Fluorescence quenching and spectrophotometric methods for the determination of 6-mercaptopurine based on carbon dots

Abstract

It is of great significance to develop an eco-friendly and sensitive platform for the detection of 6-mercaptopurine (6-MP) because of its side effects and variable activity. Herein, fluorescence quenching and spectrophotometric methods for the detection of 6-mercaptopurine (6-MP) using carbon dots (CDs) as a fluorescence probe are established. 6-MP not only serves to shelter the CDs effectively from being quenched, but also to reverse the quenching and restore the fluorescence due to its ability to remove Hg²⁺ from the surface of CDs. After the experimental conditions are optimized, the linear range for the detection of 6-MP is 0.04 to 12 μmol L⁻¹, and the detection limit is 0.01 μmol L⁻¹. When it refers to the spectrophotometric method, the linear range is 0.08 to 12 μmol L⁻¹ and the detection limit is 0.02 μmol L⁻¹. The CDs with 6-MP are systematically characterized with transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) absorption and so on. Furthermore, the proposed methods are not only expected to become a potential tool for the fast response of 6-MP but also possess the potential for practical application to be applied in the determination of 6-MP in human serum samples with satisfactory results.

---

1. Introduction

6-Mercaptopurine (6-MP), a sulfur analogue of adenine, is one of the main ingredients of anti-neoplastic agents, and uses its toxic properties to suppress cancers such as acute lymphoblastic leukemia and inflammatory bowel diseases.^1,2 Different analytical methods have been proposed for the analysis of 6-MP, such as chemiluminescence,^3,4 electrochemical methods,^5–7 high performance liquid chromatography (HPLC),^8–11 mass spectrometry,¹² capillary electrophoresis (CE)¹³ Raman assays,¹⁴ and electrochemical assays.^5,15 The electrochemical determination of 6-MP has limited application in real samples due to the poor repeatability and complex electrode modification process. HPLC and MS are always used together to detect 6-MP. Although it is selective and sensitive, it always requires expensive equipment and toxic solvents and often involves complex sample pre-treatment. CE and Raman assays also suffer from defects as with HPLC and MS. When it comes to electrochemical assays, these are the most widely reported, and numerous modified electrodes have been used in 6-MP detection. However, their poor repeatability and complexity have limited their application in real samples. Hence it is of great significance to develop a new, convenient, rapid, and sensitive method to detect trace amounts of 6-MP.

Carbon dots (CDs), a new class of fascinating carbon material, have attracted considerable attention since their first discovery in 2004.¹⁶ In striking contrast to traditional organic dyes and semiconductor quantum dots, CDs are superior fluorescent nanomaterials, and show several prominent advantages, such as low toxicity, excellent biocompatibility, strong and tunable luminescence, and high photostability.^17–20 Carbonaceous quantum dots have fuelled intensive research efforts owing to their unique optical properties and their potential applications in sensing,²¹ imaging,²² and optoelectronic devices.^23,24 As alternatives to colloidal semiconductor quantum dots (containing hazardous heavy metals, such as Cd²⁺), extensive endeavours have thus been made on the development of non- and low-toxic fluorescent nanomaterials, which can be exemplified by the persistent attention to fullerenes,²⁵ carbon nanotubes,²⁶ and graphene oxide.²⁷ However, the limited water solubility, rapid photo-bleaching, and low photoluminescence efficiencies become the main blockage for their broad applications. Among various carbon-based materials, carbon dots (CDs) are particularly encouraging because of their low cost, being easy to get, their photostability, and biocompatibility. Typical fabrication approaches, including laser ablation,²⁸ electrochemical oxidation,²⁹ confined combustion,³⁰ and chemical oxidation,³¹ are currently regarded as state-of-the-art methods to synthesize CDs, while these methods may involve extreme synthetic conditions, long times and energy consumption, expensive starting materials or apparatus, and inconvenient surface passivation treatments etc. Therefore, an approachable synthesis method suitable for an ordinary laboratory is still urgently in demand. Up to now, CDs used as sensitive fluorescence probes by fluorescence recovery sensing of 6-MP have not been reported yet.

