Design and model for ‘falling particle’ biosensors

[Display omitted] •Exploration of a new biosensor format with ‘falling’ bio-functionalized particles.•Model of system combines enzyme kinetics, mass transport and particle movement.•Model and experiment show system is largely kinetically controlled for most enzymes.•Higher signal from falling rather than settled particles due to substrate depletion.•Particle choice must balance enzyme loading capacity and particle size distribution. Particle-immobilized enzymes have proven benefits when integrated into biosensors, typically via packed-bed approaches in microfluidic channels. These benefits include dramatically improving sensitivity by increasing the effective surface area to volume ratio and enhancing shelf-life through their thermal stability. However, microfluidic approaches require complex fabrication steps to create weirs or pillars that hold the particles in place and an external pump to control sample flow. In a global trend for affordable diagnostics, there is a need to benefit from the improved performance of particle-based systems while also simplifying the fabrication and readout techniques. Here, we present a new biosensor format, where the bio-functionalized particles are moved through the fluid sample in which they are suspended. We deliver a first study into the main design considerations for this falling particle biosensor, detailing the interdependencies between the kinetics of the enzyme reaction, the mass transport of the substrate to the enzyme on the surface of the particle, and the falling behavior of the settling particles. We detail, through a mathematical model, validated by experimental results, how particle size and enzyme loading are able to influence the outcome measured and establish that this falling particle model does not deviate from the kinetic regime, but that particle size and enzyme loading can be used to tune the signal resolution and deliver simple but highly effective sensors.