s as response and recovery time, measured under different %RH levels. This sensor can be widely used in health and environment monitoring applications.
While the versatility of electrochemical sensing has made it popular among researchers working in the area of sensing, especially using MCN, PEC is another technique that is becoming one of the primary sensing techniques in analytical chemistry. [99] In PEC sensing, a photoactive material absorbs the photons, generating photo-excited charge carriers that facilitate an oxidation or reduction reaction at the electrode surface with an electroactive species in the solution. The addition of the analyte in the electrolyte either acts as an oxidant or a reductant by itself or, sometimes, interferes with the redox process resulting in a photoelectrochemical change that can be used for its detection. g-CN is used extensively as an active material in PEC sensing though only a few reports exist comparatively on the use of MCN for PEC sensing applications. In most of the MCN-based PEC sensors, MCN acts as a matrix for capturing light owing to its band gap in the visible range. The generated excited carriers, usually present at the interface, are transferred to other active sensitizing material, which then drive the electrochemical oxidation or reduction reaction resulting in signal generation. As carbon nitrides, in general, are good anchors of small organic molecules and gas molecules due to available edge orbitals, they promote interactions with the analytes. In fact, the number of edge atoms is very high in carbon nitride and perhaps the highest amongst 2D materials owing to the inherent atomic vacancies in its lattice. Additionally, doping with heteroatoms and creating composites with other semiconductors further tune its band gap and make it suitable for PEC sensing. These properties make them ideal materials for PEC biosensors. [100] MCN has been used for the PEC detection of biomolecules, heavy metal ions and even pharmaceutical compounds. Zheng et al. demonstrated a PEC detection of prostate-specific antigen (PSA) in human serum using anti-PSA, SnS 2 , and mesoporous g-CN (g-C 3 N 4 ) modified indium tin oxide (ITO) electrode. [101] SnS 2mpg-C 3 N 4 is considered as an ideal material for sensing of PSA owing to an overlapping band structure and an excellent photocurrent response due to the recombination of electron-hole pair plus specificity of anti-PSA and exhibited a PSA detection with a linear range of 50 fg mL −1 -10 ng mL −1 and a LOD of 21 fg mL −1 . The same research group also used SnO 2 /SnS 2 /mpg-C 3 N 4 as a PEC enhancer and secondary antibody conjugated with PbS/SiO 2 as a reducer for detecting the N-terminal probrain natriuretic peptide (NT-proBNP) via PEC (Figure 9a). [98] In this study, they were able to detect NT-proBNP in a linear range of 0.1 pg mL −1 to 50 ng mL −1 with a LOD of 0.05 pg mL −1 (Figure 9b,c). Following this, Bonyadi et al. showed, for the first time, a PEC sensing of epinephrine (EPI), paracetamol (PAR), mefenamic acid (MFA), and ciprofloxacin (CIP) via g-C 3 N 4 /polyaniline (PANI)/CdO nanocomposite. [102] Based on their analysis, they got the LOD of 0.011, 0.045, 0.026, and 0.005 μm for EPI, MFA, PAR, and CIP, respectively. After the study of Bonyadi and the group, Yanalak et al. demonstrated the sensing of PAR by fabricating GCE with MCN/black phosphorous (BP)-AuNPs. [103] Such a fabrication resulted in the formation of heterojunction and an increased rate of adsorption with excellent electrical properties. By using this heterojunction, they could detect PAR in a linear range of 0.3-120 μm with a LOD of 0.0425 μm. As discussed in this section, both EC and PEC sensing are popular techniques for detecting analytes using MCN. In these applications, MCN usually serves as the matrix, a signal enhancer, photon absorber or a facilitator of charge transfer. It is optimal band gap and charge separation/recombination makes it popular not only for PEC sensing but also, for optical sensing which is discussed in the next section.

