Volume 56, Number 3, 2002 APPLIED SPECTROSCOPY 2950003-7028 / 02 / 5603-0295$2.00 / 0q 2002 Society for Applied Spectroscopy

Comparison of Organic Self-Assembled Monolayers asModi� ed Substrates for Surface-Enhanced InfraredAbsorption Spectroscopy

JOHN A. SEELENBINDER* and CHRIS W. BROWN†Department of Chemistry and Partnership for Sensors and Surface Technology, University of Rhode Island, Kingston, Rhode Island02881

The effect of using self-assembled monolayers to facilitate the phys-ical and chemical adsorption of analytes onto surface-enhanced in-frared absorption (SEIRA) gold substrates has been investigated.Self-assembled monolayers of thiopropane, thiohexane, and thio-phenol were tested to determine the response for short and medium-size normal alkanes monolayers and aromatic monolayers. The an-alytes consisted of 4,6-di� uoro-nitrobenzene, o-dinitrobenzene, 4,6-dinitro-o-cresol, and tri-nitrobenzene. Spectral results for the self-assembled monolayers were compared to those obtained with anuncoated gold SEIRA surface and with typical transmission spectraof the same test samples. Some enhancement was obtained in allcases; however, the aromatic thiol monolayer produced the largestenhancements, with a maximum increase of 21 fold. As a practicalapplication of the SEIRA technique, a prototype sensor for the de-tection of trinitrotoluene (TNT) was prepared and tested.

Index Headings: Self-assembled monolayers; Surface-enhanced in-frared absorption; SEIRA; Organic sensors; Infrared spectra; Ar-omatic compounds; FT-IR.

INTRODUCTION

Surface-enhanced infrared absorption (SEIRA) spec-troscopy has been the topic of considerable recent re-search.1–16 Despite the growing interest in the technique,few practical applications have been developed. A num-ber of recent studies have demonstrated the usefulness ofSEIRA spectroscopy in practical applications;7,9–11,16 how-ever, the technique is still not commonly used.

Dif� culties with the reproducibility of SEIRA sub-strates must be addressed before SEIRA spectroscopy be-comes widely accepted. SEIRA spectra can vary greatlydepending on small variations in metal thickness andmorphology,12 heating effects on the metal � lm,2 and an-alytes adsorbing onto the metal in multiple con� gura-tions.3 Several attempts have been made to quantify theseeffects and develop new SEIRA substrates.1–4,8,9,12,14 Re-cent research has included the use of colloidal sub-strates,8,14 the effect of annealing,2,3 the addition of anti-bodies to the substrate in order to detect an analyte,8,9 theuse of new metals12 and electroplated metals,4,13 and theeffect of surface morphology.1

The present investigation approaches the substrateproblem from a new perspective by coating the metallayer with a self-assembled monolayer of organic thiols.Although the coatings do not address problems associatedwith metal irregularities, they do facilitate reproduciblebinding of the analyte to the metal surface. This technique

Received 9 June 2000; accepted 13 November 2001.* Current Address: Digilab, 68 Mazzeo Drive, Randolph, MA 02368.† Author to whom correspondence should be sent.

has been used successfully with surface-enhanced Ramanspectroscopy (SERS) to detect chlorinated hydrocarbons,16

alkali metal ions,17 and aromatic compounds.18–22 In aSERS study of aromatic hydrocarbons adsorbed onto asaturated hydrocarbon coating, Carron et al.19 found thio-propane gave a greater enhancement than dodecanethiol.A monolayer of thiopropane produced a greater enhance-ment because the analyte is held closer to the SERS sur-face.

