Academic Edi o : Qingqing Ke
Recei ed: 10 Oc obe 2025
Re ised: 10 No embe 2025
Accep ed: 17 No embe 2025
Published: 20 No embe 2025
Ci a ion: Mak i, S.P.; Kou sou ea, S.;
G igo opoulos, A.; Be kesi, K.;
Ka sinis, M.; Deligkiozi, I.;
Zoikis-Ka a hanasis, A. De elopmen
o G een Lignin–MWCNTs Hyb ids
o Sus ainable Conduc i e Ma e ials.
Elec onics 2025,14, 4539.
h ps://doi.o g/10.3390/
elec onics14224539
Copy igh : © 2025 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
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condi ions o he C ea i e Commons
A ibu ion (CC BY) license
(h ps://c ea i ecommons.o g/
licenses/by/4.0/).
A icle
De elopmen o G een Lignin–MWCNTs Hyb ids o
Sus ainable Conduc i e Ma e ials
So ia P. Mak i 1,* , S e ania Kou sou ea 1, Alexios G igo opoulos 1, Ka a Be kesi 1, Michalis Ka sinis 1,
Ioanna Deligkiozi 2and Alexand os Zoikis-Ka a hanasis 1,*
1
C ea i e Nano PC, 43 Ta oiou S ee , Me amo osi, 14451 A hens, G eece; s.kou sou ea@c ea i enano.g (S.K.);
a.g igo opoulos@c ea i enano.g (A.G.); k.be kesi@c ea i enano.g (K.B.); [email p o ec ed] (M.K.)
2AXIA Inno a ion GmbH, F i z-Hommel-Weg 4, 80805 München, Ge many; [email p o ec ed]
*Co espondence: [email p o ec ed] (S.P.M.); [email p o ec ed] (A.Z.-K.)
Abs ac
The inc easing en i onmen al impac o elec onic was e has in ensi ied he pu sui o
sus ainable ma e ials o manu ac u ing g een elec onics. This s udy p esen s he de el-
opmen o lignin-based hyb ids wi h mul i-walled ca bon nano ubes (MWCNTs) ia an
en i onmen ally iendly ul asonica ion p ocess in aqueous medium. Two hyb id ma e i-
als con aining 10 and 20 w % MWCNTs we e syn hesized and ho oughly cha ac e ized.
DLS analysis e ealed be e dispe sion and colloidal s abili y due o s ong physicochemi-
cal in e ac ions be ween lignin and MWCNTs, while SEM and TEM images con i med a
con inuous lignin ma ix embedding an in e connec ed MWCNT ne wo k. Raman spec-
oscopy indica ed s uc u al o de ing wi hin he hyb ids. The elec ical conduc i i y o
he hyb ids eached 2–5 S/cm as e alua ed by ou -poin p obe measu emen s, despi e he
high lignin con en (80–90 w %). Elec ochemical analysis sugges ed signi ican ly enhanced
edox ac i i y and elec on ans e kine ics, wi h measu ed elec oac i e su ace a eas
inc easing up o 21- old la ge compa ed wi h he unmodi ied elec ode. The syne gy
be ween lignin and MWCNTs enabled he o ma ion o a conduc i e ne wo k, highligh ing
hese hyb ids as p omising, cos -e ec i e, and sus ainable ma e ials o conduc i e and
elec ochemical applica ions in nex -gene a ion g een elec onics.
Keywo ds: lignin; MWCNTs; hyb ids; biobased; elec ical conduc i i y; modi ied
elec odes
1. In oduc ion
The apid g ow h o he elec onics indus y has led o an inc easing demand o
ad anced elec onic p oduc s, esul ing in he gene a ion o ising olumes o elec onic
was e (E-was e) [
1
]. E-was e o en con ains haza dous subs ances ha pose se ious en i-
onmen al and heal h isks i no disposed o p ope ly [
2
]. Thus, he a en ion has u ned
o he de elopmen o “g een” elec onics manu ac u ed om enewable, biodeg adable,
o en i onmen ally benign ma e ials [1].
Ma e ials o igina ing om biomass, such as cellulose, chi in, lignin, and o he s, ha e
eme ged as p omising al e na i es o de eloping g eene elec onic componen s. Lignin,
he second mos abundan biopolyme a e cellulose, is a byp oduc o he pape and
pulp indus y. I s polyphenolic s uc u e and unc ional g oups (hyd oxyl, ca bonyl, and
ca boxyl g oups) a y depending on biomass sou ce (ha dwood, so wood, o g ass) and
ex ac ion me hod [
3
,
4
]. Due o i s chemical s uc u e, abundan unc ional g oups, and
Elec onics 2025,14, 4539 h ps://doi.o g/10.3390/elec onics14224539
Elec onics 2025,14, 4539 2 o 20
biodeg adabili y, lignin is inc easingly employed as a sus ainable al e na i e o ossil-based
ma e ials o a ious applica ions [5–7].
