- Email: [email protected]
Visual and fluorescent detection of mercury ions using a dual-emission ratiometric fluorescence nanomixture of carbon dots cooperating with gold nanoclusters
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 223 (2019) 117364
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Visual and fluorescent detection of mercury ions using a dual-emission ratiometric fluorescence nanomixture of carbon dots cooperating with gold nanoclusters Jinghan Liu, Hanyue Xue, Yingnan Liu, Tong Bu, Pei Jia, Yuhang Shui, Li Wang ⁎ College of Food Science and Engineering, Northwest A&F University, Yangling, 712100, Shaanxi, China
a r t i c l e
i n f o
Article history: Received 22 January 2019 Received in revised form 16 June 2019 Accepted 7 July 2019 Available online 08 July 2019 Keywords: Ratiometric fluorescent sensor Carbon dots Gold nanoclusters Mercury (II) ions
a b s t r a c t Mercury (II) ions (Hg2+), as one of the most toxic heavy metals, can cause irreversible damage to human health even at very low concentration due to its high toxicity and bioaccumulation. Herein, a facile ratiometric fluorescence nanomixture based on carbon dots gold nanoclusters (CDs-Au NCs) was constructed for quantitative detection of Hg2+. Lysine functionalized carbon dots (CDs) were prepared by one-pot hydrothermal method, while gold nanoclusters (Au NCs) were synthesized via using chicken egg white (CEW) as reducer and stabilizer. The novel nanomixture exhibited two strong emission peaks at 450 nm and 665 nm under 390 nm excitation, and showed pink fluorescence under UV light. Interestingly, the fluorescence of the CDs-Au NCs nanomixture was selectively response to Hg2+. The fluorescence of Au NCs at 665 nm was decreased when Hg2+ was presented in the solution, while the fluorescence of CDs at 450 nm stayed constant. The fluorescence color changed from pink to blue obviously with increasing the concentration of Hg2+, which indicated that CDs-Au NCs could be used for visual detection Hg2+ by the naked eye. Under optimal conditions, this ratiometric fluorescent sensor could detect Hg2+ accurately and possess a great sensitivity with a detection limit of 63 nM. In addition, this method was applied to detect Hg2+ in real water samples with great recoveries, suggesting its potential in practical application with simplicity, environmentally friendly and low cost. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal ions are environmental pollutants and badly harmful to human health, especially mercury (II) ions (Hg2+). Unlike organic pollutants, Hg2+ is difficult to decomposed by biodegradation and could continue to accumulate in organisms and get into the food chains, causing seriously harm to creatures and human beings [1–3]. It can trigger long-term irreversible damage to human health due to its adverse effects on the vital organs and tissues even at ultratrace level [4–6]. Thus, it is an urgent demand to monitor the content level of Hg2+ in environment water and food samples. Until now, various methods have been developed for Hg2+ detection, such as atomic absorption/emission spectroscopy [7,8], mass spectroscopy (MS) [9], gas chromatography (GC) [10], high performance liquid chromatography (HPLC) [11] and so on. Although these approaches offer good performance to detect Hg2+ in most cases, the requirement of expensive instruments, complex sample processing procedure and high cost of analysis limit their wide applications. Accordingly, it is great important to develop a simple and low-cost method to detect Hg2+ with high sensitivity and selectivity.
⁎ Corresponding author. E-mail address: [email protected] (L. Wang).
https://doi.org/10.1016/j.saa.2019.117364 1386-1425/© 2019 Elsevier B.V. All rights reserved.
So far, fluorescent sensor is considered as one of the most useful technology to detect Hg2+ owning to its high sensitivity, good selectivity, low cost and simplicity [12–14]. For instance, Zhang's group [15] prepared nitrogen-doped graphene quantum dot to detect Hg2+ with sensitivity and selectivity. Rezaei et al. [16] developed water-soluble N-acetyl-L-cysteine-capped CdTe quantum dots to detect Hg2+ with a low limit of detection. Nevertheless, the single-emission fluorescence approach generally has some limitations in terms of sensitivity and resolution [17–19]. In order to eliminate those negative factors and acquire precise results, ratiometric fluorescence assays have received considerable attention nowadays. Ratiometric fluorescent method, which detect the analyte via measuring the change of the ratios of photoluminescence (PL) intensities by two different emission wavelengths, could provide a correction to the background interference, such as temperature, pH, concentration of probe, and so on [20,21]. Most of ratiometric probes were fluorescence organic dyes, which were susceptible to photobleaching, and suffered from low fluorescence quantum yield and the complexity of synthesis and purification [22]. As emerging photoluminescence materials, both carbon dots (CDs) and gold nanoclusters (Au NCs) presented some unique advantages compared with those fluorescence dyes, such as satisfactory water solubility, good biocompatibility, great optical properties and low toxicity, which
2
J. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 223 (2019) 117364
have proved to be promising candidates in construction of ratiometric fluorescent sensor [23–25]. Recently, some ratiometric fluorescent materials have been constructed by combining CDs with Au NCs. Yu's group reported a multicolorful fluorescent-nanoprobe assembled by nano-sized Bovine serum albumin (BSA)\\Au NCs and CDs, which can simultaneously detect Hg2+ and Cr6+ [1]. Xie's group synthesized a dual-emission fluorescent nanohybrid by attaching Au NCs to the BSACDs template for sensitive determination of Hg2+ [26]. However, these applications for detecting Hg2+ are currently limited by two main factors: (1) the selectivity of the probe was disappointing, which would be disturbed by one or more metal ions, including Cr3+, Ni2+, Co2+, Fe2+; (2) the fluorescence color variations with the target was not obvious enough. Thus, it is highly desirable to construct a ratiometric fluorescent sensor capable of efficient detection of Hg2+ with high selectivity and clear visualization. Herein we designed a simple, green and reliable ratiometric fluorescent nanomixture, which showed dual-emission for the visual detection of Hg2+. The ratiometric nanomixture was prepared by simply mixing CDs and Au NCs together. In the presence of Hg2+ in the sample, the PL intensity of Au NCs at 665 nm could be quenched, while the PL intensity of the CDs remained constant. The fluorescence of Au NCs was quenched gradually with the increase of Hg2+ concentration, leading the fluorescence color varied from pink to blue which could be clearly recognize by the naked eye. This ratiometric fluorescent sensor could detect Hg2+ accurately and possess a great sensitivity with the detection limit of 63 nM. Furthermore, this method was applied real water samples for Hg2+ detection. Compared with other reported fluorescence sensor for detecting Hg2+, this sensor exhibited a high selectivity for Hg2+ against most potential interfering metal ions and more obviously fluorescence color change for the determination of Hg2+ with the naked eye. 2. Material and method 2.1. Chemicals
yellow solution was cooled to room temperature and then centrifuged at 12,000 rpm for 10 min to remove large or agglomerated particles. Afterwards, the supernatant was dialyzed against redistilled water in a dialysis membrane (2000 MW CO) for 24 h. The obtained CDs solution was stored at 4 °C for further use. The Au NCs was prepared according to reported literature with a slight modification [28]. Firstly, CEW was obtained by freeze-drying the white component of chicken egg, which was then used for the next step without any further purification. A solution of HAuCl4 (10 mM, 5 mL) was mixed with CEW solution (50 mg/mL, 5 mL), the obtained mixture solution was stirred at 37 °C for 10 min. The pH of the solution was adjusted to 12 using NaOH solution (1 M). Then the mixture was incubated at 37 °C overnight in dark without any disturbance. Finally, the obtained Au NCs solution was stored at 4 °C for future use. The ratiometric fluorescent-nanomixture CDs-Au NCs was prepared by mixing CDs and Au NCs at a certain ratio of 1:1, which showed pink color. In order to achieve excellent analysis, the nanomixture was diluted 4 times. 2.4. Fluorescence assay of Hg2+ Under excitation (λex= 390 nm), the CDs-Au NCs presented two emissions peak at 450 nm and 665 nm respectively, where the peak at 450 nm was attributed to the emission of CDs, and the emission at 665 nm was belong to Au NCs. The Au NCs-CDs showed pink emission under UV light (λ = 365 nm), which was a mix color of those of CDs and Au NCs. For the determination of Hg2+, 100 μL different concentration of Hg2 + (0.1–100 μM) was added to 300 μL ratiometric fluorescentnanomixture CDs-Au NCs. The selectivity for Hg2+ was further tested by measuring fluorescent responsiveness of the nanomixture to other metal ions (including Zn2+, Pb2+, Mn2+, Mg2+, K+, Fe3+, Cu2+, Cr3+, Ca2+, Ba2+, Al3+, Fe2+, Cd2+, Ni2+, Co2+) at 200 μM in a same way. All experiments were repeated three times to ensure the stability of the experimental data. 2.5. Practicability of ratiometric fluorescent nanomixture (CDs-Au NCs)
All chemicals and materials were used directly without further purification. Disodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4) and sodium hydroxide (NaOH) were purchased from Guanghua technology Co. Ltd. (Guangdong, China). Hydrogen tetrachloroaurate (HAuCl4) was obtained from sinopharm chemical reagent co., ltd (Shanghai, China). Lysine was purchased from Aladdin Industrial Co. Ltd. (Shanghai, China). The chicken eggs were purchased from the local supermarket. Deionized water was used throughout the experiments.
The lake water and tap water were utilized to evaluate the possibility and reliability of the proposed method [29]. The lake water samples were obtained from West Lake of Northwest A&F University, and then filtered to remove the impurities using 0.22 μm membranes. The tap water samples were obtained from the residential water without further purification. Then different concentration of Hg2+ (1, 15 and 30 μM) were prepared by spiking real water with a standard solution.
