- Email: [email protected]
Long-range surface plasmons for high-resolution surface plasmon resonance sensors
Sensors and Actuators B 74 (2001) 145±151
Long-range surface plasmons for high-resolution surface plasmon resonance sensors G.G. Nenningera,*, P. TobisÏkab, J. Homolaa,1, S.S. Yeea a
Department of Electrical Engineering, University of Washington, P.O. Box 352500, Seattle, WA 98195, USA b Institute of Radio Engineering and Electronics, Chaberska 57, 18251 Prague, Czech Republic
Abstract We present the application of long-range surface plasmons to a wavelength-modulated surface plasmon resonance sensor. Theoretical design parameters and experimental data are presented for two sensor designs, using either magnesium ¯uoride or Te¯on AF-1600 as a dielectric buffer layer. The demonstrated sensitivity of the long-range surface plasmon resonance sensor in refractometric experiments is up to seven times higher than that of an equivalent conventional surface plasmon resonance (SPR) sensor, while the measured resolution is comparable. According to theoretical design calculations presented, further optimization of materials and layer thickness could reduce the resonance width while achieving even higher sensitivities, thereby creating a sensor with signi®cantly better resolution than conventional SPR sensors. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Long-range surface plasmon; Te¯on AF; Surface plasmon resonance; Optical sensors
1. Introduction Surface plasmon resonance (SPR) sensors are one of the foremost sensor types for direct, label-free observation of biomolecular interaction. For detection of biological agents in low concentrations, the sensor technology must provide high-resolution to allow for detection of small concentrations of bound molecules, where resolution is de®ned as the minimum change of the measured parameter that can be resolved. Typical refractive index resolutions on the order of 10ÿ6 RIU are possible using conventional SPR sensors [1]. In typical SPR sensor applications, light is used to excite a surface plasmon in an attenuated total re¯ection (ATR) con®guration. Light with a transverse magnetic (TM) polarization meeting the matching conditions of the surface plasma wave (SPW) is absorbed and creates a resonance absorption feature in the re¯ected angular or wavelength spectrum. For sensor applications, the refractive index change of a thin layer in contact with the metal surface of the sensor is monitored by measuring the spectral shift of the resonance dip. Although both angular and wavelength modulation have been used in SPR applications, we have used wavelength modulation throughout this paper. * Corresponding author. Tel.: 1-206-543-2178; fax: 1-206-543-3842. E-mail address: [email protected] (G.G. Nenninger). 1 On leave from Institute of Radio Engineering and Electronics, Chaberska 57, 18251 Prague, Czech Republic.
The resolution of an SPR sensor depends upon the accuracy to which the position of the resonance feature can be determined, and is ultimately limited by the system noise. To improve the resolution of the SPR sensor for a speci®c SPR con®guration, it is desirable to decrease the width of the resonance feature, thereby reducing the uncertainty of the position of the resonance. Another possible route is to increase the sensitivity of the sensor. For conventional SPR sensors, one method of achieving both of these effects is to substitute silver for gold as the SPR-active metal layer. However, the long-term stability of silver is poor in many environments, so that improved sensor performance cannot be maintained. Another method to decrease the resonance width and increase the sensitivity of the sensor is to use long-range surface plasmon resonance (LRSPR). A long-range surface plasmon (LRSP) consists of coupled surface plasma waves existing on opposite sides of a thin metal ®lm suspended between two dielectrics [2]. The LRSPR phenomenon has been observed experimentally[3]andseveral potential applicationshavebeendescribed. These applications include electrooptic modulators [4±6] and non-linear optics [7]. However, there have been few successful attempts to create a sensor based on LRSPR. Matsubara et al. presented experimental veri®cation of an anglemodulated version of an LRSPR-based sensor in 1990 [8]. In this paper, we consider the design and construction of wavelength-modulated LRSPR sensors with buffer layer
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 0 ) 0 0 7 2 4 - 3
146
G.G. Nenninger et al. / Sensors and Actuators B 74 (2001) 145±151
Fig. 1. Behavior of coupled surface plasma waves as metal thickness varies. Calculated for a wavelength of 800 nm; gold layer bound on both sides with dielectric of n 1:32. Both symmetric bound (sb) and antisymmetric bound (ab) modes are supported. Note the drop in attenuation for the symmetric bound (long-range) mode as the metal thickness decreases.
refractive indices in the vicinity of water, constructed using two different buffer layer materials: Te¯on AF-16001 (E.I. du Pont de Nemours and Company, Wilmington, Delaware, USA), and magnesium ¯uoride (MgF2). Te¯on AF-1600 has a refractive index (nd 1:31), which is less than water (nd 1:33), while MgF2 has a refractive index (nd 1:38) which is higher than water. The Te¯on-buffer LRSPR sensor would, therefore, be preferable for low molecular weight functionalization layers, while the MgF2-buffer LRSPR sensor is more appropriate for higher refractive index, multiple-layer or polymer functionalization methods. 2. LRSPR fundamentals If a thin metal layer, surrounded by dielectric and supporting a surface plasma wave (SPW) on both surfaces, is made thin enough, the two SPWs will couple and exhibit more complex behavior. Speci®cally, the effective refractive index of the single SPW splits into two bound eigenmodes as the metal thickness decreases, as illustrated in Fig. 1. These two eigenmodes have symmetric and antisymmetric bound magnetic ®eld pro®les, as seen in the ®eld pro®les of Fig. 2. These modes are characterized by low (symmetric) and high (antisymmetric) losses, and are referred to as ``long-range'' and ``short-range'' surface plasmons, a reference to their relative propagation lengths[9].Thelonger propagationlength of long-range surface plasmons reduces the width of the resonance feature. As an added bene®t, the LRSPR sensor may be designed to have a higher sensitivity than the equivalent conventional SPR sensor operating at the same wavelength. The theoretical plots of Fig. 3 illustrate the matching between the wave vector of the light incident through the prism and the wave vector of the long-range surface plasma wave (LRSPW), showing positive and negative sensitivities in different operating regions. The high sensitivity magnitude possible for the LRSPR sensor results from the low
angle between the dispersion curves for the LRSPW and the incident light. The LRSPW effective refractive index in Fig. 3 is the symmetric bound mode solution of a threelayer dispersion relationship, where the three layers are Te¯on AF-1600, 20 nm gold, and analyte (water, and water plus ethylene glycol to increase the refractive index by 0.002 RIU), so the effect of the prism has been ignored [10]. Such an approximation is valid if the active gold layer is well separated from the prism. The perturbing effect of the prism becomes more pronounced as the buffer layer becomes thinner, and may cause the resonant wavelength to shift or may prevent the LRSPW from being supported in certain wavelength ranges.
