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Interferometric nanoporous anodic alumina photonic coatings for optical sensing Yuting Chen1,2,3, Abel Santos1*,Ye Wang1, Tushar Kumeria1, Changhai Wang2*, Junsheng Li3* and Dusan Losic1*

Received 00th January 2015, Accepted 00th January 2015 DOI: 10.1039/x0xx00000x www.rsc.org/

Herein, we present a systematic study on the development, optical optimization and sensing applicability of colored photonic coatings based on nanoporous anodic alumina films grown on aluminum substrates. These optical nanostructures, so-called distributed Bragg reflectors (NAA-DBRs), are fabricated by galvanostatic pulse anodization process, in which the current density is altered in a periodic manner in order to engineer the effective medium of the resulting photonic coatings. As-prepared NAA-DBR photonic coatings present brilliant interference colors on the surface of aluminum, which can be tuned at will within the UVvisible spectrum by means of the anodization profile. A broad library of NAA-DBR colors is produced by means of different anodization profiles. Then, the effective medium of these NAA-DBR photonic coatings is systematically assessed in terms of optical sensitivity, low limit of detection and linearity by reflectometric interference spectroscopy (RIfS) in order to optimize their nanoporous structure toward optical sensors with enhanced sensing performance. Finally, we demonstrate the applicability of these photonic nanostructures as optical platforms by selectively detecting gold (III) ions in aqueous solutions. The obtained results reveal that optimized NAA-DBR photonic coatings can achieve an outstanding sensing performance for gold (III) ions, with a sensitivity of 22.16 nm µM -1, a low limit of detection of 0.156 µM (i.e. 30.7 ppb) and excellent linearity within the working range (0.9983).

Introduction Metal surface finishing based on the generation of nanoporous anodic alumina (NAA) coatings has been extensively used since the first decades of the 20th century in different industrial applications such as corrosion protection, automobile engineering and metal decoration.1,2 Among these applications, metal coloring is of great interest as a result of its broad applicability (e.g. steel and support structures, machinery and equipment, cookware, electronic devices, aerospace, etc.). Traditional metal coloring approaches are based on the use of toxic dyes or metals, which are incorporated into the oxide coating after anodization by infiltration and AC electrodeposition processes, repectively.3-9 However, these approaches present some inherent limitations as these chemical compounds degrade, release and erode with use and can be extremely dangerous for the environment and human health as they are highly toxic. Therefore, alternative coloring methods based on environment-friendly and non-toxic approaches are urgently needed in order to address these issues. Recently, some electrochemical approaches have demonstrated that photonic coatings based on nanoporous anodic alumina (i.e. layer of aluminum oxide that is generated on the surface of aluminum by electrochemical anodization) can overcome the aforementioned drawbacks of current coloring technology.10-13 In these alternative coatings, color is generated by means of the interaction between light and matter, which can be precisely

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tuned by engineering the nanoporous structure of the NAA coating through the anodization parameters.14-21 In this way, light can be selectively reflected at specific wavelengths within the UV-visible light region, endowing aluminum with color without using degradable, expensive and toxic chemical compounds. Regardless of the interesting optical properties of these photonic coatings, the potential applicability of these optical nanostructures as sensing platforms has almost been neglected. Note that NAA has unique chemical and physical chemical and physical stability, presents stable optical signals without passivation, can be easily functionalized and the interaction light-matter can be tailored by engineering its nanoporous structure. Surprisingly, to the best of our knowledge, no studies have systematically assessed the capabilities of colored aluminum coatings as optical sensing platforms. Colored aluminum can be produced by galvanostatic or potentiostatic pulse anodization approach under mild conditions in sulfuric, oxalic and phosphoric acid electrolytes. In this process, current density or voltage pulses are cyclically applied in order to engineer the nanoporous structure of NAA in depth. This electrochemical approach, known as structural engineering, makes it possible to switch the effective refractive index of NAA between high (neff-high – low anodization current density or voltage) and low (neff-low – high anodization current density or voltage) values by means of the nanopore geometry.11-13,22 Previous studies have made good use of this

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approach in order to develop some photonic nanostructures based on NAA such as distributed Bragg reflectors (DBRs) and rugate filters.11,14,22-31 Although the resulting NAA-based optical nanostructures can be combined with optical techniques such as reflectometric interference spectroscopy (RIfS) in order to develop optical sensing systems, these photonic coatings are colorless, unless the underlying aluminum substrate is removed, as they cannot reflect light efficiently due to the intrinsic limitations of NAA (i.e. low refractive index – nalumina = 1.67 and low porosity contrast between layers). This limitation, however, can be overcome by increasing the concentration of the acid electrolyte so that NAA layers feature high porosity.10 This along with a pulse anodization approach makes it possible to achieve a high effective refractive index contrast between layers (i.e. neff-high - neff-low). As a result, these nanoporous oxide coatings grown on the surface of aluminum substrates can reflect light at specific wavelengths within the UV-visible range

Nanoscale DOI: 10.1039/C5NR00369E in an efficient manner, enabling the production of colored aluminum. In this context, we present a systematic and comprehensive study on the development, assessment, optical optimization and application of colored NAA photonic coatings for metal decoration and sensing applications (Fig. 1). First, we develop fabrication procedures aimed to produce a broad library of NAA-DBR structures featuring different colors within the UVvisible range. Then, the effective medium of the resulting photonic coatings is assessed systematically by RIfS in order to establish the most optimal nanostructure in terms of optical sensitivity, low limit of detection and linearity. Finally, we demonstrate the applicability of NAA-DBR coatings as a generic optical sensing platform by developing an optical sensor for gold (III) ions.

