Self-assembled bifunctional surface mimics an enzymatic and templating protein for the synthesis of a metal oxide semiconductor

Abstract

The recent discovery and characterization of silicatein, a mineral-synthesizing enzyme that assembles to form the filamentous organic core of the glassy skeletal elements (spicules) of a marine sponge, has led to the development of new low-temperature synthetic routes to metastable semiconducting metal oxides. These protein filaments were shown in vitro to catalyze the hydrolysis and structurally direct the polycondensation of metal oxides at neutral pH and low temperature. Based on the confirmation of the catalytic mechanism and the essential participation of specific serine and histidine residues (presenting a nucleophilic hydroxyl and a nucleophilicity-enhancing hydrogen-bonding imidazole nitrogen) in silicatein's catalytic active site, we therefore sought to develop a synthetic mimic that provides both catalysis and the surface determinants necessary to template and structurally direct heterogeneous nucleation through condensation. Using lithographically patterned poly(dimethylsiloxane) stamps, bifunctional self-assembled monolayer surfaces containing the essential catalytic and templating elements were fabricated by using alkane thiols microcontact-printed on gold substrates. The interface between chemically distinct self-assembled monolayer domains provided the necessary juxtaposition of nucleophilic (hydroxyl) and hydrogen-bonding (imidazole) agents to catalyze the hydrolysis of a gallium oxide precursor and template the condensed product to form gallium oxohydroxide (GaOOH) and the defect spinel, gamma-gallium oxide (gamma-Ga(2)O(3)). Using this approach, the production of patterned substrates for catalytic synthesis and templating of semiconductors for device applications can be envisioned.

Fig. 1.

Catalytic site of serine-containing hydrolase enzymes, including silicatein. (A) Schematic of the essential chemical moieties in a serine-hydrolase active site. The proximity of the nitrogen from the imidazole ring to the hydroxyl of serine facilitates hydrogen-bonding; this enhances the nucleophilicity of the oxygen, thus potentiating catalytic hydrolysis reactions. Weakening of the O–H bond is indicated by the dashed line. (B) Ribbon model of silicatein α from an energy minimization program (insight ii). The ribbon model depicted here highlights (in green) the catalytic site in which the nucleophilic serine is presented to a hydrogen-bonding imidazole that enhances the hydrolytic activity of the enzyme (6).

Fig. 2.

Optical micrograph of the bifunctional SAM surface exposed to water vapor. Water droplets are observed condensing on the hydrophilic surface whereas none are observed on the hydrophobic surface. (Scale bar: 50 μm.)

Fig. 3.

Products of wafer-catalyzed and templated reaction. (A) Scanning electron microscopy images depicting products formed from the hydrolysis and condensation of the gallium nitrate precursor catalyzed by the bifunctional wafer containing the nucleophilic (hydroxyl) and hydrogen-bonding (imidazole) termini (NP-HB surface). (Scale bar: 10 μm.) A 10-fold greater particle number density is observed on the hydroxyl lines than on the imidazole lines. (B) Energy-dispersive spectrometry mapping of condensate on the NP-HB biomimetic catalyst revealing the product, localized on the hydroxyl lines, rich in gallium and oxygen. (C) Higher-magnification imaging reveals a dense network of layered particles condensed upon hydroxyl-printed lines. (Scale bar: 1 μm.) (D) Substitution of either essential surface functionality (nucleophile or hydrogen-bonding amine) with a nonactive methyl group renders the surface hydrolytically inactive. (Scale bar: 10 μm.) (E) Particles from hydroxyl-terminated SAMs from C. (Scale bar: 50 nm.) (F) Sample removed after a short reaction time demonstrates significantly more condensed product at the SAM interface, with a substantial decrease in particle number density away from the interface. (Scale bar: 1 μm.)

Fig. 4.

TEM images showing γ-Ga₂O₃ particles formed on the hydroxyl-terminated SAMs (A) and GaOOH particles formed on the imidazole-terminated surface (B). (Scale bars: 100 nm.) (C) Selected-area electron diffraction pattern of A confirming the γ-Ga₂O₃ structure. (D) High-resolution TEM image of A demonstrating that the larger particle is made of smaller nanocrystals that are co-aligned in the core of the particle but not aligned at the periphery. (Scale bar: 10 nm.)

Fig. 5.

Schematic depicting the formation of a bifunctional SAM. (A) A poly(dimethylsiloxane) (PDMS) stamp inked with an alkane thiolate (e.g., OH-terminated) is brought into contact with a gold surface, facilitating transfer of the thiol to the gold through adsorption with sulfur (23). (B) After the initial printing of the first alkane thiol, the wafer is then immersed in a different alkane thiol (e.g., imidazole-terminated) solution to form the second monolayer resulting in a bifunctionalized SAM surface (C) that presents the essential functionalities necessary for hydrolysis (D), similar to those found in silicatein.