Toward Accurate Photoluminescence Nanothermometry Using Rare-Earth Doped Nanoparticles for Biomedical Applications.

ConspectusPhotoluminescence nanothermometry can detect the local temperature at the submicrometer scale with minimal contact with the object under investigation. Owing to its high spatial resolution, this technique shows great potential in biomedicine in both fundamental studies as well as preclinical research. Photoluminescence nanothermometry exploits the temperature-dependent optical properties of various nanoscale optical probes including organic fluorophores, quantum dots, and carbon nanostructures. At the vanguard of these diverse optical probes, rare-earth doped nanoparticles (RENPs) have demonstrated remarkable capabilities in photoluminescence nanothermometry. They distinguish themselves from other luminescent nanoprobes owning to their unparalleled and versatile optical properties that include narrow emission bandwidths, high photostability, tunable lifetimes from microseconds to milliseconds, multicolor emissions spanning the ultraviolet, visible, and near-infrared (NIR) regions, and the ability to undergo upconversion, all with excitation of a single, biologically friendly NIR wavelength. Recent advancements in the design of novel RENPs have led to new fundamental breakthroughs in photoluminescence nanothermometry. Moreover, driven by their excellent biocompatibility, both in vitro and in vivo, their implementation in biomedical applications has also gained significant traction. However, these nanoprobes face limitations caused by the complex biological environments, including absorption and scattering of various biomolecules as well as interference from different tissues, which limit the spatial resolution and detection sensitivity in RENP temperature sensing.Among existing approaches in RENP photoluminescence nanothermometry, the most prevalent implemented mechanisms either leverage the changes in the relative intensity ratio of two emission bands or exploit the lifetimes of various excited states. Photoluminescence intensity ratio (PLIR) nanothermometry has been the mainstream method owing to the readily available spectrometers for photoluminescence acquisition. Despite offering high temperature sensitivity and spatial resolution, this technique is restricted by tedious calibration and undesirable fluctuation in photoluminescence intensity ascribed to factors such as probe concentration, excitation power density, and biochemical surroundings. Lifetime-based nanothermometry uses the lifetime of a specific transition as the contrast mechanism to infer the temperature. This modality is less susceptible to various experimental factors and is compatible with a broader range of photoluminescence nanoprobes. However, due to relatively expensive and complex instrumentation, long data acquisition, and sophisticated data analysis, lifetime-based nanothermometry is still breaking ground with recently emerging techniques lightening its path.In this Account, we provide an overview of RENP nanothermometry and their applications in biomedicine. The architectures and luminescence mechanisms of RENPs are examined, followed by the principles of PLIR and lifetime-based nanothermometry. The in-depth description of each approach starts with its basic principle of accurate temperature sensing, followed by a critical discussion of the representative techniques, applications as well as their strengths and limitations. Special emphasis is given to the emerging modality of lifetime-based nanothermometry in light of the important new developments in the field. Finally, a summary and an outlook are provided to conclude this Account.

The Coming of Age of Neodymium: Redefining Its Role in Rare Earth Doped Nanoparticles.

Among luminescent nanostructures actively investigated in the last couple of decades, rare earth (RE3+) doped nanoparticles (RENPs) are some of the most reported family of materials. The development of RENPs in the biomedical framework is quickly making its transition to the ∼800 nm excitation pathway, beneficial for both in vitro and in vivo applications to eliminate heating and facilitate higher penetration in tissues. Therefore, reports and investigations on RENPs containing the neodymium ion (Nd3+) greatly increased in number as the focus on ∼800 nm radiation absorbing Nd3+ ion gained traction. In this review, we cover the basics behind the RE3+ luminescence, the most successful Nd3+-RENP architectures, and highlight application areas. Nd3+-RENPs, particularly Nd3+-sensitized RENPs, have been scrutinized by considering the division between their upconversion and downshifting emissions. Aside from their distinctive optical properties, significant attention is paid to the diverse applications of Nd3+-RENPs, notwithstanding the pitfalls that are still to be addressed. Overall, we aim to provide a comprehensive overview on Nd3+-RENPs, discussing their developmental and applicative successes as well as challenges. We also assess future research pathways and foreseeable obstacles ahead, in a field, which we believe will continue witnessing an effervescent progress in the years to come.

Inert Shell Effect on the Quantum Yield of Neodymium-Doped Near-Infrared Nanoparticles: The Necessary Shield in an Aqueous Dispersion.

