A colorimetric sensor based on anodized aluminum oxide (AAO) substrate for the detection of nitroaromatics

Sensors and Actuators B 160 (2011) 1149–1158

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A colorimetric sensor based on anodized aluminum oxide (AAO) substrate for the detection of nitroaromatics Y. Liu a,b,∗ , H.H. Wang b , J.E. Indacochea c , M.L. Wang a a b c

Department of Civil and Environmental Engineering, Northeastern University, Boston, MA 02115, USA Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Department of Civil & Materials Engineering, University of Illinois at Chicago, IL 60607, USA

a r t i c l e

i n f o

Article history: Received 20 April 2011 Received in revised form 2 September 2011 Accepted 13 September 2011 Available online 1 October 2011 Keywords: AAO Interference color UV–vis spectra Sensor application Nitroaromatics detection

a b s t r a c t Simple and low cost colorimetric sensors for explosives detection were explored and developed. Anodized aluminum oxide (AAO) with large surface area through its porous structure and light background color was utilized as the substrate for colorimetric sensors. Fabricated thin AAO films with thickness less than ∼500 nm allowed us to observe interference colors which were used as the background color for colorimetric detection. AAO thin films with various thickness and pore-to-pore distance were prepared through anodizing aluminum foils at different voltages and times in dilute sulfuric acid. Various interference colors were observed on these samples due to their difference in structures. Accordingly, suitable anodization conditions that produce AAO samples with desired light background colors for optical applications were obtained. Thin film interference model was applied to analyze the UV–vis reflectance spectra and to estimate the thickness of the AAO membranes. We found that the thickness of produced AAO films increased linearly with anodization time in sulfuric acid. In addition, the growth rate was higher for AAO anodized using higher voltages. The thin film interference formulism was further validated with a well established layer by layer deposition technique. Coating poly(styrene sulfonate) sodium salt (PSS) and poly(allylamine hydrochloride) (PAH) layer by layer on AAO thin film consistently shifted its surface color toward red due to the increase in thickness. The red shift of UV–vis reflectance was correlated quantitatively to the number of layers been assembled. This sensitive red shift due to molecular attachment (increase in thickness) on AAO substrate was applied toward nitroaromatics detection. Aminopropyltrimethoxysilane (APTS) which can be attached onto AAO nanowells covalently through silanization and attract TNT molecules was coated and applied for TNT detection. UV–vis spectra of AAO with APTS shifted to the longer wavelength side due to TNT attachment. This red shift implied AAO thickness increased and positive detection of TNT molecules. It was also observed that both APTS and polyethyleneimine (PEI) were electron rich polymers which formed Meisenheimer complexes with TNT in solution and changed its color abruptly. This strong color change due to chemical reaction was applied as another approach for direct TNT detection. Commercial AAO films with long pores (60 ␮m) and white background color were coated with APTS or PEI and then exposed to TNT in solution. These membranes turned to pink rapidly and eventually became visibly orange after a few hours with a strong absorption around 500 nm that was consistent with the formation of Meisenheimer complexes. The visible color change can be observed by unaided eyes and is suitable for nitroaromatics detection at higher concentration while interference color red shift in AAO thin film is designed for nitroaromatics detection at monolayer (nm) level. © 2011 Elsevier B.V. All rights reserved.

1. Introduction There are more and more threats of explosive devices being set off under everyday normal peaceful life and war conditions leading to loss of life and serious injury. Therefore, there is a

∗ Corresponding author at: Department of Civil and Environmental Engineering, Northeastern University, Boston, MA 02115, USA. E-mail address: [email protected] (Y. Liu). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.09.040

great need to develop highly sensitive and reliable sensors able to detect very low levels of explosive molecules prior to detonation of the device and thus prevent loss of life. In addition to field use, some explosives, such as nitroaromatics, are blood and liver toxins which can be absorbed by the skin, lungs or gastrointestinal tract [1]. It is established that absorbing trinitrotoluene, TNT, by contact, inhalation or ingestion can cause cataracts, anemia, and abnormal liver function [2]. Therefore, detecting TNT and other nitroaromatics in soil or groundwater is necessary to reduce accidental exposure to these chemicals. Explosive detection has

