Bio-synthesis of Graphitic Carbon Nitride and ZnO/GCN Nanohybrid for Remarkable Environmental Application

doi:10.21203/rs.3.rs-5296971/v1

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Bio-synthesis of Graphitic Carbon Nitride and ZnO/GCN Nanohybrid for Remarkable Environmental Application

https://doi.org/10.21203/rs.3.rs-5296971/v1

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The current study reports on biosynthesisof pure graphitic carbon nitride (GCN), ZnO nanoparticles (NPs) and ZnO-doped gaphitic carbon nitridenanohybrids (ZnO/GCN-NHs) usingOcimumtenuiflorum(OT)leafextract.GCN synthesis using plant extract was never reported in previous studies. Under direct solar lightphotocatalytic performance of the synthesized GCN, NPs and NHs was tested on the degradation of methylene orange (MO) dye and compared. Among the samples, the ZnO/GCN-NHsexhibits superior photocatalytic activity, achieving up to 47.56% degradation in 150 min of MO dye. The cytotoxicity of the biosynthesized NPs and NHswas assessed against human dental pulp stem cells and these were found to be non-toxic, indicating their potential for biomedical applications. The antimicrobial properties were also assessed using well diffusion and disc diffusion tests against four bacterial strains i.e., two Gram-negative and two Gram-positive. The tests demonstrate significant antibacterial activity with a remarkable inhibition radius against Escherichia coli 17.5 ± 1mm, Pseudomonas aeruginosa 15.04 ± 1mm, Staphylococcus aureus 27.5 ± 1mm, and Streptococcus dysgalactiae 25 ± 1 mm. The enhanced photocatalytic and antimicrobial properties of the ZnO/GCN-NHs are hypothesized to be due to the increased production of reactive oxygen species (ROS) through the combination of ZnO NPs with biosynthesized GCN.

Physical sciences/Materials science

Physical sciences/Nanoscience and technology

• Biosynthesis of graphitic carbon nitride nanoparticles by using leaves.

• Biosynthesis of ZnO doped graphitic carbon nitride nanohybrid by one-step calcinations

process.

• Photocatalytic activity evaluation through the degradation of methylene orange dye.

• The biocompatibility of the synthesized nanoparticles and nanohybridassessed through cytotoxicity studies.

• Antibacterial activity assessedagainsttwo gram-negative and two gram-positive bacteria.

Many technologies involving have been developed to treat wastewater, such aspolymer-based nanoparticles: They are interesting owing their high surface area and tuneable properties, making them effective for capturing and removing contaminants from water [1–5].Nano-adsorbentsare materials that exhibit their enhanced surface areas and active sites, and theyare designed to adsorb pollutants from wastewater efficiently [3–7].Furthermore,nanocompositecombine different materials at the nanoscale including hybrid structures with enhanced properties for treating pollutants in water [5–8].These can be used as photo-catalyst to catalyze the breakdown of organic pollutants under light, typically sunlight, which is an eco-friendly approach to water purification. Amongstconventional water pollution-treatment methods, the elimination of organic pollutants by photo-catalytic processes is eco-friendly [4–8].Carbon derivatives such as activated carbon exfoliate graphite, graphene oxide, graphene and carbon nanotubes are the conventional materials used for water treatment [9–12].More recently, graphitic carbon nitride (GCN) and its hybrids have gained significant attention in environmental and energy-related applications due to their unique properties and versatility [11–15].These are known for their excellent photo-catalytic properties, chemical stability, and tuneable electronic properties [13–17].Unlike many conventional photo-catalysts that contain metal, GCN is a metal-free photo-catalyst, which make it eco-friendly.This not only reduces costs but also eliminates the risks associated with metal toxicity and disposal, and GCN exhibits good thermal and chemical stability, ensuring long-term performance without significant degradation.GCN can be easily modified or doped with other materials, such as ZnO to further enhance its photo-catalytic efficiency, broadening its range of applications. [18–22].Till date GCN is commonly synthesized using precursors such as dicyandiamide, cyanamide, melamine, urea, and thiourea[23–29].However, the as-prepared GCN often suffers from low photo-catalytic efficiency, primarily due to its limited surface area and the moderate generation of electron-hole pairs (e⁻/h⁺) under light irradiation. To address thisissue, green chemistry approaches are applied to enhance the photo-catalytic properties of GCN. Modifying the synthesis process to createGCN composites with increase the surface area provides more active sites for photo-catalytic reactions.Green synthesis methods are not only environmental friendly but also cost-effective and scalable [29–33].These strategies aim to utilize the photo-catalytic properties of GCN-based nanomaterials to solve environmental pollution problems, such as the degradation of organic pollutants and dyes, making them a promising solution in the field of environmental remediation[30–34].Biological molecules present in plants, algae, microorganism and fungican be used to promote the formation of NPs and their NHs through the reducing capacity of these bio-molecules [35–41].For this reason, we developed bio-synthesized GCN for contaminated water treatment. This technology is not only cost-effective but also environmental-friendly and has significant potential in the field of practical treatment with zero waste release [42–59].

