Size Control and Enhanced Stability of Silver Nanoparticles by Cyclic Poly(ethylene glycol)

Silver nanoparticles (AgNPs) are used in a wide range of applications, and the size control and stability of the nanoparticles are crucial aspects in their applications. In the present study, cyclized poly(ethylene glycol) (c-PEG) with various molecular weights, along with linear PEG with hydroxy chain ends (HO-PEG-OH) and methoxy chain ends (MeO-PEG-OMe) were applied for the Tollens' synthesis of AgNPs. The particle size was significantly affected by the topology and end groups of PEG. For example, the size determined by TEM was 40 ± 7 nm for HO-PEG_5k-OH, 21 ± 4 nm for c-PEG_5k, and 48 ± 9 nm for MeO-PEG_5k-OMe when the molar ratio of PEG to AgNO₃ (ω) was 44. The stability of AgNPs was also drastically improved by cyclization; the relative UV-Vis absorption intensity (A/A₀ × 100%) at λ_max to determine the proportion of persisting AgNPs in an aqueous NaCl solution (37.5 mM) was 58% for HO-PEG_5k-OH, 80% for c-PEG_5k, and 40% for MeO-PEG_5k-OMe, despite the fact that AgNPs with c-PEG_5k were much smaller than those with HO-PEG_5k-OH and MeO-PEG_5k-OMe.