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One-step synthesis and applications of fluorescent Cu nanoclusters stabilized by L-cysteine in aqueous solution Xiaoming Yang * ,1, Yuanjiao Feng 1, Shanshan Zhu, Yawen Luo, Yan Zhuo, Yao Dou College of Pharmaceutical Sciences, Southwest University, Chongqing 400716, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
A novel, one-step strategy for synthesizing water-soluble CuNCs was established. A simple, selective, and cost-effective assay for Hg2+ was developed. CuNCs may broaden ways for fluorescent staining and coding.
An innovative and simple strategy for synthesizing high-fluorescent Cu nanoclusters stabilized with Lcysteine has been successfully established in aqueous solution. Significantly, the Cu nanoclusters were employed for sensitive and selective detections of Hg2+, coding and fluorescent staining, suggesting their potential toward various applications.
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 May 2014 Received in revised form 14 June 2014 Accepted 15 July 2014 Available online xxx
Herein, an innovative and simple strategy for synthesizing high fluorescent Cu nanoclusters was successfully established while L-cysteine played a role as the stabilizer. Meaningfully, the current Cu nanoclusters together with a quantum yield of 14.3% were prepared in aqueous solution, indicating their extensive applications. Subsequently, the possible fluorescence mechanism was elucidated by fluorescence, UV–vis, HR-TEM, FTIR, XPS, and MS. Additionally, the CuNCs were employed for assaying Hg2+ on the basis of the interactions between Hg2+ and L-cysteine; thus facilitating the quenching of their fluorescence. The proposed analytical strategy permitted detections of Hg2+ in a linear range of 1.0 10 7 mol L 1 10 3 mol L 1, with a detection limit of 2.4 10 8 mol L 1 at a signal-to-noise ratio of 3. Significantly, this CuNCs described here were further applied for coding and fluorescent staining, suggesting may broaden avenues toward diverse applications. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Cu nanoclusters L-cysteine One-step synthesis Applications
1. Introduction Noble metal nanoclusters (NCs) usually consist of several to dozens of atoms with properties regulated by their subnanometer dimensions [1–3]. Emerging as a new type of fluorescent nanomaterial, fantastic characteristics of NCs, including satisfactory
* Corresponding author. Tel.: +86 23 68251225; fax: +86 23 68251225. E-mail addresses: [email protected], [email protected] (X. Yang). 1 Both authors contributed equally to this work.
water solubility, high quantum-yield, biocompatibility, excellent stability, and outstanding catalytic properties [4–8], have recently attracted extensive attentions, potentiating it as a novel candidate for biosensing [9,10], biolabeling [11–13] and catalysis [14]. Thus, numerous methods have been developed for synthesizing NCs during the past decades [4,6,12,15–18], especially Au and Ag nanoclusters. Nevertheless, scarce approaches have been reported toward synthesizing Cu nanoclusters (CuNCs), although copper has been widely playing critical roles in various fields as a member of transition metals [19], and CuNCs have exhibited unique fluorescent properties as reported [16,20,21].
http://dx.doi.org/10.1016/j.aca.2014.07.019 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: X. Yang, et al., One-step synthesis and applications of fluorescent Cu nanoclusters stabilized by L-cysteine in aqueous solution, Anal. Chim. Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.07.019
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Currently, there have existed kinds of methods for synthesizing CuNCs [20,22–24]. According to the literatures, polymers such as polyamidoamine dendrimer [23,25,26], serving as a template and regulating sizes and core diameters, have been applied to prepare CuNCs. Besides, another approach for synthesizing water-in-oil CuNCs was proposed to achieve different sizes by controlling the amount of reducing agents [27]. Moreover, the surfactant has been employed to encapsulate copper atoms thus forming CuNCs based on an electrochemical mechanism [28]. Again, one-pot method has been performed according to wet chemical reduction, in which 2-mercapto-5-n-propylpyrimidine played a role as protector ligand [20]. Recently, as described in the literature, CCCYYY amino acid sequence was designed as a self-oxidant reagent to obtain CuNCs as desired in alkaline conditions [13]. However, most of these methods were manipulated in organic solvents [29], leading to CuNCs as prepared showed low-fluorescence and obvious instability, thereby severely restricting their applications. In addition, the above methods have exhibited other drawbacks including tedious steps, toxic reagent, complicated instrumentations and high cost. Therefore, exploration of new strategies for synthesizing stable and highfluorescent CuNCs in aqueous phase are still desired. In this study, an innovative, simple, and economical strategy for preparing water-soluble CuNCs has been well established, while L-cysteine served as a template to encapsulate Cu atoms for the first time (Fig. 1). Meanwhile, the CuNCs here were further employed for preparing fluorescent powder and coding. Overall, the CuNCs as prepared showed obvious advantages such as one-step synthesis procedure in aqueous solution and high quantum yield of 14.3% compared with previous reported (Table S1), suggesting their potentialtobroadenavenuesforbiosensingandfantasticapplications. 2. Experimental 2.1. Chemicals All the ions (Hg2+, K+, Ag+, Zn2+, Pb2+, Eu3+, Al3+, Ca2+, Ba2+, Br , Mg2+, Fe3+, Cd2+, Ni2+, and I ), L-cysteine, and copper sulfate (CuSO4) were obtained from Shanghai Sangon Biotechnonlog Co., Ltd. (Shanghai, China). Sodium borohydride (powder, 98%) was obtained from Sigma–Aldrich (Milwaukee, WI). Disodium hydrogen phosphate (Na2HPO4) and sodium dihydrogen phosphate (NaH2PO4), sodium hydroxide (NaOH), sodium chloride (NaCl) were purchased from Dingguo Changsheng Biotechnology Co., Ltd. (Beijing, China). Ultrapure water, 18.25 MV cm, produced with an Aquapro AWL-0502-P ultrapure water system (Chongqing, China) was employed for all experiments.
