Laser Power Effects in SERS Spectroscopy at Thin Metal Films

Laser Power Effects in SERS Spectroscopy at Thin Metal Films ... enhancement above intensities of 4.5 kW/cm2 at the focus and around 54 W/cm2 at the e...
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J. Phys. Chem. B 2001, 105, 6330-6336

Laser Power Effects in SERS Spectroscopy at Thin Metal Films Carmen Viets and Wieland Hill* Institut fu¨ r Spektrochemie und Angewandte Spektroskopie (ISAS), Postfach 101352, 44013 Dortmund, Germany ReceiVed: NoVember 29, 2000; In Final Form: March 1, 2001

SERS spectroscopy requires the illumination of delicate metal structures with intense laser light. In the present paper, the effect of laser illumination on these structures is investigated for silver island films and silver films over alumina particles in focused laser beams and at the tip of fiber-optic SERS sensors. Less than proportional increases of SERS intensities with increasing laser powers indicate a reduction of SERS enhancement above intensities of 4.5 kW/cm2 at the focus and around 54 W/cm2 at the end face of fiber SERS sensors. The reduction of SERS enhancement has been attributed to irreversible changes of optical properties of the metal structures at elevated temperatures by comparison with transmission measurements. Anti-Stokes/Stokes ratios of SERS band intensities already show noticeable heating at laser powers of about 1 mW and also indicate changes in optical properties above a few milliwatts.

Introduction Raman scattering is a well-established vibrational spectroscopic method in chemical analysis. The Raman spectra generally show narrow, well-resolved vibrational bands that yield structural information on the analyte molecules and are regarded as a fingerprint of the substance. However, the signals are generally too weak to be used for sensitive probes, because most molecules exhibit low Raman scattering cross sections. Surface-enhanced Raman scattering (SERS) offers a possibility to increase the Raman intensities from molecules that are adsorbed at rough metal surfaces by about 6 orders of magnitude1 or even more. Since the first observation2 and explanation3 of this effect, theories on electromagnetic and chemical contributions to the SERS enhancement1,4 and a great variety of SERS-active surfaces have been developed. These surfaces include colloids,5 roughened electrodes,6 metal-coated nanoparticles,7 metal island films,8 silver-coated quartz posts,9 regular silver-coated grating structures,10 and others. These SERS-active surfaces were employed in an increasing number of fields such as electrochemistry,3 interface science,11 microscopy,12,13 pH-sensing,14,15 and fiber-optic chemical sensors.16 SERS spectroscopy requires the illumination of the delicate metal structures with intense laser light. Because of the absorption by the metal, illumination can lead to a noticeable heating of the sample at sufficiently high power levels. Even slightly increased temperatures can affect the structure and optical properties of delicate structures such as colloid particles and metal islands.17 Furthermore, heating by a few degrees can also clearly influence surface reactions or adsorption equilibria which are important in quantitative measurements, for example, with SERS sensors. McGlashen et al. observed that in SERS microscopy focused green laser light with powers of some 10 mW produced photophysical or photochemical damage at roughened Ag electrodes.18 Metal films over nanoparticles were shown to produce a linear increase of SERS intensities for laser powers * Correspondence to Wieland Hill, Lambda Physik AG, Hans-BoecklerStr. 12, 37079 Goettingen, Germany. Fax: +49 231 6902023; E-mail: [email protected].

up to 200 mW with unfocused illumination by an optical fiber.19 Occasionally, huge intensities above 1 MW/cm2 were applied in SERS experiments.20 However, laser power effects on delicate SERS-active metal structures have not been studied in detail yet. In the present paper, laser power effects are investigated for silver island films and silver films over alumina nanoparticles in focused laser beams and at the tip of fiber-optic SERS sensors. Experimental Section Instrumentation. SERS measurements were done using a 0.5 m triple monochromator with subtractive dispersion of the two first stages (DILOR XY) and with a nitrogen-cooled CCD camera (Wright Instruments) with a 298 × 1152 chip (EEV 88131) serving as detector. The dispersion of the spectrometer was 1.1 nm/mm. The excitation source was a CW Ti:sapphire laser (Coherent 890) pumped by an Ar+-laser (Spectraphysics 2000) and operating at a wavelength of 702 nm. The laser radiation was filtered by a laser filter monochromator (Spectrolab, Labspec III) to remove low-intensity sidebands and the spontaneous emission background. The laser intensity was varied either by rotating a circular, linear wedge, neutral density filter or by inserting different sets of metallic, neutral density filters into the laser beam. The transmission of the linear wedge filter was calibrated with respect to the rotation angle. The optical transmission of the neutral density filters in the attenuation sets was known from data by the manufacturer and checked with an UV-vis absorption spectrometer (Varian, Cary 500). Laser light was focused onto the samples through a microscope objective (Olympus, MPlan 10×/0.25). The same objective and a further lens produced a 12-fold enlarged intermediate image of the backscattered light in the plane of the 300 µm entrance slit of the spectrometer. Integration times were 10 s with five accumulations for Stokes measurements and 100 s with five accumulations for anti-Stokes measurements. For the measurement of anti-Stokes/Stokes intensity ratios, a depolarizer (Laser 2000, OFR-DPU-25) was placed in front of the spectrometer entrance to eliminate influences from the polarization dependence of the detection sensitivity. For these measurements,

