5078
J. Phys. Chem. 1996, 100, 5078-5089
Adsorption and Displacement of Melamine at the Ag/Electrolyte Interface Probed by Surface-Enhanced Raman Microprobe Spectroscopy Eckhard Koglin* Institute of Applied Physical Chemistry (IPC), Research Center Ju¨ lich, D-52425 Ju¨ lich, Germany
Bert J. Kip and Robert J. Meier DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlands ReceiVed: October 30, 1995X
Surface-enhanced Raman spectroscopy (SERS) is evaluated as a quantitative analytical tool for low concentrations of melamine and melamine derivatives in solution. Substantial variations in absolute and relative intensities of SERS bands were encountered using silver sols, which cannot be controlled. Alternatively, it was shown that SERS using a roughened silver electrode, while conditioning the applied potential, permits the acquisition of Raman spectra from electrode spots down to 1 µm in size, and the results of multiple measurements using a hard cathodic cleaning step in between each adsorption experiment gave a relative standard deviation of 15%. The high enhancement factor of the electrode micro-Raman scattering intensity creates a new trace analytical technique for obtaining high-resolution spectra of melamine from dilute aqueous solution (detection limit ∼ 10-7 mol L-1) in the opto-electrochemical cell. As an alternative for the hard cathodic cleaning step, we demonstrated that the cationic surfactant molecule cetylpyridinium chloride is able to remove preadsorbed melamine within a few seconds. The surfactant molecules can subsequently be removed from the surface by switching to a negative applied potential. This procedure results in a relative standard deviation of 10%. The effects of electrode potential on the observed SERS spectra are consistent with current ‘SERS surface selection rules’. The electrode potential and the surface concentration of the chloride counterions strongly affect the intensity of the out-of-plane modes in the adsorbed state. However, additional experiments using various excitation lines showed that an alternative theory, surface complex formation combined with charge transfer resonance Raman processes with Herzberg-Teller contributions, plays an important role.
Introduction Melamine (2,4,6-triamino-1,3,5-triazine) is a stable heterocyclic structure. Its reactions with formaldehyde to give methylol derivatives and ultimately thermosetting resins have been studied intensively, and melamine itself has therefore become a product of large-scale manufacture and use. Spectroscopic studies based on optical absorption, 13C NMR, or vibrational spectroscopies (IR, FT-IR, Raman, and FT-Raman) can provide insight into the molecular structure of these melamine-based species.1 The complexity of the melamine/ formaldehyde systems makes direct use of the spectroscopic techniques difficult. Therefore, often separation techniques like HPLC or TLC are used in combination with spectroscopic techniques. The low solubility of melamine and its derivatives requires very sensitive techniques. Great advances have been made in surface-enhanced Raman scattering (SERS).2-7 In particular, much interest has been directed toward the use of SERS spectroscopy as a tool for trace organic analysis because this technique provides both a richness of spectroscopic information and a high sensitivity.8 Considerable effort has been directed toward using metal sols as the SERS-active substrate.9 Unfortunately, the stability of the sols is strongly influenced by stirring rate, addition of chloride ions, and aging of the sols.10 In addition, the SERS intensity depends on the temperature, the ionic strength, the pH, the aggregation of the colloids in the presence of adsorbates, and the wavelength at which maximum enhancement occurs, * Author to whom correspondence should be sent. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-5078$12.00/0
which shifts to higher values with time. Problems with preparation, stability, and reproducibility seem to have inhibited the use of dispersed metal colloids in routine semiquantitative trace analysis of organic molecules.10 For melamine, the situation is even more complicated; both the enhancement and the relative intensities of bands vary from sol to sol, as will be shown in the present paper. A viable alternative to the more traditional silver colloids or silver-coated surfaces is the electrochemically controlled silver electrode that is roughened by means of an ex situ oxidationreduction cycle (ORC) with the advantage that the applied electrode potential can be used to control the amount and orientation of the species adsorbed to the electrode surface. For instance, SERS spectroscopy using an electrode has been successfully interfaced with flow injection analysis and was coupled to the HPLC separation technique.11,12 A prerequisite of this on-line monitoring with the electrode SERS detector is to clean and thus renew the electrode surface by means of the applied potential. A problem in this “potential cleaning procedure” can be the “hard” cathodic step at -2.0 V and the following roughening ORC in the presence of an analyte (in situ roughening). The extremely different spectral behavior of silver SERS substrates roughened in situ or ex situ has been shown clearly.13 Alternatively, surfactants may be used to remove the analyte from the electrode surface.14 In the current study, cetylpyridinium chloride (C16H33N+C5H5Cl-, CPC) is used. It is anticipated that the CPC molecule will displace melamine from a negatively charged electrode surface; subsequent change to a moderately negative potential will result in a © 1996 American Chemical Society
SERS of Melamine at the Ag/Electrolyte Interface desorption of the CPC molecule, and a clean electrode surface is generated. To understand the details of such surface rearrangement following displacement reactions, it is first necessary to understand as fully as possible the kinetic behavior of molecules which are undergoing surface desorption reactions in the first adsorption layer. Information from SERS experiments allows the temporal course of the displacement reaction to be followed and provides additional information about the adsorbed state. Therefore, to examine the question of reorientation and desorption of melamine by CPC molecules, surface Raman spectra were collected in real time at different stages of adsorption. In summary, the purpose of this contribution is to utilize the SERS electrode analysis15 to obtain fundamental information on the adsorption mechanism of melamine and coadsorption with chloride ions, in order to understand the observed phenomena on Ag sols. Furthermore, the electrode method will be tested as an alternative to the sol method. An effective cleaning procedure of the electrode is essential for practical application and might be accomplished either by an ORC or by using surfactants. The removal of the preadsorbed melamine molecules from the first adsorption layer by means of the cationic surfactant CPC will be studied. Before we start to discuss the results from these studies, the interpretation requires some knowledge of the character of the vibrations of the melamine and CPC molecule. These will first be discussed in the next section. Theoretical Background of Conventional and SurfaceEnhanced Raman Modes of Melamine. Two theoretical models have been developed to explain the SERS effect: the classical electromagnetic enhancement (CEME) model16,17 and the charge (CT) model.6,7 The electromagnetic surface selection rules in the CEME model and the distance dependence of the Raman scatterer from the surface18-20 are essential for the interpretation of the SERS spectra. In addition, the metalelectron-mediated resonance Raman effect (CT model)6,7 with the CT surface selection rules plays an important role. The ability to couple the dipole moment fluctuation accompanying the molecular vibrations with the electric field at the metal surface is decisive for the observed SERS intensity. The CEME related surface selection rules suggest that vibrational modes possessing polarizability tensors along the surface normal will experience the greatest intensity enhancement. Consequently, the result of these rules is that vibrational motion perpendicular to the surface will couple more effectively with the enhanced surface electromagnetic fields than vibrational motions parallel to the surface. One of the key factors here is the orientation of the molecule with respect to the metal surface. The possible interfacial orientation of melamine on the surface is determined by the interaction of the compact double-layer static electric field with the charge or induced dipole moment. It is not obvious whether bonding of melamine to the SERS substrate occurs through the π-electrons of the triazine in a parallel arrangement to the surface normal, in an end-on or edge-on manner (through the amino group and/or the triazine N-sp2 lone pair electron), largely perpendicular to the surface, or an arrangement in which the melamine molecule is tilted through one of the three amino groups. The charge transfer resonance Raman scattering via a Herzberg-Teller (vibronic) term6,7 should also be considered in the chemisorbed state in the first monolayer. This theoretical CT model for SERS is comprehensive and can predict intensity vs voltage profiles and the electron transfer in both directions from metal-to-molecule and molecule-to-metal, and that only limited numbers of SERS modes are selectively enhanced.
