Adsorption of Methimazole on Silver Nanoparticles: FTIR, Raman, and

J. Phys. Chem. C 2009, 113, 7091–7100

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Adsorption of Methimazole on Silver Nanoparticles: FTIR, Raman, and Surface-Enhanced Raman Scattering Study Aided by Density Functional Theory Nandita Biswas,*,† Susy Thomas,‡ Anjana Sarkar,† Tulsi Mukherjee,† and Sudhir Kapoor† Radiation & Photochemistry DiVision and High Pressure Physics DiVision. Bhabha Atomic Research Centre, Mumbai 400 085, India ReceiVed: January 7, 2009; ReVised Manuscript ReceiVed: February 23, 2009

FTIR, Raman, and surface-enhanced Raman scattering (SERS) of methimazole (MMI) have been investigated. MMI is an important antithyroid drug that inhibits the formation of thyroid hormone. It is widely used clinically in the treatment of hyperthyroidism, and thus it is useful to study its surface adsorption characteristics. The experimental FTIR and Raman data are supported with DFT calculations using B3LYP functional with LANL2DZ basis set. This is the first report on the vibrational analysis of the thiol and thione forms of MMI and their various possible silver complexes. pH-dependent normal Raman spectra have been recorded, which show the abundance of the thione form of MMI in acidic, neutral, and alkaline media. From the SERS spectra as well as theoretical calculations, it has been inferred that in neutral and alkaline media, the thiol form of MMI is chemisorbed to the silver surface through the ring N atom of the imidazole ring with an edge-on orientation and the imidazole ring lying in the plane of the silver surface. In contrast, it has been concluded that in the acidic medium, the thione form of MMI gets adsorbed to the silver surface. Thus, the pH-dependent SERS spectra have shown the preferential existence of thione and thiol tautomeric forms on the silver surface in acidic, neutral, and alkaline media. 1. Introduction The interaction of nanoparticles with biomolecules and microorganisms is an imminent field of research. Within this field, an area that has been largely gaining attention is the interaction of metal nanoparticles with drugs. Noble-metal nanomaterials are ideal candidates for such interactions. Among noble metals, silver nanoparticles have received considerable attention due to their attractive physicochemical properties. The surface plasmon resonance, in the visible region,1 and large effective scattering cross section of individual silver nanoparticles make them ideal candidates for molecular labeling,2 where phenomena such as surface-enhanced Raman scattering (SERS) can be exploited. A rough metal nanoparticle surface contains very complex electromagnetic (EM) modes, which have important consequences for incident light. These modes modify spectroscopic properties of an adsorbed molecule in a radical manner by changing the electromagnetic field incident on the molecule. The lifetime and emission intensity of its excited states are also altered.3 Metal surface properties are thus an important aspect for surface-enhanced Raman scattering (SERS).4 In SERS, enhancements of up to 106 in the Raman signal can be obtained for molecules adsorbed on metal surfaces.5 Apart from the increased sensitivity in Raman detection limit, SERS facilitates the determination of the sites through which the molecule interacts with the metal surface and the kind of orientation it is likely to assume with respect to the metal surface.6-13 Recently, metal nanoparticles have been exploited for delivery of drugs.14,15 It would be therefore of significance to investigate the surface adsorption characteristics of pharmacologically important drugs. * Corresponding author. Phone: (+)- 91-22-25590301. Fax: (+)- 9122-25505151. Email: [email protected]; [email protected]. † Radiation & Photochemistry Division. ‡ High Pressure Physics Division.

Figure 1. Chemical structure of MMI.

Methimazole (MMI), 2-mercapto-1-methylimidazole (Figure 1), the active metabolite of carbimazole, inhibits thyroid hormone biosynthesis by preventing the organification of iodide in the thyroid.16 It is an important antithyroid drug and is widely used clinically in the treatment of hyperthyroidism. In addition to its antithyroid effect, there is considerable evidence that MMI also acts as an immunosuppressive agent in Graves’ disease.17,18 MMI contains the thiourea pharmacophore. Because of the push-pull mechanism in MMI, where the nitrogen lone pairs donate electrons to the thiocarbonyl group, this pharmacophore possesses significant electron-donating ability at the sulfur atom. Thus, the donating property of MMI is expected to be the origin of its antithyroid activity. In this Article, we illustrate the interaction of MMI with silver nanoparticles in terms of the change in the optical properties and vibrational frequencies depending upon its binding site and orientation. DFT (B3LYP functional with LANL2DZ basis set) calculations have been carried out for a detailed interpretation of the FTIR, Raman, and SERS spectra. The pH-dependent normal Raman and SERS spectra have been investigated to study the preferential existence of thiol-thione tautomerism in aqueous solution and on the silver surface. To the best of our knowledge, this is the first report on the vibrational analysis of MMI and its silver complexes. MMI has two possible binding

10.1021/jp900134n CCC: $40.75  2009 American Chemical Society Published on Web 04/02/2009

