Thermoresponsive Polymeric Nanolenses Magnify the Thermal Sensitivity of Single Upconverting Nanoparticles.

Lanthanide-based upconverting nanoparticles (UCNPs) are trustworthy workhorses in luminescent nanothermometry. The use of UCNPs-based nanothermometers has enabled the determination of the thermal properties of cell membranes and monitoring of in vivo thermal therapies in real time. However, UCNPs boast low thermal sensitivity and brightness, which, along with the difficulty in controlling individual UCNP remotely, make them less than ideal nanothermometers at the single-particle level. In this work, it is shown how these problems can be elegantly solved using a thermoresponsive polymeric coating. Upon decorating the surface of NaYF4 :Er3+ ,Yb3+ UCNPs with poly(N-isopropylacrylamide) (PNIPAM), a >10-fold enhancement in optical forces is observed, allowing stable trapping and manipulation of a single UCNP in the physiological temperature range (20-45 °C). This optical force improvement is accompanied by a significant enhancement of the thermal sensitivity- a maximum value of 8% °C+1 at 32 °C induced by the collapse of PNIPAM. Numerical simulations reveal that the enhancement in thermal sensitivity mainly stems from the high-refractive-index polymeric coating that behaves as a nanolens of high numerical aperture. The results in this work demonstrate how UCNP nanothermometers can be further improved by an adequate surface decoration and open a new avenue toward highly sensitive single-particle nanothermometry.

Thermally induced all-optical inverter and dynamic hysteresis loops in graphene oxide dispersions.

We experimentally study the temporal dynamics of amplitude-modulated laser beams propagating through a water dispersion of graphene oxide sheets in a fiber-to-fiber U-bench. Nonlinear refraction induced in the sample by thermal effects leads to both phase reversing of the transmitted signals and dynamic hysteresis in the input-output power curves. A theoretical model including beam propagation and thermal lensing dynamics reproduces the experimental findings.

Impact of excitation pulse width on the upconversion luminescence lifetime of NaYF4:Yb3+,Er3+ nanoparticles.

The upconversion luminescence (UCL) lifetime has a wide range of applications, serving as a critical parameter for optimizing the performance of upconversion nanoparticles (UCNPs) in various fields. It is crucial to understand that this lifetime does not directly correlate with the decay time of the emission level; rather, it represents a compilation of all the physical phenomena taking place in the upconversion process. To delve deeper into this, we analyzed the dependence of the UCL lifetime on the excitation pulse width for ß-NaYF4:Yb3+,Er3+ nanoparticles. The results revealed a significant increase in the UCL lifetime with both the excitation pulse width and the excitation intensity. The laser fluence was identified as the parameter governing the UCL decay dynamics. We showcased the universality of the pulse-width-dependent UCL lifetime phenomenon by employing UCNPs of various sizes, surface coatings, host matrices, Yb3+ and Er3+ ratios, and dispersing UCNPs in different solvents. Theoretical explanations for the experimental findings were derived through a rate equation analysis. Finally, we discussed the implications of these results in UCNP-FRET (Förster resonance energy transfer)-based applications.

New Insights in Luminescence and Quenching Mechanisms of Ag2S Nanocrystals through Temperature-Dependent Spectroscopy.

Bright near-infrared-emitting Ag2S nanocrystals (NCs) are used for in vivo temperature sensing relying on a reversible variation in intensity and photoluminescence lifetime within the physiological temperature range. Here, to gain insights into the luminescence and quenching mechanisms, we investigated the temperature-dependent luminescence of Ag2S NCs from 300 to 10 K. Interestingly, both emission and lifetime measurements reveal similar and strong thermal quenching from 200 to 300 K, indicating an intrinsic quenching process that limits the photoluminescence quantum yield at room temperature, even for perfectly passivated NCs. The low thermal quenching temperature, broadband emission, and multiexponential microsecond decay behavior suggest the optical transition involves strong lattice relaxation, which is consistent with the recombination of a Ag+-trapped hole with a delocalized conduction band electron. Our findings offer valuable insights for understanding the optical properties of Ag2S NCs and the thermal quenching mechanism underlying their temperature-sensing capabilities.

