Download Tuning Dynamic Range and Sensitivity of White

Anal. Chem. 1999, 71, 5116-5122
Tuning Dynamic Range and Sensitivity of
White-Light, Multimode, Fiber-Optic Surface
Plasmon Resonance Sensors
Louis A. Obando and Karl S. Booksh*
Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287
This paper presents the advantages of modifying the
geometry of the sensing tip of a white-light, multimode
optical fiber SPR sensor to optimize the dynamic range
and sensitivity. By selectively beveling the distal end of
the fiber probe, the wavelength of resonance can be redshifted by more than 100 nm and blue-shifted by more
than 30 nm. This increases the flexibility of a white-light
SPR sensor by increasing the dynamic range of accessible
RIs and by shifting the resonance to the most sensitive
regions of the detector. Sensitivity, measured in wavelength shift per RI change, can be increased by a factor
of 4. Also, multiple-wavelength regions of SPR activity can
simultaneously be observed on the same probe, thus
increasing the information content of a SPR spectrum.
Surface plasmon resonance (SPR) spectroscopy has been
employed for quantitative and qualitative analysis in analytical
chemistry,1-3 biochemistry,4-7 physics,8,9 and engineering10-13
applications. Being a surface technique that is sensitive to changes
of 10-5-10-6 refractive index (RI) units within approximately 200
nm of the SPR sensor/sample interface, SPR spectroscopy is
becoming increasingly popular for monitoring the growth of thin
organic films deposited on the sensing layer.14-17 As little as 0.01
nm of average film deposition can be detected when the RI
(1) Brecht, A.; Gauglitz, G. Anal. Chem. Acta 1997, 347, 219-233.
(2) Marco, M.-P.; Gee, S.; Hammock, B. D. TrAC, Trends Anal. Chem. 1995,
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Chem. 1998, 70, 703-706.
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Hubscher, U.; Ducommun, B. FEBS Lett. 1996, 391, 66-70.
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229-240.
(7) Jordan, C. E.; Corn, R. M. Anal. Chem. 1997, 69, 1449-1456.
(8) Lerme, J.; Palpant, B.; Prevel, B.; Cottancin, E.; Pellerin, M.; Treilleux, M.;
Vialle, J. L.; Perez, A.; Broyer, M. Eur. Phys. J. D 1998, 4, 95-108.
(9) de Brujin, H. E.; Kooyman, R. P. H.; Greve, J. Appl. Opt. 1990, 29, 19741978.
(10) Bender, W. J.; Dessey, R. E.; Miller, M. S.; Claus, R. O. Anal. Chem. 1994,
66, 963-970.
(11) Ruiz, E. G.; Garces, I.; Aldea, C.; Lopez, M. A.; Mateo, J.; Chamarro, J. A.;
Alegret, S. Sens. Actuators A 1993, 37-38, 221-225.
(12) Matsubara, K.; Kawata, S.; Minami, S. Appl. Spectrosc. 1988, 42, 13751379.
(13) Weiss, M. N.; Srivastava, S.; Groger, H. Electron. Lett. 1996, 32, 842-843.
(14) Jung, L. S.; Campbell, C. T.; Chinowsky, T. M.; Mar, M. N.; Yee, S. S.
Langmuir 1998, 14, 5636-5648.
(15) Hausch, M.; Beyer, D.; Knoll, W.; Zentel, R. Langmuir 1998, 14, 72317216.
5116 Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
difference between the film and bulk solution is 0.1 RI unit.14 Thus,
a submonolayer of adsorbed protein-like substance (RI 1.4) from
an aqueous solution (RI 1.3) can easily be observed. However,
there is an inherent lack of selectivity of the SPR effect for
quantitative molecular analyses that must be overcome by functionalizing the metal surface with analyte-specific receptors (i.e.,
antibodies).14,18-21 Depending on the binding constant of the
receptor-ligand system, a reversible probe can be constructed
with nanomolar sensitivity.