10.1021/jp004315c CCC: $20.00 © 2001 American Chemical Society Published on Web 06/14/2001

Laser Power Effects in SERS Spectroscopy the laser power was solely adjusted by inserting different sets of metallic, neutral density filters into the beam line. Simultaneous to the SERS measurements, the transmission through the SERS-active slides was measured by placing a calibrated photodiode sensor (Coherent, LM-2) with a 1000:1 attenuator directly under the sample slides so that the power transmitted through the samples could be read on a power meter (Coherent, Fieldmaster). For the SERS measurements at the fiber-optic sensors, the above-mentioned microscope objective of the DILOR XY spectrometer was used to couple the laser light into the fiber sections. The laser was hereby focused on the uncoated end of the fibers. This way the SERS-active tip surface was illuminated through the fiber, and the backscattered light was collected by the same microscope objective after passing the same fiber. The UV-vis transmission of island films was measured with a double monochromator UV-vis/NIR-spectrometer (Varian, Cary 500). For the investigation of tempering effects, the SERS substrates were placed into an electrical oven for 20 min at the given temperature and thus subsequently exposed to increasing temperatures up to 473 K. The transmission of these tempered substrates was measured at room temperature. Preparation of SERS-Active Surfaces. SERS-active surfaces were prepared on glass slides and on freshly cut fused silica fiber tips16 according to the following techniques: (a) SilVer Metal Island Films (AgIF). Glass slides were cleaned in nitric acid vapor for 5 h, rinsed with distilled water, cleaned in distilled water vapor for another 5 h, rinsed again with distilled water, and then dried by overnight storage in a desiccator. The slides and a holder with freshly cleaved fused silica fiber sections (SpecTran, HCP-M0200T-06, i.d. 250 µm, length 10 cm) were placed in an evaporation chamber (Balzers: Hochvakuum-Bedampfungsanlage BA710V) at a distance of approximately 50 cm from the evaporation source. The fibers were positioned so that one end faced the evaporation source, whereas the opposite end was in the shadow of the fiber holder. The samples were rotated around the vertical axis of the chamber with a velocity of 4 rpm in order to achieve the same coating for all samples in the bell jar. A silver layer with a mass thickness of 5 nm was deposited at a rate of 0.006 nm/s at a pressure below 10-3 Pa. (b) SilVer Films oVer Nanoparticles (AgFON). Glass slides were cleaned in piranha solution (1:1 mixture of 30% H2O2 and concentrated H2SO4), rinsed with distilled water, dried, and spincoated with alumina particles from a 5% alumina suspension in water. The agglomerate-free alumina (SEPP03, Fa. Piepelow & Brand, D) had a grain size of 300 nm. For further details see ref 21. After drying, the slides were placed in the evaporation chamber. An adhesion layer of 2 nm of chromium was deposited at a rate of 0.03 nm/s, and a layer of 75 nm of silver was successively deposited at a rate of 0.2 nm/s using the same equipment and pressure as those used for producing metal island films and a sample rotation of 5 rpm. Adsorption of Test Substances. The SERS-active glass slides and fiber tips were immersed in a 10 mM solution of thiophenol (purity >99%, Aldrich) in ethanol for 10 min and finally rinsed with ethanol. Thiophenol was used as a reference chemical to test the SERS activity because it forms chemically bonded, stable monolayers on silver surfaces22 and exhibits a comparatively large Raman cross section. Results and Discussion Thiophenol SERS Spectra. The adsorption of thiophenol was completed after 10 min, as indicated by a constant SERS

J. Phys. Chem. B, Vol. 105, No. 27, 2001 6331

Figure 1. SERS spectrum of thiophenol at a silver island film substrate. The marked area was taken as intensity of the band at 1023 cm-1.

