From —liquid crystal thermometers

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From liquid crystal thermometers
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LC thermometer

• Hold a liquid crystal thermometer card over your
  hand and observe it as it warms up. Notice that
  it starts out black, then becomes red, yellow,
  green, blue then black once again. Liquid
  crystals can be adjusted by the manufacturer to
  change colors over different temperature ranges,
  but 30 C to 35 C seems to be a common range.
• The molecules in the liquid crystal thermometer
  are made of cholesterol, and are called
  cholesteric liquid crystals. The molecules are
  rod-shaped. The rods form layers in the liquid
  crystal, the rods in adjacent layers running in a
  slightly different direction from those in the
  layer above or below. From one layer to the next
  the rods form a structure like the stairs of a
  spiral staircase. The spiral staircase rises
  through some distance before the molecules twist
  around parallel to their original direction. When
  this distance equals half-a-wavelength of red
  light, the liquid crystal will reflect red light.
  When the distance is shorter, matching half a
  wavelength of blue light, the crystal will
  reflect blue light. (The wavelength of light is
  measured in the liquid crystal and is shorter
  than the wavelength in air.)
• This seemed strange to me at first. Why should
  cold crystals with closely spaced layers reflect
  red light and warmer more spread out layers
  reflect blue light? The resolution of this
  problem came when I found out that the
  temperature effects not only the spacing of the
  layers but the twist angle from one layer to the
  next. When the liquid crystal warms up the layers
  spread apart slightly, but, in addition, each
  layer of rods rotates a little more relative to
  the later below it. This means it takes fewer
  layers and therefore a shorter distance between
  layers of parallel rods. So when the temperature
  increases the spacing between layers in which the
  rods are parallel decreases. The color goes from
  red when cold to blue when hot.
• The liquid crystals are usually encapsulated,
  packaged in microscopic plastic spheres. This
  allows the material to be cut. they are also
  thermally rugged which allows them to be
  laminated to protect them from hard handling in a
  high temperature laminating machine.

http//www.exo.net/pauld/activities/liquidcrystal
/liquidcrystal.html
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Liquid Crystal Sensors for Chemical Detection

• Ke Zhang
• Sept. 23, 2005

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Other methods

• Spectroscopy methods (Mass spectroscopy, gas
  chromatography, infrared spectroscopy) can give
  precise and quantitative detection, however they
  are bulky, complex to provide real-time, portable
  detection.
• Electrochemical sensors (such as electronic nose)
  and optical sensors can be designed to detect gas
  analyte fast and effectively.
• Liquid crystal sensor is another option for
  chemical detection which is usually based on
  simple mechanisms and instrumentations.

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Outline

• Introduction of liquid crystals
• 4 type of liquid crystal sensors for chemical
  detection
• Type 1Nematic LC shifting for signal
  amplification
• Type 2Cholesteric LC sensors (color change)
• Type 3Discotic LC sensors (conductivity change)
• Type 4Lyotropic LC shifting for signal
  amplification
• Comparison and Conclusion

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Introduction of Liquid Crystals

• Liquid crystal phase is intermediate form between
  crystalline solid and isotropic liquid. It has
  some special properties
• Individual molecules always have anisotropic
  shape (rodlike, disk-like), dielectric constant,
  reflective indices
• Molecular arrangement posses some degree of
  positional order or orientational order
• Molecular alignment can be affected by external
  fields (electric, magnetic, shear stress or
  surface anchoring)

Thermotropic Liquid Crystals
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Calamatic Liquid Crystals
Nematic phase
smectic phase
cholesteric phase

• Nematic LC molecules have orientational order.
  The direction can be controlled by alignment film
  and external field.
• Cholesteric LC pitch length can be changed by
  temperature and interaction with chemicals.

