Adsorption Kinetics of a Photosensitive Surfactant Inside Microgels

Selective Segregation of Thermo-Responsive Microgels via Microfluidic Technology

Separation of equally sized particles distinguished solely by material properties remains still a very challenging task. Here a simple separation of differently charged, thermo-responsive polymeric particles (for example microgels) but equal in size, via the combination of pressure-driven microfluidic flow and precise temperature control is proposed. The separation principle relies on forcing thermo-responsive microgels to undergo the volume phase transition during heating and therefore changing its size and correspondingly the change in drift along a pressure driven shear flow. Different thermo-responsive particle types such as different grades of ionizable groups inside the polymer matrix have different temperature regions of volume phase transition temperature (VPTT). This enables selective control of collapsed versus swollen microgels, and accordingly, this physical principle provides a simple method for fractioning a binary mixture with at least one thermo-responsive particle, which is achieved by elution times in the sense of particle chromatography. The concepts are visualized in experimental studies, with an intend to improve the purification strategy of the broad distribution of charged microgels into fractioning to more narrow distribution microgels distinguished solely by slight differences in net charge.

Versatile and Remotely Controllable Light-Induced Coagulation of Particles Under Flow in a 2D Channel

On-demand switch on/off blood clogging is of paramount importance for the survival of mammals, for example as a quick response to seal damage wounds to minimize their bleeding rate. This mechanism is a complex chain process from initiated red blood cell aggregation at the target location (open wound) that quickly seals on a macroscopic scale the damaged flash. Inspired by nature an on-demand switchable particle clogging mechanism is developed with high spatial resolution down to micrometer size using light as an external non-invasive stimulation. Particle clogging can be adjusted on demand strong enough to even withstand pressure-driven fluid flow, additionally building up walls of aggregated particles, which stop the momentum of big particles under shear. The principle relies on a photosensitive surfactant, which induces under light illumination a long-ranged lateral attractive phoretic-osmotic activity of silica microparticles forcing them to aggregate. The strength of aggregation and therefore motion reduction or even stop of the particles against the fluid flow depends on the ratio between the aggregation strength and the velocity of the particles. The aggregation strength can be precisely controlled by the applied light intensity and adjusted particle concentration. Increasing both parameters results in a stronger aggregation tendency.

Light-Induced Material Motion Fingerprint - A Tool Toward Selective Interfacial Sensitive Fractioning of Microparticles via Microfluidic Methods

In this article, a novel strategy is presented to selectively separate a mixture of equally sized microparticles but differences in material composition and surface properties. The principle relies on a photosensitive surfactant, which makes particles under light illumination phoretically active. The latter hovers microparticles from a planar interface and together with a superimposed fluid flow, particles experience a drift motion characteristic to its interfacial properties. The drift motion is investigated as a function of applied wavelength, demonstrating that particles composed of different material show a unique spectrally resolved light-induced motion profile. Differences in those motion profile allow a selective fractioning of a desired particle from a complex particle mixture made out of more than two equally sized different particle types. Besides that, the influence of applied wavelength is systematically studied, and discussed the origin of the spectrally resolved chemical activity of microparticles from measured photo-isomerization rates.

Compartmentalized Polyampholyte Microgels by Depletion Flocculation and Coacervation of Nanogels in Emulsion Droplets

In pH-responsive drug carriers, the distribution of charges has been proven to affect delivery efficiency but is difficult to control and verify. Herein, we fabricate polyampholyte nanogel-in-microgel colloids (NiM-C) and show that the arrangement of the nanogels (NG) can easily be manipulated by adapting synthesis conditions. Positively and negatively charged pH-responsive NG are synthesized by precipitation polymerization and labelled with different fluorescent dyes. The obtained NG are integrated into microgel (MG) networks by subsequent inverse emulsion polymerization in droplet-based microfluidics. By confocal laser scanning microscopy (CLSM), we verify that depending on NG concentration, pH value and ionic strength, NiM-C with different NG arrangements are obtained, including Janus-like phase-separation of NG, statistical distribution of NG, and core-shell arrangements. Our approach is a major step towards uptake and release of oppositely charged (drug) molecules.

Photosensitive Spherical Polymer Brushes: Light-Triggered Process of Particle Repulsion

We report on a light-triggered process at which repulsive interactions between microparticles with a polyelectrolyte (PE) brush coating can be remotely controlled. The spherical polyelectrolyte brushes are loaded with photosensitive azobenzene containing surfactant which can undergo reversible photo-isomerization from trans to cis state. The surfactant hydrophilicity is altered by illumination with light of an appropriate wavelength, at which a dynamic exchange of the more surface-active trans isomer in comparison to the more water soluble cis isomer with the PE brush generates a concentration gradient of the cis isomers near a solid surface where the particle is sedimented. In this way, each spherical brush produces its local lateral diffusioosmotic flow pointing outside in a radial direction resulting in mutual long-range repulsive interactions. We demonstrate that a PE layer has a higher tendency to absorb surfactant in comparison to plain silica particles, yielding a larger flow strength. This correlation holds true up to a critical intensity, where the dynamic exchange is adsorption limited with respect to trans isomers and especially pronounced for the PE-coated particles.

