Multiphoton excited fluorescent materials for frequency upconversion emission and fluorescent probes

Structure-activity relationships of aniline-based squaraines for distinguishable staining and bright two-photon fluorescence bioimaging in plant cells

An organelle-selective vision provides insights into the physiological response of plants and crops to environmental stresses in sustainable agriculture ecosystems. Biological applications often require two-photon excited fluorophores with low phototoxicity, high brightness, deep penetration, and tuneable cell entry. We obtained three aniline-based squaraines (SQs) tuned from hydrophobic to hydrophilic characteristics by modifying terminal pendant groups and substituents, and investigated their steady-state absorption and far-red-emitting fluorescence properties. The SQs exhibited two-photon absorption (2PA) ranging from 750 to 870 nm within the first biological spectral window; their structure-property relationships, corresponding to the 2PA cross sections (δ2PA), and structure differences were demonstrated. The maximum δ2PA value was ∼1220 GM at 800 nm for hydrophilic SQ3. Distinct biological staining efficiency and selective SQ bioimaging were evaluated utilizing the onion epidermal cell model. Contrary to the hydrophobic SQ1 results in the onion epidermal cell wall, amphiphilic SQ2 tagged the vacuole and nucleus and SQ3 tagged the vacuole. Distinguishable staining profiles in the roots and leaves were achieved. We believe that this study is the first to demonstrate distinct visualisation efficiency induced by the structure differences of two-photon excited SQs. Our results can help establish the versatile roles of novel near-infrared-emitting SQs in biological applications.

Second Near-Infrared (NIR-II) Window for Imaging-Navigated Modulation of Brain Structure and Function

For a long time, optical imaging of the deep brain with high resolution has been a challenge. Recently, with the advance in second near-infrared (NIR-II) bioimaging techniques and imaging contrast agents, NIR-II window bioimaging has attracted great attention to monitoring deeper biological or pathophysiological processes with high signal-to-noise ratio (SNR) and spatiotemporal resolution. Assisted with NIR-II bioimaging, the modulation of structure and function of brain is promising to be noninvasive and more precise. Herein, in this review, first the advantage of NIR-II light in brain imaging from the interaction between NIR-II and tissue is elaborated. Then, several specific NIR-II bioimaging technologies are introduced, including NIR-II fluorescence imaging, multiphoton fluorescence imaging, and photoacoustic imaging. Furthermore, the corresponding contrast agents are summarized. Next, the application of various NIR-II bioimaging technologies in visualizing the characteristics of cerebrovascular network and monitoring the changes of the pathology signals will be presented. After that, the modulation of brain structure and function based on NIR-II bioimaging will be discussed, including treatment of glioblastoma, guidance of cell transplantation, and neuromodulation. In the end, future perspectives that would help improve the clinical translation of NIR-II light are proposed.

Perspective and Prospects on persistent luminescent nanoparticles for biological imaging and tumor therapy

Persistent luminescent nanoparticles (PLNPs) are photoluminescent materials that can still emit luminescence after the cessation of the excitation light source. In recent years, due to their unique optical properties, the PLNPs have attracted extensive attention in the biomedical field. Since the PLNPs effectively eliminate autofluorescence interference from biological tissues, many researchers have contributed a lot of work in the fields of biological imaging and tumor therapy. This article mainly introduces the synthesis methods of the PLNPs and their progress in the application of biological imaging and tumor therapy, as well as the challenges and development prospects.

Multimodal analyses of vitiligo skin identifies tissue characteristics of stable disease

Vitiligo is an autoimmune skin disease characterized by the destruction of melanocytes by autoreactive CD8⁺ T cells. Melanocyte destruction in active vitiligo is mediated by CD8⁺ T cells, but the persistence of white patches in stable disease is poorly understood. The interaction between immune cells, melanocytes, and keratinocytes in situ in human skin has been difficult to study due to the lack of proper tools. We combine noninvasive multiphoton microscopy (MPM) imaging and single-cell RNA-Seq (scRNA-Seq) to identify subpopulations of keratinocytes in stable vitiligo patients. We show that, compared with nonlesional skin, some keratinocyte subpopulations are enriched in lesional vitiligo skin and shift their energy utilization toward oxidative phosphorylation. Systematic investigation of cell-to-cell communication networks show that this small population of keratinocyte secrete CXCL9 and CXCL10 to potentially drive vitiligo persistence. Pseudotemporal dynamics analyses predict an alternative differentiation trajectory that generates this new population of keratinocytes in vitiligo skin. Further MPM imaging of patients undergoing punch grafting treatment showed that keratinocytes favoring oxidative phosphorylation persist in nonresponders but normalize in responders. In summary, we couple advanced imaging with transcriptomics and bioinformatics to discover cell-to-cell communication networks and keratinocyte cell states that can perpetuate inflammation and prevent repigmentation.

