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Boron/Nitrogen Co-doped carbon quantum dots as a high sensitive and selective fluorescent sensor for PO4 detection

Boron/Nitrogen doped carbon quantum dots (BNCQD) was synthesized and its physiochemical and optical properties were investigated by FT-IR, XRD, XPS, TEM and spectroscopic methods. The freshly prepared BNCQD shows a strong green fluorescence at 400 nm excitation, and PO4 anion can readily turn-on the fluorescence of BNCQD through forming stable complex. Then, we designed a turn-on fluorescence sensor for phosphate ion detection which indicate a high selectivity towards PO4 ions in comparison with the other anions with a limit of detection of 0.1 μM. It was approved that BNCQD is highly sensitive and selective to detect PO4 anion in wastewater.


Introduction
Excessive presence of phosphorus species in drinking water and various types of agricultural, domestic, and industrial wastewaters is a serious problem worldwide. 1 Presence of excess phosphate into the aquatic environment leads to eutrophication which can promote growth of harmful algal and decrease the amount of dissolved oxygen in water. 2 Therefore, decreasing or removing the phosphate in water is very important function for abbreviating the pollution of water bodies. Various techniques have been reported for phosphate removal successfully, such as chemical precipitation, adsorption, osmosis and biological treatment. [3][4][5][6] High efficiency, simple operation and applicability at lower concentrations in adsorption technique, make it the most widely used method for phosphate removal. 7 Therefore, trace level detection of phosphate is very beneficial from the point of view of water hygiene. There are many protocols containing doping, surface modifications, and their analytical developments have been reported. [8][9][10][11][12] Qiu et al used GQD combined with Europium ions as photoluminescent probes for phosphate sensing. 13 Jin et al. reported the detection of phosphate ions using of phosphorescence of Mn doped ZnS quantum dots. 14 Dan et al. used Ag2S QD/metal − organic shell composite as a fluorescent sensor for the detection of Phosphate ion in aqueous solutions. 15 Phosphate ion detection have also been reported using silver nanoclusters/metal-organic shell composite in aqueous solutions. 16 In continuation of our interest in preparation and using CQD and modified CQD as a reagent and/or catalyst in chemical reactions, [17][18][19] we decided to prepare B-doped CQD and examine its applicability for the sensing of some important anions.
Due to their excellent optical and electronic properties, carbon quantum dots (CQD) are shown as a very interesting compounds from various areas including sensors, bioimaging and catalysis. [20][21][22] To overcome some drawbacks such as low quantum yields of pristine CQD, which contain only carbon and oxygen atoms, surface modification as well as doping with elements such as nitrogen was performed to achieve high fluorescence efficiency. [23][24][25][26] Intrinsic properties of carbon materials are effectively tuned by doping with heteroatoms, and new phenomena and unexpected characteristics are produced. There are numerous reports on the doping in graphene by N and B substitution of C to properly change their structural, optical and chemical properties. [27][28][29] Doping these heteroatoms in graphene makes it a semimetal or a semiconductor, with possible applications in sensing. [30][31] Some reported studies have shown that doping with nitrogen can largely improve the fluorescence of doped CQD. [32][33][34] Surface modified and element-dopped photoluminescence CQD were used as biocompatible, biodetectors and biosensors. 35 Liu et al reported the using of N-doped CQD as a glucose sensor. 36 Herein, we report an efficient fluorescence sensing system for phosphate anion detection based on the boron and nitrogen-doped graphene quantum dots (BNCQD), which was synthesized by using a one-pot MW-assisted simple reaction of boric acid, citric acid and ammonia in an aqueous solution. Phosphate is a fundamental component of the nutritional chain of aquatic microorganisms, and an appropriate indicator of organic pollution in water. 33 Therefore phosphate tracing is of great important for controlling of eutrophication. Scheme 1 shows the combination of BNCQD and phosphate and the principles of this anion detection concept. Fluorescence intensity is increased by the coordination of phosphate anion to the B atoms in the BNCQD. Scheme 1. BNCQD preparation and the effect of phosphate absorption on its fluorescence intensity.