2.2. Instrumentation
3. Results and discussion
All fluorescence measurements were performed on a Horiba FluoroMax-4 spectrofluorometer (Horiba，America) with excitation and emission slit set at 5 nm band pass in 10 mm × 2 mm quartz cell. UV–vis absorption spectra were recorded on Shimadzu UV-2550 spectrophotometer (Shimadzu, Japan). The morphology and size of the CDs-Au NCs were characterized by high-resolution transmission electron microscopy (HR-TEM) using a JEM-2100plus microscope operated at an accelerating voltage of 200 kV (JEOL, Japan). Elemental and functional groups analysis were obtained by a ESCALAB 250 spectrometer (Thermo Fisher Scientific, USA). Fourier transform infrared (FT-IR) spectroscopy was obtained from a Vetex 70 FT-IR spectroscopy (Bruker Corp, Germany).
3.1. Sensor design
2.3. Synthesis of ratiometric fluorescent nanomixture (CDs-Au NCs) The CDs were synthesized via classical one-pot hydrothermal method [27]. Briefly, lysine solution (10 mM, 30 mL) was heated hydrothermally in a stainless steel autoclave at 200 °C for 6 h. The resulting
The working principle of the ratiometric fluorescent sensor was shown in Scheme 1. The sensor was constructed by simply mixing CDs and Au NCs together. As the reference signal, CDs were synthesized by one-pot hydrothermal step using lysine as the precursor. Meanwhile, the CEW were used as the reducer and stabilizer for preparing Au NCs, which worked as a report signal. The obtained ratiometric fluorescent nanomixture possessed two emission peaks under a single excitation wavelength. In the presence of Hg2+ in the sample, the fluorescence of Au NCs at 665 nm could be quenched, while the fluorescence of the CDs remained constant. According to the reported literature [30–32], dispersion forces between closed-shell metal atoms are highly strong, especially when it involved heavy metal ions such as Hg2+ (4f14 5d10) and Au+ (4f14 5d10). The metallophilic bond would quench the fluorescence of Au NCs. In addition, the Hg2+-Au+ metallophilic bond showed high specificity and affinity which permitted establishing a highly sensitive and selective sensor for Hg2+. Under UV lamp, the fluorescence of
J. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 223 (2019) 117364
3
Scheme 1. Illustration of the synthetic process of CDs-Au NCs and Hg2+ sensing.
Au NCs was quenched gradually with the increase of Hg2+, leading to the fluorescence color change from pink to blue which could be clearly recognized by naked eye. Compared with the reported dual-emission fluorescence probes [33–35], this synthetic process developed here was simple, environmentally friendly without the requirement of other coupling agents. 3.2. The synthesis and characterization of ratiometric fluorescent nanomixture (CDs-Au NCs) For the synthesis of CDs, lysine was chosen as the precursor because of its environmental-friendly feature and rich in multifunctional groups
(\\COOH, \\NH2), providing both carbon and nitrogen sources for the formation of fluorescent CDs. Au NCs was synthesized according to the previous reports, which used CEW as template and stabilizer. CEW contained rich source of proteins which were used to entrap Au3+ and reduce it to form Au NCs. The influence of different coating and reducing components of the CEW has been investigated. From Fig. S1A, it can be concluded that both OVA and lysozyme could be the coating and reducing reagent to synthesize Au NCs, and the fluorescence intensity was comparable to that of CEW-synthesized Au NCs. Among them, OVA is the main protein in CEW which may be the responsible for the formation of Au NCs. As shown in Fig. S1B, different eggs were able to synthesize Au NCs with similar fluorescence properties, demonstrating that
Fig. 1. (A) UV–vis absorption spectra for Au NCs, CDs and CDs-Au NCs. (B) PL emission spectra for Au NCs, CDs and CDs-Au NCs. The inset showed the corresponding fluorescent photograph under a UV lamp irradiation. (C) High-resolution C 1s spectra of CDs-Au NCs. (D) High-resolution Au 4f spectra of CDs-Au NCs.
4
J. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 223 (2019) 117364
the usage of chicken egg white is not susceptible to purchasing source and brand. Furthermore, lysine and chicken egg white are natural source biomass which are extremely cheap, easily available and have less environmental impact. UV–vis absorption spectra and photoluminescence spectra were measured to investigate the photophysical properties of the CDs, Au NCs and CDs-Au NCs. As shown in Fig. 1A, CDs showed obvious absorption peak at 278 nm, which was readily ascribed to n-π* transition of C_O groups on the surface of CDs [36]. The Au NCs only displayed nonsignificant absorption peaks in absorption spectrum, and did not reveal the typical surface plasma resonance (SPR) peak of larger gold nanoparticles [37,38]. CDs-Au NCs nanomixture displayed a dual-emission spectrum with two fluorescence peaks, the one located at 450 nm derived from CDs as well as the other at 665 nm derived from Au NCs (Fig. 1B). Moreover, the two emission maximum were well separated with a wavelength distance of 215 nm, which would facilitate to fabricate a ratiometirc fluorescence probe and will beneficial for visualization of the color changes during detection. The photographs of CDs, Au NCs and CDs-Au NCs under UV light showed blue, red and pink fluorescence, respectively (inset in Fig. 1B). In addition, the fluorescence quantum yields determined for Lys-CDs and CEW-AuNCs were 8.1% and 2.7%, respectively, which were measured using quinine sulfate as reference (Fig. S2). As shown in Fig. S3, there was no obvious spectral overlap region in UV–vis absorption spectra and the fluorescence emission spectra of CDs/Au NCs, which indicated that there was no energy transfer between CDs and Au NCs, and the generated pink fluorescence of the CDs-Au NCs nanomixture was a mix of blue fluorescence of CDs and red fluorescence of Au NCs [39]. The composition of CDs-Au NCs was characterized by X-ray photoelectron spectroscopy (XPS). As revealed in Fig. S4, there were carbon, oxygen, nitrogen, sodium, chloride and gold elements on the surface of CDs-Au NCs. The peaks at 1071.8, 530.9, 399.3, 285.3, 198.1 and 84.4 eV were attributed to Na 1s, O 1s, N 1s, C 1s, Cl 2p, and Au 4f, respectively. In the high resolution C 1s spectrum of the CDs-Au NCs nanomixture (Fig. 1C), the three major peaks at 288.0, 285.9 and 284.8 eV referred to C_O or C_O, C\\O or C\\N, and sp2 graphitic C or sp3 graphitic C, respectively. In the high resolution of Au 4f spectrum (Fig. 1D) showed peaks at 87.7 eV and 83.9 eV, which could be assigned to Au 4 f5/2 and Au 4 f7/2 signals, respectively. There are two peaks corresponding to Au 4 f7/2 located at 84.3 eV and 83.6 eV, demonstrating that Au (I) and Au (0) existed on the surface of CDs-Au NCs nanomixture [40,41]. Notably, FT-IR spectrum of CDs-Au NCs nanomixture illustrated that characteristic –OH, –COOH, –NH2 groups existed on the surface of CDs-Au NCs (Fig. S5), which was in excellent accord with the results from the XPS analysis. The high resolution transmission electron microscope (HR-TEM) image (Fig. 2A) of CDs-Au NCs illustrated the CDs-Au NCs system was prepared with good dispersion without aggregation. It can be seen in Fig. 2B that CDs were mostly amorphous spherical, which was consistent with the previous reported results [42]. Meanwhile, Fig. 2C
revealed that Au NCs was highly crystalized and clear lattice fringes were observed with an interspacing of 0.26 nm, corresponding to the d-spacing of the crystal plane of face centered cubic Au (111) [43]. As shown in the particle size distribution histogram (Fig. S6), the average diameters of CDs and Au NCs were 11 and 6.2 nm, separately. The HRTEM image also implied that CDs-Au NCs showed great stability after assembled, which was formed by simply mixing of the two nanomaterials. To explore the interaction of the two components of the CDs-Au NCs, the zeta potential was measured. As can be seen in Fig. S7, the zeta potentials of CDs, Au NCs and the CDs-Au NCs all exhibited negative electric potential and located at −24.3 mV, −21 mV and −26.1 mV, respectively. Thus, there were electrostatic repulsion between the CDs and the Au NCs, and particles did not disturb by other factors during the mixture process. These results explained that CDs-Au NCs were consisted of dispersed CDs and Au NCs, and there was good stability between Au NCs and CDs. 3.3. Optimization of detection conditions Interestingly, the fluorescence of CDs-Au NCs at 665 nm was dramatically decrease once Hg2+ was introduced and there was merely change of the fluorescence at 450 nm (Fig. 3). Meanwhile, Under the UV light, the fluorescent color of CDs-Au NCs changed from pink to blue, when Hg2+ was added into the nanomixture (inset in Fig. 3). In order to obtain high sensitivity of the nanomixture for the Hg2+ detection, some related factors (pH of solution and reaction time) were optimized [44]. As shown in Fig. S8A, when the pH value of the solution changed from 3.8 to 9.8, the PL intensity ratio (I450/I665) of CDs-Au NCs and CDs-Au NCs-Hg2+solution both gradually decreased. At pH 7.4, the changed of PL intensity of CDs-Au NCs system in the presence and absence of Hg2+ reached a maximum. Thus, pH 7.4 PBS buffer (100 mM) was chosen as the optimal pH. In addition, Fig. S8B indicated that the fluorescence response of CDs-Au NCs to Hg2+ was achieved within 7 min, then remained stable. Thus, the optimal reaction time was selected as 7 min. 3.4. Detection of Hg2+ by using CDs-Au NCs nanomixture To verify the fluorescence response to Hg2+, different amounts of Hg2+ were added to CDs-Au NCs system. As shown in Fig. 4A, the PL intensity at 665 nm of CDs-Au NCs was continuously decrease with the concentration of Hg2+ increased, while the emission band at 450 nm displayed no obvious changes. There was a good linear relationship between the PL intensity ratio (I450/I665) and the concentration of Hg2+ in the range of 0.1–90 μM (Fig. 4B). The regression equation was I450/I665 = 0.4783 + 0.125x, where x was the concentration of Hg2+. The corresponding regression coefficient was 0.999, and the detection limit (LOD) for Hg2+ was 63 nM. By research for first-order dynamics, the rate constant of pseudo-first-order kinetics (Kobs) was about 0.398 min-1 based on the slope of the regression equation (Fig. S9),
Fig. 2. (A) HR-TEM image of CDs-Au NCs, red circles and yellow circles represent Au NCs and CDs (B) HR-TEM image of CDs. (C) Crystalline lattices of Au NCs.
J. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 223 (2019) 117364
5
Fig. 3. Fluorescence emission spectra of CDs-Au NCs before and after the addition of 50 μM Hg2+. The inset showed the corresponding fluorescent photograph under a UV lamp irradiation.
meanwhile the dynamic range of detection system was in the range of 1–7 min (Fig. S8B), indicating that the response between CDs-Au NCs and Hg2+ was rapid. Compared with other methods for Hg(II) detection (Table S1), the sensitivity of this system was comparable. Accordingly, continuous photoluminescence color change from pink to blue could be observed under the UV lamp (Fig. 4C), which was available for the visual detection of Hg2+ by naked eye. Compared with reported fluorescence sensor including some ratiometric fluorescence probe for detecting Hg2+, this method displayed more obviously fluorescence color change for the determination of Hg2+.
3.5. Selectivity of the CDs-Au NCs nanomixture for Hg2+ detection Selectivity is an important evaluation parameter for the performance of fluorescence sensor. Thus, to demonstrate the selectivity of the CDs-Au NCs nanomixture for the detection of Hg2+, the change of
Fig. 5. (A) Selectivity of the ratiometric fluorescence system to different metal ions in PBS buffer (100 mM, pH 7.4). The final concentrations of Hg2+ and other meter ions were 50 μM and 200 μM, respectively. (B) Digital photographs of CDs-Au NCs at different metal ions under the UV lamp irradiation.
the ratiometric fluorescence intensity of the CDs-Au NCs system after the addition of other metal ions (including Zn2+, Pb2+, Mn2+, Mg2+, K+, Fe3+, Cu2+, Cr3+, Ca2+, Ba2+, Al3+, Fe2+, Cd2+, Co2+, Ni2+) were monitored (Fig. 5A). Specifically, the PL intensity ratio (I450/I665) of CDs-Au NCs system dramatically increased with the presence of Hg2+, while other ions had negligible interference on the system even at high concentration. The disturbance of potential interfering ions to the probe's ability for detect mercury have been investigated. The result indicated that the system has a great anti-interference ability (Fig. S10). Visually, as shown in Fig. 5B, the fluorescent color of CDs-Au NCs in the presence of Hg2+ changed from pink to blue under the UV light, but the fluorescent color stayed constant when other ions were existed. Therefore, this fluorescence nanomixture was suitable for selective determination of Hg2+.
Fig. 4. (A) Ratiometric fluorescence spectra for the CDs-Au NCs upon addition of different concentrations of Hg2+ in PBS buffer (100 mM) at pH 7.4. (B) Plot of I450/I665 versus Hg2+ concentration in the range of 0.1–100 μM. The plot shows a good linear relationship. (C) Digital photographs of the CDs-Au NCs at different concentrations of Hg2+ under a UV lamp irradiation.
6
J. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 223 (2019) 117364
3.6. Possible quenching mechanism of the fluorescence sensor
Acknowledgements
In order to evaluate the mechanism of the CDs-Au NCs nanomixture for Hg2+ detection, the quenching effect can be expressed well by SternVolmer's Eq. (1):
The authors gratefully acknowledge the financial supports of the Fundamental Research Funds for the Northwest A&F University (Nos. Z111021601) and Talented Program (A279021724).
F0 ¼ 1 þ K SV ½Q ¼ 1 þ K q τ0½Q F
Appendix A. Supplementary data
ð1Þ
Where F0 and F represent the fluorescence intensities of CEW-AuNCs in the absence and presence of Hg2+, respectively. KSV is the quenching constant, and [Q] is the concentration of Hg2+. Kq is the quenching rate constant and τ0 is the fluorescence lifetime of CEW-Au NCs. As shown in Fig. S11, I/I0 showed a linear relationship with the concentration of Hg2 + , the quenching constant were about 0.2358 μM−1 based on the slope of the regression equation at 25 °C. The average lifetime of the fluorescence molecule (τ0) was 10−8 s, so according to the formula (1), Kq was calculated to be 2.36 × 107 M−1 s−1, which was lower than the maximum value for dynamic quenching effect (1.0 × 1010 M−1 s−1). This result indicated that the fluorescence quenching might be ascribed to dynamic quenching effect. In order to further verify the mechanism of the system, the SternVolmer plots at different temperature have been tested. In this experiment condition, quenching constant KSV increased as the temperature was raised (Fig. S11). This result provided a support for the above speculation. Above all, the fluorescence quenching might be ascribed to dynamic quenching effect. 3.7. Determination of Hg2+ in real samples The applicability of the CDs-Au NCs were demonstrated by detecting Hg2+ in two water samples, including lake water and tap water. The results were shown in Table 1. The water samples were spiked with different amounts of Hg2+ and the emission spectra of CDs-Au NCs were recorded. The recoveries of the real water samples were in the range from 95.1% to 111%, with relative standard deviations (RSDs) from 1.02% to 2.80%, which demonstrated that the proposed method for detecting Hg2+ in real water samples are with high accuracy and precision. 4. Conclusions In conclusion, a dual-emission ratiometric fluorescent nanomixture called CDs-Au NCs has been constructed for the visual detection of Hg2 + . The ratiometric fluorescent nanomixture was composed of Au NCs and CDs, and provided a built-in correction that eliminated the environmental impacts and ensured sensor accuracy. Importantly, lysine and chicken egg white are natural source biomass which are extremely cheap, easily available and have less environmental impact. The whole synthesis process was facile and environmentally friendly. This CDsAu NCs nanomixture enabled simple and fast detection of Hg2+, with great sensitivity and excellent selectivity, and also provided a highquality visible platform for monitoring Hg2+ under UV lamp. We believe that this kind of ratiometric fluorescent strategy could be extended to other chemo/biosensors because its great optical properties and excellent biocompatibility. Table 1 Recovery text and precision of the analysis of Hg2+ in spiked real water samples. Samples Lake water
Tap water
number
Spiked (μM)
Found (μM)
Recovery(%)
RSD(%)
1 2 3 1 2 3
1 15 30 1 15 30
1.08 14.31 29.85 1.11 14.66 28.53
108 95.4 99.5 111 97.7 95.1
1.02 2.80 1.12 2.60 1.81 1.02
Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2019.117364.
