J. Phys. Chem. C 2009, 113, 6989–7002
6989
Dopamine Molecules on Aucore-Agshell Bimetallic Nanocolloids: Fourier Transform Infrared, Raman, and Surface-Enhanced Raman Spectroscopy Study Aided by Density Functional Theory Surojit Pande,† Subhra Jana,† Arun Kumar Sinha,† Sougata Sarkar,† Mrinmoyee Basu,† Mukul Pradhan,† Anjali Pal,‡ Joydeep Chowdhury,*,§ and Tarasankar Pal*,† Department of CiVil Engineering, Indian Institute of Technology, Kharagpur 721302, India, Department of Physics, Sammilani MahaVidyalaya, Baghajatin Station, E.M. Bypass, Kolkata 700 075, India, and Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India ReceiVed: NoVember 20, 2008; ReVised Manuscript ReceiVed: March 16, 2009
Adsorption of dopamine (DA) on a Aucore-Agshell bimetallic nanocolloidal surface has been investigated using surface-enhanced Raman spectroscopy (SERS). The normal Raman spectra (NRS) of DA molecules in bulk and in aqueous solution have been investigated in depth. The vibrational signatures, as observed from the Raman and FTIR spectra of the molecule, have been assigned from the potential energy distributions. The pH-dependent NRS of the DA molecule in aqueous solution has been recorded to elucidate the protonation effect and preferential existence of different forms of the molecule. The pH-dependent SERS spectra of the molecule adsorbed on the bimetallic Aucore-Agshell nanocolloidal surface are also reported. The enhancement of bands in the pH-dependent SERS spectra suggests that the molecules are adsorbed onto the bimetallic Aucore-Agshell surface with the molecular plane tilted with respect to the silver surface of Aucore-Agshell bimetallic nanoparticles. The model study authenticates the spectral disposition and orientation of the molecule. Thus, experiment and theory keep abreast of the variety of DA structures envisaged from SERS studies on a new substrate. 1. Introduction Colloidal solutions of noble metals, especially those of gold and silver, have extensively been studied because of their intense absorption band in the visible region, often coined as surface plasmon absorption. This band is attributed to the collective oscillation of the electron gas in the particles with a change in the electron density at the surface.1 Single-molecule sensitivity of Raman scattering enhanced by resonantly excited metal nanoparticles has caused a renewed interest since the discovery of surface-enhanced Raman spectroscopy (SERS) in 1974.2 Actually SERS spectroscopy offers the possibility of overcoming many of the problems of conventional Raman spectroscopy.3-5 The SERS phenomenon is accompanied by quenching of fluorescence associated with Raman bands which extend the range of molecules, crude mixtures, and extracts that can be investigated. Despite intensive theoretical work and publications of excellent papers, the exact nature of the huge enhancement in Raman intensity found in SERS is still a matter of controversy.6-8 The magnitude of the enhancement in Raman scattering cross section depends on (i) the chemical nature of the adsorbed molecules, (ii) the roughness of the surface, and (iii) the optical properties of the adsorbate. Recently, quantum chemical calculations were employed successfully to assign the vibrational bands, which are then utilized to interpret the SERS spectra of the molecules.9-11 Among many facets of quantum chemical calculations, one is density functional theory (DFT). The most intriguing features of it is that everything is obtained * Corresponding authors,
[email protected] and tpal@ chem.iitkgp.ernet.in. † Department of Chemistry, Indian Institute of Technology. ‡ Department of Civil Engineering, Indian Institute of Technology. § Department of Physics, Sammilani Mahavidyalaya.
directly from an observable and we are led to one particle theory that contains electron correlation.12 The ultrahigh sensitivity of SERS enables one to record the spectra at concentrations down to the single molecular level but also to estimate the molecular forms and their possible orientations on the metal surface.13-16 Due to these advantages, SERS has generated considerable impact in fields such as surface chemistry, electrochemistry, solid-state physics, analytical chemistry, inorganic chemistry of metals, problems of radiating multipoles near metal surfaces, generation of surface plasmon, and study of corrosion.17-21 It has proved to be a very useful tool for solving problems in biophysics, biochemistry, and molecular biology.22-24 Cathecholamines and their derivatives have received much attention in contemporary scientific research because of their remarkable industrial and biological significance.25-29 Dopamine (DA) is the major neurotransmitter for the extrapyramidal motor tracts of the central nervous system (CNS) and is centrally involved in the mechanism of psychostimulant addiction.30 Its receptors are the major sites of action of antipsychotic as well as anti-Parkinsonism drugs. DA also acts as an isotropic vasopressor agent, and it specifically works at a number of different receptor sites in the body depending on the dose at which it is administered. At lower doses it binds to dopamine receptors in the kidney, gut, brain, and heart causing the blood vessels in these organs to widen. This improves the blood flow and therefore improves the amount of oxygen supplied to the organs. When the dose of DA is increased, β-receptors, which are found on the heart muscle, are also activated. Dopamine hydrochloride is a potent drug and is extensively used in heart failure, hypotension, and treating certain types of shock. DA molecule has two ionizable functional groups (one ethylammonium and two hydroxyl), which may participate in
10.1021/jp810210a CCC: $40.75 2009 American Chemical Society Published on Web 04/08/2009
6990 J. Phys. Chem. C, Vol. 113, No. 17, 2009 acid-base equilibrium. The molecule primarily has two pKa values; the pKa1 and pKa2 values for the proton dissociation from one of the externally attached hydroxyl and ethylammonium groups of the molecule are at 8.87 and 10.64, respectively.31-34 However, the second hydroxyl group of the DA molecule has a pKa value greater than 12 and so, at pH > 12, the molecule exists in dianionic form.32-34 Depending upon the pH of the medium, the DA molecule thus may coexist as complex mixture of zwitterionic, charged, and nonionized species.35 The relative rates of penetration are not precisely known as yet.36 Theoretical results indicate that at physiological pH (pH ∼7), DA exists mostly as cation and the indirect evidence of the preferential existence as cationic form is revealed from the experimental results of site-directed mutagenesis studies.37 However, at pathological pH, the preferential cationic form of the DA molecule may undergo translocation toward the other forms of the molecule.33 In order to elucidate the variation of the structure, conformation, and hydrogen bonding ability of DA molecules upon the protonation/deprotonation effect, we present here the detailed experimental and theoretical Fourier transform infrared (FTIR), normal Raman spectra (NRS), and SERS of the molecule adsorbed on bimetallic Aucore-Agshell nanocolloid. The theoretical estimation of vibrational signatures of the molecule has been provided from the quantum chemical calculations using DFT. The adsorption and orientation of the pH-dependent form of the DA molecule on Aucore-Agshell surface at trace level (∼10-7 M), close to that encountered under physiological and pathological conditions in living system, have been elucidated from the SERS spectra. After elucidating the adsorption mechanism of the molecule, the study can be expanded to the adsorption on membranes or other interesting biological surfaces for medical or therapeutic treatments.38-42 Additionally, this paper describes complete vibrational assignment of the DA molecule, though some reports are present elsewhere.43-49 2. Experimental Section 2.1. Chemicals and Instruments. All the reagents were of analytical reagent grade. Chloroauric acid (HAuCl4), silver nitrate (AgNO3), β-cyclodextrin (β-CD), and dopamine (DA) in its hydrochloride form were purchased from Sigma-Aldrich and were used as received. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were obtained from S.D. Fine-Chem. Double distilled water was used throughout the course of the investigation. All absorption spectra were recorded in a Shimadzu UV-160 spectrophotometer (Kyoto, Japan) taking the solutions in a 1 cm quartz cuvette. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) measurements of the metal sols were performed in a Hitachi H-9000 NAR instrument on samples prepared by placing a drop of fresh metal sols on copper grids precoated with carbon films, followed by solvent evaporation under vacuum. FTIR spectral characteristics of the samples were collected in reflectance mode with a Nexus 870 ThermoNicolet instrument coupled with a Thermo-Nicolet Continuum FTIR microscope. The solid powder was placed on a KBr pellet and was dried under vacuum for 6 h before analysis. NRS and SERS spectra of dopamine molecules were obtained with a Renishaw Raman Microscope, equipped with a He-Ne laser excitation source emitting at a wavelength of 633 nm, and a Peltier cooled (-70 °C) charge coupled device (CCD) camera. A Leica microscope was attached and was fitted with three objectives (5×, 20×, 50×). For these experiments, the 20× objective was used to focus the laser beam onto a spot of 1-2 µm2. Laser power at the sample was 20 mW and the data
Pande et al.
Figure 1. Time-dependent absorption spectra of Aucore-Agshell bimetallic nanoparticles prepared from β-CD (7 mM) capped Au seed at pH ∼10-12.
