A fluorescent CQDs for the Cu2+ detection in water

In order to obtain high efficiency and high precision fluorescent detection agent of trace metal ions in solution, the CQDs were optimized with citric acid, which acted as a carbon source, as well as urea, which acted as a nitrogen source. It was a carbon nanoparticle with a particle size of around 3.16 nm, featuring numerous functional groups attached to the surface. These attributes not only facilitated its dispersion within water but also conferred increased stability, thereby resulting in a more consistent fluorescence intensity. It was a fluorescent sensor that could successfully detect copper ions (Cu2+) in the range of 3-5 mM. First, we set the excitation wavelength to 380nm. Then, the quenched CQDs fluorescence intensity would be found, which was defined as the dynamic quenching of energy transfer caused by the binding of amine groups and Cu2+. CQDs were actually applied to detect Cu2+ in the concentration range 2-6 mM, with R2=0.9565. Thus, the CQDs had high practical application value for metal ion detection and alcohol sensing.


Introduction
Copper iron (Cu 2+ ) was a heavy metal element, and copper-containing industrial wastewater was often discharged during mining and manufacturing due to the widespread utilization of copper products in fields such as the electronic industry and power transmission.At low concentrations, Cu 2+ acted as an essential human micronutrient.However, at high concentrations, Cu 2+ was not only one of the primary environmental pollutants but also had various harmful effects on human health.Therefore, the detection of Cu 2+ was critical in the industrial environment [1] .
Carbon quantum dots (the CQDs) were carbon particles with a size under 10 nm, composed of sp 2 /sp 3 hybrid carbon atoms and attached states.They are incredibly intriguing entities.In 2004, during the experiment, the Scrivens team accidentally discovered an unknown carbon nanoparticle with fluorescent properties by purifying single-walled carbon nanotubes.In 2006, Sun's group prepared carbon nanoparticles with fluorescent properties by using laser ablation of carbon targets, naming them carbon quantum dots for the first time.Due to their excellent hydrophile, high conductivity, low toxicity, and fluorescent properties, the magic CQDs have been developed for various fields, including medicine, chemistry, food, and the environment.In this paper, because of their high and stable fluorescent characteristics, the CQDs, as novel metal ion sensors, were mainly used to detect Cu2+ in water, which provided a simple method for detection [2] .
There are many raw materials for making the different types of CQDs.And the difference in raw materials will also lead to its production methods are not the same.According to the starting materials of the CQDs, whether large matter or small molecules, the synthesis methods could be divided into two major categories.The "top-down method" referred to the method of obtaining the CQDs by destroying large-sized carbon sources.For example, there was a discharge method, laser ablation method, and chemical oxidation method.The "bottom-up method" refers to making the CQDs from organic small molecules, biomaterials, and artificial complexes by hydrothermal/solvent heat, ultrasonic shock, and microwave assistance.By contrast, the latter production method was cheaper, simpler, and more widely used.The most prevalent production method was the one-step hydrothermal synthesis method because of its convenient operation and further modification.Most importantly, hydrothermal synthesis made our production process extremely simple and made the optimization of CQDs more efficient.The carbon source gradually changed from the initial carbon nanotubes, graphite, carbon black, activated carbon, candle ash, natural gas, and other carbon-containing organic chemical reagents to grass, fruit juice, fruit peel, soybean milk, and other green natural products.Non-natural substances as raw materials were more conducive to generating and applying the CQDs, because there was no complex preprocessing and purification, which resulted in a relatively stable fluorescence intensity (FL) and more accurate detection data.Various studies were looking for better CQDs for industrial production and practical testing to realize the simple, fast, and accurate detection of metal ions in water [3] .
CQDs acted as metal ion detectors, relying on their fluorescence response mechanism.The detection signal between the CQDs and Cu 2+ was based on the degree of fluorescence quenching.Generally, two main luminescence quenching mechanisms were acknowledged.One was carbon nuclear emission induced by intact carbon crystals with fewer defects and modification groups.The other was the surface-state emission, including the molecular and trap states induced by the carbon skeleton and the linked chemical groups.The widely studied and applied detection mechanism was fluorescence quenching, which could be considered from the following aspects.The emission wavelength of CQDs corresponded to the excitation wavelength since there were numerous emission wells on the surface.It would cause a change of energy and then affect the fluorescence intensity.In addition, the fluorescence was quenched because of the interaction between the CQDs and Cu 2+ , which could be related to the CQDs' structure, composition, and functional groups [4] .
In our experiment, this paper has shown a kind of CQDs made from urea and citric acid by one-step hydrothermal synthesis, which could be successfully applied for detecting metal Cu 2+ in water in the 3-5 mM range.They were precisely characterized using advanced technical means, such as FT-IR, TEM, UV-vis, and PL.It overcame the difficulties of metal ions detection by traditional technology and solved the problems related to the CQDs, such as fluorescence instability, low quantum yield, and inaccurate detection.Most importantly, without introducing heavy metal ions, the sensor production has no destructive impact on the environment.The CQDs were optimized to a great extent, and the detection was effective, which had a wide range of practical application values.

