ELSEVIER Sensors and Actuators B 38-39 (1997) 375-379

Development of a surface plasmon resonance sensor for commercial applications

JosC MeEndez ay* , Richard Carr a, Dwight Bartholomew a, Hemant Taneja a, Sinclair Yee b, Chuck Jung b, Clement Furlong b

a Sensors and Infrared Laboratory, Texas Instruments Inc,, PO Box 6.55936, MS 1.50, Dallas, TX 75265, USA b University of Washington, Seattle, WA 98195, USA

Abstract

An optical tabletop system basedon surface plasmon resonance (SPR) for refractive-index determination has been developed to demonstrate the feasibility of a miniaturized and integrated concept which is also described. The tabletop system is constructed from the ‘miniaturizable’ components required to realize a manufacturable, integrated minisensor utilizing the SPR phenomenon for transduction. The tabletop system exhibits adequate sensitivity, stability, and reproducibility while maintaining overall system simplicity. The sensor system is excited by a near-infrared light-emitting diode (LED) available in die form, since a laser source is impractical for the miniaturized sensor. The light is optically coupled into a plastic prism because the minisensor optics are readily molded using plastics or epoxy, rather than glass. The angular composition of the diverging reflected radiation is then separated and quantified by a photodiode array (also available in die form) consisting of pixels on a 63 t.r,rn pitch. A sputtered gold film is used as the SPR excitation layer. The sensor system performance is qualified using aqueous solutions containing ethylene glycol. The response to changes in concentration of the ethylene glycol is found to be on the order of one part in 104. This translates to a refractive-index change of approximately lo-‘. The stability of the system response has been investigated by quantifying the response change in water over a two-day period. The stability is excellent when temperature compensation is implemented. The components utilized in the tabietop system are consistent with the development of a low-cost miniature integrated surface plasmon sensor. Such a device has been constructed. A sketch of a minisensor is shown, along with preliminary response data.

Keywords: Surface plasmon resonance; Biosensors; Optical chemical sensors

1. Introduction

The surface plasmon resonance (SPR) phenomenon can be used to measure the refractive index of a wide variety of chemical samples. Such arefractive-index determination may be utilized as a process-control and/or monitoring tool in chemical systems which can be qualified by an acceptable refractive-index window. Quantitative determinations of analyte concentrations can only be made in limited situations where the system is closed and binary, provided the appro- priate concentration to refractive-index mapping is known. Analyte concentrations may be determined in three-compo- nent mixtures in special circumstances where the sample space includes some dispersion information, or where absorp- tive differences cause significant broadening in the SPR response. Compositional information for mixtures with four or more components is extremely difficult to achieve using the surface plasmon resonance phenomenon alone.

* Corresponding author. Tel.: + 1972 995 7914. Fax: + 1972 995 6558

0925-4005/97/$17,00 0 1997 Elsevier Science S.A. All rights reserved Pz150925-4005(97)00047-6

Sensing applications that involve multiple analytes require selective sensor schemes. These systems often involve chem- ical transduction layers such as selectively absorbing mem- branes or functionaiized surfaces that provide specific analyte binding. A chemical transduction layer is a thin film used to change the effective refractive index seen by the sensor based on average optical property changes or thickness changes, which depend upon the concentration of the analyte. Through the use of an intermediate transduction layer, SPR [l] has been demonstrated in making useful selective chemical sen- sors [ 2,3], A number of SPR configuration systems have been reported, such as immunoassay [ 45 3, liquid [ 6,7], gas [4,8] and thin film [ 91. Biosensing configurations use the fact that antibody/antigen monolayer formations lead to a net change in the effective refractive index measured by the sensor. Gas sensing may involve the use of various reversibly gas-absorbing organic thin films interfaced with the SPR layer.

The practical applications of SPR sensors towards indus- trial, environmental and biomedical sensing have been lim-

376 J. MelPndez et al. /Sensors and Actuators B 38-39 (1997) 375-379

ited in part because high-performance SPR sensors have been traditionally implemented using expensive optical instru- ments. The sensor system presented here maintains reasona- ble performance properties using components .that are compatible with a miniaturized and integrated SPR sensor concept, which is also described. This paper presents a brief overview of the SPR phenomenon, a description of our sensor system design, and experiments conducted on the system, which define the system performance. We conclude with a brief presentation of what we believe is the world’s first fully integrated SPR device using tabletop system components, and present preliminary performance data.