Figure 2. TEM (a) and HRTEM (b) images of Aucore-Agshell bimetallic nanoparticles.
acquisition time was usually 30 s. The holographic grating (1800 grooves/mm) and the slit enabled the spectral resolution of 1 cm-1. A pH meter (Systronics pH meter model MK-VI, India) was used to measure the pH. 2.2. Synthesis of Aucore-Agshell Bimetallic Nanoparticles. Aucore-Agshell bimetallic nanoparticles were synthesized by the reported method.50 In short, the synthesis may be described as follows. At first, 0.0396 g of β-CD was dissolved in 4.93 mL of water. Then 0.02 mL of HAuCl4 (10 mM) solution was mixed with it. After 2 min 0.05 mL of NaOH (1.0 M) was added into the solution so that the pH of the solution becomes ∼10-12. Then the reaction mixture was heated on a water bath at ∼90 °C. After ∼15 min, the solution turned pink indicating the formation of gold nanoparticles in the solution. Now an aliquot of 0.04 mL of AgNO3 (10 mM) was added drop by drop to the preformed colloidal gold solution. The solution was allowed to stand for 5 min and then heated on a water bath at ∼90 °C. After about half an hour, the pink color of the solution turned to reddish yellow indicating the formation of a silver shell on the gold particles, i.e., the formation of Aucore-Agshell particles. No extra β-CD or NaOH was needed to induce the chemical reduction of silver ions and their deposition onto the gold seed particles. The synthesized bimetallic nanoparticles were characterized by UV-visible spectroscopy (Figure 1) and TEM and HRTEM studies (Figure 2) and the estimated size was ∼22 nm. From the HRTEM images the core diameter and shell thickness were observed to be ∼10 nm and ∼12 nm, respectively. 2.3. SERS Studies. The sample was prepared by mixing of 0.75 × 10-4 M bimetallic nanocolloid with 1.0 × 10-7 M
Dopamine Adsorption on a Metal Surface
J. Phys. Chem. C, Vol. 113, No. 17, 2009 6991
Figure 3. Optimized molecular structure of the (a) cationic, (b1 and b2) zwitterionic, (c) neutral, and (d1 and d2) anionic forms of the molecule obtained from the B3LYP/6-31++G(d,p) level of theory with their gas phase energy values. The numbers in parentheses refer to the solvated energy values of the respective forms of the molecule.
aqueous DA. Then the pH of the solution was adjusted by 0.1 M HCl or NaOH. It took 24 h to obtain the best enhanced SERS spectra. So, the incubation time was always 24 h for for the spectral measurement. 3. Theoretical Calculations Theoretical calculations were carried out using Gaussian-03 software.51 Optimization of the cationic, zwitterionic, neutral, and anionic forms of the DA molecule and the vibrational frequencies at their respective optimized geometries were computed by the DFT calculation. The B3LYP, that is, the Becke three hybrid exchange52 and Lee-Yang-Parr correlation functional53 (LYP), along with the Pople split valence diffused and polarized basis set 6-31++G(d,p) were utilized in the calculation. It is known that the 6-31++G(d,p) basis set in conjunction with B3LYP functional are sufficient for the calculation of geometric parameters and harmonic frequencies of molecules like that of DA which contain substituted phenyl ring structures.54,55 For the molecule-metal model complexes, the geometry optimization was fruitful using only the LanL2DZ56 basis set which makes use of the ECP pseudopotentials for metal atoms. The theoretically estimated vibrational frequencies for the metal-molecule surface complex models obtained from the B3LYP/Lanl2DZ level of calculations satisfactorily agree with the observed vibrational frequencies without using a scaling factor. However, the theoretically estimated vibrational frequencies of free molecules obtained from the B3LYP/6-31++G (d,p) level of calculations were scaled by the scaling factor 0.9614. In the process of geometry optimization for the fully relaxed method, convergence of all the calculations and the absence of imaginary values in the wavenumbers confirmed the attainment of local minima on the potential energy surface. Cartesian displacement and calculated (B3LYP/6-31++G(d,p)) vibrational modes of the molecule have been displayed using Gauss View-03 software. Normal coordinate analyses of the dopamine molecules were carried out from the output of the DFT calculations using GAR2PED57 software. The isoelectric point (PI) of the DA molecule was theoretically estimated using58 Marvin 5.1.0 software.
The effects of aqueous medium as solvent on the molecular structure and on the vibrational modes of different forms of the molecule were estimated by the integral equation formalism polarized continuum model (IEFPCM), which is a special version of the polarized continuum model (PCM). In this model, the solvent, water, is treated as a continuum dielectric medium and the solute is considered as a trapped molecule in a cavity surrounded by the solvent. Detail discussions of the IEFPCM model have been reported elsewhere.59 4. Results and Discussion 4.1. Normal Raman and FTIR Spectra of the Molecule and their Vibrational Assignment. The DA hydrochloride, a prominent member of the cathecholamine family, has an externally attached ethyl ammonium group and a phenolic hydroxyl group which act as proton acceptor and proton donor, respectively. The pKa value of the second phenolic group of the molecule is greater than 12.32-34 Hence, except at extremely basic pH values (pH >12), where both hydroxyl groups of the molecule can be dissociated, DA can exist as cationic (DA+), zwitterionic (DA(), neutral (DA0), and anionic (DA-) forms. The optimized molecular structures of the DA+, DA(, DA0, and DA- forms of the molecule are shown in Figure 3. However, among the above-mentioned various forms, the intramolecular hydrogen bond formation is enviable in the DA( and DA- forms of the molecule (Figure 3b1,b2,d1,d2). The O9 · · · H22 and O9 · · · H21/O7 · · · H22 and O7 · · · H21 bond lengths are ∼2.06 Å for the DA( and DA- forms of the molecule, respectively. In order to retrieve some ideas concerning the relative stability of different forms of the molecule in the gas phase and in aqueous solution, minimum energies of the molecule at their respective optimized geometries have been computed using the DFT method. The theoretical results indicate that the cationic (DA+) form of the molecule are energetically more favorable over the other forms in the gas phase and in aqueous solution. The SCF energy of the cationic form of the DA molecule is estimated to be ca. -12.04/-12.79 eV and -24.66/-25.58 eV more stable than the zwitterionic (DA() and anionic (DA-) forms of the molecule in the gas phase/aqueous solution, respectively. All
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Figure 4. (a) Normal Raman spectrum of the DA molecule in solid state. (b) FTIR spectrum of the DA molecule of neat powder in KBr pellet.
the four different forms of the molecule belong to CS point group symmetry, and among them the DA+ molecule has 23 and DAform of the molecule 21 atoms. Thus the 63 and 57 fundamental vibrations of DA+ and DA- molecules are divided into the symmetry species as, 43A′ + 20A′′ and 39A′ 18A′′, respectively. The other two forms of the molecule (DA( and DA0) both have 22 atoms, and hence 60 fundamental vibrations for both of them are classified as 41A′ + 19A′′. On the basis of the equilibrium dissociation constant (Kz ∼ 7.83), the neutral, uncharged form of DA is less predominant than the zwitterionic form and its population is found to be 0.2%.33,35 Simple knowledge of group theory predicts that the planar (A′) and nonplanar (A′′) species for all the four different forms of the DA molecule are expected to appear both in the Raman and in the IR spectra. However, among these vibrations originating from the various forms of the molecule, some vibrations are degenerate. The NRS of the molecule in neat solid and the FTIR spectra of powdered molecule in KBr pellet are shown in parts a and b of Figure 4, respectively. The normal Raman spectra of the molecule at 0.1 M in aqueous solution recorded at various pH values are shown in Figure 5. The theoretically simulated NRS spectra of the DA+, DA(, and DA- forms of the dopamine molecule in gas phase are shown in Figure 6. The primary aim of recording the NRS and FTIR spectra of the molecule is to apprehend the existence of preferential form/forms of the molecule and there from the assignment of its vibrational signatures in solid state and also in aqueous solution. Table 1 lists the experimentally observed FTIR and NRS band frequencies of the molecule. The theoretically computed vibrational frequencies of the cationic (DA+), zwitterionic (DA(), and anionic (DA-) forms of the DA molecule in the gas phase are also shown in Table 1 along with their tentative assignments, as provided from the PED. The PED calculations in terms of internal coordinates of the molecule have been estimated from the output of the DFT calculations. The observed disagreement between the theory and the experiment could be a consequence
Figure 5. Normal Raman spectrum of the DA molecule in aqueous solution (0.1 M) at varied pH.