References [1] L. Yu, L. Zhang, G. Ren, S. Li, B. Zhu, F. Chai, F. Qu, C. Wang, Z. Su, Multicolorful fluorescent-nanoprobe composed of Au nanocluster and carbon dots for colorimetric and fluorescent sensing Hg2+ and Cr6+, Sensors Actuators B Chem. 262 (2018) 678–686. [2] X. Li, Y. Zhang, Y. Chang, B. Xue, X. Kong, C. Wei, Catalysis-reduction strategy for sensing inorganic and organic mercury based on gold nanoparticles, Biosens. Bioelectron. 92 (2016) 328. [3] Y. Qi, F.R. Xiu, G. Yu, L. Huang, B. Li, Simple and rapid chemiluminescence aptasensor for Hg(2+) in contaminated samples: a new signal amplification mechanism, Biosens. Bioelectron. 87 (2017) 439–446. [4] E.M. Nolan, S.J. Lippard, Tools and tactics for the optical detection of mercuric ion, Chem. Rev. 39 (50) (2008) 3443–3480. [5] M. Saleem, M. Rafiq, M. Hanif, Organic Material Based Fluorescent Sensor for Hg2+: A Brief Review on Recent Development, Springer International Publishing, 2017. [6] S. Zhang, D. Zhang, X. Zhang, D. Shang, Z. Xue, D. Shan, X.Q. Lu, Ultratrace naked-eye colorimetric detection of Hg(2+) in wastewater and serum utilizing mercurystimulated peroxidase mimetic activity of reduced graphene oxide-PEI-Pd nanohybrids, Anal. Chem. 89 (6) (2017) 3538. [7] I. Serafimovski, I. Karadjova, T. Stafilov, J. Cvetković, Determination of inorganic and methylmercury in fish by cold vapor atomic absorption spectrometry and inductively coupled plasma atomic emission spectrometry, Microchem. J. 89 (1) (2008) 42–47. [8] F.X. Han, W.D. Patterson, Y. Xia, B.B.M. Sridhar, Y. Su, Rapid determination of mercury in plant and soil samples using inductively coupled plasma atomic emission spectroscopy, a comparative study, Water Air Soil Pollut. 170 (1–4) (2006) 161–171. [9] M. Leermakers, W. Baeyens, P. Quevauviller, M. Horvat, Mercury in environmental samples: speciation, artifacts and validation, TrAC Trends Anal. Chem. 24 (5) (2005) 383–393. [10] V. Arancibia, M. Valderrama, K. Silva, T. Tapia, Determination of chromium in urine samples by complexation-supercritical fluid extraction and liquid or gas chromatography, Journal of Chromatography B Analytical Technologies in the Biomedical & Life Sciences 785 (2) (2003) 303. [11] H. Cheng, C. Wu, J. Liu, Z. Xu, Thiol-functionalized silica microspheres for online preconcentration and determination of mercury species in seawater by high performance liquid chromatography and inductively coupled plasma mass spectrometry, RSC Adv. 5 (25) (2015) 19082–19090. [12] G. Chen, Z. Guo, G. Zeng, L. Tang, Fluorescent and colorimetric sensors for environmental mercury detection, Analyst 140 (16) (2015) 5400. [13] F. Yan, D. Cao, M. Wang, N. Yang, Q. Yu, L. Dai, L. Chen, A new rhodamine-based "offon" fluorescent chemosensor for Hg (II) ion and its application in imaging Hg (II) in living cells, J. Fluoresc. 22 (5) (2012) 1249–1256. [14] Q. Niu, X. Wu, S. Zhang, T. Li, Y. Cui, X. Li, A highly selective and sensitive fluorescent sensor for the rapid detection of Hg(2+) based on phenylamine-oligothiophene derivative, Spectrochimica Acta Part A Molecular & Biomolecular Spectroscopy 153 (2016) 143. [15] R. Zhang, W. Chen, Nitrogen-doped carbon quantum dots: facile synthesis and application as a "turn-off" fluorescent probe for detection of Hg2+ ions, Biosens. Bioelectron. 55 (2014) 83–90. [16] B. Rezaei, M. Shahshahanipour, A.A. Ensafi, H. Farrokhpour, Development of highly selective and sensitive fluorimetric label-free mercury aptasensor based on cysteamine@CdTe/ZnS quantum dots, experimental and theoretical investigation, Sensors & Actuators B Chemical 247 (2017) 400–407. [17] Y. Zhan, F. Luo, L. Guo, B. Qiu, Y. Lin, J. Li, G. Chen, Z. Lin, Preparation of an efficient ratiometric fluorescent nanoprobe (m-CDs@[Ru(bpy)3](2+)) for visual and specific detection of hypochlorite on site and in living cells, Acs Sensors 2 (11) (2017) 1684. [18] K. Li, Q. Feng, G. Niu, W. Zhang, Y. Li, M. Kang, K. Xu, J. He, H. Hou, B.Z. Tang, Benzothiazole-based aiegen with tunable excited-state intramolecular proton transfer and restricted intramolecular rotation processes for highly sensitive physiological pH sensing, Acs Sens 3 (5) (2018) 920–928. [19] Y. Huang, P. Zhang, M. Gao, F. Zeng, A. Qin, S. Wu, B.Z. Tang, Ratiometric detection and imaging of endogenous hypochlorite in live cells and in vivo achieved by using an aggregation induced emission (AIE)-based nanoprobe, Chem. Commun. 52 (45) (2016) 7288–7291. [20] W. Sun, J. Yu, R. Deng, Y. Rong, B. Fujimoto, C. Wu, H. Zhang, D.T. Chiu, Semiconducting polymer dots doped with europium complexes showing ultranarrow emission and long luminescence lifetime for time-gated cellular imaging, Angew. Chem. Int. Ed. Engl. 125 (43) (2013) 11504–11507.
J. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 223 (2019) 117364 [21] J. Yao, K. Zhang, H. Zhu, F. Ma, M. Sun, H. Yu, J. Sun, S. Wang, Efficient ratiometric fluorescence probe based on dual-emission quantum dots hybrid for on-site determination of copper ions, Anal. Chem. 85 (13) (2013) 6461–6468. [22] M. Hua, S. Bo, Y. Wang, D. Cong, Y. Jiang, J. Yuan, Dual-emissive nanoarchitecture of lanthanide-complex-modified silica particles for in vivo ratiometric time-gated luminescence imaging of hypochlorous acid, Chem. Sci. 8 (1) (2017) 150. [23] Y. Liu, H. Xue, J. Liu, Q. Wang, L. Wang, Carbon quantum dot-based fluorometric nitrite assay by exploiting the oxidation of iron(II) to iron(III), Microchim. Acta 185 (2) (2018) 129. [24] Y. Song, S. Zhu, B. Yang, Bioimaging based on fluorescent carbon dots, RSC Adv. 4 (52) (2014) 27184–27200. [25] M. Cui, Y. Zhao, Q. Song, Synthesis, optical properties and applications of ultra-small luminescent gold nanoclusters, Trends Anal. Chem. 57 (2) (2014) 73–82. [26] H. Xie, J. Dong, J. Duan, G.I.N. Waterhouse, J. Hou, S. Ai, Visual and ratiometric fluorescence detection of Hg2+ based on a dual-emission carbon dots-gold nanoclusters nanohybrid, Sensors Actuators B Chem. 259 (2018) 1082–1089. [27] L. Wang, Y. Yin, A. Jain, H.S. Zhou, Aqueous phase synthesis of highly luminescent, nitrogen-doped carbon dots and their application as bioimaging agents, Langmuir the Acs Journal of Surfaces & Colloids 30 (47) (2014) 14270–14275. [28] X. Yan, H. Li, T. Hu, X. Su, A novel fluorimetric sensing platform for highly sensitive detection of organophosphorus pesticides by using egg white-encapsulated gold nanoclusters, Biosens. Bioelectron. 91 (2017) 232–237. [29] Y. Liu, Q. Wang, S. Guo, P. Jia, Y. Shui, S. Yao, C. Huang, M. Zhang, L. Wang, Highly selective and sensitive fluorescence detection of hydroquinone using novel silicon quantum dots, Sensors and Actuators B-Chemical 275 (2018) 415–421. [30] T. Qing, X. He, D. He, Z. Qing, K. Wang, Y. Lei, T. Liu, P. Tang, Y. Li, Oligonucleotidetemplated rapid formation of fluorescent gold nanoclusters and its application for Hg2+ ions sensing, Talanta 161 (2016) 170–176. [31] Q. Niu, P. Gao, M. Yuan, G. Zhang, Y. Zhou, C. Dong, S. Shuang, Y. Zhang, Development of sensing method for mercury ions and cell imaging based on highly fluorescent gold nanoclusters, Microchem. J. 146 (2019) 1140–1149. [32] J. Xie, Y. Zheng, J.Y. Ying, Highly selective and ultrasensitive detection of Hg2+ based on fluorescence quenching of au nanoclusters by Hg2+-Au+ interactions, Chem. Commun. 46 (6) (2010) 961–963. [33] X. Yu, C.-X. Zhang, L. Zhang, Y.-R. Xue, H.-W. Li, Y. Wu, The construction of a FRET assembly by using gold nanoclusters and carbon dots and their application as a
[34]
[35]
[36]
[37] [38]
[39]
[40]
[41]
[42]
[43]
[44]
7
ratiometric probe for cysteine detection, Sensors Actuators B Chem. 263 (2018) 327–335. Y. Shi, Z. Chen, X. Cheng, Y. Pan, H. Zhang, Z. Zhang, C.W. Li, C. Yi, A novel dualemission ratiometric fluorescent nanoprobe for sensing and intracellular imaging of Zn2+, Biosens. Bioelectron. 61 (1) (2014) 397–403. X. Wang, J. Yu, X. Wu, J. Fu, Q. Kang, D. Shen, J. Li, L. Chen, A molecular imprintingbased turn-on ratiometric fluorescence sensor for highly selective and sensitive detection of 2,4-dichlorophenoxyacetic acid (2,4-D), Biosens. Bioelectron. 81 (2016) 438–444. D. Qu, M. Zheng, P. Du, Y. Zhou, L. Zhang, D. Li, H. Tan, Z. Zhao, Z. Xie, Z. Sun, Highly luminescent S, N co-doped graphene quantum dots with broad visible absorption bands for visible light photocatalysts, Nanoscale 5 (24) (2013) 12272–12277. N.R. Jana, L. Gearheart, C.J. Murphy, Seeding growth for size control of 5−40 nm diameter gold nanoparticles, Langmuir 17 (22) (2001) 6782–6786. Z. Chen, Y. Huang, X. Li, T. Zhou, H. Ma, H. Qiang, Y. Liu, Colorimetric detection of potassium ions using aptamer-functionalized gold nanoparticles, Anal. Chim. Acta 787 (2013) 189–192. G. Chen, F. Song, J. Wang, Z. Yang, S. Sun, J. Fan, X. Qiang, X. Wang, B. Dou, X. Peng, FRET spectral unmixing: a ratiometric fluorescent nanoprobe for hypochlorite, Chem. Commun. 48 (24) (2012) 2949–2951. T. Tian, Y. Zhong, C. Deng, H. Wang, Y. He, Y. Ge, G. Song, Brightly near-infrared to blue emission tunable silver-carbon dot nanohybrid for sensing of ascorbic acid and construction of logic gate, Talanta 162 (2017) 135–142. W.J. Niu, D. Shan, R.H. Zhu, S.Y. Deng, S. Cosnier, X.J. Zhang, Dumbbell-shaped carbon quantum dots/AuNCs nanohybrid as an efficient ratiometric fluorescent probe for sensing cadmium (II) ions and l -ascorbic acid, Carbon 96 (2016) 1034–1042. J. Wang, C.F. Wang, S. Chen, Amphiphilic egg-derived carbon dots: rapid plasma fabrication, pyrolysis process, and multicolor printing patterns, Angew. Chem. Int. Ed. 51 (37) (2012) 9297–9301. J. Sun, F. Yang, D. Zhao, X. Yang, Highly sensitive real-time assay of inorganic pyrophosphatase activity based on the fluorescent gold nanoclusters, Anal. Chem. 86 (15) (2014) 7883–7889. X. Yan, H. Li, W. Zheng, X. Su, Visual and fluorescent detection of tyrosinase activity by using a dual-emission ratiometric fluorescence probe, Anal. Chem. 87 (17) (2015) 8904–8909.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Visual and fluorescent detection of mercury ions using a dual-emission ratiometric fluorescence nanomixture of carbon dots cooperating with gold nanoclusters Jinghan Liu, Hanyue Xue, Yingnan Liu, Tong Bu, Pei Jia, Yuhang Shui, Li Wang ⁎ College of Food Science and Engineering, Northwest A&F University, Yangling, 712100, Shaanxi, China
