Optical protein detection based on magnetic clusters rotation

Abstract

In this paper we present a simple method to quantify aggregates of 200nm magnetic particles. This method relies on the optical and magnetic anisotropy of particle aggregates, whereas dispersed particles are optically isotropic. We orientate aggregates by applying short pulses of a magnetic field, and we measure optical density variation directly linked to this reorientation. By computing the scattering efficiency of doublets and singlets, we demonstrate the absolute quantification of a few % of doublets in a well dispersed suspension. More generally, these optical variations are related to the aggregation state of the sample. This method can be easily applied to an agglutination assay, where target proteins induce aggregation of colloidal particles. By observing only aligned clusters, we increase sensitivity and we reduce the background noise as compared to a classical agglutination assay: we obtain a detection limit on the C-reactive protein of less than 3pM for a total assay time of 10min.

Fast Magnetic Field-Enhanced Linear Colloidal Agglutination Immunoassay

“…Actually, few aggregates are always present within the stock suspension of particles, and this signal may be interpreted as the alignment of those aggregates with external field. 21 We can observe that the OD drop is more pronounced when CRP is added in the medium; this could indicate that CRP aggregates particles even before magnetic field is applied. Nevertheless, this CRPdependent signal is of the same order of magnitude as nonspecific signal due to particles aggregation in stock solution.…”

Rotating magnetic particles for lab-on-chip applications – a comprehensive review

“…Another way to measure the concentration of target molecules is by a cluster assay. Here MPs are provided with capture molecules and form clusters in the presence of target molecules [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] (Fig. 1C).…”

“…One year later the same group published a paper showing that these rotating magnetic particle chains can be used to capture biological molecules. 9 This principle has been extensively studied in the years thereafter 11 and was used in many bio-sensing applications 35,37,38,42,43 (Fig. 1B and C) to significantly shorten assay times, typically from 60-120 min to 15-30 min.…”

Homogeneous Biosensing Based on Magnetic Particle Labels

“…Following detailed analysis of the theory of ligand-receptor interaction in chains of magnetic particles [ 227 ] and experimental investigations of the kinetics for analyte molecules with different tether lengths and numbers of binding sites [ 228 ], the group also demonstrated the method to be capable of detecting C-reactive protein (CRP) directly from serum samples with a detection limit of about 1 pM and a dynamic range of three orders of magnitude with a total cycle time of one minute [ 229 ]. Finally, the group also introduced an advanced measurement method, where the concentration of dimers is no longer determined in a randomized state, but the extinction difference of the dimers for magnetic-field induced alignment parallel and perpendicular to the optical axis is used, which further increases the signal and achievable sensitivity [ 230 ]. Here, the trick is to apply the aligning magnetic field pulse at a magnitude sufficient to rotate the particle doublets created by analyte molecule interaction in the field direction, but insufficient to induce re-chaining of particle labels by magnetic dipolar interactions, which would lead to a false unspecific signal [ 230 ].…”

“…Finally, the group also introduced an advanced measurement method, where the concentration of dimers is no longer determined in a randomized state, but the extinction difference of the dimers for magnetic-field induced alignment parallel and perpendicular to the optical axis is used, which further increases the signal and achievable sensitivity [ 230 ]. Here, the trick is to apply the aligning magnetic field pulse at a magnitude sufficient to rotate the particle doublets created by analyte molecule interaction in the field direction, but insufficient to induce re-chaining of particle labels by magnetic dipolar interactions, which would lead to a false unspecific signal [ 230 ]. For their chosen experimental conditions, the authors determined a field magnitude of 5 mT as good compromise between particle doublet alignment rate and prevention of re-chaining [ 230 ].…”