Unveiling Versatile Applications and Toxicity Considerations of Graphitic Carbon Nitride

Metal-free, low-cost, organic photocatalytic graphitic carbon nitride (g-C3N4) has become a promising and impressive material in numerous scientific fields due to its unique physical and chemical properties. As a semiconductor with a suitable band gap of ~2.7 eV, g-C3N4 is an active photocatalytic material even after irradiation with visible light. However, information regarding the toxicity of g-C3N4 is not extensively documented and there is not a comprehensive understanding of its potential adverse effects on human health or the environment. In this context, the term “toxicity” can be perceived in both a positive and a negative light, depending on whether it serves as a benefit or poses a potential risk. This review shows the applications of g-C3N4 in sensorics, electrochemistry, photocatalysis, and biomedical approaches while pointing out the potential risks of its toxicity, especially in human and environmental health. Finally, the future perspective of g-C3N4 research is addressed, highlighting the need for a comprehensive understanding of the toxicity of this material to provide safe and effective applications in various fields.


Introduction
Graphitic carbon nitride (g-C 3 N 4 ), as the most stable and resistant allotrope out of five polymorphs of carbon nitrides (α-C 3 N 4 , β-C 3 N 4 , cubic-C 3 N 4 , pseudocubic-C 3 N 4 , and graphitic-C 3 N 4 ), is a novel metal-free organic semiconductor with several notable physiochemical properties.In the last decade, g-C 3 N 4 has attracted remarkable attention in research due to its prospective application in numerous fields such as photocatalysis, energy storage, electrochemical sensors, and biomedical engineering.The attribute "graphitic" refers to the layered structure with individual heterocyclic layers bound together by weak van der Waals forces like graphite.The hexagonal network consisting of C-N covalent bonds is mainly formed by s-triazine (triazine) or tri-s-triazine (heptazine) units.The general formula C 3 N 4 could be misleading, as the theoretical C/N ratio of 0.75 is hardly obtained by synthesis and the g-C 3 N 4 also contains a considerable amount of hydrogen from amino uncondensed groups.With the convenient band gap energy of ~2.7 eV, g-C 3 N 4 is an attractive photocatalyst active under the irradiation of visible light, suitable for systems utilising sunlight [1][2][3][4].
Evaluating the resource efficiency of g-C3N4 synthesis methods is essential for assessing its overall sustainability.Green synthesis routes that minimise energy consumption, waste generation, and the use of hazardous materials are preferred.The synthesis of g-C3N4 is influenced by several key factors that determine its structural, morphological, and functional properties.These factors include the choice of precursors, synthesis method, temperature, and reaction atmosphere (Figure 1).By carefully controlling these factors, researchers can tailor the properties of g-C3N4 to suit specific applications, ranging from photocatalysis and water splitting to sensors and biomedical applications.

General Conditions and Precursors of g-C3N4 Green Synthesis
One of the most commonly used methods of g-C3N4 synthesis is the calcination of various nitrogen-and carbon-rich precursors, generally melamine, dicyandiamide, thiourea, and urea, under the O2 or N2 atmosphere [4,9].These precursors undergo thermal polymerisation or condensation reactions, occurring between 400 °C and 600 °C, while the building units and g-C3N4 polymer network fully form above 500 °C.When passing the

General Conditions and Precursors of g-C 3 N 4 Green Synthesis
One of the most commonly used methods of g-C 3 N 4 synthesis is the calcination of various nitrogen-and carbon-rich precursors, generally melamine, dicyandiamide, thiourea, and urea, under the O 2 or N 2 atmosphere [4,9].These precursors undergo thermal polymerisation or condensation reactions, occurring between 400 • C and 600 • C, while the building units and g-C 3 N 4 polymer network fully form above 500 • C. When passing the temperature of 600 • C, the polymer becomes unstable, and beyond 700 • C it is decomposed completely [10].
During the preparation of g-C 3 N 4 , the most important factors that influence the composition and properties of the end product are precursor selection and polymerisation conditions, i.e., the temperature of calcination and heating rate.Zheng et al. conducted a study in which melamine and urea were chosen and compared as reagents for g-C 3 N 4 formation solely through thermal polymerisation at varying temperatures (500, 550, 600, and 650 • C).It was found that the surface area of g-C 3 N 4 gradually increased with an increasing pyrolysis temperature in both cases; however, urea-derived g-C 3 N 4 was overall a lot less dense and lighter, with a highly porous, "sponge-like" structure in comparison to melamine-derived g-C 3 N 4 with a high order of crystallinity.This phenomenon is attributed to a much larger atmosphere of gases (NH 3 , H 2 O, CO 2 ) generated from the pyrolysis of urea, creating bubbles that may function as gas templates, which increases the distance between the layers.Samples derived from urea also showed better adsorption and photocatalytic abilities [11].
More than the calcination temperature itself, a major factor in controlling the outcome of g-C 3 N 4 products may be the heating rate during polymerisation.Sharma et al. compared g-C 3 N 4 prepared from urea and melamine at varying heating rates.They found that g-C 3 N 4 synthesised from both urea and melamine showed enhanced photocatalytic and surface properties when produced at a slower heating rate of 2 • C/min in comparison with g-C 3 N 4 prepared at a high heating rate of 10 • C/min [12].In addition to the calcination described above, i.e., the thermal polymerisation of nitrogen-and carbon-rich precursors, other methods for the preparation of g-C 3 N 4 are known, including solvothermal and hydrothermal methods, molten salt synthesis, and chemical vapour deposition (CVD).In solvothermal synthesis, g-C 3 N 4 is prepared in a solvent (either water or an organic solvent) under high pressure and temperature, which can result in various morphologies of g-C 3 N 4 , such as nanosheets or g-C 3 N 4 with a porous structure [13].Using the molten salt method, g-C 3 N 4 with a high degree of crystallinity and a large surface area can be prepared.In this technique, molten salts at high temperatures, such as chlorides, nitrates, and carbonates, are utilised as the reaction medium.This method is also often used in the preparation of g-C 3 N 4 nanosheets [14].CVD can be utilised to form a thin film of g-C 3 N 4 on a suitable substrate using gas-phase precursors at elevated temperatures.This method yields thin g-C 3 N 4 films with controlled thickness and excellent uniformity [15].

