Microwave-assisted Synthesis, Characterization, Photocatalytic Degradation of Antibiotics, and Fluorometric Selective Sensing Activity of g-C3N4 Supported CuO Composites

Herein, we have designed for the fabrication of a series of g-C3N4/CuO composite by using one-step microwave-assisted synthesis for the degradation of antibiotics and detection of nano-molar range of toxic heavy metal ions. The synthesized g-C3N4/CuO composites were analyzed and characterized to know the structure, phase, surface area, absorption region, bandgap, and size of the composites. From the observation of TEM and XRD measurements, g-C3N4/CuO composites have hexagonal shape with average diameter of the particles is 25 ± 5 nm. The observed band gap values from UV-vis DRS for g-C3N4 nanosheets and CuO NPs are 2.64 eV and 1.72 eV. The synthesized g-C3N4/CuO composite has prodigious specific surface area (32.47 m2/g), which is the evident for superior heterogeneous catalytic applications. Therefore, the synthesized g-C3N4/CuO composites were tested for the degradation of antibiotics such as tetracycline (TC) and ciprofloxacin (CIP) under UV light illumination, it shows 88.02% and 90.01% degradation was observed within 1 h due to the matching optical band gap and internal charge transfer of excitons with in the heterojunction surface among g-C3N4 and CuO in the composite than the individual components (g-C3N4 and CuO) due to the high surface area and tiny particles of CuO were randomly deposited on the surface of g-C3N4 nanosheets. The catalytic reduction reaction follows as pseudo-first order equation and reused for 5 consecutive cycles without remarkable loss of catalytic activity. Moreover, the synthesized CuO NPs and g-C3N4/CuO composites were used as a prominent fluorescence sensing probe for the selective detection of Pb2+ in nano-molar range of concentration with Ksv is 1.38 × 104 mol− 1dm3. It was observed as a linear relationship based on the change in intensity, the limit of detection was determined to be 0.184 nM.


Introduction
of multi-drug resistance microorganisms including bacteria, fungi, virus, and parasites, which cause for the development of diseases in humans / animals /crops [3]. Hereafter, it is required to evolve some definite procedures for the removal of such types of harmful organic pollutants such as antibiotic drugs from the water stream with the combination of composites due to formation of non-antibiotic drug composition to become ineffective for the prevention, control, and avoid spreading infectious diseases [4,5]. In during this procedure, the various methods were used for the removal of such antibiotic drug effluents from wastewater such as biological treatment [6], adsorption method [7], and heterogeneous photocatalysis [8][9][10][11][12]. Among these techniques, the heterogeneous photocatalysis is considered as a one of the simple and efficient method for the removal and degradation of pollutants as harmful waste materials into small harmless products such as CO 2 and H 2 O under naturally occurring light with renewable energy source and economical significant manner [12,13].
The two-dimensional material such as graphitic carbon nitride (g-C 3 N 4 ) is a captivating material has attractive research interest due to its unique properties and economic significant fabrication procedure. It is an organic n-type semiconductor based catalyst with appropriate bandgap energy is 2.72 eV, which is corresponding to visible light absorption at the wavelength of 460 nm [14,15]. As reported structure of g-C 3 N 4 , the designed framework consists N-bridged aromatic poly tri-s-triazine (heptazine) units having π-conjugates with strong C-N covalent bonds [15]. From the π-conjugated sp2 hybridized structure and heptazine rings provides an unexceptional bandgap energy to g-C 3 N 4 sheets as well as significant photophysical properties including electronic band structure and its thermal / chemical stability [16]. Moreover, the existing properties of g-C 3 N 4 are limited to low surface area, surface inertness, insufficient visible light absorption, high rate of recombination of photogenerated charge carriers, and poor mobility of charge carriers for its practical applications [16]. In order to overcome the drawbacks of g-C 3 N 4 to involve practical applications majorly investigation of catalytic properties, the development of new synthetic strategies to prepare g-C 3 N 4 based composites like combination of metals, dopants, and other semiconductor materials to improve the electronic structure, formation of heterojunction, particle diameter, specific surface area, and porosity of the materials were practiced [16].
Based on these development of fabrication of g-C 3 N 4 based composites by using the combination of other semiconductor materials including metal oxides such as TiO 2 , WO 3 , CeO 2 , CuO, ZnO, SnO 2 , and NiO has a great attention was drawn for the estimation of efficient photocatalytic activity towards the degradation or removal of organic toxic pollutants with effect of its stability, reusability, and approachable economic consequence [17][18][19][20][21][22]. Among them, the individual CuO NPs has been used as a good adsorbent and exhibited superior light absorption efficiency with the comparison of other metal oxides towards the degradation or removal of organic waste in the area of wastewater treatment [23,24]. In addition, the catalysts having their catalytic performance can affect by various parameters such as bandgap energy, particle size, and specific surface area of the catalysts [25]. Moreover, the CuO NPs as p-type semiconductor material has emerging properties of an extensive optical bandgap for the rate of recombination of photogenerated charge carriers, inevitably, it shows deprived photocatalytic performance of the CuO NPs [25].
To avoid such poor photocatalytic performance of CuO NPs, the development of innovative materials to enhance the catalytic activity by the formation of high surface area and heterojunction with the combination of g-C 3 N 4 nanosheets. By this g-C 3 N 4 based CuO heterojunction containing material with a bandgap energy of 2.74 eV is an auspicious material because of its reasonable bandgap energy and remarkable optical properties by the coupling with CuO to obtain better catalytic performance as well as sensing properties in wastewater remediation. Therefore, CuO and g-C 3 N 4 were combined to provide a p-n junction as heterojunction in the composite structure, which enhances the prominent photocatalytic performance. So far, many researchers have been reported g-C 3 N 4 based metal oxide materials has the potential application in the promising fields including gas sensing [25], lithium ion batteries, catalysis, and water splitting [2627] are still limited. To the best of our knowledge, the synthesis of g-C 3 N 4 /CuO composites, which gives the particles and desired characteristics such as preventing particle aggregation and stable as well as the strong co-ordination modes between N and Cu atoms at the heterojunction interface of g-C 3 N 4 /CuO frameworks implies that the g-C 3 N 4 /CuO composites prone to contain nanostructured CuO species, therefore, could be beneficial to improve the performance.
Therefore, in this study, we report the synthesis and characterization of g-C 3 N 4 /CuO composites to estimate the size, shape, and morphology by various techniques, and the investigation of degradation of TC and CIP by photocatalytic process under UV light illumination and detection of heavy metals in the fields of wastewater treatment. Moreover, the photodegradation efficiency, kinetics, and the effects of amount of CuO in the g-C 3 N 4 /CuO composites were systematically examined and its reusability and stability also studied. The nano-molar concentration level of heavy metal ions preferably Pb 2+ was detected by selective sensing fluorometric probe. This study is useful for further development of novel composites for wastewater treatment systems and it could be afford to utilize for the improvement of photodegradation of antibiotics and detection of heavy toxic metals by g-C 3 N 4 /CuO based composites.

