Accepted Manuscript Fluorescent graphene quantum dot nanoprobes for the sensitive and selective detection of mercury ions Baojuan Wang, Shujuan Zhuo, Luyang Chen, Yongjun Zhang PII: DOI: Reference:
S1386-1425(14)00702-1 http://dx.doi.org/10.1016/j.saa.2014.04.129 SAA 12093
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
10 December 2013 17 February 2014 21 April 2014
Please cite this article as: B. Wang, S. Zhuo, L. Chen, Y. Zhang, Fluorescent graphene quantum dot nanoprobes for the sensitive and selective detection of mercury ions, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.04.129
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 proof before it is published in its final 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.
Fluorescent graphene quantum dot nanoprobes for the sensitive and selective detection of mercury ions
Baojuan Wang a, Shujuan Zhuo b,*, Luyang Chenb, Yongjun Zhang b a College of Life Sciences, Anhui Normal University, Wuhu 241000, People's Republic of China b The key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, People's Republic of China
Abstract Graphene quantum dots were prepared by ultrasonic route and served as a highly selective water-soluble probe for sensing of Hg2+. The fluorescence emission spectrum of graphene quantum dots was at about 430 nm. In the presence of Hg2+, the fluorescence of the quantum dots significantly quenched. And the fluorescence intensity gradually decreased with the increasing concentration of Hg2+. The change of fluorescence intensity is directly proportional to the concentration of Hg2+. Under optimum conditions, the linear range for the detection of Hg2+ was 8.0 × 10-7 to 9 × 10-6 M with a detection limit of 1.0 × 10-7 M. In addition, the preliminary mechanism of fluorescence quenching was discussed in the paper. The constructed sensor with high sensitivity and selectivity, simple, rapid properties makes it valuable for further application.
1
Keywords :Graphene quantum dots; Mercury ion; Sensor; Fluorescence analysis
Corresponding author. Tel: 86-553-3883513, Fax: 86-553-3883513. E-mail: [email protected]
2
Introduction Among heavy metal ions, mercury is one of the most dangerous and ubiquitous pollutants [1,2]. Water-soluble divalent mercuric ion (Hg2+) is one of the most usual and stable form of mercury pollution, which provides a pathway for contaminating vast amounts of water and soil [3-5]. Its contamination comes from a variety of natural sources and human activities. And an annual release amount was 4400-7500 metric tons estimated by the United Nations Environment Programme (UNEP) [6,7]. It is demonstrated that Hg2+ can easily pass through skin, respiratory, and gastrointestinal tissues, leading to DNA damage, mitosis impairment, and permanent damage to the central nervous system [8-10]. Therefore, from the viewpoint of environmental protection and health concerns, developing effective analytical methods for the sensitive detection of trace amounts of Hg2+ is an increasing demand and especially important [11-13]. Methods currently available for detecting Hg2+ include
inductively
coupled
plasma
mass
spectrometry
(ICPMS)
[14],
spectrophotometry [15], biosensor [16], atomic absorption/emission spectroscopy [17] and fluorescence assay [18-20]. Among these methods, the fluorescence assay might be the best choice for Hg2+ detection due to its high sensitivity, fast analysis and being non-sample-destructing or less cell-damaging [21,22]. Recently, photoluminescent (PL) graphene quantum dots (GQDs) have attracted growing interest due to their easy fabrication, high quantum yield, low cost, low toxicity, excellent biocompatibility, good photostability and water solubility, which has shown great promise in a variety of applications, especially in the field of
3
photovoltaics devices and bioimaging [23-28]. However, much work is still needed to explore the full potential of these nanomaterials for developing advanced smart sensors. Herein, we present a convenient approach to synthesize GQDs, and demonstrate that such GQDs can serve as a very effective fluorescent probe for label-free, sensitive, and selective detection of Hg2+ by the fluorescence quenching. Furthermore, the preliminary mechanism of fluorescence quenching was proposed.
