Surface-Enhanced Vibrational Spectroscopy and Density Functional

Article pubs.acs.org/JPCC

Surface-Enhanced Vibrational Spectroscopy and Density Functional Theory Study of Isoniazid Layers Adsorbed on Silver Nanostructures Aaron R. Owen, Jon W. Golden, Adam S. Price, William A. Henry, William K. Barker, and Donald A. Perry* Department of Chemistry, University of Central Arkansas, Conway, Arkansas 72035, United States S Supporting Information *

ABSTRACT: A combination of surface-enhanced Raman spectroscopy (SERS), surfaceenhanced infrared spectroscopy (SEIRA), and density functional theory (DFT) calculations are used to study the adsorption and thin film growth of isoniazid (an antimicrobial used to treat tuberculosis) on silver nanostructures (SNS). It is known that an isoniazid crystal takes on a C1 configuration where one of the hydrogens at the end of the hydrozyl group (−NH-NH2) form an intramolecular hydrogen bond with the only oxygen atom. DFT calculations involving combinations of isoniazid dimers, trimers, and tetramers are used to model and interpret the Raman spectrum of isoniazid powder. The DFT calculations highlight the importance of accurately modeling hydrogen-bonding interactions between adjacent hydrozyl groups as well as interactions involving the ring nitrogen and the hydrozyl group. SERS, SEIRA, and DFT calculations involving isoniazid dimers and four-metal silver clusters reveal that isoniazid takes on a T conformation in a monolayer adsorbed on SNS where the hydrozyl group is rotated around a single bond so that the −NH2 is oriented down with respect to the oxygen atom. Further evidence from the SEIRA and DFT calculations illustrate that the multilayer of isoniazid deposited from acetone maintains a geometry similar to the T conformation. In addition to the polar solvent acetone, n-heptane, a solvent with nonpolar bonds, is used to study the interactions of isoniazid with the SNS. These studies reveal that isoniazid has a stronger affinity for SNS when using n-heptane as the deposition solvent. An increase in attraction between isoniazid and the SNS is probably accomplished by changing the angle of interaction of the NH2 and −NH groups with respect to the SNS. It is believed that a lower level of solvation occurs when using n-heptane as a deposition solvent, which allows for increased isoniazid attraction to the SNS.

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INTRODUCTION Isoniazid is an important antibacterial agent used as a first line treatment of tuberculosis.1 Bacterial resistance to tuberculosis drugs is increasing, and in 2012, nearly 9 million people were infected worldwide, resulting in 1.2 million deaths.2 It is wellknown that silver nanoparticles (SNPs) exhibit broad-range antibacterial properties.3,4 Perhaps more interesting is the notion that when used in concert, antibiotics and SNPs can yield synergistic inhibitory effects on bacterial growth and proliferation.5 In 2011, a study using SNPs with isoniazid reported a 95% survival rate for mice infected with tuberculosis; this was an improvement of 55% versus isoniazid therapy alone.6 Much of the pathogenic resistance to antibiotics is due to the release of antibiotics into the environment.7 Inevitably, newly synthesized types of SNPs will also be found in the environment. A looming question is what happens to pathogenic resistance when exposed to both a specific antibiotic and SNPs? In the meantime, it is becoming more imperative to understand how the most important antibiotics interact with metal nanoparticles (MNPs). There have been a number of recent spectroscopic studies looking at isoniazid and the interaction of isoniazid with MNPs. Several research groups have used various combinations of infrared, Raman, and density functional theory (DFT) calculations to study isoniazid.8−10 Two surface-enhanced © 2014 American Chemical Society

Raman spectroscopy (SERS) studies have been published exploring the interaction of isoniazid with SNPs or gold nanoparticles (AuNPs).11,12 Both of these SERS papers confirmed that isoniazid adsorbs to the SNPs/AuNPs through the ring nitrogen atom similar to pyridine adsorption to SNPs.13 Unlike pyridine, the hydrozyl side group of isoniazid is likely to allow for strong hydrogen bonding in the monolayer. However, neither of the previous SERS manuscripts explored the idea of isoniazid hydrogen bonding in the monolayer or the impact of the MNPs on isoniazid multilayer formation in conjunction with DFT calculations. It is well-known that SERS is a short-ranged phenomena providing large Raman enhancement factors primarily in the monolayer.14 On the other hand, surface-enhanced infrared absorption (SEIRA) has a longer range effect where the infrared absorption can be enhanced in both the monolayer and the multiayer.15−18 Used together, SERS and SEIRA can document all stages of thin organic and biological film growth.19−28 Special Issue: John C. Hemminger Festschrift Received: May 30, 2014 Revised: October 22, 2014 Published: October 22, 2014 28959

dx.doi.org/10.1021/jp505360b | J. Phys. Chem. C 2014, 118, 28959−28969

The Journal of Physical Chemistry C

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method. SEIRA enhancement factors observed from isoniazid on the SNS were typically about a factor of 30−40 and the SERS enhancement factor for isoniazid was approximately ×104, as determined by comparison to thick isoniazid layers deposited on bare CaF2 windows. Scanning electron microscopy and atomic force microscopy have been employed previously to investigate these silver films which are typically rounded particles with an average particle size of 50−100 nm.17

