NANO EXPRESS Open Access
The in luence o colloidal pa ame e s on he
speci ic powe abso p ion o PAA-coa ed
magne i e nanopa icles
Yolanda Piñei o-Redondo
1
, Manuel Bañob e-López
1*
, I án Pa diñas-Blanco
2
, Ge a do Goya
3
,
M A u o López-Quin ela
1
and José Ri as
1
Abs ac
The sui abili y o magne ic nanopa icles (MNPs) o ac as hea nano-sou ces by applica ion o an al e na ing
magne ic ield has ecen ly been s udied due o hei p omising applica ions in biomedicine. The unde s anding o
he magne ic elaxa ion mechanism in biocompa ible nanopa icle sys ems is c ucial in o de o op imize he
magne ic p ope ies and maximize he speci ic abso p ion a e (SAR). Wi h his aim, he SAR o magne ic
dispe sions con aining supe pa amagne ic magne i e nanopa icles bio-coa ed wi h polyac ylic acid o an a e age
pa icle size o ≈10 nm has been e alua ed sepa a ely by changing colloidal pa ame e s such as he MNP
concen a ion and he iscosi y o he sol en . A ema kable dec ease o he SAR alues wi h inc easing pa icle
concen a ion and sol en iscosi y was ound. These beha iou s ha e been discussed on he basis o he
magne ic elaxa ion mechanisms in ol ed.
PACS: 80; 87; 87.85j
In oduc ion
Biocompa ible magne ic nanopa icles (MNPs) a e
inc easingly being used in many biomedical applica-
ions, such as magne ic esonance imaging, d ug deli -
e y, cell and issue a ge ing o hype he mia [1-3]. Fo
hype he mia he apy, nano echnology o e s a powe -
ul ool o he design o nanome e hea -gene a ing
sou ces, which can be ac i a ed emo ely by he appli-
ca ion o an ex e nal al e na ing magne ic ield (AMF).
The magne ic ene gy abso p ion o nanopa icle-con-
aining issues induces a localized hea ing ha allows a
a ge ed cell dea h a a c i ical empe a u e abo e 42
o 45°C. This empe a u e inc ease can be used o
selec i ely kill cance cells [4,5]. P e ious epo s show
ha he e ec i e use o MNPs o induce magne ic
hea ing by applica ion o an ex e nal adio- equency
magne ic ield depends essen ially on se e al ac o s
ela ed o he size, shape, sol en and magne ic p ope -
ies o nanopa icles [6-9]. O special in e es is he
hea ing powe a e ha can be a ained wi h MNPs
because an inc ease o he hea ing a e would imply
lowe doses o MNPs adminis e ed o he pa ien and
lowe ime o s ay in he body o he pa ien . Fo his
eason, i is necessa y o op imize he design o he
nanopa icles in o de o achie e he equi ed s uc-
u al and magne ic p ope ies which lead o he maxi-
mum hea ing powe .
Fo single-domain pa icles, which a e below he
supe pa amagne ic (SPM) size limi , no hea ing due o
hys e esis losses occu s. The e o e, he hea ing powe
a ises om he ene gy dissipa ed in he e e sible p o-
cess o elaxa ion o he magne ic momen s o hei
equilib ium o ien a ion once he magne ic ield is
emo ed. This mechanism is cha ac e ized by he Néel
elaxa ion p ocess. In addi ion o his, he o a ional
mo ion o he pa icles wi hin he sol en due o he
o que o ces on he magne ic momen , B ownian
elaxa ion, cons i u es ano he sou ce o hea ing, as a
consequence o he ene gy libe a ed by ic ion in he
eo ien a ion o he pa icle in he su ounding ca ie
liquid. The well-known Rosensweig equa ion [10] p e-
dic s he SAR o a magne ic nanopa icle exposed o a
* Co espondence: manuel.banob [email protected]
1
Applied Physics and Physical Chemis y Depa men s, Uni e si y o San iago
de Compos ela, San iago de Compos ela, 15782, Spain
Full lis o au ho in o ma ion is a ailable a he end o he a icle
Piñei o-Redondo e al.Nanoscale Resea ch Le e s 2011, 6:383
h p://www.nanoscale esle .com/con en /6/1/383
© 2011 Piñei o-Redondo e al; licensee Sp inge . This is an Open Access a icle dis ibu ed unde he e ms o he C ea i e Commons
A ibu ion License (h p://c ea i ecommons.o g/licenses/by/2.0), which pe mi s un es ic ed use, dis ibu ion, and ep oduc ion in
any medium, p o ided he o iginal wo k is p ope ly ci ed.
a ying magne ic ield as SAR = P/( F), whe e Pis he
dissipa ed powe hea :
P=πμ0H02
χ
(1)
in which he magne ic suscep ibili y c”con ains he
ac ion o bo h elaxa ion mechanisms:
χ =2π χ0τe
1+
(
ω2π τe
)
2
.
