Utilizing lanthanide upconversion nanoparticles to convert near-infrared to visible light presents a potential way for fabricating the next generation of low-cost near-infrared imaging sensors. Integrating microlens arrays with upconversion nanoparticles has been shown to be a promising approach for improving the efficiency of upconversion nanoparticles. However, approaches suitable for prototyping and producing microlens arrays to explore optimal device designs are lacking. In this work, we report an approach to fabricating flexible near-infrared-to-visible upconversion devices incorporating upconversion nanoparticles and microlens arrays, which enables easy adjustment of device structures and lens geometry. This is achieved by fabricating flexible films containing upconversion nanoparticles using molding in combination with femtosecond laser 3D printing of lenses, facilitating rapid prototyping for different application scenarios. By adding the microlens array, the intensities of the green (525 and 540 nm) and red (654 nm) upconversion emission bands were enhanced by a factor of 3 and 10, respectively, potentially leading to much reduced detectable near-infrared light intensity.

Exploring lanthanide light upconversion (UC) has emerged as a promising strategy to enhance the near-infrared (NIR) responsive region of silicon solar cells (SSCs). However, its practical application under normal sunlight conditions has been hindered by the narrow NIR excitation bandwidth and the low UC efficiency of conventional materials. Here, we report the design of an efficient multiband UC system based on Ln3+/Yb3+-doped core-shell upconversion nanoparticles (Ln/Yb-UCNPs, Ln3+= Ho3+, Er3+, Tm3+). In our design, Ln3+ ions are incorporated into distinct layers of Ln/Yb-UCNPs to function as near-infrared (NIR) absorbers across different spectral ranges. This design achieves broad multiband absorption withtin the 1100 to 2200 nm range, with an aggregated bandwidth of ~500 nm. We have identified a synthetic electron pumping (SEP) effect involving Yb3+ ions, facilitated by the synergistic interplay of energy transfer and cross-relaxation between Yb3+ and other ions Ln3+ (Ho3+, Er3+, Tm3+). This SEP effect enhances the UC efficiency of the nanomaterials by effectively transferring electrons from the low-excited states of Ln3+ to the excited state of Yb3+, resulting in intense Yb3+ luminescence at ~980 nm within the optimal response region for SSCs, thus markedly improving their overall performance. The SSCs integrated with Ln/Yb-UCNPs with multiband excitation demonstrate the largest reported NIR response range up to 2200 nm, while enabling the highest improvement in absolute photovoltaic efficiency reported, with an increase of 0.87% (resulting in a total efficiency of 19.37%) under standard AM 1.5 G irradiation. Our work tackles the bottlenecks in UCNP-coupled SSCs and introduces a viable approach to extend the NIR response of SSCs.

Reversible dark state transitions in fluorophores represent a limiting factor in fluorescence-based ultrasensitive spectroscopy, are a necessary basis for fluorescence-based super-resolution imaging, but may also offer additional, largely orthogonal fluorescence-based readout parameters. In this work, we analyzed the blinking kinetics of Cyanine5 (Cy5) as a bar-coding feature distinguishing Cy5 from rhodamine fluorophores having largely overlapping emission spectra. First, fluorescence correlation spectroscopy (FCS) solution measurements on mixtures of free fluorophores and fluorophore-labeled small unilamellar vesicles (SUVs) showed that Cy5 could be readily distinguished from the rhodamines by its reversible, largely excitation-driven trans-cis isomerization. This was next confirmed by transient state (TRAST) spectroscopy measurements, determining the fluorophore dark state kinetics in a more robust manner, from how the time-averaged fluorescence intensity varies upon modulation of the applied excitation light. TRAST was then combined with wide-field imaging of live cells, whereby Cy5 and rhodamine fluorophores could be distinguished on a whole cell level as well as in spatially resolved, multiplexed images of the cells. Finally, we established a microfluidic TRAST concept and showed how different mixtures of free Cy5 and rhodamine fluorophores and corresponding fluorophore-labeled SUVs could be distinguished on-the-fly when passing through a microfluidic channel. In contrast to FCS, TRAST does not rely on single-molecule detection conditions or a high time resolution and is thus broadly applicable to different biological samples. Therefore, we expect that the bar-coding concept presented in this work can offer an additional useful strategy for fluorescence-based multiplexing that can be implemented on a broad range of both stationary and moving samples.

