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Highly Selective and Sensitive Detection of Hg2+ Based on Förster Resonance Energy Transfer between CdSe Quantum Dots and g-C3N4 Nanosheets
Nanoscale Research Lettersvolume 13, Article number: 235 (2018)
In the presence of Hg2+, a fluorescence resonance energy transfer (FRET) system was constructed between CdSe quantum dots (QDs) (donor) and g-C3N4 (receptors). Nanocomposites of g-C3N4 supported by CdSe QDs (CdSe QDs/g-C3N4 nanosheets) were fabricated through an electrostatic interaction route in an aqueous solution. The nanocomposites were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and transmission electron microscopy. Results showed that the g-C3N4 nanosheets were decorated randomly by CdSe QDs, with average diameter of approximately 7 nm. The feasibility of the FRET system as a sensor was demonstrated by Hg (II) detection in water. At pH 7, a linear relationship was observed between the fluorescence intensity and the concentration of Hg (II) (0–32 nmol/L), with a detection limit of 5.3 nmol/L. The new detection method was proven to be sensitive for detecting Hg2+ in water solutions. Moreover, the method showed high selectivity for Hg2+ over several metal ions, including Na+, Mg2+, Ca2+, Pb2+, Cr3+, Cd2+, Zn2+, and Cu2+. The CdSe QDs/g-C3N4 nanosheet conjugate exhibited desirable long-term stability and reversibility as a novel FRET sensor. The novel FRET-based fluorescence detection provided an attractive assay platform for quantifying Hg2+ in complex water solutions.
The main cause of mercury poisoning in humans was polluted natural waters . Hg2+ ion metabolism by aquatic microbes produces methyl mercury, which was a potent neurotoxin associated with cognitive and motion disorders . Therefore, mercury detection methods that are rapid, cost-effective, facile, and applicable to complex environments are necessary. Particularly, nanomaterials with unique optical properties can be employed to develop optical sensors with high sensitivity and selectivity . Semiconductor quantum dots (QDs), fluorescent metal nanoclusters (NCs), noble metal nanoparticles (NPs), and carbon nanodots (CDs) were commonly used in the design of Hg2+ optical sensors because of their distinct properties, such as easy synthesis, high stability, functionalization, and biocompatibility. Many fluorescent sensors for Hg2+ had been reported [4,5,6,7,8]. For example, Huang et al.  developed a time-gated Förster resonance energy transfer (FRET) sensor for Hg2+ detection. Moreover, different FRET systems had been developed for the detection for Hg2+ [10,11,12]. Notably, FRET systems could be similarly built using nanoparticles, such as QDs, as well as organic and inorganic NPs [13,14,15]. Among the nanoparticles, g-C3N4 nanosheets had attracted widespread interest [16, 17]. Although g-C3N4 nanosheets have been applied as sensors, a FRET detection system with g-C3N4 nanosheets and CdSe QDs for metal ions has not been reported. FRET-based fluorescence sensing systems offer multiple advantages .
In the present study, a new FRET-based fluorescence sensor was developed to detect mercury ions in aqueous media by using g-C3N4 nanosheets and CdSe QDs particles as vehicles. The proposed mechanism was illustrated in Fig. 1.
Mercury (II) chloride (HgCl2) was purchased from Tong Ren Chemical Research Institute (Guizhou, China). Urea and CdSe QDs were purchased from Aladdin Reagent Company (Shanghai, China). Other reagents and chemicals were of analytical reagent grade and used without further purification. All solutions were prepared using purified water from a Milli-Q gradient water purification system (Millipore Inc., USA; nominal resistivity 18.2 MÙ cm).
An X-ray diffractometer (Rigaku D/max-2400) was used to obtain diffraction patterns. Ultraviolet–visible (UV–vis) spectra were recorded on a UV–vis 800 spectrophotometer at room temperature. Fourier-transform infrared (FTIR) spectra were recorded on a Nicolet-nexus670 spectrometer using KBr. Fluorescence measurements were performed at room temperature with an RF-5301PC fluorescence spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed using a multifunctional spectrometer (Thermo Scientific).
Construction of FRET Sensor between the g-C3N4 Nanosheets and CdSe QD Particles
In a typical procedure, g-C3N4 (125 mg, which was synthesized according to our previous report ) was dispersed in 250 mL of water (1:1) and ultrasonicated for 5 h at ambient temperature. Then, CdSe QDs (1.838 g, 0.0216 mol) were dissolved in the solution by sonication for 2 h. Given that the amine group on the g-C3N4 nanosheets and CdSe QDs had a carboxyl group, g-C3N4 nanosheets and CdSe QDs nanoparticles would be combined by electrostatic interaction. All solutions were prepared in Milli-Q gradient water (pH = 7). The CdSe QDs/g-C3N4 nanosheet conjugate emission spectra were recorded. All samples were excited at 334 nm, which was near the minimal acceptor absorption.