Herein, CDs, as an eco-friendly and sensitive biosensor, were prepared using the hydrothermal method and have been utilized as a fluorescence probe for the detection of 6-MP. The simple detection strategy is presented in Scheme 1. Firstly, the formation of the CDs–Hg²⁺ ground state complex results in the fluorescence quenching of the CDs, and the fluorescence probe turns off. Then the self-assembly of 6-MP on the CD surfaces occurs, leading to competition between Hg²⁺ and 6-MP. Furthermore, it disturbs the interaction between Hg²⁺ and the CDs. When Hg²⁺ was removed from the surface of the CDs, the fluorescence of the CDs recovers, therefore, the system thus could be turned on. The intensity of the fluorescence recovery and absorbance of the CDs were both in a linear relationship with the concentration of 6-MP, which made it possible to detect 6-MP. The methods showed the advantages of high sensitivity, good selectivity and easy operation. Additionally, the preparation of CDs without the use of toxic raw materials made the probe more eco-friendly, which was pivotal for practical applications. Most importantly, the proposed methods have great potential application in the detection of 6-MP in human serum samples with satisfactory results.

Scheme 1 Schematic diagram for the detection of 6-MP by the CDs fluorescence probe.

2. Experimental

2.1. Instrumentation

The fluorescence spectrum was recorded with an F-2500 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) using a 1 cm path length. A UV-2450 spectrophotometer (Shimadzu, Japan) was used for acquiring absorption spectra and measuring absorbance. A high resolution transmission electron microscope (Tecnai G2 F20 S-TWIN, FEI Company, USA) was used to characterize the morphology of the CDs, which was operated at an accelerating voltage of 200 kV. Fourier transform infrared spectrometry (FTIR-8400S, Kyoto, Japan) was employed to identify functional groups of the as-prepared CDs. A FL-TCSPC Fluorolog-3 fluorescence spectrometer (Horiba Jobin Yvon Inc., France) is used to measure fluorescence emission decay curves of the systems. A pHS-3D pH meter (Shanghai Scientific Instruments Company, China) was used to measure the pH values.

2.2. Reagents

The stock solution of sodium citrate was obtained from Jinshan Chemical Reagent Factory, Chengdu, China, 6-mercaptopurine from Shanghai Aladdin Reagent Co., Ltd, China, and NH₄HCO₃ from Kelon Chemical Reagent Factory, Chengdu, China. The Britton–Robinson (BR) buffer solutions at different pH values were prepared by mixing the mixed acid (composed of 2.71 mL 85% H₃PO₄, 2.36 mL HAc and 2.47 g H₃BO₃) with 0.2 mol L⁻¹ NaOH in different proportions. The buffer solutions were used to control the acidity. All reagents were of analytical reagent grade and were used without further purification, and doubly-distilled water was used throughout.

2.3. Synthesis of fluorescent CDs

The pristine CDs were fabricated according to a hydrothermal method.³² Briefly, 0.2 g sodium citrate, 1.5 g NH₄HCO₃ and 10 mL double-distilled water were put into a Teflon-lined stainless steel autoclave. Furthermore, the mixture was hydrothermally treated for 4 h at 180 °C in a drying oven. After cooling to room temperature, the transparent product was subjected to dialysis (1000 Da, molecular weight cut-off) against ultrapure water for 12 h in order to obtain pure CDs. The purified homogeneous CDs were thus obtained, which were stable for several months. The final product was re-dissolved to 250 mL with double-distilled water and stored at 4 °C for further analysis.

2.4. Recommended procedure for determination of 6-mercaptopurine

The procedure for 6-mercaptopurine (6-MP) detection is described as follows: 0.5 mL CDs, 0.5 mL Hg²⁺ (with a final concentration of Hg²⁺ of 4 × 10⁻⁵ mol L⁻¹) and different amounts of 6-MP were added into a 10 mL colorimetric tube, and diluted to a volume of 5 mL with double-distilled water. Fluorescence spectra were measured at room temperature at λ_ex = 358 nm, and λ_em = 442 nm. Both the excitation and emission slit width was 10 nm.

2.5. Analysis of real samples

The human plasma samples, obtained from different healthy people in a local hospital, were evaluated to verify the accuracy of the proposed method. The analysis of real samples diluted 500 times with ultrapure water was performed. 0.1 mL of the sample was added into a 10.0 mL calibrated tube and diluted to 5.0 mL with ultrapure water. The resultant samples were spiked with standard 6-MP solution at different concentration levels and then analyzed with the proposed method.

