In the Laboratory
Surface-Enhanced Raman Spectroscopy A Novel Physical Chemistry Experiment for the Undergraduate Laboratory1 Gabriela C. Weaver* Department of Chemistry, University of Colorado at Denver, Campus Box 194, P. O. Box 173364, Denver, CO 80217-3364 Karen Norrod Department of Chemistry, University of Colorado at Boulder, Box 215, Boulder, CO 80309-0215
Raman spectroscopy is often taught in physical chemistry laboratories as an additional technique to infrared (IR) spectroscopy for studying the vibrational modes of molecules. Some vibrational modes are active in either IR or Raman but not both, making the two techniques excellent complements of one another. Raman spectroscopy, however, is based on light scattering rather than light absorption, as is the case with IR or UV–visible spectroscopies. This scattering phenomenon makes Raman a powerful and interesting tool in its own right, independent of its spectroscopic relation to IR. Raman spectra of molecules adsorbed onto metal colloids have been observed to exhibit a considerable signal enhancement (1–5). This effect has been studied repeatedly since its discovery in 1974 and is known as surface-enhanced Raman spectroscopy (SERS). The signal enhancement aspect of the technique is explored in the colloid experiment presented here, which has been adapted specifically for use in the undergraduate laboratory. It is very well suited for use in undergraduate teaching laboratories owing to the simplicity of the colloid preparation and the compatibility with any Raman spectrometer already in use. Using SERS in the physical chemistry laboratory can extend the scope of the Raman theory normally covered in undergraduate physical chemistry by including the concepts of light scattering, surface chemistry, and resonance effects. This paper, while not intended to be a tutorial on Raman spectroscopy, presents some background on SERS and Raman and details one SERS experiment intended to demonstrate the signal enhancement effect due to adsorption of a Raman-active molecule onto a metal colloid. Background of SERS and Light Scattering
The Colors of Colloids The study of light scattering by metal colloids began with Michael Faraday while he was a professor at the Royal Institution of Great Britain in the mid-19th century (1). In an effort to further clarify the interactions between matter and light, Faraday worked with gold colloids or hydrosols comprising suspended particles that were small relative to the wavelength of visible light. He observed that these hydrosols were brilliantly colored in various hues of red, green, and violet. This phenomenon first observed by Faraday is the same one that accounts for the colors of stained-glass windows made with colloidal suspensions of gold, silver, and other metals. *Corresponding author
Using John Tyndall’s scattering experiments, Lord Rayleigh, Faraday’s successor, later developed much of the theory for light scattering by small particles (1, 2). When particles are smaller than the wavelength of incoming light, the total extinction cross-section is a combination of absorption and scattering contributions (3). Both of these contributions are dependent on the dielectric constant of the particular metal used in the colloid, ε, and the ratio between the particle radius and the incident wavelength. The dielectric constant varies with the frequency of the incident radiation, and for silver the maximum extinction occurs at an incident frequency of 380 nm (2). This effect is most pronounced for silver particles with radii of approximately 20 nm (4, 5), giving these silver colloids a bright yellow color. This scattering effect gives gold colloids their deep red hues due to a maximum extinction at a slightly higher wavelength.
Raman Spectroscopy A brief description of the Raman effect is necessary in order to understand the theory behind surface-enhanced Raman. For a more extensive discussion of Raman spectroscopy, the reader is referred to a variety of excellent texts and journal articles (6–8). The Raman effect, originally observed by Lord Raman in 1928, is also a light-scattering effect. When an incident electromagnetic field, Ei, interacts with a molecule, an electric dipole is induced in the molecule, given by: p = αE i
(1)
where α is referred to as the polarizability of the molecule and p is the induced dipole. The incident field is a time-varying quantity of the form E i = E o cos(2 πυ i t)
(2)
For a vibrating molecule, the polarizability is also a timevarying term with the frequency of vibration, υ vib: α = αo + αvibcos(2 πυvib t)
(3)
Multiplication of the two time-varying terms, E i and α, gives a cross term of the form:
αvibE o cos 2πt υi + υvib + cos 2πt υi – υvib 2
(4)
This cross term in the induced dipole represents light that can be scattered at both higher and lower energy than the Rayleigh (elastic) scattering of the incident radiation. The
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increments are given by the vibrational frequencies of the molecule, υvib. These lines are referred to as the “anti-Stokes” and “Stokes” lines, respectively. The Stokes lines are generally stronger. Since the Stokes and anti-Stokes lines contain the same type of information, generally only the stronger Stokes lines are scanned. When Lord Raman initially observed this effect, it was very difficult to discern owing to an extremely faint signal. However, use of a laser as the illumination source produces enough intensity that current detection systems using either photomultiplier tubes or charge-coupled device (CCD) detectors can be used to observe the Raman effect relatively easily.
