Surface-enhanced Raman spectroscopy for trace detection on solids
Raman scattering is the inelastic scattering of light upon interaction with a sample, and is the basis of Raman spectroscopy, an analytical technique that has been very useful in the analysis of materials. One general limitation of conventional Raman spectroscopy, however, has been the weakness of Raman scattering signals. It was observed in 1978 by Fleischmann, Van Duyne, Creighton, and their coworkers that when a molecule is next to certain metallic substrate, one may observe an enhancement of the Raman scattering intensity. This observation has brought forth what is known as surface-enhanced Raman spectroscopy (or SERS), which has a significantly extended the reach of Raman spectroscopy as an analytical tool. A critical requirement in this process is the presence of nanoparticles at the surface. As such, SERS is a truly nanoscience phenomenon as it relies on nanostructures for enhancement to occur.
In this work, the SERS technique is applied to the detection of extremely little amount of defects in bone implant prototypes. The nanoparticle-based substrate is composed of colloidal gold nanostars with localized surface plasmon resonance (LSPR) at 690 nm. Spectral maps, on clean and on nanostars covered surfaces, were obtained exactly at the same position using confocal Raman spectroscopy. Comparison of the two maps shows that there are more monoclinic phases detected in the nanostars-covered surface possibly due to the lightning rod effect in the nanostar tips. While SERS is a well-established detection technique and extensively being used for the detection of trace amounts and even single-molecule of chemical species, it has not been widely applied to detection in solids. Very little work has been done to exhibit SERS in solid materials and it remains a challenge to achieve SERS enhancement on dense and polished solids, such as zirconia implants. We report an unprecedented attempt on SERS on solid zirconia, which provides early evidence of the effectivity of the technique even on non-porous materials. With further improvement in sensitivity, SERS is a promising technique for the early detection of monoclinic phase in zirconia-based implants.