This overview highlights how the high-energy ultraviolet or visible light required to drive photochemical reactions can be overcome by integrating the chromophores into supramolecular structures containing upconverting nanoparticles with trivalent lanthanide dopants (such as Tm3+ and Er3+). These nanoparticles are particularly interesting systems because they absorb multiple photons of near infrared light and convert them into higher-energy light which is emitted in the ultraviolet and visible regions of the electromagnetic spectrum. The upconverting nanoparticles effectively act as nanoscopic ‘light bulbs’, and in this way, less damaging near infrared light can be used to trigger photochemical reactions for use in imaging and small molecule release. Several examples of how this phenomenon is being used in photochemistry will be presented with the focus being on self-assembled supramolecular systems, some of which are being used in cells and small animals.

The photothermal effect is not able to break bonds and release small molecules from the surface of SiO2–Au core–shell nanoparticles unless the nanosystem is first exposed to visible light. Only after this light triggers the ring-opening reaction of dithienylethene chromophores attached to the surface of the nanoparticles can the heat generated by the NIR light induce reverse Diels–Alder reactions.

The water-insolubility of a potentially versatile photoresponsive ‘turn-on’ fluorescence probe was overcome by incorporating it into a nano-assembly containing an upconverting nanoparticle wrapped in an amphiphilic polymer. The appeal of the nano-system is not only in the ability to turn “on” and “off” the fluorescence from the organic chromophore using UV and visible light, it is in the fact that the nanoparticle acts as a static probe because it emits red and green light when excited by near infrared light, which is not effected by UV and visible light. This dual-functioning emission behaviour was demonstrated in live organisms.

Exposure to UV light generates a ring-closed isomer of a diarylethene, which undergoes very slow bond breaking and release even after the light is turned off. The rate of release is increased by exposing the isomer to UV and/or visible light.

The synthesis of water-dispersible nano-assemblies containing upconverting nanoparticles (UCNPs) and porphyrin molecules using a one-pot method is described. When the nano-assemblies are excited by 980 nm near-infrared light, the intensity of the ‘green emission’ band of the UCNP is reduced due to a combination of Föster Resonace Energy Transfer (FRET) and emission-reabsorption. This process is also responsible for the emission from the porphyrin chromophores despite the fact they do not absorb near-infrared light.

A thermally remendable polymer was synthesized by the Diels–Alder reaction between dithienylfuran and maleimide monomers to generate a photoresponsive diarylethene. UV light (312 nm) and visible light (>435 nm) “gate” the reversibility of the Diels–Alder reaction and turn the self-healing properties of the polymer “off” and “on”, respectively. After exposure to UV light, the strength of the polymer as an adhesive is enhanced. Visible light weakens the adhesive.

A photoresponsive amphiphilic polymer was synthesized and used to encapsulate upconverting lanthanide-doped nanoparticles to produce a novel water-dispersible nanoassembly with a high loading of emission quenchers. The nanoassembly exhibits fluorescent emission in the visible region upon irradiation with 980 nm light, which can be reversibly modulated by toggling the isomeric state of photoresponsive chromophores attached to the polymer’s backbone using UV and visible light. Photon counting experiments show that the quenching mechanism for this new nanoassembly is a combination of Förster resonance energy transfer (FRET) and emission-reabsorption. Compared to the similar nanoassembly prepared from a reported “plug-and-play” method, this new nanoassembly has higher overall quenching efficiency due to the increased photoswitch loading (14 times compared to the existing nanoassembly).

A convenient and versatile protocol to encapsulate lanthanide doped upconverting nanoparticles by an amphiphilic polymer shell containing photoresponsive diarylethene chromophores was developed. The assemblies are all water-soluble and fluoresce in the visible region of the spectrum when excited with 980 nm near-infrared light. The fluorescent emission can be selectively and reversibly modulated by alternatively irradiating the photoresponsive nanoparticles with UV light and visible light, which triggers ring-closing and ring-opening reactions of the chromophores, respectively. Fluorescence lifetime experiments suggest that the quenching mechanism is a combination of energy transfer and emission-reabsorption processes. These photoresponsive upconverting nanoparticles have the potential to advance bioimaging and other applications in nanophotonics.Keywords: fluorescence modulation; molecular switches; photoresponsive upconverting nanoparticles; self-assembly;

Anthracene endoperoxide ligands anchored to the surfaces of gold nanoparticles undergo bond breaking and release singlet oxygen when the nanoparticles convert 532 nm laser light to heat localized near their surfaces.

Using a photosensitive hybrid hydrogel loaded with upconversion nanoparticles (UCNPs), we show that continuous-wave near-infrared (NIR) light (980 nm) can be used to induce the gel–sol transition and release large, inactive biomacromolecules (protein and enzyme) entrapped in the hydrogel into aqueous solution “on demand”, where their bioactivity is recovered. This study is a new demonstration and development in harnessing the unique multiphoton effect of UCNPs for photosensitive materials of biomedical interest.

