Research article Received: 21 October 2013,
Revised: 17 February 2014,
Accepted: 20 February 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2670
Amino acids as novel nucleophiles for silver nanoparticle-luminol chemiluminescence Na Lia* and Shubiao Nib ABSTRACT: The use of noble metal nanoparticles (NPs) as reductants in chemiluminescence (CL) has been reported only rarely owing to their high oxidation potentials. Interestingly, nucleophiles could dramatically lower the oxidation potential of Ag NPs, such that in the presence of nucleophiles Ag NPS could be used as reductants to induce the CL emission of luminol, an important CL reagent widely used in forensic analysis for the detection of trace amounts of blood. Although nucleophiles are indispensible in Ag NP-luminol CL, only inorganic nucleophiles such as Cl-, Br-, I- and S2O3 2- have been shown to be efficient. The effects of organic nucleophiles on CL remain unexplored. In this study, 20 standard amino acids were evaluated as novel organic nucleophiles in Ag NP-luminol CL. Histidine, lysine and arginine could initiate CL emission; the others could not. It is proposed that the different behaviors of 20 standard amino acids in the CL reactions derive from the interface chemistry between Ag NPs and these amino acids. UV/vis absorption spectra were studied to validate the interface chemistry. In addition, imidazole and histidine were chosen as a model pair to compare the behavior of the monodentate nucleophile with that of the corresponding multidentate nucleophile in Ag NP-luminol CL. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: amino acids; nucleophiles; Ag nanoparticles; chemiluminescence
→O - þ CuðIIÞ LH - þ O - →AP - * þ N AP - *→AP - þ hv
Introduction
CuðIÞ þ O2
Noble metal nanoparticles (NPs), especially Ag and Au NPs, have been widely studied because of their excellent performance in microelectronics, optics, electronics, magnetic devices and catalysis (1–3). Recently, noble metal NPs have been used in a class of homogeneous chemical reactions accompanied by light emission, i.e. chemiluminescence (CL) (4–7). Noble metal NPs participate as catalysts or reductants in CL reactions; their use as catalysts has gained much more attention (8–19), while their use as reductants has been rarely reported owing to their high oxidation potentials. Interestingly, nucleophiles can drastically lower the oxidation potential of Ag NPs (20–24), paving the way for Ag NPs to be utilized as reductants in CL reactions. Lucigenin, an important CL reagent that easily produces CL emission when reduced under alkaline conditions, could be reduced by Ag NPs when potassium iodide was used as the nucleophile to lower the oxidation potential of Ag NPs (25). Unlike iodide ions, other nucleophiles such as cysteine, mercaptoacetic acid, mercaptopropionic acid and thiourea have been shown to be efficient in these reactions. Surprisingly, luminol, another important CL reagent, which easily produces CL emission when it is oxidized under alkaline conditions, can also induce CL reactions by Ag NPs in the presence of halogen ions and Cu(II) (26). The CL mechanism has been shown to be as follows: Ag NPs, with the aid of halogen ions, reduce Cu(II) to produce a Cu(I) complex; the Cu(I) complex reacts with the dissolved oxygen to form the superoxide anion; and then the superoxide anion reacts with luminol to produce CL.
CuðIIÞ þ AgNPsð0Þ
Luminescence 2014
→
Nucleophiles
CuðIÞ þ AgðIÞ
(1)
• 2
• 2
2
2
2
2
(2) (3) (4)
The Cu(I) complex is a key intermediate in the CL reaction. Halogen ions are considered to be nucleophiles, and have two important functions in the luminol CL. One is to enhance the redox capability of Ag NPs, which reduce Cu(II) to generate the Cu(I) complex. The other function is to bind to Cu(I) to avoid dismutation. Although nucleophiles are indispensable when Ag NPs are used as reductants for the CL reactions of luminol, only inorganic nucleophiles, including halogen ions and S2O3 2-, have been shown to be efficient. The effects of organic nucleophiles on such CL remain unexplored. In this study, 20 standard amino acids were evaluated as novel organic nucleophiles in Ag NPluminol CL reactions. It was found that histidine, lysine and arginine could initiate the CL of luminol. The mechanism was studied in view of the interface chemistry between the amino acids and Ag NPs using absorption spectra. Moreover, imidazole
* Correspondence to: N. Li, State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, 610059, People’s Republic of China. E-mail: [email protected] a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, 610059, People’s Republic of China
b
China National Analytical Center Guangzhou, Guangzhou, Guangdong, 510070, People’s Republic of China
Copyright © 2014 John Wiley & Sons, Ltd.
N. Li and S. Ni and histidine were selected to compare the efficacy of the monodentate nucleophile with its related multidentate nucleophile in Ag NP-luminol CL reactions.