Ultra-bright carbon quantum dots for rapid cell staining

A schematic illustration of the synthesis of G-CDs and cell imaging under one-photon and two-photon conditions.

Resonance-Enhanced Two-Photon Absorption and Optical Power Limiting Properties of Three-Dimensional Perylene Bisimide Derivatives

Push-pull organic structures characterized by an intramolecular charge transfer (ICT) process and π-electron delocalization are potentially interesting luminescent materials. A series of three-dimensional o-carborane-containing perylene bisimide derivatives (PBIs) were synthesized, and their optical properties were systematically investigated to illustrate the stereo effect, especially on the two-photon absorption (2PA) and optical power limiting (OPL) properties. Open-aperture Z-scan curves showed that all four PBIs displayed strong and broad two-photon absorptivities based on the resonance-enhanced phenomenon. The maximum degenerate two-photon absorption cross section (δ_2PA) increased with the number of PBI substituents. The derivative CB-PBI possessed a δ_2PA value of ∼2400 GM at 650 nm, a significant enhancement in comparison with that of the parent PBI (∼719 GM), ascribed to the present stereo effect. When the aromatic-donating units changed from naphthyl and pyrenyl to PBI, the generated multidimensional intramolecular charge transfer (ICT) from the aromatic units to the o-carborane cage contributed to the 2PA processes. All of the fluorophores exhibited excellent optical power limiting (OPL) performances as well as a minimum limiting threshold of ∼4.98 mJ/cm² for CB-PBI. These significant results not only allow us to get deep insight into the nature of the fundamental stereo effect and nonlinear optical (NLO) response involved but also guide us toward the design of new multifunctional luminescent materials.

Theranostic Applications of Nanoparticle-Mediated Photoactivated Therapies

Nanoparticle-mediated light-activated therapies, such as photodynamic therapy and photothermal therapy, are earnestly being viewed as efficient interventional strategies against several cancer types. Theranostics is a key hallmark of cancer nanomedicine since it allows diagnosis and therapy of both primary and metastatic cancer using a single nanoprobe. Advanced in vivo diagnostic imaging using theranostic nanoparticles not only provides precise information about the location of tumor/s but also outlines the narrow time window corresponding to the maximum tumor-specific drug accumulation. Such information plays a critical role in guiding light-activated therapies with high spatio-temporal accuracy. Furthermore, theranostics facilitates monitoring the progression of therapy in real time. Herein, we provide a general review of the application of theranostic nanoparticles for in vivo image-guided light-activated therapy in cancer. The imaging modalities considered here include fluorescence imaging, photoacoustic imaging, thermal imaging, magnetic resonance imaging, X-ray computed tomography, positron emission tomography, and single-photon emission computed tomography. The review concludes with a brief discussion about the broad scope of theranostic light-activated nanomedicine.

Discovery of Highly Specific Catalytic-Site-Targeting Fluorescent Probes for Detecting Lysosomal PDE10A in Living Cells

A challenge for sensors targeting specific enzymes of interest in their native environment for direct imaging is that they rationally exploit a highly selective fluorescent probe with a high binding affinity to provide real-time detection. Immunohistochemical staining, proteomic analysis, or recent enzymatic fluorescent probes are not optimal for tracking specific enzymes directly in living cells. Herein, we introduce the concept of designing a highly effective fluorescent probe (BVQ1814) targeting phosphodiesterase 10A with a highly potent affinity and a >1000-fold subfamily selectivity by gaining insights into the three-dimensional structural information of the active site of the catalytic pocket. BVQ1814 showed an outstanding binding affinity for PDE10A in vitro and specifically detected PDE10A in living cells, indicating that most PDE10A was probably distributed in the lysosomes. We validated the PDE10A distribution in stable mCherry-PDE10A-overexpressing HepG2 cells. This probe delineated the profile of PDE10A in tissue sections and exhibited a remarkable therapeutic effect as a PDE10A inhibitor for treating pulmonary arterial hypertension. This concept will open up a new avenue for designing a highly effective fluorescent probe for tracking receptor proteins by taking full advantage of the structural information in the ligand-binding pocket of the target of interest.

Frequency-Upconverted Stimulated Emission by Up to Six-Photon Excitation from Highly Extended Spiro-Fused Ladder-Type Oligo(p-phenylene)s

Frequency-upconverted fluorescence and stimulated emission induced by multiphoton absorption (MPA) have attracted much interest. As compared with low-order MPA processes, the construction of high-order MPA processes is highly desirable and rather attractive, yet remains a formidable challenge due to its inherent low transition probability. We report the observation of the first experimental frequency-upconverted fluorescence and stimulated emission by simultaneous six-photon excitation in an organic molecular system. The well-designed organic conjugated system based on cross-shaped spiro-fused ladder-type oligo(p-phenylene)s (SpL-z, z=1-3) manifests reasonably high MPA cross-sections and brilliant luminescence emission simultaneously. The six-photon absorption cross-section of SpL-3 with an extended π-conjugation was evaluated as 8.67×10^-169  cm¹²  s⁵  photon^-5 . Exceptionally efficient 2- to 6-photon excited stimulated emission was achieved under near-infrared laser excitation.

