Ever heard the expression: finding a needle in a haystack?
Well it is a child play if you compare finding a molecule in a teaspoon of liquid! In an average size haystack there are less than a million straws (taking into account that a needle is smaller than straw and can hide behind one multiply by 1,000 the complexity and you get one billion); A teaspoon of water contain half a million billion billion molecules (given or taken a few billions depending on the water temperature....), hence finding a single molecule in a teaspoon is bound to be half a million billion times more difficult.
Clearly, these are back of the napkins calculations but help in understanding the value of this news coming from Penn University where a team of researchers have found a way to spot a single molecule in a drop of liquid (and even in a small clump of material or in a whiff of gas).
They are exploiting a physical phenomena, Raman scattering, whereby a laser light gets "disturbed" by hitting a molecule and the disturbance differs depending on the type of molecule. By measuring this "disturbance" one can detect/identify the molecule.
Of course, it is much more complex than this. You have to focus your laser beam in such a way to hit the molecule, then you have to detect the disturbance and as it is easy to imagine such a disturbance is very weak and hence pretty tough to detect.
Here is where the value of the researchers at Penn State University lies.
They have found a way to create a surface structured in such a way to concentrate the material and at the same time amplify the disturbance.
They created a detection platform, SLIPSERS: Slippery Liquid Infused Porous Surface-Enhanced Raman Scattering, where a droplet containing the molecule is dropped. The surface is structured in such a way that the oil or water contained in the droplet evaporates in such a way that the molecule is forced to aggregate (stick) to gold nanoparticles. These, in turns, are aimed by the laser beam, enhancing the disturbance created by the molecule, hence easing detection.
If you want more details, and are prepared to digest the more technical lingo, you can read the paper.
In practice, this result opens the way to extremely sensitive sensors, that can be used in a variety of fields, including detection of explosive traces and biomolecules associated to cancer cells, thus enabling earlier detection.