This presentation is based on Kenneth S Suslick's Chem review paper on Optoelectronic nose. Sensing of different VOC and toxic gasses is possible with monitoring the change in colour of the sensitive dye.

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2.  Humans beings, who are generally more visual than olfactory
creatures, the sense of smell is one of our most basic capabilities,
and we are able to recognize and discriminate over 10000
individual scents.
 The chemical specificity of the human olfactory system does not
come from specific receptors for specific analytes (e.g., the
traditional lock-and-key model of substrate-enzyme interactions),
but rather olfaction makes use of pattern recognition of the
combined response of several hundred olfactory receptors.
 Humans have some 400 active olfactory receptor, a large fraction of
them are believed to be metalloproteins that are highly reactive to
numerous odorant molecules through coordination of the analyte
to the metal-binding site.
Human Olfactory System
* Chem. Soc. Rev., 2013, 42, 8649--8682
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3.  Colorimetric analysis is a method of determining the concentration of a chemical element or chemical compound
in a solution with the aid of a color reagent.
 It is applicable to both organic compounds and inorganic compounds.
 Colorimetric sensor devices responses are typically indicated by a colour change enabling qualitative and in some
cases semi-quantitative analyte/VOC detection, without the need for expensive spectroscopic equipment.
Colorimetric Sensor
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4. Colorimetric Sensor Array
 Colorimetric sensor array is composed of a gel flat plate containing sensitive colorimetric dyes. The array’s
responses are attributable to the targeted interactions between the volatiles of interest and chemical
dyes.
*Note: The sensors need not to have a selective response; rather, they have nonspecific reactions with a
range of chemicals.
 Optical arrays based on chemoresponsive colorants (dyes and nanoporous pigments) probe the chemical
reactivity of analytes, rather than their physical properties. This provides a high dimensionality to chemical
sensing that permits high sensitivity (often down to parts per billion (ppb).
 Impressive discrimination among very similar analytes, and fingerprinting of extremely similar mixtures
over a wide range of analyte categories, in both gaseous and liquid phases is also possible.
 Colorimetric and fluorometric sensor arrays therefore effectively overcome the limitations of traditional
array-based sensors that solely depend on physisorption or nonspecific chemical interactions.
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5. Electronic nose
Optical/ Optoelectronic nose
Optical Sensor requirements
 Two structural features are essential to the design of colorimetric or fluorometric sensors: functionality sites
to interact with analytes, and a chromophore or fluorophore to couple with the active site.
 Potential analytes vary in many aspects of their chemical properties: solubility, hydrophilicity, redox,
hydrogen bonding, Lewis donor/acceptor, and proton acidity and basicity of targets.
Figure . Strength of physical or chemical
intermolecular interactions on a semiquantitative
scale of enthalpy change.
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6. Types of Optical Sensor
Based on the types of intermolecular interactions that can induce significant colorimetric or fluorometric
changes, one may divide cross-reactive, chemoresponsive dyes into five classes:
(i) Brønsted acidic or basic dyes (e.g., various pH indicators)
(ii) Lewis acid/base dyes (e.g.,metal complexes with open coordination sites or metal-ioncontaining
chromogens)
(iii) Redox dyes
(iv) Colorants with large permanent dipoles (i.e., zwitterionic vapochromic or solvatochromic dyes) for
detection of local polarity or hydrogen bonding
(v) Chromogenic aggregative materials (e.g., plasmonic nanoparticles and nanoscale transition metal sulfides).
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7. (i) Brønsted Acid/Base Dyes
Fig: Some naturally occurring pH indicators. Litmus is from a lichen,
delphinidin from cabernet sauvignon, cyanidin from blueberries, and
rosinidin from rose periwinkle.
 The colors of Brønsted acids or bases are pH-dependent; i.e., their UV−vis absorption spectra change through
protonation or deprotonation as the pH changes.
Fig: Several some typical pH indicator
dyes.
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8. (ii) a. Lewis Acidic Dyes
 Most of the strongly odiferous compounds are Lewis bases: amines, carboxylic acids, sulfides, and
phosphines.
 These are also among the most common volatile metabolites of microbes.
 The primary function of the olfactory system is to keep our body away from high concentrations of bacteria
or other microorganisms, and hence the location of our nostrils immediately above the mouth!
 Inspired by the important role that metal-binding sites of olfactory receptors play in sensing such Lewis
bases, Lewis acid dyes are an obvious sensor choice.
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9. (ii) Lewis Acidic Dyes (Metalloporphyrins)
 Among Lewis acidic dyes, metalloporphyrins are a popular choice for the detection of metal-ligating volatiles
due to the open axial coordination sites and the large color changes from both wavelength and intensity
shifts of strong π-π* absorbances upon ligand binding.
 Metalloporphyrins can be readily modified to adjust their chemical reactivity by changing metal centers or
peripheral substituents to provide shaped pockets that restrict the access to the metal binding sites.
Figure. Structures of representative chemoresponsive dyes,
including porphyrins or porphyrinoids, and others containing
Lewis acid metal ions.
 A wide range of both ligating volatile organic
compounds (VOCs) (e.g., amines, carboxylic
acids, phosphines and phosphites, thiols) will
generate large color changes with
metalloporphyrin sensors.
 The difference in color of scarlet red arterial
blood versus the purple of venous blood is a
natural example of the colorimetric detection
of O2 using porphyrins
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10. Lewis Acidic Dyes for Anion Detection
 The use of Lewis acidic dyes for the detection of anions
has been extensively reviewed.
 The biological and medical significance of many
anions, from the simple (Cl-, F-, NO3-, PO43-, H2PO42-,
etc.) to the complex (e.g., ATP, lipid anions, nucleic acids,
and other phosphorylated biomolecular ions), received
much greater attention in recent years.
 Dye displacement assays, indicators with urea, thiourea,
or naphthalimide, and metal ion containing chromogens,
have been extensively explored in the past few years for
the colorimetric or fluorescent detection of anions.
Figure. Schematic illustration of fluorescence turn-on sensing of anions based on
disassembly of initially nonfluorescent (self-quenched) polymer aggregates upon anion
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11. Lewis Basic Dyes for Cation Detection
Figure. Complexometric indicators. (a) Several traditional indicators
used for complexometric titrations. (b) Representative metal complexes
of calcein, murexide, and EDTA, from top to bottom.
 Lewis bases include mostly chelating and macrocyclic
ligands that can have extremely high affinities for Lewis
acids.
 Modern supramolecular chemistry finds its origins in the
design of crown ethers, cryptands, cyclodextrins, and
calixarenes, whose sizes govern their specific binding of
metal ions.
Fig. EBT is blue in a buffered solution at pH 10.
It turns red when Ca2+ ions are added.
Ca2+, Mg2+, Al3+Ca2+ Ca2+, Mg2+, Al3+
Ca2+, Ni2+Cu2+ Ga3+, In3+,
Sc3+
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12. (iii) Redox Indicator Dyes
 Reduction/oxidation (redox) indicators are colorimetric reagents which provide a characteristic color
change at a specific electrode potential
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(iv) Solvatochromic or Vapochromic Dyes
 Solvatochromic or vapochromic dyes undergo color changes inresponse to changes in the polarity
of the local environment.
Figure. Structure of some representative solvatochromic dyes. (a) Three typical solvatochromic dyes. (b) solvatochromic shifts of a
merocyanine dye, in various solvents with different polarities. (c) Frontier orbital energy changes for negative solvatochromism (e.g., as in
merocyanine dyes).

