Individual nanobubbles detection using acoustic based flow cytometry

We use a novel acoustic-based flow cytometer to detect individual nanobubbles flowing in a microfluidic channel using high-frequency ultrasound and photoacoustic waves. Each individual nanobubble (or cluster of nanobubbles) flowing through the foci of high-frequency ultrasound (center frequency 375 MHz) and nanosecond laser (532 nm) pulses interacts with both pulses to generate ultrasound backscatter and photoacoustic waves. We use in-house generated nanobubbles, made of lipid shells and octafluoropropane gas core, to detect ultrasound backscatter signals using an acoustic flow cytometer. Nanobubble solutions sorted in size through differential centrifugation are diluted to 1:10,000 v/v in phosphate buffered saline solution to maximize the probability that the detected signals are from individual nanobubbles. Nanobubble populations were sized using resonant mass measurement. Results show that the amplitude of the detected ultrasound backscatter signal is dependent on the nanobubble size. The average amplitude of the ultrasound backscatter signals from at least 950 nanobubbles with an average diameter of 150 nm, 225 nm, and 350 nm was 5.1±2.5 mV, 5.3±2.3 mV, and 6.4±1.8 mV, respectively. Similarly, we detected interleaved ultrasound backscatter and photoacoustic signals from nanobubbles tagged with Sudan Black B dye. The average amplitude of the ultrasound backscatter and photoacoustic signals from these black nanobubbles with an average diameter of 238 nm is 10±11 mV and 54±75 mV, respectively. The presence of the dye on the shell suppressed unique features seen in the ultrasound backscatter from the nanobubbles without dye. At present, there is no robust commercial technique able to analyze the ultrasonic response of individual nanobubbles. The acoustic flow cytometer can potentially be used to analyze physical parameters, such as size and ultrasonic response, of individual nanobubbles.