Laser Raman Spectroscopy - Different Excitation Sources and Extension to Surface Enhanced Raman Spectroscopy

Raman Spectroscopy

Raman spectroscopy is one of the research analytical techniques used in laboratories. It involves usage of scattered light in measurement of vibrational energy modes of an analyte. The technique is named after an Indian Physicist C.V. Raman, who was the pioneer to observe Raman scattering in 1928 along with his research partner K. S. Krishnan. The process occur is Raman shift and Raman imaging is done through Raman analysis

The following article describes the usage of conventional Raman spectrometers with three different excitation sources. Then, the potential of Raman spectroscopy is utilized to achieve Surface Enhanced Raman Spectroscopy (SERS) activity.

Experimental Design

The Raman spectrometer has been modified to work with three kinds of excitation lasers. The lasers are argon-ion laser, He-Ne laser and diode laser, which are operating at 514.5 nm, 633 nm and 782 nm respectively. Out of the three sources, argon-ion laser and diode laser are preferentially used for experiments. Argon-ion laser is a kind of broadband laser, that emits simultaneously at a number of various wavelengths at the head. The different wavelengths are 454.6 nm, 488 nm, 496.5 nm and 514.5 nm. 

The strongest and well-distinguished 514.5 nm is chosen as the excitation wavelength for the Raman spectroscopy system. The broadband output of the laser is dispersed by a prism. The separation of 514.5 nm emission was done upon passing the light through a narrow homemade pinhole.

Construction of Spectrometer

The beam is focussed on a cylindrical glass vial containing the sample. The Raman scattered radiations are collected at a geometry of 45-90° through a large aperture convex lens and then, it is focussed to the entrance window of optical fiber. In front of the entrance of optical fiber, a holographic notch filter is used, which permits the desired Raman signals to pass through it and mitigating the unwanted Rayleigh scattering. 

The entire setup is used in a dark room for avoiding desensitization of sensitive detective systems by ambient light. In addition, a light housing is used to cover mirror, filter, sample, lenses and entrance slit of the spectrograph, preventing the light reaching from computer monitor and other radiant objects. The outlet of the optical fiber is linked to the entrance slit of the spectrograph. 

In the interior of the spectrograph, the frequency-shifted signal is dispersed by a triple grating turret to resolve the available Raman signatures spectrally. With a good signal to noise ratio, the collection time for acquisition of spectrum is 60 seconds.

Pros of the Designed System

The Raman spectrometer used here has possess the following pros:

• The spectrometer offers flexibility in choosing the excitation wavelength. During the course of experiments, based on the requirement, the source of the excitation can be changed easily as there are three laser excitation sources.
• Raman signal collection optics and sample irradiation are placed on the top of a spacious movale lab-jack, which enables changing the irradiation and collection geometry for the search of better Raman signal. This design facilitates to examine Raman lines upto 200 cm^-1 from the laser with a resolution of less than 2 cm^-1 in the analysis of Raman fingerprints for several components.
• The sample collection geometry is changed to optimize the Raman signal and position of the head of the optical fiber is easily changed to render compatibility with the new geometry.
• The detection system of the Raman spectrometer is equipped with a greater sensitive spectrograph. There is high-aperture optics and coma-corrected configuration for reduction of astigmatism. CCD camera is ultrasensitive and undergoes Peltier cooling, which enables detection of weak Raman signals with enough resolution.

Selection of Laser

Some hazardous organic compounds like aniline, benzene, pyridine and chlorobenzene and dyes like prussian blue, ketone red and R6G are selected for the experiment. Some of the mentioned compounds have strong absorption in UV to visible regions. Hence, diode laser is used to avoid the interference fluorescence problem as usage of argon-ion laser results in strong fluorescence. There is observation of broad, diffusion and intense fluorescence signals when argon-ion is laser is used for pyridine and Prussian blue. When diode is used, the both compounds reasonably produce structured Raman bands with negligible fluorescence.

Some of the compounds like chlorobenzene, nitrobenzene and benzene reveal no fluorescence. For these non-fluorescent compounds, higher Raman signal is obtained when argon-ion laser is used, when compared during the usage of diode laser. Hence, for lesser fluorescent or non-fluorescent compounds, argon-ion laser has better potentiality than diode laser.

The peaks in Raman spectrum are compared with literature values and are found to be in agreement with the literature with a narrow difference, exhibiting reasonable reliability and accuracy of the Raman spectrometer used in the experiment.

Use of Annexing SERS Technique

When an AgN collide is incubated with an R6G compound, the Raman band intensities of R6G across the selected spectral region encounter a larger enhancement and the average enhancement is in the fold of 1000. The Raman spectrum around the region of 0-1500 cm^-1 exhibits a larger degree of enhancement. Raman spectroscopy, when extended with SERS conditions, is affected with plasmon resonances, which produce enhancement. This is dependent on the wavelength. Consequently, different portions of the spectrum are amplified by different quantities, depending on the dispersion of underlying resonance fabricating the enhancement.

Huge dumping of industrial by-products occur in the rivers and sea everyday. This kind of continuous dumping serves as a threat to aquatic microorganisms and living bodies. Thus, it becomes a bigger challenge to the biodiversity in those areas. Proper profiling is a requisite for designing a better treatment for the dumping components. Trace detection through SERS marks a progress in the development of superior detection systems to deal with the hazardous organic pollutants in the earth.

Conclusion

A short introduction has been given to Raman spectroscopy. Then, a design of a modified Raman spectrometer with three different laser sources has been performed. Subsequently, the designed Raman spectrometer has been extended to SERS activity for the detection of dyes and organic compounds. Using chemically synthesized AgN, the Raman signal is enhanced by 1000 times. While argon-ion laser is suitable for non-fluorescence compounds, diode laser is compatible for components revealing fluorescence.