Recently, we 23 showed that self-assembled monolayersof thiophenol on gold could serve as a substrate for mea-suring SEIRA spectra of phthalates and nitro-substitutedaromatic compounds. In the present investigation, wecompare the enhancements for three different organic thi-ol coatings: thiopropane, thiohexane, and thiophenol.SEIRA spectra of di� uoro-nitrobenzene, o-dinitroben-zene, and dinitro-o-cresol were measured on conventionalgold SEIRA substrates and gold SEIRA substrates thatwere coated with the three organic thiol coatings. En-hancements due to different coatings are compared andmechanistic reasons for the enhancements are suggested.Using the results of this comparison, an organic thiolmodi� ed SEIRA substrate for the detection and quanti-� cation of trinitrotoluene (TNT) was explored.

SEIRA: Electromagnetic and Chemical Mecha-nisms. Enhancement of infrared absorption occurs whenchemicals are adsorbed onto, or in close proximity to,thin � lms of gold, silver, and other transition metals. Theenhancement is believed to originate from two distinctmechanisms: electromagnetic and chemical.1,25 The elec-tromagnetic enhancement is generally accepted to be along-range effect, enhancing the adsorption of severalmonolayers of analyte. The chemical mechanism is short-range and affects only a monolayer of analyte.24

The thin metal � lms involved in SEIRA enhancementsare known to exist primarily as metal ‘‘islands’’.24 IRradiation incident on a metal ‘‘island’’ establishes an os-cillating dipole. This dipole, in turn, produces an electro-magnetic � eld that is perpendicular to the ‘‘island’’ at allpoints. Metal ‘‘islands’’ that are close to one another willcreate in-phase � elds. The collective resonance of the in-phase � elds causes an order of magnitude enhancementin the intensity of the incident IR radiation. This increasein intensity is the basis of the electromagnetic mecha-nism.

The basis of the chemical mechanism is not well un-derstood. Osawa has suggested that the chemical mech-anism could be divided into two parts.24 First, a three-fold enhancement can be attributed to the alignment of

296 Volume 56, Number 3, 2002

FIG. 1. Spectra of 2,4-di� uoronitrobenzene (3.6 mg/cm 2): (a) SEIRAusing a thiophenol coated gold surface, (b) SEIRA using a thiopropanecoated gold surface, (c) SEIRA using an uncoated gold surface, (d )SEIRA using a thiohexane coated gold surface, and (e) transmissionusing a KBr window.

the analyte with the perpendicular electromagnetic � eld.The analyte, p-nitrobenzoic acid, adsorbs only throughthe CO2 group; the symmetric COO2 stretch and the sym-metric NO2 stretch are aligned with the � eld perpendic-ular to the metal island.24 The other ten-fold enhancementattributed to the chemical mechanism was thought to bedue to a change in vibrational polarizability or a chargetransfer mechanism.24

Grif� ths and co-workers also studied the effects ofchemisorption on SEIRA spectra.1 They noted differencesbetween SEIRA and adsorption spectra of p-nitrophenol.These changes were attributed to electron donation to theanalyte by the metal � lm.1 The changes were similar tothose observed in spectra of nitro-aromatics in electrondonating solvent. The authors note that the donor–accep-tor mechanism explains the large enhancement of ab-sorptions involving strong electron withdrawing groups.1

EXPERIMENTAL

The SEIRA spectra were measured using external re-� ection geometry with a gold coated silicon plate as abase substrate. Silicon was chosen as the substrate; how-ever, calcium � uoride or germanium could also be used.The silicon plates were washed in a four step cleaningprocess comprising a nitric acid wash, a hydro� uoricacid/water wash, a sulfuric acid/hydrogen peroxide wash,and a hydro� uoric acid/methanol wash. These plates werethen sputter coated with gold using an MRC 8667 Mul-titarget Sputtering System (Material Research Corp., Or-angeburg, NY). The sputtering rate was controlled to givea uniform gold thickness of 10 nm. Previous studies24,25

have shown that 10-nm thickness results in isolated Auislands, which leads to greater enhancements. Followingthe coating, the plates were rinsed with methanol.