Ion-induced bias in Ag2S luminescent nanothermometers.

Luminescence nanothermometry allows measuring temperature remotely and in a minimally invasive way by using the luminescence signal provided by nanosized materials. This technology has allowed, for example, the determination of intracellular temperature and in vivo monitoring of thermal processes in animal models. However, in the biomedical context, this sensing technology is crippled by the presence of bias (cross-sensitivity) that reduces the reliability of the thermal readout. Bias occurs when the impact of environmental conditions different from temperature also modifies the luminescence of the nanothermometers. Several sources that cause loss of reliability have been identified, mostly related to spectral distortions due to interaction between photons and biological tissues. In this work, we unveil an unexpected source of bias induced by metal ions. Specifically, we demonstrate that the reliability of Ag2S nanothermometers is compromised during the monitoring of photothermal processes produced by iron oxide nanoparticles. The observed bias occurs due to the heat-induced release of iron ions, which interact with the surface of the Ag2S nanothermometers, enhancing their emission. The results herein reported raise a warning to the community working on luminescence nanothermometry, since they reveal that the possible sources of bias in complex biological environments, rich in molecules and ions, are more numerous than previously expected.

Exploring the Origin of the Thermal Sensitivity of Near-Infrared-II Emitting Rare Earth Nanoparticles.

Rare-earth doped nanoparticles (RENPs) are attracting increasing interest in materials science due to their optical, magnetic, and chemical properties. RENPs can emit and absorb radiation in the second biological window (NIR-II, 1000-1400 nm) making them ideal optical probes for photoluminescence (PL) in vivo imaging. Their narrow emission bands and long PL lifetimes enable autofluorescence-free multiplexed imaging. Furthermore, the strong temperature dependence of the PL properties of some of these RENPs makes remote thermal imaging possible. This is the case of neodymium and ytterbium co-doped NPs that have been used as thermal reporters for in vivo diagnosis of, for instance, inflammatory processes. However, the lack of knowledge about how the chemical composition and architecture of these NPs influence their thermal sensitivity impedes further optimization. To shed light on this, we have systematically studied their emission intensity, PL decay time curves, absolute PL quantum yield, and thermal sensitivity as a function of the core chemical composition and size, active-shell, and outer-inert-shell thicknesses. The results revealed the crucial contribution of each of these factors in optimizing the NP thermal sensitivity. An optimal active shell thickness of around 2 nm and an outer inert shell of 3.5 nm maximize the PL lifetime and the thermal response of the NPs due to the competition between the temperature-dependent back energy transfer, the surface quenching effects, and the confinement of active ions in a thin layer. These findings pave the way for a rational design of RENPs with optimal thermal sensitivity.

Boosting the Near-Infrared Emission of Ag2S Nanoparticles by a Controllable Surface Treatment for Bioimaging Applications.

Ag2S nanoparticles are the staple for high-resolution preclinical imaging and sensing owing to their photochemical stability, low toxicity, and photoluminescence (PL) in the second near-infrared biological window. Unfortunately, Ag2S nanoparticles exhibit a low PL efficiency attributed to their defective surface chemistry, which curbs their translation into the clinics. To address this shortcoming, we present a simple methodology that allows to improve the PL quantum yield from 2 to 10%, which is accompanied by a PL lifetime lengthening from 0.7 to 3.8 µs. Elemental mapping and X-ray photoelectron spectroscopy indicate that the PL enhancement is related to the partial removal of sulfur atoms from the nanoparticle's surface, reducing surface traps responsible for nonradiative de-excitation processes. This interpretation is further backed by theoretical modeling. The acquired knowledge about the nanoparticles' surface chemistry is used to optimize the procedure to transfer the nanoparticles into aqueous media, obtaining water-dispersible Ag2S nanoparticles that maintain excellent PL properties. Finally, we compare the performance of these nanoparticles with other near-infrared luminescent probes in a set of in vitro and in vivo experiments, which demonstrates not only their cytocompatibility but also their superb optical properties when they are used in vivo, affording higher resolution images.