Fig. 1 NAA-DBR photonic coatings for coloring aluminum and optical sensing. a) Illustration of electrochemical pulse anodization process used to produce NAA-DBR photonic coatings (detail showing how the effective medium is engineered in depth by modulating the nanopores geometry). b) Geometric features of nanopores in NAA-DBR coatings (i.e. total thickness of the coating (LT), the lengths of segments with high and low effective refractive index (Lhigh and Llow, respectively) and the period length (LTp) defined as Lhigh + Llow). c) Structurally colored aluminum produced by selective reflection of light at specific wavelengths within the UV-visible range. The reflection of light can be tuned by engineering the geometric features of nanopores in NAA-DBR coatings by specific pulse anodization conditions.

Experimental Materials Aluminum (Al) foils of thickness 0.32 mm and purity 99.9997% were supplied by Goodfellow Cambridge Ltd. (UK). Sulfuric acid (H2SO4), phosphoric acid (H3PO4), perchloric acid

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(HClO4), chromium trioxide (CrO3), 3-(mercaptopropyl)trimethoxysilane (MPTMS), hydrogen peroxide (H2O2), gold (III) chloride (AuCl3), D-glucose (C6H12O6), isopropanol (C3H8O) and ethanol (C2H5OH) were purchased from SigmaAldrich (Australia) and used without further processing. Ultrapure water Option Q–Purelabs (Australia) was used for preparing all the solutions used in this study.

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Fabrication of NAA-DBR coatings with tunable colors NAA-DBR coatings were fabricated by a galvanostatic pulse anodization approach under mild conditions in sulfuric acid electrolyte. Prior to anodization, 1.5 x 1.5 cm square Al substrates were cleaned under sonication in ethanol (EtOH) and distilled water for 15 min each. Subsequently, Al substrates were electropolished in a mixture of EtOH and HClO4 4:1 (v:v) at 20 V and 5⁰C for 3 min. Then, electropolished Al substrates were first anodized in an aqueous solution 1.1 M sulfuric acid at a current density of 1.12 mA cm-2 for 1h at 3⁰C. Then, the anodization profile was switched to pulse mode, in the course of which the current density was cyclically pulsed between a high current density pulse (Jhigh = 1.12 mA cm-2) and a low current density pulse (Jlow = 0.28 mA cm-2) to engineer the effective refractive index of NAA in depth. In order to create a broad library of NAA-DBR structures and cover a wide range of colors within the UV-visible spectrum, the length of the current density pulse period (Tp) and the number of pulses (Np), were set to four and three different values (i.e. Tp – 1170, 1035, 900 and 675 s and Np – 100, 150 and 200 pulses), respectively. Note that Tp is defined as the total time length of high and low anodization current density pulses (Eq. 1): ܶ௣ = ‫ݐ‬௛௜௚௛ + ‫ݐ‬௟௢௪

(1)

where thigh and tlow are the time duration of high (Jhigh) and low (Jlow) anodization current density pulses, respectively. The ratio between thigh and tlow was set to thigh:tlow = 1:4. So, a total of 12 different types of NAA-DBR coatings were produced in this study, the fabrication characteristics of which (i.e. Tp and Np) are summarized in Table 1. Fig. 2a depicts an example of anodization profile used in this study to NAA-DBR photonic coatings, where the main parameters (i.e. Jhigh, Jlow, thigh and tlow) are defined. Table 1. NAA-DBR photonic coatings produced in this study as a function of the fabrication conditions (i.e. Tp and Np). Np Tp (s) 100 150 200 NAA-DBR(675-100) NAA-DBR(675-150) NAA-DBR(675-200) 675 NAA-DBR(900-100) NAA-DBR(900-150) NAA-DBR(900-200) 900 1035 NAA-DBR(1035-100) NAA-DBR(1035-150) NAA-DBR(1035-200) 1170 NAA-DBR(1170-100) NAA-DBR(1170-150) NAA-DBR(1170-200) Assessment of effective medium of NAA-DBR coatings The optical properties in terms of sensitivity, low limit of detection and linearity of the different NAA-DBR coatings produced in this study were assessed by RIfS. RIfS is an optical technique based on the constructive interference of reflected light that takes place when a white light beam interacts with a thin film. Light reflected by the film is enhanced at those wavelengths corresponding to the optical modes of the Fabry– Pérot cavity (i.e. film–surrounding medium). This phenomenon, known as the Fabry–Pérot effect, is translated into wellresolved and narrow fringes/oscillations in the RIfS spectrum of the film, from which the effective optical thickness (OTeff) of the film can be estimated. The wavelength of each fringe maximum in the RIfS spectrum can be estimated by the Fabry– Pérot relationship (Eq. 2): ܱܶ௘௙௙ = 2݊௘௙௙ ‫ߣ݉ = ߠݏ݋ܥ ் ܮ‬

