As DNA are negative in surface charge, the best way to couple DNA with AuNPs is to use positive charged AuNPs. You can easily synthesise positive charged AuNPs by the reduction of poly ethylene imine (PEI). The surface charge you will get around (+) 20-30 mV. So by easy electrostatic force you can couple your DNA.

Try to titrate ZETA Potential to zero. In the case dispersion is getting unstable, add non-ionic surfactant from BRIGE seria. Then see the interaction with DNA. That is Colloid Chemistry, one have to play around to succeed. 

As DNA are negative in surface charge, the best way to couple DNA with AuNPs is to use positive charged AuNPs. You can easily synthesise positive charged AuNPs by the reduction of poly ethylene imine (PEI). The surface charge you will get around (+) 20-30 mV. So by easy electrostatic force you can couple your DNA.

What size of particles are you using? Which protocol have you tried? The common protocal proposed by the Mirkin group more than 10 years ago involved slow addition of salt to the nanoparticle solution. Also, if you want stable binding, you need thiolated DNA. There are many articles out there on making DNA-gold nanoparticle conjugates.

Dear Dr. Gill

The size of gold nanoparticles are 22nm, and i follow protocol that Mirkin group published in 2006. Probe DNA is thiolated. Formerly i synthesis gold nanoparticles with size average 22nm and zeta potential -26. With these gold nanoparticles i succes to couple thiolated probe to the gold nanoparticles. Recently i am not succes to couple thiolated probe to the gold nanoparticles, even with these gold nanoparticles.

what is your capping agent? People usually make citrate coated nanoparticles, and the citrate can be easily exchanged with thiols. However if your capping agent binds the gold particles strongly, the replacement might be very slow. If your capping is strongly bound and you have functional groups, such as carboxy, you can try to covalently couple amine-modified DNA using EDC/NHS.

The capping agent is citrate. I have another question Dr. Gill. What is the role of salt ( especially concentration) in gold nanoparticle functionalization?

The salt is reducing the electrostatic repulsion between the negatively charged DNA and the negatively charged citrate-coated nanoparticles. You can also try reducing the pH. Look at my article with J. Liu from last year.