Abstract
Concentration of a nonfluorescent solution was determined in a 100-nm scale channel using a differential interference contrast thermal lens microscope (DIC-TLM). The DIC-TLM detects a refractive index change induced by sample heating as phase contrast. Concentration measurement was first performed in a nanochannel which is 21 xm wide and 500 nm deep. The limit of detection (LOD) was 2.4 μM and the LOD corresponded to 390 molecules in the detection volume of 0.25 fL. DIC-TLM is the only way to measure absorbance sensitively in such thin nanochannels.