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Abstract
Since carbon dots (CDs) with good water solubility are preferred by researchers and biological applications, a hydrothermal method was used to synthesize green fluorescent CDs with an excitation-independent peak at 526 nm using deionized water as the solvent and neutral red as the carbon source. To achieve spectral modulation, the pH of the solvent was adjusted with KOH to obtain orange CDs (O-CDs) in an alkaline environment, with the emission peak red-shifted to 630 nm. The water-soluble CDs were prepared for multidimension sensing as Fe3+ sensing (on/off). Carbon dots dispersed into a silica gel matrix can be used for fingerprint detection of various materials.
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1. Introduction
Carbon dots (CDs) have attracted a lot of attention from researchers due to their properties such as carbon-based, zero-dimensional, quasi-spherical nanoparticles with fluorescent properties and extremely small particle size (usually below 10 nm). Furthermore, it is often considered to be a combination of inorganic quantum dots and organic ligand materials and applied in the fields of biological, photovoltaic, light emitting diode, etc. [1,2]. Most solvents for the preparation of CDs are organic and the polarity of solvent has a significant effect on the performance of CDs [3]. The purification of CDs is usually complicated causing the organic solvents residues and poor water solubility, which limits the biological application of CDs. The CDs prepared using water as the solvent tends to have good water solubility, less toxic, and environmentally friendly, which causes no damage to the human body or the environment and are also beneficial for application in ion detection and bioimaging. Therefore, the preparation of water-soluble CDs with long-lived luminescence is of great research significance.
Trivalent ferric ion (Fe3+) is an essential trace element in the human body and is involved in the processes and regulation of cellular metabolism and oxygen transport in hemoglobin [4]. Organic dyes, metal nanoclusters, and nanoparticles are common fluorescent probes for the detection of Fe3+ ions [5–8]. However, the preparation of these probes usually requires specialized synthetic skills or complex purification procedures. Therefore, CDs with simple preparation and excellent photostability are more suitable as probes for Fe3+ detection. Fingerprints, as one of the unique characteristics of a person, are often used as important evidence in criminal cases [9–14]. CDs have also shown great potential in fingerprinting in recent years due to their cost-effective and non-toxic advantages.
In this paper, we select neutral red as the carbon source and deionized water as the solvent to create excitation-independent, water-soluble green CDs (G-CDs) in a one-step hydrothermal method. To achieve spectral modulation, additional KOH is mixed with the reaction solvent and orange CDs (O-CDs) will be obtained. The emission peaks of the two kinds of CDs are at 526 nm and 630 nm, respectively. The CDs are rich in functional groups and can be used for metal detection, especially, the detection of Fe3+ ions. This carbon point has strong specific detection of Fe3+ ions as well as anti-interference ability. Taking G-CDs as an example, a fluorescent powder with green fluorescence was prepared using silica as a solid dispersion matrix. The solid fluorescent powder is applied uniformly on different material carriers, and the detailed features of fingerprints can be well represented.
2. Experimental details
2.1 Materials
The neutral red (>90%) is purchased from Macklin Biochemical Co., Ltd., Shanghai, China. Glycerol (99.9%) and Ascorbic acid (99.7%) are provided by Aladdin reagent Co., Ltd., Shanghai, China. Aqueous solutions of K+, Na+, Ca2+, Fe2+, Al3+, Fe3+, Cd2+, Co2+, Mg2+, Ca2+ and Ba2+ are prepared from KCl, NaCl, CaCl2, FeCl2•4H2O, AlCl3, FeCl3•6H2O, CdCl2, CoCl2•6H2O, MgCl2•6H2O, CaCl2•2H2O and BaCl2•H2O. All chloride reagents were purchased from Aladdin chemical (Shanghai). Deionized (DI) water is provided by the laboratory. All medicines were used without further purification.
2.2 Preparation of multicolor CDs
G-CDs: In the synthesis, 0.35 g of neutral red was added to 12 mL deionized water and stirred for 15 min. Then the mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave reactor and heated constantly for a duration 4 hours at 200 °C. After that, the reactant was cooled down naturally and the obtained solution was filtered using a 0.22 µm filter membrane and centrifuged at 8000 rpm for 5 minutes. This purification step was repeated four times.
O-CDs: Neutral red (0.35 g) was dissolved in deionized water (12 mL) and stirred for 15 minutes. At the same time, 3 mL of potassium hydroxide (the concerntration of KOH is 1.012 mol/L) was added to the mixed solution (The pH approximately is 12.5) Other steps are the same for the preparation of G-CDs.
