Highly Photostable Near-IR-Excitation Upconversion Nanocapsules Based on Triplet-Triplet Annihilation for in Vivo Bioimaging Application

High-efficiency triplet-triplet annihilation upconversion microemulsion with facile preparation and decent air tolerance

Current research of triplet-triplet annihilation upconversion (TTA-UC) faces difficulty such as overuse of organic solvents and quenching of excited triplet sensitizers by molecular oxygen. Herein, we propose an efficient and facile preparation strategy of TTA-UC microemulsion to overcome these issues. With simple device and short preparation process, air-stable TTA-UC with a high upconversion efficiency of 16.52% was achieved in microemulsion coassembled from TritonX114, tetrahydrofuran and upconverting chromophores (platinum octaethyl-porphyrin and 9,10-diphenylanthracene). This is comparable to the highest UC efficiency ever reported for TTA-UC microemulsion systems. The excellent UC performance of TX114-THF could be attributed to two perspectives. Firstly, small-size micelle accommodated chromophores up to high concentrations in organic phase, which promoted efficient molecular collision. Additionally, high absorbance at 532 nm ensured full use of excitation light, getting more long wavelength photons involved in the TTA-UC process. Moreover, air-stable TTA-UC also performed well in microemulsion with various surfactants, including nonionic surfactants (Tween 20, Tween 80, Triton X-110, Triton X-114), ionic surfactants (sodium dodecyl sulfate, cetyltrimethyl ammonium bromide) and block copolymers (pluronic F127, pluronic P123), through three conjectural assembly models according to the structural characteristics of surfactant molecules (concentrated, uncompacted and scattered). These discoveries could provide estimable reference for selection of surfactants in relevant fields of TTA-UC.

Multifold Enhanced Photon Upconversion in a Composite Annihilator System Sensitized by Perovskite Nanocrystals

Photon upconversion via triplet-triplet annihilation (TTA-UC) provides a pathway to overcoming the thermodynamic efficiency limits in single-junction solar cells by allowing the harvesting of sub-bandgap photons. Here, we use mixed halide perovskite nanocrystals (CsPbX3, X = Br/I) as triplet sensitizers, with excitation transfer to 9,10-diphenylanthracene (DPA) and/or 9,10-bis[(triisopropylsilyl)ethynyl]anthracene (TIPS-An) which act as the triplet annihilators. We observe that the upconversion efficiency is five times higher with the combination of both annihilators in a composite system compared to the sum of the individual single-acceptor systems. Our work illustrates the importance of using a composite system of annihilators to enhance TTA upconversion, demonstrated in a perovskite-sensitized system, with promise for a range of potential applications in light-harvesting, biomedical imaging, biosensing, therapeutics, and photocatalysis.

Triplet-Triplet Annihilation Upconverting Liposomes: Mechanistic Insights into the Role of Membranes in Two-Dimensional TTA-UC

Triplet-triplet annihilation upconversion (TTA-UC) implemented in nanoparticle assemblies is of emerging interest in biomedical applications, including in drug delivery and imaging. As it is a bimolecular process, ensuring sufficient mobility of the sensitizer and annihilator to facilitate effective collision in the nanoparticle is key. Liposomes can provide the benefits of two-dimensional confinement and condensed concentration of the sensitizer and annihilator along with superior fluidity compared to other nanoparticle assemblies. They are also biocompatible and widely applied across drug delivery modalities. However, there are relatively few liposomal TTA-UC systems reported to date, so systematic studies of the influence of the liposomal environment on TTA-UC are currently lacking. Here, we report the first example of a BODIPY-based sensitizer TTA-UC system within liposomes and use this system to study TTA-UC generation and compare the relative intensity of the anti-Stokes signal for this system as a function of liposome composition and membrane fluidity. We report for the first time on time-resolved spectroscopic studies of TTA-UC in membranes. Nanosecond transient absorption data reveal the BODIPY-perylene dyad sensitizer has a long triplet lifetime in liposome with contributions from three triplet excited states, whose lifetimes are reduced upon coinclusion of the annihilator due to triplet-triplet energy transfer, to a greater extent than in solution. This indicates triplet energy transfer between the sensitizer and the annihilator is enhanced in the membrane system. Molecular dynamics simulations of the sensitizer and annihilator TTA collision complex are modeled in the membrane and confirm the co-orientation of the pair within the membrane structure and that the persistence time of the bound complex exceeds the TTA kinetics. Modeling also reliably predicted the diffusion coefficient for the sensitizer which matches closely with the experimental values from fluorescence correlation spectroscopy. The relative intensity of the TTA-UC output across nine liposomal systems of different lipid compositions was explored to examine the influence of membrane viscosity on upconversion (UC). UC showed the highest relative intensity for the most fluidic membranes and the weakest intensity for highly viscous membrane compositions, including a phase separation membrane. Overall, our study reveals that the co-orientation of the UC pair within the membrane is crucial for effective TTA-UC within a biomembrane and that the intensity of the TTA-UC output can be tuned in liposomal nanoparticles by modifying the phase and fluidity of the liposome. These new insights will aid in the design of liposomal TTA-UC systems for biomedical applications.

Triplet-triplet annihilation photon upconversion-mediated photochemical reactions

Photon upconversion is a method for harnessing high-energy excited states from low-energy photons. Such photons, particularly in the red and near-infrared wavelength ranges, can penetrate tissue deeply and undergo less competitive absorption in coloured reaction media, enhancing the efficiency of large-scale reactions and in vivo phototherapy. Among various upconversion methodologies, the organic-based triplet-triplet annihilation upconversion (TTA-UC) stands out - demonstrating high upconversion efficiencies, requiring low excitation power densities and featuring tunable absorption and emission wavelengths. These factors contribute to improved photochemical reactions for fields such as photoredox catalysis, photoactivation, 3D printing and immunotherapy. In this Review, we explore concepts and design principles of organic TTA-UC-mediated photochemical reactions, highlighting notable advancements in the field, as well as identify challenges and propose potential solutions. This Review sheds light on the potential of organic TTA-UC to advance beyond the traditional photochemical reactions and paves the way for research in various fields and clinical applications.

