Indefinite and bidirectional near-infrared nanocrystal photoswitching

Engineering colloidal semiconductor nanocrystals for quantum information processing

Quantum information processing-which relies on spin defects or single-photon emission-has shown quantum advantage in proof-of-principle experiments including microscopic imaging of electromagnetic fields, strain and temperature in applications ranging from battery research to neuroscience. However, critical gaps remain on the path to wider applications, including a need for improved functionalization, deterministic placement, size homogeneity and greater programmability of multifunctional properties. Colloidal semiconductor nanocrystals can close these gaps in numerous application areas, following years of rapid advances in synthesis and functionalization. In this Review, we specifically focus on three key topics: optical interfaces to long-lived spin states, deterministic placement and delivery for sensing beyond the standard quantum limit, and extensions to multifunctional colloidal quantum circuits.

Short-Wave Infrared Light-Emitting Diodes Using Colloidal CuInS2 Quantum Dots with ZnI2 Dual-Passivation

Short-wave infrared (SWIR) light-emitting diodes (LEDs) have emerged as promising technologies for diverse applications such as optical communication, biomedical imaging, surveillance, and machine vision. Colloidal quantum dots (QDs) are particularly attractive for SWIR LEDs due to their solution processability, compatibility with flexible substrates, and tunable absorption and luminescence. However, the presence of toxic elements or precious metals in most SWIR-emitting QDs poses health, environmental, and cost challenges. In this context, CuInS2 (CIS) QDs are known for low toxicity, cost-effective fabrication, and SWIR-light emitting capability. However, CIS QDs have not yet been directly utilized to fabricate SWIR LEDs to date, which is due to low particle stability, inefficient charge carrier recombination, and significantly blue-shifted luminescence after integrating into LED devices. To address challenges, we propose a dual-passivation strategy using ZnI2 as a chemical additive to enhance both the optical property of plain CIS QDs and charge carrier recombination upon LED device implementation. The resulting CIS-QD-based LEDs exhibit a stable SWIR electroluminescence (EL) peak (over 1000 nm) with a high EL radiance and a record external quantum efficiency in the SWIR region. Our study represents a significant step forward in SWIR-QLED technology, offering a pathway for the development of high-performance, low-cost, and nontoxic SWIR light sources.

The mechanisms behind the extreme susceptibility of photon avalanche emission to quenching

The photon avalanche (PA) process that emerges in lanthanide-doped crystals yields a threshold and highly nonlinear (of the power law order >5) optical response to photoexcitation. PA emission is the outcome of the excited-state absorption combined with a cross-relaxation process, which creates positive and efficient energy looping. In consequence, this combination of processes should be highly susceptible to small perturbations in energy distribution and can thus be hindered by other competitive "parasitic" processes such as energy transfer (ET) to quenching sites. Although luminescence quenching is a well-known phenomenon, exact mechanisms of the susceptibility of PA to resonant energy transfer (RET) remain poorly understood, limiting its practical applications. A deeper understanding of these mechanisms may pave the way to new areas of PA exploitation. This study focuses on the investigation of the LiYF4:3%Tm3+ PA system co-doped with Nd3+ acceptor ions, which are found to impact both the looping and emitting levels. This effectively disrupts the PA emission, causing an increase in the PA threshold (Ith) and a decrease in the PA nonlinearity (Smax). Our complementary modelling results reveal that ET from the looping level increases Ith and Smax, whereas ET from the emitting level diminishes Smax and the final emission intensity. Ultimately, significant PA emission quenching demonstrates a high relative sensitivity (SR) to infinitesimal amounts of Nd3+ acceptors, highlighting the potential for PA to be utilized as an ultra-sensitive, fluorescence-based reporting mechanism that is suitable for the detection and quantification of physical and biological phenomena or reactions.

Optical super-resolution nanothermometry via stimulated emission depletion imaging of upconverting nanoparticles

From engineering improved device performance to unraveling the breakdown of classical heat transfer laws, far-field optical temperature mapping with nanoscale spatial resolution would benefit diverse areas. However, these attributes are traditionally in opposition because conventional far-field optical temperature mapping techniques are inherently diffraction limited. Optical super-resolution imaging techniques revolutionized biological imaging, but such approaches have yet to be applied to thermometry. Here, we demonstrate a super-resolution nanothermometry technique based on highly doped upconverting nanoparticles (UCNPs) that enable stimulated emission depletion (STED) super-resolution imaging. We identify a ratiometric thermometry signal and maintain imaging resolution better than ~120 nm for the relevant spectral bands. We also form self-assembled UCNP monolayers and multilayers and implement a detection scheme with scan times >0.25 μm2/min. We further show that STED nanothermometry reveals a temperature gradient across a joule-heated microstructure that is undetectable with diffraction limited thermometry, indicating the potential of this technique to uncover local temperature variation in wide-ranging practical applications.