Fluorescence Detection of Hg2+
Hg2+ was quenched at room temperature in water. During a typical operation, 10 μL of the CdSe QDs/g-C3N4 nanosheet conjugates was added to 3 mL of ultrapure water, and then the calculated amount of Hg2+ was added. The emission spectra of the CdSe QDs/g-C3N4 nanosheet conjugates were recorded 2 mins later at room temperature.
Interference and Competition Analyses
The response of the FRET nanoprobe to other metal ions (Na+, Mg2+, Ca2+, Pb2+, Cr3+, Cd2+, Zn2+, and Cu2+) was studied through fluorescence spectroscopy. Studies were carried out using the CdSe QDs/g-C3N4 nanosheet conjugates emitting at 450 nm. The conjugate solution was placed in a 1-cm optical path quartz fluorescence cuvette. Fluorescence intensity was measured at emission wavelength of 450 nm under excitation wavelength of 334 nm in the presence of each possible interference (32 nM). Competition assays were also performed for all the possible interferences previously analyzed. For competition experiments, 32 nM Hg2+ aqueous solutions were prepared.
Results and Discussion
The structure and morphology of g-C3N4 nanosheets were characterized by TEM, XPS, and XRD. The TEM image in Fig. 2a showed that the g-C3N4 nanosheet possessed a graphene-like morphology that mainly consists of a few layers . Figure 2a showed the XRD patterns of the g-C3N4 nanosheets. The strong XRD peak centered at 27.4° corresponded to the typical graphitic interlayer stacking (002) peak of g-C3N4. The small peak at 13.1° corresponded to the periodic in-plain structural packing feature within the sheets [20, 21]. XPS measurement was used to analyze the valence states of g-C3N4 nanosheets. The XPS spectrum in Fig. 2c showed the C–C bonded to N at 284.8 and 288.0 eV, and the N 1 s spectrum was at 397.04 eV. In Fig. 2d, the peak at 811 cm−1 was attributed to the vibration of the triazine ring. The peaks around 1000 cm−1 represented the stretching modes of CN heterocycles, and the peak at 1800 cm−1 corresponded to C–NH–C. The peaks at 300–3600 cm−1 corresponded to N–H and O–H stretching vibrations .
UV-vis and Fluorescence Properties of CdSe QDs/g-C3N4 Nanosheets
Fluorescence and UV-vis absorption spectra were obtained to evaluate the optical properties of CdSe QDs/g-C3N4 nanosheets. As shown in Fig. 3a, a large peak at approximately 334 nm was observed in the UV–vis absorption spectrum. Moreover, the fluorescence emission and excitation peaks were observed at 452 and 334 nm in the synchronous fluorescence spectroscopy in Fig. 3b and were associated with the emission fluorescence and ultraviolet light excitation of nanosheets. The emission peaks showed a shift compared with the pure g-C3N4 nanosheets at 14–16 nm (emission and excitation peaks were observed at 438 and 310 nm as presented in Fig. 3c), which could be ascribed to the FRET. The influence of excitation wavelengths on fluorescence intensities was also confirmed.
Effect of pH to the Fluorescence of the CdSe QDs/g-C3N4 Nanosheet Conjugates
Figure 4 showed the fluorescence of the CdSe QDs/g-C3N4 nanosheet conjugates at different pH values. The pH value increased from 3 to 7 with the fluorescence intensity. However, the fluorescence intensity gradually decreased when the pH value varied increased from 7 to 10, which could be attributed to the effect of pH on the change in the surface charge owing to protonation–deprotonation due to the existence of amino groups in the structure of g-C3N4 nanosheets. In this study, the CdSe QDs/g-C3N4 nanosheet conjugates were conducted for the detection of Hg2+ ions, and the pH value of 7 was selected as the optimum pH value. The fluorescence emissions were measured at pH 7 containing different concentrations of NaCl to obtain the stability of the CdSe QDs/g-C3N4 nanosheet conjugates under high ionic strength circumstances. Only a slight change was observed under high ionic strength in the fluorescence intensities of the CdSe QDs/g-C3N4 nanosheet conjugates. The result showed that high ionic strength had minimal effects to the fluorescence intensities of the conjugates.
