Laser in a droplet

When you hear about a laser, you likely imagine a medium-size apparatus with a light beam coming out of it, not a bacterium in a drop of liquid. Well, Turkish and British scientists went beyond ordinary imagination – they expressed a fluorescent protein in E.coli and suspended live bacteria in droplets. Illuminated droplets served as a lens, focusing the protein light emission, creating a living laser. The group published their findings in Lab on Chip.

Lasers are everywhere. Even if you don’t own a laser in a form of a laser pointer, you certainly use one when you scan your purchases. To construct a laser, you need three things – a source of energy to get it going, a material capable of amplifying this energy (gain medium), and a feedback mechanism to amplify the initial energy even more.

In run-of-the-mill lasers, the initial electromagnetic waves are trapped by mirrors, which bounce and enhance them. One of the mirrors is semi-transparent, allowing some of the amplified energy to escape. If this “escape area” is in the form of a narrow slit and the energy is in the visible light part of the spectrum, a laser beam is created – the weapon of choice for Sci-Fi battles.

In a typical laser, you have a gas-filled cylinder or a glass rod with ions for the energy amplification. However, the internal paraphernalia can be replaced by an illuminated liquid droplet suspended in mid-air via a standing sonic wave (Whovians, rejoice!) or optical tweezers.

A suspended droplet is a perfect sphere, which allows the initial light to go inside and bounce from the internal edges of the sphere. A photon from an excitation laser can make tens of thousands of bounces.  The liquid inside the drop serves as the amplification medium and the droplet edge behaves as mirrors.

Some of the light leaks from the droplet-cavity in all directions. Because the leakage is omnidirectional, there is no beam, but the droplet size – from nano to micrometers – and variable droplet composition permit the use of the droplet lasers for various applications such as biosensors and lab-on-chip.

Once you’ve done your initial setup of an external light source and supply of droplets, you can vary the droplet composition.  Starting with a droplet made from pure water, you can add to it fluorescent dyes for a signal amplification.  Or you can use a fluorescent protein.

In the article, the scientists used yellow fluorescent protein (YFP) and were able to detect it at 49 µM. Then they went further and instead of using YFP alone, employed a live E.coli cell, which synthesized the protein. One bacterium expressing YFP in a droplet was enough to serve as a gain medium and it’s possible to create droplets containing growth medium.

This technique is not just a curiosity but may have various technological applications. The problem with many biosensors, especially in contact with complex environments, is their limited life span and low signal to noise ratio. Biomolecules such as fluorescent proteins can be engineered to activate after binding to a specific ligand present in the environment. Live cells allow stable intracellular conditions, selective uptake of the molecules and continuous synthesis of the sensor molecules. Therefore, droplet-based lasers, containing biomolecules and live cells can be used in environmental analysis.