Liquid crystals (LCs) are fluidic phases that exhibit molecular order, which have recently therefore attracted considerable research interest for the sensing of temperature, toxic gases, heavy metals, proteins, cancer cells and viruses. LC-isotropic phase transition can be induced via thermal or chemical stimuli. Recent progress showed that droplets, initially sequestered within nematic LC due to elastic distortions, can be released through their drag with the progression of the nematic-isotropic phase boundary and expulsion from the LC-bulk aqueous phase boundary. In this study, we explored light-responsive (near infrared, NIR) droplet release through photothermal effect. Specifically, we hybridized 4-cyano-4'-pentylbiphenyl (5CB), a room-temperature nematic LC, with a diimmonium-based dye showing strong light absorption and photothermal heat generation within the 700-1000 nm region. Water droplets were initially entrapped in 5CB through long-range ordering-induced elastic strain. Release was accomplished by illuminating NIR laser on a local spot of dye-hybridized 5CB leading to heating above isotropic phase transition temperature (32 °C). Droplets were ejected from the LC-aqueous interface through their drag by isotropic phase boundary. We show that the heating rate can be tuned from 0.2 to 0.7 °C/s by varying laser power intensity from 1 to 4 W/cm<sup>2</sup>. Speed of isotropic phase boundary propagation was found to be dependent on the laser beam size. Photothermal dye and 5CB are durable against several laser on/off cycles suggesting their multiple use. Use of laser as the excitation source renders hybrid LC beneficial in medical fields in terms of deep penetration in tissue, controlled, local and on-demand release of substances. New photothermal liquid crystal material presented here holds potential in drug release, environmental remediation, photothermal therapy, and photoacoustic imaging applications.

Photothermal dye-doped liquid crystals for NIR-responsive release applications

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.