Experimental Chemicals and solutions A 1.0 × 10-2 mol/L stock solution was prepared by dissolving luminol (Sigma, St. Louis, MO, USA) in 0.10 mol/L sodium hydroxide solution. Amino acid, AgNO3, CuSO4, NaBH4, sodium citrate and H2O2 were obtained from Guangzhou Reagents (Guangzhou, China). Imidazole was purchased from Sangon Biotech Co. Ltd. (Shanghai, China). All reagents were of analytical grade and redistilled water was used throughout. Preparation of Ag colloid Ag colloid was synthesized using the chemical reduction method in solution, as described in the literature (12). In brief, 20 mL of 1.0 × 10-3 mol/L AgNO3 aqueous solution was added dropwise to 75 mL of 2 × 10-3 mol/L NaBH4 aqueous solution with vigorous stirring. Five minutes later, 5 mL of 1% (w/w) sodium citrate aqueous solution was added to stabilize the colloid. The colloid was stirred for another 20 min and aged for 2 days at 4 °C before use. The resulting yellow Ag colloid exhibited surface plasmon resonance (SPR) absorption bands in the visible region (~ 400 nm), in good agreement with reported results for spherical Ag particles ~ 20 nm in diameter (12). Assuming that the Ag NPs have a density equivalent to that of bulk silver (10.5 g/cm3), the concentration of ~ 20 nm spherical Ag NPs was calculated as ~ 8.2 × 10-10 mol/L.
while the other amino acids can not generate CL under the same reaction conditions. A mechanism was proposed for the Ag NP–nucleophile– CuSO4–luminol CL reactions, as given above (eqns 1–4). Based on the proposed mechanism, Cu(I) complex is a key intermediate in the CL reactions. It was deduced that efficient nucleophiles such as histidine, lysine and arginine could decrease the oxidation potential of Ag NPs, which reduced Cu (II) to generate Cu(I) complex. To validate this hypothesis, UV/ vis absorption spectra were used to monitor the reaction between CuSO4 and Ag NPs in the presence of amino acids. Ag NP–histidine–CuSO4 was chosen as a model system in this study. Ag NPs exhibited SPR absorption bands in the visible region (~ 400 nm) before the reaction, in good agreement with reported results (12). When histidine and CuSO4 were added to the yellow Ag colloid, time-dependent SPR absorption spectra were recorded for the mixture. The SPR absorption spectra were measured at a time interval of ~ 2.4 s. Although more than 40 spectra were obtained in 1.6 min, only five typical spectra at different moments are shown in Fig. 2. Clearly, as the reaction time increases, the SPR absorbance of the mixture at 400 nm decreases
Optical measurements CL was detected using a system that included a syringe pump (KD Scientific Inc., Holliston, MA, USA), a model IFFS-A luminometer (Ruimai Electronic Science Co., China), a photomultiplier (CR105 Bingsong Electronic Co., China), a 3 mL quartz tube (used as a detection cell) and a computer. In a typical experiment, a mixture of 100 μL Ag colloid, 100 μL histidine and 100 μL CuSO4 was pipetted into a quartz tube, which was used as the test cell, and then 100 μL of luminol was injected into the mixture using the syringe pump. In the continuous injection experiment, the injection volume of luminol was 10 μL each time and the time interval of the injection was ~ 20 s. Light emission was measured by the luminometer during injection. UV/vis spectra were measured on a model G1315B Diode Array Detection (Agilent, Santa Clara, CA, USA).
Figure 1. CL signal for 20 standard amino acids in the Ag NP-luminol CL system. –3 –3 Reaction conditions: CuSO4, 1.0 × 10 mol/L; luminol, 1.0 × 10 mol/L in 0.05 mol/L –4 –2 carbonate buffer (pH 10.0); Ag, 2.0 × 10 mol/L; amino acid, 1.0 × 10 mol/L. H, histidine; K, lysine; R, arginine; E, glutamic acid; T, threonine; Y, tyrosine; A, alanine; V, valine; S, serine; M, methionine; F, phenylalanine; D, aspartic acid; L, leucine; I, isoleucine; P, proline; N, asparagine; G, glycine; W, tryptophan; C, cysteine; Q, glutamine.
Results and discussions Twenty standard amino acids as nucleophiles in the Ag colloid–CuSO4–luminol CL system Twenty standard amino acids, histidine, lysine, arginine, glutamic acid, threonine, tyrosine, alanine, valine, serine, methionine, phenylalanine, aspartic acid, leucine, isoleucine, proline, asparagine, glycine, tryptophan, cysteine and glutamine, were studied as nucleophiles in the Ag colloid–CuSO4–luminol CL system during a static injection of luminol, as described above. As shown in Fig. 1, histidine, lysine and arginine exhibit an obvious signal,
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Figure 2. Time-dependent UV/vis absorption spectra during the reaction between CuSO4 and Ag NPs in the presence of histidine. Five selected spectra were recorded at 0, 12, 36, 60 and 96 s after the addition of CuSO4 in the mixing solution of the Ag –3 colloid with histidine. Conditions: CuSO4, 1.0 × 10 mol/L; histidine, 0.01 mol/L; Ag, –4 –3 2.0 × 10 mol/L; luminol, 1.0 × 10 mol/L in 0.05 mol/L carbonate buffer (pH 10.0).