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(2)

where OTeff is the effective optical thickness of the film, neff is its effective refractive index, LT is its physical thickness, θ is the angle the light travels through the film (i.e. in this case 0⁰) and m is the order of the fringe in the RIfS spectrum, the maximum of which is located at the wavelength λ. As a result, any modification of the effective medium of the film can be estimated by means of the effective optical thickness change (∆OTeff), which can be calculated by applying fast Fourier transform (FFT) to the RIfS spectrum.32-36 In this study, we used this principle as a means of assessing the optical performance of NAA-DBR coatings and establishing the most optimal structure in terms of sensitivity, low limit of detection and linearity. To this end, the effective medium of these nanoporous structures was modified systematically by infiltrating their nanopores with different solutions (i.e. D-glucose 0.5 M, ethanol and isopropanol (IPA)) in order to achieve a broad contrast of the refractive index of the medium filling the nanopores (i.e. 1.349, 1.362 and 1.378 RIU, respectively). The effective optical thickness changes in NAA-DBR coatings were monitored in real-time using a flow cell combined with a RIfS system. A detailed explanation of this calculation process is provided in Fig. S1 – ESI. Briefly, this system is composed of a bifurcated optical probe that focuses white light from a source (LS-1LL, Ocean Optics, USA) on the surface of these photonic coatings with an illumination spot of 2 mm in diameter. Reflected light from this spot is collected by the collection fiber, which is assembled in the same optical probe. Reflected light is then transferred to a miniature spectrometer (USB4000+VIS-NIR-ES, Ocean Optics, USA). UV-visible optical spectra were acquired from 400 to 1000 nm and saved at intervals of 30 s, with an integration time of 10 ms and 10 average measurements. These RIfS spectra were processed in Igor Pro library (Wavemetrics, USA) in order to estimate ∆OTeff. The flow rate of the different solutions through the flow cell was maintained constant at 100 µL min-1 by a syringe pump (Fusion 200 Touch series, Chemyx Inc., USA). Optical sensing of gold (III) ions After assessing the effective medium of the different NAADBR coatings fabricated in this study, the most optimal NAADBR structure was used to demonstrate the capability of these photonic coatings as optical sensing platforms. To endow NAA-DBR coatings with surface chemistry selectivity toward gold (III) ions, the inner surface of nanopores was chemically modified with MPTMS following a well-established silanization protocol.37,38 Prior to functionalization, the inner surface of NAA-DBR coatings was hydroxylated in boiling H2O2 30 wt% for 10 min at 90⁰C. After this, these coatings were dried under nitrogen stream and functionalized with MPTMS via chemical vapor deposition at 135⁰C for 3 h. Subsequently, NAA-DBR structures were washed with ethanol and water in order to remove any physisorbed MPTMS molecule. A stock solution of Au3+ ions (1 mM) was prepared by dilution of AuCl3 in ultrapure water. Different analytical solutions of gold (III) ions were prepared by further dilution of the stock solution. Five different concentrations of Au3+ ranging from 0.05 to 5 µM were prepared and subsequently used for assessing the sensing performance of the proposed NAA-DBR sensors. Sensing experiments were performed in a custom designed and fabricated flow cell. Real-time sensing measurements were carried out at a flow rate of 100 µL min-1. A stable baseline with ultrapure water was established for 15

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Structural characterization The structural characteristics of the different types of NAADBR coatings were estimated from SEM images acquired by a field emission gun scanning electron microscope (FEG-SEM FEI Quanta 450). These images were subsequently analyzed by ImageJ (public domain program developed at the RSB of the NIH).40 Note that all the aforementioned experiments were repeated three times with freshly prepared samples and solutions. Furthermore, the obtained values of the different characteristic parameters were calculated as averages and standard deviations.