Optical Biosensors
Optical biosensors generally function based on the effect of analytes on the optical properties of sensing materials. Optical sensors are known for their excellent capability for providing high sensitivity, easy detection, adaptability, and biocompatibility in different conditions. The recombination of photoexcited electrons and holes in the carbon nitride results in the generation of a strong photoluminescence (PL) response. The interaction of carbon nitride with heavy metal ions or other molecules provides a non-radiative pathway for the charge carriers to lose energy, reducing or quenching the inherent PL and is used for their optical sensing. The high surface area of MCN provides a large number of sites for the interaction of analytes, thereby enhancing the PL quenching response. Lee et al. first studied the use of MCN in copper ion (Cu 2+ ) detection. [104] In this study, they showed a reduction in PL intensity of MCN proportional to the concentration of Cu 2+ . The reduction of PL activity was also selective for Cu 2+ due to its ability to capture photo-excited electrons. Based on this, they could detect Cu 2+ ions in a linear range of 100 nm to 1 mm with 12.336 nm LOD. After the successful sensing of Cu 2+ ion, the same group used this Cu 2+ -MCN conjugate as chemosensors for sensing cyanide ions (CN − ) in an aqueous solution. [105] In this study, they prepared Cu 2+ -MCN by simply mixing mpg-C 3 N 4 and Cu(NO 3 ) 2 . CN − is selective to form stable [Cu(CN) n ] (1−n) species Figure 9. A) Photocurrent-generating (left) and quenching (right) mechanism of PEC sensor for sensing of NT-proBNP. B) Calibration curve of photocurrent-logarithm of antigen concentration and C) photocurrent of immunosensor with different concentration of NT-proBNP antigen (a-h includes concentration of 0.1, 0.5, 1, 10, 100, 1000, 10 000, and 50 000 pg·mL −1 ). Reproduced with permission. [98] Copyright 2018, American Chemical Society.
with Cu 2+ , which led to the restoration of inherent PL activity of MCN. In the presence of CN − , Cu 2+ -MCN acts as a turn-on fluorescence sensor and with this, they were able to get LOD of 80 nm. Based on the fluorescence property of MCN, Yuliati and the group studied the detection of N-nitrosopyrrolidine (NPYR). [106] MCN generally shows two excitation sites (275 and 370 nm) due to terminal N─C and N═C groups. NPYR favored electrostatic interaction with MCN via N─C site; hence, the fluorescent intensity of MCN gradually decreased with the increase of NPYR up to 0.80 μmol. Above this concentration, due to the unavailability of N─C site, NPYR could not lower the fluorescent intensity; hence, by using MCN, they could detect NPYR in range of 0.35-0.80 μmol. Obregn et al. also showed the role of MCN in fluorescence-based sensing of Au +3 . [107] This study showed that the decrease in fluorescence intensity of MCN depended on the amount of Au 3+ . Based on this observation, Au 3+ was detected with a LOD of 1.1 μM with a linear range of 0-15 μm.
In addition to the inherent absorption and emission properties of carbon nitride-based materials, its inherent oxidizing ability is also used for the detection of analytes. This ability of carbon nitrides to oxidize substrates is often compared with biological enzymes and is regarded as nanozyme-like activity. Carbon nitride can depict both oxidase (direct oxidation of substrates) and peroxidase (reducing H 2 O 2 to hydroxyl radicals) like an activity that can be enhanced by creating its composite with other metals and metal oxides. The nanozyme activity of carbon ni-trides is used to oxidize various dyes for colorimetric detection of H 2 O 2 and other biological molecules by fabrication of enzymatic immunoassays. Especially, the large number of active sites in MCN facilitates the oxidation of a large number of substrate molecules and increases the overall activity. For example, Ahmed et al. developed colorimetric sensing of H 2 O 2 by using zinc (Zn) doped mesoporous graphitic carbon nitride (Zn-mpg-C 3 N 4 ) thin nanosheets. [44d] Zn doping in mpg-C 3 N 4 increased charge separation, electron transfer and band gap, which helped them to oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) aided by H 2 O 2 . By using Zn-mpg-C 3 N 4 , they were also able to detect H 2 O 2 in a linear range of 10-2000 μm with LOD of 1.4 μm. Increased peroxidase activity of MCN after doping of Zn, opens a new avenue for the doping of different metals for the detection of H 2 O 2 and other analytes associated with it. Pend et al. synthesised Fe-doped mesoporous g-C 3 N 4 (Fe-mpg-C 3 N 4 ) with high surface area and a small band gap, and found that the presence of Fe makes it peroxidase competent. [108] Owing to peroxidase mimetic activity, Fe-mpg-C 3 N 4 effectively oxidize TMB in the presence of H 2 O 2 . Based on this observation, a colorimetric sensor for the H 2 O 2 with a linear range of 0.005 to 400 μm with a LOD of 0.005 μm was developed. Since H 2 O 2 is a by-product of glucose oxidation carried out by glucose oxidase (GOx), and therefore the concentration of H 2 O 2 is directly associated with glucose concentration in the presence of GOx, they were able to sense glucose molecules in the range of 1-1000 μm with LOD of 0.5 μm.

