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J. Phys. Chem. C 2009, 113, 18304–18311
Study of Adsorption of Aminobenzoic Acid Isomers on Silver Nanostructures by Surface-Enhanced Infrared Spectroscopy Donald A. Perry,* James S. Cordova, Lauren G. Smith, Hye-Jin Son, and Elizabeth M. Schiefer Department of Chemistry, UniVersity of Central Arkansas, Conway, Arkansas 72035
Enkeleda Dervishi, Fumiya Watanabe, and Alexandru S. Biris Department 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 a silver powder. Each aminobenzoic acid (ABA) isomer was complexed with a silver ion(s) and modeled with density functional theory to aid in the interpretation of the SEIRA results. Atomic force microscopy and scanning electron microscopy compared and contrasted surface roughness between the evaporated silver film and silver powder that leads to surface-enhanced vibrations of adsorbates on the silver substrates. The ability of SEIRA to enhance the infrared signal of an adsorbate monolayer and the subsequent multilayer was essential in exploring ABA adsorption as a function of the polar properties of the deposition solvent. For m- and p-aminobenzoic acids, it was demonstrated that their deposition in alkane solvents with nonpolar bonds resulted in 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 of m-aminobenzoic acid, which had stronger adsorption than the o-aminobenzoic acid. Outcomes from this work will 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 understanding the adsorption chemistry of nanosized silver structures due to their increased use in many applications such as cosmetics, medicine, clothing, and catalysis.1–5 Their unique, size-dependent properties have many desirable physical, chemical, and biological properties. Most notably, silver nanoparticles exhibit microbial activity toward many pathogens2 but also possible deleterious effects on human health and the environment.1 The demonstrated biological activity of silver nanoparticles motivates the examination of their interaction with amino acids. Aminobenzoic acid (ABA) isomers are often used in the preparation of metal nanoparticles and assist in their functionalization.6 2-Aminobenzoic acid is vitamin L, and 4-aminobenzoic acid is part of a vitamin B complex along with bacterial vitamin H.7 ABA isomers also have antimutagenic activity ascribed to the decomposition of N-methyl-N′-nitro-N-nitrosoguanidine caused by ABA activity at bacterial cell walls.6 Because both silver nanoparticles and the ABA isomers have antimicrobial properties, work described in this paper involving the study of ABA adsorption on silver nanoparticles may be the first step toward developing a novel ABA/silver nanocomposite with enhanced activity toward pathogens. This study will help establish simple amino acids as model systems to investigate the adsorption characteristics of more complex biomolecules such as proteins or DNA. Research presented here will also aid in ascertaining the impact of silver nanoparticles and * To whom correspondence should be addressed.
silver nanoparticles functionalized with ABA isomers in the environment. Over the years there have been a number of surface-enhanced Raman spectroscopy (SERS) studies involving the ABA isomers with excitation in the visible region.8–21 The limitation of this previous work is that p-aminobenzoic acid (PABA) adsorbed onto silver nanomorphologies undergoes photoxidation in air under visible light excitation to p,p′-azidobenzoate.13 There is now evidence that suggests under certain circumstances both o-aminobenzoic acid (OABA) and m-aminobenzoic acid (MABA) can undergo photolytic chemistry in the visible region when adsorbed to certain silver nanostrucutures.22 However, there has been some SERS work conducted in the near-infrared in colloids involving PABA adsorption on silver nanostructures where photooxidation is not expected to be an issue.23–25 Several recent Fourier transform infrared (FTIR) spectroscopic studies of the ABA isomers and their alkali salts are also in the literature.26–28 Surface-enhanced infrared absorption (SEIRA) has been previously used to explore the adsorption of aromatic isomers on noble metal nanoparticles and the effect of the polarity of the deposition solvent. Badilescu and co-workers29 studied the o-, m-, and p-nitrobenzoic acid isomers while Griffiths and coworkers30 investigated the nitrophenol isomers with SEIRA. Posey et al.31 employed both SERS and SEIRA to look at the adsorption of the nitroaniline isomers, and Perry et al.32 also used the same techniques to research the hydroxybenzoic acid isomers. These last reports took advantage of the unique ability of SEIRA to probe at both the monolayer and multilayer level in order to demonstrate how the adsorption of the nitroaniline and hydroxybenzoic acid isomers depended on the deposition
10.1021/jp906871g CCC: $40.75 2009 American Chemical Society Published on Web 09/24/2009
Study of Aminobenzoic Acid Isomers
J. Phys. Chem. C, Vol. 113, No. 42, 2009 18305
Figure 1. DFT optimized structures of the following ions complexed with two silver ions: (a) o-aminobenzoate, (b) m-aminobenzoate, and (c) p-aminobenzoate.