Donor acceptor fluorophores: synthesis, optical properties, TD-DFT and cytotoxicity studies

Donor-π-acceptor (D-π-A) fluorophores consisting of a donor unit, a π linker, and an acceptor moiety have attracted attention in the last decade. In this study, we report the synthesis, characterization, optical properties, TD-DFT, and cytotoxicity studies of 17 near infrared (NIR) D-π-A analogs which have not been reported so far to the best of our knowledge. These fluorophores have chloroacrylic acid as the acceptor unit and various donor units such as indole, benzothiazole, benzo[e]indole, and quinoline. The fluorophores showed strong absorption in the NIR (700-970 nm) region due to their enhanced intramolecular charge transfer (ICT) between chloroacrylic acid and the donor moieties connected with the Vilsmeier-Haack linker. The emission wavelength maxima of the fluorophores were in between 798 and 870 nm. Compound 20 with a 4-quinoline donor moiety showed an emission wavelength above 1000 nm in the NIR II window. The synthesized fluorophores were characterized by 1H NMR and 13C NMR, and their optical properties were studied. Time dependent density functional theory (TD-DFT) calculations showed that the charge transfer occurs from the donor groups (indole, benzothiazole, benzo[e]indole, and quinoline) to the acceptor chloroacrylic acid moiety. Fluorophores with [HOMO] to [LUMO+1] transitions were shown to possess a charge separation character. The cytotoxicity of selected fluorophores, 4, 7, 10 and 12 was investigated against breast cancer cell lines and they showed better activity than the anti-cancer agent docetaxel.

Structural Variation and Switchable Nonlinear Optical Behavior of Metal-Organic Frameworks

Two europium metal-organic frameworks (MOFs) based on the same ligand, named as ZJU-23-Eu and ZJU-24-Eu, are selectively synthesized by fine-tuning solvent contents to tailor the coordination modes. Eu atoms are eight-coordinated and nine-coordinated in ZJU-23-Eu and ZJU-24-Eu respectively, and their frameworks vary in both spatial connectivity and symmetry. The ligand not only has multiphoton response but also suitable triplet energy level (19 998 cm^-1 ) to sensitize Eu³⁺ . Thus ZJU-23-Eu exhibits characteristic emission of Eu³⁺ peaking at 614 nm via the energy transfer from the two-/three-photon excited ligand to Eu³⁺ , with its bidimensional layered structure benefiting this process. In contrast, the changed spatial connectivity in tridimensional ZJU-24-Eu narrows the distances between adjacent Eu³⁺ ions and reduces the density, resulting in poor two-photon excited fluorescence. Besides, noncentrosymmetric ZJU-24-Eu shows second harmonic generation (SHG) response with an intensity of ≈6.2 times relative to KH₂ PO₄ (KDP) microcrystalline powder while centrosymmetric ZJU-23-Eu cannot. These results have established two nonlinear optical (NLO) models based on MOFs to synchronously analyze the effects of two structural variables on different NLO behaviors, and provide ingenious ways to design MOF-based NLO devices with function on demand.

Halide perovskite nanocrystals for multiphoton applications

Halide perovskite nanocrystals (NCs) are a unique class of NCs with novel properties distinct from those of traditional semiconductor NCs. These exceptional properties of defect tolerance, large absorption coefficients, high brightness, and narrow emission linewidths stem from their atypical band structure. Their facile synthesis and broad colour tunability have attracted widespread interest for application in light emitting devices and lasers. One fledging niche area is the field of multiphoton excited emission where their giant nonlinear optical action cross-sections are highly favorable for imaging applications. This Frontier article examines the state-of-the-art in perovskite NCs for multiphoton applications from the materials science and physics perspectives that include their synthesis and nonlinear optical characterization. Opportunities and challenges of these exceptional NCs as potential fluorescent labels for multiphoton deep tissue microscopy are also highlighted.

