Photoswitching in a Liquid Crystalline Pt(II) Coordination Complex

Photoswitching of photoluminescence has sparked tremendous research interests for super-resolution imaging, high-security-level anti-counterfeiting, and other high-tech applications. However, the excitation of photoluminescence is usually ready to trigger the photoswitching process, making the photoluminescence readout unreliable. Herein, we report a new photoswitch by the marriage of spiropyran with platinum(II) coordination complex. Viable photoluminescence can be achieved upon excitation by 480 nm visible light while the photoswitching can be easily triggered by 365 nm UV light. The feasible photoswitching may be benefited from the formed liquid crystalline (LC) phase of the designed photoswitch as a crystalline spiropyran is normally unable to implement photoswitching. Compared to the counterparts, this LC photoswitch can show distinct and reliable apparent colors and emission colors before and after photoswitching, which may promise the utility in high-security-level anti-counterfeiting and other advanced information technologies.

Photo-Controlled Gating of Selective Bacterial Membrane Interaction and Enhanced Antibacterial Activity for Wound Healing

Reversible biointerfaces are essential for on-demand molecular recognition to regulate stimuli-responsive bioactivity such as specific interactions with cell membranes. The reversibility on a single platform allows the smart material to kill pathogens or attach/detach cells. Herein, we introduce a 2D-MoS2 functionalized with cationic azobenzene that interacts selectively with either Gram-positive or Gram-negative bacteria in a light-gated fashion. The trans conformation (trans-Azo-MoS2 ) selectively kills Gram-negative bacteria, whereas the cis form (cis-Azo-MoS2 ), under UV light, exhibits antibacterial activity against Gram-positive strains. The mechanistic investigation indicates that the cis-Azo-MoS2 exhibits higher affinity towards the membrane of Gram-positive bacteria compared to trans-Azo-MoS2 . In case of Gram-negative bacteria, trans-Azo-MoS2 internalizes more efficiently than cis-Azo-MoS2 and generates intracellular ROS to kill the bacteria. While the trans-Azo-MoS2 exhibits strong electrostatic interactions and internalizes faster into Gram-negative bacterial cells, cis-Azo-MoS2 primarily interacts with Gram-positive bacteria through hydrophobic and H-bonding interactions. The difference in molecular mechanism leads to photo-controlled Gram-selectivity and enhanced antibacterial activity. We found strain-specific and high bactericidal activity (minimal bactericidal concentration, 0.65 μg/ml) with low cytotoxicity, which we extended to wound healing applications. This methodology provides a single platform for efficiently switching between conformers to reversibly control the strain-selective bactericidal activity regulated by light.

Photoswitching/back-switching assessment of biobased cellulose acetate/azobenzene handleable films under visible-light LED irradiation

The light-induced processes performed by photofunctional polymer films are crucial aspects of developing integrated energy storage devices properly. Herein, we report the preparation, characterization, and study of the optical properties of a series of biobased cellulose acetate/azobenzene (CA/Az1) handleable films at different compositions. The photoswitching/back-switching behavior of the samples was investigated using varied LED irradiation sources. Additionally, poly(ethylene glycol) (PEG) was deposited onto cellulose acetate/azobenzene films to study the back-switching process's effect and nature in the fabricated films. Interestingly, the melting enthalpies of PEG before and after being irradiated with blue LED light were 2.5 mJ and 0.8 mJ, respectively. Conveniently, FTIR and UV-visible spectroscopy, thermogravimetry (TGA), contact angle, differential scanning calorimetry (DSC), polarized light microscopy (PLM), and atomic force microscopy (AFM) were used for the characterization of the sample films. Complementarily, theoretical electronic calculations provided a consistent approach to the energetic change in the dihedral angles and non-covalent interaction for the trans and cis isomer in the presence of cellulose acetate monomer. The results of this study revealed that CA/Az1 films are viable photoactive materials displaying handleability attributes with potential uses in harvesting, converting, and storing light energy.

