Microwave assisted one-step green synthesis of fluorescent carbon nanoparticles from ionic liquids and their application as novel fluorescence probe for quercetin determination

Journal of Luminescence 140 (2013) 120–125

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Microwave assisted one-step green synthesis of fluorescent carbon nanoparticles from ionic liquids and their application as novel fluorescence probe for quercetin determination Deli Xiao a,1, Danhua Yuan a,1, Hua He a,b,n, Mengmeng Gao a a b

Department of Analytical Chemistry, China Pharmaceutical University, Nanjing 210009, China Key Laboratory of Drug Quality Control and Pharmacovigilance, Ministry of Education, China Pharmaceutical University, Nanjing 210009, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 September 2012 Received in revised form 14 January 2013 Accepted 14 February 2013 Available online 13 March 2013

In this study, a new sensitive and convenient method for the determination of quercetin based on the fluorescence quenching of fluorescent carbon nanoparticles (CNPs) was developed. The CNPs derived from ionic liquids were prepared using a green and rapid microwave-assisted synthetic approach for the first time. The one-step green preparation process is simple and effective, neither a strong acid solvent nor surface modification reagent is needed, which makes this approach very suitable for largescale production. The prepared CNPs were characterized by high-resolution transmission electron microscopy, Fourier transform infrared spectrometry, elemental analysis and spectrofluorometry. In NH3–NH4Cl buffer solution (pH 9.47), the fluorescence signals of CNPs decreased obviously with increase of the quercetin concentration. The effect of other coexisting foreign substances on the intensity of CNPs showed a low interference response. Under the optimum conditions, the fluorescence intensity presented a linear response versus quercetin concentration according to the Stern–Volmer equation with an excellent 0.9989 correlation coefficient. The linearity ranged from 2.87  10  6 to 31.57  10  6 mol L  1 with the detection limit (3s) of 9.88  10  8 mol L  1. The recovery of this method was in the range of 93.3–105.1%. Therefore, the CNPs could to be a promising candidate as a fluorescence probe for the detection of trace levels of quercetin due to their advantages in low-cost production, low cytotoxicity, strong fluorescence and excellent biocompatibility. & 2013 Elsevier B.V. All rights reserved.

Keywords: Microwave Fluorescent carbon nanoparticles Ionic liquids Fluorescence probe Quercetin

1. Introduction Flavonoids, commonly found in fruits, vegetables and some beverages, represent a large group of plant phenols [1]. Flavonoids are derived from heterocyclic 2-phenylbenzopyrone. Commonly all the three cycles are substituted by hydroxyl groups or methoxy groups and discrete derivatives differ in the stage of substitution and oxidation [2]. Quercetin (Fig. 1) is a typical flavonol which is abundant in fruits and vegetables. During the past years, quercetin has drawn extensive attention as it displays a variety of biological activities including cardiovascular protection, anticancer activity, antiulcer effects, antiallergy activity, cataract prevention, antiviral activity and anti-inflammatory effects. Most of these beneficial effects were known due to the antioxidant activity of quercetin [3], thus it is of significance to develop an appropriate approach to detecting it. Hitherto, n Corresponding author at: China Pharmaceutical University, 24 Tongjia Lane, Nanjing 210009, Jiangsu province, China. Tel./fax: þ 86 025 83271505. E-mail addresses: [email protected], [email protected] (H. He). 1 Authors contributed equally.

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.02.032

numerous analytical methods have been employed for the quantification of quercetin, such as high-performance liquid chromatography with UV absorption detector [4] or chemiluminescence detection [5], electrophoresis with a diode array detector [6] and electroanalytical methods [7,8]. Several fluorescence methods [9,10] have also been reported, which were considered to be the most promising methods due to their high sensitivity. As novel fluorescence indicators, fluorescent semiconductor quantum dots (QDs) [11–16] have attracted considerable attention and could be widely used in biomedical and pharmaceutical analysis. However, heavy metals which are essential elements in these conventional semiconductors, are of restrictive use for concerns about their toxicity, stability and environmental hazards [17–19]. So how to fabricate benign nanomaterials with similar optical properties is an interesting challenge and have inspired intensive research efforts. In recent years, a new type of visible emitters have been reported exclusively based on fluorescent carbon nanoparticles (CNPs) [20–25]. They appear to be a promising alternative to traditional toxic metal-based semiconductor QDs in many fields due to their advantages in strong fluorescence, low cytotoxicity and excellent biocompatibility.