Raman Signal Enhancement by Metal Colloids When electromagnetic radiation is incident on the particles of a metal colloid, a resonant field is induced. This resonance effect can be understood in terms of the bonding structure of metals. The electrons in metals behave as a sea of free negative charges, or plasma, bound by stationary cations. Excitation by an electromagnetic wave at a particular frequency can induce a resonant vibration of these free electrons. These vibrations are known as plasmons. The vibrating electrons will generate an additional electric field near the particle’s surface at their vibrational frequency (1 ). The enhancements in Raman signals observed in SERS are encountered when Raman-active molecules are adsorbed onto the surface of small, metal particles. Since the Raman intensity is proportional to the square of the induced dipole moment (6 ), an enhancement of the signal can arise from a change in either the polarizability, α , or the exciting electromagnetic field, E i (see eq 1). Although both effects can contribute to the observed enhancement, the dominant contribution in this case is the electromagnetic field effect. Because of the resonance effect between colloidal metals and incident radiation, the field that actually induces Raman scattering on adsorbed molecules is a sum of the incident field, E i, and the resonantly induced field near the surface, E ind. Although a complete mathematical treatment of this effect is beyond the scope of this paper, a comprehensive discussion of the theory is given by Kerker et al. (9, 10). The enhancement effect is seen to be dependent on the incident wavelength, and parallels the wavelength dependence of the resonant scattering as discussed above in the section on the colors of colloids. For example, a study of sodium citrate adsorbed on silver col-
Figure 2. Raman spectrum of neat pyridine.
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Figure 1. Side view of experimental setup of Raman system used. 100 mW Ar+ ion laser at 514.5 nm. Power at sample is 43 mW.
loids shows enhancements ranging from 103 to 106, depending on the incident wavelength used (1, 10). Adsorbing molecules onto the colloidal metals produces a variety of interesting effects. When a Ag colloid is prepared, the sample appears yellow. However, when a salt is added to the colloid, the metal particles tend to aggregate. As the particle size and morphology change with aggregation, the frequency dependence of ε also changes (1), causing the resonant wavelength for plasmon-induced enhancements to shift. As a result, adding adsorbates to the colloids will change the color of the sample and the frequency at which the enhancement is maximized. For repeated studies of sodium citrate on Ag, the maximum enhancement was seen at an incident wavelength of 647 nm rather than at the 380 nm characteristic of maximum scattering for bulk Ag colloid (1, 10). Sample Preparation Numerous preparations are given in the literature for colloids of silver and gold (11–15), with the first of these described in detail by M. Carey Lea in 1889 (11). The preparations are well suited for use in an undergraduate laboratory owing to their relative simplicity and ability to be completed within a laboratory period. In addition, the preparations are interesting because they result instantly in an array of colors ranging from yellow to wine-red and violet, depending on the metal used and the molecule adsorbed.
Figure 3. Raman spectrum of 0.1 M pyridine in triply distilled water.
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In the Laboratory
Of the colloid preparations that we tested from the literature, the most stable is the Ag colloid described by Creighton et al. (12). In the sample preparation described below, the stock silver colloid preparation is carried out first, and then pyridine is adsorbed onto the metal colloid for Raman studies. All of the solutions were prepared with triply distilled water, and the pyridine was fractionally distilled by instructors over zinc powder immediately before use.
