Seeing single molecules

Seeing single molecules Triangular salt crystals with gold colloids embedded. The colour represents the relative height of the colloid, yellow being the highest.

Dr Hilary Hartigan, Dr Richard Brown and Dr Martin Milton.
National Physical Laboratory.

Dr Lesley Cohen.
Imperial College.

It is a challenge worse than trying to identify a particular grain of sand on the length of the seashore of the UK, but it could soon be possible to probe the dynamic and functional behaviour of single biological molecules. This ability to make spectroscopic measurements on just one molecule of a substance represents an amazing increase in our ability to observe the 'nuts and bolts' of life. Future potential applications of such a technology include 'Star Trek' type hand-held scanners for instant medical diagnosis to devices for people with nut allergies to screen their food and scene-of-crime evidencegathering instruments.

One pioneering approach to single molecule detection that could form the basis of such devices is called Surface Enhanced Raman Spectroscopy (SERS). This is a non-invasive technique that can identify active chemical groups within the structure of complex molecules. Raman spectroscopy has been a routine analytical tool for determining the vibrational spectrum of molecules for many years. Essentially the light from a laser scattering off a sample picks up the vibrational frequencies of the sample molecules caused by its bonds stretching or bending. These frequencies can add to (or subtract from) the frequency of the incident laser light and a vibrational spectrum of the molecule can be obtained by scanning the reflected light over a range of wavelengths either side of the wavelength of the laser.

To boost the signal from just one molecule to detectable levels requires amplification by a wave moving through a collection of nanoparticles to provide a massive signal enhancement. 'With SERS two methods of enhancement help boost the signal: electromagnetic and chemical', says Hilary Hartigan. 'To achieve both enhancements the sample needs to be placed on metallic nanoparticles.'

The main electromagnetic enhancement comes from the interaction of the laser with the surface of the metal and is usually best seen using silver or gold nanoparticles. Exactly how the chemical enhancement works is still a matter for debate, but it seems that the target molecule must adsorb onto the metal nanoparticle for this to be effective. To achieve the best signal the nanoparticles must be much smaller than 100 nanometres.

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