Selectivity of CdSe QDs/g-C3N4 Nanosheet FRET System in Detecting Mercury Ion
Selectivity is an important parameter of a new sensing system. The selectivity of the CdSe QDs/g-C3N4 nanosheet FRET sensor was evaluated using various metal ions (e.g., Cu2+, Mg2+, Na+, Ca2+, Hg2+, Cr3+, Pb2+, Cd2+, and Zn2+); the results were shown in Fig. 5a. Compared with the blank sample without ions, the fluorescence ratio of Hg2+ increased obviously, while the fluorescence intensity of other metal ions changed slightly or remains the same. These results indicated that the FRET sensor showed more selectivity than the others (Fig. 5b). Thus, the CdSe QDs/g-C3N4 showed high selectivity toward Hg2+. This phenomenon was distinct in comparison with pure g-C3N4 nanosheet, which was selective for Cu2+ and Hg2+ [23, 24].
Feasibility of the FRET Fluorescence Process in Detecting Hg2+
To study the practicability of the FRET sensor, the CdSe QDs/g-C3N4 nanosheet fluorescence detection of Hg2+ was performed. The presence of Hg2+ resulted in decreased fluorescence intensity as shown in Fig. 6, which illustrated that Hg2+ could effectively quench the FRET sensor. In order to study the sensitivity, the response of the sensor to different Hg2+ concentrations was further evaluated by fluorescence spectroscopy, and the results were shown in Fig. 6a. The fluorescence intensity of g-C3N4 nanosheets gradually decreased with increasing of Hg2+ concentrations. Figure 6b explained that the I/I0 was dependent on the concentration of Hg2+, where I0 and I were the fluorescence intensity in the absence and presence, respectively, of Hg2+. Moreover, the relation of I/I0 between concentrations of Hg2+ was linear, and the equation of linear regression was I = − 9.6 × 107+ 550.5(R2 = 0.9882), as shown in the inset of Fig. 6b. Compared with recently reported luminescence methods, the proposed method had lower detection limit and higher sensitivity [25, 26]. The g-C3N4 nanosheets and CdSe QDs displayed no obvious quenching response to other metal ions apart from Hg2+, which suggested a relatively high selectivity for this method.
The other coexisting cations that affect the detection of mercury ion were detected as well. The response of the CdSe QDs/g-C3N4 nanosheet-based sensing system toward Hg2+ ion in the presence of alkali, alkaline earth, and other transition metal ions was shown in Table 1. The coexistence of most of the metal ions did not interfere with the binding of Hg2+, which indicated that the interference of these coexisting ions on the Hg2+ sensor was negligible.
In addition, long-term stability is a superior property of sensors. The absorbance and the fluorescence during the continuous investigation every 3 days within 2 weeks indicated that the activity of CdSe QDs/g-C3N4 nanosheets remained above 92% of the initial efficiency though they were stored at ambient environment. The results indicated that the CdSe QDs/g-C3N4 nanosheets as FRET sensors had good long-term stability.
Compared with previous reports concerning fluorescence assays for Hg2+ (results are listed in Table 2), the CdSe QDs/g-C3N4 nanosheet fluorescence probe based on FRET with the concentration of Hg (II) in the range of 0–32 nmol/L at pH = 7 exhibited a limit of detection at 5.3 nmol/L. Thus, our method obtained a superior detection limit and linear range.
Application of the FRET Sensor
The CdSe QDs/g-C3N4 nanosheets as a FRET sensor successfully provided a good platform for detecting Hg2+ in real samples because of their sensitivity and selectivity. Well, lake, and tap waters were selected as real samples for analysis in which the recovery of Hg2+ were in the range of 95.4–101.6% (Table 3). The relative standard deviation (RSD) of Hg2+ was in the range of 0.64–1.72%. The result stated clearly that the designed method can be efficiently used to detect Hg2+ in practical applications. The acceptable values of RSD and relative error confirmed the high sensitivity, high precision, and high reliability of the proposed FRET sensor for Hg2+ determination in practical applications.
A FRET-based system was developed for detecting Hg2+ within g-C3N4 nanosheets/CdSe QDs. The detection limit for Hg2+ ion was 5.3 nM, with a linear response ranging from 0 to 32 nM. The applicability of this sensor was demonstrated by measuring the content of Hg2+ in real samples. Given the long-term stability, low cost, and facile preparation of the CdSe QDs/g-C3N4 nanosheet conjugates, the fluorescence assay could be used as an environmental protection sensor. This strategy would provide an alternative approach for constructing FRET-based sensors for Hg2+ in aqueous media, including environmental and biological samples.
Fluorescence resonance energy transfer (FRET) system was constructed between CdTe quantum dots (QDs) (donor) and g-C3N4 (acceptor) in the presence of Hg2+ for the first time.
