Modifying the magnetic response of magnetotactic bacteria: incorporation of Gd and Tb ions into the magnetosome structure

sc.li/nanoscale-ad ances
Volume 4
Numbe 12
21 June 2022
Pages 2525–2764
ISSN 2516-0230
PAPER
E. M. Je emo as, J. Alonso e al.
Modi ying he magne ic esponse o magne o ac ic
bac e ia: inco po a ion o Gd and Tb ions in o he
magne osome s uc u e
Nanoscale
Ad ances
Modi ying he magne ic esponse o magne o ac ic
bac e ia: inco po a ion o Gd and Tb ions in o he
magne osome s uc u e†
E. M. Je emo as, *
a
L. Ganda ias,
b
L. Ma cano,
cd
A. Gac´
ıa-P ie o,
e
I. O ue,
A. Muela,
bg
M. L. Fdez-Gubieda,
cg
L. Fe n´
andez Ba qu´
ın
a
and J. Alonso*
a
Magne o ac ic bac e ia Magne ospi illum g yphiswaldense MSR-1 biosyn hesise chains o cube–oc ahed al
magne osomes, which a e 40 nm magne i e high quali y (Fe
3
O
4
) nanopa icles. The magne ic p ope ies o
hese c ys alline magne i e nanopa icles, which can be modified by he addi ion o o he elemen s in o he
magne osome s uc u e (doping), a e o p ime in e es in a ple ho a o applica ions, hose ela ed o cance
he apy being some o he mos p omising ones. Al hough p e ious s udies ha e ocused on ansi ion
me al elemen s, a e ea h (RE) elemen s a e e y in e es ing as doping agen s, bo h om a undamen al
poin o iew (e.g. significan diffe ences in ionic sizes) and o he po en ial applica ions, especially in
biomedicine (e.g. magne ic esonance imaging and luminescence). In his wo k, we ha e in es iga ed he
impac o Gd and Tb on he magne ic p ope ies o magne osomes by using diffe en complemen a y
echniques. X- ay diff ac ion, ansmission elec on mic oscopy, and X- ay abso p ion nea edge
spec oscopy analyses ha e e ealed ha a small amoun o RE ions, 3–4%, inco po a e in o he Fe
3
O
4
s uc u e as Gd
3+
and Tb
3+
ions. The expe imen al magne ic cha ac e isa ion has shown a clea Ve wey
ansi ion o he RE-doped bac e ia, loca ed a T100 K, which is sligh ly below he one co esponding
o he undoped ones (106 K). Howe e , we epo a dec ease in he coe ci i y and emanence o he RE-
doped bac e ia. Simula ions based on he S one –Wohl a h model ha e allowed us o associa e hese
changes in he magne ic esponse wi h a educ ion o he magne oc ys alline (K
C
) and, especially, he
uniaxial (K
uni
) aniso opies below he Ve wey ansi ion. In his way, K
uni
eaches a alue o 23 and 26 kJ
m
3
o he Gd- and Tb-doped bac e ia, espec i ely, whils a alue o 37 kJ m
3
is ob ained o he
undoped bac e ia.
1 In oduc ion
Magne osomes a e memb ane-enclosed single-domain
magne ic nanopa icles made o magne i e (Fe
3
O
4
) o g eigi e
(Fe
3
S
4
) syn hesised by magne o ac ic bac e ia (MTB). These
magne osomes a e a anged in one o se e al chains inside he
MTB, which allow he MTB o o ien in wa e by means o he
o que exe ed by he Ea h’s magne ic eld on he chain.
1
The
size and shape o magne osomes s ongly depend on he MTB
species, hei sizes gene ally being comp ised be ween 35 and
120 nm, and hey can be shaped as cube–oc ahed al, hexagonal
p isms, a ows, e c.
2
The po en ial ans e o magne osomes
owa ds biomedical applica ions has boos ed he in e es o
hese biosyn hesised magne ic nanopa icles in he ecen
yea s. Thei high pu i y and c ys allini y ( e e ed o as high
quali y om he eunde ), na ow size dis ibu ion, good
biocompa ibili y, and ela i ely easy unc ionaliza ion ha e
made magne osomes p omising candida es as he anos ic
agen s o magne ic hype he mia, d ug deli e y, and magne ic
esonance imaging (MRI), among o he applica ions.
3–10
a
Dp o. CITIMAC, Uni e sidad de Can ab ia, 39005 San ande , Spain. E-mail:
[email p o ec ed]; ja ie .alon[email p o ec ed]
b
Dp o. Inmunolog´
ıa, Mic obiolog´
ıa y Pa asi olog´
ıa, Uni e sidad del Pa´
ıs Vasco (UPV/
EHU), 48940 Leioa, Spain
c
Helmhol z–Zen um Be lin ¨
u Ma e ialien und Ene gie, Albe -Eins ein-S . 15, 12489
Be lin, Ge many
d
Dp o. Elec icidad y Elec ´
onica, Uni e sidad del Pa´
ıs Vasco (UPV/EHU), 48940 Leioa,
Spain
e
Dp o. F´
ısica Aplicada, Uni e sidad del Pa´
ıs Vasco (UPV/EHU), 48013 Bilbao, Spain
SGIke Medidas Magn´
e icas, Uni e sidad del Pa´
ıs Vasco (UPV/EHU), 48940 Leioa,
Spain
g
BCMa e ials, Basque Cen e o Ma e ials, Applica ions and Nanos uc u es, UPV/
EHU, Spain
†Elec onic supplemen a y in o ma ion (ESI) a ailable: Fig. S1 and S2: Collec ion
o TEM images, whe e he appea ance o R sal s a ached o he bac e ial body and
pa icula i ies o he magne osome chains can be inspec ed. Fig. S3 includes he
ZFC/FC M s. H cu es measu ed a diffe en empe a u es o he RE–doped MTB.
