Integrated optics provide a convenient platform for efficient biosensors

Sensors are vital in many areas of life: for monitoring the environment for pollution and the effects of natural disasters (e.g., oil spills, volcanic gases); for defense and anti-terrorism at airports and war zones; and in medical applications, such as the detection of pathogens and diseases (e.g., bird flu, cancer). If these devices could be made small and cheap enough to be disposable, then a small sample (e.g., of blood) could be injected into a newly opened sterile sensor, and its constituents rapidly analyzed. Such a device would effectively be a ‘lab-on-a-chip.’

A myriad of different sensors exist at present, but all rely on reading the response of a physical quantity to an external change in the environment. The quality of each sensor can be compared by considering important parameters such as the detection limit (the smallest detectable quantity of a given element), specificity (ability to respond only to the desired element), footprint, energy efficiency, and cost. Increasingly, optical sensors¹ are being used, which use changes in the properties of light (e.g., phase, intensity, or spectral component) as it passes through the sample of interest. Of key importance is how strongly light is affected by these changes.

We have developed a class of optical sensors based on mass-producible silicon technology. Known as slot photonic crystals,^2–4 they maximize the interaction of light with matter inside a nanoscale slot structure. Our use of silicon-on-insulator technology allows other optical components to be fabricated on the same chip, enabling a small integrated optical device capable of dense multiplexing.

Slot photonic crystals can be used to slow down and squeeze light into small volumes of air, as little as a few tens of cubic nanometers, thus enhancing light interaction with the contents of the slot (see Figure 1). This results from a combination of light confinement mechanisms, including slot waveguide⁵ and photonic crystal effects, which allow tailoring of light dispersion in both time and space. In addition to the low detection limit provided by this enhancement, the device can also be made to specifically target a substance by coating (‘functionalizing’) it with bio-recognition elements such as antibodies, which only bind to molecules of a particular shape. Combining with microfluidic circuits allows routing of the sample to different sensors, which given the micron-scale footprint of each sensor element allows integration of hundreds or thousands of independent sensors on the same chip.

Figure 1. Scanning electron microscopy pictures of our slot photonic crystal biosensor. The inset shows, in false color, a molecule of interest being probed by light.

For the lowest detection limit, the resolution and sensitivity must be maximized, a key contribution to this being sharp spectral features, which we can track. Photonic crystals can be fabricated in silicon with standard lithography, realizing cavities with a high quality factor, up to 50,000 in our case,³ enabling us to carry out sensitive measurements of biological substances.

To deliver analytes to the sensor elements, microfluidic channels have been integrated into the silicon chips using a rubber-like material called PDMS (polydimethylsiloxane). The channels are then flushed with chemicals that facilitate the immobilization of antibodies on the surface of the sensor. When an antigen-containing solution flows over the sensor, surface binding between the antibody and antigen cause a change in the local refractive index within the device, which is read out from the change in the central wavelength of the cavity peak. We have demonstrated⁴ detection of the protein avidin in concentrations as low as 15nM in a sensor only 10 microns long using biotin as a receptor.

Slot photonic crystals promise to play a major role in the development of optical biosensors. To deploy them in real situations, our future work will look at incorporating the source and detectors into the device, as well as trials with other biological or chemical substances of interest. The advantages offered by integrated optics, namely, small size, mass production, high sensitivity, and biocompatibility, promise much for the next generation of research-grade and commercial biosensors in a multitude of applications.