Synthesis and spectral investigations of a new dyad with spiropyran and fluorescein units: toward information processing at the single molecular level

A new dyad 1 with two spiropyran units as the photochromic acceptors and one fluorescein unit as the fluorescent donor was synthesized and characterized. External inputs (ultraviolet light, visible light, and proton) induce the reversible changes of the structure and, concomitantly, the absorption spectrum of dyad 1 due to the presence of two spiropyran units. Only the absorption spectrum of the ME form of the spiropyran units in dyad 1 has large spectral overlap with the fluorescence spectrum of the fluorescein unit. Thus, the fluorescence intensity of dyad 1 is modulated by reversible conversion among the three states of the photochromic spiropyran units and the fluorescence resonance energy transfer (FRET) between the ME form and the fluorescein unit. Based on the fact that dyad 1 could "read out" three external input signals (ultraviolet light, visible ligh,t and proton) and "write" a compatible specific output signal (fluorescence intensity), dyad 1 described here can be considered to perform an integrated circuit function with one OR and one AND interconnected logic gates. The present results demonstrate an efficient strategy for elaborating and transmitting information at the single molecular level.     

A photochromic acceptor as a reversible light-driven switch in fluorescence resonance energy transfer (FRET)

A method has been developed for the quantitative determination of fluorescence resonance energy transfer (FRET) based on the modulation of donor fluorescence upon the reversible photoconversion of a photochromic acceptor. A model system was devised, consisting of Lucifer Yellow cadaverine (LYC, donor) conjugated to the photochromic molecule, 6-nitroBIPS (1′,3′-dihydro-1′-(2-carboxyethyl)-3′,3′-dimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indoline]). Near-ultraviolet irradiation catalyzes the conversion of the colorless spiropyran (SP) to the colored merocyanine (MC) form of 6-nitroBIPS. Only the MC form absorbs at the emission wavelengths of the donor, thereby potentiating FRET, as demonstrated by quenching of the donor. Subsequent irradiation in the visible MC absorption band reverts 6-nitroBIPS to the SP form and FRET is inactivated. The acceptor exhibited high photostability under repeated cycles of alternating UV–Vis irradiation. In this model system, the intramolecular FRET efficiency was close to 100%. The observed maximal donor quenching of 34±3% was indicative of an equilibrium determined by the high quantum efficiency of forward conversion (SP→MC) induced by near-UV irradiation and a low but finite quantum efficiency of the back reaction resulting from excitation of the MC form directly as well as indirectly (by FRET via the donor). A quantitative formalism for the photokinetic scheme was developed. Photochromic FRET (pcFRET) permits repeated, quantitative, and non-destructive FRET determinations for arbitrary relative concentrations of donor and acceptor and thus offers great potential for monitoring dynamic molecular interactions in living cells over extended observation times by fluorescence microscopy.     

Micelles and reverse micelles with a photo and thermo double‐responsive block copolymer

A novel photo and thermo double‐responsive block copolymer was developed to fabricate micelles and reverse micelles in aqueous solution. The block copolymer was synthesized by ATRP block copolymerization of a spiropyran‐ containing methacrylate (SPMA) with di(ethylene glycol) methyl ether methacrylate (DEGMMA). By facile control of the photo irradiation and solution temperature, PSPMA‐core and PDEGMMA‐core micelles can be obtained, respectively. The thermo‐ and photo‐responsive micelles were used as smart polymeric nanocarriers for controlled encapsulation, triggered release, and re‐encapsulation of model drug coumarin 102. The double‐responsive self‐assembly and disassembly were tracked by dynamic light scattering, transmission electron microscopy, and fluorescence spectroscopy. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2855–2861, 2010     

Recent development in near infrared light-responsive polymeric materials for smart drug-delivery systems

