Nanoporous anodic aluminum oxide (AAO) is an important template for 1D nanomaterial synthesis. It is used as an etching template for nanopattern transfer in a variety of contexts, including nanostructured material synthesis, electrical sensors, optical sensors, photonic and electronic devices, photocatalysis, and hardness and anticorrosion improvement.
Reference | Fabrication Method/Time | Signal Intensity (RH%) |
Response (ΔC/C0) |
Response–Recovery Time | |||||
---|---|---|---|---|---|---|---|---|---|
[25] | AAO/NA | 0.08–20 nF (RH5%–95%) |
About 20,000% | NA | |||||
[31] | AAO prepared by oxalic acid from high-purity Al/4 h | 43]0.65–108.9 nF (RH15%–80%) | 16,650% | NA | |||||
Quercetin | 0.14 mg/mL | Two-step bilayered or funnel-like NAA structures | [33] | AAO prepared by oxalic acid from high-purity Al at −5 °C/NA | 1.5–60 nF | ||||
[44] | Cu2+ | (RH5%–95%) |
About 3900% | NA | |||||
0.007 ppm | Two-step vertical AAO | [35] | AAO prepared by oxalic acid and pore widening from high-purity Al/6 h | 1.8–16 nF (RH5%–75%) |
793.02% | NA | |||
[45] | Drug release | - | One-step bamboo-like AAO fabricated by Sinusoidal anodization | [36] | Al sputtering on paper and anodized by phosphoric acid from high-purity Al/NA | NA (RH20%–80%) |
About 500% | NA | |
[48] | PL | Staphylococcus aureus Cocaine | 0.5 μm | Two-step vertical AAO | [37] | Spin coating polymer material on AAO/NA | 10–38 pF (RH20%–90%) |
280% | NA |
[38] | Sputtered Al for AAO on Si from high-purity Al/NA | 2.08–2.17 pF (RH30%–90%) | About 4.4% | 289 s/286 s | |||||
[26] | AAO prepared by oxalic acid and pore widening from high-purity Al at 15 °C/8 h | NA | About 4830% | 18–188 s/NA | |||||
[39] | Silica and poly(3,4-ethylenedioxythiophene) composites/NA | About 20–1000 pF (RH11%–95%) |
About 4000% | 77 s/30 s | |||||
[40] | carbon nanofiber (CNF) and nanofibrillated cellulose (NFC)/27 h | About 231-3290 pF (RH40%- 100%) | About 100% | 41 s/50 s | |||||
[41] | bis(4-benzylpiperazine-1-carbodithioato-k2S, S′)nickel(II) complex/NA | 15.95 pF–38.1 pF (RH30%–90%) | About 138% | 25 s/30 s | |||||
[30] | AAO prepared by oxalic acid/2 h | 19.25–984.26 nF (RH20%–80%) |
5013% | Below 10 s/10 s |
Ref. | Technique | Analyte | LOD | Substrate |
---|---|---|---|---|
[42] | RIfS | Plant hormones (ABA, SA, auxins, cytokinins, gibberellins) | 0.1 μm | One-step vertical AAO on ITO glass chip |
[ | ||||
[ | ||||
49 | ||||
] | Vascular endothelial growth factor (VEGF) | 1 ng/μL | Two-step vertical AAO filled with ZnO | |
[50] | SPR/LSPR | Label-free DNA | 5 nm | Nanobowled AAO barrier |
[51] | Transmembrane protein CD63 | 1 ng/mL | Gold nano array fabricated using 2-step vertical AAO as a mask | |
[52] | IgA | 10 ng/μL | Gold-capped 2-step vertical AAO | |
[53] | Label-free DNA | 10 nM | Gold nanoantenna array fabricated using 1-step vertical AAO as a mask | |
[54] | Cell interleukin-6 | 10 ng/mL | Nanoimprinting cyclo-olefin polymer (COP) using 2-step vertical AAO mold | |
[55] | SERS | Aflatoxin B1 | 0.5 μg/L | Bipyramid-like nanoparticles in 2-step vertical AAO |
[56] | C-reactive protein, interleukin-6, serum amyloid A, and procalcitonin | 53.4, 4.72, 48.3, and 7.53 fg/mL | Gold–Silver core–shell nanoparticles on commercial AAO | |
[57] | Nitrate ion | 1.03 ppm | Gold nanoparticle clusters on 1-step AAO layer | |
[58] | Methylene blue | - | UV-curable resin nanorod arrays using the 2-step branched AAO as a mold | |
[62] | Beta-hydroxybutyric acid | 11 nM | Striped Au-Ag nanowire fabricated using vertical AAO as a shadow mask | |
[59] | Rhodamine B | 1 × 10−10 mol/L | Ag-loaded bamboo-like AAO as 1D photonic crystal and defective photonic crystals | |
[60] | Methylene blue | 1 nM | Irregular 1-step AAO pores | |
[61] | Melamine | 0.05 ppm | Irregular 1-step AAO pores in 1–2 μm cavities |
RIfS is a useful optical method for sensing the substances, especially biomolecules, which is caused by the interference of light reflected from the top and bottom of the film [46]. The interference spectroscopy depends on the effective optical thickness (EOT), which can be calculated by the eEquation below(13):
EOT = 2 neff d cosθ = mλ