Fig. 2. Modulus of the complex tangential magnetic field profile for symmetric and antisymmetric bound surface plasma waves on a 25 nm thick gold layer (symmetric mode structure: SF14 prism, 300 nm Teflon AF-1600, 25 nm gold, n 1:32 analyte, 531 nm light, y 49:5 ; antisymmetric mode structure: SF14 prism, 175 nm Teflon AF-1600, 25 nm gold, n 1:32 analyte, 702 nm light, y 70 ).
G.G. Nenninger et al. / Sensors and Actuators B 74 (2001) 145±151
147
Fig. 4. Configuration of an LRSPR sensor using a removable sensor substrate.
Fig. 3. Illustration of wave vector matching for a long-range surface plasma wave (LRSPW) supported on a 20 nm thick gold layer suspended between a Teflon AF-1600 buffer and analyte: water (dotted line) or water 0:002 RIU (dashed line): (a) operation in a region of positive sensitivity; (b) operation in a region of negative sensitivity.
3. Sensor design To create an LRSPR sensor, the metal layer must be separated from the ATR prism by a dielectric buffer layer with a lower refractive index than the prism, as illustrated in Fig. 4. Part of the dif®culty in creating an LRSPR sensor is the selection of an appropriate buffer material. Ideally, the buffer±metal±analyte structure will be symmetrical, so that the buffer and analyte have the same refractive index. However, coupled plasmons may exist on asymmetric structures [10], which has been demonstrated experimentally [11]. Indeed, controlled asymmetry of the structure may increase the propagation length of the LRSP [12]. However, there is a limit to the amount of asymmetry in a structure supporting an LRSP, so the buffer material should have a refractive index similar to that of the intended analyte. For biosensors, the analyte is typically aqueous, with a thin layer of protein on the surface of the sensor (functionalization material plus bound target molecules), therefore, a buffer layer with a refractive index near water is required. The
refractive index of the buffer layer material is an important parameter for LRSPR sensor design. Since dielectric materials are in general not available in a continuous range of refractive indices, selection of the buffer layer refractive index may be limited. Once the buffer material is chosen, the primary design parameters are the thickness of the metal layer (typically gold) and the thickness of the buffer layer. The operating angle of the instrument may be varied in order to produce the best coupling between the light and the LRSPR, as indicated by a deep resonance with low TM re¯ectivity at the minimum of the resonance. Choosing a design analyte provides an operating point for the sensor. Given these parameters, design curves may be produced displaying the best resonance depth, the wavelength at which this resonance occurs, the width of the resonance, and the sensitivity of the sensor. Design curves for sensors operating in water are provided in Fig. 5 for MgF2-buffer, and in Fig. 6 for Te¯on AF-1600 buffer. For these calculations, dispersive refractive index models for the MgF2 and Te¯on-buffer layers were created using published data [13,14]. As illustrated in Figs. 5(d) and 6(d), both buffer materials exhibit a transition between positive and negative sensitivities, so that for sensors constructed with a thick gold layer and a thin buffer layer, the resonant wavelength shifts to longer wavelengths as the refractive index of the analyte increases. For sensors constructed of a thin gold layer and a thick buffer layer, the sensitivity is negative, so that the resonant wavelength shifts to shorter wavelengths with an increase in the refractive index of the analyte. Near the transition between these sensitivity regimes, the sensitivity is very low. For speci®c combinations of gold thickness and buffer thickness, the magnitude of the sensitivity can be extremely high, on the order of 1 105 nm RIUÿ1 in the case of the Te¯on-based LRSPR sensor. This ®gure compares with a sensitivity of 1:4 104 nm RIUÿ1 for a conventional prism-coupled wavelength-interrogation SPR sensor operating at 850 nm. Another characteristic is illustrated for both buffer materials in the resonance width curves of Figs. 5(c) and 6(c). Both sensor types show a broadening of the resonance feature over a range of gold thickness and buffer thickness.
148
G.G. Nenninger et al. / Sensors and Actuators B 74 (2001) 145±151
Fig. 5. Design curves as a function of metal and buffer thickness for an LRSPR sensor using an SF2 glass substrate and a MgF2-buffer layer, with water as the analyte. The operating angle was optimized at each point to achieve the greatest coupling into the resonance (operating wavelength 500±1000 nm): (a) resonance depth; (b) resonant wavelength; (c) resonance width (full width at half-minimum); (d) sensitivity.
The broadening of the resonance is caused by matching between the wave vector of the illuminating light and the wave vector of the LRSPW over a range of wavelengths. The exact nature and magnitude of this effect is dependent upon the relative dispersion characteristics of the sensor materials and the analyte. This effect was observed experimentally in the case of the Te¯on-based LRSPR sensor. 4. Sensor construction Rectangular SF2 glass substrates were used in the construction of the MgF2-based LRSPR sensor elements. These substrates were cleaned and then coated with successive layers of MgF2 and gold in a thermal vacuum deposition system. The thickness of each layer was monitored during deposition using a crystal deposition monitor.
