Electrochemical Surface Science - American Chemical Society

Chapter 26

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Surface-Enhanced

Raman Study

Effect of pH and Electrode Potential on the Interfacial Behavior of Some Substituted Pyridines 1

Mark R. Anderson and Dennis H. Evans

2

1

Department of Chemistry, University of Utah, Salt Lake City, UT 84112 Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716

2

The surface-enhanced Raman spectra (SERS) provide information about the extent of protonation of the species adsorbed at the silver/aqueous solution interface. The compounds investigated were 4-pyridylcarbinol (1), 4-acetylpyridine (2), 3-pyridinecarboxaldehyde (3), isonicotinic acid (4), isonicotinamide (5), 4-benzoylpyridine (6), 4(aminomethyl)pyridine (7) and 4-aminopyridine (8). For 1, the fraction of the adsorbed species which was protonated at -0.20 V vs. SCE varied with pH in a manner indicating stronger adsorption of the neutral than the cationic form. The fraction protonated increased at more negative potentials. Similar results were obtained with 3. For all compounds but 4, bands due to the unprotonated species near 1600 cm and for the ring-protonated species near 1640 cm were seen in the SERS spectra. -1

-1

In 1974 Fleischmann e t a l . demonstrated t h a t Raman s p e c t r a o f p y r i d i n e adsorbed a t a s i l v e r e l e c t r o d e from an aqueous s o l u t i o n c o u l d be o b t a i n e d w i t h e x c e l l e n t s i g n a l - t o - n o i s e a t a roughened s i l v e r surface Q). Subsequently, Jeanmaire and Van Duyne (2) as w e l l as A l b r e c h t and C r e i g h t o n (3) demonstrated t h a t the Raman spectrum o f adsorbed p y r i d i n e a t the s i l v e r s u r f a c e r e p r e s e n t e d a 10 enhancement o f the s i g n a l expected c o n s i d e r i n g t y p i c a l Raman parameters. Such a d i s c o v e r y was very e x c i t i n g because i t p r e s e n t e d a r e l a t i v e l y s i m p l e means o f o b t a i n i n g i n s i t u the v i b r a t i o n a l s p e c t r a o f m o l e c u l e s adsorbed onto a s u r f a c e . S i n c e t h a t t i m e , c o n s i d e r a b l e e f f o r t has been devoted t o s t u d y i n g t h i s phenomenon. Much o f t h i s r e s e a r c h e f f o r t has been d i r e c t e d t o the study o f the fundamental b a s i s o f surface-enhanced Raman s c a t t e r i n g (SERS), i n o r d e r t o understand the u n d e r l y i n g p r i n c i p l e s . There have a l s o been many a p p l i c a t i o n s o f SERS t o s i t u a t i o n s i n which i n s i t u v i b r a t i o n a l

c

0097-6156/88/0378-0383$06.00/0 1988 American Chemical Society

Soriaga; Electrochemical Surface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