Preparation of g-C 3 N 4 Nanomaterials
Bulk g-C 3 N 4 without further treatment, activation, or modification exhibits inadequate photocatalytic properties due to a relatively low surface area and few active sites, low solarlight adsorption, and fast recombination of electron-hole pairs [16].There is an important emphasis on the preparations of g-C 3 N 4 nanomaterials mainly in the form of nanosheets or even nanotubes, which exhibit noticeably enhanced physiochemical properties compared to bulk g-C 3 N 4 .Furthermore, for improved properties of g-C 3 N 4 , elemental doping with nonmetals (B, S, O) or metals (Ni, Co, Mg) or doping with various molecules and materials (TiO 2 , carbon nanotubes) to introduce certain functional groups into the structure of g-C 3 N 4 and many other methods are also being explored as means of g-C 3 N 4 functionalisation for technological applications [17][18][19].
Nanosheets of g-C 3 N 4 with a hexagonal lattice similar to graphene are typical examples of 2D nanomaterials, and their formation can be achieved through two different approaches, regarded as the "top-down" and "bottom-up" methods (Figure 2).When utilising a "top-down" method, the layered bulk material is delaminated into nanosheets.Nanosheets of g-C 3 N 4 with a thickness of around 2 nm can be obtained by simple thermal oxidation of bulk g-C 3 N 4 in the air atmosphere [20].The other most commonly used top-down methods are chemical, ultrasound, or microwave-assisted exfoliation [21,22].In contrast, a "bottom-up" method typically relies on the use of various templates to build the desired materials out of the most elementary units-atoms and molecules.In the hard-template method, a rigid template of pre-synthesised material, e.g., silica, alumina, or clays, acts as a mould to give shape to the precursor material.On the other hand, soft templating relies on the use of surfactants, block copolymers, and ionic liquids to act as templating agents to assist in the assembly of the precursor material into a porous structure [23].Gas-template methods could also be included in bottom-up soft-template methods.Easy one-step synthesis with the use of NH 4 Cl as a gas-template agent could be used for the preparation of 3.1 nm g-C 3 N 4 nanosheets directly from dicyandiamide as a precursor [24].The main advantages of the top-down approach are scalability and reproducibility, while the bottom-up approach enables the direct synthesis of nanosheets with uniform size and crystallinity [25].In addition to 2D g-C 3 N 4 nanosheets, 1D nanotubes of g-C 3 N 4 have also shown significant performance enhancements.Mo et al. successfully prepared nitrogen-rich g-C 3 N 4 with a tubular structure by calcination of a supramolecular precursor under an NH 3 atmosphere.The precursor for g-C 3 N 4 nanotubes was prepared using the hydrothermal method through the supramolecular self-assembly of melamine with hydroxylammonium chloride.The resulting g-C 3 N 4 nanotubes exhibited a significantly increased specific surface area, approximately 16 times higher than bulk g-C 3 N 4 [26].
Int. J. Mol.Sci.2024, 25, 7634 4 of 24 template method, a rigid template of pre-synthesised material, e.g., silica, alumina, or clays, acts as a mould to give shape to the precursor material.On the other hand, soft templating relies on the use of surfactants, block copolymers, and ionic liquids to act as templating agents to assist in the assembly of the precursor material into a porous structure [23].Gas-template methods could also be included in bottom-up soft-template methods.Easy one-step synthesis with the use of NH4Cl as a gas-template agent could be used for the preparation of 3.1 nm g-C3N4 nanosheets directly from dicyandiamide as a precursor [24].The main advantages of the top-down approach are scalability and reproducibility, while the bottom-up approach enables the direct synthesis of nanosheets with uniform size and crystallinity [25].In addition to 2D g-C3N4 nanosheets, 1D nanotubes of g-C3N4 have also shown significant performance enhancements.Mo et al. successfully prepared nitrogen-rich g-C3N4 with a tubular structure by calcination of a supramolecular precursor under an NH3 atmosphere.The precursor for g-C3N4 nanotubes was prepared using the hydrothermal method through the supramolecular self-assembly of melamine with hydroxylammonium chloride.The resulting g-C3N4 nanotubes exhibited a significantly increased specific surface area, approximately 16 times higher than bulk g-C3N4 [26].As has been shown, the synthesis of g-C3N4 can be achieved through various methodologies, each affecting the material's efficiency, morphology, and purity.Each synthesis method offers distinct advantages and trade-offs, impacting the suitability of g-C3N4 for specific applications such as photocatalysis, water splitting, and sensor technologies.These are summarised in Table 1.

Thermal polymerisation
Produces bulk g-C3N4 with high crystallinity and decent photocatalytic properties.However, this method often results in a limited surface area and poor dispersion in solutions.
High purity with moderate efficiency due to limited surface area and electron-hole recombination issues.

Solvothermal and hydrothermal methods
Creates g-C3N4 with various morphologies, such as nanosheets or nanospheres, and can introduce porosity.
Higher surface area and enhanced photocatalytic activity compared to bulk g-C3N4.
Purity depends on the solvent and reaction conditions but is generally good.As has been shown, the synthesis of g-C 3 N 4 can be achieved through various methodologies, each affecting the material's efficiency, morphology, and purity.Each synthesis method offers distinct advantages and trade-offs, impacting the suitability of g-C 3 N 4 for specific applications such as photocatalysis, water splitting, and sensor technologies.These are summarised in Table 1.

Thermal polymerisation
Produces bulk g-C 3 N 4 with high crystallinity and decent photocatalytic properties.However, this method often results in a limited surface area and poor dispersion in solutions.
High purity with moderate efficiency due to limited surface area and electron-hole recombination issues.

Solvothermal and hydrothermal methods
Creates g-C 3 N 4 with various morphologies, such as nanosheets or nanospheres, and can introduce porosity.
Higher surface area and enhanced photocatalytic activity compared to bulk g-C 3 N 4 .Purity depends on the solvent and reaction conditions but is generally good.

Template-assisted synthesis
Produces porous or structured g-C 3 N 4 with high surface areas and tailored morphologies.
High surface area leads to improved catalytic performance.Template removal can impact purity if not performed thoroughly.

Synthesis Type Result Efficiency and Purity
Chemical vapour deposition (CVD) Yields thin films of g-C 3 N 4 with controlled thickness and high uniformity.
Very high purity and good control over material properties, leading to efficient catalytic and electronic applications.
Molten salt synthesis Produces g-C 3 N 4 with a high degree of crystallinity and large surface area.
High efficiency due to better dispersion and surface properties.Purity is generally high but depends on the post-synthesis washing to remove residual salts.

Doping and composite formation
Enhances electronic properties, photocatalytic activity, and stability.
Creates composites with synergistic properties.
Improved efficiency due to enhanced charge separation and increased active sites.Purity can be impacted by the choice of dopants and composite materials.

Exfoliation techniques
Produces ultrathin g-C 3 N 4 nanosheets with high surface areas and excellent catalytic properties.
High efficiency due to increased active sites and better light absorption.Purity is generally high if exfoliation is performed without introducing impurities.

Applications of g-C 3 N 4
g-C 3 N 4 showcases outstanding multifunctionality and enhanced performance in various applications, positioning it as a leading material in photocatalysis and related fields (Figure 3).

