Phosphorescence, characterized by luminescent lifetimes significantly longer than that of biological autofluorescence under ambient environment, is of great value for biomedical applications. Academic evidence of fluorescence imaging indicates that virtually all imaging metrics (sensitivity, resolution, and penetration depths) are improved when progressing into longer wavelength regions, especially the recently reported second near-infrared (NIR-II, 1000-1700 nm) window. Although the emission wavelength of probes does matter, it is not clear whether the guideline of "the longer the wavelength, the better the imaging effect" is still suitable for developing phosphorescent probes. For tissue-specific bioimaging, long-lived probes, even if they emit visible phosphorescence, enable accurate visualization of large deep tissues. For studies dealing with bioimaging of tiny biological architectures or dynamic physiopathological activities, the prerequisite is rigorous planning of long-wavelength phosphorescence, being aware of the cooperative contribution of long wavelengths and long lifetimes for improving the spatiotemporal resolution, penetration depth, and sensitivity of bioimaging. In this Review, emerging molecular engineering methods of room-temperature phosphorescence are discussed through the lens of photophysical mechanisms. We highlight the roles of phosphorescence with emission from visible to NIR-II windows toward bioapplications. To appreciate such advances, challenges and prospects in rapidly growing studies of room-temperature phosphorescence are described.

Molecularly Engineered Room-Temperature Phosphorescence for Biomedical Application: From the Visible toward Second Near-Infrared Window.

ABSTRACT Thermal annealing has been proven to be an efficient method to optimize the device performance of organic and polymeric opto-electronic materials. However, no detailed information of aggregate structures was obtained for a deeper understanding of what happens during thermal annealing. Herein, through modulation of molecular configurations by tunable linkage positions, and the amplified amplitudes of molecular motions by incorporation of additional methylene units, accurate changes of aggregated structures upon thermal annealing have been achieved, accompanying with the ‘turn-on’ room temperature phosphorescence (RTP) response by about 4800- and 177-fold increase of lifetimes. The stretching and swing motion models have been proposed, which afforded an efficient way to investigate the science of dynamic aggregation in depth.

Thermal annealing promoted room temperature phosphorescence: motion models and internal mechanism

Photo-responsive materials (PRMs) featuring remarkable variation in emission color and/or intensity under light stimulus have attracted much attention due to their potential applications in information storage, anti-counterfeit and sensing. Herein, a new kind of PRMs with tunable luminescence lifetimes manipulated by light stimulus is reported. Transparent and flexible polymer films with room temperature phosphorescence (RTP) property are facilely prepared by solution blending polymers and chromophores. Not only RTP intensity, but also RTP lifetime of the doped polymer films is reversibly regulated through consecutive UV irradiation. Particularly, RTP lifetime is amendable between 0.83 and 2.84 s by facilely regulating irradiation duration, which is related to the consumption of oxygen in the polymer films. Taking advantage of the tunable RTP lifetime and intensity, the doped polymer films are used for rewritable write-once-read-many (WORM) information storage. In addition, application of the polymer films in visual sensing of environmental moisture is demonstrated. This work offers a new kind of PRMs with modulable emission lifetimes through light stimuli, which is useful for various applications including sensing, anti-counterfeiting and data storage. 

Efficient room-temperature phosphorescence with tunable lifetime through light modulation from flexible polymer films

Color-tunable room-temperature phosphorescence (RTP) with potential in many fields is of great importance but extremely challenging. It is necessary to comprehend the correlation between the molecular structure and property to design and synthesize such materials. Metal-organic coordination polymers (CPs) with good predesignability and precise structure have become a platform to construct RTP materials. Herein, three zinc-based CPs containing halogen and a flexible tetradentate ligand are synthesized. All of these CPs present two constant emission regions and an excitation-dependent emission region. Structure-property analysis shows that these emissions originate from isolated chromophores and dimerized chromophores as well as various charge transfers. The phosphorescence colors of these CPs can be modulated by excitation and temperature. This study provides a novel strategy to construct multicolor and multiresponsive RTP materials based on metal-organic coordination polymers.

Multicolor Phosphorescence Modulated by Excitation and Temperature in Zn-Based Coordination Polymers.

