Study On The Resonance Raman Scattering Of β-carotene And The Enhancement On Stimulated Raman Scattering By Its Fluorescene

The study of biomolecules promotes the development of life science, medicine and chemistry etc. The non-biological application of biomolecules is a challenging research, and is strongly potential. Recently, biomolecules have been applied to catalyzer, molecular identification, optical memory and molecular lead etc. The study of bimolecules promotes the progress of the experimental and theoretical research for molecular interaction, resonance Raman effect, surface enhancement Raman scattering and laser physics.In this paper, the compatibility Teflon-AF 2400 liquid core optical fibre (LCOF) with resonance Raman spectroscopy was used to detect trace amount of aqueous biomolecules, and we determined the sensitivity enhancement factor (SEF), signal-to-noise ratio (S/N) and detection limit of Teflon-AF LCOF; Applying Teflon-AF waveguide cell, the absolute resonance Raman cross sections (RCS) for C=C and C-C stretching modes ofβ-carotene in aqueous solution at low concentrations were determined, and a tentative theory exploration for the experimental results was given; In addition, we studied the effect ofβ-carotene on the stimulated Raman scattering (SRS) thresholds of the first Stokes and the second Stokes line of CS2.The three parts in detail of this paper were as followed: Part 1: The application of Teflon-AF LCOF to the detection of aqueous biomoleculesRaman spectroscopy (RS) is a powerful analytical tool, which provides detailed vibrational information, and therefore has strong potential in identification of analyte. In contrast to FT-IR it can easily be used for aqueous samples owing to the strong infrared absorption of water, and is therefore suitable for biological samples. However, RS suffers from an extremely low sensitivity. Teflon-AF 2400 is quite physically and optically stable and its RI is lower than that of water. So the information about trace amount and ultra- trace amount of biomolecules in aqueous solution could be obtained by the compatibility of Raman spectroscopy and LCOF. Compared to conventional cell, we obtained the SEF, S/N and detection limit of Teflon-AF LCOF. The SEF is proportional to the effective length of fibre, and the effective length is inversely proportional to loss coefficient. The relation is given by whereεand C is the absorbance coefficient and concentration of the core solution, respectively, and K0 is a constant. Fig. 1 shows the correspondding relation of the SEF versus the concentration ofβ-carotene in aqueous solution in experiment, and it can be seen that the changing tendency of SEF with concentration fits with equation (1) approximately. The maximum sensitivity enhancement factor for concentrations greater than the detection limit in a conventional cell was about 10. With theβ-carotene concentration decreasing, the S/N decreases nonlinearly in the both two cells. The signal-to-noise ratios in LCOF cell (S/NF) are almost equal to the signal-to-noise ratios in conventional cell (S/NC) at higher concentrations due to the strong absorption for exciting laser and reabsorption for the Raman scattering light which made the Raman signals can not be enhanced. The S/NF is gradually superior to S/NC with the decreasing concentration as the absorption tends to be weak. There is an improvement in detection limits of about 1000 times for measurements performed in the LCOF in comparison to in the conventional cell. Fig. 2 shows the original Raman spectra of aqueousβ-carotene in LCOF at (a)2.5×10-9 M,(b)2.5×10-10 M. Therefore, The RRS effect in Teflon-AF LCOF cell shows its advantage at extremely low concentration (<μM) for aqueous absorbing sample. The combination of LCOF and RRS is promising in bioanalytical and biomedical applications.Part 2: Study of the resonance Raman scattering ofβ-caroteneRaman cross section (RCS) is an important parameter which can shows the molecular microscopic property as well as the Raman shift and the linewidth. It relates not only to the incident frequency and the structure of a scattered molecule but also to the environment where the scattered molecules are located. Furthermore, it indicates the light-scattering capacity of a particular molecule and the distribution of electric cloud.Applying Teflon-AF waveguide cell, the absolute resonance Raman cross sections (RCS) for C=C and C-C stretching modes ofβ-carotene in aqueous solution at low concentrations (10-6~10-10 M) were determined by measuring Raman intensity. It was unexpected that the RCS increased rapidly with the decrease of concentration at extremely low concentration range (10-8~10-10 M), and it reached the value of 1.22×10-21cm2molecule-1Sr-1 at 2.5×10-10 M larger than general RCS (10-30cm2molecule-1Sr-1) for 109 times. Fig. 3 shows the dependence of the changing tendency of RCS for C=C and C-C stretching mode of aqueousβ-Carotene on the concentration. Each point on the curve is the value of the RCS at various concentrations. When the concentration is in the 10-6~10-7 M region, the RCS is flat with respect to the concentration; when the concentration is lower than 10-8 M, the RCS increases rapidly with the decrease of concentration. A tentative theory exploration for the experimental results was given that the RCSs of aqueousβ-Carotene at low concentrations were larger than general RCS for about 106~109 times, which could be attributed to the resonance Raman effect and dipole-induced dipole interaction between solute and solvent molecules. Additionally, the anomalous increase of RCS with the decreasing concentration was due to the dipole-induced dipole interaction.Part 3: The enhancement of stimulated Raman scattering byβ- carotene's fluoresceneSRS is an important nonlinear optical effect for generating tunable coherent radiation in visible, infrared and ultra-violet, so it is important in the application of laser physics field. Combination with LCOF has the advantages such as high sensitivity, high signal-to-noise ratio, high transitional efficiency etc.The LCOF was applied to SRS, which could decrease the threshold of SRS 2~3 orders, and we obtained the first Stokes of CS2 by weak laser power (0.75~1.8 mJ). The second Stokes line of CS2 was observed at lower pump energy (1.2mJ) in the solution ofβ-Carotene in CS2 compared with that of pure CS2 (higher than 1.8mJ). It is shown from experiments that the SRS threshold of the second Stokes line of CS2 was enhanced by the fluorescence ofβ-Carotene in CS2 solution. When theβ-Carotene was added to the CS2, the strong fluorescence was created at the position of the second Stokes of CS2 (572 nm) which was within the band of fluorescence. The fluorescence was much stronger than the corresponding spontaneous Raman noise, so the spontaneous Raman noise could be increased by the fluorescence. Therefore,β-Carotene was as fluorescence seeding which could induce the enhancement of the second Stokes. The application of LCOF extends the detection methods of the trace amount of biomolecules in aqueous solution, can supply a emulating environment where the subject, especially biological samples exist, and is a potentially powerful analytical tool for life sciences, medicine and biological environment. The study of resonance RCSs of biomolecules at extremely low concentrations supplies experimental data and theoretical basis for the microscope mechanism of molecular interaction, which is helpful to known the natures of vibronic coupling and provides experimental basis for surface chemistry and application researches. In addition, the enhancement of SRS by biomolecular fluorescence in LCOF has significant value in the find of new fibre Raman amplification. As mentioned-above, these researches are not only basical research, but also applicable ones. All the studies will be beneficial to the bioanalytical and biomedical application, surface chemistry and laser physics etc.