Fabrication of calibration plate for total internal reflection fluorescence microscopy using roll-type liquid transfer imprint lithography

Abstract

A strong demand exists for the visualization of the fluid flow in microchannels, such as those in labs-on-a-chip. Multilayer nanoparticle image velocimetry (MnPIV) is a promising technique for this purpose, wherein the x- and y-positions of a nanoparticle are measured using a camera and its z-position is estimated via its fluorescence intensity. To accurately estimate the z-position, however, MnPIV requires a fluorescence intensity calibration process. Water is typically used in the microfluidic devices and a nanoscale level with a refractive index matching method is conventionally used for the calibration of MnPIV. Therefore, a calibration plate with nanoscale levels whose refractive index is matched to water is necessary to calibrate the z-position of fluorescent particles in water. In this study, a fabrication technique of the calibration plate was performed using ultraviolet (UV) nanoimprint with a UV-curable resin, whose refractive index is matched to water and which possesses a high viscosity of 2900cps. Although the residual layer thickness (RLT) of the level must be less than 100nm to perform MnPIV, it is challenging to reduce the RLT with such a high viscosity of the resin using conventional UV nanoimprint lithography. To reduce the RLT, therefore, in this study roll-type liquid-transfer imprint lithography (LTIL) was used with the high-viscosity UV-curable resin, where a two-fold repetition of the roll-type LTIL process demonstrated a calibration pattern RLT of 40nm or less. As a result, calibration of the MnPIV system was achieved with the calibration plate developed herein and fabricated by UV nanoimprint lithography.