Langmuir 1996, 12, 6389-6398
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Integrated Optics Evanescent Wave Surface Enhanced Raman Scattering (IO-EWSERS) of Mercaptopyridines on a Planar Optical Chemical Bench: Binding of Hydrogen and Copper Ion Jean Baldwin, Norbert Schu¨hler, Ian S. Butler, and Mark P. Andrews* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 2K6, Canada Received April 16, 1996X Chemical interactions at molecular interfaces have been studied by integrated optics evanescent wave surface-enhanced Raman spectroscopy (IO-EWSERS). We describe methods to decorate planar glass waveguides with two-dimensional aggregates of Rayleigh limit silver particles by covalent attachment of them through a propane thiolate moiety grafted to the glass surface in a prior step. The evanescent field of the propagating transverse electric waves is enhanced by coupling with surface plasmon modes of the metal particles. Surface tethering and orientations of 2- and 4-mercaptopyridine (MPy) on Ag particles are studied through a comparative analysis of their SER spectra on silver metal liquid-like films (MELLFs), Ag hydrosols, and the colloid functionalized waveguide. Both molecules adopt roughly perpendicular orientations when bonded through the sulfur atom to silver. Overcoating the silver adlayer on the waveguide converts the construction into a sensing “optical chemical bench” (OCB). Accordingly, reversible acidbase titration of surface-bound 4-Mpy could be examined. In contrast, Cu2+ is coordinated irreversibly. X-ray photoelectron spectroscopy gives insight into the OCB fabrication and sensing process as the structure is evaluated step by step from a silica substrate surface to the copper ion at the outermost layer. The EWSER experiment is sensitive to perturbations of the vibrational structure of the heterocycle adsorbate at sub-monolayer coverage. The experiment can be conducted without photodegradation at relatively low laser power on inexpensive waveguides.
Introduction Surface-mediated enhancements of optical field interactions at interfaces have been of interest to the scientific community for some time. Our success with metal particle optical field intensifiers to enhance nonlinear optical1 and Raman processes2 in fractal silver or gold metal nanoparticle/polymer composites and waveguides led us to consider an application of the field enhancement effect to the study of molecular structure and chemical reactivity at interfaces. This is accomplished by preparing twodimensional metal/organic/inorganic oxide heterostructures by molecular self-assembly methods that include metal colloid deposition.3 In the past2 we showed that benzenethiol could be grafted onto the surface of colloidal gold particles hosted dilutely in a poly(vinyl alcohol) (PVA) thin film optical waveguide matrix. Despite the fact that in this configuration the guided wave interrogates mainly PVA polymer, a significant enhancement of Raman scattering was observed emerging from the benzenethiolate moieties at the gold colloid/polymer interface. In the experiments described in this paper, we have built on these observations in using (mercaptopropyl)trimethoxysilane (MPTMS) as a molecular adhesive to bind silver colloid to the surface of a glass waveguide. Indeed, the recent literature contains a record of similar achievements in this area.4 Among the available nondestructive spectroscopic probes for interfaces, surface-enhanced Raman scattering (SERS) has proved its importance for characterizing organic and inorganic adsorbates, with most X Abstract published in Advance ACS Abstracts, September 1, 1996.
(1) Andrews, M. P.; Ghebremichael, F.; Kuzyk, M. G. Nonlinear Opt. 1993, 6, 103. (2) Andrews, M. P.; Kanigan, T.; Xu, W.; Kuzyk, M. G. Proc. SPIEsInt. Soc. Opt. Eng. 1994, 2042, 366. (3) See: Andrews, M. P. Abstract published in Nanocomposites and Supramolecular Materials: Design, Synthesis and Properties, An International Workshop, sponsored by the ACS Division of Polymer Chemistry, Inc., March 8-11, 1994, San Diego, CA.
S0743-7463(96)00367-8 CCC: $12.00
studies confined to interactions with Cu, Ag, and Au.5,6 For the most part, the SER effect requires the presence of roughened surfaces that can be obtained in a number of ways, including electrochemical roughening of electrodes.7 Roughening can also be simulated by colloidal sols4 and metal liquid-like films (MELLFs).8 In this paper we describe how a colloid adlayer can couple with an evanescent wave propagating in an underlying waveguide through resonant interactions with the conduction electrons of the metal particles (surface plasmon excitations). In this configuration, the colloid overlayer acts as an optical field intensifier, increasing the cross sections for Raman scattering. Propagation of a transverse electric (TE) wave occurs with small optical losses, since the thin metal/ dielectric interface effectively behaves as a TE wave polarizer. Following our earlier nomenclature,4a the multilayer structure consisting of silver colloid + 3-MPTMS + waveguide is invoked as an “optical chemical bench” (OCB). Adsorption of molecules onto the surface of the OCB alters boundary conditions for wave propagation so that the structure ends up acting as a source of information about itself. This suggests that the bench might be used as an integrated optics (IO)9 sensor by taking advantage of SER enhancement of evanescent waves. Integrated optics evanescent wave surface-enhanced Raman scattering (IO-EWSER) spectroscopy can then be defined as a method that combines the capability of integrated optics, (4) (a) Xu, W.; Andrews, M. P. Opt. Soc. Am. Tech. Digest 1995, 21, 378. (b) Freeman, G. R.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. S.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (c) Van Duyne, R. P.; Hulteen, J. C.; Treichel, D. A. J. Chem. Phys. 1993, 99, 2101. (5) Brandt, E. S.; Cotton, T. M. In Physical Methods of Chemistry Series; Rossiter, B. W., Baetzold, R. C., Eds.; John Wiley & Sons: New York, 1993; Vol. IXB, Chapter 8, p 633. (6) Chang, R. K.; Furtak, T. E. Surface-Enhanced Raman Scattering; Plenum Press: New York, 1982. (7) Fleischmann, M.; Hendra, P. J.; McQuillan, J. Chem. Phys. Lett. 1974, 26, 163. (8) Efrima, S. Heterog. Chem. Rev. 1994, 1, 339. (9) See for example: Bohn, P. W. Trends Anal. Chem. 1987, 6, 223.