J. Phys. Chem., Vol. 100, No. 12, 1996 5079
Figure 1. Raman microprobe analysis of melamine: Raman spectrum of a microparticle; particle size about 40 µm × 70 µm; excitation at 514.5 nm; 8 mW (at sample); beam spot, 1 µm diameter; integration time, 2 s; number of readings, 70.
Because the magnitude and direction of the dipole moment fluctuations in both models are directly related to the character of the normal modes, an understanding of the normal modes of the molecule under study is needed to extract information on the orientation of the adsorbed molecule from the SERS spectrum. Figure 1 shows the normal (i.e. non-SERS) Raman spectrum of melamine. The normal modes of melamine have been assigned on the basis of experiments and calculations.21-24 There is disagreement between these assignments. Accurate representation of the vibrational frequencies can be accomplished by taking into account the so-called cross terms in the force field expression, as incorporated in class II force fields.25,26 A class II force field for melamine was recently presented elsewhere, and the vibrational spectrum of melamine as calculated compares very well with the experimental Raman spectrum.27 These calculations and vibrational assignments are probably the most reliable at this moment. Results for the modes that will be used in the current study (between 600 and 1000 cm-1) are shown in Figure 2. The most intense feature in the melamine Raman spectrum appears at 676 cm-1, is assigned to the ring breathing 2 mode, and involves an in-plane deformation of the triazine ring (see Figure 2a, calculated frequency 645 cm-1). The other strong Raman band with an experimental frequency of 986 cm-1 is ring breathing mode 1 of the triazine ring with the calculated frequency of 966 cm-1 (see Figure 2b). The band observed around 625-640 cm-1 is assigned to a partly out-of-plane vibration as shown in Figure 2c (calculated band position at 578 cm-1). Regarding the interpretation of the observed band at 740-776 cm-1, the calculated frequency of the nearest possible mode is about 100 cm-1 off (Figure 2d, mainly out-of-plane), which is clearly beyond the standard deviation between calculated and observed frequencies as reported in ref 23. Ab initio calculated Raman intensities (Vanhommerig, S. A. M., Eindhoven University of Technology, unpublished calculations employing a 6-31G* basis set and a constrained planar geometry) revealed some intensity in this range. In view of the relatively low intensity of the band in the experimental spectrum, it cannot be ruled out, however, that this band is a combination band of the 378/387 cm-1
5080 J. Phys. Chem., Vol. 100, No. 12, 1996
Figure 2. Eigenvectors of the modes of melamine27 calculated in the range 600-1000 cm-1: (a) observed 676 cm-1, calculated 645 cm-1; (b) observed 986 cm-1, calculated 966 cm-1; (c) observed 624-640 cm-1, calculated 578 cm-1; (d) observed 740-776 cm-1, calculated bands at 674 and 681 cm-1; (e) observed 810 cm-1, calculated 830 cm-1.
doublet. The band observed around 810 cm-1 is attributed to the out-of-plane vibration, as indicated in Figure 2e (calculated position at 830 cm-1). In order to utilize micro-SERS spectra to study surfactant/ melamine desorption processes on charged surfaces, we need additional information on the assignments for the surfactant molecule CPC. The assignments13,28 are based on extensive comparisons with vibrational spectra of long chain molecules (polymers) and surfactants with different head groups. Figure 3a shows the micro-Raman spectrum of a microparticle of CPC in the low-wavenumber region, and Figure 3b presents the CH region. Prominent head group (N+C5H5, pyridinium ring)related bands in the crystalline CPC molecule are the in-plane ring deformation vibration at 651 cm-1, the intense symmetrical and trigonal ring breathing mode at 1028 cm-1, the ring stretching vibration at 1638 cm-1, and the CH ring stretching mode at 3084 cm-1. The characteristic tail group vibrations are located around 1062, 1132, 1300, 1458, 2852, and 2881 cm-1. The only one band of strong intensity in the midfrequency range which can be unequivocally assigned to the hydrocarbon tail is the twisting motion at 1300 cm-1. The strong hydrocarbon tail bending motion at 1452 cm-1 is also in an area where CH3 bending and CH2 bending vibrations both contribute. But for the CPC long-chain surfactant molecule this dominant band would be automatically assigned to CH2 bending. The bands at 2852 and 2881 cm-1 have been assigned to CH2 symmetric and antisymmetric modes, respectively, for different long-chain molecules with a hydrophobic hydrocarbon tail. Experimental Section Materials. Silver sols were prepared following a recipe given by Xu and Zheng (modified Lee and Meisel sols).29 Subsequently, either 20 mL (recipe B) or 30 mL (recipe C) of a 1% solution of sodium dodecyl sulphate (SDS) is added to 50 mL of the silver sol solution prepared according to the Xu and Zheng
Koglin et al.
Figure 3. Raman microprobe analysis of cetylpyridinium chloride (CPC): Raman spectrum of a microparticle; particle size about 20 µm × 30 µm; excitation at 514.5 nm; 6 mW (at sample); beam spot, 1 µm diameter; integration time, 2 s; number of readings, 30. Spectra in the low- (a) and high- (b) frequency range.
