The One-Step Preparation of Green-Emissioned Carbon Dots through Hydrothermal Route and Its Application

This paper reports the preparation of green-emissioned CDs by a one-step hydrothermal route with DL-malic acid and ethylenediamine as raw materials and further explored the impacts of the amount of reactants, the reaction time, and temperature upon the optical properties of prepared CDs, to select the optimal preparation parameters. The optical properties, surface groups, and element components of the prepared CDs have been systematically studied by UV-Vis absorption spectra, fluorescence spectra, FTIR, EDS, and XPS. The prepared CDs have been applied to realize the fingerprint development, fluorescence ink, and, specifically, the detection of Fe through an inner filter effect with its detection range at 2 0 × 10−7 − 2 3 × 10−5 mol/L.


Introduction
In recent years, carbon dots (CDs), as a novel fluorescent carbon nanomaterial, have been widely applied in the fields of biological sensor [1,2], biological imaging [3,4], drug delivery [5,6], photocatalysis [7], and light-emitting diodes [8], thanks to their unique properties, such as excellent biological compatibility, stable photoluminescence, and easy surface functionalization [1].However, most blue-emissioned CDs can cause damage to cells and organisms [9].Therefore, the preparation of long-wavelength CDs is essential for the better application of CDs in the fields of sensor and biological imaging.
Thus, researchers have developed more CDs with longwavelength emission, including green- [10][11][12][13], yellow- [14][15][16], and red-emissioned [17][18][19][20][21][22][23] CDs.Gu et al. have prepared green-emissioned CDs through a microwave route with crane sugar as carbon source and diethylene glycol (DEG) as reaction media [24].Dong and his group acquired orange-emissioned CDs through a microwave route with pphenylenediamine as raw material and ethambutol as solvent [14].Xu et al. used selenocystine as a raw material and have acquired Se-doped green-emissioned CDs with QYs at 7.6% through the hydrothermal route (60 °C, 24 h) [25].The adjustment of the emission wavelength of prepared CDs is mostly through the selection of raw materials, without noticing the impacts of other synthesis parameters upon the optical properties of prepared CDs.Some research teams have reported the adjustment of reaction solvents; reaction temperature and time could effectively change the emission wavelength of prepared CDs.Ding et al. have acquired PL emission from blue to NIR CDs with o-phenylenediamine and L-glutamic acid as raw materials, and formamide, dimethylformamide (DMF), ethanol, and H 2 SO 4 aqueous solution as solvents [26].Chen and his team used p-phenylenediamine as carbon source and acquired orange-emissioned CDs (200 °C, 1 h) and blue-emissioned ones (160 °C, 6 h) [27].Therefore, the systematic investigation of synthesis parameters is essential for adjustment of CD emission and further understanding of their emission mechanism.Some reported CDs have already exhibited excellent optical stability, pH stability, salt stability, and ability against the interference of metal ions and thus have been widely applied in the fields of fingerprint development and imaging [28], fluorescence ink [29], and LED [30].However, the application of these CDs as fluorescence sensor for metal detection are mostly based on photoinduced electron transfer (PET) [31,32], which is not sensitive enough to realize specific and selective detection to target metal ions.New detection models are thus essential.Recently, a new fluorescence sensor based on inner filter effect (IFE) has attracted the interest of the researchers [33,34].IFE quenches the fluorescence of fluorophores through the absorption of excitation light and/or emission light by the absorber, without any requirements of coupling or electrostatic binding between the detection objects and the fluorescent probe, endowing better selectivity and suitability for analysis applications.However, the low concentration of metal ions in the aqueous phase could hardly change the fluorescence of CDs through IFE.The proper developer would interact with the metal ions to form a colored complex with strong absorption ability, leading to a dramatic change in CD fluorescence.This method exhibits excellent selectivity and sensitivity.
Iron is one of the most important elements for the biological system, with its involvement in many physiological processes, such as oxygen delivery [35], enzymatic reactions [36], and electron transfer in the mitochondrial respiratory chain [37].But the excessive accumulation of iron in the human body would lead to various diseases, including hepatitis [38], Alzheimer's disease [39], and Parkinson's disease [40].Iron exists in two forms of compound forms: reductive Fe 2+ and oxidizing Fe 3+ .These two forms exhibit a dramatic difference in physiological activity.The present detections mainly focus on Fe 3+ and Fe ions [41][42][43], so the development of a fast and accurate detection method to Fe 2+ is of crucial importance.
This paper reports the preparation of green-emissioned CDs through a hydrothermal route with DL-malic acid and EDA as raw materials and explores the impacts of the ratio of reactants, reaction time, and temperature upon the fluorescence intensity of CDs to get the optimal reaction parameters.The optical properties, surface groups, and element components of the prepared CDs have been systematically studied by UV-Vis absorption spectra, fluorescence spectra, FTIR, EDS, and XPS.The CDs exhibit excellent stability and have been successfully applied in fingerprint development, fluorescence ink, and specifically detection of Fe 2+ through the inner filter effect.2.2.Instrumentation.The DHG-9030-type electric thermostatic blast drying oven (Shanghai Yiheng Technology Company) was used to synthesize CDs, and UV-Vis absorption spectra were recorded on a Lambda-35 UV/Vis spectrophotometer (PerkinElmer Company).Fluorescence spectra were acquired with a LS55 spectrofluorometer (PerkinElmer Company).Fourier transform infrared spectra were obtained on a Nicolet 6700 (FTIR) spectrometer (Thermo Fisher Scientific).EDS patterns were captured using an FEI Quanta 200 scanning electron microscope equipped with an energydispersive X-ray spectrometer (FEI Company).XPS measurements were acquired with a VG MultiLab 2000 X-ray photoelectron spectrometer (Thermo Electron Corporation).Structural investigations were carried out by X-ray diffractometry (XRD, Bruker D8 Advance, Germany).

