Anal. Chem. 1987, 59, 2559-2563
2559
Feasibility Studies for the Detection of Organic Surface and Subsurface Water Contaminants by Surface-Enhanced Raman Spectroscopy on Silver Electrodes Michael M. Carrabba,* Robert B. Edmonds, and R. David Rauh
EIC Laboratories, Inc., Norwood, Massachusetts 02062
Fundamental components of various families of organic contaminants that are found in surface and subsurface waters have been lnvestlged by surface-enhanced Raman spectroscopy (SERS). The SERS substrate was a silver etectrode maintained at various electrode potentials. The limit of detection for pyridine was calculated to be 8.5 pg. Variation of the electrode potentiat and excitation wavelength was used to qualitatively determine a two-component mixture of contaminants. The In situ type of conditions of low lonk strengths and humic materials was found not to Inhibit the SERS effect on the silver electrode.
With the increasing presence of energy, industrial, and defense-related waste, a general need exists for methods to monitor the presence, evolution, and hydrological transport of organic contaminants in surface and subsurface waters. These contaminants frequently constitute a hazard to the biological environment and eventually become entrenched in the food supply. Even if in-ground waste disposal was terminated immediately, the release and transport of contaminants in ground contact would continue for decades (1). I t is important, therefore, to assess the extent that contaminants will be transformed, immobilized, or mobilized for transport into the environment and to determine the degree to which the transport process will affect the natural subsurface systems. Presently, no in situ technique exists that provides both specificity and sensitivity for subsurface monitoring. Ideally, infrared spectroscopy could be used as a technique for monitoring both specific identity and concentration, but infrared absorption measurement would be impossible in a strongly absorbing aqueous medium. Raman spectroscopy would be more appropriate as an in situ monitor since it uses visible light to provide a “fingerprint” of a molecule. Raman spectroscopy, like infrared spectroscopy, gives information about bonding. The main disadvantage of Raman spectroscopy is its insensitivity. Although Raman spectroscopy itself could never approach the detection limits necessary for an in situ water analysis, the relatively new technique of surface-enhanced Raman spectroscopy (SERS) has indicated an enhancement in Raman signal of 3-6 order of magnitude. The SEW phenomenon was first reported by Fleischmann and co-workers in 1974 (2),who described a very large enhancement of the Raman signal from pyridine when absorbed onto an electrochemically roughened Ag surface. Although numerous reports have deal€ with the theory of SERS ( 3 , 4 ) , only a few analytical studies employing SERS have appeared in the literature (5-9). These previous analytical applications employed metal-coated microspheres and SiOz posts, silver and indium island films, and colloids as the SERS active substrates. Only Vo-Dinh and co-workers (6) examined a mixture of compounds.
The SERS technique has the potential of resolving a mixture into its different Raman active components. The SERS-related phenomena that are chemically specific, such as the adsorption of organic molecules on a metal substrate and the potential dependence of the electrosorption, can lead to selectivity ( 1 0 , I I ) . In this paper, we evaluate SERS combined with electrosorption as an analytical tool for the detection of a mixture of groundwater contaminants. EXPERIMENTAL SECTION Instrumentation. A Spectra Physics 164 argon ion laser was used as the excitation source for a Spex (1401) double-grating spectrometer equipped with a cooled RCA (C31034) GaAs photomultiplier tube operating in photon-counting mode for all the surface-enhanced Raman scattering measurements. All spectra were collected with a spectral band-pass of 5 cm-’ and a laser power of 100 mW. Substrate Preparation. The SERS substrate, a 1-mm diameter Ag wire embedded in a glass sheath, was operated in an electrochemical optical cell that had a Pt counter electrode and a Fisher (13-639-56)standard calomel electrode (SCE) as a reference. A polished substrate was electrochemically roughened in a solution containing the contaminant of interest and 0.1 M KC1 prior to data collection. The roughening was accomplished by applying five oxidation/reduction cycles (ORC) between +0.2 to -0.6 V vs SCE with a BioAnalytical Systems (CV 1B) potentiostat at a sweep rate of 20 mV/s prior to data collection. All voltages are reported vs a SCE. Materials and Reagents. The 0.1 M KC1 (Fisher-ACS Certified) electrolyte solution was prepared with distilled water. The samples were obtained from the following suppliers: pyridine and benzene from Burdick and Jackson (HPLC grade); quinoline (reagent grade) from Matheson, Coleman and Bell; humic acid (technical grade), benzofuran (99.5%),benzothiophene (97%), indole (98%), and nitrobenzene (99%) from