recipe. The melamine solution used had 9 mg/20 mL of melamine dissolved. Drops of a NaCl solution were added to induce aggegation/activation of the sol. For further details on the procedure we used we refer to ref 10. Cetylpyridinium chloride (CPC) was obtained from FLUKA-Chemie AG in the purest form. Standard commercial melamine was used (DSM). All other chemical reagents were of analytical quality from E. Merck. Equipment. An electrode SERS apparatus consists basically of a laser excitation source, the potential-controlled electrochemical cell, the optics for collecting the surface scattering, and a Raman spectrometer. The opto-electrochemical cell was designed to obtain surface Raman spectra of electrochemical adsorbed species with the use of microscope optics, which allowed unambiguous placement of the argon ion laser focus at the electrode surface with spatial resolution on the order of 1 µm. Electrochemical Equipment. The opto-electrochemical cell consisted of a Teflon cylinder with a diameter of ca. 10 mm and a capacity of ca. 1.0 mL of solution. The working electrode was a polycrystalline Ag metal disk, ca. 2 mm in diameter, enclosed in a Teflon holder. This metal electrode was prepared before each experiment by mechanical polishing with a 1 µm Al2O3 suspension, ultrasonic treatment, and electrochemical cleaning by H2 evolution. The electrochemical equipment consisted of a potentiostat (PAR, model 173) using a threeelectrode system and a function generator (PAR, model 175) as programmer for the oxidation-reduction cycle (ORC). A digital Coulomb meter was used to measure the charge transfer during the ORC, which indicates the degree of metal dissolution and recrystallization of the electrode. All potential measurements were made with respect to the saturated calomel electrode (SCE). Optoelectrode Preparation. The detection of SERS signals from the electrode surface requires specific surface roughening
SERS of Melamine at the Ag/Electrolyte Interface of the electrode surface, which was accomplished using the following roughening procedure. The surface was pretreated by running the ORC in the electrolyte solution containing the molecule studied. The voltage was stepped to +0.2 V vs SCE in a 0.1 M KCl solution in order to pass between 100 and 150 mC/cm2 and subsequently stepped back to -0.6 V. For intensity standardization the procedure was put under the digital Coulomb meter control (130 mC/cm2 for the multiple and repeating measurements) and yielded a very reproducible SERS-active electrode surface. This quantity of surface charge yielded the most reproducible SERS signals. We chose an ex situ roughening, i.e. in the electrolyte solution in the absence of the surfactant/melamine molecules with the solution being subsequently added to the opto-electrochemical cell, in order to avoid conformation changes. The SERS spectra were obtained at a negative potential in the electrochemical cell. In this region from 0.0 to -1.4 V, the non-Faradaic region, no electrochemistry occurs for melamine and CPC. Raman Spectra. Raman spectra in the visible spectral range were obtained with an Instruments S.A. MOLE-S3000 triple, computerized, spectrometer equipped with multichannel (E-IRY 1024) data acquisition. The excitation was performed with an argon ion laser (Spectra Physics, model 2020-03) operating at 457, 488, and 514 nm or via a Dye laser system at 645 nm. The microscope attached to the MOLE-S3000 provides a way of precisely focusing the laser light on the electrode surface (diameter of 1 µm, 8 mW of power). The collection of one Raman spectrum took about 5-10 s. This implies that during one ORC (5 mV s-1) 20-40 spectra could be recorded. With 600 grooves mm-1 holographic gratings, the spectral coverage was 1600 cm-1. Near infrared Fourier transform microprobe SERS spectra (micro-NIR-FT-SERS) were recorded using a Bruker FT-Raman system RFS 100 equipped with a microscope attached to the interferometer using NIR-fiber optics. The samples were excited by a 1064 nm diode pumped Nd:YAG laser with 250 mW of power at the sample. Spatial resolution for the ×10 objective used was 100 µm. Results 1. SERS Studies of Melamine Using Ag Sols. 1.1. Results for Various Sols. SERS spectra of melamine solutions were recorded using the two different Ag hydrosols B and C, respectively (see Experimental Section). The order of addition of melamine and NaCl was varied as was the amount of dropwise added NaCl. In the final solutions the melamine concentration was 600 ppm and the NaCl concentration was raised by ca. 50 ppm per drop added. In the first series of experiments the melamine solution was added to the Ag sol and subsequently drops of NaCl were added. After each drop, the SERS spectrum was recorded (see Figure 4A). Clearly, the SERS spectrum changed markedly with increasing NaCl concentration; in particular, the relative intensities of the various bands in the cluster around 700 cm-1 changed. For low NaCl concentration the band around 725 cm-1 was relatively intense but decreased in intensity with increasing NaCl concentration. In the case of reversed addition order, i.e. first adding NaCl and subsequently adding melamine to the silver sol, the intensity of both bands at 680 and 725 cm-1 was relatively low for the low NaCl concentrations (compare parts B and A of Figure 4). For high NaCl concentrations, the 680 cm-1 band was very intense and the 725 cm-1 band was almost absent. Using the same order of addition but with a hydrosol with a lower amount of SDS (hydrosol B) resulted in SERS
J. Phys. Chem., Vol. 100, No. 12, 1996 5081 spectra only exhibiting the 680 cm-1 band (see Figure 4C). Moreover, the SERS intensity for this hydrosol at low NaCl concentrations was very low. In all cases, a low intensity broad band at 625-650 cm-1 was present. In addition to the earlier observed irregular SERS intensity variations for pyridine adsorbed on silver sols,10 the results on melamine presented here showed that also relatiVe intensities of Raman bands markedly vary for various sols. 1.2. Influence of Excitation WaVelengths. Parts C-E of Figure 4 show the spectra of melamine adsorbed on the Ag sol for excitation at 514, 457, and 645 nm, respectively. For excitation at 457 and 514 nm, the results are comparable; for low Cl- concentration two bands were observed at 680 and around 710 cm-1, and for higher Cl- concentration the band at 680 cm-1 is most intense. In the case of excitation in the red, i.e. at 645 nm, a band was observed at 645 cm-1, while the bands at 680 and 710 cm-1 were not visible. 2. SERS Studies of Melamine Using Ag Electrodes. In order to understand the features observed for melamine adsorbed on Ag sols, experiments were performed on Ag electrodes. In this case the experimental parameters can be defined more accurately (surface potential, surface roughness, NaCl concentration, etc.) and studied explicitly. 2.1. Potential Dependence. Figure 5a-c shows the microSERS spectra of melamine in the wavenumber range between 150 and 1750 cm-1 at several representative electrode potentials. The potential of 0.0 V (Figure 5a) corresponds to a positively charged surface, the potential of -0.7 V (Figure 5b, expanded view in 5d) to a neutral surface (point of zero charge), and that of -1.0 V (Figure 5c) to a negatively charged surface.30 We observed that, in the non-Faradaic region of 0.0 to -1.3 V, the electrode potential has a marked effect on the SERS intensity. The most intense SERS band in the whole potential region at 671 cm-1 (Es ) -1.0 V) and 677 cm-1 (Es ) -0.1 V) is the characteristic in-plane ring breathing 2 mode of the melamine molecule. The intensity of this band increased as the potential became more negative, and the band had a maximum intensity at a potential of -1.2 V vs SCE. In contrast to this intense SERS band is the relative weak intensity of the in-plane ring breathing 1 mode of the triazine ring in the trisubstituted triazine molecule at 992 cm-1 in the SERS spectrum at Es ) -1.0 V as compared to the normal Raman spectrum. Relative intensities of modes with out-of-plane contributions like the 624, 729, and 803 cm-1 bands (see Figure 2) strongly varied with the applied potential. Going from a potential of 0.0 to -1.3 V, first the purely in-plane modes at 674 and 992 cm-1 show maximum enhancement. Going to more negative potentials, first the partly out-of-plane mode at 624 cm-1 is more enhanced (see Figure 6). At more negative potentials, the totally out-of-plane modes at 729 and 803 cm-1 exhibit the highest enhancement. In the potential range from -0.1 to -0.7 V, a peak was observed at 243 cm-1. At the electrode potential of Es ) -0.1 V the former band appeared just after the complete reduction of the adherent AgCl and Ag-melamine layer and possible assignments include a Ag-N (N from the NH2 group of melamine) or Ag-Cl vibration. Because the corresponding spectrum of the pure electrolyte solution without the melamine showed a strong broad band at 236 cm-1, we concluded that this band must mainly involve a Ag0-Cl- vibration. Figure 5 also shows that the intensity of the band at 243 cm-1 strongly decreases upon a shift of the potential from -0.1 to 0.6 V. Since chloride ions are almost completely desorbed at Es ) -0.6 V, any remaining intensity is this spectral region (viz. Figure 5a) is attributed to the Ag-N (melamine) motion in analogy to a similar band observed for Ag/pyridine.31
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A
C
B
D
E
Figure 4. SERS spectra of melamine adsorbed on Ag sols. Ag sols were prepared following the modified Lee and Meisel recipe.29 In all cases the melamine concentration was 600 ppm. The hydrosols are described in the Experimental Section. The amount of added NaCl was varied. (A) SERS spectra of melamine added to Ag hydrosol C. Subsequently NaCl was added [50 (a), 100 (b), 150 (c), 200 (d), 250 (e), 300 (f), and 350 (g) ppm NaCl]. (B) SERS spectra for case in which NaCl is added first to hydrosol C and melamine is subsequently added [50 (a), 100 (b), 150 (c), 200 (d), 250 (e), 300 (f), and 350 (g) ppm NaCl]. (C) SERS spectra for hydrosol B, order of addition same as in part B [50 (a), 150 (b), 250 (c), and 350 (d) ppm NaCl]. (D) SERS spectra recorded under same conditions as in part C but with excitation at 457 nm [350 (a), 250 (b), 150 (c), and 50 (d) ppm NaCl]. (E) SERS spectra recorded under same conditions as in part C but with excitation at 645 nm [350 (a), 250 (b), 150 (c), and 50 (d) ppm NaCl].