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2014
Amino acids in Ag nanoparticle-luminol chemiluminescence gradually. After the reaction, the yellow color of the mixture disappeared. These results indicated that Ag NPs could react with CuSO4 in the presence of histidine. Meanwhile, other amino acids were investigated under the same reaction conditions. Arginine and lysine led to fading of the Ag NPs, as did histidine, in good agreement with their responses in the CL tests (Fig. 1). Because of their importance in the clinical diagnostics of a variety of metabolic disorders and human diseases, cysteine and its analogs have been intensively evaluated in CL systems with metal NPs as catalysts, especially in luminol–H2O2 systems (27–30). In this case, aminothiols can greatly inhibit the luminol CL signals because they can bind tightly at the surface of Au NPs due to the Au–S interaction, making Au NPs inefficient in the catalytic decomposition reaction of H2O2 (27). Hence, in this study, cysteine was considered as a potential nucleophile to be evaluated in the CL test due to the similar interaction between cysteine and Ag NPs. However, although cysteine could lead to fading of the mixture of Ag colloid and CuSO4, it could not generate a significant signal in the CL test. The reason for this is likely to be the aggregation and deposition of citrate-capped Ag NPs in the presence of cysteine, which has been reported previously (31,32). When cysteine was added to the Ag colloid, the aggregation and deposition of Ag NPs might significantly decrease the active surface so that CL emission with redox reactions of Ag atoms on the surface of Ag NPs was aborted. The different behaviors of the 20 standard amino acids in the CL reactions derive from the contributions of their various side chains. The considerable CL emission seen for histidine is generated by injecting luminol, while alanine, without an obvious CL signal, could be used as a control to support the function of the imidazole group of histidine in the reactions. In addition, it was reported that imidazole, as an efficient nucleophile, could initiate the redox reaction between Cu2+ and Ag colloid (21). Absorption of imidazole on Ag NPs can dramatically alter their redox reactivity. Without imidazole, the addition of Cu2+ could not result in the oxidation of Ag particles. It has been reported that the Fermi level of Ag NPs modified by imidazole is significantly shifted toward a negative potential (approximately –
0.4 V), and is more negative than the standard redox potential of the Cu2+/Cu+ couple (0.153 V) (21). As a result, the reducing ability of Ag NPs in the presence of imidazole is enhanced so that Cu2+ could be reduced to produce Cu(I) complex, a key intermediate in the CL reactions. Thus, the behavior of histidine in the CL reactions could be derived from the interface chemistry of Ag NPs and its side chain, the imidazole moiety. The different affects of imidazole and histidine as nucleophiles in CL reactions should be the next issue of interest. Comparison between the monodentate nucleophile and its related multidentate nucleophile in the Ag NP-luminol CL Imidazole and histidine were selected as a model pair to investigate the different behaviors of the monodentate nucleophile and the related multidentate nucleophile in an Ag NP-luminol CL. The activities of imidazole and histidine in CL reactions were tested in continuous injection experiments. As shown as in Fig. 3, the CL signal of imidazole is much weaker than that of histidine. It has been reported that the reducing ability of Ag NPs in the presence of monodentate nucleophiles was closely related to the interaction of monodentate nucleophiles with the surface Ag atom (26). For example, because the interaction of Cl- with Ag NPs was less powerful than that of Br-, the reducing ability of Ag NPs in the presence of NaCl is weaker than that in the presence of NaBr at the same concentration. As a result, Ag NPs in the presence of NaBr could induce the luminol CL, but Ag NPs in the presence of NaCl could not under the same reaction conditions. Similarly, the interaction of imidazole with Ag NPs is weaker than that of histidine. The reducing ability of Ag NPs in the presence of imidazole must be weaker than that in the presence of histidine. The oxidation potential of Ag to Ag+ can be expressed by the Nernst equation:
→Ag
Ag
þ
þ e
(5) þ
E1 ¼ E0 þ ðRT=FÞ ln½Ag
(6)
where, E0 is the standard potential of Ag+/Ag. In the presence of nucleophiles, the oxidation potential of Ag to Ag+ can be expressed as:
Figure 3. CL profiles by continuously injecting luminol solution into the mixture of Ag colloid, CuSO4 and nucleophiles, such as histidine (A) and imidazole (B). Conditions: –3 –4 –4 –3 CuSO4, 1.0 × 10 mol/L; Ag, 2.0 × 10 mol/L; 2.0 × 10 mol/L of Ag, luminol, 1.0 × 10 mol/L in 0.05 mol/L carbonate buffers (pH 10.0); imidazole, 0.01 mol/L; histidine, 0.01 mol/L.
Luminescence 2014
Copyright © 2014 John Wiley & Sons, Ltd.
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N. Li and S. Ni
Conclusions
Figure 4. Effect of pH on the CL reactions of luminol–CuSO4–Ag NPs in the pres–3 –4 ence of histidine. Conditions: CuSO4, 1.0 × 10 mol/L; Ag, 2.0 × 10 mol/L; histi–3 dine, 0.01 mol/L; luminol, 1.0 × 10 mol/L in 0.05 mol/L carbonate buffers with different pH values.
