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Construction of a novel "Off-On" fluorescence sensor for highly selective sensing of selenite based on europium ions induced crosslinking of nitrogen-doped carbon dots
Journal of Luminescence 194 (2018) 768–777
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Construction of a novel "Off-On" fluorescence sensor for highly selective sensing of selenite based on europium ions induced crosslinking of nitrogendoped carbon dots
MARK
⁎
Niloufar Amin, Abbas Afkhami , Tayyebeh Madrakian Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Europium ions Selenite Nitrogen-doped carbon dots Off-On probe
A highly blue luminescent nitrogen-doped carbon dots (N-CDs) was prepared, using citric acid and L-lysine via one pot hydrothermal treatment, and applied to the determination of selenite. The synthesized N-CDs exhibited a high photostability, excellent optical features with a high quantum yield (21.5%) and average size of 3 nm. The method is based on the competition between oxygen donor atoms in selenite oxyanion with those from carboxylate and hydroxyl groups at the surface of N-carbon dots for europium ions binding. N-CDs could be readily quenched upon the addition of europium ions owing to high affinity of carboxylate and hydroxyl groups at the surface of carbon dots to Eu3+ leading to aggregation of N-CDs (state OFF). After selenite addition, disruption of the aggregated N-CDs takes place, leading to restoration of the quenched fluorescence (State ON). The repeatability was less than 3.2% for selenite in both standard and real samples (n = 3). The method provides a simple procedure enabled selective detection of selenite with a linear range of 0.078–21.4 µg mL−1 and a limit of detection of 53.0 ng mL−1 (S/N = 3). The accuracy and precision were evaluated based on the detection of selenite in health care products with satisfactory results.
1. Introduction Selenium (Se) is a nutrient and essential element for human health at a slight level, but its excessive amounts is toxic and harmful for living organism. Se is a main component of antioxidant enzymes which protecting the human body from free radicals [1–3]. In general, selenium is required by the human body in the range 50–200 µg day−1 [4]. Therefore, taking less than the recommended dosage of Se will cause a number of deficiency syndromes [5]. Furthermore, the toxicity of Selenium depends on its chemical forms. It has been reported that inorganic Se species such as selenite (SeO32 −) and selenate (SeO24−) are more toxic than the organic ones [6]. Since the concentration range between essential and toxic is very small, it has great significance to develop a selective, facile and accurate method to detection of Se in biological and environmental samples [2]. So far, several analytical methods have been developed for detection of selenium, including high performance liquid chromatography – inductively coupled plasma mass spectrometry (HPLC-ICP-MS) [7], hydride generation atomic fluorescence spectrometry [8,9], total reflection X-ray fluorescence spectroscopy [10], hydride generation atomic absorption spectrometry [11], inductively coupled plasma mass
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spectrometry [12] and spectrophotometry [13]. These common analytical methods are highly accurate, sensitive and widely used for selenium detection; however, they suffer from some inherent drawbacks such as lengthy or tedious process, laborious procedures, sophisticated and expensive instruments and they require complicated sample preparation steps which limit their applicability. Fluorescence analysis might be a good selection for detecting selenium due to cost effectiveness, simplicity of operation, and high selectivity and sensitivity. Since the methods reported on the development of fluorescence sensors for detection of selenite generally require a time consuming procedures, toxic and unsafe reagents (e.g. quinoline derivatives), large organic solvent consumption, complicated sample-pretreatment, utilization of preconcentration steps, that wide utilization of these methods is largely limited [14–19]. Hence, development and design fluorescence probes which are safe to human health, rapid, simple, cost effective and eco-friendly is highly desired. Carbon quantum dots have been captured more and more attention in recent decade because of their low toxicity, and good biocompatibility, high photostability, water-solubility, easy to surface
Corresponding author. E-mail address: [email protected] (A. Afkhami).
http://dx.doi.org/10.1016/j.jlumin.2017.09.048 Received 12 July 2017; Received in revised form 10 September 2017; Accepted 19 September 2017 Available online 22 September 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
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modification and unique excitation-dependent fluorescence, which have gained much interest as potential compared to semiconductor quantum dots and organic dyes [20–24]. Historically, the photoluminescence (PL) CDs were firstly discovered through purification of single-walled carbon nanotubes by preparative electrophoresis [25]. They are very promising in numerous possible applications such as photocatalysis [26], optoelectronic devices [27], bioimaging [28], and optical sensors [29]. Up to now, different approaches have been employed for the preparation of fluorescent C-dots, including electrochemical [30], laser ablation [31], arc-discharge [25], hydrothermal [32], solvothermal [33], microwave pyrolysis [34], plasma and ultrasonic treatment [35,36]. Among mentioned strategies, hydrothermal method has distinct advantages such as mild synthetic treatment, simple and ecofriendly reaction procedures and the more important that it does not require sophisticate tools [37]. Despite the impressive benefits, carbon dots have a low quantum yield (QY, often less than 10%) which still limits their applications [38]. In order to improving the QY of the CDs, doping process (introduction heteroatoms like N, B, and S into CDs) and surface modification are two ways that can also endow the CDs with new properties, such as sensing ability toward metal ions or biomolecules for various applications and high catalytic reactivity, based on adjusted intrinsic electronic structure and modification of the CDs surface with functional molecules [39]. Previous works report that Eu3+ possess higher affinity to pyrophosphates (PPi) and phosphates than the hydroxyl and carboxyl groups [40,41]. Therefore, we infer that another oxyanion such as selenite is able to selectively bind europium ions due to the negatively charged oxygen groups. In this work, we demonstrate one-pot, but two steps, hydrothermal carbonization route to the preparation nitrogen-doped carbon dots using citric acid and L-lysine in mild conditions. Citric acid serves as the molecular precursor and carbon source, as well as L-lysine as the surface passivation agent. It also provides nitrogen. Remarkably, it is found that the as-prepared N -CDs exhibit a high fluorescence Quantum yield (21.5%) and strong emit highly blue fluorescence under 330 nm. To the best of our knowledge, application of carbon nanodots-based fluorescent sensors in quantification of selenite has never been reported.
2.3. Preparation of N-CDs The N-doped CDs were prepared by the carbonization of citric acid as in a similar method reported previously with some modification [42]. Briefly, 2.0 g of citric acid (9.5 mmol) and 1.2 g of lysine (8.3 mmol) were mixed thoroughly with 50 mL of distilled water, by ultrasonication, to become a homogeneous solution, followed by evaporation at 70 °C until dryness within 12 h. The thermal process was continued until the colorless solution became pale pink thick syrup. After that this syrup was transferred to a poly (tetrafluoroethylene) (Teflon)-lined autoclave and heated at 200 °C for 3 h. After cooling to room temperature, the resulting brownish syrup was dispersed in ultrapure water and then subjected to dialysis in a dialysis bag (MW cutoff: 5 kDa). Then the purified N-CDs diluted to 100 mL after neutralized with 1 mol L−1 NaOH solution. The purified N-CDs were preserved at room temperature for further uses. 2.4. Measurement of quantum yield In order to measure The QY of as-prepared CDs, we carried out this experiment on the report of a previously described protocol [43]. Quinine sulfate in 0.1 M H2SO4 was chosen as a refrence ( QY = 0.54). To reduce self absorption effects, absorbance values for both the carbon dots and refrence solution were restricted below 0.1 at 340 nm. The quantum yield of the N-CQDs was determined as below:
Grad ⎞ ⎛ n2 ⎞ Q = QR ⎛ ⎜ ⎟ 2 Grad R ⎠ ⎝ nR ⎠ ⎝ ⎜
⎟
Where Q is the quantum yield of fluorescence, Grad denotes to the gradient from the plot of integrated photoluminescence intensity (excited at 330 nm) versus the absorbance, n is refractive index of the solvent and the subscript R refers standard reference. 2.5. Fluorescence sensing of selenite In a typical assay for the determination of selenite, 18 µL of N-CDs (1 mg mL−1), 600 µL of Tris-HCl buffer solution (50 mM, pH 7.0) and 6.6 µL of Eu3+ (0.1 M) were mixed thoroughly, and diluted to 3 mL using ultrapure water. Then 10 µL of serial concentrations of selenite was added. After equilibration for 1 min, the photoluminescence (PL) spectra were recorded under the fixed excitation wavelength at 330 nm and emission spectra were collected at 430 nm, respectively. In order to obtain favorable fluorescence spectra, the excitation and emission slit was adjust to 5 nm.
2. Experimental 2.1. Materials Citric acid (CA), L-lysine (97.0%), Eu(NO3)3·5H2O (99.9%), Ce (NO3)3·6H2O (99.9%), Sm (NO3)3·6H2O (99.9%) and quinine sulfate (98.0%) were purchased from Sigma-Aldrich, St. Louis, MO, USA. Sodium selenite (Na2SeO3), sodium nitrate (NaNO3), sodium nitrite (NaNO2), FeCl3·6H2O, NiCl2·6H2O, CoCl2·6H2O, Zn (NO3)3·6H2O, MnCl2·4H2O, CuSO4·5H2O, CaCl2, and other materials were obtained from Merck, Darmstadt, Germany. All reagents were of analytical grade, and doubly distilled water was used throughout.
2.6. Detection of selenite in real sample For the analysis of real samples, the selenium content of selenium sulfide shampoo was evaluated by applying the CDs-Eu3+ system prepared in this work. The pretreatment of shampoo sample was performed following the previously reported procedure [44]. Briefly, 1 g of shampoo sample was placed into a 100 mL Kjeldahl flask, then 1 mL of sulfuric acid (98%) was added. The solution mixture was heated to fuming for 15 min. After cooling to room temperature, 5 mL of 30% (w/ v) H2O2 was introduced. To discard excess hydrogen peroxide, the mixture was boiled severely and allowed to cool. Then the solution was then diluted 100-fold with water before using for the analysis.
2.2. Instrumentation Morphology and size of N-CDs were examined by a transmission electron microscopy (TEM, Philips, CM30, operating at 100 kV). UV–vis absorption and fluorescence spectra were collected with an Agilent 8453 Diode array spectrophotometer and an LS-50B fluorescence spectrometer equipped with a 1.0 cm quartz cell and a xenon lamp (PerkinElmer, USA), respectively. The Fourier Transform infrared (FTIR) spectra were recorded on a Perkin-Elmer model Spectrum GX within the range 400–4000 cm−1. X-ray photo-electron spectroscopy (XPS) analysis was obtained on an 8025-BesTec twin anode XR3E2 X-ray source system. X-ray diffraction (XRD) patterns were carried out on a 38066 Riva, d/G. via M. Misone, 11/D (TN), Italy. A Metrohm model 713 pH-meter was used for pH measurements.
3. Results and discussion 3.1. Formation mechanism and characterization of N-CDs As shown in Scheme 1, water soluble N-CDs were synthesized and an “on-off-on” platform was designed for highly selective and rapid detection of tetravalent selenium, based on a competitive reaction 769
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Scheme 1. Schematic illustration of sensing process for detection of selenite based on N-CDs/Eu3+.
