Carbon Dots from Natural‐Product: Applications as Adsorbent and Sensing of Fe3+ Ions

Freshwater contamination is a significant concern due to the increasing pollution by industrial activities. Dyes have a wide range of uses and are introduced at different stages of manufacture, raising the risk of unwanted human and environmental contact. Consequently, the demand for an effective method for removing dyes has become more important than before. In this context, carbon dots have been synthesized by the green synthesis method from coriander leaves (C-CDs) and used as a prospective adsorbent to remove (MB) methylene blue dye from aqueous solution. The as-synthesized C-CDs are characterized by HR-TEM, XRD, XPS, FTIR, Zeta potential, UV–visible, and photoluminescence (PL). Effects of different controlling parameters such as adsorbent dosage, pH, contact time, and initial MB dye concentration were investigated. The highest adsorption efficiency (82.6%) and maximum adsorption capacity (96.05 mg/g) of MB were obtained at optimum conditions (303 K). The adsorption isotherm data could be fitted well by Freundlich model, and the experimental data fitted to the Pseudo-Second-Order kinetic model. It is worth noting that C-CDs exhibited excellent sensitivity and high fluorescence quenching effect on Fe3+ ions.


Introduction
In recent years, the heightened awareness has emerged that by 2050, many countries will face significant problems related to the lack of freshwater [1].The latest projections by the (UNESCO) United Nations Educational, Scientific and Cultural Organization suggest that the world population will reach 9.3 billion in 2050, and two-thirds of people will live under water stress [2].The reuse and recycling of wastewater after contamination treatment is a serious issue in many areas to ensure water security at the national level.Contamination of water sources is increasing by many factors, such as climate, precipitation, population growth and related activities [3].Wastewater contains a variety of pollutants, and some wastewater often includes several classes of contaminants, such as anions, heavy metals, dyes, and different types of organic compounds [4].Large quantities of dyes have been used as raw materials and produced as end products in textiles, clothing, food, etc.It has been estimated that the textile industries are consumed more than 7 × 10 5 tons of dyes, which makes them the top three pollutants [5][6][7][8].The ingestion of water polluted with dyes poses a potential risk or threat to all forms of life and has negative effects on human health.Many different methods are available for removing dyes from wastewater, including ion exchange, adsorption, filtration, and chemical precipitation [9,10].Among these methods, adsorption has significant advantages over other processes because of the flexibility and simplicity of design, cost savings, and ease of operation with adaptable designs [11].Over recent years, a wide variety of adsorbent materials have been recommended for efficient dye removal, including graphene, activated carbon, metal oxides/hydroxides, polymers, and zeolites, but It should be noted these adsorbents have several problematic issues.In many cases, the adsorption capacity is quite low, with a long time taken to reach equilibrium [12,13].Accordingly, the search for a high-capacity adsorbent with low toxicity and functional ability continues under the key condition of economic feasibility.Recently, carbon dots (CDs) have emerged as efficient absorbents for the removal of dyes from wastewater [14].CDs can be generally synthesized using any carbon sources, such as chemicals, graphene, plants, and food wastes.Among all types of carbon sources, biomass and its derivatives, such as bagasse, fruit peel, tea residue, and leaves, received considerable attention due to renewability, low cost, and availability [15][16][17][18][19].
Taking into consideration the above, we report a green and novel synthesis of CDs derived from coriander leaves via a simple hydrothermal method.This approach is less time-consuming and uses water as a solvent without any additional surface passivating agent and have a high (QY) quantum yield of 12.0%.Coriander, known as Chinese parsley, is edible and rich in proteins and carbohydrates naturally, which is abundant in oxygen and carbon elements.The use of leaves as potential sources not only meets the urgent requirement for large-scale production of CDs, but also improves the development of sustainable applications [20][21][22].
Fe 3+ ion is one of the important trace elements that play predominant roles in many biochemical pathways.It acts as a key structural element for various critical biological functions, such as electron transfer, cell respiration, oxygen transport, DNA and hemoglobin synthesis because of its unique redox and coordination properties.It is well known that the large amount of Fe 3+ in human body may cause severe diseases like Alzheimer's disease, liver and kidney damage, and Parkinson's disease [23][24][25].To control all Fe 3+ ion biological functions, Fe 3+ ion concentration should be accurately monitored in biological and environmental samples.Currently, many spectroscopic techniques have been adopted for the analysis of Fe 3+ ions in various samples including atomic absorption spectrometry, mass spectrometric, fluorescence, UV-Vis and fluorescence [26][27][28].In view of the above, the selective and sensitive detection of Fe 3+ is extremely important.The synthesized C-CDs have shown good sensitivity toward Fe 3+ over other metal ions.In this study, we have presented C-CDs that possess the following significant advantages; a) Eco-friendly adsorbent for removal of MB from wastewater, b) Synthesized through the hydrothermal method, c) Excellent sensitivity for Fe 3+ ions.

