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graphene oxide-magnetite-molecularly imprinting sensor
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 153 (2016) 535–541
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
CdTe quantum dots@luminol as signal amplification system for chrysoidine with chemiluminescence-chitosan/graphene oxide-magnetite-molecularly imprinting sensor Huimin Duan, Leilei Li, Xiaojiao Wang, Yanhui Wang, Jianbo Li, Chuannan Luo ⁎ Key Laboratory of Chemical Sensing & Analysis in Universities of Shandong (University of Jinan), School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China
a r t i c l e
i n f o
Article history: Received 3 April 2015 Received in revised form 25 September 2015 Accepted 26 September 2015 Available online 28 September 2015 Keywords: CdTe QDs@luminol magnetic graphene oxide chemiluminescence resonance energy transfer molecularly imprinted polymer chrysoidine
a b s t r a c t A sensitive chemiluminescence (CL) sensor based on chemiluminescence resonance energy transfer (CRET) in CdTe quantum dots@luminol (CdTe QDs@luminol) nanomaterials combined with chitosan/graphene oxidemagnetite-molecularly imprinted polymer (Cs/GM-MIP) for sensing chrysoidine was developed. CdTe QDs@ luminol was designed to not only amplify the signal of CL but also reduce luminol consumption in the detection of chrysoidine. On the basis of the abundant hydroxy and amino, Cs and graphene oxide were introduced into the GM-MIP to improve the adsorption ability. The adsorption capacities of chrysoidine by both Cs/GM-MIP and nonimprinted polymer (Cs/GM-NIP) were investigated, and the CdTe QDs@luminol and Cs/GM-MIP were characterized by UV–vis, FTIR, SEM and TEM. The proposed sensor can detect chrysoidine within a linear range of 1.0 × 10−7 - 1.0 × 10−5 mol/L with a detection limit of 3.2 × 10−8 mol/L (3δ) due to considerable chemiluminescence signal enhancement of the CdTe quantum dots@luminol detector and the high selectivity of the Cs/GM-MIP system. Under the optimal conditions of CL, the CdTe QDs@luminol-Cs/GM-MIP-CL sensor was used for chrysoidine determination in samples with satisfactory recoveries in the range of 90-107%. © 2015 Published by Elsevier B.V.
1. Introduction The chemiluminescence resonance energy transfer (CRET) process originated from a chemiluminescence donor-acceptor pair in which there was an overlap between the emission spectrum of the donor and the absorption spectrum of acceptor [1]. CRET was occurred by the oxidation of a substrate, which was beneficial for reducing the non-specific signals caused by external light excitation [2]. More importantly, CRET could be used for homogeneous analysis of complex samples without separation. CRET was a widely applied technique for its dramatically reducing the fluorescence bleaching and lessening the autofluorescence of the system and has been applied in many fields, for example, aptamer sensors [3], microchip electrophoresis [4] and immunoassay application [5]. Quantum dots (QDs) have gained increasing interest during the past decades and were nanomaterials typically in the range of or generally in the range of 2–10 nm in diameter [6]. Their novel physical and optical properties and light of the exact wavelength could cause size-tunable emission and simultaneous excitation [7]. At present, basically because of its excellent natural instincts such as good photostability, broad absorption spectra, narrow emission range, intense brightness, high luminescence efficiency and electronic properties, QDs had attracted much ⁎ Corresponding author. E-mail address: [email protected] (C. Luo).
http://dx.doi.org/10.1016/j.saa.2015.09.016 1386-1425/© 2015 Published by Elsevier B.V.
attention and had widespread applications in photoluminescence probes [8], in vivo imaging [9], labeling in living cells [10] and biological luminescent labels [11], and it was one of the most exciting directions in nanoscience fields of the current century. CdTe QDs@luminol conjugates implied a modification of the CdTe QDs surface with luminol and in turn, that could change their properties accordingly [12,13]. In the CRET process, luminol was used to excite CdTe QDs through the luminol CL reaction. Then, CRET process could be proceeded with between chemiluminescence donor luminol and receptor CdTe QDs [14]. They were reported to be a wonderful material that could amplify CL single with higher CRET efficiency. Chemiluminescence (CL), which has been applied in several areas such as clinical, biomedical and food analysis, etc., was of high sensitivity, simple instrumentation and safe from background scattering light [15]. However, the extensive development and application of CL in practical samples were still limited owing to its poor selectivity. In recent years, molecularly imprinted polymer (MIP) possessed high selectivity for target molecule, and were widely used in medical and technological fields [16,17]. MIP was a well-established technology to mimic antibody-antigen interaction in a synthetic platform which possessed outstanding recognition capabilities [18]. However, MIP suffered from low adsorption capacity and difficult separation. At present, graphene oxide (GO) has attracted considerable attention for its unique structure and extraordinary properties, and hold great promise for potential applications in nanomaterial and nanotechnology [19,20]. GO was widely
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used as superior sorbents in metal ions [21], photocatalytic decolorization [22] and drug delivery [23], etc. Recently, chemical modification and functionalization of GO has focused on incorporating a composite material on graphene sheets to widen its applications. Magnetite (Fe3O4) nanoparticles (NPs) were commonly served as a stabilizer and segregator with merits of stable physical properties, low toxicity and eco-friendliness. For the presence of free amine groups, chitosan (Cs) has greater solubility and reactivity [24]. Then, Cs coated-Fe3O4 magnetic NPs had prospective applications in dye removal [25] and electrochemical [26] et al. Chrysoidine, which was an aromatic amine alkali ubiquitous industrial dye and used for dyeing daily necessities such as artificial fiber, leather and paper, etc., would lead to carcinogenicity [27] and teratogenicity [28] for human body. Being so hazardous to our environment, it was significant to develop a simple, direct and real-time way to detect chrysoidine. Up to now, many methods have been reported for chrysoidine detection, such as high performance liquid chromatography [29] and liquid