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One-pot fabrication of hollow cross-linked fluorescent carbon nitride nanoparticles and their application in the detection of mercuric ions
Talanta 143 (2015) 205–211
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One-pot fabrication of hollow cross-linked fluorescent carbon nitride nanoparticles and their application in the detection of mercuric ions Jie Ma a,b, Baoling Guo a,b, Xiaotong Cao a,b, Yanping Lin a,b, Bixia Yao a,b, Feiming Li a,b, Wen Weng a,b,n, Lizhang Huang c a
Department of Chemistry and Environmental Science, Minnan Normal University, Zhangzhou 363000, China Fujian Provincial Key Laboratory of Modern Analytical Science and Separation Technology, Zhangzhou 363000, China c Zhangzhou Product Quality Supervision Institute, Zhangzhou 363000, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 26 January 2015 Received in revised form 18 May 2015 Accepted 25 May 2015 Available online 27 May 2015
Hollow cross-linked fluorescent carbon nitride nanoparticles (CNNPs) were fabricated via a facile one-pot solvothermal process. The obtained CNNPs were characterized by multiple analytical techniques including transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), solid-state nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The excitation-dependent fluorescence emission spectra showed significant differences for the CNNPs derived from various proportions of citric acid monohydrate and urea. The fluorescence quantum yield of the obtained CNNPs could reach 31%. The CNNPs exhibited good fluorescence quenching selectivity to mercuric ions. Concentration experiments showed that there existed two parts of linear relationship between fluorescence intensity and concentration of Hg2 þ ions in the range of 0.1–8 and 8– 32 μM. The limit of detection (LOD) was estimated to be 0.094 μM. This method can be applied to the detection of Hg2 þ ions in tap water samples. & 2015 Elsevier B.V. All rights reserved.
Keywords: Carbon nitride nanoparticles Fluorescent probe Quenching Mercuric ions Citric acid Urea
1. Introduction Fluorescent nanoparticles have greatly attracted attention recently because of their promising applications in many fields such as bioimaging, biosensing, optoelectronic devices and photocatalysis [1–3]. This kind of material includes carbon dots, nanodiamonds, carbon nanotubes, fluorescent graphene, semiconductor quantum dots, etc. Because people have recognized the toxicity of heavy metals-containing quantum dots, seeking for novel and effective fluorescent nanopartilces has become an important tendency. A typical representative is carbon dots. Study on their preparation and application offers exciting opportunities [4– 10]. Heteroatom-doped and hollow carbon dots (CDs) are coming into our sight [11–15]. Developing novel fluorescent nanoparticles is still very meaningful. Carbon nitride is another promising candidate to complement carbon in materials applications [16]. Graphitic carbon nitride (g-C3N4), considered to be the most stable allotrope among various carbon nitrides, has been widely used in water splitting, solar energy transfer, and pollutant degradation [17]. It is usually n Corresponding author at: Department of Chemistry and Environmental Science, Minnan Normal University, Zhangzhou 363000, China. E-mail address: [email protected] (W. Weng).
http://dx.doi.org/10.1016/j.talanta.2015.05.063 0039-9140/& 2015 Elsevier B.V. All rights reserved.
prepared from nitrogen-rich precursors such as melamine, cyanamide, dicyandiamide, and urea by high-temperature pyrolysis method. The resultant carbon nitrides generally have large particle sizes, poor luminescent properties, and poor water-solubility. A few researches about the fabrication and application of fluorescent carbon nitride quantum dots or nanosheets with enhanced photoresponsive property were reported recently. Zhang et al. reported a low-temperature solid-phase method to synthesize highly fluorescent carbon nitride dots with a quantum yield of 42% from urea and sodium citrate [17]. Barman reported a microwave mediated method to prepare highly fluorescent graphitic carbon nitride quantum dots with a quantum yield of 29% from formamide. The carbon nitride quantum dots can play a dual role for selective and sensitive detection of mercuric ions as well as iodide ions in aqueous media [18]. Sun et al. reported a general strategy for the production of photoluminescent carbon nitride dots by microwave heating organic amines in the presence of acid [19,20]. Photoluminescent carbon nitride dots were also prepared from CCl4 and ethylenediamine [21]. Xie et al. reported a pathway to prepare the ultrathin graphitic-phase C3N4 nanosheets for bioimaging by a liquid exfoliation route from bulk g-C3N4 [22]. Chen et al. developed an effective and facile fluorescence sensing approach for the label-free and selective determination of Cr(VI) using graphitic carbon nitride nanosheets very recently [23]. Ultrathin g-C3N4 nanosheets were also used as an effective
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fluorescent probe for sensitive and selective detection of Cu2 þ ions [24]. Developing simple methods for synthesizing carbon nitride nanoparticles with different morphology is very meaningful. In this report, a facile one-pot solvothermal method was used to prepare the CNNPs. A green solvent, oleic acid, was used as the reaction media. The obtained CNNPs that showed hollow crosslinked morphology were successfully applied to the detection of mercuric ions.
2. Experimental 2.1. Chemicals Citric acid monohydrate (CA), urea and oleic acid were all analytical reagents (AR). They were purchased from Shanghai Sinopharm Chemical Reagent Co. Ltd. (China). Quinine sulfate and Rhodamine B (AR) were purchased from Aladdin Industrial Corporation (Shanghai, China). Ultrapure water was prepared with a Milli-Q system (Millipore, Bedford, MA, USA). Other reagents were all analytical and were used without further purification. 2.2. Preparation of the CNNPs A simple one-step solvothermal method was used to prepare the mentioned CNNPs. A typical synthesis process is as follows. CA monohydrate (0.8 g) and urea (3.2 g) were placed in a three neck flask, and then oleic acid (40 ml) was added. The precursors were completely dissolved in oleic acid at about 170 °C. The mixture was then heated at 200 °C for 30 min under vigorous magnetic stirring. The colorless solution changed to a clear brownish black solution as the reaction progressed. After the reaction the solution was cooled at room temperature, and then black solid precipitate was obtained directly. The precipitate was washed sufficiently with nhexane, dispersed in ultrapure water, and centrifuged at 6000 rpm for 30 min to remove large particles product. The supernatant was distilled under reduced pressure to obtain the end product. The CNNPs prepared from CA and urea with a weight ratio of 4:1, 1:1, and 1:4 were denoted as CNNPs-41, CNNPs-11, and CNNPs-14, respectively.
