An Application of Plastic Microchannel-Microheater Chips to a Thermal Synthetic Reaction

Fluorescence lifetime of Rhodamine B in aqueous solutions of polysaccharides and proteins as a function of viscosity and temperature

Rhodamine B (RhB) is a well known dye extensively used in thermometric studies, either considering the decrease in the fluorescence intensity or the lifetime (τ) with temperature. Lifetime measurements are preferred over intensity ones as they are more robust. In order to expand microscopy thermometry to complex food fluids, the effect of solutes on the τ of RhB was studied using fluorescence lifetime imaging microscopy (FLIM) in a two-photon microscope. Polysaccharides of different molecular weights (glucose, lactose, dextran, maltodextrin, and sodium alginate), as well as whey proteins, were considered as typical model food ingredients. A linear increase in τ with the concentration is observed in most polysaccharides, highlighting that it is not due to an increase in the macroscopic viscosity, but in maltodextrins a Langmuir-like concentration dependence is observed. There are extensive interactions between RhB and whey proteins at small concentrations that quickly increase τ up to saturation at >10 wt% proteins, with τ modelled well using an adsorption Langmuir model. Therefore, the effect of solutes on RhB τ is not related to changes in the macroscopic viscosity. The temperature sensitivity of τ, quantified using apparent activation energies, decreases at high solute contents.

Sulforhodamine Nanothermometer for Multiparametric Fluorescence Lifetime Imaging Microscopy

Live cells function within narrow limits of physiological temperature (T) and O₂ and metabolite concentrations. We have designed a cell-permeable T-sensitive fluorescence lifetime-based nanoprobe based on lipophilic sulforhodamine, which stains 2D and 3D cell models, shows cytoplasmic localization, and has a robust response to T (∼0.037 ns/K). Subsequently, we evaluated the probe and fluorescence lifetime imaging microscopy (FLIM) technique for combined imaging of T and O₂ gradients in metabolically active cells. We found that in adherent 2D culture of HCT116 cells intracellular T and O₂ are close to ambient values. However, in 3D spheroid structures having size >200 μm, T and O₂ gradients become pronounced. These microgradients can be enhanced by treatment with mitochondrial uncouplers or dissipated by drug-induced disaggregation of the spheroids. Thus, we demonstrate the existence of local microgradients of T in 3D cell models and utility of combined imaging of O₂ and T.

Temperature measurement in the microscopic regime: a comparison between fluorescence lifetime- and intensity-based methods

Thermally sensitive fluorescent indicators have been proposed to monitor temperature changes in microfluidic systems, mainly based on fluorescence intensity or lifetime. However, measuring temperature in a structured environment, such as biological tissue, presents additional challenges due to the chemical and structural complexity. Here, we investigate the potential for resolving temperature distributions within the volume of a single cell. Rhodamine B (RhB) dye was employed as a temperature indicator to compare fluorescence intensity- and lifetime-based techniques. The relationship between the fluorescence lifetime and temperature was found to be highly dependent on the biological environment. The intensity-based method allowed the temperature distribution to be mapped with partial success within the volume of a single cell. Under ideal circumstances, the temperature can be mapped pixel by pixel with a resolution better than ±0.3°C within the cell cytoplasm, but this accuracy was reduced to ±1.8°C by environmental variations. These results suggest that the fluorophore should be encapsulated and immobilized in the biological tissue in order to reduce the influence of environmental factors on temperature measurements at the cellular level.

Quantitative 3D mapping of fluidic temperatures within microchannel networks using fluorescence lifetime imaging

We describe a novel method for quantitatively mapping fluidic temperature with high spatial resolution within microchannels using fluorescence lifetime imaging in an optically sectioning microscope. Unlike intensity-based measurements, this approach is independent of experimental parameters, such as dye concentration and excitation/detection efficiency, thereby facilitating quantitative temperature mapping. Micrometer spatial resolution of 3D temperature distributions is readily achieved with an optical sectioning approach based on two-photon excitation. We demonstrate this technique for mapping of temperature variations across a microfluidic chip under different heating profiles and for mapping of the 3D temperature distribution across a single microchannel under applied flow conditions. This technique allows optimization of the chip design for miniaturized processes, such as on-chip PCR, for which precise temperature control is important.

Raman Microspectroscopy/Imaging Study on Phase-Vanishing Processes of Fluorous Biphase Systems in Microchannel-Microheater Chips

Temperature-dependent phase-vanishing processes between perfluorohexane (FC-72) and n-heptane (C7H16, volume ratio of 1:1 and phase-vanishing temperature>41 degrees C), were studied in a glass microchannel-microheater chip under solution-flow conditions. Without applying a voltage to the heater at 21 degrees C, the two liquids in the microchannel were separated into two streams at a solution flow rate (v) of 0.975 cm/s, while the phase boundary between FC-72 and C7H16 vanished in the downstream side of the heater at V=4.5 V. Then, we conducted Raman intensity imaging during the phase vanishing processes in the microchannel by monitoring the band at 756 (FC-72) or 1456 cm-1 (C7H16). At V=4.5 V and v=0.975 cm/s, although no phase boundary between the two liquids in the microchannel was observed, the relevant Raman intensity images indicated the inhomogeneous nature of the solution, and distribution of C7H16 to the FC-72 phase along with solution flow was observed.