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The continuous nonpolar phase can change the morphology of lecithin reverse micelles dramatically. In alkane solvents, increasing the hydration leads to gelation as isolated water droplets transform into an entangled tube network. In contrast, increasing the hydration for lecithin in benzene maintains isolated, largely spherical traditional reverse micelles. Solvation dynamics experiments have been carried out in the two different micellar environments. These measurements show that solvation dynamics are considerably more restricted in the tubular micelles than in the spherical droplets. This is interpreted as evidence for water pool formation and disorder at the interface in the spherical micelles and supports the hypothesis of substantial water incorporation into the tubular micelles. To perform solvation dynamics experiments in the benzene/lecithin/water reverse micelles, we have synthesized a new headgroup-labeled probe molecule. This synthesis, coupling the standard solvation dynamics probe, coumarin...

The solvation dynamics of water in lecithin/cyclohexane reverse micelles have been determined via ultrafast time-resolved fluorescence studies. At hydration levels w0 ≤ 4.8, the reverse micellar samples are nonviscous. Here a single relaxation time is observed that is much longer than the response of free or bulk water. In contrast, small additions of water to the samples produces a viscous gel, referred to by others as an organogel or “living polymer”. At hydration levels of w0 ≥ 5.8, three relaxation times are observed with approximate time constants of 0.5, 15, and 200 ps, the shortest of which correlates to free water motion. The dynamics reveal no evidence of micelle crossover or branch points associated with gel formation. In comparison to Aerosol OT reverse micelles of similar hydration, the water in the lecithin reverse micelles is significantly more restricted. It is proposed that lecithin sequesters significantly more water than previously predicted precluding formation of distinct core water po...

The mobility of water in water/Aerosol OT (AOT)/isooctane reverse micelles has been investigated via polar solvation dynamics measurements using ultrafast, time-resolved fluorescence-upconversion spectroscopy. For the smallest reverse micelles studied, w0 = 1.1, no solvation dynamics are observed on the time scale of the instrument, suggesting that the water is essentially frozen. As w0 increases, the mobility of water increases, and the time-correlation functions exhibit multiexponential decays. A subpicosecond component is observed on a time scale similar to that of diffusive bulk-water motion.1 The other components decay on time scales of a few picoseconds to hundreds of picoseconds. The amplitude of the solvent response increases as w0 increases, which is attributed to increased mobility of the water inside the reverse micelles. Because the micellar interior is highly ionic, the solvation dynamics of coumarin 343 in a 1 M Na+ (Na2SO4) solution were also investigated to compare the dynamics of water in...

We have used a white light continuum generated with ultrashort laser pulses from a Ti:sapphire laser system as the radiation source for second-harmonic generation measurements. The white light continuum provides easily tuned radiation for experiments requiring a range of wavelengths. Despite the small coherence length of the radiation, parametric processes, such as second-harmonic generation, are possible with this source. In particular, surface second-harmonic generation using the white light continuum is reasonable because the extent of the interface is much smaller than the coherence length of the radiation. We demonstrate second-harmonic generation from a gold surface and show that surface second-harmonic generation using the white light continuum can be used to measure absorption spectroscopy of molecules adsorbed to surfaces via resonance enhancement of the surface second-harmonic signals.

Nanoscale sensors can be created when an expected energetic pathway is created and then that pathway is either initiated or disrupted by a specific binding event. Constructing the sensor on the nanoscale could lead to greater sensitivity and lower limits of detection. To this end, quantum dots (QDs) can be considered prime candidates for the active components. Relative to organic chromophores, QDs have tunable spectral properties, show less susceptibility to photobleaching, have similar brightness, and have been shown to display electro-optical properties. In this review, we discuss recent articles that incorporate QDs into directed energy flow systems, some with the goal of building new and more powerful sensors and others that could lead to more powerful sensors.

This article is a review of new and versatile optical-immunoassay instrumentation for water monitoring developed through two European Union projects, River ANAlyser (RIANA) and Automated Water Analyser Computer Supported System (AWACSS). Both projects utilise immunoassay techniques to isolate the analytes and Total Internal Reflection Fluorescence (TIRF) to quantify them. Completed in 1999, the RIANA project developed a sensitive and cost-effective analytical system capable of simultaneous detection of multiple-analytes in real-world water samples. The AWACSS project has been in progress since 2001 and is developing rugged-but-sensitive instrumentation that will detect up to 30 analytes simultaneously, will operate unattended, and will have networking capability.

Individual fluorescent polystyrene nanospheres (10-100-nm diameter) and individual fluorescently labeled DNA molecules were dispersed on mica and analyzed using time-resolved fluorescence spectroscopy and atomic force microscopy (AFM). Spatial correlation of the fluorescence and AFM measurements was accomplished by (1) positioning a single fluorescent particle into the near diffraction-limited confocal excitation region of the optical microscope, (2) recording the time-resolved fluorescence emission, and (3) measuring the intensity of the excitation laser light scattered from the apex of an AFM probe tip and the AFM topography as a function of the lateral position of the tip relative to the sample substrate. The latter measurements resulted in concurrent high-resolution (approximately 10-20 nm laterally) images of the laser excitation profile of the confocal microscope and the topography of the sample. Superposition of these optical and topographical images enabled unambiguous identification of the sample topography residing within the excitation region of the optical microscope, facilitating the identification and structural characterization of the nanoparticle(s) or biomolecule(s) responsible for the fluorescence signal observed in step 2. These measurements also provided the lateral position of the particles relative to the laser excitation profile and the surrounding topography with nanometer-scale precision and the relationship between the spectroscopic and structural properties of the particles. Extension of these methods to the study of other types of nanostructured materials is discussed.

Polar solvation dynamics in various reverse micellar environments are investigated. The solvent is immobilized in the smallest micelles and becomes more mobile with increasing water content and micellar size.

A technique for micrometer-scale patterning of multiple functional biological molecules on surfaces is demonstrated. The technique is referred to as single-feature inking and stamping (SFINKS). It combines elements of dip-pen nanolithography and microcontact printing. "Inked" atomic force microscopy probes are used to ink individual features of an elastomer stamp. From a single stamp, we printed three different probe ssDNA with10 mum resolution and showed that they specifically hybridize the complementary DNA labeled with different fluorophores. As a further demonstration of SFINKS' versatility, we patterned a silane onto a silicon wafer consisting of four subpatterns separated by100 mum and composed of 2 mum lines. We discuss why patterns such as these are impractical with available techniques. Furthermore, we comment on the prospects for multiple stamping after a single inking.

We have measured the absorption spectrum of the laser dye IR125 (also known as indocyanine green) at the water/air interface using resonant enhanced surface second harmonic generation. The spectra of the dye molecules at the interface are extremely sensitive to the bulk concentration. All the surface spectra reflect aggregation of the dye at the interface, even at the smallest concentrations detectable, below 1 (mu) M. Resonant enhanced second harmonic generation appears to be a good technique for measuring spectra at liquid interfaces.

The potential uses of inorganic quantum dots in biolabelling and sensing could be expanded through their ability to function as resonant energy-transfer donors. Novel quantum dot–protein sensors may be the way ahead.