Chapte Ill! TAILORING OF METAL ION-IMPRINTED MICROSPHERES USING SURFACE-TEMPLATE POLYMERISATION: SYNTHESIS, CHARACTERISATION AND SPECIFICITY STUDIES

e molecular imprinting technique used to prepare metal ion selective polymers has

received much attention in recent years as a method to prepare support rnatenals for T metal ion separation and concentration with increased ~pecificity.'.~ Eventhough the

molecular imprinting technique was used for a wide variety of applications such as

separation of metal ions,) saccharides,'.5 separation of o rpc6 . ' and low-molecular

compounds like amino acid derivative^,^.^ its applications in all these fields are

lunited. Since these preparations are all based on solution or bulk polymerisation

techniques the imprinted structures usually formed as bulk resins and this requite

gnnding and sieving of the samples before use. This results in the partial destruction

of the imprinted structure. Presence of a residual guest in the resin phase, low

binding selectivities, slow rebinding kinetics, and the difficulty in handling the water

soluble guest molecules are other h t a t i o n s of this bulk resins.1° Microspheres from

synthetic polymers have become interest in the development of functional

materials."J2

In order to overcome all these problems associated with the desgmnp of

metal ion imprinted polymers for selective metal ion separation, a new imprinting

technique called satfan iqnnh",q has been proposed by Takagi et al.13-l8 The basic

concept of dus imprinting technique was the use of an emulsified suspension of the

h e a r polymer (latex) as the guest recognition field as shown in Scheme 111.1. A

seeded microsphere emulsion was prepared, whlch contains the linear chain

polymer, cartying the functional group to interact with the guest molecules (1).

Divinyl type monomers are added to this emulsion, the functional group interacts

with the metal ions to form the complexes (2). The obtained microspheres are then

immobdued by subsequent crosslinlung polymerisation with divinyl crosslinking

agents (3). The metal ions are then removed from the microspheres to give an

imprinted host polymer.

Scheme 111.1. Basic principle of surface imprinting

This chapter deals wth the Cu(II) ion-imprinted surface template polymers

based on acrylic acid, methacrylonitdle, styrene and a difunctional crosshking agent.

The crosslinlung agents selected are divinylbenzene (DVB), ethylene glycol

drnethactylate (EGDMA), 1,4-butanediol dimethacrylate (BDDMA) and 1,6-

hexanediol dacn~late (HDODA). The criteria for the selection of these crosslinking

agents are the dfference in heir relative rigdty-flexibhty and polarity. Introduction

of these crosslinlung agents in varying proportions would dehte ly vary the

physicochemical characteristics of the po lpe r support k e hydrophhc-hydrophobic

balance, ng~dty and flexibilitv, pore size and pore volume and hence the metal ion

rebindmg properties of the tadored systems. The developed Cu(l1)-imptinad

mcrospheres were characterized by IK, UV, EPK spectra and SEM analysis. In

order to compare the effiaency of the metal ion imprinted system in speafic metal

ion rebindmg, an unirnprinted system uras also synthesized and specificity studies

were also cartied out

III.1. Preparation of DVB-, EGDMA-, BDDMA-, and HDODA-

The preparation of Cu(I1) ion-imprinted microspheres by surface-template

polymerisaaon involves the formation of a seeded emulsion of rnicrospheres in

water. This involves the copolymerisation of styrene, methacryloniaile and acrylic

acid using potassium persulfate as an initiator at 7 K for 7 h in water to get a linear

seeded emulsion of microspheres (1) (Scheme 111. 2). The obtained seeded emulsion

was then brought to room temperature. The crosslinkmg agent and

methacryloniaile were further added and the emulsion was kept at 2OC at pH 9.5 for

24 h. By this treatment the carboxyl groups w i h the microspheres attained little

mobility and eventually migrate to the surface of the emulsion as carboxylate anions

(2). The pH of the emulsion was lowered to 5.0 using 2N HC1 solution and to h s

0.1 N Cu2SO4 solution was added. The complexation between the metal ions and

the carboxylate groups on the microsphere surface would result in the reorganization

of carboxylate groups to form a metal ion-imprinted structure on the microsphere

surface (3). The imprinted structure was then immobilized by the formation of an

interpenetrating network by polymerization with respective crosslinkmg agent. The

rnicrospheres thus obtained werc FJtered, washed with water, acetone and methanol

and dned.