2.3. Characterization Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded using Tecnai G2 F20 instruments (FEI, America). The X-ray diffraction (XRD) spectra were recorded using a Bruker DAVINCI D8 ADVANCE diffractometer (Germany). Elemental analysis was performed using a vario EL Elemental Analyzer (Elementar Analysensysteme GmbH, Germany). FTIR spectra were recorded using a Magna-IR 750 Fourier transform infrared spectrometer (Nicolet, America). An AV600 nuclear magnetic spectrometer (Bruker, Germany) was used to record the 13C NMR and 1H NMR spectra. A UV-2550 spectrophotometer (Shimadzu, Japan) was used to record the UV–vis spectra of the CNNPs in ultrapure water. The XPS spectra were recorded using a Thermo ESCALAB 250XI multifunctional imaging electron spectrometer (Thermo Fisher, America). The PL spectra of the CNNPs and the determination of mercury ions were recorded using a Cary Eclipse PL spectrophotometer (Varian, America). 2.4. Calculation of quantum yields of the CNNPs The quantum yield (Φ) of the obtained CNNPs was measured by comparing the integrated photoluminescence (PL) intensities and the absorbance values of the products with the reference quinine sulfate for CNNPs-41 and Rhodamine B for CNNPs-11 and CNNPs-14. The quinine sulfate (literature ΦR ¼ 0.53 at an excitation wavelength of 320 nm) was dissolved in 0.1 M H2SO4 (refractive index η of 1.33), Rhodamine B (literature ΦR ¼0.31 at an excitation wavelength of 420 nm) and the obtained CNNPs were dissolved in ultrapure water (η ¼1.33). Absorbencies in the 10 mm fluorescence cuvette were kept under 0.1 at the excitation wavelengths. The quantum yield of the CNNPs was determined by the following equation:
Φ = ΦR ×
AR I η2 × × 2 IR A ηR
where Φ is the quantum yield, I is the measured integrated emission intensity, η is the refractive index of the solvents, and A is the optical density. The subscript R refers to the reference of known quantum yield.
Fig. 1. The TEM images (A and B), HRTEM images (C and D) of the obtained CNNPs.
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Fig. 2. The XRD pattern of the obtained CNNPs.
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as the reaction media. The mixture turned from a colorless solution to a clear brownish black solution as the reaction progressed, suggesting the formation of carbonization products. Black solid precipitate could be obtained directly after the reaction. The end product was obtained after the precipitate was washed sufficiently with n-hexane, dispersed in ultrapure water, centrifuged to remove large particles, and distilled under reduced pressure. Compared with other synthesis process, this method has the advantages of easy fabrication, easy enlargement, high yield, and simple post-processing. The fabrication process did not use any inorganic acid or metal ion. It is worth mentioning that water generated in the reaction process may act as a “soft-template” [25], and modulate the size of the formed nanoparticles. With the further dehydration or deammoniation, hollow cross-linked nanoparticles were eventually formed. Fig. 1 shows that the product has hollow cross-linked nanosphere morphology. As estimated from the TEM images (Fig. 1A and B), the diameter of the nanosphere was about 14 nm. The representative high-resolution TEM images (Fig. 1C and D) clearly show the cross-linked and crystallized graphitic-like structures.
Fig. 3. (A) The XPS spectrum of the obtained CNNPs-14. (B) and (C) are the corresponding C1s spectrum and N1s spectrum.
2.5. Detection of Hg2 þ ions The detection of Hg2 þ ions was performed in ultrapure water solution. In a typical assay, aqueous solution of the CNNPs-14 with the final concentration of 1 μg mL 1 was mixed with different metal ions with the final concentration of 100 μM. The fluorescence intensity was recorded to evaluate the selectivity of the chemosensor. The PL response of the CNNPs-14 solution towards 4 μM Hg2 þ in the presence of 4 μM other metal ions was recorded to perform the interference tests. Several common ions including Na þ , K þ , Ca2 þ and Mg2 þ ions with higher concentrations were also used to evaluate the interference. Different concentration of Hg2 þ ions was added to the aqueous solution of the CNNPs to evaluate the linear concentration range and the limit of detection. The tap water samples obtained from our lab spiked with Hg2 þ ions at different concentrations levels were used to evaluate the CNNPs in real sample analysis. The fluorescence spectra were recorded at an excitation wavelength of 420 nm and all experiments were performed at room temperature.
3. Results and discussion 3.1. Preparation and characterization of the CNNPs CA monohydrate and urea in various proportions were used as the precursors. Unlike hydrothermal method, oleic acid was used
The lattice parameter is 0.333 nm, which is in agreement with the (002) plane of graphitic carbon nitride [17]. The crystal defects such as wavy curvatures and dislocations [26] can be clearly observed in the as-prepared nanoparticles. The XRD pattern (Fig. 2) displays a set of peaks centered at 27.2°, consistent with the previous reports on graphitic carbon nitride. Several fine splitting peaks (19.6°, 26.3°, 27.2° and 29.0°) were observed, suggested the existence of different types of crystal defects or different circumstance of surface functional groups of carbon nitride cluster. This feature is similar to the multiple transitions of carbon nitride synthesized at moderate temperatures [16]. The XPS spectrum shows three peaks at 285.1, 400.1 and 531.1 eV (Fig. 3A), which can be attributed to C1s, N1s, and O1s, respectively. The C1s spectrum can be deconvoluted into four peaks at 284.82, 285.46, 288.21 and 289.70 eV (Fig. 3B), indicating the presence of four types of carbon bonds: sp2 C ¼ C or sp3 C–C (284.82 eV), C–O or C–N (285.46 eV), sp2 N–C ¼ N (288.21 eV), and C ¼O (289.70 eV). The deconvolution of the N1s spectrum indicated the presence of three types of nitrogen bonds: C–N–C (399.58 eV), N–(C)3 (400.25 eV), and C–N–H groups (401.20 eV) (Fig. 3C) [21]. The solid-state 13C-NMR spectrum confirmed the existence of carbon nitride units. Two distinct peaks centered at 151.87 and 160.52 ppm (Fig. 4A) related to the formation of a poly(tris-striazine) structure characteristic of melem, melon and the final carbon nitride [27]. It is worth mentioning that no distinct peaks appeared around 120 ppm, further suggesting that the forming
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Fig. 4. The solid-state
13
C NMR spectrum (A) and the FT-IR spectrum (B) of CNNPs-14.
