Review: 3-Aminopropyltriethoxysilane (APTES) Deposition Methods on Oxide Surfaces in Solution and Vapor Phases for Biosensing Applications

Surface functionalization and bioreceptor immobilization are critical processes in developing a highly sensitive and selective biosensor. The silanization process with 3-aminopropyltriethoxysilane (APTES) on oxide surfaces is frequently used for surface functionalization because of beneficial characteristics such as its bifunctional nature and low cost. Optimizing the deposition process of the APTES layer to obtain a monolayer is crucial to having a stable surface and effectively immobilizing the bioreceptors, which leads to the improved repeatability and sensitivity of the biosensor. This review provides an overview of APTES deposition methods, categorized into the solution-phase and vapor-phase, and a comprehensive summary and guide for creating stable APTES monolayers on oxide surfaces for biosensing applications. A brief explanation of APTES is introduced, and the APTES deposition methods with their pre/post-treatments and characterization results are discussed. Lastly, APTES deposition methods on nanoparticles used for biosensors are briefly described.

Recent advances in metal-based nanoporous materials for sensing environmentally-related biomolecules

Highly sensitive, stable, selective, efficient, and short reaction time sensors play a substantial role in daily life/industry and are the need of the day. Due to the rising environmental issues, nanoporous carbon and metal-based materials have attracted significant attention in environmental analysis owing to their intriguing and multifunctional properties and cost-effective and rapid detection of different analytes by sensing applications. Environmental-related issues such as pollution have been a significant threat to the world. Therefore, it is necessary to fabricate highly promising performance-based sensor materials with excellent reliability, selectivity and good sensitivity for monitoring various analytes. In this regard, different methods have been employed to fabricate these sensors comprising metal, metal oxides, metal oxide carbon composites and MOFs leading to the formation of nanoporous metal and carbon composites. These composites have exceptional properties such as large surface area, distinctive porosity, and high conductivity, making them promising candidates for several versatile sensing applications. This review covers recent advances and significant studies in the sensing field of various nanoporous metal and carbon composites. Key challenges and future opportunities in this exciting field are also part of this review.

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.

Pushing the limits of nanopore transport performance by polymer functionalization

Inspired by the design and performance of biological pores, polymer functionalization of nanopores has emerged as an evolving field to advance transport performance within the last few years. This feature article outlines developments in nanopore functionalization and the resulting transport performance including gating based on electrostatic interaction, wettability and ligand binding, gradual transport controlled by polymerization as well as functionalization-based asymmetric nanopore and nanoporous material design going towards the transport direction. Pushing the limits of nanopore transport performance and thus reducing the performance gap between biological and technological pores is strongly related to advances in polymerization chemistry and their translation into nanopore functionalization. Thereby, the effect of the spatial confinement has to be considered for polymer functionalization as well as for transport regulation, and mechanistic understanding is strongly increased by combining experiment and theory. A full mechanistic understanding together with highly precise nanopore structure design and polymer functionalization is not only expected to improve existing application of nanoporous materials but also opens the door to new technologies. The latter might include out of equilibrium devices, ionic circuits, or machine learning based sensors.

High-Throughput Methods in the Discovery and Study of Biomaterials and Materiobiology

The complex interaction of cells with biomaterials (i.e., materiobiology) plays an increasingly pivotal role in the development of novel implants, biomedical devices, and tissue engineering scaffolds to treat diseases, aid in the restoration of bodily functions, construct healthy tissues, or regenerate diseased ones. However, the conventional approaches are incapable of screening the huge amount of potential material parameter combinations to identify the optimal cell responses and involve a combination of serendipity and many series of trial-and-error experiments. For advanced tissue engineering and regenerative medicine, highly efficient and complex bioanalysis platforms are expected to explore the complex interaction of cells with biomaterials using combinatorial approaches that offer desired complex microenvironments during healing, development, and homeostasis. In this review, we first introduce materiobiology and its high-throughput screening (HTS). Then we present an in-depth of the recent progress of 2D/3D HTS platforms (i.e., gradient and microarray) in the principle, preparation, screening for materiobiology, and combination with other advanced technologies. The Compendium for Biomaterial Transcriptomics and high content imaging, computational simulations, and their translation toward commercial and clinical uses are highlighted. In the final section, current challenges and future perspectives are discussed. High-throughput experimentation within the field of materiobiology enables the elucidation of the relationships between biomaterial properties and biological behavior and thereby serves as a potential tool for accelerating the development of high-performance biomaterials.

