Study of Adsorption of Aminobenzoic Acid Isomers on Silver Nanostructures bySurface-Enhanced Infrared Spectroscopy

Donald A. Perry,* James S. Cordova, Lauren G. Smith, Hye-Jin Son, andElizabeth M. SchieferDepartment of Chemistry, UniVersity of Central Arkansas, Conway, Arkansas 72035

Enkeleda Dervishi, Fumiya Watanabe, and Alexandru S. BirisDepartment of Applied Sciences, Nanotechnology Center, UniVersity of Arkansas, Little Rock, Arkansas 72204

ReceiVed: July 20, 2009; ReVised Manuscript ReceiVed: September 8, 2009

o-, m-, and p-aminobenzoic isomers were studied with surface-enhanced infrared absorption (SEIRA)spectroscopy and temperature-programmed desorption (TPD) on vacuum-evaporated silver films and on asilver powder. Each aminobenzoic acid (ABA) isomer was complexed with a silver ion(s) and modeled withdensity functional theory to aid in the interpretation of the SEIRA results. Atomic force microscopy andscanning electron microscopy compared and contrasted surface roughness between the evaporated silver filmand silver powder that leads to surface-enhanced vibrations of adsorbates on the silver substrates. The abilityof SEIRA to enhance the infrared signal of an adsorbate monolayer and the subsequent multilayer was essentialin exploring ABA adsorption as a function of the polar properties of the deposition solvent. For m- andp-aminobenzoic acids, it was demonstrated that their deposition in alkane solvents with nonpolar bonds resultedin increased intermolecular attraction between amino groups of adjacent ABA molecules in the monolayer.TPD and SEIRA results proved that p-aminobenzoic acid adsorption to silver was stronger than that ofm-aminobenzoic acid, which had stronger adsorption than the o-aminobenzoic acid. Outcomes from this workwill be important to many diverse areas such as biochemistry, bioengineering, environmental chemistry,nanotechnology, and catalysis where the adsorption of amino acids is important.

Introduction

In recent years there has been a growing interest in under-standing the adsorption chemistry of nanosized silver structuresdue to their increased use in many applications such ascosmetics, medicine, clothing, and catalysis.1–5 Their unique,size-dependent properties have many desirable physical, chemi-cal, and biological properties. Most notably, silver nanoparticlesexhibit microbial activity toward many pathogens2 but alsopossible deleterious effects on human health and the environ-ment.1

The demonstrated biological activity of silver nanoparticlesmotivates the examination of their interaction with amino acids.Aminobenzoic acid (ABA) isomers are often used in thepreparation of metal nanoparticles and assist in their function-alization.6 2-Aminobenzoic acid is vitamin L, and 4-aminoben-zoic acid is part of a vitamin B complex along with bacterialvitamin H.7 ABA isomers also have antimutagenic activityascribed to the decomposition of N-methyl-N′-nitro-N-ni-trosoguanidine caused by ABA activity at bacterial cell walls.6

Because both silver nanoparticles and the ABA isomers haveantimicrobial properties, work described in this paper involvingthe study of ABA adsorption on silver nanoparticles may bethe first step toward developing a novel ABA/silver nanocom-posite with enhanced activity toward pathogens. This study willhelp establish simple amino acids as model systems to inves-tigate the adsorption characteristics of more complex biomol-ecules such as proteins or DNA. Research presented here willalso aid in ascertaining the impact of silver nanoparticles and

silver nanoparticles functionalized with ABA isomers in theenvironment.

Over the years there have been a number of surface-enhancedRaman spectroscopy (SERS) studies involving the ABA isomerswith excitation in the visible region.8–21 The limitation of thisprevious work is that p-aminobenzoic acid (PABA) adsorbedonto silver nanomorphologies undergoes photoxidation in airunder visible light excitation to p,p′-azidobenzoate.13 There isnow evidence that suggests under certain circumstances botho-aminobenzoic acid (OABA) and m-aminobenzoic acid (MABA)can undergo photolytic chemistry in the visible region whenadsorbed to certain silver nanostrucutures.22 However, there hasbeen some SERS work conducted in the near-infrared in colloidsinvolving PABA adsorption on silver nanostructures wherephotooxidation is not expected to be an issue.23–25 Several recentFourier transform infrared (FTIR) spectroscopic studies of theABA isomers and their alkali salts are also in the literature.26–28

Surface-enhanced infrared absorption (SEIRA) has beenpreviously used to explore the adsorption of aromatic isomerson noble metal nanoparticles and the effect of the polarity ofthe deposition solvent. Badilescu and co-workers29 studied theo-, m-, and p-nitrobenzoic acid isomers while Griffiths and co-workers30 investigated the nitrophenol isomers with SEIRA.Posey et al.31 employed both SERS and SEIRA to look at theadsorption of the nitroaniline isomers, and Perry et al.32 alsoused the same techniques to research the hydroxybenzoic acidisomers. These last reports took advantage of the unique abilityof SEIRA to probe at both the monolayer and multilayer levelin order to demonstrate how the adsorption of the nitroanilineand hydroxybenzoic acid isomers depended on the deposition* To whom correspondence should be addressed.