Fig. 1. A: a. UV–vis absorption spectra of N –CDs, b. Excitation spectra, c. Emission spectra and Inset: photographs of N -CDs under daylight (right) and UV light (left), and B: fluorescence emission spectra of the as-prepared N-CDs under different excitation wavelengths.
product. N-CDs were synthesized from citric acid and L-lysine through hydrothermal carbonization treatment under high pressure at 200 °C for 3 h. L-lysine was carefully chosen to fulfill the nitrogen-doping process based on the following considerations. On the one hand, citric acid have a low carbonization temperature and contains three carboxylic groups [45]. Thus can react with the amine groups of lysine at the early stage of the hydrothermal process to form a nitrogen-containing precursor. Probably formation process for this N-CDs is that firstly citric acid carbonizes to form carboxylated CDs, and then the amidation reaction of the carboxyl CDs and L-lysine occurs to doping the CDs.
between selenium oxyanion (selenite), and carboxylic acid and hydroxyl groups on the surface of the prepared N-CDs for europium ions. In this way, upon the addition of Eu3+, aggregation of N-CDs occurs due to Eu3+ ions coordination with the carboxyl and hydroxyl groups at the surface of the N-CDs and act as a bridge between N-CDs. Upon the addition of selenite into N-CDs/Eu3+, induced the disruption of Eu3+triggered aggregation of N-CQDs because of stronger affinity of selenite than that of the carboxyl or hydroxyl groups on the surface of N-GQDs to europium ions and then fluorescence of N-CDs was recovered. We further demonstrate the applicability of this sensor through employing the developed assay strategy for the detection of selenite in health care 770
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considerable distribution of surface emissive trap sites on each N-CDs [48,49]. 3.2. Chemical and physical characterization of nitrogen-doped C-dots The formation of N-CDs was confirmed by TEM measurements. Fig. 2A exhibits the TEM image of prepared N-CDs. It could be observed that N-CDs have a spherical shape and well-dispersion. Also the size distribution histogram shows the narrow size distribution that alights in the range of 1–6 nm and the average size value is 3 nm (Fig. 2A, inset). The XRD pattern of the N-CDs (Fig. 2B) displayed a broad diffraction peak at 2θ = 23.5° which is due to the small size of the N-CDs. An interlayer spacing 0.39 nm which is contributed by the (002) plane of graphite. The corresponding interlayer spacing in graphite (0.34 nm) becomes larger in N-CDs, which can be ascribed to the introduction of rich of oxygen groups [50]. To characterize the surface functional groups, FT-IR were performed. As revealed in Fig. 2C, a broad absorption band centered at 3200–3472 cm− 1 attributed to the stretching vibrations of O–H and N–H groups, whereas the sharp peak located at 2987 cm− 1 is typically corresponded with C–H stretching vibrations of alkane groups. The band at 1668 cm−1 can be assigned to the C˭C stretching mode of the aromatic hydrocarbons and the peaks at around 1733, 1074, 1229 cm−1 attributed to stretching vibrations of C˭O and phenolic C–OH groups, respectively, implying the existence of oxygen-rich groups on the surface of N-doped CDs [47]. More importantly, the characteristic absorption peaks amido CON—H bending vibration (1501 cm−1) and amido CO—N (1405 cm−1) stretching vibration are identified in N-CDs, indicated a successful incorporation of nitrogen atoms in N-CDs [51,52]. All results demonstrate that the surfaces of CDs are surrounded by hydrophilic groups, which improve stability and hydrophilicity of the N -CDs in an aqueous system. To further confirm the chemical composition of the CDs, XPS spectrum were collected for N-doped CDs. As shown in Fig. 3A the nitrogen-doped carbon dots are predominantly composed of oxygen, carbon and nitrogen which revealed three peaks 284.7, 393.9, and 529.4 eV that related to C 1s, N 1s, and O 1s, respectively. A highresolution XPS spectrum of C1 s (Fig. 3B) can be resolved into four different peaks at 284.7, 285.2, 286.9, and 288.5 eV, which can be ascribed to C˭C, C–N, C–O, and C˭N/C˭O groups, respectively [53,54]. These results confirm the graphitic structure (sp2 C˭C) of the N-CDs. The N 1s spectrum (Fig. 3C) reveals two distinct peaks at 400, and 401.5 eV arising from the pyrrolic-like N–(C) 3 and N–H/NH2 groups, respectively [55]. The O1s spectrum (Fig. 3D) can be deconvoluted into two peaks at 533.1 and 531.8 eV, which are associated with the C–OH/ C–O–C and C˭O bands, respectively [29,56]. These results confirm once again that the N-CDs are modified with plentiful hydrophilic groups. Surface chemistry of the N-CDs determined by XPS which is consistent with the results of FT-IR.
Fig. 2. A: TEM image of the N-CDs. Inset: The size distribution of the N-CDs. B: XRD spectra of the N-CDs. C: FT-IR spectrum of the N-CDs.
As shown in Fig. 1A typical UV–visible spectra of N-CDs exhibits two absorption peaks: a shoulder at 250 nm that mainly ascribed to the π-π* energy transition of the CDs and this corresponding to the carbon core C˭C of a conjugated system and a peak at about 340 nm can be regarded as the n−π* transition of C˭O or C˭N bond [46,47]. The CDs had an emission peak at 430 nm when excited at 330 nm. Inset of Fig. 1A shows the optical images of the C-dots under the illumination of sunlight and irradiated by a 365 nm UV lamp, respectively To further explore their optical properties, photoluminescence behavior of N-CDs was studied under the different excitation wavelengths. As-prepared N-CDs, similar to the other carbon nanomaterial, exhibit excitation dependent PL behavior. So that as shown in Fig. 1B by increasing excitation wavelength, PL emission band gradually shifted to higher emission wavelengths, i.e. from 305 to 465 nm, also along with decreasing PL intensity. Previous reports indicate that this phenomenon caused by the synergistic effect of variation of particle sizes and the
3.3. The possible quenching and recovery mechanism In order to study the Off-On mechanism several experiments were performed. Fig. 4A exhibits the high fluorescence intensity of CDs at 430 nm. The fluorescence of N-CDs decreased to 53% of the initial value during 5 min in the presence of 220 µM Eu3+ ions (Fig. 4A. curves a,b). It is expected that, upon the addition of selenite to the CDs/ Eu3+ system, the fluorescence intensity at 415 nm increased along with a little shift (Fig. 4A. curve c). After Eu3+ addition, because of the coordination between Eu3+ and N-CDs, Eu3+ acts as a bridge between neighboring carboxylate and hydroxyl groups at the surface of N-CDs leads to the formation of N-CDs aggregates. Based on this point of view, possible mechanism of florescence quenching related to N-CDs was analyzed using a Stern-Volmer equation: F0/F = 1+Ksv [Eu3+], where F0 and F are the fluorescence intensities of N-CDs at 430 nm in the 771
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Fig. 3. A: XPS survey spectra of the N-CDs. High-resolution XPS spectra of C1s: B, N1s: C and O1s:D.
Fig. 4. A: Fluorescence emission spectra of CDs under different conditions. B: Resonance light scattering spectra of the N-CD solution with different conditions (I: CDs; II: CDs/Eu3+; III: CDs/Eu3+/selenite; IV: Eu3+/selenite). Concentrations of Eu3+ and selenite were 220 μM and 7.8 µg/mL, respectively.
[57]. Therefore the quenching rate constant (Kq), Kq = Ksv τ0, was obtained as 2 × 1012 M−1 s−1. These results indicate that the quenching provoked by Eu3+ is probably because of static quenching arising from the formation of complex between N-CDs and Eu3+ [57].
absence and presence of Eu3+, respectively. KSV (Stern-Volmer constant) obtained from the plots of F0 /F was found to be 2 × 103 M−1 indicating the high quenching efficiency of the N-CDs excited state. Also, the fluorescence lifetime of N-CDs that is reported as 11.06 ns 772
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Fig. 5. A: Fluorescence intensity ratio F/F0 of the N-CDs solution in the presence of various metal ions B: The relative fluorescence intensity of N-CDs in the presence of different concentrations of Eu3+. F and F0 represent the FL intensities of CDs in the presence and absence of Eu3+, respectively.
Fig. 6. A, B photoluminescence spectra of free N-CDs with different pH conditions.
After the addition of selenite, fluorescence is seen to be regenerated along with the red shift of the PL emission wavelength from 415 nm to 425 nm. The shift in emission wavelength might suggest that Eu3+induced close attachment of N-CDs affects the luminescence property of N-CDs to some degree [58]. As expected, the Eu3+ ion strongly binds to selenite. Meanwhile, there is no detectable fluorescence response when the selenite solution was added to the N-CDs solution alone (Fig. 4B, curve c). It was worth nothing that no change in fluorescence intensity was perceived in the selenite and Eu3+ solution alone (Fig. 4B, curve e). These results indicate that Eu3+ can binds with selenite preferentially, removes from the surface of N-CDs, thus effectively recovery of fluorescence was occurred (Scheme 1). Thus a switchable “on-off-on” sensor was fabricated based on the above strategy. To support the predicted mechanism in Scheme 1, resonance light scattering (RLS) technique was carried out. The RLS spectra were recorded by synchronous scanning of the excitation and emission monochromators from 200 to 800 nm. The RLS intensities of the N-CD dispersion before and after the addition of selenite are very weak (Fig. 4B, curves I and IV). After the addition of europium ions, the RLS intensities increased considerably that confirms the formation of N-CDs aggregations by Eu3+ (Fig. 4B, Curve II). By the addition of selenite to
N-CDs/Eu3+ system, the corresponding RLS intensities decreased (Fig. 4B, curve III). The cause of this is that europium ions have stronger affinity for selenite than the carboxylate and hydroxyl groups at the surface of N-CDs. Therefore, selenite acts as an anti-aggregating agent and the aggregates formed are redisposed. Also the RLS pattern of the Eu3+/selenite mixture is similar to that of the CDs/selenite mixture and N-CDs alone solution which clearly verifies the above stated mechanism. Thus based on the mechanism, N-CDs /Eu3+ were developed as sensors to respond selenite. 3.4. Optimization of the experimental conditions 3.4.1. Selection of quencher ion and its concentration The fluorescence response of N-CDs upon the addition of different cautions was investigated. The examined metal ions include Na+, K+, Mg2+, Ca2+, Zn2+, Fe2+,Ni2+, Co2+, Fe3+, Mn2+, Pb2+and some lanthanide ions such as Eu3+, Sm3+, Ce3+. As shown in Fig. 5A, negligible fluorescence signal changes were observed in the presence of common metal ions except Fe3+, and among the tested lanthanide ions, it is evident that the fluorescence intensity of N-CDs shows a significant quenching when Eu3+ was added. Since we intend to design a selenite 773
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Fig. 7. A: PL intensities of N-CDs under different conditions (a: free N-CDs, b: N-CDs in the presence of Eu3+, c: N-CDs/Eu3+in the presence of selenite. B: pH effect on the N-CDs/ Eu3+ sensing system, C: effect of N-doped CDs concentrations, D: effect of time. Concentrations of Eu3+ and selenite were 220 μM and 7.8 µg ml−1, respectively.
Fig. 8. A: Fluorescence spectra of N-CDs/Eu3+ to selenite at different concentrations under excitation at 330 nm, B: The linear relationship between ΔF and selenite concentration in the range of 0.078–54.6 µg/mL.