Figure 6. The theoretical gas-phase Raman spectrum of the (a) anionic, (b) zwitterionic, and (c) cationic forms of the DA molecule calculated using the B3LYP/6-31++G(d,p) level of theory.
of the anharmonicity and also may be due to the general tendency of the quantum chemical methods to overestimate the force constants at the exact equilibrium geometry.60,61 However,
Dopamine Adsorption on a Metal Surface
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TABLE 1: Observed and Calculated IR and Raman Bands of the Molecule in Varied pH Environment and Their Tentative Assignmentsa NRS solid NRS pH DA+ FTIR (obsd) (obsd) ∼4 (obsd) (calcd) (cm-1) (cm-1) (cm-1) (cm-1) 267 w
311 vvw 330 vw
267 w
311 vvw
330 w
assignment (PED %)
274
26R6,7,19 31[R17,3,4 R16,3,4] 35[R13,1,2 R23,1,2] 7R4,5,6
291 309
61R5,7,19 19[R13,1,2 R23,1,2] 9[R17,3,4 R16,3,4] 6R4,5,6 95[R6,7,19 R8,9,20]
317
94γ7,19
344
94γ9,20
373
99[τ5,6,7,19 τ8,6,7,19]
439 454
96[τ8,6,5,18 τ6,8,10,21] 95[R4,5,6 R5,8,6 R8,11,10]
523
62R4,5,11 25[R17,3,4 R16,3,4] 10R13,2,1
NRS pH DA( ∼9 (obsd) (calcd) (cm-1) (cm-1) 267 vvw
258
286 311
458 vvw 474 vvw 517 vvw 534 vvw 560 vvw 596 w
394 ms 458 vw
474 vw 517 vvw
474 vw 517 vvw
553 w
565
96[R4,5,6 R5,6,8 R10,11,4 R5,4,11]
596 w
596 w
605
95τ3,4,11,22
634 vw
634 w
670 vvw 692 vvw
330 vvw
335
365
267 vvw
51[R13,1,2 R21,1,2 R12,1,2] 41R14,15,2
311 vvw
51[R8,6,7 R6,8,9] 17R3,2,14 13[R13,2,1 R21,1,2] 14[R4,16,3 R4,3,17] 54[τ4,11,20,3 τ4,5,18,3] 15[R4,3,16 R4,3,17] 12[R13,2,1 R21,2,1] 95[τ11,10,19,20 τ6,5,18,7]
330 vvw
451
62[R6,5,18 R4,11,20 R11,10,19] 24[R3,16,2 R3,17,2] 5[R13,2,1 R21,2,1]
458 vvw
508 vvw
516
62[R8,10,11 R11,4,5 R4,5,6] 30r4,3 7[R2,1,13 R2,1,21 R2,1,12]
508 vvw
596 vvw
534 vvw 573
91[τ5,4,18,3 τ4,11,20,3 τ8,10,9,19]
588
52[R10,4,11 R8,10,11 R10,8,6 R6,5,4] 22[R4,16,3 R4,17,3]
94[τ8,6,7,19 τ5,6,7,19]
11γ2,13,14 74γ20,1,12 6[R4,11,10 R5,6,8] 40R6,7,21 45R10,8,9 11R16,3,15
365
66[τ19,11,4,3 τ17,5,6,7] 20r2,3 8R20,1,2
422
94[τ5,6,17,7 τ10,8,18,9]
458
71[R4,11,10 R5,6,8] 23R15,3,16
534
76[R4,11,10 R10,8,6 R6,5,4 R5,4,11] 21r3,4 82[R4,11,10 R10,8,6 R6,5,4] 12[R4,3,15 R4,3,16] 94[τ4,11,19,3 τ19,11,10,18 τ5,6,17,7]
586 596 vvw
88R20,1,12 34[τ17,5,4,3 τ3,4,11,19] 24[R2,3,15 R2,3,16] 22[R1,2,13 R1,2,14] 12[R1,2,20 R2,1,12] 53R16,3,15 39R6,7,21
596
670 vvw 688
91[τ10,11,19,20 τ6,5,7,18]
692 vvw
689
95[τ6,5,7,17 τ18,10,11,19]
702
58[R2,1,12 R2,1,13 R2,1,21] 17[R3,14,2 R3,15,2] 8[R4,16,3 R4,3,17] 7R8,10,11 45[R13,2,1 R21,2,1 R12,2,1] 41[R3,14,2 R3,15,2 R1,14,2 R1,15,2] 9[R4,3,17 R4,3,16] 51γ7,22 46[γ11,20 γ10,19]
725 vvw
719
59[R11,10,8 R4,5,6] 31r3,4 7R2,1,20
750 vvw
751
43γ7,21 21[γ11,19 γ10,18] 12R3,15,16 15R14,2,13 34γ7,21 31R14,2,13 27R3,15,16 8[R2,1,12 R2,1,20] 52γ7,21 44[γ11,19 γ10,18]
725 w
725 w
724
58R11,10,8 32[R17,3,4 R16,3,4] 8[R13,2,1 R23,2,1]
725 vw
713
750 w
750 vvs
750 vs
745
24[R12,1,2 R13,1,2 R23,1,2] 37[R15,2,14] 34R17,3,16
750 w
761
772 w
309
assignment (PED %)
625 vvw 668
772 w
224 251
283
458 vw
720 vvw
772 vvw
71[τ18,5,6,7 τ20,11,10,19] 13[R3,2,15 R3,2,14] 10[R2,1,21 R2,1,12 R2,1,13] 80R17,3,16 9R13,2,1 6R6,7,22
534 vvw 553 w
NRS pH DA∼11 (obsd) (calcd) (cm-1) (cm-1)
315
421 394 w 458 w
assignment (PED %)
772
44R4,6,5 9[R12,1,2 R13,1,2] 7R17,3,4 24R6,7,19 14r8,9
772 vvw
770
94[R13,2,1 R12,1,2 R21,1,2]
753
780
53γ7,22 44[γ5,18 γ11,20 γ10,19]
774
786
35[R13,1,2 R21,1,2] 29[R6,5,4 R8,6,5] 27[R4,3,16 R4,3,17] 6γ7,22
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TABLE 1: Continued NRS solid NRS pH DA+ FTIR (obsd) (obsd) ∼4 (obsd) (calcd) (cm-1) (cm-1) (cm-1) (cm-1) 791 w
791 s
812 ms
assignment (PED %)
791 s
798
47R23,2,1 42γ5,18 8γ10,21
812 vw
820
71[τ18,5,6,7 τ8,6,7,19] 26[R23,1,2 R13,1,2]
NRS pH DA( ∼9 (obsd) (calcd) (cm-1) (cm-1) 791 w
assignment (PED %)
805
56[R12,1,2 R13,2,1 R21,2,1] 22[R3,14,2 R3,15,2] 19[R4,16,3 R4,17,3]
809
79[γ18,5 γ7,22 γ10,19 γ11,20] 8[R12,1,2 R21,1,2 R13,1,2]
NRS pH DA∼11 (obsd) (calcd) (cm-1) (cm-1) 791 vvw
789
815 818
875 w
879 vw
879 vw
870 889 898
86[R12,1,2 R13,1,2] 14[R14,2,15 R16,3,17] 94τ11,10,21,22 84[R12,1,2 R13,1,2 R23,1,2] 21R17,3,16 9[R4,11,22 R4,11,21]
863
915 932 w
932 w
932 vw
963 w
948 w 963 ms
948 s 963 w
938
1082 w
1014 w
1082 vvw
1014 w
1082 vvw
1037
79[R12,2,1 R13,2,1] 19[R17,3,4 R16,3,4]
1077
90[R13,2,1 R23,2,1 R12,1,2] 7[R14,2,3 R15,2,3] 54[R6,7,19 R8,9,20 R22,11,10] 31R16,3,17 11[R23,1,2 R13,2,1]
1082
1115 w
1115 w
1152 w
1146 w
1146 vw 1138
1135
1149
72[R6,7,19 R8,9,20] 14[R4,5,6 R5,8,6 R8,11,10] 7[R12,1,2 R13,1,2] 74[R4,11,22 R8,10,21] 11R17,3,16 9R8,9,20 71[R4,11,22 R8,10,21 R4,5,18] 15R8,9,20 6[R3,6,4 R3,17,4]
1174 vvw
999
54[R20,1,2 R12,1,2] 15[R15,3,4 R16,3,4] 17[R14,2,3 R13,2,3] 12R8,10,18
932 vvw
925
55[R20,1,2 R12,1,2] 14[R15,3,16 R14,2,13] 09[R11,10,18 R4,5,17]
1115 vvw
992vvw
994
43[R15,16,3 R14,2,13] 30R12,1,2 18R17,5,6
1026
71[R16,3,2 R15,3,2] 11[R20,1,2 R12,1,2] 14R13,2,1
1061
52[R13,2,1 R21,2,1 R12,2,1] 39r2,3
1093
95[R10,11,20 R11,10,19]
1083
83[R19,11,10 R18,10,8] 9R3,2,14 5R2,1,20
1104
21[R20,1,2 R12,1,2] 31[R3,4,16 R3,4,15] 25[R3,2,14 R3,2,13] 20R11,19,10 65[R6,5,17 R11,10,18] 27[R2,3,16 R2,3,15] 6R1,2,14
1122
1115 vvw
1111
1157 vvw 1161
1190 vvw
1182
1204 vvw
1209 w
1226 74[R16,3,17 R15,2,14] 22[R12,1,2 R13,1,2]
71[R6,5,18 R8,10,19] 25r3,4
1146 vvw
1170 vvw
1230
56[R4,16,3 R4,17,3] 20R3,2,14 18[R12,2,1 R13,2,1 R21,2,1]
1014vvw
1190 s 1190 vvw 1209 vw
61[γ2,1,12 γ2,1,20] 21γ5,17 8R2,14,13 7R3,15,16 97τ19,11,10,18
948 vw 963 vvw
1115 ms
1174 ms
52[R4,5,6 R4,11,10 R11,10,8 R5,4,11] 16[R4,3,16 R4,3,15] 13[R1,2,14 R1,2,13] 9rc)o 85γ5,17 10[R20,1,12 R12,1,2 R20,1,2]
909
56[R4,5,6 R4,11,10] 31R11,10,19 9R4,3,17
38R17,3,4 33R12,1,2 14r6,7 12[R4,5,6 R10,4,11]
994vvw 1014 w
854
90τ10,11,19,20
assignment (PED %)
1243
47[R3,2,14 R3,2,15] 28[R4,16,3 R4,17,3] 19[R12,1,2 R13,2,1 R21,1,2] 91[R8,10,19 R6,7,22 R4,5,18]
1172
51[R4,5,17 R11,10,18] 47R6,7,21
1201
45[R4,3,16 R4,3,17] 42[R3,2,14 R3,2,15] 6[R4,5,18 R10,11,20] 44R3,2,15 40R2,3,16 11[R11,10,19 R6,7,22]
1220
41R15,3,4 37[R1,2,13 R1,2,14] 16[R2,1,20 R2,1,12] 43[R4,11,19 R11,10,18 R4,5,17] 22R4,3,16 27r6,8 6R6,7,21
Dopamine Adsorption on a Metal Surface
J. Phys. Chem. C, Vol. 113, No. 17, 2009 6995
TABLE 1: Continued NRS solid NRS pH DA+ FTIR (obsd) (obsd) ∼4 (obsd) (calcd) (cm-1) (cm-1) (cm-1) (cm-1) 1260 w
1260 vvw
1260 vvw
1256
1279
1288 vvs
1288 vs
1288 s
68R14,2,15 31[R12,1,2 R13,1,2]
1288
47[R16,3,17 R15,2,1] 19[R13,1,2 R23,1,2] 10[R4,5,18 R4,11,22]
1302
44[R6,7,19 R8,9,20] 38[R4,11,22 R8,10,21] 16[R14,2,3 R17,3,4]
1329 w
1340 w
1340 vw 1345
1397 vw
1400 s 1420 1432
1451 w
1451 w
1447 1450
1472 w 1497 vs 1516 ms 1540 vw 1560 vw 1582 vw
1472 vw 1497 vvw
51[r4,5 r5,6 rc-c] 42R4,11,22 6[R6,7,19 R8,9,20] 56[R17,3,16 R2,14,15] 28[R13,1,2 R12,1,2] 72[R15,2,14 R17,3,16] 9[R13,1,2 R12,1,2] 81[R23,1,2 R12,1,2 81[R23,1,2 R12,1,2 R13,1,2] 9R17,3,16
NRS pH DA∼11 (obsd) (calcd) (cm-1) (cm-1)
48[R2,3,14 R2,3,15] 45[R4,17,3 R4,16,3]
1256
46R6,7,21 18[R1,13,2 R1,14,2] 7R2,3,16 21[R17,5,4 R19,11,10]
1270
34[R3,14,2 R3,15,2] 33[R4,17,3 R4,16,3] 26R6,7,22
1263