solvent properties. SEIRA was also used to show how pnitrobenzoic acid adsorption changed while changing from acetone to methanol as the deposition solvent.33 With SEIRA and SERS, Smith et al. also examined how the polar properties of the deposition solvent altered the adsorption of a number of different analgesics in both the monolayer and multilayer.34 In this work scanning electron microscope (SEM) and atomic force microscopy (AFM) were used to compare and contrast the difference between silver films evaporated on CaF2 windows versus a commercial 2-3 µm silver powder. Then SEIRA and temperature-programmed desorption (TPD) were employed to study the adsorption of the ABA isomers in the monolayer and multilayer on evaporated silver films and silver powders. SEIRA also revealed how the polar properties of the deposition solvent impacted the adsorption of the ABA isomers. To our best knowledge, this is the first time intermolecular attraction between amino groups of one type of adsorbed aromatic molecule in the monolayer has been reported. Experimental Section The aminobenzoic acid isomers and the 2-3 µm silver powder were purchased from Aldrich. All solvents used for the preparation of solutions were HPLC or Optima grade (Fisher Scientific 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-Nicolet IR100 FTIR spectrometer. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were taken on a Nicolet Magna 560 FTIR spectrometer, using an International Crystal Laboratories DRIFTS attachment (4 cm-1 resolution and 16 scans were averaged). Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectra were taken with a Thermo Foundation Series Performer ATR attachment with a diamond ATR crystal (single-bounce design) on an IR100 spectrometer. Temperature-programmed desorption (TPD) experiments were carried out with a Varian Chromatoprobe inserted into a Varian 1070 temperature-programmable injector equipped with liquid CO2 cooling capacity attached to a Saturn 2200 ion-trap mass spectrometer. All TPD experiments began at 293 K and were ramped at 40 deg/min to a maximum temperature of 593 K. Both the 2-3 µm silver powder and the vacuum-evaporated silver films were imaged with JEOL JSM-7000F field emission SEM and Veeco Dimension 3100D AFM. Silver films for SEIRA studies were prepared by vapor deposition onto CaF2 substrates in a home-built vacuum chamber with a base pressure of 1 × 10-6 Torr. Film thickness was monitored with a quartz crystal microbalance (Infinicon). The CaF2 were polished with a Buehler Mastermet 2 colloidal silica suspension, sonicated and rinsed with nanopure water, and air-
dried prior to usage. It has been shown in previous work that a 7 nm silver film is optimal for SEIRA under the current experimental conditions.31 Henceforth, when referring to a silver film, this is with reference to a 7 nm silver film evaporated on a CaF2 window. As with any silver film or powder exposed to atmospheric conditions, it is assumed that some level of silver oxidation has occurred, but more in-depth studies are required.35 An ABA coverage near a monolayer for SEIRA and SERS studies was prepared by pipeting 25 µL of a 50 ppm ABA solution onto a silver film and allowing the solvent to evaporate. This approximate monolayer coverage assumed an experimentally determined average spot size of 4 cm2 for all solvents and resulted in reasonably uniform ABA films of about 200 ng/ cm.2 For the DRIFTS and TPD experiments the appropriate aliquot of a 1000 ppm solution of an ABA isomer was exposed to 0.01-0.05 g of the 2-3 µm silver powder. Throughout the remainder of the paper the phrase “silver powder” refers to the 2-3 µm silver powder. Brunauer-Emmett-Teller (BET) and Langmuir surface area analyses of the 2-3 µm silver powder were determined by recording nitrogen adsorption/desorption isotherms at 77 K, using a static volumetric technique (Micromeritics ASAP 2020). Before the physisorption measurements, the catalyst systems were degassed at 623 K for 4 h under vacuum. The pore size distributions were calculated by using the adsorption branch of the N2 adsorption/desorption isotherm and the Barret-JoynerHalenda (BJH) method. Density functional theory (DFT) calculations were performed with the Gaussian 2003 suite at the B3LYP level of theory.36 A LANL2DZ basis set was used for calculations of all aminobenzoate ions complexed with silver ions, and frequencies are reported without correction. For each ABA isomer, separate calculations were performed with one silver ion associated with the benzoate group, and another calculation with a silver ion close to the benzoate group and a second silver ion in proximity to the amino group. Results and Discussion Structures a-c in Figure 1 shows the DFT optimized structures of the aminobenzoate ions OABA, MABA, and PABA, respectively, where two silver ions are complexed with the benzoate and amino groups of each of the ABA isomer ions. The resulting vibrational frequencies obtained from these optimized structures and the structures resulting from one silver ion associated with the benzoate group of each ABA isomer ion are used later to aid in the interpretation of the infrared spectra of the ABA isomers adsorbed on the silver films and the 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,
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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.