Fig. 5. (A) The UV–vis spectra of the obtained CNNPs (a, CNNPs-41; b, CNNPs-11; c, CNNPs-14. Inset: photographs taken under 365 nm UV light). The excitation-dependent emission spectra of CNNPs-41 (B), CNNPs-11 (C), and CNNPs-14 (D).
conjugate units were carbon nitride, rather than graphene. The FTIR spectrum (Fig. 4B) exhibited characteristic bands at 1370 and 1400 cm 1, which can be assigned to aromatic C ¼ N stretches. The characteristic band at 764 cm 1 in the fingerprint region can be assigned to the breathing mode of s-triazine rings [18]. The intense band at 1600 and 1710 cm 1 can be assigned to asymmetric C¼ N and C ¼O stretching vibrations, respectively. The broad peaks between 3000 and 3450 cm 1 are attributed to N–H and O–H stretching vibrations. The elemental analysis showed that the product contained 39.58% of carbon, 22.08% of nitrogen, 4.23% of hydrogen, and 34.11% of oxygen by weight. It can be seen that the as-prepared CNNPs contain rich oxygen. The oxygen containing functional groups such as carboxyl group and hydroxyl group may locate on
the edge of the graphitic carbon nitride units. These functional groups endow the product with good water-solubility. The solubility of the products prepared at higher temperature decreased obviously probably due to the decrease of the number of polar functional groups. The UV–vis absorption spectra revealed some important information (Fig. 5). For CNNPs-41 (CA:urea¼4:1), only a characteristic peak at 344 nm was shown (Fig. 5A-a). This peak can be ascribed to the amide-containing fluorophores, which are formed in the dehydration process. With the increase of the amount of urea, the intensity of the peak decreased and several new characteristic peaks appeared at 248, 272, 324, and 408 nm (Fig. 5A-b and c). The adsorption peaks at 248 and 272 nm can be ascribed to n–π* electronic transitions of C ¼O groups and π–π* electronic
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Fig. 6. (A) Normalized fluorescence intensity of CNNPs-14 (1 μg mL 1) at an excitation wavelength of 420 nm in the presence of 100 μM of various metal ions. (B and C) The linear relationship between the fluorescence intensity and the concentration of Hg2 þ ions. (D) The overlay fluorescence spectra of CNNPs-14 upon addition of various concentrations of Hg2 þ (from the top to down: 0, 0.1, 0.5, 1.5, 3, 5, 7, 8, 9 and 10 μM).
Fig. 7. Selective PL response of aqueous CNNPs-14 solutions towards 4 μM Hg2 þ (black bars), and interference of 4 μM other metal ions.
transitions of carbon nitrides containing s-triazine rings [28]. The peak centered at 324 nm can be ascribed to the typical adsorption pattern of carbon nitrides semiconductor [16,17,24]. The adsorption peak centered at 408 nm should be ascribed to surface/molecular center [29]. Excess urea is favorable to promote the formation of carbon nitride nanoparticles due to its low pyrolysis temperature. The corresponding PL spectra conformed to the UV–vis
absorption features. For CNNPs-41, the emission wavelength exhibited an excitation-independent characteristic in the wavelength range of 300–360 nm, and then had a red-shift above 360 nm. The maximum emission peak was observed at 440 nm, at an excitation wavelength of 360 nm. For CNNPs-11, an uncommon excitation-dependent emission feature was observed. The emission wavelength experienced a little blue-shift, and then had a red-shift with the PL intensity of rising first, and then failling and rising again. Two maximum emission peaks appeared at 455 and 520 nm, at excitation wavelengths of 360 and 420 nm, respectively. For CNNPs-14, two distinct emission peaks appeared at 360 and 520 nm at an excitation wavelength of 300 nm. The PL intensity of left peak reached its peak at 320 nm excitation wavelength, then decreased at 340 nm and faded away above 380 nm. The PL intensity of right peak decreased first, and then increased and decreased again with the increase of excitation wavelength. The maximum emission peak was observed at 515 nm, at an excitation wavelength of 420 nm. The two maximum excitation wavelengths were consistent with the UV–vis absorption feature. In this context, the products can be regarded as functional carbon nitride nanoparticles, or polymer/carbon nitride composite. Amide-containing polymer was the main component when excess CA was used (in the case of CNNPs-41). Due to low thermal decomposition temperature of urea, the carbon nitride component increased with the increase of the content of urea precursor (in the case of CNNPs-11 and CNNPs-14). Especially for CNNPs-14, the fluorescent peak centered at 360 nm which was excited at 320 nm was prominent, suggesting the formation of carbon nitride component. The quantum yield (QY) of CNNPs-41 was calculated to be 19%
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Fig. 8. (A) Fluorescence emission spectra of CNNPs-14 in tap water upon addition of various concentrations of Hg2 þ (from top to bottom: 0, 0.1, 0.5, 1.5, 3, 5, 7, 8, 9 and 10 μM), at an excitation wavelength of 420 nm. (B) The dependence of PL intensity on the concentration of Hg2 þ ions within the range of 0–8 μM.
