Modifying colloidal gold particles with DNA is a new interesting approach in the development of genetic biosensors. Normally the modification is designed to consist of a covalent gold−sulfur bond mediated by a thiol group on one end of a single-stranded oligonucleotide. Here we investigate to what extent the binding actually consists of only the sulfur bridge or if some other nonspecific binding mechanism is present as well. We report on an electrophoresis study showing high amounts of strong, non-thiol-mediated (nonspecific) binding of both single- and double-stranded DNA to gold nanoparticles. Interestingly, even the double strands, lacking interacting groups from the exposed bases of single-stranded DNA, interact nonspecifically with the gold particles. We suggest the mechanism for this to be ion-induced dipole dispersive interactions, where the negatively charged phosphate groups on the DNA induce dipoles in the highly polarizable gold particles. Moreover, we show that particles with nonspecifically adsorbed DNA can be separated from the specifically modified and unmodified ones by gel electrophoresis.

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Oligonucleotide-modified gold nanoparticles are used in various kinds of colorimetric DNA targeting biosensors and nanoparticle assembly techniques. Herein we focus on how the size of 13 nm gold colloids changes upon DNA modification. We have performed a series of electrophoresis experiments of particles modified both thiol specifically and nonspecifically with single- and double-stranded oligonucleotides of different lengths (12- and 25-mers). Both unmodified and DNA-modified particles migrated at constant velocity in different concentrations of Metaphor agarose gels. Linear Ferguson plots were obtained for all samples, and on the basis of the Ogston model approach, we present how the particle size increases in different amounts depending on the oligonucleotide length, secondary structure, and type of modification (specific or nonspecific). Thiol specifically modified particles obtain a thicker DNA layer since the oligonucleotides are only anchored to the particle in one end and thus stand up from the surface more compared to nonspecifically modified ones, where the oligonucleotides tend to lay more or less flat on the surface with multiple adsorption points. However the thickness of the DNA layer for the thiol specifically modified particles is smaller than the length of a corresponding stretched oligonucleotide, suggesting a flexibility of the thiol-bound strands allowing them to tilt relative to the particle surface.

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