A simple s a egy o imp o e he in e acial ac i i y o ue
Janus gold nanopa icles: a sho e hyd ophilic capping
ligand†
Miguel Angel Fe nandez-Rod iguez,aLimei Chen,bCh is ophe P. Deming,bMiguel An-
gel Rod iguez-Val e de,aShaowei Chen,bMiguel Angel Cab e izo-Vilchez,aand Roque
Hidalgo-Al a ez∗a
Janus gold nanopa icles (JPs) o ∼4 nm-diame e hal unc ionalized wi h 1-hexane hiol as hy-
d ophobic capping ligand exhibi signi ican ly highe in e acial ac i i y, ep oducibili y and heo-
logical esponse when he o he hal is unc ionalized wi h 1,2-me cap op opanediol (JPs-MPD)
han wi h 2-(2-me cap oe hoxy)e hanol (JPs-MEE), bo h ac ing as he hyd ophilic capping ligand.
The in e acial p essu e measu ed by pendan d op ensiome y eaches 50 mN/m and 35 mN/m
o he JPs-MPD a he wa e /ai and wa e /decane in e ace, espec i ely. A he same a ea pe
pa icle, he JPs-MEE e eal signi ican ly lowe in e acial p essu e: 15 mN/m and 5 mN/m a he
wa e /ai and wa e /decane in e ace, espec i ely. In e acial dila a ional heology measu emen s
also show an elas ic shell beha iou a highe comp ession s a es o JPs-MPD while he JPs-
MEE p esen nea -ze o elas ici y. The enhanced in e acial ac i i y o JPs-MPD is explained in
e ms o chemical and hyd a ion di e ences be ween he MPD and MEE ligands, whe e MPD has
a sho e hyd oca bon chain and wice mo e hyd oxyl e minal g oups han MEE.
Picke ing emulsions can be he modynamically s abilized by
amphiphilic Janus nanopa icles (JPs) wi h a we abili y
aniso opy1–3.I is known ha JPs show h ee imes mo e adso p-
ion ene gy han homogeneous nanopa icles2,4. S ong e o s
ha e been made o syn hesize and simula e JPs wi h di e en
mo phologies and su ace chemis y o con ol he way in which
hese pa icles sel -assemble a luid in e aces5–9.
Gold nanopa icles andmonly unc ionalized wi h 1-
undecane hiol and N,N,N- ime hyl (11-me cap oundecyl)
ammonium chlo ide a e epo ed o become Janus-like when
he capping ligands ea ange a he wa e /ai in e ace10.
Ne e heless, ecen ly Regue a e al.11 demons a ed by neu on
e lec i i y ha such ea angemen does no happen wi h gold
nanopa icles unc ionalized wi h 1-oc ane hiol and 6-me cap o-
1-hexanol a he wa e /ai in e ace. A way o ob ain gold
JPs wi h ue sepa a e domains is o selec i ely unc ionalize
each hemisphe e o he co e wi h he desi ed capping ligands
immobilizing he nanopa icles in a Langmui balance12,13.
I is undamen al o selec he app opia e capping ligands ha
aBiocolloid and Fluid Physics G oup, Applied Physics Depa men , Facul y o Sciences,
Uni e si y o G anada, G anada, Spain; E-mail: hidalgo@ug .es
bDepa men o Chemis y and Biochemis y, Uni e si y o Cali o nia, 1156 High
S ee , San a C uz, CA 95064, USA.
†Elec onic Supplemen a y In o ma ion (ESI) a ailable: HRTEM mic og aphies and
pendan d ops pic u es. See DOI: 10.1039/b000000x/
will con e he Janus cha ac e o he nanopa icles because o he
signi ican dependence be ween hese capping ligands and he in-
e acial ac i i y o he inal JPs. We p opose a simple s a egy o
enhance he in e acial ac i i y, heological esponse and ep o-
ducibili y h ough colloidal s abili y o ue gold Janus nanopa -
icles. We syn hesized Janus gold nanopa icles hal capped by 1-
hexane hiol and he o he hal by 2-(2-me cap o-e hoxy)e hanol
(JPs-MEE) o 1,2-me cap op opanediol (JPs-MPD) in su ac an -
ee condi ions as desc ibed in p e ious wo ks (see Fig. 1)12–14.