Results and discussion Fabrication and structural characterization of NAA-DBR photonic coatings Fig. 2b depicts the basic structure of NAA-DBR photonic coatings assessed in this study. The geometric features of these photonic nanostructures are the total thickness of the coating (LT), the lengths of the segments in the structure of the NAADBR with high and low effective refractive index (Lhigh and Llow, respectively) and the period length (LTp), which is defined as Lhigh + Llow. As commented above, NAA-DBR photonic coatings were fabricated by electrochemical anodization of aluminum substrates through galvanostatic pulse anodization. This electrochemical approach makes it possible to engineer the structure of these photonic coatings in depth by modulating the pore diameter through the anodization profile (Fig. 2c). In this way, the interaction light-matter can be tuned by switching the effective refractive index of the nanoporous material between high (neff-high – low anodization current density) and low (neff-low – high anodization current density) values. Similar approaches have been extensively used to develop a variety of biomimetic photonic materials.41-43 As a result of their optical properties, NAA-DBR photonic coatings experience sharp changes in their effective optical thickness when their nanoporous structure is filled with different media (Figs. 2d and e). To gain insight into the different parameters affecting the optical properties of NAA-DBR structures (i.e. color and sensing performance) and create a complete library of colored NAA-DBR coatings, the anodization parameters Tp and Np (vide supra) were broadly modified. While Tp was set to four different values (Tp – 1170, 1035, 900 and 675 s), Np was set to three different values for each of these current density pulse periods (Np – 100, 150 and 200 pulses). Geometrically, NAA-DBRs can be described as a stack of layers of NAA nanopores featuring periodic increments and decrements of porosity in depth. While the periodicity of these increments and decrements of porosity can be established by the anodization profile, other geometric features such as pore density, pore diameter and porosity can be precisely controlled by other anodization parameters such as the ratios Jhigh:Jlow and thigh:tlow, the type of acid electrolyte and its temperature and concentration. Fig. 3 compiles a set of anodization profiles of four types of NAA-DBR photonic

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coatings produced in this study along with the corresponding cross-section SEM images. In these nanostructures, the effective medium is structurally engineered in depth by means of a stepwise anodization current density profile. As a result, the color of the resulting NAA-DBR coating can be precisely engineered by the anodization profile and the anodization conditions. Fig. 4 depicts a set of digital images of the resulting NAA-DBR photonic coatings as a function of the fabrication parameters. These images reveal that NAA-DBR coatings can be produced featuring bright colors such as yellow, brown, pink and purple simply by tuning the anodization parameters. The position of the wavelength at which the corresponding NAA-DBR photonic coating reflects light more efficiently can be estimated by the Bragg’s law44 adapted to NAA-DBR coatings (Eq. 3): ଶ ଶ ݉ߣ = 2(‫ܮ‬௛௜௚௛ ට݊௘௙௙ି௛௜௚௛ − ܵ݅݊ߠ + ‫ܮ‬௟௢௪ ට݊௘௙௙ି௟௢௪ − ܵ݅݊ߠ)

(3)

where m is the diffraction order, the maximum of which is located at the wavelength λ, Lhigh and Llow are the lengths of the segments in the structure of the NAA-DBR with high and low effective refractive index, respectively. In the case of RIfS, light is shinned at normal incidence (i.e. θ = 0⁰). Therefore, the Bragg’s law can be simplified as Eq. 4: ݉ߣ = 2(‫ܮ‬௛௜௚௛ ݊௘௙௙ି௛௜௚௛ + ‫ܮ‬௟௢௪ ݊௘௙௙ି௟௢௪ )

(4)

This equation denotes that NAA-DBR photonic coatings can reflect light at longer wavelengths when they are produced with longer pulse periods given that Tp is directly proportional to Lhigh and Llow (i.e. ↑Tp → ↑(Lhigh + Llow) → ↑λ). However, it is worthwhile noting that anodisation under mild conditions is an electrochemical process controlled by the diffusion of ionic species (i.e. Al3+ and O2-) through the oxide barrier layer at the pore bottom tips. In particular, when the anodisation current density is switched from high to low a pulse anodisation approach, there is a slow potential recovery process due to the thickness of the oxide barrier layer. As a result, the growth rate of the different layers does not change linearly with the pulse period length. Nevertheless, the longer the pulse period (↑Tp) the thicker the period length (↑(Lhigh + Llow)). Therefore, according to the Bragg’s law, NAA-DBR photonic coatings can be produced featuring a broad range of colors by means of Tp. This approach makes it possible to design the color of the resulting coating at will within the UV-visible range. Our study also reveals that the color intensity of these coatings increases with the number of cycles as light can be reflected more efficiently when the number of layers in the NAA-DBR structure increases. In addition, Fig. S2 (ESI) demonstrates that these photonic coatings experience a sharp red shift in color when their nanopores are filled with a medium with higher refractive index than air (e.g. isopropanol – nisopropanol = 1.378 RIU). This can be explained by the Fabry–Pérot relationship (Eq. 2). This property indicates that these nanostructures are highly sensitive toward changes in the effective medium, opening new opportunities for these photonic structures to be directly used as platforms for visual/colorimetric sensing. Another interesting property of these nanoporous structure is that they allow analyte molecules to be selectively immobilized inside the nanopores when the surface chemistry of alumina is modified by functional molecules such as thiol groups. Motivated by these results, we decided to explore the potential of NAA-DBR photonic coatings as optical sensing platforms.

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min prior to injection of the different analyte solutions into the flow cell. The contact of Au3+ ions with thiol groups on MPTMS-functionalized NAA-DBR coatings produced sharp changes in the OTeff of the film and continued till total saturation of the thiol groups on the inner surface of nanopores.39 Finally, water was flowed again in order to remove physisorbed gold (III) ions and establish the total ∆OTeff associated with immobilized Au3+ ions.

