Analytical Sciences

Acoustic Sensing Based on Density Shift of Microspheres by Surface Binding of Gold Nanoparticles

Akihisa MIYAGAWA,* Yoshinori INOUE,*,** Makoto HARADA,* and Tetsuo OKADA*

*Department of Chemistry, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
**Department of Applied Chemistry, Aichi Institute of Technology, 1247 Yachigusa, Yakusa-cho, Toyota 470-0392, Japan

Herein, we propose a concept for sensing based on density changes of microparticles (MPs) caused by a biochemical reaction. The MPs are levitated by a combined acoustic–gravitational force at a position determined by the density and compressibility. Importantly, the levitation is independent of the MPs sizes. When gold nanoparticles (AuNPs) are bound on the surface of polymer MPs through a reaction, the density of the MPs dramatically increases, and their levitation position in the acoustic–gravitational field is lowered. Because the shift of the levitation position is proportional to the number of AuNPs bound on one MP, we can determine the number of molecules involved in the reaction. The avidin–biotin reaction is used to demonstrate the effectiveness of this concept. The number of molecules involved in the reaction is very small because the reaction space is small for an MP; thus, the method has potential for highly sensitive detection.

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