Fig. S4 offe s a compa ison be ween he he mal e olu ion o DH
C
and jDM
/M
s
jo
he undoped, Gd [100 : 100], Tb [100 : 100], Mn [480 : 100] and Mn [100 : 100]
bac e ial samples. See h ps://doi.o g/10.1039/d2na00094
Ci e his: Nanoscale Ad ., 2022, 4,
2649
Recei ed 9 h Feb ua y 2022
Accep ed 1s Ap il 2022
DOI: 10.1039/d2na00094
sc.li/nanoscale-ad ances
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Fu he mo e, magne osomes ha e also been conside ed as
eliable models o in es iga e he ela ionship be ween he
s uc u al and magne ic p ope ies o magne i e a he nano-
scale,
11–14
a ma e o deba e ha has a ac ed g ea in e es o
many yea s in he scien ic communi y. To his espec , mag-
ne osomes can be employed o in es iga e diffe en ele an
issues, including he su i al o he Ve wey ansi ion o
magne i e a he nanoscale o he ole o shape aniso opy in
ace ed nanopa icles.
15–17
Ne e heless, despi e hese e y p omising ea u es, mag-
ne osomes p esen some d awbacks, especially when compa ed
wi h hei chemically syn hesised coun e pa s. These include,
o example, he es ic ed unabili y o hei shape, size and
chemical composi ion, as hese ea u es a e s ic ly gene ically
de e mined.
18,19
These es ic ions cons i u e a nuisance when
ying o modi y he magne ic esponse o magne osomes o
diffe en applica ions.
20–23
Howe e , al e na i e ou es ha e
been de ised o o e come some o hese limi a ions. MTB
exhibi a high affini y and specici y owa ds i on, which hey
ex ac om he medium in o de o syn hesise magne osomes.
In he same way, i has been demons a ed ha MTB can also
syn hesise magne osomes doped wi h some ansi ion me als
such as manganese, i anium, coppe , o cobal ,
23–28
by adding
limi ed amoun s o hese me als o he g ow h medium. The e
a e howe e e y ew s udies desc ibing he inco po a ion o
o he elemen s,
29
which unde lines he inhe en complexi y
associa ed wi h he doping p ocess.
Among all he possible doping candida es, he inco po a ion
o Ra e Ea h (RE) ions in o magne osomes would be consid-
e ably appealing. RE doping opens he doo o modi ying he
in e nal s uc u e and he magne ic p ope ies o he nano-
pa icles, bo h being accomplished a he same ime. Mo eo e ,
RE elemen s a e cu en ly used in se e al op-no ch elds, such
as biomedicine, ca alysis, and/o sola cells.
30,31
The ascina ion
owa ds RE does no s op a hei po en ial biomedical and
echnological ans e , ye he e is also oom o he eme gence
o new magne ic phenomena. In his way, om a undamen al
poin o iew, he la ge unquenched o bi al angula momen um
and high spin–o bi coupling o he 4 elec ons in some RE
ions can gi e ise o mo e p onounced magne ic ea u es in
compa ison o ansi ion me al ions.
32,33
The e o e, he e is also
a g ea po en ial o in es iga ion on RE-doped magne osomes,
apa om he ones doped wi h ansi ion me als. To he bes o
ou knowledge, he only wo k ha has been published in his
a ea is he one by Shimoshige e al.
34
Specically, hey doped
Magne ospi illum magne icum RSS-1 wi h Sm ions, ob aining
co e/shell magne osomes made o magne i e in he co e and
sama ium oxide in he shell.
In ou wo k, we ha e been able o inco po a e, o he  s
ime, Gd
3+
and Tb
3+
ions in o magne osomes om he Magne-
ospi illum g yphiswaldense s ain MSR-1. Gd
3+
is a S-s a e ion
(L¼0) wi h se en unpai ed elec ons, which has been in es i-
ga ed, among o he hings, o de elop gadolinium-doped i on
oxide nanopa icles exhibi ing a T
1
–T
2
dual-model MRI
con as .
35,36
On he o he hand, Tb
3+
is an ion wi h six unpai ed
elec ons, which has a ac ed a en ion o he possibili y o
p o iding magne i e nanopa icles wi h luminescence
p ope ies, which can be use ul o moni o ing he nano-
pa icles wi hin he con ex o se e al biomedical applica-
ions.
36,37
Fu he mo e, he inco po a ion o Gd and Tb ions in o
he magne i e s uc u e has also a ac ed a en ion due o he
modula ion o he magne ic p ope ies o magne i e when he
la ge Gd
3+
and Tb
3+
ions a e inco po a ed in o i s in e se
spinel s uc u e.
38
Bea ing all hese conside a ions in mind, we p esen he e
a combina ion o expe imen al and heo e ical esul s o
in es iga e he ole o Gd
3+
and Tb
3+
ca ions in he magne ic
esponse o magne osomes. The mo phological and s uc u al
p ope ies o hese RE-doped magne osomes ha e been s udied
by ansmission elec on mic oscopy (TEM) and X- ay diff ac-
ion (XRD). The inco po a ion o he RE ions in o he magne-
osome s uc u e has been in es iga ed by X- ay abso p ion nea
edge spec oscopy (XANES) expe imen s, ca ied ou in la ge
scale Synch o on acili ies. In addi ion, he magne ic esponse
o hese doped magne osomes has been ho oughly analysed,
and compa ed wi h undoped magne osomes, by using diffe en
expe imen al magne ic measu emen s, including ze o-eld
cooling/eld-cooling (ZFC/FC) cu es and hys e esis loops (M
s. H). Finally, a modied S one –Wohl a h model has been
employed o simula e he expe imen al M s. H loops. This has
allowed us o pinpoin he specic magne ic changes aking
place, and o ela e hese changes o he in insic modica ion
o he effec i e aniso opies o hese Gd- and Tb-doped
magne osomes.