Chemicals and Solutions
Freshly harvested coriander leaves were bought from the local market of Aligarh and thoroughly washed before use.Methylene blue dye was purchased from Merck chemical Co. Figure 1

Synthesis of C-CDs
Briefly, 5 g of the fresh coriander leaves were finely crushed using a mortar and pestle and added to 50 mL of DDW by stirring at room temperature for 30 min.The solution was transferred to hydrothermal process for 5 h at 210 °C.The mixture was allowed to cool at room temperature, and the large black insoluble portion was filtered through a 0.22 μm filter membrane.The brownish-yellow C-CDs solution obtained then was kept at 4 °C for further studies and characterizations [29].

Characterization
The optical properties of the C-CDs were measured by using UV-Vis spectroscopy (Perkin Elmer LAMBDA-45) and PL spectra (HITACHI-F-2500 FL).The main functional groups of C-CDs were identified by Fourier transform spectroscopy (Perkin Elmer IR spectrometer) before and after adsorption.X-ray diffraction or XRD (Rigaku TTRAX III) was used to determine the crystallographic structure of the adsorbent.X-ray photoelectron spectroscopy (XPS) (PHI 5000 Versa Probe III) is widely applied to determine the elemental composition.The surface charge of the C-CDs was determined by zeta potential (ZEECOM ZC-3000).

Batch Equilibrium Studies
The impact of adsorbent dose (0.01-0.05 g), pH (2-10), temperature (303-343 K), and initial MB concentration (10-100 mg/L) for the removal of a basic dye (MB) onto C-CDs were studied.In each experiment, we added a desired amount of C-CDs to 50 mL of dye solution in a 100 mL conical flask under stirring to achieve equilibrium.At equilibrium, the concentration of MB was measured at wavelength of 664 nm using UV-vis Spectrophotometer.The adsorption efficiency (q%) and capacity (q e ) of the adsorbent can be calculated by the following equations: where V and M are volume of solution (L) and mass of adsorbent used (g), respectively.C e is the equilibrium concentration (mg/L), and C i is the initial concentration of adsorbate in the solution (mg/L).

Effect of Temperature (Thermodynamic and Kinetics Isotherms)
The impact of temperature on uptake of MB dye onto the surface of C-CDs was studied through a series of experiments at 303 K, 313 K, 323 K, and 343 K. Thermodynamic calculations were performed through the following equations: where K d is distribution coefficient, ΔG° is Gibb's free energy (kJ/mol), ΔH° and ΔS° are obtained from the slope and intercept of Van't Hoff plot of ln K d versus 1/T, respectively.Values of K L can be determined from the relation ln q e /C e .The experimental data were analyzed with the pseudofirst-order (PFO) and (PSO) pseudo-second-order models to improve understanding of kinetics of adsorption process.
The PFO model assumes that molecules of dye filled sites have linear relationship with rate of adsorption.The PFO equation can be expressed as follows: where k 1 is rate constant of PFO (1/h), q e is the equilibrium adsorption capacity (mg/g) and q t is quantity of adsorbateadsorbed at time (mg/g).
The PSO model is the most suitable for explaining the adsorption kinetics.The PSO equation is generally expressed as follows: The intercept and slope of linear plot of t/q t vs t is used to calculate the values of q e and k 2 , where k 2 is rate constant of PSO.