In the present study, we investigated the biosynthesis of pure ZnO-NPs, pure GCN-NPs, and ZnO/GCN-NHs using Ocimumtenuiflorum (OT) plant leaves extract, which is renowned for its aromatic and medicinal properties. The chemical composition of OT includes ursolic acid, alkaloids, glycosides, carvacrol, linalool, tannins, rosmarinic acid, and various aromatic compounds [29].To the best of our knowledge, the application of plant extracts in the synthesis of GCN and its hybrids has not been previously reported. This study highlights that biosynthesis is more effective for large-scale production and the development of thenext-generation of nanotechnology. Thephoto-catalytic properties, cytotoxicity, and antibacterial activity of the synthesized nanomaterialswere studied for potential applications. Overall, the study demonstrates that the biosynthesized photo-catalysts not only have excellent photo-catalytic properties but also possess significant antibacterial properties based on four bacterial i.e.,two gram-negative and twogram-positive that were tested, making them promising candidates for environmental and biomedical applications. The use of OTplant extract in the synthesis process presents a green chemistry approach that is effective, scalable and environmental-friendly, paving the way for next-generation nanotechnology solutions.

2.1. Materials: The chemicals employed are all of analytical grades. Thermo Fischer chemicals include zinc acetate hexahydrate (Zn(CH₃COO)₂,6H₂O)precursor, freshOcimumtenuiflorum(OT)leavescollected from the herbal garden of D.C.R. University of Science and Technology, India. Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Streptococcus dysgalactiae were obtained from the National Centre for Laboratory Research and Risk Assessment (LABRIS) in Estonia. All bacterial isolates were identified by MALDI-TOF mass spectrometry method.Human dental pulp stem cells (HDPSC) for cytotoxicity were obtained from the permanent teeth of patients from Health Sciences Faculty of odontology at the University of Montpellier (France).

2.2. Preparation of plant Extract: 30 g of OT leaves werewashedfirst with tapwater to remove dirt and soil, and thenwashed again with distilled water (DW) to finalize thecleaning. Then leaves were chopped into smaller pieces and placed in a 200 mL borosilicate beaker. Subsequently, 50 mL of double-distilled water (DDW) was added to the beaker containing the chopped leaves. The mixture of chopped leaves and DDW was boiled until the solution turned dark green. After boiling, the supernatant was separated from the solid residue by pouring it through filter paper. The resulting filtrate, was then stored at 4˚C for subsequent use. As shown in [Fig -1].

2.3Biosynthesis of nanomaterials:

2.3.1 Pure ZnO-NPs: 0.02 M zinc acetate hexahydrate was dissolved in 50 mL of DDW. The zinc acetate solution was stirred at room temperature for 1 h. After that, 15 mL of the plant extract was added drop by drop to the zinc acetate solution. The appearance of a slight yellow colour after the addition of the plant extract suggests the initiation of ZnO-NPs synthesis. The solution was stirred at room temperature for an additional 2 h. After synthesis, the solution was centrifuged at 5000 rpm for 15 minutes. The pellet formed by the synthesized ZnO-NPs was collected after centrifugation. It was then dried in a hot air oven at 60˚C for 12 h as shown in [Fig -1].

2.3.2Pure GCN-NPs: The biosynthesis of GCN was performed by using 30 g of OTleaves that had been kept in a hot air oven at a temperature of 70˚C for 36 h. The dryleaves were then ground into a powder. The powder was then added to a closed ceramic crucible and heated in a muffle furnace at 550˚C for 3 h at a heating rate of 2˚C/min. GCN-NPspellets were crushed into a fine powder with a mortar and pestle[Fig-2 (a)].

2.3.3ZnO doped GCN nanohybrid (ZnO/GCN-NHs): 0.44 g Zinc acetate precursor was added into 100 mLDW and stirred at room temperature for 1 h. Then, 20 mLof OTleaf extract was added drop-wise and stirred for 1 h. Then 0.44 g of pre-synthesized GCN powder was added in the beaker, and again stirred for another 2 h. The resulting precipitate was centrifuged for 15 min at 5000 rpm, followed by drying in a hot air oven at80˚C for 8 h. After collecting the samples, they were calcinated at 300˚C for 2h in a muffle furnace. The resulting powder was hermetically sealed to avoid possible oxidation and for later use in experiments [Fig-2 (b)].