SERS of Melamine at the Ag/Electrolyte Interface
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Figure 6. Relative intensity of the 624 to 980 cm-1 band vs the electrode potential. Experimental conditions as in Figure 5.
(d)
Figure 5. Micro-SERS spectra of melamine: Ag electrode; ex situ roughening; Es at -1.0 V (a), -0.7 V (b), and 0.0 V (c) vs SCE; concentration of melamine, 10-2 M; excitation at 514.5 nm; 10 mW; integration time, 1.5 s; number of readings, 60. Part of part b is expanded in part d.
Figure 7. Micro-SERS spectra of melamine: Ag electrode; ex situ roughening; Es of -0.2 V vs SCE; concentration of melamine, 10-2 M; excitation at 514.5 nm; laser power, 4 mW; laser spot focus, 1 µm; integration time, 1.5 s; number of readings, 80. (a) 1 M KCl electrolyte concentration; (b) 10-3 M KCl electrolyte concentration.
2.2. Effect of Counterions. In order to further elucidate the nature of the melamine-halide complex, the effect of bulk chloride concentration on the adsorption process was studied. The melamine concentration was kept constant at 10-2 M, and the chloride concentration was varied from 0.001 M KCl to 1 M KCl aqueous solutions. The roughening pretreatment was carried out in a very low electrolyte concentration (0.01 M), and this pretreatment solution was replaced by purified water. The corresponding melamine/electrolyte solution was subsequently added to the electrochemical cell (ex situ roughening). Increasing the Cl- concentration from 0.001 M to 1 M affects the adsorption of the active surface complex and markedly changes the SERS spectra in the whole potential range from
0.0 to -1.4 V. Especially the relative intensity of the partly out-of-plane vibration at 624 cm-1 drastically depended on the bulk chloride ion concentration. Figure 7 shows, as an example, two typical micro-SERS spectra at a high bulk chloride concentration of shows, as an example, two typical micro-SERS spectra at a high bulk chloride concentration of 1 M and the micro-SERS bands of melamine dissolved in a 10-3 M KCl solution. At a fixed potential of -0.2 V vs SCE, the intensity of the 624 cm-1 band was much larger at the lower chloride concentration. This result indicates that the Cl- bulk concentration influences the partly out-of-plane 624 cm-1 mode more strongly than the in-plane 985 cm-1 mode. In 0.1 M KCl, the 624 cm-1 band intensity from the interfacial melamine reached
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Figure 8. Relative intensity of the 624 to 980 cm-1 band vs the electrode potential for two electrolyte concentrations of 0.1 M KCl and 0.01 M KCl. Experimental conditions as in Figure 7.
a maximum at -0.5 V, while in 0.01 M KCl, the intensity is maximized at -0.75 V (c.f. Figure 8). The adsorption of the Cl- anions was examined by recording the Ag0-Cl- stretching band at 243 cm-1. In the 10-2 M melamine solution in 1 M KCl the SERS Ag0-Cl- band reached a maximum intensity and decreased strongly at lower bulk chloride concentrations (c.f. Figure 7). These results strongly suggest that, at sufficiently low bulk Cl- concentration, the adsorbed melamine molecules cover more metal surface places, resulting in a change in the melamine spectrum. 2.3. Near Infrared Excitation (NIR-FT-SERS Microprobe). NIR-FT-SERS microprobe spectroscopy has a particular advantage over visible SERS because near infrared radiation is less likely to cause photochemical changes at the surface, fluorescence from the sample is possibly avoided, and the SERS enhancement factor is about one order of magnitude larger than for SERS with visible excitation.32 In NIR-FT-SERS, the Raman excitation is performed using a Nd:YAG laser operating at 1064 nm. The spectrum is recorded using a Fourier Transform IR spectrometer working in the near infrared spectral range. Because of the mentioned advantages, there has been considerable interest in the use of NIR-FT-SERS spectroscopy as a new analytical technique and for obtaining structural information about adsorption and reactivity in the first adsorption layer. Figure 9 shows the NIR-FT-SERS microprobe spectra of melamine adsorbed on a roughened silver electrode in 0.1 M KCl aqueous solution taken with 1064 nm excitation at different characteristic potentials. To our knowledge, this is the first report on electrode NIR-FT-SERS microprobe measurements over a fiber-optic-coupled NIR microscope. A comparison with the visible excitation SERS of melamine (viz. Figure 5) shows that the band intensities especially below the potential of -0.9 V in the NIR-FT-SERS spectra differ significantly. The ratio of the relative intensities of the two 627 and 678 cm-1 bands changes from 0.3 (I627/I678) for melamine in the visible SERS spectrum to 2.2 for the same sample in the NIR-FT-SERS spectrum at a potential of -0.2 V. At the negatively charged surface (Es ) -1.3 V) this ratio
Koglin et al.
Figure 9. NIR-FT-SERS microprobe spectra of melamine at different applied potentials excited at 1064 nm: 10-2 M melamine dissolved in 0.1 M KCl; Ag electrode roughened with 120 mC/cm2; laser power, 300 mW; 20 scans; objective 10×.
changes only from 0.2 to 0.5. This means that in the NIR-FTSERS spectra between Es ) -0.1 V and Es ) -0.9 V the partly out-of-plane 627 cm-1 band is more enhanced than the in-plane ring breathing 2 mode at 676 cm-1. 3. Cleaning of Electrode Surface. 3.1. Cleaning the Electrode Surface by the in Situ ORC Roughening Procedure. If the analyte of interest (in this case melamine) can be adsorbed and desorbed reproducibly from the silver electrode substrate surface, then SERS microprobe spectroscopy at the electrode has the potential to be exploited as a detection technique for trace analysis. For increasing the sensitivity of the SERS method, we used a modified cleaning procedure of the silver electrode surface before running the ORC. The ex situ electrochemical cleaning by a “hard” cathodic hydrogen evolution step at -2.0 V vs SCE for 20 s was carried out in alkaline solution (mixture of 25 g of NaOH, 35 g of Na2CO3, and 15 g of Na3PO4 per 1 L of tridistilled water solution) with a current density of 6 A/cm2. In this way the hydrogen bubbles mechanically remove the remainders of Al2O3 and graphitic carbon. The alkaline solution dissolves fatty impurities. After that, the in situ ORC roughening procedure was carried out and the cycle was stopped if a 130 mC/cm2 of anodic charge has been passed across the electrode surface. This cleaning and roughening procedure gave the best results with respect to maximum SERS intensity and reproducibility. The degree of reproducibility was measured by means of the integrated intensity of the prominent ring breathing 2 mode at 670 cm-1 at a potential of -1.2 V. The laser beam power from the microscope with a diameter of 1 µm was kept at 5 mW at the electrode surface for the multiple and repeated measurements. To examine the reproducibility under these conditions, we performed a series of tests with a 10-4 melamine solution in 0.1 M KCl on different days with fresh test solutions. Each test series of melamine consisted of ten measurements at twenty different selected spots on the electrode surface. Between the SERS measurements all parts of the electrochemical cell were cleaned and the working electrode was roughened according to the procedure described above. The results of multiple mea-
SERS of Melamine at the Ag/Electrolyte Interface
Figure 10. Microelectrode SERS spectrum of cetylpyridinium chloride: Ag electrode; ex situ roughening; Es of -0.6 V vs SCE; concentration 5 × 10-5 M; excitation at 514.5 nm; 10 mW; integration time, 1.5 s; number of readings, 60. Spectra in the low- (a) and high (b) frequency range.