chromatography-mass spectrum [30], etc. Nevertheless, these methods were more or less limited by complicated processes, expensive equipments or high cost during the procedures. Hence, development of an efficient method for the detection of chrysoidine was of important significance. In this work, a sensitive CL sensor based on CRET in CdTe QDs@ luminol nanomaterials combined with Cs/GM-MIP for detection of chrysoidine was proposed. CdTe QDs@luminol, which was designed in the preparing process of the sensor, could amplify the signal of CL with saving luminol consumption in the detection of chrysoidine. On the basis of the abundant hydroxy and amino, Cs and graphene oxide were introduced into the GM-MIP to improve the adsorption ability. The adsorption capacities of the chrysoidine-Cs/GM-MIP and Cs/GMNIP were researched, and the CdTe QDs@luminol and Cs/GM-MIP were confirmed by UV–vis, FTIR, SEM and TEM. Under the optimal conditions, the CdTe QDs@luminol-Cs/GM-MIP-CL sensor was used for chrysoidine determination in practical samples with high selectivity, sensitivity and reagent economized way. 2. Experiment 2.1. Chemicals and materials Chrysoidine (A.R) was purchased from Aladdin Industrial Co.; Acrylamide (A.R), CdCl2°2H2O (A.R) and KBH4 (A.R) were purchased from Sinopharm Chemical Reagent Co. Ltd; Sodium thioglycolate, Tellurium powder (99.99%), Luminol (98%), Boric acid, Acetic acid (99.8%) and Ethylene glycol dimethacrylate (EGDMA, A.R) was supplied by Aladdin Reagent Co. Ltd.; Methacrylic acid (A.R), Ethanol (A.R), Acetic acid (98%), 2,2-azobisisobutyronitrile (AIBN, A.R), Chitosan (90%) was supplied by Sigma-Aldrich; The Methanol(99%, Damao Co. Ltd), Nitric acid (65%-68%, Kande Co. Ltd), Sulfuric acid (98%, Far Eastern Group), Graphite (66%-68%, Hongyan Co. Ltd), Hydrogen peroxide (30%), Sodium hydroxide (97%, Xiya Co. Ltd), Potassium permanganate and all the other chemicals unless specified were of analytical reagent grade and used without further purification unless specified. 2.2. The CdTe QDs@luminol-Cs/GM-MIP-CL sensor The IFFM-E flow injection CL analyzer (Xi’an Remex Electronic instrument High-Tech Ltd., China) was equipped with an automatic injection system and a detection system. Glass capillary filling with CdTe QDs@luminol was positioned on the CL detection window. Glass tube filling with a certain amount Cs/GM-MIP (Cs/GM-NIP) which used magnetism to fill and fix was collected with the pump as recognition elements shown in Fig. 1(A). When chrysoidine solution ran through the Glass tube, chrysoidine could be absorbed by Cs/GM-MIP selectivity while Cs/GM-NIP could not absorb chrysoidine. Fourier transform infrared spectroscopy (FTIR) analyses were recorded on a PerkinElmer
spectrometer (America) using KBr pellets. TEM measurement was made on JEM1400 Transmission electron microscope. The morphology of nanoparticles was analyzed with a scanning electron microscope (SEM, FEI, America). 2.3. Preparation of CdTe QDs@luminol CdTe QDs and CdTe QDs@luminol were synthesized according to a modified procedure described in the previous literatures [31,14] and the preparing process was shown in Fig. 1(B) briefly. NaHTe solution was prepared by adding 0.0310 g Te powder and 0.0540 g KBH4 to freshly prepared N2-saturated 0.1142 g CdCl2 · 2H2O solution in the presence of sodium thioglycolate and degassed for 5 min with N2 bubbling. Subsequently, the value of pH of the solution was adjusted to 10.5 with 0.1 mol/L NaOH solution. After dispersed well, the mixing solution was heated to 100 °С and refluxed for 4 h. The products (thioglycolic acid-capped CdTe QDs) were washed with methanol, and dried at 50°С under vacuum. CdTe QDs@luminol was synthesized using EGDMA as a coupling reagent to conjugate luminol to thioglycolic acid-capped CdTe QDs. Firstly, 0.1 g of as-prepared thioglycolic acid-capped CdTe QDs was precipitated with KOH solution (0.1 mol/L) and methanol, and then 10.00 mL 0.01 mol/L luminol H3BO3-KOH buffer solutions and 0.01 g of EGDMA were added into the solution to prepare CdTe QDs@luminol conjugates. Then, the mixture was constantly stirred under room temperature for 2 h. Finally, the prepared solutions were purified by precipitating with KOH solution (0.1 mol/L) and methanol, respectively. The superfluous luminol and EGDMA was removed by centrifuging (10 min, 8000 r/min). The final products (CdTe QDs@luminol) were dried in an oven for use at 50°С. 2.4. Preparation of GO, Cs/GM-MIP and GM-NIP GO was prepared from nature graphite powders by a modified Hummers method [32] and our previous work with some modification, respectively. Firstly, 6.0 g graphite powder was added into a 500 mL flask. Subsequently, 120 mL H2SO4 (98%) and 80 mL HNO3 (65-68%) mixture solution were added into the flask and then cooled in an ice bath. Then, 15 g KMnO4 was added gradually under vigorous stirring. When the mixture became pasty and the color turned into brownish, 200 mL of H2O2 (30%) was added to the paste with agitation, and the temperature of the mixture was kept to be below 20°С. The diluted suspension was stirred at 90°С till the color of the mixture became light yellow. Finally, the mixture was separated with centrifugation and washed with water and ethanol until the pH = 7.0 while ultrasonication and dried under vacuum to get GO nanosheets. Cs/GM was prepared according to the previous literature [33,26]. 0.5 g of Cs was dissolved in 100 mL of 2% acetic acid by ultrasonication and agitation to disperse well. Subsquently, 0.1 g Fe3O4 particles and 0.1 g as prepared GO were added to the molten chitosan colloidal solution, and then stirred for 1 h. Then, 6 mL 50% glutaraldehyde was added to the mixed solution. After the solution was stirred for 1.5 h at 40 °С, 1 mol/L NaOH was added till the pH = 10.5. The reaction was carried out at 80 °С for 60 min under constant mechanical stirring. The precipitate was isolated in the magnetic field and washed with doubledistilled water and isolated by a permanent magnet. The obtained Cs/GM composites were then dried under vacuum. Chrysoidine-Cs/GM-MIP was prepared according to report [20] and the schematic of the preparing process was shown in Fig. 1(C). By dispersing 1.0 mmol methacrylic acid and 0.25 mmol chrysoidine into 40 mL ethanol, the mixture was stirred for 0.5 h. After shaking, 0.07 g Cs/GM, 10.0 mmol EGDMA and 37 mg AIBN was added in the mixture under nitrogen protection at 65 °С for 0.5 h. And then, the mixture was shaked for 12 h at 60 °С. The obtained product was washed times with methanol-acetic acid (9:1, V/V) solution. Finally, the resultant product was dried under vacuum.
