pubs.acs.org/Langmuir © 2010 American Chemical Society
Metal-Metal-Interaction-Facilitated Coordination Polymer as a Sensing Ensemble: A Case Study for Cysteine Sensing Jiang-Shan Shen, Dong-Hua Li, Ming-Bo Zhang, Jun Zhou, Hui Zhang, and Yun-Bao Jiang* Department of Chemistry, College of Chemistry and Chemical Engineering, and the MOE Key Laboratory of Analytical Sciences, Xiamen University, Xiamen 361005, China Received August 7, 2010. Revised Manuscript Received November 15, 2010 A detailed investigation of the absorption and CD signals of Ag(I)-cysteine (Cys) aqueous solutions at buffered or varying pH has allowed us to suggest that coordination polymers are formed upon mixing Ag(I) and Cys bearing a Ag(I)-Cys repeat unit. The formation of the coordination polymers are shown to be facilitated by both the Ag(I) 3 3 3 Ag(I) interaction and the interaction between the side chains in the polymeric backbone. The former allows for an immediate spectral sensing of Cys with enantiomeric discrimination capacity with both high sensitivity and selectivity, and the contribution of the side-chain/side-chain interaction serves to guide extended sensing applications by means of modulating this interaction. With our preliminary data on the corresponding Cu(I)-Cys and Au(I)-Cys systems that exhibited similar spectral signals, we conclude that the M(I)-SR coordination polymers (M = Cu, Ag, or Au) could in general function as spectral sensing ensembles for extended applications. This sensing ensemble involves the formation of coordination polymers with practically no spectral background, thus affording high sensing sensitivity and selectivity.
1. Introduction Recent developments in the fabrication of nanostructures of coinage metals (Au, Ag, and Cu) and related coinage metal sulfides have made rewarding use of thiol compounds as capping ligands and/or reductants, in which procedure-metallophilicity-facilitated M(I)-SR polymers were suggested to be the synthesis precursors.1 In fact, related investigations have become active in the past several years. For example, two crystal structures of thiolate-protected Au nanocrystals have recently been obtained.2 Crystal structure analyses indicated that these nanocrystals were well protected by thiolate-Au(I) polymers, although the roles and properties of these polymers remain unclear. By applying a similar strategy and employing chiral ligands, chiral Au or Ag nanostructures with observable optical activity have been successfully constructed.3 The origin of the observed chiroptical activity, however, has not been fully understood. In addition, SH-containing amino acids and peptides such as cysteine (Cys), homocysteine (Hcys), and glutathione (GSH) adsorbed on the surface of Au or Ag nanoparticles have been employed to mediate the assembly and disassembly of nanoparticles via modulating the zwitterionic interaction between *Corresponding author. E-mail:
[email protected]. (1) (a) Bri~nas, R. P.; Hu, M.; Qian, L.; Lymar, E. S.; Hainfeld, J. F. J. Am. Chem. Soc. 2008, 130, 975–982. (b) Zhu, M.; Lanni, E.; Garg, N.; Bier, M. E.; Jin, R. J. Am. Chem. Soc. 2008, 130, 1138–1139. (c) Corbierre, M. K.; Lennox, R. B. Chem. Mater. 2005, 17, 5691–5696. (d) Negishi, Y.; Nobusada, K.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 5261– 5270. (e) Sandhyarani, N.; Resmi, M. R.; Unnikrishnan, R.; Vidyasagar, K.; Ma, S.; Antony, M. P.; Selvam, G. P.; Visalakshi, V.; Chandrakumar, N.; Pandian, K.; Tao, Y. -T.; Pradeep, T. Chem. Mater. 2000, 12, 104–113. (f ) Zhang, J.; Geddes, C. D.; Lakowicz, J. R. Anal. Biochem. 2004, 332, 253–260. (g) Viau, G.; Piquemal, J.-Y.; Esparrica, M.; Ung, D.; Chakroune, N.; Warmont, F.; Fievet, F. Chem. Commun. 2003, 2216–2217. (h) Pradeep, T.; Mitra, S.; Nair, A. S.; Mukhopadhyay, R. J. Phys. Chem. B 2004, 108, 7012–7020. (i) Rosemary, M. J.; Pradeep, T. J. Colloid Interface Sci. 2003, 268, 81–84. ( j) Ang, T. P.; Wee, T. S. A.; Chin, W. S. J. Phys. Chem. B 2004, 108, 11001–11010. (2) (a) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Science 2007, 318, 430–433. (b) Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. J. Am. Chem. Soc. 2008, 130, 3754–3755. (3) (a) Yao, H.; Miki, K.; Nishida, N.; Sasaki, A.; Kimura, K. J. Am. Chem. Soc. 2005, 127, 15536–15543. (b) Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 2000, 104, 2630–2641. (c) Gautier, C.; B€urgi, T J. Am. Chem. Soc. 2006, 128, 11079–11087. (d) Tamura, M.; Fujihara, H. J. Am. Chem. Soc. 2003, 125, 15742–15743. (e) Nishida, N.; Yao, H.; Kimura, K. Langmuir 2008, 24, 2759–2766. (f ) Nishida, N.; Yao, H.; Ueda, T.; Sasaki, A.; Kimura, K. Chem. Mater. 2007, 19, 2831–2841. (g) Gautier, C.; B€urgi, T. ChemPhysChem 2009, 10, 483–492. (h) Kitaev, V. J. Mater. Chem. 2008, 18, 4745–4749.