J. Phys. Chem. C 2009, 113, 18304–1831118304

10.1021/jp906871g CCC: $40.75 2009 American Chemical SocietyPublished on Web 09/24/2009

solvent properties. SEIRA was also used to show how p-nitrobenzoic acid adsorption changed while changing fromacetone to methanol as the deposition solvent.33 With SEIRAand SERS, Smith et al. also examined how the polar propertiesof the deposition solvent altered the adsorption of a number ofdifferent analgesics in both the monolayer and multilayer.34

In this work scanning electron microscope (SEM) and atomicforce microscopy (AFM) were used to compare and contrastthe difference between silver films evaporated on CaF2 windowsversus a commercial 2-3 µm silver powder. Then SEIRA andtemperature-programmed desorption (TPD) were employed tostudy the adsorption of the ABA isomers in the monolayer andmultilayer on evaporated silver films and silver powders. SEIRAalso revealed how the polar properties of the deposition solventimpacted the adsorption of the ABA isomers. To our bestknowledge, this is the first time intermolecular attractionbetween amino groups of one type of adsorbed aromaticmolecule in the monolayer has been reported.

Experimental Section

The aminobenzoic acid isomers and the 2-3 µm silverpowder were purchased from Aldrich. All solvents used for thepreparation of solutions were HPLC or Optima grade (FisherScientific or Aldrich). Polished CaF2 windows (25 × 4 mm2)were purchased from International Crystal Laboratories.

SEIRA spectra were obtained in transmission mode (4 cm-1

resolution and 16 scans were averaged) with a Thermo-NicoletIR100 FTIR spectrometer. Diffuse reflectance infrared Fouriertransform spectroscopy (DRIFTS) spectra were taken on aNicolet Magna 560 FTIR spectrometer, using an InternationalCrystal Laboratories DRIFTS attachment (4 cm-1 resolution and16 scans were averaged). Fourier transform infrared attenuatedtotal reflectance (FTIR-ATR) spectra were taken with a ThermoFoundation Series Performer ATR attachment with a diamondATR crystal (single-bounce design) on an IR100 spectrometer.Temperature-programmed desorption (TPD) experiments werecarried out with a Varian Chromatoprobe inserted into a Varian1070 temperature-programmable injector equipped with liquidCO2 cooling capacity attached to a Saturn 2200 ion-trap massspectrometer. All TPD experiments began at 293 K and wereramped at 40 deg/min to a maximum temperature of 593 K.Both the 2-3 µm silver powder and the vacuum-evaporatedsilver films were imaged with JEOL JSM-7000F field emissionSEM and Veeco Dimension 3100D AFM.

Silver films for SEIRA studies were prepared by vapordeposition onto CaF2 substrates in a home-built vacuum chamberwith a base pressure of 1 × 10-6 Torr. Film thickness wasmonitored with a quartz crystal microbalance (Infinicon). TheCaF2 were polished with a Buehler Mastermet 2 colloidal silicasuspension, sonicated and rinsed with nanopure water, and air-

dried prior to usage. It has been shown in previous work that a7 nm silver film is optimal for SEIRA under the currentexperimental conditions.31 Henceforth, when referring to a silverfilm, this is with reference to a 7 nm silver film evaporated ona CaF2 window. As with any silver film or powder exposed toatmospheric conditions, it is assumed that some level of silveroxidation has occurred, but more in-depth studies are required.35

An ABA coverage near a monolayer for SEIRA and SERSstudies was prepared by pipeting 25 µL of a 50 ppm ABAsolution onto a silver film and allowing the solvent to evaporate.This approximate monolayer coverage assumed an experimen-tally determined average spot size of 4 cm2 for all solvents andresulted in reasonably uniform ABA films of about 200 ng/cm.2 For the DRIFTS and TPD experiments the appropriatealiquot of a 1000 ppm solution of an ABA isomer was exposedto 0.01-0.05 g of the 2-3 µm silver powder. Throughout theremainder of the paper the phrase “silver powder” refers to the2-3 µm silver powder.

Brunauer-Emmett-Teller (BET) and Langmuir surface areaanalyses of the 2-3 µm silver powder were determined byrecording nitrogen adsorption/desorption isotherms at 77 K,using a static volumetric technique (Micromeritics ASAP 2020).Before the physisorption measurements, the catalyst systemswere degassed at 623 K for 4 h under vacuum. The pore sizedistributions were calculated by using the adsorption branch ofthe N2 adsorption/desorption isotherm and the Barret-Joyner-Halenda (BJH) method.

Density functional theory (DFT) calculations were performedwith the Gaussian 2003 suite at the B3LYP level of theory.36 ALANL2DZ basis set was used for calculations of all aminoben-zoate ions complexed with silver ions, and frequencies arereported without correction. For each ABA isomer, separatecalculations were performed with one silver ion associated withthe benzoate group, and another calculation with a silver ionclose to the benzoate group and a second silver ion in proximityto the amino group.

Results and Discussion

Structures a-c in Figure 1 shows the DFT optimizedstructures of the aminobenzoate ions OABA, MABA, andPABA, respectively, where two silver ions are complexed withthe benzoate and amino groups of each of the ABA isomer ions.The resulting vibrational frequencies obtained from theseoptimized structures and the structures resulting from one silverion associated with the benzoate group of each ABA isomerion are used later to aid in the interpretation of the infraredspectra of the ABA isomers adsorbed on the silver films andthe silver powder.

Figure 2 shows the AFM analysis (left) and an SEM image(right) of a 7 nm thick film of silver evaporated on a polished,

Figure 1. DFT optimized structures of the following ions complexed with two silver ions: (a) o-aminobenzoate, (b) m-aminobenzoate, and (c)p-aminobenzoate.