Materials and Methods
The double distilled water was used as a solvent for throughout the experiment. The received chemicals were used without further purification. The antibiotic drugs, tetracycline and ciprofloxacin were collected from Aurobindo Chemicals Pvt. Ltd. The analytical grade reagents of melamine, Cu(NO 3 ) 2 .3H 2 O, methanol, ethanol, and different metal salts (Al 3+ , K + , Ca 2+ , Ba 2+ , Cu 2+ , Pb 2+ , Ni 2+ , Zn 2+ , Cd 2+ , and Ag + ) as chlorides were procured from Merck Chemical Reagent Co. Ltd., India.

Synthesis of g-C 3 N 4 nanosheets
In the typical procedure for the synthesis of g-C 3 N 4 nanosheets by a single step polymerization of melamine (3 g) under the thermal treatment at 550 o C for 5 h with heating rate is 5 o C/min in muffle furnace under inert atmosphere. The obtained pale yellow colored product was grinded, purified, and collected. Further as per the previous report [28], the g-C 3 N 4 ultrathin nanosheets were fabricated by the liquid exfoliation of g-C 3 N 4 powder in an aqueous medium. The elaborated procedure, 50 mg of g-C 3 N 4 powder was dispersed in appropriate quantity of water under ultrasonicator stirring continuously for 24 h. The formed suspension was centrifuged to remove large unreacted particles and final product was washed a number of times with double distilled water and dried at 100 o C for 5 h in an oven. The obtained final g-C 3 N 4 nanosheets were collected and used for further analysis.

Synthesis of Pristine CuO NPs
In the preparation of pristine CuO NPs by using unpretentious microwave-assisted method [29], wherein the preparation, Cu(CH 3 COO) 2 .5H 2 O (0.2 M, 50 mL) was mixed with NaOH solution (2.25 M, 10 mL) under continuous magnetic stirring. After vigorous stirring for 1 h, the reaction mixture was collected and transferred in to a microwave oven with the operating conditions at 60 MHz for 30 s, and washed with solvents for several times to reach pH = 7. Consequently, the obtained reaction suspension was centrifuged and separated, and dried in an oven at 100 o C for 24 h to avoid unreacted precursor and impurities. The final resulted powder as CuO NPs was further estimated thermal stability by the heat treatment at 450 o C with heating arte of 5 o C/min for 3 h in a muffle furnace.