2.4Characterization

The optical absorbance of the biosynthesized pure ZnO-NPs, pure-GCN-NPs and ZnO/GCN-NHs was measured with UV-Vis spectrometer (Ocean optics, USA). Fourier transform infrared (FTIR)Spectrometer (Nicolet is10 Thermo Scientific, Driesch, Germany) was used to study the chemical bonds in the samples in the range of 360 to 1100 cm^-1.X-ray diffraction patterns were collected using a Bruker D8 Advance diffractometer equipped with CuKα1radiation (λ = 0.15406 nm) selected by a Ge (111) monochromator.Thermal stability of all composites was studiedby “Perkin Elmer STA 6000” in an air atmosphere at a range from 30 to 600 ℃ at a ramp rate of 10℃ min⁻¹.Photoluminescence (PL) spectroscopy was carried out on ZnO powders at room temperature with an excitation wavelength of 365 nm of a LSM-365A LED (Ocean insight, Orlando, FL, USA) with a specified output power of 10 mW. The emission was collected using a FLAME UV-Vis spectrometer (Ocean optics, Orlando, FL, USA) with spectral resolution of 1.34 nm.Surface morphology and elemental analyses onGCN-NPs, ZnO-NPs and NHs samples were performed using a high-resolution scanning electron microscope (SEM) HR-SEM Zeiss Merlin (Carl Zeiss Microscopy, Munich, Germany), and energy dispersive X-ray analysis (EDS) system Bruker EDX-XFlash6/30 detector (Bruker, Oxford, UK) with an acceleration voltage of 4 kV for SEM and 10 kV for EDX analysis.

2.5Cytotoxicityassay: The biocompatibility of the synthesized NPs and NHs was assessed through cytotoxicity studies. Human dental pulp stem cells (HDPSC) were obtained from the permanent teeth of patients under approval of the Ethical Committee of Health Sciences Faculty of odontology at the University of Montpellier (France). HDPSC were grown under standard cell culture conditions (37 °C, 5 % CO₂)in 24 well plates (Falcon), and in complete culture medium consisting of α-MEM(Gibco) supplemented with 10% Foetal bovine serum (Sigma) and 1% penicillin-streptomycin (sigma) until they reached 80% confluence. After that, cells were exposed to 50 ug/ml concentrations of the biosynthesized materials. Cell viability is measured using trypan blue(Invitrogen)to determine the cytotoxic effects. Briefly, for each condition, cells were detached using Trypsin-EDTA (Gibco) which was inactivated by complete culture medium. From each cell suspension 10µ1 were mixed with an equal volume of trypan blue and deposited in Countess chamber slides (Invitrogen). Cell counting was performed using the automated cell counter Countess 3 (Thermo Fisher).

2.6 Antibacterial studies: The antibacterial studies were conducted using the well diffusion assay and disk diffusionassay methods. 30 ml of sterile nutrient agar were added onto sterile Petri dishes, and a sterilized spreader was used to consistently spread the agar over the plate.100 µLof a bacterial suspension was homogeneously spread onto the sterilized agar plate using a sterilized spreader. The wells were created using a sterilized borer. 5 mg and 10 mg (liquid) of biosynthesized pure ZnO-NPs, pure-GCN-NPsand ZnO/GCN-NHs added into the wells on the agar plates. The plates were then incubated for 16 h at 37°C. The plates were visually assessed and the inhibition zone in millimetres (mm) was measured.

2.7 Photo catalytic studies: The experiments were performed under direct solar light exposure with an average intensity of 1.2x105 Lux, at D.C.R. University of Science and Technology, Murthal, Sonepat, India (28.990N; 77.022E).For each experiment, 10 mg photo-catalyst was used in 100 mL of solution containing 10 ppm of the methylene orange (MO) stirred uniformly using a magnetic stirrer for 30 min in dark condition. Then the first reading (i.e.,dark reading) was performed using 5 mL in a centrifuge tube. Ten additional readings were performed atintervals of 15minof sunlight exposure to test the photo-catalytic degradation. All measured samples were previously centrifuged at 2000 rpm for 3 min before measure the absorption with a UV-Vis spectrophotometer (Shimadzu UV-2450, Japan) in the range of 200nm to 800nm in order to estimate the residual dye concentration in the sample.

Langmuir-Hinshelwood kinetic model was applied to rate the photo-degradation reaction with each photo-catalyst sample independently with corresponding dye solutions.