2.2. Instrumentation All fluorescence measurements were performed on a Hitachi F-7000 fluorescence spectrophotometer (Tokyo, Japan) with excitation slit set at 5 nm band pass and emission at 5 nm band pass in 1 cm 1 cm quartz cell. Meanwhile, UV–vis absorption spectra were recorded by a Shimadzu UV-2450 spectrophotometer (Tokyo, Japan). The high-resolution transmission electron microscopy (HR-TEM) images were taken using a TECNAI G2 F20 microscope (FEI, USA) at 200 KV. Elemental and functional groups analysis were obtained by ESCALAB 250 X-ray photoelectron spectrometer and Fourier transform infrared spectrometer (Tokyo, Japan), respectively. The quantum yields were obtained by using Absolute PL quantum yield spectrometer C11347 (Hamamatsu, Japan). Mass spectra were obtained by UltrafleXtreme Matrixassisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI TOF/TOF MS, Bruker Daltonics Inc., USA). A Fangzhong pHS-3C digital pH meter (Chengdu, China) was used to measure the pH values of the aqueous solutions and a vortex mixer QL-901 (Haimen, China) was used to blend the solution. The thermostatic water bath (DF-101s) was purchased from Gongyi Instrument Co., Ltd. (Henan, China). 2.3. Synthesis of CuNCs Basically, as a typical experiment, 0.25 mL of CuSO4 solution (1 mM) was added into 2.5 mL of L-cysteine solution (35 mg mL 1) with NaOH (0.4 M). After votexing, this solution was incubated under vigorous stirring at 55 C water bath for 4.5 h. Then, this aqueous solution was filtered with 0.22 mm filter membrane to remove the larger product. Finally, the fluorescent CuNCs were further collected by dialysis against deionized water through a dialysis membrane (1000 MWCO) for 24 h. The CuNCs obtained here were stable for 3 months when stored in the dark at 4 C. 2.4. Detection of Hg2+ Firstly, 50 mL CuNCs and 50 mL PBS buffer (0.1 M, pH 9.0) were pipetted into a 1.5-mL vial. Subsequently, an appropriate volume of Hg2+ working solution or sample solution was added, diluted to 500 mL with Milli-Q purified water and vortex-mixed thoroughly. The mixture was then left to react at 55 C for 10 min and transferred for fluorescence measurements. Finally, interferences originated from other metal ions were investigated individually in the presence of the CuNCs prepared here.
Fig. 1. Schematic illusion of synthesizing water-soluble CuNCs.
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2.5. Preparing urine samples Fresh human urine sample was collected from Southwest University Hospital. For the recovery experiments, impurities of the urine sample were precipitated by appropriate speed of centrifugation (2000 rpm, 10 min). Then, the supernatant was collected into 3 tubes and supplemented with standard Hg2+ solutions (0.8 10 4 M, 1.0 10 4 M, 1.2 10 4 M). 3. Results and discussion 3.1. Characterization of CuNCs To characterize this synthesized CuNCs, the maximum excitation and emission spectra were initially recorded as 375 nm and 480 nm (Fig. 2A) respectively, and the fluorescent properties of the CuNCs solution were subsequently investigated. As shown in Fig. 2A, the CuNCs aqueous solution emitted obvious cyan fluorescence (photograph II) under UV light (365 nm) while appearing as yellowish and transparent under daylight (photograph I). Additionally, the powder of CuNCs as described by lyophilization (photograph IV) showed striking fluorescence under UV light, whereas yellowish was observed under daylight (photograph III). Significantly, the fluorescent powder has been preserved for more than 3 months without marked changes, indicating that it may provide the possibility for matching the requirement of commercial scale. In Fig. 2B, only CuNCs exhibited distinct fluorescence, indicating that the fluorescence was originated from the CuNCs rather than CuSO4 or L-cysteine. To explore the surface groups of CuNCs obtained here, the spectra of Fourier transform infrared spectroscopy (FTIR) of were recorded. As Fig. 2C revealing, disappearance of –SH stretching vibrations at 2511.32 cm 1 suggested that Cu atoms of CuNCs were covered by L-cysteine through Cu S bonding. Subsequently,
3