© 1996 American Chemical Society
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Figure 1. Schematic diagram of the heterostructure assembly glass/3-MPTMS/Ag colloid of the IO-EWSER substrate with an adsorbed layer of sensor coating, 4-MPy. The propagating evanescent wave, TE mode (- - -), leaks into the 3-MPTMS/Ag colloid overlayers, increasing the Raman scattering cross section and enhancing the surface 4-MPy adsorbate vibrations. The selective sensor function of 4-MPy is depicted by the H+ ions of MPyH+ represented by (0) which are replaced by Cu2+ ions (9).
evanescent waves, and surface-enhanced Raman scattering on a single platform to detect analytes. The present paper describes the use of this technique to bind H+ and Cu2+ analytes. A schematic of the waveguide structure is shown in Figure 1. With IOEWSERS we retain the attractive features of SERS with colloids but gain the advantage of being able to work in media that would normally cause catastrophic aggregation of the colloid. Covalent binding of the colloid to 3-MPTMS at the waveguide surface is tenacious. This broadens the range of compounds that can be adsorbed without damaging the metal overlayer. In the future, research may reveal how robust structures such as these might lend themselves as sensing elements in a variety of demanding environments including organic fluids, vapors, and variable-temperature settings. Our approach underscores an advantage of IO-EWSERS over the use of metal liquidlike films (MELLFs): in our hands, optical chemical benches are demonstratively more robust and easier to make more reproducibly than are MELLFs. Furthermore, planar waveguides offer a convenient format for mass production of channel and slab configurations that can be multiplexed. Planar symmetry can lead to simplifications over cylindrical symmetry waveguides (fibers) for purposes of modeling and optimization. Potential uses of optical OCB’s include applications to the study of organized molecular assemblies, protective coatings, electrochemical transduction, lubrication, wetting, and biological recognitionsin short, many situations in which interfacial processes perturb light traveling in an optical circuit.10 Chemical sensors usually consist of a sensing layer which responds selectively to a particular analyte.11 Practical difficulties with selective coatings might be overcome by using thin films consisting of self-assembling molecules with simple ion-responsive terminal functional (10) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvilli, I.; McCarty, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. L.; Yu, H. Langmuir 1987, 3, 932. (11) Bard, A. J.; Abruna, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. R.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147.
groups.12 Since mercaptopyridine compounds for sensing are known to self-assemble on metal surfaces,13-17 we selected these for binding to the OCB. For the purpose of Raman spectroscopy, the choice of a para-substituted pyridine molecule holds a number of advantages. Apart from being a strong Raman scatterer, the vibrational fingerprint of pyridine and its congeners is well characterized. Perturbations of their relatively simple spectra are easily detected. In addition, mercaptopyridine offers both sulfur and nitrogen binding sites. Bifunctionality can be turned to advantage by selective attachment of one end of the molecule (sulfur) to the metal overlayer of the OCB. This paper consists of two parts. In part 1 we develop a comparative SER study of 2- and 4-mercaptopyridine (2- and 4-MPy) adsorbed on colloidal silver sol, metal liquid-like films (MELLFs), and an optical chemical bench (OCB). The purpose of this comparison is to establish the extent to which the SER effect is coextensive from one Ag system to another. Since 2- and 4-MPy are at least ambidentate,18 they offer several possible modes of adsorption and orientation on a metal surface. Investigations of 2- and 4-MPy on modified Au substrates have concluded that both 2- and 4-MPy bind through the sulfur atom, with the pyridine ring oriented approximately normal to the metal surface.14,15,19 Through a careful analysis of the spectral data collected in our IO-EWSER study we conclude that both compounds adopt a nearly upright orientation when they adsorb on silver through (12) Keplay, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1992, 64, 3191. (13) Gui, J. Y.; Lu, F.; Stern, D. A.; Hubbard, A. T. J. Electroanal. Chem. 1990, 292, 245. (14) Takahashi, M.; Fujita, M.; Ito, M. Surf. Sci. 1985, 158, 307. (15) Taniguichi, I.; Iseki, M.; Yamaguchi, H.; Yasukouchi, K. J. Electroanal. Chem. 1985, 186, 299. (16) Bryant, M. A.; Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 753. (17) Jones, T. A.; Perez, G. P.; Johnson, B. J.; Crooks, R. M. Langmuir 1995, 11, 1318. (18) Kennedy, B. P.; Lever, A. B. P. Can. J. Chem. 1972, 50, 3489. (19) Xu, H.; Tseng, C. H.; Vickers, T. J.; Mann, C. K.; Schelenoff, J. B. Surf. Sci. 1994, 311, L707.
Mercaptopyridines on a Planar Optical Chemical Bench
the sulfur substituent. Finally, since accessibility of the coordinating substituent is important for ion binding, a study that builds a steric barrier into the sensing molecule (as in 2-MPy) can be helpful. Binding of H+ and Cu2+ to a 4-MPy sensing layer attached to the OCB is studied with IO-EWSERS in the second part of this paper. Interactions between adsorbed 4-MPy and analyte occur through the pyridine nitrogen. This is anticipated from studies of binding of cytochrome c and sulfonated anthroquinones to 4-MPy-modified Au electrodes.17,20,21 The IO-EWSER technique is sensitive to metal ion induced perturbations of ligand vibrational modes at submonolayer coverage. Raman spectra of selected modes can be recorded in roughly 45 s of detector integration time, despite the fact that the majority of the power resides in the waveguide. With TE polarization, this is accomplished with no optical damage. Finally, we use X-ray photoelectron spectroscopy (XPS) to construct a layerby-layer interpretation of the OCB from the substrate to the Cu2+ ion. Experimental Section Materials. 2- and 4-MPy (Aldrich) were sublimed in vacuo immediately prior to use. Other chemicals were used as received. Sodium borohydride (99+%, Janssen Chimica) and silver nitrate (p.a. Aldrich) were used to prepare Ag colloid. 