Spontaneous Fluorescence Research Articles

Harnessing LRET in a rationally designed "sandwich" fluorescent probe for selective ClO- sensing.

Upconversion nanoparticles (UCNPs), as a type of light-emitting nanomaterials excited by near-infrared (NIR) light, can circumvent interference from spontaneous fluorescence and scattered light emitted by biological molecules in sensing applications. Traditional homogeneous core-shell UCNPs struggle to locate the position of luminescent doped ions (such as Tm3+) within the NaYF4 matrix, only the luminescent ions close to the surface of the particles can be efficiently quenched, with a low bursting efficiency that produces a considerable background. The full effectiveness energy transfer exists only by sufficiently close proximity between donor and acceptor. Here, a distance dependence "sandwich" structured NIR fluorescent probe UCNPs-IR820 was developed based on luminescence resonance energy transfer (LRET), where the luminescent ions (Tm3+) were secured in a intermediate layer of a core-middle-shell UCNPs structure, which could be quenched by the specific distance external adjacent receptor molecule (cyanine NIR dye IR820). In the presence of the analyte ClO-, the double bonds of IR820 were oxidized, unable to absorb the luminescence from the core-middle-shell "sandwich" UCNPs, thus an "off to on" luminescence was restored. This core-middle-shell design could effectively enhance the effect of LRET-based detection strategies through physically meaningful distance constraints, providing new ideas for the design of future UCNP-based NIR fluorescent probes.

Emission induced by strong coupling between hybrid Tamm–surface plasmon polariton mode and rhodamine 6G dye exciton

otal internal reflection ellipsometry (TIRE) and leakage microscopy were applied for the study of photonic-plasmonic nanostructures supporting hybrid Tamm–surface plasmon modes and their strong coupling with Rhodamine 6G organic dye excitons. The optical response of TIRE has shown that Tamm and surface plasmon polaritons interact strongly and the formed hybrid plasmonic mode alters resonances in the energy spectra. Moreover, both TPP (Tamm plasmon polaritons) and SPP (surface plasmon polaritons) components in the hybrid mode are strongly coupled with R6G-PMMA (poly(methyl methacrylate)) layers at the inner and outer interfaces of the 50 nm gold layer, respectively. Leakage microscopy in the back focal plane optical configuration proves the energy transfer of excited emitters through the 50 nm gold layer in the strong coupling regime. Polaritonic emission in the strong coupling has better coherence properties than conventional spontaneous fluorescence emission from pure Rhodamine 6G organic dye molecules.

Recent advances of photoresponsive nanomaterials for diagnosis and treatment of acute kidney injury.

Non-invasive imaging in the near-infrared region (NIR) offers enhanced tissue penetration, reduced spontaneous fluorescence of biological tissues, and improved signal-to-noise ratio (SNR), rendering it more suitable for in vivo deep tissue imaging. In recent years, a plethora of NIR photoresponsive materials have been employed for disease diagnosis, particularly acute kidney injury (AKI). These encompass inorganic nonmetallic materials such as carbon (C), silicon (Si), phosphorus (P), and upconversion nanoparticles (UCNPs); precious metal nanoparticles like gold and silver; as well as small molecule and organic semiconductor polymer nanoparticles with near infrared responsiveness. These materials enable effective therapy triggered by NIR light and serve as valuable tools for monitoring AKI in living systems. The review provides a concise overview of the current state and pathological characteristics of AKI, followed by an exploration of the application of nanomaterials and photoresponsive nanomaterials in AKI. Finally, it presents the design challenges and prospects associated with NIR photoresponsive materials in AKI.

Effect of Alkyl Chain Length on Room-Temperature Phosphorescent Probes for Mitochondria-Targeted Imaging.

Room temperature phosphorescent (RTP) probes have significant advantages in the field of cellular imaging, as their long lifetimes can prevent interference from the spontaneous fluorescence of organisms. Persulfurated arenes are a typical RTP molecular parent nucleus. However, most of the applied research on them is concentrated in anti-counterfeiting, and relatively few are applied in bioimaging. The molecular structure and structure-property relationship of them applied in bioimaging are still in the exploration stage. In this work, we have designed and synthesized a series of RTP probes with long alkyl chains, all of which can be targeted to mitochondria with good water solubility for mitochondria-targeted imaging. Further, we investigated the effect of alkyl chains on the luminescence properties of these probes, and found that the moderate length of alkyl chains can realize the enhancement of phosphorescence intensity. We believe this finding is of guiding significance for the design of molecular structures in the field of RTP probes.