45R2,3,16 53[R1,2,13 R1,2,14]
1285
42[R4,3,16 R4,3,15] 26[R10,11,19 R2,10,18] 23R6,7,21 6[R2,1,12 R2,1,20] 34R4,3,15 31[R3,2,14 R3,2,13] 26[R19,11,10 R18,10,11] 7[R12,1,2 R20,1,2]
1275
35[R3,14,2 R3,15,2] 31[R4,17,3 R4,16,3] 28R6,7,22
1313
99[R12,13,1 R12,21,1 R13,21,1]
1324
73[R10,11,20 R8,10,19 R6,7,22] 24[R4,3,17 R4,3,16]
1288 vvw
1421
65[R8,10,19 R6,7,22 R6,5,18] 31[R4,3,17 R4,3,16] 98R14,15,2
1431
98R16,3,17
1409
1290
1329 vvw
1353
92[R1,2,13 R1,2,14] 6[R2,1,20 R2,1,12]
1426
96γ15,2,16
1455
66R6,7,21 27[R19,11,10 R18,10,8 R17,5,4] 6[R4,11,19 R11,10,18] 96R2,13,14
1400 ms
1459 1472
77[R18,4,5 R22,11,10 R21,10,8 R18,5,6] 19[R6,7,19]
assignment (PED %)
1247
1451 vvw
1472 w 1497 vw 1509
assignment (PED %)
44[r4,11 r10,8... rc-c] 29[R6,7,19 R8,9,20] 21[R15,2,14 R3,2,16] 87R14,2,15 9R17,3,16
1397 vw
1437 vvw
1288 w
1329 vvw
1347 1390 vw
NRS pH DA( ∼9 (obsd) (calcd) (cm-1) (cm-1)
52[R19,7,6 R20,9,8] 35R17,3,4 7R15,2,1 6[R4,6,5 R6,8,10 R4,11,10] 59[R17,3,4 R16,3,4] 33[R5,4,18 R11,10,21]
1284
1319 s 1329 w 1340 vvw
assignment (PED %)
1506
34[R20,11,4 R6,5,18] 62R6,7,22 84[r4,11 r5,6... rc-c] 11[R4,16,3 R4,17,3]
1472 vvw
1480
1480 77[r4,11 r4,5] 21R6,7,21
1527
49[r4,5 r10,8] 48R6,7,21
1580
51[r5,6 r11,10] 44R6,7,21
1586
97R12,1,20
1540 vw 1560 vw 1574 1582 w
1582 w
1581 1591
1601 1601 1601 w vvw w 1612 s 1617 s 1617 s
1601 1607
1582 vw 85[r4,5 r5,6... rc-c] 13R8,9,20 91[r4,5... rc-c] 6[R6,7,19 1592 R8,9,20] 94[R23,1,2 R12,1,2 1598 R13,1,2 R12,1,23 R13,1,12] 77[R23,1,2 R12,1,2 1603 R13,1,2]
72rc)o 16R6,7,22 8[R6,5,18 R11,10,19]
1560 vvw 1582 vvw
96[R12,21,1 R12,13,1 R13,21,1] 70[r6,5 r4,5] 18R22,7,6 9[R4,5,18 R10,11,20] 96[R12,1,21 R21,1,13 R13,1,12]
Key: vs, very strong; s, strong; ms, medium strong; w, weak; vw, very weak; vvw, very very weak; r, stretching; R, in-plane bending; γ, out-of-plane bending; τ, torsion. Only contributions g5 are reported. a
it is to be emphasized that the calculated Raman spectrum represents the vibrational signatures of the molecules in the gas phase. Hence, the experimentally observed NRS of the molecule recorded in the solid state and in solution may differ significantly from the calculated spectrum. Despite this fact, one can observe that there is a general concordance regarding the Raman
intensities as well as the position of the peaks between the experimental and calculated spectra.62-64 The FTIR spectra of the powdered sample and the NRS spectrum of the molecule in solid state are characterized by sharp and well-resolved vibrational bands (Figure 4a,b). Compared to the NRS spectrum of the molecule recorded in the solid state,
6996 J. Phys. Chem. C, Vol. 113, No. 17, 2009 the spectrum of the molecule in aqueous solution at pH ∼4 (Figure 5) exhibits almost the same nature of Raman bands. However, with the increase in pH (pH ∼9) the Raman bands (Figure 5) in general are broadened and are accompanied by the variation of the relative intensity pattern. Finally at pH ∼11, the diminution in intensities (Figure 5) of almost all the bands is observed. The modes arise principally from the stretching and bending vibrations of the cathechol ring moiety of the molecule. In the assignment of the vibrational frequencies, literature concerning the vibrational analyses of the dopamine and related cathecholamine molecules have been considered.43-49 Here, in this report, we have extended the band assignment of the DA molecule using DFT and quantified the band positions from PED calculation in view of the reported information. An interesting observation has been presented for the appearance of weak but well-resolved bands at around 267, 311, and 330 cm-1 in the NRS spectrum of the molecule in the solid state and in aqueous solution at pH ∼4. The band at ∼267 cm -1 (calcd at 274 cm-1 for DA+, 258 cm-1 for DA(, and 251 cm-1 for DA- form of the dopamine molecule) has considerable contributions from in-plane R(C6-O7-H19); R(H17-C3-C4) bending vibrations originating from the DA+ molecule and/or torsional motions τ(H18-C5-C6-O7); τ(H20-C11-C10-H19) and τ(H17-C5-C4-C3); τ(H19-C11-C4-C3) arising from DA( and DA- forms of the dopamine molecule, respectively. However, at this specified pH of the medium (pH ∼4), the DA+ form of the molecule is known to be preponderant, so the vibrational signature of 267 cm-1 may apparently be considered to arise from the in-plane bending vibrations of the cationic form of the molecule. A similar conclusion can be drawn for the 311 cm-1 band (calcd at 309 cm-1 for DA+/DA- and at 311 cm-1 for DA( forms of the molecule), which has been assigned to have prevailing contribution from in-plane R(C6-O7-H19); R(C8-O9-H20) bending vibrations originating from the DA+ molecule. There is a discrepancy in the assignment of the 330 cm-1 band (calcd at 344 cm-1 for DA+ and at 335 cm-1 for DA( forms of the molecule). The vibrational signature of this band has prominent contribution from the out-of-plane γ(O9-H20) bending mode and/or in-plane R(C8-C6-O7) and R(C6-C8-O9) bending vibrations originating from the DA+ and DA( forms of the molecule, respectively. Interestingly, all these bands disappear completely at higher pH (pH ∼9 and 11). The disappearance of the above-mentioned bands at alkaline pH may be due to the possible deprotonation of the O9 atom of the DA molecule. The perturbation of vibrations involving the deprotonated oxygen (O9) atom of the cathechol moiety of the DA molecule with pH of the medium may result in the variations of Raman intensities of 267, 311, and 330 cm-1 bands. These results primarily connote the existence of the cationic form of the DA molecule at acidic pH (pH ∼4). Considerable attention can be drawn regarding the bands centered at ∼725 (calcd at 724/713/719 cm-1 for DA+/DA(/ DA- forms of the dopamine molecule), 750 (calcd at 745/761/ 751 cm-1 for DA+/DA(/DA- forms of the dopamine molecule), 772 (calcd at 772/780/774 cm-1 for DA+/DA(/DA- forms of the dopamine molecule), and 791 cm-1 (calcd at 798/805/789 cm-1 for DA+/DA(/DA- forms of the dopamine molecule) in the FTIR and in the NRS spectra of the molecule. Among these modes, the intensity of the 750 cm-1 band is much more pronounced in comparison to others. This band, though weak in FTIR, appears as a very strong signal in the NRS spectra of the molecule recorded in solid state and in aqueous solution at pH ∼4. The vibration of this band has been assigned to have