D. Xiao et al. / Journal of Luminescence 140 (2013) 120–125

Several methods of preparing eco-friendly fluorescent CDs or CNPs have been reported, and can be generally classified into two main groups: top-down and bottom–up methods [26]. The topdown method is to etch a larger carbon structure into individual nanoparticles, such as arc-discharge single-walled carbon nanotubes [27], laser ablation of graphite [20], electrochemical oxidation of graphite and multiwalled carbon nanotubes [28,29], carbonizing polymerized resols on silica spheres [30], chemical oxidation soots of candles, natural gas, commercially activated carbon and lampblack [31–33] and chemical oxidation of oxide graphene [34]. While the bottom–up method is to form nanoparticles from molecular precursors through chemical and thermal oxidation or microwave pyrolysis of carbonaceous compounds [25,35–39]. However, most of these synthesis methods involve expensive starting materials, great energy-consuming devices, intricate processes and the as-synthesized CDs or CNPs. Typically, the CDs or CNPs are always required to be oxidized by strong acid and further surface-passivated by surface modification reagent to improve the water solubility of these nanoparticles and modify the PL properties. Therefore, it is still a critical issue to design an economical, facile, effective and green synthetic route to produce strong fluorescent CDs or CNPs on a large scale. Herein, we present an economical, facile, effective and green microwave pyrolysis approach to synthesize fluorescent CNPs. A characteristic feature of this one-step approach is that the formation and functionalization of CNPs are accomplished simultaneously through the microwave pyrolysis of the ionic liquids, neither a strong acid solvent nor surface modification reagent is needed. The synthetic process occurs in a domestic microwave oven using inexpensive ionic liquids as the necessary source of carbon which has the advantage of being relatively cheap and absolutely ‘‘green’’. Recently, many methods for the determination of metallic ions, such as Hg2 þ , Ag þ , Cd2 þ , Fe2 þ , Cu2 þ and Pb2 þ using fluorescent nanoparticles have been reported, which are based on the quenching or enhancement of the fluorescent intensity of the fluorescent nanoparticles. To broaden applicability of fluorescent nanoparticles in new areas, we explored a method based on the fluorescent quenching of CNPs by quercetin. In this study, we proposed a new kind of nanometer-sized fluorescent particles by a microwave-heating route. To the best of our knowledge, microwave assisted one-step green synthesis of CNPs derived from ionic liquids has not been reported so far. It was found that the fluorescence intensity of CNPs quenched in the presence of quercetin, and the quenched intensity of fluorescence was

121

proportional to the concentration of quercetin. Based on this phenomenon, CNPs were employed as fluorescence probes for the determination of quercetin. This method was sensitive, rapid, accurate and simple, and could be used as a new and reliable means for the quantitative determination of therapeutic agents.

2. Experimental 2.1. Apparatus UV–vis absorption was characterized by a UV1800 UV–vis spectrophotometer (Shimadzu Corporation, Japan). Photoluminescence (PL) emission measurements were performed using a RF-5301PC fluorescence spectrophotometer (Shimadzu Corporation, Japan). The morphology of the as-synthesized nanoparticles was studied using a FEI Tecnai G2 F20 transmission electron microscope (TEM) and a IX71 inverted research microscope (Olympus, Japan). The surface groups on CNPs were measured with a 8400s FTIR spectrometer (Shimadzu Corporation, Japan). Elemental analysis was acquired with a Elementar Vario ELIII.

2.2. Materials and reagents 1-butyl-3-methylimidazolium tetrafluoroborate was purchased from Lanzhou Green hem ILS, LICP. CAS. China. Quercetin was purchased from shanghai China. All the chemicals were used as received without further purification.