384

ELECTROCHEMICAL SURFACE SCIENCE

s p e c t r a would p r o v i d e v a l u a b l e i n f o r m a t i o n . An e l e c t r o c h e m i c a l e n v i r o n m e n t i s o n e s u c h s y s t e m w h e r e SERS h a s b e e n p a r t i c u l a r l y valuable. SERS h a s b e e n u s e d t o s t u d y a number o f e l e c t r o c h e m i c a l phenomena, i n c l u d i n g the a d s o r p t i o n o f s i m p l e a n i o n i c a d s o r b a t e s and t h e mechanisms o f e l e c t r o d e r e a c t i o n s ( 4 , 5 ) . It i s a l s o p o s s i b l e to s t u d y p r o p e r t i e s o f s u r f a c e s p e c i e s by s y s t e m a t i c a l l y a l t e r i n g t h e c o n d i t i o n s under which s p e c t r a are o b t a i n e d . In t h i s manner, s p e c t r o s c o p i c c h a n g e s may b e c o r r e l a t e d w i t h t h e e n v i r o n m e n t a l p e r t u r b a t i o n s and i n f o r m a t i o n about the p r o p e r t i e s o f the m o l e c u l e a t t h e s u r f a c e may b e d e d u c e d . I n a p r e v i o u s s t u d y ( 6 ) we i n v e s t i g a t e d t h e s p e c t r a o f a d s o r b e d 4 - p y r i d i n e c a r b o x a l d e h y d e as a f u n c t i o n o f a p p l i e d e l e c t r o d e p o t e n t i a l and as a f u n c t i o n o f b u l k s o l u t i o n pH. T h i s s t u d y demonstrated t h a t t h e s p e c t r u m o f 4 - p y r i d i n e c a r b o x a l d e h y d e was d r a m a t i c a l l y d e p e n d e n t upon t h e s e e x p e r i m e n t a l v a r i a b l e s . Separate spectroscopic features w e r e i d e n t i f i e d w h i c h c o u l d be a t t r i b u t e d t o t h e p r o t o n a t e d a n d t h e unprotonated 4-pyridinecarboxaldehyde species. Interestingly, in s o l u t i o n s n e a r pH 7 , w h e r e t h e c o n c e n t r a t i o n o f p r o t o n a t e d 4 p y r i d i n e c a r b o x a l d e h y d e i n s o l u t i o n i s n e g l i g i b l e , bands a t t r i b u t a b l e t o the p r o t o n a t e d s p e c i e s on the s u r f a c e appeared i n the s p e c t r u m a t negative potentials. T h i s b e h a v i o r was t h o u g h t t o be c a u s e d l a r g e l y by t h e p o t e n t i a l dependence o f t h e a d s o r p t i o n c o e f f i c i e n t s , t h a t o f t h e c a t i o n b e i n g r e l a t i v e l y l a r g e r a t more n e g a t i v e p o t e n t i a l s . A s m a l l e r c o n t r i b u t i o n a r i s e s f r o m l o w e r i n g o f t h e s o l u t i o n pH i n t h e r e g i o n l o c a l to the e l e c t r o d e s u r f a c e . S u c h a phenomenon h a d p r e v i o u s l y b e e n o b s e r v e d i n a SERS s t u d y o f p h o s p h a t e s p e c i e s ( 7 ) a n d i s s u p p o r t e d by e l e c t r o c h e m i c a l t h e o r y . In a d d i t i o n , as the e l e c t r o d e p o t e n t i a l was made more n e g a t i v e , t h e c a r b o n y l f e a t u r e g r a d u a l l y d i m i n i s h e d i n s i z e and e v e n t u a l l y d i s a p p e a r e d . This o b s e r v a t i o n was a t t r i b u t e d t o a n i n c r e a s i n g d e g r e e o f h y d r a t i o n o f t h e 4 - p y r i d i n e c a r b o x a l d e h y d e a s t h e e l e c t r o d e p o t e n t i a l was made m o r e n e g a t i v e , i n consonance w i t h the f a c t t h a t the c a r b o n y l group o f p r o t o n a t e d 4 - p y r i d i n e c a r b o x a l d e h y d e i s 94? h y d r a t e d i n s o l u t i o n . T h e s e i n t e r p r e t a t i o n s d i f f e r e d somewhat f r o m t h o s e o f B u n d i n g a n d B e l l ( 8 ) i n a s i m i l a r SERS s t u d y o f 4 - p y r i d i n e c a r b o x a l d e h y d e . These w o r k e r s c o n c l u d e d t h a t e l e c t r o n w i t h d r a w a l by t h e e l e c t r o d e f r o m t h e adsorbed 4-pyridinecarboxaldehyde induced the t o t a l h y d r a t i o n o f the formyl substituent. S u p p o r t f o r t h i s i n t e r p r e t a t i o n was g a i n e d b y c o n s i d e r i n g t h e s p e c t r o s c o p i c b e h a v i o r o f s i m i l a r compounds w i t h i n the c o n t e x t o f the 4-pyridinecarboxaldehyde o b s e r v a t i o n s . Bunding and B e l l , however, o n l y i n v e s t i g a t e d the s p e c t r o s c o p i c b e h a v i o r a t a single, negative potential. I t i s the purpose o f the c u r r e n t study t o i n v e s t i g a t e s y s t e m a t i c a l l y the s p e c t r o s c o p i c b e h a v i o r o f o t h e r p y r i d i n e d e r i v a t i v e s as a f u n c t i o n o f e l e c t r o d e p o t e n t i a l and b u l k s o l u t i o n pH a n d t o c o m p a r e t h e o b s e r v a t i o n s t o t h o s e p r e v i o u s l y presented for 4-pyridinecarboxaldehyde. R e s u l t s and D i s c u s s i o n Compounds 1-8 h a v e b e e n i n v e s t i g a t e d . 4 - P y r i d y l c a r b i n o l , 1, i s o f i n t e r e s t b e c a u s e i t i s one o f t h e e l e c t r o c h e m i c a l r e d u c t i o n p r o d u c t s o f 4 - p y r i d i n e c a r b o x a l d e h y d e , the species f o r which surface p r o t o n a t i o n r e a c t i o n s were d i s c o v e r e d and c h a r a c t e r i z e d i n e a r l i e r work ( 6 ) .


