Real time and label-free detection of C-reactive protein in serum by long period grating in double cladding fiber

Here, we present a novel label-free biosensor based on fiber optic technology which was tested for the detection of a serum inflammatory marker, the C-reactive protein (CRP). The biosensor is based on a long period grating (LPG) inscribed in a double cladding fiber (DCF) having a W-type refractive index profile. Such DCF fiber permits to tune the sensor working point to the so-called mode transition region through etching of the fiber outer cladding. Therefore, a significant enhancement of the refractive index sensitivity, as well as visibility of the grating spectral features were attained since the mode transition was induced in all-silica fiber structure. Subsequently, the so-prepared LPG was coated with a nano-scale layer of graphene oxide, providing carboxylic functional groups for the covalent immobilization of the biological recognition element for the CRP. As a result, a remarkable limit of detection of 320 pg/mL and a large working range of clinical relevance (0.002-100 μg/mL) were achieved during the real time detection of CRP in human serum.