Novel biosensors from photonic crystals

Abstract

Screening the biochemical interaction of potential pharmaceutical compounds against a wide array of proteins and cells is a critical early step in drug discovery. In this way, costly failures can be avoided before the drug is introduced to animals and humans. Similarly, testing patients’ blood or tissue samples for expression of a gene profile will become common practice in decisions regarding the most promising course of treatment. Reliable assays are also essential to applications such as environmental monitoring, crop protection, and the detection of biological warfare agents. Biological substances may be most simply sensed directly through their dielectric permittivity, a measure of how they affect electric fields, in so-called label-free detection. Even greater sensitivity may be obtained by attaching a fluorescent compound to the substance of interest, and then exciting the florescent molecule. We have developed a new class of biosensors based on optical devices known as photonic crystals (PCs) that can be used for both label-free and fluorescence-based detection.1–6 The key attributes for acceptance of new technology in these fields are sensitivity (how low a concentration of a chemical, protein, or gene may be detected), cost per test, and throughput (the number of tests that can be performed at once). PCs represent a unique and versatile class of optical devices for manipulating the electromagnetic fields associated with light. Through the proper application of PC design and fabrication, electromagnetic fields may be confined and concentrated to enhance the interaction between light and biological material in contact with the PC. We have developed methods that enable PC biosensors to be inexpensively produced in plastic, as well as associated instrumentation.7 Combined, the biosensors and detection instruments enable high-throughput detection of biochemical binding kinetics, imaging large arrays of biochemical tests,8–11 and imaging detection of cells.12–14 Figure 1. Device structure for the PC surface. (A) Schematic crosssection of the device structure. (B) Scanning electron microscope photo of top view and cross-section of a replica-molded 2D grating comprising an array of bumps coated with titanium dioxide (TiO2) highrefractive-index dielectric material. (C) Photo of the fabricated device attached to an ordinary glass microscope slide, where the entire surface of the slide is populated by the PC sensor.