Slow light in molecular-aggregate nanofilms.

We study slow-light performance of molecular aggregates arranged in nanofilms by means of coherent population oscillations. The molecular cooperative behavior inside the aggregate enhances the delay of input signals in the gigahertz range in comparison with other coherent population oscillation-based devices. Moreover, the problem of residual absorption present in coherent population oscillation processes is removed. We also propose an optical switch between different delays by exploiting the optical bistability of these aggregates.

Oligonucleotide sensor based on magnetic capture and photoligation of upconverting nanoparticles in solid surfaces.

In this work, we present a luminescence platform that can be used as point of care system for determining the presence and concentration of specific oligonucleotide sequences. This sensor exhibited a limit of detection as low as 50 fM by means of: (i) the use of single-stranded DNA (ssDNA) functionalized magnetic microparticles that captured and concentrated ssDNA-upconverting nanoparticles (ssDNA-UCNPs) on a solid support, when the target sequence (miR-21-5p DNA-analogue) was in the sample, and (ii) a photoligation reaction that covalently linked the ssDNA-UCNPs and the ssDNA magnetic microparticles, allowing stringent washes. The presented sensor showed a similar limit of detection when the assays were conducted in samples containing total miRNA extracted from human serum, demonstrating its suitability for detecting small specific oligonucleotide sequences under real-like conditions. The strategy of combining UCNPs, magnetic microparticles, and a photoligation reaction provides new insight into low-cost, rapid, and ultra-sensitive detection of oligonucleotide sequences.

Contribution of resonance energy transfer to the luminescence quenching of upconversion nanoparticles with graphene oxide.

Upconversion nanoparticles (UCNP) are increasingly used due to their advantages over conventional fluorophores, and their use as resonance energy transfer (RET) donors has permitted their application as biosensors when they are combined with appropriate RET acceptors such as graphene oxide (GO). However, there is a lack of knowledge about the design and influence that GO composition produces over the quenching of these nanoparticles that in turn will define their performance as sensors. In this work, we have analysed the total quenching efficiency, as well as the actual values corresponding to the RET process between UCNPs and GO sheets with three different chemical compositions. Our findings indicate that excitation and emission absorption by GO sheets are the major contributor to the observed luminescence quenching in these systems. This challenges the general assumption that UCNPs luminescence deactivation by GO is caused by RET. Furthermore, RET efficiency has been theoretically calculated by means of a semiclassical model considering the different nonradiative energy transfer rates from each Er3+ ion to the GO thin film. These theoretical results highlight the relevance of the relative positions of the Er3+ ions inside the UCNP with respect to the GO sheet in order to explain the RET-induced efficiency measurements.

10-Fold Quantum Yield Improvement of Ag2S Nanoparticles by Fine Compositional Tuning.

Ag2S semiconductor nanoparticles (NPs) are near-infrared luminescent probes with outstanding properties (good biocompatibility, optimum spectral operation range, and easy biofunctionalization) that make them ideal probes for in vivo imaging. Ag2S NPs have, indeed, made possible amazing challenges including in vivo brain imaging and advanced diagnosis of the cardiovascular system. Despite the continuous redesign of synthesis routes, the emission quantum yield (QY) of Ag2S NPs is typically below 0.2%. This leads to a low luminescent brightness that avoids their translation into the clinics. In this work, an innovative synthetic methodology that permits a 10-fold increment in the absolute QY from 0.2 up to 2.3% is presented. Such an increment in the QY is accompanied by an enlargement of photoluminescence lifetimes from 184 to 1200 ns. The optimized synthetic route presented here is based on a fine control over both the Ag core and the Ag/S ratio within the NPs. Such control reduces the density of structural defects and decreases the nonradiative pathways. In addition, we demonstrate that the superior performance of the Ag2S NPs allows for high-contrast in vivo bioimaging.

