The concept of dlgltal molecular detectlon, the aMllty to count lndlvldual molecules, b described and the mathematlcal framework for deflnlng detedlon llmlts under varlow experbnental conrtralnts presented. We show that wlth modest dgnaCt+noI"tbfordetectbnofrhgk~,tlgnYlccmt advantages accrue when wlng a dlgltal detectlon strategy versus a conventlonal approach. Concentration detectlon lbnlts decrease Inversely wlth total sample volume for dlgltal detection versus reductlon as the Inverse square root for conventbnal measurements. Thk advantage could ykld detedlon lbntts that are redmod by orders of magnltude If (IMIpk voknn b Ihrlted. I n add#bn, we presenl exp.mwMtal r # u l b~r c l # n g t h e~d d l g l t a l m o k c u b r d e t~ wlng fl-phycowythrln molecules and levttated mkrodropld fluorhetry. Single fl-phycoerythrln molecules are detected wlth a signal-to-ndre ratlo greater than 4. (8) Stevenson, C. L.; Winefordner, J. D. Appl. Spectrosc. 1992, 46, 407-419.
We describe a new method for probing phase separation of polymer-blend systems in
spherical microparticles generated from microdroplets of dilute polymer solution. Two-dimensional optical
diffractiona technique sensitive to material inhomogeneity on a length scale of ≈30 nmis used to
probe phase-separation behavior of bulk-immiscible polymers in attoliter and femtoliter volumes. Under
conditions of rapid solvent evaporation (≈2−10 ms) and relatively low polymer mobility, homogeneous
composite particles can be formed using different polymers that ordinarily undergo phase separation in
bulk preparations. We show that, for homogeneous particles, the refractive index (related to material
dielectric constant) can be tuned by adjusting the relative weight fractions of polymers in the blend.
Molecular dynamics simulations of polymer-blend microparticles are presented to illustrate at a molecular
level effects of polymer interactions and boundary conditions on phase separation in polymer-blend
nanoparticles. Finally, we show that polymer mobility affects phase-separation behavior in microparticles
using mixtures of poly(vinyl alcohol)s with low molecular weight (high-mobility) poly(ethylene glycol)
oligomers. For higher molecular weight polymers (>10 K), surface energy constraints inhibit phase
separation, and the polymer-blend particles are observed to be homogeneous to within experimental
resolution. Conversely, blends of low molecular weight PEG with PVA phase-separate on a time scale of
several minutes to form heterogeneous composite particles.
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