Experimental section Materials Graphene was purchased from Ningbo Institute of Materials Technology & Engineering Chinese Academy of Sciences (China). HgCl2 was purchased from Shanghai Sinopharm Chemical Reagent Co. Ltd (China). Stock solution of HgCl2 (1.0 × 10-3 M) was prepared by dissolving the commercial products directly in doubly distilled water. The working solutions were then prepared by appropriate dilution of this stock solution. Tris(hydroxymethyl)aminomethane-HCl (Tris-HCl) buffer solution (pH = 8.6, composed of 0.1 M HCl and 0.1 M Tris) was used to control the acidity. Other reagents were of analytical reagent grade and were used without further purification. Water used throughout was doubly distilled water. Preparation of GQDs GQDs were prepared by ultrasonic route as previous report with slight modification [29]. Briefly, graphene was oxidized in concentrated H2SO4 and HNO3 at room
4
temperature, then the mixed solution was treated ultrasonically for 12 h with a ultrasonic instrument (Model: KQ-300 TDE, 300 W, 80 kHz). The mixture was calcined in a furnace installed with an exhaust gas recovery at 350 °C to remove the concentrated H2SO4 and HNO3. The as-prepared products were re-dispersed in water. Then the suspension was filtered through a 0.22 μm microporous membrane to get a brown filter solution. This solution was further centrifuged at 12000 rpm for 10 min to obtain GQDs. Characterization The transmission electron microscopy (TEM) was taken with Tecnai G2 20 S-TWIN transmission electron microscope operating at 200 kV. Fluorescence spectra and
light
scattering
spectroscopy were
measured
with
a
Hitachi-F-4600
spectrofluorimeter. Raman spectrum was collected on an HR 800 Raman spectroscopy (J Y, France) equipped with a synapse CCD detector and a confocal Olympus microscope. The spectrograph uses 600 g/mm gratings and a 633 nm He-Ne laser. Standard procedures Into a 5 mL volumetric flask was transferred 0.05 mL of Tris-HCl buffer solution (pH = 8.6), 1.0 mL of GQDs, and an appropriate quantity of working solution of HgCl2 was added. The mixture was diluted to 5 mL with water and thoroughly mixed. Then the fluorescence intensities of the sample (F) and the blank (F0) solutions were measured with the following settings of the spectrofluorimeter: excitation wavelength, λex = 310 nm; emission wavelength, λem = 430 nm; excitation and emission
5
band-passes, 10 nm.
Results and Discussion TEM image of as-prepared monodisperse GQDs is showed in Fig. 1. The GQDs oxidized by concentrated acids (H2SO4 and HNO3) contain plenty of carboxylate moieties on their surface, imparting their excellent water solubility. So the present GQDs can freely disperse in water with transparent appearance, and exhibit good photostability. The appearance and luminescence properties remain invariable after storing at least eight months in air at room temperature. Fig. 2 shows the Raman spectroscopy of the GQDs. which confirms the quality of the as-prepared GQDs. The peak at 1586 cm-1 (G band) corresponds to the E2g mode of the graphite and is related to the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice. The D band at around 1337 cm-1 is associated with the vibrations of carbon atoms with dangling bonds in the termination plane of disordered graphite [30]. The as-prepared GQDs show an excitation-independent PL behavior. When the excitation wavelength changes from 270 to 340 nm in steps of 10 nm, the PL spectra are almost unchanged and show an emission peak at ca. 430 nm (Fig. 3), which might attribute to the fluorescence of tiny nanoribbons came from the protrudent edge of GQDs synthesized by ultrasonic route. We then tested whether the as-prepared GQDs can be used to fabricate fluorescence sensor for Hg2+. As can be seen from Fig. 4a, the FL intensity of GQDs at 430 nm
6
decreased gradually with increasing concentration of Hg2+. Whereas the intrinsic spectral peak of GQDs-Hg2+ was almost unchanged in comparison with that of free GQDs. The quenching fluorescence is proportional to the concentration of Hg2+. The calibration graph is linear over the range of 8 × 10-7 to 9 × 10-6 M with a detection limit of 1.0 × 10-7 M (Fig. 4b). The detection limit was given by the equation, DL = KS0/S, where K was a numerical factor chosen according to the confidence level desired, S0 was the standard deviation of the blank measurements (n = 9) and S was the sensitivity of the calibration graph [31]. Here a value of 3 for K was used. The fluorescence change of the system was greatly dependent on