We developed a rapid, convenient and inexpensive method
to quantify the number of label-free DNA strands attached
to AuNPs of large or small core sizes. The number of strands
per nanoparticle can easily be determined from solutions of
DNA-NPs at concentrations typically used in sensing assays.
The UV-visible spectroscopy assay was used in concert
with a conventional Oligreen dye assay to determine two different
DNA sequences bound to AuNPs, without the need for
labeled DNA. The results of our mixed ligand shell analysis
support a model for disulfide-terminated DNA adsorption in
which there is fast non-specific adsorption of DNA to the gold
surface dictated by chain length and base composition, followed
by rearrangement and additional specific binding to the
gold surface.
The generality of our approach means that in principle,
this method can be extended to determine the number of
DNA, complementary DNA, RNA or synthetic peptide
strands (whose UV-visible signatures overlap with that of
decomposed gold nanoparticles)51–53 bound to gold or silver
nanoparticles.These materials are of interest due to their ability
to form versatile nanoparticle assemblies,54,55 specifically
induce apoptosis in tumor cells,56 and act as sensitive analytical
probes in single molecule experiments.55 The concentration
of silver nanoparticles can be determined from their UVvisible
Aλmax and empirically determined extinction coefficients.
57 Solutions of silver nanoparticles, at the concentration
used for in vivo toxicity assays,58 undergo oxidative decomposition
by KCN59 to form salts that absorb light at 260 nm.38
Using this method in concert with a dye that specifically binds
double-stranded DNA would allow the number of bound
complementary DNA strands to be determined.
The simplicity and wide applicability of this method
makes it well suited for determining the number of recognition
and diluent DNA strands bound to gold nanoparticles.
This information is essential to understanding the relationship
between the structure of a nanoparticle’s ligand shell and its
analytical and biosensing properties. We anticipate that information
gained using this method will lead to design of nanomaterials
with enhanced properties.