absorption spectra of CuNCs, L-cysteine, and CuSO4 were displayed in Fig. 2D respectively. Only CuNCs exhibited a weak absorption peak around 375 nm, as well as both L-cysteine and CuSO4 showed no obvious peaks. To further elucidate the nanostructures of CuNCs, a high resolution transmission electron microscope (HR-TEM) was employed to directly observe the morphology and particle size distributions. As shown in Fig. 3A and B, CuNCs prepared here existed as a majority population within the size range of 2–3 nm and no aggregation emerged, depicting their satisfactory dispersity. In addition, the size distribution of CuNCs was obtained from HR-TEM image analysis (Fig. S1). toward the purpose of gaining insight into components of the current CuNCs, XPS survey spectra were performed. As shown in Fig. 3C, six major peaks of S(1s), C(1s), N(1s), Na, O(1s), Cu (2p), and Na(1s) obviously emerged in the spectrum, indicating that CuNCs prepared here were mainly composed of C, O, N, S, Cu, and Na. To fairly describe the peak of Cu, the amplified peak was showed in Fig. 3D. The binding energy position for Cu 2p3/2 and Cu 2p1/2 were located at 932.05 eV and 952.05 eV, indicating that CuNCs were composed of Cu0 and Cu+. Furthermore, there were no peak displayed around 942 eV (Fig. 3D), demonstrating the absence of Cu2+ for CuNCs, which agreed with the reported [20,22]. Next, for determining the Cu atom number of CuNCs, the representative matrix-assisted laser-desorption ionization time of flight mass spectrum (MALDI-TOF MS) were obtained by the positive-ion mode (Fig. 4). As shown in Fig. 4A and B, the dominant m/z peak at 911.065 eV was originated from [Cu4L5 + H + 2Na]+ (L = C3H7O2NS) together with Na adducts. To clarify the components in detail, spectra in the range of m/z = 600–650, 725–745 were further amplified (Fig. 4C and D). The lower peaks located at 616.843 eV and 735.366 eV were ascribed to [Cu4L3–H]+ and [Cu4L4–H]+. This result demonstrated that the CuNCs as synthesized were composed of four copper atoms.
Fig. 2. (A) Flourescence exitation and emission spectra of CuNCs. Inset: photographs of CuNCs solution (I, II) and powder (III, IV); (B) fluorecence spectra of CuNCs, L-cysteine and CuSO4; (C) FTIR spectra of CuNCs and L-cysteine; (D) UV–vis spectra of CuNCs, L-cysteine and CuSO4.
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Fig. 3. (A), (B) HR-TEM images of CuNCs; (C) XPS of CuNCs thus obtained; (D) amplified XPS of Cu 2p electrons.
Fig. 4. (A) Positive mode MALDI-TOF MS of CuNCs; (B), (C), (D) amplified views of peaks originated from [Cu4L5 + H + 2Na]+ (m/z = 911.065), [Cu4L3–H]+ (m/z = 616.843), [Cu4L4– H]+ (m/z = 735.366), respectively.
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Fig. 5. (A) Effect of different metal ions on the fluorescence intensity of CuNCs; (B) fluorescence spectra of CuNCs in the presence of various concentrations of Hg2+. Insert: plot of (F0–F) versus the logarithm of concentrations of Hg2+ introduced; the word “CuNCs” written by using CuNCs as ink under daylight (C) and UV light (D); photograghs of the absorbent cotton and a paper craft stained by the current CuNCs under daylight (E and G) and UV light (F and H).
Taking all the above characteristic data together, a brand-new type of CuNCs were successfully synthesized by a developed novel and simple method. 3.2. Optimization of synthesis conditions Again, to identify the optimized conditions for synthesizing CuNCs, various experiments were performed. As revealed (Fig. S2A, S2B, and S2C), the fluorescent intensities of CuNCs exhibited variations along with varying reaction time, temperature and concentrations of L-cysteine, demonstrating that synthesis of CuNCs were dependent on these selected conditions; thus, 4.5 h, 55 C and 35 mg mL 1 of L-cysteine served as the optimal conditions toward the following experiments. Moreover, to address whether this current condition was the optimal condition or not, other synthesis approaches including high reaction– temperature, NaBH4 replacing of NaOH, and low concentration of Cu2+ without NaOH were further investigated for comparison. As Fig. S2D indicated, the highest-fluorescent CuNCs were obtained only under this optimized condition. 3.3. Stability of CuNCs Ultimately, to identify the stability of CuNCs for bearing circumstances, further experiments were performed. As revealed (Figs. S3 and S4), the fluorescent intensities of CuNCs scarcely exhibited alteration along with time-lapse and varying organic solvents, demonstrating the CuNCs satisfactory stability.