Simultaneous Nanolocal Polymer and In Situ Readout Unit Placement in Mesoporous Separation Layers

Bioinspired solid-state nanopores and nanochannels have attracted interest in the last two decades, as they are envisioned to advance future sensing, energy conversion, and separation concepts. Although much effort has been made regarding functionalization of these materials, multifunctionality and accurate positioning of functionalities with nanoscale precision still remain challenging. However, this precision is necessary to meet transport performance and complexity of natural pores in living systems, which are often based on nonequilibrium states and compartmentalization. In this work, a nanolocal functionalization and simultaneous localized sensing strategy inside a filtering mesoporous film using precisely placed plasmonic metal nanoparticles inside mesoporous films with pore accessibility control is demonstrated. A single layer of gold nanoparticles is incorporated into mesoporous thin films with precise spatial control along the nanoscale layer thickness. The local surface plasmon resonance is applied to induce a photopolymerization leading to a nanoscopic polymer shell around the particles and thus nanolocal polymer placement inside the mesoporous material. As near-field modes are sensitive to the dielectric properties of their surrounding, the in situ sensing capability is demonstrated using UV-vis spectroscopy. It is demonstrated that the sensing sensitivity only slightly decreases upon functionalization. The presented nanolocal placement of responsive functional polymers into nanopores offers a simultaneous filtering and nanoscopic readout function. Such a nanoscale local control is envisioned to have a strong impact onto the development of new transport and sensor concepts, especially as the system can be developed into higher complexity using different metal nanoparticles and additional design of mesoporous film filtering properties.

A Stable and Indurative Superhydrophobic Film with Excellent Anti-Bioadhesive Performance for 6061 Al Protection

Superhydrophobic surfaces have attracted intensive attention in the antifouling field because of their excellent anti-bioadhesive performance and environmental friendliness. However, promising surfaces have met great challenges of poor mechanical robustness under harsh serving conditions. Herein, an organic-inorganic composite strategy, that the silane-modified TiO2 nanoparticles are compounded into the porous framework provided by the stable and indurative aluminum oxide film, is proposed to address the common serious problem in superhydrophobic surfaces. Different from the traditional superhydrophobic surfaces, this composite film possesses a ~18 μm thick layer which can provide strong support to silane-modified TiO2 nanoparticles. The resulting film can reserve superhydrophobicity to the surface even after a thickness loss of ~15 μm under continuous abrasion. At the same time, the results of the bacterial adhesive tests also verify that the film has the same long-term anti-bioadhesive performance. The film with superhydrophobicity, excellent anti-bioadhesive property, and stable robustness will make it a promising candidate for serving in a harsh environment, and the design concept of this film could be applied to various substrates.

The toolbox of porous anodic aluminum oxide–based nanocomposites: from preparation to application

Anodic aluminum oxide (AAO) templates have been intensively investigated during the past decades and have meanwhile been widely applied through both sacrificial and non-sacrificial pathways. In numerous non-sacrificial applications, the AAO membrane is maintained as part of the obtained composite materials; hence, the template structure and topography determine to a great extent the potential applications. Through-hole isotropic AAO features nanochannels that promote transfer of matter, while anisotropic AAO with barrier layer exhibits nanocavities suitable as independent and homogenous containers. By combining the two kinds of AAO membranes with diverse organic and inorganic materials through physical interactions or chemical bonds, AAO composites are designed and applied in versatile fields such as catalysis, drug release platform, separation membrane, optical appliances, sensors, cell culture, energy, and electronic devices. Therefore, within this review, a perspective on exhilarating prospect for complementary advancement on AAO composites both in preparation and application is provided.

Wetting-Controlled Localized Placement of Surface Functionalities within Nanopores

In the context of sensing and transport control, nanopores play an essential role. Designing multifunctional nanopores and placing multiple surface functionalities with nanoscale precision remains challenging. Interface effects together with a combination of different materials are used to obtain local multifunctionalization of nanoscale pores within a model pore system prepared by colloidal templating. Silica inverse colloidal monolayers are first functionalized with a gold layer to create a hybrid porous architecture with two distinct gold nanostructures on the top surface as well as at the pore bottom. Using orthogonal silane- and thiol-based chemistry together with a control of the wetting state allows individual addressing of the different locations within each pore resulting in nanoscale localized functional placement of three different functional units. Ring-opening metathesis polymerization is used for inner silica-pore wall functionalization. The hydrophobized pores create a Cassie-Baxter wetting state with aqueous solutions of thiols, which enables an exclusive functionalization of the outer gold structures. In a third step, an ethanolic solution able to wet the pores is used to self-assemble a thiol-containing initiator at the pore bottom. Subsequent controlled radical polymerization provides functionalization of the pore bottom. It is demonstrated that the combination of orthogonal surface chemistry and controlled wetting states can be used for the localized functionalization of porous materials.

Regulating Ion Transport in a Nanochannel with Tandem and Parallel Structures via Concentration Polarization

The unique phenomena of ion selectivity and ion current rectification (ICR) in nanofluidics have been widely used to construct bioinspired channels and organs, sensors, and power generators. However, the excellent performance of a single nanochannel does not show a linear increase when it is scaled up into multiple nanochannels in tandem and parallel structure, and in some cases, it even shows a reverse trend. Understanding of this scaling-up inconsistency in nanofluidics is essential to the design of functional devices. Here, we provide a method for investigating the ion transport properties in multiple nanochannels in tandem and parallel connections. We find that interfacial resistance caused by ion concentration polarization (ICP) in tandem and parallel nanochannels has a significant impact on ICR, showing a nonlinear scaling-up feature with the tandem number and a decreased trend with the parallel number, which is not expected in electronic devices. We further verify that it is feasible to regulate ion transport in tandem and parallel nanochannels by adding gap distances between nanochannels in tandem and parallel structures to decouple the ICP region between nanochannels. This study provides fundamental insights into the ion transport properties in nanofluidic circuits, which hold promise for the design of high-performance nanofluidic devices in the fields of separation, energy, and sensors.

