Lanthanide-doped Nanocrystals Research Articles

Microwave-assisted Green Synthetic Approach towards Water Dispersible Luminescent PVP-coated Tb3+ and Ce3+/Tb3+-doped KZnF3 Nanocrystals

Background: Perovskite fluoride nanomaterials are an interesting research topic in material science due to their exciting properties like high-temperature superconductivity, magnetic behaviour, piezoelectric behaviour, etc. Doping of lanthanide ions into the perovskite fluoride nanomaterials makes them more promising as they have applications from biological label-ling to multicolor optical devices. Objective: This study aimed to carry out the synthesis of perovskite KZnF3 nanocrystals in an eco-friendly environment with the help of a microwave-assisted route in a shorter reaction time and at low temperatures. Moreover, it aimed to make the nanocrystals water dispersible, illuminating brighter photoluminescence, which was achieved by coating nanocrystals surface with poly(N-vi-nyl-2-pyrrolidone) and doping of different lanthanide ions (Ln= Tb3+ and Ce3+/Tb3+) respectively, into the KZnF3 nanocrystals matrix. Methods: The synthesis of nanocrystals was performed in an environment-friendly microwave-assisted way and under green conditions. For example, in the preparation of Tb3+(5mol%)-doped KZnF3 nanocrystals, 0.95 mmol of Zn(NO3)2 and 0.05 mmol of Tb(NO3)3 were dissolved in 8 mL of distilled water. Then, an 8 mL aqueous solution of KF (3 mmol) was added to it. The entire mixture was stirred well for 15 minutes. About 60 mg of PVP was added to the mixture and stirred for another 15 minutes. Then, a microwave reaction vessel was made by transferring the final reaction mixture into it and kept under microwave irradiation at 90°C temperature for 15 minutes. Finally, the product was cooled to room temperature and collected by centrifugation. Results: Both Tb3+(5mol%)-doped and Ce3+(15mol%)/Tb3+(5mol%) co-doped KZnF3 nanocrystals exhibit very strong green photoluminescence. The structural and optical properties of as-obtained nanocrystals were characterized by PXRD, field emission scanning electron microscopy, Fourier infrared spectroscopy, transmission electron microscopy, thermogravimetric analysis, and photoluminescence spectra. Conclusion: The nanocrystals with uniform cubical morphology having ~60 nm sizes were successfully synthesized. The high photoluminescence efficiency, together with the water dispersibility of the nanocrystals, makes the material useful in many fields of optical devices and offers several biological applications. Moreover, this method could be used to make other lanthanide-doped perovskite fluoride nanocrystals.

Effect of Pr3+ concentration in luminescence properties & upconversion mechanism of triple doped NaYF4: Yb3+, Er3+, Pr3+

Lanthanide-doped fluoride nanocrystals (NCs) exhibit excellent optical features, including upconversion and downconversion luminescence (UCL and DCL), that can be utilized in a variety of applications. In this study, we have successfully demonstrated the photoluminescence behavior of triple-doped NaYF4: Yb3+, Er3+, Pr3+ NCs in the Vis-NIR region. Herein, highly monodisperse hexagonal phase NaYF4: Yb0.2, Er0.02, Prx nanocrystals in various Pr3+ (x = 0, 0.1, 0.5, and 1 mol %) concentration with ∼22 nm diameter synthesized by thermal decomposition technique. The photoluminescence studies for all samples were performed under 980 nm laser excitation. The luminescence intensity of Er3+ including blue (407 nm), green (520 and 540 nm), red (654 nm), and near-infrared (845 nm and 1530 nm) emissions was significantly quenched and Pr3+ emission intensity at 1290 nm (Pr3+:1G4→3H5) changes irregularly upon doping with Pr3+ ions. Furthermore, we performed the excitation power dependence and decay time analysis to investigate the energy transfer and upconversion mechanisms of samples. These findings indicate that the presence of praseodymium strongly reduces emission intensities due to abundant cross-relaxation channels. In addition, particle size is an efficient factor, shedding light on the influence of Pr3+ on the energy transfer and upconversion mechanisms of the fluorides.