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amino acid residues.4 This provides new insight into precisely manipulating the interfacial interaction among anchoring amino acids and potential applications in chemo- and biosensing. However, the synthesis precursors of these metal nanostructures were paid relatively less attention. We envisaged that the in-situ-formed M(I)-SR coordination polymers in aqueous solutions may act as a feasible signaling ensemble. Because metallophilicity-facilitated M(I)-thiolate can be featured by its characteristic absorption and photoluminescence (PL) signals, reportedly because of the ligandto-metal charge-transfer transition mixed with the metal-centered state that is modified by metallophilic interaction (LMMCT),5 the polymeric “chromophore” formed in situ from the optically transparent ligands (HSR) can therefore act as a good signal reporter for spectral signaling, with practically no spectral background. The formation of M(I)-SR coordination polymers in solution from M(I) and HSR species is expected to afford an amplified signal for sensing.6 Considering the biological significance of cysteine,7 (4) (a) Mandal, S.; Gole, A.; Lala, N.; Gonnade, R.; Ganvir, V.; Sastry, M. Langmuir 2001, 17, 6262–6268. (b) Li, T.; Park, H. G.; Lee, H.-S.; Choi, S.-H. Nanotechnology 2004, 15, S660–S663. (c) Sudeep, P. K.; Joseph, S. T. S.; Thomas, K. G. J. Am. Chem. Soc. 2005, 127, 6516–6517. (d) Lim, I.-I. S.; Mott, D.; Ip, W.; Njoki, P. N.; Pan, Y.; Zhou, S.; Zhong, C.-J. Langmuir 2008, 24, 8857–8863. (e) Lim, I-I. S.; Ip, W.; Crew, E.; Njoki, P. N.; Mott, D.; Zhong, C.-J.; Pan, Y.; Zhou, S. Langmuir 2007, 23, 826–833. (f ) Lim, I.-I. S.; Mott, D.; Engelhard, M. H.; Pan, Y.; Kamodia, S.; Luo, J.; Njoki, P. N.; Zhou, S.; Wang, L.; Zhong, C. J. Anal. Chem. 2009, 81, 689–698. (5) (a) Narayanaswamy, R.; Young, M. A.; Parkhurst, E.; Ouellette, M.; Kerr, M. E.; Ho, D. M.; Elder, R. C.; Bruce, A. E.; Bruce, M. R. M. Inorg. Chem. 1993, 32, 2506–2517. (b) Yam, V. W.-W.; Cheng, E. C.-C.; Zhou, Z. -Y. Angew. Chem., Int. Ed. 2000, 39, 1683–1685. (c) Yam, V. W.-W.; Cheng, E. C.-C.; Cheung, K.-K. Angew. Chem., Int. Ed. 1999, 38, 197–199. (d) Li, C.-K.; Chan, C.-L.; Yam, V. W.-W. Angew. Chem., Int. Ed. 1998, 37, 2857–2859. (e) Yam, V. W.-W.; Chan, C.-L.; Li, C.-K.; Wong, K. M.-C. Coord. Chem. Rev. 2001, 216-217, 173–194. (f ) Guyon, F.; Hameau, A.; Khatyr, A.; Knorr, M.; Amrouche, H.; Fortin, D.; Harvey, P. D.; Strohmann, C.; Ndiaye, A. L.; Huch, V.; Veith, M.; Avarvari, N. Inorg. Chem. 2008, 47, 7483–7492. (6) Ruan, Y.-B.; Li, A.-F.; Zhao, J.-S.; Shen, J.-S.; Jiang, Y.-B. Chem. Commun. 2010, 46, 4938–4940. (7) (a) Droge, W.; Holm, E. FASEB J. 1997, 11, 1077–1089. (b) Zafarullaha, M.; Li, W. Q.; Sylvester, J.; Ahmad, M. Cell. Mol. Life Sci. 2003, 60, 6–20. (c) PukaSundvall, M.; Eriksson, P.; Nilsson, M.; Sandberg, M.; Lehmann, A. Brain Res. 1995, 705, 65–70. (d) Janaky, R.; Varga, V.; Hermann, A.; Saransaari, P.; Oja, S. S. Neurochem. Res. 2000, 25, 1397–1405. (e) Wang, X. F.; Cynader, M. S. J. Neurosci. 2001, 21, 3322–3331. (f ) Goodman, M. T.; McDuffie, K.; Hernandez, B.; Wilkens, € L. R.; Selhub, J. Cancer 2000, 89, 376–382. (g) Liu, J.; Yeo, H. C.; Overvik-Douki, E.; Hagen, T.; Doniger, S. J.; Chu, D. W.; Brooks, G. A.; Ames, B. N. J. Appl. Physiol. 2000, 89, 21–28.