Stimuli-responsive drug delivery systems (DDS) may overcome the drawbacks of conventional chemotherapy for cancer treatment. In particular, light-responsive polymer-based DDS may ensure spatio and temporal control in drug delivery. In this regard, near infrared (NIR) light triggered drug nanocarriers present several advantages when compared to UV–visible light triggered nanocarriers. This review surveys the recent development on the design, synthesis, functions, and applications of NIR photo-sensitive compounds in the development of long-wavelength light-responsive nanocarriers. Diverse NIR light responsive groups such as coumarin (CM), ortho-nitrobenzyl (ONB), 2-diazo-1,2-naphthoquinone (DNQ) and spiropyran (SP) derivatives and their photo-cleavage reaction mechanisms are discussed, as well as the use of indocyanine green (ICG) and its photo-thermal application. The loading into polymeric nanocarriers of up converting nanoparticles (UCNPs) which can convert NIR light into UV or visible light is also discussed. The described DDS are classified on the basis on the photo responsive groups. In details, the behavior of different polymeric materials such as micelles, hydrogels bearing photo responsive groups linked to bioactive molecules which are released under NIR light irradiation is reviewed and discussed. A section relative to commonly used instrument setup for drug release studies by NIR light irradiation is also presented for better understanding how the light has been used to irradiate in various experimental situations.     

AIE Ligand Constructed Zn(Ⅱ)Complex with Reversible Photo-induced Color and Emission Changes

Fluoran salicylaldehyde hydrazone metal complex(FSHMC)is a kind of recently reported photo-responsive system,which has the advantages of simple synthesis,multiple colors as well as distinct color change before and after UV light irradiation.However,the emission property of FSHMC is relatively unitary.In solid state,especially,only fluorescence quench is induced after UV light irradiation,which limits their applications.In this work,a typical aggregation-induced emission(AIE)moiety of tetraphenylethene(TPE)was introduced to the design of FSHMC.The obtained FSHMC,2-Zn,exhibited reversible color and fluorescence changes upon UV light irradiation.Due to the AIE feature of compound 2,2-Zn exhibited different emission changes upon UV light irradiation in THF and in solid matrix,because of the fluorescence resonance energy transfer(FRET)process from TPE moiety to rhodamine B moiety.     

Spiropyran‐Conjugated Pluronic as a Dual Responsive Colorimetric Detector

Novel spiropyran‐conjugated Pluronic [polyethylene oxide (PEO)‐b‐polypropylene oxide (PPO)‐b‐polyethylene oxide (PEO)] micelles are developed as a new colorimetric detector showing photo‐ or thermo‐switchable behavior. Facile conjugation of spiropyran to Pluronic was confirmed by ¹H NMR, UV–Vis, and Fluorescence spectroscopy. A switchable photoluminescence is found depending on the irradiation with either UV or visible light, and temperature resulting from structural isomerization of spiropyran between spiropyran (SP) and merocyanine (MC) form. Cytotoxicity of the spiropyran‐conjugated Pluronic (SP‐PL) was evaluated following an MTT assay, whereas photo responsiveness of spiropyran within the micelles was determined by confocal laser scanning microscopy.     

Photo-, thermally, and pH-responsive microgels

Microgels with photo-, thermally, and pH-responsive properties in aqueous suspension have been synthesized and characterized using dynamic light scattering and UV-visible spectroscopy. The new route involved first preparing poly(N-isopropylacrylamide) (PNIPAM)-allylamine copolymer microgels and a spiropyran photochrome (SP) bearing a carboxylic acid group. Then the functionalized spiropyran was coupled to the microgel via an amide bond. The dark-equilibrated gel particles feature spiropyran molecules in the polar, merocyanine form. After irradiation of visible light, the particle size becomes smaller because spiropyran changes to the relatively nonpolar, closed spiro form. The PNIPAM-SP microgels undergo a volume phase transition in water from a swollen state to a collapsed state with increasing temperature under all light conditions. However, the transition temperature range of the PNIPAM-SP is much broader than that for the PNIPAM without SP. The PNIPAM-SP microgels are monodisperse and self-assemble into a crystalline lattice while in suspension. The UV-visible spectra of an aqueous suspension of PNIPAM-SP microgel in the dark-adapted, merocyanine form showed both an absorption peak around 512 nm due to the merocyanine (giving a reddish color to the suspension) and two sharp peaks from Bragg diffraction of colloidal crystallites. Upon visible irradiation, the 512-nm band bleached significantly due to spiropyran photoisomerization. The spiropyran photoisomerization and accompanying color changes of the suspension were reversible upon alternating dark, UV, and visible light irradiation. Due to the residues of amine groups, the swelling capability of PNIPAM-SP microgels reduces as the pH value is changed from 7 to 10.     