Table 1 Comparison of sensing performance of carbon-based nanoparticles in the detection of Hg2 þ ions. Fluorescent probe
Limit of detection (μM) Linear range (μM) Reference
N-CQDs 0.23 Graphene QDs 0.1 Carbon dots 1.3 Functionalilzed CDs 2.69 CNNPs 0.094
0–25 0.8–9 0–2.69 0.1–2.69 0.1–8 and 8–32
[30] [31] [32] [33] This work
using quinine sulfate as a reference, and the QY value of CNNPs-11 and CNNPs-14 was 31%, using Rhodamine B as the reference. CNNPs-41 exhibited strong blue fluorescence under 365 nm UV light, but CNNPs-11 and CNNPs-14 exhibited cyan fluorescence. This illustrates that the luminescent property can be regulated conveniently by varying the ratio of CA and urea. 3.2. Detection of Hg2 þ ions based on the obtained CNNPs The as-prepared product was then applied in the ion detection. Different ions were added to the aqueous solution of CNNPs-14. As shown in Fig. 6A, the PL intensity decreases remarkably with the addition of Hg2 þ , whereas other ions had negligible (Cr6 þ , Cr3 þ , Cu2 þ , Pb2 þ , Co2 þ , Sr2 þ , Na þ , K þ , Mg2 þ , Ni2 þ , Ba2 þ , Cd2 þ , Zn2 þ , Al3 þ and Fe2 þ ) or less impact (Fe3 þ ) on the intensity. This suggests that the obtained nanoparticles have potential application on the Hg2 þ detection. The PL intensity decreases with the increase of Hg2 þ concentration. The data-fitting result shows that there exists two parts of linear relationship in the concentration range of 0.1–8 and 8–32 μM (Fig. 6B and C). The reason maybe lies in that there are at least two different interaction sites for Hg2 þ ions on the nanoparticles. At low concentrations, Hg2 þ ions interact preferentially with the “strong” binding site, and lead to fluorescence quenching quickly. The limit of detection (LOD) was estimated to be 0.094 μM based on three times the standard deviation rule. The obtained LOD was lower or comparable to those reported with other fluorescent probes [30]. We also performed the pH effect on the determination of mercuric ions. The fluorescence intensity of the CNNPs and fluorescence quenching by Hg2 þ ions remained about the same in the pH range of 6.0–10.0. When the pH value of the solution is less than 6.0, the fluorescence quenching of the CNNPs by Hg2 þ ions became less obvious. We think that the amino groups would transform into ammonium ions in the acidic condition, and then the complexation interaction between the free amino groups and
Hg2 þ ions would decrease. Similar results could be obtained in a long time interval, and the relative standard deviation (RSD) was less than 2%. The sensitivity and selectivity for Hg2 þ ions towards quenching of fluorescence of the CNNPs are possibly due to several reasons, such as greater affinity of Hg2 þ ions towards nitrogen, larger radius of Hg2 þ ions and its ability to form a stable non-fluorescent complex with the CNNPs [18]. Another possible explanation is aggregate-induced quenching [31]. The Hg2 þ ion may simultaneously bind to multiple nitrogen atoms of the CNNPs and oxygencontained groups. This complexation process may induce the formation of aggregates, change the electronic structure of the CNNPs and then lead to non-radiative electron/hole recombination annihilation. 3.3. Detection of Hg2 þ ions in tap water samples Interference tests (Fig. 7) show that the coexistence of other ions (4 μM) only has a negligible effect on the fluorescence quenching. Also, 300-fold Na þ and K þ ions, and 50-fold Ca2 þ and Mg2 þ ions had a small influence on the fluorescence intensity of the CNNPs towards Hg2 þ ions. Then, the method was used to assay the concentration of Hg2 þ in real water samples. The tap water samples obtained from our lab without any pretreatment were spiked with Hg2 þ ions at different concentrations levels. The fluorescence response of tap water is shown in Fig. 8A. The PL intensity decreases with the increase of concentration of Hg2 þ ions. Good linear relationship between (F0 F)/F0 and the concentrations of Hg2 þ was observed in the range of 0–8 μM (Fig. 8B). The method was also used to assay the concentration of Hg2 þ in the 10 times concentrated tape water. Similar results were obtained. Good linear relationship existed between (F0 F)/F0 and the concentrations of Hg2 þ in the range of 0–8 μM (R2 ¼0.9997). This suggests that the obtained CNNPs have a promising prospect for the detection of Hg2 þ ions in real samples. Several traditional methods including atomic absorption/ emission spectroscopy, selective cold vapor atomic fluorescence spectrometry, X-ray fluorescence spectrometry and inductively coupled plasma mass spectrometry (ICPMS) can be used to the detection of mercuric ions. These methods are sensitive and selective but they require sophisticated instruments and complicated sample preparation [30]. It is still of great challenge to develop simple methods for the detection of mercuric ions. The photoluminescent carbon-based nanoparticles have the advantages of easy fabrication, high quantum yield, low cost, low toxicity, excellent biocompatibility, good photostability and water solubility. Their application in the determination of Hg2 þ ions has
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attracted growing interest. Some significative progress has been made recently. From Table 1 we can find that the obtained LOD for the as-prepared CNNPs was lower than those reported with other carbon-based nanoparticles. The two parts of linear relationship in the detection of Hg2 þ ions were first reported. The linear range was wide relatively. Besides, good linear relationship between the fluorescent intensity and the concentration of Hg2 þ was observed for the concentrated tape water sample, further suggesting that the obtained CNNPs have a promising prospect for the detection of Hg2 þ ions in real samples.
4. Conclusions In summary, we have prepared a kind of hollow cross-linked fluorescent carbon nitride nanoparticles from CA and urea by a facile one-pot solvothermal process. The as-prepared CNNPs possessed moderate fluorescence quantum yields (up to 31%) without further passivation treatment. The CNNPs displayed an uncommon photoluminescent feature, and could be used as a promising probe to assay the concentration of mercuric ions.
Acknowledgments This work was supported by Natural Science Foundation of Fujian Province (No. 2012J06005), Education Bureau of Fujian Province of China (Nos. JK2011030 and JA13195), the College Students’ Innovative and Entrepreneurial Training Program (No. 20141040), and the Foundation of Fujian Provincial Bureau of Quality and Technical Supervision (No. fjqi2013109).