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Fig. 2 Structural engineering and optical sensing principle in NAA-DBR photonic coatings. a) Example of anodization profile used to produce different NAADBR photonic coatings by modifying the fabrication parameters (i.e. Jhigh, Jlow, thigh and tlow). b) Definition of the basic geometric features in NAA-DBR photonic coatings (i.e. LT, LTp, Lhigh and Llow). c) Illustration showing how the effective medium is engineered in depth by means of the geometric features in NAA-DBRs. d) Example of sensing principle based on effective optical thickness changes used in NAA-DBR photonic coatings. e) Illustration depicting the visual effective optical thickness change and digital picture showing a NAA-DBR structure with color change after selective infiltration of its nanopores with isopropanol (IPA)).

Assessment of optical characteristics of NAA-DBR photonic coatings by RIfS The optical properties of composite materials such as NAADBR photonic coatings can be described by the effective medium approximation, where the macroscopic properties of composite materials can be estimated from averaging the multiple values of individual constituents that compose the composite material.45 NAA-DBR photonic coatings can be described as a composite material based on stacks of NAA layers featuring different porosity levels. Therefore, a systematic assessment of the effective medium of these composite photonic structures makes it possible to quantify how sensitive a NAA-DBR structure is toward changes in the refractive index of the medium filling the nanopores. Following this principle, we assessed the optical characteristics of the resulting NAA-DBRs by measuring changes in the effective optical thickness associated with different levels of refractive

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index of the medium filling the nanopores. This was accomplished by filling the nanoporous matrix of these photonic nanostructures with three different solutions (i.e. Dglucose 0.5 M, ethanol and isopropanol). As mentioned above, an increment of the refractive index of the medium filling the nanopores of NAA-DBRs yields a red shift in its effective optical thickness (Figs. S1 and S2). Therefore, the optical properties of the NAA-DBR photonic coatings were evaluated by measuring ∆OTeff in RIfS using as a reference a stable baseline obtained with water. This process was carried out in a flow cell, through which the different solutions were flowed at a constant rate of 100 µL min-1 (Fig. S1). In this way, we established the most sensitive NAA-DBR structure toward changes in the refractive index of the medium filling the nanopores. The obtained results are depicted in Fig. 5 and the optical characteristics of each NAA-DBR coating are summarized in Table 2.

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Fig. 3 Examples of anodization profiles, cross-section images of NAA-DBR photonic coatings and illustration showing the preferential wavelength at which these photonic nanostructures reflect light more efficiently when the pulse period is changed (scale bars = 500 nm) (insets show digital pictures of real NAADBR photonic coatings of 1 cm in diameter). a-c) NAA-DBR675-100. d-f) NAA-DBR900-100. g-h) NAA-DBR1035-100. k-l) NAA-DBR1170-100.

These results reveal that the most sensitive NAA-DBR structure toward changes in the effective medium was NAADBR1035-150, which presented a sensitivity (S) of 37931 ± 3001 nm RIU-1, a low limit of detection (LLoD) of 0.352 ± 0.04 RIU and a linearity (R2) of 0.9876 (Table 2 and Table S1). Contour plots of these results are shown in Figs. 6 and S3, which provide a more comprehensive insight into the existing relationship between the geometric characteristics of NAADBR photonic coatings (i.e. Tp and NP) and the different optical

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characteristics (i.e. S, LLoD and R2). Likewise in topographic or isobaric surface maps, contour plots make it easier to visualize the paths with the highest or lowest slope between the highest and the lowest values of the corresponding sensing parameter. Fig. 6a illustrates the relationship between sensitivity and the periodicity and the number of pulses in the studied NAA-DBR photonic coatings. This analysis reveals that the distribution of sensitivity with Tp and Np is fairly homogeneous and lines between color fields are closer to each other in a similar manner

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Nanoscale throughout the contour plot. The distribution of color fields is homogeneous around the maximum value, which was achieved by the photonic coating NAA-DBR1035-150 (i.e. 37931 ± 3001 nm RIU-1). Fig. 6b shows the results obtained for the relationship between the low limit of detection of NAA-DBR photonic coatings, which was calculated as 3σ and expressed in refractive index units, and Tp and Np. This result denotes that the dependence of LLoD with the geometric features of NAADBR photonic coatings is more heterogeneous than that shown for S, with a more variant distribution of color fields throughout the contour plot. The lowest LLoD was achieved by the photonic coating NAA-DBR1035-200 (i.e. 0.147 ± 0.01 RIU). Finally, Fig. S3 depicts the dependence of the linear response of NAA-DBR toward changes in the medium filling their nanopores with Tp and Np. The obtained results demonstrate that the different NAA-DBRs analysed in this study present small differences in linearity, which are a logical consequence of the RIfS method (Table S1). Nevertheless, it is worthwhile mentioning that the highest linearity was achieved by NAADBR1035-200 (i.e. 0.9978). In summary, these results reveal that NAA-DBR produced with a pulse period of 1035 s provide the best optical characteristics in terms of sensitivity, low limit of detection and linearity. Nevertheless, these properties are dependent on the number of pulses as well. So, whereas the NAA-DBR structure with 150 pulses provides the best sensitivity, the same type of NAA-DBR photonic coating produced with 200 pulses achieves the lowest limit of detection and the most linear response. From these findings, we decided to assess the sensing performance of the most sensitive photonic nanostructure (i.e. NAA-DBR1035-150) when detecting gold (III) ions in a chemically selective manner.

