chapter 2 ion imprinted polymers: a...

Chapter 2

ION IMPRINTED POLYMERS: A REVIEW

Ion Imprinted Polymers: A ReviewIon Imprinted Polymers: A ReviewIon Imprinted Polymers: A ReviewIon Imprinted Polymers: A Review 13

2.1 Introduction

The design of synthetic materials, which are able to mimic the

recognition processes found in nature, became an important and active area of

research1-3. Molecular imprinting technology (MIT) is today a viable synthetic

approach to design robust molecular recognition materials able to mimic natural

recognition entities, such as antibodies and biological receptors4-6. MIT is

considered a versatile and promising technique which is able to recognize both

biological and chemical molecules including amino acids and proteins7-9,

pollutants, drugs and food10-11. Further, application areas include separation

sciences and purification12-14, chemical sensors15, catalysis16, drug delivery17,

and biological antibodies and receptor systems18. MIT is based on the formation

of a complex between an analyte (template) and a functional monomer. In the

presence of a large excess of crosslinking agent, a three-dimensional polymer

network is formed. After polymerization process, the template is removed from

the polymer leaving specific recognition sites complementary in shape, size and

Fig.II.1. Schematic representation of the molecular imprinting

process by non-covalent imprinting method

14 Chapter 2Chapter 2Chapter 2Chapter 2

chemical functionality to molecule (Fig.II.1). Usually, intermolecular

interactions like hydrogen bonds, dipole-dipole and ionic interactions between

the template molecule and functional groups present in the polymer matrix drive

the molecular recognition phenomena. Thus the resultant polymer recognizes

and binds selectively only the template molecules.

Selective recognition of metal ions is a real challenge for a wide range

of applications in analytical science. In that purpose, ion-imprinted polymers

(IIPs) have been increasingly developed during the last 15 years on the principle

of molecularly imprinted polymers. Metal ion extraction in the aqueous

environment remains a major issue because of their high toxicity, their

persistence and susceptible carcinogenic effect19. Removal of metal ions from

diverse water resources is essential. Various methods can be used for metal ion

removal20 from the most common liquid-liquid extraction and solid-liquid

extraction or solid-phase extraction (SPE)21 to chemical precipitation22,

membrane filtration23, flotation24, electrochemical25 and biofiltration methods26.

Solid phase extraction using ion exchange or chelating materials offers

many advantages for metal ion separation since it simplifies the separation

process and reduces the disposal costs as well as the solvent uses and

exposure27. Many synthetic polymers bearing various chelating functions have

been studied28. They can be prepared either by impregnation29, by grafting of

commercial sorbents or home-made copolymers, or by direct copolymerization

of a functional monomer with a crosslinker30.

Ion-imprinted polymers (IIPs) have been introduced by Nishide et al. by

crosslinking of poly(4-vinyl pyridine) with 1,4-dibromobutane in the presence

of a metal ion31. This conception appears to be similar to the MIPs’ by changing

the template molecule to a metal ion (Fig.II.2). However, whereas MIPs

generally interact with template molecules via classical functional monomers


through hydrogen bonds or van der Waals interactions, a slight variation is the

resort to coordination chemistry when IIPs are concerned. The preparation of

IIPs generally requires a ligand to form a complex with the metal ion to produce

selective binding sites after metal leaching. The first IIPs were described by

Nishide et al. in 197632 not so long after Wulff and Klotz prepared the first

molecularly imprinted polymeric materials in 19722.

Fig.II.2. Schematic representation of synthesis of metal ion imprinted interpenetrating polymer networks

2.2 Ion imprinted polymers: General features

The general procedure for IIP elaboration consists in the preparation of a

ligand-metal complex followed by copolymerization with a cross-linker in order

to create three dimensional recognition cavities inside the polymer networks.

Denizli et al. have detailed this process in three steps as follows33: (i)

complexation of the metal ion to a polymerizable ligand, (ii) polymerization of

this complex, and (iii) removal of the template ion after polymerization. The

polymerizable ligands are often named as bi-functional reagents: one

functionality coming from their chelating ability and the other one from the

vinyl function. A more simple method to prepare IIPs consists in using non-

polymerizable ligand in which the ligand is embedded inside the polymer

matrix through some trapping process34. Whatever the choice to introduce the

ligand, the interactions between the polymer framework and the complexed ion


are based on coordinative bonds from some electron donating hetero atoms to

the unfilled orbitals of the outer sphere of the metal ions.

