Ultrasensitive DNA monitoring by Au–Fe3O4 nanocomplex

Quantitative detection of biomolecules (e.g. DNA and proteins) has become increasingly important in a variety of fields, including medical diagnostics, food safety, and anti-bioterrorism. We report a simple, sensitive, and inexpensive quantitative approach for DNA detection based on the optical properties of gold nanoparticles and gold-coated magnetic nanoparticle hybrids. We employed a simple one-step reaction to synthesize gold-coated iron oxide nanoparticles (Fe3O4–Au NP). Fe3O4 nanoparticles were used as the central core for preparation of Fe3O4–Au in an aqueous state without precipitation and aggregation of nanoparticles. Citrate-coated Fe3O4 nanoparticles were prepared and subsequently coated with Au layers through reduction of HAuCl4 by citrate on the surface Fe3O4. The resulting Fe3O4–Au nanoparticles showed good paramagnetic properties and were coated with thin layers of gold atoms (∼10 nm) having an average diameter of ca. 20 nm. These magnetic nanoparticles were well-dispersed in water and stable at physiological pH without precipitation. Nucleic acid-functionalized Fe3O4–Au nanoparticles were then hybridized with complementary target DNA molecules. This resulted in nucleic acid-functionalized gold nanoparticles. To achieve high sensitivity, Fe3O4–Au nanoparticles were employed for the collection of gold nanoprobes that hybridized with complementary target DNA molecules. The Au–Fe3O4–Au nanocomplex remains in solution at a concentration proportional to the concentration of the target DNA and its optical properties allow it to be easily quantified using UV/Vis absorption spectroscopy. The limit of detection for this method was as low as 0.1 fM. This selectable detection system is easy to operate, inexpensive, and versatile, making it a candidate for future use in predicting the seafood safety risks through detection of different DNA sequences and any related genetic mutations.