In recent years, DFT calculations with small metal clusters (typically 3−20 metal atoms) have been used to successfully model SERS spectra of molecules such as pyridine.29−34 Small metal cluster calculations have even been effective at characterizing such subtleties as shifts in CH2 and NH2 wagging modes in the side chains of aromatic molecules adsorbed to MNPs.35 Most of these studies involve the adsorption of molecules that do not have hydrogen-bonding in the monolayer. When considering the Raman and infrared spectra of pure organic molecules, DFT simulations with dimers and more complex arrangements of molecules based on crystal structures have proved suitable in characterizing complex hydrogen-bonding networks. However, reasonable reproduction of Raman and infrared spectra can often be hit or miss depending on the complexity of the intermolecular attractions.8,16 Here we will use a combination of SERS, SEIRA, and DFT calculations to characterize the growth of thin layers of isoniazid on silver nanostructures (SNS) formed through vacuum deposition of silver onto salt plates. SERS will be used both to characterize the initial formation of the monolayer and to document any changes in the monolayer during multilayer deposition. SEIRA spectra will be used to monitor all stages of isoniazid film growth while being cognizant of the fact that the SEIRA spectra represent a composite of both the mono- and multilayer. Because of strong hydrogen bonding between the hydrozyl (−NH-NH2 group) side bands in isoniazid, DFT simulations of the SERS spectra will involve isoniazid dimers in proximity to small four-atom silver nanoclusters. It will also be demonstrated that more intricate structures than dimers are necessary to begin to truly model bulk vibrational spectra of something like isoniazid powder because of multifaceted hydrogen-bonding interactions.

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THEORETICAL BASIS DFT simulation of vibrational spectra used the Gaussian 2009 suite at the B3LYP level of theory.36−38 A cc-PVDZ basis set was used for the C, H, O, and N atoms in calculations, and a ccPVDZ-PP basis set was used for silver atoms in the clusters. All vibrational frequencies were scaled by a factor of 0.97 in order to account for anharmonicity and other issues inherent to approximations made in the calculation.

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RESULTS AND DISCUSSION Figure 1 displays the three DFT optimized geometries of isoniazid. We borrow the structure labels C1, C2, and T from

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EXPERIMENTAL SECTION Isoniazid was purchased from Aldrich. Twenty-five ×4 mm polished CaF2 windows were purchased from International Crystal Laboratories. 99.99% pure silver wire for silver film preparation was from Myron Toback. SEIRA and attenuated total reflectance Fourier transform infrared spectroscopy (ATRFTIR) spectra were obtained on a Thermo-Nicolet IR100 FTIR spectrometer. All infrared spectra were conducted in transmission mode (4 cm−1 resolution and 16 scans were averaged). ATR-FTIR spectra were collected with a Thermo Foundation Series Performer ATR Attachment with a diamond ATR crystal (single-bounce design). Raman and SERS spectra were taken with a SciAps. Inc. Advantage 633 Raman spectrometer with a 633 nm excitation line in back scattering mode. SNS were grown on CaF2 windows by thermal evaporation of 7 nm of silver as measured with an Infinicon quartz crystal microbalance in a home-built vacuum chamber with a base pressure better than 1 × 10−6 Torr. CaF2 crystals were polished with a silica suspension rinsed and sonicated in methanol, and air-dried. A monolayer exposure of isoniazid was prepared as a cast film for SEIRA and SERS analysis by pipetting 25 μL of a 50 ppm of solution onto SNS on CaF2 and allowing the solvent to evaporate. This is approximately a monolayer assuming an experimentally determined average spot size of four cm2 for all solvents resulting in reasonably uniform layer of about 250 ng/ cm2. The monolayer approximation accounts for the number of isoniazid molecules in the given area on a flat surface and does not specifically account for surface roughness. A multilayer of isoniazid will be formed with multiple aliquots of the same drop

Figure 1. Three DFT optimized geometries of isoniazid with the SCF energy of each isomer. The distance between O(15) and H(12) is highlighted in the figure for the C1 (2.36 Å) and C2 (3.24 Å) isomers. The O(15) to H(12) distance is 4.20 Å for the T isomer.

other researchers.8 In Figure 1, the top geometry C1 is thought to be the dominant geometry in the X-ray crystal structure of isoniazid.8,39 C1 is the lowest energy optimized DFT structure of the three isoniazid structures. A key observation in the C1 structure is that the end −NH2 is oriented such that one of the −NH2 hydrogen atoms is within hydrogen-bonding distance to the lone oxygen in isoniazid (in the C1 geometry in Figure 1 this is O-15 hydrogen bonded to H-12). The T conformation is the second most stable structure where the −NH2 group is rotated such that it is pointing away with respect to the lone oxygen atom−out of hydrogen bonding range. C2 is the highest energy structure and is similar to C1 except that the hydrogen atoms of the end −NH2 group are pointing down with respect to the lone oxygen atom. For future reference, interactions with the ring nitrogen of isoniazid will be referred to as at position 1, while interactions with the lone oxygen will be position 2 and interactions with the hydrozyl end group will be position 3. 28960



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Figure 2. From the bottom are the Raman spectra obtained from isoniazid powder (red) and the DFT optimized C1 structures of the single molecule (1−3) dimer, (2−3) dimer, (3−3) dimer, (1−3−3) trimer, and the (1−3−3−1) tetramer. DFT Raman simulations are actually scattering activities (A4/AMU) and have been scaled to match the Raman powder.