Rational design of light-controllable polymer micelles

Amphiphilic block copolymer (BCP) micelles are nanocarriers that hold promise for controlled delivery applications. This account highlights our recent works on light-dissociable BCP micelles. We have designed and developed light-responsive amphiphilic BCPs whose micellar aggregates (core-shell micelles and vesicles) can be disrupted by light exposure. The basic strategy is to incorporate a chromophore into the structure of the hydrophobic block, whose photoreaction can result in a conformational or structural change that shifts the hydrophilic/hydrophobic balance toward the destabilization of the micelles. Using various chromophores including azobenzene, pyrene and nitrobenzene, we have achieved both reversible and irreversible dissociation of BCP micelles upon illumination with UV/visible or near infrared light. The demonstrated rational design principle based on light-changeable or light-switchable amphiphilicity is general and can be applied to many polymer/chromophore combinations. This opens the door to developing photocontrollable polymer nanocarriers offering control over when and where the release of loaded agents takes place.     

Polymer micelles as building blocks for the incorporation of azobenzene: enhancing the photochromic properties in layer-by-layer films

We described the use of block copolymer micelles as building blocks for the incorporation of water-insoluble photochromic species of azobenzene and the fabrication of multilayer films by alternating the deposition of the block copolymer micelles of poly(styrene-b-acrylic acid), incorporating azobenzene and poly(diallyl-dimethylammonium chloride). The azobenzene incorporated into the block copolymer micelles can undergo a reversible photoisomerization under the irradiation of UV and visible light sources. An interesting finding is that the photoisomerization of the azobenzene in the multilayer film is faster than it is in its normal solid film, but very similar to that in its diluted solution. Furthermore, the amount of azobenzene incorporated into the micelles can influence the photoisomerization rates in the films. Therefore, we expect that the block copolymer micelles may provide a proper microenvironment for the photoisomerization of azobenzene and the as-prepared polyelectrolyte/block copolymer micelle thin films will be useful for photoswitching materials.     

Reversible photorheological fluids based on spiropyran-doped reverse micelles

We describe a new class of photorheological (PR) fluids whose rheological properties can be reversibly tuned by light. The fluids were obtained by doping lecithin/sodium deoxycholate (SDC) reverse micelles with a photochromic spiropyran (SP) compound. Initially, the lecithin/SDC/SP mixtures formed highly viscoelastic fluids, reflecting the presence of long, wormlike reverse micelles. Under UV irradiation, the SP was isomerized to the open merocyanine (MC) form, causing the fluid viscosity to decrease 10-fold. When the UV irradiation was switched off, the MC reverted to the SP form, and the viscosity recovered its initial value. This cycle could be repeated several times without loss of response. The rheological transitions are believed to reflect changes in the lengths of the reverse worms. To our knowledge, this is the first example of a simple, reversible PR fluid that can be made entirely from commercially available components.     