Study of Aminobenzoic Acid Isomers J. Phys. Chem. C, Vol. 113, No. 42, 2009 18305

CaF2 window. A polydispersed silver film was formed with anaverage size of the individual Ag grains of 30 nm × 40 nmand a rms roughness of 5.7 nm. The first image in Figure 3represents the AFM scanning of a particulate of the silverpowder, while Figure 3b shows the corresponding SEM imageof the silver powder with a magnification of a surface asperity.For the silver powder the rms surface roughness was found tobe about 1.8 nm and the particles have an average size of 48nm × 19 nm. It is apparent from the AFM work that the particlesin both the silver film and silver powder have particles of aboutthe same 2D dimensions with the silver films having a slightlylarger height on average.

BET surface area of the silver powder was 0.4026 m2/g andthe Langmuir surface area was 0.5757 m2/g. Considering theBET surface area of silver powder and the fact that in our TPDand DRIFTS experiments we used from 0.01 to 0.05 g of silverpowder, and assuming a 4 cm2 area of an evaporated silver filmexposed to the deposition solution, this equates to approximatelythe same surface area in our experiments on silver films versussilver powders. Surface area and pore analysis are importantfactors since they relate to the samples’ surface nanomorpholo-gies and indirectly to the spacing between the Ag surfaces. Theadsorption average pore width (4 V/A by BET) was determinedto be 4.3 nm, which is a value that is known to providesignificant surface enhancement of the spectroscopic signal.Localization of surface plasmons (electromagnetic wave inducedcoherent charge density oscillations) in such Ag nanostructurescan lead to strong increase in the electromagnetic fields withinthe surface asperities of the nanoparticles and the narrowinterparticle gaps, forming the so-called spectroscopic hot spots.Enhancement of the electromagnetic fields can take place invarious spectral ranges and therefore it can induce a strongenhancement of the spectroscopic signal of up to several million

times in SERS and several thousand times in SEIRA.31 Thisprocess, can lead to the ability of detecting and studying singlemolecules or single layers of molecules adsorbed over thesurface of the Ag nanostructures by both SERS and SEIRA.31

Figure 4 shows the TPD spectra of equal exposures of 25µL of a 1000 ppm solution in acetone of each of the ABAisomers (equivalent to a multilayer) desorbing from silverpowder. The desorption maximum for the OABA, MABA, andPABA multilayer are about 450, 490, and 510 K, respectively.This suggests that the strength of adsorption of the ABA isomersto the silver powder is PABA > MABA > OABA. TPD spectraof OABA and MABA each have a high-temperature tailassociated with desorption of the monolayer. For PABA, themultilayer portion of the TPD spectrum is a more symmetricpeak than that associated with OABA and MABA. However,note a peak in the PABA spectrum centered at around 600 K.

Figure 2. An AFM image of a 7 nm silver film on the left, and an SEM image of the same film on the right.

Figure 3. On the left is a 1 µm scale AFM image of silver powder, with a 100 nm scale SEM image of silver powder on the right.

Figure 4. TPD spectra of an equivalent multilayer coverage (50 µLof a 1000 ppm acetone solution) each of OABA, MABA, and PABAdesorption from silver powder.

18306 J. Phys. Chem. C, Vol. 113, No. 42, 2009 Perry et al.

It is important to remember that the maximum temperature towhich the silver powders were heated in the TPD experimentswas 593 K. Hence, the high-temperature desorption peak in thePABA TPD spectrum centered at about 600 K actuallyrepresents PABA desorption time (associated with a 40 deg/min ramp rate) after the silver powder reached a maximum of593 K. We associate this high-temperature desorption peak withthe PABA monolayer.

SEIRA enhancement factors were obtained for the each ofthe ABA isomers by comparison of thick ABA layers on a cleanCaF2 for the silver films and on KBr powder for the silverpowder. Enhancement factors for all the ABA isomers werefound to be ×20-50 when adsorbed on the silver films. MABAtypically showed the least SEIRA enhancement while PABAexhibited the most. These enhancement factors were less onsilver powder. These lower SEIRA enhancement factors madeit more difficult to consistently obtain high-quality spectra of amonolayer of the ABA isomers on silver powder. Hence, it wasdecided not to include the monolayer coverage of the ABAisomers on silver powder here. In Figure 5a is presented theFTIR-ATR spectrum of the OABA powder. Next, spectra b-ein Figure 5 represent the SEIRA spectra of 5, 25, 100, and 400µL exposures of OABA, respectively, to a silver film, using a50 ppm ether solution. In these experiments for ABA isomerdeposition on silver films each 25 µL exposure represents abouta monolayer. Figure 5f shows a DRIFTS spectrum of 25 µL of1000 ppm OABA in acetone exposed to 0.05 g of silver powder.A 25 µL exposure of a 1000 ppm solution of an ABA isomerto 0.05 g of silver powder will result in several ABA layers.Table 1 shows the mode assignments, DFT calculated frequen-cies, the SEIRA frequencies for OABA deposited onto a silverfilm from ether, and the DRIFTS frequencies for OABAdeposited onto silver powder from acetone. Frequencies fromthe DFT calculations of the OABA ion complexed to one silverion (in Table 1) matched the experimental data better thanOABA complexed with two silver ions.

