Sensitive determination of concentration of nonfluorescent species in an extended-nano channel by differential interference contrast thermal lens microscope.

A photothermal detector, named differential interference contrast thermal lens microscope (DIC-TLM), was used to determine the concentration of a nonfluorescent solution in a nanochannel. This method exploits a local change in the refractive index of a solution caused by light absorption. A solution was introduced into a 100-nm scale channel by a pressure-driven nanofluidic control system and its concentration was determined by DIC-TLM. The limit of detection (LOD) was 2.4 microM in a nanochannel that was 21 microm wide and 500 nm deep. The LOD was 3 orders of magnitude smaller than that of conventional method. Moreover, the detection volume was accurately determined to be merely 0.25 fL by using a nanochannel with an optical path length of 500 nm. Based on these results, the number of detected molecules was calculated to be 390. In addition, the concentration of a solution in a nanochannel that was 790 nm wide and 500 nm deep could be determined. Finally, the relationship between sensitivity and channel size was investigated and the sensitivity was found to decrease with decreasing nanochannel size, which indicates that the changes in the refractive indices of water and silica cancel each other out. The DIC-TLM realizes sensitive detection of nonfluorescent species in nanochannels without requiring any special fabrication techniques. Therefore, DIC-TLM is expected to be a highly useful analytical technique in nanofluidics.