RESEARCH ARTICLE
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Ele a ing Pla inum o Volume ic Capaci ance: High Su ace
A ea Elec odes h ough Reac i e P Spu e ing
Maciej G yszel, Ma ie Jakešo á, Xuan Thang Vu, S en Ingeb and ,
and E ic Daniel Głowacki*
Pla inum is he mos widesp ead elec ode ma e ial used o implan able
biomedical and neu oelec onic de ices, mo i a ing explo ing ways o
imp o e i s pe o mance and unde s and i s undamen al p ope ies. Using
eac i e magne on spu e ing, P Oxis p epa ed, which upon pa ial educ ion
yields a po ous hin-film o m o pla inum wi h a o able p ope ies, no ably
eco d-low impedance alues ou compe ing o he epo s o pla inum-based
elec odes. I is es ablished ha i s high elec ochemical capaci ance scales
wi h hickness, in he way o olume ic capaci o ma e ials like I Oxand
poly(3,4-e hylenedioxy hiophene), PEDOT. Unlike hese wo well-known
analogs, howe e , i is ound ha P Oxcapaci ance is no caused by e e sible
pseudo a adaic eac ions bu a he due o high su ace a ea. In con as o
I Ox,P O
xis no a e e sible alence-change oxide, bu a he a po ous o m
o pla inum. The findings show ha his oxygen-con aining o m o P can
place P elec odes on a le el compe i i e wi h I Oxand PEDOT. Due o i s
ela i ely low cos and ease o p epa a ion, P Oxcan be a good choice o
mic o ab ica ed bioelec onic de ices.
1. In oduc ion
Pla inum is he mos common bioin e ace elec ode ma e ial
o neu os imula ion and neu al eco ding elec odes. P esen ly,
M. G yszel
Labo a o y o O ganic Elec onics
Depa men o Science and Technology
Linköping Uni e si y
B edga an 33, No köping 60174, Sweden
M. Jakešo á, E. D. Głowacki
Bioelec onics Ma e ials and De ices Labo a o y
Cen al Eu opean Ins i u e o Technology
B no Uni e si y o Technology
Pu kyˇ
no a 123, B no 61200, Czech Republic
E-mail: [email p o ec ed].cz
X. T. Vu, S. Ingeb and
Ins i u e o Ma e ials in Elec ical Enginee ing 1
RWTH Aachen Uni e si y
52074 Aachen, Ge many
The ORCID iden ifica ion numbe (s) o he au ho (s) o his a icle
can be ound unde h ps://doi.o g/10.1002/adhm.202302400
© 2024 The Au ho (s). Ad anced Heal hca e Ma e ials published by
Wiley-VCH GmbH. This is an open access a icle unde he e ms o he
C ea i e Commons A ibu ion License, which pe mi s 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.
DOI: 10.1002/adhm.202302400
nea ly all ch onically implan ed bioelec-
onic medicine de ices, such as deep b ain
s imula o s, pe iphe al ne e in e aces,
implan ed encephalog aphy elec odes, and
spinal co d s imula o s u ilize pla inum
as he ac i e elec ode.[1–3]Pla inum is e-
ga ded as a noble me al wi h sui able bio-
compa ibili y. I s double-laye capaci ance
and cha ge s o age/injec ion capaci y a e,
howe e , ela i ely low compa ed o many
nex -gene a ion neu al in e ace elec ode
ma e ials such as TiN, conduc ing poly-
me s, and I Ox.[4,5]To boos he in e a-
cial capaci ance o P o mo e compe i i e
le els, a ious echniques o mic o- and
nano-s uc u ing o pla inum ha e been ex-
plo ed. Pla inum elec odes can be ough-
ened and po osified by lase abla ion,[6]
o by elec ochemical ea men s.[7]These
p ocedu es enhance he exposed su ace
a ea o he pla inum, inc easing capaci ance
and leading o a d op in elec ochemical
impedance. The mos -explo ed me hod o p epa a ion o
high su ace-a ea ul a-low-impedance pla inum is elec ochem-
ical deposi ion o s uc u ed pla inum om pla inum sal
solu ions.[8,9]These po ous s uc u es a e some imes e e ed
o as pla inum black o g ey, and can ea u e a ious shapes
o nano o mic os uc u ed P . P ominen examples include P
nanog ass[9]o nanoP [10]coa ings epo ed by Asplund and co-
wo ke s, demons a ed o in i o and in i o applica ions.[10]
These coa ings ep esen he o-da e bes epo s in e ms o low
impedance pla inum (e.g., |Z| @ 1 kHz be ween 20–50 kΩ o
35 μm diame e elec odes). Ne e heless, he key pe o mance
me ics o pla inum elec odes lag behind he a o emen ioned
eme ging neu al in e ace ma e ials.
