Magnetic actuation has been introduced to an optical immunosensor technology resulting in improvements in both rapidity and limit of detection for an assay quantitating low concentrations of a representative protein biomarker. For purposes of demonstration, an assay was designed for monocyte chemotactic protein 1 (MCP-1), a small cytokine which regulates migration and infiltration of monocytes and macrophages, and is an emerging biomarker for several diseases. The immunosensor is based on arrays of highly multiplexed silicon photonic microring resonators. A one-step sandwich immunoassay was performed and the signal was further enhanced through a tertiary recognition event between biotinylated tracer antibodies and streptavidin-coated magnetic beads. By integrating a magnet under the sensor chip, magnetic beads were rapidly directed towards the sensor surface resulting in improved assay performance metrics. Notably, the time required in the bead binding step was reduced by a factor of 11 (4 vs 45 min), leading to an overall decrease in assay time from 73 min to 32 min. The magnetically-actuated assay also lowered the limit of detection (LOD) for MCP-1 from 124 pg mL-1 down to 57 pg mL-1. In sum, the addition of magnetic actuation into bead-enhanced sandwich assays on a silicon photonic biosensor platform might facilitate improved detection of biomarkers in point-of-care diagnostics settings.
Sensor chips (6×6 mm) and read out instrumentation were obtained from Genalyte, Inc. (San Diego, CA) and details of their operation in sensing experiments have bene previously described.7, 8 Briefly, arrays of 32-individually addressable microring sensors (30 μm diameter) were fabricated into the top layer of silicon-on-insulator at a commercial-scale silicon foundry (IMEC). Proximal to each microring resonator is a linear interrogation waveguide, through which the optical properties of the microring can be probed by coupling light through input and output diffractive grating couplers. Post fabrication, the entire chip surface is coated with a perfluoropolymer and then photolithography and reactive ion etching were used to open annual openings over 24 of the microrings. These microrings come into contact with the flowing solution during a detection experiments and the remaining 8 occluded rings serve as thermal controls.
In a sensing experiment, sensor chips are loaded into a previously described fluidic cartridge. Solutions are then flowed directly across the surface through microfluidic channels defined through a laser cut Mylar gasket (0.007″ thick) under the control of a syringe pump. The assembly was loaded into the instrumentation and light was coupled into the sensor grating coupler from and tunable external cavity laser. Each sensor was monitored in a serial fashion with resonance wavelengths monitored in real-time as a dip in transmittance as the laser wavelength is scanned across a suitable spectral range.