Published on Web 12/06/2010
DOI: 10.1021/la103153e
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we report here a case study on the Ag(I) 3 3 3 Ag(I) interactionfacilitated Ag(I)-Cys coordination polymers8 for the spectral sensing of Cys. This study shows that the metallophilicity-facilitated M(I)-SR coordination polymers could indeed act as a spectral sensing ensemble for broad applications via modulating the metalmetal interaction in coordination polymers. It should be pointed out that although many examples have reportedly shown the metallophilic interactions,9 applying such interactions to spectral sensing remains largely unexplored.5d,e,10,11
2. Experimental Section Absorption spectra were recorded on a Varian Cary 300 absorption spectrophotometer using a 1 cm quartz cell. Photoluminescence spectra were taken on a Hitachi F-4500 fluorescence spectrophotometer using excitation and emission slits of 10 nm each. CD spectra were recorded on a Jasco-810 CD spectrophotometer. Dynamic light scattering (DLS) data were collected from a ZetaPALS zeta potential analyzer. Infrared (IR) spectra were recorded on Nicolet Avatar FT-IR360 spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Quantum 2000 scanning ESCA microprobe with a minimum X-ray beam size of less than 10 μm. Glutathione (GSH, >98%), L-cysteine (L-Cys, > 99%), and AgNO3 (>99.8%) were purchased from Sangon (Shanghai, China). D-Cysteine 3 HCl (D-Cys, >99%) and homocysteine (Hcys, > 99%) were purchased from Sigma and TCI, respectively. Other chemicals (AR) were used as received from Guoyao (Shanghai, China).
Figure 1. Absorption (a, b) and CD (c, d) spectra of Cys of varying concentration in the presence (a, c) and absence (b, d) of Ag(I) in a 10 mM pH 5.0 NaAc-HAc buffer solution. [Cys] = 0-2.5 10-5 M (a-d) and [Ag(I)] = 2.5 10-5 M (a, c).
3. Results and Discussion Because of the strong complexation ability of organothiolate anions (RS-) toward Ag(I),12 the interaction of Ag(I) with Cys occurs immediately upon mixing in a 10 mM pH 5.0 NaAc-HAc buffer solution. This is indicated by the appearance of two absorption bands at 280 and 360 nm, respectively, in which spectral windows both Cys and Ag(I) are optically transparent (Figure 1). Although the absorption spectra of Ag(I)-L-Cys and Ag(I)-D-Cys solutions are almost the same, splitting CD signals in the corresponding absorption regions shows perfect mirror images (Figure 1c). To understand the intrinsic origin of these spectral signals, structural information was explored. It has been difficult to crystallize Ag(I)-Cys for detailed structural characterization, although polymeric structures of several Ag(I)-thiolates including Ag(I)-Cys have been preliminarily probed by amperometric and spectrophotometric methods.12d In fact, only a few Ag(I)-thiolates have hitherto been unambiguously characterized by X-ray crystallography, which (8) (a) Dance, I. G.; Fitzpatrick, L. J.; Rae, A. D.; Scudder, M. L. Inorg. Chem. 1983, 22, 3785–3788. (b) Tang, K.; Yang, J.; Yang, Q.; Tang, Y. J. Chem. Soc., Dalton Trans. 1989, 2297–2302. (c) Howard-Lock, H. E. Met.-Based Drugs 1999, 6, 201– 209. (d) Jin, X.; Xie, X.; Qian, H.; Tang, K.; Liu, C.; Wang, X.; Gong, Q. Chem. Commun. 2002, 600–601. (e) Liu, X.; Yang, H.; Zheng, N.; Zheng, L. Eur. J. Inorg. Chem. 2010, 2084–2087. (9) (a) Schmidbaur, H. Gold Bull. 2000, 33, 3–10. (b) Pyykk€o, P. Chem. Rev. 1997, 97, 597–636. (c) Yam, V. W.-W.; Cheng, E. C.-C. Chem. Soc. Rev. 2008, 37, 1806– 1813. (d) Schmidbaur, H.; Schier, A. Chem. Soc. Rev. 2008, 37, 1931–1951. (e) Katz, M. J.; Sakai, K.; Leznoff, D. B. Chem. Soc. Rev. 2008, 37, 1884–895. (10) (a) Caballero, A.; Garcı´ a, R.; Espinosa, A.; Tarraga, A.; Molina, P. J. Org. Chem. 2007, 72, 1161–1173. (b) Mansour, M. A.; Connick, W. B.; Lachicotte, R. J.; Gysling, H. J.; Eisenberg, R. J. Am. Chem. Soc. 1998, 120, 1329–1330. (c) Li, C.-K.; Lu, X.-X.; Wong, K. M.-C.; Chan, C.-L.; Zhu, N.; Yam, V. W.-W. Inorg. Chem. 2004, 43, 7421–7430. (d) He, X.; Cheng, E. C.-C.; Zhu, N.; Yam, V. W.-W. Chem. Commun. 2009, 4016–4018. (11) Shen, J.-S.; Li, D.-H.; Cai, Q.-G.; Jiang, Y.-B. J. Mater. Chem. 2009, 19, 6129–6224. (12) (a) Dance, I. G.; Fisher, K. J.; Banda, R. M. H.; Scudder, M. L. Inorg. Chem. 1991, 30, 183–187. (b) Fijolek, H. G.; Gonzalez-Duarte, P.; Park, S. H.; Suib, S. L.; Natan, M. J. Inorg. Chem. 1997, 36, 5299–5305. (c) Bensebaa, F.; Ellis, T. H.; Kruus, E.; Voicu, R.; Zhou, Y. Langmuir 1998, 14, 6579–6587. (d) Andersson, L. -O. J. Polym. Sci., Part A-1: Polym. Chem. 1972, 10, 1963–1973.