Utilizing Infrared Spectroscopy to Analyze the Interfacial Structures of Ionic Liquids/Al₂O₃ and Ionic Liquids/Mica Mixtures under High Pressures

The interfacial interactions between ionic liquids (1,3-dimethylimidazolium methyl sulfate and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate) and solid surfaces (mesoporous aluminum oxide and mica) have been studied by infrared spectroscopy at high pressures (up to 2.5 GPa). Under ambient pressure, the spectroscopic features of pure ionic liquids and mixtures of ionic liquids/solid particles (Al₂O₃ and mica) are similar. As the pressure is increased, the cooperative effect in the local structure of pure 1,3-dimethylimidazolium methyl sulfate becomes significantly enhanced as the imidazolium C–H absorptions of the ionic liquid are red-shifted. However, this pressure-enhanced effect is reduced by adding the solid particles (Al₂O₃ and mica) to 1,3-dimethylimidazolium methyl sulfate. Although high-pressure IR can detect the interactions between 1,3-dimethylimidazolium methyl sulfate and particle surfaces, the difference in the interfacial interactions in the mixtures of Al₂O₃ and mica is not clear. By changing the type of ionic liquid to 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, the interfacial interactions become more sensitive to the type of solid surfaces. The mica particles in the mixture perturb the local structure of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate under high pressures, forcing 1-ethyl-3-methylimidazolium trifluoromethanesulfonate to form into an isolated structure. For Al₂O₃, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate tends to form an associated structure under high pressures.

Spatially Controlled Surface Modification of Porous Silicon for Sustained Drug Delivery Applications

A new and facile approach to selectively functionalize the internal and external surfaces of porous silicon (pSi) for drug delivery applications is reported. To provide a surface that is suitable for sustained drug release of the hydrophobic cancer chemotherapy drug camptothecin (CPT), the internal surfaces of pSi films were first modified with 1-dodecene. To further modify the external surface of the pSi samples, an interlayer was applied by silanization with (3-aminopropyl)triethoxysilane (APTES) following air plasma treatment. In addition, copolymers of N-(2-hydroxypropyl) acrylamide (HPAm) and N-benzophenone acrylamide (BPAm) were grafted onto the external pSi surfaces by spin-coating and UV crosslinking. Each modification step was verified using attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, water contact angle (WCA) measurements, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). In order to confirm that the air plasma treatment and silanization step only occurred on the top surface of pSi samples, confocal microscopy was employed after fluorescein isothiocyanate (FITC) conjugation. Drug release studies carried out over 17 h in PBS demonstrated that the modified pSi reservoirs released CPT continuously, while showing excellent stability. Furthermore, protein adsorption and cell attachment studies demonstrated the ability of the graft polymer layer to reduce both significantly. In combination with the biocompatible pSi substrate material, the facile modification strategy described in this study provides access to new multifunctional drug delivery systems (DDS) for applications in cancer therapy.

Differential functionalisation of the internal and external surfaces of carbon-stabilised nanoporous silicon

A versatile strategy to differentiate the surface chemistry of the internal and external pore walls of highly-stable nanoporous silicon.

Integrated Solid-State Nanopore Electrochemistry Array for Sensitive, Specific, and Label-Free Biodetection

Nanopore ionic current measurement is currently a prevailing readout and offers considerable opportunities for bioassays. Extending conventional electrochemistry to nanoscale space, albeit noteworthy, remains challenging. Here, we report a versatile electrochemistry array established on a nanofluidic platform by controllably depositing gold layers on the two outer sides of anodic aluminum oxide (AAO) nanopores, leading to form an electrochemical microdevice capable of performing amperometry in a label-free manner. Electroactive species ferricyanide ions passing through gold-decorated nanopores act as electrochemical indicator to generate electrolytic current signal. The electroactive species flux that dominates current signal response is closely related to the nanopore permeability. Such well-characteristic electrolytic current-species flux correlation lays a premise for quantitative electrochemical analysis. As a proof-of-concept demonstration, we preliminarily verify the analytical utility by detection of nucleic acid and protein at picomolar concentration levels. Universal surface modification and molecule assembly, specific target recognition and reliable signal output in nanopore enable direct electrochemical detection of biomolecules without the need of cumbersome probe labeling and signal amplification.

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.

Engineering of Surface Chemistry for Enhanced Sensitivity in Nanoporous Interferometric Sensing Platforms

We explore new approaches to engineering the surface chemistry of interferometric sensing platforms based on nanoporous anodic alumina (NAA) and reflectometric interference spectroscopy (RIfS). Two surface engineering strategies are presented, namely (i) selective chemical functionalization of the inner surface of NAA pores with amine-terminated thiol molecules and (ii) selective chemical functionalization of the top surface of NAA with dithiol molecules. The strong molecular interaction of Au³⁺ ions with thiol-containing functional molecules of alkane chain or peptide character provides a model sensing system with which to assess the sensitivity of these NAA platforms by both molecular feature and surface engineering. Changes in the effective optical thickness of the functionalized NAA photonic films (i.e., sensing principle), in response to gold ions, are monitored in real-time by RIfS. 6-Amino-1-hexanethiol (inner surface) and 1,6-hexanedithiol (top surface), the most sensitive functional molecules from approaches i and ii, respectively, were combined into a third sensing strategy whereby the NAA platforms are functionalized on both the top and inner surfaces concurrently. Engineering of the surface according to this approach resulted in an additive enhancement in sensitivity of up to 5-fold compared to previously reported systems. This study advances the rational engineering of surface chemistry for interferometric sensing on nanoporous platforms with potential applications for real-time monitoring of multiple analytes in dynamic environments.

