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Project: Sensitive probing of surface charging processes based on the electrokinetics of single colloidal particles

2010-05-01 – 2017-04-30

Abstract

The Center for Nano- and Biophotonics sets up research lines in the area of nanoparticles, microlaser technology and optical switches, biosensors, bio-spectroscopy, scanning of biomaterials, electro-optical particle manipulation and active nanophotonic implants.

Objectives
A. Fundamental research on charging mechanisms in nonpolar colloidal suspensions
In nonpolar liquids such as oil, colloidal particles carry almost no charge because of the low dielectric constant.
Surfactant stabilizes the charge on the surface of the particles and acts as a charging agent [16]. This principle is
used in the stabilization of soot in petroleum [17], the study of colloidal crystals [18], and in electrophoretic
displays [19]. The processes that are responsible for the charge of colloidal particles in a nonpolar liquid with
surfactant are still poorly understood [20].
In this project we will investigate the elementary charging mechanisms for micro- and nanoparticles in
nonpolar liquids. Different kinds of particles (silica, PMMA, polystyrene) in a range of sizes (10-1000 nm) will
be studied in solutions of various concentrations of a number of surfactants (poly-isobutylene succinimide,
AOT,…). Correlating the statistics of elementary fluctuations of the particle charge over time, with the
properties of particles and surfactants, will make it possible to test hypotheses for the mechanisms involved in
the charging of colloidal particles.
B. Fast, sensitive quantitative label-free detection of bio-molecules
In classic microcarrier assays for protein or bio-molecule detection, the binding of the antigen of interest to the
antibodies on the surface of the microcarriers is quantified by a ‘sandwich’ fluorescence detection scheme.
Microcarriers are coated with antibodies that can bind specifically to antigens that we want to detect. When the
microcarriers are added to the bio-liquid sample (e.g. blood), antigens in the sample bind to the microcarriers
that are coated with corresponding antibodies. To detect if binding did occur on certain microcarriers,
fluorescently labeled detection antibodies (which can bind to the antigens) are added to the sample. If antigen
was bound to a certain microcarrier, a fluorescence signal will be seen on the surface of those carriers with
intensity proportional to the amount of antigen bound to the surface. The use of detection antibodies is
cumbersome, complicates quantification due to unspecific binding, and can interfere with microcarrier colorencoding
schemes. Label-free detection of binding to the surface of nanoparticles is of interest since it would
make the use of detection antibodies unnecessary. Molecules of interest in medical diagnostics are, for example
cytokines, such as tumor necrosis factors and interleucines, which are related to a number of inflammatory
conditions of immune or infectious origin.
In this project, we will use colloidal polystyrene nanoparticles coated with antibodies, proteins or DNA
molecules for the label-free detection of antigen, protein or complementary DNA strand [21]. The binding of
specific molecules on the nanoparticle surface will cause changes in the nanoparticle size and/or charge. The
electrical force experienced by an elementary charge can be significant compared to other forces that act on the
nanoparticles, and therefore electrical detection systems can be extremely sensitive. In this project we will
develop optical tracking electrophoresis of individual particles to detect the binding of specific bio-molecules
on nanoparticles. This method provides a label-free, fast, sensitive and quantitative alternative to existing
methods such as gel-electrophoresis and Western-blotting [22]. The use of individual colloidal nanoparticles as
a probe for molecules has many advantages over existing methods [23]. It is fast and efficient, because the
detector-particles can be dispersed in the bulk of the liquid, which leads to short diffusion times compared to
surface-based detection systems. The microscope-based particle tracking system can measure the properties of
tens of particles in a fraction of a second. This enables the detection of different bio-molecules simultaneously
in one sample [6]. The sensitivity is high because the binding of just a few bio-molecules leads to an important
electrical force, and therefore very sample volumes and low concentrations should be sufficient. Finally, once
specific bio-molecules are bound to the receptors on the nanoparticles, these could be transported by
electrophoresis to a desired reservoir for further analysis.