Gaining control on optical force by the stimulated-emission resonance effect.

The resonance between an electronic transition of a micro/nanoscale object and an incident photon flux can modify the radiation force exerted on that object, especially at an interface. It has been theoretically proposed that a non-linear stimulated emission process can also induce an optical force, however its direction will be opposite to conventional photon scattering/absorption processes. In this work, we experimentally and theoretically demonstrate that a stimulated emission process can induce a repulsive pulling optical force on a single trapped dye-doped particle. Moreover, we successfully integrate both attractive pushing (excited state absorption) and repulsive pulling (stimulated emission) resonance forces to control the overall exerted optical force on an object, validating the proposed non-linear optical resonance theory. Indeed, the results presented here will enable the optical manipulation of the exerted optical force with exquisite control and ultimately enable single particle manipulation.

Synthetic fibrous hydrogels as a platform to decipher cell-matrix mechanical interactions.

Cells continuously sense external forces from their microenvironment, the extracellular matrix (ECM). In turn, they generate contractile forces, which stiffen and remodel this matrix. Although this bidirectional mechanical exchange is crucial for many cell functions, it remains poorly understood. Key challenges are that the majority of available matrices for such studies, either natural or synthetic, are difficult to control or lack biological relevance. Here, we use a synthetic, yet highly biomimetic hydrogel based on polyisocyanide (PIC) polymers to investigate the effects of the fibrous architecture and the nonlinear mechanics on cell-matrix interactions. Live-cell rheology was combined with advanced microscopy-based approaches to understand the mechanisms behind cell-induced matrix stiffening and plastic remodeling. We demonstrate how cell-mediated fiber remodeling and the propagation of fiber displacements are modulated by adjusting the biological and mechanical properties of this material. Moreover, we validate the biological relevance of our results by demonstrating that cellular tractions in PIC gels develop analogously to those in the natural ECM. This study highlights the potential of PIC gels to disentangle complex bidirectional cell-matrix interactions and to improve the design of materials for mechanobiology studies.

Unravelling 3D Dynamics and Hydrodynamics during Incorporation of Dielectric Particles to an Optical Trapping Site.

Mapping of the spatial and temporal motion of particles inside an optical field is critical for understanding and further improvement of the 3D spatio-temporal control over their optical trapping dynamics. However, it is not trivial to capture the 3D motion, and most imaging systems only capture a 2D projection of the 3D motion, in which the information about the axial movement is not directly available. In this work, we resolve the 3D incorporation trajectories of 200 nm fluorescent polystyrene particles in an optical trapping site under different optical experimental conditions using a recently developed widefield multiplane microscope (imaging volume of 50 × 50 × 4 µm3). The particles are gathered at the focus following some preferential 3D channels that show a shallow cone distribution. We demonstrate that the radial and the axial flow speed components depend on the axial distance from the focus, which is directly related to the scattering/gradient optical forces. While particle velocities and trajectories are mainly determined by the trapping laser profile, they cannot be completely explained without considering collective effects resulting from hydrodynamic forces.

The primeval optical evolving matter by optical binding inside and outside the photon beam.

Optical binding has recently gained considerable attention because it enables the light-induced assembly of many-body systems; however, this phenomenon has only been described between directly irradiated particles. Here, we demonstrate that optical binding can occur outside the focal spot of a single tightly focused laser beam. By trapping at an interface, we assemble up to three gold nanoparticles with a linear arrangement which fully-occupies the laser focus. The trapping laser is efficiently scattered by this linear alignment and interacts with particles outside the focus area, generating several discrete arc-shape potential wells with a half-wavelength periodicity. Those external nanoparticles inside the arcs show a correlated motion not only with the linear aligned particles, but also between themselves even both are not directly illuminated. We propose that the particles are optically bound outside the focal spot by the back-scattered light and multi-channel light scattering, forming a dynamic optical binding network.

Expansion microscopy allows high resolution single cell analysis of epigenetic readers.

Interactions between epigenetic readers and histone modifications play a pivotal role in gene expression regulation and aberrations can enact etiopathogenic roles in both developmental and acquired disorders like cancer. Typically, epigenetic interactions are studied by mass spectrometry or chromatin immunoprecipitation sequencing. However, in these methods, spatial information is completely lost. Here, we devise an expansion microscopy based method, termed Expansion Microscopy for Epigenetics or ExEpi, to preserve spatial information and improve resolution. We calculated relative co-localization ratios for two epigenetic readers, lens epithelium derived growth factor (LEDGF) and bromodomain containing protein 4 (BRD4), with marks for heterochromatin (H3K9me3 and H3K27me3) and euchromatin (H3K36me2, H3K36me3 and H3K9/14ac). ExEpi confirmed their preferred epigenetic interactions, showing co-localization for LEDGF with H3K36me3/me2 and for BRD4 with H3K9/14ac. Moreover addition of JQ1, a known BET-inhibitor, abolished BRD4 interaction with H3K9/14ac with an IC50 of 137 nM, indicating ExEpi could serve as a platform for epigenetic drug discovery. Since ExEpi retains spatial information, the nuclear localization of marks and readers was determined, which is one of the main advantages of ExEpi. The heterochromatin mark, H3K9me3, is located in the nuclear rim whereas LEDGF co-localization with H3K36me3 and BRD4 co-localization with H3K9/14ac occur further inside the nucleus.