2. Background

Surface plasmon waves (SPWs) are excited at the inter- face between a thin metal and a sample by coupling through a high-refractive-index substrate [ lo], as illustrated inFig. 1. SPWs are transverse waves with an oscillating electric field normal to the surface. Since surface plasmons only have an eiectric-field component which is normal to the surface, p- polarized light is required to satisfy the boundary conditions necessary to excite SPR. When the wave vector of the radi- ation in the surface normal is made to match the SPR condi- tion by a specific combination of wavelength and incidence angle, a surface plasmon is excited, which results in energy loss from the reflected intensity. The optical material para- meters, which include the real and imaginary refractive indi- ces of the metal and substrate, as well as the metal thickness, describe the optical system. The specific combination of the excitation wavelength and angle that result in SPR excitation then determine the refractive index of the sample material at the given wavelength.

The SPR can be observed as a minimum of the reflectance when the angle of incidence is varied and the wavelength of the light is held constant. Such an angle modulation approach simplifies refractive-index determination over wavelength modulation [ 111 in that the optical material parameters, par- ticularly those of the metal, can be a significant function of wavelength, while they do not depend on the angle. Due to the wavelength dispersion of the system parameters, 100% coupling of p-polarized light with a surface plasmon requires a monochromatic light source when the angular modulation approach is employed. For practical applications, a HeNe

R&B) ti Refractive Index of Sample

Fig. 1. Surface plasmon resonance excitation configuration.

Fig. 2. Theoretical transverse magnetic reflected intensity curves excited by Gaussian distributed light sources with FWHM values of 100, 40, and 12 nm compared with monochromatic excitation.

laser [ 121 has been traditionally used to excite SPR, since it has a narrow bandwidth and is widely available. However, a theoretical analysis of SPR excitation by light sources of wider bandwidths indicates that even light sources with band- widths around 50 nm are capable of yielding high levels of SPR excitation. Emission bandwidths less than 50 nm can be obtained routinely in the near-infrared (NIR) portion of the spectrum with AlGaAs-based light-emitting diodes (LEDs) . Fig. 2 is a theoretical plot of the SPR reflected intensity curves for full width at half maximum (FWHM) values of 100,40, and 12 nm compared with the theoretical monochromatic excitation. The system comprised an 820 nm peak excitation, 535 A of Au, substrate refractive index of 1.63 and assumed a water sample. The resulting solution was calculated using a custom software solution of the Fresnel equations for a multi-layer system. The effect of the broadening is included by a weighted average of the reflection solution, r, for each given wavelength, and is given by

where h is the wavelength, B is the angle, d is the metal thickness, and the n are the refractive indices of the substrate (p) , metal (m) , and sample (s) , The distribution function is given by a Gaussian of the appropriate FWHM determined by a,

p(h) =exp[ -a(h-&)‘I (2)

The observed dependence of the SPR coupling with the FWHM of the excitation source suggests that an IR LED is sufficient to obtain SPR coupling where the minimum in the normalized reflected intensity is less than 0.2, which we shall show is sufficient to obtain excellent resolution.

3. SPR sensor configuration

Fig. 3 shows a schematic design of the SPR sensor system presented in this paper. The divergent radiation from an LED is collimated and passed through a polarizer which can be

J. Melhdez et al. /Sensors and Actuators 3 38-39 (1997) 375-379 377

Lens Aoerture Liquid Sample Holder

intensity Control

Comouter

-11.1 p

Detect{ )r

r

Fig. 3. Schematic of the surface plasmon resonance sensor system.

rotated 360” to obtain any combination of p and s linear polarizations. Collimation of the light beam ensures linear polarization, which is important for the maximization of SPR excitation at the metal/sample dielectric interface. The polar- ized light is then apertured down to set the desired input cone dimension from the converging lens. The light propagates through the prism in a converging angular distribution with a spot size which is roughly minimized at the SPR layer/ prism interface. The range of the divergent light beam inci- dent on the detector array may be set by using a convex lens. The detector array signal is captured using an A/D converter with a FIFO memory buffer coupled to a TI TravelMateTM 4000 notebook computer using a PCMCIA card. The raw signal is automatically reference corrected and an algorithm can be used to determine the refractive index.