Figure 2. SERS spectra of thiophenol SERS bands for different laser powers. For high laser powers, a new unassigned feature appeared at 960 cm-1.

intensity after this time. Therefore, the formation of monolayers with a homogeneous coverage of the substrates could be assumed, and SERS intensities of the prepared substrates could be used as a measure of the SERS enhancement. Thiophenol shows three characteristic, strong bands at 1001, 1023, and 1074 cm-1 (Figure 1). Therefore, Raman spectra were mostly recorded in the shown spectral window centered at 1000 cm-1. The baseline-corrected area of the band at 1023 cm-1 (marked in Figure 1) was used as a measure of the SERS enhancement of the substrates. Figure 2 shows that the SERS spectra of thiophenol-coated AgFON substrates changed at laser powers above 10 mW: an additional band appeared at 960 cm-1. This band disappeared when the laser power was again reduced to 0.9 mW. It showed up for all measurements at AgFON substrates and was also observed for two out of six investigated island film samples. Maybe a new, unstable surface species was formed at elevated temperatures. However, chemically reversible formation of such a species is questionable. The excitation of additional Raman modes at high laser power may be an alternate explanation of the appearance of the 960 cm-1 band. Unfortunately, the nature of this band could not be completely analyzed using the present experimental data. Power Dependence of SERS Intensities from Metal Films on Slides. Raman spectra of thiophenol on AgIF and AgFON substrates were recorded with laser powers increasing from 0.9 to 37.0 mW at the sample. The power variation was achieved by rotating the circular, linear wedge, neutral density filter. At higher laser powers, the SERS enhancement of the substrates changed with irradiation time. To determine the power dependence of the enhancement in equilibrium, irradiation at the given

6332 J. Phys. Chem. B, Vol. 105, No. 27, 2001

Figure 3. Laser power dependencies of thiophenol SERS band intensities measured at a AgFON surface (9) and at a silver island film (2). Lines serve as a guide for the eye only.

Viets and Hill properties are quite important in the electromagnetic enhancement mechanism. Therefore, it can be concluded that laser powers of a few milliwatts cause changes of the metal layer leading to a reduction of SERS enhancement. The high SERS intensities at the higher laser powers could originate from the rim of the laser focus, whereas the very center of the focus area could exhibit a widely reduced SERS enhancement at these power levels. For laser powers above 2.5 mW, the SERS intensity depended not only on the coverage with analyte molecules and on the type of SERS substrate, but also on the degree of damage caused by the present or previous laser illumination. Therefore, the laser-induced changes in the focus area can strongly affect quantitative measurements. Nowadays, many SERS measurements are carried out using microscopes with 50× or 100× magnifying objectives, because they are suited for the analysis of single SERS-active particles and because they offer large collection angles. The diameter of the laser focus is clearly smaller at higher magnifications, and consequently the laser intensity is much higher for the same power. Therefore, a clearly smaller laser power range would be useful for quantitative SERS analyses with strongly magnifying microscope objectives. The intensity in the laser focus can be estimated as follows: Assuming diffraction-limited focusing, the focal intensity distribution for a circular aperture is determined by the Airy function23

I(r) )

Figure 4. Laser power dependencies of thiophenol SERS band intensities measured at a AgFON surface (9) and at a silver island film (2). For both curves, the initial range is fitted by linear dependencies.

powers was applied for 15 min when no further changes were observable before recording the spectra used for calculating the figures. This approach was suited for estimating the useful laser powers for practical applications, where in general a long-term irradiation was required. Figure 3 shows the power dependence of the intensity thiophenol band at 1023 cm-1 for both types of substrates. Both types of substrates showed a nearly asymptotic power dependence of the SERS band intensities. Figure 4 demonstrates that the expected proportionality between SERS band intensity and laser power could only be observed at comparatively low powers up to approximately 2.5 mW for both types of substrates. For higher laser powers, the SERS intensities from both types of substrates still increased with the applied power, but this response was weaker than that in the range of proportionality and further weakened toward higher powers. Finally, any power increase above 10 mW gave only a minor increase in the SERS intensities for both types of substrates. The loss of proportionality between SERS intensity and laser power may principally be related to a reduction of the SERS enhancement by the metal layer or desorption of analyte molecules. Thiophenol is covalently bonded to the silver surface, and this strong bond is probably not broken at moderately elevated temperatures. Moreover, measurements of transmission and anti-Stokes/Stokes ratios presented below give clear indications for substantial changes of the optical properties of the metal layers at moderate temperatures and laser powers. These optical