New Type Annihilator of π-Expanded Diketopyrrolopyrrole for Robust Photostable NIR-Excitable Triplet-Triplet Annihilation Upconversion

Near-infrared light excitable triplet-triplet annihilation upconversion (NIR TTA-UC) materials have attracted interest in a variety of emerging applications such as photoredox catalysis, optogenetics, and stereoscopic 3D printing. Currently, the practical application of NIR TTA-UC materials requires substantial improvement in photostability. Here, we found that the new annihilator of π-expanded diketopyrrolopyrrole (π-DPP) cannot activate oxygen to generate superoxide anion via photoinduced electron transfer, and its electron-deficient characteristics prevent the singlet oxygen-mediated [2 + 2] cycloaddition reaction; thus, π-DPP exhibited superior resistance to photobleaching. In conjunction with the NIR photosensitizer PdTNP, the upconversion efficiency of π-DPP is as high as 8.9%, which is eight times of the previously reported PdPc/Furan-DPP. Importantly, after polystyrene film encapsulation, less than 10% photobleaching was observed for this PdTNP/π-DPP-based NIR TTA-UC material after four hours of intensive NIR light exposure. These findings provide a type of annihilator with extraordinary photostability, facilitating the development of NIR TTA-UC materials for practical photonics.

Enhancing NIR-to-visible upconversion in a rigidly coupled tetracene dimer: approaching statistical limits for triplet-triplet annihilation using intramolecular multiexciton states

Important applications of photon upconversion through triplet-triplet annihilation require conversion of near-IR photons to visible light. Generally, however, efficiencies in this spectral region lag behind bluer analogues. Herein we consider potential benefits from a conformationally well-defined covalent dimer annihilator TIPS-BTX in studies that systematically compare function to a related monomer model TIPS-tetracene (TIPS-Tc). TIPS-BTX exhibits weak electronic coupling between chromophores juxtaposed about a polycyclic bridge. We report an upconversion yield ϕUC for TIPS-BTX that is more than 20× larger than TIPS-Tc under comparable conditions (0.16%). While the dimer ϕUC is low compared to bluer champion systems, this yield is amongst the largest so-far reported for a tetracenic dimer system and is achieved under unoptimized conditions suggesting a significantly higher ceiling. Further investigation shows the ϕUC enhancement for the dimer is due exclusively to the TTA process with an effective yield more that 30× larger for TIPS-BTX compared to TIPS-Tc. The ϕTTA enhancement for TIPS-BTX relative to TIPS-Tc is indicative of participation by intramolecular multiexciton states with evidence presented in spin statistical arguments that the 5TT is involved in productive channels. For TIPS-BTX we report a spin-statistical factor f = 0.42 that matches or exceeds values found in champion annihilator systems such as DPA. At the same time, the poor relative efficiency of TIPS-Tc suggests involvement of non-productive bimolecular channels and excimeric states are suspected. Broadly these studies indicate that funneling of photogenerated electronic states into productive pathways, and avoiding parasitic ones, remains central to the development of champion upconversion systems.

Coulomb interactions for mediator-enhanced sensitized triplet-triplet annihilation upconversion in solution

Sensitized triplet-triplet annihilation upconversion offers an attractive possibility to replace a high-energy photon by two photons with lower energy through the combination of a light-harvesting triplet sensitizer and an annihilator for the formation of a fluorescent singlet state. Typically, high annihilator concentrations are required to achieve an efficient initial energy transfer and as a direct consequence the most highly energetic emission is often not detectable due to intrinsic reabsorption by the annihilator itself. Herein, we demonstrate that the addition of a charge-adapted mediator drastically improves the energy transfer efficiency at low annihilator concentrations via an energy transfer cascade. Inspired by molecular dyads and recent developments in nanocrystal-sensitized upconversion, our system exploits a concept to minimize intrinsic filter effects, while boosting the upconversion quantum yield in solution. A sensitizer-annihilator combination consisting of a ruthenium-based complex and 9,10-diphenylanthracene (DPA) is explored as model system and a sulfonated pyrene serves as mediator. The impact of opposite charges between sensitizer and mediator - to induce coulombic attraction and subsequently result in accelerated energy transfer rate constants - is analyzed in detail by different spectroscopic methods. Ion pairing and the resulting static energy transfer in both directions is a minor process, resulting in an improved overall performance. Finally, the more intense upconverted emission in the presence of the mediator is used to drive two catalytic photoreactions in a two-chamber setup, illustrating the advantages of our approach, in particular for photoreactions requiring oxygen that would interfere with the upconversion system.

Near-infrared light-triggered prodrug photolysis by one-step energy transfer

Prodrug photolysis enables spatiotemporal control of drug release at the desired lesions. For photoactivated therapy, near-infrared (NIR) light is preferable due to its deep tissue penetration and low phototoxicity. However, most of the photocleavable groups cannot be directly activated by NIR light. Here, we report a upconversion-like process via only one step of energy transfer for NIR light-triggered prodrug photolysis. We utilize a photosensitizer (PS) that can be activated via singlet-triplet (S-T) absorption and achieve photolysis of boron-dipyrromethene (BODIPY)-based prodrugs via triplet-triplet energy transfer. Using the strategy, NIR light can achieve green light-responsive photolysis with a single-photon process. A wide range of drugs and bioactive molecules are designed and demonstrated to be released under low-irradiance NIR light (100 mW/cm2, 5 min) with high yields (up to 87%). Moreover, a micellar nanosystem encapsulating both PS and prodrug is developed to demonstrate the practicality of our strategy in normoxia aqueous environment for cancer therapy. This study may advance the development of photocleavable prodrugs and photoresponsive drug delivery systems for photo-activated therapy.