In Figure 2 the bottom Raman spectrum (red) is from isoniazid powder. In an attempt to model the Raman spectrum of isoniazid powder, various DFT Raman simulations are performed assuming a C1 isoniazid configuration and are stacked vertically in Figure 2. Starting from the second spectrum and going from the bottom to the top is a Raman DFT simulation of a lone C1 isoniazid molecule. Next is the C1 dimer where the ring nitrogen (position 1) is interacting with the hydrozyl group (position 3) of an adjacent isoniazid molecule. This is referred to as the (1−3) dimer. Henceforth, in the description of DFT structures with more than one isoniazid molecule in parentheses will be the ordering of the molecular interactions by position. Accordingly, above the (1−3) dimer are the (2−3) dimer and the (3−3) dimer. Pictures of the three different DFT optimized dimers are below the spectra in Figure 2. The top two DFT simulations in Figure 2 involve the (1−3− 3) trimer and the (1−3−3−1) tetramer. These trimer and tetramer complexes are depicted on the right in Figure 2. All DFT simulations in Figure 2 and future figures are up-scaled on the Y-axis to match the intensity of experimental spectra for visual comparison in the figure.

Table 1 demonstrates vibrational data relating to the C1 configuration. The first three columns of data are the vibrational frequencies obtained from the Raman spectrum of isoniazid powder and the DFT simulations of the C1 molecule and the C1 (3−3) dimer. The last column contains mode assignments. For DFT simulations of dimers in the tables, many of the vibrational modes have two vibrational frequencies because vibrational motion is often duplicated in each molecule of the dimer. Frequencies involving the same vibrational motion in each of the molecules in the dimer are often different because of variations in geometry and attractive forces. Inspection of Figure 2 and Table 1 shows that the various DFT simulations in Figure 2 are reasonable facsimiles of the Raman spectrum of isoniazid powder in the regions below 1250 cm−1 and above 1500 cm−1. Some of the theoretical bands in these regions are shifted from the experimental values, but it is apparent that the simulations have at least accounted for all the experimental bands in these two regions. These small shifts in the theoretical bands are not a surprise given the limitations of all quantum metal cluster calculations such as these and the fact that the hydrogen bonding in a collection of isoniazid molecules is sure to be complex.40,41 Hence, it is not a surprise 28961



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Table 1. Isoniazid C1 Configuration Vibrational Dataa Raman powder

DFT C1 molecule

DFT C1 (3−3) dimer

DFT C1 (3−3) dimer 4 × 2 Ag @pos 1

838

837

893 1006

869 981 1051 1094

837 839 884 980 1051 1104 1114 1176 1178 1200 1201 1261 1312 1315 1391 1469 1470 1499 1507 1560 1596 1634 1636 1695 1697

842 844 884 999 1050 1104 1115 1175 1177 1200 1201 1262 1312 1315 1399 1470 1471 1502 1512 1557 1609 1634 1635 1700 1702

1112 1135 1189 1221

1335

1184 1202 1260 1271 1317 1390 1444 1477

a

1556 1604

1560 1596 1642

1672

1705

ATR powder

modes CH (OP) bend

869 1060 1090

NH2 wag ring breathing ring def. CN str./NH bend

1140

NN str./NH bend

1194 1220 1263 1330

CH/NH bend

1410

ring def. NH bend NH2 rock ring def. ring def./NH bend

1490

ring def./NH bend

1550 1601 1632

ring def. ring def. NH2 scissor

1662

CO str.

Stretch modes (str.), deformation (def.), out of plane (OP).

Figure 3. On the left from bottom to top are Raman/SERS spectra of: the T (3−3) dimer, the T (3−3) dimer 4 × 2 Ag @pos1, SERS from a monolayer of isoniazid deposited from acetone downscaled by a factor of 2, isoniazid powder, the C1 (3−3) dimer, and the C1 dimer (3−3) 4 × 2 Ag @pos1. DFT Raman simulations are actually scattering activities (A4/AMU) and have been scaled to match the Raman powder.