we asked whether the CuNCs described here could potentially serve as a fluorescent probe to detect Hg2+ or not. Toward that purpose, Hg2+ was introduced into the CuNCs as prepared. Fig. S5 showed that fluorescence intensity of the CuNCs at 480 nm decreased to about 50%, when 10 mM Hg2+ was added. Simultaneously, no obvious fluorescence was observed under UV light (photograph II), indicating that the CuNCs may be applied for detecting Hg2+. To determine Hg2+ effectively, conditions including pH, reaction temperature and time were optimized. Considering the speed, easy-operation and selectivity of this assay, pH 9.0, 45 C, and 50 min served as the optimal conditions during the following experiments (Fig. S6). 3.5. Selectivity Next, the selectivity of this analytical strategy was evaluated by testing the response to other ions (Hg2+, K+, Ag+, Zn2+, Pb2+, Eu3+, Al3 + , Ca2+, Ba2+, Br , Mg2+, Fe3+, Cd2+, Ni2+, I , and 100 mM for each) under optimum conditions for the case of 10 mM Hg2+. Fig. 5A revealed that the fluorescence probe responded selectively toward over the other metal ions, indicating the excellent selectivity of the CuNCs for Hg2+ detection. Furthermore, the fluorescent intensity decrease (F0–F) versus the logarithmic plot of Hg2+ concentrations displayed a linear range from 1.0 10 7 mol L 1 to 1.0 10 3 mol L 1 with a correlation coefficient of 0.9598 (Fig. 5B), demonstrating an excellent precision of the fluorescent probe. The detection limit of Hg2+ was 2.4 10 8 mol L 1 at a signal-to-noise ratio of 3. Overall, the results showed a promising method for assaying Hg2+. 3.6. Detection of Hg2+in urine samples
3.4. Assaying Hg
2+
Hg2+, as one of highly toxic metal ions, has exhibited manifold damage on human health by invading respiratory passage, skin, or digestive system [30,31]. Recently, fluorescent probes have provided new ways for detections of Hg2+ [31–33]. Considering that the NCs were generally applied for sensing metal ions [34,35],
For testing the practicality of the developed approach, standard recovery experiments were performed in human urine samples (Table 1). As listed, the recoveries of all samples were 102.5%, 98.6%, and 101.1% respectively, and exhibited little inference from substrates of urine, indicating the proposed method may broaden avenues for practical detections of Hg2+ in real samples.
Table 1 The recovery of Hg2+ in supplemented urine samples detected by this proposed method. Sample
Hg2+ supplemented (10
1 2 3
1.2 1.0 0.8
4
M)
Hg2+ measured (10 1.23 0.986 0.809
4
M)
Recovery (%) 102.5 98.6 101.1
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3.7. Fantastic applications Interestingly, benefited from their excellent fluorescent properties, CuNCs were further employed for meaningful applications. As shown in Fig. 5D, the fluorescent word under UV light was readily obtained, and no visible characters appearing on the paper under daylight (Fig. 5C). Moreover, both absorbent cotton and a paper craft with staining by the CuNCs implied dramatic fluorescence under UV light (Fig. 5F and H) compared with that under daylight (Fig. 5E and G). Hence, these photographs depicted that the CuNCs described here may play a role for text encryption and fluorescent staining. 4. Conclusions In summary, we have creatively synthesized CuNCs stabilized with L-cysteine via a simple method for the first time. During the whole procedure, only two common chemicals of CuSO4 and L-cysteine were introduced, thus novel and water-soluble CuNCs obtained, indicating unique advantages of this synthesis approach. Subsequently, the CuNCs was employed to selectively detect Hg2+ on the basis of interactions between Hg2+ and L-cysteine [36,37], thus facilitating to quench their fluorescence. Again, the practicability of this fluorescent probe has been validated by assaying Hg2+ in human urine samples. Significantly, this new CuNCs described here were applied for coding and fluorescent staining, demonstrating that it may widen potential roads for biosensing and commercial purpose. Acknowledgments We gratefully acknowledge financial support by National Natural Science Foundation of China (31100981), Research Fund for the Doctoral Program of Higher Education of China (20110182120014), Natural Science Foundation Project of CQ CSTC (cstc2013jcyjA10117), Fundamental Research Funds for the Central Universities (XDJK2013B038), and Program for Innovative Research Team in University of Chongqing (2013). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.aca.2014.07.019. References [1] J.P. Wilcoxon, B.L. Abrams, Synthesis, structure and properties of metal nanoclusters, Chem. Soc. Rev. 35 (2006) 1162–1194. [2] R.C. Jin, Quantum sized, thiolate-protected gold nanoclusters, Nanoscale 2 (2010) 343–362. [3] Y.Z. Lu, W. Chen, Sub-nanometre sized metal clusters: from synthetic challenges to the unique property discoveries, Chem. Soc. Rev. 41 (2012) 3594–3623. [4] S.A. Patel, C.I. Richards, J.C. Hsiang, R.M. Dickson, Water-soluble Ag nanoclusters exhibit strong two-photon-induced fluorescence, J. Am. Chem. Soc. 130 (2008) 11602–11603. [5] I. Diez, R.H.A. Ras, Fluorescent silver nanoclusters, Nanoscale 3 (2011) 1963–1970. [6] H.C. Yeh, J. Sharma, J.J. Han, J.S. Martinez, J.H. Werner, A DNA-silver nanocluster probe that fluoresces upon hybridization, Nano Lett. 10 (2010) 3106–3110. [7] H. Zhang, X. Huang, L. Li, G.W. Zhang, I. Hussain, Z. Li, B. Tan, Photoreductive synthesis of water-soluble fluorescent metal nanoclusters, Chem. Commun. 48 (2012) 567–569. [8] J. Zheng, P.R. Nicovich, R.M. Dickson, Highly fluorescent noble metal quantum dots, Annu. Rev. Phys. Chem. 58 (2007) 409–431. [9] S.J. Guo, E.K. Wang, Noble metal nanomaterials: controllable synthesis and application in fuel cells and analytical sensors, Nano Today 6 (2011) 240–264.