Dual-Color Photoconvertible Fluorescent Probes Based on Directed Photooxidation Induced Conversion for Bioimaging

We herein present a new concept to produce dual-color photoconvertible probes based on a mechanism called Directed Photooxidation Induced Conversion (DPIC). As a support of this mechanism, styryl-coumarins (SCs) bearing Aromatic Singlet Oxygen Reactive Moieties (ASORMs) like furan and pyrrole have been synthesized. SCs are bright fluorophores, which undergo a hypsochromic conversion upon visible light irradiation due to directed photooxidation of the ASORM that leads to the disruption of conjugation. SC-P, a yellow emitting probe bearing a pyrrole moiety, converts to a stable blue emitting coumarin with a 68 nm shift allowing the photoconversion and tracking of lipid droplet in live cells. This new approach might pave the way to a new generation of photoconvertible dyes for advanced bioimaging applications.

Photochemical Mechanisms of Fluorophores Employed in Single-Molecule Localization Microscopy

Decoding cellular processes requires visualization of the spatial distribution and dynamic interactions of biomolecules. It is therefore not surprising that innovations in imaging technologies have facilitated advances in biomedical research. The advent of super-resolution imaging technologies has empowered biomedical researchers with the ability to answer long-standing questions about cellular processes at an entirely new level. Fluorescent probes greatly enhance the specificity and resolution of super-resolution imaging experiments. Here, we introduce key super-resolution imaging technologies, with a brief discussion on single-molecule localization microscopy (SMLM). We evaluate the chemistry and photochemical mechanisms of fluorescent probes employed in SMLM. This Review provides guidance on the identification and adoption of fluorescent probes in single molecule localization microscopy to inspire the design of next-generation fluorescent probes amenable to single-molecule imaging.

Photoswitchable carbon-dot liposomes mediate catalytic cascade reactions for amplified dynamic treatment of tumor cells

Most biochemical reactions that occur in living organisms are catalyzed by a series of enzymes and proceed in a tightly controlled manner. The development of artificial enzyme cascades that resemble multienzyme complexes in nature is of current interest due to their potential in various applications. In this study, a nanozyme based on photoswitchable carbon-dot liposomes (CDsomes) was developed for use in programmable catalytic cascade reactions. These CDsomes prepared from triolein are amphiphilic and self-assemble into liposome-like structures in an aqueous environment. CDsomes feature excitation-dependent photoluminescence and, notably, can undergo reversible switching between a fluorescent on-state and nonfluorescent off-state under different wavelengths of light irradiation. This switching ability enables the CDsomes to exert photocatalytic oxidase- and peroxidase-like activities in their on- (bright) and off- (dark) states, respectively, resulting in the conversion of oxygen molecules into hydrogen peroxide (H₂O₂), followed by the generation of active hydroxyl radicals (OH). The two steps of oxygen activation can be precisely controlled in a sequential manner by photoirradiation at different wavelengths. Catalytic reversibility also enables the CDsomes to produce sufficient reactive oxygen species (ROS) to effectively kill tumor cells. Our results reveal that CDsomes is a promising photo-cycling nanozyme for precise tumor phototherapy through regulated programmable cascade reactions.

New tricks and emerging applications from contemporary azobenzene research

Azobenzenes have many faces. They are well-known as dyes, but most of all, azobenzenes are versatile photoswitchable molecules with powerful photochemical properties. Azobenzene photochemistry has been extensively studied for decades, but only relatively recently research has taken a steer towards applications, ranging from photonics and robotics to photobiology. In this perspective, after an overview of the recent trends in the molecular design of azobenzenes, we highlight three research areas where the azobenzene photoswitches may bring about promising technological innovations: chemical sensing, organic transistors, and cell signaling. Ingenious molecular designs have enabled versatile control of azobenzene photochemical properties, which has in turn facilitated the development of chemical sensors and photoswitchable organic transistors. Finally, the power of azobenzenes in biology is exemplified by vision restoration and photactivation of neural signaling. Although the selected examples reveal only some of the faces of azobenzenes, we expect the fields presented to develop rapidly in the near future, and that azobenzenes will play a central role in this development.