980 cm−1. The tetramer has the ring breathing modes closer to 990 cm−1 because that model begins to account for all of the more important hydrogen-bonding interactions in the isoniazid crystal structure including (3−3) and (1−3) hydrogen-bonding interactions.8,35 Another strong band in the Raman spectrum of isoniazid powder is a −NH2 wag mode at 893 cm−1.

that small clusters of 2−4 isoniazid molecules will not be able to exactly reproduce the multifaceted hydrogen-bonding networks that are sure to be present in the Raman spectrum of isoniazid powder. For example, the experimental ring breathing mode for isoniazid powder is at 1006 cm−1, but all of the dimer calculations have the breathing mode at close to 28962



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Table 2. Isoniazid T Configuration Vibrational Dataa SERS acetone

SERS n-heptane

SEIRA acetone monolayer

DFT T molecule

DFT T (3−3) dimer

687

702

843 (weak)

841 (strong)

843

919 1010

924 1014

699 716 837 849 939 980

1147

1142

902 983

1065

1059

1066

1060

1090

1105

1075

1145

1150

1208

1214

1261

1341 1376

1339 1371

SEIRA multilayer n-heptane

1012

1045 1105

SEIRA acetone multilayer

1165 1213

1200

1260

1265

1260

1295

1295

1278

1343 1373 1406

1370

1336 1373

1351

1410

1390

1440

1448

1435

1428

1502

1504

1473

1572

1580

1514

1554

1607 1631

1608 1633

1508 1528 1602

1456 1473 1491 1533 1602

1597

1594 1626

1663

1670

1660

1702

a

1063 1067 1076 1162 1165 1198 1200 1258 1260 1279 1319 1342 1357 1389 1391 1418 1434 1471 1473 1551 1556 1594 1627 1630 1693 1722

DFT T (3−3) dimer 4 × 2 Ag @pos 1 692 708 834 847 926 999 1000 1068 1072 1080 1083 1161 1163 1198 1263 1265 1281 1302 1346 1365 1398 1399 1420 1442 1473 1477 1549 1553 1607 1627 1633 1686 1723

modes ring/NH (OP) bend CH (OP) bend NH2 wag ring breathing ring def./NH bend ring def./NH bend NN str./NH bend CH/NH bend ring def. NH2 rock CN str./NH bend ring def./NH bend ring def./NH bend ring def. ring def. ring def. NH2 scissor CO str.

Stretch modes (str.), deformation (def.), out of plane (OP).

intense −NH bend mode at 1271 cm−1 and a very weak −NH2 rock mode at 1317 cm−1. The C1 (1−3) and (2−3) dimer simulations, with strong −NH bend modes at 1271 and 1274 cm−1, respectively, are no better than the C1 molecule simulation at reproducing the experimental band at 1335 cm−1. The Raman simulation of the C1 (3−3) dimer has a strong set of doublet −NH2 rock modes at 1312/1315 cm−1, which is more representative of the 1335 cm−1 than bands from the C1 (1−3) or (2−3) dimers. This result agrees with X-ray data35 showing interactions between −NH/−NH2 hydrozyl groups of adjacent isoniazid molecules are the key hydrogenbonding interactions in isoniazid powder. Note in Figure 2 that the most intense −NH bend modes further blue shift to 1320 cm−1 in the (1−3−3) trimer and to 1325 cm−1 in the (1−3− 3−1) tetramer as more isoniazid molecules are added to better model all of the hydrogen-bonding interactions present in the crystal structure of isoniazid. On the left of Figure 3 are various experimental and theoretical Raman and SERS spectra involving isoniazid. In the two center spectra (colored in black) are the Raman spectrum of isoniazid powder (reproduced from Figure 2) and the SERS spectrum of an isoniazid monolayer deposited on SNS from acetone. In the top blue spectra are the simulated Raman spectra from just the C1 (3−3) dimer and the C1 (3−3) dimer

Frequencies of −NH2 and CH2 wag modes are known to be quite susceptible to bonding environments.31 Accordingly, the simulation of a single C1 isoniazid molecule places the −NH2 wag at 869 cm−1m while the simulation of the isoniazid (3−3) dimer has it at a more reasonable 884 cm−1 due to hydrogen bonding between the hydrozyl groups. Below 1250 cm−1, two other protuberant bands in the Raman spectrum of isoniazid powder at 1189 and 1221 cm−1 are assigned as combination bands. The protuberant bands involve NH bend modes by doublets in the C1 (3−3) dimer spectrum at 1176/1178 cm−1 and 1200/1201 cm−1. In the region above 1500 cm−1, bands in the Raman of isoniazid powder at 1556, 1604, and 1672 cm−1 are represented in the Raman C1 (3−3) dimer simulation by ring deformation modes at 1560 and 1596 cm−1 and a CO stretch at 1695/1697 cm−1. In the Raman spectrum of isoniazid powder, the largest peak at 1335 cm−1 is associated with −NH bend modes where the phrase −NH bend mode will be generic for any motion involving the −NH and −NH2 groups in isoniazid. It is here where the combination of various Raman simulations in Figure 2 reveal the most important hydrogen-bonding effects in the isoniazid crystal structure. A Raman simulation of a single C1 isoniazid molecule does a poor job of reproducing the experimental band at 1335 cm−1 since it contains only one 28963



