Raman spectroscopy has traditionally had a limited capacity to detect molecules in diluted samples because of low signal yield. Now a team of researchers from the University of Hyderabad says it has improved molecular detection at low concentration levels by arranging nanoparticles on nanowires.
The team decorated vertically aligned silicon nanowires with varying densities of silver nanoparticles, utilizing and enhancing the structure's 3D shape. Their results, published in the Journal of Applied Physics, show that their device was able to enhance the Raman signals for cytosine by a factor of 100,000.
"The beauty is that we can improve the density of these nanowires using simple chemistry," said Soma Venugopal Rao, one of the paper's authors. "If you have a large density of nanowires, you can put more silver nanoparticles into the substrate and can increase the sensitivity of the substrate."
The team was also able to find a cheaper way to make silicon nanowires, using a technique called electroless etching. They "decorated" these wires with silver nanoparticles with variable and controlled densities, which increased the nanowires' surface area.
"Optimizing these vertically aligned structures took a lot of time in the beginning," explained Nageswara Rao, another of the paper's authors. "We increased the surface area and to do this we needed to change the aspect ratio."
The results have given the team reason to believe that it might soon be possible to detect compounds in concentrations on the scale of nanomolar or even picomolar, Nageswara Rao said.
Image: Detection of a low concentration analyte molecule using silicon nanowires decorated with silver nanoparticles and surface enhanced Raman scattering measurements. Image courtesy of V.S. Vendamani.