Light is used to ‘gate’ the Diels–Alder reaction using a photoresponsive dithienylfuran backbone and turn the reversibility of the Diels–Alder reaction ‘off’ and ‘on’ at 100 °C. These features make the reported system an excellent candidate for developing the next generation of self-healing polymers and photothermal drug delivery vehicles.

The intensity and colour of the light emitted from upconverting nanoparticles is controlled by the state of photoresponsive dithienylethene ligands decorated onto the surface of the nanoparticles. By selectively activating one or both ligands in a mixed, 3-component system, a multimodal read-out of the emitted light is achieved.

The ability of gold (Au) nanoparticles (NPs) to generate heat efficiently by absorbing visible and near-infrared (NIR) light holds great promise as a means to trigger chemical and biochemical events near the NPs. Previous demonstrations show that pulsed laser irradiation can selectively elicit the release of a fluorescent dye covalently anchored to the NP surface through a heat-labile linker without measurably changing the temperature of the surroundings. This article reports that the authors demonstrate the biological efficacy of this approach to photodelivery by showing that the decorated Au NPs are rapidly internalized by cells, are stable under physiological conditions, are nontoxic, and exhibit nonlethal photorelease following exposure to pulsed laser radiation. These observations, further supported by the versatility of our delivery motif, reaffirm the potential for further development of nonlethal photothermal therapeutics and their future relevance to such fields as gene therapy and stem-cell differentiation.From the Clinical EditorThe authors further refine previous observations suggesting that Au NP-s may be useful in targeted drug or gene delivery systems. Due to a strong photothermal release effect and their generally low toxicity, Au NP-s may become an important subject of choice in targeted delivery systems.Spherical 15-nm gold nanoparticles (Au NPs) were decorated with a modified fluorescent dye anchored to the particle surface by a mercaptoalkyl chain and an intervening, heat-labile oxabicycloheptene linker. Solutions containing the decorated NPs were injected into live oocytes and introduced to Chinese hamster ovary cells via their growth media. Both cell cultures were shown to be viable following exposure to the NP solutions and subsequent pulsed laser irradiation, which resulted in visible increases in fluorescence attributable to diffusion of the dye away from the NP surface following cleavage of the oxabicycloheptene structure elicited by photothermal heating of the Au NPs.

We demonstrate a novel strategy enabling the use of a continuous-wave diode near-infrared (NIR) laser to disrupt block copolymer (BCP) micelles and trigger the release of their “payloads”. By encapsulating NaYF4:TmYb upconverting nanoparticles (UCNPs) inside micelles of poly(ethylene oxide)-block-poly(4,5-dimethoxy-2-nitrobenzyl methacrylate) and exposing the micellar solution to 980 nm light, photons in the UV region are emitted by the UCNPs, which in turn are absorbed by o-nitrobenzyl groups on the micelle core-forming block, activating the photocleavage reaction and leading to the dissociation of BCP micelles and release of co-loaded hydrophobic species. Our strategy of using UCNPs as an internal UV or visible light source upon NIR light excitation represents a general and efficient method to circumvent the need for UV or visible light excitation that is a common drawback for light-responsive polymeric systems developed for potential biomedical applications.

We describe a versatile and convenient visible detection method for organophosphorus compounds based on a colorless ‘pro-photoresponsive’ organic molecule that undergoes photochemical ring-closing to produce a colored isomer only after it reacts with vapors of the phosphorylating agent.

The photophysical behavior for two photochromic Pt-terpyridine acetylide complexes containing pendant dithienylethenes (DTEs) bound to the metal through the alkynyl linkage is presented. Selective excitation of the Pt complex with visible light resulted in the metal-sensitized ring closing of the DTE unit. The central purpose of this study was to understand how excited state interactions govern the photophysics by correlating differences in the linkage of the two components with differences in the intramolecular energy transfer processes that occur between the Pt complex and the DTE. A series of model complexes without photochromic ligands were prepared and studied to elucidate the contributions of the triplet metal-to-ligand charge transfer and triplet intraligand states. It is demonstrated that reducing the orbital overlap of the metal-based and intraligand states by lengthening the linkage and eliminating a conjugated pathway is effective at dramatically decreasing the efficiency of intramolecular energy transfer. This is evidenced by the appearance of Pt-terpyridine based phosphorescence and a significant decrease in the observed rate of metal-sensitized ring closing of the DTE.

