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Surface Plasmon Resonance is an advanced and highly sensitive optical technology that can measure refractive index changes on a sensor chip’s gold surface due to a change in mass that occurs during a binding event. This change can be used to monitor biological interactions such as the concentration of target molecules, kinetic rates and affinity constants.

BiOptix’ technology is a novel ultra-sensitive detection platform known as enhanced Surface Plasmon Resonance (eSPR) which combines high sensitivity with microarray detection capability.

eSPR technology was invented by BiOptix engineers in collaboration with Dr. Jan Hall, the 2005 Nobel Laureate in Physics. US Patent 7,233,396, “Polarization-Based Interferometric Detector” was issued on June 19, 2007. eSPR is an advanced detection method that measures relative change in phase of the P and S components of polarized light when reflected off a surface coated with a transducer material. This differential phase shift is caused by a local change in refractive index due to binding of an analyte to the chemically specific transducer material.

Classical interferometry (dual path) is known to be one of the most sensitive techniques to measure refractive index changes, but is impractical to use in laboratories and commercial environments because it is susceptible to environmentally induced signal disruptions and drift. Any change between separated beams used in classical interferometry leads to noise and instability problems. Common path interferometry negates this susceptibility because the phase changes are measured and derived from the same laser beam, allowing a more robust configuration with the inherent sensitivity advantage of an interferometric detection technique. Furthermore, by employing the surface plasmon excitation effect, the response from the transducer element is amplified, so that an ultimate sensitivity level of 2 x 10-8 RIU is theoretically achievable.


Principles of enhanced surface plasmon resonance (eSPR). Left graphic panel shows the covalently linked antibodies present on the gold surface of the transducer slide in a multiplex format. As the sample is exposed to the transducer, accumulation of biomass at the surface due to specific affinity interactions causes an optical phase shift of the P and S components of the laser, which is measured with the photo detectors. The response is then analyzed and the output delivered as a voltage measurement proportional to changes in refractive index at the surface, which in turn is proportional to concentration of the target molecule in solution. Right Panel B illustrates the phase differential induced by refractive index change at surface.

The design of BiOptix surface plasmon resonance spectrometers allows discrete areas within the sample cell to be interrogated simultaneously by use of photo-diode arrays aligned to the reflected beam from the sample cell. Thus, microarrays of capture agents can be built into the flow cell so that samples can be screened for multiple analytes simultaneously with multiplexed readout.

Summary of eSPR Technical Advantages

  • Interferometry using a single beam
    • Internal reference within the single beam
    • Highly stable regardless of environmental conditions
    • High sensitivity with low noise
  • Surface Plasmon Enhanced (SPE) Linear response to mass change
    • Increased dynamic range
    • Well-characterized Au surface chemistry
  • Multiplex configuration
    • All chip features are read simultaneously
    • No moving parts in the detector
    • Allows for a reference channel for background subtraction


The following demonstrations outline the process of Enhanced Surface Plasmon Resonance (eSPR) and the phase-based technology of the BiOptix 404pi. Click on an image to launch the video.

Meet the 404pi

Overview of the 404pi SPR system and its features and technology.

BiOptix Software for the 404pi

See the capabilities and get a feel for the latest software for the 404pi.

SPR: Measurement Principle

The surface plasmon resonance measurement principle is based on measuring index of refraction changes at an affinity layer during exposure to an analyte. This is accomplished by measuring relative phase changes of two components of polarized light within a single beam.

SPR: Optics

The optical system transforms laser-diode emitted light into s- and p- polarized components within the same beam. Amplitude and phase of each component are tuned for maximum sensitivity.

SPR: Phase Changes

These phase changes are induced by mass increase at the surface that results from the specific interaction of an analyte with the ligand attached to the chip under Total Internal Reflection (TIR) conditions. Any change in the effective index of refraction of the affinity layer, such as through the addition/binding of chemical or biological molecules, generates an optical phase shift, and alters the laser beam’s polarization state.

SPR: Detection of Minute Phase Changes

Comparison of the beam’s two polarization states (s and p) enables detection of minute phase changes, which characterize the rate and extent of the surface interaction to thereby yield information for accurate bio-molecule identification and quantification.

For additional information or to request a product demonstration, please contact a BiOptix product specialist at: 303-545-5550 or

BiOptix: Kinetics with Confidence.