Research On Femtosecond Laser Microfabrication Based Fiber SERS Sensor

Combined with all the advantages of fiber optic sensing technology and Raman spectroscopy technique,the fiber surface enhanced Raman scattering(SERS)sensor offers sufficient spectral information of analyte molecules with low concentration,and can be used for remote,real-time,in situ,in vivo,on-line SERS spectral detection,without the drawbacks of weak signal,strong background interference,inability for trace detection by the common Raman spectra.Fiber SERS sensor has attractted more and more concerns due to its important advantages in biochemical analysis fields,such as life sciences,food safety,and environmental monitoring.In this thesis,with combination of femtosecond(fs)laser micromachining technology and photochemical deposition technology,we have explored a series of high-performance fiber SERS sensors.The main contents are as follows:1.A silica fiber SERS sensor based on fs laser microfabrication technology is fabricated.In this thesis,the fs laser microfabrication fiber system is set up,and the fiber is roughened by fs laser ablation.The effect of fs laser power and ablated width on the surface morphology and SERS enhancement of ablated optical fiber is studied.The feasibility of using the excitation laser with the wavelength at 785 nm to modify the processed fiber surface as the SERS active substrate is analyzed.The mechanism of photochemical deposition is analyzed theoretically and the excitation power and deposition time are optimized during the deposition process.The preparation method is more simple,rapid and low cost.The surface of the fiber after roughening is more efficient and fast for the deposition of silver nanoparticles than that of the flat fiber surface under the same deposition time,and in the fabrication process,the optimized laser power and the ablated width can be used to prepare a high sensitivity fiber SERS sensor for detection of rhodamine 6G solution.2.A novel polymer fiber SERS sensor based on fs laser microfabrication technology is developed.In this thesis,the characteristics of polymer fiber abated by fs laser are studied.The PMMA material is much softer than the silica fiber,and with the same pulse energy and scanning speed,the laser ablated slit is much deeper than that of silica fiber,and low pulse energy can achieve a good surface morphology.The SERS activity of PMMA polymer fiber is verified by rhodamine 6G as the target molecule after photochemical deposition of silver nanoparticles.The SERS signal intensity varies with the pulse power and femtosecond laser ablated width used in fs laser fabrication process.For a certain laser pulse energy,there exists an optimized scan surface period.The PMMA polymer SERS sensor proposed in this study has a good performance for the detection of R6 G solution.Given a slightly prolonged integral time and better cleaved end faces of PMMA fiber,the performance of the sensor can be improved,which can be compatible to silica fiber based SERS fiber.3.A novel U-shaped silica fiber SERS sensor based on fs laser micromachining is developed.A series of silica fibers with different widths of U-shaped structure are prepared.The U-shaped structure fabricated by fs laser maintains a uniform shape and good morphology.The relationship between the excitation power received by the far surface of the U-shaped structure and its width is analyzed theoretically.The U-shaped fiber-optic probe is modified with silver nanoparticles by the photochemical deposition method.The U-shaped fiber SERS sensor is used to detect the 10-8 M R6 G solution.With the increase of U-shaped structure width,the relative intensity of Raman peak at 1509.7 cm-1 decreases nonlinearly,which indicates that the Raman enhancement effect of the fiber probe is weakened with the increase of U-shaped structure width.When the width of U-shaped structure is 12 ?m,the relative intensity of Raman peak at 1509.7 cm-1 is the largest,which is four times larger than that of the single roughened surface fiber SERS probe.Compared with a single roughened surface fiber SERS sensor,the prepared SERS sensor increases the SERS active area on the one hand,resulting in more "hot spots" with SERS enhancement effect.On the other hand,the presence of the far face of the U-shaped structure improves the efficiency of the Raman light collection.The U-shaped structure based fiber SERS sensor has the advantages of core mode field excitation,achieving a highly sensitive,real-time and in situ detection of R6 G molecules with a low concentration.4.The D-shaped silica fiber SERS sensor based on fs laser micromachining and photochemical deposition of silver nanoparticles is carried out.This thesis presents a D-shaped silica fiber SERS sensor that belongs to the evanescent field excitation type,which can flexibly increase the SERS active area to improve the SERS signal.The D-shaped silica fiber with different length of active area is prepared by using fs laser ablation.Then,a layer of silver nanoparticles is modified by photochemical deposition method as SERS active substrate.The surface morphology of the sensor is studied by SEM image.The detection results of 10-7 M rhodamine 6G(R6G)solution show that the length of the D-shaped sensing zone has a great effect on the enhancement effect.When the sensor length is in the range of 100 ?m to 500 ?m,the detected Raman intensity is linearly increased as the length increases,and the linearity is 0.998.It can be used for high senstivity,real-time and in situ detection of rhodamine 6G dye molecules.