Preparation of water-soluble asphalt carbon dots and hyperchromic effect to Cr (VI)

Water-soluble asphalt carbon dots(W-CDs)was prepared from medium temperature coal tar pitch as raw material, via mixed acid oxidation,alkali solubilization and acid precipitation method. It was observed that as-prepared W-CDs were irregular particles with good water dispersibility and the particle size was controlled by the pH of the acid precipitation solution by a polarizing microscope . Infrared spectroscopy confirmed that the surface of the W-CDs was functionalized by hydrophilic and color-promoting functional groups, which had good water solubility; the thermal weight loss proved to be good thermal stability below 200°C; the fluorescence spectrum proved that the fluorescence emission peak position of the carbon dots appears red shift with increase of excitation wavelength, with fluorescence conversion property, and its emission light intensity decreases, belonging to non-resonant fluorescence type of direct jump fluorescence; UV spectrum proves that the addition of W-CDs can be chromium (VI) CrO4 content The detection limit was increased from milligram to microgram; the molar absorptivity of CrO4 was increased by 4 times. The experimentally optimized W-CDs-ultraviolet-viscometry method for measuring CrO4 conditions: W-CDs to chromium ratio of 8:2, pH of 8, hexamethylenetetramine as a surfactant, and the solution was allowed to stand at room temperature for 45 min.


Introduction
Carbon nanomaterials refer to carbon materials with at least one dimension of less than 100 nm in the dispersed phase.Carbon quantum dots (CQDs) is a new member of the carbon nanomaterial family. Wang [1] reviewed the application fields and detection methods on CQDs. CQDs as a rising star of carbon nanomaterials, by virtue of their in water solubility, chemical inertness, low toxicity, easy functionalization and high fluorescence intensity, photobleaching resistance, adjustable color of light and other optical properties, have displayed tremendous momentum in numerous fields such as biosensing, bioimaging, drug delivery, optoelectronics, photovoltaics and photocatalysis, as well as in metal ions, anions, small organic molecules and biomolecules. Mao [2] used fluorescent CQDs as probes to establish a new method for the determination of Cu 2+ based on the fluorescence quenching effect of Cu 2+ on carbon quantum dots. Niu [3] used a bottom-up electrochemical (EC) method to prepare green fluorescent nitrogen-doped carbon quantum dots (N-CQDs), based on the principle of on-off fluorescence (FL), the photoluminescence intensity of N-CQDs is further extended to ON-OFF-ON and ON-OFF-ON-OFF by the nature of Fe 3+ quenching, used for the detection of Fe 3+ in specific environments. Gao [4] produced an on-off-on alternately controlled CQDs fluorescent nanosensor to detect iron Fe 3+ in living cells. At the same time, glutathione (GSH) can increase the fluorescence of CDs/Fe 3+ mixed solution, and effectively distinguish cancer cells from normal cells based on the difference in the content of glutathione between cancer cells and normal cells. Li [5] studied a novel electrochemical sensor for simultaneous determination of Cd 2+ and Pb 2+ by anodic stripping voltammetry used by NCQDs-GO. In the hybrid system, both N-CQDs and GO have a large electroactive surface area and rich oxygen-containing functional groups, which providing the active site for the compound effectively adsorbs heavy metal ions through electrostatic interaction, improving sensor sensitivity. Li [6] using CQDs composite TiO2 under visible light reduced Cr (Ⅵ), after 120 min light reaction, the reduction of chromium (Ⅵ) rate reached 99. 2%, which is the 1.5 times of the reduction efficiency of TiO 2. Jiang [7] synthesized TiO2 nanocomposite photocatalysts incorporating CQDs by sol-gel method, CQDs have up-conversion fluorescence characteristics, and the visible light response of TiO2 samples doped with CQDs is significantly improved. Chen [8] constructed a novel sensor based on Pt nanoparticle-CQDs/ionic liquid functionalized graphene oxide (PtNPs-CDs/IL-GO) nanocomposites to detect H2O2. Zhang [9] used CQDs to improve the charge transfer and electrocatalytic properties of the BiVO4-SnO2 interface. Ye [10] demonstrated that CQDs, as an environmentally friendly photosensitizer, can extend the range of light absorption to the entire visible range significantly. Wang [11] synthesized a novel full-spectrum-driven CQDs (CQDs)/Bi2WO6 (CBW) hybrid material to provide full spectrum utilization for enhanced photoactive CQDs heterojunctions in contaminant degradation and energy conversion for reference. Zhang [12] synthesized a CQDs/BiPO4 nanocomposite photocatalytic material with enhanced visible light absorption and charge separation, the photocatalytic performance of CQDs/BiPO4 composites is better than that of pure under simulated sunlight or visible light. Hua [13] prepared α-BO / N-CQDs composites by low temperature hydrothermal synthesis in one step. When N-CQDs are combined with α-Bi2O3 nanotubes, the absorbance increases. The introduction of N-CQDs can inhibit the photogenerated electron-hole recombination, promote the production of hydroxyl radicals and superoxide radicals, and improve the degradation rate of rhodamine B by the catalyst under simulated sunlight. Wei [14] prepared NCDs/TiO2 composites using physical mixing and aging methods. enhance the utilization efficiency of light by the system, improve the photocatalytic performance, but also enhance the stability of the catalyst [15] . The chromium in natural waters is mainly in the valence state of Cr (III) and Cr (VI), of which Cr (VI) is hundreds of times more toxic than Cr (III) [16] . With the continuous development of industry, a large amount of chromium-containing waste is produced in the production process, which directly damages the health and safety of aquatic products and human bodies. Therefore, it is practical to establish a method of detecting Cr (VI) in water with high density and low detection limit. In this paper, Water-soluble asphalt carbon dots(W-CDs)was prepared from medium /V (nitric acid) ) mixed acid as oxidant; the ratio of carbon source asphalt to mixed acid 1g:20mL, Constant temperature 38℃ for 7.5 h; The following，the contents of the flask were poured into deionized water to terminate the reaction, and place for the night at room temperature; then was filtered using Buchner funnel and washed with water until the filter cake was neutral. The filter cake was transferred to 500mL flask, mixed with 1 mol/L NaOH until the pH of the solution was greater than 12, and stirred at 80℃ for 1 h to fully dissolve; and filtered while hot. The filtrate was added with 1 mol/L hydrochloric acid to adjust the pH of the solution to 6, and vacuum filtered, and the filter cake was recorded as W-CDs 1#; the filtrate was further precipitated with 1 mol/L hydrochloric acid to adjust the pH of the solution to 4, and vacuum filtered, and the filter cake was recorded as W-CDs 2#; the filtrate is further precipitated with 1 mol/L hydrochloric acid to adjust the pH of the solution to 2, and vacuum filtered, and the filter cake was recorded as W-CDs 3#.