[

2J1

]

(2πRλsin φ)

2πR sin φ λ

2

I0

(1)

where φ is the angle of diffraction, R is the radius of the circular aperture, λ represents the laser wavelength, and J1 is the first Bessel function. I0 is the intensity in the center of the diffraction pattern given by

I0 )

πR2P λ2f 2

(2)

where P is the laser power incident upon the aperture and f is the focal length of the microscope objective. This distribution results in a diffraction pattern with one bright central disk that is surrounded by concentric rings. The radius r0 of this central disk (defined by the middle of the inner dark ring) is given by

r0 ) sin φ0 f ) 0.61

λf24 R

(3)

where φ0 is the smallest zero point angle of eq 1. About 83.8% of the laser light is concentrated within the central disk of the diffraction pattern.23 For the experimental parameters used here (f ) 18 mm for the microscope objective, R ) 0.95 mm, and λ ) 702 nm), the radius of the central disk turns out to be r0 ) 8.1 µm and the intensity in the center of the disk is I0 ) 1.8 kW/cm2 at a laser power of 1 mW. The average intensity across the central disk is Ia ) 0.4 kW/cm2 at the same laser power. On the basis of the results with the focusing used, the maximum allowable laser intensities for damage-free SERS measurements can be estimated to be around 4.5 kW/cm2 in the center of the focus. For 50× or 100× magnifying objectives, this intensity is already reached at clearly lower laser powers (about 0.1 or 0.025 mW) because the intensity is roughly inversely proportional to the square of the focus radius.

Laser Power Effects in SERS Spectroscopy

J. Phys. Chem. B, Vol. 105, No. 27, 2001 6333

Figure 5. Thiophenol SERS band intensity recorded through a fiberoptic sensor with an island film coated tip in air (2) and in ethanol (9).

However, somewhat higher powers may be allowable for such objectives because the smaller focal spots are more efficiently cooled by thermal conduction into the support. It should be noted that the estimations are based on the assumption of perfect focusing. The real maximum allowable intensities may therefore be somewhat lower. Laser Power Effects for SERS Fiber Sensors. At slides, the laser power is not evenly distributed across the focus but is concentrated in its center according to the Airy distribution (eq 1). Therefore, the results from slides cannot be simply transferred to fiber-optic SERS sensors,16 where the laser power is assumed to be more evenly distributed across the tip’s surface. The investigated fiber-optic SERS sensors had a core diameter of d ) 200 µm. Thus the average intensity at the fiber tips was

IF )

4E ) 3.2 W/cm2 2 πd

(4)

at a laser power of 1 mW. This intensity is about 560 times smaller than that in the center of the microscope focus mentioned above. Figure 5 shows the power dependence of the SERS intensity from an island film coated fiber tip in air and in ethanol. For fiber tips in air, the SERS intensity was proportional to the laser power up to about 17 mW. At larger laser powers, the slope of the SERS intensity was lowered. When the island film coated fiber tip was immersed in ethanol during the power dependence measurements, the SERS intensities were somewhat lower than in air. The proportionality range was extended to about 20 mW, and the deviation from proportionality was weaker up to the maximum available power of 37 mW (Figure 5). On one hand, the extended range of proportionality may be related to the cooling of the tip’s surface by the surrounding liquid. On the other hand, the lowered SERS intensities in the proportionality range might indicate a reduced enhancement of electromagnetic fields at the metal/ethanol interface which may reduce photophysical or -chemical degradation effects. Similar observations were made for the immersion of differently coated SERS-active fiber tips in water.25 In comparison to the results for island films on slides, the proportionality range on the power scale was about 6 times larger for fiber sensors with the same coating. This was obviously caused by the smaller laser intensities at the fiber tips. Calculating the maximum allowable intensity for damagefree SERS at fiber sensors according to eq 4, a value of 54

Figure 6. Transmission of laser light through a silver island film (9) and for a AgFON substrate (b). For better illustration, the data for AgFON were multiplied by a factor of 10. The lines serve as a guide for the eye only.

