Thermo-optical Characterization of Photothermal Optical Phase Shift Detection in Extended-Nano Channels and UV Detection of Biomolecules

Abstract

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.

Review of ultrasensitive readout for micro-/nanofluidic devices by thermal lens microscopy

“…The flow rate may be optimized to minimize the heat transfer from the solvent to the walls of the nanochannel, which can also be done by increasing the modulation frequency at the expense of sensitivity. 77 This is particularly important for a system in which the temperature gradients (dn∕dT) of the solvent and chip material are of the opposite polarity. For example, the dn∕dT of water is −9.1 × 10 −5 K −1 , while it is þ9.8 × 10 −6 K −1 for fused silica.…”

Detection and Characterization of Individual Nanoparticles in a Liquid by Photothermal Optical Diffraction and Nanofluidics

“…Therefore, signal intensity depends on the light absorption characteristics of nanoparticles rather than positions and trajectories of nanoparticles. However, for 10 1 –10 3 nm channels, the sensitivity of photothermal detection methods rapidly decreases because the effect of thermal diffusion from the channels to the substrates (e.g., glass) becomes dominant . For example, thermal diffusion length in the aqueous solution at 1 kHz modulation is calculated to be ∼7 μm, which is 1 or 2 orders larger than the size of the nanochannels.…”

“…However, for 10 1 −10 3 nm channels, the sensitivity of photothermal detection methods rapidly decreases because the effect of thermal diffusion from the channels to the substrates (e.g., glass) becomes dominant. 26 For example, thermal diffusion length in the aqueous solution at 1 kHz modulation is calculated to be ∼7 μm, which is 1 or 2 orders larger than the size of the nanochannels. Then, most of the heat generated by the photothermal effect does not contribute to the signal, and detection performance deteriorates.…”

Nonfluorescent Molecule Detection in 102 nm Nanofluidic Channels by Photothermal Optical Diffraction

“…The principle of the DIC-TLM is based on wave optics (an interference effect) and thus applicable to nanochannels. By utilizing the DIC-TLM, a label-free protein molecule detection in a 21 μm × 900 nm channel has been achieved with the LOD of 600 molecules . Also, it was applied to an enzyme-linked immunosorbent assay (ELISA) in a 700 nm channel and realized the detection of a countable-number of protein molecules .…”

“…Also, it was applied to an enzyme-linked immunosorbent assay (ELISA) in a 700 nm channel and realized the detection of a countable-number of protein molecules . However, for sizes smaller than 900 nm, the sensitivity of the DIC-TLM rapidly decreased due to thermal diffusion to the glass substrates . For 10 1 –10 2 nm channels, the effect of thermal diffusion on the glass substrates becomes dominant.…”