3-MPTMS (Petrarch) was used to functionalize waveguides. For the pH studies, HCl (99.99%) and NaOH (99.99%) (Aldrich) were used. Dichloromethane and acetonitrile (Aldrich) were of spectroscopic grade. High-purity water was obtained from a Milli-Q Ultrapure Millipore water system. Glass cover slides (24 × 30 mm3 × 150 µm, Fisher Scientific) or 2 µm sol-gel derived films were selected as waveguide substrates. Silicon (110) wafers for XPS (Vackers Chemitronics) exhibited a conductivity of 1-10 (Ω) cm. Preparation Procedures. Ag colloids used to prepare waveguides, MELLFs, and colloidal sols were produced by reduction of AgNO3 with NaBH4, following established methods.4a,22 3-MPTMS and Ag colloid modifications to the surface of the glass waveguide have been detailed in a previous paper.4a The 4- and 2-MPy compounds were adsorbed onto freshly prepared Ag colloid-derivatized waveguides by immersion for 10 min in a 2 mM acetonitrile solution containing the adsorbate. Cu2+ binding to OCB’s conjugated with 2- and 4-MPy was studied by immersing them in a 2 mM aqueous solution of Cu(NO3)2 for 10 min followed by repeated washing with Millipore water. Spectral changes were monitored as a function of time as a given waveguide was repeatedly immersed in the copper ion solution. MELLFs were best prepared22 by mixing equal volumes (2 mL) of an aqueous Ag colloid and a dichloromethane solution containing 2 mM of the adsorbate in a tall, cylindrical, stoppered vial. The vial was then shaken vigorously for 1 min. A metallic film formed at the interface of the aqueous and dichloromethane layers. This film was removed from the interface by carefully immersing a small piece of glass slide (0.7 × 0.7 cm2) through the solvent phase, transferring the film from the aqueous layer onto the glass, and then withdrawing the glass slowly through the solvent phase. The 2-MPy Ag sol was obtained by adding an aqueous solution of 2-MPy (60 µL of 10 mM) to 2 mL of the Ag colloid. The colloid aggregated immediately, giving a purple solution. This was transferred immediately to a capillary sample cell for SERS measurement. Similarly, 4-MPy Ag sol was obtained by adding an aqueous solution of 4-MPy (10 µL of 0.2 mM) to 2 mL of Ag colloid. In this case, aggregation occurred slowly, eventually yielding a purple-brown solution. The SER spectrum was collected after 12 h. (20) Xie, Y.; Dong, S. Electroanalysis 1994, 6, 567. (21) Sagara, T.; Niwa, K.; Hinnrn, C.; Niki, K. Langmuir 1990, 6, 254. (22) Vlckova´, B.; Barnett, S. M.; Kanigan, T.; Butler, I. S. Langmuir 1993, 9, 3234.
Langmuir, Vol. 12, No. 26, 1996 6391 Silver 2- and 4-MPy salts were prepared for comparative Raman studies. Stoichiometric quantities of Ag(NO3)2 and 2and 4-MPy (0.05 mol) were dissolved in ethanol/water solution and mixed in a 1:1 volume ratio. A pale-yellow precipitate of silver cation/4-MPy and a white precipitate of silver cation/2MPy polymer were obtained. Each precipitate was filtered and rinsed with ethanol and water and then dried under vacuum. FT-Raman spectra of 2- and 4-MPy and the polymeric silver complexes were recorded from the solid state compounds. For pH-dependent studies, aliquots of HCl or NaOH were used until the desired pH was reached. Acidic pH studies were also repeated with HClO4. FT-Raman spectra of 4-MPy solutions were recorded immediately after preparing solutions. Instrumentation. All SER spectral data were obtained with 514.5 nm (50 mW at the waveguide) radiation from a Spectra Physics Model 164 Ar+ laser in combination with an integrated optics spectrometer bench constructed in-house.23 The detector system consisted of a liquid N2-cooled charged coupled device (CCD) detector interfaced to an Instruments S.A. HR640 spectrograph equipped with a holographic grating (1800 grooves mm-1). An acquisition time of 30 s was typically employed for each 500 cm-1 window. FT-Raman spectra were recorded with a Bruker IFS-88 spectrometer equipped with an FRA-105 Raman module and a liquid N2-cooled Ge detector. Spectra were excited with a Nd3+: YAG diode laser operating at 1064.1 nm (150 mW). Two hundred fifty spectral scans were typically collected. X-ray Photoelectron Spectroscopy (XPS). For layer-bylayer analysis of the multilayer slab waveguide by XPS, samples were prepared on the silica overlayer of a (110) wafer. Sample preparation followed the same recipe as that used to prepare the OCBs. We identify each sample with the following labels: sample A, Si wafer; sample B, Si wafer modified with 3-MPTMS (Si/3MPTMS); sample C, Si/3-MPTMS with adsorbed Ag colloid (Si/ 3-MPTMS/Ag); sample D, Si/3-MPTMS/Ag with adsorbed 4-MPy (Si/3-MPTMS/Ag/4-MPy); and sample E, Si/3-MPTMS/Ag/4-MPy immersed in aqueous Cu(NO3)2 solution (Si/3-MPTMS/Ag/4-MPy/ Cu). XPS measurements were performed with a Fisons VG ESCALAB 220i-XL spectrometer using monochromatized Al KR radiation. All spectra were collected in constant analyzer energy (CAE) mode. High-resolution spectra were collected at a constant pass energy of 20 eV. The pressure of the UHV chamber during XPS analysis was typically 4 × 10-10 Torr. Binding energies were referenced to the Au 4f7/2 line at 84.0 eV. XPS data were analyzed with Eclipse v1.7p, software provided with the instrument.
Results and Discussion Part 1. Comparison of SERS of 2- and 4-Mercaptopyridine on Different Substrates. 1.a. SERS of 4-Mercatopyridine. SER spectra obtained for 4-MPy adsorbed on MELLF, IO-EWSERS, and Ag hydrosol substrates are shown in Figure 2a-c, respectively. Peak positions and band assignments for 4-MPy are given in Table 1. Assignments are based on those for parasubstituted pyridine and thiophenol.24-27 3-MPTMS interposed between the silver adlayer and the waveguide is a poor Raman scatterer, so its contributions to the total 4-MPy SER spectrum are negligible. Assuming local C2v symmetry, substituted pyridine ring vibrations are distributed among the four symmetry species, a1, a2, b1, and b2, all of which are Raman active. In-plane motions are described by a1 and b2 modes, with out-of-plane vibrational motions carrying a2 and b1 symmetry. The unenhanced Raman spectrum in the region 1800-400 cm-1 is domi(23) Andrews, M. P.; Kanigan, T.; Xu, W.; Kuzyk, M. Proc. SPIEsInt. Soc. Opt. Eng. 1993, 2042, 366. (24) Green, J. H. S.; Kynaston, W.; Paisley, H. M. Spectrochim. Acta 1963, 19, 549. (25) Spinner, E. J. Chem. Soc. 1963, 3860. (26) Joo, H. T.; Kim, M. S.; Kim, K. J. Raman Spectrosc. 1987, 18, 57. (27) Dollish, F. R.; Fateley, W. G.; Bentley, F. F. Characteristic Raman Frequencies of Organic Compounds; John Wiley & Sons: New York, 1974.