CaF2 window. A polydispersed silver film was formed with an average size of the individual Ag grains of 30 nm × 40 nm and a rms roughness of 5.7 nm. The first image in Figure 3 represents the AFM scanning of a particulate of the silver powder, while Figure 3b shows the corresponding SEM image of the silver powder with a magnification of a surface asperity. For the silver powder the rms surface roughness was found to be about 1.8 nm and the particles have an average size of 48 nm × 19 nm. It is apparent from the AFM work that the particles in both the silver film and silver powder have particles of about the same 2D dimensions with the silver films having a slightly larger height on average. BET surface area of the silver powder was 0.4026 m2/g and the Langmuir surface area was 0.5757 m2/g. Considering the BET surface area of silver powder and the fact that in our TPD and DRIFTS experiments we used from 0.01 to 0.05 g of silver powder, and assuming a 4 cm2 area of an evaporated silver film exposed to the deposition solution, this equates to approximately the same surface area in our experiments on silver films versus silver powders. Surface area and pore analysis are important factors since they relate to the samples’ surface nanomorphologies and indirectly to the spacing between the Ag surfaces. The adsorption average pore width (4 V/A by BET) was determined to be 4.3 nm, which is a value that is known to provide significant surface enhancement of the spectroscopic signal. Localization of surface plasmons (electromagnetic wave induced coherent charge density oscillations) in such Ag nanostructures can lead to strong increase in the electromagnetic fields within the surface asperities of the nanoparticles and the narrow interparticle gaps, forming the so-called spectroscopic hot spots. Enhancement of the electromagnetic fields can take place in various spectral ranges and therefore it can induce a strong enhancement of the spectroscopic signal of up to several million
Figure 4. TPD spectra of an equivalent multilayer coverage (50 µL of a 1000 ppm acetone solution) each of OABA, MABA, and PABA desorption from silver powder.
times in SERS and several thousand times in SEIRA.31 This process, can lead to the ability of detecting and studying single molecules or single layers of molecules adsorbed over the surface of the Ag nanostructures by both SERS and SEIRA.31 Figure 4 shows the TPD spectra of equal exposures of 25 µL of a 1000 ppm solution in acetone of each of the ABA isomers (equivalent to a multilayer) desorbing from silver powder. The desorption maximum for the OABA, MABA, and PABA multilayer are about 450, 490, and 510 K, respectively. This suggests that the strength of adsorption of the ABA isomers to the silver powder is PABA > MABA > OABA. TPD spectra of OABA and MABA each have a high-temperature tail associated with desorption of the monolayer. For PABA, the multilayer portion of the TPD spectrum is a more symmetric peak than that associated with OABA and MABA. However, note a peak in the PABA spectrum centered at around 600 K.
Study of Aminobenzoic Acid Isomers
J. Phys. Chem. C, Vol. 113, No. 42, 2009 18307 TABLE 1: Frequencies and Mode Assignments Associated with OABA Adsorption
Figure 5. (a) FTIR-ATR of OABA powder, (b-e) SEIRA spectra of 5, 25, 100, and 400 µL exposures of 50 ppm OABA in ether to a silver film, and (f) DRIFTS spectrum of a 25 µL exposure of a 1000 ppm OABA solution in methanol to 0.05 g of silver powder.
