Quantitative, precise and multi-wavelength evaluation of the light-to-heat conversion efficiency for nanoparticular photothermal agents with calibrated photoacoustic spectroscopy.

Biomedical photothermal therapy with optical nanoparticles is based on the conversion of optical energy into heat through three steps: optical absorption, thermal conversion of the absorbed energy and heat transfer to the surrounding medium. The light-to-heat conversion efficiency (LHCE) has become one of the main metrics to quantitatively characterize the last two steps and evaluate the merit of nanoparticules for photothermal therapy. The estimation of the LHCE is mostly performed by monitoring the temperature evolution of a solution under laser irradiation. However, this estimation strongly depends on the experimental set-up and the heat balance model used. We demonstrate here, theoretically and experimentally, that the LHCE at multiple wavelengths can be efficiently and directly determined, without the use of models, by calibrated photoacoustic spectroscopy. The method was validated using already characterized colloidal suspensions of silver sulfide nanoparticles and maghemite nanoflowers and an uncertainty of 3 to 7% was estimated for the LHCE determination. Photoacoustic spectroscopy provides a new, precise and robust method of analysis of the photothermal capabilities of aqueous solutions of nanoagents.

N-Heterocyclic Carbene Boranes: Dual Reagents for the Synthesis of Gold Nanoparticles.

N-Heterocyclic carbenes (NHCs) have drawn considerable interest in the field of nanomaterials chemistry as highly stabilizing ligands enabling the formation of strong and covalent carbon-metal bonds. Applied to gold nanoparticles synthesis, the most common strategy consists of the reduction of a preformed NHC-AuI complex with a large excess of a reducing agent that makes the particle size difficult to control. In this paper, we report the straightforward synthesis of NHC-coated gold nanoparticles (NHC-AuNPs) by treating a commercially available gold(I) precursor with an easy-to-synthesize NHC-BH3 reagent. The latter acts as both the reducing agent and the source of surface ligands operating under mild conditions. Mechanistic studies including NMR spectroscopy and mass spectrometry demonstrate that the reduction of gold(I) generates NHC-BH2 Cl as a by-product. This strategy gives efficient control over the nucleation and growth of gold particles by varying the NHC-borane/gold(I) ratio, allowing unparalleled particle size variation over the range of 4.9±0.9 to 10.0±2.7 nm. Our strategy also allows an unprecedented precise and controlled seeded growth of gold nanoparticles. In addition, the as-prepared NHC-AuNPs exhibit narrow size distributions without the need for extensive purification or size-selectivity techniques, and are stable over months.

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.

Risk Analysis and Technology Assessment of Emerging (Gd,Ce)2O2S Multifunctional Nanoparticles: An Attempt for Early Safer-by-Design Approach.

Acceptability and relevance of nanoparticles in the society is greatly improved using a safer-by-design strategy. However, this is difficult to implement when too late in the development process or when nanoparticles are already on the market (e.g., TiO2). We employ this strategy for emerging nanoparticles of lanthanide oxysulfide of formula (Gd,Ce)2O2S, relevant for photocatalysis as well as for multimodal imaging, as the bandgap of the nanoparticles, related to their Ce content, impacts their ability to absorb visible light. As a first step, we investigated the production of reactive oxygen species (ROS) as a function of cerium content, in abiotic conditions and in vitro using murine macrophage RAW 264.7 cell line. We demonstrate that, at sub-lethal doses, Ce-containing oxysulfide nanoparticles are responsible for a higher ROS intracellular formation than cerium-free Gd2O2S nanoparticles, although no significant inflammatory response or oxidative stress was measured. Moreover, there was no significant loss of cerium as free cation from the nanoparticles, as evidenced by X-ray fluorescence mapping. Based on these results, we propose a risk analysis for lanthanide oxysulfide nanoparticles, leading to a technology assessment that fulfills the safer-by-design strategy.

Experimental measurement of local high temperature at the surface of gold nanorods using doped ZnGa2O4 as a nanothermometer.