The gold � lms on the silicon plates were coated witha self-assembled monolayer of thiopropane, thiohexane,or thiophenol. All organic thiols were purchased fromAldrich Chemical Company (Milwaukee, WI) and dilutedto 5 mg/mL in absolute ethanol (Quantum Chemical Co.,Tuscola, IL). The silicon–gold plates were soaked in therespective organic thiol solution for three days, thenrinsed with ethanol and allowed to air dry before use.Thiophenol is highly toxic through both skin adsorptionand inhalation. Care was taken to insure that workerswere not exposed to the liquid or vapor.

4,6-Dinitro-o-cresol (DNOC) was obtained from ChemServices (West Chester, PA). o-Dinitrobenzene (DNB)and 2,4-di� uoro-nitrobenzene (DFNB) were obtainedfrom Aldrich Chemical Company (Milwaukee, WI). Mil-itary grade trinitrotoluene (TNT) was obtained from thelaboratory of J. Smith, University of Rhode Island. Theanalytes were diluted to 0.03 mg/mL in cyclohexane (EMScience, Gibbstown, NJ) or acetone (Fischer, Fair Lawn,NJ) in the case of TNT. The solutions of analytes weredispensed in quantities of 100 to 400 mL onto the thiolcoated substrates. The solvent was allowed to evaporate,leaving either a thin layer of solid analyte, as in the caseof DNOC and DNB, or an evenly dispersed layer of liq-uid analyte, as in the case of DFNB. Spin-coating wasused to prepare a uniform distribution of the samples.While the solvent was evaporating, the sample plate wasspun at 15 rpm.

SEIRA spectra were measured using the organic thiolcoated gold–silicon plates in an external re� ection con-� guration with a sample size of 4 cm 2. The samples wereilluminated with p-polarized radiation. Re� ection mea-surements were made using a Foxboro–Wilks (Foxboro,MA) Model 9D re� ection accessory with an incident an-gle of 728 from normal. The Brewster angle for Si is73.68. Previously, it has been shown that optimum en-hancements for p-polarized radiation are obtained at an-gles slightly less than the Brewster angle.25 Moreover, inour studies of antibodies on gold coated silicon win-dows,9 we found that 728 produced optimum enhance-ments. For comparison, transmission spectra were mea-sured of � lms cast on potassium bromide windows. Spec-tra were measured on a Bio-Rad FTS-40 FT-IR spectrom-eter (Cambridge, MA), coadding 65 scans at 4 cm21

resolution.

RESULTS AND DISCUSSION

4,6-Di� uoro-nitrobenzene (DFNB). Spectra of DFNBmeasured in the transmission mode, as SEIRA spectrausing thiopropane, thiohexane, and thiophenol coatedsubstrates and as a SEIRA spectrum using an uncoatedgold covered substrate, are shown in Fig. 1. Bands fromthe aromatic ring are observed near 1560, 1500, 945, 830,and 780 cm21. The anti-symmetric NO2 band is observedat 1540 cm21 while the symmetric stretch is observednear 1350 cm21. Based on the anti-symmetric NO2

stretch, the uncoated gold � lm, thiopropane, and thio-hexane coated gold � lms all enhanced the absorption bya factor of approximately 1.5 fold. The thiophenol coated� lm, however, enhanced the spectra by over 10 fold.Compared to the other two analytes (discussed below),few orientation effects are observed for the DFNB onthiophenol.

DFNB is a slightly volatile hydrophobic liquid. Onboth the KBr transmission window and the uncoated gold� lm, the liquid beads up as small droplets. The coveragewas only marginally better with both the thiopropane andthiohexane coated substrates; however, the analyte ap-peared as an evenly dispersed liquid on the thiophenolcoated SEIRA substrate. In the case of DFNB, the thio-

APPLIED SPECTROSCOPY 297

FIG. 2. Spectra of o-dinitrobenzene (2.8 mg/cm 2). (a) SEIRA on thio-phenol coated gold surface, (b) SEIRA on thiopropane coated gold sur-face, (c) SEIRA on thiohexane coated gold surface, (d ) SEIRA on un-coated gold surface, and (e) transmission on a KBr window.

