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Highly selective detection of p-nitrophenol using fluorescence assay based on boron, nitrogen co-doped carbon dots
Author’s Accepted Manuscript Highly selective detection of p-nitrophenol using fluorescence assay based on boron, nitrogen codoped carbon dots Na Xiao, Shi Gang Liu, Shi Mo, Na Li, Yan Jun Ju, Yu Ling, Nian Bing Li, Hong Qun Luo www.elsevier.com/locate/talanta
PII: DOI: Reference:
S0039-9140(18)30232-7 https://doi.org/10.1016/j.talanta.2018.02.114 TAL18433
To appear in: Talanta Received date: 17 November 2017 Revised date: 22 February 2018 Accepted date: 28 February 2018 Cite this article as: Na Xiao, Shi Gang Liu, Shi Mo, Na Li, Yan Jun Ju, Yu Ling, Nian Bing Li and Hong Qun Luo, Highly selective detection of p-nitrophenol using fluorescence assay based on boron, nitrogen co-doped carbon dots, Talanta, https://doi.org/10.1016/j.talanta.2018.02.114 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highly
selective
detection
of
p-nitrophenol
using
fluorescence assay based on boron, nitrogen co-doped carbon dots Na Xiaoa, Shi Gang Liua, Shi Mob, Na Lia, Yan Jun Jua, Yu Linga, Nian Bing Lia,*, Hong Qun Luoa,* a
Key Laboratory of Eco-Environments in Three Gorges Reservoir Region (Ministry
of Education), School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, PR China b
Department of Physics, City University of Hong Kong, Tat Chee Avenue, Kowloon,
Hong Kong, China [email protected] (NB Li) [email protected] (HQ Luo) *
Corresponding Author. Tel: +86 23 68253237; fax: +86 23 68253237; .
Abstract p-Nitrophenol (p-NP) contaminants seriously endanger environmental and living beings health, hence to establish a sensitive and selective method is of great importance for the determination of p-NP. In this work, boron and nitrogen co-doped carbon dots (B,N-CDs) were synthesized by one-step hydrothermal method using 3-aminophenylboronic acid as the sole precursor. The product was characterized through high-resolution transmission electron microscopy, fluorescence spectroscopy, UV-visible absorption spectroscopy, X-ray photoelectron spectroscopy, and Fourier 1
transform infrared spectroscopy. Without any functionalized modification, B,N-CDs can be directly applied as a ‘turn-off’ fluorescent probe for rapid, highly selective, and sensitive detection of p-NP. The fluorescent sensor based on the B,N-CDs exhibited a broad linear response to the concentration of p-NP in the range of 0.5 60 μM and 60 200 μM, respectively, and provided a detection limit of 0.2 μM. It was found that only the absorption spectrum of p-NP has a wide overlap with the fluorescence excitation and emission spectra of B,N-CDs compared to those of other representative analogues. The response mechanism was due to the inner filter effect and the formation of dynamic covalent B-O bonds between B,N-CDs and p-NP, which endowed the sensing platform with the rapid response and high selectivity to p-NP. Finally, the sensor showed the practicability of p-NP determination in environmental water samples.
Graphical Abstract
2
Keywords: Boron, nitrogen co-doped carbon dots; Fluorescent sensor; p-Nitrophenol; Inner filter effect
1. Introduction p-Nitrophenol (p-NP) is an indispensable material for the manufacture of pharmaceuticals and fine chemicals, and can be widely found in soils and aquatic environments [1]. However, as a chemical industrial wastewater contaminant [2-4], p-NP can stably exist in different environments and continues to gather in the food chain, potentially leading to damages of environment and living beings owing to its toxicity [5]. Thus, the US Environmental Protection Agency has listed p-NP as a priority pollutant [6]. Although there are many methods for the detection of p-NP, such as capillary electrophoresis [7], chromatographic technique [8], and electrochemical methods [9], some deficiencies including electrode instability, sophisticated and expensive instruments or complicated pretreatment procedures make them unsuitable for rapid monitoring. In contrast, fluorescence analysis exhibits superiority because of its simple sample pretreatment, less solvent consumption, rapid response, low expense, and high sensitivity. Thus, some fluorescence probes have been used for p-NP detection, for instance,
mono-6-SH-β-cyclodextrin-capped
CdTe
quantum
dots
[10],
3
β-cyclodextrin-capped
ZnO
quantum
dots
[11],
and
carbon
dots
(CDs)
(tris(hydroxymethyl)aminomethane (Tris) and ethyleneglycol bis-(2-aminoethyl ether)-N,N,N’,N’-tetraacetic acid (EGTA) as carbon sources) [12].Unfortunately, the widespread applications of these methods are hampered by the toxicity, heavy metal pollution, sensors fabrication through two-step processes, using more than one precursor, and relatively high cost reagents. All in all, the development of rapid, inexpensive, environment-friendly, and simple fluorescence analysis method for p-NP is desperately necessary. CDs as an effective alternative to toxic quantum dots have attracted much attention in recent years due to low toxicity, biocompatibility, easy preparation, and high sensitivity to the target [13,14]. Recently, there are numerous works dealing with quenching of CDs fluorescence for analytical purposes based on different quenching mechanisms, including photoinduced electron transfer (PET), Förster resonance energy transfer (FRET), and inner filter effect (IFE). Through the PET and FRET quenching mechanisms of CDs, Pb2+, Co2+, Cu2+, Hg2+, S2-, adenosine 5′-triphosphate, phytic acid, thiamine, glyphosate, folate receptor, glutathione, and carcinoembryonic antigen can be detected [15]. Tang et al. employed β-cyclodextrin-modified CDs to detect alkaline phosphatase (ALP) activity based on PET through host-guest recognition between p-NP and β-cyclodextrin [16]. García et al. reported the CDs synthesized by EGTA and Tris via thermal carbonization, which can be used to detect p-NP based on the energy transfer [12]. However, IFE has grown to be a useful and efficient tactics for the construction and development of novel fluorescent sensors by 4
translating the analytical absorption signals into fluorescence signals, which possess enhanced selectivity and sensitivity compared to other quenching mechanisms (e.g., PET and FRET) [17-19]. The IFE refers to the absorption of an absorber to the excitation and/or emission of light within the detection system when the fluorescence excitation or emission spectra of fluorophores overlap with the absorption spectra of the absorbers [20]. Based on the IFE, S2-, Cr6+, ascorbic acid, picric acid, hemoglobin, fluazinam, alkaline phosphatase, and β-glucuronidase can be detected with CDs [15]. Li et al. reported that the N-CDs were synthesized with catechol and ethanediamine by a hydrothermal method and applied to ALP sensing based on IFE by catalysis of ALP towards p-nitrophenylphosphate to yield p-NP [20]. Therefore, the IFE-based sensors are simple to implement, and can be potentially applied in other fields. In this work, we synthesized N and B co-doped CDs (B,N-CDs) using a single starting material containing both B and N, which not only simplifies the synthesis step but also reduces the by-product compared with most of the B,N-CDs synthesized from at least two compounds as the raw materials. Herein, through comparing the fluorescence
properties
aminophenylboronic 3-aminophenylboronic
of
acid acid
B,N-CDs isomers
synthesized
including
monohydrate,
and
from
three
different
2-aminophenylboronic 4-aminophenylboronic
acid, acid
hydrochloride under same condition, we finally adopted 3-aminophenylboronic acid monohydrate as the sole precursor with a favorable chemical structure. The synthesized B,N-CDs exhibited a strong blue fluorescence emission at 430 nm under 350 nm excitation, and some other properties such as excellent water solubility, 5
oxidation resistance and high stability even under a wide pH range, long-time storage and high ionic strength. B,N-CDs without any functionalized modification can be directly applied as a rapid, highly selective and sensitive sensor for p-NP detection (Scheme 1), which is simpler and more straightforward compared to the previously reported nanoparticle-based sensors fabrication through two-step processes [11,21,22]. Furthermore, the developed sensor was applied to p-NP detection in real water samples (tap water or river water) with satisfactory results, suggesting that the B,N-CDs have great potential for the detection of p-NP in real samples.