Assessment of Binding Affinity between Drugs and Human Serum Albumin Using Nanoporous Anodic Alumina Photonic Crystals

In this study, we report an innovative approach aiming to assess the binding affinity between drug molecules and human serum albumin by combining nanoporous anodic alumina rugate filters (NAA-RFs) modified with human serum albumin (HSA) and reflectometric interference spectroscopy (RIfS). NAA-RFs are photonic crystal structures produced by sinusoidal pulse anodization of aluminum that present two characteristic optical parameters, the characteristic reflection peak (λPeak), and the effective optical thickness of the film (OTeff), which can be readily used as sensing parameters. A design of experiments strategy and an ANOVA analysis are used to establish the effect of the anodization parameters (i.e., anodization period and anodization offset) on the sensitivity of HSA-modified NAA-RFs toward indomethacin, a model drug. To this end, two sensing parameters are used, that is, shifts in the characteristic reflection peak (ΔλPeak) and changes in the effective optical thickness of the film (ΔOTeff). Subsequently, optimized NAA-RFs are used as sensing platforms to determine the binding affinity between a set of drugs (i.e., indomethacin, coumarin, sulfadymethoxine, warfarin, and salicylic acid) and HSA molecules. Our results verify that the combination of HSA-modified NAA-RFs with RIfS can be used as a portable, low-cost, and simple system for establishing the binding affinity between drugs and plasma proteins, which is a critical factor to develop efficient medicines for treating a broad range of diseases and medical conditions.

Realisation and advanced engineering of true optical rugate filters based on nanoporous anodic alumina by sinusoidal pulse anodisation

This study is the first realisation of true optical rugate filters (RFs) based on nanoporous anodic alumina (NAA) by sinusoidal waves. An innovative and rationally designed sinusoidal pulse anodisation (SPA) approach in galvanostatic mode is used with the aim of engineering the effective medium of NAA in a sinusoidal fashion. A precise control over the different anodisation parameters (i.e. anodisation period, anodisation amplitude, anodisation offset, number of pulses, anodisation temperature and pore widening time) makes it possible to engineer the characteristic reflection peaks and interferometric colours of NAA-RFs, which can be finely tuned across the UV-visible-NIR spectrum. The effect of the aforementioned anodisation parameters on the photonic properties of NAA-RFs (i.e. characteristic reflection peaks and interferometric colours) is systematically assessed in order to establish for the first time a comprehensive rationale towards NAA-RFs with fully controllable photonic properties. The experimental results are correlated with a theoretical model (Looyenga-Landau-Lifshitz - LLL), demonstrating that the effective medium of these photonic nanostructures can be precisely described by the effective medium approximation. NAA-RFs are also demonstrated as chemically selective photonic platforms combined with reflectometric interference spectroscopy (RIfS). The resulting optical sensing system is used to assess the reversible binding affinity between a model drug (i.e. indomethacin) and human serum albumin (HSA) in real-time. Our results demonstrate that this system can be used to determine the overall pharmacokinetic profile of drugs, which is a critical aspect to be considered for the implementation of efficient medical therapies.

Biomimetic Nanoporous Anodic Alumina Distributed Bragg Reflectors in the Form of Films and Microsized Particles for Sensing Applications

In this study, we produce for the first time biomimetic films and microsized particles based on nanoporous anodic alumina distributed Bragg reflectors (NAA-DBRs) by a rational galvanostatic pulse-anodization approach. These biomimetic photonic structures can feature a broad range of vivid bright colors, which can be tuned across the UV-visible spectrum by engineering their nanoporous structure through different anodization parameters. The effective medium of NAA-DBRs films is systematically assessed as a function of the anodization period, the anodization temperature, and the current density ratio by reflectometric interference spectroscopy (RIfS). This analysis makes it possible to establish the most sensitive structure toward changes in its effective medium. Subsequently, specific detection of vitamin C molecules is demonstrated. The obtained results reveal that NAA-DBRs with optimized structure can achieve a low limit of detection for vitamin C molecules as low as 20 nM, a sensitivity of 227±4 nm μM(-1), and a linearity of 0.9985. Finally, as proof of concept, we developed a new photonic nanomaterial based on NAA-DBR microsized particles, which could provide new opportunities to produce microsized photonic analytical tools.