Gravimetric Sensing Using Quartz Crystal Microbalance
Greater industrialization and urbanization have led to an increase in the emission of volatile organic gases (VOCs). Generation of these gases by transportation, fuel combustion, furniture, building and decorative materials causes numerous health hazards, including nausea, headaches as well as other lethal diseases like cancers. The presence of VOCs in the atmosphere is, thus, extremely dangerous for community health and the environment and therefore, there is an urgent need for developing techniques for selective sensing of VOCs. [109] Many sensors are available in the current market, and quartz crystal microbalance (QCM) is among them. QCM is an ultrasensitive thickness shear mode (TSM) device having thin film metal electrodes and a single crystal quartz plate on its surface. [110] This works on the principle of the Saurbrey equation wherein the difference of mass added and removed from the electrode surface (Δm) leads to a frequency shift (Δf). [111] Based on this principle, the sensing of VOC gases was first demonstrated by Vinu and the group using MCN. [112] They assessed the sensing capability of novel nitrogen-containing mesoporous carbon with well-ordered pores (NMC-G) for different VOCs such as acetic acid, ethanol, and toluene. The same group also synthesized MCN with a different precursor, 3-amino-1,2,4-triazole (3-AT), to selectively sense formic acid. [34e] Synthesized material showed a surface area of 472 m 2 g −1 with a pore diameter of 5.5 nm, which plays a major role in enhancing the sensing of formic acid. The massive uptake of formic acid was attributed to the presence of pyridine molecules with weak bases and a dipole moment of 2.26 in NMC-G. Formic acid (1.4 D) interacts with this MCN more strongly due to the higher dipole moment difference compared with acetic acid (1.7 D). [14e] Jia et al. prepared meso-microporous carbon nitride film via a combination of hard and soft templating approaches using polystyrene spheres and polyethyleneimine. [113] In this study, they used several VOC gases such as propanol, acetone, acetic acid, ethyl acetate, toluene, and cyclohexane and showed higher specificity for acetic acid in the presence of surface NH and NH 2 groups. It was shown that the oxidation of the mesoporous framework of MCN in UV/ozone could switch the selectivity of the MCN toward basic molecules such as aniline owing to the presence of a large number of carboxyl, hydroxyl, and N-oxides that may strongly interact with amines, resulting in a high selectivity of sensing of aniline. Srinivasan et al. also showed the use of metal-free MCN nanosheet in selective sensing of ethanol at room temperature. [114] This nanosheet had shown a sensing response rate of 182.4 for 50 ppm of ethanol with a linear detection range of 0.5-50 ppm at room temperature. Tomer et al. demonstrated the sensing of ethanol via novel silver nanoparticles (Ag NPs)-loaded MCN in ppm level with a linear range of 1-50 ppm. [115] Their material showed a higher response rate (R a /R g = 49.2), full recovery within 7 s, and long-term stability, showing their real-time application in the environment. Loading of Ag NPs increases MCN's electronic conductivity, which in turn enhances the selectivity of ethanol sensing. They also demonstrated the sensing of ethanol at different temperatures to show the effect of temperature