Light-Driven, Dynamic Assembly of Micron-To-Centimeter Parts, Micromachines and Microbot Swarms

This work describes light-driven assembly of dynamic formations and functional particle swarms controlled by appropriately programmed light patterns. The system capitalizes on the use of a fluidic bed whose low thermal conductivity assures that light-generated heat remains "localized" and sets strong convective flows in the immediate vicinity of the particles being irradiated. In this way, even low-power laser light or light from a desktop slide projector can be used to organize dynamic formations of objects spanning four orders of magnitude in size (from microns to centimeters) and over nine orders of magnitude in terms of mass. These dynamic assemblies include open-lattice structures with individual particles performing intricate translational and/or rotational motions, density-gradient particle arrays, nested architectures of mechanical components (e.g., planetary gears), or swarms of light-actuated microbots controlling assembly of other objects.

Multicolor Long-Term Single-Particle Tracking Using 10 nm Upconverting Nanoparticles

Single-particle tracking (SPT) is a powerful technique to unveil molecular behaviors crucial to the understanding of many biological processes, but it is limited by factors such as probe photostability and spectral orthogonality. To overcome these limitations, we develop upconverting nanoparticles (UCNPs), which are photostable over several hours at the single-particle level, enabling long-term multicolor SPT. We investigate the brightness of core-shell UCNPs as a function of inert shell thickness to minimize particle size while maintaining sufficient signal for SPT. We explore different rare-earth dopants to optimize for the brightest probes and find that UCNPs doped with 2% Tm3+/30% Yb3+, 10% Er3+/90% Yb3+, and 15% Tm3+/85% Yb3+ represent the optimal probes for blue, green, and near-infrared emission, respectively. The multiplexed 10 nm probes enable three-color single-particle tracking on live HeLa cells for tens of minutes using a single, near-infrared excitation source. These photostable and multiplexed probes open new avenues for numerous biological applications.

Recent advances in super-resolution optical imaging based on aggregation-induced emission

Super-resolution imaging has rapidly emerged as an optical microscopy technique, offering advantages of high optical resolution over the past two decades; achieving improved imaging resolution requires significant efforts in developing super-resolution imaging agents characterized by high brightness, high contrast and high sensitivity to fluorescence switching. Apart from technical requirements in optical systems and algorithms, super-resolution imaging relies on fluorescent dyes with special photophysical or photochemical properties. The concept of aggregation-induced emission (AIE) was proposed in 2001, coinciding with unprecedented advancements and innovations in super-resolution imaging technology. AIE probes offer many advantages, including high brightness in the aggregated state, low background signal, a larger Stokes shift, ultra-high photostability, and excellent biocompatibility, making them highly promising for applications in super-resolution imaging. In this review, we summarize the progress in implementation methods and provide insights into the mechanism of AIE-based super-resolution imaging, including fluorescence switching resulting from photochemically-converted aggregation-induced emission, electrostatically controlled aggregation-induced emission and specific binding-regulated aggregation-induced emission. Particularly, the aggregation-induced emission principle has been proposed to achieve spontaneous fluorescence switching, expanding the selection and application scenarios of super-resolution imaging probes. By combining the aggregation-induced emission principle and specific molecular design, we offer some comprehensive insights to facilitate the applications of AIEgens (AIE-active molecules) in super-resolution imaging.

Bridging Smart Nanosystems with Clinically Relevant Models and Advanced Imaging for Precision Drug Delivery

Intracellular delivery of nano-drug-carriers (NDC) to specific cells, diseased regions, or solid tumors has entered the era of precision medicine that requires systematic knowledge of nano-biological interactions from multidisciplinary perspectives. To this end, this review first provides an overview of membrane-disruption methods such as electroporation, sonoporation, photoporation, microfluidic delivery, and microinjection with the merits of high-throughput and enhanced efficiency for in vitro NDC delivery. The impact of NDC characteristics including particle size, shape, charge, hydrophobicity, and elasticity on cellular uptake are elaborated and several types of NDC systems aiming for hierarchical targeting and delivery in vivo are reviewed. Emerging in vitro or ex vivo human/animal-derived pathophysiological models are further explored and highly recommended for use in NDC studies since they might mimic in vivo delivery features and fill the translational gaps from animals to humans. The exploration of modern microscopy techniques for precise nanoparticle (NP) tracking at the cellular, organ, and organismal levels informs the tailored development of NDCs for in vivo application and clinical translation. Overall, the review integrates the latest insights into smart nanosystem engineering, physiological models, imaging-based validation tools, all directed towards enhancing the precise and efficient intracellular delivery of NDCs.