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Figure 2. SER spectra of 4-mercaptopyridine (4-MPy) adsorbed on (a) MELLF, 2 mL of 2 × 10-3 M in 2 mL of colloid; (b) IOEWSERS, 2 × 10-3 M of 4-MPy in acetonitrile; and (c) silver hydrosol, 10 µL of 2 × 10-4 M in 2 mL of colloid.
nated by the more intense a1 modes, with the exception of a strong band at 1394 cm-1, assigned to a 14b2 ring mode. The a2 modes are inherently very weak in aromatic ring systems, and bands assigned to these modes, as well as the b1 modes, are understandably weak in the normal Raman spectrum of neat 4-MPy. Turning to the SER spectra, the a2 modes are absent and the b1 modes remain very weak for 4-MPy adsorbed on any substrate. In general, the SER spectra of 4-MPy are dominated by bands belonging to b2 and a1 modes. For comparison, the unenhanced FT-Raman spectra of the bulk Ag+/4-MPy complex and pure solid 4-MPy are displayed in Figure 3. SER spectra of 4-MPy and the normal Raman spectrum of the silver/4-MPy complex show remarkable similarity in their vibrational band profile. Bands at 430, 714, 1007, 1107, 1586, and 1616 cm-1 of the Ag+/4-MPy complex have counterparts, though with different intensity, in the SER spectra of 4-MPy adsorbed on different substrates, e.g., bands at 435, 709, 1015, 1098, 1583, and 1617 cm-1 in the IO-EWSER spectrum (Figure 2b). The 1a1 ring-breathing mode near 1000 cm-1 remains intense in both the normal Raman spectrum of the Ag+/ 4-MPy complex and the SER spectra. The nitrate counterion is the carrier of the 1041 cm-1 band in the Raman spectrum of the Ag+/4-MPy complex in Figure 3a. The shoulder at 1051 cm-1 on the higher wavenumber side of the nitrate band belongs to the ν18a1 β(CH) bending mode. This feature is absent in the IO-EWSER and colloidal SER spectra. It is replaced by a band at 1065 cm-1, which we assign to the ν18b2 β(CH) mode. By way of comparison, both ν18a1 and ν18b2 modes are detected as weak bands in the MELLF spectrum. All SER spectra and the normal Raman spectrum of the Ag+/4-MPy complex exhibit the same dramatic increase in intensity of the 12a1 mode near 1098 cm-1. This is a so-called X-sensitive mode.24,25 X-sensitive modes are described as modes that are strongly coupled substituent and aromatic ring modes. Coupling is modulated by the local environment of the X substituent. This coupling accounts for the intensity modulation of the 12a1 mode when 4-MPy is converted to Ag+/4-MPy or when 4-MPy is adsorbed on a silver surface. This effect has already been noted in the SERS of thiophenol,26,28 in electrochemical SERS of 4-MPy on Au electrodes,15 and
Baldwin et al.
to a lesser extent in the normal Raman spectrum of pyridine/metal complexes.28,29 Coupling of the 12a1 ring breathing mode with the ν(C-S) stretching mode is responsible for this effect.16,26,28 All SER spectra exhibit a 1220 cm-1 band for which there is no counterpart in the normal Raman spectrum of neat 4-MPy or the Ag+/4-MPy complex. We note that a band similar in intensity and frequency position has been found in the colloidal Ag SERS of pyridine and pyrazine. This has been attributed to a 9a1 β(CH) mode.30 Where the normal spectrum of the Ag+/4-MPy complex does not show well-resolved features between 1250 and 1580 cm-1, this is not so for the SERS spectra of 4-MPy, where there is a dependence on the method of fabricating the SER substrate. Colloidal sol SERS gives peaks at 1305, 1338, 1407, 1433, and 1470 cm-1. Weak vibrational features are observed in the MELLF spectrum at 1390, 1458, and 1515 cm-1. Interestingly, only one band at 1478 cm-1 dominates the IO-EWSER spectrum in this region. It is not clear what causes these differences on changing from one SERS substrate to another. Arguments relating to differences in surface potential, adsorbate polarizability, and scale of roughness have been advanced to rationalize similar observations in the past.1,31,32 1.b. SERS of 2-Mercaptopyridine. Figure 4 compares SER spectra of 2-MPy on MELLF, IO-EWSER, and Ag hydrosol substrates. Normal Raman spectra of neat 2-MPy and the Ag+/2-MPy complex are compared in Figure 5. The vibrational assignments collected in Table 2 follow those of a previous report and assume that the 2-MPy molecule retains Cs symmetry.33,34 In the solid state, 2-MPy exists as a weak, intramoleculary H-bonded dimer of C2h symmetry. The normal Raman spectrum of the Ag+/2-MPy complex is similar to that of the Ag+/4-MPy complex; i.e., the spectrum is dominated by the more intense bands belonging to the ν(C-S) modes (727 and 1135 cm-1) and the in-plane ring modes (1001 and 1589 cm-1). The X-sensitive mode at 1135 cm-1 exhibits an increase in relative intensity when the Ag+/2-MPy complex is formed. This behavior is similar to what we observe for the Ag+/4-MPy complex. This band shifts to 1114 cm-1 and remains unenhanced for 2-MPy on the OCB and the colloidal sol. By contrast in Figure 4, considerable enhancement of this feature is seen in the MELLF spectrum. A broad low-intensity band near 1627 cm-1 is tentatively assigned to adsorbed water in the hydrosol system. The band near 1230 cm-1 in the IO-EWSER and hydrosol spectra may belong to the coupled mode δ(NH)/ β(CH).33 This may be evidence that the basic nitrogen atom picks up a proton when the minor thiol component of the thiol-thione tautomer adsorbs on silver. Focusing again on the 1250-1450 cm-1 region, we observe that the SER spectra of 2-MPy obtained on the collidal sol and the OCB are more complicated than those of the Ag+/2-MPy complex. The SERS from the 2-MPy on the MELLF substrate is noticeably featureless in this region. The appearance of these bands is attributed to orientation effects. The following section discusses orientations of 2and 4-MPy on these various SER substrates. (28) Carron, K.; Hurley, L. G. J. Phys. Chem. 1991, 95, 9979. (29) Akyuz, S.; Dempster, B.; Morehouse, R. L.; Suzuchi, S. J. Mol. Struct. 1973, 17, 105. (30) Moskovits, M.; DiLella, D. P.; Maynard, K. J. Langmuir 1988, 4, 67. (31) Cotton, T. M. In Advances in Infrared and Raman Spectroscopy; Clark, J. H., Hester, R. E., Eds.; John Wiley & Sons: New York, 1988; Vol. 16, Chapter 3, p 91. (32) Creighton, J. A. Surf. Sci. 1986, 173, 665. (33) Shunmugam, R.; Sathyanarayana, D. N. Spectrochim. Acta 1984, 40A, 757. (34) Lautie, A.; Hervieu, J.; Beloc, J. Spectrochim. Acta 1983, 39A, 367.
