Biomedical subjects

John T Fourkas

Publications and source records attributed to John T Fourkas.

12 recordsLinked to original sources

PubMed2006

Soft-lithographic replication of 3D microstructures with closed loops.

There is growing interest in lithographic technologies for creating 3D microstructures. Such techniques are generally serial in nature, prohibiting the mass production of devices. Soft-lithographic techniques show great promise for simple and rapid replication of arrays of microstructures but have heretofore not been capable of direct replication of structures with closed loops. We demonstrate that 3D microstructures created with multiphoton absorption polymerization can be replicated by using microtransfer molding to afford complex daughter structures containing closed loops. This method relieves many of the topological constraints of soft lithography, paving the way for the large-scale replication of true 3D microstructures.

Journal Article↗

PubMed2006

Temperature-dependent optical Kerr effect spectroscopy of aromatic liquids.

Ultrafast optical Kerr effect (OKE) spectroscopy has been used to study the temperature-dependent dynamics of five aromatic liquids: benzene, benzene-d(6), hexafluorobenzene, mesitylene, and 1,3,5-trifluorobenzene. The intermediate response time of all of the liquids was found to scale with the collective orientational correlation time, as has been observed for other simple liquids. The spectra of hexafluorobenzene, 1,3,5-trifluorobenzene, and mesitylene are qualitatively different from those of the other liquids and exhibit different behavior with temperature. These spectra allow us to assess the influence of different molecular parameters on the shape of the OKE spectrum. On the basis of these data, we propose a model that links the differences in the OKE spectra to corresponding differences in the local ordering of the liquids.

Journal Article↗

PubMed2006

Selective functionalization of 3-D polymer microstructures.

We demonstrate the selective functionalization of 3-D polymer microstructures that were created using multiphoton absorption polymerization. By fabricating different portions of the structures with acrylic and methacrylic polymers, we are able to take advantage of the differential reactivities of these materials to perform functionalization chemistry on a single polymeric component. We demonstrate the selective deposition of metal to create structures, such as a functional microinductor. Our strategy is quite general and can be extended readily to the deposition of materials, such as metal oxides and biomolecules.

Journal Article↗

PubMed2005

Effects of molecular association on polarizability relaxation in liquid mixtures of benzene and hexafluorobenzene.

In this work we have studied the relaxation dynamics of the many-body polarizability anisotropy in liquid mixtures of benzene (Bz) and hexafluorobenzene (Hf) at room temperature by femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) experiments and molecular dynamics (MD) simulations. The computed polarizability response arising from intermolecular interactions was included using the first-order dipole-induced-dipole model with the molecular polarizability distributed over the carbon sites of each molecule. We found good qualitative agreement between experiments and simulations in the features exhibited by the nuclear response function R(t) for pure liquids and mixtures. The long-time diffusive decay of R(t) was observed to vary substantially with composition, slowing down noticeably with dilution of each of the species as compared with that in the corresponding pure liquids. MD simulation shows that the effect on R(t) is due to the formation of strong and localized intermolecular association between Bz and Hf species that hinder the rotational diffusive dynamics. The formation of these Bz-Hf complexes in the liquid mixtures also modifies the rotational diffusive dynamics of the component species in such a way that cannot be explained solely in terms of a viscosity effect. Even though the computed orientational diffusive relaxation times associated with Bz and Hf are larger by a factor of approximately 2 than those from experiments, we found similar trends in experiments and simulations for these characteristic times as a function of composition. Namely, the collective and single-molecule orientational correlation times associated with Bz are observed to grow monotonically with the dilution of Bz, while those corresponding to Hf species exhibit a maximum at the equimolar composition. We attribute the quantitative discrepancy between experiments and simulations to the use of the Williams potential, which seems to overestimate the intermolecular interactions and thus predicts not only a slower translational dynamics but also a slower rotational diffusion dynamics than in real fluids.

Journal Article↗

PubMed2005

Ultrafast orientational dynamics of nanoconfined benzene.

Ultrafast optical Kerr effect spectroscopy has been used to study the orientational dynamics of benzene and benzene-d(6) confined in nanoporous sol-gel glass monoliths with a range of average pore sizes. All of the observed orientational diffusion of confined benzene is found to occur on a slower time scale than in the bulk, even in pores with diameters that are significantly larger than a benzene molecule. The orientational dynamics of benzene-d(6) are found to be inhibited to a lesser extent than those of benzene, which is attributed to the differences in wetting properties of the two liquids on silica. The decays are fit well by a sum of two exponentials, the faster of which depends on pore size. Similar results are found in pores that have been modified with trimethylsilyl groups, although the relaxation is faster than in unmodified pores. Comparison to Raman line width data for confined benzene-d(6) suggests that the liquid exhibits significant structuring at the pore walls, with the benzene molecules lying flat on the surfaces of unmodified pores.

