Fig. 1. SEM and TEM images of various categories of gold nanocrystals. (a) Gold nanospheres^[17]; (b) gold nanorods^[18]; (c) gold nanocages^[20]; (d) gold nanostars^[23]; (e) gold nanodisks^[25]; (f) gold nanoshells^[27

Fig. 2. Diagram of gold nanocrystalline polymer assembly^[40]

Fig. 3. SEM image, side view of structure, and photoacoustic signal of two-dimensional gold nanostructure. (a) SEM image of two-dimensional gold nanostructure; (b) side view of two-dimensional gold nanostructure; (c) acoustic signal from gold nanostructure and carbon black film versus incident wavelength^[43]

Fig. 4. Fabrication procedures, SEM images, and photoacoustic signal comparisons of 3D Ag nanostructures. (a) Fabrication procedures for 3D silver nanostructure (upper) and their SEM images (lower); (b) photoacoustic signals from 3D Ag nanostructures, 2D Ag nanostructures, and polymer^[44]

Fig. 5. Mechanism of matrix metalloproteinases (MMPs) activating CuS-peptide-BHQ3 (CPQ) contrast agent and changes of PA signals of BHQ3 and CuS. (a) Mechanism of MMPs activating CPQ contrast agent; (b) changes of PA signals of BHQ3 and CuS after adding MMPs^[50]

Fig. 6. Broadband all-optical ultrasound transducers. (a) Side view of gold nanostructure; (b) side view of all-optical transducer; (c) SEM image of polymer with holes; (d) frequency response curve of ultrasound transducer^[2]

Fig. 7. Schematic of phase cancellation of photoacoustic signal^[52]

Table 1. Summary of fabrication methods, modulation methods, properties, and applications of different gold nanocrystals