Fig. 4 Digital pictures of NAA-DBR photonic coatings produced in this study. Note that aluminum substrates were electrochemically anodized through a circular window of 1 cm in diameter.

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PAPER DOI: 10.1039/C5NR00369E Demonstration of NAA-DBR photonic coatings and RIfS as an optical sensing system for gold (III) ions As a proof of applicability and general concept, the most sensitive NAA-DBR photonic coating structure (i.e. NAADBR1035-150) was evaluated as a potential optical sensing platform for detecting gold (III) ions (Au3+) in water under specific adsorption conditions. To endow NAA-DBR structures with chemical selectivity toward Au3+ ions, the surface chemistry of these photonic nanoporous structures was functionalized with thiol functional groups through a wellestablished silanization process reported elsewhere (Fig. 7a).37 Note that thiol groups have strong affinity toward gold ions and mercapto-silane functionality was specifically chosen because the thiol functional group is known to bind selectively and specifically to Au3+ ions.

Fig. 5 Optical assessment of NAA-DBR photonic coatings by RIfS as a function of the fabrication parameters (i.e. Tp and Np) and the refractive index of the medium filling the nanopores (i.e. nmedium). a) NAA-DBR photonic coatings with 100 anodization pulses. b) NAA-DBR photonic coatings with 150 anodization pulses. c) NAA-DBR photonic coatings with 200 anodization pulses.

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Nanoscale DOI: 10.1039/C5NR00369E freshly prepared thiol-functionalized NAA-DBR structures were used to characterize the sensing capabilities of these nanoporous platforms combined with RIfS. Fig. 7b depicts the sensing performance of these NAA-DBR structures when detecting gold ions in aqueous solution. This optical sensing system presented a sensitivity of 22.16 ± 0.46 nm µM-1, with a low limit of detection of 0.156 µM (i.e. 30.7 ppb) and excellent linear response with the analyte concentration (i.e. 0.9983). Therefore, these photonic structures could be used as sensing platforms in optical sensing systems aimed at detecting heavy metal ions in water sources with mining, environmental and safety purposes. Finally, Table 3 compiles a summary of the most representative studies on NAA-based optical structures combined with RIfS. As these results show, the presented photonic structures based on NAA-DBRs can achieve outstanding sensitivities as compared to previously reported systems.

Fig. 6 Contour plots showing the distribution of the optical characteristics of NAA-DBR photonic coatings (i.e. sensitivity (S) and low limit of detection (LLoD)) as a function of the pulse period and the number of pulses. a) Sensitivity in terms of change in sensing parameter (i.e. ∆OTeff) expressed in percentage per refractive index unit. b) Low limit of detection calculated as 3σ and expressed in refractive index units.

Table 2. Optical characteristics of NAA-DBR photonic coatings (i.e. sensitivity – S and low limit of detection – LLoD) assessed by RIfS after infiltration with glucose, ethanol and isopropanol. NAA-DBR

S (nm RIU-1)

LLoD (RIU)

NAA-DBR(675-100)

19346 ± 2268

0.522 ± 0.05

NAA-DBR(900-100)

31233 ± 1450

0.207 ± 0.05

NAA-DBR(1035-100)

25765 ± 3465

0.599 ± 0.02

NAA-DBR(1170-100)

34943 ± 3941

0.502 ± 0.03

NAA-DBR(675-150)

25810 ± 1167

0.201 ± 0.02

NAA-DBR(900-150)

30600 ± 2962

0.431 ± 0.04

NAA-DBR(1035-150)

37931 ± 3001

0.352 ± 0.04

NAA-DBR(1170-150)

26266 ± 1571

0.266 ± 0.02

NAA-DBR(675-200)

18787 ± 1269

0.294 ± 0.02

NAA-DBR(900-200)

27317 ± 900

0.149 ± 0.01

NAA-DBR(1035-200)

28803 ± 962

0.147 ± 0.01

NAA-DBR(1170-200)

13799 ± 912

0.301 ± 0.01

Fig. 7 Demonstration of NAA-DBR photonic coatings as optical sensing platforms for detecting gold ions (Au3+) in water. a) Scheme illustrating the surface chemistry modification using thiol chemistry. b) Calibration curve showing the sensing performance of NAA-DBR photonic coatings as a function of the analyte concentration ([Au3+]).

A total of five different concentrations of gold ions were used in this study (i.e. 0.05, 1, 2, 3, 4 and 5 µM) and a set of

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PAPER DOI: 10.1039/C5NR00369E

S

LLoD

(nm µM-1)

(µM)

0.01

10.00

R2

Ref.