(2)
Th ough an e ec i e elaxa ion ime o he wo
mechanisms wo king in pa allel:
1
τ
e
=
1
τN
+
1
τB
,
(3)
whe e
τB=3ηVH
k
B
T(4)
is he B own elaxa ion ime depending on he sol en
iscosi y hand he hyd odynamic adius o he NP,
V
H=

1+ δ
R

34πR3
3, and
τN=τ0
exp KanVM/kBT
(
KanVM/kBT
)
(5)
is he Néel elaxa ion ime depending on he magne ic
olume o he NP,
V
M=4πR3
3
and K
an
is he magne ic
aniso opy ene gy cons an o he magne ic co e o he
NP.
The e o e, he hea dissipa ion o a magne ic
hype he mia expe imen pe o med on a e o luid will
depend on: (1) he applied magne ic ield s eng h and
equency and (2) he physical p ope ies o he e o-
luid: sol en iscosi y, magne ic and hyd odynamic
adiuso heNPs,and hemagne icaniso opyene gy
cons an o he magne ic co e o he NP.
Adequa ely coa ed i on oxide-based nanopa icles ha e
been he mos ex ensi ely s udied ma e ial in hype he -
mia expe imen s because hey ha e e y low oxici y,
making hem sui able o in i o applica ions [11,12]. In
pa icula , he polyac ylic acid (PAA) coa ing is an aqu-
eous soluble polyme wi h a high densi y o eac i e
unc ional g oups which make i e y a ac i e in bio-
medicine due mainly o i s capabili y o o m lexible
polyme chain-p o ein complexes ough elec os a ic,
hyd ogen bonding o hyd ophobic in e ac ions. Fu he -
mo e, he biochemical ac i i y o he p o ein is main-
ained in he esul ing p o ein-polyme complexes [13].
The e o e, he use o biocompa ible SPM nanopa i-
cles capable o esiding inside he human body o a ea-
sonable ime is highly desi able o biomedical
applica ions. The absence o coe ci e o ces and ema-
nence p e en s he magne ic in e ac ion be ween pa i-
cles and he o ma ion o pa icle agg ega es and small
clus e s [1].
Bo h mechanisms depend on pa icle size, whe eas
only he B ownian con ibu ion depends on he iscos-
i y, h,o heca ie sol en .Howe e ,al hough hesize
dependence o he hea ing powe has been al eady
in es iga ed and indica es he exis ence o an op imal
pa icle size in which he hea ing powe is maximum
[14], he ea enosys ema icda aon hein luenceo
pa icle concen a ion o sol en p ope ies in he same
magne ic sys em and in a simul aneous way. As deduced
om he Rosensweig equa ion and unde ce ain expe i-
men al condi ions, bo h Néel and B ownian elaxa ion
imes a e compa able o SPM nanopa icles a ound 10
nm; he e o e, changes in he pa icle concen a ion, sol-
en iscosi y o pa icle su ace modi ica ion could lead
o impo an di e ences in he SAR obse ed. To ou
bes knowledge, no hea ing p ope ies o PAA-modi ied
high quali y magne i e MNPs ha e been p e iously
epo ed. Such combina ion o he chemical ea u es
desc ibed abo e makes colloidal PAA-magne i e a p o-
mising sys em in ad anced bionano echnologies. Fo
his eason, da a abou i s hea ing p ope ies unde spe-
ci ic expe imen al condi ions, which could ep oduce
physiological condi ions in an in- i o expe imen , a e
highly desi ed.
Ou app oach in his esea ch includes he syn hesis
o di e en biocompa ible and monodispe se high qual-
i y single-domain magne i e NPs based e o luids and
has been ocused on he speci ic abso p ion a e (SAR)
dependence o ac o s ela ed o he pa icle concen a-
ion and sol en p ope ies, c ucial pa ame e s o he
biomedical applica ions in o de o p o ide he pa ien s
wi h an op imal dosage.
