366 Volume 54, Number 3, 2000 APPLIED SPECTROSCOPY0003-7028 / 00 / 5403-0366$2.00 / 0

q 2000 Society for Applied Spectroscopy

Self-Assembled Monolayers of Thiophenol on Gold as aNovel Substrate for Surface-Enhanced Infrared Absorption

JOHN A. SEELENBINDER, CHRIS W. BROWN, * and DANIEL W. URISHPartnership for Sensor and Surface Technology (J.A.S., C.W.B.), Department of Chemistry (J.A.S., C.W.B.), and Department of

Civil and Environmental Engineering (D.W.U.), University of Rhode Island, Kingston, Rhode Island 02881

A unique method of obtaining surface-enhanced infrared absorp-

tion (SEIRA) spectra for chemicals that will not chemically attach

to a metal surface has been investigated. Surface enhancements aregreatest for molecules that bind to metals. In order to achieve great-

er enhancement for those analytes that do not bind to SEIRA met-

als, we have investigated self-assembled monolayers as a means oflinking analytes to a gold substrate. Monolayers of thiophenol were

formed onto sputter-coated gold± silicon substrates. Analytes were

deposited onto the thiophenol-coated gold± silicon wafers, and ex-ternal re¯ ection SEIRA spectra were then measured. Enhancement

factors as high as 30-fold compared to those for conventional SEIRA

substrates are demonstrated. The self-assembled monolayers ongold substrates are shown to change both relative intensities and

band positions of the adsorbed analyte. These intensity changes and

frequency shifts show strong interaction of selected analytes withthe self-assembled monolayer. This study of phthalates and nitro-

substituted aromatic compounds demonstrates the usefulness of the

technique. Spectral changes evident through the use of the thio-phenol monolayer are discussed.

Index Headings: Surface-enhanced infrared absorption; SEIRA;

Self-assembled monolayers; Thiophenol; Gold substrates.

INTRODUCTION

The spectral selectivity of infrared spectroscopy allowsfor positive identi® cation of many chemical contaminantsin industrial, biological, and environmental samples;however, the relatively low molar absorptivities in theinfrared region limit its use in many applications. For thisreason, enhancement techniques that lower the level ofdetection of infrared spectroscopy are of great interest.Surface-enhanced infrared absorption spectroscopy(SEIRA) was ® rst reported as an enhancement techniquein 1980.1 Since then there have been several advances inthe use 2±13 and theory14±20 of this technique.

The SEIRA phenomenon has been observed on thinlayers of several metal surfaces including silver, 2±5,15±19

gold,8±10 copper,7,19 and tin.6 Current theory on the mech-anism of the enhancement suggests that polarized IR lightcauses an oscillating dipole within metal islands of thecoating. This dipole establishes an electric ® eld that isperpendicular to every point on the island surface. Theelectric ® eld serves to channel the infrared energy intoanalytes that are close to or adsorbed onto the metal sur-face, causing vibrations that are perpendicular to the sur-face to absorb an enhanced amount of IR energy. Thesurface selectivity rule for SEIRA is similar to that ofre¯ ection-absorption spectroscopy (RAS) in that only vi-brations that have a change in dipole moment perpendic-ular to the metal surface are enhanced.

Received 24 May 1999; accepted 21 October 1999.* Author to whom correspondence should be sent.

In addition to the enhancement caused by the electric® eld surrounding each metal island, several investiga-tors11,16 have proposed a contribution due to chemical in-teraction. Although studies6,20,21 have shown that chemicalattachment is not required for surface enhancement, thecontribution due to chemical interaction may explain thelarge enhancement factors for chemicals that bind to themetal surface. Unfortunately, this condition limits thelargest surface enhancements to chemicals that have afunctional group that will bind to a metal. Our previouswork with SEIRA biosensors8,9 demonstrated the abilityto detect pathogens that were bound to a gold surfacethrough the use of antibodies. Although it is not clearwhether the spectral changes are due to functional groupson the antigen or due to a rearrangement of the antibod-ies, the success of this work led us to study the attach-ment of chemical analytes to SEIRA surfaces using or-ganic monolayers.

Selective enhancement for surface-enhanced Ramanspectra (SERS) has been obtained through the use of thi-ol-terminated hydrocarbon coatings on SERS active met-al surfaces. 22±27 These coatings bind the analytes to themetal surface and achieve greater enhancement than canbe observed with uncoated metal surfaces. Carron andco-workers have used this technique to analyze chlori-nated hydrocarbons,22 alkali metal ions,23 and aromaticcompounds.24±27 With the use of a propane thiol coating,detection limits of 10 ppm have been obtained.27 Thecoatings have been used in a SERS detector for high-performance liquid chromatography (HPLC)25 and for¯ ow injection analysis (FIA),26 which demonstrates thedurability of the coatings.