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One-pot fabrication of hollow cross-linked fluorescent carbon nitride nanoparticles and their application in the detection of mercuric ions Jie Ma a,b, Baoling Guo a,b, Xiaotong Cao a,b, Yanping Lin a,b, Bixia Yao a,b, Feiming Li a,b, Wen Weng a,b,n, Lizhang Huang c a
Department of Chemistry and Environmental Science, Minnan Normal University, Zhangzhou 363000, China Fujian Provincial Key Laboratory of Modern Analytical Science and Separation Technology, Zhangzhou 363000, China c Zhangzhou Product Quality Supervision Institute, Zhangzhou 363000, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 26 January 2015 Received in revised form 18 May 2015 Accepted 25 May 2015 Available online 27 May 2015
Hollow cross-linked fluorescent carbon nitride nanoparticles (CNNPs) were fabricated via a facile one-pot solvothermal process. The obtained CNNPs were characterized by multiple analytical techniques including transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), solid-state nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The excitation-dependent fluorescence emission spectra showed significant differences for the CNNPs derived from various proportions of citric acid monohydrate and urea. The fluorescence quantum yield of the obtained CNNPs could reach 31%. The CNNPs exhibited good fluorescence quenching selectivity to mercuric ions. Concentration experiments showed that there existed two parts of linear relationship between fluorescence intensity and concentration of Hg2 þ ions in the range of 0.1–8 and 8– 32 μM. The limit of detection (LOD) was estimated to be 0.094 μM. This method can be applied to the detection of Hg2 þ ions in tap water samples. & 2015 Elsevier B.V. All rights reserved.
Keywords: Carbon nitride nanoparticles Fluorescent probe Quenching Mercuric ions Citric acid Urea
1. Introduction Fluorescent nanoparticles have greatly attracted attention recently because of their promising applications in many fields such as bioimaging, biosensing, optoelectronic devices and photocatalysis [1–3]. This kind of material includes carbon dots, nanodiamonds, carbon nanotubes, fluorescent graphene, semiconductor quantum dots, etc. Because people have recognized the toxicity of heavy metals-containing quantum dots, seeking for novel and effective fluorescent nanopartilces has become an important tendency. A typical representative is carbon dots. Study on their preparation and application offers exciting opportunities [4– 10]. Heteroatom-doped and hollow carbon dots (CDs) are coming into our sight [11–15]. Developing novel fluorescent nanoparticles is still very meaningful. Carbon nitride is another promising candidate to complement carbon in materials applications [16]. Graphitic carbon nitride (g-C3N4), considered to be the most stable allotrope among various carbon nitrides, has been widely used in water splitting, solar energy transfer, and pollutant degradation [17]. It is usually n Corresponding author at: Department of Chemistry and Environmental Science, Minnan Normal University, Zhangzhou 363000, China. E-mail address: [email protected] (W. Weng).
http://dx.doi.org/10.1016/j.talanta.2015.05.063 0039-9140/& 2015 Elsevier B.V. All rights reserved.
prepared from nitrogen-rich precursors such as melamine, cyanamide, dicyandiamide, and urea by high-temperature pyrolysis method. The resultant carbon nitrides generally have large particle sizes, poor luminescent properties, and poor water-solubility. A few researches about the fabrication and application of fluorescent carbon nitride quantum dots or nanosheets with enhanced photoresponsive property were reported recently. Zhang et al. reported a low-temperature solid-phase method to synthesize highly fluorescent carbon nitride dots with a quantum yield of 42% from urea and sodium citrate [17]. Barman reported a microwave mediated method to prepare highly fluorescent graphitic carbon nitride quantum dots with a quantum yield of 29% from formamide. The carbon nitride quantum dots can play a dual role for selective and sensitive detection of mercuric ions as well as iodide ions in aqueous media [18]. Sun et al. reported a general strategy for the production of photoluminescent carbon nitride dots by microwave heating organic amines in the presence of acid [19,20]. Photoluminescent carbon nitride dots were also prepared from CCl4 and ethylenediamine [21]. Xie et al. reported a pathway to prepare the ultrathin graphitic-phase C3N4 nanosheets for bioimaging by a liquid exfoliation route from bulk g-C3N4 [22]. Chen et al. developed an effective and facile fluorescence sensing approach for the label-free and selective determination of Cr(VI) using graphitic carbon nitride nanosheets very recently [23]. Ultrathin g-C3N4 nanosheets were also used as an effective
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fluorescent probe for sensitive and selective detection of Cu2 þ ions [24]. Developing simple methods for synthesizing carbon nitride nanoparticles with different morphology is very meaningful. In this report, a facile one-pot solvothermal method was used to prepare the CNNPs. A green solvent, oleic acid, was used as the reaction media. The obtained CNNPs that showed hollow crosslinked morphology were successfully applied to the detection of mercuric ions.
2. Experimental 2.1. Chemicals Citric acid monohydrate (CA), urea and oleic acid were all analytical reagents (AR). They were purchased from Shanghai Sinopharm Chemical Reagent Co. Ltd. (China). Quinine sulfate and Rhodamine B (AR) were purchased from Aladdin Industrial Corporation (Shanghai, China). Ultrapure water was prepared with a Milli-Q system (Millipore, Bedford, MA, USA). Other reagents were all analytical and were used without further purification. 2.2. Preparation of the CNNPs A simple one-step solvothermal method was used to prepare the mentioned CNNPs. A typical synthesis process is as follows. CA monohydrate (0.8 g) and urea (3.2 g) were placed in a three neck flask, and then oleic acid (40 ml) was added. The precursors were completely dissolved in oleic acid at about 170 °C. The mixture was then heated at 200 °C for 30 min under vigorous magnetic stirring. The colorless solution changed to a clear brownish black solution as the reaction progressed. After the reaction the solution was cooled at room temperature, and then black solid precipitate was obtained directly. The precipitate was washed sufficiently with nhexane, dispersed in ultrapure water, and centrifuged at 6000 rpm for 30 min to remove large particles product. The supernatant was distilled under reduced pressure to obtain the end product. The CNNPs prepared from CA and urea with a weight ratio of 4:1, 1:1, and 1:4 were denoted as CNNPs-41, CNNPs-11, and CNNPs-14, respectively.