Rectangular SF14 glass substrates were used in the construction of the Te¯on-based LRSPR sensor elements. These substrates were cleaned and then coated with a ¯uorosilane solution to promote adhesion of the Te¯on AF-1600 to the glass, as recommended by the Te¯on AF manufacturer. Following a baking step, the slides were spin-coated with a solution of 3% Te¯on AF-1600, diluted in FC-77 Fluorinert solvent (3M Chemicals, Saint Paul, Minnesota, USA). This dilute solution produced a smoother surface ®nish than the stock 6% Te¯on solution. The substrate was then baked in a series of steps, as recommended by the manufacturer, to drive off the solvent and provide some leveling of the surface. Te¯on thickness was estimated based on spin speed and comparison between theory and experiment for samples coated with the same thickness of Te¯on but a different thickness of gold. Gold was deposited on the Te¯on surface using a thermal vacuum deposition system. Gold thickness
G.G. Nenninger et al. / Sensors and Actuators B 74 (2001) 145±151
149
Fig. 6. Design curves as a function of metal and buffer thickness for an LRSPR sensor using an SF14 glass substrate and a Teflon AF-1600 buffer layer, with water as the analyte. The operating angle was optimized at each point to achieve the greatest coupling into the resonance (operating wavelength 500± 1000 nm): (a) resonance depth; (b) resonant wavelength; (c) resonance width (full width at half-minimum); (d) sensitivity.
was monitored during deposition using a crystal deposition monitor. 5. Experimental configuration The samples were index-matched to a glass prism of the same type of glass as the substrate, part of a wavelengthmodulated SPR instrument used for conventional SPR sensing. Light from a tungsten±halogen light source was carried via an optical ®ber to a collimator and aperture to illuminate the sensor, and the ATR re¯ected light was collected using a second collimator, then carried via an optical ®ber to an optical spectrometer. The operating range of the spectrometer was approximately 500±1000 nm. For the MgF2buffer LRSPR sensors, a low-noise MSC500 spectrometer from Carl Zeiss was used, while a higher-noise Ocean Optics S2000 spectrometer was used for testing of the Te¯on-buffer
sensors. The operating angle of the instrument was adjusted to provide the best coupling of light into the LRSPR sensor. The internal operating angle of the instrument was approximately 608 for the MgF2-buffer sensor and 508 for the Te¯on-buffer sensor. 6. Experimental results Spectral plots of re¯ectivity of an LRSPR sensor, using a 360 nm MgF2-buffer layer and a 40 nm gold layer, are shown in Fig. 7 for several refractive indices of analyte. Fig. 8 demonstrates the sensitivity of a sensor with identical layers during a refractometric experiment. The average sensitivity over the range of 1.332±1.334 RIU was 5:0 103 nm RIUÿ1, slightly higher than the equivalent conventional SPR sensor (wavelength modulation, prism coupled, using 50 nm gold) operating at the same wave-
150
G.G. Nenninger et al. / Sensors and Actuators B 74 (2001) 145±151
Fig. 7. Measured spectra for an LRSPR sensor consisting of a layered structure of SF2 glass, 360 nm MgF2, and 40 nm gold as the refractive index of the analyte was varied in steps from 1.333 to 1.342 RIU.
Fig. 9. Measured spectra for an LRSPR sensor consisting of a layered structure of SF14 glass, 500 nm Teflon AF-1600, and 30 nm gold. The refractive index of the water analyte was varied in 0.001 RIU steps from 1.334 to 1.337 RIU via the addition of ethylene glycol.
length. The measured resolution based on noise levels was 1:2 10ÿ3 nm, for an average refractive index resolution of 2:4 10ÿ7 RIU. Fig. 9 illustrates measured spectral plots of re¯ectivity for an LRSPR sensor using a 500 nm Te¯on layer and a 30 nm gold layer. The four curves show the effect of 0.001 RIU steps in the refractive index of the analyte, made through the addition of ethylene glycol to the water analyte. The average sensitivity of this sensor over a range of 1.334±1.337 RIU was 3:8 103 nm RIUÿ1, consistent with theoretical predictions. An equivalent conventional SPR sensor operating in the same wavelength range would have a theoretical sensitivity of 2:6 103 nm RIUÿ1. The measured resolution based on noise levels was 1:5 10ÿ2 nm, for an average refractive index resolution of 4:0 10ÿ6 RIU.
Another sensor, constructed using a 700 nm Te¯on-buffer layer and a 24 nm thick gold layer, showed a broader resonance feature but extremely high sensitivity of 3:0 104 nm RIUÿ1 over a range of 1.334±1.335 RIU, as illustrated in Fig. 10. Both the high sensitivity and broad resonance are predicted by the design curves discussed above. Since the equivalent conventional SPR sensor would have a sensitivity of 4:3 103 nm RIUÿ1 at this wavelength range, this LRSPR sensor represents a seven-fold increase in sensitivity. Due to the broad resonance curve, the measured resolution based on noise levels was 7:4 10ÿ2 , corresponding to an average refractive index resolution of 2:5 10ÿ6 RIU. Results of sensor characterizations for a number of LRSPR sensor elements are summarized in Table 1.
Fig. 8. Measured response during a refractometric experiment for an LRSPR sensor consisting of a layered structure of SF2 glass, 360 nm MgF2, and 40 nm gold. The refractive index of the water analyte was varied in steps from 1.3323 to 1.3340 RIU via the addition of ethylene glycol.
Fig. 10. Measured response during a refractometric experiment for an LRSPR sensor consisting of a layered structure of SF14 glass, 700 nm Teflon AF-1600, and 24 nm gold. The refractive index of the water analyte was varied in equal steps from 1.334 to 1.335 RIU via the addition of ethylene glycol.
