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J. Phys. Chem. B 2002, 106, 5408-5412
Comparison of Resonant and Non Resonant Conditions on the Concentration Dependence of Surface Enhanced Raman Scattering from a Dye Adsorbed on Silver Colloid C. McLaughlin, D. Graham, and W. E. Smith* Department of Pure and Applied Chemistry, 295 Cathedral Street, Glasgow, G1 1XL, United Kingdom ReceiVed: October 3, 2001; In Final Form: January 4, 2002
Surface enhanced Raman scattering (SERS) and surface enhanced resonance Raman scattering (SERRS) from a silver colloid suspension are compared using a dye designed to bond strongly to a silver surface. Titration of the dye into a silver colloid suspension caused aggregation in a controlled manner without an aggregating agent being added. Concentrations of dye equivalent to between 5 × 10-9 and 10-3 M in the final dilution before adsorption to the silver were used. The results suggest that monolayer coverage of the surface occurs at approximately 10-6 M. Above this concentration, the suspensions are less stable, and the relationship between intensity and concentration is complex. Below this concentration, three main regions can be identified by electronic absorption spectroscopy. At dye concentrations up to 7.5 × 10-8 M, there is little evidence of aggregation, although there are changes in the spectra ascribed here to surface changes caused by dye adsorption. Between 7.5 × 10-8 M and 2.5 × 10-7 M, a well-defined small aggregate appears to occur and above 2.5 × 10-7 M larger less well-defined aggregates form. SERRS gave linear concentration dependence below 7.5 × 10-8 M suggesting scattering from single particles. A changeover region occurs close to where the first evidence of aggregation is detected by electronic spectroscopy, and at higher concentrations up to about monolayer coverage a second linear region was obtained. SERS below the concentration at which aggregation is detected by electronic spectroscopy was weak and difficult to obtain. At higher concentrations, the SERS gradients are steeper and the maximum enhancement observed is within a factor of 4 of that obtained in SERRS. The study shows that there is a different mechanism in SERRS compared to SERS with single particle enhancement being much greater in SERRS.
Introduction Surface enhanced resonance Raman scattering (SERRS) is an efficient process and single molecule detection has been demonstrated.1,2 It has the advantage for sensitive analytical methodologies that molecularly specific and sharp signals are obtained, thus enabling discrimination of components within a matrix without separation. However, a major problem in developing the technique is that the effect is not well understood theoretically. To obtain SERRS, an analyte containing a chromophore is adsorbed onto a suitably roughened metal surface. A laser excitation frequency is chosen to coincide with the absorption frequency of the chromophore to give molecular resonance and the surface roughness of the metal is set so that the surface plasmon is in resonance at the same frequency to create surface enhancement.3 These conditions are usually only approximately fulfilled but the effect is different experimentally from surface enhanced Raman scattering (SERS) in which there is no molecular resonance and only surface enhancement occurs. For example, in SERS, surface selection rules4 can cause the intensities of the bands to be very different from those observed in normal Raman scattering, whereas in SERRS, the spectra are usually similar to the equivalent resonance spectra with some intensity differences.3 The most commonly used substrates are silver and gold because both have suitable plasmon resonance frequencies and absorption characteristics to give effective enhancement with visible excitation. Many different surfaces have been used, including electrodes, deposited films, gratings, etched surfaces
and colloidal suspensions.6-11 Colloidal particles have been used to demonstrate single molecule detection1,2 and the enhancement possible with dimers12 and fractal clusters.13 These studies show that SERRS enhancement is complex with some surface structures giving much greater enhancement of the scattering than others. To improve understanding of the effect, particularly for practical applications in analysis, a study was carried out with a silver colloidal suspension under as simple and reliable conditions as possible. Three different excitation wavelengths were chosen to span a range of frequencies in which either SERRS or SERS would be expected to dominate. The chosen analyte binds strongly to the silver surface and therefore eliminates problems with desorption of the analyte. It causes aggregation and in this study no other aggregation agent was used. The aggregating agents added in most studies are usually present in amounts far larger than those required for monolayer surface coverage and the surface chemistry is complex. The selfaggregation method used here links the degree of aggregation to the concentration and the extent of surface coverage of the analyte. This eliminates one of the greatest uncertainties in SERS/SERRS studies of colloidal suspensions. Experimental Section The colloid was prepared using a modified Lee & Miesel method14 colloid with a maximum absorbance within the range 403-410 nm and a full-width, half-maximum of 60 nm or less was used throughout this study, as it has been found to provide a consistent particle size and a small size distribution.3 4(5′-
10.1021/jp0136819 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/03/2002
SERS from a Dye Adsorbed on Silver Colloid
Figure 1. 4(5′-Azobenzotriazoyl)-3,5-dimethoxyphenylamine (Dye 1) used in this study.