2.3. Preparation of CNPs As shown in Scheme 1, the fluorescent CNPs were prepared by a facile green route of microwave-assisted synthetic approach. In a typical synthesis, 0.5 g of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4) was mixed with 20 mL of distilled water under stirring to form a clear transparent solution in a beaker. Then, the mixed solution was put into a domestic microwave oven (700 W) and heated for different time periods. The color-changed solution was centrifuged at 13,000 rpm for 30 min to remove less-fluorescent deposit. Finally, a clear yellowbrown aqueous solution containing CNPs was obtained.

2.4. Determination of quercetin

Fig. 1. Structure of quercetin.

In a 10 mL volumetric flask was successively placed 10 mL of 3.69  10  6 mol L  1 CNPs and an appropriate volume of quercetin solution. The mixture was then diluted to the mark with NH3–NH4Cl buffer solution (0.10 mol L  1, pH 9.47) and mixed thoroughly. The fluorescent spectra were obtained by scanning the emission from 300 to 800 nm on the spectrofluorimeter (with 5 and 5 nm slit width for excitation and emission, respectively), and F and F0 which are the FL intensity of the CNPs at a given quercetin concentration and in a quercetin-free solution were measured at 354 nm.

Scheme 1. A schematic illustration of the preparation procedure of CNPs by microwave pyrolysis.

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3. Results and discussion 3.1. Preparation of CNPs The synthetic approach included the following procedures: (1) mixing [Bmim]BF4 and distilled water thoroughly; and (2) heating the mixture in the microwave oven. The synthetic process occured in a domestic microwave oven using just [Bmim]BF4 as the necessary source of carbon and nitrogen, which has the advantage of being relatively cheap and absolutely ‘‘green’’. In this experiment, several CNPs were designed by varying microwave irradiation time, and Table 1 showed the photoluminescence quantum yield of the fluorescent CNPs for various periods of microwave pyrolysis time. It could be seen that the photoluminescence quantum yield changed when microwave pyrolysis time varied from 3.5 to 8.5 min and the maximum quantum yield was achieved at 7.5 min. These data indicated that the formation of CNPs is fast and facile.

3.2. Characterization of CNPs The UV–vis absorption and fluorescence spectra of CNPs are obtained and shown in Fig. 2. The fluorescence intensity at 354 nm was used for quantitative analysis. It can be seen that the line width of the fluorescence spectra was relatively narrow and symmetric, which indicated that the as-prepared CNPs were nearly monodisperse and homogenous. Using quinine sulfate as a standard, the quantum yield of the CNPs was 0.0518 (Table 2), and these values were comparable to previous reports [37,40]. Table 1 Photoluminescence quantum yield of fluorescent carbon nanoparticles for various periods of microwave pyrolysis time. Time (min)

3.5

4.5

5.5

6.5

7.5

8.0

8.5

Quantum yield

0

0.0165

0.0349

0.0463

0.0518

0.0517

0.0514

200

300

400

500

3.3. The fluorescence quenching of CNPs by quercetin Fig. 6 illustrated the emission spectra of CNPs and their fluorescence titration with quercetin. It was observed that the fluorescence intensity of CNPs at 435 nm (excitation 354 nm) decreased significantly with the increase of quercetin concentrations. Considering this significant quenching of fluorescence intensity, the possibility of developing sensitive methods for quercetin based on spectrofluorometry were evaluated. 3.4. Effect of pH