7092 J. Phys. Chem. C, Vol. 113, No. 17, 2009

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sites, electron-rich nitrogen/sulfur atoms and the ring π-electrons, which can bind to the electron-deficient Ag ion or the neutral silver atom. From the SERS spectra, and detailed theoretical calculations of the thiol and thione forms of MMI as well as their charged and neutral silver complexes, it has been inferred that at neutral and alkaline media, the thiol form of MMI is chemisorbed to the silver surface through the ring nitrogen atom with an edge-on orientation and the imidazole ring lying in the plane of the silver surface, while at acidic medium (pH 2), the thione form of MMI is adsorbed on the silver surface. 2. Experimental Section Methimazole (MMI), AgNO3, and sodium borohydride used for the UV-vis, FTIR, Raman, and TEM measurements as well as preparation of silver hydrosol were from Aldrich chemicals and S. D. fine chemicals, India, and were used without further purification. Aqueous silver sol was prepared by the chemical reduction of AgNO3, with NaBH4.19 In brief, to 100 mL of a N2 bubbled solution of AgNO3 [10-4 (M)] was added 2.6 × 10-3 (M) NaBH4. The solution was shaken vigorously until a clear yellow silver sol was obtained. MMI was added to the aqueous silver sol and characterized using UV-vis absorption, Raman, and transmission electron microscopic (TEM) techniques. The pH of the aqueous solution and sol was adjusted with the addition of dilute HCl and NaOH. Silver films were prepared on glass slides using the method reported by Sarkar et al.20 Slides were dipped in different concentrations of MMI in water. The UV-vis absorption spectra were recorded using a Chemito UV2600 spectrophotometer. Raman and SERS spectra of MMI were recorded at room temperature using the 532 nm line, from a diode pumped Nd3+:YAG laser (SUWTECH laser, model G-SLM-020 from Shanghai Uniwave Technology Co. Ltd.). The laser power used to record the Raman spectrum was 25 mW. The Raman scattered light was collected at the backscattered geometry and detected using a CCD-based, home-built monochromator21 together with a super notch filter, covering a spectral range of 200-1750 cm-1. The FTIR spectrum of methimazole was obtained using a BOMEM DA3.008 model FT infrared spectrometer with a KBr beam splitter and MCT detector. A pellet of methimazole in KBr matrix (2 mg in 200 mg) was prepared, and spectra were recorded at an unapodized resolution of 2 cm-1. TEM was performed on JEM model CM (200 kV), Philips. One drop of sample solution was cast onto a copper grid for TEM characterization. The evaporation of the solvent leaves the nanoparticle on the surface for TEM measurement. 3. Computational Details To get an insight into the experimental results, the geometry optimization was performed for MMI, its anion, and its silver complexes using the density functional theory (DFT) with B3LYP functional22 and LANL2DZ basis sets using Gaussian 98 program.23 MMI can exist in two possible tautomeric forms, the thiol and the thione forms. So geometry optimization was carried out for both tautomeric forms of MMI and the calculated vibrations, and their infrared and Raman intensities were compared to the experimental FTIR and Raman spectra. Geometry optimization was also carried out for the thiolate anion and protonated forms of MMI, and the various possible charged and neutral silver complexes of MMI. No symmetry restriction was applied during geometry optimization. The vibrational frequencies for the thiolate anion and protonated forms of MMI and all possible MMI-Ag complexes were computed at the

Figure 2. Surface plasmon absorption band of silver sol and silver sol with added MMI (10-8 to 10-5 M).

optimized geometry to ensure that it corresponds to a local minimum on the potential energy surface and not to a saddle point. The computed vibrations at the optimized geometry were compared to the normal Raman and the SERS spectra. 4. Results and Discussion 4.1. Absorption Studies. The UV-visible absorption spectra of metal colloids are related to the surface plasmon resonance, whose frequencies depend on the aggregation pattern of the sol. The position and shape of the surface plasmon absorption band are also strongly dependent on the size and shape of the particle, dielectric constant of the medium, and surface-adsorbed species. The UV-visible absorption spectra of the silver sol before and after the addition of MMI were monitored. Varying concentration of MMI was added to the silver sol to study its effect on the absorption spectra. As shown in Figure 2, the absorption spectrum of the silver sol (yellow) showed a single sharp peak at 380 nm due to the surface plasmon resonance band. The absorption spectra of MMI at various concentrations in the hydrosol (10-8, 10-7, 10-6, and 10-5 M) are also shown in Figure 2. It is observed that with the addition of MMI (10-8 M) to the silver sol, the absorption spectrum becomes very broad with a decrease in the absorbance at 380 nm and a shoulder appearing around 485 nm. The observed change in the intensity of the surface plasmon absorption band is attributed to the disturbance of the electron gas on the surface of the particle. With an increase in the concentration of MMI (10-7 and 10-6 M), the color of the sol changes from yellow to pink due to adsorption of MMI on the silver surface, and peaks appear at 535 and 507 nm, respectively. The appearance of the red-shifted peaks is attributed to the aggregation of the silver particles in the presence of the adsorbed molecule.24 With further increase in MMI concentration, the absorption spectrum becomes broad, as shown in the figure. The optimum concentration for the formation of MMI-silver complex is 10-6 and 10-7 M. 4.2. TEM Analysis. TEM analysis was carried out to check the effect of addition of MMI on the size and shape of the silver nanoparticles. Silver nanoparticles formed by NaBH4 reduction leads to the formation of well-separated 15 nm particles. On addition of 10-7 M MMI to the silver sol, aggregation of the particles takes place. A representative TEM image of the

Adsorption of Methimazole on Silver Nanoparticles

Figure 3. Transmission electron micrograph of aggregated silver nanoparticles obtained on the addition of 1 × 10-7 (M) MMI to the silver sol.

aggregated particles is shown in Figure 3. It is evident from the picture that addition of MMI facilitates the aggregation of silver nanoparticles. The average size of the particles upon aggregation was 25 nm, although the particles retained their spherical shape. 4.3. Computational Results. The optimized structures of the thiol, thione, and anionic forms of MMI are shown in Figure 4a-c, respectively. To get an idea regarding the relative stability of different forms of the molecule in the gas phase, we have computed the minimum energies of the molecule at their respective optimized geometries using B3LYP/LANL2DZ method. The computed energies at the B3LYP/LANL2DZ level of theory for the thiol, thione, and thiolate anion forms of MMI are -172 857.24, -172 878.85, and -172 527.57 kcal mol-1. The computed energies suggest that both the thiol and the thione forms of MMI are energetically more favorable than the thiolate anion form. The computed results also show that the thione form is more stable (by 21.6 kcal mol-1) than the thiol form in the gas phase. The optimized structural parameters of the thiol, thione, and thiolate anion forms of MMI are tabulated in Table 1. The DFT results indicate that the imidazole moiety is planar in the thiol, thione, and thiolate anion forms of MMI, and the C atom of the methyl group is sp3-hybridized with relevant HCH bond angles being 108-109°. In thiol, the S6H10 bond distance is 1.373 Å. The thiol H atom (H10) lies in the plane of the imidazole moiety with the C1S6H10 bond angle being 93.7° and the N2C1S6H10 and N5C1S6H10 dihedral angles being ∼175.4° and 4.8°, respectively. The main difference is observed in the bond lengths C1N5 and C1S6 of the thiol and thione forms of MMI. In thione, the C1N5 bond is elongated by 0.054 Å, while the C1S6 bond is compressed by 0.093 Å. This is expected because these are the atoms that are involved in tautomerism. In the anion, the C1N5 and C1S6 bond distances are intermediate between the thiol and thione forms. For the MMI-silver complex, various possible conformers are possible, one with the charged silver ion, and the second with neutral silver atom. For the thiol and thione forms of MMI, the silver ion or the silver atom can bind through the ring N atom and the S atom, respectively, as shown in Figure 5a-d. The optimized structures of MMI (thiol)-Ag+ (Figure 5a) and MMI (thiol)-Ag (Figure 5b) show that while Ag+ is directly bound to the free N of the imidazole ring, Ag is partially bound to the ring N atom. In MMI (thiol)-Ag+ and MMI (thiol)-Ag, the N5-Ag bond distances are 2.167 and 2.425 Å, respectively.