Synthesis of g-C 3 N 4 /CuO Composites
In the preparation of the series of g-C 3 N 4 /CuO composites by the amount variation of CuO NPs were loaded on the surface of g-C 3 N 4 nanosheets via microwave-assisted method. In the typical synthesis of g-C 3 N 4 /CuO composites as follows: 1 g of g-C 3 N 4 and optimized by various quantities of the synthesized CuO NPs such as 0.5 wt% (g-C 3 N 4 /CuO-1), 2.5 wt% (g-C 3 N 4 /CuO-2), and 5 wt% (g-C 3 N 4 /CuO-3) were dispersed and ultrasonically stirred for 60 min in 150 mL of double distilled water. The optimized amount of CuO in the composite is 5 wt% (g-C 3 N 4 /CuO-3 composite) was higher than that of other catalysts such as 0.5 wt%, 2.5 wt%, and 10 wt% (in this case, the weight percentages of g-C 3 N 4 is less than the weight% of CuO, it may cause the structure of composite is quash and affects the electron transfer and absorption capacity of g-C 3 N 4 to reduction when compared to 5 wt%). Therefore, we chose g-C 3 N 4 /CeO 2 -3 as the optimal content to carry out our further investigations. Then, the reaction mixture was stirring for 1 h, the reaction mixture was collected and transferred in to a microwave oven with the operating conditions at 60 MHz for 30 s, and washed with solvents for several times to reach pH = 7. Consequently, the obtained reaction suspension was centrifuged and separated, and dried in an oven at 100 o C for 24 h to avoid unreacted precursor and impurities. The final resulted powder as g-C 3 N 4 /CuO composites was further estimated thermal stability by the heat treatment at 450 o C with heating arte of 5 o C/min for 3 h in a muffle furnace.

Characterizations
The crystal nature and phase of the synthesized composites were analyzed by the powder XRD patterns (Xpert Philips, CuKα radiation source, 40 kV generator voltage, 40 mA generator current) at room temperature. TEM images (Tecnai G2) with 80 kV acceleration voltage were captured of the synthesized composites for the detection of morphology, shape, and average diameter of the particle in the composites. The specific surface area of the composites were recorded by using N 2 adsorption-desorption isotherms with ASAP-2000 instrument. The optical absorbance and emission behavior of the synthesized g-C 3 N 4 /CeO 2 composites the stock solutions of synthesized g-C 3 N 4 /CuOcomposites and various metal ion salts solutions (Al 3+ , K + , Ca 2+ , Ba 2+ , Cu 2+ , Pb 2+ , Ni 2+ , Zn 2+ , Cd 2+ , and Ag + ) were prepared with particular concentration by using double distilled water as a solvent. To examine the fluorometric sensing selectivity of CuO and g-C 3 N 4 /CuO composites for selective metal ion Pb 2+ , 0.5 mL of every metal ion solutions (10 µM) were added into 2.5 mL of individual CuO and g-C 3 N 4 /CuO composite solution at room temperature under the same conditions, and CuO and g-C 3 N 4 /CuO composite solution was used as a blank or control, respectively. In order to effect the concentration of Pb 2+ in selective sensing activity, the various concentrations (10-100 µM) of Pb 2+ (0.5 mL) were added to the g-C 3 N 4 /CuO composite(2.5 mL) for in detail detection of the sensitivity limits of g-C 3 N 4 /CuO composite. The fluorescence emission spectra of the reaction solutions were recorded and measured their intensities after mixing the metal ion solution by a fluorescence spectrophotometer (λ em = 512 nm, λ ex = 340 nm) and analysed their detection limits in nano-molar range of concentration.

UV-vis Absorption Analysis
The optical absorption properties including bandgap energy and understand the effect of the heterojunctions on light absorption of the synthesized g-C 3 N 4 , CuO, and g-C 3 N 4 /CuO composites were characterized by using UVvis DRS and the obtained results were displayed in Fig. 1. It can be seen from Fig. 1, the obtained UV-vis spectra of both g-C 3 N 4 nanosheets and CuO NPs were exhibited a strong broad absorption band in the UV-vis region. In general, the bandgap energies of the synthesized g-C 3 N 4 , CuO, and g-C 3 N 4 /CuO composites were calculated as 2.64 eV, 1.72 eV, and 1.88-2.01 eV, respectively by the following equation: E g = 1240/λ [31]. In addition as shown in Fig. 1, the absorption spectrum of the synthesized g-C 3 N 4 /CuO composite is observed as a combination of UV-vis spectra of the individual components as g-C 3 N 4 and CuO. The higher concentration of CuOin the composite, the light absorbance was exhibited more in the visible region due to the low bandgap energy. Moreover, the obtained results were suggesting that the formation of g-C3N4/CuO composite has absorption edge shifted to higher wavelengths (red shift) in the visible region with the increasing the concentration of CuO in the composite, as a result, the synthesized composites having better capability of light absorption, which could be enhance the photocatalytic activity.
were recorded in the wavelength range of 200-800 nm using UV-vis absorption spectrophotometer(UV-3600, Shimadzu) and photoluminescence (PL) spectra (RF-5001 PC supported fluorescence spectrometer) with an excitation wavelength of 330 nm. The identification of organic group of moieties involved various functional groups and its vibrational frequencies were analyzed by attenuated total reflectance ATR-FTIR spectra (Perkin Elmer).