the pH. The effect of pH on the fluorescence intensity of the system was studied in the range of pH 7.1 to 9.0 by employing Tris-HCl buffer solution. The experiments indicated that the fluorescence intensity of GQDs and GQDs-Hg2+ were changed little in the selected pH range (Fig. 5). In this work, we chose the pH 8.6 Tris-HCl as the buffer system. In order to evaluate the selectivity of the proposed method, the influence of various cations was studied. It can be seen that those tested cations such as Ca2+, Zn2+, Fe2+,Co2+ and so on are scarcely interfered (Fig. 6). The results showed that the present method had a satisfactory selectivity. Generally speaking, the mechanism of fluorescence quenching by Hg2+ can be attributed to facilitate non-radiative electron/hole recombination annihilation through an effective electron transfer process [32]. Another possible explanation is aggregate-induced quenching. Graphene was oxidized in concentrated H2SO4 and
7
HNO3, thus the surface of the as-prepared GQDs is attached with a lot of carboxylate groups. Hg2+ might exhibit a certain affinity to carboxylate groups on the surface of GQDs and induce the GQDs aggregated to some extent. As a consequence, the fluorescence of GQDs gets quenched with Hg2+ ion adding into the GQDs solution gradually. To confirm the assumption, resonance light scattering (RLS) spectroscopy was measured to investigate the interaction of GQDs and Hg2+. RLS is the phenomenon that the wavelength of the incident beam is close to that of the absorption band of the molecular particles which exist as aggregates, Rayleigh scattering will deviate from the law and the intensity of some wavelengths will rapidly increase [33]. As can be seen from Fig. 7, the RLS signal of GQDs is very weak. With the addition of Hg2+, the RLS intensity increases strongly, indicating that the interaction occurs between GQDs and Hg2+, the GQDs-Hg2+complex may be formed in the process. Another possible reason is the formation of aggregates of GQDs induced by Hg2+, leading to enhanced light scattering signal. The detailed mechanisms need to be studied further, relevant work is in progress.
Conclusions In summary, we have demonstrated a convenient and novel type of GQDs based sensor for the detection of Hg2+ with high sensitivity and selectivity. The mechanism of fluorescence quenching might be attributed to the formation of aggregates of GQDs in the presence of Hg2+ through investigation of RLS spectra. Combined with excellent photoluminescence and highly water-soluble, the as-prepared GQDs will have potential applications in many other fields. 8
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC, 21303003) and Natural Science Foundation of Anhui Province College (KJ2012Z126 and KJ2013A129).
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Figure Caption
Fig. 1. TEM image of as-prepared GQDs. Fig. 2. Raman spectrum of the as-prepared GQDs. Fig. 3. (a) PL spectra of the GQDs at different excitation wavelengths; (b) The corresponding normalized PL spectra. Fig. 4. (a) PL spectra of GQDs quenched by Hg2+ ion and (b) PL intensity of the GQDs vs. Hg2+. I0 and I are the fluorescence intensity in the absence and presence of Hg2+, respectively. The error bars represent the standard deviation of three measurements. Fig. 5. Influence of pH in the range of 7.1-9.0. Concentration: Hg2+, 12 μM. Other conditions are the same as those described in the procedure. Fig. 6. Selectivity of the Hg2+ sensor. All competing ion solutions were 5 μM. Fig. 7. RLS spectra of GQD and GQD-Hg2+.
12
Fig. 1. TEM image of as-prepared GQDs.
13
Fig. 2. Raman spectrum of the as-prepared GQDs.
14
Fig. 3. (a) PL spectra of the GQDs at different excitation wavelengths; (b) The corresponding normalized PL spectra.
15
Fig. 4. (a) PL spectra of GQDs quenched by Hg2+ ion and (b) PL intensity of the GQDs vs. Hg2+. I0 and I are the fluorescence intensity in the absence and presence of Hg2+, respectively. The error bars represent the standard deviation of three measurements.
16
Fig. 5. Influence of pH in the range of 7.1-9.0. Concentration: Hg2+, 12 μM. Other conditions are the same as those described in the procedure.
17
Fig. 6. Selectivity of the Hg2+ sensor. All competing ion solutions were 5 μM.
18
Fig. 7. RLS spectra of GQD and GQD-Hg2+.
19
► Graphene quantum dots were prepared and chosen to detect Hg2+ via fluorescence quenching. ► The aggregates of GQDs might be formed in the presence of Hg2+. ► The light scattering signal enhancement of graphene quantum dots was caused by Hg2+.