He ein we ake an al e na i e app oach o imp o ing he
elec ochemical p ope ies o P : p epa a ion o pa ially bulk-
oxidized P Oxfilms by eac i e magne on spu e ing. This builds
upon ou ecen ly published me hod o p epa a ion o po ous no-
ble me al films by (elec o)chemical educ ion o spu e ed noble
me al oxides (Au, Pd, and P ).[11]In his ea lie wo k we ound
ha unlike he o he “noble” me als, educ ion o P Oxp oceeded
only pa ially and a high le el o oxygen con en always emained
in he bulk o he films ega dless o ea men condi ions. In-
spi ed by his finding, he ein we ha e se ou o cha ac e ize
spu e ed P Oxas an elec ode ma e ial in he con ex o bioelec-
onics applica ions. To his end, we p epa e elec odes based on
eac i ely spu e ed P Ox(Figu e 1A,B) and compa e hem wi h
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Figu e 1. A) C oss sec ion schema ic o mic oelec odes es ed in his s udy: Ø 40 μm Au mic oelec odes coa ed wi h P Ox. B) Scanning elec on
mic oscopy (SEM) mic og aphs o a plana P film, e sus a P Oxfilm. The P Oxshows a fine mic opo ous s uc u e. C) Elec ochemical impedance
spec a (solid line – modulus, dashed line – phase angle) o Ø 40 μm Au mic oelec odes coa ed wi h 240 nm o P Ox, PEDOT:PSS (c osslinked wi h
GOPS), o I Ox(a e elec ochemical ac i a ion desc ibed in he expe imen al sec ion). Impedance o plana P elec odes o he same size (no shown),
gi e |Z| (≈450 kΩ@ 1 kHz). D) A eal and olume ic capaci ance o P Ox(135 W DC spu e ing a 47 mTo o A wi h 20% O2) laye s o diffe en
hickness, deposi ed on Ø 40 μm Au mic oelec odes. Calcula ions we e done based on cyclic ol amme y da a. A eal capaci ance linea ly scales up
wi h P Ox hickness up o 600 nm, hicke samples ha e only sligh ly inc eased a eal capaci ance and lowe olume ic capaci ance. Inc easing hickness
beyond his poin leads o an inc ease in se ies esis ance, which may o igina e om he poo conduc i i y o he P Oxbulk film,[12]and/o so-called
po e esis ance, and small elec oly e-filled channels in oduce elec ochemical impedance.
wo well-known high-capaci y/low-impedance elec odes used
in bioelec onics: poly(3,4-e hylenedioxy hiophene):poly(s y ene
sul ona e), PEDOT:PSS, and I Ox. These ma e ials se e as
poin s o e e ence o help unde s and bo h he pe o mance and
elec ochemical na u e o P Oxelec odes.
2. Resul s and Discussion
2.1. Capaci i e P ope ies o P Ox
We cha ac e ized he elec ochemical impedance o Ø 40 μm
P Oxmic oelec odes, and es ima ed he elec ochemical capac-
i ance o P Oxfilms using cyclic ol amme y (de ails on film
p epa a ion can be ound in Sec ion 2.2). The impedance o
hese is significan ly lowe han plana P , and lowe han p e i-
ous epo s on bes -o s uc u ed P mic oelec odes (we achie e
≈8.6 kΩ@ 1 kHz, compa ed wi h alues >20 kΩ epo ed in
he li e a u e o he same sized elec odes, Ø 35–40 μm).[13]In a
side-by-side compa ison, ou measu ed impedance is on-pa wi h
pseudo a adaic coa ings PEDOT:PSS and I Ox(Figu e 1C). Mo e-
o e , he mos ema kable esul was ha he wo-dimensional
a eal capaci ance o P Oxwas no only ela i ely high (Figu e 1D),
bu he measu ed capaci ance scaled linea ly wi h he hickness
o he deposi ed P Oxlaye , in he same manne as he so-called
olume ic capaci o ma e ials PEDOT and I Ox.Wecalcula e
a olume ic capaci ance o 200 mF cm−3, a alue on-pa wi h
he bes op imized PEDOT o mula ions.[14]This finding begs
he ques ion as o he mechanism behind his high capaci y. PE-
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Figu e 2. Cyclic ol amme y scans (100 mV s−1, deoxygena ed PBS o pH 7.4) egis e ed in −1.0 o +1.0 V e sus Ag/AgCl ange o esh samples o :
A) P Ox,B)I O
x. While he I Oxsample shows a p og essi e inc ease in elec ochemical capaci ance, known as elec ode “ac i a ion,” P Oxshows he
opposi e beha io whe e he ea u es o he CV e ol e o esemble hose o P . By 40 scans, he scan s abilizes, and P Oxfilms beha e essen ially as P
o a high su ace a ea. Film hickness =240 nm, a ea o he samples =0.05 cm2. Impedance o P Oxand olume ic capaci ance emain unchanged ia
his p ocedu e.
