Visual Nanosensor for the Detection of Heavy Metals in Water

Introduction

It has been known for decades that metals with a thin oxide coating can exhibit interference colours by a Fabry-Perot interference phenomenon.¹ The colour produced by constructive interference of light reflecting from the air/oxide and oxide/metal interfaces is directly related to the oxide film thickness, and this property has been used as a convenient way to determine oxide film thickness by the unaided eye. Recently, interference colours of nanostructured metal oxides formed at Al, Ti, and Si have been reported. The observed colour of these nanostructured oxide films is dependent on film thickness, surface modification, and porosity of the nanostructures.  These parameters have been investigated for potential applications in sensing and displays.^2,3 In this study, the interference colours of Ta oxide nanotubular films have been used to create a sensor that allows for the visual detection of heavy metals in water (ppm to ppb levels).

Methods and Results

We have previously reported that thin films of highly ordered Ta₂O₅ nanotubes (NTs) can be formed by the anodization of Ta foil.⁴ The Ta₂O₅ NTs were prepared by anodization of Ta in a solution containing HF, H₂SO­₄ and H₂O in a two-electrode cell configuration. Ta₂O₅ NT diameter was controlled by the applied voltage, and NT length was varied by controlling the anodization time and HF concentration,⁵ as shown in Figure 1 (a-d). 

The colour of the NT film changes with NT length, progressing through brown, deep purple, deep blue, yellow, orange, purple, and blue as NT length increases from 20-500 nm. Beyond a length of ~500 nm, teal and pink colours dominate the visible appearance of the oxide films.

The peaks and valleys present in the diffuse reflectance spectrum vary with oxide film thickness, and with the refractive index of the nanotubular film. This was seen to change upon NT infiltration with different analytes (as shown in Figure 1e), allowing for visual sensing applications. 

Due to the conductive Ta metal NT support, heavy metals, such as Cu, Cd, and Pb, can be electrodeposited inside the NT film, resulting in refractive index change, and thus, a visible colour change. This results in a rapid visual sensor that does not require costly or bulky instrumentation, making it ideal for detecting heavy metals in water in remote locations.

Acknowledgements

We gratefully acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), and scholarship support from NSERC and Alberta Innovates Technology Futures for CH. We also thank Drs. Dimitre Karpuzov and Shihong Xu (Alberta Centre for Surface Engineering and Science, Edmonton, Alberta) for assistance with the FE-SEM imaging.

References

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(2)         Zhao, B. X.; Meng, G.; Xu, Q.; Han, F. Adv. Mater. 2010, 22, 2637–2641.

(3)         Lezec, H. J.; McMahon, J. J.; Nalamasu, O.; Ajayan, P. M. Nano Lett. 2007, 7, 329–333.

(4)         El-Sayed, H.; Singh, S.; Greiner, M. T.; Kruse, P. Nano Lett. 2006, 6, 2995–2999.

(5)         Horwood, C.; El-Sayed, H.; Birss, V. Electrochim. Acta 2014, 132, 91–97.