Photocatalytic Degradation Test of Antibiotics
The photocatalytic degradation test of the synthesized g-C 3 N 4 /CuO composites were designed for the degradation of antibiotic drugs including TC and CIP in the glass reactor equipped with UV filter (EasyMax 102 Advanced Thermostat system model: A9616-07) under 200 W Hg-Xe lamp. In a distinctive procedure as mentioned in the previous work [30], prior to exposure of light, the reaction suspension (25 mg of catalyst + 15 mg/L of antibiotic solution in 100 mL) was stirred under magnetic stirrer in the dark for 1 h to reach the adsorption-desorption equilibrium. After that, the reaction suspension was exposed to light as initiating the photocatalytic degradation reactions, the equal portions (1 mL) of the reaction suspension was taken at different time intervals to analyze the concentration of reaction solution. The retrieved reaction suspension was centrifuged and separated from the solution by filtration through a 0.2 micromolar Millipore filter. The whole degradation reaction pathway was monitored by using UV-vis absorption spectroscopy in the range from 200 to 550 nm. The maximum absorbance values were recorded for the TC and CIP degradation using the synthesized composites at their corresponding wavelengths at 357 nm; and 274 and 322 nm, respectively. The photocatalytic degradation performance of the synthesized composites by the degradation of TC and CIP antibiotic drugs was estimated using the following equation: Where,C 0 and C are the initial and final concentrations of antibiotic drug at different time intervals, respectively.

Fluorometric Selective Sensing Test
Detection and quantification of heavy metal ions and toxic organic molecules were found in the wastewater and environment by fluorescence sensing analysis is active area of research, so it may quite beneficial to human health. All

FTIR Analysis
The synthesizedg-C 3 N 4 /CuO composites were characterized by FTIR spectra for the analysis of presented functional groups with the interaction of composites. The functional groups and structures of the synthesized g-C 3 N 4 nanosheets, CuO NPs, and g-C 3 N 4 /CuO composites were investigated by FTIR and the obtained spectral results were displayed in Fig. 3. In FTIR spectrum of g-C 3 N 4 nanosheets, the strong absorption bands appeared between at the wavenumber of 1240 cm − 1 and 1750 cm − 1 , which is ascribed to the C = N/C-N stretching vibration bands in g-C 3 N 4 and these are related with the skeletal stretching vibrations of various triazine rings [34,35]. The extended broad absorption band at around 800 cm − 1 is also corresponded to the stretching vibrations of triazine ring [36]. The broad absorption band situated around at 3513 cm − 1 is resembles to N-H stretching vibration of amine related groups and O-H stretching vibration due to the adsorbed water on the surface [37]. In the FTIR spectrum of CuO NPs, the strong broad absorption peaks were observed at 948 cm − 1 and 675 cm − 1 , it can be ascribed to the characteristic peaks of Cu-O bond. Moreover, the all characteristic absorption peaks of g-C 3 N 4 and CuO were presented in the characteristic peaks of the g-C 3 N 4 /CuO composites with slight changes in the intensities, it can be seen that the main characteristic peaks of individual components were well matched with the structure of composite.

Photoluminescence Analysis
In sight of the more detailed optical emission properties of the synthesized g-C 3 N 4 /CuO composites were analyzed by using photoluminescence properties. The recorded PL spectra of the synthesized g-C 3 N 4 , CuO and g-C 3 N 4 /CuO composites were displayed in Fig. 2. It can be seen as the synthesized all samples were approximately exhibited a broad emission peak in the range between 500 and 520 nm at an excitation wavelength of 365 nm. For the comparison of PL spectra of g-C 3 N 4 /CuO composites with g-C 3 N 4 , the extreme PL intensity was exhibited at 476 nm, it is suggesting that the rate of recombination of photo-generated electron-hole charge carriers is relatively high, the formed composite was shown prominent catalytic activity [32]. However, the synthesized CuO NPs were exhibited lowest PL intensity, it could be suggesting to promote better catalytic activity due to the high rate of recombination of charge carriers [33]. Therefore, in addition the combination of g-C 3 N 4 with CuO in the composite, it would provide extreme rate of recombination and exhibited as well as catalytic activity based on their parameters e.g. particle size, interface between g-C 3 N 4 and CuO particles, etc. [33]. However, the positive impact of CuO NPs in the g-C 3 N 4 /CuO composite enhances the separation of charge carriers and shows prominent catalytic activity.   0 2) crystalline planes, which was matching to hexagonal phase of g-C 3 N 4 nanosheets and well g-C 3 N 4 /CuO composites were identified by using the recorded powder XRD patterns and shown in Fig. 4. The crystalline diffraction peaks of g-C 3 N 4 nanosheets were Fig. 4 The powder XRD patterns of the synthesized g-C 3 N 4 nanosheets, CuO NPs, and g-C 3 N 4 /CuO composites Fig. 3 FTIR spectra of the synthesized g-C 3 N 4 nanosheets, CuO NPs, and g-C 3 N 4 /CuO composites 12 ± 1 nm using Debye-Scherrer's equation. In addition, the diffraction peaks of pristine CuO NPs has less intense with large size of the particles, when compared to the diffraction peaks of CuO NPs with the combination of g-C 3 N 4 , which is relatively narrow width of diffraction peaks and suggesting to the formed g-C 3 N 4 /CuO composite has high crystallinity, purity, and small size of the particles for the better catalytic activity.

