In a method for detecting analyte which is well-known in this field, latex or erythrocyte is sensitized with antigen or antibody beforehand, and, then, agglutination reaction with an analyte which exists in biological fluid such as protein or a certain specific organic material is utilized for detecting the analyte. An example of latex which has widely been employed for the above-mentioned method is polystyrene latex, which is being studied also as to what influence is exerted on analyte-detecting rate or the sensitivity of detection (e.g., non-specific adsorption of materials other than analyte, or the like) by factors such as particle size and functional group (carboxyl group and the like) on the surface of particle (see Journal of the Japan Society for Clinical Laboratory Automation, 12, 2, 121-124, 1987).
Agglutination of latex particles via analyte has a major influence on detecting rate. It takes, however, a considerable time for the agglutination, since the speed of said agglutination is determined by diffusion. Hence, it has been tried, for instance, to make latex particles carry magnetic particles and to raise the rate of agglutination or precipitation of latex particles by magnetic force (see, e.g., J. Applied Polymer Science, Vol. 50, 765-776, 1993).
Polystyrene latex particles and the like which are mentioned in the former of the above-mentioned documents are liable to agglutinate non-specifically, and are sometimes slightly inferior in dispersion stability. Furthermore, the surface of such particles is apt to non-specifically adsorb impure protein other than analyte. Thus, a part of the inventors of this invention has provided, as a quantum dot to detect analyte in biological sample, composite particles which are composed, for instance, of semiconductor-ultra fine particles-encapsulating fine particles whose surface is covered with chain of polyethylene glycol (hereinafter referred to also as PEG) or poly(ethylene oxide) (or wherein PEG having high mobility in an aqueous solution takes plural brush-like structures) (see pamphlet of WO 02/056020, pages 16-17). Although this type of quantum dot prevents non-specific adsorption of impure protein, it has generally a particle size up to hundreds of nanometers, and has high-mobility PEG on its surface. Thus, quantum dot makes a very stable dispersion in an aqueous solution, and has therefore not necessarily been considered to be suitable for use in a system to rapidly detect analyte with use of agglutination reaction.