Ivan R. Piletic

Orientational dynamics of water confined on a nanometer length scale in reverse micelles.

The time-resolved orientational anisotropies of the OD hydroxyl stretch of dilute HOD in H(2)O confined on a nanometer length scale in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles are studied using ultrafast infrared polarization and spectrally resolved pump-probe spectroscopy, and the results are compared to the same experiments on bulk water. The orientational anisotropy data for three water nanopool sizes (4.0, 2.4, and 1.7 nm) can be fitted well with biexponential decays. The biexponential decays are analyzed using a wobbling-in-a-cone model that involves fast orientational diffusion within a cone followed by slower, full orientational relaxation. The data provide the cone angles, the diffusion constants for motion within the cones, and the final diffusion constants as a function of the nanopool size. The two processes can be interpreted as a local angular fluctuation of the OD and a global hydrogen bond network rearrangement process. The trend in the relative amplitudes of the long and short exponential decays suggest an increasing rigidity as the nanopool size decreases. The trend in the long decay constants indicates a longer hydrogen bond network rearrangement time with decreasing reverse micelle size. The anisotropy measurements for the reverse micelles studied extrapolate to approximately 0.33 rather than the ideal value of 0.4, suggesting the presence of an initial inertial component in the anisotropy decay that is too fast to resolve. The very fast decay component is consistent with initial inertial orientational motion that is seen in published molecular-dynamics simulations of water in AOT reverse micelles. The angle over which the inertial orientational motion occurs is determined. The results are in semiquantitative agreement with the molecular-dynamics simulations.

Epoxide Pathways Improve Model Predictions of Isoprene Markers and Reveal Key Role of Acidity in Aerosol Formation

Isoprene significantly contributes to organic aerosol in the southeastern United States where biogenic hydrocarbons mix with anthropogenic emissions. In this work, the Community Multiscale Air Quality model is updated to predict isoprene aerosol from epoxides produced under both high- and low-NOx conditions. The new aqueous aerosol pathways allow for explicit predictions of two key isoprene-derived species, 2-methyltetrols and 2-methylglyceric acid, that are more consistent with observations than estimates based on semivolatile partitioning. The new mechanism represents a significant source of organic carbon in the lower 2 km of the atmosphere and captures the abundance of 2-methyltetrols relative to organosulfates during the simulation period. For the parametrization considered here, a 25% reduction in SOx emissions effectively reduces isoprene aerosol, while a similar reduction in NOx leads to small increases in isoprene aerosol.

Pump-Probe Imaging Differentiates Melanoma from Melanocytic Nevi

Multiphoton imaging reveals chemical changes in melanoma compared to benign nevi and could enhance current clinical diagnostic protocols. A Wolf in Sheep’s Clothing It is frequently difficult to distinguish whether something is dangerous or harmless. In the case of melanoma diagnosis, a misdiagnosed lesion could have deadly consequences. Rightly, doctors err on the side of caution; however, false-positive diagnoses result in unnecessary surgeries and biopsies, as well as emotional distress for the patient. Matthews et al. have developed a new imaging technique that can distinguish melanoma from benign lesions, which in concert with current techniques could improve patient diagnosis and decrease the need for unnecessary tests. The pigment melanin is the primary determinant of skin color. There are two dominant types of melanin in melanocytic lesions: eumelanin and pheomelanin. Eumelanin, which is brown/black, is the most common biological form of melanin, whereas pheomelanin is largely responsible for red hair and freckles. The authors use a multiphoton imaging technique, pump-probe spectroscopy, to determine the ratio of these different melanins in melanocytic lesions in the context of lesion architectural and cytological features. Eumelanin was found at higher levels in melanoma compared with both dysplastic and benign nevi. When combined with pathological examination, imaging-based determination of the melanin ratio decreased the number of false-positive diagnoses compared with pathological examination alone. Moreover, this imaging technique could be used on hematoxylin and eosin–stained slides, which are currently used by pathologists for melanoma diagnosis, and may even be able to be adapted for noninvasive diagnostics. In conjunction with traditional diagnostic methods, melanocytic imaging should greatly improve doctor’s ability to sort the wolves from the sheep. Melanoma diagnosis is clinically challenging: the accuracy of visual inspection by dermatologists is highly variable and heavily weighted toward false positives. Even the current gold standard of biopsy results in varying diagnoses among pathologists. We have developed a multiphoton technique (based on pump-probe spectroscopy) that directly determines the microscopic distribution of eumelanin and pheomelanin in pigmented lesions of human skin. Our initial results showed a marked difference in the chemical variety of melanin between nonmalignant nevi and melanoma, as well as a number of substantial architectural differences. We examined slices from 42 pigmented lesions and found that melanomas had an increased eumelanin content compared to nonmalignant nevi. When used as a diagnostic criterion, the ratio of eumelanin to pheomelanin captured all investigated melanomas but excluded three-quarters of dysplastic nevi and all benign dermal nevi. Additional evaluation of architectural and cytological features revealed by multiphoton imaging, including the maturation of melanocytes, presence of pigmented melanocytes in the dermis, number and location of melanocytic nests, and confluency of pigmented cells in the epidermis, further increased specificity, allowing rejection of more than half of the remaining false-positive results. We then adapted this multiphoton imaging technique to hematoxylin and eosin (H&E)–stained slides. By adding melanin chemical contrast to H&E-stained slides, pathologists will gain complementary information to increase the ease and accuracy of melanoma diagnosis.