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been an active research area for many years [3]. Military explosives including TNT, RDX (1,3,5-trinitroperhydro-1,3,5-triazine), and PETN ([3-nitrooxy-2,2-bis (nitrooxymethyl)propyl] nitrate) contain common nitro functional group. The difficulty in detecting these explosives is that their vapor pressures at room temperature are very low (vapor pressure of TNT is 10−5 Torr at 300 K [3]). Hence, developing explosive sensors with extremely high sensitivity and standoff detection remains a major challenge. Current explosive detectors, such as X-ray screening and nuclear imaging, are large, expensive and require skilled technicians to operate. Here, we report a simple, straightforward and low cost method for detecting explosives. The sensor is made of an anodized aluminum oxide (AAO) thin film which changes its color upon exposure to explosives. The associated color change may be observed directly by unaided eyes when nitroaromatics concentration is high, or may be detected spectroscopically through interference color red shift at monolayer adsorption level. Compared to other explosive detectors, the colorimetric sensors require minimum training before use. Nitroaromatics and related compounds, including TNT, DNT (dinitrotoluene), are electron-deficient due to the strong electronwithdrawing effect of the nitro groups. The electron-deficient aromatic rings can form anionic ␴-complexes with nucleophiles [4]. The as-formed Meisenheimer complexes show a purple-reddish color which can be used for nitroaromatics detection. Several chemicals which can react with nitroaromatics and change their color have been demonstrated for colorimetric detection of explosives [4–8]. Normally, the reagent to produce this color change is embedded in resin exchange matrices, polymeric membranes or porous thin films. These materials have high surface to volume ratio which allows for numerous contact sites between the reagent and nitroaromatics. After reacting with explosives, the color change of the reagent can be visually identified and measured by UV–vis absorption spectrometry. Heller et al. developed a tube detector made up of two sections. One section contained basic oxides which convert TNT into Meisenheimer anions; the other section was composed of ion exchange resin which collected the colored anions. Ammonium chloride was added in this section to prevent any discoloration of the purplish resin. They reported a detecting limit of TNT which was about 0.1 ppm [5]. Pamula et al. developed a droplet based microfluidic chip utilizing electrowetting to mix potassium hydroxide (KOH) with TNT dissolved in dimethyl sulfoxide (DMSO) [7]. The colored Meisenheimer complex was detected by a simple absorbance measurement system. The transport, mixing, reaction, and detection of TNT were all performed in a single microfluidic chip. The absorption spectra of Meisenheimer complex formed by TNT/DNT and KOH were obtained [7]. They have different peaks within visible range (400–700 nm), and thus can be differentiated by the color change. A linear range of detection for TNT from 4 to 20 ␮g/ml is demonstrated, and the time for each reaction-to-detection is about 2.5 min. Linear relationship between absorbance and the concentration of TNT measured on the chip was obtained. Xie et al. reported a surface molecular self-assembly strategy for imprinting polymer (APTS, aminopropyltrimethoxysilane) nanowires/nanotube arrays in porous alumina template [8]. The amino groups of APTS can form Meisenheimer complexes with TNT molecules which has specific absorption peaks in UV–vis spectrum. The absorption of TNT solution after adding APTS was around 500 nm and increased with the amount of APTS added [8]. Commercial detection kits for explosives based on colorimetry are available. Their detection limit is advertised as 5–10 ng [9]. ExprayTM detection kits developed by Plexus Scientific Company can locate and identify three types of explosives. Group A are nitroaromatics (including TNT, TNB, DNT, tetryl and picric acid) which can be detected by a strong base solution. This solution will be sprayed on any suspicious object. The colored Meisenheimer complexes are then swiped off with a white paper towel. This

requires direct contact with explosives which will expose the people collecting samples to potential risk. Besides, it is not an efficient method. Alumina thin films with various colors and self-ordered porous structures are promising optical parts in photoelectronics, optical integrated circuits, and sensing devices [10–13]. The color of AAO is produced through thin film interference which is controlled by the thickness and refractive index of this film [14]. The film properties are affected by anodization temperature, etching solution, time and voltage applied during AAO fabrication. For a given etching solution, voltage is the most crucial factor to be considered. AAO film grows faster (the pores get deeper for the same time) under higher voltage. Meanwhile, larger pore diameter and pore to pore distance will be created [15]. For samples anodized in sulfuric acid, the optimum long range ordering of porous array was achieved with a voltage of 25 V [16]. The optical properties of AAO membranes anodized in sulfuric acid have attracted intensive attention from researchers due to their relatively smaller pore diameter compared to those anodized in oxalic acid and phosphoric acid [17]. This smaller pore diameter results in higher porosity and larger surface area. Here, we fabricated AAO samples in sulfuric acid using four different DC voltages (5, 10, 15 and 25 V) and five different anodization times (2, 4, 6, 8, and 10 min.). Their optical properties were analyzed with use of UV–vis reflectance spectrometry. Various colors were observed on samples anodized with different voltages for different times. These colors were produced by interference of light reflecting from the top surface of alumina and light reflecting from the interface surface of alumina and aluminum substrate. Here we assume the porous alumina membrane is a uniform film without any microstructure but with a modified refractive index. In fact, there are three layers inside the AAO alumina nano-structure and each of them has a specific refractive index: the refractive index of inner high purity layer is about 1.7; the refractive index of the rather pure outer layer is about 1.6; and for the highly anion contaminated layer, the refractive index is about 1.4 [18,19]. The averaged refractive index of porous alumina was measured to be 1.63 [20]. Thus we applied thin film interference model and this effective refractive index to estimate the reflectance of AAO thin films. The calculated interference curves agreed well with the measured UV–vis spectra. In addition, we calculated the thicknesses of AAO membranes using thin film interference model. The linear growth of porous alumina with time in sulfuric acid was observed. The suitable anodization conditions that can be used to fabricate AAO with desired color and structure for various applications were obtained. Since the color of AAO thin film can be altered by its thickness and refractive index, molecular attachment on AAO substrate can shift its color through the increase in thickness. This color shift was demonstrated through dip-coated PSS (polystyrene sulfonate sodium salt) and PAH (polyallylamine hydrochloride) layer by layer on AAO substrate. Consistent color shifts due to increase in thickness were observed. These make AAO thin film a potential substrate for colorimetric sensors. Coating thin films (usually polymer) with desired properties on AAO substrates have been widely applied in nanofiltration and membrane based-separation systems [21–24]. In these applications, AAO usually functions as the substrate which provides large contact area through its porous structure. The amino group of APTS can form Meisenheimer complex with TNT molecules. In addition, it can bind covalently on AAO substrate through silanization [8]. The porous structure of AAO produces a large surface area, leading to a better contact with TNT molecules. The coated APTS attracts TNT molecules in solution which results in increased thickness of AAO substrate. This will shift the interference color of AAO upon TNT attachment. The corresponding UV–vis spectra of AAO–APTS thin film shifted to longer wavelength side after attaching TNT molecules which indicated that

