Adsorption of 4-(N,N-Dimethylamino)-4'-nitrostilbene on an Amorphous Silica Glass Surface

Stilbenes are a compelling class of organic photoswitches with a high degree of tunability that sensitively depend on their environment. In this study, we investigate the adsorption properties of 4-(N,N-dimethylamino)-4'-nitrostilbene (DANS), a push-pull stilbene, on amorphous silica glass. Plane-wave density functional theory (DFT) calculations are used to understand how the trans and cis isomers of DANS interact with the amorphous surface and which are the most preferred modes of adsorption. Our calculations revealed that the O-H···O hydrogen bonds between the nitro group and hydroxyl groups of the silica surface dominate the intramolecular interaction. In addition to hydrogen bonding, O-H···π interactions with the aromatic ring and double bond play a critical role in adsorption, whereas C-H···O interactions are present, but contribute little. Therefore, both isomers of DANS favor parallel orientations such that not only the functional groups but also the aromatic parts can strongly interact with the glass surface.

All-in-One Photoactivated Inhibition of Butyrylcholinesterase Combined with Luminescence as an Activation and Localization Indicator: Carbon Quantum Dots@Phosphonate Hybrids

Photopharmacology is a booming research area requiring a new generation of agents possessing simultaneous functions of photoswitching and pharmacophore. It is important that any practical implementation of photopharmacology ideally requires spatial control of the medicinal treatment zone. Thus, advances in the study of substances meeting all the listed requirements will lead to breakthrough research in the coming years. In this study, CQDs@phosphonate nanohybrids are presented for the first time and combine biocompatible and nontoxic luminescent carbon quantum dots (CQDs) with photoactive phosphonate enabling inhibition of butyrylcholinesterase (BChE), which is a prognostic marker of numerous diseases. The conjunction of these components in hybrids maintains photoswitching and provides enhancement of BChE inhibition. After laser irradiation with a wavelength of 266 nm, CQDs@phosphonate hybrids demonstrate a drastic increase of butyrylcholinesterase inhibition from 38% up to almost 100% and a simultaneous luminescence decrease. All the listed hybrid properties are demonstrated not only for in vitro experiments but also for complex biological samples, i.e., chicken breast. Thus, the most important achievement is the demonstration of hybrids characterized by a remarkable combination of all-in-one properties important for photopharmacology: (i) bioactivity toward butyrylcholinesterase inhibition, (ii) strong change of inhibition degree as a result of laser irradiation, luminescence as an indicator of (iii) bioactivity state, and of (iv) spatial localization on the surface of a sample.

Small-molecule photoswitches for fluorescence bioimaging: engineering and applications

Fluorescence microscopy has revolutionised our understanding of biological systems, enabling the visualisation of biomolecular structures and dynamics in complex systems. The possibility to reversibly control the optical or biochemical properties of fluorophores can unlock advanced applications ranging from super-resolution microscopy to the design of multi-stimuli responsive and functional biosensors. In this Highlight, we review recent progress in small-molecule photoswitches applied to biological imaging with an emphasis on molecular engineering strategies and promising applications, while underlining the main challenges in their design and implementation.

Reversible Near-Infrared Fluorescence Photoswitching in Aqueous Media by Diarylethene: Toward High-Accuracy Live Optical Imaging

Fluorescence imaging is an indispensable tool in modern biological research, allowing simple and inexpensive color-coded visualizations of real-time events in living cells and animals, as well as of fixed states of ex vivo specimens. The accuracy of fluorescence imaging in living systems is, however, impeded by autofluorescence, light scattering, and limited penetration depth of light. Nevertheless, the clinical use of fluorescence imaging is expected to grow along with advances in imaging equipment, and will increasingly demand high-accuracy probes to avoid false-positive results in disease detection. To this end, a water-soluble and relatively safe diarylethene (DAE)-based reversible near-infrared (NIR) fluorescence photoswitch for living systems is prepared here. Furthermore, to facilitate excellent switching performance, the photoirradiation results obtained is compared using three different visible light sources to turn on NIR fluorescence through cycloreversion of DAE. While photoswitching using 589 nm light leads to slightly higher cell viability, fluorescence quenching efficiency and fatigue resistance are higher when 532 nm light with low photobleaching is used in both aqueous solution and living systems. The authors anticipate that their reversible NIR fluorescence photoswitch mediated by DAE can be beneficial for fluorescence imaging in aqueous media requiring accurate detection, such as in the autofluorescence-rich living environment.