New Chemical Biology Tools for the Histamine Receptor Family

The histamine research community has in the last decade been very active and generated a number of exciting new chemical biology tools for the study of histamine receptors, their ligands, and their pharmacology. In this paper we describe the development of histamine receptor structural biology, the use of receptor conformational biosensors, and the development of new ligands for covalent or fluorescent labeling or for photopharmacological approaches (photocaging and photoswitching). These new tools allow new approaches to study histamine receptors and hopefully will lead to better insights in the molecular aspects of histamine receptors and their ligands.

A practical guide to photoswitching optoacoustics tomography

Photochromic proteins and photoswitching optoacoustics (OA) are a promising combination, that allows OA imaging of even small numbers of cells in whole live animals and thus can facilitate a more wide-spread use of OA in life-science and preclinical research. The concept relies on exploiting the modulation achieved by the photoswitching to discriminate the agents' signal from the non-modulating background. Here we share our analysis approaches that can be readily used on data generated with commercial OA tomography imaging instrumentation allowing-depending on the used photoswitching agent and sample-routine visualizations of as little as several hundreds of transgene labeled cells per imaging volume in the live animal.

Photo-transformable genetically-encoded optical probes for functional highlighting in vivo

Studying the brain requires knowledge about both structure (i.e., connectome) and function of its constituents (neurons and glia alike). This need has prompted the development of novel tools and techniques, in particular optical techniques to examine cells remotely. Early works (1900's) led to the development of novel cell-staining techniques that, when combined with the use of a very simple light microscope, visualized individual neurons and their subcellular compartments in fixed tissues. Today, highlighting of structure and function can be performed on live cells, notably in vivo, owing to discovery of GFP and subsequent development of genetically encoded fluorescent optical tools. In this review, we focus our attention on a subset of optical biosensors, namely probes whose emission can be modified by light. We designate them photo-transformable genetically encoded probes. The family of photo-transformable probes embraces current probes that undergo photoactivation (PA), photoconversion (PC) or photoswitching (PS). We argue that these are particularly suited for studying multiple features of neurons, such as structure, connectivity and function concomitantly, for functional highlighting of neurons in vivo.

Implementation of Blue Light Switchable Bacterial Adhesion for Design of Biofilms

Control of bacterial adhesions to a substrate with high precision in space and time is important to form a well-defined biofilm. Here, we present a method to engineer bacteria such that they adhere specifically to substrates under blue light through the photoswitchable proteins nMag and pMag. This provides exquisite spatiotemporal remote control over these interactions. The engineered bacteria express pMag protein on the surface so that they can adhere to substrates with nMag protein immobilization under blue light, and reversibly detach in the dark. This process can be repeatedly turned on and off. In addition, the bacterial adhesion property can be adjusted by expressing different pMag proteins on the bacterial surface and altering light intensity. This protocol provides light switchable, reversible and tunable control of bacteria adhesion with high spatial and temporal resolution, which enables us to pattern bacteria on substrates with great flexibility.

Excitation wavelength optimization improves photostability of ASAP-family GEVIs

Recent interest in high-throughput recording of neuronal activity has motivated rapid improvements in genetically encoded calcium or voltage indicators (GECIs or GEVIs) for all-optical electrophysiology. Among these probes, the ASAPs, a series of voltage indicators based on a variant of circularly permuted green fluorescent protein (cpGFP) and a conjugated voltage sensitive domain (VSD), are capable of detecting both action potentials and subthreshold neuronal activities. Here we show that the ASAPs, when excited by blue light, undergo reversible photobleaching. We find that this fluorescence loss induced by excitation with 470-nm light can be substantially reversed by low-intensity 405-nm light. We demonstrate that 405-nm and 470-nm co-illumination significantly improved brightness and thereby signal-to-noise ratios during voltage imaging compared to 470-nm illumination alone. Illumination with a single wavelength of 440-nm light also produced similar improvements. We hypothesize that reversible photobleaching is related to cis-trans isomerization and protonation of the GFP chromophore of ASAP proteins. Amino acids that influence chromophore isomerization are potential targets of point mutations for future improvements.