26.

ANDERSON AND EVANS

Interfacial Behavior of Substituted Pyridines

385

Characterization of Surface Species by SERS. Before presenting the results obtained with 1, the spectral features which have proven to be useful in identifying surface species w i l l be reviewed. Both in solution and by SERS, pyridines show a ring mode in the Raman spectrum near 1600 cm"'. When the ring nitrogen i s protonated, thi; band disappears and i s replaced by a band near 1640 cm" . The 1 : R = -CH 0H

R

2

1-2,

4-8

3

7 :R

•NH2

8

•COC6H5

: R

r e l a t i v e intensities of these two bands provide information about the extent of protonation. If one assumes that the scattering cross sections for unprotonated species (1600 cm" band) and protonated species (1640 cm band) are equal, the fraction of the pyridine compound which is protonated is given by Ii640 ( 1640 ^600) where 11600 11640 heights of the two bands. In the present research, i t was confirmed for compounds 1-3 that, in the normal Raman spectra of aqueous solutions of various pH, a band near 1600 cm appears for the neutral pyridine and a band near 1640 cm"' exists for the ring-protonated species. In cases where the substituent on the pyridine ring contains a carbonyl group, a weak band for the C-0 stretch can be detected in the SERS spectra, generally near 1700 cm" . The SERS spectra of pyridines contain many other bands but those mentioned above have proven to be particularly useful in characterizing the surface species. /

a n d

a r e

I

+

t n e

1

4-Pyridylcarbinol, 1. This compound has been investigated previously (8) and i t was noted that i t s SERS spectra were almost i d e n t i c a l to those of 4-pyridinecarboxaldehyde. SERS spectra in the 1500-1700 cm" region are shown in Figure 1. Buffers with pH bracketing the pK of 4-pyridylcarbihol (5.76 (9)) were employed and i t can be seen that the band for the unprotonated species near 1600 cm" i s predominant at pH 6.88 but decreases as the band due to the protonated species (near 1640 cm" ) grows in when the pH i s lowered. A variety of buffers was used and the intensity r a t i o was measured from SERS spectra obtained at -0.20 V vs. SCE. As mentioned above, t h i s intensity ratio is a measure of the fraction of the surface compound which i s protonated. The fraction i s plotted in Figure 2 along with the fraction protonated in solution (calculated from the pK ). It i s apparent that the fraction protonated at the surface lags the fraction protonated in solution as the pH i s lowered. This may be caused by r e l a t i v e l y stronger adsorption of the neutral pyridine compared to the protonated species at this potential. Considering competitive adsorption of two species according to the Langmuir isotherm at close-to-saturation surface coverage, the fraction of the surface species which i s protonated, ( X ) , w i l l be given by Equation 1, where [H*] i s the hydronium ion concentration 1

a

a

p

s


386


Figure 1. SERS spectra of 0.050 M 4-pyridylcarbinol in solutions of various pH. A l l spectra were obtained at -0.20 V vs. SCE.



Figure 2. Surface mole fraction of protonated 4-pyridylcarbinol vs. pH. Points are from SERS spectra obtained at -0.20 V vs. SCE. Dashed: mole fraction of the protonated form i n solution. S o l i d : mole fraction of protonated form on surface from Equation 1 with K = 30.

pH

388


(Xp)

+

s

+

= [H ]/([H ] + K K)