Lanthanide-doped nanoparticles (LnNPs) are versatile near-infrared (NIR) emitting nanoprobes that have led to their growing interest for use in biomedicine-related imaging. Toward the brightest LnNPs, high photoluminescence quantum yield (PLQY) values are attained by implementing core/shell engineering, particularly with an optically inert shell. In this work, a thorough investigation is performed to quantify how an outer inert shell maintains the PLQY of Nd3+-doped LnNPs dispersed in an aqueous environment. Three relevant quantitative findings affecting the PLQY of Nd3+-doped LnNPs are identified: (i) the PLQY of core LnNPs is improved 3-fold upon inert shell coating; (ii) PLQY decreases with increasing Nd3+ doping despite the inert shell; and (iii) solvent quenching has a major influence on the PLQY of the LnNPs, though it is relatively lessened for high Nd3+ doping. Overall, we shed new light on the impact of the LnNP architecture on the NIR emission, as well as on the quenching effects caused by doping concentration and solvent molecules.

Effect of light scattering on upconversion photoluminescence quantum yield in microscale-to-nanoscale materials.

Scattering affects excitation power density, penetration depth and upconversion emission self-absorption, resulting in particle size -dependent modifications of the external photoluminescence quantum yield (ePLQY) and net emission. Micron-size NaYF4:Yb3+, Er3+ encapsulated phosphors (∼4.2 µm) showed ePLQY enhancements of >402%, with particle-media refractive index disparity (Δn): 0.4969, and net emission increases of >70%. In sub-micron phosphor encapsulants (∼406 nm), self-absorption limited ePLQY and emission as particle concentration increases, while appearing negligible in nanoparticle dispersions (∼31.8 nm). These dependencies are important for standardising PLQY measurements and optimising UC devices, since the encapsulant can drastically enhance UC emission.

Single-shot compressed optical-streaking ultra-high-speed photography.

Single-shot ultra-high-speed imaging is of great significance to capture transient phenomena in physics, biology, and chemistry in real time. Existing techniques, however, have a restricted application scope, a low sequence depth, or a limited pixel count. To overcome these limitations, we developed single-shot compressed optical-streaking ultra-high-speed photography (COSUP) with an imaging speed of 1.5 million frames per second, a sequence depth of 500 frames, and an (x,y) pixel count of 0.5 megapixels per frame. COSUP's single-shot ultra-high-speed imaging ability was demonstrated by recording single laser pulses illuminating through transmissive targets and by tracing a fast-moving object. As a universal imaging platform, COSUP is capable of increasing imaging speeds of a wide range of CCD and complementary metal-oxide-semiconductor cameras by four orders of magnitude. We envision COSUP to be applied in widespread applications in biomedicine and materials science.

Nd3+ doped Gd3Sc2Al3O12 nanoparticles: towards efficient nanoprobes for temperature sensing.

Development of contactless temperature-probing nanoplatforms based on thermosensitive near-infrared (NIR) light-emitting nanoparticles opens up new horizons for biomedical theranostics at a deep tissue level. Here, we report on the crystallinity and relative thermal sensitivity of NIR emitting Nd3+ doped Gd3Sc2Al3O12 (GSAG:Nd3+) nanoparticles synthesized by a solvothermal method. The obtained nanoparticles are well-crystallized, with sizes less than 100 nm, and can be dispersed in water without any additional functionalization. Upon excitation at 806 nm, the nanoparticles exhibit emission in the first and second biological optical transparency windows. The temperature sensing properties were evaluated from the luminescence intensity ratio of the thermally coupled emission lines corresponding to the R1, R2→Z5 transitions between the Stark sublevels of the 4F3/2 and 4I9/2 electronic states of Nd3+ in the physiological temperature range of 20-50 °C. GSAG:Nd3+ nanoparticles exhibit a maximal relative thermal sensitivity of 0.20% °C-1, higher than that of YAG:Nd3+ nanoparticles used as a control, due to the difference in the crystal field of the host matrices. A higher synthesis temperature in the range of 300-400 °C was also provided to improve the crystallinity of the GSAG:Nd3+ nanoparticles which results in a higher relative thermal sensitivity. Our results demonstrate the potential of GSAG:Nd3+ nanoparticles as luminescence nanothermometers and emphasize the interest of the GSAG matrix itself, which with the presence of Gd, could lead to multimodal diagnostic applications in nanothermometry and magnetic resonance imaging (MRI).

Small and Bright Lithium-Based Upconverting Nanoparticles.