G.G. Nenninger et al. / Sensors and Actuators B 74 (2001) 145±151
151
Table 1 Summary of experimental results for several LRSPR sensor elements, compared with the sensitivity of an equivalent conventional SPR sensor Buffer material
Buffer thickness (nm)
Gold thickness (nm)
Resonant wavelength n 1.334 (nm)
Sensitivity of long-range SPR sensor (nm RIUÿ1)
Teflon Teflon Teflon Teflon MgF2
500 700 800 700 360
30 30 30 24 40
650 720 725 700 693
3.8 7.2 9.2 3.0 5.0
a
103 103 103 104 103
Sensitivity of equivalent conventional SPR sensora (nm RIUÿ1) 2.6 5.2 5.2 4.3 3.8
103 103 103 103 103
Conventional SPR sensor: spectrally-modulated, SF14 prism (SF2 for MgF2-buffer), 50 nm gold at same wavelength.
7. Conclusions We have demonstrated the successful application of longrange surface plasmons to a wavelength-modulated sensor design. Both magnesium ¯uoride and Te¯on AF-1600 were successfully used as dielectric buffer materials. The demonstrated sensitivity was up to seven times higher than that of an equivalent conventional SPR sensor, while the resolution is comparable. According to theoretical design calculations presented, further optimization of materials and layer thickness could reduce the resonance width while achieving even higher sensitivities, thereby creating a sensor with signi®cantly better resolution than current conventional SPR sensors. Acknowledgements This research was supported by the United States Department of Defense under contract DAAD13-99-C-0032 and by the Grant Agency of the Czech Republic under contracts 102/99/0549 and 102/00/1536. References [1] J. Homola, S.S. Yee, G. Gauglitz, Surface plasmon resonance sensors: review, Sens. Actuators B 54 (1999) 3±15. [2] D. Sarid, Long-range surface-plasma waves in very thin metal films, Phys. Rev. Lett. 47 (1981) 1927±1930. [3] J.C. Quail, J.G. Rako, H.J. Simon, Long-range surface-plasmon modes in silver and aluminum films, Optics Lett. 8 (1983) 377±379. [4] C. Plumereau, A. Bouchoux, A. Cachard, Electrooptic light modulator using long-range surface plasmons, novel optoelectronic devices, Proc. SPIE 800 (1987) 79±83. [5] J.S. Schildkraut, Long-range surface plasmon electrooptic modulator, Appl. Optics 27 (1988) 4587±4590. [6] P.J. Kajenski, Tunable optical filter using long-range surface plasmons, Opt. Eng. 36 (1997) 1537±1541. [7] G.I. Stegeman, J.J. Burke, D.G. Hall, Non-linear optics of long-range surface plasmons, Appl. Phys. Lett. 41 (1982) 906±908. [8] K. Matsubara, S. Kawata, S. Minami, Multilayer system for a highprecision surface plasmon resonance sensor, Optics Lett. 15 (1990) 75±77. [9] J. CtrokyÂ, J. Homola, P.V. Lambeck, S. Musa, H.J.W.M. Hoekstra, R.D. Harris, J.S. Wilkinson, B. Usievich, N.M. Lyndin, Theory and modelling of optical waveguide sensors utilising surface plasmon resonance, Sens. Actuators B 54 (1999) 66±73.
[10] J.J. Burke, G.I. Stegeman, T. Tamir, Surface-polariton-like waves guided by thin, lossy metal films, Phys. Rev. B 33 (1986) 5186±5201. [11] N.M. Lyndin, I.F. Salakhutdinov, V.A. Sychugov, B.A. Usievich, F.A. Pudonin, O. Parriaux, Long-range surface plasmons in asymmetric layered metal±dielectric structures, Sens. Actuators B 54 (1999) 37±42. [12] L. Wendler, R. Haupt, An improved virtual mode theory of ATR experiments on surface polaritons, Phys. Stat. Sol. B 143 (1987) 131±148. [13] W.J. Tropf, T.J. Harris, M.E. Thomas, in: R.W. Waynant, M.N. Ediger (Eds.), Electro-optics Handbook, McGraw-Hill, New York, pp. 11.1±11.97 (Chapter 11). [14] J.H. Lowry, J.S. Mendlowitz, N.S. Subramanian, Optical characteristics of Teflon AF fluoroplastic materials, Opt. Eng. 31 (1992) 1982±1984.
Biographies Garet G. Nenninger received a B.S. degree in electrical engineering from the Massachusetts Institute of Technology, Cambridge, USA in 1991 and an M.S. degree in electrical engineering from the University of Washington, Seattle, USA in 1998. From 1991±1996, he worked as an instrumentation and control engineer for the Naval Sea Systems Command in Washington, DC. He is currently a Ph.D. degree candidate in the Department of Electrical Engineering at the University of Washington. His research interests include optical sensors, instrumentation, and sensor systems. Petr TobisÏka received his M.Sc. degree in physics from Charles University, Prague, Czech Republic in 1996. He is currently a graduate student at Charles University, Prague and works at the Institute of Radio Engineering and Electronics, Prague. His research interest is in surface plasmons and their application in (bio)chemical sensing. JirõÂ Homola received his diploma in physical electronics from the Czech Technical University, Prague, Czech Republic in 1988, and a Ph.D. degree in the field of guided-wave optics from the Academy of Sciences of the Czech Republic in 1993. Since 1993, he has been with the Institute of Radio Engineering and Electronics of the Academy of Sciences of the Czech Republic, Prague, working on fiber optic sensors and surface plasmon resonance-based sensing devices. Currently, he is a research assistant professor at the University of Washington, Seattle. His research interests are in guided-wave optics, and optical sensors and biosensors. Sinclair S. Yee received B.S., M.S., and Ph.D. degrees in electrical engineering from the University of California, Berkeley, in 1959, 1961, and 1965, respectively. In 1964, he joined the Lawrence Livermore Laboratory as a research engineer working on semiconductor devices. Since 1966, he has been on the faculty of the Department of Electrical Engineering at the University of Washington, where in 1974, he became a full professor. In 1972±1974, he was a NIH special research fellow, and since 1979, he has been an IEEE fellow. His main research interests are concerned with silicon-based and GaAs quantum well optical modulator devices, surface plasmon resonance, and microsensor arrays.