PL Intensity

Absorbance

Excitation 354 375 395 405

The transmission electron microscopic (TEM) image and FL images of CNPs were showed in Fig. 3. The TEM image suggested that these nanoparticles were uniform and monodispersed with spherical shape, and the size of the particle was 4.2571.85 nm (for CNPs) in diameter. The chemical composition of these nanoparticles was further determined by collecting the corresponding energy-dispersed spectroscopy (EDS) results, as shown in Fig. 4. The peaks of C, N and O elements were observed, indicating that these nanoparticles were formed by [Bmim]BF4 and H2O. Furthermore, the peaks of F, S, Si and Cu elements were also observed, which were originated from [Bmim]BF4 adsorbed on the surface of CNPs and glass substrate used for EDS analysis. The elemental analysis implied that the composition of the CNPs was C 48.97 wt%, N 8.56 wt% and H 5.72 wt% (Table 3). The carbon content of the CNPs increased obviously after carbonization, which was mainly attributed to the loss of nitrogen and hydrogen in the microwave pyrolysis process. Fourier transform infrared (FT-IR) spectra were taken by using KBr to observe the functional groups of CNPs (A) and ionic liquids (B). As shown in Fig. 5, both A and B displayed many common characteristics in their spectra. FTIR spectra showed that the peaks at 3445 cm  1 and 3142 cm  1 were attributed to the stretching vibrations of aromatic C–H and aliphatic C–H respectively. The peaks at 2962 cm  1, 2870 cm  1 and 1300 cm  1 corresponded to the asymmetric and symmetric stretching vibrations of C–H bonds.There were typical peaks around 1060 cm  1, which were ascribed to stretching vibrations of B–F. The characteristic absorption bands of aromatic C–N and C–C heterocycles at 1465 cm  1 and 1572 cm  1 were observed, which revealed that the local structure of nanoparticles obtained is composed of CN units [25,41].

600

Wavelength(nm) Fig. 2. UV–vis absorption and fluorescence spectra (excited at 354 nm) of CNPs.

Table 2 The quantum yields of CNPs. Substance Integrated emission intensity (I)

Abs. at 355 nm

Refractive index

Quantum yield of solvent (Z) (%)

Quinine sulfate CNPs

51424.233

0.006

1.33

0.5400 (known)

18101.791

0.022

1.33

0.0518

In order to develop a sensitive spectrophotometric method for determination of quercetin, we studied the effect of the buffer solution. The influence of various buffers on the relative fluorescence intensity of the system was studied by using Tris–HCl, HAc– NaAc, KH2PO4–Na2B4O7 and NH3–NH4Cl buffers at various pH values, and a satisfactory result for the detection of quercetin was obtained in NH3–NH4Cl buffers. At the same time, the use of different buffer pH could lead to a drastic change of fluorescence intensity of CNPs, which could influence the sensitivity and selectivity of target materials. In order to develop a sensitive spectrophotometric method for determination of quercetin, the effect of pH value of the solution on the relative fluorescence intensity (F0/F) was studied and the results were shown in Fig. 7. The effect of pH between 8.94 and 10.15 was studied in order to select the optimum conditions for the determination. Since F0/F was the highest at pH 9.47, the optimal pH was chosen to be 9.47 in this study. 3.5. Effect of Ionic strength The effect of different concentrations of NH3–NH4Cl buffer solution was invested and the results were shown in Fig. 8. It exhibited that the fluorescence intensity of the complex changed

D. Xiao et al. / Journal of Luminescence 140 (2013) 120–125

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Fig. 3. TEM images and their size distributions for CNPs (A) and fluorescence microscopy images of CNPs (B).

Flu o rescence In ten si ty

600

1 400

13 3 200

0

400

500

600

Wavelength(nm) Fig. 6. Fluorescence emission spectra of CNPs in the absence and presence of different concentrations of quercetin; lem ¼354 nm; concentration of CNPs, 0.369 mmol L  1; quercetin from (1) to (13): 0, 2.87, 5.74, 8.61, 11.48, 14.35, 17.22, 20.09, 22.96, 25.83, 28.70 31.57 and 34.44  10  6 mol L  1.

Fig. 4. EDS spectrum of CNPs thus formed.

Table 3 Elemental analysis of [Bmim]BF4 and CNPs.