482 DOI: 10.1021/la103153e
Figure 2. Plots of CD ellipticity (a) and absorbance (b) at 360 nm vs the [Cys]/[Ag(I)] ratio in a 10 mM pH 5.0 NaAc-HAc buffer solution. [Ag(I)] = 2.5 10-5 M.
Figure 3. Job plot of absorbance at 280 nm vs [Ag(I)]/([Ag(I)] þ [L-Cys]). The total concentration of [Ag(I)] and [L-Cys] was 5.0 10-5 M. The buffer used was 10 mM pH 5.0 NaAc-HAc.
shows polymeric structures with the -Ag(I)-S(R)- repeat unit.8 Because of the hydrophilic character of the Cys ligand employed here, a detailed characterization of Ag(I)-Cys in aqueous solutions was possible. Spectral titrations (Figure 2) and a Job plot (Figure 3) showed 1:1 Ag(I)/Cys binding stoichiometry. IR spectral data supported the formation of Ag(I)-thiolate in that the stretching band of S-H of free Cys at 2555 cm-1 disappeared for Ag(I)-Cys (Figure S1). Referring to an analogous structure,1a a chainlike polymeric structure of Ag(I)-Cys with a -Ag(I)-S(R)- repeat unit was proposed to exist in the aqueous solutions (Figure 4). Langmuir 2011, 27(1), 481–486
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Figure 4. Switching argentophilic attraction by the pH-modulated electrostatic interaction between adjacent ligands along the Ag(I)-Cys polymeric backbone.1a R1 and R2 are the Ag-S-Ag angles at low and high pH, respectively, R1 < R2. The argentophilic interaction is shown by the dashed line.
Figure 5. Hydrodynamic diameters (Dh) of Ag(I)-L-Cys polymers at pH 4.8 (red) and 8.0 (blue). [Ag(I)] = [L-Cys] = 2.5 10-5 M.
Note that a double-stranded helical polymeric structure has been probed for Ag(I)-D-penicillamine with zigzag chains of the -Ag(I)-S(R)- repeat unit8d in which D-penicillamine is structurally similar to Cys. A similar structure was also found in the crystals of 3-methylpentane-3-thiolato-Ag(I).8a More importantly, argentophilicity has been suggested to promote the formation of Ag(I)-thiolate aggregates from the basic zigzag chains.8 Dynamic light scattering data further supported the Ag(I)-Cys polymeric structure by indicating average hydrodynamic diameters of ca. 700 nm at pH 5 and ca. 400 nm at pH 8 (Figure 5). The chemical state of the Ag species in the coordination polymer was then clarified. Although X-ray photoelectron spectroscopy (XPS) represents a powerful means of probing the elemental chemical state, it is unable to discriminate M(0) and M(I) in coinage metal thiolates because the difference between the binding energies of M(0) and M(I) is too small (Figures S2-S3 and Table S1).13 The Ag(I) chemical state of the Ag species in Ag(I)-Cys polymers was therefore probed by examining the absorption spectra of Ag(I)-Cys upon NaBH4 reduction, a step typically employed in the fabrication of coinage metal nanocrystals.1 It was observed that upon NaBH4 reduction the original absorption of Ag(I)-Cys polymers disappeared but a new band at 450 nm appeared, which is indicative of the Ag nanostructures (Figure S4).1g,14 Recent investigations have revealed that Au or Ag metal nanoclusters are extraordinarily stable even in the presence of strong reduction or oxidation agents.15 This means that the absorption spectra would hardly undergo any change (13) Bensebaa, F.; Zhou, Y.; Deslandes, Y.; Kruus, E.; Ellis, T. H. Surf. Sci. 1998, 405, L472–L476. (14) (a) Petty, J. T.; Zheng, J.; Hud, N. V.; Dickson, R. M. J. Am. Chem. Soc. 2004, 126, 5207–5212. (b) Ritchie, C. M.; Johnsen, K. R.; Kiser, J. R.; Antoku, Y.; Dickson, R. M.; Petty, J. T. J. Phys. Chem. C 2007, 111, 175–181. (15) (a) Negishi, Y.; Chaki, N. K.; Shichibu, Y.; Whetten, R. L.; Tsukuda, T. J. Am. Chem. Soc. 2007, 129, 11322–11323. (b) Zheng, J.; Petty, J. T.; Dickson, R. M. J. Am. Chem. Soc. 2003, 125, 7780–7781.
