What happens to charge during laser ablation in water? One of my friends from lab makes gold nanoparticles in the following manner:


*

*He puts a gold plate in water (without surfactants or other additives)

*He fires laser at the plate

*After 5 minutes solution is purple (because of AuNPs)


Somehow AuNP's don't agglomerate. My friend says it's because nanoparticles are negatively charged and repel each other. However he couldn't answer me how do charge conservation rule is fulfilled.
So my question is: where does positive charge go?
My hypothesis is that positively charged gas just bubbles out of the water. Is this correct?
 A: According to "Surface chemistry of gold nanoparticles produced by laser ablation in aqueous media." Sylvestre J-P, Poulin S, Kabashin AV, Sacher E, Meunier M, Luong JHT  http://pubs.acs.org/doi/pdf/10.1021/jp047134%2B 

A detailed chemical analysis showed that the nanoparticles formed were
  partially oxidized by the oxygen present in solution. The
  hydroxylation of these Au−O compounds, followed by a proton loss to
  give surface Au−O-, resulted in the negative charging of the
  nanoparticles.

Your friend might be interested to learn that, 

The partial oxidation of the gold nanoparticle surface enhances its
  chemical reactivity and consequently has a strong impact on its
  growth. In particular, the oxidized surface reacted efficiently with
  Cl- and OH- to augment its net surface charge.

so adding a bit of salt might improve the process, although the effect may be happening already if the nanoparticles are reacting with existing chlorine (if he's using tap water rather than distilled). 
