Silica aerogels are ideal materials for active and passive components in optical sensors. Their visible transparency, high surface area, facile transport of gases through the material, thermal and chemical stability, and ability to be filled with additional active phases are the key properties that aerogels bring to sensor applications. The Microstructured Materials Group has recently discovered a new process that induces a permanent, visible photoluminescence in silica aerogels (see the section on aerogel composite materials). Shortly after these materials were prepared, it was observed that the intensity of the photoluminescence was indirectly proportional to the amount of gaseous oxygen within the aerogel. The quenching of photoluminescence by oxygen is a phenomenon that is frequently observed in many luminescent materials.
In simple terms, photoluminescence occurs when a material absorbs a photon of sufficient energy. The entity that absorbs the photon may be a discrete molecule, or a defect center in a solid-state material, and is often referred to a "carrier". When the photon has been absorbed, the carrier is moved into a high energy, "excited" state. The carrier will then relax back to its ground state after a certain length of time. This "lifetime" of the excited state is usually on the order of nanoseconds to microseconds. The mechanism by which the carrier relaxes determines whether the photoluminescence is termed "fluorescence" or "photoluminescence." If an oxygen molecule collides with a carrier while it is in its excited state, the oxygen molecule will absorb the excess energy of the carrier and quench the photoluminescence. The oxygen molecule absorbs the energy and undergoes a triplet-to-singlet transition, while the carrier undergoes a non-radiative relaxation. The efficiency of the photoluminescence quenching is, therefore, determined by the number of collisions between the material containing the carrier, and oxygen molecules. As the collision frequency of gases is determined by, the pressure (P), temperature (T), and the number of molecules present, at a certain P and T, the quenching efficiency, and, consequently, the photoluminescence intensity will be determined by the concentration of oxygen in the atmosphere surrounding the material.
Oxygen sensors based on this principle have been extensively studied. The most common sensor elements studied are those based on an organic or inorganic compound suspended in a thin silicone membrane. Advantages of using an aerogel-based sensor element over these systems include a more rapid response time (due to rapid diffusion of gases through the aerogel pore network), and improved resistance to photo-bleaching (as the photoluminescence is caused by stable defect centers in SiO2). The Microstructured Materials Group has built a prototype oxygen sensor based on this technology. The sensor is intended to perform as low cost, moderate sensitivity device operating most effectively in the concentration range of 0-30% oxygen. The sensor operates independently of the nature of the other gases present in the feed gas and of the feed gas flow rate. The prototype sensor has been successfully operated over a temperature range of -25 to +85 degrees C (this range is based on other experimental limitations of the system, the actual usable range is larger). The highest sensitivity is observed at lower temperatures.
The prototype sensor uses a Hg-arc lamp for excitation (330 nm), and a Si photodiode for detection of the emission (500nm). The prototype design can be easily miniaturized, and a device can be designed with built-in pressure and temperature compensation.
This sensor is available for technology-transfer (see the Aerogel Technology Transfer Page).
The graphic below plots the measured photoluminescence intensity (irradiance) vs oxygen pressure (concentration gives a similar plot) at two temperatures using the prototype sensor.