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Table 1. 4-Mercaptopyridine Raman Shifts (cm-1) assignment26,27 7a1, δ(C-S)/γ(CCC)39 16b1, γ(CCC) 6b2, β(CCC) 6a1, (β(CC)/ν(C-S)24,38 10b1, γ(CH)
solid
silver (4-mercaptopyridine)
hydrosol
IO-EWSERS
415 497
430
435 490
707 811
705 813
709 810
1008 1049 1064 1097
1007
1015
1065 1098 1135
1062 1098
1a1 (ring breathing) 18a1, β(CH) 18b2, β(CH) 12a1 (ring breathing)/(C-S)15,24,26 β(CH)/δ(NH)34 9a1, β(CH) 3b2, β(CH)
1200 1250 1290
1204 1218
14b2, ν(CC) 19b2, ν(CdC/CdN) 19a1, ν(CdC/CdN)
1394 1459 1478
1390 1458
8b2, ν(CC) 8a1, ν(CC)
1604 1617
430
MELLF
431 471 647 721 790 901 990 1045 1080 1106
646 714 1007 1051 (sh) 1107
1586 1616
1525 1583 1611
1207 1221
1222 1305 1338 1407 1433 1470
1478
1573 1615
1583 1617
Figure 3. Unenhanced FT-Raman spectra of (a) the silver 4-mercaptopyridine complex and (b) 4-mercaptopyridine in the solid state. λex ) 1064.1 nm at 150 mW.
Figure 4. SER spectra of 2-mercaptopyridine (2-MPy) adsorbed on (a) MELLF, 2 mL of 2 × 10-3 M 2-MPy in 2 mL of colloid; (b) IO-EWSER, 2 × 10-3 M of 2-MPy in acetonitrile; and (c) silver hydrosol, 60 µL of 1 × 10-2 M in 2 mL of colloid.
1.c. Orientation. We have already established that the major spectral features of 4-MPy are preserved irrespective of the method of preparing the SERS substrate. Clearly, it is important to know that the binding site nitrogen is free to coordinate an ion from solution. Therefore, an understanding of the orientation of the 4-MPy ligand on the OCB is crucial. Surface selection rules are often applied to molecules on SERS-active substrates to deduce orientations.35-37 Modes that are perpendicular to the surface are preferentially enhanced. For C2v molecules, we require all four symmetry species to define orientation because each contributes information through the polarizability tensor components. For perpendicular orientation, the relative enhancement of bands is in the order of species b1 ) b2 > a2. Molecules lying flat on the surface will experience enhancements such that a2 ) b1 > b2. The a1 totally symmetric vibrational modes cannot be used alone to assign orientation.28 Unfortunately, b1 modes
are weak and a2 modes are not observed at all from 4-MPy on any of the SER substrates. While the most significant enhancements appear in the in-plane ring vibrations, 12a1/ ν(C-S) and 8b2 ν(CC) and 9a1 β(CH), the inherent weakness of the b1 and absence of the a2 vibrations makes comparison of their relative intensities with those of the a1 and b2 modes impossible.28 Some insight may be gained, nevertheless, by considering planes of vibrations in the molecular frame.38 Normal modes that involve in-plane motion of the ring CdC and CdN bond stretches would be expected to be most enhanced for more or less perpendicular orientations of molecules with respect to the surface. Out-of-plane a2 and b1 vibrations should experience little enhancement. Alternatively, were molecules to lie flat, the totally symmetric a1 and nontotally symmetric b2 in-plane vibrations would experience little or no enhancement. Some out-of-plane vibrations would be enhanced. In the SER spectra of 4-MPy, the 1a1 inplane totally symmetric ring vibration appears as an intense band. Preferential enhancement of the in-plane
(35) Moscovits, M.; Shu, S. J. Phys. Chem. 1984, 88, 5526. (36) Creighton, J. A. Surf. Sci. 1985, 158, 211. (37) Creighton, J. A. Surf. Sci. 1983, 124, 211.
(38) Stekas, T.; Diamandopoulos, P. J. Phys. Chem. 1990, 94, 1986.
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Figure 5. Unenhanced FT-Raman spectra of (a) the silver 2-mercaptopyridine complex and (b) 2-mercaptopyridine in the solid state. λex ) 1064.1 nm at 150 mW.
12a1, 9a1, and 8b2 vibrations suggests that the plane of the pyridine moiety is roughly perpendicular to the surface. 2-MPy probably binds with a predominately perpendicular orientation to the surface of the colloid attached to the waveguide as well. Bands appear near 1272, 1413, and 1451 cm-1 in the IO-EWSER and colloidal SER spectra that are absent in the normal Raman spectrum of the Ag+/2-MPy complex. These bands are assigned to ν14b, ν19b, and ν19a in-plane ν(CdC/CdN) vibrations. Enhancement of modes that include vibrational motions of CdN ring bonds suggests that the pyridine ring plane is normal to the surface. Enhancement is seen in the a′ ν(CC) mode at 1551 cm-1 and the a′ ν(CH) stretching vibrations at 3061 and 3109 cm-1. Figure 6 shows the CH stretching region, comparing 2- and 4-MPy. Observe that there is only marginal enhancement in this region for 4-MPy, providing further evidence that both molecules adopt a near perpendicular orientation.38 Previous SERS studies of 2- and 4-MPy on modified Au, Ag, and Pt surfaces also concluded that the pyridine ring is normal to the surface,14-16 and this has been supported by EELS, LEED, and Auger surface studies of 2- and 4-MPy on Ag(111) electrodes.13 1.d. N- or S-Bonded Adsorption of 2- and 4-MPy on the SERS Substrate? As discussed above, the pyridine ring most probably resides perpendicular to the metal surface for both 2- and 4-MPy. For this orientation, adsorption is expected to occur via σ donation; yet σ donation can occur through the nitrogen and/or the sulfur atom. Our analysis of the IO-EWSER data suggests that 2- and 4-MPy are bonded to the Ag colloid surface via the sulfur atom. This mode of adsorption is not surprising considering the strong affinity of thiols for Au surfaces.39 Reaction of aromatic and aliphatic thiols with Cu, Ag, and Au surfaces occurs with simultaneous disappearance of the intense polarized ν(S-H) stretch between 2590 and 2560 cm-1 following cleavage of the S-H bond.26 Unfortunately, this diagnostic mode is broadened and weakened for both 2- and 4-MPy because of the thione/thiol tautomerism40,41 shown in Scheme 1. For recourse we turn to the lowwavenumber region, where the X-sensitive mixed β(CC)/ ν(CS) vibrations are located.25,33,42 These undergo the (39) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, G. M. J. Am. Chem. Soc. 1989, 111, 321. (40) Beak, P.; Fry, F. S.; Lee, J. J.; Steele, F. J. Am. Chem. Soc. 1976, 98, 171. (41) Stoyanov, S.; Petkov, I.; Antonov, L.; Stoyanova, T.; Karagiannidis, P.; Aslanidis, P. Can. J. Chem. 1989, 68, 1482.
