#101
Principle of SPR detection: Intensity profile and shift of the SPR angle
When a light beam impinges onto a sensor (made from metal film coated on a glass slide) at a specificangle (resonance angle), the electrons in the metal film (called surface plasmons) are set to resonate withthe light wave. The resonance results in the absorption of light. In a typical SPR detection setup a beam
is focused onto a sensor. The focused light provides a range of incident angles, and the reflected beam
will cover the same range of the angles and the projection of the beam forms a rectangular shaped bandof light. When the SPR angle occurs within the spread angles, a dark line will appear in the band. Anintensity profile of this band is plotted against the range of angles as shown in Figure 1.

Figure 1. Principle of SPR detection in the mode of measuring the SPR angular shift.

This profile is often used in a typical SPR experiment to indicate that the metal film (or the solution
above the metal film) matches the SPR resonance condition, and that the apparatus is set up properly.
However, the actual measurement of molecular adsorption kinetics is only related to the shift of theabsorption dip in the intensity profile corresponding to the SPR angle change (Figure 2). One has tosacrifice the resolution of the measurement to acquire the whole intensity profile curves and comparethem to find the shift of the dip (minimum).

Copyright Biosensing Instrument Inc. 2014

Rev 3

December 2014

Figure 2. A laser is beamed upon a metal film through a prism and the reflected beam image shows a dark line due to SPR.
The intensity profile of the reflected beam exhibits a dip or minimal intensity at the resonance angle. An SPR experimentmeasures the position shift of the dip (the angle shift) upon molecular adsorption, and this shift represents the adsorption
kinetics when plotted as a function of time.

Biosensing Instrument Incorporated uses a different approach to detect the SPR angle change by using
an enhanced position-sensitive device with advanced algorithm to detect the SPR absorption dip shift.
Since it only measures the position shift of the dip, it offers a highly sensitive detection scheme tomeasure extremely small angle changes of the SPR. As long as the SPR angle resides inside the anglespread, the system delivers exceptionally high angular resolution in its measurement.
Even though intensity profile is commonly used for demonstrating the proper alignment and the setup ofan SPR system, it only offers limited status check of a metal film at a single position. For a multichannel detection scheme used in the BI SPR system, a single intensity profile check is not sufficient toensure that the system is in proper SPR detection condition. One has to ensure that all the illuminatedareas of the metal film are in good condition. BI SPR provides a simple but sufficient solution by giving
the user a visualized image of the SPR absorption dip. By examining the quality of the dark line in thisway (as shown in figure 3-5), one could ensure that all conditions of the SPR are met and a successful
experiment can be conducted.

Figure 3. A good metal film gives a clean and straight SPR dark line in air (left) and solution (right).

As illustrated in Figure 3, a good metal film gives a well-defined, clean, and straight dark line. Thisguarantees a good intensity profile at any point along the dark line.

Copyright Biosensing Instrument Inc. 2014

Rev 3December 2014

Figure 4. A contaminated film gives fuzzy and indistinct dark line.

Figure 5. A trapped air bubble underneath of the metal film sensor will break the dark line. However, at the drawn line, darkline will still gives a good intensity profile of the SPR.

Figure 4 and 5 demonstrates that a partially contaminated metal film can still give a reasonable intensity
profile of SPR at a given point. However, it will not lead to a reliable measurement, since the film
quality across a large area is bad.

Biosensing	  Instrument	  Inc.	  
1 West Elliot Rd., Suite 111, Tempe, AZ 85284
Tel: (480) 491-2777, Fax: (866) 897-8741
info2bi@BiosensingUSA.com
www.BiosensingUSA.com

Copyright Biosensing Instrument Inc. 2014

Rev 3

December 2014