A range of deposition solvents with varying polarity wereused for SEIRA experiments involving OABA adsorption onsilver films: methanol, acetone, CH2Cl2, ether, and n-heptane.The choice of deposition solvent was not seen to have an impacton OABA adsorption in the SEIRA experiments on silver films,presumably because the strong intramolecular hydrogen bondinginhibited any impact from a change in polarity of the depositionsolvent. Previous TPD results presented in Figure 4 supportedthis observation showing that OABA adsorption is weaker thanMABA and PABA adsorption most likely because the strongintramolecular hydrogen bonding in OABA hindered a largedegree of interaction with the solvent. Thus it obstructed anyconsiderable intermolecular attractions and limited more pro-nounced interaction with the silver film.

We have consistently observed in our work that less polarsolvents such as ether and CH2Cl2 result in slightly better definedspectra because more polar deposition solvents such as acetoneand methanol can damage the silver film after high solventexposures. This explains why we have shown results using anether solvent for the OABA experiments collected on the silverfilms. However, it was necessary to use an extremely polarsolvent such as methanol or acetone as the deposition solventfor the OABA experiments on silver powder (as well as forensuing results entailing MABA and PABA adsorption on silverpowder) due to the low solubility of OABA in less polar solventssuch as ether,CH2Cl2, and n-heptane because higher OABAconcentrations are required in the DRIFTS experiments on silverpowder versus the SEIRA experiments on silver films.

Strong ring deformation/COO stretch bands in the 1375-1390cm-1 range imply that OABA ionized when adsorbed on eitherthe silver film or powder in the monolayer and possibly intothe multilayer. This suggests that OABA adsorbed at least inpart through the carboxylate group. No new bands or significantband shifts were observed as the OABA coverage progressedfrom the monolayer into the multilayer over a silver film. In

Figure 5. (a) FTIR-ATR of OABA powder, (b-e) SEIRA spectra of5, 25, 100, and 400 µL exposures of 50 ppm OABA in ether to a silverfilm, and (f) DRIFTS spectrum of a 25 µL exposure of a 1000 ppmOABA solution in methanol to 0.05 g of silver powder.

TABLE 1: Frequencies and Mode Assignments Associatedwith OABA Adsorption

mode DFT SEIRA etherDRIFTSmethanol

ring def.; NH2 bend 1043 1061ring def.; NH2 bend 1080 1113 1110ring def.; NH2 bend;COO str.

1129 1161 1159

CH, NH bend 1176 (doublet) (doublet)CH, NH bend 1205 1246 1213ring def.; C-NH2

str.1308 1300 1302

ring def.; COOasym. str.

1320 1325 1326

ring def.; COO sym.str.

1379 1376 1379

ring def.; COOasym. str.

1391 1390

ring def.; COOasym. str.

1426 1423 1460

1453ring def.; COOasym. str.

1503 1486 1490

ring def.; COOasym. str.

1525 1558 1535

ring def.; NH2

scissor1607 1585 1587

ring def.; NH2

scissor1646 1615 1611

ring def.; NH2

scissor1666 1673 1682

Study of Aminobenzoic Acid Isomers J. Phys. Chem. C, Vol. 113, No. 42, 2009 18307

Figure 5b-e there is a noticeably large increase in the intensityof the ring deformations in the 1400-1600 cm-1 range relativeto other bands as a function of growing OABA coverage. InTable 1 we have tentatively assigned these ring deformationmodes to ionized OABA, but in fact their presence and stronggrowth with respect to other OABA bands may signify the onsetof molecular OABA adsorption into a multilayer.

One must also note the similarities upon comparing the FTIR-ATR spectrum of OABA powder in Figure 5a to the submono-layer and monolayer SEIRA spectra of OABA adsorbed onsilver films (Figure 5b,c). It is not uncommon that the infraredspectrum of an ortho-aromatic isomer such as OABA withstrong intramolecular hydrogen bonding will not look that muchdifferent from the infrared spectrum of its correspondingbenzoate ion.31,32 This likeness between a molecular ortho-aromatic and its ion makes it sometimes difficult to morecompletely interpret results of vibrational experiments whereionization is occurring.

All spectra of OABA deposited on silver powder and on silverfilms in Figure 5 have many similarities with regard to the peaksthat are present but not always the relative peak intensities. Onedifference is the presence of a second ring deformation/COOstretch band at about 1390 cm-1 for OABA adsorbed on silverpowder (Figure 5f) that is not clearly defined when OABA isadsorbed on a silver film. It may be that this band is not resolvedfrom the other ring deformation/COO stretch band at 1376 cm-1

when OABA is absorbed on a silver film. There are alsodifferences in the ring deformation modes in the 1400-1600cm-1 range when comparing OABA adsorbed on silver powderto a silver film. In general, there is a decrease in intensity ofthe NH bending modes (most notably in the 1200 cm-1) rangeand the C-NH2 stretch at 1300 cm-1 when OABA is adsorbedon a silver powder versus a silver film. These differencesbetween OABA adsorbed on a silver film versus a silver powdersuggest that OABA may adsorb with a different orientation onsilver powder versus a silver film. If we assume a more uprightorientation for OABA adsorbed to silver through the benzoategroup, there would be a change in OABA tilt when adsorbedonto the silver film versus adsorption on silver powder. We drawthis conclusion based predominately on the decrease in NHbending mode intensity and the C-NH2 stretch when OABAadsorbs on silver powder.

