Fig. 1. Common photoacoustic contrast agents. (a) Comparison of noise equivalent concentrations and molar extinction coefficients of main endogenous and exogenous contrast agents^[3]; (b) organic dyes (e.g., ICG, melanin nanoparticles) and inorganic dyes (e.g., gold nanoparticles)^[12,14,33]; (c) comparison of the performance of different exogenous photoacoustic contrast agents^[12,14,33]

Fig. 2. Preparation methods of liposomal photoacoustic contrast agents. (a) Conventional preparation method; (b) microfluidic preparation method^[21]

Fig. 3. Preparation of liposomes via microfluidic technology. (a) 3D-printed microfluidic flow focusing device^[26]; (b) 3D-printed microfluidic vortex focusing device^[27]; (c) one-step preparation of aptamer-modified ICG liposomes using a microfluidic mixing device^[21]; (d) in situ preparation and purification of ICG liposomes via a dialysis-functionalized microfluidic device^[29]; (e) preparation of aptamer-modified MB liposomes with a microfluidic device^[28]; (f) preparation of small-sized ICG liposomes using a cavitation-on-a-chip device^[30]

Fig. 4. Applications of liposomal photoacoustic contrast agents. (a) Cardiovascular PAI^[43]; (b) brain photoacoustic PAI^[44]; (c)‒(g) tumor PAI^[8,18,45-46]