Label-free immunosensing by long period fiber gratings at the lowest order cladding mode and near turn around point

•Fabrication of long period fiber gratings with lowest order cladding mode near turn around point.•Maximum enhancement of the evanescent field of the sensor.•Microfluidic thermostated flow-cell.•Label free IgG/anti-IgG immunoassay performed in human serum.•Limit of detection 0.16 ng/ml (1.06 pM).•Information on the binding kinetics.