Fluorinated Hafnium and Zirconium Coenable the Tunable Biodegradability of Core-Multishell Heterogeneous Nanocrystals for Bioimaging.

Upconversion (UC)/downconversion (DC)-luminescent lanthanide-doped nanocrystals (LDNCs) with near-infrared (NIR, 650-1700 nm) excitation have been gaining increasing popularity in bioimaging. However, conventional NIR-excited LDNCs cannot be degraded and eliminated eventually in vivo owing to intrinsic "rigid" lattices, thus constraining clinical applications. A biodegradability-tunable heterogeneous core-shell-shell luminescent LDNC of Na3HfF7:Yb,Er@Na3ZrF7:Yb,Er@CaF2:Yb,Zr (abbreviated as HZC) was developed and modified with oxidized sodium alginate (OSA) for multimode bioimaging. The dynamic "soft" lattice-Na3Hf(Zr)F7 host and the varying Zr4+ doping content in the outmoster CaF2 shell endowed HZC with tunable degradability. Through elaborated core-shell-shell coating, Yb3+/Er3+-coupled UC red and green and DC second near-infrared (NIR-II) emissions were, respectively, enhanced by 31.23-, 150.60-, and 19.42-fold when compared with core nanocrystals. HZC generated computed tomography (CT) imaging contrast effects, thus enabling NIR-II/CT/UC trimodal imaging. OSA modification not only ensured the exemplary biocompatibility of HZC but also enabled tumor-specific diagnosis. The findings would benefit the clinical imaging translation of LDNCs.

Luminescence enhancement through co-sensitization in lanthanide composites for efficient photocatalysis.

Lanthanide-doped nanocrystals that convert near-infrared (NIR) irradiation into shorter wavelength emission (ultraviolet-C) offer many exciting opportunities for biomedicine, bioimaging, and environmental catalysis. However, developing lanthanide-doped nanocrystals with high UVC brightness for efficient photocatalysis is a formidable challenge due to the complexity of the multiphoton process. Here, we report a series of heterogeneous core-multishell structures based on a co-sensitization strategy with multi-band enhanced emission profiles under 980 nm excitation. Interestingly, the multiphoton processes involving two to six-photon upconversion are highly promoted via a co-sensitization strategy. More importantly, through growth layers of TiO2 and CdS photocatalysts, these lanthanide nanocomposites with efficient multi-upconverted emission show efficient photocatalytic activity. This study provides a new perspective for mechanistic understanding of multiphoton processes in heterostructures and also offers exciting opportunities for highly efficient photocatalytic applications.

Simultaneously excited downshifting/upconversion luminescence from lanthanide-doped core–shell lead-free perovskite nanocrystals for encryption and data storage

The constructed Cs2AgInCl6-based core–shell NCs with separated optically active centers can not only prevent the adverse energy interactions between dopants, but also achieve dual-modal PL in the visible region and improved photostability.

Topological Control of Lanthanide-Doped Upconversion Nanocrystals for Improved FRET Coupling Efficiency in Second Near-Infrared-Triggered Photodynamic Therapy

Topological Control of Lanthanide-Doped Upconversion Nanocrystals for Improved FRET Coupling Efficiency in Second Near-Infrared-Triggered Photodynamic Therapy

Bone disease imaging through the near-infrared-II window

Skeletal disorders are commonly diagnosed by X-ray imaging, but the radiation limits its use. Optical imaging through the near-infrared-II window (NIR-II, 1000–1700 nm) can penetrate deep tissues without radiation risk, but the targeting of contrast agent is non-specific. Here, we report that lanthanide-doped nanocrystals can passively target the bone marrow, which can be effective for over two months. We therefore develop the high-resolution NIR-II imaging method for bone disease diagnosis, including the 3D bone imaging instrumentation to show the intravital bone morphology. We demonstrate the monitoring of 1 mm bone defects with spatial resolution comparable to the X-ray imaging result. Moreover, NIR-II imaging can reveal the early onset inflammation as the synovitis in the early stage of rheumatoid arthritis, comparable to micro computed tomography (μCT) in diagnosis of osteoarthritis, including the symptoms of osteophyte and hyperostosis in the knee joint.

