Future perspectives and recent advances in stimuli-responsive materials

Stimuli-responsive polymers have been attracting great interest within the scientific community for several decades. The unique feature to respond to small changes in the environmental conditions has made this class of materials very promising for several applications in the field of nanoscience, nanotechnology and nanomedicine. So far, several different chemical, physical or biochemical stimuli have been investigated within natural or synthetic polymers. Very interesting and appealing seems to be the combination of several stimuli to tune the properties of these materials in manifold ways. Within this present review, we want to highlight the recent progress in the field of synthetic stimuli-responsive polymers combining temperature and light responsiveness.

Temperature- and light-responsive smart polymer materials

A great deal of research has been carried out on lanthanide organic complex-based organic-inorganic hybrid materials in the last decade. This critical review begins with a formulation of the fundamentals of lanthanide organic complex luminescence, and presents various current designed ideas, synthetic routes, luminescence behaviors and potentials of the latest advanced (a) sol-gel materials, (b) mesoporous materials, (c) titania materials, (d) ionic liquids and ionogels, (e) polymers, and (f) bifunctional magnetic-optical composites based on lanthanide organic complexes. Finally, challenges and opportunities for further improvement of organic-inorganic hybrid luminescent materials based on lanthanide organic complexes will be discussed.

Hybrid materials based on lanthanide organic complexes: a review

Cells are integral to all forms of life due to their compartmentalization by the plasma membrane. However, living organisms are immensely complex. Thus there is a need for simplified and controllable models of life for a deeper understanding of fundamental biological processes and man-made applications. This is where the bottom-up approach of synthetic biology comes from: a stepwise assembly of biomimetic functionalities ultimately into a protocell. A fundamental feature of such an endeavor is the generation and control of model membranes such as liposomes and polymersomes. We compare and contrast liposomes and polymersomes for a better a priori choice and design of vesicles and try to understand the advantages and shortcomings associated with using one or the other in many different aspects (properties, synthesis, self-assembly, applications) and which aspects have been studied and developed with each type and update the current development in the field.

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Liposomes and polymersomes: a comparative review towards cell mimicking.

Photo-responsive systems and biomaterials: photochromic polymers, light-triggered self-assembly, surface modification, fluorescence modulation and beyond

Poly(N-isopropylacrylamide)-block-poly{6-[4-(4-methylphenyl-azo) phenoxy] hexylacrylate} (PNIPAM-b-PAzoM) was synthesized by successive reversible addition-fragmentation chain transfer (RAFT) polymerization. In H 2 O/THF mixture, amphiphilic PNIPAM-b-PAzoM self-assembles into giant micro-vesicles. Upon irradiation of light at 365 nm, fusion of the vesicles was observed directly under an optical microscope. The real-time fusion process is presented and the derivation is preliminarily due to the perturbation by the photoinduced trans-to-cis isomerization of azobenzene units in the vesicles.

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Photoinduced Fusion of Micro‐Vesicles Self‐Assembled from Azobenzene‐Containing Amphiphilic Diblock Copolymers

Extremely stable, high-density information storage media were prepared by the connection of two azobenzene groups via hydrogen bonding to form supramolecular bisazopolymers. The supramolecular bisazopolymers contain 4-((4-hydroxyphenyl)diazenyl)benzonitrile (AzoCN) as a hydrogen bonding donor and poly(6-(4-(pyridin-4-yldiazenyl)phenoxy)hexyl methacrylate) (pAzopy) as a hydrogen bonding acceptor. High quality films of the supramolecular bisazopolymers pAzopy/(AzoCN)x (x = 0.25, 0.5, 0.75, 1.0) with different molar ratios of donor/acceptor were prepared by spin-casting. The supramolecular bisazopolymers spontaneously form lamellar structures with a periodic thickness of 7.1 nm. These samples exhibit optically induced birefringence. The birefringence and proportion of remnant birefringence increase from 0.0265 to 0.1 and 50.5% to 108%, respectively, as the content of AzoCN in the samples increases, which indicates a larger proportion of AzoCN enhances both the birefringence and its stability. We also use an azopolymer without pyridine groups (pAzoCH3) and AzoCN to do a control experiment, which shows that pAzoCH3/(AzoCN)1.0 do not show significant enhanced birefringence and its stability. The enhancements in pAzopy/(AzoCN)x are because of the unique structure of the supramolecular bisazopolymers. A new laser direct writing system has been developed for optical recording on the azopolymers. The pAzopy/(AzoCN)1.0 film is significantly better at image recording than the pAzopy film, because the optically recorded images on the pAzopy/(AzoCN)1.0 film are very clear after storage for four months whereas the optically recorded images on the pAzopy one disappear after just one day. Four-dimensional optical recording has been achieved by integrating the polarization and the intensity of the laser and the two dimensions of a plane. An information density of about 0.93 Gbit cm−2 could be optically recorded on the pAzopy/(AzoCN)1.0 film, which is about 20 times the information density of a normal DVD.

Supramolecular bisazopolymers exhibiting enhanced photoinduced birefringence and enhanced stability of birefringence for four-dimensional optical recording

Poly(N-isopropylacrylamide)-block-poly{6-[4-(4-pyridyazo)phenoxy] hexylmethacrylate} (PNIPAM-b-PAzPy) was synthesized by successive reversible addition-fragmentation chain transfer (RAFT) polymerization. In a water/tetrahydrofuran (H 2 O/THF) mixture, amphiphilic PNIPAM-b-PAzPy self-assembles into giant micro-vesicles. Upon alternate ultraviolet (UV) and visible light irradiation, obvious reversible swelling-shrinking of the vesicles was observed directly under an optical microscope. The maximum percentage increase in volume, caused by the UV light, reached 17%. Moreover, the swelling could be adjusted using the UV light power density. The derivation of this effect is due to photoinduced reversible isomerization of azopyridine units in the vesicles.

Reversible Photocontrolled Swelling-Shrinking Behavior of Micron Vesicles Self-Assembled from Azopyridine-Containing Diblock Copolymer

Diblock copolymers composed of poly(acrylic acid) and poly{6-[4-(4-methylphenylazo)-phenoxy]hexyl acrylate} (PAA-block-PAzoM) were synthesized by RAFT polymerization. In a mixture of H 2 O and THF, PAA-block-PAzoM self-assembled into giant spherical microvesicles, which were dispersed in the mixture. Using UV-vis spectroscopy and optical microscopy, it is demonstrated that the spherical shape of the vesicles results from a framework composed of the azobenzene H-aggregate skeleton. A plausible structural model of the microvesicles is proposed, and a discussion on the formation of H-aggregates and the photoresponsive properties is presented.

Wei Su

Papers

Photoinduced Fusion of Micro‐Vesicles Self‐Assembled from Azobenzene‐Containing Amphiphilic Diblock Copolymers

Supramolecular bisazopolymers exhibiting enhanced photoinduced birefringence and enhanced stability of birefringence for four-dimensional optical recording

Reversible Photocontrolled Swelling-Shrinking Behavior of Micron Vesicles Self-Assembled from Azopyridine-Containing Diblock Copolymer

Formation and Photoresponsive Properties of Giant Microvesicles Assembled from Azobenzene‐Containing Amphiphilic Diblock Copolymers

Sphere to disk transformation of micro-particle composed of azobenzene-containing amphiphilic diblock copolymers under irradiation at 436 nm