To ou knowledge, we p o ide o he i s ime use ul
in o ma ion in o de o co ec ly in e p e and design
PAA-coa ed magne i e based biomedical applica ions in
which he a ge issues may ha e di e en iscosi ies
and di e en capaci y o e ain low o high concen a-
ions o NP inside, yielding unexpec ed esul s.
Expe imen al
I on oxide MNPs we e ob ained in o de o s udy he
e ec o some colloidal pa ame e s on hei hype he -
mia p ope ies. Magne i e MNPs o ≈10 nm we e
syn hesized by chemical co-p ecipi a ion o an aqueous
solu ion con aining Fe
2+
(FeSO
4
·7H
2
O, 99%) and Fe
3+
(FeCl
3
·6H
2
O, 97%) sal s in he mola a io Fe
2+
/Fe
3+
=
0.67 wi h ammonium hyd oxide (NH
4
OH, 28%). To
ob ain Fe
3
O
4
@PAA MNPs, immedia ely a e magne i e
p ecipi a ion an excess o PAA (Mn = 1800) was added
o he solu ion. The PAA coa ing educes he
Piñei o-Redondo e al.Nanoscale Resea ch Le e s 2011, 6:383
h p://www.nanoscale esle .com/con en /6/1/383
Page 2 o 7
elec os a ic pa icle in e ac ions and he e o e g ea ly
inc eases he colloidal s abili y o he dispe sion. Finally,
he pH o he solu ion was adjus ed o pH = 10 by add-
ing e ame hylammonium hyd oxide (TMAOH) 10% in
o de o imp o e he s abili y o he e o luid as much
as possible.
Speci ic abso p ion a e o he samples was measu ed
by means o a home-made magne ic adio- equency
(RF) powe gene a o ope a ing a a ixed equency o ν
= 308 KHz and an induced magne ic ield o B=15
mT. A cylind ical Te lon sample holde was placed in
he midpoin o an e hylene glycol cooled hollow coil
(maximum o RF magne ic ield), inside a he mally iso-
la ed cylind ical Dewa glass unde high acuum condi-
ions (10
-6
mba ). Measu emen s we e ca ied ou by
placing 140 μL o e o luid in he sample holde and
eco ding he empe a u e inc ease e sus ime wi h a
ib e-op ic he mome e (Neop ix) du ing app ox. 5 min
o applied magne ic ield.
Resul s and discussion
The c ys alline phase o i on oxide nanopa icles was
iden i ied by powde X- ay di ac ion (XRD) using a
PHILIPHS di ac ome e wi h Cu Ka adia ion l=
1.5406 Å. The posi ion and ela i e in ensi ies o he
e lec ion peaks con i m he p esence o a magne i e/
maghemi e phase wi h espinel s uc u e (JCPDS 19-
0629). The c ys alli e size, d
(hkl)
, was calcula ed om he
b oadening (FWHM) o he (311) e lec ion ollowing
he Debye-Sche e equa ion, esul ing o be d
(311)
≈12
nm. I is impo an o ema k ha he absence o ex a
e lec ions indica ed ha no o he i on oxides as sec-
onda y phases a e p esen .
The a achmen o he polyme o he magne i e pa i-
cle su ace was con i med by a - ansmission-in a- ed
(FTIR) spec oscopy using a The mo Scien i ic-Nicole
6700 spec oscope. D ied powde samples we e measu ed
di ec ly using he a enua ed o al e lec ance (ATR)
op ion. The cha ac e is ic abso p ion equencies o PAA
ela ed o he ib a ional modes o he ee ca bonyl
g oups we e iden i ied in he PAA spec um: C = O
s e ch a 1709 cm
-1
, C-O-H in-plane de o ma ion a
1452 and 1415 cm
-1
and C-O s e ch a 1250 cm
-1
.The
posi ion o hese IR bands is in good ag eemen wi h p e-
ious expe imen al epo ed da a [15]. A e eac ion
be ween he PAA and magne i e NPs, a d as ic in ensi y
dec eases o he C = O s e ching peak a 1709 cm
-1
was
obse ed. This s ong in ensi y dec ease o he C = O
s e ching peak and he appea ance o new bands a 1547
and 1404 cm
-1
, which a e due o he asymme ic and
symme ic s e ching o he COO
-
ca boxyla e g oup,
espec i ely, sugges s ha an e icien a achmen
be ween he polyme and he pa icle su ace has aken
place h ough he ca bonyl g oup. By examining he e-
quency sepa a ion be ween he symme ic and he asym-
me ic COO
-
s e ching ib a ions, Δν≈150 cm
-1
,and
aken in o accoun he c i e ia es ablished by Deacon and
Phillips [16], he ca boxyla e g oup ha e been ound o
ac as b idging complex. On he o he hand, om he -
mog a ime ic analysis (Pe kin Elme TGA 7 analyze )
he amoun o PAA co e ing he magne i e nanopa icles
was ound o be 25% o he o al mass. Taking hese
esul s in o accoun , he es ima ed polyme shell hick-
ness su ounding he magne i e NPs was a ound 1 nm.