Probing biological processes in living organisms that could provide one-of-a-kind insights into real-time alterations of significant physiological parameters is a formidable task that calls for specialized analytic devices. Classical biochemical methods have significantly aided our understanding of the mechanisms that regulate essential biological processes. These methods, however, are typically insufficient for investigating transient molecular events since they focus primarily on the end outcome. Fluorescence resonance energy transfer (FRET) microscopy is a potent tool used for exploring non-invasively real-time dynamic interactions between proteins and a variety of biochemical signaling events using sensors that have been meticulously constructed. Due to their versatility, FRET-based sensors have enabled the rapid and standardized assessment of a large array of biological variables, facilitating both high-throughput research and precise subcellular measurements with exceptional temporal and spatial resolution. This review commences with a brief introduction to FRET theory and a discussion of the fluorescent molecules that can serve as tags in different sensing modalities for studies in chemical biology, followed by an outlining of the imaging techniques currently utilized to quantify FRET highlighting their strengths and shortcomings. The article also discusses the various donor-acceptor combinations that can be utilized to construct FRET scaffolds. Specifically, the review provides insights into the latest real-time bioimaging applications of FRET-based sensors and discusses the common architectures of such devices. There has also been discussion of FRET systems with multiplexing capabilities and multi-step FRET protocols for use in dual/multi-analyte detections. Future research directions in this exciting field are also mentioned, along with the obstacles and opportunities that lie ahead. 

Exploring the landscape of FRET-based molecular sensors: Design strategies and recent advances in emerging applications

The stimulus-responsive room-temperature phosphorescence (RTP) materials have become an increasingly significant topic in the fields of bioimaging, sensing, and anticounterfeiting. However, this kind of materials is scarce to date, especially for the ones with delicate stimulus-responsive behavior. Herein, a universal strategy for multilevel thermal erasure of RTP via chromatographic separation of host-guest doping RTP systems is proposed. The tunable host-guest systems, matrix materials, heating temperature, and time are demonstrated to allow precise six-level data encryption, QR code encryption, and thermochromic phosphorescence encryption. Mechanistic study reveals that the thermal-responsive property might be attributed to molecular thermal motion and the separation effect of the silica gel, which provides expanded applications of host-guest RTP materials such as cold chain break detection. This work offers a simple yet universal way to construct advanced responsive RTP materials.

https://downloads.spj.sciencemag.org/research/2022/9782713.pdf

Molecular Thermal Motion Modulated Room-Temperature Phosphorescence for Multilevel Encryption

The application of nanostructures to design fluorescence resonance energy transfer (FRET) based sensing platforms has been greatly concerned with the demand for sensitive and selective detection of biomolecules. Here, a novel sensitive turn-on fluorescence strategy based on the FRET mechanism has been designed for hyaluronidase (HAase) detection through the modulation of Cu2-x[email protected] nanoprobe fabricated by self-assembly of rhodamine 6G (Rh6G) together with Cu2-x[email protected] nanoparticles through electrostatic adsorption. The Cu2-x[email protected] had extensive localized surface plasma resonance (LSPR) absorption in the wide range of ultraviolet (UV) to near-infrared (NIR) wavelengths and showed good light capture characteristics, which can be acted as good acceptors in the FRET interactions with Rh6G, inducing its efficient fluorescence quenching. In the presence of HAase, the FRET process was disrupted and the fluorescence signal was recovered. In the range of 0.1–10.0 U/mL, the fluorescence recovery of Rh6G showed a good linear relationship with the concentration of HAase, and the detection limit was 0.06 U/mL. The sensing platform has been used for HAase detection in real urine samples and cancer cells imaging. 

Fluorescence turn-on Cu2-xSe@HA-rhodamine 6G FRET nanoprobe for hyaluronidase detection and imaging.

Plasmonic nanomaterials have attracted great attention in the field of catalysis and sensing for their outstanding electrical and optical properties. Here, a representative type of nonstoichiometric Cu2−xSe nanoparticles with typical near-infrared (NIR) localized surface plasma resonance (LSPR) properties originating from their copper deficiency was applied to catalyze the oxidation of colorless TMB into their blue product in the presence of H2O2, indicating they had good peroxidase-like activity. However, glutathione (GSH) inhibited the catalytic oxidation of TMB, as it can consume the reactive oxygen species. Meanwhile, it can induce the reduction of Cu(II) in Cu2−xSe, resulting in a decrease in the degree of copper deficiency, which can lead to a reduction in the LSPR. Therefore, the catalytic ability and photothermal responses of Cu2−xSe were decreased. Thus, in our work, a colorimetric/photothermal dual-readout array was developed for the detection of GSH. The linear calibration for GSH concentration was in the range of 1–50 μM with the LOD as 0.13 μM and 50–800 μM with the LOD as 39.27 μM. To evaluate the practicability of the assay, tomatoes and cucumbers were selected as real samples, and good recoveries indicated that the developed assay had great potential in real applications.

https://www.mdpi.com/2079-4991/13/11/1787/pdf?version=1685616484

Plasmonic Cu2−xSe Mediated Colorimetric/Photothermal Dual-Readout Detection of Glutathione

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Molecular Thermal Motion Modulated Room-Temperature Phosphorescence for Multilevel Encryption

Fluorescence turn-on Cu2-xSe@HA-rhodamine 6G FRET nanoprobe for hyaluronidase detection and imaging.

Plasmonic Cu2−xSe Mediated Colorimetric/Photothermal Dual-Readout Detection of Glutathione