Nanoporous Anodic Alumina Surface Modification by Electrostatic, Covalent, and Immune Complexation Binding Investigated by Capillary Filling

Solid-state nanopore: chemical modifications, interactions, and functionalities

Nanopore technology is a burgeoning detection technology for single-molecular sensing and ion rectification. Solid-state nanopores have attracted more and more attention because of their higher stability and tunability than biological nanopores. However, solid-state nanopores still suffer the drawbacks of low signal-to-noise ratio and low resolution, which hinders their practical applications. Thus, developing operatical and useful methods to overcome the shortages of solid-state nanopores is urgently needed. Here, we summarize the recent research on nanopore modification to achieve this goal. Modifying solid-state nanopores with different coating molecules can improve the selectivity, sensitivity, and stability of nanopores. The modified molecules can introduce different functions into the nanopores, greatly expanding the applications of this novel detection technology. We hope that this review of nanopore modification will provide new ideas for this field.

Recent advance in solid state nanopores modification and characterization

Nanopore, due to its advantages of modifiable, controllability and sensitivity, has made a splash in recent years in the fields of biomolecular sequencing, small molecule detection, salt differential power generation, and biomimetic ion channels, etc. In these applications, the role of chemical or biological modification is indispensable. Compared with small molecules, the modification of polymers is more difficult and the methods are more diverse. Choosing appropriate modification method directly determines the success or not of the research, therefore, it is necessary to summarize the polymer modification methods toward nanopores. In addition, it is also important to provide clear and convincing evidence that the nanopore modification is successful, the corresponding characterization methods are also indispensable. Therefore, this review will summarize the methods of polymer modification of nanopores and efficient characterization methods. And we hope that this review will provide some reference value for like-minded researchers.

Fabrication and application of nanoporous anodic aluminum oxide: a review

Due to the unique optical and electrochemical properties, large surface area, tunable properties, and high thermal stability, nanoporous anodic aluminum oxide (AAO) has become one of the most popular materials with a large potential to develop emerging applications in numerous areas, including biosensors, desalination, high-risk pollutants detection, capacitors, solar cell devices, photonic crystals, template-assisted fabrication of nanostructures, and so on. This review covers the mechanism of AAO formation, manufacturing technology, the relationship between the properties of AAO and fabrication conditions, and applications of AAO. Properties of AAO, like pore diameter, interpore distance, wall thickness, and anodized aluminum layer thickness, can be fully controlled by fabrication conditions, including electrolyte, applied voltage, anodizing and widening time. Generally speaking, the pore diameter of AAO will affect its specific application to a large extent. Moreover, manufacturing technology like one/two/multi step anodization, nanoimprint lithography anodization, and pulse/cyclic anodization also have a major impact on overall array arrangement. The review aims to provide a perspective overview of the relationship between applications and their corresponding AAO pore sizes, systematically. And the review also focuses on the strategies by which the structures and functions of AAO can be utilized.

Recent Advances in Nanoporous Anodic Alumina: Principles, Engineering, and Applications

The development of aluminum anodization technology features many stages. With the story stretching for almost a century, rather straightforward-from current perspective-technology, raised into an iconic nanofabrication technique. The intrinsic properties of alumina porous structures constitute the vast utility in distinct fields. Nanoporous anodic alumina can be a starting point for: Templates, photonic structures, membranes, drug delivery platforms or nanoparticles, and more. Current state of the art would not be possible without decades of consecutive findings, during which, step by step, the technique was more understood. This review aims at providing an update regarding recent discoveries-improvements in the fabrication technology, a deeper understanding of the process, and a practical application of the material-providing a narrative supported with a proper background.

Magnetic nanoparticle decorated anodic alumina nanotubes for fluorescent detection of cathepsin B

In this work, we present the process to provide anodic alumina nanotubes with magnetic responsivity based on magnetic nanoparticles. We demonstrate the possibility to cause the motion of these composite nanotubes under magnetic field, providing them with guided mobility. The obtained magnetic anodic alumina nanotubes are completely characterized and their potential to undergo selective and effective functionalization, and stimuli-responsive load release is demonstrated. For this purpose, protease-triggered release of fluorescent molecules loaded inside the magnetic anodic alumina nanotubes (MAANTs) by selective functionalization is performed. The inner walls of the MAANTs were selectively covered with protein padding of albumin-fluorescein isothiocyanate conjugate (FITC-BSA) through means of silanization. Protein functionalization was designed to undergo proteolytic hydrolysis in presence of cathepsin B- protease highly expressed during growth and initial stages of tumor metastasis - in order to cleave peptide bond of albumin and release fluorescent fragments of the protein. Proteolytic reaction with the enzyme is performed under acidic conditions. Presented arrangement is an exemplary combination of functionalities - which are vast - and of value for applications like drug delivery and biosensing applications.