Aldrich; carbon tetrachloride (certified), methanol (certified), napthalene (purified), phenol (reagent), and toluene (certified) from Fisher; Ag wire from Alfa (99.9%); and aniline (analytical grade) from Mallinckrodt. All chemicals were used without further purification. RESULTS AND DISCUSSION The SERS spectra were recorded for 14 compounds listed in Table I. The organic compounds that we studied were the fundamental components of various families (amines, phenols, nitroaromatics, aromatic N-heterocycle, both basic and neutral; neutral aromatic hydrocarbons) as well as organic solvents and humic acid. These compounds are representative of the type of contaminants that are found in groundwater (12). The SERS effect on Ag electrodes in aqueous solution has been previously reported for a few these compounds, as indicated in Table I. The SERS spectra were obtained a t excitation wavelengths of 488 and 514.5 nm on the surface of a 1-mm diameter Ag electrode at a 100-mW power level and a t various electrode potentials (w SCE) for all of the compounds in Table I. For all the spectra examined at 514.5 nm, the peak at 1122 cm-l is an unfiltered mercury line. All of the compounds exhibit the SERS effect at 514.5 nm with the exception of CC14
0003-2700/87/0359-2559$01.50/00 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 21, NOVEMBER 1, 1987
Table I. Contaminants and Wavelengths of Excitation Examined by SERS on Ag Electrodes"
*il_
ELECTRODE POTENTI A? (VOLTS vs SCE) --/
obsd at compound
488 nm
513.5 nm
lit.
ani1ine benzene benzofuran benzothiophene carbon tetrachloride humic acid indole methanol naphthalene nitrobenzene phenol pyridine toluene quinoline Contaminants were examined at various electrode potentials (0.0 to -1.6 V vs. SCE) on a Ag electrode.
- 0.8
-0.6
-0.4
-0.2
L 1500
000
-
GO
I530
and humic acid. Since humic materials are naturally present in surface and subsurface waters, the absence of humic acid interference is promising for in situ applications. At 488 nm only eight of the compounds exhibited the SERS effect on Ag (Table I). This result suggests that a charge-transfer mechanism (13) may be important and may add to the selectivity of the technique. Au and Cu were also examined as SERS substrates, but no spectra were obtained a t either excitation wavelength. In accordance with other researchers, it is likely that longer wavelengths are required for SERS excitation on these substrates (14, 15). The adsorption of organic molecules on a metal substrate can be chemically specific due to its free energy of adsorption (AGab). The adsorption of a variety of organic molecules has been evaluated on numerous metal substrates (16). Large negative A G d are commonly observed for gas-olid processes. However, in the aqueous phase all values fall between -5 and -10 kcal/mol and mostly around -7 kcal/mol. I t has been proposed that the energy-determining step is the displacement of water from the metal surface in all cases-hence the relative constancy of AGads (16). Although relatively constant, the values still vary enough to provide selectivity on different substrates. The structural selectivity of adsorption and the SERS activation is a key issue in employing SERS in the analysis of mixtures. Since the AGab change as a function of electrode potential (17-19), another means of characterization of the contaminants would be to change the electrode potential. At different electrode potentials the SERS spectrum for an adsorbed compound appears to change. The electrode potential dependence is induced by changes of the surface adsorbates a t the electrode surface. The electrode potential is a thermodynamic variable of the surface interface. The image dipole theory indicates that the vibrational modes should be dependent on the dipolar orientation of the molecule on the metal surface (20). For the case of pyridine, Shin and Kim observed the potential dependence of the SERS spectra (10). They concluded that the change in the SERS spectra corresponded to two different adsorption states of pyridine and that the adsorption states rather than the adsorption species are responsible for the potential dependence. This result indicates that a particular vibrational band of a molecule can be highlighted by a change in the electrode potential. This change as a function of the Ag electrode potential at 514.5 nm for 0.05 M pyridine and 0.05 M quinoline in 0.1 M KCl electrolyte is shown in Figure 1. In both cases, no electrochemical reactions were observed. For example, in the case of pyridine and quinoline, at a certain electrode potential of
5c;
l00C CM-I
CM
Figure 1. The SERS spectra on Ag electrode as a function of electrode
potential for pyridine (left)and quinoline (right). The quinoline spectra have been multiplied by a factor of 3.3. l-
I
-
l5C"
IC?-
0300 CPS
5-2
CCI
Flgwe 2. The SERS spectra of a mixture of contaminantsat electrode
potentials of -1.0 (top)and -0.6 (bottom)V and an excitation wavelength of 514.5 nm. The arrows and the dots indicate the major peaks due to pyridine and quinoline, respectively.