2 Ma e ials and me hods
2.1 Magne o ac ic bac e ia: cul u e and magne osome
isola ion
Magne ospi illum g yphiswaldense MSR-1 (DMSZ 6631) was
g own wi hou shaking a 28 C in Flask S anda d Medium
(FSM) (Heyen and Sch¨
ule
39
) con aining (pe li e o deionized
wa e ) 0.1 g KH
2
PO
4
, 0.15 g MgSO
4
$7H
2
O, 2.38 g HEPES, 0.34 g
NaNO
3
, 0.1 g yeas ex ac , 3 g soybean pep one, 0.3% (w / ol) o
sodium py u a e as he ca bon sou ce and 100 mM o Fe(III)-
ci a e. Fo gadolinium and e bium doping o bac e ia, 100 mM
o Gd(III)-quina e and Tb(III)-quina e we e added, espec i ely.
Bac e ia we e g own in 100 mL bo les lled wi h 80 mL o
he cul u e media o ob ain he desi ed oxygen concen a ion
condi ions. The inocula ion in FSM con aining 100 mM o Gd/
Tb-quina e is made om a 48 hou cul u e g own in FSM in
a 1/10 dilu ion. In o de o ensu e ha he bac e ia a e in
con ac wi h he dopan long enough, wo subcul u es o 48
hou s a e made in FSM con aining 100 mM o Gd/Tb-quina e.
The ep oducibili y was assu ed wi h mo e han 10 eplica es
o he RE doped bac e ia ob ained a diffe en imes since he
beginning o he s udies.
Two diffe en samples we e employed in he subsequen
expe imen al measu emen s: whole cells and isola ed magne-
osomes om he bac e ia. Fi s , he whole cell samples we e
ha es ed by cen i uga ion, xed in 2% glu a aldehyde, and
washed h ee imes in Milli Q wa e . Second, he isola ion o
magne osomes was pe o med ollowing he p o ocol desc ibed
by G ¨
unbe g e al.
30
wi h mino modica ions. The cells,
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suspended in 20 mM HEPES–4 mM EDTA (pH ¼7.4), we e
dis up ed using a F ench p ess (P¼1.4 kba ). To p omo e he
sepa a ion o he magne osomes, he lysa ed cells we e soni-
ca ed and cen i uged a 600 g o 5 min o emo e cell deb is.
Then, magne ic sepa a ion was employed o collec he mag-
ne osomes om he supe na an , and ae wa ds, hey we e
insed 10 imes wi h 10 mM Hepes–200 mM NaCl (pH ¼7.4).
2.2 T ansmission elec on mic oscopy
T ansmission elec on mic oscopy (TEM) was ca ied ou on
bo h uns ained whole bac e ia (i.e., whole cells) and he isola ed
magne osomes ex ac ed om he bac e ia. In bo h cases,
samples we e adso bed on o 300 mesh ca bon-coa ed coppe
g ids. The images we e ob ained wi h a JEOL JEM-14000 Plus
elec on mic oscope a an accele a ing ol age o 120 kV. The
pa icle size dis ibu ion was analysed by using ImageJ so-
wa e.
40
Mo e han 130 magne osomes om diffe en cells we e
measu ed one-by-one in o de o ensu e good s a is ics.
2.3 X- ay diff ac ion
X-Ray diff ac ion (XRD) measu emen s we e pe o med on Gd-
and Tb-doped whole bac e ia (whole cells) using a B uke D8
Ad ance diff ac ome e wo king in B agg–Ben ano geome y
wi h Cu-K
a
(l¼1.5418 ˚
A) adia ion. The selec ed ange o he
2qB agg angle was 18 o 95, wi h an angula s ep o 0.02a
a coun ing a e o 1 second/s ep. The ob ained XRD pa e ns
we e analysed using Rie eld enemen s. All he measu e-
men s we e ca ied ou in he whole cells in o de o minimise
possible oxida ion o he magne osomes ae ex ac ion.
2.4 X- ay abso p ion nea edge spec oscopy
X- ay abso p ion nea edge spec oscopy (XANES) was pe -
o med on Gd- and Tb-doped magne osomes ex ac ed om he
bac e ia a bo h he Fe–K and RE-L
3
edges (7112 eV o Fe–K,
7514 eV o Tb-L
3
, and 7243 eV o Gd-L
3
). Measu emen s we e
ca ied ou a he CLAESS beamline o he ALBA synch o on a
oom empe a u e. Fe K-edge and Tb L
3
-edge measu emen s
we e ca ied ou in ansmission mode, and Gd L
3
-edge
measu emen s we e ca ied ou in uo escence mode. In all
cases, he measu emen s we e pe o med using a double Si
c ys al monoch oma o o ien ed in he [111] di ec ion.
2.5 Magne ic measu emen s
The magne ic cha ac e isa ion was ca ied ou on he whole
bac e ia (whole cells). The samples we e eeze-d ied and
encapsula ed in gela in capsules. Magne ic measu emen s we e
pe o med in a supe conduc ing quan um in e e ence de ice
magne ome e (Quan um Design MPMS-5). Magne isa ion s.
empe a u e (M s. T) cu es we e measu ed ollowing he usual
ze o-eld-cooling/eld-cooling (ZFC/FC) p o ocol, wi h an
applied magne ic eld o 5 mT. Magne isa ion s. magne ic eld
(M s. H) loops we e measu ed a diffe en empe a u es, 10–300
K, applying magne ic elds up o 1 T.
