A flexible thermal lens microscope for highly sensitive detection in microfluidic chips

A flexible thermal lens microscope (TLM), which combines the advantages of both conventional thermal lens spectrometry and TLM, is reported in this paper. The flexibility lies in the fact that the TLM can be configured in diffraction-limited (DL) or non-DL excitation mode: in small micro spaces (<50 µ m), the setup can work in DL mode (as in previous TLMs) and shows a limit of detection (LOD) of 2  ×  10^‒5 cm^‒1; in large micro spaces (50–1000 µ m),the setup working in non-DL mode was demonstrated to be much more advantageous. Compared with the case in DL mode, the TLM in non-DL mode shows up to an eight times lower LOD under 100–1000-fold less intense excitation, and better resistivity to flow-induced signal reduction and fluctuation to beam misalignment and background scattering, making it particularly suitable for detecting photolabile samples in highly flowing scattering mediums. For photostable samples, excitation laser powers of watt scale (in contrast to 100 mW in DL mode) could be employed to further decrease the LOD. This flexible TLM can therefore be applied for highly sensitive on-chip analysis of a broad range of analytes in different mediums. (letter)