Figure 2. Representative electron micrograph showing the typical hexagonal andneedle shaped particles.
Azobenzotriazoyl)-3,5-dimethoxyphenylamine (Dye 1) as prepared previously was used in this study.15 Samples were prepared for electronic spectroscopy measurements by adding 100 µL of colloid containing dye at the appropriate concentration to 1 mL of distilled water. Renishaw 2000 spectrometers equipped with a macro sampler and a Renishaw microprobe 100 were used to record the Raman spectra. They were equipped with Spectra Physics argon and helium neon lasers set to give 3mW of power at the sample at 457.9, 514.5, and 632 nm. The initial Raman spectra in Figure 4 were recorded from a cuvette containing 10 µL of the appropriate concentration of dye and 1 mL of colloid 8 min after mixing For the concentration-dependent measurements a simple flow cell was used.16 1.6 mL min-1 of colloid, 1.6 mL min-1 of water and 0.32 mL min -1 of the appropriate concentration of dye solution were pumped through three arms of the flow cell using a multichannel peristaltic pump. Because the pump operated at a single speed, different diameters of tubing were selected to achieve the different flow rates The concentration dependent spectra were taken with a 30 s accumulation and each point is the average of 5 measurements. A methanol spectrum was recorded before each set of 5 measurements, and used to normalize the intensities of the SERRS/SERS spectra. Results and Discussion Dye Properties. The dye used (Figure 1) contains the benzotriazole group which displaces citrate from the surface of the colloid to bind directly to the silver surface to form
J. Phys. Chem. B, Vol. 106, No. 21, 2002 5409 polymeric layers.17 This ensures that adsorption to the metal surface is thermodynamically favored and almost 100% adsorption occurs. To confirm this, the dye was added to the colloid at 10-6 M and after incubation it was centrifuged to remove all silver particles. Attempts to obtain SERRS from the supernatant by adding fresh colloid and aggregating with sodium chloride were unsuccessful despite the detection limit of below 10-9 M expected under these conditions, confirming this conclusion. Thus, effectively one molecule of aggregating agent was added to the surface for every one SERRS/SERS chromophore making for a much simpler model than in previous studies from which to interpret the spectra obtained. In contrast, the most commonly used dye for SERRS, rhodamine 6G, is expected to desorb appreciably at lower concentrations and the surface chemistry is complex and not understood. Colloid. TEM studies of the colloid indicate that it consists mainly of hexagonally shaped particles, the longest dimension of a typical particle being approximately 36 nm (Figure 2). The colloid was prepared a number of times. Some needle shaped particles were found in most preparations but in one particular case, no needles could be detected in any TEM sample measured despite surveying a large area of the sample under the electron microscope. The intensity of SERRS from this colloid was comparable to that obtained from other colloids suggesting that special shapes of particles are not responsible for the major part of the enhancement. Further, the approximate surface area of the silver in the suspension can be calculated and from an estimate of the area occupied by the dye aligned vertically and horizontally to the surface. This suggests complete surface coverage of all particles lies at about 10-6 M. This concentration is shown later to be the highest concentration at which linear concentration dependence can be obtained. This suggests that all or most molecules can take part in the scattering mechanism rather than a few molecules adsorbed on “special sites”. Electronic Absorption Spectra. A plot of the electronic absorption spectrum from the plasmon on the silver surface is shown for different concentrations of dye added in Figure 3. Three distinct concentration regions can be discerned. At low concentrations, there is no change in the peak position, a slight drop in band intensity, and a slight rise in the broad baseline absorption toward the low energy side. There is also a slight change in the shape of the peak. Because there is no detectable change in the frequency at the peak maximum, the most likely explanation for these changes is that the surface plasmon absorption characteristics are altered by adsorption of the dye as expected and used in surface plasmon resonance detection. There is little evidence in the spectra of aggregation occurring. Above this concentration between 7.5 × 10-8 M and 2.5 × 10-7 M, there is a large change and a second peak at approximately 660 nm occurs. This peak appears broader than the peak in the unaggregated colliod but these spectra are plotted on a wavelength scale. Using an energy scale this peak is of approximately the same half width or slightly greater than that of the main peak at about 400 nm (spectrum c, Figure 3). This suggests that a structured association between particles is occurring producing an aggregate of a specific size. This is likely to be a dimer and the midpoint of the region corresponds to approximately 1/10th of surface coverage.12 Above this concentration and up to 10-6 M, a broader band forms at the longer wavelengths suggesting larger aggregates. This can be seen visually as a change in color in the colloid. Excitation Frequency. The absorption maximum of the dye is 435 nm in methanol diluted with water which means 457.9 nm excitation can be considered to produce resonance as well
