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Lysozyme-stabilized gold nanoclusters as a novel fluorescence probe for cyanide recognition
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 77–80
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Lysozyme-stabilized gold nanoclusters as a novel fluorescence probe for cyanide recognition Dongtao Lu a, Lili Liu b, Fengxia Li b, Shaomin Shuang b, Yingfu Li a, Matin M.F. Choi c, Chuan Dong a,⇑ a
Research Center for Environmental Science and Engineering, Shanxi University, Taiyuan 030006, China College of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China c Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong Special Administrative Region b
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 Lys-AuNCs fluorescence
probe for cyanide anion detection was developed. Lys-AuNCs could be served as a ‘‘naked-eye’’ optical probe for cyanide anion. The probe was environmentallyfriendly and synthesized conveniently.
a r t i c l e
i n f o
Article history: Received 24 May 2013 Received in revised form 12 August 2013 Accepted 4 October 2013 Available online 11 October 2013 Keywords: Gold nanoclusters Cyanide recognition Fluorescence probe
a b s t r a c t Lysozyme-stabilized gold nanoclusters (Lys-AuNCs) have been synthesized and utilized as a fluorescent probe for selective detection of cyanide (CN). Lys-AuNCs had an average size of 4 nm and showed a red emission at 650 nm (kex = 370 nm). The fluorescence of Lys-AuNCs could be quenched by CN. An excellent sensitivity and selectivity toward the detection of CN in aqueous solution was observed. The fluorescence intensity was linear with the CN concentration in the range of 5.00 106 M–1.20 104 M with a detection limit as low as 1.9 107 M. Also, the addition of CN to Lys-AuNCs could induce an obvious color change from light yellow to colorless. Correspondingly, a bright red fluorescence disappeared and a blue fluorescence appeared. The results indicated that Lys-AuNCs could be applied in detection of cyanide on environmental aspects. Ó 2013 Published by Elsevier B.V.
Introduction Anion recognition has been a challenging area in nanotechnology due to its extensive research activities including environmental, clinical, chemical, and biological application in recent decades [1]. Among the various anions, cyanide was an extremely toxic contaminant that could bind heme cofactors to inhibit the terminal respiratory chain enzyme cytochrome c oxidase and lead to the death of human or other organisms [2]. The hazardous toxic pollutant cyanide was widely released from the industrial settings (1.5 million tons per year) and made it significant research efforts directed toward the detection of cyanide by relevant biological ⇑ Corresponding author. Tel.: +86 351 7018842; fax: +86 351 7011011. E-mail address: [email protected] (C. Dong). 1386-1425/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.saa.2013.10.009
technology and means. Several strategies for detecting cyanide have been developed based on spectrophotometry [3], potentiometry with cyanide-selective electrodes [4] and flow injection amperometry [5], etc. Fluorescence spectroscopy for the detection of cyanide might be favoriate choice in ultratrace quantity [6] by virtue of high sensitivity and easy operation. Additionally, most current fluorescent probes involved were operationally complex, complicated synthetic procedure and utilized in organic solvents for detecting cyanide [7,8]. Consequently developing fluorescent probes with high sensitivity, low cost and green synthesis to directly measure cyanide in aqueous media was competitive. Recently, gold nanoclusters (AuNCs) possessed distinct physical and chemical attributes that made them excellent optical probes for the fabrication of novel chemical and biological sensing [9]. These features broadened the application of AuNCs toward fluores-
78
D. Lu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 77–80
cence sensing. Dong’s group [10] monitored cyanide from dissolution of Rhodamine B-adsorbed Au nanoparticles (RB-AuNPs). Fluorescence of RB-AuNPs was enhanced in the presence of cyanide based on the fact that Au NPs were etched by cyanide. Liu et al [11] developed a novel BSA-stabilized AuNCs for detecting cyanide by etching-induced fluorescence quenching. Up to now, AuNCs could be stablized with amine [12], DNA [13], peptide [14], protein [15], polymers [16], dendrimers [17], and thiolate ligands [18] as scaffolds. Inspired by natural strategy to fabricate biominerals, using biomolecules as scaffold for developing green synthesis protocols to prepare AuNCs have attracted a great deal of interest. Proteins played an important role in the synthesis of AuNCs because the amino, carboxyl and thiol groups in proteins could be served as effective stabilizing agents [19,20]. In this work, water-soluble lysozyme stabilized AuNCs (LysAuNCs) could be facilely synthesized and characterized. Interestingly, the synthetic Lys-AuNCs could be utilized as a fluorescence probe for detection of CN. Furthermore, the fluorescence of LysAuNCs was quenched linearly by CN. Thus, a cost-effective and sensitive fluorescence probe for the monitoring of CN was constructed.
