Effects o f the conditions o f the microemulsion preparation on the properties o f Fe

VNU journal of Science, Natural Scicnccs and T echnology 24 (2008) 9-15

Effects o f the conditions o f the microemulsion preparation on the properties o f Fe

³

Ơ

⁴

nanoparticles

Nguyen Thai Ha', Nguyen Hoang Hai'’*

Nguyen Hoang Luong', Nguyen Chau’, Huynh Dang Chinh^

' C e n te r f o r M a te r ia ls S cien ce, F a c u lty o f P h ysics, C o lle g e o f S cien ce, V N U 3 3 4 N g u yen Trai, H an oi, V ietnam

^ F a c u lty o f C h e m ic a l T ech n olo gy, H a n o i U n iv e r s ity o f T e c h n o lo g y ] D a i C o Viet, H an oi, Vietnam

R eceived 12 March 2008

A b stract. Fe3 0 4 nanoparticles have b een prepared by the niicroem ulsion technique w ith water as the aqueous phase, n-hexane as the o il phase and Span 80 as the surfactant. The reaction occurred under air, N2 or high temperature and high pressure atmosphere. Particle size can be confrolled by the concentration o f the reactants d issolved in water, the ratio o f water/surfactant and the atm ospheric conditions. The particle size is o f 6 - 20 nm. T hey are supeqDaramagnetic with the saturation m agnetization o f 50 em u/g. Functionalization o f the particle surface has b een caư ied out by using a single layer o f o leic acid for hydrophilic surface and double layer o f o leic acid and sodium d odecyl sulfate for hydrophilic surface to disperse them in non-aqueous and aqueous solvents, respectively. Changing the conditions o f the preparation affected to the properties o f the product. This technique sh ow ed advantages such as sim ple, sm all size particles, m onodisperse over the coprecipitation m ethods.

K eyw ords'. M agnetite nanoparticles, m icroem ulsion, superparamagnetic, F c3 0 4.

1. In tro d u c tio n n a n o p a r tic le s to d is p e r s e in th e carrier liq u id . T h e carrier liq u id ca n b e p o la r iz e d or n o n - M a g n e tic flu id s are o f in te r e st o f m a n y p o la r iz e d d e p e n d in g o n a p p lic a tio n s . A s a resea rc h e rs d u e to th eir p o te n tia l a p p lic a tio n s in re su lt, it is n e c e s s a r y to c h o o s e a proper p h y s ic s an d b io l o g y [1 ,2 ] . M a g n e tic flu id s su rfa cta n t fo r n a n o p a r tic le s d isp e r se in the c o n s is t o f m a g n e tic n a n o p a r tic le s , a su rfa c ta n t carrier liq u id . M a g n e tic p a r tic le s are n o rm a lly and a carrier liq u id . T h e m a g n e tic p r o p e r tie s o f req u ired to h a v e a h ig h sa tu ra tio n m a g n e tiz a tio n m a g n e tic flu id s are d e te r m in e d b y m a g n e tic Ịự[^^ b io c o m p a tib ility , lo w - c o s t and sta b ility n a n o p a r tic le s (N P s ) . T h e su rfa c ta n t h e lp s u n d er th e w o r k in g e n v ir o n m e n t. M a g n e tite F e3 0 4 are w id e ly u se d to m a k e m a g n e tic flu id ---.. . T’ 1 . ccon^i^ b e c a u s e that m a teria l c a n fu lfill a b o v e Coưesponding author. Tel.: 84-4-5582216.

E-mail: nhhai@vnu.cdu.vn r e q u ir e m e n ts. F o r b io lo g ic a l a p p lic a tio n s, tw o

10 N.T. Ha et al. / V N U journal o f Science, Natural Sciences and Technology 24 (2008) 9-15

nano effects have been taken into account, which are high surface area and superparamagnetic propertỵ Super- paramagnetic NPs have no coercive field and no remanent magnetization but they do have high magnetization under a magnetic field. This fact is important for biological applications when it is desired to have high magnetization when a magnetic field is applied and to have no magnetization when the magnetic field is off.

W hile magnetite particles are required to have the diameter less than about ^{2 0} nm in order to be superparamagnetic at room temperature, the surface effect is sữonger when particle size is smaller. In ađition, particle size distribution is very important for ensuring all particles have the same magnetic properties. The simplest way to make magnetite fluids is coprecipitation and ions by O H ' at room temperature [3].

However, this m ethod has a problem to obtain particles with diam eter o f less than ^{1 0} nm and with small size disfribution.

