Nanoporous anodic aluminum oxide films for UV/vis detection of noble and non-noble metals

Abstract

In this study, a simple, rapid and inexpensive approach for the screening of heavy metals with photometric reagents was developed based on porous, anodic aluminium oxide (AAO) films, with detection limits of 0.45 mg L⁻¹ (Co²⁺), 0.25 mg L⁻¹ (Pb²⁺) and 0.59 mg L⁻¹ (Ni²⁺). Noble metal ions Ag⁺ and Pd²⁺, as well as Cu²⁺, formed nanoparticles within the AAO channels during micro-solid phase extraction driven by galvanic electroless displacement followed by UV detection.

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Introduction

Industrial galvanic production often results in severe environmental pollution by heavy metals.¹ Moreover, the environmental protection laws of many countries do not regulate the maximum concentrations of metal nanoparticles (NPs) in matrices such as waste water in the vicinity of galvanic manufacturing plants.^2–5

Routine analytical procedures for monitoring heavy metals are often based on inductively coupled plasma optical emission spectroscopy (ICP-OES),⁶ which are strongly influenced by matrix interferents and usually require elaborate sample preparation routines for environmental samples.^2,6 Solid-phase spectrophotometry techniques are often less affected by interferents, as they perform metal sorption and direct spectrophotometric detection of the trapped analytes from the same material.^7–12 The solid materials applied for sorption are, for example, gelatin gels, polyacrylamide fibres and carbon nanotubes.^8–12 Their utility in the direct spectrophotometric measurement is often restricted by the limited sorption capacity for metals, however, which then requires very sensitive instrumental detection techniques, e.g. X-ray fluorescence or ultraviolet spectroscopy.⁷ Furthermore, for rapid detection of heavy metals in environmental samples, whole-cell biosensors based on metallothionein promoters as well as DNA sensors using metal-mediated DNA duplexes have been proposed.^13–19 Unfortunately, the required homogeneity of the reactions and the elaborate detection equipment make them unsuitable for rapid on-site, in situ analysis.

In this study, we have developed a simple and rapid method for screening heavy metals from water samples by solid phase micro-extraction of metals with subsequent visual detection. We propose the use of anodic aluminium oxide (AAO) nanoporous films for this purpose. AAO can be readily formed by anodic oxidation of aluminium, resulting in highly porous materials with adjustable structures. AAO has been utilized in many applications, including molecular separations, catalysis, energy generation/storage, electronics and photonics, sensors, hybrid nanomaterials and templates, drug delivery and template syntheses.^20–27 AAO exhibits both active acidic and basic centres on the surface, making non-covalent attachment of analytes readily possible through donor–acceptor interactions and hydrogen bonding to the AAO surface via appropriate anchor groups.²⁶ Also, AAO matrices have been successfully utilized as templates for biosensors^28–30 and as substrate materials for analysis of peptides by surface assisted laser desorption/ionisation mass spectrometry (SALDI-MS).³¹

Furthermore, metals such as Au, Ag, Cu, etc. are readily deposited within the pores of AAO substrates by electrochemical deposition on anodic aluminium oxide templates with high efficiency to obtain protective and decorative coatings.^32–34 No particular attention has been paid so far to the use of both electrochemical and chemical deposition of metals on anodic aluminium oxide surfaces as a means of analytical sampling and detection. We have investigated the analysis of metal ions with different reduction potentials (E⁰); viz., Ag⁺, Pd²⁺, Cu²⁺, Co²⁺, Ni²⁺ and Pb²⁺. The approach allowed the screening of non-noble metals (Ni²⁺, Co²⁺, and Pb²⁺) with photometric reagents and UV-detection of noble Ag⁺, Pd²⁺ and Cu²⁺ due to their chemical reduction to nanoparticles in the solid AAO matrix.

