Label-free detection of 3-nitro-_L-tyrosine with nickel-doped graphene localized surface plasmon resonance biosensor

3-nitro-L-tyrosine (3-NT) is believed to be a biomarker of neurodegenerative diseases and metal doped graphene possess exceptionally high binding energy of 3-NT with metal-nitro chemisorption. Here we report a novel label-free detection scheme of 3-NT via nickel-doped graphene (NDG) as the functionalized receptor on our phase detecting localized surface plasmon resonance (LSPR) biosensor. When compared with reported 3-NT immunoassay with enzyme-linked immunosorbent assay (ELISA), our NDG-LSPR platform offers two advantages i.e. 1) label-free and 2) capture of 3-NT by direct chemisorption. Our limit of detection for 3-NT in PBS was found to be 0.13 pg/ml and the linear dynamic range of response was from 0.5 pg/ml to 1 ng/ml, i.e. four orders of magnitude. The specificity of our NDG receptor to 3-NT was also verified with L-tyrosine of equivalent concentrations in PBS and diluted human serum, for which the NDG receptor shows negligible responses. In addition, the adsorption of 3-NT and L-tyrosine to the NDG receptor were also investigated by atomic force microscopy and further verified by surface enhanced Raman spectroscopy. Therefore, our NDG-LSPR biosensor competes favorably against ELISA and we believe it should be an attractive and economical solution to early diagnostic of 3-NT related disorders for clinical applications.