Degradationpercentage:\(\:1-\left[\frac{C}{C0}\right]*100\:\)

Pseudo first order reaction kinetics:\(\:In\frac{C0}{C}=Kt\)

Where, C is Concentration of dye at t, C₀ is the initial concentration of the dye at t = 0, k is the rate constant in s^-1.

3.1 UV-VisAnalysis: In a first approach, the absorption properties of bio-synthesizedsamples werestudied using a UV-Vis spectrophotometer. Pure GCN-NPs,ZnO-NPsand ZnO/GCN-NHsband gaps were calculated through UV-Vis absorption spectroscopy followed by Tauc plots[3, 5, 8, 9, 12, 16, 23]. Thedirect band gap energies of 3.32 eV forZnO-NPs, 3.16 eVfor GCN-NPs,2.78 eV for indirect band gap and 2.61 eV for ZnO/GCN-NHs were obtained, as shown in [Fig-3 (a, b i,ii, and c)]and Table I. GCN exhibits indirect band gap ranging from 1.8 eV to 2.7 eV, depending on its structural and chemical properties. Additionally, GCN can also exist in a cubic crystalline form that exhibits a direct band gap at 3.16 eV. The observed very low reduction in band gap from 2.78 eV to 2.61 eV in ZnO-doped GCN nanohybrids is likely due to the electronic interactions between ZnO and GCN, which modify the band structure, increase the electron and hole displacements from valance band to conduction band which creates new energy states and facilitate easier electron transitions under visible light.Moreover,ZnO/GCN-NHssample allowsa more efficient absorption under visible light exposure. This promotes the generation of more electron-hole pairs under visible light illumination. Consequently, this leads to higher photo-catalytic and antibacterial activities due to the production of larger number of electron-hole pairs that can effectively participate in redox reactions that degrade pollutants and promote the production of ROS that kill bacteria. [8, 9, 12, 16, 29, 30, 37, 39, 46–48].

NPs and NHs	Band gap
Pure ZnO-NPs	3.32 eV
Pure GCN-NPs	3.16 eV and 2.71
ZnO/GCN-NHs	2.61 eV

Table I

Band gap of biosynthesised NPs and NHs

3.2 XRD study:XRDpatterns of the biosynthesized ZnO-NPs, GCN-NPs and ZnO/GCN-NHs [Fig-4 (a,band c)] show thatGCN-NPsexhibit a strong XRD peak at 2θ of 27.7˚that corresponds to the (002) planethat is common to all GCN phases, whichtherefore confirms the graphitic stacking of C₃N₄ in pure-GCN-NPs and ZnO/GCN-NHs samples[8, 16, 23, 37–39]. The identification of GCN pattern is known to be challenging due to its unique nature, and XRD peak positions can varies from one phase to another [40]. The intensity of the (002) XRD peak is lower in the case ofZnO/GCN-NHs sample, which is most probably due to the presence of GCN crystallites in the case of the NHs. The decrease in intensity could be due to thestrong anchoring between the GCN-NPs host, whereZnO-NPs induce thedistortion of the nitride porous structure and alter the distance between the pores. The XRD pattern of ZnO-NPs shows XRD peaks at 2θ values of 31.82˚ (100), 34.52˚ (002), 36.38˚ (101), 47.67˚ (102), 56.8˚ (110), 63.07˚ (103), 68.2˚ (112) and 69.28˚ (201)(JCPDS file no 36-1451) [8, 16, 29, 30, 42, 45] thatcorrespond to zinc oxide wurtzite hexagonal structure.The sharp peaks observed at (100), (002), and (101) planes indicate a good crystallinityof ZnO-NPs and the existence of a long-range order [Fig-4(a)] [8, 45, 47]. The intense peak at 27.7˚ visible in the pure GCN-NPs powder sample has a comparable interplanar spacing of 0.332 nm with the reported GCN, which confirms the layered structure of aromatic conjugates [Fig-4(b)] [8, 23, 29, 30, 46, 47].

