Surface plasmon resonance enhanced optical absorption

REVIEW OF SCIENTIFIC INSTRUMENTS

VOLUME 72, NUMBER 7

JULY 2001

Surface plasmon resonance enhanced optical absorption spectroscopy for studying molecular adsorbates S. Wang, S. Boussaad, and N. J. Taoa) Department of Physics, Florida International University, Miami, Florida 33199

共Received 20 February 2001; accepted for publication 23 April 2001兲 We present an automated setup to measure the surface plasmon resonance 共SPR兲-enhanced optical absorption spectra of molecular adsorbates. The setup detects the reflectivity at the SPR resonance angle as a function of the incident light wavelength. Because the resonance angle is wavelength dependent, a feedback mechanism adjusts the photodetector position to follow the resonance angle when the wavelength varies. Both theoretical calculations and experimental measurements show a signal enhancement of up to ⬃40 times over the conventional absorption spectroscopy. The SPR-based absorption spectroscopy is surface specific because the optical field is localized near the surface at resonance. In addition, the SPR angular shift is simultaneously measured, which provides adsorbate coverage and adsorption kinetic information. We anticipate that with our automated system, the method could be used in the study of adsorbed molecules and in chemical and biosensor applications. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1379604兴

I. INTRODUCTION

tional approach, as shown by recent theoretical calculations.15,16 The previous SPR experiments were performed with either a rotational plate12 or photodiode array setup,16 which measure one data at a time. We present here a fully automated, SPR enhanced absorption spectroscopy setup. Our method is based on our previously reported high-resolution bicell detection method,17 which can detect the SPR angular shift precisely. We scan the wavelength and follow the consequent shift in the SPR resonance angle continuously with a feedback loop, which allows us to measure the reflectivity at the SPR resonance angle as a function of wavelength. We have studied zinc phthalocynine–tetrasulfonic acid tetrasodium salts and cytochrome c to demonstrate the capability of our setup. We found a 40 times enhancement in sensitivity over the conventional approach. The SPR-based spectroscopy is surface specific because the SPR field is localized near the surface. In addition, the conventional SPR ability to study adsorption kinetics and coverage is preserved in our setup.

The ability to measure optical absorption spectra of molecules adsorbed on a surface is important because it provides valuable information about the adsorbed species and it is relevant to biosensor applications.1 The conventional methods measure the change in the optical reflectivity 共or transmission兲 of the surface due to the optical absorption by the adsorbed molecules, which is difficult when the surface coverage is 1 monolayer or less.2,3 Improved methods,4 such as attenuated total reflectance 共ATR兲,5 wavelength modulated reflectance 共WMR兲,6 and electrochemical potential modulated reflectance 共EMR兲,7–11 have been developed. ATR spectroscopy uses multiple reflections to increase the sensitivity, which requires a large area of the sample surface. WMR improves the signal-to-noise-ratio via wavelength modulation. Neither ATR nor WMR is surface specific because the molecules in the bulk solution contribute to the change of the reflectance. EMR is surface specific, but limited to molecules with electrochemical or Stark properties. It has been shown that surface plasmon resonance 共SPR兲 can be used to obtain the spectroscopic properties of organic films on silver.12,13 By measuring the SPR angular shift, dip width, and dip intensity as a function of wavelength, Pockrand et al.12 extracted the real and imaginary parts of the dielectric constants of the films. Boussaad et al.13 measured the SPR angular shift versus wavelength using a high angular resolution SPR and obtained optical absorption spectra of redox proteins using the Kramers–Kronig relation. The method was later used to obtain Stark spectroscopy.14 Absorption spectroscopy can also be obtained by measuring the reflectivity at the SPR resonance angle in a fashion similar to the conventional absorption spectroscopy. The SPR-based approach has a large signal enhancement over the conven-

II. PRINCIPLE

The surface absorption spectroscopy is based on SPR, a technique that has emerged as a powerful technique for a variety of chemical and biological sensor applications.18–25 Surface plasmons are collective oscillations of free electrons in a metallic film. The most widely used SPR method is the so-called Kretschmann configuration,26,27 in which a p-polarized light is incident through a prism upon the metal film 共Fig. 1兲. When the angle of the incident light reaches an appropriate value ( ␪ R ), the reflection decreases to a minimum, corresponding to the resonance of surface plasmons. ␪ R is extremely sensitive to molecules on or near the metal film, which has been widely used to determine the coverage and average thickness of adsorbed molecular films. Most previous works determine ␪ R at one or two28 wavelengths, which do not provide direct information about the

a兲

Author to whom correspondence should be addressed; electronic mail: [email protected]

