Detection of Volatile Organic Compounds through Spectroscopic Signatures in Nanoporous Fabry-Pérot Optical Microcavities.

Increasingly complex modern gas-monitoring scenarios necessitate advanced sensing capabilities to detect and identify a diverse range of gases under varying conditions. There is a rising demand for individual sensors with multiple responses capable of recognizing gases, identifying components in mixtures, and providing stable responses. Inspired by gas sensors employing multivariable response principles, we develop a nanoporous anodic alumina high-order microcavity (NAA-HOµCV) gas sensor with multiple optical outputs for discriminative gas detection. The NAA-HOµCV architecture, formed by a Fabry-Pérot microcavity with distributed Bragg reflector (DBR) mirrors and an extended-length microcavity layer supporting multiple resonant modes, serves as an effective solid-state fingerprint platform for distinguishing volatile organic compound (VOC) gases. Our research reveals that the coupling strength of light into resonant modes and their evolution depend on the thickness of the DBR mirrors and the dimension of the microcavity layer, which allows us to optimize the discriminative sensing capability of the NAA-HOµCV sensor through structural engineering of the microcavity and photonic crystal mirrors. Gas-sensing experiments conducted on the NAA-HOµCV sensor demonstrate real-time discrimination between physiosorbed VOC gases (isopropanol, ethanol, or acetone) in reversible gas sensing. It also achieves superior ppb-level sensing in irreversible gas sensing of model silane molecules. Our study presents promising avenues for designing compact, cost-effective, and highly efficient gas sensors with tailored properties for discriminative gas detection.

Engineering of Solid-State Nanoporous Laser through Dendrimer-Encapsulated Fluorophores.

Dendrimers─nanosized macromolecules that can function as hosts for encapsulation of guest molecules─provide new avenues to engineer gain media for lasing systems. In this context, this study investigates the interplay between the geometric features of a model porous scattering medium, nanoporous anodic alumina (NAA), and the chemical features of a model fluorophore-dendrimer encapsulation system to maximize random lasing. The inner surface of the NAA platforms is functionalized with fluorophore molecules encapsulated within dendrimers via an electrostatic interaction. The resulting solid-state composite structures emit well-resolved, intense random lasing when subjected to optical pumping. By engineering fluorophore-dendrimer and geometric features of scattering medium, we can precisely tune the characteristics of random lasing emissions. It is found that lasing structures with low porosity and thickness functionalized with fluorophore molecules encapsulated in second-generation dendrimers provide the best platforms for lasing generation, resulting in a strongly polarized laser at ∼594 nm that has a high quality-gain product of ∼1588 au, a polarization quality of ∼0.86, and a lasing threshold of ∼0.05 mJ pulse-1. Comparative analysis indicates that dendrimers achieve 2.5 times better random lasing than conventional surfactants due to improved encapsulation and minimization of photobleaching. Our results reveal the importance of the fluorophore encapsulation method and design of scattering media in the engineering of random lasing platforms for applications in optical and optoelectrical systems.

Tailoring Tamm Plasmon Resonances in Dielectric Nanoporous Photonic Crystals.

The fields of plasmonics and photonic crystals (PCs) have been combined to generate model light-confining Tamm plasmon (TMM) cavities. This approach effectively overcomes the intrinsic limit of diffraction faced by dielectric cavities and mitigates losses associated with the inherent properties of plasmonic materials. In this study, nanoporous anodic alumina PCs, produced by two-step sinusoidal pulse anodization, are used as a model dielectric platform to establish the methodology for tailoring light confinement through TMM resonances. These model dielectric mirrors feature highly organized nanopores and narrow bandwidth photonic stopbands (PSBs) across different positions of the spectrum. Different types of metallic films (gold, silver, and aluminum) were coated on the top of these model dielectric mirrors. By structuring the features of the plasmonic and photonic components of these hybrid structures, the characteristics of TMM resonances were studied to elucidate effective approaches to optimize the light-confining capability of this hybrid TMM model system. Our findings indicate that the coupling of photonic and plasmonic modes is maximized when the PSB of the model dielectric mirror is broad and located within the midvisible region. It was also found that thicker metal films enhance the quality of the confined light. Gas sensing experiments were performed on optimized TMM systems, and their sensitivity was assessed in real time to demonstrate their applicability. Ag films provide superior performance in achieving the highest sensitivity (S = 0.038 ± 0.001 nm ppm-1) based on specific binding interactions between thiol-containing molecules and metal films.