In the context of light-mediated tumor treatment, the application of ultraviolet (UV) radiation can initiate drug release and photodynamic therapy. However, its limited penetration depth in tissues impedes the subcutaneous applicability of such radiation. On the contrary, near-infrared (NIR) light is not energetic enough to initiate secondary photochemical processes, but can pierce tissues at a significantly greater depth. Upconverting nanoparticles (UCNPs) unify the advantages of both extremes of the optical spectrum, they can be excited by NIR irradiation and emit UV light through the process of upconversion, effective NIR-to-UV generation being attained with UCNPs as large as 100 nm. However, in anticipation of biomedical applications, the size of UCNPs must be greatly minimized to favor their cellular internalization; yet straightforward size reduction negatively affects the NIR-to-UV upconversion efficiency. Herein, we propose a two-step strategy to obtain small yet bright lithium-based UCNPs. First, we synthesized UCNPs as small as 5 nm by controlling the relative amount of coordinating ligands, namely oleylamine (OM) and oleic acid (OA). Although these UCNPs were chemically unstable, particle coarsening via an annealing process in the presence of fresh OA yielded structurally stable and highly monodisperse sub-10 nm crystals. Second, we grew a shell with controlled thickness on these stabilized cores of UCNPs, improving the NIR-to-UV upconversion by orders of magnitude. Particularly in the case of LiYbF4:Tm3+/LiYF4 UCNPs, their NIR-to-UV upconversion surpassed the gold standard 90 nm-sized LiYF4:Tm3+, Yb3+ UCNPs. All in all, these UCNPs show great potential within the biomedical framework as they successfully combine the requirements of small size, deep tissue NIR penetration and bright UV emission.

Seeing, Targeting and Delivering with Upconverting Nanoparticles.

Efficient control over drug release is critical to increasing drug efficacy and avoiding side effects. An ideal drug delivery system would deliver drugs in the right amount, at the right location and at the right time noninvasively. This can be achieved using light-triggered delivery: light is noninvasive, spatially precise and safe if appropriate wavelengths are chosen. However, the use of light-controlled delivery systems has been limited to areas that are not too deep inside the body because ultraviolet (UV) or visible (Vis) light, the typical wavelengths used for photoreactions, have limited penetration and are toxic to biological tissues. The advent of upconverting nanoparticles (UCNPs) has made it possible to overcome this crucial challenge. UCNPs can convert near-infrared (NIR) radiation, which can penetrate deeper inside the body, to shorter wavelength NIR, Vis and UV radiation. UCNPs have been used as bright, in situ sources of light for on-demand drug release and bioimaging applications. These remote-controlled, NIR-triggered drug delivery systems are especially attractive in applications where a drug is required at a specific location and time such as in anesthetics, postwound healing, cardiothoracic surgery and cancer treatment. In this Perspective, we discuss recent progress and challenges as well as propose potential solutions and future directions, especially with regard to their translation to the clinic.

Highly Efficient Copper Sulfide-Based Near-Infrared Photothermal Agents: Exploring the Limits of Macroscopic Heat Conversion.

Among the foreseeable therapeutic approaches at the cellular level, nanoplatform-driven photothermal therapy is a thriving tool for the selective eradication of malignant tissues with minimal side effects to healthy ones. Hence, chemically versatile, near-infrared absorbing plasmonic nanoparticles are distinctly appealing and most sought after as efficient photothermal agents. In this work, a straightforward method to synthesize monodisperse PEGylated copper sulfide nanoparticles of pure covellite (CuS) phase, featuring strong localized surface plasmonic resonance absorption in the near-infrared and flexible surface chemistry, imparted by monomethyl ether polyethylene glycol molecules, is developed and optimized. These nanoparticles show a remarkable photothermal heat conversion efficiency (HCE) of 71.4%, which is among the highest for CuS systems and rivals that of plasmonic noble metal nanostructures. Moreover, through critical evaluation and mathematical modeling of the material's properties and measurement methodology, it is assessed that the calculated HCE values drastically depend on experimental conditions such as wavelength-dependent solvent absorption properties, sol concentration, and optical path. These findings are of paramount relevance to the photothermal community, since they call for a standardization of the procedure for the evaluation of the HCE of proposed photothermal agents, in order to make the reported values universally and reliably comparable.

Photocleavable Hydrogel-Coated Upconverting Nanoparticles: A Multifunctional Theranostic Platform for NIR Imaging and On-Demand Macromolecular Delivery.

