Although these established advantages in the previous methods are well known, they still show at least two drawbacks. First, for the third case, it is believed that one of the main reasons for the superquenching arises from the �ШC�� interaction among the aromatic segments, which is driven by a combination of electrostatic and hydrophobic attraction between cationic conjugated polymers and negatively charged DNA. However, almost all the CCPs reported previously contain small aromatic units consisting of 5- or 6-membered ring structures, leading to an inefficient backbone �ШC�� stacking and thus reducing their quenching efficiency. We speculate that the superquenching efficiency of the water-soluble conjugated polymers might be increased by introducing dense aromatic units into their backbones.

Nevertheless, only a limited number of conjugated polymers or oligomers with dense aromatic units have been synthesized, and few of them have been applied for ultra DNA detection. Second, all the three methods are widely employed for sensing single nucleotide polymorphism (SNP), but they are often inadequate to directly discriminate between single-stranded DNA (ss-DNA) and double-stranded DNA (ds-DNA) by sensing the probe signal since they specifically require harsh denaturation conditions for hybridization to a complementary single-stranded DNA [15-23]. It is known that DNA damage has been determined to be responsible for the induction of cell lethality, mutagenesis and carcinogenesis [24]. In some cases, this damage will render a weakened polymer structure at the site of the attack (alkali labile site (ALS)) and give rise to strand breaks or cleavage [25,26].

Therefore, it is significant for discrimination between ss-DNA and ds-DNA. The conventional technique for this discrimination, using reversed-phase high-performance liquid chromatography (RP-HPLC), is expensive and time-consuming [27]. Several optical sensors were also developed for this purpose. One method relies on measuring the lifetime changes of the probe molecule upon binding to DNA [28]; another depends on measuring the changes in fluorescence intensity of the small molecular probe upon intercalating into DNA [23]. However, most of these available probes cannot efficiently differentiate between ds-DNA and ss-DNA [28].

Although studies on the size-specific interactions between CCP and ss- or ds-DNA by measuring its optical responses in the presence of the chromophore-labeled DNA probe have been reported GSK-3 [3], there is scant literature concerning the discrimination between ss-DNA and ds-DNA using a label-free DNA assay combining with CCP as a direct signal reporter.In this paper, we report the sensing and discriminating of ss-DNA and ds-DNA by the use of a unique water soluble conjugated oligopyrene derivate, OHPBDB, as the probe molecule.