indicating the formation of Ag NCs. The solution of Lys-AuNCs emitted the intense red luminescence under UV light (365 nm). Fluorescence measurements A typical CN detection process was conducted as follows. The as-prepared Lys-AuNCs solutions were 1.25 mg/mL for CN detection. NaOH–NaHCO3 buffer solutions were used to adjust the acidity of solution at pH 11.0 for detection. The fluorescence spectra of Lys-AuNCs upon titration with CN were recorded by Edinburgh F900 spectrophotometer. To evaluate the selectivity of CN fluorescence detection by Lys-AuNCs, other anions such as F, Cl, 2 Br, I, CO2 , and 3 , CH3COO , NO3 , NO2 , ClO4 , SCN , EDTA 3 C6 H5 O7 were also tested and the response recorded and analyzed at pH 11.0. Results and discussion Characterization of Lys-AuNCs
Absorption spectra were obtained with a TU-1901 spectrophotometer (Persee, Peking, China). An Edinburgh F900 spectrophotometer (UK) was used for fluorescence emission spectra recording. Transmission electron microscopy (TEM) was performed with a JEOL 2100 at 200 kV (Japan). The sample was freeze-dried and provided by infrared spectroscopy between 4000 and 500 cm1 on a Shimadzu FTIR-8400S spectrometer (Japan). A PB10 pH meter (Sartorius, German) was used to adjust pH values.
Highly fluorescent Lys-AuNCs were obtained via one-pot, ‘‘green’’ synthetic route. As shown in Fig. 1A, Lys-AuNCs were approximately spherical in shape and about 4 nm in diameter. Infrared spectroscopy (FTIR) was a valuable method to analyze the protein secondary structures [21,22]. Secondary structure analysis of proteins was nearly exclusively done using the amide I band (1650 cm1), but the amide II (1550 cm1) and amide III bands (1400–1200 cm1) have also been shown useful [21]. Fig. 1B showed the amide I band was no shifted but decreased in the intensity of the peak after assembled to AuNCs. This trend indicated that unordered structures increased and fewer helical structures were present. The appearance of a band at 1500 cm1 in Lys-AuNCs indicated a deprotonation of Tyr–OH [21]. From the absorption and fluorescence spectra of Lys-AuNCs (Fig. 2), it could be concluded that no apparent surface plasmon resonance absorption peak in 520 nm was observed. When excited at 370 nm, the solution of Lys-AuNCs showed an emission peak centered at 650 nm. The fluorescence quantum yield (QY) of the Lys-AuNCs was calculated to be 5.2% by comparison with rhodamine 6G in ethanol solution (with a standard QY of 95%). The photoluminescence was stable in the pH range of 1–13.
Synthesis of Lys-AuNCs
Fluorescence quenching of Lys-AuNCs by CN
Lys-AuNCs were synthesized according to reference [15]. Briefly, 2 mL of 4 mM HAuCl43H2O solution was added to 2 mL lysozyme solution (16 mg/mL) with vigorous stirring. About 0.2 mL NaOH (1 M) was introduced to adjust the acidity of solution at pH 12. The reaction should proceed at 37 °C for 8 h. Simultaneously, the solution color turned from light yellow to deep brown,
The fluorescence responses of Lys-AuNCs to CN were investigated in Fig. 3A. Upon the addition of 3.05 104 M CN, the emission intensity of Lys-AuNCs at 650 nm decreased. The pH of buffer solution was investigated in order to improve the sensitive detection of CN by Lys-AuNCs. CN was inclined to capture the available protons in the solution under lower pH conditions
Experimental Chemicals and reagents Aurichlorohydric acid (HAuCl43H2O) was purchased from Aldrich (>99.9%, US). Lysozyme was ordered from Shanghai Sangon Biotechnology Co. Ltd (Shanghai, China). Water (>18.2 MX cm) used for the experiments was purified by a Milli-Q system. Apparatus
Fig. 1. (A) TEM images of Lys-AuNCs. (B) The FTIR spectra of lysozyme and Lys-AuNCs.