M icroemulsion (inverse micelle) is suitable way for obtaining the uniform and size conừollable nanoparticles [4], A microemulsion may be defined as a thermodynamically stable dispersion o f two immiscible liquids consisting o f small droplets o f one or both liquids stabilized by an interfacial film o f surface active molecules (surfactant, stabilizer). In water-in-oil microemulsions, the aqueous (water) phase is dispersed as microdroplets suưounded by a monolayer o f surfactant molecules in the continuous non-aqueous (hydrocarbon) phasẹ If a soluble metal salt is incorporated in the aqueous phase o f the microemulsion, it will reside within the aqueous droplets surrounded by oil. These microdroplets continuously collide, coalesce and break again.

If two identical microemulsions are produced

with a reactant p dissolved in the aqueous cores o f one microemulsion and a reactant Q in the other microemulsion, upon mixing, they will form precipitate PQ, which will be contained entirely within the aqueous cores o f the microemulsions. The growth o f these particles in microemulsions is suggested to involve inter­

droplet exchange and nuclei aggregation.

2. Experiment

The synthesis process occuưed via the mixing o f two m icroemulsion systems with identical compositions but different aqueous phase types - one containing metal ions (reactant A), the other, a precipitating agent (reactant B). The first oríề consisted o f an

a q u e o u s s o lu tio n o f iro n c h lo r id e sa lts

(FeCl².⁶H^{2 0} and FeCl³.⁶H^{2 0} ) dispersed in the Serbian monooleate (Span 80)/n-hexanẹ The second system comprised a precipitating agent NH⁴OH dispersed in the span 80/n-hexanẹ The two microemulsions were mixed together under continuous stiưing (typically ² hr) to obtain nanoparticles. We obtained a water-in-oil reverse microemulsion system, in which Span 80 as surfactant to stabilize the emulsion state, n-Hexane as the continuous oil phase (o), and the aqueous phase (w) containing c = 0.2 - 0.4 M (the concenfration o f was adjusted to keep the ratio o f Fê^/Fế" to be ^{2 : 1} - reactant A), was used for synthesis o f magnetite NPs.

Particle size could be adjusted by changing concentration c o f the reactant in the aqueous phase, changing the volume ratio o f water and surfactant (w^{/ 5} = ^{2 0} - ^{1 0 0}), and the reaction atmosphere (300 K/1.0 at and 450 K/1.5 at).

There were three types o f samples: (A) mixing in air, (B) after mixing in air, the system was

N.T. Ha et a l Ị V N U Journal o f Science, Natural Sciences and Technology 24 (2008) 9-15 11

2 0 (d e g r« « )

Fig. 1. X R D patterns o f m agnetite pow der with concentration o f o f 0.2 M in the aqueous phase.

The solid squares present the theoretical reflections o fF e 3 0 4 (p d f # 79 04 18).

submitted to an atmosphere with temperature of

180°c

and pressure o f 1.5 at for a time o f ⁸ hr, and (C) mixing in N². High temperature and pressure in case B fostered the reaction to form nanoparticles. In type B, we combined the microemulsion and the hydrothermal technique.

When reaction completed, magnetic decantation was applied to remove N Ps from the excess solution. Then oleic acid (OA) as a surfactant was mixed to coat NPs. Using magnetic decantation and washing by n-Hexane four times, OA-coated NPs dispersed in n-Hexane was made. The fact that span 80 could not be used to coat NPs was due to the molecule o f this surfactant could not create a chemisorption with magnetite surface while OA could [5]. For dispersing in water, Sodium dodecyl sulfate (SDS) \\as used as a second layer o f surfactant.

The hydrophobic part o f SDS tended to the hydrophobic part o f OA, which created a hydrophilic surface on nanoparticles (SDS/OA- coated nanoparticle).

Fig. 2. Typical SEM im age o f m agnetite nanoparticles (type C).

Structure analysis o f the dried powder o f non-coated NPs was conducted by using a D5005 X-ray diffractometer with Cu K a radiation. Magnetic properties were measured by a DMS 880 vibrating sample magnetometer.

Morphology o f NPs was examined by a JEOL 5410 LV scanning electron microscope. Weight loss (Thermal Gravity Analysis) as a function o f temperature (heating rate o f ^{1 0}°c/m in ) was studied by a DSC SDT 2960 TA Instraments.