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(v) Chromogenic Aggregative Indicators
 Chromogenic indicators: whose color is induced by particle aggregation.
 Example: simple precipitation of metal salts or formation of metal nanoclusters upon reactions with
sulfides,
 Nanomaterials of all sorts including metallic and metal oxides, carbon-based nanotubes, grapheme variants,
carbon dots, or quantum dots, have been employed as chromogenic aggregative probes and widely applied
to biosensing or immunosensing.
 Aggregation, dispersion, or formation of colloidal materials generate changes in color or fluorescence
through multiple optical mechanisms.

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Fabrication & Application of the colorimetric sensors

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Figure: Colour change profiles of a metalloporphyrin sensor
array. A, Colour change profiles of the metalloporphyrin
sensor array as a function of exposure time to n-
butylamine vapour. Metalloporphyrins were immobilized
on reverse phase silica gel plates. Colour images were
obtained with a flatbed scanner (HP Scanjet 3c) at 200
d.p.i. resolution. Subtraction of the initial scan from a scan
after 5 min of N2 exposure was used as a control, giving a
black response, as shown. 9.3% n-butylamine in N2 was
then passed over the array and scans made after exposure
for 30 s, 5 min and 15 min. The RGB mode images were
subtracted (absolute value) using Adobe Photoshop
software, with contrast enhancement by expanding the
pixel range (a 32-value range was expanded to 256 each for
the R, G and B values). B, Colour change profiles for a series
of vapours; the degree of ligand softness (roughly their
polarizability) increases from left to right, top to
bottom. Each analyte was delivered to the array as a
nitrogen stream saturated with the analyte vapour at 20
deg C (to ensure complete saturation, vapour exposures of
15 min or greater were used). DMF, dimethylformamide;
THF, tetrahydrofuran.
Zn(TPP)

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 Triacetone triperoxide (TATP), one of the most dangerous primary explosives.
 Owing to the lack of UV absorbance, fluorescence, or facile ionization, TATP is
extremely difficult to detect directly.

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Figure 1. (a) Acid catalyzed decomposition of
TATP. (b) Color difference maps of TATP vapor at
concentrations specified after 5 min (top row) and
10 min (bottom row) of exposure at 50% relative
humidity and 298 K

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Figure. The direct responses of color change profiles of the 20 representative volatile organic compounds
(VOCs) at their IDLH concentration without pre-oxidation compared to the color change profiles with pre-
oxidation; 5 min exposure at 50% relative humidity and 298K.

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Fig. Representative explosives and related compounds targeted for identification
using the colorimetric sensor array.

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Fig. Difference maps of the 40-element
colorimetric sensor array showing signal-
to-noise of 16 explosives, related analytes,
and the control.

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Figure 10. Multiplexed detection using a 1-mm diameter fiber optic bundle containing ∼50 000 individual 3-µm
optical fibers, each capable of containing an oligonucleotide-functionalized bead. The remaining images show a
portion of the fiber bundle and the response of each bead type as well as their collective response. Each bead type is
marked using a different color. The blue circle is a positional marker and the same for all images

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 Array-based optical sensing is useful in addressing a wide range of analytical challenges.
 By using chemically diverse chemoresponsive dyes or biological receptors in combination with
chemometric analysis, arrays can be designed to discriminate various structurally similar analytes and
analyte mixtures.
 Another essential feature of colorimetric or fluorometric sensor arrays is that they probe chemical
properties of analytes, rather than physical properties, giving highly discriminatory, specific responses to
an enormous range of analytes.
 The colorimetric sensors can be made unresponsive to humidity using hydrophobic dyes.
Conclusion