Eletrostati Dipole F ormation by Assoiation between Composite Miro-

between Composite Mirogels and Gold

Nanopartiles

4.2.1 Introdution

Biologial olloidal systems like virus and proteins often present a omplex and dened

struture with dierent funtionalities whih is the prerequisite of their spei and

tar-geted appliations. They an be onsidered in many ases as omplex polyeletrolytes

with a pathy struture, amosai of negative,positiveor neutralareas.

Obtaining suh strutures synthetially is muh more hallenging. Complex geometries

and dened lusters ould be obtained via pikering emulsion and evaporation of the

solventasinitiatedbyPine etal. onmirometripartiles[257℄and onbidisperseolloids

[258, 259℄. This approah was reently extended by Wagner et al. to olloidal system

[260℄. Another strategy onsistsonthe use of a templateas shown reently by Xiaet al.

[261℄. Nevertheless most ofthe time these lusters are not speiallyfuntionalized and

lak of long ranged interations.

At the same time many eorts have been put into the synthesis and haraterization of

Janus systems [262268℄, whih present a versatile behavior and interesting properties

as stabilizersfor instane [265, 267℄. Most of these new systems relyon their ambivalent

nature,neverthelessitremainsdiulttosynthesizedstableolloidaleletrostatidipoles.

Indeed mixing oppositely harged olloids most of the time leads to the

destabiliza-tion of the system [269℄. In this setion we onsidered the assoiation of ationi

gold nanopartileswiththermosensitive anioniore-shell mirogelspolystyrene/poly(

N

-isopropylarylamide)in the dilute regime.

For this purpose ationi gold nanopartiles were prepared as desribed formerlyby

Ni-idome,where theauthors illustratedtheir appliationinthe funtionalgene deliveryand

anerous ells detetion [270℄. The use of inorganinanopartiles for gene therapy suh

as amino-modied silia nanopartiles and ationi gold nanopartiles was reently

re-ported[270272℄. Thespeial haraterofthese nanopartilesdiers fromthat oforgani

gene arrier moleules, and they are expeted to be a novel base material for the next

generation of funtional gene arriers. Gold nanopartiles have the advantages of easy

preparation and the possibility of hemial modiation of the surfae. Moreover they

havebeenfoundsuitableinmanyappliationsspanningfromatalysisandnano-eletroni

totreatment and detetion of anerogeni ells [273275℄.

On the other hand omposite mirogels have been intensively studied and used inmany

appliationsfrom theprotein adsorption[19,20℄tothe use as templateforthe redution

of metal nano-partiles [2123℄. Inreasing the temperature these mirogels swell and

deswell in a ontinuous or disontinuous fashion following their degree of ross-linking

as shown in the setion 2.1.4. Not only the size of the partiles an be ontrolled with

the temperature, but also their potential. As presented in the former setion, systems

synthesized by seeded emulsion polymerization with an anioni initiator were found to

present someeletrostati originatedfromremaininginitiatorfragmentsmostly based on

the surfae of the ore partiles. This ensures the stability of the system even after the

volume phase transition. Thus the interation potential an be adjusted ontrolling the

size andsaltonentrationof thedispersion. In anexess of salt,the system isnotstable

anymore above the volume phase transition and was found to reversibly oagulate (see

hapter 4.1).

The eletrostati stabilization relatedto the presene of negativeharges ombined with

the steristabilizationensured bythe shell has alreadybeenused toabsorbationigold

nanorods [31, 32℄ and silver nanopartiles [276℄. Whereas most of the studies foussed

on the adsorption of small partiles, we investigate the adsorption of larger partiles

(size ratio in the range 1 to 4). We present a simple way to suessfully adsorbed the

gold on our omposite mirogels to obtain anorgani/organi doublet or triplet keeping

the dipolar harater of the assoiation. The inuene on the size ratio and on the

preparation is rst disussed. Afterwards we investigate the orrelation between both

partiles by turbidimetri titration, UV light spetrosopy, zeta potential measurement,

dynami light sattering and mirosopi methods. The struture and stability of the

assoiationis onsidered atthe end of this setion.

4.2.2 Experimental

Materials

The system used in this study onsists on a polystyrene ore with a rosslinked

PNI-PAM shell ontaining 2.5

mol%

^{BIS (KS2) (see setion 2.1.2 for further details). The}

gold nanopartiles were synthesized following the reipe desribed by Niidome et al.