General Information
All solvents and reagents are industrial and used without further purification. Infrared (FT-IR) spectra of samples in KBr were recorded by a Bruker PS-15 spectrometer. UV-Vis absorption spectrum was carried out using a UV/VIS/NIR spectrometer (Perkin Elmer lambda 650). The morphology of BNCQDs was observed on a Zeiss EM10C transmission electron microscope (TEM). Analysis of X-ray photoelectron spectroscopy (XPS) were done with an ESCALAB 250.Xi from Thermo Scientific using Mg X-ray resource. The structure of the nanoparticle was determined using X-ray diffraction pattern (XRD, Bruker, AXS model D8). Fluorescence spectra were measured on a Jasco FP-6500 fluorescence spectrophotometer. MW experiments were conducted in a Milestone MicroSynth apparatus.

Synthesis of BNCQD
Citric acid (1g, 0.052 mol), boric acid (0.125g, 0.0053 mol) and ammonia (0.80 mL, 18.66 mol) were added in 5mL water and stirred vigorously. Then the mixture was heated with full power MWI (1000 W) for 2 minutes. Then the dark-green reaction mixture was cooled to room temperature and deionized water (5 mL) was added and stirred thoroughly. Then, water was evaporated under the vacuum at 50 ᵒ C and the residual (BNCQD) was collected for further analysis.

Synthesis of NCQD
Citric acid (1 g, 0.052 mol) and ammonia (0.3 g, 0.017 mol) were add in 5mL water and heated with full power microwave (1000 W) for 3 minutes. The light brown liquid obtained was placed in oven under 50 ᵒ C temperature and NCQD were collected.

Experimental Conditions
For sensitivity examination, Na3PO4 solutions in deionized water were prepared in various concentrations (0.1-120 µM). Then, the resultant solution (5 mL) was added into the deionized aqueous solution of BNCQD (0.0025 mg/mL) at 25 ᵒ C with the pH range of 6-8 and immediately mixed thoroughly. After incubation for 10 min, the fluorescence spectrum of the resulting solution was recorded at λex =400 nm. Described procedure was carried out for all of other salts such as Na2CO3, KOH, NaOH, NaNO3, HCN, Na2SO4, AcOH, HF, HCl, HBr and HI.

Measurement of the Quantum Yield
The quantum yield of the synthesized BNCQD and NCQD were determined according to below equation using quinine sulfate as a standard fluorescent.
In this equation Y is the quantum yield, I is the measured integrated emission intensity, n is the refractive index, and A is the extinction. The "u" and "st" are refered for sample with unknown QY and for standard respectively.

Characterization
B-and N-doped carbon quantum dots (BNCQD) was prepared under MW irradiation by using boric acid as the boron source and ammonia as the source of nitrogen (Step 1, Scheme 1). Transmission electron microscopy (TEM) image in Figure 1 shows that the particle size distribution of BNCQD is in the range of 5-30 nm and that the average size is approximately 15 nm.   The FT-IR spectra of the CQD and BNCQD sample are given in Figure S1 (see supporting information). Broad absorption band at 3100-3500 cm -1 from BNCQD spectrum is assigned to O-H and N-H. Carbonyl C=O groups show an absorption band at approximately 1710 cm -1 . The hydrophilicity of BNCQD is achieved via the presence of these functional groups. 39 Two strong absorption peaks at 1432 and 1192 cm -1 are attributed to B-O stretching vibration and B-C absorption band respectively and this is a strong evidence for the presence of B atom in the BNCQD structure. Two particular peaks at 1340 and 817 cm -1 are observed, which are associated with the B-N bond stretching vibration and the B-N-B bending vibration, respectively and therefore the presence of N atom in structure is approved.