3. Results and discussion

3.1. Characterization of the CDs

In order to characterize the as-prepared CDs, several methods such as fluorescence spectroscopy, UV-vis absorption spectroscopy, transmission electron microscopy (TEM), and FTIR spectrometry were used to evaluate its characteristics from different aspects. The size and morphology of the as-prepared CDs were observed by TEM. Fig. 1 displays a TEM image of the CDs. It can be clearly seen that the as-synthesized CDs were uniform in size and possess an early spherical shape. The average size of the obtained CDs was about 8 nm, which was comparable to most of the previous work.^19,33

Fig. 1 TEM image of the as-prepared CDs.

Fourier transform infrared spectrometry (FTIR) was measured to provide further evidence for the components and the surface functional groups of the as-prepared CDs. As shown in Fig. 2a, the absorption bands around 3402 cm⁻¹ were accounted for by the stretching vibrations of O–H and those at 2929 cm⁻¹ were related to the stretching vibration of C–H. The peak at 1584 cm⁻¹ corresponded to the C=C and C=O stretching vibrations, while C–N stretching vibrations at 1412 cm⁻¹ and 949 cm⁻¹ were attributed to the bending vibrations of C–H. Peaks at 1189 and 728 cm⁻¹ were related to the stretching vibration of C–O and N–H, respectively. Meanwhile, in the Raman spectrum shown in Fig. 2b, the peaks of C–O, C–N, and C–C/C=O are obvious, which were attributed to the D band (sp³-hybridized) and G band (sp²-hybridized) of the CDs. The D band was associated with the vibrations of carbon atoms with dangling bonds in the termination plane of disordered CDs. The G band corresponded to the E_2g mode of the CDs and was related to the vibration of the sp²-hybridized carbon atoms in a two-dimensional hexagonal lattice. The coexistence of both bands indicated that the C dots are amorphous.^34,35 Both the FTIR and Raman spectra revealed that multiple functional groups like –COOH and a small amount of N-containing groups were present on the surface of the synthesized CDs, which reveals that the as-prepared CDs were mainly surrounded by carboxylate and hydroxyl groups. Furthermore, a typical X-ray diffraction (XRD) pattern of the CDs presented a diffraction peak centered at 30°, and when comparing peak a with the standard card of carbon, it indicated the formation of carbon nanostructures as amorphous carbon (Fig. 2c).

Fig. 2 FTIR (a), Raman (b) and XRD (c) spectra of the fluorescent CDs.

As to the optical properties of CDs, the UV-vis absorption spectrum and fluorescence emission spectrum (Fig. 3) of the CDs were investigated. As shown in Fig. 3, the maximal absorption of the CDs was at 340 nm, which was very different from most of the previous reports.^36–38 The fluorescence spectrum of the CDs showed a 442 nm emission peak under excitation of 358 nm, with a Stokes shift of 102 nm. The sodium citrate solution itself was non-emitting when excited at 358 nm, which indicated that the bright blue fluorescence came from the CDs. When excited at 358 nm, the full width at half maximum of the CDs emission spectrum was about 65 nm, which was narrower than those reported (70 nm).^39,40 The quantum yield (Φ_u) of the CDs was calculated using quinine sulfate (Φ_s = 0.55) as the reference standard.⁴¹

where F_u and F_s refer to the areas under the fluorescence curves of the CDs and the standard, respectively. A_u and A_s are the absorbance of the sample and the standard at the excitation wavelength, respectively. The Φ_u value of the CDs was found to be 70%.

Fig. 3 Characteristic optical spectra of CDs. Overlapping of absorption, excitation and emission spectra of CDs in aqueous solution.

3.2. Fluorescence and absorption spectra sensing of 6-MP with CDs as the prober

The fluorescence and absorption spectra of the CDs are shown in Fig. 4, respectively. As shown in the two figures, Hg²⁺ has a dramatic quenching effect on the fluorescence of CDs. Meanwhile, after 6-MP was added, due to the competition reaction between Hg²⁺ and 6-MP, the fluorescence intensity and absorption coefficient of the analytical system were gradually enhanced with the increase of the concentration of 6-MP. As a result, trace amounts of 6-MP could be detected based on the enhancement of the fluorescence intensity and absorption coefficient of the analytical system. Herein, the fluorescence of the analytical system after 6-MP was added could not be totally restored. It is because the coordination between Hg²⁺ and the CDs could not be complete and part of Hg²⁺ would still inhibit the fluorescence of the CDs.

Fig. 4 Effects of 6-MP on the fluorescence (A) and absorption (B) spectra of the CDs–Hg²⁺ system. c(Hg²⁺): 4 × 10⁻⁵ mol L⁻¹; (a)–(f) c(6-MP): 2.0, 4.0, 6.0, 8.0, 10.0, and 12.0 μmol L⁻¹. pH = 7.5.

