A new technique that could lead to chemical sensors that detect single molecules has been unveiled by researchers in the UK and France. It involves creating arrays of tiny nanostructures that interact strongly with light at a specific wavelength. When as few as one molecule of interest attaches to a nanostructure, the optical properties of the material change dramatically – signalling the presence of that chemical species.

The technique involves using "plasmonic metamaterials", which are arrays of tiny metal structures that can be fine-tuned to interact with light in very specific ways. With these materials, it is possible to tweak the shape, size and arrangement of the structures to support collective oscillations of conduction electrons – called plasmons – at specific frequencies.

In this latest work, Sasha Grigorenko and colleagues at the University of Manchester and Aix-Marseille University have made metamaterials that are arrays of gold structures on a glass substrate. Each structure is about 90 nm tall and 100 nm across and separated by about 300 nm. The most basic array comprised a single gold pillar as the unit cell, but other arrays had double pillars or dumbbell shapes as unit cells – while others were indentations in sheets of gold.

Zero reflection

The arrays are designed to reflect no red light at a wavelength of about 710 nm and it is this property that changes if a tiny number of target molecules stick to the gold nanostructures. To make the array sensitive to a specific molecular species, it is coated with a tiny amount of a special chemical that will grab onto the molecules of interest. Each nanostructure in an array has about 2000 such binding sites.

The team used its device to detect the presence of streptavidin – a particular protein that will bind to the nanostructures. This involved firing 710 nm light at the surface of the array at an incidence angle of 53 degrees before measuring the intensity of the reflected light along with the relative phase of its S and P polarization – the phase being extremely sensitive to tiny changes in the reflective properties of the array. When the array was exposed to a dilute solution of streptavidin for just a few minutes, the team saw a jump in the amount of light reflected accompanied by a sharp phase shift.

Detecting one molecule per nanostructure

The team reckons that the devices have a detection sensitivity of about 1–4 streptavidin molecules per nanostructure. But the researchers say that by using advanced phase detection techniques, this could be improved to as little as 0.004 molecules per nanostructure. This is less than one molecule per square micron of the device and between two and three orders of magnitude better than plasmonic sensors based on changes in light intensity rather than phase.

One interesting aspect of the work is that the response of the arrays was calibrated by overlaying them with graphene – a sheet of carbon just one atom thick. Once the graphene is in place it is bombarded with hydrogen, which can then bind to nanostructures. The presence of hydrogen on the graphene can be measured using Raman spectroscopy, which provides an independent calibration of the number of molecules bound to the nanostructures.

Grigorenko told physicsworld.com that the team is now looking at how the arrays could be produced more cheaply. The first devices were made using the expensive process of electron lithography, while the team is now working on much cheaper self-assembly techniques. Grigorenko and his colleagues are also looking at employing better techniques for phase measurement with a long-term goal of creating prototype sensors that could be used as analytical tools for detecting trace amounts of chemicals.

The devices are described in Nature Materials.