3.3 FTIR analysis: The absorption peak at 800 cm⁻¹ corresponds to the triazine ring vibrations, a distinctive feature of graphitic carbon nitride. This peak confirms the successful formation of GCN-NPs.The peak at 1436 cm⁻¹ represents the stretching vibration modes of the triazine rings[8, 9, 37, 39].The peaks at 1607 cm⁻¹ and 1698 cm⁻¹ are associated with the stretching vibrations of C–N bonds in the extended GCNstructure[8, 9, 23, 39].The broad absorption band ranging from 2895 cm⁻¹ to 3364 cm⁻¹ is due to the presence of N-H groups and adsorbed hydroxyl groups or H_₂O, indicating surface hydroxylation or hydration[8, 9, 39]. For pure ZnO-NPs, characteristic bands fromZn-O bond vibrations arevisible in the range of 550 − 450 cm⁻¹, and the band around 3437cm⁻¹ corresponds to hydroxyl group stretching vibrationsthat are present on the surface of ZnO-NPs [8, 42, 46, 47]. For the ZnO/GCN-NHs sample, the intensities of peaks ranging from 1656 cm⁻¹ to 1000 cm⁻¹ decrease and merge into a broad absorption band due to the addition of ZnO. These peaks correspond to various organic functional groups, specifically C = O (carbonyl), C-O (carboxyl or ester), and C-H (alkyl or aromatic) bonds. The broadening and merging of these peaks suggest that interactions occur between GCN and ZnO, likely affecting the vibrational modes of these organic groups.The 800 to 780 cm⁻¹ band is red-shifted, suggesting weakening of the C-N bond strength, indicating stretching in the conjugated system of GCN and ZnO[8, 42, 47]. The new peak at 3100 cm⁻¹ could indeed indicate the formation of C-N bonds with a different hybridization state (likely sp³), as these bonds would have a weaker bond strength and thus absorb at a higher wave number compared to the typical sp² C-N bonds. This alteration is possibly due to the presence of ZnO-NPs, which may cause breakage in the triazine units of GCN. The simultaneous growth of ZnO during the synthesis process interferes with the thermal polymerization of GCN, leading to a partially deteriorated graphitic structure of GCN. The combined analysis from FTIR and XRD thus provides a comprehensive understanding of the structural dynamics and interactions between ZnO-NPs and GCN. This highlights the beneficial effects of these modifications indicated by the new C-N bonds and changes in the graphitic structure, likely contributes to the improved photo-catalytic and antimicrobial properties observed in the ZnO/GCN-NHs [8, 39, 46, 47][Fig-5 (a,b and c)].

3.4 TGA study: Thermal stability of the biosynthesized pure GCN-NPsand ZnO/GCN-NHs samples were done through thermogravimetric analysis (TGA) as shown in [Fig-6 (a and b)] [8, 16].Initially, aweight loss: in the temperature range of 100˚C to 180˚C, is observed due to the evaporation of physically adsorbed water molecules. This stage also involves the decomposition of phytochemicals and organic moieties that act as capping agents and reductants during the biosynthesis process. A secondary weight loss in the temperature range of 150˚C to 440˚C is attributed to the decomposition of the remaining phenolic acids, flavonoids, and carbohydrates originating from the Ocimumtenuiflorum extract. These organic compounds play a crucial role in the growth and stabilization of the NPs and NHs. The TGA analysis indicates that the biosynthesized materials undergo significant thermal decomposition in these temperature ranges due to the loss of organic components from the plant extract. The higher weight loss decrease in the case of ZnO/GCN-NHs could be to the degradation of the organic molecules attached to ZnO NPs surface that are not present in the case of GCN-NPs. The understanding of thermal behaviour is important for potential practical applications in photocatalysis and biomedicine [8].

3.5 Photoluminescence emission spectroscopy:In this study, the PL spectra of pure GCN-NPs and ZnO/GCN-NHs were compared using an excitation wavelength of 365 nm. Pure GCN exhibits a transition band at 411 nm, attributed to electron transitions from anti-bonding (σ*) states to lone pair electron states of nitrogen atoms in the GCN structure.A broad band centered ~ 470 nm is also observed as shown in [Fig-7(a)], which is associated with transitions involving electrons from antibonding (π*) states to lone pair states, as well as from antibonding (π*) to bonding (π) states. This broad band suggests the presence of multiple electronic relaxation pathways.In the ZnO/GCN-NHs, the main transition band shifts to 380 nm, indicates the incorporation of ZnO in the GCN-NPs. This shift likely result from interactions between ZnO and GCN, such as electron transfer or band coupling which alters the energy levels within the hybrid material. The broad band is also present in the ZnO/GCN-NHs as shown in [Fig-7 (b)]. Suggesting the formation of new electronic states at the ZnO-GCN interface, which could improve the separation of photo-generated electron-hole pairs, a critical factor for enhanced photocatalysis [3, 5, 10, 23, 37, 38, 45, 46].

3.6 SEM study andEDX analyses: The SEM study provided detailed insights into the morphology and size of the biosynthesized materials, revealing that pure GCN-NPs exhibit a layered, graphite-like structure with uniformly separated plate-like sheets at a microscale, averaging around 34 nm in size as shown in [Fig. 8 (a,d)]. In contrast, ZnO/GCN-NHs display a distinct morphology with a reduced average size of approximately 24 nm, indicating that the simultaneous growth of ZnO-NPs with GCN limits their growth, resulting in smaller particle sizes as shown in [Fig. 8 (b,e)].