Multipolar Porphyrin-Triazatruxene Arrays for Two-Photon Fluorescence Cell Imaging

Two-photon excited fluorescent (TPEF) materials are highly desirable for bioimaging applications owing to their unique characteristics of deep-tissue penetration and high spatiotemporal resolution. Herein, by connecting one, two, or three electron-deficient zinc porphyrin units to an electron-rich triazatruxene core via ethynyl π-bridges, conjugated multipolar molecules TAT-(ZnP)_n (n=1-3) were developed as TPEF materials for cell imaging. The three new dyes present high fluorescence quantum yields (0.40-0.47) and rationally improved two-photon absorption (TPA) properties. In particular, the peak TPA cross section of TAT-ZnP (436 GM) is significantly larger than that of the ZnP reference (59 GM). The δ_TPA values of TAT-(ZnP)₂ and TAT-(ZnP)₃ further increase to 1031 and up to 1496 GM, respectively, indicating the effect of incorporated ZnP units on the TPA properties. The substantial improvement of the TPEF properties is attributed to the formation of π-conjugated quadrapole/octupole molecules and the extension of D-π-A-D systems, which has been rationalized by density function theory (DFT) calculations. Moreover, all of the three new dyes display good biocompatibility and preferential targeting ability toward cytomembrane, thus can be superior candidates for TPEF imaging of living cells. Overall, this work demonstrated a promising strategy for the development of porphyrin-based TPEF materials by the construction and extension of D-π-A-D multipolar array.

Tuning molecular aggregation to achieve highly bright AIE dots for NIR-II fluorescence imaging and NIR-I photoacoustic imaging

Currently, bright aggregation-induced emission luminogens (AIEgens) with high photoluminescence quantum yields (PLQYs) in the NIR-II region are still limited, and thus an efficient strategy to enhance NIR-II fluorescence performance through tuning molecular aggregation is proposed here. The synthesized donor-acceptor tailored AIEgen (DTPA-TBZ) not only exhibits an excellent absorptivity in the NIR-I region, but also good fluorescence signals in the NIR-II region with an emission extending to 1200 nm. Benefiting from such improved intramolecular restriction and aggregation, a significant absolute PLQY value of 8.98% was obtained in solid DTPA-TBZ. Encouragingly, the resulting AIE dots also exhibit a high relative PLQY of up to 11.1% with IR 26 as the reference (PLQY = 0.5%). Finally, the AIE dots were applied in high performance NIR-II fluorescence imaging and NIR-I photoacoustic (PA) imaging: visualization of abdominal vessels, hind limb vasculature, and cerebral vessels with high signal to background ratios was performed via NIR-II imaging; Moreover, PA imaging has also been performed to clearly observe tumors in vivo. These results demonstrate that by finely tuning molecular aggregation in DTPA-TBZ, a good NIR-I absorptivity and a highly emissive fluorescence in the NIR-II region can be achieved simultaneously, finally resulting in a promising dual-modal imaging platform for real-world applications to achieve precise cancer diagnostics.

Functionalized Acrylonitriles with Aggregation-Induced Emission: Structure Tuning by Simple Reaction-Condition Variation, Efficient Red Emission, and Two-Photon Bioimaging

Acrylonitriles with aggregation-induced emission (AIE) characteristics have been found to show promising applications in two-photon biomedical imaging. Generally, elaborate synthetic efforts are required to achieve different acrylonitriles with distinct functionalities. In this work, we first reported the synthesis of two different group-functionalized AIE-active acrylonitriles (TPAT-AN-XF and 2TPAT-AN) obtained simply by mixing the same reactants at different temperatures using a facile and transition metal-free synthetic method. These two AIE luminogens (AIEgens) exhibit unique properties such as bright red emission in the solid state, large Stokes shift, and large two-photon absorption cross section. Water-soluble nanoparticles (NPs) of 2TPAT-AN were prepared by a nanoprecipitation method. In vitro imaging data show that 2TPAT-AN NPs can selectively stain lysosome in live cells. Besides one-photon imaging, remarkable two-photon imaging of live tumor tissues can be achieved with high resolution and deep tissue penetration. 2TPAT-AN NPs show high biocompatibility and are successfully utilized in in vivo long-term imaging of mouse tumors with a high signal-to-noise ratio. Thus, the present work is anticipated to shed light on the preparation of a library of AIE-active functionalized acrylonitriles with intriguing properties for biomedical applications.

Highly photostable two-photon NIR AIEgens with tunable organelle specificity and deep tissue penetration

Photostability is a particularly important parameter for fluorescence imaging especially long-term dynamic tracking in live samples. However, many organic fluorophores show poor photostability under one-photon and two-photon continuous irradiation. In addition, these traditional fluorophores also suffer from aggregation-caused quenching (ACQ) in aggregate state in insolvable water environment. Therefore, it remains challenging to develop photostable and ACQ-free fluorophores for biological imaging. In this work, we developed two highly photostable aggregation-induced emission luminogens (AIEgens) based on the cyanostilbene core for in vitro and ex vivo bioimaging. These AIEgens named CS-Py⁺SO₃^- and CS-Py⁺ exhibit near-infrared solid-state emission, large Stokes shift (>180 nm), high fluorescence quantum yield (12.8%-13.7%) and good two-photon absorption cross section (up to 88 GM). CS-Py⁺SO₃^- and CS-Py⁺ show specific organelle staining with high biocompatibility in membrane and mitochondria in live cells, respectively. In addition, selective two-photon mitochondria visualization in live rat skeletal muscle tissues with deep-tissue penetration (about 100 μm) is successfully realized by using CS-Py⁺. Furthermore, these AIEgens especially CS-Py⁺ exhibit remarkably high resistance to photobleaching under one-photon and two-photon continuous irradiation. These highly photostable AIEgens could be potentially utilized in visualizing and tracking specific organelle-associated dynamic changes in live systems.