It is important to remember that the maximum temperature to which the silver powders were heated in the TPD experiments was 593 K. Hence, the high-temperature desorption peak in the PABA TPD spectrum centered at about 600 K actually represents PABA desorption time (associated with a 40 deg/ min ramp rate) after the silver powder reached a maximum of 593 K. We associate this high-temperature desorption peak with the PABA monolayer. SEIRA enhancement factors were obtained for the each of the ABA isomers by comparison of thick ABA layers on a clean CaF2 for the silver films and on KBr powder for the silver powder. Enhancement factors for all the ABA isomers were found to be ×20-50 when adsorbed on the silver films. MABA typically showed the least SEIRA enhancement while PABA exhibited the most. These enhancement factors were less on silver powder. These lower SEIRA enhancement factors made it more difficult to consistently obtain high-quality spectra of a monolayer of the ABA isomers on silver powder. Hence, it was decided not to include the monolayer coverage of the ABA isomers on silver powder here. In Figure 5a is presented the FTIR-ATR spectrum of the OABA powder. Next, spectra b-e in Figure 5 represent the SEIRA spectra of 5, 25, 100, and 400 µL exposures of OABA, respectively, to a silver film, using a 50 ppm ether solution. In these experiments for ABA isomer deposition on silver films each 25 µL exposure represents about a monolayer. Figure 5f shows a DRIFTS spectrum of 25 µL of 1000 ppm OABA in acetone exposed to 0.05 g of silver powder. A 25 µL exposure of a 1000 ppm solution of an ABA isomer to 0.05 g of silver powder will result in several ABA layers. Table 1 shows the mode assignments, DFT calculated frequencies, the SEIRA frequencies for OABA deposited onto a silver film from ether, and the DRIFTS frequencies for OABA deposited onto silver powder from acetone. Frequencies from the DFT calculations of the OABA ion complexed to one silver ion (in Table 1) matched the experimental data better than OABA complexed with two silver ions.
DFT
ring def.; NH2 bend ring def.; NH2 bend ring def.; NH2 bend; COO str. CH, NH bend CH, NH bend ring def.; C-NH2 str. ring def.; COO asym. str. ring def.; COO sym. str. ring def.; COO asym. str. ring def.; COO asym. str.
1043 1080 1129
1061 1113 1161
1110 1159
1176 1205 1308
(doublet) 1246 1300
(doublet) 1213 1302
1320
1325
1326
1379
1376
1379
ring def.; COO asym. str. ring def.; COO asym. str. ring def.; NH2 scissor ring def.; NH2 scissor ring def.; NH2 scissor
SEIRA ether
DRIFTS methanol
mode
1391
1390
1426
1423
1460
1503
1453 1486
1490
1525
1558
1535
1607
1585
1587
1646
1615
1611
1666
1673
1682
A range of deposition solvents with varying polarity were used for SEIRA experiments involving OABA adsorption on silver films: methanol, acetone, CH2Cl2, ether, and n-heptane. The choice of deposition solvent was not seen to have an impact on OABA adsorption in the SEIRA experiments on silver films, presumably because the strong intramolecular hydrogen bonding inhibited any impact from a change in polarity of the deposition solvent. Previous TPD results presented in Figure 4 supported this observation showing that OABA adsorption is weaker than MABA and PABA adsorption most likely because the strong intramolecular hydrogen bonding in OABA hindered a large degree of interaction with the solvent. Thus it obstructed any considerable intermolecular attractions and limited more pronounced interaction with the silver film. We have consistently observed in our work that less polar solvents such as ether and CH2Cl2 result in slightly better defined spectra because more polar deposition solvents such as acetone and methanol can damage the silver film after high solvent exposures. This explains why we have shown results using an ether solvent for the OABA experiments collected on the silver films. However, it was necessary to use an extremely polar solvent such as methanol or acetone as the deposition solvent for the OABA experiments on silver powder (as well as for ensuing results entailing MABA and PABA adsorption on silver powder) due to the low solubility of OABA in less polar solvents such as ether,CH2Cl2, and n-heptane because higher OABA concentrations are required in the DRIFTS experiments on silver powder versus the SEIRA experiments on silver films. Strong ring deformation/COO stretch bands in the 1375-1390 cm-1 range imply that OABA ionized when adsorbed on either the silver film or powder in the monolayer and possibly into the multilayer. This suggests that OABA adsorbed at least in part through the carboxylate group. No new bands or significant band shifts were observed as the OABA coverage progressed from the monolayer into the multilayer over a silver film. In
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Perry et al. TABLE 2: Frequencies and Mode Assignments Associated with MABA Adsorption
Figure 6. (a) FTIR-ATR of MABA powder, (b, c) SEIRA spectra of 25 and 400 µL exposures of a 50 ppm solution of MABA in ether to a silver film, (d) SEIRA spectrum of a 25 µL exposure of a 50 ppm solution of MABA in n-heptane to a silver film, and (e) DRIFTS spectrum of 25 µL of a 1000 ppm solution of MABA in methanol to 0.05 g of silver powder.