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D. Lu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 77–80
4Au þ 8CN þ O2 þ 2H2 O ¼ 4½AuðCNÞ2 þ 4OH
ð1Þ
Although gold nanoparticles are etched, their core size decreases, the change of the emission peak position could not be observed. Quenching of fluorescence Lys-AuNCs could be described by the Stern–Volmer equation: I0/I = 1 + KSV[Q], Where I and I0 denote the fluorescent intensity at 650 nm with and without CN, respectively. As shown in Fig. 4B, the results could be described by Stern–Volmer equation.
F 0 =F ¼ 1:12 þ 15473½CN
ð2Þ 2
Fig. 2. The absorption (a) and emission spetra (b) of Lys-AuNCs.
(pKa = 9.36) [11]. Thus, the fluorescence quenching effect was obtained in the pH range of 7.0–13.0. As shown in Fig. 3B, the fluorescence quenching effect reached a maximum at pH 11.0, so the pH 11.0 was selected as optimum condition.
Detection of CN based on Lys-AuNCs The sensitivity and linearity of the Lys-AuNCs system were evaluated by the varying CN concentration. Fig. 4A showed the fluorescence spectra titration of Lys-AuNCs in the presence of CN anion at pH 11.0. The free Lys-AuNCs exhibited a red emission at 650 nm. A strong coordination is established between the Au+ and CN. The formation of [Au(CN)2] could significantly quench the fluorescence of Lys-AuNCs. Upon the addition of CN anion, the emission spectra of Lys-AuNCs decreased linearly at 650 nm. Au could be oxidized in the presence of O2 and cyanide according to Eq. (1). [23]
An excellent linear relation with r of 0.9958 was obtained over the concentration ranges of 5–120 lM. Under the current experimental condition, the detection limit was measured to be 1.9 107 M. which was much lower than the World Health Organization (WHO)’s limit (1.9 lM) for drinking water. Selectivity for CN detection The interference of foreign anions (F, Cl, Br, I, CO2 3 , 2 CH3 COO , NO , and C6 H5 O3 3 , NO2 , ClO4 , SCN , EDTA 7 ) to LysAuNCs was examined with the same concentration as CN of 50 lM. Fig. 5A showed the effect of anions on fluorescence intensity of Lys-AuNCs at pH 11.0. As could be seen, these competitive anions showed weak interference to cyanide detection and did not lead to any significant fluorescence changes at all. The detection selectivity could be visualized with the naked eye. As shown in Fig. 5B, the photograph took under visible light and UV light. CN could react with Lys-AuNCs resulted in an obvious color change from light yellow to colorless under visible light. Correspondingly, a bright red fluorescence disappeared and a blue fluorescence appeared under UV light. Because the blue fluorescence of lysozyme was covered by the red fluorescence of LysAuNCs, Thus, in the presence of cyanide, AuNCs would be gradually
Fig. 3. (A) The fluorescence spectra of Lys-AuNCs in the absence (a) and presence (b) of 3.05 104 M CN. (B) The effects of pH on the fluorescence response of Lys-AuNCs in the presence of 50 lM CN.
Fig. 4. (A) Fluorescence response of Lys-AuNCs upon addition of CN under optimum conditions. CN concentration (from 1 to 16, lM): 0, 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 120. (B) Stern–Volmer plot of fluorescence quenching of Lys-AuNCs by CN.
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Fig. 5. (A) The fluorescence response of Lys-AuNCs in the presence of 50 lM various anions. (B) The color (top) and the fluorescent (bottom) changes of Lys-AuNCs solution to different anions ions (concentration of all the anions ions was 3 mM).
etched; the red fluorescence of AuNCs disappeared. Numerous anions were subjected to Lys-AuNCs, unique color change of CN was found and easy to be recognized. Conclusions In summary, a facile method of lysozyme assembled AuNCs for monitoring CN was proposed. Upon the addition of CN, the fluorescence emission of Lys-AuNCs was steadily quenched. The fluorescent probe of Lys-AuNCs allowed for the sensing of CN in the range of 5.00 106 M to 1.20 104 M with a detection limit as low as 1.9 107 M. The Lys-AuNCs could be visualized with the naked eye for CN. Lys-AuNCs will be served as a probe for the further design and application in fluorescence imaging, environmental analysis, and so on. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21175086 and 21175087) and Hundred Talents Program of Shanxi province. References [1] G.J. Mohr, Sens. Actuators. B. 107 (2005) 2–13. [2] R. Bhattacharya, S.J.S. Flora, in: R.C. Gupta (Ed.), Handbook of Toxicology of Chemical Warfare Agents, Academic Press, Boston, 2009, pp. 255–270.