2.3 Detection of metal ions
The G-CDs solution was divided into equal volumes, and 80 µmol/L Al3+, Ba2+, Ca2+, Cd2+, Fe2+, Co2+, K+, Mg2+, and Na+ standard solutions were added to them. The mixed solutions were sonicated for 5 min, and the emission spectra at 452 nm were tested at room temperature, and the changes in fluorescence intensity were recorded. To test the anti-interference ability of G-CDs solutions with Fe3+, the same volume of Fe3+ solution was added to it, and the fluorescence intensity was recorded.
2.4 Detection of fingerprints
Fingerprint detection using ordinary brushing method: The surface of the material was cleaned with alcohol, and then the experimenter repeatedly rubbed the forehead using fingers or palm so that it was stained with part of the grease. After that, the fingerprints were printed on different materials, and the as-prepared CDs were coated on the fingerprint. In this case, the solid CDs powder is prepared as follows: 3 mL-CDs were dispersed into 0.5 g of silica, stirred thoroughly, and then placed in a drying oven and dried at 70 °C for 3 hours to obtain the desired solid carbon dot powder. Finally, the obvious fingerprints can be observed under the ultraviolet lamp irradiation.
2.4 Characterizations
JEOL's JEM-2100 Plus transmission electron microscope was utilized to obtain transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) patterns of the carbon dots. X-ray photoelectron spectroscopy (XPS) measurements were taken on an Axisultra DLD, Kratos. Raman spectra were recorded using a excitation of 532 nm laser. Fourier transform infrared spectra (FTIR) were carried out by Nicolet IS 10 in the range 400 to 4000 cm-1. Ultraviolet-visible (UV-Vis) absorption spectra were recorded on a Lambda 950 UV/VIS/NIR spectrophotometer. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were performed using a Hitachi F-7000 fluorescence spectrophotometer. The fluorescence lifetime spectra were obtained by using a Horiba spectrophotometer (FL-1057).
3. Results and discussion
Water-soluble CDs are easily and quickly prepared, as in Supplement 1, Fig. S1, by simple filtration and purification to obtain the sample required for testing. The morphologies of the two kinds of CDs are characterized by TEM patterns. As shown in Figs. 1(a) and (b), G-CDs and O-CDs samples have good dispersion, uniform particle size, and a regular circular shape. The inset shows that the particle size distribution of CDs exhibits better monodispersed with an average particle size of 5.0 nm and 4.6 nm, which is very close to the observation from their corresponding TEM images. Insets of Figs. 1(a) and (b) show the corresponding HRTEM images and the crystallographic characteristics of the CDs. Both CDs have a lattice spacing of 0.21 nm attributing to the (100) lattice plane of graphite [15,16]. The O-CDs prepared by adding KOH are more densely distributed and appear to overlap, which may be due to the high concentration of the sample.
Fig. 1. Morphologies and structural characterization of CDs. TEM images of (a) G-CDs and (b) O-CDs, inserts are the corresponding particle size distribution at the top left corner and HRTEM at the bottom right corner. (c) Raman spectra of CDs. (d) FTIR spectra of CDs.
Figure 1(c) shows typical Raman spectra of the two kinds of CDs. It shows two peaks at 1354 cm-1 and 1564 cm-1 corresponding to the disordered (D band) and graphite bands (G band) of carbon materials, respectively [17–19]. The integrated area is calculated for the disordered D-band and the crystalline G-band, and the intensity ratio (ID/IG) of the two represents the crystallinity of the sample. The value of ID/IG is around 0.98 for G-CDs, indicating that the G-CDs exhibit a similar structure to graphite. The value of ID/IG for O-CDs increases to 2.07, which indicates the decrease of the graphitic carbon content. The FTIR spectrum can identify the difference in functional groups in the two kinds of CDs. As shown in Fig. 1(d), the two CDs have contain the similar functional groups information. The absorption peak at 3444 cm-1 is attributed to O-H stretching vibrations, the peak at 3222 cm-1 is due to N-H stretching vibrations, the peaks at 2028cm-1 and 2710 cm-1 correspond to C-H stretching vibrations, the peaks at 2710 to 2603 cm-1 broad peaks were attributed to C-O stretching vibrations, the peaks at 1593 cm-1 was attributed to C = N stretching vibrations, the peaks at 1596 cm-1 and 1471 cm-1 indicated the presence of COOH, respectively [20–26]. The results of the above analysis show that the -OH content and C = O content of the O-CDs are significantly more than that of the G-CDs, suggesting that the KOH treatment can increase their surface oxidation, improve the surface state of the CDs, increase the number of oxygen-containing functional groups, and modulate the performance of the samples, thus causing an emission redshift.
