Photochemical Bubble Generation from Polymer Films: Dependence on Molecular Structure and Application for Ultrasound Imaging

Photochemical generation of N2 gas by aromatic azide derivatives dissolved in transparent polymers provides a way to generate bubbles without direct heating. In this work, it is shown that molecules 2-azidoanthracene (2N3-AN), 2-(azidomethyl)anthracene (2N3-CH2-AN), 1-azidopyrene (N3-PY), and 1-(azidomethyl)pyrene (N3-CH2-PY) are all capable of generating stable surface layers of N2 bubble after exposure to 365 nm light. Bubble formation is modeled as a multistep kinetic process that involves molecular photolysis, gas transport through the polymer, and bubble nucleation in water. Direct conjugation of the azide substituent to the aromatic core leads to more rapid photolysis and facile bubble formation, but even azides with relatively slow reaction rates can generate dense bubble layers if high light intensities are used. Rapid transport of the photogenerated N2 gas through the polymer appears to be general, with poly(methyl methacrylate), polystyrene and polycarbonate all supporting robust bubble growth. The photoinduced bubble layer was shown to significantly enhance the visibility of a coated glass pipette when imaged by an ultrasound instrument. The ability to prepare polymer coatings that undergo photochemical gas evolution provides a new functionality that may be useful in medical imaging applications.