DOT and I Oxs o e cha ge in hei olume due o pseudo a adaic
eac ions.[4]The PEDOT chains can be e e sibly oxidized and
educed, and coun e ions can be anspo ed in and ou o he
PEDOT laye .[15]In I Ox, e e sible edox eac ions a e a ailable,
especially he I 3+/I 4+couple. This makes I Oxa alence-change
oxide which can e e sibly s o e cha ge in i s bulk. We pe o med
a se ies o expe imen s o es i he bulk o he P Oxbeha es anal-
ogously o I Ox, wi h bulk elec ochemical e e sibili y. I is well-
es ablished ha he su ace o pla inum is e e sibly oxidized, as
an oxide o ms du ing anodic pola iza ion and is subsequen ly
educed du ing a ca hodic scan.[16]The ques ion is i pla inum
oxide in he bulk is elec ochemically accessible o edox ac i -
i y in a way analogous o I Ox. Ou expe imen s show ha in
a po en ial window ele an o bioelec onics, i.e., neu os im-
ula ion, he e is no e idence o bulk e e sibili y. In epea ed
cyclic ol amme y scans om +1 o−1 V e sus Ag/AgCl in de-
oxygena ed PBS (Figu e 2) we see a dec ease in peak ampli ude
and a ea on P Ox, o yield a CV eminiscen o no mal P , albei
wi h high capaci ance (Figu e 2A). The fi s ew scans show high
educ ion cu en s, which we in e p e as elec ochemically ac-
cessible pla inum oxides being educed o pla inum. O e he
cou se o 40 cycles, he CV s abilizes o a amilia one o P .
E en he highly cha ac e is ic p o on adso p ion peaks[16] om
he o ma ion o P -H a e appa en . This e e sion o he CV
cha ac e is ics o hose esembling pla inum would indica e ha
he P Oxsample is, ne , elec ochemically beha ing as P i sel ,
and he oxide is bu ied wi hin elec ochemically inaccessible ac-
ions o he sample. I Oxelec odes, unde he same scanning
condi ions, show p og essi ely highe ampli ude peaks, bo h ca-
hodic and anodic, o e he whole scan ange (Figu e 2B).[4]This
p ocess o inc easing cha ge s o age capaci y is indica i e o in-
c easing hyd a ion o he laye and high e e sibili y o he I e-
dox eac ions, and is in he li e a u e on such elec odes known
as ac i a ion o p econdi ioning. The exchange o H+and OH−
in o he laye is epo ed o accompany he alence change eac-
ions o I .[17]Thus, well-hyd a ed I Oxlaye s a e associa ed wi h
high elec ochemical capaci ance. This analogous “ac i a ion”
p ocess does no appea in P Oxfilms. We can su mise ha hese
P Oxlaye s consis s o po ous conduc i e suppo , which om
ene gy-dispe si e X- ay spec oscopy (EDX) we can conclude
is la gely oxidized, wi h elec ochemically ac i e pla inum a
he su ace.
The i e e sibili y o P Ox edox beha io is co obo a ed by
measu ing oxygen con en in he films be o e and a e gal-
anos a ic educ ion and hen eoxida ion. I he eac ion we e
e e sible, educ ion is expec ed o yield lowe measu ed oxy-
gen con en while eoxida ion should es o e he oxygen con-
en . While he e is a small deg ee o e e sibili y o oxygen con-
en appa en in P Oxdu ing educ ion/ eoxida ion due o well-
known su ace oxida ion/ educ ion, he le el o e e sibili y is
much highe in I Oxsamples (Table 1). This dis inc ion in e-
e sibili y we ound is also appa en in expe imen s o epea ed
CVs (2600 cycles in −0.3 o +0.1 V s Ag/AgCl ange). While
he oxygen con en o P Ox alls, I Ox e ains i s o iginal s oi-
chiome y (Table 1). The oxygen s oichiome y imbalance can be
mechanis ically a ionalized in he ollowing way: The gal anos-
a ic applica ion o nega i e cu en can d i e a adaic eac ions
like wa e educ ion, while gal anos a ic applica ion o posi i e
cu en leads o oxida ion o exposed me allic pla inum, bu only
a he elec ochemically accessible su ace. Mos o he cu en
goes in o wa e oxida ion. I is well-es ablished ha wa e oxi-
da ion p oceeds elec oca aly ically om he oxidized pla inum
su ace.[18]The o ma ion o a su ace oxide is well-es ablished,
bu he oxide hickness is e y limi ed and does no ing ess in o
he film wi h highe applied ol age. Meanwhile, in I Ox, elec o-
chemical edox eac ions occu h oughou he bulk o he sam-
ple, allowing he oxygen con en o change.
