The influence of colloidal parameters on the specific power absorption of PAA-coated magnetite nanoparticles

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 Re e ences 1. Pankhu s QA, Connolly J, Jones SK, Dobson J: Applica ions o magne ic nanopa icles in biomedicine. J Phys D Appl Phys 2003, 36:R167. 2. Lau en S, Fo ge D, Po M, Roch A, Robic C, Vande Els L, Mulle RN: Magne ic i on oxide nanopa icles: syn hesis, s abiliza ion, ec o iza ion, physicochemical cha ac e iza ions, and biological applica ions. Chem Re 2008, 108:2064. 3. Hadjipanayis CG, Bonde MJ, Balak ishnan S, Wang X, Mao H, Hadjipanayis GC: Me allic I on Nanopa icles o MRI Con as Enhancemen and Local Hype he mia. 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J Magn Magn Ma e 2007, 311:228. 25. Fo in J-P, Gazeau F, Wilhelm C: In acellula hea ing o li ing cells h ough Néel elaxa ion o magne ic nanopa icles. Biophys Le 2008, 37:223. 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. Submi you manusc ip o a jou nal and bene i om: 7 Con enien online submission 7 Rigo ous pee e iew 7 Immedia e publica ion on accep ance 7 Open access: a icles eely a ailable online 7 High isibili y wi hin he i eld 7 Re aining he copy igh o you a icle Submi you nex manusc ip a 7 sp inge open.com 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 7 o 7