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Hollow fluorescent carbon nanoparticles catalyzed chemiluminescence and its application
Journal of Luminescence 179 (2016) 595–601
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Hollow fluorescent carbon nanoparticles catalyzed chemiluminescence and its application Yunfang Wu, Suqin Han n School of Chemistry and Material Science, Shanxi Normal University, Linfen 041004, Shanxi, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 November 2015 Received in revised form 16 July 2016 Accepted 28 July 2016
In this paper, the chemiluminescence (CL) of luminol-H2O2 with cross-linked hollow fluorescent carbon nanoparticles (HFCNs) was reported. The possible CL reaction mechanism was elucidated by means of CL spectra, UV–vis spectra and some radical scavenger experiments. The CL luminophor was 3-aminophthalate, the CL enhancement of luminol-H2O2 system was supposed to originate from the intrinsic catalytic activity of HFCNs, which efficiently catalyzed the decomposition of H2O2 to generate superoxide radical anion in luminol solution. Dipyridamole (DIP) had an inhibitory effect for the luminol-H2O2HFCNs CL system. The decrease CL intensity was linear with the logarithm of DIP concentration in the range of 2.0 10–8–1.0 10–5 mol/L. The detection limit was 3.6 10–9 mol/L. The proposed method was applied for the determination of DIP in tablets and urine samples with satisfactory results. & 2016 Elsevier B.V. All rights reserved.
Keywords: Chemiluminescence Hollow fluorescent carbon nanoparticles Luminol Dipyridamole
1. Introduction Chemiluminescence (CL) is defined as the production of light through a chemical reaction, in which some excited species are formed and deactivated to the ground state with light emission. Additionally, CL has proved to be a useful phenomenon in the laboratory, finding ever increasing applications in analytical chemistry for its high sensitivity, wide linear range, simple instrumentation and lack of background scattering light interference. Fluorescent carbon nanoparticles (FCNs) are generally small oxygenous carbon nanoparticles with good water solubility, low toxicity, high chemical stability and low environmental hazard [1,2]. This new nanoparticles have been successfully used as bioimaging [3], photocatalysis [4] and fluorescence probe [5]. Due to the superior emitting properties of FCNs, they have been applied in CL including ultraweak CL systems [6–10] and oxidants induced direct CL systems [11–13]. These investigations open new sight into the optical characteristics of the FCNs and widen their potential optical application. Dipyridamole (DIP) is widely used for the treatment of cardiovascular diseases because it stimulates a rise in the blood flow through the coronary circulation. It can improve efficiency and decrease tiredness in certain sports, so it is one of the forbidden substances by the International Olympic Committee [14]. Various analytical methods including spectrophotometry [15], photoluminescence [16–18], CL n
Corresponding author. E-mail address: [email protected] (S. Han).
http://dx.doi.org/10.1016/j.jlumin.2016.07.062 0022-2313/& 2016 Elsevier B.V. All rights reserved.
[19,20], electrochemistry [21–23] and chromatography [24–27] have been reported for this purpose. Water-soluble cross-linked hollow fluorescent carbon nanoparticles (HFCNs) and solid fluorescent carbon nanoparticles (SFCNs) were prepared in an automatic method without external heat treatment by simply mixing glacial acetic acid, water and diphosphorus pentoxide in minutes. Luminol-H2O2 CL reaction, a popular CL reaction, has been widely applied for the detection of various substances. In this work, the luminol-H2O2 CL reaction was chosen as a model system and the effect of HFCNs and SFCNs on the CL was explored. The results showed that HFCNs and SFCNs could enhance the CL of the luminol-H2O2 system and the enhanced capability of HFCNs was superior to SFCNs. The enhancement mechanism of HFCNs on luminol CL was investigated. Furthermore, DIP could inhibit the CL intensity of the luminol-H2O2-HFCNs. The analytical application potential for the determination of DIP was exploited.
2. Experimental section 2.1. Materials All the chemicals were analytical reagent grade. 0.01 mol/L luminol stock solution was prepared by dissolving 0.1771 g luminol (Shaanxi Normal University, Xi’an, China) in 0.1 mol/L NaOH, diluting to 100 mL in a brown calibrated flask. The stock solution of 1.0 10–3 mol/L DIP (National Institutes for Food and Drug
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Control, Beijing, China) prepared by dissolving 0.0252 g DIP with a small amount of 0.1 mol/L HCl, diluting with water in a 50 mL volumetric flask. Working solutions of H2O2 were prepared daily with 30% (v/v) H2O2 (Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd., Tianjin, China). Other reagents including glacial CH3COOH, P2O5, NaOH and HCl were purchased from Luoyang Chemical Reagent Co. Ltd. All the solutions were prepared with pure water (Chengdu Ulupure Technology Company, China). 2.2. Apparatus The experiments were performed on a BPCL ultraweak CL analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China) equipping with the IFIS-C intelligent flow injection sampler (Xi’an Remax Analytical Instrument, Shannxi, China). The UV‐vis absorption spectra were studied using a Cary5000 spectrophotometer (Varian, USA). The CL and fluorescence (FL) spectra were measured with a Cary Eclipse fluorescence spectrophotometer (Agilent, Australia). A Fourier transform infrared (FT-IR) spectrum was carried out on a Varian 660-IR spectrometer in the range of 500–4000 cm–1 (Varian, USA). Transmission electron microscopy (TEM) image was recorded by a JEM-2100 transmission electron microscopy (JEOL, Japan). 2.3. Synthesis of HFCNs and SFCNs HFCNs and SFCNs were synthesized according to the literature [28] with some modifications. The reactants for producing HFCNs and SFCNs were the same. For HFCNs, the homogeneous mixture solution of 1 mL glacial acetic acid and 80 μL water was quickly added to 2.5 g P2O5 in a 50 mL beaker without stirring. In this system, the upper temperature was mainly controlled by vaporizing the glacial acetic acid at its boiling point (117 °C). The nanobubbles of glacial acetic acid vapor then served as the templates for hollow structures. Finally, the HFCNs in dark brown were collected by dispersing in water, followed by adjustment of the pH to 7.0 with NaOH, diluting to 100 mL in a brown volumetric flask. The obtained HFCNs solution was stable for at least one month in the refrigerator. For SFCNs, a homogeneous mixture solution of 1 mL glacial acetic acid and 5 μL water was quickly added to 1.5 g P2O5 in a 5 mL tube with shake. In room temperature, the SFCNs in yellow were collected by dispersing in water, followed by adjustment of the pH to 7.0 with NaOH, diluting to 100 mL in a brown volumetric flask. The concentrations of HFCNs and SFCNs were 0.35 mol/L calculated by C element in glacial acetic acid. 2.4. Procedures The manifolds of the flow injection CL system were shown in Fig. 1. All solutions were delivered by two peristaltic pumps (P1, P2). P1 was used to deliver HFCNs or SFCNs solution (channel a) and luminol in NaOH solution (channel b) at a flow rate of 2.0 mL/min.