In this work, we have observed surface-enhanced in-frared spectra of several analytes. These analytes werenot bound directly to the gold SEIRA surface, but ratherthey were weakly bound to a thiol-terminated organiccoating, as is shown schematically in Fig. 1. The self-assembled monolayer of thiophenol (benzenethiol) wasused as a coating to isolate phthalates and nitro-substi-tuted aromatic compounds onto a gold sputter-coated sur-face. These modi® ed SEIRA surfaces displayed spectralenhancement over both conventional, uncoated SEIRAsurfaces and RAS-type surfaces.

EXPERIMENTAL

All SEIRA measurements were conducted in the ex-ternal re¯ ection mode with the use of gold-coated siliconplates as the SEIRA active surface. Silicon substrateswere used for convenience; however, barium ¯ uoride orcalcium ¯ uoride substrates could also be used. The sili-con plates were washed in a four-step cleaning process

APPLIED SPECTROSCOPY 367

FIG. 1. A schematic diagram of the proposed SEIRA substrate design.The picture represents a surface of a sputtered gold ``island’ ’ .

comprised of a nitric acid wash, 1:1 hydro¯ uoric acid/water wash, 1:1 sulfuric acid/hydrogen peroxide wash,and 1:1 hydro¯ uoric acid/methanol wash. Following thewash, the plates were sputter coated with gold by usingan MRC 8667 multitarget sputtering system (MaterialResearch Corp., Orangeburg, NY). The sputtering ratewas controlled at 6.0 AÊ/s to give a gold thickness ofapproximately 100 AÊ. The gold-coated plates were thenrinsed with methanol.

The silicon±gold plates were coated with a self-assem-bled monolayer of thiophenol. Ninety-seven percent thio-phenol was purchased from Aldrich Chemical Company(Milwaukee, WI) and diluted to 1 m g/mL in analytical-grade methanol (EM Science, Gibbstown, NJ). The sili-con±gold plates were soaked in thiophenol solution for® ve days, then rinsed with methanol and allowed to airdry before use. Thiophenol is both ¯ ammable and highlytoxic through both inhalation and skin absorption. Carewas taken to ensure that workers were not exposed to theliquid or fumes.

Dimethyl isophthalate (DMIP) and didecyl phthalate(DDP) were obtained from Chem Services (West Chester,PA). 2,4-Di¯ uoronitrobenzene (DFNB) and o-dinitroben-zene (DNB) were obtained from Aldrich Chemical Com-pany. All analytes were diluted to their respective con-centrations in analytical-grade cyclohexane (EM Sci-ence). Analytes were dispensed in the desired quantitiesonto the thiophenol±gold surfaces. The solvent was al-lowed to evaporate, leaving either a thin layer of solidanalyte as in the case of DMIP and DNB or an evenlydispersed layer of liquid analyte as in the case of DDPand DFNB.

SEIRA spectra were measured by using the thiophen-ol-coated silicon±gold plates in an external re¯ ectioncon® guration. The sample size was approximately 4 cm 2.Re¯ ection measurements were made with a Foxboro Cor-poration (Foxboro, MA) re¯ ection accessory with an in-cident angle of 72 8 from normal. SEIRA intensity is de-pendent on incident angle, with angles near the Brewsterangle (74 8 for silicon) yielding the greatest enhancement.Spectra were measured on a Bio-Rad FTS-40 Fouriertransform infrared (FT-IR) instrument (Cambridge, MA)with coaddition of 64 scans at 4 cm 2 1 resolution.

RESULTS AND DISCUSSION

Preliminary work on this project was focused on ob-taining surface-enhanced spectra of aromatic hydrocar-

bons. The goal was to develop an environmental sensorthat would have enhanced sensitivity as well as the spec-tral selectivity obtained only with vibrational spectros-copy. A technique combining surface enhancement withanalyte isolating self-assembled monolayers was exploredas a method to obtain infrared spectra of these com-pounds. This study led to new information regarding therequirements for surface enhancements.

