Experimental techniques for imaging and measuring transient vapor nanobubbles

Abstract

Imaging and measuring transient vapor bubbles at nanoscale pose certain experimental challenges due to their reduced dimensions and lifetimes, especially in a single event experiment. Here, we analyze three techniques that employ optical scattering and acoustic detection in identifying and quantifying individual photothermally induced vapor nanobubbles (NBs) at a wide range of excitation energies. In optically transparent media, the best quantitative detection can be achieved by measuring the duration of the optical scattering time-response, while in an opaque media, the amplitude of the acoustic time-response well describes NBs in the absence of stress waves. Transient thermal evaporation and the generation of vapor bubbles are one of the basic processes accompanying non-stationary high-temperature heat-and mass-transfer in liquids. The science and the methods for the detection of inertial vapor bubbles of various origins were well developed for macro-and micro-sized bubbles and mainly employ their ability to emit pressure and to scatter the incident light. Recent developments in nanoscience reduced the spatial and temporal scale of vapor bubbles to nanometers and nanoseconds. [7][8][9][10]23,[27][28][29][30][31][32][33] Unlike their larger analogs, vapor nanobubbles (NBs) require much higher sensitivity and resolution of the detection methods for their imaging, quantification, and identification among other phenomena, such as transient heating and the generation of stress waves. Here, we analyze several experimental techniques for the imaging and quantitative analysis of transient vapor nanobubbles as single events and we troubleshoot some related errors. Due to the multiple biomedical applications of NBs and related phenomena, [34][35][36][37][38] it should be noted that we consider the transient events, but not the materials (particles) that are often also called nanobubbles.39, 40 We also do not consider the cavitation of preexisting bubbles that is well studied elsewhere. 41 While NBs may have various sources of energy (the heating of liquid above the boiling threshold, local rarefaction, and plasma discharge), we employed an experimental model of a single NB in water. Such a model provides maximal precision, control, and reproducibility in NB generation through the localized transient photothermal heating of liquid above the evaporation point. This was achieved through the optical excitation of individual 60 nm gold nanospheres in water with single short laser pulses (70 ps and 532 nm) at specific fluences above the NB generation threshold. We used the plasmonic conversion of optical energy into heat to control the maximal diameter and lifetime of the NBs through the fluence of a single laser pulse, as described in detail previously. [8][9][10]33 This experimental model includes an internal metal nanoparticle (NP) that acts as the source of the bubble energy during bubble generation and prevents the development of extreme temperatures and sonoluminescence at the collapse stage, 8,42 unlike "classical" bu...

Hemozoin-generated vapor nanobubbles for transdermal reagent- and needle-free detection of malaria

“…2A) returned all three responses typical for a transient vapor nanobbuble (14)(15)(16): a bright flash in the time-resolved optical scattering image ( Fig. 2 A, III); the optical scattering trace (showing the expansion and collapse of the nanobubble), which is quantified through the trace lifetime ( Fig.…”

“…S1B) and acoustic (Fig. S1D) traces show the nanobubble dynamics and measure its maximal size through the lifetime of the optical trace or the maximal amplitude of the acoustic trace (14,16). Free space delivery of laser pulses and optical detection of nanobubbles were used for isolated objects by focusing the excitation pulses and probing continuous laser beams on the object (Fig.…”

“…2, IV). This parameter characterizes the maximal diameter of expanding and collapsing nanobubbles (14,16). The maximal amplitude of each acoustic trace was measured in mV as a peak-to-peak signal (Fig.…”

“…Detection, imaging, and quantification of H-VNBs were performed simultaneously with the excitation laser pulse using three independent methods (16). Time-resolved optical scattering imaging (Fig.…”

“…1A). Using our experience in the generation and detection of vapor nanobubbles of other origins in cells and animals (15,16), we hypothesized that hemozoin-induced vapor nanobubbles (H-VNBs) can act as highly sensitive optical and acoustic probes for malaria detection ( Fig. 1 B and C).…”