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7093 In the charged complex, the SH bond is perpendicular to the imidazole ring, while in the neutral complex, the SH bond lies in the molecular plane. The binding energies of the charged and the neutral MMI (thiol)-Ag complexes are -67.0 and -9.3 kcal mol-1, respectively. The B3LYP/LANL2DZ optimized structures of the charged and neutral complexes of MMI (thione)-Ag are shown in Figure 5c and d. The figures show that in the charged complex, Ag+ is directly bound to the free S atom of MMI with the S6-Ag bond distance being 2.502 Å and the C1S6Ag bond angle being 105.6°. The S6-Ag bond lies perpendicular to the plane of the imidazole ring. In the neutral MMI (thione)-Ag complex, Ag is partially bound to the S6 atom with the S6-Ag bond length being 2.852 Å. In the neutral complex, the Ag atom is partially bound to S6, and it lies in the plane of the imidazole ring. The binding energies of the charged and the neutral MMI (thione)-Ag complexes are -66.1 and -8.2 kcal mol-1, respectively. The vibrational frequencies were calculated with B3LYP/ LANL2DZ basis set for MMI (thiol and thione forms) and its thiolate anion at their respective optimized geometries in vacuum (Tables 2 and 3) and compared to the FTIR and Raman spectra in the solid state and saturated solution. The Raman vibrations for all possible silver complexes of MMI (charged and neutral) were computed at the optimized geometry and compared to the SERS spectra. The pKa of MMI is known to be 11.3825/12.37.26 Thus, MMI can exist in the thiolate anion form at pH > 12. Under our experimental conditions of alkaline, neutral, and acidic pH of 9, 7, 4, and 2, MMI is expected to exist either in thiol or in thione forms. Thus, adsorption of thiol or thione forms of MMI is mainly observed on the silver surface. 4.4. Raman and FTIR Spectra of MMI and Its Vibrational Assignments. The thiol and thione forms of MMI with 13 atoms have 33 fundamental modes of vibrations. Both of these tautomeric forms of MMI belong to the Cs point group and have 24 in-plane (A′) and 9 out-of-plane (A′′) fundamental vibrations. All of these modes of vibrations are expected to appear in Raman as well as in FTIR spectra. The Raman spectra of MMI in the solid state and in saturated solution are shown in Figure 6a for the region 200-1700 cm-1 and in Figure 6b for the region 2600-3900 cm-1. All of the observed Raman vibrations are tabulated in Table 2. The FTIR spectrum of powdered sample of MMI in a KBr pellet is shown in Figure 6c. The assignments of the Raman bands to the stretching and bending vibrations of the imidazole moiety and the methyl group are based on comparison of the Raman and FTIR spectra of the solid with the calculated vibrations of the thione and thiol forms of MMI. The Raman spectrum of solid MMI exhibits eight strong bands in the frequency region 200-1600 cm-1 [Figure 6a]. The Raman bands are observed at 1575, 1464, 1340, 1278, 1254, 914, 693, and 531 cm-1. The 1575 and 1464 cm-1 vibrations are assigned to the C3C4 stretch and C1S6 stretch. The bands at 1340 and 1278 cm-1 are attributed to the ring CN stretch with contributions from ring bending and ring CH bending vibrations. The 1254 and 914 cm-1 bands are assigned to in-plane (ip) ring breathing and ring bending modes. The Raman bands at 693 and 531 cm-1 are attributed to CN2C7 bend and NC1S6 bend, respectively. Few medium intensity bands are observed at 1248, 1095, and 408 cm-1 and are assigned to ring breathing, ring CN stretch, and ring rotation, respectively. Weak bands are observed at 1451, 1407, 1151, 1016, 853, 841, 737, 680, and 602 cm-1. Of these vibrations, the modes observed at 1451 and 1151 cm-1 correspond to C1S6 stretch, while the band at 1407 cm-1 corresponds to ring CN stretch. The modes at


Biswas et al.

Figure 4. Optimized structure along with the atom numbering of (a) thiol, (b) thione, (c) thiolate anion, and (d) protonated forms of MMI.

TABLE 1: Optimized Structural Parameters of (a) Thiol, (b) Thione, and (c) Thiolate Anion Forms of MMI (a) MMI thiol

(b) MMI thione

(c) MMI anion

C1N2 ) 1.385 C1N5 ) 1.335 C4N5 ) 1.402 C3C4 ) 1.386 N2C3 ) 1.404 N2C7 ) 1.466 C1S6 ) 1.821 C4H9 ) 1.079 C3H8 ) 1.078 S6H10 ) 1.373

Bond Lengths (Å) C1N2 ) 1.394 C1N5 ) 1.389 C4N5 ) 1.402 C3C4 ) 1.370 N2C3 ) 1.407 N2C7 ) 1.464 C1S6 ) 1.728 N5H10 ) 1.010 C3H8 ) 1.078 C4H9 ) 1.077

C1N2 ) 1.420 C1N5 ) 1.366 C4N5 ) 1.397 C3C4 ) 1.387 N2C3 ) 1.405 N2C7 ) 1.454 C1S6 ) 1.783 C4H9 ) 1.083 C3H8 ) 1.081

N2C1N5 ) 112.3 C1N5C4 ) 105.2 N5C4C3 ) 110.2 C4C3N2 ) 106.1 C3N2C1 ) 106.2 C1N2C7 ) 126.9 C3N2C7 ) 126.8 N2C1S6 ) 121.9 N5C1S6 ) 125.8

Bond Angles (in deg) N2C1N5 ) 104.0 C1N5C4 ) 111.5 N5C4C3 ) 106.4 C4C3N2 ) 107.7 C3N2C1 ) 110.4 C1N2C7 ) 124.3 C3N2C7 ) 125.3 N2C1S6 ) 129.0 N5C1S6 ) 127.0

N2C1N5 ) 108.4 C1N5C4 ) 106.8 N5C4C3 ) 111.1 C4C3N2 ) 105.2 C3N2C1 ) 108.5 C1N2C7 ) 125.4 C3N2C7 ) 126.1 N2C1S6 ) 122.6 N5C1S6 ) 129.0

N2C1S6H10 ) -175.4 N5C1S6H10 ) 4.8 C4N5C1S6 ) 179.8 C3N2C1S6 ) -179.6 C7N2C1S6 ) -1.1 C7N2C3C4 ) -178.8 C7N2C3H8 ) 1.3 C7N2C1N5 ) 178.8

