  We present direct evidence for a reversible phase transition of DNA-linked
colloidal gold assemblies. Transmission electron microscopy and optical
absorption spectroscopy are used to monitor the colloidal gold phase
transition, whose behavior is dominated by DNA interactions. We use
single-stranded DNA-capped colloidal gold that is linked by complementary
linker DNA to form the assemblies. We found that, compared to free DNA, a sharp
melting transition is observed for the DNA-linked colloidal gold assemblies.
The structure of the assemblies is non-crystalline, much like a gel phase,
consistent with theoretical predictions. Optical spectra and melting curves
provide additional evidence of gelation of the colloidal system. The phase
transition and separation are examples of percolation in a dilute solvent.
