J. Phys. Chem. 1984, 88, 2641-2650
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-
t i
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the OH. e 0 distances have been estimated by the relation of Nakamoto et The koH and k, values are listed in Table VIII. As can be seen from this table and from Figure 7, the k, values are related to the enthalpies of formation but more interesting is the comparison between other systems; in Figure 7, k, has been plotted against -AH for OH. -N’ and OH.. -S4IH bonds. It clearly appears that the k , values are markedly higher for the OH. 23 bonds. The theory of Allen24predicts that the degree to which the lone pair of the base overlaps A H (in our case OH) is the principal factor governing k,. In order to compare this theory with the experimental data, the k, values-for a given -AH value-have been compared with the extension of the lone pair of the base. For a -AH value of 22 kJ mol-’, the k , values are as follows: OH. .O bonds, k , = 9 N m-I; OH. .N bonds, k , = 17 N m-l; OH. .S bonds, k, = 40 N m-I. The lone-pair extensions of the 0, N, and S atoms are respectively 1.58, 1.77, and 2.13 A. The right quarter of Figure 7 indicates that k, (at constant -AH) is very nicely related to the lone-pair extension.
30 1
15
20
25
Figure 7. k, (N m-l) as a function of -AH (kJ mol-’) for OH...O, OH-. -N, and OH.* 43 hydrogen bonds. Right quarter of the figure: k, (N m-I) as a function of the lone-pair extent I (A) of the base.
involving two or more phenol molecules. The vu valles listed in Table VI11 are very insensitive to the strength of the interaction and do not allow one to distinguish between the OH. -0and OH. N bonds; indeed, for the complexes between pyrimidine and the same proton donors,l the experimental values varied only between 113 and 120 cm-I. The force constant k, of the intermolecular vibration and that of the vOH vibraiton, koH, have been computed by the Lippincott-Schroeder unidimensional f u n ~ t i o n ,using ~ ~ , ~for~ the dissociation energy of the OH bond a value of 447 kJ mol-’; further,
-
Acknowledgment. We are indebted to the University of Leuven for financial support and to Dr. J. P. Dekerk from the laboratory of Professor G. L’abb6 for assistance during the methylation of the bases. O.K. thanks the ABOS for a fellowship. Registry No. I, 6104-45-6;11, 3739-81-9; 111, 2228-27-5; IV, 87414-6; V, 289-95-2; phenol, 108-95-2; 3,4,5-trichlorophenol,609-19-8. (40) Nakamoto, K.; Margoshes, M.; Rundle, R. E. J. A m . Chem. Soc. 1955, 77, 640.
(41) Reyntjens-Van Damme, D.; Zeegers-Huyskens, Th. Adu. Mol. Relaxation Interact. Processes 1980, 16, 15.
Protonation of the Methyl Orange Derivative of Aspartate Adsorbed on Colloidal Silver: A Surface-Enhanced Resonance Raman Scatterlng and Fluorescence Emission Study Olavi Siiman* and Adam Lepp Department of Chemistry, Clarkson University, Potsdam, New York 13676 (Received: June 27, 1983; In Final Form: December 1, 1983)
Absorption, resonance Raman, and fluorescence emission spectra of the protonated form of the azo dye, dabsyl aspartate, DABS-ASP, were measured both in aqueous solution and on colloidal silver particles. Spectra taken as a function of pH were used to estimate pK, values of the conjugate acid, HDABS-ASP, in solution and on colloidal silver. Surface enhancement of resonance Raman scattering and fluorescence emission intensity in HDABS-ASP ranged from 20 to 300 and 1 to 20, respectively, for total silver concentrationsof 0.02-0.30 g/L. Surface-enhancedresonance Raman scattering (SERRS) excitation profiles peaked at 540 nm for HDABS-ASP-silver hydrosols at pH -4.6 and matched the absorption peak position at the same pH. Both the red-shifted absorption maximum and the higher enhancementsunder more acidic conditions were correlated with more extensive aggregation in the silver hydrosols. Lower SERRS ( 102-103)and fluorescence emission (1-10) enhancements for chromophoric adsorbates on colloidal silver than SERS enhancements ( 105-106)for nonchromophoric adsorbates were attributed to shorter excited-state lifetimes for DABS-ASP and HDABS-ASP near a lossy silver surface and possibly longer surface-to-chromophore distances.
Introduction The detection of Raman band enhancements up to 106-foldfor molecules adsorbed on silver and other metal surfaces has precipitated a flurry of activity in the area of surface-enhanced Raman scattering’” (SERS). Other surface-enhanced phenomena such (1) Van Duyne, R. P. In “Chemical and Biological Applications of Lasers”, Moore, C. B., Ed.; Academic Press: New York, 1979; Vol. 4, p 101. (2) Furtak, T. E.; Reyes, J. Surf. Sci. 1980, 93, 251. (3) Otto, A. Appl. Surf.Sci. 1980, 6, 309. (4) “Surface Enhanced Raman Scattering”, Chang, R. K., Furtak, T. E., Eds.; Plenum Press: New York, 1982. (5) Cooney, R. R.; Mahoney, M. R.; McQuillan, A. J. In “Advances in Infrared and Raman Spectroscopy”;Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1982; Vol. 9, p 188. (6) Otto, A. In ”Light Scattering in Solids”;Cardona, M., Giintherodt, G., Eds.; Springer: West Berlin, in press; Vol. IV.
0022-3654/84/2088-2641$01.50/0
as surface-enhanced resonance Raman ~ c a t t e r i n g ~(SERRS) -’~ and surface-enhanced luminescence emission*6-20from chromo(7) Jeanmaire, D. L.; Van Duyne, R. P. J . Electroanal. Chem. 1977,84, 1.
(8) Hagen, G.; Glavaski, B. S.;Yeager, E. J . Electroanal. Chem. 1978, 88, 269. (9) Cotton, T. M.; Schultz, S.G.; Van Duyne, R. P. J . A m . Chem. Soc. 1980,102, 7962. (10) Pemberton, J. E.; Buck, R. P. J . Phys. Chem. 1981, 85, 248. (11) Pemberton, J. E.; Buck, R. P. Anal. Chem. 1981, 53, 2263. (12) Pemberton, J. E.; Buck, R. P. J. Electroanal. Chem. 1982, 132, 291. (13) Weitz, D. A.; Garoff, S.; Gramila, T. J., Opt. Lett. 1982, 7, 168. (14) Siiman, 0.; Bumm, L. A.; Callaghan, R.; Kerker, M. In “Proceedings of International Conference on Time-Resolved Vibrational Spectroscopy”, Aug 16-20, 1982, Lake Placid, New York. (15) Siiman, 0.; Lepp, A,; Kerker, M. J. Phys. Chem. 1983, 87, 5319.
