The hemispherical barrier oxide layer (BOL) closing the bottom tips of hexagonally distributed arrays of cylindrical nanochannels in nanoporous anodic alumina (NAA) membranes is structurally engineered by anodizing aluminum substrates in three distinct acid electrolytes at their corresponding self-ordering anodizing potentials. These nanochannels display a characteristic ionic current rectification (ICR) signal between high and low ionic conduction states, which is determined by the thickness and chemical composition of the BOL and the pH of the ionic electrolyte solution. The rectification efficiency of the ionic current associated with the flow of ions across the anodic BOL increases with its thickness, under optimal pH conditions. The inner surface of the nanopores in NAA membranes was chemically modified with thiol-terminated functional molecules. The resultant NAA-based iontronic system provides a model platform to selectively detect gold metal ions (Au3+) by harnessing dynamic ICR signal shifts as the core sensing principle. The sensitivity of the system is proportional to the thickness of the barrier oxide layer, where NAA membranes produced in phosphoric acid at 195 V with a BOL thickness of 232 ± 6 nm achieve the highest sensitivity and low limit of detection in the sub-picomolar range. This study provides exciting opportunities to engineer NAA structures with tailorable ICR signals for specific applications across iontronic sensing and other nanofluidic disciplines.
Porous biodegradable scaffolds have many applications in bioengineering, ranging from cell culture and transplantation, to support structures, to induce blood vessel and tissue formation in vivo. While numerous strategies have been developed for the manufacture of porous scaffolds, it remains challenging to control the spatial organization of the pores. In this study, we introduce the use of the granular convection effect, also known as the muesli or brazil nut effect, to rapidly engineer particulate templates with a vertical size gradient. These templates can then be used to prepare scaffolds with pore size gradients. To demonstrate this approach, we prepared templates with particle size gradients, which were then infused with a prepolymer solution consisting of the pentaerythritol ethoxylate (polyol), sebacoyl chloride (acid chloride), and poly(caprolactone). Following curing, the template was dissolved to yield biodegradable polyester‐ether scaffolds with pore size gradients that could be tuned depending on the size range of the particulates used. The application of these scaffolds was demonstrated using pancreatic islets, which were loaded via centrifugation and retained within the scaffold's pores without a decrease in viability. The proposed strategy provides a facile approach to prepare templates with spatially organized pores that could potentially be used for cell transplantation, or guided tissue formation.
Random lasing provides new opportunities to engineer cost-competitive, highly controllable, and integrable light sources for a broad range of photonic technologies such as sensing, hyperspectral imaging, high-resolution spectroscopic analysis, and photonic circuits. In this study, we engineer the self-organized structure of nanoporous anodic alumina (NAA) through the electrochemical oxidation of aluminum to generate a palette of model nanoporous platforms with tailored, hexagonally distributed, straight cylindrical nanopores. The inner surface of these platforms is functionalized with a model organic fluorophore via micellar solubilization of a surfactant. The resultant organic–inorganic composite structures provide model platforms to develop optically pumped solid-state random lasers with well-resolved, intense lasing bands. The effect of NAA’s geometric features on the random lasing characteristics of these model platforms is elucidated by precisely engineering its nanopore diameter, nanopore length, interpore distance, and ordering. Structural engineering of NAA makes it possible to tune and maximize random-lasing emissions, resulting in strong, polarized lasing at ∼628 nm characterized by a remarkably high-quality-gain product of ∼1433, a polarization quality of ∼0.9, and a lasing threshold of ∼0.87 mJ pulse–1.
Nanoporous anodic alumina (NAA) is an emerging platform material for photonics and light-based applications. However, demonstrations of narrow bandwidth lasing emissions from this optical material remain limited. Here, we demonstrate that narrow bandwidth NAA-based gradient-index filters (NAA-GIFs) can be optically engineered to achieve high-quality visible lasing. NAA-GIFs fabricated by a modified sinusoidal pulse anodization approach feature a well-resolved, intense, high-quality photonic stopband (PSB). The inner surface of NAA-GIFs is functionalized with rhodamine B (RhoB) fluorophore molecules through micellar solubilization of sodium dodecyl sulfate (SDS) surfactant. Systematic variation of the ratio of SDS and RhoB enables the precise engineering of the light-emitting functional layer to maximize light-driven lasing associated with the slow photon effect at the red edge of NAA-GIFs’ PSB. It is found that the optimal surfactant-to-fluorophore ratio, namely, 20 mM SDS to 0.81 mM RhoB, results in a strong, polarized lasing at ∼612 nm. This lasing was characterized by a remarkably high-quality–gain product of ∼536, a Purcell factor of 2.2, a lasing threshold of ∼0.15 mJ per pulse, and a high-quality polarization ratio of ∼0.7. Our results benefit the advancement of the NAA-based lasing technology for a variety of photonic disciplines such as sensing, tweezing, light harvesting, and photodetection.
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