Electro-optic grating-coupled surface plasmon resonance (EOSPR)

Abstract

An instrument for measuring and analyzing surface plasmon resonance (SPR) and/or surface plasmon coupled emission on an electro-optic grating-coupled sensor surface is described herein. The sensor chip achieves SPR through a grating-coupled approach, with variations in the local dielectric constant at regions of interest (ROI) at the sensor surface detected as a function of the intensity of light reflecting from these ROI. Unlike other grating-based approaches, the metal surface is sufficiently thin that resonant conditions are sensitive to dielectric constant changes both above and below the metal surface (like the Kretschmann configuration). Dielectric constant shifts that occur as mass accumulates on the surface can be returned to reference intensities by applying voltage across the underlying electro-optic polymer. Approaches to the development of the sensor surfaces are described, as are software and hardware features facilitating sample handling, data gathering, and data analysis by this solid-state approach.

Claims

1. An instrument for detecting changes in the optical properties of a sensor surface defining an image plane, said instrument comprising: an electro-optic material having a surface including a diffraction grating coincident with said image plane, said electro-optic material having a dielectric constant and refractive index that change as a function of the strength of an electric-field to which the electro-optic material is exposed; an SPR-active layer on said diffraction grating; a voltage generator arranged to deliver a variable voltage to said electro-optic polymer, thereby exposing said electro-optic material to a variable electric field which changes the dielectric constant and refractive index of said electro-optic material; a source of collimated polarized light of a predetermined wavelength arranged to project said collimated polarized light onto said image plane at a defined angle of incidence; an imaging detector arranged to form an image of said sensor surface from said polarized light reflected from said sensor surface; wherein at least a portion of said polarized light is coupled into surface plasmons at said sensor surface and the intensity of the reflected light received by said imaging detector varies as a function of the intensity of said electric field.

2. The instrument of claim 1, wherein said electro-optic material, diffraction grating, SPR-active layer, and predetermined wavelength result in a resonant angle at which a majority of the polarized light is coupled into surface plasmons at said SPR-active layer and said resonant angle changes as a function of the strength of said electric field.

3. The instrument of claim 2, wherein said resonant angle changes in response to a mass of material bound to said SPR-active layer, the applied voltage can be varied to change the strength of said electric field to correct for changes in said resonant angle due to material bound to said SPR-active layer and the change in voltage can be used to measure said mass of material bound to said SPR-active surface.

4. A method of measuring changes in the optical properties at a sensor surface defining an image plane comprising the steps of: providing an electro-optic material with a conductive layer on one side and a diffraction grating opposite the conductive layer and an SPR-active layer on said diffraction grating forming said sensor surface, said electro-optic material having a dielectric constant and refractive index that vary as a function of an electric field established between said conductive layer and said SPR-active layer; connecting a variable voltage generator to said conductive layer and said SPR-active layer; arranging a source of collimated polarized light of a predetermined wavelength to project said collimated polarized light onto said sensor surface at a defined angle of incidence; positioning an image detector to form an image of said sensor surface from said polarized light reflected from said sensor surface; monitoring the reflected intensity as a function of a variable voltage applied across said conductive layer and said SPR-active layer.

5. The method of claim 4, comprising: establishing a reference image of said sensor surface at a first voltage applied across said EO material; causing material to bind to said sensor surface, thereby altering the optical properties of the sensor surface; and varying the voltage applied across said EO material to re-establish said reference image at a second voltage; and employing a change in voltage between said first voltage and said second voltage to determine the mass of material bound to said sensor surface.

6. The method of claim 4, wherein said step of connecting a variable voltage generator comprises connecting said SPR-active layer to ground.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1 is a schematic drawing showing one possible optical path for the instrument; and

(2) FIG. 2 shows several views of one possible EOSPR chip.