Dihedral Angles (in deg) N2C1N5H10 ) 180.0 C4N5C1S6 ) 180.0 H10N5C1S6 ) 0.0 C7N2C1S6 ) 0.0 C3N2C1S6 ) 179.9 C3C4N5H10 ) -179.9

C4N5C1S6 ) 180.0 C7N2C1S6 ) 0.0 C7N2C3H8 ) 0.0 C3N2C1S6 ) -179.9 C1N5C4H9 ) 180.0

1016 and 853 cm-1 are attributed to ip ring bend and ip C1S6H bend, respectively. The Raman bands at 841 and 737 cm-1 are assigned to out-of-plane (oop) ring CH bends. The modes at 680 and 602 cm-1 are attributed to ring oop bend. In the FTIR spectrum, strong peaks are observed at 1572, 1465, 1276, 1246,

742, and 678 cm-1. Similarly, medium intensity peaks are observed at 1401, 1337, 1263, 1150, 1094, 1015, and 530 cm-1. In the FTIR spectrum, weak peaks are observed at 916, 852, 690, and 602 cm-1. The methyl CH stretch is observed at 2940, 3104, and 3132 cm-1 in the Raman spectrum and at 2940, 3108, and 3127 cm-1 in the FTIR spectrum. The imidazole ring CH stretch is observed at 3169 and 3160 cm-1 in the Raman and FTIR spectrum, respectively. The Raman and FTIR spectra of the solid MMI (Figure 6a and c and Table 2) show strong peaks at 1575 and 1572 cm-1, respectively, that can be correlated to the 1565 cm-1 (calculated) mode of thione, assigned to C3C4 stretching vibration of the imidazole ring coupled to ring CH and NH bending vibrations. In addition, the absence of the strong SH peak (expected around 2550 cm-1) in the Raman spectra indicates the preferential existence of the thione form of MMI in the solid state, although the presence of the thiol form cannot be ruled out completely. The Raman spectrum of MMI saturated aqueous solution is shown in Figure 6a for the 200-1700 cm-1 region and in Figure 6b for the 2600-3900 cm-1 region. A broad feature at 1636 cm-1 is attributed to ip bending vibration of H2O. As shown in Figure 6a, intense peaks are observed at 1484, 1352, 1286, and 692 cm-1. Of these modes, 1484 cm-1 is assigned to methyl CH scissoring. The vibrations observed at 1352 and 1286 cm-1 are attributed to ring CN stretch, and the 692 cm-1 band is assigned to CN2C7 bend. Medium intensity peaks are observed at 1583, 1463, 921, and 526 cm-1. The peaks are assigned to C3C4 stretch, C1S6 stretch, ip ring bend, and NC1S6 bend, respectively. As shown in Figure 6a and b, few weak bands are observed at 3160, 2954, 1412, 1269, 1160, 1109, 1091, 1021, 608, 412, and 258 cm-1. The 3160 and 2954 cm-1 peaks are almost obscured by the broad OH stretching vibration of H2O observed at 3600 cm-1. These peaks are assigned to ring CH


J. Phys. Chem. C, Vol. 113, No. 17, 2009 7095

Figure 5. Optimized structure of (a) MMI (thiol)-Ag+, (b) MMI(thiol)-Ag, (c) MMI(thione)-Ag+, and (d) MMI(thione)-Ag complexes.

TABLE 2: Assignment of FTIR and Raman Spectra of Solid MMI and B3LYP/LANL2DZ Calculated Vibrations (Scaled by 0.97) in cm-1 of Its Thiol and Thione Formsa calculated vibrations FTIR spectra

Raman spectra

thiol

thione

3160s 3127s 3108s 2940m 1572s 1465s

3169s 3132s 3104s 2940m 1575s 1464s 1451sh 1407w 1340s 1278s 1254s 1248m 1151w 1095m 1016w 914s 853w 841w 737w 693s 680w 602w 531s 408m

3196 3078 3047 2966

3210 3075 3045 2962 1565 1426

ring CH str. CHMe str. CHMe str. CHMe str. C3C4 str., ring CH(NH) bend C1S6 str., ring CN str., ring NH bend

1301 1256

ring CN str., N2C7 str., ring CH bend ring CN str., ring bend, ring CH bend ring CN str., ring bend, ring CH (NH) bend ring breathing, C7N2 str., ring CH(NH) bend

1401m 1337m 1276s 1263m 1246s 1150m 1094m 1015m 916w 852w 742s 690w 678s 602w 530m a

1402 1360 1264 1133 1069 1016 891 868 734 659 670 614

1131 1073 895 839 709 666 659 625 501 393

approximate assignments

C1S6 str., ring CH bend ring CN str., ring CH(NH) bend ring bend, ring CH bend, CHMe bend, C1S6H bend ring bend, ring CH (NH) bend, C1S6H bend C1S6H ip bend ring CH oop bend ring CH oop bend C1N2C7 bend, C3N2C7 bend ring oop bend, ring CH (NH) oop bend ring oop bend, ring CH oop bend N2C1S6 bend, N5C1S6 bend ring rot., N2C1S6 bend, N5C1S6 bend

Abbreviations used: w, weak; m, medium; s, strong; str., stretch; ip, in-plane; oop, out-of-plane; rot, rotation; Me, methyl.

stretch and methyl CH stretch, respectively. The weak bands observed at 1412 and 1160 cm-1 are assigned to ring CN stretch and C1S6 stretch, respectively. The peak at 1269 cm-1 is assigned to ring breathing mode. The weak peaks observed at 1109 and

1091 cm-1 are assigned to ring CN stretch. The 1021, 608, 412, and 258 cm-1 bands are attributed to ip ring bend, ring oop bend, ring rotation, and ip CS wag, respectively. The Raman bands of MMI saturated solution along with their assignments


Biswas et al.