FIG. 3. A pictorial representation of possible orientations of o-di� uo-ronitrobenzene on an organic thiol coated gold surface.

FIG. 4. Expansion of the symmetric NO2 vibrational band of o-dini-trobenzene: (a) thiophenol coated gold, (b) thiopropane coated gold, (c)thiohexane coated gold, and (d ) uncoated gold.

phenol coating serves to hold the analyte to the plate. Onan uncoated surface, the DFNB was too volatile to showsigni� cant enhancement, while the thiopropane andthiohexane coatings did not bind the DFNB suf� cientlyto reduce the volatility.

o-Dinitrobenzene (DNB). Spectra of DNB are shownin Fig. 2. Two major bands are observed for the anti-symmetric and symmetric NO 2 stretches of DNB near1530 cm21 and 1360 cm21, respectively. Different en-hancement factors are seen for each of the SEIRA sub-strates. Based on the anti-symmetric NO2 stretch, an un-coated gold substrate enhances the infrared absorptionover 3 fold. The thiohexane coated gold substrate alsohas an enhancement factor of 3 fold, whereas thiopropanewas slightly better, at over 11 fold, and thiophenol at 21fold.

Differences in relative intensities of the anti-symmetricand symmetric stretches are also observed. In the trans-mission and uncoated SEIRA spectra, the anti-symmetricstretch is observed to have twice the intensity of the sym-metric stretch. When organic thiol coatings are used, theintensity of the anti-symmetric NO2 stretch is nearlyequal to that of the symmetric stretch. Since DNB has nofunctional groups that will preferentially attach to goldand the spectrum of DNB on uncoated gold matches thetransmission spectrum of a cast � lm, the geometry of theanalyte on the gold surface is assumed to be random.

The ratio of the anti-symmetric NO2 band to the sym-metric NO2 band of DNB adsorbed onto organic thiolcoated SEIRA surfaces is near 1:1 in all three cases. InSEIRA spectra,26 vibrations that are perpendicular to thesurface of a metal island will be preferentially enhanced.Two possibilities for the attachment geometry of DNBon an organic thiol surface are shown in Fig. 3. The ge-ometry of Fig. 3A allows for a larger enhancement ofthe symmetric stretch over the anti-symmetric stretch,whereas in the geometry of Fig. 3B, both the symmetricand anti-symmetric NO2 stretches are expected to be en-hanced by an equal amount. Furthermore, in the case ofa thiophenol coating, the DNB would preferentially at-tach by the geometry given in Fig. 3A, as this wouldproduce the optimum pi–pi coupling. Since the relativeintensities of the NO2 stretches are similar for all organic

thiol coated SEIRA spectra of DNB, the adsorption isbelieved to occur with the geometry given in Fig. 3A.

Comparison of the symmetric NO 2 stretch near 1360cm21 in the uncoated gold and gold coated with thio-hexane, thiopropane, and thiophenol show that the bandbecomes sharper and shifts to higher energy. An enlarge-ment of the symmetric stretching region for the four sep-arate SEIRA substrates is shown in Fig. 4. The SEIRAspectrum of DNB on an uncoated gold substrate has atriplet band for the symmetric NO2 stretch near 1360cm21. Two bands of the triplet arise from the in-phaseand out-of-phase symmetric stretches of the adjacent NO2

groups, while the lowest frequency band of the triplet isdue to self-association or Fermi resonance.26 The SEIRAspectrum of DNB on a thiohexane coated gold–siliconwafer contains a doublet with one maximum at 1368cm21 and the other maximum at 1357 cm21; the higher-frequency band of the doublet is 25% stronger than thelower-frequency band. The SEIRA spectrum of DNB ona thiopropane coated gold–silicon wafer also contains thedoublet with the higher-frequency band twice as large asthe lower-frequency band. The thiophenol coated SEIRAsubstrate spectrum of DNB is observed to contain mainlythe high-frequency component of the symmetric NO2

stretching band, with only a small shoulder correspondingto the lower frequency component.