3. Results and discussion

The mechanism o f formation o f particles was understood as a short single burst o f nucleation occuưed when the concenữation o f constituent species reached critical supersaturation. Then, the nuclei so obtained were allowed to grow uniformly by diffusion o f solutes from the solution or/and aggregation o f other nuclei to their surface until the final size was attained. In conventional coprecipitation, size (d) can be controlled by concentration o f reactants [5], pH and ionic strength [⁶], Size o f

1 2 - ^{1 0 0} nm could be made by this technique.

12 N.T. Ha et a i / V N U Journal o f Science, Natural Sciences and Technology 24 (2008) 9-Ĩ5

-2 0-

-4 0 -

-10000 -5000 10000

H(Oe)

Fig. 3. Magnetization curves o f o f type c sam ples with different concentrations o f the reactant.

Smaller particle size is difficult to obtain.

Microemulsion can produce small particles with diameter can be less than ^{1 0} nm, which coprecipitation technique cannot do [7]. In microemulsion, amount o f reactant is limited in a volume o f the microdroplet, which can be controlled by water/surfactant ratio and atmospheric conditions.

XRD patterns o f the dried non-coated NPs o f type c sample with different concentration (0.2 and 0,4 M) o f reactant (w/s == 20) were shown in Fig. 1. All reflections are o f magnetite Fe³Ơ⁴. These indicated that the particles have the invert spinel crystalline structure as in the bulk phase. The width o f peaks o f the sample with higher concentration was broader than that o f the peaks o f sample with lower concentration. That means high concentration produced large particles. By controlling concentration, we could control the particle size. It suggested a way to obtain desired particles. Particle diameter can be detenĩiined by Cherrer formula [⁸]:

T(C)

Fig. 4. W eight loss as a function o f temperature o f O A -coated N P s o f type c sam ple with c = 0.2 M.

0

⁾

ổ s i n Ỡ

w h e r e Ẫ is th e w a v e len g th o f th e X -r a y , 0 is the

reflection angle, and B is the full w id th at half maximum o f the peak. The particle diameter obtained from that for all samples was in the range from 7 nm to 22 nm.

A typical scanning electron microscope (SEM) image o f magnetite sample (type

c,

^w/s

= 20) coated by OA was presented in Fig. 2.

Particle size was less than 10 nm which is in agreement with a value from XRD results.

Some features o f this image showed particle size can be 5-6 nm. Similar images were obtained for other samples.

Magnetic properties o f sample of type A prepared under ambient conditions were non­

ferromagnetic at room temperature, which can be understood by the fact that the reaction could not complete under these conditions. Whereas, magnetic properties o f samples o f type B were ferromagnetic with the saturation magnetization Ms o f 50 emu/g and the coercive field He o f 50 Oe at room temperature for sample with c = 0.1

M . M s and H e reduced when the concentration

N.T. Ha et n l Ị V N U journal of Science, Natural Sciences and Technoỉogỵ 24 (2008) 9-15 13

o f reactant lo w e r e d an d r e a c h e d 2 0 e m u /g an d 5 O e , r e s p e c tiv e ly , for sa m p le w ith c = 0 .0 2 5 M . T h e critical d ia m e te r dc at w h ic h fe ư o m a g n e tic prop erty b e c o m e s s u p e r p a r a m a g n e tic w a s d eterm in ed fr o m th e e q u iv a le n t c o n d itio n o f m a g n etic e n e r g y a n d th erm a l en er g y :

KV - 25kT (2 )

w h er e K is a n is o ư o p y c o n s ta n t o f m a teria l that m a k es N P s (m a g n e tite ), V is th e v o lu m e o f particle. V is p ro p o rtio n a l to i / / , k is the B o ltz m a n c o n s ta n t an d T is th e a b so lu te tem p eratu re. F o r m a g n e tite , c r itic a l d ia m e te r is ab o u t 2 0 n m . T h e f e ư o m a g n e t is m in ty p e B sa m p les m a y c o m e fro m th e p a r tic le s w ith th e s iz e d la rg er th a n th e c r itic a l d im e n s io n . L arge p a rticles w e r e f o n n e d w h e n th e m ic r o e m u ls io n sy s te m s w a s u n d e r h ig h tem p era tu re an d h ig h p ressu re, w h ic h m a d e th e m ic r o d r o p le ts b e c o m e b ig g e r b e c a u s e th e in te r fa c ia l e n e r g y in c r e a se d w ith the te m p era tu re an d p r e ssu r e . In s o m e b io a p p lic a tio n s su c h a s h y p e r th e n n ia ,

