The absorption of light by plasmonic nanostructures and their associated temperature increase are exquisitely sensitive to the shape and composition of the structure and to the wavelength of light. Therefore, much effort is put into synthesizing novel nanostructures for optimized interaction with the incident light. The successful synthesis and characterization of high quality and biocompatible plasmonic colloidal nanoparticles has fostered numerous and expanding applications, especially in biomedical contexts, where such particles are highly promising for general drug delivery and for tomorrow’s cancer treatment. We review the thermoplasmonic properties of the most commonly used plasmonic nanoparticles, including solid or composite metallic nanoparticles of various dimensions and geometries. Common methods for synthesizing plasmonic particles are presented with the overall goal of providing the reader with a guide for designing or choosing nanostructures with optimal thermoplasmonic properties for a given application. Finally, the biocompatibility and biological tolerance of structures are critically discussed along with novel applications of plasmonic nanoparticles in the life sciences.

We show that individual colloidal CdSe-core quantum dots can be optically trapped and manipulated in three dimensions by an infrared continuous wave laser operated at low laser powers. This makes possible utilizing quantum dots not only for visualization but also for manipulation, an important advantage for single molecule experiments. Moreover, we provide quantitative information about the magnitude of forces applicable to a single quantum dot and of the polarizability of an individual quantum dot.Colloidal quantum dots (QDs) are fluorescent semiconductor nanocrystals.1 They are bright and photostable with a broad excitation spectrum and a narrow emission spectrum, normally distributed around a specific wavelength, dependent on the size of the QD. Absorption of any photons with wavelengths above this specific wavelength causes the formation of an electron-hole pair, the recombination of which results in photon emission. The fluorescent blinking of nanocrystal quantum dots is the result of a bistability between an emitting state where the quantum dot is described as on and the nonemitting off state.2 The extreme brightness and photostability of QDs make them excellent choices as markers to visualize biological systems. For instance they have been used to mark individual receptors in cell membranes 3 or to label living embryos at different stages. 4 It has long been a goal to optically trap or otherwise control quantum dots 5 to establish a combined visualization and optical manipulation technique. Optical trapping of aggregates of colloidal quantum dots in two dimensions was recently proved possible 6 using a pulsed YLF laser, and it was claimed that to trap quantum dots with a continuous wave (CW) laser one would need extremely high powers on the order of 20 W. In this Letter, we prove that optical trapping of individual quantum dots using a CW infrared laser operated at only 0.5 W is, in fact, possible. By observing the Brownian motion of the trapped quantum dots, we deduced the strength of the optical trap and thus found the magnitude of the optical forces acting on a single quantum dot. We used two independent approaches to render probable that it was indeed a single quantum dot in the trap and not an aggregate.An inducible dipole in an inhomogeneous field experiences a force in the direction of the field gradient, the gradient force, F b grad . A particle with an induced dipole moment will be forced toward the laser focus by this three-dimensional restoring force. Hence, the existence of an induced QD dipole moment is essential for optical trapping. Opposing the gradient force are the scattering force, F b scat , and the absorption force, F b abs , which are proportional to the scattering and absorption cross sections, respectively. If infrared laser light, with a wavelength that exceeds the maximum emitted wavelength of the QDs by far, is used for trapping and if the QDs are physically very small in comparison to the focus area of the trapping laser light, then the scattering and absorption forces ...

A single CW infrared laser beam can simultaneously trap and excite an individual colloidal quantum dot. Though the laser light is relatively weak, the excitation occurs through two-photon absorption. This finding eliminates the demand for an excitation light source in addition to a trapping laser in nanoscale experiments with simultaneous force-manipulation and quantum dot visualization. Also, we demonstrate that optical trapping efficiencies of individual quantum dots do not correlate with their emission wavelength or physical size.

Absorption of near infrared (NIR) light by metallic nanoparticles can cause extreme heating and is of interest for instance in cancer treatment since NIR light has a relatively large penetration depth into biological tissue. Here, we quantify the extraordinary thermoplasmonic properties of platinum nanoparticles and demonstrate their efficiency in photothermal cancer therapy. Although platinum nanoparticles are extensively used for catalysis, they are much overlooked in a biological context. Via direct measurements based on a biological matrix we show that individual irradiated platinum nanoparticles with diameters of 50-70 nm can easily reach surface temperatures up to 900 K. In contrast to gold nanoshells, which are often used for photothermal purposes, we demonstrate that the platinum particles remain stable at these extreme temperatures. The experiments are paralleled by finite element modeling confirming the experimental results and establishing a theoretical understanding of the particles' thermoplasmonic properties. At extreme temperatures it is likely that a vapor layer will form around the plasmonic particle, and we show this scenario to be consistent with direct measurements and simulations. Viability studies demonstrate that platinum nanoparticles themselves are non-toxic at therapeutically relevant concentrations, however, upon laser irradiation we show that they efficiently kill human cancer cells. Therefore, platinum nanoparticles are highly promising candidates for thermoplasmonic applications in the life sciences, in nano-medicine, and for bio-medical engineering.

Most progress on optical nanoparticle control has been in liquids, while optical control in air has proven more challenging. By utilizing an air chamber designed to have a minimum of turbulence and a single laser beam with a minimum of aberration, we trapped individual 200 to 80 nm gold nanoparticles in air and quantified the corresponding trapping strengths. These results pave the way for construction of metallic nanostructures in air away from surfaces.