Area density of streptavidin can be evaluated by the number density of biotinylated microbubbles

Abstract

Targeted microbubbles (TMBs) that specifically accumulate on target sites via biochemical bonds have been studied for using ultrasound diagnoses and therapies (e.g., ultrasound molecular imaging) in the research field. To understand the specific interactions between TMBs and their target molecules, a biosensor system with a quartz crystal microbalance (QCM) was constructed. In this system, TMBs become absorbed on their target molecule, which was fixed to the QCM surface via a self-assembled monolayer. Our previous studies showed that the system allowed the evaluation of the interaction between biotinylated MBs and the target molecule, streptavidin, by monitoring changes in the resonant frequency of QCM [Muramoto et al., Ultrasound Med. Biol., 40(5), 1027–1033 (2014)]. This paper investigates how the amount of streptavidin relates to the amount of absorbed biotinylated MBs. The amount of streptavidin on the QCM surface was evaluated by measuring the difference in its resonant frequency before and after the fixation of streptavidin. After which, the amount of absorbed MBs was also evaluated by measuring the frequency shift during the interaction between MBs and the target molecule. Our results showed a weak correlation between the amounts of bound MB and the density of streptavidin (correlation coefficient, r = 0.44), suggesting that the area density of target molecule can be evaluated by estimating the number density of TMBs.