-1.4 V the SERS spectrum was strong for quinoline, but there was very little signal for pyridine. This effect adds further analytical selectivity to the SERS technique. We obtained the SERS spectra at 488 and 514.5 nm for all the compounds in Table I at various electrode potentials (0.0 to -1.4 V). All of the compounds that exhibited the SERS effect showed some potential-dependent effects of peak positions and intensities that could be useful in identification. In the interest of brevity, only the quinoline and pyridine spectra are shown in detail in Figure 1. On the basis of the results of the electrode potential study, we chose to study a mixture of pyridine and quinoline. They were selected on the basis of the structural similarities of the two compounds. Both compounds have aromatic nitrogen heterocycles as their bases, but quinoline contains an additional aromatic ring. The compounds were dissolved in various proportions in the aqueous electrolyte. Figure 2 shows the SERS spectra on a Ag electrode of the mixture (0.03 M pyridine and of 0.02 M quinoline in a 0.1 M KCl electrolyte) at potentials of -0.6 and -1.0 V. The arrows indicate peaks that are due to pyridine, and the dots indicate peaks that are due to quinoline. Table I1 contains a list of peak positions and possible vibrational assignments. The results in Figure 2 and Table I1 verify that the SERS technique on Ag electrodes can be used as a one-dimensional identification tool
ANALYTICAL CHEMISTRY, VOL. 59, NO. 21, NOVEMBER 1, 1987
1
Table 11. Spectral Identification by Peak Position of a Mixture of Pyridine and Quinoline at Various Electrode Potentials Deak aosition -0.6 V mix quinolineb py
py”
622 ~3 650 vi2
518 622 650
518 ~ 2 9 622 v ~ O 692 vi0
752 ~ 764
2 3
752 764 782 950
764 ~ 2 782 V26 950 ~
7
1004 VI
518 620 650 696
762
762
942
942
696 740 762 780 942
1000
996
1014 1030 1040 1064
1014 1034 1040
1032 v6 1064 vg 1140 vi6 1212v, 1272 1370~15
1477 vg 1564 vi4 1593 ~4 1620
1035 1045 1064 1144 1212 1230 1272 1324 1370 1380 1436 1460 1477 1574 1593 1620
518 620
2 5
1004
1003 1018 ~ 1038 ~ 1045
2 4 2 3
1028 1064
1072 1144 ~
2 0
1146 1210
1230
1370 ~ 1 1388 ~ 1 1436 vi3 1460
5
1372
4
1476 1500 vi1 1574 vi0 1593 vg 1624 VS
1570 1586
1076 1140
1146 1210 1230
1230
1312 1330 1372 1390 1431 1458 1476
1312 1372 1390 1431 1458 1496 1570 1586 1624
1570 1586 1620
aAssignmentsfrom ref 4. *Assignments from ref 31.
