Investigation of the effects of design parameters on sensitivity of surface plasmon resonance biosensors

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Abstract

Surface based analytical tools have gained more importance for rapid, sensitive and label-free monitoring of molecular recognition events. The surface plasmon resonance (SPR) approach has played a prominent role in real time monitoring of surface binding events. This paper first provides a brief description of the existing biosensing methods. Next, an investigation of the role of thin films of gold, silver, and aluminum for protein detection in SPR biosensors is presented. It is shown that the sensitivity, which is indicated by the shift of plasmon dip, is not linearly related to the thickness of protein but quadratic over a specific range. The approach involves plotting the reflectivity curve as a function of the angle of incidence.

Introduction

Biosensors convert biological events into measurable signals such as electrical signals or optical emissions. They involve three main elements: recognition, transduction, and signal processing. Designing biosensors requires an investigation of the following tasks: growing substrate, designing interface, and introducing functional groups. An illustration of these tasks is presented in Fig. 1. The figure shows a biomolecular layer of receptor molecules (red) that recognize a target analyte (green) without uniting background molecules (blue) where the receptor molecules are attached to the physical transducer (grey). The device provides an output relative to the change in the interaction between the sensing layer and the target analyte. The following conditions need to be met in a biosensor: (i) biocatalyst needs to be highly specific to a particular analyte, be stable under normal storage conditions, and show good stability over a large number of assays, (ii) reaction needs to take place independent of such physical parameters as pH and temperature, (iii) probe needs to be biocompatible with no toxic/antigenic effects, and (iv) response (e.g., electrical, optical, mechanical) needs to be accurate, low noise, and linear.

The first biosensor was developed by Clark and Lyons in 1962 [1] which is now called the first-generation biosensor. Various biosensing platforms based on the use of different receptor molecules [2], immobilization techniques [3], and transducer mechanisms [4] have been developed. Therefore, there exists different categorization of biosensors in the literature. For example, one particular categorization of biosensors is based on their detection principle and include optical [5], [6] and non-optical [7], [8] groups.

This paper presents an overview of recent biosensing approaches, first. Next, it considers a surface plasmon resonance (SPR) biosensor that is an optical biosensor capable of providing ultra sensitive (within femtomolar limit of detection) and label-free detection. A numerical investigation of the effects of design parameters on sensitivity of the SPR biosensor is presented. The biomolecular interactions (e.g., protein and gold nanoparticles) based on the shift of plasmon dip at a certain angle of incidence of optical coupling of signal using a reflectivity graph are studied. In a SPR biosensor with a planar metal thin film, a propagating surface plasmon resonance takes place when the surface plasmons are excited. The use of nanoparticles (e.g., gold, silver and aluminum) produces a localized surface plasmon resonance, and enhances the electromagnetic field near the surface of nanoparticles. In addition, the shift of resonance dip due to the immobilization of target molecules on an evaporated gold film (lipid protein in this work) is investigated.

Section snippets

Categorization of biosensors

In general, biosensors are categorized in terms of their biological element, immobilization method, or transducer mechanism. Here we employ the latter parameter, the transducer mechanism, to categorize and describe biosensors. An overview of different types biosensors together with an analysis of their performances are given in the following.

Principle of SPR detection

From the theory of optics, above a certain incident critical angle, no light is refracted across the interface. While the light is totally reflected back to the medium of higher refractive index, the electromagnetic field component pierces several nanometers distance into a lower refractive index producing an exponentially extenuating evanescent wave (Fig. 2). When the interface between the media is coated with a thin layer of material (e.g., silver) and the light is monochromatic and

Theory

The theory behind the actual SPR signal can be elucidated by the electromagnetic coupling of the incident light with the surface plasmon of the covering layer such as gold, aluminum, etc. As mentioned earlier, when the optical beam reaches the interface of metal and dielectric, part of the beam is reflected and part of the beam is transmitted which leads to an interaction between the light and the electrons in the metal. This interaction generates a collective movement of the nearly free

Experimental study

A simulation experiment was carried out in which a laser beam (wavelength 633 nm) is reflected from the base of a prism [56]. In this study, we have taken in to account three thin films of gold, silver and aluminum to cover the base. Finally, protein is immobilized on the surface. The reflected light which is normal to the surface is then collected as a function of the angle of incidence. The refractive index profile and other properties of the examined materials were used in this investigation.

Results and discussions

The sensitivity of a biosensing system as a whole depends on a number of factors (e.g., coated materials, and target probe) but in the context of this study, sensitivity improvements has resulted from proper selection of the metal thin films and the specificity of the captured target molecule. It is outlined in the earlier discussions that gold films provide better sensitivity for the detection purpose than many other films. From the experimental data (Table 2), with gold coating, Δθres was

Conclusions

This paper investigated biomolecular interaction of protein and gold nanoparticles in a SPR biosensor. The investigation was carried out based upon the shift of plasmon dip at a certain angle of incidence of the optical coupling of signal using a reflectivity graph. The use of nanoparticles in the SPR biosensor can produce a localized surface plasmon resonance, and enhance the electromagnetic field near the surface of nanoparticles. The paper discussed a theoretical analysis of the SPR sensing.

Acknowledgments

The first author would like to acknowledge the financial support of the Australian Government through the IPRS scholarship. Also, we would like to thank Mr. Helmut Schiretz for providing assistance and information.

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