Photoswitchable fluorescent nanoparticles and their emerging applications

Photoswitching the fluorescence of nanoparticles for advanced optical applications

The dynamic optical response properties and the distinct features of nanomaterials make photoswitchable fluorescent nanoparticles (PF NPs) attractive candidates for advanced optical applications. Over the past few decades, the design of PF NPs by coupling photochromic and fluorescent motifs at the nanoscale has been actively pursued, and substantial efforts have been made to exploit their potential applications. In this perspective, we critically summarize various design principles for fabricating these PF NPs. Then, we discuss their distinct optical properties from different aspects by highlighting the capability of NPs in fabricating new, robust photoswitch systems. Afterwards, we introduce the pivotal role of PF NPs in advanced optical applications, including sensing, anti-counterfeiting and imaging. Finally, current challenges and future development of PF NPs are briefly discussed.

Photoluminescence switching in quantum dots connected with fluorinated and hydrogenated photochromic molecules

We investigate switching of photoluminescence (PL) from PbS quantum dots (QDs) crosslinked with two different types of photochromic diarylethene molecules, 4,4'-(1-cyclopentene-1,2-diyl)bis[5-methyl-2-thiophenecarboxylic acid] (1H) and 4,4'-(1-perfluorocyclopentene-1,2-diyl)bis[5-methyl-2-thiophenecarboxylic acid] (2F). Our results show that the QDs crosslinked with the hydrogenated molecule (1H) exhibit a greater amount of switching in photoluminescence intensity compared to QDs crosslinked with the fluorinated molecule (2F). With a combination of differential pulse voltammetry and density functional theory, we attribute the different amount of PL switching to the different energy levels between 1H and 2F molecules which result in different potential barrier heights across adjacent QDs. Our findings provide a deeper understanding of how the energy levels of bridge molecules influence charge tunneling and PL switching performance in QD systems and offer deeper insights for the future design and development of QD based photo-switches.

Ligand-protected nanocluster-mediated photoswitchable fluorescent nanoprobes towards dual-color cellular imaging

Development of robust multi-color photoswitchable fluorescent probes is critical for many optical applications, but it remains a challenge to rationally design these probes. Here, we report a new design of Förster resonance energy transfer-based dual-color photoswitchable fluorescent nanoparticles (DPF NPs) by taking advantage of the distinct properties of ligand-protected gold nanoclusters (AuNCs). Detailed photophysical studies revealed that ultrasmall-sized AuNCs not only act as the FRET donors due to their intrinsic fluorescence properties, but also play a significant role in regulating the photochromic and aggregate properties of spiropyran through ligand-spiropyran interactions. These DPF NPs exhibit a high fluorescence on/off ratio (∼90%) for both green and red fluorescence emission, and good reversibility during cycled photo-stimulation. Cell imaging experiments showed that DPF NPs could specifically accumulate in lipid droplets, and enable photoswitchable dual-color imaging in living cells. Moreover, by labeling mitochondria with a green-emitting marker, we demonstrated that DPF NPs can distinguish different targets based on dynamic and static fluorescence signals at the sub-cellular level in two emission channels reliably. This study provides a new strategy for designing robust photoswitchable fluorescent probes by modulating the properties of photochromic dyes through ligand-protected nanoclusters, which can be generalized for the development of other photoswitch systems towards advanced optical applications.

Sterically Hindered Diarylethenes with a Benzobis(thiadiazole) Bridge: Enantiospecific Transformation and Reversible Photosuperstructures

ConspectusPhotochromic diarylethenes featuring reversible regulation by external light irradiation have attracted increasing attention in versatile applications such as logic gates, supramolecular systems, liquid crystals, and super-resolution imaging because of their outstanding bistability and fatigue resistance. However, for typical diarylethene systems, there always exist three typical unsolved issues. The first is how to modulate the bistability between the open and closed forms from the viewpoint of ethene bridge aromaticity. The second is how to decrease and avoid the photoinactive parallel conformer in order to achieve a high quantum yield, since the open form possesses the photoactive antiparallel (ap) conformation and the photoinactive parallel (p) conformation. Because of the typical rapid rotation of the flexible side aryl groups, the two conformers cannot be separated efficiently, thereby resulting in a relatively low photocyclization quantum yield. The third is how to fulfill the enantiospecific transformation with reversibility to photomodulate the chirality. Stereochemically, the ap conformer with C₂ symmetry can be further subdivided into a pair of enantiomers with P and M helicity originating from the central hexatriene moiety. Similarly, the rapid rotation can also lead to the loss of intrinsic chirality, restricting the development and application of light-driven chiroptical switches. Accordingly, it is desirable to construct a specific diarylethene system to break through these bottlenecks for real versatile applications.Our group has recently developed a unique sterically hindered diarylethene system based on benzobis(thiadiazole) as the ethene bridge for completely solving these issues. We introduce a low-aromaticity benzobis(thiadiazole) unit into the diarylethene as a central ethene bridge with incomparably high bistability. To block or freeze the rotation of flexible side aryls, we further incorporate a large bulky benzothiophene unit to induce a large steric hindrance, or rotation barrier, between the ethene bridge and side aryls, thereby successfully separating multiple conformers of the diarylethenes with high photocyclization quantum yields and enantiospecific photoreaction. Consequently, given such a fantastic building block, we enhance its performance by means of supramolecular self-assembly, thereby realizing unique conformer-dependent self-assembly as well as unprecedented concerted isomerization and enantiospecific photoreaction of photoresponsive metallacycles. In addition, decoration of the intrinsically chiral diarylethenes with mesogenic units can enable us to manipulate the helical superstructure of liquid crystals, thus achieving a multiple anticounterfeiting technique and a quadridimensional manipulable laser. We also unravel the dual aggregation-induced emission (AIE) behavior of the sterically hindered diarylethene, especially as applied in super-resolution imaging.

