Table 2.

Examples of PA contrast agent explored in PA imaging.

Materials		Types of nanoagents	Advantages (+)/Disadvantages (-)	References
Inorganic	Metallic nanomaterials	Au nanorods; Au nanostars; Au nanocages; Au nanoshell; Au nanovesicles; Au nanoflowers; Ag nanoplates; Palladium nanoplates; antimony nanoparticles	(+) tunable physiochemical properties; chemically inert element with reasonable biocompatibility; able to carry cargoes. (–) non-biodegradability; suboptimal photothermal stability	Chen et al., 2015b; Zhong et al., 2015 Wang et al., 2015a Zhang et al., 2013 Topete et al., 2014 Song et al., 2015a Huang et al., 2014a Homan et al., 2012 Nie et al., 2014 Li and Chen, 2015
	Carbon-based nanomaterials	Carbon nanotubes; Graphenes; Carbon dots	(+) able to carry cargoes; good photothermal stability. (–) non-biodegradability; heterogeneity	Zhang et al., 2015 Lalwani et al., 2013; Sheng et al., 2013; Ge et al., 2015
	Transition metal chalcogenides (TMC)-based nanomaterials	CuS; WS2; MoS2; FeS; Bi2S3; CuSe; Co9Se8; Bi2Se3	(+) high photothermal conversion efficiency; good photothermal stability; low cost. (–) non-biodegradability; contain heavy metal elements	Cheng et al., 2014a; Yin et al., 2014 Cui et al., 2015; Yang et al., 2015b Hessel et al., 2011; Liu et al., 2015e Song et al., 2015b
Organic	Dyes	Porphyrin- and Cyanine-based dyes, e.g., Indocyanine green (ICG), IR780, IR825, etc.	(+) good biocompatibility/biodegradability. (–) poor aqueous solubility, low photothermal stability, short bloodstream circulation half-life	Sheng et al., 2014; Wang et al., 2015b Lovell et al., 2011; Song et al., 2015c Huang et al., 2014b; Chen et al., 2015a; Rong et al., 2015
	Polymer-based nanomaterials	Polypyrrole; Polyaniline; Polydopamine; Semiconducting polymers	(+) good biocompatibility and photothermal stability; able to carry cargoes. (–) their biodegradation behaviors remain unknown	Yang et al., 2011; Lin et al., 2014 Yang et al., 2012; Pu et al., 2015