The sizes ob ained by high esolu ion TEM measu emen s (see
Fig. S1†) a e 3.5±0.9nm and 3.7±1.9nm o he JPs-MEE and
JPs-MPD, espec i ely. The elec opho e ic mobili y o bo h JPs
was measu ed wi h a Ze aSize Nano (Mal e n) in a 10−2M
sodium ci a e MilliQ wa e solu ion o s abilize he elec ical
double laye ob aining µe,JPs−MEE = (−2.2±1.5)·10−8m2/(V·s)
and µe,JPs−MPD = (−2.9±0.4)·10−8m2/(V·s). F om p e ious
wo ks, he JPs-MEE showed an a e age mac oscopic con ac an-
gle o (56.1±1.8)◦and (49.0±1.3)◦in each hemisphe e, whe eas
he JPs-MPD showed (63.3±2.7)◦and (53.4±2.9)◦in hei espec-
i e hyd ophobic and hyd ophilic hemisphe es12,13. The con ac
angles a e sligh ly lowe o he JPs-MEE likely due o he ab i-
ca ion p ocess: o MEE adso p ion, he close-packed monolaye
o 1-hexane hiol co e ed gold nanopa icles was imme sed in a
wa e solu ion con aining MEE13 and his migh p oduce g ea e
1–5 | 1
SH
OOH
CH3
SH
OH
OH
Fig. 1 MEE (le ) and MPD ( igh ) capping ligands. The SH g oup is he
ancho g oup a he gold nanopa icle su ace.
56
60
64
68
72
0 200 400 600 800 1000 1200
Su ace Tension γ (mN/m)
Time (s)
1.6⋅1012 JPs−MPD
3.2⋅1012 JPs−MPD
4.8⋅1012 JPs−MPD
8.0⋅1012 JPs−MPD
Fig. 2 Su ace ension e olu ion o e he ime a e he deposi ion o
di e en numbe o JPs-MPD in THF on he su ace o an ini ial 5µL
MilliQ wa e pendan d op and g ow h up o 45 µL. Same colo cu es
co espond o di e en expe imen s wi h he same numbe o deposi ed
JPs-MPD. A e he sol en e apo a ion, he su ace ension emained
s able.
hyd ophilic capping ligand exchange and hus lowe con ac an-
gle han o he JPs-MPD in which he unc ionaliza ion wi h
MPD was pe o med di ec ly in he Langmui balance, exchanging
he MPD ligands om he wa e subphase12. The con ac angle
alues in he hyd ophilic hemisphe es a e simila wi hin e o s,
which e lec s ha he di e ences be ween MPD and MEA a e no
e lec ed mac oscopically in he con ac angle and he we abili y
con as (i.e. con ac angle di e ences be ween hemisphe es) is
simila in bo h JPs-MEE and JPs-MPD wi hin e o s. The pendan
d op ensiome y was conduc ed as ollows: di e en amoun s o
Janus nanopa icles (JPs-MEE and JPs-MPD) dispe sed in e ahy-
d o u an (THF, HPLC g ade) we e deposi ed on a wa e pendan
d op wi h a handheld mic osy inge and a mic oposi ione . The
su ace ension was ob ained by axisymme ic d op shape anal-
ysis upon THF e apo a ion, while he pendan d op olume was
kep cons an . The esul s in Fig. 2 show a dec ease in he inal
su ace ension a e e apo a ion o THF which is highe as he
concen a ion o JPS-MPD is inc eased ( e e o p e ious wo k
o he JPs-MEE simila cha ac e iza ion15). The expe imen s a e
highly ep oducible as can be seen in he di e en uns o a ixed
concen a ion o JPs (di e en cu es wi h same colo in Fig. 2).
A e he THF e apo a ion, g owing and sh inking expe imen s
we e pe o med a 0.08 µL/s o each JP concen a ion. Nex ,
he pendan d op was imme sed in decane and he g owing and
sh inking expe imen s we e pe o med again. We plo he in e a-
cial p essu e (Π=γ0−γ, whe e γ0is 72.5mN/m o he wa e /ai
(W/A) and 52.3mN/m o he wa e /decane (W/O) in e aces and
γis he measu ed in e acial ension) agains he d op a ea pe
pa icle (Ap, he a ea o he pendan d op di ided by he num-
be o deposi ed JPs). A piecewise comp ession iso he m can be
seen in Fig. 3a and Fig. 3b o W/A and W/O in e aces, espec-
i ely. The i s ema kable ac is he lowe in e acial ac i i y
o JPs-MEE compa ed o JPs-MPD a he same Ap alues. A he
lowes Ap eached o JPs-MPD, Πis 50mN/mand 35mN/mand
15mN/mand 5mN/m o he JPs-MEE a he W/A and W/O in e -
aces, espec i ely. The highes Π alues ob ained o JPs-MEE,
a e u he comp ession, a e 30mN/mand 20mN/ma he W/A
