Introduction: The amino acid valine is one of the twenty-two essential amino acids of living cells and the human diet. The amino acid valine is also effective in making some neurotransmitters. If very low levels of the amino acid valine (C₅H₁₁NO₂) are present in the blood, a defect is probable to exist in the body's metabolic system, which can lead to metabolic diseases. The surface-enhanced Raman spectroscopy (SERS) method is a precise technique for identifying extremely low concentrations of chemicals and biological materials. In the present paper, for the purpose of controlling the mentioned metabolic diseases, some substrates coated with silver nanoparticles have been applied to detect the amino acid valine.
Analysis Method: The silver nanoparticles were fabricated by a simple chemical method (Lee and Misel) using silver salt and reducing agent tri-sodium citrate dihydrate and silver nanoparticles were coated on glass substrates by a spinning method (spin-coat). Finally, Raman signal enhancement of molecular vibrations of the amino acid valine was identified by the SERS substrates as plasmonic sensors and Raman spectroscopy.
Results: The fabricated silver nanoparticles are spherical and quasi-spherical nanoparticles that confirm the formation of silver nanoparticles by observing the plasmon resonance peak at 410 nm. The active SERS substrates have been coated with nanoparticles, which enhances the Raman signal. The enhancement of the Raman signal is due to the resonance of the surface plasmons of the nanoparticles. The active SERS substrates, silver nanoparticles deposited on a glass substrate, were made for the detection of the amino acid valine, the detection limit was equal to 10^-9 M, and the relative standard deviation (RSD) for six iterative measurements at a concentration of 10^-9 M was obtained to be 4.30%. Therefore, the obtained Raman results indicate that the active SERS substrates, silver nanoparticles for the detection of the amino acid valine, with the developed methods, demonstrate promising results for SERS-based studies and can lead to the development of Micron sensors.
Conclusion: In Raman spectroscopy, SERS active substrates coated with silver nanoparticles are of researchers' interests due to the resonance of surface plasmons of silver nanoparticles and larger light scattering from silver particles since the Raman signal amplifies the molecular vibrations of the amino acid valine. By decreasing the concentration of the amino acid valine deposited on the active SERS substrates, the Raman signal is also attenuated with the reduction in the number of molecular vibrations; by increasing the surface roughness of SERS active substrates, the Raman signal can be amplified due to the increase in light scattering from rough centers. These rough centers are the larger particles that are made during the deposition process by the spin-coat method, and as a result, they lead to a signal enhancement by increasing the rate of light scattering from themselves, and molecular vibrations of the amino acid valine were then detected by SERS substrates as plasmonic sensors and Raman spectroscopy.