2.3. Characterization Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded using Tecnai G2 F20 instruments (FEI, America). The X-ray diffraction (XRD) spectra were recorded using a Bruker DAVINCI D8 ADVANCE diffractometer (Germany). Elemental analysis was performed using a vario EL Elemental Analyzer (Elementar Analysensysteme GmbH, Germany). FTIR spectra were recorded using a Magna-IR 750 Fourier transform infrared spectrometer (Nicolet, America). An AV600 nuclear magnetic spectrometer (Bruker, Germany) was used to record the 13C NMR and 1H NMR spectra. A UV-2550 spectrophotometer (Shimadzu, Japan) was used to record the UV–vis spectra of the CNNPs in ultrapure water. The XPS spectra were recorded using a Thermo ESCALAB 250XI multifunctional imaging electron spectrometer (Thermo Fisher, America). The PL spectra of the CNNPs and the determination of mercury ions were recorded using a Cary Eclipse PL spectrophotometer (Varian, America). 2.4. Calculation of quantum yields of the CNNPs The quantum yield (Φ) of the obtained CNNPs was measured by comparing the integrated photoluminescence (PL) intensities and the absorbance values of the products with the reference quinine sulfate for CNNPs-41 and Rhodamine B for CNNPs-11 and CNNPs-14. The quinine sulfate (literature ΦR ¼ 0.53 at an excitation wavelength of 320 nm) was dissolved in 0.1 M H2SO4 (refractive index η of 1.33), Rhodamine B (literature ΦR ¼0.31 at an excitation wavelength of 420 nm) and the obtained CNNPs were dissolved in ultrapure water (η ¼1.33). Absorbencies in the 10 mm fluorescence cuvette were kept under 0.1 at the excitation wavelengths. The quantum yield of the CNNPs was determined by the following equation:
Φ = ΦR ×
AR I η2 × × 2 IR A ηR
where Φ is the quantum yield, I is the measured integrated emission intensity, η is the refractive index of the solvents, and A is the optical density. The subscript R refers to the reference of known quantum yield.
Fig. 1. The TEM images (A and B), HRTEM images (C and D) of the obtained CNNPs.
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Fig. 2. The XRD pattern of the obtained CNNPs.
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as the reaction media. The mixture turned from a colorless solution to a clear brownish black solution as the reaction progressed, suggesting the formation of carbonization products. Black solid precipitate could be obtained directly after the reaction. The end product was obtained after the precipitate was washed sufficiently with n-hexane, dispersed in ultrapure water, centrifuged to remove large particles, and distilled under reduced pressure. Compared with other synthesis process, this method has the advantages of easy fabrication, easy enlargement, high yield, and simple post-processing. The fabrication process did not use any inorganic acid or metal ion. It is worth mentioning that water generated in the reaction process may act as a “soft-template” [25], and modulate the size of the formed nanoparticles. With the further dehydration or deammoniation, hollow cross-linked nanoparticles were eventually formed. Fig. 1 shows that the product has hollow cross-linked nanosphere morphology. As estimated from the TEM images (Fig. 1A and B), the diameter of the nanosphere was about 14 nm. The representative high-resolution TEM images (Fig. 1C and D) clearly show the cross-linked and crystallized graphitic-like structures.
Fig. 3. (A) The XPS spectrum of the obtained CNNPs-14. (B) and (C) are the corresponding C1s spectrum and N1s spectrum.
2.5. Detection of Hg2 þ ions The detection of Hg2 þ ions was performed in ultrapure water solution. In a typical assay, aqueous solution of the CNNPs-14 with the final concentration of 1 μg mL 1 was mixed with different metal ions with the final concentration of 100 μM. The fluorescence intensity was recorded to evaluate the selectivity of the chemosensor. The PL response of the CNNPs-14 solution towards 4 μM Hg2 þ in the presence of 4 μM other metal ions was recorded to perform the interference tests. Several common ions including Na þ , K þ , Ca2 þ and Mg2 þ ions with higher concentrations were also used to evaluate the interference. Different concentration of Hg2 þ ions was added to the aqueous solution of the CNNPs to evaluate the linear concentration range and the limit of detection. The tap water samples obtained from our lab spiked with Hg2 þ ions at different concentrations levels were used to evaluate the CNNPs in real sample analysis. The fluorescence spectra were recorded at an excitation wavelength of 420 nm and all experiments were performed at room temperature.
3. Results and discussion 3.1. Preparation and characterization of the CNNPs CA monohydrate and urea in various proportions were used as the precursors. Unlike hydrothermal method, oleic acid was used
The lattice parameter is 0.333 nm, which is in agreement with the (002) plane of graphitic carbon nitride [17]. The crystal defects such as wavy curvatures and dislocations [26] can be clearly observed in the as-prepared nanoparticles. The XRD pattern (Fig. 2) displays a set of peaks centered at 27.2°, consistent with the previous reports on graphitic carbon nitride. Several fine splitting peaks (19.6°, 26.3°, 27.2° and 29.0°) were observed, suggested the existence of different types of crystal defects or different circumstance of surface functional groups of carbon nitride cluster. This feature is similar to the multiple transitions of carbon nitride synthesized at moderate temperatures [16]. The XPS spectrum shows three peaks at 285.1, 400.1 and 531.1 eV (Fig. 3A), which can be attributed to C1s, N1s, and O1s, respectively. The C1s spectrum can be deconvoluted into four peaks at 284.82, 285.46, 288.21 and 289.70 eV (Fig. 3B), indicating the presence of four types of carbon bonds: sp2 C ¼ C or sp3 C–C (284.82 eV), C–O or C–N (285.46 eV), sp2 N–C ¼ N (288.21 eV), and C ¼O (289.70 eV). The deconvolution of the N1s spectrum indicated the presence of three types of nitrogen bonds: C–N–C (399.58 eV), N–(C)3 (400.25 eV), and C–N–H groups (401.20 eV) (Fig. 3C) [21]. The solid-state 13C-NMR spectrum confirmed the existence of carbon nitride units. Two distinct peaks centered at 151.87 and 160.52 ppm (Fig. 4A) related to the formation of a poly(tris-striazine) structure characteristic of melem, melon and the final carbon nitride [27]. It is worth mentioning that no distinct peaks appeared around 120 ppm, further suggesting that the forming
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Fig. 4. The solid-state
13
C NMR spectrum (A) and the FT-IR spectrum (B) of CNNPs-14.