Spectra of several analytes were measured after beingdispersed onto gold-coated silicon wafers that had beenovercoated with self-assembled monolayers of thiophen-ol. Aliphatic substituted benzenes (toluene, o-xylene, m-xylene, p-xylene, trimethyl benzene, t-butyl benzene),hetercyclic aromatics (aniline, pyridine), and polycyclicaromatics (methyl napthlene, pyrene) were all found tohave no spectral enhancement when dispersed onto thethiophenol-coated gold±silicon wafers as compared totransmission spectra of the same mole amount. However,aromatic compounds that contained strong electron-with-drawing groups show large spectral enhancement on thio-phenol-coated gold±silicon wafers as compared to bothtransmission and re¯ ectance-absorption spectra. Phthal-ates (dimethyl isophthalate and didecyl phthalate) and ni-tro-aromatics (di¯ uoronitrobenzene and o-dinitroben-zene) exhibited strong spectral enhancements when re-¯ ectance spectra were measured with the thiophenol-coated gold±silicon wafers. This disparity was likely dueto differences in electron-withdrawing character of theanalytes. Other researchers19 have observed preferentialenhancement of vibrations involving electron-withdraw-ing functional groups. Chemical enhancement of infraredabsorptions has been linked to an electron donor±accep-tor relationship between the metal surface and the ana-lyte.19 The electron-withdrawing character of the morepolar analytes led to greater enhancement even thoughthe less polar aliphatic substituted heterocyclic and poly-cyclic aromatics bound with greater af® nity to the thio-phenol coating.

SEIRA spectroscopy has been shown to be polarizationdependent, similar to RAS.5 Re¯ ectance spectra of ana-lytes adsorbed onto organic monolayers, which are at-tached to SEIRA substrates, show the same polarizationdependence. The polarization dependence can be seen inthe spectra of dimethyl isophthalate shown in Fig. 2. ARAS spectrum is displayed for comparison along withthe s- and p-polarized re¯ ectance spectrum measured ona thiophenol-coated gold±silicon (TPG) SEIRA substrate.The p-polarized SEIRA spectrum shows many bands thatare very weak in the s-polarized SEIRA spectrum. Thecarbonyl band near 1720 cm 2 1 is quite strong in the p-polarized SEIRA spectrum. Strong absorbances are alsoobserved for the CH3 deformation near 1450 cm 2 1, theCO stretch near 1280 cm 2 1, the aromatic ring modes from1200 to 1000 cm 2 1, and the out-of-plane CH bending onthe aromatic ring at 730 cm 2 1. The s-polarized spectrumof the same DMIP sample on the thiophenol-coated gold±silicon wafer displays far smaller absorbances. The s-po-larized spectrum exhibits a weak carbonyl band near1720 cm 2 1 and the CH3 deformation near 1450 cm 2 1. Thelargest positive band evident in the s-polarized spectrumis due to the out-of-plane CH bending at 730 cm 2 1. Thep-polarized RAS spectrum of DMIP on a gold mirror isalso far less intense than the p-polarized SEIRA spectrum

368 Volume 54, Number 3, 2000

FIG. 2. Re¯ ectance spectra of 0.3 mg dimethyl isophthalate. (A) The p-polarized SEIRA spectrum of DMIP on a self-assembled monolayer ofthiophenol on gold (TPG); (B) s-polarized spectrum of DMIP on a self-assembled monolayer of thiophenol on gold (TPG); (C ) p-polarized RASspectrum of DMIP on a thick gold mirror.

FIG. 3. (A) The p-polarized SEIRA re¯ ectance spectrum of 5.0 mg didecyl phthalate on a self-assembled monolayer of thiophenol on gold (TPG);(B) digitized absorbance spectrum of neat DDP.

measured on a thiophenol-coated gold±silicon wafer; thep-polarized SEIRA spectrum exhibits a sevenfold en-hancement over the p-polarized RAS spectrum. Relativeintensities for the carbonyl, CO stretch, CH deformations,and aromatic ring modes are similar to those for the s-polarized SEIRA spectrum. The relative intensity of theout-of-plane CH bending at 730 cm 2 1 is much greater inthe RAS spectrum as compared to the s-polarized SEIRAspectrum.

Other phthalates also show surface-enhanced absorp-tions when adsorbed onto a self-assembled thiophenol-coated SEIRA substrate. A p-polarized spectrum of di-decyl phthalate on a thiophenol-coated gold±silicon waferis shown in Fig. 3. When the SEIRA spectrum was com-pared to a library spectrum 28 of neat DDP, several dif-

ferences were observed. The carbonyl band centered at1732 cm 2 1 is a doublet in the SEIRA spectrum, whereasthe library spectrum contains only a single band. Fur-thermore, the SEIRA spectrum has two separate bands at1304 and 1258 cm 2 1, whereas the library spectrum hasonly a single broad band in this region. A large band isobserved near 720 cm 2 1 in the SEIRA spectrum of DDPon a thiophenol-coated surface. This band is due to theadjacent CH 2 rock of the long hydrocarbon chains. Dueto the highly hydrophobic nature of the aliphatic sidechains, DDP orients with the chains parallel to the sur-face. With the chains parallel to the surface, the directionof the rock is perpendicular, which allows for enhance-ment of the 720 cm 2 1 band. This orientation also placesthe out-of-plane aromatic C±H wags perpendicular to the

APPLIED SPECTROSCOPY 369

FIG. 4. Re¯ ectance spectra of 5.8 mg 2,4-di¯ uoronitrobenzene. (A) The p-polarized SEIRA spectrum of DFNB on a self-assembled monolayer ofthiophenol on gold (TPG); (B) p-polarized spectrum of DFNB on an uncoated gold±silicon wafer.

monolayer surface, resulting in an enhanced band at 735cm 2 1. Other differences include a more pronouncedshoulder at 1137 cm 2 1 and a higher relative intensity forthe band at 1040 cm 2 1 compared to the 1073 cm 2 1 band.