Electrochemical Sensing
The basic principle of any electrochemical sensor or biosensor is the ability to convert chemical signals into easy-to-interpret electrical signals, therefore offering a fast, cheap, and selective in situ analysis.To reach even lower limits of detection, elevated sensitivity, and selectivity, extensive research is being put into the development of modified commercially available electrodes.Properties such as high specific surface area and conductivity, facile surface functionalisation, tunable bandgap, and exceptional thermal and chemical stability are only some that are typically highlighted within the topic of g-C3N4 as an electrode modifier.Several formats of electrodes with g-C3N4 modifications have recently In electrochemical sensing, g-C 3 N 4 -modified electrodes exhibit high sensitivity and selectivity for detecting organic compounds and heavy metals.The enhanced detection limits are attributed to its high surface area and conductivity, making g-C 3 N 4 -modified electrodes more effective than traditional ones.This performance improvement ensures faster, cheaper, and more reliable in situ analysis, critical for applications in environmental monitoring and healthcare diagnostics.
Photoelectrochemical (PEC) sensing benefits from g-C 3 N 4 's role as a photoactive electrode.It enhances PEC system efficiency by generating effective photogenerated electronhole pairs.This results in higher sensitivity and specificity compared to many conventional materials.g-C 3 N 4 's ability to improve PEC sensing performance is particularly advantageous in detecting low concentrations of analytes, making it suitable for applications in food safety testing and pollution monitoring.
In photocatalytic applications, g-C 3 N 4 is effective in pollutant degradation and water disinfection, offering an eco-friendly alternative to traditional methods.While bulk g-C 3 N 4 has limitations, its nanostructured forms exhibit enhanced photocatalytic activity, often surpassing other photocatalysts in efficiency.This makes g-C 3 N 4 an excellent choice for environmental remediation, where sustainable and efficient solutions are needed.
For water splitting, g-C 3 N 4 nanocomposites demonstrate high efficiency in hydrogen production.Their tunable bandgap and stability make them competitive with other photocatalytic materials.The ability to efficiently split water into hydrogen and oxygen positions g-C 3 N 4 as a key material in the development of the hydrogen economy, which is crucial for addressing climate change and reducing reliance on fossil fuels.
In drug delivery systems, g-C 3 N 4 nanoparticles are valued for their biocompatibility and functionalisation capability.These properties enable targeted drug delivery with controlled release, offering improved therapeutic efficacy over some traditional drug delivery systems.The use of g-C 3 N 4 in biomedical applications highlights its potential to enhance the effectiveness and precision of treatments.
Phototherapy is another area where g-C 3 N 4 excels due to its superior light absorption and photothermal conversion efficiency.This material provides a non-invasive treatment option with potential advantages over conventional photothermal agents, particularly in cancer treatment and other medical applications.g-C 3 N 4 's properties ensure effective and safe phototherapy, promoting better patient outcomes.

Electrochemical Sensing
The basic principle of any electrochemical sensor or biosensor is the ability to convert chemical signals into easy-to-interpret electrical signals, therefore offering a fast, cheap, and selective in situ analysis.To reach even lower limits of detection, elevated sensitivity, and selectivity, extensive research is being put into the development of modified commercially available electrodes.Properties such as high specific surface area and conductivity, facile surface functionalisation, tunable bandgap, and exceptional thermal and chemical stability are only some that are typically highlighted within the topic of g-C 3 N 4 as an electrode modifier.Several formats of electrodes with g-C 3 N 4 modifications have recently been prepared for the detection of a plethora of substances [28].
A carbon paste electrode (CPE) consisting of graphite and paraffin wax, along with g-C 3 N 4 as an attractive modifier for the heightened signal response, was used for the detection of tryptophan [29] and acyclovir [30].Highly accurate determination of heavy metals (Cr, Ni) in tap water was accomplished by potentiometric titration with g-C 3 N 4 and multiwalled carbon nanotube (MWCNT)-modified CPE [31].
Screen-printed electrodes (SPE) modified solely with nanoscale g-C 3 N 4 doped with additional oxygen-containing functional groups were utilised for dibenzofuran analysis via impedimetric detention to achieve an exceptionally low LOD (pM range) [32].In another study, Cu, Zn, and Fe metal-organic frameworks (MOFs) enriched with melamine-derived g-C 3 N 4 were evaluated towards 2,4-dichlorophenol sensing on a modified SPE.The Fe-MOF/g-C 3 N 4 nanocomposite showed the most fitting electrochemical behaviour and was used for real sample analysis, achieving nanomolar detection limits [33].Niaz et al. used cathodic stripping voltammetry for the detection of iodide ions (I − ) on a chitosan/g-C 3 N 4 composite-modified SPE.After applying a cleaning potential, the modifying layer remained unchanged for several repeated measurements, which stands out among many single-use sensing methods utilising SPEs [34].
Glassy carbon electrodes (GCE) are arguably the most widely used electrodes in electroanalytical sensing, with endless potential for surface modification to suit the specific choice of analyte.Metronidazole was successfully detected with appropriate selectivity with no interference from other electroactive compounds, using a MoS 2 /g-C 3 N 4 composite anchored with Nafion on the electrode's surface [35].A GCE modified with a nanocomposite of Bi 2 Te 3 /g-C 3 N 4 achieved some of the best limits of detection (calculated as 0.7 µM) reported to date for the electrochemical analysis of epinephrine, also known as adrenaline [36].Impressive results for perphenazine determination were also achieved via a LaCoO 3 /g-C 3 N 4 -modified GCE.In this work, the nanocomposite resembled "belts" of individual sheet-like layers of g-C 3 N 4 dotted with circular nanoparticles of the lanthanum compound [37].
Each electrochemical sensing application has unique requirements and challenges.Optimising g-C 3 N 4 -modified electrodes for specific analytes, matrix compositions, and environmental conditions remains a challenge.Tailoring the properties of g-C 3 N 4 to address these specific application needs requires further investigation.One of the primary challenges associated with g-C 3 N 4 -modified electrodes is the complexity and variability of synthesis methods.Achieving consistent and reproducible g-C 3 N 4 structures with desired properties can be challenging, impacting the reliability and reproducibility of electrode performance.While g-C 3 N 4 shows promise for electrochemical sensing applications in research settings, scaling up production and commercialisation remain hurdles.Developing cost-effective synthesis methods and ensuring scalability while maintaining performance is crucial for transitioning g-C 3 N 4 -modified electrodes from the lab to practical applications.