[270℄. The gold nanopartileswere prepared by NaBH

4

^{redution of HAuCl}

4

^inthe

pres-ene of 2-aminoethanethiol at a Au/NaBH

4

^/ 2-aminoethanethiol ratio of 56 : 0.1 : 85 (mol/mol/mol). Immediately after the NaBH

4

^{redution, the solution was opaque and}

beomearedwine olor. The partileswere thenusedforthe assoiationwithoutfurther

puriation.

Thesamplewerepreparedbyslowadditionofthe0.28

gL ⁻¹

^{goldsolutionondilute}

miro-gelsolutions. Themirogelonentrationwaseitherset at0.2

gL ⁻¹

^{forthe zetapotential}

and for the UV light spetrometry measurement, and between 4.10

−2

and 2.6.10

−2 gL ⁻¹

for the sample prepared by titration. The gold nanopartiles were stable for

approxi-mately two weeks, afterwards they started to get adsorbed at the surfae of the glass

ontainer. Thusthe haraterization ofthe gold nanopartilesandthe preparationof the

dierentsolutions were done with a freshgold solution withinone week.

Methods

Sanningforemirosopy(SFM)experimentswerearriedonaommerialSFM(Model

Dimension3100,fromVeeoInstrumentsIn.) (seesetion4.1.3forfurtherdetails).

Field-emission sanning eletron mirosopy (FESEM) was performed using a LEO Gemini

mirosope equipped with a eld emission athode. The samples have been prepared

by spin oating at 2000

rpm

^{on siliium waver for the FESEM} experiments, whereas the SFM samples have been prepared by dipoating on mia. Transmission eletron

mirosopy(TEM) andryogenized TransmissionEletronMirosopy(Cryo-TEM) have

0

Figure4.10: Transmission eletron mirograph of a 0.2 wt.aqueous suspension of ationi

gold partiles obtained by NaBH

4

^{redution of HAuCL}

4

^{in the presene of}

aminoethanethiol. The average radius of the gold partiles was determined at 22.7

±

^3.7nm(see histogram). Thestabilizationof thepartiles by theaminoethanethiol has been evidene by the presene of a 5-8

nm

^{thin shell around the partiles.}

been performed on a Zeiss EM922 EFTEM (Zeiss NTS GmbH, Oberkohen, Germany).

A few miroliters of the solution were plaed on a bare opper TEM grid (Plano, 600

mesh) and the exess of liquid wasremoved with lter paper.

The turbidity measurement of the adsorption of the gold nanopartilesonthe omposite

mirogel was performed on a titrator (Titrando 809, Metrohm, Herisau, Switzerland)

equipped with aturbidity sensor (

λ ₀ = 523 nm

^, Spetrosense, Metrohm). The 0.28

gL ⁻¹

goldsolutionwasaddedatarateof0.25

mL/min

^ona16

mL

^0.04

gL ⁻¹

^{mirogelsolution}

at25

o C

^.

Dynami lightsattering was done using a Peters ALV 5000 lightsattering goniometer.

The samples were highly diluted(

c = 2.5.10 ⁻³ wt%

^{)to prevent multiple sattering. The}

deay rate

Γ

^{wasobtainedfromtheseondumulantanalysisforsatteringanglefrom30}

^o

to 150

o

with aninrement of 10

o

. The zeta potential was determined with the Zetasizer

withastrit ontrolof thetemperatureset by Peltierelementsat

±

^0.1

^o

^{C. Forthediret}

observationof thestabilityof thedierentsolutions,thesamplewere equilibrated30

min

in athermostated hamber.

4.2.3 Results and Disussion

Cationi gold nanopartiles

Transmissioneletronmirosopyhas beenperformedonthe ationigoldpartiles

stabi-lized by the aminoethanethiol (see g. 4.10). The radius of the partiles alulated from

the TEM has been determined at 22.7

±

^3.7

nm

^{. The} stabilization atthe surfae of the partileanbediretlyimage,byathinlayerofbetween5and 8

nm

^{aroundthepartiles.}

The thiolgroups have astronganity withthe gold,under neutralonditions theamine

groups are hydrolyzed into NH

⊕

3

^{providingthe ationiharater of the partiles.}

More-over the size of the orona respet to the size of the moleulesupposes the organization

ina multilayer. In ordernot to damagethis layer the piturehave been taken underlow

dose onditions. Nonetheless the surfatant is still very sensitive to the radiation and

get easily damageafter too long exposure. Thusit is not easy todetermine its thikness

preisely. The average hydrodynami radius has been also measured by dynami light

1 2 3 4 5

Figure4.11: Adsorption of the ationi gold nanopartiles on the omposite mirogels. The 0.28

gL ⁻¹

^{goldsolutionwasadded to16}

mL

^4.10

⁻² gL ⁻¹

^{latexsolution. Theturbidityof}

the solution wasmeasured during the addition proess. The turbidity wasexpressed

intermoftheabsorbane

A = − ln(I/I ₀ )