The impact of surface chemistry on the performance of localized solar-driven evaporation system

This report investigates the influence of surface chemistry (or wettability) on the evaporation performance of free-standing double-layered thin film on the surface of water. Such newly developed evaporation system is composed of top plasmonic light-to-heat conversion layer and bottom porous supporting layer. Under solar light illumination, the induced plasmonic heat will be localized within the film. By modulating the wettability of such evaporation system through the control of surface chemistry, the evaporation rates are differentiated between hydrophilized and hydrophobized anodic aluminum oxide membrane-based double layered thin films. Additionally, this work demonstrated that the evaporation rate mainly depends on the wettability of bottom supporting layer rather than that of top light-to-heat conversion layer. The findings in this study not only elucidate the role of surface chemistry of each layer of such double-layered evaporation system, but also provide additional design guidelines for such localized evaporation system in applications including desalination, distillation and power generation.

Label-Free real-time quantification of enzyme levels by interferometric spectroscopy combined with gelatin-modified nanoporous anodic alumina photonic films

Herein, we present an interferometric sensor based on the combination of chemically functionalized nanoporous anodic alumina photonic films (NAA-PFs) and reflectometric interference spectroscopy (RIfS) aimed to detect trace levels of enzymes by selective digestion of gelatin. The fabrication and sensing performance of the proposed sensor were characterized in real-time by estimating the changes in effective optical thickness (i.e., sensing principle) of gelatin-modified NAA-PFs (i.e., sensing element) during enzymatic digestion. The working range (WR), sensitivity (S), low limit of detection (LLoD), and linearity (R(2)) of this enzymatic sensor were established by a series of experiments with different concentrations of gelatin (i.e., specific chemical sensing element) and trypsin (i.e., analyte), a model protease enzyme with relevant implications as a biomarker in the diagnosis of several diseases. The chemical selectivity of the sensor was demonstrated by comparison of gelatin digestion by other nonspecific enzyme models such as chymotrypsin and horseradish peroxidase. Furthermore, the role of the chemical sensing element (i.e., gelatin) was assessed by using hemoglobin instead of gelatin. Finally, we demonstrated that this sensor can be readily used to establish the kinetic parameters of enzymatic reactions. The obtained results revealed that the presented sensor has a promising potential to be used as a point-of-care system for fast detection of gastrointestinal diseases at early stages.

Rational Design of Photonic Dust from Nanoporous Anodic Alumina Films: A Versatile Photonic Nanotool for Visual Sensing

Herein, we present a systematic study on the development, optimisation and applicability of interferometrically coloured distributed Bragg reflectors based on nanoporous anodic alumina (NAA-DBRs) in the form of films and nanoporous microparticles as visual/colorimetric analytical tools. Firstly, we synthesise a complete palette of NAA-DBRs by galvanostatic pulse anodisation approach, in which the current density is altered in a periodic fashion in order to engineer the effective medium of the resulting photonic films in depth. NAA-DBR photonic films feature vivid colours that can be tuned across the UV-visible-NIR spectrum by structural engineering. Secondly, the effective medium of the resulting photonic films is assessed systematically by visual analysis and reflectometric interference spectroscopy (RIfS) in order to establish the most optimal nanoporous platforms to develop visual/colorimetric tools. Then, we demonstrate the applicability of NAA-DBR photonic films as a chemically selective sensing platform for visual detection of mercury(II) ions. Finally, we generate a new nanomaterial, so-called photonic dust, by breaking down NAA-DBRs films into nanoporous microparticles. The resulting microparticles (μP-NAA-DBRs) display vivid colours and are sensitive towards changes in their effective medium, opening new opportunities for developing advanced photonic nanotools for a broad range of applications.

Interferometric nanoporous anodic alumina photonic coatings for optical sensing

Herein, we present a systematic study on the development, optical optimization and sensing applicability of colored photonic coatings based on nanoporous anodic alumina films grown on aluminum substrates. These optical nanostructures, so-called distributed Bragg reflectors (NAA-DBRs), are fabricated by galvanostatic pulse anodization process, in which the current density is altered in a periodic manner in order to engineer the effective medium of the resulting photonic coatings. As-prepared NAA-DBR photonic coatings present brilliant interference colors on the surface of aluminum, which can be tuned at will within the UV-visible spectrum by means of the anodization profile. A broad library of NAA-DBR colors is produced by means of different anodization profiles. Then, the effective medium of these NAA-DBR photonic coatings is systematically assessed in terms of optical sensitivity, low limit of detection and linearity by reflectometric interference spectroscopy (RIfS) in order to optimize their nanoporous structure toward optical sensors with enhanced sensing performance. Finally, we demonstrate the applicability of these photonic nanostructures as optical platforms by selectively detecting gold(iii) ions in aqueous solutions. The obtained results reveal that optimized NAA-DBR photonic coatings can achieve an outstanding sensing performance for gold(iii) ions, with a sensitivity of 22.16 nm μM(-1), a low limit of detection of 0.156 μM (i.e. 30.7 ppb) and excellent linearity within the working range (0.9983).

Effect of pore diameter in nanoporous anodic alumina optical biosensors

This study demonstrates how it is possible to tune the sensitivity of nanoporous anodic alumina optical biosensors by adjusting pore diameter.