The nanocomposites of g-C3N4 supported by CdSe QDs (CdSe QDs /g-C3N4) were fabricated through a simple electrostatic interaction route in an aqueous solution.
The feasibility of the FRET system as a sensor was demonstrated for detecting Hg (II) in water solution. At pH 7, a linear relationship was observed between the quenched fluorescence intensity of the concentration of Hg (II) in the range of 0–32 nmol/L. The detection limit was 5.3 nmol/L.
The novel FRET-based fluorescence detection may provide an attractive assay platform for quantifying Hg2+ in complex water solutions.
Förster resonance energy transfer
X-ray photoelectron spectroscopy
Yari A, Papi F (2009) Highly selective sensing of mercury (II) by development and characterization of a PVC-based optical sensor. Sensors Actuators B Chem 138:467–473
Clarkson TW, Magos L, Myers GJ (2003) The Toxicology of mercury-current exposures and clinical manifestations. Engl J Med 349:1731–1737
Zhang LD, Fang M (2010) Nanomaterials in pollution trace detection and environmental improvement. Nano Today 5:128–142
Shiravand G, Badiei A, Ziarani GM (2017) Carboxyl-rich g-C3N4 nanoparticles: synthesis, characterization and their application for selective fluorescence sensing of Hg2+ and Fe3+ in aqueous media. Sensors Actuators B Chem 242:244–252
Li J, Wang H, Guo Z, Wang Y, Ma H, Ren X, Du B, Wei Q (2017) A “turn-off” fluorescent biosensor for the detection of mercury (II) based on graphite carbon nitride. Talanta 162(1):46–51
Duan J, Zhang Y, Yin Y, Li H, Wang J, Zhu L (2018) A novel “on-off-on” fluorescent sensor for 6-thioguanine and Hg2+ based on g-C3N4 nanosheets. Sensors Actuators B Chem 257:504–510
Bao Y, Chen K (2016) AgCl/Ag/g-C3N4 hybrid composites: preparation, visible light-driven photocatalytic activity and mechanism. Nano-Micro Lett 8(2):182–192
Li H, Yin S, Sato T, Wang Y (2016) Enhanced photocatalytic performance of luminescent g-C3N4 photocatalyst in darkroom. Nanoscale Res Lett 11:91
Huang DW, Niu CG, Ruan M et al (2013) Highly sensitive strategy for Hg2+ detection in environmental water samples using long lifetime fluorescence quantum dots and gold nanoparticles. Environ Sci Technol 47:4392–4398
Wang G, Chang X, Peng J, Liu K, Zhao K, Yu C, Fang Y (2015) Towards a new FRET system via combination of pyrene and perylene bisimide: synthesis, self-assembly and fluorescence behavior. Phys Chem Chem Phys 17:5441–5449
Liu Y, Qin O, Li H, Chen M, Zhang Z, Chen Q (2018) Turn-on fluoresence sensor nanoparticles and gold nanoparticles. J Agric Food Chem 255:836–844
Yu C, Zhang J, Li JH, Liu P, Wei P, Chen L (2011) Fluorescent probe for copper (II) ion based on a rhodamine spirolactame derivative, and its application to fluorescent imaging in living cells. Microchim Acta 174:247–255
Chen J, Zeng F, Wu S, Zhao JJ, Chen Q, Tong Z (2008) Reversible fluorescence modulation through energy transfer with ABC triblock copolymer micelles as scaffolds. Chem Commun 43:5580–5582
Frigoli M, Ouadahi K, Larpent C (2009) A cascade FRET mediated ratiometric sensor for Cu2+ ions based on dual fluorescent ligand-coated polymer nanoparticles. Chem Eur J 15:8319–8330
Bednarkiewicz A, Nyk M, Samoc M (2010) Up-conversion FRET from Er3+/Yb3+: NaYF4 nanophosphor to CdSe quantum dots. J Phys Chem C 114:17535–17541
Yang G, Zhu C, Du D, Zhu J, Lin Y (2015) Graphene-like two-dimensional layered nanomaterials: applications in biosensors and nanomedicine. Nanoscale 7(34):14217–14231
Zhai Y, Zhu Z, Zhu C, Ren J, Wang E, Dong S (2014) Multifunctional water-soluble luminescent carbon dots for imaging and Hg2+ sensing. J Mater Chem B 2:6995–6999
Kikuchi K (2010) Design, synthesis and biological application of chemical probes for bio-imaging. Chem Soc Rev 39:2048–2053
Wang S, Lu Q, Yan X, Yang M, Ye R, Du D, Lin Y (2017) “On-off-on” fluorescence sensor based on g-C3N4 nanosheets for selective and sequential detection Ag+ and S2. Talanta 168:168–173
