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A differential surface plasmon resonance sensor
Sensors and Actuators B 159 (2011) 33–38
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A differential surface plasmon resonance sensor David J.L. Graham, Lionel R. Watkins ∗ Department of Physics, University of Auckland, P.B. 92019, Auckland, New Zealand
a r t i c l e
i n f o
Article history: Received 21 February 2011 Received in revised form 30 May 2011 Accepted 2 June 2011 Available online 13 June 2011 Keywords: Surface plasmon resonance SPR sensors Refractive index
a b s t r a c t A novel technique that detects only the difference in refractive index between two areas of a surface plasmon resonance (SPR) sensor is presented. Since the measurement is independent of the common index, errors due to changes in the bulk index or temperature of the sensed medium are largely eliminated. A single, monochromatic beam is twice reflected from the SPR surface, with the p- and s-polarisations interchanged between reflections. A linear response to refractive index difference is obtained by measuring the change in angle of incidence at the second reflection required to maintain minimum transmission. Measurements with salt solutions of known refractive index are used to demonstrate that this instrument can detect refractive index differences while being insensitive to changes in the refractive index common to both areas of the SPR surface. A typical noise of 1.5 × 10−6 RIU with a ±0.005 RIU refractive index difference range was achieved. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Optical sensors based on surface plasmon resonance (SPR) are widely used in the study of biomolecular interactions since they provide both a label-free technique and high sensitivity. SPR sensors also find application in the detection of chemical and biological analytes [1]. However, a major limitation to the accuracy of these sensors arises from drifts due to changes in the bulk refractive index or temperature of the sensed medium. One technique commonly used in imaging SPR instruments to minimise these effects is to compare the return from a sensitised area of the sensor to that from an unsensitised reference area [2–4]. Simpler, two channel systems have also been developed and are useful for measuring a single binding reaction, with a reference to correct for bulk index or temperature changes. One approach is to use one of the standard single channel techniques but with two beams and two detectors [5–7]. A two channel measurement can also be made with a single detector by shifting the resonance of one of the channels so that the two resonances can be distinguished. This can be achieved by coating part of the sensor surface with a thin, high refractive index layer [8] which will shift the angle or wavelength at which resonance occurs for that area. An angle or wavelength scan will show two intensity minima corresponding to the two channels. Provided the high index layer is much thinner than the penetration depth of the surface plasmon, the resonance will still change in response to
∗ Corresponding author. Tel.: +64 9 373 7599; fax: +64 9 373 7445. E-mail address: [email protected] (L.R. Watkins). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.06.014
refractive index changes at the surface. Refractive index changes at the coated and uncoated areas can thus be measured separately. Alternatively, one can use a collimated white light source and a specially designed coupling prism that causes the beam to be reflected from two areas of the SPR sensor surface at two different angles of incidence [9], thereby allowing the two resonance to be separated in wavelength. Heterodyne interferometry has also been used to measure the difference in phase for two separate beams reflected from different areas of a SPR surface [10]. Here we present a novel differential SPR instrument that measures the refractive index difference between two areas of a sensor surface while being insensitive to the common index. Our approach is to use a single collimated beam of monochromatic polarised light that is reflected from the SPR surface before being returned, via a quarter-wave plate and mirror, and reflecting from an adjacent area of the same surface. The transmission of our optical system is proportional to the square of the difference of the ppolarisation reflection coefficients for the two areas and hence to the refractive indices at these two areas. As changes in the refractive index have the same effect as changing the angle of incidence at which resonance occurs, the angle of incidence for the second reflection can be adjusted by the mirror so that the reflection coefficients are approximately equal. The angle of incidence difference that makes the reflection coefficients approximately equal can be found as the point of minimum transmission and this difference is approximately proportional to the refractive index difference between the two areas. The instrument presented here detects changes in the angle of incidence difference for minimum transmission.
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D.J.L. Graham, L.R. Watkins / Sensors and Actuators B 159 (2011) 33–38
Detector
0.04 Δθ = 0
Mirror
Analyser -45°
°
0.035
Δθ = −0.3 Δθ = 0.3°
0.03 0.025
T
Quarter-wave plate 45° Polariser 45° (1)
(2)
0.015
Fig. 1. Schematic of the differential SPR system.
0.01 0.005
2. Theory Fig. 1 shows the concept of our differential surface plasmon resonance sensor. A beam of polarised light is reflected from one area of a SPR surface, with reflection coefficients rp1 and rs1 . Two passes through a quarter-wave plate at 45◦ exchanges the p and s-polarised light and the beam is then reflected from a second area of the SPR surface with reflection coefficients rp2 and rs2 . The beam finally passes through an analyser and is incident on a photodetector. We employ the Jones matrix method [11] to determine the response of our instrument. Suppose that light polarised at 45◦ is reflected from the first area of the SPR surface. The electric field E1 leaving this surface is readily found:
E1 =
rp1 0
0 rs1
1 √ 2
1 1
1 = √ 2
rp1 rs1
.
(1)
This beam makes two passes through the quarter-wave plate which is equivalent to a single pass through a half-wave plate and has the effect of exchanging the p- and s-polarised states. A further reflection from the second area of the SPR surface then ensues:
E2 =
rp2 0
0 rs2
1 √ 2
rs1 rp1
1 = √ 2
rp2 rs1 rp1 rs2
.
(2)
If the two surface are identical, rp1 = rp2 = rp and rs1 = rs2 = rs and so the final polarisation state is rp rs E2 = √ 2
1 1
.
