Ultrasound triggered organic mechanoluminescence materials

Abstract

Ultrasound induced organic mechanoluminescence materials have become one of the focal topics in wireless light sources since they exhibit high spatiotemporal resolution, biocompatibility and excellent tissue penetration depth. These properties promote great potential in ultrahigh sensitive bioimaging with no background noise and noninvasive nanodevices. Recent advances in chemistry, nanotechnology and biomedical research are revolutionizing ultrasound induced organic mechanoluminescence. Herein, we try to summarize some recent researches in ultrasound induced mechanoluminescence that use various materials design strategies based on the molecular conformational changes and cycloreversion reaction. Practical applications, like noninvasive bioimaging and noninvasive optogenetics, are also presented and prospected.

Keywords: Acoustic cavitation; Bioimaging; Focused ultrasound; Mechanoluminescence; Photocatalysis; ROS; Sonochemistry.

Fig. 1.

Mechanism of ultrasound stimulation. The energy of sound can be used to trigger physical and chemical effects through the process of acoustic cavitation, which includes nucleation, growth, and collapse. Nucleation begins in gas-filled crevices resulting from inhomogeneities in the solution or surface imperfections. The microbubble size then oscillates non-linearly, with the growth of the microbubble occurring during the negative acoustic pressure phase (rarefaction). Upon reaching a critical size threshold, the bubble collapses. Note that the bubble dimensions are on the μm scale and the time it takes for the process to occur is on the μs scale. The implosive collapse results in several notable effects, including microjetting, which is the asymmetrical collapse of a bubble due to pressure gradients near surfaces, hotspot conditions of 4,000 – 25,000 K and 800 atm, sonoluminescence, reactive free radical formation, and shockwaves.[–43,56].

Fig. 2.

ROS-based mechanism of ultrasound. The reactive free radicals could be generated via the sonosensitizer-based method and thermolysis of water. Due to the acoustic cavitation effect, the energy of ultrasound was released and sensed via a near sonosensitizer, thus activating oxygen to generate reactive oxygen species. Moreover, the local hotspot will also lysis the surrounding water molecules to generate ・OH and ・H.[46,56,90].

Fig. 3.

Sonochemiluminescence materials based on naphthopyran change the color via conformation conversion under the force/ultrasound. (i) different naphthopyran regioisomers, (ii) the conformation conversion under the force, (iii) the color changes upon the naphthopyran decorated PDMS materials under the force. Reproduced from Ref.[112] with permission from American Chemical Society, Copyright 2016. (iv) bisvinyl-terminated naphthopyran (1.5 wt%) is crosslinked into the PDMS and isomerizes under the ultrasound to generate orange-colored merocyanine. Reproduced from Ref.[54] with permission from PNAS, Copyright 2017.

Fig. 4.

(i) The chemical structure of ITPADA, (ii) the fluorescence quantum yield of ITPADA in THF-H₂O solution with various water fractions with/without ultrasound,(iii) the photoluminescence spectrum of ITPADA in different ultrasound powers with a fixed frequency of 40 kHz. Reproduced from Ref.[124] with permission from Royal Society of Chemistry, Copyright 2014.

Fig. 5.

(i)Dioxetane (1.5 wt%) is crosslinked into the PDMS, and cycloelimination reaction is triggered via ultrasound to generate blue light in the presence of sensitizer 9,10-diphenylanthracene, (ii) the blue light emission from dioxetane decorated PDMS, and (iii) optical images, which show FUS irradiation dependent performance due to the irreversible cycloelimination reaction. Reproduced from Ref.[54] with permission from PNAS, Copyright 2017.

Fig. 6.

(i) the activation of anthracene-maleimide mechanophore results in the generation of coumarin fluorescent moiety, (ii) photoluminescence spectra under the ultrasound stimulation. Reproduced from Ref.[147] with permission from American Chemical Society, Copyright 2021.

Fig. 7.

Schematic of anthracene-maleimide cycloreversion under the force. The fluorescence is generated via the liberation of anthracene. Reproduced from Ref.[156] with permission from Royal Society of Chemistry, Copyright 2019.

Fig. 8.

Schematic of BCH-Naphs and the fluorescence emission under the ultrasound. Reproduced from Ref.[167] with permission from American Chemical Society, Copyright 2020.