surements showed that the absolute SERS intensities were reproduced over a period of weeks with a standard deviation of 15%. The SERS microprobe limit of detection for melamine turned out to be 5 × 10-8 M (at 0.1 M KCl). This corresponds to 5 ng in the 1 mL solution in the electrochemical cell. While the surface area of the polished electrode was on the order of 3 mm2, which is estimated to increase by a factor of eight or more for the roughened surface, and the diameter of the focused laser beam was on the order of 1 µm, we were able to probe only 1 femtogram of melamine. As compared with previous colloid SERS investigations,10 roughened electrodes remain the most versatile SERS substrate, because the charge on the metal surface is easily manipulated by changing the electrode potential providing an extra degree of freedom which can be used to enhance sensitivity or selectivity. 3.2. Cleaning the Electrode Surface by Exchange with CPC. 3.2.1. Exchange Reactions of Melamine and CPC on the Ag Electrode. As turned out in the previous paragraphs, the reproducibility and controllability of the SERS enhancement was far better for Ag electrodes than for Ag sols. In order to make an analytical tool from the SERS Ag electrode technique, an effective cleaning procedure of the electrode is essential for practical application and might be accomplished either by an ORC or by using surfactants. We have tried to remove the preadsorbed melamine molecules from the first adsorption layer by means of the cationic surfactant cetylpyridinium chloride (C16H33N+C5H5Cl- (CPC). In order to understand the phenomena during exchange reactions of CPC and melamine, the electrode SERS spectra of CPC are studied first. Figure 10 shows the micro-SERS spectra in the wavenumber range between 100 and 1800 cm-1 at an electrode potential of -0.3 V. The potential of -0.3 V vs SCE corresponds to a slightly positively charged electrode surface. There are two intensity maxima of the SERS vibrational modes at this electrode potential. The first maximum is around 1030 cm-1, and this
J. Phys. Chem., Vol. 100, No. 12, 1996 5085
Figure 11. Micro-SERS spectra of displacement reactions of preadsorbed melamine by CPC. (a) Microelectrode SERS spectrum of adsorbed melamine: Ag electrode surface; ex situ roughening; Es ) -0.6 V vs SCE; melamine concentration, 10-2 M; 0.1 M KCl; excitation at 514 nm; laser power, 5 mW; laser focus 1 µm; integration time, 0.8 s; number of readings, 20. (b) Micro-SERS spectrum of the displacement reaction: 1 µL of 10-3 M CPC was added to the optoelectrochemical cell. The SERS spectrum was taken 30 s after injection of the displacing CPC molecules. Experimental conditions as in part a. (c) Micro-SERS spectrum of the displacement process. The SERS spectrum was taken 90 s after injection of the CPC surfactant molecules. Experimental conditions as in part a.
SERS band can be attributed to the enhanced ring breathing mode of the pyridinium head group. The second SERS peak is at 1626 cm-1 and is identified with the ring stretching vibration. Comparing the relative intensities of the micro-SERS spectrum and the microprobe normal Raman spectrum, we can clearly see the strong enhancement of the pyridinium ring breathing mode of the pyridinium head group. In addition, we can see that the enhancements of the pyridinium ring vibrations and the hydrocarbon modes are quite different. The characteristic tail vibration at 1301 cm-1 (CH2 twisting vibration of hydrocarbon tail) is completely absent in the adsorbed state. The CH2-scissoring vibration at 1452 cm-1 shows a very low intensity. On the basis of the short-range sensitivity of the SERS enhancement, we can conclude that the pyridinium head group is attached to the surface, leaving the hydrocarbon chain directed away from the surface. Because vibrations with atomic motion perpendicular to the surface couple more effectively to the surface electromagnetic waves, the strong intensity of the enhanced ring breathing mode (1030 cm-1) and the intense ring stretching band at 1626 cm-1 suggest that the pyridinium head group is in a standing up configuration (3,4 edgewise orientation with coadsorption of Cl-). Perhaps the most striking evidence for this molecular orientation with the pyridinium ring adsorption is the SERS behavior in the (C-H) stretch region, shown in Figure 10b. The pyridinium CH ring stretching mode at 3080 cm-1 was more enhanced relative to other micro-SERS CH bands in this spectral range (compare with Figure 3b). The SERS activation of the Ag electrode surface was carried out ex situ, in pure supporting electrolyte (0.1 M KCl). This electrolyte solution was removed and the same electrolyte with a 1 × 10-2 M concentration of melamine was added. The SERS spectrum of melamine measured at -0.6 V is shown in Figure 11a. One
5086 J. Phys. Chem., Vol. 100, No. 12, 1996 microliter of a 10-3 M CPC solution was then added to the 1 mL opto-electrochemical cell volume. First excellent results were obtained from SERS measurements even at early times. The first data point was taken 20 s after injection of the displacing CPC molecules (Figure 11b). The partial disappearance of the melamine bands at 763 and 989 cm-1 indicates a replacement of melamine by CPC. Apart from the decrease in the melamine surface scattering intensity, the coadsorption effect of CPC is clearly demonstrated by the strong scattering intensity of the CPC vibration modes at 1032 and 1624 cm-1. From this observation it is corroborated that CPC is able to displace a fraction of the preadsorbed melamine in the first 20 s after injection. This fraction decreases with increasing rearrangement time, which suggests that CPC becomes more strongly attached at the surface. After 90 s a complete displacement of the preadsorbed melamine had already taken place (Figure 11c). If the displacement time of melamine from the surface is also electrostatically determined, it is expected that the melamine desorption kinetics can be strongly influenced by the electrode surface charge. It appears, therefore, that electrostatic considerations of the melamine and CPC adsorption on the charged metal surface play a crucial role in determining the desorption time of melamine by CPC. Electrochemical measurements showed that neutral molecules like the melamine molecule (pH of about 6.5) generally adsorb very strongly on an electrode surface of the zero charged potential with a high surface coverage. These investigations are in agreement with the long desorption time of 90 s at this electrode potential in the SERS replacement experiments, as shown in Figure 11c. After pretreating of the melamine adsorption at -0.6 V, the surface potential was scanned in the negative direction from -0.6 to -1.0 V. At this potential neutral melamine molecules in the absence of CPC are adsorbed, as was shown in Figure 6. The rapid melamine desorption process at this potential is attributed to the strong electrostatic interaction of the positively charged surfactant head group with the negatively charged electrode surface. This strong CPC adsorption process results in a very fast desorption time in the CPC/melamine displacement reaction: if the preadsorbed melamine molecules are adsorbed at the negative surface, the complete replacement takes only 40 s. The strong potential dependence of the desorption process can be observed in case the potential is shifted to 0.0 V. At this highly positively charged substrate surface (c.f. Figure 5) the adsorption of the melamine molecules increases by the coadsorption of Cl- ions to the surface (Ag/melamine/Clsurface complex) and we obtain a high population of melamine and Cl- ions which are strongly attached to the surface. As mentioned before, the displacement of the preadsorbed melamine by the secondly added CPC is strongly influenced by the electrostatic interaction between the adsorbent surface and the surfactant. At potentials between -0.6 and 0.0 V, i.e. at a positive surface charge, cationic surfactants can desorb from the surface because of the electrostatic repulsion. Specifically coadsorbed Cl- counterions near the positively charged surfactant head group are necessary to explain the strong observed binding of cationic surfactants on positively charged surfaces.33 Therefore, the adsorption, rearrangements, and desorption of melamine molecules from positively charged surfaces follow a more complicated displacement mechanism. The SERS desorption measurements at this surface charge are quantitatively similar to those at the neutral and negatively charged surfaces, but everything now occurs much more slowly: the CPC adsorbs at a much slower rate, and the melamine molecules leave the surface much more slowly. These slower desorption kinetics result in a complete replacement time of 3 min.