Fig. 5. (A) The UV–vis spectra of the obtained CNNPs (a, CNNPs-41; b, CNNPs-11; c, CNNPs-14. Inset: photographs taken under 365 nm UV light). The excitation-dependent emission spectra of CNNPs-41 (B), CNNPs-11 (C), and CNNPs-14 (D).
conjugate units were carbon nitride, rather than graphene. The FTIR spectrum (Fig. 4B) exhibited characteristic bands at 1370 and 1400 cm 1, which can be assigned to aromatic C ¼ N stretches. The characteristic band at 764 cm 1 in the fingerprint region can be assigned to the breathing mode of s-triazine rings [18]. The intense band at 1600 and 1710 cm 1 can be assigned to asymmetric C¼ N and C ¼O stretching vibrations, respectively. The broad peaks between 3000 and 3450 cm 1 are attributed to N–H and O–H stretching vibrations. The elemental analysis showed that the product contained 39.58% of carbon, 22.08% of nitrogen, 4.23% of hydrogen, and 34.11% of oxygen by weight. It can be seen that the as-prepared CNNPs contain rich oxygen. The oxygen containing functional groups such as carboxyl group and hydroxyl group may locate on
the edge of the graphitic carbon nitride units. These functional groups endow the product with good water-solubility. The solubility of the products prepared at higher temperature decreased obviously probably due to the decrease of the number of polar functional groups. The UV–vis absorption spectra revealed some important information (Fig. 5). For CNNPs-41 (CA:urea¼4:1), only a characteristic peak at 344 nm was shown (Fig. 5A-a). This peak can be ascribed to the amide-containing fluorophores, which are formed in the dehydration process. With the increase of the amount of urea, the intensity of the peak decreased and several new characteristic peaks appeared at 248, 272, 324, and 408 nm (Fig. 5A-b and c). The adsorption peaks at 248 and 272 nm can be ascribed to n–π* electronic transitions of C ¼O groups and π–π* electronic
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Fig. 6. (A) Normalized fluorescence intensity of CNNPs-14 (1 μg mL 1) at an excitation wavelength of 420 nm in the presence of 100 μM of various metal ions. (B and C) The linear relationship between the fluorescence intensity and the concentration of Hg2 þ ions. (D) The overlay fluorescence spectra of CNNPs-14 upon addition of various concentrations of Hg2 þ (from the top to down: 0, 0.1, 0.5, 1.5, 3, 5, 7, 8, 9 and 10 μM).
Fig. 7. Selective PL response of aqueous CNNPs-14 solutions towards 4 μM Hg2 þ (black bars), and interference of 4 μM other metal ions.
transitions of carbon nitrides containing s-triazine rings [28]. The peak centered at 324 nm can be ascribed to the typical adsorption pattern of carbon nitrides semiconductor [16,17,24]. The adsorption peak centered at 408 nm should be ascribed to surface/molecular center [29]. Excess urea is favorable to promote the formation of carbon nitride nanoparticles due to its low pyrolysis temperature. The corresponding PL spectra conformed to the UV–vis
absorption features. For CNNPs-41, the emission wavelength exhibited an excitation-independent characteristic in the wavelength range of 300–360 nm, and then had a red-shift above 360 nm. The maximum emission peak was observed at 440 nm, at an excitation wavelength of 360 nm. For CNNPs-11, an uncommon excitation-dependent emission feature was observed. The emission wavelength experienced a little blue-shift, and then had a red-shift with the PL intensity of rising first, and then failling and rising again. Two maximum emission peaks appeared at 455 and 520 nm, at excitation wavelengths of 360 and 420 nm, respectively. For CNNPs-14, two distinct emission peaks appeared at 360 and 520 nm at an excitation wavelength of 300 nm. The PL intensity of left peak reached its peak at 320 nm excitation wavelength, then decreased at 340 nm and faded away above 380 nm. The PL intensity of right peak decreased first, and then increased and decreased again with the increase of excitation wavelength. The maximum emission peak was observed at 515 nm, at an excitation wavelength of 420 nm. The two maximum excitation wavelengths were consistent with the UV–vis absorption feature. In this context, the products can be regarded as functional carbon nitride nanoparticles, or polymer/carbon nitride composite. Amide-containing polymer was the main component when excess CA was used (in the case of CNNPs-41). Due to low thermal decomposition temperature of urea, the carbon nitride component increased with the increase of the content of urea precursor (in the case of CNNPs-11 and CNNPs-14). Especially for CNNPs-14, the fluorescent peak centered at 360 nm which was excited at 320 nm was prominent, suggesting the formation of carbon nitride component. The quantum yield (QY) of CNNPs-41 was calculated to be 19%
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Fig. 8. (A) Fluorescence emission spectra of CNNPs-14 in tap water upon addition of various concentrations of Hg2 þ (from top to bottom: 0, 0.1, 0.5, 1.5, 3, 5, 7, 8, 9 and 10 μM), at an excitation wavelength of 420 nm. (B) The dependence of PL intensity on the concentration of Hg2 þ ions within the range of 0–8 μM.
