1

Chapter 1

Chemosensors for ions: A review of literature

In the host guest chemistry the term chemosensor is more closely associated with

a molecular event. Sensor is a system that on stimulation by any form of energy

undergoes change in its own state and thus one or more of its characteristics.1 This

change is used to analyze the stimulant both qualitatively and quantitatively. The optical

and photo-physical changes in a molecule are found more valuable in this regard. These

receptor molecules exhibit selective response to specific ions or neutral species to be used

as chromosensors. An appropriate definition of a chemical sensor is the “Cambridge

definition”2

Chemical sensors are miniaturized devices that can deliver real time and on-

line information on the presence of specific compounds or ions in even complex samples.

Chemical sensors employ specific transduction techniques to yield analyte information.

The most widely used techniques employed in chemical sensors are optical absorption,

luminescence, redox potential etc. but sensors based on other spectroscopies as well as on

optical parameters, such as refractive index and reflectivity, have also been developed.3

Need for chemosensors

Various neutral and ionic species find widespread use in physiology, medical

diagnostics, catalysis and environmental chemistry.4, 5

As cations and anions are

prevalent in both heavy industry and in farming and as such in the environment,

chemosensors are beginning to find many applications more so because the role of ions is

being better understood now. Different cations such as Ag+, Cu

2+, Co

2+, Hg

2+ etc. are

relevant in different fields. Just to cite a few examples, Ag+ is useful in radio-

immunotherapy and photographic technology. It inhibits the growth of bacteria and fungi

and reduces the risk of bacterial and fungal infection and also

finds use in cancer

immunotherapy, deodorizing clothes and agricultural sterilizing agent. Hg2+

causes

environmental and health problems. A wide variety of symptoms are observed upon

exposure including digestive, kidney and especially neurological diseases. The level of

this ion is therefore an object of strict regulation and should not exceed 1 µg L-1

. Cobalt

2

is an essential element, required for the coenzyme vitamin B12 and also believed to be

cardiotoxic and may cause lung damage. Copper is necessary for the growth,

development, and maintenance of bone, connective tissue, brain, heart and many other

body organs. It is involved in the formation of red blood cells, the absorption and

utilization of iron and the synthesis and release of life-sustaining proteins and enzymes.

These enzymes in turn produce cellular energy and regulate nerve transmission, blood

clotting and oxygen transport. It stimulates the immune system to fight infections, repair

injured tissues and promote healing. It also helps to neutralize "free-radicals" which can

cause severe damage to cells.

Similarly in case of anions few examples may include chloride sensing which is

now being used for environmental monitoring. Chloride ion measurements can aid the

monitoring of landfills for leaks, tracing the movement of pollutants within a natural

water body and detection of salt water intrusion into drinkable ground or surface waters.

The monitoring of nitrate levels is used to trace pollution from agriculture (nitrate

fertilizers), for checking fish farms and other aquaculture for waste build-up and for

surveying nutrient levels in natural water bodies. The excess of fluoride can lead to

fluorosis which is fluoride toxicity and results in increase in bone density. The F- ion is

important in clinical treatment for osteoporosis and detection of fluoride toxicity resulting

from over accumulation of F- in bone.

6 There are many anions either used as, or are the

products of, the degradation or hydrolysis of chemical warfare agents for example

fluoride, which is the decomposition product of Sarin gas found in many nerve agents.7

Cyanide is highly toxic to animals any type of release into the environment can lead to

serious problems.8,9

But it is used in various industrial processes, including gold mining

and electroplating, production of organic chemicals and polymers, such as nitriles, nylon,

and acrylic plastics enhancing the chance of unwanted release.10

From the above it is clear that detection of ions is vital as many industrial and

agricultural processes can lead to the release of ions to the environment, if left unchecked

these can have devastating effects. A major research effort is being focused towards

finding inexpensive, reliable and simple ways of detecting ions in solution. Therefore

finding new selective ion receptor systems is an important goal which involves sensor

3

development, environmental remediation, selective separation and extraction of chemical

species. The future envisages that greater interest and legal requirements for

environmental, food and water monitoring will increase the need of detection of anionic

species, selectively at more and more lower concentrations.

Cation vs. anion sensing- Though booming in a big way, the research in anion-selective

receptors design is still less developed than that reported for its cation counterpart. This

lack of development can be related to the following differences that exist between anions

and cations.

- Ionic size – The anionic sizes are generally larger than cations and consequently

necessitate larger binding sites. For e.g. 0.167 nm ionic radius of Cl- is much larger than

0.133 nm of K+. The latter is same as the smallest F

– anion.

- Shapes- Anions come in different geometrical shapes e.g. spherical (Cl-), linear (CN

-),

tetrahedral (SO42-

) and trigonal planar (NO3-).

– The pH range for existence of anions is narrower in comparison to cations as in the case

of phosphate and also ionization states can be variable, e.g. carbonate vs bicarbonate.

- Anions are more heavily hydrated than the cations of comparable sizes.

Design of the chemosensors consist of three components as shown in Fig. 1; a

chemical receptor capable of recognizing the guest of interest usually with high

selectivity; a transducer or signaling unit which converts that binding event into a

measureable physical change and finally a method of measuring this change and

converting it to useful information.

Figure 1. Schematic diagram showing binding of an analyte (guest) by a chemosensor (host),

producing a complex with altered optical properties.

4

There are three different approaches11

which have been used by various groups in

pursuing the synthetic receptors, which differ in the way the first two units are arranged

with respect to each other.

- Binding site-signalling approach

- Displacement approach

- Chemodosimeter approach

These differ in the way the first two units are arranged with respect to each other. In the

“binding site-signalling subunit” approach the two parts are linked through a covalent

bond.12

The interaction of the analyte with the binding site induces changes in the

electronic properties of the signalling subunit resulting in sensing of the target anion. The

displacement approach is based in the formation of molecular assemblies of binding site-

signalling subunit, which on coordination of a certain anion with the binding site results

in the release of the signaling subunit into the solution with a concomitant change in their

optical properties.13

In the chemodosimeter approach a specific anion-induced chemical

reaction occurs which results in an optical signal.14

Out of these three approaches the first

one has been widely exploited and this is the one which would be pursued in the present

review.

Depending on the type of signals produced on the binding event, sensors may be

put into two categories; Electronic sensors or Optical sensors.15

The former produce

signals in the form of changes in the electrochemical properties whereas the latter bring

changes in the optical properties.

Electronic Sensors

These sensors are mainly categorized into five categories.

1. Ion-Selective Electrodes (ISEs)

2. Field-effect transistors (FETs)

3. Electroactive Sensors

4. Biosensors

5. Microelectrodes

5

Optical sensors

The optical sensors further can be classified into two categories.

(A) Chromogenic chemosensors. In such type of chemosensors the coordination site

binds the guest in such a way that signaling unit shows the changes in color.

(B) Fluorogenic chemosensors. In fluorogenic chemosensors the interaction between the

coordination site and the guest moiety shows the changes in fluorescence behavior of the

signaling unit.

The development of colorimetric sensors is increasingly appreciated since naked

eye detection can offer qualitative and quantitative information without resort to any

spectroscopic instrumentation. The fluorescence measurement on the other hand is

usually very sensitive, versatile and offer sub-micromolar estimation of guest species. A

wide variety of optical chemosensors have been reported for the cation, anion and neutral

molecules. Based on the nature of analyte being detected, irrespective of the

photophysical phenomenon the receptors follows, the chemosensors may be broadly

classified into 3 categories; Cations sensors, Anions sensors , Neutral sensors. The

present review is restricted to the optical chemosensors for cations and anions, which can

be further classified as :

1. Cation sensors:

Optical cation sensing, which was originally started for metal ions not having a

spectra of their own in the visible region, has been extensively studied. With the better

understanding of host-guest chemistry of crown ethers, it became easier to develop

chromogenic sensors for metal ions. Later many excellent examples of chromophoric and

fluorophoric receptors for different cations have been prepared and studied.