The properties of ion-imprinted polymers are remarkable because of

their high selectivity towards the target ion due to a memory effect resulting

from their preparation process. The affinity of the ligand for the imprinted metal

ion, the size and shape of the generated cavities are the two main factors

responsible for high selectivity35. As recognition sites are generated from the

self assembly of some ligand(s) around the template ion and subsequent

crosslinking, this arrangement enables the binding sites to match the charge,

size and coordination number of the ion36. Moreover, the complex geometry can

be preserved through the crosslinking and leaching steps generating a favorable

environment for the template ion rebinding. The functioning principle of IIP

could be compared with ion-mediated biological self assembly processes such

as intercellular junctions. In such ion-mediated biological self assembly

processes, the recognition is mediated by calcium ions37. Because they are

highly crosslinked polymers, IIPs are durable and can be reused without loss of

activity38.

2.3. Reagents for molecular imprinting

Polymerization reaction is known as a very complex process, which

could be affected by many factors, such as nature of monomer, crosslinker,

initiator, temperature and time of polymerization, the presence or absence of

magnetic field, and volume of the polymerization mixture. In order to obtain the

ideal imprinted polymer, a variety of factors should be optimized. Thus

synthesis of imprinted polymer is a time-consuming process. In order to prepare

imprinted polymer with perfect properties, numerous attempts have been made

to investigate such effects on the recognition properties of the polymeric

materials39. The selection of appropriate reagents is a crucial step in the


molecular imprinting process. The functioning of an imprinted polymer is

controlled by the type of template, ligand, functional monomer, crosslinkers,

solvent and initiator used 40.

(i)Templates

Generally, template molecules are target compounds in analytical

processes. An ideal template molecule should satisfy the following three

requirements. First, it should not contain groups involved in or preventing

polymerization. Second, it should exhibit excellent chemical stability during the

polymerization reaction. Finally, it should contain functional groups well

adapted to assemble with functional monomers. Most templates applied in

molecular imprinting are derived from widespread environmental pollutants,

which can be grouped into three broad categories, including endocrine

disrupting chemicals (EDC), pharmaceuticals and toxic metal ions41. Endocrine

disrupting chemicals have attracted extensive attention when referring to global

environmental problems because they can lead to the disturbance of central

regulatory functions in humans and wild life. Worse still, some endocrine

disrupting chemicals are even suspected to cause human infertility or stunt

growth of children. Among those endocrine disrupting chemicals, hormone

drugs,42 triazine pesticides,43 and bisphenol A44 are the most widely concerning

pollutants used in preparation of imprinted polymers. Pharmaceuticals have

gained increasing attention and interest in the last few years due to their

continuous release in the environment. Water compatible imprinted polymers

have a significant development in the field of molecular imprinting technology.

At present, water compatible imprinted polymers for determination of

quinolones in urine,45 milk,46 river water47 have been synthesized. Heavy metal

ions are always the focus of attention owing to their difficulty of degradation

and ease of bioenrichment48.


(ii) Ligand

The role of the ligand is the basic ctiterion since ion chelation is

involved in the process of recognition. Since the template in ion imprinted

polymers is an ion, reacting via its unfilled orbitals of the outer sphere, the

major interactions with the polymer matrix will occur with some electron

donating heteroatom. The general trend is to use ligand bearing one or more

chelating groups. Some of the functionalized ligands used in the chemical

immobilization approach are given in Fig.II.3.

Fig. II.3. Functionalized ligands used in the synthesis of IIP

(iii) Functional monomer

The role of the monomer is to provide functional groups which can form

a complex with the template by covalent or non-covalent interactions. The use

of a mono vinylated monomer, such as 4-vinylpyridine, has proved to be


essential in the trapping approach to form a ternary complex with the metal ion.

However, even when a functionalized ligand is used in the chemical approach,

some other monomers may be introduced in the polymerization mixture.

