Plasmonic nano-particles sense chemicals better than ever

At the heart of the sensor is an array of gold discs  – the nano-particles – deposited on a transparent substrate.

A laser (~760nm) is used to excite ‘surface lattice resonances’ (SLRs) amongst the particles – resonances which are sensitive to refractive index changes near the surface of the particles (surface sensitivity) and in the volume between them (bulk sensitivity) – according to the paper ‘Second harmonic spectroscopy of surface lattice resonances‘, which describes the work in Nano Letters.

The system is non-linear – it has a power law dependence to electric fields – and therefore creates ultra-violet products at the second-harmonic (~380nm).

During resonance, second-harmonic generation is greatly enhanced – up to 450x, according to Bath, allowing UV spectroscopy to analyse the refractive index environment of the particles with high sensitivity.

Various resonances exist in the system – affected by pump wavelength, angle of incidence, particle material and array lattice constant, for example, and the team has identified a particular resonance that is highly sensitive to molecules in the near-field of the array.

“This mechanism for generating UV light is affected by molecules binding to the surface of the nanoparticles, providing a means of sensing a very small amount of material,” said the University.

“When molecules bind to the surface of a gold nano-particle, they affect the electrons at the gold surface, causing them to change the amount of UV light they emit,” said Bath physicist Dr Ventsislav Valev. “This technique could enable ultra-sensitive detection of molecules in tiny volumes. It is 100 times more sensitive than current methods and could in the future be used for detecting very low concentrations of biological markers for the early diagnostic screening for diseases.”

Nano-particles were fabricated by researchers at Northwestern University, Illinois.