Adsorption Isotherm Studies
Batch adsorption study was performed in 50 mL beaker having 0.04 g C-CDs with different initial dye concentrations (10,20,40,60, 80 and 100 mg/L).In this study, the Langmuir, Freundlich, and Temkin models were fitted using the following Eqs: (3) where K L and K f are the Langmuir and Freundlich constant, respectively, q m is the maximum adsorption capacity, 1 n f is adsorption intensity.B = (R T /b T ) related to the heat of adsorption (J/mol), where b T and K T are Temkin isotherm constant and equilibrium binding constant (L/mol), respectively.

Calculation of Quantum Yield
Quantum Yield (QY) of C-CDs can be measured by comparing (QY) quinine sulfate as a reference.C-CDs were dissolved in DDW, while quinine sulfate was dissolved in in 0.5 M H 2 SO 4 (QY = 0.54, η = 1.33).The QY of CQDs was obtained based on Eq: where QY represents the Quantum yield, A represents the optical density at the excitation wavelength of 350 nm, η (η C-CDs = 1.33 and η R = 1.76) represents the refractive index of solvent, and I represents the integrated fluorescence intensity.The subscript "C-CDs" and "R" refers to C-CDs sample and reference substance (Quinine sulfate), respectively.

Sensing of Fe 3+ Ion
Fluorescent quenching of Fe 3+ ions was conducted in DDW at room temperature.For that, C-CDs solution (1 ml in 300 µM) reacted with 300 µM concentration of various metal cations, include Mg 2+ , Pb 2+ , Na + , Ca 2+ , Mn 2+ , Cu 2+ , Zn 2+ , Ni 2+ , Co 2+ , K + , Cd 2+ , Cr 2+ , Fe 3+ and Sr 2+ .The resulting solution was lightly shaken and incubated at room temperature for 20 min.The quenching efficiency has been determined using a modified Stern-Volmer equation: The LOD (limit of detection) was calculated by the formula: where Ksv is the Stern-Volmer constant, [Q] is Fe 3+ ion concentration, F and F o are the fluorescence intensities in the presence and absence of quencher, respectively.S and σ are the slope of curve and the standard deviation of the response, respectively.

Detection of Fe 3+ in Actual Water Sample
To detect Fe 3+ ions with C-CDs in the actual water sample, the concentration of Fe 3+ was studied by river water samples nearby our university campus.The water sample was first filtered through 0.22 µM membrane and centrifuged at 8000 rpm for 20 min for the removal of any suspended particles.The concentration of Fe 3+ in the actual water sample was calculated with the proposed method and then the fluorescence intensity of the solution was measured after adding different concentrations of Fe 3+ .The recovery rate of the samples was obtained from equation: where C o is the concentration of Fe 3+ added to the actual water sample, C 1 and C 2 are the concentration of Fe 3+ in the actual water sample before and after the addition of standard Fe 3+ .