Choosing proper fluorescent dyes, proteins, and imaging techniques to study mitochondrial dynamics in mammalian cells

Mitochondrial dynamics refers to the processes maintaining mitochondrial homeostasis, including mitochondrial fission, fusion, transport, biogenesis, and mitophagy. Mitochondrial dynamics is essential for maintaining the metabolic function of mitochondria as well as their regulatory roles in cell signaling. In this review, we summarize the recently developed imaging techniques for studying mitochondrial dynamics including: mitochondrial-targeted fluorescent proteins and dyes, live-cell imaging using photoactivation, photoswitching and cell fusion, mitochondrial transcription and replication imaging by in situ hybridization, and imaging mitochondrial dynamics by super-resolution microscopy. Moreover, we discuss examples of how to choose and combine proper fluorescent dyes and/or proteins.

Photostable and photoswitching fluorescent dyes for super-resolution imaging

Super-resolution fluorescence microscopy is a recently developed imaging tool for biological researches. Several methods have been developed for detection of fluorescence signals from molecules in a subdiffraction-limited area, breaking the diffraction limit of the conventional optical microscopies and allowing visualization of detailed macromolecular structures in cells. As objectives are exposed to intense laser in the optical systems, fluorophores for super-resolution microscopy must be tolerated even under severe light irradiation conditions. The fluorophores must also be photoactivatable and photoswitchable for single-molecule localization-based super-resolution microscopy, because the number of active fluorophores must be controlled by light irradiation. This has led to growing interest in these properties in the development of fluorophores. In this mini-review, we focus on the development of photostable and photoswitching fluorescent dyes for super-resolution microscopy. We introduce recent efforts, including improvement of fluorophore photostability and control of photoswitching behaviors of fluorophores based on photochemical and photophysical processes. Understanding and manipulation of chemical reactions in excited fluorophores can develop highly photostable and efficiently photoswitchable fluorophores that are suitable for super-resolution imaging applications.

Dual channel RESOLFT nanoscopy by using fluorescent state kinetics

We show that RESOLFT fluorescence nanoscopy, a low light level scanning superresolution technique employing reversibly switchable fluorescent proteins (rsFPs), is capable of dual-channel live-cell imaging that is virtually free of chromatic errors and temporal offsets. This is accomplished using rsEGFP and Dronpa, two rsFPs having similar spectra but different kinetics of switching and fluorescence emission. Our approach is demonstrated by imaging protein distributions and dynamics in living neurons and neuronal tissues.

Imaging the assembly, structure, and function of the nuclear pore inside cells

The nuclear pore complex (NPC) mediates selective transport across the nuclear envelope (NE) and plays crucial roles in several additional cellular functions. In higher eukaryotes, the NPC and the NE disassemble and reassemble during cell division and live-cell imaging has been a powerful tool to analyze these dynamic processes. Here, we present a method for the kinetic analysis of postmitotic NPC assembly and reestablishment of transport competence in intact cells by multicolor 4D imaging and photoswitching. By applying the methods we have established previously using normal rat kidney to HeLa cells, we demonstrate the conservation of NPC assembly in different mammalian cells. We recently showed that the molecular organization of the NPC can be studied by combining stochastic super-resolution microscopy with single-particle averaging and present this method here in detail.

Fluorescence Modulation and Photochromism in Azobismaleimide Derivatives

A fluorophore-photochrome system incorporating an aryleneimine type fluorophore and an azobismaleimide photochrome was developed and the photochemical properties of this system were investigated. The photoisomerization of trans-azoaromatic chromophore leads to the increase of the fluorescence intensity of fluorophore. The cis azobismaleimide isomers revert photochemically to the trans form and the emission intensity decreases. The fluorescence intensity of the imine fluorophore can be modulated under irradiation with UV and visible (436 nm) light due to reversible trans-cis-trans photoisomerization of azobismaleimide partner. The photoisomerization kinetics was obeyed a first-order relationship with a rate constant of 1.95 × 10(-2) s(-1) for azobismaleimide/imine fluorophore system and for polyazothioetherimide/imine derivative the kinetics was described by a biexponential equation.