An ultra-thin MXene film with high conversion efficiency for broadband ultrasonic photoacoustic transducer

High-pressure, broadband, and miniatured ultrasound emitters are urgently needed in biomedical imaging and treatment as well as non-destructive detection. In this work, we report a laser generated ultrasonic photoacoustic transducer (LGUPT) based on an ultra-thin layer of MXene (Ti${}_3$C${}_2$T${}_x$) nanosheets. Under the excitation of $532\mathrm{nm}$ nanosecond laser pulses, the amplitude of the generated sound pressure can reach $8.7\mathrm{MPa}$, with a bandwidth of $17.4\mathrm{MHz}$ at the irradiation intensity of $17.72\mathrm{mJ}/{\mathrm{cm}}^2$. The photoacoustic conversion efficiency of the $1.2\mu m$-thick MXene film/PDMS transducer was found to be $1.21\times 10^{-2}$, which is among the highest values reported to date. The MXene thin film can also be drop-casted on the curved surface of a focusing lens. The amplitude of the sound pressure signal can reach 25.3 $\mathrm{MPa}$ and the bandwidth $19.7\mathrm{MHz}$ at a pulse laser energy of $28.12\mathrm{mJ}/{\mathrm{cm}}^2$. The width of the focal spot at −3 dB of maximum amplitude was found in the range of $\mathrm{0.14}\mathrm{mm}$ for the optical lens based LGUPT under the condition of a laser spot diameter of $\mathrm{15}\mathrm{mm}$ by theoretical simulation. The water processable focusing LGUPT demonstrated excellent ultrasonic cavitation effect on the tissue mimicking agar plate. Our experimental and theoretical work highlights the potential of ultra-thin MXene film based LGUPTs for high precision photoacoustic therapy, integrated imaging and sensing instruments.