3.3. Optimum conditions of the reaction

3.3.1 Effect of pH. The relationship between the quenching effect and pH is investigated by Britton–Robinson (BR) buffer solutions, with the pH ranging from 5.0 to 8.5. Fig. 5 shows the influence of the solution acidity on the fluorescence intensities of the system with BR buffer. It was observed by keeping the CD and Hg²⁺ concentrations constant, while changing the pH of BR buffer solution. We found that the enhanced intensities reached the highest and remained constant at the pH range of 7.3–7.7. When the acidity was lower than the optimum range, the fluorescence intensity decreased gradually, while when it reached a higher value, the quenching effect of Hg²⁺ decreased. Therefore, subsequent studies were performed at pH 7.5.

Fig. 5 Effects of acidity. c(Hg²⁺): 4 × 10⁻⁵ mol L⁻¹; c(6-MP): 8.0 μmol L⁻¹.

3.3.2 Effect of concentration of Hg²⁺. Based on the above fluorescence properties of the CDs, different concentrations of Hg²⁺ were added to the aqueous solution of CDs and the fluorescence emission intensity was measured to study the sensitivity of the CD nanosensor. As shown in Fig. 6, there was a phenomenon of Hg²⁺ concentration-dependent quenching of the fluorescence of the CDs. With the increase of the concentration of Hg²⁺, the fluorescence intensity of the CDs decreased gradually. The fluorescence of the CDs could be almost quenched when the concentration of Hg²⁺ was 4 × 10⁻⁵ mol L⁻¹. Taking all the above situations into account, the concentration of 4 × 10⁻⁵ mol L⁻¹ Hg²⁺ was chosen as the optimum concentration for further study.

Fig. 6 Effect of concentration of Hg²⁺. pH = 7.5.

3.3.3 Selectivity study. Subsequently, in order to evaluate the selectivity of the sensing system, different interfering substances (5 μmol L⁻¹) were used in a series of experiments, such as inorganics (NaCl, KCl, CaCl₂, MgCl₂, CdCl₂, NH₄Fe(SO₄)₂, ZnCl₂), vitamins (ascorbic acid, pyridoxine), amino acids (glycine, tryptophan, aspartic acid, arginine), and carbohydrates (glucose, sucrose, cyclodextrin). As shown in Table 1, the influence of those interfering substances was almost negligible, which kept in the range of ±5.0%.

Table 1 Effects of coexisting substances (c_6-MP = 8.0 μmol L⁻¹)

Foreign substance	Concentration (μg L^−1)	Relative error (%)	Foreign substance	Concentration (μg L^−1)	Relative error (%)
NaCl	292.2	1.7	Glucose	360.32	2.6
KBr	595.0	2.3	Malt sugar	720.64	−2.4
CaCl2	554.9	−2.7	Sucrose	684.6	−3.3
MgCl2	476.1	3.1	L-Tyrosine	906.0	−2.3
NH4Cl	267.5	1.4	Phenylalanine	825.95	−0.8
CuSO4	798.4	−4.3	L-Arginine	871.0	−4.3
ZnCl2	681.5	3.5	Methionine	746.09	−4.8
MnSO4	755.0	3.9	Glycine	375.35	−3.1
NH4Fe (SO4)2	964.36	4.7	L-Aspartic acid	665.5	−2.2
NaF	209.95	2.4	Vitamin B1	300.81	3.3
NiSO4	788.52	−4.6	Vitamin C	176.12	4.3
CoSO4	843.45	−3.7	β-Cyclodextrin	1134.98	4.8
CdCl2	799.44	3.2	NH4SCN	380.6	3.9
KNO3	505.5	−1.6	Na2SO3	630.2	3.6

---

3.4. Mechanism for the recognition of 6-MP

Scheme 1 shows the mechanism of the detection of 6-MP using CDs as fluorescence probes. The surface states of the CDs can be influenced by metal ions, which exhibit a strong effect on the fluorescence intensity of the CDs. Due to this effect, CDs are used to detect Hg²⁺. Initially, when CDs were free in aqueous solution, they showed a strong fluorescence intensity. However, in the presence of Hg²⁺, the fluorescence of the CDs was quenched significantly through the charge transfer process, which suggests that Hg²⁺ binds to CDs and the surface state of the CDs changes. One key feature of 6-MP lies in its thiol group, which makes 6-MP have a strong binding preference toward Hg²⁺ by forming a Hg²⁺–S bond. Upon the addition of 6-MP, the competitive interaction between Hg²⁺, 6-MP, and the CDs disturbs the interaction between Hg²⁺ and the CDs. Hg²⁺ was then able to be removed from the surface of the CDs, and thus the fluorescence of the system could be recovered. The different fluorescent responses upon the addition of Hg²⁺ and 6-MP make the CDs able to be used for 6-MP detection.