^where

I ₀

^{orrespondstotheinitialintensity}

of the latexsolution(see text forfurther details). The dierent number orresponds

to the dierent solutions prepared following the same proedure for dierent gold

onentrations.

sattering between 25 and 45

o

C in bak sattering at 173

o

with the Zetasizer. No

signif-iant dependene on the temperature was observed. The average hydrodynami radius

obtainedfromtheseondumulantanalysiswasfoundequalto27

nm

^{witharelativehigh}

polydispersity index (0.28). On the ontrary of the evaluation of the size by TEM, the

DLSismoresensitivetothe preseneofaggregatesinthesolution. Nonetheless thisvalue

presents anieagreementwiththe TEMif weonsider the ontributionofthe surfatant

layer. The eletrophoreti mobility has been expressed by the way of a Zeta potential

alulated from the Smoluhowski approximation between 25 and 45

o

C. This value was

also not sensitive to the temperature and the average zeta potential was found equal to

27

mV

^{. This onrms the ationi harater of the partiles,} nevertheless this value is lowerthan the onereportedin theliterature forthe same samplepreparation(36.2

mV

^).

Turbidimetri titration

Whereas a slow addition of latex on an exess of gold partiles diretly leaded to the

destabilizationof the solution,itwas possible toadd goldon anexess of latexpartiles.

Forthisreasonthe solutionshavebeenpreparedbyslowadditionofthe goldonthe

ore-shell partiles. Weperformed turbidimetrititrationin order tofollowthe assoiationof

theationigoldnanopartilesontotheompositemigrogels. Tothispurposeweprepared

a16

mL

^{latexsolutionwithaninitial}onentrationof4.10

−2 gL ⁻¹

^{andaddedatarateof}

0.25

mL/min

^a0.28

gL ⁻¹

^{goldsolution. Theturbidityofthesolutionwasmeasuredduring}

the wholeadditionproess. Thegold solutionasastrongabsorption at525

nm

^{losed to}

the wavelength ofthe probe(523

nm)

^{. Forthis reasonwe used the} transmittedintensity

I 0

^{ofthelatexsolutionasreferenetonormalizethe}transmittedintensity

I

^{ofthesolution}

and we followed the evolution of the absorbane dened by

A = − ln(I/I 0 )

^{as funtion}

of the gold onentration (see g. 4.11). As expeted, the absorbane inreases rst

-50

Figure4.12: Zeta potential of the gold nanopartiles (hollow squares), of ore (full irles) and

Core-Shell (full squares) partiles alulated with the Smoluhowski approximation

as funtion of the temperature with and without addition of gold for a 0.2

gL ⁻¹

latex solution. Remaining eletrostati interations explain the stability of the latex

at high temperature. Addition of the gold solution is sreening the Zeta potential

(see the dierent onentrations in the legend). An aggregation an be observed for

a onentration of 0.160

gL ⁻¹

^{at 35}

^o C

^.

linearly untila gold onentration of 0.03

gL ⁻¹

^{. After this} onentration the absorbane still inreased linearly but with a lower slope. This deviation was attributed to a fast

aggregation proess followed by the destabilization of the solution, to the shift of the

plasmon maximum towards higher wavelengthes onrmed by a hange of the solution

from red to blue and to the presene of free gold partiles. Four dierent samples were

preparedfollowingthesameproedure(seeg. 4.11). Foraonentrationhigherthan0.03

gL ⁻¹

^{,the solutionwere not stableand sedimented withinanhour. We}approximatedthe orresponding number ratio between the gold and latexpartilesonsidering the density

of gold (19.3

gcm ⁻³

^{), the size of the gold partiles determined from the} transmission eletron mirosopy and the number of omposite mirogel partile present in solution

(asdesribed in setion 2.1.2). We found out a ratio of 1.05, whihbasially means that

in average no more than one gold partiles ould be adsorbed onto the mirogelsunder

this onditions. This nding was onrmed afterkeeping the solutionsfor more than six

months. Below this ritial onentration the solution ould be very easily redispersed

andnogoldwasabsorbedonthesurfaeoftheglassontainerwhihisastrongindiation

oftheorrelationofthegoldwiththemirogel. Ontheontraryforhigheronentrations

the solutionsould not be redispersed anymore after two months and a part of the gold

was absorbed on the surfae of the glass ontainer, whih onrmed the assumption of

free gold partilesin the solution.