Experimental

Chemicals and materials

CoSO₄, Pb(NO₃)₂, NiSO₄, CuSO₄, H₂PdO₂, AgNO₃, 1-nitroso-2-naphthol-3,6-disulfonic acid disodium salt (nitroso-R-salt, NRS), dithizone, murexide, alizarin red, KI, dimethylglyoxime, and sulfosalicylic acid were purchased from Merck (Darmstadt, Germany), 60 μm Al foil (nominal composition of Al98Mg2%) and platinum plates (99.99%) were from VWR (Darmstadt, Germany), granular Al₂O₃ (TU 2163-011-51444844-2005, grain size 2–4 mm, pore diameter 0.2–0.5 μm) was from AM3 Ltd. (Chelyabinsk, Russia), and isopropanol, HCl, acetone, and ethanol were from Sigma-Aldrich (Steinheim, Germany). Organic-free, deionized water was generated by an Elga PureLab (Celle, Germany) water purification system.

Preparation of the AAO matrix

The properties of the synthesized AAO films, such as morphology, fragility, inherent colour, and porosity, prepared from aluminium foil, strongly depend on the nature of the electrolyte solution as well as parameters such as electrolysis time (t_el) and current (I_a). For the preparation of anodic films, a two-step procedure was applied using oxidation of aluminium foil in 0.6 M sulfosalicylic acid as it was optimized in our previous study involving analytical application of AAO.²⁶ Al foil was initially polished using a series of abrasive papers (9 → 6 → 3 μm). To remove the native layer, an anodic current density of I_a = 1 A dm⁻² for t_el(1) = 10 min (first oxidation) was initially applied; during the second oxidation step, the current density ranged from I_a = 0.5 to 1.5 A dm⁻² for t_el(2) = 40 min. The anode was aluminium, while the cathode was a platinum plate. Anodizing of samples was carried out in a glass bottle using a two-electrode electrochemical cell and a P-5827 M potentiostat (Gomel, Belarus) in galvanostatic mode. The prepared anodic films were cleaned in acetone and ethanol, and stored under room conditions not more than 30 days before metal immobilization (see below).

Scanning electron microscopy (SEM) and X-ray spectral analysis (EDX)

For the determination of pore diameters (D_por, nm) and verifying that noble metals were reduced to nanoparticles during solid phase extraction in the AAO channels, a FEI (Hillsboro, OR, USA) Quanta 400 FEG scanning electron microscope equipped with an EDAX (Mahwah, NJ, USA) Genesis V 6.04 EDX X-ray spectral analysis system was used. SEM images were taken at an accelerating voltage of 10 kV and image size of 1024 × 884 pixels.

Inductively coupled plasma mass spectrometry (ICP MS)

To verify the chemical stability of the produced anodic films, ICP MS analysis with ELEMENT XR (Thermo Fisher Scientific, Bremen, Germany) of 5 mL water in high resolution mode (HR) before and after AAO (2 × 0.5 cm²) immersion for 30 minutes was used with the following source parameters: cool gas: 16.00 L min⁻¹; sample gas: 1.24 L min⁻¹; faraday deflection: −217 V; plasma power: 1250 W; peri. pump speed: 10 rpm; torch X-Pos.: 1.9 mm; torch Y-Pos.: 0.8 mm; torch Z-Pos.: −3.3 mm.

Micro-solid phase extraction in AAO channels

Metal stock solutions were prepared at 10 mg mL⁻¹. After dilution of the stock solutions down to concentration levels near the limit of detection (LOD), immobilization within the AAO matrix under static (24 h) and dynamic (ultrasonic bath for 30 min) conditions was carried out using several approaches:

(1) initial treatment of AAO (plate size 1 × 2 cm²) with photometric reagents (NRS, dithizone, murexide, alizarin red, KI, and dimethylglyoxime) followed by sorption of the targeted ions;

(2) sorption (saturation) of the pre-formed photometric complex from water by the AAO surface;

(3) initial sorption of ions by the AAO surface followed by treatment with photometric reagents.