Heat measurement induced by photoexcitation of a plasmonic metal nanoparticle assembly under environmental conditions is of primary importance for the further development of applications in the fields of (photo)catalysis, nanoelectronics and nanomedicine. Nevertheless, the fine control of the rise in temperature remains difficult and limits the use of this technology due to the lack of local temperature measurement tools working under environmental conditions. Luminescence nanothermometers are an alternative solution to the limitations of conventional contact thermometers since they are able to give an absolute temperature value with high spatial resolution using common optical equipment. As a proof of concept of this nanothermometry approach, a high local temperature exceeding one hundred degrees is measured on the thermalized photoexcited aggregate of gold nanorods using ZnGa2O4:Cr3+,Bi3+ nanothermometers that have a strong temperature dependence on the luminescence lifetime of chromium(iii) and high sensitivity over an extensive range of temperatures. A study on the influence of the average distance between nanosensors and nanoheaters on the measured temperature is carried out by coating the nanosensors with a silica layer of tunable thickness, highlighting the temperature gradient at the vicinity of the nanoheater as the theory predicts. The variation of the optical nanosensor response is relevant and promising, and it could be further envisioned as a potential candidate for local temperature measurement at the nanoscale since no plasmonic effect on Cr3+ lifetime is observed. The results reported here open up an even wider field of application for high temperature nanothermometry on real samples such as aggregate particles for many applications including catalysis and nanoelectronics. Thermometry using luminescent nanoprobes, which is complementary to thermal microscopy techniques, will allow in situ and in operando temperature monitoring at very small scales.

Silica-clay nanocomposites for the removal of antibiotics in the water usage cycle.

The increasingly frequent detection of resistant organic micropollutants in waters calls for better treatment of these molecules that are recognized to be dangerous for human health and the environment. As an alternative to conventional adsorbent material such as activated carbon, silica-clay nanocomposites were synthesized for the removal of pharmaceuticals in contaminated water. Their efficiency with respect to carbamazepine, ciprofloxacin, danofloxacin, doxycycline, and sulfamethoxazole was assessed in model water and real groundwater spiked with the five contaminants. Results showed that the efficacy of contaminant removal depends on the chemical properties of the micropollutants. Among the adsorbents tested, the nanocomposite made of 95% clay and 5% SiO2 NPs was the most efficient and was easily recovered from solution after treatment compared with pure clay, for example. The composite is thus a good candidate in terms of operating costs and environmental sustainability for the removal of organic contaminants.

Nanoconfined water vapour as a probe to evaluate plasmonic heating.

Engineering photothermal effects in plasmonic materials is of paramount importance for many applications, such as cancer therapy, chemical synthesis, cold catalysis and, more recently, metasurfaces. The evaluation of plasmonic heating at the nanoscale is challenging and generally requires sophisticated equipments and/or temperature-sensitive probes such as fluorescent molecules or materials. Here, we propose to use water vapor as a probe to evaluate the local heating around plasmonic nanoparticles. We demonstrate the concept for the case of a plasmonic colloidal film characterized by bi-modal nanoporosity. In particular, we exploit the thermal and light water liquid-vapor phase transitions taking place in the nanoporous medium that can be triggered by external stimuli, such as heating or irradiation, to obtain structural and optical variations in the film. The local temperature is then estimated using spectroscopic ellipsometry data acquired by a multimodal chamber. More generally, this method offers a simple and general approach to determine the local temperature that only requires a nanoporous material and water vapor, such as environmental humidity. In addition, this approach can be further generalized to other materials, vapor molecules or optical techniques.

Following in Situ the Degradation of Mesoporous Silica in Biorelevant Conditions: At Last, a Good Comprehension of the Structure Influence.

Mesoporous silica nanoparticles (MSNs) have seen a fast development as drug delivery carriers thanks to their tunable porosity and high loading capacity. The employ of MSNs in biomedical applications requires a good understanding of their degradation behavior both to control drug release and to assess possible toxicity issues on human health. In this work, we study mesoporous silica degradation in biologically relevant conditions through in situ ellipsometry on model mesoporous nanoparticle or continuous thin films, in buffer solution and in media containing proteins. In order to shed light on the structure/dissolution relationship, we performed dissolution experiments far from soluble silicate species saturation. Via a complete decorrelation of dissolution and diffusion contributions, we proved unambiguously that surface area of silica vectors is the main parameter influencing dissolution kinetics, while thermal treatment and open mesoporous network architecture have a minor impact. As a logical consequence of our dissolution model, we proved that the dissolution lag-time can be promoted by selective blocking of the mesopores that limits the access to the mesoporous internal surface. This study was broadened by studying the impact of the organosilanes in the silica structure, of the presence of residual structuring agents, and of the chemical composition of the dissolution medium. The presence of albumin at blood concentration was found affecting drastically the dissolution kinetics of the mesoporous structure, acting as a diffusion barrier. Globally, we could identify the main factors affecting mesoporous silica materials degradation and proved that we can tune their structure and composition for adjusting dissolution kinetics in order to achieve efficient drug delivery.

