The Surface Plasmon Resonance

Published Date: 02 Nov 2017

INTRODUCTION

Surface plasmon resonance is an optical phenomenon that occurs when a light beam passes from a material having a high refractive index(a glass prism) into a material with a lower index. In this case, the light is bent towards the interference plane.

Total internal reflection occurs when light strikes the surface at a greater angle than the critical one. Total internal reflection is necessary for surface plasmon resonance to occur. A technical solution for this was the use of a free electron metal such as gold or silver for the prism coating. The free electrons on the surface of gold or silver can oscillate and create surface plasmons. Surface plasmon resonance occurs when the wavelength of the incident light matches the one of the surface plasmons at a particular angle of incidence. At this angle there is a minimum of the reflected light and is called the surface plasmon resonance level.

An evanescent electrical field is also generated when surface plasmon resonance occurs. This evanescent electrical field extends from the metal to the sample medium in contact with it. As a result of this, the amplitude of the surface plasmon waves changes, in turn modifying the refractive index of the sample medium. Consequently the angle at which surface plasmon resonance occurs also changes.

Surface plasmon resonance sensors are in principle thin-film refractometers that measure changes in the refractive index that appear at the surface of a metal film supporting a surface plasmon.

Depending on what characteristic of the light is modulated , surface plasmon resonance sensors are classified as sensors with angular, intensity, wavelength and phase modulation.

In surface plasmon resonance sensors with angular modulation, a monochromatic light wave is used to excite a surface plasmon. The strength of coupling between the incident wave and the surface plasmon is observed at multiple angles of incidence, typically by employing a convergent light beam. The excitation of surface plasmons is observed as a dip in the angular spectrum of reflected light. The angle of incidence yielding the strongest coupling is measured and used as a sensor output.

In surface plasmon resonance sensors with wavelength modulation, a surface plasmon is excited by a collimated light wave containing multiple wavelengths, typically a beam of polychromatic light. The excitation of surface plasmons is observed as a dip in the wavelength spectrum of reflected light. The wavelength yielding the strongest coupling is measured and used as a sensor output.

Surface plasmon resonance sensors with intensity modulation are based on measuring the strength of the coupling between the light wave and the surface plasmon at a single angle of incidence and wavelength, and the intensity of light wave serves as a sensor output. In surface plasmon resonance sensors with phase modulation the shift in phase of the light wave coupled to the surface plasmon is measured at a single angle of incidence and wavelength of the light wave and used as a sensor output.

Most of the SPR sensors developed to date, including the first reported SPR sensor, use a prism coupler to couple light to a surface plasmon. Prism coupling is convenient and can be realized with simple and conventional optical elements. Moreover, it can be readily combined with any type of modulation.

Content

Several experiments concluded that SPR biosensors are capable of detecting various analytes depending on the surface functionalization. Here are a few examples of the experiments involving the use of SPR biosensors:

Detection of gastric carcinoma-associated antigen MG7-Ag in human sera

MG7-Ab is a gastric cancer-specific tumor-associated antigen investigated to serve for the early diagnosis of gastric cancer.

In this experiment the MG7 antigen was detected in the sera of gastric patients with the use of a surface plasmon resonance sensor in order to develop a rapid and simple assay for early diagnosis. Specific monoclonal M7 antibodies were used as capture and detection receptors immobilized on the surface of surface plasmon resonance sensor chips for MG7 antigen identification in the human sera.

A wavelength modulation surface plasmon resonance sensor was used for this experiment.

MKN45 cancer cell lysate, gastric cancer patients and healthy blood donors sera were used in order to perform this method.

Mouse monoclonal MG7 antibodies were immobilized on an activated surface consisting of an 3-Mercaptopropionic acid monolayer activated by a solution of N-hydroxysuccinimide and N-ethyl-N-(dimethylaminopropyl)-carbodiimide.

Analysis of the MG7 antigen in te gastric cancer patients sera was performed using the surface plasmon resonance sesnsor chips, after dilution of 1:300 in sterile phosfphate buffer solution(pH 7.4).

During the tests wavelength resonance shifts were monitored in order to determine the binding process between the MG7 antigen and the MG7 antibody immobilized on the gold surface of the surface plasmon resonance chips.

The wavelength resonance shifts between the PBS and the sample observed durin the tests were: 11.9 nm for MKN45 cancer cell lysate, 10.9 nm for cancer patients sera and 5.4 nm for healthy blood donors sera.

This method proves that spectral surface plasmon resonance biosensor can be used for rapid detection of the MG7 antigen in the human serum in order to offer a rapid diagnosis for gastric cancer.

Surface plasmon resonance-based highly sensitive immunosensing for Brain Natriuretic Peptide using nanobeads for signal amplification

Brain Natriuretic Peptide(BNP) is a polypeptide secreted by the heart cells located in the ventricles in response to excessive stretching of the cardiomyocytes. BNP levels measurement is a useful indicator of heart failure. The normal concentration of BNP is ~20pg/ml, but in the case of chronic or acute cardiac failure it can rise up to ~1000 pg/ml.

In this experiment pictogram levels of BNP were detected usig two types of monoclonal antibodies(BC203 and KY-BNP-II) which recognize different epitopes of BNP. The primary antibody was covalently immobilized on an activated gold coated glass plate with the use of 11-mercapto-1-undecanoic acid and carbodiimide hydrochloride and N-hydroxylsuccinimide. The secondary monoclonal antibody was labeled with biotin with the use of a sulfo- N-hydroxylsuccinimide solution.

The assay was realized using a flow cell assembly in which a solution of BNP(0-100 ng/ml) was flown.

A sandwich type immunoassay was realized by the use of the primary antibody immobilized on the gold coated glass prism that reacted with the BNP, the secondary biotin labeled antibody used to enhance signal by binding with a different epitope of BNP and the subsequent adding of streptavidin nanobeads.