affecting the performance and found out that <100°C is not suitable for sensing 1 ppm ethanol but 100-250°C is ideal for the sensing of 5-100 ppm ethanol. The reason behind the selective detection of ethanol is the higher sur-face area and ordered pore channels as well as the presence of oxygen sites on Ag NPs at higher temperatures, which increases ethanol adsorption. After showing the importance of Ag-MCN in ethanol sensing, Chaudhary et al. showed the ability of Ag-V 2 O 5 -MCN prepared by the hard templating method for effective sensing of xylene with sensing response values of 4.9 and 12.7 at 40°C for 50 and 500 ppm, respectively. [116] Like MCN, V 2 O 5 is also an n-type semiconductor with a unique layered structure, narrow band gap, non-toxic nature, high chemical stability, and excellent electrochemical attributes. Due to the absorption of oxygen on Ag NPs in the presence of xylene gas, an upsurge of the electrons in the conduction band of Ag NPs and sensor surface leads to enhancement of response rate even at lower working temperatures. The same group also demonstrated the sensing of acetone using platinum and MoO 3 doped MCN (Pt-MoO 3 /mpg-CN). [117] The synthesized hybridized gas sensors exhibited good linearity, selectivity, fast response recovery time, reusability, and long-time stability. Pt-MoO 3 /mpg-CN also showed temperaturedependent sensing for acetone (175°C), toluene (250°C), ethanol (225°C), n-butanol (275°C) with fast response and recovery times in the range of 8-16 s and 5-12 s, respectively. The sensing of different analytes at different temperatures was due to dissimilar molecular interactions and divergent partial pressure of gases. Among different analytes, nanohybrid sensor shows a selective sensitivity for acetone (25 ppm) with different temperature range (40-150°C). In another report, Malik et al. showed the sensing of VOCs (including formaldehyde, acetone, toluene, and ethanol) at low temperature using palladium-tungsten trioxide (Pd-WO 3 ) loaded MCNs which were synthesized via hard templating method. [118] WO 3 was selected as the material of choice owing to its tunable bandgap and good resistance to photo corrosion whereas palladium nanoparticles were used due to their synergistic chemical and electronic sensitization in gas sensors. Apart from this, the formation of heterojunction between MCN and WO 3 as well as, two homo junctions in as-synthesised material play an important role in sensing response. Pd-WO 3 loaded MCN sensors showed a temperature dependent sensing and was able to sense formaldehyde (120°C), toluene (160°C), ethanol (200°C), and acetone (200°C) with an average response time ranging between 6-9 s. The same group also showed the selective sensing of toluene by In(III)-SnO 2 loaded MSN synthesized via hard templating method using KIT-6 silica template. [119] This sensor exhibited excellent selectivity for 50 ppm toluene at 200°C with selective response of 61.4 (R a /R g ) along with response time of 4 s. This sensor is also capable to detect 50 and 100 ppm of toluene gas at 90°C. Selectivity of these sensors is majorly attributed to the formation of heterojunction. Overall, MCNs offer exciting properties such as high surface area, high and tunable nitrogen content, good thermal and chemical stability, surface functionalization, and good band gap which render them a good platform for detection of various analytes.

