Abstract
We present a new class of ultrasound molecular imaging agents that extend upon the design of micromotors that are designed to move through fluids by catalyzing hydrogen peroxide (H&sub2;O&sub2;) and propelling forward by escaping oxygen microbubbles. Micromotor converters require 62 mm of H&sub2;O&sub2; to move - 1000-fold higher than is expected in vivo. Here, we aim to prove that ultrasound can detect the expelled microbubbles, to determine the minimum H&sub2;O&sub2; concentration needed for microbubble detection, explore alternate designs to detect the H&sub2;O&sub2; produced by activated neutrophils and perform preliminary in vivo testing. Oxygen microbubbles were detected by ultrasound at 2.5 mm H&sub2;O&sub2;. Best results were achieved with a 400-500 nm spherical design with alternating surface coatings of catalase and PSS over a silica core. The lowest detection limit of 10-100 μm was achieved when assays were done in plasma. Using this design, we detected the H&sub2;O&sub2; produced by freshly isolated PMA-activated neutrophils allowing their distinction from naïve neutrophils. Finally, we were also able to show that direct injection of these nanospheres into an abscess in vivo enhanced ultrasound signal only when they contained catalase, and only when injected into an abscess, likely because of the elevated levels of H&sub2;O&sub2; produced by inflammatory mediators.