W/cm2 is obtained which is about 80 times smaller than that for the center of the focused beam used at the SERS-active slides. This may be related to the more efficient heat dissipation from a small spot in comparison to the larger fiber tip surface. Furthermore, damage of the island film in the very center of the laser focus might not lead to an observable deviation from proportionality, because a considerable portion of the total SERS intensity arises from outer regions of the focal area. Finally, the maximum intensity in the focus may have been somewhat overestimated by assuming perfect focusing. Irreversible Decrease of SERS Enhancement. During measurements of the laser power dependency for SERS-active slides, the reversibility was checked occasionally by repeating low power measurements after increasing the laser power to higher values. Laser powers above a certain level irreversibly reduced the SERS activity in the irradiated area of the samples. This happened in the same power range where the proportionality between laser power and SERS intensity (Figure 4) was lost. Although the substrates were irreversibly damaged at larger laser powers, the SERS activity was not lost completely. For example, the SERS intensity from a AgFON substrate at 0.9 mW was reduced by a factor of 2.5 after illumination with a power of 37 mW. For island films, the SERS intensities were reduced by a factor of 4.5 by the same treatment. This irreversible loss of SERS activity at higher laser powers confirms the previous discussion on the power dependence of SERS intensities. Transmission of SERS Substrates. The power dependence of the transmission of AgFON and AgIF substrates (Figure 6) was recorded during the power dependence measurements of SERS intensities (Figures 3 and 4). Up to a power of 7 mW, the transmission of the AgFON substrate was constant at a value of about 0.7%. At higher laser powers, the transmission clearly increased and approached 7% above 30 mW. The transmission curve for a AgIF substrate is somewhat different: starting from 32% for the lowest laser powers, the transmission slowly increases up to 45% for laser powers of about 6 mW. Above 6 mW, the transmission curve shows a steep increase up to 65% at about 10 mW, and this increase is followed by a nearly asymptotic behavior at even larger powers. The maximum transmission of 70% is already reached at about 15 mW. These results show that the silver island films are more delicate than the AgFON substrates: Even for low intensities, the transmission of the island film slowly increased with the laser power, while the AgFON substrate initially showed a

6334 J. Phys. Chem. B, Vol. 105, No. 27, 2001

Figure 7. UV-vis absorbance spectra of a thiophenol-coated silver island film after 20 min of heating to different temperatures.

constant transmission. Therefore, it is obvious that the optical properties of the island films were already slightly changed at lower laser powers than those suggested by the SERS measurements (Figure 4). The sharp increase in the island film’s transmission was at lower laser powers and was steeper than that observed at AgFON substrates. After this sharp increase, the island film reached its maximum transmission at lower laser powers than the AgFON substrate. Island film structure and herewith its optical absorption and SERS activity are known to depend on temperature during its formation process.26 These properties can also be permanently changed by tempering after the evaporation of the metal. The tempering results in a blue shift of the absorption band, narrowing of its bandwidth, and reduction of the optical density.17,26 It should also be noted that chemical coating of island films results in slightly different absorption spectra in comparison with those for bare island films.27,28 UV-vis absorption measurements for the investigated island films showed a red shift of 70 nm of the absorption band after coating the film with thiophenol. Temperature-induced changes of UV-vis absorption spectra of thiophenol coated silver island films are shown in Figure 7. A slight broadening of the absorption band around 650 nm, an increase of its absorbance, and a red shift of its peak position were already observed at 353 K and reach their maximum values at 393 K. These continuous changes may be related to the annealing of defects, crystallization, or changes of the shape of the metal islands. At even higher temperatures, the band showed a blue shift, and its absorbance is dramatically reduced to a complete disappearance of the band at 200°. A new band around 410 nm was formed at temperatures above 433 K. The isosbestic point observed for the spectra measured after treatments between 433 and 473 K indicates that a conversion between two types of metal film structures with different absorption spectra took place in this temperature range. Figure 8 shows the temperature dependence of the absorbance of the island film for the laser wavelength (702 nm) as well as for the wavelengths corresponding to Stokes (755 nm) and antiStokes (656 nm) shifts by 1000 cm-1 from the laser wavelength. At room temperature, the absorbance at the anti-Stokes shifted wavelength was somewhat larger than that at the Stokes shifted wavelength. Such a large absorbance causes a somewhat stronger SERS enhancement of the anti-Stokes bands.29 It should be noted, however, that the SERS enhancement is also proportional to ||2, which strongly increases with the wavelength in

