Synthesis of Carbon Dots from butterfly pea (Cliptoria ternatea) as detection of the heavy metal ion Cu 2+

The synthesis of carbon dots nanomaterials based on butterfly pea (Cliptoria ternatea) with urea doped (N-CDs) has been successfully carried out using the hydrothermal method. The N-CDs material shows a dark brown solution which indicates that the carbon dots have been successfully synthesized. In this study, we analyzed the influence of optical properties before and after adding Cu2+ ions to the sensitivity of N-CD. The characterization of N-CDs was analyzed using a UV-Vis Spectrophotometer and Photoluminescence (PL) Spectroscopy, as well as using Fourier Transform Infra-Red. The research results show that the absorption spectrum range is from 200 to 800 nm, with an absorption peak at 206.58 nm and the band gap energy obtained is 4.95 eV. The CD solution produces good fluorescence when observed under ultraviolet light (395 nm) and emits a turquoise green color. Based on the IR spectrum, there is an increase in transmittance in the carbon dots spectrum after the addition of urea and at the wave number 1404 cm−1 there is a C-N group which indicates that the process of doping nitrogen atoms on the carbon dots has been successful. The sensitivity of N-CD when sensing Cu2+ ions with a minimum Limit of Detection (LOD) is as low as 183.33 nm. The N-CD can be applied as a biosensor friendly and environmentally method.


Introduction
Carbon dots (CD) are zero-dimensional carbon nanoparticles that have a size smaller than 10 nm and have fluorescent capabilities (Barus et al., 2023;Raveendran et al., 2019).Since its discovery in 2004, this material has attracted the attention of many researchers because of its excellent characteristics such as environmental friendliness, easy synthesis, biocompatibility, good photostability, and easy soluble in water (Pudza et al., 2020; Emami et al., 2021 ).In general, CD materials can be synthesized from carbonrich sources such as biomass sources.Butterfly pea flower (Clitoria ternatea) is the biomass of a blue flower species that grows widely in Asia and is widely used as medicine.Butterfly pea flowers contain compounds that are rich in carbon so these flowers can potentially be used as CD material (Shinde et al., 2023;Tak et al., 2020).Currently, carbon dots synthesis can be carried out using various methods such as chemical oxidation, electrochemistry, laser ablation, ultrasonification and hydrothermal (Yoo et al., 2019).However, among the methods previously mentioned, the hydrothermal method is an effective method for synthesizing CDs because it is environmentally friendly, does not use dangerous materials, is cheap, and saves energy while also being able to produce CDs with surfaces that have hydrophilic groups such as carboxyl and hydroxyl groups., and amines which have the potential to detect the presence of heavy metals (Wang et al., 2016;Sun et al., 2016).The current application of CDs is very massive, used as sensors to detect the presence of heavy metal ions such as Cu +2.Cu +2 metal ions are generally produced from industrial and agricultural activities which can have a negative impact on human health if they accumulate in the human body, because they can cause diseases such as liver, kidney disorders and mental disorders such as Alzheimer's.(Bandi et al., 2020;Sun et al., 2017;Rong et al., 2017) so that according to the regulations of the Minister of Health of the Republic of Indonesia the threshold value for Cu +2 metal ions is 2 ppm (Alkian et al., 2022).Furthermore, the CD surface can be functionalized by adding a certain number of doping heteroatoms such as nitrogen, boron, sulfur and others to substitute carbon atoms in the sp2/sp3 network through covalent bonds.Functionalization of CD materials by adding doping such as nitrogen can be easily done because the atomic size is almost similar to carbon atoms and the electronegativity of the nitrogen atom.The addition of nitrogen atomic dopants to the surface of the CD material can be done by adding a number of materials containing nitrogen.Urea is a compound with the chemical formula CO(NH2)2 which is generally used as fertilizer to provide sufficient nitrogen intake for plants.Urea can be used as a nitrogen source in the synthesis of N-CDs because the material is environmentally friendly, easy to obtain, non-toxic, and the price is quite cheap (Barus et., al 2023).So from this research, N-CDs were synthesized based on butterfly pea flowers and urea as sensors to detect the presence of Cu+2 metal ions.

Synthesis of butterfly pea flower CDs
The butterfly pea flowers that have been obtained are first washed using clean water and dried in the oven for 12 hours at a temperature of 60 o C. Next, the dried butterfly pea flowers are blended until smooth and filtered using a 200 mesh sieve to obtain fine powder of butterfly pea leaves.The fine powder of telang flower leaves was weighed as much as 1 gram and mixed into 50 mL of distilled water and ultrasonified for 20 minutes until the solution changed color to dark purple.The butterfly pea leaf powder solution was then put into an autoclave and heated in the oven for 6 hours and then cooled to room temperature.Next, the butterfly pea flower solution obtained was then centrifuged at a speed of 11,000 rpm for 25 minutes to separate the precipitate and supernatant.The butterfly pea flower supernatant that was obtained was then filtered through a PES membrane.After that, the butterfly pea leaf filtrate was then stored at 4 o C for testing.

Cu 2+ ion detection test
Detection of Cu ions was carried out based on previous research, namely Mu et al., 2020, where 1990 µL of Cu solution with a concentration of 0-5 µM was added into the cuvette, then 10 µL of butterfly pea flower N-CDs were added and stirred until the solution was homogeneous, then tested with a Photoluminescence Spectrophotometer.