[3] S. Hajizadeh, K. Farhadi, M. Forough, R.E. Sabzi, Anal. Methods 3 (2011) 2599– 2603. [4] American Public Health Association, Standard Methods for the Examination of Water and Wastewater, 21st ed., Washington, DC, Method 4500-CN-, 2005. [5] A.R. Surleva, V.D. Nikolova, M.T. Neshkova, Anal. Chim. Acta. 583 (2007) 174– 181. [6] X. Lv, J. Liu, Y. Liu, Y. Zhao, M. Chen, P. Wang, W. Guo, Sens. Actuators. B. 158 (2011) 405–410. [7] Y. Sun, G. Wang, W. Guo, Tetrahedron 65 (2009) 3480–3485. [8] A.O. El-Ballouli, Y. Zhang, S. Barlow, S.R. Marder, M.H. Al-Sayah, B.R. Kaafarani, Tetrahedron Lett. 53 (2012) 661–665. [9] E. Boisselier, D. Astruc, Chem. Soc. Rev. 38 (2009) 1759–1782. [10] L. Shang, L. Jin, S.J. Dong, Chem. Commun. (2009) 3077–3079. [11] Y. Liu, K. Ai, X. Cheng, L. Huo, L. Lu, Adv. Funct. Mater. 20 (2010) 951–956. [12] X. Lu, H.Y. Tnan, B.A. Korgel, Y. Xia, Chem.-Eur. J. 14 (2008) 1584–1591. [13] J.Y. Kim, J.S. Lee, Nano Lett. 9 (2009) 4564–4569. [14] Y. Negishi, K. Nobusada, T. Tsukuda, J. Am. Chem. Soc. 127 (2005) 5261–5270. [15] J.P. Xie, Y.G. Zheng, J.Y. Ying, J. Am. Chem. Soc. 131 (2009) 888–889. [16] N. Schaeffer, B. Tan, C. Dickinson, M.J. Rosseinsky, A. Laromaine, D.W. McComb, M.M. Stevens, Y. Wang, L. Petit, C. Barentin, D.G. Spiller, A.I. Cooper, R. Levy, Chem. Commun. 34 (2008) 3986–3988. [17] J. Zheng, J.T. Petty, R.M. Dickson, J. Am. Chem. Soc. 125 (2003) 7780–7781. [18] O. Varnavski, G. Ramakrishna, J. Kim, D. Lee, T. Goodson, ACS Nano. 4 (2010) 3406–3412. [19] H. Wei, Z.D. Wang, L.M. Yang, S.L. Tian, C.J. Hou, Y. Lu, Analyst. 135 (2010) 1406–1410. [20] Y.H. Lin, W.L. Tseng, Anal. Chem. 82 (2010) 9194–9200. [21] A. Barth, Infrared spectroscopy of proteins, BBA-Bioenergetics 1767 (2007) 1011–1073. [22] L. Shang, Y.Z. Wang, J.G. Jiang, S.J. Dong, Langmuir 23 (2007) 2714–2721. [23] J. Zhai, Y. Zhai, S. Dong, Colloids and Surfaces A: Physicochem, Eng. Aspects. 335 (2009) 207–210.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Lysozyme-stabilized gold nanoclusters as a novel fluorescence probe for cyanide recognition Dongtao Lu a, Lili Liu b, Fengxia Li b, Shaomin Shuang b, Yingfu Li a, Matin M.F. Choi c, Chuan Dong a,⇑ a
Research Center for Environmental Science and Engineering, Shanxi University, Taiyuan 030006, China College of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China c Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong Special Administrative Region b
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 Lys-AuNCs fluorescence
probe for cyanide anion detection was developed. Lys-AuNCs could be served as a ‘‘naked-eye’’ optical probe for cyanide anion. The probe was environmentallyfriendly and synthesized conveniently.