2. Experimental 2.1. Chemicals 3-Aminophenylboronic
acid
monohydrate
(3-APBA⋅H2O),
2-aminophenylboronic acid (2-APBA), 4-carboxyphenylboronic acid (4-CPBA), benzoic acid (BA), aniline (AN), p-nitrotoluene (p-NT), o-dihydroxybenzene (o-DHB), p-chlorophenol (p-CP), m-dinitrobenzene (m-DNB), methylbenzene (MB), 2,4,6-trinitrotoluene p-nitrophenol
(TNT),
(p-NP),
m-nitrophenol hydroquinone
(m-NP), (HQ),
o-nitrophenol phenol
(PHE),
(o-NP), and
tetra-n-butylammonium hexafluorophosphate (Bu4NPF6) were supplied by Aladdin Reagent Co., Ltd., Shanghai, China. 4-Aminophenylboronic acid hydrochloride (4-APBA⋅HCl) was obtained from Macklin Biochemical Co., Ltd., Shanghai, China. Ethylenediaminetetra-acetic acid disodium salt (EDTA), NaOH, and NaCl were purchased from Kelong Chemical Reagent Co., Ltd., Chengdu, China. Ultrapure 6
water (18.2 MΩ cm) was used throughout the experiments. 2.2. Apparatus The high-resolution transmission electron microscopy (HRTEM) images were observed from a JEM-2100 (JEOL Ltd., Japan). UV-vis absorption spectra were measured with a UV-2450 spectrophotometer (Suzhou Shimadzu Instrument Co., Ltd., China). Fluorescence spectra were recorded on an F-2700 spectrofluorophotometer (Hitachi Ltd., Japan) with 10 nm slit widths for both excitation and emission. X-ray photoelectron spectroscopy (XPS) measurements were conducted using an ESCALab 250Xi (Thermo Fisher Scientific, USA). Fourier transform infrared (FTIR) spectrum was recorded on a Bruker IFS 113v spectrometer (Bruker, Germany). The pH values of solutions were obtained from METTLER TOLEDO FE28 pH meter (Mettler Toledo, China). The fluorescence lifetimes were determined using an FLSP920 fluorescence spectrometer (Edinburgh, UK). 2.3. Synthesis of B,N-CDs A simple hydrothermal method was used to the one-pot synthesis of B,N-CDs on the basis of previous literature with minor modification [23]. Typically, 5.0 mg of 3-APBA⋅H2O was dissolved in 5.0 mL of ultrapure water, and subsequently the pH was adjusted to 10.0 with 1.0 M NaOH under agitation. Next, the solution was transferred to the Teflon-lined autoclave and heated to 160 °C for 8 h. After being cooled to room temperature naturally, the resultant yellowish-brown solution was centrifuged to remove large aggregates at 10, 000 rpm for 20 min. Finally, the as-prepared B,N-CDs were stored at 4 °C for later use. In addition, other control CDs 7
were synthesized by using the same method except that 3-APBA⋅H2O was replaced by BA, 4-CPBA, AN, 2-APBA, and 4-APBA⋅HCl, respectively. The diluted 100-fold B,N-CDs (10 μL mL-1) aqueous solutions were used as the fluorescent probe to establish the sensing platform for p-NP in this research. 2.4. Quantum yield measurement The quantum yield (QY) of the B,N-CDs was measured according to a slope method [24]. Quinine sulfate with QY = 0.54 dissolved in 0.1 M H2SO4 was chosen as a standard sample. In this work, quinine sulfate and B,N-CDs solutions were prepared with five different concentrations, and all of their absorbance at 350 nm is less than 0.1. The QY can be measured by the following equation:
Grad x 2X 2 x ST GradST ST
(1)
where ϕ is the quantum yield, the subscript “st” and “x” refer to the standard and samples, respectively, Grad is the slope, and η is the refractive index. For these aqueous solutions, ηx/ηst = 1. 2.5. Procedures for p-NP sensing 5 μL of CDs and 150 μL of BR buffer (pH 7.7) were added to a polypropylene microcentrifuge tube. Subsequently, various concentrations of p-NP were added and the mixtures were diluted to 500 μL with ultrapure water. After the mixtures were incubated for 1 min at room temperature, the fluorescence emission signal was monitored at an excitation wavelength of 350 nm. In order to analyze the real water samples, river water obtained from the Jialing River in Chongqing was filtered 8
through a 0.22 μm membrane to remove particles and tap water from our laboratory was not pretreated. No p-NP was detected in the real samples and then the samples were spiked with 1.0 mM EDTA and different concentrations (20.0, 50.0, 120.0, 160.0 μM) of standard p-NP solutions. Other steps were consistent with those described above. Fluorescence quenched efficiency (F0-F)/F0 was analyzed (F and F0 are the fluorescence intensity of the B,N-CDs in the presence and absence of p-NP, respectively).
3. Results and discussion 3.1.Optimization of synthesis conditions The B,N-CDs were prepared by a facile one-pot hydrothermal method using 3-APBA⋅H2O as a sole B and N source. To achieve high luminescent B,N-CDs, the following experimental conditions towards 5.0 mg of 3-APBA⋅H2O in 5.0 mL of ultrapure water were optimized (Fig. S1): (A) the pH value of the solution; (B) reaction temperature; (C) reaction time. As shown in Fig. S1, the carbonization conditions seriously affect the fluorescence intensity of B,N-CDs. Consequently, 5.0 mg of 3-APBA⋅H2O in 5.0 mL of ultrapure water with the reaction temperature 160 °C for 8 h at pH 10.0 was specified as the optimum condition for synthesizing B,N-CDs. 3.2.Optical properties of B,N-CDs As a result of π-π* transition of C=C bond [23,25], a broad UV-visible absorption band of B, N-CDs is observed at 220-320 nm with strong peaks at 229 and 268 nm (Fig. 1A). Furthermore, the B,N-CDs have a maximum emission peak at 430 9
nm when excited at 350 nm, and the solution emitted strong blue fluorescence under 365 nm UV light (Fig. 1A, inset 2). As shown in Fig. 1B, the fluorescence emission spectra of B,N-CDs remained unchanged when the excitation wavelength ranges from 330 420 nm. The results suggest that the B,N-CDs have an excitation-independent fluorescent characteristic, which was probably ascribed to narrow size distribution and relatively uniform surface [26,27]. Meanwhile, the QY of B,N-CDs was measured to be 7.0% using quinine sulfate as the standard (Fig. S2), which exceeds that of B-CDs (2.3%) [28]. Furthermore, we also used BA, 4-CPBA, and AN to synthesize CDs, B-CDs, and N-CDs under identical conditions. As shown in Fig. S3A, the CDs synthesized from BA (curve 1), AN (curve 2), and 4-CPBA (curve 3) did not display obvious fluorescence emission compared to that synthesized from 3-APBA⋅H2O (curve 4) with an excitation wavelength of 350 nm, indicating that the B and N co-doping can enhance the emission intensity of B,N-CDs. The co-doping of heteroatoms can introduce more active sites and reduce nonradiative recombination, and this phenomenon has been intensively investigated [29-31]. Jiang et al. reported that phenylenediamine isomers can synthesize CDs with red, green, and blue emissions by a solvothermal method [32]. In order to explore whether the chemical structure will affect the fluorescence properties, 3-APBA⋅H2O was replaced by 2-APBA (curve 5) and 4-APBA⋅HCl (curve 6), only CDs prepared by 3-APBA⋅H2O showed a strong fluorescence emission signal. Nevertheless, CDs prepared by 4-APBA⋅HCl showed no emission, 2-APBA showed very weak maximum emission at 483 nm under 370 10
