Polyvalent DNA gold nanoparticles are colloidal gold whose surface is modified with thiol capped synthetic DNA sequences. They were co-discovered by Chad Mirkin et al. at Northwestern University in 1996 and Paul Alivisatos at University of California, Berkeley. Due to the strong interaction between gold and thiols (-SH), a single monolayer of DNA around the gold particle can be obtained. The negative charge repulsion of the phosphate backbone of DNA orients the DNA strands out into solution with a unique footprint that is dependent on gold nanoparticle size and packing density. (Schematic Image)
Simply by positioning many strands into close proximity, these particles possess novel properties in which the independent DNA strands do not. Due to the cooperative effects of the polyvalency of multiple DNA strands, DNA on a particle has a stronger and sharper melting temperature when binding to its complement compared to DNA free in solution. Because of this effect and the unique optical properties of colloidal gold, various DNA detection technologies have been developed.
Synthesis
Gold nanoparticles can either be purchased or synthesized from a variety of methods.
- Reduce the DNA into thiols by adding 0.1M solution of DTT and 0.18M phosphate buffer (PB) (pH=8) for 1 hour
- Purify the DNA via a NAP-5 column
- Add DNA to gold nanoparticles at a concentration of 1 OD/mL
- Bring the concentration of sodium dodecyl sulfate (SDS) and PB to a final concentration of 0.01% and 0.01M, respectively
- After 20 minutes, bring the concentration of NaCl to 0.05M using a 2M NaCl and 0.01M PB stock solution while maintaining 0.01% SDS and incubate for 20 minutes
- Repeat step 5
- Increase the NaCl concentration at increments of 0.1M until a final concentration of 1M is reached over 20 minute increments
- Incubate overnight
- Centrifuge down the gold nanoparticles and remove supernatant and resuspend in a 0.1% SDS solution
- Repeat step 9 four times