→AgLn
Agþ þ nL
β ¼ K1 K2 Kn ¼
þ
(7)
½AgLnþ ½Agþ ½Ln
(8)
E2 ¼ E0 þ ðRT=FÞ lnð½AgLnþ =β½Ln Þ
(9)
where, L is the ligand as nucleophiles; β is the cumulative or overall constant; K is the stability constant of Ag+–L and refers to formation of the complexes on step at a time. One can see that if β increases, the oxidation potential of Ag decreases, according to eqn (9.) Therefore, an increase in β promotes the tendency for the oxidation of Ag NPs by Cu(II). It is known that the stability constant for a 1: 2 complex of Ag+–histidine (β = 109.21) (33) is higher than that for Ag+–imidazole (β = 106.94) (34). Thus, the oxidation potential of Ag in the presence of histidine is lower than that of Ag in the presence of imidazole. It is reasonable that the CL signal of histidine is stronger than that of imidazole. In addition, as shown in Fig. 3, when the luminol solution was successively injected into the mixture, the CL intensity of histidine increased, whereas the CL intensity of imidazole decreased. The increase in the CL intensity for histidine was likely due to differences in the pH of the CL reactions. The effects of pH on the CL reactions of luminol–CuSO4–Ag NPs in the presence of histidine were investigated. As shown in Fig. 4, when the pH of the reaction mixture was > 9.3–12.5, the CL intensity increased. In the continuous injection experiment, the pH of the mixture, which was considered to be the pH of the CL reactions, was measured once the luminol solution was injected. The pH values for the mixture were 7.0, 9.6, 10.4, 10.5, 10.6 and 10.7, respectively, for the six injections. An obvious increase in the pH of the mixture appeared with the first two injections of the luminol solution, whereas the pH of the mixture increased only slightly with further injections. Consequently, obvious increases in the CL intensity were obtained with the first two injections, and the CL intensity was steady on further additions. By contrast, the decrease in CL intensity of imidazole might result from a decrease in Cu(I). Cu(I) must be consumed when luminol is injected into the system. The production of Cu(I) is probably slow with imidazole as the nucleophile such that the freshly generated Cu(I) was not be enough to compensate for the loss of Cu(I) with the previous injection. In fact, the time interval of the injection was always 20 s. Thus, the accumulated time of the Cu(I) is also 20 s after the first injection and the CL intensity tend to be steady with further injections.
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Twenty standard amino acids were employed as novel organic nucleophiles in Ag NP-luminol CL reactions. Histidine, lysine and arginine could initiate the CL of the luminol–CuSO4–Ag colloid system and others could not. The different behaviors of the 20 amino acids in CL reactions were proposed to derive from interface chemistry between the amino acids and Ag NPs. Analyses of the interface chemistry between amino acids and Ag NPs by virtue of absorption spectra were in good agreement with the CL data. Furthermore, imidazole and histidine were chosen to investigate the different behaviors of the monodentate nucleophile and the related multidentate nucleophile. The CL signal of histidine was much stronger than that of imidazole because the interaction of histidine with Ag NPs was much stronger than that of imidazole. This work is of great importance in gaining a better understanding of the special properties of Ag NPs in the presence of nucleophiles, and also greatly expands the scope of reagents in Ag NP-luminol CL reactions, which may find future application in the development of analytical methods for the selective determination of those organic nucleophiles such as histidine, lysine and arginine. Acknowledgements The financial support of this research by China National Analytical Center Guangzhou is gratefully acknowledged.
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Amino acids in Ag nanoparticle-luminol chemiluminescence 15. Li N, Wang W, Tian D, Cui H. pH-dependent catalytic properties of Pd–Ag nanoparticles in luminol chemiluminescence. Chem Commun 2010;46:1520–2. 16. Liu B, He Y, Duan C, Li N, Cui H. Platinum nanoparticle-catalyzed lucigenin–hydrazine chemiluminescence. J Photochem Photobio A 2011;217:62–7. 17. Kamruzzaman M, Alam AM, Kim KM, Lee SH, Kim YH, Kabir ANMH, et al. Chemiluminescence microfluidic system of gold nanoparticles enhanced luminol–silver nitrate for the determination of vitamin B12. Biomed Microdevice 2013;15:195–202. 18. Ge L, Wang SM, Song XR, Ge SG, Yu JH. 3D origami-based multifunction-integrated immunodevice: low-cost and multiplexed sandwich chemiluminescence immunoassay on microfluidic paperbased analytical device. Lab Chip 2012;12:3150–8. 19. Liu C, Li BX. Silver nanoparticle-initiated chemiluminescence reaction of luminol–AgNO3 and its analytical application. Anal Bioanal Chem 2011;401:229–35. 20. Mulvaney P, Linnert T, Henglein A. Surface-chemistry of colloidal silver in aqueous-solution – observations on chemisorption and reactivity. J Phys Chem 1991;95:7843–6. 21. Vukovic VV, Nedeljkovic JM. Surface modification of nanometerscale silver particles by imidazole. Langmuir 1993;9:980–3. 22. Sibbald MS, Chumanov G, Cotton TM. Reduction of cytochrome c by halide-modified, laser-ablated silver colloids. J Phys Chem 1996;100:4672–8. 23. Bonifacio A, Keizers PHJ, Vermeulen NPE, Commandeur JNM, Gooijer C, Zwan G. Surface-enhanced resonance Raman scattering of cytochrome P450-2D6 on coated silver hydrosols. Langmuir 2007;23:1860–6. 24. McMahon JJ, Barry M, Breen KJ, Radziwon AK, Brooks LD, Blair MR. Photocatalysis of the oxygen reduction reaction at adsorbatecovered silver. J Phys Chem C 2008;112:1158–66.
