Numerical modeling of photothermal effect in nanofluidic channels

A numerical model was established for photothermal detection of non-fluorescent molecules confined in micro/nanofluidic channels. Recently, nanofluidics has been accepted as a methodology to analyze the behavior of single molecules. The extremely high surface-to-volume ratio of the nanospace is also promising for biochemical reactors. However, detection of the solute molecules confined in the nanospace is still difficult unless the molecules are fluorescently labeled. To date, our group has studied photothermal spectroscopy, which can detect non-fluorescent molecules using thermal relaxation, and developed photothermal optical phase shift (POPS) detection. The POPS detector has realized a detection limit of 30 protein molecules, but heat transfer from the sample solution to the glass nanochannel is a barrier to achieve single-molecule sensitivity. In this study, a finite element method (FEM) analysis was performed to simulate the photothermal effect and heat transfer in the nanochannels. The POPS signal calculated from the temperature distribution was compared with experimental values to validate the numerical model. The influences of the detection parameters, including channel depth, modulation frequency, and misalignment of laser beams, are all discussed for improvement of the sensitivity.