TABLE 3: Assignment of Raman (Saturated Solution), SERS, and B3LYP/LANL2DZ Calculated Vibrations (Scaled by 0.97) of MMI (Thiol and Thione Forms) and MMI-Silver Complexes [(A) MMI (thiol)-Ag+, (B) MMI (thione)-Ag+] in cm-1a Raman

calculated

solution

thiol

thione

3160w 2954w 1636br 1583m

3196 2966

3210 2962

1484s 1463m 1412w 1352s 1286s 1269w 1160w 1109w 1091w 1021w 921m 692s

1471

608w 526m 412w 258w a

SERS

calculated

hydro-sol

film

A

B

approximate assignments

3086 2991

3083 2990

1638br

3136s 2936s 1639br

1521m

1521m

1511

1455m 1406w 1364s 1318m 1281w 1142m

1455m 1408w 1365s 1320s 1281w 1136m

1086m 1034m 936m 694w 676w 617m 500m 430m

1085m 1034m 938m 695m 680m 617m 502m 432m

1085 1019 940 664 654 626

202s

190

ring CH str. CHMe str. ip bend of H2O C3C4 str., ring CH(NH) bend C3C4 str., C1N2 str., ring CH bend CHMe scissoring C1S6 str., ring CN str., ring NH bend ring CN str., N2C7 str., ring CH bend ring CN str., ring bend, ring CH bend ring CN str., ring bend, ring CH(NH) bend ring breathing, C1S6 str., ring CH(NH) bend C1S6 str., ring CH bend, C4N5 str. ring C4N5 str., ring CH bend, C1S6H bend ring CN str., ring CH bend ring bend, ring CH bend, CHMe bend, C1S6H bend ring bend, ring CH(NH) bend, C1S6H bend C1N2C7 bend, C3N2C7 bend ring oop bend, ring CH(NH) oop bend ring oop bend, ring CH(NH) oop bend N2C1S6 bend, N5C1S6 bend ring rot., N2C1S6 bend, N5C1S6 bend ring rot., N2C1S6 bend, N5C1S6 bend ip CS wag V(Ag-N)

1565 1477 1426

1402 1360 1264 1133 1111 1069 1016 891 659 614

1301 1256 1131 1073 895 666 625 501

1414 1398 1342 1262 1143

411

393 247

1329 1254 1137 1085 911 656 676 612 471

Abbreviations used: s, strong; m, medium; w, weak; br, broad; str., stretch; ip, in-plane; oop, out-of-plane; Me, methyl.

imidazole ring stretching/bending vibrations of both the thione and the thiol forms of the molecule. The appearance of these bands indicates the coexistence of both the thiol and the thione forms of MMI in solution. 4.5. pH-Dependent Normal Raman Spectra of MMI in Aqueous Solution. The pH-dependent normal Raman spectra of 0.1 M MMI in aqueous solution are shown in Figure 7. The Raman spectra at pH 2, 4, 7, and 9 have been normalized with respect to the 692 cm-1 peak, which corresponds to the CN2C7 bending vibration and appears as a strong band in the entire

Figure 6. The normal Raman spectra of (a) solid and saturated solution of MMI in the region 200-1750 cm-1, (b) solid and saturated solution in the 2800-3900 cm-1 region, and (c) FTIR spectra.

are tabulated in Table 3. The appearance of bands at ∼1484, 1269, 1160, 1091, 921, 692, and 608 cm-1 in the normal Raman spectrum in solution suggests significant contribution from the

Figure 7. pH-dependent Raman spectra of 0.1 M MMI in aqueous solution.



pH range. Comparison of the normalized Raman spectra in the entire pH range shows that Raman spectra are strikingly similar at neutral (pH 7) and alkaline pH (pH 9). At slightly acidic pH 4, a few weak bands start appearing in the normalized Raman spectrum at 472, 500, and 1318 cm-1. These Raman bands at 472, 500, 1318, and also a band at 1371 cm-1 gain intensity at acidic (pH 2) medium as compared to the normalized Raman spectra at pH 4. Moreover, at pH 2 the intensities of the 1463 and 1484 cm-1 bands are almost comparable, while the 1484 cm-1 band is stronger than the 1463 cm-1 band at pH 4, 7, and 9. Similarly, the Raman bands observed at 1091 and 1109 cm-1 appear as a doublet at pH 4, 7, and 9, while at pH 2, the 1109 cm-1 band decreases in intensity and appears as a shoulder to the 1091 cm-1 band. The respective population of the thione-thiol tautomeric forms at various pH can be estimated from the ratio of the sum of the integrated intensities Ithione and Ithiol of the assigned experimental bands divided by the theoretically predicted sums of the absolute intensities (Athione and Athiol) of the respective Raman bands:27,28

[thiol] ) [thione]

∑ Ithiol ∑ Athione ∑ Athiol ∑ Ithione

Figure 8. Plots of frontier molecular orbitals, HOMO and LUMO, of thiol, thione, and protonated forms of MMI.

(1)

The results indicate that 72% of the thione species of MMI exists at pH 2, whereas at pH 4, 7, and 9, the percentage of thione species varies from 55% to 65%. These results corroborate the earlier literature data on spectroscopic and structural details of organic molecules existing in thione-thiol tautomerism.28,29 It also shows the relative abundance of the thione form of the molecule in acidic, neutral, and alkaline media. The appearance of intense Raman bands was observed at 472, 500, 1318, and 1371 cm-1 at pH 2, but no significant shift was observed for the Raman bands in the entire pH range. These Raman bands at 472, 500, 1318, and 1371 cm-1 are expected to be due to the protonation of the thione form of methimazole. It is to be noted that the optimized structure of the protonated forms of both thiol and thione resulted in the same structure as shown in Figure 4d. The optimized structure of the protonated form of MMI shows that H14+ is lying perpendicular to the imidazole moiety with the S6-H14+ distance being 1.383 Å and the C1S6H14 bond angle being 96.3°. The dihedral angles, N2C1S6H14 and N5C1S6H14, are 85.6° and 100.8°, respectively. The vibrational frequencies corresponding to the protonated form of MMI show good agreement with the experimental Raman bands at pH 2 and 4. The observed Raman bands (calculated vibrations) at 472 (465) and 500 (501) are assigned to the NC1S6 bend with contributions from the N2C3H and N2C7H bend. Similarly, the Raman bands at 1318 (1341) and 1371 (1403) cm-1 are attributed to ring CN stretch along with contributions from ring bend and ring CH(NH) bend. The effect of protonation of MMI can be explained from the plots of frontier molecular orbitals, HOMO and LUMO, of the thiol, thione, and the protonated forms of MMI as shown in Figure 8. It is observed from the figure that there is slight perturbation in the electronic charge distribution of the protonated form of MMI as compared to the thione and thiol forms, mainly because the H14+ lies in the plane perpendicular to the imidazole moiety. Thus, no shifts in the Raman bands are observed upon protonation. 4.6. Surfaced-Enhanced Raman Scattering (SERS) Spectra of MMI. The SERS spectra of MMI at different concentrations (10-4 M to 10-8 M) in silver hydrosol (pH 9) are presented

Figure 9. The SERS spectra of MMI [10-8 (M) to 10-4 (M)] adsorbed in silver sol.

