Antiphase dual-color correlation in a reactant-product pair imparts ultrasensitivity in reaction-linked double-photoswitching fluorescence imaging

Full-Quantum Treatment of Molecular Systems Confirms Novel Supracence Photonic Properties

Our understanding of molecules has stagnated at a single quantum system, with atoms as Newtonian particles and electrons as quantum particles. Here, however, we reveal that both atoms and electrons in a molecule are quantum particles, and their quantum-quantum interactions create a previously unknown, newfangled molecular property-supracence. Molecular supracence is a phenomenon in which the molecule transfers its potential energy from quantum atoms to photo-excited electrons so that the emitted photon has more energy than that of the absorbed one. Importantly, experiments reveal such quantum energy exchanges are independent of temperature. When quantum fluctuation results in absorbing low-energy photons, yet still emitting high-energy photons, supracence occurs. This report, therefore, reveals novel principles governing molecular supracence via experiments that were rationalized by full quantum (FQ) theory. This advancement in understanding predicts the super-spectral resolution of supracence, and molecular imaging confirms such innovative forecasts using closely emitting rhodamine 123 and rhodamine B in living cell imaging of mitochondria and endosomes.

Composite Mesoporous Silica Nanoparticles with Dual-color Afterglow for Cross-correlation-based Living Cell Imaging

Room temperature phosphorescence (RTP) materials are characterized with emission after removing the excitation source. Such long-lived emission feature possesses great potential in biological fluorescence imaging because it enables a way regarding temporal dimension for separating the interference of autofluorescence and common noises typically encountered in conventional fluorescence imaging. Herein, we constructed a new type of mesoporous silica nanoparticles (MSNs)-based composite nanoparticles (NPs) with dual-color long-lived emission, namely millisecond-level green phosphorescence and sub-millisecond-level delayed red fluorescence by encapsulating a typical RTP dye and Rhodamine dye in the cavities of the MSNs with the former acting as energy donor (D) while the latter as acceptor (A). Benefiting from the close D-A proximity, energy match between the donor and the acceptor and the optimized D/A ratio in the composite NPs, efficient triplet-to-singlet Förster resonance energy transfer (TS-FRET) in the NPs occurred upon exciting the donor, which enabled dual-color long-lived emission. The preliminary results of dual-color correlation imaging of live cells based on such emission feature unequivocally verified the unique ability of such NPs for distinguishing the false positive generated by common emitters with single-color emission feature.

Fluorescence detection and imaging of intracellular sulphite using a remote light activatable photochromic nanoprobe

The development of a responsive fluorescent probe for the detection of a particular biomolecule in a specific site at the desired moment is important in the fields of bioanalysis and imaging, molecular biology and biomedical research. In this work, we report the development of a remote-light activatable nanoprobe for the fluorescence detection of sulphite in pure aqueous solution and its imaging applications in living cells. The nanoprobe, Poly-Cm-SP, is fabricated simply by wrapping photochromic molecules (Cm-SP) into a polymer nanoparticle. Upon alternate UV/Vis light irradiation for several seconds, the Poly-Cm-SP nanoprobe exhibits red/blue fluorescence switch due to the inactive/active FRET processes from coumarins to the SP/MR isomers of the photochromic molecule. In the presence of sulphite, the specific reaction of sulphite with the electron deficit "CC" bond of the MR isomer occurs, resulting in an inefficient FRET process and thus exhibiting a constant "ON" blue channel fluorescence signal. After UV-light irradiation, the formation of activated Poly-Cm-MRin situ thus enables the detection of sulphite through recording the ratiometric changes of fluorescence signals at both blue and red channels. The Poly-Cm-SP nanoprobe possesses excellent biocompatibility and lysosome distribution capability, allowing it to be used for photochromic imaging and sulphite detection in the lysosomes of living macrophage cells. This work thus offers a new remote-light activatable nanoprobe for the detection and imaging of sulphite in biological systems.

Bacterial Nanocompartments: Structures, Functions and Applications

Increasing efficiency is an important driving force behind cellular organization and often achieved through compartmentalization. Long recognized as a core principle of eukaryotic cell organization, its widespread occurrence in prokaryotes has only recently come to light. Despite the early discovery of a few microcompartments such as gas vesicles and carboxysomes, the vast majority of these structures in prokaryotes are less than 100 nm in diameter - too small for conventional light microscopy and electron microscopic thin sectioning. Consequently, these smaller-sized nanocompartments have therefore been discovered serendipitously and then through bioinformatics shown to be broadly distributed. Their small uniform size, robust self-assembly, high stability, excellent biocompatibility, and large cargo capacity make them excellent candidates for biotechnology applications. This review will highlight our current knowledge of nanocompartments, the prospects for applications as well as open question and challenges that need to be addressed to fully understand these important structures.