1I
1500
IO00 CM-l
0030 CPS
-1.0 v mix quinoline
620 650 696
1000 v32
i
2501
!O,OOO CP5
500
The SERS spdctra of 0.05 M pyridine (top) and 0.05 M quinoline (bottom) at an excitation wavelength of 488 nm and an electrode potential of -0.6 V. Figure 3.
similar to the classic identification methods of infrared or normal Raman spectroscopy. However, the implementation of varying the electrode potential would add another dimension to the identification process. The use of multiple wavelengths, which would add another dimension to the analysis, was also demonstrated on a mixture. When the excitation wavelength was changed to 488 nm, the peaks that belong to quinoline were not observed. This is possible since quinoline does not exhibit the SERS effect at an excitation wavelength of 488 nm as shown in Figure 3. A major concern in using SERS to detect groundwater contamination is whether there will be interferences. Interferences caused by the interaction of several contaminants on the SERS substrate could be overcome by using a matrix
1500
1000 CM-I
500
Flgure 4. The SERS spectra of a mixture of humic acid and pyridine (top) and of a solution of humic acid (bottom). The excitation wavelength is 514.5 nm.
of SERS conditions (i.e., potential, wavelength, substrate) combined with a computerized spectral identification method. Computerized methods have been used in infrared analysis by Puskar and co-workers to identify a five-component mixture of organic compounds (21). The presence of naturally occurring humic material is also of concern as an interference to the SERS technique. We mixed various concentrations of pyridine with a 1 ppm concentration of humic acid. Since the humic acid structure is ill-defined, the mixture was made with a typical subsurface water concentration of 0.3 ppm of total organic carbon (TOC) which corresponds to 1 ppm of the Aldrich sample if it contains 30% TOC. Figure 4 shows the SERS spectra of the mixture (1ppm humic acid and 1580 ppm pyridine) and of 10000 ppm humic acid both in a 0.1 M KCl electrolyte. As evident from the figure, the SERS spectrum of pyridine seems not to be inhibited by the presence of humic acid. The lack of interference is probably related to more negative AGads for the small aromatic molecule as well as a large concentration difference due to the large molecular size of the humic compound (i.e., smaller number of moles at similar parts per million levels). A concentration study was done on pyridine and quinoline, two typical contaminants. For this study, the signal from a strong peak was monitored as a function of concentration in the solution on a Ag electrode at a potential of -0.6 V. In previous concentration studies of Vo-Dinh and co-workers (5-7), a specific volume (3 pL) of various concentrations of material in solution was evaporated on the SERS substrate. In our experiment, the sample is in a dynamic equilibrium with the SERS substrate. Chen and co-workers have shown that the SERS signal for pyridine can be related to a simple Langmuir adsorption isotherm (22). They determined that the AGads for pyridine was -5.7 kcal/mol on a Ag electrode. The simplified Langmuir isotherm relates the surface coverage (e),concentration (C), and the free energy of adsorption by the following equation:
e/(l - e) = (C/55.7)e-AG/RT
(1) The fractional coverage is the ratio of the measured coverage (r)to the maximum coverage (rmax) e = r/rmaXR where R is the roughness factor which has been determined for Ag electrodes to be between 1.5 and 2.5 (23). The quantity rmax is usually calculated on the basis of the molecular size and assumed orientation. For a pyridine molecule lying flat on a surface, rmax for a monolayer of coverage can be estimated from the rmaX for benzene which is 5.0 X mol/cm2 (18). For quinoline at 1372 cm-’, the lowest concentration that we were able to observe was 5.7 X 10+ M (700 ppb). The signal versus concentration curve for quinoline is shown in Figure 5. The lowest concentration of pyridine at 1006 cm-l that
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 21, NOVEMBER 1, 1987
*
3
4
2
6
PPM
Figure 5. The SERS signal of qulnollne for the 1372 cm-' peak on a Ag electrode (-0.6 V) vs concentration. The bars indicate the range of the data points, which are an average over a 4-9 period at an electrode potential of -0.6 V.