Generation of 1 MHz 15 fs pulses in the near-infrared with high energy and their application to time-resolved spectroscopy.

Ultrashort pulses with a duration of ∼10 fs are crucial to fully resolve nuclear motions in molecules and materials. Here, we employ nonlinear pulse compression using photonic crystal fiber in the near-infrared region to generate ultrashort pulses at a high repetition rate with significant pulse energy. Femtosecond pulses centered around 1200 nm from a cavity-dumped optical parametric oscillator are compressed to a duration of 15 fs. The pulse energy reaches several tens of nanojoules, sufficient for wavelength conversion via nonlinear processes. Second harmonic generation produces clean 13 fs pulses at around 600 nm with pulse energy of 4 nJ. The effectiveness of nonlinear pulse compression using photonic crystal fiber for ultrafast time-resolved spectroscopy is demonstrated through time-resolved fluorescence (photoluminescence) and transient absorption apparatus, utilizing the second harmonic as pump pulses. Specifically, a time resolution of ∼20 fs is achieved in a time-resolved fluorescence experiment, enabling the recording of nuclear wave packets over 1200 cm-1 by spontaneous fluorescence. The high repetition rate at the megahertz level significantly improved the signal quality.

Multimodal imaging diagnosis and analysis of prognostic factors in patients with adult-onset Coats disease.

To describe the multimodal imaging features, treatment, and outcomes of patients diagnosed with adult-onset Coats disease. This retrospective study included patients first diagnosed with Coats disease at ≥18 years of age between September 2017 and September 2021. Some patients received anti-vascular endothelial growth factor (VEGF) therapy (conbercept, 0.5 mg) as the initial treatment, which was combined with laser photocoagulation as needed. All the patients underwent best corrected visual acuity (BCVA) and intraocular pressure examinations, fundus color photography, spontaneous fluorescence tests, fundus fluorescein angiography, optical coherence tomography (OCT), OCT angiography, and other examinations. BCVA alterations and multimodal image findings in the affected eyes following treatment were compared and the prognostic factors were analyzed. The study included 15 patients who were aged 24-72 (57.33±12.61)y at presentation. Systemic hypertension was the most common associated systemic condition, occurring in 13 (86.7%) patients. Baseline BCVA ranged from 2.0 to 5.0 (4.0±1.1), which showed improvement following treatment (4.2±1.0). Multimodal imaging revealed retinal telangiectasis in 13 patients (86.7%), patchy hemorrhage in 5 patients (33.3%), and stage 2B disease (Shield's staging criteria) in 11 patients (73.3%). OCT revealed that the baseline central macular thickness (CMT) ranged from 129 to 964 µm (473.0±230.1 µm), with 13 patients (86.7%) exhibiting a baseline CMT exceeding 250 µm. Furthermore, 8 patients (53.3%) presented with an epiretinal membrane at baseline or during follow-up. Hyper-reflective scars were observed on OCT in five patients (33.3%) with poor visual prognosis. Vision deteriorated in one patient who did not receive treatment. Final vision was stable in three patients who received laser treatment, whereas improvement was observed in one of two patients who received anti-VEGF therapy alone. In addition, 8 of 9 patients (88.9%) who received laser treatment and conbercept exhibited stable or improved BCVA. Multimodal imaging can help diagnose adult-onset Coats disease. Anti-VEGF treatment combined with laser therapy can be an option for improving or maintaining BCVA and resolving macular edema. The final visual outcome depends on macular involvement and the disease stage.

Water-soluble near-infrared AgInS2 quantum dots for Ca2+ detection and bioimaging

Calcium ions (Ca2+) are key players in intracellular signaling as second messengers and play a pivotal role in various physiological processes. In this study, near-infrared water-soluble AgInS2 quantum dots (AIS QDs) for Ca2+ detection were synthesized by a one-step hydrothermal method. The fluorescence quantum yield (PL QY) of the quantum dots was as high as 23.99 %. With low cytotoxicity and good fluorescence properties, as well as short reaction time, the ternary AIS QDs have excellent synthesis efficiency and quantum yield, which are advantageous for Ca2+ detection and bioimaging applications. The fluorescence quenching of the quantum dots showed a clear linear relationship with calcium ion concentration in the range of 0–250 μM (detection limit: 0.65 μM). Confocal imaging experiments demonstrate the excellent biofluorescence imaging capability of AIS QDs. By tuning the Ag/In molar ratio, AIS QDs can achieve fluorescence emission in the near-infrared wavelength band (620–700 nm), and the near-infrared fluorescence imaging has deeper tissue penetration, less tissue absorption and photodamage, and lower interference of spontaneous fluorescence, which further expands the potential of QDs for bioimaging applications.