Proof of crystal-field-perturbation-enhanced luminescence of lanthanide-doped nanocrystals through interstitial H+ doping

Crystal-field perturbation is theoretically the most direct and effective method of achieving highly efficient photoluminescence from trivalent lanthanide (Ln3+) ions through breaking the parity-forbidden nature of their 4f-transitions. However, exerting such crystal-field perturbation remains an arduous task even in well-developed Ln3+-doped luminescent nanocrystals (NCs). Herein, we report crystal-field perturbation through interstitial H+-doping in orthorhombic-phase NaMgF3:Ln3+ NCs and achieve a three-orders-of-magnitude emission amplification without a distinct lattice distortion. Mechanistic studies reveal that the interstitial H+ ions perturb the local charge density distribution, leading to anisotropic polarization of the F− ligand, which affects the highly symmetric Ln3+-substituted [MgF6]4− octahedral clusters. This effectively alleviates the parity-forbidden selective rule to enhance the 4f–4 f radiative transition rate of the Ln3+ emitter and is directly corroborated by the apparent shortening of the radiative recombination lifetime. The interstitially H+-doped NaMgF3:Yb/Er NCs are successfully used as bioimaging agents for real-time vascular imaging. These findings provide concrete evidence for crystal-field perturbation effects and promote the design of Ln3+-doped luminescent NCs with high brightness.

Mechano-luminescence Behavior of Lanthanide-Doped Fluoride Nanocrystals for Three-Dimensional Stress Imaging.

Pervasive mechanical force in nature and human activities is closely related to intriguing physics and widespread applications. However, describing stress distribution timely and precisely in three dimensions to avoid "groping in the dark" is still a formidable challenge, especially for nonplanar structures. Herein, we realize three-dimensional (3D) stress imaging for sharp arbitrary targets via advanced 3D printing, owing to the use of fluoride nanocrystal(NC)-based ink. Notably, a fascinating mechano-luminescence (ML) is observed for the homogeneously dispersed NaLuF4:Tb3+ NCs (∼25 nm) with rationally designed deep traps (at 0.88 and 1.02 eV) via incorporating Cs+ ions and using X-ray irradiation. Carriers captured in the corresponding traps are steadily released under mechanical stimulations, which enables a ratio metric luminescence intensity based on the applied force. As a result, a significant mechano-optical conversion and superior optical waveguide of the corresponding transparent printed targets demonstrate stress in 3D with a high spatial and temporal resolution based on stereovision. These results highlight the optical function of the 3D-printed fluoride NCs, which cast light into the black boxes of stress described in space, benefiting us in understanding the ubiquitous force relevant to most natural and engineering processes.

Distinct Luminescent Thermal Behaviors of Yb3+- and Nd3+-Sensitized Core/Shell Upconversion Nanocrystals