Characterization of C-CDs
To investigate the optical properties of the prepared C-CDs, UV-Vis and photoluminescence (PL) spectroscopy were recorded.The UV-vis absorbance curve shows two bands at 250 nm and 323 nm.The absorption peaks at 250 nm and 325 nm, which could be assigned to (π-π*) transitions of C=C bonds and (n-π*) transitions of C=O bonds, respectively (Fig. 2a).The PL spectra of C-CDs under various excitation wavelengths (Fig. 2b).The PL behaviour of C-CDs was dependent on the excitation wavelength when the excitation wavelength was changed from 300 to 390 nm in 10 nm increments.The emission peak toward a longer wavelength is due to the photoinduced electrons and holes present in C-CDs at different surfaces of energy traps.The maximum emission was noted at 460 nm (cyan colour), with an excitation wavelength of 370 nm.A smaller particle will have a visible PL effect at a lower wavelength, while a larger particle will exhibit PL at a higher wavelength [30].Figure 2c presents the decay curve of the obtained C-CDs with excitation and emission wavelengths in 370 and 420 nm respectively.The decay curve can be fitted by a single exponential function: The fluorescence lifetime of C-CDs was 6.45, and such a short lifetime shows that the luminescence mechanism of C-CDs is the radiative recombination nature.
FTIR spectra has been used to identify the functional groups of C-CDs before and after adsorption, as shown in Fig. 3a.The peaks around 3440 cm −1 (attributed to the O-H stretching vibrations), 2918 cm −1 (correspond to C-H stretching vibrations), 1610 cm −1 (corresponded to C=O stretching), 1321 cm −1 (ascribed to C-O stretching vibrations of carboxylic ester group) and the peak observed around 1017 cm −1 (attributed to C=O vibrations).Figure 3b displays the FTIR spectrum of C-CDs after the adsorption of MB at the surface of C-CDs.These minor peaks changes after MB dye adsorption are due to their involvement in the adsorption process through Van der Waals forces (weak electrostatic interaction) [31].X-ray diffraction (XRD) pattern depicted a broad peak at 2θ = 21.5° and a weak peak ( 15) at 2θ = 43.3°that are assigned to (002) and ( 101) diffraction patterns of graphitic carbon respectively, (Fig. 3c).The former peak corresponds to interlayer spacing of 3.77 Å which is slightly more than the spacing between (002) planes in bulk graphite (3.44 Å).The XPS results showed that these C-CDs are composed of atomic C (282.8 eV) and O (529.4 eV), as can be seen in Fig. 4a.The measured spectrum of C1s consisted of three noticeable peaks: 282.9 eV, 284.1 eV and 285.7 eV, which attributed to the C-H, C=C sp 2 and sp 3 C (C-OH), respectively (Fig. 4b).In addition, the deconvolution spectrum of O1s showed three binding energy peaks: 529.1 eV, 529.7 eV and 531.9, which may be ascribed to the C-O, C-O-C and O-C=O/C=O, respectively (Fig. 4c).
HRTEM is used to study microporous materials at atomic scale.Figure 5a shows the presence of spherical C-CDs ranging below 10 nm.The distribution curve displayed that nearly 15% C-CDs had diameter in the range of 4.5 nm, while 7.5% of the C-CDs had diameter in the range of 5.5 nm, as shown in Fig. 5b.The zeta potential measurement of C-CDs exhibits that the C-CDs were negatively

Effect of pH
The pH is a significant parameter determining the adsorption efficiency since it can control the adsorbent surface charge and the pollutants' ionization.Small amounts of dilute HCl or NaOH were applied to adjust the pH.The adsorption efficiency increases with increasing initial pH in the ranges of 2-10 (Fig. 6a).The low adsorption efficiency observed at pH 2 is due to less number of negatively charged adsorbent sites and extra H + sites presented at the C-CDs surface.The presence of the excess H + ions competing for adsorption sites with the cations groups on the dye [32,33].The maximum adsorption of the MB dye was observed at pH 10.Based on the results, pH = 10 was chosen as the desired pH for further studies.

Effect of Adsorbent Dose and Time
In order to study the effect of C-CDs dose (g) on the removal of MB, the experimentation was carried out with an initial concentration of 100 mg/L and pH 10 under the contact time between 30 to 120 min, while the amount of adsorbent added was varied.Figure 6b illustrates the plot between the percent adsorption efficiency against the dose of adsorbent at a different time.It was observed that the adsorption efficiency % of MB was increased with increasing adsorbent dose up to 0.04 g, then increased slowly with a further increase of adsorbent dose and afterward remained unchanged.The increase in adsorption efficiency was because of the increase in the available absorption surface sites for C-CDs.Thus, to get a better percent of adsorption efficiency of MB and to not use more quantity of adsorbent, 0.04 g was selected as the best mass of the adsorbent in the following studies.