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25. Guo JZ, Cui H. Lucigenin chemiluminescence induced by noble metal nanoparticles in the presence of adsorbates. J Phys Chem C 2007;111:12254–9. 26. Li N, Gu J, Cui H. Luminol chemiluminescence induced by silver 2+ nanoparticles in the presence of nucleophiles and Cu . J Photochem Photobio A 2010;215:185–90. 27. Li QQ, Liu F, Lu C, Lin JM. Aminothiols sensing based on fluorosurfactant-mediated triangular gold nanoparticle-catalyzed luminol chemiluminescence. J Phys Chem C 2011;115:10964–70. 28. Li QQ, Shang F, Lu C, Zheng ZX, Lin JM. Fluorosurfactant-prepared triangular gold nanoparticles as postcolumn chemiluminescence reagents for high-performance liquid chromatography assay of low molecular weight aminothiols in biological fluids. J Chromatogr A 2011;1218:9064–70. 29. Bai SL, Chen QS, Lu C, Lin JM. Automated high performance liquid chromatography with on-line reduction of disulfides and chemiluminescence detection for determination of thiols and disulfides in biological fluids. Anal Chim Acta 2013;768:96–101. 30. Zhang LJ, Lu BQ, Lu C. Chemiluminescence sensing of aminothiols in biological fluids using peroxymonocarbonate-prepared networked gold nanoparticles. Analyst 2013;138:850–5. 31. Chen Z, He Y, Luo S, Lin H, Chen Y, Sheng P, Li J, Chen B, Liu C, Cai Q. Label-free colorimetric assay for biological thiols based on ssDNA/silver nanoparticle system by salt amplification. Analyst 2010;135:1066–9. 32. Wang GL, Dong YM, Zhu XY, Zhang WJ, Wang C, Jiao HJ. Ultrasensitive and selective colorimetric detection of thiourea using silver nanoprobes. Analyst 2011;136:5256–60. 33. You YH, Zhang CP. Interaction between silver ions and histidine. J Clin Rehabil Tissue Eng Res 2010;14:5498–504. 34. Miyajima T, Nishimura H, Kodama H, Ishiguro SI. On the complexation of Ag(I) and Cu(II) ions with poly(N-vinylimidazole). React Funct Polym 1998;38:183–95.
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence
Revised: 17 February 2014,
Accepted: 20 February 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2670
Amino acids as novel nucleophiles for silver nanoparticle-luminol chemiluminescence Na Lia* and Shubiao Nib ABSTRACT: The use of noble metal nanoparticles (NPs) as reductants in chemiluminescence (CL) has been reported only rarely owing to their high oxidation potentials. Interestingly, nucleophiles could dramatically lower the oxidation potential of Ag NPs, such that in the presence of nucleophiles Ag NPS could be used as reductants to induce the CL emission of luminol, an important CL reagent widely used in forensic analysis for the detection of trace amounts of blood. Although nucleophiles are indispensible in Ag NP-luminol CL, only inorganic nucleophiles such as Cl-, Br-, I- and S2O3 2- have been shown to be efficient. The effects of organic nucleophiles on CL remain unexplored. In this study, 20 standard amino acids were evaluated as novel organic nucleophiles in Ag NP-luminol CL. Histidine, lysine and arginine could initiate CL emission; the others could not. It is proposed that the different behaviors of 20 standard amino acids in the CL reactions derive from the interface chemistry between Ag NPs and these amino acids. UV/vis absorption spectra were studied to validate the interface chemistry. In addition, imidazole and histidine were chosen as a model pair to compare the behavior of the monodentate nucleophile with that of the corresponding multidentate nucleophile in Ag NP-luminol CL. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: amino acids; nucleophiles; Ag nanoparticles; chemiluminescence
→O - þ CuðIIÞ LH - þ O - →AP - * þ N AP - *→AP - þ hv
Introduction
CuðIÞ þ O2
Noble metal nanoparticles (NPs), especially Ag and Au NPs, have been widely studied because of their excellent performance in microelectronics, optics, electronics, magnetic devices and catalysis (1–3). Recently, noble metal NPs have been used in a class of homogeneous chemical reactions accompanied by light emission, i.e. chemiluminescence (CL) (4–7). Noble metal NPs participate as catalysts or reductants in CL reactions; their use as catalysts has gained much more attention (8–19), while their use as reductants has been rarely reported owing to their high oxidation potentials. Interestingly, nucleophiles can drastically lower the oxidation potential of Ag NPs (20–24), paving the way for Ag NPs to be utilized as reductants in CL reactions. Lucigenin, an important CL reagent that easily produces CL emission when reduced under alkaline conditions, could be reduced by Ag NPs when potassium iodide was used as the nucleophile to lower the oxidation potential of Ag NPs (25). Unlike iodide ions, other nucleophiles such as cysteine, mercaptoacetic acid, mercaptopropionic acid and thiourea have been shown to be efficient in these reactions. Surprisingly, luminol, another important CL reagent, which easily produces CL emission when it is oxidized under alkaline conditions, can also induce CL reactions by Ag NPs in the presence of halogen ions and Cu(II) (26). The CL mechanism has been shown to be as follows: Ag NPs, with the aid of halogen ions, reduce Cu(II) to produce a Cu(I) complex; the Cu(I) complex reacts with the dissolved oxygen to form the superoxide anion; and then the superoxide anion reacts with luminol to produce CL.