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Figure 3. Electronic spectrum of the colloid with different concentrations of dye added. (a) 5 × 10-8 M, (b) 1 × 10-7 M, (c) 2.5 × 10-7 M, (d) 5 × 10-7 M, and (e) 1 × 10-6 M.
Figure 4. SERS spectra of the dye at (a) 457.9 nm 5 × 10-8 M, (b) 457.9 nm 5 × 10-7 M, (c) 632.8 nm 5 × 10-8 M, and (d) 632.8 nm 5 × 10-7 M. Absolute spectral intensities have been offset by an arbitrary amount in order to show the changes in relative intensity.
as surface enhancement (SERRS), whereas 514.5 and 632.8 nm excitation will produce surface enhancement. However, some molecular resonance contribution to scattering can occur at frequencies well away from true resonance but the enhancement factors are small. Thus, there may be a small resonance component particularly with 514.5 nm. In contrast in the aggregated colloid the UV-visible spectra indicate surface plasmon resonances throughout the visible region and into the near infrared so that surface enhancement is expected for all three but SERRS only for 457.9 nm excitation. Comparison of SERRS and SERS. The dye concentration reported throughout is the final dye concentration following addition to the colloidal suspension, but before any adsorption of the dye on the surface takes place. In this way, the dilution caused by the colloidal suspension is taken into account. Spectra were taken at final dye concentrations of 5 × 10-8 M and 5 × 10-7 M using three different excitation frequencies. Although
there was a considerable drop in absolute intensity at the lower concentration with all three excitation frequencies, with 457.9 nm excitation(i.e., SERRS) there was little to no variation in the relative peak intensities between the two concentrations. With 514.5 nm excitation, there were only minor variations in intensity between the main peaks but with 632 nm excitation, the spectra vary considerably (i.e., SERS). These results are consistent with a gradual change from SERRS obtained with 457.9 nm excitation to SERS with 632 nm excitation. Thus with SERRS, where molecular resonance plays a larger part in the enhancement the scattering is less sensitive to changes in molecular orientation on the surface. This is illustrated from the data for 457.9 and 632.8 nm excitation in Figure 4. The data for 514.5 nm excitation, reflects conditions which are effectively preresonant enhancement. Concentration Dependence. The effect of concentration was investigated for each excitation frequency by plotting the intensity for the two main peaks in each spectrum against concentration (Figure 5). There was a significant difference in concentration dependence between the two peaks only for 632 nm excitation. The intensity of the most intense peak was plotted for the widest possible range of concentrations for all three wavelengths (Figure 5). To accommodate the wide concentration range, the intensities are plotted on log scales. However, this obscures the fact that there is a change in gradient in the spectra excited with 457.9 nm excitation. The two straight-line regions are shown in Figure 6. At low concentrations, a gradient of 6.4 10-8 with an error of 6.1% is obtained whereas at high concentration the gradient is 1.4 × 10-9 and the error is 8.4%. The different gradients shown in Figure 6 for the three different cases indicate the difference between SERRS and SERS. In the case of SERRS but not SERS significant scattering is obtained at low concentrations where less than 1/1000 of the available surface area is covered. This indicates that for SERRS, a process must exist in which the unaggregated particle provides enhancement. The slope is close to linear which also suggests a single particle absorption process. This mechanism is effective until appreciable surface coverage occurs(about 1/10 of a monolayer). The UV-visible spectra show the growth of a peak probably due mainly to dimers and any associated small
SERS from a Dye Adsorbed on Silver Colloid
J. Phys. Chem. B, Vol. 106, No. 21, 2002 5411
Figure 6. Plot of normalized SERRS intensity (1373 cm-1) vs ABTDMOPA concentration over 2 regions (A) 4.84 × 10-9 M to 7.10 × 10-8 M and (B) 1.01 × 10-7 M to 7.57 × 10-7 M.