Luminescent materials generally exhibit the thermal quenching due to accelerated nonradiative transitions at elevated temperatures. Recently, the discovery of anomalous thermally induced luminescence enhancement in lanthanide-doped upconversion nanocrystals (UCNCs) has aroused wide attention. The research on this topic not only deepens the understanding on the upconversion luminescence (UCL) mechanism but also promotes frontier applications based on UCNCs. Herein, comparative studies on temperature-dependent UCL properties were performed between Yb3+- and Nd3+-sensitized active-core/active-shell UCNCs. Opposite luminescent thermal behaviors were revealed for these two types of core/shell NCs, and the underlying mechanisms were elucidated with the assistance of temperature-dependent steady/transient spectral analysis under various atmospheres. Yb3+-sensitized active-core/active-shell NCs exhibited an anomalous luminescence enhancement with increasing temperature, which was attributed to the thermally alleviated quenching of surface-adsorbed water molecules. By contrast, Nd3+-sensitized active-core/active-shell NCs showed a normal thermal quenching due to rapidly increased deactivation rates of Nd3+ ions at elevated temperatures, although their UCL intensity was also affected by the quenching effect of surface-adsorbed water molecules. Highly secure anti-counterfeiting patterns were also demonstrated using the nanohybrids of Yb3+- and Nd3+-sensitized core/shell UCNCs as luminescent inks on the basis of their opposite luminescent thermal behaviors. This work provides new insights into luminescent thermal behaviors and energy loss pathways of lanthanide-doped UCNCs.

YAG:Ln3+ (Ln = Nd, Yb, Ce) nanocrystals for application in luminescent fiber-based temperature sensors

We developed and characterized luminescent temperature sensors with a simple construction based on YAG : Ln3 + (Ln = Nd, Yb, Ce) nanocrystals and silica multimode optical fibers. Lanthanide-doped nanocrystals 40 to 60 nm in size were synthesized in the form of powders using the modified Pechini method. The obtained materials exhibited high sensitivity of luminescence intensity to temperature variations at wavelengths of 550 nm (YAG : Ce3 + ), 1030 nm (YAG : Yb3 + ), and 1064 nm (YAG : Nd3 + ) in the temperature range 50°C to 600°C. Additionally, we offered a method to eliminate influence of vibration on accuracy of temperature measurements by adding SiO2 sol to powders after their synthesis in sensitive elements.

Probing nearby molecular vibrations with lanthanide-doped nanocrystals.

The photoluminescence (PL) of lanthanide-doped nanocrystals can be quenched by energy transfer to vibrations of molecules located within a few nanometers from the dopants. Such short-range electronic-to-vibrational energy transfer (EVET) is often undesired as it reduces the photoluminescence efficiency. On the other hand, EVET may be exploited to extract information about molecular vibrations in the local environment of the nanocrystals. Here, we investigate the influence of solvent and gas environments on the PL properties of NaYF4:Er3+,Yb3+ upconversion nanocrystals. We relate changes in the PL spectrum and excited-state lifetimes in different solvents and their deuterated analogues to quenching of specific lanthanide levels by EVET to molecular vibrations. Similar but weaker changes are induced when we expose a film of nanocrystals to a gas environment with different amounts of H2O or D2O vapor. Quenching of green- and red-emitting levels of Er3+ can be explained in terms of EVET-mediated quenching that involves molecular vibrations with energies resonant with the gap between the energy levels of the lanthanide. Quenching of the near-infrared-emitting level is more complex and may involve EVET to combination-vibrations or defect-mediated quenching. EVET-mediated quenching holds promise as a mechanism to probe the local chemical environment-both for nanocrystals dispersed in a liquid and for nanocrystals exposed to gaseous molecules that adsorb onto the nanocrystal surface.

Tunable near-infrared emission and three-photon absorption in lanthanide-doped double perovskite nanocrystals.

