UV causes dramatic changes in aggregation with mixtures of photoactive and inert surfactants

Light induced micelle to vesicle transition in an aqueous solution of a surface active ionic liquid

A new simple surface active ionic liquid displayed reversible micelle–vesicle transition under alternative UV/vis irradiation without additives.

Photo-actuation of liquids for light-driven microfluidics: state of the art and perspectives

Using light to control liquid motion is a new paradigm for the actuation of microfluidic systems. We review here the different principles and strategies to induce or control liquid motion using light, which includes the use of radiation pressure, optical tweezers, light-induced wettability gradients, the thermocapillary effect, photosensitive surfactants, the chromocapillary effect, optoelectrowetting, photocontrolled electroosmotic flows and optical dielectrophoresis. We analyze the performance of these approaches to control using light many kinds of microfluidic operations involving discrete pL- to μL-sized droplets (generation, driving, mixing, reaction, sorting) or fluid flows in microchannels (valve operation, injection, pumping, flow rate control). We show that a complete toolbox is now available to control microfluidic systems by light. We finally discuss the perspectives of digital optofluidics as well as microfluidics based on all optical fluidic chips and optically reconfigurable devices.

Spatial and temporal control of surfactant systems

This paper reviews some recent progress on approaches leading to spatial and temporal control of surfactant systems. The approaches revolve around the use of redox-active and light-sensitive surfactants. Perspectives are presented on experiments that have realized approaches for active control of interfacial properties of aqueous surfactant systems, reversible control of microstructures and nanostructures formed within bulk solutions, and in situ manipulation of the interactions of surfactants with polymers, DNA and proteins. A particular focus of this review is devoted to studies of amphiphiles that contain the redox-active group ferrocene - reversible control of the oxidation state of ferrocene leads to changes in the charge/hydrophobicity of these amphiphiles, resulting in substantial changes in their self-assembly. Light-sensitive surfactants containing azobenzene, which undergo changes in shape/polarity upon illumination with light, are a second focus of this review. Examples of both redox-active and light-sensitive surfactants that lead to large (>20mN/m) and spatially localized ( approximately mm) changes in surface tensions on a time scale of seconds are presented. Systems that permit reversible transformations of bulk solution nanostructures - such as micelle-to-vesicle transitions or monomer-to-micelle transitions - are also described. The broad potential utility of these emerging classes of amphiphiles are illustrated by the ability to drive changes in functional properties of surfactant systems, such as rheological properties and reversible solubilization of oils, as well as the ability to control interactions of surfactants with biomolecules to modulate their transport into cells.

Formation and stability of nanoemulsions with mixed ionic-nonionic surfactants

A simple, low-energy two-step dilution process has been applied with binary mixtures of ionic-nonionic surfactants to prepare nanoemulsions. The systems consist of water/DDAB-C(12)E(5)/decane. Nanoemulsions were obtained by dilution of concentrates located in bicontinuous microemulsion or lamellar liquid crystal phase regions. The nanoemulsions generated were investigated both by contrast-variation small-angle neutron scattering (SANS) and dynamic light scattering (DLS). The SANS profiles show that C(12)E(5) nanodroplets suffer essentially no structural change on incorporation of the cationic DDAB surfactant, except for increased electrostatic repulsive interactions. Interestingly, SANS indicated that the preferred droplet sizes were hardly affected by the surfactant mixture composition (up to a DDAB molar ratio (m(DDAB)/(m(DDAB) + m(C(12)E(5))) of 0.40) and droplet volume fraction, phi, between 0.006 and 0.120. No notable changes in the structure or radius of nanoemulsion droplets were observed by SANS over the test period of 1 d, although the droplet number intensity decreased significantly in systems stabilized by C(12)E(5) only. However, the DLS sizing shows a marked increase with time, with higher droplet volume fractions giving rise to the largest changes. The discrepancy between apparent nanoemulsion droplet size determined by DLS and SANS data can be attributed to long-range droplet interactions occurring outside of the SANS sensitivity range. The combined SANS and DLS results suggest flocculation is the main mechanism of instability for these nanoemulsions. The flocculation rate is shown to be significantly retarded by addition of the charged DDAB, which may be due to enhanced electrostatic repulsive forces between droplets, leading to improved stability of the nanoemulsions.

Low energy methods of phase separation in colloidal dispersions and microemulsions

The majority of work on phase separation of colloidal systems has been concerned with the energy intensive approaches such as ultracentrifugation, solvent evaporation, changes of temperature and pressure etc. However, in modern nanotechnology it is desirable to minimize environmental impact in order to achieve separation and recovery of colloidal products. In this review recent research on phase separation methods, requiring relatively lower energy consumption are summarized. These include polymer-, solvent- and photo-induced approaches to phase separation.