2.2

Element contents

[Bmim]BF4 (calculated) CNPs

2.0

C (%)

N (%)

H (%)

42.47 50.32

12.39 8.21

6.64 5.58

F0/F

Sample

1.8 1.6 1.4 1.2 8.6

8.8

9.0

9.2

9.4

9.6

9.8 10.0 10.2

pH Fig. 7. Influence of pH on the relative fluorescence intensity, concentration of CNPs, 0.369 mmol L  1; and quercetin 8.61  10  6 mol L  1.

significantly when concentrating ranging from 0.05 to 0.20 mol L  1 at pH 9.47. Since the intensity at 0.10 mol L  1 was the highest and remained stable, the optimal concentration of NH3–NH4Cl was chosen as 0.10 mol L  1 in this study. 3.6. Effect of reaction time

Fig. 5. FT-IR spectrum of CNPs (A) and ionic liquids (B).

At room temperature, the effect of time on the fluorescence intensity of the system was detected. As shown in Fig. 9, the

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Table 4 Interference of coexisting foreign substances on the relative fluorescence intensity.

1.6

F0/F

1.5 1.4 1.3 1.2 1.1 0.04

0.08 0.12 0.16 CNH3-NH4Cl(mol/L)

0.20

Fig. 8. Influence of the concentration of NH3–NH4Cl on the relative fluorescence intensity, NH3–NH4Cl buffer( pH 9.47 ), from left to right, the concentration of the NH3–NH4Cl is 0.05, 0.08, 0.10, 0.12, 0.15, and 0.20 mol L  1.Concentration of CNPs, 0.369 mmol L  1; quercetin 8.61  10  6 mol L  1.

Coexisting substance

Coexisting concentration (mol L  1)

Change of fluorescence intensity (%)

NaCl NaBr KI KNO3 KBr CaCl2 BaCl2 ZnSO4 NaMoO4 CONO3 BSA leucine tyrosine Penicillin V K

3.3  10  3 3.3  10  3 3.3  10  4 3.3  10  3 3.3  10  3 3.0  10  4 3.3  10  4 3.0  10  5 3.0  10  5 3.0  10  5 5.0  10  5 2.7  10  5 6.3  10  6 6.7  10  6

þ0.93 þ 1.50 þ 1.17 þ 1.99 þ 2.72 þ0.89 þ0.97 þ0.64  0.02 þ 3.06 þ0.75  0.49 þ 1.14  2.43

Concentrations: CNPs, lem ¼ 354 nm; pH ¼9.47.

0.369 mmol L  1;

quercetin

2.87  10  6 mol L  1,

2.0 Table 5 Determination of quercetin Sample by proposed method.

F0/F

1.5

1.0

Quercetin sample

Added (mol L  1)

Average founded (mol L  1)

Recovery (%) (n¼ 10)

RSD (%)

1 2 3

1.80  10  6 2.00  10  5 2.90  10  5

1.74  10  6 1.92  10  5 2.65  10  5

93.3–102.3 94.6–105.1 96.3–104.6

2.9 2.5 2.2

0.5

0.0

obtained from quercetin measurement fit the following equation:

0

5

10

15

20

25

30

35

F 0 =F ¼ 1þ 9:39  106 ½S

40

Time/min Fig. 9. Effect of the reaction time on fluorescence intensity of CNPs in the absence and presence of quercetin.

fluorescence relative intensity changed slightly and tended to be stable for more than 30 min. Therefore, this system exhibited rapid reaction rate and good stability.

In our study, the good linear relationship was obtained using a modified Stern–Volmer plot. A good linear relationship was observed up to quercetin concentration ranging from 2.87  10  6 to 31.57  10  6 mol L  1 with a correlation coefficient of 0.9989. KSV was found to be 9.39  106 L mol–1. And the detection limit, calculated following the 3s IUPAC criteria, was 9.88  10  8 mol L  1. 3.9. Precision and accuracy

3.7. Interference of coexisting foreign substances In order to assess the selectivity of the proposed method, the tolerance of levels of coexisting foreign substances was researched. As seen in Table 4, this method was free from interference of most of metal ions and bovine serum albumin (BSA). Hence, the present procedure could be used for direct determination of quercetin in biological fluids and environmental samples. The experimental results manifested that the method had a high selectivity. 3.8. Calibration curves and sensitivity Fig. 6 exhibits the PL quenching responses of quercetin to the CNPs. The PL quenching followed the Stern–Volmer equation below: F 0 =F ¼ 1 þK SV ½S

