We report the results of an experiment and numerical simulations that demonstrate the large spatial extent and the effect of the so-called energy reservoir during the filamentation of femtosecond laser pulses in air. By inserting pinholes of different sizes in the filament path we observe different stages of development ranging from the termination of the filament, through its partial survival, to undisturbed propagation. A background containing up to 50% of the pulse energy is found to be necessary to maintain the filament formation, including a first refocusing.

A fluorescent polymeric hybridization transducer supported on magnetic microbeads was investigated for the rapid, ultrasensitive, and sequence-specific detection of DNA. We show that the polymer derivative can be used to detect target DNA directly on magnetic particles by preparing “target-ready” microbeads grafted with the polymer and suitable DNA probes. A detection limit of ∼200 target copies in a probed volume of 150 μL (1.4 copies/μL) was obtained for a DNA sequence specific to Candida albicans This detection scheme does not require the release of the hybridized target DNA prior to its detection or the labeling or amplification of the nucleic acids. Furthermore, we show that the fluorescence from these biosensing magnetic beads can be read while magnetically confined in a small volume by a microelectromagnetic trap, which offers the possibility of performing both the preconcentration and detection steps simultaneously on the same support. The combination of the fluorescent polymer biosensor with magnetic particle-assisted DNA preconcentration extends the application of this ultrasensitive biosensor to biological samples with complex matrixes and to integrated lab-on-a-chip platforms, where it could be used for fast multitarget DNA detection in point-of-care diagnostics and field analysis.

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Self-assembled molecular structures immobilized on solid substrates and composed of fluorophore-tagged oligonucleotide probes and an optical polymeric transducer were investigated for the trace level detection of DNA target molecules. Rapid and efficient energy transfer between the polymeric transducer and fluorophores within the molecular aggregates leads to a massive intrinsic amplification of the fluorescence signal and to the label-free detection of as little as 300 DNA molecules, with the specificity required for the detection of single-nucleotide mismatches. This capacity for attomolar detection levels while the sensing structures are attached onto solid supports could lead to the development of biochip platforms for fast and simple PCR-free multitarget DNA detection.