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upon NaBH4 reduction if the Ag species in the Ag(I)-Cys solution exists in the Ag(0) state. We therefore concluded that in the Ag(I)-Cys polymeric backbone the Ag species existed in the þ1 state rather than in a neutral chemical state. Referring to the argentophilicity-facilitated polymeric structures8 and the reported absorption spectral assignments,5 absorptions at 280 and 360 nm observed in the Ag(I)-Cys polymers were similarly assigned to the ligand-to-metal charge-transfer transition mixed with the metalcentered (ds/dp) state that is modified by the argentophilic interaction (LMMCT).9,10 With the CD signals of the Ag(I)-Cys solutions, a similar observation upon NaBH4 reduction was made in which the CD signals disappeared after NaBH4 reduction (Figure S5), suggesting that they are also associated with the LMMCT transition. It is worth pointing out that although the chiral center in Cys is one saturated -CH2- away from the coordinating S atom (Figure 4), strong splitting CD signals were observed in the Ag(I)-Cys solution at wavelengths at which the argentophilic-interaction-related LMMCT transition absorbs (Figure 1).5 This is an indication of the synergetic interplay of the electrostatic interaction among the -SR side chains and the Ag(I) 3 3 3 Ag(I) argentophilic attraction in the Ag(I)-Cys polymeric backbone, which likely results in a certain helicity.8a,d It was therefore expected that this helicity may disappear at higher pH because the electrostatic repulsion among the -SR side chains now bearing only -CO2- negative charges (Figure 4) would weaken the Ag(I) 3 3 3 Ag(I) argentophilic attraction, which was indeed verified later. Because the argentophilic interaction is a weak interaction that is close in strength to the strongest hydrogen bonding16 and the related absorption is very often interfered by the intraligand transition, the spectroscopic observation of the argentophilic interaction has hitherto remained rare in the literature.16,17 To the best of our knowledge, our CD evidence for the direct observation of argentophilicity represents the first of its kind. To indentify the contribution of the -SR/-SR side-chain interaction, we monitored the pH dependence of the absorption and CD spectra of the Ag(I)-Cys solution. It was found that with increasing pH the absorption of Ag(I)-Cys solution at 280 and 360 nm gradually decreased and eventually disappeared whereas the absorption below 260 nm was gradually enhanced, with an isosbestic point at 260 nm (Figure 6a). This means that there are two forms of polymeric structures existing in the aqueous solution, which is in agreement with that indicated by the dynamic light scattering data (Figure 5). A similar pH profile was observed in the traces of CD spectra (Figure 6b). More importantly, these processes could be reversed by lowering the solution pH, and such pHswitching behavior could be repeated for several cycles without an (16) (a) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H. J. Am. Chem. Soc. 2000, 122, 10371–10380. (b) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H.; Fackler, J. P., Jr. J. Am. Chem. Soc. 2001, 123, 11237–11247. (17) (a) Che, C.-M.; Tse, M.-C.; Chan, M.-C. W.; Cheung, K.-K.; Phillips, D.-L.; Leung, K.-H. J. Am. Chem. Soc. 2000, 122, 2464–2468. (b) Che, C.-M.; Lai, S.-W. Coord. Chem. Rev. 2005, 249, 1296–1309.
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Figure 7. PL spectra of the Ag(I)-GSH solution at various pH values. [Ag(I)] = [GSH] = 7.5 10-5 M and λex = 290 nm. Slits for both excitation and emission monochromators are 10 nm.
Figure 6. Absorption (a) and CD (b) spectra of the Ag(I)-L-Cys solution at various pH values. Evolution of CD ellipticity (red) and absorbance (blue) at 360 nm for the Ag(I)-L-Cys system as the solution pH was switched between 4.8 and 9.0 (c). N is the number of cycles. [Ag(I)] = [L-Cys] = 2.5 10-5 M.
obvious loss of signal (Figures 6c and S6). This intriguing pH dependence and reversibility in the absorption and CD signals of the Ag(I)-Cys solution further supported the existence of polymeric structures and the attribution of new absorption and CD signals to the origin of the argentophilicity (Figure 4). This is understandable in terms of the electrostatic interactions among Cys residues in the Ag(I)-Cys polymeric backbone. The electrostatic attraction of the Cys residues at low pH would afford a smaller Ag-S-Ag angle (R1) than that (R2) at high pH when electrostatic repulsion occurs (Figures 4 and S7).1a The argentophilic attraction was accordingly expected to be weakened at high pH, and it could be enhanced when the pH was lowered. The observed reversible switching in the absorption and CD signals was hence attributed to the pH switching of argentophilic attraction in the Ag(I)-Cys polymeric backbone. A similar aurophilicity switching has been realized by employing a cation-binding interaction.5d,e,10c,10d The relatively featureless absorption observed from Ag(I)-Cys solution at high pH probably originated from an LMCT transition with no argentophilic interaction. This may serve as additional evidence of the þ1 Ag state in the Ag(I)-Cys polymeric backbone rather than the metallic Ag(0) covered by thiols.18 Although the quantum yield of photoluminescence (PL) from the Ag(I)-Cys polymeric solution was too low to allow us to record a credible PL spectrum, a PL spectrum of Ag(I)-GSH was obtained with an emission maximum of 545 nm that features an extraordinarily large Stokes shift of 9429 cm-1 (Figures 7 and S8). GSH is a naturally occurring tripeptide containing a cysteine residue and is structurally similar to cysteine.11 The emission of Ag(I)GSH could be analogously assigned to the LMMCT transition related to the argentophilic interaction.5 This observation is also (18) Nobusada, K. J. Phys. Chem. B 2004, 108, 11904–11908.