Preparation of the CDs.
In a typical preparation procedure, 1.0 g DL-malic acid and 0.4 mL EDA were mixed in 20 mL of water under stirring, followed by deaerating with high-purity nitrogen gas for 20 min.Finally, the solution was transferred into a 25 mL Teflon-lined autoclave.It was loaded in an oven at 200 °C for 4 h and then cooled to room temperature naturally.The QY of CDs was measured according to the literature [44].Rhodamine 6G in ethanol was chosen as the reference standard (QY = 95%).The PL intensity was measured by an LS55 spectrofluorometer with λ ex = 460 nm.The measured wavelength range was 470-700 nm.Excitation and emission slits were 10 nm and 6 nm, respectively.During fluorescence measurement, the concentration of CDs was 1/10 of the original concentration, unless otherwise specified.
The purification of CDs was as follows.Acetonitrile was added to the CD solution (volume ratio of CDs to acetonitrile was 1: 3).The solution was then centrifuged at 5000 r/min for 5 min.The precipitate after centrifugation was freeze-dried to a solid and stored for future use.

Synthesis and Characterization of CDs
3.1.1.The Impact of EDA Amount upon Prepared CDs.CDs have been prepared through a hydrothermal route with DLmalic acid and EDA.The impacts of the amount of reactants, reaction time, and temperature upon the optical properties of prepared CDs have been investigated.With other reaction parameters fixed (1.0 g DL-malic acid, 200 °C, 4 h), the amount of EDA was optimized.As shown in Figure 1(a), the fluorescence intensity increases with the increased amount of EDA and reaches the maximum when the EDA amount is at 0.4 mL.Further increase of EDA would cause fluorescence decrease.Moreover, the emission peak of prepared CDs red-shifts from 498 nm to 528 nm with the increased EDA amount.EDA works both as the doping agent 2 Journal of Nanomaterials of N element and as the surface passivation agent [45].
According to the previous reports, the doping of the N element could effectively enhance the fluorescence intensity of prepared CDs [46], and the existence of the passivation agent could decrease the surface defects of CDs [47].Therefore, the enhanced amount of EDA could lead to improved fluorescence intensity, while, on the other hand, because EDA is strongly alkaline, the excessive amount of EDA would enhance the pH value of the precursor solution, which is not conducive to the synthesis of CDs.Therefore, the optimal EDA amount is 0.4 mL.With other experimental factors fixed, the pH value of the precursor solution was investigated.As illustrated in Figure 1(b), the pH value greatly impacts the PL intensity of prepared CDs.The PL intensity gradually enhances with the increase in pH from 3 to 4.5.However, when the pH value increases from 4.5 to 9, the PL intensity of the CDs obviously decreases.The optimal pH value is thus set at 4.5.

