Hazelnut proteins detection in cookies using an immersible label-free photonic chip sensor

Rapid and sensitive methods to detect allergenic proteins in foods at the point-of-need could help to protect allergic consumers from life-threatening accidental exposure. In this context, the development of a label-free optical immunosensor for detecting hazelnut proteins in cookies is presented. The sensor is based on silicon photonic chips containing two integrated Mach-Zehnder interferometers (MZI) with light input and output on one side of the chip, and sensing window openings on the other side. Coupling with a white-light LED and a spectrometer for signal recording is achieved via a bifurcated fiber. The sensing window of one of the MZIs is modified with a mouse monoclonal antibody against hazelnut proteins, while the other sensing window is modified with bovine serum albumin to serve as blank. The assay follows a two-step sandwich immunoassay format, involving a 10-min reaction with the hazelnut proteins calibrator or cookie sample, followed by a 2-min reaction with a mixture of monoclonal antibodies. All reactions were performed by immersing the chip side where the MZIs windows are located into the solutions. The recorded spectrum is processed in real-time to transform the spectrum shifts due to immunoreactions taking place on the sensing windows of the two MZIs into phase shifts. The developed assay detects hazelnut proteins at concentrations as low as 25 ng/mL in calibrators prepared in buffer or in aqueous extract of hazelnut-free cookies. Additionally, cookies containing hazelnuts were analyzed demonstrating the sensor ability for fast and sensitive detection of the hazelnut proteins in commercial products.