Koglin et al.
Figure 12. Removal of adsorbed melamine by CPC (soft cathodic step cleaning). (a) Microelectrode SERS spectrum of adsorbed melamine: Ag electrode surface; ex situ roughening; Es ) -0.9 V vs SCE; melamine concentration, 10-2 M; 0.1 M KCl; excitation at 632 nm; laser power at the sample, 3 mW; laser focus, 1 µm. (b) MicroSERS spectrum of the displacement reaction: 1 µL of 10-3 M CPC was added to the opto-electrochemical cell. The SERS spectrum was taken 10 s after injection of the displacing CPC molecules. (c) SERS spectrum taken 40 s after injection of the CPC molecules. Experimental conditions as in part b. (d) Electrode surface cleaning by a “soft cathodic sweeping step” to Es ) -1.3 V. (e) Microelectrode SERS spectrum of melamine taken after the cleaning step described in part d with subsequent refreshing of the electrode solution and refilling of the electrode cell with a fresh melamine solution as described in part a.
3.2.2. Reproducibility by Cleaning the Surface Using CPC. Once the melamine was removed by CPC, the CPC molecules could be removed from the electrode surface by sweeping the potential to -1.3 V (soft cathodic step). This effect can be clearly seen in Figure 12. Experiments were performed to determine the SERS intensity reproducibility by adsorbing melamine, displacing the melamine at -0.8 V electrode potential by CPC, and subsequently removing the CPC by the potential sweep to -1.3 V. This series of steps was cycled 10 times without using a hard cathodic step to clean the electrode surface. The reproducibility of the intensity of the 671 cm-1 melamine band was better than 10%. This showed that the chosen procedure really resulted in a very good reproducibility, making it possible to use electrode SERS as a quantitative analytical tool. Discussion SERS as an Analytical Tool Using Ag Sols and Ag Electrodes. The SERS results obtained for melamine adsorbed on Ag sols clearly showed that the use of sols for quantitative analytical work will be very limited.10 On the basis of the work performed on electrode SERS, the wide variations in the (relative) intensities of the various bands in the SERS spectrum of adsorbed melamine were ascribed to uncontrollable variations in the surface potential of the Ag sols. In the case of electrode SERS, one can control this surface potential. The problem here is the cleaning of the surface in order to prepare the electrode
SERS of Melamine at the Ag/Electrolyte Interface for a subsequent experiment. Two ways were proposed and tested: (1) a hard cathodic step to clean the surface of the potential [The drawback of this method is that the surface roughness of the electrode might be changed by this step, possibly resulting in a lower reproducibility of the quantitative measurement.] and (2) a displacement of the adsorbed melamine by CPC followed by a sweep to a slightly negative surface potential. The first method resulted in a reproducibility of ca. 15%; the second method resulted in a reproducibility of ca. 10%. The investigations have also shown that by using a commercially available FT-Raman microscope it is possible to obtain SERS spectra from silver electrode surfaces which are of better quality than the spectra obtained using excitation in the visible region. Regarding analytical application in trace analysis of melamine using NIR excitation, the recording of high-intensity ring breathing mode 2 at a Ag electrode potential of Es ) -1.3 V vs SCE was particularly suitable. Interpretation of the Influence of Surface Potential and Excitation Frequency. Since it has been well established that the classical electromagnetic enhancement (CEME) mechanism largely contributes to the SERS enhancement, we first try to interpret the SERS spectra of melamine with the “surface selection rules” from this CEME model6,7,17 and limit ourselves to the SERS spectra obtained with 514 nm excitation. To evaluate the orientation of the adsorbed melamine molecules with respect to the charged silver metal surface, and following information from the micro-SERS data can be used: (i) frequency shifts of the bands in the micro-SERS spectra relative to the corresponding normal micro-Raman spectra; (ii) relative intensities of the in-plane and out-of-plane bands in the SERS spectra, to be examined on the basis of surface selection rules; and (iii) the occurrence of new bands that can be ascribed to adsorbate-silver interaction. In general, the frequency shift of molecules that adsorb via π-orbitals (adsorption via the ring π-system in a parallel surface orientation) is opposite to that observed for nitrogen lone pair adsorbed molecules (adsorption in this manner would result in a near vertical orientation33-36). In the π-bound orientation the SERS frequencies including the ring breathing modes decrease on adsorption and then increase at more negative electrode potentials and the molecule would lie flat with the intensities of the out-of-plane modes significantly enhanced relative to the in-plane mode intensities. Both the frequency of the 677 cm-1 ring breathing 2 mode and the 983 cm-1 ring breathing 1 mode decreased upon adsorption to the Ag electrode at -0.1 V (∆ννSERS ) ν-0.1V νsolution ) -7 and -5, respectively), suggesting that adsorption occurs through π-interaction (flat adsorption). The SERS frequency of the ring breathing 2 band increased from 671 to 684 cm-1 in the potential range from 0.0 to -0.4 V (more edgeon), and at -0.5 V a downshift to 671 cm-1 occurs (more flatwise). This SERS frequency then remained constant up to a potential of -1.4 V. In contrast, the ring breathing 1 mode shifts from 981 to 1003 cm-1 between 0.0 and -1.4 V (edgeon amino-N adsorption). The frequencies of most other modes decreased at more negative potentials, consistent with edge-on adsorption through a N lone pair. The inconsistency of the conclusions drawn from these observations can be explained by the assumption that the interaction of melamine with the surface is via both the N lone pair (amino group, ring triazine nitrogen atoms) and the π-system of the triazine ring. This close proximity of the NH2 amino group, the lone pair of the triazine nitrogen atoms, and the π-electrons of the ring make this plausible if the melamine is slightly tilted from a pure perpendicular orientation. The fact that the intensities of the in-plane ring breathing modes are strongly increased due to the