Advances in Optical Biosensors and Sensors Using Nanoporous Anodic Alumina

This review paper focuses on recent progress in optical biosensors using self-ordered nanoporous anodic alumina. We present the fabrication of self-ordered nanoporous anodic alumina, surface functionalization, and optical sensor applications. We show that self-ordered nanoporous anodic alumina has good potential for use in the fabrication of antibody-based (immunosensor), aptamer-based (aptasensor), gene-based (genosensor), peptide-based, and enzyme-based optical biosensors. The fabricated optical biosensors presented high sensitivity and selectivity. In addition, we also showed that the performance of the biosensors and the self-ordered nanoporous anodic alumina can be used for assessing biomolecules, heavy ions, and gas molecules.

Remote biosensor for the determination of trypsin by using nanoporous anodic alumina as a three-dimensional nanostructured material

The determination of trypsin in the human real sample is a routine medical investigation to assess the pancreatic disease. Herein, we fabricated an interferometric reflectance spectroscopy based biosensor for the determination trypsin. For this purpose, urease and fluorescein 5(6)-isothiocyanate (FLITC) were immobilized on the nanoporous anodic alumina (NAA). The operation principle of the proposed biosensor is based on the change in the pH of the solution during the reaction of urease and urea and therefore change in the light-absorbing ability of FLITC in the presence of trypsin. The reaction of the urease enzyme with urea increased the pH of the solution because of producing ammonia. This increase in the pH of solution increased the light-absorbing ability of the immobilized FLITC on NAA and therefore the intensity of the reflected light from the NAA to the charge-coupled device detector decreased. In the presence of trypsin, the catalytic activity of immobilized urease on NAA decreased. This decrease in the activity of urease enzyme consequent on the decrease in the amount of the generated ammonia. Therefore, the immobilized FLITC on the NAA did not absorb more light and consciously, the intensity of the light reflected light into the detector increased. The proposed biosensor exhibited a good response to the concentration of trypsin in the range of 0.25–20 μg.mL⁻¹ with the limit of detection of 0.06 μg.mL⁻¹.

Aptamer-Based Nanoporous Anodic Alumina Interferometric Biosensor for Real-Time Thrombin Detection

Aptamer biosensors are one of the most powerful techniques in biosensing. Achieving the best platform to use in aptamer biosensors typically includes crucial chemical modifications that enable aptamer immobilization on the surface in the most efficient manner. These chemical modifications must be well defined. In this work we propose nanoporous anodic alumina (NAA) chemically modified with streptavidin as a platform for aptamer immobilization. The immobilization of biotinylated thrombin binding aptamer (TBA) was monitored in real time by means of reflective interferometric spectroscopy (RIfS). The study has permitted to characterize in real time the path to immobilize TBA on the inner pore walls of NAA. Furthermore, this study provides an accurate label-free method to detect thrombin in real-time with high affinity and specificity.

Environmental Copper Sensor Based on Polyethylenimine-Functionalized Nanoporous Anodic Alumina Interferometers

Anthropogenic copper pollution of environmental waters from sources such as acid mine drainage, antifouling paints, and industrial waste discharge is a major threat to our environment and human health. This study presents an optical sensing system that combines self-assembled glutaraldehyde-cross-linked double-layered polyethylenimine (PEI-GA-PEI)-modified nanoporous anodic alumina (NAA) interferometers with reflectometric interference spectroscopy (RIfS) for label-free, selective monitoring of ionic copper in environmental waters. Calibration of the sensing system with analytical solutions of copper shows a linear working range between 1 and 100 mg L-1, and a low limit of detection of 0.007 ± 0.001 mg L-1 (i.e., ∼0.007 ppm). Changes in the effective optical thickness (ΔOTeff) of PEI-GA-PEI-functionalized NAA interferometers are monitored in real-time by RIfS, and correlated with the amount of ionic copper present in aqueous solutions. The system performance is validated through X-ray photoelectron spectroscopy (XPS) and the spatial distribution of copper within the nanoporous films is characterized by time-of-flight-secondary ion mass spectroscopy (TOF-SIMS). The specificity and chemical selectivity of the PEI-GA-PEI-NAA sensor to Cu2+ ions is verified by screening six different metal ion solutions containing potentially interfering ions such as Al3+, Cd2+, Fe3+, Pb2+, Ni2+, and Zn2+. Finally, the performance of the PEI-GA-PEI-NAA sensor for real-life applications is demonstrated using legacy acid mine drainage liquid and tap water for qualitative and quantitative detection of copper ions. This study provides new opportunities to develop portable, cost-competitive, and ultrasensitive sensing systems for real-life environmental applications.