Cs2AgInCl6 double perovskite (DP) nanocrystals (NCs) are an emerging class of materials with promising application potential in photonics/optoelectronics owing to their nontoxicity, direct bandgap, and high thermal and moisture stability. These NCs are, however, rarely explored for nonlinear optical (NLO) applications. Herein, we present a comprehensive investigation of the photophysical and nonlinear optical properties of erbium- (Er) and ytterbium (Yb)-doped Cs2AgInCl6 nanocrystals (denoted as Er-DP and Yb-DP, respectively). Temperature-dependent photoluminescence of these NCs was analyzed to estimate their exciton binding energy, Huang-Rhys parameter (S), and electron-phonon coupling strength, which are of fundamental interest to gain an in-depth understanding of the material systems. Femtosecond Z-scan experiments with 800 nm excitation revealed the reverse saturable absorption (RSA) behavior owing to three-photon absorption (3PA). The obtained values of the 3PA coefficients were 1.35 × 10-4 and 1.64 × 10-4 cm3 GW-2, respectively, and the nonlinear refractive indices were estimated to be 1.02 × 10-15 and 1.15 × 10-15 cm2 W-1, respectively, for Er-DP and Yb-DP. These values are superior to those obtained in undoped Cs2AgInCl6 NCs. The physical parameter, Kane energy, which is closely related to the magnitude of the oscillator strength, was estimated to be 25 eV and 26 eV for Er-DP and Yb-DP, respectively. As a proof-of-concept application, we further obtained the optical limiting onset and figure of merit to reveal their prospect as an optical limiter and in photonic switching application. With such emission and nonlinear optical properties, we anticipate that lanthanide-doped Cs2AgInCl6 NCs can be used for designing eco-friendly nonlinear optoelectronic/photonic devices.

Temperature-sensitive lanthanide-doped core-multishell nanocrystals with excitation-wavelength-dependent bimodal luminescence thermo-behaviors and their application in dynamic anticounterfeiting

Herein, we constructed a sort of particular lanthanide-doped core-multishell nanocrystals, NaErF4:0.5%Tm@NaYF4 @NaYF4:50%Yb,2%Ho@NaYF4:50%Yb (CSSS-UCNPs), which first integrated bimodal temperature-dependent UC emissions and orthogonal luminescence triggered by dual NIR lasers. Specifically, CSSS-UCNPs respectively presented strong yellowish green (Ho3++Er3+) and red luminescence (Er3+) under 980 nm and 1550 nm excitations via regulating constructional parameters. Moreover, the photoluminescence of CSSS-UCNPs showed remarkable temperature dependence, and luminescence thermo-behaviors changed with excitation wavelengths: thermo-enhanced Ho3+ green luminescence under 980 nm irradiation, and thermo-quenched Er3+ red luminescence under 1550 nm excitation. The unexpected thermal intensification of Ho3+ luminescence in CSSS-UCNPs of such large dimension (>>50 nm) violated the available literature involving size-dependent thermal enhancement. It’s discovered that the heat-induced vibration attenuation of oleate ligands mainly contributed to 980 nm-excited thermal intensification, which had never been reported before. Thanks to their unique excitation/temperature-responsive luminescence, the printed patterns by CSSS-UCNPs displayed noticeable changes in brightness and chromaticity under NIR excitations when raising the temperature. Briefly, this work affords a promising candidate for high-security-level anticounterfeiting, and, more importantly, supplies new perspectives on anomalous thermal enhancement in large-sized Yb3+-sensitized UCNPs.

Dual-resonance enhancement upconversion luminescence by elongated Ag semi-shell coated on NaYF4:Yb,Er@SiO2 nanorod

Plasmonic resonance induced by metallic structures is a feasible solution to enhance the upconversion luminescence (UCL) of lanthanide-doped nanocrystals. A simulation investigation on the optical properties of an elongated Ag semi-shell coated on nanorod is carried out in this paper. Nondegenerate longitudinal and radial void modes in the Ag cavity can be tuned almost independently and controllably to resonate with excitation and emission simultaneously via adjusting the length and diameter of the nanorod respectively. Moreover, both mode volumes are concentrated in the central region of the cavity. Spectral-plus spatial-overlapping ensures that the elongated Ag semi-shell is an efficient dual-resonance enhancement plasmonic structure. Under dual-resonance conditions, green UCL can achieve 718-fold enhancement when radial resonance overlapping with green UCL, while red UCL can achieve 141-fold enhancement when overlapping with red UCL, with original quantum efficiency (η0) of 1%, both values are higher than that of gold nanorod. Since Purcell effect, resonating with different emission bands can realize color modulation, the intensity ratio of green UCL to red can raise to 9.1 or lower to 1.1 from the original 2.0. Although enhancement gradually weakens with the increase of η0, more than 20-fold enhancement can be still obtained even with η0 up to 33%.