Fluorescent graphene quantum dot nanoprobes for the sensitive and selective detection of mercury ions
Baojuan Wang a, Shujuan Zhuo b,*, Luyang Chenb, Yongjun Zhang b
Graphical Abstract
RLS spectra of GQD and GQD-Hg2+
S1386-1425(14)00702-1 http://dx.doi.org/10.1016/j.saa.2014.04.129 SAA 12093
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
10 December 2013 17 February 2014 21 April 2014
Please cite this article as: B. Wang, S. Zhuo, L. Chen, Y. Zhang, Fluorescent graphene quantum dot nanoprobes for the sensitive and selective detection of mercury ions, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.04.129
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 proof before it is published in its final 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.
Fluorescent graphene quantum dot nanoprobes for the sensitive and selective detection of mercury ions
Baojuan Wang a, Shujuan Zhuo b,*, Luyang Chenb, Yongjun Zhang b a College of Life Sciences, Anhui Normal University, Wuhu 241000, People's Republic of China b The key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, People's Republic of China
Abstract Graphene quantum dots were prepared by ultrasonic route and served as a highly selective water-soluble probe for sensing of Hg2+. The fluorescence emission spectrum of graphene quantum dots was at about 430 nm. In the presence of Hg2+, the fluorescence of the quantum dots significantly quenched. And the fluorescence intensity gradually decreased with the increasing concentration of Hg2+. The change of fluorescence intensity is directly proportional to the concentration of Hg2+. Under optimum conditions, the linear range for the detection of Hg2+ was 8.0 × 10-7 to 9 × 10-6 M with a detection limit of 1.0 × 10-7 M. In addition, the preliminary mechanism of fluorescence quenching was discussed in the paper. The constructed sensor with high sensitivity and selectivity, simple, rapid properties makes it valuable for further application.
1
Keywords :Graphene quantum dots; Mercury ion; Sensor; Fluorescence analysis
Corresponding author. Tel: 86-553-3883513, Fax: 86-553-3883513. E-mail: [email protected]
2
Introduction Among heavy metal ions, mercury is one of the most dangerous and ubiquitous pollutants [1,2]. Water-soluble divalent mercuric ion (Hg2+) is one of the most usual and stable form of mercury pollution, which provides a pathway for contaminating vast amounts of water and soil [3-5]. Its contamination comes from a variety of natural sources and human activities. And an annual release amount was 4400-7500 metric tons estimated by the United Nations Environment Programme (UNEP) [6,7]. It is demonstrated that Hg2+ can easily pass through skin, respiratory, and gastrointestinal tissues, leading to DNA damage, mitosis impairment, and permanent damage to the central nervous system [8-10]. Therefore, from the viewpoint of environmental protection and health concerns, developing effective analytical methods for the sensitive detection of trace amounts of Hg2+ is an increasing demand and especially important [11-13]. Methods currently available for detecting Hg2+ include
inductively
coupled
plasma
mass
spectrometry
(ICPMS)
[14],
spectrophotometry [15], biosensor [16], atomic absorption/emission spectroscopy [17] and fluorescence assay [18-20]. Among these methods, the fluorescence assay might be the best choice for Hg2+ detection due to its high sensitivity, fast analysis and being non-sample-destructing or less cell-damaging [21,22]. Recently, photoluminescent (PL) graphene quantum dots (GQDs) have attracted growing interest due to their easy fabrication, high quantum yield, low cost, low toxicity, excellent biocompatibility, good photostability and water solubility, which has shown great promise in a variety of applications, especially in the field of
3
photovoltaics devices and bioimaging [23-28]. However, much work is still needed to explore the full potential of these nanomaterials for developing advanced smart sensors. Herein, we present a convenient approach to synthesize GQDs, and demonstrate that such GQDs can serve as a very effective fluorescent probe for label-free, sensitive, and selective detection of Hg2+ by the fluorescence quenching. Furthermore, the preliminary mechanism of fluorescence quenching was proposed.