Tang Y, Song H, Su Y, Lv Y (2013) Turn-on persistent luminescence probe based on graphitic carbon nitride for imaging detection of biothiols in biological fluids. Anal Chem 85:11876–11884
Wang X, Maeda K, Thomas A, Takanabe K, Xin G, Carlsson JM, Domen K, Antonietti M (2009) A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat Mater 8:76–80
Zhang X, Xie X, Wang H, Zhang J, Pan B, Xie Y (2013) Enhanced photoresponsive ultrathin graphitic-phase C3N nanosheets for bioimaging. J Am Chem Soc 135:18–21
She X, Xu H, Xu Y, Yan J, Xia J, Xu L, Song Y, Jiang Y, Zhang Q, Li H (2014) Exfoliated graphene-like carbon nitride in organic solvents: enhanced photocatalytic activity and highly selective and sensitive sensor for the detection of trace amounts of Cu2+. J Mater Chem A 2:2563–2570
Tian J, Liu Q, Asiri AM, Al-Youbi AO, Sun X (2013) Ultrathin graphitic carbon nitride nanosheet: a highly efficient fluorosensor for rapid, ultrasensitive detection of Cu2+. Anal Chem 85:5595
Xuan F, Luo X, Hsing IM (2013) Conformation-dependent exonuclease III activity mediated by metal ions reshuffling on thymine-rich DNA duplexes for an ultrasensitive electrochemical method for Hg2+ detection. Anal Chem 85(9):4586–4593
Huang J, Gao X, Jia J, Kim JK, Li Z (2014) Graphene oxide-based amplified fluorescent biosensor for Hg2+ detection through hybridization chain reactions. Anal Chem 86(6):3209–3215
Liu B, Zeng F, Wu S, Wang J, Tang F (2013) Ratiometric sensing of mercury (II) based on a FRET process on silica core-shell nanoparticles acting as vehicles. Microchim Acta 180:845–853
Li T, Zhou YY, Sun JY, Tang DB, Guo SX, Ding XP (2011) Ultrasensitive detection of mercury (II) ion using CdTe quantum dots in sol–gel-derived silica spheres coated with calixarene  as fluorescent probes. Microchim Acta 175:113–119
Murkovic I, Wolfbeis OS (1997) Fluorescence-based sensor membrane for mercury (II) detection. Sensor Actuators B 39:246–251
Shi W, Ma H (2008) Rhodamine B thiolactone: a simple chemosensor for Hg2+ in aqueous media. Chem Commun 16:1856–1858
Koneswaran M, Narayanaswamy R (2012) CdS/ZnS core-shell quantum dots capped with mercaptoacertic acid as fluorescent probes for Hg (II) ions. Microchim Acta 178:171–178
Zheng AF, Chen JL, Wu GN, Wei HP, He CY, Kai XM, Wu GH, Chen YC (2009) Optimization of a sensitive method for the “swith-on” determination of mercury (II) in waters using rhodamine B capped gold nanoparticles as a fluorescent sensor. Microchim Acta 164:17–27
Lee MH, Lee SJ, Jung JH (2010) Luminophore-immobilized mesoporous silica for selective Hg2+ sensing. Tetrahedron 63:12087–12092
Huang D, Niu C, Wang X, Lv X, Zeng G (2013) “Turn-on” fluorescent sensor for Hg2+ based on single-stranded DNA functionalized Mn:CdS/ZnS quantum dots and gold nanoparticles by time-gated mode. Anal Chem 85(2):1164–1170
Chen KH, Wang HW, Kang BS, Chang CY, Wang YL, Lele TP, Ren F, Pearton SJ, Dabiran A, Osinsky A, Chow PP (2008) Low Hg (II) ion concentration electrical detection with AlGaN/GaN high electron mobility transistors. Sens Actuators B 134:386–389
Li J, Mei F, Li WY, He XW, Zhang YK (2008) Study on the fluorescence resonance energy transfer between CdTe QDs and butyl-rhodamine B in the presence of CTMAB and its application on the detection of Hg (II). Spectrochimica Acta Part A 70(4):811–817
This work was supported by the National Natural Science Foundation of China (No. 21703189) and Xianyang Normal University “blue talent project” (XSYQL201709); Shaanxi Provincial Natural Science Foundation (2018JM2047); Xianyang Normal University Innovative entrepreneurship project (XSYHGKZ1705); and Xianyang Normal University students’ innovative and entrepreneurial projects (2017078).
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