Studies on diffusion dynamics of single molecules (SMs) have been useful in revealing inhomogeneity of polymer thin films near and above the glass-transition temperature (T(g)). However, despite several applications of polymer thin films where exposure to solvent (or vapor) is common, the effect of absorbed solvent molecules on local morphology and rigidity of polymer matrices is yet to be explored in detail. High-T(g) hydrophilic polymers such as poly(vinylpyrrolidone) (PVP) are used as pharmaceutical coatings for drug release in aqueous medium, as they readily absorb moisture, which results in effective lowering of the T(g) and thereby leads to plasticization. The effect of moisture absorption on swelling and softening of PVP thin films was investigated by visualizing the diffusion dynamics of rhodamine 6G (Rh6G) tracer molecules at various ambient relative humidities (RH). Wide-field epifluorescence microscopy, in conjunction with high-resolution SM tracking, was used to monitor the spatiotemporal evolution of individual tracers under varied moisture contents of the matrix. In the absence of atmospheric moisture, Rh6G molecules in dry PVP films are translationally inactive, suggestive of rigid local environments. Under low moisture contents (RH 30-50%), translational mobility remains arrested but rotational motion is augmented, indicating slight swelling of the polymer network which marks the onset of plasticization. The translational mobility of Rh6G was found to be triggered only at a threshold ambient RH, beyond which a large proportion of tracers exhibit extensive diffusion dynamics. Interestingly, SM tracking data at higher moisture contents of the film (RH ≥ 60%) reveal that the distributions of dynamic parameters (such as diffusivity) are remarkably broad, spanning several orders of magnitude. Furthermore, Rh6G molecules display a wide variety of translational motion even at a fixed ambient RH, clearly pointing out the extremely inhomogeneous environment of plasticized PVP network. Intriguingly, it is observed that a majority of tracers undergo anomalous subdiffusion even under high moisture contents of the matrix. Analyses of SM trajectories using velocity autocorrelation function reveal that subdiffusive behaviors of Rh6G are likely to originate from fractional Brownian motion, a signature of tracer dynamics in viscoelastic medium.

The nature of the polarization-field in disorder induced nanoscale potential fluctuations (radiative traps) within (In,Ga)N based quantum-well (QW) heterostructures remains ambiguous. Spectrally resolved photoluminescence microscopy has been utilized to probe the local polarization field by monitoring the extent of quantum-confined Stark effect (QCSE) in radiative trap centers spontaneously formed within an (In,Ga)N QW based light emitting diode. Interestingly, two distinct categories of nanoscale radiative domains, which arise from indium compositional and interface-morphology related fluctuations of the active layers, are found to have very different degree of built-in polarization fields. Screening of QCSE in indium-rich emission centers results in blue-shift of transition energies by up to 400 meV, significantly higher than that reported previously for group III-nitride based semiconductor heterostructures. A lack of correlation between the extent of QCSE and local indium mole-fractions suggests that size, shape, and strain of individual localization centers play a crucial role in modulating the local polarization field.

While dynamics of single-molecule (SM) fluorescent probes have been used to investigate the structure and relaxation processes in polymers near the glass transition temperature (T), it is difficult to perform SM imaging at elevated temperatures which restricts such studies to a limited number of polymers for which T is close to room temperature (RT). Plasticization, solvent (or additive) induced lowering of T, offers an alternate avenue to access various effective temperatures in the glassy and rubbery phases of polymers under ambient conditions. By investigation of the reorientational propensity of individual Rhodamine 6G (Rh6G) probes, which is governed by rigidity/dynamics of the polymer cavities, we have explored the extent of spatiotemporal heterogeneity during moisture induced plasticization of poly(vinylpyrrolidone) (PVP), far below and near (below and above) bulk T. Lack of any probe reorientation suggests that the matrix remains extremely rigid up to a certain level of hydration, as expected for probes buried deep within the glassy state. At intermediate levels of hydration, SMs undergo a wide variety of rotational dynamics ranging from being static/wobbling motion to slow, hindered large-angle reorientation, as well as facile, intermittently hindered fast rotation, which reflects that swelling/softening of network cavities is spatiotemporally extremely diverse as the effective T approaches RT. SM probes exhibit temporally nonuniform rotational mobility even at relatively high moisture contents of the matrix beyond which probes can undergo translational motion, which indicates that relatively slow time scale polymer segmental motion can be operational for plasticized PVP (in the rubbery state). Our inferences are supported by the non-Gaussian nature of angular jump distributions for dipolar reorientation, similar to those reported for translational diffusion of SM tracers in polymers and cellular media, suggesting the existence of slow time-varying local environmental changes around individual probe molecules during plasticization.

