Photoacoustic flow measurements based on wash-in analysis of gold nanorods

In this study, photoacoustic flow measurement methods based on wash-in analysis are presented. These methods use the rod-to-sphere shape transformations of gold nanorods induced by pulsed-laser irradiation. Due to the shape dependence of the optical absorption of the gold nanorods, these shape transitions are associated with a change in the peak optical absorption wavelength. Pulsed-laser irradiation at the wavelength corresponding to the peak optical absorption of the original gold nanorods allows the particles that undergo shape changes to be viewed as "being destructed" by the laser irradiation at that wavelength, hence, flow information can be derived from the change in ultrasound intensity that is directly related to the wash-in rate of the gold nanorods and the laser intensity. Two flow estimation methods based on the wash-in analysis are described. The first method first applies high-energy laser pulses that induce shape changes in all the nanorods. A series of low-energy pulses then are applied to monitor the acoustic signal change as new nanorods flow into the region of interest. The second method uses single-energy laser pulses such that the "destruction" and "detection" are performed simultaneously. The simulation results show that it is valid to fit the time-intensity curves by exponential models. To demonstrate the validity of the proposed methods, an Nd:YAG pulsed laser operating at 1064 nm was used for optical irradiation, and a 1-MHz ultrasonic transducer was used for acoustic detection. Gold nanorods with a peak optical absorption at 1018 nm and a concentration of 0.26 nM were used to estimate flow velocities ranging from 0.35 to 2.83 mm/s. The linear regression results show that the correlation coefficients between the measured velocities and the true values are close to unity (> 0.94), thus demonstrating the feasibility of the proposed photoacoustic techniques for relative flow estimation.