Engineering optical properties of gold-coated nanoporous anodic alumina for biosensing
© Hernández-Eguía et al.; licensee Springer. 2014
Received: 16 May 2014
Accepted: 8 July 2014
Published: 21 August 2014
The effect in the Fabry-Pérot optical interferences of nanoporous anodic alumina films coated with gold is studied as a function of the porosity and of the gold thickness by means of reflectance spectroscopy. Samples with porosities between 14 and 70% and gold thicknesses (10 and 20 nm) were considered. The sputtering of gold on the nanoporous anodic alumina (NAA) films results in an increase of the fringe intensity of the oscillations in the spectra resulting from Fabry-Pérot interferences in the porous layer, with a reduction in the maximum reflectance in the UV-visible region. For the thicker gold layer, sharp valleys appear in the near-infrared (IR) range that can be useful for accurate spectral shift measurements in optical biosensing. A theoretical model for the optical behavior has also been proposed. The model shows a very good agreement with the experimental measurements, what makes it useful for design and optimization of devices based on this material. This material capability is enormous for using it as an accurate and sensitive optical sensor, since gold owns a well-known surface chemistry with certain molecules, most of them biomolecules.
KeywordsNanoporous anodic alumina Gold coating Thin-film Reflectance spectroscopy Pore widening Effective medium approximation Modeling Fabry-Pérot interferences
Nanoporous anodic alumina (NAA) is one of the smartest materials in which scientists have centered their research with considerable interest in recent years[1, 2] due to their physicochemical properties like thermal stability, environmental toughness, and biocompatibility. Alumina has been studied for decades. The fabrication technology permits to obtain highly ordered and customized porous nanostructures that makes NAA very attractive for different applications such as nanomaterial synthesis[4, 5], photonics, or sensors[7–9].
In particular, NAA has demonstrated its sensing capabilities: a great wealth of work has been carried out with this material in biotechnology areas, and it presents reliable possibilities of working as portable chemical and biochemical sensors, as well as label-free biosensors. Furthermore, if the optical waveguide properties of NAA are exploited, much higher sensitivities than conventional surface plasmon resonance (SPR) sensors[2, 13, 14] can be achieved. Sensors based on alumina improve their sensitivity by the measurement of the oscillations in the reflectance spectrum produced by the Fabry-Pérot (F-P) interferences in a NAA thin film[15, 16]. More specifically, sensors based on reflection interference spectroscopy (RIfS) have been developed with favorable results[17, 18]. The variation of these oscillations upon analyte detection is the sensing principle. It is well known that the fringe intensity (FI) of the F-P interference pattern depends on the internal reflectivity of the mirrors composing the F-P cavity. A F-P interferometer consists essentially of two plates with parallel reflecting plane surfaces (with some small transmittivity). When illuminated at near-normal incidence, a multiple-beam interference is generated that results in the maxima and minima in the reflectance or transmittance spectra.
In this work, a technique to improve the FI and consequently the sensitivity of NAA-based sensors is studied, and a model to predict the optical response and evaluate the material sensitivity has been developed. For this purpose, the UV-visible-infrared (IR) spectra of different NAA thin films obtained with different pore diameters (Dp) were investigated before and after the deposition of a thin gold layer on its surface. This optical characterization will allow determining the geometric properties of the porous alumina. The gold layer increases the reflection coefficient at the NAA-medium interface and improves the FI. The measured spectra were compared with numerical simulations in order to establish a model based on the effective medium approximation to account for the porous nature of the material and to obtain a tool for the evaluation of the structure sensitivity.
NAA sample fabrication
The NAA samples were fabricated by the well-known two-step anodization process[21, 22]. First, samples were cleaned employing deionized (DI) H2O, EtOH, and again DI H2O and electropolished in a mixture of EtOH and HClO4 4:1 (v/v) at 20 V and 5°C for 4 min. During the electropolishing process, the stirring rotation was alternated from clockwise to counterclockwise every 60 s in order to avoid stripes in the samples due to the stirring direction. Immediately after, the first anodization step was carried out in an aqueous solution of H2C2O4 0.3 M as electrolyte at 40 V and 5°C for 20 h in order to obtain 10% porosity for maximum self-ordering of pores. The obtained alumina film in the first step was dissolved by wet chemical etching in a mixture of H3PO4 0.4 M and chromic acid H2CrO7 0.2 M at 70°C for 3 h 30 min. The second anodization step was performed under the same conditions as the previous one. Finally, the pore diameter was modulated by applying a wet chemical etching after the anodization procedure in an aqueous solution of H3PO4 5 wt% for a given time tPW of 0, 6, 12, and 18 min.
Surface coating of NAA samples and thickness calibration
Gold was sputtered on the samples at 0.05 mbar and 30 mA during 21 or 45 s, to obtain 10- or 20-nm gold overlayers on the NAA, respectively, employing a sputter coater Bal-Tec SCD 004 (Bal-Tec, Balzers, Liechtenstein). X-ray reflectometry was used in advance to calibrate the sputtering process and estimate the average deposited thickness. These X-ray reflectometry measurements were made using a Bruker-AXS D8-Discover diffractometer (Bruker AXS, Inc., Madison, WI, USA) with parallel incident beam (Göbel mirror) and vertical theta-theta goniometer, XYZ motorized stage mounted on an Eulerian cradle, incident and diffracted-beam Soller slits, a 0.01° receiving slit, and a scintillation counter as a detector. The angular 2 T diffraction range was between 0.4 and 5°. The data were collected with an angular step of 0.004° at 10 s per step. Cukα radiation was obtained from a copper X-ray tube operated with variable voltage (kV) and current (mA).
