Structural Engineering of the Barrier Oxide Layer of Nanoporous Anodic Alumina for Iontronic Sensing

Vol. 14 pp 21181-21197The 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.Juan Wang, Cheryl Suwen Law, Satyathiran Gunenthiran, Huong Nguyen Que Tran, Khoa Nhu Tran, Siew Yee Lim, Andrew D. Abell, and Abel Santo

Understanding the Intrinsic Rectification Properties of Nanoporous Anodic Alumina by Selective Chemical Etching

The distribution of oxygen and aluminum vacancies across the hemispherical barrier oxide layer (BOL) of nanoporous anodic alumina (NAA) relies intrinsically on the electric field-driven flow of electrolytic species and the incorporation of electrolyte impurities during the growth of anodic oxide through anodization. This phenomenon provides new opportunities to engineer BOL's inherited ionic current rectification (ICR) fingerprints. NAA's characteristic ICR signals are associated with the space charge density gradient across BOL and electric field-induced ion migration through hopping from vacancy to vacancy. In this study, we engineer the intrinsic space charge density gradient of the BOL of NAA under a range of anodizing potentials in hard and mild anodization regimes. Real-time characterization of the ICR fingerprints of NAA during selective etching of the BOL makes it possible to unravel the distribution pattern of vacancies through rectification signals as a function of etching direction and time. Our analysis demonstrates that the space charge density gradient varies across the BOL of NAA, where the magnitude and distribution of the space charge density gradient are revealed to be critically determined by anodizing the electrolyte, regime, and potential. This study provides a comprehensive understanding of the engineering of ion transport behavior across blind-hole NAA membranes by tuning the distribution of defects across BOL through anodization conditions. This method has the potential to be harnessed for developing nanofluidic devices with tailored ionic rectification properties for energy generation and storage and sensing applications.Juan Wang, Cheryl Suwen Law, Satyathiran Gunenthiran, Siew Yee Lim, Khanh Nhien Vu, Van Truc Ngo, Kornelius Nielsch, Andrew D. Abell, and Abel Santo

Lasing from Narrow Bandwidth Light-Emitting One-Dimensional Nanoporous Photonic Crystals

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 (NAAGIFs) 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.Satyathiran Gunenthiran, Juan Wang, Wanxia Zhao, Cheryl Suwen Law, Siew Yee Lim, Jamie A. McInnes, Heike Ebendorff-Heidepriem, Andrew D. Abell, Zeyad T. Alwahabi, and Abel Santo

Engineering of Solid-State Random Lasing in Nanoporous Anodic Alumina Photonic Crystals

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.Satyathiran Gunenthiran, Juan Wang, Huong Nguyen Que Tran, Khoa Nhu Tran, Siew Yee Lim, Cheryl Suwen Law, Andrew D. Abell, Zeyad T. Alwahabi, and Abel Santo

Real-time detection of per-fluoroalkyl substance (PFAS) self-assembled monolayers in nanoporous interferometers

Identification and quantification of per- and polyfluoroalkyl substances (PFASs) remain challenging due to their chemical diversity, and their inert optical and chemical nature. Here, we present an optical system integrating perfluorosilane-functionalized nanoporous anodic alumina (NAA) interferometers with reflectometric interference spectroscopy (RIfS) for real-time, label-free detection of self-assembled monolayers (SAMs) of perfluorooctanoic acid (PFOA) as a model PFAS. Measured changes in the effective optical thickness (ΔOT(eff)) of NAA interferometers made it possible to study the fluorous interaction-induced self-assembly of PFOA molecules with perfluorosilane functional molecules of varying length, in real time and in situ. Analysis of key sensing parameters—sensitivity, low limit of detection and linearity—allowed us to determine the most optimal molecular length of perfluorosilanes to maximize immobilization of PFOA onto functional surfaces. Freundlich and Langmuir isotherm models were adapted to experimentally acquired values of ΔOT(eff) to elucidate the mechanism of PFOA–perfluorosilane interactions. Interpretation of these models suggests that PFOA binds to perfluorosilanes functional groups immobilized onto the inner surface of NAA interferometers through a fluorous interaction-induced Freundlich mechanism. The potential real-life applicability of this system was demonstrated by detecting the formation of PFOA-based SAMs in aqueous matrices of varying complexity (i.e. ultrapure, deionized, tap, and river water). This study provides new insights into how functional surface chemistries can be engineered to maximize sensitivity and selectivity to PFAS, harnessing fluorous interactions—with implications for future deployable systems to detect and remove these emerging toxicants.Cheryl Suwen Law, Juan Wang, Satyathiran Gunenthiran, Siew Yee Lim, Andrew D. Abell, Lutz Ahrens, Tushar Kumeria, Abel Santos, Nicolas H. Voelcke