nm excitation with a red-shift (Fig. S3B) compared to 3-APBA⋅H2O. The fluorescence (Fig. S3A) and UV-vis absorption spectra (Fig. S4) of B,N-CDs synthesized from the other two aminophenylboronic acid isomers, B-CDs, N-CDs, and CDs are different from that of B,N-CDs synthesized from 3-APBA⋅H2O. These findings indicated that B,N co-doping and the different molecular structures of aminophenylboronic acid play significant roles in controlling fluorescence properties of CDs. The detailed information of the sole precursor for synthesizing CDs including BA, AN, 4-CPBA, 3-APBA⋅H2O, 2-APBA, 4-APBA⋅HCl are shown in Table S1. 3.3.Characterization of B,N-CDs The TEM image (Fig. 1C) reveals that the B,N-CDs possess a good monodispersity and narrow size distribution (1.9 2.5 nm), with an average diameter of 2.1 nm (Fig. 1D). The FT-IR spectrum of as-prepared B,N-CDs is shown in Fig. S5 (black curve), and the distinct characteristic peaks at 3385, 1651, 1402, 1340, 1267, 1123, 1094, 1024, and 650-930 cm−1 are ascribed to NH, C=C, CN, BO, COC, COH, CB, BOH, and OH [23,25], respectively. The XPS spectra further state the composition of B,N-CDs. Fig. 2A shows that the B,N-CDs contain four dominant peaks at 531.6, 399.6, 285.6, and 191.6 eV which were corresponded to O 1s, N 1s, C 1s, and B 1s, respectively. The high-resolution spectrum of C 1s (Fig. 2B) reveals that there are C=O (288.2 eV), CO/CN (286.2 eV), CC/C=C (284.8 eV), and CB (283.9 eV) bonds [23,25,33,34]. From the N 1s spectrum (Fig. 2C), three peaks are ascribed to graphitic N (400.9 eV), pyrrolic N (399.6 eV), and pyridinic N (398.9 eV) [33,34]. Moreover, the BO bond (192.7 eV) and BC bond (191.8 eV) were 11
observed from the B 1s spectrum as displayed in Fig. 2D. All the mentioned data show that the B,N-CDs possess abundant functional groups on the surface, such as COOH, C=O, OH, NH, etc. 3.4.Stability of B,N-CDs The as-obtained B,N-CDs are relatively stable toward a broad pH ranges from 3 to 12 and a continuous UV lamp irradiation for 45 min (Fig. S6A and S6B). Furthermore, Fig. S7 shows that the B,N-CDs can remain stable for at least two months when stored at 4 °C. The fluorescence intensity of B,N-CDs changes slightly even when the concentrations of NaCl are up to 4.0 M in Fig. S8A, suggesting that the B,N-CDs are extremely stable even under a solution of extreme ionic strength. Since majority of the CDs are easily oxidized [35,36], the B,N-CDs antioxidant capacity was investigated (Fig. S8B). It turned out that the B,N-CDs have good oxidation resistance, although the hydrogen peroxide concentration in the medium is higher than 0.4 M. All the results manifest that the B,N-CDs are comparatively stable, which guarantees their applications for p-NP determination in sophisticated samples. 3.5.Establishment of sensing platform for p-NP B,N-CDs possess good aqueous solubility, intrinsic fluorescence, and ample surface groups, suggesting that it would have greatly hopeful prospects in biological and chemical field applications. We noticed that the fluorescence of B,N-CDs was progressively decreased with increasing p-NP concentration, promoting us to establish a sensing platform to detect p-NP. To achieve better fluorescence response of the B,N-CDs for p-NP, several assay conditions were optimized (Fig. 3): (A) probe 12
concentration; (B) buffer solution; (C) sample pH value; (D) interaction time. As presented in Fig. 3A, 3B, and 3C, we chose 10 μL mL-1 the probe, BR buffer solutions at pH 7.7 as the optimum detection conditions. Under such conditions, the B,N-CDs show a higher detection sensitivity for p-NP over m-NP and o-NP. In addition, interaction time is critical for a sensor in practical application. The fluorescence response of the B,N-CDs to reaction time was continuously monitored after the addition of p-NP. As shown in Fig. 3D, the fluorescence can be completely quenched 1 min later. The result shows that the interaction between B,N-CDs and p-NP is quick, which means that the B,N-CDs are expected to be applied to real-time tracking of p-NP in a practical system. Under optimum conditions, the sensitivity for detecting p-NP was estimated by recording the fluorescence intensity change of the B,N-CDs probe at 430 nm. Fig. 4A shows that the fluorescence intensity is gradually decreased with increasing p-NP concentration ranging from 0 to 200 μM, accompanied by an apparent red-shift in fluorescence emission. Fig. 4B shows that the fluorescence quenching efficiency shows two good linear relationships against the p-NP concentration over the range of 0.5 60 and 60 200 μM with the regression equation: (F0-F)/F0 = 0.00961C + 0.0562 (R2 = 0.9906) and (F0-F)/F0 = 0.00204C + 0.497 (R2 = 0.9932), respectively, where F and F0 denote the fluorescence intensity of B,N-CDs in the presence and absence of p-NP, respectively, and C denotes the concentration of p-NP. The detection limit for p-NP was 0.2 μM, based on the 3σ/slope (where σ is the standard deviation). Although there have been some reports on two linear relationships between the 13
analytical signal and the concentration [34,37,38], it is not clear why another linear relationship occurs in the high concentration range at present, and further exploration is needed. The selectivity of B,N-CDs for detection of p-NP was evaluated. The detailed information of p-NP and other structurally similar compounds are depicted in Table S2. Fig. 5 shows that except for p-NP, m-NP, and o-NP, the other representative analogues have no remarkable interferences, including m-DNB, MB, HQ, p-CP, PHE, o-DHB, AN, BA, p-NT, and TNT even at a high concentration (200 μM). The fluorescence quenching efficiency of p-NP (90%) is over four times that of m-NP (22%) and o-NP (19%), which may result from the different molecular structures of m-NP, o-NP, and p-NP, leading to the high selectivity of the B,N-CDs to p-NP. In view of the fact that some common ions may have potential interferences in practical water samples detection, the fluorescence quenching efficiency of common cations, such as Pb2+, K+, Li+, Ca2+, Sr2+, Fe3+, Ag+, Mn2+, Cu2+, Mg2+, Ba2+, Co2+, Hg2+, Zn2+, Al3+, Ni2+, and Cr3+ were also evaluated. In Fig. S9A, the fluorescence intensity was nearly not affected by most metal ions except for Hg2+. Even so, the interferences from Hg2+ can be effectively masked when enough EDTA was added. As shown in Fig. S9B, the fluorescence quenching efficiency of p-NP coexisting with common metal ions is nearly the same as that of p-NP alone, indicating that the interference of the cations is negligible. Moreover, we tested the fluorescence of B,N-CDs in the presence of 11 kinds of anions (NO3-, NO2-, Br-, Cl-, F-, I-, Ac-, S2-, CO32-, SO42-, and SO32- ) and they barely interfere with the detection of p-NP even when coexisting with 14
p-NP, respectively, as shown in Fig. S10. All these results imply that the fluorescent probe has good specificity toward p-NP. The synthesis of B,N-CDs using 3-APBA as the sole precursor is more simple and straightforward compared to other methods, which not only simplifies the synthesis step but also reduces the by-product. B,N-CDs without any modification can be directly applied as a fluorescent probe for a rapid and high selective detection of p-NP. Furthermore, the linear range and detection limit of our method are comparable to those of other analytical methods, as summarized in Table 1.