The sensor system utilizes a near-IR LED and a linear photodiode array, TSL 402 (Texas Instruments Inc.), with 256 pixels. Each pixel is 50 pm wide and the detector array has a pitch of 63 pm. An equilateral prism of 25 mm dimen- sion was used. The sample solutions were held in a liquid holder of size 27 mm X 27 mm X 30 mm (LX WX H) . This holder was attached to the prism using RTV rubber cement.

The SPR metal layer was a gold thin film sputtered onto the prism surface. Sputtering was chosen as the deposition technique to minimize the microstructure of the SPR metal. Poor metal and surface quality can lead to contamination centers, which result in drift and non-reproducibility of the sensor response. Gold was specifically chosen for the SPR layer because of its very small dielectric constant, which results in a sharp absorption peak. Also, gold is resistive to oxidation, leading to a more stable sensor.

4. Experimental results

The resolution of the sensor system was investigated under room-temperature conditions. Aqueous solutions of ethylene glycol [ 131, which has a refractive index of 1.429 at 589.3 nm, were used to quantify the sensor system resolution. The SPR reflected intensity curve is obtained using the following procedure. A reflected intensity reference spectrum is obtained by shining s-polarized light, which does not excite SPR. For an accurate normalization, it is essential that the

change in reflectance obtained from s and p polarizations can be solely accounted for by SPR excitation.

S-normalized reflected intensity curves were obtained using various ethylene glycol dilutions (O.Ol%, O.l%, l%, 10%) as test samples. These reflected intensity curves were compared to those obtained by using DI water as the calibra- tion standard. The difference in the curves determines the change in refractive index. In the experimental results shown in Fig. 4 it can be seen that the sensor was able to distinguish a 0.01% ethylene glycol solution from pure DI water. Fig. 4(a) shows the shifting of the resonance curve to higher coupling angles as the percentage of ethylene glycol increases, indicative of increasing refractive index. The min- ima may be extracted from the data by a straightforward smoothing/derivative method. Fig. 4(b) quantifies the differences by subtracting the water-normalized reflected intensity from each of the dilutions and taking the absolute value. It is clear from the Figure that the sensor is performing near its highest sensitivity in detecting 0.01% ethylene glycol, since the signal-to-noise ratio in the difference becomes low.

Drift experiments were also conducted to investigate the stability of the sensor when measuring a water sample. Since the refractive index of water is a significant function of tem- perature, a thermistor was used to measure the temperature at each sample reading. The results of these experiments are shown in Fig. 5, for continuous system operation of 50 h. During this period, the sample temperature was allowed to vary with the external influences of the room. As can be seen in the Figure, this resulted in a rise in temperature to 24.7”C at the third hour, followed by a downward trend to 22°C after

F 1.0

& 0.8

8 0.6 s 2 0.4 G $! 0.2

0.0 1

1 o”

10

Pixel #

1 o- 4 1 10 100

Pixel # Fig. 4. (a) Ratio of the p to s polarizations of the reflected intensity curves of aqueous solutions with various ethylene glycol contents. (b) Absolute value of difference of reflected intensity curves of water and various ethylene glycol solutions.

378 J. Mel&dez et al. /Sensors and Actuators 3 38-39 (1997) 375-379

110.2 5

E i g 110.1 3 .- .G

4:

; 110.0 ii

.a 2

i 109.9 3a

0

j 109.8 2

0 10 20 30 40 50

Time (h)

Fig. 5. The surface plasmon resonance effective pixel number, temperature, and compensated data over a two-day period in water.

two days. The change in refractive index is monitored by the SPR system as a change in the minimum effective pixel num- ber. In the Figure, this value is seen to go to a minimum at 3 h, and towards a maximum near two days. Indeed, the mirror image of these functions is a statement of their interrelation- ship. When the effect of temperature is included in the SPR data, it is seen that the data are quite reproducible over the two-day period, indicating the stability of the sensor system. This stability corresponds to & 1.3 X lo-’ refractive-index units.