Figure 6a presents the FTIR-ATR spectrum of MABApowder. Spectra b and c in Figure 6 are SEIRA spectra of 25

and 400 µL exposures of a 50 ppm solution of MABA in ether,respectively, to a silver film, and Figure 6d is a spectrum of 25µL of a 50 ppm exposure of MABA in n-heptane to a silverfilm. A DRIFTS spectrum of a 25 µL exposure of a 1000 ppmsolution of MABA in methanol onto 0.05 g of silver powder isshown in Figure 6e. Table 2 has the MABA/silver complexmode assignments, calculated DFT frequencies, frequencies fora SEIRA spectrum of MABA deposited from ether andn-heptane on a silver film, and the DRIFTS frequencies forMABA deposited from acetone on silver powder. For MABA,mode assignments and DFT calculated frequencies for theMABA ion complexed to two silver ions in Table 2 were foundto best match the experimental results.

In discussing the results of Figure 6, we will temporarilybypass Figure 6d where MABA was adsorbed to a silver filmin an n-heptane solution and focus on MABA adsorption usingsolvents with some bond polarity. There are many similaritieswhen comparing MABA adsorption with OABA adsorption ina polar solvent to a silver film or powder. In both cases, strongring deformation/COO stretch modes in the 1375-1390 cm-1

range again suggest ionization of MABA similar to OABA. Justas with OABA, when MABA was adsorbed on a silver powderin methanol, a second ring deformation/COO stretch mode (1412cm-1) appeared that was not present in an MABA multilayerdeposited from ether on a silver film. When going from amonolayer coverage of MABA in Figure 6b to a multilayerMABA coverage in Figure 6c on a silver film, strong ringdeformation modes appear in the 1450-1600 cm-1 just as wasthe case for OABA. These ring deformation modes wereassigned to the interactions between MABA/silver ion complex.However, the similarity in the 1450-1600 cm-1 range betweena multilayer of MABA deposited from ether in Figure 6c versusthe ring deformation of the FTIR-ATR of MABA powder inFigure 6a might suggest the MABA is starting to adsorb withoutionization at a 400 µL multilayer exposure. A decrease ofMABA ionization in the multilayer spectrum is further supportedby the disappearance in Figure 6c of the NH2 scissor mode at1684 cm-1 present in Figure 6b. It is noted that there is still alarge increase in the intensity of the 1381 cm-1 ring deformation/COO stretch mode while going from a monolayer MABAcoverage in Figure 6b to a multilayer coverage in Figure 6cindicating a significant MABA ionization in the multilayer.

It was seen in experiments performed on silver films thatdifferent solvents with polar bonds including CCl4, CH2Cl2,methanol, and acetone reproduced MABA adsorption observed

Figure 6. (a) FTIR-ATR of MABA powder, (b, c) SEIRA spectra of25 and 400 µL exposures of a 50 ppm solution of MABA in ether toa silver film, (d) SEIRA spectrum of a 25 µL exposure of a 50 ppmsolution of MABA in n-heptane to a silver film, and (e) DRIFTSspectrum of 25 µL of a 1000 ppm solution of MABA in methanol to0.05 g of silver powder.

TABLE 2: Frequencies and Mode Assignments Associatedwith MABA Adsorption

mode DFTSEIRAether

SEIRAheptane

DRIFTSmethanol

CH bend 1053 1073 1078ring def.; NH bend 1102 1100NH bend 1118 1124CH bend 1160 1168 1170CH, NH bend 1212 1220C-N str. 1230 1279CH, NH bend 1341 1308 1345 1305ring def.; COO sym. str. 1384 1381 1388 1390ring def.; COO sym. str. 1390 1412ring def.; COO asym. str. 1458 1455 1457 1473ring def.; COO asym. str. 1500 1522 1510ring def.; NH bend 1523 1559 1558ring def.; NH2 scissor 1638 1633 1640NH2 scissor 1684 1680CdO from NH2

intermolecular interaction1733

18308 J. Phys. Chem. C, Vol. 113, No. 42, 2009 Perry et al.

with ether on silver films. Acetone gave the same result asmethanol for MABA adsorption on silver powder. Whencomparing MABA adsorption on silver powder with that on asilver film, in Figure 6e a multilayer exposure of MABA inmethanol to a silver powder lacks many of the bands below1300 cm-1 in Figure 6c for a multilayer exposure of MABA insolvents with polar bonds to a silver film. Presumably this isbecause of the weak surface-enhancement for MABA adsorbedon silver powder. Bands below 1300 cm-1 grow in only at muchlarger MABA exposures to silver powder.

Spectra a-c in Figure 7 are SEIRA spectra which consist of5, 25, and 400 µL exposures of a 50 ppm PABA solution inCH2Cl2 on silver films. Figure 7d shows the FTIR-ATR spectraof the PABA powder. A SEIRA spectrum of a 25 µL exposureof a 50 ppm solution of PABA in n-heptane to a silver film ispresented in Figure 7e while a corresponding 100 µL exposureis shown in Figure 7f. Figure 7g represents a DRIFTS spectrumof a 25 µL exposure of a 1000 ppm PABA solution in methanolonto the silver powder. When assigning the vibrational modesof adsorbed PABA to silver, it was found that the DFTsimulations of PABA, like MABA, complexed to two silverions appeared more like the experimental results than a PABAion complexed to one silver ion. Table 3 contains modeassignments, DFT frequencies for the PABA ion/2 silver ioncomplex, SEIRA frequencies for PABA adsorbed to a silverfilm using CH2Cl2 as the deposition solvent, SEIRA frequenciesfor PABA adsorbed to a silver film using n-heptane, and theDRIFTS frequencies of PABA adsorbed to silver powder withacetone.