TEM Micrographs
The in detail surface morphology, microstructure, and size of the synthesized g-C 3 N 4 /CuO composites were investigated by TEM images as displayed in Fig. 5. The TEM image of g-C 3 N 4 nanosheets was exhibited that the g-C 3 N 4 nanosheetshave approximately 130 nm of length and 20 ± 5 nm of diameter as shown in Fig. 5(a). It shows and confirmed as that of obtained nanosheets has a smooth and stacking flat sheets with regular surface area and porous structure. The particles of monometallic CuO NPs shows as an irregular spherical shape with the size in the range of 5-25 nm and the results were displayed in Fig. 5(b). matched with JCPDS file No. 87-1526 [36]. In addition, theses crystalline planes were related to the in-planer structural packing motif of tris-triazine units and interlayer stacking of conjugated aromatic systems, respectively [38]. In those crystal planes, the diffraction peak was shifted to slightly higher diffraction angle as reported which is at 2θ of 28.2 o , which is cause for effective exfoliation of g-C 3 N 4 nanosheets [38]. In the XRD pattern of CuO NPs shows in Fig. 4 as the obtained peaks was suggesting to the comparable with the crystal structure of hexagonal phase. The obtained crystal diffraction peaks are observed at the 2θ of 32.  2 2), which is good agreement with the previous report [39]. In the XRD pattern of g-C 3 N 4 /CuO composite exhibited the combination diffraction peaks of the obtained diffraction peaks of CuO NPs and g-C 3 N 4 nanosheets, which might be due to the welldispersed CuO particles on the surface of g-C 3 N 4 sheets and it is confirmed the formation of g-C 3 N 4 /CuO composite. The estimated particle size of the synthesized CuO NPs and the g-C 3 N 4 /CuO composite is approximately 18 ± 2 nm and CuO NPs, and g-C 3 N 4 /CuO composites were 16.28 m 2 /g − 1 , 11.86 m 2 /g − 1 and 32.47 m 2 /g − 1 , respectively. In this regard, the maximum surface area having composites is assigned to the high surface roughness of the g-C 3 N 4 /CuO composite, which may be beneficial for the prominent heterogeneous catalytic response for the various reactions. Moreover, the maximum surface area provides more number of reaction sites on their surface to adsorb more reactants to convert products, it could be favorable to the enhancement of photocatalytic activity of the composites.

Photocatalytic Activity
The photocatalytic degradation of selected antibiotics by using g-C 3 N 4 based composite catalysts will be more challenging and interesting area of research. In the part of degradation of antibiotics in aqueous medium, it can be degraded or reduced to the small molecules under light exposure. In this catalytic process, the selected antibiotics were expected to act as a co-catalyst for the reduction and it get oxidized during photocatalytic reaction [41][42][43]. In this regard, we have selected to study included an evaluation of the photocatalytic activity of the g-C 3 N 4 /CuOcomposites in the degradation of TC and CIP under UV light irradiation. The evaluation of photocatalytic activity of the synthesized g-C 3 N 4 , CuO, and g-C 3 N 4 /CuO compositeswas performed for all samples and the results were presented in Figs. 7 and 8. The superior photocatalytic activities of the synthesized g-C 3 N 4 /CuO composite catalysts was exhibited due to the formation of heterojunction of g-C 3 N 4 with other compatible semiconductors such as CuO NPs is an active way to enhance the transfer of charge carriers in the photocatalytic reactions.
In the Fig. 5(c), the synthesized CuO NPs were quite homogeneously distributed on g-C 3 N 4 nanosheets in the g-C 3 N 4 /CuO composite, but the maximum concentration of CuO NPs in the composite was observed at the perimeter of large sheets of g-C 3 N 4 and size of the particles were smaller than the pristine CuO NPs. It can be suggesting to that the g-C 3 N 4 nanosheets were act as supporter for the prevention of agglomeration and enhance the dispersibility nature with small size of the particles. Moreover, the formed heterostructure type of structure and morphology of the g-C 3 N 4 /CuO composite is prominent beneficial to the effective separation of photogenerated electron-hole pairs and resulting to the enhancement of catalytic nature of the composites. Finally, the results provided by TEM images, it was clearly suggesting that the formation of g-C 3 N 4 /CuO heterostructure.