Photo-assisted gene delivery using light-responsive catanionic vesicles

Photoresponsive catanionic vesicles have been developed as a novel gene delivery vector combining enhanced cellular uptake with phototriggered release of vesicle payload following entry into cells. Vesicles with diameters ranging from 50 to 200 nm [measured using cryo-transmission electron microscopy (TEM) and light-scattering techniques] form spontaneously, following mixing of positively charged azobenzene-containing surfactant and negatively charged alkyl surfactant species. Fluorescent probe measurements showed that the catanionic vesicles at a cation/anion ratio of 7:3 formed at surfactant concentrations as low as 10 microM of the azobenzene surfactant under visible light (with the azobenzene surfactant species principally in the trans configuration), while 50-60 microM of the azobenzene surfactant is required to form vesicles under UV illumination (with the azobenzene surfactant species principally in the cis configuration). At intermediate surfactant concentrations (ca. 15-45 microM) under visible light conditions, transport of DNA-vesicle complexes occurred past the cell membrane of murine fibroblast NIH 3T3 cells through endocytosis. Subsequent UV illumination induced rupture of the vesicles and release of uncomplexed DNA into the cell interiors, where it was capable of passing through the nuclear membrane and thereby contributing to enhanced expression. Single-molecule fluorescent images of T4-DNA demonstrated that the formation of vesicles with a net positive charge led to compaction of DNA molecules via complex formation within a few seconds, while UV-induced disruption of the vesicle-DNA complexes led to DNA re-expansion to the elongated-coil state, also within a few seconds. Transfection experiments with eGFP DNA revealed that photoresponsive catanionic vesicles are more effectively taken up by cells compared to otherwise identical alkyl (i.e., nonazobenzene-containing and thus nonlight-responsive) catanionic vesicles, presumably because of pi-pi stacking interactions that enhance bilayer rigidity in the photoresponsive vesicles. Subsequent UV illumination following endocytosis leads to further dramatic enhancements in the transfection efficiencies, demonstrating that vector unpacking and release of DNA from the carrier complex can be the limiting step in the overall process of gene delivery.

Nanoemulsions prepared by a two-step low-energy process

A simple low-energy two-step dilution process has been applied in oil/surfactant/water systems with pentaoxyethylene lauryl ether (C12E5), dodecyldimethylammonium bromide, sodium bis(2-ethylhexyl)sulfosuccinate, sodium n-dodecyl sulfate-pentanol, and hexadecyltrimethylammonium bromide-pentanol. Appropriate formulations were chosen for the concentrate to be diluted with water to generate oil-in-water (O/W) emulsions or nanoemulsions. For the system of decane/C12E5/water, bluish, transparent nanoemulsions having droplet radii of the order of 15 nm were formed, only when the initial concentrate was a bicontinuous microemulsion, whereas opaque emulsions were generated if the concentrate began in an emulsion-phase region. Nanoemulsions generated in the system decane/C12E5/water have been investigated both by dynamic light scattering (DLS) and contrast-variation small-angle neutron scattering (SANS). The SANS profiles show that nanodroplets exist as spherical core-shell (decane-C12E5) particles, which suffer essentially no structural change on dilution with water, at least for volume fractions phi down to 0.060. These results suggest that the nanoemulsion droplet structure is mainly controlled by the phase behavior of the initial concentrate and is largely independent of dilution. A discrepancy between apparent nanoemulsion droplet sizes was observed by comparing DLS and SANS data, which is consistent with long-range droplet interactions occurring outside of the SANS sensitivity range. These combined phase behavior, SANS, and DLS results suggest a different reason for the stability/instability of nanoemulsions compared with earlier studies, and here it is proposed that a general mechanism for nanoemulsion formation is homogeneous nucleation of oil droplets during the emulsification.

Surface and micelle properties of novel multi-dentate surfactants

Novel multi-dentate surfactants, based on alkyl amines of varying hydrophobicity were synthesized, and molecular structures were characterized by IR, UV-vis, NMR and FAB-MS. The new surfactants have good water solubility and are highly efficient at reducing aqueous surface tension. Small-angle neutron scattering (SANS) studies were carried out with aqueous solutions in D(2)O to study aggregation. Spherical micelles were shown to form, and these grow with increasing alkyl chain length; their conformation is unusual compared to conventional linear chain surfactants.

Photodestructible vesicles

Stable vesicles are formed in a 1.4 wt % solution comprising a 1:3 mixture of the anionic photodestructible surfactant sodium 4-hexylphenylazosulfonate (C6PAS) and inert cationic cetyltrimethylamonium bromide (CTAB). UV irradiation drives an irreversible breakdown of the mixed C6PAS/CATB vesicles, owing to the selective degradation of C6PAS. A light-induced transition from small spherical-like polydisperse vesicles to long needle-like aggregation is observed.

Photosensitive gelatin

Employing photodestructible surfactants in gelatin-based aqueous gels presents novel possibilities for controlling colloidal and aggregation properties of surfactant gelatin complexes. Light-triggered breakdown of the gelatin-bound photosurfactant aggregates causes dramatic changes in viscosity and aggregation.

Self-assembly of light-sensitive surfactants

This review covers recent advances with an intriguing class of functionalised light-sensitive surfactants. The main chemical classes are described, and the photo-responses in interfacial and aggregation systems are discussed.

Reversible photoswitching in a cell-sized vesicle

A photosensitive amphiphilic molecule can switch the shape of an assembled vesicle as determined by microscopic observation. Photoisomerization induces a change in membrane fluctuation behavior or a morphological transition between ellipsoid and bud shapes, depending on the asymmetrical degree of the initial shape. The mechanism of this reversible photoswitching in the vesicle morphology is interpreted in terms of a change in the effective cross-sectional area of the photosensitive molecule.

Photo-stabilised microemulsions

Light-induced stabilisation of water-in-heptane microemulsions has been achieved with a UV-sensitive gemini photo-surfactant.