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© 2001 American Institute of Physics

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Rev. Sci. Instrum., Vol. 72, No. 7, July 2001

Wang, Boussaad, and Tao

FIG. 1. Schematic of the multiwavelength SPR enhanced absorption spectroscopy.

electronic states and, thus the identity of the probed molecules. By measuring ␪ R as a function of the wavelength,23,24 the electronic states of the adsorbed molecules can be obtained.12 When tuning the wavelength of the incident light to the absorption band, the resonance angle oscillates because the index of refraction of the molecules oscillates according to the Kramers–Kronig relation. By accurately determining the angular oscillation with a high-resolution SPR setup,17 absorption spectroscopy of a monolayer of redox protein was obtained.13 In addition to angular oscillation, tuning the wavelength to the absorption band also changes the reflectivity at the resonance, as shown by both theoretical calculations13,16 and experimental measurements 共Fig. 2兲.12,16 This change is due

to two effects. The first one is direct absorption of light by the adsorbed molecules, which always decreases the reflectivity. Because the SPR intensifies the optical field near the surface, the reflectivity decrease is much greater than that of a conventional reflectance measurement. The second effect is that the absorption of light by molecules perturbs the resonance condition and causes an increase in the reflectivity. So the net reflectivity change is either positive or negative, depending on which effect dominates, which is controlled by the thickness of the metal film. In any event, one can obtain the absorption spectrum by measuring R( ␪ R ) versus wavelength. We note that this method measures a change of small quantity 共reflectivity at resonance兲, which can be done more accurately than measuring the same change on top of a large quantity 共reflectivity in conventional absorption spectroscopy兲. Therefore, this method should be more sensitive than the conventional approaches even without the SPR enhancement. III. MATERIALS AND METHODS A. Theoretical simulation

FIG. 2. SPR reflectivity at a given wavelength of incident light upon molecular adsorption. If the wavelength lies outside of the absorption bands of the molecule, the position of the SPR dip shifts to a higher angle but the reflectivity of the SPR dip changes little. However, if the wavelength falls into an absorption band, both the position and reflectivity change. The position change is proportional to the molecular coverage, and the reflectivity change provides absorption spectroscopy.

We calculated the SPR reflectivity as a function of wavelength using the Fresnel optics. The calculation included four phases: a BK7 prism, silver film, molecular layer, and buffer solution 共water兲. The refractive indexes of the BK7 prism as a function of wavelength used in the calculation are determined with the equations provided by the manufacturer 共Melles Griot兲. The refractive indices of silver and water are obtained from the CRC Handbook. For the adsorbed molecular layer, we assumed the real part of the refractive to be 1.5, and extracted the imaginary part n i according to n i (␭)⫽␭ ⑀ (␭)/2␲ , where ␭ is the wavelength of the incident light. ⑀共␭兲 in the above relation is the molar extinction coefficient, which was determined experimentally from the solution-phase absorption spectra. The calculated changes in the SPR reflectivity (⌬R( ␪ R )) due to the optical absorption of the sample molecules is approximately propor-



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tional to the absorption spectrum, ⌬R ␪ (␭)⫽c⌬ ⑀ (␭). The constant in the expression represents the SPR enhancement factor. Its value depends on the strength of the surface plasmon, and therefore depends on the thickness of the metal film and the distance between the sample and metal surface. Quantitative study29 shows that SPR will decrease with the distance of sample from the surface distance exponentially, and the SPR enhancement factor will behave in a similar fashion. B. Chemicals

Sodium perchlorate, sodium phosphate, and 11mercaptoundecanoic acid 共HS-共CH2兲10 –COOH兲 were purchased from Sigma-Aldrich. Ascorbic acid sodium salt was purchased from Fluka. Zinc phthalocynine-tetrasulfonic acid tetrasodium salts 共Znph兲 were from Porphyrin Products, Inc. 共Logan, Utah兲. All chemicals were used without further purification. Horse heart cytochrome c from Sigma was purified by cation exchange chromatography using a Whatman CM-52 column as described in the literature.30 C. Instrumentation