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Fig. 1. (a) Schematic illustration of the two step anodization procedure. (b) AFM image (400 nm × 400 nm scan) of fabricated AAO nanowells.

TNT attachment increased the thickness of AAO thin film as expected. Detecting agents such as APTS and PEI (polyethyleneimine) can react with TNT to form Meisenheimer complexes that show a visible purple-reddish color. This reaction took place in a few seconds and turned the TNT solution into purple immediately. Commercial AAO with thickness of 60 ␮m is very thick comparing to fabricated AAO thin films which provides more contact sites for TNT detection. Since the film is very thick, no interference color can be observed. Instead, it has a white background color for visual colorimetric detection. Larger surface area (longer pores) can attract more TNT molecules that result in a more dramatic color change. We coated PEI or APTS on commercial AAO films and then exposed them to dilute TNT solution. The color of AAO membranes turned from white to orange. AAO sample with 100 nm pore diameter produced a stronger color change than AAO with 200 nm pore diameters. 2. Experimental procedures 2.1. Procedures for AAO fabrication We used a two step anodization process to fabricate AAO thin films. It is a modified “Masuda” process [25]. The schematic illustration for this two step anodization procedure is shown in Fig. 1a. First, aluminum foil was electropolished in 60% perchloric acid and EtOH (1:8, v/v) for 3 min. The temperature was set to 5 ◦ C in an ice/water bath. Aluminum was connected to the anode while platinum was connected to the cathode. The voltage between these two electrodes was set to be 30 V. The surface oxide layer and unevenness of aluminum were removed through electropolishing. First anodization was carried out in a 0.3 M sulfuric acid at 3 ◦ C for 12 h. Again Al worked as the anode and Pt worked as the cathode. The samples were anodized using four different voltages: 10, 15, 20, and 25 V. Then the anodized AAO samples were soaked in 80 ◦ C chromic/phosphoric acid for over 6 h to dissolve the alumina layer formed in first anodization. However, the nanowells pattern was created on the surface of aluminum which acted as sites to grow

new porous alumina with a more regular structure. The second anodization was carried out under the same conditions as the first anodization except that the times were set to be: 2, 4, 6, 8, and 10 min, respectively. Thin AAO films with interference colors were fabricated. The samples were rinsed with water and ethanol after each step. Then, 8 nm Ag was thermally evaporated onto the front side to enhance the interference color of AAO. AFM image of fabricated AAO shows the ordered nanoporous structure formed in two step anodization (Fig. 1b). 2.2. Layer by layer deposition Poly(styrene sulfonate) sodium salt (PSS) and poly(allylamine hydrochloride) (PAH) (Aldrich) are negatively and positively charged, respectively. Alumina membrane is positively charged below pH 9 [26]. The PSS molecules are attached to the AAO substrate through electrical force. Then positively charged PAH molecules were attracted by the surface assembled PSS molecules. By repeating this process, we coated PSS/PAH layer by layer on AAO substrate. The electrolyte solution for assembly contained 0.02 mmol/L polyelectrolyte and 0.5 M NaCl. The pH values of PSS and PAH solutions were 2.1 and 2.3, respectively. Here, we dipped AAO into PSS/PAH solution for 30 min, and then rinsed with water for 5 min. UV–vis reflectance was recorded each time after coating two PSS/PAH bilayers. AAO substrates used in this experiment were fabricated using 20 V at 3 ◦ C and the second anodization times were 4, 6, and 8 min. Silver was deposited on the front side of AAO to enhance the interference colors. 2.3. Procedures for silanization and polymer coating Alumina membrane was cleaned in an ultrasonic bath with distilled water and ethanol for several minutes. Annealing was carried out at 150 ◦ C for 1 h in the oven to expose Al–O bonds on the surface of AAO nanopores. Then we prepared a solution of 1 ml 3-aminopropyltrimetroxysilane (APTS), 6 ml ethanol and 0.2 ml sodium acetate buffer (pH 5.0). The AAO samples were soaked in

