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Science  25 Aug 2006:
Vol. 313, Issue 5790, pp. 1021
DOI: 10.1126/science.313.5790.1021a

Protease activity can be quantified in fluorescent or colorimetric assays, but such assays can require substrate modification, lengthy incubation times, and extensive workup procedures, along with relatively large sample volumes. Orosco et al. have fabricated a silicon-based photonic crystal device that exhibits picomolar sensitivity in analyzing microliter aliquots of pepsin and produces a rapid response visible to the eye. Furthermore, no molecular labeling is necessary.

Photonic crystals block transmission of specific wavelengths of light through periodic alternation of high-refractive-index solid regions and low-refractive-index pores. The authors etched p-doped silicon to create the pores and then methylated the pore surfaces to keep out polar buffer solution. The top surface was then coated with a hydrophobic layer of zein protein. Application of pepsin caused the protein to fragment and fall into the pores, which in turn raised the refractive index in the pore region and led to a redshift in the reflected light. — PDS

Adv. Mater. 18, 1393 (2006).

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