Quantitative detection of potassium ions and adenosine triphosphate via a nanochannel-based electrochemical platform coupled with G-quadruplex aptamers

The development of synthetic nanopores and nanochannels that mimick ion channels in living organisms for biosensing applications has been, and still remains, a great challenge. Although the biological applications of nanopores and nanochannels have achieved considerable development as a result of nanotechnology advancements, there are few reports of a facile way to realize those applications. Herein, a nanochannel-based electrochemical platform was developed for the quantitative detection of biorelated small molecules such as potassium ions (K(+)) and adenosine triphosphate (ATP) in a facile way. For this purpose, K(+) or ATP G-quadruplex aptamers were covalently assembled onto the inner wall of porous anodic alumina (PAA) nanochannels through a Schiff reaction between -CHO groups in the aptamer and amino groups on the inner wall of the PAA nanochannels under mild reaction conditions. Conformational switching of the aptamers confined in the nanochannels occurs in the presence of the target molecules, resulting in increased steric hindrance in the nanochannels. Changes in steric hindrance in the nanochannels were monitored by the anodic current of indicator molecules transported through the nanochannels. As a result, quantitative detection of K(+) and ATP was realized with a concentration ranging from 0.005 to 1.0 mM for K(+) and 0.05 to 10.0 mM for ATP. The proposed platform displayed significant selectivity, good reproducibility, and universality. Moreover, this platform showed its potential for use in the detection of other aptamer-based analytes, which could promote its development for use in biological detection and clinical diagnosis.

Nanoporous anodic alumina rugate filters for sensing of ionic mercury: toward environmental point-of-analysis systems

Herein, we present an ultrasensitive, cost-competitive, and portable optical sensing system for detecting ionic mercury in environmental water. This analytical system combines structurally engineered and chemically modified nanoporous anodic alumina rugate filters (NAA-RFs) with reflection spectroscopy (RfS). The sensing performance of the proposed system is assessed through several tests, establishing its sensing performance (i.e., linear working range from 1 to 100 μM of Hg(2+), low limit of detection 1 μM of Hg(2+) ions (i.e., 200 ppb), and sensitivity of 0.072 nm μM(-1)), chemical selectivity (i.e., exposure to different metal ions Co(2+), Mg(2+), Ni(2+), Cu(2+), Pb(2+), Fe(3+), Ca(2+), Cr(6+), and Ag(+)) and metal ions binding mechanism (i.e., fitting to Langmuir and Freundlich isotherm models). Furthermore, the detection of Hg(2+) ions in tap and environmental water (River Torrens) is successfully carried out, demonstrating the suitability of this system for developing environmental point-of-analysis systems.

Nanoporous anodic alumina platforms: engineered surface chemistry and structure for optical sensing applications

Electrochemical anodization of pure aluminum enables the growth of highly ordered nanoporous anodic alumina (NAA) structures. This has made NAA one of the most popular nanomaterials with applications including molecular separation, catalysis, photonics, optoelectronics, sensing, drug delivery, and template synthesis. Over the past decades, the ability to engineer the structure and surface chemistry of NAA and its optical properties has led to the establishment of distinctive photonic structures that can be explored for developing low-cost, portable, rapid-response and highly sensitive sensing devices in combination with surface plasmon resonance (SPR) and reflective interference spectroscopy (RIfS) techniques. This review article highlights the recent advances on fabrication, surface modification and structural engineering of NAA and its application and performance as a platform for SPR- and RIfS-based sensing and biosensing devices.

Nanoporous Anodic Alumina: A Versatile Platform for Optical Biosensors

Nanoporous anodic alumina (NAA) has become one of the most promising nanomaterials in optical biosensing as a result of its unique physical and chemical properties. Many studies have demonstrated the outstanding capabilities of NAA for developing optical biosensors in combination with different optical techniques. These results reveal that NAA is a promising alternative to other widely explored nanoporous platforms, such as porous silicon. This review is aimed at reporting on the recent advances and current stage of development of NAA-based optical biosensing devices. The different optical detection techniques, principles and concepts are described in detail along with relevant examples of optical biosensing devices using NAA sensing platforms. Furthermore, we summarise the performance of these devices and provide a future perspective on this promising research field.

Overlimiting current and shock electrodialysis in porous media

Most electrochemical processes, such as electrodialysis, are limited by diffusion, but in porous media, surface conduction and electroosmotic flow also contribute to ionic flux. In this article, we report experimental evidence for surface-driven overlimiting current (faster than diffusion) and deionization shocks (propagating salt removal) in a porous medium. The apparatus consists of a silica glass frit (1 mm thick with a 500 nm mean pore size) in an aqueous electrolyte (CuSO4 or AgNO3) passing ionic current from a reservoir to a cation-selective membrane (Nafion). The current-voltage relation of the whole system is consistent with a proposed theory based on the electroosmotic flow mechanism over a broad range of reservoir salt concentrations (0.1 mM to 1.0 M) after accounting for (Cu) electrode polarization and pH-regulated silica charge. Above the limiting current, deionized water (≈10 μM) can be continuously extracted from the frit, which implies the existence of a stable shock propagating against the flow, bordering a depleted region that extends more than 0.5 mm across the outlet. The results suggest the feasibility of shock electrodialysis as a new approach to water desalination and other electrochemical separations.