Photoelectrochemical Sensing
Photoelectrochemical (PEC) detection utilises light for excitation and measures current or voltage as the output signal.In the construction of PEC platforms, g-C 3 N 4 can have different roles, such as a photoactive electrode material, sensitiser, and either electron donor or acceptor.Additionally, owing to diverse surface functional groups, g-C 3 N 4 can act as an immobiliser for bioreceptors such as aptamers [38,39].
A PEC system was developed on an F-doped SnO 2 (FTO) glass platform first modified with g-C 3 N 4 and BiOI on top for accurate detection of ascorbic acid.The continuously decreasing cathodic photocurrent was attributed to an increasing ascorbic acid concentration.The research states that this combination of electrode modifiers leads to the formation of abundant charge transfer pathways.Their reordering or subsequent layering creates a template for task-specific modulation of these pathways based on the intended use [40].
Surface-modified g-C 3 N 4 nanosheets decorated with clusters of Cu (2%) provided an accurate photoelectrochemical sensing platform for the detection of 4-nitrotoluene.Cu (II) ions effectively worked in synergy with the exfoliated g-C 3 N 4 nanosheets to promote separation and migration of the photogenerated electron-hole pairs, therefore greatly enhancing the photocurrent response to the analyte.The copper ions in this system served as an electron-trapping medium to facilitate the reduction of p-nitrotoluene.The surface modification proved to be crucial in this system, as g-C 3 N 4 nanosheets themselves showed minimal change in photocurrent in response to increasing 4-nitrotoluene concentration [41].
A photoelectrochemical aptasensing platform for the detection of extremely low concentrations of atrazine was developed by Yan et al.In this study, additional cyano functional groups, as well as nitrogen deficiencies, were introduced into the bulk g-C 3 N 4 material to trap photoinduced electrons, improve the adsorption capacity of visible light, and therefore improve PEC performance.Atrazine-specific aptamer as the identification element was further used to modify g-C 3 N 4 , and atrazine concentration was relative to the decrease in detected photocurrent [42].Tian et al. prepared an aptasensor based on the electrochemiluminescence (ECL) of g-C 3 N 4 for the accurate detection of aflatoxin B1.The biosensor was formed by deposition of g-C 3 N 4 modified with aflatoxin-specific aptamer on GCE.By itself, g-C 3 N 4 exhibited intense ECL signals; therefore, growing aflatoxin concentration was directly proportional to a decrease in ECL signal response, as the aflatoxin-aptamer complex blocked active sites on the material [43].
Achieving precise control over surface functionalisation and maintaining the stability of modified g-C 3 N 4 structures are significant challenges that warrant attention in PEC sensing applications.Potential interference from other compounds and optimised selectivity for real-world applications are critical considerations for the practical implementation of g-C 3 N 4 -based PEC sensors.As with electrochemical sensing, scaling up the production and commercialisation of g-C 3 N 4 -modified PEC platforms presents challenges.Developing cost-effective synthesis methods and ensuring scalability while maintaining performance and reproducibility are essential for translating laboratory-scale research into practical sensing devices.
Table 2 summarises the aforementioned applications of several g-C 3 N 4 -based sensors including their comparison to methods using different modifiers on the electrode surface.In most cases, sensors utilising g-C 3 N 4 were able to achieve higher sensitivity and significantly lower limits of detection to the same analyte.Additionally, in several examples, the modifier containing g-C 3 N 4 requires fewer steps and simpler synthesis approaches, proving the practicality of this material in electrochemical and photoelectrochemical sensing.

Photocatalytic Applications
Among the first discovered and most prominently researched attributes of g-C 3 N 4 is its photocatalytic properties; however, as has been mentioned, g-C 3 N 4 in its bulk form is severely hindered for these applications, showing a relatively small specific surface area and therefore insufficient active sites as well as the rapid recombination of excited electron-hole pairs.Numerous studies over the years show that the limitations of bulk g-C 3 N 4 can be easily worked around with the introduction of g-C 3 N 4 -based nanostructures or by engineering electron redistribution by introducing vacancies or doping g-C 3 N 4 with other elements.It has been noted that many approaches to modifying g-C 3 N 4 for enhanced photocatalytic properties rely on creating structures with various metals, which may be detrimental to the environment in the long run due to the low availability of some metals in nature and their inherent toxicity.Special attention is being paid to developing g-C 3 N 4 catalysts using methods and materials with sustainability in mind [56,57].
A novelty approach to 1,2 amino alcohol synthesis has been developed through ultrathin porous g-C 3 N 4 nanosheets-enabled photocatalysis under visible light by Xu et al.The 1,2-amino alcohol component serves as a crucial chemical backbone in numerous natural products, biologically active compounds, and ligands.This approach enables simultaneous reductive and oxidative reactions, leading to the formation of two radical intermediates that couple to produce the final product with almost 100% yields.Most notably, the synthesised photocatalyst exhibited satisfactory recyclability, remaining effective for up to seven cycles.Additionally, the reaction mechanism was explored for its applicability to other complex molecules, suggesting potential industrial applications for ultrathin porous g-C 3 N 4 nanosheets in the synthesis of valuable pharmaceutical intermediates [58].
Ozonation refers to the production of reactive oxygen species (ROS) from ozone (O 3 ) molecules, capable of attacking and degrading a wide range of organic compounds and microorganisms.Nanoclustered g-C 3 N 4 functionalised with additional oxygen-containing (mainly C=O and C-O) groups was found to be an exceptional catalyst, capable of generating primarily • OH and O 2 •− radicals in the presence of O 3 , as confirmed by EPR analysis.When tested, atrazine removal of a system containing oxygen-functionalised g-C 3 N 4 and O 3 reached nearly 93% compared to O 3 alone (63%).The material's catalytic abilities remained constant under both acidic and alkaline conditions, suggesting a wide applicability [59].Another study reports significantly lower (around 30%) usage of O 3 and complete inactivation of Escherichia coli bacteria and JC virus in water, utilising a dual system of g-C 3 N 4 /O 3 , greatly reducing the cost of such water treatment.The main ROS generated were identified to be • OH [60].
Xu et al. prepared g-C 3 N 4 via a multiple-step calcination route resulting in a material with a combined crystalline and amorphous structure and a narrower bandgap than its bulk g-C 3 N 4 counterpart, making it more conductive.Upon stimulation with xenon lamplight in an environment abundant in O 2 , g-C 3 N 4 with a combined crystalline and amorphous structure was capable of efficient H 2 O 2 production and nearly 100% removal of methylene blue under simulated conditions.The key ROS generated and important in the mechanism of pollutant degradation reaction were identified to be O 2 •− radicals and singlet oxygen 1 O 2 [61].Photocatalytic disinfection strategies against microbial and bacterial pollution have been considered recently as a preferable solution in contrast to traditional bacterial inactivation methods because of their ecological sustainability.Pure g-C 3 N 4 without any additives shows certain antibacterial properties under visible light illumination on account of its unique photocatalytic properties.As previously mentioned, the photoexcited free radicals ( • OH, O 2 •− ) can attack cell walls and membranes, causing them to rupture, leak intracellular materials, and kill bacteria [62].
These approaches demonstrate innovative solutions to enhance the photocatalytic properties of g-C 3 N 4 , enabling its application in diverse environmental remediation and synthesis processes.However, concerns about the environmental impact of using metalbased modifications to enhance g-C 3 N 4 photocatalytic properties are rising.It is important to develop sustainable g-C 3 N 4 catalysts using methods and materials that minimise en-vironmental harm.This reflects a growing awareness within the scientific community regarding the need for eco-friendly approaches to material synthesis and catalysis.Developing scalable and reproducible synthesis routes for g-C 3 N 4 -based nanostructures or doped materials is essential for practical applications but often presents challenges in terms of cost and complexity.Assessing the stability of these materials over extended operation periods and under different environmental conditions is crucial for evaluating their practical viability for industrial applications.Further research is needed to elucidate the underlying processes governing photocatalytic reactions on g-C 3 N 4 surfaces and to identify key factors influencing catalyst activity and selectivity.

