Nano Sensors Group | Illinois

Surface Enhanced Raman Spectroscopy

Surface Enhanced Raman Spectroscopy (SERS) is a label-free detection approach that enables specific identification of chemical molecules through their unique vibrational modes when the molecules scatter light.  In order to detect the wavelengths of light that interact most strongly with a particular molecule, a high intensity, single wavelength light source (a laser) provides a large number of photons.  The chance that a photon will interact with a molecule is enhanced by several orders of magnitude when the molecule is adsorbed to a metal surface that can couple electromagnetic fields from the photons excitation more efficiently.

PC-SERS
By developing photonic crystal (PC) surfaces that incorporate metals and metal nanoparticles, the Nano-Sensors Group demonstrated PC-SERS as a means for increasing the electric field intensity exposed to metal surfaces, thereby increasing the detection sensitivity of SERS (1).  More recently, we have been developing fabrication approaches for SERS nanoparticle surfaces that do not require photolithography, so the surfaces can be applied inexpensively over large surface areas, and incorporated into single-use disposable laboratory items like plastic tubing, microscope slides, and microplates.SERSFigure 1. Schematic of a photonic crystal with gold nano-rods deposited on its surface (left), and the computer simulated enhanced electromagnetic fields generated when a laser light excites its resonant mode (right).


The amplification of the signals in SERS comes mainly through the electromagnetic interaction of light with certain metals (Ag, Au, Cu), which produces large amplifications of the laser field through excitations generally known as localized surface plasmon resonances.
Photonic crystal surfaces coupled with metal nanoparticles provide a new mechanism for enhancing the electric field available for SERS. Our line-scanner microscope allows us shining a laser at an angle on the photonic crystal surface in order to match its resonance mode.
SERS
Figure 2. Comparison between OFF resonance PC mode and ON resonance with clear enhancement of SERS signal from Rhodamine 6g. The enhancement is of about 10x.

Nanodomes
This group developed a new SERS substrate consisting of a closely spaced metal nanodome array fabricated on flexible plastic film. We use a low cost, large area replica molding process to produce a 2-dimensional periodic array of cylinders that is subsequently coated with SiO2 and gold (or silver) thin films to form dome-shaped structures with plasmonic properties.
SERS
Figure 3. Nanodomes fabrication by replica molding process, cross section (top left); computer simulation of localized surface plasmon between gaps or hot spots (top right).


We have experimentally demonstrated that an in-line SERS sensor has a great potential for enhanced safety of current smart infusion systems through point-of-care detection and real-time monitoring of intravenous drug delivery within biomedical tubing.


SERS
Figure 4. Real view of a set of sensor devices (a); SEM image of the plasmonic structure (b); flow cell with sensor device (c); schematic of in-line fluidic sensing system (d)


The ability of SERS sensors to quantitatively identify drug compounds, to kinetically monitor changes in drug concentration, and to simultaneously detect a combination of two drugs has been demonstrated:
SERS
Figure 5. SERS spectra of ten pharmaceutical drugs measured at concentrations used in clinical drug delivery, spectra are vertically offset for clarity (top left); SERS spectra of dopamine solution at various concentrations showing quantitive measurement capability and low limit of detection.

References

  1.  (*W) "Photonic crystals with SiO2-Ag "post-cap" nanostructure coatings for surface enhanced Raman spectroscopy," S.-M. Kim, W. Zhang, and B.T. Cunningham, Applied Physics Letters, Vol. 93, p. 143112, DOI: 10.1063/1.2998695, Published Online October 9, 2008.
  2. "Surface-enhanced raman nanodomes," C.J. Choi, A. Xu, H.-Y. Wu, L. Liu, and B.T. Cunningham, Nanotechnology, Vol 21, p. 415301 (2010) DOI: 10.1088/0957-4484/21/41/415301.
  3. "Plasmonic coupling of SiO2-Ag "post-cap" nanostructures and silver film for surface enhanced Raman scattering," H.-Y. Wu and B.T. Cunningham, Applied Physics Letters, Vol. 98, Issue 15, p. 153103, 2011 (Featured on Cover).
  4. "Quick detection of contaminants leaching from polypropylene centrifuge tube with surface enhanced raman spectroscopy and ultra violet absorption spectroscopy," Z. Xu, C.J. Choi, B.T. Cunningham, and G.L. Liu, Journal of Raman Spectroscopy, Published Online April 19, 2011, DOI:10.1002/jrs.2950.
  5. "Biochemical Sensor Tubing for Point-of-Care Monitoring of Intravenous Drugs and Metabolites," C.J. Choi, J. Weyhenmayer, H.-Y. Wu, and B.T. Cunningham, Lab on a Chip, Vol. 12, No. 3, p. 574-581, 2012.
  6. "Nanoreplicated positive and inverted submicron polymer pyramids array for Surface-Enhanced Raman Spectroscopy," Z. Xu, H.-Y. Wu, S.U. Ali, J. Jiang, B.T. Cunningham, and G.L. Liu, Journal of Nanophotonics, Vol. 5, p. 053526, 2011. (doi: 10.1117/1.3663259)
  7. "Plasmonic nanogap-enhanced Raman scattering using a resonant nanodome array," H.-Y. Wu, C. J. Choi and B.T. Cunningham, Small, Vol. 8, No. 18, p. 2769, 2012 (Featured on Cover).
  8. Point-of-care detection and real-time monitoring of intravenously delivered drugs via tubing with an integrated SERS sensor, H.-Y. Wu and B.T. Cunningham, Nanoscale, Vol. 6, p. 5162, 2014 (DOI: 10.1039/C4NR00027G)