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Figure 8. CD (a) and absorption (b) spectra of the Ag(I)-GSH solution at various pH values. [Ag(I)] = [GSH] = 7.5 10-5 M.
significant because luminescent coinage M(I) complexes in aqueous solutions are rare.16b,19 A sensitive pH dependence in the PL spectrum of Ag(I)-GSH was also observed (Figure 7). With increasing pH, the PL intensity at 545 nm decreased whereas an emission at shorter wavelength gradually evolved, with an isoemissive point at 445 nm (Figure 7). This may similarly suggests that a conversion of the emitting species of the Ag(I)-GSH polymers occurs upon pH variation. A high PL polarization of 0.30 was measured from the Ag(I)-GSH solution at pH 3.8, which is further indicative of the bulky polymeric structure of the luminophore existing in aqueous solution.20 The absorption and CD spectra of Ag(I)-GSH were found to be similar to those of the Ag(I)-Cys counterpart, with similar pH dependences and switching behaviors (Figures 8, S9, and S10). Upon NaBH4 reduction, the absorption, CD, and PL signals of Ag(I)-GSH eventually disappeared (Figure 9), again similar to those observed for the Ag(I)-Cys system, suggesting that Ag species in the Ag(I)-GSH system also exists in the þ1 chemical state. It is important to note that although all of these data support the argentophilic interaction origin of the spectral signals in Ag(I)Cys and Ag(I)-GSH systems the pH dependence is not the same for these two systems. At a low pH of 5.0, hardly any absorption or CD signals were observed from Ag(I)-GSH, whereas appreciable signals were found in the Ag(I)-Cys solution (Figure 10). This can be attributed to the differed isoelectric points of GSH and Cys (Figure S7) so that at the selected pH of 5 the repulsion among the (19) (a) Forward, J. M.; Assefa, Z.; Fackler, J. P., Jr. J. Am. Chem. Soc. 1995, 117, 9103–9104. (b) Stillman, M. J.; Zelazowski, A. J.; Gasyna, Z. FEBS Lett. 1988, 240, 157–162. (20) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer: New York, 1999; p 291.
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Figure 10. Plots of CD (a) and absorption (b) signals at 360 nm for Ag(I)-GSH and Ag(I)-L-Cys vs solution pH.
Figure 9. CD (a), absorption (b), and PL (c) spectra of Ag(I)GSH in a 10 mM NaAc-HAc buffer at pH 3.8 upon reduction by NaBH4 for increasing duration of up to 20 min. [Ag(I)]=[GSH]= 3.75 10-5 M (a, b), [Ag(I)] = [GSH] = 7.5 10-5 M (c), and [NaBH4]=ca. 1.510-4 M. λex=360 nm (c). Slits for both excitation and emission monochromators are 10 nm.
GSH residues in the side chains weakens or even breaks the Ag(I) 3 3 3 Ag(I) interaction in the Ag(I)-GSH coordination polymers, thereby leading to no new absorption and CD signals. This difference allows for a reliable entry for the selective signaling of Cys in the presence of GSH by controlling the solution pH (Figure 11). The aforementioned observations established that the spectral signals of the Ag(I)-Cys coordination polymers are related to the Ag(I)-Ag(I) interaction in the polymeric backbone that is very sensitive to the Cys/Cys side-chain interactions. The presence of Cys in the solution is thus necessary for the observation of these signals, which actually points to the possibility of Cys sensing using these reporting signals. The contribution of the side-chain interaction to these signals suggests a method for extended sensing by means of tuning this interaction. As a proof of principle, we showed that these signals could indeed allow the sensing of Cys with high sensitivity and selectivity. It was already made obvious in Figure 1 that both the absorption and CD signals increased with increasing Cys concentration in the presence of Ag(I). Because there is no absorption or CD signals in either Ag(I) or Cys solution beyond 250 nm, at which these spectral signals of the Ag(I)-Cys solution were monitored, this signaling protocol features a zero spectral background (Figure 1), which is a characteristic preferred for higher sensitivity in spectral sensing. Good linear relationships were established between the absorbance or CD ellipticity at 360 nm and the D- or L-Cys concentration (Figure 11). Limits of detection (3σ/k, n = 11) for D- or L-Cys were obtained to be at the 1 μM level by means of absorption and CD spectra (Table 1). On the basis of the molar extinction coefficient, the signaling sensitivity of this system is 1 order of Langmuir 2011, 27(1), 481–486
Figure 11. Plots of absorbance (a) and CD ellipticity (b) at 360 nm vs [Cys] and [GSH] in 10 mM pH 5.0 NaAc-HAc buffer in the presence of Ag(I). [Ag(I)] = 2.5 10-5 M.