AA 0 0 0 S US& M'2N.WB.W

MCN - o" ___t

H~CJ pH2.2,7W, 7h "' pHS.5,Z.C,24h 000 oOb.

Scheme 111.2. Preparation of Cu(I1) ion-imprinted microspheres

It is noteworthy that the metal ion imprinting and reorganization of the

carboxvlate groups were carried our at pH 5.0, rather than at pH 9.5, where carboxyl

group migration into the microsphere surface was induced. This was done in order

to avoid the precipitation of metal hydroxide. The pH of the suspension should not

be lowered below pH 5.0, which would lead to the formation of free cat'boxyl

groups. This would result in the formation of microspheres with reduced metal ion

binding.

Polymers with varying degrees of crosshkmg (9, 17 and 25 molYo) were

prepared by varying the amount of DVB, EGDMA, BDDMA and HDODA Fables

V.l-4). The CuQ1)-imprinted microspheres prepared at pH 5.0 were obtained in

79.20 - 93.40% isolable yield. In order to increase the q d i t y of the Cu(I1) ion-

i m p ~ t e d structure, a metal ion imprinted polymer with 19 mol% DVB crosslinking

was synthesized. In this case, methacrylonitrile in the second stage was not added to

the seeded emulsion. Thls resulted in increased rigidity of the microspheres. The

yield of the obtained microspheres was found to be 84.25%.

To compare the efficiency of the imprinted system for the specific and

selective concenaation of metal ion, an unimprinted surface-template polymer as a

reference 6.e. one without any imprinted metal ion) was also synthesized. The

degree of DVB crosshking in dus system was 17%. The surface carboxylate groups

are arranged in a random fashlon here. The details for the preparation of

unimpdnted microspheres are given Table V.1. The yield was 84.0%.

111.2. Desorption of Imprinted Cu(I1) Ions fonn the Imprinted

Microspheres and the Cu(11) Rebinding Studies

The Cu(II) ion desorption and the estimation of metal ion complexed on the

microsphere were carried out as follows. The microspheres were equilibrated with

0.2 N HCI and shaken for 8 h in a thermostatically controlled water bath shaker at

room temperature. The metal ion concentration in the solution phase was estimated

spectrophotometrically. The metal ion in the imprinted polymer was exchanged

with proton on treatment with HCI. The microsphere thus obtained was neutralized

using 0.2 N KOH and washed with water. The microspheres obtained after

neuuahation was used for the metal ion rebindmg experiments. The amount of

CuQI) ion imprinted on the 9, 17 and 25 mol% DVB-, EGDMA-, BDDMA- and

HD0D.A-crosslinked microspheres are given in Table 111.1.

Table 111.1. Amount of Cu(I1) ion imprinted on DVB-, EGDMA-, BDDMA- and HDODA-crosslinked Cu(I1) ion-imprinted microspheres

111.3. Metal Ion Specificities of DVB-, E G D U - , BDDMA-and

HDODA-crosslinked Cu(I1) Ion Desorbed Imprinted and

Unimprinted Microspheres

To investigate the specificity characteristics of the various Cu@Q ion-

imprinted Cu(Ir) ion-desorbed systems, defmite amount of the various crosslinked

systems were equhbrated with Co(II), Ni(II), Cup9 and Zn(Ir) ion solutions at pH

5.0 and at its n a n d pH, (natural pH of 0.05 N metal salt solution: Co(II) = 5.4;

NifJI) = 6.1; Cu(II) = 5.4 and Zn(II) = 5.6. An interesting observation is the

specific rebindmg of Cu(II) ions by all the Cu(II)-desorbed systems. Thls results

from the memoy of the Cu(II) ion imprinted system for the desorbed Cu(II) ions, as

shown in Scheme 111.3. During metal ion imprinting, the coordination g e o m e q of

the copper complex is immobilized on the surface of the three-dimensional solid

polymeric microsphere by interpenetrating polymer networks. O n metal ion

desorption, these immobilized l ipnd did not undergo any deformation from its

immobilized orientation resdnng in the specific rebinding of the same metal ion.