EDS analyses confirm a homogeneous distribution of ZnO elements within the ZnO/GCN-NHs, highlighting the successful integration of ZnO as shown in [Fig. 8 (c)]. This integration not only modifies the morphology and reduces the size of the GCN-NPs but also ensures a uniform distribution of ZnO across the GCN surface, enhancing the photocatalytic and antimicrobial properties of the NHs [5, 8, 23, 45–47].

4.1 Photocatalytic Activity: Pure ZnO-NPs, pure GCN-NPs and ZnO/GCN-NHs were examined under direct sunlight to evaluate their photo-catalytic potential toward the photo-degradation of methylene orange (MO) dye solution.ZnO/GCN-NHs expressed enhanced photocatalytic activity over pure GCN-NPs and pure-ZnO-NPs. The degradation shown by pure ZnO-NPs, pure GCN-NPs and ZnO/GCN-NHs was 27.03%, 40.01% and 47.56% after 150 min toward MO dye solution as noticeable strong peak equivalent to absorbance of MO observed at about 461–464 nm. Degradation could possibly be the result of the low band gap of ZnO/GCN-NHs, as evident from the results of the UV-Vis,Tauc plot and PL analysis, and supports the process of heterogeneous photocatalysis. The photo-degradation efficiency was calculated by considering the initial amount of dye left after adsorption-desorption equilibrium. Photocatalytic degradation curves of MO dye solution using, pure ZnO-NPs, pure GCN-NPs and ZnO/GCN-NHs are shown in [Fig-9(a,b and c)]. In the case of pure ZnO-NPs, the photocatalytic degradation efficiency was found to be only 27.03%, which could be probably due to a shielding of the surface of the ZnO nanoparticles owing to the adsorption of MO dye molecules. Pure GCN-NPs exhibit good photocatalytic properties and degraded the MO dye up to 40.01%. ZnO/GCN-NHs showed an enhanced efficiency, and degradation increased to47.56%[45–48]. The observed order of photocatalytic activityZnO/GCN-NHs > pure GCN-NPs > pure ZnO-NPsalso aligns with the sequence of their calculated band gap energies. ZnO/GCN-NHs exhibit the highest photocatalytic efficiency due to the synergistic effect of combining ZnO and GCN, which results in an optimal band gap that enhances light absorption across a broader spectrum. In addition, the combination and anchoring with GCN probably prevent MO dye to attach on ZnO-NPs surface, which prevent photocatalytic properties shielding[48]. Pure GCN, with a narrower band gap, performs better than pure ZnO-NPs under visible light but is still less efficient than the NHs. Pure ZnO-NPs, with its wider band gap, is primarily effective under UV light, but MO dye adsorption on ZnO-NPs surface results in lower overall photocatalytic activity compared to the hybrids and pure GCN.ZnO/GCN-NHs showed the highest value for the apparent rate constant, as shown [Fig. 9 (d and e)].

4.2. Trypan blue assessment of cell viability: 24 hours post-seeding, cells were treated with 50 µg/mL of pure ZnO-NPs, pure GCN-NPs and ZnO/GCN-NHs (all dissolved in DMSO) in triplicate. After the incubation period, the cells were stained with trypan blue. Viable cells exclude trypan blue, while non-viable cells absorb the dye, allowing for differentiation and counting under a microscope as shown in [Fig-10 (a,c,e,and g are before incubation);(b,d,f,and h are after incubation). The control group, with 90–100% living cells, sets a benchmark for cell viability in the absence of nanoparticle treatment, indicating healthy cell growth conditions as shown in [Fig-10 (b,i)].Furthermore, the slight reduction in cell viability (80–90%) suggests that sinceZnO-NPs andGCN-NPsonlyhave small cytotoxic effects, the majority of the cells remain viable, indicates moderate biocompatibility. The decrease could be due to cellular stress or NPs uptake leading to minor cytotoxic effects as shown in [Fig-10 (d,f,i)]. The 100% cell viability observed with ZnO/GCN-NHs as shown in [Fig-10 (h,i)] compared to the 80–90% viability with ZnO-NPs and GCN-NPs can be attributed tosynergistic effectsthat improve stability, dispersion, and reduce individual cytotoxic effects. The surface properties and interactions of ZnO/GCN-NHs are optimized for enhanced biocompatibility whichreduces the risk of rapid ion-induced cytotoxicity, while the nanoparticles optimized size and morphology result in less toxic interactions with cells.Additionally, their optimized size and morphology promote less toxic interaction with cells along with their differential cellular uptake, minimizes harmful internalization pathways. Together, these factors enhance the biocompatibility and safer of ZnO/GCN-NHs,making them apromising candidate for potential biomedical applications.