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One-step synthesis and applications of fluorescent Cu nanoclusters stabilized by L-cysteine in aqueous solution Xiaoming Yang * ,1, Yuanjiao Feng 1, Shanshan Zhu, Yawen Luo, Yan Zhuo, Yao Dou College of Pharmaceutical Sciences, Southwest University, Chongqing 400716, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
A novel, one-step strategy for synthesizing water-soluble CuNCs was established. A simple, selective, and cost-effective assay for Hg2+ was developed. CuNCs may broaden ways for fluorescent staining and coding.
An innovative and simple strategy for synthesizing high-fluorescent Cu nanoclusters stabilized with Lcysteine has been successfully established in aqueous solution. Significantly, the Cu nanoclusters were employed for sensitive and selective detections of Hg2+, coding and fluorescent staining, suggesting their potential toward various applications.
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 May 2014 Received in revised form 14 June 2014 Accepted 15 July 2014 Available online xxx
Herein, an innovative and simple strategy for synthesizing high fluorescent Cu nanoclusters was successfully established while L-cysteine played a role as the stabilizer. Meaningfully, the current Cu nanoclusters together with a quantum yield of 14.3% were prepared in aqueous solution, indicating their extensive applications. Subsequently, the possible fluorescence mechanism was elucidated by fluorescence, UV–vis, HR-TEM, FTIR, XPS, and MS. Additionally, the CuNCs were employed for assaying Hg2+ on the basis of the interactions between Hg2+ and L-cysteine; thus facilitating the quenching of their fluorescence. The proposed analytical strategy permitted detections of Hg2+ in a linear range of 1.0 10 7 mol L 1 10 3 mol L 1, with a detection limit of 2.4 10 8 mol L 1 at a signal-to-noise ratio of 3. Significantly, this CuNCs described here were further applied for coding and fluorescent staining, suggesting may broaden avenues toward diverse applications. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Cu nanoclusters L-cysteine One-step synthesis Applications
1. Introduction Noble metal nanoclusters (NCs) usually consist of several to dozens of atoms with properties regulated by their subnanometer dimensions [1–3]. Emerging as a new type of fluorescent nanomaterial, fantastic characteristics of NCs, including satisfactory
* Corresponding author. Tel.: +86 23 68251225; fax: +86 23 68251225. E-mail addresses: [email protected], [email protected] (X. Yang). 1 Both authors contributed equally to this work.
water solubility, high quantum-yield, biocompatibility, excellent stability, and outstanding catalytic properties [4–8], have recently attracted extensive attentions, potentiating it as a novel candidate for biosensing [9,10], biolabeling [11–13] and catalysis [14]. Thus, numerous methods have been developed for synthesizing NCs during the past decades [4,6,12,15–18], especially Au and Ag nanoclusters. Nevertheless, scarce approaches have been reported toward synthesizing Cu nanoclusters (CuNCs), although copper has been widely playing critical roles in various fields as a member of transition metals [19], and CuNCs have exhibited unique fluorescent properties as reported [16,20,21].
http://dx.doi.org/10.1016/j.aca.2014.07.019 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: X. Yang, et al., One-step synthesis and applications of fluorescent Cu nanoclusters stabilized by L-cysteine in aqueous solution, Anal. Chim. Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.07.019
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Currently, there have existed kinds of methods for synthesizing CuNCs [20,22–24]. According to the literatures, polymers such as polyamidoamine dendrimer [23,25,26], serving as a template and regulating sizes and core diameters, have been applied to prepare CuNCs. Besides, another approach for synthesizing water-in-oil CuNCs was proposed to achieve different sizes by controlling the amount of reducing agents [27]. Moreover, the surfactant has been employed to encapsulate copper atoms thus forming CuNCs based on an electrochemical mechanism [28]. Again, one-pot method has been performed according to wet chemical reduction, in which 2-mercapto-5-n-propylpyrimidine played a role as protector ligand [20]. Recently, as described in the literature, CCCYYY amino acid sequence was designed as a self-oxidant reagent to obtain CuNCs as desired in alkaline conditions [13]. However, most of these methods were manipulated in organic solvents [29], leading to CuNCs as prepared showed low-fluorescence and obvious instability, thereby severely restricting their applications. In addition, the above methods have exhibited other drawbacks including tedious steps, toxic reagent, complicated instrumentations and high cost. Therefore, exploration of new strategies for synthesizing stable and highfluorescent CuNCs in aqueous phase are still desired. In this study, an innovative, simple, and economical strategy for preparing water-soluble CuNCs has been well established, while L-cysteine served as a template to encapsulate Cu atoms for the first time (Fig. 1). Meanwhile, the CuNCs here were further employed for preparing fluorescent powder and coding. Overall, the CuNCs as prepared showed obvious advantages such as one-step synthesis procedure in aqueous solution and high quantum yield of 14.3% compared with previous reported (Table S1), suggesting their potentialtobroadenavenuesforbiosensingandfantasticapplications. 2. Experimental 2.1. Chemicals All the ions (Hg2+, K+, Ag+, Zn2+, Pb2+, Eu3+, Al3+, Ca2+, Ba2+, Br , Mg2+, Fe3+, Cd2+, Ni2+, and I ), L-cysteine, and copper sulfate (CuSO4) were obtained from Shanghai Sangon Biotechnonlog Co., Ltd. (Shanghai, China). Sodium borohydride (powder, 98%) was obtained from Sigma–Aldrich (Milwaukee, WI). Disodium hydrogen phosphate (Na2HPO4) and sodium dihydrogen phosphate (NaH2PO4), sodium hydroxide (NaOH), sodium chloride (NaCl) were purchased from Dingguo Changsheng Biotechnology Co., Ltd. (Beijing, China). Ultrapure water, 18.25 MV cm, produced with an Aquapro AWL-0502-P ultrapure water system (Chongqing, China) was employed for all experiments.