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investigated the effect of pH on the performance of the CDs-Eu3+ sensing system. (Fig. 7A and B). Fig. 7 exhibits the fluorescence response changes of CDs-Eu3+ without and with selenite at different pH values. This behavior caused by dependent on coordination between NCDs and Eu3+ at different pH values. At pH < 4, owing to coordination between protonated N-CDs and Eu3+, quenching of FL is not completed. At pH > 9, since the Eu3+ ions belong to hard acids, formation of insoluble complex with OH- (Eu(OH)3) is more favorable than with carboxylate groups, then making the fluorescence quenched incompletely [41]. Thus the fluorescence quenching of the N-CDs was impressive for pH ranged between 4 and 9. Maximum extent of fluorescence recovery was achieved by selenite at pH 7. The possible reason for this can be due to that the dominant form of selenite in this pH is HSeO−3 and negligible SeO32 − is present [60]. The positively charged surface of the probe (N-CDs/Eu3+) caused the coordination between probe and selenite, thus the possibility of Eu3+ removal by selenite from the surface of N-CDs is provided. Therefore, the pH was adjusted using a Tris-HCl buffer (pH 7, 10 mM) for subsequent detection experiments.
Fig. 9. Selectivity of the N-CDs/Eu3+ toward selenite; the concentration of all interference anions were 7.8 μg mL−1. All experiments were carried out in Tris-HCl (7, 10 mM) buffer.
Table 1 Tolerance limits of the foreign ions in the analysis of selenite by CDs/Eu3+ system. Added ions
Tolerance limit (MFIa/MSelenite)
CH3COOBr-, Cl-, Na+, K+ S2-, Pb2+
600 500 200 100
SO24− , NO− 3 Ca2+, Mg2+ − − NO2 , ClO3
50 30 20
SO32 − , Zn2+ FFe3+ a
3.4.3. Effect of N-CDs dosage The restoration efficiency [F–F0] (where F0 and F are the fluorescence intensities of the CDs/Eu3+ mixture at 430 nm in the absence and presence of selenite, respectively) in the presence of 7.8 µg ml−1 selenite as a function of N-CDs concentration was evaluated. As shown in Fig. 7C, by increasing concentration of N-doped CDs, the restoration efficiency was gradually increased and then decreased which may be attributed to the self-absorption and aggregation of CDs in the high concentrations of them. Thus, 0.17 mg mL−1 of N-doped CDs was selected for the following experiment.
10 2
Foreign ion.
3.4.4. Incubation time effect The effect of the incubation time of selenite was investigated as well. As shown in Fig. 7D, by the addition of selenite, the fluorescence recovery reached its maximum value after 5 min and then remained stable, showing that the interaction between Eu3+ and selenite reached equilibrium rapidly. Therefore the reaction time was fixed at 5 min.
Table 2 The application of the proposed method for analysis of shampoo samples spiked with different amounts of selenite. Se(µg/mL) Added
Found
Recovery (%)
(%) RSD (n = 3)
0 4.9 9.9 15.8
1.6 6.4 12.3 18.6
– 97.9 108 107.6
– 3.13 2.9 2.25
3.5. Analytical performance Under the optimized conditions, N-CDs/Eu3+ could be used as the fluorescence probe for the sensing of selenite. The fluorescent spectra of N-CDs/Eu3+ in the presence of different amounts of selenite were exhibited in Fig. 8A. Therefore, by increasing the concentration of selenite (0.078–54.6 µg mL−1) to the probe solution (N-CDs/Eu3+), fluorescence intensity of the system gradually increased. As indicated in Fig. 8B, the enhancement of the fluorescence recovery was closely related to the concentration of selenite. The recovery efficiency (ΔF), exhibited a good linear relationship as a function of the concentration of selenite over the range of 0.078–21.4 µg mL−1 with a detection limit of 54 ng mL−1 (Fig. 6B). The calibration equation, with the favorable correlation coefficient (R2 = 0.993), was Y= 0.0508 + 0.2072C, in which Y ascribes to the recovery efficiency (F–F0) and C was the concentration of selenite in µg mL−1.
sensor, in accordance with the HSAB theory, Eu3+ is harder than Fe3+ ion, therefore we predicted that it forms stronger bond caused by formation potent coordination between a hard acid (Eu3+) and a hard base (HSeO3– or SeO32 −) than Fe3+ [59]. Eu3+ concentration should be chosen so that the fluorescence intensity of N-CDs could be quenched in the absence of selenite while not excess, and impressive FL recovery could be observed with the addition of selenite. For this purpose, as shown in Fig. 5B, the quenching efficiency could reach over 50% when 220 µM Eu3+ was added. Moreover, excess Eu3+, existed as free site in the solution, will cause error in the procedure of detection due to their coordination ability with selenite. Therefore, 220 µM Eu3+ was chosen in subsequent experiments.
3.6. Selectivity To evaluate the selectivity of the CDs/Eu3+ sensing system for selenite, the effect of equal concentrations of various anions on the changes of fluorescence intensity was investigated. The fluorescence recovery efficiency [(F - F0)/F0] (where F0 and F are the PL intensities before and after adding selenite, respectively) of each anions is displayed in Fig. 9. It can be seen that selenite shows the largest recovery among the anions tested, whereas other anions have weak effects on the fluorescence of CDs/Eu3+ probe. This excellent selectivity is attributed to the strong interaction between Eu3+ ions and the oxygen atoms of
3.4.2. pH effect Fig. 6A and B exhibit the change in FL intensities of free N-CDs at different pH values. As could be seen, the fluorescence intensity of CDs in the pH range 4–8 remained relatively constant which offers the feasibility to construct a sensor over this range. Under acidic (pH < 3) and basic conditions (pH > 9), protonation – deprotonation of functional groups at the surface of N-CDs (carboxylic acid, hydroxyl) take place and consequently decreases the FL intensity [59]. We also 775
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[13] A. Safavi, A. Afkhami, A. Massoumi, Spectrophotometric catalytic determination of ultra-trace amounts of selenium based on the reduction of resazurin by sulphide, Anal. Chim. Acta 232 (1990) 351–356. [14] F. Costa-Silva, G. Marques, C.C. Matos, A.I.R.N.A. Barros, F.M. Nunes, Selenium contents of Portuguese commercial and wild edible mushrooms, Food Chem. 126 (2011) 91–96. [15] S. Liang, J. Chen, D.T. Pierce, X. Zhao, A turn-on fluorescent nanoprobe for selective determination of selenium (IV), ACS Appl. Mater. Interfaces 5 (2013) 165–5173. [16] W.B. Dye, E. Bretthauer, H.J. Seim, C. Blincoe, Fluorometric determination of selenium in plants and animals with 3,3-diaminobenzidine, Anal. Chem. 35 (1963) 1687–1693. [17] J.H. Watkinson, Fluorometric determination of traces of selenium, Anal. Chem. 32 (1960) 981–983. [18] J. Pedro, F. Andrade, D. Magni, M. Tudino, A. Bonivardi, On-line submicellar enhanced fluorometric determination of Se (IV) with 2, 3-diaminonaphthalene, Anal. Chim. Acta 516 (2004) 229–236. [19] J. Zhu, S. Liu, Z. Liu, Y. Li, M. Qiao, X. Hu, Highly selective speciation and fluorimetric determination of Se(IV) in infant formulas using micelle-capped nile blue A, RSC Adv. 3 (2013) 21570–21575. [20] Y.P. Sun, B. Zhou, Y. Lin, W. Wang, K.A.S. Fernando, P. Pathak, M.J. Meziani, B.A. Harruff, X. Wang, H. Wang, P. Luo, H. Yang, M.E. Kose, B.L. Chen, L.M. Veca, S. Xie, Quantum-sized carbon dots for bright and colorful photoluminescence, J. Am. Chem. Soc. 128 (2006) 7756–7762. [21] G.E. LeCroy, S.K. Sonkar, F. Yang, L.M. Veca, P. Wang, K.N. Tackett, J.J. Yu, E. Vasile, H. Qian, Y. Liu, P. Luo, Y.P. Sun, Toward structurally defined carbon dots as ultracompact fluorescent probes, ACS Nano 8 (2014) 4522–4529. [22] S.N. Baker, G.A. Baker, Luminescent carbon nanodots: emergent nanolights, Angew. Chem. Int. Ed. 49 (2010) 6726–6744. [23] H. Li, Z. Kang, Y. Liu, S.T. Lee, Carbon nanodots: synthesis, properties and applications, J. Mater. Chem. 22 (2012) 24230–24253. [24] C. Ding, A. Zhu, Y. Tian, Functional surface engineering of Cdots for fluorescent biosensing and in vivo bioimaging, Acc. Chem. Res. 47 (2014) 20–30. [25] X. Xu, R. Ray, Y. Gu, H.J. Ploehn, L. Gearheart, K. Raker, W.A. Scrivens, Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments, J. Am. Chem. Soc. 126 (2004) 12736–12737. [26] Y.Q. Zhang, D.K. Ma, Y.G. Zhang, W. Chen, S.M. Huang, N-doped carbon, quantum dots for TiO2 -based photocatalysts and dye-sensitized solar cells, Nano Energy 2 (2013) 545–552. [27] W. Kwon, S. Do, J. Lee, S. Hwang, J.K. Kim, S.W. Rhee, Free standing luminescent films of nitrogen-rich carbon nanodots toward large-scale phosphor-based whitelight-emitting devices, Chem. Mater. 25 (2013) 1893–1899. [28] H. Ding, J.S. Wei, H.M. Xiong, Nitrogen and sulfur co-doped carbon dots with strong blue luminescence, Nanoscale 6 (2014) 13817–13823. [29] W. Wang, Y.C. Lu, H. Huang, A.J. Wang, J.R. Chen, J.J. Feng, Facile synthesis of N,S-co doped fluorescent carbon nanodots for fluorescent resonance energy transfer recognition of methotrexate with high sensitivity and selectivity, Biosens. Bioelectron. 64 (2015) 517–522. [30] H. Li, H. Ming, Y. Liu, H. Yu, X. He, H. Huang, K. Pan, Z. Kang, S.-T. Lee, Fluorescent carbon nanoparticles: electrochemical synthesis and their pH sensitive photoluminescence properties, New J. Chem. 35 (2011) 2666–2670. [31] L. Cao, X. Wang, M.J. Meziani, F. Lu, H. Wang, P.G. Luo, Y. Lin, B.A. Harruff, L.M. Veca, D. Murray, S.Y. Xie, Y.P. Sun, Carbon dots for multiphoton bioimaging, J. Am. Chem. Soc. 129 (2007) 11318–11319. [32] S. Sahu, B. Behera, T.K. Maiti, S. Mohapatra, Simple one-step synthesis of highly luminescent carbon dots from orange juice: application as excellent bio-imaging agents, Chem. Commun. 48 (2012) 8835–8837. [33] B. Han, W. Wang, H. Wu, F. Fang, N. Wang, X. Zhang, S. Xu, Polyethyleneimine modified fluorescent carbon dots and their application in cell labeling, Colloids Surf. B 100 (2012) 209–214. [34] C. Liu, P. Zhang, X. Zhai, F. Tian, W. Li, J. Yang, Y. Liu, H. Wang, W. Wang, W. Liu, Nano-carrier for gene delivery and bioimaging based on carbon dots with PEIpassivation enhanced fluorescence, Biomaterials 33 (2012) 3604–3613. [35] J. Wang, C.F. Wang, S. Chen, Amphiphilic egg-derived carbon dots: rapid plasma fabrication, pyrolysis process, and multicolor printing patterns, Angew. Chem. Int. Ed. 37 (2012) 9297–9301. [36] Z. Ma, H. Ming, H. Huang, Y. Liu, Z. Kang, One-step ultrasonic synthesis of fluorescent N-doped carbon dots from glucose and their visible-light sensitive photocatalytic ability, New J. Chem. 36 (2012) 861–864. [37] B.B. Chen, Z.X. Liu, W.C. Deng, L. Zhan, M. Liu, C.Z. Huang, A large-scale synthesis of photoluminescent carbon quantum dots: a self-exothermic reaction driving the formation of the nanocrystalline core at room temperature, Green Chem. 18 (2016) 5127–5132. [38] C. Liu, P. Zhang, F. Tian, W. Li, F. Li, W. Liu, One-step synthesis of surface passivated carbon nanodots by microwave assisted pyrolysis for enhanced multicolor photoluminescence and bioimaging, J. Mater. Chem. 21 (2011) 13163–13167. [39] S. Gao, Y. Chen, H. Fan, X. Wei, C. Hu, L. Wang, L. Qu, A green one-arrow-twohawks strategy for nitrogen-doped carbon dots as fluorescent ink and oxygen reduction electro catalysts, J. Mater. Chem. A 2 (2014) 6320–6325. [40] L. Lin, X. Song, Y. Chen, M. Rong, T. Zhao, Y. Jiang, Y. Wang, X. Chen, One-pot synthesis of highly greenish-yellow fluorescent nitrogen-doped graphene quantum dots for pyrophosphate sensing via competitive coordination with Eu3+ ions, Nanoscale 7 (2015) 15427–15433. [41] J.M. Bai, L. Zhang, R.P. Liang, J.D. Qiu, Graphene quantum dots combined with europium ions as photoluminescent probes for phosphate sensing, Chem. Eur. J. 19 (2013) 3822–3826. [42] Y. Dong, H. Pang, H.B. Yang, C. Guo, J. Shao, Y. Chi, C.M. Li, T. Yu, Carbon-based