Lanthanide-doped upconverting nanoparticles (UCNPs) have emerged as excellent nanotransducers for converting longer wavelength near-infrared (NIR) light to shorter wavelengths spanning the ultraviolet (UV) to the visible (Vis) regions of the spectrum via a multiphoton absorption process, known as upconversion. Here, we report the development of NIR to UV-Vis-NIR UCNPs consisting of LiYF4:Yb(3+)/Tm(3+)@SiO2 individually coated with a 10 ± 2 nm layer of chitosan (CH) hydrogel cross-linked with a photocleavable cross-linker (PhL). We encapsulated fluorescent-bovine serum albumin (FITC-BSA) inside the gel. Under 980 nm excitation, the upconverted UV emission cleaves the PhL cross-links and instantaneously liberates the FITC-BSA under 2 cm thick tissue. The release is immediately arrested if the excitation source is switched off. The upconverted NIR light allows for the tracking of particles under the tissue. Nucleus pulposus (NP) cells cultured with UCNPs are viable both in the presence and in the absence of laser irradiation. Controlled drug delivery of large biomolecules and deep tissue imaging make this system an excellent theranostic platform for tissue engineering, biomapping, and cellular imaging applications.

Towards understanding the unusual photoluminescence intensity variation of ultrasmall colloidal PbS quantum dots with the formation of a thin CdS shell.

In this study, we report anomalous size-dependent photoluminescence (PL) intensity variation of PbS quantum dots (QDs) with the formation of a thin CdS shell via a microwave-assisted cation exchange approach. Thin shell formation has been established as an effective strategy for increasing the PL of QDs. Nonetheless, herein we observed an unusual PL decrease in ultrasmall QDs upon shell formation. We attempted to understand this abnormal phenomenon from the perspective of trap density variation and the probability of electrons and holes reaching surface defects. To this end, the quantum yield (QY) and PL lifetime (on the ns-µs time scales) of pristine PbS QDs and PbS/CdS core/shell QDs were measured and the radiative and non-radiative recombination rates were derived and compared. Moreover, transient absorption (TA) analysis (on the fs-ns time scale) was performed to better understand exciton dynamics at early times that lead to and affect longer time dynamics and optical properties such as PL. These experimental results, in conjunction with theoretical calculations of electron and hole wave functions, provide a complete picture of the photophysics governing the core/shell system. A model was proposed to explain the size-dependent optical and dynamic properties observed.

Tunable chemical release from polyester thin film by photocatalytic zinc oxide and doped LiYF4 upconverting nanoparticles.

Once manufactured or implanted, polyester release kinetics tend to be fixed with little modulation possible for optimal local chemical concentrations. Here, a typical implantable polyester was fabricated into thin films (∼50 µm thick) with additives of photocatalytic ZnO nanoparticles, lanthanide-doped LiYF4 nanoparticle upconverting nanoparticles, or a combination thereof and irradiated with either 6 mW ultraviolet (365 nm) light emitting diodes or 50 mW near-infrared (980 nm) laser diodes to induce polymer photooxidation. Irradiated polyester films with the aforementioned photoadditives had enhanced release kinetics up to 30 times more than nonirradiated, neat films with extended release times of 28 days. Near-infrared, ZnO-mediated photocatalysis had the highest light on/light off ratio release kinetics of 15.4, while doped LiYF4 upconversion nanoparticles paired with ZnO nanoparticles had the highest linear R(2) correlation of 0.98 with respect to duty cycle and release kinetics. Future applications of the technology will aim toward modulation of previously developed polymeric reagents/drugs for real-time, feedback-optimized release.

Short-wave Infrared Photoluminescence Lifetime Mapping of Rare-Earth Doped Nanoparticles Using All-Optical Streak Imaging.

The short-wave infrared (SWIR) photoluminescence lifetimes of rare-earth doped nanoparticles (RENPs) have found diverse applications in fundamental and applied research. Despite dazzling progress in the novel design and synthesis of RENPs with attractive optical properties, existing optical systems for SWIR photoluminescence lifetime imaging are still considerably restricted by inefficient photon detection, limited imaging speed, and low sensitivity. To overcome these challenges, SWIR photoluminescence lifetime imaging microscopy using an all-optical streak camera (PLIMASC) is developed. Synergizing scanning optics and a high-sensitivity InGaAs CMOS camera, SWIR-PLIMASC has a 1D imaging speed of up to 138.9 kHz in the spectral range of 900-1700 nm, which quantifies the photoluminescence lifetime of RENPs in a single shot. A 2D photoluminescence lifetime map can be acquired by 1D scanning of the sample. To showcase the power of SWIR-PLIMASC, a series of core-shell RENPs with distinct SWIR photoluminescence lifetimes is synthesized. In particular, using Er3+ -doped RENPs, SWIR-PLIMASC enables multiplexed anti-counterfeiting. Leveraging Ho3+ -doped RENPs as temperature indicators, this system is applied to SWIR photoluminescence lifetime-based thermometry. Opening up a new avenue for efficient SWIR photoluminescence lifetime mapping, this work is envisaged to contribute to advanced materials characterization, information science, and biomedicine.