magnitude greater than that obtained by directly measuring the spectral signals of cysteine itself at the troublesome wavelength of around 215 nm (Table S2 and Figure 1b,d). A signaling amplification in the Ag(I)-Cys system was therefore shown.6 In comparison with previous reports for Cys sensing,4f,21,22 the present system (21) (a) Lee, K.-S.; Kim, T.-K.; Lee, J. H.; Kim, H.-J.; Hong, J.-I. Chem. Commun. 2008, 6173–6175. (b) Tanaka, F.; Mase, N.; Barbas, C. F., III. Chem. Commun. 2004, 1762–1763. (c) Shao, N.; Jin, J. Y.; Cheung, S. M.; Yang, R. H.; Chan, W. H.; Mo, T. Angew. Chem., Int. Ed. 2006, 45, 4944–4948. (d) Zhang, M.; Yu, M.; Li, F.; Zhu, M.; Li, M.; Gao, Y.; Li, L.; Liu, Z.; Zhang, J.; Zhang, D.; Yi, T.; Huang, C. J. Am. Chem. Soc. 2007, 129, 10322–10323. (e) Wang, W.; Rusin, O.; Xu, X.; Kim, K. K.; Escobedo, J. O.; Fakayode, S. O.; Fletcher, K. A.; Lowry, M.; Schowalter, C. M.; Lawrence, C. M.; Fronczek, F. R.; Warner, I. M.; Strongin, R. M. J. Am. Chem. Soc. 2005, 127, 15949–15958. (f ) Wang, W.; Escobedo, J. O.; Lawrence, C. M.; Strongin, R. M. J. Am. Chem. Soc. 2004, 126, 3400–3401. (g) Rusin, O.; St. Luce, N. N.; Agbaria, R. A.; Escobedo, J. O.; Jiang, S.; Warner, I. M.; Dawan, F. B.; Lian, K.; Strongin, R. M. J. Am. Chem. Soc. 2004, 126, 438–439. (h) Li, S.-H.; Yu, C.-W.; Xu, J.-G. Chem. Commun. 2005, 450–452. (i) Kim, T.-K.; Lee, D.-N.; Kim, H.-J. Tetrahedron Lett. 2008, 49, 4879–4881. ( j) Zhang, D.; Zhang, M.; Liu, Z.; Yu, M.; Li, F.; Yi, T.; Huang, C. Tetrahedron Lett. 2006, 47, 7093–7096. (k) Zeng, Y.; Zhang, G.; Zhang, D.; Zhu, D. Tetrahedron Lett. 2008, 49, 7391–7394. (l) Zeng, Y.; Zhang, G.; Zhang, D. Anal. Chim. Acta 2008, 627, 254–257. (m) Zhang, X.; Ren, X.; Xu, Q.-H.; Loh, K. P.; Chen, Z. -K. Org. Lett. 2009, 11, 1257–1260. (n) Lin, W.; Long, L.; Yuan, L.; Cao, Z.; Chen, B.; Tan, W. Org. Lett. 2008, 10, 5577–5580. (o) Duan, L.; Xu, Y.; Qian, X.; Wang, F.; Liu, J.; Cheng, T. Tetrahedron Lett. 2008, 49, 6624–6627. (p) Lin, W.; Yuan, L.; Cao, Z.; Feng, Y.; Long, L. Chem.;Eur. J. 2009, 15, 5096–5103. (q) Chen, X.; Zhou, Y.; Peng, X.; Yoon, J. Chem. Soc. Rev. 2010, 39, 2120–2135. (r) Chen, X.; Ko, S.-K.; Kim, M. J.; Shin, I.; Yoon, J. Chem. Commun. 2010, 46, 2751–2753. (22) (a) Sudeep, P. K.; Joseph, S. T. S.; Thomas, K. G. J. Am. Chem. Soc. 2005, 127, 6516–6517. (b) Negi, D. P. S.; Chanu, T. I. Nanotechnology 2008, 19, 1–5. (c) Lee, J. -S.; Ulmann, P. A.; Han, M. S.; Mirkin, C. A. Nano Lett. 2008, 8, 529–533. (d) Shimada, T.; Ookubo, K.; Komuro, N.; Shimizu, T.; Uehara, N. Langmuir 2007, 23, 11225–1232. (e) Lu, C.; Zu, Y. Chem. Commun. 2007, 3871–3873. (f ) Shang, L.; Qin, C.; Wang, T.; Wang, M.; Wang, L.; Dong, S. J. Phys. Chem. C 2007, 111, 13414– 13417. (g) Lu, C.; Zu, Y.; Yam, V. W.-W. Anal. Chem. 2007, 79, 666–672. (h) Shang, L.; Dong, S. Biosens. Bioelectron. 2009, 24, 1569–1573.
DOI: 10.1021/la103153e
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Table 1. Limits of Detection (LOD) of L-Cys and D-Cys a
LODUV-vis, M
a
LODCD, M
1.55 10-6 1.27 10-6 1.55 10-6 1.05 10-6 The LOD was calculated by employing the equation LOD = 3σ/k,6 in which σ is standard deviation from 11 blank solutions and k is the linear slope fitted from Figure 11a,b. Fitted equations are A360 nm =-0.0068 þ 2775.7[L-Cys] (r2 =0.9774) and A360 nm =-0.0049 þ 2774.3[D-Cys] (r2 = 0.9938) and CD360 nm=4.00 - 260398.9[L-Cys] (r2=0.9990) and CD360 nm= -3.68 þ 314285.3[D-Cys] (r2 = 0.9838). L-Cys
D-Cys a
The observation that racemic Cys did not produce any new absorption signal in the presence of Ag(I), whereas new absorption signals were observed in the case of racemic Hcys under the same condition, provides a way of discriminating Cys from Hcys. Methionine and cystine, as R-amino acids containing sulfur but not SH, and other naturally occurring R-amino acids were found to lead to no noticeable changes in either absorption or CD spectra when mixed with Ag(I) under the same conditions (Figure 12). This means that the sensing is very selective for Cys as well.
