Imprinting of metal receptors into multilayer polyelectrolyte films: fabrication and applications in marine antifouling

Abstract

Polymeric films constructed using the layer-by-layer (LbL) fabrication process were employed as a platform for metal ion immobilization and applied as a marine antifouling coating. The novel Cu²⁺ ion imprinting process described is based on the use of metal ion templates and LbL multilayer covalent cross-linking. Custom synthesized, peptide mimicking polycations composed of histidine grafted poly(allylamine) (PAH) to bind metal ions, and methyl ester containing polyanions for convenient cross-linking were used in the fabrication process. Two methods of LbL film formation have been investigated using alternate polyelectrolyte deposition namely non-imprinted LbL_A, and imprinted LbL_B. Both LbL films were cross linked at mild temperature to yield covalent bridging of the layers for improved stability in a sea water environment. A comparative study of the non-imprinted LbL_A films and imprinted LbL_B films for Cu²⁺ ion binding capacity, leaching rate and stability of the films was performed. The results reveal that the imprinted films possess enhanced affinity to retain metal ions due to the preorganization of imidazole bearing histidine receptors. As a result the binding capacity of the films for Cu²⁺ could be improved by seven fold. Antifouling properties of the resulting materials in a marine environment have been demonstrated against the settlement of barnacle larvae, indicating that controlled release of Cu ions was achieved.

Scheme 1. The synthesis of PIAMA-Me.

Scheme 2. The synthesis of PAH-His.

Fig. 1. (A) UV-VIS absorption spectra of PAH-His coordinated with Cu. (B) Example of absorption spectra fitted with three different signals for 2.3 mL of PAH-His (6 mM) added to the 2 mL of Cu(NO₃)₂ (15 mM). (C) Integration of the signal intensity at 620 nm, 770 nm and 820 nm, fitting data to the binding model.

Fig. 2. Schematic illustration of PIAMA-Me, PAH-His and PAH-His(Cu).

Fig. 3. LbL film formation by different processing methods, (non-imprinted LbL_A refers to assembly of PIAMA-Me and PAH-His, and imprinted LbL_B refers to assembly of PIAMA-Me and PAH-His(Cu)). Cross linking of the films as employed followed by loading and releasing of copper within the LbL films. Schematic representation of the polymer PIAMA-Me, PAH-His and PAH-His(Cu) are shown in Fig. 2.

Fig. 4. FTIR spectra of non-imprinted LbL_A film before and after cross linking.

Fig. 5. Thickness data of non-imprinted LbL_A (A) and imprinted LbL_B (B) films from ellipsometry and the AFM scratch test.

Fig. 6. Frequency changes monitored by QCM-D for (A) non-imprinted LbL_A (7 bi-layers) and (B) imprinted LbL_B (10 bi-layers) films.

Fig. 7. (A) Saturation isotherms monitored by QCM-D for Cu²⁺ loading in non-imprinted LbL_A and imprinted LbL_B films. The surface proved fully saturable and followed normal Langmuir-like adsorption behavior. ΔF represents the frequency change (Hz). (B) Adsorption isotherms of non-imprinted LbL_A and imprinted LbL_B films for Cu²⁺ ions fitted by the Langmuir model. [M] denotes the molar concentration of Cu²⁺.

Fig. 8. Cu 2p XPS spectra of the LbL film before and after loading of copper in the (A) non-imprinted LbL_A films and (B) imprinted LbL_B films.

Fig. 9. Cu 2p XPS spectra of the non-imprinted LbL_A (A) and imprinted LbL_B films (B) against sea salts.

Fig. 10. Cyprid settlement and toxicity of imprinted and non-imprinted LbL films with copper loading. Each value is the mean of 8 replicate measurements. The error bars here are standard deviations.