2.2. Instrumentation All fluorescence measurements were performed on a Hitachi F-7000 fluorescence spectrophotometer (Tokyo, Japan) with excitation slit set at 5 nm band pass and emission at 5 nm band pass in 1 cm 1 cm quartz cell. Meanwhile, UV–vis absorption spectra were recorded by a Shimadzu UV-2450 spectrophotometer (Tokyo, Japan). The high-resolution transmission electron microscopy (HR-TEM) images were taken using a TECNAI G2 F20 microscope (FEI, USA) at 200 KV. Elemental and functional groups analysis were obtained by ESCALAB 250 X-ray photoelectron spectrometer and Fourier transform infrared spectrometer (Tokyo, Japan), respectively. The quantum yields were obtained by using Absolute PL quantum yield spectrometer C11347 (Hamamatsu, Japan). Mass spectra were obtained by UltrafleXtreme Matrixassisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI TOF/TOF MS, Bruker Daltonics Inc., USA). A Fangzhong pHS-3C digital pH meter (Chengdu, China) was used to measure the pH values of the aqueous solutions and a vortex mixer QL-901 (Haimen, China) was used to blend the solution. The thermostatic water bath (DF-101s) was purchased from Gongyi Instrument Co., Ltd. (Henan, China). 2.3. Synthesis of CuNCs Basically, as a typical experiment, 0.25 mL of CuSO4 solution (1 mM) was added into 2.5 mL of L-cysteine solution (35 mg mL 1) with NaOH (0.4 M). After votexing, this solution was incubated under vigorous stirring at 55 C water bath for 4.5 h. Then, this aqueous solution was filtered with 0.22 mm filter membrane to remove the larger product. Finally, the fluorescent CuNCs were further collected by dialysis against deionized water through a dialysis membrane (1000 MWCO) for 24 h. The CuNCs obtained here were stable for 3 months when stored in the dark at 4 C. 2.4. Detection of Hg2+ Firstly, 50 mL CuNCs and 50 mL PBS buffer (0.1 M, pH 9.0) were pipetted into a 1.5-mL vial. Subsequently, an appropriate volume of Hg2+ working solution or sample solution was added, diluted to 500 mL with Milli-Q purified water and vortex-mixed thoroughly. The mixture was then left to react at 55 C for 10 min and transferred for fluorescence measurements. Finally, interferences originated from other metal ions were investigated individually in the presence of the CuNCs prepared here.
Fig. 1. Schematic illusion of synthesizing water-soluble CuNCs.
Please cite this article in press as: X. Yang, et al., One-step synthesis and applications of fluorescent Cu nanoclusters stabilized by L-cysteine in aqueous solution, Anal. Chim. Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.07.019
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2.5. Preparing urine samples Fresh human urine sample was collected from Southwest University Hospital. For the recovery experiments, impurities of the urine sample were precipitated by appropriate speed of centrifugation (2000 rpm, 10 min). Then, the supernatant was collected into 3 tubes and supplemented with standard Hg2+ solutions (0.8 10 4 M, 1.0 10 4 M, 1.2 10 4 M). 3. Results and discussion 3.1. Characterization of CuNCs To characterize this synthesized CuNCs, the maximum excitation and emission spectra were initially recorded as 375 nm and 480 nm (Fig. 2A) respectively, and the fluorescent properties of the CuNCs solution were subsequently investigated. As shown in Fig. 2A, the CuNCs aqueous solution emitted obvious cyan fluorescence (photograph II) under UV light (365 nm) while appearing as yellowish and transparent under daylight (photograph I). Additionally, the powder of CuNCs as described by lyophilization (photograph IV) showed striking fluorescence under UV light, whereas yellowish was observed under daylight (photograph III). Significantly, the fluorescent powder has been preserved for more than 3 months without marked changes, indicating that it may provide the possibility for matching the requirement of commercial scale. In Fig. 2B, only CuNCs exhibited distinct fluorescence, indicating that the fluorescence was originated from the CuNCs rather than CuSO4 or L-cysteine. To explore the surface groups of CuNCs obtained here, the spectra of Fourier transform infrared spectroscopy (FTIR) of were recorded. As Fig. 2C revealing, disappearance of –SH stretching vibrations at 2511.32 cm 1 suggested that Cu atoms of CuNCs were covered by L-cysteine through Cu S bonding. Subsequently,
3