4.3 Study of Antibacterial Activity: The antimicrobial properties of the pureGCN-NPs, pure ZnO-NPs andZnO/GCN-NHs have been assessed using two different concentrations (5 mg (i) and 10 mg (ii))against E. coli, P. aeruginosa, S. aureus andS.dysgalactiae strains using disk and well diffusionassaymethods as shown in [Fig-11].The inhibition area is described in Table II. No antimicrobial activity was observed from pure GCN-NPs. In the case of pure ZnO-NPs, a small inhibition area was visible around the well and disk diffusion (see table II). ZnO-NPs are known photo-catalysts that exhibit antimicrobial properties through the production of ROS species under light exposure (preferably near UV wavelengths). In the case ofZnO/GCN-NHs, a larger inhibition area was observed (see Table II). Thelarger inhibition area indicates a larger production of ROS species that can inhibit the proliferation of bacteria and kill them. The highest efficiency can only be explained by an improvement of the photocatalytic properties of the ZnO-NPs through the anchoring of the nanoparticles on GCN nanostructuresurface.This enhanced activity is attributed to the increased surface-to-volume concentration of ZnO/GCN-NHs, which is speculated to produce ROS and consequently destroy the bacteria. Furthermore, the study suggests that the incorporation of ZnO-NPs into GCN enhances the charge separation of electron-hole pairs by reducing the band gap of the photo-catalyst[16, 29]. This, in turn, creates a barrier against recombination, thereby increasing the antimicrobial activity [29, 30, 32]. However, gram-negative bacteria exhibit less sensitivity compared to gram-positive bacteria when exposed to biosynthesized NPs and NHs. This differential sensitivity can be attributed to the structural differences in their cell walls, as well as ROS species are negatively charged, so gram negative will be less sensitive due to charge repulsion in addition to membrane nature. Gram-positive bacteria have a thick peptidoglycan layer in their cell walls, which is relatively porous and allows NPs and NHs to penetrate more easily as shown in [Fig-11(b,c,f,g,j and k)]. In contrast, the cell wall of gram-negative bacteria includes an additional outer membrane composed of lippolysaccharides. This outer membrane acts as a protective barrier, improving the barrier properties and increasing bacterial resistance to the penetration of NPs and NHs as shown in [Fig-11(a,d,e,h,i and l)]. Due to these structural differences, a larger zone of inhibition was observed for gram-positive bacteria compared to gram-negative bacteria when treated with the biosynthesized NPs and NHs. Despite the thicker peptidoglycan layer in gram-positive bacteria, it is less effective against ROS due to the positively charged nature of the membrane, whichresults in a more significant antibacterial effect. However, both gram-positive and gram-negative bacteria are affected at different degreesby the antimicrobial properties of ZnO/GCN-NHs, the unique structural characteristics of their cell walls lead to varying degrees of sensitivity. This understanding of bacterial resistance mechanisms is crucial for optimizing the design and application of nonmaterials for antimicrobial purposes [16, 29, 32].

Stains 100 µl	Biosynthesised Material	Zone Inhibition (Radius mm)
		Well diffusion method		Disc Diffusion methods
		5 mg	10 mg	Disc Diffusion methods
Escherichia coli	ZnO/GCN-NHs	15 mm	17.5 mm	17 mm
Streptococcus dysgalactiae		23 mm	25 mm	24.4 mm
Staphylococcus aureus		24.3 mm	27.5 mm	25 mm
Pseudomonas aeruginosa		15.3 mm	14.4 mm	11 mm

Table II

Inhibition zone of ZnO/GCN-NHs towards Escherichia coli; Streptococcus dysgalactiae; Staphylococcus aureus; Pseudomonas aeruginosabacteria.

Dye degradation Mechanism

The photocatalytic degradation mechanism of methylene orange (MO) using ZnO/GCN-NHs can be explained by the inhibited electron-hole (e-h) pair recombination and the superior photocatalytic efficiency observed in the study. When ZnO/GCN-NHs is irradiated by sunlight, photogenerated electrons on the GCN are excited from the valence band (VB) to the conduction band (CB). These excited electrons are then transferred from the CB of GCN to the CB of ZnO. This transfer is facilitated by the interface between ZnO and GCN and the more negative CB edge potential of GCN compared to ZnO[8, 16, 23, 29, 31–33, 37, 38]. Conversely, the photo-induced holes from the VB of ZnO move to the VB of GCN due to the lower positive potential of the GCN valence band. The transferred electrons and holes react with chemisorbed oxygen and water molecules (H₂O) on the composite surface to yield superoxide anions (O₂⁻) and hydroxyl radicals (•OH). These ROS are highly reactive and can oxidize and degrade MO. The holes in the VB of GCN can directly oxidize MO, along with the production of additional •OH radicals, as illustrated in [Fig. 12 (a, b)]. This efficient separation and transfer of photogenerated charges inhibit the recombination of e-h pairs, leading to enhanced photocatalytic activity [5, 10, 23, 45, 46, 48].