Herein, in order to clarify the fluorescence quenching mechanism, the interaction of 6-MP with the CDs was studied by spectrofluorometry at two temperatures (288 K and 298 K) (Fig. 7) with the Stern–Volmer equation:^42–44

F₀/F = 1 + K_qτ₀[Q] = 1 + K_sv[Q]

where F and F₀ represent the fluorescence intensity of the CDs in the absence and presence of Hg²⁺, respectively. K_sv is the Stern–Volmer quenching rate constant, which is a quenching efficiency measurement, and [Q] is the concentration of the quencher. τ₀ is the lifetime of the fluorophore (taken as 10⁻⁸ s), and K_q is the quenching rate constant, and it could be calculated by the equation K_q = K_sv/τ₀.

Fig. 7 The Stern–Volmer curves of Hg²⁺-quenched CDs at different temperatures. c(Hg²⁺): 4 × 10⁻⁵ mol L⁻¹. c(6-MP): 2.0, 4.0, 6.0, 8.0, and 10.0 μmol L⁻¹. pH = 7.5.

Fig. 7 shows the Stern–Volmer plots of F₀/F versus [Q] at two different temperatures, from which it can be seen that with the rise of temperature, the quenching constant decreases. It can be concluded that the quenching process is probably by static quenching resulting from the formation of the CDs–Hg²⁺ complex. In order to verify the real quenching process further, the fluorescence lifetime and UV-visible absorption spectra are employed to give some more evidence for the actual quenching process.

A powerful method to distinguish weather it is a dynamic or static quenching mechanism is the fluorescence lifetime. If it is dynamic quenching, τ₀/τ = F₀/F, and the fluorescence lifetime can be cut down by the quenching medium; if it is static quenching, τ₀/τ = 1, and the quenching medium can’t change the fluorescence lifetime of the excitation state fluorescence molecule.^45,46 The fluorescence emission decay curves of the CDs and CDs–Hg²⁺ are performed on a FL-TCSPC Fluorolog-3 fluorescence spectrometer at room temperature, as shown in Fig. 8. The fluorescence lifetimes of these two systems are 1.03 ns and 1.20 ns, respectively, which demonstrates that the quenching of fluorescence of the CDs by Hg²⁺ is a static quenching process.

Fig. 8 Fluorescence emission decay curves of CDs (a) and the CDs–Hg²⁺ system (b). c(Hg²⁺): 4 × 10⁻⁵ mol L⁻¹. pH = 7.5.

Meanwhile, the UV-vis absorption spectra of the CDs in the absence and presence of Hg²⁺ were recorded. As shown in Fig. 9, the absorption spectrum of the CDs (curve a) is quite different from the spectrum of CDs–Hg²⁺ (curve b), which indicates that Hg²⁺ attached to the surface of the CDs. Furthermore, when 6-MP was added, the absorption spectrum (curve c) changed, and a bigger valley appeared at 286 nm. This might be because the binding force between the CDs and Hg²⁺ was weak, while the thiol group of 6-MP makes 6-MP have a strong binding preference toward Hg²⁺ by forming a Hg²⁺–S bond. In conclusion, the formation of these binary and ternary complexes could cause absorption spectral changes to different extents.^47,48

Fig. 9 Absorption spectra of the CDs–Hg²⁺–6-MP system: (a) CDs, (b) CDs–Hg²⁺, and (c) CDs–Hg²⁺–6-MP. c(Hg²⁺): 4 × 10⁻⁵ mol L⁻¹. c(6-MP): 8.0 μmol L⁻¹. pH = 7.5.