CuðIIÞ þ AgNPsð0Þ
Luminescence 2014
→
Nucleophiles
CuðIÞ þ AgðIÞ
(1)
• 2
• 2
2
2
2
2
(2) (3) (4)
The Cu(I) complex is a key intermediate in the CL reaction. Halogen ions are considered to be nucleophiles, and have two important functions in the luminol CL. One is to enhance the redox capability of Ag NPs, which reduce Cu(II) to generate the Cu(I) complex. The other function is to bind to Cu(I) to avoid dismutation. Although nucleophiles are indispensable when Ag NPs are used as reductants for the CL reactions of luminol, only inorganic nucleophiles, including halogen ions and S2O3 2-, have been shown to be efficient. The effects of organic nucleophiles on such CL remain unexplored. In this study, 20 standard amino acids were evaluated as novel organic nucleophiles in Ag NPluminol CL reactions. It was found that histidine, lysine and arginine could initiate the CL of luminol. The mechanism was studied in view of the interface chemistry between the amino acids and Ag NPs using absorption spectra. Moreover, imidazole
* Correspondence to: N. Li, State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, 610059, People’s Republic of China. E-mail: [email protected] a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, 610059, People’s Republic of China
b
China National Analytical Center Guangzhou, Guangzhou, Guangdong, 510070, People’s Republic of China
Copyright © 2014 John Wiley & Sons, Ltd.
N. Li and S. Ni and histidine were selected to compare the efficacy of the monodentate nucleophile with its related multidentate nucleophile in Ag NP-luminol CL reactions.
Experimental Chemicals and solutions A 1.0 × 10-2 mol/L stock solution was prepared by dissolving luminol (Sigma, St. Louis, MO, USA) in 0.10 mol/L sodium hydroxide solution. Amino acid, AgNO3, CuSO4, NaBH4, sodium citrate and H2O2 were obtained from Guangzhou Reagents (Guangzhou, China). Imidazole was purchased from Sangon Biotech Co. Ltd. (Shanghai, China). All reagents were of analytical grade and redistilled water was used throughout. Preparation of Ag colloid Ag colloid was synthesized using the chemical reduction method in solution, as described in the literature (12). In brief, 20 mL of 1.0 × 10-3 mol/L AgNO3 aqueous solution was added dropwise to 75 mL of 2 × 10-3 mol/L NaBH4 aqueous solution with vigorous stirring. Five minutes later, 5 mL of 1% (w/w) sodium citrate aqueous solution was added to stabilize the colloid. The colloid was stirred for another 20 min and aged for 2 days at 4 °C before use. The resulting yellow Ag colloid exhibited surface plasmon resonance (SPR) absorption bands in the visible region (~ 400 nm), in good agreement with reported results for spherical Ag particles ~ 20 nm in diameter (12). Assuming that the Ag NPs have a density equivalent to that of bulk silver (10.5 g/cm3), the concentration of ~ 20 nm spherical Ag NPs was calculated as ~ 8.2 × 10-10 mol/L.
while the other amino acids can not generate CL under the same reaction conditions. A mechanism was proposed for the Ag NP–nucleophile– CuSO4–luminol CL reactions, as given above (eqns 1–4). Based on the proposed mechanism, Cu(I) complex is a key intermediate in the CL reactions. It was deduced that efficient nucleophiles such as histidine, lysine and arginine could decrease the oxidation potential of Ag NPs, which reduced Cu (II) to generate Cu(I) complex. To validate this hypothesis, UV/ vis absorption spectra were used to monitor the reaction between CuSO4 and Ag NPs in the presence of amino acids. Ag NP–histidine–CuSO4 was chosen as a model system in this study. Ag NPs exhibited SPR absorption bands in the visible region (~ 400 nm) before the reaction, in good agreement with reported results (12). When histidine and CuSO4 were added to the yellow Ag colloid, time-dependent SPR absorption spectra were recorded for the mixture. The SPR absorption spectra were measured at a time interval of ~ 2.4 s. Although more than 40 spectra were obtained in 1.6 min, only five typical spectra at different moments are shown in Fig. 2. Clearly, as the reaction time increases, the SPR absorbance of the mixture at 400 nm decreases
Optical measurements CL was detected using a system that included a syringe pump (KD Scientific Inc., Holliston, MA, USA), a model IFFS-A luminometer (Ruimai Electronic Science Co., China), a photomultiplier (CR105 Bingsong Electronic Co., China), a 3 mL quartz tube (used as a detection cell) and a computer. In a typical experiment, a mixture of 100 μL Ag colloid, 100 μL histidine and 100 μL CuSO4 was pipetted into a quartz tube, which was used as the test cell, and then 100 μL of luminol was injected into the mixture using the syringe pump. In the continuous injection experiment, the injection volume of luminol was 10 μL each time and the time interval of the injection was ~ 20 s. Light emission was measured by the luminometer during injection. UV/vis spectra were measured on a model G1315B Diode Array Detection (Agilent, Santa Clara, CA, USA).
