Photoacoustics is a powerful biomedical imaging and detection technique, because it is a noninvasive, nonionizing, and low‐cost method facilitating deep tissue penetration. However, suitable contrast agents need to be developed to increase the contrast for in vivo imaging. Gold nanoparticles are often discussed as potential sonophores due to their large absorption cross‐section and their tunable plasmon resonance. However, disadvantages such as toxicity and low photoacoustic efficiency in the tissue transparency window prevail, preventing their clinical application. As a result, there remains a strong need to develop colloidal photoacoustic contrast agents which absorb in the tissue transparency window, exhibit high photoacoustic signal, and are biocompatible. Here, a facile synthetic approach is presented to produce melanin shells around various gold nanoparticle geometries, from spheres to stars and rods. These hybrid particles show excellent dispersability, better biocompatibility, and augmented photoacoustic responses over the pure melanin or pristine gold particles, with a rod‐shape geometry leading to the highest performance. These experimental results are corroborated using numerical calculations and explain the improved photoacoustic performance with a thermal confinement effect. The applicability of melanin coated gold nanorods as gastrointestinal imaging probes in mouse intestine is showcased.

Photoacoustics is an upcoming modality for biomedical imaging, which promises minimal invasiveness at high penetration depths of several centimeters. For superior photoacoustic contrast, imaging probes with high photothermal conversion efficiency are required. Gold nanoparticles are among the best performing photoacoustic imaging probes. However, the geometry and size of the nanoparticles determine their photothermal efficiency. We present a systematic theoretical analysis to determine the optimum nanoparticle geometry with respect to photoacoustic efficiency in the near-infrared spectral range, for superior photoacoustic contrast. Theoretical predictions are illustrated by experimental results for two of the most promising nanoparticle geometries, namely, high aspect ratio gold nanorods and gold nanostars.

A one-step soft lithography based pathway to manufacture aligned gold nanorod@poly-(N-isopropylacrylamide) (GNR@pNIPAM) hybrid chains with hexagonal arrangement of the nanorods and with an anisotropic optical response is presented. After demonstration of an efficient synthesis protocol, yielding uniform composite microgels in high concentration, a micromolding procedure using wrinkled polydimethylsiloxane (PDMS) templates to fabricate aligned hybrid chains is introduced. It is found that the self-assembled GNR@pNIPAM microgels inside the PDMS wrinkle grooves can be transferred onto solid substrates, on which they exhibit a hexagonal order, as confirmed by small-angle X-ray scattering. Further, it is shown that the application of minimized PDMS wrinkle dimensions aligns GNRs inside the pNIPAM microgels, and that the optical response of such molded assemblies is anisotropic.

The optical properties of 3D metacrystals made of gold nanoparticles in protein nanocages are studied. These metacrystals have sizes of tens of micrometers and are of high structural and optical quality. Through microspectroscopy measurements and model calculations it is demonstrated that the metacrystals show plasmonic absorption in the green wavelength range and are largely transparent in the red and infrared ranges. By using empty nanocages as placeholders in the metacrystal lattice, it is possible to control how strongly the metamaterial absorbs. Measurements on a pyramidal metacrystal show that it deflects visible light. The deflection shows evidence for anomalous refraction at short wavelengths and normal refraction at long wavelengths. The refractive dispersion is ascribed to the optical dispersion relation of the plasmonic metamaterial.