4. Conclusions
Figure 12. Absorption (a) and CD (b) response of Ag(I) solution to amino acids and GSH in 10 mM pH 5.0 NaAc-HAc buffer. Cys is in the L and D forms, Hcys is in its racemic form, and the other tested amino acids are in the L form. [Ag(I)] = 2.5 10-5 M and [amino acid] = [GSH] = 2.5 10-5 M.
affords not only a low limit of detection but also a unique enantiomeric signaling capacity for Cys by using CD spectroscopy.23,24 Under the same buffer condition of pH 5.0, GSH did not produce any absorption or a CD spectral signal in the presence of Ag(I) (Figure 11). Homocysteine (Hcys), another SH-containing homologous amino acid that differs from Cys only by an additional methylene (-CH2-) group and is often observed to be associated with cardiovascular disease,25 was found to result in similar absorption signals in the presence of Ag(I) in the same pH 5.0 buffer (Figures S11 and 12a). However, Hcys, which is commercially available only in racemic form, did not produce any new CD spectral signal in the presence of Ag(I) (Figure 12b). (23) (a) Gao, X.; Xing, G.; Yang, Y.; Shi, X.; Liu, R.; Chu, W.; Jing, L.; Zhao, F.; Ye, C.; Yuan, H.; Fang, X.; Wang, C.; Zhao, Y. J. Am. Chem. Soc. 2008, 130, 13810–13810. (b) Shimpuku, C.; Ozawa, R.; Sasaki, A.; Sato, F.; Hashimoto, T.; Yamauchi, A.; Suzuki, I.; Hayashita, T. Chem. Commun. 2009, 1709–1711. (c) Nieto, S.; Lynch, Vi. M.; Anslyn, E. V.; Kim, H.; Chin, J. Org. Lett. 2008, 10, 5167–5170. (d) Nieto, S.; Lynch, V. M.; Anslyn, E. V.; Kim, H.; Chin, J. J. Am. Chem. Soc. 2008, 130, 9232–9233. (24) During the expansion of this version of this article, we became aware of related work that was independently conducted. See Nan, J.; Yan, X.-P. Chem.; Eur. J. 2010, 16, 423–427. While their work focused on analytical chemistry aspects, our work attempts to show a detailed mechanism for the establishment of a general spectral sensing platform.11 (25) (a) Jacobsen, D. W. Clin. Chem. 1998, 44, 1833–1843. (b) Rasmussen, K.; Møller, J. Ann. Clin. Biochem. 2000, 37, 627–648.
486 DOI: 10.1021/la103153e
We developed a conceptually new yet feasible spectral signaling ensemble that is highly selective for and sensitive to Cys, which operates in a “mix-and-measure” mode. The in-situ-formed Ag(I)-Cys coordination polymers exhibited argentophilicity-related absorption and CD signals with practically no spectral background because both Ag(I) and Cys are spectrally transparent in these spectral windows. The experiments indicated that the insitu-formed Ag(I)-Cys or Ag(I)-GSH coordinating polymers were facilitated by the argentophilicity. The high sensitivity and selectivity for Cys sensing likely results from the formation of Ag(I)-Cys coordination polymers, which affords signal amplification. The observed interesting pH-switching character of the spectral signals of the Ag(I)-Cys polymers shows that the electrostatic interaction among the Cys side chains plays an important role in maintaining the Ag(I)-SR polymeric structure as facilitated by the argentophilic interaction, and extended sensing systems can be constructed by manipulating the interactions among the -SR chains. In addition, the red-shifted absorption of the coordination polymers can in principle be employed as a source of photoexcitation for resonance-energy-transfer-based photoluminescence sensing. It is therefore concluded that the present case study not only provides a feasible and simple sensing system for Cys but also establishes a metal-metal-interactionbased spectral sensing platform with enhanced performance. These results are also expected to be significant in controlling the structures of the M(I)-SR polymers for the syntheses of metal nanostructures. Our preliminary investigations on the corresponding Cu(I)-Cys and Au(I)-Cys systems have shown similar spectral signals, and the M(I)-SR coordination polymers (M = Au, Ag, or Cu) are thus assumed to be able to serve as spectral sensing platforms. Acknowledgment. We greatly appreciate the support of the NSF of China (grants 20675069, 20835005, J0630429, and J1030415). Thanks are due to Prof. Shui-Ju Wang and Prof. Guo-Bin Han of the Department of Chemistry, Xiamen University for instructive discussions on XPS data and for generous help with DLS experiments, respectively. Supporting Information Available: Figures S1-S11 and Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2011, 27(1), 481–486
