Machine Olfaction Device (MOD) Sensors (Part Three)

Quartz Crystal MicrobalanceOne of the IDT surfaces will expand and contract
The Quartz Crystal Microbalance (QCM/QMB) is anwhen an alternating current applied to it. The
extremely sensitive mass sensor, capable ofmovement of the surface generates a wave (some
measuring mass changes in the nanogram range [1].scientists/researchers call it a "Rayleigh Wave"), which
QCMs are piezoelectric devices fabricated from a thinwill pass through the substrate. A frequency counter
plate of quartz with electrodes affixed to each sidelocated in the IDT receiver will then record the
of the plate.frequency of the wave.
A QCM-D (Quartz Crystal Microbalance withTo minimise noise and temperature, as well as lower
Dissipation monitoring) consists of a thin quartz discthe frequency to be measured, a dual SAW set up
sandwiched between a pair of electrodes.may be constructed, and therefore, the reference
Due to the piezoelectric properties of quartz, it issignal from the SAW (uncoated) will be mixed with
possible to excite the crystal into oscillation bythe sensor signal.b. Sensing Mechanism
applying an AC voltage across the electrodes.The physical properties of the surface can affect the
Changes to this oscillation are directly proportional towavelength/frequency of the surface wave itself. A
mass changes on the crystal [1].thin layer of polymer coats the substrate, which is
Various sorbent coatings can be used on the crystallocated between the two IDTs. The absorption of
surface in order to add element of selectivity to thegas changes the mass of the polymer, and
sensor [2]. A number of different types of sensorconsequently the properties of the sensitive layer.
operate under similar basic principles, such as "BulkThe surface wave is not just affected by the
Acoustic Wave (BAW)" and "Surface Acoustic Wavechange of mass; it is affected by other factors, such
(SAW) sensors". Both sensors require an A.C. voltageas temperature, pressure, dielectric constant and
for configurations/operation. BAW sensors use theviscosity.
electric field in order to excite the quartz crystal toSmart Sensors
oscillate, and SAW sensors use wave propagation onSmart sensors are simply sensors with
the surface sensor [1].a. Manufacturing Processmicroprocessors attached to them. When it comes
After being cut along certain crystallographic axis, theto a system design, a smart sensor can be:
thin plates of the single piezoelectric crystal quartzEasier.
are covered with thin gold electrodes on both sidesCheaper.
[4].More reliable and more scaleable.
The two sides of the crystal are then coated withHigher performance.
polymer films. The coating technique could be any ofMore rapid to design.
the following [4]:Obviously, these benefits are all obtained when
microprocessors or computing resources are
1. Spray coating.embedded on the sensor. Therefore, the processing
2. Growth of Langmuir-Blodgett films.of data is performed on the spot i.e. within each
3. Self-Assembled Monolayers (SAMs).individual sensor, instead of using a central system
The coating will provide the conductivity and changingcontroller. In addition to this, ordinary sensors output
of mass.b. Sensing Mechanismraw data; but only useful data is produced by a
The QCM is basically a thin quartz wafer withsmart sensor. Many of the smart sensors can be
electrode pads on each side [5].easily programmed and/or reprogrammed, thus
The QCM oscillates mechanically, when connected tosaving time and expense.
an amplifier.The feasibility of using such kind of sensors in any
At the same time the amplifier oscillates electronically,MOD depends on how small the device will be and on
with a certain frequency.the final application(s), as well as the final cost of the
On the surface of the QCM there is a coating of adevice itself.
sensitive chemical. Exposure of which to analyteNajib Altawell
vapour, cause the molecules of the analyte inter intoReferences
the coating. The result will be an increase in mass,[1] Lee-Davey, J., (2004) "Application Of Machine
which causes a slowing in the frequency of oscillation.Olfaction Principles For The Detection Of High
QCM are very sensitive to any minute changes inVoltage Transformer Oil Degradation"Cranfield
their mass, and for this reason the QCM can measureUniversity.
changes in its frequency to 1 part in 108 [5]. Normal[2] Perera, A., Sundic T., Pardo A., Gutierrez-Osuna R.,
operating frequencies are in the range from 10 to 30Marco S., (2002)"A Portable Electronic Nose Based on
MHz. [4].Embedded PC Technology and GNU/Linux: Hardware,
Surface Acoustic Wave Sensors (SAW)Software and Applications"
As in the QMB (i.e. QMC) this sensor is based on theIEEE SENSORS JOURNAL, VOL. 2, NO. 3, JUNE 2002
same principle i.e. when mass changes, frequency235
changes. The device utilises surface acoustic waves,[3] K. Persaud, G. Dodd, Nature 1982, 299, 352-355.
with a frequency of about 600 MHz [4].a.[4] Nose Office (2003) "NOSE II - The Second
Manufacturing ProcessNetwork on Artificial Olfactory Sensing" University of
Two inter-digital transducers (IDT) are usually madeTuebingen Dec 2003 - Germany
up from thin metal electrodes and fitted on "a[5] Finklea, H. O., lecture notes ( ) "Gas Phase
polished piezoelectric substrate", located in the centreSensors"
and enclosed by resonators [4].Department of Chemistry West Virginia University
The wavelength is determined by the spacing of theMorgantown, WV 26506-6045.
IDT fingers.