Residual nanoparticle label immunosensor for wash-free C-reactive protein detection in blood

• Detection of residual nanoparticle labels provides new detection schemes.
• Residual nanoparticle label detection outperformed soluble labels.
• Wash-free systems can be built for rapid diagnostics.

Current diagnostic immunotechnologies are universally based on the measurement of the bound label-antibody fraction in direct binding or sandwich-assay type approaches with various detection techniques (e.g. enzyme-linked immunosorbent assay or ELISA) on solid stationary phase surface. Here an alternative reciprocal approach is presented based on the detection of the non-bound fraction of nanoparticle-labelled antibodies using microparticles as solid support. The advantage of detecting the non-bound fraction of the labelled antibody instead of the bound fraction is the high dynamics and the suggested increased flexibility in the selection of the detection mode. No actual washing steps are required as the bound and non-bound fractions of the detection nanoparticle label are separated using physical separation rather than consecutive washing repeats. The quantitative proof-of-concept set-up was demonstrated through blood-based detection of C-reactive protein (CRP). A blood sample containing CRP was diluted 1/50 and measured in 15-min resulting in a linear response at a range from 1 to 30 μg/ml. The lowest limit of detection was below 0.03 μg/ml and the assay coefficient of variation ranged from 0.3 to 9%. The nanoparticle-based residual label detection outperformed the corresponding molecular label method providing wider applicability with nearly an order of magnitude higher signal-to-background ratio for novel assay configurations in clinical diagnostics practices.