The increase in the higher frequency portion of thesymmetric NO2 stretch follows the increase in enhance-

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FIG. 5. Spectra of 4,6-dinitro-o-cresol (3 mg/cm 2): (a) SEIRA on thio-phenol coated gold � lm, (b) SEIRA on thiopropane coated gold � lm,(c) SEIRA on uncoated gold � lm, (d ) SEIRA on thiohexane coatedgold � lm, and (e) transmission on a KBr window.

FIG. 6. A pictorial representation of possible geometries of 4,6-dinitro-o-cresol on an organic thiol coated gold surface.

ment factors between different organic thiol coated SEI-RA substrates. In the case of the thiophenol, the DNBmolecule is held strongly to the coating through pi–pistacking. Since the rotation of thiophenol molecules on agold surface is limited due to pi–pi stacking between ad-jacent coating molecules, the DNB analyte is held in arelatively rigid position with respect to the gold surface.If the geometry given in Fig. 3A is assumed, the sym-metric vibration of the NO2 group in the 1 position willbe enhanced over the symmetric vibration of the NO2

group in the 2 position, as the former group is perpen-dicular to the surface, whereas the latter group is at anangle of 608 to the gold surface. Only a single band isobserved for the symmetric NO2 stretch of DNB on thio-phenol because only one stretch (of the two possiblestretches) is enhanced. In the case of the thiopropane,limited rotation of the coating around single bonds is pos-sible; therefore, both stretches experience some enhance-ment, although the single stretch in the 1 position is stillenhanced to a greater extent. The thiohexane coating con-tains the most � exibility; therefore, the doublet is of near-ly equal intensity.

4,6 Dinitro-o-cresol (DNOC). Spectra of DNOC areshown in Fig. 5. Several prominent bands are observedin the DNOC spectra. The band near 1605 cm21 is due

to the aromatic ring quadrant stretch; this band is en-hanced in the case of the uncoated, thiopropane, and thio-phenol SEIRA substrates, but it remains unchanged in thespectrum measured on a thiohexane coated gold surface.The anti-symmetric NO2 stretch is observed near 1550cm21 in all of the SEIRA spectra but remains weak inthe transmission spectrum. The symmetric NO2 stretchingband at 1343 cm21 is observed in all spectra, althoughthe band shape and position changes slightly. A broadband is observed in the SEIRA spectra centered near1246 cm21. This band is assigned to the CO stretch andis observed only weakly in the transmission spectrum.

The anti-symmetric NO2 stretching and the CO stretch-ing vibrations experience the greatest enhancements. Aswith the other two analytes, the SEIRA spectrum ofDNOC was enhanced the largest amount using a thio-phenol coated gold substrate. Based on the anti-symmet-ric NO2 stretch, the thiophenol coated surface enhancedthe adsorption 20 fold. The thiopropane coated surfaceprovided an 11-fold enhancement, while the thiohexanecoated surface provided a 4.5-fold enhancement. The un-coated gold surface provided a larger enhancement thanthe thiohexane, at 9 fold. DNOC has an –OH functionalgroup, which can dissociate a proton and bind to a goldsurface.1 Binding through the CO– group holds theDNOC close to the surface, resulting in a larger enhance-ment.

The CO stretching vibration of DNOC is also mostgreatly enhanced with the thiophenol coating. Enhance-ment factors for the CO stretch on thiohexane, uncoatedgold, thiopropane, and thiophenol are 1.2, 3.1, 3.5, and6.3, respectively. Again, the hexane reduced the enhance-ment while the thiophenol doubled the enhancement.Thiopropane and uncoated gold produced nearly the sameenhancement.