(I) Chromogenic cation sensors. The chromogenic cation sensors may be grouped into

three broad categories.16

i) Chemosensors for photometry in organic media: Such types of chemosensors function

only in organic media. This category includes neutral and some protonic species

depending on their molecular charge when they form complex with metal ions. Neutral

chemosensors cause a net electronic charge transfer from the donor end to the acceptor

6

end with in the chromophore. Such type of chemosensors involves intramolecular and

intermolecular electron donor acceptor interactions, resulting in metal induced

hypsochromic or bathochromic spectral shifts. Thus the metal binding to the chemosensor

affects the electronic ground and excited state of chromophore with respect to the

corresponding electronic state of the uncomplexed chromophore. In case of protonic

chemosensors, the groups such as nitrophenol, azophenol having acidic protons form an

integral part of the chemosensor skeleton. The color change is attributed to the

deprotonation in these receptors. Although the organic base is not enough to cause the

color change by itself, it facilitates the metal induced proton dissociation of the

chromophore, which is followed by the intramolecular charge transfer through

conjugation leading to color change.

ii) Chemosensors for extraction photometry: This type includes the anionic type

chemosensors in which a proton-dissociable group is a part of the chromophore. In the

presence of metal ion the interaction of the particular chemosensor is displayed in the

form of optical properties. If the anion produced upon deprotonation is singly charged,

the ionophore formed is a charge-neutralized complex with monovalent metal ion which

is extractable from aqueous solution to a water-immiscible organic solvent. Sometimes

receptors contain two anionic side arms the resulting molecules extracts the divalent

metal ions. The performance of such chemosensors depends upon the steric orientation of

the anionic group and cavity of the receptor to accommodate the particular metal ion.

iii) Chemosensors for photometry in aqueous media: Such types of chemosensors are

specially designed for the metal photometry in aqueous media. This type of photometry

eliminates the use of toxic and volatile organic solvents and also eliminates the phase

separation step required in extraction photometry.

II. Fluorogenic cation sensors; A large number of fluorescent signaling systems

exploiting the photo induced intramolecular electron transfer (PET) mechanism and metal

ions as input are known. These systems are designed to covalently link fluorophores to

electron donating atoms of acyclic or macrocyclic receptors. In guest free state, the

binding subunit of the receptor donates a lone pair to the excited fluorophore via PET

(Figure 2) and the excited fluorophore comes to the ground state through a non-radiative

7

pathway, causing quenching of fluorescence. The presence of a metal ion engages this

lone pair through bonding, the PET is blocked, leading to the recovery of fluorescence. A

system with fluorescent on/off capability can potentially act as a molecular photonic

switch, which is the most important component of any true photonic device.

Figure 2. Schematic diagram showing mechanism of ion recognition by fluorescent PET sensors

Based upon the above techniques, excellent examples of receptors for various

analytes have been prepared and studied for application in physiology, medical

diagnostics and in environmental chemistry. A number of chromogenic receptors based

on crown ethers,17

calixarenes; thiacalixarenes; azacrown-calixarenes,18

and podands19

have been reported in literature.

(a) Podand based azo-coupled colorimetric cation sensors

A number of azobenzene-based colorimetric receptors have been synthesized and

its ion recognition studies were performed. In solution, receptor 1 gives rise to a large

cation-induced hypsochromic shift for Cu2+

resulting in a change from red to pale-yellow

that was clearly visible to the naked eye.20

Azo 8-hydroxyquinoline benzoate 2a gives21

distinct color change in the presence of

Hg2+

in CH3CN, which produces naked eye detection of Hg(II). In order to investigate

the effect of substituents, the receptor 2a has been modified to synthesize a series of 8-

hydroxyquinoline benzoates with diverse azo groups. Bathochromic effect of 2 a-c occurs

as substituent changes from -H, -OCH3, to -N(CH3)2 in CH3CN solution upon

complexation with Hg2+

ion.

8

NN

N

O

HN NH

OR R

NO2

1a R=CH(CH3)NHCO(CH2)10CH3

1b R=CH(CH3)NHCOCH3

1c R=(CH2)10CH3

N

O

NN

O

R

H3C

2a R = -N(CH3)2

2b R = -H

2c R = -OCH3

N

N

N

R

CO2KCO2K

NO2

3a R=OCH3

3b R= H

1 2 3

A naked eye, highly selective and reversible colorimetric detection of Cu2+

has

been achieved with 3 under physiological condition. The internal charge transfer (ICT)

sensors are highly colored, absorbing in the green. For 3a, the Cu2+

recognition gives rise

to red-to-yellow color changes that are visible to the naked-eye and reversible upon

addition of EDTA, whereas for 3b, which lacks the aromatic o-methoxy chelating group,

no such changes were observed.

Callan et al. has reported Quantum Dot–Schiff base conjugate 4 whereby the

incorporation of a receptor onto the surface of a Quantum Dot (QD) alters its

selectivity.22

The three-dimensional surfaces of the nanoparticle provide a particular

organization to the receptor, which can recognize the Cu (II) and Fe(III) simultaneously,

at physiological pH in a buffered solution. A significant shift in the absorption spectrum

was observed in 4 on the addition of Cu(II) and Fe(III) in semi aqueous medium, in

contrast the Schiff base itself displayed no selectivity.

NOH

S

R =

R RR

R

R

R

RRR

R

R

R

R

R

ZnS

CdSe

4

9

(b) Calix based azo-coupled colorimetric cation sensors

The metal ion induced deprotonation of azophenol moieties23

produces visual

optical changes in the electronic spectra of the ligands 5, 6, 7, 8, 9. Upon the addition of

100 equiv. of Hg2+

the color of the solution changed distinctly from yellow to red. In

another series 1,3-alternate chromogenic azo-coupled calix[4]biscrowns 6 there is a

regioselective complexation of metal ion. From the results of UV/vis band shift upon

metal ion complexation, it was concluded that the metal ions were entrapped only by the

upper crown loop, causing the hypsochromic shift on the UV/vis spectra. Calix- [4]

bis(crown-5)(crown-6) revealed K+ ion selectivity while calix [4]bis(crown-6)(crown-6)

showed Cs+ ion selectivity caused by a size complementarity between hosts and guest

ions.

OH OHO

N Bu1NBu1

O

NH HN

N N

NO2

NO2

O2N

O2N

O

N N

O

O O

O

O

O

OO

O

O

N N

n

R R

n R6 a 1 NO26 b 2 NO26 c 1 NH2

OO

O O

NH

O

OHN

O

O

5 6 7

Upper rim allyl- and arylazo-coupled calix[4] arenes based chromogenic sensors 8

is highly selective for Hg(II) ion.24

This shows visual color change in the presence of

Hg(II) ion. A marked bathochromic shift in the UV/vis spectra from λmax 359 nm to 520

nm (Δ λmax = 161 nm) was observed upon titration by Hg(ClO4)2 in a methanol-

chloroform solvent system. Job‟s plot shows 1:1 stoichiometry between 8 and Hg(II) ion.