selenite ion, as previously discussed. For more investigation of selectivity, the interference from some anions and various cations in the presence of selenite with excess amounts of coexisting ions was examined. The summary of the obtained results is presented in Table 1. It can be clearly seen that all the examined ions, except Fe3+, have no serious interfering effect on the fluorescence response even when the concentration of the tested ions was 10–600-fold excess over selenite. The tolerance limit was defined as the concentration of coexisting species that produces a relative error of ≤ 5.0% for selenite detection. The results demonstrate that the developed Eu3+-adjusted sensor exhibits excellent selectivity for the determination of selenite. 3.7. Application to real samples To demonstrate the applicability of N-CDs/Eu3+ system to sensing selenite, selenium sulfide shampoo 2.5% (for treatment of dandruff) as real sample was selected. The results are summarized in Table 2. the relative standard deviation of the results for selenite were lower than 3.2%, and the recoveries of selenite were found to be in the range of 97.9–108.0%, indicating that our sensing system was highly reproducible and accurate for rapid sensing of selenite in such real samples. 4. Conclusion Briefly, in this work, nitrogen-doped carbon quantum dots, with highly quantum yield (21.5%), were synthesized and utilized to construct an efficient off-on fluorometric assay for highly selective detection of selenite. To the best of our knowledge, this is the first report on the construction of a sensor for selenite based on aggregation of CDs and europium ions. Compared with the previous fluorimetric works, the proposed method does not require preconcentration step, time consuming procedures and complexed operatory. Meanwhile, take advantages such as simplicity in method, environmentally-friendly, lowcost, fast response and easy operation have caused our offered strategy become an easy, fast and reproducible method for the determination of selenite. References [1] M. Navarro-Alarcon, M.C. Lopez-Martinez, Essentiality of selenium in the humanbody: relationship with different diseases, Sci. Total Environ. 249 (2000) 347–371. [2] T. Klapec, M.L. Mandic, J. Grgic, L. Primorac, A. Perl, V. Krstanovic, Selenium in selected foods grown or purchased in eastern Croatia, Food Chem. 85 (2004) 445–452. [3] B. Izgi, S. Gucer, R. Jacimovic, Determination of selenium in garlic (Allium sativum) and onion (Allium cepa) by electro thermal atomic absorption spectrometry, Food Chem. 99 (2006) 630–637. [4] H. Robberecht, R.V. Grieken, Selenium in environmental waters: determination, speciation and concentration levels, Talanta 29 (1982) 823–844. [5] J.B. Loṕez-Saez, A. Senra-Varela, L. Pousa-Estevez, Selenium in breast cancer, Oncology 64 (2003) 227–231. [6] K. Agrawala, K.S. Patela, K. Shrivasb, Development of surfactant assisted spectrophotometric method for determination of selenium in waste water samples, J. Hazard. Mater. 161 (2009) 1245–1249. [7] B. Chen, M. He, X. Mao, R. Cui, D. Pang, B. Hu, Ionic liquids improved reversedphase HPLC on-line coupled with ICP-MS for selenium speciation, Talanta 83 (2011) 724–731. [8] F. Wang, G. Zhang, Simultaneous quantitative analysis of arsenic, bismuth, selenium, and tellurium in soil samples using multi-channel hydride-generation atomic fluorescence spectrometry, Appl. Spectrosc. 65 (2011) 315–319. [9] V. Stibilj, P. Smrkolj, A. Krbavcic, Investigation of the Declared Value of Selenium in Food Supplements by HG-AFS, Microchim. Acta 150 (2005) 323–327. [10] H. Stosnach, Analytical determination of selenium in medical samples, staple food and dietary supplements by means of total reflection X-ray fluorescence spectroscopy, Spectrochim. Acta B 65 (2010) 859–863. [11] J.Y. Cabon, W. Erler, Determination of selenium species in seawater by flow injection hydride generation in situ trapping followed by electrothermal atomic absorption spectrometry, Analyst 123 (1998) 1565–1569. [12] W.T. Buckley, J.J. Budac, D.V. Godfrey, K.M. Koenig, Determination of selenium by inductively coupled plasma mass spectrometry utilizing a new hydride generation sample introduction system, Anal. Chem. 64 (1992) 724–729.
776
Journal of Luminescence 194 (2018) 768–777
N. Amin et al.
[43]
[44]
[45] [46]
[47]
[48]
[49]
[50]
[51]
[52]
dots co-doped with nitrogen and sulfur for high quantum yield and excitation-independent emission, Angew. Chem. Int. Ed. 52 (2013) 7800–7804. S. Sahu, B. Behera, T.K. Maiti, S. Mohapatra, Simple one-step synthesis of highly luminescent carbon dots from orange juice: application as excellent bio-imaging agents, Chem. Commun. 48 (2012) 8835–8837. A. Afkhami, A. Safavi, A. Massoumi, Spectrophotometric determination of trace amounts of selenium with catalytic reduction of bromate by hydrazine in hydrochloric acid media, Talanta 39 (1992) 993–996. Y. Dong, R. Wang, H. Li, J. Shao, Y. Chi, X. Lin, G. Chen, Polyamine-functionalized carbon quantum dots for chemical sensing, Carbon 50 (2012) 2810–2815. S. Zhu, Q. Meng, L. Wang, J. Zhang, Y. Song, H. Jin, K. Zhang, H. Sun, H. Wang, B. Yang, Highly photoluminescent carbon dots for multicolor patterning, sensors, and bioimaging, Angew. Chem. Int. Ed. Engl. 52 (2013) 3953–3957. D. Pan, J. Zhang, Z. Li, C. Wu, X. Yana, M. Wu, Observation of pH-, solvent-, spin-, and excitation-dependent blue photoluminescence from carbon nanoparticles, Chem. Commun. 46 (2010) 3681–3683. H. Zhu, X. Wang, Y. Li, Z. Wang, F. Yanga, X. Yang, Microwave synthesis of fluorescent carbon nanoparticles with electrochemiluminescence properties, Chem. Commun. 34 (2009) 5118–5120. H. Huang, C.G. Li, S.J. Zhu, H.L. Wang, C.L. Chen, Z.R. Wang, T.Y. Bai, Z. Shi, S.H. Feng, Histidine-derived nontoxic nitrogen-doped carbon dots for sensing and bioimaging applications, Langmuir 30 (2014) 13542–13548. Y.P. Hu, J. Yang, J.W. Tian, L. Jia, J.S. Yu, Waste frying oil as a precursor for onestep synthesis of sulfur-doped carbon dots with pH-sensitive photoluminescence, Carbon 77 (2014) 775–782. X. Gong, W. Lu, M.C. Paau, Q. Hu, X. Wu, S. Shuang, C. Donga, M.M.F. Choi, Facile synthesis of nitrogen-doped carbon dots for Fe3+ sensing and cellular imaging, Anal. Chim. Acta 861 (2015) 74–84. J. Duan, J. Yu, S. Feng, L. Su, A rapid microwave synthesis of nitrogen–sulfur co-
[53]
[54]
[55]
[56]
[57]
[58]
[59]
[60]
777
doped carbon nanodots as highly sensitive and selective fluorescence probes for ascorbic acid, Talanta 153 (2016) 332–339. C.H. Choi, M.W. Chung, S.H. Park, S.I. Woo, Additional doping of phosphorus and/ or sulfur into nitrogen-doped carbon for efficient oxygen reduction reaction in acidic media Phys, Chem. Chem. Phys. 15 (2013) 1802–1805. S. Wohlgemuth, R.J. White, M. Willinger, M. Titirici, M. Antonietti, A one-pot hydrothermal synthesis of sulfur and nitrogen-doped carbon aerogels with enhanced electrocatalytic activity in the oxygen reduction reaction, Green Chem. 14 (2012) 1515–1523. Z. Yang, M. Xu, Y. Liu, F. He, F. Gao, Y. Su, H. Wei, Y. Zhang, Nitrogen-doped, carbon-rich, highly photoluminescent carbon dots from ammonium citrate, Nanoscale 6 (2014) 1890–1895. W. Wang, Y.C. Lu, H. Huang, A.J. Wang, J. Chen, J.J. Feng, Solvent-free synthesis of sulfur- and nitrogen-co-doped fluorescent carbon nanoparticles from glutathione for highly selective and sensitive detection of mercury (II) ions, Sens. Actuators B: Chem. 202 (2014) 741–747. M. Lin, H.Y. Zou, T. Yang, Z.X. Liu, H. Liu, C.Z. Huang, An inner filter effect based sensor of tetracycline hydrochloride as developed by loading photoluminescent carbon nanodots in the electrospun nanofibers, Nanoscale 8 (2016) 2999–3007. Q. Zhang, C. Song, T. Zhao, H.W. Fu, H.Z. Wang, Y.J. Wang, D.M. Kong, Photoluminescent sensing for acidic amino acids based on the disruption of graphene quantum dots/europium ions aggregates, Biosens. Bioelectron. 65 (2015) 204–210. R.S. Dickins, A.S. Batsanov, J.A.K. Howard, D. Parker, H. Puschmanna, S. Salaman, Structural and NMR investigations of the ternary adducts of twenty α-amino acids and selected dipeptides with a chiral, diaqua–ytterbium complex, Dalton Trans. (2004) 70–80. V.R. Preedy, Selenium: Chemistry, Analysis, Function and Effects, Royal Society of Chemistry, School of Medicine, King's College London, UK, 2015.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Construction of a novel "Off-On" fluorescence sensor for highly selective sensing of selenite based on europium ions induced crosslinking of nitrogendoped carbon dots
MARK
⁎
Niloufar Amin, Abbas Afkhami , Tayyebeh Madrakian Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Europium ions Selenite Nitrogen-doped carbon dots Off-On probe
A highly blue luminescent nitrogen-doped carbon dots (N-CDs) was prepared, using citric acid and L-lysine via one pot hydrothermal treatment, and applied to the determination of selenite. The synthesized N-CDs exhibited a high photostability, excellent optical features with a high quantum yield (21.5%) and average size of 3 nm. The method is based on the competition between oxygen donor atoms in selenite oxyanion with those from carboxylate and hydroxyl groups at the surface of N-carbon dots for europium ions binding. N-CDs could be readily quenched upon the addition of europium ions owing to high affinity of carboxylate and hydroxyl groups at the surface of carbon dots to Eu3+ leading to aggregation of N-CDs (state OFF). After selenite addition, disruption of the aggregated N-CDs takes place, leading to restoration of the quenched fluorescence (State ON). The repeatability was less than 3.2% for selenite in both standard and real samples (n = 3). The method provides a simple procedure enabled selective detection of selenite with a linear range of 0.078–21.4 µg mL−1 and a limit of detection of 53.0 ng mL−1 (S/N = 3). The accuracy and precision were evaluated based on the detection of selenite in health care products with satisfactory results.