A combination of super-resolution fluorescence and magnetic resonance imaging using a Mn(ii) compound

Two manganese(ii) complexes supported by terpyridyl-based ligands were synthesized; they showed an enhanced fluorescence, including a two-photon signal and magnetic contrast, and were used in multi-modal imaging.

Selective two-photon absorption in carbon dots: a piece of the photoluminescence emission puzzle

Carbon nanodots (Cdots) are now emerging as promising nonlinear fluorophores for applications in biological environments. A thorough and systematic approach to the two-photon induced emission of Cdots that could provide design guidelines to control their nonlinear emission properties is still missing. In this work, we address the nonlinear optical spectroscopy of Cdots prepared by controlled chemical cutting of graphene oxide (GO). The two-photon absorption in the 700-1000 nm region and the corresponding emission spectrum are carefully investigated. The highest two-photon absorption cross-section estimated was 130 GM at 720 nm. This value is comparable with the one reported for graphene nanoribbons with push-pull architecture. The emission spectrum depends on the excitation mode. At the same excitation energy, nonlinear excitation results in excitation-wavelength independent emission, while upon linear excitation the emission is excitation-wavelength dependent. The biphotonic interaction seems to be selective towards sp2 clusters bearing electron donor and acceptor groups found in push-pull architectures. Both linear and nonlinear emission can be understood based on the existence of isolated sp2 clusters involved in π-π stacking interactions with clusters in adjacent layers.

Tetraphenylethylene-Interweaving Conjugated Macrocycle Polymer Materials as Two-Photon Fluorescence Sensors for Metal Ions and Organic Molecules

A luminescent conjugated macrocycle polymer (CMP) with strong two-photon fluorescence property, namely, P[5]-TPE-CMP, is constructed from ditriflate-functionalized pillar[5]arene and a 1,1,2,2-tetrakis(4-ethynylphenyl)ethylene (TPE) linker through a Sonogashira-Hagihara cross-coupling reaction. Significantly, in sharp contrast with the corresponding conjugated microporous polymer without synthetic macrocycles, P[5]-TPE-CMP shows an outstanding stability against photobleaching and exhibits highly selective cation sensing capability toward Fe³⁺ at different excitation wavelengths (both UV and red-near-infrared regions). Meanwhile, its fluorescence could also be sufficiently quenched by 4-amino azobenzene, a frequently used organic dye that is certified to be carcinogenic, as compared with a group of common organic compounds. This work paves a new way for enhancing the properties of porous organic polymers through the introduction of supramolecular macrocycles like macrocyclic arenes.

Enantiopure distorted ribbon-shaped nanographene combining two-photon absorption-based upconversion and circularly polarized luminescence

Herein we describe a distorted ribbon-shaped nanographene exhibiting unprecedented combination of optical properties in graphene-related materials, namely upconversion based on two-photon absorption (TPA-UC) together with circularly polarized luminescence (CPL). The compound is a graphene molecule of ca. 2 nm length and 1 nm width with edge defects that promote the distortion of the otherwise planar lattice. The edge defects are an aromatic saddle-shaped ketone unit and a [5]carbohelicene moiety. This system is shown to combine two-photon absorption and circularly polarized luminescence and a remarkably long emission lifetime of 21.5 ns. The [5]helicene is responsible for the chiroptical activity while the push-pull geometry and the extended network of sp2 carbons are factors favoring the nonlinear absorption. Electronic structure theoretical calculations support the interpretation of the results.

Nonlinear Absorption Response Correlated to Embedded Ag Nanoparticles in BGO Single Crystal: From Two-Photon to Three-Photon Absorption

We report on the embedded silver (Ag) nanoparticles fabricated by Ag+ ion implantation into the Bi4Ge3O12 (BGO) crystal. Localized surface plasmon resonance (LSPR) phenomenon has been observed by linear optical absorption spectrum, which is accordance with the expectation based on Mie theory calculation. Further proofs are given by SRIM, TEM and SAED analysis, which explain the slight difference between experiment and calculation. Based on the z-scan system, it is found that the nonlinear optical response is converted from two-photon absorption to three-photon absorption under the 515 nm femtosecond pulse excitation within the LSPR band. The nonlinear absorption coefficient is measured to be ~3.1 × 10-9 cm/W (two-photon absorption coefficient) and ~8.9 × 10-14cm3/W2 (three-photon absorption coefficient) for pure BGO crystal and the sample embedded with Ag nanoparticles (Ag:BGO), respectively. Finally, we have proposed a model to explain the asymmetric nonlinear transmittance, which is in good agreement with the experimental results.

