Enhancing silver: Hybrids for SERS

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  • Published: Dec 1, 2013
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Enhancing silver: Hybrids for SERS

Straightforward hybrids

A relatively straightforward method for making core-shell silver-molecularly imprinted polymer Credit: Elsevier/Biosensors and Bioelectronics

A relatively straightforward method for making core-shell silver-molecularly imprinted polymer (Ag@MIP) hybrids has been developed and initial experiments show it has great potential for use in surface-enhanced Raman spectroscopy (SERS) for ultrasensitive analytical applications.

Chemists Limin Chang, Yan Ding and Xin Li of the Jilin Normal University, Siping, China, reiterate the huge benefits of SERS over many other techniques, not least the fact that it is a non-destructive analytical tool but also has what they refer to as "highly sensitive characterization features." Of course, the enhancement observed is at the whim of the surface features of any given active substrate used in the measurements. Moreover, many highly active substances are intrinsically reactive and unstable, which somewhat limits their use. In addition, active elements such as copper, gold and silver also suffer from a particularly limiting characteristic when one is hoping to analyse cells and tissue samples, they are not biocompatible.

Imprinted solution

The team has turned to molecular imprinting as a possible solution to the various problems seen with many SERS active compounds. Molecular imprinting involves moulding a substance, usually a polymer, to a target molecule, or analyte, and removing this "template" to leave behind a structure complementary to the original target. Indeed, the imprinted material will match precisely the size and shape and every contour of the target molecule so that such molecular imprinted polymers (MIPs) have been developed for highly specific sensors over the last several years. Not surprisingly, several teams have investigated ways to couple MIPs to SERS to exploit the materials in highly specific and sensitive analyses.

While the MIPs have been used to make the analysis more specific, the focus has not been on what effect the use of a MIP would have on the SERS enhancement itself. Now, Chang and colleagues have discovered that, surprisingly, the use of a MIP on hybrid SERS active silver particles can not only make the technique highly specific but boosts the signals they observe.

Core-shell enhancement

The team's relatively simple route to core-shell silver-molecularly imprinted polymer (Ag@MIP) hybrid materials involves first fabricating silver microspheres using an acid-directed self-assembly approach. These particles are then modified using a coupling agent to install the MIP. The researchers used X-ray diffraction, energy dispersive spectroscopy, transmission and scanning electron microscopy, and Fourier transform infrared spectroscopy (FTRI) to characterize the morphology and structure of the resulting hybrid materials.

Tests of the Ag@MIP hybrids thus obtained showed an extremely high SERS effect in a proof of principle experiment with 4-mercaptobenzoic acid (4-MBA) as the target molecule. Sensitivity as low as a 10 –15 molar detection limit was possible. The team adds that the SERS signals for 4-MBA were reproducible and selective against other substances. They allude to an explanation for the SERS boost lying with the molecular imprinting giving rise to a "gate effect", although the details are not entirely clear and the mechanism will be the subject of further research the team hints. "Our experimental results show that the Ag@MIP hybrid as SERS platform is potential for ultrasensitive sensing and analytical applications," the team concludes.

Related Links

Biosens Bioelectroics, 2013, 50, 106-110: "Surface molecular imprinting onto silver microspheres for surface enhanced Raman scattering applications"

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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