1. Introduction Selenium (Se) is a nutrient and essential element for human health at a slight level, but its excessive amounts is toxic and harmful for living organism. Se is a main component of antioxidant enzymes which protecting the human body from free radicals [1–3]. In general, selenium is required by the human body in the range 50–200 µg day−1 [4]. Therefore, taking less than the recommended dosage of Se will cause a number of deficiency syndromes [5]. Furthermore, the toxicity of Selenium depends on its chemical forms. It has been reported that inorganic Se species such as selenite (SeO32 −) and selenate (SeO24−) are more toxic than the organic ones [6]. Since the concentration range between essential and toxic is very small, it has great significance to develop a selective, facile and accurate method to detection of Se in biological and environmental samples [2]. So far, several analytical methods have been developed for detection of selenium, including high performance liquid chromatography – inductively coupled plasma mass spectrometry (HPLC-ICP-MS) [7], hydride generation atomic fluorescence spectrometry [8,9], total reflection X-ray fluorescence spectroscopy [10], hydride generation atomic absorption spectrometry [11], inductively coupled plasma mass
⁎
spectrometry [12] and spectrophotometry [13]. These common analytical methods are highly accurate, sensitive and widely used for selenium detection; however, they suffer from some inherent drawbacks such as lengthy or tedious process, laborious procedures, sophisticated and expensive instruments and they require complicated sample preparation steps which limit their applicability. Fluorescence analysis might be a good selection for detecting selenium due to cost effectiveness, simplicity of operation, and high selectivity and sensitivity. Since the methods reported on the development of fluorescence sensors for detection of selenite generally require a time consuming procedures, toxic and unsafe reagents (e.g. quinoline derivatives), large organic solvent consumption, complicated sample-pretreatment, utilization of preconcentration steps, that wide utilization of these methods is largely limited [14–19]. Hence, development and design fluorescence probes which are safe to human health, rapid, simple, cost effective and eco-friendly is highly desired. Carbon quantum dots have been captured more and more attention in recent decade because of their low toxicity, and good biocompatibility, high photostability, water-solubility, easy to surface
Corresponding author. E-mail address: [email protected] (A. Afkhami).
http://dx.doi.org/10.1016/j.jlumin.2017.09.048 Received 12 July 2017; Received in revised form 10 September 2017; Accepted 19 September 2017 Available online 22 September 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
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modification and unique excitation-dependent fluorescence, which have gained much interest as potential compared to semiconductor quantum dots and organic dyes [20–24]. Historically, the photoluminescence (PL) CDs were firstly discovered through purification of single-walled carbon nanotubes by preparative electrophoresis [25]. They are very promising in numerous possible applications such as photocatalysis [26], optoelectronic devices [27], bioimaging [28], and optical sensors [29]. Up to now, different approaches have been employed for the preparation of fluorescent C-dots, including electrochemical [30], laser ablation [31], arc-discharge [25], hydrothermal [32], solvothermal [33], microwave pyrolysis [34], plasma and ultrasonic treatment [35,36]. Among mentioned strategies, hydrothermal method has distinct advantages such as mild synthetic treatment, simple and ecofriendly reaction procedures and the more important that it does not require sophisticate tools [37]. Despite the impressive benefits, carbon dots have a low quantum yield (QY, often less than 10%) which still limits their applications [38]. In order to improving the QY of the CDs, doping process (introduction heteroatoms like N, B, and S into CDs) and surface modification are two ways that can also endow the CDs with new properties, such as sensing ability toward metal ions or biomolecules for various applications and high catalytic reactivity, based on adjusted intrinsic electronic structure and modification of the CDs surface with functional molecules [39]. Previous works report that Eu3+ possess higher affinity to pyrophosphates (PPi) and phosphates than the hydroxyl and carboxyl groups [40,41]. Therefore, we infer that another oxyanion such as selenite is able to selectively bind europium ions due to the negatively charged oxygen groups. In this work, we demonstrate one-pot, but two steps, hydrothermal carbonization route to the preparation nitrogen-doped carbon dots using citric acid and L-lysine in mild conditions. Citric acid serves as the molecular precursor and carbon source, as well as L-lysine as the surface passivation agent. It also provides nitrogen. Remarkably, it is found that the as-prepared N -CDs exhibit a high fluorescence Quantum yield (21.5%) and strong emit highly blue fluorescence under 330 nm. To the best of our knowledge, application of carbon nanodots-based fluorescent sensors in quantification of selenite has never been reported.
2.3. Preparation of N-CDs The N-doped CDs were prepared by the carbonization of citric acid as in a similar method reported previously with some modification [42]. Briefly, 2.0 g of citric acid (9.5 mmol) and 1.2 g of lysine (8.3 mmol) were mixed thoroughly with 50 mL of distilled water, by ultrasonication, to become a homogeneous solution, followed by evaporation at 70 °C until dryness within 12 h. The thermal process was continued until the colorless solution became pale pink thick syrup. After that this syrup was transferred to a poly (tetrafluoroethylene) (Teflon)-lined autoclave and heated at 200 °C for 3 h. After cooling to room temperature, the resulting brownish syrup was dispersed in ultrapure water and then subjected to dialysis in a dialysis bag (MW cutoff: 5 kDa). Then the purified N-CDs diluted to 100 mL after neutralized with 1 mol L−1 NaOH solution. The purified N-CDs were preserved at room temperature for further uses. 2.4. Measurement of quantum yield In order to measure The QY of as-prepared CDs, we carried out this experiment on the report of a previously described protocol [43]. Quinine sulfate in 0.1 M H2SO4 was chosen as a refrence ( QY = 0.54). To reduce self absorption effects, absorbance values for both the carbon dots and refrence solution were restricted below 0.1 at 340 nm. The quantum yield of the N-CQDs was determined as below:
Grad ⎞ ⎛ n2 ⎞ Q = QR ⎛ ⎜ ⎟ 2 Grad R ⎠ ⎝ nR ⎠ ⎝ ⎜
⎟
Where Q is the quantum yield of fluorescence, Grad denotes to the gradient from the plot of integrated photoluminescence intensity (excited at 330 nm) versus the absorbance, n is refractive index of the solvent and the subscript R refers standard reference. 2.5. Fluorescence sensing of selenite In a typical assay for the determination of selenite, 18 µL of N-CDs (1 mg mL−1), 600 µL of Tris-HCl buffer solution (50 mM, pH 7.0) and 6.6 µL of Eu3+ (0.1 M) were mixed thoroughly, and diluted to 3 mL using ultrapure water. Then 10 µL of serial concentrations of selenite was added. After equilibration for 1 min, the photoluminescence (PL) spectra were recorded under the fixed excitation wavelength at 330 nm and emission spectra were collected at 430 nm, respectively. In order to obtain favorable fluorescence spectra, the excitation and emission slit was adjust to 5 nm.
2. Experimental 2.1. Materials Citric acid (CA), L-lysine (97.0%), Eu(NO3)3·5H2O (99.9%), Ce (NO3)3·6H2O (99.9%), Sm (NO3)3·6H2O (99.9%) and quinine sulfate (98.0%) were purchased from Sigma-Aldrich, St. Louis, MO, USA. Sodium selenite (Na2SeO3), sodium nitrate (NaNO3), sodium nitrite (NaNO2), FeCl3·6H2O, NiCl2·6H2O, CoCl2·6H2O, Zn (NO3)3·6H2O, MnCl2·4H2O, CuSO4·5H2O, CaCl2, and other materials were obtained from Merck, Darmstadt, Germany. All reagents were of analytical grade, and doubly distilled water was used throughout.
2.6. Detection of selenite in real sample For the analysis of real samples, the selenium content of selenium sulfide shampoo was evaluated by applying the CDs-Eu3+ system prepared in this work. The pretreatment of shampoo sample was performed following the previously reported procedure [44]. Briefly, 1 g of shampoo sample was placed into a 100 mL Kjeldahl flask, then 1 mL of sulfuric acid (98%) was added. The solution mixture was heated to fuming for 15 min. After cooling to room temperature, 5 mL of 30% (w/ v) H2O2 was introduced. To discard excess hydrogen peroxide, the mixture was boiled severely and allowed to cool. Then the solution was then diluted 100-fold with water before using for the analysis.
2.2. Instrumentation Morphology and size of N-CDs were examined by a transmission electron microscopy (TEM, Philips, CM30, operating at 100 kV). UV–vis absorption and fluorescence spectra were collected with an Agilent 8453 Diode array spectrophotometer and an LS-50B fluorescence spectrometer equipped with a 1.0 cm quartz cell and a xenon lamp (PerkinElmer, USA), respectively. The Fourier Transform infrared (FTIR) spectra were recorded on a Perkin-Elmer model Spectrum GX within the range 400–4000 cm−1. X-ray photo-electron spectroscopy (XPS) analysis was obtained on an 8025-BesTec twin anode XR3E2 X-ray source system. X-ray diffraction (XRD) patterns were carried out on a 38066 Riva, d/G. via M. Misone, 11/D (TN), Italy. A Metrohm model 713 pH-meter was used for pH measurements.
3. Results and discussion 3.1. Formation mechanism and characterization of N-CDs As shown in Scheme 1, water soluble N-CDs were synthesized and an “on-off-on” platform was designed for highly selective and rapid detection of tetravalent selenium, based on a competitive reaction 769
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Scheme 1. Schematic illustration of sensing process for detection of selenite based on N-CDs/Eu3+.
