Abstract
A general method for assembling patterned interfaces of uniform, flexible mesoporous iron oxide nanopyramid islands (NPIs) is presented. The three-dimensional (3D) mesoporous iron oxide–NPI interfaces possess a unique mesostructure that features a large surface area (~158 m2 g−1), a large pore size (~18 nm) and excellent flexibility (can be folded 100 times). Furthermore, the 3D mesoporous Au–NPI interfaces allow efficient immobilization of cytochrome c (Cyt c; more than 165-fold increase) and a significant enhancement of localized surface plasmon resonance (~26-fold at 625 nm) compared with that of two-dimensional (2D) planar iron oxide films without nanopores. More importantly, the ultrasensitive integrated interfaces demonstrate over 1000-fold enhancement of the photocurrent variation on the 3D mesostructures based on the switchable direct electrochemistry of Cyt c. The strategy of interfacial assembly offers new possibilities for the chemical design of patterned mesoporous semiconductors with high flexibility and tailored photocatalytic characteristics. This investigation provides a novel paradigm for an unconventional 3D porous biointerface that can be used for sub-nanomolar level recognition of biomolecules (~0.2 nM for H2O2) and suggests the new concept of large-surface-area 3D mesostructure–protein interfaces as a step toward using direct electrochemistry for biomedical applications. Flexible lab-on-a-chip devices containing iron oxide nanopyramids may kick start new approaches to sub-nanomolar detection of biomolecules. A team of Chinese and Australian researchers led by Dongyuan Zhao from Fudan University used lithographic techniques to pattern surfaces with pointy iron oxide nanopyramids and gold nanoparticles and then loaded the protein cytochrome c (Cyt c). The three-dimensional, mesoporous cavities inside the nanopyramids boosted both cell attachment onto the device and the surface plasmon signals used for biosensing. When the team exposed the biointerface to solar light to initiate the Cyt c redox cycle, they saw a 1,000-fold enhancement in the photoelectrochemical current over typical interfaces. The magnitude of such solar-driven currents makes the device extremely sensitive — the researchers foresee direct monitoring of electrochemical transfer between proteins for real-time disease diagnostics. A general method for assembling patterned interfaces of uniform, flexible mesoporous iron oxide nanopyramid islands is presented. The 3D porous interfaces possess a unique mesostructure that features a large surface area, a large pore size and excellent flexibility. Furthermore, the 3D porous Au–NPI interfaces allow efficient immobilization of cytochrome c and a significant enhancement of localized surface plasmon resonance. More importantly, the ultrasensitive integrated interfaces demonstrate over 1000-fold enhancement of the photocurrent variation on the 3D mesostructures based on the switchable direct electrochemistry cytochrome c. The strategy of interfacial assembly offers new possibilities for the chemical design of patterned mesoporous semiconductors with high flexibility and tailored photocatalytic characteristics.
Highlights
The 3D patterned Ti foils with SiO2 patterns (Ti-SiO2) film template with a uniform square space for the growth of Prussian blue (PB) was obtained by electron beam physical vapor deposition of a thin layer of SiO2 (~50 nm) on the Ti foils (Supplementary Figure 2)
High-resolution transmission electron microscopy images show that PB nanopyramid islands (NPIs) with patterned arrays are solid and crystalline (Supplementary Figures 5b and c)
When subjected to an interface-constrained thermal pyrolysis at 400 °C, the color of the 3D PB NPI films changes from blue to iron red (Supplementary Figure 6a)
Summary
Nanodevice interfaces functionalized with various signal-responsive molecules have been designed to provide these devices with tunable properties that make them suitable for diverse applications.[1,2,3,4]External signals of different types (for example, optical, electrical, magnetic, mechanical and chemical/biochemical inputs5–9) are applied to reversibly activate nanodevice interfaces upon demand.[10,11,12,13]From these studies, molecular–semiconductor hetero-interfaces (such as protein–TiO2,14,15 DNA–quantum dots,[16,17] organic molecule–silicon,[18] redox molecule–nanowires[19] and photoactive molecule–nanowires20) have been applied to adjust photoelectrochemical (PEC) activities by enhancing the efficiency of PEC conversion.[21,22,23,24] In PEC-based bioanalysis (for example, DNA analysis,[25,26,27] immunoassays[28,29,30,31] and enzymatic sensing32), PEC-enzymatic sensing has been the focus of considerable research because of the facile and inexpensive fabrication of appropriate interfaces and, more importantly, their high sensitivities and specificities.[33,34]In addition, third-generation PEC enzyme biosensors based on direct electrochemistry have been shown to be capable of direct electron transfer between their active sites and electrodes.[33,35,36] A number of enzymes/redox proteins, such as horseradish peroxidase,[37] hemoglobin,[38] microperoxidase[39] and glucose oxidase,[40,41] have been integrated into enzyme-based, third-generation biosensors. External signals of different types (for example, optical, electrical, magnetic, mechanical and chemical/biochemical inputs5–9) are applied to reversibly activate nanodevice interfaces upon demand.[10,11,12,13] From these studies, molecular–semiconductor hetero-interfaces (such as protein–TiO2,14,15 DNA–quantum dots,[16,17] organic molecule–silicon,[18] redox molecule–nanowires[19] and photoactive molecule–nanowires20) have been applied to adjust photoelectrochemical (PEC) activities by enhancing the efficiency of PEC conversion.[21,22,23,24] In PEC-based bioanalysis (for example, DNA analysis,[25,26,27] immunoassays[28,29,30,31] and enzymatic sensing32), PEC-. Solar-driven interface interactions between living cells and semiconductors are still inefficient because of the limited number of active sites on conventional 2D structures.[15,33] Second, direct electron transfer between redox proteins and semiconductors is typically prevented by the thickness of the protein layer and is permitted only at interfaces of a few layers or a single-layer of Received 14 March 2015; revised 1 May 2015; accepted 25 May 2015 proteins.[30,38] Passing this bottleneck requires being able to monitor cells by using three-dimensional (3D) micro-structures, as well as ultra-high interaction surfaces/interfaces for direct electron transfer between proteins and semiconductors, and ultrasensitive methods to capture trace signals from cellular activities.[42,43,44]
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