Understanding and controlling the performance of engineered nanoparticle (NP) systems is greatly assisted by quantitative characterization of their coatings. Useful measurements methods have been described for NPs in liquid environment, but NP aggregation often represents a limiting factor which impairs the accuracy of techniques such as dynamic light scattering for quantification purposes. Here, the authors show how differential centrifugal sedimentation (DCS) and x-ray photoelectron spectroscopy (XPS) can provide quantitative information on the NP coating thickness, molecular conformation, and grafting density of aggregated NP samples. The authors find that thiol-terminated methoxy polyethylene glycol (mPEG) coating thickness on gold NPs increases with increasing particle size and mPEG molecular weight. The hydration of the mPEG shell was estimated by comparing the shell thickness measured in liquid by DCS and vacuum by XPS and was found to increase with the mPEG molecular weight. Finally, the authors used XPS to measure the grafting density of the mPEG molecules. This was found to depend on the mPEG molecular volume and decreased for larger mPEG molecules, suggesting that the grafting density is determined by the conformation of the mPEG molecules in liquid. This analysis provides practical measurement methods for optimizing the design of engineered NP systems and ultimately enhance and control their performance.

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See supplementary material at http://dx.doi.org/10.1116/1.4968882 for representative DLS and XPS measurements of citrate-coated and mPEG-coated nanoparticles; average thiol spacing between adjacent mPEG molecules as a function of NPs' size and their incubation time with the mPEG molecules, as inferred from XPS measurements; shell thickness measured in liquid by DCS and in vacuum by XPS of 40 nm gold NPs coated with 2 kDa mPEG using different time durations for the incubation; relative amount of water contained in the mPEG shell of 40 nm NPs as a function of the PEG molecular weight and an estimation of mPEG surface molecular density in liquids.

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