Ligand-Assisted Direct Lithography of Upconverting and Avalanching Nanoparticles for Nonlinear Photonics

Upconverting nanoparticles (UCNPs) exhibit unique nonlinear optical properties that can be harnessed in microscopy, sensing, and photonics. However, forming high-resolution nano- and micropatterns of UCNPs with large packing fractions is still challenging. Additionally, there is limited understanding of how nanoparticle patterning chemistries are affected by the particle size. Here, we explore direct patterning chemistries for 6-18 nm Tm3+-, Yb3+/Tm3+-, and Yb3+/Er3+-based UCNPs using ligands that form either new ionic linkages or covalent bonds between UCNPs under ultraviolet (UV), electron-beam (e-beam), and near-infrared (NIR) exposure. We study the effect of UCNP size on these patterning approaches and find that 6 nm UCNPs can be patterned with compact ionic-based ligands. In contrast, patterning larger UCNPs requires long-chain, cross-linkable ligands that provide sufficient interparticle spacing to prevent irreversible aggregation upon film casting. Compared to approaches that use a cross-linkable liquid monomer, our patterning method limits the cross-linking reaction to the ligands bound on UCNPs deposited as a thin film. This highly localized photo-/electron-initiated chemistry enables the fabrication of densely packed UCNP patterns with high resolutions (∼1 μm with UV and NIR exposure; <100 nm with e-beam). Our upconversion NIR lithography approach demonstrates the potential to use inexpensive continuous-wave lasers for high-resolution 2D and 3D lithography of colloidal materials. The deposited UCNP patterns retain their upconverting, avalanching, and photoswitching behaviors, which can be exploited in patterned optical devices for next-generation UCNP applications.

Quantum Dot Fluorescent Imaging: Using Atomic Structure Correlation Studies to Improve Photophysical Properties

Efforts to study intricate, higher-order cellular functions have called for fluorescence imaging under physiologically relevant conditions such as tissue systems in simulated native buffers. This endeavor has presented novel challenges for fluorescent probes initially designed for use in simple buffers and monolayer cell culture. Among current fluorescent probes, semiconductor nanocrystals, or quantum dots (QDs), offer superior photophysical properties that are the products of their nanoscale architectures and chemical formulations. While their high brightness and photostability are ideal for these biological environments, even state of the art QDs can struggle under certain physiological conditions. A recent method correlating electron microscopy ultrastructure with single-QD fluorescence has begun to highlight subtle structural defects in QDs once believed to have no significant impact on photoluminescence (PL). Specific defects, such as exposed core facets, have been shown to quench QD PL in physiologically accurate conditions. For QD-based imaging in complex cellular systems to be fully realized, mechanistic insight and structural optimization of size and PL should be established. Insight from single QD resolution atomic structure and photophysical correlative studies provides a direct course to synthetically tune QDs to match these challenging environments.

Luminescence Thermometry Beyond the Biological Realm

As the field of luminescence thermometry has matured, practical applications of luminescence thermometry techniques have grown in both frequency and scope. Due to the biocompatibility of most luminescent thermometers, many of these applications fall within the realm of biology. However, luminescence thermometry is increasingly employed beyond the biological realm, with expanding applications in areas such as thermal characterization of microelectronics, catalysis, and plasmonics. Here, we review the motivations, methodologies, and advances linked to nonbiological applications of luminescence thermometry. We begin with a brief overview of luminescence thermometry probes and techniques, focusing on those most commonly used for nonbiological applications. We then address measurement capabilities that are particularly relevant for these applications and provide a detailed survey of results across various application categories. Throughout the review, we highlight measurement challenges and requirements that are distinct from those of biological applications. Finally, we discuss emerging areas and future directions that present opportunities for continued research.