and W/O in e aces, espec i ely. Con a y o JPs-MEE, he com-
p ession iso he ms o JPs-MPD a he W/A in e ace exhibi open
cycles a he beginning o he expe imen s poin ing ou ha he
colloidal monolaye ea anges in o a inal s a e ha is p ese ed
in u he comp ession cycles. This beha iou is a enua ed a he
W/O in e ace as he hys e esis cycles a e much smalle (i.e. he
uppe comp ession and lowe expansion cu es a e close ) which
migh be due o he ac ha enough ene gy is p o ided o each
a mo e elaxed s a e when i is imme sed in decane. Mo eo e ,
he low hys e esis o he JPs-MEE comp ession cycles compa ed
o he JPs-MPD migh be due o he lowe in e acial ac i i y o
hese pa icles. Since he ab ica ion p ocess, hyd ophobic cap-
ping ligand, we abili y con as , size and cha ge we e simila o
bo h JPs-MEE and JPs-MPD, he di e ences be ween in e acial
ac i i y mus come om he in e acial ac i i y o he MEE and
MPD hyd ophilic capping ligands. Whe eas MEE has a longe hy-
d oca bon chain ( ou CH2and one oxygen) and one hyd oxyl
e minal g oup, he MPD has a sho e hyd oca bon chain ( h ee
CH2) and wo hyd oxyl e minal g oups (see Fig. 1). The lowe
numbe o hyd oca bon g oups and highe numbe o hyd oxyl
g oups o MPD migh esul in highe hyd a ion o he hyd ophilic
hemisphe e o he JPs (i.e. es ablishing hyd ogen bonds be ween
wa e and he hyd ophilic capping ligands). These chemical di -
e ences migh play a decisi e ole in he inal in e acial ac i i y
o hese JPs.
The in e acial dila a ional heology o he JPs was e alua ed
by en pe iodic olume a ia ions o 1µL o di e en pe iods.
When a pe iodic injec ion/ex ac ion o olume is pe o med o
he pendan d op, he in e ace ies o e-es ablish he equilib-
ium. This coun e ac ion is ep esen ed by a complex quan i y
composed by a s o age pa and a loss pa :
E=Ed+iωηd(1)
whe e Eis he su ace dila a ional modulus ha accoun s o he
change in su ace ension p oduced by a small change in a su ace
a ea, Edis he in e acial dila a ional elas ici y, ωis he oscilla ion
equency and ηdis he in e acial dila a ional iscosi y16. I he
iscosi y is negligible du ing he elaxa ion p ocess a e pe u -
ba ion o he in e ace, he in e ace p esen an essen ially elas ic
beha io . The ex ao dina y in e acial ac i i y o JPs-MPD is e-
lec ed in he heology esul s in Fig. 4a and 4b (see Fig. S2
and S3†). Fo bo h W/A and W/O in e aces, E dec eases sligh ly
and ηinc eases o inc easing pe iods. Fo he W/A in e ace,
Edand ηdinc eases clea ly wi h he comp ession s a e o he col-
loidal monolaye . This end is also obse ed o he W/O in e -
2 | 1–5
(a) Wa e /ai in e ace
0
10
20
30
40
50
0 5 10 15 20 25 30 35
Π (mN/m)
A eapa icle (nm2/pa icle)
1.6⋅1012 JPs−MEE
3.3⋅1012 JPs−MEE
5.0⋅1012 JPs−MEE
8.3⋅1012 JPs−MEE
16.6⋅1012 JPs−MEE
33.1⋅1012 JPs−MEE
49.7⋅1012 JPs−MEE
1.6⋅1012 JPs−MPD
3.2⋅1012 JPs−MPD
4.8⋅1012 JPs−MPD
8.0⋅1012 JPs−MPD
(b) Wa e /decane in e ace
0
5
10
15
20
25
30
35
0 5 10 15 20 25 30 35
Π (mN/m)
A eapa icle (nm2/pa icle)
1.7⋅1012 JPs−MEE
5.0⋅1012 JPs−MEE
16.6⋅1012 JPs−MEE
1.6⋅1012 JPs−MPD
3.2⋅1012 JPs−MPD
4.8⋅1012 JPs−MPD
8.0⋅1012 JPs−MPD
Fig. 3 Su ace p essu e agains he a ea pe pa icle o di e en numbe o JPs-MEE and JPs-MPD deposi ed a he (a) W/A and (b) W/O in e aces.
Fo a mo e de ailed cha ac e iza ion o he JPs-MEE, please e e o Fe nandez e al.15
ace bu wi h lowe alues o bo h Edand ηd. The high Ed alue
is a signal ha he colloidal monolaye c ea es an elas ic shell
on he pendan d op a highe comp ession s a es. This elas ic
beha iou again sugges s he abili y o he JPs-MPD as emulsi-
ie s. Fu he heology expe imen s we e pe o med o a ixed
pe iod o 10scompa ing he esponse o JPs-MEE and JPs-MPD.