a r t i c l e
i n f o
Article history: Received 24 May 2013 Received in revised form 12 August 2013 Accepted 4 October 2013 Available online 11 October 2013 Keywords: Gold nanoclusters Cyanide recognition Fluorescence probe
a b s t r a c t Lysozyme-stabilized gold nanoclusters (Lys-AuNCs) have been synthesized and utilized as a fluorescent probe for selective detection of cyanide (CN). Lys-AuNCs had an average size of 4 nm and showed a red emission at 650 nm (kex = 370 nm). The fluorescence of Lys-AuNCs could be quenched by CN. An excellent sensitivity and selectivity toward the detection of CN in aqueous solution was observed. The fluorescence intensity was linear with the CN concentration in the range of 5.00 106 M–1.20 104 M with a detection limit as low as 1.9 107 M. Also, the addition of CN to Lys-AuNCs could induce an obvious color change from light yellow to colorless. Correspondingly, a bright red fluorescence disappeared and a blue fluorescence appeared. The results indicated that Lys-AuNCs could be applied in detection of cyanide on environmental aspects. Ó 2013 Published by Elsevier B.V.
Introduction Anion recognition has been a challenging area in nanotechnology due to its extensive research activities including environmental, clinical, chemical, and biological application in recent decades [1]. Among the various anions, cyanide was an extremely toxic contaminant that could bind heme cofactors to inhibit the terminal respiratory chain enzyme cytochrome c oxidase and lead to the death of human or other organisms [2]. The hazardous toxic pollutant cyanide was widely released from the industrial settings (1.5 million tons per year) and made it significant research efforts directed toward the detection of cyanide by relevant biological ⇑ Corresponding author. Tel.: +86 351 7018842; fax: +86 351 7011011. E-mail address: [email protected] (C. Dong). 1386-1425/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.saa.2013.10.009
technology and means. Several strategies for detecting cyanide have been developed based on spectrophotometry [3], potentiometry with cyanide-selective electrodes [4] and flow injection amperometry [5], etc. Fluorescence spectroscopy for the detection of cyanide might be favoriate choice in ultratrace quantity [6] by virtue of high sensitivity and easy operation. Additionally, most current fluorescent probes involved were operationally complex, complicated synthetic procedure and utilized in organic solvents for detecting cyanide [7,8]. Consequently developing fluorescent probes with high sensitivity, low cost and green synthesis to directly measure cyanide in aqueous media was competitive. Recently, gold nanoclusters (AuNCs) possessed distinct physical and chemical attributes that made them excellent optical probes for the fabrication of novel chemical and biological sensing [9]. These features broadened the application of AuNCs toward fluores-
78
D. Lu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 77–80
cence sensing. Dong’s group [10] monitored cyanide from dissolution of Rhodamine B-adsorbed Au nanoparticles (RB-AuNPs). Fluorescence of RB-AuNPs was enhanced in the presence of cyanide based on the fact that Au NPs were etched by cyanide. Liu et al [11] developed a novel BSA-stabilized AuNCs for detecting cyanide by etching-induced fluorescence quenching. Up to now, AuNCs could be stablized with amine [12], DNA [13], peptide [14], protein [15], polymers [16], dendrimers [17], and thiolate ligands [18] as scaffolds. Inspired by natural strategy to fabricate biominerals, using biomolecules as scaffold for developing green synthesis protocols to prepare AuNCs have attracted a great deal of interest. Proteins played an important role in the synthesis of AuNCs because the amino, carboxyl and thiol groups in proteins could be served as effective stabilizing agents [19,20]. In this work, water-soluble lysozyme stabilized AuNCs (LysAuNCs) could be facilely synthesized and characterized. Interestingly, the synthetic Lys-AuNCs could be utilized as a fluorescence probe for detection of CN. Furthermore, the fluorescence of LysAuNCs was quenched linearly by CN. Thus, a cost-effective and sensitive fluorescence probe for the monitoring of CN was constructed.