The carrier water (channel c), DIP standard or sample solution (channel d) and H2O2 solution (channel e) were delivered by P2 at a flow rate of 2.5 mL/min. Polytetrafluoroethylene tube was employed to connect all components in the flow system. HFCNs were firstly mixed with luminol at the three-way channel; H2O was used as the carrier for the mixing of HFCNs and luminol. The mixture of HFCNs and luminol finally mixed with DIP and H2O2 solution at the flow cell, in which the CL reaction occurred. The CL signal was detected by the photomultiplier tube (PMT) and recorded with computer. The concentration of DIP was quantified by the relative CL intensity, ΔI¼I0–I, where I0 and I denoted CL intensity in the absence and presence of DIP.
3. Results and discussion 3.1. Spectra of HFCNs and SFCNs Fig. 2(A) showed the UV–vis and FL spectra of HFCNs. A broad absorption around 297 nm and a sharp absorption at 247 nm were observed. The peak at 247 nm was ascribed to π–π* transition of aromatic C¼ C bonds [28], while the shoulder at 297 nm attributed to n–π* transition of C ¼O bonds [28,29]. The FL spectra of the HFCNs had no shift as the excitation wavelength varied. The maximum FL intensity ( 500 nm) was obtained with an excitation wavelength of 400 nm. Fig. 2(B) showed the FT-IR spectrum of the HFCNs. An apparent absorption peak of the OH group at about 3449 cm–1 and an absorption peak of the C¼ O group conjugated with aromatic carbons at 1658 cm–1 appeared. These data showed that the HFCNs were rich in carboxylic groups. A peak at 1542 cm–1 from a conjugated C ¼C stretching vibration was observed, indicating unsaturated carbon bonds formed during the carbonization process. TEM image of HFCNs was displayed in Fig. 2 (C) and the HFCNs were cross-linked with each other. Fig. 2 (D) showed the UV–vis and FL spectra of SFCNs. There were two absorption peaks of SFCNs, one was at 297 nm and another at 247 nm. This was consistent with the absorption of HFCNs. The maximum FL emission ( 500 nm) was obtained with an excitation wavelength of 400 nm and the emission shifted with the excitation wavelength. The sizes of prepared SFCNs were less than 10 nm (Fig. 2(E)). 3.2. The effect of HFCNs and SFCNs for luminol CL system The enhanced effect of HFCNs and SFCNs for luminol-H2O2 CL was studied (Fig. 3). Added SFCNs into the luminol-H2O2 CL system brought about 2-fold CL intensity enhancement and the HFCNs could enhance the CL intensity about 26 times. The results showed that HFCNs exhibited higher sensitive effect on the luminol-H2O2 CL reaction. So, HFCNs was selected as enhancer of luminol-H2O2 system in subsequent measurements. 3.3. Measurement of quantum yield of HFCNs The quantum yield (QY) of the HFCNs was calculated using following function:
Φ ¼ ΦR
Fig. 1. Schematic diagram of the FI-CL system. a: HFCNs or SFCNs solution; b: luminol solution; c: carrier stream; d: DIP standard or sample solution; and e: H2O2 solution. P1 and P2: peristaltic pumps; and F: CL flow-cell.
I AR η2 2 IR A ηR
Quinine sulfate in 0.1 mol/L H2SO4 (literature quantum yield 0.54 at 360 nm) was chose as a standard. Where Φ was the quantum yield, I was the measured integrated emission intensity, η and A was the refractive index and optical density, respectively. The subscript R referred to the reference fluorophore of known quantum yield. In order to minimize re-absorption effects the
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Fig. 2. UV–vis and FL spectra of HFCNs (A) and SFCNs (B). FT-IR spectra of HFCNs (C).