Watter Splitting
Another possible application of a photoelectrochemical system based on g-C 3 N 4 is utilising the photogenerated electron-hole pairs to drive redox processes associated with water splitting.Simply put, water splitting is a chemical reaction in which hydrogen (H 2 ) and oxygen (O 2 ) are generated from water molecules.The development of economical and efficient means of water splitting is regarded to be one of the major building blocks for the hydrogen economy, considered to be one of the most promising ways to mitigate ongoing climate change caused by the use of fossil fuels [63].Water splitting is divided into three main routes, photocatalytic, electrocatalytic, and photoelectrocatalytic.Photocatalytically, water splitting occurs through the formation of bound electron-hole pairs (excitrons) by adsorption of light.For electrocatalytic water splitting, O 2 is generated on the anode and H 2 is generated on the cathode at an electrode potential of 1.23 V, assuming the electrochemical cell has been assembled with appropriate materials.When these two principles combine, water splitting can also occur via a photoelectrocatalytic route, meaning a photocatalyst material is bound to an electrode and potential is applied to the system in the presence of light [64].
Wang et al. presented an interesting new approach to photoelectrocatalytic water splitting with the use of g-C 3 N 4 nanofibers decorated with black phosphorous nanoparticles.Black phosphorous has the properties of a p-type semiconductor with notable optoelectrical qualities, with an adjustable bandgap ranging from 0.3 eV to 2 eV, depending on the specific morphology of the material, while g-C 3 N 4 functions as a n-type semiconductor with a defined bandgap of 2.7 eV.The combination of these two materials as a p-n heterojunction led to the formation of a highly functional photoresponsive material, achieving a high evolution rate of hydrogen and being stable for over 8 h [65].
Sun et al. achieved exceptional H 2 production rates through photocatalytically driven water-splitting reactions on a complex of g-C 3 N 4 nanosheets deposited with single-atom cerium sites.In addition to the water splitting reaction, the prepared nanocomposite was able to oxidize amines after irradiation with visible light.The lifetime of photoinduced electrons was improved by 5 ns in comparison to bare g-C 3 N 4 , and the production of superoxide radicals was proven to be the driving force of the reaction mechanisms.The addition of cerium also improved charge transfer resistance and showed significant photoluminescence quenching compared to the bare g-C 3 N 4 material [66].
The reaction of O 2 evolution has been remarked on as highly energy-demanding ad not particularly fast due to its mechanism requiring the transfer of four electrons.Iridium oxide (IrO 2 ) is the most commonly used catalyst in water electrolysis, but its main disadvantage is the low availability of the rare metal and, therefore, an exceptionally high price.Aiming to improve on these facts, Torres-Pinto et al. developed an electrocatalytic system consisting of g-C 3 N 4 deposited onto Ni foam substrates.Upon extensive performance evaluation of this system, for the first time it was noted that a g-C 3 N 4 -based catalyst was able to surpass the point-of-reference catalyst IrO 2 in terms of rates of generated O 2 in an alkaline medium [67].
Table 3 summarises some advantages and disadvantages of g-C 3 N 4 materials compared to other common catalysts.However, it should be noted that even though significant progress has already been made in this research, the use of g-C 3 N 4 in photoelectrocatalysis is still in its beginning stages.Current trends focus on environmental friendliness and cost efficiency in technological materials suggesting great potential for g-C 3 N 4 -based catalysts.Perovskite materials Adjustable, depending on the composition.
Prone to degradation from oxygen, moisture, UV exposure, and high temperatures (above 80).
Low cost of materials, faces challenges in large-scale processing.
Some perovskites contain lead.Additionally, toxic solvents are employed in processing.
Low catalytic performance when used alone.

Porous organic polymers (POPs)
Adjustable, depending on composition.Generally stable.
Inexpensive precursors, scalable.Estimated production cost lower than 10 USD/kg.
Relatively large pore size (>1 nm), which may be a limiting factor in some applications.

Drug Delivery Systems and Phototherapy
Low biotoxicity and good biocompatibility make g-C 3 N 4 a favourable material for biomedical applications, especially in the field of cancer treatment.It can be used as a nanocarrier by loading the therapeutic agent or drug into the cavities to make a complex.The drug molecules or therapeutic agents are loaded on the nanocarrier by weak forces of attraction, e.g., London dispersion force [72].Assuming the acidic microenvironment of the tumour (a pH value around 5), both the anticancer drug and its carrier (g-C 3 N 4 ) become negatively charged once absorbed by the tumour.Hence, the electrostatic repulsion facilitates the release of the anticancer drug.Recent insights into g-C 3 N 4 in drug delivery testing have been summarised in more detail by Pourmadadi et al. [17].
Among many types of water-splitting materials, g-C 3 N 4 has attracted considerable attention for its adjustable bandgap and band position.After the modification, water splitting can be driven under a high penetrable laser, which makes g-C 3 N 4 suitable for phototherapy.Unfortunately, some disadvantages still restrict the application of g-C 3 N 4 in water splitting, including a high recombination rate of photogenerated electron-hole pairs and lower light utilisation.Therefore, the modification design of g-C 3 N 4 for further widening the light absorption range and improving stability is necessary to meet the requirements of practical water splitting [73].

Toxicity Considerations of g-C 3 N 4
The mechanism of any phenol-like compound degradation using g-C 3 N 4 typically involves a photocatalytic process wherein g-C 3 N 4 serves as a catalyst to facilitate the decomposition of phenolic compounds under light irradiation [74].When exposed to light, g-C 3 N 4 absorbs photons, promoting electrons from the valence band to the conduction band, creating electron-hole pairs.The photoexcited electrons (e − ) and holes (h + ) can react with water and oxygen molecules adsorbed on the surface of g-C 3 N 4 to generate already mentioned ROS.Phenol molecules present in the solution are adsorbed onto the surface of g-C 3 N 4 through π-π interactions or hydrogen bonding.The ROS generated on the surface of g-C 3 N 4 reacts with the adsorbed phenol molecules, leading to the oxidation of phenolic compounds into intermediate products such as hydroquinone, benzoquinone, and eventually, smaller organic acids, aldehydes, and carbon dioxide.During this process, several risks that can negatively impact the environment or human health need to be considered.These are summarised in Table 4.

Toxic intermediates
As phenol undergoes degradation, various intermediate products are formed.Some of these intermediates may exhibit toxicity to humans and aquatic organisms, potentially posing risks to environmental and human health.