Materials
Urea sodium hydroxide, hydrochloric acid and citric acid [C 6 H 8 O 7 •H 2 O], as well as anhydrous ethanol (CH 3 CH 2 OH), were provided by Beijing Tongguang Company.

Production of CQDs
Urea, which acted as a nitrogen source, and citric acid, which acted as a carbon source, were used to obtain the CQDs by a hydrothermal synthesis method.The production process of CQDs was simple and easy, without the complex purification process and the use of toxic and harmful substances, which greatly simplified the purification step.At the same time, for the optimization of CQDs, fluorescent sensors with specific detection functions could be made without the introduction of heavy metal ions.
The process of making CQDs could be divided into five steps: hydrothermal synthesis method, filtration, centrifugation, alcohol sinking, and drying.Briefly, we mixed citric acid and urea and dissolved them for ten minutes.Then, the mixture was slowly heated to the temperature of 180 °C for 4 hours.Afterward, we should take the reactants out of the oven and cool them to the right temperature to open the reactor safely.Subsequently, the large particles should be removed with a filter membrane.After the removal of large particles, the carbon point solution could be alcohol-sunk to separate the carbon point.We could centrifuge the carbon point solution in a centrifuge, rotating at 5,000 rpm, and this operation took ten minutes.Finally, we dried the CQDs at 60 ℃ for 4 h.Therefore, the CQDs powder was obtained, which could be applied for the following experiments.

Detection of metal ions
The detection process of Cu 2+ in water could be divided into two main steps: one was to establish a linear curve, while the other was to detect the fluorescence intensity of the unknown solution and Cu 2+ and calculate the specific concentration of Cu 2+ .CQDs were dissolved in ultrapure water for the following experimental detection.Firstly, the CQDs solution was mixed with Cu 2+ solution.Secondly, five minutes later, the fluorescent date was written down, and the linear curve was established between the FL intensity of CQDs and the concentration of Cu 2+ .Thirdly, under the experimental conditions, we mixed the CQDs solution with unknown Cu 2+ solutions, measured the FL intensity of the mixture, and then calculated the concentration of Cu 2+ .In Figure 2a, three prominent peaks were observed at 285.0 eV, 401.0 eV, and 532.0 eV, corresponding to three crucial elements: carbon, nitrogen, and oxygen atoms.The content analysis revealed approximate percentages of 55.68% for carbon (C), 11.21% for nitrogen (N), and 34.21% for oxygen (O).From the C1s spectrum (Figure 2b), distinct peaks were observed: (C=C, C-C) at 286.54 eV, (C-OH, C-OR) at 284.82 eV, and (COOH) at 277.95 eV.Furthermore, in the N1s spectrum (Figure 2c), peaks corresponding to (C-N-C) appeared at 399.93 eV, (C≡N) at 400.57eV, and (N-H) at 399.95 eV.Likewise, in the O1s spectrum (Figure 2d), distinctive peaks were identified: (O=C-O) at 538.09 eV, (C-O) at approximately 531.79 eV, and (C=O) at around 523.95 eV.These findings strongly suggest the successful introduction of nitrogen (N) elements into the structure of carbon nanoparticles during the synthesis process.Functional groups had a significant influence on the fluorescence intensity of CQDs, which was indispensable, and it was also an important research object that directly affected the quantum yield of fluorescence.The introduction of nitrogen sources greatly enriches the functional groups of carbon points.From Figure 3, we would find demonstrated major chemical functional groups.For example, the peaks at 956 cm -1 corresponded to carboxylic groups.It could demonstrate the C=C at 1407 cm -1 .A symbol of C=O was observed at 1545 cm⁻¹.The peaks ranging from 2500 to 2000 cm⁻¹ indicated C-H stretching, while O-H stretching occurred at 3181 cm⁻¹.Furthermore, the N-H stretching at 3347 cm⁻¹ was attributed to the doping of nitrogen atoms derived from urea [6] .There were two peaks in UV-vis absorption.The first peak (240 nm) could be the n -π* transition for the C=O, and the other peak (340 nm) might be the π-π* transition of aromatic sp 2 domains.As described in Figure 4, after adding Cu 2+ , the peak at about 240 nm has changed.Therefore, it could be said that the quenching mechanism was the dynamic quenching caused by the combination of the CQDs and Cu 2+ [7] .Compared with various CQDs optimization methods, N doping into the carbon structure and its effect on the optical properties had been explored.Due to its rich nitrogen content, Urea has been recognized as an excellent nitrogen source for forming N-doped carbon materials.From Figure 5a, the emission spectra of the CQDs showed a specific photoluminescence feature with different excitation wavelengths.Obviously, a change in FL could be seen, and a significant red-shift phenomenon was observed.This was a unique phenomenon of fluorescent materials attributed to their particular surface state and size effect.In addition, to explore whether the CQDs could be stable for detection, we have tested their stability through experiments.Figure 5b shows the stability of the CQDs solution in 300 min.After the CQDs powder was dissolved in water, the FL of the CQDs was relatively stable within 100 min, then decreased gradually by about 16% till 300 min.It could be explained by the poor stability of the small particle size of the CQDs, which was prone to spontaneously aggregate for lower surface energy.Moreover, other fluorescent substances would also agglomerate and quench due to instability.Therefore, we said that the CQDs had accurate application values within one hour after they were dissolved in water.However, experiments demonstrated that the CQDs were stored in a powder state; they could be kept valid for 45 days [8] .To explore the effect of pH on the fluorescence intensity of the fluorescent sensor, in Figure 6, the CQDs exhibited pH-dependent fluorescence responses from 2 to 12.Because of the protonation or deprotonation of their functional groups under acidic conditions, there was a decrease in fluorescence.

