Temperature-Driven Oxygenation Rate Control by Polymeric Photosensitizer

Temperature- and pH-Responsive Polymeric Photocatalysts for Enhanced Control and Recovery

The emergence of heterogeneous photocatalysis has facilitated redox reactions with high efficiency, without compromising the recyclability of the photocatalyst. Recently, stimuli-responsive heterogeneous photocatalytic materials have emerged as a powerful synthetic tool, with simple and rapid recovery, as well as an enhanced dynamic control over reactions. Stimuli-responsive polymers are often inexpensive and easy to produce. They can be switched from an active "on" state to an inert "off" state in response to external stimuli, allowing the production of photocatalyst with adaptability, recyclability, and orthogonal control on different chemical reactions. Despite this versatility, the application of artificial smart material in the field of heterogeneous photocatalysis has not yet been maximized. In this Minireview, we will examine the recent developments of this emerging class of stimuli-responsive heterogeneous photocatalytic systems. We will discuss the synthesis route of appending photoactive components into different triggerable systems and, in particular, the controlled activation and recovery of the materials.

NIR-Triggered "OFF/ON" Photodynamic Therapy through a Upper Critical Solution Temperature Block Copolymer

Activatable photodynamic therapy (A-PDT) has attracted great attention in precision medicine, which can be activated by endogenous or exogenous stimuli to selectively produce reactive oxygen species (ROS) at the disease site. Thermal responsive polymers with a lower critical solution temperature (LCST) have normally been utilized for constructing A-PDT system. Herein, we fabricated a photothermal activatable photosensitizer (A-PS) by the combination of thermal responsive porphyrin-containing P(AAm-co-AN-co-TPP)-b-POEGMA amphiphilic block copolymer with an upper critical solution temperature (UCST) of 42 °C and a cyanine dye of IR780. The photoactivity of porphyrin units could be severely inhibited by IR780 due to the fluorescence resonance energy transfer (FRET) from TPP to IR780 during blood circulation process ("OFF" state). After an uptake by A549 cells and then irradiated with 808 nm laser, A-PS nanoparticles were subsequently dissociated owing to the increased local temperature above the UCST of the polymer chains by excellent photothermal conversion of IR780, resulting in the enhanced photoactivity of TPP ("ON" state) and the remarkable antitumor effect. Therefore, the UCST-based A-PS extended the biological application of thermal responsive polymers, which may provide a new insight into the design of smart cancer therapeutic systems.

Advanced smart-photosensitizers for more effective cancer treatment

Photodynamic therapy (PDT) based upon the use of light and photosensitizers (PSs) has been used as a novel treatment approach for a variety of tumors. It, however, has several major limitations in the clinic: poor water solubility, long-term phototoxicity, low tumor targeting efficacy, and limited light penetration. With advances in nanotechnology, materials science, and clinical interventional imaging procedures, various smart-PSs have been developed for improving their cancer-therapeutic efficacy while reducing the adverse effects. Here, we briefly review state-of-the-art smart-PSs and discuss the future directions of PDT technology.

Photoreductive synthesis of monodispersed Au nanoparticles with citric acid as reductant and surface stabilizing reagent

UV irradiation of water containing HAuCl₄ and citric acid at room temperature successfully produces monodispersed Au nanoparticles. The size of Au particles is easily tuned by the intensity of UV light and the amount of citric acid.

Intermolecular Structural Change for Thermoswitchable Polymeric Photosensitizer

We developed a thermoswitchable polymeric photosensitizer (T-PPS) by conjugating PS (Pheophorbide-a, PPb-a) to a temperature-responsive polymer backbone of biocompatible hydroxypropyl cellulose. Self-quenched PS molecules linked in close proximity by π-π stacking in T-PPS were easily transited to an active monomeric state by the temperature-induced phase transition of polymer backbones. The temperature-responsive intermolecular interaction changes of PS molecules in T-PPS were demonstrated in synchrotron small-angle X-ray scattering and UV-vis spectrophotometer analysis. The T-PPS allowed switchable activation and synergistically enhanced cancer cell killing effect at the hyperthermia temperature (45 °C). Our developed T-PPS has the considerable potential not only as a new class of photomedicine in clinics but also as a biosensor based on temperature responsiveness.

Amino-substituted spirothiopyran as an initiator for self-assembly of gold nanoparticles

Amino-substituted spirothiopyran promotes spontaneous aggregation of gold nanoparticles, producing the aggregates with tunable sizes and narrow size distributions.