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Nematic LC Sensors
Fig. 3 (A) Schematic illustration of the change
in surface roughness caused by the binding of
molecules of Av (left) or lgG (right) to ligands
hosted within a SAM of molecules supported on
gold film. (B) Scanning tunneling microscopy
image of the surface of a thin (10nm)
semitransparent, obliquely deposited (50? from
normal) gold film prepared by electron beam
evaporation onto a glass substate at 0.02 nm/s.
(C) Profile of the surface of the gold along the
black line shown in (B).
V. K. Gupta, N. L. Abbot, Langmuir, 12, 2587,
1996 V. K. Gupta, J. J. Skaife, T. B. Dubrovsky,
N. L. Abbot, Science, 279, 2077, 1998
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Nematic LC Sensors
Nematic liquid crystals can be used to amplify
and image transducer receptor-mediated binding of
proteins at surfaces. Nanometer scale
topographies on the surfaces could be overwhelmed
by the specific binding of proteins to surface
immobilized ligands, thus leading to microscopic
changes in the orientations of LCs aligned by
these surfaces.
Fig. 4. Optical images (light transmitted through
crossed polarizers) of nematic LC sandwiched
between mixed SAMs formed from BiSH and C8SH with
or without bound proteins.
V. K. Gupta, J. J. Skaife, T. B. Dubrovsky, N. L.
Abbot, Science, 279, 2077, 1998
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Nematic LC Sensors

• Selective binding of analyte and receptor
  immobilized on the surface can also change
  interaction (binding) between LC and surface.
• The resulting optical signal can easily be
  observed by the naked eye due to amplification by
  the LC molecules long-range order.
• Combination of surface design and receptor
  selection can make this sensor applicable to
  various chemicals.

R. R. Shah, N.L. Abbott, Science, 293, 1296, 2001
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Nematic LC Sensors
R. R. Shah, N.L. Abbott, Science, 293, 1296, 2001
12
Nematic LC Sensors

• Because LC molecules have high mobility of
  liquids, information about the binding of analyte
  and receptor at surfaces propagates rapidly from
  the surface into the bulk of the LC (transduction
  and amplification can occur in a few seconds).
• The ligands on the substrate can be tailored to
  reversibly bind with the LC molecules and
  analytes.

R. R. Shah, N.L. Abbott, Science, 293, 1296, 2001
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Cholesteric LC Sensors
Cholesteric LCs can selectively reflect light of
wavelengths equal to the pitch length (?Rnavep).
Organic vapors can be incorporated between the
layers of a cholesteric liquid crystal, changing
its pitch length and, therefore, its color.
A glass disk was used as substrate for
preparation of the CLC films. Its reverse side
was made black to absorb unscattered light. In
order to prolong the lifetime of liquid crystal
sensor (decrease evaporation and mechanical
drifting), the liquid crystal films can also be
immobilized in PVC and silicone rubber.
I.M. Raimundo Jr., R. Narayanaswamy, Analyst, 124
(1999) 16231627.
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Cholesteric LC Sensors

• The optical properties of the sensing phase
  can be tuned by changing the composition of the
  CLC mixture, altering the sensitivity and
  selectivity for different organic vapors.
• The lifetime of the CLC layer is not long
  (lt200 mins). To overcome this drawback, the
  liquid crystal films could be encapsulated in PVC
  or silicone rubber, if the CLC is maintained in
  small domains dispersed in the polymeric film.

Fig. 6 Effect of the solvents on the reflectance
spectra of the 0.18 CC 0.82 CN films (top)and
0.22 CC0.78 CN films (bottom).
D. A. Winterbottom, R. Narayanaswamy, I. M.
Raimundo Jr., Sensors and Actuators B, 90, 52,
2003
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Discotic Liquid Crystals
discotic liquid crystals

• Discotic LC molecules typically consist of an
  aromatic core surrounded by hydrocarbon side
  chains.
• The center of the molecular stack is electron
  rich and relatively conductive in nature, whereas
  the outer layer is relatively insulating.

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Discotic LC Sensors
Gold electrode

• Charge carrier can move easily along the
  columns, with relatively high mobilities, but
  charge carrier motion between columns is greatly
  hindered by the intervening hydrocarbon chain
  region.
• Molecules at the surface have many more degrees
  of freedom and are able to undergo orientational
  and positional fluctuations. This leads to a
  considerably enhanced conductivity compared with
  the bulk.
• Disturbances such as the absorption of
  volatiles will have a very strong effect on these
  fluctuations, giving discotic liquid crystals
  their sensitivity to the presence of volatiles.

Silicon Substrate
DLC film
J. Clement, N. Boden, T. D. Gibson, R. C.
Chandler, J. N. Hulbert, E. A. Ruck-Keene,
Sensors and Actuators B, 47, 37, 1998
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Discotic LC Sensors

• DR R0 is positive for all alkane hydrocarbons.
  The tentative explanation is that the alkane
  volatile dissolves into the alkyl side chain
  region of the discotic surface, disrupts the
  reorientational and positional fluctuations of
  the molecules, thus increasing the surface
  resistance. The longer the alkane chain length,
  the greater the disruption of the fluctuations,
  and the greater the measured resistance.
• Other volatiles, such as benzene for instance,
  decrease the surface resistance of the discotic
  liquid crystal. This is presumably due to their
  affinity for the aromatic core region where they
  assist the transport of charge from aromatic core
  to aromatic core.