MWCNTs consis o mul iple g aphene shee s in a coaxial a angemen and can exhibi
semiconduc ing o me allic beha io , depending on hei s uc u e [
8
,
9
]. Owing o hei
good elec ical, he mal, and mechanical p ope ies, hey a e widely used in elec onics
and sensing applica ions [
10
,
11
]. Howe e , CNTs end o agg ega e in mos sol en s due o
s ong
π
–
π
s acking and an de Waals o ces, which signi ican ly limi hei p ocessabili y
and pe o mance [
12
]. To imp o e hei dispe sion in wa e media, ossil-based su ac an s
such as ionic SDS, ca ionic CTAB, and non-ionic T i on X-100 a e o en used [13–18].
A ew s udies ha e demons a ed ha lignin can se e as an e icien dispe san o
CNTs, o e ing compa able o e en supe io pe o mance o con en ional su ac an s. I s
amphiphilic na u e and a oma ic s uc u e enable s ong
π
-
π
in e ac ions wi h CNTs, im-
p o ing dispe sion in aqueous and o he pola media. Goodman e al. demons a ed ha al-
kali lignin dispe sed MWCNTs mo e e ec i ely han SDS and CTAB a lignin:CNT a ios be-
ween 1:20 and 5:1 [
19
]. Simila ly, Rochez e al. achie ed long- e m s able lignin/MWCNT
dispe sions (1:1–6:1), wi h he 2:1 a io showing op imum s abili y. When inco po a ed in o
a PP-g-MA la ex, hese dispe sions yielded uni o m composi e ilms wi h su ace esis i i y
o app oxima ely 10
5Ω
/sq [
20
]. Teng e al. [
21
] u he con i med ha ac iona ed lignin
dispe sed MWCNTs mo e e ec i ely han aw lignin in DMF a a lignin:MWCNT a io o
1:3.1, wi h he low-molecula -weigh ac ion yielding he mos homogeneous dispe sion
due o s onge π–πin e ac ions.
The enhanced dispe sion p o ided by lignin and he syne gis ic in e ac ions be ween
he wo componen s in lignin–CNT sys ems ha e also been shown o imp o e he elec ical
conduc i i y and he moelec ic pe o mance o CNT-based ma e ials, as epo ed in se e al
s udies. Zhang e al. epo ed ha inc easing lignin con en (11.8–23.1 w %, co esponding
o lignin:CNT a ios o 1:7.5–1:3.3) imp o ed CNT ex olia ion and cha ge anspo , yielding
ilms wi h a powe ac o up o 198
µ
W m
−1
K
−2
[
7
]. Likewise, Culeb as e al. demons a ed
ha lignin-doped CNT ya ns achie ed conduc i i ies up o 157.6 S cm
−1
and a powe
ac o o 132
µ
W m
−1
K
−2
a 23 w % lignin, a ibu ed o
π
–
π
in e ac ions ha densi ied
CNT bundles and p omo ed e icien ca ie anspo [22].
Lignin has also been inco po a ed in o CNT inks and coa ings, whe e i se es a dual
ole as a dispe san and s uc u al s abilize . Kama udin e al. o mula ed an aqueous
MWCNT ink using alkali lignin, which educed nano ube agglome a ion and imp o ed
dispe sion s abili y [
23
]. Mo e ecen ly, Wang e al. ab ica ed conduc i e and supe hy-
d ophobic lignin/MWCNT coa ings wi h a 1:3 lignin:MWCNT a io, eaching conduc i i y
close o 0.289 S cm
−1
. Lignin was uni o mly embedded wi hin he CNT ne wo k h ough
hyd ogen bonding and
π
–
π
in e ac ions, e ec i ely p e en ing agg ega ion while imp o -
ing adhesion, ab asion esis ance, and long- e m s abili y [24].