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this solution for 30 min to assemble APTS on the pore wall. Then we took the membrane out and rinsed it with ethanol. Silanization is accomplished by curing the sample in an oven at 150 ◦ C for 2 h under argon atmosphere. Polyethyleneimine can also form Meisenheimer complex with TNT and is very sticky which can directly attach onto AAO sample. AAO films were soaked in PEI solution with the same composition as APTS for 30 min. UV–vis spectrum was measured to analyze the color change of AAO after APTS or PEI coating. 2.4. Colorimetric detection of nitroaromatics AAO thin films made by a two step anodization method and coated with APTS were dipped in 0.88 mM TNT acetonitrile solution (Aldrich) for 1 h, rinsed with ethanol and dried in air. UV–vis reflectance was measured after TNT attachment. The color change due to Meisenheimer complex was also tested in liquid phase using 0.44 mM TNT solution. 50, 100, 150, 250, 350, and 450 ␮l PEI solution made in the previous step was gradually added into 1.5 ml TNT solution. The color changed from transparent to purple instantly. UV–vis absorption was measured after each step. Commercial AAO coated with APTS or PEI was soaked in 1 mM TNT solution for 6 h in order to fully expose the nano-pores to TNT. Then the samples were taken out, rinsed with water and ethanol, and dried in air. The color of commercial AAO turned from white to orange. UV–vis spectrum (taken with use of Cary 5000 at 45◦ ) was measured and a strong absorption around 500 nm was observed. 3. Results and discussion 3.1. Color and UV–vis spectra AAO thin films with interference colors were fabricated under different conditions. The as-made samples showed faint colors. These colors are due to interference as described in detail in Section 3.2. A free standing thin AAO film made from sulfuric acid is transparent and colorless. Scattering due to the air filled pores in AAO is known to cause absorption but the effect is strong only in large pores sample prepared in phosphoric acid under high voltage (>120 V) [27]. Scattering can also cause a baseline drop in the UV region (below 350 nm). The positive identification of target molecules is based on entire spectrum shift from UV to vis regions. After enhancing the surface colors through thermally evaporating 8 nm Ag on the surface, the colors of AAO samples can be visually observed and described in Table 1.

Table 1 Colors of AAO samples fabricated under different conditions. 8 nm silver has been thermally evaporated onto the front surface to enhance the color. The initial color of aluminum foil before anodization is silver/white.

Voltage

Time

2 min

4 min

Light

Brown

6 min

8 min

10 min

Dark

Dark blue

Blue

As shown in this table, the colors of the alumina membranes anodized for longer period of time or using higher anodization voltages tend to become brighter due to their increased thicknesses. The strategy was to start with a nonconspicuous color as the background color and to develop an intense color after chemical attachment. Molecules attached on the surfaces of AAO nano-pores can increase the thickness of AAO thin films and modify their refractive indexes. Thus the color change due to these effects was applied for colorimetric detection. Using low voltage and short time scale for AAO fabrication, the starting color was dark as visually observed which is not suitable for colorimetric detection. Using high voltage and long time in anodization, the AAO films represented several strips of colors. That means different areas on the sample have different colors. In those cases, the color change depends on the spot location that has been chosen on the sample, which makes the detection unreliable. Therefore, we chose the fabrication conditions within the red frame in Table 1 which are expected to produce AAO samples with applicable color detection. Fig. 2 presents the UV–vis spectrum of AAO with silver deposited on the front side and the peak/valley shows larger intensity after Ag deposition but also shifted slightly to the longer wavelength side. This indicates that silver coating slightly increases its thickness but enhances the color of AAO dramatically. UV–vis reflectances of AAO samples anodized using 10, 15, 20 and 25 V are displayed in Fig. 3. The times for the second anodization were 0, 2, 4, 6, 8, and 10 min. It is clear that alumina thin films anodized under higher voltages and longer times show more oscillations in their UV–vis spectra, which are due to thicker films been produced. In principle a simple bilayer structure consists of aluminum with a thin alumina coating can create interference colors. The simple bilayer structure can be represented by the 0 min samples in Fig. 3 (10, 15, and 25 V). As can be seen from Fig. 3, the 0 min samples reveal a shallow dip in the UV region (below 350 nm) with featureless tail in the visible region. The featureless tail cannot serve as a good handle to identify spectrum shift. In contrast, except for the 10 V anodization potential, all other 4–10 min samples show multiple oscillations due to the nanopore formation. These oscillations with sharp peaks and valleys are beneficial for the detection of small shift when target molecules are captured. 3.2. Estimation of film thickness through interference model

10 V

brown

15 V

Middle brown

Blue

Light blue

Very light blue

Light yellow

20 V

Blue

Light blue

Yellow

Purple

Green

25 V

Light blue

Purple, yellow

Green, purple

Green, Red

Red, light green

brown

Fig. 2. UV–vis spectra of AAO anodized using 25 V for 4 min before (solid) and after (dash dotted) Ag deposited.