Ladder-like Polymer Brushes Containing Conjugated Poly(Propylenedioxythiophene) Chains.

The high stability and conductivity of 3,4-disubstituted polythiophenes such as poly(3,4-ethylenedioxythiophene) (PEDOT) make them attractive candidates for commercial applications. However, next-generation nanoelectronic devices require novel macromolecular strategies for the precise synthesis of advanced polymer structures as well as their arrangement. In this report, we present a synthetic route to make ladder-like polymer brushes with poly(3,4-propylenedioxythiophene) (PProDOT)-conjugated chains. The brushes were prepared via a self-templating surface-initiated technique (ST-SIP) that combines the surface-initiated atom transfer radical polymerization (SI-ATRP) of bifunctional ProDOT-based monomers and subsequent oxidative polymerization of the pendant ProDOT groups in the parent brushes. The brushes prepared in this way were characterized by grazing-angle FTIR, XPS spectroscopy, and AFM. Steady-state and time-resolved photoluminescence measurements were used to extract the information about the structure and effective conjugation length of PProDOT-based chains. Stability tests performed in ambient conditions and under exposure to standardized solar light revealed the remarkable stability of the obtained materials.

Fast-tracking of single emitters in large volumes with nanometer precision.

Multifocal plane microscopy allows for capturing images at different focal planes simultaneously. Using a proprietary prism which splits the emitted light into paths of different lengths, images at 8 different focal depths were obtained, covering a volume of 50x50x4 µm3. The position of single emitters was retrieved using a phasor-based approach across the different imaging planes, with better than 10 nm precision in the axial direction. We validated the accuracy of this approach by tracking fluorescent beads in 3D to calculate water viscosity. The fast acquisition rate (>100 fps) also enabled us to follow the capturing of 0.2 µm fluorescent beads into an optical trap.

Structural characterization of fibrous synthetic hydrogels using fluorescence microscopy.

The structural features of the matrix surrounding the cells play a crucial role in regulating their behavior. Here, we used fluorescence microscopy and customized analysis algorithms to characterize the architecture of fibrous hydrogel networks. As a model system, we investigated a new class of synthetic biomimetic material, hydrogels prepared from polyisocyanides. Our results show that these synthetic gels present a highly heterogeneous fibrous network, with pores reaching a few micrometers in diameter. By encapsulating HeLa cells in different hydrogels, we show that a more porous structure is linked to a higher proliferation rate. The approach described here, for the characterization of the network of fibrous hydrogels, can be easily applied to other polymer-based materials and provide new insights into the influence of structural features in cell behavior. This knowledge is crucial to develop the next generation of biomimetic materials for 3D cell models and tissue engineering applications.

Microscopic insight into non-radiative decay in perovskite semiconductors from temperature-dependent luminescence blinking.

Organo-metal halide perovskites are promising solution-processed semiconductors, however, they possess diverse and largely not understood non-radiative mechanisms. Here, we resolve contributions of individual non-radiative recombination centers (quenchers) in nanocrystals of methylammonium lead iodide by studying their photoluminescence blinking caused by random switching of quenchers between active and passive states. We propose a model to describe the observed reduction of blinking upon cooling and determine energetic barriers of 0.2 to 0.8 eV for enabling the switching process, which points to ion migration as the underlying mechanism. Moreover, due to the strong influence of individual quenchers, the crystals show very individually-shaped photoluminescence enhancement upon cooling, suggesting that the high variety of activation energies of the PL enhancement reported in literature is not related to intrinsic properties but rather to the defect chemistry. Stabilizing the fluctuating quenchers in their passive states thus appears to be a promising strategy for improving the material quality.

Light and oxygen induce chain scission of conjugated polymers in solution.

Conjugated polymers have been widely studied as flexible, versatile semiconductors in organic electronics. However, the material stability is one of the problems limiting their applications. Thus, understanding the degradation process of conjugated polymers is crucial. In this work, we monitored the chain scission of the model polymer MEH-PPV in chloroform solutions under different conditions by assessing its molecular weight using gel permeation chromatography and optical spectral measurements. We showed that changes in the UV-VIS spectrum can be seen only when the degradation has already progressed substantially. The fluorescence spectrum was found to be almost totally insensitive to the degradation stage of the polymers. We demonstrate that chain scission in solutions happens even in the dark leading to a 15% decrease of the molecular weight after just one day of storage. If exposed to room light, the chain length decreases by about 10 times over one day of exposure. Using stronger light intensity or enriching the solution with oxygen accelerates the degradation process dramatically. The rate of the reaction follows approximately a square root dependence with light intensity and oxygen concentration. We conclude that some extent of polymer degradation is difficult to avoid in common laboratory practices since to prevent it, one needs to work in an oxygen-free atmosphere in the dark. Preparation of polymer films from partially degraded solutions might lead not only to losing the connection between the molecular weight and the opto-electronic properties but also to unintentional doping of the semiconductor by products of chain scission reactions.