In addi ion o hei s uc u al and conduc i e oles, lignin–CNT hyb ids ha e ecen ly
gained a en ion in elec ochemical and ene gy- ela ed applica ions [
25
,
26
]. Dege u e al.
epo ed a lignin-modi ied glassy ca bon elec ode (GCE) capable o ca alyzing his amine
oxida ion in phospha e bu e solu ion, demons a ing sensi i e de ec ion in biological
and ood samples [
27
]. Simila ly, CNT composi es wi h o he biopolyme s such as cellu-
lose and chi osan ha e been ex ensi ely s udied o elec ochemical sensing [
28
,
29
]. Fo
ins ance, CNT–cellulose and CNT–chi osan composi es ha e been widely employed in enzy-
ma ic bio uel cells and glucose biosenso s, demons a ing enhanced sensi i i y and powe
densi y [
30
–
32
]. Howe e , s udies employing elec odes modi ied wi h lignin/MWCNT
hyb ids emain limi ed. Fo example, Chokka eddy e al. de eloped lignin–MWCNT–CuO
nanocomposi es o elec ochemical sensing, which exhibi ed enhanced edox ac i i y due
o syne gis ic in e ac ions among he componen s [
33
]. Simila ly, Liu e al. designed lignin
Elec onics 2025,14, 4539 3 o 20
ibe –CNT–g aphene elec odes o Li–S ba e ies, achie ing imp o ed conduc i i y and
ene gy s o age pe o mance [30].
Some o us ecen ly epo ed ha only he inco po a ion o lignin–MWCNT hyb ids
as ille s in PLA composi es success ully impa ed elec ical conduc i i y, in con as o
PLA/lignin and PLA/MWCNT sys ems, which showed insula ing beha io [
34
]. Build-
ing on hese indings, his wo k in es iga es he in insic physicochemical and elec ical
p ope ies o he lignin–MWCNTs hyb ids hemsel es. Lignin, i sel a na u al biopolyme ,
was used as he ma ix o de eloping ca bon-based, g een, conduc i e hyb id ma e ial.
Lignin-based hyb ids con aining 10 and 20 w % MWCNTs we e p epa ed ia a simple,
one-s ep, en i onmen ally iendly ul asonica ion p ocess in aqueous medium. Lignin
se ed a dual unc ion: i ac ed as a na u al dispe san h ough non-co alen in e ac ions
wi h MWCNTs as well as a binding ma ix acili a ing he o ma ion o in e connec ed
conduc i e pa hways.
Small hyd odynamic diame e and s able ze a po en ial alues we e measu ed by DLS,
sugges ing good dispe sion and colloidal s abili y o he hyb ids. SEM and TEM images
e ealed a con inuous ne wo k o MWCNTs embedded wi hin he lignin ma ix. B oadband
dielec ic spec oscopy (BDS) and ou -poin p obe (4PP) measu emen s indica ed ha he
newly p epa ed hyb ids show excellen elec ical conduc i i y despi e hei high lignin
con en , an insula ing biopolyme . The ob ained alues we e signi ican ly highe compa ed
o hose epo ed o o he lignin–CNT sys ems wi h simila o e en highe MWCNTs
loadings and compa able o hose epo ed o composi es con aining polyme ic ma ices
syn hesized om biobased monome s. Mo eo e , he hyb ids achie ed high elec oac i i y
and cha ge- ans e e iciency e en a low CNT loadings as e idenced by cyclic ol amme y
(CV) measu emen s, highligh ing hei po en ial o he manu ac u ing o elec odes o
sensing applica ions.
2. Ma e ials and Me hods
2.1. Ma e ials
Soda lignin (P o obind 1000) was pu chased om Tano is AG (Ruschlikon, Swi ze -
land). Mul i-walled ca bon nano ubes wi h >96% pu i y and ou side diame e 8–18 nm we e
pu chased om Nanog a i (Anka a, Tu key). The po assium e icyanide (K
3
[Fe(CN)
6
]),
po assium chlo ide (KCl), and N,N-Dime hyl o mamide (DMF) we e ob ained om PENTA
(P ague, Czech Republic), while poly( e a luo oe hylene) (CF
2
CF
2
)
n
) and ca bon black
om Sigma-Ald ich (S . Louis, MO, USA). Elec ochemical cha ac e iza ion was pe o med
using a s anda d h ee-elec ode glass cell se up, wi h a glassy ca bon wo king elec ode
(
d=3mm
) in a Te lon body, a pla inum mesh coun e elec ode, and an Ag/AgCl ( illed
wi h 3 M KCl) e e ence elec ode (d = 6 mm), all sou ced om S ony Lab (New Yo k, NY,
USA). Re e se osmosis (RO) wa e ob ained om an in-house pu i ica ion sys em was used
o all dispe sions and sample p epa a ions.