The Impacts of Reaction Temperature and Time upon
CDs.With other reaction parameters fixed (1.0 g DL-malic acid and 0.4 mL EDA, 4 h), the impact of reaction temperature upon the optical properties of prepared CDs was explored.As shown in Figure 2(a), the optimal reaction temperature is at 200 °C.The low reaction temperature is unbeneficial to the formation of the core and the decrease in surface defects [48], while the excessively high temperature would cause serious carbonization of prepared CDs and destroy the surface fluorophores of prepared CDs (the CD solution prepared at 210 °C exhibits a darker color), leading to a decrease in fluorescence intensity.
With other parameters fixed (1.0 g DL-malic acid and 0.4 mL EDA, 200 °C), the impact of reaction time (2 h-7 h) upon the fluorescence intensity of the products was also investigated.As shown in Figure 2(b), with the increase in reaction time from 2 h to 4 h, the fluorescence intensity of prepared CDs enhances and then stays stable (4 h-5 h).However, when the reaction time surpasses 5 h, the color of the product turns dark with an obvious decrease in PL intensity.The optimal reaction time is thus set at 4 h.Under high temperature, DLmalic acid and EDA first undergo dehydration condensation reactions and form polymers, and the polymer then is carbonized into the carbon core [49].The long reaction time is beneficial to the decomposition of EDA and thus increased the doping ratio of the N element in the carbon core.Through the above experiments, the optimal reaction parameters are 1.0 g DL-malic acid and 0.4 mL EDA which react at 200 °C for 4 h.With rhodamine 6G as the reference, the QY of CDs prepared at optimal conditions is 38.2%.