Incomplete degradation
Incomplete degradation of phenol or its intermediates could result in the accumulation of persistent organic pollutants in the environment, leading to long-term ecological impacts and potential bioaccumulation in the food chain.
Unwanted ROS Uncontrolled generation of ROS during the photocatalytic process may cause oxidative stress in aquatic organisms and disrupt ecosystems, particularly in sensitive aquatic environments.
Release of nanoparticles While g-C 3 N 4 is generally considered to be biocompatible, the long-term effects of nanoparticle exposure on human health and the environment are still not fully understood.
To mitigate these risks, thorough research and risk assessments are necessary to understand the fate and toxicity of intermediate products, optimise photocatalytic conditions to minimise the formation of toxic by-products, and implement appropriate wastewater treatment strategies to ensure the safe disposal of treated effluents.Additionally, proper monitoring and regulation of g-C 3 N 4 -based photocatalytic processes are essential to prevent potential adverse impacts on human health and the environment.
Various experimental data show that g-C 3 N 4 -based materials are biocompatible only under the investigated doses.Biodegradability, chronic toxicity, and larger animal model testing have not been systematically and deeply studied yet.Therefore, the biocompatibility of g-C 3 N 4 -based photocatalysts still requires detailed biosafety assessments.Only when the biocompatibility of g-C 3 N 4 -based photocatalysts is fully demonstrated will it be possible to proceed with subsequent clinical applications [75].For example, nanosheets of g-C 3 N 4 exhibit promising applications as efficient photosensitizers for photodynamic tumour therapy and as pH-responsive nanocarriers for drug delivery.As photosensitizers, these nanosheets demonstrate the capability to generate ROS, effectively targeting and eliminating cancer cells under low-intensity light irradiation.Despite this mechanism holding significant potential for enhancing cancer treatment outcomes, the adverse effects on surrounding healthy tissues and cells need to be considered.However, when this material undergoes unintended processes such as the spontaneous release of certain components (e.g., metals) and the generation of ROS, its toxicity can be deemed a negative outcome [76].
Table 5 summarises possible mechanisms through which g-C 3 N 4 could exhibit toxicity during biomedical applications (i.e., cancer therapy).

Consideration Impact
Particle size and shape Nanoparticles of g-C 3 N 4 may exhibit different properties and behaviours compared to bulk materials.Their small size and high surface area-to-volume ratio could increase interactions with biological systems, potentially leading to adverse effects such as cellular uptake, oxidative stress, and inflammation.

Chemical composition
The chemical composition of g-C 3 N 4 , including any surface functionalisation or impurities, could influence its toxicity profile.For example, surface groups or contaminants may enhance cellular uptake or trigger immune responses, leading to cytotoxic or immunotoxic effects.

Biological interactions
When introduced into biological systems, g-C 3 N 4 nanoparticles may interact with cellular components such as proteins, lipids, and nucleic acids.These interactions could disrupt cellular processes, interfere with signalling pathways, or induce cellular damage, ultimately leading to cytotoxicity or genotoxicity.

Oxidative stress
Nanoparticles of g-C 3 N 4 have the potential to generate ROS through photoactivation or chemical reactions.Excessive ROS production can overwhelm cellular antioxidant defences, leading to oxidative stress and cellular damage.

Biodegradation and clearance
The biodegradation and clearance of g-C 3 N 4 nanoparticles from the body are critical factors in determining their long-term toxicity.If nanoparticles persist in biological tissues or accumulate in organs over time, they may elicit adverse effects such as chronic inflammation, fibrosis, or organ damage.

Aggregation and agglomeration
Nanoparticles of g-C 3 N 4 may agglomerate or aggregate in biological fluids or tissues, altering their physicochemical properties and biological interactions.Aggregated nanoparticles could lead to localised toxicity, impaired cellular uptake, or obstruction of biological pathways.

Assessment of g-C 3 N 4 Toxicity
As mentioned above, in research for future practical use, the more attractive form of g-C 3 N 4 has become the g-C 3 N 4 nanosheets due to its improved properties such as higher photocatalytic activity or enhanced photoluminescence compared to bulk g-C 3 N 4 material.Until now, g-C 3 N 4 has been considered a nontoxic material, according to the following studies.

In Vitro Cytotoxicity Assessment on Human Cells
Thin nanosheets of g-C 3 N 4 are considered a promising candidate within the field of bioimaging based on their various favourable attributes including enhanced photoluminescence, metal-free nature, and relatively low-cost preparation.An important step in researching the practical use of materials in bioimaging involves assessing cell toxicity.A summary of biocompatibility tests for g-C 3 N 4 can be found in Table 6.Zhang et al. reported that ultrathin g-C 3 N 4 nanosheets, prepared by ultrasound-assisted liquid exfoliation of bulk g-C 3 N 4 in water, showed no evident reduction of HeLa cell viability even at a high incubation concentration of 600 µg/mL.The size distribution of tested exfoliated g-C 3 N 4 was assessed by atomic force microscopy imaging, revealing a range from 70 to 160 nm, centred around 100 nm, and a thickness of nanosheets around 2.5 nm.The biocompatibility of prepared ultrathin nanosheets was further validated through a bioimaging test using an excitation wavelength λ = 405 nm, while the activity and morphology of HeLa cells were not disrupted [77].
A comparison of bulk material toxicity with the toxicity of nanosheets was tested on A549 cells, a lung epithelial carcinoma cell line, by Duan et al.The thickness of g-C 3 N 4 nanosheets with a porous structure prepared by thermal exfoliation at 500 • C was determined as 1.0 nm.As the concentration of tested photocatalyst increased from 5 up to 100 µg/mL, the percentage of cell viability decreased after a 24-h incubation period irrespective of whether bulk or nanosheet material was used.Interestingly, g-C 3 N 4 nanosheets were less destructive to A549 cells as the decrease in cell viability with increasing concentration was lower than bulk material.Even at the highest tested concentration of g-C 3 N 4 nanosheets, the decrease in the A549 cell viability after 24 h was relatively low, which suggests good biocompatibility of g-C 3 N 4 nanosheets toward tested cells within the tests [78].
Another study investigating the toxicity of bulk and g-C 3 N 4 nanosheets was conducted by Davardoostmanesh et al.Nanosheets of a size under 100 nm were prepared by electrophoresis fractionation of bulk g-C 3 N 4 synthesised from urea.The cytotoxicity tests of prepared materials were performed on Saos-2, a bone carcinoma cell line, and foreskin fibroblast HFF cells.Bulk g-C 3 N 4 exhibited lower toxicity against Saos-2 cells after 24 and 48 h of incubation compared to g-C 3 N 4 nanosheets.Determined half maximal inhibitory concentrations (IC 50 ) were 66.8 ± 3.3 µg/mL and 27.0 ± 4.2 µg/mL for nanosheets and 214.4 ± 13.7 µg/mL and 104.0 ± 8.5 µg/mL for bulk after 24 and 48 h of incubation.Fluorescence microscopy revealed that g-C 3 N 4 nanosheets were able to penetrate the tumour cells, distribute widely within the cytoplasm, and accumulate near the nuclear membrane.This finding was not observed with bulk material.There was no significant cell viability difference observed after treating normal HFF skin cells with 48-h IC 50 concentrations of g-C 3 N 4 in bulk or nanosheet form.Based on these discoveries, g-C 3 N 4 nanosheets were proved to be potential candidates in bioimaging, drug delivery, and also anticancer therapy [79].
In another study, Dong et al. examined and compared the cytotoxicity of g-C 3 N 4 with triazine (t-C 3 N 4 ) and heptazine (h-C 3 N 4 ) structure moieties on the A549 cell line.According to the results obtained by MTT assay, t-C 3 N 4 exhibits higher toxicity toward A549 cells compared to h-C 3 N 4 across all tested concentrations ranging from 25 to 500 µg/mL.The same result was also achieved using the WST-8 assay after the same incubation time of 24 h as in the MTT assay.The suggested mechanism of t-C 3 N 4 and h-C 3 N 4 toxicity toward A549 cells via oxidative stress caused by ROS formation was confirmed by measurements of total glutathione in cells after incubation with both t-C 3 N 4 and h-C 3 N 4 at two different concentrations (50 and 100 µg/mL).It was proved that t-C 3 N 4 induced higher levels of oxidative stress in A549 cells compared to h-C 3 N 4 , but both t-C 3 N 4 and h-C 3 N 4 showed significantly lower toxicity when compared to graphene oxide under similar experimental conditions [80].Surface charge represents another important factor affecting the biological properties of g-C 3 N 4 .The relation between g-C 3 N 4 surface charge and toxicity towards MCF-7 breast adenocarcinoma cells was investigated by Huang et al.Alteration in surface charge can be achieved through functionalisation via chemical oxidation to produce g-C 3 N 4 rich in carboxyl groups (Cg-C 3 N 4 ) or by chemical hydrolysis to create a nitrogen-rich derivate with amino groups (Ag-C 3 N 4 ).Despite using the different synthesis methods, the obtained nanosheets were similar in size with 40-50 nm diameters.Neither of the prepared materials showed any toxicity effect and thus a significant reduction in cell viability after 24 h up to the concentration of 400 µg/mL.Cellular uptake examination by fluorescence spectroscopy and field emission scanning electron microscopy (FE-SEM) revealed that at equal concentrations, the Ag-C 3 N 4 exhibited faster rates of cellular entry, which resulted in a higher accumulation of Ag-C 3 N 4 within the cells.Additionally, at the lower incubation concentrations of g-C 3 N 4 , a larger contrast in cellular uptake was observed, suggesting that cationic Ag-C 3 N 4 displays a greater affinity for cellular interactions because of the existing electrostatic interaction with the cell membrane charged negatively [81].
One of the other methods used for modifying the g-C 3 N 4 structure is metal doping or codoping.The effect on the toxicity of g-C 3 N 4 catalyst after metal nanoparticle doping was studied by Pieta et al.Cytotoxicity of prepared g-C 3 N 4 and Ni-, Cu-, and Cu-Nidoped g-C 3 N 4 nanosheets, with sizes ranging from 1.1 to 1.5 nm, was investigated on the HaCaT human skin cell line.Microscopic analysis revealed that HaCaT cells incubated for 24 h with metal-doped g-C 3 N 4 catalyst samples exhibited dose-dependent changes in morphology, while there was no evident change in morphology of cells observed after incubation with pure g-C 3 N 4 nanosheets, even at the highest concentration of 500 µg/mL.In addition to the examination of HaCaT cell morphology, the impact of catalysts on the cell viability/proliferation was also determined using the MTS assay.A statistically significant reduction in cell viability was noted at concentrations of 25 µg/mL in the Nidoped sample, 50 µg/mL in the Cu-doped sample, and 12.5 µg/mL in the Cu-Ni-codoped sample.In contrast, g-C 3 N 4 nanosheets without modifications did not show any cytotoxic effect on HaCaT cells.The calculated IC 50 values were 53.93 µg/mL for Ni-doped catalyst, 157.00 µg/mL for Cu-doped catalyst, and 40.10 µg/mL for Cu-Ni samples [82].