Investigation of the Energy Loss in Upconversion Luminescence of Lanthanide-Doped Nanocrystals for Anticounterfeiting and a Nanoheater

Investigation of the Energy Loss in Upconversion Luminescence of Lanthanide-Doped Nanocrystals for Anticounterfeiting and a Nanoheater

(Invited) The Evolution of Inorganic Nanocrystals for Bioimaging

The first applications of luminescent nanocrystals to bioimaging were semiconductor quantum dots with optoelectronic properties that largely mirror those of organics and proteins, but with substantially increased stability and brightness that have enabled single molecule and other challenging imaging applications. Building on this success, newer nanocrystals have been engineered with optical properties unlike anything found in traditional probes, including perfect photostability,1,2 anti-Stokes emission a billion-fold more efficient than 2-photon excitation,3 and most recently, photon avalanches hosted within nanostructures.4 Avalanches are steeply nonlinear events in which outsized responses arise from a series of minute inputs. With light, photon avalanching (PA) had been observed only in bulk materials and aggregates, often at cryogenic temperatures, preventing its application to bioimaging. We recently reported the realization of PA at room temperature in sub-30 nm Tm3+-doped NaYF4 upconverting nanoparticles (UCNPs) and demonstrated their use in high-resolution imaging at wavelengths that fall within NIR spectral windows of maximal biological transparency. Avalanching nanoparticles (ANPs) can be pumped by either continuous-wave or pulsed lasers and exhibit all of the defining features of PA: clear excitation power thresholds, exceptionally long rise time at threshold, and a dominant excited-state absorption that is >10,000 times larger than ground-state absorption. Beyond the avalanching threshold, ANP emission scales with up to the 31st power of pump intensity, an extreme nonlinearity caused by the induced positive optical feedback within each nanocrystal. This enables sub-70 nm spatial resolution using only simple scanning confocal microscopy and before any computational data analysis. NaYF4 ANPs with 8-20% Tm3+content can be excited at either 1064 or 1450 nm, with avalanching emission at 800 nm. Pairing the steep nonlinearity of ANPs with existing superresolution techniques and computational methods allows for imaging with higher resolution and at ca. 100-fold lower excitation intensities than is possible with other probes. For application of ANPs to live-cell imaging, we have developed synthetic chemistry-free methods for conjugating engineered antibodies to NP-surface SpyCatcher proteins,5 which bind and spontaneously form covalent isopeptide bonds with cognate SpyTag peptides. This enables controlled and irreversible attachment of antibodies to nanoparticle surfaces, for specific targeting of cell-surface receptors in quantitative live-cell study of their distribution, trafficking, and physiology. Wu, S. et al. Non-blinking and photostable upconverted luminescence from single lanthanide-doped nanocrystals. Proc. Natl. Acad. Sci. 106, 10917–10921 (2009).Fernandez-Bravo, A. et al. Continuous-wave upconverting nanoparticle microlasers. Nat. Nanotechnol. 13, 572–577 (2018).Tian, B. et al. Low irradiance multiphoton imaging with alloyed lanthanide nanocrystals. Nat. Commun. 9, 3082 (2018).Lee, C. et al. Giant nonlinear optical responses from photon-avalanching nanoparticles. Nature 589, 230–235 (2021).Pedroso, C. C. S. et al. Immunotargeting of nanocrystals by SpyCatcher conjugation of engineered antibodies. ACS Nano 15, 18374–18384 (2021). Figure 1

Boltzmann- and Non-Boltzmann-Based Thermometers in the First, Second and Third Biological Windows for the SrF2:Yb3+, Ho3+ Nanocrystals Under 980, 940 and 915 nm Excitations

