Last Month's Most Accessed Feature: Raman boost: Low-cost solutions

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  • Published: Mar 1, 2018
  • Categories: Raman
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Nano boost

Detection of a low concentration analyte molecule using silicon nanowires decorated with silver nanoparticles and surface enhanced Raman scattering measurements. Credit: V.S. Vendamani

Arranging nanoparticles on nanowires can enhance surface-enhanced Raman spectroscopy (SERS) by exploiting electromagnetic fields to boost scattering and thus sensitivity of standard dyes, such as R6G, by many times, according to researchers in India.

As medicine and pharmacology increasingly need to investigate nanoscale processes, the role of Raman spectroscopy in this endeavour could widen. However, its sensitivity limitations when it comes to detecting and characterizing molecules in dilute samples is well known. The new approach by a team of researchers at the University of Hyderabad could remedy that situation. The team has decorated vertically aligned silicon nanowires with varying densities of silver nanoparticles, ultimately tuning the overall three-dimensional structure of the system. They provide more details in the Journal of Applied Physics and demonstrated that their device can boost Raman signals for cytosine protein and ammonium perchlorate by a factor of 100,000.

Beautiful nanowires

"The beauty [of this approach] is that we can improve the density of these nanowires using simple chemistry," explains team member Soma Venugopal Rao. "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."

Assembling nanostructures for SERS devices has been a significant challenge in this field but with many and wide-ranging efforts being undertaken to do so. An approach to building these structures in three dimensions with silicon nanowires has recently gained attention because of the large surface area to volume ratio of nanowires. The potential was there for superior performance but making pristine silicon nanowires is an expensive procedure. The Hyderabad team searched for a workaround that would be less costly. In the end, they focused on the technique of electroless etching which would allow them to fabricate a wide range of nanowires. They could then decorate these nanowires with silver nanoparticles under controlled conditions to further increase the surface area of the nanowires at little additional expense.

Optimization

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

Once the team had optimized their system to detect Rhodamine dye at the nanomolar concentration level, they could demonstrated a Raman boost of sensitivity by a factor of 10,000 to 100,000. They were able to detect concentrations of cytosine, a nucleotide found in DNA, and ammonium perchlorate, a molecule with potential for detecting explosives, at dilute concentrations as low as 50 and 10 micromolar, respectively.

These results suggest to the team that it might even be possible with further optimization and development to detect some compounds at concentrations as lower as nanomolar or even picomolar, Rao adds. Future work might turn to gold rather than silver nanoparticles as well as making the nanowires thinner with a view to detecting a range of other types of compound at low concentration.

Related Links

J Appl Phys 2018, 123, 014301: "Three-dimensional hybrid silicon nanostructures for surface enhanced Raman spectroscopy based molecular detection"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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