a r t i c l e
i n f o
Article history: Received 22 January 2019 Received in revised form 16 June 2019 Accepted 7 July 2019 Available online 08 July 2019 Keywords: Ratiometric fluorescent sensor Carbon dots Gold nanoclusters Mercury (II) ions
a b s t r a c t Mercury (II) ions (Hg2+), as one of the most toxic heavy metals, can cause irreversible damage to human health even at very low concentration due to its high toxicity and bioaccumulation. Herein, a facile ratiometric fluorescence nanomixture based on carbon dots gold nanoclusters (CDs-Au NCs) was constructed for quantitative detection of Hg2+. Lysine functionalized carbon dots (CDs) were prepared by one-pot hydrothermal method, while gold nanoclusters (Au NCs) were synthesized via using chicken egg white (CEW) as reducer and stabilizer. The novel nanomixture exhibited two strong emission peaks at 450 nm and 665 nm under 390 nm excitation, and showed pink fluorescence under UV light. Interestingly, the fluorescence of the CDs-Au NCs nanomixture was selectively response to Hg2+. The fluorescence of Au NCs at 665 nm was decreased when Hg2+ was presented in the solution, while the fluorescence of CDs at 450 nm stayed constant. The fluorescence color changed from pink to blue obviously with increasing the concentration of Hg2+, which indicated that CDs-Au NCs could be used for visual detection Hg2+ by the naked eye. Under optimal conditions, this ratiometric fluorescent sensor could detect Hg2+ accurately and possess a great sensitivity with a detection limit of 63 nM. In addition, this method was applied to detect Hg2+ in real water samples with great recoveries, suggesting its potential in practical application with simplicity, environmentally friendly and low cost. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal ions are environmental pollutants and badly harmful to human health, especially mercury (II) ions (Hg2+). Unlike organic pollutants, Hg2+ is difficult to decomposed by biodegradation and could continue to accumulate in organisms and get into the food chains, causing seriously harm to creatures and human beings [1–3]. It can trigger long-term irreversible damage to human health due to its adverse effects on the vital organs and tissues even at ultratrace level [4–6]. Thus, it is an urgent demand to monitor the content level of Hg2+ in environment water and food samples. Until now, various methods have been developed for Hg2+ detection, such as atomic absorption/emission spectroscopy [7,8], mass spectroscopy (MS) [9], gas chromatography (GC) [10], high performance liquid chromatography (HPLC) [11] and so on. Although these approaches offer good performance to detect Hg2+ in most cases, the requirement of expensive instruments, complex sample processing procedure and high cost of analysis limit their wide applications. Accordingly, it is great important to develop a simple and low-cost method to detect Hg2+ with high sensitivity and selectivity.
⁎ Corresponding author. E-mail address: [email protected] (L. Wang).
https://doi.org/10.1016/j.saa.2019.117364 1386-1425/© 2019 Elsevier B.V. All rights reserved.
So far, fluorescent sensor is considered as one of the most useful technology to detect Hg2+ owning to its high sensitivity, good selectivity, low cost and simplicity [12–14]. For instance, Zhang's group [15] prepared nitrogen-doped graphene quantum dot to detect Hg2+ with sensitivity and selectivity. Rezaei et al. [16] developed water-soluble N-acetyl-L-cysteine-capped CdTe quantum dots to detect Hg2+ with a low limit of detection. Nevertheless, the single-emission fluorescence approach generally has some limitations in terms of sensitivity and resolution [17–19]. In order to eliminate those negative factors and acquire precise results, ratiometric fluorescence assays have received considerable attention nowadays. Ratiometric fluorescent method, which detect the analyte via measuring the change of the ratios of photoluminescence (PL) intensities by two different emission wavelengths, could provide a correction to the background interference, such as temperature, pH, concentration of probe, and so on [20,21]. Most of ratiometric probes were fluorescence organic dyes, which were susceptible to photobleaching, and suffered from low fluorescence quantum yield and the complexity of synthesis and purification [22]. As emerging photoluminescence materials, both carbon dots (CDs) and gold nanoclusters (Au NCs) presented some unique advantages compared with those fluorescence dyes, such as satisfactory water solubility, good biocompatibility, great optical properties and low toxicity, which
2
J. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 223 (2019) 117364
have proved to be promising candidates in construction of ratiometric fluorescent sensor [23–25]. Recently, some ratiometric fluorescent materials have been constructed by combining CDs with Au NCs. Yu's group reported a multicolorful fluorescent-nanoprobe assembled by nano-sized Bovine serum albumin (BSA)\\Au NCs and CDs, which can simultaneously detect Hg2+ and Cr6+ [1]. Xie's group synthesized a dual-emission fluorescent nanohybrid by attaching Au NCs to the BSACDs template for sensitive determination of Hg2+ [26]. However, these applications for detecting Hg2+ are currently limited by two main factors: (1) the selectivity of the probe was disappointing, which would be disturbed by one or more metal ions, including Cr3+, Ni2+, Co2+, Fe2+; (2) the fluorescence color variations with the target was not obvious enough. Thus, it is highly desirable to construct a ratiometric fluorescent sensor capable of efficient detection of Hg2+ with high selectivity and clear visualization. Herein we designed a simple, green and reliable ratiometric fluorescent nanomixture, which showed dual-emission for the visual detection of Hg2+. The ratiometric nanomixture was prepared by simply mixing CDs and Au NCs together. In the presence of Hg2+ in the sample, the PL intensity of Au NCs at 665 nm could be quenched, while the PL intensity of the CDs remained constant. The fluorescence of Au NCs was quenched gradually with the increase of Hg2+ concentration, leading the fluorescence color varied from pink to blue which could be clearly recognize by the naked eye. This ratiometric fluorescent sensor could detect Hg2+ accurately and possess a great sensitivity with the detection limit of 63 nM. Furthermore, this method was applied real water samples for Hg2+ detection. Compared with other reported fluorescence sensor for detecting Hg2+, this sensor exhibited a high selectivity for Hg2+ against most potential interfering metal ions and more obviously fluorescence color change for the determination of Hg2+ with the naked eye. 2. Material and method 2.1. Chemicals
yellow solution was cooled to room temperature and then centrifuged at 12,000 rpm for 10 min to remove large or agglomerated particles. Afterwards, the supernatant was dialyzed against redistilled water in a dialysis membrane (2000 MW CO) for 24 h. The obtained CDs solution was stored at 4 °C for further use. The Au NCs was prepared according to reported literature with a slight modification [28]. Firstly, CEW was obtained by freeze-drying the white component of chicken egg, which was then used for the next step without any further purification. A solution of HAuCl4 (10 mM, 5 mL) was mixed with CEW solution (50 mg/mL, 5 mL), the obtained mixture solution was stirred at 37 °C for 10 min. The pH of the solution was adjusted to 12 using NaOH solution (1 M). Then the mixture was incubated at 37 °C overnight in dark without any disturbance. Finally, the obtained Au NCs solution was stored at 4 °C for future use. The ratiometric fluorescent-nanomixture CDs-Au NCs was prepared by mixing CDs and Au NCs at a certain ratio of 1:1, which showed pink color. In order to achieve excellent analysis, the nanomixture was diluted 4 times. 2.4. Fluorescence assay of Hg2+ Under excitation (λex= 390 nm), the CDs-Au NCs presented two emissions peak at 450 nm and 665 nm respectively, where the peak at 450 nm was attributed to the emission of CDs, and the emission at 665 nm was belong to Au NCs. The Au NCs-CDs showed pink emission under UV light (λ = 365 nm), which was a mix color of those of CDs and Au NCs. For the determination of Hg2+, 100 μL different concentration of Hg2 + (0.1–100 μM) was added to 300 μL ratiometric fluorescentnanomixture CDs-Au NCs. The selectivity for Hg2+ was further tested by measuring fluorescent responsiveness of the nanomixture to other metal ions (including Zn2+, Pb2+, Mn2+, Mg2+, K+, Fe3+, Cu2+, Cr3+, Ca2+, Ba2+, Al3+, Fe2+, Cd2+, Ni2+, Co2+) at 200 μM in a same way. All experiments were repeated three times to ensure the stability of the experimental data. 2.5. Practicability of ratiometric fluorescent nanomixture (CDs-Au NCs)
All chemicals and materials were used directly without further purification. Disodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4) and sodium hydroxide (NaOH) were purchased from Guanghua technology Co. Ltd. (Guangdong, China). Hydrogen tetrachloroaurate (HAuCl4) was obtained from sinopharm chemical reagent co., ltd (Shanghai, China). Lysine was purchased from Aladdin Industrial Co. Ltd. (Shanghai, China). The chicken eggs were purchased from the local supermarket. Deionized water was used throughout the experiments.
The lake water and tap water were utilized to evaluate the possibility and reliability of the proposed method [29]. The lake water samples were obtained from West Lake of Northwest A&F University, and then filtered to remove the impurities using 0.22 μm membranes. The tap water samples were obtained from the residential water without further purification. Then different concentration of Hg2+ (1, 15 and 30 μM) were prepared by spiking real water with a standard solution.