3.6.Possible mechanism of B,N-CDs sensor for p-NP detection To illuminate the possible mechanism of the high sensitivity and selectivity of B,N-CDs for p-NP detection, the UV-vis absorption spectra of p-NP and other representative analogues were determined. Fig. 6A shows that only the absorption spectrum of p-NP has a wide overlap with the fluorescence excitation and emission spectra of B,N-CDs compared to those of other representative analogues (m-DNB, MB, HQ, p-CP, PHE, o-DHB, AN, BA, p-NT, TNT, m-NP, and o-NP) in the BR buffer (pH 7.7), which indicates that the fluorescence quenching of B,N-CDs result from the IFE or FRET [44]. The fluorescence quenching caused by the IFE is temperature-independent. As shown in Fig. S11, the effect of temperature on fluorescence reduction is ignorable, indicating that the fluorescence decrease of B,N-CDs is mainly IFE. The fluorescence lifetime of fluorophore is not altered by the quencher in the IFE process [45]. This statement can be further affirmed from that the 15
fluorescence lifetimes of B,N-CDs have ignorable change before and after the addition of p-NP in Fig. 6B (τ = 5.06 ns) and 6C (τ = 4.90 ns), respectively. This phenomenon suggests that there is no FRET between B,N-CDs and p-NP [46] and the fluorescence quenching process is static, because of the formation of a non-fluorescent complex or compound [21]. Moreover, Tang et al [47] reported that the maximal absorption of p-NP can be shifted to a longer wavelength (400 nm) with the increase in pH values. The absorption band of p-NP has an overlap with the emission spectrum of poly(p-phenylenes) functionalized with oligo(oxyethylene) side chains (PPP-OR10) and the emission peak of PPP-OR10/p-NP red-shifted as a result of the IFE. Fig. 4A shows that the emission peak position of the B, N-CDs is red-shifted with the increasing p-NP concentration and the absorption spectrum of p-NP has a wide overlap with the fluorescence emission spectrum of B,N-CDs in Fig. 6A. Thus, we deduced that the emission wavelength of B,N-CDs red-shifted is due to the IFE. To further verify the fluorescence quenching mechanism, we explored the electrochemical properties of B,N-CDs by the cyclic voltammetry (CV). The detailed calculation process is shown in Fig. S12. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of B,N-CDs are determined as −3.82 and −8.78 eV, respectively, on the basis of the empirical formula [48]. Moreover, the ELUMO and EHOMO of p-NP were calculated to be –2.92 and –7.16 eV, respectively, according to the B3LYP method in Dmol3 module. The lower the ELUMO value of the analyte is, the easier it is to accept electrons [44]. As can 16
be seen from Fig. 6D, the electrons in the photo-excited B,N-CDs were not allowed to transfer from the LUMO of B,N-CDs to the LUMO of p-NP, suggesting that PET between B,N-CDs and p-NP is not feasible. In addition, as shown in Fig. 3C, the pH has an important effect on the detection of the p-NP especially when the pH ranges from 6.0 to 8.0. This phenomenon can be attributed to the existence of boronic acid groups in B,N-CDs and the phenolic hydroxyl group in p-NP. It has been reported that a significant association between boronic acid and hydroxyl group of p-NP can form a dynamic covalent B-O bond under neutral or weakly alkaline conditions [21]. Thus, the B,N-CDs probe binds to p-NP to form a non-fluorescent conjugate, resulting in obvious fluorescence quenching. Hence, the B,N-CDs exhibited high sensitivity and selectivity for p-NP, which is attributed to the IFE and the formation of dynamic covalent B-O bonds between B,N-CDs and p-NP.
3.7.Environmental water samples detection It remains challenging to analyze the real samples for a presented fluorescent sensor, due to the presence of potentially unknown interferents in real samples. In order to trace the feasibility of the sensor, we applied the synthesized B,N-CDs to detect p-NP in the Jialing River and tap water samples. No p-NP was detected in the water samples. The accuracy of the method was identified when the samples were spiked with different concentrations of p-NP standard solution. The results are exhibited in Table S3, it can be seen that the recoveries of p-NP range from 97.02 to 17
107.84% and the relative standard deviations (RSD) of three times repeated determination for each sample vary from 0.55% to 1.60%, which indicates that our method has a good recovery, accuracy, and repeatability. All the results above imply that the sensor has great application potential for the detection of p-NP in practical samples.
4. Conclusion In conclusion, a rapid, facile, less sample consumption, and highly selective fluorescence sensing platform based on B,N-CDs was successfully used for p-NP detection. The B,N-CDs were prepared through the one-pot hydrothermal method using a sole precursor 3-aminophenylboronic acid monohydrate as the nitrogen, boron and carbon source, simultaneously. Without any extra surface modification, the synthesized B,N-CDs can be directly used as a fluorescent probe for p-NP sensing with high selectivity based on the IFE and the formation of dynamic covalent B-O bonds between them. Moreover, the interference from other analogues is negligible. This work provides a potential application of B,N-CDs for the monitoring of p-NP in the environmental field.
Acknowledgment This work was financially supported by the National Natural Science Foundation of China (No. 21675131) and the Municipal Science Foundation of Chongqing City (No. CSTC-2015jcyjB50001).
Appendix A. Supporting information 18
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j. Talanta.201 X.XX.XXX.
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Figure captions: Scheme 1. Synthetic strategy for fluorescent B,N-CDs and the detection schematic of p-NP. Fig. 1. (A) The UV-vis absorption and fluorescence spectra of B,N-CDs, inset: photos 26
of B,N-CDs under visible light (1) and UV light (365 nm, 2). (B) Excitation-independent fluorescence spectra of the B,N-CDs. TEM image (C) and the corresponding particles size distribution (D) of B,N-CDs. C(B,N-CDs) = 10 μL mL-1. Fig. 2. (A) XPS full scan spectrum. High-resolution C1S (B), N1S (C), and B1S (D) spectra of the B,N-CDs. Fig. 3. Fluorescence responses of B,N-CDs in the presence of p-NP, m-NP, and o-NP, respectively, at different concentrations of B,N-CDs (A), different buffer solutions (B), and in BR buffer at different pH value (C). (D) Fluorescence intensities of the B,N-CDs/p-NP with the variation of time. C(p-NP, m-NP, o-NP) = 60 μM. Fig. 4. (A) Fluorescence spectra of B,N-CDs (10 μL mL-1) after addition of different concentrations of p-NP in BR buffer (pH 7.7). (B) Fluorescence response of B,N-CDs with the p-NP concentration increased. Insets: Linear relationships between (F0 F)/F0 and p-NP level in the range of 0.5 - 60 μM and 60 - 200 μM, respectively. Fig. 5. Fluorescence response of B,N-CDs (10 μL mL-1) to 200 μM p-NP and other selected analogues in BR buffer (pH 7.7). Fig. 6. (A) The UV-vis absorption spectra of p-NP and potentially interfering substances (60 μM), and fluorescence excitation and emission spectra of B,N-CDs (10 μL mL-1). Time-resolved fluorescence spectrum of B,N-CDs (10 μL mL-1) in the absence (B) and presence of 200 µM p-NP (C), respectively. (D) The EHOMO and ELUMO of B,N-CDs and p-NP. All the solutions pH value is 7.7 in BR buffer medium.
27
28
29
30
Table 1. Comparison of analytical performance for p-NP using different methods. Probe
Three dimensional electrochemical
Method
Electrochemistry
Linear
LOD
Ref.
range (μM)
(μM)
10 200
1.10
[39]
1.16
[40]
paper —
CeO2-ZnO nanoellipsoids
Electrochemistry
CdSe QDs and DNA composite film
Photoelectrochemistry 0.7 50
0.27
[22]
modified electrode PEI a stabilized Ag NCs b
Colorimetry
5 140
1.28
[41]
Carbon dots
Fluorometry
0.1 50
0.028
[12]
Cyclodextrin-capped CdTe QDs c
Fluorometry
20 100
0.30
[10]
Nitrogen-doped carbon dots
Fluorometry
0.72 79
0.16
[42]
Coumarin derivative
Fluorometry
0.7 143
0.70
[43]
31
-Cyclodextrin-capped ZnO QDs
Fluorometry
1 40
0.34
[11]
B,N-CDs
Fluorometry
0.5 200
0.20
This work
a
PEI: polyethyleneimine, b Ag NCs: Ag nanoclusters, c QDs: quantum dots.
Highlights
The synthesis of B,N-CDs using one precursor is simple and straightforward.