The physics of the SPR effect has been extensively described.22,23 The evanescent field of a photon can optically excite
a standing charge-density wave on a thin, metal surface. The wave
function of this SP wave is governed by the angle and energy of
the incident photon as well as the complex dielectric constants of
both the metal film and the substrate on which the film is
deposited. If the wave function of the SP matches the wave
function of the environment at the metal surface, largely determined by the complex dielectric constant (refractive index) of the
medium in contact with the metal layer, the energy of the photon
is transferred from the SP wave to the medium. As the RI of the
medium in contact with the SP wave increases, the wavelength
and angle from normal for optimal SPR coupling increases.
To exploit the SP effect for analytical applications, two types
of SPR sensors have been employed: constant-angle sensors and
constant-wavelength sensors. Constant-angle SPR sensors employ
the traditional “Kretschmann configuration” where a metal-coated
BK7 glass prism is used for the sensing area. Monochromatic
light is passed through the prism and reflected off the metal-coated
sensing area. The normalized intensity of reflected light (versus
an air or water reference) is plotted against the incident angle of
the photon.17,24 This system is easily modified to perform white(16) Lawrence, C. R.; Martin, A. S.; Sambles, R. Thin Solid Films 1992, 208,
269-273.
(17) Frutos, A. G.; Corn, R. M. Anal. Chem. 1998, 70, 449A-455A.
(18) Sasaki, S.; Nagata, R.; Hock, B.; Karube, I. Anal. Chem. Acta 1998, 368,
71-76.
(19) Lofas, S.; Johnsson, B. J. Chem. Soc., Chem. Commun. 1990, 1526-1528.
(20) Lofas, S.; Johnsson, B.; Edstrom, A.; Hannson, A.; Lindquist, G.; Hillgren,
R.-M. M.; Stigh, L. Biosens. Bioelectron. 1995, 10, 813-822.
(21) Berger, C. E. H.; Beumer, T. A. M.; Kooyman, R. P. H.; Greve, J. Anal.
Chem. 1998, 70, 703-706.
(22) Raether, H. Surface Plasmons on Smooth and Rough Surfaces and on Gratings;
Springer-Verlag: Berlin, 1988.
(23) Liebsch, A. Electronic Excitations at Metal Surfaces; Plennum Press: New
York, 1997.
(24) Lenferink, A. T. M.; Kooyman, R. P. H.; Greve, J. Sens. Actuators B 1991,
3, 261-265.
10.1021/ac990470f CCC: $18.00
© 1999 American Chemical Society
Published on Web 10/09/1999
Figure 1. Simulation showing how λSPR increases nonlinearly with increasing θinc and the wavelength of light. Reflection intensity is lowest
(maximum energy transfer) at the darkest areas. The two smaller graphs show how λSPR is red-shifted when θinc is increased by 10°.
light SPR measurements.14,25-28 Collimated white light is reflected
off the metal layer and dispersed across an array detector. The
collected spectrum is normalized (versus an air or water reference)
and plotted as a function of wavelength. Both the laser and whitelight systems are readily conformed to automated, benchtop, flowthrough instruments. In fact, such a laser-based instrument is
marketed and optimized for bioanalytical analyses.19,20 However,
these prism or planar substrate-based sensors are not well-suited
for field or in situ analysis; to do this, optical fiber-based SPR
sensors are needed.
Consequently, fiber-optic-based SPR sensors have recently
attracted much attention.10,13,29-32 Numerous sensor geometries
have been proposed for the fiber sensors. Some have attached
prisms to the end of an optical fiber probe.14,26,33 In general, singlemode optical fibers have been employed. Either a small patch of
cladding is removed from the center of a length of optical fiber to
expose the sensing area10,13,30,31 or a mirror is affixed to the distal
(25) Johnston, K. S.; Chinowsky, T. M.; Yee, S. S. Proc. SPIE 1996, 2836, 178185 (Chemical, Biochemical, and Environmental Chemical Sensors VIII).