2.2. Instrumentation
3. Results and discussion
All fluorescence measurements were performed on a Horiba FluoroMax-4 spectrofluorometer (Horiba，America) with excitation and emission slit set at 5 nm band pass in 10 mm × 2 mm quartz cell. UV–vis absorption spectra were recorded on Shimadzu UV-2550 spectrophotometer (Shimadzu, Japan). The morphology and size of the CDs-Au NCs were characterized by high-resolution transmission electron microscopy (HR-TEM) using a JEM-2100plus microscope operated at an accelerating voltage of 200 kV (JEOL, Japan). Elemental and functional groups analysis were obtained by a ESCALAB 250 spectrometer (Thermo Fisher Scientific, USA). Fourier transform infrared (FT-IR) spectroscopy was obtained from a Vetex 70 FT-IR spectroscopy (Bruker Corp, Germany).
3.1. Sensor design
2.3. Synthesis of ratiometric fluorescent nanomixture (CDs-Au NCs) The CDs were synthesized via classical one-pot hydrothermal method [27]. Briefly, lysine solution (10 mM, 30 mL) was heated hydrothermally in a stainless steel autoclave at 200 °C for 6 h. The resulting
The working principle of the ratiometric fluorescent sensor was shown in Scheme 1. The sensor was constructed by simply mixing CDs and Au NCs together. As the reference signal, CDs were synthesized by one-pot hydrothermal step using lysine as the precursor. Meanwhile, the CEW were used as the reducer and stabilizer for preparing Au NCs, which worked as a report signal. The obtained ratiometric fluorescent nanomixture possessed two emission peaks under a single excitation wavelength. In the presence of Hg2+ in the sample, the fluorescence of Au NCs at 665 nm could be quenched, while the fluorescence of the CDs remained constant. According to the reported literature [30–32], dispersion forces between closed-shell metal atoms are highly strong, especially when it involved heavy metal ions such as Hg2+ (4f14 5d10) and Au+ (4f14 5d10). The metallophilic bond would quench the fluorescence of Au NCs. In addition, the Hg2+-Au+ metallophilic bond showed high specificity and affinity which permitted establishing a highly sensitive and selective sensor for Hg2+. Under UV lamp, the fluorescence of
J. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 223 (2019) 117364
3
Scheme 1. Illustration of the synthetic process of CDs-Au NCs and Hg2+ sensing.
Au NCs was quenched gradually with the increase of Hg2+, leading to the fluorescence color change from pink to blue which could be clearly recognized by naked eye. Compared with the reported dual-emission fluorescence probes [33–35], this synthetic process developed here was simple, environmentally friendly without the requirement of other coupling agents. 3.2. The synthesis and characterization of ratiometric fluorescent nanomixture (CDs-Au NCs) For the synthesis of CDs, lysine was chosen as the precursor because of its environmental-friendly feature and rich in multifunctional groups
(\\COOH, \\NH2), providing both carbon and nitrogen sources for the formation of fluorescent CDs. Au NCs was synthesized according to the previous reports, which used CEW as template and stabilizer. CEW contained rich source of proteins which were used to entrap Au3+ and reduce it to form Au NCs. The influence of different coating and reducing components of the CEW has been investigated. From Fig. S1A, it can be concluded that both OVA and lysozyme could be the coating and reducing reagent to synthesize Au NCs, and the fluorescence intensity was comparable to that of CEW-synthesized Au NCs. Among them, OVA is the main protein in CEW which may be the responsible for the formation of Au NCs. As shown in Fig. S1B, different eggs were able to synthesize Au NCs with similar fluorescence properties, demonstrating that
Fig. 1. (A) UV–vis absorption spectra for Au NCs, CDs and CDs-Au NCs. (B) PL emission spectra for Au NCs, CDs and CDs-Au NCs. The inset showed the corresponding fluorescent photograph under a UV lamp irradiation. (C) High-resolution C 1s spectra of CDs-Au NCs. (D) High-resolution Au 4f spectra of CDs-Au NCs.
4
J. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 223 (2019) 117364
the usage of chicken egg white is not susceptible to purchasing source and brand. Furthermore, lysine and chicken egg white are natural source biomass which are extremely cheap, easily available and have less environmental impact. UV–vis absorption spectra and photoluminescence spectra were measured to investigate the photophysical properties of the CDs, Au NCs and CDs-Au NCs. As shown in Fig. 1A, CDs showed obvious absorption peak at 278 nm, which was readily ascribed to n-π* transition of C_O groups on the surface of CDs [36]. The Au NCs only displayed nonsignificant absorption peaks in absorption spectrum, and did not reveal the typical surface plasma resonance (SPR) peak of larger gold nanoparticles [37,38]. CDs-Au NCs nanomixture displayed a dual-emission spectrum with two fluorescence peaks, the one located at 450 nm derived from CDs as well as the other at 665 nm derived from Au NCs (Fig. 1B). Moreover, the two emission maximum were well separated with a wavelength distance of 215 nm, which would facilitate to fabricate a ratiometirc fluorescence probe and will beneficial for visualization of the color changes during detection. The photographs of CDs, Au NCs and CDs-Au NCs under UV light showed blue, red and pink fluorescence, respectively (inset in Fig. 1B). In addition, the fluorescence quantum yields determined for Lys-CDs and CEW-AuNCs were 8.1% and 2.7%, respectively, which were measured using quinine sulfate as reference (Fig. S2). As shown in Fig. S3, there was no obvious spectral overlap region in UV–vis absorption spectra and the fluorescence emission spectra of CDs/Au NCs, which indicated that there was no energy transfer between CDs and Au NCs, and the generated pink fluorescence of the CDs-Au NCs nanomixture was a mix of blue fluorescence of CDs and red fluorescence of Au NCs [39]. The composition of CDs-Au NCs was characterized by X-ray photoelectron spectroscopy (XPS). As revealed in Fig. S4, there were carbon, oxygen, nitrogen, sodium, chloride and gold elements on the surface of CDs-Au NCs. The peaks at 1071.8, 530.9, 399.3, 285.3, 198.1 and 84.4 eV were attributed to Na 1s, O 1s, N 1s, C 1s, Cl 2p, and Au 4f, respectively. In the high resolution C 1s spectrum of the CDs-Au NCs nanomixture (Fig. 1C), the three major peaks at 288.0, 285.9 and 284.8 eV referred to C_O or C_O, C\\O or C\\N, and sp2 graphitic C or sp3 graphitic C, respectively. In the high resolution of Au 4f spectrum (Fig. 1D) showed peaks at 87.7 eV and 83.9 eV, which could be assigned to Au 4 f5/2 and Au 4 f7/2 signals, respectively. There are two peaks corresponding to Au 4 f7/2 located at 84.3 eV and 83.6 eV, demonstrating that Au (I) and Au (0) existed on the surface of CDs-Au NCs nanomixture [40,41]. Notably, FT-IR spectrum of CDs-Au NCs nanomixture illustrated that characteristic –OH, –COOH, –NH2 groups existed on the surface of CDs-Au NCs (Fig. S5), which was in excellent accord with the results from the XPS analysis. The high resolution transmission electron microscope (HR-TEM) image (Fig. 2A) of CDs-Au NCs illustrated the CDs-Au NCs system was prepared with good dispersion without aggregation. It can be seen in Fig. 2B that CDs were mostly amorphous spherical, which was consistent with the previous reported results [42]. Meanwhile, Fig. 2C
revealed that Au NCs was highly crystalized and clear lattice fringes were observed with an interspacing of 0.26 nm, corresponding to the d-spacing of the crystal plane of face centered cubic Au (111) [43]. As shown in the particle size distribution histogram (Fig. S6), the average diameters of CDs and Au NCs were 11 and 6.2 nm, separately. The HRTEM image also implied that CDs-Au NCs showed great stability after assembled, which was formed by simply mixing of the two nanomaterials. To explore the interaction of the two components of the CDs-Au NCs, the zeta potential was measured. As can be seen in Fig. S7, the zeta potentials of CDs, Au NCs and the CDs-Au NCs all exhibited negative electric potential and located at −24.3 mV, −21 mV and −26.1 mV, respectively. Thus, there were electrostatic repulsion between the CDs and the Au NCs, and particles did not disturb by other factors during the mixture process. These results explained that CDs-Au NCs were consisted of dispersed CDs and Au NCs, and there was good stability between Au NCs and CDs. 3.3. Optimization of detection conditions Interestingly, the fluorescence of CDs-Au NCs at 665 nm was dramatically decrease once Hg2+ was introduced and there was merely change of the fluorescence at 450 nm (Fig. 3). Meanwhile, Under the UV light, the fluorescent color of CDs-Au NCs changed from pink to blue, when Hg2+ was added into the nanomixture (inset in Fig. 3). In order to obtain high sensitivity of the nanomixture for the Hg2+ detection, some related factors (pH of solution and reaction time) were optimized [44]. As shown in Fig. S8A, when the pH value of the solution changed from 3.8 to 9.8, the PL intensity ratio (I450/I665) of CDs-Au NCs and CDs-Au NCs-Hg2+solution both gradually decreased. At pH 7.4, the changed of PL intensity of CDs-Au NCs system in the presence and absence of Hg2+ reached a maximum. Thus, pH 7.4 PBS buffer (100 mM) was chosen as the optimal pH. In addition, Fig. S8B indicated that the fluorescence response of CDs-Au NCs to Hg2+ was achieved within 7 min, then remained stable. Thus, the optimal reaction time was selected as 7 min. 3.4. Detection of Hg2+ by using CDs-Au NCs nanomixture To verify the fluorescence response to Hg2+, different amounts of Hg2+ were added to CDs-Au NCs system. As shown in Fig. 4A, the PL intensity at 665 nm of CDs-Au NCs was continuously decrease with the concentration of Hg2+ increased, while the emission band at 450 nm displayed no obvious changes. There was a good linear relationship between the PL intensity ratio (I450/I665) and the concentration of Hg2+ in the range of 0.1–90 μM (Fig. 4B). The regression equation was I450/I665 = 0.4783 + 0.125x, where x was the concentration of Hg2+. The corresponding regression coefficient was 0.999, and the detection limit (LOD) for Hg2+ was 63 nM. By research for first-order dynamics, the rate constant of pseudo-first-order kinetics (Kobs) was about 0.398 min-1 based on the slope of the regression equation (Fig. S9),
Fig. 2. (A) HR-TEM image of CDs-Au NCs, red circles and yellow circles represent Au NCs and CDs (B) HR-TEM image of CDs. (C) Crystalline lattices of Au NCs.
J. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 223 (2019) 117364
5
Fig. 3. Fluorescence emission spectra of CDs-Au NCs before and after the addition of 50 μM Hg2+. The inset showed the corresponding fluorescent photograph under a UV lamp irradiation.
meanwhile the dynamic range of detection system was in the range of 1–7 min (Fig. S8B), indicating that the response between CDs-Au NCs and Hg2+ was rapid. Compared with other methods for Hg(II) detection (Table S1), the sensitivity of this system was comparable. Accordingly, continuous photoluminescence color change from pink to blue could be observed under the UV lamp (Fig. 4C), which was available for the visual detection of Hg2+ by naked eye. Compared with reported fluorescence sensor including some ratiometric fluorescence probe for detecting Hg2+, this method displayed more obviously fluorescence color change for the determination of Hg2+.
3.5. Selectivity of the CDs-Au NCs nanomixture for Hg2+ detection Selectivity is an important evaluation parameter for the performance of fluorescence sensor. Thus, to demonstrate the selectivity of the CDs-Au NCs nanomixture for the detection of Hg2+, the change of
Fig. 5. (A) Selectivity of the ratiometric fluorescence system to different metal ions in PBS buffer (100 mM, pH 7.4). The final concentrations of Hg2+ and other meter ions were 50 μM and 200 μM, respectively. (B) Digital photographs of CDs-Au NCs at different metal ions under the UV lamp irradiation.
the ratiometric fluorescence intensity of the CDs-Au NCs system after the addition of other metal ions (including Zn2+, Pb2+, Mn2+, Mg2+, K+, Fe3+, Cu2+, Cr3+, Ca2+, Ba2+, Al3+, Fe2+, Cd2+, Co2+, Ni2+) were monitored (Fig. 5A). Specifically, the PL intensity ratio (I450/I665) of CDs-Au NCs system dramatically increased with the presence of Hg2+, while other ions had negligible interference on the system even at high concentration. The disturbance of potential interfering ions to the probe's ability for detect mercury have been investigated. The result indicated that the system has a great anti-interference ability (Fig. S10). Visually, as shown in Fig. 5B, the fluorescent color of CDs-Au NCs in the presence of Hg2+ changed from pink to blue under the UV light, but the fluorescent color stayed constant when other ions were existed. Therefore, this fluorescence nanomixture was suitable for selective determination of Hg2+.
Fig. 4. (A) Ratiometric fluorescence spectra for the CDs-Au NCs upon addition of different concentrations of Hg2+ in PBS buffer (100 mM) at pH 7.4. (B) Plot of I450/I665 versus Hg2+ concentration in the range of 0.1–100 μM. The plot shows a good linear relationship. (C) Digital photographs of the CDs-Au NCs at different concentrations of Hg2+ under a UV lamp irradiation.
6
J. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 223 (2019) 117364
3.6. Possible quenching mechanism of the fluorescence sensor
Acknowledgements
In order to evaluate the mechanism of the CDs-Au NCs nanomixture for Hg2+ detection, the quenching effect can be expressed well by SternVolmer's Eq. (1):
The authors gratefully acknowledge the financial supports of the Fundamental Research Funds for the Northwest A&F University (Nos. Z111021601) and Talented Program (A279021724).
F0 ¼ 1 þ K SV ½Q ¼ 1 þ K q τ0½Q F
Appendix A. Supplementary data
ð1Þ
Where F0 and F represent the fluorescence intensities of CEW-AuNCs in the absence and presence of Hg2+, respectively. KSV is the quenching constant, and [Q] is the concentration of Hg2+. Kq is the quenching rate constant and τ0 is the fluorescence lifetime of CEW-Au NCs. As shown in Fig. S11, I/I0 showed a linear relationship with the concentration of Hg2 + , the quenching constant were about 0.2358 μM−1 based on the slope of the regression equation at 25 °C. The average lifetime of the fluorescence molecule (τ0) was 10−8 s, so according to the formula (1), Kq was calculated to be 2.36 × 107 M−1 s−1, which was lower than the maximum value for dynamic quenching effect (1.0 × 1010 M−1 s−1). This result indicated that the fluorescence quenching might be ascribed to dynamic quenching effect. In order to further verify the mechanism of the system, the SternVolmer plots at different temperature have been tested. In this experiment condition, quenching constant KSV increased as the temperature was raised (Fig. S11). This result provided a support for the above speculation. Above all, the fluorescence quenching might be ascribed to dynamic quenching effect. 3.7. Determination of Hg2+ in real samples The applicability of the CDs-Au NCs were demonstrated by detecting Hg2+ in two water samples, including lake water and tap water. The results were shown in Table 1. The water samples were spiked with different amounts of Hg2+ and the emission spectra of CDs-Au NCs were recorded. The recoveries of the real water samples were in the range from 95.1% to 111%, with relative standard deviations (RSDs) from 1.02% to 2.80%, which demonstrated that the proposed method for detecting Hg2+ in real water samples are with high accuracy and precision. 4. Conclusions In conclusion, a dual-emission ratiometric fluorescent nanomixture called CDs-Au NCs has been constructed for the visual detection of Hg2 + . The ratiometric fluorescent nanomixture was composed of Au NCs and CDs, and provided a built-in correction that eliminated the environmental impacts and ensured sensor accuracy. Importantly, lysine and chicken egg white are natural source biomass which are extremely cheap, easily available and have less environmental impact. The whole synthesis process was facile and environmentally friendly. This CDsAu NCs nanomixture enabled simple and fast detection of Hg2+, with great sensitivity and excellent selectivity, and also provided a highquality visible platform for monitoring Hg2+ under UV lamp. We believe that this kind of ratiometric fluorescent strategy could be extended to other chemo/biosensors because its great optical properties and excellent biocompatibility. Table 1 Recovery text and precision of the analysis of Hg2+ in spiked real water samples. Samples Lake water
Tap water
number
Spiked (μM)
Found (μM)
Recovery(%)
RSD(%)
1 2 3 1 2 3
1 15 30 1 15 30
1.08 14.31 29.85 1.11 14.66 28.53
108 95.4 99.5 111 97.7 95.1
1.02 2.80 1.12 2.60 1.81 1.02
Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2019.117364.