Ultrafast photochemistry produces superbright short-wave infrared dots for low-dose in vivo imaging.

Optical probes operating in the second near-infrared window (NIR-II, 1,000-1,700 nm), where tissues are highly transparent, have expanded the applicability of fluorescence in the biomedical field. NIR-II fluorescence enables deep-tissue imaging with micrometric resolution in animal models, but is limited by the low brightness of NIR-II probes, which prevents imaging at low excitation intensities and fluorophore concentrations. Here, we present a new generation of probes (Ag2S superdots) derived from chemically synthesized Ag2S dots, on which a protective shell is grown by femtosecond laser irradiation. This shell reduces the structural defects, causing an 80-fold enhancement of the quantum yield. PEGylated Ag2S superdots enable deep-tissue in vivo imaging at low excitation intensities (<10 mW cm-2) and doses (<0.5 mg kg-1), emerging as unrivaled contrast agents for NIR-II preclinical bioimaging. These results establish an approach for developing superbright NIR-II contrast agents based on the synergy between chemical synthesis and ultrafast laser processing.

Control of upconversion luminescence by gold nanoparticle size: from quenching to enhancement.

Metallic nanostructures have the potential to modify the anti-Stokes emission of upconverting nanoparticles (UCNPs) by coupling their plasmon resonance with either the excitation or the emission wavelength of the UCNPs. In this regard gold nanoparticles (AuNPs) have often been used in sensors for UCNP luminescence quenching or enhancement, although systematic studies are still needed in order to design optimal UCNP-AuNP based biosensors. Amidst mixed experimental evidence of quenching or enhancement, two key factors arise: the nanoparticle distance and nanoparticle size. In this work, we synthesize AuNPs of different sizes to assess their influence on the luminescence of UCNPs. We find that strong luminescence quenching due to resonance energy transfer is preferentially achieved for small AuNPs, peaking at an optimal size. A further increase in the AuNP size is accompanied by a reduction of luminescence quenching due to an incipient plasmonic enhancement effect. This enhancement counterbalances the luminescence quenching effect at the biggest tested AuNP size. The experimental findings are theoretically validated by studying the decay rate of the UCNP emitters near a gold nanoparticle using both a classical phenomenological model and the finite-difference time-domain method. Results from this study establish general guidelines to consider when designing sensors based on UCNPs-AuNPs as donor-quencher pairs, and suggest the potential of plasmon-induced luminescence enhancement as a sensing strategy.

Observation of large 10-Gb/s SBS slow light delay with low distortion using an optimized gain profile.

An optimum SBS gain profile is designed to achieve better slow-light performance. It consists of a nearly flat-top profile with sharp edges. Tunable delays up to 3 pulse widths for 100-ps-long input pulses, corresponding to 10 Gb/s data rates, are found while keeping an output-input pulse-width ratio below 1.8. Bit-error-rate (BER) measurements performed for a non-return-to-zero modulation format demonstrates 28 ps of delay under error-free operation.

Oligonucleotide Sensor Based on Selective Capture of Upconversion Nanoparticles Triggered by Target-Induced DNA Interstrand Ligand Reaction.