Optical Properties
Optical properties of obtained BNCQD were investigated using UV-Vis and Fluorescence techniques. Figure 4 shows the UV/Vis absorption spectrum and the PL spectra of BNCQD. Typical absorption peaks at 220 nm and 330 nm are observed which assigned to the π-π* transition of aromatic sp 2 domains and n-π* transition of functional groups on the BNCQD respectively. 40 As shown in Figure  4b, BNCQD exhibits an optimal emission peak at 503 nm under excitation at 400 nm, thus it shows a strong green light emission under PL irradiation. In addition, the fluorescence quantum yield (QY) of the resulted BNCQD was about 28.4% using quinine sulfate as the calculated standard reference.

Mechanism of Phosphate Ions Detection
For comparison of the effect of PO4 3on fluorescence of BNCQD and NCQD, the fluorescence spectra of these compounds are shown in the presence and absence of PO4 3in Figure 5. BNCQD shows a strong fluorescence emission peak at 503 nm, whereas NCQD (with quantum yield about 26.8) exhibit a weak emission peak at 503 nm and it results that fluorescence quantum yield of BNCQD is considerably higher than that of NCQD. It can be suggested that the coordination of carboxylate groups on the BNCQD surfaces to B atom in the structure of BNCQD leads to the formation of nonfluorescent BNCQD aggregates and decreasing of fluorescence. It is interesting to note that an obvious increase in fluorescence intensity of BNCQD is observed in the presence of PO4 3-, whereas it has only a negligible influence on the florescence of NCQD. The capability of a BNCQD sensing system towards the detection of PO4 3was evaluated by measuring the changes in fluorescence intensity of this compound at 503 nm with varying PO4 3concentration ( Figure 6). It is clearly seen that BNCQD has a positive response towards PO4 3and the fluorescence intensity at 503 nm increases by the PO4 3concentration addition. As can be seen, the increasing in concentration of PO4 3from 0 to 120 µM leads an obvious enhancement in fluorescence intensity in the emission band at 503 nm. In fact, in the absence of phosphate ions, B 3 + ions can coordinate with the carboxylate groups on the surface and doped nitrogen atoms in the carbon quantum dots, as a result, fluorescence of BNCQD are quenched through energy transfer or electron transfer processes.
In the present of PO4 3ions, they can donate their electron to vacancy orbitals of B atoms to create stable bonds and resulting increased fluorescence intensity of BNCQDs, again. Because B 3+ ions exhibition a more tend to phosphor donor electron groups in PO4 3ions toward the oxygen in carboxylate groups and nitrogen of carbon dots. A linear relationship between the increased fluorescence intensity of BNCQDs and the concentration of PO4 3is observed (inset of Figure 6, 0.1 to 120 μm, R 2 = 0.9989 with the calibration equation is Y= 2.4911+ 0.0479 [PO4 3-]). The limit of detection for phosphate ion was calculated to be 0.1 µM according to Sm equation which is comparable to the world health organization standard (0.2 µM) for drinking water. 41 To compare the selectivity of BNCQD against PO4 3ions, we perform titration with others anions. The selectivity of the fluorescent sensor for PO4 3over other substances such as CO3 2-, NaOH and KOH was evaluated and the results are shown in Figure 8. As shown in Figure 7, no clear increase is observed with other ions in comparison with PO4 3which demonstrate high selectivity for PO4 3-. Therefore, BNCQD can be introduced as an excellent sensitive and high specific sensor for PO4 3detection in real samples.

Conclusion
In conclusion, we wish to introduce a specific, highly selective and rapid assay for the detection of PO4 3anion. Fluorescent B-doped carbon quantum dots is synthesized via a simple and one-pot MWassisted method and is used as environmentally friendly and biocompatible sensor of PO4 3for the first time. The sensitivity is based on the coordination of PO4 3anion via its oxygen-donors to B atoms in the BNCQD structure. Fluorescence intensity of BNCQD is increased by the coordination of PO4 3-. The fluorescence increasing efficiency is linearly correlated with the PO4 3concentration.