Boosting Dye-Sensitized Luminescence by Enhanced Short-Range Triplet Energy Transfer

Dye-sensitization can enhance lanthanide-based upconversion luminescence, but is hindered by interfacial energy transfer from organic dye to lanthanide ion Yb3+ . To overcome these limitations, modifying coordination sites on dye conjugated structures and minimizing the distance between fluorescence cores and Yb3+ in upconversion nanoparticles (UCNPs) are proposed. The specially designed near-infrared (NIR) dye, disulfo-indocyanine green (disulfo-ICG), acts as the antenna molecule and exhibits a 2413-fold increase in luminescence under 808 nm excitation compared to UCNPs alone using 980 nm irradiation. The significant improvement is attributed to the high energy transfer efficiency of 72.1% from disulfo-ICG to Yb3+ in UCNPs, with majority of energy originating from triplet state (T1 ) of disulfo-ICG. Shortening the distance between the dye and lanthanide ions increases the probability of energy transfer and strengthens the heavy atom effect, leading to enhanced T1 generation and improved dye-triplet sensitization upconversion. Importantly, this approach also applies to 730 nm excitation Cy7-SO3 sensitization system, overcoming the spectral mismatch between Cy7 and Yb3+ and achieving a 52-fold enhancement in luminescence. Furthermore, the enhancement of upconversion at single particle level through dye-sensitization is demonstrated. This strategy expands the range of NIR dyes for sensitization and opens new avenues for highly efficient dye-sensitized upconversion systems.

The statistical probability factor in triplet mediated photon upconversion: a case study with perylene

Triplet-triplet annihilation photon upconversion (TTA-UC) is a process where two low-energy photons are converted into one higher-energy photon. A crucial component for an efficient upconversion process is the statistical probability factor (f), defined as the probability of the formation of a high-energy singlet state upon coupling of two low-energy triplet states. Theoretically, f depends on the energy level distribution, molecular orientation, inter-triplet exchange coupling of triplet dyads, and spin-mixing of resulting spin states (singlet, triplet, and quintet). However, experimental values of f for acene-based annihilators have been subject to large variations due to many factors that have resulted in the reporting of different f values for the same molecule. In this work, we discuss these factors by studying perylene as a case study annihilator, for which by far the largest variation in f = 16 to 100% has been reported. We systematically investigated the TTA-UC of PdTPBP:perylene, as a sensitizer-annihilator pair and obtained the experimental f = 17.9 ± 2.1% for perylene in THF solution. This limits the maximum TTA-UC quantum yield to 9.0% (out of 50%) for this annihilator. We found that such a low f value for perylene is largely governed by the energy-gap law where higher non-radiative losses due to the small energy gap between 2 × T1 and T2 affect the probability of singlet formation. Interestingly, we found this observation true for other acene-based annihilators whose emission ranges from the UV to the yellow region, thus providing a blueprint for future design of efficient TTA-UC systems.

Efficient Red-to-Blue Triplet-Triplet Annihilation Upconversion Using the C70-Bodipy-Triphenylamine Triad as a Heavy-Atom-Free Triplet Photosensitizer

Triplet-triplet annihilation upconversion (TTA-UC) with heavy-atom-free organic triplet photosensitizers has attracted extensive attention recently, however, the successful examples with absorption in red and first near-infrared (NIR-I, 650-900 nm) region are still insufficient. Herein, we conducted a new TTA-UC system of perylene using C70-bodipy-triphenylamine triad (C70-BDP-T) as the heavy-atom-free photosensitizer. Efficient red-to-blue (663 to 450 nm) TTA-UC was achieved in this system with an anti-Stokes shift of 0.88 eV and a quantum yield up to 5.2% (out of a 50% maximum) in deaerated toluene. Notably, this is the highest quantum yield to date in similar TTA-UC systems with heavy-atom-free organic photosensitizers. Using steady-state and transient absorption spectroscopy, together with cyclic voltammogram and quantum chemical calculations, photophysical and photochemical mechanisms were elucidated. Specifically, two triplet triads, C70-3BDP*-T and 3C70*-BDP-T, were produced efficiently upon photoexcitation, with lifetimes of 2.0 ± 0.1 and 32.2 ± 0.3 μs, respectively. Electron transfer and recombination mechanisms were confirmed to play crucial roles in the formation of these triplets, instead of intersystem crossing. Our results shed light on the superiority of fullerenes in the development of heavy-atom-free photosensitizers.

Multi-wavelength excited triplet-triplet upconversion microcrystals based on hot-band excitation for optical information encryption

Multi-wavelength hot-band excitation, forbidden in the conventional Stokes fluorescence mechanism, is found to be available with cascading triplet-triplet annihilation upconversion (TTA-UC). Selective excitation of Pt(II)octaethylporphyrin (PtOEP) by diode lasers with wavelengths of 532 nm, 589 nm, 635 nm, 655 nm, and 671 nm respectively can all induce 9,10-diphenylanthracene (DPA) to emit blue upconversion, with the maximum anti-Stokes shift of 0.95 eV in the microcrystals exposed to air. Whether the zero-vibrational energy level excitation or the hot-vibrational energy level excitation in the ground state, the PtOEP/DPA pair showed triplet-triplet energy transfer (TTET) efficiencies approaching ∼95%. The doped microcrystal samples without encapsulation can emit blue upconversion from green/yellow/red excitation with stability for ∼20 days under atmospheric conditions, demonstrating their potential applications in multiple information encryption.

Plasmonic Metal Nanostructures Meet Triplet-Triplet Annihilation-Based Photon Upconversion Systems: Performance Improvements and Application Trends

Improving the performance of upconversion systems based on triplet-triplet annihilation (TTA-UC) can have far-reaching implications for various fields, including solar devices, nano-bioimaging, and nanotherapy. This review focuses on the use of localized surface plasmon (LSP) resonance of metal nanostructures to enhance the performance of TTA-UC systems and explores their potential applications. After introducing the basic driving mechanism of TTA-UC and typical sensitizers used in these systems, we discuss recent studies that have utilized new sensitizers with distinct characteristics. Furthermore, we confirm that the enhancement in upconverted emission can be explained, at least in part, by the mechanism of "metal-enhanced fluorescence", which is attributed to LSP resonance-induced fluorescence enhancement. Next, we describe selected experiments that demonstrate the enhancement in upconverted emission in plasmonic TTA-UC systems, as well as the emerging trends in their application. We present specific examples of studies in which the enhancement in upconverted emission has significantly improved the performance of photocatalysts under both sunlight and indoor lighting. Additionally, we discuss the potential for future developments in plasmonic TTA-UC systems.