Characterization and discussion
However, it showed the strongest FL under neutral and alkaline environments, which indicated that the CQDs were conducive to fluorescence emission in neutral conditions.Therefore, ultrapure water was chosen as the solvent, which has realized the advantage of low price and environmental protection [9] .As can be seen above, the CQDs could be used to detect Cu 2+ .It could even detect concentrations in the 3-5 mM range to get fast, simple, and accurate detection results.In Figure 7a, with increasing the concentration of Cu 2+ in the CQDs solutions, the FL of the CQDs was sensitively and linearly decreased.From Figure 7b, the plot indicated a good linearity (R 2 =0.9986), which showed excellent practicability.The linear relation was calculated by the Stern-Volmer equation [10] : Considering that there were many impurities in the water and other metal ions that could potentially affect the detection of Cu 2+ , CQDs were used to detect Cu 2+ in actual water.From Figure 8, CQDs exhibited the capability to detect Cu 2+ within the 2-6 mM range, with an R 2 value of 0.9565.This underscores the high practical utility of CQDs as metal ion detectors, enabling rapid, efficient, and accurate detection of metal ions in water samples.

Conclusions
In conclusion, the magic CQDs were prepared using urea as a nitrogen source and citric acid as a carbon source via the hydrothermal method.CQDs represent a carbon nanoparticle with a particle size of approximately 3.16 nm, featuring multiple functional groups attached to its surface.As a metal ion detector for detecting Cu 2+ , the quenching mechanism of the CQDs was defined as the dynamic quenching of energy transfer caused by the binding of amine groups and Cu 2+ .The CQDs could be used to detect Cu 2+ ; it could even detect concentrations in the range of 3-5 mM, and the plot indicated a favorable linearity (R 2 =0.9986) between the quenching degree of the CQDs fluorescence and Cu 2+ concentration.Furthermore, it also showed a good correlation coefficient of 0.9565 for Cu 2+ detection (2-6 mM) in water.Therefore, it could be asserted that CQDs exhibited great practical application value.They offer a simple, rapid, and accurate method for detecting metal ions in water, thereby serving as a valuable reference for employing CQDs as metal ion detectors.

Figure 6 .
Figure 6.The effect of pH.

Figure 7 .
Figure 7. FL spectrum of Cu 2+ in the CQDs aqueous solution(a); Linear relationship between Cu 2+ and the CQDs (b).

Figure 8 .
Figure 8.The linear relationship between Cu 2+ and the CQDs in actual samples.