Molecular Immobilization and Resonant Raman Amplification by Complex-Loaded Enhancers (MIRRACLE) on copper (II)-chitosan-modified SERS-active metallic nanostructured substrates for multiplex determination of dopamine, norepinephrine, and epinephrine.

A unique approach based on Molecular Immobilization and Resonant Raman Amplification by Complex-Loaded Enhancers (MIRRACLE) on copper (II)-chitosan-modified SERS-active metallic nanostructured substrates is proposed for sensitive and rapid determination of the catecholamines (CA) dopamine, norepinephrine, and epinephrine. The ternary (CA)2Cu(4AAP)2 complexes were characterized by the appearance of new absorbance bands at 555, 600, and 500 nm for dopamine, norepinephrine, and epinephrine, respectively. The new absorbance band matched with a broad surface plasmon resonance band of utilized silver nanoparticles: 450-600 nm, and 633 excitation wavelength. We observed enhancement factors up to 3.6·106 due to the additional resonant enhancement. The multiplexing capabilities of quantitative spectral unmixing for Raman spectra of a group of CAs, which differ by only either hydroxy or methyl group, at the fingerprint region were successfully demonstrated with the direct classic least squares model. The achieved nM limits of detection with only 1.5 mW laser power and analysis of spiked human blood plasma samples proved the possibility of the multiplex determination of the catecholamines at the level of reference concentrations in the blood of healthy people as well as promise for the future facilitation in the precision diagnosis of neuroendocrine tumors and neurodegenerative diseases.