Signal limitations in tip-enhanced Raman scattering: the challenge to become a routine analytical technique

The fundamental parameters and limitations that determine the signal strength in tip-enhanced Raman scattering (TERS) are discussed. A semiquantitative analysis of the Raman signal expected in different experimental geometries and with different sample systems is presented, taking into account experimental parameters including Fresnel factor, numerical aperture of the illumination and collection optics, detection efficiency, and the Raman scattering cross section of the material. A top/side-on illumination geometry is essential for the study of non-transparent samples. It can yield the highest signal levels when strong tip-sample coupling using a metallic substrate provides large field enhancement. In contrast, axial/through-sample illumination is limited to transparent sample materials. Although conceptually simpler in experimental implementation and despite high numerical aperture signal collection efficiency, signals are generally weaker due to limited field enhancement. Crystalline solids with small Raman cross sections and dense molecular/biological systems with unavoidable far-field background provide the biggest challenge for TERS analysis yet at the same time hold the most exciting outstanding scientific questions TERS has the potential to answer.