Table 1 Comparison of sensing performance of carbon-based nanoparticles in the detection of Hg2 þ ions. Fluorescent probe
Limit of detection (μM) Linear range (μM) Reference
N-CQDs 0.23 Graphene QDs 0.1 Carbon dots 1.3 Functionalilzed CDs 2.69 CNNPs 0.094
0–25 0.8–9 0–2.69 0.1–2.69 0.1–8 and 8–32
[30] [31] [32] [33] This work
using quinine sulfate as a reference, and the QY value of CNNPs-11 and CNNPs-14 was 31%, using Rhodamine B as the reference. CNNPs-41 exhibited strong blue fluorescence under 365 nm UV light, but CNNPs-11 and CNNPs-14 exhibited cyan fluorescence. This illustrates that the luminescent property can be regulated conveniently by varying the ratio of CA and urea. 3.2. Detection of Hg2 þ ions based on the obtained CNNPs The as-prepared product was then applied in the ion detection. Different ions were added to the aqueous solution of CNNPs-14. As shown in Fig. 6A, the PL intensity decreases remarkably with the addition of Hg2 þ , whereas other ions had negligible (Cr6 þ , Cr3 þ , Cu2 þ , Pb2 þ , Co2 þ , Sr2 þ , Na þ , K þ , Mg2 þ , Ni2 þ , Ba2 þ , Cd2 þ , Zn2 þ , Al3 þ and Fe2 þ ) or less impact (Fe3 þ ) on the intensity. This suggests that the obtained nanoparticles have potential application on the Hg2 þ detection. The PL intensity decreases with the increase of Hg2 þ concentration. The data-fitting result shows that there exists two parts of linear relationship in the concentration range of 0.1–8 and 8–32 μM (Fig. 6B and C). The reason maybe lies in that there are at least two different interaction sites for Hg2 þ ions on the nanoparticles. At low concentrations, Hg2 þ ions interact preferentially with the “strong” binding site, and lead to fluorescence quenching quickly. The limit of detection (LOD) was estimated to be 0.094 μM based on three times the standard deviation rule. The obtained LOD was lower or comparable to those reported with other fluorescent probes [30]. We also performed the pH effect on the determination of mercuric ions. The fluorescence intensity of the CNNPs and fluorescence quenching by Hg2 þ ions remained about the same in the pH range of 6.0–10.0. When the pH value of the solution is less than 6.0, the fluorescence quenching of the CNNPs by Hg2 þ ions became less obvious. We think that the amino groups would transform into ammonium ions in the acidic condition, and then the complexation interaction between the free amino groups and
Hg2 þ ions would decrease. Similar results could be obtained in a long time interval, and the relative standard deviation (RSD) was less than 2%. The sensitivity and selectivity for Hg2 þ ions towards quenching of fluorescence of the CNNPs are possibly due to several reasons, such as greater affinity of Hg2 þ ions towards nitrogen, larger radius of Hg2 þ ions and its ability to form a stable non-fluorescent complex with the CNNPs [18]. Another possible explanation is aggregate-induced quenching [31]. The Hg2 þ ion may simultaneously bind to multiple nitrogen atoms of the CNNPs and oxygencontained groups. This complexation process may induce the formation of aggregates, change the electronic structure of the CNNPs and then lead to non-radiative electron/hole recombination annihilation. 3.3. Detection of Hg2 þ ions in tap water samples Interference tests (Fig. 7) show that the coexistence of other ions (4 μM) only has a negligible effect on the fluorescence quenching. Also, 300-fold Na þ and K þ ions, and 50-fold Ca2 þ and Mg2 þ ions had a small influence on the fluorescence intensity of the CNNPs towards Hg2 þ ions. Then, the method was used to assay the concentration of Hg2 þ in real water samples. The tap water samples obtained from our lab without any pretreatment were spiked with Hg2 þ ions at different concentrations levels. The fluorescence response of tap water is shown in Fig. 8A. The PL intensity decreases with the increase of concentration of Hg2 þ ions. Good linear relationship between (F0 F)/F0 and the concentrations of Hg2 þ was observed in the range of 0–8 μM (Fig. 8B). The method was also used to assay the concentration of Hg2 þ in the 10 times concentrated tape water. Similar results were obtained. Good linear relationship existed between (F0 F)/F0 and the concentrations of Hg2 þ in the range of 0–8 μM (R2 ¼0.9997). This suggests that the obtained CNNPs have a promising prospect for the detection of Hg2 þ ions in real samples. Several traditional methods including atomic absorption/ emission spectroscopy, selective cold vapor atomic fluorescence spectrometry, X-ray fluorescence spectrometry and inductively coupled plasma mass spectrometry (ICPMS) can be used to the detection of mercuric ions. These methods are sensitive and selective but they require sophisticated instruments and complicated sample preparation [30]. It is still of great challenge to develop simple methods for the detection of mercuric ions. The photoluminescent carbon-based nanoparticles have the advantages of easy fabrication, high quantum yield, low cost, low toxicity, excellent biocompatibility, good photostability and water solubility. Their application in the determination of Hg2 þ ions has
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attracted growing interest. Some significative progress has been made recently. From Table 1 we can find that the obtained LOD for the as-prepared CNNPs was lower than those reported with other carbon-based nanoparticles. The two parts of linear relationship in the detection of Hg2 þ ions were first reported. The linear range was wide relatively. Besides, good linear relationship between the fluorescent intensity and the concentration of Hg2 þ was observed for the concentrated tape water sample, further suggesting that the obtained CNNPs have a promising prospect for the detection of Hg2 þ ions in real samples.
4. Conclusions In summary, we have prepared a kind of hollow cross-linked fluorescent carbon nitride nanoparticles from CA and urea by a facile one-pot solvothermal process. The as-prepared CNNPs possessed moderate fluorescence quantum yields (up to 31%) without further passivation treatment. The CNNPs displayed an uncommon photoluminescent feature, and could be used as a promising probe to assay the concentration of mercuric ions.
Acknowledgments This work was supported by Natural Science Foundation of Fujian Province (No. 2012J06005), Education Bureau of Fujian Province of China (Nos. JK2011030 and JA13195), the College Students’ Innovative and Entrepreneurial Training Program (No. 20141040), and the Foundation of Fujian Provincial Bureau of Quality and Technical Supervision (No. fjqi2013109).
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