optical densities in the 20 nm fluorescence cuvette were kept under 0.1 at the excitation wavelength. An excitation slit width of 2.5 nm was used to excite the HFCNs samples and to record their photoluminescence spectra, which exhibited excellent optical properties (15.5% FL quantum yields). 3.4. Inhibition of DIP on the HFCNs-enhanced luminol CL The effect of DIP on luminol-H2O2 CL with and without HFCNs was studied. Fig. 4 showed the kinetic curves of the CL system in the static injection mode. A weak and rapid CL emission was observed when luminol was mixed with H2O2. Whereas, the addition of HFCNs could highly enhance the CL intensity and the maximum CL signal appeared about 2.5 s after reaction. In addition to that,
luminol-H2O2 and luminol-H2O2-HFCNs in the presence of DIP caused decrease of CL intensity for the original reaction, respectively. Therefore, the luminol-H2O2-HFCNs CL system was chosen to establish the method for the determination of DIP in pharmaceutical preparations. The strongest inhibition was obtained by optimizing the CL parameters. Fig. 4 also showed that HFCNs could generate CL with H2O2, but the CL signal was very weak, therefore, it was unsuitable for the determination of DIP. 3.5. Optimization of reaction conditions Fig. 5(A) showed the effect of H2O2 concentration on the CL reaction. The CL increased with H2O2 concentration up to 0.07 mol/L and decreased above 0.085 mol/L. When the concentration of H2O2
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CL intensity
1500
3.6. Interference The influence of potentially interfering species was studied using a standard solution of 2.0 10–6 mol/L DIP under the optimum working conditions. When the influence of interference on the peak height was not higher than 5%, it was not considered to interfere with the determination of DIP. Substances that did not interfere with the determination of DIP included 1000-fold K þ , Na þ , Li þ , NH4þ , Ca2 þ , Cl , NO3 , SO24 and PO34 ; 500-fold Pb2 þ , Mg2 þ , Zn2 þ , magnesium stearate, lactose, starch and dextrin; 100fold CO23 , EDTA, citric acid, lactic acid, salicylic acid, sucrose and glucose; 50-fold HCO3 , Ac , Fe3 þ , Fe2 þ , carbamide, uric acid and mannitol, and 10-fold Cu2 þ , Co2 þ , leucine, tryptophan, proline, tyrosine, lysine and phenylalanine. The amount of the potentially interfering species in the samples was below their tolerable levels or could be decreased with diluting, so there would be no interference from these species in DIP determination.
1000
500
0 0
100
200
300
Time (s) Fig. 3. CL intensity of (a) luminol-H2O2, (b) luminol-H2O2-SFCNs and (c) luminolH2O2-HFCNs.
Under the optimum conditions, the CL response was found to be linear in the DIP concentration range of 2.0 10–8–1.0 10– 5 mol/L. The regression equation was ΔI ¼2667.4log C þ4880.9 (C ¼10–6 mol/L, r ¼0.9998). The detection limit according to IUPAC was 3.6 10–9 mol/L and the relative standard deviation (RSD) was 2.7% for eleven replicate determination of 2.0 10–6 mol/L DIP. Fig. 6 showed the calibration graph and typical CL responses for the determination of DIP. The results indicated that this CL system had a good linearity, relatively high sensitivity and suitable precision.
16000 e
12000
CL intensity
3.7. Analytical performances
8000 a
3.8. Determination of DIP in tablets and human urine
4000
0 0
10
20
30
Times (s) Fig. 4. CL kinetic curves: (a) H2O2-HFCNs; (b) luminol-H2O2-DIP; (c) luminol-H2O2; (d) luminol-H2O2-HFCNs-DIP and (e) luminol-H2O2-HFCNs.
was 0.08 mol/L, the most suitable CL signal was recorded and the CL emission was the strongest. The concentrations of H2O2 higher than 0.085 mol/L caused low CL intensity and many gaseous bubbles appeared in the waste solution. The phenomenon could be explained by the H2O2 decomposing in alkaline solution, resulting in unstable CL signals. Therefore, the system was operated at 0.08 mol/L H2O2 throughout this study. For the NaOH concentration, the strongest CL signal was obtained at 0.05 mol/L (Fig. 5(B)). Thus, 0.05 mol/L NaOH was employed. The concentration of HFCNs in the range of 0.007–0.03 mol/L was investigated (Fig. 5(C)). The result indicated that the CL increased with the concentration of HFCNs up to 0.02 mol/L, and then remained constant. So, 0.02 mol/L HFCNs was adopted. The influence of luminol concentration was tested. As shown in Fig. 5(D), the CL increased by the luminol concentration up to 7.0 10–7 mol/L and remained constant. Therefore, the luminol concentration of 7.0 10–7 mol/L was adopted. The effects of the flow rate of reagents on the CL were investigated. The strongest relative CL signal was obtained when the flow rate of P1 up to 2.0 mL/min, and P2 up to 2.5 mL/min. Hence, flow rate of 2.0 mL/ min for P1 and 2.5 mL/min for P2 were selected for the following experiment.
The developed method was applied to the determination of DIP in tablets. The commercial tablets from different manufacturers were purchased from the local pharmacy. Ten DIP tablets were carefully weighed and ground powder. A portion equivalent to one tablet was accurately weighed and dissolved in 25 mL 0.01 mol/L HCl. The solution was sonicated for 15 min and centrifuged for 10 min at 6000 rpm. A 2.5 mL of supernatant solution was transferred to a 50 mL flask and diluted to the mark with water. The results were collected in Table 1 and the recoveries were between 98.0% and 101.6%. Human urine samples were obtained from healthy volunteers. A certain amount of standard DIP solution was added to 1.0 mL of urine sample in a centrifuge tube and mixed for 2 min. This mixture was diluted to 10 mL with water and then centrifuged for 15 min at 5000 rpm. The supernatant was diluted to the appropriate concentration for the determination of DIP. The recoveries were in the range of 98.5–102% in Table 2. 3.9. Possible CL mechanism The role of nanoparticles in a liquid-phase CL reaction could be as catalysts or emitters [6]. In order to identify the role of the HFCNs in the luminol-H2O2 CL system, CL spectra were measured by a FL spectrometer with the xenon lamp turned off. As shown in Fig. 7(A), the maximum CL emissions for the luminol-H2O2 system in the absence of HFCNs or the presence of HFCNs/DIP were around in 425 nm, which was characteristic wavelength of excited 3-aminophthalate anion (3-APAn) [30]. The CL was produced in this way and the HFCNs themselves did not produce CL. As a result, the enhancement of CL signal derived from the catalytic effects of HFCNs. The UV–vis absorption spectra were recorded. As shown in Fig. 7(B), the light absorption of the luminol-H2O2-HFCNs system
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6000
Relative CL intensity
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Relative CL intensity
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4000
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2000 0.04
0.06
0.08
0.02
0.10
Concentration of H2O2(mol/L)
0.04
0.06
0.08
Concentration of NaOH (mol/L)
6000
Relative CL intensity
Relative CL intensity
6000
5000
4000
5000
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3000 2000 0.007
0.014
0.021
0.028
0.035
2
Concentration of HFCNs (mol/L)
4
6
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10
-7
Concentration of luminol (10 mol/L)
Fig. 5. Effects of reagent concentrations on the CL intensity of (A) H2O2, (B) NaOH, (C) HFCNs and (D) luminol.