The esul s in Fig. 4c and 4d poin ou ha he JPs-MPD each sig-
ni ican ly highe Edand ηd alues upon comp ession (i.e. lowe
Ap) han he JPs-MEE (∼10 imes highe Edand ηd o JPs-MPD
han JPs-MEE a he W/A in e ace and 2 imes a he W/O in-
e ace), sugges ing ha he elas ic shell beha iou is no p esen
o he JPs-MEE. A inal conside a ion mus be aken in o accoun
o gold nanopa icles in he ange o a ew nanome e s (i.e. less
han 10nm), he adso p ion ene gy a he in e ace is o he o de
o KBT17 and hey a e expec ed o easily lea e he in e ace. Ne -
e heless, he s able in e acial ension o e ime a e he THF
e apo a ion, he closed g owing/sh inking cycles and he dila a-
ional heology seem o poin ou ha he JPs-MEE and JPs-MPD
a e i e e sibly ancho ed a he W/A and W/O in e aces, p oba-
bly due o i s Janus cha ac e .
In conclusion, he JPs-MEE and JPs-MPD a e simila in ab i-
ca ion p ocess, hyd ophobic capping ligand, we abili y con as ,
size and cha ge, bu a e unc ionalized wi h di e en hyd ophilic
capping ligand. The JPs-MPD exhibi a signi ican ly highe in e -
acial ac i i y a W/A and W/O in e aces. Mo eo e , he dila a-
ional heology sugges s an elas ic shell-like beha iou o he pen-
dan d op when he JPs-MPD a e deposi ed a W/A and W/O in-
e aces. This elas ic shell beha iou seems o be absen wi h he
JPs-MEE. This poin s ou he impo ance o he chemical s uc u e
o he capping ligands in JPs o p edic he in e acial ac i i y and
he e o e hei abili y as emulsi ie s. Sho e hyd oca bon chain
and mo e hyd oxyl e minal g oups in he hyd ophilic capping
ligands seems o be a ou e o ob ain enhanced in e acial ac i -
i y o his kind o JPs ia enhanced hyd a ion o he hyd ophilic
hemisphe e o he JPs. To he bes o ou knowledge, i is he i s
ime ha such high in e acial ac i i y is ob ained wi h ∼4nm-
diame e gold nanopa icles in su ac an - ee condi ions.
Acknowledgemen s
This wo k was suppo ed by he Spanish MINECO (p ojec s
MAT2013-44429-R and MAT2014-60615R), by “Jun a de An-
dalucía” and FEDER (p ojec s P10-FQM-5977 and P12-FQM-
1443) and by US Na ional Science Founda ion (DMR-1409396).
Au ho s hank o D . J.A. Holgado-Te iza o he so wa e
Con ac o c
used o su ace ension measu emen s.
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(a) In e acial dila a ional elas ici y Ed
0
50
100
150
200
250
300
0 10 20 30 40 50
Ed (mN/m)
Pe iod (s)
35 nm2/pa . W/A
35 nm2/pa . W/O
15 nm2/pa . W/A
15 nm2/pa . W/O
8 nm2/pa . W/A
8 nm2/pa . W/O
5 nm2/pa . W/A
5 nm2/pa . W/O
3 nm2/pa . W/A
3 nm2/pa . W/O
(b) In e acial dila a ional iscosi y ηd
0
10
20
30
40
50
60
0 10 20 30 40 50
ηd (mN/(m⋅s))
Pe iod (s)
35 nm2/pa . W/A
35 nm2/pa . W/O
15 nm2/pa . W/A
15 nm2/pa . W/O
8 nm2/pa . W/A
8 nm2/pa . W/O
5 nm2/pa . W/A
5 nm2/pa . W/O
3 nm2/pa . W/A
3 nm2/pa . W/O
(c) In e acial dila a ional elas ici y Ed o 10spe iod
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0 5 10 15 20 25 30 35
Ed (mN/m)
Apa icle (nm2/pa icle)
W/A JPs−MEE
W/A JPs−MPD
W/O JPs−MEE
W/O JPs−MPD
(d) In e acial dila a ional iscosi y η o 10spe iod
0
2
4
6
8
10
12
0 5 10 15 20 25 30 35
ηd (mN/(m⋅s))
Apa icle (nm2/pa icle)
W/A JPs−MEE
W/A JPs−MPD
W/O JPs−MEE
W/O JPs−MPD
Fig. 4 (a) In e acial dila a ional elas ic modulus (Ed) and (b) iscosi y (ηd) o JPs-MPD agains di e en pe iods o di e en Apcomp ession s a es a
he W/A and W/O in e aces. (c) Edand (d) ηdo JPs-MEE and JPs-MPD agains he Apa he W/A and W/O in e aces, o 10spe iod.
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