DETAILED DESCRIPTION OF THE INVENTION

(3) As used herein, the term electro-optic polymer or EO polymer refers to polymers or other materials whose dielectric constant varies as a function of applied voltage. SEO100 from Soluxra, LLC of Seattle, Wash. is an example of an electro-optic polymer potentially compatible with the disclosed embodiments. The acronym GCSPR stands for Grating-Coupled Surface Plasmon Resonance, and EOSPR stands for Electro-Optic grating-coupled Surface Plasmon Resonance. SPCE stands for Surface Plasmon Coupled Emission. The EOSPR sensor surface is alternatively called the chip, the sensor chip, or the EOSPR chip, and will support an electro-optic grating-coupled approach to both SPR and SPCE.

(4) An EOSPR sensor chip and complementary detection schema providing surface plasmon resonance analysis with or without concomitant SPCE measurements are described below.

(5) In a dielectric-metal-dielectric arrangement, Equation 1 holds true, and .sub.sp therefore depends on the dielectric constant of all three layers (.sub.1, .sub.2, and .sub.3). In the Kretschmann configuration, local changes in the dielectric constant due to bound mass (.sub.1) affect .sub.sp while the values of .sub.2 and .sub.3 do not vary. The change in .sub.sp in these instruments is detected by adjusting the momentum of the source light, often by varying the incident angle () until resonance is achieved. In the grating-coupled approach, coupling into the GCSPR system is governed by Equation 2. When .sub.source and .sub.grating are fixed, as occurs readily in classic GCSPR instruments, the mass-dependent changes in .sub.1 shape .sub.sp and are observed as changes in the coupling angle (). Equations that define .sub.sp and the coupling angle for Kretschmann instruments are similar. However, the EOSPR platform departs from these detection schema by restricting to a given angle (e.g. the SPR angle for a water-gold-plastic construct), or a small range of angles and instead interrogating accumulation or depletion of bound mass by changing .sub.3. To clarify, one approach to detection would measure the effect of accumulating mass (manifesting .sub.1) not by changing the properties of the incident light (e.g. angle), but instead by changing the properties of the underlying grating itself (.sub.3). As alluded to above, classic GCSPR instruments are not thought of as dielectric-metal-dielectric systems because the metal layer is typically thick enough to obscure any impact of .sub.3. For .sub.3 to be relevant, as is proposed in this EOSPR approach, the thickness of the metal layer must be reduced to thicknesses such as those found in Kretschmann e.g., less than approximately 50 nm. In this case, the controlled change in .sub.3 will be accomplished by replacing the underlying dielectric with a polymer displaying a significant electro-optic (EO) effect.

(6) Integrating an EO polymer into a GCSPR-style chip permits a detection scheme where departures from resonant conditions (.sub.1) will be measured solely by varying the dielectric constant of the EO polymer. The dielectric constant change at the surface due to binding mass (typically no larger than 10.sup.5 RIU) is readily matched by changes in the dielectric constant of the polymer (which can modulate by as much as 10.sup.3 RIU). The precise match between incident light and the plasmon mode would therefore not be a function of angle, but a function of applied voltage.

(7) One embodiment of the EOSPR chip (20) is sketched as FIG. 2, the components of which are built upon an inert substrate (21). Above this substrate is a conductive layer (25) whose properties support the application of voltage and adhesion of the polymer layer (26). This conductive layer need not be metal nor SPR active, but along with the SPR-active metal surface (24) acts to promote the application of a uniform electric field across the EO material. The EO material (26) is a layer that has been patterned to act as a diffraction grating, but whose optical properties continue to display a significant electro-optic effect. An SPR-active metal surface (24) overlies this particular embodiment, and contact pads (22 and 23) are continuous with the two conductive layers (24 & 25).

(8) The instrumentation portion is presented here as a standalone device interfacing with the EOSPR chip, providing control of input voltage, monitoring reflected light intensity at the sensor surface, controlling fluidics, and interpreting data. Devices offering any and all of these functionalies in a package designed to support this grating-coupled electro-optic approach to SPR or SPCE sensing shall comprise an EOSPR instrument for the purposes of this discussion. As presented, the minimum components for such a system include a light source (30), a polarizer and collimated lens (31), the chip (20, perhaps positioned on a removable holding apparatus, 35), suitable filters and lenses (32), a detector (33), the voltage generator (34), and a computer. Fluidics, their interfaces, and controls may be a component of the chip, a component of the instrument, or may stand alone, but are not represented in the figures for clarity.