Effect of Temperature
The effect of temperature on uptake of MB onto C-CDs was studied in the temperature range of 303 to 343 K at varying initial dye concentrations.As shown in Fig. 7a, an increase in temperature (303 K to 343 K) leads to a gradual reduction in the MB adsorption efficiency at different initial concentrations.Thermodynamic behaviors are applied to understand better the effect of temperature on the removal of dyes on adsorbents.Table 1 displays the thermodynamic parameters for the adsorption of MB (initial dye concentration = 100 mg/L) onto C-CDs.The negative value of ΔH o showed that the process is exothermic in nature (Fig. 7b).∆G o values obtained are negative, suggesting that the adsorption process is spontaneous when the temperature changes from 303 K-343 K without external energy.The ΔSׄ ° value achieved is − 0.0063 (J/mol K).The negative  sign revealed an decreasing randomness at the solid/liquid interface during the adsorption process [34].

Effects of Initial MB Concentration and Isotherm Studies
The effect of initial MB concentration is closely related to the sites present on the adsorbent surface.The adsorption capacity of C-CDs was shown to increase with increasing initial MB concentration from 10 to 100 (mg/L) due to the adsorption sites on the surface of C-CDs becoming saturated (Fig. 8) [35].The adsorption capacity of the C-CDs was expressively dependent on the initial dye concentrations in the solution.A comparison of the maximum adsorption capacities of MB by various adsorbents is shown in Table 2.
The adsorption isotherms describe how the molecules of the adsorbate are dispersed among the solid and liquid phases when the adsorption process reaches an equilibrium state.In this study, we employed Langmuir, Freundlich, and Temkin isotherm model to analyze isotherm data (Fig. 9a and b).The Langmuir isotherm is a simple type of adsorption equilibrium model for single-layer adsorption on a homogeneous surface when no interaction occurs between adjacent adsorbed molecules [36].Freundlich isotherm is used to describe reversible and non-ideal adsorption systems on heterogeneous surfaces [37].Temkin model is based on the assumption that the heat of adsorption decreases linearly because of interactions between adsorbent and adsorbate [38].A summary of the isotherm parameters calculated for the Langmuir,   Freundlich, and Temkin models for each temperature (303-343 K) is given in Table 3.Based on the results, the value of R 2 according to the Freundlich model is higher than the Langmuir model.The slope range for the Freundlich isotherm model was determined between 0-1, showing the adsorption intensity and surface heterogeneity.If n = 1, the adsorption is linear, n < 1 a chemical process, or n > 1 a physical process [39].The high K F value is favorable at 303 K, which gives the adsorption of aggregated molecules.Therefore, experimental data for the adsorption of MB using C-CDs with respect to R 2 followed the order; Freundlich > Temkin > Langmuir.

Kinetics of Adsorption
The experimental data were calculated based on the (PFO) Pseudo-First-Order and (PSO) Pseudo-Second-Order models to understand the kinetics of the adsorption process.Table 4 displays the parameters of the PFO and PSO models.The PSO model provided a better explanation of adsorption for MB from adsorbent.In this study, the R 2 value of PSO for every initial dye concentration was observed to be higher than 0.97.At PSO, dye concentration increased with increasing the value of qe might be due to the high competition for vacant sites, leading to higher adsorption rates.A plot between (t/qt) versus time was drawn to express the PSO kinetic model (Fig. 10).

