Nanocantilevers (tiny “diving boards” of material that vibrate at certain resonant frequencies) can be used to detect a variety of biological and chemical molecules, including chemical vapours. When the vapour molecule is absorbed onto the cantilever, it alters the frequency at which the cantilever vibrates. Monitoring this change, by measuring laser light reflected off the structure, for example, then allows the mass of the particle to be calculated. The smaller the device, the more sensitive it is and researchers have already succeeded in detecting molecules that are as light as attograms (10–18 g) at room temperature and pressure, and below the zeptogram 10–21 scale in vacuum.

Coating the cantilevers with a polymer film is a good way to further increase these devices’ sensitivity. It also allows the sensors to be functionalized so that they can detect specific vapour molecules thanks to various chemical interactions between the polymer and the molecules.

Previous techniques to coat nanocantilevers involved drop-casting to apply films that were between 2–10 nm thick. While effective, such thin coatings do limit the types of molecule the sensors can detect. Other methods include top-down processes, such as microcapillary-pipette-assisted drop-casting and ink-jet printing but these produce films that are irregular, with most sensors ending up not being very well coated at all.

To overcome these problems, a team led by Michael Roukes has now tried a technique called SI-ATRP (surface-initiated atom-transfer radical polymerization) to grow uniformly thick films on nanocantilever sensors (made of gold-coated silicon nitride). The researchers directly grew PMMA onto the surface of the devices in a two-step process. First, they chemically bound a small molecule polymerization initiator containing a thiol group onto the surface using self-assembly. Next, they transferred the sensor to a solution containing monomers and the polymerization catalyst.

“The polymer then grows from the monolayer of the polymerization initiator bound to the nanocantilever surface and we can simply control the thickness of the polymer film by choosing how long the polymerization reaction time lasts,” explains team member Heather McCaig.

PMMA films up to about 100 nm thick

SI-ATRP allows us to grow PMMA films up to about 100 nm thick, she told nanotechweb.org. This is much thicker than that possible using drop-casting, which is limited to depositing coatings with a maximum thickness of just 10 nm. “Sensors coated with the thicker films are able to detect both lower and higher concentrations of vapour molecules than those that are bare or coated with a drop-cast polymer film. And that is not all, SI-ATRP is also a more reliable process than drop-casting and compatible with wafer-scale processing.”

The Caltech researchers tested their devices by exposing them to seven types of organic vapour (hexane, toluene, heptane, ethyl acetate, chloroform, tetrahydrofuran, and isopropanol). The analytes were delivered at concentrations of P/Po = 0.0050–0.080 (where P is the partial pressure and Po is the saturated vapour pressure of the analyte at room temperature). The sensors were exposed to pure carrier gas for 70 s, to the analyte vapour itself for 400 s and a final carrier gas to purge the system for 630 s.

These sensors could be used to detect a wider range of vapour molecules than we studied here, said McCaig, including dangerous substances like nerve agents.

The team details its experiments in Nano Letters DOI: 10.1021/nl500475b.