Mo phology and c ys al s uc u e o PAA-coa ed mag-
ne i e nanopa icles we e cha ac e ized by ansmission
elec on mic oscopy (TEM) and scanning ansmission
elec on mic oscopy (STEM) echniques using a PHI-
LIPS CM-12 (100 kV) and a Hi achi S-5500 (30 kV)
mic oscopes, espec i ely. Figu e 1 (le ) shows he
Figu e 1 (Le ) TEM image o Fe
3
O
4
@PAA NPs. Inse shows a b illian ield HR-STEM image o a single Fe
3
O
4
@PAA pa icle. (Righ ) His og am
co esponding o he Fe
3
O
4
@PAA NPs.
Piñei o-Redondo e al.Nanoscale Resea ch Le e s 2011, 6:383
h p://www.nanoscale esle .com/con en /6/1/383
Page 3 o 7
uni o m pseudo sphe ical shape o magne i e@PAA
MNPs. The a e age pa icle size and dis ibu ion is
shown in he co esponding his og am on he igh and
esul ed o be highly monodispe se wi h d=9±2nm
(85% o he o al amoun o pa icles), in good ag ee-
men wi h he c ys alline domain size calcula ed om
XRD esul s. Inse o Figu e 1 (le ) shows a ep esen a-
i e high- esolu ion (HR) b illian ield (BF) STEM
mic og apho asinglepa icle egion,showinghigh
c ys allini y and he s uc u al homogenei y o he pa i-
cles. The long ange domain s uc u e and he absence
o mul i-domains sugges ha hese nanopa icles can
be conside ed as small single c ys als. I is also e i-
denced ha he PAA coa ing p e en s he o ma ion o
agg ega es, since hey a e ac ually well sepa a ed om
each o he (as deduced om he dis ance be ween he
whole pa icle in he middle o he pic u e and he su -
ounding ones shown a he edges).
Figu e 2 shows he magne iza ion cu es as a unc ion
o he applied magne ic ield up o 2 T o PAA-coa ed
magne i e NPs pe o med in a supe conduc ing quan-
um in e e ence de ice (SQUID) magne ome e . A clea
SPM beha iou is obse ed whe e coe ci e o ces and
emanence a e elusi e. This is in good conco dance
wi h he XRD and TEM/STEM esul s which e idenced
ha magne i e co es a e wi hin he size egion below
he single- o mul i-domain limi , in which FM pa icles
show a SPM-like beha iou . Magne iza ion o sa u a ion,
M
s
, is abou 60 emu g
-1
a oom empe a u e. Howe e ,
a e co ec ion o he magne ic da a by sub ac ing he
non-magne ic mass co esponding o he PAA shell
( ha ep esen s a 25% o he o al mass, as deduced
om he he mal analysis), he sa u a ion inc eases
again un il 80 emu g
-1
, which is e y close o he bulk
magne iza ion o magne i e (90 emu g
-1
). This indica es
ha he in insic magne ic p ope ies o he magne i e
nuclei ha e no been a ec ed by he coa ing.
Magne ic hype he mia esul s
The SAR o magne ic hype he mia expe imen s has
been calcula ed by using [14]
S
AR = 
T

cliqρliq

,
(6)
whe e c
liq
and
liq
is hespeci ichea capaci yand
densi y o he liquid, espec i ely, and F he weigh con-
cen a ion o he MNPs in hecolloid.Bype o minga
linea i o he hype he mia da a ( empe a u e e sus
ime) in he ini ial ime in e al, = [1-10] s, we ob ain
he expe imen al alue o T

. In his way, he SAR can
be calcula ed using Equa ion 6, since all he emaining
pa ame e s a e known.