J. Phys. Chem., Vol. 100, No. 12, 1996 5087 SERS effect and application of selection rules provides additional support for the almost perpendicular orientation of the adsorbed melamine molecule. In fact the more tilted orientation required for a strong interaction of the π and N lone pair electrons with the metal would give rise to a strong enhancement of the out-of-plane vibrations. The SERS intensities in the 200-1200 cm-1 region were generally large over the whole surface potential range, and this section of the ring breathing 2 and ring breathing 1 band region is displayed in an extended view in Figure 5d at the point of zero charge (Es ) -0.7 V). This region also contains the partly-out-of-plane mode at 624 cm-1 and the totally-out-of-plane modes at 729 and 803 cm-1. The drastic increase in the intensity of these modes at this potential as compared to the positive surface (0.0 V) indicated the reorientation of the ring with surface potential. Going from a positive to a more negatively charged surface, first the in-plane mode at 671 cm-1 is mostly enhanced. Subsequently, the partly-outof-plane mode at 624 cm-1 increased in intensity, and finally also the relative intensity of the totally-out-of-plane modes at 729 and 803 cm-1 increased. These results suggest that the triazine ring is reoriented from a perpendicular orientation to an orientation more parallel to the surface, going from a negatively charged to a neutral charged surface. Moreover, the out-of-plane bands observed in the micro-SERS spectrum at 624 cm-1 (Es ) -0.7 V) and at 803 cm-1 (Es ) -0.7 V) were not seen in the normal Raman spectrum. Especially the intensity of these bands drastically depended on the applied potential, suggesting that the orientation changes with potential. When reasoning along the CEME lines presented above, a problem arose when one considers the influence of excitation frequency. Both in the case of 645 nm excitation for the Ag sols and for 1064 nm excitation for the Ag electrode, the 625650 cm-1 melamine band (λex ) 1064 nm: 627 cm-1 at -1.3 V; 631 cm-1 at -0.7 V, 649 cm-1 at 0.0 V) was very intense whereas at identical potentials excitation at 457 and 514 nm resulted in a 625-650 cm-1 band with low relative intensity. This put serious question marks at the above followed line of reasoning using the adsorbate orientation concept based on the CEME surface selection rules. Otto et al. also showed a breakdown of the CEME selection rules, using isolated silver islands on posts.7 One of the two mechanisms directed toward understanding the origin of SERS is based on the photoinduced charge transfer (CT) process which creates electron-hole pairs that generate Raman scattered photons under recombination and is described by Birke et al.6 and Otto et al.7 This theoretical model is based on tuning of the CT excitation into and out of resonance by changing either the Fermi level, Ef, of the metal or the energy of excitation (hω). The electronic levels of the adsorbed molecule are easily changed relative to Ef in electrochemical environments by changing the applied potential, and the SERS intensity becomes large at a certain potential where the energy gap ∆E ) |Emol - Emet| becomes equal to the excitation energy. The CT process is expected to become less favorable when the excitation energy and ∆E are mismatched. Therefore, on electrode surfaces, SERS spectra show resonance-shaped intensity profiles as a function of applied potential. When an electron is transferred from Ef to an affinity level of the molecule (metal-to-molecule CT), a positive slope is observed in the incident photon energy vs the applied voltage plot. Figure 5 and Figure 9 show that the SERS intensity of melamine changed dramatically with applied potential and photon energy. For 514 nm excitation, the SERS of totally symmetric ring breathing mode 2 at 677 cm-1 was the most
5088 J. Phys. Chem., Vol. 100, No. 12, 1996
Koglin et al. In interpreting the potential dependence for different excitation wavelengths, the energy position and width of the CT band can be determined using a procedure reported by Furtak37 and Ingram et al.38 As a result of the SERS intensity potential profile to the applied potential, the energy of the vacant melamine level for melamine on Ag relative to the reference potential of 0.0 V vs SCE (ECT) was estimated using the data at the shortest excitation wavelength. The maximum in the intensity potential profile corresponds to a ECT value of 2.84 eV. The energy width ∆(hω) of the charge-transfer excitation resonance can be estimated from the full width at half maximum (fwhm) of the SERS intensity potential prophile ∆V and the slope from the plot of the excitation energy vs the maximum potential obtained from the SERS intensity potential profiles as follows:37,38
∆(hω) ) (d∆E/dVapp)∆V
Figure 13. Comparison of the SERS spectra of a 10-2 M melamine solution recorded at 1064 nm and -0.6 V (a), at 514 nm and -0.6 V (b), and at 1064 nm and -1.4 V (c).
Figure 14. Effects of tuning the CT process into and out of resonance by changing the excitation energy from 514 to 1064 nm.
intense band in the whole potential range, while for the 1064 nm excitation up to a potential of about -0.8 V the band at 628 cm-1 was the most intense SERS band in the spectrum. In contrast ring breathing mode 1 at 986 cm-1 shows the same potential dependence for different excitation energies. A comparison of the SERS spectra for 1064 and 514 nm excitation shows that at the same electrode potential the SERS spectra are completely different (Figure 13a,b) but a very good similarity in the SERS spectra is observed at the potential of -0.7 V for 514 nm excitation and of -1.4 V for 1064 nm excitation (Figure 13b,c). The maximum in the SERS intensity potential prophile (10-2 M melamine in 0.1 M KCl) for the 628, 677, and 805 cm-1 bands shifts to lower surface potentials with increasing excitation wavelengths (514, 632, and 1064 nm, corresponding to hω ) 2.41, 1.96, and 1.16 eV, respectively); e.g., the maximum in the SERS intensity potential for the 628 cm-1 band shifts from -0.6 V for 514 nm excitation to -0.9 V for 1064 nm excitation. The slope of the photon energy vs applied voltage plot is about 1.24 eV/V. This is well interpreted by a resonance Raman-like process associated with the photon-driven charge-transfer mechanism from the metal to an affinity level of the adsorbed molecule.
(1)
The fwhm of the SERS intensity potential prophile ∆V was calculated to be 0.6 V. According to eq 1, and using the slope d∆E/dVapp obtained for the 0.1 M KCl electrolyte of 1.24 eV/ V, this corresponds to a width ∆(hω) of ∼0.75 eV (fwhm). Thus, increasing the applied potential to a more negative value (Ef increases) in the NIR SERS spectra caused tuning into resonance. Adding the energy 1.4 eV corresponding to the applied potential of -1.4 V to the NIR excitation energy of 1.16 eV for 1064 nm excitation, the resulting energy of 2.56 eV falls nicely within the CT resonance band (2.84 ( 0.32 eV), as indicated in Figure 14. This explains the similarity between the SERS spectrum recorded at -0.6 V and 514 nm and the spectrum recorded at -1.4 V and 1064 nm (Figure 13b,c). The same charge-transfer enhancement model can be used to explain the effect of the counterions in the SERS spectra. The decrease in the negative charge (Cl-) on the solution side is compensated by a decrease in the positive charge on the Ag colloid surface, which leads to a decrease in the charge-transfer excitation energy, just as if the externally applied potential Vapp between the metal and the bulk solution had been made more negative. Therefore, the SERS spectra at low Cl- bulk concentration are comparable with the SERS spectra at electrode potentials to a more negative value. The SERS spectrum at Vapp ) -0.2 V in a 10-3 M KCl electrolyte solution (cf. Figure 7b) is comparable with the SERS signals at Vapp ) -0.7 in a 10-1 KCl electrolyte solution (c.f. Figure 5b). Now that we have established that the charge transfer mechanism is important for the description of the resonance SERS effect of melamine on Ag, the SERS selection rules can be established by analyzing the three different charge transfer mechanisms involved under resonance conditions:39 RFσ ) A + B + C, where F and σ represent x, y, or z. Term A represents the Franck-Condon contribution, and only totally symmetric modes are surface enhanced by this mechanism. Terms B and C arise from the Herzberg-Teller term and represent the SERS via molecule-to-metal CT and metal-to-molecule CT, respectively. Both totally and nontotally symmetric modes are enhanced by these terms. Since the CT is from metal-tomolecule for melamine on silver surfaces, as described above, the nontotally symmetric modes (e.g. the mode at 803 cm-1) are believed to be enhanced via the C term. The A′1 modes are believed to be enhanced via the A term. Especially the total symmetric ring breathing mode (A′1) is selectively enhanced under CT resonance by the A term mechanism. A detailed group theoretical discussion will be published separately. Conclusions The results on melamine adsorption at Ag sols clearly showed that these sols cannot be used to perform quantitative analysis