Nanoparticle-based single molecule fluorescent probes

Single molecule fluorescent probes have attracted considerable attention duet to their ultimate sensitivity, fast response, low sample consumption, and high signal-to-noise ratio. Nanoparticles with outstanding optical properties make them perfect candidates for probes in application of single molecule detection. In this review, we focus on various kinds of nanoparticles acting as single molecule fluorescent probes, including quantum dots, upconverting fluorescent nanoparticles, carbon dots, single-wall carbon nanotubes, fluorescent nanodiamonds, polymeric nanoparticles, nanoclusters, and metallic nanoparticles. Optical properties of various nanoparticles and their recent application in single molecule fluorescent probes are explored. How nanoparticles boost the sensitivity of detection is emphasized in combination with different sensing strategies. Future trends of nanoparticles in single molecule detection are also discussed. We hope this review can provide practical guidance for researchers who work on nanoparticle-based single molecule fluorescent probes.

Immunophenotyping: Analytical approaches and role in preclinical development of nanomedicines

Pharmaceutical products can activate immune cells, suppress their function, or change the immune responses to traditional immunologically active agonists such as those present in microbes. Therefore, the assessment of immunostimulation, immunosuppression, and immunomodulation comprises the backbone of immunotoxicity studies of new drug entities. Depending on physicochemical properties (e.g., size, charge, surface functionalities, hydrophobicity), nanoparticles can be immunostimulatory, immunosuppressive, and immunomodulatory. Various methods and experimental frameworks have been established to support preclinical translational studies of nanotechnology-based drug products. Immunophenotyping after the exposure of cells or preclinical animal models to nanoparticles can provide critical information about the changes in both the numbers of immune cells and their activation status. However, this methodology is underutilized in preclinical studies of engineered nanomaterials. Herein, we review current literature about varieties of instrumentation and methods utilized for immunophenotyping, discuss their advantages and limitations, and propose a roadmap for applying immunophenotyping to support preclinical immunological characterization of nanotechnology-based formulations.

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.

Photochromism and photoswitchable luminescence in a Zn7 cluster-based metal-organic framework with an organic guest

Photochromic materials coupled with photoswitchable luminescence functionalities are of particular interest due to their potential applications in switches and optical memory devices. However, the construction of such materials, especially those with two-color emission states, is still challenging. In this context, a rare Zn₇ cluster-based host-guest MOF material, (bbmp)[Zn₇(IPA)₆(OH)₄(H₂O)₂] (1) (H₂IPA = isophthalic acid, bbmp·2I = 4,4'-([1,1'-biphenyl]-4,4'-diyl)bis(1-methylpyridin-1-ium) diiodide), was prepared by encapsulating an organic cation into an anionic MOF produced from zinc cations and isophthalic acid ligands, which exhibits reversible naked detectable photochromic properties varying from yellow to green upon UV-Vis light irradiation. The photoactive guest bbmp²⁺ and the short O⋯N⁺ distances between the oxygen atoms of the carboxylate groups and the pyridine ring play a crucial role in the photochromism of this compound. More interestingly, the luminescence color of this cluster-based host-guest material can be reversibly switched from green to blue upon irradiation, exhibiting photoswitchable luminescence properties.