Concen a ion e ec s
When he concen a ion o a e o luid is inc eased, he
i s ob ious consequence is ha he mean in e -pa icle
dis ance is educed. I he sys em is u he exposed o
an ex e nal RF magne ic ield ha magne izes he SPM
nanopa icles, magne ic dipola in e ac ion will become
ele an and con ibu e o he magne ic p ope ies o
he e o luid. Since some con o e sies exis s in heo e-
ical s udies abou he in luence o he dipola in e ac-
ion on he in insic magne ic p ope ies o he MNPs
[17], expe imen al measu emen s showing concen a ion
e ec s on SAR p ope ies o MNPs will help o elucida e
he ques ion.
In o de o s udy he e ec o he magne i e concen-
a ion on he hype he mia p ope ies o aqueous e o-
luids and o achie e an e icien empe a u e inc ease
in he samples, we p epa ed wo se ies o aqueous
Fe
3
O
4
and Fe
3
O
4
@PAA NPs based dispe sions a di e -
en magne i e concen a ions, anging om 0.6 o 20 g
L
-1
. Figu e 3 shows he e olu ion o he SAR wi h mag-
ne i e concen a ion. The e olu ion o he SAR coe i-
cien e eals ha he hea p oduc ion e iciency
dec eases wi h magne i e concen a ion o Fe
3
O
4
@PAA
NPs, while a di e en beha iou is obse ed o ba e
Fe
3
O
4
NPs. We associa e his beha iou o he in e -
pa icle dipole-dipole in e ac ions, which a e p opo -
ional o he pa icle concen a ion in he ca ie luid.
Fo Fe
3
O
4
@PAA NPs, as he pa icle concen a ion
inc eases, pa icles ge close o each o he inc easing
hei dipola magne ic momen in e ac ion in p esence
o a RF ex e nal magne ic ield. The ene gy dissipa ion
mechanism di ec ly in ol ed and s ongly dependen on
he dipole-dipole in e ac ion is he Néel elaxa ion ime,
since B ownian elaxa ion is much less sensi i e o he
concen a ion o magne i e momen s because he in e -
Figu e 2 Magne iza ion cu es as a unc ion o he applied
magne ic ield up o 2 T o Fe
3
O
4
@PAA NPs a oom
empe a u e.
Piñei o-Redondo e al.Nanoscale Resea ch Le e s 2011, 6:383
h p://www.nanoscale esle .com/con en /6/1/383
Page 4 o 7
pa icle o ceismainlyhyd odynamicinna u e[18].
The highe dipola in e ac ions he longe Néel elaxa-
ion imes. The e o e, his long- ange collec i e mag-
ne ic beha iou a inc easing pa icle concen a ions
appea s o play a majo ole in dec easing he SAR. In
con as , a e y low pa icle concen a ions he pa icles
a e mo e isola ed om each o he . In his scena io, he
in e -pa icle dipola in e ac ion dec eases d ama ically
wi h dis ance, ∝1/
6
, and he e iciency o powe dissipa-
ion o he medium is highly op imized. Al hough simi-
la esul s ha e been p e iously epo ed in he
li e a u e in o he magne ic sys ems, he e a e ew
wo ks dealing wi h he e ec s o magne ic in e ac ions
on SAR, being mos ly no compa able o con o e sial:
U izbe ea e al. [19] showed a SAR inc ease wi h dilu-
ion o ≈11 nm maghemi a nanopa icles based e o-
luids, al hough he s udy was ca ied ou h ough AC
suscep ibili y measu emen s pe o med below ≈100 kHz;
and while [20] epo ed a highe SAR o igh ly asso-
cia ed dex an-coa ed i on oxide nanopa icles (d≈90
nm) han o a mo e loosely associa ed ones, in [9], no
concen a ion e ec s we e de ec ed. Figu e 3 includes
expe imen al da a om Linh e al. [21] o ela i ely
compa able colloidal magne i e based e o luid. A simi-
la SAR dependence o he pa icle concen a ion is
obse ed, al hough di e ences in he absolu e alues
could de i ed om he sligh ly di e en pa icle size,
pa icle dis ibu ion, coa ing agen o expe imen al con-
di ions o equency and applied magne ic ield. I is
impo an o men ion ha such a simila concen a ion
hea ing e iciency was also obse ed in a di e en han
magne i e sys em based on Ni-Zn e i e nanopa icles
dispe sed in a shape memo y polyme [22].