The strength of the interactions between template and monomer affects

the affinity of imprinted polymers49 and determines the accuracy and selectivity

of recognition sites.50 The stronger the interaction is, the more stable the

complex is, resulting in the higher binding capacity of the imprinted polymers,

therefore correct selection of the functional monomer is very important. Tedious

trial and error tests are often required to select a suitable monomer.

Fig.II.4 Monomers used in molecular imprinting

Commonly used monomers for molecular imprinting include

methacrylic acid (MAA), acrylic acid (AA), 2- or 4-vinylpyridine (2- or 4-VP),

acrylamide, trifluoro methacrylic acid and 2-hydroxyethyl methacrylate

(HEMA). Structures of several typical monomers are listed in Fig.II.4.


Methacrylic acid has been used as a “universal” functional monomer due to its

unique characteristics, being capable to act as a hydrogen-bond donor and

acceptor, and showing good suitability for ionic interactions51.

(iv) Crosslinkers

The role of the crosslinker is to fix functional groups of functional

monomers around imprinted molecules, and thereby form highly crosslinked

rigid polymer. The crosslinker plays a major role in the ion imprinted polymers

elaboration since it sparks off the binding sites creation. After removal of

templates, the formed holes should completely complement to target molecules

in shape and functional groups. Types and amounts of crosslinkers have

profound influences on selectivity and binding capacity of molecularly

imprinted polymers. When the dosage of crosslinkers is too low, molecularly

imprinted polymers cannot maintain stable cavity configurations because of the

low crosslinking degree. However, over-high amounts of crosslinkers will

reduce the number of recognition sites per unit mass of molecularly imprinted

polymers52. Commonly used crosslinkers involve ethylene glycol dimethacrylate

(EGDMA), trimethylolpropane trimethacrylate , divinylbenzene (DVB), etc. Their

structures are displayed in Fig. II. 5.


Fig. II. 5. Crosslinkers used in molecular imprinting

(v) Solvent

Solvent is introduced during the synthesis of ion imprinted polymers in

order to generate the porous structure. This porosity is essential for making a

high number of binding sites accessible to the analytes. The porogen properties

of such a solvent, commonly named a “porogen”, are depending on the

difference between its solubility parameter and that of the monomers and

polymer chains53. The solvating power of the medium will be varying along the

polymerization process while monomers are consumed. All the polymerization

components must be soluble in the porogen. For IIP preparation, this concerns

not only the monomers (including the crosslinker) and the initiator but also the

ion template in a free form or entrapped in a complex. Porogenic solvent plays

an important role in polymerization. It acts as not only a porogen but also

solvent in preparation process. Besides, it also influences the bonding strength

between functional monomers and templates, the property and morphology of

polymer, especially in non-covalent interaction system. Aprotic and low polar

organic solvents, such as toluene, acetonitrile and chloroform are often used in


non-covalent polymerization processes in order to obtain good imprinting

efficiency. It is notable that imprinted polymers prepared in organic solvent

work poorly in aqueous media because of the ‘‘solvent memory’’.

(vi) Initiators

The role of the initiator in radical polymerization is well-known as it is

responsible for the creation of monomer radicals in order to propagate the

polymer formation. The initiator is homolytically cleaved by thermal

decomposition, photolysis or ionizing radiation. These two latter techniques are

rarely used for ion imprinted polymer preparation. Any of the methods of

initiation can be used to initiate free radical polymerization in presence of the

template. If the template is photochemically or thermally unstable then initiators

that can be triggered photochemically and thermally respectively would not be

attractive. When complexation is driven by H-bonding then lower

polymerization temperatures are preferred and under such circumstances

photochemically active initiators are preferred. The rate of radical

polymerization depends on the nature of the concentration of the initiator. 2,

2’-azo-bis-isobutyronitrile (AIBN) and potassium persulphate are the most

efficient initiators. The choice of the initiator depends on the nature of the

template.

2.4 Polymerization techniques

Ion imprinted polymers are generally synthesized by polycondensation,

while organic ion imprinted polymers are mainly prepared by free radical

polymerization. The formats of these ion imprinted polymers can vary

according to the polymerization process. Bulk polymerization will lead to

monolithic materials, whereas well-defined particles can be produced by

heterogeneous (suspension or emulsion) or homogeneous (dispersion or

precipitation) polymerization.