Figure 1 shows the SPR absorption spectroscopy setup. A BK7 plano-cylindrical lens 共Melles Griot兲 was used as a prism. In the prism, a BK7 glass slide, coated with a silver film by a sputtering coater, was placed with an index matching fluid. On the silver film, a Teflon sample cell was mounted to hold the sample solution. White light from a 150 W xenon lamp 共Oriel兲 was sent to a monochromator, from which a beam of monochromatic light with a bandwidth of ⬃0.5 nm was collimated. After passing through a polarizer and a beam splitter, the collimated beam was focused through the prism onto the silver film. A portion of the collimated beam reflected from the beam splitter was detected by a photodiode 共reference detector兲. Light reflected from the silver film was detected with a bicell photodiode detector 共Hamamatsu Corp., model S2721-02兲, which was mounted on a precision translation stage driven by a stepping motor. At each wavelength, the bicell detector follows the SPR dip position to maintain a zero differential signal 共A–B兲 from the bicell. The SPR reflectivity R ␪ (␭) was obtained from the ratio between the total signal of the bicell detector 共A⫹B兲 and the reference signal. Compared with a laser, after passing the monochromator, the white light source is much weaker, and it contains some 60 Hz noise from the power source. In order to improve the signal-to-noise ratio, the intensity of the incident light was modulated at 300 Hz with an optical chopper. The sample and the reference signals were detected with lock-in amplifiers 共Model SR830, Stanford Research Systems, and Model 5110, Princeton Applied Research兲. R ␪ (␭) versus ␭ was measured by scanning the wavelength with the computer-controlled monochromator. For each experiment, a (␭) vs ␭兴 was first scanned with background spectrum 关R buffer ␪ the metal film exposed in the buffer, followed by the sample scan 关R ␪sample(␭) vs ␭兴 after the sample molecules were adsorbed onto the film 共Fig. 3兲. The absorption spectrum of the

FIG. 3. SPR reflectivity as a function of wavelength, with 共solid line兲 and without 共dash line兲 the ZnPh adsorbed on a 40 nm thick silver film. The difference reflects the absorption spectrum of the adsorbed ZnPh. The step in the wavelength scan was 0.125 nm. The time constant for the lock-in amplifiers was set to 300 ms, and the final data was smoothed by the adjacent averaging method over 5 nm interval.

adsorbed molecules was obtained by subtracting the background from the sample data 关⌬R ␪ (␭)⫽R sample (␭) ␪ ⫺R ␪buffer(␭)]. The integrated control software is programmed with LABVIEW for Windows version 5.0 共National Instruments兲 and running on a Pentium computer with WINDOWS 98. The program controls the wavelength of the monochromator and the position of the differential detector and collects all the data.

D. Solution-phase absorption spectroscopy

Solution-phase absorption spectra were measured using Shimadzu UV-2101PC ultraviolet-visible scanning spectrophotometer. The spectra of both oxidized and reduced cytochrome c were determined. Cytochrome c is reduced by adding 100 mM ascorbic acid sodium salt to a 0.2 mM protein solution.

E. Electrochemistry

For the cytochrome c experiments, the silver film was precoated with a self-assembled monolayer of HS–共CH2) 10 – COOH, which allowed cytochrome c to absorb onto the surface with no sign of denaturation. In order to control the redox state of the protein, the electrochemical potential of the metal film was controlled with Pt and Ag wires as counter- and quasireference electrodes, respectively. The quasireference electrode was calibrated and the potential is quoted here with respect to Ag/AgCl 共3 M KCl兲 reference electrode. The adsorbed cytochrome c was kept in the reduced and oxidized states by holding the electrochemical potential of the silver film at ⫺100 and ⫹240 mV, respectively. The potential was controlled using a Bi-Potentiostat from Pine Instrument Company 共model AFRDE5兲.


