An assortment of hydrothermal reactions of chromic(III) nitrate and 2-aminoterephthalic acid was systematically studied to yield high-quality amine-functionalized MIL-101(Cr) nanoparticles (MIL-101(Cr)-NH2). A comprehensive understanding of the role that synthesis conditions and basic modulators have on the formation of MIL-101(Cr)-NH2 in aqueous media was extracted and reported herein. With the aid of a NaOH modulator at optimized concentration, sub-20 nm MIL-101(Cr)-NH2 nanoparticles were prepared with good yield, minimized particle agglomeration, and a high Brunauer-Emmett-Teller (BET) surface area of 2800 +/- 200 m(2)/g. To the best of our knowledge, these are the smallest particle sizes and the highest surface areas reported for directly synthesized MIL-101(Cr)-NH2. Owing to their superior surface area and Lewis basic amine functionality, the MIL-101(Cr)-NH2 nanoparticles exhibit a high CO2 adsorption of up to 3.4 mmol/g at 288 K and 1 bar and a superior CO2/N-2 selectivity of 26.5 at 308 K and 0.1 bar. A high isosteric heat of -54.6 kJ/mol for CO2 adsorption implies the strong interaction between CO2 and the amine-functionalized framework. The successful synthesis of sub-20 nm amine-functionalized MIL-101(Cr) nanoparticles offers a great opportunity to engineer advanced MIL-101(Cr)-based functional adsorbents and membranes for CO2 capture and separation.