The fluorescence of a polyanionic conjugated polymer can be quenched by extremely low concentrations of cationic electron acceptors in aqueous solutions. We report a greater than millionfold amplification of the sensitivity to fluorescence quenching compared with corresponding ''molecular excited states.'' Using a combination of steady-state and ultrafast spectroscopy, we have established that the dramatic quenching results from weak complex formation [polymer (؊) ͞quencher (؉) ], followed by ultrafast electron transfer from excitations on the entire polymer chain to the quencher, with a time constant of 650 fs. Because of the weak complex formation, the quenching can be selectively reversed by using a quencher-recognition diad. We have constructed such a diad and demonstrate that the fluorescence is fully recovered on binding between the recognition site and a specific analyte protein. In both solutions and thin films, this reversible fluorescence quenching provides the basis for a new class of highly sensitive biological and chemical sensors. With the rising awareness of the public vulnerability to chemical and biological terrorism, there is a heightened need for detection techniques that show both high sensitivity and selectivity. Such techniques also would find wide use in medical diagnostics and biomedical research applications. Methods of identifying biological molecules such as the enzyme-linked immunosorbant assay (ELISA) achieve selectivity by using specific antibody͞antigen interactions to anchor the antigen to a substrate, with a subsequent colorimetric change or fluorescence signal on addition of secondary reagents; these techniques can be time-consuming and require multistep procedures. Other approaches have used molecular recognition ligands to link to specific receptor sites on a biological species, usually as a means also of fixing the biomolecule to a substrate or membrane (1-6) It has remained a challenge to incorporate the selectivity offered by ligand͞receptor interactions into a sensor that can be extremely sensitive, robust, and versatile.We have recently explored the photophysical properties of a fluorescent, water-soluble polyanionic conjugated polymer [poly (2-methoxy-5-propyloxy sulfonate phenylene vinylene (MPS-PPV)] (Fig. 1B), one of a larger class of related molecules [poly phenylene vinylene (PPV)] (Fig. 1 A and derivatives) that has been the subject of almost explosive recent interest (7-13). Although much attention has focused on the well known potential for use of PPV derivatives as electronic materials [e.g., electrochemical sensors (14-16) light-emitting diodes (17, 18), and integrated circuits (19,20)], the highly charged backbone of MPS-PPV (with charge density approximating that of polynucleic acids such as DNA and RNA), also makes it a model polymer for understanding the interactions and self-assembly properties of charged biopolymers. In this paper, we report a striking discovery: the use of this fluorescent anionic polymer leads to a greater than million-fold amplificatio...

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nanotube is observed and the lower side of the wall corresponds to the outside of the nanotube. The thickness of the wall is about 3.3 nm and it consists of many parallel graphene layers. Each layer, however, curves and wrinkles to some extent, indicating lower crystallinity of the present nanotubes than the ones prepared by other methods, such as arc discharge synthesis. It should be noted that this image does not exhibit any clear difference in crystallinity between pure carbon layers (upper half of the wall) and N-doped layers (the lower half). In the case of nanotubes from P-A CVD, their HRTEM images (not shown here) were found to be very similar to the image of Figure 4, and again there was no crystallinity difference between N-doped and undoped multiwalls.In conclusion, this study has demonstrated the fabrication of aligned carbon nanotubes with double coaxial structure of N-doped and undoped multiwalls. It can be determined whether the N-doped layer belongs to the inner or outer multiwalls by changing the sequence of the two-step CVD process. Moreover, the thickness of both the N-doped and pure carbon layers is controllable by changing each CVD period. The use of the AAO film as a template enables us for the first time to precisely control the nitrogen location in N-doped carbon nanotubes. Since nitrogen doping would enhance the electron-conducting properties of carbon nanotubes, the present carbon nanotubes may exhibit excellent performance as field electron emitters. The present technique opens up a novel route for the synthesis of heteroatom-doped carbon nanotubes with double coaxial structure and furthermore this will lead to the production of coaxial heterojunctions (pn, npn, or pnp) by stacking N-and B-doped layers. ExperimentalBy anodic oxidation of an aluminum plate, an AAO film with a channel diameter of 30 nm and a thickness of about 70 lm was prepared. Details are given elsewhere [13]. The resultant AAO film was placed on a quartz boat in a horizontal quartz reactor (inside diameter 55 mm). The reactor temperature was then increased to 800 C under N 2 flow. When the temperature reached 800 C, propylene gas (1.2 % in N 2 ) was passed through the reactor at a total flow rate of 1000 cm 3 (STP)/min. After the 2 h carbon deposition from propylene, the reactor was cooled down to room temperature and the carbon-coated AAO film taken out. In the second step, the carbon-coated film was placed in the reactor again and acetonitrile vapor (4.2 % in N 2 of 500 cm 3 (STP)/min) was allowed to flow over the film at 800 C. The vapor was generated by bubbling N 2 through acetonitrile liquid in a saturator kept at 0 C. This acetonitrile CVD was performed for 5 h. After this two-step sequential CVD process, the doubly coated AAO film was treated with 10 M NaOH solution at 150 C for 6 h to remove the alumina template, thereby liberating the nanotubes from the template AAO film.The carbon-coated AAO films and the corresponding carbon nanotubes were analyzed by X-ray photoelectron spectroscopy (XPS). The samples were...

Nucleophilic catalysis is a general strategy for accelerating ester and amide hydrolysis. In natural active sites, nucleophilic elements such as catalytic dyads and triads are usually paired with oxyanion-holes for substrate activation, but it is difficult to parse out the independent contributions of these elements or to understand how they emerged in the course of evolution. Here we explore the minimal requirements for esterase activity by computationally designing artificial catalysts using catalytic dyads and oxyanion holes. We found much higher success rates using designed oxyanion holes formed by backbone NH groups rather than by sidechains or bridging water molecules and obtained four active designs in different scaffolds by combining this motif with a Cys-His dyad. Following active site optimization, the most active of the variants exhibited a catalytic efficiency (kcat/KM) of 400 M−1s−1 for the cleavage of a p-nitrophenyl ester. Kinetic experiments indicate that the active site cysteines are rapidly acylated as programmed by design, but the subsequent slow hydrolysis of the acyl-enzyme intermediate limits overall catalytic efficiency. Moreover, the Cys-His dyads are not properly formed in crystal structures of the designed enzymes. These results highlight the challenges that computational design must overcome to achieve high levels of activity.