Journal Article↗

PubMed2005

Optical Kerr effect spectroscopy using time-delayed pairs of pump pulses with orthogonal polarizations.

We characterize in detail a recently introduced technique in which perpendicularly polarized pulses with controllable intensities and timing are used for the excitation step in optical Kerr effect spectroscopy. We examine the ratio of pump pulse intensities required to cancel the contribution of reorientational diffusion or of a Raman-active intramolecular vibration to the signal as a function of the delay time between excitation pulses. These results indicate that the signal can be described well as arising from the sum of independent third-order responses initiated by each pump pulse. This conclusion is further supported by using data obtained with a single pump pulse to model decays obtained with two pump pulses.

Journal Article↗

PubMed2005

Highly efficient multiphoton-absorption-induced luminescence from gold nanoparticles.

We demonstrate that highly efficient photoluminescence is generated from gold nanoparticles as small as a few nanometers in diameter upon irradiation with sub-100-fs pulses of 790-nm light. Strong emission is observed at excitation intensities comparable to or less than those typically used for multiphoton imaging of fluorescently labeled biological samples. The particles have polarized emission, can radiate more efficiently than single molecules, do not exhibit significant blinking, and are photostable under hours of continuous excitation. These observations suggest that metal nanoparticles are a viable alternative to fluorophores or semiconductor nanoparticles for biological labeling and imaging.

Absorption↗

PubMed2003

Orientational dynamics of liquids confined in nanoporous sol-gel glasses studied by optical kerr effect spectroscopy.

When a liquid is restricted to volumes on a molecular distance scale, its orientational and translational dynamics are perturbed strongly by the confinement. Nanoporous sol-gel glasses are an excellent model system for studying the effects of confinement on the behavior of liquids, and in this Account we review experiments in which ultrafast optical Kerr effect spectroscopy has been used to study the orientational dynamics of liquids confined in these media. We contrast the effects of confinement on the orientational dynamics of weakly wetting, strongly wetting, and networked liquids, and we discuss the influence of factors such as pore size, molecular shape, and surface chemistry.

Journal Article↗

PubMed2002

Production, analysis, and application of spatially resolved shells in solid-phase polymer spheres.

Two-photon fluorescence microscopy has been used to interrogate the interior functionality of polymer resin beads. By employing this technique, the spatial distribution of the initial functionality contained within the polymer matrix has been determined. Spatially resolved, concentric shells were then produced synthetically in these polymer spheres via a series of protection/deprotection reactions in which two-photon fluorescence microscopy was employed to monitor each successive step. To demonstrate the potential utility of these techniques in combinatorial screening, a set of beads was prepared containing a unique tripeptide sequence in each of the three concentric shells within each individual bead. The set was then screened for the binding affinity of each tripeptide toward a fluorescent ligand.

Journal Article↗

PubMed2002

Efficient and robust multiphoton data storage in molecular glasses and highly crosslinked polymers.

As electronics become ever faster and more powerful, there is growing interest in three-dimensional laser-based optical data storage techniques, which can potentially provide efficient storage at densities significantly higher than those that are likely to be available from magnetic media. The development of inexpensive, efficient and robust media has been a major obstacle in optical data storage. However, we have discovered a class of materials that become highly fluorescent on multiphoton absorption of pulses of 800-nm light from a Ti:sapphire oscillator, making them excellent candidate storage media. The materials are inexpensive, of high optical quality, can be processed readily, and can take a number of useful forms, including molecular glasses and highly crosslinked polymers. Three-dimensional data can be stored at high densities in these materials, and are highly robust to readout.

Electronics↗

PubMed2001

Higher-order optical correlation spectroscopy in liquids.

Linear optical spectroscopies have long been used to study the behavior of liquids. Laser technology has progressed to the point that it has become possible to perform nonlinear optical experiments that probe higher-order correlation functions in liquids, opening a new window into our understanding of the microscopic details of solution-phase processes. Here we review advances that have been made in recent years in employing higher-order electronic and vibrational spectroscopies to study liquid-state dynamics and structure.

Spectrophotometry, Infrared↗

PubMedDate not supplied

In situ observation of molecular diffusion in solid supports using two-photon fluorescence microscopy.

We demonstrate that two-photon fluorescence microscopy is a useful tool for monitoring the diffusion of molecules through polymeric solid supports. As a proof of principle, TentaGel beads were bound to a cover slip, and diffusion of Rhodamine 6G through single beads was observed in real time. Diffusion experiments performed in 40 different beads indicate that there is a considerable degree of heterogeneity in diffusion rates from bead to bead.

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