To u he p obe he dis inc ion in elec ochemical eac ions
occu ing a P Ox e sus I Ox, we measu ed elec ochemical ca-
paci ance in aqueous and nonaqueous elec oly e using 0.05 m
LiClO4, as his elec oly e sal is soluble in bo h wa e and ace-
oni ile. The esul s a e shown in Table 2. In he case o I Ox, he
measu ed capaci ance dec eases by a ac o o h ee when mea-
su ed in nonaqueous elec oly e. This can be unde s ood since
wi hou wa e , he pseudo a adaic eac ions affo ding he high ca-
paci y a e blocked. These eac ions equi e exchange o H+and
H2O wi h he bulk o he film. We do no obse e his kind o
d op in capaci ance o P Ox, howe e . In ac , he measu ed ca-
paci ance alue is abou 10% highe in ace oni ile han in wa-
e . This signals ha he cha ge s o age mechanism in P Oxdoes
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Table 1. Change in he oxygen con en in laye s (100 nm) subjec ed o elec ochemical educ ion and eoxida ion ea men s. Due o limi a ions o he
EDX assay,[11] he esul s should be ea ed as semi-quan i a i e.
O con en [a om%] by EDX
Elec ochemical expe imen S age o he expe imen P OxI Ox
Gal anos a ic educ ion/ eoxida ion:
high cu en densi y
F esh sample 45.2 47.3
A e educ ion: −5mAcm
−2, 60 s 22.9 40.9
A e eoxida ion: +5mAcm
−2, 60 s 26.3 46.0
Reco e y o ini ial O con en (a om%) 15.0% 80.0%
Gal anos a ic educ ion/ eoxida ion:
low cu en densi y
F esh sample 45.2 48.1
A e educ ion: −70 μAcm
−2, 2500 s (3.5 mC) 37.4 41.5
A e eoxida ion: +40 μAcm
−2, 4750 s (3.8 mC) 39.6 47.3
Reco e y o ini ial O con en (a om%) 27.1% 87.5%
CV cycling: −0.3 o +0.1 V,
100 mV s−1
F esh sample 45.2 47.6
A e 2600 CV cycles (5 h 45´) 37.6 47.4
Dec ease o he O con en (a om%) 17.5% 0.4%
no equi e aqueous ion exchange as in I Ox. The highe mea-
su ed capaci ance o nonaqueous condi ions in P Oxcan be ex-
plained by he lowe iscosi y o ace oni ile, which allows be e
elec oly e pene a ion in o he po ous s uc u e o P Ox. Taken
oge he , hese esul s all implica e he P Oxfilm as a po ous
double-laye capaci ance P ma e ial wi h a adaic p ope ies e-
sembling P , and no he pseudo a adaic mechanism pos ula ed
o he alence-change oxide I Ox.
2.2. Op imizing Deposi ion Condi ions o P OxElec odes
We uned he eac i e spu e ing condi ions (spu e ing p essu e,
powe , DC e sus RF plasma, and oxygen:a gon a io) o yield
an op imized P Oxwi h espec o high elec ochemical capac-
i y and low impedance. We also es ima ed he oxygen con en
in he films. Va ying he spu e ing condi ions, when conside -
ing he final oxygen con en o he film as es ima ed by EDX
analysis, se e al ends a e appa en : Fi s , mo e oxygen added
in he spu e ing mix u e always co ela es wi h highe oxygen
con en . Fo a fixed A :O2 a io, DC spu e ing always yields less
oxygen con en han he analogous p ocess using RF. Rega dless
o plasma ype, inc easing powe always co ela es wi h less oxy-
gen con en . O e all wo king p essu e does inc ease oxygen con-
en , also he wo king p essu e has a subs an ial effec on film
oughness and mo phology. While hese diffe en pa ame e s
may yield po ous P Oxfilms sui able o many in e es ing appli-
ca ions, anging om ca alysis o biosenso s, ou p esen ocus
Table 2. Capaci ance alues calcula ed om CV scans in aqueous e sus
nonaqueous elec oly es. Film hickness =100 nm.
Ma e ial Elec oly e sol en A eal capaci ance
[mF cm−2]
I OxAce oni ile 0.41
H2O 1.17
P OxAce oni ile 1.17
H2O 1.06
is on low-impedance/high capaci y elec odes o bioelec onics.