in Figure 9. The SERS spectra show almost no enhancement at concentrations of 10-4, 10-5, and 10-8 M MMI in silver sol. Good enhancement is observed at the concentrations of 10-6 and 10-7 M MMI in silver sol, with maximum enhancement for 10-6 M MMI sol probably due to monolayer coverage. From Figure 9 it is also observed that the change in concentration of MMI is followed by the change in intensity of the Raman bands with the peak positions showing absolutely no dependence on the concentration. This indicates that the adsorbed MMI does not undergo any change in the binding mechanism or orientation with the change in concentration. The differences in the spectrum of the species in solution and surface adsorbed state, based on frequency shifts, intensities, and band shapes, provide crucial information on the interfacial interactions and the geometry of the adsorbate on the surface. The changes observed in the SERS spectrum may be correlated with the normal Raman spectrum of MMI in aqueous solution recorded under similar conditions (pH 9). The SERS spectrum shows selective enhancement and considerable shifts for various Raman vibrations. The vibrational frequencies observed in the SERS spectrum along with the calculated values obtained from B3LYP/LANL2DZ basis set for MMI (thiol)-Ag+ and MMI (thione)-Ag+ complexes are listed in Table 3. The SERS activity of MMI as shown in Figure 9 is mainly centered in the region 400-1500 cm-1. The most significant feature in the SERS spectrum is the emergence of a very strong band at 1364


Figure 10. The SERS spectra of different concentrations of MMI [10-3 (M)–10-7(M)] adsorbed over silver films. The inset shows the 2600–3800 cm-1 region.

cm-1. This peak is observed at 1352 cm-1 in the solution spectrum. This Raman band is assigned to the ring CN stretch along with contributions from imidazole ring bend and ring CH bend. The enhancement in the intensity of the ring CN stretching and ring bending vibration and an observed blue shift of 12 cm-1 clearly suggests that the imidazole ring is directly involved in the interaction with silver surface, mainly through the lone pair on the N atom. In the SERS spectrum, moderate intensity peaks are observed at 1521, 1455, 1318, 1142, 1086, 1034, 936, 617, 500, and 430 cm-1. The 1521 and 1455 cm-1 Raman bands correspond to the C3C4 stretch and C1S6 stretch, respectively. The 1318 and 1086 cm-1 modes are assigned to the ring CN stretches, while the 1142 cm-1 mode is attributed to the C1S6 stretch. The 1034 and 936 cm-1 modes correspond to the imidazole ring bending. The 617, 500, and 430 cm-1 Raman bands are attributed to the ring oop bend, ip NC1S6 bend, and ring rotation, respectively. In the SERS spectrum, weak Raman bands are observed at 1406, 1281, 694, and 676 cm-1. In addition, a broadband is observed at 1638 cm-1 similar to that observed in solution spectra and is attributed to ip bending mode of water. The weak bands at 1406 and 1281 cm-1 are assigned to ring CN stretch and ring breathing, respectively. The 694 and 676 cm-1 Raman peaks correspond to CN2C7 and ring oop bending vibration, respectively. The Raman bands observed in the SERS spectrum along with the calculated vibrations and their assignments are tabulated in Table 3. The optimized structures and the calculated vibrations of the silver complexes of MMI(thiol)-Ag+ and MMI (thione)-Ag+ as shown in Figure 5a and c support the experimental observation. The SERS spectra of MMI adsorbed on silver films recorded at different concentrations (10-3 M to 10-7 M) are presented in Figure 10. The SERS spectrum for the 10-5 M MMI in silver shows maximum enhancement, suggesting the formation of a monolayer.7 The increase in the concentration of MMI from 10-3 to 10-5 M is followed by an increase in intensity of the Raman bands with absolutely no change in the peak position, indicating no change in the binding mechanism or orientation with the change in MMI concentration, as was observed in case of silver hydrosol. The SERS spectrum for the MMI adsorbed silver film has features almost similar to that of the silver hydrosol with strong

Biswas et al.

Figure 11. pH-dependent SERS spectra of MMI [10-6 (M)] adsorbed in silver sol.

peaks appearing at 1365 and 1320 cm.-1 Medium intensity Raman peaks are observed at 1521, 1455, 1136, 1085, 1034, 938, 695, 680, 617, 502, and 432 cm-1. Weak bands are observed at 1408 and 1281 cm-1. An additional peak is observed in the SERS spectra of MMI adsorbed silver films at 202 cm-1. The strong band at 202 cm-1 is attributed to Ag-N stretch. The appearance of the intense band arising due to Ag-N stretching vibration confirms that MMI is bound to the silver surface through the ring N atom as corroborated by the calculated results of MMI (thiol)-Ag+ complex [Figure 5a]. The observed Raman vibrations are tabulated in Table 3 along with the calculated frequencies and their assignments. The inset of figure 10 shows two strong Raman bands at 3136 and 2936 cm-1 assigned to the ring CH stretch and methyl CH stretch, respectively. 4.7. pH-Dependent SERS Spectra of MMI. The pHdependent SERS spectra of MMI at the adsorbate concentration of 10-6 M in colloidal silver are shown in Figure 11. The SERS spectra at pH 4, 7, and 9 are almost similar with slight changes being observed at pH 4. Significant changes are observed in terms of shifts as well as intensities in the Raman bands at pH 2 as compared to pH 4, 7, and 9. The Raman band observed at 430 cm-1 in neutral and alkaline pH shifts to 411 cm-1 at pH 2. The 617 cm-1 band is intense at pH 9, but it is weak at pH 7, 4, and 2. The 676 cm-1 band gains intensity with respect to 694 cm-1 at pH 2. The Raman band at 922 starts appearing at pH 4 and gains intensity at pH 2 as observed in the figure. The 936 and the 1034 cm-1 bands, which are observed at pH 4, 7, and 9, shift to 1025 cm-1 at pH 2. The 1086 and 1098 cm-1 bands appear as a doublet at pH 2, whereas only the 1086 cm-1 band is observed at pH 4, 7, and 9. The 1142 cm-1 band is shifted to 1156 cm-1 at pH 2. The 1260 cm-1 band that is not present at neutral and alkaline pH appears as a weak feature at pH 4 and 2. Moreover, the Raman band at 1281 cm-1 is shifted to 1289 cm-1 at pH 4 and 2. This band gains intensity at pH 2. The 1318 cm-1 band disappears completely at pH 2, and the 1364 cm-1 band, which is the most strong band at pH 4, 7, and 9, shifts to 1355 cm-1 with moderate intensity at pH 2. The 1455 cm-1 band gradually gains intensity at pH 4 and is the most intense SERS band at pH 2. The 1521 cm-1 band