Stock Ag Colloid Preparation The preparation of the Ag colloid stock involves a reduction of AgNO3 with NaBH4. A 2 mM aqueous solution of NaBH4 was prepared immediately before use (as it decomposes rapidly) and N2 was bubbled through the solution before carrying out the reaction. Ice-cold NaBH4 solution was added to a 1 mM solution of AgNO3 in a 3:1 v/v ratio (NaBH4/ AgNO3) with rapid stirring. The mixture turns dark yellow immediately upon combination of the two solutions. Stirring continues until the colloid reaches room temperature. In our preparations, we used 12 mL of the NaBH4 solution and 4 mL of the AgNO3, yielding a 16-mL sample, which required only 10–15 min to equilibrate thermally. Larger samples can be prepared as long as the correct ratios and sample temperatures are maintained. Adsorption of Pyridine onto the Colloid To adsorb pyridine onto the Ag, 0.1 M pyridine was added to the colloid stock in a 1:15 v/v ratio (pyridine/stock) with vigorous stirring. The sample exhibits a variety of color changes upon adding the pyridine, becoming darker yellow after a few minutes, then proceeding rapidly through reddish brown, brown, and eventually back to dark yellow. The yellow colloid is stable for months, even if exposed to room light and air. This sample of adsorbed pyridine is used for Raman spectroscopy and is treated in the same way as any liquid Raman sample. Experimental Setup The Raman system used to analyze our samples is shown in a side view in Figure 1. The excitation light source is an air-cooled Ar+ laser system at 514.5 nm with a SPEX halfmeter single monochromator. A charge-coupled device (CCD) array was used for detection. However, a system with a standard photomultiplier tube would also work (16). Light
Figure 4. Raman spectrum of pyridine adsorbed onto Ag colloid. Total pyridine concentration is 6.25 mM.
from the laser was focused onto the sample via a premonochromator, a 40-cm focal length lens, and an aluminumcoated mirror with an incident angle of approximately 60° to the sample. The power at the sample surface was measured to be 43 mW. The sample was held by a laboratory jack and it was possible to focus the scattered light through the stationary optics onto the monochromator by adjusting the height of the jack. Collection of the scattered light was achieved with a 10× microscope objective and a camera lens for collimating the scattered light. The collimated beam was directed with a second aluminum mirror into the monochromator and detected with a liquid-nitrogen-cooled CCD. The spectrum was scanned from the laser line (514.5 nm; 19,436 cm᎑1) out to lower energies to observe the Stokes lines. Although this is the Raman system that was used in our work, other possible configurations for Raman spectroscopy have been described in the literature (16–18). Results Raman spectra were taken of neat pyridine and of the Ag colloids with adsorbed pyridine following the preparation described above. The peak areas were compared and corrected for the volume of pyridine on the Ag particles compared with the volume in the neat sample. Comparison of these peak areas shows a large enhancement in signal strength, demonstrating the SERS effect as described below.
Neat Pyridine Samples Raman spectra were first taken of neat pyridine in order to use these for comparison with the colloid samples. The spectrum of pure, distilled pyridine is shown in Figure 2. The four peaks visible correspond to those reported at 988, 1028, 1214, and 1580 cm᎑1 in the library of Raman spectra (19). These values are wavenumber shifts to lower energy from the Rayleigh scattered peak from the laser line. Also, the peaks shown are only the Stokes peaks. Once the sample is diluted in water to 0.1 M, only the two strongest peaks remain discernible, as shown in Figure 3. Pyridine Adsorbed on Ag Colloid The spectrum for pyridine adsorbed on the Ag colloid is shown in Figure 4. The peak positions have not changed, but the relative intensities of the two peaks are different. This is due to the effect of the polarizability on the dipole moment
Figure 5. Comparison of peaks at 1032 cm᎑1 and 1004 cm᎑1 for 0.1 M pyridine and pyridine adsorbed on Ag colloid.
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of the molecule for each vibrational mode. Different vibrational modes of the molecule will be affected differently by a charge-transfer effect between the adsorbate and the metal particle (6 ), which will change the polarizability characteristics. This is a small effect compared to the overall enhancement of the signal intensity by the resonantly scattered field from the metal particle.