3.5. Calibration curve

Under the optimum conditions, the linear range for the determination of 6-MP was evaluated by the fluorescence quenching method and spectrophotometry. The intensity of the mixture was enhanced along with elevating the concentrations of the analyte. As shown in Fig. 10A and B, the fluorescence enhancement can also be described by the following equations: ΔF = 688.91c + 43.19 (ΔF = F − F₀), and ΔA = 0.333c + 0.043 (ΔA = A − A₀). The enhancement of fluorescence is proportional to the concentration of 6-MP in the range from 0.04 to 12 μmol L⁻¹ (R² = 0.999), with a detection limit of 0.01 μmol L⁻¹ (3σ/k) for the system, while for the spectrophotometry, it was 0.08 to 12 μmol L⁻¹ (R² = 0.998) and the detection limit was 0.02 μmol L⁻¹ (3σ/k). Obviously, the fluorescence quenching method is much more sensitive than spectrophotometry, but both of them are faster and more facile than the previous works, which are shown in Table 2.

Fig. 10 Calibration curves for 6-MP: (A) fluorescence, and (B) absorption. c(Hg²⁺): 4 × 10⁻⁵ mol L⁻¹. c(6-MP): 2.0, 4.0, 6.0, 8.0, 10.0, and 12.0 μmol L⁻¹. pH = 7.5.

Table 2 Analytical features of some typical methods employed for 6-MP determination

Method	Linearity (μmol L^−1)	Detection limit (nmol L^−1)	Remarks
a FL: fluorescence; EC: electrode chemistry; HPLC: high performance liquid chromatography.
FL^a^49,50	0.2–66.6	75.5	Rapid, effective, and selective, but the sensitivity is lower
	0.08–49.13	0.02
EC^a^51–53	0.02–0.25	0.03	Low cost and sensitive yet requires much time and several complicated steps to fabricate functionalized electrodes
	0.4–100.0	0.2
	0.5–900.0	0.1
HPLC^a^54	0.001–0.02	0.4	Effective and selective, but needs expensive equipment, and uses toxic organic solvents
Gold nanoparticles^55	0.1–120.0	0.02	Effective, and selective but the reagents are expensive
CdTe quantum dots^56	0.2–3.2	0.08	Complicated steps and the reagents are toxic
FL^a (this work) spectrophotometric	0.04–12.0	0.01	Eco-friendly, fast, facile and sensitive
	0.08–3.2	0.02

---

3.6. Determination of 6-MP in blood serum samples

In order to evaluate the practicality of the proposed methods, blood serum samples were chosen for investigation. The analysis of real samples with 500 times diluted serum samples was performed using the proposed methods. 500 μL sample solution was used based on the procedure described in Section 2.3. The recovery was detected by the standard addition method and the results of the above determination are listed in Table 3, from which it can be seen that the proposed methods had good accuracy (recovery of 98.6–103.0% and 96.7–101.1%, respectively) and repeatability (RSD of 2.8–3.3% and 2.7–3.1%, respectively), and could be successfully applied to the analysis of 6-MP in blood serum samples.

Table 3 Determination of 6-MP in blood serum samples

Method	Serum sample	Found (μmol L^−1)	Added (μmol L^−1)	Total found (μmol L^−1)	Recovery (%)	RSD (%, n = 5)
a ND: not found. FL: fluorescence.
Spectrophotometric	1	ND^a	0.4	0.41	102.5	2.8
	2	ND^a	0.7	0.68	98.6	3.1
	3	ND^a	1.0	1.03	103.0	3.3
FL^a	1	ND^a	0.3	0.29	96.7	2.7
	2	ND^a	0.6	0.59	98.3	2.9
	3	ND^a	0.9	0.91	101.1	3.1

---

4. Conclusions

In conclusion, fluorescence quenching and spectrophotometric methods for the detection of 6-mercaptopurine (6-MP) using carbon dots (CDs) as a fluorescence probe is established. With the proposed CD-based fluorescence probe, the prepared sensor exhibits a lower detection limit and wider linear range compared with most other 6-MP sensors. Furthermore, this sensor possesses simplicity of preparation, high sensitivity, good stability and reproducibility. The excellent performance of the proposed CD-based fluorescence probe would provide a promising platform for the potential for practical application.

Acknowledgements

The authors gratefully acknowledge financial support for this study by grants from the National Natural Science Foundation of China (Grant no. 21575117) and the Special Fund of Chongqing Key Laboratory (CSTC).