Reversible Photoswitching in Poly(2-oxazoline) Nanoreactors

This contribution reports light responsive catalytic nanoreactors based on poly(2-oxazoline) diblock copolymers. The hydrophobic block of the copolymer is a random copolymer consisting of a spiropyran functionalized 2-oxazoline (SPOx) and 2-(but-3-yn-1-yl)-4,5-dihydrooxazole (ButynOx), while the hydrophilic block is based on 2-methyl-2-oxazoline (MeOx). The block copolymer is terminated with tris(2-aminoethyl) amine (TREN) that serves as catalyst in a Knoevenagel condensation. Four block copolymers with different ButynOx/SPOx and hydrophilic/hydrophobic ratios are synthesized and self-assembled through solvent exchange. Micelles and vesicles of various sizes are observed by TEM, which undergo morphological and size changes in response to irradiation with UV light. We hypothesize that these transformations in the nanostructures are caused by increases in the hydrophilicity of the hydrophobic block when spiropyran (SP) isomerizes to merocyanine (MC) in the presence of UV light. The reversible transition from micellar to vesicular nanoreactors resulted in increased reaction kinetics through improved substrate accessibility to the catalytic site, or termination of the catalytic reaction due to polymer precipitation. These nanoreactors present a promising platform towards photoregulating reaction outcomes based on changes in nanostructure morphology.     

Photocleavable and tunable thermoresponsive amphiphilic random copolymer: Self‐assembly into micelles,dye encapsulation,and triggered release

A double‐responsive amphiphilic random copolymer (P(OEtOxA)‐ran‐PNBA) composed of thermoresponsive poly(oligo(2‐ethyl‐2‐oxazoline)acrylate) (P(OEtOxA)) segments and photocleavable poly(2‐nitrobenzyl acrylate) (PNBA) segments is synthesized via combination of cationic ring‐opening polymerization (CROP) and reversible addition‐fragmentation chain transfer (RAFT) polymerization techniques. The P(OEtOxA)‐ran‐PNBA copolymer exhibits lower critical solution (LCST)‐type soluble‐to‐turbid phase transition in water with tunable cloud point (T_cp) with respect to chain length of P(OEtOxA) segment present. The photocleavage of PNBA segments by UV irradiation transforms amphiphilic P(OEtOxA)‐ran‐PNBA to fully hydrophilic P(OEtOxA)‐ran‐poly(acrylic acid) resulting in the appreciable increase of T_cp of copolymer in aqueous solution. Owing to the amphiphilic nature, the P(OEtOxA)‐ran‐PNBA copolymer molecules self‐assemble into well‐dispersed spherical micelles in water. There is a disruption of the copolymer micelles with UV light irradiation as well as shrinkage of micellar size with increasing temperature above the LCST of copolymer in solution. Finally, the encapsulation of hydrophobic guest molecule (nile red) into P(OEtOxA)‐ran‐PNBA copolymer micelles and thermo‐ and photo‐triggered release of nile red are demonstrated. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1714–1729     

Design and proof of reversible micelle‐to‐vesicle multistimuli‐responsive morphological regulations

Multistimuli‐responsive precise morphological control over self‐assembled polymers is of great importance for applications in nanoscience as drug delivery system. A novel pH, photoresponsive, and cyclodextrin‐responsive block copolymer were developed to investigate the reversible morphological transition from micelles to vesicles. The azobenzene‐containing block copolymer poly(ethylene oxide)‐b‐poly(2‐(diethylamino)ethyl methacrylate‐co‐6‐(4‐phenylazo phenoxy)hexyl methacrylate) [PEO‐b‐P(DEAEMA‐co‐PPHMA)] was synthesized by atom transfer radical polymerization. This system can self‐assemble into vesicles in aqueous solution at pH 8. On adjusting the solution pH to 3, there was a transition from vesicles to micelles. The same behavior, that is, transition from vesicles to micelles was also realizable on addition of β‐cyclodextrin (β‐CD) to the PEO‐b‐P(DEAEMA‐co‐PPHMA) solution at pH 8. Furthermore, after β‐CD was added, alternating irradiation of the solution with UV and visible light can also induce the reversible micelle‐to‐vesicle transition because of the photoinduced trans‐to‐cis isomerization of azobenzene units. The multistimuli‐responsive precise morphological changes were studied by laser light scattering, transmission electron microscopy, and UV–vis spectra. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

A Reversible Multi‐Stimuli‐Responsive Fluorescence Probe and the Design for Combinational Logic Gate Operations

