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In recent years, thermal lensing (TL) technology, as ancient as the laser technology, has been reconsidered as a detection technology to meet sensitivity and miniaturization requirements. TL detects a local change of the refractive index induced by local heating that results from optical absorption of a laser beam (excitation) focused within this small volume of sample. To achieve better sensitivity, a separate laser (probe) is used to probe the lensing effect. The sensitivity TL can offer has been demonstrated to be extraordinary. In a side-by-side comparison [1] reported by us, TL shows 140 times better sensitivity than a state-of-art commercial UV-Vis detector. In some special cases, TL can even detect single molecules.
Most of the reported TL works are still lingering in the visible wavelength range. For wider TL applications, a UV-excitation laser is desired for detection of various ‘colorless’ chemicals and biomolecules without labeling. We have built a TL setup using a Q-switched ND-YAG laser (λ=1064 nm) with subsequent frequency quadrupling crystal set that yields an output wavelength of 266 nm. The direct detection of non-aromatic amino acids was tested. In excitation power dependence studies, we discovered that two-photon absorption dominates the UV TL signal from all of the non-aromatic amino acids and photobleaching results in discrepancies between the experimental signal and the expected signal from quadratic extrapolation.
Figure 1. Comparison between experimental thermal lensing signal (light gray) and expected thermal lensing signal (dark gray) based on a quadratic fit of the excitation power dependent curve of 13 non-aromatic amino acids. The vulnerability to photobleaching of each amino acid is indicated above, ranked by negative (-), weak positive (+), and strongly positive (++), respectively.
1. F. Li, A. A. Kachanov, and R. N. Zare, "Detection of Separated Analytes in Subnanoliter Volumes Using Coaxial Thermal Lensing," Anal. Chem. 79, 5264-5271 (2007).