Justin Gooding, Till Böcking and Kris Kilian of the University of New South Wales discuss how surface chemistry lets porous silicon biosensors fulfil their promise.

Porous silicon photonic crystals can be tuned to reflect different colours of light by altering their periodicity

A biosensor is a device for detecting an analyte that combines biological molecules selective for the analyte with a detector known as a signal transducer. The biological molecules provide the sensor with its selectivity while the transducer determines the extent of the interaction between the biomolecules and the analyte. The transducer converts this information into a signal that a person can read, such as a colour change. Examples of successful biosensors include glucose meters used by diabetics and home pregnancy test kits.

Apart from being selective for the target analyte, biosensors should be miniaturisable, portable, robust and sensitive. In vivo biosensors also need to be biocompatible and non-toxic, so ideally a biosensor would not need an implanted power source to operate it. Porous silicon has emerged as an attractive option for biosensors. In fact, it would be the perfect material, in particular for in vivo biosensors, if it could be stabilised against degradation.

Porous silicon is made by electrochemically drilling nanoscale pores into silicon wafers in ethanolic hydrogen fluoride solutions, a process known as etching. The pore size can be adjusted from microporous to macroporous by varying the etching conditions; most biosensing applications use mesoporous materials, which have a pore size between two and 50 nanometres. By varying the current density during etching, the silicon's porosity can be altered to produce 1D periodic structures, known as photonic crystals, that reflect or transmit light at precisely defined wavelengths.

Absorption or desorption molecules on to the pore walls in a photonic crystal alters the wavelength of light that the crystal reflects, making porous silicon an ideal transducer for biosensors. In addition, the wavelength range can be tuned to the visible or infrared range simply by altering the crystal's periodicity. Porous silicon transducers require no power to operate and, if tuned to reflect near infrared light, which can penetrate living tissues, can be monitored directly through the skin using silicon diode detectors.

Porous silicon has an even more attractive feature over other photonic crystals for biosensing. In the body, mesoporous silicon degrades to orthosilicic acid, the most common naturally occurring form of silicon. Because this product is benign, porous silicon biosensors do not have to be removed from the body. But the degradation of porous silicon also results in a change in optical properties, which of has severely compromised the application of porous silicon in biosensing.

An elegant solution to this hurdle involves using surface chemistry to stabilise the materials. For example, by forming a monolayer of alkyl chains linked to the surface by strong silicon-carbon bonds, scientists have dramatically increased the stability of porous silicon. The same surface chemistry has also allowed researchers to explore a host of strategies for immobilising biological molecules onto the pore surfaces. These advances take us a significant step forward in applying this powerful material to biosensing and revitalise the quest for implantable smart materials.

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