Hydrogen bonded multilayers of poly(2-ethyl-2-oxazoline) stabilized silver nanoparticles and tannic acid

Abstract

The formation of H-bonded multilayers of PEOX stabilized Ag-NPs and TA by layer-by-layer (LbL) process is reported. Ag-NPs were synthesized in aqueous solutions by chemical reduction. Four different molar mass PEOX (5K, 50K, 200K, 500K) was used to stabilize the Ag-NPs in dispersion. The effect of PEOX molar mass, PEOX concentration, pH and time on the temporal stability of Ag-NP dispersions was investigated. All dispersions showed bimodal size distribution. Individual PEOX stabilized Ag-NPs had sizes less than 10 nm. The size of agglomerates depended on the PEOX molar mass and was larger than 50 nm. The agglomerates consisted of individual Ag-NPs in the mesh of bridging PEOX chains. The kinetic stability of the dispersions also depended on PEOX molar mass. Ag-NP dispersions without any PEOX were not stable. PEOX-5K and PEOX-50K stabilized Ag-NP dispersions were stable for 6 days, while PEOX-200K and PEOX-500K stabilized dispersions were stable up to 2 weeks. The amount of Ag-NPs in the dispersion increased with PEOX concentration. Decreasing pH from 9.9 in as-prepared dispersion to 4.5 increased the individual Ag-NP size slightly from 10 nm to 16 nm. All PEOX stabilized dispersions were stable up to 8 days at pH < 9.9. PEOX(Ag-NP)s were used as H-accepting component and TA as H-donating component in the preparation of LbL films. The growth profiles and the pH-induced disintegration of the multilayers were similar to those of PEOX/TA multilayers. All multilayers showed linear growth profiles and were stable up to pH 8.5. SEM and AFM images of the multilayers showed homogeneous distribution of Ag-NPs in the films. The loading capacity of Ag-NPs in the multilayers was determined by XPS to be 1-3%. The understanding of the effect of different molar mass PEOX in stabilizing Ag-NPs in dispersions and in forming uniform Ag-NP loaded pH-responsive LbL films are important in controlling the size and loading capacity of Ag-NPs in multilayers. Ag-NP loaded LbL films of PEOX and TA have potential biomedical applications as antibacterial coatings which can release Ag+ ions from stable multilayers below the critical pH and disintegrate by releasing all Ag-NPs at the critical pH.