Conclusion and Future Perspective
MCNs are a unique class of materials that are finding increasing interest in the field of sensors due to their attractive features, including low-cost synthesis, preparation ease, high surface area, tunable pore size, shape and surface functionalities, basicity, conductivity, and an appropriate band gap (Figure 10). From the materials point of view, the emergence of the latest research in MCNs has led to a greater understanding of their synthesis and physico-chemical features. The discovery of the new nitrogen-rich forms of MCNs such as C 3 N 5 , C 3 N 6 , and C 3 N 7 has proved remarkable for several fields in the energy sector owing to the enhanced absorption, electronic, basic, and semiconducting properties. In this review, we have discussed various conventional methods for the synthesis of MCNs, such as hard and soft templating, sol-gel, and template-free approaches by including recently reported materials and their properties. The various physico-chemical features of MCNs can be suitably manipulated through variations of the experimental parameters such as silica template, precursor, synthesis temperature, synthesis residence time, and so on. One of the challenges in synthesis lies in introducing a high surface area at high temperatures with the preservation of high nitrogen content in the structure. Controlling the mesoporosity and specific morphology also requires significant attention. The research can also be directed to explore more stable and environmentally sustainable silica templates for the synthesis of MCNs. [57,120] The use of biodegradable templates may avoid the use of hydrogen fluoride or ammonium bifluoride for template removal. Soft templating and template-free methods have shown advantages of cost-effectiveness, reduced use of hazardous chemicals, and could be more suitable for large-scale production of MCNs.
From a sensing point of view, MCNs could become frontrunner materials in the future. With increased research interest, the use of MCNs for real-time detection of analytes in minimal time using a pocket-size device is not far from realization. More research into the functionalization of MCNs with metals/metal oxides for improved sensing activity will add new insights into the field. Similarly, even though multiple forms of carbon nitride with high nitrogen content have been discovered and used for other applications, their use in sensing applications has not been explored to a large extent due to certain challenges (Figure 10). High nitrogen content along with a high surface area of MCN in mesoporous domains, can act as a potential adsorbent for sev-eral analytes, and loaded materials can react with adsorbed material to achieve desired reactions for enhanced sensing abilities. Even though the enzymatic activity of graphitic carbon nitride has been explored extensively, MCN has not been used extensively to take advantage of this activity. One of the challenges in owing to its large surface area. This also leads to agglomeration of already large-sized MCN which, interferes with the measurement. Thus, improving the dispersion of MCN is one of the major requirements that need further research. In this review, we focused on the use of MCNs for various sensing techniques, including photo-electrochemical, optical, and quartz microbalance. The very recent literature was cited, discussed, and compared to uncover the potential for MCNs for sensing. Even though graphene, due to high electronic mobility, is highly sensitive, however it being semi-metallic is not apt for various excitonic based sensors. Doped graphene systems have band gap opening, however at the expense of electronic mobility. MCN excellently fits in well in this role, owing to its band gap in the visible range and the immense feasibility of band gap engineering in a seamless manner. The integration of the graphene-based nanostructures with MCN and their applications in sensing has not been explored at all even though they have unique capabilities of semiconducting and conducting properties in a single system. It is also surprising that the use of MCN with other 2D nanostructures, including Mxene, carbon dots, Xenes, and other layered nanostructures for sensing applications is not explored. The introduction of these nanostructures in MCN is expected to offer unique sensing capabilities, which will widen the detection capabilities of these materials. Finally, integrating the MCNs-based electrodes into miniature devices is a feasible strategy for fabricating smart sensors for various sensing applications. Further research is warranted for improving its dispersion, agglomeration, and electrical and optical activities.
www.advancedsciencenews.com www.advsensorres.com Vaishwik Patel is a final-year Ph.D. candidate at the Global Innovative Centre for Advanced Nanomaterials (GICAN), University of Newcastle, where he is working in the field of bio-nanotechnology. His research primarily focuses on the development and functionalization of different metal/metal oxide and porous materials for selective sensing applications. He is also involved in the study of cancer immunotherapy and drug delivery using various nanomaterials, to improve the efficacy and specificity of targeted therapies while minimizing side effects. Ajayan Vinu is the Global Innovation Chair Professor and director of the Global Innovative Centre for Advanced Nanomaterials (GICAN) at the University of Newcastle. He introduced a new field of research on nanoporous nitrides and developed novel methods for making nanoporous materials culminating in multiple reports of the world's first mesoporous carbon nitride, boron nitrides, boron carbon nitrides, biomolecules, and fullerenes for applications in sensing, energy storage, fuel cells, adsorption and separation, and catalysis.