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.

Generation of Tamm Plasmon Resonances for Light Confinement Applications in Narrowband Gradient-Index Filters Based on Nanoporous Anodic Alumina.

Gold-coated gradient-index filters based on nanoporous anodic alumina (Au-coated NAA-GIFs) were used as model platforms to elucidate how Tamm plasmons can be tailored by engineering the geometric features of the plasmonic and photonic components of these hybrid structures. NAA-GIFs with well-resolved, intense photonic stopbands at two positions of the visible spectrum were fabricated through sinusoidal pulse anodization. These model photonic crystals were used to assess how the quality of Tamm plasmon resonances can be enhanced by tuning the features of the dielectric mirror and the thickness of the porous gold coating layer. It is found that the highest value of the quality factor of Tamm resonance (Q Tamm = 237) is obtained for 11 nm of gold on a dielectric mirror with low porosity corresponding to the resonant spectral position of λTamm of ∼698 nm. Our analysis indicates that Tamm resonances in as-produced Au-coated NAA-GIFs are weak due to the constrained range of wavelengths (narrow bands) at which these photonic crystal structures reflect light. However, after broadening of their photonic stopband upon pore widening, Tamm resonances become better resolved, with higher intensity. It is also observed that the quality of light confinement worsens progressively with the thickness of the porous gold coating layer after a critical value. In contrast to conventional surface plasmon resonance systems, this hybrid Tamm porous system does not require complex coupling systems and provides a nanoporous structure that can be readily tailored for a range of photonic technologies such as sensing and lasing.

A Miniaturized Device Coupled with Digital Image Correlation for Mechanical Testing.

Miniaturized mechanical testing based on small sample testing technology is a powerful technique to characterize the mechanical properties of different materials, and it is being used in different application fields. However, the small size of the specimens poses several challenges because the results are highly sensitive to measurement accuracy and the corresponding mechanical properties can change substantially due to the so-called specimen size effect. In this work, a novel testing device based on miniaturized specimens is presented. The equipment is designed to test materials in tensile and compressive loadings, but it is also capable of performing reverse-loading tests. Buckling of the specimen is an inherent phenomenon in compression loadings, especially for thin materials. Therefore, specimen geometry is properly studied and optimized to mitigate this effect. To evaluate the deformation of the specimen, the digital image correlation (DIC) technique is used to capture the full-field strain in the central gauge section of the sample. A sensitivity analysis of the DIC setting parameters was performed for this application. To evaluate the performance of the developed system, experimental results of monotonic tests and tests with reverse loadings (tension-compression) are presented, considering two high-strength steels (DP500 and DP780).

Analysis of ESAFORM 2021 cup drawing benchmark of an Al alloy, critical factors for accuracy and efficiency of FE simulations.

This article details the ESAFORM Benchmark 2021. The deep drawing cup of a 1 mm thick, AA 6016-T4 sheet with a strong cube texture was simulated by 11 teams relying on phenomenological or crystal plasticity approaches, using commercial or self-developed Finite Element (FE) codes, with solid, continuum or classical shell elements and different contact models. The material characterization (tensile tests, biaxial tensile tests, monotonic and reverse shear tests, EBSD measurements) and the cup forming steps were performed with care (redundancy of measurements). The Benchmark organizers identified some constitutive laws but each team could perform its own identification. The methodology to reach material data is systematically described as well as the final data set. The ability of the constitutive law and of the FE model to predict Lankford and yield stress in different directions is verified. Then, the simulation results such as the earing (number and average height and amplitude), the punch force evolution and thickness in the cup wall are evaluated and analysed. The CPU time, the manpower for each step as well as the required tests versus the final prediction accuracy of more than 20 FE simulations are commented. The article aims to guide students and engineers in their choice of a constitutive law (yield locus, hardening law or plasticity approach) and data set used in the identification, without neglecting the other FE features, such as software, explicit or implicit strategy, element type and contact model.