Pande et al. significant contribution either from the in-plane or from the outof-plane modes arising from DA+ and/or DA(/DA- forms of the dopamine molecule, respectively. Considering the intensity profile of this band in the NRS spectra of the molecule in the solid state and in the aqueous solution in acidic pH, its assignment as the in-plane modes arising from the cationic (DA+) form of the molecule is more justified. This result again substantiates the presence of cationic form of the DA molecule in the solid state and in aqueous solution at acidic pH (pH ∼4). However, with the increase in pH of the medium, the intensity of this band drops significantly, and at pH ∼11, the band disappears completely. A similar trend is also observed for the bands centered at ∼725, 772, and 791 cm-1. The 725 cm-1 band has significant contribution from the in-plane R(C11-C10-C8); R(H13-N1-C2); R(C3-C2-H14), and R(C11-C10-C8); R(C4-C5-C6) bending vibrations descending from the DA+, DA(, and DA- forms of the molecule, respectively. The 772 cm-1 band, which is weak but well resolved both in the FTIR and in the NRS spectrum of the molecule, has prevailing contribution from the in-plane phenolic ring bending mode of the cationic (DA+) form of the molecule and/or out-of-plane γ(O7-H22); γ(C5-H18) and γ(O7-H21); γ(C11-H19) bending modes of the zwitterionic (DA() and anionic (DA-) forms of the molecule, respectively. The 791 cm-1 band which is moderately strong in the NRS but weak in FTIR has been assigned to have almost equivalent contributions from the inplane R(H23-N1-C2) as well as from the out-of-plane γ(C5-H18) bending vibrations of the DA+ form of the molecule. The vibrational signature of the above-mentioned band may also originate from the in-plane R(H12-N1-C2) and R(C4-C5-C6) bending vibrations of the DA( and DA- forms of the molecule, respectively. However, all the above-mentioned bands thus representing different vibrational signatures in the same frequency region appear weakly in the NRS spectrum of the molecule at pH ∼9. Weak but distinct appearance of these modes in the NRS spectrum of the molecule at pH ∼9 may not only signify that these bands are perceptive upon deprotonation but also mark the concomitance of the cationic, zwitterionic, and anionic forms of the DA molecule in the specified pH (pH ∼9). This result is in accordance with the literature reported elsewhere.43-49 A moderately intense band at ∼812 cm-1 (calcd at 820/809/ 815 cm-1 for DA+/DA(/DA- forms of the molecule) observed in the FTIR spectra but absent in the NRS has been ascribed to have prevailing contribution from the out-of-plane mode of vibration arising from the DA+, DA(, and DA- forms of the molecule. However, it is to be mentioned that in general the out-of-plane modes show strongly in the infrared and weakly in the Raman,65 and so the assignment of the band under this class of vibration is justified. This vibration is degenerate and so determining the preferential form/forms of the dopamine molecule in the solid state become ambiguous. Considerable attention can be drawn regarding the presence of a very strong and well-resolved band centered at ∼1288 cm-1 (calcd at 1288 cm -1 for DA+, 1275 cm -1 for DA(, and 1290 cm-1 for DA- form of the dopamine molecule), in the NRS and FTIR spectra of the molecule recoded in the solid state. This band has been ascribed to in-plane -CH2 bending vibration arising from the cationic, zwitterionic, and anionic forms of the molecule. Previously, this band was ascribed to ν(C-O) stretching vibration of the DA molecule.31,49 The band is fairly intense at pH ∼4, weakens at pH ∼9, and almost disappears at pH ∼11. The variation in intensity of this band may be due to the deprotonation effect of the DA molecule at higher pH (pH
Dopamine Adsorption on a Metal Surface ∼9 and 11). However, the weak appearance of this band in the NRS spectra of the molecule at pH ∼9 and 11 may further indicate the presence of all the three forms of the molecule in the above-mentioned range of pH. Moderately intense bands at ∼1451 cm-1 (calcd at 1447 cm-1 for DA+ and at 1455 cm-1 for DA- form of the molecule) in the NRS spectra of the molecule recorded in solid state and in aqueous solution at acidic pH (pH ∼4) has prevailing contribution from the in-plane R(H14-C2-H15); R(H17-C3-H16) and R(C6-O7-H21) bending modes originating from the DA+ and DA- forms of the molecule, respectively. The assignment of this band is in accordance with the report of Kneipp et al.,49 and the vibration was identified with the benzene mode 19a (ν19a). Considering the presence of the above-mentioned band only at acidic pH, it is plausible that in aqueous solution at pH ∼4, the band may represent a vibrational signature originating from the cationic form of the molecule. This is again in accordance with the general consensus regarding the preferential existence of the cationic form of the DA molecule at acidic pH.35 An interesting observation can be drawn concerning the appearance of three bands at ∼1582, 1601, and 1617 cm-1 in the NRS of the molecule recorded in the solid state and in the aqueous solution at acidic pH (pH ∼4). At pH ∼9 and 11, the 1601 and 1617 cm-1 bands disappear and/or may remain overlapped under the broad band profile of the 1582 cm-1 vibrational mode. The band centered at ∼1601 cm-1 (calcd at 1601 cm-1 for DA+ and at 1598 cm-1 for DA( form of the molecule) has significant contribution from the in-plane R(H23-N1-C2) bending vibrations originating from the DA+ and ν(C4-C5); ν(C5-C6); bending vibrations arising from the DA( forms of the dopamine molecule. The 1617 cm-1 (calcd at 1607 cm-1 for DA+ and at 1603 cm-1 for DA( form of the molecule) band has prevailing contribution from the in-plane R(H23-N1-C2), R(H12-N1-C2), R(H13-N1-C2) and R(H12-N1H21), R(H21-N1-H13), R(H13-N1-H12) bending vibrations of the DA+ and DA( forms of the dopamine molecule, respectively. However, the variation in intensities of 1601 and 1617 cm-1 bands with pH may indicate that these two bands are sensitive with the extent of protonation. The 1582 cm-1 band (calcd at 1581/1580 cm-1 for DA+/DA- forms of the molecule) has been assigned to the stretching vibrations of the cathechol ring moiety of the cationic and the anionic forms of the molecule. The assignment of this band is in accordance with the reported literature of Spiro et al.46 who identified this band with benzene mode 8a (ν8a). The substantial broadening and overall increase in intensity of this band with pH may further presage the presence of more than one species of the DA molecule at pH ∼9 and 11. Thus from the above vibrational analysis in solid as well as in solution phase, it is presumed that the cationic forms of the molecule is preponderant in the solid state and in aqueous solution at pH ∼4, while all the three forms (cationic, anionic, and zwitterionic) of the dopamine molecule coexist in the pH range between 9 and 11. In this connection, it may be mentioned that the presence of very weak but obtrusive bands at ∼932, 1115, 1329, and 1472 cm-1 in the NRS spectra of the molecule recorded in the solid state and in aqueous solution at pH ∼4 connote the presence of the zwitterionic and anionic species of the molecule (though in trace proportion) even in the solid state and at acidic pH. However, the overall variation in intensities of 750, 772, 791, 1288, 1397, 1451, 1472, 1582, and 1601 cm-1 bands in the NRS spectra with the increase in pH of the medium not only signify the presence of zwitterionic and/or anionic
J. Phys. Chem. C, Vol. 113, No. 17, 2009 6997
Figure 7. Theoretical spectrum of isoelectric point of DA molecule.