Characteristic Instruments
The W-CDs were characterized by Fourier transform infrared spectrometer, UV-visible spectrophotometer, fluorescence spectrophotometer, and thermal weight loss tester.

Preparation of the W-CDs solution
Accurate weighing 0.2000 g the as-prepared W-CDs Put into a 100mL bottle, dissolved using NaOH aqueous solution( pH 10 ), and made up to a volumetric flask of 100mL to obtain 2.0 mg/mL W-CDs solution.

Characterization of as-prepared W-CDs
To study the apparent characteristic and optical behavior of the W-CDs, FTIR and UV-vis were applied in detail. The TG/DTG curves of the as-prepared W-CDs are presented in Fig.3. As shown in Fig.3 Fig. 4(c). When the excitation wavelength is increased from 254 nm to 320 nm, it can be seen that the maximum emission wavelength is almost constant ， meanwhile the intensity gradually heighten; When the excitation wavelength is increased from 320 nm to 420 nm, it can be seen that the maximum emission wavelength is red-shifted, meanwhile the intensity gradually decreases. CQDs as reported by others. which may arise from the π-π* transition of the nanocarbon particles and from the excitation energy sink of the surface states. According to Beer's law, ε increases from 0.140 to 0.600.

Effect of the W-CDs on CrO4 2molar absorption coefficient
Generally, ε becomes larger, indicating that the absorption band of the measured substance becomes stronger, which is favorable for the meas urement of the absorbance, and it can be seen that the W-CDs can enhance the CrO4 2molar absorption coefficient. The solution for the experiment were prepared .The  (1) Carbon dots with hydrophilic and color-promoting functional groups on the surface can be prepared by the water-insoluble medium-temperature asphalt as raw material, via mixed acid oxidation，alkali solubilization and acid precipitation method; the particle size of the carbon dots can be controlled by the pH value of the acid precipitation solution, and the particle size distribution range is 70nm to 280nm at the pH value of 6, with a mean value of 115nm, the carbon dots are irregular particles and disperse well in water.

Optimization of conditions for measuring
(2) When the particle size of the as-prepared carbon dots is larger than 300nm, the UV absorbance decreases with the increase of the absorption wavelength, and there is no maximum absorption peak; and the fluorescence emission peak red-shifted with the increase of the excitation wavelength, which has fluorescence conversion property; emission intensity reduced, which is a direct linear fluorescence of non-resonant fluorescent type. The addition of as-prepared W-CDs can increase the UV molar absorption coefficient of CrO4 2by 4 times, and the density of detection increases from mg level to ug level.