Long-range surface plasmons for high-resolution surface plasmon resonance sensors G.G. Nenningera,*, P. TobisÏkab, J. Homolaa,1, S.S. Yeea a
Department of Electrical Engineering, University of Washington, P.O. Box 352500, Seattle, WA 98195, USA b Institute of Radio Engineering and Electronics, Chaberska 57, 18251 Prague, Czech Republic
Abstract We present the application of long-range surface plasmons to a wavelength-modulated surface plasmon resonance sensor. Theoretical design parameters and experimental data are presented for two sensor designs, using either magnesium ¯uoride or Te¯on AF-1600 as a dielectric buffer layer. The demonstrated sensitivity of the long-range surface plasmon resonance sensor in refractometric experiments is up to seven times higher than that of an equivalent conventional surface plasmon resonance (SPR) sensor, while the measured resolution is comparable. According to theoretical design calculations presented, further optimization of materials and layer thickness could reduce the resonance width while achieving even higher sensitivities, thereby creating a sensor with signi®cantly better resolution than conventional SPR sensors. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Long-range surface plasmon; Te¯on AF; Surface plasmon resonance; Optical sensors
1. Introduction Surface plasmon resonance (SPR) sensors are one of the foremost sensor types for direct, label-free observation of biomolecular interaction. For detection of biological agents in low concentrations, the sensor technology must provide high-resolution to allow for detection of small concentrations of bound molecules, where resolution is de®ned as the minimum change of the measured parameter that can be resolved. Typical refractive index resolutions on the order of 10ÿ6 RIU are possible using conventional SPR sensors [1]. In typical SPR sensor applications, light is used to excite a surface plasmon in an attenuated total re¯ection (ATR) con®guration. Light with a transverse magnetic (TM) polarization meeting the matching conditions of the surface plasma wave (SPW) is absorbed and creates a resonance absorption feature in the re¯ected angular or wavelength spectrum. For sensor applications, the refractive index change of a thin layer in contact with the metal surface of the sensor is monitored by measuring the spectral shift of the resonance dip. Although both angular and wavelength modulation have been used in SPR applications, we have used wavelength modulation throughout this paper. * Corresponding author. Tel.: 1-206-543-2178; fax: 1-206-543-3842. E-mail address: [email protected] (G.G. Nenninger). 1 On leave from Institute of Radio Engineering and Electronics, Chaberska 57, 18251 Prague, Czech Republic.
The resolution of an SPR sensor depends upon the accuracy to which the position of the resonance feature can be determined, and is ultimately limited by the system noise. To improve the resolution of the SPR sensor for a speci®c SPR con®guration, it is desirable to decrease the width of the resonance feature, thereby reducing the uncertainty of the position of the resonance. Another possible route is to increase the sensitivity of the sensor. For conventional SPR sensors, one method of achieving both of these effects is to substitute silver for gold as the SPR-active metal layer. However, the long-term stability of silver is poor in many environments, so that improved sensor performance cannot be maintained. Another method to decrease the resonance width and increase the sensitivity of the sensor is to use long-range surface plasmon resonance (LRSPR). A long-range surface plasmon (LRSP) consists of coupled surface plasma waves existing on opposite sides of a thin metal ®lm suspended between two dielectrics [2]. The LRSPR phenomenon has been observed experimentally[3]andseveral potential applicationshavebeendescribed. These applications include electrooptic modulators [4±6] and non-linear optics [7]. However, there have been few successful attempts to create a sensor based on LRSPR. Matsubara et al. presented experimental veri®cation of an anglemodulated version of an LRSPR-based sensor in 1990 [8]. In this paper, we consider the design and construction of wavelength-modulated LRSPR sensors with buffer layer
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 0 ) 0 0 7 2 4 - 3
146
G.G. Nenninger et al. / Sensors and Actuators B 74 (2001) 145±151
Fig. 1. Behavior of coupled surface plasma waves as metal thickness varies. Calculated for a wavelength of 800 nm; gold layer bound on both sides with dielectric of n 1:32. Both symmetric bound (sb) and antisymmetric bound (ab) modes are supported. Note the drop in attenuation for the symmetric bound (long-range) mode as the metal thickness decreases.
refractive indices in the vicinity of water, constructed using two different buffer layer materials: Te¯on AF-16001 (E.I. du Pont de Nemours and Company, Wilmington, Delaware, USA), and magnesium ¯uoride (MgF2). Te¯on AF-1600 has a refractive index (nd 1:31), which is less than water (nd 1:33), while MgF2 has a refractive index (nd 1:38) which is higher than water. The Te¯on-buffer LRSPR sensor would, therefore, be preferable for low molecular weight functionalization layers, while the MgF2-buffer LRSPR sensor is more appropriate for higher refractive index, multiple-layer or polymer functionalization methods. 2. LRSPR fundamentals If a thin metal layer, surrounded by dielectric and supporting a surface plasma wave (SPW) on both surfaces, is made thin enough, the two SPWs will couple and exhibit more complex behavior. Speci®cally, the effective refractive index of the single SPW splits into two bound eigenmodes as the metal thickness decreases, as illustrated in Fig. 1. These two eigenmodes have symmetric and antisymmetric bound magnetic ®eld pro®les, as seen in the ®eld pro®les of Fig. 2. These modes are characterized by low (symmetric) and high (antisymmetric) losses, and are referred to as ``long-range'' and ``short-range'' surface plasmons, a reference to their relative propagation lengths[9].Thelonger propagationlength of long-range surface plasmons reduces the width of the resonance feature. As an added bene®t, the LRSPR sensor may be designed to have a higher sensitivity than the equivalent conventional SPR sensor operating at the same wavelength. The theoretical plots of Fig. 3 illustrate the matching between the wave vector of the light incident through the prism and the wave vector of the long-range surface plasma wave (LRSPW), showing positive and negative sensitivities in different operating regions. The high sensitivity magnitude possible for the LRSPR sensor results from the low
angle between the dispersion curves for the LRSPW and the incident light. The LRSPW effective refractive index in Fig. 3 is the symmetric bound mode solution of a threelayer dispersion relationship, where the three layers are Te¯on AF-1600, 20 nm gold, and analyte (water, and water plus ethylene glycol to increase the refractive index by 0.002 RIU), so the effect of the prism has been ignored [10]. Such an approximation is valid if the active gold layer is well separated from the prism. The perturbing effect of the prism becomes more pronounced as the buffer layer becomes thinner, and may cause the resonant wavelength to shift or may prevent the LRSPW from being supported in certain wavelength ranges.