References [1] L. Yu, L. Zhang, G. Ren, S. Li, B. Zhu, F. Chai, F. Qu, C. Wang, Z. Su, Multicolorful fluorescent-nanoprobe composed of Au nanocluster and carbon dots for colorimetric and fluorescent sensing Hg2+ and Cr6+, Sensors Actuators B Chem. 262 (2018) 678–686. [2] X. Li, Y. Zhang, Y. Chang, B. Xue, X. Kong, C. Wei, Catalysis-reduction strategy for sensing inorganic and organic mercury based on gold nanoparticles, Biosens. Bioelectron. 92 (2016) 328. [3] Y. Qi, F.R. Xiu, G. Yu, L. Huang, B. Li, Simple and rapid chemiluminescence aptasensor for Hg(2+) in contaminated samples: a new signal amplification mechanism, Biosens. Bioelectron. 87 (2017) 439–446. [4] E.M. Nolan, S.J. Lippard, Tools and tactics for the optical detection of mercuric ion, Chem. Rev. 39 (50) (2008) 3443–3480. [5] M. Saleem, M. Rafiq, M. Hanif, Organic Material Based Fluorescent Sensor for Hg2+: A Brief Review on Recent Development, Springer International Publishing, 2017. [6] S. Zhang, D. Zhang, X. Zhang, D. Shang, Z. Xue, D. Shan, X.Q. Lu, Ultratrace naked-eye colorimetric detection of Hg(2+) in wastewater and serum utilizing mercurystimulated peroxidase mimetic activity of reduced graphene oxide-PEI-Pd nanohybrids, Anal. Chem. 89 (6) (2017) 3538. [7] I. Serafimovski, I. Karadjova, T. Stafilov, J. Cvetković, Determination of inorganic and methylmercury in fish by cold vapor atomic absorption spectrometry and inductively coupled plasma atomic emission spectrometry, Microchem. J. 89 (1) (2008) 42–47. [8] F.X. Han, W.D. Patterson, Y. Xia, B.B.M. Sridhar, Y. Su, Rapid determination of mercury in plant and soil samples using inductively coupled plasma atomic emission spectroscopy, a comparative study, Water Air Soil Pollut. 170 (1–4) (2006) 161–171. [9] M. Leermakers, W. Baeyens, P. Quevauviller, M. Horvat, Mercury in environmental samples: speciation, artifacts and validation, TrAC Trends Anal. Chem. 24 (5) (2005) 383–393. [10] V. Arancibia, M. Valderrama, K. Silva, T. Tapia, Determination of chromium in urine samples by complexation-supercritical fluid extraction and liquid or gas chromatography, Journal of Chromatography B Analytical Technologies in the Biomedical & Life Sciences 785 (2) (2003) 303. [11] H. Cheng, C. Wu, J. Liu, Z. Xu, Thiol-functionalized silica microspheres for online preconcentration and determination of mercury species in seawater by high performance liquid chromatography and inductively coupled plasma mass spectrometry, RSC Adv. 5 (25) (2015) 19082–19090. [12] G. Chen, Z. Guo, G. Zeng, L. Tang, Fluorescent and colorimetric sensors for environmental mercury detection, Analyst 140 (16) (2015) 5400. [13] F. Yan, D. Cao, M. Wang, N. Yang, Q. Yu, L. Dai, L. Chen, A new rhodamine-based "offon" fluorescent chemosensor for Hg (II) ion and its application in imaging Hg (II) in living cells, J. Fluoresc. 22 (5) (2012) 1249–1256. [14] Q. Niu, X. Wu, S. Zhang, T. Li, Y. Cui, X. Li, A highly selective and sensitive fluorescent sensor for the rapid detection of Hg(2+) based on phenylamine-oligothiophene derivative, Spectrochimica Acta Part A Molecular & Biomolecular Spectroscopy 153 (2016) 143. [15] R. Zhang, W. Chen, Nitrogen-doped carbon quantum dots: facile synthesis and application as a "turn-off" fluorescent probe for detection of Hg2+ ions, Biosens. Bioelectron. 55 (2014) 83–90. [16] B. Rezaei, M. Shahshahanipour, A.A. Ensafi, H. Farrokhpour, Development of highly selective and sensitive fluorimetric label-free mercury aptasensor based on cysteamine@CdTe/ZnS quantum dots, experimental and theoretical investigation, Sensors & Actuators B Chemical 247 (2017) 400–407. [17] Y. Zhan, F. Luo, L. Guo, B. Qiu, Y. Lin, J. Li, G. Chen, Z. Lin, Preparation of an efficient ratiometric fluorescent nanoprobe (m-CDs@[Ru(bpy)3](2+)) for visual and specific detection of hypochlorite on site and in living cells, Acs Sensors 2 (11) (2017) 1684. [18] K. Li, Q. Feng, G. Niu, W. Zhang, Y. Li, M. Kang, K. Xu, J. He, H. Hou, B.Z. Tang, Benzothiazole-based aiegen with tunable excited-state intramolecular proton transfer and restricted intramolecular rotation processes for highly sensitive physiological pH sensing, Acs Sens 3 (5) (2018) 920–928. [19] Y. Huang, P. Zhang, M. Gao, F. Zeng, A. Qin, S. Wu, B.Z. Tang, Ratiometric detection and imaging of endogenous hypochlorite in live cells and in vivo achieved by using an aggregation induced emission (AIE)-based nanoprobe, Chem. Commun. 52 (45) (2016) 7288–7291. [20] W. Sun, J. Yu, R. Deng, Y. Rong, B. Fujimoto, C. Wu, H. Zhang, D.T. Chiu, Semiconducting polymer dots doped with europium complexes showing ultranarrow emission and long luminescence lifetime for time-gated cellular imaging, Angew. Chem. Int. Ed. Engl. 125 (43) (2013) 11504–11507.
J. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 223 (2019) 117364 [21] J. Yao, K. Zhang, H. Zhu, F. Ma, M. Sun, H. Yu, J. Sun, S. Wang, Efficient ratiometric fluorescence probe based on dual-emission quantum dots hybrid for on-site determination of copper ions, Anal. Chem. 85 (13) (2013) 6461–6468. [22] M. Hua, S. Bo, Y. Wang, D. Cong, Y. Jiang, J. Yuan, Dual-emissive nanoarchitecture of lanthanide-complex-modified silica particles for in vivo ratiometric time-gated luminescence imaging of hypochlorous acid, Chem. Sci. 8 (1) (2017) 150. [23] Y. Liu, H. Xue, J. Liu, Q. Wang, L. Wang, Carbon quantum dot-based fluorometric nitrite assay by exploiting the oxidation of iron(II) to iron(III), Microchim. Acta 185 (2) (2018) 129. [24] Y. Song, S. Zhu, B. Yang, Bioimaging based on fluorescent carbon dots, RSC Adv. 4 (52) (2014) 27184–27200. [25] M. Cui, Y. Zhao, Q. Song, Synthesis, optical properties and applications of ultra-small luminescent gold nanoclusters, Trends Anal. Chem. 57 (2) (2014) 73–82. [26] H. Xie, J. Dong, J. Duan, G.I.N. Waterhouse, J. Hou, S. Ai, Visual and ratiometric fluorescence detection of Hg2+ based on a dual-emission carbon dots-gold nanoclusters nanohybrid, Sensors Actuators B Chem. 259 (2018) 1082–1089. [27] L. Wang, Y. Yin, A. Jain, H.S. Zhou, Aqueous phase synthesis of highly luminescent, nitrogen-doped carbon dots and their application as bioimaging agents, Langmuir the Acs Journal of Surfaces & Colloids 30 (47) (2014) 14270–14275. [28] X. Yan, H. Li, T. Hu, X. Su, A novel fluorimetric sensing platform for highly sensitive detection of organophosphorus pesticides by using egg white-encapsulated gold nanoclusters, Biosens. Bioelectron. 91 (2017) 232–237. [29] Y. Liu, Q. Wang, S. Guo, P. Jia, Y. Shui, S. Yao, C. Huang, M. Zhang, L. Wang, Highly selective and sensitive fluorescence detection of hydroquinone using novel silicon quantum dots, Sensors and Actuators B-Chemical 275 (2018) 415–421. [30] T. Qing, X. He, D. He, Z. Qing, K. Wang, Y. Lei, T. Liu, P. Tang, Y. Li, Oligonucleotidetemplated rapid formation of fluorescent gold nanoclusters and its application for Hg2+ ions sensing, Talanta 161 (2016) 170–176. [31] Q. Niu, P. Gao, M. Yuan, G. Zhang, Y. Zhou, C. Dong, S. Shuang, Y. Zhang, Development of sensing method for mercury ions and cell imaging based on highly fluorescent gold nanoclusters, Microchem. J. 146 (2019) 1140–1149. [32] J. Xie, Y. Zheng, J.Y. Ying, Highly selective and ultrasensitive detection of Hg2+ based on fluorescence quenching of au nanoclusters by Hg2+-Au+ interactions, Chem. Commun. 46 (6) (2010) 961–963. [33] X. Yu, C.-X. Zhang, L. Zhang, Y.-R. Xue, H.-W. Li, Y. Wu, The construction of a FRET assembly by using gold nanoclusters and carbon dots and their application as a
[34]
[35]
[36]
[37] [38]
[39]
[40]
[41]
[42]
[43]
[44]
7
ratiometric probe for cysteine detection, Sensors Actuators B Chem. 263 (2018) 327–335. Y. Shi, Z. Chen, X. Cheng, Y. Pan, H. Zhang, Z. Zhang, C.W. Li, C. Yi, A novel dualemission ratiometric fluorescent nanoprobe for sensing and intracellular imaging of Zn2+, Biosens. Bioelectron. 61 (1) (2014) 397–403. X. Wang, J. Yu, X. Wu, J. Fu, Q. Kang, D. Shen, J. Li, L. Chen, A molecular imprintingbased turn-on ratiometric fluorescence sensor for highly selective and sensitive detection of 2,4-dichlorophenoxyacetic acid (2,4-D), Biosens. Bioelectron. 81 (2016) 438–444. D. Qu, M. Zheng, P. Du, Y. Zhou, L. Zhang, D. Li, H. Tan, Z. Zhao, Z. Xie, Z. Sun, Highly luminescent S, N co-doped graphene quantum dots with broad visible absorption bands for visible light photocatalysts, Nanoscale 5 (24) (2013) 12272–12277. N.R. Jana, L. Gearheart, C.J. Murphy, Seeding growth for size control of 5−40 nm diameter gold nanoparticles, Langmuir 17 (22) (2001) 6782–6786. Z. Chen, Y. Huang, X. Li, T. Zhou, H. Ma, H. Qiang, Y. Liu, Colorimetric detection of potassium ions using aptamer-functionalized gold nanoparticles, Anal. Chim. Acta 787 (2013) 189–192. G. Chen, F. Song, J. Wang, Z. Yang, S. Sun, J. Fan, X. Qiang, X. Wang, B. Dou, X. Peng, FRET spectral unmixing: a ratiometric fluorescent nanoprobe for hypochlorite, Chem. Commun. 48 (24) (2012) 2949–2951. T. Tian, Y. Zhong, C. Deng, H. Wang, Y. He, Y. Ge, G. Song, Brightly near-infrared to blue emission tunable silver-carbon dot nanohybrid for sensing of ascorbic acid and construction of logic gate, Talanta 162 (2017) 135–142. W.J. Niu, D. Shan, R.H. Zhu, S.Y. Deng, S. Cosnier, X.J. Zhang, Dumbbell-shaped carbon quantum dots/AuNCs nanohybrid as an efficient ratiometric fluorescent probe for sensing cadmium (II) ions and l -ascorbic acid, Carbon 96 (2016) 1034–1042. J. Wang, C.F. Wang, S. Chen, Amphiphilic egg-derived carbon dots: rapid plasma fabrication, pyrolysis process, and multicolor printing patterns, Angew. Chem. Int. Ed. 51 (37) (2012) 9297–9301. J. Sun, F. Yang, D. Zhao, X. Yang, Highly sensitive real-time assay of inorganic pyrophosphatase activity based on the fluorescent gold nanoclusters, Anal. Chem. 86 (15) (2014) 7883–7889. X. Yan, H. Li, W. Zheng, X. Su, Visual and fluorescent detection of tyrosinase activity by using a dual-emission ratiometric fluorescence probe, Anal. Chem. 87 (17) (2015) 8904–8909.