0 1984 American Chemical Society
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phoric adsorbates have received relatively little attention. Any observation of the enhancement of luminescence emission near a metal surface suffers from the possible loss of emission intensity via nonradiative channels to the metal. Some controversy presently exists over energy transfer mechanisms2’ between molecules and metal surfaces such as in the pyrazine-silver as well as over fluorescence enhancements from high quantum yield emitters such as typical laser dyes,lbZ0rhodamine 6G,rhodamine B, and nile blue, adsorbed on thin silver films. Since the latter dyes were in direct contact with “lossy” surfaces and the quantum efficiencies of the emitters were very high, the net result was a decrease in relative emission intensity by quenching. The latter effect enabled the observation of SERS spectra of rhodamine 6G,which would ordinarily have been eclipsed by the much higher fluorescence emission intensity. The lack of observed surface enhancements of 103-104, as might be expected for luminescence emission from the electrodynamic theory of small particles,26was disappointing. Nevertheless, as emitters with low quantum efficiencies and/or with longer distances from the surface were not studied, further work on surface-enhanced luminescence is required. A derivative of methyl orange (MO) which absorbs at 472 (basic form) or 503 nm (acidic form) and fluoresces weakly at 563 nm provides the possibility of observing simultaneously its SERRS and surface-enhanced fluorescence emission spectra. At the same time its protonation reaction on a colloidal silver particle surface can be monitored. MO, 4-[[4-(dimethylamino)phenyl]azo]benzenesulfonic acid, can exist in a basic form
or in several acidic forms, protonated at the a- or @-azonitrogen atom and/or the dimethylamino nitrogen atom. Each of the latter three structures has been proposed for monoprotonated HMO. In general, the nitrogen atom of a tertiary amine behaves as a stronger base than either azo nitrogen atom; however, a contribution from the resonance form
indicates that the @azo nitrogen atom can act as a stronger base. MO and HMO absorb maximally at 465 and 508 nm, respectively, and resonance Raman (RR) spectra of both have been (16) Ritchie, G.; Chen, C. Y.;Burstein, E. Bull. Am. Phys. SOC.1980, 25, 260. (17) Chen, C. Y.; Davoli, I.; Ritchie, G.; Burstein, E. Surf. Sci. 1980, 101, 363. (18) Glass, A. M.; Liao, P. F.; Bergman, J. G.; Olson, D. H. Opt. Lett. 1980, 5, 368. (19) Ritchie, G . ; Burstein, E. Phys. Rev. B 1981, 24, 4843. (20) Weitz, D. A,; Garoff, S.; Hanson, C. D.; Gramila, T.J.; Gersten, J. I. J. Lumin. 1981, 24/25, 83. (21) Chance, R. R.; Prock, A.; Silbey, R. Adv. Chem. Phys. 1978, 37, 1. (22) Whitmore, P. M.; Robota, H. J.; Harris, C. B. J. Chem. Phys. 1982, 76, 740. (23) Rossetti, R.; Brus, E. J. Chem. Phys. 1980, 73, 572. (24) Rossetti, R.; Brus, E. J. Chem. Phys. 1982, 76, 1146. (25) Whitmore, P. M.; Robota, H. J.; Harris, C. B. J. Chem. Phys. 1982, 77, 1560. (26) Wang, D.-S.; Kerker, M. Phys. Rev. B 1982, 25, 2433. (27) Carey, P. R.; Schneider, H.; Bernstein, H. J. Biochem. Eiophys. Res. Commun. 1972, 47, 588. (28) Kumar, K.; Carey, P. R. Can. J. Chem. 1977, 55, 1444. (29) Lorriaux, J. L.; Merlin, J. C.; Dupaix, A.; Thomas, E. W. J. Raman Spectrosc. 1979, 8, 81. (30) Machida, K.; Kim, B.-K.;Saito, Y . ;Igarashi, K.; Uno, T.Bull. Chem. SOC.Jpn. 1974, 47, 18. (31) Machida, K.; Kim, B.-K.;Saito, Y.; Igarashi, K.; Uno, T. Bull. Chem. SOC.Jpn. 1975, 48, 1394. (32) Machida, K.; Lee, H.; Kuwae, A. J. Raman Spectrosc. 1980, 9, 198.
Siiman and Lepp
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Figure 1. Absorption spectra of DABS-ASP and HDABS-ASP in aqueous solution at various pH values: 1, 6.49; 2, 3.81; 3, 3.33; 4, 3.1 1; 5, 2.85; 6,2.29; 7, 1.46. Resonance Raman excitation profile (---) for HDABS-ASP at pH 2.5.
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reported previously. R R spectra3w34indicated that the species azoof H M O that absorbs at 508 nm assumes the quinoid benzene structures protonated at the @-azonitrogen atom, though coexistence with the amino-protonated tautomer has been suggested3’ from detection of an absorption peak at 318 nm and R R spectra obtained by excitation at 325.0nm. R R ~ p e c t r aof~ ~ . ~ ~ the diprotonated form of M O and some of its deuterated derivatives in concentrated sulfuric acid show that this dicationic azo form is protonated at the dimethylamino and a-azo nitrogen atoms. The pK, of H M O has been reported36 as 3.4 at 25 “C in aqueous solution and its zwitterionic structure, which allows head-to-tail dimer configurations, has been s h o ~ n ~to~produce - ~ * aggregation M and of pH below in solutions of concentration above 1 X 2.75. Chosen for its superior adsorption properties, the N-substituted methyl orange derivative of aspartic acid, DABS-ASP, has already been used by us in SEW-SERRS studies of its basic form in silver hydros01s’~J~ and in silver o r g a n o ~ o l s . ~ The ~ intense electronic absorption band of DABS-ASP in both basic and acidic (HDABS-ASP) forms overlaps with the silver particle dipolar surface plasmon, which gives rise to enhancement of R R scattering from both protonated and unprotonated forms of DABS-ASP. Luminescence emission from neither M O nor HMO has been reported previously, although some of the published R R spectraz7-34do show steep backgrounds which can be indicative of interference from luminescence emission. Thus, DABS-ASP, which does luminesce weakly, can serve as a probe on colloidal silver for both surface-enhanced resonance Raman and luminescence emission. Since it is possible to observe, simultaneously, Raman and luminescence emission spectra of DABS-ASP in a range of pH values over which protonation of an azo nitrogen atom occurs, this allows the application of SERRS and fluorescence spectroscopy to the surface titration of this azo dye. Excitation profiles for both SERRS and RRS of the azo dye can also be compared to discern any perturbation on dye excited states by the silver surface. The pH titration of the silver hydrosols is limited by the point of zero charge of the particles, at which coagulation of the sol is expected, and by the protonation of the carboxylate functional ~~~~~~
~
(33) Merlin, J. C.; Lorriaux, J. L.; Dupaix, A,; Thomas, E. W. J. Raman Spectrosc. 1981, 1 1 , 131. (34) Lee, H.; Machida, K.; Kuwae, A,; Saito, Y . J . Mol. Struct. 1980,68, 51. (35) Tsuda, M.; Oikawa, S.; Watanabe, T. In ‘Abstracts of Papers”, 96th Annual Meeting of Pharmaceutical Society of Japan, Nagoya, April, 1976, p 108. (36) Meites, L. “Handbook of Analytical Chemistry”;McGraw-Hill: New York, 1963; Section 3. (37) DeVijlder, M.; DeKeukeleire, D. Bull. SOC.Chim. Belg. 1978,87, 9. (38) DeVijlder, M. J. Chem. SOC.,Faraday Trans. I 1981, 77, 129. (39) Siiman, 0.;Lepp, A,; Kerker, M. Chem. Phys. Lett. 1983, 100, 163.
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 2643
MO Derivatives of Aspartate Adsorbed on Ag group in dabsyl aspartate which provides for adsorption of the molecule to the silver surface. The pK, of this group for aspartic acid in aqueous solution is 3.85. Even if the silver hydrosol were to coagulate near pH 3.85, this would still allow partial protonation of the azo group in DABS-ASP adsorbed on colloidal silver.