Fig. 1. A: a. UV–vis absorption spectra of N –CDs, b. Excitation spectra, c. Emission spectra and Inset: photographs of N -CDs under daylight (right) and UV light (left), and B: fluorescence emission spectra of the as-prepared N-CDs under different excitation wavelengths.
product. N-CDs were synthesized from citric acid and L-lysine through hydrothermal carbonization treatment under high pressure at 200 °C for 3 h. L-lysine was carefully chosen to fulfill the nitrogen-doping process based on the following considerations. On the one hand, citric acid have a low carbonization temperature and contains three carboxylic groups [45]. Thus can react with the amine groups of lysine at the early stage of the hydrothermal process to form a nitrogen-containing precursor. Probably formation process for this N-CDs is that firstly citric acid carbonizes to form carboxylated CDs, and then the amidation reaction of the carboxyl CDs and L-lysine occurs to doping the CDs.
between selenium oxyanion (selenite), and carboxylic acid and hydroxyl groups on the surface of the prepared N-CDs for europium ions. In this way, upon the addition of Eu3+, aggregation of N-CDs occurs due to Eu3+ ions coordination with the carboxyl and hydroxyl groups at the surface of the N-CDs and act as a bridge between N-CDs. Upon the addition of selenite into N-CDs/Eu3+, induced the disruption of Eu3+triggered aggregation of N-CQDs because of stronger affinity of selenite than that of the carboxyl or hydroxyl groups on the surface of N-GQDs to europium ions and then fluorescence of N-CDs was recovered. We further demonstrate the applicability of this sensor through employing the developed assay strategy for the detection of selenite in health care 770
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considerable distribution of surface emissive trap sites on each N-CDs [48,49]. 3.2. Chemical and physical characterization of nitrogen-doped C-dots The formation of N-CDs was confirmed by TEM measurements. Fig. 2A exhibits the TEM image of prepared N-CDs. It could be observed that N-CDs have a spherical shape and well-dispersion. Also the size distribution histogram shows the narrow size distribution that alights in the range of 1–6 nm and the average size value is 3 nm (Fig. 2A, inset). The XRD pattern of the N-CDs (Fig. 2B) displayed a broad diffraction peak at 2θ = 23.5° which is due to the small size of the N-CDs. An interlayer spacing 0.39 nm which is contributed by the (002) plane of graphite. The corresponding interlayer spacing in graphite (0.34 nm) becomes larger in N-CDs, which can be ascribed to the introduction of rich of oxygen groups [50]. To characterize the surface functional groups, FT-IR were performed. As revealed in Fig. 2C, a broad absorption band centered at 3200–3472 cm− 1 attributed to the stretching vibrations of O–H and N–H groups, whereas the sharp peak located at 2987 cm− 1 is typically corresponded with C–H stretching vibrations of alkane groups. The band at 1668 cm−1 can be assigned to the C˭C stretching mode of the aromatic hydrocarbons and the peaks at around 1733, 1074, 1229 cm−1 attributed to stretching vibrations of C˭O and phenolic C–OH groups, respectively, implying the existence of oxygen-rich groups on the surface of N-doped CDs [47]. More importantly, the characteristic absorption peaks amido CON—H bending vibration (1501 cm−1) and amido CO—N (1405 cm−1) stretching vibration are identified in N-CDs, indicated a successful incorporation of nitrogen atoms in N-CDs [51,52]. All results demonstrate that the surfaces of CDs are surrounded by hydrophilic groups, which improve stability and hydrophilicity of the N -CDs in an aqueous system. To further confirm the chemical composition of the CDs, XPS spectrum were collected for N-doped CDs. As shown in Fig. 3A the nitrogen-doped carbon dots are predominantly composed of oxygen, carbon and nitrogen which revealed three peaks 284.7, 393.9, and 529.4 eV that related to C 1s, N 1s, and O 1s, respectively. A highresolution XPS spectrum of C1 s (Fig. 3B) can be resolved into four different peaks at 284.7, 285.2, 286.9, and 288.5 eV, which can be ascribed to C˭C, C–N, C–O, and C˭N/C˭O groups, respectively [53,54]. These results confirm the graphitic structure (sp2 C˭C) of the N-CDs. The N 1s spectrum (Fig. 3C) reveals two distinct peaks at 400, and 401.5 eV arising from the pyrrolic-like N–(C) 3 and N–H/NH2 groups, respectively [55]. The O1s spectrum (Fig. 3D) can be deconvoluted into two peaks at 533.1 and 531.8 eV, which are associated with the C–OH/ C–O–C and C˭O bands, respectively [29,56]. These results confirm once again that the N-CDs are modified with plentiful hydrophilic groups. Surface chemistry of the N-CDs determined by XPS which is consistent with the results of FT-IR.
Fig. 2. A: TEM image of the N-CDs. Inset: The size distribution of the N-CDs. B: XRD spectra of the N-CDs. C: FT-IR spectrum of the N-CDs.
As shown in Fig. 1A typical UV–visible spectra of N-CDs exhibits two absorption peaks: a shoulder at 250 nm that mainly ascribed to the π-π* energy transition of the CDs and this corresponding to the carbon core C˭C of a conjugated system and a peak at about 340 nm can be regarded as the n−π* transition of C˭O or C˭N bond [46,47]. The CDs had an emission peak at 430 nm when excited at 330 nm. Inset of Fig. 1A shows the optical images of the C-dots under the illumination of sunlight and irradiated by a 365 nm UV lamp, respectively To further explore their optical properties, photoluminescence behavior of N-CDs was studied under the different excitation wavelengths. As-prepared N-CDs, similar to the other carbon nanomaterial, exhibit excitation dependent PL behavior. So that as shown in Fig. 1B by increasing excitation wavelength, PL emission band gradually shifted to higher emission wavelengths, i.e. from 305 to 465 nm, also along with decreasing PL intensity. Previous reports indicate that this phenomenon caused by the synergistic effect of variation of particle sizes and the
3.3. The possible quenching and recovery mechanism In order to study the Off-On mechanism several experiments were performed. Fig. 4A exhibits the high fluorescence intensity of CDs at 430 nm. The fluorescence of N-CDs decreased to 53% of the initial value during 5 min in the presence of 220 µM Eu3+ ions (Fig. 4A. curves a,b). It is expected that, upon the addition of selenite to the CDs/ Eu3+ system, the fluorescence intensity at 415 nm increased along with a little shift (Fig. 4A. curve c). After Eu3+ addition, because of the coordination between Eu3+ and N-CDs, Eu3+ acts as a bridge between neighboring carboxylate and hydroxyl groups at the surface of N-CDs leads to the formation of N-CDs aggregates. Based on this point of view, possible mechanism of florescence quenching related to N-CDs was analyzed using a Stern-Volmer equation: F0/F = 1+Ksv [Eu3+], where F0 and F are the fluorescence intensities of N-CDs at 430 nm in the 771
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Fig. 3. A: XPS survey spectra of the N-CDs. High-resolution XPS spectra of C1s: B, N1s: C and O1s:D.
Fig. 4. A: Fluorescence emission spectra of CDs under different conditions. B: Resonance light scattering spectra of the N-CD solution with different conditions (I: CDs; II: CDs/Eu3+; III: CDs/Eu3+/selenite; IV: Eu3+/selenite). Concentrations of Eu3+ and selenite were 220 μM and 7.8 µg/mL, respectively.
[57]. Therefore the quenching rate constant (Kq), Kq = Ksv τ0, was obtained as 2 × 1012 M−1 s−1. These results indicate that the quenching provoked by Eu3+ is probably because of static quenching arising from the formation of complex between N-CDs and Eu3+ [57].
absence and presence of Eu3+, respectively. KSV (Stern-Volmer constant) obtained from the plots of F0 /F was found to be 2 × 103 M−1 indicating the high quenching efficiency of the N-CDs excited state. Also, the fluorescence lifetime of N-CDs that is reported as 11.06 ns 772
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Fig. 5. A: Fluorescence intensity ratio F/F0 of the N-CDs solution in the presence of various metal ions B: The relative fluorescence intensity of N-CDs in the presence of different concentrations of Eu3+. F and F0 represent the FL intensities of CDs in the presence and absence of Eu3+, respectively.
Fig. 6. A, B photoluminescence spectra of free N-CDs with different pH conditions.
After the addition of selenite, fluorescence is seen to be regenerated along with the red shift of the PL emission wavelength from 415 nm to 425 nm. The shift in emission wavelength might suggest that Eu3+induced close attachment of N-CDs affects the luminescence property of N-CDs to some degree [58]. As expected, the Eu3+ ion strongly binds to selenite. Meanwhile, there is no detectable fluorescence response when the selenite solution was added to the N-CDs solution alone (Fig. 4B, curve c). It was worth nothing that no change in fluorescence intensity was perceived in the selenite and Eu3+ solution alone (Fig. 4B, curve e). These results indicate that Eu3+ can binds with selenite preferentially, removes from the surface of N-CDs, thus effectively recovery of fluorescence was occurred (Scheme 1). Thus a switchable “on-off-on” sensor was fabricated based on the above strategy. To support the predicted mechanism in Scheme 1, resonance light scattering (RLS) technique was carried out. The RLS spectra were recorded by synchronous scanning of the excitation and emission monochromators from 200 to 800 nm. The RLS intensities of the N-CD dispersion before and after the addition of selenite are very weak (Fig. 4B, curves I and IV). After the addition of europium ions, the RLS intensities increased considerably that confirms the formation of N-CDs aggregations by Eu3+ (Fig. 4B, Curve II). By the addition of selenite to
N-CDs/Eu3+ system, the corresponding RLS intensities decreased (Fig. 4B, curve III). The cause of this is that europium ions have stronger affinity for selenite than the carboxylate and hydroxyl groups at the surface of N-CDs. Therefore, selenite acts as an anti-aggregating agent and the aggregates formed are redisposed. Also the RLS pattern of the Eu3+/selenite mixture is similar to that of the CDs/selenite mixture and N-CDs alone solution which clearly verifies the above stated mechanism. Thus based on the mechanism, N-CDs /Eu3+ were developed as sensors to respond selenite. 3.4. Optimization of the experimental conditions 3.4.1. Selection of quencher ion and its concentration The fluorescence response of N-CDs upon the addition of different cautions was investigated. The examined metal ions include Na+, K+, Mg2+, Ca2+, Zn2+, Fe2+,Ni2+, Co2+, Fe3+, Mn2+, Pb2+and some lanthanide ions such as Eu3+, Sm3+, Ce3+. As shown in Fig. 5A, negligible fluorescence signal changes were observed in the presence of common metal ions except Fe3+, and among the tested lanthanide ions, it is evident that the fluorescence intensity of N-CDs shows a significant quenching when Eu3+ was added. Since we intend to design a selenite 773
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Fig. 7. A: PL intensities of N-CDs under different conditions (a: free N-CDs, b: N-CDs in the presence of Eu3+, c: N-CDs/Eu3+in the presence of selenite. B: pH effect on the N-CDs/ Eu3+ sensing system, C: effect of N-doped CDs concentrations, D: effect of time. Concentrations of Eu3+ and selenite were 220 μM and 7.8 µg ml−1, respectively.
Fig. 8. A: Fluorescence spectra of N-CDs/Eu3+ to selenite at different concentrations under excitation at 330 nm, B: The linear relationship between ΔF and selenite concentration in the range of 0.078–54.6 µg/mL.