7500
7500
CL intensity
Relative CL intensity
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a
6000 4500 3000
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-0.5
0.0
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Time (s)
lgC (10 mol/L)
Fig. 6. The calibration graphs (A) and typical CL responses for the determination of DIP (B). Unit: 10–6 mol/L, from a–p: 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 4.0, 6.0, 8.0, 10.0. Table 1 The determination of DIP in tablets (n¼3). Sample
Nominal content (mg/tablet)
This method (mg)
Added (mg)
Found (mg)
Recovery (%)
RSD (%)
Tablet 1
25
24.77 0.75
– 10 25
– 34.6 70.21 50.17 0.12
– 99.0 101.6
3.04 0.61 0.24
Tablet 2
25
25.17 0.21
– 10 25
– 34.9 70.21 50.2 70.17
– 98.0 100
0.84 0.60 0.34
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CL reaction probably happened in a radical way, in which the generation of free radicals appeared to be the key factors. Thiourea [31] and mannitol [32] were effective radical scavengers for OH. When 0.01 mol/L thiourea or 4.0 10–3 mol/L mannitol was added to the CL system, a distinct inhibition was observed by a factor of 72% or 60%. This indicated that OH was generated in the CL process. When NaN3 was added to the luminol-H2O2-HFCNs system, the CL inhibition percentage increased with the NaN3 concentration. Therefore, it was concluded that singlet oxygen (1O2) participated in the CL reaction [33]. The mechanism of luminol-H2O2 system under alkaline condition in aqueous solution had been widely studied. Also, it had been well confirmed that reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), OH and superoxide radical anion (O2 ) are the most important oxidizing species, and the reaction of luminol-H2O2 in alkaline medium in the absence of a catalyst is relatively slow, so the produced CL emission is weak. It was also reported that H2O2 decomposition on supported metal catalysts such as gold/silver alloy, copper nanoclusters and ZnO nanoparticles [34–36] attributed to the fact that they could accelerate the breakdown of H2O2 to generate more OH. Shi et al. [37] had found carbon nanodots possessed intrinsic peroxidase-like activity. Thus under the effect of HFCNs as peroxidase mimetics H2O2 might be broken up into double OH radicals. The OH then reacted with luminol to facilitate the generation of luminol radical (L ) and O2 , which further reacted with each other to form the 3-APAn, generated CL [38,39]. DIP on the luminol-H2O2-HFCNs CL
was equal to the sum of the light absorption of individual system of HFCNs and luminol-H2O2, which also implied that the catalysis of HFCNs for the reaction between luminol and H2O2. When DIP was added to the mixing solution of luminol-H2O2-HFCNs, the absorption peak of DIP at 400 nm had an obvious red shift, which showed that DIP participated in the reaction. FL spectra were displayed in Fig. 7(C). After mixing HFCNs with luminol-H2O2, the characteristic FL peak of HFCNs at 500 nm decreased and the peak at 425 nm was stronger than that of luminol-H2O2 system, which showed that the reaction of luminol with H2O2 catalyzed by HFCNs. In the luminol-H2O2-HFCNs-DIP system, the FL intensity of DIP at 500 nm severely decreased and the FL peak at 425 nm of luminol-H2O2-HFCNs system almost disappeared. These results demonstrated that the DIP was involved in the CL reaction. The effect of radical scavengers on the CL was studied. When 1.0 10–4 mol/L ascorbic acid was added to the luminol-H2O2HFCNs system, about 65% CL signal was quenched. Therefore, the Table 2 The determination of DIP in fortified human urine samples. Sample
Added (10–7 mol/L)
Determined (10–7 mol/L)
Recovery (%)
RSD
1 2 3
3.0 5.0 7.0
3.2 70.31 5.17 0.21 6.9 70.054
101.6 102.0 98.5
9.69 4.12 0.78
20
1.5
c
h
1.0 Absorbance
CL intensity
15
10
0.5 a
5
b a
0 350
400
0.0 450
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200
400
500
Wavlength (nm)
Wavlength (nm)
e
d
FL intensity
150
100 c 50 b a
0 300
400
500
600
Wavelength (nm) Fig. 7. (A) CL spectra of (a) luminol-H2O2; (b) luminol-H2O2-HFCNs-DIP and (c) luminol-H2O2-HFCNs. (B) UV–vis absorption spectra of (a) H2O2-HFCNs; (b) HFCNs; (c) DIP; (d) H2O2-DIP; (e) luminol-H2O2; (f) luminol-H2O2-HFCNs; (g) luminol-H2O2-DIP and (h) luminol-H2O2-HFCNs-DIP. (C) FL spectra of (a) luminol-H2O2-DIP; (b) luminol-H2O2; (c) luminol-H2O2-HFCNs-DIP; (d) luminol-H2O2-HFCNs; (e) DIP.
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Scheme 1. The CL mechanism of luminol catalyzed by traditional catalyst (A) and HFCNs (B).
system could react with active oxygen species such as OH, O2 and 1O2, and then led to a decrease in CL intensity. Based on the above results, the enhanced mechanism was summarized in Scheme 1.
4. Conclusion HFCNs were prepared without external heat treatment, and it had an enhance effect for the luminol-H2O2 CL system. The enhancement of CL was suggested to attribute to the catalysis of HFCNs as peroxidase mimetics on the radical generation and electron-transfer processed during the luminol CL reaction. DIP containing OH group could inhibit the CL signal of the luminolH2O2-HFCNs system under the optimized experimental conditions, which could be potentially used to detect this compound in tablet and human fluids samples. This work was of great importance for the investigation of new and efficient catalysts for CL system and helpful for understanding of CL mechanism correspondingly.