References

1. S. Sahasranaman, D. Howard and S. Roy, Eur. J. Clin. Pharmacol., 2008, 64, 753–767.
2. O. H. Nielsen, B. Vainer and J. Rask-Madsen, Aliment. Pharmacol. Ther., 2001, 15, 1699–1708.
3. X. C. Shen, L. F. Jiang, H. Liang, X. Lu, L. J. Zhang and X. Y. Liu, Talanta, 2006, 69, 456–462.
4. H. W. Sun, T. Wang, X. Y. Liu and P. Y. Chen, J. Lumin., 2013, 134, 154–159.
5. P. N. Zhou, L. Q. He, G. L. Gan and S. Y. Ni, J. Electroanal. Chem., 2012, 665, 63–69.
6. S. Shahrokhian, F. Ghorbani-Bidkorbeh, A. Mohammadi and R. Dinarvand, J. Solid State Electrochem., 2012, 16, 1643–1650.
7. M. Keyvanfarda, V. Khosravi, H. Karimi-Maleh, K. Alizad and B. Rezaei, J. Mol. Liq., 2013, 177, 182–187.
8. X. N. Cao, L. Lin, Y. Y. Zhou, G. Y. Shi, W. Zhang, K. Yamamoto and L. T. Jin, Talanta, 2003, 60, 1063–1070.
9. A. F. Hawwa, J. S. Millership, P. S. Collier and J. C. McElnay, J. Pharm. Biomed. Anal., 2009, 49, 401–409.
10. K. M. Naik and S. T. Nandibewoor, J. Anal. Chem., 2013, 68, 1085–1088.
11. R. Zakrzewski, K. Borowczyk, A. Luczak, W. Mlynarski, J. Trelinska and A. Luczak, Bioanalysis, 2013, 5, 869–877.
12. J. M. Rosenfeld, V. Y. Taguchi, B. L. Hillcoat and M. Kawai, Anal. Chem., 1977, 49, 725–727.
13. S. R. Rabel, J. F. Stobaugh and R. Trueworthy, Anal. Biochem., 1995, 224, 315–322.
14. H. F. Yang, Y. L. Liu, Z. M. Liu, Y. J. Yang, H. Jiang, Z. R. Zhang, G. L. Shen and R. Q. Yu, J. Phys. Chem. B, 2005, 109, 2739–2744.
15. S. Shahrokhian, F. Ghorbani-Bidkorbeh, A. Mohammadi and R. Dinarvand, J. Solid State Electrochem., 2012, 16, 1643–1650.
16. X. Xu, R. Ray, Y. Gu, H. J. Ploehn, L. Gearheart, K. Raker and W. A. Scrivens, J. Am. Chem. Soc., 2004, 126, 12736–12737.
17. B. Yin, J. Deng, X. Peng, Q. Long, J. Zhao, Q. Lu, Q. Chen, H. Li, H. Tang, Y. Zhang and S. Yao, Analyst, 2013, 138, 6551–6557.
18. S. Qu, X. Wang, Q. Lu, X. Liu and L. Wang, Angew. Chem., Int. Ed., 2012, 124, 12215–12218.
19. P. J. Ni, H. C. Dai, Z. Li, Y. J. Sun, J. T. Hu, S. Jiang, Y. L. Wang and Z. Li, Talanta, 2015, 144, 258–262.
20. Y. Shi, C. Y. Li, S. P. Liu, Z. F. Liu, J. H. Zhu, J. D. Yang and X. L. Hu, RSC Adv., 2015, 5, 64790–64796.
21. Z. Lin, W. Xue, H. Chen and J. M. Lin, Anal. Chem., 2011, 83, 8245–8251.
22. F. Yan, Y. Zou, M. Wang, X. Mu, N. Yang and L. Chen, Sens. Actuators, B, 2014, 192, 488–495.
23. J. M. Luther and J. L. Blackburn, Nat. Photonics, 2013, 7, 675–677.
24. X. Guo, C. F. Wang, Z. Y. Yu, L. Chen and S. Chen, Chem. Commun., 2012, 48, 2692–2694.
25. K. Xu, F. Liu, J. Ma and B. Tang, Analyst, 2011, 136, 1199–1203.
26. X. L. He, L. Zhang, H. T. Qi, P. Yu, J. J. Fei and L. Q. Mao, Analyst, 2014, 139, 2114–2117.
27. Q. Xi, J. J. Li, W. F. Du, R. Q. Yu and J. H. Jiang, Analyst, 2016, 141, 96–99.
28. H. Goncalves, P. A. S. Jorge, J. R. A. Fernandes and J. C. G. Esteves da Silva, Sens. Actuators, B, 2010, 145, 702–707.