Ultrafast Super-Resolution Imaging Exploiting Spontaneous Blinking of Static Excimer Aggregates.

Single-molecule localization methods have been popularly exploited to obtain super-resolved images of biological structures. However, the low blinking frequency of randomly switching emission states of individual fluorophores greatly limits the imaging speed of single-molecule localization microscopy (SMLM). Here we present an ultrafast SMLM technique exploiting spontaneous fluorescence blinking of cyanine dye aggregates confined to DNA framework nanostructures. The DNA template guides the formation of static excimer aggregates as a "light-harvesting nanoantenna", whereas intermolecular excitation energy transfer (EET) between static excimers causes collective ultrafast fluorescence blinking of fluorophore aggregates. This DNA framework-based strategy enables the imaging of DNA nanostructures with 12.5-fold improvement in speed compared to conventional SMLM. Further, we demonstrate the use of this strategy to track the movement of super-resolved DNA nanostructures for over 20 min in a microfluidic system. Thus, this ultrafast SMLM holds great potential for revealing the dynamic processes of biomacromolecules in living cells.

Physical reality

The principle of non-locality, or the existence of systems of particles with properties that relate them even at great distances, are called entangled systems and defy the intuition that requires all relationships to be described by means of energy exchange or by material links, quantum physics presents non-locality as a consequence of objects being described by a single wave function and as such unless they decohere (losses its nonphysical link), the relationship remains, and each cannot be understood separately. Recently, there have been many technological models that can produce entangled systems. In addition to the examples that submicroscopic physics can illustrate, the simplest is spontaneous parametric fluorescence, which requires a laser and a parametric crystal that has allowed very elaborate experiments to be carried out and shows the relevance of quantum physics and the limitations of our perception. The examples described here have emerged over the years as attempts to make this concept more acceptable and try to guide the imagination to situations where these types of phenomena can be plausible and point to human perception with the duality of bringing us closer to the appreciation of nature, but at the same time, it is the main limitation to appreciate and understand nature. The relevance of this concept was recognized with the Nobel Prize 2022 in physics, and it can be summarized as the Proof of the Bell inequality using anecdotes from a Nobel non-recipient.

Squaraine Dyes Exhibit Spontaneous Fluorescence Blinking That Enables Live-Cell Nanoscopy.

Hampered by their susceptibility to nucleophilic attack and chemical bleaching, electron-deficient squaraine dyes have long been considered unsuitable for biological imaging. This study unveils a surprising twist: in aqueous environments, bleaching is not irreversible but rather a reversible spontaneous quenching process. Leveraging this new discovery, we introduce a novel deep-red squaraine probe tailored for live-cell super-resolution imaging. This probe enables single-molecule localization microscopy (SMLM) under physiological conditions without harmful additives or intense lasers and exhibits spontaneous blinking orchestrated by biological nucleophiles, such as glutathione or hydroxide anion. With a low duty cycle (∼0.1%) and high-emission rate (∼6 × 104 photons/s under 400 W/cm2), the squaraine probe surpasses the benchmark Cy5 dye by 4-fold and Si-rhodamine by a factor of 1.7 times. Live-cell SMLM with the probe reveals intricate structural details of cell membranes, which demonstrates the high potential of squaraine dyes for next-generation super-resolution imaging.

Construction of an autofluorescence interference-free phosphorescence biosensor for the specific detection of TK1 mRNA

The anti-interference ability of biosensors is critical for detection in biological samples. Fluorescence-based sensors are subject to interference from self-luminescent substances in biological matrices. Therefore, phosphorescent sensors stand out among biosensors due to their lack of self-luminescence background. In this study, a phosphorescent sensor was constructed, which can accurately detect thymidine kinase 1 (TK1) mRNA in biological samples and avoid autofluorescence interference. When there is no target, polydopamine (PDA) is used as the phosphorescence resonance energy transfer (PRET) acceptor to quench the phosphorescence of the persistently luminescent (PL) nanomaterial. When there is a target, the DNA modified by the PL nanomaterial is replaced by the hairpin H and removed away from the PDA, resulting in a rebound in phosphorescence. The phosphorescent sensor exhibits a good linear relationship in the TK1 mRNA concentration range of 0–200 nM, and the detection limit was 1.74 nM. The sensor fabricated in this study can effectively avoid interference from spontaneous fluorescence in complex biological samples, and sensitively and precisely detect TK1 mRNA in serum samples, providing a powerful tool to more accurately detect biomarkers in biological samples.