Figure 5b-e there is a noticeably large increase in the intensity of the ring deformations in the 1400-1600 cm-1 range relative to other bands as a function of growing OABA coverage. In Table 1 we have tentatively assigned these ring deformation modes to ionized OABA, but in fact their presence and strong growth with respect to other OABA bands may signify the onset of molecular OABA adsorption into a multilayer. One must also note the similarities upon comparing the FTIRATR spectrum of OABA powder in Figure 5a to the submonolayer and monolayer SEIRA spectra of OABA adsorbed on silver films (Figure 5b,c). It is not uncommon that the infrared spectrum of an ortho-aromatic isomer such as OABA with strong intramolecular hydrogen bonding will not look that much different from the infrared spectrum of its corresponding benzoate ion.31,32 This likeness between a molecular orthoaromatic and its ion makes it sometimes difficult to more completely interpret results of vibrational experiments where ionization is occurring. All spectra of OABA deposited on silver powder and on silver films in Figure 5 have many similarities with regard to the peaks that are present but not always the relative peak intensities. One difference is the presence of a second ring deformation/COO stretch band at about 1390 cm-1 for OABA adsorbed on silver powder (Figure 5f) that is not clearly defined when OABA is adsorbed on a silver film. It may be that this band is not resolved from the other ring deformation/COO stretch band at 1376 cm-1 when OABA is absorbed on a silver film. There are also differences in the ring deformation modes in the 1400-1600 cm-1 range when comparing OABA adsorbed on silver powder to a silver film. In general, there is a decrease in intensity of the NH bending modes (most notably in the 1200 cm-1) range and the C-NH2 stretch at 1300 cm-1 when OABA is adsorbed on a silver powder versus a silver film. These differences between OABA adsorbed on a silver film versus a silver powder suggest that OABA may adsorb with a different orientation on silver powder versus a silver film. If we assume a more upright orientation for OABA adsorbed to silver through the benzoate group, there would be a change in OABA tilt when adsorbed onto the silver film versus adsorption on silver powder. We draw this conclusion based predominately on the decrease in NH bending mode intensity and the C-NH2 stretch when OABA adsorbs on silver powder. Figure 6a presents the FTIR-ATR spectrum of MABA powder. Spectra b and c in Figure 6 are SEIRA spectra of 25
mode
DFT
CH bend ring def.; NH bend NH bend CH bend CH, NH bend C-N str. CH, NH bend ring def.; COO sym. str. ring def.; COO sym. str. ring def.; COO asym. str. ring def.; COO asym. str. ring def.; NH bend ring def.; NH2 scissor NH2 scissor CdO from NH2 intermolecular interaction
1053 1102 1118 1160 1212 1230 1341 1384 1390 1458 1500 1523 1638
SEIRA ether
SEIRA heptane
1073 1100
1078
1168 1220 1308 1381 1455 1522 1559 1633 1684
DRIFTS methanol
1124 1170 1279 1345 1388 1457 1510
1305 1390 1412 1473 1558 1640 1680
1733
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 25 µL of a 50 ppm exposure of MABA in n-heptane to a silver film. A DRIFTS spectrum of a 25 µL exposure of a 1000 ppm solution of MABA in methanol onto 0.05 g of silver powder is shown in Figure 6e. Table 2 has the MABA/silver complex mode assignments, calculated DFT frequencies, frequencies for a SEIRA spectrum of MABA deposited from ether and n-heptane on a silver film, and the DRIFTS frequencies for MABA deposited from acetone on silver powder. For MABA, mode assignments and DFT calculated frequencies for the MABA ion complexed to two silver ions in Table 2 were found to best match the experimental results. In discussing the results of Figure 6, we will temporarily bypass Figure 6d where MABA was adsorbed to a silver film in an n-heptane solution and focus on MABA adsorption using solvents with some bond polarity. There are many similarities when comparing MABA adsorption with OABA adsorption in a polar solvent to a silver film or powder. In both cases, strong ring deformation/COO stretch modes in the 1375-1390 cm-1 range again suggest ionization of MABA similar to OABA. Just as with OABA, when MABA was adsorbed on a silver powder in methanol, a second ring deformation/COO stretch mode (1412 cm-1) appeared that was not present in an MABA multilayer deposited from ether on a silver