To understand the effect of internal structure and surface groups on the PL emission of CDs. The survey scans XPS spectrum presented in Fig. 2(a) shows three peaks: C 1s (285 eV), N 1s (400 eV), and O 1s (531 eV), which indicates that CDs mainly consist of C, N, and O elements [18]. The element content can be obtained by the XPS analysis and the results are shown in Supplement 1, Table S1. The N content decreases from 13.24% to 8.01% for the G-CDs and O-CDs. Conversely, the O content increases from 5.34% to 12.38%, which is further evidence that the alkaline environment enhances the degree of oxidation of CDs.
Fig. 2. (a) XPS survey spectra of G-CDs and O-CDs. (b-d) High-resolution C 1s, N 1s, and O 1s XPS spectra of G-CDs and O-CDs.
To further confirm the chemical bands on the survey of as-prepared CDs, the high-resolution XPS spectra are also carried out. In the high-resolution spectra C 1s peak (Fig. 2(b)), the C 1s band has been mainly resolved into four peaks, representing, which includes C-C/C = C (284.8 eV), C = O (287.35 eV), C-N/C-O (286.05 eV) and COOH (288.15 eV) [27]. The N 1s spectra are measured and shown in Fig. 2(c), in which three peaks corresponding to pyridine nitrogen (399.1 eV), amino nitrogen (399.75 eV), and pyrrolic nitrogen (400.5 eV) can be fitted [28]. The O 1s spectrum (Fig. 2(d)) can be divided into two peaks located at 531.25 eV and 532.1 eV by spilled peak fitting, corresponding to C-O and C = O, respectively [29]. The XPS results show the amino content decrease, suggesting that the Neutral Red is more reactive after adding KOH. The increase of carbonation and the number of oxygen-containing functional groups is attributed to the breaking of N-H bonds for condensation.
UV-Vis and fluorescence measurements are utilized to characterize the absorption and PL properties of G-CDs and O-CDs. The corresponding data are shown in Fig. 3(a). The UV-Vis absorption of G-CDs exhibits two peaks at 255 nm and 321 nm, which is attributed to the π-π* transitions of C-C due to sp2 hybridization in the carbon nucleus [30–32]. The absorption peak at the high-energy region 420 nm is attributed to the n-π* transitions of C = N on the surface of CDs, which is believed to be the source of fluorescence generation [33]. While for O-CDs, there is a strong absorption at about 450 nm, which is responsible for the orange fluorescence emission. The energy gap was calculated by the TAUC method (Supplement 1, Fig. S2) using the 255 nm deconvolution component of the UV-Vis absorption spectrum [34]. The energy gap of G-CDs is 4.55 eV in comparison with the 4.27 eV measured for the O-CDs. Fig. S3 in Supplement 1 shows the fluorescence spectrum of G-CDs and O-CDs, respectively.
Fig. 3. (a) UV-Vis absorption spectra and emission spectra of G-CDs and O-CDs. G-CDs (green line, λex = 452 nm) and O-CDs (orange line, λex = 500 nm). (b) PL spectra of G-CDs excited at different wavelengths. (c) PL spectra of O-CDs excited at different wavelengths. (d) Time-resolved PL spectra of G-CDs and O-CDs.
The optimum emission peak corresponds to the excitation wavelength as follows: for G-CDs, Ex (excitation wavelength) is 452 nm, Em (emission wavelength) is 527 nm (Supplement 1, Fig. S3a); For O-CDs, Ex is 500 nm, Em is 630 nm (Fig. S3b). The PL properties of the two CDs are also shown in Fig. 3(a). When the excitation wavelength changes from 372 nm to 422 nm, the maximum emission wavelength of G-CDs remains 527 nm, indicating the G-CDs have excitation-independent properties (Fig. 3(b)). The PL spectra of O-CDs are prepared by adding KOH located at about 630 nm and it covers the region from 550 nm to 800 nm. The maximum emission wavelength of O-CDs is maintained at 630 nm when the excitation wavelength varies from 420 nm to 540 nm, indicating the O-CDs have excitation-independent properties (Fig. 3(c)).
