IN A NUTSHELL: A new type of polyhedral oligosilsesquioxane (POSS) composite material for use in resonant gravimetric vapor sensors such as surface acoustic wave (SAW) sensors has been developed.  SAW sensor arrays are comprised of a set of piezoelectric quartz substrates, each coated with a different sensing polymer.  Upon interaction with a vapor, the mass of the polymer increases, and a detectable change in the frequency of the acoustic wave occurs.  Hence a given vapor generates a unique set of frequency changes, or ‘fingerprint’.

The array of polymers must cover the full range of solubility interactions and must include non-polar, polarizable, dipolar, hydrogen-bond basic and hydrogen-bond acidic polymers.  SAW sensor polymers must contribute suitable sensing capabilities to the array, but they must also be physically robust and able to withstand multiple vapor challenges over time.  POSS nanosensors dispersed in a non-sensing polycarbosilane carrier polymer offer a unique combination of strong sensor response, robust physical properties and minimal loss of sensor response after many repeated vapor exposures over time.

These materials may also be used as coatings in other resonant vapor sensor systems such as quartz microbalances and silicon microtubes.  They may also be applied in any sensor technology where a coating with a specific solubility interaction is required, e.g., a hydrogen-bond acidic coating can be used to create optical fibers, carbon-polymer composite array components (for electronic noses) or pre-concentrator materials with a specificity for hydrogen-bond basic analytes such as nerve agents.  Pre-concentrators are used to increase the concentration of a vapor reaching a sensor, so that less sensitive sensor systems (e.g., FTIR) can detect it.

THE PROBLEM: Currently, almost all polymers for resonant sensor arrays have linear architectures, and it is difficult to attain a combination of good sensor properties and robust coating properties in a single linear polymeric material.  In order for vapors to easily diffuse in and out of a SAW polymer, it is desirable that the polymers have low glass transition (Tg) temperatures.  However, low-Tg polymers tend to be fluids at room temperature.  Many SAW polymers lose their response over time because they either cease to wet the substrate, they are gradually eroded from the substrate or they undergo irreversible chemical changes that decrease (or prevent) the reversible interaction of their sensing groups with the analyte.  In addition, polymers that contribute hydrogen-bond acidity to an array are not commercially available.

THE STATE OF THE TECHNOLOGY: Right now, SAW arrays are based on commercial off-the-shelf linear polymers (e.g., polyisobutylene) and specialty linear hydrogen-bond acidic polymers such as BSP3, and their SAW sensor performance has not been fully optimized.  Some attempts to improve physical properties by cross-linking SAW sensor coatings have been made, but this had a detrimental effect upon the diffusion of the analyte into the coating, and the sensor response.  In contrast, MMI has pioneered the improvement of SAW performance through the use of innovative polymer architectures of comparable cost to the existing specialty linear materials.

OUR NOVELTY: MMI has developed a POSS nanosensor platform for resonant vapor sensor arrays that forms robust coatings and maintains a strong sensor response over time.  The current range of POSS nanosensors is hydrogen-bond acidic (reflecting the lack of commercial availability of this class of material), but POSS nanosensors carrying sensor groups associated with any solubility interaction, or specific to any analyte of interest, may be easily produced.