Upper rim modification of tetrathiacalix[4]arenes bearing various azo groups have been

reported 9 and their ionic recognition has been studied in a hope to obtain new molecular

filters and devices for specific use.25

10

OO O

NN

O

H H

HN NH

H3CO OCH3

Hg2+

OHOH HO

R4

S

R1R2

R3

OHSS S

R1 = NSH2C3-N=N-

R2, R3, R4 = H

8 9

Kim et al. has synthesized azo-coupled calix[4]azacrown-5 10 having an azo

chromophoric pendant group.26

Absorption spectra showed hypsochromic shift upon

cation complexation, alkali and alkaline metal ions. Receptor has selectivity for K+ not

only due to the size compatability between the K+ ion and the azacrown-5 loop but also

due to a significant K+···π interaction between the two aromatic rings and the K

+ ion. The

UV band shifting is also dependent on the lipophilicity of the species of counter anion

used. Azo-coupled calix [4] crown based receptors 11 showed visual color change on the

addition of Ca2+

and Mg2+

in the cone conformation.27 However cesium and other alkali

metals metal ions were not entrapped in the crown-6-ring because ligand was not in 1,3-

alternate conformation. Metal ions entrapped by the crown loop induced deprotonation of

the receptor easily even at neutral pH causing a charge density shift in the direction of the

acceptor substituent‟s (nitro group) of the chromoionophore.

A dipodal calix[4]arene derivative28

12 was found to be highly selective for Na+.

A bathochromic shift in the UV–Vis spectra was shown in the presence of Na+

with a

visual color change in the solution from yellow to red. The binding ability and

photophysical behavior of alkali cations by calix[4]arene derivative 12 was explained on

the basis of substituted effects at the lower rim of parent calix[4]arene and size-fit

concept between host calix[4]arenes and guest cations.

11

OO

O O

O O

N

NN

NO2

O

N N

O O O

O

O

O

O

H H

N N

O2N NO2

OHOH OO

N N

HCHC

OHHO

N

N N

N

NO2 NO2

10 11 12

(c) Crown based azo-coupled colorimetric cation sensors

In literature Hg2+

selectivie azo-coupled macrocyclic chromoionophores were

synthesized by incorporating benzene 13 and pyridine 14 subunits.29

In a cation induced

color change experiment, 13 gave a larger cation-induced hypsochromic shift than 14,

suggesting that the presence of the pyridine unit in 14 may inhibit the Hg···N-azo

interaction. The observed color changes for Hg2+

in 13 and 14 were found to be

controlled by anion-coordination ability.

O O

S S

N

NN

NO2

N

O O

S S

N

NN

NO2

NN

N

NO2

O O

OO

O

OO

O

N

NN

NO2

O

13 14 15 16

12

An N-azo-coupled macrocycle 15 has been used as a chromoionophore30

and

shown to exhibit Hg2+

selectivity where the color of the metal complex is controlled by

the anion. The two different colors for the complexes with iodide and perchlorate anions

have been attributed to two different types of endo and exo complexes. The azo dye based

chemosensor31

16 displays extremely good sensitivity and selectivity for Na+ as

compared to other physiologically important alkali and alkaline earth metal ions, which is

pH independent above pH 3.9. This sensitivity is in the same range as found in blood for

Na+

ion concentration. Receptor showed an absorption band at max 485 nm, which

showed a hypsochromic shifts with Δmax 110 nm on addition of Na+. This also showed a

strong visual color change from red to yellow in aqueous solution even at low metal ion

concentrations.

Chromogenic N2S2-donor macrocycles functionalized with p-nitroazobenzene 17

and phenyltricyanovinyl 18 units were synthesized.32

These two receptors exhibit

excellent Hg2+

selectivity by showing the drastic metal-induced color change from red to

colorless (λmax = 137 - 140 nm). The sensing ability for Hg2+

with the proposed

chromoionophores is due to the stable 1:1 metal-ligand complexation formation.

N N

S

S

N

N NO2

N N

S

S

CN

CN

NC

17 18

(d) Acyclic fluorescent cation receptors

Duan et al. have synthesized a new Ag(I) selective fluorescent chemodosimeter

19 by incorporating imidazolium units and naphthyl lumophores into a two-arm

dipodand.33

A conformational switching „„off–on‟‟ signalling process was shown by the

receptor in the presence of Ag (I), through a combined signaling by both PET and

excimer formation mechanisms.

13

N

N

N

N

N

N

N

N

Ag+

Ag+

OFF

ON

N NH

NH

H

Cu, -H+

N N

N

H Cu2+

19 20

A 1,8-diaminonaphthalene based Cu2+

selective ratiometric and colorimetric

fluoroionophore receptor 20 recognizes Cu2+

selectively on the basis of ICT in which the

metal has been incorporated in the electron donor moiety of the deprotonated

fluorophore UV-vis and emission spectra of the chemosensor showed remarkable red

shift in acetonitrile-water solution upon Cu2+

complexation. A dual color change from

colorless (receptor) to purple followed by blue and large red shifts in emission were also

observed.34

Molecule 21 is a chromogenic and off-on fluorogenic probe35

for the naked-eye

detection of copper (II) in water-acetonitrile. The receptor can detect Cu2+

from

environmental samples by UV and fluorescence spectroscopy, in the range of trace

amounts (μgL-1

), as required by organisms. With the stepwise addition of Cu2+

the

absorption band at 550 nm disappeared with simultaneous appearance of a new band at

393 nm. Similarly fluorescence titration of receptor showed 5-fold enhancement in the

intensity of emission at 650 nm upon the addition of Cu2+

solution.

Pyrene-based Cu2+

indicator 22 that functions both as a naked-eyed and

fluorogenic probe36

for Cu2+

with 2:1 complex stoichiometries. Upon addition of Cu2+

the

receptor shows a strong static excimer emission at 460 nm, along with a weak monomer

emission at 388 nm. The excimer emission intensity induced by the Cu2+

ion depends on

the spacer length between the pyrene and quinolinylamide unit. For Cu2+

ion sensing the

intramolecular distance between the pyrene and quinoline amide groups should be less,

this allows for eminent π···π interactions.

14

N

NC

CNCl

N

NN

NH

O

N

N

NH HN

N N

OO

21 22 23

A Cu2+-

sensing fluorescent sensor 23 senses the metal ion on the basis of the

mechanism of internal charge transfer (ICT ) with high sensitivity and selectivity.37

This

resulted in the reduction of electron donating ability of the two amino groups conjugated

to the naphthalene ring. This molecule detects the Cu2+

ratiometrically with blue shifts of

50 nm. In ethanol water solutions it exhibits a strong fluorescent emission at 525 nm. The

formation of 1:1 metal-ligand complex has been observed with a shift in the emission

band from 525 nm to 475 nm. Dithiadicarbamates have been used to create the metal ion

receptor38

24 that has a high affinity towards Hg2+

. More attention has been paid to

carbamodithioate and related structures for the molecular recognition of heavy metals.

These sensors add novel interesting tools for environmental and biological detection of

Hg2+

.

O O

ClCl

HO

N

S

S S

SN N

S

S

S

SS

S

S

S

SS

N

a b

24 25

TTF-pyridyl derivatives 25a and 25b along with their phenyl analogues

have been designed and synthesized (where TTF = tetrathiafulvalene).39

In these

compounds, the TTF moiety and pyridyl group or phenyl group are bridged via a double

15

bond, which was designed to optimize the communication between the TTF unit and the

pyridyl or phenyl group. All these compounds exhibit strong absorption bands in the

range of 370 to 550 nm, which are assigned to the ICT from the HOMO in TTF to the

LUMO in the pyridyl or phenyl group. The interaction between pyridyl group and Pb2+

enhances the electron-accepting ability of the pyridyl group. 25 show remarkable sensing

and coordinating properties to Pb2+

as compared to their phenyl analogues, with visual

color change from yellow to deep purple.