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Fig. 1. Schematic diagram of CdTe QDs@luminol-Cs/GM-MIP-CL sensor (A); the preparing process of CdTe QDs@luminol which was used to amplify the CL signal (B) and Cs/GM-MIP (C) which was used to recognize chrysoidine selectivity.
The Cs/GM-NIP was prepared and processed in the same way, but without any chrysoidine.
3. Results and discussion 3.1. Characterization of CdTe QDs and CdTe QDs@luminol and Cs/GM-MIP
2.5. Adsorption performance of Cs/GM-MIP and Cs/GM-NIP Batch adsorption experiments were conducted to investigate the effect the initial chrysoidine concentration. 10.0 mg Cs/GM-MIP and Cs/ GM-NIP nanoparticles were placed into 5 mL centrifuge tubes, respectively. Then, 2.0 mL of different chrysoidine concentration solution was added into the tube and the dispersion liquid was incubated at 60 °C for 1 h. After magnetic separation, the residual chrysoidine concentration of the supernatant in the tube was determined by CL instrument. The amount of protein adsorbed by the particles was calculated from the following formula. Q¼
ðc0 −ce ÞV m
Where Q (mg/g) was the mass of protein adsorbed by unit mass of dry particles, c0 (mg/mL) and ce (mg/mL) were the concentrations of chrysoidine in the initial and final solutions, respectively, V (mL) was the volume of the adsorption mixture, and m (g) was the mass of the Cs/GM-MIP (Cs/GM-NIP).
In the UV–vis absorption and fluorescence spectrum of CdTe QDs shown in Fig. 2(A), the first excitonic absorption peak at 545 nm could be observed, and the purified CdTe QDs have a symmetric fluorescence emission peak with the full width at half maximum 24 nm which indicated a sufficiently narrow size distribution of the as-prepared CdTe QDs. Fig. 2(B) showed the FTIR spectra of CdTe QDs, CdTe QDs@luminol nanoparticles and luminol respectively. It could be seen that the stretching vibration of -CH2-S contributed to the strong absorption at 1200 cm−1 [34]. The clearly observed complicate and strong absorption peaks ranging from 1400-1600 cm−1 were for the vibration of the benzene ring, and the broad peak at around 3500 cm−1 corresponded to the stretching vibration of secondary amine and hydroxyl together in the spectra of CdTe QDs@luminol. In the spectra, peaks at 1380 cm− 1 were due to the bending vibration of C-H, which provided direct evidence of the successful preparation of CdTe QDs@luminol. SEM was used to characterize the microstructures of GO, GM, Cs/GM-MIP and Cs/GM-NIP nanocomposites. As shown in Fig. 3(A), the crumpled silk waves-like carbon sheets, a characteristic feature of
Fig. 2. The UV–vis (red line) and fluorescence (green line) spectra of CdTe QDs (A); the FTIR spectra of CdTe QDs and CdTe QDs@luminol (B).
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Fig. 3. The SEM images of GO (A), GM (B), Cs/GM-MIP (C) and Cs/GM-NIP (D).
the single-layer GO, which could reduce the free energy of system, was observed clearly. Fig. 3(B) illustrated that the GO surface was well firmly coated with Fe3O4 nanoparticles, which densely attached to both sides of GO sheets to form a sandwich-like composite structure against the aggregation of GO and enable the separation of the products easy. Fig. 3(C) indicates that the surface of MIP was rough and uneven while the surface of Cs/GM-NIP (D) was smooth, and it strongly confirmed the modification of GM with MIP. In the TEM images, shown in Fig. 4(A), the naturally aggregation of GO, the smooth surface and wrinkled edge was seen apparently. It could be indicated that the Fe3O4 nanoparticles were distributed on GO sheets, which were nearly flat and had a big area up to several square micrometers from Fig. 4(B). Therefore, it was reasonable to say GO and GM was synthesized satisfactorily. 3.2. Binding affinity of Cs/GM-MIP and Cs/GM-NIP Binding affinity of Cs/GM-MIP and Cs/GM-NIP to chrysoidine were researched and shown in Fig. 5(A). With chrysoidine concentration increased, competition for the active adsorption sites increased and the adsorption process then was gradually slowed down. The Cs/GM-MIP
showed higher binding affinity to chrysoidine and the adsorption capacity was 8.9 × 10−5 mol/g compared to that of Cs/GM-MIP which was 2.1 × 10−5 mol/g. The result could be explained by that special cavities for chrysoidine were formed during the imprinting process. On the contrary, as for Cs/GM-NIP, there was no chrysoidine molecules in the preparing process and no structure recognition sites were formed accordingly, and then adsorption capacity to chrysoidine was much lower. And the great difference between Cs/GM-MIP and Cs/GM-NIP directly demonstrated a good recognition ability and selectivity of the receptor. 3.3. Optimization of CdTe QDs@luminol-Cs/GM-MIP-CL sensor Some important experimental parameters, including the concentrations of H2O2 and NaOH and pump speed, influenced the detection of chrysoidine greatly and were chosen. The pump speed was an important factor due to its direct influence on the CRET process in CdTe QDs@luminol and the amount of chrysoidine absorbed on the Cs/GMMIP. The lower the pump speed was, the more effective the CRET occurred. But at the same time, the other substances in the samples would be absorbed by Cs/GM-MIP. Thus, pumps speed 30 r/min was
Fig. 4. The TEM images of GO (A) and GM (B).