(1)

a

in solution, K i s the acid dissociation constant in solution and K = rn,s&n/rp Bp ^s B are saturation coverages and adsorption c o e f f i c i e n t s , respectively, for the neutral (n) and protonated (p) species. At the concentration used in this work, close-to-saturation coverage i s expected based on adsorption studies of other pyridines at a mercury electrode (10, 11). Equation 1 i s plotted in Figure 2 ( s o l i d curve) for K = 30 which matches the experimental data reasonably well. Introduction of interaction parameters through the Frumkin isotherm brought a perceptible improvement in the agreement but, in view of the scatter in the experimental data, this refinement i s probably not meaningful. Thus, at -0.20 V, the adsorption coefficient of the neutral form of 4-pyridylcarbinol i s about 30 times that of the protonated form (assuming equal saturation coverages). This quantity i s dependent upon the electrode potential, however. In Figure 3 are shown SERS spectra obtained at constant solution pH (6.88) but variable electrode potential. At -0.2 V only a small fraction i s protonated (weak feature at 1640 cm" ) but the fraction increases substantially as the potential i s made more negative u n t i l at the most negative potential (-0.60 V) i t decreases again. The same trend was observed with 4-pyridinecarboxaldehyde (6) and can be explained by r e l a t i v e l y stronger adsorption of the cation (protonated form) which i s expected at more negative potentials. A smaller contributor to the effect i s the increase in the effective pH in the i n t e r f a c i a l region as the potential i s made more negative (6 7). The intensity r a t i o i s given in Table I along with the solution pH which would be necessary to cause the same fraction of 1 to be protonated as i s present on the surface. a

w

n

e

r

e

a n d

> s

t

Table I. Fraction of 4-Pyridylcarbinol which i s Protonated as Calculated from SERS Spectra a

Potential ( V vs. S C E ) -0.20 -0.30 -0.40 -0.50

(x ) p

pH

s

b

6.6 5.0 4.7 4.7

0.15 0.22 0.36 0.36

*Data from Figure 3, pH 6.88. pH in solution which produces fraction protonated equal to that seen on the surface.

4-Acetylpyridine 2. Relatively intense SERS spectra were obtained for 2 using 0.10 M KC1 (8) but there was l i t t l e dependence of the r e l a t i v e intensities of the bands on potential. A band due to the unprotonated pyridine was seen near 1600 cm" and, at pH < 6, a band due to the protonated compound appeared near 1640 cm and increased at the expense of the 1600 cm"' band u n t i l only the protonated species could be detected at pH = 1.3. The pK of 2 i s 3.51 (9), t

a


ANDERSON AND EVANS


Figure 3. SERS spectra of 0.050 M 4-pyridylcarbinol i n pH 6.88 buffer at various electrode potentials (vs. SCE).


389

390


about two units smaller than 1, and correspondingly lower pH values are required to generate large populations of the protonated form of 2 on the surface. Unlike 1 and 4-pyridinecarboxaldehyde, the f r a c t i o n protonated at constant pH hardly changes with p o t e n t i a l . In 0 . 1 0 M K C 1 , the SERS spectra of 2 show a small band a t about 1690 cm" , attributable to the carbonyl stretch. Allen and Van Duyne ( 1 2 ) have considered orientational effects on band i n t e n s i t i e s and have concluded that the r e l a t i v e i n t e n s i t i e s should be given by Equation 2 , where the intensity of the carbonyl band d^o^ioooJsERs

I

I

= ( I69O/ IOOO)NRS

# C O S

0

2

< >

i s expressed r e l a t i v e to the intense symmetrical ring-breathing band near 1 0 0 0 cm"', i n both the SERS and normal Raman solution spectra (NRS). 0 i s the angle of the carbonyl group with respect to the surface normal. For v e r t i c a l orientation of 4-acetylpyridine (adsorption v i a the rine nitrogen atom), 0 w i l l be 6 0 ° . The data for the 1 6 9 0 and 1 0 0 0 cm" bands i n the SERS and NRS ( 0 . 1 0 M K C 1 ) were analyzed according to Equation 2 and gave 0 = 6 1 , 5 9 and 6 3 ° for - 0 . 2 0 , - 0 . 4 0 and - 0 . 6 0 V, respectively. Thus, the results are consistent with a v e r t i c a l orientation which i s independent of p o t e n t i a l . T