Light-Activated Nanoprobes for Biosensing and Imaging

Fluorescent nanoprobes are indispensable tools to monitor and analyze biological species and dynamic biochemical processes in cells and living bodies. Conventional nanoprobes have limitations in obtaining imaging signals with high precision and resolution because of the interference with biological autofluorescence, off-target effects, and lack of spatiotemporal control. As a newly developed paradigm, light-activated nanoprobes, whose imaging and sensing activity can be remotely regulated with light irradiation, show good potential to overcome these limitations. Herein, recent research progress on the design and construction of light-activated nanoprobes to improve bioimaging and sensing performance in complex biological systems is introduced. First, recent innovative strategies and their underlying mechanisms for light-controlled imaging are reviewed, including photoswitchable nanoprobes and phototargeted nanosystems. Subsequently, a short highlight is provided on the development of light-activatable nanoprobes for biosensing, which offer possibilities for the remote control of biorecognition and sensing activity in a precise manner both temporally and spatially. Finally, perspectives and challenges in light-activated nanoprobes are commented.

Dual-color fluorescent nanoparticles showing perfect color-specific photoswitching for bioimaging and super-resolution microscopy

Dual-emissive systems showing color-specific photoswitching are promising in bioimaging and super-resolution microscopy. However, their switching efficiency has been limited because a delicate manipulation of all the energy transfer crosstalks in the systems is unfeasible. Here, we report a perfect color-specific photoswitching, which is rationally designed by combining the complete off-to-on fluorescence switching capability of a fluorescent photochromic diarylethene and the frustrated energy transfer to the other fluorescent dye based on the excited-state intramolecular proton transfer (ESIPT) process. Upon alternation of UV and visible light irradiations, the system achieves 100% switching on/off of blue emission from the diarylethene while orange emission from the ESIPT dye is unchanged in the polymer film. By fabricating this system into biocompatible polymer nanoparticles, we demonstrate microscopic imaging of RAW264.7 macrophage cells with reversible blue-color specific fluorescence switching that enables super-resolution imaging with a resolution of 70 nm.

Photoswitchable nanoparticles can be used for selective imaging in biological systems but usually have only one color. Here the authors develop a two-color fluorescent emissive system that allows full on-off switching of one component color of the system while the other color is unaffected, which has implications for super-resolution imaging.

Photochromism into nanosystems: towards lighting up the future nanoworld

The ability to manipulate the structure and function of promising nanosystems via energy input and external stimuli is emerging as an attractive paradigm for developing reconfigurable and programmable nanomaterials and multifunctional devices. Light stimulus manifestly represents a preferred external physical and chemical tool for in situ remote command of the functional attributes of nanomaterials and nanosystems due to its unique advantages of high spatial and temporal resolution and digital controllability. Photochromic moieties are known to undergo reversible photochemical transformations between different states with distinct properties, which have been extensively introduced into various functional nanosystems such as nanomachines, nanoparticles, nanoelectronics, supramolecular nanoassemblies, and biological nanosystems. The integration of photochromism into these nanosystems has endowed the resultant nanostructures or advanced materials with intriguing photoresponsive behaviors and more sophisticated functions. In this Review, we provide an account of the recent advancements in reversible photocontrol of the structures and functions of photochromic nanosystems and their applications. The important design concepts of such truly advanced materials are discussed, their fabrication methods are emphasized, and their applications are highlighted. The Review is concluded by briefly outlining the challenges that need to be addressed and the opportunities that can be tapped into. We hope that the review of the flourishing and vibrant topic with myriad possibilities would shine light on exploring the future nanoworld by encouraging and opening the windows to meaningful multidisciplinary cooperation of engineers from different backgrounds and scientists from the fields such as chemistry, physics, engineering, biology, nanotechnology and materials science.

