Fig. 1. (a) 3D schematic of the microfluidic diffusional sizing (MDS) device used in this study.18 (b) The Y-junction showing the protein mixing with fluorogenic labelling solution after the diffusion step. (c) The labelling region for o-phthalaldehyde (OPA) react on-chip with primary amine containing residues on the protein.24–27 (d) The observation region for monitoring the fluorescence intensity of the labelled protein. (e) The T-junction showing the flow of labelled protein and unlabelled protein solution.