Long-term switching of single photochromic triads based on dithienylcyclopentene and fluorophores at cryogenic temperatures

Photochromic molecules can be reversibly converted between two bistable forms by light. These systems have been intensively studied for applications as molecular memories, sensing devices, or super-resolution optical microscopy. Here, we study the long-term switching behavior of single photochromic triads under oxygen-free conditions at 10 K. The triads consist of a photochromic unit that is covalently linked to two strong fluorophores that were employed for monitoring the light-induced conversions of the switch via changes in the fluorescence intensity from the fluorophores. As dyes we use either perylene bisimide or boron-dipyrromethen, and as photochromic switch we use dithienylcyclopentene (DCP). Both types of triads showed high fatigue resistance allowing for up to 6000 switching cycles of a single triad corresponding to time durations in the order of 80 min without deterioration. Long-term analysis of the switching cycles reveals that the probability that an intensity change in the emission from the dyes can be assigned to an externally stimulated conversion of the DCP (rather than to stochastic blinking of the dye molecules) amounts to 0.7 ± 0.1 for both types of triads. This number is far too low for optical data storage using single triads and implications concerning the miniaturization of optical memories based on such systems will be discussed. Yet, together with the high fatigue resistance, this number is encouraging for applications in super-resolution optical microscopy on frozen biological samples.

Light-Triggered Switching of Quantum Dot Photoluminescence through Excited-State Electron Transfer to Surface-Bound Photochromic Molecules

This paper describes reversible "on-off" switching of the photoluminescence (PL) intensity of CdSe quantum dots (QDs), mediated by photochromic furylfulgide carboxylate (FFC) molecules chemisorbed to the surfaces of the QDs. Repeated cycles of UV and visible illumination switch the FFC between "closed" and "open" isomers. Reversible switching of the QDs' PL intensity by >80% is enabled by different rates and yields of PL-quenching photoinduced electron transfer (PET) from the QDs to the respective isomers. This difference is consistent with cyclic voltammetry measurements and density functional calculations of the isomers' frontier orbital energies. This work demonstrates fatigue-resistant modulation of the PL of a QD-molecule complex through remote control of PET. Such control potentially enables applications, such as all-optical memory, sensing, and imaging, that benefit from a fast, tunable, and reversible response to light stimuli.

Dual Emission: Classes, Mechanisms, and Conditions

There has been much interest in dual-emission materials in the past few years for materials and life science applications; however, a systematic overview of the underlying processes is so-far missing. We resolve this issue herein by classifying dual-emission (DE) phenomena as relying on one emitter with two emitting states (DE1), two independent emitters (DE2), or two correlated emitters (DE3). Relevant DE mechanisms for materials science are then briefly described together with the electronic and/or geometrical conditions under which they occur. For further reading, references are given that offer detailed insight into the complex mechanistic aspects of the various DE processes or provide overviews on materials families or their applications. By avoiding ambiguities and misinterpretations, this systematic, insightful Review might inspire future targeted designs of DE materials.

Manipulating the fluorescence lifetime at the sub-cellular scale via photo-switchable barcoding

Fluorescent barcoding is a pivotal technique for the investigation of the microscale world, from information storage to the monitoring of dynamic biochemical processes. Using fluorescence lifetime as the readout modality offers more reproducible and quantitative outputs compared to conventional fluorescent barcoding, being independent of sample concentration and measurement methods. However, the use of fluorescence lifetime in this area has been limited by the lack of strategies that provide spatiotemporal manipulation of the coding process. In this study, we design a two-component photo-switchable nanogel that exhibits variable fluorescence lifetime upon photoisomerization-induced energy transfer processes through light irradiation. This remotely manipulated fluorescence lifetime property could be visually mapped using fluorescence lifetime imaging microscopy (FLIM), allowing selective storage and display of information at the microscale. Most importantly, the reversibility of this system further provides a strategy for minimizing the background influence in fluorescence lifetime imaging of live cells and sub-cellular organelles.

A simplicity-guided cocktail approach toward multicolor fluorescent systems

A molecular cocktail containing two photochromic diarylethene derivatives that displays multicolor emission spanning blue-green to orange in a color-correlated fashion has been devised. The function does not rely on excited state communication such as energy transfer reactions, which is the typical case for similar systems. Instead, harnessing the intrinsic fluorescent and photochromic properties of the two individual diarylethene derivatives run in parallel is enough to realize the color changes. This offers an extremely flexible situation as for the choice of the fluorophores and their respective concentrations. The function is conveniently demonstrated in bulk solution at μM concentrations, where a single light source serves as the color changing stimulus.

The first example of “turn-off” red fluorescence photoswitching for the representatives of nitrile-rich negative photochromes

The first example of reversible fluorescence photoswitching by visible light was shown for the representatives of negative photochromes containing a nitrile-rich acceptor.