Antimicrobial Mechanism: The antimicrobial mechanism involves the interaction of bacterial species with the ROS produced by the biosynthesized ZnO/GCN-NHs, which is depicted in [Fig. 12 (c)]: The photogenerated holes, electrons, superoxide anions (O₂⁻), and hydroxyl radicals (•OH) invade the bacterial cell wall. These reactive species cause the rupture of the bacterial cell wall, leading to spoilage and outflow of the cell cytoplasm. The damages to the cell membrane and leakage of cell contents ultimately lead to bacterial death. The prolonged lifespan of e-h pairs in ZnO/GCN-NHs, facilitated by the conjugation of photogenerated electrons on GCN layers, as confirmed by photoluminescence (PL) studies, enhances both bactericidal and photocatalytic activities[3,5.8,9,23]. The synergistic effect between the layered GCN and ZnO-NPs is responsible for this improved ROS production performance. As shown in [Fig. 12 (c)].The ZnO/GCN-NHs shows significant potential for antimicrobial applications due to its enhanced charge separation, broader wavelength absorption, and higher surface area. These properties lead to the efficient generation of reactive species under solar light irradiation, making the composite a promising candidate for various environmental and biomedical applications[8, 9, 12, 33, 42, 47, 48, 53–59].

This manuscript reports the successful biosynthesis of highly efficient visible-light-driven ZnO/GCN-NHs photo-catalysts using a one-step calcination approach with Ocimumtenuiflorum (OT) leaves. For the first time, GCN has been bio-synthesized using plant extract from leaves. The photo-degradation experiments with methyl orange (MO) and photoluminescence (PL) analysis demonstrated that ZnO/GCN-NHs significantly enhance the separation of photo-generated charge carriers, resulting in superior visible light photocatalytic activity. The ZnO/GCN-NHs achieved nearly 47.56% degradation of MO within 150 minutes.GCN photocatalyst nanohybrid exhibitshigh stability under visible light irradiation. The formation of a well-defined heterojunction layered structure between ZnO and GCN was confirmed by XRD and SEM.This structure enhances the separation of photo-generated charge carriers, contributing to the superior photocatalytic performance. ZnO/GCN-NHs displayed high antibacterial activity against both gram-positive and gram-negative bacterial strains. The enhanced antibacterial properties are attributed to the increased surface-to-volume ratio and effective production of ROS that can interact with bacterial cell walls.

Implications and Further Research

This study presents an efficient and stable photocatalytic nanomaterial that can be used for environmental remediation. The one-step biosynthesis approach using Ocimumtenuiflorum leaves offers an easy and green preparation procedure for fabricating efficient GCN based nanohybrid photo-catalysts. Such approach will enable the development of sustainable and scalable production methods for large scale applications.However, further research is needed toclarify the specific contributions of different antimicrobial processes (e.g., ROS generation, protein mutations) to the overall antibacterial activitythat can then be applied to other pathogens like viruses, and investigate the combination of metal oxide nanoparticle with GCN to further enhance light absorption and charge separation. The broadening of the wavelengthabsorption range of GCN-based nanocomposites will enable to improve the photocatalytic properties and photo-degradation efficiency.

Note

I declare that the authors have no competing interests as defined by Nature Portfolio, or other interests that might be perceived to influence the results and/or discussion reported in this paper.

Author Contribution

A. Main writer of the manuscript, Methodology, Investigator.B. Review the manuscript, Suggested for the figures 3rd and 7th.C. Characterizations helps D. lab, Idea and review of the manuscript E. lab for cytotoxicity activity and resourcesF. Helps in whole manuscriptG. Supervised the whole work, Data analysis, Data Security, editing and correcting manuscript, funding, Validation etc.

Acknowledgement

This work is supported by Mobilitas + postdoctoral grant MOBJD1204 and ETAg Research Team Grant PRG2115 fundings.

Data Availability

Data is provided within the manuscript

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No competing interests reported.

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Bio-synthesis of Graphitic Carbon Nitride and ZnO/GCN Nanohybrid for Remarkable Environmental Application

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