We present a sensor that exploits the phenomenon of upconversion luminescence to detect the presence of specific sequences of small oligonucleotides such as miRNAs among others. The sensor is based on NaYF4:Yb,Er@SiO2 nanoparticles functionalized with ssDNA that contain azide groups on the 3' ends. In the presence of a target sequence, interstrand ligation is possible via the click-reaction between one azide of the upconversion probe and a DBCO-ssDNA-biotin probe present in the solution. As a result of this specific and selective process, biotin is covalently attached to the surface of the upconversion nanoparticles. The presence of biotin on the surface of the nanoparticles allows their selective capture on a streptavidin-coated support, giving a luminescent signal proportional to the amount of target strands present in the test samples. With the aim of studying the analytical properties of the sensor, total RNA samples were extracted from healthy mosquitoes and were spiked-in with a specific target sequence at different concentrations. The result of these experiments revealed that the sensor was able to detect 10-17 moles per well (100 fM) of the target sequence in mixtures containing 100 ng of total RNA per well. A similar limit of detection was found for spiked human serum samples, demonstrating the suitability of the sensor for detecting specific sequences of small oligonucleotides under real conditions. In contrast, in the presence of noncomplementary sequences or sequences having mismatches, the luminescent signal was negligible or conspicuously reduced.

Resonantlike phenomenon in short-pulse free-electron-laser oscillators with modulated desynchronism.

Recently a new method of controlling the pulse length of a short-pulse free-electron laser (FEL) has been developed. By modulating the synchronism between the optical and electron pulses in the FEL cavity, it was found that the output power and the micropulse length of the FEL beam oscillates at the modulation frequency. In this paper, we study theoretically the behavior of the micropulse length, both in the high loss (steady state) regime and the low loss (limit cycle) regime, when a modulated desynchronism is applied. In order to do this, we analyze the dynamics of a short-pulse FEL oscillator. The modulation frequency value plays an important role in the dynamics. We find that there is a resonantlike phenomenon between the externally applied desynchronism modulation and the limit cycle oscillation without modulation of a free-electron laser.

Microstructure evolution in magnetorheological suspensions governed by Mason number.

The spatiotemporal evolution of field-induced structures in very dilute polarizable colloidal suspensions subject to rotating magnetic fields has been experimentally studied using video microscopy. We found that there is a crossover Mason number (ratio of viscous to magnetic forces) above which the rotation of the field prevents the particle aggregation to form chains. Therefore, at these high Mason numbers, more isotropic clusters and isolated particles appear. The same behavior was also found in recent scattering dichroism experiments developed in more concentrated suspensions, which seems to indicate that the dynamics does not depend on the volume fraction. Scattering dichroism experiments have been used to study the role played by the volume fraction in suspensions with low concentration. As expected, we found that the crossover Mason number does not depend on the volume fraction. Brownian particle dynamics simulations are also reported, showing good agreement with the experiments.

Polarizable particle aggregation under rotating magnetic fields using scattering dichroism.

We used scattering dichroism to study the dynamics of dipolar chains induced in magnetorheological suspensions under rotating magnetic fields. Both the dichroism (proportional to the total number of aggregated particles) and the phase lag show different behavior below and above a cross-over frequency. The cross-over frequency depends linearly on both the square of the magnetization and the inverse of the viscosity. The Mason number (ratio of viscous to magnetic forces) governs the dynamics. Therefore, there is a cross-over Mason number below which the dichroism remains almost constant and above which the rotation of the field prevents the particle aggregation process from taking place. Our experimental results have been compared with particle dynamics simulations showing good agreement.

Modulation-frequency-controlled change from sub- to superluminal regime in highly doped erbium fibers.

We report a change from sub- to superluminal propagation upon increasing the modulation frequency of an amplitude-modulated 1,550 nm signal when propagating through highly doped erbium fibers pumped at 980 nm. We show that the interplay between the pump absorption and the pump-power broadening of the spectral hole induced by coherent population oscillations may drastically affect the fractional advancement or delay of the signal for the considered fibers.

Dynamic transition from modelike patterns to turbulentlike patterns in a broad-area Nd:YAG laser.

We report the first experimental observation to our knowledge of a dynamic transition from modelike patterns to completely disordered patterns in a large-aspect-ratio Nd:YAG laser. Recordings of near-field patterns with an integration time as small as 1 ns allow us to follow the evolution of the transverse intensity profile along the output pulse of the laser.