As illustrated in Fig. 4, the colors of AAO thin films were resulted from interference of light waves reflected from the top surface and the interfacial alumina/aluminum surface. The pattern of interference between the two light waves depends on the difference in their phases, which is controlled by the film thickness, refractive index of the film and the angle of incident light (in this experiment, we use a fixed 45◦ ). Thus analyzing the interference of thin film

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Fig. 3. UV–vis spectra of AAO samples fabricated using (a) 10 V, (b) 15 V, (c) 20 V, and (d) 25 V with 2, 4, 6, 8, and 10 min anodization times.

Fig. 4. Schematic illustration of thin film interference on porous alumina/aluminum film.

can reveal its properties, including the film thickness and effective refractive index. The AAO membrane which has ordered porous structure can be approximated as a uniform layer without considering any nanostructure but a modified refractive index (n ≈ 1.63) [20]. Applying thin film interference model we obtained a reasonably good fitting for UV–vis spectrum. UV–vis reflectance can be approximated by the following equations [14]: ı(, ) =

4d 



nf 2 − n2 sin2  ± ,

I(, ) = Io cos2 (ı) + I1

n = 1,  = 45◦

(1) (2)

here d is the thickness of thin film, nf is the modified refractive index of porous alumina which is 1.63, and n is refractive index of air, which is 1. The incident angle of UV–vis light was always kept at 45◦ .  is the wavelength of incident light. The intensity of reflecting light can be calculated with use of Eq. (2). Next the calculated reflectance was plotted together with measured UV–vis spectrum as shown in Fig. 5. The calculated curves using thin film interference model fit reasonably well with the measured UV–vis spectra which meant that the interference model is adequate to analyze the reflectance of these AAO samples for the current study. The thin film interference model assumes that the intensity of interference light is constant, but the real intensity of UV–vis spectrum taken from AAO sample is not constant due to absorption. As shown in Fig. 5, the reflectance intensities toward longer wavelength become smaller and no longer overlapped with the calculated curve. However, the frequency of oscillation estimated from simple interference model fit the measured UV–vis spectrum quite well. According to thin film interference model, the wavelength that showed maximum (peak p ) or minimum intensity (valley v ) was determined by the thickness of the film. Thus, the thickness of the membrane was calculated by the wavelength of the peak using Eq. (3) or the wavelength of

Fig. 5. Experimental UV–vis spectra (solid) and the calculated curves with use of thin film interference model (dash dotted) for samples made using 20 V for 4 min and 10 min. 8 nm Ag was deposited on the front surface.

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Fig. 6. The calculated thicknesses of AAO fabricated under different conditions before and after Ag deposition. For consistency, all the thicknesses were calculated by wavelength of the peak using Eq. (3). The solid lines represent the thicknesses of alumina membranes before silver deposition, while the dotted dash lines represent the thicknesses of the membranes with silver on the top.

the valley with Eq. (4). It is obvious in Eqs. (3) and (4) that the frequency of UV–vis oscillation and refractive index of AAO film need to be considered for thickness estimation, while the intensity of UV–vis reflectance was insignificant in thickness estimation: d=

d=

mP



nf 2 − 0.5

4

(m − 0.5)V



4

nf 2 − 0.5

m = 1, 2, 3 . . .

(3)

m = 1, 2, 3 . . .

(4)

Fig. 8. The reflectance spectra of AAO samples after 6, 8 and 10 PSS/PAH bilayers coating. The AAO sample was second anodized using 20 V for 6 min with no silver deposition.