(3)
That is, the light is linearly polarised at 45◦ and attenuated by a factor of |rp rs | 2 . If the light is then incident on an analyser set with its transmission axis at −45◦ , the transmission will be zero. If the reflection coefficients of the two surfaces are not identical, then the transmitted light will not be zero but can be calculated from E2
=
1 2
1 −1 −1 1
1 = √ 2 2 where J−45 =
1 2
1 √ 2
rp2 rs1 rp1 rs2
rp2 rs1 − rp1 rs2 rp1 rs2 − rp2 rs1
1 −1 −1 1
, (4)
,
(5)
is the Jones matrix for a polariser with its transmission axis oriented at −45◦ . The transmitted intensity T is then T=
0.02
1 |rp2 rs1 − rp1 rs2 |2 . 4
(6)
The s-polarised light does not couple to the plasmon and so the change in rs with refractive index is much smaller than
0 −5
−4
−3
−2
−1
0
Δn
1
2
3
4
5 −3
x 10
Fig. 2. Transmission vs. refractive index difference for different angle of incidence differences. SPR surface is 50 nm of gold on glass as described in the text. 1 = 71◦ .
for p-polarised light and hence we can use the approximation rs1 ≈ rs2 ≈ rs . The transmission is therefore T≈
1 |rs (rp2 − rp1 )|2 , 4
(7)
and is approximately proportional to the square of the difference between the p-reflection coefficients for the two SPR surfaces. Simulations of a typical SPR system comprising 50 nm of gold (n = 0.161 + 3.64i [12]) on glass (n = 1.51) were made using the scattering matrix method of thin film modelling [11]. The wavelength was 680 nm for all the modelling results presented here. The angle of incidence for the first reflection was 1 = 71◦ , which is approximately the value to give minimum reflectivity for an aqueous external medium (n = 1.33). The refractive index at the first SPR surface is defined to be the common index, n1 . The refractive index at the second surface is defined to be the common index plus the refractive index difference between the two surfaces, n2 = n1 + n. The transmission as a function of the refractive index difference n is shown in Fig. 2. Also shown is the transmission in which there is a difference in the angle of incidence for the two surfaces with 2 = 1 + as this will be useful later. The transmission is zero at n = 0, = 0 and increases as |n | increases. As shown in the figure, changes in the refractive index difference n could be detected from changes in the transmission. However the response is approximately quadratic. Fig. 2 also shows that changing the angle of incidence of the second reflection shifts the n value at which the intensity is a minimum. This is because the main effect of changing the refractive index for a SPR system is to shift the angle of incidence at which the resonance occurs [13]. A more linear measurement of the refractive index difference n can be obtained by measuring the angle of incidence difference at which the minimum transmission occurs. Fig. 3 shows the angle of incidence difference for minimum transmission m vs. refractive index difference for different common indices n1 . The angle of incidence difference for minimum transmission was found numerically for each value of n by calculating the transmission for a range of values and finding the minimum. As can be seen in the figure, m has an almost linear response to the index difference and is almost independent of the common index. The sensitivity of m to the refractive index difference is approximately 140◦ RIU−1 , the same sensitivity to refractive index as a single channel angle scanned instrument using this SPR system. The measurement of m is the basis for the experimental apparatus to be described in the following section. Note that if the angle of incidence or n1 is changed, moving the operating point off resonance, the transmission at n = 0, = 0 would still be zero, but the transmission
D.J.L. Graham, L.R. Watkins / Sensors and Actuators B 159 (2011) 33–38
0.8 0.6 0.4
Δθ
m
0.2 0 −0.2 n = 1.330 1
−0.4
n = 1.335 1
n1 = 1.340
−0.6 −0.8 −5
−4
−3
−2
−1
0
Δn
1
2
3
4
5 −3
x 10
Fig. 3. Angle of incidence difference for minimum transmission (m ) vs. refractive index difference (n) for different common indices (n1 ).
would change more slowly as |n | increased as the reflection coefficients change less rapidly away from resonance. This will reduce the sensitivity with which changes in m can be detected in the presence of optical noise. The simulations presented here have assumed that the SPR surfaces at the location of the first and second reflections are identical, except for the refractive index of the sensed medium. If this is not the case, for example, the thickness or optical properties of the gold film at the two areas of the surface are different, then the angle of incidence for minimum transmission will become sensitive to the common index n1 . Suppose though, that we adjust the analyser azimuthal angle . The transmission T is then readily found to be T≈
1 |rs1 (rp2 cos + rp1 sin )|2 . 2
(8)
If rp for one of the areas changes more quickly as a function of n1 , then can be adjust to make T independent of n1 again [14]. The difference in the rate at which the reflection coefficients change will depend on n1 and the angle of incidence, so this compensation will only be effective over a limited range. Note that an equivalent compensation can be achieved by adjusting the polariser azimuthal angle. This compensation was not necessary for the experimental results presented in the next section. 3. Experimental apparatus Fig. 4 shows a schematic diagram of our differential SPR instrument. To measure m with high accuracy, the angle of incidence difference was modulated sinusoidally using a mirror glued onto a piezo-bender. The ac component of the resulting photocurrent is proportional to the gradient of the intensity as a function of the angle [15]. This signal can be used as the input to a feedback loop that applies an offset to the modulator in order to drive the ac component to zero. The point at which the gradient is zero is the minimum transmission as a function of , that is m . The value of the offset applied by the feedback loop to the mirror is thus proportional to m . With this technique, it is possible to detect changes in the resonance angle with a sensitivity better than 10−3 degrees [15]. Not shown in the figure is an additional beam used to monitor the mirror on the piezo-bender, as will be described later in this section. The light source was a QSDM680-9 super luminescent diode (SLD), centre wavelength 677 nm, spectral width 7.5 nm FWHM. The diode was mounted in a temperature controlled laser diode mount with an aspheric collimation lens. The polariser was
35