(i) Bulk polymerization

Bulk polymerization technique is convenient and simple and applied for

the development of new imprinting strategies and for mechanistic studies54. The

bulk materials usually need to be crushed, ground and sieved to obtain particles

of the desired dimensions for further investigations. This process is tedious and

time-consuming with usually less than 50% recovery of the ground polymer is

possible which causes problems. The irregular size and shape of the particles

are not favorable for many chromatographic and separation applications. Some

binding sites can also be destroyed during the grinding which leads to a

considerable loss of loading capacity of the IIPs. Because of these major

limiting drawbacks, many efforts have been made to prepare IIPs directly in a

bead format through heterogeneous or homogeneous polymerization.

(ii) Suspension and emulsion polymerization

Suspension as well as emulsion polymerization, is a heterogeneous type

polymerization since it requires two non miscible phases, one of them being the

continuous phase and the other the dispersed phase. When IIPs are prepared by

suspension polymerization, the dispersed phase contains the monomers, the

initiator, the porogen and the template ion. This is typically an organic phase in

contact with an aqueous continuous phase, stabilized by gelatin, hydroxyl ethyl

cellulose and eventually containing sodium chloride salt. The polymerization

occurs in the droplets of the dispersed phase acting as small sized reactors, thus

generating polymer beads. This technique makes it possible to control the

shape, size and the porosity of the polymer particles.

(iii) Dispersion and precipitation polymerization

Dispersion and precipitation polymerization are classified as

homogeneous polymerization since, in the initial stage, the medium is

composed of a unique phase. All the components of polymerisation are soluble


in the porogen and the polymerization is initiated in homogeneous solution.

Whereas the primary particles, formed by dispersion polymerization, are

swollen by the polymerization medium and/or the monomer, they do not swell

in the medium and precipitate in the precipitation polymerization process55.

Dispersion polymerization involves the resort to a stabilizer since the particle

dispersions produced in the absence of any stabilizer are not sufficiently stable

and coagulation is possible during their formation. There are very few examples

of IIPs produced by dispersion polymerization56.

2.5. Different stages in the preparation of imprinted polymers

(i) Preorganization stage

Preorganized approach57 which uses covalent reversible bonds giving a

rather homogeneous population of binding sites and reducing the non-specific

sites. However, to remove the template from the polymer matrix, it is necessary

to cleave the covalent bonds. The structure and stability of this complex indicate

the behavior of the resulting molecular imprinted polymer.

(ii) Covalent interactions

Covalent imprinting58 is distinguished by the use of templates which are

covalently bound to one or more polymerizable groups. After polymerization

the template is cleaved and the functionality left in the binding site is capable of

binding the target molecule by re-establishment of the covalent bond. The

advantage of this approach is that the functional groups are only associated with

the template site. However only a limited number of compounds (alcohols,

aldehydes, ketones, amines and carboxylic acids) can be imprinted with this

approach.


(iii) Non-covalent interactions

In non-covalent imprinting the interactions between functional monomer

and template during polymerization are the same as those between polymer and

template in the rebinding step58. These are based on non-covalent forces such as

H-bonding, ion-pairing and dipole-dipole interactions. This method was first

introduced in organic polymers by the group of Mosbach57. Due to its simplicity

this method is the most widely used to create imprinted polymers. To date the

most common functional monomer employed is methacrylic acid, first reported

by Mosbach et al59 and adapted by others for the imprinting of 2,4-D,

hydroquinidine,60,61 hormones,62 nucleotides63 and cyclic peptides64.

(iv) The semi-covalent approach

Fig. II.6. Different approaches for IIP elaboration

As the template is covalently bound to a polymerizable group, the

functionality which is recovered after cleavage of the template should only be

found in the binding site. This also results in a more uniform distribution in

binding site affinities65. The rebinding, which takes place via non-covalent

interactions, has no kinetic restrictions except diffusion. Two variations in the


semi-covalent approach can be distinguished (FigII.6). These are: (i) the

template and the monomer are connected directly, or (ii) the template and the

monomer are connected using a spacer group. The first true semi-covalent

approach was reported by Sellergren and Andersson66 for the imprinting of

p-aminophenylalanine ethyl ester. The idea of the semi-covalent approach was

introduced by the group of Wulff, when it was used in combination with the

(reversible) covalent approach67.

