Water inside and between cells with
dimensions on the order of
101–103 nm such as synaptic clefts and
mitochondria is thought to be important to biological functions, such
as signal transmissions and energy production. However, the characterization
of water in such spaces has been difficult owing to the small size
and complexity of cellular environments. To this end, we proposed
and fabricated a biomimetic nanospace exploiting nanofluidic channels
with defined dimensions of hundreds of nanometers and controlled environments.
A method of modifying a glass nanochannel with a unilamellar lipid
bilayer was developed. We revealed that 2.1–5.6 times higher
viscosity of water arises in a 200 nm sized biomimetic nanospace by
interactions between water molecules and the lipid bilayer surface
and significantly affects the molecular/ion transport that is required
for the biological functions. The proposed method provides both a
technical breakthrough and new findings to the fields of physical
chemistry and biology.
The expansion of microfluidics research to nanofluidics requires absolutely sensitive and universal detection methods. Photothermal detection, which utilizes optical absorption and nonradiative relaxation, is promising for the sensitive detection of nonlabeled biomolecules in nanofluidic channels. We have previously developed a photothermal optical phase shift (POPS) detection method to detect nonfluorescent molecules sensitively, while a rapid decrease of the sensitivity in nanochannels and the introduction of an ultraviolet (UV) excitation system were issues to be addressed. In the present study, our primary aim is to characterize the POPS signal in terms of the thermo-optical properties and quantitatively evaluate the causes for the decrease in sensitivity. The UV excitation system is then introduced into the POPS detector to realize the sensitive detection of nonlabeled biomolecules. The UV-POPS detection system is designed and constructed from scratch based on a symmetric microscope. The results of simulations and experiments reveal that the sensitivity decreases due to a reduction of the detection volume, dissipation of the heat, and cancellation of the changes in the refractive indices. Finally, determination of the concentration of a nonlabeled protein (bovine serum albumin) is performed in a very thin 900 nm deep nanochannel. As a result, the limit of detection (LOD) is 2.3 μM (600 molecules in the 440 attoliter detection volume), which is as low as that previously obtained for our visible POPS detector. UV-POPS detection is thus expected be a powerful technique for the study of biomolecules, including DNAs and proteins confined in nanofluidic channels.
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