Using the sole precursor simplifies the synthesis step and reduces the by-product.
Without any modification, B,N-CDs can be directly applied as a fluorescent probe.
A rapid and highly selective method was proposed to detect p-nitrophenol.
32
PII: DOI: Reference:
S0039-9140(18)30232-7 https://doi.org/10.1016/j.talanta.2018.02.114 TAL18433
To appear in: Talanta Received date: 17 November 2017 Revised date: 22 February 2018 Accepted date: 28 February 2018 Cite this article as: Na Xiao, Shi Gang Liu, Shi Mo, Na Li, Yan Jun Ju, Yu Ling, Nian Bing Li and Hong Qun Luo, Highly selective detection of p-nitrophenol using fluorescence assay based on boron, nitrogen co-doped carbon dots, Talanta, https://doi.org/10.1016/j.talanta.2018.02.114 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highly
selective
detection
of
p-nitrophenol
using
fluorescence assay based on boron, nitrogen co-doped carbon dots Na Xiaoa, Shi Gang Liua, Shi Mob, Na Lia, Yan Jun Jua, Yu Linga, Nian Bing Lia,*, Hong Qun Luoa,* a
Key Laboratory of Eco-Environments in Three Gorges Reservoir Region (Ministry
of Education), School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, PR China b
Department of Physics, City University of Hong Kong, Tat Chee Avenue, Kowloon,
Hong Kong, China [email protected] (NB Li) [email protected] (HQ Luo) *
Corresponding Author. Tel: +86 23 68253237; fax: +86 23 68253237; .
Abstract p-Nitrophenol (p-NP) contaminants seriously endanger environmental and living beings health, hence to establish a sensitive and selective method is of great importance for the determination of p-NP. In this work, boron and nitrogen co-doped carbon dots (B,N-CDs) were synthesized by one-step hydrothermal method using 3-aminophenylboronic acid as the sole precursor. The product was characterized through high-resolution transmission electron microscopy, fluorescence spectroscopy, UV-visible absorption spectroscopy, X-ray photoelectron spectroscopy, and Fourier 1
transform infrared spectroscopy. Without any functionalized modification, B,N-CDs can be directly applied as a ‘turn-off’ fluorescent probe for rapid, highly selective, and sensitive detection of p-NP. The fluorescent sensor based on the B,N-CDs exhibited a broad linear response to the concentration of p-NP in the range of 0.5 60 μM and 60 200 μM, respectively, and provided a detection limit of 0.2 μM. It was found that only the absorption spectrum of p-NP has a wide overlap with the fluorescence excitation and emission spectra of B,N-CDs compared to those of other representative analogues. The response mechanism was due to the inner filter effect and the formation of dynamic covalent B-O bonds between B,N-CDs and p-NP, which endowed the sensing platform with the rapid response and high selectivity to p-NP. Finally, the sensor showed the practicability of p-NP determination in environmental water samples.
Graphical Abstract
2
Keywords: Boron, nitrogen co-doped carbon dots; Fluorescent sensor; p-Nitrophenol; Inner filter effect
1. Introduction p-Nitrophenol (p-NP) is an indispensable material for the manufacture of pharmaceuticals and fine chemicals, and can be widely found in soils and aquatic environments [1]. However, as a chemical industrial wastewater contaminant [2-4], p-NP can stably exist in different environments and continues to gather in the food chain, potentially leading to damages of environment and living beings owing to its toxicity [5]. Thus, the US Environmental Protection Agency has listed p-NP as a priority pollutant [6]. Although there are many methods for the detection of p-NP, such as capillary electrophoresis [7], chromatographic technique [8], and electrochemical methods [9], some deficiencies including electrode instability, sophisticated and expensive instruments or complicated pretreatment procedures make them unsuitable for rapid monitoring. In contrast, fluorescence analysis exhibits superiority because of its simple sample pretreatment, less solvent consumption, rapid response, low expense, and high sensitivity. Thus, some fluorescence probes have been used for p-NP detection, for instance,
mono-6-SH-β-cyclodextrin-capped
CdTe
quantum
dots
[10],
3
β-cyclodextrin-capped
ZnO
quantum
dots
[11],
and
carbon
dots
(CDs)
(tris(hydroxymethyl)aminomethane (Tris) and ethyleneglycol bis-(2-aminoethyl ether)-N,N,N’,N’-tetraacetic acid (EGTA) as carbon sources) [12].Unfortunately, the widespread applications of these methods are hampered by the toxicity, heavy metal pollution, sensors fabrication through two-step processes, using more than one precursor, and relatively high cost reagents. All in all, the development of rapid, inexpensive, environment-friendly, and simple fluorescence analysis method for p-NP is desperately necessary. CDs as an effective alternative to toxic quantum dots have attracted much attention in recent years due to low toxicity, biocompatibility, easy preparation, and high sensitivity to the target [13,14]. Recently, there are numerous works dealing with quenching of CDs fluorescence for analytical purposes based on different quenching mechanisms, including photoinduced electron transfer (PET), Förster resonance energy transfer (FRET), and inner filter effect (IFE). Through the PET and FRET quenching mechanisms of CDs, Pb2+, Co2+, Cu2+, Hg2+, S2-, adenosine 5′-triphosphate, phytic acid, thiamine, glyphosate, folate receptor, glutathione, and carcinoembryonic antigen can be detected [15]. Tang et al. employed β-cyclodextrin-modified CDs to detect alkaline phosphatase (ALP) activity based on PET through host-guest recognition between p-NP and β-cyclodextrin [16]. García et al. reported the CDs synthesized by EGTA and Tris via thermal carbonization, which can be used to detect p-NP based on the energy transfer [12]. However, IFE has grown to be a useful and efficient tactics for the construction and development of novel fluorescent sensors by 4
translating the analytical absorption signals into fluorescence signals, which possess enhanced selectivity and sensitivity compared to other quenching mechanisms (e.g., PET and FRET) [17-19]. The IFE refers to the absorption of an absorber to the excitation and/or emission of light within the detection system when the fluorescence excitation or emission spectra of fluorophores overlap with the absorption spectra of the absorbers [20]. Based on the IFE, S2-, Cr6+, ascorbic acid, picric acid, hemoglobin, fluazinam, alkaline phosphatase, and β-glucuronidase can be detected with CDs [15]. Li et al. reported that the N-CDs were synthesized with catechol and ethanediamine by a hydrothermal method and applied to ALP sensing based on IFE by catalysis of ALP towards p-nitrophenylphosphate to yield p-NP [20]. Therefore, the IFE-based sensors are simple to implement, and can be potentially applied in other fields. In this work, we synthesized N and B co-doped CDs (B,N-CDs) using a single starting material containing both B and N, which not only simplifies the synthesis step but also reduces the by-product compared with most of the B,N-CDs synthesized from at least two compounds as the raw materials. Herein, through comparing the fluorescence
properties
aminophenylboronic 3-aminophenylboronic
of
acid acid
B,N-CDs isomers
synthesized
including
monohydrate,
and
from
three
different
2-aminophenylboronic 4-aminophenylboronic
acid, acid
hydrochloride under same condition, we finally adopted 3-aminophenylboronic acid monohydrate as the sole precursor with a favorable chemical structure. The synthesized B,N-CDs exhibited a strong blue fluorescence emission at 430 nm under 350 nm excitation, and some other properties such as excellent water solubility, 5
oxidation resistance and high stability even under a wide pH range, long-time storage and high ionic strength. B,N-CDs without any functionalized modification can be directly applied as a rapid, highly selective and sensitive sensor for p-NP detection (Scheme 1), which is simpler and more straightforward compared to the previously reported nanoparticle-based sensors fabrication through two-step processes [11,21,22]. Furthermore, the developed sensor was applied to p-NP detection in real water samples (tap water or river water) with satisfactory results, suggesting that the B,N-CDs have great potential for the detection of p-NP in real samples.