An AceDAN–porphyrin(Zn) dyad for fluorescence imaging and photodynamic therapy via two-photon excited FRET

A FRET based AceDAN–porphyrin(Zn) dyad was designed to generate red emission and singlet oxygen (¹O₂) simultaneously, which were utilized successfully for two-photon excited fluorescence imaging and PDT of cancer cells.

Shaping Luminescent Properties of Yb3+ and Ho3+ Co-Doped Upconverting Core-Shell β-NaYF4 Nanoparticles by Dopant Distribution and Spacing

At the core of luminescence color and lifetime tuning of rare earth doped upconverting nanoparticles (UCNPs), is the understanding of the impact of the particle architecture for commonly used sensitizer (S) and activator (A) ions. In this respect, a series of core@shell NaYF₄ UCNPs doped with Yb³⁺ and Ho³⁺ ions are presented here, where the same dopant concentrations are distributed in different particle architectures following the scheme: YbHo core and YbHo@…, …@YbHo, Yb@Ho, Ho@Yb, YbHo@Yb, and Yb@YbHo core-shell NPs. As revealed by quantitative steady-state and time-resolved luminescence studies, the relative spatial distribution of the A and S ions in the UCNPs and their protection from surface quenching has a critical impact on their luminescence characteristics. Although the increased amount of Yb³⁺ ions boosts UCNP performance by amplifying the absorption, the Yb³⁺ ions can also efficiently dissipate the energy stored in the material through energy migration to the surface, thereby reducing the overall energy transfer efficiency to the activator ions. The results provide yet another proof that UC phosphor chemistry combined with materials engineering through intentional core@shell structures may help to fine-tune the luminescence features of UCNPs for their specific future applications in biosensing, bioimaging, photovoltaics, and display technologies.

Peptide-Decorated Tunable-Fluorescence Graphene Quantum Dots

We report here the synthesis of graphene quantum dots with tunable size, surface chemistry, and fluorescence properties. In the size regime 15-35 nm, these quantum dots maintain strong visible light fluorescence (mean quantum yield of 0.64) and a high two-photon absorption (TPA) cross section (6500 Göppert-Mayer units). Furthermore, through noncovalent tailoring of the chemistry of these quantum dots, we obtain water-stable quantum dots. For example, quantum dots with lysine groups bind strongly to DNA in solution and inhibit polymerase-based DNA strand synthesis. Finally, by virtue of their mesoscopic size, the quantum dots exhibit good cell permeability into living epithelial cells, but they do not enter the cell nucleus.

A series of terpyridine-based zinc(ii) complexes assembled for third-order nonlinear optical responses in the near-infrared region and recognizing lipid membranes

Two-photon (TP) microscopy has advantages for biological imaging in that it allows deeper tissue-penetration and excellent resolution compared with one-photon (OP) microscopy.

Open-Chain Crown-Ether-Derived Two-Photon Fluorescence Probe for Real-Time Dynamic Biopsy of Mercury Ions

A novel two-photon fluorescence probe for Hg2+ derived from bis(styryl)terephthalonitrile, as a two-photon fluorophore, and bis[2-(2-hydroxyethyl sulfanyl) ethyl]amino group (ionophore), as a novel Hg2+ ligand, was developed. The probe possesses small molecule size, large two-photon absorption cross-section (1067 GM) in H2O, non-cytotoxic effect, long wavelength emission at 588 nm, large Stokes shift (121 nm), excellent photostability, high water solubility, good cell permeability, and pH insensitivity in the biologically relevant range. The probe can selectively detect Hg2+ ions in live cells and living tissues without interference from other metal ions and the membrane-bound probes, and its quenching constant is 8.73 × 105 M–1.

Simultaneous Near-Infrared and Two-Photon In Vivo Imaging of H2 O2 Using a Ratiometric Fluorescent Probe based on the Unique Oxidative Rearrangement of Oxonium

A new ratiometric fluorescent H₂ O₂ probe, benzopyrylium-coumarin (BC), is designed by using an oxonium moiety as the unique H₂ O₂ response site. The BC probe exhibits an extremely large emission shift of 221 nm in response to H₂ O₂ , and is successfully applied for the simultaneous near-infrared and two-photon imaging of H₂ O₂ in living cells, mouse-liver tissues, and zebrafish.