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IV. RESULTS

We have chosen two different molecules: an organic dye molecule 共ZnPh兲 and a redox protein 共cytochrome c兲, to demonstrate our automated SPR enhanced absorption spectroscopy. These molecules have pronounced absorption peaks in the visible range. In the case of cytochrome c, the absorption depends on its redox state. In each case, we calculated the SPR absorption spectra with a different thickness of silver films and measured the spectra experimentally under conditions using the setup described above. A. ZnPh „Zinc phthalocynine-tetrasulfonic acid tetrasodium salts…

Figure 4共a兲 shows the absorption spectrum of ZnPh in aqueous solution, which shows a strong absorption band near 635 nm and a smaller shoulder near 675 nm. Using the solution-phase absorption spectrum as an input, we calculated the SPR absorption spectra for 40, 50, and 55 nm thick silver films, shown in Fig. 4共b兲. The SPR absorption spectra are similar in shape to the solution-phase absorption spectrum, but the SPR reflectivity is either positive or negative, depending on the thickness of the silver film. The reflectivity at the absorption peak 共635 nm兲 versus the film thickness 关inset of Fig. 4共b兲兴 shows that the SPR reflectivity crosses zero around 45 nm. We note that 45 nm is the thickness that gives the best SPR resonance 共nearly 100% absorption of light by surface plasmons兲. Above ⬃45 nm, the SPR reflectivity change is positive and reaches a maximum near 60 nm. The positive change in the SPR reflectivity corresponds to a less pronounced surface plasmon resonance, due to the change in the resonance condition by ZnPh. Below ⬃45 nm, the SPR reflectivity changes to the negative direction and reaches its negative maximum near 30 nm. The decrease in the reflectivity is due to the absorption of light by ZnPh, which is more dominant than the effect of changing the resonance condition when the silver film is very thin. The SPR reflectivity change due to the absorption of light by adsorbed molecules is ⬃0.04 in the case of a ⬃30 nm silver film, which is about 40 times greater than the corresponding reflectivity change in a conventional surface reflectivity measurement. The enhancement factor is about 25 times for a 60 nm silver film. For higher sensitivity, ⬃30 or ⬃60 nm are the best. For films thinner than 30 nm or thicker than 60 nm, the net change in the reflectivity is still much greater than the conventional absorption spectroscopy, but less desirable because the SPR dip becomes less pronounced. Our simulations are in good agreement with the calculations reported by Kano et al.13 and Kolomenskii et al.,16 which show two maximum enhancements at two different thicknesses of the metal film. In addition to the enhancement, SPR absorption spectroscopy has an advantage in the dynamic range of the photodetectors. The conventional approach measures light reflected from 共or transmitted through兲 a surface with adsorbed molecules. The intensity change in the reflected 共transmitted light due to optical absorption of the molecules is usually very small compared to the total intensity. In order to detect the small change on top of the intense reflection 共transmission兲, the photodetector and the associated electronics must

FIG. 4. 共a兲 Solution-phase absorption spectrum of ZnPh in 0.1 M NaClO4. 共b兲 Theoretical SPR absorption spectra of ZnPh adsorbed on silver films of different thickness. 共c兲 The corresponding experimental SPR absorption spectra obtained under the similar conditions as in 共b兲.

have a large dynamic range. In contrast, SPR absorption spectroscopy measures the intensity change at the SPR dip, which is much weaker than the direct reflection. Therefore, for a given dynamic range, SPR absorption spectroscopy is



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more sensitive than the conventional approach even without the enhancement. In order to confirm the model calculations, we measured the SPR absorption spectra experimentally for silver films at various thicknesses 共40, 50, and 55 nm兲. We started each experiment by measuring the SPR reflectivity in buffer 共0.1 M NaClO4) as a function of wavelength. We then added 10⫺4 M aqueous solution of ZnPh into the SPR cell and monitored the adsorption process by measuring the angular shift of the SPR dip. The angle increased initially and reached a plateau in ⬃30 min. The amount of shift is about 0.15°, corresponding to a monolayer coverage of the adsorbed molecules. Replacing the ZnPh solution with buffer caused no further change in the SPR angle, indicating that the adsorbed molecules were rather stable on the surface. These conclusions were confirmed by directly imaging the adsorbed molecules with scanning tunneling microscopy.24 After reaching a monolayer coverage, we then measured the SPR reflectivity as a function of the wavelength. We obtained the SPR absorption spectra by subtracting the buffer background, as shown in Fig. 4共c兲. The dependence of the SPR reflectivity on the silver film thickness is in excellent agreement with the theoretical calculations. The peak position and the overall shape of the experimental SPR absorption spectra are also compared to the corresponding theoretical calculations. However, the peaks are much broader than the calculated spectra. The broadening is much greater than the wavelength resolution of our spectrometer, which rules out the instrumentation resolution as a possible cause. The calculated spectra used in solution-phase spectrum as an input, so the deviation may come from the difference between molecules in bulk solution and on the silver surface. For example, the strong interaction between the ZnPh and the metal surface can alter the electronic states of the molecules and thus change the optical absorption spectra. B. Cytochrome c