A large ring deformation/COO stretch mode at 1385 cm-1

in the submonolayer and monolayer PABA exposures to a silverfilm in the CH2Cl2 solvent in Figure 7a,b confirmed that PABAis ionizing when in direct contact with the silver film. However,PABA film growth in the multilayer is different than that ofOABA and MABA in that the intensities of most of the peaks

have changed when comparing a PABA monolayer in Figure7b to a PABA multilayer in Figure 7c. A similar trend inmultilayer growth of the p-hydroxybenozic acid isomer versuso- and m-hydroxybenzoic acid has been previously docu-mented.32 Evidence in spectra a-c in Figure 7 suggests thatthe degree of PABA ionization has significantly decreased inthe multilayer coverage in Figure 7c. First, there is a decreasein the intensity of the ring deformation/COO stretch mode at1385 cm-1 relative to the same peak in the PABA monolayerin Figure 7b. A decrease in the relative intensity of a peakassociated with an ion after that ion has been covered with themolecular species has been previously observed.31,32,34 Second,the decreased intensity of the ring deformation/COO stretchmode at 1385 cm-1 relative to other bands in the multilayerPABA spectrum in Figure 7c also suggests that the degree ofPABA ionization has decreased. Finally, when comparing amultilayer coverage of PABA deposited from CH2Cl2 in Figure7c to the FTIR-ATR spectrum of PABA in powder in Figure7d, it is apparent that the two spectra are similar. All three ofthese occurrences suggest that PABA ionization in the multilayerhas decreased significantly as compared to the 400 µL exposurepresented in Figure 7c. Just as with OABA and MABA, wehave tentatively assigned all modes in the PABA spectra a-cand e-g in Figure 7 to the PABA ion/silver complex in Table3 even though it is clear that the amount of PABA ionizationhas decreased in the multilayer. It is worth stating that thedecrease of ionization of an aromatic acid in the next severallayers deposited after the monolayer is not a foregone conclu-sion. We have previously shown the degree of ionization of anumber of aromatic acids in the multilayer is impacted in partby the polar properties of the deposition solvent.32,34

Upon comparing a multilayer coverage of PABA adsorbedin methanol to silver powder in Figure 7g to a multilayercoverage of PABA adsorbed to a silver film in CH2Cl2 in Figure7c, most of the same bands are present in spectra c and g inFigure 7, spare differences in ring deformation modes in the1400-1600 cm-1 range. These differences suggest, just as withMABA, that a multilayer of PABA forms with slightly differentgeometric characteristics on the silver powder versus a silverfilm.

Now we must be more specific about the polar properties ofthe deposition solvent. When PABA or MABA was adsorbed

Figure 7. (a-c) SEIRA spectra of 5, 25, and 400 µL exposures of a50 ppm solution of PABA in CH2Cl2 to a silver film, (d) FTIR-ATRof PABA powder, (e, f) SEIRA spectra of 25 and 100 µL exposures ofa 50 ppm solution of PABA in n-heptane to a silver film, and (g)DRIFTS spectrum of a 25 µL exposure of a 1000 ppm solution ofPABA in methanol to 0.05 g of silver powder.

TABLE 3: Frequencies and Mode Assignments Associatedwith PABA Adsorption

ModeDFTfreq

SEIRACH2Cl2

SEIRAheptane

DRIFTSmethanol

CH bend 1046 1081ring def.; NH bend 1114 1128 1140 1130CH bend 1164 1174 1174 1175CH, NH bend 1213 1253 1263C-N str. 1223 1290 1280 1285 (broad)CH, NH bend 1347 1311 1345 1310

1327 1325ring def.; COO sym. str. 1387 1385 1398 1390

1391ring def.; NH bend 1442 1423a 1465 1425a

1442a

ring def.; COO asym. str. 1488 1516 1540 (broad)ring def. 1537 1574 1570ring def. 1646 1603 1608 1610ring def.; NH bend 1651 1630 1630NH2 scissor 1690 1665 1660 (broad)CdO from NH2

intermolecular interaction1740

a Bands associated with intact PABA adsorption

Study of Aminobenzoic Acid Isomers J. Phys. Chem. C, Vol. 113, No. 42, 2009 18309

to a silver film with CCl4, results were obtained that mirroredthe outcome with other solvents with polar bonds includingmethanol, ether, acetone, and CH2Cl2. In Figure 6d and Figure7e,f for MABA and PABA adsorption to a silver film withn-heptane, a solvent with nonpolar bonds, the result is clearlydifferent than when MABA/PABA were adsorbed to a silverfilm with use of a deposition solvent without polar bonds. Theoutcome of Figure 7e-f for PABA was also duplicated withn-pentane as the deposition solvent. We have previously reportedsuch an impact on adsorbate geometry/surface interactiondependent on the polar properties of the deposition solvent instudies involving the nitroaniline isomers,31 hydroxybenzoic acidisomers,32 and acetylsalicylic acid and ibuprofen adsorption.34

