Science - August 12, 2010

Get set for better biosensors

Wageningen chemists can produce perfect monolayers on oxide-free silicon, a discovery which opens the way for new and better biosensors.

Printing patterns on monolayers: shown here is an amine on a monolayer with acid fluoride.
Biosensors are gadgets for detecting molecules. The sensitive element of such a sensor, the molecular receptor, is fixed to a surface - call it the base - which is often a silicon plate. If a particle is detected, the electrical properties of the silicon undergo a change. This has the makings of a meter.
Silicon oxidizes just as immediately when exposed to air or to water. The oxide layer on the surface hinders the production of a sensitive and stable biosensor. Therefore, it pays to develop a sensor based on oxide-free silicon. PhD student Luc Scheres has come up with a process for this purpose. He graduated cum laude just before the summer vacation in the work he performed for Professor Han Zuilhof (Organic Chemistry).
Scheres' work centres on developing a method to produce almost perfect monolayers on silicon substrates under mild reactive circumstances (room temperature, in a vacuum and in the dark). These layers are only one molecule thick. In this case, alkynes are involved: simple hydrocarbons with triple bonds at the ends for reacting with silicon. After binding to the silicon, an alkene with a double bond is left over. The perfection is in the occupancy factor of that thin layer. 'Theoretically, an occupancy factor of 69 percent of the surface is possible. I have obtained 65 percent', says Scheres. 'The higher the occupancy factor, the more stable the layer.' The molecule layer protects the silicon from oxidation. 'These layers are now ready for use.'
Scheres also demonstrates what you can do with such a layer. He produces layers with acid fluoride at the ends of hydrocarbon chains. Such a functionalized group is extremely reactive to amines, and this enables complex biomolecules to attach easily to the surface. These in turn are able to selectively pick out certain substances from a solution: the principle of the biosensor. The monolayer acts thus as a kind of interface which makes work fast, effective and oxide-free.
Blood analysis
In his thesis, Scheres also shows that patterns can be printed easily on the monolayer.  Using laser beams, even patterns with extremely thin lines (100 nm thin) are possible. Laser beams burn off the monolayer at required points, creating space for new functionalized groups. These groups can bind, for example, proteins, DNA, sugars or antibodies. 'The aim is of course to develop a chip which, for example, can analyze blood at high speeds. You place a drop of blood on a sensor which can detect characteristic substances of several hundred disease symptoms.' This may sound futuristic currently. 'But we have already set forth several big steps',  Scheres remarks.