2.6. Characterization

For their insoluble nature, MIPs are notoriously difficult to characterize.

However, some analytical techniques can be utilized for their chemical and

morphological characterization, including solid-state NMR techniques68,

elemental micro-analysis and Fourier-Transform infrared spectroscopy69 which

can be applied to obtain chemical information on the composition of the

polymer.

(i) FT-IR

FT-IR spectra provide the fundamental analytical basis for rationalizing

the mechanism of interactions for selective binding site formation at the

molecular level. The interaction between the functional monomer and template

during pre-polymerisation complex formation and the template incorporation

into the imprinted polymer during rebinding can be confirmed by the

characteristic FT-IR absorption analysis70.

(ii) 1H NMR

Proton NMR titration experiments facilitate observation of hydrogen

bond formation between bases and carboxylic acid through hydrogen bonding.

These studies have been introduced in molecular imprinting for investigating

the extent of complex formation in pre-polymerization solutions and to identify

the specific sites in interacting structures that engage in complexation. Thus


evaluating the shift of proton signals due to participation in hydrogen bonding

and other interactions were used as the criteria for complex formation, M/T

ratio and interacting forces.

(iii) 13C CP-MAS-NMR

Being rigid solids, neither usual NMR spectroscopy nor X-ray

diffraction methods can be applied successfully to follow rebinding with

imprinted polymers. Therefore it is not possible to obtain reliable structural

information of the interactions occurring in the cavities between binding site

and template. The polymer analysis using cross-polarization magic angle

spinning Carbon-13 nuclear magnetic resonance (13C CP-MAS-NMR)

techniques can give information about the polymer backbone.

(iv) Scanning electron microscopy (SEM)

SEM can be used in distinct ways to probe imprinted polymers on a

variety of length scales. Scanning electron microscopy is the most widely used

technique to study the shape, size, morphology and porosity of polymers71. As

for all metal-ion sorbents, the properties of IIPs are related to their morphology,

shape and porous structure. When IIPs are expected to be produced in the form

of individual particles, SEM imaging turns out to be indispensable. It is usually

observed that the NIP surface is smoother than the IIP’s and that the roughness

of the surface of imprinted polymer is increased by the removal of the template

ion72. The presence or the absence of a metal ion in a polymer matrix can be

assessed by X-ray diffraction and energy dispersive X-ray (SEM-EDAX)

spectroscopy analyses, which was proved to be very efficient73.

2.7. Optimization of binding properties of IIPs

Binding isotherms measure the binding efficiency of a material over a

range of analyte concentrations and are usually plotted as the binding capacity

versus the concentration of free analyte remaining in solution. These


equilibrium data can be achieved by various sorption isotherms such as

Langmuir, Freundlich or Langmuir-Freundlich. The Langmuir adsorption

isotherm is valid for a monolayer adsorption onto a surface containing a finite

number of identical sites74. It is applicable to homogeneous binding where each

metal ion-imprinted particle binding process has equal binding activation

energy. It is consistent with specific and strong binding onto specific sites75.

The Freundlich equation is basically empirical and assumes that the adsorption

occurs on heterogeneous surfaces. It describes reversible adsorption and is not

restricted to the formation of a monolayer76. The Langmuir-Freundlich model

takes into account the heterogeneous character of imprinted polymers and is

capable of interpreting the saturation behavior of the adsorption isotherms in

high concentration regions77.

2.8. Advantages of molecular imprinted polymers

The main advantages of molecularly imprinted polymers are their high

selectivity and affinity for the target molecule used in the imprinting

procedure78,79. Imprinted polymers compared to biological systems such as

proteins and nucleic acids, have higher physical robustness, strength, resistance

to elevated temperature and pressure, and inertness towards acids, bases, metal

ions and organic solvents. In addition, they are also less expensive to be

synthesized, and the storage life of the polymers is very high, keeping their

recognition capacity for several years at room temperature.