NAA

Analyte

[11]

Rugate Filter

Glucose

[12]

Rugate Filter

Mercury Ions

0.07

1.00

0.9920

[13]

Straight Pore

Mercury Ions

0.69

22.82

0.8540

0.9980

[13]

NAA-DBR

8.76

4.20

0.9940

[39]

Straight Pore

Gold Ions

1.09

0.10

0.9900

NAA-DBR

Gold Ions

22.2

0.16

0.9983

This Study

Conclusions To summarize, this study has reported on the structural engineering of colored photonic coatings based on nanoporous anodic alumina distributed Bragg reflectors produced by galvanostatic pulse anodization in sulfuric acid electrolyte. NAA was selected as the material of choice to develop these photonic nanostructures due to its unique chemical and physical properties, including a cost-effective and fully scalable fabrication process, versatile nanopore geometry, chemical and physical stability under harsh media, stable optical signals, mechanical robustness, easy chemical functionalization and capability to tune the interaction light-matter by engineering its nanoporous structure through electrochemical pulse anodization approach. Twelve different types of NAA-DBR photonic coatings were fabricated by modifying the anodization conditions. This made it possible to engineer the effective medium of NAA-DBRs and thus generate a broad library of colored aluminum substrates. Subsequently, the effective medium of these nanoporous photonic coatings was assessed by reflectometric interference spectroscopy in order to discern which NAA-DBR structure is more suitable to develop optical sensing systems. The obtained results revealed that the most sensitive NAA-DBR structure toward changes in the effective medium was NAA-DBR1035-150, which was produced with a periodicity of 1035 s and 150 anodization pulses. This analysis enabled the development of an optimal sensing system, which was demonstrated by modifying the surface chemistry of these NAA-DBR photonic coatings with functional groups with affinity toward gold ions. This optical system presented an outstanding sensing performance for gold (III) ions, with a sensitivity of 22.16 nm µM-1, a low limit of detection of 0.156 µM (i.e. 30.7 ppb) and excellent linearity within the working range (0.9983). The proposed optical sensing system combining colored aluminum and reflectometric interference spectroscopy is envisaged for portable and low-cost detection systems with applications in environmental, health safety and mining technologies.

High Technology Research and Development Program of China (2012AA021706).

Notes and references 1 School of Chemical Engineering, The University of Adelaide, Engineering North Building, 5005 Adelaide, Australia. 2 Jiangsu Key Laboratory of Marine Biology, College of Resources and Environmental Science, Nanjing Agricultural University, 210095 Nanjing, P. R. China. 3 College of Food Science and Technology, Nanjing Agricultural University, 210095 Nanjing, P. R. China. * E-mails: [email protected] / [email protected] / [email protected] / [email protected]

Electronic Supplementary Information (ESI) available: The Supporting Information file provides further information about real-time monitoring of ∆OTeff with changes in the refractive index of the medium filling the nanopores, demonstration of visual red shift in a NAA-DBR sample after infiltration with isopropanol and calculations of linearity (R2) for each NAA-DBR coating. See DOI: 10.1039/b000000x/ 1 2 3 4 5

6 7 8 9 10 11 12

13

Acknowledgements Authors thank the support provided by the Australian Research Council (ARC) through the grants number DE140100549, DP120101680 and FT110100711 and the School of Chemical Engineering (UoA). Authors thank the Adelaide Microscopy (AM) centre for FEG-SEM characterization. Authors also thank the support from Nanjing Agricultural College dominant disciplines Innovation Fund 080-80900211-52, Natural Science Foundation of Jiangsu Province (BK20140713) and National

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W. Lee and J. S. Park, Chem. Rev., 2014, 114, 7487-7556. W. Lee, R. Ji, U. Gösele and K. Nielsch, Nat. Mater., 2006, 5, 741-747. J. J. Zhang, Z. Y. Li, Z. J. Zhang, T. S. Wu and H. Y. J. Sun, Appl. Phys., 2013, 113, 244305. M. Zemanova, M. Chovancova, Z. Galikova and P. Krivosik, Renewable Energy, 2008, 33, 2303-2310. I. Tsangaraki-Kaplanoglou, S. Theohari, Th. Dimogerontakis, N. Kallithrakas-Kontos, Y. M. Wang, H. H. Kuo and S. Kia, Surf. Coat. Technol., 2006, 201, 2749-2759. C. J. Donahue and J. A. Exline, J. Chem. Educ., 2014, 91, 711715. A. Hakimizad, K. Raeissi and F. Sahrafizadeh, Surf. Coat. Technol., 2012, 206, 2438-2445. D. G. W. Goad and M. Moskovits, J. Appl. Phys., 1978, 49, 2929-2934. S. Van Gils, P. Mast, E. Stijns and H. Terryn, Surf. Coat. Technol., 2004, 185, 303-310. L. Yisen, C. Yi, L. Zhiyuan, H. Xing and L. Yi, Electrochem. Commun., 2011, 13, 1336-1339. T. Kumeria, M. M. Rahman, A. Santos, J. Ferré-Borrull, L. F. Marsal and D. Losic, Anal. Chem., 2014, 86, 1837-1844. T. Kumeria, M. M. Rahman, A. Santos, J. Ferré-Borrull, L. F. Marsal and D. Losic, ACS Appl. Mater. Interfaces, 2014, 6, 12971-12978. T. Kumeria, A. Santos, M. M. Rahman, J. Ferré-Borrull, L. F. Marsal and D. Losic, ACS Photonics, 2014, 1, 1298-1306. K. Schwirn, W. Lee, R. Hillebrand, M. Steinhart, K. Nielsch and U. Gösele, ACS Nano, 2008, 2, 302-310. W. Lee, K. Schwirn, M. Steinhart, E. Pippel, R. Scholz and U. Gösele, Nat. Nanotechnol., 2008, 3, 234-239. W. Lee, J. C. Kim, Nanotechnology, 2010, 21, 485304. W. Lee, J. C. Kim and U. Gösele, Adv. Funct. Mater., 2009, 19, 1-7. H. Masuda and K. Fukuda, Science, 1995, 268, 1466-1468.

