Surface-sensitive quantitative studies of competitive molecular adsorption on nanoparticles were conducted using a modified Attenuated attenuated Total total Reflectionreflectance-Fourier Transform transform Infrared infrared (ATR-FTIR) spectroscopy method. Adsorption isotherms for thiolated polyethylene glycol (SH-PEG) on AuNPs as a function of three different molecular mass values (1 kDa, 5 kDa, and 20 kDa) were characterized. We find that surface density of SH-PEG on AuNPs is inversely proportional to the molecular mass (Mm). Equilibrium binding constants for SH-PEG, obtained using the Langmuir adsorption model, show the binding affinity for SH-PEG is proportional to Mm. Simultaneous competitive adsorption between mercaptopropionic acid (MPA) and 5 kDa SH-PEG (SH-PEG5K) was investigated, and we find that MPA concentration is the dominant factor influencing the surface density of both SH-PEG5K and MPA; whereas, the concentration of SH-PEG5K effects only SH-PEG5K surface density. Electrospray differential mobility analysis (ES-DMA) was employed as an orthogonal characterization technique. ES-DMA results are consistent with the results obtained by ATR-FTIR, confirming our conclusions about the adsorption process in this system. Ligand displacement competitive adsorption, (where the displacing molecular species is added after completion of the ligand surface reaction,) was also interrogated by ATR-FTIR. Results indicate that SH-PEG, with a larger Mm, exhibits better stability on AuNPs than a smaller-Mm SH-PEG against displacement by bovine serum albumin (BSA) as a model serum protein. In addition, the binding affinity of BSA to AuNPs is inhibited for SH-PEG conjugated AuNPs, an effect that is enhanced at higher SH-PEG Mm values.
Citation: Analytical Chemistry
Pub Type: Journals
gold, nanoparticle, infrared, electrospray, mobility, mercaptopropionic, ethylene glycol, albumin