Zeta potential measurement

Wemeasured theZetapotentialofsolutiononsistingon0.2

gL ⁻¹

^{latexsolutionwith}

dif-ferentonentrationsofgoldvaryingbetween5.4.10

−3

and1.6.10

−1 gL ⁻¹

^{. The}polystyrene orepartilesusedforthe synthesisof theompositemirogelshas beenmeasured aswell

asthepure goldnanopartilessolutionand thepure ore-shellpartilesasfuntionofthe

temperature(see g. 4.12). The orepartilespresented arelativelyonstant zeta

poten-0 0.2 0.4 0.6 0.8

300 400 500 600 700 800

l [nm]

300 400 500 600 700 800

45°C

Figure4.13: A) UV visible extintion spetrumof a pure 0.2

gL ⁻¹

^{ore-shell dispersion (dashed}

line) and of 0.2

gL ⁻¹

^{ore-shell dispersion with 0.0280, 0.120 and 0.160}

gL ⁻¹

ationi gold measured at 20

o C

^{. The inset display the adsorption of the pure gold}

(dashed line) and of the three solutions after substration of the latex ontribution.

The position of the maximum adsorption

λ _max

^{of the gold shifts from 525 to 535}

nm

^{after adsorption on the omposite mirogels as shown by the thin dotted lines).}

B) Adsorption of a 0.02

gL ⁻¹

^{ore-shell dispersion with 0.120}

gL ⁻¹

^{ationi gold}

solution measured with inreasing temperature. C) Maximum of the orresponding

plasmon band of the gold nanopatiles as a funtion of temperature (full irles)

ompared with the evolution of polymer volume fration of the PNIPAMshell

(hol-low squares). The full line presents the t following the Flory-Rehner theory (see

setion 2.1.4).

tialat -43

mV

^{over allthe} temperaturerange. Conerningthe pure ore-shell dispersion below32

o

C, theeletrophoretimobilitywasfoundtoaround-15

mV

^{,reetingboththe}

low surfae harge density and the high frition oeient of the swollen partiles.

How-ever, a dramatihange inthe eletrophoreti mobilityversus temperaturewas observed

above the volume transition temperature. As expeted, the absolute values of the zeta

potentialinreasedwithinreasingtemperatureduetothe thermalsensitivityand

shrink-ing of the PNIPAM shell to reah approximately the value of the ore zeta potential at

high temperatures asdesribed by López-León et al. [72℄. The temperature dependene

ofthezetapotentialouldbeinterpretedbytheinrease inthesurfaehargedensitydue

to the redution in the partile size, the enhanement of the loal harge onentration

onthe partile's surfae and the fritional oeientredution of the ollapsed partiles.

This eet ould bediretly visualizedonsidering the strong bukling up of the shell at

low temperature and the ollapsed form of the shell at higher temperature as shown in

gure 2.2.

Addinggold induedthe derease of the absolutevalueof the zetapotentialtobealmost

equal to zero for a onentration for a onentration of 1.6.10

−1 gL ⁻¹

^{below the volume}

phase transition. At higher temperatures the thermosensitivity was maintained with a

transition around 32

o C

^{. All the} measurements presented a mononodale distribution of the zeta values onrming the assoiation of the gold with the mirogel. For the higher

onentrationorrespondingtoanumberratioofone goldforonemirogelaoaggulation

proesshas beenobservedfortemperatureshigherthan40

o C

^{. Thediminutionofthezeta}

potentialouldbeinterpretedasanindiationoftheassoiationofthe oppositelyharged

partiles leading rst to the distortion of the harge distribution, to the inrease in the

size of the partilesand tothe inrease of the surfatant onentration in the solution.

Thermoresponsive optial properties

The assoiation of the gold nanopartiles with the mirogelswas investigated by UV-vis

spetrosopy. The dependene ofthe plasmonadsorption onthe size and temperatureof

olloidalgoldpartilesinaqueoussolutionhasbeenalreadydisussedbyLinketal. [277℄.

They foundout thatmonodisperse 21.7

nm

^{goldpartileshaveamaximum adsorptionat}

521

nm

^{. The maximuminthe adsorption wasobserved at525}

nm

^{forour solutionwhih}

isingoodresultswith the literatureif weonsiderthe inuene ofthe polydispersityand

of the surfatant adsorbed atthe surfae of the gold partiles.

