50 mm γ irradiated glass bottom dish

Vapor nanobubbles (VNBs) were generated by irradiating samples of ca. 1 × 10⁹ NPs/mL in double-distilled water (ddH₂O), present in a 50 mm γ-irradiated glass bottom dish (MatTek Corporation, Ashland, MA, USA), with ca. 3 ns pulsed 532 nm laser light (Cobolt TorTM Series, Cobolt AB, Solna, Sweden). Lasers pulses were generated on demand using a 25 MHz pulse generator (TGP3121, Aim-TTi, Huntingdon, UK), with control over the pulse energy being provided by an adjustable DC power supply (HQ Power PS23023, Velleman Group, Gavere, Belgium). Energies were registered using an Ophir Starlite energy meter (MKS Instruments, Andover, MA, USA). The VNBs were visualized using dark field microscopy, where the increased scatter of VNBs resulted in bright white spots [19 (link)]. Short movies of this phenomenon were captured using a cMOS camera (Blackfly S GigE-Mono, FLIR, Wilsonville, OR, USA) and screen recording software, which allowed the counting of individual VNBs. The number of generated VNBs was determined in the irradiated area as a function of the applied laser fluence. A Boltzmann sigmoid curve was fitted to the data normalized against the maximal number of counted VNBs in GraphPad Prism version 8.0.0 (GraphPad Software, San Diego, CA, USA), allowing quantification of the VNB-threshold as the laser pulse fluence at which 90% of the irradiated particles generate a detectable VNB.

Different in-house developed optical set-ups were used to determine the laser pulse fluence threshold, which is defined as the laser fluence of a single laser pulse at which 90% of the irradiated NBs generate a VB. In short, NBs (stock: $~$ 1.3 × 10⁹ NBs/mL) were first diluted 100× in PBS and transferred to a 50 mm γ-irradiated glass bottom dish (MatTek Corporation, Ashland, MA, USA). After sedimentation of the NBs, the samples were studied on the different set ups.