(26) Homola, J.; Schwotzer, G.; Lehman, H.; Willsch, R.; Ecke, W.; Bartelt, H.
Proc. SPIE 1995, 2508, 324-333 (Chemical, Biochemical, and Environmental Fiber Sensors VII).
(27) Johnston, K. S.; Yee, S. S.; Booksh, K. S. Anal. Chem. 1997, 69, 18441851.
(28) Jory, M. J.; Bradberry, G. W.; Cann, P. S.; Sambles, J. R. Meas. Sci. Technol.
1995, 6, 1193-1200.
(29) Fontana, E.; Dulman, H. D.; Doggett, D. E.; Pantell, R. H. IEEE Trans.
Instrum. Meas. 1998, 47, 168-173.
(30) Ronot-Trioli, C.; Trouillet, A.; Veillas, C.; El-Shaikh, A.; Gagnaire, H. Anal.
Chem. Acta 1996, 319, 121-127.
(31) Alonso, R.; Villuendas, F.; Tornos, J.; Pelayo, J. Sens. Actuators A 1993,
37-38, 187-192.
(32) Jorgenson, R. C.; Yee, S. S. Sens. Actuators B 1993, 12, 213-220.
(33) Garces, I.; Aldea, C.; Mateo, J. Sens. Actuators B 1992, 7, 771-774.
tip of the optic fiber and the exposed sensing area is near the
tip.29,32 Single-mode fibers have the advantage of maintaining the
polarization of the exciting light and propagating only a single
angle of reflection through the fiber. Together this maintains a
sharp SPR dip without an excessive reflective background.
Concurrently, multimode optical fibers have been largely avoided
for true SPR fiber-based sensors. The mixing of modes and loss
of polarization leads to relatively shallow, broad reflectance dips
compared to prism-based sensors.27 However, any loss of calibration precision stemming from inability to reliably determine the
exact minimum of the SPR spectra can be overcome by employing
multivariate calibration.27,34
The location of λSPR can be predicted for a given incident angle,
and vice versa, using a simulation that calculates the reflection
coefficients for a multilayer structure based on Fresnel and
Maxwell equations (Figure 1).35-37 It was assumed that the
incident angle at the fiber interface was the average propagation
angle in the fiber and it was not necessary to include the effects
of the angular distribution of light rays in multimode fibers. The
simulation predicts that an increase in the incident angle will excite
SPR at a longer wavelength. By tapering the probes, the angle of
incidence is increased by the degree of the angle of taper. SPR
minimum are increasingly red-shifted as the amount of the taper
is increased. The simulation also shows that the number of
(34) Johnston, K. S.; Booksh, K. S.; Chinowsky, T.; Lee, S. S. Sens. Actuators B
1999, 54, 80-88.
(35) Ishimaru, A. Electromagnetic Wave Propagation, Radiation, and Scattering;
Prentice Hall: Englewood Cliffs, NJ, 1991.
(36) Griffiths, D. J. Introduction to Electrodynamics, 2nd ed.; Prentice Hall:
Englewood Cliffs, NJ, 1981.
(37) Johnston, K. S. Characterization of Thin Films Using Surface Plasmon
Resonance. Master Thesis, University of Washington, 1995.
Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
5117
incident angle-wavelength combinations that excite SPR increases at longer wavelengths. The resulting SPR spectrum is a
sum of those excitations, and consequently, SPR features become
broader as the resonant wavelength increases. These features are
broader than those produced by techniques that use a single
incident wavelength or angle.
Coupling multivariate calibration with white-light, multimode
fiber-optic SPR systems presents the possibility of constructing
small, field-portable SPR sensors. It has been shown that multivariate calibration with 32-nm resolution yields the same analytical
accuracy and precision as “minimum-hunt” calibration with 1-nm
resolution.27 White-light, fiber-optic systems can be constructed
with small battery-powered lamps and low-resolution spectrographs that fit onto PC cards. Readily replaceable and interchangeable sensing areas can be affixed onto the end of the optical fibers
with SMA-type connectors.
