The application of surface-enhanced Raman spectroscopy for the detection of excitatory amino acids

Abstract

The release of excitatory amino acids (EAAs) from injured neurons has been associated with secondary injury following head trauma. The development of a rapid and sensitive method for the quantification of EAAs may provide a novel means for clinical management of patients affected by head trauma. Of the amino acids L-glutamic (Glu) and L-aspartic acid (Asp), the concentration of Glu is more indicative of injury to the central nervous system. This study explores the detection limit of Glu using surface enhanced Raman spectroscopy (SERS) on aqueous silver colloids. This study examines methods to monitor the colloidal reactions for the calibration of the enhancement observed. Thirty second Raman spectral scans were taken utilizing a 50 mW argon laser. Isolated aqueous Glu was quantifiable from 0.4 []mol/L - 5 []mol/L from an examination of a unique spectral feature at 830cm⁻¹. Asp exhibits a unique but comparable shifted spectral feature at 785cm⁻¹ using the same probing parameters. In order to test this technique in vivo, rat extracellular brain fluid microanalysis samples were collected before and after a localized brain infarct was produced using a middle cerebral artery occlusion (MCAO). SERS spectral analysis of these samples showed an intriguing correlation with expected changes in EAA levels as influenced by the MCAO damage model. We show that SERS techniques may have the promise of providing the sensitivity and selectivity required to quantify EAAs in a phantom medium at physiological concentrations. Compared with conventional detection techniques such as high performance liquid chromatography, this research employs no sample preparation techniques, thus reducing assay time. The controlled studies and limited animal studies suggest that the SERS technique described here may become a viable prelude for providing diagnostic information about nervous system injury in anticipation of clinical intervention.