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Fig. 5. Adsorption capacities of Cs/GM-MIP and Cs/GM-NIP in different chrysoidine concentrations (A); interferences study of CdTe QDs@luminol-Cs/GM-MIP-CL sensor (B). Interferences substance: 1. Sunset Yellow 2. Alizarin Yellow R 3. Epinephrine 4. Acid Orange A.
chosen to achieve the optimal sensitivity and selectivity for the determination of chrysoidine shown in Fig. 6(A). Also it was reported [35] that the pH of the solutions would great effects on the CL intensity in luminol CL system. So the effects of NaOH and H2O2 concentrations were examined to be 0.005 and 0.05 mol/L respectively as Fig. 6 (B-C) showed. 3.4. The analytical performance of CdTe QDs@luminol-Cs/GM-MIP-CL sensor Under the optimal conditions of CL, the CL intensity responded linearly to the concentration of chrysoidine with the range from 1.0 × 10−7 to 1.0 × 10− 5 (mol/L) with detection limit of 3.2 × 10− 8 mol/L (3δ). The regression equation was ΔI = 281 + 4.4 × 108 c (mol/L) shown in Fig. 6(D) with a correlation coefficient of 0.9967. Compared to the Cs/GM-MIP-CL method: the regression equation was ΔI = 193 + 8.3 × 10 7 c (mol/L) (R2 = 0.9904) and linear range was from 3.0 × 10 − 6 - 1.0 × 10 − 4 mol/L with the detection limit 9.7 × 10− 7 mol/L, the proposed CdTe QDs@luminol-Cs/GM-MIP-CL sensor exhibited higher sensitivity and lower detection limit when using CdTe QDs@luminol as CL single amplifier. The results showed
that CdTe quantum dots@luminol was used as signal amplification system for CL detection was feasible. 3.5. Interferences study Specificity was an important criterion for sensors. Under the chosen conditions, interferences from some coexisting substances were investigated. The tolerable fold of interfering substances in sample with Cs/GM-MIP and Cs/GM-NIP column was compared when relative error was less than ± 5% and the tolerance times were shown in Fig. 5(B). In CL, 100 times of Sunset Yellow (compared to chrysoidine) have interference, but when in CdTe QDs@luminol-Cs/GM-MIP-CL, 200 times of Sunset Yellow (compared to chrysoidine) has interference on the detection of chrysoidine. Serious interferences were observed after adding Sunset Yellow, Alizarin Yellow R, Epinephrine and Acid Orange A to the standard solution of chrysoidine in CL system. The times of concentration of the four substances relative to 1.0 × 10− 6 mol/L chrysoidine, which would infect the detection, were relatively lower than pure CL method. But these results suggested the acceptable specificity of the sensor for chrysoidine when in CdTe QDs@luminol-Cs/GM-
Fig. 6. Optimization results: (A) Effect of pump speed on CL intensity; (B) Effect of H2O2 concentration on CL intensity; (C) Effect of NaOH concentration on CL intensity; (D) The equation of linear regression of the CdTe QDs@luminol-Cs/GM-MIP-CL sensor.
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that CdTe QDs@luminol were remarkably effective on the detection of chrysoidine.
Table 1 Application of the sensor in real sample analysis. Sample
c/10−6 mol/L
Added/10−6 mol/L
Found/10−6 mol/L (n = 6)
Recovery%
RSD%
Paper Fabric
1.1 3.7
3.0 3.0
4.3 6.4
107 90
3.4 3.7
4. Conclusion
MIP-CL system, and the interference could be reduced and eliminated greatly. 3.6. Application in real sample analysis To further investigate the feasibility of the sensor for the real sample applications, the sensing of chrysoidine in paper and fabric cloth was carried out. Before the determination, samples were diluted and obtained appropriately step by step to be in the linear range. The assay results of two samples using the proposed method were obtained and shown in Table 1.
In present study, a sensitive CL sensor combined with Cs/GM-MIP for sensing chrysoidine was developed based on CRET in CdTe QDs@ luminol nanomaterials as signal amplification label. CdTe QDs@luminol and Cs/GM-MIP were synthesized and characterized by UV–vis, FTIR, TEM and SEM. CdTe QDs@luminol was designed to not only amplify the signal of CL but also reduce luminol consumption in the detection of chrysoidine. Subsequently, the absorption capacity of Cs/GM-MIP was studied to be 8.9 mol/g. Then, the effects of pump speed and luminous reagents’ concentrations on CL intensity were explored and the possible mechanism of CL reaction was discussed. On the basis of the considerably amplified CL signal and high selectivity of Cs/GM-MIP, the proposed method successfully fulfilled the sensitive detection of chrysoidine in samples with reagent economized way. Acknowledgements
3.7. Chrysoidine detection with CdTe QDs@luminol based CRET The possible chemiluminescence mechanism of CL reaction was discussed in Fig. Generally, the H2O2, acting as electron donors in the redox reaction, could transfer electron to luminol, generating oxidizedstate luminol* (Rreactions 1). As a high energy holder, luminol* returned to the stable state and released photons (Reactions 2).
(1)
(2)
On the other hand, the energy from the luminol* could transfer to CdTe QDs preferentially, producing oxidized-state CdTe QDs* (Reactions 3). And then CdTe QDs* returned to the ground state, which led to the generation of photons (Reactions 4).
