Colloids and Surfaces A: Physicochemical and Engineering Aspects
Rigid DNA chains near nanoparticles
Introduction
Polyelectrolyte adsorption onto nanoparticles is an important theme not only in fundamental science [1] but also in numerous applications such as stabilization of colloidal particles [2], [3], [4] and various templates for microcapsules and immunoassays [5], [6]. While the structure and conformations of adsorbed polyelectrolytes have been extensively studied mainly by using light scattering [7], [8], [9], the conformation of free polyelectrolytes near nanoparticle solid surfaces have been more problematical, as they are more difficult to measure by direct experiment.
Tohver et al. [10] reported interesting results related to stabilization of colloid suspensions using nanoparticles. When negligibly charged microspheres and strongly (but oppositely) charged nanoparticles were mixed, nanoparticles built up near the microspheres and stabilized them by electrostatic repulsion between microspheres. It is interesting to consider the possibility that an analogous situation might occur when colloidal particles are mixed with oppositely charged polyelectrolytes. Strong electrostatic interaction binds them tightly in deionized water. However, the addition of salt reduces the attractive force and the salt concentration can be controlled to vary the separation. While there is no concern about conformation in the case of hard nanoparticles, a flexible polyelectrolytes may adapt its conformations.
Here, we studied the conformation of rhodamine-labeled DNA oligomers near positively charged polystyrene (PS) latex particles using two-photon excited time-resolved fluorescence technique. Since both the DNA backbone and PS particles are quenchers of the rhodamine dyes [11], [12], the conformation of DNA oligomers governed by electrostatic interaction can be deduced by measuring the fluorescence lifetime.
Section snippets
Materials
Amidine-terminated polystyrene latex particles were purchased from Interfacial Dynamics Corporation (Tualatin, Or) and used as received without further purification. They are positively-charged in deionized water, with surface charge density 3.2 μC/cm2. The dry diameter of the latex particle is stated by the manufacturer to be 40±10 nm. Rhodamine-labeled DNA oligomer, 5′-RhG-GATGATGAGAAGAAC-3′, was custom-synthesized and purchased from TriLink BioTechnologies Inc. (San Diego, CA). Rhodamine-G dye
Results and discussion
As polystyrene nanoparticles were added to the DNA solution, strong electrostatic interaction attracted DNA onto the surface since amine groups on polystyrene nanoparticles carry positive charges and DNA has a negatively-charged phosphate backbone. The ratio of the number of DNA molecules to the number of nanoparticles was fixed at 5 throughout this paper. Fig. 1 compares the fluorescence anisotropy decays of free DNA and adsorbed DNA on the nanosecond time scale. Both decays were best fitted
Prospects
The main point of this study is that, although the separation between DNA and nanoparticles was increased by adding salt, the DNA molecules continued to be electrostatically affected by the nanoparticles, adopting conformations less flexible than those of unattached DNA. Note also that, in principle (if there were no fluorescence quenchers present apart from DNA bases and nanoparticles), the root mean square distance (〈δ2〉) between the dye and the nanoparticles at various NaCl concentration
References (12)
- et al.
Colloid Interface Sci.
(2001) - et al.
Macromolecules
(2003) - et al.
Langmuir
(2001) - et al.
J. Phys. Chem. B
(2003) - et al.
Langmuir
(2001) - et al.
Langmuir
(2000)