we were able to obtain with our systems was 5 X M (4 ppm). Combining the previous result with eq 1and the AG,* of pyridine, the surface coverage of the electrode was determined to be 0.014 monolayer. This coverage level is slightly higher than the 1/100 of a monolayer which was reported as the minimum amount of surface coverage for SERS (3). When eq 2 is applied with the surface coverage we determined for pyridine, the r value would be 1.4 x lo-" mol/cm2 if an average value of 2 is used for the surface roughness (R)and the molecule is lying flat on the electrode. Taking into account the area of our electrode (7.8 X cm2), a detection limit of 8.5 pg is possible with this technique for pyridine. Since the collection efficiency (0.1 sr) of our Raman system is one factor that limits our detection limit, the detection limit may be extended even lower with improved collection optics. A major concern in using SERS with an electrochemical surface for an in situ application is whether the electrode surface can be renewed at a low ionic strength. To test this question, a solution of 0.025 M pyridine in local drinking water was examined. The solution had conductance of 7.9 X lo* S, and the water had a conductance of 6.0 X lo4 S . These values are two orders of magnitude less than our typical laboratory samples which have 0.1M KC1 added as the supporting electrolyte. In comparison, distilled or deionized water usually has a conductance of lo4 S cm-l while surface and groundwaters have a range of conductances of 5 X to 5 X S cm-' (24). The renewability is dependent on the geometry of the electrode and on the potentiostat power supply. Figure 6 shows the electrochemical recycling of a Ag electrode in a low ionic strength solution. The cycling monitors the intensity of the 1006 cm-' peak of a 0.025 M solution of pyridine in tap water as a function of oxidation/reduction cycle (-0.6 to +0.2 V). The small size of our electrode (7.85 x cm2) allows it to be recycled with only 7 pA of current from our potentiostat (maximum current 500 FA) in a low ionic strength medium. Another concern is using the SERS to detect groundwater contamination is whether there will be interference of the SERS signal of a contaminant by background fluorescence. During the course of experiments, we had no interference due to fluorescence at 488 or 514.5 nm. And if longer wavelengths are used in future studies, then the chances of fluorescence will be diminished since most organics absorb toward the UV region of the spectrum.
1
CONCLUSION Our experiments indicate that SERS on Ag electrodes could be used as a multivariable analysis technique to detect typical organic contaminants in H20. Other applications of the technique could be in the determination of hydrological transport and rates of movement in a subsurface system.
I
l
l
-0.6 t0.2 -0.6
ELECTRODE POTENTIAL Figure 8. The electrochemical recycling of ionic strength solution.
a Ag
electrode in a low