(3)
(4)
(5)
On the basis of the CRET in CdTe QDs@luminol, the decomposition of chrysoidine would consume the energy according to the technique chemically induced dynamic nuclear polarization (CIDNP) [36] (Reactions 5). This route could reduce the CL emission. The results have clearly indicated
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
CdTe quantum dots@luminol as signal amplification system for chrysoidine with chemiluminescence-chitosan/graphene oxide-magnetite-molecularly imprinting sensor Huimin Duan, Leilei Li, Xiaojiao Wang, Yanhui Wang, Jianbo Li, Chuannan Luo ⁎ Key Laboratory of Chemical Sensing & Analysis in Universities of Shandong (University of Jinan), School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China
a r t i c l e
i n f o
Article history: Received 3 April 2015 Received in revised form 25 September 2015 Accepted 26 September 2015 Available online 28 September 2015 Keywords: CdTe QDs@luminol magnetic graphene oxide chemiluminescence resonance energy transfer molecularly imprinted polymer chrysoidine
a b s t r a c t A sensitive chemiluminescence (CL) sensor based on chemiluminescence resonance energy transfer (CRET) in CdTe quantum dots@luminol (CdTe QDs@luminol) nanomaterials combined with chitosan/graphene oxidemagnetite-molecularly imprinted polymer (Cs/GM-MIP) for sensing chrysoidine was developed. CdTe QDs@ luminol was designed to not only amplify the signal of CL but also reduce luminol consumption in the detection of chrysoidine. On the basis of the abundant hydroxy and amino, Cs and graphene oxide were introduced into the GM-MIP to improve the adsorption ability. The adsorption capacities of chrysoidine by both Cs/GM-MIP and nonimprinted polymer (Cs/GM-NIP) were investigated, and the CdTe QDs@luminol and Cs/GM-MIP were characterized by UV–vis, FTIR, SEM and TEM. The proposed sensor can detect chrysoidine within a linear range of 1.0 × 10−7 - 1.0 × 10−5 mol/L with a detection limit of 3.2 × 10−8 mol/L (3δ) due to considerable chemiluminescence signal enhancement of the CdTe quantum dots@luminol detector and the high selectivity of the Cs/GM-MIP system. Under the optimal conditions of CL, the CdTe QDs@luminol-Cs/GM-MIP-CL sensor was used for chrysoidine determination in samples with satisfactory recoveries in the range of 90-107%. © 2015 Published by Elsevier B.V.
1. Introduction The chemiluminescence resonance energy transfer (CRET) process originated from a chemiluminescence donor-acceptor pair in which there was an overlap between the emission spectrum of the donor and the absorption spectrum of acceptor [1]. CRET was occurred by the oxidation of a substrate, which was beneficial for reducing the non-specific signals caused by external light excitation [2]. More importantly, CRET could be used for homogeneous analysis of complex samples without separation. CRET was a widely applied technique for its dramatically reducing the fluorescence bleaching and lessening the autofluorescence of the system and has been applied in many fields, for example, aptamer sensors [3], microchip electrophoresis [4] and immunoassay application [5]. Quantum dots (QDs) have gained increasing interest during the past decades and were nanomaterials typically in the range of or generally in the range of 2–10 nm in diameter [6]. Their novel physical and optical properties and light of the exact wavelength could cause size-tunable emission and simultaneous excitation [7]. At present, basically because of its excellent natural instincts such as good photostability, broad absorption spectra, narrow emission range, intense brightness, high luminescence efficiency and electronic properties, QDs had attracted much ⁎ Corresponding author. E-mail address: [email protected] (C. Luo).
http://dx.doi.org/10.1016/j.saa.2015.09.016 1386-1425/© 2015 Published by Elsevier B.V.
attention and had widespread applications in photoluminescence probes [8], in vivo imaging [9], labeling in living cells [10] and biological luminescent labels [11], and it was one of the most exciting directions in nanoscience fields of the current century. CdTe QDs@luminol conjugates implied a modification of the CdTe QDs surface with luminol and in turn, that could change their properties accordingly [12,13]. In the CRET process, luminol was used to excite CdTe QDs through the luminol CL reaction. Then, CRET process could be proceeded with between chemiluminescence donor luminol and receptor CdTe QDs [14]. They were reported to be a wonderful material that could amplify CL single with higher CRET efficiency. Chemiluminescence (CL), which has been applied in several areas such as clinical, biomedical and food analysis, etc., was of high sensitivity, simple instrumentation and safe from background scattering light [15]. However, the extensive development and application of CL in practical samples were still limited owing to its poor selectivity. In recent years, molecularly imprinted polymer (MIP) possessed high selectivity for target molecule, and were widely used in medical and technological fields [16,17]. MIP was a well-established technology to mimic antibody-antigen interaction in a synthetic platform which possessed outstanding recognition capabilities [18]. However, MIP suffered from low adsorption capacity and difficult separation. At present, graphene oxide (GO) has attracted considerable attention for its unique structure and extraordinary properties, and hold great promise for potential applications in nanomaterial and nanotechnology [19,20]. GO was widely
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used as superior sorbents in metal ions [21], photocatalytic decolorization [22] and drug delivery [23], etc. Recently, chemical modification and functionalization of GO has focused on incorporating a composite material on graphene sheets to widen its applications. Magnetite (Fe3O4) nanoparticles (NPs) were commonly served as a stabilizer and segregator with merits of stable physical properties, low toxicity and eco-friendliness. For the presence of free amine groups, chitosan (Cs) has greater solubility and reactivity [24]. Then, Cs coated-Fe3O4 magnetic NPs had prospective applications in dye removal [25] and electrochemical [26] et al. Chrysoidine, which was an aromatic amine alkali ubiquitous industrial dye and used for dyeing daily necessities such as artificial fiber, leather and paper, etc., would lead to carcinogenicity [27] and teratogenicity [28] for human body. Being so hazardous to our environment, it was significant to develop a simple, direct and real-time way to detect chrysoidine. Up to now, many methods have been reported for chrysoidine detection, such as high