Lab on a bubble: SERS on spheres

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  • Published: Dec 1, 2011
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Lab on a bubble: SERS on spheres

Bubble up

A new class of materials for surface-enhanced Raman spectroscopy (SERS) consisting of hollow, buoyant silica microspheres coated with gold nanoparticles has been developed. These new materials allow for a novel type of molecular assay designated as a lab-on-a-bubble (LoB).

The team - Virginia Schmit, Brandon Scott and Keith Carron of the University of Wyoming, Laramie, Richard Martoglio of DePauw University, Indiana, Aaron Strickland of iFyber LLC in Ithaca, New York, Carron is also based at Snowy Range Instruments in Laramie, Wyoming - explain that the new assay is "a convenient platform for the detection of analytes in solution and offer several advantages over traditional colloidal gold and planar SERS substrates."

Micro- and nano-electromechanical systems, MEMS and NEMS, have taken enormous steps in small-scale analytics and diagnostics and the associated term "lab-on-a-chip" has even diffused into the non-specialist vernacular. Miniaturised and novel components are constructed on supporting materials complete with input, separation technology, detectors and outputs. The ingenuity of the designers of such devices is on a par with the creativity of the designers of the electronics analogues from which they garner the LoC epithet. There is a continued drive towards such miniaturization, paralleled only by the urge to exploit these systems as single-step assays and diagnostic tools.

The US team has now built on earlier work with paramagnetic particles that have been used as the "engines" to drive LoC devices. Their approach to nanopropulsion employs hollow, and hence buoyant, silica "bubbles" on the surface of which useful materials can be deposited. "Our initial work with paramagnetic nanoparticles was driven by a fundamental limitation to surface enhanced Raman scattering (SERS) analysis with colloidal nanoparticles," the team explains. The limitation is due to the inverse relationship between spectral resolution and the spectrometer's optical throughput, so-called étendue. This leads to a conflict of interests: to sample a nanoparticle colloidal suspension requires constant Brownian motion of the particles involved to allow assay separation but this movement means that they are only fleetingly in the focus of the required narrowly focused laser beam. The SERS signals thus fluctuate due to Brownian motion and lower limits on detection are therefore not as low as one might hope.

Buoyant science

The lab-on-a-bubble concept utilises the buoyancy of the particles to enhance assay separation while gold nanoparticles on the surface of the bubbles act as the requisite SERS nanosensors. The team has now demonstrated efficacy in the detection of a generic thiol-containing Raman-active small molecule 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB). DTNB represents a neutral adsorbate with a high k value and has been used as the succinimate derivative in femtomolar studies of prostate-specific antigen, for instance.

They also tested detection of cyanide, which they explain is a useful model analyte for environmental testing studies. At a concentration of 5 micromolar DTNB, the team could obtain a SERS signal 28 times larger on their LoB system than with the more conventional approach of using colloidal gold nanoparticles, which bodes well for the wider use of the LoB approach. The detection limit for cyanide is approximately 170 parts per trillion (ppt), which is on a par with detection with a more conventional gold colloid.


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

 A new class of materials for surface-enhanced Raman spectroscopy (SERS) consisting of hollow, buoyant silica microspheres coated with gold nanoparticles has been developed. These new materials allow for a novel type of molecular assay designated as a lab-on-a-bubble (LoB).
A gold-coated LoB (schematic top, 50x micrograph bottom)

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