Figure 5. Plot of signal intensity for the peak at 1373 cm-1 against concentration for the three excitation wavelengths. (A) 457.9 nm, (B) 514.5 nm, and (C) 632.8 nm.
aggregates in the same concentration range. However, these species will gradually form larger clusters as the dye concentration is increased and this gives a second shorter region with a line of different slope. With 632 nm excitation no signals can be observed below 10-7M. At 514 nm, some spectra can be obtained with dye concentrations below 10-7M although the range over which the signal is obtained is short and the signals are poor. This suggests that the small resonant contribution at 514 nm is providing some enhancement. Surface Coverage. From the calculation of the surface coverage, a rough estimation of the concentration required for complete coverage can be made. This should be in the region
between 10-5 and 10-6 M. All the concentration dependent data indicates that this concentration region corresponds to the maximum SERRS or SERS intensity. Above this concentration, the aggregated suspensions are less stable and tend to precipitate out as larger clusters are formed, and this process was observed in some runs. However, where data has been taken before precipitation occurred a much reduced slope is obtained at higher concentrations corresponding to multilayer coverage. It indicates that most of the enhancement is associated with the first layer. Different Enhancement Mechanisms in SERRS and SERS. This work provides a clear pattern to distinguish the difference between SERRS and SERS. At higher concentrations SERRS intensities are higher than those for SERS but the advantage is only about a factor of 4. A similar effect has been found for rhodamine where the difference in enhancement between rhodamine at 514 nm, where it is truly in resonance and rhodamine at 632 nm is again about 4.3 The large advantage of SERRS occurs at low concentrations where single particle enhancement occurs only in SERRS allowing lower concentrations to be detected. The identification of this pattern helps to explain some of the apparent contradictions found in previous experiments. When SERS is obtained from colloid most of the enhancement usually comes from aggregation whereas two processes appear to be involved in SERRS and one does not require aggregation. This explains how a single molecule on a single particle can be detected with SERRS even if no signal can be obtained in SERS without aggregation. A further difference between SERS and SERRS is that much larger changes in relative intensity can occur when surface enhancement is the dominant process. If the spectra of a number of similar benzotriazole azo dyes is recorded using SERRS, the relative intensities of the main bands are similar. However, with SERS, appreciable differences in intensity between the bands appear.
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Conclusions
References and Notes
The experiments in this study involved a dye molecule strongly attached to the surface with no aggregating agent added. There seem to be no “special” adsorption sites with all molecules adsorbed able to contribute to the signal. The surface chemistry is therefore more predictable than rhodamine and a better ligand to study the activity of SERRS. The mechanism of SERRS appear to be different from that of SERS. SERRS appears to operate more effectively for a single particle and is more effective at low concentrations. There is also an additional intensity from aggregation. However, SERS is more molecularly specific and nearly as effective at enhancing the scattering process at higher concentrations. In quantitative studies, it is clear that the method of aggregation as well as the degree of attachment of the ligand to the surface is critical, and control over the amount of aggregating agent, and its interaction with the surface is necessary. SERRS has an advantage over SERS in this regard in that the signals appear to be less prone to relative intensity changes.
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Acknowledgment. We thank EPSRC for support for this project and a student for one of us (C. McL).