O n the contrary, the same microspheres bind other metal ions m e Co(II),

Ni(I1) and Zn(II) ions to a lesser extent than the desorbed Cu(II) ion. The size

Afference between the imprinted copper ion with other metal ions and the

Scheme 111.3, Specific metal ion rebinding of imprinted microspheres

difference m their coordmadon geomeuy irnmobhzed on the microsphere surface

conaibute the specific rebindmg of Cu(lI) ion. The carboxylate groups of the

polymer chains are placed on the Cu(lI) ion-imprinted microspheres in such a way

that they rnatch the optimum coordmadon geomeq for Cu(I1) ion and these are

fixed as such during the c r o s s h h g polymerisation in the next step. On metal ion

desorption if the coordmadon geomeuy of the Cu(lI) ion is not disturbed it would

result in its specific rebinchg.

m.3(4 Comparison of Cu (II] ion rebinding of Cu(Uj ion-

imprinted and unimprinted microspheres

The primary objective of surface-imprinted methodology is to synthesize

polymers which can selectively concentrate the imprinted metal ion. The binding

studies of metal ion-unimprinted resins revealed a random incorporation of ligand

sites. This random polymer binds nearly equal amount of metal ions. This is in

contrast to the polymer prepared in the presence of metal ions, which showed

specificity for the imprinted metal ions. Thls point to a favorable stable geometry

kept in the microspheres for the imprinted metal ion and these cavities are fixed

during crosslinking reaction. In order to investigate this, the specificity studies were

carried out using Cu(I1) ion imprinted and unimprinted 17 molO/o DVB-crosslinked

systems. Interestingly the Cu(II) ion desorbed system specifically rebinds Cu (II) ion

than other metal ions, as evidenced from the very low uptake of other metal ions of

this system as represented in Fig. 111.1. The Cu(II) ion binding of the unirnprinted

resin is !ess than the imprinted one and also the binding of Co(II), Ni(I1) and Zn(II)

ions are higher than the imprinted system. This also supports the fixing of the

geometry in the d o r e d imprinted system for Cu(II) ion than other metal ions.

Fig.III.1. Metal ion specificty studies of 17 mo1V0 DVB-crosslinked Cu(I1) ion-imprinted and unimprinted microspheres

lU. 3 @) w e c t of pH dependence on metal ion rebinding

'The imprinted microspheres adsorbed the corresponding Cu(Il) ions more

effectively than the unimprinted microspheres; however the metal ion uptake of the

CoQI), Ni(I1) and Zn(1I) ions was found to be less at pH 5.0 than its natural pH.

The resr~lts of the effect of pH dependence on metal ion rebinding of the various

crosslinked systems are given in Table 111. 2.

0.3(c) Effect of the nature and degree of the crosslinkhg

agent on the metal ion specwcity

The ~nmduction of a crosslinlung agent into a polymer c h i n h p m

msolubility and +dry to the system. The rigidiry and s w e h b ~ t y of these

crossbked systems vary "th the nature as well as the degree of the crossblvng.

I h u s the introduction of rigd and hydrophobic DVB cross ldng , increrres he

rigidity and hydrophobicity of the tailored systems, where as the inooduction of the

flexible EGDMA, BDDMA and HDODA-crossblung wodd increase the

flexibility and polarity of the developed systems.