3 Resul s and discussion
3.1 S uc u al cha ac e isa ion
T ansmission elec on mic oscopy (TEM) was employed o
s udy he size, shape, and a angemen o he RE doped mag-
ne osomes. Fig. 1(a)–(c) show ep esen a i e images o he
magne osomes ex ac ed om he bac e ia co esponding o
he undoped, Gd-, and Tb-doped bac e ia, espec i ely. We ha e
included in Fig. S1 and S2 o he ESI†addi ional TEM images o
he MTB and hei magne osome chains. In bo h Gd- and Tb-
doped bac e ia, he magne osomes clea ly exhibi he ace ed
cube-oc ahed al mo phology ypical o M. g yphiswaldense (see
Fig. 1(a)). Howe e , some o hese RE doped magne osomes
seem o p esen a less ace ed mo phology compa ed o hei
undoped coun e pa s (see Fig. S1 in he ESI†). Along hese
lines, i has been epo ed ha he p esence o doping sal s in
he cul u e medium and he inco po a ion o he doping
elemen s in o he magne osome s uc u e can impose s ess in
he biomine alisa ion p ocess.
25,27,28
In ac , simila shape
i egula i ies ha e also been epo ed, o example, in Mn-
doped magne osomes.
27
Indeed, high- esolu ion ansmission
elec on mic oscopy (HRTEM) o simila high esolu ion
imaging echniques would be needed o quan i a i e analyses.
We ha e also obse ed ha he chains o magne osomes inside
he RE-doped MTB occasionally p esen mino i egula i ies
and de o ma ions, as depic ed in Fig. S1 and S2 in he ESI.†
Mo eo e , he RE doped bac e ia end o o m la ge chains
(27 magne osomes/chain) compa ed o he undoped bac e ia
(20) (see Table 1). His og ams accoun ing o he size-
dis ibu ion o he magne osomes a e shown in Fig. 1(d)–( ),
oge he wi h he co esponding Gaussian  s. Fo he undoped
magne osomes, wo size dis ibu ions can be obse ed, one
cen e ed a ound 47(8) nm and he o he one cen e ed a
22(8) nm. This double size dis ibu ion is ypical o hese M.
g yphiswaldense bac e ia, and accoun s o he diffe ence in size
be ween he magne osomes loca ed a he ends o he chain
(smalle ) and hose loca ed a he inne posi ions (la ge ). Jus
in he same way, wo size dis ibu ions a e also obse ed o he
Tb-doped magne osomes, cen e ed a 42(6) nm and 29(2) nm.
Howe e , only a single size dis ibu ion cen e ed a 33(9) nm is
ob ained o he Gd-doped magne osomes. Wha is clea
acco ding o hese TEM analyses, is ha he Gd- and Tb-doped
bac e ia end o syn hesise longe chains wi h smalle magne-
osomes. A simila size educ ion was also ound o M. g y-
phiswaldense bac e ia doped wi h o he elemen s, such as Mn
and Co.
26–28
A possible explana ion o he p esence o longe
chains in RE doped bac e ia could be ha an inc ease in he
magne osomes/chain a io would compensa e o he educ ion
o he magne ic momen pe magne osome, gi en he smalle
a e age size o he RE doped magne osomes compa ed o he
undoped ones. As a esul , he ne magne ic momen pe chain
would emain simila in bo h cases. Howe e , u he wo k will
be needed o con m his.
X- ay diff ac ion (XRD) analyses we e pe o med o de ec he
possible p esence o in e nal s uc u al changes in he RE-
doped magne osomes. Fig. 1(g)–(i) show he XRD pa e ns o
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he undoped, Gd- and Tb-doped bac e ial samples, espec i ely,
oge he wi h he co esponding Rie eld enemen s
41
(back-
g ound was effec i ely sub ac ed du ing he analysis). The ob-
ained B agg e o s R
B
a e always below 4% o Gd-doped, and
15% o Tb-doped samples, ensu ing he eliabili y o he pe -
o med analysis. The peak iden ica ion o he XRD pa e ns, as
shown by he e ical g een ba s below hem, has con med he
p esence o magne i e (Fe
3
O
4
) in bo h RE-doped bac e ia
(25.2(3)% con en o Gd-, and 12.31(1)% o Tb-doped
bac e ia). Apa om he Fe
3
O
4
phase, he XRD pa e ns also
p esen some eec ions co esponding o NaCl (66.0(2)% o
Gd-, and 68.66(1)% Tb-doped bac e ia) and KCl sal s (8.5(1)%
o Tb-doped bac e ia). These con ibu ions come om he PBS
medium employed o washing he ha es ed bac e ia. Besides,
a poo ly c ys allised con ibu ion ela ed o he GdCl
3
sal
(8.8%) has also been shown in Fig. 1(h). We mus cla i y ha he
XRD con ibu ions o hese addi ional sal s a e well diffe en i-
a ed om he one co esponding o he magne osomes, and
he e o e hey do no affec he analysis o he Fe
3
O
4
phase.
Rie eld enemen s shown in Fig. 1(g)–(i) (black colou )
co obo a e he p esence o well- o med c ys alline magne o-
somes in he undoped and RE doped bac e ia. The ob ained
la ice pa ame e s o each ensemble a e a¼8.3598(3) ˚
A o Gd-
doped, and a¼8.3815(1.1) ˚
A o Tb-doped samples. These alues
a e sligh ly educed (<0.4%) wi h espec o he one ypically e-
po ed o bulk Fe
3
O
4
(a¼8.397 ˚
A)
42
and undoped magne o-
somes (a¼8.3985(2) ˚
A).
