Reversible cationic gemini surfactant-induced aggregation of anionic gold nanoparticles for sensing biomolecules

We report a practical, economical and sensitive method for DNA and lysozyme biomolecule detection based on previous aggregation of an anionic citrate gold nanoparticle, Au@citrate, induced by N,N'-bis(dimethyldodecyl)-alpha,omega-alkanediammonium dibromide series (m-s-m, m = 12, s = 3 and 6) gemini surfactants. Starting from Au@citrate/12-s-12 aggregated complexes, disaggregation processes were induced, effectively and immediately, by increasing biopolymer concentrations. This phenomenon that changes the optical properties of the systems was originated by the strong, kinetically rapid interactions of Au@citrate with the biopolymer, favoring the formation of different highly stabilized Au@citrate/biopolymer/12-s-12 complexes, which particular structures were visualized via the AFM technique. In all cases, the order of addition of the reagents was demonstrated to be crucial for the success of the reaction, and therefore effective biomolecule detection. The new approach highlights a way to avoid problems derived from the time lapse observed in previously reported sensors based on gold nanoparticle' aggregation phenomena, with a limit of detection in the nanomolar range. The results showed a better linear response in sensors prepared from 12-6-12 surfactant, in line with both the greater difference observed between the equilibrium binding constants for 12-6-12/biopolymer and Au@citrate/biopolymer interactions, and the significant interparticle distance among Au@citrate nanoparticles through the aggregates.