The nature and degree of the crosslinkmg agents would definitely influence

the memory of the d o r e d system in keeping the stable geomeu). for Cu(II) ions.

Surulatly with other metal ions b i n h g increases with increasing flrxibhty of the

crosshkmg agents. In general a system with rigid crosslinking favors specific metal

ion biding and the specificity increases with increase in degree of crosshking. With

increasing flexibility of the crosslinking agent, thls stability would decrease resulting

in decreased specificity. Fig. 111.2 shows the dependence of the nature of the

crosshkmg on the specificty studies 17 mol% DVB-, EGDMA-, BDDMA- and

Fig. 111.2. Effect of the nature of the crosslinking agent on the metal ion specificity of 17 mol% crosslinked Cu(I1) ion-imprinted microspheres

HDODA-crosslinked Cu(I1) ion desorbed microspheres. The increased rigidity of

the DVR crosslinkmg keeps the geomeuy of the metal ion centers more stable

resuldng in increased rebinding of Cu(I1) ions. With increasing flexibility of the

crosshlung agent, the stability of the tadored metal centers decreases resulting in

decreased specificity with increasing flexibility of the crosslinking agent. The

binding characteristics of other metal ions are less than as expected.

From the results (Fig. 111.2) it is clear that the rebinding of the rigid DVB-

crosshkeci sj-stem is higher compared to the other crosshked systems. With

EGDMA- , BDDMA- and HDODA-crosslinked system, as the spacing between the

connecdng polymer chains increases, the extent of specificity decreases. This arises

from the lack of rigid~ty contributed by the flexibility of the crosslinking agent In

general the optimum coordmation geometry required for the selective b idmg of the

desorbed of metal ion cannot be maintained unless the crosshked polymer matrix is

adequately crosslinked.

In order to investigate the effect of the deeee of crosslinking on specific

metal ion rebinding, the rebk~ding studies of the Cu(II) ion desorbed 9, 17, and 25

molO/o of IIVB-, EGDhfl-, BDDMA- and HDODA-crosslmked systems were

followed. From the results it is clear tbat all systems specifically rebind Cu(1I) ion.

Compared to copper rebinding, the rebindmg of other metal ions are much less as

represented in Fig. 111. 3(a-d). For each case with increasing crosshk density, the

specifidq characteristics increased.

'&Q

Fig. III.3(a) Effect of the degree of DVB crosslinking on the metal ion specificity of Cu(1I) ion-imprinted microspheres

Fig. III.3@) Effect of the degree of EGDMA crosslinking on the metal ion specificity of Cu(I1) ion-imprinted microspheres

Fig. III.3(c) Effect of the degree of BDDMA crosslinking on the metal ion specificity of Cu(I1) ion-imprinted microspheres

Fig. III.3(d) Effect of the degree of HDODA crosslinking on the metal ion specificity of Cu(I1) ion-imprinted microspheres

In all cases, the specificity of the Cu(I1)-desorbed systems increased with

increasing crosshk density. Thus the 25 mol% crosslinked system has the highest

specificity. T h s also supports the stabiity of the polymer microsphere to keep the

geometry of the desorbed copper ion more favourably. As expected, the specificity

with increasing crosslink density is higher for the DVB-crosslinked system.

The metal ion specificity studes of 9, 17, 19 and 25 mol% DVB-crosslinked

systems indicated higher specificity for the 19 mol% system (Fig. 111.4). In this, the

methacrylonitrile in the second stage was not added and this would result in an

overall increase in the rigdity of the system. The cavities of the desorbed metal ions

are kept more stable in tlus system than the other systems.

Fig.III.4. Metal ion specificities of Cu(I1) ion-imprinted DVB-crosslinktd microspheres

111.4. Characterization of Cu(I1) ion-imprinted and Unimprinted

Microspheres

The Cu(I1) ion-imprinted microspheres were characterised by PT-IK, UV

and IiPK spectra, and by SEM analysis.