BET Surface Area Analysis
In general, the photocatalytic reactions of the composites as catalysts is related to the structure, size of the particles, as well as its specific surface area and porosity as pore size distribution of the catalyst. Therefore, the surface area and structure of the synthesized g-C 3 N 4 , CuO NPs, and g-C 3 N 4 /CuO composites were investigated by N 2 adsorption-desorption isotherms and specific surface area as well as pore size distributions were estimated by the BET equation and the obtained results were shown in Fig. 6. From this results, the obtained isotherm of the g-C 3 N 4 /CuO composite exhibits a type IV with a hysteresis loop, which indicates the presence of mesopores [40] and the obtained results have pore diameter in the range of < 10 nm. The BET surface area (S BET ) of the synthesized g-C 3 N 4 nanosheets, Fig. 6 N 2 adsorption-desorption isotherms and pore size distributions of the synthesized g-C 3 N 4 nanosheets, CuO NPs, and g-C 3 N 4 /CuO composites the particles, and high specific surface area for exhibited prominent adsorption of antibiotics due to the monometallic CuO with different concentrations and results were shown in Fig. 7(b) and Fig. 8(b). By the comparison of TC and CIP adsorption on the surface of maximum concentration of CuO (0.5 wt%) involved in the g-C 3 N 4 /CuO composite due to the monometallic CuO NPs as noted that both catalysts have CuO particles of similar size. In addition, 0.5 wt% of CuO in the composite which can be found as main component as responsible for efficient adsorption of TC and CIP due to its bandgap energy. The degradation efficiency of prepared g-C 3 N 4 /CuO-3 composite was enormously increased as 88.02% and 90.12% of TC and CIP antibiotics in 60 min of photocatalytic reaction by exposure of UV light irradiation. It was also found that g-C 3 N 4 /CuO-3 composites exhibited better ability to absorb TC and CIP than g-C 3 N 4 , CuO, g-C 3 N 4 CuO-1 and g-C 3 N 4 /CuO-2 composites. In view of this observation, we hypothesize that the formation of g-C 3 N 4 /CuO heterostructures can have some positive impact on the ability of metal oxides to adsorb TC and CIP molecules due to the interaction between g-C 3 N 4 and CuO NPs.

Kinetic Study
The evaluation of the reaction mechanism and apparent rate constants for the photocatalytic degradation of selected antibiotics such as TC and CIP were calculated by the following equation [44]: Where, A o and At are the absorbance of the antibiotics before the reaction and after the reaction at certain time intervals, respectively; k denotes the rate constant, and t is the reaction time for the antibiotic degradation under UV light exposure. In Fig. 7(c) and Fig. 8(c) displays the plot drawn between ln [A 0 / A t ] vs. time, the slope of the linear curve is calculated from the graph as apparent rate constants. The obtained results were suggesting to the linear fitting of plots confirms the photocatalytic degradation of TC and CIP according to pseudo-first order kinetics. According to the obtained results, the k value determined for g-C 3 N 4 /CuO-3 composite was much times higher than that established for g-C 3 N 4 , CuO, g-C 3 N 4 /CuO-1 and g-C 3 N 4 /CuO-2 composites. The photocatalytic degradation parameters for the degradation of TC and CIP including degradation efficiency values and rate constants with correlation factors (R 2 ) of these antibiotics using g-C 3 N 4 , CuO, and g-C 3 N 4 /CuO composites under UV light irradiation were summarized in Tables 1 and 2.
In the prior photocatalytic degradation of selected antibiotics (TC and CIP) by using the synthesized g-C 3 N 4 , CuO, and g-C 3 N 4 /CuO composites in 60 min of the reaction under the dark conditions as adsorption only and upon light exposure with monochromatic UV light and the results were displayed in Fig. 7(a) and Fig. 8(a). From theses obtained results, the contribution of TC and CIP degradation in overall antibiotic degradation in the presence of catalysts such as g-C 3 N 4 , CuO, and g-C 3 N 4 /CuO composites was negligible under dark conditions, which is approximately less than 10% after 60 min of the reaction time. In view of the observation, it can be concluded that the catalysts such as g-C 3 N 4 , CuO, and g-C 3 N 4 /CuO composites were responsible for the adsorption of the antibiotic on the surface of catalyst. Interestingly, the synthesized g-C 3 N 4 /CuO composites have chemical structure, composition, small size of Fig. 7 (a) UV-vis absorption spectra of the photocatalytic degradation of TC, (b) percentage of degradation, (c) kinetic plot, and (d) stability and recycling plot for the degradation of TC using the synthesized g-C 3 N 4 , CuO, and g-C 3 N 4 /CuO composites under exposure of UV-light degradation of antibiotics TC and CIP, we have investigated the recyclability experiment and the obtained results were displayed in Fig. 7(d) and Fig. 8(d), respectively. It is indicating that the synthesized g-C 3 N 4 /CuO-3 composite was