Orientational relaxation and vibrational excitation transfer in methanol–carbon tetrachloride solutions

Time and polarization resolved ultrafast infrared vibrational spectroscopy of the hydroxyl stretch of methanol dissolved in carbon tetrachloride has been utilized to investigate orientational relaxation and vibrational excitation transfer. The anisotropy decay of the deuterated hydroxyl stretch of methanol-d was measured in two solutions: Isotopically mixed 0.8 mol % methanol-d 23 mol % methanol-h in CCl4 and isotopically pure methanol-d at 26 mol % in CCl4. The anisotropy decay in the isotopically mixed methanol solution is a biexponential characterized by 1.7±0.7 ps and 17±3 ps time constants, with 40±10% of the decay occurring with the slower time constant. The biexponential anisotropy decay has been analyzed with a restricted orientational diffusion model that involves fast orientational diffusion within a cone of semi-angle θc, followed by slower, full orientational relaxation. The fast orientational relaxation occurs within a cone semi-angle of θc=45°±5°, with a diffusion coefficient of Dc−1=13±5 ps...

Hydrogen bond breaking probed with multidimensional stimulated vibrational echo correlation spectroscopy

H bond population dynamics are extricated with exceptional detail using ultrafast (<50 fs) IR multidimensional stimulated vibrational echo correlation spectroscopy with full phase information and frequency resolved IR pump-probe expts. performed on the hydroxyl stretch of MeOH-OD oligomers in CCl4. H bond breaking makes it possible to acquire data for times much greater than the hydroxyl stretch vibrational lifetime. The correlation spectra and detailed calcns. demonstrate that vibrational relaxation leads to H bond breaking for oligomers that have hydroxyl stretch frequencies on the low energy (red) side of the hydroxyl stretch spectrum, the spectral region that is assocd. with the strongest H bonds. Frequency resolved pump-probe data support the conclusions drawn from the correlation spectra. Using a global fit to the pump-probe spectra, in conjunction with assignments made possible through the correlation spectra, the residual ground state and photoproduct of H bond breaking were prepd. near their thermal equil. distribution. The spectrum of the H bond breaking photoproduct and the residual ground state approach the steady-state temp. difference spectrum on the tens of picoseconds time scale, indicating the system thermalizes on this time scale. [on SciFinder(R)]

Ultrafast heterodyne detected infrared multidimensional vibrational stimulated echo studies of hydrogen bond dynamics

Multidimensional vibrational stimulated echo correlation spectra with full phase information are presented for the broad hydroxyl stretch band of methanol-OD oligomers in CCl4 using ultrashort ( 1 ps) shows that there is frequency correlation between the initially excited hydroxyl stretch and the frequency shifted hydroxyl stretch formed by hydrogen bond breaking.

Structural dynamics of hydrogen bonded methanol oligomers: Vibrational transient hole burning studies of spectral diffusion

Frequency resolved pump-probe experiments have been conducted on the deuterated hydroxyl stretch of methanol-d in a solution containing 0.8% methanol-d/23% methanol-h in carbon tetrachloride. Methanol-d molecules that both donate and receive hydrogen bonds have an inhomogeneously broadened hydroxyl stretch absorption line centered at 2487 cm−1. With a laser tuned to 2513 cm−1, the high-frequency side of the absorption spectrum is excited. The equilibration of the excited state peak and the ground-state hole results in the time-dependent shift in the frequency of the signal, which is used to monitor the dynamics of spectral diffusion. Model calculations were conducted to address the influence of spectral diffusion in the ground and excited states on the experimental observables when the vibrational lifetime is comparable to the spectral diffusion time. The model calculations illustrate the influence on the signal of absorbers in the ground state that have relaxed from the excited state. This aspect of the ...

Label-free in vivo optical imaging of microvasculature and oxygenation level

The ability to perform high-resolution imaging of microvasculature and its oxygenation is very important in studying early tumor development. Toward this goal, we improved upon our excited state absorption (ESA)-based imaging technique to allow us to not only image hemoglobin directly but also differentiate between oxy- and deoxyhemoglobin in tissue. We demonstrate the separation of arterioles from venules in a live nude mouse ear using our imaging technique.

Probing near-infrared photorelaxation pathways in eumelanins and pheomelanins.

Ultraviolet-visible spectroscopy readily discerns the two types of melanin pigments (eumelanin and pheomelanin), although fundamental details regarding the optical properties and pigment heterogeneity are more difficult to disentangle via analysis of the broad featureless absorption spectrum alone. We employed nonlinear transient absorption spectroscopy to study different melanin pigments at near-infrared wavelengths. Excited-state absorption, ground-state depletion, and stimulated emission signal contributions were distinguished for natural and synthetic eumelanins and pheomelanins. A starker contrast among the pigments is observed in the nonlinear excitation regime because they all exhibit distinct transient absorptive amplitudes, phase shifts, and nonexponential population dynamics spanning the femtosecond-nanosecond range. In this manner, different pigments within the pheomelanin subclass were distinguished in synthetic and human hair samples. These results highlight the potential of nonlinear spectroscopies to deliver an in situ analysis of natural melanins in tissue that are otherwise difficult to extract and purify.

Simultaneous self-phase modulation and two-photon absorption measurement by a spectral homodyne Z-scan method

We developed a technique to simultaneously measure self-phase modulation and two-photon absorption using shaped femtosecond laser pulses. In the conventional Z-scan measurement technique the amount of nonlinearity is determined by measuring the change in shape and intensity of a transmitted laser beam. In contrast, our method sensitively measures nonlinearity-induced changes in the pulse spectrum. In this work we demonstrate the technique in nonlinear absorptive and dispersive samples, quantify the obtained signal, and compare the measurements with traditional Z-scans. This technique is capable of measuring these nonlinearities in highly scattering samples.