2.9. Disadvantages of molecular imprinted polymers

Molecular imprinting process presents various advantages, as described

above, but some drawbacks must also be considered. Design of a new MIP

system suitable for a specific template molecule often requires a lot of time and

work for synthesis, washing and testing. Generally many attempts need to be

made by changing various experimental parameters, before finding the optimum


conditions. Combinatorial chemistry has been recently adopted in order to

accelerate the optimization of MIPs to attain the desired performance80,81.

Combinatorial approach also allows finding the best MIP composition through

the simultaneous synthesis and evaluation of tens or hundreds of imprinted

polymers prepared on a small scale (mini-MIPs). Sellergren80 introduced a high-

throughput synthesis and screening system in a 96-well plate format that allows

rapid optimization and fine tuning of the molecular recognition properties of

MIPs library by the use of filter plates for rapid template removal and a

multifunctional plate reader for a parallel analysis of the supernatant fractions.

Another important critical point is the design of water-compatible MIPs. For

various applications especially in clinical and environmental fields the

imprinting capability of imprinted polymers in water medium is required82,83.

Unfortunately water molecules will compete with the template making weaker

or destroying non-covalent interactions between template and functional

monomer.

2.10. Applications of molecular imprinted polymers

The peculiar properties of MIPs have made them a highly interesting

tool for different application areas, including separation sciences and

purification, sensors and biosensors, catalysis and drug delivery. Molecular

imprinting chromatography is one of the most extensively studied application

areas of MIPs, which are highly suitable for chromatographic separation, chiral

separations, solid phase extraction, antibody and receptor mimicking, enzyme

mimetic catalysis, organic synthesis and drug delivery84.

(i) MIPs in separation techniques

Molecularly imprinted chromatography is one of the most traditional

applications of molecularly imprinted polymers85 especially for liquid

chromatography86,87 with imprinted polymers usually synthesized by bulk


polymerization, ground and sieved mechanically and subsequently packed in a

chromatographic column88. However, the mechanical processing leads to

irregular particles with relatively broad size distribution, resulting in packing of

irreproducible quality. For this reason monolithic molecular imprinting columns

have been recently prepared directly inside stainless steel columns or capillary

columns89. Experimental data suggest that not always uniform imprinted

particles allow better chromatographic performances. For instance, precipitation

polymerization was used to prepare spherical beads but with a total pore volume

still lower compared to irregular particles obtained by bulk polymerization and

it has been seen that the porosity of the beads strongly influences the

chromatographic performance of these systems90. The imprinted polymer

sorbents, due to selective interactions exhibited, retain the analyte more strongly

than the non-imprinted polymer materials91. Imprinted polymers are frequently

used as chiral stationary phases in high performance liquid chromatography to

obtain enantiomeric resolution of racemic mixtures such as amino acid

derivatives92 and drugs93.

MIP based micro-columns for capillary electro chromatography (CEC)

are recently realized for separation of several compounds94. Solid phase

extraction (SPE) is another important area of application of imprinted polymers

in analytical chemistry95. Molecular imprinted solid phase extraction (MISPE)

was applied for extraction of several compounds in different sample matrices

such as biological, 96 environmental97 and also in food analysis98. The first

application of MISPE was made by Sellergren in 1994 who performed a

selective extraction of pentamidine, a drug used for the treatment of acquired

immune deficiency syndrome related disorders, in urine samples99.


(ii) In analytical chemistry

To overcome the template bleeding problem the imprinted polymers can

be synthesized using a close structural analogue of the analyte as template (also

called dummy template) that can capture also the target analyte since very

similar cavities are generated100. In a recent work101 imprinted polymers for 5-

nitroimidazole recognition was prepared using a template analogue which was

different from the analytes studied, methacrylic acid as monomer and

divinylbenzene as crosslinker in chloroform. For the first time a MISPE

protocol was developed for the selective extraction of four different 5-nitro

imidazoles and three of their metabolites in egg powder. The first work on solid

phase micro extraction appeared in 1990102 and, since then, many different

studies have looked at using this simple method to extract different analytes

from many different matrices.