Direct Synthesis of N-Heterocyclic Carbene-Stabilized Copper Nanoparticles from an N-Heterocyclic Carbene-Borane.

N-Heterocyclic carbene (NHC)-stabilized copper nanoparticles (NPs) were synthesized from an NHC-borane adduct and mesitylcopper(I) under thermal conditions (refluxing toluene for 2.5 h). NPs with a size distribution of 11.6±1.8 nm were obtained. The interaction between Cu NPs and NHC ligands was probed by X-ray photoelectron spectroscopy, which showed covalent binding of the NHC to the surface of the NPs. Mechanistic studies suggested that NHC-borane plays two roles: contributing to the reduction of [CuMes]2 to release Cu0 species and providing NHC ligands to stabilize the copper NPs.

Mechanism and kinetics of magnetite oxidation under hydrothermal conditions.

The stability of magnetite under oxidizing hydrothermal conditions was evaluated at temperatures of 120, 150, 180 and 275 °C. A well-characterized sample of commercially-available magnetite with a particle size of approximately 690 nm was oxidized by dissolved oxygen (DO) under alkaline hydrothermal conditions in titanium autoclaves. In these trials, the DO was always in equilibrium with the gas phase oxygen that was air-derived and was located above the hydrothermal solution, which contained ammonium hydroxide at a pH25 °C of approximately 9.5. Samples recovered by filtration were analysed by X-ray diffraction and scanning electron microscopy, while Fe(ii)/Fe ratios were determined by titration in conjunction with spectrophotometry. Oxidation between 120 and 180 °C was found to generate high concentrations of maghemite and hematite in the product, with the latter compound having either a hexagonal bipyramidal or rhombohedral morphology. The oxidation kinetics was consistent with a diffusion controlled process. The reaction probably proceeded via the outward diffusion of ferrous ions from the magnetite, forming a magnetite/maghemite core/shell structure in conjunction with the dissolution of maghemite and reprecipitation of hematite. Oxidation at 275 °C presented different characteristics from those observed at the lower temperatures. Negligible amounts of maghemite were found, and the primary oxidation product was hematite with no specific morphologies. Moreover, the kinetics was slower than at 180 °C. This unexpected temperature effect is attributed to the rapid growth, at 275 °C, of a dense layer of hematite on the surface of the magnetite that impeded the oxidation of magnetite.

A high pressure pathway toward boron-based nanostructured solids.

Inorganic nanocomposites made of an inorganic matrix containing nanoparticle inclusions provide materials of advanced mechanical, magnetic, electrical properties and multifunctionality. The range of compounds that can be implemented in nanocomposites is still narrow and new preparation methods are required to design such advanced materials. Herein, we describe how the combination of nanocrystal synthesis in molten salts with subsequent heat treatment at a pressure in the GPa range gives access to a new family of boron-based nanocomposites. With the case studies of HfB2/ß-HfB2O5 and CaB6/CaB2O4(iv), we demonstrate by X-ray diffraction and through (scanning) transmission electron microscopy the crystallization of borate matrices into rare compounds and unique nanostructured solids, while metal boride nanocrystals remain dispersed in the matrix and maintain small sizes below 30 nm, thus demonstrating a new multidisciplinary approach toward nanoscaled heterostructures.

Dispersions of Goethite Nanorods in Aprotic Polar Solvents.

Colloidal suspensions of anisotropic nanoparticles can spontaneously self-organize in liquid-crystalline phases beyond some concentration threshold. These phases often respond to electric and magnetic fields. At lower concentrations, usual isotropic liquids are observed but they can display very strong Kerr and Cotton-Mouton effects (i.e., field-induced particle orientation). For many examples of these colloidal suspensions, the solvent is water, which hinders most electro-optic applications. Here, for goethite (α-FeOOH) nanorod dispersions, we show that water can be replaced by polar aprotic solvents, such as N-methyl-2-pyrrolidone (NMP) and dimethylsulfoxide (DMSO), without loss of colloidal stability. By polarized-light microscopy, small-angle X-ray scattering and electro-optic measurements, we found that the nematic phase, with its field-response properties, is retained. Moreover, a strong Kerr effect was also observed with isotropic goethite suspensions in these polar aprotic solvents. Furthermore, we found no significant difference in the behavior of both the nematic and isotropic phases between the aqueous and non-aqueous dispersions. Our work shows that goethite nanorod suspensions in polar aprotic solvents, suitable for electro-optic applications, can easily be produced and that they keep all their outstanding properties. It also suggests that this solvent replacement method could be extended to the aqueous colloidal suspensions of other kinds of charged anisotropic nanoparticles.