One problem that needs to be addressed is optimizing the
dynamic range and sensitivity of these field-portable systems.
Currently, the desired wavelength of SPR resonance for a given
refractive index is tuned by adding SiOx, or other dielectric
coatings, either above or below the metal sensing area.10,13,33 These
overlays are not ideal for three reasons. First, coating the metal
surface with the dielectric prevents functionalization of the metal
with analyte-specific complexing agents (i.e., thiol binding of
antibodies to a gold layer). Second, applying a dielectric coating
requires expensive equipment such as an electron beam evaporator. And third, overlays can only increase the dynamic range of a
sensor at the expense of sensitivity; by the nature of an overlay,
the analyte is moved away from the sensor, minimizing the effect
of the sample coupling with the SP wave.
EXPERIMENTAL SECTION
A white-light, fiber-optic SPR refractometer was constructed
similarly to that proposed by Jorgenson and Yee.32 Full-spectrum
light from a 50-W QTH source (Oriel) was collected by a 0.38
numerical aperture (NA), 400 mm core, low OH optical fiber (3M;
FT-400-EMT). Light exiting this optical fiber jumper was collimated and passed through a 50/50 broad-band, nonpolarizing
beam splitter (Oriel). Half of the light was then focused into a
fiber jumper that led to the SPR probe tip, and half was lost. The
distal end of this fiber jumper and the SPR probe tip were each
connected with a SMA connector. The jumper and the probe were
thus joined by a SMA to SMA adapter (Thor Labs). Light reflected
from the tip of the probe returned up the second fiber jumper
and passed through the beam splitter with half being lost and
half being focused onto a third fiber-optic jumper. The distal end
of the third jumper was SMA connected and attached to a f/2.2
holographic spectrograph (Kaiser). The spectrograph provided
400 nm of spectral coverage from 530 to 930 nm. Dispersed light
was collected by a 256 × 1024 pixel CCD camera (Princeton
Instruments). The camera was thermoelectrically cooled to -38
°C.
Rough probe tips were prepared by securing approximately
5.5 cm of optical fiber into the SMA connector with 48-h epoxy.
The Tefzel buffer was removed with a mechanical stripper and
the cladding dissolved in acetone. This exposed 0.5 cm of fiber
core for dual tapering with shaft and straight probes and
approximately 0.25 cm for the dual-tapered probes. A polishing
chuck was machined that permits polishing the fiber tip 90, 80,
5118 Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
Figure 2. Fiber probe tip archetypes investigated for SPR analyses: (A) straight (ST), (B) dual-tapered (DT), (C) dual-tapered with
shaft (DS), and (D) chiseled/truncated chiseled.
75, 70, 60, 30, 20, 15, and 10° from normal. Each polished surface
of the fiber was polished on 5-, 3-, 1-, and 0.3-mm lapping films
until visually smooth. The probe tips were then sputter-coated with
1 nm of Cr and 50 nm of Au (Cressington). Coating thickness
was monitored electronically by a quartz crystal microbalance
placed in the sputtering chamber (Cressington).
One hundred sucrose (Aldrich) solutions from 0 to 50% by
mass were prepared by dilutions from a stock sucrose solution.
This resulted in samples having RIs from 1.3298 to 1.4148 as
verified by a low-cost refractometer (Fisher Scientific). Although
the analyses and solutions could be equivalently presented in
terms of RI, the analytical precision of the balance (MettlerToledo), 0.1 mg, is greater than the analytical precision of the
available refractometer, 1 × 10-4 RI, for the volume prepared.
Spectra were collected using Winspec software included with the
CCD camera and controller (Princeton Instruments). A 100-ms
integration time was chosen for data collection, and each spectrum
represents the average of 25 integrations. House-written code was
employed to convert the Winspec files to ASCII text files and
consequently to import the text files into Matlab (MathWorks Inc.)