Strong multiphoton absorption properties of one styrylpyridinium salt in a highly polar solvent

Multiphoton absorption (MPA) effects have become useful for real applications as well as conceptual predictions. However, most of organic molecules exhibit small Stokes shift and reduced MPA in the highly polar solvents, which may seriously hinder their related applications. In this work, one styrylpyridinium salt has been synthesized, which exhibits outstanding properties such as bright red fluorescence at the wavelength of 626 nm in a highly polar solvent (DMSO). Importantly, it is noted that the material also exhibits strong two- and three-photon absorption action cross-section (δ_2PA = 597 GM and δ_3PA = 18 × 10^-80 cm⁶∙s²∙photon^-2, respectively), which can be excited in near-infrared (NIR) window I (650-900 nm) and NIR window II (1000-1450 nm). Meanwhile, two-photon in vitro bioimaging and MPA induced optical limiting behavior have been successfully demonstrated based on the chromophore.

Polarized three-photon-pumped laser in a single MOF microcrystal

Higher order multiphoton-pumped polarized lasers have fundamental technological importance. Although they can be used to in vivo imaging, their application has yet to be realized. Here we show the first polarized three-photon-pumped (3PP) microcavity laser in a single host-guest composite metal-organic framework (MOF) crystal, via a controllable in situ self-assembly strategy. The highly oriented assembly of dye molecules within the MOF provides an opportunity to achieve 3PP lasing with a low lasing threshold and a very high-quality factor on excitation. Furthermore, the 3PP lasing generated from composite MOF is perfectly polarized. These findings may eventually open up a new route to the exploitation of multiphoton-pumped solid-state laser in single MOF microcrystal (or nanocrystal) for future optoelectronic and biomedical applications.

Nucleic acid-selective light-up fluorescent biosensors for ratiometric two-photon imaging of the viscosity of live cells and tissues

Rational design of specific ratiometric viscosity probes with small molecular weight is a challenge in practical biotechnology applications. Herein two novel water-soluble, small-molecular ratiometric probes, bearing N-methyl benzothiazolium moiety (DSF and DBF), are designed for two-photon fluorescent imaging as a functional of local viscosity. The dye DSF, a light-up fluorescent probe, is sensitive to local viscosity and selectively stains nuclear DNA, which can be used to inspect asynchronous cells under confocal microscopy. While the dye DBF as a molecular rotor displays strong fluorescence enhancement in viscous media or binding to RNA. It exhibits dual absorption and emission as well, and only the red emission is markedly sensitive to viscosity changes, providing a ratiometric response and selectively imaging nucleolic and cytosolic RNA. Interestingly it is shown, for the first time, that the intracellular targeting and localization (DNA and RNA) of the two dyes are entirely realized simply by modifying the substituent attached to the benzothiazolium.

Highly Selective Two-Photon Fluorescent Probe for Ratiometric Sensing and Imaging Cysteine in Mitochondria

A novel ratiometric mitochondrial cysteine (Cys)-selective two-photon fluorescence probe has been developed on the basis of a merocyanine as the fluorophore and an acrylate moiety as the biothiol reaction site. The biocompatible and photostable acrylate-functionalized merocyanine probe shows not only a mitochondria-targeting property but also highly selective detection and monitoring of Cys over other biothiols such as homocysteine (Hcy) and glutathione (GSH) and hydrogen sulfide (H2S) in live cells. In addition, this probe exhibits ratiometric fluorescence emission characteristics (F518/F452), which are linearly proportional to Cys concentrations in the range of 0.5-40 μM. More importantly, the probe and its released fluorophore, merocyanine, exhibit strong two-photon excited fluorescence (TPEF) with two-photon action cross-section (Φσmax) of 65.2 GM at 740 nm and 72.6 GM at 760 nm in aqueous medium, respectively, which is highly desirable for high contrast and brightness ratiometric two-photon fluorescence imaging of the living samples. The probe has been successfully applied to ratiometrically image and detect mitochondrial Cys in live cells and intact tissues down to a depth of 150 μm by two-photon fluorescence microscopy. Thus, this ratiometric two-photon fluorescent probe is practically useful for an investigation of Cys in living biological systems.

Photon upconversion: from two-photon absorption (TPA) to triplet–triplet annihilation (TTA)

Recent advances and remaining challenges are presented in the areas of TPA/TTA-UC, with particular emphasis on molecular engineering of these two upconversion materials.

A TP-FRET-based two-photon fluorescent probe for ratiometric visualization of endogenous sulfur dioxide derivatives in mitochondria of living cells and tissues

A TP-FRET-based ratiometric two-photon fluorescent probe for endogenous SO₂ derivative imaging in mitochondria has been developed for the first time.