The strong enhancement of the anti-symmetric NO2

stretch compared to the weaker enhancement of the sym-metric NO 2 stretch determines the orientation of theDNOC on the SEIRA active surfaces. Four possible ori-entations are given in Fig. 6. Due to the surface selectionrule, orientations represented by A and B are expected toproduce greater enhancement for the symmetric NO2

stretch than the anti-symmetric stretch. Additionally, ori-entation A places the polar NO 2 group within the non-

APPLIED SPECTROSCOPY 299

FIG. 7. Spectra of trinitrotoluene (TNT) (5 mg/cm 2). (a) SEIRA onthiophenol coated gold � lm, (b) SEIRA on uncoated gold � lm, and (c)transmission on a KBr window.

polar coating. Orientations represented by C and D areexpected to produce the greatest enhancement for theanti-symmetric NO2 stretch, as the change in dipole mo-ment for the anti-symmetric stretch is perpendicular tothe surface. Orientation C still places two polar NO2

groups within the nonpolar coating. Another possible ori-entation places the aromatic ring parallel to the coatingsurface; however, this would exclude both the anti-sym-metric and symmetric NO2 stretches. Also, the parallelgeometry would allow enhancement of out-of-plane ar-omatic CH bending, which is not observed. The mostlikely orientation of DNOC on the three organic thiolcoatings, considering both the SEIRA spectra and the po-larity of the coating and analyte, is given in Fig. 6D.

Detection of TNT. In each of the above cases, thegreatest enhancement was obtained using a thiophenolcoated gold substrate. For the detection of TNT, the thio-phenol coated gold substrate was compared to an un-coated gold substrate and standard transmission geome-try. Spectra of 5 mg/cm 2 military grade TNT are shownin Fig. 7. The largest features observed were the anti-symmetric NO2 stretch near 1545 cm21 and the symmetricNO2 stretch near 1350 cm21. Small bands were also notedfor the aromatic ring modes near 900 and 720 cm21.Based on the anti-symmetric NO2 stretch, the thiophenolcoated gold substrate enhanced the adsorption by 4 fold.The uncoated gold substrate enhanced the absorption byonly 2 fold for the TNT analyte. Enhancements wereslightly less for the symmetric NO 2 stretch, at 3.5 foldfor the thiophenol coated substrate and 2 fold for theuncoated substrate. The enhancement factors for TNT aresigni� cantly less than similar nitro-aromatic compounds.Unlike the other compounds tested, TNT has hydrophilicNO2 groups symmetrically around the aromatic ring,which may have limited the binding to the hydrophobicthiophenol coating. Additionally, no signi� cant spectralchanges were noted between the transmission spectrumand the SEIRA on the thiophenol coated gold spectrum,indicating that there was no preferred adsorption geom-etry for the TNT.

CONCLUSION

The effect of self-assembled monolayer coatings onSEIRA spectroscopy was measured. Monolayers of or-ganic thiol coatings were found to generally increase thesurface enhancement of nitro substituted aromatic anal-ytes compared to a bare gold surface. Thiophenol wasfound to give the largest enhancement of the three coat-ings tested. Due to pi–pi stacking, analytes tested in thisstudy bound to the thiophenol coating best. The increasedbinding serves to limit evaporation, orient the analyte onthe surface, and increase infrared absorption through ashort-ranged chemical mechanism. A thiohexane coatingwas found to give nearly equal enhancement to an un-coated substrate, while a thiopropane coating gave slight-ly better enhancement in all cases.

The use of monolayer coatings on SEIRA substratescould extend the technique to applications where it orig-inally was not viable. For each class of analyte, a propercoating must be designed. This coating should maximizebinding while holding the analyte close to the surface.With the proper coating, the infrared absorption of anal-ytes may be increased by an additional 10 to 20 fold.

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