indicating the formation of Ag NCs. The solution of Lys-AuNCs emitted the intense red luminescence under UV light (365 nm). Fluorescence measurements A typical CN detection process was conducted as follows. The as-prepared Lys-AuNCs solutions were 1.25 mg/mL for CN detection. NaOH–NaHCO3 buffer solutions were used to adjust the acidity of solution at pH 11.0 for detection. The fluorescence spectra of Lys-AuNCs upon titration with CN were recorded by Edinburgh F900 spectrophotometer. To evaluate the selectivity of CN fluorescence detection by Lys-AuNCs, other anions such as F, Cl, 2 Br, I, CO2 , and 3 , CH3COO , NO3 , NO2 , ClO4 , SCN , EDTA 3 C6 H5 O7 were also tested and the response recorded and analyzed at pH 11.0. Results and discussion Characterization of Lys-AuNCs
Absorption spectra were obtained with a TU-1901 spectrophotometer (Persee, Peking, China). An Edinburgh F900 spectrophotometer (UK) was used for fluorescence emission spectra recording. Transmission electron microscopy (TEM) was performed with a JEOL 2100 at 200 kV (Japan). The sample was freeze-dried and provided by infrared spectroscopy between 4000 and 500 cm1 on a Shimadzu FTIR-8400S spectrometer (Japan). A PB10 pH meter (Sartorius, German) was used to adjust pH values.
Highly fluorescent Lys-AuNCs were obtained via one-pot, ‘‘green’’ synthetic route. As shown in Fig. 1A, Lys-AuNCs were approximately spherical in shape and about 4 nm in diameter. Infrared spectroscopy (FTIR) was a valuable method to analyze the protein secondary structures [21,22]. Secondary structure analysis of proteins was nearly exclusively done using the amide I band (1650 cm1), but the amide II (1550 cm1) and amide III bands (1400–1200 cm1) have also been shown useful [21]. Fig. 1B showed the amide I band was no shifted but decreased in the intensity of the peak after assembled to AuNCs. This trend indicated that unordered structures increased and fewer helical structures were present. The appearance of a band at 1500 cm1 in Lys-AuNCs indicated a deprotonation of Tyr–OH [21]. From the absorption and fluorescence spectra of Lys-AuNCs (Fig. 2), it could be concluded that no apparent surface plasmon resonance absorption peak in 520 nm was observed. When excited at 370 nm, the solution of Lys-AuNCs showed an emission peak centered at 650 nm. The fluorescence quantum yield (QY) of the Lys-AuNCs was calculated to be 5.2% by comparison with rhodamine 6G in ethanol solution (with a standard QY of 95%). The photoluminescence was stable in the pH range of 1–13.
Synthesis of Lys-AuNCs
Fluorescence quenching of Lys-AuNCs by CN
Lys-AuNCs were synthesized according to reference [15]. Briefly, 2 mL of 4 mM HAuCl43H2O solution was added to 2 mL lysozyme solution (16 mg/mL) with vigorous stirring. About 0.2 mL NaOH (1 M) was introduced to adjust the acidity of solution at pH 12. The reaction should proceed at 37 °C for 8 h. Simultaneously, the solution color turned from light yellow to deep brown,
The fluorescence responses of Lys-AuNCs to CN were investigated in Fig. 3A. Upon the addition of 3.05 104 M CN, the emission intensity of Lys-AuNCs at 650 nm decreased. The pH of buffer solution was investigated in order to improve the sensitive detection of CN by Lys-AuNCs. CN was inclined to capture the available protons in the solution under lower pH conditions
Experimental Chemicals and reagents Aurichlorohydric acid (HAuCl43H2O) was purchased from Aldrich (>99.9%, US). Lysozyme was ordered from Shanghai Sangon Biotechnology Co. Ltd (Shanghai, China). Water (>18.2 MX cm) used for the experiments was purified by a Milli-Q system. Apparatus
Fig. 1. (A) TEM images of Lys-AuNCs. (B) The FTIR spectra of lysozyme and Lys-AuNCs.
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D. Lu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 77–80
4Au þ 8CN þ O2 þ 2H2 O ¼ 4½AuðCNÞ2 þ 4OH
ð1Þ
Although gold nanoparticles are etched, their core size decreases, the change of the emission peak position could not be observed. Quenching of fluorescence Lys-AuNCs could be described by the Stern–Volmer equation: I0/I = 1 + KSV[Q], Where I and I0 denote the fluorescent intensity at 650 nm with and without CN, respectively. As shown in Fig. 4B, the results could be described by Stern–Volmer equation.
