Table 1. Comparison of inorganic materials for delivery purposes.

Summarized advantages and potential issues of gold, silica, iron oxide, and UCNP platforms for in vivo delivery of therapeutics, with key references for delivery of each form of therapeutic cargo.

Material	Advantages	Challenges	References
Gold	tunable size and shape, optical reactivity, flexible monolayer design, inherently non-toxic	uncertain biological fate, nuanced synthetic process	Small molecule 71–78, 81–86, 88, 122–127, 130–136, 165 Protein 93, 99, 100, 138–140, 142, 143 Nucleic acid 105–118, 146–153, 156, 159, 160, 173
Silica	high loading capacity, controllable release rate, flexible platforms for triggered release	toxicity from surface-exposed silanol groups, increased ROS production	Small molecule 225, 242, 243, 245, 248, 205–213, 220, 221 Protein 230, 240, 244, 214, 215 Nucleic acid 237, 217–219
Iron Oxide	hyperthermic properties, potential for contrast imaging or magnetic localization	particle accumulation/degradation in vivo produces toxic byproducts	Small molecule 308, 309, 313–315 Protein 310, 325 Nucleic acid 311, 312, 316–323
Lanthanide Upconversion Particles	NIR-triggered release for localized drug release, low toxicity	limited available excitation wavelengths, need for silica components	Small molecule 348–355, 363, 364, 365, 367, 371 Protein 366 Nucleic acid 356, 368, 372

NIR: Near-infrared radiation

ROS: Reactive oxygen species