a Lambda Research Optics broadband near infra-red polarising beam-splitter cube. It was clamped in a v-mount with its transmission axis at 45◦ to set the initial polarisation state. The intensity of the beam incident on the prism was approximately 2 mW. The arrangement of the SPR system and the flow cell is shown in the insert of Fig. 4. A high index (SF2 glass n = 1.64) prism was used to reduce the angle of incidence for coupling to the plasmon and to make the alignment easier. An SPR sensor surface from Ssens was used. This was a 25 mm diameter, 1 mm thick glass (n = 1.51) disc coated with a 2 nm titanium (typical n = 2.81 + 3.90i at 680 nm [16]) adhesion layer and 50 nm of gold (typical n = 0.161 + 3.64i at 680 nm [12]). Index matching oil from Cargile (n = 1.51) was used between the sensor disc and prism to reduce reflections from this interface. The prism and flow cell were mounted on a rotation stage to allow the angle of incidence to be controlled. The angle of incidence at the prism was 1.3◦ for all the experimental presented here. Applying Snell’s law to the air–prism and prism–disc interfaces indicates that the angle of incidence for the first reflection from the SPR surface was 71.4◦ . An aluminium block with two flow cells was clamped to the SPR sensor disc. The bottom flow cell (1) covers the area of the SPR surface where the first reflection occurs and the top flow cell (2) covers the area where the second reflection occurs. The flow cells were both approximately 0.1 ml in volume. Two holes drilled through the block into each flow cell, with plastic pipe fittings on the outside, formed the inlet and outlet. Each flow cell inlet was connected through a 4-1 selector valve to four plastic 60 ml syringes that could contain distilled water or salt solutions with different refractive indices. A tube from the outlet of each flow cell led to a waste bucket. When changing the solution in a flow cell, 5 ml of the new solution was flowed through the cell to ensure that the previous solution was completely replaced. A Peltier thermoelectric cooler (TEC) driven by a PID controller was used to stabilise the temperature of the flow cell at 20 ◦ C. The angle of incidence of the second reflection from the SPR surface was controlled using a movable mirror. The mirror was a 6 × 3 mm piece of microscope cover slip with a 40 nm gold coating. The mirror was glued to one end of a Physik Instrumente PL112.11 piezo bender actuator, the other end of which was clamped to a mirror mount on top of a rotation stage. The actuator was driven by a 240 Hz sinusoid and with an amplitude such that the angle of the beam reflected from the mirror changed by approximately ±0.1◦ . The mirror was placed 30 cm from the prism and a 7.5 cm focal length cylindrical lens was placed halfway between them to reduce the movement of the beam across the SPR surface as the angle of the mirror about the vertical axis changed due to the bending of the piezo. The mirror was set slightly tilted about a horizontal axis to steer the beam so that the second reflection from the SPR surface occurs at the part of the surface covered by flow cell (2). A Thorlabs Fresnel rhomb was used as the quarter-wave plate, mounted in a manual rotation stage and set at 45◦ . After the second reflection from the SPR surface, the beam was focused onto a Glan–Thompson polariser with a f = 70 mm lens. The axis of the polariser was set at −45◦ and the transmitted light focused onto the detector with a f = 35 mm doublet lens. The ac component of the photocurrent was amplified and fed to a simple phase sensitive detector constructed by switching the gain of an amplifier between +1 and −1 at the modulation frequency with a CMOS switch. The output was filtered with a 7 Hz low pass RC filter and used to drive a proportional–integral controller which maintained the mean angle of incidence at the point where the amplitude of the photocurrent ac signal, and thus the output of the phase sensitive detector, was zero. The system described above maintains the angle of incidence at the second SPR surface at the point where the ac signal is zero. To make differential SPR measurements, changes in the angle of
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D.J.L. Graham, L.R. Watkins / Sensors and Actuators B 159 (2011) 33–38
Photodiode Fresnel rhomb
Analyser Prism
Mirror on pizeo bender
Cylindrical lens SLD
Polariser Aluminium block with two flow cells
O-ring
Flow cell (2)
Prism Gold film
Flow cell (1)
Glass disc
Fig. 4. Schematic of the experimental apparatus for the differential SPR measurements. The beam and components shown as dashed lines are above the incident beam and the insert shows a cross section of the prism and flow cells along the dotted line.
incidence must be monitored and it is not sufficient to simply monitor the drive voltage to the piezo bender as there is substantial hysteresis in the relationship between the angle of the mirror and the applied voltage. The mirror movement was therefore measured directly using an auxiliary beam from the SLD and a position sensitive detector (PSD). The auxiliary beam was created by reflecting a small portion of the main beam with a glass wedge prism and directing it towards the mirror. An f = 400 mm lens was used to focus the light reflected from the mirror onto a <1 mm spot on a Hamamatsu S3931 PSD. This is a photodiode with a 6 × 1 mm active area. The anode has a resistive layer and two connections, one at each end of the active area. The photo-current splits between the two connections. The position of the light spot on the detector, P, can be found from the two photo-currents i1 and i2 by P=
i1 − i2 , i1 + i2
(9)
where P = 0 is the centre of the detector and P = ± 1 is full scale (±3 mm). The cathode is common and is used to reverse bias the diode. As the angle of the mirror changes, the beam will move across the PSD and the position change can be detected. Two photodiode front-end amplifiers were used to convert the PSD photo-currents to voltages which were detected using a 12 bit data acquisition card. The bandwidth of the amplifiers was limited to 2.3 Hz (3 dB) to remove the signal due to the angle of incidence modulation. When making differential SPR measurements the average value of P for each 1-s period was recorded. The PSD was mounted 13.5 cm from the moving mirror. So the full scale (±3 mm) of the PSD corresponds to a ±0.0222 radian change in the angle of the reflected beam. For this small angle, the paraxial approximation holds and the measured P will be proportional to the angle of the reflected beam and thus m . The size of the light spot on the detector and the angle modulation does not affect the observed average P assuming all of the beam is detected.
As the beam reaches the edge of the detector only part of it will be detected and the observed P will no longer be linearly dependent on m . By scanning the rotation stage that the movable mirror was mounted on it was found that P was linear over the range | P |<0.7 and all refractive index measurements will be limited to this range. As was shown in Fig. 3 in the theoretical section of this paper m and thus P are expected to be proportional to the refractive index difference between the two areas of the SPR surface. The exact conversion between m and P depends on the exact position of the cylindrical lens and is therefore difficult to calibrate exactly. In the experimental results P will therefore be presented directly as measured from the photo-currents.
3.1. Refractive index measurements In order to measure the response of the instrument to refractive index changes, solutions of known index were needed. Three solutions were prepared by mixing distilled water with salt (sodium chloride, Ajax Finechem 99.9% pure). The salt and mixed solution were weighed with a precision balance to calculate the concentration. The solutions were stirred until all visible salt crystals had dissolved and then left for at least a day in covered containers to ensure that the salt had completely dissolved and the solutions were at ambient temperature. The difference in refractive index between the solutions and pure water (n = 1.33) was calculated from linear fitting to data from the CRC handbook [17]. The concentration and refractive indices of the solutions used are given in Table 1. Two similar measurements made with the three solutions are shown in Fig. 5. For the measurement shown in the top half of the figure, both of the flow cells were filled with water at the start, as (0) labelled . Solution (1) was then added to flow cell 2, as labelled (0) (0) . The measured position P can be seen to change as there is now (1)
D.J.L. Graham, L.R. Watkins / Sensors and Actuators B 159 (2011) 33–38 Table 1 Salt solutions used to characterise the differential instrument.