Upconverting Nanoparticles Coated with Light-Breakable Mesoporous Silica for NIR-Triggered Release of Hydrophobic Molecules.

Upconverting nanoparticles (UCNPs) doped with Yb3+ and Tm3+ are near-infrared (NIR) to ultraviolet (UV) transducers that can be used for NIR-controlled drug delivery. However, due to the low quantum yield of upconversion, high laser powers and long irradiation times are required to trigger this drug release. In this work, we report the one-step synthesis of a nanocomposite consisting of a LiYbF4:Tm3+@LiYF4 UCNP coated with mesoporous UV-breakable organosilica shells of various thicknesses. We demonstrate that a thin shell accelerates the breakage of the shell at 1 W/cm2 NIR light exposure, a laser power up to 9 times lower than that of conventional systems. When the mesopores are loaded with hydrophobic vitamin D3 precursor 7-dehydrocholesterol (7-DH), shell breakage results in subsequent cargo release. Its minimal toxicity in HeLa cells and successful internalization into the cell cytoplasm demonstrate its biocompatibility and potential application in biological systems. The tunability of this system due to its simple, one-step synthesis process and its ability to operate at low laser powers opens up avenues in UCNP-powered NIR-triggered drug delivery toward a more scalable, flexible, and ultimately translational option.

Enhancing upconverted white light in Tm3+/Yb3+/Ho3+-doped GdVO4 nanocrystals via incorporation of Li+ ions.

The white light emission of Tm3+/Yb3+/Ho3+-doped GdVO4 nanocrystals, following excitation with near-infrared light (λexc = 980 nm), via a multiphoton upconversion process is presented. Upconverted blue emission from the Tm3+ ions as well as green/red emissions from the Ho3+ ions contributes to the observed white light. The calculated Commission internationale de l'éclairage (CIE) color coordinates were calculated to be x = 0.34; y = 0.32 and lie at the center of the white region. Furthermore, the intensity of the upconverted white light was enhanced by the incorporation of monovalent Li+ ions into the GdVO4 matrix. An explanation for this enhancement is proposed based on X-ray diffraction and fluorescence lifetime measurements.

Synthesis of ligand-free colloidally stable water dispersible brightly luminescent lanthanide-doped upconverting nanoparticles.

The synthesis using the thermal decomposition of metal trifluoroacetates is being widely used to prepare oleate-capped lanthanide-doped upconverting NaYF(4):Er(3+)/Yb(3+) nanoparticles (Ln-UCNPs). These nanoparticles have no inherent aqueous dispersibility and inconvenient postsynthesis treatments are required to render them water dispersible. Here, we have developed a novel and facile approach to obtain water-dispersible, ligand-free, brightly upconverting Ln-UCNPs. We show that the upconversion luminescence is affected by the local environment of the lanthanide ions at the surface of the Ln-UCNPs. We observe a dramatic difference of the integrated upconverted red:green emission ratio for Ln-UCNPs dispersed in toluene compared to Ln-UCNPs dispersed in water. We can enhance or deactivate the upconversion luminescence by pH and H/D isotope vibronic control over the competitive radiative and nonradiative relaxation pathways for the red and green excited states. Direct biofunctionalization of the ligand-free, water-dispersible Ln-UCNPs will enable myriad new opportunities in targeting and drug delivery applications.

Polymer brush coated upconverting nanoparticles with improved colloidal stability and cellular labeling.

