In this work, multiwall carbon nanotubes (MWCNTs) and titanume dioxide nanoparticles (TiO2) were used in the preparation of DNA based biosensor. One of the problems of DNA based biosensors preparation is DNA immobilization on the surface of nanomaterials. DNA is an important biological polymer, which is classi?ed as a natural, negatively charged polyelectrolyte due to its phosphate groups. It can be immobilized on the surface of carbon nanotubes via covalent and noncovalent interactions. However, the results are unsatisfactory because negatively charged CNTs repulse the negatively charged DNA. For solve of this problem, we used positively charged polyelectrolytes as dispersant of MWCNTs and TiO2. At first, we compared two positively charged polyelectrolyte, poly diallyldimethylammonium chloride (PDDA) and chitosan, for DNA immobilization on the surface of MWCNTs, TiO2 and pencil graphite electrode (PGE). The MWCNTs and TiO2 not only display unique electron transfer properties, but also increase the amount of positively charged polyelectrolyte deposited on the surface of` electrode. At second steps, to recognize the effect of nanoparticle on the DNA immobilization on the electrode surface, a comparision was done between MWCNTs and TiO2. For this purpose, differential pulse voltammetry was used. After interaction of dsDNA with methylen blue by following the oxidation signals of guanine and adenine, the detection limits of PGE/PDDA-MWCNTs/dsDNA and PGE/PDDA-MWCNTs-TiO2/dsDNA were obtained 85.0 and 12.0 nM, respectively. At finally, by using modified pencil graphite electrode with dsDNA a new voltammetric method was given for measuring atropine sulfate. After interaction of dsDNA with atropine sulfate by using of redaction of oxidation signals of guanine and adenine was measured atropine sulfate. The detection limit was obtained 30.0 nM. The proposed method was used for determination of atropine sulfate in plasma and urine.