OO

NH

NH

N

N

N

O

NHCH2(PO) (OEt)2

NHCH2(PO) (OEt)2

O

NHCH2(PO) (OEt)2

O

26

(e) Cyclic fluorescent cation receptors

Chromogenic sensors based on chromogenic macrocyclic backbone are

particularly convenient for the development of selective sensors for different metal ions.

In fact, by changing the nature and the number of side arms this backbone can be adapted

to the nature of the metal analyte. In addition, the modification of solubility properties of

the chosen receptor by the introduction of a neutral (diethoxyphosphoryl) methyl group is

also interesting and can be widely applied for creating water-soluble receptors. A new

anthraquinone-based colorimetric molecular sensor 26 exhibits efficient binding for lead

ion in water and allows naked-eye detection.40

A 1,2,3-triazole based Al3+

selective chemosensor 27 was synthesized and studied

for its optical/ electrochemical properties.41

A significant enhancement in fluorescence

intensity was seen at 556 nm, on the addition of Al3+

ions due to the combined effect of

ICT and CHEF caused by 1,2,3-triazole ring and carbonyl groups within a 2:1 complex

mode.

16

O

O

O O

N

NN NN

N

OO

27 (Adapted from reference 41, Kim et al. Org. Lett. 2010, 12, 560)

An On-Off fluorescent chemosensor 28 for Cu2+

works via intramolecular

fluorescence resonance energy transfer (FRET-On). In the presence of Cu2+

fluorescence

of pyrene is strongly quenched (FRET-Off) whereas that of naphthalene group is

revived.42

28

(Adapted from reference 42, Kim et al. Tetrahedron 2008, 64, 1294)

(f) Calix based fluorescent cation receptors

The calix[4]arene derivate 29 carrying two spirobenzopyran moieties have been

reported43

as lanthanide ions sensor. Significant shifts in UV–vis spectrum and the

emission spectra have been observed in the presence of lanthanide ions. An apparent

color change could be observed with the „naked eye‟ over other cations including Na+,

K+, Mg

2+, Ca

2+, Fe

3+, Cu

2+ and Zn

2+. A triazole-modified calix[4]crown in the 1,3-

alternate conformation based fluorescent on-off switchable chemosensor 30 with two

different types of cationic binding sites has been synthesized by Chung et al.44

Fluorescence studies showed strong quenching by Hg2+

, Cu2+

, Cr3+

, and Pb2+

; however,

17

the revival of emission from the strongly quenched Pb2+

complex was observed by the

addition of K+, Ba

2+, or Zn

2+ ions.

R OHOH R

O NO2

O

OO

R =

O O

N N

N N

N N

O O

OO

O

N O

N

N

SO

Cl

Fe

Fe

N

N

N

S

O

O

NN

NS

O

O

29 30 31 32

(g) Metal based cation sensors

Two ferrocene derivatives 31, 32 bearing two donor-acceptor systems are

capable of selectively sensing cations by cooperative binding of the two ,‟ groups

bonded to the ferrocene nucleus, thus permitting the naked-eye selective colorimetric

detection of Cu2+

cation.45

A novel colorimetric detection of Hg2+

based on a mesoporous

nanocrystalline TiO2 film sensitized with a ruthenium dye 33 has been affected in

aqueous solution with high selectivity.46

Using an entirely different approach Hg2+

has

been sensed by using a coordination complex of Ru(III) with substituted bipyridine and

thiocyanate ions.

N N

N

N

O

O

O

OH O

OH

OO

Ru N C S

NC

S

TiO2

N Ru

HOOC

HOOC

NCSSCN

NCS

HOOC O O

N

NH2

CN

CN

N

C2H5OOC

C2H5OOC

33 34 35

Based on ruthenium complexes mercury selective colorimetric sensors 34 has been

reported.47

Hg2+

coordinates reversibly to the sulfur atom of the dye‟s NCS groups and a

color change was observed even at submicromolar level concentration of mercury.

18

Moreover, by adsorbing the sensor molecules onto high-surface-area mesoporous metal

oxide films, an easy-to-use system for the dip sensing of mercury (II) in aqueous solution

has been developed. These films can be reused for sensing if they are washed with an

aqueous solution of KI which presumably removes the mercury from the surface by

forming a stable iodide complex of it.

(h) Miscellaneous examples of cation sensors

A coumarin-based copper selective colorimetric chemosensor 35 shows good

sensitivity and selectivity for the Cu2+

both in aqueous solution and on paper-made test

kits. The quantitative detection of Cu2+

was preliminarily examined.48

A

phosphorodithionate-based ligand 36 communicates a color change with Hg2+

owing to a

charge transfer and a solubility change.49

The receptor 37 is colorimetric mercury

selective ion sensor.50

It was observed that when commercially available 2-amino

isoquinoline was mixed with mercuric ion no color changes were observed, whereas in

case of receptor 37 color and fluorescence changes upon binding. A base/buffer was

required to bind with mercury ion in a 1:1 stoichiometry.

HO

O2N

NO O

PS-S

NHAc

N

S N

HO

O2C

N NH2

36 37 38

Recently an inexpensive, OMP (organic molecular probe) 38 consisting of a

charge-transfer complex, has been developed for the detection of small quantities of

mercury.51

OMP is a hemicyanine dye composed of an electron-donor aniline moiety and

an electron acceptor benzothiazolium species, showing a detection limit of 100 ppb for

mercury.

19

2. Anion sensors

Confining here to chromogenic anion sensors, these may be broadly divided into two

main categories, following the classification put forth by both P. A. Gale52

and Tuntulani

et al.53

(I) Metal and Lewis acid based chromogenic hosts

(II) Nonmetal based chromogenic hosts

(I) Metal and Lewis acid based chromogenic hosts : As put forth by Gale52

for metal-

involved chromogenic hosts, the changing color comes from the color of metals or metal

complexes, especially transition metal ions, whose electronic properties are changed upon

coordination to anions. The anions may also displace the coordinated chromophore to

produce a colorimetric response. Besides coordination aspects, the color change can

originate from reactions between chromogenic hosts or indicators and anions. This type

of chromogenic anion sensor can be called a “chromoreactand”. When the reaction

occurs, the conformed host will definitely change its electronic properties. This,

therefore, results in an observable color change. Metal ions in these receptors can play

one or many out of the following roles; act as a coordination site for the anions, as non-

coordinating moiety but changing its physical properties in the presence of anion or as an

electron withdrawing component in a -electron system so as to facilitate attack of anion

on it. It may also preorganize the receptor for the anion or behave as a co-guest in an ion

pair receptor.54-56

Many excellent reviews are also available in the literature on metal ions

based57-61

anionic sensors.

(II) Nonmetal based chromogenic hosts : Anion sensors based on nonmetal

chromogenic hosts employ H-bonding and/or electrostatic interactions for carrying out

the job. Confining here to this category of anion sensors, many such systems have been

reported which are based upon amide,62

pyrrole,63-64

urea/thiourea,65-66

azo dye units,67

naphthalene/naphthalimide units,68

porphyrin units containing neutral hosts,69

polyammonium and ammonium containing,70

guanidinium, amidinium or thiouronium71

containing cationic hosts. Many excellent reviews are also available in the literature on

fluorogenic/chromogenic,11,52,53,72

pyrrole based,63

guanidinium based,73

calixarene

20

based,74

and urea based65

receptors. In all of these neutral or positively charged receptors,

polarized N-H fragments are there which behave as H-bond donors towards anions.