Achieving enhanced solid-state photochromism and mechanochromism by introducing a rigid steric hindrance group

For photochromic molecules, effective isomerization usually requires conformational freedom, which is usually unavailable under solvent-free conditions. In this work, we report a new method, which can realize the reversible switching of spiropyran molecules by introducing a rigid aromatic ring group and this method can provide the required free volume to transform from a closed-ring to an open-ring form. This new molecule can quickly change color in the solid state under ultraviolet light, and can be erased after being heated at 60 °C for about 5 minutes. Furthermore, this new compound presents mechanochromicity when a mechanical force is applied. What is more, it can be used for at least 30 cycles of print-erase operations without apparent fatigue. This new molecule exhibits improved photochromic and anti-fatigue properties in the solid state, which can promote its application in both ultraviolet printing and anti-counterfeiting materials.

Molecular Supracence Resolving Eight Colors in 300-nm Width: Unprecedented Spectral Resolution

Monitoring multiple molecular probes simultaneously establishes their correlations and reveals the holistic mechanism. Current fluorescence imaging, however, is limited to about four colors because of typically circa 100-nm spectral width. Herein, we show that molecular supracence imparts superior spectral resolution, resolving eight colors in 300-nm width, about 37.5-nm per color. A recently discovered light-molecule interacting phenomenon, supracence only measures molecular emission above its excitation energy due to entanglement between atomic quantum system and electronic quantum system. As such, supracence takes advantage of sharp spectral edge of a single pathway and excitation specificity to produce narrow bands, whereas fluorescence has to deal with multiple pathways spilling out low-energy long tail, that causes poor resolution. Thus, supracence enables myriad innovative molecular spectroscopy and microscopic imaging with profound impact broadly.

A hydrogen peroxide activatable nanoprobe for light-controlled "double-check" multi-colour fluorescence imaging

A new probe for precise and accurate bioimaging contributes significantly to advancing biomedical research for early disease diagnosis and treatment monitoring. Through wrapping a photochromic molecule (SP-Np-B) within a polymer nanoparticle, a new light-controlled multicolour fluorescence nanoprobe (Poly-SP-Np-B) is developed for precise fluorescence subcellular bioimaging. Poly-SP-Np-B shows an "OFF-ON" red-emitting fluorescence response upon alternate UV/Vis light irradiation. After activation by hydrogen peroxide (H2O2), a green-emitting Poly-SP-Np nanoparticle is generated, thus allowing light-controlled fluorescence response simultaneously, i.e., green and yellow switch upon alternate UV/Vis light irradiation for 10 and 20 s, respectively. Such a "blinking" fluorescence signal change is not possible by only using a photochromic molecule probe (SP-Np-B) with alternate UV/vis light irradiation for over 5 min. Poly-SP-Np-B has large isomerization kinetic constants (kSP-MR = 0.4543 s-1 and kMR-SP = 0.0809 s-1), excellent biocompatibility and lysosome distribution capability, enabling multicolour fluorescence imaging in live cells. With exo-/endogenous H2O2 activation in lysosomes, light-controlled "double-check" fluorescence imaging at the subcellular level is successfully achieved. More specifically, the change in fluorescence imaging is reversible in green, red and yellow channels in live cells upon excitation under alternate UV and visible light. This work thus provides a new strategy to develop switchable photochromic probes for precise fluorescence bioassay and bioimaging.

Correlative dual-alternating-color photoswitching fluorescence imaging and AFM enable ultrastructural analyses of complex structures with nanoscale resolution

There is a practical motivation for correlating different types of microscopy for revealing complementary information of ultrastructures with resolution beyond the diffraction limit. The correlative microscopy strategy based on the combination of super-resolution fluorescence imaging with atomic force microscopy (AFM) is expected to provide both the specificity and three-dimensional structural information of nanomaterials. Herein we synthesized a dual-alternating-color photoswitchable fluorescent probe based on a naphthalimide-spiropyran dyad (NI-SP) and explored the capability of such correlative microscopy for visualizing nanostructures with complex structural hierarchy. NI-SP underwent reversible photoswitching between green and red fluorescence based on a reversible photochemical reaction and such reaction-linked correlation between two distinct types of fluorescence signals intrinsically enabled mutual authentication in super-resolution fluorescence imaging. Additionally, such correlative microscopy also demonstrated mutual complementation between different pieces of structural information of the target acquired via fluorescence imaging and AFM, respectively, in which the former reveals spatial distribution of fluorescent dyes in the nanoscale polymer fibroid micelles while the latter maps the topographical structure of the target with complex structural hierarchy. The results obtained in this work proclaimed that the combination of such correlative microscopy with our NI-SP probe is an effective modality for ultrastructural analysis and has future applications in various complex systems such as tissue/organ imaging.