3-Pyridinecarboxaldehyde 3 . Possible hydration of the aldehyde group makes the aqueous solution chemistry of 3 p o t e n t i a l l y more complex and interesting than the other compounds. Hydration i s l e s s extensive with 3 than 4-pyridinecarboxaldehyde but upon protonation, about 80% w i l l e x i s t as the hydrate (gem-diol). The calculated d i s t r i b u t i o n of species as a function of pH i s given i n Figure 4 based on the equilibrium constants determined by Laviron ( 9 ) . In comparison to 1 and 2 , the SERS spectra of 3-pyridinecarboxaldehyde ( 3 ) are r e l a t i v e l y featureless ( 8 ) . The spectra are dominated by the symmetrical ring-breathing mode at 1 0 3 0 cm" but the features associated with the unprotonated species (about 1 6 0 0 cm" ) and the protonated species (about 1 6 4 0 cm"') are d e f i n i t e l y present along with a weak carbonyl band at about 1 7 1 0 cm" . The v a r i a t i o n i n the r e l a t i v e population of protonated species i s as expected (Figure 5 ) though a detailed analysis reveals some surprises. As can be seen in Figure 5 , about equal i n t e n s i t i e s of the 1 6 0 0 and 1 6 4 0 cm" bands are obtained at pH = 3 . 8 6 , near the pK ( 3 . 7 3 ( 9 ) ) . However, the band associated with the unprotonated pyridine persists at pH = 1 . 3 , where less than \% of the solution species remains unprotonated. When the intensity ratios are measured for a variety of buffers, and the values are plotted vs. pH, the values approach 0 . 8 at low pH (Figure 6 ) , the same as the f r a c t i o n of 3 e x i s t i n g i n solution as the protonated, hydrated form ( c f . Figure 4 ) . This could mean that the residual 1 6 0 0 cm" band seen at low pH i s due to protonated, unhydrated 3 on the surface. As seen with 4-acetylpyridine, the r e l a t i v e i n t e n s i t i e s of the 1600 and 1 6 4 0 cm features, at constant solution pH, are i n s e n s i t i v e to electrode potential. Again, t h i s behavior may be associated with the r e l a t i v e l y weaker b a s i c i t y of 3 and 2 (pK = 3 . 5 1 and 3 . 7 3 , respectively) compared to 1 ( 5 . 7 6 ) and 4-pyridinecarboxaldehyde ( 4 . 7 8 ) though the exact reason i s not known. The l a t t e r two show t

1

a

a



392


Figure 5. SERS spectra of 0.050 M 3-pyridinecarboxaldehyde i n solutions o f various pH. A l l spectra were obtained a t -0.20 V vs. SCE.



1. O -

2

3

4

5

6

pH

7

8

9

10

/

I

11

12

13

14

F i g u r e 6. Experimental i n t e n s i t y r a t i o ( I i 6 4 0 ( 1 6 4 0 + *1600)) f r o m SERS s p e c t r a o f 0 . 0 5 0 M 3 - p y r i d i n e c a r b o x a l d e h y d e o b t a i n e d a t - 0 . 2 0 V v s . S C E . Curve i s mole f r a c t i o n o f p r o t o n a t e d / h y d r a t e d species present i n s o l u t i o n .