In he opposi e, he SAR beha iou o ba e Fe
3
O
4
NPs
is comple ely di e en . F om he ob ained esul s, we
deduce ha he di e ences obse ed in he SAR depen-
dence o he pa icle concen a ion be ween he ba e
and PAA coa ed pa icles can be a ibu ed o he ac i e
ole played by he PAA shell. The PAA coa ing no only
s abilizes he SPM nanopa icles in he aqueous medium
media ing he in e -pa icle dipola in e ac ion (di ec ly
ela ed o he Néel elaxa ion ime), bu also changes
he hyd odynamic adius o he pa icles and modi y he
B ownian elaxa ion ime by ic ion o he nanopa icle
su ace in he ca ie luid. In he case o ba e magne i e
nanopa icles, signi ican dipola in e ac ions a e s ill
p esen a low pa icle concen a ions, while agg ega ion
phenomena and clus e o ma ion occu s a high pa i-
cle concen a ions. Howe e , u he wo k is needed in
o de o add ess in mo e de ail his issue. A simila
beha iou has been also epo ed by Ve ges e al. [23]
o highe magne i e pa icle sizes, al hough he SAR
alues a e signi ican ly lowe .
Sol en iscosi y e ec
In o de o e alua e sepa a ely he B ownian con ibu-
ion o he gene al hype he mia mechanism in SPM
magne i e nanopa icles, he hea ing p ope ies o mag-
ne ic dispe sions a a ixed pa icle concen a ion ha e
been e alua ed as a unc ion o he sol en iscosi y, h,
which is di ec ly ela ed o he B ownian elaxa ion
h ough Equa ion 4. In he p esence o an AMF, he
MNP will o a e ying o align i s magne ic dipola
momen o he di ec ion o he magne ic ield. The ic-
ion o he pa icle wi h he sol en will gene a e hea
and his mechanism is known as B ownian elaxa ion. I
con ibu es o he o al hea ing in compe ence wi h he
Néel elaxa ion, in which he magne ic momen o he
pa icle eo ien s in e nally wi hou he physical o a ion
o he pa icle. B own elaxa ion ime inc eases wi h NP
size and sol en iscosi y gi ing ise o an inc ease in
SAR alues. Howe e , when τ
B
becomes oo much high
τ
e
=τ
Néel
and only Néel elaxa ion con ibu es o he
hea dissipa ion mechanism. The e o e, o e y iscous
sol en s, he B ownian con ibu ion is blocked and only
Néel elaxa ion con ibu es, dec easing he SAR.
Figu e 4 shows he e olu ion o SAR o PAA-coa ed
magne i e e o luids wi h iscosi y. Di e en alues o
iscosi y anging om 1 o 90 mPa s we e achie ed by
using di e en sol en s (wa e , e hylene glycol, 1-2-
p opanediol and poly-e hylene glycol). I is impo an
o men ion ha he magne i e concen a ion was kep
cons an in all he samples, which showed a e y good
s abili y o all he sol en s used. The e ec o chan-
ging he sol en iscosi y e eals ha B ownian elaxa-
ion con ibu ion is also signi ican in small SPM
nanopa icles. A sligh SAR inc ease om 36.5 o 37.3
Wg
-1
akes place as he sol en iscosi y inc eases
Figu e 3 E olu ion o he speci ic abso p ion a e (SAR) o
aqueous Fe
3
O
4
@PAA NPs dispe sions a se e al concen a ions
be ween 0.6 and 20 g L
-1
unde an applied AC magne ic ield
o B= 15 mT and ν= 308 kHz. Solid line is a guide o he eye.
Piñei o-Redondo e al.Nanoscale Resea ch Le e s 2011, 6:383
h p://www.nanoscale esle .com/con en /6/1/383
Page 5 o 7

om h= 1 mP s (wa e ) o h= 17 mP s (e hylene gly-
col). Howe e , he use o sol en s o highe iscosi ies
causes signi ican SAR dec eases. This endency ag ees
wi h heo e ical p edic ions [10] and expe imen al
esul s ound in dex an-coa ed magne i e e o luids,
whe e a maximum SAR is obse ed in he in e al o 1
<h<3mPs[24].
The maximum o hea dissipa ion occu s o Equa ion
1 when he ma hema ical condi ion 2π τ
e
= 1 is ul illed
[6]. The e o e, conce ning iscosi y, he maximum will
be obse ed o a ce ain alue:
η∼
(
kBTτN
)
/
(
3VH
(
2π τN−1
)).