Spectra of nitro-substituted aromatic compounds withthe use of the self-assembled thiophenol-coated SEIRAsubstrate were also measured. Spectra of 2,5-di¯ uoroni-trobenzene on both an uncoated and a thiophenol-coatedgold±silicon wafer are shown in Fig. 4. As can be seenfrom the spectra, the thiophenol-coated substrate exhibitsa 30-fold enhancement over the uncoated gold±siliconsubstrate. In addition to having a greater enhancement,the DFNB on a thiophenol-coated substrate shows chang-es in relative intensities of two bands along with slightspectral shifts. The largest change in relative intensitiesis observed for the doublet aromatic CF stretch at 1260cm 2 1. The spectrum measured with the use of the un-coated SEIRA substrate shows a doublet centered at 1260cm 2 1 with equal relative intensities, whereas the spectrummeasured with the thiophenol-coated SEIRA substrateshows the lower energy branch of the doublet nearlytwice as intense as the higher energy branch. The doubletcentered at 820 cm 2 1, due to the aromatic NO2 scissor,also changed in relative intensity with the lower energyband, becoming nearly equal in absorbance to the higherenergy band.

The differences in relative intensities for DFNB aresimilar to changes found previously on other SEIRA sys-tems.13 Merklin and Grif® ths13 attribute similar shifts inthe SEIRA spectrum of nitrophenol on silver to the silvercoating donating electron density to the nitrophenol.Along similar logic, the thiophenol coating in this studyappears to donate electron density to the DFNB analyte.It should be also noted that the out-of-plane CH bendnear 775 cm 2 1 is very strong in the SEIRA spectrum ofDFNB on a thiophenol-coated substrate, while the sameband is absent in the spectrum of DFNB on an uncoatedsubstrate.

The thiophenol-coated gold±silicon wafers were also

used to obtain surface-enhanced spectra of o-dinitroben-zene. Both the anti-symmetr ic and sym metric NO 2

stretches near 1540 and 1360 cm 2 1, respectively, alongwith the aromatic out-of-plane C±H bends between 880and 700 cm 2 1 are observed, as is shown in Fig. 5. As theamount of DNB on the surface is increased from 1.4 to5.5 m g/cm 2, a change in relative intensities is observedbetween the in-phase and out-of-phase symmetric NO 2

stretches at 1370 and 1355 cm 2 1. For the anti-symmetricstretch, the in-phase stretch at 1550 cm 2 1 broadens as theamount of DNB increases. The lower concentrationsshow largest effects due to the analyte-coating interac-tion. As the amount of DNB on the surface increases,multiple layers of analyte are formed, and the interactionsbetween the analyte and coating are lost. In the case ofDNB on a thiophenol coating, it appears that completecoverage occurs between 1.4 and 2.8 m g/cm 2. These re-sults, along with the results of the DFNB, show that thecoating affects not only the degree of enhancement forcertain compounds but also the shape and frequency ofseveral bands. The effects of different thiol coatings willbe the topic of a later paper.

CONCLUSION

Surface-enhanced infrared absorption has been studiedfor several years; however, few practical uses for thistechnique have been found. Enhancement is obtainedwith analytes that are not bound to the metal surface, butfor the technique to become well used, additional waysto bind additional analytes to the metal surface must befound. This investigation shows that a self-assembledmonolayer can be used to weakly bind and enhance theinfrared spectra of certain analytes. Dramatic enhance-ments can be obtained through the use of organic coat-ings for chemicals that show no enhancement on un-coated substrates. Spectral changes are observed for an-alytes adsorbed onto the organic coatings, and selectivebands are enhanced relative to other bands in the spec-

370 Volume 54, Number 3, 2000

FIG. 5. The p-polarized SEIRA re¯ ectance spectra of o-dinitrobenzene: (A) 5.5 m g/cm 2; (B) 4.1 m g/cm 2; (C ) 2.8 m g/cm 2; (D ) 1.4 m g/cm 2.

trum. Through the use of organic coatings to bind ana-lytes to thin metal substrates, surface enhancement canbe observed for a wide variety of analytes.

ACKNOWLEDGMENT

This work was supported by the United States Department of DefenseEnvironmental Training Fellowship.

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