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Altering the Surface Characteristics of Coated Silver Surfaces. Soft Donors Allow the Direct Detection of Isolated Porphyrins Using Surface-Enhanced Resonance Raman Spectroscopy John Reglinski,* Mark D. Spicer, Jonathan F. Ojo, Gerard D. McAnally, Agnieszka Sko´rska, Susan J. Smith, and W. Ewen Smith Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, United Kingdom Received February 18, 2003. In Final Form: April 17, 2003
Introduction The use of colloidal silver suspensions as roughened surfaces for surface-enhanced resonance Raman spectroscopy (SERRS)1,2 allows the retrieval of vibrational spectra from a wide variety of species in an aqueous suspension. The technique relies on efficient adsorption of the analyte onto the roughened surface, a process which is dependent on the properties of the surface itself. For example, only a limited number of azo dyes adsorb effectively on citrate-reduced colloid surfaces whereas 25 such dyes used in one study gave good SERRS when polyL-lysine was used as a surface coating.3 Problems of surface charge, encountered when observing analytes such as DNA, can be overcome using spermine4,5 which reduces the charge on the DNA and the silver. Although there have been great advances in colloid technology for SERRS, certain species such as the metalloporphyrins do not deposit effectively on the colloid surfaces currently available.6,7 Most surface modifications leave a surface which is essentially hard with oxygen or nitrogen donors at the surface. The alternative is to use self-assembled monolayers (SAMs) such that a lipophilic or surfactant type layer7,8 can be created. Thus colloids modified in such a way as to make the metal surface soft, making them compatible with larger more polarizable species, would be of considerable value for the adsorption of soft species which are typically difficult to study. Surface-modifying species of this type would extend the range of SERRSactive molecules that can be used in analysis and in particular allow us to study a range of porphyrins in situ. This outcome would be of significant value in biomimetic and catalytic studies. Concurrent with our interest in SERRS technology, we have an established interest in design and synthesis of soft tripodal ligands, namely, the hydrotris(methimazolyl)borate anion (Figure 1).9,10 This ligand system has been * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: 141-548-2349. Fax: 141-552-0876. (1) Isola, N. R.; Stokes, D. L.; Vodinh, T. Anal. Chem. 1998, 70, 1352. (2) Vodinh, T.; Hiromoto, M. Y. K.; Begum, G. M.; Moody, R. L. Anal. Chem. 1984, 56, 1667. (3) Munro, C. H.; Smith, W. E.; White, P. C. Analyst 1993, 118, 731. (4) Graham, D.; Smith, W. E.; Linacre, A. M. T.; Munro, C. H.; Watson, N. D.; White, P. C. Anal. Chem. 1997, 69, 4703. (5) Graham, D.; Mallinder, B. J.; Smith, W. E. Biopolymers (Biospect.) 2000, 57, 85. (6) Vlckova, B.; Solecka-Cermakova, K.; Matejka, P.; Baumruk, V. J. Mol. Struct. 1997, 408, 149. (7) Song, O. K.; Shin, E. J.; Lee, M. Y.; Kim, D.; Han, J. T.; Jee, J. G.; Yoon, M. J. J. Raman Spectrosc. 1992, 23, 667. (8) Imahori, H.; Norieda, H.; Nishimura, Y.; Yamazaki, I.; Higuchi, K.; Kato, N.; Motohiro, T.; Yamada, H.; Tamaki, K.; Arimura, M.; Sakata, Y. J. Phys. Chem. B 2000, 104, 1253.