Experimental Section Silver hydrosols with adsorbed citrate and DABS-ASP were prepared by procedures already described in earlier report^^^,^^ in which details of the Raman measurements were also given. Absorption spectra were measured with a Perkin-Elmer Model 551A spectrophotometer. Titrations of DABS-ASP solutions and silver sols with adsorbed DABS-ASP were carried out with micropipette additions of 0.010,0.10, or 1.0 N sulfuric acid. The resulting mixtures were recirculated through either a 3.0 X 3.0 mm2 cross-section Raman flow cell or a 1.0-cm path length absorption flow cell by an LKB peristaltic pump. To obtain absorption corrections in the Raman measurements, laser intensity in the presence and absence of sample was measured with a Scientech 1-in. disk calorimeter laser power meter. A combination saturated calomel-glass electrode and Chemtrix pH meter were used to monitor pH at ambient temperature (24 "C). Results Absorption Spectra of DABS-ASP in Solution. The absorption spectra of DABS-ASP solutions (9.00 mL of water 400 WLof 6.2 X M DABS-ASP in 0.01 M Na3cit) adjusted from pH 7 to 1 with additions of sulfuric acid are shown between 600 and 300 nm in Figure 1. The 472 nm ( E = 35 000) band maximum of DABS-ASP in its basic form increased in intensity and was red shifted to 503 nm (e = 70 000) in its acidic form, HDABSASP. Very little shift in the 503-nm peak occurred below pH 2.3, though the absorbance at 503 nm continued to rise with addition of more sulfuric acid. The volume-corrected absorbances a t 503 nm were used to obtain an estimate of 3.1 for the pK, of HDABS-ASP. The increasing absorbance at 503 nm beyond pH 2.3 tended to make determination of the inflection point in the pH vs. absorbance curve difficult and prevented an accurate evaluation of the pK, from the spectrophotometric titration. This problem was not apparent in the pH vs. resonance Raman band and fluorescence emission band intensity curves (vide infra). In addition, the absorption spectra of HDABS-ASP showed no evidence of a band near 318 nm which could be associated with a dimethylamino-protonated tautomer. Fluorescence Emission Spectra of DABS-ASP in Solution. When the R R spectra of DABS-ASP were recorded at various pH values it became evident that HDABS-ASP, in particular, also showed a broad emission background. So that both this broad luminescence emission band which peaked at 563 nm and the relatively sharp Raman bands could be seen, a wide Raman shift range from 4500 to 100 cm-' is covered in Figure 2. Excitation (501.7-nm Ar+) as close as possible to the absorption band maximum (503 nm) of HDABS-ASP was chosen to obtain spectra that were most sensitive to the addition of sulfuric acid to the solutions (400 pL of 6.2 X lo4 M DABS-ASP in 0.01 M Na,cit -t 10.00 mL of water). Eight spectra of DABS-ASP solutions ranging in pH from 6.47 to 0.68 are shown in Figure 2. When the excitation wavelength was changed, the shift for the broad band peak changed but its absolute position remained at 563 nm. Thus, it was identified as a luminescence emission band and not as a Raman band. The integrated emission band intensity increased steadily as the pH of DABS-ASP solutions was lowered. An intense electronic absorption band at 314 nm ( e 22000) in trans-azobenzene has been identified41,42 as a a a* transition with its moment parallel to the long axis of the molecule. A weaker broad absorption band near 450 nm ( e 500) has been
+
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(40) Siiman, 0.; Bumm, L. A.; Callaghan, R.; Blatchford, C. G.; Kerker, M. J. Phys. Chem. 1983.87, 1014. (41) Jaffz, H. H.; Yeh, S. J.; Gardner, R. W. J . Mol. Spectrosc. 1958, 2, 120. (42) Beveridge, D. L.; Jaff6, H. H. J . Am. Chem. Soc. 1966, 88, 1948.
TABLE I: Vibrational Band Frequencies (em-') and Mode Assignments for HDABS-ASP RRS" RRS SERSb SERS' assignmt
+
1623 vs 1500 mw 1407 m 1387 w, sh 1370 w 1272 m 1183 m A,,
1623 vs 1500 mw 1409 m, sh 1393 s 1370 ms 1269 ms 1179 s
1623 s 1500 mw 1409 w, sh 1393 s 1370 s 1269 ms 1179 s
v(C-C), N' ring
4Ph) v(Ph) v(C-N') v(C-C), N' ring 4C-Np) v(N,-N,)
"pH 3.16, A,, = 488.0 nm. bpH 4.41, &, = 488.0 nm. cpH 4.44, = 647.1 nm.
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assigned43to an n a* transition. Only recently has fluorescence emission from the singlet '(r,a*)and '(n,a*) excited states of trans-azobenzene been o b s e r ~ e d . Picosecond ~ ~ ~ ~ ~ excitation at 354 and 531 nm unveiled emission spectra with an S2 So band So band between 580 and 800 nm. at 420 nm and an SI Phosphorescence46 from azo compounds has not yet been detected. Intense fluorescence emission from the protonated species of trans-azobenzene and trans-(dimethy1amino)azobenzene in aqueous H2S04has been reported.47 These emission properties were also compared4* with those of o-diaza aromatic molecules of C,, symmetry. The intense absorption band of azobenzenes at 314 nm in neutral solution is replaced by an intense band at 426 nm in aqueous H2S04. The protonated species emits at 495 nm in 15% H2S04and at 508 nm in 66% HzSO4 solution. These investigators have proposed that the order of singlet '(n,?r*) and '(n,?r*) states is reversed in the protonated species; Le., the ?r a* transition occurs at lower energy, so that excitation into an intense '(r,a*)absorption band at 426 nm yielded intense fluorescence emission from an SI So band at 495-508 nm. The higher energy of the '(n,a*) excited state (one electron transition from a nonbonding level associated with an azo nitrogen atom) in the protonated species
-
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Y
might originate from the lower basicity of the azo nitrogen atoms. Thus, the weak emission at 563 nm from HDABS-ASP is tentatively assigned to fluorescence from a singlet I(a,a*)excited state similar to the one in protonated trans-azobenzene. That the wavelength onset of emission from HDABS-ASP (-500 cm-' red shifted from 501.7 nm) overlaps well with the absorption band 70000) supports the singlet assignment centered at 503 nm (e of this excited state. The smaller normalized Stokes shift [AD = (-Y, - -Y , ) / P ~ ] , 0.11, for HDABS-ASP compared to 0.14 for trans-azoben~ene~~ in 15% aqueous H2SO4 suggests a slightly more rigid structure for HDABS-ASP. Asymmetry in the absorption bands of DABS-ASP and HDABS-ASP and vibrational structure in the R R excitation profile of DABS-ASP also indicate a rigid structure in HDABS-ASP. Resonance Raman Spectra of DABS-ASP in Solution. R R spectra of DABS-ASP solutions, prepared by adding aliquots of 0.01 N HzSO4 to 10.00 mL of water 200 pL of 6.2 X M DABS-ASP in 0.01 M Na3cit, are shown in Figure 3, right-hand side. As the pH of these solutions was adjusted from 5.62 to 3.16, the RR bands of HDABS-ASP successively dominated the spectra to a greater degree. Both DABS-ASP and HDABS-ASP were simultaneously excited by the 488.0-nm Ar+ laser line. Since only two sets of R R bands were observed to vary in relative intensity with pH, they are assignable to only two species in the equilibrium, DABS-ASP H+ G HDABS-ASP, Le., the basic form and a single monoprotonated form.