−0.118
NaCl %w/w
(n − 1.33)
(0) (1) (2) (3)
0 1.00 2.00 3.00
0 0.00175 0.00350 0.00526
−0.12
−0.122 (0) (0)
P
Solution
a refractive index difference between the two areas and so m (1) changes. Solution (1) was then added to flow cell 1, as labelled . (1) Note that the position P and thus m returns to approximately its starting value as the refractive index difference is zero again. The other two solutions were then injected in order of increasing refractive index with each solution being added to flow cell 2 first, then flow cell 1. At the end of the experiment the flow cells were filled in turn with water. A second set of measurements was made with the solutions now being added to flow cell 1 first and then to flow cell 2 and is shown in the lower half of Fig. 5. As can be seen from the figure, the sensor output changes in response to refractive index differences and is almost independent of the common index. The spikes seen in the data are caused by temperature differences as the solutions were added. The difference in (0) (0) P between and corresponds to a change in m of approx(0) (3) ◦ imately 0.7 . The PSD was mounted so that when P = 0, m ≈ 0. (0) The offset observed in the P value for is therefore believed to (0) be mostly due to differences in the properties of the surfaces at the areas covered by flow cells (1) and (2). Fig. 6 is the same measurement as that shown in the top half of Fig. 5 but with a greatly enlarged scale in order to show the noise and the response to the common index. The small response to the
0.2 0
P
−0.2 −0.4
(0) (0)
(0) (1)
(1) (1)
(1) (2)
(2) (2)
(2) (3)
(3) (3)
8
10
12
37
(3) (0)
(0) (0)
−0.6
(1) (1)
(2) (2)
(3) (3)
(0) (0)
−0.124
−0.126
−0.128
−0.13 0
2
4
6
8
10
12
14
16
18
Time minutes Fig. 6. Sensor response as the salt solutions were added in order of increasing refractive index, then water. For the top graph the solutions were added to flow cell (2) first and for the bottom graph the solutions were added to flow cell (1) first. The labels show the solutions in the two flow cells at different points during the experiment.
common index visible in this figure is expected to be caused by differences in the properties of the gold surface at the location of the first and second reflections. Some drift is seen as the difference (0) at the start and end of the experiment. between the P for (0) A measurement of the instrument’s response to all 16 combinations of the three solutions and water was also made and is shown in Fig. 7. The contents of the two flow cells during the experiment are labelled in the same way as for the previous figure. Again, the sensor response clearly depends on the refractive index difference of the solutions in the two flow cells and not on the common index. The noise and response to the common index for this experiment was very similar to that experienced in the first and illustrated in Fig. 6. For each of the 16 combinations of solutions, the mean P value for the half minute before the next solution was added was calculated. The mean P value vs. the refractive index difference is plotted for all 16 results in Fig. 8. A linear least squares fit to all 16 points is shown to illustrate the linearity of the response. It can be seen that P is linearly dependent on the refractive index difference n and almost independent of the common index n1 . The noise in P 0.6
−0.8 0
2
4
6
14
16
18
Time minutes
0.4
0.6 0.2
0.4 (0) (0)
(1) (0)
(1) (1)
(2) (1)
(2) (2)
(3) (2)
(3) (3)
(0) (3)
(0) (0)
0
P
0.2
P
−0.2
0 −0.4
−0.2 −0.4 0
(0) (0) (0) (0) (1) (1) (1) (1) (2) (2) (2) (2) (3) (3) (3) (3) (0) −0.6 (0) (1) (2) (3) (3) (2) (1) (0) (0) (1) (2) (3) (3) (2) (1) (0) (0)
2
4
6
8
10
12
14
16
18
Time minutes Fig. 5. Sensor response as the salt solutions were added in order of increasing refractive index, then water. For the top graph the solutions were added to flow cell (2) first and for the bottom graph the solutions were added to flow cell (1) first. The labels show the solutions in the two flow cells at different points during the experiment.
−0.8 0
5
10
15
20
25
30
35
Time minutes Fig. 7. Sensor response for all 16 combinations of the salt solutions and water in the flow cells.
38
D.J.L. Graham, L.R. Watkins / Sensors and Actuators B 159 (2011) 33–38
suspected to be due to vibrations and fluctuations at the SPR surface. The noise from the position sensitive detector is also close to limiting the accuracy of the instrument and if the accuracy of refractive index difference measurements is to be improved a more accurate way of monitoring the angle of the second reflection is needed. The range of the instrument was limited to the ±0.005 RIU range demonstrated by the range of the position sensitive detector. Improving the method of monitoring the angle of the second reflection could also increase the measurement range.
0.6 0.4 0.2
P
0 −0.2 n =n 1
References
n1 = nw + 0.0035
−0.6 −0.8 −0.006
w
n1 = nw + 0.00175
−0.4
n1 = nw + 0.00526
−0.004
−0.002
0
0.002
0.004
0.006
Δn Fig. 8. Mean sensor response P vs. refractive index difference (n) for different values of the common index (n1 ). The line shown is a least mean squares linear fit to all 16 points.
was calculated from the standard deviation for the first minute ( P = 0.000125) and together with the sensitivity obtained from the gradient of the best fit line (dP/dn = 103.9), this corresponds to a noise in the measured refractive index difference of 1.2 × 10−6 RIU. This is a noise in m of approximately (1.7 × 10−4 )◦ . The noise was also calculated using the standard deviations for the first minute of the two experiments shown in Fig. 5, yielding 1.4 × 10−6 RIU and 1.5 × 10−6 RIU. 4. Conclusion We have presented a novel technique for measuring the refractive index difference between two areas of an SPR sensor surface. The measurement is insensitive to the common index and so should therefore be largely free of errors due to changes in the bulk index or temperature of the sensed medium. A single, monochromatic beam is incident on the SPR surface and a small mirror mounted on a piezo bender used to effect the return reflection. A quarterwave plate ensures that the p- and s-polarisations are exchanged for the returned beam. The difference in angle of incidence for the two reflections is directly proportional to the refractive index difference and is readily measured by monitoring the movement of the mirror on the piezo bender. Unlike standard SPR sensors which measure the refractive index for particular areas this instrument directly measures the difference in refractive index between two areas. We have demonstrated that this sensor can detect refractive index differences over a ±0.005 RIU range with high linearity and little sensitivity to refractive index changes common to both areas. It is expected that the response to the common index could be reduced by using a more uniform gold surface. A typical noise of 1.5 × 10−6 RIU in the measured refractive index difference was observed. The noise limit observed is
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Biographies David Graham received an MSc degree in physics from the University of Auckland in 2005 and a PhD in 2011. His thesis research topic was SPR sensors for chemical and biological sensing. Lionel Watkins received his PhD from the University of Wales, UK. His research interests include ellipsometry and optical interferometry.