According to Amendola and coworkers65a

there is a remarkable correspondence between

transition metal coordination chemistry and anion coordination chemistry. Neutral

ligands donate electron pairs to the metal and neutral receptors donate H-bonds to the

anion. Thus, whereas formation of metal-ligand complex can be studied in any solvent,

which ensures solubility of the reactants, the study of the anions is preferably carried out

in aprotic media to avoid competition for the anion by the H-bonding solvent. The

recognition phenomenon in these systems typically involves either H-bonding (incipient

proton transfer) or complete deprotonation of -N-H protons (frozen proton transfer)75

or a

stepwise process leading from H-bonding to proton dissociation. Sometimes it leads to

the stepwise deprotonation of two protons, excluding the intermediate step corresponding

to the H-bonding. As proposed by Fabbrizzi et al.76

during this host-guest complexation,

following consecutive equilibria may take place in solution, involving a neutral receptor

LH2 and the anion X.

LH2 + X- [LH2 X]- 1

[LH2 X]- + X- [LH]- + [HX2]- 2

[LH]- + 2X- L- + [HX2]- 3

Here the first step corresponds to a genuine H-bonded complex and the second step

involves abstraction of a proton from the complex to leave a mono deprotonated

derivative LH-

which may further get fully deprotonated according to step 3 at higher

concentrations of the anion. The second step, akin to the Bronsted acid-base reaction

takes the recognition process out of the realm of the supramolecular chemistry.65a

Nevertheless it is an efficient and sensitive method of anion sensing and has been amply

employed for this purpose.77

The occurrence of deprotonation is highly dependent on the

intrinsic acidity of LH, stability of [HX2]- and the polarity of the solvent and thus is not

observed in all the cases. Hence based upon above interactions, the efforts have been

directed toward the design and implementation of new tools for the anion sensing. Anion

21

sensors can be classified into 2 categories: neutral receptors and positively charged

receptors.

(i) Neutral non-metal based anion receptors

(a) Amide-based receptors. Amide NH groups have been employed to design a wide

range of receptors for anion recognition. Recently receptors 39a,b possessing nitro

aromatic moieties as signaling units and amide and/or amines as binding units have been

reported.78

The receptor gives a spectacular naked eye detection of F- at ppm level and is

highly selective for it even in the presence of other halide ions. An addition of a few

drops of a protic solvent reverses the color change thus establishing the role of H-bonding

interaction in the recognition.

C

O

NH HN

O NH

O

HN

O

C

NO2NO2

C

O

NH HN

O NH

O

HN

O

C

NO2

O2N

NO2

NO2

39 a 39 b

Szumna et al. prepared an acetate selective macrocyclic receptor 40 containing two

linked Pyridine clefts.79

The receptor showed 1:1 stoichiometry in solution form whereas

a 2:1 complex formed in solid state, which was confirmed by X-ray crystal strcture.

Anion binding properties of a series of macrocyclic tetraamides 41a-d containing

two 2,6- diamidopyridine groups linked via short aliphatic chains, were compared by

Chmielewski and co-workers.80

It was observed that receptor‟s size play a significant role

in the strength and selectivity of its anion complexes. It was found that binding constant

for chloride increases with increase in the size of ring. Further increase in the size of ring

causes a decrease in binding constant for chloride. A good size complementarity between

chloride and 41b was found as compared to 41d, later showed strong binding affinity for

large sized anions such as phosphate, sulfate or carboxylate anions. An isophthalic acid

based amide receptor 42 shows significant selectivity for sulfate and phosphate.81

Both

22

amide and bridging amine sites are responsible for anion recognition. Complementarity

between the receptor and oxo-anion as well deprotonation of the anion by the amine

groups of receptor play a significant role in the binding strength.

N

HNNH

O

NH

O

HN

O

O

N

HN

HN

O

NH

O

N

NH

O On

a n = 0 b n = 1

c n = 2 d n = 3

NH

NH

O

N

N HN

HN

O

O

O

CH3

CH3

40 41 a-d 42

Neutral receptors 43 with six amide groups in a pseudotetrahedral cleft

arrangement and optically fluorescent moieties pyrene groups have been designed82

which display a high selectivity for phosphates. In a macrobicyclic ion-pair receptor 44

with adjacent anion and cation binding sites,83

it was found that steric selectivity of the

receptor for alkylammonium cations increases with simultaneous binding of the anion.

N

NH

NH

O

ON

NO

N

NO

O

O

R

H

H

R

R

H

H

R

a R = Cyclohexyl

b R = (1-prenyl) methyl

NH HN

OO

O

N

OO

O

N

43 44

A series of sulfonamides based receptors have been designed by Kondo et al. The role of

hydroxyl group in sulfonamide cleft receptors 45 and 47 have been investigated and their

recognition behavior was compared with the non-hydroxyl containing compounds 46 and

48.84

1H NMR titration experiments have been performed to calculate the association

23

constants. The higher affinity of the hydroxyl functionalized compounds 45 for anionic

guests was observed as compared to non-hydroxyl containing compounds 46. Thus it was

evident that -OH group is involved in the anion complexation.85

S

NH H

N

S

OH HO

OO

OO

S

NH H

N

S

NH H

N

OO

OO

n-Bu Bu-n

HN

S

HO

OO

HN

S

HN

OO

Bu-n

45 46 47 48

(b) Urea-thiourea based receptors. Receptors based on urea and thiourea are good

hydrogen bond donors and are excellent receptors for anions such as fluoride,

carboxylates and phosphates ions. These receptors can form strongly hydrogen bonded

complexes with biologically important anions because of high hydrogen-bonding ability

of these functional groups. Anions such as fluoride and acetate cause the deprotonation of

urea and thiourea NH by the formation of stable species like HF2-. A new chromogenic

azophenol-thiourea based anion sensor 49 has been developed.67a

This system allows for

the selective visual colorimetric detection of F-, H2PO4

-, and AcO

- by means of hydrogen

bonding interactions. In 1H NMR, large downfield shifts in thiourea NH protons were

seen upon complexation. Broadening of phenolic OH indicated its involvement in the

hydrogen-bonding interactions. These selectivity trends turned out to be dependent upon

guest basicity and conformational complementarity between ligand and the guest.

A dual-chromophore sensor 50 with two chromophoric moieties, azophenol and

p-nitrophenyl has been developed. A colorimetric differentiation was seen for different

anions such as H2PO4,- F

-, CH3COO

- of similar basicity, whereas there was no

colorimetric differentiation in case of receptors 51 with only one chromophoric moiety

(azophenol group only). Thus the cooperativity of two chromophoric moieties in a dual-

chromophore receptor plays a significant role in color discrimination of anions of similar

basicity. It has been noticed that both the chromophores have been affected with four

oxygens of H2PO4- via multitopic hydrogen bonding interaction. This causes a significant

color change with H2PO4- as compared to F

- and CH3COO

- , which have a relatively

24

weaker effect on the p-nitrophenyl group. This enables the color discrimination between

H2PO4-, F

- and CH3COO

- .

67b

NN

OH

NO2

HNNH

NH HN

S

n-Bu Bu-n

S

NN

OH

NO2

HNNH

NH HN

S S

NN

OH

NO2

HNNH

NH HN

S S

O2N NO2

49 50 51

A chromogenic indoaniline–thiourea-based sensor 52 has been reported67c

for the

recognition of different tetrahedral oxoanions (H2PO4−, HSO4

−) in an organic solvent

through complementary intermolecular hydrogen bonding. A selective colorimetric

response was shown by these oxoanions by disturbing the intramolecular hydrogen-

bonded structure of the sensor. A hypsochromic band shifts was observed from 678 nm

to 632 nm upon the addition of H2PO4−. However a less intense color change was

observed in the presence of AcO−

or F− with higher basicity, while no color change was

seen in case of Cl−, Br

− and I

− even upon the addition of exess of anions to receptor.