where EOT is the effective optical thickness, neff is the effective refractive index of AAO, d is the thickness of the AAO, m is the order of the interference, and λ is the wavelength of the light. After adding the analyte on the AAO film, a change in the EOT causes a shift of the interference spectroscopy. In general, to increase the sensitivity and selectivity of the measurement, the binding reaction between the analyte and the film is essential [46]. The nanostructured porous films compared with the planar polymer films have higher surface area for increasing the sensitivity [47]. Recently, Feng et al. presented an AAO-based RIFS biosensor to detect the different plant hormones including ABA, SA, auxins, cytokinins, and gibberellins by coating an aptamer on the AAO nanofilm [42]. Nemati et al. presented bilayered AAO films with hierarchical funnel-like structures for optimizing the optical sensing performance toward multi-analyte biosensing [43]. Kaur et al. combined self-assembled glutaraldehyde-cross-linked, double-layered, polyethylenimine (PEI-GA-PEI)-modified AAO interferometers and reflectometric interference spectroscopy (RIfS) as an optical sensing system for detecting ionic copper in environmental waters [44]. Kapruwan et al. exploited the optical properties of AAO gradient index filters, which have been used to measure the release dynamics of the cargo molecule in real time [45]. PL refers to light emission after the absorption of photons, which causes photon excitation and produces the electronic transition. The absorbed light causes the electrons of the material to jump to a higher state and then return back to a lower energy level with a light emission [47]. Surface plasmon resonance (SPR) is the oscillation of electrons on a metallic surface stimulated by an extra incident light. The analyte is detected based on the changes in the absorbance of SPR of the substrate [50]. Furthermore, AAO can be used to fabricate some nanostructure to produce localized surface plasmon resonance (LSPR). Compared with SPR, LSPR is more easily excited, which means complex optical configurations are unnecessary. Thus, some LSPR sensing researches using simple, even portable, optical devices have been reported [50][51][52][53][54]. Recently, SERS has been the most-published topic and can be applied for trace detection of various substances. SERS is a sensitive technique for detecting trace substances in industry, environment, wastewater, and air pollution. The enhancement comes from the overall effect of the electromagnetic mechanism (EM) and the chemical mechanism (CM). The EM is attributed to the local electrical field enhanced by the plasma resonance, and the CM is attributed to the electrons transfer between the analyte and the SERS substrate [60]. Here, the SERS substrates majorly based on the EM are discussed because the enhancement of EM is substantially higher than that of CM. The porous materials with properties of simple manufacturing and high aspect ratio are regarded as good candidates for forming the nanostructures of SERS substrates. Therefore, AAO with tunable nanopores is a popular material for fabricating the SERS substrate with various approaches. The current reported AAO-based SERS substrates can be divided into MNPs in the AAO pores [55][56][57], using AAO as a mold for fabricating the metal nanostructure array, and the metal film on the nanopores. First, AAO can be the support for the nanoparticles: Lin et al. used a drop-dry approach to deposit gold nanobipyramids (Au NBPs) into vertical two-step AAO pores as the SERS substrate to detect Aflatoxin B1. For Aflatoxin B1 sensing, the Au NBPs–AAO substrate has a corresponding linear range from 1.5 μg/L to 1.5 mg/L and an LOD of 0.5 μg/L [55]. Second, AAO can be the mold or the mask to fabricate the nanoparticles, nanowires, and nanorods. Chung et al. demonstrated the UV-curable resin nanorods array deposited with Ag MNPs as the SERS substrate. The UV-curable resin was filled into the nanopores of the two-step vertical and branched AAO to transfer the morphology of the nanopores to the UV-curable resin and generate the nanorods array.