Hwang et al. They obtained the growth rate of AAO nano-pores in 0.3 M oxalic acid via measuring the pore length from SEM images [15]. For a given voltage (20–60 V) and temperature (5–30 ◦ C), the AAO film thickness increases linearly with time. In addition, the growth rate increases at a much faster rate at higher anodization potential. Therefore, thin film interference model is a simple and effective method to obtain the film thickness of AAO. 3.3. Color shift from coating PSS/PAH layers

The calculated thicknesses of fabricated AAO films vs. time under different anodization voltages are shown in Fig. 6. The films are slightly thicker after nominal 8 nm silver deposition (the dash dotted lines). The calculated thickness of silver layer on AAO was about 4.7 nm. This was slightly thinner than the direct thickness reading from a quartz crystal microbalance sensor during silver thermal-evaporation (8 nm). Since the structure of quartz crystal (solid) is different from that of AAO substrate (porous), and the exact Ag layer thickness will not affect our sensing measurements (vide infra), the exact Ag thin film thickness needs to be better analyzed with an effective medium theory [28] and is beyond the scope of current study. As shown in Fig. 6, the AAO film thickness increases linearly with time during the initial anodization for voltages between 10 and 25 V. The growth rates of AAO film increase from 4 nm/min at 10 V to ∼40 nm/min at 25 V and are plotted in Fig. 7. Our results are consistent with what has been reported by

Fig. 7. AAO film growth rate at 3 ◦ C and 0.3 M sulfuric acid with different anodization potentials.

Interference color of porous alumina is affected by its thickness and refractive index. Thus, organic molecules stacking onto AAO substrate will change its color. These AAO thin films could be applied for colorimetric sensors. This color shift methodology was validated by coating PSS/PAH layer by layer over AAO thin films. 2, 4, 6, 8 and 10 PSS/PAH bilayers were dip-coated over AAO samples. UV–vis reflectance was measured after each coating process. It was observed that UV–vis spectra shifted consistently to longer wavelength side after each coating. This was due to the overall thickness of AAO film increased after PSS/PAH assembly. The UV–vis reflectances of AAO after coating with 6, 8, and 10 PSS/PAH bilayers are compared in Fig. 8. In the next experiment, one side of AAO sample was covered with silver to produce a visible color before PSS/PAH coating. The other side of AAO sample was bare porous alumina. The reflective spectra from both sides shifted to the longer wavelength side after coating PSS/PAH polymers. Therefore, we believe that silver deposition did not affect the attachment of PSS on alumina significantly. Using thin film interference model to calculate the thicknesses of the overall AAO films after each coating step, we obtained linear increase of thickness with the number of layers. Here, the refractive index of PSS/PAH polymers was approximated to be 1.5 [29]. The calculated thicknesses of alumina after coating PSS/PAH bilayers are plotted in Fig. 9 (the front side with silver and the back side with no silver deposition were compared in this graph). As shown in Fig. 9, the AAO samples without Ag coating show a consistent increase of their thickness, 13.89, 13.43, and 13.88 nm for 10 bilayers over the 4, 6, and 8 min samples, respectively, while AAO samples with Ag coating show thickness increase of 4.87, 18.11, and 24.33 nm. The thickness of PSS/PAH film is known to be sensitive to the solvent, pH, electrolyte/buffer, etc. [30]. The Ag coating and the presence of NaCl electrolyte may affect the resulting film thickness. Despite the larger variation in the Ag coated AAO samples, both sets of AAO samples show PSS/PAH film thickness linearly increases with the number of bilayers. In addition, the AAO samples without Ag coating give a 1.37 ± 0.03 nm/bilayer thickness

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Fig. 9. The estimated overall thicknesses of AAO after coating PSS/PAH layer by layer. Here the samples were second anodized for 4, 6, and 8 min. The left picture shows the side without Ag deposition while the right picture shows the side with Ag deposition.

Table 2 The changes in color for AAO films after coating 10 PSS/PAH bilayers. Here, three samples with different initial colors were fabricated in sulfuric acid for 4, 6, and 8 min anodization times and 8 nm silver layer was deposited to enhance the surface color of AAO samples. 4 min

6 min

8 min

AAO/Ag AAO/Ag with PSS/PAH

in excellent agreement with the 0.80–1.50 nm/bilayer thickness measured using ellipsometry [31]. The results from PSS/PAH layer by layer coating validate our interference color method and indicate a precision of better than 0.1 nm can be achieved. The interference color measurement enables us to detect monolayer attachment of the target molecules. After repeating the layer by layer assembly ten times, the color of alumina membrane has changed significantly. The photographs taken from the samples before and after coating are shown in the following Table 2. 3.4. Nitroaromatics detection by AAO thin film The shift of interference color upon deposition of organic molecules on AAO template was demonstrated in the previous section. We next coated AAO thin film with aminosilane APTS, placed it in a TNT solution, and used the color shift to detect targeted explosives. APTS can be attached covalently on alumina through silanization and will attract TNT in dilute solution. UV–vis reflectance spectra of as prepared AAO thin film, AAO with APTS, and AAO/APTS after exposure to TNT were measured. As shown in Fig. 10, after coating APTS on alumina substrate, the UV–vis