F 0 =F ¼ 1:12 þ 15473½CN
ð2Þ 2
Fig. 2. The absorption (a) and emission spetra (b) of Lys-AuNCs.
(pKa = 9.36) [11]. Thus, the fluorescence quenching effect was obtained in the pH range of 7.0–13.0. As shown in Fig. 3B, the fluorescence quenching effect reached a maximum at pH 11.0, so the pH 11.0 was selected as optimum condition.
Detection of CN based on Lys-AuNCs The sensitivity and linearity of the Lys-AuNCs system were evaluated by the varying CN concentration. Fig. 4A showed the fluorescence spectra titration of Lys-AuNCs in the presence of CN anion at pH 11.0. The free Lys-AuNCs exhibited a red emission at 650 nm. A strong coordination is established between the Au+ and CN. The formation of [Au(CN)2] could significantly quench the fluorescence of Lys-AuNCs. Upon the addition of CN anion, the emission spectra of Lys-AuNCs decreased linearly at 650 nm. Au could be oxidized in the presence of O2 and cyanide according to Eq. (1). [23]
An excellent linear relation with r of 0.9958 was obtained over the concentration ranges of 5–120 lM. Under the current experimental condition, the detection limit was measured to be 1.9 107 M. which was much lower than the World Health Organization (WHO)’s limit (1.9 lM) for drinking water. Selectivity for CN detection The interference of foreign anions (F, Cl, Br, I, CO2 3 , 2 CH3 COO , NO , and C6 H5 O3 3 , NO2 , ClO4 , SCN , EDTA 7 ) to LysAuNCs was examined with the same concentration as CN of 50 lM. Fig. 5A showed the effect of anions on fluorescence intensity of Lys-AuNCs at pH 11.0. As could be seen, these competitive anions showed weak interference to cyanide detection and did not lead to any significant fluorescence changes at all. The detection selectivity could be visualized with the naked eye. As shown in Fig. 5B, the photograph took under visible light and UV light. CN could react with Lys-AuNCs resulted in an obvious color change from light yellow to colorless under visible light. Correspondingly, a bright red fluorescence disappeared and a blue fluorescence appeared under UV light. Because the blue fluorescence of lysozyme was covered by the red fluorescence of LysAuNCs, Thus, in the presence of cyanide, AuNCs would be gradually
Fig. 3. (A) The fluorescence spectra of Lys-AuNCs in the absence (a) and presence (b) of 3.05 104 M CN. (B) The effects of pH on the fluorescence response of Lys-AuNCs in the presence of 50 lM CN.
Fig. 4. (A) Fluorescence response of Lys-AuNCs upon addition of CN under optimum conditions. CN concentration (from 1 to 16, lM): 0, 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 120. (B) Stern–Volmer plot of fluorescence quenching of Lys-AuNCs by CN.
80
D. Lu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 77–80
Fig. 5. (A) The fluorescence response of Lys-AuNCs in the presence of 50 lM various anions. (B) The color (top) and the fluorescent (bottom) changes of Lys-AuNCs solution to different anions ions (concentration of all the anions ions was 3 mM).
etched; the red fluorescence of AuNCs disappeared. Numerous anions were subjected to Lys-AuNCs, unique color change of CN was found and easy to be recognized. Conclusions In summary, a facile method of lysozyme assembled AuNCs for monitoring CN was proposed. Upon the addition of CN, the fluorescence emission of Lys-AuNCs was steadily quenched. The fluorescent probe of Lys-AuNCs allowed for the sensing of CN in the range of 5.00 106 M to 1.20 104 M with a detection limit as low as 1.9 107 M. The Lys-AuNCs could be visualized with the naked eye for CN. Lys-AuNCs will be served as a probe for the further design and application in fluorescence imaging, environmental analysis, and so on. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21175086 and 21175087) and Hundred Talents Program of Shanxi province. References [1] G.J. Mohr, Sens. Actuators. B. 107 (2005) 2–13. [2] R. Bhattacharya, S.J.S. Flora, in: R.C. Gupta (Ed.), Handbook of Toxicology of Chemical Warfare Agents, Academic Press, Boston, 2009, pp. 255–270.
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