All data manipulations were performed in the Matlab operating
environment. Reported SPR spectra are determined by taking the
ratio of each reflectance spectrum to an air reflectance spectrum
collected at the beginning of a sample run. Subpixel wavelength
location of each SPR dip was found by the roots of a second-order
polynomial fit about the local minima of each dip.
RESULTS AND DISCUSSION
Initial Investigation of Probe Designs. Three probe designs
were selected for presentation in this paper. The traditional
straight (ST) probe (Figure 2A) is of the type employed in other
white-light, multimode optical fiber applications.27,33 The dualtapered (DT) probe (Figure 2B) has one side and the front mirror
polished at complimentary angles to maintain orthogonality
between the two surfaces. Thus if angle A were polished 75° from
normal to the long axis of the fiber, the front mirror would be
polished 90° - 75° ) 15° from normal. The dual-tapered plus shaft
(DS) probe (Figure 2C) maintains the tapered sensing area but
adds a length of straight, unaltered sensing area to the probe tip.
Other probe designs, such as a chisel-tipped probe, where the
top and bottom of the probe were polished to the same angle,
Figure 3. Relative SPR spectra of sucrose samples collected with the ST probe design.
and a truncated chisel-tipped probe (Figure 2D), were investigated.
It was determined that these probe designs do not return sufficient
light to be of use. Incorporating one 90° angle in the probe tip is
required to maintain a majority of the reflected light below the
acceptance angle of the fiber.
More complicated three-dimensional probe tip designs based
on conical or truncated conical geometries were rejected. Although
these designs may present useful properties, the difficulty of
constructing such a probe is prohibitive. Forming a conical tip
on an optical fiber would require a pipet puller, a HF acid bath,
or an expensive automatic polisher. Instead, the scope of this
investigation is limited to designs that can be easily and reproducibly constructed in most laboratories.
Straight Probes. A collection of SPR spectra of aqueous
sucrose solutions spanning 0-50% sucrose (by weight) is presented in Figure 3. These are the reflection spectra that have been
normalized by the reflection spectrum of air. A number of features
that are particular of white light SPR sensors are evident in Figure
3. First, the relative reflection intensity never drops below 50%.
Only the light p-polarized with respect to the sensor surface is
SPR active; thus, half the photon flux down the sensor has no
probability of absorbance. Second, the reflectance dips are rather
broad compared to prism-based white-light refractometers. This
is caused by the multimode nature of the fiber; photons traveling
down the fiber will interact with the sensing surface with a
distribution of angles equal to the acceptance cone of the fiber.
Thus, each angle will have a slightly different wavelength of
maximum coupling for a given solution. The resulting SPR
spectrum is an ensemble over the range of propagating angles.
Third, the information content is most readily visible in the
spectral shift, not in intensity changes, across all samples.
However, over small RI ranges univariate intensity calibration is
possible,10,21,33 but multivariate calibration methods are needed for
global intensity-based analyses.27 Fourth, the shift in wavelength
coupling is not linear with respect to analyte concentration or RI
changes. The SPR spectra shift only 75 nm from 1.329 to 1.372
RI units (0-25% sucrose), but shift 225 nm from 1.372 to 1.415 RI
units (25-50% sucrose).
Figure 4. Calibration curve for minimum fit of white-light, fiber-optic
SPR probe.
The third and fourth points from above are shown in Figure 4
and Table 1, respectively. The calibration curve for 100 sucrose
samples is shown in a plot of solution refractive index versus
wavelength of minimum. There is a strong, systematic, albeit
nonlinear, trend that is easily exploited for calibration. Taking the
derivative of the calibration curve demonstrates the differential
in sensitivity at lower and higher RI (Table 1). The sensitivity of
the spectral shift is approximately 2000 nm/RI unit for the most
dilute solutions and almost 10 000 nm/RI unit for the most
concentrated solutions analyzed (Table 1).
With this probe geometry, refractive indexes above 1.42 (50%
sucrose) and below 1.33 (water) are not reasonably accessible.