Fig. 2. Modulus of the complex tangential magnetic field profile for symmetric and antisymmetric bound surface plasma waves on a 25 nm thick gold layer (symmetric mode structure: SF14 prism, 300 nm Teflon AF-1600, 25 nm gold, n 1:32 analyte, 531 nm light, y 49:5 ; antisymmetric mode structure: SF14 prism, 175 nm Teflon AF-1600, 25 nm gold, n 1:32 analyte, 702 nm light, y 70 ).
G.G. Nenninger et al. / Sensors and Actuators B 74 (2001) 145±151
147
Fig. 4. Configuration of an LRSPR sensor using a removable sensor substrate.
Fig. 3. Illustration of wave vector matching for a long-range surface plasma wave (LRSPW) supported on a 20 nm thick gold layer suspended between a Teflon AF-1600 buffer and analyte: water (dotted line) or water 0:002 RIU (dashed line): (a) operation in a region of positive sensitivity; (b) operation in a region of negative sensitivity.
3. Sensor design To create an LRSPR sensor, the metal layer must be separated from the ATR prism by a dielectric buffer layer with a lower refractive index than the prism, as illustrated in Fig. 4. Part of the dif®culty in creating an LRSPR sensor is the selection of an appropriate buffer material. Ideally, the buffer±metal±analyte structure will be symmetrical, so that the buffer and analyte have the same refractive index. However, coupled plasmons may exist on asymmetric structures [10], which has been demonstrated experimentally [11]. Indeed, controlled asymmetry of the structure may increase the propagation length of the LRSP [12]. However, there is a limit to the amount of asymmetry in a structure supporting an LRSP, so the buffer material should have a refractive index similar to that of the intended analyte. For biosensors, the analyte is typically aqueous, with a thin layer of protein on the surface of the sensor (functionalization material plus bound target molecules), therefore, a buffer layer with a refractive index near water is required. The
refractive index of the buffer layer material is an important parameter for LRSPR sensor design. Since dielectric materials are in general not available in a continuous range of refractive indices, selection of the buffer layer refractive index may be limited. Once the buffer material is chosen, the primary design parameters are the thickness of the metal layer (typically gold) and the thickness of the buffer layer. The operating angle of the instrument may be varied in order to produce the best coupling between the light and the LRSPR, as indicated by a deep resonance with low TM re¯ectivity at the minimum of the resonance. Choosing a design analyte provides an operating point for the sensor. Given these parameters, design curves may be produced displaying the best resonance depth, the wavelength at which this resonance occurs, the width of the resonance, and the sensitivity of the sensor. Design curves for sensors operating in water are provided in Fig. 5 for MgF2-buffer, and in Fig. 6 for Te¯on AF-1600 buffer. For these calculations, dispersive refractive index models for the MgF2 and Te¯on-buffer layers were created using published data [13,14]. As illustrated in Figs. 5(d) and 6(d), both buffer materials exhibit a transition between positive and negative sensitivities, so that for sensors constructed with a thick gold layer and a thin buffer layer, the resonant wavelength shifts to longer wavelengths as the refractive index of the analyte increases. For sensors constructed of a thin gold layer and a thick buffer layer, the sensitivity is negative, so that the resonant wavelength shifts to shorter wavelengths with an increase in the refractive index of the analyte. Near the transition between these sensitivity regimes, the sensitivity is very low. For speci®c combinations of gold thickness and buffer thickness, the magnitude of the sensitivity can be extremely high, on the order of 1 105 nm RIUÿ1 in the case of the Te¯on-based LRSPR sensor. This ®gure compares with a sensitivity of 1:4 104 nm RIUÿ1 for a conventional prism-coupled wavelength-interrogation SPR sensor operating at 850 nm. Another characteristic is illustrated for both buffer materials in the resonance width curves of Figs. 5(c) and 6(c). Both sensor types show a broadening of the resonance feature over a range of gold thickness and buffer thickness.
148
G.G. Nenninger et al. / Sensors and Actuators B 74 (2001) 145±151
Fig. 5. Design curves as a function of metal and buffer thickness for an LRSPR sensor using an SF2 glass substrate and a MgF2-buffer layer, with water as the analyte. The operating angle was optimized at each point to achieve the greatest coupling into the resonance (operating wavelength 500±1000 nm): (a) resonance depth; (b) resonant wavelength; (c) resonance width (full width at half-minimum); (d) sensitivity.
The broadening of the resonance is caused by matching between the wave vector of the illuminating light and the wave vector of the LRSPW over a range of wavelengths. The exact nature and magnitude of this effect is dependent upon the relative dispersion characteristics of the sensor materials and the analyte. This effect was observed experimentally in the case of the Te¯on-based LRSPR sensor. 4. Sensor construction Rectangular SF2 glass substrates were used in the construction of the MgF2-based LRSPR sensor elements. These substrates were cleaned and then coated with successive layers of MgF2 and gold in a thermal vacuum deposition system. The thickness of each layer was monitored during deposition using a crystal deposition monitor.