Photometric analysis

To qualitatively confirm the presence of nanoparticles of noble metals after immobilization in AAO pores, photometric analyses were carried out using a CV-2000-02 (Spectra Ltd., St. Petersburg, Russia) spectrophotometer. To transform AAO into the transparent form, the porous layer of AAO (area 1 × 2 cm) was separated from the aluminium barrier layer by placing the film for 30 s in 10 mM HCl. The transparent AAO films were removed from the solution, deposited on quartz plates by the Schaeffer method (horizontal lift³⁵), placed into cuvettes for photometry and investigated in the wavelength range from 200 to 1100 nm (step size, 1 nm). Blank, empty AAO matrices, prepared under the same conditions, were used in these experiments.

Sorption capacities of non-transparent AAO films were evaluated using adsorption values (a, mol g⁻¹):²⁵

where c₁ and c₂ are the initial and final concentrations of detected metal ions (mol mL⁻¹), respectively, m is the mass of the AAO plate (g) and V is the volume of solution (mL).

Residual concentrations of metal ions in solution after immersion of AAO matrices were determined by conventional photometry: by reacting Pb²⁺ and Ag⁺ with dithizone, Co²⁺ with NRS, and Cu²⁺ with alizarin red.^36,37 To compare the sorption capacity of AAO films granular Al₂O₃ was used.

Potentiometric pH measurements

As the reference system to measure the pH values of the solutions, a potentiometric sensor Multitest IPL-301 with combined electrode ESC-10603 (Cemiko Ltd., Novosibirsk, Russia) was used. The acidity of the solutions was adjusted using pH buffer mixtures.

Results and discussion

Under the applied electrochemical conditions, regular porous structures were generated inside the utilized Al foils, with average diameters of 40–60 nm and channel lengths of up to 25 μm (Fig. 1). The pore density of AAO films was 10¹⁰ to 10¹¹ pores per cm², with no visible external structure defects on the surface. EDX analysis of Al-films after anodization revealed the presence of Mg (due to the used Al alloy) and S (sulfosalicylic electrolyte) besides Al and O lines (ESI, Fig. S1†). Subsequent ICP MS analysis showed significant migration of Al (approx. 2.5 ppb) and Mg (approx. 300 ppt) from the films after anodization in water (ESI, Table S1†). We interpret the increased amounts of Al and Mg in water after AAO immersion as a result of ion transport/migration through the micro-cracks occurring between the barrier layer and porous matrix (ESI, Fig. S2†). Due to the combined presence of oxidized and free metallic Al within the AAO matrices, interesting properties such as reduction/oxidation abilities during metal immobilisation were expected.

Fig. 1 SEM images of the AAO matrix: (left) surface; (right) cross-section showing 25 μm length of porous channels.

Optimization of micro-solid phase extraction

As summarized in the Experimental section, three different experimental approaches for metal sorption on the AAO micro-solid extraction material were compared. The first two strategies, i.e., preliminary modification of AAO with the photometric reagents followed by metal ion sorption, and sorption of the preformed photometric complex from water, unfortunately did not exhibit sufficient efficiency.

In the first approach, dithizone produced a complex with the free Al³⁺ ions that was accompanied by unstable colouring of the film surface. AAO modification with reagents containing fewer functional groups (murexide) exhibited indeed a colour change of the material, showing that a stable complex was created between the target surface and photometric agent; unfortunately, after applying metal ions onto this modified surface, no visual analytical effects were seen. Similarly, immobilized NRS (3%) exhibited significant changes in the UV spectrum of the three main lines (β-, π) of the naphthoquinone forms. Moreover, the additional line at λ = 527 nm along with an increased molar extinction coefficient (Fig. 2, top) can be linked to intramolecular charge transfer initiated by the AAO matrix, readily demonstrating complex formation between Al³⁺ and functional NRS groups. NRS has a branched structure with functional groups still available after immobilization on the AAO surface. After immersion of this modified AAO matrix in CoSO₄ solution, a new line at λ₄₃₆ indicated the complex formation of Co(II) and NRS (Fig. 2, bottom) was observed. Despite the complex formation of Co(II), useful visual changes of the colour were not obtained below 0.01 mg mL⁻¹; we speculate that this was due to the partial shielding of the photometric reagent's functional groups by Al³⁺ (see above).