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

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.

Predicting Fitness Centre Dropout.

The phenomenon of dropout is often found among customers of sports services. In this study we intend to evaluate the performance of machine learning algorithms in predicting dropout using available data about their historic use of facilities. The data relating to a sample of 5209 members was taken from a Portuguese fitness centre and included the variables registration data, payments and frequency, age, sex, non-attendance days, amount billed, average weekly visits, total number of visits, visits hired per week, number of registration renewals, number of members referrals, total monthly registrations, and total member enrolment time, which may be indicative of members' commitment. Whilst the Gradient Boosting Classifier had the best performance in predicting dropout (sensitivity = 0.986), the Random Forest Classifier was the best at predicting non-dropout (specificity = 0.790); the overall performance of the Gradient Boosting Classifier was superior to the Random Forest Classifier (accuracy 0.955 against 0.920). The most relevant variables predicting dropout were "non-attendance days", "total length of stay", and "total amount billed". The use of decision trees provides information that can be readily acted upon to identify member profiles of those at risk of dropout, giving also guidelines for measures and policies to reduce it.

Spectral Engineering of Tamm Plasmon Resonances in Dielectric Nanoporous Photonic Crystal Sensors.

Model light-confining Tamm plasmon cavities based on gold-coated nanoporous anodic alumina photonic crystals (TMM-NAA-PCs) with spectrally tunable resonance bands were engineered. Laplacian and Lorentzian NAA-PCs produced by a modified Gaussian-like pulse anodization approach showed well-resolved, high-quality photonic stopbands, the position of which was precisely controlled across the visible spectrum by the periodicity in the input anodization profile. These PC structures were used as a platform material to develop highly reflective distributed Bragg mirrors, the top sides of which were coated with a thin gold film. The resulting nanoporous hybrid plasmonic-photonic crystals showed strong light-confining properties attributed to Tamm plasmon resonances at three specific positions of the visible spectrum. These structures achieved high sensitivity to changes in refractive index, with a sensitivity of ∼106 nm RIU-1. The optical sensitivity of TMM-NAA-PCs was assessed in real time, using a model chemically selective binding interaction between thiol-containing molecules and gold. The optical sensitivity was found to rely linearly on the spectral position of the Tamm resonance band, for both Laplacian and Lorentzian TMM-NAA-PCs. The density of self-assembled monolayers of thiol-containing analyte molecules formed on the surface of the metallic film directly contributes to the dependence of sensitivity on TMM resonance position in these optical transducers. Our findings provide opportunities to integrate TMM modes in NAA-based photonic crystal structures, with promising potential for optical technologies and applications requiring high-quality surface plasmon resonance bands.

Role of Spectral Resonance Features and Surface Chemistry in the Optical Sensitivity of Light-Confining Nanoporous Photonic Crystals.

Nanoporous anodic alumina optical microcavities (NAA-µQVs) with spectrally tunable resonance band and surface chemistry are used as model light-confining photonic crystal (PC) platforms to elucidate the combined effect of spectral light confinement features and surface chemistry on optical sensitivity. These model nanoporous PCs show well-resolved, spectrally tunable resonance bands (RBs), the central wavelength of which is engineered from ∼400 to 800 nm by the period of the input anodization profile. The optical sensitivity of the as-produced (hydrophilic) and dichlorodimethylsilane-functionalized (hydrophobic) NAA-µQVs is studied by monitoring dynamic spectral shifts of their RB upon infiltration with organic- and aqueous-based analytical solutions of equally varying refractive index, from 1.333 to 1.345 RIU. Our findings demonstrate that hydrophilic NAA-µQVs show ∼81 and 35% superior sensitivity to their hydrophobic counterparts for organic- and aqueous-based analytical solutions, respectively. Interestingly, the sensitivity of hydrophilic NAA-µQVs per unit of spectral shift is more than 3-fold higher in organic than in aqueous matrices upon equal change of refractive index, with values of 0.347 ± 0.002 and 0.109 ± 0.001 (nm RIU-1) nm-1, respectively. Conversely, hydrophobic NAA-µQVs are found to be slightly more sensitive toward changes of refractive index in aqueous medium, with sensitivities of 0.072 ± 0.002 and 0.066 ± 0.006 (nm RIU-1) nm-1 in water- and organic-based analytical solutions, respectively. Our advances provide insights into critical factors determining optical sensitivity in light-confining nanoporous PC structures, with implications across optical sensing applications, and other photonic technologies.