111.414 FT-LR spectm

The surface-template polymers obtained after the desorption of the copper

Ion gave typical absorption for the dimeric carboxyl group at 1730 cm-1. The

incorporation of methacrylonitrile in the polymer was confirmed by the

charactenstic nitnle absorption at 2360 cm-'. In addition to these, the spectra

showed characteristic absorptions of the respective crosslinking agents. In the case

of Cu(I1) ion-imprinted resii, the carboxylate peak shlfted to lower frequency region

indicating complexation with metal ion (Fig. 111.5).

Wavenumber (m-1)

Fig. 111.5. FT-IR spectra of (a) Cu(I1) ion-imprinted ; and @) unirnprinted tnicrospheres

m.*) W-vis spectra

'The electronic specua of Cu(I1) ion-imprinted surface template polymers

contain two bands, whch are assigned to the d-d transieon correspondmg to the

Ti?, --TQ transition and to a symmetry forbidden ligand charge transfer.I9 ?'he fust

band was obtained in the region of 13500-14750 cm-' and the second at 26000-

26500 cm-1 (Fig. 111.6).

200 Wavelengm (nm) 900

Pig . 111.6. UV-Vis spectra of Cu(I1) ion-imprinted microspherr

m . 4 ( ~ ) EPR spectm

Tnc EPR spectra of 17 molO/o DVB-crosslinked CufJI) ion imprinted

microsphere is gven in Fig. 111.7. The EPR parameters are: g , = 2.4610; = 2.0350;

A = 172; A,= 57 and aZCu = 0.7962. This indcated a &toned tetragonal type

symmetry for the CufJI) ion-imprinted microspheres.

I 2000 2600 30W 3500

Gauss

Fig.III.7. EPR spectra of 17 mol% DVB-crosslinked Cu(I1) ion-imprinted microsphere

m.4(4 Scanning electron microscopy

Both the CufJI) ion-imprinted and unimptinted resins were spherical fine

particles. 'The SEM photographs of 17 molO/o DVB-crosslinked CufJI) ion

Imprinted microspheres showed that their surface is rough and corrugated when

compared with DVB-crosslinked Cu(lI) ion imptinted microspheres pig. 111. 8).

The SEM photographs of 17 molO/o EGDMA-, BDDMA- and HDODA-crosshked

mcrospheres showed almost s d a r pattern in their surface structural features

characterisdc of submicron spheres (Fig. 111. 9(a-c)).

Fig.IXI.8. Scanning electron micrograph of 17 molO/o DVB-crosslinked (a)unimprinted microspheres and (b) Cu(1I) ion-imprinted microspheres

Fig.III.9 (a) Scanning electron micrograph of 17 mol% EGDMA-crosslinked Cu(1X) ion-imprinted microspheres

Fig.III.9 (b) Scanning electron micrograph of 17 mol% BDDMA-crosslinked Cu(I1) ion-imprinted microspheres

Pig.III.9 (c) Scanning electron micrograph of 17 molO/o WDODA-crosslinked Cu(1I) ion-imprinted microspheres

111.5. Metal Ion Selectivity Studies of Cu(1I) Ion-imprinted and

Unimprinted Microspheres

The easy and rapid access of the surface carboxylate groups in the imprinted

polymers with predetermined cavities was used for the selective concentration of

metal ions. To investigate thls, competitive rebindmg experiments were carried out

using pairs of metal ions. For th~s Cue-Co(II) and CuQI)-Ni(II) mixtures were

used. The amounts of metal ion complexed were estimated using the multi-

component UV-analysis. As seen in Tables IFI.3(a & b), the Cu(1I) ion-imprinted

microspheres selectively binds its desorbed metal ion viz. Cu(Y2) ion from a mixture

of metal ions.