Fig. 2 (top) Absorbance spectra of pure NRS solution (red); spectrum after immersion of AAO in NRS solution (black) (pH = 4). (bottom) Absorbance spectra of CoSO₄ (0.000765 mg mL⁻¹) before AAO + NRS contact (red); the same CoSO₄ (0.000765 mg mL⁻¹) after immersion of AAO + NRS (black), pH = 4.

The second approach (immobilization of the preformed photometric complexes of the metal and reagent from water) did not give any visible effects, probably due to geometrical size restrictions, where the complex did not properly enter the AAO pores.

The third approach (sorption of metal ions on the AAO film, followed by treatment with photometric reagents) provided efficient immobilization of metal ions within the AAO pores regardless of whether the saturation of the AAO was performed under static or dynamic conditions (see the Experimental section). The AAO films gave beneficial colour changes that were readily detected for the quantification of the metal ions. Co(II) immobilized within AAO after dithizone treatment was measured down to 0.00059 mg mL⁻¹; NRS gave 0.00045 mg mL⁻¹; Pb(II) was determined at a limit of 0.00026 mg mL⁻¹ using KI for visualization (Fig. 3, shown for films saturated under dynamic conditions). The obtained colours were stable for at least 6 months. The limits of visual detection for Co(II), Ni(II) and Pb(II) with different photometric reagents are summarized in Table 1.

Fig. 3 Images of AAO films after immobilization of non-noble ions at different concentration levels followed by photometric reagent visualization.

Table 1 Visible changes and obtained limits of detection on the AAO films after metal ion immobilization followed by photometric reagent treatment

Metal ion	Photometric reagent	Color	LOD^a (mg mL^−1)
a LOD defined as the limit concentration of element in waste water.
Co^2+	Dithizone	Yellow-red	0.00059
Co^2+	Nitroso-R-salts	Rose	0.00045
Pb^2+	Dithizone	Bright rose	0.00025
Pb^2+	Alizarin red	Violet (in ammonium buffer)	0.00030
Pb^2+	KI	Bright yellow	0.00026
Ni^2+	Dimethylglyoxime	Rose	0.00059

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Equally no visible changes for the reported above concentrations during non-noble metal immobilisation to blank aluminium foil (before anodization) or granular Al₂O₃ were detected.

Sorption of noble metals

In the next experiment, AAO films were applied for micro-solid phase extraction of metal ions that are located right of hydrogen in the Beketov metal displacement series; i.e., noble metal ions that are readily reduced to the metallic elements, for example, Ag⁺, Cu²⁺ and Pd²⁺. In our experiments, these ions were chemically reduced within the AAO channels, formed nanoparticular structures (Fig. 4) and could not be visualized by using photometric reagents as it was described above for non-noble metals.

Fig. 4 SEM images of AAO films after saturation (dynamic conditions) in 10 mM CuSO₄ (left) and AgNO₃ (right). The small white spots visible on both images are Cu and Ag particles after Cu²⁺ and Ag⁺ reduction.