Figure 1. CL signal for 20 standard amino acids in the Ag NP-luminol CL system. –3 –3 Reaction conditions: CuSO4, 1.0 × 10 mol/L; luminol, 1.0 × 10 mol/L in 0.05 mol/L –4 –2 carbonate buffer (pH 10.0); Ag, 2.0 × 10 mol/L; amino acid, 1.0 × 10 mol/L. H, histidine; K, lysine; R, arginine; E, glutamic acid; T, threonine; Y, tyrosine; A, alanine; V, valine; S, serine; M, methionine; F, phenylalanine; D, aspartic acid; L, leucine; I, isoleucine; P, proline; N, asparagine; G, glycine; W, tryptophan; C, cysteine; Q, glutamine.
Results and discussions Twenty standard amino acids as nucleophiles in the Ag colloid–CuSO4–luminol CL system Twenty standard amino acids, histidine, lysine, arginine, glutamic acid, threonine, tyrosine, alanine, valine, serine, methionine, phenylalanine, aspartic acid, leucine, isoleucine, proline, asparagine, glycine, tryptophan, cysteine and glutamine, were studied as nucleophiles in the Ag colloid–CuSO4–luminol CL system during a static injection of luminol, as described above. As shown in Fig. 1, histidine, lysine and arginine exhibit an obvious signal,
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Figure 2. Time-dependent UV/vis absorption spectra during the reaction between CuSO4 and Ag NPs in the presence of histidine. Five selected spectra were recorded at 0, 12, 36, 60 and 96 s after the addition of CuSO4 in the mixing solution of the Ag –3 colloid with histidine. Conditions: CuSO4, 1.0 × 10 mol/L; histidine, 0.01 mol/L; Ag, –4 –3 2.0 × 10 mol/L; luminol, 1.0 × 10 mol/L in 0.05 mol/L carbonate buffer (pH 10.0).
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Luminescence 2014
Amino acids in Ag nanoparticle-luminol chemiluminescence gradually. After the reaction, the yellow color of the mixture disappeared. These results indicated that Ag NPs could react with CuSO4 in the presence of histidine. Meanwhile, other amino acids were investigated under the same reaction conditions. Arginine and lysine led to fading of the Ag NPs, as did histidine, in good agreement with their responses in the CL tests (Fig. 1). Because of their importance in the clinical diagnostics of a variety of metabolic disorders and human diseases, cysteine and its analogs have been intensively evaluated in CL systems with metal NPs as catalysts, especially in luminol–H2O2 systems (27–30). In this case, aminothiols can greatly inhibit the luminol CL signals because they can bind tightly at the surface of Au NPs due to the Au–S interaction, making Au NPs inefficient in the catalytic decomposition reaction of H2O2 (27). Hence, in this study, cysteine was considered as a potential nucleophile to be evaluated in the CL test due to the similar interaction between cysteine and Ag NPs. However, although cysteine could lead to fading of the mixture of Ag colloid and CuSO4, it could not generate a significant signal in the CL test. The reason for this is likely to be the aggregation and deposition of citrate-capped Ag NPs in the presence of cysteine, which has been reported previously (31,32). When cysteine was added to the Ag colloid, the aggregation and deposition of Ag NPs might significantly decrease the active surface so that CL emission with redox reactions of Ag atoms on the surface of Ag NPs was aborted. The different behaviors of the 20 standard amino acids in the CL reactions derive from the contributions of their various side chains. The considerable CL emission seen for histidine is generated by injecting luminol, while alanine, without an obvious CL signal, could be used as a control to support the function of the imidazole group of histidine in the reactions. In addition, it was reported that imidazole, as an efficient nucleophile, could initiate the redox reaction between Cu2+ and Ag colloid (21). Absorption of imidazole on Ag NPs can dramatically alter their redox reactivity. Without imidazole, the addition of Cu2+ could not result in the oxidation of Ag particles. It has been reported that the Fermi level of Ag NPs modified by imidazole is significantly shifted toward a negative potential (approximately –
0.4 V), and is more negative than the standard redox potential of the Cu2+/Cu+ couple (0.153 V) (21). As a result, the reducing ability of Ag NPs in the presence of imidazole is enhanced so that Cu2+ could be reduced to produce Cu(I) complex, a key intermediate in the CL reactions. Thus, the behavior of histidine in the CL reactions could be derived from the interface chemistry of Ag NPs and its side chain, the imidazole moiety. The different affects of imidazole and histidine as nucleophiles in CL reactions should be the next issue of interest. Comparison between the monodentate nucleophile and its related multidentate nucleophile in the Ag NP-luminol CL Imidazole and histidine were selected as a model pair to investigate the different behaviors of the monodentate nucleophile and the related multidentate nucleophile in an Ag NP-luminol CL. The activities of imidazole and histidine in CL reactions were tested in continuous injection experiments. As shown as in Fig. 3, the CL signal of imidazole is much weaker than that of histidine. It has been reported that the reducing ability of Ag NPs in the presence of monodentate nucleophiles was closely related to the interaction of monodentate nucleophiles with the surface Ag atom (26). For example, because the interaction of Cl- with Ag NPs was less powerful than that of Br-, the reducing ability of Ag NPs in the presence of NaCl is weaker than that in the presence of NaBr at the same concentration. As a result, Ag NPs in the presence of NaBr could induce the luminol CL, but Ag NPs in the presence of NaCl could not under the same reaction conditions. Similarly, the interaction of imidazole with Ag NPs is weaker than that of histidine. The reducing ability of Ag NPs in the presence of imidazole must be weaker than that in the presence of histidine. The oxidation potential of Ag to Ag+ can be expressed by the Nernst equation:
→Ag
Ag
þ
þ e
(5) þ
E1 ¼ E0 þ ðRT=FÞ ln½Ag
(6)
where, E0 is the standard potential of Ag+/Ag. In the presence of nucleophiles, the oxidation potential of Ag to Ag+ can be expressed as:
Figure 3. CL profiles by continuously injecting luminol solution into the mixture of Ag colloid, CuSO4 and nucleophiles, such as histidine (A) and imidazole (B). Conditions: –3 –4 –4 –3 CuSO4, 1.0 × 10 mol/L; Ag, 2.0 × 10 mol/L; 2.0 × 10 mol/L of Ag, luminol, 1.0 × 10 mol/L in 0.05 mol/L carbonate buffers (pH 10.0); imidazole, 0.01 mol/L; histidine, 0.01 mol/L.