2.2. Syn hesis o MWCNTs–Soda Lignin Hyb ids
Fo he p epa a ion o 10% MWCNTs-soda Lignin (sL) hyb id, 0.5 g o MWCNTs was
added in a double laye jacke ed beake con aining 500 mL RO wa e unde magne ic
s i ing. The mix u e was subjec ed o ul asonica ion (US) using an Ul asound p ocesso
(UP200S , 26 kHz, 200 W; Hielsche Ul asonics GmbH, Tel ow, Ge many) equipped wi h
a sono ode (S26d14; Hielsche Ul asonics GmbH, Tel ow, Ge many) o 1 h. Then, 4.5 g
o soda lignin was added and US was applied o 2 mo e hou s. The empe a u e o he
dispe sion du ing he en i e US p ocess was main ained a 25
◦
C using an ex e nal cooling
ci cula o connec ed o he double laye beake jacke . A e US, he aqueous dispe sion was
eeze-d ied o ob ain MWCNTs–sL hyb id ma e ial in powde o m. Fo he p epa a ion
Elec onics 2025,14, 4539 4 o 20
o 20% MWCNTs-sL hyb id, he same p ocess was ollowed using 1.0 g o MWCNTs and
4.0 g o sL. US was ca ied ou unde ixed se ings: 200 W powe , 80% ampli ude, and a
50% pulse du y cycle.
2.3. Expe imen al Me hods
2.3.1. Dynamic Ligh Sca e ing (DLS)
The hyd odynamic diame e (HDD) and ze a po en ial (ZP) alues we e de e mined
using DLS wi h a Li esize 500 (An on Paa GmbH, G az, Aus ia) ins umen . The samples
we e dilu ed o a concen a ion o 100–200 ppm and we e sonica ed in a ba h o 5 min
o ensu e uni o m dispe sion be o e measu emen . Addi ionally, he su ace cha ge o
he dispe sion was de e mined h ough ZP measu emen . The measu emen s we e aken
in iplica e.
2.3.2. Scanning Elec on Mic oscopy (SEM)
The su ace mo phology o s a ing ma e ials and hyb ids we e analyzed using SEM
analysis wi h a TESCAN VEGA COMPACT (TESCAN GROUP, a.s., B no, Czech Republic)
ins umen a an accele a ing ol age o 10–15 keV. P io o SEM imaging, he samples we e
gold-coa ed using he SC7620 ‘Mini’ Spu e Coa e /Glow Discha ge Sys em (Quo um
Technologies L d., Laugh on, UK).
2.3.3. T ansmission Elec on Mic oscopy (TEM)
Nanoscale in es iga ion was pe o med wi h a high esolu ion JEOL JEM-2100 LaB6
ansmission elec on mic oscope (HRTEM) (JEOL L d., Tokyo, Japan), ope a ing a 200 kV.
The samples (~0.1 g) we e suspended in deionized wa e and ea ed wi h ul asound o
disagg ega e he agglome a ed pa icles. A d op om he suspension was hen placed on
a 300-mesh ca bon coa ed coppe g id and ai -d ied o e nigh . Elemen al analyses we e
ca ied ou using an Ox o d X-Max 100 Silicon D i Ene gy Dispe si e X- ay spec ome e
(Ox o d Ins umen s, Abingdon, UK), connec ed o TEM, wi h a p obe size anging om
2 o 5 nm in STEM mode.
2.3.4. B oadband Dielec ic Spec iscopy (BDS)
The elec ical conduc i i y a RT~20
◦
C was e alua ed by dielec ic spec oscopy
and by means o No ocon ol BDS se up (No ocon ol GmbH, Mon abau , Ge many)
on he sample in he o m o hin disks (~1 mm in hickness). Upon applica ion o an
al e na e o he sample capaci o , he complex dielec ic pe mi i i y,
ε
* =
ε′−
i
ε′′
, was
eco ded iso he mally as a unc ion o equency, , in he ange om 10
−1
o 10
6
Hz. The
dependence o complex elec ical conduc i i y,
σ
*, was es ima ed om
ε
* ia Equa ion (1),
σ∗(ω)=i×ω×ε0×ε∗(ω)(1)
whe e
ω
= 2
π·
is he angula equency and
ε0
is he dielec ic pe mi i i y o he acuum.