forms of the DA molecule (as mentioned earlier) but also may result from the considerable intramolecular hydrogen bond interaction as evinced in the molecular structures of DA( and DA- (Figure 3b1,b2,d1,d2). In this connection, it may be mentioned that the isoelectric point (pI) of biological amphoteric molecules like dopamine is of considerable interest. The pI is the pH at which a particular molecule or surface carries no net electrical charge. Amphoteric molecules which bear both positive and negative charges at the same time depending on the functional groups present in the molecule are called zwitterionic. Below and above the pI, molecules carry a net positive and negative charge, respectively. The two pKa values of dopamine molecules (cathechol hydroxyl and ethylammonium, respectively) are reported to be at 8.87 and 10.63.31-34 The cathecholate/ethylammonium zwitterion is an important species between pH 8.6 and pH 10. The experimentally obtained pI of dopamine is 9.75 and the theoretically calculated pI of dopamine molecule has been estimated to be 9.22 (Figure 7) which closely resembles the experimental value. The dissociation of proton from the -OH group starts with the increase of pH and above pH ∼9 reasonable concentrations of anionic form of the dopamine molecules are expected. At the intermediate pH range we envisage the presence of both the cationic as well as the anionic forms of the DA molecule. In fact, at pH ∼8, the dopamine cation is found to be by far the most dominant form (ca. 95%) while the relative populations of the zwitterionic and neutral species are found to be only 3% and 0.2%, respectively. The anionic form is only significantly populated at pH greater than 11.35,66,67 There remains a dynamic equilibrium between the cationic and anionic form of the dopamine molecules. So, from the above discussion we can conclude that in general there is a possibility for the existence of all the three forms of the DA molecule in the varied range of pH. This conjecture also supports the experimentally observed vibrational analyses of the molecule recorded in the solid state and in aqueous solution at varied range of pH. 4.2. SERS Spectra of DA Adsorbed on Aucore-Agshell Bimetallic Nanocolloids. In comparison to the NRS spectra of the DA molecule recorded in the solid state and in 0.1 M aqueous solution at pH ∼4, the vibrational signatures representing the SERS spectra of the molecule at 1.0 × 10-7 M adsorbate concentration are in general broadened, considerably enhanced, and also accompanied by the variation of the relative intensity pattern. The Raman bands in NRS spectra of the molecule recorded in aqueous solution at pH ∼9 and 11 are in general weak; however, the corresponding SERS spectra at the abovementioned pH values exhibit a large number of distinct and structured vibrational bands. Moreover, compared to the NRS spectra, some of the band positions of the SERS spectra of the molecule are either blue- or red-shifted, which are described in the succeeding section.
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Figure 8. pH-dependent SERS spectra of the DA molecule in Aucore-Agshell bimetallic nanoparticles. Condition: [DA] ) 1.0 × 10-7 M and [Aucore-Agshell] ) 0.75 × 10-4 M.
Figure 9. Cartesian displacement and calculated [B3LYP/6-31++G(d,p)] vibrational modes of various forms of DA molecules. The numbers in parentheses refer to the experimental value of the assigned band.
4.3. pH-Dependent SERS Spectra of the Molecules. The pH-dependent SERS spectra of DA at 1.0 × 10-7 M adsorbate concentration are shown in Figure 8. From the figure, it is seen that with the increase in pH of the medium, some of the SER bands exhibit dramatic variations in relative intensities. For example the bands centered at ∼1462 and 1329 cm-1 undergo significant variations in Raman intensities with pH. The 1462 cm-1 band may have its NRS counterpart at ∼1472 cm-1 (calcd at ∼1472 cm-1 for DA( and at 1480 cm-1 for DA- forms of the dopamine molecule) in the solution and in solid state. This band has prevailing contribution from R(C6-O7-H22) in-plane bending vibrations originating from the DA( form of the DA molecule. Interestingly, this mode also has 77% contribution from the ring stretching ν(C4-C11), ν(C4-C5), and R(C6-O7-H21) 21% contribution from in-plane bending vibrations arising from the DA- form of the molecule. The general broad nature of the above-mentioned band in the SERS spectra of the molecule and its significant enhancement with pH not only signify the presence of both the DA( and DA- forms of the molecule at acidic, neutral, and alkaline pH but may also indicate considerable interaction with the oxygen (O7) atom of the cathechol moiety of the DA molecule with the Aucore-Agshell nanocolloidal surface. The involvement of oxygen (O7) atom of the molecule in the adsorption process is further characterized by the significant enhancement of 1329 cm-1 band in the entire pH-dependent SER spectral profile. This band has prevailing contribution from R(C6-O7-H22) in-plane bending vibrations originating from the DA( form of the DA molecule. The Cartesian displacements of the above-mentioned vibrations arising from the cationic, zwitterionic, and/or anionic forms of the DA molecule are shown in Figure 9.
Broadening and distinct splitting is observed for the band centered at ∼1258 cm-1 in the pH-dependent SERS spectra. A significantly enhanced band is observed at ∼1258 cm-1 in the SERS spectra of the molecule recorded at pH ∼4. With the increase in pH, distinct splitting of the band is recorded with decrease in intensity. The splitting of broad and well-resolved 1258 cm-1 (calcd at 1256 cm-1 for the DA+ forms of the DA molecule, respectively) mode into 1249 (calcd at 1243/1256 cm-1 for DA(/DA- forms of the molecule) and 1260 cm-1 (calcd at 1247/1263 cm-1 for DA(/DA- forms of the molecule) bands at higher pH may further signify the presence of DA( and DA- forms of the molecule in the surface adsorbed state. An interesting conclusion can be drawn regarding the presence of 915 (calcd at 915 cm-1 for the DA( form of the molecule) and 930 cm-1 (calcd at 938/925 cm-1 for the DA+/DA- forms of the molecule) bands in the entire pH-dependent SERS spectral profile. The 930 cm-1 band has been ascribed to a mixed vibration principally contributing from the in-plane bend and the ring stretch of the cathechol moiety of the DA+ form of the DA molecule. The 915 cm-1 band has been ascribed to in-plane angle bending of the cathechol moiety of the DA( form of the molecule. However, in general, the enhancement of both 915 and 930 cm-1 bands in the SERS spectra of the molecule recorded at pH ∼4, 9, and 11 mark the presence of all the three different forms of the DA molecule in the surface-adsorbed state at the tested range of pH. Remarkable variations in the spectral band profile and SER intensities are observed for 1113, 1126, and 1144 cm-1 bands with pH. The 1113, 1144 cm-1 bands observed in the pHdependent SERS spectra of the molecule may have their NRS counterpart at ∼1115 (calcd 1111 cm-1 for DA- form of the molecule) and 1146 cm-1 (calcd 1138 cm-1 for DA+ form of
Dopamine Adsorption on a Metal Surface
Figure 10. Optimized molecular structures of three different Ag2-dopamine interaction models with their SCF energies (Gibbs free energies) values. The numbers in the parentheses refer to the Gibbs free energy values of the respective models.
the molecule) and have been ascribed to the vibrational signatures principally contributing from the in-plane bending of the anionic (e.g., DA-) and cationic (e.g., DA+) forms of the molecule, respectively. The 1126 cm-1 band observed in the SERS spectra of the molecule is however absent in the NRS spectra of the molecule recorded in the solid state or in solution, though DFT calculations predict a band at 1122 cm-1 for the DA( form of the molecule. This band is thus ascribed to have predominant contribution from the in-plane R(C6-C5-H18); R(C8-C10-H19) bending vibration arising from DA( form of the DA molecule. The distinct presence and significant variation in intensities of all the above-mentioned bands in the entire pHdependent SER spectral profile further signify the presence of all the three forms of the DA molecule in the surface-adsorbed state. This observation is in accordance with our earlier conjecture. Intensity reversal is observed for the pair of bands centered at around 843 and 864 cm-1 in the SERS spectra of the molecule recorded at pH ∼4. The former band has been ascribed to have a predominant contribution from the torsional motion τ(H19-C11-C10-H18) arising from the anionic form (DA-) of the DA molecule, while the later band at ∼864 cm-1 has prevailing contribution originating either from the in-plane R(H12-N1-C2) and R(H13-N1-C2) bending vibration from the cationic (DA+) form of the molecule and/or torsional motion τ(C10-C11-H19-H20) of the zwitterionic (DA( form) of the molecule. With the increase in pH, the 843 cm-1 band gains in intensity; this may indicate the expected preponderance of the anionic species in the surface adsorbed state at pH ∼11. Alternatively, the variation in relative intensities of the abovementioned bands may also be due to the changes in relative orientations of the various forms of the DA molecule on the bimetallic nanocolloidal surface. Thus from the above discussion, the presence of all the three different forms of the DA molecule in the surface adsorbed state have been suggested. The presence of all three different forms
J. Phys. Chem. C, Vol. 113, No. 17, 2009 6999
Figure 11. Simulated SERS spectra of three different DA models: (A) model A, (B) model B, and (C) model C.