Characterization of CDs.
As shown in Figure 3, there exists an obvious absorption peak at 324 nm in CDs' UV-Vis absorption spectra, which originates from the n − π * transition of the surface group.The prepared CDs emit a bright green fluorescence under the illumination of UV light (365 nm).
The element composition of prepared CDs set a great impact upon the fluorescence properties.IR and EDS were used to analyze the surface groups and element composition of prepared CDs.As shown in Figure 4(a), the absorption peak at 3300 cm -1 originates from the stretching vibration of C-OH and N-H, the peaks at 3061 cm -1 and 2936 cm -1 from the stretching vibration of C-H, the peak at 1650 cm -1 from the stretching vibration of C=O, the peak at 1565 cm -1 from the bending vibration of N-H, the peak at 1390 cm -1 from the stretching vibration of C-N, and the peaks at 1182 cm -1 and 1086 cm -1 from the bending vibration of C-H.These surface groups are similar to those of CDs prepared by a microwave route [13].The -COOH and -NH 2 3 Journal of Nanomaterials groups show excellent water solubility of CDs, beneficial to their application in biochemical detections.
As shown in Figure 4(b), CDs are mainly composed of three elements, C, N, and O, with their ratios at 88.4%, 10.9%, and 0.7% wt, respectively.The N content of CDs prepared by the hydrothermal route is higher than that of CDs prepared by the microwave route (4.07%) [13].The low temperature during the synthesis of CDs prepared by the microwave route (800w, 7 min) is not beneficial to the decomposition of EDA and the doping of the N element, while the N element has a positive effect on QYs [45].This may be the reason why the QY of hydrothermal synthesis of CDs is higher than that of microwave synthesis of CDs.The crystallinity state of CDs is judged from the XRD pattern (Figure S1), which displays a clear broad peak at around 23 °, demonstrating an amorphous property [50] of the CDs prepared in our case.The HRTEM image of the CDs is shown in Figure S8.The CD particles are spherical and monodispersed.The particle size distribution is 2.73-4.27nm, and the average particle size is about 3.51 nm.
XPS measurements were performed to identify the detail of the chemical bond compositions.As shown in Figure 5(a), the full XPS spectra display three typical peaks at 285, 400, and 531 eV for C1s, N1s, and O1s, respectively, indicating that the CDs consisted of C, N, and O elements.In the high-resolution XPS spectra (Figure 5(b)), the C1s band can be deconvoluted into three binding energy peaks at 284.6, 286.1, and 288.3 eV, which could be assigned to C-C/C=C, C-O, and C=O, respectively.The N1s spectra Figure 5(c) display two peaks at 399.8 and 399.5 eV, attributed to N-C and -NH groups, respectively.O1s (Figure 5(d)) exhibits two main bands at 531.9 eV and 530.6 eV, confirming C-OH/C-O-C [51] and C=O [52], respectively.All these results of XPS are in agreement with FTIR results.
Because of the long-wavelength emission and strong luminescence of CDs, CDs are used to enhance the visualization of latent fingerprints.Figure 6(a) shows the PL images of sebaceous fingerprints (sebum-rich) developed by CDs.Sebaceous fingerprints were obtained by volunteers gently rubbing their fingertips over the forehead or nose, followed by stamping them on the aluminum foil with minimal pressure.The foil was immersed in the solution of the CDs under stirring for 30 min, followed by washing with water and drying under air.Photographs were taken under 365 nm UV light using a multimode microplate reader.The luminescence of fingerprints is sufficiently strong such that it can even overcome the reflection light from the blue background luminescence from the aluminum foil.Such results confirm our hypothesis that the large amount of -COOH on the CD's surface can promote the electrostatic interaction between CDs and fatty residues of latent fingerprints, enhancing the visualization of latent fingerprints.The excellent PL properties of CDs have ensured their potential application as fluorescence ink.The aqueous solution of CDs was directly injected into a pen without any chemical modification.Moreover, it was   2+ Based on IFE 3.2.1.The Detection of Fe 2+ and Its Mechanism.The prepared CDs (λ em = 488 nm) can serve as the fluorescence probe of the detection of Fe 2+ in the aqueous phase (λ ex = 430 nm, the concentration of CDs is 0.5 mg/mL).Since the pH stability and salt resistance of CDs would interfere the selectivity and sensitivity of the detection system, the anti-interference ability was thus investigated.At the same time, as shown in Figure S2, the separate addition of 2 × 10 −5 mol/L Hg 2+ , Pb 2+ , Co 3+ , Ag + , Ca 2+ , Zn 2+ , Mn 2+ , Fe 3+ , and Fe 2+ into the CD solution set almost no impact upon the fluorescence of CDs.In order to achieve the detection of Fe 2+ , IFE is thus applied in the detection.