The e o e, we e alua ed hose wo pa ame e s only, and in a di-
ec compa ison (Table 3) he bes -pe o ming films a e p epa ed
ia he ollowing condi ions: (DC, 135 W, 47 mTo , 20% O2). This
op imized coa ing o P Oxwas compa ed in mic oelec odes wi h
wo well-known low impedance coa ings: PEDOT:PSS and I Ox.
This yielded he compa ison o impedance shown in Figu e 1C.
2.3. Bio ouling Tes s and Compa a i e S abili y
Du ing ope a ion o bioelec onics de ices, physiological condi-
ions ep esen a ha sh en i onmen and he issue o bio ouling
is impo an . Adhesion o p o eins and o he biomolecules on he
su ace can se e ely deg ade pe o mance o elec ode ma e ials.
We compa ed samples made using he op imized p o ocol (DC
135 W, 47 mTo , 20% O2) wi h I Oxand PEDOT as e e ences.
In hese expe imen s, we p epa ed MEAs wi h Ø 40 μm mic o-
Table 3. Es ima ed oxygen con en in P Oxlaye s (120 nm) and elec o-
chemical capaci y as a unc ion o diffe en spu e ing pa ame e s. Due
o sa e y limi a ions o he spu e ing sys em, deposi ions a e limi ed o
<20% O2in he spu e ing gas mix u e. *Sample hickness homogenei y
was low, he e o e easonable es ima ion o capaci ance was N/A.
P Oxp epa a ion p ocedu e O con en [a om%]
by EDX
Capaci ance
[mF cm−2]
Re . [11] p ocedu e, Elec ochemical
educ ion −0.8 V 60 s
44.9 0.612
RF 60 W, 25 mTo , 15% O256.1 N/A*
RF 135 W, 25 mTo , 15% O250.3 1.159
RF 135 W, 30 mTo , 15% O251.3 1.220
RF, 135 W, 47 mTo , 20% O255.5 1.305
DC 135 W, 28 mTo , 20% O240.5 0.763
DC 135 W, 37 mTo , 20% O244.5 1.216
DC,60W,47mTo ,20%O
250.6 N/A*
DC 135 W, 47 mTo , 20% O245.2 1.517
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Figu e 3. Rela i e s abili y o P Ox,I O
x, and PEDOT:PSS laye s (240 nm, on Ø40 μm Au mic oelec odes) subjec ed o bio ouling (A, C, and E) and
accele a ed aging es s a 60°(B, D, and F). A,B) Change in absolu e impedance a 1 kHz. C,D) Change in peak ol age alues o ca hodic phase
o 200 μCcm
−2phase−1biphasic pulse. These a e ca hodic ol age excu sions om baseline measu ed e sus an AgCl e e ence elec ode in h ee-
elec ode configu a ion. E,F) Change in olume ic capaci ance. All esul s o gi en ma e ial a e a e aged o e 3–4 diffe en mic oelec odes. Samples
a e emo ed om he aging chambe and measu ed, hen e u ned o he gi en aging condi ion.
elec odes made o plana gold, coa ed wi h 240 nm o each o he
espec i e coa ings.
The s abili y o he MEA samples was e alua ed in wo se s o
s abili y es s: The fi s add essed longe - e m bio ouling, whe e
he samples we e s o ed in 2% bo ine se um albumin in 1×PBS
a 37 °C o e he cou se o 30 days. The second was an accele a ed
aging es , whe e he samples we e s o ed in 1×PBS a 60 °C.
O e he cou se o bo h o hese p o ocols, samples we e egu-
la ly emo ed and es ed ia impedance measu emen s, CV cy-
cles, and a measu emen o ol age excu sions du ing applica ion
o biphasic cu en s imula ion pulses wi h 200 μCcm
−2phase−1.
The esul s o key figu es o me i : impedance alue a 1 kHz, ol-
ume ic capaci ance alues, and ca hodic ol age excu sion du -
ing pulsing show a c i ical compa ison be ween he h ee coa -
ings (Figu e 3). PEDOT and I Oxgene ally ou pe o m P Oxin
e ms o % change o e ime, hough P Oxlaye s demons a ed
some su p ising quali ies such as s able olume ic capaci ance
which was consis en ly mo e han wice as high as ha o PE-
DOT. O e all, he long- e m s abili y o P Ox, howe e , was poo e
han he e e ence samples I Oxand PEDOT. Though he pe o -
mance o P Oxdec eased, impedance emains an o de o mag-
ni ude be e han P alone. We would specula e ha he decline
in P Oxpe o mance is due o agg ega ion effec s which lead o
a dec ease in effec i e su ace a ea o some ex en .