Experimental section Materials Graphene was purchased from Ningbo Institute of Materials Technology & Engineering Chinese Academy of Sciences (China). HgCl2 was purchased from Shanghai Sinopharm Chemical Reagent Co. Ltd (China). Stock solution of HgCl2 (1.0 × 10-3 M) was prepared by dissolving the commercial products directly in doubly distilled water. The working solutions were then prepared by appropriate dilution of this stock solution. Tris(hydroxymethyl)aminomethane-HCl (Tris-HCl) buffer solution (pH = 8.6, composed of 0.1 M HCl and 0.1 M Tris) was used to control the acidity. Other reagents were of analytical reagent grade and were used without further purification. Water used throughout was doubly distilled water. Preparation of GQDs GQDs were prepared by ultrasonic route as previous report with slight modification [29]. Briefly, graphene was oxidized in concentrated H2SO4 and HNO3 at room
4
temperature, then the mixed solution was treated ultrasonically for 12 h with a ultrasonic instrument (Model: KQ-300 TDE, 300 W, 80 kHz). The mixture was calcined in a furnace installed with an exhaust gas recovery at 350 °C to remove the concentrated H2SO4 and HNO3. The as-prepared products were re-dispersed in water. Then the suspension was filtered through a 0.22 μm microporous membrane to get a brown filter solution. This solution was further centrifuged at 12000 rpm for 10 min to obtain GQDs. Characterization The transmission electron microscopy (TEM) was taken with Tecnai G2 20 S-TWIN transmission electron microscope operating at 200 kV. Fluorescence spectra and
light
scattering
spectroscopy were
measured
with
a
Hitachi-F-4600
spectrofluorimeter. Raman spectrum was collected on an HR 800 Raman spectroscopy (J Y, France) equipped with a synapse CCD detector and a confocal Olympus microscope. The spectrograph uses 600 g/mm gratings and a 633 nm He-Ne laser. Standard procedures Into a 5 mL volumetric flask was transferred 0.05 mL of Tris-HCl buffer solution (pH = 8.6), 1.0 mL of GQDs, and an appropriate quantity of working solution of HgCl2 was added. The mixture was diluted to 5 mL with water and thoroughly mixed. Then the fluorescence intensities of the sample (F) and the blank (F0) solutions were measured with the following settings of the spectrofluorimeter: excitation wavelength, λex = 310 nm; emission wavelength, λem = 430 nm; excitation and emission
5
band-passes, 10 nm.
Results and Discussion TEM image of as-prepared monodisperse GQDs is showed in Fig. 1. The GQDs oxidized by concentrated acids (H2SO4 and HNO3) contain plenty of carboxylate moieties on their surface, imparting their excellent water solubility. So the present GQDs can freely disperse in water with transparent appearance, and exhibit good photostability. The appearance and luminescence properties remain invariable after storing at least eight months in air at room temperature. Fig. 2 shows the Raman spectroscopy of the GQDs. which confirms the quality of the as-prepared GQDs. The peak at 1586 cm-1 (G band) corresponds to the E2g mode of the graphite and is related to the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice. The D band at around 1337 cm-1 is associated with the vibrations of carbon atoms with dangling bonds in the termination plane of disordered graphite [30]. The as-prepared GQDs show an excitation-independent PL behavior. When the excitation wavelength changes from 270 to 340 nm in steps of 10 nm, the PL spectra are almost unchanged and show an emission peak at ca. 430 nm (Fig. 3), which might attribute to the fluorescence of tiny nanoribbons came from the protrudent edge of GQDs synthesized by ultrasonic route. We then tested whether the as-prepared GQDs can be used to fabricate fluorescence sensor for Hg2+. As can be seen from Fig. 4a, the FL intensity of GQDs at 430 nm
6
decreased gradually with increasing concentration of Hg2+. Whereas the intrinsic spectral peak of GQDs-Hg2+ was almost unchanged in comparison with that of free GQDs. The quenching fluorescence is proportional to the concentration of Hg2+. The calibration graph is linear over the range of 8 × 10-7 to 9 × 10-6 M with a detection limit of 1.0 × 10-7 M (Fig. 4b). The detection limit was given by the equation, DL = KS0/S, where K was a numerical factor chosen according to the confidence level desired, S0 was the standard deviation of the blank measurements (n = 9) and S was the sensitivity of the calibration graph [31]. Here a value of 3 for K was used. The fluorescence change of the system was greatly dependent on the pH. The effect of pH on the fluorescence intensity of the system was studied in the range of pH 7.1 to 9.0 by employing Tris-HCl buffer solution. The experiments indicated that the fluorescence intensity of GQDs and GQDs-Hg2+ were changed little in the selected pH range (Fig. 5). In this work, we chose the pH 8.6 Tris-HCl as the buffer system. In order to evaluate the selectivity of the proposed method, the influence of various cations was studied. It can be seen that those tested cations such as Ca2+, Zn2+, Fe2+,Co2+ and so on are scarcely interfered (Fig. 6). The results showed that the present method had a satisfactory selectivity. Generally speaking, the mechanism of fluorescence quenching by Hg2+ can be attributed to facilitate non-radiative electron/hole recombination annihilation through an effective electron transfer process [32]. Another possible explanation is aggregate-induced quenching. Graphene was oxidized in concentrated H2SO4 and
