BIOP Research Program

Biomedical bio-sensing

In the field of biochemical quantification, it is essential to develop state-of-the-art affinity sensors capable of detecting biological active substances in extremely small concentrations. In BIOP, biosensors based on optical techniques are developed. The purpose of the development is to apply these new biosensors in measurements of enzymes, pesticides, and glucose in blood samples.

Absolute refractive index determination by micro-interferometric backscatter detection

It is of great interest to be able to measure small changes in the refractive index in small volumes of fluid. This may be achieved by using the micro-interferometric backscatter detection (MIBD) scheme,¹ which is based on a simple optical system. MIBD is universal since refractive index varies with a wide range of parameters. Changes in temperature, concentration and pressure may be detected inside a small volume of the liquid by using this method. MIBD has previously been shown capable of measuring changes in the refractive index of liquids on the order of 10^-7.² The MIBD technique is based on interference of laser light after it is reflected from different regions in a capillary. These reflections generate an interference pattern that moves upon changing refractive index of the liquid in the capillary. The small angle interference pattern traditionally considered has a repetition frequency in the refractive index space that limits the ability to measure refractive index to refractive index changes causing such one repetition. Such refractive index changes are typically on the order of three decades. Recent modeling and experiments with the MIBD technique has shown that other intensity variations in the pattern are present for larger backscattered angles.³

By considering these variations we have shown two methods by which it is possible to extend the dynamic measurement range to make an absolute refractive index measurement. One method utilizes variations in the Fresnel coefficients while the second approach is based on the refractive index dependent onset of total internal reflection angles.³ The model³ predicts an abrupt change in intensity moving towards lower backscatter angles as the refractive index of the liquid approaches the one of the glass tubing, see Fig. 1A. This feature of the interference pattern is also observed experimentally, see Fig. 1B, and it agrees with the predicted feature in position- refractive index space within experimental error. From our experiments, the precision of the absolute determination of the refractive index is found to be 2.5 10^-4 with the refractive index in the range of 1.33 to 1.5. With our current technique and setup, we are able to perform an absolute refractive index measurement with accuracy on this level on a 180 nL volume. The main limitations for accuracy such as temperature control and detector resolution are the same as conventional MIBD. The theoretical limit using this approach is therefore similar to the limit achievable by conventional MIBD and it is possible to perform a conventional MIBD measurement simultaneously to our newly proposed method. In principle if the dimensions and refractive index of the capillary tube are known, then there is a one to one relationship between the backscatter angle and the refractive index of the liquid enabling the determination of the absolute refractive index.

Fig. 1. Simulated (A) and experimental (B) interference pattern.

References

1. D. J. Bornhop, Appl. Opt. 34, 3234-3239 (1995).
2. K. Swinney, D. Markov, and D. J. Bornhop, Rev. of Scien. Instruments 71, 2684-2692 (2000).
3. H. S. Sørensen, H. Pranov, P. E. Andersen, and D. J. Bornhop, UK patent appl.: 0220684.5.

The main goal for this focus area:

• to demonstrate on-chip optical biosensors and show their improved sensitivity.

Our current activities are concentrated on:

• micro-interferometric back-scatter detection systems integrated with microfluidic systems.

Contact person:

Peter E. Andersen