# SPR curve (dip curve) and Sensorgram

Updated: Jul 29

글쓴이: Alex Jang

작성일: 2014년 2월 5일 (수정일: 2020년 5월 31일)

I would like to explain the process of acquiring a sensorgram that the actual user observes. To do so, it is necessary to first understand the SPR curve or dip curve

Figure 1 shows an SPR curve obtained from an angle-variable SPR device. The black curve represents the SPR curve at time 0, and the red curve represents the curve at time 1. Between time 0 and 1, some changes occurred on the sensor chip surface, which is reflected in the changes of the curve

<Figure 1. SPR curve>

The x-axis represents the angle of incidence, and the y-axis represents the reflectivity.

Since the light entering the prism undergoes refraction, the actual angle of incidence should be considered as the angle of incidence inside the prism. Of course, if the prism is semi-circular, there will be no refraction.

Reflectivity refers to the ratio of the reflected light to the incident light. If 50 units of light are incident and 50 units are reflected, the reflectivity will be 1, indicating total reflection. On the other hand, if 50 units of light are incident and 0 units are reflected, the reflectivity will be 0. Have you understood the concept of reflectivity?

Let's observe from low to high angles on the x-axis. The first point of inflection occurs, which is the angle at which total internal reflection begins. We call this angle the critical angle. If a prism and a metal layer are fixed, this angle varies depending on the refractive index of the material on the surface. The critical angle follows Snell's law. (You can look up Snell's law for more information.)

Shall we observe a little further to the right of the x-axis? Instead of total internal reflection, a sudden deep valley appears. Yes, that's when the SPR phenomenon occurs. As you can see, a parabola has appeared.

We can obtain a real-time graph, called a sensorgram in two ways using this curve.

(Sensorgram shows the change in intensity over time)

Now let's observe the black and red curves in Figure 1. When changes occur on the metal surface over time, the SPR curve also changes. We need to determine how to obtain a sensorgram from these changing curves.

Currently available commercial equipment obtains sensorgrams using the following two methods:

The first is the fixed angle mode or reflectivity measurement mode, in which the angle is fixed and changes in reflectivity are observed in real-time. This corresponds to dR(%) in Figure 1.

The second method is to observe the real-time changes in the SPR angle. This corresponds to dθSPR.

<Figure 2>

Figure 2 represents the sensorgram, with RU on the y-axis corresponding to dR(%) or dθSPR.

I hope you have now understood how sensorgrams can be obtained using SPR.