2. Experimental 2.1. Chemicals 3-Aminophenylboronic
acid
monohydrate
(3-APBA⋅H2O),
2-aminophenylboronic acid (2-APBA), 4-carboxyphenylboronic acid (4-CPBA), benzoic acid (BA), aniline (AN), p-nitrotoluene (p-NT), o-dihydroxybenzene (o-DHB), p-chlorophenol (p-CP), m-dinitrobenzene (m-DNB), methylbenzene (MB), 2,4,6-trinitrotoluene p-nitrophenol
(TNT),
(p-NP),
m-nitrophenol hydroquinone
(m-NP), (HQ),
o-nitrophenol phenol
(PHE),
(o-NP), and
tetra-n-butylammonium hexafluorophosphate (Bu4NPF6) were supplied by Aladdin Reagent Co., Ltd., Shanghai, China. 4-Aminophenylboronic acid hydrochloride (4-APBA⋅HCl) was obtained from Macklin Biochemical Co., Ltd., Shanghai, China. Ethylenediaminetetra-acetic acid disodium salt (EDTA), NaOH, and NaCl were purchased from Kelong Chemical Reagent Co., Ltd., Chengdu, China. Ultrapure 6
water (18.2 MΩ cm) was used throughout the experiments. 2.2. Apparatus The high-resolution transmission electron microscopy (HRTEM) images were observed from a JEM-2100 (JEOL Ltd., Japan). UV-vis absorption spectra were measured with a UV-2450 spectrophotometer (Suzhou Shimadzu Instrument Co., Ltd., China). Fluorescence spectra were recorded on an F-2700 spectrofluorophotometer (Hitachi Ltd., Japan) with 10 nm slit widths for both excitation and emission. X-ray photoelectron spectroscopy (XPS) measurements were conducted using an ESCALab 250Xi (Thermo Fisher Scientific, USA). Fourier transform infrared (FTIR) spectrum was recorded on a Bruker IFS 113v spectrometer (Bruker, Germany). The pH values of solutions were obtained from METTLER TOLEDO FE28 pH meter (Mettler Toledo, China). The fluorescence lifetimes were determined using an FLSP920 fluorescence spectrometer (Edinburgh, UK). 2.3. Synthesis of B,N-CDs A simple hydrothermal method was used to the one-pot synthesis of B,N-CDs on the basis of previous literature with minor modification [23]. Typically, 5.0 mg of 3-APBA⋅H2O was dissolved in 5.0 mL of ultrapure water, and subsequently the pH was adjusted to 10.0 with 1.0 M NaOH under agitation. Next, the solution was transferred to the Teflon-lined autoclave and heated to 160 °C for 8 h. After being cooled to room temperature naturally, the resultant yellowish-brown solution was centrifuged to remove large aggregates at 10, 000 rpm for 20 min. Finally, the as-prepared B,N-CDs were stored at 4 °C for later use. In addition, other control CDs 7
were synthesized by using the same method except that 3-APBA⋅H2O was replaced by BA, 4-CPBA, AN, 2-APBA, and 4-APBA⋅HCl, respectively. The diluted 100-fold B,N-CDs (10 μL mL-1) aqueous solutions were used as the fluorescent probe to establish the sensing platform for p-NP in this research. 2.4. Quantum yield measurement The quantum yield (QY) of the B,N-CDs was measured according to a slope method [24]. Quinine sulfate with QY = 0.54 dissolved in 0.1 M H2SO4 was chosen as a standard sample. In this work, quinine sulfate and B,N-CDs solutions were prepared with five different concentrations, and all of their absorbance at 350 nm is less than 0.1. The QY can be measured by the following equation:
Grad x 2X 2 x ST GradST ST
(1)
where ϕ is the quantum yield, the subscript “st” and “x” refer to the standard and samples, respectively, Grad is the slope, and η is the refractive index. For these aqueous solutions, ηx/ηst = 1. 2.5. Procedures for p-NP sensing 5 μL of CDs and 150 μL of BR buffer (pH 7.7) were added to a polypropylene microcentrifuge tube. Subsequently, various concentrations of p-NP were added and the mixtures were diluted to 500 μL with ultrapure water. After the mixtures were incubated for 1 min at room temperature, the fluorescence emission signal was monitored at an excitation wavelength of 350 nm. In order to analyze the real water samples, river water obtained from the Jialing River in Chongqing was filtered 8
through a 0.22 μm membrane to remove particles and tap water from our laboratory was not pretreated. No p-NP was detected in the real samples and then the samples were spiked with 1.0 mM EDTA and different concentrations (20.0, 50.0, 120.0, 160.0 μM) of standard p-NP solutions. Other steps were consistent with those described above. Fluorescence quenched efficiency (F0-F)/F0 was analyzed (F and F0 are the fluorescence intensity of the B,N-CDs in the presence and absence of p-NP, respectively).
3. Results and discussion 3.1.Optimization of synthesis conditions The B,N-CDs were prepared by a facile one-pot hydrothermal method using 3-APBA⋅H2O as a sole B and N source. To achieve high luminescent B,N-CDs, the following experimental conditions towards 5.0 mg of 3-APBA⋅H2O in 5.0 mL of ultrapure water were optimized (Fig. S1): (A) the pH value of the solution; (B) reaction temperature; (C) reaction time. As shown in Fig. S1, the carbonization conditions seriously affect the fluorescence intensity of B,N-CDs. Consequently, 5.0 mg of 3-APBA⋅H2O in 5.0 mL of ultrapure water with the reaction temperature 160 °C for 8 h at pH 10.0 was specified as the optimum condition for synthesizing B,N-CDs. 3.2.Optical properties of B,N-CDs As a result of π-π* transition of C=C bond [23,25], a broad UV-visible absorption band of B, N-CDs is observed at 220-320 nm with strong peaks at 229 and 268 nm (Fig. 1A). Furthermore, the B,N-CDs have a maximum emission peak at 430 9
nm when excited at 350 nm, and the solution emitted strong blue fluorescence under 365 nm UV light (Fig. 1A, inset 2). As shown in Fig. 1B, the fluorescence emission spectra of B,N-CDs remained unchanged when the excitation wavelength ranges from 330 420 nm. The results suggest that the B,N-CDs have an excitation-independent fluorescent characteristic, which was probably ascribed to narrow size distribution and relatively uniform surface [26,27]. Meanwhile, the QY of B,N-CDs was measured to be 7.0% using quinine sulfate as the standard (Fig. S2), which exceeds that of B-CDs (2.3%) [28]. Furthermore, we also used BA, 4-CPBA, and AN to synthesize CDs, B-CDs, and N-CDs under identical conditions. As shown in Fig. S3A, the CDs synthesized from BA (curve 1), AN (curve 2), and 4-CPBA (curve 3) did not display obvious fluorescence emission compared to that synthesized from 3-APBA⋅H2O (curve 4) with an excitation wavelength of 350 nm, indicating that the B and N co-doping can enhance the emission intensity of B,N-CDs. The co-doping of heteroatoms can introduce more active sites and reduce nonradiative recombination, and this phenomenon has been intensively investigated [29-31]. Jiang et al. reported that phenylenediamine isomers can synthesize CDs with red, green, and blue emissions by a solvothermal method [32]. In order to explore whether the chemical structure will affect the fluorescence properties, 3-APBA⋅H2O was replaced by 2-APBA (curve 5) and 4-APBA⋅HCl (curve 6), only CDs prepared by 3-APBA⋅H2O showed a strong fluorescence emission signal. Nevertheless, CDs prepared by 4-APBA⋅HCl showed no emission, 2-APBA showed very weak maximum emission at 483 nm under 370 10