Flyscan opportunities in medicine: the case of quantum rattle based on gold quantum dots.

The new rapid scan method, Flyscan mode, implemented on the DiffAbs beamline at Synchrotron SOLEIL, allows fast micro-X-ray fluorescence data acquisition. It paves the way for applications in the biomedical field where a large amount of data is needed to generate meaningful information for the clinician. This study presents a complete set of data acquired after injection of gold-cluster-enriched mesoporous silica nanospheres, used as potential theranostic vectors, into rats. While classical X-ray fluorescence investigations (using step-by-step acquisitions) are based on a limited number of samples (approximately one per day at the DiffAbs beamline), the Flyscan mode has enabled gathering information on the interaction of nanometer-scale vectors in different organs such as liver, spleen and kidney at the micrometer scale, for five rats, in only a single five-day synchrotron shift. Moreover, numerous X-ray absorption near-edge structure spectra, which are beam-time-consuming taking into account the low concentration of these theranostic vectors, were collected.

Nanophase Segregation of Self-Assembled Monolayers on Gold Nanoparticles.

Nanophase segregation of a bicomponent thiol self-assembled monolayer is predicted using atomistic molecular dynamics simulations and experimentally confirmed. The simulations suggest the formation of domains rich in acid-terminated chains, on one hand, and of domains rich in amide-functionalized ethylene glycol oligomers, on the other hand. In particular, within the amide-ethylene glycol oligomers region, a key role is played by the formation of interchain hydrogen bonds. The predicted phase segregation is experimentally confirmed by the synthesis of 35 and 15 nm gold nanoparticles functionalized with several binary mixtures of ligands. An extensive study by transmission electron microscopy and electron tomography, using silica selective heterogeneous nucleation on acid-rich domains to provide electron contrast, supports simulations and highlights patchy nanoparticles with a trend toward Janus nano-objects depending on the nature of the ligands and the particle size. These results validate our computational platform as an effective tool to predict nanophase separation in organic mixtures on a surface and drive further exploration of advanced nanoparticle functionalization.

Quantified Binding Scale of Competing Ligands at the Surface of Gold Nanoparticles: The Role of Entropy and Intermolecular Forces.

A basic understanding of the driving forces for the formation of multiligand coronas or self-assembled monolayers over metal nanoparticles is mandatory to control and predict the properties of ligand-protected nanoparticles. Herein, 1 H nuclear magnetic resonance experiments and advanced density functional theory (DFT) modeling are combined to highlight the key parameters defining the efficiency of ligand exchange on dispersed gold nanoparticles. The compositions of the surface and of the liquid reaction medium are quantitatively correlated for bifunctional gold nanoparticles protected by a range of competing thiols, including an alkylthiol, arylthiols of varying chain length, thiols functionalized by ethyleneglycol units, and amide groups. These partitions are used to build scales that quantify the ability of a ligand to exchange dodecanethiol. Such scales can be used to target a specific surface composition by choosing the right exchange conditions (ligand ratio, concentrations, and particle size). In the specific case of arylthiols, the exchange ability scale is exploited with the help of DFT modeling to unveil the roles of intermolecular forces and entropic effects in driving ligand exchange. It is finally suggested that similar considerations may apply to other ligands and to direct biligand synthesis.

Interaction of TiO2 nanoparticles with proteins from aquatic organisms: the case of gill mucus from blue mussel.

