Building highly sensitive dye assemblies for biosensing from molecular building blocks.

Fluorescence superquenching is investigated for polyelectrolytes consisting of cyanine dye pendant polylysines ranging in number of polymer repeat units (N(PRU)) from 1 to 900, both in solution and after adsorption onto silica nanoparticles. As N(PRU) increases, the absorption and fluorescence evolve from monomer spectra to red-shifted features indicative of molecular J aggregates. In solution, the superquenching sensitivity toward an anionic electron acceptor increases by more than a millionfold over the N(PRU) range from 1 to 900. The dramatic increase is attributed to enhanced equilibrium constants for binding the quenchers, and the amplified quenching of a delocalized exciton of approximately 100 polymer repeat units. The self-assembly of monomer onto silica and clay nanoparticles leads to formation of J aggregates, and surface-activated superquenching enhanced 10,000x over the monomer in solution, indicating the formation of "self-assembled polymers" on the nanoparticle surface. Utilization of these self-assembled polymers as high-sensitivity biosensors is demonstrated.

Highly sensitive biological and chemical sensors based on reversible fluorescence quenching in a conjugated polymer.

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 million-fold 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.

Femtosecond high-sensitivity, chirp-free transient absorption spectroscopy using kilohertz lasers.

A simple method is demonstrated for high-sensitivity, chirp-free measurements of femtosecond (fs) transient absorption over the entire bandwidth of a white-light continuum probe. This technique uses phase-sensitive detection, with spectral scanning and simultaneous adjustment of the time delay between pump and probe pulses; it permits a direct measurement of spectra undistorted by chirp at all time scales, limited only by the resolution of the fs source. The method is applied to study the ultrafast relaxation dynamics of pi-conjugated oligomers and semiconductor nanocrystals.

Enhanced optical limiting in derivatized fullerenes.

We have observed enhanced optical limiting behavior in solutions of a derivatized fullerene (phenyl-C(61)-butyric acid cholesteryl ester) from 532 to 700 nm. Transient absorption measurements determined the spectral and temporal regions of interest for optical limiting in C(60) and in C(60) derivatives that are due to a reverse saturable absorption mechanism and predicted enhanced limiting at longer wavelengths. Intensity-dependent transmission measurements made at several wavelengths confirmed these results. The increased solubility and the broadened ground-state absorption of the functionalized C(60) make it suitable for use as an optical limiter in the red and the near infrared.