(7)
I one changes expe imen al condi ions in ol ed in
Equa ion 7 (pa icle size, s eng h and equency o
applied magne ic ield, coa ing agen o magne ic ma e-
ial o he NPs), he loca ion, heigh and wid h o he
maximum o hea dissipa ion cu e can change comple-
ely, gi ing ise o a a ie y o magne ic SAR ela ion-
ships wi h iscosi y. This explains why in li e a u e one
can ind di e en beha iou s o SAR wi h iscosi y: a
iscosi y independen cu e, a decaying one o e en an
inc easing one, jus only by a ying he pa icle sizes
and composi ion [25]. Also a Lo en zian cu e, wi h a
maximum loca ed a ce ain alues o iscosi y, has been
epo ed [24].
In his sense, he maximum o ou SAR cu e is
ob ained o a highe iscosi y alue han [19] because
he chemical/physical cha ac e is ics o ou MNPs (size,
mo phology, coa ing, e c.) and he expe imen al condi-
ions o he applied RF magne ic ield a e di e en .
Conclusions
Biocompa ible PAA-coa ed magne i e based e o luids
con aining SPM nanopa icles o ≈10 nm ha e been
chemically syn hesized. The in luence o se e al colloidal
pa ame e s on he speci ic powe abso p ion o hese
magne ic dispe sions has been s udied. Pa icle concen-
a ion dependence o SAR has been mainly obse ed a
low magne i e concen a ions and a maximum in he
SAR has been sugges ed as a unc ion o he sol en
iscosi y a ound 22 mPa s.
Abb e ia ions
ATR: a enua ed ansmission e lec ance; BF: b illian ield; FTIR: a
ansmission in a- ed; HR: high esolu ion; MNPs: magne ic nanopa icles;
Ms: magne iza ion o sa u a ion; PAA: poly(ac ylic acid); RF: adio equency;
SAR: speci ic abso p ion a e; SPA: speci ic powe abso p ion; SPM:
supe pa amagne ic; STEM: scanning ansmission elec on mic oscopy;
SQUID: supe conduc ing quan um in e e ence de ice; TEM: ansmission
elec on mic oscopy; TMAOH: e ame hylammonium hyd oxide; XRD: X- ay
di ac ion.
Acknowledgemen s
This wo k is suppo ed by he Eu opean Communi y’s unde he FP7-
Coope a ion P og amme h ough he MAGISTER p ojec ‘Magne ic Sca olds
o in i o Tissue Enginee ing’La ge Collabo a i e P ojec FP7 - 21468.
h p://www.magis e -p ojec .eu/
Au ho de ails
1
Applied Physics and Physical Chemis y Depa men s, Uni e si y o San iago
de Compos ela, San iago de Compos ela, 15782, Spain
2
R&D Depa men ,
Nanogap Subnmpowde SA, Milladoi o, Ames, A Co uña, 15985, Spain
3
Ins i u o de Nanociencia de A agón and Condensed Ma e Physics
Depa men , Uni e si y o Za agoza, Za agoza, 50018, Spain
Au ho s’con ibu ions
YP-R ca ied ou he hype he mia/SAR measu emen s, pa icipa ed in he
discussion and helped o d a he manusc ip . MB-L pa icipa ed in he
design o he s udy, in he syn hesis and chemical/physical cha ac e iza ion
o he samples, in he discussion and d a ed he manusc ip . IP-B
pa icipa ed in he syn hesis and chemical cha ac e iza ion o he samples.
GG was in ol ed in he design and ab ica ion o he hype he mia
equipmen , pa icipa ed in he discussion and e ised he manusc ip . MAL-
Q pa icipa ed in he discussion and e ised he manusc ip . JR pa icipa ed
in i s design, coo dina ion and e ised he manusc ip . All he au ho s ead
and app o ed he inal manusc ip .
Compe ing in e es s
The au ho s decla e ha hey ha e no compe ing in e es s.
Recei ed: 5 No embe 2010 Accep ed: 16 May 2011
Published: 16 May 2011
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doi:10.1186/1556-276X-6-383
Ci e his a icle as: Piñei o-Redondo e al.: The in luence o colloidal
pa ame e s on he speci ic powe abso p ion o PAA-coa ed magne i e
nanopa icles. Nanoscale Resea ch Le e s 2011 6:383.
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