film. When going from a monolayer coverage of MABA in Figure 6b to a multilayer MABA coverage in Figure 6c on a silver film, strong ring deformation modes appear in the 1450-1600 cm-1 just as was the case for OABA. These ring deformation modes were assigned to the interactions between MABA/silver ion complex. However, the similarity in the 1450-1600 cm-1 range between a multilayer of MABA deposited from ether in Figure 6c versus the ring deformation of the FTIR-ATR of MABA powder in Figure 6a might suggest the MABA is starting to adsorb without ionization at a 400 µL multilayer exposure. A decrease of MABA ionization in the multilayer spectrum is further supported by the disappearance in Figure 6c of the NH2 scissor mode at 1684 cm-1 present in Figure 6b. It is noted that there is still a large increase in the intensity of the 1381 cm-1 ring deformation/ COO stretch mode while going from a monolayer MABA coverage in Figure 6b to a multilayer coverage in Figure 6c indicating a significant MABA ionization in the multilayer. It was seen in experiments performed on silver films that different solvents with polar bonds including CCl4, CH2Cl2, methanol, and acetone reproduced MABA adsorption observed
Study of Aminobenzoic Acid Isomers
J. Phys. Chem. C, Vol. 113, No. 42, 2009 18309 TABLE 3: Frequencies and Mode Assignments Associated with PABA Adsorption Mode CH bend ring def.; NH bend CH bend CH, NH bend C-N str. CH, NH bend
1046 1114 1164 1213 1223 1347
ring def.; COO sym. str.
1387 1391 1442
ring def.; NH bend ring def.; COO asym. str. ring def. ring def. ring def.; NH bend NH2 scissor CdO from NH2 intermolecular interaction a
Figure 7. (a-c) SEIRA spectra of 5, 25, and 400 µL exposures of a 50 ppm solution of PABA in CH2Cl2 to a silver film, (d) FTIR-ATR of PABA powder, (e, f) SEIRA spectra of 25 and 100 µL exposures of a 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 of PABA in methanol to 0.05 g of silver powder.
with ether on silver films. Acetone gave the same result as methanol for MABA adsorption on silver powder. When comparing MABA adsorption on silver powder with that on a silver film, in Figure 6e a multilayer exposure of MABA in methanol to a silver powder lacks many of the bands below 1300 cm-1 in Figure 6c for a multilayer exposure of MABA in solvents with polar bonds to a silver film. Presumably this is because of the weak surface-enhancement for MABA adsorbed on silver powder. Bands below 1300 cm-1 grow in only at much larger MABA exposures to silver powder. Spectra a-c in Figure 7 are SEIRA spectra which consist of 5, 25, and 400 µL exposures of a 50 ppm PABA solution in CH2Cl2 on silver films. Figure 7d shows the FTIR-ATR spectra of the PABA powder. A SEIRA spectrum of a 25 µL exposure of a 50 ppm solution of PABA in n-heptane to a silver film is presented in Figure 7e while a corresponding 100 µL exposure is shown in Figure 7f. Figure 7g represents a DRIFTS spectrum of a 25 µL exposure of a 1000 ppm PABA solution in methanol onto the silver powder. When assigning the vibrational modes of adsorbed PABA to silver, it was found that the DFT simulations of PABA, like MABA, complexed to two silver ions appeared more like the experimental results than a PABA ion complexed to one silver ion. Table 3 contains mode assignments, DFT frequencies for the PABA ion/2 silver ion complex, SEIRA frequencies for PABA adsorbed to a silver film using CH2Cl2 as the deposition solvent, SEIRA frequencies for PABA adsorbed to a silver film using n-heptane, and the DRIFTS frequencies of PABA adsorbed to silver powder with acetone. A large ring deformation/COO stretch mode at 1385 cm-1 in the submonolayer and monolayer PABA exposures to a silver film in the CH2Cl2 solvent in Figure 7a,b confirmed that PABA is ionizing when in direct contact with the silver film. However, PABA film growth in the multilayer is different than that of OABA and MABA in that the intensities of most of the peaks
DFT SEIRA SEIRA freq CH2Cl2 heptane
1488 1537 1646 1651 1690
1128 1174 1253 1290 1311 1327 1385 1423a 1442a 1516 1574 1603 1630 1665
1081 1140 1174 1263 1280 1345
DRIFTS methanol 1130 1175
1398
1285 (broad) 1310 1325 1390
1465
1425a
1608
1540 (broad) 1570 1610 1630 1660 (broad)
1740
Bands associated with intact PABA adsorption