Photoswitchable fluorescent nanoparticles and their emerging applications

Although fluorescence offers ultrasensitivity, real-world applications of fluorescence techniques encounter many practical problems. As a noninvasive means to investigate biomolecular mechanisms, pathways, and regulations in living cells, the intrinsic heterogeneity and inherent complexity of biological samples always generates optical interferences such as autofluorescence. Therefore, innovative fluorescence technologies are needed to enhance measurement reliability while not compromising sensitivity. In this review, we present current strategies that use photoswitchable nanoparticles to address these real-world challenges. The unique feature in these photoswitchable nanoparticles is that fundamental molecular photoswitches are playing the critical role of fluorescence modulation rather than traditional methods like modulating the light source. As a result, new innovative technologies that have recently emerged include super-resolution imaging, frequency-domain imaging, antiphase dual-color correlation, etc. Some of these methods improve imaging resolution down to the nanometer level, while others boost the detection sensitivity by orders of magnitude and confirm the nanoparticle probes unambiguously. These enhancements, which are not possible with non-photoswitching molecular probes, are the central topics of this review.

Photoregulation of α-Chymotrypsin Activity by Spiropyran-Based Inhibitors in Solution and Attached to an Optical Fiber

Here the synthesis and characterization of a new class of spiropyran-based protease inhibitor is reported that can be reversibly photoswitched between an active spiropyran (SP) isomer and a less active merocyanine (MC) isomer upon irradiation with UV and visible light, respectively, both in solution and on a surface of a microstructured optical fiber (MOF). The most potent inhibitor in the series (SP-3 b) has a C-terminal phenylalanyl-based α-ketoester group and inhibits α-chymotrypsin with a Ki of 115 nM. An analogue containing a C-terminal Weinreb amide (SP-2 d) demonstrated excellent stability and photoswitching in solution and was attached to the surface of a MOF. The SP isomer of Weinreb amide 2 d is a competitive reversible inhibitor in solution and also on fiber, while the corresponding MC isomer was significantly less active in both media. The ability of this new class of spiropyran-based protease inhibitor to modulate enzyme activity on a MOF paves the way for sensing applications.

Photoswitchable semiconductor nanocrystals with self-regulating photochromic Förster resonance energy transfer acceptors

Photoswitchable molecules and nanoparticles constitute superior biosensors for a wide range of industrial, research and biomedical applications. Rendered reversible by spontaneous or deterministic means, such probes facilitate many of the techniques in fluorescence microscopy that surpass the optical resolution dictated by diffraction. Here we have devised a family of photoswitchable quantum dots (psQDs) in which the semiconductor core functions as a fluorescence donor in Förster resonance energy transfer (FRET), and multiple photochromic diheteroarylethene groups function as acceptors upon activation by ultraviolet light. The QDs were coated with a polymer bearing photochromic groups attached via linkers of different length. Despite the resulting nominal differences in donor-acceptor separation and anticipated FRET efficiencies, the maximum quenching of all psQD preparations was 38±2%. This result was attributable to the large ultraviolet absorption cross-section of the QDs, leading to preferential cycloreversion of photochromic groups situated closer to the nanoparticle surface and/or with a more favourable orientation.

The effect of a “push–pull” structure on the turn-on fluorescence of photochromic thio-ketone type diarylethenes

A diarylethene with a six-membered ring carrying an electron-donating sulfur atom and an electron-withdrawing carbonyl group shows fluorescence in its ring-closed state.

Visible-light triggered photoswitching systems based on fluorescent azulenyl-substituted dithienylcyclopentenes

Novel azulenyl functionalized dithienylethenes were synthesized, and their photochromic reactivity, fluorescent and electrochemical properties were investigated.