You could try Advion Gel. The active ingredient is indoxacarb as opposed to borax. Works the same way, but is a more powerful poison. Some studies have been critical of its ability to wipe out the colony, as the poison can kill the ants before they return.
You could try experimenting with different ratios of sugar water and borax. Be aware ants can detect borax and will ignore your concoction if the concentration is too high.