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isoniazid adsorbed on SNS. There are a number of bands present in the SERS spectrum of isoniazid that are not observed in the Raman spectrum of isoniazid powder or have undergone a frequency shift. Some of the most obvious and important changes can be seen in the −NH bend mode region from 1150 to 1400 cm−1. For the −NN str./−NH bend at 1189 cm−1 in the Raman spectrum of isoniazid powder in Figure 3 and Table 1, C1 calculations place the frequency at 1184 cm−1 for the simulation of one gas-phase isoniazid and at 1176/1178 cm−1 for the C1 (3−3) dimer. However, Figure 3 and Table 2 illustrate that the SERS monolayer of isoniazid this −NN str./− NH bend mode shifted down to 1147 cm−1, which is more consistent with DFT T conformation frequencies of 1165 and 1162/1165 cm−1 observed for a single isoniazid molecule and the T (3−3) dimer, respectively. The broad band associated with −NH bend modes at 1335 cm−1 in the Raman spectrum of isoniazid powder gives way to a large, broad doublet at 1341 and 1376 cm−1 in the SERS spectrum of an isoniazid monolayer adsorbed on SNS. From the Raman simulations of the T (3−3) dimer this doublet is described by a −CN str./−NH bend modes at 1342/1357 cm−1. Figure 4 shows for a single T

with four-atom silver clusters at position 1 of each molecule in the dimer. Pictures of the C1 (3−3) dimer and C1 (3−3) dimer with silver clusters are to the right of the spectra in Figure 3. Dimers with metal clusters will henceforth have a designation at the end such as 4 × 2 (two four-atom silver clusters) and @pos 1 describing which of the three isoniazid positions is interacting with the metal cluster. Therefore, the complete name of the C1 (3−3) dimer with two 4-atom silver clusters in Figure 3 is C1 dimer (3−3) 4 × 2 Ag @pos 1. The C1 dimer (3−3) 4 × 2 Ag @pos 1 frequency data can be found in column 4 of Table 1. At the bottom of Figure 3 in red are Raman simulations of the T (3−3) dimer and T (3−3) dimer 4 × 2 Ag @pos 1 with the corresponding pictures of the DFT optimized geometries to the right of the theoretical spectra. Table 2 contains Raman data relating to the T configuration of isoniazid. The first column comprises vibrational frequencies from the SERS of a monolayer of isoniazid deposited from acetone. In columns 6−8 of Table 2 are vibrational frequencies obtained from the Raman simulation of one isoniazid molecule, and the Raman simulations from the T (3−3) dimer and T (3− 3) 4 × 2 Ag @pos1. Mode assignments are in column 9 of Table 2 based on the T (3−3) dimer and T (3−3) 4 × 2 Ag @ pos1 calculations and through comparison to previous isoniazid mode assignments.8−12 In Table 2, many, but not all, frequencies containing −NH bend modes are quite similar when comparing the isoniazid T (3−3) dimer with the T (3−3) dimer 4 × 2 Ag @pos 1. A parallel result occurs in Table 1 when relating theoretical frequencies linked to the C1 (3−3) dimer and the C1 (3−3) dimer 4 × 2 Ag @pos 1. This is not a surprise because a metal cluster interacting with the ring nitrogen of isoniazid cannot alter vibrational frequencies associated with −NH bend modes in the hydrozyl side chain through resonance. However, the DFT calculations do show that the metal clusters optimized at position 1 in both the C1 (3−3) and T (3−3) dimers alter frequencies and relative intensities of various ring modes such as the deformation modes around 1600 cm−1. Such a result can be seen when inspecting the Raman spectra obtained from the T (3−3) dimer and the T (3−3) dimer 4 × 2 Ag @pos 1 in the two bottom spectra of Figure 3 and Table 2. On a first glance, it also appears in Figure 3 that all of the NH bend modes in the 1200−1400 cm−1 are smaller in the T (3−3) dimer 4 × 2 Ag @ pos 1 spectrum versus the T (3−3) dimer spectrum. The issue is that the ring deformation modes around 1600 cm−1 were so large in the Raman simulation of the T (3−3) dimer 4 × 2 Ag @pos 1 that the deformation makes some of the other vibrational modes appear small when the Raman spectrum of the T (3−3) dimer 4 × 2 Ag @pos 1 is scaled to match the relative intensities of the experimental spectra in Figure 3. Hence, vibrational intensities are approximately the same for the −NH bend modes with and without the metal clusters at position 1 in the Raman simulation of the T (3−3) dimer. In Figure 3 and Table 2 the ring breathing mode of isoniazid has shifted from 1006 cm−1 in the Raman spectrum of isoniazid powder to 1010 cm−1 in the SERS spectrum of an isoniazid monolayer. This frequency shift has been observed in other isoniazid SERS studies and confirms that the isoniazid molecule is adsorbing to the SNS through the ring nitrogen in a manner similar to pyridine.11−13 Such an experimental result supports DFT models of the SERS spectrum of isoniazid where the metal cluster is in proximity to ring nitrogen. Evidence will now be offered that points to the T conformation as the primary geometry of a monolayer of

Figure 4. Select normal modes for the T conformation of isoniazid with frequencies and dimer frequencies.