Luminescence 2014
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N. Li and S. Ni
Conclusions
Figure 4. Effect of pH on the CL reactions of luminol–CuSO4–Ag NPs in the pres–3 –4 ence of histidine. Conditions: CuSO4, 1.0 × 10 mol/L; Ag, 2.0 × 10 mol/L; histi–3 dine, 0.01 mol/L; luminol, 1.0 × 10 mol/L in 0.05 mol/L carbonate buffers with different pH values.
→AgLn
Agþ þ nL
β ¼ K1 K2 Kn ¼
þ
(7)
½AgLnþ ½Agþ ½Ln
(8)
E2 ¼ E0 þ ðRT=FÞ lnð½AgLnþ =β½Ln Þ
(9)
where, L is the ligand as nucleophiles; β is the cumulative or overall constant; K is the stability constant of Ag+–L and refers to formation of the complexes on step at a time. One can see that if β increases, the oxidation potential of Ag decreases, according to eqn (9.) Therefore, an increase in β promotes the tendency for the oxidation of Ag NPs by Cu(II). It is known that the stability constant for a 1: 2 complex of Ag+–histidine (β = 109.21) (33) is higher than that for Ag+–imidazole (β = 106.94) (34). Thus, the oxidation potential of Ag in the presence of histidine is lower than that of Ag in the presence of imidazole. It is reasonable that the CL signal of histidine is stronger than that of imidazole. In addition, as shown in Fig. 3, when the luminol solution was successively injected into the mixture, the CL intensity of histidine increased, whereas the CL intensity of imidazole decreased. The increase in the CL intensity for histidine was likely due to differences in the pH of the CL reactions. The effects of pH on the CL reactions of luminol–CuSO4–Ag NPs in the presence of histidine were investigated. As shown in Fig. 4, when the pH of the reaction mixture was > 9.3–12.5, the CL intensity increased. In the continuous injection experiment, the pH of the mixture, which was considered to be the pH of the CL reactions, was measured once the luminol solution was injected. The pH values for the mixture were 7.0, 9.6, 10.4, 10.5, 10.6 and 10.7, respectively, for the six injections. An obvious increase in the pH of the mixture appeared with the first two injections of the luminol solution, whereas the pH of the mixture increased only slightly with further injections. Consequently, obvious increases in the CL intensity were obtained with the first two injections, and the CL intensity was steady on further additions. By contrast, the decrease in CL intensity of imidazole might result from a decrease in Cu(I). Cu(I) must be consumed when luminol is injected into the system. The production of Cu(I) is probably slow with imidazole as the nucleophile such that the freshly generated Cu(I) was not be enough to compensate for the loss of Cu(I) with the previous injection. In fact, the time interval of the injection was always 20 s. Thus, the accumulated time of the Cu(I) is also 20 s after the first injection and the CL intensity tend to be steady with further injections.
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Twenty standard amino acids were employed as novel organic nucleophiles in Ag NP-luminol CL reactions. Histidine, lysine and arginine could initiate the CL of the luminol–CuSO4–Ag colloid system and others could not. The different behaviors of the 20 amino acids in CL reactions were proposed to derive from interface chemistry between the amino acids and Ag NPs. Analyses of the interface chemistry between amino acids and Ag NPs by virtue of absorption spectra were in good agreement with the CL data. Furthermore, imidazole and histidine were chosen to investigate the different behaviors of the monodentate nucleophile and the related multidentate nucleophile. The CL signal of histidine was much stronger than that of imidazole because the interaction of histidine with Ag NPs was much stronger than that of imidazole. This work is of great importance in gaining a better understanding of the special properties of Ag NPs in the presence of nucleophiles, and also greatly expands the scope of reagents in Ag NP-luminol CL reactions, which may find future application in the development of analytical methods for the selective determination of those organic nucleophiles such as histidine, lysine and arginine. Acknowledgements The financial support of this research by China National Analytical Center Guangzhou is gratefully acknowledged.
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