nm excitation with a red-shift (Fig. S3B) compared to 3-APBA⋅H2O. The fluorescence (Fig. S3A) and UV-vis absorption spectra (Fig. S4) of B,N-CDs synthesized from the other two aminophenylboronic acid isomers, B-CDs, N-CDs, and CDs are different from that of B,N-CDs synthesized from 3-APBA⋅H2O. These findings indicated that B,N co-doping and the different molecular structures of aminophenylboronic acid play significant roles in controlling fluorescence properties of CDs. The detailed information of the sole precursor for synthesizing CDs including BA, AN, 4-CPBA, 3-APBA⋅H2O, 2-APBA, 4-APBA⋅HCl are shown in Table S1. 3.3.Characterization of B,N-CDs The TEM image (Fig. 1C) reveals that the B,N-CDs possess a good monodispersity and narrow size distribution (1.9 2.5 nm), with an average diameter of 2.1 nm (Fig. 1D). The FT-IR spectrum of as-prepared B,N-CDs is shown in Fig. S5 (black curve), and the distinct characteristic peaks at 3385, 1651, 1402, 1340, 1267, 1123, 1094, 1024, and 650-930 cm−1 are ascribed to NH, C=C, CN, BO, COC, COH, CB, BOH, and OH [23,25], respectively. The XPS spectra further state the composition of B,N-CDs. Fig. 2A shows that the B,N-CDs contain four dominant peaks at 531.6, 399.6, 285.6, and 191.6 eV which were corresponded to O 1s, N 1s, C 1s, and B 1s, respectively. The high-resolution spectrum of C 1s (Fig. 2B) reveals that there are C=O (288.2 eV), CO/CN (286.2 eV), CC/C=C (284.8 eV), and CB (283.9 eV) bonds [23,25,33,34]. From the N 1s spectrum (Fig. 2C), three peaks are ascribed to graphitic N (400.9 eV), pyrrolic N (399.6 eV), and pyridinic N (398.9 eV) [33,34]. Moreover, the BO bond (192.7 eV) and BC bond (191.8 eV) were 11
observed from the B 1s spectrum as displayed in Fig. 2D. All the mentioned data show that the B,N-CDs possess abundant functional groups on the surface, such as COOH, C=O, OH, NH, etc. 3.4.Stability of B,N-CDs The as-obtained B,N-CDs are relatively stable toward a broad pH ranges from 3 to 12 and a continuous UV lamp irradiation for 45 min (Fig. S6A and S6B). Furthermore, Fig. S7 shows that the B,N-CDs can remain stable for at least two months when stored at 4 °C. The fluorescence intensity of B,N-CDs changes slightly even when the concentrations of NaCl are up to 4.0 M in Fig. S8A, suggesting that the B,N-CDs are extremely stable even under a solution of extreme ionic strength. Since majority of the CDs are easily oxidized [35,36], the B,N-CDs antioxidant capacity was investigated (Fig. S8B). It turned out that the B,N-CDs have good oxidation resistance, although the hydrogen peroxide concentration in the medium is higher than 0.4 M. All the results manifest that the B,N-CDs are comparatively stable, which guarantees their applications for p-NP determination in sophisticated samples. 3.5.Establishment of sensing platform for p-NP B,N-CDs possess good aqueous solubility, intrinsic fluorescence, and ample surface groups, suggesting that it would have greatly hopeful prospects in biological and chemical field applications. We noticed that the fluorescence of B,N-CDs was progressively decreased with increasing p-NP concentration, promoting us to establish a sensing platform to detect p-NP. To achieve better fluorescence response of the B,N-CDs for p-NP, several assay conditions were optimized (Fig. 3): (A) probe 12
concentration; (B) buffer solution; (C) sample pH value; (D) interaction time. As presented in Fig. 3A, 3B, and 3C, we chose 10 μL mL-1 the probe, BR buffer solutions at pH 7.7 as the optimum detection conditions. Under such conditions, the B,N-CDs show a higher detection sensitivity for p-NP over m-NP and o-NP. In addition, interaction time is critical for a sensor in practical application. The fluorescence response of the B,N-CDs to reaction time was continuously monitored after the addition of p-NP. As shown in Fig. 3D, the fluorescence can be completely quenched 1 min later. The result shows that the interaction between B,N-CDs and p-NP is quick, which means that the B,N-CDs are expected to be applied to real-time tracking of p-NP in a practical system. Under optimum conditions, the sensitivity for detecting p-NP was estimated by recording the fluorescence intensity change of the B,N-CDs probe at 430 nm. Fig. 4A shows that the fluorescence intensity is gradually decreased with increasing p-NP concentration ranging from 0 to 200 μM, accompanied by an apparent red-shift in fluorescence emission. Fig. 4B shows that the fluorescence quenching efficiency shows two good linear relationships against the p-NP concentration over the range of 0.5 60 and 60 200 μM with the regression equation: (F0-F)/F0 = 0.00961C + 0.0562 (R2 = 0.9906) and (F0-F)/F0 = 0.00204C + 0.497 (R2 = 0.9932), respectively, where F and F0 denote the fluorescence intensity of B,N-CDs in the presence and absence of p-NP, respectively, and C denotes the concentration of p-NP. The detection limit for p-NP was 0.2 μM, based on the 3σ/slope (where σ is the standard deviation). Although there have been some reports on two linear relationships between the 13
analytical signal and the concentration [34,37,38], it is not clear why another linear relationship occurs in the high concentration range at present, and further exploration is needed. The selectivity of B,N-CDs for detection of p-NP was evaluated. The detailed information of p-NP and other structurally similar compounds are depicted in Table S2. Fig. 5 shows that except for p-NP, m-NP, and o-NP, the other representative analogues have no remarkable interferences, including m-DNB, MB, HQ, p-CP, PHE, o-DHB, AN, BA, p-NT, and TNT even at a high concentration (200 μM). The fluorescence quenching efficiency of p-NP (90%) is over four times that of m-NP (22%) and o-NP (19%), which may result from the different molecular structures of m-NP, o-NP, and p-NP, leading to the high selectivity of the B,N-CDs to p-NP. In view of the fact that some common ions may have potential interferences in practical water samples detection, the fluorescence quenching efficiency of common cations, such as Pb2+, K+, Li+, Ca2+, Sr2+, Fe3+, Ag+, Mn2+, Cu2+, Mg2+, Ba2+, Co2+, Hg2+, Zn2+, Al3+, Ni2+, and Cr3+ were also evaluated. In Fig. S9A, the fluorescence intensity was nearly not affected by most metal ions except for Hg2+. Even so, the interferences from Hg2+ can be effectively masked when enough EDTA was added. As shown in Fig. S9B, the fluorescence quenching efficiency of p-NP coexisting with common metal ions is nearly the same as that of p-NP alone, indicating that the interference of the cations is negligible. Moreover, we tested the fluorescence of B,N-CDs in the presence of 11 kinds of anions (NO3-, NO2-, Br-, Cl-, F-, I-, Ac-, S2-, CO32-, SO42-, and SO32- ) and they barely interfere with the detection of p-NP even when coexisting with 14
p-NP, respectively, as shown in Fig. S10. All these results imply that the fluorescent probe has good specificity toward p-NP. The synthesis of B,N-CDs using 3-APBA as the sole precursor is more simple and straightforward compared to other methods, which not only simplifies the synthesis step but also reduces the by-product. B,N-CDs without any modification can be directly applied as a fluorescent probe for a rapid and high selective detection of p-NP. Furthermore, the linear range and detection limit of our method are comparable to those of other analytical methods, as summarized in Table 1.