Catalyst Stability & Reusability Test
To evaluate the stability and reusability of the synthesized g-C 3 N 4 /CuO-3 composite as a catalyst for the photocatalytic Fig. 8 (a) UV-vis absorption spectra of the photocatalytic degradation of CIP, (b) percentage of degradation, (c) kinetic plot, and (d) stability and recycling plot for the degradation of TC using the synthesized g-C 3 N 4 , CuO, and g-C 3 N 4 /CuO composites under exposure of UV-light Where, χ denotes the electro-negativity, E e (4.5 eV) and E g shows as the energy of free electron on standard hydrogen scale of the optical bandgap of g-C 3 N 4 /CuO composite. The VB (1.55 eV) and CB (-1.09 eV) values of the g-C3N4 is lying above than the VB (0.88 eV) and CB (-0.91 eV) values of CuO NPs, which facilitates the easy transfer of photogenerated charge carriers such as electrons and holes. From these data, the band positions in the band structure of the synthesized g-C 3 N 4 /CuO composite behaves like a typical type-II heterostructutred photocatalyst. According this type of catalyst, the transfer of electrons from CB of g-C 3 N 4 to the CB of CuO and holes from the VB of CuO to the VB of g-C 3 N 4 ,which may not have high reduction potential (only-0.91 eV for CB of CuO) and oxidation potential (1.55 eV for VB of g-C 3 N 4 ) for generating O 2 − radicals (-0.33 eV) and OH radicals (2.40 eV). Based on the staggered band structure of the synthesized composite for the photocatalytic degradation mechanism of antibiotics follows the S-scheme (step scheme) mechanism [47]. In this regard, the present synthesized g-C 3 N 4 /CuO composites following the S-scheme mechanism by the interactions of chemical bonds between the interfaces of the individual components in the composite [48,49]. In addition, the mechanism follows as the electrons from CB of CuO as well as electrons in the CB of g-C 3 N 4 ; and holes from the VB of g-C 3 N 4 as well as holes in the VB of CuO were relatively combined with each other due to the effect of internal electric field and resulting maximum redox potential are retained. The photogenerated electrons in the CB of g-C 3 N 4 could reduce O 2 to O 2 •and holes in the VB of CuO could oxidize OHto OH • , which are the major reactive species for the degradation of antibiotics in during the photocatalytic reactions [50].

Detection of Heavy Metals
In the fluorescent detection of metal ions, the synthesized g-C 3 N 4 /CuO composite was used as a fluorescent indicator for the selective detection of heavy metal ions in nanomolar range of concentrations. To evaluate the fluorescent selectivity sensing activity of the synthesized g-C 3 N 4 /CuO composites and CuO NPs for the detection of metal ions, the concentration of the selected metal ions solutions in the range of 10-100 micromolar level (Figs. 9 and 10). Prior to that, the investigation of fluorometric response of the metal ions to the effect of the concentration of g-C 3 N 4 /CuO degrades as much as in the similar amount of antibiotics of TC and CIP up to the 3rd cycle, and only 5% decrement was observed in the 5th cycle. The photocatalytic performance of the synthesized g-C 3 N 4 /CuO-3 composite has a certain extent decrease with increasing number of recycling times, which may be due to the loss of catalyst during the washing, drying, sedimentation and transferring process [45]. It shows that the g-C 3 N 4 /CuO-3 composite is stable with prominent photocatalytic degradation efficiency for the degradation of TC and CIP antibiotics over at least 5 cycles under UV-light irradiation. Moreover, these obtained results were indicated that the g-C 3 N 4 /CuO composite catalysts are objectively photocatalytically stable and retain the potential for practical applications.

Photocatalytic Degradation Mechanism
The photocatalytic degradation mechanism for the degradation of antibiotics of TC and CIP by using the synthesized g-C 3 N 4 /CuO composite can be explained by the following mechanism, which shown as the transfer mode of electrons and alignment of band positions of individual components in the g-C 3 N 4 /CuO-3 composite. The following equations were explain to calculate the values of conduction band (CB) and valence band (VB) edges for better understanding the mechanism, respectively [45,46]. ions was shown in Fig. 9(b) and Fig. 10(b). It can be seen from Fig. 9(b) and Fig. 10(b), the relative PL intensity was low, which is suggesting to the composite act as a sensor for better selectivity detection of Pb 2+ over the other metal ions under similar reaction conditions. Moreover, the drastic change as decrease in the PL intensity and its color of the synthesized g-C 3 N 4 /CuO composite upon the addition of Pb 2+ ion solution, it is confirming that the CuO particles were dispersed on the surface of g-C 3 N 4 nanosheets is play key role to interact with the Pb 2+ via coordinated covalent bonds, resulting to get prominent sensing activity.
In general for the practical applications, it is an indispensable to set up the fluorescent sensing response and composite and CuO NPs as a control experiment, 10 µL of an aqueous solution of Al 3+ , K + , Ba 2+ , Ca 2+ , Cu 2+ , Cd 2+ , Ag + , Ni 2+ , and Zn 2+ ions to the g-C 3 N 4 /CuO composite and CuO NPs, did not produce any significant fluorescence intensity changes. However, upon the addition of Pb 2+ ion solution was added to the g-C 3 N 4 /CuO composite and CuO NPs solution, the fluorescence intensity was immediately decreases in the emission spectra was observed. The in detail fluorescence intensity changes in the emission spectra for the selective detection of metal ions were displayed in Fig. 9(a) and Fig. 10(a). Similarly, the bar schematic diagram for the selective sensing probe as the synthesized g-C 3 N 4 /CuO composite and CuO NPs among various metal  curve was found to be 1.38 × 10 4 mol − 1 dm 3 . The linear relationship phenomenon of the Stern-Volmer plot suggests to the PL quenching can be used for the determination of quencher. These results of the fluorescence quenching study indicated that the behaviour of quenching of Pb 2+ on the fluorescence intensity of g-C 3 N 4 /CuO composite is found to be concentration dependent. It was observed that behaviour of quenching value upon addition of Pb 2+ was linear in the range of 10 to 100 µM with the limit of detection (LOD) is 0.184 nM. However, the g-C 3 N 4 /CuO composites exhibited great fast simultaneous detection of Pb 2+ in actual samples because of the remarkable surface area, interaction of Pb 2+ with composite surface and improbable adsorptive capability of g-C 3 N 4 and CuO in the g-C 3 N 4 /CuO composites.

