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

Abstract

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.

Figure 1.

(a) Schematic describing the structural characteristics of NAA (i.e., pore diameter: D_p, pore length: L_p, barrier layer thickness: L_b, and interpore distance: D_int); (b) Top and (c) cross-sectional SEM images of NAA.

Figure 2.

(a) Schematic of top and cross-sectional view showing the distribution of impurities in a NAA pore cell; (b) Four different stages of dissolution of NAA alumina layer under acidic conditions (5 v % H₃PO₄ at 35 °C) (adapted with permission from reference [32]).

Figure 3.

A summary of typical wet chemical and gas phase techniques used to modify the surface of NAA (reprinted with permission from [29]).

Figure 4.

SEM images of DC-magnetron Ag sputter-coated NAA membranes fabricated at different voltages and sputtering time set to 10 min (scale bar = 100 nm): (a) 20 V; (b) 30 V; (c) 40 V; (d) 50 V; (e) 60 V. (Adapted with permission from [37]).

Figure 5.

(a) A schematic illustration of n-heptylamine coating on NAA; (b) Scheme showing the shadowing effect resulting in blocking of pores with long time deposition; (c) AFM images of plasma-coated NAA for different time periods. (Adapted with permission from [42]).

Figure 6.

(a) A schematic illustration of fabrication of modulated pore NAA and subsequent deposition of magnetic nanotubes; (b) SEM and TEM images of the prepared magnetic nanotubes. (Adapted with permission from [53]).

Figure 7.

(a) A scheme showing the setup used to grow CNTs inside NAA templates by CVD; (b) Schematic illustration of recycling process of plastic by CVD process to yield CNTs. (Adapted with permission from [58]).

Figure 8.

Schematic of silanization process used for modifying NAA. (Adapted with permission from [62]).

Figure 9.

Schematic illustrating of silanization process for functionalizing the surface of NAA with multiple silanes (Adapted with permission from [77]).

Figure 10.

Functionalization path of NAA surfaces with n-alkanoic acid. (Adapted with permission from [82]).

Figure 11.

Schematic presenting the step-wise process used to fabricate lipid-bilayers inside NAA pores using PEG triggering. (Adapted with permission from [73]).

Figure 12.

(a) A general scheme showing steps for layer-by-layer deposition of polyelectrolytes; (b) LbL modification of NAA pores used to immobilize antibodies. (Adapted with permission from [96] and [101], respectively).

Figure 13.

(a) A schematic process for templating NAA structure to replicate silica nanotubes via sol-gel process; (b) TEM images of the resulting silica nanotubes templated from NAA. (Adapted with permission from [109]).

Figure 14.

SEM images of multisegmented metal nanotubes with a stacked configuration of metal inside NAA templates. (a) Cross-sectional SEM image of Au-Ni-Au-Ni-Au along the nanotube axis (Au: yellow and Ni: purple) (b) and (c) SEM images of multisegmented metal nanotubes after dissolution of the NAA in NaOH. (Adapted with permission from [128]).

Figure 15.

(a) Cross-sectional SEM images of pore diameter modulations in NAA produced by switching the anodization between MA and HA regimes; (b) Schematic illustration of cyclic anodization process introduced by Losic et al.; (c) Cross-sectional SEM images of high aspect ratio funnel-like NAA produced by Santos et al. (Adapted with permission from [8,24,25]).

Figure 16.

(a) Schematic illustration of the SPR-NAA waveguide sensor developed by Hotta et al. and its typical reflection spectrum; (b) Schematic of NAA-LPR setup developed by Kim et al. by coating the top surface of NAA with gold to form nano-caps. (Adapted with permission from [150,151], respectively).

Figure 17.

(a) A schematic of NAA-RIfS sensor for detection of Au(III) ions developed by Kumeria et al. and plot showing its ability for real-time detection; (b) Real-time monitoring of drug release from NAA using RIfS; (c) The comparative study between RIfS-NAA and PLS-NAA sensors by Santos et al. showing ability of the two systems to generate barcodes. (Adapted with permission from [59,60,168]).