When exited with a pulsed laser at 850 nm, a strong ultrasound signal can be detected. The signal intensity decreases exponentially with dropping concentrations of our P1 particles, as is expected from photoacoustic theory (see Figure , e, and Figure ). Remarkably, even at low concentrations of 400 µg/mL, we are still able to observe a photoacoustic signal. This shows that the incorporation of a drug into polyimidazole particles has no detrimental effect on the contrast performance compared to conventional polyimidazole particles, qualifying our prodrug loaded nanoparticles as effective theranostic agents. Next, we analyze how well the particles can be imaged in biological tissue. We inject 50 µL of a 1 mg/mL aqueous solution of P1 into a chicken breast phantom at increasing depths, while recording the photoacoustic signal. Even at large depths of 1.4 cm, we are able to locate our theranostic particles (see Figure ). The gained results corroborate the great potential of the particles in biomedical imaging applications.