Acknowledgments This work was financially supported by the Natural Science Foundation of Shanxi Province (Grant no. 2013011013-3).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2016.07.062.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Hollow fluorescent carbon nanoparticles catalyzed chemiluminescence and its application Yunfang Wu, Suqin Han n School of Chemistry and Material Science, Shanxi Normal University, Linfen 041004, Shanxi, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 November 2015 Received in revised form 16 July 2016 Accepted 28 July 2016
In this paper, the chemiluminescence (CL) of luminol-H2O2 with cross-linked hollow fluorescent carbon nanoparticles (HFCNs) was reported. The possible CL reaction mechanism was elucidated by means of CL spectra, UV–vis spectra and some radical scavenger experiments. The CL luminophor was 3-aminophthalate, the CL enhancement of luminol-H2O2 system was supposed to originate from the intrinsic catalytic activity of HFCNs, which efficiently catalyzed the decomposition of H2O2 to generate superoxide radical anion in luminol solution. Dipyridamole (DIP) had an inhibitory effect for the luminol-H2O2HFCNs CL system. The decrease CL intensity was linear with the logarithm of DIP concentration in the range of 2.0 10–8–1.0 10–5 mol/L. The detection limit was 3.6 10–9 mol/L. The proposed method was applied for the determination of DIP in tablets and urine samples with satisfactory results. & 2016 Elsevier B.V. All rights reserved.
Keywords: Chemiluminescence Hollow fluorescent carbon nanoparticles Luminol Dipyridamole
1. Introduction Chemiluminescence (CL) is defined as the production of light through a chemical reaction, in which some excited species are formed and deactivated to the ground state with light emission. Additionally, CL has proved to be a useful phenomenon in the laboratory, finding ever increasing applications in analytical chemistry for its high sensitivity, wide linear range, simple instrumentation and lack of background scattering light interference. Fluorescent carbon nanoparticles (FCNs) are generally small oxygenous carbon nanoparticles with good water solubility, low toxicity, high chemical stability and low environmental hazard [1,2]. This new nanoparticles have been successfully used as bioimaging [3], photocatalysis [4] and fluorescence probe [5]. Due to the superior emitting properties of FCNs, they have been applied in CL including ultraweak CL systems [6–10] and oxidants induced direct CL systems [11–13]. These investigations open new sight into the optical characteristics of the FCNs and widen their potential optical application. Dipyridamole (DIP) is widely used for the treatment of cardiovascular diseases because it stimulates a rise in the blood flow through the coronary circulation. It can improve efficiency and decrease tiredness in certain sports, so it is one of the forbidden substances by the International Olympic Committee [14]. Various analytical methods including spectrophotometry [15], photoluminescence [16–18], CL n
Corresponding author. E-mail address: [email protected] (S. Han).
http://dx.doi.org/10.1016/j.jlumin.2016.07.062 0022-2313/& 2016 Elsevier B.V. All rights reserved.
[19,20], electrochemistry [21–23] and chromatography [24–27] have been reported for this purpose. Water-soluble cross-linked hollow fluorescent carbon nanoparticles (HFCNs) and solid fluorescent carbon nanoparticles (SFCNs) were prepared in an automatic method without external heat treatment by simply mixing glacial acetic acid, water and diphosphorus pentoxide in minutes. Luminol-H2O2 CL reaction, a popular CL reaction, has been widely applied for the detection of various substances. In this work, the luminol-H2O2 CL reaction was chosen as a model system and the effect of HFCNs and SFCNs on the CL was explored. The results showed that HFCNs and SFCNs could enhance the CL of the luminol-H2O2 system and the enhanced capability of HFCNs was superior to SFCNs. The enhancement mechanism of HFCNs on luminol CL was investigated. Furthermore, DIP could inhibit the CL intensity of the luminol-H2O2-HFCNs. The analytical application potential for the determination of DIP was exploited.
2. Experimental section 2.1. Materials All the chemicals were analytical reagent grade. 0.01 mol/L luminol stock solution was prepared by dissolving 0.1771 g luminol (Shaanxi Normal University, Xi’an, China) in 0.1 mol/L NaOH, diluting to 100 mL in a brown calibrated flask. The stock solution of 1.0 10–3 mol/L DIP (National Institutes for Food and Drug
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Control, Beijing, China) prepared by dissolving 0.0252 g DIP with a small amount of 0.1 mol/L HCl, diluting with water in a 50 mL volumetric flask. Working solutions of H2O2 were prepared daily with 30% (v/v) H2O2 (Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd., Tianjin, China). Other reagents including glacial CH3COOH, P2O5, NaOH and HCl were purchased from Luoyang Chemical Reagent Co. Ltd. All the solutions were prepared with pure water (Chengdu Ulupure Technology Company, China). 2.2. Apparatus The experiments were performed on a BPCL ultraweak CL analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China) equipping with the IFIS-C intelligent flow injection sampler (Xi’an Remax Analytical Instrument, Shannxi, China). The UV‐vis absorption spectra were studied using a Cary5000 spectrophotometer (Varian, USA). The CL and fluorescence (FL) spectra were measured with a Cary Eclipse fluorescence spectrophotometer (Agilent, Australia). A Fourier transform infrared (FT-IR) spectrum was carried out on a Varian 660-IR spectrometer in the range of 500–4000 cm–1 (Varian, USA). Transmission electron microscopy (TEM) image was recorded by a JEM-2100 transmission electron microscopy (JEOL, Japan). 2.3. Synthesis of HFCNs and SFCNs HFCNs and SFCNs were synthesized according to the literature [28] with some modifications. The reactants for producing HFCNs and SFCNs were the same. For HFCNs, the homogeneous mixture solution of 1 mL glacial acetic acid and 80 μL water was quickly added to 2.5 g P2O5 in a 50 mL beaker without stirring. In this system, the upper temperature was mainly controlled by vaporizing the glacial acetic acid at its boiling point (117 °C). The nanobubbles of glacial acetic acid vapor then served as the templates for hollow structures. Finally, the HFCNs in dark brown were collected by dispersing in water, followed by adjustment of the pH to 7.0 with NaOH, diluting to 100 mL in a brown volumetric flask. The obtained HFCNs solution was stable for at least one month in the refrigerator. For SFCNs, a homogeneous mixture solution of 1 mL glacial acetic acid and 5 μL water was quickly added to 1.5 g P2O5 in a 5 mL tube with shake. In room temperature, the SFCNs in yellow were collected by dispersing in water, followed by adjustment of the pH to 7.0 with NaOH, diluting to 100 mL in a brown volumetric flask. The concentrations of HFCNs and SFCNs were 0.35 mol/L calculated by C element in glacial acetic acid. 2.4. Procedures The manifolds of the flow injection CL system were shown in Fig. 1. All solutions were delivered by two peristaltic pumps (P1, P2). P1 was used to deliver HFCNs or SFCNs solution (channel a) and luminol in NaOH solution (channel b) at a flow rate of 2.0 mL/min.