Increasing the refractive index above 1.43 would push the SPR
dip outside the range of the spectrograph. Even if a moveable
grating spectrograph were employed, the sensitivity of the detector
decreases significantly at wavelengths greater than 900 nm.
Similarly, the SPR dip of a sample with a RI less than 1.32 would
fall in the noisy region of the spectra between 530 and 550 nm or
would lie off the detector altogether.
Dual-Tapered Probes. Applying a dual taper to the tip of the
fiber probe can dramatically shift the wavelength of maximum
Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
5119
Table 1. Sensitivity and Dynamic Range of the SPR Probesa
sensitivity [δλ/δRI (×1.0 × 103)]
dual taper + shaftb
dual taperb
refractive index
(% sucrose by mass)
straight
1.3298 (0)
1.3468 (10)
1.3638 (20)
1.3723 (25)
1.3808 (30)
1.3893 (35)
1.3978 (40)
1.4148 (50)
1.15
1.59
2.02
2.45
3.11
4.11
5.51
9.86
80/10
DT
75/15
DT
80/10
DS
80/10
DS
75/15
DS 1
75/15
DS 2
70/20
DS 1
60/30
DS 1
1.38
2.29
3.23
4.35
6.22
9.12
1.52
3.74
5.12
7.04
0.19
1.04
1.68
2.17
2.90
3.99
5.52
10.31
0.81
2.16
2.86
3.52
4.67
6.53
0.29
1.27
1.80
2.21
2.88
3.96
5.56
10.83
0.63
4.89
8.52
0.63
1.24
1.65
2.03
2.66
3.64
5.07
9.65
0.72
1.03
1.46
1.91
2.63
3.71
5.21
9.84
a Sensitivity is given as the change in wavelength (nm) per change of 1 refractive index unit. b DT, dual taper; DS, dual taper shaft; 1, first dip;
2, second dip.
Figure 5. Relative SPR spectra of distilled water for ST and two
DT probe designs.
SPR coupling. However, with this red-shift comes a substantial
broadening of the SPR dip (Figure 5). An 80°/10° DT shifts the
resonance of water 73 nm and a 75°/15° DT shifts the water
resonance an additional 36 nm. Applying a 60°/30° dual taper
shifted the water resonance to the edge of the detector and the
resonance dip was so broad that efficient location of the minimums
was difficult. Small-angle tapers, such as 85°/5°, would yield redshifts less than 70 nm and largely maintain the sharp SPR dip.
With the increasing red-shift in the SPR dip comes increasing
sensitivity and decreasing dynamic range of the SPR probe. Figure
6 presents the derivatives of the nonlinear calibration curves for
an ST and two DT probes. The 75°/15° DT probe has 2-3 times
the sensitivity of the ST probe over the range of 0-25% sucrose.
Concurrently, the ST probe is sensitive to changes above 25%
sucrose while the 75°/15° DT probe can monitor changes below
1.33 RI. The dynamic range and sensitivity of the 80°/10° DT
probe lies between that of the ST and the 75°/15° DT probes.
It is important to note that there is not a straight trade between
sensitivity and dynamic range, as there is with most linear
calibration applications or with adding a dielectric overlay to the
sensing area. Tapering the fiber shifts the dynamic range of the
sensor to lower RI regions; the greater the taper, the greater the
shift. However, the dynamic range is most sensitive to changes
in the tapered angle for small deviations from the ST probe. Also,
there is a relative loss of sensitivity at the highest accessible RI
with the dual-tapered probes. For each of the probes geometries
presented in Figure 6, the most concentrated sucrose solution
5120 Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
Figure 6. Minimum-fit calibration curve for ST and two DT probe
designs.
analyzed with each probe displayed a SPR minimum near 875 nm.
However, the sensitivity at 875 nm for the ST probe is 9860 nm/
RI unit, for the 80°/10° DT probe is 9120 nm/RI unit, and is only
7040 nm/RI unit for the 75°/15° DT probe.
