Fast Light-Driven Motion of Polydopamine Nanomembranes.

The actuation of micro- and nanostructures controlled by external stimuli remains one of the exciting challenges in nanotechnology due to the wealth of fundamental questions and potential applications in energy harvesting, robotics, sensing, biomedicine, and tunable metamaterials. Photoactuation utilizes the conversion of light into motion through reversible chemical and physical processes and enables remote and spatiotemporal control of the actuation. Here, we report a fast light-to-motion conversion in few-nanometer thick bare polydopamine (PDA) membranes stimulated by visible light. Light-induced heating of PDA leads to desorption of water molecules and contraction of membranes in less than 140 µs. Switching off the light leads to a spontaneous expansion in less than 20 ms due to heat dissipation and water adsorption. Our findings demonstrate that pristine PDA membranes are multiresponsive materials that can be harnessed as robust building blocks for soft, micro-, and nanoscale actuators stimulated by light, temperature, and moisture level.

Temperature dependent FCS studies using a long working distance objective: Viscosities of supercooled liquids and particle size.

In this work, we describe new experimental setups for Fluorescence Correlation Spectroscopy (FCS) where a long working distance objective is used. Using these setups, FCS measurements in a broad temperature range for a small sample volume of about 50 µl can be performed. The use of specially designed cells and a dry long working distance objective was essential for avoiding temperature gradients in the sample. The performance of the new setups and a traditional FCS setup with immersion objectives is compared. The FCS data in combination with the Stokes-Einstein (SE) relation were used to obtain the values of the nanoviscosity of a fluid. We show for selected molecular van der Waals supercooled liquids that despite the fact that in these systems, a characteristic length scale can be defined, the nanoviscosity obtained from FCS is in a very good agreement with the macroscopic (rheometric) viscosity of the sample in a broad temperature range. This result corroborates the applicability of the SE relation to supercooled liquids at temperatures above 1.2 Tg. We also show that the temperature dependent size of thermoresponsive microgel particles can be determined by FCS using the designed cells and a long working distance objective in a broader size range without a need to use the correction procedure since the size correction is proportional to the square of the ratio of the hydrodynamic radius to the confocal volume size.

Simple way to analyze Brillouin spectra from turbid liquids.

The shape of the Brillouin light-scattering spectrum recorded from turbid liquids is distinctly distorted compared to that from a transparent sample. The reason for this is the multiple scattering of light within the medium. The usual expression for the Brillouin spectrum does not apply to the multiple scattering situation. In this Letter, we consider a Brillouin spectrum from opaque samples composed of a distribution of spectra resulting from elementary scattering events, each occurring in single scattering vector conditions. We introduce a one-parameter test function to define the probability distribution of scattering events occurring at a given value of the scattering vector. The proposed procedure was tested on model liquids that consisted of suspensions of sub-micrometer spherical particles of different size and concentration, dispersed in different carrier liquids and studied as a function of temperature. Our analysis made it possible to account for the effect of multiple scattering and to recover the values of mechanical parameters describing the pure solvents.

Fast Photoactuation and Environmental Response of Humidity-Sensitive pDAP-Silicon Nanocantilevers.

Multi-responsive nanomembranes are a new class of advanced materials that can be harnessed in complex architectures for micro and nano-manipulators, artificial muscles, energy harvesting, soft robotics, and sensors. The design and fabrication of responsive membranes must meet such challenges as trade-offs between responsiveness and mechanical durability, volumetric low-cost production ensuring low environmental impact, and compatibility with standard technologies or biological systems This work demonstrates the fabrication of multi-responsive, mechanically robust poly(1,3-diaminopropane) (pDAP) nanomembranes and their application in fast photoactuators. The pDAP films are developed using a plasma-assisted polymerization technique that offers large-scale production and versatility of potential industrial relevance. The pDAP layers exhibit high elasticity with the Young's modulus of ≈7 GPa and remarkable mechanical durability across 20-80 °C temperatures. Notably, pDAP membranes reveal immediate and reversible contraction triggered by light, rising temperature, or reducing relative humidity underpinned by a reversible water sorption mechanism. These features enable the fabrication of photoactuators composed of pDAP-coated Si nanocantilevers, demonstrating ms timescale response to light, tens of µm deflections, and robust performance up to kHz frequencies. These results advance fundamental research on multi-responsive nanomembranes and hold the potential to boost versatile applications in light-to-motion conversion and sensing toward the industrial level.

Multi-responsive poly-catecholamine nanomembranes.