Rectangular SF14 glass substrates were used in the construction of the Te¯on-based LRSPR sensor elements. These substrates were cleaned and then coated with a ¯uorosilane solution to promote adhesion of the Te¯on AF-1600 to the glass, as recommended by the Te¯on AF manufacturer. Following a baking step, the slides were spin-coated with a solution of 3% Te¯on AF-1600, diluted in FC-77 Fluorinert solvent (3M Chemicals, Saint Paul, Minnesota, USA). This dilute solution produced a smoother surface ®nish than the stock 6% Te¯on solution. The substrate was then baked in a series of steps, as recommended by the manufacturer, to drive off the solvent and provide some leveling of the surface. Te¯on thickness was estimated based on spin speed and comparison between theory and experiment for samples coated with the same thickness of Te¯on but a different thickness of gold. Gold was deposited on the Te¯on surface using a thermal vacuum deposition system. Gold thickness
G.G. Nenninger et al. / Sensors and Actuators B 74 (2001) 145±151
149
Fig. 6. Design curves as a function of metal and buffer thickness for an LRSPR sensor using an SF14 glass substrate and a Teflon AF-1600 buffer layer, with water as the analyte. The operating angle was optimized at each point to achieve the greatest coupling into the resonance (operating wavelength 500± 1000 nm): (a) resonance depth; (b) resonant wavelength; (c) resonance width (full width at half-minimum); (d) sensitivity.
was monitored during deposition using a crystal deposition monitor. 5. Experimental configuration The samples were index-matched to a glass prism of the same type of glass as the substrate, part of a wavelengthmodulated SPR instrument used for conventional SPR sensing. Light from a tungsten±halogen light source was carried via an optical ®ber to a collimator and aperture to illuminate the sensor, and the ATR re¯ected light was collected using a second collimator, then carried via an optical ®ber to an optical spectrometer. The operating range of the spectrometer was approximately 500±1000 nm. For the MgF2buffer LRSPR sensors, a low-noise MSC500 spectrometer from Carl Zeiss was used, while a higher-noise Ocean Optics S2000 spectrometer was used for testing of the Te¯on-buffer
sensors. The operating angle of the instrument was adjusted to provide the best coupling of light into the LRSPR sensor. The internal operating angle of the instrument was approximately 608 for the MgF2-buffer sensor and 508 for the Te¯on-buffer sensor. 6. Experimental results Spectral plots of re¯ectivity of an LRSPR sensor, using a 360 nm MgF2-buffer layer and a 40 nm gold layer, are shown in Fig. 7 for several refractive indices of analyte. Fig. 8 demonstrates the sensitivity of a sensor with identical layers during a refractometric experiment. The average sensitivity over the range of 1.332±1.334 RIU was 5:0 103 nm RIUÿ1, slightly higher than the equivalent conventional SPR sensor (wavelength modulation, prism coupled, using 50 nm gold) operating at the same wave-
150
G.G. Nenninger et al. / Sensors and Actuators B 74 (2001) 145±151
Fig. 7. Measured spectra for an LRSPR sensor consisting of a layered structure of SF2 glass, 360 nm MgF2, and 40 nm gold as the refractive index of the analyte was varied in steps from 1.333 to 1.342 RIU.
Fig. 9. Measured spectra for an LRSPR sensor consisting of a layered structure of SF14 glass, 500 nm Teflon AF-1600, and 30 nm gold. The refractive index of the water analyte was varied in 0.001 RIU steps from 1.334 to 1.337 RIU via the addition of ethylene glycol.
length. The measured resolution based on noise levels was 1:2 10ÿ3 nm, for an average refractive index resolution of 2:4 10ÿ7 RIU. Fig. 9 illustrates measured spectral plots of re¯ectivity for an LRSPR sensor using a 500 nm Te¯on layer and a 30 nm gold layer. The four curves show the effect of 0.001 RIU steps in the refractive index of the analyte, made through the addition of ethylene glycol to the water analyte. The average sensitivity of this sensor over a range of 1.334±1.337 RIU was 3:8 103 nm RIUÿ1, consistent with theoretical predictions. An equivalent conventional SPR sensor operating in the same wavelength range would have a theoretical sensitivity of 2:6 103 nm RIUÿ1. The measured resolution based on noise levels was 1:5 10ÿ2 nm, for an average refractive index resolution of 4:0 10ÿ6 RIU.
Another sensor, constructed using a 700 nm Te¯on-buffer layer and a 24 nm thick gold layer, showed a broader resonance feature but extremely high sensitivity of 3:0 104 nm RIUÿ1 over a range of 1.334±1.335 RIU, as illustrated in Fig. 10. Both the high sensitivity and broad resonance are predicted by the design curves discussed above. Since the equivalent conventional SPR sensor would have a sensitivity of 4:3 103 nm RIUÿ1 at this wavelength range, this LRSPR sensor represents a seven-fold increase in sensitivity. Due to the broad resonance curve, the measured resolution based on noise levels was 7:4 10ÿ2 , corresponding to an average refractive index resolution of 2:5 10ÿ6 RIU. Results of sensor characterizations for a number of LRSPR sensor elements are summarized in Table 1.
Fig. 8. Measured response during a refractometric experiment for an LRSPR sensor consisting of a layered structure of SF2 glass, 360 nm MgF2, and 40 nm gold. The refractive index of the water analyte was varied in steps from 1.3323 to 1.3340 RIU via the addition of ethylene glycol.
Fig. 10. Measured response during a refractometric experiment for an LRSPR sensor consisting of a layered structure of SF14 glass, 700 nm Teflon AF-1600, and 24 nm gold. The refractive index of the water analyte was varied in equal steps from 1.334 to 1.335 RIU via the addition of ethylene glycol.