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A differential surface plasmon resonance sensor David J.L. Graham, Lionel R. Watkins ∗ Department of Physics, University of Auckland, P.B. 92019, Auckland, New Zealand
a r t i c l e
i n f o
Article history: Received 21 February 2011 Received in revised form 30 May 2011 Accepted 2 June 2011 Available online 13 June 2011 Keywords: Surface plasmon resonance SPR sensors Refractive index
a b s t r a c t A novel technique that detects only the difference in refractive index between two areas of a surface plasmon resonance (SPR) sensor is presented. Since the measurement is independent of the common index, errors due to changes in the bulk index or temperature of the sensed medium are largely eliminated. A single, monochromatic beam is twice reflected from the SPR surface, with the p- and s-polarisations interchanged between reflections. A linear response to refractive index difference is obtained by measuring the change in angle of incidence at the second reflection required to maintain minimum transmission. Measurements with salt solutions of known refractive index are used to demonstrate that this instrument can detect refractive index differences while being insensitive to changes in the refractive index common to both areas of the SPR surface. A typical noise of 1.5 × 10−6 RIU with a ±0.005 RIU refractive index difference range was achieved. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Optical sensors based on surface plasmon resonance (SPR) are widely used in the study of biomolecular interactions since they provide both a label-free technique and high sensitivity. SPR sensors also find application in the detection of chemical and biological analytes [1]. However, a major limitation to the accuracy of these sensors arises from drifts due to changes in the bulk refractive index or temperature of the sensed medium. One technique commonly used in imaging SPR instruments to minimise these effects is to compare the return from a sensitised area of the sensor to that from an unsensitised reference area [2–4]. Simpler, two channel systems have also been developed and are useful for measuring a single binding reaction, with a reference to correct for bulk index or temperature changes. One approach is to use one of the standard single channel techniques but with two beams and two detectors [5–7]. A two channel measurement can also be made with a single detector by shifting the resonance of one of the channels so that the two resonances can be distinguished. This can be achieved by coating part of the sensor surface with a thin, high refractive index layer [8] which will shift the angle or wavelength at which resonance occurs for that area. An angle or wavelength scan will show two intensity minima corresponding to the two channels. Provided the high index layer is much thinner than the penetration depth of the surface plasmon, the resonance will still change in response to
∗ Corresponding author. Tel.: +64 9 373 7599; fax: +64 9 373 7445. E-mail address: [email protected] (L.R. Watkins). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.06.014
refractive index changes at the surface. Refractive index changes at the coated and uncoated areas can thus be measured separately. Alternatively, one can use a collimated white light source and a specially designed coupling prism that causes the beam to be reflected from two areas of the SPR sensor surface at two different angles of incidence [9], thereby allowing the two resonance to be separated in wavelength. Heterodyne interferometry has also been used to measure the difference in phase for two separate beams reflected from different areas of a SPR surface [10]. Here we present a novel differential SPR instrument that measures the refractive index difference between two areas of a sensor surface while being insensitive to the common index. Our approach is to use a single collimated beam of monochromatic polarised light that is reflected from the SPR surface before being returned, via a quarter-wave plate and mirror, and reflecting from an adjacent area of the same surface. The transmission of our optical system is proportional to the square of the difference of the ppolarisation reflection coefficients for the two areas and hence to the refractive indices at these two areas. As changes in the refractive index have the same effect as changing the angle of incidence at which resonance occurs, the angle of incidence for the second reflection can be adjusted by the mirror so that the reflection coefficients are approximately equal. The angle of incidence difference that makes the reflection coefficients approximately equal can be found as the point of minimum transmission and this difference is approximately proportional to the refractive index difference between the two areas. The instrument presented here detects changes in the angle of incidence difference for minimum transmission.
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D.J.L. Graham, L.R. Watkins / Sensors and Actuators B 159 (2011) 33–38
Detector
0.04 Δθ = 0
Mirror
Analyser -45°
°
0.035
Δθ = −0.3 Δθ = 0.3°
0.03 0.025
T
Quarter-wave plate 45° Polariser 45° (1)
(2)
0.015
Fig. 1. Schematic of the differential SPR system.
0.01 0.005
2. Theory Fig. 1 shows the concept of our differential surface plasmon resonance sensor. A beam of polarised light is reflected from one area of a SPR surface, with reflection coefficients rp1 and rs1 . Two passes through a quarter-wave plate at 45◦ exchanges the p and s-polarised light and the beam is then reflected from a second area of the SPR surface with reflection coefficients rp2 and rs2 . The beam finally passes through an analyser and is incident on a photodetector. We employ the Jones matrix method [11] to determine the response of our instrument. Suppose that light polarised at 45◦ is reflected from the first area of the SPR surface. The electric field E1 leaving this surface is readily found:
E1 =
rp1 0
0 rs1
1 √ 2
1 1
1 = √ 2
rp1 rs1
.
(1)
This beam makes two passes through the quarter-wave plate which is equivalent to a single pass through a half-wave plate and has the effect of exchanging the p- and s-polarised states. A further reflection from the second area of the SPR surface then ensues:
E2 =
rp2 0
0 rs2
1 √ 2
rs1 rp1
1 = √ 2
rp2 rs1 rp1 rs2
.
(2)
If the two surface are identical, rp1 = rp2 = rp and rs1 = rs2 = rs and so the final polarisation state is rp rs E2 = √ 2
1 1
.
(3)
That is, the light is linearly polarised at 45◦ and attenuated by a factor of |rp rs | 2 . If the light is then incident on an analyser set with its transmission axis at −45◦ , the transmission will be zero. If the reflection coefficients of the two surfaces are not identical, then the transmitted light will not be zero but can be calculated from E2
=
1 2
1 −1 −1 1
1 = √ 2 2 where J−45 =
1 2
1 √ 2
rp2 rs1 rp1 rs2
rp2 rs1 − rp1 rs2 rp1 rs2 − rp2 rs1
1 −1 −1 1
, (4)
,
(5)
is the Jones matrix for a polariser with its transmission axis oriented at −45◦ . The transmitted intensity T is then T=
0.02
1 |rp2 rs1 − rp1 rs2 |2 . 4
(6)
The s-polarised light does not couple to the plasmon and so the change in rs with refractive index is much smaller than
0 −5
−4
−3
−2
−1
0
Δn
1
2
3
4
5 −3
x 10
Fig. 2. Transmission vs. refractive index difference for different angle of incidence differences. SPR surface is 50 nm of gold on glass as described in the text. 1 = 71◦ .