Signal Enhancement The spectra of the two peaks of the pyridine adsorbed on Ag and the pyridine in solution are compared in Figure 5. A large enhancement is immediately apparent from the figure. The concentration of pyridine in the colloid sample is 16 times less than in the 0.1 M sample. Using calculations of the peak areas for each of these peaks, the enhancement effect is between 100 and 300 times. Peak areas were calculated simply by carrying out a least-squares Gaussian fit in a spreadsheet program and then integrating the area under that curve. The enhancements we observed are larger than those reported by some researchers (12) but are in good agreement with other SERS studies of pyridine on Ag (20). However, the enhancements are not as large as those calculated theoretically or those reported for colloid–adsorbate systems that are more difficult to prepare (1, 4). Summary The experiment reported here is well suited for use in the undergraduate physical chemistry laboratory. The procedures, including preparation of the colloids and collection of the SERS spectra, are relatively straightforward and can be carried out by students during a laboratory session. The theory behind SERS can be expanded to many levels of complexity in order to extend the normal discussions of light scattering by matter and of Raman spectroscopy. SERS can be carried out with a variety of samples in addition to those described in this paper. Numerous adsorbates have been reported in the literature for these systems in addition to the pyridine reported here, such as sodium citrate and CN ᎑ ions (13, 14 ), polypeptides and proteins (15), organophosphorus compounds and sulfur oxide catalysts (6 ). Also, SERS substrates are not limited to colloids in aqueous media but can range from polymer films to roughened Ag and Au electrodes as reported in the literature (6, 21–24 ). The experiment described in this paper is used to demonstrate one important aspect of SERS: the very noticeable signal enhancements due to adsorption onto a metal colloid. This type of study looks at the effects of the colloid on the Raman spectrum of the adsorbate. However, there are a variety of types of experiments that could be carried out. As an advanced project for students, a complementary study examining the effect of the adsorbate on the colloid resonant frequency can be done. Such a study would require inducing Raman scattering with a variety of wavelengths of light and then determining the wavelength of maximum enhancement. This would definitely be a longer-term project for one or a
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group of students, but it is suggested as a way of demonstrating the multitude of possibilities that exist for SERS experiments in the undergraduate curriculum. Acknowledgments We would like to expresses our gratitude to Richard Chang of Yale University for his information regarding the background work in SERS. Note 1. Paper presented at the 14th Biennial Conference on Chemical Education; American Chemical Society, Division of Chemical Education; August, 1996.
Literature Cited 1. Kerker, M. Proc. R. Inst. GB 1989, 61, 229–250. 2. Kerker, M. Appl. Opt. 1991, 30, 4699–4705. 3. Creighton, J. A. In Surface Enhanced Raman Spectroscopy; Chang, R. K.; Furtak, T. E., Eds.; Plenum: New York, 1982; pp 315–338. 4. Wang, D. S.; Chew, H.; Kerker, M. Appl. Opt. 1980, 19, 2256– 2257. 5. Kerker, M. Acc. Chem. Res. 1984, 17, 271–277. 6. Ferraro, J. R.; Nakamoto, K. Introductory Raman Spectroscopy; Academic: Boston, 1994. 7. Surface Enhanced Raman Spectroscopy; Chang, R. K.; Furtak, T. E., Eds.; Plenum: New York, 1982. 8. Diem, M. Introduction to Modern Vibrational Spectroscopy; Wiley: New York, 1993. 9. Kerker, M.; Wang, D. S.; Chew, H. Appl. Opt. 1980, 19, 3373– 3388. 10. Kerker, M.; Wang, D. S.; Chew, H.; Siiman, O.; Bumm, L. A. In Surface Enhanced Raman Spectroscopy; Chang, R. K.; Furtak, T. E., Eds.; Plenum: New York, 1982; pp 109–128. 11. Lea, M. C. Am. J. Sci. 1889, 137, 476–491. 12. Creighton, J. A.; Blatchford, G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790–798. 13. von Raben, K. U.; Chang, R. K.; Laube, B. L. Chem. Phys. Lett. 1981, 79, 465–469. 14. von Raben, K. U.; Chang, R. K; Laube, B. L.; Barber, P. W. J. Phys. Chem. 1984, 88, 5290–5296. 15. Cermakova, K.; Sestak, O.; Matejka, P.; Baumruk, V.; Vlckova, B. Collect. Czech. Chem. Commun. 1993, 58, 2682–2694. 16. Physical Chemistry: Developing a Dynamic Curriculum; Schwenz, R. W.; Moore, R. J., Eds.; American Chemical Society: Washington, DC, 1993. 17. Galloway, D. B.; Ciolkowski, E. L.; Dallinger, R. F. J. Chem. Educ. 1992, 69, 79–83. 18. Steehler, J. K. J. Chem. Educ. 1990, 67, A65–A71. 19. Raman/Infrared Atlas of Organic Compounds; Shrader, B., Ed.; VCH: Weinheim, Germany, 1989; p I7-01. 20. Wetzel, H; Gerscher, H. Chem. Phys. Lett. 1980, 76, 460–464. 21. Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735–743. 22. Mullen, K.; Carron, K. Anal. Chem. 1994, 66, 478–483. 23. Ni, F.; Cotton, T. M. Anal. Chem. 1986, 58, 3159–3163. 24. Sudnik, L. M.; Norrod, K. L.; Rowlen, K. L. Appl. Spectrosc. 1996, 50, 422–424.
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