ferromagnetic behavior is required. So this type

o f sa m p le ca n b e a p p lie d fo r s u c h a p p lic a tio n s . S a m p le s o f ty p e c s h o w e d su p e rp a ra m a g n etic b eh a v io r. T h e m a g n e tiz a tio n c u r v e o f th e se sa m p le s w ith c o n c e n tr a tio n o f 0 .2 M - 0 .4 M w a s g iv e n in F ig . 3 . H ig h e s t M s o f 5 0 e m u /g w a s re a ch ed fo r s a m p le w ith c = 0 .4 M . T h e v a lu e o f M s r e d u c e d to 3 5 , 3 0 , an d 2 5 e m u /g w h e n th e c o n c e n fr a tio n w a s 0 .3 , 0 .2 4 , a n d 0 .2 0 M , r e s p e c tiv e ly . T h is ca n b e a s c r ib e d to the sm a ller p a r tic le s iz e in th e s a m p le w ith lo w c o n c e n fr a tio n in w h ic h , a m o u n t o f reactan t lim ite d in a d ro p le t o f m ic r o e m u ls io n w a s sm a lle r than that in th e d r o p le t o f h ig h co n c e n tr a tio n . A s a r e s u lt, s m a lle r p a r tic le s w e r e f o n n e d in th e l o w c o n c e n ừ a t io n s a m p le s . S m a ll p a r tic le p o s s e s s e s la rg er su r fa c e la y er w h o s e m a g n e tiz a tio n w a s n o r m a lly lo w e r than

that o f th e b u lk m a teria l. W ith ty p e c s a m p le s, v a lu e o f M s w a s a ls o d e p e n d e n t o n the ratio w /s in a w a y w h ic h w a s s im ila r to oth er ty p e s o f s a m p le s . T h e sa tu ra tion m a g n e tiz a tio n red u ced w ith w /s . W ith w /s s m a lle r than 6 0 , M s o f a b ou t 5 0 e m u /g d o e s n o t c h a n g e s ig n ific a n tly . H o w e v e r , at h ig h e r w /s , th e v a lu e o f M s re d u ce s fa ster an d lo w e r s to 3 5 e m u /g at w /s = 1 0 0 . T h e e x p la n a tio n for th at is th e sa m e a s th e arg u m en t a b o v e . T h e r e fo r e , th e o p tim u m ratio is c h o s e n to b e 2 0.

A m o n g th ree w a y s fo r the p rep aration o f m a g n e tic n a n o p a r tic le s , m ic r o e m u ls io n in N2 a tm o sp h e r e w a s th e b e s t w a y to p ro d u ce su p e rp a ra m a g n etic p a r tic le s. T h e p a rticle s iz e ca n b e c o n tr o lle d b y a d ju stin g th e c o n c e n ừ a tio n o f re a cta n ts, v o lu m e ratio o f w a ter/su rfacta n t.

M a g n e tic n a n o p a r tic le s te n d to form

clusters to reduce surface energy. To disperse NPs in a solvent, we need a stabilizer. There are two types of solvents: polarized (such as water) and non-polarized (such as n-hexane). Each type o f solvent requires suitable stabilizer (known as another name “surfactant”).

P o la r iz e d and non-polarized solvent only allow hydrophilic and hydrophobic particles to be dispersed, respectively. Therefore, the particles must be coated by a surfactant which makes

th em h y d r o p h ilic or h y d r o p h o b ic . T h at su rfa cta n t m u st h a v e a stro n g co n ta c t w ith the p a r tic le s. T h e c o n ta c t th at c o m e s from h y d r o p h o b ic a ffin ity in s u c h th e c a s e o f S pan

80 was much weaker than that came from chemisorption in such the case of OA. With OA, the hydrophilic carboxyl group attached to

p a rticle su r fa c e an d le ft th e h y d ro ca rb o n ch a in ou tw a rd [5 ]. S o that, O A -c o a te d N P s h a v e h y d r o p h o b ic su rfa c e w h ic h m a k e s th em b e d isp e r s e d in n o n -p o la r iz e d h e x a n e . W e ig h t lo s s

14 N.T. Ha et al. Ỉ V N U Journal of Science, Natural Sciences and Technology 24 (2008) 9-15

o f a typical OA-coated NPs o f type c sample with c = 0.2 M was presented in Fig. 4. In the temperature range lower than ^{2 0 0}° c , the loss was about ²% w hich can be explained by the

ev a p o ration o f r e m a in e d w ater. T h ere w a s a 17 %

weight loss appeared in the range ^{2 0 0}° c - 250"^C, which resulted from the evaporation of OA coating NPs. From the weight loss o f OA- coated NPs (17%) and supposing that there was a single layer o f OA molecules around particles and the area of a OA molecule took place on the particle surface was about 0.3 nm^ [9], we can estimate particle size o f NPs was about ⁸ nm.