J. Clement, N. Boden, T. D. Gibson, R. C.
Chandler, J. N. Hulbert, E. A. Ruck-Keene,
Sensors and Actuators B, 47, 37, 1998
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Discotic LC Sensors
The response to saturated water vapor is
negligible. This insensitivity to the presence of
water is primarily due to the hydrophobic nature
of the surface of the discotic liquid crystals.
J. Clement, N. Boden, T. D. Gibson, R. C.
Chandler, J. N. Hulbert, E. A. Ruck-Keene,
Sensors and Actuators B, 47, 37, 1998
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Lyotropic Liquid Crystals
Lyotropic liquid crystals (such as surfactants)
can form a variety of liquid crystalline phases
in solvents. This ordering is due to
hydrophobic/hydrophilic competitions.
micelle
hexagonal columnar
lamellar
Concentration increase
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Lyotropic LC Sensors
Most biologic receptors possess both hydrophilic
and hydrophobic regions and thus readily
incorporate into biphilic lyotropic liquid
crystals. When the receptor-ligand complex
forms, the uniform order is disrupted producing
an optical response.
Fig. 10 schematic representation of the
amplification mechanism with the specific ligand
bound to its receptor causing deformation of the
liquid crystal and alternation of the
transmission of polarized light.
C.J. Woolverton, G.D. Niehaus, K.J. Doane, O.D.
Lavrentovich, S.P. Schmidt, S.A. Signs, US
Patent, US 6171802, 2001.
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Lyotropic LC Sensors
The elasticity of the liquid crystal amplifies
the local distortions in the vicinity of the
receptor-ligand complex, and rapidly expands it
to an optically detectable, supramicron
scale. Lyotropic LCs offer a system that
rapidly, reliably and automatically detects
ligands, especially when present in very small
quantities. It is further envisioned that a
multiwell system, each well having a
predetermined receptor, can be used to detect
different kind of ligands individually.
Fig. 5 A profound increase in transmission of
propagating light occurs when liquid crystals
amplify the binding of antibody to ligands.
C.J. Woolverton, G.D. Niehaus, K.J. Doane, O.D.
Lavrentovich, S.P. Schmidt, S.A. Signs, US
Patent, US 6171802, 2001.
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Comparison of 4 types of LC sensors
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Comparison of 4 types of LC sensors
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Conclusions

• 4 types of liquid crystal sensors and their
  working mechanisms have been introduced.
• Compared to the bulky and complex spectroscopy,
  these LC sensors can not only provide real-time,
  portable detection at reasonable sensitivity, but
  also can be easily operated and manufactured.

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Selected References

• P. G. de Gennes, J. Prost, The Physics Of Liquid
  Crystals, 2 ed., Oxford Science Publication,
  Oxford, 1993.
• http//www.barrettresearch.ca/teaching/liquid_crys
  tal/
• V. K. Gupta, J. J. Skaife, T. B. Dubrovsky, N. L.
  Abbot, Science, 279, 2077, 1998.
• R. R. Shah, N.L. Abbott, Science, 293, 1296,
  2001.
• C.J. Woolverton, G.D. Niehaus, K.J. Doane, O.D.
  Lavrentovich, S.P. Schmidt, S.A. Signs, US
  Patent, US 6171802, 2001.
• N. Abbott, R. Shah, World Patent, WO 02/075294
  A1, 2002.
• D. A. Winterbottom, R. Narayanaswamy, I. M.
  Raimundo Jr., Sensors and Actuators B, 90, 52,
  2003.
• J. Clement, N. Boden, T. D. Gibson, R. C.
  Chandler, J. N. Hulbert, E. A. Ruck-Keene,
  Sensors and Actuators B, 47, 37, 1998.
• F.L. Dickert, A. Haunschild, P. Hofmann, J. Anal.
  Chem., 350, 577 (1994).
• F.J. Poziomek, T.J. Novak, R.A. Mackay, Mol.
  Cryst. Liq. Cryst., 27, 175, (1973).