Triplet Fusion Upconversion for Photocuring 3D-Printed Particle-Reinforced Composite Networks

High energy photons (λ < 400 nm) are frequently used to initiate free radical polymerizations to form polymer networks, but are only effective for transparent objects. This phenomenon poses a major challenge to additive manufacturing of particle-reinforced composite networks since deep light penetration of short-wavelength photons limits the homogeneous modification of physicochemical and mechanical properties. Herein, the unconventional, yet versatile, multiexciton process of triplet-triplet annihilation upconversion (TTA-UC) is employed for curing opaque hydrogel composites created by direct-ink-write (DIW) 3D printing. TTA-UC converts low energy red light (λ_max  = 660 nm) for deep penetration into higher-energy blue light to initiate free radical polymerizations within opaque objects. As proof-of-principle, hydrogels containing up to 15 wt.% TiO₂ filler particles and doped with TTA-UC chromophores are readily cured with red light, while composites without the chromophores and TiO₂ loadings as little as 1-2 wt.% remain uncured. Importantly, this method has wide potential to modify the chemical and mechanical properties of complex DIW 3D-printed composite polymer networks.

Nanoengineering Triplet-Triplet Annihilation Upconversion: From Materials to Real-World Applications

Using light to control matter has captured the imagination of scientists for generations, as there is an abundance of photons at our disposal. Yet delivering photons beyond the surface to many photoresponsive systems has proven challenging, particularly at scale, due to light attenuation via absorption and scattering losses. Triplet-triplet annihilation upconversion (TTA-UC), a process which allows for low energy photons to be converted to high energy photons, is poised to overcome these challenges by allowing for precise spatial generation of high energy photons due to its nonlinear nature. With a wide range of sensitizer and annihilator motifs available for TTA-UC, many researchers seek to integrate these materials in solution or solid-state applications. In this Review, we discuss nanoengineering deployment strategies and highlight their uses in recent state-of-the-art examples of TTA-UC integrated in both solution and solid-state applications. Considering both implementation tactics and application-specific requirements, we identify critical needs to push TTA-UC-based applications from an academic curiosity to a scalable technology.

Recent advances in lanthanide-doped up-conversion probes for theranostics

Up-conversion (or anti-Stokes) luminescence refers to the phenomenon whereby materials emit high energy, short-wavelength light upon excitation at longer wavelengths. Lanthanide-doped up-conversion nanoparticles (Ln-UCNPs) are widely used in biomedicine due to their excellent physical and chemical properties such as high penetration depth, low damage threshold and light conversion ability. Here, the latest developments in the synthesis and application of Ln-UCNPs are reviewed. First, methods used to synthesize Ln-UCNPs are introduced, and four strategies for enhancing up-conversion luminescence are analyzed, followed by an overview of the applications in phototherapy, bioimaging and biosensing. Finally, the challenges and future prospects of Ln-UCNPs are summarized.

Enhanced chiroptic properties of nanocomposites of achiral plasmonic nanoparticles decorated with chiral dye-loaded micelles

The development of circularly polarized luminescence (CPL)-active materials with both large luminescence dissymmetry factor (g_lum) and high emission efficiency continues to be a major challenge. Here, we present an approach to improve the overall CPL performance by integrating triplet-triplet annihilation-based photon upconversion (TTA-UC) with localized surface plasmon resonance. Dye-loaded chiral micelles possessing TTA-UC ability are designed and attached on the surface of achiral gold nanorods (AuNRs). The longitudinal and transversal resonance peaks of AuNRs overlap with the absorption and emission of dye-loaded chiral micelles, respectively. Typically, 43-fold amplification of g_lum value accompanied by 3-fold enhancement of upconversion are obtained simultaneously when Au@Ag nanorods are employed in the composites. More importantly, transient absorption spectra reveal a fast accumulation of spin-polarized triplet excitons in the composites. Therefore, the enhancement of chirality-induced spin polarization should be in charge of the amplification of g_lum value. Our design strategy suggests that combining plasmonic nanomaterials with chiral organic materials could aid in the development of chiroptical nanomaterials.

Red Light-Responsive Upconverting Nanoparticles for Quantitative and Controlled Release of a Coumarin-Based Prodrug

Photolytic reactions allow the optical control of the liberation of biological effectors by photolabile protecting groups. The development of versatile technologies enabling the use of deep-red or NIR light excitation still represents a challenging issue, in particular for light-induced drug release (e.g., light-induced prodrug activation). Here, light-sensitive biocompatible lipid nanocapsules able to liberate an antitumoral drug through photolysis are presented. It is demonstrated that original photon upconverting nanoparticles (LNC-UCs) chemically conjugated to a coumarin-based photocleavable linker can quantitatively and efficiently release a drug by upconversion luminescence-assisted photolysis using a deep-red excitation wavelength. In addition, it is also able to demonstrate that such nanoparticles are stable in the dark, without any drug leakage in the absence of light. These findings open new avenues to specifically liberate diverse drugs using deep-red or NIR excitations for future therapeutic applications in nanomedicine.