Developed Process Circuit Flowsheet of Al Amar Ore for Production of Nanocrystalline Ferrite and Improving Gold Recovery.

Al Amar gold ore is rich in sulfides of base metals and is commercially applied for the production of copper concentrate via floatation and gold bullion by cyanidation of tailing. The current process flowsheet suffers from low gold recovery (∼60%) and loss of metals in the hazardous stockpiled residue. This work addresses these drawbacks by a newly experimental redesign of the process circuit. The innovative flowsheet comprises a sequence of operations, including acid leaching of the roasted ore, gold recovery from the leach residue, and preparation of a valuable zinc-copper-lead ferrite from the filtrate by coprecipitation followed by heat treatment. The ore is roasted at 650 °C and then leached in 20% HCl, where most of Zn, Cu, Pb, and Fe contents are dissolved, while pristine gold remains in the residue. Most of the gold (∼93%) can be recovered by cyanidation of the acid leach residue. Stoichiometric ratios of dissolved Zn, Cu, Pb, and Fe in the acid leach solution can be kept at 0.6:0.3:0.1:2.0, respectively, only by adding a small amount of ferric chloride. These metals are coprecipitated at varying pH values from 8 to 10, and the produced powders are annealed at temperatures from 600 to 1100 °C. X-ray diffraction (XRD) charts reveal sharp peaks of the targeted Zn0.6Cu0.3Pb0.1Fe2O4 phase at 600 °C, while a highly crystalline single phase is obtained at 1100 °C, independently of precipitation pH. The crystalline size of the produced powders increases with annealing temperatures (from 18-27 nm at 600 °C to 85-105 nm at 1100 °C). The finest size is found at pH 12. Scanning electron microscopy (SEM) investigation shows uniform cubic microstructures of samples annealed at 1100 °C. The produced ferrite powders exhibit soft magnetic characteristics. Saturation magnetization, M s, substantially increases with pH. Coercivity, H c, increases with increasing annealing temperatures, from 600 to 800 °C, and decreases above 800 °C. Preliminary cost-benefit analysis revealed that the profit margin of the proposed process flowsheet is promising. The wastewater is almost free of heavy metals. Our advances in high gold recovery and preparation of valuable magnetic nanocrystalline ferrite provide exciting opportunities to enhance and maximize Al Amar ore production for practical applications.

Engineering of Broadband Nanoporous Semiconductor Photonic Crystals for Visible-Light-Driven Photocatalysis.

A new class of semiconductor photonic crystals composed of titanium dioxide (TiO2)-functionalized nanoporous anodic alumina (NAA) broadband-distributed Bragg reflectors (BDBRs) for visible-light-driven photocatalysis is presented. NAA-BDBRs produced by double exponential pulse anodization (DEPA) show well-resolved, spectrally tunable, broad photonic stop bands (PSBs), the width of which can be precisely tuned from 70 ± 6 to 153 ± 9 nm (in air) by progressive modification of the anodization period in the input DEPA profile. Photocatalytic efficiency of TiO2-NAA-BDBRs with tunable PSB width upon visible-NIR illumination is studied using three model photodegradation reactions of organics with absorbance bands across the visible spectral regions. Analysis of these reactions allows us to elucidate the interplay of spectral distance between red edge of TiO2-NAA-BDBRs' PSB, electronic bandgap, and absorbance band of model organics in harnessing visible photons for photocatalysis. Photodegradation reaction efficiency is optimal when the PSB's red edge is spectrally close to the electronic bandgap of the functional semiconductor coating. Photocatalytic performance decreases dramatically when the red edge of the PSB is shifted toward visible wavelengths. However, a photocatalytic recovery is observed when the PSB's red edge is judiciously positioned within the proximity of the absorption band of model organics, indicating that TiO2-NAA-BDBRs can harness visible electromagnetic waves to speed up photocatalytic reactions by drastically slowing the group velocity of incident photons at specific spectral regions. Our advances provide new opportunities to better understand and engineer light-matter interactions for photocatalysis, using TiO2-NAA-BDBRs as model nanoporous semiconductor platforms. These high-performing photocatalysts could find broad applicability in visible-NIR light harvesting for environmental remediation, green energy generation, and chemical synthesis.