(iii) As chemical sensors and biosensors

Recently chemical sensors and biosensors are of increasing interest in

the field of modern analytical chemistry. This is due to the new demands that

are appearing particularly in clinical diagnostics, environmental analysis and

also in food analysis. A sensor is characterized by two important element,

which has a specific interaction with an analyte which converts this interaction

into a measuring effect. Recently the synthesis of artificial receptors capable of

binding a target analyte with comparable affinity and selectivity to natural

antibodies or enzymes has been done. Imprinted technology can also be used

as antibody-like materials with high selectivity and sensitivity, owing to their

long-term stability, chemical inertness and insolubility in water and organic

solvents103. In particular, the integration of imprinted polymers with sensors can

be realized by in situ polymerization, using a photochemical or thermal initiator 104, or by surface grafting with chemical or UV initiation105.


The first generation of imprinted polymer sensors was prepared using

imprinted polymers synthesized in the form of monoliths.106 The imprinted

polymer sensor obtained was successfully employed to determine atropine

concentrations in human serum and urine107. Imprinted polymers were also

widely employed as sensors for enantiomeric separation of different

compounds108. An interesting area of research was the preparation of mass

sensitive imprinted polymer sensors for cells and viruses. Molecular imprinted

polymer technology has been extensively studied for its potential applications in

sensing109.

(iv) Catalysis

Considerable efforts have been made to investigate the possible use of

imprinted polymer for catalytic applications110. The high selectivity and

strength of these polymers make them suitable to be used at elevated

temperatures and pressures, in the presence of several organic solvents, and also

under acidic and basic reaction conditions. For these reasons, MIPs can be

employed instead of biomolecules, such as enzymes and natural catalytic

antibodies111 which are highly vulnerable to certain conditions. The use of MIP

for catalytic application is very important because MIP catalysts are able to

mimic the selectivity and stereospecificity of the binding domains of antibodies

and enzymes which are generally utilized as catalysts in several reactions.

Catalytically active imprinted materials can be obtained using analogues of

substrates, transition states or products as templates in the imprinting protocol.

The polymer matrix obtained has cavities with a shape similar to the shape of

the template used but the imprinting technology must also ensure the right

placement of the functional groups in the binding sites in consideration of the

type of catalytic process that they will assist112. The strategy using substrate

analogues as template, involves the use of compounds that mimic the reaction

complex between the substrate and the matrix. The catalytic groups will be


introduced in the right positions into the cavities of the polymer and

subsequently they will act as catalyst in presence of the true substrate. Finally,

thermodynamic and kinetic studies were also carried out on MIPs catalytic

systems with the aim of understanding molecular imprinting and its

specificity113. Thus, the increasing rate and the larger induction, in logic, may

be responsible for the specific recognition and catalysis.

(v) Drug delivery

Recently an area of great challenge in molecular imprinting technology

is that of therapeutic agents, various imprinted polymers have been used as

unusual synthetic polymeric carriers to prepare drug delivery systems114.

Imprinted polymers for drug delivery applications should have specific

characteristics. The imprinted cavities should be stable to maintain the

conformation in the absence of the template, but also flexible to facilitate the

realization of a fast equilibrium between the release and re-uptake of the

template in the cavity. To this end, the non-covalent imprinting provides faster

equilibrium kinetics than the covalent imprinting approach115. Recently,

molecularly imprinted hydrogels for the recognition of cholesterol have been

prepared by molecular design of methacrylate-based structures containing

poly(ethylene glycol) in moderately and highly cross-linked networks. In recent

years, many researchers have published about the use of MIPs in drug delivery

applications116 given their physicochemical properties to protect the drug from

degradation by enzymes during systemic trafficking in the body. The first report

of imprinted polymers used as sustained release devices has been presented by

Norell and co-workers117, 118.

(vi) Selective concentration of metal ions

An important application of the metal ion imprinted polymer is in the

selective concentration of metal ions. The efforts in the selective concentration


of the metal ions from aqueous solutions using IIP are steadly increasing. The

selectivity depends on various factors: (i) the specific interaction of the ligands

with metal ions; (ii) coordination number and coordination geometry of the

metal ion; (iii) charge on the metal ion, and (iv) to some extent the size of the

metal ion. However the conventional chelating ligands have encountered

problems including low binding selectivity, slow rebinding kinetics and loss of

selectivity with time.