Influence of the oxazole ring connection on the fluorescence of oxazoyl-triphenylamine biphotonic DNA probes

On the basis of our previous work on DNA fluorophores derived from vinylpyridinium-triphenylamine, we explored the structure space around the electron-rich triphenylamine (TP) core by changing the vinyl bond to an oxazole ring. As 2,5-diaryloxazoles are known to be highly fluorescent and efficient two photon absorbers, we synthesized analogues with two different connections of the oxazole to the triphenylamine core: TP-Ox2Py and TP-Ox5Py sets. Since the benzimidazolium group was proven to be more effective in the TP series than the pyridinium, we also synthesized a TP-Ox5Bzim set. The TP-Ox5Py series retains the TP-Py properties: on/off behavior on DNA, good two-photon cross-section and bright staining of nuclear DNA by microscopy under both one or two-photon excitation. On the other hand, the TP-Ox2Py series does not display fluorescence upon binding to DNA. The TP-Ox5Bzim set is fluorescent even in the absence of DNA and displays lower affinity than the corresponding TP-Ox5Py. CD experiments and docking were performed to understand these different behaviors.

Ultralow-threshold multiphoton-pumped lasing from colloidal nanoplatelets in solution

Although multiphoton-pumped lasing from a solution of chromophores is important in the emerging fields of nonlinear optofluidics and bio-photonics, conventionally used organic dyes are often rendered unsuitable because of relatively small multiphoton absorption cross-sections and low photostability. Here, we demonstrate highly photostable, ultralow-threshold multiphoton-pumped biexcitonic lasing from a solution of colloidal CdSe/CdS nanoplatelets within a cuvette-based Fabry–Pérot optical resonator. We find that colloidal nanoplatelets surprisingly exhibit an optimal lateral size that minimizes lasing threshold. These nanoplatelets possess very large gain cross-sections of 7.3 × 10⁻¹⁴ cm² and ultralow lasing thresholds of 1.2 and 4.3 mJ cm⁻² under two-photon (λ_exc=800 nm) and three-photon (λ_exc=1.3 μm) excitation, respectively. The highly polarized emission from the nanoplatelet laser shows no significant photodegradation over 10⁷ laser shots. These findings constitute a more comprehensive understanding of the utility of colloidal semiconductor nanoparticles as the gain medium in high-performance frequency-upconversion liquid lasers.

Multiphoton-pumped lasing from semiconductor nanocrystals in solution is difficult due to Auger recombination, low volume fraction and high threshold. Here, Li et al. demonstrate photostable, ultralow threshold multi-photon pumped lasing from colloidal CdSe/CdS nanoplatelets in a Fabry-Pérot optical resonator.

Indole-based cyanine as a nuclear RNA-selective two-photon fluorescent probe for live cell imaging

We have demonstrated that the subcellular targeting properties of the indole-based cyanines can be tuned by the functional substituent attached onto the indole moiety in which the first example of a highly RNA-selective and two-photon active fluorescent light-up probe for high contrast and brightness TPEF images of rRNA in the nucleolus of live cells has been developed. It is important to find that this cyanine binds much stronger toward RNA than DNA in a buffer solution as well as selectively stains and targets to rRNA in the nucleolus. Remarkably, the TPEF brightness (Φσmax) is dramatically increased with 11-fold enhancement in the presence of rRNA, leading to the record high Φσmax of 228 GM for RNA. This probe not only shows good biocompatibility and superior photostability but also offers general applicability to various live cell lines including HeLa, HepG2, MCF-7, and KB cells and excellent counterstaining compatibility with commercially available DNA or protein trackers.

Robust two-photon visualized nanocarrier with dual targeting ability for controlled chemo-photodynamic synergistic treatment of cancer

In consideration of the intrinsic complexity of cancer, just being a delivery nanovehicle for the nanocarrier is no longer enough to fulfill requirements of dealing with cancer. In this regard, the multifunctional nanocarrier appears to be an appealing choice in cancer treatment. Herein, the novel multifunctional nanocarrier (Fe3O4-NS-C3N4@mSiO2-PEG-RGD) possessing properties of dual targeting (the peptide- and magnetism-mediated targeting), imaging (one- and two-photon modes), pH-triggered release of loaded anticancer drug, and synergistic treatment (photodynamic therapy (PDT) combined with chemotherapy) are successfully developed. The nanocarrier specifically centralizes within cancer cells with the enhanced amount through the dual targeting ability and is facilely tracked under one- and two-photon imaging modes attributed to the autofluorescence. Then, visible light irradiation-induced PDT combined with low pH-triggered chemotherapy synergistically cooperate to efficiently kill cancer cells. Following the above process, the multifunctional nanocarrier demonstrates effective inhibition of the growth of A549 and HeLa cancer cells. The efficient manipulation of Fe3O4-NS-C3N4@mSiO2-PEG-RGD also implies potential applications of the multifunctional nanocarrier in delivery of different agents. Furthermore, it might also broaden the scope of fabrication of the multifunctional nanocarrier for inhibiting the growth of cancer cells.

Hexaazatriphenylene (HAT) derivatives: from synthesis to molecular design, self-organization and device applications

The creativity and inventiveness of chemists working with the 1,4,5,8,9,12-hexaazatriphenylene (HAT) building block is highlighted in this review.