N

O HNNH

NH HN

S

n-Bu Bu-n

S

N

Fe

O O

O

O

O

O

HN

HN

NO2

O

Fe

HN

O

HN

SS

Q D

52 53 54

A remarkable anion and cation induced On-Off colour switching was shown by a

ferrocene-based ditopic receptor 53 having a urea and a benzocrown ether unit.86

The

receptor showed absorption Maxima at λmax 304, 332 nm, after 10 molar equivalents

addition of F- a new a band appeared at 472 with a significant color change from

25

colorless to yellow. However upon the addition of K+ the chromogenic process reversed

and the yellow colour induced by the presence of fluoride was now no longer observable

and the absorption maximum at 472 nm had disappeared. Thus the ditopic inputs of

cation and anion color changes in the receptor controlled by switching on (in the presence

of F-) and off (in the presence of K

+) the optical outputs.

Callan and co-worker have designed ferrocenyl urea receptor 54, attached to the

surface of a pre-formed CdSe/ZnS Quantum dot (QD) via a dihydrolipoic acid linker,87

in

which QD fluorescence is quenched by an electron transfer mechanism. The receptor

showed good selectivity for fluoride over other monovalent anions. In the presence of

fluoride ion, the fluorescence of receptor again enhanced due to the change in the PET

between the ferrocene units and the QD.

N N

ON+

O-

N+

O

O-

H H

O

N N

O

H H

O NN

O O

O

55 56

Fabbrizzi et al. have reported bright yellow 1:1 complexes75

of 1,3-bis(4-

nitrophenyl) urea based receptor 55 with a variety of oxoanions in an CH3CN solution,

the stability of these anions decreases with the decreasing basicity of the anion. Fluoride

displays a more intricate behavior; upon the stepwise addition of F- the absorption band

of receptor at 345 nm disappeared, with a simultaneous appearance of a new band at 370

nm which reached its limiting value after the addition of 1 equiv. of F- to give the stable

1:1 complex through a hydrogen-bonding interaction. Upon the further addition of a

second equivalent of F- , a new band at 475 nm (orange red color) was obtained which

could be assigned to urea deprotonation, due to the formation of HF2-. Another Urea-

based receptor 56 has been reported having napthalimide like electron-withdrawing

chromogenic substituents.88

Upon the addition of fluoride stepwise deprotonation of the

two N-H fragments was observed with significant color change, whereas only one N-H

fragment deprotonated in the presence of less basic anions such as CH3COO- and H2PO4

-

. In the presence of F- ion the band at 400 nm decreased and a new band developed at 540

26

nm with a visual color change from yellow to red, which was assigned to the

deprotonation of one of the N-H fragments of urea subunit. On further addition of F- ion

the color of the solution turned red to blue and the band at 540 nm disappeared with the

simultaneous appearance of a new band at 600 nm. This was assigned to the double

deprotonation of the N-H fragments of receptor. A similar deprotonation behavior was

also observed in the presence of OH- ion.

O

NH HN

O

NH HN

O

NH HN

O

NH HN

O

NH HN

O

NH HN

ClCl

NO2O2N

a b c

57

Ortho-phenylenediamine based bis-urea compounds 57 have been synthesized89

for the

recognition of carboxylate anions in DMSO- water solution. This is also supported by the

crystal structure of benzoate complex of receptor 57 a, which shows that receptor bound

to carboxylates via four hydrogen bonds. The binding affinity for anions could be

increased by introducing an electron withdrawing group to the bis –urea skeleton of the

receptor and by converting urea group to thiourea the carboxylate / dihydrogen selectivity

could be tuned further.

NH HN

NH

NH

HN

HN

O

O

S S

NO2 NO2

NH HN

NH

NH

HN

HN

O

O

S S

NO2 CF3

NH

S

NH

NH

CF3

NHS

CF3

a b

58 59

27

Anthraquinone based thiourea receptors 58 have been used to give good sensitivity and

selectivity for discrimination of maleate vs. fumarate or malate vs tartrate.90

Receptor 58a

can also distinguish cis-aconitate or trans-aconitate with a unique colour change and can

be used as colorimetric sensors for recognition of chiral dicarboxylates. Gunnlaugsson

and co-workers have synthesized PET based chemosensor 59 for the recognition of

anions having two binding sides such as dicarboxylates and pyrophosphate.91

Thiourea

receptor sites with concomitant PET quenching of the anthracene moiety were

responsible for anion recognition.

A fluoride selective, urea-activated phthalimide based anion sensor 60 with a

stereogenic centre was synthesized.92

Upon the addition of fluoride a new CT complex

emission at a longer wavelength was observed with no changes in the singlet lifetime of

the receptor. This suggests a fluorescence static quenching mechanism in the presence of

fluoride. In order to study the binding behavior of receptor with fluoride 1H-NMR

titration experiments were also performed in CD3CN. Two signals at 5.89 ppm and 7.81

ppm appeared corresponding to urea protons, which shifted to downfield by 2 ppm and

4.5–5 ppm respectively, suggesting H-bonding interactions between the receptor and

anion rather than receptor deprotonation. Upon addition of a protic solvent the

reversibility of this process was also confirmed.

N N

H H

ON

O

O*

NH

NH

O

HN

HN

Xn

R

a n = 1 X = O R = H

b n = 1 X = S R = CF3

c n = 5 X = S R = NO2

60 61

Indole based receptors 61a-c with amide and urea or thiourea moieties as

additional NH donor groups have been reported by Pfeffer and co-workers.93

All four N–

H bond donors in these receptors strongly interact with the anions. 1H NMR titrations

suggest that all the receptors show deprotonation in the presence of F- in DMSO-d6 and

weak interactions with chloride. In the presence of chloride a large shift in proton

28

resonances was observed in the urea or thiourea NH groups as compared to indole and

amide NH groups. This suggests that urea or thiourea NH groups predominantly interact

with the chloride ion. Whereas in the presence of oxo-anions in particular dihydrogen

phosphate a co-operative binding behavior was observed, in which protons of the entire

hydrogen bond donor i.e. urea or thiourea, indole and amide show a significant downfield

shift in the proton resonances.

O

HN O

HN O

HN

NO2

OHOH OO

HN

HN

O O

NH

NH

HN

O

NH

O

NO2 O2N

N

HNNH

O

NH

CF3

O O

62 63 64

Gunnlaugsson and co-workers reported the amidourea-based compound 62

bearing a 4-hydrazine 1,8-naphthalimides that is able to act as colorimetric sensor94

for

different anion. The receptors showed bathochromic shifts in the absorption spectra upon

the addition of various anions such as acetate, dihydrogenphosphate and fluoride, either

due to hydrogen bonding or deprotonation of, the amidourea. Unique changes in the

absorption spectra were shown with each of the three anions. This can be considered as a

„fingerprint‟ identity for different anions.

The amido urea-based colorimetric anion sensors95

63 and 64 gave rise to red

shifts in their absorption spectra upon anion recognition, the sensing of F- and

hydrogenpyrophosphate gave rise to large changes with concomitant color changes from

yellow to purple, which were visible to the naked eye.

(c ) Indole and pyrrole based anion receptors. The Indole and pyrrole groups

provide a highly effective moiety for anion complexation in synthetic receptor systems.

Oxidized bis(indolyl)methane 65 a simple chromophore containing an acidic H-bond

donor moiety and a basic H-bond acceptor moiety, can act as a selective colorimetric

29

sensor96

either for F- in aprotic solvent or for HSO4

- and weak acidic species in water-

containing medium. The deprotonation/protonation of oxidized bis(indolyl)methane 65 is

responsible for the dramatic color change.