In Figure 6d for a 25 µL exposure of MABA in n-heptane toa silver film is a band at 1733 cm-1, and in Figure 7e there isa similar band at 1740 cm-1 for a 25 exposure of PABA inn-heptane to a silver film. This is assigned as a mode indicativeof a free carbonyl stretch caused by hydrogen bonding inducedreorientation between two PABA or MABA molecules contain-ing amino groups. A number of different researchers haveobserved intermolecular attraction between molecules withamino groups (or amino/NO2 groups) and amino/C-O interac-tions only in the multilayer.31,37 For example, in previous workinvolving the adsorption of m-nitroaniline, bands indicative ofintermolecular hydrogen bonding were not detected until at leasta 100 µL exposure of a 50 ppm solution on a silver film.31 Figure6d for MABA adsorption with an n-heptane deposition solventand Figure 7e for PABA adsorption with n-heptane are the firstreports of hydrogen bonding between amino groups of one typeof adsorbed aromatic molecule in the monolayer. Figure 8 isan illustration of proposed models for PABA adsorption insolvents with and without polar bonds on rough silver substrates.First is a PABA molecule adsorbed in a more upright positionto a silver film through the carboxylate group when using adeposition solvent with polar bonds. There are also two tiltedPABA molecules adsorbed to a silver film with use of adeposition solvent with nonpolar bonds where the potentialhydrogen bonding is highlighted and where one oxygen of thecarboxylate group is interacting less with the silver film thanthe other oxygen. This PABA adsorption configuration couldpresent in an infrared spectrum with a carbonyl stretch. A similarmodel is proposed for MABA adsorption.

Besides the emergence of the carbonyl stretch when MABAand PABA are adsorbed to a silver film with use of n-heptane,there are a number of other changes in MABA and PABAadsorption that occur upon switching from a deposition solventwith polar bonds to one without polar bonds. First, there aresignificant changes in all the COO symmetric and asymmetric

stretch bands for both MABA and PABA that indicate aconsiderable alteration of the carboxylate interaction with thesilver film. Second, the NH2 scissor modes present at 1585,1615, and 1673 cm-1 for MABA and at 1684 cm-1 for PABAdisappear when n-heptane is the deposition solvent. This is tobe expected when the NH2 is involved in hydrogen bondingwith an adjacent group.37 Third, there are dramatic frequencychanges for every mode involving either an NH bend or an C-Nstretch in the PABA and MABA monolayer spectra whenswitching from a deposition solvent with bond polarity ton-heptane without bond polarity.

NH bend modes in the 1100-1350 cm-1 range for PABAadsorption to a silver film with n-heptane in Figure 7e,f aremuch smaller than the corresponding NH bend modes for PABAadsorption to a silver film with CH2Cl2 in Figure 7b-d. Adecrease in the NH bend modes does not suggest a PABAorientation where the amino group is interacting with the silverfilm. The decrease in these NH bend modes when PABA isadsorbed to a silver film by using a deposition solvent withnonpolar bonds such as n-heptane versus PABA deposition witha solvent with polar bonds is consistent with reorientation ofadsorbed PABA and interaction between adjacent PABA aminogroups. It is proposed that when deposition occurs by using asolvent with nonpolar bonds such as n-heptane there is lessPABA solvation during the adsorption process than when adeposition solvent with polar bonds is used. This lower levelof solvation thus allows for the increased intermolecularattraction between MABA and PABA amino groups in themonolayer.

Conclusion

SEM and AFM showed that the silver powder and silver filmsboth have similar surface roughness that led to surface enhance-ment in SEIRA. TPD results highlighted the fact that each ofthe ABA isomers desorbed from the silver powder with peaksfrom the monolayer and multilayer. The TPD results alsoindicated that the strength of adsorption to silver powder wasPABA > MABA > OABA. SEIRA revealed that OABA ionizedin the monolayer with a subsequent decrease of ionization inthe multilayer when OABA was adsorbed on silver filmsindependent of the polar properties of the deposition solvent.Because of the strong intramolecular hydrogen bonding inOABA films, a change in the polar properties of the depositionsolvent was not seen to impact OABA adsorption. OABAmultilayer adsorption to both silver films and the silver powderexhibited similar infrared spectra spare differences in the ringdeforrmation modes in the 1400-1600 cm-1 range. MABAdemonstrated multilayer adsorption on a silver film and silverpowder that closely mimicked OABA adsorption with use of apolar deposition solvent although PABA formed quite differ-ently. When MABA and PABA were deposited on silver filmsby using an alkane deposition solvent with nonpolar bonds, itwas shown that there was a strong intermolecular interactionin the monolayer most likely due to a hydrogen bonding networkdeveloped between amino groups on adjacent MABA/PABAmolecules in the monolayer.

Acknowledgment. We acknowledge the Arkansas Nano-technology Center for use of the AFM, Raman, and BETisotherm equipment. We thank Dr. Pat Desrochers, Dr. JerryManion, Dr. Karen Weaver, and Dr. Brian Gilbert for manyfruitful discussions.

References and Notes

(1) Panyala, N.; Pena-Mendez, E.; Havel, J. J. Appl. Biomed. 2008, 6,117–129.

Figure 8. Schematic showing the proposed PABA adsorption to asilver after deposition in a solvent with and without polar bonds.

18310 J. Phys. Chem. C, Vol. 113, No. 42, 2009 Perry et al.

(2) Syed, A.; Suresha, S. Chem. EnViron. Res. 2006, 15, 5–25.(3) Shamra, V.; Yngard, R.; Lin, A. AdV. Colloid Interface Sci. 2009,

145, 83–96.(4) Satokawa, S. Ryusan to Kogyo 2007, 60, 209–214.(5) Kramer, J.; Benoit, J.; Bowles, K.; DiToro, D.; Herrin, R.; Luther,

G. SilVer in the EnVironment: Transport, Fate, and Effects; SETAC Press:Boca Raton, FL, 2002.

(6) Dong, X.; Zuo, F.; Zhong, W.; Li, Z.; Chen, P. Gongneng Cailiao2005, 36, 1558–1560.