The carrier water (channel c), DIP standard or sample solution (channel d) and H2O2 solution (channel e) were delivered by P2 at a flow rate of 2.5 mL/min. Polytetrafluoroethylene tube was employed to connect all components in the flow system. HFCNs were firstly mixed with luminol at the three-way channel; H2O was used as the carrier for the mixing of HFCNs and luminol. The mixture of HFCNs and luminol finally mixed with DIP and H2O2 solution at the flow cell, in which the CL reaction occurred. The CL signal was detected by the photomultiplier tube (PMT) and recorded with computer. The concentration of DIP was quantified by the relative CL intensity, ΔI¼I0–I, where I0 and I denoted CL intensity in the absence and presence of DIP.
3. Results and discussion 3.1. Spectra of HFCNs and SFCNs Fig. 2(A) showed the UV–vis and FL spectra of HFCNs. A broad absorption around 297 nm and a sharp absorption at 247 nm were observed. The peak at 247 nm was ascribed to π–π* transition of aromatic C¼ C bonds [28], while the shoulder at 297 nm attributed to n–π* transition of C ¼O bonds [28,29]. The FL spectra of the HFCNs had no shift as the excitation wavelength varied. The maximum FL intensity ( 500 nm) was obtained with an excitation wavelength of 400 nm. Fig. 2(B) showed the FT-IR spectrum of the HFCNs. An apparent absorption peak of the OH group at about 3449 cm–1 and an absorption peak of the C¼ O group conjugated with aromatic carbons at 1658 cm–1 appeared. These data showed that the HFCNs were rich in carboxylic groups. A peak at 1542 cm–1 from a conjugated C ¼C stretching vibration was observed, indicating unsaturated carbon bonds formed during the carbonization process. TEM image of HFCNs was displayed in Fig. 2 (C) and the HFCNs were cross-linked with each other. Fig. 2 (D) showed the UV–vis and FL spectra of SFCNs. There were two absorption peaks of SFCNs, one was at 297 nm and another at 247 nm. This was consistent with the absorption of HFCNs. The maximum FL emission ( 500 nm) was obtained with an excitation wavelength of 400 nm and the emission shifted with the excitation wavelength. The sizes of prepared SFCNs were less than 10 nm (Fig. 2(E)). 3.2. The effect of HFCNs and SFCNs for luminol CL system The enhanced effect of HFCNs and SFCNs for luminol-H2O2 CL was studied (Fig. 3). Added SFCNs into the luminol-H2O2 CL system brought about 2-fold CL intensity enhancement and the HFCNs could enhance the CL intensity about 26 times. The results showed that HFCNs exhibited higher sensitive effect on the luminol-H2O2 CL reaction. So, HFCNs was selected as enhancer of luminol-H2O2 system in subsequent measurements. 3.3. Measurement of quantum yield of HFCNs The quantum yield (QY) of the HFCNs was calculated using following function:
Φ ¼ ΦR
Fig. 1. Schematic diagram of the FI-CL system. a: HFCNs or SFCNs solution; b: luminol solution; c: carrier stream; d: DIP standard or sample solution; and e: H2O2 solution. P1 and P2: peristaltic pumps; and F: CL flow-cell.
I AR η2 2 IR A ηR
Quinine sulfate in 0.1 mol/L H2SO4 (literature quantum yield 0.54 at 360 nm) was chose as a standard. Where Φ was the quantum yield, I was the measured integrated emission intensity, η and A was the refractive index and optical density, respectively. The subscript R referred to the reference fluorophore of known quantum yield. In order to minimize re-absorption effects the
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Fig. 2. UV–vis and FL spectra of HFCNs (A) and SFCNs (B). FT-IR spectra of HFCNs (C).
