A rotating molecular ruler : determining nanometer-scale particle-particle distances in an optomagnetic cluster assay

We investigate a fast and sensitive optomagnetic biomarker detection technology based on magnetic particles. Antibody-coated superparamagnetic particles capture biomarker molecules and form clusters with a biomarker molecule sandwiched between two particles. The particle clusters are actuated using a rotating magnetic field, which induces an oscillating light scattering cross-section (see Fig. 1a). Sub-picomolar biomarker concentrations can be resolved by the light scattering signals [Ranzoni et al, Nanoletters 2011; ACS Nano 2012]. In this paper we report a method to quantify inter-particle distances with nanometer resolution. The light scattering data show high-frequency signal components (see Fig. 1b). Simulations show that high-frequency components hold detailed information about the geometry of the particle clusters, including a strong dependence on the inter-particle distance (see Fig. 1c). We will report the simulation results and experimental data of corresponding model cluster assays

Optimization of the current extracted from an ultracold ion source

Photoionization of trapped atoms is a recent technique for creating ion beams with low transverse temperature. The temporal behavior of the current that can be extracted from such an ultracold ion source is measured when operating in the pulsed mode. A number of experimental parameters are varied to find the conditions under which the time-averaged current is maximized. A dynamic model of the source is developed that agrees quite well with the experimental observations. The radiation pressure exerted by the excitation laser beam is found to substantially increase the extracted current. For a source volume with a typical root-mean-square radius of 20 µm, a maximum peak current of 88 pA is observed, limited by the available ionization laser power of 46 mW. The optimum time-averaged current is 13 pA at a 36% duty cycle. Particle-tracking simulations show that stochastic heating strongly reduces the brightness of the ion beam at higher current for the experimental conditions

Distance within colloidal dimers probed by rotation-induced oscillations of scattered light

Aggregation processes of colloidal particles are of broad scientific and technological relevance. The earliest stage of aggregation, when dimers appear in an ensemble of single particles, is very important to characterize because it opens routes for further aggregation processes. Furthermore, it represents the most sensitive phase of diagnostic aggregation assays. Here, we characterize dimers by rotating them in a magnetic field and by recording the angle dependence of light scattering. At small scattering angles, the scattering cross section can be approximated by the total cross-sectional area of the dimer. In contrast, at scattering angles around 90 degrees, we reveal that the dependence of the scattering cross section on the dimer angle shows a series of peaks per single 2π rotation of the dimers. These characteristics originate from optical interactions between the two particles, as we have verified with two-particle Mie scattering simulations. We have studied in detail the angular positions of the peaks. It appears from simulations that the influence of particle size polydispersity, Brownian rotation and refractive index on the angular positions of the peaks is relatively small. However, the angular positions of the peaks strongly depend on the distance between the particles. We find a good correspondence between measured data and calculations for a gap of 180 nm between particles having a diameter of 1 micrometer. The experiment and simulations pave the way for extracting distance-specific data from ensembles of dimerizing colloidal particles, with application for sensitive diagnostic aggregation assays

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), 3134-314

Nanoscale interparticle distance within dimers in solution measured by light scattering

We demonstrate a novel approach to quantify the interparticle distance in colloidal dimers using Mie scattering. The interparticle distance is varied in a controlled way by changing the ionic strength of the solution and the magnetic attraction between the particles. The measured scaling behavior is interpreted using an energy-distance model that includes the repulsive electrostatic and attractive magnetic interactions. The center-to-center distances of particles with a 525 nm radius can be determined with a root-mean-square accuracy of 12 nm. The data show that the center-to-center distance is larger by 83 nm compared to perfect spheres. The underlying distance offset can be attributed to repulsion by charged protrusions caused by particle surface roughness. The measurement method accurately quantifies interparticle distances that can be used to study cluster formation and colloid aggregation in complex systems, e.g., in biosensing applications

On characterization of an ultracold ion source

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On characterization of an ultracold ion source

Abstract only

On characterization of an ultracold ion source

Abstract only

On characterization of an ultracold ion source

Abstract only