performance liquid chromatography [29] and liquid chromatography-mass spectrum [30], etc. Nevertheless, these methods were more or less limited by complicated processes, expensive equipments or high cost during the procedures. Hence, development of an efficient method for the detection of chrysoidine was of important significance. In this work, a sensitive CL sensor based on CRET in CdTe QDs@ luminol nanomaterials combined with Cs/GM-MIP for detection of chrysoidine was proposed. CdTe QDs@luminol, which was designed in the preparing process of the sensor, could amplify the signal of CL with saving luminol consumption in the detection of chrysoidine. On the basis of the abundant hydroxy and amino, Cs and graphene oxide were introduced into the GM-MIP to improve the adsorption ability. The adsorption capacities of the chrysoidine-Cs/GM-MIP and Cs/GMNIP were researched, and the CdTe QDs@luminol and Cs/GM-MIP were confirmed by UV–vis, FTIR, SEM and TEM. Under the optimal conditions, the CdTe QDs@luminol-Cs/GM-MIP-CL sensor was used for chrysoidine determination in practical samples with high selectivity, sensitivity and reagent economized way. 2. Experiment 2.1. Chemicals and materials Chrysoidine (A.R) was purchased from Aladdin Industrial Co.; Acrylamide (A.R), CdCl2°2H2O (A.R) and KBH4 (A.R) were purchased from Sinopharm Chemical Reagent Co. Ltd; Sodium thioglycolate, Tellurium powder (99.99%), Luminol (98%), Boric acid, Acetic acid (99.8%) and Ethylene glycol dimethacrylate (EGDMA, A.R) was supplied by Aladdin Reagent Co. Ltd.; Methacrylic acid (A.R), Ethanol (A.R), Acetic acid (98%), 2,2-azobisisobutyronitrile (AIBN, A.R), Chitosan (90%) was supplied by Sigma-Aldrich; The Methanol(99%, Damao Co. Ltd), Nitric acid (65%-68%, Kande Co. Ltd), Sulfuric acid (98%, Far Eastern Group), Graphite (66%-68%, Hongyan Co. Ltd), Hydrogen peroxide (30%), Sodium hydroxide (97%, Xiya Co. Ltd), Potassium permanganate and all the other chemicals unless specified were of analytical reagent grade and used without further purification unless specified. 2.2. The CdTe QDs@luminol-Cs/GM-MIP-CL sensor The IFFM-E flow injection CL analyzer (Xi’an Remex Electronic instrument High-Tech Ltd., China) was equipped with an automatic injection system and a detection system. Glass capillary filling with CdTe QDs@luminol was positioned on the CL detection window. Glass tube filling with a certain amount Cs/GM-MIP (Cs/GM-NIP) which used magnetism to fill and fix was collected with the pump as recognition elements shown in Fig. 1(A). When chrysoidine solution ran through the Glass tube, chrysoidine could be absorbed by Cs/GM-MIP selectivity while Cs/GM-NIP could not absorb chrysoidine. Fourier transform infrared spectroscopy (FTIR) analyses were recorded on a PerkinElmer
spectrometer (America) using KBr pellets. TEM measurement was made on JEM1400 Transmission electron microscope. The morphology of nanoparticles was analyzed with a scanning electron microscope (SEM, FEI, America). 2.3. Preparation of CdTe QDs@luminol CdTe QDs and CdTe QDs@luminol were synthesized according to a modified procedure described in the previous literatures [31,14] and the preparing process was shown in Fig. 1(B) briefly. NaHTe solution was prepared by adding 0.0310 g Te powder and 0.0540 g KBH4 to freshly prepared N2-saturated 0.1142 g CdCl2 · 2H2O solution in the presence of sodium thioglycolate and degassed for 5 min with N2 bubbling. Subsequently, the value of pH of the solution was adjusted to 10.5 with 0.1 mol/L NaOH solution. After dispersed well, the mixing solution was heated to 100 °С and refluxed for 4 h. The products (thioglycolic acid-capped CdTe QDs) were washed with methanol, and dried at 50°С under vacuum. CdTe QDs@luminol was synthesized using EGDMA as a coupling reagent to conjugate luminol to thioglycolic acid-capped CdTe QDs. Firstly, 0.1 g of as-prepared thioglycolic acid-capped CdTe QDs was precipitated with KOH solution (0.1 mol/L) and methanol, and then 10.00 mL 0.01 mol/L luminol H3BO3-KOH buffer solutions and 0.01 g of EGDMA were added into the solution to prepare CdTe QDs@luminol conjugates. Then, the mixture was constantly stirred under room temperature for 2 h. Finally, the prepared solutions were purified by precipitating with KOH solution (0.1 mol/L) and methanol, respectively. The superfluous luminol and EGDMA was removed by centrifuging (10 min, 8000 r/min). The final products (CdTe QDs@luminol) were dried in an oven for use at 50°С. 2.4. Preparation of GO, Cs/GM-MIP and GM-NIP GO was prepared from nature graphite powders by a modified Hummers method [32] and our previous work with some modification, respectively. Firstly, 6.0 g graphite powder was added into a 500 mL flask. Subsequently, 120 mL H2SO4 (98%) and 80 mL HNO3 (65-68%) mixture solution were added into the flask and then cooled in an ice bath. Then, 15 g KMnO4 was added gradually under vigorous stirring. When the mixture became pasty and the color turned into brownish, 200 mL of H2O2 (30%) was added to the paste with agitation, and the temperature of the mixture was kept to be below 20°С. The diluted suspension was stirred at 90°С till the color of the mixture became light yellow. Finally, the mixture was separated with centrifugation and washed with water and ethanol until the pH = 7.0 while ultrasonication and dried under vacuum to get GO nanosheets. Cs/GM was prepared according to the previous literature [33,26]. 0.5 g of Cs was dissolved in 100 mL of 2% acetic acid by ultrasonication and agitation to disperse well. Subsquently, 0.1 g Fe3O4 particles and 0.1 g as prepared GO were added to the molten chitosan colloidal solution, and then stirred for 1 h. Then, 6 mL 50% glutaraldehyde was added to the mixed solution. After the solution was stirred for 1.5 h at 40 °С, 1 mol/L NaOH was added till the pH = 10.5. The reaction was carried out at 80 °С for 60 min under constant mechanical stirring. The precipitate was isolated in the magnetic field and washed with doubledistilled water and isolated by a permanent magnet. The obtained Cs/GM composites were then dried under vacuum. Chrysoidine-Cs/GM-MIP was prepared according to report [20] and the schematic of the preparing process was shown in Fig. 1(C). By dispersing 1.0 mmol methacrylic acid and 0.25 mmol chrysoidine into 40 mL ethanol, the mixture was stirred for 0.5 h. After shaking, 0.07 g Cs/GM, 10.0 mmol EGDMA and 37 mg AIBN was added in the mixture under nitrogen protection at 65 °С for 0.5 h. And then, the mixture was shaked for 12 h at 60 °С. The obtained product was washed times with methanol-acetic acid (9:1, V/V) solution. Finally, the resultant product was dried under vacuum.