The N = 34 isotope Sc-55 has been investigated using in-beam gamma-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. Spectra from the direct (p, pn) reaction as well as indirect reaction channels have been investigated. gamma rays with energies 496(10), 570(12), 682(14), 1510(30), 1780(36), 2345(57) and 2470(50) keV have been observed. A level scheme was constructed based on gamma gamma coincidence analysis and relative intensities. The results have been compared to the level scheme already reported in literature, as well as to large-scale shell model calculations in the sd - pf model space. A new level at 1510keV, decaying directly to the ground state, has been proposed and spin-parity J(pi) = 7/2(-) was tentatively assigned. The effect of including the nu g(9/2) orbital is discussed. It can be concluded that the main low-energy properties of Sc-55 seem to be included in the original sd - pf model space.

This work details the synthesis of paramagnetic upconversion nanoparticles doped with Fe3+ in various morphologies via the thermal decomposition method, followed by comprehensive characterization of their structures, optical properties and magnetism using diverse analytical techniques. Our findings demonstrate that by precisely modulating the ratio of oleic acid to octadecene in the solvent, one can successfully obtain hexagonal nanodiscs with a consistent and well-defined morphology. Further adjustments in the oleic acid to octadecene ratio, coupled with fine-tuning of the Na+/F− ratio, led to the production of small-sized nanorods with uniform morphology. Significantly, all Fe3+-doped nanoparticles displayed pronounced paramagnetism, with magnetic susceptibility measurements at 1 T and room temperature of 0.15 emu g−1 and 0.14 emu g−1 for the nanodiscs and nanorods, respectively. To further enhance their magnetic properties, we replaced the Y-matrix with a Gd-matrix, and by fine-tuning the oleic acid/octadecene and Na+/F− ratios, we achieved nanoparticles with uniform morphology. The magnetic susceptibility was 0.82 emu g−1 at 1 T and room temperature. Simultaneously, we could control the nanoparticle size by altering the synthesis temperature. These upconversion nanostructures, characterized by both paramagnetic properties and regular morphology, represent promising dual-mode nanoprobe candidates for optical biological imaging and magnetic resonance imaging.

Pure organic ultralong room temperature phosphorescent (URTP) materials have garnered significant attention for applications in luminescent materials, biosensing, and information encryption. These materials offer advantages over heavy metal phosphorescent materials, such as lower cost, reduced biological toxicity, and minimal environmental impact. Herein, for the first time, we demonstrate a series of organic RTP materials based on spiro[fluorene-9,9′-xanthene] (SFX) hole-transporting molecules, specifically X59 and X55. Our research presents that incorporating more rigid SFX units significantly extends RTP lifetime and enhances photoluminescence quantum yield (PLQY). The large steric hindrance of the rigid SFX structures suppresses nonradiative molecular motions, thereby prolonging phosphorescence emission. Compared to the baseline molecule X1, experimental results show that molecule X59 extends the phosphorescence lifetime by 230 ms, while X55 achieves an extension of 260 ms. Furthermore, we highlight the potential of this series of RTP molecules for use in transparent, programmable luminescent tags. Our work not only expands the molecular types of organic RTP materials but also provides innovative strategies for designing long-lived, high-quantum-yield RTP molecules. We envision that this will advance the smart device field of organic phosphorescent materials and their practical applications, such as intelligent labels, tags, and optical sensors.