Developments of this method that are now in progess include extending the analysis to different SERS substrates, construction and testing of a fiber optic probe for remote SERS analysis, and the development of a rule-based decision tree program to identify components of a complex mixture. ACKNOWLEDGMENT We express our appreciation to Robert Carter, University of Massachusetts-Boston, for technical assistance. Registry NO.HZO, 7732-185; C&$N"H,62-53-3;C&, 71-43-2; CCld, 56-23-5; CH30H, 67-56-1; CBH,jOH, 108-95-2; CeH,jN02, 98-953; CsH&H3,10888-3; benzofuran, 271-89-6;benzothiophene, 11095-43-5; indole, 120-72-9; naphthalene, 91-20-3; pyridine, 110-86-1;quinoline, 91-22-5. LITERATURE CITED (1) "Research Summary Subsurface Transport Program", DOE Report DOE/ER-0156/3, Sept 1985. (2) Fleischmann, M.; Hendra, P.; Mcqulllan, A. Chem. Phys. Lett. 1974, 2 6 , 163. (3) Chang, R., Furtak, T., Eds. In Surface Enhanced Raman Scattering; Plenum: New York, 1982. (4) Pockrand, I. In Surface Enhanced Raman Vibrat&nal Studies and SolidlGas Interfaces; Springer-Verlag: Berlin, 1964. (5) Vo-Dinh, T.; Hiromoto, M.; Begun, G.; Moody, R. Anal. Chem. 1984, 56, 1667. (6) Vo-Dinh, T.; Meier, M.; Wokaum, A. Anal. Chim. Acta 1986, 787, 139. (7) Enlow, P.; Buncick, M.; Warmack, R.; Vo-Dinh, T. Anal. Chem. 1986, 5 8 , 1119. (8) Jennlng, C.; Aaroca, R.; Hor. A.; Loutfy, R. Anal Chem. 1984, 5 6 , 2033. (9) Tran, C. Anal. Chem. 1984, 56, 824. (IO) Shin, 0.; Kim, J. Chem. Phys. Lett. 1985, 720, 569. (11) Shin, G.; Kim, J. Surf. Sci. 1985, 758, 286. (12) "The Selection of Organic Chemicals for Subsurface Transport Research", DOE Report DOE/ER-0217, Dec 1984. (13) Otto. A. In Light Scattering In SolMs, Cardona, M., Guntherodt. G., Eds.; Springer-Verlag: Berlin, 1984; p 289. (14) Gao, P.; Patterson, M.; Tadayyoni, M.; Weaver, M. Langmuir 1985, 7 , 173. (15) Loo, B.; Lee, Y. Chem. Phys. Lett. 1984, 772, 580. (16) Piersma, 8. In Electrosorption: Glleadi, E., Ed.; Plenum: New York, 1977; Chapter 2. (17) Dahms. H.; Green, M. J . Electrochem. SOC. 1963, 110, 1075. (18) O'M. Bockrls, J.; Green, M.; Swinkels, D. J . Electrochem. SOC. 1984, 1 7 7 , 743. (19) O'M. Bockrls, J.; Swinkel. D. J . Electrochem. SOC. 1964, 7 1 7 , 736. (20) Crelghton, J. Surf. Scl. 1983, 724, 209. (21) Puskar, M.; Levine, S.; Lowry, S. Anal. Chem. 1986, 5 8 , 1156. (22) Chen, C.; Helm T.; Rlcard, D.; Shen, Y. Cbem. Phys. Lett. 1961. 8 3 , 455. (23) Weaver, M.; Farquharson, S.; Tadayyoni, M. J . Chem. Phys. 1985, 8 2 , 4867. (24) Hem, J. Study and Interpretation of Chemical Characteristics of Natura/ Waters, 2nd ed.; U.S. Government Printing Office: Washington, 1970; p 102. (25) Shlndo, H. J . Chem. Soc., Faraday Trans. 1 1986, 8 2 , 45. (26) Howard, M.; Cooney, R. Chem. Phys. Lett. 1982, 8 7 , 299. (27) Nishlhara, C.; Shindo, H. J . Nectroanal. Chem. 1986, 202, 231. (28) Marinyuk, V.; Lararenko-Manevich, R.; Kolotyrkin, Y. J . Electroanal. Chem. 1980, 110, 111. (29) Fleischmann, M.; Hill, I. In Surface Enhanced Raman Scattering; Chang, R., Furtak, T., Eds : Plenum: New York, 1982; p 275.
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Anal. Chem. 1987, 59, 2563-2566 (30) Girlando. A.; Gordon, J.: k i t m n n , D.; Philpott, M.; Seki, H.: Swalen, J. Surf. Sci. 1980, 101, 417. (31) Wait, S., Jr.; McNerney, J. J . Mol. Spechosc. 1970, 3 4 , 56.
RECEIVED for review February 6,1987. Accepted July 13,1987.
