Single-stranded DNA with G-rich sequences can fold into secondary structures, G-quadruplexes, via intramolecular hydrogen-bonding interactions. This conformational change can be detected by a homogeneous assay method based on fluorescence resonance energy transfer (FRET) from a water-soluble cationic conjugated polymer (CCP) to a fluorescein chromophore labeled at the terminus of the G-quadruplex DNA. The space charge density around the DNA controls the efficiency of FRET from the CCP to the fluorescein. The higher FRET efficiency for the CCP/G-quadruplex pair is correlated to the stronger electrostatic interactions between the more condensed G-quadruplex and the CCP in comparison to the CCP/ssDNA pair. Since the potassium ion can specifically bind to the G-quadruplex DNA, the G-quartet-DNA/CCPs assembly can also be used as a platform to sense the potassium ion in water with high selectivity and sensitivity.

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The bactericidal mechanisms of poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPE) and oligo-phenylene ethynylenes (OPE) were investigated using electron/optical microscopy and small-angle X-ray scattering (SAXS). The ultrastructural analysis shows that polymeric PPE-Th can significantly remodel the bacterial outer membrane and/or the peptidoglycan layer, followed by the possible collapse of the bacterial cytoplasm membrane. In contrast, oligomeric end-only OPE (EO-OPE) possesses potent bacteriolysis activity, which efficiently disintegrates the bacterial cytoplasm membrane and induces the release of bacterial cell content. Using single giant vesicles and SAXS, we demonstrated that the membrane perturbation mechanism of EO-OPE against model bacterial membranes results from a 3D membrane phase transition or perturbation.

It is essential to develop alternative strategies to treat infections, especially those infections caused by Staphylococcus aureus, which is responsible for most skin infections. Among those strategies, light-induced inactivation of pathogens appears to be a promising candidate. We present four novel “end only” oligo(phenylene ethynylene)s (EO-OPE-1s) that have the ends functionalized with cationic groups and are powerful light-activated biocides against Escherichia coli, Staphylococcus epidermidis, and S. aureus. We have correlated the light-induced biocidal activities with singlet oxygen quantum yields Φ (1O2) of EO-OPE-1s, and a higher Φ (1O2) correlates with a better light-induced biocidal activity. Coupled with our previous work on the interactions of EO-OPE-1s with dioleoyl-sn-glycero-3-phosphocholine (DOPC)/cholesterol vesicles, we believe the biocidal process involves the following: (1) EO-OPE-1s penetrate the bacterial membrane, (2) EO-OPE-1s photosensitize the generation of singlet oxygen and/or other reactive oxygen species, and (3) singlet oxygen and/or reactive oxygen species trigger the cytotoxicity.

Three series of cationic oligo p-phenyleneethynylenes (OPEs) have been synthesized to study their structure-property relationships and gain insights into the transition from molecular to macromolecular properties. The absorbance maxima and molar extinction coefficients in all three sets increase with increasing number of repeat units; however, the increase in λ(max) between the oligomers having 2 and 3 repeat units is very small, and the oligomer having 3 repeat units shows virtually the same spectra as a p-phenyleneethynylene polymer having 49 repeat units. A computational study of the oligomers using density functional theory calculations indicates that while the simplest oligomers (OPE-1) are fully conjugated, the larger oligomers are nonplanar and the limiting "segment chromophore" may be confined to a near-planar segment extending over three or four phenyl rings. Several of the OPEs self-assemble on anionic "scaffolds", with pronounced changes in absorption and fluorescence. Both experimental and computational results suggest that the planarization of discrete conjugated segments along the phenylene-ethynylene backbone is predominantly responsible for the photophysical characteristics of the assemblies formed from the larger oligomers. The striking differences in fluorescence between methanol and water are attributed to reversible nucleophilic attack of structured interfacial water on the excited singlet state.

A conjugated‐polymer‐based “mix‐and‐detect” optical sensor for mercury ions is fabricated by using a water‐soluble poly[3‐(3′‐N,N,N‐triethylamino‐1′‐propyloxy)‐4‐methyl‐2,5‐thiophene hydrochloride] (PMNT) and a label‐free, mercury‐specific oligonucleotide (MSO) probe. PMNT binds to the Hg2+‐free MSO and the Hg2+–MSO complex in different ways, and exhibits distinguishable and specific optical responses to the target‐induced conformation change.

By making use of the aggregation-induced emission feature of silole, compound 1 with an ammonium group is designed and synthesized with a view to developing a new optical probe for fluorescence turn-on detection of DNA and label-free fluorescence nuclease assay. The fluorescence of 1 increases largely upon mixing with DNA, in particular for long DNA, indicating that 1 can be used for fluorescence turn-on detection of DNA. More interestingly, 1 can be employed to follow the DNA cleavage process by nuclease. Therefore, a label-free fluorescence nuclease assay method is successfully established with 1. Furthermore, this label-free fluorescence assay can also be used for inhibitor screening of nucleases. Given its simplicity, easy operation, sensitivity and cost-effectiveness, this method can be extended to other nuclease assays and high-throughput screening of nuclease inhibitors.

A new method has been developed for the label-free, convenient, and real-time monitoring of the cleavage of single-stranded DNA by single-strand-specific S1 nuclease and hydroxyl radical based on cationic water-soluble poly[3-(3'-N,N,N-triethylamino-1'-propyloxy)-4-methyl-2,5-thiophene hydrochloride](PMNT). The PMNT can form an interpolyelectrolyte complex with ssDNA (duplex) through electrostatic interactions, in which PMNT takes a highly conjugated and planar conformation, and thus PMNT exhibits a relatively red-shifted absorption wavelength. When ssDNA is hydrolyzed by S1 nuclease or hydroxyl radical into small fragments, the PMNT/ssDNA duplex cannot form. In this case, the PMNT remains in random-coil conformation and exhibits a relatively short absorption wavelength. The nuclease digestion or oxidative damage by hydroxyl radical of DNA can be monitored by absorption spectra or just visualized by the "naked-eye" in view of the observed PMNT color changes in aqueous solutions. This assay is simple and rapid, and there is no need to label DNA substrates. The most important characteristic of the assay is direct visualization of the DNA cleavage by the "naked-eye", which makes it more convenient than other methods that rely on instrumentation. The assay also provides a promising application in drug screening based on the inhibition of oxidative damage of DNA.