Photoisomerization of PtII Complexes Containing Two Different Photochromic Chromophores: Boron Chromophore versus Dithienylethene Chromophore

In order to examine competitive photoisomerization, a series of novel photochromic Pt^II molecules that contain both dithienylethene (DTE) and B(ppy)Mes₂ units (ppy=2-phenylpyridine, Mes=mesityl) were successfully synthesized and fully structurally characterized. Their photochromic properties were examined by UV/Vis, emission and NMR spectroscopy. It was found that the DTE unit in all three compounds is the preferred photoisomerization site, exhibiting reversible photochromism with irradiation. The B(ppy)Mes₂ unit does not undergo photoisomerization in these molecules, but likely enhances the photoisomerization quantum efficiency of the DTE moiety through the antenna effect. Extended irradiation with UV light leads to the rearrangement of the ring-closed isomers of DTE. TD-DFT computational studies indicate that the DTE photocyclization proceeds via a triplet pathway through an efficient energy transfer process.

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.

Switchable Fluorophores for Single-Molecule Localization Microscopy

The past decade has witnessed an explosion in the use of super-resolution fluorescence microscopy methods in biology and other fields. Single-molecule localization microscopy (SMLM) is one of the most widespread of these methods and owes its success in large part to the ability to control the on-off state of fluorophores through various chemical, photochemical, or binding-unbinding mechanisms. We provide here a comprehensive overview of switchable fluorophores in SMLM including a detailed review of all major classes of SMLM fluorophores, and we also address strategies for labeling specimens, considerations for multichannel and live-cell imaging, potential pitfalls, and areas for future development.

Solid-State Photoinduced Luminescence Switch for Advanced Anticounterfeiting and Super-Resolution Imaging Applications

Solid-state organic photoswitches with reversible luminescence modulation property are highly attractive because of their wide prospects in advanced photonic applications, such as optical data storage, anticounterfeiting and bioimaging. Yet, developing such materials has long been a significant challenge. In this work, we construct an efficient solid-state photoswitch based on a spiropyran-functionalized distyrylanthracene derivative (DSA-2SP) that exhibits exceptional reversible absorption/luminescence modulation ability. Efficient photoswitching between DSA-2SP and its photoisomer DSA-2MC are facilitated by large free volumes induced by nonplanar molecular structures of DSA moieties, as well as the intramolecular hydrogen bonds between the DSA and MC moieties. Consequently, the excellent solid-state photochromic property of DSA-2SP is highly applicable as both anticounterfeiting inks and super-resolution imaging agents.

Photoswitchable chromic behavior of conjugated polymer films for reversible patterning and construction of a logic gate

Photoswitchable fluorescent films were constructed using a conjugated polymer as a fluorescence-color changeable unit and a photochromic molecule as a phototriggered, selectively energy-accepting component.

“Blinking” silica nanoparticles for optical super resolution imaging of cancer cells

A strategy to fabricate blinking silica nanoparticles is presented, which can be used in single molecule localization imaging.

Photochromic RAFT reagent helps construct superior photoswitchable fluorescent polymer nanoparticles for rewritable fluorescence patterning and intracellular dual-color imaging

Photoswitchable fluorescent polymeric nanoparticles with photochromic RAFT chain transfer reagent were synthesized for rewritable fluorescence patterning and intracellular dual color imaging.

Preparation of Gold-Carbon Dots and Ratiometric Fluorescence Cellular Imaging

In this study, we synthesized novel gold-carbon dots (GCDs) with unique properties by microwave-assisted method. The characterization of high-resolution transmission electron microscope (HRTEM), XRD, high-angle annular dark field scanning transmission electron microscope (HAADF-STEM), and energy dispersive spectrometer demonstrates that GCDs are composed of carbon and Au. Tiny Au clusters are dispersed in a 2 nm-size carbon skeleton, which integrates the properties of typical CDs and gold nanoclusters (AuNCs), displaying fascinating peroxidase-like activity and single excitation/dual emission. Dual emission of the GCDs exhibits different fluorescent response to the target species and enables the GCDs to be exploited for sensing and bioimaging. The highly photostable and biocompatible GCDs were applied to dual fluorescent imaging for breast cancer cells and normal rat osteoblast cells under a single excitation. Moreover, ratiometric fluorescence imaging was used to monitor Fe(3+) level in normal rat osteoblast cells.