Adsorption of Methimazole on Silver Nanoparticles disappears at pH 2, and a band at 1573 cm-1 appears. The differences observed in the SERS spectra at acidic, neutral, and alkaline pH and the appearance of the 411, 922, 1025, 1156, 1289, and 1573 cm-1 bands in the SERS spectra at pH 2, could be correlated to the solution spectra at pH 2 with similar Raman bands observed at 412, 921, 1021, 1160, 1286, and 1583 cm-1. This correlation of the solution and SERS spectra at pH 2 indicates the existence of the thione form of MMI in acidic medium. 4.8. Adsorption of MMI on Silver and Its Orientation. There are two possibilities of the adsorption of MMI on the metal surface, physisorption and chemisorption. In case of physisorption, the SERS spectra of the adsorbed molecules are almost similar to that of the free molecules with only slight changes observed in the Raman bandwidths. Yet in case of chemisorption, due to the overlap of the molecular and metal orbitals of the molecule, the molecular structure is modified, and, in consequence, the position of the Raman bands and their intensities are dramatically changed.30 The SERS spectra of MMI in silver hydrosol and on silver films as shown in Figures 9 and 10 clearly show significant changes as compared to the solution spectra at pH 9, indicating the possibility of chemisorption of MMI on the silver surface. The shifts observed in the SERS spectra are supported by the calculated vibrations of the charged MMI-Ag complexes (chemisorption) instead of the neutral MMI-Ag complexes (physisorption). This can also be clearly explained from the plots of frontier molecular orbitals, HOMO and LUMO, of the charged and neutral silver complexes of the thiol and thione forms of MMI as shown in Figure 12. From the figure, it is observed that there is negligible interaction between the imidazole ring and the neutral silver in case of neutral complexes of the thiol and thione forms of MMI, because in this case the neutral silver atom is loosely bound to the ring N atom or the S atom in MMI (thiol)-Ag and MMI (thione)-Ag, respectively. In the charged Ag complexes of MMI (thiol) and MMI (thione) as seen from the figure, there is significant interaction between the occupied imidazole ring and the unoccupied silver ion leading to the formation of stable MMI (thiol)-Ag+ and MMI (thione)-Ag+ complexes. The adsorption of MMI on the metal (silver) surface also depends on the active sites of the molecule through which the binding takes place and can be estimated from the enhancement of the relevant Raman bands according to the electromagnetic surface selection rules. Moreover, the orientation of the molecule can also be estimated on the basis of the surface selection rules, as predicted by Moskovits31 and Creighton.32 According to the surface selection rule, the normal modes of vibrations with the polarizability derivative components perpendicular to the surface will be enhanced. The possible active sites of the thiol and thione forms of MMI are the free ring nitrogen (N5) in thiol and sulfur (S6) in thione and the delocalized π-electron cloud of the imidazole ring in both thiol and thione forms. The binding through the lone pairs of the N and S atoms usually leads to a blue shift in the ring breathing and the CN stretching vibrations,33,34 whereas binding through the π-electrons usually leads to a red shift in the Raman vibrations. For MMI adsorbed on the silver surface at neutral and alkaline media, the mode at 1364 cm-1 (ring CN stretch with imidazole ring bend) shows huge enhancement, followed by 1318 and 500 cm-1 modes that are assigned to ring CN stretching and NC1S6 bending modes, respectively. The huge enhancement in the intensity and the remarkable blue shift of the ring CN stretching and imidazole ring bending vibration (1364 cm-1) observed in the SERS spectrum clearly suggest that the free ring N5 is


Figure 12. Plots of frontier molecular orbitals, HOMO and LUMO, of the charged and neutral silver complexes of thiol and thione forms of MMI.

directly involved in the interaction with the silver surface. The Raman peaks observed at 1318, 1281, 1034, 936, and 617 cm-1 in the SERS spectrum also undergo considerable blue shift as compared to the solution spectrum. The majority of these Raman peaks are assigned to the ring CN stretches, ring bending, and ring breathing modes, which infers that the MMI is adsorbed on the silver surface through the free N atom of the imidazole ring. These results are well supported by the calculated vibrations corresponding to the optimized structure of the MMI (thiol)-Ag+ complex [Figure 5a]. In addition, the appearance of an intense peak in the SERS spectrum at 202 cm-1, attributed to Ag-N stretching vibration, further provides evidence for the direct attachment of the imidazole ring N of the thiol form of MMI to the silver surface. In the SERS spectrum of MMI, interesting features are observed for the C1S6 and ring CN stretching modes observed at 1455 and 1406 cm-1. Both of these modes showed a red shift of 8 and 6 cm-1, respectively. Additional bands observed at 1142, 1086, and 500 cm-1 in the SERS spectrum, assigned to C1S6 stretch, ring CN stretch, and NC1S6 bend, also show red shifts. It is observed from the figure that mostly the modes undergoing enhancement in intensity are the in-plane stretching and bending vibrations with the exception of 676 and 617 cm-1 that are attributed to the out-of-plane ring bending vibrations. The experimental data are well supported with theoretical models of the charged silver complexes of MMI. Thus, from the theoretical and experimental results, we can

7100 J. Phys. Chem. C, Vol. 113, No. 17, 2009 conclude that on the surface of silver, MMI preferentially exists in the thiol form at neutral and alkaline pH and is bound to the silver surface directly through the ring nitrogen atom of the imidazole ring with the ring lying in the plane of the silver surface. The orientation of MMI on the silver surface could be either edge-on or flat-on through the ring nitrogen atom or the ring π system, respectively. According to the surface-selection rules, the evidence for the edge-on modes is the appearance of inplane modes in comparison to out-of-plane modes, which may be weak or absent. Second, in case of edge-on orientation, the metal-N band can be observed in the low frequency region for the adsorbed molecule. In addition, the ring breathing modes of the adsorbate usually increase in intensity upon adsorption. Similarly, for the flat-on adsorption mode via the π electrons, the frequency shifts of the adsorbate show a very different behavior as compared to the edge-on modes. In the latter case, the out-of-plane modes are greatly enhanced. In the case of MMI adsorbed on the silver surface, the enhancement of mostly the in-plane ring stretching and bending vibrations and only a few out-of-plane bending vibrations in the SERS spectrum as well as the presence of Ag-N stretching band in the low frequency region indicates edge-on orientation of MMI on silver with the imidazole ring lying in the plane of the silver surface as shown in Figure 5a. The differences observed in the pH-dependent SERS spectra give clear evidence for preferential existence of the thiol and thione tautomeric forms on the silver surface with the thione form being dominant at acidic pH 2 and the thiol form being present at slightly acidic, neutral, and alkaline pH. 5. Summary Silver nanoparticles undergo aggregation on the addition of MMI to the silver hydrosol. The surface plasmon absorption band of the aggregated silver nanoparticles in aqueous solution shows a considerable red shift as compared to the original silver nanoparticles, as is desirable for Raman measurements and can be exploited for detecting biomolecules by surface-enhanced Raman scattering. The SERS technique has been exploited to record the vibrational spectra of MMI adsorbed on silver nanoparticles. The geometry optimization and the vibrational assignments of the two tautomeric (thiol and thione) forms of MMI, its thiolate anion and protonated forms, and their charged and neutral silver complexes [MMI (thiol)-Ag and MMI (thione)-Ag] have been carried out using DFT method. The Raman spectrum in solid and solution indicates the abundance of the thione form of MMI in solid and in acidic, neutral, and alkaline solutions. From the interpretation of the observed features in the SERS spectrum, the presence of Ag-N stretching band in the low frequency region, the observed enhancements of various in-plane ring bending and stretching vibrations, in addition to a few out-of-plane bending vibrations and the observed shifts of the Raman bands along with the application of “surface selection rules”, it has been inferred that preferentially the thiol form of MMI is chemisorbed to the silver surface at neutral and alkaline media, assuming an edge-on orientation with the imidazole ring nitrogen (N5) attached directly to the