Beam-profile compensation for quantum yield characterisation of Yb-Tm codoped upconverting nanoparticles emitting at 474 nm, 650 nm and 804 nm

Upconverting nanoparticles (UCNPs) have found widespread applications in biophotonics and energy harvesting due to their unique non-linear optical properties arising from energy transfer upconversion (ETU) mechanisms. However, accurately characterising the power density-dependent efficiency of UCNPs using the internal quantum yield (iQY) is challenging due to the lack of methods that account for excitation beam-profile distortions. This limitation hinders the engineering of optimal UCNPs for diverse applications. To address this, this work present a novel beam profile compensation strategy based on a general analytical rate-equations model, enabling the evaluation of iQY for ETU processes of arbitrary order, such as ETU2, ETU3, and beyond. The method was applied to characterise the ETU2 and ETU3 processes corresponding to the main emission peaks (474 nm, 650 nm, and 804 nm) of a Yb-Tm codoped core-shell β-UCNP. Through this approach, the transition power density points (which delimit the distinct non-linear regimes of the upconversion luminescence (UCL)), and the saturation iQY values (which are reached at high excitation power densities above the transition points) were determined. The ETU2 process exhibits a single transition power density point, denoted as ρ2, while the ETU3 processes involve two transition points, ρ2 and ρ3. By compensating for the beam profile, we evaluate the iQY of individual lines across a wide dynamic range of excitation power densities (up to 105 W cm-2), encompassing both non-linear and linear regimes of UCL. This study introduces a valuable approach for accurately characterising the iQY of UCNPs, facilitating a deeper understanding of the upconversion and its performance. By addressing excitation beam-profile distortions, this method provides a comprehensive and reliable assessment of the power density-dependent iQY. The results highlight the applicability and effectiveness of this beam profile compensation strategy, which can be employed for a wide range of UCNPs. This advancement opens new avenues for the tailored design and application of UCNPs in various fields, especially for biophotonics.

Cross Relaxation Enables Spatiotemporal Color-Switchable Upconversion in a Single Sandwich Nanoparticle for Information Security

Smart control of ionic interaction dynamics offers new possibilities for tuning and editing luminescence properties of lanthanide-based materials. However, it remains a daunting challenge to achieve the dynamic control of cross relaxation mediated photon upconversion, and in particular the involved intrinsic photophysics is still unclear. Herein, this work reports a conceptual model to realize the color-switchable upconversion of Tm3+ through spatiotemporal control of cross relaxation in the design of NaYF4 :Gd@NaYbF4 :Tm@NaYF4 sandwich nanostructure. It shows that cross relaxation plays a key role in modulating upconversion dynamics and tuning emission colors of Tm3+ . Interestingly, it is found that there is a short temporal delay for the occurrence of cross relaxation in contrast to the spontaneous emission as a result of the slight energy mismatch between relevant energy levels. This further enables a fine emission color tuning upon non-steady state excitation. Moreover, a characteristic quenching time is proposed to describe the temporal evolution of cross relaxation quantitatively. These findings present a deep insight into the physics of ionic interactions in heavy doping systems, and also show great promise in frontier applications including information security, anti-counterfeiting and nanophotonics.

Accelerating the Design of Multishell Upconverting Nanoparticles through Bayesian Optimization

The photon upconverting properties of lanthanide-doped nanoparticles drive their applications in imaging, optoelectronics, and additive manufacturing. To maximize their brightness, these upconverting nanoparticles (UCNPs) are often synthesized as core/shell heterostructures. However, the large numbers of compositional and structural parameters in multishell heterostructures make optimizing optical properties challenging. Here, we demonstrate the use of Bayesian optimization (BO) to learn the structure and design rules for multishell UCNPs with bright ultraviolet and violet emission. We leverage an automated workflow that iteratively recommends candidate UCNP structures and then simulates their emission spectra using kinetic Monte Carlo. Yb3+/Er3+- and Yb3+/Er3+/Tm3+-codoped UCNP nanostructures optimized with this BO workflow achieve 10- and 110-fold brighter emission within 22 and 40 iterations, respectively. This workflow can be expanded to structures with higher compositional and structural complexity, accelerating the discovery of novel UCNPs while domain-specific knowledge is being developed.

Understanding Yb3+-sensitized photon avalanche in Pr3+ co-doped nanocrystals: modelling and optimization