Cyanine fluorophores are extensively used in fluorescence spectroscopy and imaging. Upon continuous excitation, especially at excitation conditions used in single-molecule and super-resolution experiments, photo-isomerized states of cyanines easily reach population probabilities of around 50%. Still, effects of photo-isomerization are largely ignored in such experiments. Here, we studied the photo-isomerization of the pentamethine cyanine 5 (Cy5) by two similar, yet complementary means to follow fluorophore blinking dynamics: fluorescence correlation spectroscopy (FCS) and transient-state (TRAST) excitation-modulation spectroscopy. Additionally, we combined TRAST and spectrofluorimetry (spectral-TRAST), whereby the emission spectra of Cy5 were recorded upon different rectangular pulse-train excitations. We also developed a framework for analyzing transitions between multiple emissive states in FCS and TRAST experiments, how the brightness of the different states is weighted, and what initial conditions that apply. Our FCS, TRAST, and spectral-TRAST experiments showed significant differences in dark-state relaxation amplitudes for different spectral detection ranges, which we attribute to an additional red-shifted, emissive photo-isomerized state of Cy5, not previously considered in FCS and single-molecule experiments. The photo-isomerization kinetics of this state indicate that it is formed under moderate excitation conditions, and its population and emission may thus deserve also more general consideration in fluorescence imaging and spectroscopy experiments.

The frequency-domain (FD) method provides an alternative to the commonly used time-domain (TD) approach in characterizing the luminescence kinetics of luminophores, with its own strengths, e.g., the capability to decouple multiple lifetime components with higher reliability and accuracy. While extensively explored for characterizing luminophores with down-shifted emission, this method has not been investigated for studying nonlinear luminescent materials such as lanthanide-doped upconversion nanoparticles (UCNPs), featuring more complicated kinetics. In this work, employing a simplified rate-equation model representing a standard two-photon energy-transfer upconversion process, we thoroughly analyzed the response of the luminescence of UCNPs in the FD method. We found that the FD method can potentially obtain from a single experiment the effective decay rates of three critical energy states of the sensitizer/activator ions involved in the upconversion process. The validity of the FD method is demonstrated by experimental data, agreeing reasonably well with the results obtained by TD methods.

Photodynamic therapy (PDT) fundamentally relies on local generation of PDT precursor states in added photosensitizers (PS), particularly triplet and photo-radical states. Monitoring these states in situ can provide important feedback but is difficult in practice. The states are strongly influenced by local oxygenation, pH and redox conditions, often varying significantly at PDT treatment sites. To overcome this problem, we followed local PDT precursor state populations of PS compounds, via their fluorescence intensity response to systematically varied excitation light modulation. Thereby, we could demonstrate local monitoring of PDT precursor states of methylene blue (MB) and IRdye700DX (IR700), and determined their transitions rates under different oxygenation, pH and redox conditions. By fiber-optics, using one fiber for both excitation and fluorescence detection, the triplet and photo-radical state kinetics of locally applied MB and IR700 could then be monitored in a tissue sample. Finally, potassium iodide and ascorbate were added as possible PDT adjuvants, enhancing intersystem crossing and photoreduction, respectively, and their effects on the PDT precursor states of MB and IR700 could be locally monitored. Taken together, the presented procedure overcomes current methodological limitations and can offer feedback, guiding both excitation and PDT adjuvant application, and thereby more efficient and targeted PDT treatments.

In lanthanide-doped upconversion nanoparticles (UCNPs), the concentration of emitter ions, also known as activator ions, is usually limited to 1 - 5 mol% due to concentration quenching effects. This circumstance limits the luminescent efficiency of UCNPs' and their use in a variety of application areas. Earlier studies have attributed the activator concentration quenching to migration of energy to the nanoparticle surface, while indicating that cross-relaxation between activator ions had a minor role therein. In this work, we carried out comparative studies on Er3+-doped and Yb3+-Er3+ codoped UCNPs and could, in contrast to this notion, prove a general adverse effect of cross-relaxation between activator ions, here Er3+ ions, on up -conversion luminescence (UCL). The direct result of the cross-relaxation is that the energy of the excitation light is accumulated into a low-lying excited state of Er3+ in the infrared region, so forming a "low-lying excited state energy trap ". As a result, the excitation energy is used for generating down-conversion lu-minescence or for indirectly facilitating UCL channels that are directly related to the low-lying excited state energy trap. The identified effect can be used to regulate UCL channels to achieve a concentrated UCL band that is more favorable for certain applications, e.g., biological imaging.