for p-polarised light and hence we can use the approximation rs1 ≈ rs2 ≈ rs . The transmission is therefore T≈
1 |rs (rp2 − rp1 )|2 , 4
(7)
and is approximately proportional to the square of the difference between the p-reflection coefficients for the two SPR surfaces. Simulations of a typical SPR system comprising 50 nm of gold (n = 0.161 + 3.64i [12]) on glass (n = 1.51) were made using the scattering matrix method of thin film modelling [11]. The wavelength was 680 nm for all the modelling results presented here. The angle of incidence for the first reflection was 1 = 71◦ , which is approximately the value to give minimum reflectivity for an aqueous external medium (n = 1.33). The refractive index at the first SPR surface is defined to be the common index, n1 . The refractive index at the second surface is defined to be the common index plus the refractive index difference between the two surfaces, n2 = n1 + n. The transmission as a function of the refractive index difference n is shown in Fig. 2. Also shown is the transmission in which there is a difference in the angle of incidence for the two surfaces with 2 = 1 + as this will be useful later. The transmission is zero at n = 0, = 0 and increases as |n | increases. As shown in the figure, changes in the refractive index difference n could be detected from changes in the transmission. However the response is approximately quadratic. Fig. 2 also shows that changing the angle of incidence of the second reflection shifts the n value at which the intensity is a minimum. This is because the main effect of changing the refractive index for a SPR system is to shift the angle of incidence at which the resonance occurs [13]. A more linear measurement of the refractive index difference n can be obtained by measuring the angle of incidence difference at which the minimum transmission occurs. Fig. 3 shows the angle of incidence difference for minimum transmission m vs. refractive index difference for different common indices n1 . The angle of incidence difference for minimum transmission was found numerically for each value of n by calculating the transmission for a range of values and finding the minimum. As can be seen in the figure, m has an almost linear response to the index difference and is almost independent of the common index. The sensitivity of m to the refractive index difference is approximately 140◦ RIU−1 , the same sensitivity to refractive index as a single channel angle scanned instrument using this SPR system. The measurement of m is the basis for the experimental apparatus to be described in the following section. Note that if the angle of incidence or n1 is changed, moving the operating point off resonance, the transmission at n = 0, = 0 would still be zero, but the transmission
D.J.L. Graham, L.R. Watkins / Sensors and Actuators B 159 (2011) 33–38
0.8 0.6 0.4
Δθ
m
0.2 0 −0.2 n = 1.330 1
−0.4
n = 1.335 1
n1 = 1.340
−0.6 −0.8 −5
−4
−3
−2
−1
0
Δn
1
2
3
4
5 −3
x 10
Fig. 3. Angle of incidence difference for minimum transmission (m ) vs. refractive index difference (n) for different common indices (n1 ).
would change more slowly as |n | increased as the reflection coefficients change less rapidly away from resonance. This will reduce the sensitivity with which changes in m can be detected in the presence of optical noise. The simulations presented here have assumed that the SPR surfaces at the location of the first and second reflections are identical, except for the refractive index of the sensed medium. If this is not the case, for example, the thickness or optical properties of the gold film at the two areas of the surface are different, then the angle of incidence for minimum transmission will become sensitive to the common index n1 . Suppose though, that we adjust the analyser azimuthal angle . The transmission T is then readily found to be T≈
1 |rs1 (rp2 cos + rp1 sin )|2 . 2
(8)
If rp for one of the areas changes more quickly as a function of n1 , then can be adjust to make T independent of n1 again [14]. The difference in the rate at which the reflection coefficients change will depend on n1 and the angle of incidence, so this compensation will only be effective over a limited range. Note that an equivalent compensation can be achieved by adjusting the polariser azimuthal angle. This compensation was not necessary for the experimental results presented in the next section. 3. Experimental apparatus Fig. 4 shows a schematic diagram of our differential SPR instrument. To measure m with high accuracy, the angle of incidence difference was modulated sinusoidally using a mirror glued onto a piezo-bender. The ac component of the resulting photocurrent is proportional to the gradient of the intensity as a function of the angle [15]. This signal can be used as the input to a feedback loop that applies an offset to the modulator in order to drive the ac component to zero. The point at which the gradient is zero is the minimum transmission as a function of , that is m . The value of the offset applied by the feedback loop to the mirror is thus proportional to m . With this technique, it is possible to detect changes in the resonance angle with a sensitivity better than 10−3 degrees [15]. Not shown in the figure is an additional beam used to monitor the mirror on the piezo-bender, as will be described later in this section. The light source was a QSDM680-9 super luminescent diode (SLD), centre wavelength 677 nm, spectral width 7.5 nm FWHM. The diode was mounted in a temperature controlled laser diode mount with an aspheric collimation lens. The polariser was
35
a Lambda Research Optics broadband near infra-red polarising beam-splitter cube. It was clamped in a v-mount with its transmission axis at 45◦ to set the initial polarisation state. The intensity of the beam incident on the prism was approximately 2 mW. The arrangement of the SPR system and the flow cell is shown in the insert of Fig. 4. A high index (SF2 glass n = 1.64) prism was used to reduce the angle of incidence for coupling to the plasmon and to make the alignment easier. An SPR sensor surface from Ssens was used. This was a 25 mm diameter, 1 mm thick glass (n = 1.51) disc coated with a 2 nm titanium (typical n = 2.81 + 3.90i at 680 nm [16]) adhesion layer and 50 nm of gold (typical n = 0.161 + 3.64i at 680 nm [12]). Index matching oil from Cargile (n = 1.51) was used between the sensor disc and prism to reduce reflections from this interface. The prism and flow cell were mounted on a rotation stage to allow the angle of incidence to be controlled. The angle of incidence at the prism was 1.3◦ for all the experimental presented here. Applying Snell’s law to the air–prism and prism–disc interfaces indicates that the angle of incidence for the first reflection from the SPR surface was 71.4◦ . An aluminium block with two flow cells was clamped to the SPR sensor disc. The bottom flow cell (1) covers the area of the SPR surface where the first reflection occurs and the top flow cell (2) covers the area where the second reflection occurs. The flow cells were both approximately 0.1 ml in volume. Two holes drilled through the block into each flow cell, with plastic pipe fittings on the