The nature and the degree o f the crosslinking agrnts are crucial in

m:~~nr:liriing rhe .;talilit\. of the ilnprinred system. . \ s the spcci6c structure of the

cayit\- is detcrrnined not only by the incorporated nletal ions, but also by the fixed

nrranj:crncnt o f the polymer chai~ls, the opumizauon o f the polymcr matrix structure

and s r ;~b~i~ t ! I S Imporrant in deciding rhc srlecrivinr characteristics uf the developed

s r n s . 'l'hc good a c c c s s ~ h i l i ~ of rhc cavitics is thc highly dccisivc factor for thc

selectivity, which in turn dep ends c,n me n'NrE mil amount of the crosslinUng

a8nt usd in production o f mctd im se1e~"'e p'h'mcfii'

fie resul, of %ie'tive cu(I9 ion reb i id ig of 9, 17 and 25 mol% D m - .

EGDMA-, BDDMA- and HDODA-crosslu*ed "rosphera from miamre of

Cupn and CopQ ions are given in Fig 111. lO(a-d).

1.2

v

Cmaslnk densh (mol%)

. 0 ) Selective Cu(I1) ion rebinding of CuQI) io1l-impdnted crosslinked microsphere

Fig. 111.10@) Seiective Cu(L1) ion rebinding of Cu(11) EGDMA-crosslinlcrd microsphere

ion-imprinted

Y

Crosslink density (mol%)

~ l g . 111.10(~) Selective CU(LI) ion rebindily of Cu(1I) ionimptinted BDDMA-crosslinked microsphere

Crossl~nk density (mob%)

111.10(d) Selective Cu(1I) ion rebinding of Cu(11) ion-im~dnted HDODA-crosslinked microsphere

On correlating the selectivity studies with the nature as weu as the debvee of

crosslinking, it was found chat the selectivity characteristics decreased in the order:

DVB- > EGUbLi- > BDDMA- > HDODA-crosshked system. T h ~ s arises from

the increased stability of the imprinted structure in the ng~d DVB-crosslinked

system. With increasing flexibhty of the crosslinking agent, the stability of the

imprinted system decreases and this decrease is much hgher in the HDODA-

crosslinked system. With increasing degree of crosslinktng, the selectivity

characteristics also increased. This arises from the increased stabhty of the

imprinted system to keep its geometry as such for the imprinted ion.

Crwdlink density (mol%)

Fig. 111.11. Cu(I1) ion selectivity of imprinted and unimprinted 17 moloh DVB-crosslinked microspheres

Fig.III.11 is a comparison of the selectivity studies of 17 mola/o DVB-

crosslinked Cu(II) ion unimprinted and imprinted microsphers. It is evident from

the figure that the CuQI) ion-imprinted system has enhanced selectivity towards

<;u(II) ion than the unimprinted system resulting in a clear concentration of the

Cu(II) ions.

in conclusion, Cu@Q ion-imprinted microspheres designed and synthesied

here by surface imprindng can be used as selective Cu(I1) ion adsorbents. The

i m p ~ t e d srnlcture was constructed on the microsphere surface with the aid of the

interaction between the metal ions and the surface mobile carboxyl groups. The

coordtnated structure was then imrnobhed using crosslinking polymerisation. The

metal ion-imprinted microspheres, adsorbed the corresponding guest metal ions

more effectively than the unimprinted microsphers. The uniform and neat spherical

beads did not require any pretreatment, such as grindmg and sieving before use.

The nature and degree of the crosslinking agent employed have a signtficant effect

on specificity and selectivity of the tailored systems. Specificity and selectivity

characteristics, whlch are measure of the stabhty of the polymeric system, increased

with increasing rigidtty of the crosslinking agent. Also in all systems with increasing

degree of c r o s s h h g , the specificity as well as selectivity characteristics increased.

Thus the tailoring of metal ion specific polymers would be possible by the suitable

selection of the monomers and crosshking agents. The surface imprinted

microspheres is therefore expected to be useful materials for the selective separation

and concentration of metal ions. In addition, this technique would improve the

complexadon behaviour, includmg the reaction rate, specificity and selectivity, based

on the unique features of the rnicrospheres.

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