CO2 separation across polyethylene glycol (PEG)-based ultra-thin film composite mixed matrix membranes (UTFC-MMMs) containing surface-functionalized SiO2 nanoparticles (SFSNPs) were investigated. The SFSNPs were prepared by physicochemical adsorption of polyethyleneimine (PEI), polydopamine (PDA) and codeposition of PEI and PDA onto the porous SiO2 nanoparticles (diameter of similar to 10 nm). A cross-linked PEG-based polymer incorporating the SFSNPs was formed into an ultra-thin, CO2 selective layer of similar to 55 nm via a novel nano-coating technology, namely continuous assembly of polymers (CAP). The resulting PEG-based UTFC-MMMs demonstrate the potential of ultra thin films prepared by the CAP nanotechnology to enhance CO2 separation. The mixed matrix membranes achieved a CO2 permeance of similar to 1300 GPU (1 GPU=10(-6) cm(3) (STP) cm(-2) S-1 cmHg(-1)) and a favorable CO2/N-2 selectivity of 27, which is a 30% and 25% rise in CO2 permeance and CO2/N-2 selectivity respectively above the values obtained from simple PEG-based UTFC membranes. (C) 2016 Elsevier B.V. All rights reserved.