To better understand the mechanisms of TiO2 nanoparticle (NP) uptake and toxicity in aquatic organisms, we investigated the interaction of NPs with the proteins found in gill mucus from blue mussels. Mucus is secreted by many aquatic organisms and is often their first line of defense against pathogens, xenobiotics, and other sources of environmental stress. Here, five TiO2 NPs and one SiO2 NP were incubated with gill mucus and run out on a one-dimensional polyacrylamide gel for a comparative qualitative analysis of the free proteins in the mucosal solution and the proteins bound to NPs. We then used nanoscale liquid chromatography coupled with tandem mass spectrometry to identify proteins of interest. Our data demonstrated dissimilar protein profiles between the crude mucosal solution and proteins adsorbed on NPs. In particular, extrapallial protein (EP), one of the most abundant mucus proteins, was absent from the adsorbed proteins. After thermal denaturation experiments, this absence was attributed to the EP content in aromatic amino acids that prevents protein unfolding and thus adsorption on the NP. Moreover, although the majority of the protein corona was qualitatively similar across the NPs tested here (SiO2 and TiO2), a few proteins in the corona showed a specific recruitment pattern according to the NP oxide (TiO2 vs SiO2) or crystal structure (anatase TiO2 vs rutile TiO2). Therefore, protein adsorption may vary with the type of NP. Graphical abstract Proteins with adsorption selectivity as identified from isolated bands.

Benefit of Rare-Earth "Smart Doping" and Material Nanostructuring for the Next Generation of Er-Doped Fibers.

Erbium-doped fiber amplifiers (EDFAs) for harsh environments require to develop specific fabrication methods of Er 3+-doped fibers (EDFs) so as to limit the impact of radiation-induced absorption. In this context, a compromise has to be found between the concentration of Erbium and the glass composition. On the one hand, high concentration of Er 3+ ions helps to reduce the length of the EDF and hence the cumulated attenuation but generally leads to luminescence quenching mechanisms that limit the performances. On the other hand, so as to avoid such quenching effect, glass modifiers like Al 3+ or P 5+ ions are used in the fabrication of commercial EDFs but are not suitable for applications in harsh environment because these glass modifiers are precursors of radiation-induced structural defects and consequently of optical losses. In this work, we investigate the concept of smart doping via material nanostructuring as a way to fabricate more efficient optical devices. This approach aims at optimizing the glass composition of the fiber core in order to use the minimal content of glass modifiers needed to reach the suited level of performances for EDFA. Er 3+-doped alumina nanoparticles (NPs), as precursor of Er 3+ ions in the preform fabrication process, were used to control the environment of rare-earth ions and their optical properties. Structural and optical characterizations of NP-doped preforms and optical fibers drawn from such preforms demonstrate the interest of this approach for small concentrations of aluminum in comparison to similar glass compositions obtained by a conventional technique.

Design Defines the Effects of Nanoceria at a Low Dose on Soil Microbiota and the Potentiation of Impacts by the Canola Plant.

Soils act as nanoceria sinks via agricultural spreading and surface waters. Canola plants were grown for one month in soil spiked with nanoceria (1 mg·kg(-1)). To define the role of nanomaterials design on environmental impacts, we studied nanoceria with different sizes (3.5 or 31 nm) and coating (citrate). We measured microbial activities involved in C, N, and P cycling in the rhizosphere and unplanted soil. Bacterial community structure was analyzed in unplanted soil, rhizosphere, and plant roots by 454-pyrosequencing of the 16S rRNA gene. This revealed an impact gradient dependent on nanomaterials design, ranging from decreased microbial enzymatic activities in planted soil to alterations in bacterial community structure in roots. Particle size/aggregation was a key parameter in modulating nanoceria effects on root communities. Citrate coating lowered the impact on microbial enzymatic activities but triggered variability in the bacterial community structure near the plant root. Some nanoceria favored taxa whose closest relatives are hydrocarbon-degrading bacteria and disadvantaged taxa frequently associated in consortia with disease-suppressive activity toward plant pathogens. This work provides a basis to determine outcomes of nanoceria in soil, at a dose close to predicted environmental concentrations, and to design them to minimize these impacts.

Charge Transfer at Hybrid Interfaces: Plasmonics of Aromatic Thiol-Capped Gold Nanoparticles.

Although gold nanoparticles stabilized by organic thiols are the building blocks in a wide range of applications, the role of the ligands on the plasmon resonance of the metal core has been mostly ignored until now. Herein, a methodology based on the combination of spectroscopic ellipsometry and UV-vis spectroscopy is applied to extract dielectric functions of the different components. It is shown that aromatic thiols allow a significant charge transfer at the hybrid interface with the s and d bands of the gold core that yields "giant" red shifts of the plasmon band, up to 40 nm for spherical particles in the size range of 3-5 nm. These results suggest that hybrid nanoplasmonic devices may be designed through the suitable choice of metal core and organic components for optimized charge exchange.