have changed when comparing a PABA monolayer in Figure 7b to a PABA multilayer in Figure 7c. A similar trend in multilayer growth of the p-hydroxybenozic acid isomer versus o- and m-hydroxybenzoic acid has been previously documented.32 Evidence in spectra a-c in Figure 7 suggests that the degree of PABA ionization has significantly decreased in the multilayer coverage in Figure 7c. First, there is a decrease in the intensity of the ring deformation/COO stretch mode at 1385 cm-1 relative to the same peak in the PABA monolayer in Figure 7b. A decrease in the relative intensity of a peak associated with an ion after that ion has been covered with the molecular species has been previously observed.31,32,34 Second, the decreased intensity of the ring deformation/COO stretch mode at 1385 cm-1 relative to other bands in the multilayer PABA spectrum in Figure 7c also suggests that the degree of PABA ionization has decreased. Finally, when comparing a multilayer coverage of PABA deposited from CH2Cl2 in Figure 7c to the FTIR-ATR spectrum of PABA in powder in Figure 7d, it is apparent that the two spectra are similar. All three of these occurrences suggest that PABA ionization in the multilayer has decreased significantly as compared to the 400 µL exposure presented in Figure 7c. Just as with OABA and MABA, we have tentatively assigned all modes in the PABA spectra a-c and e-g in Figure 7 to the PABA ion/silver complex in Table 3 even though it is clear that the amount of PABA ionization has decreased in the multilayer. It is worth stating that the decrease of ionization of an aromatic acid in the next several layers deposited after the monolayer is not a foregone conclusion. We have previously shown the degree of ionization of a number of aromatic acids in the multilayer is impacted in part by the polar properties of the deposition solvent.32,34 Upon comparing a multilayer coverage of PABA adsorbed in methanol to silver powder in Figure 7g to a multilayer coverage of PABA adsorbed to a silver film in CH2Cl2 in Figure 7c, most of the same bands are present in spectra c and g in Figure 7, spare differences in ring deformation modes in the 1400-1600 cm-1 range. These differences suggest, just as with MABA, that a multilayer of PABA forms with slightly different geometric characteristics on the silver powder versus a silver film. Now we must be more specific about the polar properties of the deposition solvent. When PABA or MABA was adsorbed
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Figure 8. Schematic showing the proposed PABA adsorption to a silver after deposition in a solvent with and without polar bonds.
to a silver film with CCl4, results were obtained that mirrored the outcome with other solvents with polar bonds including methanol, ether, acetone, and CH2Cl2. In Figure 6d and Figure 7e,f for MABA and PABA adsorption to a silver film with n-heptane, a solvent with nonpolar bonds, the result is clearly different than when MABA/PABA were adsorbed to a silver film with use of a deposition solvent without polar bonds. The outcome of Figure 7e-f for PABA was also duplicated with n-pentane as the deposition solvent. We have previously reported such an impact on adsorbate geometry/surface interaction dependent on the polar properties of the deposition solvent in studies involving the nitroaniline isomers,31 hydroxybenzoic acid isomers,32 and acetylsalicylic acid and ibuprofen adsorption.34 In Figure 6d for a 25 µL exposure of MABA in n-heptane to a silver film is a band at 1733 cm-1, and in Figure 7e there is a similar band at 1740 cm-1 for a 25 exposure of PABA in n-heptane to a silver film. This is assigned as a mode indicative of a free carbonyl stretch caused by hydrogen bonding induced reorientation between two PABA or MABA molecules containing amino groups. A number of different researchers have observed intermolecular attraction between molecules with amino groups (or amino/NO2 groups) and amino/C-O interactions only in the multilayer.31,37 For example, in previous work involving the adsorption of m-nitroaniline, bands indicative of intermolecular hydrogen bonding were not detected until at least a 100 µL exposure of a 50 ppm solution on a silver film.31 Figure 6d for MABA adsorption with an n-heptane deposition solvent and Figure 7e for PABA adsorption with n-heptane are the first reports of hydrogen bonding between amino groups of one type of adsorbed aromatic molecule in the monolayer. Figure 8 is an illustration of proposed models for PABA adsorption in solvents with and without polar bonds on rough silver