Label-free detection methods are important in (bio)sensing, since without them, "on-line" monitoring is not possible. Indeed label-free detection methods exist, most notably Surface Plasmon Resonance (SPR) and Quartz Crystal Microbalance (QCM). The problem with these methods is that they are not specific: they report on accumulation of material on the sensor surface.

Raman spectroscopy provides chemical information about specific bonds, and is thus a very attractive method for specific label-free detection. To test the limits of sensitivity of this approach we prepared