Viets and Hill

Figure 8. Temperature dependence of the absorbance of a thiophenolcoated island film for the wavelengths used in Stokes and anti-Stokes SERS measurements. The absorbance was measured at room temperature after heating the sample to the given temperature.

the considered range.29 Therefore, Stokes bands may be more strongly enhanced than the corresponding anti-Stokes bands. After a slight increase at 393 K, the absorbance at the wavelengths relevant in SERS measurements rapidly decreased at temperatures above 413 K. Therefore, heating of the island films to these moderately increased temperatures should result in a clear decrease of the SERS enhancement with the used wavelength. Comparing laser transmission for different powers (Figure 6) and absorbance at the laser wavelength after tempering (Figure 8), it is evident that laser powers as low as 6-10 mW are sufficient to cause increases in transmission as they are found after tempering at 413 K-473 K. Anti-Stokes/Stokes Ratios. The large SERS enhancement of the investigated substrates also permitted the measurement of anti-Stokes SERS. The ratio of anti-Stokes and Stokes Raman intensities is known to be a measure of the occupation of vibrationally excited states and thus of the temperature of the molecules. The anti-Stokes/Stokes ratio of SERS band intensities also depends on the vibrational excitation of the sample and thus on its temperature. However, it is somewhat more complicated to obtain the exact temperature from the SERS intensities, because their ratio is also determined by the wavelength-dependent SERS enhancement.30 Nevertheless, temperature changes can be monitored by the ratio of anti-Stokes and Stokes SERS bands as long as the wavelength-dependency of the SERS enhancement remains unchanged. The anti-Stokes and Stokes Raman intensities from island film substrates were measured according to their dependence on laser power. The ratio of both intensities was normalized according to the sensitivity curve of the spectrometer with the CCD detector and then used to calculate apparent temperatures T of the SERS substrates:

T)

-hc∆ν k ln(V(λAS/λSt)4)

(5)

V is the normalized anti-Stokes/Stokes intensity ratio, ∆ν represents the Raman shift, and λAS and λSt are the wavelengths at the anti-Stokes and Stokes band positions. The constants in the formula are the Planck constant h, the velocity of light c, and the Boltzmann constant k. The intensities of the three thiophenol bands at 1001 cm-1/ -1001 cm-1, 1023 cm-1/-1023 cm-1, and 1074 cm-1/-1074 cm-1 were used for these calculations. Figures 9 and 10 show the dependence of the calculated temperatures for AgIF and AgFON substrates on the laser power.

Laser Power Effects in SERS Spectroscopy

Figure 9. Calculated temperatures at a thiophenol-coated island film with dependence on the laser power. Temperatures were calculated from the Stokes and anti-Stokes SERS band intensities at 1001 cm-1 (9), 1023 cm-1 (b), and 1074 cm-1 (2). The line serves a guide for the eye only.

Figure 10. Temperatures of a thiophenol-coated AgFON surface for different excitation laser powers. The values were calculated from the Stokes and anti-Stokes SERS band intensities at 1001 cm-1 (9), 1023 cm-1 (b), and 1074 cm-1 (2). The line serves a guide for the eye only.

For island films (Figure 9), the anti-Stokes/Stokes intensity ratio indicates a temperature of approximately 290 K for the lowest laser power. This value is in good agreement with room temperature, which is a bit unexpected since the wavelength dependence of the SERS enhancement was completely disregarded in the calculation mentioned above. For laser powers up to 4 mW, the calculated temperature increases approximately linearly with the laser power and reaches a maximum temperature of 335 K. Between 4 and 6 mW, the calculated temperature suddenly drops to a value of about 240 K which remains nearly constant for even higher laser powers. The increase in the calculated temperatures at laser powers below 4 mW shows that such small laser powers were sufficient to heat the sample significantly. According to the UV-vis absorbance data (Figure 7), the calculated temperature increase of about 45 K would not result in such changes of the island films as they are perceptible from the absorbance spectra. However, the calculated temperature derived from the antiStokes/Stokes intensity ratios represents an average value for the whole focal region. In the very center of the focus, the laser intensity is about 4.5 times larger than the average intensity in the focal spot (see above). Therefore, clearly larger temperature