specifically designed to preferentially bind to soft metal cations such as bismuth(III),11 thallium(I),12 and mercury(II)13 in preference to hard metal cations such as sodium.9 Subsequent structural studies on a variety of metals9-14 have confirmed a preference for tetrahedral and octahedral complexes. However, a report by Santini et al. clearly demonstrated that with silver, the ligand adopts a didentate structure (Figure 1).14 This weakens one of the methimazolyl contacts creating a vacancy, which can be easily filled by a suitable monodentate donor from the solute. The relevance of the complex reported by Santini et al.14 to SERRS technology is evident. Were this structural arrangement to be repeated at the silver oxide layer present on the silver particles used in SERRS, we would generate an uncomplexed methimazolyl unit which would engender the desired soft binding site for the efficient chemisorption of highly polarizable analytes (Figure 1). Thus we report here the use of the hydrotris(methimazolyl)borate and hydrotris(thiazolyl)borate anions as surface modifiers for colloidal silver particles thus producing a novel soft coating. As an example of the use of these species in SERRS, we have recorded the SERRS spectra of hemin. Materials and Methods Unless otherwise stated, the chemicals were commercially obtained. Hemin was purchased from Sigma-Aldrich Chemical Co. Ltd., Dorset, U.K. Sodium hydrotris(methimazolyl)borate11 (NaTm) and sodium hydrotris(thiazolyl)borate anions10 (NaTz) were synthesized as previously reported. AgTm and AgTz were prepared using the method of Santini.14 Silver colloid was prepared using a variation of the Lee and Meisel procedure.15,16 Raman scattering and SERRS data on solid NaTm, NaTz, AgTm, and AgTz were collected with a Renishaw Ramascope 2000 spectrometer using the 632.8 and 514.5 nm lines of a HeNe laser in combination with a ×50 microscope objective. Each spectrum is the sum of three 10-s accumulations. SERRS spectra from Hemin were collected with a Renishaw Ramascope 2000 spectrometer using the 514.5 nm line of a HeNe laser in combination with a ×50 microscope objective. Each spectrum is the sum of three 10-s accumulations. The spectra were obtained from aqueous solution in silver colloid in a 96-well microtiter plate. Silver colloid (300 µL) approximately 30 nm in the long direction17 was preaggregated with 1% NaTz (50 µL) inoculated with the relevant hemin solution (1.9 mg/50 mL in EtOH stock: 5.8 × 10-5 M diluted in water, 50 µL). The concentrations stated in Figure 3 (10-9, 10-10 M) are the final concentrations in the colloidal solution.
Results and Discussion Figure 2 shows the Raman spectra of the NaTm (B) and AgTm (C) and the SERS spectrum of the adduct formed via the reaction of NaTm at the silver colloid surface (D). (9) Reglinski, J.; Garner, M.; Cassidy, I. D.; Slavin, P. A.; Spicer, M. D.; Armstrong, D. R. J. Chem. Soc., Dalton Trans. 1999, 2119. (10) Ojo, J. F.; Slavin, P. A.; Reglinski, J.; Garner, M.; Spicer, M. D.; Kennedy, A. R.; Teat, S. J. Inorg. Chim. Acta 2001, 313, 15. (11) Reglinski, J.; Spicer, M. D.; Garner, M.; Kennedy, A. R. J. Am. Chem. Soc. 1999, 121, 2317. (12) Slavin, P. A.; Reglinski, J.; Spicer, M. D.; Kennedy, A.R. J. Chem. Soc., Dalton Trans. 2000, 239. (13) Cassidy, I.; Garner, M.; Kennedy, A. R.; Potts, G. B. S.; Reglinski, J.; Slavin, P. A.; Spicer, M. D. Eur. J. Inorg. Chem. 2002, 1235. (14) Santini, C.; Lobbia, G. G.; Pettinari, C.; Pellei, M.; Valle, G.; Calogero, S. Inorg. Chem. 1998, 37, 890. (15) Lee, P. C.; Meisel, T. J. J. Phys. Chem. 1982, 86, 3391. (16) Rodger, C.; Smith, W. E.; Dent, G.; Edmonson, M. J. Chem. Soc., Dalton Trans. 1996, 791. (17) Munro, C. H.; Smith, W. E.; Garner, M.; Clarkson, J.; White, P. C. Langmuir 1995, 11, 3712.
10.1021/la034279j CCC: $25.00 © 2003 American Chemical Society Published on Web 06/17/2003
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Figure 1. The hydrotris(methimazolyl)borate anion (left) in its identified tripodal configuration (refs 9, 11-13), its silver adduct (ref 14) (middle), and the hydrotris(thiazolyl)borate anion (right) (ref 10).
Figure 2. The Raman spectra (632.8 nm, 40 mW) of (A) solid AgTz, (B) solid NaTm, and (C) solid AgTm and (D) the SERS spectrum of Tm deposited on silver colloid. The spectra were collected from 200 to 2000 cm-1 with a ×50 microscope objective using three 10-s accumulations. A direct comparison between the two surface modifiers can be made from spectra A and C, which shows that the saturated asymmetric thiazole moiety has the simpler spectrum.