Dual-Tapered Probes with Shaft. Incorporating a dual taper,
while leaving a length of shaft unaltered, constructs a probe that
exhibits bimodal SPR spectra. (Figure 7) The first resonance dip
is relatively narrow and slightly blue-shifted from the resonance
minimum of the ST probe. The second resonance dip is very broad
and red shifted relative to the ST probe. The second resonance
is easily noticeable in the 80°/10° DS probe. For the 75°/15° DS
probe, the second resonance is very broad and location of the
minimum is difficult, even in low RI solutions. For fibers with more
radical tapers, 70°/20° and 60°/30°, the second minimum is shifted
beyond the range of the detector for the most dilute samples.
The dynamic ranges of the DS probes are seen in Table 1.
The first minimum of the 80°/10° DS probe is blue-shifted by
approximately 40 nm relative to the ST probe for the more
concentrated samples. This permits analysis beyond 50% sucrose
with the 80°/10° DS probe. However, the gain in upper end
dynamic range is small; the probe sensitivity of 10 030 nm/RI at
50% sucrose extrapolates to an increase in the dynamic range of
3% or 0.0025 refractive index unit. The second dip has a much
shorter dynamic range. For the 75°/15° DS probe, the dip shifts
beyond the range of the detector around 20% sucrose. For the
80°/10° DS probe, the dynamic range of the second dip ends at
35% sucrose.
Figure 7. Relative SPR spectra of 5% sucrose solutions collected with the ST and three DS probe designs.
The advantages associated with this class of probe geometry
are seen in comparing the sensitivities of the DS probes (Table
1). While the sensitivity of the first dip is constant throughout all
geometries, the second dip of the 80°/10° DS probe has approximately twice the sensitivity to changes in RI than the straight,
unaltered, probe. The second dip of the 75°/15° DS probe has up
to 4 times the sensitivity of the ST probe. The differential
sensitivity of one probe increases the information content of the
collected SPR spectra. With one SPR dip, or two dips that respond
identically, it would be impossible to resolve spectral shifts
associated with RI changes from spectral shifts associated with
alignment problems. Both shifts would be of constant wavelength
throughout the spectrum. However, with two dips, each with a
different sensitivity, RI information can be validated by their
relative location. Finally, the dual-minimum spectra combine the
best of dynamic range and sensitivity from the ST and DT probes.
The first minimum has the dynamic range of the ST probes,
0-50% sucrose. The second minimum increases the sensitivity
of the probe at the lower RI where the sensitivity of the ST probe
is low.
Impact On Probe Design and Calibration. Five sundry
aspects of modifying the optical fiber geometry for SPR probes
merit mention. The ability to tune the wavelength of resonance
without dielectric overlays allows customization of SPR probes
for antibody-facilitated analyses. First, all modifications exist
beneath the metal coating; therefore, the efficient thiol binding
of antibodies to the gold layer is readily accomplished. Second,
these modifications are simple and easily performed in any
laboratory that would make their own SPR probe tips. Unlike SiOx
overlays, expensive equipment is not needed to construct custom
probe tips. The most expensive consumable is the SMA connector.
Third, a finer array of polishing angles is accessible with electric
fiber polishers. Electric fiber polishers are capable of polishing
the fiber tips at shallower angles than the polishing chuck. (The
limitation of the polishing chuck is derived from the inability of
drilling 400-mm holes through the chuck at greater than 80° from
normal.)
Fourth, probes can be constructed such that the wavelength
of resonance for the RI of interest occurs near the wavelength of
maximum reflectance signal to noise. This is useful when the
probe is employed for screening or alarm applications (i.e. when
a decision is to be made at a given RI). Were the resonance to
occur near 750 nm (peak intensity), the signal to noise of the ratio
spectra around the wavelengths of maximum information would
be much greater. This effect can be seen in Figures 2 and 4 where
the signal is most noisy at the ends of the spectra and relatively
clean in the middle. As the RI approaches the target RI, the signal
becomes cleaner and the precision of analysis consequently
improves.