In Vitro Cytotoxicity Assessment on Animal Cells
In addition to the toxicity studies on human cell lines, a few studies have also examined the cytotoxic effects of g-C 3 N 4 and its modifications on animal cells.Chung et al. studied the toxicity of g-C 3 N 4 and metal (Fe, Cu, Zn)-doped g-C 3 N 4 on rat pheochromocytoma PC12 cells.It was found that neither g-C 3 N 4 nor its metal derivates caused a notable decrease in cell viability across the broad range of tested concentrations (0-100 µg/mL) under dark or under white LED irradiation conditions.Achieved results confirm good biocompatibility of g-C 3 N 4 and metal-doped g-C 3 N 4 , as well as the negligible impact of ROS generated after irradiation of photocatalysts on animal cells [83].
The cytotoxicity of pyrrolic nitrogen-rich Cu-doped g-C 3 N 4 nanosheets, in conjunction with upconversion nanoparticles (UCNPSs) acting as artificial enzymes, was tested on mouse cancerogenic 4 T1 and healthy L929 fibroblast cells.No apparent toxic effects of the prepared enzyme-like nanocomposite were observed in L929 cells after 24 h of incubation across a wide range of concentrations (0-500 µg/mL).Due to its good biocompatibility and ability to be taken up by 4 T1 cells, the nanocomposite with g-C 3 N 4 was further investigated as a promising photosensitive drug against cancerous 4 T1 cells.Following irradiation with a 980 nm laser at varying concentrations of g-C 3 N 4 nanocomposite, the 4 T1 cells were eliminated, while NIR laser irradiation alone resulted in a high survival of cells, indicating that 980 nm laser irradiation alone was ineffective in killing the cells [84].

Ecotoxicity Tests on Microorganisms, Fishes, and Plant Seeds
Besides the tests evaluating toxicity on various types of cells, it is crucial to investigate the ecotoxicity of g-C 3 N 4 on various organisms, especially considering that a significant amount of g-C 3 N 4 application research is focused on water treatment and environmental remediation.Rosa et al. evaluated the acute toxicity of g-C 3 N 4 and magnetic nanocomposite consisting of g-C 3 N 4 and Fe 3 O 4 nanoparticles on zebrafish (Danio rerio) embryos.Bulk g-C 3 N 4 used in the study was prepared by calcination of the mixture of melamine and urea, and the magnetic nanocomposites were prepared by the coprecipitation method.In the case of prepared samples of g-C 3 N 4 at all tested concentrations (0-100 mg/L), no statistically significant changes were observed in all monitored parameters indicating lethality, developmental variations, or malformations during the development of the zebrafish embryo-larva 24, 48, 72, and 96 h after fertilisation.The results indicate that both g-C 3 N 4 alone and in combination with magnetic nanoparticles do not cause teratogenic or acute toxic effects on the embryos of zebrafish [85].
The acute toxicity of ternary nanocomposite consisting of ZnO, CdO, and g-C N 4 was evaluated in Nile tilapia (Oreochromis niloticus) by Berhanu et al.Based on the observed results, g-C 3 N 4 in combination with ZnO and CdO semiconductors does not cause any change in experimental toxicological parameters at low exposure concentrations (10 mg/L).However, at the higher concentrations (500 and 1000 mg/L) a few hours after exposure, tested fish started to show signs of respiratory distress such as dyspnoea, swimming to the water surface, or rapid movement of the operculum.The determined 50% lethal concentration (LC 50 ) was at 12 h 113 mg/L [86].
Abdel-Moniem et al. analysed the toxicity of g-C 3 N 4 and a nanocomposite consisting of Bi 2 S 3 and g-C 3 N 4 nanosheets using the Microtox model 500 analyser, capable of conducting in vitro tests of acute toxicity on the nonpathogenic bacterium Vibrio fisheri.The calculated effective 50% concentrations (EC 50 -15 min) of g-C 3 N 4 nanosheets and Bi 2 S 3 -x@C 3 N 4 nanosheets were ≥100, which indicates that these materials are non-toxic, environmentally friendly, and suitable for water treatment processes.Prepared samples were also tested on various microorganisms, specifically Escherichia coli, Staphylococcus aureus, and Candida albicans, for disinfecting properties under dark conditions.Out of all tested materials, g-C 3 N 4 nanosheets displayed the weakest antimicrobial properties against all selected microorganisms.Its higher antimicrobial activity was observed against E. coli, with the value almost reaching 80%.However, the antimicrobial activity against S. aureus and C. albicans was negligible, not even reaching 20%.Meanwhile, the antimicrobial activity of the g-C 3 N 4 nanosheets modified with Bi 2 S 3 nanosheets significantly improved, with the values almost reaching up to 100% toward all tested microorganisms [87].
The possible phytotoxicity of g-C 3 N 4 toward rice seedlings (Oryza sativa L.) was evaluated by Hao et al.Tested g-C 3 N 4 , with an average size spanning several hundred nanometres and thickness of about 3-7 nm, was synthesised by urea calcination followed by freeze-drying.The obtained weights of rice biomass, treated with g-C 3 N 4 (250 mg/kg), after 30 days of exposure showed increases of 29.9% (rice roots), and 22.4% (rice shoots) compared to untreated control rice seedlings.The utilisation of a low dose of g-C 3 N 4 did not affect the rice seedlings' growth and it even mitigated the phytotoxicity induced by heavy metals (As, Cd) present in the soil through adsorption of metals on g-C 3 N 4 , which caused inhibition of As and Cd distribution and accumulation in rice plants [88].