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Fig. 1. Schematic diagram of CdTe QDs@luminol-Cs/GM-MIP-CL sensor (A); the preparing process of CdTe QDs@luminol which was used to amplify the CL signal (B) and Cs/GM-MIP (C) which was used to recognize chrysoidine selectivity.
The Cs/GM-NIP was prepared and processed in the same way, but without any chrysoidine.
3. Results and discussion 3.1. Characterization of CdTe QDs and CdTe QDs@luminol and Cs/GM-MIP
2.5. Adsorption performance of Cs/GM-MIP and Cs/GM-NIP Batch adsorption experiments were conducted to investigate the effect the initial chrysoidine concentration. 10.0 mg Cs/GM-MIP and Cs/ GM-NIP nanoparticles were placed into 5 mL centrifuge tubes, respectively. Then, 2.0 mL of different chrysoidine concentration solution was added into the tube and the dispersion liquid was incubated at 60 °C for 1 h. After magnetic separation, the residual chrysoidine concentration of the supernatant in the tube was determined by CL instrument. The amount of protein adsorbed by the particles was calculated from the following formula. Q¼
ðc0 −ce ÞV m
Where Q (mg/g) was the mass of protein adsorbed by unit mass of dry particles, c0 (mg/mL) and ce (mg/mL) were the concentrations of chrysoidine in the initial and final solutions, respectively, V (mL) was the volume of the adsorption mixture, and m (g) was the mass of the Cs/GM-MIP (Cs/GM-NIP).
In the UV–vis absorption and fluorescence spectrum of CdTe QDs shown in Fig. 2(A), the first excitonic absorption peak at 545 nm could be observed, and the purified CdTe QDs have a symmetric fluorescence emission peak with the full width at half maximum 24 nm which indicated a sufficiently narrow size distribution of the as-prepared CdTe QDs. Fig. 2(B) showed the FTIR spectra of CdTe QDs, CdTe QDs@luminol nanoparticles and luminol respectively. It could be seen that the stretching vibration of -CH2-S contributed to the strong absorption at 1200 cm−1 [34]. The clearly observed complicate and strong absorption peaks ranging from 1400-1600 cm−1 were for the vibration of the benzene ring, and the broad peak at around 3500 cm−1 corresponded to the stretching vibration of secondary amine and hydroxyl together in the spectra of CdTe QDs@luminol. In the spectra, peaks at 1380 cm− 1 were due to the bending vibration of C-H, which provided direct evidence of the successful preparation of CdTe QDs@luminol. SEM was used to characterize the microstructures of GO, GM, Cs/GM-MIP and Cs/GM-NIP nanocomposites. As shown in Fig. 3(A), the crumpled silk waves-like carbon sheets, a characteristic feature of
Fig. 2. The UV–vis (red line) and fluorescence (green line) spectra of CdTe QDs (A); the FTIR spectra of CdTe QDs and CdTe QDs@luminol (B).
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Fig. 3. The SEM images of GO (A), GM (B), Cs/GM-MIP (C) and Cs/GM-NIP (D).
the single-layer GO, which could reduce the free energy of system, was observed clearly. Fig. 3(B) illustrated that the GO surface was well firmly coated with Fe3O4 nanoparticles, which densely attached to both sides of GO sheets to form a sandwich-like composite structure against the aggregation of GO and enable the separation of the products easy. Fig. 3(C) indicates that the surface of MIP was rough and uneven while the surface of Cs/GM-NIP (D) was smooth, and it strongly confirmed the modification of GM with MIP. In the TEM images, shown in Fig. 4(A), the naturally aggregation of GO, the smooth surface and wrinkled edge was seen apparently. It could be indicated that the Fe3O4 nanoparticles were distributed on GO sheets, which were nearly flat and had a big area up to several square micrometers from Fig. 4(B). Therefore, it was reasonable to say GO and GM was synthesized satisfactorily. 3.2. Binding affinity of Cs/GM-MIP and Cs/GM-NIP Binding affinity of Cs/GM-MIP and Cs/GM-NIP to chrysoidine were researched and shown in Fig. 5(A). With chrysoidine concentration increased, competition for the active adsorption sites increased and the adsorption process then was gradually slowed down. The Cs/GM-MIP
showed higher binding affinity to chrysoidine and the adsorption capacity was 8.9 × 10−5 mol/g compared to that of Cs/GM-MIP which was 2.1 × 10−5 mol/g. The result could be explained by that special cavities for chrysoidine were formed during the imprinting process. On the contrary, as for Cs/GM-NIP, there was no chrysoidine molecules in the preparing process and no structure recognition sites were formed accordingly, and then adsorption capacity to chrysoidine was much lower. And the great difference between Cs/GM-MIP and Cs/GM-NIP directly demonstrated a good recognition ability and selectivity of the receptor. 3.3. Optimization of CdTe QDs@luminol-Cs/GM-MIP-CL sensor Some important experimental parameters, including the concentrations of H2O2 and NaOH and pump speed, influenced the detection of chrysoidine greatly and were chosen. The pump speed was an important factor due to its direct influence on the CRET process in CdTe QDs@luminol and the amount of chrysoidine absorbed on the Cs/GMMIP. The lower the pump speed was, the more effective the CRET occurred. But at the same time, the other substances in the samples would be absorbed by Cs/GM-MIP. Thus, pumps speed 30 r/min was
Fig. 4. The TEM images of GO (A) and GM (B).