-
+
+
(43) (44) (45) (46) (47) (48)
Hochstrasser, R. M.; Lower, S. K. J. Chem. Phys. 1962, 36, 3505. Struve, W. S. Chem. Phys. Lett. 1977, 46, 15. Morgante, C. G.; Struve, W. S. Chem. Phys. Lett. 1979, 68,267. Engel, P. S.; Steel, C. Acc. Chem. Res. 1973, 6, 275. Rau, H. Ber. Bunsenges. Phys. Chem. 1967, 71, 48. Rau, H. Ber. Bunsenges. Phys. Chem. 1968, 72, 408.
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The Journal of Physical Chemistry, Vol. 88, No. 12, 1984
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RAMAN SHIFT, cm-' Figure 2. Resonance Raman and fluorescence emission spectra of DABS-ASP and HDABS-ASP at various pH values: A, 6.47; B, 4.77; C, 4.25; D, 3.82; E, 3.33; F, 2.85; G, 1.46; H, 0.68. Instrumental conditions: excitation wavelength, 501.7-nm Ar'; incident laser power, 25 mW; spectral slit width at 1000 cm-I, 7.5 cm-I; photon counting time interval, 0.05 s, for spectrum H, 0.05 s from 100 to 1500 cm-', 0.04 s from 1500 to 4500 cm-'.
On the basis of previous R R studies with methyl orange and related dyes, some of the relevant R R band assignments for HDABS-ASP are summarized in Table I. Assignments for DABS-ASP were made in our previous paper.I5 Since only the chromophoric region of the molecules contributes to the R R spectra and this region (the (dimethy1amino)azobenzene portion) is the same in HDABS-ASP and HMO, it follows that RR spectra of these two species are very similar. The sharp, intense 1623-cm-l RR band of HDABS-ASP is separated from other DABS-ASP and HDABS-ASP bands and was used to obtain excitation profiles and to monitor the population of HDABS-ASP in mixtures containing both acidic and basic forms. The RR excitation profile for 2.3 X low5M HDABS-ASP at pH 2.5, referenced to an internal standard, 0.93 M Na2S04,is shown as the dashed line at the bottom of Figure 1 . Integrated Raman band intensities of HDABS-ASP at 1623 cm-I and sulfate ion at 980 cm-' were
compared at each excitation wavelength. The peak in the R R excitation profile has a 1:l correspondence with the one at 503 nm in the absorption spectrum of HDABS-ASP. The integrated emission band and 1623-cm-I R R band intensities were obtained from the spectra in Figure 2, corrected for volume of acid that was added, and placed on a relative scale with maximum values of R R intensity and fluorescence emission intensity at pH 0.68. Since DABS-ASP also showed weak emission near 563 nm, the relative fluorescence intensity necessarily does not begin with 0 at pH 6.47. These relative intensities plotted against pH are shown in Figure 4. Both curves show half-titration points (50% R R intensity; 6 3 % fluorescence emission intensity) near pH 3.6 which to a first approximation gives the pK, of the conjugate acid of DABS-ASP. With this pK, value a linear dependence of relative R R (fluorescence) band intensity on the mole fraction of HDABS-ASP was found. These results are again
MO Derivatives of Aspartate Adsorbed on Ag
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RAMAN SHIFT, cm-' Figure 3. Resonance Raman spectra of DABS-ASP (right-hand side) and surface-enhancedresonance Raman spectra of DABS-ASP on colloidal silver (left-hand side) at various pH values and photon-counting time intervals (s). Right-hand side: A, 5.62, 0.300; B, 4.52, 0.300; C, 4.00, 0.300; D, 3.50,0.200;E, 3.16,O.lSO. Left-hand side: A, 6.83,0.0700; B, 6.00,0.0500;C, 5.49,0.0500; D, 4.91,0.0500;E, 4.41,0.0300. Instrumental conditions: excitation wavelength, 488.0-nm Ar'; incident laser power, 25 mW; spectral slit width at 1000 cm-I, 7.0 cm-'. I
,
consistent with a two-state system, DABS-ASP + H+ s HDABS-ASP, in which there is only one monoprotonated species. Absorption Spectra of DABS-ASP and HDABS-ASP on Colloidal Silver. The absorption spectra of DABS-ASP (9.00 mL of water 0.200 mL of 6.2 X M DABS-ASP in 0.01 M Na3cit) with varying amounts of colloidal silver from a stock silver sol stabilized with citrate (2.0 g/L = [Ag]) at pH near 6.0, at which DABS-ASP is in its basic form, all showed a single intense band maximum due to colloidal silver near 400 nm. It has been shown earlier that DABS-ASP displaced citrate at least partially on the surface." Each of these sols are titrated with sulfuric acid to pH -4.6, and the absorption spectra taken immediately after the titrations are shown in Figure 5 . If they were taken to pH lower than -4.4, they coagulated within 10 min. The hydrosols, initially with DABS-ASP totally in its basic azo form, showed a silver particle absorption peak a t 400 nm and a very weak shoulder near 500 nm as reported previously.1s As they were titrated with sulfuric acid from pH -7.0 to pH -3.9, the sols
+
became deeper red and then purple in color and developed a definite second absorption maximum in the 500-550-nm region. The appearance of additional lower-energy bands in sol absorption spectra has been reported in other silver and gold hydrosol syst e m ~ . ~ ~These , ' ~ lower-energy bands can be attributed to the formation of coagula of the smaller primary silver particles. Surface-Enhanced Resonance Raman Spectra of HDABS-ASP on Colloidal Silver. SERRS spectra of DABS-ASP on colloidal silver ~ h o w e d ' ~an , ' ~excitation profile peak at 490 nm and an absolute enhancement of Raman band intensity of the order lo3. In Figure 3, left-hand side, are shown the SERRS spectra (488.0-nm Ar+ excitation) of DABS-ASP and HDABS-ASP on colloidal silver (10.0 cm3 of IO-fold diluted sol 200 pL of
+
(49) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J . Chem. SOC., Faraday Trans. 2 1919, 75, 190. (50) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf.Sci. 1982, 120, 435.
