Quantum dots (QDs) and nanoparticles (NPs) are made of metal and/or semiconductor materials with diameters ranging from 5 to 100 nm. The properties of these nanomaterials, which depend on their size and the material they are made from, are usually completely different than the properties of their corresponding bulk materials. There may be anywhere from 1 to 1,000 electrons in a single QD, providing numerous possibilities for their opti-

cal and electrical properties.

The unique optical properties of NPs can be beneficial to their biological applications. NPs have broad absorption spectra, ranging from the visible region into the ultraviolet, and narrow, size-tunable photoluminescence spectra. Both of these spectra depend on the size of the NPs. The most attractive optical properties in NPs, from a biological aspect, appear in those that are approximately 10 nm in diameter or smaller. When illuminated with ultraviolet (UV) light, such NPs emit extremely bright fluorescent light.

NPs have been used to sense deoxyri-bonucleicacid (DNA). Positively charged NP-DNA complexes have been created as probes themselves, for the detection of nucleic acids. However, the fundamentals of the interactions between NPs themselves — without any mediating linker — and DNA have yet to be fully understood. Given the vast optical, magnetic, and electrical properties of NPs, knowledge of their interaction with DNA will create new insight into the world of nanobiotechnology.

In this research, the interactions between gold (Au) NPs and single-strand DNA were studied to develop a fundamental understanding and to identify potential future applications for NP-DNA complexes. These applications include gene therapy, drug delivery to DNA, and an improved method for decoding DNA. A detailed study of NP interactions with DNA is also expected to provide an enhanced understanding of NP interactions with other biomolecules, such as ribonucleic acid (RNA), proteins, and enzymes.

Colloidal Au NPs were synthesized by reducing hydroaurochloric acid by trisodium citrate dihydrate. The suspended NPs were sonicated for 15 minutes to obtain a uniform distribution and cluster-free nanoparticles. Fairly uniform-sized Au NPs capped by citrate ions were obtained. About 25 μl of the suspension was spin-coated at a rate of 120 rpm for 6 minutes onto a silicon substrate and dried in a dessicator under a clean environment. A 1 mM DNA solution was prepared using 5 ml of phosphate buffer. One drop of the solution was then spin-coated onto an Au substrate at a rate of 120 rpm for 3 minutes. The surface characterization of the 3 Au substrate with DNA was performed using tapping mode atomic force microscopy (AFM).

The Au NP and the DNA solutions were mixed at a 1:1 ratio and spin-coated on a freshly cleaved mica substrate. The dried substrate containing the NPs and DNA was imaged using tapping-mode AFM. A series of images was obtained that showed prominent interaction between Au NPs and DNA.

The broad and overlapping emission spectra of Au NPs and DNA reveal that Au NPs at lower intensity have strong affinity to DNA. The possible reason of positive shift in the wavelengths of both Au NPs and DNA for Au NP-DNA mixture is unknown and requires further investigation. However, the broad and overlapping emission spectra of Au NPs and DNA reveal the strong interaction between the Au and DNA.

The morphology and surface properties of the Au NPs, DNA, and the Au NP-DNA complex on different substrates reveal the uniform heights; the diameter, however, varied considerably. The heights of the NPs ranged between 2 and 6 nm, with average height to be 4 nm. The diameter ranged between 30 and 250 nm, with an average of 100 nm. This observation suggested the clustering/agglomeration of small NPs and nanorods into larger particles. Similarly, the height and the diameter of the single-strand DNA ranged between 1 and 3 nm and 25 to 75 nm, respectively, suggesting that there were possibility of more than one single-stranded DNA lined parallel to each other. It is also worth noting that AFM tends to exaggerate the measurement in the x or y direction due to the x-y movement of the scanner.

The Au NP-DNA sample showed both Au NPs and DNA oligomer strands. AFM images show that the DNA and Au NPs do, indeed, interact with one another. The DNA appears to link to larger Au NPs of an overall average size of 150 nm. Smaller NPs appear more frequently in the images than Au NP-DNA linked complexes.

The results indicate that the citrate-capped Au NPs naturally link with single-strand DNA. The exact nature of the interaction needs to be determined, though it can be speculated that the attractive forces between the Au NPs and single-strand DNA are a type of intermolecular attractive force, rather than a type of chemical bond.

This work was done by Molly Karna, Govind Mallick, and Shashi P. Karna of the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp  under the Materials category. ARL-0112

This Brief includes a Technical Support Package (TSP).
Fundamental Interaction Between Au Nanoparticles and Deoxyribonucleic Acid (DNA)

(reference ARL-0112) is currently available for download from the TSP library.

Don't have an account? Sign up here.