Decorated nanowires produce big improvement in SERS

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  • Published: Jan 5, 2018
  • Source: American Institute of Physics
  • Channels: Raman

Techniques for identifying and characterizing different molecules, such as Raman spectroscopy, are increasingly important for medicine and pharmacology. But the problem with Raman spectroscopy, which identifies molecules based on their scattering of laser light, is that it has a limited capacity to detect molecules in diluted samples because of low signal yield.

A team of researchers from the University of Hyderabad in India has now come up with a solution to this problem. They have developed a novel version of surface-enhanced Raman spectroscopy (SERS) that utilizes nanoparticles arranged on nanowires to detect molecules at low concentrations.

The team decorated vertically aligned silicon nanowires with varying densities of silver nanoparticles, utilizing and enhancing the structure's three-dimensional shape. Their results, published in the Journal of Applied Physics, show that their device was able to enhance the Raman signals for cytosine protein and ammonium perchlorate 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."

Applying the necessary nanostructures to SERS devices remains a challenge for the field. Building these structures in three dimensions with silicon nanowires has garnered attention for their higher surface area and superior performance, but silicon nanowires are still expensive to produce.

Instead, the team was able to find a cheaper way to make silicon nanowires and used a technique called electroless etching to make a wide range of nanowires. They ‘decorated’ these wires with silver nanoparticles with variable and controlled densities to increase the nanowires' surface area.

"Optimizing these vertically aligned structures took a lot of time in the beginning," said Nageswara Rao, another of the paper's authors. "We increased the surface area and to do this we needed to change the aspect ratio."

After optimizing their system to detect Rhodamine dye, they found that the decorated nanowires could enhance Raman sensitivity by a factor of 10,000 to 100,000. This allowed SERS to detect cytosine, a nucleotide found in DNA, and ammonium perchlorate, a molecule with potential for detecting explosives, at concentrations as low as 50µM and 10µM, respectively.

The results have given the team reason to believe that it might soon be possible to detect compounds at nanomolar or even picomolar concentrations, Nageswara Rao said. The team's work has opened several avenues for future research, from experimenting with different nanoparticles such as gold, increasing the sharpness of the nanowires or testing these devices across several types of molecules.

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