Structural and optical characterization of samples
The NAA samples were characterized by an environmental scanning electron microscope (ESEM; FEI Quanta 600, Hillsboro, OR, USA) and field emission SEM (Schottky FE) 4 pA to 20 nA, 0.1 to 30 kV and 1.1 nm. The specular reflectance measurements were performed in a PerkinElmer Lambda 950 UV/VIS/NIR spectrometer (PerkinElmer, Waltham, MA, USA) with a tungsten lamp used as excitation light source. The standard image-processing package (ImageJ, public domain program developed at the RSB of the NIH, USA) was used to carry out the SEM image analysis.
Results and discussion
Results from the SEM image characterization of the samples after the pore widening and before the deposition of gold
Pore widening time (min)
Estimated pore diameter, Dp(nm)
Standard deviation (nm)
These volume fractions are related to the porosity P of the porous layer, P being the volume fraction of air and 1 - P the volume fraction of aluminum oxide. The calculated reflectance spectra shown in Figure 2 correspond to the best least-square fit obtained by varying the porosity of the layer.
Results from the optical characterization of the samples after the pore widening and before the deposition of gold
Pore widening time (min)
NAA film porosity, P(%)
NAA film effective refractive index, neff
Estimated pore diameter, Dp(nm)
Comparing the Dp obtained from this optical characterization method with the approximate estimation from SEM, it can be seen that both show an increasing trend but that pore size determinations are not very precise from image analysis of surface pictures.
In the near-IR range, the spectra show bigger differences: the reflectance for the samples with 10 nm of gold show symmetric oscillations with respect to the reflectance minima, while for 20 nm of gold, the oscillations are asymmetric. Furthermore, the position of the minima is clearly blue shifted in the samples with 20 nm of gold with respect to the samples without and with 10 nm of gold. It is important to remark that this asymmetry and blue shift decrease with increasing tPW and that for the two lower porosities (corresponding to tPW = 0 min and tPW = 6 min), this asymmetry results in narrow valleys with small width and a well-defined minimum wavelength that can be useful in the detection of spectral shifts.
If the FI between the samples with 10 and 20 nm of deposited gold is compared, it can be concluded that the relation of the FI with the gold thickness is strongly dependent on the porosity of the NAA film: for the lower porosities, the FI for the 10 nm gold-coated samples is bigger, but this trend is reversed as the porosity increases.
Results from the optical characterization of the samples with t PW = 0 min and t PW = 18 min after the deposition of 20 nm of gold
Pore widening time (min)
NAA film porosity, P1(%)
Volume fraction of gold in the NAA film, fAu(%)
NAA film thickness, d1(nm)
Gold film porosity, P2(%)
Gold film thickness, d2(nm)
The model is able to explain the reduction of the reflectance maxima in the UV-visible range by the small amount of gold that can penetrate into the pores (0.1% for tPW = 0 min and 1.2% for tPW = 18 min). These results are consistent with the pore size, as a bigger amount of gold can penetrate for bigger pores. Nevertheless, the model predicts a smaller reflectance reduction than what is observed in the measurements. This is due to the fact that there possibly exist other sources of loss in this spectral range than the absorption from the gold in the inner pore walls. Such losses can arise from scattering or plasmonic effects that the model cannot take into account. In the near-IR range, the changes in the shape of the oscillations are explained by the differences in thickness and porosity of the gold layer on the NAA film. The obtained gold film porosities are also consistent with the porosity of the NAA film (P2 = 55.3% for tPW = 0 min and P2 = 59.5% for tPW = 18 min), bigger for the bigger NAA film porosity. This result is in good agreement with previous works where a 10-nm-thickness gold layer is sputtered onto NAA. Cross-sectional FE-SEM pictures in this work show that sputtered gold does not penetrate into the NAA pores and forms a superficial film. With just these two parameters (thickness and porosity), it is possible to account for all the features observed in the spectra in the near-IR range: the narrow asymmetric valleys for the low-porosity NAA that become more symmetric as the porosity increases and the differences in blue shift of the reflectance minima.
In this work, we have shown the effect on the reflectance spectra of nanoporous anodic alumina films of the sputtering of a gold overlayer, as a function of the NAA porosity and of the gold thickness. The results show that the gold overlayer improves dramatically the contrast of the oscillations in the reflectance spectrum, what would result in an improvement of NAA-based optical sensors. By adequately tuning the gold thickness, sharp valleys in the reflectance can be obtained in the near-IR range that can further contribute to a more accurate determination of spectral shifts and a consequent sensitivity improvement. A model based on the effective medium approximation for the NAA layer and for the deposited gold thin film has been proposed and shows a good agreement with the experimental measurements. In particular, the model is able to explain the shape of the sharp reflectance valleys in the near-IR for the different gold thicknesses and NAA porosities.
This work shows that nanoporous anodic alumina coated with gold is a promising structure for future biosensing applications because of the improved sensitivity in any pore geometry due to the enhancement in the reflectance FI. Specific applications could then benefit from a big surface-to-volume ratio in big porosity structures to sense biomolecules, whereas for filtering purposes, the pore diameter can be tuned to match the molecule size to be transported through the membrane.
nanoporous anodic alumina
surface plasmon resonance
reflection interference spectroscopy
effective optical thickness
- n eff :
porous layer effective refractive index
- D int :
- D p :
- t PW :
pore widening time
This research was supported by the Spanish Ministerio de Economía y Competitividad through the grant number TEC2012-34397 and the Generalitat de Catalunya through the grant number 2009-SGR-549.
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