NNH

NH

NH

HNNH

ONH

NH

O

NH

O

NH

65 66 67

1, 3-diindolylurea based receptor 66 showed a high affinity for dihydrogen

phosphate in polar solvent.97

The 1H NMR titrations suggest that oxo-anions interact

strongly with urea and indole groups as compared to amide group as in 67. Jurczak and

co-workers have synthesized diindolylmethane based new acyclic anion receptor98

68.

The receptors can bind HPO42-

via hydrogen bonding interactions even in pure methanol.

An unprecedented stable dimer was formed by interaction of two ligand molecules with

phosphate anion, which was supported by X-ray structural analysis. Oxygen atoms of the

anion form eight complementary hydrogen bonds with NH groups of 68. It was evident

from the N-O distances in the X-ray structural that indole moieties bind more strongly

with anion than urea groups.

NH

NH

NH HN

NH HNHN

HN

OO

N N

NH

NH

R

a R = H, b R = NO2

68 69

Two indole-based receptors 69 a and 69 b have been designed for sensing of

fluoride and acetate anions.99

Highly flat rigid structure of indole-based system with a

large system was responsible for high binding affinity and sensitivity for acetate and

30

fluoride anions. Combined use of receptors 69a and 69b on the basis of their visual color

change in the presence of acetate and fluoride anions offers a simple way for

distinguishing these two anions with the naked-eye.

An indole conjugated bisthiocarbonohydrazone based receptor 70 has been

developed100

for the selective recognition of fluoride ion by visual color change along

with a near-infrared (NIR) band at 936 nm. Receptor showed an absorption band at 342

nm corresponding to the Ar-CH=N–NH conjugation. Upon the stepwise addition of F-

absorption maxima at 342 nm disappeared with simultaneous appearance of new bands at

522, 572, and 936 nm. A large shift in the NIR region is due to the high conjugation and

planarity of 70, which favors maximum negative charge distribution of the deprotonated

receptor in the presence of fluoride.

HN

HN

N

S

N

HN NH

NH

NH HN

OO

70 71

Zhu and co-workers have synthesized colorimetric fluoride sensor 71 by

combining pyrrole and hemiquinone units.101

The receptor contains two kinds of NH

protons with different chemical environments. Dideprotonation was observed in the

presence of fluoride, which was responsible for the selective discrimination of fluoride

from other anions. Other anions like CN-, CH3COO

- or H2PO4

induce

monodeprotonation only with the formation of an absorption band at 740 nm. Different

deprotonation behavior between these anions and F-

is due to the unique stability of

[HF2]-.

Two amidopyrroles 72 and 73 containing imidazole groups, have been

reported.102

These receptors have ability to bind and co-transport H+/Cl

- across vesicle

membranes similar to the prodigiosin. X-ray crystal structures showed that in the solid

state receptor 72 formed a „2+2‟ complex with HCl and with each chloride bound by

31

three hydrogen bonds; two from the pyrrole and amide groups of one receptor and one

from the imidazolium group of another receptors in the dimer.

HN

HN

NN

O

Ph Ph

HN

HN

O

Ph Ph

N

HN

O

NH

Ph Ph

HN

HN

NO2

S

72 73 74

Deprotonation was observed in the pyrrolylamidothiourea receptor 74 upon

addition of only one equivalent of anions such as acetate, fluoride and dihydrogen

phosphate, which was also confirmed by X-ray crystallography. The receptor 74 showed

a significant shift in the absorption band at 430 nm with a distinct color change from

yellow to red.103

(d) Hydroxyl group containing receptors. In recent years Hydroxyl groups have been

employed to design different anion sensors and have proven to be effective via hydrogen

bonding or deprotonation of –OH group. Hong and co-workers used azophenol dyes as an

optical-signaling chromophoric unit in the receptors 2-4, which gave a significant color

change in the presence of fluoride ion.104

Azophenolic OH of these receptors function as

an anion binding site while the steric effects of alkyl groups (H, methyl, tert-butyl) at 2

and 6 positions affect the anion selectivity. Fluoride ion strongly binds to 75, 76 and 77

due to the formation of the salt complex. In the presence of 1equiv. of fluoride receptor

76 and 77 undergo significant color change from yellow to blue. Steric hindrance by the

dimethyl groups at 2 and 6 positions in receptor 76 prohibits the formation of effective

hydrogen bonds and salt complexes for the larger anions. This causes the discrimination

for fluoride ion in receptor 76 as compared to 75. Although better selectivity for F- ion

is

expected for 77 due to the larger t-Bu groups ortho to the phenolic OH, but no color

discrimination was observed for the fluoride anion due to the existence of hydrazone–

azophenol equilibrium.

32

NN

OH

NO2

Bu-nn-Bu

NN

OH

NO2

OH

NO2

NN

OH

NO2

NNH

75 76 77 78

A chromogenic receptor 78 containing a nitro group as a signalling unit and OH

and NH groups as binding sites has been reported105

by Kandaswamy and co-workers for

the colorimetric sensing of fluoride ions. Upon the addition of fluoride ion, solutions of

the receptor became orange. In 1H NMR titrations two signal corresponding to the OH

and NH protons were observed at 11.65 and 10.64 ppm respectively. An upfield shift was

observed upon the addition of 1 equiv of F-

due to the hydrogen bonding between the

fluoride ions and the OH and NH groups of receptor 78. Upon further addition of F- more

than 2 equiv. deprotonation was observed in the receptor with the appearance of a new

signal at 16 ppm corresponding to HF2-.

NH N

HNNHN

NH

NH

NH

NHO

O

HN

HNO

HN O

NN

NO2

OH

R =

H N

O

OH

Br Br

HO

R

R

R

R

79 80

A selective coloration for F-, H2PO4

- and AcO

- was shown by an azophenolurea-

introduced porphyrin based colorimetric sensor 79.106

The binding interactions between

the azophenolic OH of the receptor and the different anions as well as the basicity of the

anions were responsible for the observed color changes. The absorption intensities of the

receptor with different anions were correlated to the intensity of color change, in the

33

order of F- > AcO

- > H2PO4

- . Depending upon the basicity F

-, H2PO4

- and AcO

- form

stronger complexes than other anions with significant color changes.

A phosphate selective ortho-hydroxyphenyl oxadiazole based fluorescent

chemosensor 80 have been reported by Lee and co-workers.107

Fluorescent emission

bands were observed at 330 nm and around 500 nm in DMF corresponding to the enol

form and keto form, respectively. In the presence of dihydrogen phosphate both the

fluorescence emission bands are quenched with a slight red shift, due to the

intermolecular hydrogen bonding between hydroxyl groups of the receptor and anion. In

the presence of basic anion like fluoride ion, the OH group of the receptor was

deprotonated with the formation of a new emission band at 450 nm.

(e) Imidazole and benzimidazole based anion receptors. NH group of imidazole and

benzimidazole provide efficient binding sites for recognition of different anions. Raposo

and co-workers have reported bithienyl-imidazo-anthraquinone based chromogenic and

fluorescent chemosensor 81.108

Upon the addition of fluoride ion a red shift was observed

in the absorption spectra with an enhancement of the fluorescence emission intensity. The

deprotonation of the NH in the imidazole ring with fluoride ion was responsible for a

significant color change from yellow to pink. Upon addition of metal ions such as Zn2+

,

Cu2+

and Hg2+

the original yellow color of receptor-fluoride complex is restored with a

quenching in the fluorescence intensity. This reversible colorimetric reaction upon metal

complexation gives rise to an Off-On-Off (yellow-pink-yellow) system.