1


394

r e l a t i v e l y higher populations of protonated pyridine in the SERS spectra as the potential i s made more negative. At the most negative potentials, the protonated form of 3 appears to desorb as seen also with 1 and 2. For 4-pyridinecarboxaldehyde, this desorption has been correlated with the desorption of chloride (6) suggesting that the cation and chloride ion are coadsorbed. Isonicotinic Acid, 4. It i s d i f f i c u l t to obtain a spectrum of 4 because the neutral form i s not very soluble. At low pH, however, the ring nitrogen i s protonated (J3) and the cationic i s o n i c o t i n i c acid i s s u f f i c i e n t l y soluble to obtain SERS spectra. A r e l a t i v e l y intense spectrum was obtained at -0.20 V with 0.050 M i s o n i c o t i n i c acid, 0.10 M KC1 and 0.10 M HC1. Many of the spectral features seen with other pyridines are present but the i n a b i l i t y to vary solution pH made i t impossible to investigate the relative surface populations of protonated and unprotonated forms. Isonicotinamide, 5. This compound was s u f f i c i e n t l y soluble to allow SERS spectra to be obtained at the 50 mM level in 0.10 M KC1 and 0.10 M KC1 + 0.10 M HC1 at -0.20 V. The spectra resembled those seen with other pyridines. In particular, an intense band at 1600 cm was seen with the neutral electrolyte and i t was replaced by a band at 1640 cm"' i n the acidic electrolyte. Of the two basic s i t e s , only the ring nitrogen w i l l be protonated in 0.10 M HC1 (J_3) so, with this compound also, the 1640 cm band appears to be due to the protonated pyridine. No carbonyl band was seen in either spectrum. 4-Benzoylpyridine, 6. Of possible interest here i s the fact that 6 contains both a pyridyl and a phenyl group. Pyridine and benzene have very similar vibrational modes which should produce bands due to both aromatic groups at approximately the same positions in the spectra of 6. Like isonicotinic acid, 6 was r e l a t i v e l y insoluble i n water but i t was s u f f i c i e n t l y soluble i n 50 % (v/v) ethanol/water to allow spectra to be obtained at the 50 mM l e v e l . Again, spectra were recorded at -0.20 V i n 0.10 M KC1 and i n 0.10 M KC1 + 0.10 M HC1 (Figure 7). Under both conditions, strong bands, characteristic of the aromatic ring systems, are seen near 1000, 1200 and 1600 cm" . A carbonyl band appears at approximately 1660 c m . The main difference between the spectrum obtained at low pH and that recorded from neutral solution, i s the band near 1640 cm which i s again attributed to the protonated pyridyl ring. In this case, the strong band remaining at about 1600 cm"' i s probably due to the phenyl group. 4-(Aminomethyl)pyridine, 7. The SERS spectra of most pyridines show a broad, r e l a t i v e l y featureless background over the range of 12001700 cm" . This background scattering i s immense for 7 i n 0.10 M KC1 at -0.20 V. Atop the background i s the strong band near 1600 cm" , characteristic of the neutral pyridyl ring. The spectrum obtained in 0.10 M KC1 + 0.10 M HC1 has no band at 1600 cm" but, instead, a very strong band at about 1640 cm" i s seen. In a l l of the other pyridines this band i s associated with the protonated pyridyl ring. The aminomethyl group i s the more basic s i t e on 7 so the appearance 1

1

1


ANDERSON AND EVANS


600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

Raman S h i f t ,

cm"

1

Figure 7. SERS spectra of 0.050 M 4-benzoylpyridine obtained at -0.20 V vs. SCE. 50J (v/v) ethanol/water. A: 0.10 M KC1. B: 0.10 M KC1 + 0.10 M HC1. (Sharp lines at 738, 1040 and 1060 cm" are from scattered argon ion emission). 1


395

396


1

of the band at 1640 cm" probably means that both the aminomethyl group and the pyridyl nitrogen are protonated in the acidic medium. 4-Aminopyridine, 8. Like 4-(aminomethyl)pyridine, 8 has two basic s i t e s . The pK of the ring nitrogen i s 9.12 while that of the amino group i s estimated to be -6 (14, 15). The SERS spectrum in 0.10 M KC1 has a very large background between 1200 and 1700 cm" ( l i k e that seen with 7) with a weak band due to the unprotonated pyridine at about 1600 c m . In this case, protonation of the pyridyl nitrogen occurs readily so that even with a buffer of pH = 6.19, the 1600 cm" band has been replaced by a band near 1640 cm" . (The broad background between 1200 and 1700 cm"' i s also much weaker). The spectrum obtained with 0.10 M KC1 + 0.10 M HC1 i s almost identical to that seen with the pH = 6.19 buffer. In view of the pK values for 8, protonation of the amino group i s not expected in 0.10 M HC1. a