The SEM images clearly showed visible globules of Cu-NPs and Ag-NPs of 100–1000 nm sizes on the AAO surface. UV measurements exhibited additional bands in the absorbance spectra, corresponding to metallic NPs (Table 2). The presence of multiple additional lines in the spectra of each of the tested ions probably corresponded to nanoparticle species with different diameters on the solid AAO matrix. We explain the observed effect by the presence of free Al and Mg ions (ESI, Table S1†), resulting in chemical reduction of noble metal ions. The existence of metallic NPs on AAO was confirmed by means of SEM (Fig. 4), EDX analysis (Fig. 5e) and UV-spectroscopy (Table 2) independently. The sorption capacity of the AAO film for noble metal ions (Cu²⁺, Ag⁺, and Pd²⁺) was estimated. It was found that in comparison to granular Al₂O₃ the sorption capacity (RSD ≤ 10%) of AAO was approx. 13-fold larger under static, and almost 18-fold larger under dynamic conditions (Table 3). The behaviour of AAO films for noble metal ions can be explained by considering the internal matrix charge transfer between Al⁰ ↔ Al³⁺ from the presence of free Al. Free Al is expected to promote surface electrochemistry of AAO and stimulate Cu²⁺, Ag⁺, and Pd²⁺ reduction to NPs by galvanic displacement (electroless deposition).^38–40 During electroless deposition, self-assembled metallic structures are formed by galvanic displacement on the semiconductor surface. The reaction is based on in situ electrochemistry which resulted in the reduction of metal ions to NPs on the substrate in the absence of external sources of electric current or chemical reducing agents.⁴⁰

Table 2 Absorbance (A) of noble metals: solution spectra in comparison to immobilized species within solid transparent AAO films

Ion	A (Solution) (λmax, nm)	A (AAO + Me^n+) (λmax, nm)	Note
Cu^2+	1.21 (515)	0.21 (355)	Additional lines at λ355,387 nm (Cu^2+ → Cu^0), λ475–697,943 nm (Ag^+ → Ag^0), λ496,690,894 nm (Pd^2+ → Pd^0), indicate formation of NPs within the solid phase/AAO matrix
		0.19 (387)
Ag^+	1.16 (520)	0.13 (475)
		0.24 (697)
		0.16 (943)
Pd^2+	1.03 (530)	0.10 (496)
		0.16 (690)
		0.18 (894)

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Fig. 5 Illustration of Pd-NP chemical reduction in the AAO matrix during galvanic displacement: (a) pure AAO matrix; (b) etched AAO in HCI (10 s); (c) reduced Pd-NPs after immersion in precursor solution (10 s); (d) cross-section of AAO matrix + Pd-NPs (10 s); (e) EDX spectra of the AAO matrix after Pd-NP reduction; (f) Pd nanoflowers (120 s duration).

Table 3 Comparison of sorption capacity (a, mol g⁻¹) of granulated Al₂O₃ and AAO films

Me^2+^a	a , mol g^−1 (% RSD)
	Granular Al2O3	AAO (static saturation)	AAO (dynamic saturation)
a The metal ion concentration was chosen near the LOD value for these experiments. b Estimated in solutions after contact within non-transparent AAO films.
Cu^2+ (515 nm)	0.6 (4.5)	6.9 (3.1)	9.8 (8.6)
Ag^+ (520 nm)	0.2 (7.8)	4.1 (5.9)	6.8 (4.2)
Pd^2+ (530 nm)	0.7 (2.3)	8.3 (10.1)	10.1 (7.3)

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To demonstrate the observed effects occurring in the system in dynamic conditions, we simulated the galvanic displacement of AAO by immersing the anodic films first in 10 mM HCl for 10 s and then directly in the metal ion precursor solutions (Fig. 5, shown for Pd). Immersion of AAO in HCl generated characteristic etched structures, enhancing electron exchange between the Al matrix and precursor solution (Fig. 5b). Direct saturation of AAO with metal precursor solutions (20 mM of Pd²⁺) leads to spontaneous metal ion reduction to NPs, with regular Pd-NPs and nanoflowers (40–100 nm) visible on the AAO (Fig. 5c). Metal NPs were homogeneously distributed mostly within the AAO surface without deep penetration into the porous structure (Fig. 5d). Increasing the electrolysis time leads to intensive growth of larger NPs (500–800 nm), with the morphology changing from nano- to micro-structures (Fig. 5f).