N

HN

S

S

O

O

ON

O

O

NH

NH

NH

N NH

N NH

N NH

81 82

Benzimidazole based tripodal fluorescent receptor109

82 showed significant

changes in its fluorescent behavior in the presence of iodide ion over the other anion such

as F-, Cl

-, Br

-, HSO4

-, NO3

-, CH3COO

-, and H2PO4

- in 10% aqueous CH3CN. A role of

34

benzylic N-H in the binding with iodide is suggested. The ion recognition behavior of 83

was investigated using UV–vis, fluorescence and 1H NMR spectroscopy. In the

fluorescence spectra large quenching of intensity of the band at 375 nm was observed on

the addition of fluoride ion, the further addition of F- ion slowly shifted the band to 434

nm.110

NN

NH N

HN

N

NH

N

NH

HN HNSS

O

O O

O

Cl Cl

83 84

A benzimidazole-based fluorescent receptor 84 has been developed111

for the

recognition of the adipate. A 1:1 receptor-adipate complex formed, which restricts the

rotation around the single bond between two benzimidazoles. This results in two emitting

states responsible for the ratiometric recognition of adipate.

NH HNN

NH N

HN

O2N NO2

O O

N

N

HO

O

R

a R = -H

b R = -OCH3

c R = -NO2

85 86

Jang and co-worker have designed benzimidazole based neutral dipodal

colorimetric anion chemosensor 85 for the detection of F-and AcO

-.112

In the pure

DMSO, receptor can bind to a DMSO molecule and electron-deficient sulfur of bounded

DMSO act as a binding site for anion recognition. Han et al. have reported fluoride

selective colorimetric and ratiometric fluorescent chemosensors 86.113

The anion-induced

proton transfer was responsible for the colorimetric and ratiometric properties. In the first

step receptor formed a hydrogen bonded complex with the fluoride ion, whereas in the

35

second step deprotonation of the complex was observed to form L-

and FHF-. The

fluorescence behavior and ion interaction with fluoride depends upon the electron push-

pull properties of the substituents on the phenyl para position. Receptor 86a showed the

best selectivity for F- ion as compared to other receptors, which was due to the fitness of

anion in the acidity of NH-group of receptor.

(ii) Positively charged, non-metal based anion sensors

Anion binding behavior of tren-based tripodal species 87 was studied by

Bowman-James and co-workers.114

The receptor has high affinity for dihydrogen

phosphate and hydrogen sulfate with log Ka = 3.25 and 3.20 respectively. It was evident

from the X-ray crystal structure that the ligand form a triprotonated complex with

phosphate.

N+N

N+

N+H

H

H

HH

H

N+ N+

N N

N N

N+ N+

I

N

N

N+

N+

NH2

NH2

PF6-

PF6-

87 88 89

In the receptor 88 four imidazolium groups are appended to a napthalene scaffold

through methylene bridging.115

The receptor showed high selectivity for iodide as

compared to other halides. The selective binding was evident from the 1H NMR titrations

and fluorescence quenching. A benzimidazole based fluorescent receptor 89 shows a

ratiometric fluorescent sensing for acetate ion over a wide range of anions.116

Upon the

addition of acetate ion, quenching in fluorescence intensity at 443 nm was observed, with

the appearance of a new band at 339 nm.

36

N+ N+

N+

N+

N+

N+

N+ N+

N+

N+

N+

N+

HN

O

HN

R

O

NH

R

NH

NH

O

NH

R

O

HN

R

HN

R = p-tolyl

90 91

By incorporating two tripodal binding domains two viologen-based anion

receptors 90 and 91, have been designed.117

The receptor 90 has high affinity for

carboxylate anions such as acetate, malonate and succinate with a significant color

change from colorless to intense purple. These absorption changes arise because of

charge-transfer from anion to the bipyridinium unit. In case of receptor 91, initially no

color change was seen as the anions bind at the periphery of the receptor, however further

addition of more than two equivalents of acetate and more than one equivalent

dicarboxylate anions, receptor exhibit significant color change. Thus the identity of the

peripheral binding groups plays an important role to select the amount of anion required

for particular colorimetric response.

NH HN

N+N+

O O

PF6-

2 PF6-

NH HN

N N

N+ N+

O O

PF6-

92 93

The receptor 92 exhibits good recognition ability towards different anions118

such

as H2PO4-, F

- and AcO

- through hydrogen bonding and electrostatic interactions.

37

Dihydrogen phosphate bound selectively to the receptor through strong excimer

formation. The combined effects of semi-rigid structures, electrostatic interactions and

the participation of both N–H···O and C–H···O hydrogen bonds are responsible for the

high affinity and selectivity of receptor. Another anthracene appended o-

phenylenediamine-based receptor 93 exhibit strong interaction with dihydrogen

phosphate and fluoride ion in CH3CN.119

In the presence of these anions quenching of

anthracene emissions upon complexation was observed as compared to other anions.

A chloride selective fluorescent Off–On chemical sensor 94 exhibits a guest-

induced conformational switching process.120

The benzoimidazolium moieties form

hydrogen bonded complex with anion which was evident from 1H NMR experiments. In

the presence of chloride ion protons attached to the electron-deficient carbon atoms in the

benzoimidazolium group shifted downfield, suggesting the formation of CH+···anion

hydrogen bonds.

N

N

N

N

N

NH

H

H

N

N N

N N

2 PF6-

94 95

Yoon and co-workers have reported fluorescent receptor 95 the binding properties

of which have been studied from the changes in the fluorescent behavior of pyrene.121

Receptor 95 displayed a quite selective fluorescent quenching effect in the presence of

H2PO4-

due to PET process. By the use of positively charged H-bond donating

carbolinium ion 96 and neutral H-bond donating receptors 97, it has been demonstrated

that the stability of 1:1 complexes is strictly related to the acidic tendencies of the

receptor and to the basic properties of the anion. 65a

38

RR

R

NH

N+

1

2 3 4 5

68

7R =

NO2

NO2

H

H

NNO

NO2

N

H H

NR1 R2

O

a R1 = R2 = H

b R1 = R2 =

c R1 = R2 =

96 97

For biologically important anions like HSO4- and H2PO4

- amide groups held on various

scaffolds like isophthalamide, dipicolinic and pyrrole bisamide have been reported with

the most recent being 98 with azulene bisthioamide moiety.122

The latter might enhance

the anion interaction due to large dipole of azulene. For the detection of micromolar

concentration of cyanide ion a receptor containing oxazine ring 99 has been designed123

which opens to form a 4-nitrophenylazophenolate chromophore in response to cyanide

ion.

HNNH

S S

Bu Bu

N

N

O

NO2

N

MeMe

Me

98 99

Conclusions-

Among the various types of sensors, optical sensors have very promising future.

Since the various cations or anions are the essential part of the environmental, food and

water, so their recognition and selective removal with high sensitivity (recognition at

39

lower concentrations up to µm level) forms an important part of chemical research. The

technical problems in the designing of these sensors need to be overcome, which include

the following considerations.

The sensor designed for the ion recognition should be stable, having sufficient

lifetime and the ability to operate in water and the low sensitivity to the operative pH are

important under normal conditions. The sensors having the capability to sense/recognize

the various analytes in aerobic and aqueous medium are highly required.

On the other hand to deal with the problem of sensor lifetime is to avoid it

completely by using each sensor once. Hence the design and synthesis of inexpensive and

disposable sensor such as disposable sensor strips or nanoparticle based optical beads

highly desired. These strips or beads display significant color changes in the presence of

particular ion solution. The optical sensors show high selectivity for a particular ion in

the presence of other ions of similar nature. The design and synthesis of sensors that can

detect small concentrations of the ion of interest in the presence of other interfering ions

is of considerable interest.

40

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