Concomitant with our previous results that the partially condensed DNA remain exposed and thus available to the buffer solution (Fig. ), the divalent ions (Mg 2+ ) in the buffer act as counterions that can attract uncondensed negatively charged domains of the bound DNA leading to aggregation. Two fold increase of the DNA resulted in precipitation of DNA-PEI 30 -SWCNT out of the solution (Fig. ), whereas, reducing the Mg 2+ concentration in the half-strength MES buffer decreased the aggregation and enhanced the expression efficiency (see Fig. ).