5. Discussion

The SPR system presented demonstrates high performance using relatively simple low-cost components. As anticipated based on our theoretical calculations, using a near-IR LED instead of a laser diode resulted in coupling efficiencies which did not seem to impact negatively on the use of the SPR phenomenon for refractive-index sensing. While the mono- chromiciiy of the source is known to be important, and LEDs have been used in commercial systems, the authors are not aware of any publication that quantifies through calculation the impact of non-monochromatic light on the expected SPR curve. The advantages of using an LED come from its smaller size, simpler system implementation, and lower cost.

Traditionally a 1024-pixel detector array of very small pitch is used for detecting the SPR phenomenon [ 8,14-161, though some commercial systems are reported to use sub- stantially less pixels. The 256-pixel Si diode array of 63 km pitch has displayed high levels of resolution, allowing adeter- mination of refractive-index changes on the order of lo-‘. For the purposes of making a low-cost system, the number of pixels can be further reduced to only a few, since the entire reflectance curve is not required for sensing a change in the index of refraction. This is highlighted by noting the change in just the first few pixels in Fig. 4 (which is better noted through the use of a log scale). The results in Fig. 4(b) show excellent resolution using only pixel number 1 when a dif- ference signal-processing scheme is used. However, to min- imize the effects of drift and instability on the refractive-index

determination, as well as the susceptibility to imaginary refractive-index changes of the sample, we do not recommend such an extreme implementation.

The components utilized in the SPR system we devetoped are completely compatible with the construction of a fully integrated miniature SPR device. A conceptual sketch ofsuch a device is shown in Fig. 6. Many optical configurations are possible. The minisensor is based on the integrally molded optical (bio)chemical sensor concept invented at Texas Instruments. It comprises a narrow-band W AlGaAs LED light source enclosed in, a light-absorbing optical housing. The housing includes a silicon light-to-voltage chip to mon- itor LED intensity variations. The top of the housing has an aperture to allow a controlled light spread to enter the system. A polarizing material is attached to the aperture to limit the introduction of transverse electric radiation into the system, since only transverse magnetic radiation can excite SPR. Reflecting optics are used to guide the light through the encapsulating optical material, to the SPR layer, and its final projection onto a linear silicon photodector array. We have successfully constructed such a device. Data recorded from the first sensor are shown in Fig. 7, where theresonance angle is monitored versus time as isopropanol is added at two dif- ferent instances. Future work will establish the performance metrics of the miniature device, and explore opportunities for applications in chemical and biochemical sensing. As addi- tional sensors arefabricatedandtheirperformancequantified, we expect to elaborate in future publications on the minisen- sor fabrication procedures and performance results.

Fig. 6. The miniature integrated surface plasmon device concept.

100 200 300 400

Time (seconds)

Fig. 7. The response of the first fully integrated surface plasmon device to the addition of isopropanoi to water.

J. Mel&dez et al. /Sensors andActuators B 38-39 (1997) 375-379 379

6. Summary and conclusions

An SPR-based tabletop optical system was developed using materials compatible with the manufacturing of a min- iature integrated SPR-based sensor. The use of a near-IR LED and 256-pixel photodiode array reduces cost while maintain- ing high performance levels. At its minimum, the normalized reflected intensity was below 0.2, which is near theoretical IeveIs for a system using an LED to excite surface plasmons. The excellent performance can be attributed to a stable trans- duction layer. The resolution of the sensor system was on the order of 10m5 refractive-index units, or one part in 10 000 alcohol in water. An investigation of system drift indicated the sensor maintained a consistent response over a two-day period. A miniature integrated SPR device has been fabri- cated, and the first data from the device shown.

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Biographies

Jose’ Mel&de2 received his Ph.D. in 1994 from the Solid State Laboratory at Stanford University. His dissertation was on the development of process models and simulation for mercury cadmium telluride. Dr MelCndez received his BS ( 1990) and MS ( 1991) degrees in electrical engineering from the Massachusetts Institute of Technology. His BS the- sis was entitled ‘1 /f Noise in Integrating Infrared MIS Sensor Devices’ and his MS thesis was ‘Extraction of Insulator Trap Densities from 1 /f Noise Measurements’. Dr MelBndez has been a member of the Sensors and Infrared Laboratory since 1988 when he joined the group as an MIT VI-A intern. In early 1994 he joined the laboratory as a member of the tech- nical staff. He is presently manager of analytical sensor sys- tems, responsible for research and development activities related to advanced sensor systems.