9
This sligh con ac ion o he uni cell
pa ame e could in p inciple seem coun e in ui i e, since he
ionic adius o Gd
3+
(1.08 ˚
A) and Tb
3+
(1.06 ˚
A) is la ge han ha
o Fe
3+
(0.63–0.78 ˚
A) o Fe
2+
(0.92 ˚
A).
36,43
Ne e heless, simila
educ ions in he la ice pa ame e ha e been epo ed in o he
RE-doped Fe
3
O
4
nanopa icles, and unde s ood in e ms o he
RE-media ed s ain
44
and/o su ace s ess.
45
Rie eld enemen s also p o ide in o ma ion on he mean
diame e and mic os ain. The ob ained alues o he mean
diame e hDio he magne osomes a e 34.8(2) nm o he Gd-
Table 1 A e age (TEM) diame e , hDi, numbe o magne osomes pe
chain, N, and la ice pa ame e , a, o he undoped, Gd- and Tb-doped
samples. The e o in he a e age diame e co esponds o he s an-
da d de ia ion, s
Sample Undoped Gd-doped Tb-doped
hDi(nm) 47(8), 22(8) 33(9) 42(6), 29(2)
N20 27 27
a(˚
A) 8.3985(2) 8.3598(3) 8.3815(1.1)
Fig. 1 Rep esen a i e TEM images (a)–(c) o he magne osomes (ex ac ed om he bac e ia), size-dis ibu ion his og ams (d)–( ) and XRD
pa e ns (g)–(i), oge he wi h Rie eld efinemen s, co esponding o he undoped, Gd- and Tb-doped bac e ia, espec i ely. The size-dis i-
bu ions a e fi ed wi h Gaussian dis ibu ion. In (g)–(i), he posi ion o he hkl eflec ions a e ma ked below he XRD pa e ns in g een lines. In all
cases, he Fe
3
O
4
phase gi es ise o he mos in ense peaks. XRD efinemen s o he undoped bac e ia a e ep oduced om e . 9 wi h
pe mission. Inse s in XRD show a ep esen a i e TEM image o M. g yphiswaldense bac e ia.
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doped, and 32.7(3) nm o Tb-doped samples, alues ha a e in
good ag eemen wi h he esul s ob ained by TEM (see abo e),
con ming he single c ys alline na u e o he magne osomes.
On he o he hand, mic os ain alues o h¼1.92(9)% (Gd) and
h¼3.9(1)% (Tb) ha e been ob ained. These s ain alues indica e
ha he p esence o bo h doping ions dis o s he c ys alline
s uc u e o he Fe
3
O
4
magne osomes. Simila esul s ha e been
epo ed o o he doped magne osomes.
22,25,26
A his poin , TEM and XRD esul s ha e e ealed ha he
c ys alline s uc u e o he magne osome is mos ly main ained
despi e he p esence o Gd and Tb ions inside he Fe
3
O
4
la ice.
This s uc u al cha ac e isa ion has been comple ed by in es-
iga ing he inco po a ion o he Gd and Tb ions in o he
magne osomes using XANES. XANES is a e y powe ul elemen -
sensi i e synch o on echnique ha has p o ided us accu a e
in o ma ion on he oxida ion s a e and si e occupancy o he Gd
and Tb ions in he spinel s uc u e o magne i e.
26,46
XANES expe imen s we e ca ied ou on Gd- and Tb-doped
magne osomes, ex ac ed om he bac e ia, bo h a he Fe–K
and RE-L
3
edges. Since he XANES signal o he RE sal s
a ached o he bac e ial body is so la ge ha i masks any signal
due o he RE doped magne osomes, his ime we ha e wo ked
wi h isola ed magne osomes ins ead o he whole bac e ia in
o de o a oid his effec . Fig. 2(a) and (b) show he XANES
spec a o he Gd- and Tb-doped magne osomes a he Gd-L
3
(7243 eV) and Tb-L
3
(7514 eV) edges, espec i ely. The p esence
o a clea abso p ion edge o bo h samples is an indica o o he
inco po a ion o bo h Tb and Gd in o he magne osome s uc-
u e. Ne e heless, we canno comple ely disca d he possibili y
o he p esence o some Gd/Tb sal s a ached o he memb ane
o he magne osomes, despi e he mul iple washings o emo e
any emaining sal s ae ex ac ion. I should also be no ed
ha , ega dless he low Tb-con en would, in p inciple, ha e led
o measu ing he Tb-L
3
edge in uo escence mode, he emis-
sion lines o Tb-L
3
o e lap wi h he Fe–K ones, imposing he use
o ansmission measu ing mode. Hence, he no malised
ansmission spec um shown in Fig. 2(b) co esponds o an
ex emely low abso p ion jump. I is also no iceable ha he
high abso p ion whi e line o he RE-doped magne osomes,
associa ed wi h he numbe o holes in he 5d band ( alence)
and he loca ion o he 5d s a es. The o e all shapes o he
XANES spec a esemble hose o he e e ence compounds
shown in Fig. 2, i.e., GdCl
3
and Tb(NO
3
)
3
. The o e lapping edge
posi ion is a clea -cu indica o o he oxida ion s a e o he
abso bing a om,
47
indica ing ha he oxida ion s a e o he RE
ions inside magne osomes is ha o RE
3+
.