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7
PH Figure 4. Relative resonance Raman and relative fluorescence emission (---) band intensities of HDABS-ASP vs. pH. (-.-a)
DABS-ASP in 0.01 M Na3cit) at pH 6.83,6.00, 5.49, 4.91, and 4.41. On the right-hand side of the same figure are R R spectra of DABS-ASP solutions without colloidal silver at pH 5.62,4.52, 4.00, 3.50, and 3.16. The volume of added 0.01 M H2S04was 0.0, 0.400, 0.750, 1.150, and 1.500 on the left-hand side and 0.0, 0.100, 0.150, 0.300, and 0.650 on the right-hand side. Spectral changes that occur with the protonation of DABS-ASP are similar for DABS-ASP in solution and for DABS-ASP adsorbed to the silver particle surface. The sol spectra, however, are out of step in pH with the solution spectra. The pH 5.5,4.9, and 4.4 sol spectra, Figure 3C-E, left-hand side, are very similar to the solution spectra at pH 4.5,4.0, and 3.5, respectively, Figure 3B-D, right-hand side, between 1300 and 1000 cm-'. In the region between 1300 and 1450 cm-' comparison of these same pairs of spectra, one unit apart, shows that the relative Raman band intensity of v(N=N) at 1419 cm-' has decreased with lower pH for DABS-ASP, HDABS-ASP on the silver surface, whereas a band near 1360 cm-I assigned to v(Ph-NMe2) decreased in relative band intensity for DABS-ASP, HDGBS-ASP in more acidic solution. Near 1600 cm-' spectra of the sol at pH 4.9 and 4.4, Figure 3, D and E, left-hand side, most resemble spectra in solution at pH 4.5 and 4.0, respectively, Figure 3, B and C, right-hand side. SERRS and SERS spectra of HDABS-ASP and DABSASP coadsorbed on colloidal silver at pH 4.4-4.5 are compared in Figure 6 at several excitation wavelengths, 647.1, 590.3, 514.5, and 488.0 nm, spanning the region of SERRS spectra (overlap of absorption bands at Am, = 503 nm for HDABS-ASP and ,A, = 472 nm for DABS-ASP with A, = 540 nm for silver sol) to a SERS-only spectrum at A,, = 647.1 nm. In Table I are tabulated some Raman band positions and relative intensities of HDABS-ASP and DABS-ASP on colloidal silver at pH 4.4-4.5 for excitation at 488.0 nm in the region of SERRS spectra and for excitation at 647.1 nm in the region of SERS-only spectra. As in the Raman spectral5 of DABS-ASP on colloidal silver at pH -6.0 relative band intensity changes do occur giving characteristic RRS-like spectra at bXc = 488.0 nm and normal Raman type spectra at A,, = 647.1 nm. SERRS enhancement factors, Zsol/Zref, at A,, = 488.0 nm (Figure 3) for comparable pH of DABS-ASP Raman bands in the silver sol vs. those in solution (ref) are 750 for the 1623-cm-' band at pH -4.5 and 60 for bands near 1400 cm-' at pH -5.5. Enhancement factors are the same at a particular pH irrespective of whether DABS-ASP or HDABS-ASP is compared in solution and on colloidal silver. From Figure 6, in which Raman spectra at pH -4.5 for various excitation wavelengths (A, nm) are presented, enhancement factors (ZRo,/Zref, in parentheses) are as follows: 488.0 (226), 514.5 (576), 509.3 (2370), 647.1 (1020), for the 1623-cm-' band. The Zsa,/Zref values at A,, = 590.3 and 647.1 nm are necessarily crude estimates since HDABS-ASP does not absorb strongly or give RR spectra in this wavelength region.
I 300
I 400
b
coo
I XK)
U
WVELENGTH, nm
Figure 5. Absorption spectra of DABS-ASP and HDABS-ASP (pH of colloidal silver (mL of silver sol): 2,O.lOO; 3, 0.250; 4, 0.500; 5, 1.00; 6, 1.50; 7, 2.00. SERRS excitation profile
4.6) with varying amounts
(---)
for sol in spectrum 4.
It should be stressed that these enhancement values represent a lower limit since they are calculated with the assumption that all the added DABS-ASP is adsorbed on the colloidal silver. Typically, about 30% of the added DABS-ASP was adsorbed to the colloidal silver at these concentrations of DABS-ASP (1.2 X M) and silver (0.2 g/L) so that the actual enhancements are three to four times greater than the values tabulated above. Another SERRS excitation profile for HDABS-ASP on silver (0.5 mL of sol + 9.0 cm3 of water + 200 p L of DABS-ASP in 0.01 M Na,cit titrated with 0.01 N sulfuric acid to pH 4.6) is shown as the dashed line at the bottom of Figure 5 for excitation wavelengths from 465.8 to 623.6 nm. In this case the 1623-cm-' HDABS-ASP SERRS band was compared at each wavelength with the 1415-cm-' Raman band of a 1.36 M aqueous sodium citrate solution. The peak in the SERRS profile between 514.5 and 577.0 nm corresponds to the absorption peak for a parallel run, Le., spectrum 4 in Figure 5, which shows a second silver
The Journal of Physical Chemistry, Vol. 88. No. 12, 1984 2647
MO Derivatives of Aspartate Adsorbed on Ag I
I
I
I
I
I 1
L
300
z
U I200
z W
0
0.10
0.20
0.30
0.40
Figure 7. Surface enhancement of resonance Raman intensity 1400-cm-' bands at pH -6.0; ---, 1625-cm-' band at pH -4.5) vs. total silver concentration. (---e,
w
t
/"
i
Figure 8. Surface enhancement of luminescence emission intensity pH -6.0; ---, pH -4.5) vs. total silver concentration.
I 1600
I
I
1400
I
I 1200
0
RAMAN SHIFT, cm" Figure 6. Surface-enhanced resonance Raman spectra of HDABS-ASP M) on colloidal silver (10 mL of 10-fold diluted (200 p L of 6.2 X stock Ag sol) at pH -4.5. Instrumental conditions: excitation wavelength, nm; incident laser power, mW; spectral slit width at 1000 cm-', cm-'; photon counting time interval, s: A, 647.1, 81, 7, 0.150; B, 590.3, 22, 7, 0.100; C, 514.5, 26, 7, 0.0100; D, 488.0, 25, 7, 0.0300.
particle absorption band maximum at 540 nm. Surface-Enhanced Fluorescence Emission of DABS-ASP and HDABS-ASP on Colloidal Silver. Combined SERRS and fluorescence emission spectra of DABS-ASP and HDABS-ASP on colloidal silver were obtained over the same 100-4500 cm-I Raman shift range (A,, = 501.7 nm) as shown in Figure 2 for the solution species. To obtain these spectra, we added varying amounts of silver sol (0.10, 0.50, 1.0, and 2.0 mL of a 2 g/L Ag sol) to DABS-ASP in solution (9.0 mL of water 0.200 mL of 6.2 X M DABS-ASP in 0.01 M Na,cit). Spectra of sols at pH 6.0 before addition of sulfuric acid and spectra of sols titrated to pH -4.5 were recorded. Fluorescence emission band and SERRS enhancements of the 1400- and 1623-cm-l bands of DABS-ASP and HDABS-ASP at pH 6.0 and pH 4.5, respectively, were obtained from the spectra by comparison with a
+
(-e-.,
reference containing no silver. SERRS enhancements are plotted against total Ag concentration in Figure 7 while corresponding fluorescence enhancements are compared in Figure 8. At pH 6.0 the SERRS enhancement increases from 16 at 0.02 g/L Ag to 66 at 0.20-0.36 g/L Ag while fluorescence enhancements vary from 1.2 to 4.0 in the same Ag concentration range. The fluorescence intensity lags the R R intensity of DABS-ASP on colloidal silver by about an order of magnitude. At pH 4.5,both SERRS and fluorescence emission enhancements of HDABSASP on colloidal silver vary over a much wider range: SERRS from 12 to 320, fluorescence emission from 0.90 to 21. The fluorescence intensity still lags the SERRS intensity by an order of magnitude. The upper limit in enhancements is nearly five times larger for sols in more acidic media, pH 4.5 vs. pH 6.0.
Discussion One of the tests for validity of the electrodynamic theorys1+s2 (molecular dipole-surface plasmon coupling) for SERS from molecules on small silver particles is the predicted coincidence in absorption and SERS peak wavelengths for particles much smaller than the wavelength. The peak wavelength itself will vary with particle shape and structure, e.g., concentric spheres.53 For particle sizes comparable to and larger than the wavelength, the absorption spectrum and SERS excitation profile may be de(51) Kerker, M.; Wang, D.-S.; Chew, H. Appl. Opt. 1980,19,3373,4159. (52) Wang, D.-S.; Kerker, M. Phys. Rev. B 1981, 24, 1777. (53) Kerker, M.;Blatchford, C. G. Phys. Rev. B 1982, 26, 4052.