Generally, the optical property of CDs is associated with their structure and the surface state. We can see from Fig. 1 that the size of O-CDs is smaller than that of G-CDs, which can be deduced that the PL spectra of O-CDs should present a blue shift according to the quantum size effect. In this case, the emission peak positions of O-CDs present redshift, which is attributed to the increasing oxidation degree. The FTIR and XPS analysis results prove our inference. It is noteworthy that the as-prepared CDs have a concentration-dependent phenomenon, and the results show the PL intensity increases with the increasing concentration of CDs in the experimental test range in Supplement 1, Fig. S4. To visualize the effect of concentration on the luminescent intensity of the carbon dots, the intensity change is represented by a line graph, and the results are displayed in Fig. S3(c). It can be seen that in the test range, the intensity has an overall upward trend, with slight fluctuations in some concentration ranges. Especially, the G-CDs rose even more.
Furthermore, we explored the fluorescence lifetimes of G-CDs and O-CDs. As can be seen in Fig. 3(d), both CDs have similar fluorescence lifetime curves in the nanosecond order. The curve can be fitted by the double exponential equation:
The specific values for τ1 and τ2 in the equations are given in Table S2. The curve can be fitted by the double exponential equation and τG-CDs and τO-CDs are calculated to be 3.65 ns and 3.99 ns, respectively. The curve was fitted and the vertical coordinates were treated by taking the natural logarithm. In an alkaline environment, the electron transfer between the excited state and solvent will occur when the sample is excited, which extends the fluorescence lifetime of CDs.
Because G-CDs and O-CDs have better water-soluble properties, accompanied by luminescent stability, low toxicity, and harmlessness, which make them can be utilized for ion detection. It was determined by FTIR and XPS tests that the surface of CDs is rich in functional groups, which enable the CDs to undergo rapid complexation reactions with metal ions, effectively suppressing the fluorescence of CDs and causing their quenching [35–37]. To search the application of as-prepared CDs in the ions probe, we measured the fluorescence responses of G-CDs by the addition of nine kinds of metal ions including Al3+, Ba2+, Ca2+, Cd2+, Fe2+, Co2+, K+, Mg2+ and Na+ ions. It can be perceived from Fig. 4(a), that the intensity of G-CDs hardly changes with doping different ions. However, the addition of Fe3+ has the most significant effects on its quenching, reducing the fluorescence intensity by about 80%. It is found that the fluorescent probe can accurately identify the presence of Fe3+ in the presence of coexisting other metal ions and has strong anti-interference ability. The fluorescence spectra of G-CDs with the addition of different concentrations of Fe3+ are investigated and shown in Fig. 4(b), from which the PL intensity of G-CDs decreases with the concentration of added Fe3+ in the region of 0 µmol- 1 mmol. Similarly, the O-CDs are detected for Fe3 + and the corresponding concentration spectra are shown in Supplement 1, Fig. S5.
Fig. 4. (a) Fluorescence spectra of G-CDs solutions with the addition of different ions and fluorescence intensity change spectra of Fe3+ resistance to interference. (b) Fluorescence intensity changes the spectra of G-CDs solutions with the concentration of Fe3+ ions (λex = 452 nm). (c) UV-Vis absorption of Fe3+ ion solution. (d) The spectrum of change in fluorescence intensity of O-CDs solution by Fe3+ ion concentration.
The quenching process can be analyzed using the Stern-Volmer equation [38].
where F0 and F represent the intensities of CDs before and after adding the quencher, and Ksυ is the Stern-Volmer constant of the quencher and fluorophore. [Q] expresses the concentration of metal ions, and Kq is the quenching rate constant of the biological macromolecule; and ${\mathrm{\tau }_0}$ is the average lifetime of a molecule without adding a quencher. To further quantify the relationship between the fluorescence intensity with Fe3+ concentration, we construct a cartesian coordinate system using Q and F0/F as horizontal and vertical coordinates as shown in Fig. 4(c). The relationship between F0/F and concentration Fe3+ shows a good linear relationship within the range of 0-300 µmol, which accords with regression equation F0/F = 4.04*10−3 [Fe3+] + 0.894. Moreover, the constant Kq is much greater than the static quenching (2*1010 L·mol-1s-1) [39], which indicates that the process belongs to dynamic quenching. The limit of detection (LOD) (S/N = 3) for Fe3+ is 0.28 µmol. To investigate the quenching mechanism of G-CDs&Fe3+ ions solution, the absorption spectrum of Fe3+ solution and CDs&Fe3+ solution is tested and shown in Fig. 4(d). From the absorption spectrum of the Fe3+ solution, no obvious absorption peak in the UV-Vis region. While, the absorption of the CDs&Fe3+ mixture shows a distinct absorption peak at 295 nm, and the absorption peak of CDs originally located at 420 nm disappears. The interaction of Fe3+ with the carboxyl groups contained on the surface of CDs results in significant changes in the functional groups of CDs, which cause the fluorescence quenching and the disappearance of the absorption peak [40].The fluorescence spectrum of CDs&Fe3+ solution with additional ascorbic acid (AA) is shown in Fig. 5(a). With the increase of AA solution concentration, the fluorescence of CDs gradually recovers. The absorption spectrum of Fig. 5(b) revealed that the absorption peak in the visible region of G-CDs appears after AA is added to CDs&Fe3+ solution, indicating that AA can inhibit the rapid electron transfer process between Fe3+ ions and CDs, so the fluorescence of G-CDs does not disappear and the optical properties unchanged. Based on the fluorescence detection properties of G-CDs, they can be used for the repeat detection of Fe3+. A schematic diagram of G-CDs as a fluorescent switch is briefly in Fig. 5(c).