spectrum shifted to longer wavelength. Additional red shift upon reacting with TNT molecules was clearly noticed. This shift in reflectance confirms the positive detection of TNT and is used to estimate the increase in thickness due to the attachment of TNT molecules. Dinitrotoluene (DNT) of comparable concentration was also exposed to APTS solution and no color change was observed. This is likely due to the less electron deficient nature of the aromatic ring and no Meisenheimer complex was formed. This observation indicates good selectivity toward TNT. Applying the thin film interference model, the thickness of AAO was calculated after coating APTS and after coating TNT on AAO with APTS. The calculated thickness of bare AAO sample was 235.26 nm, AAO with APTS was 244.27 nm, and AAO/APTS with TNT was 249.14 nm. The increase in thickness after coating APTS was 9.01 nm and the increase in thickness after coating TNT was 4.87 nm. So the ratio between the thickness of APTS and the thickness of TNT been absorbed was around 1.85. The shift of UV–vis spectrum implied a change in surface color. However, the small red shift is not sufficient for human eye detection. Although the small increase (4.87 nm) of thickness did not produce an obvious color shift, a hand held colorimetric detector for explosives could be developed through this mechanism. With suitable preconcentration process on TNT and silver enhanced surface color, visible color change of AAO upon TNT attachment is expected. The concentration of TNT can be quantified in terms of the increase in thickness of AAO–APTS film. 3.5. Detecting TNT in solution through Meisenheimer complex

Fig. 10. UV–vis reflectance of as prepared AAO substrate, AAO coated with APTS, and AAO/APTS after TNT explosure.

It is observed that TNT in a solution can be detected by introducing APTS or PEI into the solution which forms Meisenheimer complexes with TNT molecules and turns the solution from transparent to purple abruptly. PEI was added into a 0.44 mM TNT solution and UV–vis absorption was measured (Fig. 11). A strong absorption around 500 nm and a weak absorption around 360 nm were observed and the solution turned to pink abruptly after adding a 50 ␮l PEI solution. The color got (Fig. 12) darker after adding more PEI solution and finally turned to dark purple. As seen in Fig. 11, the absorption around 500 nm was getting stronger after adding more PEI solution. This was due to more and more imine groups of PEI forming Meisenheimer complexes with TNT molecules. The

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Fig. 11. UV–vis absorption of 0.44 mM TNT solution after adding 50, 100, 150, 250, 350, and 450 ␮l PEI solution. The absorption of this solution was detected again after exposure to air for 12 h. (The solvent for TNT is ethanol/acetonitrile, 8:2, v/v.)

absorption of PEI mixed with TNT was consistent with the absorption of APTS mixed with TNT which has been reported by Xie et al. [8]. Accordingly, we noted that both PEI and APTS can be used for colorimetric detection of TNT solution. After the purple solution was left overnight, it turned from purple to orange and UV–vis spectrum showed an even stronger and sharper peak around 500 nm. This might due to the decomposition of TNT or instability of Meisenheimer complexes in air. 3.6. Coating polymer on thick AAO film for TNT detection The pores of commercial AAO (from Whatman Ltd.) are irregular in structure. The pore length of commercial AAO is 60 ␮m, while thickness of anodized AAO film fabricated for interference colorimetry is on the scale of a few hundred nanometers. These thicker membranes do not show any interference colors. We believed that longer pores would attract more TNT molecules and thus gave us a dramatic visible color change. Therefore we coated PEI and APTS on commercial AAO discs for TNT detection. Two types of AAO discs were used as the substrates. One of them had 100 nm pore diameter, while the other one 200 nm. This porous structure of AAO discs produced large surface area for TNT detection. After coating with PEI or APTS, the discs remain white. The discs were then soaked in TNT solution. These AAO discs turn from white to pink after a few minutes and eventually turned to orange after a few hours. The formation of Meisenheimer complex shows purple to reddish color (vide supra). But the color will turn to orange after being exposed to air for long time. The pictures of commercial AAO discs before and after soaking in TNT solution are shown side by side in Fig. 13.

Fig. 12. The color of 0.44 mM TNT solution turned from clear to pink after adding 50 ␮l PEI solution.

Fig. 13. The AAO discs before and after soaking in 1 mM TNT solution. The upper white discs were commercial AAO, while the lower orange discs were commercial AAO with APTS and TNT attached on the surface. The pore diameter of the right half disc was 100 nm and that of the circular disc on the left was 200 nm.