Here, a novel multi‐stimuli‐responsive fluorescence probe is developed by incorporating spiropyran group into the coumarin‐substituted polydiacetylene (PDA) vesicles. The fluorescence of PDA can be turned on upon heating, and can be quenched upon exposure to UV light irradiation or pH stimuli owing to the fluorescene resonance energy transfer (FRET) between the red‐phase PDA and the open merocyanine (MC) form of spiropyran. Moreover, we have designed and experimentally realized a set of logic gate operations for the first time based on the fluorescence modulation of the designed system upon thermal, photo, and pH stimuli. This novel type of resettable logic gates augur well for practical applications in information storage, optical recording, and sensing in complicated microenvironments.

     

Fabrication of Novel Reversible Photoswitchable Fluorescent Nanoparticles

Novel photoswitchable fluorescent nanoparticles were fabricated by a facile one-step mini-emulsion polymerization, in which fluorescence resonance energy transfer (FRET) donor, 9,10-diphenylanthracene (DPA), and photoswitchable acceptor, spiropyran derivate, were simultaneously embedded in polymer matrix during the polymerization process. The prepared fluorescent nanoparticles exhibit the typical absorption properties of both DPA dye and spiropyran moiety, indicating that the two chromophores have been incorporated into the polymer nanoparticles. The obtained fluorescent nanoparticles exhibited superior photoswitchable fluorescent performance due to the effective photoinduced interparticle FRET. Moreover, the novel photoswitchable fluorescent nanoparticles also revealed small size (ca. 60 nm), high intensity, relatively fast photoresponsive property and good photoreversibility in aqueous media.     

In Situ Probing Intracellular Drug Release from Redox‐Responsive Micelles by United FRET and AIE

Redox‐responsive micelles are versatile nanoplatforms for on‐demand drug delivery, but the in situ evaluation of drug release is challenging. Fluorescence resonance energy transfer (FRET) technique shows potential for addressing this, while the aggregation‐caused quenching effect limits the assay sensitivity. The aim of the current work is to combine aggregation‐induced emission (AIE) probe with FRET to realize drug release assessment from micelles. Tetraphenylethene (TPE) is selected as AIE dye and curcumin (Cur) is chosen as the model drug as well as FRET receptor. The drug is covalently linked to a block copolymer via the disulfide bond linker and TPE is also chemically linked to the polymer via an amide bond; the obtained amphiphilic polymer conjugate self‐assembles into micelles with a hydrodynamic size of ≈125 nm. Upon the supplement of glutathione or tris(2‐carboxyethyl)phosphine) trigger (10 × 10⁻³m ), the drug release induces the fluorescence increase of both TPE and Cur. Accompanied with the FRET decay, absorption enhancement and particle size increase are observed. The same phenomenon is observed in MCF‐7 cells. The FRET–AIE approach can be a useful addition to the spectrum of available methods for monitoring drug release from stimuli‐responsive nanomedicine.     

Preparation of Photoresponsive PAN-NH2@EPESP Fiber Films with Mechanical Stability for Regulating Wettability and Micro-environment Humidity

In view of the current living micro-environment and requirements of green economy, intelligent light-responsive humidification materials have become a hot spot in intelligent polymer mate-rials. In this work, reversible photoresponsive films with a mechanical stability and an excellent humidity adjustment performance are developed by modifying epoxy-ether-spiropyran(EPESP) on amino-nanofibers(PAN-NH₂), which was produced through electrospinning. Such smart films for regulating wettability and micro-environment humidity can be reversibly manipulated by the simple switch of UV and visible light irradiation because of the unique transformation between polar ring-opened status and nonpolar ring-closed status of the spiropyran units. The effects of EPESP modification amount on the morphology, wettability, tensile strength and the ability to regulate humidity were investigated in detail. The results show that with the increase of modification amount of EPESP, both the humi-dity regulation and tensile strength increased; when the modification amount exceeds 7 mg, the range of regulating humidity increases slowly.     

