Direct-absorption solar collectors (DASCs) that employ plasmonic nanofluids with engineered optical properties have drawn much attention for solar thermal applications. One of the major issues limiting further development of DASCs is the long-term dispersion of nanoparticles within the plasmonic nanofluids, which should be sustainable at high-temperature conditions. In this research, we propose surface-modified metal@SiO2 core/shell nanoparticles (CSNPs) to improve the dispersion stability and tune the absorption coefficient of nanofluids. The Au@SiO2 and Ag@SiO2 CSNPs are synthesized using a low-temperature two-step solution process. The plasmonic nanofluids with the synthesized metal@SiO2 CSNPs exhibit excellent dispersion stability of 93.7% for Au@SiO2 and 100% for Ag@SiO2 in 6 months without using any surfactants, and they also present a good thermal stability after thermal exposure at 150 degrees C for an hour. The absorption and scattering coefficients of a plasmonic nanofluid should be known precisely to properly analyze its photothermal conversion. Here, we also develop a new measurement system to separately determine the absorption and scattering coefficients of nanofluid. The Au@SiO2 CSNPs-dispersed nanofluid is observed to exhibit an extremely low scattering albedo (i.e., omega = 0.011) in comparison with that of the Ag@SiO2 CSNPs-dispersed nanofluid (omega = 0:3).