Low temperature thermal dependent Filgrastim adsorption behavior detected with ToF-SIMS

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) detected changes in Filgrastim (granulocyte colony stimulating growth factor, G-CSF) adsorption behavior at a solid interface when exposed to temperatures as low as 35 °C, i.e., before thermal denaturation, was detected by circular dichroism (CD) or dynamic light scattering (DLS). Biopharmaceuticals rely on maintaining sufficient conformation to impart correct biological function in vivo. Stability of such molecules is critical during synthesis, storage, transport, and administration. CD analysis indicated loss of structure at temperatures greater than ~60 °C, while DLS detected aggregation at ~42 °C. Furthermore, we demonstrate the nature of G-CSF interaction with a surface was altered rapidly and at relatively low temperatures. Specifically, after 10 min thermal treatment, changes in adsorption behavior occurred at 35 °C indicated by principal component analysis of spectra as primarily due to increasing yields of methionine fragments. This was likely to be due to either altering the preferential protein orientation upon adsorption or greater denaturation exposing the hydrophobic core. This investigation demonstrates the sensitivity of ToF-SIMS in studying biopharmaceutical adsorption and conformational change and can assist with studies into promoting their stability.

Gold-coated ordered nanoporous anodic alumina bilayers for future label-free interferometric biosensors

A cost-effective label-free optical biosensor based on gold-coated self-ordered nanoporous anodic alumina bilayers is presented. The structure is formed by two uniform nanoporous layers of different porosity (i.e., a top layer with large pores and a bottom layer with smaller pores). Each layer presents uniform pore size, regular pore distribution, and regular diameter along its pore length. To increase and improve the output sensing signals, a thin gold layer on the top surface was deposited. The gold layer increases the refractive index contrast between the nanoporous alumina layer and the analytical aqueous solution, and it results in a greater contrast in the interferometric spectrum and a higher sensitivity of the structure. From this structurally engineered architecture, the resulting reflectivity spectrum shows a complex series of Fabry-Pérot interference fringes, which was analyzed by the reflective interferometric Fourier transform spectroscopy (RIFTS) method. To determine the performance of this structure for biosensing applications, we tested bovine serum albumin (BSA) as the target protein. The results show a significant enhancement of the RIFTS peak intensity and position when a gold layer is on the top surface.

In situ determination of the pore opening point during wet-chemical etching of the barrier layer of porous anodic aluminum oxide: nonuniform impurity distribution in anodic oxide

Wet-chemical etching of the barrier oxide layer of anodic aluminum oxide (AAO) was systematically investigated by using scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS), and a newly devised experimental setup that allows accurate in situ determination of the pore opening point during chemical etching of the barrier oxide layer. We found that opening of the barrier oxide layer by wet-chemical etching can be significantly influenced by anodization time (tanodi). According to secondary ion mass spectrometry (SIMS) analysis, porous anodic aluminum oxide (AAO) samples formed by long-term anodization contained a lower level of anionic impurity in the barrier oxide layer compared to the short-term anodized one and consequently exhibited retarded opening of the barrier oxide layer during the wet-chemical etching. The observed compositional dependence on the anodization time (tanodi) in the barrier oxide layer is attributed to the progressive decrease of the electrolyte concentration upon anodization. The etching rate of the outer pore wall at the bottom part is lower than that of the one at the top part due to the lower level of impurity content in that region. This indicates that a concentration gradient of anionic impurity in the outer pore wall oxide may be established along both the vertical and radial directions of cylindrical pores. Apart from the effect of electrolyte concentration on the chemical composition of the barrier oxide layer, significantly decreased current density arising from the lowered concentration of electrolyte during the long-term anodization (~120 h) was found to cause disordering of pores. The results of the present work are expected to provide viable information not only for practical applications of nanoporous AAO in nanotechnology but also for thorough understanding of the self-organized formation of oxide nanopores during anodization.

A comparison of washing methods for hair mineral analysis: internal versus external effects

A major difficulty in hair elemental (mineral) analysis for biomonitoring is adequate understanding of the effectiveness of washing procedures. A review of washing protocols used in hair analysis publications showed little consensus with regard to solvents and surfactants used, washing times, and number of washing stages. Two washing approaches were subsequently used to compare their influence on internal and external surface elemental signals determined with time-of-flight secondary ion mass spectrometry. Na, K, Ca, Mg, and Fe were assessed with regard to their relative signal compared to carbon. Both washing methods had similar effect. All elements except for Fe appear to be removed from the surface of the hair as well as from inside the hair. Only the internal Fe content changed with washing and could indicate that external surface bound Fe may not be removed with most washing procedures. It is shown that washing procedures can have a significant effect on reducing the internal elemental signal levels in hair.

Local drug delivery to the bone by drug-releasing implants: perspectives of nano-engineered titania nanotube arrays

Titania nanotube (TNT) arrays fabricated by electrochemical anodization of titanium are currently one of the most attractive nanomaterials due to their remarkable properties. In this review, we highlight recent research activities that are focused on the application of the TNT arrays for local drug delivery, specifically for addressing problems associated with orthopedic implants. The advantages of drug-releasing implants based on TNT arrays for local delivery of therapeutics in bone related to these challenging problems including inflammation, infection and osseointegration are discussed. An overview of recent research to advance the drug-releasing performance of TNT arrays and the potential of their future applications and development are presented.