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Table 3. Summary of optical characteristics of different NAAbased photonic photonic structures combined with RIfS to detect different analytes.

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Nanoscale DOI: 10.1039/C5NR00369E H. Masuda and F. J. Hasegwa, Electrochem. Soc., 1997, 144, L127-L130. H. Masuda, K. Yada and A. Osaka, Jpn. J. Appl. Phys., 1998, 37, L 1340-L 1342. K. Nielsch, J. Choi, K. Schwirn, R. B. Wehspohn and U. Gösele, Nano Lett., 2002, 2, 677-680. O. Jessensky, F. Müller and U. Gösele, Appl. Phys. Lett., 1998, 72, 1173–1175. M. M. Rahman, L. F. Marsal, J. Pallarès and J. Ferré-Borrull, ACS Appl. Mater. Interfaces, 2013, 5, 13375-13381. A. Santos, T. Kumeria and D. Losic, Materials, 2014, 7, 42974320. T. Kumeria, A. Santos and D. Losic, Sensors, 2014, 14, 1187811918. A. Santos, T. Kumeria and D. Losic, TrAC, Trends Anal. Chem., 2013, 44, 25-38. W. J. Zheng, G. T. Fei, B. Wang, Z. Jin and L. D. Zhang, Mater. Lett., 2009, 63, 706-708. Q. Xu, H. Y. Sun, Y. H. Yang, L. H. Liu and Z. Y. Li, Appl. Surf. Sci., 2011, 258, 1826-1830. Y. Liu, H. H. Wang, J. E. Indacochea and M. L. Wang, Sens. Actuators, B, 2011, 160, 1149-1158. G. L. Shang, G. T. Fei, Y. Zhang, P. Yan, S. H. Xu and L. D. Zhang, J. Mater. Chem. C, 2013, 1, 5285-5291. B. Wang, G. T. Fei, M. Wang, M. G. Kong and L. D. Zhang, Nanotechnology, 2007, 18, 365601. R. Dronov, A. Jane, J. G. Shapter, A. Hodges and N. H. Voelcker, Nanoscale, 2011, 3, 3109-3114. M. M. Orosco, C. Pacholski and M. J. Sailor, Nat. Nanotechnol., 2009, 4, 255-258. S. D. Alvarez, C. P. Li, C. E. Chiang, I. K. Schuller and M. J. Sailor, ACS Nano, 2009, 10, 3301-3307. A. Santos, T. Kumeria, Y. Wang and D. Losic, Nanoscale, 2014, 6, 9991-9999. A. Santos, V. S. Balderrama, M. Alba, P. Formentín, J. FerréBorrull, J. Pallarès and L. F. Marsal, Adv. Mater., 2012, 24, 1050-1054. A. M. Md Jani, I. M. Kempson, D. Losic and N. H. Voelcker, Angew. Chem., Int. Ed., 2010, 49, 7933-7937. A. Debrassi, A. Ribbera, W. M. de Vos, T. Wennekes and H. Zuilhof, Langmuir, 2014, 30, 1311-1320. T. Kumeria, A. Santos and D. Losic, ACS Appl. Mater. Interfaces, 2013, 5, 11783-11790. M. D. Abràmoff, P. J. Magalhaes and S. J. Ram, Biophotonics Int., 2004, 11, 36-42. J. Ge and Y. Yin, Angew. Chem. Int. Ed., 2011, 50, 1492-1522. Y. Zhao, Z. Xie, H. Gu, C. Zhu and Z. Gu, Chem. Soc. Rev., 2012, 41, 3297-3317. C. Fenzl, T. Hirsch and O. S. Wolfbeis, Angew. Chem. Int. Ed., 2014, 53, 3318-3335. P. A. Snow, E. K. Squire, P. St. J. Russell and L. T. Canham, J. Appl. Phys., 1999, 86, 1781-1784. W. Theiβ, S. Henkel and M. Arntzen, Thin Solid Films, 1995, 255, 177-180.

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Interferometric nanoporous anodic alumina photonic coatings for optical sensing.

Herein, we present a systematic study on the development, optical optimization and sensing applicability of colored photonic coatings based on nanopor...
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