G.G. Nenninger et al. / Sensors and Actuators B 74 (2001) 145±151
151
Table 1 Summary of experimental results for several LRSPR sensor elements, compared with the sensitivity of an equivalent conventional SPR sensor Buffer material
Buffer thickness (nm)
Gold thickness (nm)
Resonant wavelength n 1.334 (nm)
Sensitivity of long-range SPR sensor (nm RIUÿ1)
Teflon Teflon Teflon Teflon MgF2
500 700 800 700 360
30 30 30 24 40
650 720 725 700 693
3.8 7.2 9.2 3.0 5.0
a
103 103 103 104 103
Sensitivity of equivalent conventional SPR sensora (nm RIUÿ1) 2.6 5.2 5.2 4.3 3.8
103 103 103 103 103
Conventional SPR sensor: spectrally-modulated, SF14 prism (SF2 for MgF2-buffer), 50 nm gold at same wavelength.
7. Conclusions We have demonstrated the successful application of longrange surface plasmons to a wavelength-modulated sensor design. Both magnesium ¯uoride and Te¯on AF-1600 were successfully used as dielectric buffer materials. The demonstrated sensitivity was up to seven times higher than that of an equivalent conventional SPR sensor, while the resolution is comparable. According to theoretical design calculations presented, further optimization of materials and layer thickness could reduce the resonance width while achieving even higher sensitivities, thereby creating a sensor with signi®cantly better resolution than current conventional SPR sensors. Acknowledgements This research was supported by the United States Department of Defense under contract DAAD13-99-C-0032 and by the Grant Agency of the Czech Republic under contracts 102/99/0549 and 102/00/1536. References [1] J. Homola, S.S. Yee, G. Gauglitz, Surface plasmon resonance sensors: review, Sens. Actuators B 54 (1999) 3±15. [2] D. Sarid, Long-range surface-plasma waves in very thin metal films, Phys. Rev. Lett. 47 (1981) 1927±1930. [3] J.C. Quail, J.G. Rako, H.J. Simon, Long-range surface-plasmon modes in silver and aluminum films, Optics Lett. 8 (1983) 377±379. [4] C. Plumereau, A. Bouchoux, A. Cachard, Electrooptic light modulator using long-range surface plasmons, novel optoelectronic devices, Proc. SPIE 800 (1987) 79±83. [5] J.S. Schildkraut, Long-range surface plasmon electrooptic modulator, Appl. Optics 27 (1988) 4587±4590. [6] P.J. Kajenski, Tunable optical filter using long-range surface plasmons, Opt. Eng. 36 (1997) 1537±1541. [7] G.I. Stegeman, J.J. Burke, D.G. Hall, Non-linear optics of long-range surface plasmons, Appl. Phys. Lett. 41 (1982) 906±908. [8] K. Matsubara, S. Kawata, S. Minami, Multilayer system for a highprecision surface plasmon resonance sensor, Optics Lett. 15 (1990) 75±77. [9] J. CtrokyÂ, J. Homola, P.V. Lambeck, S. Musa, H.J.W.M. Hoekstra, R.D. Harris, J.S. Wilkinson, B. Usievich, N.M. Lyndin, Theory and modelling of optical waveguide sensors utilising surface plasmon resonance, Sens. Actuators B 54 (1999) 66±73.
[10] J.J. Burke, G.I. Stegeman, T. Tamir, Surface-polariton-like waves guided by thin, lossy metal films, Phys. Rev. B 33 (1986) 5186±5201. [11] N.M. Lyndin, I.F. Salakhutdinov, V.A. Sychugov, B.A. Usievich, F.A. Pudonin, O. Parriaux, Long-range surface plasmons in asymmetric layered metal±dielectric structures, Sens. Actuators B 54 (1999) 37±42. [12] L. Wendler, R. Haupt, An improved virtual mode theory of ATR experiments on surface polaritons, Phys. Stat. Sol. B 143 (1987) 131±148. [13] W.J. Tropf, T.J. Harris, M.E. Thomas, in: R.W. Waynant, M.N. Ediger (Eds.), Electro-optics Handbook, McGraw-Hill, New York, pp. 11.1±11.97 (Chapter 11). [14] J.H. Lowry, J.S. Mendlowitz, N.S. Subramanian, Optical characteristics of Teflon AF fluoroplastic materials, Opt. Eng. 31 (1992) 1982±1984.
Biographies Garet G. Nenninger received a B.S. degree in electrical engineering from the Massachusetts Institute of Technology, Cambridge, USA in 1991 and an M.S. degree in electrical engineering from the University of Washington, Seattle, USA in 1998. From 1991±1996, he worked as an instrumentation and control engineer for the Naval Sea Systems Command in Washington, DC. He is currently a Ph.D. degree candidate in the Department of Electrical Engineering at the University of Washington. His research interests include optical sensors, instrumentation, and sensor systems. Petr TobisÏka received his M.Sc. degree in physics from Charles University, Prague, Czech Republic in 1996. He is currently a graduate student at Charles University, Prague and works at the Institute of Radio Engineering and Electronics, Prague. His research interest is in surface plasmons and their application in (bio)chemical sensing. JirõÂ Homola received his diploma in physical electronics from the Czech Technical University, Prague, Czech Republic in 1988, and a Ph.D. degree in the field of guided-wave optics from the Academy of Sciences of the Czech Republic in 1993. Since 1993, he has been with the Institute of Radio Engineering and Electronics of the Academy of Sciences of the Czech Republic, Prague, working on fiber optic sensors and surface plasmon resonance-based sensing devices. Currently, he is a research assistant professor at the University of Washington, Seattle. His research interests are in guided-wave optics, and optical sensors and biosensors. Sinclair S. Yee received B.S., M.S., and Ph.D. degrees in electrical engineering from the University of California, Berkeley, in 1959, 1961, and 1965, respectively. In 1964, he joined the Lawrence Livermore Laboratory as a research engineer working on semiconductor devices. Since 1966, he has been on the faculty of the Department of Electrical Engineering at the University of Washington, where in 1974, he became a full professor. In 1972±1974, he was a NIH special research fellow, and since 1979, he has been an IEEE fellow. His main research interests are concerned with silicon-based and GaAs quantum well optical modulator devices, surface plasmon resonance, and microsensor arrays.