J. Phys. Chem. B, Vol. 105, No. 27, 2001 6335 increases may occur in the very center and can be sufficient to cause the changes observed during the tempering. The sudden drop of the calculated temperature (Figure 9) occurred at approximately the same laser powers as the pronounced increase in transmission (Figure 6) and where the SERS intensity ceases to be proportional to the laser power. Therefore, it can be assumed that fundamental changes of the film structure occur at this power level. The nonrealistic, low calculated temperatures at high laser powers indicate a severe influence of the wavelength dependence of the SERS enhancement on the anti-Stokes/Stokes intensity ratios. The structure of the island film was apparently modified so profoundly by the laser irradiation that the wavelength dependence of the SERS enhancement changed considerably. For AgFON substrates (Figure 10), the anti-Stokes/Stokes intensity ratio indicated a calculated temperature of about 280 K for the lowest laser power. This value is apparently somewhat low because of the mentioned reasons. For laser powers up to 2 mW, the temperature approximately showed the expected linear increase with the laser power up to about 320 K. For higher laser powers, the temperature increased at a lower rate up to 350 K at 12 mW. For lower powers, the temperature curve is similar to that of the island films. The temperatures in this power range do not apparently cause observable damage of the SERS-active structures. However, the calculated temperatures must again be considered as averaged values for the whole focal region, and the temperature in the very center should clearly be higher. The lower slope of the temperature curve above 2 mW indicates changes of the wavelength dependence of the SERS enhancement which are not as severe as those for the island films. The change in the wavelength dependence can more directly be observed from the dependence of the calculated temperature on the Raman shift of the considered band (Figure 10): For laser powers above 2 mW, the anti-Stokes/Stokes intensity ratios and thus the calculated temperatures increase with the Raman shift. This implies a higher SERS enhancement for shorter wavelengths. Temperature increases by some 10 K as they were observed for laser powers of a few milliwatts were not sufficient to alter the SERS enhancement of the substrates noticeably. However, it should be noted that even such temperature changes can seriously influence chemical equilibria, for example, the adsorption of analyte molecules at the surfaces. Therefore, temperature effects of laser illumination should be thoroughly investigated before applying SERS measurements in quantitative analyses. Conclusion An irreversible decrease of SERS enhancement was observed for laser powers above 2.5 mW in a focused laser beam on SERS substrates and above 20 mW at the tip of fiber-optic SERS sensors. The maximum allowable intensities for damage-free SERS measurements are estimated to be around 4.5 kW/cm2 for the focused laser beam and 54 W/cm2 for fiber-optic sensors. Anti-Stokes/Stokes ratios of SERS bands indicate laser heating of the substrates and changes of their optical properties at higher laser powers. Increased laser transmission and the temperatureinduced shift of the spectral absorption peak of island films also show that these changes occur at moderate powers and temperatures. Therefore, the effect of laser power on the SERS enhancement can be related to changes in optical properties of the SERS substrates at moderately elevated temperatures. The SERS intensity further increases underproportionally at higher

6336 J. Phys. Chem. B, Vol. 105, No. 27, 2001 laser powers. However, the observed decrease in enhancement should be considered in quantitative measurements. Acknowledgment. This project was financially supported by the Bundesministerium fu¨r Bildung und Forschung (BMBF), Germany (Grant 13N6886). References and Notes (1) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783. (2) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (3) Jeanmaire, D. J.; VanDuyne, R. P. J. Electroanal. Chem. 1977, 84, 1. (4) Otto, A.; Mrozek, I.; Grabhorn, H.; Akeman, W. J. Phys.: Condens. Matter 1992, 4, 1143. (5) Hildebrandt, P.; Stockburger, M. J. Phys. Chem. 1984, 88, 5935. (6) Pettinger, B.; Wetzel, H. In Surface-Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum Press: New York, 1982; pp 293-314. (7) Goudonnet, J. P.; Begun, G. M.; Arakawa, E. T. Chem. Phys. Lett. 1982, 92, 197. (8) Bergman, J. G.; Chemla, D. S.; Liao, P. F.; Glass, A. M.; Pinczuk, A.; Hart, R. M.; Olson, D. H. Opt. Lett. 1981, 6, 33. (9) Liao, P. F. In Surface-Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum Press: New York, 1982; pp 379-390. (10) Kahl, M.; Voges, E.; Kostrewa, S.; Viets, C.; Hill, W. Sens. Actuators, B 1998, 51, 285. (11) Weaver, M. J.; Zou, S.; Chan, H. Y. H. Anal. Chem. 2000, 72, 38A.

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