Conclusions and Future Perspectives
The prospects of g-C 3 N 4 lie in its further development and improvement for a variety of applications such as photocatalysis, water splitting, environmental remediation, and even sensor applications.The sensitivity of g-C 3 N 4 towards various analytes, including organic compounds and heavy metals, has been demonstrated, making g-C 3 N 4 a suitable material useful in processes such as environmental monitoring, healthcare diagnostics, and food safety testing.Overall, while g-C 3 N 4 is considered to be relatively safe for many applications, it is also essential to carefully evaluate the potential toxicity risks associated with its use (Figure 4) in sensing and electronics applications.The potential toxicity of these nanocomposites may depend on factors such as the size, shape, surface chemistry, and dose of the g-C 3 N 4 nanoparticles, as well as the specific application and exposure scenario.Understanding whether g-C 3 N 4 -containing materials can be recycled or safely disposed of at the end of their lifecycle is important for sustainable product stewardship.Proper waste management practices should be followed to minimise the release of g-C 3 N 4 nanoparticles into the environment and mitigate potential ecological risks.In biomedical applications, such as drug delivery systems or biomedical imaging, the potential toxicity of g-C 3 N 4 nanoparticles to human cells or tissues needs to be thoroughly evaluated.This includes assessing cytotoxicity, genotoxicity, immunotoxicity, and any potential long-term health effects associated with exposure to g-C 3 N 4 materials.
applications, such as drug delivery systems or biomedical imaging, the potential toxicity of g-C3N4 nanoparticles to human cells or tissues needs to be thoroughly evaluated.This includes assessing cytotoxicity, genotoxicity, immunotoxicity, and any potential longterm health effects associated with exposure to g-C3N4 materials.
g-C3N4 is often used in photocatalytic coatings for environmental remediation purposes, such as air purification or water treatment.While g-C3N4 itself is considered to be relatively inert, there may be concerns about the release of nanoparticles, metals, or degradation products from these coatings into the environment, which could potentially impact human health or ecosystems.Once g-C3N4 is doped with heavy metals such as cadmium to enhance its photocatalytic activity for water purification, there is a risk of leaching of toxic metal ions into the environment during the catalytic process.These metal ions can pose significant environmental and health hazards due to their toxicity and potential for bioaccumulation in organisms.If released into water bodies, the metal ions could contaminate the aquatic environment, posing risks to aquatic organisms and potentially entering the food chain.Understanding the mechanical properties of g-C3N4 is essential for ensuring the robustness, proper design, and optimal functioning of coatings, devices, and systems based g-C 3 N 4 is often used in photocatalytic coatings for environmental remediation purposes, such as air purification or water treatment.While g-C 3 N 4 itself is considered to be relatively inert, there may be concerns about the release of nanoparticles, metals, or degradation products from these coatings into the environment, which could potentially impact human health or ecosystems.Once g-C 3 N 4 is doped with heavy metals such as cadmium to enhance its photocatalytic activity for water purification, there is a risk of leaching of toxic metal ions into the environment during the catalytic process.These metal ions can pose significant environmental and health hazards due to their toxicity and potential for bioaccumulation in organisms.If released into water bodies, the metal ions could contaminate the aquatic environment, posing risks to aquatic organisms and potentially entering the food chain.
Understanding the mechanical properties of g-C 3 N 4 is essential for ensuring the robustness, proper design, and optimal functioning of coatings, devices, and systems based on this material.The mechanical characterisation of g-C 3 N 4 reveals a material with high stiffness, moderate hardness, and good wear resistance, suitable for a range of applications from coatings to structural components.However, its brittle nature and anisotropic properties need careful consideration in design and application.Researchers and engineers can optimise g-C 3 N 4 for specific uses by understanding and leveraging these mechanical properties, ensuring robustness and longevity in practical applications.Exploring modifications, heterojunctions, novel composite structures, and nanostructures for enhancing g-C 3 N 4 efficiency will allow the expansion of its practical applications in sustainable energy and environmental management.Overall, the perspectives of g-C 3 N 4 are vast and multidisciplinary.Continued research efforts aimed at the examination of its properties and synthesis methods as well as toxicity investigations are necessary for further progress.

Figure 1 .
Figure 1.The key synthesis factors influencing the final properties of g-C3N4.

Figure 1 .
Figure 1.The key synthesis factors influencing the final properties of g-C 3 N 4 .

Figure 2 .
Figure 2. Schematic illustration of the synthesis process of g-C3N4 resulting in different types of nanomaterial.

Figure 2 .
Figure 2. Schematic illustration of the synthesis process of g-C 3 N 4 resulting in different types of nanomaterial.

Figure 4 .
Figure 4. Schematic illustration of g-C3N4 potential toxicity routes and evaluation methods.

Figure 4 .
Figure 4. Schematic illustration of g-C 3 N 4 potential toxicity routes and evaluation methods.

Funding:
This work was supported by the Scientific Grant Agency VEGA of the Slovak Academy of Sciences and the Ministry of Education of the Slovak Republic 1/0436/23 and the Slovak Research and Development Agency APVV 19-0250 and APVV-21-0076.Funding from the European Union's

Table 2 .
Sensing applications of g-C 3 N 4 (mentioned in Sections 3.1 and 3.2) and their performance comparison to other photocatalytic materials.

Table 4 .
g-C 3 N 4 mechanisms and toxicity considerations during photocatalytic processes.

Table 5 .
g-C 3 N 4 mechanisms and toxicity considerations in biomedical applications.

Table 6 .
Summary of g-C 3 N 4 biocompatibility tests on human and animal cells.