Fifth, the modified probe designs are not as compatible with
traditional minimum-hunt-based calibration compared to the ST
probe design. As the reflectance spectra exhibit broader dips,
estimation of the true minimums becomes increasingly difficult.
For the DS probes, often the first or second minimum becomes
an inflection point in the ensemble SPR reflection spectra.
However, the effective broadening of the spectra is not a hindrance
to multivariate calibration methods. Multivariate methods rely on
intensity changes at individual wavelengths for calibration. Therefore, the broadening of the spectra actually improves the information distribution for multivariate analysis. The robustness and
applicability of multivariate analysis for calibration of broad SPR
spectra has been demonstrated.27,34 In these studies, the effective
width of the SRP spectral dip has been increased by a factor of
32-64 without any degradation of predictive performance of the
SPR system when multivariate methods were employed.
Other Factors Impacting SPR Spectra. In the acquired
spectra, SPR features dip from approximately 80 to 65% of the
incident light intensity. Two reasons for this are the TE component
of white light and stray light at the interface of two SMA
connectors. Only the TM component of white light will couple
into the surface plasmon wave (SPW), and consequently, the
minimum reflection intensity should be 50% of the incident light
intensity. Stray light from reflections at the interface of two coupled
SMA connectors produced a background that was not accounted
Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
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for in the ratio of spectra. These two combined effects account
for only 35% of the incident light coupling into the SPW.
Not addressed in this study are two factors that impact the
dynamic range and sensitivity of white-light, fiber-optic SPR
probes. The most important is the optical fiber employed. The RI
of the core and cladding of the fiber define the range of angles
that propagate down the fiber. Fibers with lower NA propagate
modes closer to normal to the fiber-optic interface, on average,
down the fiber. Assuming that the decrease in NA is accomplished
while preserving the RI of the fiber core, decreasing the NA
simultaneously narrows the SPR dip and blue-shifts the ensemble
SPR spectra by eliminating the higher-wavelength couplings.
Concurrently, increasing the RI of the fiber core blue-shifts the
SPR dip. This shifts the dynamic range of the sensor equal to the
increase in RI of the fiber core. Therefore, much can be done to
optimally tune a SPR probe by also adjusting the manufactured
properties of the optical fiber itself; however, it is less expensive
to alter the geometry of the probe tip than to engineer specialty
fibers for each SPR application. The most economical strategy
for optimizing the SPR probe design is to choose a commercial
fiber that is most appropriate for a given application, keeping in
mind that it is easier to red-shift the SPR than to blue-shift the
dip, and to modify the geometry of the commercial fiber.
The second issue not addressed is the mode distribution of
light propagating down the fiber. It is observed that, when the
sensor was out of alignment, the spectra were slightly blue-shifted.
Further investigation showed that when the blue-shift cor-
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Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
responded to an increased proportion of skew rays propagating
down the fiber. Increasing the contribution of skew rays resulted
in a blue shift of up to 15 nm. Conceivably, adjusting the skew
ray distribution, along with fiber composition and geometry, will
result in even broader control of the sensitivity on dynamic range
of the white-light, optical fiber SPR probes.
CONCLUSIONS
The ability to selectively tune the wavelength of resonance with
a white-light, multimode, optical fiber SPR sensor by modifying
the geometry of the fiber tip has been demonstrated. Advantages
of this method compared to SiOx overlays include maintaining
accessibility of the gold surface for antibody bonding and
maximizing sensitivity of the probe. Coupling these probe tip
modifications with multivariate calibration methods should enable
construction of inexpensive, field-portable SPR sensors of environmental contaminants.
ACKNOWLEDGMENT
This work is supported by the National Science Foundation.
Thanks to Nathan Wilken ([email protected]) for developing
software used in manipulating file formats.
Received for review May 4, 1999. Accepted August 27,
1999.
AC990470F