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investigated the effect of pH on the performance of the CDs-Eu3+ sensing system. (Fig. 7A and B). Fig. 7 exhibits the fluorescence response changes of CDs-Eu3+ without and with selenite at different pH values. This behavior caused by dependent on coordination between NCDs and Eu3+ at different pH values. At pH < 4, owing to coordination between protonated N-CDs and Eu3+, quenching of FL is not completed. At pH > 9, since the Eu3+ ions belong to hard acids, formation of insoluble complex with OH- (Eu(OH)3) is more favorable than with carboxylate groups, then making the fluorescence quenched incompletely [41]. Thus the fluorescence quenching of the N-CDs was impressive for pH ranged between 4 and 9. Maximum extent of fluorescence recovery was achieved by selenite at pH 7. The possible reason for this can be due to that the dominant form of selenite in this pH is HSeO−3 and negligible SeO32 − is present [60]. The positively charged surface of the probe (N-CDs/Eu3+) caused the coordination between probe and selenite, thus the possibility of Eu3+ removal by selenite from the surface of N-CDs is provided. Therefore, the pH was adjusted using a Tris-HCl buffer (pH 7, 10 mM) for subsequent detection experiments.
Fig. 9. Selectivity of the N-CDs/Eu3+ toward selenite; the concentration of all interference anions were 7.8 μg mL−1. All experiments were carried out in Tris-HCl (7, 10 mM) buffer.
Table 1 Tolerance limits of the foreign ions in the analysis of selenite by CDs/Eu3+ system. Added ions
Tolerance limit (MFIa/MSelenite)
CH3COOBr-, Cl-, Na+, K+ S2-, Pb2+
600 500 200 100
SO24− , NO− 3 Ca2+, Mg2+ − − NO2 , ClO3
50 30 20
SO32 − , Zn2+ FFe3+ a
3.4.3. Effect of N-CDs dosage The restoration efficiency [F–F0] (where F0 and F are the fluorescence intensities of the CDs/Eu3+ mixture at 430 nm in the absence and presence of selenite, respectively) in the presence of 7.8 µg ml−1 selenite as a function of N-CDs concentration was evaluated. As shown in Fig. 7C, by increasing concentration of N-doped CDs, the restoration efficiency was gradually increased and then decreased which may be attributed to the self-absorption and aggregation of CDs in the high concentrations of them. Thus, 0.17 mg mL−1 of N-doped CDs was selected for the following experiment.
10 2
Foreign ion.
3.4.4. Incubation time effect The effect of the incubation time of selenite was investigated as well. As shown in Fig. 7D, by the addition of selenite, the fluorescence recovery reached its maximum value after 5 min and then remained stable, showing that the interaction between Eu3+ and selenite reached equilibrium rapidly. Therefore the reaction time was fixed at 5 min.
Table 2 The application of the proposed method for analysis of shampoo samples spiked with different amounts of selenite. Se(µg/mL) Added
Found
Recovery (%)
(%) RSD (n = 3)
0 4.9 9.9 15.8
1.6 6.4 12.3 18.6
– 97.9 108 107.6
– 3.13 2.9 2.25
3.5. Analytical performance Under the optimized conditions, N-CDs/Eu3+ could be used as the fluorescence probe for the sensing of selenite. The fluorescent spectra of N-CDs/Eu3+ in the presence of different amounts of selenite were exhibited in Fig. 8A. Therefore, by increasing the concentration of selenite (0.078–54.6 µg mL−1) to the probe solution (N-CDs/Eu3+), fluorescence intensity of the system gradually increased. As indicated in Fig. 8B, the enhancement of the fluorescence recovery was closely related to the concentration of selenite. The recovery efficiency (ΔF), exhibited a good linear relationship as a function of the concentration of selenite over the range of 0.078–21.4 µg mL−1 with a detection limit of 54 ng mL−1 (Fig. 6B). The calibration equation, with the favorable correlation coefficient (R2 = 0.993), was Y= 0.0508 + 0.2072C, in which Y ascribes to the recovery efficiency (F–F0) and C was the concentration of selenite in µg mL−1.
sensor, in accordance with the HSAB theory, Eu3+ is harder than Fe3+ ion, therefore we predicted that it forms stronger bond caused by formation potent coordination between a hard acid (Eu3+) and a hard base (HSeO3– or SeO32 −) than Fe3+ [59]. Eu3+ concentration should be chosen so that the fluorescence intensity of N-CDs could be quenched in the absence of selenite while not excess, and impressive FL recovery could be observed with the addition of selenite. For this purpose, as shown in Fig. 5B, the quenching efficiency could reach over 50% when 220 µM Eu3+ was added. Moreover, excess Eu3+, existed as free site in the solution, will cause error in the procedure of detection due to their coordination ability with selenite. Therefore, 220 µM Eu3+ was chosen in subsequent experiments.
3.6. Selectivity To evaluate the selectivity of the CDs/Eu3+ sensing system for selenite, the effect of equal concentrations of various anions on the changes of fluorescence intensity was investigated. The fluorescence recovery efficiency [(F - F0)/F0] (where F0 and F are the PL intensities before and after adding selenite, respectively) of each anions is displayed in Fig. 9. It can be seen that selenite shows the largest recovery among the anions tested, whereas other anions have weak effects on the fluorescence of CDs/Eu3+ probe. This excellent selectivity is attributed to the strong interaction between Eu3+ ions and the oxygen atoms of
3.4.2. pH effect Fig. 6A and B exhibit the change in FL intensities of free N-CDs at different pH values. As could be seen, the fluorescence intensity of CDs in the pH range 4–8 remained relatively constant which offers the feasibility to construct a sensor over this range. Under acidic (pH < 3) and basic conditions (pH > 9), protonation – deprotonation of functional groups at the surface of N-CDs (carboxylic acid, hydroxyl) take place and consequently decreases the FL intensity [59]. We also 775
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[13] A. Safavi, A. Afkhami, A. Massoumi, Spectrophotometric catalytic determination of ultra-trace amounts of selenium based on the reduction of resazurin by sulphide, Anal. Chim. Acta 232 (1990) 351–356. [14] F. Costa-Silva, G. Marques, C.C. Matos, A.I.R.N.A. Barros, F.M. Nunes, Selenium contents of Portuguese commercial and wild edible mushrooms, Food Chem. 126 (2011) 91–96. [15] S. Liang, J. Chen, D.T. Pierce, X. Zhao, A turn-on fluorescent nanoprobe for selective determination of selenium (IV), ACS Appl. Mater. Interfaces 5 (2013) 165–5173. [16] W.B. Dye, E. Bretthauer, H.J. Seim, C. Blincoe, Fluorometric determination of selenium in plants and animals with 3,3-diaminobenzidine, Anal. Chem. 35 (1963) 1687–1693. [17] J.H. Watkinson, Fluorometric determination of traces of selenium, Anal. Chem. 32 (1960) 981–983. [18] J. Pedro, F. Andrade, D. Magni, M. Tudino, A. Bonivardi, On-line submicellar enhanced fluorometric determination of Se (IV) with 2, 3-diaminonaphthalene, Anal. Chim. Acta 516 (2004) 229–236. [19] J. Zhu, S. Liu, Z. Liu, Y. Li, M. Qiao, X. Hu, Highly selective speciation and fluorimetric determination of Se(IV) in infant formulas using micelle-capped nile blue A, RSC Adv. 3 (2013) 21570–21575. [20] Y.P. Sun, B. Zhou, Y. Lin, W. Wang, K.A.S. Fernando, P. Pathak, M.J. Meziani, B.A. Harruff, X. Wang, H. Wang, P. Luo, H. Yang, M.E. Kose, B.L. Chen, L.M. Veca, S. Xie, Quantum-sized carbon dots for bright and colorful photoluminescence, J. Am. Chem. Soc. 128 (2006) 7756–7762. [21] G.E. LeCroy, S.K. Sonkar, F. Yang, L.M. Veca, P. Wang, K.N. Tackett, J.J. Yu, E. Vasile, H. Qian, Y. Liu, P. Luo, Y.P. Sun, Toward structurally defined carbon dots as ultracompact fluorescent probes, ACS Nano 8 (2014) 4522–4529. [22] S.N. Baker, G.A. Baker, Luminescent carbon nanodots: emergent nanolights, Angew. Chem. Int. Ed. 49 (2010) 6726–6744. [23] H. Li, Z. Kang, Y. Liu, S.T. Lee, Carbon nanodots: synthesis, properties and applications, J. Mater. Chem. 22 (2012) 24230–24253. [24] C. Ding, A. Zhu, Y. Tian, Functional surface engineering of Cdots for fluorescent biosensing and in vivo bioimaging, Acc. Chem. Res. 47 (2014) 20–30. [25] X. Xu, R. Ray, Y. Gu, H.J. Ploehn, L. Gearheart, K. Raker, W.A. Scrivens, Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments, J. Am. Chem. Soc. 126 (2004) 12736–12737. [26] Y.Q. Zhang, D.K. Ma, Y.G. Zhang, W. Chen, S.M. Huang, N-doped carbon, quantum dots for TiO2 -based photocatalysts and dye-sensitized solar cells, Nano Energy 2 (2013) 545–552. [27] W. Kwon, S. Do, J. Lee, S. Hwang, J.K. Kim, S.W. Rhee, Free standing luminescent films of nitrogen-rich carbon nanodots toward large-scale phosphor-based whitelight-emitting devices, Chem. Mater. 25 (2013) 1893–1899. [28] H. Ding, J.S. Wei, H.M. Xiong, Nitrogen and sulfur co-doped carbon dots with strong blue luminescence, Nanoscale 6 (2014) 13817–13823. [29] W. Wang, Y.C. Lu, H. Huang, A.J. Wang, J.R. Chen, J.J. Feng, Facile synthesis of N,S-co doped fluorescent carbon nanodots for fluorescent resonance energy transfer recognition of methotrexate with high sensitivity and selectivity, Biosens. Bioelectron. 64 (2015) 517–522. [30] H. Li, H. Ming, Y. Liu, H. Yu, X. He, H. Huang, K. Pan, Z. Kang, S.-T. Lee, Fluorescent carbon nanoparticles: electrochemical synthesis and their pH sensitive photoluminescence properties, New J. Chem. 35 (2011) 2666–2670. [31] L. Cao, X. Wang, M.J. Meziani, F. Lu, H. Wang, P.G. Luo, Y. Lin, B.A. Harruff, L.M. Veca, D. Murray, S.Y. Xie, Y.P. Sun, Carbon dots for multiphoton bioimaging, J. Am. Chem. Soc. 129 (2007) 11318–11319. [32] S. Sahu, B. Behera, T.K. Maiti, S. Mohapatra, Simple one-step synthesis of highly luminescent carbon dots from orange juice: application as excellent bio-imaging agents, Chem. Commun. 