Compared to this the metal ion-imprinted polymers exhibit rapid and

usually strong complexation with metal ion resulting in high selectivity for the

imprinted ion. The imprinted cavities in the polymers containing functional

groups whose arrangement corresponds to that of the imprinted molecule. The

selectivity for the optical resolution of racemates and separation of different

diastereoisomers, configurational isomers and metal ion are quiet high. It is also

possible to perform stereoselective reaction with the substrate bound inside the

cavities. The advantages of the metal ion imprinted polymers over the

conventional chelating polymers are: (i) the selectivity was further improved in

every case; and (ii) the imprinted binding sites should be able to fix the growing

polymer chain in a defined topology. After the removal of the imprinted

molecule, it should be able to rebind the imprinted molecule. In all cases the

structure of the polymer matrix and the macromolecular characteristics are of

crucial importance in specificity and selectivity.

Ion imprinted polymers are generally used for treatment or analysis of

water from tap, river, lake and ground waters119. Their use can be extended to

complex matrices such as sea water120, industrial waste water121-125. After some

sample treatment, IIPs can be used to extract metal ions from solid samples

such as food126, soil and mine tailing samples,127 and human hair128. As

separation materials, IIP particles can also be incorporated in chromatographic

columns129. The applications of IIP are now extending to sensors. IIP particles


can thus be mixed with some carbon paste130 or included in a polyvinyl chloride

membrane to prepare ion-selective electrode. Recently IIPs have been combined

with quartz crystal microbalances to design sensor devices131.

2.11. Biosorption

Heavy metals are widespread pollutants of great environmental concern

as they are non-degradable and thus persistent. It is well perceived that there is a

permissible limit of each metal, above which they are generally toxic132,133 and

some are even hazardous. The conventional processes used for effluent

treatment are precipitation as hydroxides/sulphides, oxidation/reduction and ion

exchange. The processes are expensive and not eco-friendly. Microbial biomass

can passively bind large amounts of metal ions, a phenomenon commonly

referred to as biosorption thus providing a cost-effective solution for industrial

waste water management134. Biosorption mainly involves cell surface

complexation, ion exchange and micro precipitation. Different microbes have

been found to vary in their affinity for different heavy metal(s) and hence differ

in their metal binding capacities. Some biomasses exhibit preference for certain

heavy metal ions whereas others do not show any specific binding and are broad

range. Biosorbents, especially those derived from seaweed (macroscopic algae)

and alginate derivatives, exhibit high affinity for many metal ions. Because

biosorbents are widely abundant and less expensive than industrial synthetic

adsorbents, they hold great potential for the removal of toxic metals from

industrial effluents. Various studies have demonstrated the efficiency of living

and nonliving micro-organisms, such as bacteria, yeasts, moulds, micro-algae,

cyanobacteria and biomass from water treatment sewage to remove metals from

solution. Several types of organic and inorganic biomass have also been used as

sorbent materials135-137.


2.12. Conclusion

Heavy metal exposures and poisoning are considered as hazardous

which need to take the utmost care and precautions to prevent these toxins from

entering our bodies. Thus biosorption offers an economically feasible

technology for efficient removal and recovery of metals from aqueous solutions.

The process of biosorption has many attractive features including the selective

removal of metal ions over a broad range of pH and temperature, its rapid

kinetics of sorption and desorption and low capital and operational costs. The

biosorbents can easily be produced using inexpensive growth media or obtained

as a byproduct from some industry. The judicious choice of biosorbents can also

out complete the commercial ion exchange polymers which have conventionally

been used in the removal of metal ions. The increasing demand for sample

treatments and quality control will be a major cause for the future development

of the ion imprinting technology either as SPE sorbents or sensing devices in

biomimetic sensors. The understanding of the mechanisms of interaction

between the template metal ion and the polymer matrix is a fundamental

challenge in ion imprinting method. It could be a useful tool in the design of

performing ion imprinted biosorbents for the removal of hazardous metals from

aqueous environment which is the focus of present investigation.


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chapter 2 ion imprinted polymers: a...

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