2648 The Journal of Physical Chemistry, Vol. 88, No. 12, 1984
coupled. The sols under study here are comprised of particles much smaller than the wavelengths. SERRS excitation profile^'^ for silver particles in which DABS-ASP has displaced some or all of the citrate reached a maximum which corresponded to a weak absorption band shoulder of the sols near 500 nm in agreement with the model. SERS profiles of the parent yellow sol with adsorbed citrate40 peaked at about the same position at 490 nm, but a distinct absorption band shoulder at this wavelength was absent. The coincidence in excitation and absorption of DABS-ASP-containing sols was maintained as the pH of the sols was adjusted from 6.0 to 4.5, and the secondary, lower-energy absorption band maximum as well as the peak in the SERS profile shifted‘from 500 to 540 nm. Also,higher SERRS enhancements were observed for DABS-ASP on silver at pH 4.5 compared to pH 6.0. These observations are in qualitative agreement with the predictions of electrodynamic theory for small silver particles. A red shift in both absorption ’ * particles ~~ and SERS enhancement maxima was p r e d i ~ t e d ~for of larger size and/or nonspherical shape, as would occur in clusters or strings of silver particles. As well, nonspherical particles such as prolate spheroids are predicted to give higher SERS enhancements. That extensive aggregation ’existed in our acidified sols could also be surmised from their eventual coagulation at pH 54.5.
We now also believe that particle morphology in the yellow sol with adsorbed citrate4 played a dominant role in determining the large SERS enhancement and its peak Wavelength. A number of small clusters of particles was always detected in electron micrographs of this sol. While their contribution to the sol absorption spectrum was small, their effect on the SERS enhancement can be very large. Thus, the large SERS enhancement ((2-3) X los) is presumed to originate from small nonspherical clusters of primary silver particles with adsorbed citrate. These clusters do not necessarily require direct metal contact between individual particles. As long as the distance between particles in the clusters is about equal to or less than the size of the primary particles, then ihterparticle interactions that broaden and red shift both absorption and SERS excitation bands become operative. To support this contention are experimental results (vide infra) that were recently obtained54for molecules adsorbed on wellcharacterized films consisting of silver islands of nearly circular cross section. The diameters of these islands, -200 A, were roughly the same as those of our colloidal silver particles. A similar relationship between SERS excitation profile and absorption band maxima was found for molecules adsorbed on silver-island films.54 The islands, separated by distances comparable to their size, showed an absorption band at 440 nm, to slightly longer wavelength from that anticipated for an isolated individual island 6f the same shape and size. However, SERS and SERRS excitation profiles for p-nitrobenzoate and rhodamine 6G on these films peaked at 490 and 510 nm, respectively. Although this noncoincidence in absorption (440 nm) and SERS or SERRS peaks was not discussed by the authors, it does agree with our experiments with citrate and DABS-ASP on colloidal silver. Parallel behavior was noted in gold hydrosols with adsorbed citrate. No measurable SERS spectra were detected from citrate on colloidal gold that showed only the single-particle absorption band at 530 nm. When aggregation was induced in these gold sols by addition of pyridineSo or by aging,s5 SERS spectra of pyridine or citrate were observed. The SERS excitation profile maxima for pyridine on colloidal gold matched the wavelengths of maximum absorption for various amounts of added pyridine. These maxima were red shifted from the 530-nm single-particle peak for colloidal gold. Surface enhancement of fluorescence emission intensity of HDABS-ASP on colloidal silver lagged behind enhancement of
Siiman and Lepp resonance Raman scattering intensity for the same adsorbed molecule at the same excitation frequency by about an order of magnitude. Theoretical calculations for dye-coated silver spheres in the small-particle limit showedz6 that peak enhancement of fluorescence intensity would be in the 103-104 range for silver in air. In water the enhancement is expected to be an order of magnitude larger. Calculated SERS enhancements for nonabsorbing molecules on similar silver spheres5’ reached a maximum of 105-106. The magnitudes of these enhancements do not agree with the experimental values, lo2 for fluorescence and lo3 for resonance Raman. It is well-known that emitting molecules in the proximity of a heavy metal atom surface have an effective decay channel through nonradiative energy transfer from the emitting molecule to the metal surface. Smooth surfaces of gold, silver, and copper were s h o ~ n ~to~ decrease ?~’ the lifetime of an excited molecule near the metal surface. Layers of polymer5*or solid argon2z-25 were used as spacers to obtain the distance dependence. For example, for the phosphorescent emitter, pyra~ine,2*-*~ T decreased with shorter distances to the silver surface and followed a 83 dependence at distances less than 100 A. The connection between the frequency dependence of this energy transfer and surface plasmon modes has been previously considered theoretically but has not been experimentally verified. For short emitter-to-metal distances within -500 A of the surface, these effects can be large and energy transfer would compete very well with emission. These effects were omitted in the electrodynamic calculationsz6for small, dye-coated silver spheres and are expected to contribute largely to the 102-103-fold difference between observed and calculated fluorescence emission enhancements. Similar results for emitters near “rough” metal surfaces have been lacking until very recently. Fluorescence emission (lifetimes and quantum yields) from a Eu3+ complex with thenoyl trifluoroacetonate on silver-island films was measured.59 The fluorescence intensity of Eu3+ on silver-island films increased ---fold and the excited-state lifetime decreased about 103-fold from their respective values of -0.4 and 280 /.LS on a bare silica substrate. In more recent experimentss4 with silver-island films as the substrate, SERS spectra of p-nitrobenzoate and SERRS and surface-enhanced fluorescence emission spectra of rhodamine 6G and basic fuchsin were measured. The authors also offered an interpretation for their “hierarchy” of enhancement values: los for SERS, lo3 for SERRS, and lo-’ to 10 for surface-enhanced fluorescence emission. The 102-folddecrease in RR enhancement near the silver surface was explained in terms of an additional damping caused by coupling of the electronic excited state to the electronic plasma resonance as Au (= uL- u1 up)becomes small or the resonance condition is approached. Both near field and scattered field intensity enhancements were considered to be modified by an increased width in the molecular absorbing or emitting state due to the additional nonradiative decay channels to the metal. A broadening of the absorption or emission band width serves to decrease its intensity and, in turn, the magnitude of enhancement. This is evident when the uncertainty principle is cast in the form
+
Au =
AE - e-
1
hc 2acAt in terms of an uncertainty in frequency. Since the lifetime, 7 , of the excited state is a good approximation for the uncertainty in t, then the bandwidth is expected to increase with shorter excited state lifetimes which are synonymous with shorter distances to the silver surface. Larger bandwidths, in turn, cause a drop in absorption band peak intensity as well as in luminescence emission (56) Drexhage, K. H.; Fleck, M.; Schafer, F. P.; Sperling, W. Ber. Bunsenges. Phys. Chem. 1966, 20, 1179.
(54) Weitz, D. A,; Garoff, S.; Gersten, J. I.; Nitzan, A. J . Chem. Phys. 1983, 78, 5324. (55) Mabuchi, M.; Takenaka, T.; Fujiyoshi, Y.;Uyeda, N . Surj Sci. 1982, 119, 150.
(57) Drexhage, K. H.; Kuhn, H.; Schafer, F. P. Be?. Bunsenges. Phys. Chem. 1968, 72, 329. ( 5 8 ) Murray, C . A.; Allara, D. L. J . Chem. Phys. 1982, 76, 1290. (59) Weitz, D. A,; Garoff, S.;Hanson, C. D.; Gramila, T.J.; Gersten, J. I. Opr. Lett. 1982, 7, 89.