3. Conclusions
P Ox, p epa ed by eac i e spu e ing, ep esen s an in e es ing
and po en ially compe i i e a ia ion o he ange o pla inum
modifica ions ha a e being pu sued in he bioelec onics field.
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I s p epa a ion is ully compa ible wi h mic op ocessing, easily
in eg a able wi h li hog aphy o ins ance. The many a iables
a ailable in eac i e spu e ing echniques, o which we ha e only
explo ed a ew, can po en ially p o ide a wide pale e o unc-
ional p ope ies o P Ox hin films. F om a undamen al poin o
iew, we ha e c i ically add essed he ques ion o cha ge s o age
mechanism in hese films: hey a e essen ially a highly po ous a-
ie y o pla inum. Mac oscopically, hese films show a ema kable
„ olume ic capaci ance″, wi h capaci ance scaling wi h hick-
ness. The olume ic capaci ance alues exceed, by mo e han a
ac o o wo, he well-known ma e ial PEDOT:PSS. Th ough a se-
ies o elec ochemical and elemen al analysis we compa e P Ox
wi h I Ox. Using hese compa a i e me hods, we show, ha he
high olume ic capaci ance effec in P Oxo igina es om high
po osi y o he films a he han a bulk e e sible edox chemis y
as is he case o I Ox(o PEDOT:PSS). The obse a ion sugges s
ha he concep o olume ic capaci ance does no necessa ily
apply only o pseudo a adaic ma e ials. In e ms o bioelec onic
mic oelec odes, he P Oxb ings pla inum o he le el o com-
pe i i e pa ame e s o impedance and capaci ance o he nex -
gene a ion ma e ials like I Oxand PEDOT. In con as o I Ox,
P Oxwo ks well al eady “as- ab ica ed” and does no equi e ex-
ensi e elec ochemical condi ioning. By i ue o ease o p ocess-
abili y and especially much lowe cos compa ed o I Ox, he use
o P Oxmay be indica ed in many applica ions o mic o ab i-
ca ed bioelec onics de ices.
4. Expe imen al Sec ion
Ma e ials:Spu e ing a ge s o Vaksis 3 m sys em (2.00″Dia. ×
0.125″ hick) we e supplied by Tes bou ne L d (P , 99.99%) and No dic
High Vacuum AB (Au, 99.99%). PEDOT:PSS (Cle ios PH1000 o mula-
ion) was supplied by He eaus. Dichlo o[2.2]pa acyclophane (Pa ylene-C
p ecu so ) was supplied by TiXX Coa ings L d. All o he chemicals we e
supplied by Sigma-Ald ich and used wi hou any pu ifica ion. Bo ine albu-
min se um used in he s abili y es s was he hea shock ac ion, pH 5.2,
≥96%.
P epa a ion o Au Coa ed Subs a es o Elec odes o 0.05 cm2Ac i e A ea:
PET oil (Polic om sc eens, 120 μm hick) was selec ed as a subs a e due
o he ac ha i p o ides good adhesion o laye s o spu e ed Au wi hou
he need o any addi ional s icking laye . Be o e Au deposi ion, he oil was
cu o ings o Ø 11.5 cm and cleaned by subsequen sonica ion (10 min)
in isop opanol and DI wa e . A e d ying, i was loaded in o a Vaksis 3 m
spu e ing sys em wi hou any p e-ac i a ion wi h oxygen plasma o ozone.
A e e acua ion o he acuum chambe o p essu e <1×10−5To , Au
laye (100 nm) was spu e ed a 3.8–4.0 mTo p essu e o pu e a gon us-
ingaDCmagne ona 100W.
Reac i e Spu e ing o P Ox:All deposi ions o me al oxides we e done
by magne on spu e ing using an A :O2mix u e. The p essu e was egu-
la ed wi h he ga e al e and o a ion eloci y o he u bomolecula pump.
The subs a es we e ne e hea ed. To ab ica e 0.05 cm2elec odes, Au-
coa ed PET oil was cu wi h scisso s o pieces o ap. 2 ×2cmsize(ap-
p op ia e o he cell used o he elec ochemical cha ac e iza ion) which
we e moun ed by he edges o he sample holde wi h Kap on ape. A -
e P Oxdeposi ion, ac i e a ea o he laye was limi ed o 0.05 cm2wi h
a sel -adhesi e oil (wi h Ø 2.5 mm opening). In he case o mic oelec-
odes, be o e loading o he spu e ing chambe , subs a es we e ea ed
wi h oxygen plasma (50 W, 120 s, Diene elec onic GmbH). Deposi ion
o he P Oxwas p eceded (in he same acuum cycle) by spu e ing o a
smoo h laye o P (10 nm) o imp o e adhesion o he oxide coa ing. In he
case o MEA samples in he final s ep o he ab ica ion sac ificial Pa ylene-
C laye was peeled off wi h he assis ance o Kap on ape, ai s eam and
i needed, mild sonica ion in DI wa e .
