Liquid crystal droplet lasers for chemical sensing

Abstract

Liquid crystal (LC) based chemical sensors have been an area of strong interest due to their ability to convert the presence of an analyte into an optical change brought about by analyte-induced LC molecular reorientation at a LC interface. In both film and droplet formats, LC sensors have been demonstrated in a diverse range of ap- plications and have been identified as a potential low-cost versatile sensing platform for uses such as healthcare, point-of-care diagnostics or environmental monitoring. However, challenges remain in the quantification and instrumentation of LC sensor outputs, particularly given the majority developed to-date require microscope imaging, and often a human eye, to interpret the sensor output.

A possible solution to these challenges exists in the use of LC lasers. Rather than providing a solely image-based output, lasing LC phases, when doped with a suitable gain medium, act as optical resonators and emit light which is characteristic of the LC molecular alignment. LC lasers can therefore be used to indicate the presence of a target analyte and offer opportunities for quantitative measurement and enhanced sensing performance due to the readily measurable and multi-metric output. Further- more, it is anticipated that LC laser emission is simpler to instrument and convert into an automated, digital sensor output compared to image-based sensing.

Presented in this thesis is the application of dye-doped chiral nematic (DDN*LC) droplet lasers for detection of LC interfacial reorientation in LC-based sensors. Firstly, the light emission in planar surface aligned DDN*LC droplets in a “Bragg onion” con- figuration is characterised, revealing complex multimodal laser emission originating from multiple distinct resonant light paths within the droplet. Sodium Dodecyl Sulfate (SDS) is used to initiate a transition from planar to homeotropic LC anchoring at the droplet surface; replicating the LC alignment transition which underpins LC-based sensors. The response of DDN*LC laser emission to this reorientation is presented, indicating a quantitative sensor response with analyte concentration measurement capabilities. By demonstrating a wavelength agnostic emission intensity response, a possible future pathway for quantitative yet low-cost LC-based sensors is highlighted. The merits of such a sensor output relative to other lasing and non-lasing LC-based chemical sensors is also discussed.

To further demonstrate feasibility, tailored microfluidic chip devices were fabricated and successfully used for lab-on-chip surfactant detection using lasing DDN*LC droplets. In addition, a device was developed to compress laser droplets in situ into an oblate shape yielding significant enhancements in emission intensity in planar aligned droplets which could further improve the practicality of LC laser-based sensors going forwards. However initial results in homeotropic droplet surface alignment configurations were not promising. Methods to improve performance in compressed droplets so that they can be used in laser-based sensing are discussed.

By addressing some of the key existing challenges in LC-based chemical sensing surrounding output quantification and instrumentation, it is anticipated that DDN*LC droplets could provide a bridge from the current state of LC sensing to real-world devices.