Many enzymes and other molecules are secreted from cells into the extracellular matrix. Once outside the cells, the enzymes play important roles in directing cell movements, forming bodily tissues and supplying vital immune responses.

Now, thanks to the advent of nanostructured materials, researchers can monitor these important processes in real time. Kris Kilian, Justin Gooding and co-workers1 at the University of New South Wales in Sydney, Australia, have used a photonic crystal to monitor enzyme secretion from living cells for the first time.

Fig. 1: Scanning electron microscopy images of a photonic crystal film with pores containing gelatin (left), and a photonic crystal covered with macrophage cells (right). Enzymes secreted from the cells digest gelatin in the crystal pores, changing the optical properties of the crystal.Copyright © Justin Gooding 2009

The researchers developed silicon-crystal films containing thousands of pores just 50 nanometers in diameter. They modified the crystals by adding several biomolecules including gelatin, which fills the entire volume of the pores (Fig. 1, left). The idea behind this design is that enzymes will digest the gelatin, changing the refractive index of the crystal.

The researchers tested their sensors by placing live white blood cells called macrophages on the crystals (Fig. 1, right). They added chemicals known to induce an immune response, whereby the macrophages secrete enzymes called matrix metalloproteases (MMPs).

After one hour, the crystal showed a clear shift in reflectivity caused by the MMPs. This accurate method for sensing MMPs is an important breakthrough because MMPs are believed to contribute to wound healing and tissue growth.

This enzyme-sensing technique is far more sensitive than previous photonic sensors because the gelatin fills the entire pores, not just the pore walls. In fact, the researchers managed to detect just one picogram (one trillionth of a gram) of protease.

“The main benefit of this work is the ease with which [we can monitor] extracellular signaling in real-time cell culture,” says Gooding. “Cell biologists have not had this capability previously.”

What’s more, Gooding believes the technology could soon be tailored for use in living animals, not just cell cultures. “The films can be fabricated from microns to centimeters depending on the application,” he says. “In fact we have engineered the nanostructure such that the optical response lies within the so-called ‘tissue window’ which will allow non-invasive monitoring through tissue. We believe this approach will make an impact in drug discovery, cancer research, inflammation research and studies of cell migration, proliferation and differentiation.”