(9) The archetypal detection scheme proposed herein involves optimizing the instrument so that the incident angle and the detector (camera, photo-diode array, etc.) are fixed at approximately the SPR angle for the bare gold surface. Room for adjustment of this angle can be engineered into the design, permitting user calibration and/or increased instrument tolerance for varied environmental conditions. Instead of monitoring reflected intensity as a function of incident angle, the instrument would measure reflected intensity as a function of voltage applied to the bottom electrode. In the most straightforward scanning protocol, the incident angle and the location of the light detector will not move. Since electro-optic polymers change dielectric constant predictably in the presence of an electric field, applying a voltage to this basal electrode while maintaining the surface metal at ground would generate a change in the dielectric constant of the sandwiched polymer (.sub.3). With the surface grounded, interference with biological interactions is not expected. The assay would monitor the binding of mass to the surface by measuring the voltage required to return all ROI to resonance or a reference value. The changes in local dielectric constant imparted by the bound mass on the surface would be in essence nullified by changes in the dielectric constant in the polymer layer. Several related works appear in the literature (including prior art from Ciencia), but none appear as well suited for commercial adoption as the EOSPR platform, primarily due to the epi-illumination architecture.

(10) The addition of an electro-optic layer in between conductive layers adds slight complexity to the chip, yet greatly reduces the requirements for instrumentation. The EOSPR chip eliminates the need for moving parts and significantly shortens the optical pathway. The proposed device would be able to simultaneously measure the shifts for the same number of ROI (1,000/cm.sup.2) as allowed by extant GCSPR instrumentation. The number of spots is constrained by the active area of the chip, since the density of spots is limited to prevent an overlap of plasmon waves. There is no fundamental reason why smaller or larger chips with tens or millions of spots could not be developed for future instruments. In any implementation, the reduced weight, cost, and fragility would make EOSPR instrumentation more portable and affordable, while increasing sensitivity over comparable GCSPR platforms. By selecting small components and a smaller chip (active area of 4 mm4 mm, enough for 100 spots) we have calculated that an optimized and sensitive EOSPR instrument could be about the size and weight of an average hardcover novel. Bringing a sensitive and high-content assay into the realm of hand-held and battery-operated devices invites enticing market opportunities.

(11) Although it is hard to quantify the sensitivity increase expected from this design ab initio, several factors inherent to this system imply that the inclusion of EO polymers will boost instrument performance beyond today's standards. Implicit in the EOSPR design are increases in the quality of the signal and decreases in the noise compared to other SPR platforms. Applied voltage can be measured more accurately than the mechanical changes of incident angle present in current systems, thus locating the SPR with high precision. In addition, thousands of measurements defining the SPR minimum could be conducted in the time it takes to perform a single scan on current instrumentation. This increased scanning velocity would also permit direct measure of faster reaction kinetics. Finally, voltages could be applied in a pseudo-random fashion with collected data interpreted in silico, thus reducing systematic error in measurements.

(12) Besides boosting the quality of the signal, direct reduction of instrument noise is possible with such a rapidly scanning instrument. Even with an inexpensive 30 Hz camera, we can employ a signal-chopping scheme to subtract noise as background. Essentially, the voltage applied to the EO material would switch from a near-resonant voltage to an off-resonant voltage at a rate of 30 Hz synchronized to the camera. The near-resonant image provides the signal, while the off-resonant image provides the background. The difference between these two images would be immune to many sources of noise, such as stray light, changes in temperature, changes in light intensity, etc. For a 30 Hz frame rate, 15 background subtracted images can be obtained per second, still allowing for extensive averaging at a reasonable data acquisition rate. The enhanced quantities of high-quality data collected by the electronic detection scheme and the expected reduction of noise encountered by chopping the signal combine to strongly suggest an overall increase in instrument sensitivity. High-sensitivity measurements and a compact, stable, and no-moving-parts design strongly suggest this platform would be ideal for field use.