Labelling Bacterial Nanocages with Photo-switchable Fluorophores

The robustness and biocompatibility of bacterial nanocages holds promise for bio‐nanotechnologies. The propensity of these nano‐carriers to penetrate cells has been demonstrated, which calls for the development of tracking strategies, both in vitro and in vivo. Here, we label bacterial nanocages with photo‐switchable fluorophores, to facilitate their imaging by super‐resolution microscopy. We demonstrate the functionalization of the encapsulin from Brevibacterium linens with a spiropyran, which is not fluorescent, by covalent attachment to the amine residues at the outer encapsulin shell. Upon alternating irradiation with ultraviolet and visible light, the spiropyran switches forth and back to its fluorescent merocyanine photo‐isomer and thus the fluorescence can be switched on and off, reversibly. We also show that the bacterial compartments preserve their structural integrity upon covalent modification and over at least five irradiation cycles.

Kinetics of Reactive Modules Adds Discriminative Dimensions for Selective Cell Imaging

Living cells are chemical mixtures of exceptional interest and significance, whose investigation requires the development of powerful analytical tools fulfilling the demanding constraints resulting from their singular features. In particular, multiplexed observation of a large number of molecular targets with high spatiotemporal resolution appears highly desirable. One attractive road to address this analytical challenge relies on engaging the targets in reactions and exploiting the rich kinetic signature of the resulting reactive module, which originates from its topology and its rate constants. This review explores the various facets of this promising strategy. We first emphasize the singularity of the content of a living cell as a chemical mixture and suggest that its multiplexed observation is significant and timely. Then, we show that exploiting the kinetics of analytical processes is relevant to selectively detect a given analyte: upon perturbing the system, the kinetic window associated to response read-out has to be matched with that of the targeted reactive module. Eventually, we introduce the state-of-the-art of cell imaging exploiting protocols based on reaction kinetics and draw some promising perspectives.

Photoswitching Near-Infrared Fluorescence from Polymer Nanoparticles Catapults Signals over the Region of Noises and Interferences for Enhanced Sensitivity

As a very sensitive technique, photoswitchable fluorescence not only gains ultrasensitivity but also imparts many novel and unexpected applications. Applications of near-infrared (NIR) fluorescence have demonstrated low background noises, high tissue-penetrating ability, and an ability to reduce photodamage to live cells. Because of these desired features, NIR-fluorescent dyes have been the premium among fluorescent dyes, and probes with photoswitchable NIR fluorescence are even more desirable for enhanced signal quality in the emerging optical imaging modalities but rarely used because they are extremely challenging to design and construct. Using a spiropyran derivative functioning as both a photoswitch and a fluorophore to launch its periodically modulated red fluorescence excitation energy into a NIR acceptor, we fabricated core-shell polymer nanoparticles exhibiting a photoswitchable fluorescence signal within the biological window (∼700-1000 nm) with a peak maximum of 776 nm. Live cells constantly synthesize new molecules, including fluorescent molecules, and also endocytose exogenous particles, including fluorescent particles. Upon excitation at different wavelengths, these fluorescent species bring about background noises and interferences covering nearly the whole visible region and therefore render many intracellular targets unaddressable. The oscillating NIR fluorescence signal with an on/off ratio of up to 67 that the polymer nanoparticles display is beyond the typical background noises and interferences, thus producing superior sharpness, reliability, and signal-to-noise ratios in cellular imaging. Taking these salient features, we anticipate that these types of nanoparticles will be useful for in vivo imaging of biological tissue and other complex specimens, where two-photon activation and excitation are used in combination with NIR-fluorescence photoswitching.

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.

Reversible Photoisomerization of Spiropyran on the Surfaces of Au25 Nanoclusters

Au25 nanoclusters functionalized with a spiropyran molecular switch are synthesized via a ligand-exchange reaction at low temperature. The resulting nanoclusters are characterized by optical and NMR spectroscopies as well as by mass spectrometry. Spiropyran bound to nanoclusters isomerizes in a reversible fashion when exposed to UV and visible light, and its properties are similar to those of free spiropyran molecules in solution. The reversible photoisomerization entails the modulation of fluorescence as well as the light-controlled self-assembly of nanoclusters.

Fluorescence modulation of a pyrazolone dye in the solid state based on energy transfer

A photochromic pyrazolone is exploited as a modulator to toggle the fluorescence of a dye “on” and “off” based on energy transfer.