<Target Substance Sensor>
The human blood contains specific markers for diseases such as cancer, hepatitis, diabetes, and osteoporosis. The disease will increase the concentration of a specific protein as a marker above the normal level. Monitoring of the markers enables early detection of the incurable disease. Therefore, the technique for the apparatus and method for monitoring such markers is promising as a next-generation medical technique. Further, transfer of cancer after removal of tumor can be detected in an early stage by monitoring the marker. Therefore, the technique of marker monitoring is promising in improving the medical treatment.
One method for analyzing an unprocessed unpurified protein employs, in principle, a sensor for detecting a specific compound by a biological interaction between a ligand and an analyte. Such a sensor is typically based on fluoroimmunoassay, plasmon resonance, optical interference, or the like. In any of these methods, a ligand is immobilized on the surface of the sensor substrate; the analyte only in the specimen is allowed to bond selectively to the ligand to exclude contaminants; and the target protein only is allowed to react to be adsorbed effectively on the surface of the sensor substrate.
<Localized Surface Plasmon Resonance Sensor>
The sensor utilizing the localized surface plasmon resonance is based on high sensitivity of metal plasmon to a refractive index change of an interfacial substance. In the analysis with this type of sensor, a ligand capable of bonding specifically to an analyte is immobilized on a thin metal film or fine metal particles; a refractive index change caused by bonding of the analyte to the ligand is sensed optically by utilizing localized surface plasmon resonance; and the concentration of the analyte is derived by analysis of the obtained spectrum. The immobilization of the ligand may be conducted chemically or physically. With this technique, information can be obtained also on a change with time (kinetics) of the reaction between the ligand and the analyte. This is regarded as an advantage of LSPR as well as the characteristics of needlessness of labeling.
The LSPR sensor can be evaluated for sensitivity by the detection limit of the analyte bonding to the ligand. However, the bonding constant depends on the kinds of the ligand and analyte, so that the detection sensitivity depends on the combination of the ligand and the analyte.
The LSPR sensor conducts the detecting by utilizing the change of resonance conditions of the metal plasmon caused by change of refractivity in the periphery of metal fine particles. Therefore, the responsiveness of the sensor to the refractive index change is the characteristic index of the sensor: the higher responsiveness to the refractivity gives a higher sensitivity of the sensor.
Japanese Patent 3452837 discloses preparation of a localized surface plasmon sensor (LSPR) by mono-dispersing metal fine particles by self organization and fixing the metal fine particles on a glass substrate surface modified with amino groups. This patent document also describes the fundamental characteristics of this sensor, and a method for measuring the concentration and kinetics of the target substance by detection of a change of the absorption spectrum intensity.
Anal. Chem. 2002, 74, 504-509: A Colorimetric Gold Nanoparticle Sensor to Interrogate Biomolecular Interactions in Real Time on a Surface discloses an LSPR sensor in which metal fine particles are fixed to a chemically modified glass substrate similarly as disclosed in the above Japanese Patent. This document discloses also observation of the reaction between streptoavidin and biotin by an intensity change of the absorption spectrum. According to this document, the refractivity responsiveness to the environmental medium change was 76.4 nm/index.
J. Phys. Chem. B 1999, 103, 9846-9853; Nanosphere Lithography Effect of External Dielectric Medium on the Surface Plasmon Resonance Spectrum of a Periodic Array of Silver Nanoparticle discloses a technique of preparing an LSPR sensor by arranging Ag dots by nanosphere lithography (lithography employing polystyrene nano-beads). According to this document, the refractivity responsiveness to the environmental medium change (RIU) was 200 nm/index.