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Fig. 5. Adsorption capacities of Cs/GM-MIP and Cs/GM-NIP in different chrysoidine concentrations (A); interferences study of CdTe QDs@luminol-Cs/GM-MIP-CL sensor (B). Interferences substance: 1. Sunset Yellow 2. Alizarin Yellow R 3. Epinephrine 4. Acid Orange A.
chosen to achieve the optimal sensitivity and selectivity for the determination of chrysoidine shown in Fig. 6(A). Also it was reported [35] that the pH of the solutions would great effects on the CL intensity in luminol CL system. So the effects of NaOH and H2O2 concentrations were examined to be 0.005 and 0.05 mol/L respectively as Fig. 6 (B-C) showed. 3.4. The analytical performance of CdTe QDs@luminol-Cs/GM-MIP-CL sensor Under the optimal conditions of CL, the CL intensity responded linearly to the concentration of chrysoidine with the range from 1.0 × 10−7 to 1.0 × 10− 5 (mol/L) with detection limit of 3.2 × 10− 8 mol/L (3δ). The regression equation was ΔI = 281 + 4.4 × 108 c (mol/L) shown in Fig. 6(D) with a correlation coefficient of 0.9967. Compared to the Cs/GM-MIP-CL method: the regression equation was ΔI = 193 + 8.3 × 10 7 c (mol/L) (R2 = 0.9904) and linear range was from 3.0 × 10 − 6 - 1.0 × 10 − 4 mol/L with the detection limit 9.7 × 10− 7 mol/L, the proposed CdTe QDs@luminol-Cs/GM-MIP-CL sensor exhibited higher sensitivity and lower detection limit when using CdTe QDs@luminol as CL single amplifier. The results showed
that CdTe quantum dots@luminol was used as signal amplification system for CL detection was feasible. 3.5. Interferences study Specificity was an important criterion for sensors. Under the chosen conditions, interferences from some coexisting substances were investigated. The tolerable fold of interfering substances in sample with Cs/GM-MIP and Cs/GM-NIP column was compared when relative error was less than ± 5% and the tolerance times were shown in Fig. 5(B). In CL, 100 times of Sunset Yellow (compared to chrysoidine) have interference, but when in CdTe QDs@luminol-Cs/GM-MIP-CL, 200 times of Sunset Yellow (compared to chrysoidine) has interference on the detection of chrysoidine. Serious interferences were observed after adding Sunset Yellow, Alizarin Yellow R, Epinephrine and Acid Orange A to the standard solution of chrysoidine in CL system. The times of concentration of the four substances relative to 1.0 × 10− 6 mol/L chrysoidine, which would infect the detection, were relatively lower than pure CL method. But these results suggested the acceptable specificity of the sensor for chrysoidine when in CdTe QDs@luminol-Cs/GM-
Fig. 6. Optimization results: (A) Effect of pump speed on CL intensity; (B) Effect of H2O2 concentration on CL intensity; (C) Effect of NaOH concentration on CL intensity; (D) The equation of linear regression of the CdTe QDs@luminol-Cs/GM-MIP-CL sensor.
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that CdTe QDs@luminol were remarkably effective on the detection of chrysoidine.
Table 1 Application of the sensor in real sample analysis. Sample
c/10−6 mol/L
Added/10−6 mol/L
Found/10−6 mol/L (n = 6)
Recovery%
RSD%
Paper Fabric
1.1 3.7
3.0 3.0
4.3 6.4
107 90
3.4 3.7
4. Conclusion
MIP-CL system, and the interference could be reduced and eliminated greatly. 3.6. Application in real sample analysis To further investigate the feasibility of the sensor for the real sample applications, the sensing of chrysoidine in paper and fabric cloth was carried out. Before the determination, samples were diluted and obtained appropriately step by step to be in the linear range. The assay results of two samples using the proposed method were obtained and shown in Table 1.
In present study, a sensitive CL sensor combined with Cs/GM-MIP for sensing chrysoidine was developed based on CRET in CdTe QDs@ luminol nanomaterials as signal amplification label. CdTe QDs@luminol and Cs/GM-MIP were synthesized and characterized by UV–vis, FTIR, TEM and SEM. CdTe QDs@luminol was designed to not only amplify the signal of CL but also reduce luminol consumption in the detection of chrysoidine. Subsequently, the absorption capacity of Cs/GM-MIP was studied to be 8.9 mol/g. Then, the effects of pump speed and luminous reagents’ concentrations on CL intensity were explored and the possible mechanism of CL reaction was discussed. On the basis of the considerably amplified CL signal and high selectivity of Cs/GM-MIP, the proposed method successfully fulfilled the sensitive detection of chrysoidine in samples with reagent economized way. Acknowledgements
3.7. Chrysoidine detection with CdTe QDs@luminol based CRET The possible chemiluminescence mechanism of CL reaction was discussed in Fig. Generally, the H2O2, acting as electron donors in the redox reaction, could transfer electron to luminol, generating oxidizedstate luminol* (Rreactions 1). As a high energy holder, luminol* returned to the stable state and released photons (Reactions 2).
(1)
(2)
On the other hand, the energy from the luminol* could transfer to CdTe QDs preferentially, producing oxidized-state CdTe QDs* (Reactions 3). And then CdTe QDs* returned to the ground state, which led to the generation of photons (Reactions 4).
(3)
(4)
(5)
On the basis of the CRET in CdTe QDs@luminol, the decomposition of chrysoidine would consume the energy according to the technique chemically induced dynamic nuclear polarization (CIDNP) [36] (Reactions 5). This route could reduce the CL emission. The results have clearly indicated
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