We have employed two third-order femtosecond spectroscopic methods, stimulated-photon-echo peak-shift (3PEPS) and transient-grating (TG) spectroscopy, to characterize solvation dynamics and interexciton-state radiationless decay in the α subunit of C-phycocyanin and in allophycocyanin. The α subunit contains a single phycocyanobilin chromophore in an isolated protein-matrix environment. Allophycocyanin contains exciton-coupled pairs of phycocyanobilins in the same type of binding site found in the α subunit. The results show that both systems exhibit a biphasic solvation response:  the inertial phase, arising from librational motions of the amino acids or included water molecules in the phycocyanobilin-binding site, contributes a 80−100-fs component to the 3PEPS profile and appears as a rapidly damped 72-cm-1 modulation of the TG signal; the diffusive phase, arising from collective protein-matrix motions, contributes a component in the TG signal and 3PEPS profile on the 5−20-ps time scale. Both systems exhibit nearly instantaneous (16-fs) components in the 3PEPS profiles that arise from intrachromophore vibrational modes. The 3PEPS profile observed with allophycocyanin exhibits additional fast decay components, with time constants of 56 and 220 fs, that apparently report the contributions to electronic dephasing arising from radiationless decay between imperfectly correlated exciton states. The TG signal evidences vibrational relaxation in the lower exciton state and incoherent energy transfer between the chromophores in a given pair. The results present complementary details on solvation and interexciton-state radiationless decay dynamics that were first observed in this laboratory using time-resolved pump−probe and anisotropy methods.

We have characterized the femtosecond dynamic-absorption spectrum from the tricarbocyanine dye IR144 with the aid of a new approach that uses time-probe-wavelength contour lines to project the phase relationships between the coherent wave-packet motions on the ground-state and excited-state potential-energy surfaces. The distinct phase of the waveform carried by a dynamic-absorption contour line reports the motion of a single wave packet. The spectrum from IR144 exhibits four regions of alternating contour phase; simulations show that this interference pattern results from the antiphase motion and partial spectral overlap of the groundstate depletion and stimulated-emission spectra and the narrowing of the spectra that occurs when the excitedstate and ground-state wave packets reach their turning points. The Fourier-magnitude spectra of contour lines and intensity transients observed in the spectral region, which are assigned to the turning point of the ground-state wave packet, have been compared to those obtained in the long-wavelength-limit spectral region that is associated with the excited-state wave packet. The intensities and frequencies of eleven modes observed over the 15-634 cm -1 region suggest that the excited-state wave packet rapidly moves from the Franck-Condon geometry to a region of the potential-energy surface that is relatively flat with respect to the global normal coordinates that bend and twist the conjugated polyene backbone of IR144. These coordinates are evidently anharmonically coupled to high-frequency modes that are not impulsively excited by the 12-fs pulses used in the dynamic-absorption experiments.

The phenomenon of enrichment of charged analytes due to the presence of an electric field barrier at the micro-nanofluidic interconnect can be harnessed to enhance sensitivity and limit-of-detection in sensor instruments. We present a numerical analysis framework to investigate two critical electrokinetic phenomena underlying the experimental observation in Plecis et al. (Micro Total Analysis Systems, pp 1038-1041, 2005b: (1) ion transport of background electrolytes (BGE) and (2) enrichment of analytes in the micro-nanofluidic devices that operate under hydrodynamic flow. The analysis is based on the full, coupled solution of the Poisson-Nernst-Planck (PNP) and Naviér-Stokes equations, and the results are validated against analytical models of simple canonical geometry. Parametric simulation is performed to capture the critical effects of pressure head and BGE ion concentration on the electrokinetics and ion transport. Key findings obtained from the numerical analysis indicate that the hydrodynamic flow and overlapped electrical double layer induce concentration-polarization at the interfaces; significant electric field barrier arising from the Donnan potential forms at the micro-nano interfaces; and streaming potential and overall potential are effectively established across the micro-nanofluidic device. The simulation to examine analyte enrichment and its dependence on the hydrodynamic flow and analyte properties, demonstrates that order-of-magnitude enrichment can be achieved using properly configured hydrodynamic flow. The results can be used to guide practical design and operational protocol development of novel micro-nanofluidic interconnect-based analyte preconcentrators.

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