outside, formed the inlet and outlet. Each flow cell inlet was connected through a 4-1 selector valve to four plastic 60 ml syringes that could contain distilled water or salt solutions with different refractive indices. A tube from the outlet of each flow cell led to a waste bucket. When changing the solution in a flow cell, 5 ml of the new solution was flowed through the cell to ensure that the previous solution was completely replaced. A Peltier thermoelectric cooler (TEC) driven by a PID controller was used to stabilise the temperature of the flow cell at 20 ◦ C. The angle of incidence of the second reflection from the SPR surface was controlled using a movable mirror. The mirror was a 6 × 3 mm piece of microscope cover slip with a 40 nm gold coating. The mirror was glued to one end of a Physik Instrumente PL112.11 piezo bender actuator, the other end of which was clamped to a mirror mount on top of a rotation stage. The actuator was driven by a 240 Hz sinusoid and with an amplitude such that the angle of the beam reflected from the mirror changed by approximately ±0.1◦ . The mirror was placed 30 cm from the prism and a 7.5 cm focal length cylindrical lens was placed halfway between them to reduce the movement of the beam across the SPR surface as the angle of the mirror about the vertical axis changed due to the bending of the piezo. The mirror was set slightly tilted about a horizontal axis to steer the beam so that the second reflection from the SPR surface occurs at the part of the surface covered by flow cell (2). A Thorlabs Fresnel rhomb was used as the quarter-wave plate, mounted in a manual rotation stage and set at 45◦ . After the second reflection from the SPR surface, the beam was focused onto a Glan–Thompson polariser with a f = 70 mm lens. The axis of the polariser was set at −45◦ and the transmitted light focused onto the detector with a f = 35 mm doublet lens. The ac component of the photocurrent was amplified and fed to a simple phase sensitive detector constructed by switching the gain of an amplifier between +1 and −1 at the modulation frequency with a CMOS switch. The output was filtered with a 7 Hz low pass RC filter and used to drive a proportional–integral controller which maintained the mean angle of incidence at the point where the amplitude of the photocurrent ac signal, and thus the output of the phase sensitive detector, was zero. The system described above maintains the angle of incidence at the second SPR surface at the point where the ac signal is zero. To make differential SPR measurements, changes in the angle of
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D.J.L. Graham, L.R. Watkins / Sensors and Actuators B 159 (2011) 33–38
Photodiode Fresnel rhomb
Analyser Prism
Mirror on pizeo bender
Cylindrical lens SLD
Polariser Aluminium block with two flow cells
O-ring
Flow cell (2)
Prism Gold film
Flow cell (1)
Glass disc
Fig. 4. Schematic of the experimental apparatus for the differential SPR measurements. The beam and components shown as dashed lines are above the incident beam and the insert shows a cross section of the prism and flow cells along the dotted line.
incidence must be monitored and it is not sufficient to simply monitor the drive voltage to the piezo bender as there is substantial hysteresis in the relationship between the angle of the mirror and the applied voltage. The mirror movement was therefore measured directly using an auxiliary beam from the SLD and a position sensitive detector (PSD). The auxiliary beam was created by reflecting a small portion of the main beam with a glass wedge prism and directing it towards the mirror. An f = 400 mm lens was used to focus the light reflected from the mirror onto a <1 mm spot on a Hamamatsu S3931 PSD. This is a photodiode with a 6 × 1 mm active area. The anode has a resistive layer and two connections, one at each end of the active area. The photo-current splits between the two connections. The position of the light spot on the detector, P, can be found from the two photo-currents i1 and i2 by P=
i1 − i2 , i1 + i2
(9)
where P = 0 is the centre of the detector and P = ± 1 is full scale (±3 mm). The cathode is common and is used to reverse bias the diode. As the angle of the mirror changes, the beam will move across the PSD and the position change can be detected. Two photodiode front-end amplifiers were used to convert the PSD photo-currents to voltages which were detected using a 12 bit data acquisition card. The bandwidth of the amplifiers was limited to 2.3 Hz (3 dB) to remove the signal due to the angle of incidence modulation. When making differential SPR measurements the average value of P for each 1-s period was recorded. The PSD was mounted 13.5 cm from the moving mirror. So the full scale (±3 mm) of the PSD corresponds to a ±0.0222 radian change in the angle of the reflected beam. For this small angle, the paraxial approximation holds and the measured P will be proportional to the angle of the reflected beam and thus m . The size of the light spot on the detector and the angle modulation does not affect the observed average P assuming all of the beam is detected.
As the beam reaches the edge of the detector only part of it will be detected and the observed P will no longer be linearly dependent on m . By scanning the rotation stage that the movable mirror was mounted on it was found that P was linear over the range | P |<0.7 and all refractive index measurements will be limited to this range. As was shown in Fig. 3 in the theoretical section of this paper m and thus P are expected to be proportional to the refractive index difference between the two areas of the SPR surface. The exact conversion between m and P depends on the exact position of the cylindrical lens and is therefore difficult to calibrate exactly. In the experimental results P will therefore be presented directly as measured from the photo-currents.
3.1. Refractive index measurements In order to measure the response of the instrument to refractive index changes, solutions of known index were needed. Three solutions were prepared by mixing distilled water with salt (sodium chloride, Ajax Finechem 99.9% pure). The salt and mixed solution were weighed with a precision balance to calculate the concentration. The solutions were stirred until all visible salt crystals had dissolved and then left for at least a day in covered containers to ensure that the salt had completely dissolved and the solutions were at ambient temperature. The difference in refractive index between the solutions and pure water (n = 1.33) was calculated from linear fitting to data from the CRC handbook [17]. The concentration and refractive indices of the solutions used are given in Table 1. Two similar measurements made with the three solutions are shown in Fig. 5. For the measurement shown in the top half of the figure, both of the flow cells were filled with water at the start, as (0) labelled . Solution (1) was then added to flow cell 2, as labelled (0) (0) . The measured position P can be seen to change as there is now (1)
D.J.L. Graham, L.R. Watkins / Sensors and Actuators B 159 (2011) 33–38 Table 1 Salt solutions used to characterise the differential instrument.