3.6.Possible mechanism of B,N-CDs sensor for p-NP detection To illuminate the possible mechanism of the high sensitivity and selectivity of B,N-CDs for p-NP detection, the UV-vis absorption spectra of p-NP and other representative analogues were determined. Fig. 6A shows that only the absorption spectrum of p-NP has a wide overlap with the fluorescence excitation and emission spectra of B,N-CDs compared to those of other representative analogues (m-DNB, MB, HQ, p-CP, PHE, o-DHB, AN, BA, p-NT, TNT, m-NP, and o-NP) in the BR buffer (pH 7.7), which indicates that the fluorescence quenching of B,N-CDs result from the IFE or FRET [44]. The fluorescence quenching caused by the IFE is temperature-independent. As shown in Fig. S11, the effect of temperature on fluorescence reduction is ignorable, indicating that the fluorescence decrease of B,N-CDs is mainly IFE. The fluorescence lifetime of fluorophore is not altered by the quencher in the IFE process [45]. This statement can be further affirmed from that the 15
fluorescence lifetimes of B,N-CDs have ignorable change before and after the addition of p-NP in Fig. 6B (τ = 5.06 ns) and 6C (τ = 4.90 ns), respectively. This phenomenon suggests that there is no FRET between B,N-CDs and p-NP [46] and the fluorescence quenching process is static, because of the formation of a non-fluorescent complex or compound [21]. Moreover, Tang et al [47] reported that the maximal absorption of p-NP can be shifted to a longer wavelength (400 nm) with the increase in pH values. The absorption band of p-NP has an overlap with the emission spectrum of poly(p-phenylenes) functionalized with oligo(oxyethylene) side chains (PPP-OR10) and the emission peak of PPP-OR10/p-NP red-shifted as a result of the IFE. Fig. 4A shows that the emission peak position of the B, N-CDs is red-shifted with the increasing p-NP concentration and the absorption spectrum of p-NP has a wide overlap with the fluorescence emission spectrum of B,N-CDs in Fig. 6A. Thus, we deduced that the emission wavelength of B,N-CDs red-shifted is due to the IFE. To further verify the fluorescence quenching mechanism, we explored the electrochemical properties of B,N-CDs by the cyclic voltammetry (CV). The detailed calculation process is shown in Fig. S12. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of B,N-CDs are determined as −3.82 and −8.78 eV, respectively, on the basis of the empirical formula [48]. Moreover, the ELUMO and EHOMO of p-NP were calculated to be –2.92 and –7.16 eV, respectively, according to the B3LYP method in Dmol3 module. The lower the ELUMO value of the analyte is, the easier it is to accept electrons [44]. As can 16
be seen from Fig. 6D, the electrons in the photo-excited B,N-CDs were not allowed to transfer from the LUMO of B,N-CDs to the LUMO of p-NP, suggesting that PET between B,N-CDs and p-NP is not feasible. In addition, as shown in Fig. 3C, the pH has an important effect on the detection of the p-NP especially when the pH ranges from 6.0 to 8.0. This phenomenon can be attributed to the existence of boronic acid groups in B,N-CDs and the phenolic hydroxyl group in p-NP. It has been reported that a significant association between boronic acid and hydroxyl group of p-NP can form a dynamic covalent B-O bond under neutral or weakly alkaline conditions [21]. Thus, the B,N-CDs probe binds to p-NP to form a non-fluorescent conjugate, resulting in obvious fluorescence quenching. Hence, the B,N-CDs exhibited high sensitivity and selectivity for p-NP, which is attributed to the IFE and the formation of dynamic covalent B-O bonds between B,N-CDs and p-NP.
3.7.Environmental water samples detection It remains challenging to analyze the real samples for a presented fluorescent sensor, due to the presence of potentially unknown interferents in real samples. In order to trace the feasibility of the sensor, we applied the synthesized B,N-CDs to detect p-NP in the Jialing River and tap water samples. No p-NP was detected in the water samples. The accuracy of the method was identified when the samples were spiked with different concentrations of p-NP standard solution. The results are exhibited in Table S3, it can be seen that the recoveries of p-NP range from 97.02 to 17
107.84% and the relative standard deviations (RSD) of three times repeated determination for each sample vary from 0.55% to 1.60%, which indicates that our method has a good recovery, accuracy, and repeatability. All the results above imply that the sensor has great application potential for the detection of p-NP in practical samples.
4. Conclusion In conclusion, a rapid, facile, less sample consumption, and highly selective fluorescence sensing platform based on B,N-CDs was successfully used for p-NP detection. The B,N-CDs were prepared through the one-pot hydrothermal method using a sole precursor 3-aminophenylboronic acid monohydrate as the nitrogen, boron and carbon source, simultaneously. Without any extra surface modification, the synthesized B,N-CDs can be directly used as a fluorescent probe for p-NP sensing with high selectivity based on the IFE and the formation of dynamic covalent B-O bonds between them. Moreover, the interference from other analogues is negligible. This work provides a potential application of B,N-CDs for the monitoring of p-NP in the environmental field.
Acknowledgment This work was financially supported by the National Natural Science Foundation of China (No. 21675131) and the Municipal Science Foundation of Chongqing City (No. CSTC-2015jcyjB50001).
Appendix A. Supporting information 18
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j. Talanta.201 X.XX.XXX.
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Figure captions: Scheme 1. Synthetic strategy for fluorescent B,N-CDs and the detection schematic of p-NP. Fig. 1. (A) The UV-vis absorption and fluorescence spectra of B,N-CDs, inset: photos 26
of B,N-CDs under visible light (1) and UV light (365 nm, 2). (B) Excitation-independent fluorescence spectra of the B,N-CDs. TEM image (C) and the corresponding particles size distribution (D) of B,N-CDs. C(B,N-CDs) = 10 μL mL-1. Fig. 2. (A) XPS full scan spectrum. High-resolution C1S (B), N1S (C), and B1S (D) spectra of the B,N-CDs. Fig. 3. Fluorescence responses of B,N-CDs in the presence of p-NP, m-NP, and o-NP, respectively, at different concentrations of B,N-CDs (A), different buffer solutions (B), and in BR buffer at different pH value (C). (D) Fluorescence intensities of the B,N-CDs/p-NP with the variation of time. C(p-NP, m-NP, o-NP) = 60 μM. Fig. 4. (A) Fluorescence spectra of B,N-CDs (10 μL mL-1) after addition of different concentrations of p-NP in BR buffer (pH 7.7). (B) Fluorescence response of B,N-CDs with the p-NP concentration increased. Insets: Linear relationships between (F0 F)/F0 and p-NP level in the range of 0.5 - 60 μM and 60 - 200 μM, respectively. Fig. 5. Fluorescence response of B,N-CDs (10 μL mL-1) to 200 μM p-NP and other selected analogues in BR buffer (pH 7.7). Fig. 6. (A) The UV-vis absorption spectra of p-NP and potentially interfering substances (60 μM), and fluorescence excitation and emission spectra of B,N-CDs (10 μL mL-1). Time-resolved fluorescence spectrum of B,N-CDs (10 μL mL-1) in the absence (B) and presence of 200 µM p-NP (C), respectively. (D) The EHOMO and ELUMO of B,N-CDs and p-NP. All the solutions pH value is 7.7 in BR buffer medium.
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29
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Table 1. Comparison of analytical performance for p-NP using different methods. Probe
Three dimensional electrochemical
Method
Electrochemistry
Linear
LOD
Ref.
range (μM)
(μM)
10 200
1.10
[39]
1.16
[40]
paper —
CeO2-ZnO nanoellipsoids
Electrochemistry
CdSe QDs and DNA composite film
Photoelectrochemistry 0.7 50
0.27
[22]
modified electrode PEI a stabilized Ag NCs b
Colorimetry
5 140
1.28
[41]
Carbon dots
Fluorometry
0.1 50
0.028
[12]
Cyclodextrin-capped CdTe QDs c
Fluorometry
20 100
0.30
[10]
Nitrogen-doped carbon dots
Fluorometry
0.72 79
0.16
[42]
Coumarin derivative
Fluorometry
0.7 143
0.70
[43]
31
-Cyclodextrin-capped ZnO QDs
Fluorometry
1 40
0.34
[11]
B,N-CDs
Fluorometry
0.5 200
0.20
This work
a
PEI: polyethyleneimine, b Ag NCs: Ag nanoclusters, c QDs: quantum dots.
Highlights
The synthesis of B,N-CDs using one precursor is simple and straightforward.
Using the sole precursor simplifies the synthesis step and reduces the by-product.
Without any modification, B,N-CDs can be directly applied as a fluorescent probe.
A rapid and highly selective method was proposed to detect p-nitrophenol.
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