The Detection of Fe
In order to choose suitable complexing agents, reagents that are easy to complex with Fe 2+ were tested, such as HAc, PAA, CA, GI, urea, EDTA-2Na, and o-phen.Figure S3 exhibits that the existence of complexing agents (1 8 × 10 −4 mol/L) does not cause obvious fluorescence intensity decrease after the addition of 2 × 10 −5 mol/L Fe 2+ into the CD solution, except that of o-phen.This occurs because the orange complex formed by Fe 2+ and o-phen have an obvious absorption peak at 512 nm, while the PL spectrum of CDs exhibit an absorption peak at 488 nm (Figure 7).The overlapping of the UV absorption peak of the Fe 2+ -o-phen complex and the fluorescence emission peak of CDs occurred.Based on the IFE process, the complex can effectively quench the fluorescence of CDs [53].
The mechanism for detecting Fe 2+ based on IFE is shown in Figure 8.There is no effect on the fluorescence of CDs when only Fe 2+ or o-phen is present in the system.The fluorescence of CDs was quenched in the presence of Fe 2+ and o-phen.In this system, Fe 2+ and o-phen form an orange complex with a distinct absorption peak at 512 nm (as shown in Figure 7), which overlaps with the fluorescence emission peak of CD.Therefore, the complex formed by Fe 2+ and o-phen can absorb the fluorescence emitted by CDs, thereby effectively quenching the fluorescence of CDs.
Meanwhile, the impacts of other metal ions that can easily complex with o-phen such as Co 2+ , Mn 2+ , Cd 2+ , Cu 2+ , Zn 2+ , Fe 3+ , and Ag + (2 × 10 −5 mol/L) upon the fluorescence intensity of CDs were also investigated (Figure S4); the effects of complexes formed by other metal ions and ophen on the fluorescence intensity of CDs are negligible.Therefore, our system of o-phen-CDs can specifically detect the existence of Fe 2+ and can also effectively distinguish Fe 2+ from Fe 3+ , which cannot be realized by some methods (such as atomic absorption spectrometry).
where F 0 and F are the fluorescence intensities of CDs before and after the addition of Fe 2+ , K sv is the quenching constant, and c is the concentration of the detection target.The system exhibits good linearity in the range of 2 0 × 10 −7 − 2 3 × 10 −5 mol/L, F 0 /F = 0 99647 + 0 00481 c , R = 0 9995.In order to verify the practical application value of this method, we applied it to the quantification of Fe 2+ in commercial drug ferrous sulfate syrup.Ferrous sulfate syrup is an iron supplement.If it is not properly preserved, Fe 2+ will be oxidized to Fe 3+ , so it is necessary to detect the concentration of Fe 2+ .We used a simple dilution method to dilute the ferrous sulfate syrup to a concentration of Fe 2+ of 1 2 × 10 −6 mol/L and calculate its detection concentration of 1 15 × 10 −6 mol/L through the Stern-Volmer equation.Finally, we calculated a recovery of 95.8% and a relative standard deviation of 1.8% (n = 3).

Conclusion
This paper reports the simple preparation method of greenemissioned CDs through the hydrothermal route with DLmalic acid and EDA as the raw material, and the optical properties and structure of the prepared CDs have been further studied through UV-Vis absorption spectra, fluorescence spectra, FTIR, EDS, and XPS.The QYs of prepared   7 Journal of Nanomaterials CDs reached 38.2% under optimal synthesis environment, and the CDs exhibit excellent pH stability and salt resistance, eligible for fingerprint development and fluorescence ink.Moreover, a new detection method with CDs as a fluorescence probe to realize the direct, effective, and specific detection of Fe 2+ through IFE have been proposed, and the system is proven to be effective in the range of 2 0 × 10 −7 − 2 3 × 10 −5 mol/L with good interference immunity, setting a solid foundation for its future application in the detection of Fe 2+ in cells and other biological systems.

Figure 6 :
Figure 6: Green-emitting CDs formed a fingerprint on aluminum foil under UV lamp irradiation (a); photographs of PL characters written with CDs on filter paper captured under room light (b) and UV light (c).

3. 2 . 2 .
The Detection of Fe 2+ .The effects of reaction time, optimal pH value of the buffer, and concentration of o-phen were determined, and the results are shown in Figure S5-S7.Shown in Figure 9 are the PL spectra of CDs in the presence of various concentrations of Fe 2+ in the Tris-HCl buffer (20 mmol/L, pH = 7).As shown from the standard curve of fluorescence intensity to Fe 2+ concentration (λ ex = 430 nm, inset of Figure 9), the fluorescence intensity of CDs decreases with the enhancement of Fe 2+ concentration.The quenching can be expressed with the Stern-Volmer equation:

Figure 8 :
Figure 8: Schematic illustration for the detection of Fe 2+ based on IFE.