Thickness Con ol o he Spu e ing Deposi ed Laye s in Vaksis 3 mSys em:
Thickness o all laye s was con olled by he du a ion o he p ocess (based
on he known and well- ep oducible alues o a e) and always in good ac-
co dance wi h eadings o qua z c ys al moni o o he spu e ing sys em.
Tooling ac o s, as well as he alues o a e, we e p ede e mined by ime-
con olled spu e ing on glass subs a es, wi h hickness measu ed wi h a
Dek ak XT (B uke ) p ofilome e .
Fab ica ion o Mic oelec ode Samples:Silicon wa e s wi h 2.5 μm he -
mally g own oxide laye we e used as he subs a e. The me al leads and
in e connec s consis ing o 20 nm Ti and 50 nm Au we e deposi ed by an
elec on-beam e apo a ion sys em (BESTEC), pa e ned by pho oli hog a-
phy (AZ 1518) and e ched wi h KI:I2:H2O gold e chan and HF:H2O2:H2O
i anium e chan . The encapsula ion was p o ided wi h a 2 μmlaye o
Pa ylene-C (SCS Labco e ) g own by chemical apo deposi ion in p es-
ence o an adhesion p omo ing silane A-174. The me al pads we e exposed
by pho oli hog aphy (AZ 1518) and eac i e ion e ching (RIE, O2plasma,
200 W). To acili a e he pa e ning o he elec ode coa ing ma e ials (I Ox,
P Ox, and PEDOT:PSS), ano he , sac ificial, laye o 2 μm Pa ylene-C was
deposi ed on op o he encapsula ion laye wi h a 2% Mic o-90 an i-
adhesi e coa ing in be ween. The elec ode a ea (40 μm diame e ) was
exposed by pho oli hog aphy (AZ 9260) and eac i e ion e ching (RIE, O2
plasma, 200 W). Thus p epa ed subs a es we e ac i a ed by an O2plasma
jus be o e deposi ion o he elec ode ma e ial. The sac ificial Pa ylene-C
laye was hen peeled off o yield he mic oelec ode a ay.
PEDOT:PSS was deposi ed by spin coa ing om a sonica ed dispe -
sion o PEDOT:PSS (Cle ios PH 1000, He aeus), 5 w % e hylene glycol,
0.1 w % dodecyl benzene sul onic acid, and 1 w % o (3-glycidyloxyp opyl)-
ime hoxysilane (added and mixed jus be o e he spin coa ing). A e he
peel-off, he PEDOT:PSS laye was annealed a 140 °C o 45min.
I Oxwas deposi ed by magne on spu e ing acco ding o p e iously
published me hods,[17]and pa e ned ia Pa ylene-C peel-off as desc ibed
abo e.
SEM Imaging and EDX Analysis:SEM imaging and EDX analysis we e
pe o med wi h SEM Zeiss Sigma-500, equipped wi h B uke Nano XFlash
610 m De ec o (silicon d i de ec o (SDD), wi h he moelec ic (Pel ie )
cooling and ligh elemen window), se a ele a ion angle 35°. All samples
we e con ac ed o he mic oscope s age using coppe ape, a e washing
wi h DI wa e and d ying in a s eam o N2. Images we e aken wi h he
in-lens seconda y elec on de ec o a an accele a ion ol age o 7–15 kV,
30 μm beam ape u e, and 3.3–3.6 mm wo king dis ance, wi h he chambe
e acua ed o p essu e <1×10−5mba . Elec on beam shi was used o
mo e he imaging a ea wi hou in oducing ib a ions.
In EDX analysis, he P/B-ZAF s anda dless me hod was always used
wi h ca bon decon olu ion, as quan ifica ion co ec ion, and ollowing
spec al lines: 0.525 keV o O (K𝛼) and 2.048 keV o P (M). To maximize
he de ec o coun a e, he wo king dis ance was 10–11 mm, he beam
ape u e was se o 120 μm wi h he high cu en mode enabled. To com-
pensa e o su ace oughness and possible sensi i i y o he noble me al
oxides o he elec on beam, low image magnifica ion (×1000) and accel-
e a ion ol age (6 kV) we e always used. Resul s, gi en as P /O a om%,
we e a e aged o e 6 measu emen s, done in diffe en pa s o he spec-
imen. Va iabili y (as ela i e s anda d de ia ion) was ela i ely low (lowe
han he ela i e assay e o , calcula ed by he so wa e). Using highe ac-
cele a ion ol ages (up o 12 kV) usually ga e simila alues o a om% bu
wi h highe a iabili y. Fo he pu pose o he analysis, in o de no o o e -
es ima e me al con en in he oxide, o elimina e signal coming om he
subs a e a designa ed se o samples was p epa ed, in an analogous way
like o he elec ochemical cha ac e iza ion wi hou 10 nm o smoo h P
in e laye .