48 (2012) 8835–8837. [33] B. Han, W. Wang, H. Wu, F. Fang, N. Wang, X. Zhang, S. Xu, Polyethyleneimine modified fluorescent carbon dots and their application in cell labeling, Colloids Surf. B 100 (2012) 209–214. [34] C. Liu, P. Zhang, X. Zhai, F. Tian, W. Li, J. Yang, Y. Liu, H. Wang, W. Wang, W. Liu, Nano-carrier for gene delivery and bioimaging based on carbon dots with PEIpassivation enhanced fluorescence, Biomaterials 33 (2012) 3604–3613. [35] J. Wang, C.F. Wang, S. Chen, Amphiphilic egg-derived carbon dots: rapid plasma fabrication, pyrolysis process, and multicolor printing patterns, Angew. Chem. Int. Ed. 37 (2012) 9297–9301. [36] Z. Ma, H. Ming, H. Huang, Y. Liu, Z. Kang, One-step ultrasonic synthesis of fluorescent N-doped carbon dots from glucose and their visible-light sensitive photocatalytic ability, New J. Chem. 36 (2012) 861–864. [37] B.B. Chen, Z.X. Liu, W.C. Deng, L. Zhan, M. Liu, C.Z. Huang, A large-scale synthesis of photoluminescent carbon quantum dots: a self-exothermic reaction driving the formation of the nanocrystalline core at room temperature, Green Chem. 18 (2016) 5127–5132. [38] C. Liu, P. Zhang, F. Tian, W. Li, F. Li, W. Liu, One-step synthesis of surface passivated carbon nanodots by microwave assisted pyrolysis for enhanced multicolor photoluminescence and bioimaging, J. Mater. Chem. 21 (2011) 13163–13167. [39] S. Gao, Y. Chen, H. Fan, X. Wei, C. Hu, L. Wang, L. Qu, A green one-arrow-twohawks strategy for nitrogen-doped carbon dots as fluorescent ink and oxygen reduction electro catalysts, J. Mater. Chem. A 2 (2014) 6320–6325. [40] L. Lin, X. Song, Y. Chen, M. Rong, T. Zhao, Y. Jiang, Y. Wang, X. Chen, One-pot synthesis of highly greenish-yellow fluorescent nitrogen-doped graphene quantum dots for pyrophosphate sensing via competitive coordination with Eu3+ ions, Nanoscale 7 (2015) 15427–15433. [41] J.M. Bai, L. Zhang, R.P. Liang, J.D. Qiu, Graphene quantum dots combined with europium ions as photoluminescent probes for phosphate sensing, Chem. Eur. J. 19 (2013) 3822–3826. [42] Y. Dong, H. Pang, H.B. Yang, C. Guo, J. Shao, Y. Chi, C.M. Li, T. Yu, Carbon-based
selenite ion, as previously discussed. For more investigation of selectivity, the interference from some anions and various cations in the presence of selenite with excess amounts of coexisting ions was examined. The summary of the obtained results is presented in Table 1. It can be clearly seen that all the examined ions, except Fe3+, have no serious interfering effect on the fluorescence response even when the concentration of the tested ions was 10–600-fold excess over selenite. The tolerance limit was defined as the concentration of coexisting species that produces a relative error of ≤ 5.0% for selenite detection. The results demonstrate that the developed Eu3+-adjusted sensor exhibits excellent selectivity for the determination of selenite. 3.7. Application to real samples To demonstrate the applicability of N-CDs/Eu3+ system to sensing selenite, selenium sulfide shampoo 2.5% (for treatment of dandruff) as real sample was selected. The results are summarized in Table 2. the relative standard deviation of the results for selenite were lower than 3.2%, and the recoveries of selenite were found to be in the range of 97.9–108.0%, indicating that our sensing system was highly reproducible and accurate for rapid sensing of selenite in such real samples. 4. Conclusion Briefly, in this work, nitrogen-doped carbon quantum dots, with highly quantum yield (21.5%), were synthesized and utilized to construct an efficient off-on fluorometric assay for highly selective detection of selenite. To the best of our knowledge, this is the first report on the construction of a sensor for selenite based on aggregation of CDs and europium ions. Compared with the previous fluorimetric works, the proposed method does not require preconcentration step, time consuming procedures and complexed operatory. Meanwhile, take advantages such as simplicity in method, environmentally-friendly, lowcost, fast response and easy operation have caused our offered strategy become an easy, fast and reproducible method for the determination of selenite. References [1] M. Navarro-Alarcon, M.C. Lopez-Martinez, Essentiality of selenium in the humanbody: relationship with different diseases, Sci. Total Environ. 249 (2000) 347–371. [2] T. Klapec, M.L. Mandic, J. Grgic, L. Primorac, A. Perl, V. Krstanovic, Selenium in selected foods grown or purchased in eastern Croatia, Food Chem. 85 (2004) 445–452. [3] B. Izgi, S. Gucer, R. Jacimovic, Determination of selenium in garlic (Allium sativum) and onion (Allium cepa) by electro thermal atomic absorption spectrometry, Food Chem. 99 (2006) 630–637. [4] H. Robberecht, R.V. Grieken, Selenium in environmental waters: determination, speciation and concentration levels, Talanta 29 (1982) 823–844. [5] J.B. Loṕez-Saez, A. Senra-Varela, L. Pousa-Estevez, Selenium in breast cancer, Oncology 64 (2003) 227–231. [6] K. Agrawala, K.S. Patela, K. Shrivasb, Development of surfactant assisted spectrophotometric method for determination of selenium in waste water samples, J. Hazard. Mater. 161 (2009) 1245–1249. [7] B. Chen, M. He, X. Mao, R. Cui, D. Pang, B. Hu, Ionic liquids improved reversedphase HPLC on-line coupled with ICP-MS for selenium speciation, Talanta 83 (2011) 724–731. [8] F. Wang, G. Zhang, Simultaneous quantitative analysis of arsenic, bismuth, selenium, and tellurium in soil samples using multi-channel hydride-generation atomic fluorescence spectrometry, Appl. Spectrosc. 65 (2011) 315–319. [9] V. Stibilj, P. Smrkolj, A. Krbavcic, Investigation of the Declared Value of Selenium in Food Supplements by HG-AFS, Microchim. Acta 150 (2005) 323–327. [10] H. Stosnach, Analytical determination of selenium in medical samples, staple food and dietary supplements by means of total reflection X-ray fluorescence spectroscopy, Spectrochim. Acta B 65 (2010) 859–863. [11] J.Y. Cabon, W. Erler, Determination of selenium species in seawater by flow injection hydride generation in situ trapping followed by electrothermal atomic absorption spectrometry, Analyst 123 (1998) 1565–1569. [12] W.T. Buckley, J.J. Budac, D.V. Godfrey, K.M. Koenig, Determination of selenium by inductively coupled plasma mass spectrometry utilizing a new hydride generation sample introduction system, Anal. Chem. 64 (1992) 724–729.
776
Journal of Luminescence 194 (2018) 768–777
N. Amin et al.
[43]
[44]
[45] [46]
[47]
[48]
[49]
[50]
[51]
[52]
dots co-doped with nitrogen and sulfur for high quantum yield and excitation-independent emission, Angew. Chem. Int. Ed. 52 (2013) 7800–7804. S. Sahu, B. Behera, T.K. Maiti, S. Mohapatra, Simple one-step synthesis of highly luminescent carbon dots from orange juice: application as excellent bio-imaging agents, Chem. Commun. 48 (2012) 8835–8837. A. Afkhami, A. Safavi, A. Massoumi, Spectrophotometric determination of trace amounts of selenium with catalytic reduction of bromate by hydrazine in hydrochloric acid media, Talanta 39 (1992) 993–996. Y. Dong, R. Wang, H. Li, J. Shao, Y. Chi, X. Lin, G. Chen, Polyamine-functionalized carbon quantum dots for chemical sensing, Carbon 50 (2012) 2810–2815. S. Zhu, Q. Meng, L. Wang, J. Zhang, Y. Song, H. Jin, K. Zhang, H. Sun, H. Wang, B. Yang, Highly photoluminescent carbon dots for multicolor patterning, sensors, and bioimaging, Angew. Chem. Int. Ed. Engl. 52 (2013) 3953–3957. D. Pan, J. Zhang, Z. Li, C. Wu, X. Yana, M. Wu, Observation of pH-, solvent-, spin-, and excitation-dependent blue photoluminescence from carbon nanoparticles, Chem. Commun. 46 (2010) 3681–3683. H. Zhu, X. Wang, Y. Li, Z. Wang, F. Yanga, X. Yang, Microwave synthesis of fluorescent carbon nanoparticles with electrochemiluminescence properties, Chem. Commun. 34 (2009) 5118–5120. H. Huang, C.G. Li, S.J. Zhu, H.L. Wang, C.L. Chen, Z.R. Wang, T.Y. Bai, Z. Shi, S.H. Feng, Histidine-derived nontoxic nitrogen-doped carbon dots for sensing and bioimaging applications, Langmuir 30 (2014) 13542–13548. Y.P. Hu, J. Yang, J.W. Tian, L. Jia, J.S. Yu, Waste frying oil as a precursor for onestep synthesis of sulfur-doped carbon dots with pH-sensitive photoluminescence, Carbon 77 (2014) 775–782. X. Gong, W. Lu, M.C. Paau, Q. Hu, X. Wu, S. Shuang, C. Donga, M.M.F. Choi, Facile synthesis of nitrogen-doped carbon dots for Fe3+ sensing and cellular imaging, Anal. Chim. Acta 861 (2015) 74–84. J. Duan, J. Yu, S. Feng, L. Su, A rapid microwave synthesis of nitrogen–sulfur co-
[53]
[54]
[55]
[56]
[57]
[58]
[59]
[60]
777
doped carbon nanodots as highly sensitive and selective fluorescence probes for ascorbic acid, Talanta 153 (2016) 332–339. C.H. Choi, M.W. Chung, S.H. Park, S.I. Woo, Additional doping of phosphorus and/ or sulfur into nitrogen-doped carbon for efficient oxygen reduction reaction in acidic media Phys, Chem. Chem. Phys. 15 (2013) 1802–1805. S. Wohlgemuth, R.J. White, M. Willinger, M. Titirici, M. Antonietti, A one-pot hydrothermal synthesis of sulfur and nitrogen-doped carbon aerogels with enhanced electrocatalytic activity in the oxygen reduction reaction, Green Chem. 14 (2012) 1515–1523. Z. Yang, M. Xu, Y. Liu, F. He, F. Gao, Y. Su, H. Wei, Y. Zhang, Nitrogen-doped, carbon-rich, highly photoluminescent carbon dots from ammonium citrate, Nanoscale 6 (2014) 1890–1895. W. Wang, Y.C. Lu, H. Huang, A.J. Wang, J. Chen, J.J. Feng, Solvent-free synthesis of sulfur- and nitrogen-co-doped fluorescent carbon nanoparticles from glutathione for highly selective and sensitive detection of mercury (II) ions, Sens. Actuators B: Chem. 202 (2014) 741–747. M. Lin, H.Y. Zou, T. Yang, Z.X. Liu, H. Liu, C.Z. Huang, An inner filter effect based sensor of tetracycline hydrochloride as developed by loading photoluminescent carbon nanodots in the electrospun nanofibers, Nanoscale 8 (2016) 2999–3007. Q. Zhang, C. Song, T. Zhao, H.W. Fu, H.Z. Wang, Y.J. Wang, D.M. Kong, Photoluminescent sensing for acidic amino acids based on the disruption of graphene quantum dots/europium ions aggregates, Biosens. Bioelectron. 65 (2015) 204–210. R.S. Dickins, A.S. Batsanov, J.A.K. Howard, D. Parker, H. Puschmanna, S. Salaman, Structural and NMR investigations of the ternary adducts of twenty α-amino acids and selected dipeptides with a chiral, diaqua–ytterbium complex, Dalton Trans. (2004) 70–80. V.R. Preedy, Selenium: Chemistry, Analysis, Function and Effects, Royal Society of Chemistry, School of Medicine, King's College London, UK, 2015.