optical densities in the 20 nm fluorescence cuvette were kept under 0.1 at the excitation wavelength. An excitation slit width of 2.5 nm was used to excite the HFCNs samples and to record their photoluminescence spectra, which exhibited excellent optical properties (15.5% FL quantum yields). 3.4. Inhibition of DIP on the HFCNs-enhanced luminol CL The effect of DIP on luminol-H2O2 CL with and without HFCNs was studied. Fig. 4 showed the kinetic curves of the CL system in the static injection mode. A weak and rapid CL emission was observed when luminol was mixed with H2O2. Whereas, the addition of HFCNs could highly enhance the CL intensity and the maximum CL signal appeared about 2.5 s after reaction. In addition to that,
luminol-H2O2 and luminol-H2O2-HFCNs in the presence of DIP caused decrease of CL intensity for the original reaction, respectively. Therefore, the luminol-H2O2-HFCNs CL system was chosen to establish the method for the determination of DIP in pharmaceutical preparations. The strongest inhibition was obtained by optimizing the CL parameters. Fig. 4 also showed that HFCNs could generate CL with H2O2, but the CL signal was very weak, therefore, it was unsuitable for the determination of DIP. 3.5. Optimization of reaction conditions Fig. 5(A) showed the effect of H2O2 concentration on the CL reaction. The CL increased with H2O2 concentration up to 0.07 mol/L and decreased above 0.085 mol/L. When the concentration of H2O2
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CL intensity
1500
3.6. Interference The influence of potentially interfering species was studied using a standard solution of 2.0 10–6 mol/L DIP under the optimum working conditions. When the influence of interference on the peak height was not higher than 5%, it was not considered to interfere with the determination of DIP. Substances that did not interfere with the determination of DIP included 1000-fold K þ , Na þ , Li þ , NH4þ , Ca2 þ , Cl , NO3 , SO24 and PO34 ; 500-fold Pb2 þ , Mg2 þ , Zn2 þ , magnesium stearate, lactose, starch and dextrin; 100fold CO23 , EDTA, citric acid, lactic acid, salicylic acid, sucrose and glucose; 50-fold HCO3 , Ac , Fe3 þ , Fe2 þ , carbamide, uric acid and mannitol, and 10-fold Cu2 þ , Co2 þ , leucine, tryptophan, proline, tyrosine, lysine and phenylalanine. The amount of the potentially interfering species in the samples was below their tolerable levels or could be decreased with diluting, so there would be no interference from these species in DIP determination.
1000
500
0 0
100
200
300
Time (s) Fig. 3. CL intensity of (a) luminol-H2O2, (b) luminol-H2O2-SFCNs and (c) luminolH2O2-HFCNs.
Under the optimum conditions, the CL response was found to be linear in the DIP concentration range of 2.0 10–8–1.0 10– 5 mol/L. The regression equation was ΔI ¼2667.4log C þ4880.9 (C ¼10–6 mol/L, r ¼0.9998). The detection limit according to IUPAC was 3.6 10–9 mol/L and the relative standard deviation (RSD) was 2.7% for eleven replicate determination of 2.0 10–6 mol/L DIP. Fig. 6 showed the calibration graph and typical CL responses for the determination of DIP. The results indicated that this CL system had a good linearity, relatively high sensitivity and suitable precision.
16000 e
12000
CL intensity
3.7. Analytical performances
8000 a
3.8. Determination of DIP in tablets and human urine
4000
0 0
10
20
30
Times (s) Fig. 4. CL kinetic curves: (a) H2O2-HFCNs; (b) luminol-H2O2-DIP; (c) luminol-H2O2; (d) luminol-H2O2-HFCNs-DIP and (e) luminol-H2O2-HFCNs.
was 0.08 mol/L, the most suitable CL signal was recorded and the CL emission was the strongest. The concentrations of H2O2 higher than 0.085 mol/L caused low CL intensity and many gaseous bubbles appeared in the waste solution. The phenomenon could be explained by the H2O2 decomposing in alkaline solution, resulting in unstable CL signals. Therefore, the system was operated at 0.08 mol/L H2O2 throughout this study. For the NaOH concentration, the strongest CL signal was obtained at 0.05 mol/L (Fig. 5(B)). Thus, 0.05 mol/L NaOH was employed. The concentration of HFCNs in the range of 0.007–0.03 mol/L was investigated (Fig. 5(C)). The result indicated that the CL increased with the concentration of HFCNs up to 0.02 mol/L, and then remained constant. So, 0.02 mol/L HFCNs was adopted. The influence of luminol concentration was tested. As shown in Fig. 5(D), the CL increased by the luminol concentration up to 7.0 10–7 mol/L and remained constant. Therefore, the luminol concentration of 7.0 10–7 mol/L was adopted. The effects of the flow rate of reagents on the CL were investigated. The strongest relative CL signal was obtained when the flow rate of P1 up to 2.0 mL/min, and P2 up to 2.5 mL/min. Hence, flow rate of 2.0 mL/ min for P1 and 2.5 mL/min for P2 were selected for the following experiment.
The developed method was applied to the determination of DIP in tablets. The commercial tablets from different manufacturers were purchased from the local pharmacy. Ten DIP tablets were carefully weighed and ground powder. A portion equivalent to one tablet was accurately weighed and dissolved in 25 mL 0.01 mol/L HCl. The solution was sonicated for 15 min and centrifuged for 10 min at 6000 rpm. A 2.5 mL of supernatant solution was transferred to a 50 mL flask and diluted to the mark with water. The results were collected in Table 1 and the recoveries were between 98.0% and 101.6%. Human urine samples were obtained from healthy volunteers. A certain amount of standard DIP solution was added to 1.0 mL of urine sample in a centrifuge tube and mixed for 2 min. This mixture was diluted to 10 mL with water and then centrifuged for 15 min at 5000 rpm. The supernatant was diluted to the appropriate concentration for the determination of DIP. The recoveries were in the range of 98.5–102% in Table 2. 3.9. Possible CL mechanism The role of nanoparticles in a liquid-phase CL reaction could be as catalysts or emitters [6]. In order to identify the role of the HFCNs in the luminol-H2O2 CL system, CL spectra were measured by a FL spectrometer with the xenon lamp turned off. As shown in Fig. 7(A), the maximum CL emissions for the luminol-H2O2 system in the absence of HFCNs or the presence of HFCNs/DIP were around in 425 nm, which was characteristic wavelength of excited 3-aminophthalate anion (3-APAn) [30]. The CL was produced in this way and the HFCNs themselves did not produce CL. As a result, the enhancement of CL signal derived from the catalytic effects of HFCNs. The UV–vis absorption spectra were recorded. As shown in Fig. 7(B), the light absorption of the luminol-H2O2-HFCNs system
Y. Wu, S. Han / Journal of Luminescence 179 (2016) 595–601
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Relative CL intensity
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Relative CL intensity
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Concentration of luminol (10 mol/L)
Fig. 5. Effects of reagent concentrations on the CL intensity of (A) H2O2, (B) NaOH, (C) HFCNs and (D) luminol.
