Determination of particle-particle distances in an optomagnetic cluster assay : a rotating molecular ruler

We investigate a fast and sensitive optomagnetic bionanotechnology for biomarker detection. Antibody-coated superparamagnetic particles capture biomarker molecules and form clusters with a biomarker molecule sandwiched between two particles. These particle clusters can be analyzed using a rotating magnetic field which induces an oscillating light scattering cross-section, resolving sub-picomolar biomarker concentrations [1,2]. In this paper we report a method to quantify the particle-particle distance with nanometer resolution. Figure 1 shows the light scattering measurement setup and Figure 2 the time-dependent signals measured at two scattering angles. Simulations of the light scattering by rotating clusters show that the high-frequency Fourier components hold detailed information about the geometry of the particle clusters, including a strong dependence on the inter-particle distance, see Figure 3. We will report the simulation results and experimental data of model cluster assays with controlled particle-particle distances. 1: Ranzoni et al, Nano Letters 2011 11 (5), 2017-2022 2: Ranzoni et al, ACS Nano 2012 6 (4), 3134-3141