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Open Access Nano Express

Monitoring of degradation of porous silicon photonic crystals using digital photography

Maria Ariza-Avidad12, Alejandra Nieto1, Alfonso Salinas-Castillo2, Luis F Capitan-Vallvey2, Gordon M Miskelly3 and Michael J Sailor1*

Author Affiliations

1 Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0358, USA

2 Department of Analytical Chemistry, University of Granada, Faculty of Sciences, Avda. Fuentenueva s/n, Granada E-18071, Spain

3 School of Chemical Sciences, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand

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Nanoscale Research Letters 2014, 9:410  doi:10.1186/1556-276X-9-410

Published: 21 August 2014

Abstract

We report the monitoring of porous silicon (pSi) degradation in aqueous solutions using a consumer-grade digital camera. To facilitate optical monitoring, the pSi samples were prepared as one-dimensional photonic crystals (rugate filters) by electrochemical etching of highly doped p-type Si wafers using a periodic etch waveform. Two pSi formulations, representing chemistries relevant for self-reporting drug delivery applications, were tested: freshly etched pSi (fpSi) and fpSi coated with the biodegradable polymer chitosan (pSi-ch). Accelerated degradation of the samples in an ethanol-containing pH 10 aqueous basic buffer was monitored in situ by digital imaging with a consumer-grade digital camera with simultaneous optical reflectance spectrophotometric point measurements. As the nanostructured porous silicon matrix dissolved, a hypsochromic shift in the wavelength of the rugate reflectance peak resulted in visible color changes from red to green. While the H coordinate in the hue, saturation, and value (HSV) color space calculated using the as-acquired photographs was a good monitor of degradation at short times (t < 100 min), it was not a useful monitor of sample degradation at longer times since it was influenced by reflections of the broad spectral output of the lamp as well as from the narrow rugate reflectance band. A monotonic relationship was observed between the wavelength of the rugate reflectance peak and an H parameter value calculated from the average red-green-blue (RGB) values of each image by first independently normalizing each channel (R, G, and B) using their maximum and minimum value over the time course of the degradation process. Spectrophotometric measurements and digital image analysis using this H parameter gave consistent relative stabilities of the samples as fpSi > pSi-ch.

Keywords:
Porous silicon; Photonic crystal; Degradation; Digital photography; Image processing; Hue color coordinate