−0.118
NaCl %w/w
(n − 1.33)
(0) (1) (2) (3)
0 1.00 2.00 3.00
0 0.00175 0.00350 0.00526
−0.12
−0.122 (0) (0)
P
Solution
a refractive index difference between the two areas and so m (1) changes. Solution (1) was then added to flow cell 1, as labelled . (1) Note that the position P and thus m returns to approximately its starting value as the refractive index difference is zero again. The other two solutions were then injected in order of increasing refractive index with each solution being added to flow cell 2 first, then flow cell 1. At the end of the experiment the flow cells were filled in turn with water. A second set of measurements was made with the solutions now being added to flow cell 1 first and then to flow cell 2 and is shown in the lower half of Fig. 5. As can be seen from the figure, the sensor output changes in response to refractive index differences and is almost independent of the common index. The spikes seen in the data are caused by temperature differences as the solutions were added. The difference in (0) (0) P between and corresponds to a change in m of approx(0) (3) ◦ imately 0.7 . The PSD was mounted so that when P = 0, m ≈ 0. (0) The offset observed in the P value for is therefore believed to (0) be mostly due to differences in the properties of the surfaces at the areas covered by flow cells (1) and (2). Fig. 6 is the same measurement as that shown in the top half of Fig. 5 but with a greatly enlarged scale in order to show the noise and the response to the common index. The small response to the
0.2 0
P
−0.2 −0.4
(0) (0)
(0) (1)
(1) (1)
(1) (2)
(2) (2)
(2) (3)
(3) (3)
8
10
12
37
(3) (0)
(0) (0)
−0.6
(1) (1)
(2) (2)
(3) (3)
(0) (0)
−0.124
−0.126
−0.128
−0.13 0
2
4
6
8
10
12
14
16
18
Time minutes Fig. 6. Sensor response as the salt solutions were added in order of increasing refractive index, then water. For the top graph the solutions were added to flow cell (2) first and for the bottom graph the solutions were added to flow cell (1) first. The labels show the solutions in the two flow cells at different points during the experiment.
common index visible in this figure is expected to be caused by differences in the properties of the gold surface at the location of the first and second reflections. Some drift is seen as the difference (0) at the start and end of the experiment. between the P for (0) A measurement of the instrument’s response to all 16 combinations of the three solutions and water was also made and is shown in Fig. 7. The contents of the two flow cells during the experiment are labelled in the same way as for the previous figure. Again, the sensor response clearly depends on the refractive index difference of the solutions in the two flow cells and not on the common index. The noise and response to the common index for this experiment was very similar to that experienced in the first and illustrated in Fig. 6. For each of the 16 combinations of solutions, the mean P value for the half minute before the next solution was added was calculated. The mean P value vs. the refractive index difference is plotted for all 16 results in Fig. 8. A linear least squares fit to all 16 points is shown to illustrate the linearity of the response. It can be seen that P is linearly dependent on the refractive index difference n and almost independent of the common index n1 . The noise in P 0.6
−0.8 0
2
4
6
14
16
18
Time minutes
0.4
0.6 0.2
0.4 (0) (0)
(1) (0)
(1) (1)
(2) (1)
(2) (2)
(3) (2)
(3) (3)
(0) (3)
(0) (0)
0
P
0.2
P
−0.2
0 −0.4
−0.2 −0.4 0
(0) (0) (0) (0) (1) (1) (1) (1) (2) (2) (2) (2) (3) (3) (3) (3) (0) −0.6 (0) (1) (2) (3) (3) (2) (1) (0) (0) (1) (2) (3) (3) (2) (1) (0) (0)
2
4
6
8
10
12
14
16
18
Time minutes Fig. 5. Sensor response as the salt solutions were added in order of increasing refractive index, then water. For the top graph the solutions were added to flow cell (2) first and for the bottom graph the solutions were added to flow cell (1) first. The labels show the solutions in the two flow cells at different points during the experiment.
−0.8 0
5
10
15
20
25
30
35
Time minutes Fig. 7. Sensor response for all 16 combinations of the salt solutions and water in the flow cells.
38
D.J.L. Graham, L.R. Watkins / Sensors and Actuators B 159 (2011) 33–38
suspected to be due to vibrations and fluctuations at the SPR surface. The noise from the position sensitive detector is also close to limiting the accuracy of the instrument and if the accuracy of refractive index difference measurements is to be improved a more accurate way of monitoring the angle of the second reflection is needed. The range of the instrument was limited to the ±0.005 RIU range demonstrated by the range of the position sensitive detector. Improving the method of monitoring the angle of the second reflection could also increase the measurement range.
0.6 0.4 0.2
P
0 −0.2 n =n 1
References
n1 = nw + 0.0035
−0.6 −0.8 −0.006
w
n1 = nw + 0.00175
−0.4
n1 = nw + 0.00526
−0.004
−0.002
0
0.002
0.004
0.006
Δn Fig. 8. Mean sensor response P vs. refractive index difference (n) for different values of the common index (n1 ). The line shown is a least mean squares linear fit to all 16 points.
was calculated from the standard deviation for the first minute ( P = 0.000125) and together with the sensitivity obtained from the gradient of the best fit line (dP/dn = 103.9), this corresponds to a noise in the measured refractive index difference of 1.2 × 10−6 RIU. This is a noise in m of approximately (1.7 × 10−4 )◦ . The noise was also calculated using the standard deviations for the first minute of the two experiments shown in Fig. 5, yielding 1.4 × 10−6 RIU and 1.5 × 10−6 RIU. 4. Conclusion We have presented a novel technique for measuring the refractive index difference between two areas of an SPR sensor surface. The measurement is insensitive to the common index and so should therefore be largely free of errors due to changes in the bulk index or temperature of the sensed medium. A single, monochromatic beam is incident on the SPR surface and a small mirror mounted on a piezo bender used to effect the return reflection. A quarterwave plate ensures that the p- and s-polarisations are exchanged for the returned beam. The difference in angle of incidence for the two reflections is directly proportional to the refractive index difference and is readily measured by monitoring the movement of the mirror on the piezo bender. Unlike standard SPR sensors which measure the refractive index for particular areas this instrument directly measures the difference in refractive index between two areas. We have demonstrated that this sensor can detect refractive index differences over a ±0.005 RIU range with high linearity and little sensitivity to refractive index changes common to both areas. It is expected that the response to the common index could be reduced by using a more uniform gold surface. A typical noise of 1.5 × 10−6 RIU in the measured refractive index difference was observed. The noise limit observed is
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Biographies David Graham received an MSc degree in physics from the University of Auckland in 2005 and a PhD in 2011. His thesis research topic was SPR sensors for chemical and biological sensing. Lionel Watkins received his PhD from the University of Wales, UK. His research interests include ellipsometry and optical interferometry.