Fig. 5. (a) The spectrum of changes in fluorescence intensity of CDs solutions by AA. (b) UV-Vis spectra of mixed CDs/Fe3+ solutions after the addition of AA. (c) Schematic illustration of the system.
The performance tests of the above-mentioned work are tested in the liquid state. To realize the application of CDs for fingerprint detection, the product is dispersed into silica and the desired substance is obtained by drying. As shown in Fig. 6(a), the Chromaticity diagram of a powder sample made of carbon dots dispersed in silica under 352 nm excitation with color coordinates (0.34, 0.57), emitting light in the green range with a correlated color temperature of 5518 K. To assess the ability of CDs to be detected on different material surfaces, several substances are selected for the experiments. These include materials such as plastic petri dishes, tin paper, black plastic bottle caps, slides, etc.
Fig. 6. (a) CIE coordinates of carbon points dispersed in silica. (b), (e), and (f) Fluorescent photos of fingerprints on different materials. (b) Desktop. (e) Slide. (f) Black plastic bottle caps. (c) and (d) are partially enlarged images of Fig. (6b).
Figure 6(b)-(f) shows the fluorescence photographs obtained from the surface of materials prepared using the coating method under 365 nm excitation light irradiation. There are distinct fingerprint marks on the different material substrates. Fig. 6(b) is partially enlarged using PS software, and its fluorescence photo is displayed in Figs. 6(c) and (d), from which the clear lines of fingerprints can be seen. Fingerprint identification photos of other materials are placed in the support material, including weighing paper, mouse, etc. The photo information is displayed in Supplement 1, Fig. S6. As can be seen in the figure, the experimenter's fingerprints are visible on different materials, but it is worth noting that on the material of weighing paper, due to the influence of paper texture and stray light from the excitation light source, which results in the appearance of only the general outline, there are no fingerprints with clear texture. Fluorescent photographs of fingerprints on different materials as well as detailed drawings demonstrate a wide range of utility in practical application scenarios at the crime scene.
4. Conclusion
In summary, multicolor CDs were prepared using a one-step hydrothermal method, and the spectral red shift was achieved by a simple KOH treatment. The experimental results showed that the alkaline environment made the reaction more adequate and increased the degree of oxidation, suggesting that the surface state mechanism caused the change in fluorescence properties. The application of ion detection to water-soluble CDs achieved specific detection of Fe3+ ions and the addition of AA enabled the recovery of their fluorescence switch probe. The G-CDs could be used for the unique selectivity of the Fe3+ according to the Stern-Volmer plots in the range of 0-300 µmol with a LOD of 0.28 µmol. The carbon dots have fluorescence recovery properties, so they can be used multiple times, reducing detection costs. The solid CDs obtained by dispersion in the matric can be used for fingerprinting a wide range of materials. The CDs are prepared in the fields of ion detection, fingerprint identification, etc.
Funding
National Natural Science Foundation of China (62075055, 62175075); the Central Project Guide local science and technology for development of Hebei Province (226Z1103G); Ningxia Sinostar Display Material Co., Ltd. (No. 20230171).
Acknowledgements
This work was partly funded by the National Natural Science Foundation of China (No.62175075, No.62075055), the Central Project Guide local science and technology for development of Hebei Province (226Z1103 G), Ningxia Sinostar Display Material Co., Ltd. (No. 20230171). We also appreciate the technology support from the high-performance supercomputer center of Hebei University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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