Recyclable optical bioplastics platform for solid state red light harvesting via triplet-triplet annihilation photon upconversion

Sustainable photonics applications of solid-state triplet-triplet annihilation photon upconversion (TTA-UC) are limited by a small UC spectral window, low UC efficiency in air, and non-recyclability of polymeric materials used. In a step to overcome these issues, we have developed new recyclable TTA-UC bioplastics by encapsulating TTA-UC chromophores liquid inside the semicrystalline gelatin films showing broad-spectrum upconversion (red/far-red to blue) with high UC efficiency in air. For this, we synthesized a new anionic annihilator, sodium-TIPS-anthracene-2-sulfonate (TIPS-AnS), that combined with red/far-red sensitizers (PdTPBP/Os(m-peptpy)₂(TFSI)₂), a liquid surfactant Triton X-100 reduced (TXr) and protein gelatin (G) formed red/far-red to blue TTA-UC bioplastic films just by air drying of their aqueous solutions. The G-TXr-TIPS-AnS-PdTPBP film showed record red to blue (633 to 478 nm) TTA-UC quantum yield of 8.5% in air. The high UC quantum yield has been obtained due to the fluidity of dispersed TXr containing chromophores and oxygen blockage by gelatin fibers that allowed efficient diffusion of triplet excited chromophores. Further, the G-TXr-TIPS-AnS-Os(m-peptpy)₂(TFSI)₂ bioplastic film displayed far-red to blue (700-730 nm to 478 nm) TTA-UC, demonstrating broad-spectrum photon harvesting. Finally, we demonstrated the recycling of G-TXr-TIPS-AnS-PdTPBP bioplastics by developing a downstream approach that gives new directions for designing future recyclable photonics bioplastic materials.

Upconversion nanomaterials and delivery systems for smart photonic medicines and healthcare devices

In the past decade, upconversion (UC) nanomaterials have been extensively investigated for the applications to photomedicines with their unique features including biocompatibility, near-infrared (NIR) to visible conversion, photostability, controllable emission bands, and facile multi-functionality. These characteristics of UC nanomaterials enable versatile light delivery for deep tissue biophotonic applications. Among various stimuli-responsive delivery systems, the light-responsive delivery process has been greatly advantageous to develop spatiotemporally controllable on-demand "smart" photonic medicines. UC nanomaterials are classified largely to two groups depending on the photon UC pathway and compositions: inorganic lanthanide-doped UC nanoparticles and organic triplet-triplet annihilation UC (TTA-UC) nanomaterials. Here, we review the current-state-of-art inorganic and organic UC nanomaterials for photo-medicinal applications including photothermal therapy (PTT), photodynamic therapy (PDT), photo-triggered chemo and gene therapy, multimodal immunotherapy, NIR mediated neuromodulations, and photochemical tissue bonding (PTB). We also discuss the future research direction of this field and the challenges for further clinical development.

Recent progress in upconversion nanomaterials for emerging optical biological applications

The recent advances of upconversion nanoparticles (UCNPs) have made them the ideal "partner" for a variety of biological applications. In this review, we describe the emerging biological optical applications of UCNPs, focus on their potential therapeutic advantages. Firstly, we briefly review the development and mechanisms of upconversion luminescence, including organic and inorganic UCNPs. Next, in the section on UCNPs for imaging and detection, we list the development of UCNPs in visualization, temperature sensing, and detection. In the section on therapy, recent results are described concerning optogenetics and neurotherapy. Tumor therapy is another major part of this section, including the synergistic application of phototherapy such as photoimmunotherapy. In a special section, we briefly cover the integration of UCNPs in therapeutics. Finally, we present our understanding of the limitations and prospects of applications of UCNPs in biological fields, hoping to provide a more comprehensive understanding of UCNPs and attract more attention.

Triplet-triplet annihilation-based photon-upconversion to broaden the wavelength spectrum for photobiocatalysis

Photobiocatalysis is a growing field of biocatalysis. Especially light-driven enzyme catalysis has contributed significantly to expanding the scope of synthetic organic chemistry. However, photoenzymes usually utilise a rather narrow wavelength range of visible (sun)light. Triplet-triplet annihilation-based upconversion (TTA-UC) of long wavelength light to shorter wavelength light may broaden the wavelength range. To demonstrate the feasibility of light upconversion we prepared TTA-UC poly(styrene) (PS) nanoparticles doped with platinum(II) octaethylporphyrin (PtOEP) photosensitizer and 9,10-diphenylanthracene (DPA) annihilator (PtOEP:DPA@PS) for application in aqueous solutions. Photoexcitation of PtOEP:DPA@PS nanoparticles with 550 nm light led to upconverted emission of DPA 418 nm. The TTA-UC emission could photoactivate flavin-dependent photodecarboxylases with a high energy transfer efficiency. This allowed the photodecarboxylase from Chlorella variabilis NC64A to catalyse the decarboxylation of fatty acids into long chain secondary alcohols under green light (λ = 550 nm).

Bypassing the statistical limit of singlet generation in sensitized upconversion using fluorinated conjugated systems

The photon upconversion based on triplet-triplet annihilation (TTA) is a mechanism that converts the absorbed low-energy electromagnetic radiation into higher energy photons also at extremely low excitation intensities, but its use in actual technologies is still hindered by the limited availability of efficient annihilator moieties. We present here the results obtained by the synthesis and application of two new fluorinated chromophores based on phenazine and acridine structures, respectively. Both compounds show upconverted emission demonstrating their ability as TTA annihilator. More interesting, the acridine-based chromophore shows an excellent TTA yield that overcomes the one of some of best model systems. By correlating the experimental data and the quantum mechanical modeling of the investigated compound, we propose an alternative efficient pathway for the generation of the upconverted emissive states involving the peculiar high-energy triplet levels of the dye, thus suggesting a new development strategy for TTA annihilators based on the fine tuning of their high-energy excited states properties.

Triplet fusion upconversion nanocapsules for volumetric 3D printing

Three-dimensional (3D) printing has exploded in interest as new technologies have opened up a multitude of applications^1-6, with stereolithography a particularly successful approach^4,7-9. However, owing to the linear absorption of light, this technique requires photopolymerization to occur at the surface of the printing volume, imparting fundamental limitations on resin choice and shape gamut. One promising way to circumvent this interfacial paradigm is to move beyond linear processes, with many groups using two-photon absorption to print in a truly volumetric fashion^3,7-9. Using two-photon absorption, many groups and companies have been able to create remarkable nanoscale structures^4,5, but the laser power required to drive this process has limited print size and speed, preventing widespread application beyond the nanoscale. Here we use triplet fusion upconversion^10-13 to print volumetrically with less than 4 milliwatt continuous-wave excitation. Upconversion is introduced to the resin by means of encapsulation with a silica shell and solubilizing ligands. We further introduce an excitonic strategy to systematically control the upconversion threshold to support either monovoxel or parallelized printing schemes, printing at power densities several orders of magnitude lower than the power densities required for two-photon-based 3D printing.