absorption spectra of CuNCs, L-cysteine, and CuSO4 were displayed in Fig. 2D respectively. Only CuNCs exhibited a weak absorption peak around 375 nm, as well as both L-cysteine and CuSO4 showed no obvious peaks. To further elucidate the nanostructures of CuNCs, a high resolution transmission electron microscope (HR-TEM) was employed to directly observe the morphology and particle size distributions. As shown in Fig. 3A and B, CuNCs prepared here existed as a majority population within the size range of 2–3 nm and no aggregation emerged, depicting their satisfactory dispersity. In addition, the size distribution of CuNCs was obtained from HR-TEM image analysis (Fig. S1). toward the purpose of gaining insight into components of the current CuNCs, XPS survey spectra were performed. As shown in Fig. 3C, six major peaks of S(1s), C(1s), N(1s), Na, O(1s), Cu (2p), and Na(1s) obviously emerged in the spectrum, indicating that CuNCs prepared here were mainly composed of C, O, N, S, Cu, and Na. To fairly describe the peak of Cu, the amplified peak was showed in Fig. 3D. The binding energy position for Cu 2p3/2 and Cu 2p1/2 were located at 932.05 eV and 952.05 eV, indicating that CuNCs were composed of Cu0 and Cu+. Furthermore, there were no peak displayed around 942 eV (Fig. 3D), demonstrating the absence of Cu2+ for CuNCs, which agreed with the reported [20,22]. Next, for determining the Cu atom number of CuNCs, the representative matrix-assisted laser-desorption ionization time of flight mass spectrum (MALDI-TOF MS) were obtained by the positive-ion mode (Fig. 4). As shown in Fig. 4A and B, the dominant m/z peak at 911.065 eV was originated from [Cu4L5 + H + 2Na]+ (L = C3H7O2NS) together with Na adducts. To clarify the components in detail, spectra in the range of m/z = 600–650, 725–745 were further amplified (Fig. 4C and D). The lower peaks located at 616.843 eV and 735.366 eV were ascribed to [Cu4L3–H]+ and [Cu4L4–H]+. This result demonstrated that the CuNCs as synthesized were composed of four copper atoms.
Fig. 2. (A) Flourescence exitation and emission spectra of CuNCs. Inset: photographs of CuNCs solution (I, II) and powder (III, IV); (B) fluorecence spectra of CuNCs, L-cysteine and CuSO4; (C) FTIR spectra of CuNCs and L-cysteine; (D) UV–vis spectra of CuNCs, L-cysteine and CuSO4.
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Fig. 3. (A), (B) HR-TEM images of CuNCs; (C) XPS of CuNCs thus obtained; (D) amplified XPS of Cu 2p electrons.
Fig. 4. (A) Positive mode MALDI-TOF MS of CuNCs; (B), (C), (D) amplified views of peaks originated from [Cu4L5 + H + 2Na]+ (m/z = 911.065), [Cu4L3–H]+ (m/z = 616.843), [Cu4L4– H]+ (m/z = 735.366), respectively.
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Fig. 5. (A) Effect of different metal ions on the fluorescence intensity of CuNCs; (B) fluorescence spectra of CuNCs in the presence of various concentrations of Hg2+. Insert: plot of (F0–F) versus the logarithm of concentrations of Hg2+ introduced; the word “CuNCs” written by using CuNCs as ink under daylight (C) and UV light (D); photograghs of the absorbent cotton and a paper craft stained by the current CuNCs under daylight (E and G) and UV light (F and H).
Taking all the above characteristic data together, a brand-new type of CuNCs were successfully synthesized by a developed novel and simple method. 3.2. Optimization of synthesis conditions Again, to identify the optimized conditions for synthesizing CuNCs, various experiments were performed. As revealed (Fig. S2A, S2B, and S2C), the fluorescent intensities of CuNCs exhibited variations along with varying reaction time, temperature and concentrations of L-cysteine, demonstrating that synthesis of CuNCs were dependent on these selected conditions; thus, 4.5 h, 55 C and 35 mg mL 1 of L-cysteine served as the optimal conditions toward the following experiments. Moreover, to address whether this current condition was the optimal condition or not, other synthesis approaches including high reaction– temperature, NaBH4 replacing of NaOH, and low concentration of Cu2+ without NaOH were further investigated for comparison. As Fig. S2D indicated, the highest-fluorescent CuNCs were obtained only under this optimized condition. 3.3. Stability of CuNCs Ultimately, to identify the stability of CuNCs for bearing circumstances, further experiments were performed. As revealed (Figs. S3 and S4), the fluorescent intensities of CuNCs scarcely exhibited alteration along with time-lapse and varying organic solvents, demonstrating the CuNCs satisfactory stability.