This work was supported by the Office of Health and Environmental Research and Ecological Research Division of the Department of Energy under SBIR Contract No. DE-ACO186ER80333
Long Path Length Absorption Measurements in Thin Dielectric Films Dennis A. Stephens and Paul W.Bohn* Department of Chemistry, University of Illinois, Urbana, Illinois 61801
Configuring a thin flim sample as an optical waveguide enhanced the path length in a transmission-based absorption experiment by ca. lo5 over that obtained by passing the radiation perpendicular to the fllm plane. Multiple wavelength measurements were used to remove the v4 scattering background and permitted quantitative determinations of Pia+ by uslng the E,, eigenmode in micrometer-sized poly(vinyipyrroiidone) (PVP) films. Conditions under which the absorbance measurements can be related to concentrations were ascertalned. A ray optics model of the scattering losses was validated by measuring the surface and volume scattering correlation lengths to be 0.14X parallel to the plate and 0.87X perpendicular to the plate with a surface-to-volume scattering ratio of 0.10 in a typical PVP film. Measurements at several different modes were then used to separate the surface and volume contributknsto the scattering loss. Good agreement was obtained between values predicted from the ray optics model and experimentally measured values for undoped films. For doped films the model failed due to the nonhomogeneous spatlai distribution of absorbers.
Absorption measurements have traditionally been extremely difficult to perform on thin film samples, primarily due to the short path lengths involved in transmission-type experiments. As an example consider the measurement of the absorption of localized states caused by trace chemical impurities in unintentionally doped GaAs. A typical sample grown by molecular beam epitaxy might contain 2 X 1014cm-3 CAS acceptors and have a thickness of 1 hm. In the usual transmission measurement made on etched substrates the localized states would require an absorption cross section of 5 X cm2 (equivalent to a huge molar absorptivity of 1.44 X lo7 L/(mol cm)) in order to generate a barely detectable linear absorption of Thus it is no surprise that to date only free exciton formation has been observed in absorption below the band-gap absorption edge (1-4). Clearly the standard method of passing radiation perpendicular to the thin film is inadequate for measuring localized state absorption. Another example in which path length is a limiting factor is the measurement of the active site concentration in polymercoated electrodes (5, 6). An increase in the path length available for absorption experiments would lower the detection limits of, for example, redox sites used in polymer-mediated electrocatalysis (7). Two approaches have traditionally been taken to enhance the absorption of weakly absorbing samples in transmission-type measurements: increasing the inherent ,0003-2700/87/0359-2563$01.50/0
absorption cross section through chemical transformation, or simply increasing the path length (8). The former approach is untenable in most thin films, because the sites to be measured are distributed throughout the solid matrix and are not available for chemical reaction, so efforts must be directed toward either increasing the path length or using calorimetric methods (9-12). In this work we increase the path length by using the thin film of interest as the active layer in a threelayer dielectric optical waveguide (13-15). Because the radiation propagates parallel, rather than perpendicular, to the film plane, path length enhancements of 10'106 are obtained. The increase in path length is gained a t a cost however. Although methods have been proposed (16), it is extremely difficult to incorporate a reference path in a thin film sample. Thus, new techniques must be used to correct for scattering losses. Our work has focused upon two separate approaches to the problem. The first is a multiple wavelength technique, in which total loss is determined in both absorbing and nonabsorbing regions of the spectrum and the scattering background eliminated by fitting it to a v4 function. The other approach allows a separate estimate of the volume and surface contributions to the total scatter to be made and is based on a ray optics approximation (17). Since this model assumes that the correlation lengths of the random fluctuations producing the scattered light are shorter than the separation between successive interface reflections in the ray optics model (see Figure l ) , it is important to measure these correlation lengths. We have obtained them by fitting measurements of the angular distribution of the in-plane scattered radiation in the air region to the theory developed by Imai and coworkers (18-21). Work reported here had as its goal the development of long path length absorption techniques for thin films, so all work has been carried out on a model system composed of poly(viny1pyrrolidone) (PVP),both undoped and doped with Pr3+.
THEORY The irradiance of the electric field traveling in the + x direction within an optical waveguide is observed to decay exponentially
I = Ioe-aTx
(1)
where x = the distance from the incoupling spot, and Io = the incident intensity of the incoupled radiation. The total loss coefficient, aT, can be expressed as
+ ass + avs
(2) showing that the total loss is due to the sum of absorptive, surface scatter, and volume scatter terms (17,22,23). The aT = a A
0 1987 American Chemical Society
