Label-free detection of vitamin D by optical biosensing based on long period fiber grating

In this work, we report a fiber optic biosensor for the label-free detection of vitamin D, by targeting the major circulating form of D3 in human body, i.e., 25-hydroxyvitamin D3 (25(OH)D3). The sensing platform relies on a long period grating (LPG) written into an unconventional fiber, i.e., double cladding fiber (DCF) with W-type refractive index profile. The sensitivity to surrounding medium is enhanced by chemical etching of the outer cladding as the working point of the device is tuned to the mode transition region. The DCF structure allows to combine the enhancement of the sensitivity (up to -1400 nm/RIU) with a good visibility of the grating resonance band (>10 dB) and long-term stability due to an all-silica structure. Moreover, the LPG transducer is coated with a nanosized layer of graphene oxide to provide carboxylic functional groups (-COOH) for the grafting of the biological recognition element for vitamin D3, i.e., a 25(OH)D3 specific antibody. To the best of our knowledge, this is the first evidence of a fiber optic biosensor for the detection of vitamin D3, which was performed using concentrations within the clinically relevant range of 1-1000 ng/mL and achieving a low limit of detection below 1.0 ng/mL in buffer solution that is well comparable with the state of the art. Finally, the performance of the biosensor was also evaluated in a complex medium with interfering proteins at physiological concentration obtaining promising results.