7
HNO3, thus the surface of the as-prepared GQDs is attached with a lot of carboxylate groups. Hg2+ might exhibit a certain affinity to carboxylate groups on the surface of GQDs and induce the GQDs aggregated to some extent. As a consequence, the fluorescence of GQDs gets quenched with Hg2+ ion adding into the GQDs solution gradually. To confirm the assumption, resonance light scattering (RLS) spectroscopy was measured to investigate the interaction of GQDs and Hg2+. RLS is the phenomenon that the wavelength of the incident beam is close to that of the absorption band of the molecular particles which exist as aggregates, Rayleigh scattering will deviate from the law and the intensity of some wavelengths will rapidly increase [33]. As can be seen from Fig. 7, the RLS signal of GQDs is very weak. With the addition of Hg2+, the RLS intensity increases strongly, indicating that the interaction occurs between GQDs and Hg2+, the GQDs-Hg2+complex may be formed in the process. Another possible reason is the formation of aggregates of GQDs induced by Hg2+, leading to enhanced light scattering signal. The detailed mechanisms need to be studied further, relevant work is in progress.
Conclusions In summary, we have demonstrated a convenient and novel type of GQDs based sensor for the detection of Hg2+ with high sensitivity and selectivity. The mechanism of fluorescence quenching might be attributed to the formation of aggregates of GQDs in the presence of Hg2+ through investigation of RLS spectra. Combined with excellent photoluminescence and highly water-soluble, the as-prepared GQDs will have potential applications in many other fields. 8
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC, 21303003) and Natural Science Foundation of Anhui Province College (KJ2012Z126 and KJ2013A129).
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Figure Caption
Fig. 1. TEM image of as-prepared GQDs. Fig. 2. Raman spectrum of the as-prepared GQDs. Fig. 3. (a) PL spectra of the GQDs at different excitation wavelengths; (b) The corresponding normalized PL spectra. Fig. 4. (a) PL spectra of GQDs quenched by Hg2+ ion and (b) PL intensity of the GQDs vs. Hg2+. I0 and I are the fluorescence intensity in the absence and presence of Hg2+, respectively. The error bars represent the standard deviation of three measurements. Fig. 5. Influence of pH in the range of 7.1-9.0. Concentration: Hg2+, 12 μM. Other conditions are the same as those described in the procedure. Fig. 6. Selectivity of the Hg2+ sensor. All competing ion solutions were 5 μM. Fig. 7. RLS spectra of GQD and GQD-Hg2+.
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Fig. 1. TEM image of as-prepared GQDs.
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Fig. 2. Raman spectrum of the as-prepared GQDs.
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Fig. 3. (a) PL spectra of the GQDs at different excitation wavelengths; (b) The corresponding normalized PL spectra.
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Fig. 4. (a) PL spectra of GQDs quenched by Hg2+ ion and (b) PL intensity of the GQDs vs. Hg2+. I0 and I are the fluorescence intensity in the absence and presence of Hg2+, respectively. The error bars represent the standard deviation of three measurements.
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Fig. 5. Influence of pH in the range of 7.1-9.0. Concentration: Hg2+, 12 μM. Other conditions are the same as those described in the procedure.
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Fig. 6. Selectivity of the Hg2+ sensor. All competing ion solutions were 5 μM.
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Fig. 7. RLS spectra of GQD and GQD-Hg2+.
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► Graphene quantum dots were prepared and chosen to detect Hg2+ via fluorescence quenching. ► The aggregates of GQDs might be formed in the presence of Hg2+. ► The light scattering signal enhancement of graphene quantum dots was caused by Hg2+.
Fluorescent graphene quantum dot nanoprobes for the sensitive and selective detection of mercury ions
Baojuan Wang a, Shujuan Zhuo b,*, Luyang Chenb, Yongjun Zhang b
Graphical Abstract
RLS spectra of GQD and GQD-Hg2+
