Thermometries for Single Nanoparticles Heated with Light.

The development of efficient nanoscale photon absorbers, such as plasmonic or high-index dielectric nanostructures, allows the remotely controlled release of heat on the nanoscale using light. These photothermal nanomaterials have found applications in various research and technological fields, ranging from materials science to biology. However, measuring the nanoscale thermal fields remains an open challenge, hindering full comprehension and control of nanoscale photothermal phenomena. Here, we review and discuss existent thermometries suitable for single nanoparticles heated under illumination. These methods are classified in four categories according to the region where they assess temperature: (1) the average temperature within a diffraction-limited volume, (2) the average temperature at the immediate vicinity of the nanoparticle surface, (3) the temperature of the nanoparticle itself, and (4) a map of the temperature around the nanoparticle with nanoscale spatial resolution. In the latter, because it is the most challenging and informative type of method, we also envisage new combinations of technologies that could be helpful in retrieving nanoscale temperature maps. Finally, we analyze and provide examples of strategies to validate the results obtained using different thermometry methods.

Impact of bimetallic interface design on heat generation in plasmonic Au/Pd nanostructures studied by single-particle thermometry.

Localized surface plasmons are lossy and generate heat. However, accurate measurement of the temperature of metallic nanoparticles under illumination remains an open challenge, creating difficulties in the interpretation of results across plasmonic applications. Particularly, there is a quest for understanding the role of temperature in plasmon-assisted catalysis. Bimetallic nanoparticles combining plasmonic with catalytic metals are raising increasing interest in artificial photosynthesis and the production of solar fuels. Here, we perform single-particle thermometry measurements to investigate the link between morphology and light-to-heat conversion of colloidal Au/Pd nanoparticles with two different configurations: core-shell and core-satellite. It is observed that the inclusion of Pd as a shell strongly reduces the photothermal response in comparison to the bare cores, while the inclusion of Pd as satellites keeps photothermal properties almost unaffected. These results contribute to a better understanding of energy conversion processes in plasmon-assisted catalysis.

Optical Printing of Single Au Nanostars.

Obtaining arrays of single nanoparticles with three-dimensional complex shapes is still an open challenge. Current nanolithography methods do not allow for the preparation of nanoparticles with complex features like nanostars. In this work, we investigate the optical printing of gold nanostars of different sizes as a function of laser wavelength and power. We found that tuning the laser to the main resonances of the nanostars in the near-infrared makes it possible to avoid nanoparticles reshaping due to plasmonic heating, enabling their deposition at the single particle level and in ordered arrays.

Challenges on optical printing of colloidal nanoparticles.

While colloidal chemistry provides ways to obtain a great variety of nanoparticles with different shapes, sizes, material compositions, and surface functions, their controlled deposition and combination on arbitrary positions of substrates remain a considerable challenge. Over the last ten years, optical printing arose as a versatile method to achieve this purpose for different kinds of nanoparticles. In this article, we review the state of the art of optical printing of single nanoparticles and discuss its strengths, limitations, and future perspectives by focusing on four main challenges: printing accuracy, resolution, selectivity, and nanoparticle photostability.

In Situ Photothermal Response of Single Gold Nanoparticles through Hyperspectral Imaging Anti-Stokes Thermometry.

Several fields of applications require a reliable characterization of the photothermal response and heat dissipation of nanoscopic systems, which remains a challenging task for both modeling and experimental measurements. Here, we present an implementation of anti-Stokes thermometry that enables the in situ photothermal characterization of individual nanoparticles (NPs) from a single hyperspectral photoluminescence confocal image. The method is label-free, potentially applicable to any NP with detectable anti-Stokes emission, and does not require any prior information about the NP itself or the surrounding media. With it, we first studied the photothermal response of spherical gold NPs of different sizes on glass substrates, immersed in water, and found that heat dissipation is mainly dominated by the water for NPs larger than 50 nm. Then, the role of the substrate was studied by comparing the photothermal response of 80 nm gold NPs on glass with sapphire and graphene, two materials with high thermal conductivity. For a given irradiance level, the NPs reach temperatures 18% lower on sapphire and 24% higher on graphene than on bare glass. The fact that the presence of a highly conductive material such as graphene leads to a poorer thermal dissipation demonstrates that interfacial thermal resistances play a very significant role in nanoscopic systems and emphasize the need for in situ experimental thermometry techniques. The developed method will allow addressing several open questions about the role of temperature in plasmon-assisted applications, especially ones where NPs of arbitrary shapes are present in complex matrixes and environments.

Fibronectin rescues estrogen receptor α from lysosomal degradation in breast cancer cells.

Estrogen receptor α (ERα) is expressed in tissues as diverse as brains and mammary glands. In breast cancer, ERα is a key regulator of tumor progression. Therefore, understanding what activates ERα is critical for cancer treatment in particular and cell biology in general. Using biochemical approaches and superresolution microscopy, we show that estrogen drives membrane ERα into endosomes in breast cancer cells and that its fate is determined by the presence of fibronectin (FN) in the extracellular matrix; it is trafficked to lysosomes in the absence of FN and avoids the lysosomal compartment in its presence. In this context, FN prolongs ERα half-life and strengthens its transcriptional activity. We show that ERα is associated with ß1-integrin at the membrane, and this integrin follows the same endocytosis and subcellular trafficking pathway triggered by estrogen. Moreover, ERα+ vesicles are present within human breast tissues, and colocalization with ß1-integrin is detected primarily in tumors. Our work unravels a key, clinically relevant mechanism of microenvironmental regulation of ERα signaling.