Figure 5共a兲 shows the solution spectra of oxidized and reduced cytochrome c, as well as the difference spectrum 共reduced minus oxidized兲. The reduced form has a sharp peak at 550 nm and a small peak at 520 nm. In sharp contrast, the oxidized form has a broad peak around 530 nm. The difference spectrum clearly shows the 550 and 520 nm peaks of the reduced cytochrome c. We calculated the SPR absorption spectra of the reduced and oxidized cytochrome c and plotted the difference spectra at various silver film thickness in Fig. 5共b兲. Similar to ZnPh, the SPR reflectivity is either positive or negative depending on the silver film thickness. However, the reflectivity crosses from negative to positive around 37 nm, instead of 45 nm as found in the case of ZnPh 关insets of Figs. 4共b兲 and 5共b兲兴. This is because the absorption peaks of cytochrome c occur at shorter wavelengths 共520 and 550 nm兲 than that of ZnPh 共635 nm兲. The thickness that gives the optimal SPR resonance depends on the wavelength of the incident light. The optimal SPR resonance occurs at a thickness of ⬃45 nm for 635 nm incident light, but it shifts to ⬃37 nm for 550 nm. Similar to that of ZnPh, a large enhancement in the SPR

FIG. 5. 共a兲 Solution-phase absorption spectrum of cytochrome c in 10 mM phosphate buffer pH 7. 共b兲 Theoretical SPR absorption spectra of cytochrome c. For clarity, the difference spectra between the oxidized and reduced cytochrome c were shown. 共c兲 The corresponding experimental SPR absorption spectra, obtained under the similar conditions as in 共b兲.

absorption spectroscopy is also observed for cytochrome c. The maximum enhancement occurs at thickness of ⬃25 and ⬃45 nm, and the corresponding enhancement factors are 35 and 15 times, respectively, over the conventional absorption spectroscopy. We measured the SPR absorption spectra of cytochrome c under conditions similar to the theoretical simulations. For


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each experiment, we injected 0.2 mM cytochrome c solution into the sample cell, and monitored the SPR angular shift as a function of time. The maximum angular shift reached after ⬃30 min is about 0.5°, corresponding to a monolayer coverage. This coverage was confirmed independently by measuring the redox peak area in the cyclic voltammogram. We recorded the SPR absorption spectra by holding the adsorbed cytochrome c at the reduced and oxidized states with the electrochemical potential. The differences of the reduced and oxidized spectra are shown in Fig. 5共c兲. Again, the overall spectral shape and the dependence of the SPR reflectivity on the silver film thickness are in excellent agreement with the theoretical simulations. The experimental absorption spectra are also somewhat broader than the calculated spectra. It may be attributed to the heterogeneous distribution and difference in the local environment of the adsorbed cytochrome c. V. DISCUSSION

In addition to higher sensitivity than the conventional absorption spectroscopy, the SPR enhanced method is surface specific because the SPR field is localized within ⬃200 nm of the surface region. This feature is important if one wants to minimize interference from molecules present in the bulk solution. Electrochemical potential modulated spectroscopy also has excellent surface specificity, but requires molecules with electrochemical or elecrochromism properties. We have previously extracted information about the electronic states of adsorbed molecules by measuring SPR dip position versus wavelength. The result relies on the change in the real part of the index of refraction when tuning the incident light to the absorption band of the molecules.23,24 In contrast, the present SPR absorption spectroscopy measures the intensity of the SPR dip. This technique has two advantages. First, it can be easily compared with the corresponding solution-phase absorption spectroscopy. Second, the present setup can simultaneously measure the SPR dip intensity and angular position. The former is the absorption spectroscopy, and the latter provides information about coverage and thickness of the adsorbed molecules as in conventional SPR applications. A related spectroscopy technique is surface-plasmon enhanced fluorescence spectroscopy.31,32 An enhancement factor of 50 for silver film on the fluorescence emission intensity has been achieved.31 The effect is also due to the enhancement in the surface light intensity at the metal/ dielectric interface caused by the surface plasmon resonance.


ACKNOWLEDGMENTS

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