A novel amphiphilic photochromic copolymer P1-o consisting of a hydrophilic poly(N-isopropylacrylamide) backbone and thermally irreversible photochromic and hydrophobic 1,2-dithienylethene derivative pendants was designed, synthesized and used to fabricate self-assembled copolymer nanoparticles in an aqueous condition. The hydrodynamic diameters of the multifunctional copolymer P1-o nanoparticles reversibly switched between two stable states with two stimuli: light and temperature in an aqueous suspension.Graphical abstract:Figure options

Only one type of lanthanide-doped upconverting nanoparticle (UCNP) is needed to reversibly toggle photoresponsive organic compounds between their two unique optical, electronic, and structural states by modulating merely the intensity of the 980 nm excitation light. This reversible “remote-control” photoswitching employs an excitation wavelength not directly absorbed by the organic chromophores and takes advantage of the fact that designer core−shell−shell NaYF4 nanoparticles containing Er3+/Yb3+ and Tm3+/Yb3+ ions doped into separate layers change the type of light they emit when the power density of the near-infrared light is increased or decreased. At high power densities, the dominant emissions are ultraviolet and are appropriate to drive the ring-closing, forward reactions of dithienylethene (DTE) photoswitches. The visible light generated from the same core−shell−shell UCNPs at low power densities triggers the reverse, ring-opening reactions and regenerates the original photoisomers. The “remote-control” photoswitching using NIR light is as equally effective as the direct switching with UV and visible light, albeit the reaction rates are slower. This technology offers a highly convenient and versatile method to spatially and temporally regulate photochemical reactions using a single light source and changing either its power or its focal point.

The morphology of calcium oxalate monohydrate (COM) kidney stones is studied using polarized light microscopy and X-ray diffraction. We show that polycrystalline structure of COM stones exhibits spherulitic texture where the arrangement of crystallites indicates that their fast growth direction is perpendicular to the corresponding radius of spherulite, resulting in the layered morphology. This is in contrast to “normal” spherulites, where the crystal growth process leads to the formation of a radiating array of fiber crystallites. We demonstrate that COM stones consist of spherulitic domains. The domains have the shape of comparatively narrow randomly distorted cones in which the crystallites form strong texture, so that their crystallographic axes have almost the same directions and the [100] crystallographic planes are nearly perpendicular to the radial direction of the domain. However, the order among the domains does not exist. Deviations of their radial directions from the corresponding radial directions of the whole stone are not large as a rule, while the other crystallographic directions of the domains are randomly distributed. A model of layered spherulitic growth explaining the observed morphology is proposed. The model suggests that every domain is formed by means of a continuous crystallization process periodically inhibited by precipitation of organic material so that alternating organic and polycrystalline layers appear. Fine crystalline channels remaining in organic layers connect neighboring crystalline layers and maintain, thereby, the coherence of crystal structure all over the domain. Preformed COM microcrystals occasionally adsorbed from urine on the surface of the growing organic layer serve as seeds for new spherulitic domains. The results are important for understanding the general principles of biomineralization, and spherulitic crystallization and could lead to the development of new strategies for preventing kidney stone formation.

Photoisomerization of a coordinating, photochromic dithienylethene bearing a pyridine and a methylpyridinium group was investigated as a means to reversibly modulate the luminescence from CdSe–ZnS core–shell quantum dots. Resonance energy transfer and electron transfer are both plausible quenching mechanisms based on an increase in the spectral donor–acceptor overlap and an anodic shift in the reduction potential accompanying the isomerization reaction of the dithienylethene photoswitch. Photochemical degradation of both the quantum dot and photochromic quencher was observed after repeated cycling between the two isomers, suggesting irreversible electron transfer from the quantum dot to the dithienylethene as the dominant luminescence quenching mechanism.

Photothermal release of DNA from gold nanoparticles either by thermolysis of the Au−S bonds used to anchor the oligonucleotides to the nanoparticle or by thermal denaturation has great therapeutic potential, however, both processes have limitations (a decreased particle stability for the former process and a prohibitively slow rate of release for the latter). Here we show that these two mechanisms are not mutually exclusive and can be controlled by adjusting laser power and ionic strength. We show this using two different double-stranded (ds)DNA−nanoparticle conjugates, in which either the anchored sense strand or the complementary antisense strand was labeled with a fluorescent marker. The amounts of release due to the two mechanisms were evaluated using fluorescence spectroscopy and capillary electrophoresis, which showed that irradiation of the decorated particles in 200 mM NaOAc containing 10 mM Mg(OAc)2 with a pulsed 532 nm laser operating at 100 mW favors denaturation over Au−S cleavage to an extent of more than six-to-one. Due to the use of a pulsed laser, the process occurs on the order of minutes rather than hours, which is typical for continuous wave lasers. These findings encourage continued research toward developing photothermal gene therapeutics.Keywords: controlled release; drug delivery; nanostructures; oligonucleotides; photothermal

Near-infrared (NIR) light is used to toggle photoswitches back and forth between their two isomers even though the chromophores do not significantly absorb this type of light. The reactions are achieved through a “remote control” process by using photon upconverting hexagonal NaYF4 nanocrystals doped with lanthanide ions. These nanoparticles absorb 980 nm light and convert it to wavelengths that can be used to trigger the photoswitches offering a practical means to potentially achieve 3D-data storage, drug-delivery, and photolithography.