Baldwin et al.
telltale shift from 721 (731) cm-1 in the free 4-MPy (2MPy) to 709 cm-1 (714) when complexed with the silver surface. Such shifts are anticipated for bonding through sulfur to a metal substrate.26,28,43,44 Part 2. Cation Sensing by IO-EWSERS. 2.a. IOEWSERS of Proton Binding. A simple test of the sensing ability of the 4-MPy overlayer is to quaternize the pyridine nitrogen.45,46 A signature for quaterized pyridine (PyH+) in the normal Raman spectrum is a definitive band near 1630 cm-1.27 The corresponding band for 4-MPyH+ is found at 1626 cm-1 for the molecule in acidic solution (Figure 7). Titration with base erases this peak, resurrecting the 8b ν(CC) band characteristic of deprotonated 4-MPy. Acidification of the waveguide coated with 4-MPy likewise produces a band for 4-MPyH+ located at 1617 cm-1. This is shown in Figure 8c. Titrating the OCB with hydroxide (Figure 8b) recovers the 8b ν(CC) band of protonated 4-MPy. Note that the titration is incomplete. This indicates that a portion of 4-MPyH+ remains inaccessible to the hydroxide ion. 2.b. IO-EWSERS of Cu2+ Binding. To demonstrate the principle of the IO-EWSER substrate as a metal ion sensor, the 4-MPy sensing layer was exposed to an aqueous solution of Cu(NO3)2. IO-EWSER spectra of 4-MPy collected as a function of time after immersion in the Cu2+ ion solution are shown in Figure 9. We observe that some 4-MPy bands increase in intensity with immersion time relative to the 1a1 and 12a1 bands. Major changes can be singled out for two principal regions of the Raman spectrum of 4-MPy. In the first place, the 8a1 ν(CdC) band at 1617 cm-1 diminishes in intensity as the copper content on the waveguide increases. In addition, the intensity of the ν8b2 mode at 1585 cm-1 grows at the expense of the former. Secondly, the 1a1 in-plane ring breathing mode at 1015 cm-1 develops a shoulder on the higher wavenumber side. The intensity of the 1a1 vibration decreases substantially with further exposure to Cu2+ ions (Figure 10). The shoulder eventually evolves as a distinct but weak peak at 1034 cm-1. This band finds correspondence with the minor spectral shifts observed for the ring vibrational frequencies when pyridine undergoes complexation with metal ions.29,47,48 Among the kinds of changes observed on complexation is an increase of the 1a1 ring breathing vibrational frequency.49 For example, we observe50 that the 1a1 band in free 4-methylpyridine shifts from 996 to 1036 cm-1 in tetrahedral CuCl2(4-methylpyridine)2 and from 991 cm-1 in free pyridine to 1020 cm-1 in Cu(NO3)2(pyridine)x, where x ) 2, 3, and 4. With this information, we assign the band at 1034 cm-1 in the IO-EWSER spectrum of 4-MPy to the ring breathing mode of a coordinated pyridine nitrogen. In analogy to the discrete Cu/pyridine complexes mentioned above, the appearance the 1034 cm-1 band from the OCB/Cu2+ interaction probably represents a blue shift from the 1015 cm-1 position of the 1a1 in-plane ring breathing mode. That the latter band does not completely disappear is again likely because not all of the 4-MPy is (42) Colthrup, N. B.; Daly, L. H.; Wiberly, S. E.; Fateley, W. G.; Grasselli, J. G. Handbook of Characteristic Infrared and Raman Frequencies of Inorganic Compounds; Academic Press: New York, 1991. (43) Garrell, R. L.; Szafrranski, C.; Tanner, W. SPIE Vol. 1336 Raman and Luminescence Spectrosc. Technol. 1990, 264. (44) Joo, T. H.; Yim, Y. H.; Kim, K.; Kim, M. S. J. Phys. Chem. 1989, 93, 1422. (45) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385. (46) Mullen, K. I.; Wang, D.; Crane, G. L.; Carron, K. T. Anal. Chem. 1992, 64, 930. (47) Thornton, D. A. Coord. Chem. Rev. 1990, 104, 251. (48) Akyuz, S. J. Mol. Struct. 1977, 42, 59. (49) Jeanmaire, D. L.; Van Duyne, J. J. Electroanal. Chem. 1977, 84, 1. (50) Unpublished results.