Spiropyran-modified gold nanoparticles: reversible size control of aggregates by UV and visible light irradiations

UV or visible light irradiation of gold nanoparticles (AuNPs) modified with a thiol-terminated spiropyran dye promotes reversible aggregation or dispersion of AuNPs. This is facilitated by the electrostatic repulsion/attraction between the AuNPs controlled by the ring-opening/closing photoisomerization of the surface dyes. This photochemical method successfully produces aggregates of AuNPs with tunable sizes (20-340 nm) and narrow size distributions (standard deviation <34%) in a reversible manner. In addition, the formed aggregates, even when left in the dark condition, scarcely change their sizes because the stacking interaction between the ring-opened forms of surface dyes suppresses thermal reverse isomerization and maintains the attractive force between the AuNPs.

Thermoresponsive copolymer containing a coumarin-spiropyran conjugate: reusable fluorescent sensor for cyanide anion detection in water

A simple copolymer consisting of N-isopropylacrylamide and coumarin-conjugated spiropyran (CS) units, poly(NIPAM-co-CS), has been synthesized. This polymer enables selective fluorometric detection of cyanide anion (CN(-)) in water at room temperature. The polymer itself shows almost no fluorescence, but shows a strong blue fluorescence in the presence of CN(-) under irradiation of UV light. The fluorescence enhancement occurs via a nucleophilic interaction between CN(-) and the photoformed merocyanine form of the CS unit, leading to a localization of π-electrons on the coumarin moiety. The polymer enables accurate determination of very low levels of CN(-) (>0.5 μM). The polymer can be recovered from water by simple centrifugation at high temperature (>40 °C), due to the heat-induced aggregation of the polymer. In addition, the polymer is regenerated by simple acid treatment, and the resulting polymer is successfully reused for further CN(-) sensing without loss of sensitivity.

Optical-scattering method for the determination of the local polymer concentration inside nanoparticles

We have developed a method based on the Mie theory for determining the local polymer concentration inside spherical nanoparticles, thereby obtaining vital information about whether the particles are swelling in the solvent or if they are contracted into a more compact structure. In addition, this method can be used to calculate the number density of the particles, the molecular weight of the particles, and (if M(n) of the polymer is known) the aggregation number. The calculations are based on the relationship between the size of the nanoparticles and the turbidity of the sample.

Spiropyran-conjugated thermoresponsive copolymer as a colorimetric thermometer with linear and reversible color change

A simple copolymer, poly(NIPAM-co-SP), consisting of N-isopropylacrylamide and spiropyran units, behaves as a colorimetric thermometer exhibiting temperature-responsive linear and reversible bathochromic/hypsochromic shift of the absorption spectra under UV irradiation.

Selective photocatalytic transformations on microporous titanosilicate ETS-10 driven by size and polarity of molecules

Photocatalytic activity of microporous titanosilicate ETS-10 has been studied in water. The photoactivated ETS-10 shows catalytic activity driven by size and polarity of substrates. ETS-10 efficiently catalyzes a conversion of substrates with a size larger than the pore diameter of ETS-10. In contrast, the reactivity of small substrates depends strongly on substrate polarity; less polar substrates show higher reactivity on ETS-10. Electron spin resonance analysis reveals that large substrates or less polar substrates scarcely diffuse inside the highly polarized micropores of ETS-10 and, hence, react efficiently with hydroxyl radicals (*OH) formed on titanol (Ti-OH) groups exposed on the external surface of ETS-10. In contrast, small polar substrates diffuse easily inside the micropores of ETS-10 and scarcely react with *OH, resulting in low reactivity. The photocatalytic activity of ETS-10 is successfully applicable to selective transformations of large reactants or less polar reactants to small polar products, enabling highly selective dehalogenation and hydroxylation of aromatics.

Temperature-controlled photooxygenation with polymer nanocapsules encapsulating an organic photosensitizer

A cross-linked poly- N-isopropylacrylamide (polyNIPAM) nanocapsule, TH@PC, containing thionine (TH), an organic photosensitizer, has been synthesized. This capsulated polymeric photosensitizer promotes a singlet oxygen oxygenation ( (1)O 2) accurately controlled by temperature: it shows high oxygenation activity at low temperature, but shows activity decrease with a rise in temperature, resulting in almost zero activity at >40 degrees C. The clear on-off activity control is driven by a heat-induced structure change of the capsule from the swollen single capsule to contracted state, and then to aggregate, behaving as an intelligent (1)O 2 filter. At low temperature, the capsule exists as the swollen single capsule, which allows (1)O 2 diffusion to bulk water, resulting in high oxygenation activity. A rise in temperature leads to contraction of the capsule, reducing the mesh size of the capsule wall. This suppresses (1)O 2 diffusion to bulk water and shows decreased activity. Intercapsule aggregation at >30 degrees C further suppresses (1)O 2 diffusion and shows almost no activity. The capsule promotes reversible activity control regardless of the heating/cooling process and can be reused with a simple recovery process.