isoniazid molecule the normal modes associated with the −NN str./−NH bend mode, the −CN str./−NH bend mode, and an example of the nearest combination band close to 1400 cm−1. A vibrational frequency associated with this −CN str./−NH bend mode is extraordinarily susceptible to the geometry/bonding environment. In the T conformation of isoniazid, the vibrational frequencies (1342/1357 cm−1) in the (3−3) isoniazid dimer associated with the −CN str./−NH bend mode are separated by 15 cm−1 because of the different orientations of each molecule in the dimer. Even more surprising is that this −CN str./−NH bend mode shifts down to 1104/1114 cm−1 for the C1 (3−3) dimer. These results suggest a geometry close to the T conformation for isoniazid in the monolayer adsorbed on SNS. It is not that difficult to eliminate the C1 geometry from the monolayer argument. For the C1 (3−3) dimer, one isoniazid molecule points up and the other one has to point down for 28964



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because some of these bands are probably from the multilayers. Columns 3 and 4 in Table 2 contain the experimental vibrational frequencies from SEIRA experiments of an isoniazid monolayer and multilayer, respectively. The vibrational frequencies extracted from the ATR-FTIR of isoniazid powder are in column 5 of Table 1 because the C1 configuration proliferates in bulk isoniazid. It is intriguing that the 1343 cm−1 peak in the SEIRA spectra in Figure 5 from the isoniazid monolayer begins to disappear with increasing isoniazid exposure and is replaced by a band that first appears at 1373 cm−1 and grows in to around 1370 cm−1. This begs the question: is the isoniazid monolayer reorganizing during multilayer formation or is the 1343 cm−1 band simply getting buried in SEIRA spectra of thick isoniazid layers? To answer this query, in Figure 6, SERS was used to

hydrogen-bonding to occur at the (3−3) positions. Assuming isoniazid adsorbs upright to SNS when adsorption occurs using a polar deposition such as acetone, it is improbable for a C1 geometry to exist at a real interface where SNS have to interact with position 1 (the ring nitrogen) of both molecules in the dimer. No evidence has been found to even consider the C2 geometry of isoniazid in adsorption studies on SNS in as it is much higher in energy than the C1 and T conformations. Figure 5 contains SEIRA spectra from bottom to top in red for increasing exposures of isoniazid deposited on SNS from

Figure 6. At the bottom in black is the background spectrum for the SNS. In red from bottom to top are SERS spectra for increasing isoniazid coverage deposited from acetone (∼1, 2, 4, 8, 16, and 36 layers) analogues to the SEIRA spectra in Figure 5

Figure 5. From bottom to top are SEIRA spectra for increasing exposures (∼1, 2, 4, 8, 16, and 36 layers) of isoniazid deposited from acetone on SNS. The top spectrum is the ATR-FTIR of isoniazid powder downscaled by 8.

acetone. Recall that individual monolayers were formed via the drop method whereby a 25 μL aliquot of a 50 ppm isoniazid/ acetone was deposited on the SNS and allowing the solvent to evaporate. Multiple aliquots of the 50 ppm isoniazid/acetone solution were deposited to form the multilayer. In the top spectrum is the ATR-FTIR of isoniazid powder (shown in black). When considering the bottom SEIRA spectrum of a monolayer of isoniazid in Figure 5, there are a number of small peaks and one dominant peak at 1343 cm−1. This band is believed to be the same −CN str./−NH bend mode observed in the SERS spectrum of a monolayer of isoniazid at 1341 cm−1 in Figure 3. It is also presumed based upon previous SERS data in Figure 3 that the monolayer organizes in the T configuration of isoniazid in all SEIRA spectra in Figure 5. One of the issues with the formation of cast films from the drop method where the solvent is allowed to evaporate is that, even at a monolayer exposure, it is likely that there are regions where multilayer formation is occurring due to uneven deposition. Hence, it is difficult to definitely assign small bands in the SEIRA spectrum of an isoniazid monolayer in Figure 5 to the actual monolayer

monitor changes in the isoniazid monolayer as a function of increasing multilayer exposures identical to Figure 5. In the bottom spectrum is the background SERS spectrum for just the silver film where bands due to low-level contamination are observed at ∼930 and 1050 cm−1. The background band at 1050 cm−1 appears to dissipate, while the 930 cm−1 band remains constant with increasing isoniazid exposure, suggesting that these two bands are originating from different contaminants. What is ascertained from Figure 6 is that the SERS spectrum from a monolayer of isoniazid does not change in any appreciable way during the buildup of isoniazid layers. Some of the bands increase in intensity with higher isoniazid exposure. In particular, ring deformation modes around 1600 cm−1 become larger with cumulative isoniazid layers. This might happen because the isoniazid molecules are packing tighter in the monolayer or are still filling in gaps in the monolayer during the onset of multilayer formation. A theory of tighter packing in the monolayer as a function of increased isoniazid film thickness would also account for the slow 28965



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Figure 7. (Left) SERS spectra from separate experiments representing a monolayer of isoniazid deposited from n-heptane on SNS. (Right) From bottom to top SEIRA spectra of increasing coverage (∼1, 2, 4, 8, 16, and 32 layers) of isoniazid deposited from n-heptane on SNS.