Light-induced destabilisation of oil-in-water emulsions using light-active bolaform surfactants

External stimuli-induced destabilisation of oil-in-water emulsions is of both fundamental and technological importance. In this work we synthesize light-active bolaform-type surfactants (LABSs) and show the preparation of decane-in-water emulsions over a range of surfactant and salt concentrations. Under ultraviolet (UV) illumination, LABSs undergo trans to cis isomerization affecting their interfacial activity. Therefore when stable emulsions stabilized by LABSs are exposed to UV light, they undergo partial destabilization. To induce interfacial flow, a small amount of volatile solvent (methanol, ethanol, tetrahydrofuran, etc.) is added at the emulsification stage and in this case complete phase separation is observed. This study demonstrates a facile route to induce destabilization of surfactant-stabilized emulsions using benign solvents and minimal use of energy (UV light) and this method could be of importance in wastewater treatment, enhanced oil recovery, protein separation, etc. where emulsion destabilization is desired.

Spiropyran/Merocyanine Amphiphile in Various Solvents: A Joint Experimental–Theoretical Approach to Photophysical Properties and Self-Assembly

This joint experimental-theoretical work focuses on molecular and photophysical properties of the spiropyran-containing amphiphilic molecule in organic and aqueous solutions. Being dissolved in tested organic solvents, the system demonstrates positive photochromism, i.e., upon UV stimulus the colorless spiropyran form is transformed into colorful merocyanine isomer. However, the aqueous solution of the amphiphile possesses a negative photochromism: the orange-red merocyanine form becomes thermodynamically more stable in water, and both UV and vis stimuli lead to the partial or complete photobleaching of the solution. The explanation of this phenomenon is given on the basis of density functional theory calculations and classical modeling including thermodynamic integration. The simulations reveal that stabilization of merocyanine in water proceeds with the energy of ca. 70 kJ mol−1, and that the Helmholtz free energy of hydration of merocyanine form is 100 kJ mol−1 lower as compared to the behavior of SP isomer in water. The explanation of such a difference lies in the molecular properties of the merocyanine: after ring-opening reaction this molecule transforms into a zwitterionic form, as evidenced by the electrostatic potential plotted around the opened form. The presence of three charged groups on the periphery of a flat conjugated backbone stimulates the self-assembly of merocyanine molecules in water, ending up with the formation of elongated associates with stack-like building blocks, as shown in molecular dynamics simulations of the aqueous solution with the concentration above critical micelle concentration. Our quantitative evaluation of the hydrophilicity switching in spiropyran/merocyanine containing surfactants may prompt the search for new systems, including colloidal and polymeric ones, aiming at remote tuning of their morphology, which could give new promising shapes and patterns for the needs of modern nanotechnology.

Generation of Local Diffusioosmotic Flow by Light Responsive Microgels

Here we show that microgels trapped at a solid wall can issue liquid flow and transport over distances several times larger than the particle size. The microgel consists of cross-linked poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-AA) polymer chains loaded with cationic azobenzene-containing surfactant, which can assume either a trans- or a cis-state depending on the wavelength of the applied irradiation. The microgel, being a selective absorber of trans-isomers, responds by changing its volume under irradiation with light of appropriate wavelength at which the cis-isomers of the surfactant molecules diffuse out of the particle interior. Together with the change in particle size, the expelled cis-isomers form an excess of the concentration and subsequent gradient in osmotic pressure generating a halo of local light-driven diffusioosmotic (l-LDDO) flow. The direction and the strength of the l-LDDO depends on the intensity and irradiation wavelength, as well as on the amount of surfactant absorbed by the microgel. The flow pattern around a microgel is directed radially outward and can be maintained quasi-indefinitely under exposure to blue light when the trans-/cis-ratio is 2/1, establishing a photostationary state. Irradiation with UV light, on the other hand, generates a radially transient flow pattern, which inverts from inward to outward over time at low intensities. By measuring the displacement of tracer particles around neutral microgels during a temperature-induced collapse, we can exclude that a change in particle shape itself causes the flow, i.e., just by expulsion or uptake of water. Ultimately, it is its ability to selectively absorb two isomers of photosensitive surfactant under different irradiation conditions that leads to an effective pumping caused by a self-induced diffusioosmotic flow.