The result is reasonably in agreement with SEM observation. To make NPs hydrophilic, we used double layer o f surfactant by coating another layer o f SDS on the OA-coated NPs. The

h y d r o c a r b o n c h a in o f S D S te n d e d in w a rd to th e

hydrocarbon chain o f OA and gave the particle a hydrophilic surface. These SDS/OA-coated NPs can be dispersed in polarized liquid such as water. In many bioapplications, NPs are required to be dispersed in water, this way functionalizing o f N Ps is a potential for that.

Especially, the double layer coated NPs have a hydrophobic space between the two layers. This space can be used as a carrier to load hydrophobic drug and with an assistance o f an external magnetic field, the double layer coated NPs can be applied for magnetic drug delivery

[10].

4. Conclusion

By adj usting concenfration o f reactant, water/surfactant ratio, reaction atmosphere in m icroemulsion method, we can produce magnetic nanoparticles with particle size o f less than 10 nm. M icroemulsion technique under N²

atmosphere is a versatile way to produce magnetic nanoparticles. The particles can be dispersed in polarized or non-polarized solvents by coating a single layer or double layer of relevant surfactant around NPs. The nanoparticles are suitable for biological applications.

A cknow ledgem ent

This work is financially supported by the Vietnam National Fundamental Research Program for Natural Sciences, project 406506.

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Soc. in press.

[6] P. Tartaj, M.d.p. Morales, s. Veintemilias- Verdaguer, T. Gonza!ez-Caưeno, C J. Sema, The preparation o f magnetic nanoparticles for applications in biomedicine, J. Phys. D: A ppl Phys. 36 (2003) R182.

[7] M.p. Pileni, Reverse micelles as microreactors, J, Phys. Chem. 97 (1993) 6961.

[8] B.D. Cullity, Elements o f X-Rray Diffraction, Addison-W esley Publishing, Reading, MA (1978).

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[9] L.P.R. Rios, Superpara- and param agnetic polym er colloids by miniemuision processes^

PhD thesis, Posdam University (2004).

[lOJ T.K. Jain, M .A. M orales, S.K . Sahoo, D.L.

L eslie-P eleck y, V. Labhasetwar, Iron O xide Nanoparticles for Sustained D eliv ery o f Anticancer Agents, M o l Pharm . 2 (20 05) 194.

9 > '

Anh hưởng của các điêu kiện chê tạo trong phương pháp nhũ

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tương lên tính chât của hạt nano Fe

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Nguyễn Thái Hà', Nguyễn Hoàng Hải', Trần Quốc Tuấn', Nguyễn Hoàng Lương', Nguyễn Châu', Huỳnh Đăng Chính^

' T ru n g tãm K h o a h ọ c Vậí liệu, K h o a Vật lý, T rư ờ n g Đ ạ i h ọ c K h o a h ọ c Tự nhiên, Đ ạ i h ọ c Q u ố c g i a H à N ội, 3 3 4 N g u yê n Trãi, H à N ộ i, Việt N a m

“ K h o a C ô n g n g h ệ H ó a học, T rư ờn g Đ ạ i h ọ c B á c h k h o a H à N ộ i ĩ Đ ạ i C ồ Việt, H à N ội, Việt N am

Hạt nano Fe^{3 0 4} đã được chế tạo bằng phương pháp nhũ tương sử dụng nước, hexane và chất hoạt hóa bề mặt Span 80. Phản ứng tạo hạt nano xảy ra trong môi trường không khí (với áp suất và nhiệt độ khí quyển và áp suất và nhiệt độ cao) và khí nitơ. Kích thước hạt nano từ ⁶ đến 20 nm. Hạt có tính siêu thuận từ với từ độ bão hòa đạt đến 50 emu/g. Việc chức năng hóa bề mặt kỵ nước được thực hiện nhờ olecic acid, chức năng hóa bề mặt ưa nước bàng lớp hoạt hóa bề mặt kép gồm oleic acid và sodium dodecyl sulfate. Thay đổi điều kiện chế tạo ảnh hường nhiều đến tính chất hạt nano. Hạt nano tạo bằng phương pháp này có những tính chất ưu việt so với phương pháp đồng kết tủa là hạt nhò, độ đồng nhất cao.