The inco po a ion o he RE
3+
ions in o he Fe
3
O
4
s uc u e
o he magne osomes is u he con med by abso p ion
measu emen s on he Fe K-edge. Fig. 2(c) and (d) show he Fe K-
edge XANES spec a o he Gd- and Tb-doped magne osomes,
Fig. 2 (a) and (b) no malised Gd- and Tb-L
3
-edge XANES spec a o Gd- and Tb-doped magne osomes, espec i ely. The co esponding
XANES spec um o he GdCl
3
and Tb(NO
3
)
3
e e ence samples ha e been included o compa ison. (c) and (d) no malised Fe–K edge XANES
spec a o magne osomes om Gd- and Tb-doped magne osomes, espec i ely. The con ol spec um (i.e., undoped magne osomes) has been
included as he e e ence. The inse s show he p e-edge and edge egions in mo e de ail.
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oge he wi h undoped magne osomes. As shown, he Fe K-edge
spec a o bo h Gd- and Tb-doped magne osomes a e e y
alike, and hey quali a i ely ep oduce he shape o he spec-
um eco ded o he con ol magne osomes. Howe e , o
bo h he Gd- and Tb- doped magne osomes, he e is a mino
damping o he oscilla ions o he XANES spec a, ha can be
aced o ins ance, in he whi e line (7131 eV) and he alley
(7160 eV) ampli udes. A simila damping has been obse ed in
XANES o magne i e nanopa icles when he pu i y and/o
c ys allini y o magne i e is sligh ly educed.
47,48
On op o
ha , a displacemen owa ds lowe ene gies o he abso p ion
edge can be obse ed, while o he undoped magne osomes,
he abso p ion edge is loca ed a 7123.1 eV, being i s posi ion
shied owa ds 7122.5 eV o bo h RE-doped magne osomes.
The e o e, his nega i e shio 0.6 eV could indica e a educ-
ion o he a e age alence s a e o Fe in he RE-doped magne-
osomes. Conside ing ha he diffe ence be ween he edge
posi ion o Fe
2+
and Fe
3+
is 7 eV, we can es ima e he alence
o he RE-doped magne osomes o be z2.55–2.57, while in he
undoped MTB, he alence is 2.66. This es ima ion has been
made aking in o accoun he ac ha he edge posi ion
depends linea ly on he alence, he e o e, a subs i u ion o
app oxima ely 3–4% o Fe
3+
ions by Gd
3+
o Tb
3+
can accoun o
he a o emen ioned alence educ ion. In o de o ob ain
a mo e accu a e es ima e, addi ional echniques such as X- ay
magne ic ci cula dich oism may p o e use ul. A he same
ime, an inc ease in he p e-edge peak ampli ude can also be
obse ed o he RE-doped magne osomes. This modica ion
eec s a change o he symme y a ound he Fe a oms, owa ds
a mo e non-cen osymme ic si e. The e o e, hese esul s
sugges a educ ion in he numbe o he cen osymme ic
oc ahed al si es occupied by Fe
3+
ions, as a consequence o hei
subs i u ion by Gd
3+
and Tb
3+
ions.
3.2 Magne ic cha ac e isa ion
The magne ic esponse o he doped-MTB has been analysed by
acing he M s.Tand M s.Hdependence. All he
measu emen s we e pe o med in whole bac e ia in o de o
minimise he effec o he in e chain in e ac ions, bu also o
allow a be e compa ison wi h he esul s ob ained o undoped
bac e ia.
26,49
Fig. 3(a) shows he ZFC–FC cu es o he RE-doped
and undoped bac e ia, shied in he Y-axis o cla i y pu poses.
S a ing wi h he undoped sample, he M s.Tcu es p esen
a s ong i e e sibili y in he whole empe a u e ange s udied,
and a sha p ansi ion in he ZFC cu e a ound T
V
105 K, which
is also accompanied by a smalle peak in he FC cu e. This
ansi ion co esponds o he well-known Ve wey ansi ion,
cons i u ing a nge p in o he p esence o s oichiome ic
magne i e.
13,50
Conce ning he Gd- and Tb-doped bac e ia, hei
o e all M s.Te olu ion is e y simila o he one o he undoped
bac e ia. The ZFC–FC cu es e idence clea i e e sibili y, and
he p esence o he Ve wey ansi ion is also e iden , al hough i
seems o be now sligh ly displaced owa ds lowe T alues, 95 K
o he Gd-doped and 99 K o Tb-doped bac e ia. This
displacemen becomes mo e e iden by compa ing he de i a-
i es o he ZFC cu es o he h ee samples, as shown in
Fig. 3(b). The e, we can obse e ha he T
V
, ma ked by he poin
a which he de i a i e becomes null, is sligh ly shied owa ds
lowe alues o bo h he Gd- and Tb-doped bac e ia. In addi ion,
he peak o he de i a i e, which ma ks he onse o he ansi-
ion, is b oade , less in ense, and also displaced owa ds lowe
empe a u es o he RE-doped bac e ia. I mus be no ed ha he
su i al o he Ve wey ansi ion in magne i e nanopa icles is
s ongly dependen on he c ys allini y and s oichiome y. Small
changes in he magne i e s uc u e, o example by doping wi h
o he elemen s o by c ea ing de ec s/ acancies,
12,26,51
can quickly
lead o he displacemen and disappea ance o his ansi ion.
Finally, on he low- empe a u e side, a s ong pa amagne ic
con ibu ion appea s in bo h RE doped bac e ia below T25 K.
This is caused by he p esence o he Gd and Tb sal s a ached o
he bac e ial body, as shown by he TEM images (see Fig. S1 and
S2 in he ESI†).
To u he explo e he magne ic beha iou o he Gd- and Tb-
doped magne osomes, we ha e also analysed hei magne ic
Fig. 3 (a) M–Tcu es measu ed ollowing he ZFC–FC p o ocol o he undoped (cyan), Gd- ( ed) and Tb-doped (blue) MTB. No e ha he
cu es a e displaced in he Y-axis o cla i y pu poses. (b) De i a i es o he ZFC magne isa ion cu es (dMdT
1
) o hese h ee samples. In bo h (a)
and (b), he measu emen s o undoped MTB a e included o compa ison pu poses, and he posi ion o he Ve wey ansi ion co esponding o
he undoped bac e ia is ma ked wi h a g ay line.