Tunable Nanoporous Anodic Alumina Photonic Crystals by Gaussian Pulse Anodization.

This study presents a Gaussian pulse anodization approach to generate nanoporous photonic crystals with highly tunable and controllable optical properties across the visible-NIR spectrum. Nanoporous anodic alumina Gaussian photonic crystals (NAA-GPCs) are fabricated in oxalic acid electrolyte by Gaussian pulse anodization, a novel form of pulse-like anodization. The effect of the Gaussian pulse width in the anodization profile on the optical properties of these photonic crystals is assessed by systematically varying this fabrication parameter from 5 to 60 s. The optical features of the characteristic photonic stopband (PSB) of NAA-GPCs-the position of the central wavelength, full width at half-maximum, and intensity-are found to be highly dependent on the Gaussian pulse width, the angle of incidence of incoming photons, and the nanopore diameter of NAA-GPCs. The effective medium of NAA-GPCs is assessed by monitoring spectral shifts in their characteristic PSB upon infiltration of their nanoporous structure with analytical solutions of d-glucose of varying concentration (0.0125-1 M). Experimental results are validated and mechanistically described by theoretical simulations, using the Looyenga-Landau-Lifshitz effective medium approximation model. Our findings demonstrate that Gaussian pulse anodization is an effective nanofabrication approach to producing highly sensitive NAA-based PC structures with versatile and tunable PSBs across the spectral regions. The findings provide new exiting opportunities to integrate these unique PC structures into photonic sensors and other platform materials for light-based technologies.

Realization of high-quality optical nanoporous gradient-index filters by optimal combination of anodization conditions.

High-quality nanoporous anodic alumina gradient-index filters (NAA-GIFs) are realized by sinusoidal pulse anodisation (SPA) of aluminum. A three-level factorial design of experiments is used to determine the effect of three critical anodization parameters -electrolyte temperature, concentration of the electrolyte and anodization time- on the quality of light control in these photonic crystal (PC) structures. Quantitative analysis of the effect of these anodization parameters on the quality of the characteristic photonic stopband (PSB) of NAA-GIFs reveals that all three anodization parameters and their respective combinations have statistically significant effects. However, anodization time is found to have the highest impact on the quality of light control in NAA-GIFs, followed by the electrolyte concentration and its temperature. Our findings demonstrate that NAA-GIFs fabricated under optimal conditions achieve an outstanding quality factor of ∼86 (i.e.∼18% superior to that of other NAA-based PCs reported in the literature). This study provides new insight into optimal anodization conditions to fabricate high-quality NAA-based PC structures, opening new exciting opportunities to integrate these nanoporous PCs as platform materials for light-based technologies requiring a precise control over photons such as ultra-sensitive optical sensors and biosensors, photocatalysts for green energy generation and environmental remediation, optical encoding and lasing.

Luminescent Porous Silicon Nanoparticles for Continuous Wave and Time-Gated Photoluminescence Imaging.

Luminescent porous silicon nanoparticles (LpSiNPs) display red-orange photoluminescence (PL) that provides large penetration depth for precise deep-tissue imaging and diagnostics. Herein, we describe in detail the fabrication process of porous silicon nanoparticles (pSiNPs), activation of photoluminescence, quantum yield measurement, and PL imaging. LpSiNPs perform as imaging probe for both the continuous wave and time-gated PL imaging.

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.

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.