The contraction of nanomaterials triggered by stimuli can be harnessed for micro- and nanoscale energy harvesting, sensing, and artificial muscles toward manipulation and directional motion. The search for these materials is dictated by optimizing several factors, such as stimulus type, conversion efficiency, kinetics and dynamics, mechanical strength, compatibility with other materials, production cost and environmental impact. Here, we report the results of studies on bio-inspired nanomembranes made of poly-catecholamines such as polydopamine, polynorepinephrine, and polydextrodopa. Our findings reveal robust mechanical features and remarkable multi-responsive properties of these materials. In particular, their immediate contraction can be triggered globally by atmospheric moisture reduction and temperature rise and locally by laser or white light irradiation. For each scenario, the process is fully reversible, i.e., membranes spontaneously expand upon removing the stimulus. Our results unveil the universal multi-responsive nature of the considered polycatecholamine membranes, albeit with distinct differences in their mechanical features and response times to light stimulus. We attribute the light-triggered contraction to photothermal heating, leading to water desorption and subsequent contraction of the membranes. The combination of multi-responsiveness, mechanical robustness, remote control via light, low-cost and large-scale fabrication, biocompatibility, and low-environment impact makes polycatecholamine materials promising candidates for advancing technologies.

Volume crossover in deeply supercooled water adiabatically freezing under isobaric conditions.

The irreversible return of a supercooled liquid to stable thermodynamic equilibrium often begins as a fast process which adiabatically drives the system to solid-liquid coexistence. Only at a later stage will solidification proceed with the expected exchange of thermal energy with the external bath. In this paper we discuss some aspects of the adiabatic freezing of metastable water at constant pressure. In particular, we investigated the thermal behavior of the isobaric gap between the molar volume of supercooled water and that of the warmer ice-water mixture which eventually forms at equilibrium. The available experimental data at ambient pressure, extrapolated into the metastable region within the scheme provided by the reference IAPWS-95 formulation, show that water ordinarily expands upon (partially) freezing under isenthalpic conditions. However, the same scheme also suggests that, for increasing undercoolings, the volume gap is gradually reduced and eventually vanishes at a temperature close to the currently estimated homogeneous ice nucleation temperature. This behavior is contrasted with that of substances which do not display a volumetric anomaly. The effect of increasing pressures on the alleged volume crossover from an expanded to a contracted ice-water mixture is also discussed.

Fano meets Stokes: Four-order-of-magnitude enhancement of asymmetric Brillouin light scattering spectra.

Observation of Fano resonances in various physical phenomena is usually ascribed to the coupling of discrete states with background continuum, as it has already been reported for various physical phenomena. Here, we report on Fano lineshapes of nonthermal GHz phonons generated and observed with pumped Brillouin light scattering in gold-silicon thin membranes, overlapping the broad zero-shift background of yet questionable origin. The system's broken mid-plane symmetry enabled the generation of coherent quasi-symmetric and quasi-antisymmetric Lamb acoustic waves/phonons, leading to the four orders-of-magnitude enhancement of Brillouin light scattering. Notably, the membrane asymmetry resulted also in the mode-dependent Stokes and anti-Stokes Fano lineshapes asymmetry.

Size-dependent nanoscale soldering of polystyrene colloidal crystals by supercritical fluids.

HYPOTHESIS: Polymer particles self-assembled into colloidal crystals have exciting applications in photonics, phononics, templates for nanolithography, and coatings. Cold soldering utilizing polymer plasticization by supercritical fluids enables a novel, low-cost, low-effort, chemical-free means for uniform mechanical strengthening of fragile polymer colloidal crystals at moderate temperatures. Here, we aim to elucidate the role of particle size and gas-specific response for the most efficient soldering, exploring the full potential of this method. EXPERIMENTS: We investigate the elastic properties of polystyrene colloidal crystals made of nanoparticles with different diameters (143 to 830 nm) upon treatment with supercritical Ar and He at room temperature. By employing Brillouin light scattering, we quantify the effect of nanoparticle size on the strengthening of interparticle contacts, evaluating the permanent change in the effective elastic modulus upon cold soldering. FINDINGS: The relative change in the effective elastic modulus reveals nonmonotonic dependence on the particle size with the most efficient soldering for mid-sized nanoparticles (about 610 nm diameter). We attribute this behavior to the crucial role of intrinsic fabrication impurities, which reduces the nanoparticles' free surface exposed to plasticization by supercritical fluids. Supercritical Ar, a good solvent for polystyrene, enabled effective soldering of nanoparticles, whereas high-pressure He treatment is entirely reversible.

Control of Intermolecular Interactions toward the Production of Free-Standing Interfacial Polydopamine Films.