Real-Time Monitoring of Biotinylated Molecules Detection Dynamics in Nanoporous Anodic Alumina for Bio-Sensing

The chemical modification, or functionalization, of the surfaces of nanomaterials is a key step to achieve biosensors with the best sensitivity and selectivity. The surface modification of biosensors usually comprises several modification steps that have to be optimized. Real-time monitoring of all the reactions taking place during such modification steps can be a highly helpful tool for optimization. In this work, we propose nanoporous anodic alumina (NAA) functionalized with the streptavidin-biotin complex as a platform towards label-free biosensors. Using reflective interferometric spectroscopy (RIfS), the streptavidin-biotin complex formation, using biotinylated thrombin as a molecule model, was monitored in real-time. The study compared the performance of different NAA pore sizes in order to achieve the highest response. Furthermore, the optimal streptavidin concentration that enabled the efficient detection of the biotinylated thrombin attachment was estimated. Finally, the ability of the NAA-RIfS system to quantify the concentration of biotinylated thrombin was evaluated. This study provides an optimized characterization method to monitor the chemical reactions that take place during the biotinylated molecules attachment within the NAA pores.

Stacked Nanoporous Anodic Alumina Gradient-Index Filters with Tunable Multispectral Photonic Stopbands as Sensing Platforms

This study presents the development and optical engineering of stacked nanoporous anodic alumina gradient-index (NAA-GIFs) filters with tunable multispectral photonic stopbands for sensing applications. The structure of these photonic crystals (PC) is formed by stacked layers of NAA produced with sinusoidally modified effective medium. The progressive modification of the sinusoidal period during the anodization process enables the generation and precise tuning of the characteristic photonic stopbands (PSB) (i.e., one per sinusoidal period in the anodization profile) of these PC structures. Four types of NAA-GIFs featuring three distinctive PSBs positioned within the visible spectral region are developed. The sensitivity of the effective medium of these NAA-GIFs is systematically assessed by measuring spectral shifts in the characteristic PSBs upon infiltration of their nanoporous structure with analytical solutions of d-glucose with several concentrations (0.025-1 M). This study provides new insights into the intrinsic relationship between the nanoporous architecture of these PCs and their optical properties, generating opportunities to fabricate advanced optical sensing systems for high-throughput and multiplexed detection of analytes in a single sensing platform.

Nanoporous Anodic Alumina Photonic Crystals for Optical Chemo- and Biosensing: Fundamentals, Advances, and Perspectives

Optical sensors are a class of devices that enable the identification and/or quantification of analyte molecules across multiple fields and disciplines such as environmental protection, medical diagnosis, security, food technology, biotechnology, and animal welfare. Nanoporous photonic crystal (PC) structures provide excellent platforms to develop such systems for a plethora of applications since these engineered materials enable precise and versatile control of light⁻matter interactions at the nanoscale. Nanoporous PCs provide both high sensitivity to monitor in real-time molecular binding events and a nanoporous matrix for selective immobilization of molecules of interest over increased surface areas. Nanoporous anodic alumina (NAA), a nanomaterial long envisaged as a PC, is an outstanding platform material to develop optical sensing systems in combination with multiple photonic technologies. Nanoporous anodic alumina photonic crystals (NAA-PCs) provide a versatile nanoporous structure that can be engineered in a multidimensional fashion to create unique PC sensing platforms such as Fabry⁻Pérot interferometers, distributed Bragg reflectors, gradient-index filters, optical microcavities, and others. The effective medium of NAA-PCs undergoes changes upon interactions with analyte molecules. These changes modify the NAA-PCs' spectral fingerprints, which can be readily quantified to develop different sensing systems. This review introduces the fundamental development of NAA-PCs, compiling the most significant advances in the use of these optical materials for chemo- and biosensing applications, with a final prospective outlook about this exciting and dynamic field.