Conclusion
In conclusions, we have successfully synthesized and characterized of g-C 3 N 4 nanosheets, CuO NPs, and g-C 3 N 4 /CuO composites via a simple microwave-assisted irradiation method for evaluation of their photocatalytic degradation of antibiotics and fluorometric sensing activities. From XRD and TEM measurements, the synthesized composites have monoclinic phase structure with 10-20 nm size of the particles were observed. The CuO particles were well dispersed and closely interacted with the surface of g-C 3 N 4 nanosheets was investigated by N 2 adsorption-desorption isotherms. In the photocatalytic activity, the degradation of antibiotics such as TC and CIP under exposure of UV light by using the synthesized g-C 3 N 4 , CuO, and g-C 3 N 4 /CuO composites was studied. The obtained results were exhibited as g-C 3 N 4 /CuO composites show enhanced photocatalytic activities for the complete degradation of TC and CIP in 60 min of reaction time, when compared to the g-C 3 N 4 and CuO NPs under similar reaction conditions. In addition, the composite of g-C 3 N 4 /CuO-3 has much higher activity (> 90%) due to the small size of the particles and high surface area and also exhibited without significant loss in the activity up to five cycles. The photocatalytic reaction mechanism is followed as pseudo-first order kinetics for all reactions. Moreover, the synthesized CuO NPs and g-C 3 N 4 /CuO composites were used as a prominent fluorescence sensing probe for the selective detection of Pb 2+ in nano-molar range of concentration with K sv is 1.38 × 10 4 mol − 1 dm 3 . It was observed as a linear relationship based on the change in intensity, the limit of detection was determined to be 0.184 nM. The obtained results were clearly suggesting to that the development of g-C 3 N 4 based metal oxide heterojunction materials is a prominent synthetic strategy to enhance the catalytic and sensing activities. minimum detection limit of the sensor system. Therefore, the establishment of the quantitative dynamic detection of Pb 2+ with different concentrations by using the synthesized g-C 3 N 4 /CuO composite. For this study, several repeated fluorometric sensing experiments were conducted with the varying dilution s of Pb 2+ ions in an aqueous medium in the range of concentration is 10 µM to 100 µM, as shown in Fig. 9(c). It can be seen in Fig. 9(c), the gradually PL intensity was decreases upon the increasing the concentration of Pb 2+ ion solution to the synthesized g-C 3 N 4 /CuO composite and large intensity decline in PL intensity with distinct colour changes was observed in the concentration of 100 µM of Pb 2+ ion solution. It is leads to the interaction of Pb 2+ is disintegration of the synthesized g-C 3 N 4 /CuO composite and resulting decrease in the concentration of CuO particles, then the limit of detection (LOD = 0.174 µM) is low. Moreover, it might be due to that the excellent dispersal of CuO particles on the surface of g-C 3 N 4 nanosheets with the addition of small amount of Pb 2+ ions is enough for etching of accessible particles [46]. From these observations, it can be clinched that more dilution of CuO particles on the surface of g-C 3 N 4 nanosheets, it may result in the detection of Pb 2+ to further lower limit.
The interaction phenomenon arises from the binding nature of Pb 2+ ions with the surface of g-C 3 N 4 nanosheets in the synthesized g-C 3 N 4 /CuO composite as an acceptor and changing the surface state of the composite, which can be clearly described by the Stern-Volmer equation [46]. Where in the F 0 and F are the fluorescence intensity of the synthesized g-C 3 N 4 /CuO composite in the absence and presence of quencher [Q] as Pb 2+ , K sv is the Stern-Volmer quenching constant, Q is the quencher. The quenching analysis of the synthesized g-C 3 N 4 /CuO composite by using the concentration from 10 µM to 100 µM of Pb 2+ was studied and the observed results were displayed in Fig. 9(d). It can be seen from Fig. 9(d), the obtained fluorescence quenching results were follows Stern-Volmer equation and a good linear correlation factor (R 2 = 0.99) was observed in the Pb 2+ concentration range from 10 µM to 100 µM. The modified Stern-Volmer equation for the same can be represented as: From the above equation, the Stern-Volmer quenching constant Ksv is calculated from the slope of the linear fitting