An all-photonic full color RGB system based on molecular photoswitches

On-command changes in the emission color of functional materials is a sought-after property in many contexts. Of particular interest are systems using light as the external trigger to induce the color changes. Here we report on a tri-component cocktail consisting of a fluorescent donor molecule and two photochromic acceptor molecules encapsulated in polymer micelles and we show that the color of the emitted fluorescence can be continuously changed from blue-to-green and from blue-to-red upon selective light-induced isomerization of the photochromic acceptors to the fluorescent forms. Interestingly, isomerization of both acceptors to different degrees allows for the generation of all emission colors within the red-green-blue (RGB) color system. The function relies on orthogonally controlled FRET reactions between the blue emitting donor and the green and red emitting acceptors, respectively.

Stimuli-responsive multicolor luminescent materials typically react on external triggers of physical nature, but photonically controlled systems which allow for remote operation were not realized. Here the authors use light as the stimulus of a responsive luminescent material which eliminates the need for physical access.

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.

Quantum Coherent Modulation-Enhanced Single-Molecule Imaging Microscopy

In fluorescence imaging and detection, undesired fluorescence interference (such as autofluorescence) often hampers the contrast of the image and even prevents the identification of structures of interest. Here, we develop a quantum coherent modulation-enhanced (QCME) single-molecule imaging microscopy (SMIM) to substantially eliminate the strong fluorescence interference, based on manipulation of the excited-state population probability of a single molecule. By periodically modulating the phase difference between the ultrashort pulse pairs and performing a discrete Fourier transform of the arrival time of emitted photons, the decimation of single molecules from strong interference in QCME-SMIM has been clearly determined, where the signal-to-interference ratio is enhanced by more than 2 orders of magnitude. This technique, confirmed to be universal to organic dyes and linked with biomacromolecules, paves the way to high-contrast bioimaging under unfavorable conditions.

Efficient “turn-off” fluorescence photoswitching in a highly fluorescent diarylethene single crystal

Efficient “turn-off” fluorescence photoswitching with full reversibility was successfully demonstrated in a fluorescent diarylethene single crystal.

Development of fluorescent nanoparticles with aggregation-induced delayed fluorescence features, improved brightness and photostability for living cells imaging

Bright, photostable fluorescent nanoparticles with long fluorescence lifetimes were fabricated based on fluorophores with AIE and TADF characteristics for bioimaging.

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.

Stable and Photoswitchable Carbon-Dot Liposome

Carbon-dot (C-dot) liposome consisting of several thousands of C-dots shows interesting photoswitching properties. The water-dispersible C-dot liposome possesses intrinsic photoluminescence (PL) and is stable against salt and photoirradiation. The PL of C-dot liposome can be turned off and then on under photoirradiation over the wavelength regions of 510-540 nm and 365-420 nm, respectively. Like reported C-dots, the C-dot liposome emits various colors when excited at different wavelengths. Having great stability and high contrast, images of individual C-dot liposome have been recorded, showing negligible photoblinking. Through a simple photolithographic approach, micropatterns of C-dot liposomes emitting different colors have been fabricated.

Photoswitchable fluorescent polymeric nanoparticles for rewritable fluorescence patterning and intracellular dual-color imaging with AIE-based fluorogens as FRET donors

Photoswitchable fluorescent polymeric nanoparticles with AIE-based fluorogens as FRET donors were prepared for rewritable fluorescence patterning and intracellular dual-color imaging.

Photoswitchable non-fluorescent thermochromic dye-nanoparticle hybrid probes

Photoswitchable fluorescent proteins with controllable light-dark states and spectral shifts in emission in response to light have led to breakthroughs in the study of cell biology. Nevertheless, conventional photoswitching is not applicable for weakly fluorescent proteins and requires UV light with low depth penetration in bio-tissue. Here we introduce a novel concept of photoswitchable hybrid probes consisting of thermochromic dye and absorbing nanoparticles, in which temperature-sensitive light-dark states and spectral shifts in absorption can be switched through controllable photothermal heating of doped nanoparticles. The proof-of-concept is demonstrated through the use of two different types of temperature-sensitive dyes doped with magnetic nanoparticles and reversibly photoswitched by a near-infrared laser. Photoacoustic imaging revealed the high contrast of these probes, which is sufficient for their visualization in cells and deep tissue. Our results suggest that these new photoswitchable multicolour probes can be used for multimodal cellular diagnostics and potentially for magnetic and photothermal therapy.

Selective molecular recognition on calixarene-functionalized 3D surfaces

Calixarene based various 3D surface materials with unique signal amplification in molecular recognition are presented, including quantum dots (QDs), metal nanoparticles (NPs), nanotubes, and mesoporous silica.

Visible light photoswitching of conjugated polymer nanoparticle fluorescence

Conjugated polymer nanoparticles doped with a reverse photochromic dye exhibit highly quenched fluorescence that can be reversibly activated by controlling the form of the photochrome with visible light.