In vitro and in vivo biolasing of fluorescent proteins suspended in liquid microdroplet cavities

Abstract

Fluorescent proteins are indispensable for selective, quantitative visualization of localization, dynamics, and interactions of key molecular constituents of live cells. Incorporation of fluorescent proteins into an optical cavity can lead to a significant increase in fluorescence signal levels due to stimulated emission and light amplification in the cavity, forming a laser with biological gain medium. Utilization of lasing emission from fluorescent biological molecules can then greatly enhance the performance of fluorescence-based biosensors benefiting from the high sensitivity of non-linear lasing processes to small perturbations in the cavity and the gain medium. Here we study optofluidic biolasers that exploit active liquid optical resonators formed by surface-supported aqueous microdroplets containing purified yellow fluorescent protein or a suspension of live E. coli bacterial cells expressing the fluorescent protein. We first demonstrate lasing in fluorescent protein solutions at concentrations as low as 49 μM. Subsequently, we show that a single fluorescent bacterial cell of micrometre size confined in a droplet-based cavity can serve as a laser gain medium. Aqueous droplet microcavities allow the maintenance of the bacterial cells under conditions compatible with unimpeded growth. Therefore, our results also suggest a direct route to microscopic sources of laser light with self-regenerating gain media.

Cellular dye lasers: lasing thresholds and sensing in a planar resonator

“…No cell damage has been observed at the pump energy levels used throughout most of this study (less than 100 nJ). This agrees with the previous studies, which showed cell damage above approximately 1 µJ [8,12].…”

“…An even more recently formed strand of research aspires to generate laser light from biomaterials in situ in live cells. Exploiting the fact that GFP and other fluorescent proteins can be produced by a wide variety of live organisms, live cells have been incorporated into cavities to enable lasing: For instance, GFP expressing E. coli bacteria were used as biological gain medium in Fabry-Perot [11] and microdroplet cavities [12], and we have shown the first biological lasers based on single human cell expressing GFP [8] or cells containing fluorescent dyes [13], using a Fabry-Perot type cavity. We have also demonstrated microcavity lasers inside cells in the form of fluorescent solid beads [14][15][16] or droplets, including naturally occurring lipid droplets inside adipocyte cells [14].…”

Droplet lasers: a review of current progress

“…Scale bar is 5µm. Reproduced from [60] with permission from The Royal Society of Chemistry. Image of an array of microdroplets in a microfluidic cavity capable of acting as a distributed feedback mirror.…”

“…This results in a large number of droplets with which to carry out experiments, but effectively means that each laser droplet is a single shot device. Other methods for making droplets include coagulation on a wire [59], resulting in droplets in the 100s of micrometer range and nebulizers [60] which produce droplets typically in the sub-ten-micrometre diameter range when designed for medical applications.…”

“…The wells can then be capped to produce a small device. 60] by making use of common fluorescent proteins used in biological imaging. Superhydrophobic surfaces are loaded with droplets with the Venus yellow fluorescent protein; lasing is shown to be possible using a 488nm femtosecond pump laser.…”