Highly monodisperse, biocompatible and functionalizable sub-10-nm citrate-stabilized gold nanoparticles (Au NPs) have been synthesized following a kinetically controlled seeded-growth strategy. The use of traces of tannic acid together with an excess of sodium citrate during nucleation is fundamental in the formation of a high number (7 × 1013 NPs/mL) of small ∼3.5 nm Au seeds with a very narrow distribution. A homogeneous nanometric growth of these seeds is then achieved by adjusting the reaction parameters: pH, temperature, sodium citrate concentration and gold precursor to seed ratio. We use this method to produce Au NPs with a precise control over their sizes between 3.5 and 10 nm and a versatile surface chemistry allowing studying the size-dependent optical properties in this transition size regime lying between clusters and nanoparticles. Interestingly, an inflection point is observed for Au NPs smaller than 8 nm in which the sensitivity of the localized surface plasmon resonance (LSPR) peak positio...