Figure 12. A comparison between the theoretical and experimental results of the three models A, B, and C: r, correlation coefficient; SD, standard deviation; RMSED, root-mean-square error of deviation.
of the molecule in the surface-adsorbed state is expected considering their existence in the free form of the molecule at various pH values. Alternatively, the entire pH-dependent SER
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TABLE 2: Apparent Enhancement Factors (AEF), Probable Tensor Element, and Symmetry Species of Some Selected Raman Bands of the DA Molecule at Various pHa apparent enhancement factor (AEF) at band positions in the SERS (NRS) spectra at pH ∼4, 9, and 11 (in cm-1)
probable tensor element
symmetry species
pH ∼4
pH ∼9
pH ∼11
440 (458), 440 (BR), 440 (BR) 750 (750), 750 (750), 757 (AB) 1329 (1325), 1329 (1332), 1329 (AB)
RXX:RYY RXX:RYY RXX:RYY
A′ A′ A′
1.73 × 107 2.6 × 108 1.73 × 106
H.E (CNM) 8.2 × 108 H.E (CNM)
H.E (CNM) H.E (CNM) H.E (CNM)
a
CNM, could not measure; H.E, huge enhancement; BR, broad; AB, absent.
spectral profile may also signify that the DA molecule is ionized on the nanocolloidal surface when the pH value is much lower or higher than the pKa values of the molecule.68,69 The interaction of the oxygen atom of the cathechol moiety of the DA molecule has been predicted from the enhancement of the Raman bands (vide ante), though the ν(Ag-O) stretching vibrations are not observed in the pH-dependent SERS spectra at the ∼220 cm-1 region.70 In order to contemplate the binding arrangement of the cationic, zwitterionic, and anionic species of the DA molecule with the bimetallic nanocolloidal surface, we envisage simple models comprising three different forms of the molecule and the Ag2 dimer cluster. The synthesized Aucore-Agshell is not an alloyed but a core-shell morphology. Since the core-shell nanoparticle has a quite thick silver shell, the gold core may be neglected to understand the surface chemical property of nanoparticles. The optimized molecular structure thus showing the interaction of the cationic, anionic, and zwitterionic forms of the DA molecule with Ag2 dimer cluster are represented as models A, B, and C, respectively. The models are shown in Figure 10. The main aim of these types of calculations is to identify the species really adsorbed on the silver surface by the best agreement with the SERS bands (Figure 11). The theoretically simulated Raman spectra of different surface complex models are shown in Figure 11 and the vibrational frequencies are presented in the Supporting Information. From Figure 10, we find that the Ag · · · O7/Ag · · · O9 distances for models A and C are 2.7036/2.66212 Å and 2.68952/2.24095 Å, respectively. The Ag · · · O9 distance for model B is 2.22856 Å. The atomic distances between the silver (of the bimetallic nanocolloidal surface) and the oxygen atom/atoms (of the different forms of DA molecule) are too appreciable to favor the bond formation. This preferred binding arrangement of the different forms of DA molecule with bimetallic nanocolloidal surface thus exhibit SER enhancements without detection of the ν(Ag-O) stretching vibration. Correlation plots of the experimental and calculated vibrational wavenumbers for the three model complexes are shown in Figure 12. The correlation coefficient (r) for all the three scatter plots is nearly equal to +1.0. The standard deviation (SD) and root mean square error of deviation (RMSED) values for all three model complexes are also nearly equal. These results signify that the theoretically predicted frequencies for all the models of surface complex almost match the experimental results within tolerable limits. This result may further portend that all the three (cationic, zwitterionic, and anionic) forms of the molecule take active part in the adsorption process, which is in accordance with our experimental observation. In order to understand the stability of various surface complex models, we estimated the binding energies (BEs) of the various models. The BE decides the stability of the model complexes. The model that has lower BE is more stable. The BE for model C is -1.3 eV while those for the models A and B are -0.42 and -1.08
eV, respectively. These results indicate that model C is more stable than models A and B in its electronic ground state though the experimentally observed SERS spectra together with the theoretically simulated Raman spectra of the molecule for various model complexes suggest the presence of all the three forms of the molecule. The SCF energies (Gibbs free energies) for models A, B, and C as obtained from the DFT calculations are -21977.31 eV (-21974.04 eV), -21990.25 eV (-21986.46 eV), and -22001.48 eV (-21996.12 eV), respectively. These results are also in agreement with our conjecture as interpreted from the BEs of the model complexes. 4.4. Orientation of DA Molecule on the Bimetallic Nanocolloidal Surface. The orientations of molecules at interfaces are of fundamental importance in surface chemistry for the understanding of metal-adsorbate binding and reactivity of adsorbed species. The presence of cationic, zwitterionic, and anionic forms of the DA molecule and their relative orientation on the bimetallic nanocolloidal surface make the prediction, concerning the orientation of respective species, truly enigmatic. However in order to have a general idea regarding the orientation of the DA molecule at various pH values, we estimate the AEF of some selected Raman bands using the relation we used elsewhere.71-74 Accordingly
AEF ) σSERS[CNRS] ⁄ σNRS[CSERS]
(1)
C and σ represent the concentration and peak area of the Raman bands measured from baseline. They are shown in Table 2. The enhancement factor becomes higher due to the adsorbed molecules on the bimetallic colloidal surface. The orientation of the molecule has been estimated from the surface selection rule, as predicted by Moskovits75 and Creighton.76 According to this rule, those vibrations having larger component of polarizability in the direction normal to the surface will be enhanced significantly. If DA+, DA(, and DA- forms of the DA molecule are considered to be lying in the xy plane and z is perpendicular to the molecular plane, then for the different stance of tilted adsorption of the molecule on Aucore-Agshell bimetallic nanocolloidal surface, the vibrations of both the in-plane A′ species spanning as xx or yy (depending upon the stance of the molecule on the bimetallic nanocolloidal surface) as well as the out-ofplane A′′ species spanning as yz and xz are expected to undergo significant enhancement. The least intense band should belong to the in-plane A′ species transforming as xy. It is clearly seen from Table 2 that we obtain an enormous 6-8 orders of magnitude enhancement of 440, 750, and 1329 cm-1 Raman bands principally representing the in-plane vibrations (A′ species) originating from DA+ and DA( forms of the DA molecule, respectively. Moreover, the other in-plane modes centered at ∼915, 930, 1113, 1126, 1144 1258, and 1462 cm-1 in the SERS spectra of the molecule, whose exact enhancement could not be predicted, are also significantly enhanced. The pHdependent SERS spectra of the molecule are also characterized
Dopamine Adsorption on a Metal Surface by noticeable enhancement of 843 and 864 cm-1 bands ascribed to the out-of plane vibrational modes arising from the DA+, DA(, and/or DA- forms of the DA molecule. Thus, in general, the overall enhancement of the in-plane and out-of plane modes arising from the DA+, DA(, and/or DA- forms of the DA molecule in the entire pH-dependent SER spectral profile may signify the tilted adsorption of the cationic, zwitterionic, and anionic forms of the DA molecule with the bimetallic nanocolloidal surface. Alternatively, the transmutation of different species of the DA molecule with the pH of the surrounding medium and their relative orientations together may result in the variation in the SER band intensities with pH.31 5. Conclusions The IR and Raman spectra of DA in the solid state and in aqueous solution at different pH values have been reported. The Raman spectrum of DA molecule is also estimated theoretically using DFT. The observed vibrational bands have been assigned from the PED. The adsorption behavior of biologically significant DA molecules on the bimetallic nanocolloidal surface has been investigated by SERS aided by density functional theory. The pH-dependent normal Raman spectra of the molecule in aqueous solution reveal the protonation effect and preferential existence of different forms, i.e., cationic, zwitterionic, and anionic form of the molecule in acidic (pH ∼4), lower (pH ∼9), and higher (pH ∼11) alkaline media, respectively. The preferred binding arrangements of the different forms of DA-Ag2 model complexes have been investigated. This result may further portend that all the three (cationic, zwitterionic, and anionic) forms of the molecule take an active part in the adsorption process, which is in accordance with our experimental observation. The orientations of the adsorbed species on a bimetallic colloidal surface have been estimated using the surface selection rule which relates to the pH-dependent SERS spectral studies. The AEF has been calculated for some selected bands which help us to know the adsorption of DA on Aucore-Agshell and the tilted orientation of the molecule with respect to silver shell. Acknowledgment. The authors are thankful to the UGC, DST, NST, and CSIR, New Delhi, and the IIT Kharagpur for financial assistance. Supporting Information Available: Observed and calculated vibrational frequencies of pH-dependent SERS and from the three model study. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (2) Fleischmannn, M.; Hendra, P. J.; MacQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163–166. (3) Alak, A. M.; Vo-Dinh, T. Anal. Chem. 1987, 459, 2149–2153. (4) Carrabba, M. M.; Edmonds, R. B.; Rauh, R. D. Anal. Chem. 1987, 59, 2559–2563. (5) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Chem. ReV. 1999, 99, 2957–2976. (6) Sanchez-Gil, J. A.; Garcia-Ramos, J. V.; Mendez, E. R. Phys. ReV. B 2000, 62, 10515–10525. (7) Sackmann, M.; Materny, A. J. Raman Spectrosc. 2006, 37, 305– 310. (8) Campion, A.; Kambhampati, P. Chem. Soc. ReV. 1998, 27, 241– 250. (9) Iliescu, T.; Maniu, D.; Chis, V.; Irimie, F. D.; Paizs, Cs.; Tosa, M. Chem. Phys. 2005, 310, 189–199. (10) Johansson, P. Phys. Chem. Chem. Phys. 2005, 7, 475–482. (11) Bolboaca, M.; Iliescu, T.; Kiefer, W. Chem. Phys. 2004, 298, 87– 95.