we asked whether the CuNCs described here could potentially serve as a fluorescent probe to detect Hg2+ or not. Toward that purpose, Hg2+ was introduced into the CuNCs as prepared. Fig. S5 showed that fluorescence intensity of the CuNCs at 480 nm decreased to about 50%, when 10 mM Hg2+ was added. Simultaneously, no obvious fluorescence was observed under UV light (photograph II), indicating that the CuNCs may be applied for detecting Hg2+. To determine Hg2+ effectively, conditions including pH, reaction temperature and time were optimized. Considering the speed, easy-operation and selectivity of this assay, pH 9.0, 45 C, and 50 min served as the optimal conditions during the following experiments (Fig. S6). 3.5. Selectivity Next, the selectivity of this analytical strategy was evaluated by testing the response to other ions (Hg2+, K+, Ag+, Zn2+, Pb2+, Eu3+, Al3 + , Ca2+, Ba2+, Br , Mg2+, Fe3+, Cd2+, Ni2+, I , and 100 mM for each) under optimum conditions for the case of 10 mM Hg2+. Fig. 5A revealed that the fluorescence probe responded selectively toward over the other metal ions, indicating the excellent selectivity of the CuNCs for Hg2+ detection. Furthermore, the fluorescent intensity decrease (F0–F) versus the logarithmic plot of Hg2+ concentrations displayed a linear range from 1.0 10 7 mol L 1 to 1.0 10 3 mol L 1 with a correlation coefficient of 0.9598 (Fig. 5B), demonstrating an excellent precision of the fluorescent probe. The detection limit of Hg2+ was 2.4 10 8 mol L 1 at a signal-to-noise ratio of 3. Overall, the results showed a promising method for assaying Hg2+. 3.6. Detection of Hg2+in urine samples
3.4. Assaying Hg
2+
Hg2+, as one of highly toxic metal ions, has exhibited manifold damage on human health by invading respiratory passage, skin, or digestive system [30,31]. Recently, fluorescent probes have provided new ways for detections of Hg2+ [31–33]. Considering that the NCs were generally applied for sensing metal ions [34,35],
For testing the practicality of the developed approach, standard recovery experiments were performed in human urine samples (Table 1). As listed, the recoveries of all samples were 102.5%, 98.6%, and 101.1% respectively, and exhibited little inference from substrates of urine, indicating the proposed method may broaden avenues for practical detections of Hg2+ in real samples.
Table 1 The recovery of Hg2+ in supplemented urine samples detected by this proposed method. Sample
Hg2+ supplemented (10
1 2 3
1.2 1.0 0.8
4
M)
Hg2+ measured (10 1.23 0.986 0.809
4
M)
Recovery (%) 102.5 98.6 101.1
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3.7. Fantastic applications Interestingly, benefited from their excellent fluorescent properties, CuNCs were further employed for meaningful applications. As shown in Fig. 5D, the fluorescent word under UV light was readily obtained, and no visible characters appearing on the paper under daylight (Fig. 5C). Moreover, both absorbent cotton and a paper craft with staining by the CuNCs implied dramatic fluorescence under UV light (Fig. 5F and H) compared with that under daylight (Fig. 5E and G). Hence, these photographs depicted that the CuNCs described here may play a role for text encryption and fluorescent staining. 4. Conclusions In summary, we have creatively synthesized CuNCs stabilized with L-cysteine via a simple method for the first time. During the whole procedure, only two common chemicals of CuSO4 and L-cysteine were introduced, thus novel and water-soluble CuNCs obtained, indicating unique advantages of this synthesis approach. Subsequently, the CuNCs was employed to selectively detect Hg2+ on the basis of interactions between Hg2+ and L-cysteine [36,37], thus facilitating to quench their fluorescence. Again, the practicability of this fluorescent probe has been validated by assaying Hg2+ in human urine samples. Significantly, this new CuNCs described here were applied for coding and fluorescent staining, demonstrating that it may widen potential roads for biosensing and commercial purpose. Acknowledgments We gratefully acknowledge financial support by National Natural Science Foundation of China (31100981), Research Fund for the Doctoral Program of Higher Education of China (20110182120014), Natural Science Foundation Project of CQ CSTC (cstc2013jcyjA10117), Fundamental Research Funds for the Central Universities (XDJK2013B038), and Program for Innovative Research Team in University of Chongqing (2013). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.aca.2014.07.019. References [1] J.P. Wilcoxon, B.L. Abrams, Synthesis, structure and properties of metal nanoclusters, Chem. Soc. Rev. 35 (2006) 1162–1194. [2] R.C. Jin, Quantum sized, thiolate-protected gold nanoclusters, Nanoscale 2 (2010) 343–362. [3] Y.Z. Lu, W. Chen, Sub-nanometre sized metal clusters: from synthetic challenges to the unique property discoveries, Chem. Soc. Rev. 41 (2012) 3594–3623. [4] S.A. Patel, C.I. Richards, J.C. Hsiang, R.M. Dickson, Water-soluble Ag nanoclusters exhibit strong two-photon-induced fluorescence, J. Am. Chem. Soc. 130 (2008) 11602–11603. [5] I. Diez, R.H.A. Ras, Fluorescent silver nanoclusters, Nanoscale 3 (2011) 1963–1970. [6] H.C. Yeh, J. Sharma, J.J. Han, J.S. Martinez, J.H. Werner, A DNA-silver nanocluster probe that fluoresces upon hybridization, Nano Lett. 10 (2010) 3106–3110. [7] H. Zhang, X. Huang, L. Li, G.W. Zhang, I. Hussain, Z. Li, B. Tan, Photoreductive synthesis of water-soluble fluorescent metal nanoclusters, Chem. Commun. 48 (2012) 567–569. [8] J. Zheng, P.R. Nicovich, R.M. Dickson, Highly fluorescent noble metal quantum dots, Annu. Rev. Phys. Chem. 58 (2007) 409–431. [9] S.J. Guo, E.K. Wang, Noble metal nanomaterials: controllable synthesis and application in fuel cells and analytical sensors, Nano Today 6 (2011) 240–264.
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