The ability to direct subsurface fluid flow or achieve a high level of control on permeability in subsurface environments necessitates the development of tunable novel subsurface fluids. These multifunctional fluids should have the potential to form hydrogels to divert flow or enhance fracture networks at elevated pressures, ability to transport proppants, undergo reversible transformations from gel-like to fluid-like in response to chemical- or pressure-based perturbations, and have improved CO2 carrying capacity for enhancing the miscibility and flowability of oil and gas. In this paper, we discuss the development of multifunctional nanofluids constructed from silica (SiO2) nanoparticles and poly(allylamine) (PAA), amine bearing polymer chains with high affinity for CO2. A 2-fold increase in CO2 absorption in SiO2-PAA nanofluids compared to the pure polymer was noted. Upon CO2 absorption, the weakly interacting polymeric chains around the nanoparticles formed relatively compact hydrogels. Time-resolved ultrasmall-angle X-ray scattering/small-angle X-ray scattering measurements showed the transition from swollen branched polymers to Gaussian coils with an increased exposure to CO2. Further, CO2-induced hydrogel formation in aqueous fluids bearing 1 wt % SiO2-PAA nanofluids occurred at room temperature, unlike in fluids bearing 1 wt % PAA. These observations point to the feasibility of forming hydrogels at lower temperatures and pressures using novel nanofluids as opposed to using the pure polymer. The ability to tune the structures and morphologies of these fluids expands the potential applications of these fluids in a wide range of subsurface environments.