Spiropyran Photoswitches in the Context of DNA: Synthesis and Photochromic Properties

A new design is presented for the incorporation of spiropyran photoswitches into nucleic acids by oligonucleotide solid phase synthesis. This design enables interaction of the 6‐nitrospirobenzopyran (NitroBIPS) photoswitch with the adjacent nucleobases and leaves the photochemical properties of NitroBIPS intact. UV/Vis spectroscopy and HPLC revealed that NitroBIPS incorporated into DNA consists of up to 40 % merocyanine in its thermal equilibrium and undergoes reversible switching between the photoisomeric spiropyran (SP) and merocyanin (MC) state by alternating excitation using visible light or heat for at least fifteen switching cycles. Exchanging the chromene part of NitroBIPS on the DNA level gives access to differently substituted spiropyran derivatives allowing the screening for spiropyrans with suitable properties in a straightforward manner. Thus, by incorporating the highly hydrolysis‐stable pyrido‐spiropyran derivative PyBIPS pure light‐triggered reversible switching of a spiropyran in DNA has been realized for the first time. Therefore, this design represents a new useful platform for investigating the photochromic behavior of different spiropyran photoswitches in a nucleic acid environment and for using spiropyrans to induce light‐ or heat‐triggered changes in conformations or in fluorescence quenching properties of oligonucleotides.     

“Turn-On” Activatable AIE Dots for Tumor Hypoxia Imaging

A hypoxia-responsive fluorescence probe of amphiphilic PEGylated azobenzene caged tetraphenylethene (TPE) for tumor cell imaging is reported; it possesses excellent solubility in aqueous medium due to the easy formation of micelles by self-assembly. The fluorescence resonance energy transfer (FRET) process ensures that the fluorescence of the azobenene caged AIE fluorogen is quenched efficiently. When cultured with tumor cells, the azo-bond is reduced under hypoxia conditions and the fluorescence of AIE fluorogen recovers dramatically. Besides using UV light, NIR light can also be used as the excited light resource to generate the fluorescence due to the two-photon fluorescence imaging process.     

Both core- and shell-cross-linked nanogels: photoinduced size change, intraparticle LCST, and interparticle UCST thermal behaviors

New thermal- and photoresponsive core-shell nanogel particles were obtained from self-assembly in aqueous solution of a double-hydrophilic block copolymer (DHBCP) of which the two blocks could be photo-cross-linked via the reversible photodimerization and photocleavage of coumarin moieties. The diblock copolymer, consisting of poly[N,N-dimethylacrylamide-co-4-methyl-[7-(methacryloyl)oxyethyloxy]coumarin] and poly[N-isopropylacrylamide-co-4-methyl-[7-(methacryloyl)oxyethyloxy]coumarin] (P(DMA-co-CMA)-b-P(NIPAM-co-CMA)), was synthesized by using reversible addition-fragmentation chain transfer (RAFT) polymerization. At T > LCST of the P(NIPAM-co-CMA) block, core-shell micelles were formed and UV light irradiation at λ > 310 nm resulted in cross-linking of both the micelle core of P(NIPAM-co-CMA) and the micelle shell of P(DMA-co-CMA); subsequent cooling of the solution to T < LCST gave rise to water-soluble, swollen nanogel particles. Upon UV light irradiation at λ < 260 nm, the decrease of cross-linking density could increase the swelling of nanogel particles by ～23% in diameter. By alternating irradiation with the different wavelengths, the average hydrodynamic diameter of nanogel particles was tunable between ～58 and ～47 nm. Interestingly, upon further cooling of the solution, aggregation occurred for nanogel particles with a moderate cross-linking density (10%-40% dimerization of coumarin moieties). Therefore, such core- and shell-cross-linked nanogel could display both "intraparticle" LCST (solubility of polymer chains forming the core) and "interparticle" UCST (solubility of particles). The possible mechanism and the effect of dimerization degree on the UCST behavior were discussed.