Understanding and Reducing Photothermal Forces for the Fabrication of Au Nanoparticle Dimers by Optical Printing.

Optical printing holds great potential to enable the use of the vast variety of colloidal nanoparticles (NPs) in nano- and microdevices and circuits. By means of optical forces, it enables the direct assembly of NPs, one by one, onto specific positions of solid surfaces with great flexibility of pattern design and no need of previous surface patterning. However, for unclear causes it was not possible to print identical NPs closer to each other than 300 nm. Here, we show that the repulsion restricting the optical printing of close by NPs arises from light absorption by the printed NPs and subsequent local heating. By optimizing heat dissipation, it is possible to reduce the minimum separation between NPs. Using a reduced graphene oxide layer on a sapphire substrate, we demonstrate for the first time the optical printing of Au-Au NP dimers. Modeling the experiments considering optical, thermophoretic, and thermo-osmotic forces we obtain a detailed understanding and a clear pathway for the optical printing fabrication of complex nano structures and circuits based on connected colloidal NPs.

Accuracy and Mechanistic Details of Optical Printing of Single Au and Ag Nanoparticles.

Optical printing is a powerful all-optical method that allows the incorporation of colloidal nanoparticles (NPs) onto substrates with nanometric precision. Here, we present a systematic study of the accuracy of optical printing of Au and Ag NPs, using different laser powers and wavelengths. When using light of wavelength tuned to the localized surface plasmon resonance (LSPR) of the NPs, the accuracy improves as the laser power is reduced, whereas for wavelengths off the LSPR, the accuracy is independent of the laser power. Complementary studies of the printing times of the NPs reveal the roles of Brownian and deterministic motion. Calculated trajectories of the NPs, taking into account the interplay between optical forces, electrostatic forces, and Brownian motion, allowed us to rationalize the experimental results and gain a detailed insight into the mechanism of the printing process. A clear framework is laid out for future optimizations of optical printing and optical manipulation of NPs near substrates.

Light-Induced Polarization-Directed Growth of Optically Printed Gold Nanoparticles.

Optical printing has been proved a versatile and simple method to fabricate arbitrary arrays of colloidal nanoparticles (NPs) on substrates. Here, we show that is also a powerful tool for studying chemical reactions at the single NP level. We demonstrate that 60 nm gold NPs immobilized by optical printing can be used as seeds to obtain larger NPs by plasmon-assisted reduction of aqueous HAuCl4. The final size of each NP is simply controlled by the irradiation time. Moreover, we show conditions for which the growth occurs preferentially in the direction of light polarization, enabling the in situ anisotropic reshaping of the NPs in predetermined orientations.

Connecting Metallic Nanoparticles by Optical Printing.

Optical printing is a simple and flexible method to bring colloidal nanoparticles from suspension to specific locations of a substrate. However, its application has been limited to the fabrication of arrays of isolated nanoparticles because, until now, it was never possible to bring nanoparticles closer together than approximately 300 nm. Here, we propose this limitation is due to thermophoretic repulsive forces generated by plasmonic heating of the NPs. We show how to overcome this obstacle and demonstrate the optical printing of connected nanoparticles with well-defined orientation. These experiments constitute a key step toward the fabrication by optical printing of functional nanostructures and microcircuits based on colloidal nanoparticles.

Gold nanoparticles supported in zirconia-ceria mesoporous thin films: a highly active reusable heterogeneous nanocatalyst.

Gold nanoparticles (NP) trapped in the mesopores of mixed zirconia-ceria thin films are prepared in a straightforward and reproducible way. The films exhibit enhanced stability and excellent catalytic activity in nitro-group reduction by borohydride and electrocatalytic activity in CO and ethanol oxidation and oxygen reduction.

Highly ordered, accessible and nanocrystalline mesoporous TiO2 thin films on transparent conductive substrates.

Highly porous (V(mesopore) = 25-50%) and ordered mesoporous titania thin films (MTTF) were prepared on ITO (indium tin oxide)-covered glass by a fast two-step method. The effects of substrate surface modification and thermal treatment on pore order, accessibility and crystallinity of the MTTF were systematically studied for MTTF deposited onto bare and titania-modified ITO. MTTF exposed briefly to 550 °C resulted in highly ordered films with grid-like structures, enlarged pore size, and increased accessible pore volume when prepared onto the modified ITO substrate. Mesostructure collapse and no significant change in pore volume were observed for MTTF deposited on bare ITO substrates. Highly crystalline anatase was obtained for MTTF prepared on the modified-ITO treated at high temperatures, establishing the relationship between grid-like structures and titania crystallization. Photocatalytic activity was maximized for samples with increased crystallization and high accessible pore volume. In this manner, a simple way of designing materials with optimized characteristics for optoelectronic applications was achieved through the modification of the ITO surface and a controlled thermal treatment.