Mercaptopyridines on a Planar Optical Chemical Bench
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Table 2. 2-Mercaptopyridine Raman Shifts (cm-1) assignment34 a′′ 16a, γ(CCC) a′′ δ(C-S)/β(CCC)33
solid
silver (2-mercaptopyridine)
389 445
446
a′′ 6a, γ(CCC) a′′ β(CC) a′ (C-S)33 a′′ γ(CH) a′′ γ(NH) a′′ γ(CH) a′′ γ(CH) a′ 1a (ring breathing) a′ 18a, β(CH) a′ 18b, β(CH) a′ 12a, (ring breathing)/ν(C-S)24-26
618 711 731 743 893 949 980 988 1024 1092 1133
β(CH)/δ(NH) a′ 14b, ν(CdC/CdN) a′′ γ(CH) a′ 19b, ν(CdC/CdN) a′ 19a, ν(CdC/CdN) a′ 8b, ν(CdC) a′ 8a, ν(CdC)
1230 1261 1372 1444 1502 1569 1614
MELLF
727
1001 1042 1135 1164 1244 1269
1004 1054 1082 1117
1417 1589
1554 1578
hydrosol
IO-EWSERS
435 485 634
432
720
714
998 1048 1084 1117 1150 1228 1274 1411 1448
1001 1050 1081 1114 1150 1230 1272 1413 1451
1549 1579
1551 1576
630
Figure 6. IO-EWSER spectra of (a) 2-mercaptopyridine and (b) 4-mercaptopyridine ν(C-H) stretching vibrations. Scheme 1. Thiol-Thione Tautomerism of 4-Mercaptopyridine Figure 7. Unenhanced FT-Raman spectra of 4-mercaptopyridine in different aqueous pH solutions: (a) pH 14; (b) pH 8.3; and (c) pH 1.
accessible or is located in regions on the colloid where insufficient numbers of ligands can be mustered to form stable coordination complexes. Steric effects account for the observation that the IO-EWSER spectrum of 2-MPy is unaffected after exposure to Cu2+. Copper ion competes more strongly than H+ for binding at nitrogen and most 4-MPyH+ is replaced by Cu2+ when an acidified bench is exposed to the metal ion. In summary, IO-EWSERS can detect binding of protons or copper ions to a sensing overlayer of 4-MPy adsorbed onto the surface of a silver colloid adlayer, chemically bonded to an optical waveguide. Binding of either ion occurs with characteristic modifications in the SER spectrum of 4-MPy. Proton binding is largely reversible, and the waveguide can be titrated nearly to equivalence with base. A fraction of protons remain inaccessible to titrant. Coordination of Cu2+ to the optical chemical bench
occurs irreversibly. Steric crowding of nitrogen in the 2-position of the ring totally inhibits Cu2+ binding. Although the stoichiometry of the final Cu2+/4-MPy complex on the OCB is not known at this time, both the IO-EWSERS and XPS data show that nitrate anion is not present in the complex. 2.c. X-ray Photoelectron Spectroscopy. With XPS, we are able to follow the layer-by-layer construction of the waveguide. Results are collected in Table 3 for samples A-E (see Experimental Section). Shifts in binding energies and changes in the full width at half maximum (FWHM) of the peaks obtained by curve-fitting for S 2p, N 1s, and Cu 2p are also tabulated there. The first step in preparing the OCB to attach silver particles is to functionalize the waveguide with a layer of 3-MPTMS. Figure 11B exemplifies this step, showing also that the S 2p1/2,3/2 band is strongly overlapped by a band due to excitations of bulk Si plasmons. After correcting for this contribution to the sulfur signal, spectrum B in Figure 12 clearly shows the data set of Figure 11 for the sulfur 2p1/2,3/2 region of the molecular adhesive overlayer
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Baldwin et al.
Figure 8. IO-EWSER spectra of 4-mercaptopyridine immersed in different aqueous pH solutions: (a) pH 8.3; (b) pH 14; and (c) pH 1.
Figure 10. IO-EWSER spectra of the 1a1 ring breathing mode of 4-mercaptopyridine (a) before exposure to Cu2+ and (b) after long time exposure to Cu2+.
Figure 9. IO-EWSER spectra of 4-mercaptopyridine after immersion in a 2 × 10-3 M aqueous solution of Cu(NO3)2: (a) after 30 min immersion; (b) after 10 min immersion; and (c) after no exposure to Cu2+.
after subtraction of the plasmon. The sulfhydryl sulfur atom of 3-MPTMS exhibits a peak located at 163.7 eV for the S 2p3/2 peak, as expected for an unreacted thiol51 The next step in building up the OCB is to attach silver particles by covalent bonds to sulfur. The outcome of this reaction is to attenuate the XPS signal due to sulfhydryl sulfur. The binding energy for the S 2p3/2 transition due to an Ag-S interaction then appears at 162.4 eV (Figure 11C). Studies of alkanethiols and thiophenol on various metal surfaces report similar shifts in binding energy due to the formation of a thiolate species (RS-M).52-55 This interpretation of the S 2p region supports the assignment made (IO-EWSERS) for the ν(C-S) bands that were selectively enhanced after loss of thiol hydrogen at the Ag/3-MPTMS interface. Overcoating the silver colloid layer with 4-MPy (51) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (52) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (53) Uvdal, K.; Bodo, P.; Liedberg, B. J. Colloid Interface Sci. 1992, 149, 163. (54) Rufael, T. S.; Huntley, D. R.; Gland, J. L. J. Phys. Chem. 1994, 98, 13022. (55) Golzhauser, A.; Panov, S.; Woll, C. Surf. Sci. 1994, 314, L849.
is vividly indicated in Figure 11D by the enhanced intensity maximizing at 163.4 eV. This band envelope was well fitted with six curves, as required by the distribution of sulfur between free and bound 3-MPTMS and bound 4-MPy. Coordination of copper in the final stage (Figure 11E) resulted in a shoulder seen to emerge at 162.3 eV on the lower binding energy side of the envelope. Coordination of Cu2+ to the nitrogen of 4-MPy is confirmed by comparing spectra D and E in Figure 13 for the N 1s region. This region of uncoordinated 4-MPy (sample D) consists of two peaks located at 401.2 and 399.5 eV. The observation of two N 1s peaks in sample D suggests that nitrogen resides in two different environments. A clue to the origin of one of the peaks can be found by returning to the IO-EWSERS study of 4-MPy, where we located a Raman band at 1617 cm-1 belonging to a pyridinium, 4-MPyH+, species. We assign the two N 1s binding energies in sample D to thiolate bound 4-MPy (399.5 eV) and 4-MPyH+ (401.2 eV) species coexisting on the colloid surface. Coordination of Cu2+ to the OCB results in the appearance of a peak at 399.0 eV dominating the N 1s region. A second contribution is located at 400.0 and 399.0 eV, with relative intensity 1:3.5. Comparable shifts have been reported for the chemisorption of pyridine (through nitrogen) onto metal surfaces.56 Copper coordinates to the surface without nitrate as a counterion, since no nitrate N 1s peak (406.0 eV) was observed. This same conclusion was reached from the IO-EWSERS study. We were able to provide evidence of formation of an Ag-S bond by analyzing the Ag 3d spectrum (Figure 14). The maximum of the Ag 3d5/2 signal falls at 368.3 eV, similar to the position observed after adsorption of thiophenolate anion on silver.57 Addition of 4-MPy attenuates the Ag 3d region, and this is shown in Figure 14D. The Cu 2p bands obtained from sample E are shown in Figure 15. The curve fit for Cu 2p displays two sets of doublets. The first set at 933.0 eV (Cu 2p3/2) and 952.7 eV (Cu 2p1/2) shows shoulders on the high binding energy (56) Easely, G. L.; Burkstrand, J. M. Phys. Rev. B. 1981, 24, 582. (57) Xue, G.; Zhang, J.; Ma, M.; Lu, Y.; Carron, K. T. J. Colloid Interface Sci. 1992, 150, 1.