BODIPY-pyrene donor-acceptor sensitizers for triplet-triplet annihilation upconversion: the impact of the BODIPY-core on upconversion efficiency

Triplet-triplet annihilation upconversion (TTA-UC) is an important type of optical process with applications in biophotonics, solar energy harvesting and photochemistry. In most of the TTA-UC systems, the formation of triplet excited states takes place via spin-orbital interactions promoted by heavy atoms. Given the crucial role of heavy atoms (especially noble metals, such as Pd and Pt) in promoting intersystem crossing (ISC) and, therefore, in production of UC luminescence, the feasibility of using more readily available and inexpensive sensitizers without heavy atoms remains a challenge. Here, we investigated sensitization of TTA-UC using BODIPY-pyrene heavy-atom-free donor-acceptor dyads with different numbers of alkyl groups in the BODIPY scaffold. The molecules with four and six alkyl groups are unable to sensitize TTA-UC in the investigated solvents (tetrahydrofuran (THF) and dichloromethane (DCM)) due to negligible ISC. In contrast, the dyad with two methyl groups in the BODIPY scaffold and the dyad with unsubstituted BODIPY demonstrate efficient intersystem crossing (ISC) of 49-58%, resulting in TTA-UC with quantum yields of 4.7% and 6.9%, respectively. The analysis of the elementary steps of the TTA-UC process indicates that heavy-atom-free donor-acceptor dyads are less effective than their noble metal counterparts, but may equal them in the future if the right combination of solvent, donor-acceptor sensitizer structure, and new luminescent molecules as TTA-UC emitters can be found.

Nanoencapsulated Phase-Change Materials: Versatile and Air-Tolerant Platforms for Triplet-Triplet Annihilation Upconversion

Efficient and long-term stable triplet-triplet annihilation upconversion (TTA-UC) can be achieved by effectively protecting the excited organic triplet ensembles from photoinduced oxygen quenching, and discovery of a new material platform that promotes TTA-UC in ambient conditions is of paramount importance for practical applications. In this study, we present the first demonstration of an organic nonparaffin phase-change material (PCM) as an air-tolerant medium for TTA-UC with a unique solid-liquid phase transition in response to temperature variation. For the proposed concept, 2,4-hexadien-1-ol is used and extensively characterized with several key features, including good solvation capacity, mild melting point (30.5 °C), and exclusive antioxidant property, enabling a high-efficiency, low-threshold, and photostable TTA-UC system without energy-intensive degassing processes. In-depth characterization reveals that the triplet diffusion among the transient species, i.e., ³sensitizer* and ³acceptor*, is efficient and well protected from oxygen quenching in both aerated liquid- and solid-phase 2,4-hexadien-1-ol. We also propose a new strategy for the nanoencapsulation of PCM by employing hollow mesoporous silica nanoparticles as vehicles. This scheme is applicable to both aqueous- and solid-phase TTA-UC systems as well as suitable for various applications, such as thermal energy storage and smart drug delivery.

Efficient Triplet-Triplet Annihilation Upconversion in Solution and Hydrogel Enabled by an S-T Absorption Os(II) Complex Dyad with an Elongated Triplet Lifetime

A new Os(II) complex dyad featuring direct singlet-to-triplet (S-T) absorption and intramolecular triplet energy transfer (ITET) with lifetime up to 7.0 μs was designed to enhance triplet energy transfer efficiency during triplet-triplet annihilation upconversion (TTA-UC). By pairing with 9,10-bis(phenylethynyl)anthracene (BPEA) as a triplet acceptor, intense upconverted green emission in deaerated solution was observed with unprecedented TTA-UC emission efficiency up to 26.3% (with a theoretical maximum efficiency of 100%) under photoexcitation in the first biological transparency window (650-900 nm). Meanwhile, a 7.1% TTA-UC emission efficiency was acquired in an air-saturated hydrogel containing the photosensitizer and a newly designed hydrophilic BPEA derivative. This ITET mechanism would inspire further development of a highly efficient TTA-UC system for biological fields and renewable energy production.

Thermally Activated Upconversion with Metal-Free Sensitizers Enabling Exceptional Anti-Stokes Shift and Anti-counterfeiting Application

Photochemical upconversion (UC) via triplet-triplet annihilation (TTA) has attracted considerable attention for its potential applications in solar energy conversion, photocatalysis, and bioimaging. Achieving a large anti-Stokes shift in photochemical UC is appealing but still a great challenge, especially for purely organic sensitizers. Here, we develop solid-state TTA UC systems with metal- and heavy atom-free dyes as the sensitizers, which sensitize the 9,10-diphenylanthracene acceptor through thermally activated triplet-triplet energy transfer. Solid-state UC emission with remarkable anti-Stokes shifts up to 1.10 eV is achieved owing to an evident enthalpy gain by the endothermic sensitization. The solid upconverter shows air-stable UC emission and potentials in dual-mode anti-counterfeiting and encryption applications. The present UC approach involving thermally activated sensitization enabled by purely organic dyes provides a versatile strategy to develop TTA UC materials with large anti-Stokes shift, air-tolerant emission, and environmental compatibility, which would have promising applications in information encryption, photochemical conversion, and bioimaging.

Triplet-triplet annihilation upconversion with reversible emission-tunability induced by chemical-stimuli: a remote modulator for photocontrol isomerization

Triplet-triplet annihilation upconversion (TTA-UC) has been widely studied, but a color-tunable TTA-UC system triggered by chemical stimuli has not yet been proposed. Herein, reversible acid/base switching of the TTA-UC emission wavelength is achieved for the first time by a simple platform, composed of a direct singlet-triplet (S₀-T₁) absorption photosensitizer, and proton-responsive 9,10-di(pyridin-4-yl)anthracene (DPyA) as an acceptor. The photosensitizer-acceptor pair exhibits efficient UC emission (quantum yield up to 3.3%, and anti-Stokes shift up to 0.92 eV) with remarkable contrast upon base/acid treatment (Δλ_em,max = 82 nm, 0.46 eV). In a proof-of-concept study, the color-adjustable TTA-UC emission was applied as a remote modulator to photo-control reversible chemical reactions for the first time. This platform enriches the portfolio of color-switchable TTA-UC, and the mechanism would inspire further development of smart UC systems and extend the application field of upconversion.