disappearance of the contaminant peak at 1050 cm−1. However, results in Figure 6 do show that there is no dramatic rearrangement of isoniazid in the monolayer during multilayer development, which suggests that the 1343 cm−1 band from an isoniazid monolayer in the SEIRA spectra of Figure 5 is simply getting buried by the signal from the SEIRA multilayer. It is also necessary to state that the relative ratios of the two bands representing −CN str./−NH bend mode modes at 1341/1376 cm−1 in the SERS spectrum of an isoniazid monolayer in Figures 3 and 6 are not constant. In many reproductions of the SERS spectrum of a monolayer of isoniazid adsorbed on SNS, the band at 1376 cm−1 is more intense than the 1341 cm−1 peak. In other spectra, the band at 1376 cm−1 is no more than a shoulder of the peak at 1341 cm−1. All other bands in the SERS spectra of isoniazid reproduce consistently. There is no clear rational to explain this variance in the ratio of the intensities of the −CN str./−NH bend mode doublet other than the possibility that an isoniazid monolayer adsorbed on SNS can be somewhat amorphous. Thus, subtle variations in the structure and bonding in different isoniazid monolayers adsorbed on SNS account for fluctuations in the relative band intensities of the doublet. Experimental and theoretical evidence suggest that a conformation similar to the T geometry of isoniazid develops in an isoniazid multilayer accrued on SNS. First, it is quite apparent in Figure 5 and a casual comparison of frequencies in Tables 1 and 2 that the SEIRA spectra involving a multilayer of isoniazid on SNS are quite different than the ATR-FTIR of isoniazid powder. Essentially all bands involving a −NH bend or the −CN stretch mode have changed when comparing the two sets of spectra. In Table 2, DFT simulations involving the isoniazid T (3−3) dimer are a better representation of the experimental SEIRA spectra than calculations involving the isoniazid C1 (3−3) dimer data in Table 1. None of the C1 dimer simulations even have bands in the range 1315−1390 cm−1 that could account for the bands at 1343/1370 cm−1 in the SEIRA experiments. On the other hand, DFT calculations involving the T (3−3) dimer were able to account for the

frequencies of all the vibrational bands in an isoniazid multilayer adsorbed on SNS. It is most likely that the 1341 cm−1 is a −CN str./−NH bend of isoniazid induced by direct interaction with SNS in the monolayer. As the multilayer forms, the 1341 cm−1 band does not grow and begins to get buried by the broad bands emerging between 1300 and 1400 cm−1. The second −CN str./−NH bend mode growing in around 1370 cm−1 increases in intensity as a function of isoniazid coverage. Similar to the SERS spectra of an isoniazid monolayer, these two bands in the SEIRA spectra at 1343/1370 cm−1 are represented in theoretical spectra by the −CN str./−NH doublet mode at 1342/1357 cm−1 from the T (3−3) dimer and at 1346/1365 cm−1 for the T (3−3) dimer 4 × 2 Ag @pos 1. All the SERS and SEIRA worked discussed up until this point in Figures 3, 5, and 6 use acetone as the deposition solvent. In fact, isoniazid thin films formed on SNS are quite vulnerable to the polar properties of the deposition solvent. Such an effect has precedence. For example, it has been shown that some substituted benzoic acids when deposited on SNS from a solvent with nonpolar bonds such as n-heptane ionize to benzoate ions not just in the monolayer but also in the multilayer.22,23 This ionization occurs because n-heptane does not solvate the substituted benzoic acids as effectively as polar solvents such as acetone during deposition on SNS. Figure 7 contains SERS and SEIRA data where isoniazid was deposited from a 50 ppm n-heptane solution. On the left are SERS spectra from two different isoniazid monolayers adsorbed on SNS. At the right are SEIRA spectra for an increasing coverage of isoniazid ranging from the monolayer into the multilayer. Columns 2 and 5 in Table 2 comprise the SERS data for an isoniazid monolayer and a SEIRA multilayer on SNS (deposited from n-heptane), respectively. There are numerous observed changes when comparing the SERS spectra for isoniazid deposited from acetone versus nheptane as highlighted in columns 1 and 2 of Table 2. Note some of the bands, such as those between 1000 and 1100 cm−1, even change relative intensity between the two different SERS spectra in Figure 7 due to variations in the isoniazid monolayer. 28966