Sensing of Fe 3+ Ions with C-CDs
The fluorescence quenching effect of C-CDs upon the addition of different metal ions was investigated.It could be concluded from Fig. 11a that there is a decrease in fluorescence intensity after the addition of C-CDs and Fe 3+ .At the same time, other metal ions show a slight change in fluorescence intensity compared to free C-CDs.According to the results, the synthesis of C-CDs was shown good selectivity towards Fe 3+ over other metal ions and could be used as an efficient fluorescence probe for Fe 3+ ions.It can be seen that no obvious interference was observed in detection of Fe 3+ in the occupancy of other metal ions.Figure 11b shows the possible mechanism of fluorescence quenching and strong interaction between the Fe 3+ and the surface groups of C-CDs, which transfer the photoelectron from C-CDs to the metal ions.The Fe 3+ ions absorbed on the surface of C-CDs and coordinated with these hydroxyl groups was due to the presence of phenolic hydroxyl group on the edges of C-CDs.This coordination interaction, electrons are transferred from the excited state of C-CDs to the empty d orbital of Fe 3+ and the formation of non-radiative electron/ hole pair [45,46].The sensitive and selective response of C-CDs to Fe 3+ ions at different concentrations is depicted in .The value of R 2 was 0.991 for a concentration ranging from 0.01 to 100 µM.The plot didn't fit the linear equation over the entire concentration range of 0.01 to 200 µM, indicating that both dynamic and static quenching processes occur in this sensor system.The LOD value of this proposed sensor is about 0.16 µM.

Determination of Fe 3+ in River Water Samples
Based on the high sensitivity and good selectivity, we calculated its potential application in determining Fe 3+ in river water samples.Table 5 shows the recovery results of all the samples were between 81.09% to 96.98%, and the value of relative standard deviation (RSDs) was very low (0.98% to 1.97%), which indicated that the proposed method had achieved high accuracy.

Conclusion
The novel CDs were successfully synthesized from coriander leaves through a facile hydrothermal method as a plausible adsorbent for the removal of a cationic dye (MB) from wastewater.The optical property, crystalline structure, surface morphology, and adsorption behavior of C-CDs are all analyzed.The amount of MB adsorbed was studied by varying initial MB concentration, C-CDs dose, pH, and contact time.It is evident that the percentage adsorption increased with an increase in initial concentration of dye, and pH values also increased with an increase in MB adsorption percentages.The adsorption isotherms and kinetic models were fitted well with Freundlich and PSO model.Moreover, thermodynamic parameters indicated that the adsorption process is exothermic in nature, spontaneous and feasible.It was validated that the obtained C-CDs had been utilized as an environmental-friendly adsorbent for removing MB in wastewater.From this study, C-CDs could effectively detect Fe 3+ with the highest selectivity by fluorescence quenching at a limit of detection (LOD) of 0.16 µM.The PL quenching of C-CDs was linearly correlated with the wide range of Fe 3+ concentration, ranging from 0.01 to 100 µM (R 2 = 0.991).
Eventually, the low-toxicity C-CDs demonstrated high selectivity sensitivity towards Fe 3+ over other metal ions.

Fig. 2 Fig. 3
Fig. 2 UV-visible absorption spectrum (a), PL spectra (b) of C-CDs and c decay curve of the C-CDs under 370 nm excitation

Fig. 4 Fig. 5 aFig. 6 a
Fig. 4 XPS spectra of a C-CDs and high resolution XPS spectra of b C1s of C-CDs and c O1s of C-CDs

Fig. 7 a
Fig. 7 a Effect of temperature on the MB adsorption by C-CDs, b Van't Hoff plot for MB adsorption

Fig. 8
Fig. 8 Effect of initial MB concentration on adsorption capacity by C-CDs at different temperatures

Fig. 9
Fig. 9 Adsorption isotherm models a Freundlich model and b Temkin model

Fig. 11 a
Fig. 10 PSO kinetic plots for the MB adsorption by C-CDs (dose = 0.04 g, temperature = 303 K)

Table 1
Thermodynamic parameters for the MB adsorption on C-CDs

Table 2
Comparison of adsorption capacities for removal of MB onto different adsorbents

Table 5
Determination of Fe 3+ in the environmental water sample