Using light to modulate biochemical agents in living organisms has a significant impact on photodynamic therapy and drug release. We demonstrate that a photoresponsive system can reversibly induce paralysis in nematodes as a model for living organisms when two different wavelengths of light are used to toggle the molecular switch between its two structural forms. This example illustrates how photoswitches offer great potential for advancing biomedical technologies.

Here we report the amount and location of fluid water absorbed by calcium oxalate monohydrate (COM) kidney stones evaluated using controlled humidity and magnetic resonance imaging (MRI). COM stones subjected to rehydration in a controlled humidity chamber for 16 h at 37 °C absorb approximately 3 wt % water, most of which resides only in thin outer shells of the stones and not within their cores. Although scanning electron microscopy (SEM) shows that there are differences in texture between the inner surface of the water-absorbing shell and the outer surface of the nonabsorbing core, energy-dispersive X-ray (EDX) spectroscopy does not reveal any differences in elemental composition between the two surfaces, and X-ray diffraction (XRD) of powered samples shows them to be similar in crystalline nature. The fact that COM stones do not absorb a significant amount of fluid water beyond their thin outer coatings, which have a different (albeit unidentified) interfacial material separating them from the bulk of the stones, may be the reason for the differences in sensitivity to common medical treatment options such as shock wave lithotripsy.

Intramolecular energy-transfer results in sensitized ring closing of a pendant dithienylethene from a platinum terpyridyl complex only when the two components are connected with a short π-conjugated linker.

The ability of a pyridine ligand on the photoresponsive 1,2-dithienylethene backbone to coordinate to a ruthenium porphyrin is modulated by interconverting the compound between its electronically insulated ring-open and electronically connected ring-closed form.

An unprecedented combination of photochromism and electrochromism is observed for two 1,2-bis(dithienyl)cyclopentene derivatives; the ring-opening reactions are photochemically driven while the ring-closing reactions can be triggered by electrochemical oxidation.

Linear π-conjugation is reversibly re-routed by irradiation of a photochromic bis(terthiophene).

We describe a versatile and convenient visible detection method for organophosphorus compounds based on a colorless ‘pro-photoresponsive’ organic molecule that undergoes photochemical ring-closing to produce a colored isomer only after it reacts with vapors of the phosphorylating agent.

Anthracene endoperoxide ligands anchored to the surfaces of gold nanoparticles undergo bond breaking and release singlet oxygen when the nanoparticles convert 532 nm laser light to heat localized near their surfaces.

The intensity and colour of the light emitted from upconverting nanoparticles is controlled by the state of photoresponsive dithienylethene ligands decorated onto the surface of the nanoparticles. By selectively activating one or both ligands in a mixed, 3-component system, a multimodal read-out of the emitted light is achieved.

Light is used to ‘gate’ the Diels–Alder reaction using a photoresponsive dithienylfuran backbone and turn the reversibility of the Diels–Alder reaction ‘off’ and ‘on’ at 100 °C. These features make the reported system an excellent candidate for developing the next generation of self-healing polymers and photothermal drug delivery vehicles.

Exposure to UV light generates a ring-closed isomer of a diarylethene, which undergoes very slow bond breaking and release even after the light is turned off. The rate of release is increased by exposing the isomer to UV and/or visible light.

This overview highlights how the high-energy ultraviolet or visible light required to drive photochemical reactions can be overcome by integrating the chromophores into supramolecular structures containing upconverting nanoparticles with trivalent lanthanide dopants (such as Tm3+ and Er3+). These nanoparticles are particularly interesting systems because they absorb multiple photons of near infrared light and convert them into higher-energy light which is emitted in the ultraviolet and visible regions of the electromagnetic spectrum. The upconverting nanoparticles effectively act as nanoscopic ‘light bulbs’, and in this way, less damaging near infrared light can be used to trigger photochemical reactions for use in imaging and small molecule release. Several examples of how this phenomenon is being used in photochemistry will be presented with the focus being on self-assembled supramolecular systems, some of which are being used in cells and small animals.

The synthesis of water-dispersible nano-assemblies containing upconverting nanoparticles (UCNPs) and porphyrin molecules using a one-pot method is described. When the nano-assemblies are excited by 980 nm near-infrared light, the intensity of the ‘green emission’ band of the UCNP is reduced due to a combination of Föster Resonace Energy Transfer (FRET) and emission-reabsorption. This process is also responsible for the emission from the porphyrin chromophores despite the fact they do not absorb near-infrared light.