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Table 3. XPS Binding Energy Shifts and FWHM (in-Parentheses) for Samples A-E (eV) element Si (plasmon) S 2p3/2 S 2p1/2
sample A Si (plasmon)
sample B Si/3MPTMS
sample C Si/3MPTMS/Ag
sample D Si/3MPTMS/Ag/4-MPy
sample E Si/3MPTMS/Ag/4-MPy/Cu
163.7 (1.2)
163.7 (1.6) 162.4 (1.6)
164.9 (1.2)
164.9 (1.6) 163.6 (1.6)
163.6 (1.7) 163.4 (1.2) 162.4 (1.6) 164.8 (1.7) 164.6 (1.2) 163.6 (1.6) 401.2 (1.8) 399.5 (1.8)
163.6 (1.5) 163.4 (1.2) 162.3 (1.8) 164.8 (1.5) 164.6 (1.2) 163.5 (1.8) 401.0 (2.5) 399.0 (1.6) 934.9 (2.0) 933.0 (1.6) 954.7 (2.2) 952.7 (1.9)
167.9 (5.3)
N 1s Cu 2p3/2 Cu 2p1/2
Figure 11. XPS of the sulfur 2p region with surface bulk Si plasmon: sample A, Si wafer; sample B, Si wafer modified with 3-MPTMS (Si/3-MPTMS); sample C, Si/3-MPTMS with adsorbed Ag colloid (Si/3-MPTMS/Ag); sample D, Si/3-MPTMS/ Ag with adsorbed 4-MPy (Si/3-MPTMS/Ag/4-MPy); and sample E, Si/3-MPTMS/AG/4-MPY immersed in aqueous Cu(NO3)2 solution (Si/3-MPTMS/Ag/4-MPy/Cu).
Figure 12. XPS of the sulfur 2p region after bulk plasmon subtraction.
side. These can be resolved into two peaks at 934.9 and 954.7 eV, respectively. The oxidation of copper, however, is not distinguishable solely on the basis of the Cu 2p binding energy. A possible interpretation might include both a Cu-N interaction and a Cu-O interaction to account for the two sets of doublets.
Figure 13. XPS of the nitrogen 1s region: sample D, Si/3MPTMS/Ag with adsorbed 4-MPy (Si/3-MPTMS/Ag/4-MPy); and sample E, Si/3-MPTMS/Ag/4-MPy immersed in aqueous Cu(NO3)2 solution (Si/3-MPTMS/Ag/4-MPy/Cu).
Figure 14. XPS of the silver 3d region: sample A, Si wafer; sample B, Si wafer modified with 3-MPTMS (Si/3-MPTMS); sample C, Si/3-MPTMS with adsorbed Ag colloid (Si/3-MPTMS/ Ag); sample D, Si/3-MPTMS/Ag with adsorbed 4-MPy (Si/3MPTMS/Ag/4-MPy); and sample E, Si/3-MPTMS/Ag/4-MPy immersed in aqueous Cu(NO3)2 solution (Si/3-MPTMS/Ag/4MPy/Cu).
Conclusions We have shown that reactions at interfaces can be studied by enhancing evanescent optical field coupling to surface plasmon modes in small silver particles covalently attached to the surface of a simple optical waveguide.
6398 Langmuir, Vol. 12, No. 26, 1996
Figure 15. XPS of the copper 2p region: sample A, Si wafer; sample B, Si wafer modified with 3-MPTMS (Si/3-MPTMS); sample C, Si/3-MPTMS with adsorbed Ag colloid (Si/3-MPTMS/ Ag); sample D, Si/3-MPTMS/Ag with adsorbed 4-MPy (Si/3MPTMS/Ag/4-MPy); and sample E, Si/3-MPTMS/Ag/4-MPy immersed in aqueous Cu(NO3)2 solution (Si/3-MPTMS/Ag/4MPy/Cu).
This is the basis of integrated optics evanescent wave surface-enhanced Raman spectroscopy (IO-EWSERS) of modes of binding and orientations of mercaptopyridine isomers on the surface of a silver colloid. The technique is sensitive to sub-monolayer coverage of surface adsorbate nitrogen heterocycles and to perturbations of the vibrational structure accompanying quaternization with H+ or
Baldwin et al.
Cu2+. With TE polarized light, no photodegradation of surface adsorbates is observed. The waveguide heterostructure consisting of substrate, mercaptopropyl molecular adhesive interlayer, and silver particles is offered as an optical chemical bench for this purpose. The OCB has been modified to study how 2- and 4-mercaptopyridine (2-MPy, 4-MPy) behave as sensing layers for binding proton or Cu2+ at the waveguide surface. Both 2- and 4-MPy species are tethered through the sulfur terminus to the surface, leaving the heterocycle approximately in an upright position. These conclusions are reached through a comparative study of silver salts of the molecules and their SER spectra on the OCB, metal liquid-like films (MELLFS), and Ag hydrosols. The pyridine nitrogen of 4-MPy is shown to be available to bind cations from solution, whereas nitrogen in the 2-MPy species cannot do so because N is sterically hindered. The sensing layer of 4-MPy can be titrated with acid. Quaternization with proton is nearly reversible on the waveguide, although some proton remains inaccessible to base. Copper cation is bound irreversibly. X-ray photoelectron spectroscopy gives insight into the layerby-layer assembly of the OCB, beginning with silica and extending stepwise through mercaptopropyl, silver colloid, and 4-MPy to Cu2+ as the terminating species. Acknowledgment. The authors wish to acknowledge the financial support of NSERC (Canada) and FCAR (Quebec), and the Swiss National Science Foundation (N.S.). LA960367A