SERS of Melamine at the Ag/Electrolyte Interface of low melamine concentrations. The intensities of the various melamine bands varied seriously using various Ag sols. Electrode SERS work unambiguously showed that this is a result of variations in the surface potential of the Ag surface. By using microelectrodes it turned out to be possible to accurately control the surface potential and consequently to be possible to obtain reproducible SERS spectra of adsorbed melamine. In order to use the microelectrode SERS technique, it is a prerequisite to be able to clean the electrode surface after each adsorption step. The first technique used to clean the electrode surface is a hard-cathodic cleaning step. Although this method may result in changes of the surface roughness of the electrode, the reproducibility turned out to be ca. 15% over weeks. A more subtle method was developed using an exchange reaction with CPC to remove the adsorbed melamine. At the proper surface potential, the exchange is very rapid. In a subsequent step, the adsorbed CPC molecules were removed from the surface by a moderate negative electrode potential. This procedure resulted in a reproducibility of ca. 10%. We tried to understand the effects of metal surface potential and excitation wavelength using the adsorbate orientation concept. It turned out that it is impossible to understand all the results in terms of changes in adsorbate orientation in the classic electromagnetic enhancement model (CEME). Another concept based on charge transfer resonance Raman processes associated with the photon-induced charge transfer from the metal to an affinity level of the adsorbed melamine molecule could explain the identical SERS bands at the applied potential of Es ) -0.6 V using the visible excitation and at Es ) -1.4 V using NIR excitation. By changing the excitation wavelength, the CT process can be in or out of resonance with the affinity level of the molecule. This can be compensated by using different surface potentials. In conclusion, the utility of micro-SERS spectroscopy for the evaluation of potential-dependent interfacial competitive and displacement reactions at charged surfaces has been demonstrated. The examples of these measurements in the field of surfactants and melamine shown in this article were selected to illustrate the sensitivity, the molecular specificity of the adsorption processes, accuracy, and the ease of substrate preparation. The spatial resolution of the laser micropobe, coupled with the SERS enhancement of the Raman cross section, means that subpicogram quantities of material localized to micrometer size surface areas can be detected and identified by SERS vibrational spectroscopy. Acknowledgment. The authors are indebted to M. J. Schwuger for his continuous encouragement and for valuable discussions. We also gratefully acknowledge S. Dohmen, S. Kreisig, and J. Cook for their skillful assistance regarding part of the experimental work. L. Markwort is acknowledged for his suggestion of the soft cathodic step cleaning procedure. The
J. Phys. Chem., Vol. 100, No. 12, 1996 5089 DSM management is acknowledged for their permission to publish this work. References and Notes (1) Scheepers, M. L.; Gelan, J. M.; Carleer, R. A.; Adriaensen, P. J.; Vanderzande, D. J.; Kip, B. J.; Brandts, P. M. Vib. Spectrosc. 1993, 6, 55. (2) Otto, A. In Light scattering in solids IV; Cardano, M., Gu¨ntherodt, G., Eds.; Springer-Verlag: Berlin, 1984; Vol. 54, p 289. (3) Chang, R.; Laube, B. CRC Crit. ReV. Mater. Sci. 1984, 12, 1. (4) Moskovits, M. ReV. Mod. Phys. 1985, 57, 83. (5) Koglin, E.; Sequaris, J.-M. In Topics in Current Chemistry; Dewar, M., et al., Eds.; Springer-Verlag: Berlin, 1986; Vol. 134, p 1. (6) Birke, R. L.; Lombardi, J. R. In Spectroelectrochemistry: Theory and Practice; Gale, R. J., Ed.; Plenum Press: New York, 1988; p 301317. (7) Otto, A.; Mrozek, I.; Grabhorn, H.; Akermann, W. J. Phys.: Condens. Matter 1992, 4, 1143. (8) Laserna, J. J. Anal. Chim. Acta 1993, 283, 607. (9) Wentrup-Byrne, E.; Sabrinas, S.; Fredericks, P. M. Appl. Spectrosc. 1993, 47, 1192. (10) Cook, J. C.; Cuypers, C. M. P.; Kip, B. J.; Meier, R. J.; Koglin, E. J. Raman Spectrosc. 1993, 24, 609. (11) Ni, F.; Thomas, L.; Cotton, T. M. Anal. Chem. 1989, 61, 888. (12) Pothier, N. J.; Force, R. K. Appl. Spectrosc. 1994, 48, 421. (13) Markwort, L.; Hendra, P. J. Spectrochim. Acta 1993, 49A, 837. (14) Koglin, E.; Laumen, B.; Borgarello, E. Prog. Colloid Polym. Sci. 1994, 95, 143. (15) Koglin, E.; Schwuger, M. J. Faraday Discuss. 1992, 94, 213. (16) Moskovits, M. J. Chem. Phys. 1982, 77, 4408. (17) Creighton, J. In Spectroscopy of Surfaces 2; Clark, R., Hester, R., Eds.; John Wiley & Sons: London, 1988; p 37. (18) Gersten, J.; Nitzan, A. J. Chem. Phys. 1980, 73, 3023. (19) Furtak, T. E.; Roy, D. Surf. Sci. 1985, 158, 126. (20) Koglin, E.; Lewinsky, H. H.; Sequaris, J.-M. Surf. Sci. 1985, 158, 370. (21) Jones, W. J.; Orville-Thomas, W. J. Trans. Faraday Soc. 1959, 55, 203. (22) Sawodny, W.; Niedenzu, K.; Dawson, J. W. J. Chem. Phys. 1966, 45, 3155. (23) Schneider, R. J. Thesis, Universita¨t Dortmund, 1974. (24) Schneider, R. J.; Schrader, B. J. Mol. Struct. 1975, 19, 1. (25) Maple, J.; Dinur, U.; Hagler, A. T. Proc. Natl. Acad. Sci. USA 1988, 85, 5250. (26) Maple, J. R.; Thacher, T. S.; Dinur, U.; Hagler, A. T. Chem. Des. Autom. News 1990, 5, 5. (27) Meier, R. J.; Maple, J. R.; Hwang, M. J.; Hagler, A. T. J. Phys. Chem. 1995, 99, 5445. (28) Sun, S.; Birke, R.; Lombardi, J. J. Phys. Chem. 1990, 94, 2005. (29) Xu, Y.; Zheng, Y. Anal. Chim. Acta 1989, 225, 227. (30) Valette, G. J. Electroanal. Chem. 1982, 139, 285. (31) Pettinger, B.; Wetzel, H. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 473. (32) Liang, E. J.; Engert, C.; Kiefer, W. J. Raman Spectrosc. 1993, 24, 775. (33) Gao, P.; Weaver, M. J. J. Phys. Chem. 1985, 89, 5040. (34) Takahashi, M.; Niwa, M.; Ito, M. J. Phys. Chem. 1987, 91, 11. (35) Carter, D. A.; Pemberton, J. E. Langmuir 1992, 8, 1218. (36) Bukowska, J.; Jackowska, K. J. Electroanal. Chem. 1994, 367, 41. (37) Furtak, T. E.; Macomber, S. H. Chem. Phys. Lett. 1983, 95, 328. (38) Ingram, J. C.; Pemberton, J. E. Langmuir 1992, 8, 2034. (39) Lombardi, J. R.; Birke, R. L.; Lu, T.; Xu, J. J. Chem. Phys. 1986, 84, 4174.
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