(7) Samsonowicz, M.; Hrynaskiewicz, T.; Wisocka, R.; Regulska, E.;Lewandowski, W. J. Mol. Struct. 2005, 744-747, 345–352.

(8) Suh, J. S.; DiLelle, D. P.; Moskovits, M. J. Phys. Chem. 1983, 87,1540–1544.

(9) Suh, J. S.; Moskovits, M. J. Am. Chem. Soc. 1986, 108, 4711–4718.

(10) Sun, S.; Birke, R. L.; Lombardi, J. R. J. Phys. Chem. 1988, 92,5965–5972.

(11) Berthod, A.; Laserna, J. J.; Winefordner, J. D. Appl. Spectrosc.1987, 41, 1137–1141.

(12) Park, H.; Lee, S. B.; Kim, K.; Kim, M. S. J. Phys. Chem. 1990,94, 7576–7580.

(13) Venkatachalam, R. S.; Boerio, F. J.; Roth, P. G. J. Raman Spectrosc.1988, 19, 281–287.

(14) Bello, J. M.; Narayanan, V. A.; Stokes, D. L.; Vo-Dinh, T. Anal.Chem. 1990, 62, 2437–2441.

(15) Dawson, P.; Hass, J. W.; Alexander, K. B.; Thompson, J.; Ferrell,T. L. Surf. Sci. Lett. 1991, 250, L383–L388.

(16) Fu, S.; Zhang, P. J. Raman Spectrosc. 1992, 23, 93–97.(17) Roth, E.; Engert, C.; Kiefer, W. J. Mol. Struct. 1995, 349, 89–92.(18) Volkan, M.; Stokes, D. L.; Vo-Dinh, T. J. Raman Spectrosc. 1999,

30, 1057–1065.(19) Stokes, D. L.; Chi, Z.; Vo-Dinh, T. Appl. Spectrosc. 2004, 58, 292–

298.(20) Wang, K.; Li, Y. Vib. Spectrosc. 1997, 14, 183–188.(21) Li, Y.; Cheng, J.; Wang, Y. Spectrochim. Acta, Part A 2000, 56,

2067–2072.(22) Under preparation for submission.(23) Ibarhim, A.; Oldham, P. B.; Stokes, D. L.; Vo-Dinh, T.; Loo, B. H.

J. Mol. Struct. 2005, 735-736, 69–73.

(24) Liang, E. J.; Engert, C.; Kiefer, W. Vib. Spectrosc. 1993, 6, 79–85.

(25) Ibrahim, A.; Oldham, P. B.; Stokes, D. l.; Vo-Dinh, T. J. RamanSpectrosc. 1996, 27, 887–891.

(26) Stalin, T.; Rajendiran, N. J. Photochem. Photobiol. A 2006, 182,137–150.

(27) Wisocka, R.; Regulska, E.; Samsonowicz, M.; Hrynaszkiewicz, T.;Lewandowski, W. Spectrochim. Acta, Part A 2005, 61, 2566–2573.

(28) Wisocka, R.; Samsonowicz, M.; Regulska, E.; Lewandowski, W.J. Mol. Struct. 2006, 792-793, 227–238.

(29) Badilescu, S.; Ashrit, P. V.; Truong, V.; Badilescu, I. I. Appl.Spectrosc. 1989, 43, 549–552.

(30) Merklin, G. T.; Griffiths, P. R. Langmuir 1997, 13, 6159–6163.(31) Posey, K. L.; Viegas, M. G.; Boucher, A. J.; Wang, C.; Stambaugh,

K. R.; Smith, M. M.; Carpenter, B. G.; Bridges, B. L.; Baker, S. L.; Perry,D. A. J. Phys. Chem. C 2007, 111, 12352–12360.

(32) Perry, D. A., Boucher, A. J.; Posey, K. L.; Cordova, J. S.; Smith,L. G.; Son, H.; Pandey, M.; Biris, A. S. Spectrochim. Acta, Part A 2009,74, 104-112.

(33) He, L.; Griffiths, P. AIP Conf. Proc. 1998, (n430), 602.(34) Smith, M.; Perry, D.; Stambaugh, K.; Smith, L.; Son, H.; Garner,

A.; Cordova, S.; Posey, K.; Biris, A. S. Vib. Spectrosc. 2009, 49, 288–297.(35) Leverette, C. L.; Jacobs, S. A.; Shanmukh, S.; Chaney, S. B.; Dluhy,

R. A.; Zhao, Y. P. Appl. Spectrosc. 2006, 60, 196A–216A.(36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,

M. A.; Cheeseman, J. R.; Zakrzewski, J. A.; Montgomery, J. A.; Stratmann,R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;Peterson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ciolowski, J.; Ortiz,J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,C. Y.; Nanyakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 03, Revision B.05;Gaussian Inc., Pittsburg, PA, 2003.

(37) Lavoie, S.; Laliberte, M. A.; McBreen, P. H. J. Am. Chem. Soc.2003, 125, 15756–15757.

JP906871G

Study of Aminobenzoic Acid Isomers J. Phys. Chem. C, Vol. 113, No. 42, 2009 18311