MO Derivatives of Aspartate Adsorbed on Ag or resonance Raman intensity. This is in agreement with experimental results for luminescent emitters. However, direct observation of similar effects on the lifetimes of resonant Raman excited states in the picosecond to subpicosecond regime are lacking. Another possible contributor to the lower SERRS enhancement for DABS-ASP on colloidal silver may be the longer distance between the surface and chromophore in our experiments than that for the molecules studied in this way on silver-island films. A rapid falloff of SERS intensity with distances8 is predicted from the small particle limit of the electromagnetic theorys1 and is also expected from simple dipole-dipole coupling theory. On these grounds the SERS intensity should be proportional to (a/r)12, where a is the radius of the silver sphere and r is the distance from the equivalent dipole at the center of the sphere to the Raman active oscillating dipole on or near the surface of the sphere. Thus, on electrostatic grounds a distance of -35 A from the surface of the silver sphere is required to produce a 102-fold decrease in SERS intensity. This distance is not unreasonable in the event that DABS-ASP has simply added as an overlayer to the citrate already adsorbed on the colloidal silver particles. If, on the other hand, the aspartate carboxylate groups of DABS-ASP are directly adsorbed to the silver surface, then silver particles of diameter 15 nm, and a separation of 12 A between the silver surface and the chromophoric end of the DABS-ASP molecule give a 6-fold SERS intensity decrease compared to the chromophoric group being attached directly to the surface. On this argument, only part of the 100-fold decline in SERS intensity can originate from a distance effect for DABS-ASP directly adsorbed to spherical silver particles. The effect of adsorption of dye molecules, DABS-ASP and HDABS-ASP, on or near silver particles upon the electronic absorption band positions of the dye in aqueous media was not directly observed since, for the concentrations of silver and dye that were used, the absorption band intensities of small silver particles were always much greater than those of the dye. The fluorescence emission band of HDABS-ASP was, however, always observed in HDABS-ASP-colloidal silver mixtures. The position of its maximum did not change significantly with increasing total silver concentration although emission and Raman band intensities varied considerably. Excitation profiles that were obtained for the dye on colloidal silver reflected a combined RRS-SERS profile when a nonchromophoric molecule, citrate, was used as a reference. Separate profile maxima for RRS-only and SERS-only were not resolved in our experiments since these would be expected to lie very close together near 472, 510 nm (RRS profilels for DABS-ASP in solution) or 503 nm (RRS profile of HDABS-ASP in solution) and 490 nm (SERS profile40 for parent silver hydrosol with adsorbed citrate). In fact, the excitation profilels for DABS-ASP on colloidal silver at pH -6.0 peaked at 510 nm while the one obtained (Figure 5) for a mixture of DABS-ASP and HDABSASP on colloidal silver at pH 4.5peaked at 540 nm. No evidence of multiple peaks was observed in either case. The position and width of the profiles matched those of absorption bands for the corresponding silver sols with adsorbed dye very closely. The positions of DABS-ASP and HDABS-ASP absorption band maxima in solution are also very close to the silver particle absorption bands at 5 10 and 540 nm, respectively. Thus, there is no evidence from either fluorescence emission or Raman spectra of DABS-ASP or HDABS-ASP that their electronic absorption spectra have been significantly perturbed by adsorption on or near colloidal silver. A shift from 472, 500 to -510 nm for DABSASP and from 503 to -540 nm for HDABS-ASP would go undetected in our experiments. SERRS intensity enhancement related to resonance with the electronic levels of DABS-ASP or HDABS-ASP could be distinguished from that related to resonance with the surface plasmon modes of silver particles in several ways. Firstly, the Raman spectra of DABS-ASP on colloidal silver measured with excitation wavelengths from 465.8 to 676.4nm spanned the region of mainly R R S in the blue to mainly SERS in the red. Vibrational band
The Journal of Physical Chemistry, Vol. 88. No. 12, 1984 2649 assignments were discussed in our previous paper.lS In particular, the intensity of the azo stretching band, v(N=N), at 1419 cm-l was very strong in the RRS region but very weak in the SERS region. On the other hand, the relative intensity of the PhN2-stretching band at 1139 cm-I grew steadily as hXc was changed from 465.8 to 676.4nm. Thus, the ratio of integrated Raman band intensities, 11419/Z1 139, which reached an asymptotic value of 0.25in the red at X 5 600 nm and a value of 2.4 at our lowest excitation wavelength (close to the value of 2.8 for DABS-ASP in solution) in the blue at 465.8 nm, can be used to estimate the relative contributions of R R S and SERS to SERRS at intermediate excitation wavelengths. The intensity ratios (I1419/Z1,39, in parentheses) for each wavelength (Aexc, nm) are as follows: 465.8 (2.4),488.0(1.8),572.5 (0.48),591.5 (0.36),624.7 (0.33), 647.1 (0.31),676.4 (0.25). For example, with values of 0.25and 2.8 for the extremes, the relative contributions from RRS and SERS at 515 nm (11419/11139 = 1.3) are 41% and 59%, respectively. Secondly, when Raman intensity enhancements were obtained for DABS-ASP and HDABS-ASP on colloidal silver, the Raman band intensity of the dye in the sol was compared to the same band intensity in a blank reference with the same total concentration of dye but no silver. If the electronic absorption spectrum of the dye has not been significantly perturbed as has been shown herein, then the ratio of Raman band intensity of dye in the sol to band intensity in the blank, Im,/Iref,gives the enhancement due to SERS, Le., resonance with the surface plasmon modes of silver particles. It may be possible to observe separate RRS and SERS excitation profile peaks for DABS-ASP or HDABS-ASP adsorbed on silver if the position of the peaks in the two profiles were further apart in energy. This situation can occur when DABS-ASP is adsorbed on a silver electrode for which SERS excitation profiles have been shown to reach a maximum between 600 and 700 nm, shifted considerably to the red of the DABS-ASP and HDABS-ASP lowest energy electronic absorption band maxima. Alternatively, the DABS-ASP absorption band can be blue shifted to 428 nm in nonaqueous solvents such as ethanol. Silver organosols in ethanol with PVP-protecting polymer and adsorbed DABS-ASP would therefore be suitable for investigating the situation in which RRS and SERS excitation profile peaks are well separated. Both of these systems, DABS-ASP on a silver electrode and on colloidal silver in ethanol, are under current i n v e s t i g a t i ~ nin ~ ~our * ~ laboratory. ~ Lastly, we consider the effect of the silver surface on the acid dissociation constant of HDABS-ASP. By inspection of Raman spectra of HDABS-ASP and DABS-ASP in solution and on colloidal silver the pK, for the azo group of HDABS-ASP was observed to increase 0.5-1 .O unit upon adsorption of the molecule onto colloidal silver particles. The same conclusion could be reached by taking the ratio of Raman band intensities, 11623/1113& from Figure 3, right-hand side and left-hand side, at various pH values. Since this ratio is proportional to [HDABS-ASP]/ [DABS-ASP], its value of 3.4 near p H 3.6in solution gives the pKa = 4.4 of HDABS-ASP adsorbed on colloidal silver. A possible source for this increase is that individual DABS-ASP molecules adsorbed on the silver surface are concentrated in a surface monolayer much more than in solution. Thus, the particles are highly negatively charged from adsorption to silver through the ionized carboxylate groups of the DABS-ASP molecule. Protons will be attracted to the most accessible basic site available, Le., the P-azo nitrogen atom of the surface-bound DABS-ASP rather than the adsorbed carboxylate groups. Due to attractive forces between positively charged hydrogen ions and negatively charged particles, a higher pH would be necessary to deprotonate surface-bound DABS-ASP at the azo nitrogen atom. Thus, the observed increase in pKa can be rationalized.