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Below 1300 cm−1, a number of bands have shifted, appeared, or disappeared upon switching from acetone to a n-heptane deposition solvent. First, notice that the isoniazid ring breathing mode moved from 1010 to 1014 cm−1 when going from an acetone to a n-heptane. This frequency shift suggests that isoniazid has a stronger attraction to the SNS using a n-heptane solvent. Presumably, this is because n-heptane does not solvate isoniazid as efficiently as acetone during the adsorption process. Second, when using n-heptane as the deposition solvent, there is a strong isoniazid C−H out-of-plane bend present at 687 cm−1 that is not present when acetone is the solvent. Another isoniazid C−H out-of plane bend mode located just above 840 cm−1 is also more intense using a n-heptane solvent. Surface vibrational selection rules state than normal modes oriented perpendicular to the surface will be more intense than modes arranged parallel to the substrate.27 Hence, an increase in the intensity of C−H out-of-plane bend modes suggests that isoniazid orients with an angle closer to perpendicular with the surface using a n-heptane solvent. The other noteworthy difference observed in the SERS spectra of isoniazid going from an acetone to a n-heptane deposition solvent is that the −CN str./−NH bend modes now located at 1339 and 1371 cm−1 are broader and more difficult to resolve. When considering the SEIRA spectra in Figure 7, the first thing to notice is that the ring breathing mode is present using a n-heptane deposition solvent. This ring breathing mode was not evident in SEIRA spectra of isoniazid using acetone as a deposition solvent. It is likely that the isoniazid ring breathing mode appears using n-heptane as the deposition solvent because of the stronger attraction of isoniazid to the SNS and increased charge transfer. The ring breathing mode first appears at 1017 cm−1 in an isoniazid monolayer and shifts to as low as 1012 cm−1 in the multilayer. Not surprisingly, this shift to lower frequency with increasing number of isoniazid layers implies that the attraction of isoniazid to the SNS is decreasing with increasing distance between isoniazid layers and the SNS. In Figure 7 all of the isoniazid SEIRA spectra are dominated by the 1336/1373 cm−1 −CN str./−NH bend doublet that grows from an absorbance of about 0.002 in the monolayer to >0.03 in the multilayer. This SEIRA doublet in Figure 7 is larger than any other bands in the spectra including the ring deformation modes around 1600 cm−1. Ring deformation modes were the largest peaks in the SEIRA spectra in Figure 5 (acetone deposition solvent). Surface selection rules explain the decrease in intensity of ring deformation modes as the isoniazid geometry tilts from perpendicular to the SNS using an acetone solvent to more parallel to the SNS using n-heptane. Because nheptane does not solvate isoniazid as effectively during the adsorption process, the −NH2 and −NH groups are free to interact more with SNS, which would also explain the origin of the increased isoniazid tilt. Figure 8 is a hypothetical picture showing how separate acetone and n-heptane deposition solvents might alter the adsorption of isoniazid on SNS. The huge increase in absorbance of the −CN str./−NH bend doublet at the expense of ring deformation modes can also be described by a tilted isoniazid geometry. In SERS and SEIRA experiments, it is often the case that polar groups in proximity to SNS see the largest enhancement factors because of polarization and charge transfer effects. Hence, as the −NH2 and −NH groups interact more with the SNS, in SEIRA experiments an enormous increase in absorbance of the −CN str./−NH bend mode doublet would be expected.

Figure 8. Possible impact of acetone and n-heptane deposition solvents on isoniazid adsorption on SNS.

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CONCLUSIONS DFT calculations involving the C1 (3−3) dimer along with trimer and tetramer conformations allowed for the detailed reproduction and interpretation of the Raman spectrum of isoniazid powder. These calculations emphasized the key (3− 3), (1−3−3), and (1−3−3−1) hydrogen-bonding interactions between adjacent isoniazid molecules in the bulk C1 conformation of isoniazid. It was shown that when isoniazid adsorbed to SNS using acetone as a deposition solvent, isoniazid took on a geometry similar to the T configuration oriented upright with respect to the SNS in the mono- and probably the multilayer. Many bands changed going from the bulk to thin film on SNS forms of isoniazid. In particular, −NH bend modes in bulk isoniazid observed at 1335 cm−1 in the Raman spectrum and at 1330 cm−1 in the infrared spectrum gave way to a −CN str./−NH mode doublet in the SERS spectra at 1341/1376 cm−1 and in the SEIRA spectra at 1343∼1370 cm−1. DFT calculations showed how the presence of SNS adsorbed at position 1 on isoniazid alter the resulting vibrational spectra with a more pronounced impact on modes dominated by ring deformations at the expense of largely −NH bend combination modes. Overall, T (3−3) dimer simulations adequately described the structures of mono- and multilayer isoniazid adsorbed on SNS. When n-heptane was the deposition solvent, shifts in the ring breathing modes revealed that isoniazid adsorbed more strongly to SNS versus an acetone deposition solvent. Other changes in both the SERS and SEIRA spectra going from an acetone to a n-heptane deposition solvent suggest isoniazid adsorbs with an orientation closer to perpendicular to the surface so that the −NH2 and −NH groups could interact more readily with the SNS. In summary, this paper uses a unique combination of both SERS and SEIRA to study the impact of SNS on isoniazid film growth. The paper also gives a good demonstration as to how various molecular cluster simulations can begin to model complex hydrogenbonding interactions in solids, and how small molecular dimer/ metal clusters can model hydrogen bonding in the monolayer on SNS in conjunction with SERS and SEIRA.

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ASSOCIATED CONTENT

S Supporting Information *

Plot comparing the experimental SERS spectra of isoniazid to DFT simulated spectra using a cc-pvdz basis set for O, N, and H atoms and using a cc-pvdz-PP versus a LANL2DZ basis set for Ag atoms. This material is available free of charge via the Internet at http://pubs.acs.org. 28967



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AUTHOR INFORMATION

Corresponding Author

*Phone: 501-450-5937. Fax: 501-450-3623. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Dr. Gerald Manion for important discussions and the National Science Foundation Grant Nos. CHE1008096 and CHE-1306420 for funding.

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REFERENCES

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Surface-Enhanced Vibrational Spectroscopy and Density Functional

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