A Fluorescent Molecular Switch Driven by the Input Sequence of Metal Cations: An Azamacrocyclic Ligand Containing Bipolar Anthracene Fragments

An azamacrocyclic ligand (L) containing two anthracene (AN) fragments connected through two triethylenetetramine bridges has been synthesized, in which each of the bridges can coordinate with one metal cation. The effects of pH and metal cations (Zn2+ and Cd2+) on the emission properties of L were studied in water. Without metal cations, L does not show any emission at basic pH values. The addition of Zn2+ leads to the production of excimer emission, which is due to a static excimer formed by direct excitation of the intramolecular ground-state dimer of the bipolar AN fragments that approach each other by Zn2+ binding. In contrast, Cd2+ addition does not result in excimer emission because the Cd2+-AN pi complex, formed by donation of a pi electron of the AN fragments to the adjacent Cd2+, suppresses pi-stacking interactions of the AN fragments. The most notable feature is the appearance of excimer emission controlled by the input sequence of metal cations: Zn2+-->Cd2+ sequential addition (each one equivalent) allows excimer emission, whereas the reverse sequence (Cd2+-->Zn2+) does not. In the Zn2+-->Cd2+ sequence, Cd2+ coordination is structurally restricted by the first Zn2+ coordination with the other polyamine bridge, leading to the formation of a weak Cd2+-AN pi complex. In contrast, for the reverse sequence, the first Cd2+ coordination forms a stable Cd2+-AN pi complex, which is not weakened by sequential Zn2+ coordination, resulting in no excimer emission.

Effects of Metal Cation Coordination on Fluorescence Properties of a Diethylenetriamine Bearing Two End Pyrene Fragments

Fluorescence properties of a diethylenetriamine bearing two end pyrene fragments (L) have been studied in water, where effects of adding metal cations (Zn2+, Cd2+, Cu2+, Hg2+, Ag+) on the emission properties of L have been studied. Without metal cations, L shows dual-mode fluorescence consisting of monomer and excimer emissions. The monomer emission intensity (I(M)) is strong at acidic pH but decreases with a pH increase because of an electron transfer (ET) from the unprotonated nitrogen atoms to the excited pyrene fragment. The excimer emission is due to the static excimer formed via a direct photoexcitation of the intramolecular ground-state dimer (GSD) of the end pyrene fragments. The excimer emission intensity (I(E)) is weak at acidic pH but increases with a pH increase because of the GSD stability increase associated with the deprotonation of the polyamine chain. Addition of metal cations leads to I(M) decrease, where chelation-driven I(M) enhancement does not occur even with diamagnetic Zn2+ and Cd2+ at any pH. This is because a pyrene-metal cation pi-complex, formed via a donation of pi-electron of the pyrene fragment to the adjacent metal center, suppresses the monomer photoexcitation. I(E) also decreases upon addition of metal cations because the pyrene-metal cation pi-complex weakens pi-stacking interaction of the end pyrene fragments, leading to GSD stability decrease. The emission properties of L-Zn2+ complexes were studied by means of time-resolved fluorescence decay measurements, and the effects of adding a less-polar organic solvent were also studied to clarify the detailed emission properties.

Solvent-Driven Multiply Configurable On/Off Fluorescent Indicator of the pH Window: A Diethylenetriamine Bearing Two End Pyrene Fragments

Fluorescence behaviors of a simple-structured molecule (L), a diethylenetriamine bearing two end pyrene fragments, have been investigated in water. Effects of adding a less-polar organic solvent (acetonitrile: MeCN) on the emission behaviors have been studied by means of steady-state and time-resolved fluorescence measurements. L dissolved in water shows dual-mode fluorescence consisting of monomer and excimer emissions. The monomer emission shows an "on-off" intensity profile against the pH window (pH 2-12), whereas the excimer emission shows an "off-on" profile. Upon MeCN addition, the monomer emission maintains the "on-off" profile. In contrast, the "off-on" profile of the excimer emission is drastically changed: L shows two more types of profiles, "off-on-off-on" and "off-on-off", along with the MeCN concentration increase, thus behaving as a multiply configurable fluorescent indicator of the pH window. The MeCN-driven excimer emission switching of L is triggered by (i) the decrease in stability of the intramolecular ground-state dimer (GSD) formed between the end pyrene fragments, which suppresses the direct photoexcitation of GSD (suppression of the "static" excimer formation), leading to a decrease in the excimer emission intensity at basic pH; and (ii) the decrease in polarity of solution, which allows formation of a "dynamic" excimer via a monomer-to-excimer transition, resulting in an enhancement of the excimer emission intensity at acidic-neutral pH.