In order toonrm the assoiationbetween the gold and the mirogelwe investigate the

dierent solutions by UV-vis spetrosopy. The pure gold solution was rst measured.

A maximum in the adsorption was obtained at 525

nm

^{whih is ommon for partiles}

in this size range [277℄. On the ontrary the latex solution did not present any spei

adsorption. Afteradsorption of the gold (g. 4.13 A)), the plasmonmaximum measured

aftersubstrationofthelatexontributionshifted from525to535

nm

^{(see insetg. 4.13}

A)).

Theadsorptionwasmeasuredfora0.2

gL ⁻ 1

^{ore-shelldispersionwith0.120}

gL ⁻¹

^ationi

goldvaryingthetemperaturebetween10and45

o C

^{(seeg. 4.13B))asdesribedreently}

[31,32℄. Theadsorptionwasfoundtoinreasewiththetemperaturefollowedbyaredshift

from535to543

nm

^{between10and45}

^o C

^{whihhavebeenattributedtotheinreaseofthe}

loal refrative index upon mirogel ollapse for low surfae overage [31℄. The position

ofthe maximum

λ max

^{has been monitoredfor thedierent}temperatureandompared to the variation of polymer volume fration

φ

^{of the shell of unoated omposite mirogel}

taken from the setion 2.1.4 (see g. 4.13 C)). The linedisplays the theoretial tusing

the Flory-Rehner theory. The transition observed in the variation of

λ max

^{follows the}

same feature asthe transition inthe PNIPAM network exept that the transitionours

about2

o C

^{beforethe LCSTofthepure mirogel. Thisagain}orroboratesthe assoiation of the gold with the mirogels.

Colloidal stability and oagulation

The stabilityof the partileshas been heked by adding dierent onentrationsof gold

on a 2

gL ⁻¹

^{mirogels solution (see g. 4.14). At 32}

^o C

^{the system was perfetly stable.}

Inreasingthe temperatureat35

o C

^{leads tothe oagulation ofthe system for}

onentra-tionshigherthan0.107

gL ⁻¹

^{. Darkred aggregateswereobserved, whihquiklysediment}

lettingalear solutioninomparison tothe puregold and ore-shellsolutions. This

on-rmstheassoiationofthe goldwiththe mirogel. Thesystem reovereditsoriginalform

25

o C

^{after a small} redispersion. The understanding of the aggregation proess is quite hallenging. First we hek if it was orrelated to the ratio between gold and mirogels.

For the same partileratio but aftera dilutionby ten the system did not aggregate. We

onsidered the inueneof the gold onentrationfor dierent latexonentration. Fora

onentration of 0.160

gL ⁻¹

^{the system was found to aggregate for 2 and 0.2}

gL ⁻¹

^latex

onentrations. Thus, the onentration ofgold andnot the numberratiogold/mirogels

seems to be the determinant fator for the reversible oagulation at high temperatures.

The omposite mirogelsare sterially stabilized by the PNIPAM shell and

eletrostati-allydue tothe use of ananioni initiatorand to the remainingSDSproviding fromthe

32°C

35°C

25°C

Figure4.14: Stabilityandoagulationofthesolutionsasfuntionofthegoldonentrationandof

thetemperature. The dierentsolutions arefromtherighttotheleft: apure 2

gL ⁻¹

ore-shell solution, a2

gL ⁻¹

^{ore-shell solution+ 0.054, 0.107, 0.160,0.220, 0.280}

gL

−1

goldrespetivelyandapure 0.2

gL ⁻¹

^{goldsolution. Allsolutionswereobserved}

rstat32

o

C,thenat35

o

Cand25

o

C.Thesamplesat25

o C

^wereredispersed topoint out the reversibility of the oagulation proess.

synthesis. As disussed in the setion 4.1, addition of salt redues the Debye length and

thestabilityofthesystemathightemperature. Fig. 4.12learlyshows thiseet interm

of a diminution of the zeta potential with inreasing gold onentration. It is still not

fully understood how the gold nanopartiles and the remaining surfatant ontribute to

the sreening of the eletrostatis.

Eletri dipoles formation and dynami lusters

Theformerexperimentslearlyshowtheassoiationofthegoldwiththemirogels.

Never-thelessthestrutureofthe omplexhasnotbeen investigateduntilnow. Forthispurpose

Never-thelessthestrutureofthe omplexhasnotbeen investigateduntilnow. Forthispurpose

Im Dokument Structure, Dynamics and Association of Thermosensitive Core-Shell Particles (Seite 118-151)