CAPÍTULO 2. MARCO TEÓRICO
2.3 ZONAS DE DESARROLLO
2.4.7 MODELO VAN HIELE
biopolymer transfer transducer signal
reading
m r--- B a r c h e m i c a l th er m a l .. X o p t ic a l e le c t r ic a l |_ o u t p u t s ig n a lFigure 4.10: Functional components of biosensor (reproduced from [21])
Biological sensors can be divided into three principal classes.
Enzymatic sensors are biological catalysts. Their prim ary role is to lower selectively the activation energy of a biochemical reaction. Using this nom enclature the selectivity is defined as the difference of the reaction velocity between enzymatically catalysed and non catalysed reactions [49].
4.2. INTRODUCTION TO GAS SENSING
nitioii element. The expression immimosensor comes from the fact th a t the antibodies or inimunoglobnlins are an essential p art of the immune system [34]. The antibodies are biological complexing agents whose prim ary purpose is to recognize and bind antigens (haptens for low molecular weight).
Receptors are highly selective biological agents, they are part of the regulatory and infor m ation system of the body. T hey mainly react on the surface of membranes [49]. Biological membranes are typically bilipid layers approximately 10 nm thick and form a barrier be tween two aqueous solutions w ith different ionic constituents [59]. The mem brane contains channels which allow certain species to pass and others not (this fact specifies the selectiv ity of the biological sensors). The channels are not open all the time and the opening and closing depends on the stim uli a t the membrane. This opening and closing of the channels in the membrane takes place in a fraction of a millisecond. Each channel can be permeable to a certain species, thus the composition of the membrane can be designed for a certain species [59]. A schematic view of a mem brane is shown in figure 4.11.
' % s C d & (
Figure 4.11: Schematic view of a bilayer membrane w ith proteins and ion conducting channels (reproduced from [26])
This shows one ionic channel which goes through a bilayer membrane. For instance, there are channels which are selectively permeable to potassium ions whose tendency to open is dependent on the concentration of calcium ions on one particular side of the membrane.
4.3. CHOICE O F GAS SENSING TE C H N IQ U E
Sp ecification s o f B iological Sensors
T he specifications for biological sensors are in th e m ajority of th e cases in th e same range as for the above m entioned electrochemical gas cells. Kreuzer and Kimmich [51] give an extensive overview of sensors which utilize biological m aterials for gas detection.
For reasons already m ention in section 4.2.3 and for an actual lack of commercially available biological sensors, they are not going to be used in this project.
4 .3
C h o ice o f G as S e n sin g T ech n iq u e
In addition to th e above represented gas sensing techniques there are m any more physical or chemical phenom ena which are used for gas detection. However, not all of th em could be investigated in the tim e of th e Ph.D . project. Thus, in th e following table th eir basic principles are mentioned and their p o ten tial for th e dynam ic m easurem ent of th e transfer factor is illustrated.
T he table gives a comprehensive overview of several existing gas sensing techniques. It was compiled at the beginning of this PhD project to get an inside into th e diverse field of gas analysis.
type response time range selectivity stability cost potential
solid state msec-sec PPb highly selective unstable inexpensive-medium yes, low detection range
and fast response
semiconductor sec-min [81] ppm non-selective [71] not guaranteed [81] inexpensive-medium no, very hard to
manufacture and slow
biological secs-min PPb [46] non-select ive dependent on reversibility
of adsorption process
unknown no, non-selective
and slow
solid electrolyte 100’s msec-sec ppm highly selective not sufficient, e.g.
nation depends on humidity [98]
medium-expensive no, hard to manufacture
and stability problem
electrochemical secs-min
[73, 63]
ppm [63] highly selective limited electrode
life time
medium-expensive no, life time
is limited
gas chroma. msec-sec ppm medium long term medium not enough selectivity
optical msec ppm highly selective guaranteed inexpensive yes, well known
technology
mass spec. msec ppm highly selective long term,
except vacuum
expensive no, too expensive and
bulky, difficult maintenance
ultrasonic [72] msec-sec % 0.1 vol % long term inexpensive no,only useable for
binary gas mixtures
photoacoustical min [74] ppm
[17, 74, 75]
highly selective not guaranteed,
drift with temperature
expensive no, too expensive
quartz [36] microbalance
< 1 min ppm highly selective not guaranteed due to [43]
absortpion and desorption cycles
inexpensive [43] no, because reactions are
very often irreversible [43]
w O X
o
I—I o M O •n O œ ra z œ2
Q H H O ffi %§
CHAPTER 5
In f r a r e d
G
a s
A
n a l y s i s- S
p e c t r o s c o p y
5.1
In tr o d u c tio n
Spectroscopic techniques are widely used in the physical sciences to analyse various static and dynam ic processes, and w hat is more relevant to this thesis, can be used for the detection of several gases. All polyatom ic molecules and heteronuclear diatom ic molecules absorb infrared radiation. This is due to th eir dipole moment, so IR radiation can interact w ith these molecules. W hen IR radiation is absorbed th e vibrational and rotatio n al energies of th e molecule are changed. The absorption p a tte rn s m easured depend on th e physical properties of th e molecules such as the num ber and type of atom s and th e stren g th of the bonds betw een th e atom s in th e molecule. T hus each molecule exhibits a unique spectrum which is called th e ’’signature” of the molecule. T he region betw een 3700 - 500 cm~^
[89] (2.7 - 50 fim) includes almost all th e signatures of th e most commonly m easured gases. This region corresponds to th e energies which can excite th e vibrational and ro tatio n al energies of th e molecules.
T he following overview of spectroscopic techniques will be confined to th e description of th e IR p a rt of th e electrom agnetic spectrum .
5.2. RASTC S P E C T R O SC O PY