29. H. Ming, Z. Ma, Y. Liu, K. Pan, H. Yu, F. Wang and Z. Kang, Dalton Trans., 2012, 41, 9526–9531.
30. A. Rahy, C. Zhou, J. Zheng, S. Y. Park, M. J. Kim, I. Jang and D. J. Yang, Carbon, 2012, 50, 1298–1302.
31. Z. A. Qiao, Y. Wang, Y. Gao, H. Li, T. Dai, Y. Liu and Q. Huo, Chem. Commun., 2010, 46, 8812–8814.
32. Y. M. Guo, Z. Wang, H. W. Shao and X. Y. Jiang, Carbon, 2013, 52, 583–589.
33. H. L. Tao, X. F. Liao, Q. Y. Wu, X. L. Xie, F. X. Zhong, Z. S. Yi, M. Qin and Z. L. Wu, Spectrochim. Acta, Part A, 2016, 153, 268–272.
34. F. Wang, Z. Xie, H. Zhang, C. Y. Liu and Y. G. Zhang, Adv. Funct. Mater., 2011, 21, 1027–1031.
35. J. H. Zhu, M. M. Li, S. P. Liu, Z. F. Liu, Y. F. Li and X. L. Hu, Sens. Actuators, B, 2015, 219, 261–267.
36. F. Wang, Y. H. Chen, C. Y. Liu and D. G. Ma, Chem. Commun., 2011, 47, 3502–3504.
37. Y. H. Wang, L. Bao, Z. H. Liu and D. W. Pang, Anal. Chem., 2011, 83, 8130–8137.
38. J. Zong, X. L. Yang, A. Trinchi, S. Hardin, I. Cole, Y. H. Zhu, C. Z. Li, T. Muster and G. Wei, Biosens. Bioelectron., 2014, 51, 330–335.
39. S. S. Wee, Y. H. Ng and S. M. Ng, Talanta, 2013, 116, 71–76.
40. L. P. Lin, X. X. Wang, S. Q. Lin, L. H. Zhang, C. Q. Lin, Z. M. Li and J. M. Liu, Spectrochim. Acta, Part A, 2012, 95, 555–561.
41. D. A. Tekdas, M. Durmus, H. Yanıka and V. Ahsena, Spectrochim. Acta, Part A, 2012, 93, 313–320.
42. R. L. Duan, C. Y. Li, S. P. Liu, Z. F. Liu, Y. F. Li, J. H. Zhu and X. L. Hu, J. Taiwan Inst. Chem. Eng., 2015, 50, 43–48.
43. E. Vaishnavi and R. Renganathan, Spectrochim. Acta, Part A, 2013, 115, 603–609.
44. C. X. Hao, S. P. Liu, W. J. Liang, D. Li, L. L. Wang and Y. Q. He, Microchim. Acta, 2015, 182, 2009–2017.
45. Y. Guan, W. Zhou, X. H. Yao, M. P. Zhao and Y. Z. Li, Anal. Chim. Acta, 2006, 570, 21–28.
46. Y. Y. Yuan, X. Huang, S. P. Liu, J. D. Yang, R. L. Duan and X. L. Hu, RSC Adv., 2016, 6, 3393–3398.
47. Z. Li, Y. N. Ni and S. Kokot, Biosens. Bioelectron., 2015, 74, 91–97.
48. J. Wang, L. Kong, W. Shen, X. L. Hu, Y. Z. Shen and S. P. Liu, Anal. Methods, 2014, 6, 4343–4352.
49. X. Y. Li, Z. Shang, H. Wei and J. D. Yang, Luminescence, 2013, 28, 294–301.
50. L. Wang and Z. J. Zhang, Talanta, 2008, 76, 768–771.
51. H. W. Sun, T. Wang, X. Y. Liu and P. Y. Chen, Luminescence, 2013, 134, 154–159.
52. X. N. Cao, L. Lin, Y. Y. Zhou, G. Y. Shi, W. Zhang, K. Yamamoto and L. T. Jin, Talanta, 2003, 60, 1063–1070.
53. M. Keyvanfarda, V. Khosravi, H. Karimi-Maleh, K. Alizad and B. Rezaei, J. Mol. Liq., 2013, 177, 82–187.
54. A. P. Li, J. D. Peng, M. Q. Zhou and J. Zhang, Spectrochim. Acta, Part A, 2016, 154, 1–7.
55. Z. Li, Y. N. Ni and S. Kokot, Biosens. Bioelectron., 2015, 74, 91–97.
56. M. X. Gao, J. L. Xu, Y. F. Li and C. Z. Huang, Anal. Methods, 2013, 5, 673–677.

---