Upconverting nanoparticles (UCNPs) possess great potential for biomedical application. UCNPs absorb and convert near-infrared (NIR) radiation in the biological imaging window to visible (Vis) and even ultraviolet (UV) radiation. NIR excitation offers reduced scattering and diminished autofluorescence in biological samples, whereas the emitted UV-Vis and NIR photons can be used for cancer treatment and imaging, respectively. However, UCNPs are usually synthesized in organic solvents and are not readily suitable for biomedical application due to the hydrophobic nature of their surface. Herein, we have removed the hydrophobic ligands from the synthesized UCNPs and coated the bare UCNPs with two custom-made hydrophilic polyelectrolytes (synthesized via the reversible addition-fragmentation chain transfer (RAFT) polymerization method). Polymers containing different amounts of PEGylated and carboxylic groups were studied. Coating with both polymers increased the upconversion (UC) emission intensity and photoluminescence lifetime values of the UCNPs, which directly translates to more efficient cancer cell labeling nanoprobes. The polymer composition plays a crucial role in the modification of UCNPs, not only with respect to their colloidal stability, but also with respect to the cellular uptake. Colloidally unstable bare UCNPs aggregate in cell culture media and precipitate, rendering themselves unsuitable for any biomedical use. However, stabilization with polymers prevents UCNPs from aggregation, increases their uptake in cells, and improves the quality of cellular labeling. This investigation sheds light on the appropriate coating for UCNPs and provides relevant insights for the rational development of imaging and therapeutic tools.

Decoupled Rare-Earth Nanoparticles for On-Demand Upconversion Photodynamic Therapy and High-Contrast Near Infrared Imaging in NIR IIb.

Rare-earth doped multi-shell nanoparticles slated for theranostic applications produce a variety of emission bands upon near-infrared (NIR) excitation. Their downshifting emission is useful for high-contrast NIR imaging, while the upconversion light can induce photodynamic therapy (PDT). Unfortunately, integration of imaging and therapy is challenging. These modalities are better to be controlled independently so that, with the help of imaging, selective delivery of a theranostic agent at the site of interest could be ensured prior to on-demand PDT initiation. We introduce here multi-shell rare-earth doped nanoparticles (RENPs) arranged in a manner to produce only downshifting emission for NIR imaging when excited at one NIR wavelength and upconversion emission for therapeutic action by using a different excitation wavelength. In this work, multi-shell RENPs with a surface-bound sensitizer have been synthesized for decoupled 1550 nm downshifting emission upon 800 nm excitation and 550 nm upconversion emission caused by 980 nm irradiation. The independently controlled emission bands allow for high-contrast NIR imaging in NIR-IIb of optical transparency that gives high-contrast images due to significantly reduced light scattering. This can be conducted prior to PDT using 980 nm to produce upconverted light at 550 nm that excites the RENP surface-bound photosensitizer, Rose Bengal (RB), to effect photodynamic therapy with high specificity and safer theranostics.

Cubic versus hexagonal - phase, size and morphology effects on the photoluminescence quantum yield of NaGdF4:Er3+/Yb3+ upconverting nanoparticles.

Upconverting nanoparticles (UCNPs) are well-known for their capacity to convert near-infrared light into UV/visible light, benefitting various applications where light triggering is required. At the nanoscale, loss of luminescence intensity is observed and thus, a decrease in photoluminescence quantum yield (PLQY), usually ascribed to surface quenching. We evaluate this by measuring the PLQY of NaGdF4:Er3+,Yb3+ UCNPs as a function of size (ca. 15 to 100 nm) and shape (spheres, cubes, hexagons). Our results show that the PLQY of α-phase NaGdF4 Er3+,Yb3+ surpasses that of ß-NaGdF4 for sizes below 20 nm, an observation related to distortion of the crystal lattice when the UCNPs become smaller. The present study also underlines that particle shape must not be neglected as a relevant parameter for PLQY. In fact, based on a mathematical nucleus/hull volumetric model, shape was found to be particularly relevant in the 20 to 60 nm size range of the investigated UCNPs.

CdSe quantum dots for two-photon fluorescence thermal imaging.

The technological development of quantum dots has ushered in a new era in fluorescence bioimaging, which was propelled with the advent of novel multiphoton fluorescence microscopes. Here, the potential use of CdSe quantum dots has been evaluated as fluorescent nanothermometers for two-photon fluorescence microscopy. In addition to the enhancement in spatial resolution inherent to any multiphoton excitation processes, two-photon (near-infrared) excitation leads to a temperature sensitivity of the emission intensity much higher than that achieved under one-photon (visible) excitation. The peak emission wavelength is also temperature sensitive, providing an additional approach for thermal imaging, which is particularly interesting for systems where nanoparticles are not homogeneously dispersed. On the basis of these superior thermal sensitivity properties of the two-photon excited fluorescence, we have demonstrated the ability of CdSe quantum dots to image a temperature gradient artificially created in a biocompatible fluid (phosphate-buffered saline) and also their ability to measure an intracellular temperature increase externally induced in a single living cell.