The effect of complexing silver to the ligand can be seen by comparing the bulk spectra of NaTm (B) and AgTm (C). There is an additional band in AgTm at 1314.8 cm-1, a shift in the frequency for a small number of peaks, most notably those at 517.8 and 610 cm-1, and an increase in intensity of some of the bands (e.g., 720.7 and 1563.0 cm-1). By comparison, and despite the surface selection rules expected in SERS, there is a greater similarity between the AgTm bulk spectrum (C) and the SERS spectrum (D) than between the bulk spectrum of NaTm and the SERS spectrum (Figure 2). This suggests that the AgTm is formed on the surface with a configuration analagous to the didentate form reported by Santini14 (Figure 1). The greatest changes between the two are the appearance of an extra band at 441.8 and 839.2 cm-1 (such behavior is a common feature of SERS), in the low frequency probably caused by the differences in correction for the broader Raleigh scattering obtained in SERS. One drawback in the Tm-modified colloid surface for the chemisorption of other analytes is the number and intensities of the bands derived from the substrate itself which might complicate or dominate the spectrum of any analyte at low concentrations. Since many of these bands arise from unsaturation, conjugation, and symmetry in the methimazolyl units, the homologous, saturated,
asymmetric, thiazoline (Tz) ligand10 (Figure 1) which has a much simpler spectrum was applied as the sodium salt to the surface. The SERS spectra shown of solid AgTz (spectrum A, Figure 2) and NaTz adsorbed onto silver colloid (spectrum A, Figure 3) demonstrate that only the bands at 1300, 1351, and 1533 cm-1 can be detected from this surface modifier. The adsorption of metalloporphyrins on modified colloids and their vibrational analysis has been an area of great interest for many years.6-8,18 However, for porphyrins without specific complexing groups, these have been difficult to study by SERS/SERRS. One strategy which has provided good spectra for a very limited range is to modify the porphyrin by the addition of pyridyl groups18 which are not present in native porphyrins. However, this means bonding to the surface in a specific manner through the edge of the porphyrin ring making the axial (proximal) coordination site on the metal less accessible. When chloroprotoporphyrin IX iron(III) (hemin) was added to the new soft surface, good SERRS spectra were obtained easily (Figure 3). By contrast, no effective spectra were obtained from the colloid without the modifier. This (18) Qu, J.; Arnold, D. P.; Fredericks, P. M. J. Raman Spectrosc. 2000, 31, 469.
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Figure 3. The SERRS spectra (514.5 nm, 100 mW) of (A) Tz deposited on silver colloid, (B) + hemin at 10-10 M, and (C) + hemin at 10-9 M. The spectra were collected from 200 to 2000 cm-1 with a ×50 microscope objective using three 10-s accumulations. The intensities of the vibrational bands from Tz are markedly lower than those of the corresponding Tm adsorbate. The control spectrum hemin + colloid with no modifier added (not shown) displays no resonances above the signal-to-noise. The bands 1550.0-1587.3 cm-1 in (C) are derived from the porphyrin ring modes such as ν2 and the vinyl groups on the rings. The residual Tz bands are identified (*).
Figure 4. A possible representation of the assembly that facilitates the coupling of hemin (left) to the silver colloid. The chloride ligand is lost on complexation.
indicates effective adsorption to the modified surface and confirms the presence of complexing groups on the surface to react with the iron (Figure 4). By comparison with our previous studies on heme proteins,19 the extremely intense hemin spectra from 10-9 and 10-10 M solutions suggest concentrations of less than 100th of a monolayer can be detected. Thus we are able to study isolated porphyrin species on these surfaces. The scattering from the hemin bands is intense due to preresonance with the Q-bands of the heme. Consequently, there is little sign of scattering from the modifier in the spectra after addition of the (19) Macdonald, I. D. G.; Smith, W. E. Langmuir 1996, 12, 706.
porphyrin (Figure 3). Clearly present in the hemin spectra (Figure 3) are the established spin (ν10, ν3) and oxidation band markers (ν4).20 At 10-9 M, we also see the appearance of weaker hemin bands at 1124.7 cm-1 (ν6 + ν8 vinyl)20 and 1164.3 cm-1. The positions of ν10 at 1623.4 cm-1 and ν3 at 1487.7 cm-1 indicate the hemin has adopted a high spin configuration. The position of ν4 (1368.8 cm-1) is indicative of the ferric state. Overall, the SERRS spectra indicate hemin in a five coordinate iron(III) environment. Since this is likely to be complexed on the proximal site through the sulfur, this suggests that the halide is no longer within the coordination sphere of the iron and the reaction face of the porphyrin is vacant (Figure 4). The strong SERRS from the hemin indicates that there is efficient adsorption of the porphyrin, and the ease with which the common marker bands can be assigned19 suggests that this system will be a good model for the study of porphyrin orientation and reactivity. Overall, the results suggest that the Tm and Tz ligands have provided a new type of surface for SERS/SERRS, which will enable new and important classes of adsorbates to be studied effectively. Acknowledgment. J.F.O., S.J.S., and A.S. thank the Royal Society, the EPSRC, and the Socrates program, respectively, for financial support. LA034279J (20) Hildebrandt, P.; Stockburger, M. Vib. Spectra Struct. 1989, 17, 443.