Aggregation of the polydopamine (PDA) molecular building blocks at the air/water interface leads to obtaining large surface nanometric-thin films. This mechanism follows two possible pathways, namely, covalent or non-covalent self-assembly, which result in a different degree of structure order and, consequently, different structural properties. Control of this mechanism could be vital for applications that require true self-support PDA free-standing films, for example, electrochemical sensing or membrane technology. Here, we are considering the impact of boric acid (BA) and Cu2+ ions on the mentioned mechanism exclusively for the free-standing films from the air/water interface. We have employed and refined our own spectroscopic reflectometry method to achieve an exceptionally high real-time control over the thickness growth. It turned out that BA and Cu2+ ions significantly impact the film growth process. Reduction of the nanoparticles size and their number was examined via UV-vis spectroscopy and transmission electron microscopy, showing a colossal reduction in the mean diameter of nanoparticles in the case of BA and a moderate reduction in the case of Cu2+. This modification is leading to significant enhancement of the process efficiency through moderation of the topological properties of the films, as revealed by atomic force microscopy. Next, applying infrared, Raman, and X-ray photoelectron spectroscopy, we presented small amounts of metal (B or Cu) in the final structure of PDA and simultaneously their vital role in the oxidation mechanism and cross-linking through covalent or non-covalent bonds. Therefore, we revealed the possibility of synthesizing films via the expected self-assembly mechanism which has hitherto been out of control. Moreover, modification of mechanical properties toward exceptionally elastic films through the BA-assisted synthesis pathway was shown by achieving Young's modulus value up to 24.1 ± 5.6 and 18.3 ± 6.4 GPa, using nanoindentation and Brillouin light scattering, respectively.

Emerging dynamics in surfactant-based liquid mixtures: octanoic acid/bis(2-ethylhexyl) amine systems.

This work focuses on the dynamic phenomena emerging in self-assembled transient intermolecular networks formed when two different surfactants are mixed. In particular, the relaxation processes in liquid mixtures composed by bis(2-ethylhexyl)amine (BEEA) and octanoic acid (OA) in the whole composition range has been investigated by dielectric spectroscopy and Brillouin spectroscopy. A thorough analysis of all the experimental data consistently suggests that, mainly driven by acid-base interactions arising when the two surfactants are mixed, supra-molecular aggregates formation causes the slowing down of molecular dynamics. This, in turn, reflects to longer-range relaxations. These changes have been found to be composition-dependent, involving strong departures of the mixture physico-chemical properties from an ideal behaviour, and reflecting the structural and dynamical features of local structures. In particular, the peculiar dynamic processes occurring in these local inter-molecular structures, have been found to be the factors responsible for the observed and quite surprising increase of direct-current conductivity which occurs when two different (and pretty non-conductive) surfactants are mixed. The discovery can be used not only to design novel materials for application purposes but also to shed more light on the basic principles regulating charge migration in structured liquid systems.

2D Short-Time Fourier Transform for local morphological analysis of meibomian gland images.

Meibography is becoming an integral part of dry eye diagnosis. Being objective and repeatable this imaging technique is used to guide treatment decisions and determine the disease status. Especially desirable is the possibility of automatic (or semi-automatic) analysis of a meibomian image for quantification of a particular gland's feature. Recent reports suggest that in addition to the measure of gland atrophy (quantified by the well-established "drop-out area" parameter), the gland's morphological changes may carry equally clinically useful information. Here we demonstrate the novel image analysis method providing detailed information on local deformation of meibomian gland pattern. The developed approach extracts from every Meibomian image a set of six morphometric color-coded maps, each visualizing spatial behavior of different morphometric parameter. A more detailed analysis of those maps was used to perform automatic classification of Meibomian glands images. The method for isolating individual morphometric components from the original meibomian image can be helpful in the diagnostic process. It may help clinicians to see in which part of the eyelid the disturbance is taking place and also to quantify it with a numerical value providing essential insight into Meibomian gland dysfunction pathophysiology.

2D fourier transform for global analysis and classification of meibomian gland images.

In recent years, significant progress has been made in the Meibography technique resulting from the use of advanced image analysis methods allowing a quantitative description of the Meibomian gland structures. Many objective measures of gland distortion were previously proposed allowing for user-independent classification of acquired gland images. However, due to the complicated nature of gland deformation, none of the single-valued parameters can fully describe the analyzed gland images. There is a need to increase the number of descriptive factors, selectively sensitive to different gland features. Here we show that global 2D Fourier transform analysis of infra-red gland images provides values of two new such parameters: mean gland frequency and anisotropy in gland periodicity. We show that their values correlate with gland dysfunction and can be used to automatically categorize the images into the three subjective classes (healthy, intermediate and unhealthy). We also demonstrated that classification performance can be improved by dimensionality reduction approach using principal component analysis.

Mechanical reinforcement of polymer colloidal crystals by supercritical fluids.