Photoluminescent enzymatic sensor based on nanoporous anodic alumina

Herein, we present a smart enzymatic sensor based on nanoporous anodic alumina (NAA) and its photoluminescence (PL) in the UV-visible range. The as-produced structure of NAA is functionalized and activated in order to perform the enzyme immobilization in a controlled manner. The whole process is monitored through the PL spectrum and each stage is characterized by an exclusive barcode, which is associated with the PL oscillations. This characteristic property allows us to calculate the change in the effective optical thickness that takes place after each stage. This makes it possible to accurately detect and quantify the immobilized enzyme within the NAA structure. Finally, the NAA geometry (i.e., the pore length and its diameter) is optimized to improve the enzyme immobilization and its detection inside the pores. This enzymatic sensor can give quick and accurate measurements of enzyme levels, what is crucial in clinical enzymology to prevent and detect diseases at their primary stage.

Label-free reflectometric interference microchip biosensor based on nanoporous alumina for detection of circulating tumour cells

In this report, a label-free reflectometric interference spectroscopy (RIfS) based microchip biosensor for the detection of circulating tumour cells (CTCs) is demonstrated. Highly ordered nanoporous anodic aluminium oxide (AAO) fabricated by electrochemical anodization of aluminium foil was used as the RIfS sensing platform. Biotinylated anti-EpCAM antibody that specifically binds to human cancer cells of epithelial origin such as pancreatic cancer cells (PANC-1) was covalently attached to the AAO surface through multiple surface functionalization steps. Whole blood or phosphate buffer saline spiked with low numbers of pancreatic cancer cells were successfully detected by specially designed microfluidic device incorporating an AAO RIfS sensor, without labour intensive fluorescence labelling and/or pre-enhancement process. Our results show that the developed device is capable of selectively detecting of cancer cells, within a concentrations range of 1000-100,000 cells/mL, with a detection limit of <1000 cells/mL, a response time of <5 min and sample volume of 50 μL of. The presented RIfS method shows considerable promise for translation to a rapid and cost-effective point-of-care diagnostic device for the detection of CTCs in patients with metastatic cancer.

A nanoporous interferometric micro-sensor for biomedical detection of volatile sulphur compounds

This work presents the use of nanoporous anodic aluminium oxide [AAO] for reflective interferometric sensing of volatile sulphur compounds and hydrogen sulphide [H2S] gas. Detection is based on changes of the interference signal from AAO porous layer as a result of specific adsorption of gas molecules with sulphur functional groups on a gold-coated surface. A nanoporous AAO sensing platform with optimised pore diameters (30 nm) and length (4 µm) was fabricated using a two-step anodization process in 0.3 M oxalic, followed by coating with a thin gold film (8 nm). The AAO is assembled in a specially designed microfluidic chip supported with a miniature fibre optic system that is able to measure changes of reflective interference signal (Fabry-Perrot fringes). When the sensor is exposed to a small concentration of H2S gas, the interference signal showed a concentration-dependent wavelength shifting of the Fabry-Perot interference fringe spectrum, as a result of the adsorption of H2S molecules on the Au surface and changes in the refractive index of the AAO. A practical biomedical application of reflectometric interference spectroscopy [RIfS] Au-AAO sensor for malodour measurement was successfully shown. The RIfS method based on a nanoporous AAO platform is simple, easy to miniaturise, inexpensive and has great potential for development of gas sensing devices for a range of medical and environmental applications.

Where bio meets nano: the many uses for nanoporous aluminum oxide in biotechnology

Porous aluminum oxide (PAO) is a ceramic formed by an anodization process of pure aluminum that enables the controllable assembly of exceptionally dense and regular nanopores in a planar membrane. As a consequence, PAO has a high porosity, nanopores with high aspect ratio, biocompatibility and the potential for high sensitivity imaging and diverse surface modifications. These properties have made this unusual material attractive to a disparate set of applications. This review examines how the structure and properties of PAO connect with its present and potential uses within research and biotechnology. The role of PAO is covered in areas including microbiology, mammalian cell culture, sensitive detection methods, microarrays and other molecular assays, and in creating new nanostructures with further uses within biology.

Hair analysis as a biomonitor for toxicology, disease and health status

Hair analysis receives a large amount of academic and commercial interest for wide-ranging applications. However, in many instances, especially for elemental or 'mineral' analysis, the degree of success of analytical interpretation has been quite minimal with respect to the extent of such endeavors. In this critical review we address the questions surrounding hair analysis with specific intent of discovering what hair concentrations can actually relate to in a biogenic sense. This is done from a chemistry perspective to explain why and how elements are incorporated into hair and their meaning. This includes an overview of variables attributed to altering hair concentrations, such as age, gender, melanin content, and other less reported factors. Hair elemental concentrations are reviewed with regard to morbidity, with specific examples of disease related effects summarized. The application of hair analysis for epidemiology and etiology studies is enforced. A section is dedicated specifically to the area of population studies with regards to mercury, which highlights how endogenous and exogenous incorporation relies on species dependant metabolism and metabolic products. Many of the considerations are relevant to other areas of interest in hair analysis, such as for drug and isotopic analysis. Inclusion of a table of elemental concentrations in hair should act as a valuable reference (298 references).