J. Phys. Chem. C, Vol. 113, No. 17, 2009 7001 (12) Schlucker, S.; Koster, J.; Nissum, M.; Popp, J.; Kiefer, W. J. Phys. Chem. A 2001, 105, 9482–9488. (13) Nie, S.; Emory, S. R. Science 1997, 275, 1102–1106. (14) Kneipp, K.; Wang, Y.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. ReV. Lett. 1996, 76, 2444–2447. (15) Goulet, P. J. G.; Aroca, R. F. Anal. Chem. 2007, 79, 2728–2734. (16) Mandal, M.; Jana, N. R.; Kundu, S.; Ghosh, S. K.; Panigrahi, M.; Pal, T. J. Nanoparticle Res. 2004, 6, 53–61. (17) Meulemans, A.; Poulain, B.; Baux, G.; Tauc, L.; Henzel, D. Anal. Chem. 1988, 58, 2088–2091. (18) Wightman, R. M.; May, L. J.; Michael, A. C. Anal. Chem. 1988, 60, 769–779. (19) Vo-Dinh, T.; Houch, K.; Stokes, D. L. Anal. Chem. 1994, 66, 3379– 3383. (20) Isola, N. R.; Stokes, D. L.; Vo-Dinh, T. Anal. Chem. 1998, 70, 1352–1356. (21) Narayanan, V.; Stump, A.; Nathan, A.; Cul, D.; Guillermo, D.; VoDinh, T. J. Raman. Spectrosc. 1999, 30, 435–439. (22) Kneipp, K.; Moskovits, M.; Kneipp, H. Surface-Enhanced Raman Scattering: Physics and Applications; Springer: Berlin, 2006. (23) Kneipp, J.; Kneipp, H.; Rice, W. L.; Kneipp, K. Anal. Chem. 2005, 77, 2381–2385. (24) Lion, L. A.; Musick, M. D.; Natan, M. J. Anal. Chem. 1998, 70, 5177–5183. (25) Jill, V. B.; Kevin, P. T.; Mark, W. R. Anal. Chem. 2002, 74, 539– 546. (26) Frederic, R.; Lionel, B.; Luc, D.; Bernard, R. Anal. Chem. 1995, 67, 1838–1844. (27) Akitane, A.; Takeshi, K.; Kayoko, I.; Eiichi, Y. Anal. Chem. 1985, 57, 1518–1522. (28) Lyne, P. D.; O’Neill, R. D. Anal. Chem. 1990, 62, 2347–2351. (29) Zimmerman, J. B.; Mark, W. R. Anal. Chem. 1991, 63, 24–28. (30) Zhang, A.; Neumeyer, J. L.; Baldessarini, R. J. Chem. ReV. 2007, 107, 274–302. (31) Mcglashen, M. L.; Davis, K. L.; Morris, M. D. Anal. Chem. 1990, 62, 846–849. (32) Armstrong, J.; Barlow, R. B. Br. J. Pharmacol. 1976, 57, 501– 516. (33) Berfield, J. L.; Wang, L. C.; Reith, E. A. J. Biol. Chem. 1999, 274, 4876–4882. (34) Lewis, G. P. Br. J. Pharmacol. Chemother. 1954, 9, 488–493. (35) Faustoa, R.; Joa˜o, M.; Ribeirob, S.; Pedroso de Lima, J. J. J. Mol. Struct. 1999, 484, 181–196. (36) Vecchia, B. E.; Bunge, A. L. Skin absorption databases and predictive equations. In Transdermal drug deliVery, 2nd ed.; Guy, R., Hadgraft, J., Eds.; Drugs and the Pharmaceutical Sciences 123; Marcel Dekker: New York, 2003; pp 57-141. (37) Kitayama, S.; Shimada, S.; Xu, H.; Markham, L.; Donovan, D. M.; Uhl, G. R. Proc. Natl. Acad. Sci.U.S.A. 1992, 89, 7782–7785. (38) Dryhurst, C. G. Electrochemistry of Biological Molecules; Academic Press: New York, 1977; p 473. (39) Pavel, I.; Moigno, D.; Cinta, S.; Kiefer, W. J. Phys. Chem. A 2002, 106, 3337–3344. (40) Baia, M.; Baia, L.; Kiefer, W.; Popp, J. J. Phys. Chem. B 2004, 108, 17491–17496. (41) Elekes, K.; Kemenes, G.; Hiripi, L.; Geffard, M.; Benjamin, P. R. J. Comp. Neurol. 1991, 307, 214–224. (42) Bello, J. M.; Vo-Dinh, T. Appl. Spectrosc. 1989, 43, 1180–1187. (43) Park, S.-K.; Lee, N.-S.; Lee, S.-H. Bull. Korean Chem. Soc. 2000, 21, 959–968. (44) Choi, Y.; Lubman, D. M. Anal. Chem. 1992, 64, 2726–2734. (45) Barreto, W. J.; Ponzoni, S.; Sassi, P. Spectrochim. Acta 1999, 55, 65–72. (46) Salama, S.; Stong, J. D.; Neilands, J. B.; Spiro, T. G. Biochemistry 1978, 17, 3781–3785. (47) Sheng, R.-S.; Zhu, L.; Morris, M. D. Anal. Chem. 1986, 58, 1116– 1119. (48) Morris, M. D. Anal. Chem. 1975, 47, 2453–2454. (49) Kneipp, K.; Wang, Y.; Dasari, R. R.; Feld, M. S. Spectrochim. Acta 1995, 51, 481–487. (50) Pande, S.; Ghosh, S. K.; Praharaj, S.; Panigrahi, S.; Basu, S.; Jana, S.; Pal, A.; Tsukuda, T.; Pal, T. J. Phys. Chem. C 2007, 111, 10806–10813. (51) Frisch, M. J.; Trucks, G. W.; Schiegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P. J.; Dannenberg, J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
7002 J. Phys. Chem. C, Vol. 113, No. 17, 2009 Rahgavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Gaussian, Inc.: Pittsburgh, PA, 2003. (52) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5653. (53) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–790. (54) Giese, B.; McNaughton, D. J. Phys. Chem. B 2002, 106, 101–112. (55) Giese, B.; McNaughton, D. J. Phys. Chem. B 2002, 106, 1461– 1470. (56) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265–3268. (57) Martin, J. M. L.; Alsenoy, C. V. GAR2PED, University of Antwerp, 1995. (58) MarVin 5.1.0, ChemAxon, http://www.chemaxon.com. (59) Cances, E.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032–3041. (60) Sarkar, J.; Chowdhury, J.; Ghosh, M.; De, R.; Talapatra, G. B. J. Phys. Chem. B 2005, 109, 12861–12867. (61) Sarkar, J.; Chowdhury, J.; Ghosh, M.; De, R.; Talapatra, G. B. J. Phys. Chem. B 2005, 109, 22536–22544. (62) Aroca, R. F.; Clavijo, R. E.; Halls, M. D.; Schlegel, H. B. J. Phys. Chem. A 2000, 104, 9500–9505.
Pande et al. (63) Bolboaca, M.; Iliescu, T.; Paizs, Cs.; Irimie, F. D.; Kiefer, W. J. Phys. Chem. A 2003, 107, 1811–1818. (64) Peica, N.; Lehene, C.; Leopold, N.; Schlu¨cker, S.; Kiefer, W. Spectrochim. Acta, Part A 2007, 66, 604–615. (65) Chowdhury, J.; Ghosh, M.; Mishra, T. N. Spectrochim. Acta 2000, 56, 2107–2115. (66) Grgas-Kuznar, B.; Simeon, V.; Weber, O. A. J. Inorg. Nucl. Chem. 1974, 36, 2151–2154. (67) Granot, J. FEBS Lett. 1976, 67, 271–275. (68) Moskovits, M.; Suh, J. J. Phys. Chem. 1984, 88, 1293–1298. (69) Tourwe´, E.; Baert, K.; Hubin, A. Vib. Spectrosc. 2006, 40, 25–32. (70) Chowdhury, J.; Mukherjee, K. M.; Misra, T. N. J. Raman Spectrosc. 2000, 31, 427–431. (71) Sarkar, J.; Chowdhury, J.; Talapatra, G. B. J. Phys. Chem. C 2007, 111, 10049–10061. (72) Chowdhury, J.; Sarkar, J.; Tanaka, T.; Talapatra, G. B. J. Phys. Chem. C 2008, 112, 227–239. (73) Basu, S.; Pande, S.; Jana, S.; Bolisetty, S.; Pal, T. Langmuir 2008, 24, 5562–5568. (74) Sarkar, S.; Pande, S.; Jana, S.; Sinha, A. K.; Pradhan, M.; Basu, M.; Chowdhury, J.; Pal, T. J. Phys. Chem. C 2008, 112, 17862–17876. (75) Moskovits, M. J. Chem. Phys. 1982, 77, 4408–4416. (76) Creighton, J. A. Surf. Sci. 1983, 124, 209–219.
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