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esponse as a unc ion o he applied magne ic eld. Hys e esis
loops, M s. H, ha e been measu ed a diffe en empe a u es,
om 5 o 300 K, ae a cooling p ocess wi h ei he no applied
magne ic eld (ze o-eld-cooling, ZFC) o wi h an applied eld
o 1 T (eld-cooling, FC). We ha e included se e al o hese M s.
Hloops in Fig. S3 o he ESI.†He e i can be seen ha a 300 K,
he M s. H loops o he 3 samples (undoped, Gd-doped, and Tb-
doped) a e e y simila , whe eas clea diffe ences eme ge when
dec easing he empe a u e, especially, below he Ve wey
empe a u e (100 K). Fig. 4 shows he he mal e olu ion o he
mos ele an hys e esis pa ame e s, i.e., he coe ci e eld,
m
0
H
C
(lepanels), and he magne isa ion emanence, no mal-
ised by he sa u a ion magne isa ion, M
/M
s
( igh panels).
These ha e been measu ed unde ZFC ( op), and FC (middle)
p o ocols, o nally compa e hem by plo ing he diffe ence (in
absolu e alue) be ween he FC and ZFC alues (bo om). The e,
in all cases (ZFC, FC and diffe ence), i can be seen ha ei he
he coe ci e eld o he emanen magne isa ion co esponding
o he doped and undoped bac e ia no longe o e lap below T
V
,
ge ing mo e and mo e diffe en ia ed wi h dec easing
empe a u e, all he way down o 5 K. The same happens o he
M s. H loops shown in Fig. S3 o he ESI†.
We will now analyse he coe ci e eld and emanence mag-
ne isa ion in g ea e de ail. Conce ning he coe ci e eld, bo h
he ZFC and FC coe ci e eld cu es o he doped bac e ia
[Fig. 4 (a) and (c)] emain nea ly cons an down o 100 K, wi h
a lowe m
0
H
C
alue (0.017 T) han he undoped bac e ia
(0.023 T). Then, he m
0
H
C
slowly inc eases up o 0.024 T a 50
K, and nally ises mo e s eeply eaching a alue o 0.045 T a
5 K, again smalle han he one ob ained o he undoped
bac e ia. The diffe ences in coe ci i y be ween RE-doped and
undoped magne osomes can be seen mo e clea ly i we ocus on
Fig. 4(e), whe e he diffe ence be ween ZFC and FC alues,
jDm
0
H
C
jcu es, is shown. The Ve wey ansi ion, delimi ed by
he non-ze o alue o jDm
0
H
C
j, is clea ly dened a ound 107 K
o he undoped bac e ia, while in he case o he Gd- and Tb-
doped samples, his ansi ion is less ab up and smoo he .
This esul ag ees well wi h he magne ic beha iou obse ed in
he ZFC/FC M s. T cu es, indica ing ha he RE ions inside he
magne osomes, on one hand, educe he effec i e aniso opy,
Fig. 4 E olu ion wi h To he coe ci e field, m
0
H
C
, ((a), (c) and (e)), and he no malised emanence, M
/M
s
((b), (d) and ( )) o he undoped, Gd-,
and Tb-doped bac e ia. Samples we e cooled unde no field (ZFC, (a) and (b)), and unde a field o 1 T (FC, (c) and (d)). (e) and ( ) depic he
diffe ence be ween FC and ZFC measu emen s, in absolu e alue, o he m
0
H
C
and he M
/M
s
alues.
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and, on he o he hand, sligh ly modi y he Ve wey ansi ion
due o mino s uc u al changes.
These esul s a e u he suppo ed by he M
/M
s
cu es
shown in Fig. 4(b), (d), and ( ). The e, i can be seen how he
shoulde s ound a T107 and 50 K o he undoped bac e ia
become smoo h and b oadened in he case o he RE-doped
bac e ia. Al hough some small diffe ences can be obse ed
be ween he alues o he M
/M
s
cu es o he Gd- and Tb-doped
samples, when plo ing he change o emanence jDM
/M
s
jin
Fig. 4( ), bo h cu es o e lap, as o jDm
0
H
C
j. Following a simila
e olu ion o ha o he jDm
0
H
C
jcu es shown in (e), jDM
/M
s
j o
he RE-doped magne osomes emains e y small (<0.005) down
o 90 K. Then, i slowly s a s inc easing up o 0.01 a 50 K, and
below ha empe a u e, he inc ease becomes mo e ab up ,
al hough he maximum alues eached (0.18) a e again
smalle han hose ob ained o he undoped magne osomes
(0.24). A compa ison be ween he he mal e olu ion o jDm
0
-
H
C
jand jDM
/M
s
jo he undoped, Gd-doped, Tb-doped, and
Mn-doped bac e ial samples is p esen ed in Fig. S4 o he ESI†.
In o de o shed ligh on he specic changes ha a e aking
place in he in insic magne ic p ope ies o he Gd- and Tb-
doped samples, we ha e ca ied ou magne ic simula ions o
he M s. H loops measu ed a diffe en empe a u es. Fo his,
we ha e employed a modied S one –Wohl a h app oach,
which has been ex ensi ely desc ibed in ou p e ious s udies.