7500
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CL intensity
Relative CL intensity
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Fig. 6. The calibration graphs (A) and typical CL responses for the determination of DIP (B). Unit: 10–6 mol/L, from a–p: 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 4.0, 6.0, 8.0, 10.0. Table 1 The determination of DIP in tablets (n¼3). Sample
Nominal content (mg/tablet)
This method (mg)
Added (mg)
Found (mg)
Recovery (%)
RSD (%)
Tablet 1
25
24.77 0.75
– 10 25
– 34.6 70.21 50.17 0.12
– 99.0 101.6
3.04 0.61 0.24
Tablet 2
25
25.17 0.21
– 10 25
– 34.9 70.21 50.2 70.17
– 98.0 100
0.84 0.60 0.34
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CL reaction probably happened in a radical way, in which the generation of free radicals appeared to be the key factors. Thiourea [31] and mannitol [32] were effective radical scavengers for OH. When 0.01 mol/L thiourea or 4.0 10–3 mol/L mannitol was added to the CL system, a distinct inhibition was observed by a factor of 72% or 60%. This indicated that OH was generated in the CL process. When NaN3 was added to the luminol-H2O2-HFCNs system, the CL inhibition percentage increased with the NaN3 concentration. Therefore, it was concluded that singlet oxygen (1O2) participated in the CL reaction [33]. The mechanism of luminol-H2O2 system under alkaline condition in aqueous solution had been widely studied. Also, it had been well confirmed that reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), OH and superoxide radical anion (O2 ) are the most important oxidizing species, and the reaction of luminol-H2O2 in alkaline medium in the absence of a catalyst is relatively slow, so the produced CL emission is weak. It was also reported that H2O2 decomposition on supported metal catalysts such as gold/silver alloy, copper nanoclusters and ZnO nanoparticles [34–36] attributed to the fact that they could accelerate the breakdown of H2O2 to generate more OH. Shi et al. [37] had found carbon nanodots possessed intrinsic peroxidase-like activity. Thus under the effect of HFCNs as peroxidase mimetics H2O2 might be broken up into double OH radicals. The OH then reacted with luminol to facilitate the generation of luminol radical (L ) and O2 , which further reacted with each other to form the 3-APAn, generated CL [38,39]. DIP on the luminol-H2O2-HFCNs CL
was equal to the sum of the light absorption of individual system of HFCNs and luminol-H2O2, which also implied that the catalysis of HFCNs for the reaction between luminol and H2O2. When DIP was added to the mixing solution of luminol-H2O2-HFCNs, the absorption peak of DIP at 400 nm had an obvious red shift, which showed that DIP participated in the reaction. FL spectra were displayed in Fig. 7(C). After mixing HFCNs with luminol-H2O2, the characteristic FL peak of HFCNs at 500 nm decreased and the peak at 425 nm was stronger than that of luminol-H2O2 system, which showed that the reaction of luminol with H2O2 catalyzed by HFCNs. In the luminol-H2O2-HFCNs-DIP system, the FL intensity of DIP at 500 nm severely decreased and the FL peak at 425 nm of luminol-H2O2-HFCNs system almost disappeared. These results demonstrated that the DIP was involved in the CL reaction. The effect of radical scavengers on the CL was studied. When 1.0 10–4 mol/L ascorbic acid was added to the luminol-H2O2HFCNs system, about 65% CL signal was quenched. Therefore, the Table 2 The determination of DIP in fortified human urine samples. Sample
Added (10–7 mol/L)
Determined (10–7 mol/L)
Recovery (%)
RSD
1 2 3
3.0 5.0 7.0
3.2 70.31 5.17 0.21 6.9 70.054
101.6 102.0 98.5
9.69 4.12 0.78
20
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c
h
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CL intensity
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5
b a
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Wavlength (nm)
Wavlength (nm)
e
d
FL intensity
150
100 c 50 b a
0 300
400
500
600
Wavelength (nm) Fig. 7. (A) CL spectra of (a) luminol-H2O2; (b) luminol-H2O2-HFCNs-DIP and (c) luminol-H2O2-HFCNs. (B) UV–vis absorption spectra of (a) H2O2-HFCNs; (b) HFCNs; (c) DIP; (d) H2O2-DIP; (e) luminol-H2O2; (f) luminol-H2O2-HFCNs; (g) luminol-H2O2-DIP and (h) luminol-H2O2-HFCNs-DIP. (C) FL spectra of (a) luminol-H2O2-DIP; (b) luminol-H2O2; (c) luminol-H2O2-HFCNs-DIP; (d) luminol-H2O2-HFCNs; (e) DIP.
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Scheme 1. The CL mechanism of luminol catalyzed by traditional catalyst (A) and HFCNs (B).
system could react with active oxygen species such as OH, O2 and 1O2, and then led to a decrease in CL intensity. Based on the above results, the enhanced mechanism was summarized in Scheme 1.
4. Conclusion HFCNs were prepared without external heat treatment, and it had an enhance effect for the luminol-H2O2 CL system. The enhancement of CL was suggested to attribute to the catalysis of HFCNs as peroxidase mimetics on the radical generation and electron-transfer processed during the luminol CL reaction. DIP containing OH group could inhibit the CL signal of the luminolH2O2-HFCNs system under the optimized experimental conditions, which could be potentially used to detect this compound in tablet and human fluids samples. This work was of great importance for the investigation of new and efficient catalysts for CL system and helpful for understanding of CL mechanism correspondingly.
Acknowledgments This work was financially supported by the Natural Science Foundation of Shanxi Province (Grant no. 2013011013-3).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2016.07.062.
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