Paper craft: Simpler SERS

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  • Published: Jun 1, 2017
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Paper craft: Simpler SERS

Office stock

(A) Microscopic image of the SERS substrate on office paper. (B) Distribution of R6G molecules (red colour) across the entire area of the well in the plasmonic paper substrate obtained by Raman Imaging. Due to the extremely fast drying of the drop, R6G molecules are uniformly distributed on the well. The intensity of the red colour is proportional to the 1509 cm−1 vibrational line intensity from R6G. The green shows the absence of R6G signal. (Credit Sci Rep/Nature/Oliveira et al)

Portable sensors used at point of need could be facilitated if everyday office paper were amenable to use as a base substrate for surface-enhanced Raman spectroscopy, according to work published in the journal Scientific Reports.

SERS has been widely used as a sensitive and non-destructive analytical tool for molecular detection. It combines the specificity of vibrational Raman spectroscopy with a sensitivity boost due to plasmon assisted scattering caused by metal nanostructures. Much research has been carried out into the pros and cons of different metal nanostructures allowing single molecules to be detected. As such SERS has become an important tool for the determination of trace analytes in chemical detection, reaction dynamics, biosensing, testing for explosives, and in art conservation, as we have discussed on the site on many occasions.

Now, Maria João Oliveira, Pedro Quaresma, Miguel Peixoto de Almeida, Andreia Araújo, Eulália Pereira, Elvira Fortunato, Rodrigo Martins, Ricardo Franco, and Hugo Águas of Universidade NOVA de Lisboa and Universidade do Porto, Portugal, suggest that for point-of-need and portable devices, a readily available and inexpensive base as a substrate for those metal nanoparticles is much needed. The team has now tested the use of two different types of paper as a base for the deposition of spherical silver nanoparticles (AgNPs) and silver nanostars (AgNSs). The first paper type, Whatman no. 1, has high porosity, while a commercially available office paper, Portucel Soporcel, offers low porosity.

Flexible friend

The idea of using paper in SERS experiments in this way was first suggested by Chieu Tran of Brookhaven National Laboratory in 1984, but work with more conventional rigid and planar supports such as glass, silicon wafers and aluminium films overtook this suggestion. Despite the concept having fallen out of favour, there is renewed interest because paper is inexpensive and has the advantages of being recyclable, biodegradable, and sourced from abundant renewable source. Paper also has wicking capability due to capillary action that is lacking in non-porous bases such as silicon wafers. It is also flexible and thin, which means it can be used to swab samples that would be inaccessible to a rigid material and so reaches the parts other substrates cannot reach. Paper is also amenable to concentration of analytes trough lateral-flow paper microfluidics and can be used as a facile sample storage medium.

Paper aggregation

In their investigation of paper as a SERS base, the team separately drop-casted the AgNPs and the AGNSs into hydrophilic wells patterned on to each type of paper. Porosity, they found, played a critical role in how the particles became distributed along the cellulose fibres of each type of paper. Most of the silver nanoparticles were retained at the illuminated surface of the office paper substrate and this gave the researchers the biggest SERS enhancement with the AgNSs. They report how they could achieve a limit of detection for a standard test compound, rhodamine-6G, as low as 11.4 ± 0.2 picograms; the enhancement was 10 million fold for this analyte.

Importantly, the well patterning technique gave them uniform substrates with the particles and even after five weeks of storage the substrates were still active for SERS experiments. Thus, the team concludes: "Paper-induced aggregation of AgNPs was found to be a viable alternative to the classical salt-induced aggregation, to obtain a highly sensitive SERS substrates."

Related Links

Sci Rep 2017, 7, 2480: "Office paper decorated with silver nanostars - an alternative cost effective platform for trace analyte detection by SERS"

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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