On gold fiducials

Apparently in theory there's

• Au₁₁ (0.8mn diameter) a.k.a. undecagold
• Au₂₅ (0.9mn)
• Au₃₈ (1.1nm)
• Au₆₈ (1.3nm)
• 1.4nm, often referred to as nanogold(verify)
• Au₁₀₂ (1.5nm)
• 1.8nm (a.k.a. Ni-NTA–Nanogold(verify))
• Au₁₁₄ (2.0nm)

• Much larger is colloidal gold. These are larger clumps and (not being ordered crystals as the above are(verify)) the sizes are a little more approximate, but they often aim for one
  • 5nm
  • 10nm
  • 15nm
  • 20nm
  • 25nm

Apparently most commonly it's 5nm or 10nm, sometimes nanogold(verify).

See also

"Site-specific biomolecule labeling with gold clusters." (PMC3568671)

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/nanogold