Biswas et al. silver surface and the ring lying in the plane of the metal surface. This inference is supported by the optimized structure of the charged MMI (thiol)-Ag complex. The significant differences observed in the pH-dependent SERS spectra indicate the preferential existence of the thione and thiol forms of MMI on the silver surface in acidic, neutral, and alkaline media. Acknowledgment. We thank Dr. S. K. Sarkar, Head, RPCD, for his support and encouragement and Dr. Sanjay Wategaonkar of TIFR, Mumbai, for providing the Gaussian 98 computational facilities. We also thank Dr. T. Sakuntala of HPPD, BARC, for helping us in recording the Raman spectra. References and Notes (1) Garrell, R. L. Anal. Chem. 1989, 61, 401A–411A. (2) Schultz, S.; Smith, D. R.; Mock, J. J.; Schultz, D. A. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 996; FIELD Journal Code: 7505876. (3) Metiu, H. Prog. Surf. Sci. 1984, 17, 153. (4) Fleischmann, M. Chem. Phys. Lett. 1974, 26, 163. (5) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783. (6) Biswas, N.; Kapoor, S.; Mahal, H. S.; Mukherjee, T. Chem. Phys. Lett. 2007, 444, 338. (7) Biswas, N.; Thomas, S.; Kapoor, S.; Misra, A.; Wategaonkar, S.; Mukherjee, T. J. Chem. Phys. 2008, 129, 184702. Biswas, N.; Thomas, S.; Kapoor, S.; Misra, A.; Wategaonkar, S.; Venkateswaran, S.; Mukherjee, T. J. Phys. Chem. A 2006, 110, 1805. (8) Thomas, S.; Biswas, N.; Venkateswaran, S.; Kapoor, S.; Naumov, S.; Mukherjee, T. J. Phys. Chem. A 2005, 109, 9928. (9) Thomas, S.; Biswas, N.; Venkateswaran, S.; Kapoor, S.; D’Cunha, R.; Mukherjee, T. Chem. Phys. Lett. 2005, 402, 361. (10) Cao, P.; Gu, R.; Tian, Z. J. Phys. Chem. B 2003, 107, 769. (11) Liang, E. J.; Engert, C.; Kiefer, W. J. Raman Spectrosc. 1993, 24, 775. (12) Cho, K.-H.; Choo, J.; Joo, S.-W. Spectrochim. Acta, Part A 2005, 61, 1141. (13) Liu, X.; Yuan, H.; Pang, D.; Cai, R. Spectrochim. Acta, Part A 2004, 60, 385. (14) Kulak, A.; Hall, S. K.; Mann, S. Chem. Commun. 2000, 506. (15) Kimshita, T.; Seino, S.; Okitsu, T.; Nakayama, T.; Nakagawa, T.; Yamamota, A. J. Alloys Compd. 2003, 359, 46. (16) Merchant, B.; Lees, J. F.; Alexander, W. D. Pharmacol. Ther. (B) 1978, 3, 305. (17) Weetman, A. P.; McGregor, A. M.; Hall, R. Clin. Endocrinol. 1984, 21, 163. (18) Kendall-Taylor, P. Br. Med. J. 1984, 288, 509. (19) Kapoor, S. Langmuir 1998, 14, 1021. (20) Sarkar, A.; Kapoor, S.; Mukherjee, T. J. Phys. Chem. B 2005, 109, 7698. (21) Roy, A. P.; Deb, S. K.; Rekha, M. A.; Sinha, A. K. Ind. J. Pure Appl. Phys. 1992, 30, 724. (22) Becke, A. D. J. Chem. Phys. 1993, 98, 1372. (23) Frisch, M. J.; Gaussian 98, revision A.11.2; Gaussian, Inc.: Pittsburgh, PA, 2001. (24) Felidj, N.; Levi, G.; Pantigny, J.; Aubard, J. New J. Chem. 1998, 725. (25) Hanlon, D. P.; Shuman, S. Cell. Mol. Life Sci. 1975, 31, 1005. (26) Kamel, A.; Colizza, K., www.thermo.com/eThermo/CMA/PDFs/ Articles/articles File_4275.pdf. (27) Lapinski, L.; Nowak, M. J.; Kwaitkowski, J. S.; Leszczynski, J. J. Phys. Chem. A 1999, 103, 280. (28) Sarkar, J.; Chowdhury, J.; Talapatra, G. B. J. Phys. Chem. C 2007, 111, 10049. (29) Martos-Calvente, R.; de la Pena O’Shea, V. A.; Campos-Martin, J. M.; Fierro, J. L. G. J. Phys. Chem. A 2003, 107, 7490. (30) Campion, A.; Kambhampati, P. Chem. Soc. ReV. 1998, 27, 241. (31) Moskovits, M. J. Chem. Phys. 1982, 77, 4408. (32) Creighton, J. A. Surf. Sci. 1983, 124, 209. (33) Gao, P.; Weaver, M. J. J. Phys. Chem. 1985, 89, 5040. (34) Gao, X.; Davis, J. P.; Weaver, M. J. J. Phys. Chem. 1990, 94, 6858.

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Adsorption of Methimazole on Silver Nanoparticles: FTIR, Raman, and

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