Results and Discussion
The UV-Vis spectrophotometer was used to determine the absorbance of butterfly pea flower CDs and butterfly pea flower N-CDs at the same concentration and is presented in Figure 3.1 below.Figure 3

Energy Gap Bands
The figure below shows the band gap energy of CDs and N-CDs samples from butterfly pea flowers using the Tauc plot method based on the absorption spectrum from test results using a UV-Vis spectrometer (Chandra et al., 2013).The equation used to determine the band gap energy is formulated as Where α is the absorption coefficient, hυ is the photon energy, B is the proportionality constant and Eg is the band gap energy.The band gap energy can be determined by plotting the curve (αhυ) 2 against hυ.The results obtained are that the band gap energy for N-CDs in this study is 4.95 eV and 5.14 eV for CDs.
From the results of the band gap test, it was found that the band gap energy of the carbon dots after the addition of urea doping decreased in value when compared to the band gap energy of the carbon dots sample before the addition of urea.Similar results were also obtained in research conducted by Barus et al., 2023 who succeeded in synthesizing soybean-based carbon dots with the addition of urea doping, where the band gap value for the carbon dots sample that had been doped with urea was lower than the band gap value for the carbon dots.before adding urea.This indicates that the nitrogen atom doping process in the telang flower-based carbon dots matrix has been successfully carried out.To study the emission reduction properties of carbon dots after being added to the Cu 2+ solution, the Stern-Vølmer plot method was used to observe the relationship between Cu 2+ concentration and the intensity ratio of N-CDs at various Cu 2+ ion concentrations.The Stern-Vølmer plot method is quite accurate so that many related studies use this method.The peak intensities of the photoluminescence spectrum are the values taken for F and Fo in all Stern-Vølmer plots (Mintz et al., 2018).As can be seen in Figure 3.6 there is good linearity over a wide range of Cu 2+ concentrations (1-5 μM) in water.Next, to determine the linear regression value of the Cu2+ ion, the Stern Volmer equation is used with the equation: Where Ksv is the Stern -Volmer constant, C is the measured concentration of Cu 2+ ions, Fo and F respectively express the intensity of N-CDs at 490 nm before and after the addition of varying concentrations of Cu 2+ .The regression value obtained in this study was Fo/F = 1 + 0.0934[Cu 2+ ] with a correlation coefficient (R 2 ) of 0.9987.

Figure3.6 Concentration Cu 2+
To determine the limit of detection concentration for ion Cu 2+ by the N-CDs sample, it is obtained based on the following equation Where Sy represents the standard deviation of the blank signal and s is the slope value obtained on the linear graph of Fo/F against Cu 2+ concentration.From the research that has been carried out, the LoD value of the N-CDs sample of butterfly pea flower was obtained at 183.33 nM.Based on the results of previous research conducted by Bhatt et al., 2020, carbon dots based on the synthetic material ascorbic acid have a LoD value of 200 nM.These results indicate that the butterfly pea flower N-CDs sample has good sensitivity for detecting the presence of Cu 2+ ions.

FT-IR
FT-IR characterization was carried out to observe the functional groups formed in the butterfly pea flower carbon dots samples without the addition of urea (CDs) and carbon dots with the addition of urea (NCDs).
The sample was scanned with FT-IR with a spectrum range of 4000 cm -1 -600 cm -1 .Based on the IR spectrum, in samples of butterfly pea flower CDs, absorption peaks were observed at wave numbers 3206 cm -1 , 1624 cm -1 , and 1002 cm -  The IR spectrum appears that the transmittance of the IR spectrum of the N-CDs sample is higher than that of the pure CDs sample, this indicates that the process of doping nitrogen atoms into the CDs matrix has been successfully carried out (Arul et al., 2019).The presence of the C-N group at wave number 1404 cm -1 also strengthens the evidence that nitrogen atom doping by adding urea to the CDs matrix has been successful.

Conclusion
In this research, carbon dots from telang flowers were synthesized with urea doping to produce N-CDs through a hydrothermal process.The N-CDs solution shows a dark brown solution under visible light and emits a turquoise green color when observed under UV light (395 nm) and has a higher absorption peak around 200 nm indicating that N doping has been successfully incorporated into the CD.The absorption spectrum range is observed from 200 to 800 nm and the absorption peak is observed at 206.58 nm.The band gap energy obtained is 4.95 eV, which indicates that N-CD is a semiconductor.Based on this study, the photoluminescence intensity of CDs decreases when the Cu 2+ concentration increases at 1ߤm to 5 ߤm,

Figure 3 . 1
Figure 3.1 UV-Vis Spectrum of N-CDs and CDs of butterfly pea flowers

Figure 3 .
Figure 3.2 N-CDs butterfly pea flowers before UV irradiation and after UV irradiation

Figure 3 . 3 Figure 3 . 4 3 . 2 .Figure 3 . 5
Figure 3.3 Band gap energy curve of butterfly pea flower CDs the N-CDs sample, more absorption peaks were observed compared to the pure butterfly pea flower CDs sample.Based on the IR spectrum, in the N-CDs sample, absorption peaks were observed at wave numbers 3206 cm -1 ,1624 cm -1 , 1536 cm -1 , 1404 cm -1 , and 1002 cm -1 , which respectively represent the -OH group, water bonds, C=C groups, C-N groups, and C-O groups (Kossy et al., 2021).