Colloidal crystals realized by self-assembled polymer nanoparticles have prominent attraction as a platform for various applications from assembling photonic and phononic crystals, acoustic metamaterials to coating applications. However, the fragility of these systems limits their application horizon. In this work the uniform mechanical reinforcement and tunability of 3D polystyrene colloidal crystals by means of cold soldering are reported. This structural strengthening is achieved by high pressure gas (N2 or Ar) plasticization at temperatures well below the glass transition. Brillouin light scattering is employed to monitor in-situ the mechanical vibrations of the crystal and thereby determine preferential pressure, temperature and time ranges for soldering, i.e. formation of physical bonding among the nanoparticles while maintaining the shape and translational order. This low-cost method is potentially useful for fabrication and tuning of durable devices including applications in photonics, phononics, acoustic metamaterials, optomechanics, surface coatings and nanolithography.

Structural relaxation in the wave-vector dependence of the longitudinal rigidity modulus.

Brillouin microscopy recently attracted much attention for being a promising tool for all-optical label-free determination of mechanical properties of biological samples. Before its widespread utilization for biomedical applications, numbers of nuances related with this technique need to be recognized. In this article, we discuss the process of structural relaxation, the phenomena not commonly addressed by the emerging bio-Brillouin community, and its effect on longitudinal rigidity modulus. Using a model aqueous polymer mixture, we show how scattering measurements performed on the same specimen using different experimental geometries can lead to different results.

Morphology and Magnetic Structure of the Ferritin Core during Iron Loading and Release by Magnetooptical and NMR Methods.

Ferritins are proteins, which serve as a storage and transportation capsule for iron inside living organisms. Continuously charging the proteins with iron and releasing it from the ferritin is necessary to assure proper management of these important ions within the organism. On the other hand, synthetic ferritins have great potential for biomedical and technological applications. In this work, the behavior of ferritin during the processes of iron loading and release was examined using multiplicity of the experimental technique. The quality of the protein's shell was monitored using circular dichroism, whereas the average size and its distribution were estimated from dynamic light scattering and transmission electron microscopy images, respectively. Because of the magnetic behavior of the iron mineral, a number of magnetooptical methods were used to gain information on the iron core of the ferritin. Faraday rotation and magnetic linear birefringence studies provide evidence that the iron loading and the iron-release processes are not symmetrical. The spatial organization of the mineral within the protein's core changes depending on whether the iron was incorporated into or removed from the ferritin's shell. Magnetic optical rotatory dispersion spectra exclude the contribution of the Fe(II)-composed mineral, whereas joined magnetooptical and nuclear magnetic resonance results indicate that no mineral with high magnetization appear at any stage of the loading/release process. These findings suggest that the iron core of loaded/released ferritin consists of single-phase, that is, ferrihydrite. The presented results demonstrate the usefulness of emerging magnetooptical methods in biomedical research and applications.

Magnetic properties of ferritin and akaganeite nanoparticles in aqueous suspension.

We have studied the magnetically induced optical birefringence Δn of horse spleen ferritin (HSF) and aqueous suspensions of several different-sized iron oxyhydroxide nanoparticles coated with different polysaccharides mimicking ferritin. The structure and dimensions of the akaganeite mineral core were characterized by XRD and TEM, respectively. The stability of the suspensions in the measurement temperature range from 278 to 358 K was confirmed by UV-Vis absorption spectroscopy. The values of optical polarizability anisotropy Δα, magnetic susceptibility anisotropy Δχ, and permanent magnetic dipole moment µm of the akaganeite nanoparticles have been estimated on the basis of the temperature dependence of the Cotton-Mouton (C-M) constant. The magnetic birefringence of Fe-sucrose has been described tentatively by different types of Langevin function allowing another estimation of Δχ and µm. The obtained permanent magnetic dipole moment µm of the studied akaganeite nanoparticles proves small and comparable to that of HSF. The value of µm is found to increase with decreasing nanoparticle diameter. Observed in a range spanning more than five orders of magnitude, the linear relation between the C-M constant and the iron concentration provides a basis for possible analytical application of the C-M effect in biomedicine. The established relation between the C-M constant and the nanoparticle diameter confirms that the dominant contribution to the measured magnetic birefringence comes from the magnetic susceptibility anisotropy Δχ. A comparison of the C-M constants of the studied akaganeite nanoparticles with the data obtained for HSF provides evidence that the ferritin core behaves as a non-Euclidian solid.

Brillouin scattering study of polyethylene glycol/water system below crystallization temperature.

We present the results of a Brillouin scattering experiment on aqueous mixtures of polyethylene glycol polymer cooled below their crystallization temperature. The shape of the registered spectrum changed considerably when the phase transition occurs. The Brillouin profile consisted of two peaks which has been interpreted as a result of the scattering of light on the local regions exhibiting different compressibility. Examination of the temperature dependence of the two Brillouin peak positions allows the identification of the specific microstructures present in the studied system. The existence of a complexlike hydration structure, stable against dilution and temperature agitation, was suggested. The nature and the behavior of the hydration complex is discussed in view of the earlier investigations in similar systems.