Determination of Trace Phosphoprotein in Food Based on Fluorescent Probe-Triggered Target-Induced Quench by Electrochemiluminescence

Evaluation of the nutrition and determination of phosphoproteins is of great importance in different foods as aberrant phosphorylation changes many biological processes and can relate to health conditions. In this study, an ultrafast (5 min) and sensitive electrochemiluminescence (ECL) sensor was innovatively fabricated for the determination of phosphoproteins in foods on the basis of fluorescent probe NH₂-TiO₂/upconversion nanomaterials (UCNPs). Impressively, the ECL intensity of NH₂-TiO₂/UCNPs-rGO/GCE was remarkably enhanced by 29 times. Furthermore, the photoactive NH₂-TiO₂ layer provided not only specific selectivity but also a large surface area as well as an unprecedented photocatalytic activity for the NH₂-TiO₂/UCNPs-rGO/GCE ECL sensor (TIECLS), which could serve as an identification element for trace phosphoproteins. Under optimal conditions, the TIECLS achieved a relatively low detection limit of 9.2 × 10^-5 mg/mL (S/N = 3). Practical application of this TIECLS was carried out in different food samples with satisfying results, which were validated by laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS).

Triplet-triplet annihilation based near infrared to visible molecular photon upconversion

Triplet-triplet annihilation based molecular photon upconversion (TTA-UC) is an exciting research area for a broad range of photonic applications due to its tunable spectral range and possible operation at non-coherent solar irradiance. Most of the TTA-UC studies are limited to Visible to Visible (Vis to Vis) energy upconversion. However, for several practical photonic applications, efficient near infrared (NIR) to Vis upconversion is preferred. Examples include, (i) photovoltaics where TTA-UC could lead to utilization of a larger part of the solar spectrum and (ii) in NIR stimulated biological applications where the deep penetration and non-invasive nature of NIR light coupled to TTA-UC offers new opportunities. Although, NIR to Vis TTA-UC is known since 2007, the recent five years have witnessed quite a progress in terms of the development of new chromophores, hybrid systems and fabrication techniques to increase the UC quantum yield at low excitation intensity. With this tutorial review we are reviewing recent progress, identifying existing challenges and discus possible future directions and opportunities.

Spin Fine Structure Reveals Biexciton Geometry in an Organic Semiconductor

In organic semiconductors, biexcitons are key intermediates in carrier multiplication and exciton annihilation. Their local geometry governs their electronic properties and yet has been challenging to determine. Here, we access the structure of the recently discovered S=2 quintet biexciton state in an organic semiconductor using broadband optically detected magnetic resonance. We correlate the experimentally extracted spin structure with the molecular crystal geometry to identify the specific molecular pairings on which biexciton states reside.

Tuning the Quantum Dot (QD)/Mediator Interface for Optimal Efficiency of QD-Sensitized Near-Infrared-to-Visible Photon Upconversion Systems

Lead sulfide (PbS) quantum dots (QDs) have shown promising performance as a sensitizer in infrared-to-visible photon upconversion systems. To investigate the key design rules, we compare three PbS-sensitized upconversion systems using three mediator molecules with the same tetracene triplet acceptor at different distances from the QD. Using transient absorption spectroscopy, we directly measure the triplet energy-transfer rates and efficiencies from the QD to the mediator and from the mediator to the emitter. With increasing distance between the mediator and PbS QD, the efficiency of the first triplet energy transfer from the QD to the mediator decreases because of a decrease in the rate of this triplet energy-transfer step, while the efficiency of the second triplet energy transfer from the mediator to the emitter increases because of a reduction in the QD-induced mediator triplet state decay. The latter effect is a result of the slow rate constant of the second triplet energy-transfer process, which is 3 orders of magnitude slower than the diffusion-limited value. The combined results lead to a net decrease of the steady-state upconversion quantum yield with distance, which could be predicted by our kinetic model. Our result shows that the QD/mediator interface affects both the first and second triplet energy transfer processes in the photon upconversion system, and the QD/mediator distance has an opposite effect on the efficiencies of the first and second triplet energy transfer. These findings provide important insight for the further rational improvement of the overall efficiency of QD-based upconversion systems.

Peptide assembly assisted triplet-triplet annihilation photon upconversion in non-deoxygenated water

Triplet-triplet annihilation upconversion (TTA-UC) has great potential in many fields. However, a stable TTA-UC system with adjustable UC efficiency in non-deoxygenated water is still in urgent demand. Here, the first example of short peptide-tuned UC luminescence in water is reported. With only a small amount of peptides, UC chromophores can assemble into tetrahedral microrods with adjustable size and UC efficiency. Successful TTA-UC luminescence of these microrods in water is achieved due to the regular and dense organization of molecular upconversion chromophores tuned by peptides, which allows rapid triplet exciton migration, avoids aggregation-induced quenching and screens molecular oxygen to upconversion chromophores.

Visible-to-ultraviolet (<340 nm) photon upconversion by triplet–triplet annihilation in solvents

Visible-to-UV upconversion shorter than 340 nm was achieved in organic solvents and the solvent dependent efficiency and photostability were elucidated.