Quick Large‐Area Detection of Thin Silicone Films with Coherent Raman Scattering Imaging

Coherent Raman Scattering Imaging offers quick and surface‐sensitive large‐area scans of specimens including fingerprinting of the substances, combined with low detection limits. By using a multi‐photon process, it offers orders of magnitude stronger surface signal in comparison to spontaneous Raman (SR) and fluorescence signals of the substrate are avoided by the detection principle and the near‐infrared laser wavelength. By exciting at a specific wavelength instead of measuring whole spectra the measurement time is remarkably reduced. Here, detecting organic thin films on glass surfaces is investigated. The detection of those thin films is relevant for production processes where the interfacial layer is influenced by the low surface energy of contaminants, especially polysiloxanes. The samples are manufactured by pipetting diluted polydimethylsiloxane (PDMS) in heptane in various concentrations (1%, 0.1%, 0.01%, and 0.001%) on glass substrates. After evaporation of the solvent, thin films of the silicone remain. The resulting average coverage of the thin films equals 100–0.1 μg cm−2. Measurements determine the detectability threshold with SR to 10 μg cm−2, while the acquired spectrum is used to determine characteristic peaks for the pump laser. Afterwards the samples are imaged at those wavenumbers with Coherent Raman Scattering (CRS) Imaging techniques providing a chemically‐specific contrast.

Single-Molecule Time-Resolved Spectroscopy in a Tunable STM Nanocavity.

Spontaneous fluorescence rates of single-molecule emitters are typically on the order of nanoseconds. However, coupling them with plasmonic nanostructures can substantially increase their fluorescence yields. The confinement between a tip and sample in a scanning tunneling microscope creates a tunable nanocavity, an ideal platform for exploring the yields and excitation decay rates of single-molecule emitters, depending on their coupling strength to the nanocavity. With such a setup, we determine the excitation lifetimes from the direct time-resolved measurements of phthalocyanine fluorescence decays, decoupled from the metal substrates by ultrathin NaCl layers. We find that when the tip is approached to single molecules, their lifetimes are reduced to the picosecond range due to the effect of coupling with the tip-sample nanocavity. On the other hand, ensembles of the adsorbed molecules measured without the nanocavity manifest nanosecond-range lifetimes. This approach overcomes the drawbacks associated with the estimation of lifetimes for single molecules from their respective emission line widths.

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.

Low-frequency background estimation and noise separation from high-frequency for background and noise subtraction.

In fluorescence microscopy, background blur and noise are two main factors preventing the achievement of high-signal-to-noise ratio (SNR) imaging. Background blur primarily emanates from inherent factors including the spontaneous fluorescence of biological samples and out-of-focus backgrounds, while noise encompasses Gaussian and Poisson noise components. To achieve background blur subtraction and denoising simultaneously, a pioneering algorithm based on low-frequency background estimation and noise separation from high-frequency (LBNH-BNS) is presented, which effectively disentangles noise from the desired signal. Furthermore, it seamlessly integrates low-frequency features derived from background blur estimation, leading to the effective elimination of noise and background blur in wide-field fluorescence images. In comparisons with other state-of-the-art background removal algorithms, LBNH-BNS demonstrates significant advantages in key quantitative metrics such as peak signal-to-noise ratio (PSNR) and manifests substantial visual enhancements. LBNH-BNS holds immense potential for advancing the overall performance and quality of wide-field fluorescence imaging techniques.

Lanthanide‐Doped Nanomaterials for Tumor Diagnosis and Treatment by Second Near‐Infrared Fluorescence Imaging

AbstractLanthanides have unique electronic configurations and stepped energy states of [Xe]4fN, which endows them with advantages including stable luminescence, large stokes shift, wide excitation spectral bands, and long fluorescence lifetime. NIR‐II fluorescence imaging overcomes the strong tissue absorption, scattering, and spontaneous fluorescence interference of traditional fluorescence and can achieve deeper tissue penetration and higher spatial and temporal resolution in vivo imaging. Lanthanides hold great potential for in vivo biosensing and bioimaging. In this work, the design and preparation of nanomaterials doped with different lanthanide elements are reviewed in this paper. Applications of lanthanide‐doped nanomaterials to different tumor therapeutics through NIR‐II fluorescence imaging and the unique potential of NIR lanthanide nanomaterials in tumor diagnosis and therapy are also discussed.