Summary Silver sols with adsorbed DABS-ASP were titrated with sulfuric acid to give sols with coadsorbed DABS-ASP and HDABS-ASP, its conjugate acid. (60) Siiman, 0.; Smith, R.; Blatchford, C.; Kerker, M., to be submitted.
J. Phys. Chem. 1984, 88, 2650-2655
2650
The RRS and fluorescence emission spectra of DABS-ASP and HDABS-ASP in solution were simultaneously observed at intervals of pH from 6.5 to 0.7. When HDABS-ASP was adsorbed on colloidal silver, its surface-enhanced R R S and fluorescence emission spectra were observed. Both its combined RRS-SERS spectrum in the region of large overlap between absorption spectra of dye and colloidal silver and its SERS-only spectrum in a region where the dye did not absorb strongly were obtained. Absolute enhancements of RRS and fluorescence emission band intensities of HDABS-ASP on colloidal silver were obtained. Dependence of the surface enhancement of RRS and fluorescence emission varied nearly linearly with total silver concentration for HDABS-ASP on colloidal silver at pH -4.5 and reached maximum values at 320 and 21, respectively. Surface enhancement of RRS and fluorescence band intensity reached a plateau near 0.20 and 0.30 g/L of silver for DABS-ASP on colloidal silver at pH -6.0 with values of 66 and 4, respectively. Absorption spectra of silver sols with adsorbed dye showed an intense band maximum at 400 nm and a very weak shoulder near 500 nm at pH 6.0. Spectra a t pH 4.5 showed the 400-nm band and a distinct lower energy band between 450 and 550 nm. The lower-energy absorption band was assigned to small clusters or aggregates of silver particles.
The peak positions in SERRS excitation profiles for DABSASP or HDABS-ASP on colloidal silver matched the peaks in absorption band intensity for the aggregated silver particles. The relative contributions of RRS and SERS to the SERRS enhancement of Raman bands of the dye on colloidal silver at any excitation wavelength could be distinguished by comparing the ratios of Raman band intensity in spectra taken in the blue (strong overlap of RRS-SERS) and red (SERS-only) regions, respectively. The positions of fluorescence emission band maxima of HDABS-ASP with increasing silver concentration did not change. Single excitation profile peaks of DABS-ASP and HDABS-ASP on colloidal silver at 490 and 540 nm, respectively, showed that absorption bands of colloidal silver near 500 and 540 nm overlapped well with excitation profile peaks of the dyes, alone, in solution at 472, 510, and 505 nm. Little perturbation of the electronic levels of the adsorbed dye molecules from those of the dyes in solution was observed.
Acknowledgment. This work was supported in part by N I H Grant GM-30904, by N S F Grant CHE-801144, and by Army Research Office Grant DAAG-29-82-K-0062. We thank Professor M. Kerker for reading the manuscript and helpful discussions. Registry No. DABS-ASP, 87667-33-2; Ag, 7440-22-4.
Hydrogen-Bonded Specles of Pyridinlurn Halogenoacetates. 2. Thermometric Behavior in Aprotic Solvents B. Chawla* and S. K. Mehta Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India (Received: July 7 , 1983; In Final Form: November 8, 1983)
The enthalpies of reaction of a series of substituted pyridines and of triethylamine, quinoline, and isoquinoline (for the sake of comparison) with trifluoroacetic acid (TFA) in chloroform were measured at 25 “C at final solution concentrations ranging to 5 X lo-’ M. The enthalpies of reaction of 2,4,6-trimethylpyridine,pyridine, and 4-cyanopyridine with from 1 X trichloroacetic acid (TCA), dichloroaceticacid (DCA), monochloroacetic acid (MCA), and acetic acid (AcOH) in chloroform were also measured at 25 OC. The concentration dependence of enthalpies of reaction was examined by measuring them for 2,4,6-trimethylpyridine-TFA, pyridine-TFA, and 4-cyanopyridine-TFA in chloroform over the entire possible solution concentration range at 25 OC. In order to study the energetic behavior of H-bonded species of the pyridine-TFA system in media of different dielectric constants, the enthalpy measurements were made in p-dioxane, o-xylene, and carbon tetrachloride. The thermometric results have been discussed in terms of interactions present in the H-bonded species of these systems in nonaqueous solutions.
Introduction Even though thermometric titration is a powerful tool for acid-base only a few thermometric titrations of amines in nonaqueous media- have been carried out. The only deliberate study of ion pairs by this method available in the literature is that of Goldshtein et al.1° The recent study of Arnett and Chawla” on the thermometric titration of a series of substituted pyridines with trifluoroacetic acid (TFA) in carbon tetrachloride is of direct relevance to the investigations presented in this paper. In their study, they have reported the enthalpies of reaction of pyridines with TFA a t stoichiometries corresponding to formation of the 1:1 (B+H--A-) complex, the 1:2 (B’H---AHA) complex, and the 2:l (B’H- -BA-) complex. In continuation of our systematic study on the hydrogen-bonded salts of a series of substituted pyridines and halogenoacetic acids in various nonaqueous solvents’* (chloroform in particular), the enthalpies of reaction (B + HA B’H- -A-) for these systems
-
* Address correspondenceto this author at the Department of Chemistry, Duke University, Durham, NC 27706. 0022-3654/84/2088-2650$01 .50/0
at stoichiometries corresponding to the formation of the 1:l (B’H- -A-) complex have been measured at 25 OC. the enthalpies of three of these systems over a wide range of final solution concentration have also been measured in order to study the effect of concentration on the behavior of hydrogen-bonded species present in solution. In order to study the effect of solvent on the (1) Gyenes, I. “Titration in Nonaqueous Media”; Cohen, D., Millar, I. T., Eds.; Van Nostrand: Princeton, NJ, 1968. (2) Huber, W. “Titrations in Nonaqueous Solvents”, translated by Express Translations Service, London; Academic Press: New York, 1968. (3) Kucharsky, J.; Sajarik, L. “Titrations in Nonaqueous Solvents”, translated by K. Sumbera; Elsevier: Amsterdam, 1965. (4) Fritz, J. S. “Acid-Base Titrations in Nonaqueous Solvents”;Allyn and Bacon: Boston, MA, 1973 and the references cited therein. ( 5 ) Forman, E. J.; Hume, D. N. J . Phys. Chem. 1959, 63, 1949. (6) Forman, E. J.; Hume, D. N. Talenta 1964, 1 1 , 129. (7) Keily, J.; Hume, D. N. Anal. Chem. 1964, 36, 543. (8) Keily, J.; Hume, D. N. Anal. Chem. 1956, 28, 1294. (9) Mead, T. E. J. Phys. Chem. 1962, 66, 2149. (IO) Goldshtein, I. P.; Guryanova, E. N.; Prepelkova, T. I. Zh. Obshch. Khim. 1972,42, 2091. (11) Arnett, E. M.; Chawla, B. J . Am. Chem. SOC.1978, 100, 217. (12) Mehta, S . K.; Chawla, B. Elecfrochim.Acta 1982, 27, 9.
0 1984 American Chemical Society