Among different upconversion processes where the emitted photon has higher energy than the one absorbed, photon avalanche (PA) is unique, because the luminescence intensity increases by 2-3 orders of magnitude in response to a tiny increase in excitation intensity. Since its discovery in 1979, PA has been observed in bulk materials but until recently, obtaining it at the nanoscale has been a significant challenge. In the present work, the PA phenomenon in β-NaYF4 colloidal nanocrystals co-doped with Pr3+ and Yb3+ ions was successfully observed at 482 nm (3P0 → 3H4) and 607 nm (3P0 → 3H6) under excitation at 852 nm. The impact of Pr3+ ion concentration and pump power dependence on PA behavior was investigated, i.e. PA non-linearity slopes of luminescence intensity curves as a function of pump power density as well as PA thresholds. The highest slopes, namely 8.6 and 9.0, and the smallest thresholds equal to 286 kW cm-2 and 281 kW cm-2, observed for emission bands at 607 nm and 482 nm, respectively, were obtained for NaYF4:0.5%Pr3+,15%Yb3+@NaYF4 colloidal nanocrystals. Besides experimental research, simulations of PA behavior in Pr3+, Yb3+ co-doped materials were performed based on differential rate equations describing the phenomena that contribute to the existence of PA. The influence of different processes leading to PA, e.g. the rates of nonradiative and radiative transitions as well as energy transfers, on PA performance was simulated aiming to understand their roles in this complex sensitized system.

A supramolecular polymer network constructed using a pillararene-based multi-functional monomer and its application as a rewritable fluorescent paper

A simple and mild stimulus-responsive fluorescent supramolecular polymer network was constructed from a pillararene-based multi-functional monomer through multiple noncovalent interactions and used as a rewritable paper.

All-Optical Data Processing with Photon-Avalanching Nanocrystalline Photonic Synapse

Data processing and storage in electronic devices are typically performed as a sequence of elementary binary operations. Alternative approaches, such as neuromorphic or reservoir computing, are rapidly gaining interest where data processing is relatively slow, but can be performed in a more comprehensive way or massively in parallel, like in neuronal circuits. Here, time-domain all-optical information processing capabilities of photon-avalanching (PA) nanoparticles at room temperature are discovered. Demonstrated functionality resembles properties found in neuronal synapses, such as: paired-pulse facilitation and short-term internal memory, in situ plasticity, multiple inputs processing, and all-or-nothing threshold response. The PA-memory-like behavior shows capability of machine-learning-algorithm-free feature extraction and further recognition of 2D patterns with simple 2 input artificial neural network. Additionally, high nonlinearity of luminescence intensity in response to photoexcitation mimics and enhances spike-timing-dependent plasticity that is coherent in nature with the way a sound source is localized in animal neuronal circuits. Not only are yet unexplored fundamental properties of photon-avalanche luminescence kinetics studied, but this approach, combined with recent achievements in photonics, light confinement and guiding, promises all-optical data processing, control, adaptive responsivity, and storage on photonic chips.

Lanthanide-Doped KMgF3 Upconversion Nanoparticles for Photon Avalanche Luminescence with Giant Nonlinearities

Lanthanide (Ln3+)-doped photon avalanche (PA) upconversion nanoparticles (UCNPs) have great prospects in many advanced technologies; however, realizing efficient PA luminescence in Ln3+-doped UCNPs remains challenging due to the deleterious surface and lattice quenching effect. Herein, we report a unique strategy based on the pyrolysis of KHF2 for the controlled synthesis of aliovalent Ln3+-doped KMgF3 UCNPs, which can effectively protect Ln3+ from luminescence quenching by surface and internal OH- defects and thereby boost upconversion luminescence. This enables us to realize efficient PA luminescence from Tm3+ at 802 nm in KMgF3: Tm3+ UCNPs upon 1064 nm excitation, with a giant nonlinearity of ∼27, a PA response time of 281 ms, and an excitation threshold of 16.6 kW cm-2. This work may open up a new avenue for exploring highly nonlinear PA luminescence through aliovalent Ln3+ doping and crystal lattice engineering toward diverse emerging applications.

A Generalized Approach to Photon Avalanche Upconversion in Luminescent Nanocrystals

Photon avalanching nanoparticles (ANPs) exhibit extremely nonlinear upconverted emission valuable for subdiffraction imaging, nanoscale sensing, and optical computing. Avalanching has been demonstrated with Tm3+-, Pr3+-, or Nd3+-doped nanocrystals, but their emission is limited to a few wavelengths and materials. Here, we utilize Gd3+-assisted energy migration to tune the emission wavelengths of Tm3+-sensitized ANPs and generate highly nonlinear emission from Eu3+, Tb3+, Ho3+, and Er3+ ions. The upconversion intensities of these spectrally discrete ANPs scale with nonlinearity factor s = 10-17 under 1064 nm excitation at power densities as low as 7 kW cm-2. This strategy for imprinting avalanche behavior on remote emitters can be extended to fluorophores adjacent to ANPs, as we demonstrate with CdS/CdSe/CdS core/shell/shell quantum dots. ANPs with rationally designed energy transfer networks provide the means to transform conventional linear emitters into a highly nonlinear ones, expanding the use of photon avalanching in biological, chemical, and photonic applications.