Method for detecting a target particle

A method for indirectly detecting a particle dispersed and moving randomly in a solution using a luminescent probe, comprising:

• (a) preparing a solution containing a target particle to be detected and a first luminescent probe that directly or indirectly binds to the target particle;
• (b) forming a complex containing the target particle and the first luminescent probe in the solution;
• (c) removing first luminescent probes not bound to the target particles from the solution containing the complex, thereby recovering the complex;
• (d) dissociating the first luminescent probe from the recovered complex and separately separating the free first luminescent probe and the target particle, thereby separately recovering the free first luminescent probe and the target particle;
• (e) repeating one or more times a cycle consisting of the following operations (a′) to (d′):
• (a′) preparing a solution by adding a second luminescent probe to the target particle recovered by separating from the free first luminescent probe in the (d);
• (b′) forming a complex containing the target particles and the second luminescent probe in the solution following the (a′);
• (c′) separating second luminescent probes not bound to the target particle, thereby recovering the complex from the solution following the (b′);
• and,
• (d′) dissociating the second luminescent probe from the complex recovered in the (c′) followed by mutually separating and recovering free second luminescent probes and the target particle;
• followed by preparing a single measurement sample solution containing the total amount of the free first and second luminescent probes recovered in the (d) and the (d′);
• and,
• (f) calculating the number of molecules of the first and second luminescent probes in the measurement sample solution by a method consisting of detecting light emitted from the first and second luminescent probes in a photodetection region of an optical system of a confocal microscope or multi-photon microscope while moving the location of the photodetection region in the measurement sample solution using the optical system.