As seen in this figure, the color of the right disc (with 100 nm pore diameter) was stronger than the color of the left disc (with 200 nm pore diameter). In addition, both of them turned from white to orange after soaking in TNT solution. For the same volume, the disc with 100 nm pore diameter had a relatively larger surface area (higher porosity) than the disc with 200 nm pore diameter. It turned out that 100 nm AAO disc had more contact sites coated with APTS or PEI for TNT attraction which showed a stronger orange color after detection. Besides, the color difference between these two types of AAO discs proved that the TNT molecules attached on the pore wall, not only on the top surface of AAO. The samples were next analyzed with a UV–vis spectrometer to quantify the color change upon TNT explosure. The UV–vis absorptions displayed in Fig. 14 have been calibrated by subtracting the background absorption (UV–vis absorption of AAO disc without TNT was used as the background). As seen in Fig. 14, there was a strong and broad absorption around 500 nm after soaking in TNT solution. Obviously this was due to the Meisenheimer complexes forming between polymer and TNT. The AAO discs with 100 nm pore diameter showed a much stronger absorption around 500 nm than the 200 nm AAO. These UV–vis results were consistent with the photograph shown in Fig. 13. There were small oscillations in addition to the strong absorption around 500 nm. These oscillations were due to the interference effect. In our thin film interference model, the thickness and refractive index of the membrane can affect the interference light wave. In this experiment, polymer and TNT molecules attached on the pore wall surface which increased the effective refractive index of AAO membrane. After subtracting the UV–vis absorption background from AAO disc, the UV–vis absorption was generated from a thin layer of TNT combined with polymer (APTS or PEI) on the pore walls. This thin layer attached on AAO sample exhibited some thin film interference features. So small oscillations combined with a strong broad absorption around 500 nm were expected. These results indicate that AAO is a promising substrate for colorimetric detection of TNT molecules. Although it requires several minutes to collect enough TNT molecules to provide a dramatic color change, it could be a visible detector which does not need any external circuitries and signal processors.

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Fig. 14. UV–vis absorption of commercial AAO disc with 0.1 ␮m (solid) and 0.2 ␮m (dash dotted) pore diameter coated with APTS or PEI, and then soaked in 1 mM TNT solution.

4. Conclusions We reported the colors and measured UV–vis spectra of AAO thin films fabricated using different voltages and times in sulfuric acid. The suitable fabrication conditions for AAO samples with desired color for visible detection are obtained. In addition, 8 nm Ag was deposited to enhance the interference color of AAO. Thin film interference model was used to analyze the UV–vis spectra and calculate the thicknesses of AAO films. We found that the growth rates of AAO increased with higher voltages and the thicknesses of AAO were linearly proportional to anodization time. After coating PSS/PAH polymers layer by layer on the porous AAO substrate, there was a linear increase in thicknesses on both sides (one side with Ag and the other side with no Ag deposition). So silver does not affect the attachment of polyelectrolyte organic molecules on AAO substrate and can enhance the intensities of reflecting light. After coating multiple layers of PSS/PAH on AAO thin film, a significant color change was observed. This mechanism was applied for TNT detection. AAO was coated with APTS to attract TNT molecules in the solution. We found that both APTS and TNT attached onto AAO films increased its thickness, the resulting interference spectra redshifted after each attachment which implied that the color change was related to the thickness increase. Our interference colorimetric sensing allows us to detect monolayer attachment of the target molecules on the nm thickness level. It has also been observed that both PEI and APTS can form Meisenheimer complexes with TNT and turn the color of TNT solution from transparent to purple. Commercial AAO films with long porous structure (60 ␮m) and white background color were used as the substrates for colorimetric detection of TNT. The membranes became orange after the attachment of nitroaromatics. This color change shows great potential for visible detection of TNT on a solid substrate. Acknowledgements Work at MSD, Argonne National Laboratory as well as FESEM carried out at EMC is supported by UChicago, Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. This material is based upon work supported by the National Science Foundation under Grant No. 0731102. References [1] W.D. McNally, Toxicity, Industrial Medicine, Chicago, IL, 1937. [2] Agency for Toxic Substances and Disease Registry, Toxicological Profile for 2,4,6-Trinitrotoluene, U.S. Department of Health and Human Services, Atlanta, GA, 1995. [3] D.S. Moore, Instrumentation for trace detection of high explosives, Rev. Sci. Instrum. 75 (8) (2004) 2499–2512.

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Biographies Y. Liu obtained her MS degree in materials engineering from the University of Illinois at Chicago in 2009 and is currently working to obtain her PhD degree in the Electrical and Computer Engineering Department at Northeastern University. Her research interests include nano materials, micro-nano fabrication, and integration of nano materials onto micro devices for gas sensing

applications and water quality monitoring. She is a student member of IEEE and MRS. H.H. Wang received his PhD degree in inorganic chemistry in 1981 from the University of Minnesota, Minneapolis, MN. He is a staff and principal investigator in the Materials Science Division, Argonne National Laboratory. His research interests include template based nanoscale synthesis, novel chemical sensors, and nanoscale materials for energy and electronics applications. J.E. Indacochea is a professor of materials science and engineering at the University of Illinois at Chicago. He obtained his PhD degree in metallurgical engineering from the Colorado School of Mines in 1981. His research interests include joining and welding science, advanced non-destructive evaluation of structural materials, and development of nanostructured gas sensors and composite materials. M.L. Wang is a Professor of civil and environmental engineering at the Northeastern University. He obtained his PhD degree in civil engineering from the University of New Mexico in 1983. His research interests include health monitoring of civil infrastructures and development of new sensor technologies for civil infrastructure applications.