ð1Þ

F and F0 are the fluorescent intensities of the CNPs at a given quercetin concentration and in a quercetin free solution respectively. [S] is the concentration of quercetin, and KSV is the quenching constant of the quencher. The experimental data

To assess the precision and accuracy of this method, determinations were carried out for a set of 10 measurements of 1.80  10  6, 2.00  10  5 and 2.90  10  5 mol L  1 quercetin under the same condition respectively. Table 5 showed that the quantification results of quercetin were in good agreement with the declared values, and the RSD was 2.9%, 2.5% and 2.2%. Table 6 summarized the detection limit, linear range and recovery with different methods for the determination of quercetin. Our method was comparable with voltammetric determination [8], electrophoresis with electrochemical detection [42], HPLC–UV [43] and chemiluminescence sensor [44]. 3.10. Mechanism of the interaction of CNPs with quercetin Several mechanisms have been proposed to explain how quercetin quenched fluorescence of CNPs. Quenching of FL emission from CNPs may occur by several mechanisms: inner filter effects, non-radiative recombination pathways, electron transfer processes and ion binding interaction [45]. Fig. 6 showed that quercetin greatly decreased the fluorescence intensity of the CNPs (F0/F from 1.00 to 4.12) and a red-shift (lem from 435 to 440 nm)

D. Xiao et al. / Journal of Luminescence 140 (2013) 120–125

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Table 6 Detection of quercetin in samples with different methods. Methods

Linear range (mol L  1)

LOD (mol L  1)

Recovery (%) (n¼ 10)

RSD (%)

Reference

Voltammetric determination CE-ED IDLLME–HPLC–UV FI-CL This method

1.0  10  7–2.0  10  5 0.5  10  6–1.0  10  3 0.165  10  8–3.31  10  6 1.4  10  6–1.6  10  4 2.87  10  6–31.57  10  6

2.0  10  9 2.25  10  7 8.60  10  10 9.3  10  7 9.88  10  8

99.2–102.6 96.84 97 96.0–101.2 93.3—105.1

1.7 2.42 2.12 2.72 2.5

[6] [40] [41] [42]

with increase in concentration of quercetin on the emission spectra maximum. We believed that the binding of quercetin to the CNPs made the surface of the particles change, which induced the quench of CNPs fluorescence. Besides, as the concentration of NH3–NH4Cl buffer solution had no significant effect on the system, the ionic strength had a vital effect on the interaction between CNPs and quercetin. We deduced that sulfadiazine may interact with the CNPs through electrostatic forces. For these reasons, we assumed that the binding of quercetin to the CNPs made the surface of the particles change which induced the quench of CNPs fluorescence [46].

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4. Conclusion In conclusion, a green and rapid microwave-assisted approach is successfully applied in the synthesis of CNPs derived from ionic liquids with high monodispersity, strong fluorescence and good water solubility for the first time. This method possesses two obvious advantages over current synthetic techniques. Firstly, the process is facile without any complex or post-treatment procedures. Secondly, the starting materials are green, eco-friendly and economical. The as-prepared CNPs have been developed for the spectrofluorometric determination of quercetin based on the quenching effect of quercetin on the fluorescence of these CNPs. Under the optimum conditions, a linear relationship between fluorescence intensity ratio of the system and concentration of quercetin in the range from 2.87  10  6 to 31.57  10  6 mol L  1 can be achieved. The results might help in developing new theories and applications for novel fluorescence probe. We have reason to believe that these CNPs derived from ionic liquids would be one of the most promising alternatives to traditional toxic metal-based semiconductor QDs and can be widely used in pharmaceutical, biomedical and environmental analysis.

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This work was supported by Graduate Students Innovative Projects of Jiangsu Province (No. CXZZ11_0812), Zhe Jiang Provincial Natural Science Foundation of China (Grant no. Y4110235) and the Fundamental Research Funds for the Central Universities (Program no. JKY2011008). We are delighted to acknowledge discussions with colleagues in our research group. References [1] [2] [3] [4]

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