Dual-emitting N-doped carbon dots/AuNCs nanohybrids as an efficient ratiometric fluorescent probe and a paper sensor for Pb2+ sensing

Ratiometric fluorescent sensors could effectively eliminate the interference of spontaneous fluorescence or instrument fluctuation to improve the analysis sensitivity and accuracy. In this work, a dual-emission nitrogen-doped carbon dots/AuNCs (NCDs-AuNCs) nanohybrid was fabricated under mild conditions by the hydrothermal method, without additional coupling agents, which is facile and environmentally friendly. The prepared nanohybrid was employed for the sensitive detection of Pb2+. The sensor exhibited dual emission peaks at 460 and 625 nm with efficient fluorescence resonance energy transfer under a single excitation wavelength of 360 nm, in which the red fluorescence of AuNCs was enhanced and the blue fluorescence of NCDs was slightly quenched with the presence of Pb2+. The quantitative analysis of Pb2+ was performed upon the change of both fluorescence intensities. The sensor was successfully used for Pb2+ detection in real water samples with satisfactory spiking recoveries in a range of 91.0%–101.3% and low relatively standard deviations (RSDs < 3%), which exhibited good selectivity and anti-interference ability. In addition, a paper-based sensor was fabricated by immobilizing the nanohybrids on filter paper, which enables the visual semi-quantitative determination of Pb2+ by the naked eye through the color transition from blue to pink. The prepared NCDs-AuNCs nanohybrids showed great application potential in environmental water monitoring in an efficient and green way.

Relationship between fluorescence characteristics of coal macerals and excitation time

The fluorescence characteristics of coal macerals can be used as one of the indexes to evaluate the properties of coking coal. In this work, a single-wavelength laser with a wavelength of 360 nm was used as the excitation source to excite the surface of particulate block under a polarizing microscope. Effect of excitation time on fluorescence characteristics of the macerals was studied. The relationship between spontaneous fluorescence intensity and the excitation time of each maceral of six kinds of coking coals show that the fluorescence characteristics of coal macerals are related to the type and metamorphism of coal. The excitation time has a certain effect on the fluorescence parameters of the macerals. By comparing the relative fluorescence intensity values under different excitation times, it is found that the mean relative fluorescence intensity within 15 s can be used as an optical parameter to characterize the structure and metamorphic grade of different macerals. The essence of this method is to express movement of electrons in outer layer of nucleus by macroscopic fluorescence spectrum and relative fluorescence intensity of the initial state value and simplify microscopic complexity into macroscopic and numerical form generally accepted.

Phonon engineering in Yb:La2CaB10O19 crystal for extended lasing beyond the fluorescence spectrum

Since the first invention of the laser in 1960, direct lasing outside the fluorescence spectrum is deemed impossible owing to the “zero-gain” cross-section. However, when electron-phonon coupling meets laser oscillation, an energy modulation by the quantized phonon can tailor the electronic transitions, thus directly creating some unprecedented lasers with extended wavelengths by phonon engineering. Here, we demonstrate a broadband lasing (1000–1280 nm) in a Yb-doped La2CaB10O19 (Yb:LCB) crystal, far beyond its spontaneous fluorescence spectrum. Numerical calculations and in situ Raman verify that such a substantial laser emission is devoted to the multiphonon coupling to lattice vibrations of a dangling “quasi-free-oxygen” site, with the increasing phonon numbers step-by-step (n = 1–6). This new structural motif provides more alternative candidates with strong-coupling laser materials. Moreover, the quantitative relations between phonon density distribution and laser wavelength extension are discussed. These results give rise to the search for on-demand lasers in the darkness and pave a reliable guideline to study those intriguing electron-phonon-photon coupled systems for integrated photonic applications.

Direct phoxim sensing based on fluorescent metal-organic framework of Nu-1000 induced FRET

To control and on-site evaluate the contamination of organophosphorus pesticides, one direct sensing approach based on spontaneous fluorescence resonance energy transfer (FRET) was developed in this study. The luminescent metal-organic frameworks (LMOFs), a novel achievement of MOFs, could provide adjustable adsorption capacity and fluorescence index to scientific analysis was first employed to evaluate phoxim molecules in vegetables and fruits. Through the π-π recognition and FRET sensing between phoxim molecules and skeleton of Nu-1000 LMOFs probes, an ultra-low detection limit of 1.65 pg/L in the range of 5×10−7 to 5×10−3 mg/mL (R2=0.998) was achieved in the presence of phoxim. Such simple and accurate LMOFs- FRET-based sensing strategy could offer valuable reference for developing sensitive FRET sensors and/or on-site evaluation approaches in the areas of food quality control and food safety.