Quantitative UV-detection of noble metals within AAO

Initially, several different approaches for UV-detection of noble metal ions on AAO films were utilized:

(1) saturation of non-transparent AAO in noble metal solutions (accompanied by reduction of noble ions to NPs) followed by removal of the Al-barrier layer and subsequent UV-detection of immobilized NPs within the transparent solid matrix;

(2) prior removal of the Al-barrier layer followed by saturation of transparent AAO in noble metal ion solutions and photometric analyses of the solid matrix;

(3) immersion of non-transparent AAO in the noble metal solutions for 30 min (allowed reduction of noble metal ions to NPs) followed by conventional photometric analysis of the residual ion concentration;

(4) prior immobilization of photometric reagents within transparent AAO films followed by immersion in noble metal ion solutions and subsequent UV-detection of the conventional photometric complex in the transparent solid matrix.

Unfortunately, we have not obtained an accurate calibration for immobilizing NPs directly in the AAO due to the partial dissolution of the reduced NPs during HCl treatment of the matrix (approach 1). Furthermore, prior removal of the Al-barrier layer (approach 2) leading to transparent AAO films and subsequent saturation in noble metal ion solutions also didn't show an appropriate analytical effect (Fig. 6a, shown for Cu²⁺). This example demonstrated again a role of matrix surface electrochemistry (free Al from the barrier layer) in the NP formation.

Fig. 6 (a) Dependence of UV absorbance of transparent AAO films on Cu²⁺ concentration; (λ(Cu⁰) = 387 nm). (b) Dependence of UV absorbance of Cu²⁺ solution (by reaction with alizarin red) on Cu²⁺ concentration; (λ(Cu²⁺) = 515 nm); (pH = 4, immersion time of AAO in metal solution, 30 min). Note: ratio errors (±σ) are too small to be shown. (c) UV spectra of previously modified transparent AAO within NRS after emersion in Cu²⁺ solutions: 1 – 0 mg L⁻¹; 2 – 0.15 mg L⁻¹; 3 – 0.25 mg L⁻¹; 4 – 0.5 mg L⁻¹; 5 – 1.0 mg L⁻¹.

Quantitative analysis of noble metal NPs was readily possible after immersion of non-transparent AAO films in the metal model solutions for 30 min (approach 3) followed by conventional photometric analysis (Fig. 6b). Remarkably, the sorption of the noble metals by AAO exhibited a linear trend over the range from 0.3 to 3.0 mg L⁻¹.

In contrast to the approaches described above, it is also highly interesting that a prior immobilization of photometric reagents within transparent AAO films followed by a further immersion in aqueous solutions of noble metals (approach 4) allowed a quantitative analysis of targeted ions in solid matrices (Fig. 6c, shown for NRS and Cu²⁺).

We believe that the proposed method of noble metal UV-detection within AAO matrices may have important application in controlling environmental NPs in the future.

Conclusions

In this study, a new approach for rapid analysis of non-noble metals (Co²⁺, Pb²⁺, and Ni²⁺) with visual detection based on anodic aluminium oxide films was introduced. Selective electroless galvanic reduction of noble metal ions (Ag⁺, Cu²⁺, and Pd²⁺) within the AAO matrix allowed simultaneous UV-detection of their nanoparticles making AAO films a promising analytical tool for environmental NP monitoring.

The next step in our study will involve investigation of sorption behavior of non-noble metals in the presence of interfering noble ions as well as UV-quantification of NPs from the multi model solutions and waste water systems.

Acknowledgements

The authors thank Prof. Eduard Arzt (Leibniz Institute for New Materials, Saarbrücken, Germany) for continuing support of their work at INM. The authors also thank Dr Claudia Fink-Straube (Leibniz Institute for New Materials, Saarbrücken, Germany) and Prof. Dietrich A. Volmer (Institute for Bioanalytical Chemistry, Saarbrücken, Germany) for fruitful discussion and proof-reading the paper.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ay02498f

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