Engineering Porous Emitting Framework Nanoparticles with Integrated Sensitizers for Low-Power Photon Upconversion by Triplet Fusion

The conversion of low-energy light into photons of higher energy based on sensitized triplet-triplet annihilation (sTTA) upconversion is emerging as the most promising wavelength-shifting methodology because it operates efficiently at excitation powers as low as the solar irradiance. However, the production of solid-state upconverters suited for direct integration in devices is still an ongoing challenge owing to the difficulties concerning the organization of two complementary moieties, the triplet sensitizer, and the annihilator, which must interact efficiently. This problem is solved by fabricating porous fluorescent nanoparticles wherein the emitters are integrated into robust covalent architectures. These emitting porous aromatic framework (ePAF) nanoparticles allow intimate interaction with the included metallo-porphyrin as triplet sensitizers. Remarkably, the high concentration of framed chromophores ensures hopping-mediated triplet diffusion required for TTA, yet the low density of the framework promotes their high optical features without quenching effects, typical of the solid state. A green-to-blue photon upconversion yield as high as 15% is achieved: a record performance among annihilators in a condensed phase. Furthermore, the engineered ePAF architecture containing covalently linked sensitizers produces full-fledge solid-state bicomponent particles that behave as autonomous nanodevices.

Upconverting Oil-Laden Hollow Mesoporous Silica Microcapsules for Anti-Stokes-Based Biophotonic Applications

A recyclable, aqueous phase functioning and biocompatible photon upconverting system is developed. Hollow mesoporous silica microcapsules (HMSMs) with ordered radial mesochannels were employed, for the first time, as vehicles for the post-encapsulation of oil phase triplet-triplet annihilation upconversion (TTA-UC), with the capability of homogeneous suspension in water. In-depth characterization of such upconverting oil-laden HMSMs (UC-HMSMs) showed that the mesoporous silica shells reversibly stabilized the encapsulated UC oil in water to allow efficient upconverted emission, even under aerated conditions. In addition, the UC-HMSMs were found to actively bind to the surface of human mesenchymal stem cells without significant cytotoxicity and displayed upconverted bright blue emission under 640 nm excitation, indicating a potential of our new TTA-UC system in biophotonic applications. These findings reveal the great promise of UC-HMSMs to serve as ideal vehicles not only for ultralow-power in vivo imaging but also for stem cell labeling, to facilitate the tracking of tumor cells in animal models.

Excitation of erbium-doped nanoparticles in 1550-nm wavelength region for deep tissue imaging with reduced degradation of spatial resolution

Rare-earth-doped nanoparticles are one of the emerging probes for bioimaging due to their visible-to-near-infrared (NIR) upconversion emission via sequential single-photon absorption at NIR wavelengths. The NIR-excited upconversion property and high photostability make this probe appealing for deep tissue imaging. So far, upconversion nanoparticles include ytterbium ions (Yb3  +  ) codoped with other rare earth ions, such as erbium (Er3  +  ) and thulium (Tm3  +  ). In these types of upconversion nanoparticles, through energy transfer from Yb3  +   excited with continuous wave light at a wavelength of 980 nm, upconversion emission of the other rare earth dopants is induced. We have found that the use of the excitation of Er3  +   in the 1550-nm wavelength region allows us to perform deep tissue imaging with reduced degradation of spatial resolution. In this excitation–emission process, three and four photons of 1550-nm light are sequentially absorbed, and Er3  +   emits photons in the 550- and 660-nm wavelength regions. We demonstrate that, compared with the case using 980-nm wavelength excitation, the use of 1550-nm light enables us to moderate degradation of spatial resolution in deep tissue imaging due to the lower light scattering coefficient compared with 980-nm light. We also demonstrate that live cell imaging is feasible with this 1550 nm excitation.

Designing next generation of photon upconversion: Recent advances in organic triplet-triplet annihilation upconversion nanoparticles

Organic triplet-triplet annihilation upconversion (TTA-UC) nanoparticles have emerged as exciting therapeutic agents and imaging probes in recent years due to their unique chemical and optical properties such as outstanding biocompatibility and low power excitation density. In this review, we focus on the latest breakthroughs in such new version of upconversion nanoparticle, including their design, preparation, and applications. First, we will discuss the key principles and design concept of these organic-based photon upconversion in regard to the methods of selection of the related triplet TTA dye pairs (photosensitizer and emitter). Then, we will discuss the recent approaches s to construct TTA-UCNPs including silica TTA-UCNPs, lipid-coated TTA-UCNPs, polymer encapsulated TTA-UCNPs, nano-droplet TTA-UCNPs and metal-organic frameworks (MOFs) constructed TTA-UCNPs. In addition, the applications of TTA-UCNPs will be discussed. Finally, we will discuss the challenges posed by current TTA-UCNP development.

Nanomized tumor-microenvironment-active NIR fluorescent prodrug for ensuring synchronous occurrences of drug release and fluorescence tracing

Improving the bioavailability and tumor-targeting ability of a prodrug, as well as monitoring its active ingredient release in vivo, is still a challenge in cancer diagnosis and therapy. Herein, a specific nanomized tumor-microenvironment-active near-infrared (NIR) fluorescent DCM-S-GEM/PEG prodrug was developed as a potent monitoring platform, wherein we conjugated antitumor drug gemcitabine (GEM) and NIR fluorescent chromophore dicyanomethylene-4H-pyran (DCM) via glutathione (GSH)-activatable disulfide linker and encapsulated DCM-S-GEM into an amphiphilic polymer DSPE-mPEG by self-assembly. The nanomized DCM-S-GEM/PEG prodrug exhibits excellent photostability and high biocompatibility, significantly improving the therapeutic efficacy toward lung tumor cells with fewer side-effects toward normal cells. Furthermore, when compared with the standalone DCM-S-GEM prodrug, the micellization with diblock DSPE-mPEG avoids fast metabolism, facilitates the accumulation of drugs in lung tumor tissues, displays longer tumor retention, and realizes precise drug release in lung tumors. The nanomized DCM-S-GEM/PEG prodrug can be developed as a promising tool to monitor prodrug delivery and activation processes in vivo.

Triplet–triplet annihilation based photon up-conversion in hybrid molecule–semiconductor nanocrystal systems

Photon up-conversion based on triplet–triplet annihilation (TTA) in a hybrid system exploits the annihilation of optically dark triplets of an organic emitter, sensitized by a semiconductor nanocrystal, to produce high-energy singlets that generate high energy emission.