4,27
B iey, he equilib ium congu a ion o he magne ic momen
o each magne osome is calcula ed as he sum o h ee con i-
bu ions: (i) he magne oc ys alline aniso opy ene gy, E
C
; (ii) he
effec i e uniaxial aniso opy ene gy, E
uni
, a ising om he
compe i ion be ween he magne osome shape aniso opy and
he dipola in e ac ions be ween magne osomes inside he
chain; and (iii) he Zeeman ene gy e m, E
Z
.
26,52
In sphe ical
coo dina es, conside ing he h100ic ys allog aphic di ec ions
o magne i e as he e e ence sys em, he o al ene gy densi y is
gi en by:
E(q, )¼E
C
(q, )+E
uni
(q, )+E
Z
(q, )(1)
being
ECðq; Þ¼KCsin4qsin2 þsin22q
4
Euniðq; Þ¼Kunih1ð
^
um$
^
uuniÞ2i
EZðq; Þ¼m0MHð^
um$
^
uHÞ
(2)
whe e qand accoun o he pola and azimu hal angles o he
magne ic momen o each magne osome, espec i ely. K
C
and
K
uni
s and o he magne oc ys alline and uniaxial aniso opy
cons an s, espec i ely. The ˆ
u
i
ep esen s he uni a y ec o
along he magne ic momen (ˆ
u
m
), he uniaxial aniso opy ec o
(ˆ
u
uni
) and he ex e nal magne ic eld (ˆ
u
H
) di ec ions, espec-
i ely. As p o ed in p e ious s udies, by SANS and elec on
c yo omog aphy imaging, among o he echniques, he ˆ
u
m
o ms an angle o 20wi h he chain axis di ec ion, h111i.
15,52
Based on hese conside a ions, he ZFC M s. H loops a
diffe en empe a u es ha e been simula ed employing he
dynamical app oach al eady desc ibed in e . 52 and 53. The
aniso opy e ms, K
C
and K
uni
, ha e been adjus ed o a ain he
bes ma ch be ween expe imen al M s. H loops and he co e-
sponding simula ions. As shown in Fig. 5(a)–( ), he calcula ed
loops closely ollow he expe imen al ones. The he mal e olu-
ion o K
C
and K
uni
o he undoped, Gd- and Tb-doped samples
is shown in Fig. 5(g) and (h). A oom empe a u e, he alues o
K
C
o he h ee samples a e simila : 11.0 kJ m
3
o he
undoped and Gd-doped samples, and 12.0 kJ m
3
o he Tb-
doped sample. These alues a e close o he heo e ical K
C
alue
o bulk magne i e, 10.8 kJ m
3
. Wi h dec easing empe a u e,
jK
C
j(in absolu e alue) sligh ly inc eases o he undoped and
Gd-doped bac e ia, while i ins ead dec eases o he Tb-doped
sample, bu o e all, he change is small, emaining a ound jK
C
j
z11–12 kJ m
3
. Howe e , below 180 K, jK
C
j ends o dec ease
o he undoped sample, becoming null a 110 K, a ound he
Ve wey ansi ion. This indica es ha he ole o he cubic
magne oc ys alline aniso opy becomes negligible below T
V
o
he undoped magne osomes, as epo ed be o e.
26,27
A simila
beha iou is also obse ed o he RE doped bac e ia, bu he
d op in jK
C
jis displaced owa ds lowe empe a u es o bo h
he Gd- and Tb-doped bac e ia, becoming p ac ically null a 90
and 100 K, espec i ely. This ollows he same end obse ed
in he ZFC–FC cu es, which indica es, again, ha he inco -
po a ion o RE ions in o he magne i e s uc u e is modi ying
he Ve wey ansi ion. The pa icula diffe ences in he K
C
alues and e olu ion be ween Gd- and Tb-doped bac e ia may
be associa ed wi h diffe ences in he inco po a ion o he Gd
3+
and Tb
3+
-ions in o he magne osome s uc u e.
Conce ning he uniaxial aniso opy e m, K
uni
, i emains
almos cons an o he h ee samples down o T
V
:11.5 kJ m
3
o he undoped and Gd-doped, and 10 kJ m
3
o he Tb-
doped bac e ia. In bo h cases, he K
uni
alue is smalle han
he one ob ained o he undoped magne osomes (12 kJ m
3
).
Being abo e he Ve wey ansi ion, K
uni
is mainly ela ed o he
effec o shape aniso opy and dipola in e ac ions, and his
dec ease could be asc ibed o diffe ences in he size dis ibu-
ion and/o mo phology o he magne osomes, as al eady
obse ed in he TEM images. These diffe ences a e mo e
app eciable in he case o he Tb-doped bac e ia. Below T
V
,K
uni
inc eases subs an ially o he undoped bac e ia, up o 37 kJ
m
3
. Howe e , o he Gd- and Tb-doped bac e ia, he inc ease
is slowe , and he onse is no so well dened (90–100 K).
Besides, he change in he slope ob ained o K
uni
o he
undoped magne osomes below 50 K is also p esen in he Gd-
and Tb-doped bac e ia, bu is less ob ious, especially in he case
o he Gd-doped bac e ia. In he end, a 10 K, a maximum K
uni
alue o 23 and 26 kJ m
3
is eached o he Gd- and Tb-doped
bac e ia, espec i ely. Quali a i ely simila esul s we e ob-
ained in he case o Mn-doped bac e ia.
27
All hese esul s clea ly indica e ha he e is amodica ion
o bo h he magne oc ys alline and uniaxial aniso opies in RE-
doped magne osomes. The changes abo e he Ve wey ansi ion
can be mos likely associa ed o modica ions in he shape/size
o he RE-doped magne osomes in compa ison o he undoped
ones. On he o he hand, a low empe a u es, he obse ed
2656 |Nanoscale Ad ., 2022, 4,2649–2659 © 2022 The Au ho (s). Published by he Royal Socie y o Chemis y
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