Dynamics of small probe molecules have been routinely used to unravel the intrinsic details of charged ion transport in polymer brushes and polyelectrolyte multilayer (PEM) thin films. However, corresponding morphological properties affected with absorption of moisture have been hardly dealt with despite numerous applications of isotropic thin films in material chemistry and medical purposes. We have explored the overall structural changes associated with plasticization of PVP thin films by probing dynamics of small reporter (rhodamine 6G, Rh6G) molecules using fluorescence correlation spectroscopy (FCS). It was observed that under lesser amounts of absorbed moisture, the rigidity of the film matrix was high enough to inhibit appreciable molecular mobility. Nonetheless, with gradual increase in the moisture level within the film, molecular movement became extremely facile, so much so that it almost attained close to a solution like state. Molecular mobility was found to be dependent on both the method of preparation and the thickness of the thin films. The diffusivities mostly followed anomalous subdiffusive behaviors, reminiscent of dynamics of tracers in crowded cellular environments. The mobility was found to be independent of any electrostatic interaction between probe and polymer thin film. Hence, the tracer dynamics was attributed most likely to the viscoelasticity of the thin film matrix.

Tracking the movement of fluorescent single-molecule (SM) tracers has provided several new insights on the local structure and dynamics in complex environments such as soft materials and biological systems. However,...

<div>Tracking the movement of fluorescent single-molecule (SM) tracers has provided several new insights on the local structure and dynamics in complex environments such as soft materials and biological systems. However, SM tracking (SMT) remains unreliable at molecular length scales, as the localization-error (LE) of SM trajectories (~30-50 nm) is considerably larger than size of molecular tracers (~1-3 nm). Thus, instances of tracer (im)mobility in heterogeneous media, which provide indicator for underlying anomalous-transport mechanisms, remains obscured within the realms of SMT. Since translation of passive tracers in an isotropic network is associated with fast dipolar rotation, we propose authentic pauses within LE can be revealed upon probing SM reorientational dynamics. Here, we demonstrate how polarization-resolved SMT (PR-SMT) can provide emission-anisotropy at each super-localized position, thereby revealing tumbling propensity of SMs during random walk. For Rhodamine 6G tracers undergoing heterogeneous transport in a hydrated polyvinylpyrrolidone (PVP) network, analyses of PR-SMT trajectories enabled us to discern instances of genuine immobility and localized motion within the LE. Our investigations on 100 SMs in hydrated (plasticized) PVP films reveal a wide distribution of dwell-times and pause-frequencies, which demonstrate that majority of probes intermittently experience complete translational and rotational immobilization. This indicates tracers serendipitously encounter compact, rigid polymer cavities during transport, implying the existence of nanoscale glass-like domains sparsely distributed in a redominantly deep-rubbery polymer network far above the glass transition. PR-SMT is simple to implement and opens up alternate avenues to interrogate transient (bio)molecular interactions leading to anomalous transport in inhomogeneous media.</div>

<div>Tracking the movement of fluorescent single-molecule (SM) tracers has provided several new insights on the local structure and dynamics in complex environments such as soft materials and biological systems. However, SM tracking (SMT) remains unreliable at molecular length scales, as the localization-error (LE) of SM trajectories (~30-50 nm) is considerably larger than size of molecular tracers (~1-3 nm). Thus, instances of tracer (im)mobility in heterogeneous media, which provide indicator for underlying anomalous-transport mechanisms, remains obscured within the realms of SMT. Since translation of passive tracers in an isotropic network is associated with fast dipolar rotation, we propose authentic pauses within LE can be revealed upon probing SM reorientational dynamics. Here, we demonstrate how polarization-resolved SMT (PR-SMT) can provide emission-anisotropy at each super-localized position, thereby revealing tumbling propensity of SMs during random walk. For Rhodamine 6G tracers undergoing heterogeneous transport in a hydrated polyvinylpyrrolidone (PVP) network, analyses of PR-SMT trajectories enabled us to discern instances of genuine immobility and localized motion within the LE. Our investigations on 100 SMs in hydrated (plasticized) PVP films reveal a wide distribution of dwell-times and pause-frequencies, which demonstrate that majority of probes intermittently experience complete translational and rotational immobilization. This indicates tracers serendipitously encounter compact, rigid polymer cavities during transport, implying the existence of nanoscale glass-like domains sparsely distributed in a redominantly deep-rubbery polymer network far above the glass transition. PR-SMT is simple to implement and opens up alternate avenues to interrogate transient (bio)molecular interactions leading to anomalous transport in inhomogeneous media.</div>