1

1

a

Experimental 4-Pyridylcarbinol, 4-acetylpyridine, 3-pyridine-carboxaldehyde and 4aminomethylpyridine were obtained from Aldrich Chemical Company (Milwaukee, Wisconsin) and were purified by d i s t i l l a t i o n at reduced pressure. 4-Benzoylpyridine was recrystallized from ethanol. 4-Aminopyridine (G. Frederick Smith Chemical Company), i s o n i c o t i n i c acid (Aldrich) and isonicotinamide (Aldrich) were used as received. T r i p l y d i s t i l l e d water was used. A l l other reagents were a n a l y t i c a l reagent grade. Acetate buffers were used for pH values between 3.5 and 5.5 while phosphate buffers were used for pH 5.5-7. The pH of solutions below pH 3.5 was adjusted with HC1. A l l solutions contained 0.10 M KC1 and the ionic strength of the buffers was adjusted to 0.90 M by addition of potassium nitrate. A l l SERS experiments were conducted with a polycrystalline s i l v e r working electrode prepared by p r e s s - f i t t i n g a 6 mm diameter cylinder of s i l v e r into one end of a 0.375 inch diameter Teflon rod through which a 6 mm diameter concentric hole had been d r i l l e d . E l e c t r i c a l contact was made via a copper wire soldered to the s i l v e r . The geometric area of the s i l v e r disk was 0.28 cm . The SERS c e l l was b u i l t following the design of Brandt (16). Prior to the experiment, the s i l v e r electrode was polished with 5, 0.3 and 0.05 mu alumina. After polishing, the electrode surface was rinsed with copious amounts of t r i p l y d i s t i l l e d water followed by sonication in t r i p l y d i s t i l l e d water. The SERS c e l l was then assembled and f i l l e d with an analyte solution which had been previously purged with nitrogen. The s i l v e r electrode was then subjected to an oxidation-reduction cycle (ORC), v i z . , oxidation at 0.20 V for ten seconds followed by reduction of the generated s i l v e r s a l t at -0.30 V u n t i l the current decreased to about zero. The spectra were taken using the 488.0 nm l i n e of a Spectra Physics model 164-00 argon ion laser with an incident power of 100 mW at the electrode surface. The laser l i g h t was focused to a l i n e image at the electrode with a c y l i n d r i c a l lens. The scattered l i g h t was focused onto the entrance s l i t of a Spex model 1401 double monochromator (2.0 cm resolution) and detection was by photon counting (using an RCA model C31034-02 photomultiplier tube). Data


26. A N D E R S O N A N D E V A N S


397

c o l l e c t i o n was performed with a microcomputer system. The intensity of the SERS spectra varied considerably with changes in conditions. A l l spectra have been plotted with an arbitrary scale for the ordinate. The electrode potential was controlled with an EG & G Princeton Applied Research (PAR) model 173 potentiostat/galvanostat and i s referenced to a saturated calomel electrode (SCE). A PAR model 276 current-to-voltage converter allowed monitoring of current during the ORC and SERS experiments and i t also provided for positive feedback iR compensation for accurate potential control. Acknowledgment This research was supported bv the National Science Foundation, Grant CHE-8722764. Literature Cited 1. Fleischmann, M.; Hendra, P. J . ; McQuillan, A. J . Chem. Phys. Lett. 1974, 26, 163-166. 2. Jeanmaire, D. L.; Van Duyne, R. P. J . Electroanal. Chem. 1977, 84, 1-20. 3. Albrecht, M. G.; Creighton, J . A. J . Am. Chem. Soc. 1977, 99, 5215-5217. 4. Weaver, M. J . ; Hupp, J. T.; Barz, F.; Gordon, J . G.; Philpott, M. R. J. Electroanal. Chem. 1984, 160, 321-333. 5. Rubim, J . C. J. Electroanal. Chem. 1987, 220, 339-350. 6. Anderson, M. R.; Evans, D. H. (submitted to J . Am. Chem. Soc.) 7. Dorain, P. B.; Von Raben, K. U.; Chang R. K. Surf. S c i . 1984, 148, 439-452. 8. Bunding, K. A.; B e l l , M. I. Surf. S c i . 1983, 118, 329-344. 9. Laviron, E. B u l l . Soc. Chim. Fr. 1961, 2325-2349. 10. Barradas, R. G.; Conway, B. E. Electrochim. Acta 1961, 5, 319348. 11. Barradas, R. G.; Conway, B. E. Electrochim. Acta 1961, 5, 349361. 12. Allen, C. S.; Van Duyne, R. P. Chem. Phys. Lett. 1979, 63, 455459. 13. Jellinek, H. H. G.; Urwin, J . R. J . Phys. Chem. 1954, 58, 548550. 14. Fischer, A.; Galloway, W. J . ; Vaughan, J . J. Chem. Soc. 1964, 3591-3596. 15. Brignell, P. J . ; Johnson, C. D.; Katritzky, A. R.; Shakir, N.; Tarhan, H. O.; Walker, G. J . Chem. Soc., B 1967, 1233-1235. 16. Brandt, E. S. Anal. Chem. 1985, 57, 1276-1280. RECEIVED July 11,1988


Electrochemical Surface Science - American Chemical Society

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