Spectrally determination of temperature based on the lanthanide-doped nanocrystals (NCs) is a vital strategy to noninvasively measure the temperature in practical applications. Here, we synthesized a series of SrF2:Yb3+/Ho3+ NCs and simultaneously observed the efficient visible upconversion luminescence (UCL) and near-infrared (NIR) downconversion luminescence (DCL) under 980, 940 and 915 nm excitations. Subsequently, these NCs were further utilized for thermometers based on the Boltzmann (thermally coupled levels, TCLs) and non-Boltzmann (non-thermally coupled levels, NTCLs) of Ho3+ ions in the first (~ 650 nm), second (~ 1012 nm) and third (~ 2020 nm) biological windows (BW-I, BW-II and BW-III) under tri-wavelength excitations. The thermometric parameters including the relative sensitivity (S_{{text{r}}}) and temperature uncertainty (delta T) are quantitatively determined on the I648/I541 (BW-I), I1186/I1012 (BW-II), and I1950/I2020 (BW-III) transitions of Ho3+ ions in the temperature range of 303–573 K. Comparative experimental results demonstrated that the thermometer has superior performances.

Zwitterionic Polymers toward the Development of Orientation-Sensitive Bioprobes

Dynamics with an orientational degree of freedom are fundamental in biological events. Probes with polarized luminescence enable a determination of the orientation. Lanthanide-doped nanocrystals can provide more precise analysis than quantum dots due to the nonphotoblinking/bleaching nature and the multiple line-shaped emission. However, the intrinsic polarization property of the original nanocrystals often deteriorates in complex physiological environments because the colloidal stability easily breaks and the probes aggregate in the media with abundant salts and macromolecules. Engineering the surface chemistry of the probes is thus essential to be compatible with biosystems, which has remained a challenging task that should be exclusively addressed for each specific probe. Here, we demonstrate a facile and efficient surface functionalization of lanthanide-doped nanorods by zwitterionic block copolymers. Due to the steric interaction and the intrinsic zwitterionic nature of the polymers, high colloidal stability of the zwitterionic nanorod suspension is achieved over wide ranges of pH and concentration of salts, even giving rise to the lyotropic liquid crystalline behavior of the nanorods in physiological media. The shear-aligned ability is shown to be unaltered by the coated polymers, and thus, the strongly polarized emission of Eu3+ is preserved. Besides, biological experiments reveal good biocompatibility of the zwitterionic nanorods with negligible nonspecific binding. This study is a stepping stone for the use of the nanorods as orientation probes in biofluids and validates the strategy of coupling zwitterions to lanthanide-doped nanocrystals for various bioapplications.

Quantitative Comparison of the Light-to-Heat Conversion Efficiency in Nanomaterials Suitable for Photothermal Therapy.

Functional colloidal nanoparticles capable of converting between various energy types are finding an increasing number of applications. One of the relevant examples concerns light-to-heat-converting colloidal nanoparticles that may be useful for localized photothermal therapy of cancers. Unfortunately, quantitative comparison and ranking of nanoheaters are not straightforward as materials of different compositions and structures have different photophysical and chemical properties and may interact differently with the biological environment. In terms of photophysical properties, the most relevant information to rank these nanoheaters is the light-to-heat conversion efficiency, which, along with information on the absorption capacity of the material, can be used to directly compare materials. In this work, we evaluate the light-to-heat conversion properties of 17 different nanoheaters belonging to different groups (plasmonic, semiconductor, lanthanide-doped nanocrystals, carbon nanocrystals, and metal oxides). We conclude that the light-to-heat conversion efficiency alone is not meaningful enough as many materials have similar conversion efficiencies—in the range of 80–99%—while they significantly differ in their extinction coefficient. We therefore constructed their qualitative ranking based on the external conversion efficiency, which takes into account the conventionally defined light-to-heat conversion efficiency and its absorption capacity. This ranking demonstrated the differences between the samples more meaningfully. Among the studied systems, the top-ranking materials were black porous silicon and CuS nanocrystals. These results allow us to select the most favorable materials for photo-based theranostics and set a new standard in the characterization of nanoheaters.