Elec ochemical Cha ac e iza ion:Cha ac e iza ion o all samples was
done wi h an I ium echnologies Ve ex One po en ios a and phospha e
buffe saline (1×PBS: 10 mm phospha e, pH 7.4; 138 mm NaCl, 2.7 mm
KCl in DI wa e ) as he elec oly e. 0.05 cm2samples we e elec ically con-
ac ed wi h coppe ape (a ached o he uncoa ed a ea o he subs a e)
and placed in a 15 mL double-sided elec ochemical cell (Redox.me),
equipped wi h a P coil as a coun e elec ode, and an Ag/AgCl wi e as a
pseudo e e ence elec ode. The elec oly e was deoxygena ed be o e mea-
su emen s by pu ging wi h a gon. Mic oelec odes we e measu ed wi h
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pla inum pla e as coun e elec ode and Ag/AgCl pseudo e e ence elec-
ode confined in a sy inge, filled wi h he elec oly e by adjus ing he p es-
su e wi h ano he sy inge connec ed by a flexible ubing. The sample was
p ecisely posi ioned unde he sy inge ip o achie e he elec oly ic con-
ac wi hou any mechanical connec ion. The elec ical con ac was ca e-
ully es ablished wi h a needle p obe placed on he con ac pad. In case o
mic oelec ode measu emen s, deoxygena ion o he elec oly e was no
possible. The cyclic ol amme y expe imen s we e pe o med wi h he
100 mV s−1scan a e and +0.1 o +0.3 V scanning ange o he pu pose
o es ima ing capaci ance o he elec odes. Scans a e conduc ed in his
ange o ob ain a s able capaci i e-cha ging ype ansien cu en , his be-
ha io s abilizes a e he fi s scan. Elec ochemical double-laye capaci-
ance was calcula ed by in eg a ing he cu en in his cha ging/discha ge
egion, acco ding o he equa ion desc ibed in Chap e 1 o he Ba d and
Faulkne ex book.[19]Elec ochemical impedance spec a we e eco ded
using a single-sine p obing AC ol age o 10 mV ampli ude a a DC bias
o 0 V e sus he e e ence elec ode wi h 20 poin s pe decade. Biphasic
pulsing s ess es was done using he same equipmen o a cha ge densi y
o 200 μCcm
−2phase−1in he ollowing sequence: 50 μs es ime(wi h
I=0 A), 250 μs ca hodic-leading squa e wa e o m, 50 μs in e pulse ime
(I=0 A), 250 μs anodic squa e wa e o m, 50 ms es ime (wi h I=0A).
The esul (as V= ( ) cu e, measu ed s Ag/AgCl pseudo e e ence elec-
ode wi h 10 μs in e al ime) was a e aged o e 10 consecu i e pulses.
The peak alue o he ca hodic phase is gi en as ol age e sus baseline
(a e aged V eco ded du ing ini ial 50 μs o he pulsing sequence).
Acknowledgemen s
The au ho s hank Ma y Donahue o help wi h PEDOT sample p epa a-
ion. This wo k has been suppo ed by he Eu opean Resea ch Council
(ERC) unde he Eu opean Union’s Ho izon 2020 esea ch and inno a-
ion p og am (E.D.G. g an ag eemen No. 949191), by he G an Agency
o he Czech Republic unde con ac 23-07432S, and by unding om
he Na ional Cen e o Neu ological Resea ch, suppo ed by he MEYS
CR (LX22NPO5107). Sample ab ica ion was suppo ed by CzechNanoLab
Resea ch In as uc u e suppo ed by MEYS CR (LM2023051).
Open access publishing acili a ed by Vysoke uceni echnicke B ne, as
pa o he Wiley - CzechELib ag eemen .
Conflic o In e es
The au ho s decla e no conflic o in e es .
Da a A ailabili y S a emen
The da a ha suppo he findings o his s udy a e a ailable om he co -
esponding au ho upon easonable eques .
Keywo ds
bioelec onics, biomedical mic ode ices, elec ochemis y, pla inum, eac-
i e spu e ing
Recei ed: July 26, 2023
Re ised: May 14, 2024
Published online:
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