substrates. First is a PABA molecule adsorbed in a more upright position to a silver film through the carboxylate group when using a deposition solvent with polar bonds. There are also two tilted PABA molecules adsorbed to a silver film with use of a deposition solvent with nonpolar bonds where the potential hydrogen bonding is highlighted and where one oxygen of the carboxylate group is interacting less with the silver film than the other oxygen. This PABA adsorption configuration could present in an infrared spectrum with a carbonyl stretch. A similar model is proposed for MABA adsorption. Besides the emergence of the carbonyl stretch when MABA and PABA are adsorbed to a silver film with use of n-heptane, there are a number of other changes in MABA and PABA adsorption that occur upon switching from a deposition solvent with polar bonds to one without polar bonds. First, there are significant changes in all the COO symmetric and asymmetric
Perry et al. stretch bands for both MABA and PABA that indicate a considerable alteration of the carboxylate interaction with the silver film. Second, the NH2 scissor modes present at 1585, 1615, and 1673 cm-1 for MABA and at 1684 cm-1 for PABA disappear when n-heptane is the deposition solvent. This is to be expected when the NH2 is involved in hydrogen bonding with an adjacent group.37 Third, there are dramatic frequency changes for every mode involving either an NH bend or an C-N stretch in the PABA and MABA monolayer spectra when switching from a deposition solvent with bond polarity to n-heptane without bond polarity. NH bend modes in the 1100-1350 cm-1 range for PABA adsorption to a silver film with n-heptane in Figure 7e,f are much smaller than the corresponding NH bend modes for PABA adsorption to a silver film with CH2Cl2 in Figure 7b-d. A decrease in the NH bend modes does not suggest a PABA orientation where the amino group is interacting with the silver film. The decrease in these NH bend modes when PABA is adsorbed to a silver film by using a deposition solvent with nonpolar bonds such as n-heptane versus PABA deposition with a solvent with polar bonds is consistent with reorientation of adsorbed PABA and interaction between adjacent PABA amino groups. It is proposed that when deposition occurs by using a solvent with nonpolar bonds such as n-heptane there is less PABA solvation during the adsorption process than when a deposition solvent with polar bonds is used. This lower level of solvation thus allows for the increased intermolecular attraction between MABA and PABA amino groups in the monolayer. Conclusion SEM and AFM showed that the silver powder and silver films both have similar surface roughness that led to surface enhancement in SEIRA. TPD results highlighted the fact that each of the ABA isomers desorbed from the silver powder with peaks from the monolayer and multilayer. The TPD results also indicated that the strength of adsorption to silver powder was PABA > MABA > OABA. SEIRA revealed that OABA ionized in the monolayer with a subsequent decrease of ionization in the multilayer when OABA was adsorbed on silver films independent of the polar properties of the deposition solvent. Because of the strong intramolecular hydrogen bonding in OABA films, a change in the polar properties of the deposition solvent was not seen to impact OABA adsorption. OABA multilayer adsorption to both silver films and the silver powder exhibited similar infrared spectra spare differences in the ring deforrmation modes in the 1400-1600 cm-1 range. MABA demonstrated multilayer adsorption on a silver film and silver powder that closely mimicked OABA adsorption with use of a polar deposition solvent although PABA formed quite differently. When MABA and PABA were deposited on silver films by using an alkane deposition solvent with nonpolar bonds, it was shown that there was a strong intermolecular interaction in the monolayer most likely due to a hydrogen bonding network developed between amino groups on adjacent MABA/PABA molecules in the monolayer. Acknowledgment. We acknowledge the Arkansas Nanotechnology Center for use of the AFM, Raman, and BET isotherm equipment. We thank Dr. Pat Desrochers, Dr. Jerry Manion, Dr. Karen Weaver, and Dr. Brian Gilbert for many fruitful discussions. References and Notes (1) Panyala, N.; Pena-Mendez, E.; Havel, J. J. Appl. Biomed. 2008, 6, 117–129.
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