2.3.5. Fou -Poin P obe (4PP)
Fou -poin p obe measu emen s we e ca ied ou using an equally spaced p obe head
and a Kei hley 2612 Sys em sou ce-me e (Kei hley Ins umen s, Solon, Ohio, USA). The
pelle s p epa ed o BDS measu emen s we e also used o 4PP cha ac e iza ion. Each
pelle had a diame e o d = 1.2 cm and a hickness o = 1 mm. The ou e p obes we e used
o apply a cu en anging om
−
5 mA o +5 mA, while he ol age was measu ed ac oss
he inne p obes. This cu en ange was selec ed o minimize Joule hea ing e ec s. The
cu en was swep om
−
5 mA o +5 mA and hen e e sed om +5 mA o
−
5 mA. The
spacing be ween p obes was s = 1.6 mm. Fo each pelle , he cu en – ol age (I–V) esponse
Elec onics 2025,14, 4539 5 o 20
was acqui ed, and he esis ance was ob ained om he linea i o he I–V slope. The
ini ial shee esis ance (Rs) was calcula ed using he s anda d ou -poin p obe equa ion:
RS=π
ln(2)
∆V
I(2)
whe e R
s
is he shee esis ance,
∆
V is he ol age measu ed be ween he inne p obes, and
I is he applied cu en be ween he ou e p obes.
The s anda d equa ion assumes ha he sample dimensions a e la ge ( ypically a
leas 40 imes g ea e ) han he p obe spacing and he sample hickness is less han 40%
o he p obe spacing. In he cu en case, nei he condi ion is sa is ied, necessi a ing a
co ec ion based on he sample’s geome y. Fo ci cula samples o diame e d and p obe
spacing s, he geome ic co ec ion ac o C is compu ed as ollows:
C=ln(2)
ln (2)+ln d2
s2+3−lnd2
s2−3(3)
Fo d = 1.2 cm and s = 1.6 mm, he geome ic co ec ion ac o is C = 0.867.
Gi en ha he samples a e hicke han 0.64 mm (i.e., 40% o he p obe spacing), a
u he co ec ion mus be applied o accoun o sample hickness. The hickness co -
ec ion ac o depends on he a io /s, which in his case is 0.625. Based on his a io, a
co ec ion ac o o 0.9898 should be applied. The inal shee esis ance alues inco po a e
bo h geome ic and hickness co ec ions. Elec ic conduc i i y was es ima ed using he
ollowing equa ion ( = 1 mm):
σ=1
RS× (4)
2.3.6. Raman Spec oscopy
Raman measu emen s we e pe o med using a Raman spec oelec ochemical sys em
(DRP-SPELECRAMAN532, Me ohm D opSens, O iedo, Spain) equipped wi h a 532 nm
exci a ion lase and con olled by D opView SPELEC so wa e ( e sion 1.0). A Raman
e lec ion p obe was employed in combina ion wi h a Raman cell designed o expe imen s
wi h sc een-p in ed elec odes (DRP-RAMANCELL, Me ohm D opSens, O iedo, Spain).
The cell was i ed wi h aluminum c ucible holde s (DRP-ALCRUCIBPACK, Me ohm
D opSens) o acili a e op ical cha ac e iza ion o bo h solid and liquid samples.
Fo elec ode modi ica ion (DRP-110), 1 mg o each sample was dispe sed in 5 mL
DMF. MWCNT-con aining samples we e sonica ed o 20 min (20 s on/10 s o ) o ensu e
p ope dispe sion, while lignin samples we e used wi hou sonica ion. The dispe sions
we e dilu ed 1:1 wi h wa e , and 5
µ
L was d op-cas on o he wo king elec ode, d ied
a oom empe a u e, and coa ed h ee imes o uni o mi y. The modi ied elec odes
we e hen placed in he Raman cell, and spec a we e eco ded a ixed lase powe and
in eg a ion ime.
2.3.7. Elec ochemical Measu emen s
Elec ochemical assessmen was pe o med using Ve sas a 3 (P ince on Applied
Resea ch, AMETEK Inc., Oak Ridge, TN, USA) po en ios a . A s anda d h ee-elec ode
glass body elec ochemical cell was used, whe e he wo king elec ode (WE) was a GCE
(d = 3 mm) in a Te lon body, he e e ence elec ode (RE) was Ag/AgCl in sa u a ed KCl
(expec ed po en ial, E
◦
= +0.197 V s. s anda d hyd ogen elec ode (SHE) a 25
◦
C), and
he coun e elec ode (CE) was a P -mesh. The cyclic ol amme ic s udies we e ca ied
ou in 1 mM o K
3
[Fe(CN)
6
] in 0.1 M KCl aqueous suppo ing elec oly e in he po en ial
Elec onics 2025,14, 4539 6 o 20
window o
−
0.2 o 0.6 V, and he scan a es anged om 10 o 200 mV/s. All elec ochemical
measu emen s we e pe o med wi hou applying au oma ic po en ial co ec ion.
Be o e each expe imen , he GCE was polished wi h 0.06
µ
m Al
2
O
3
slu y on polishing
clo h and insed wi h dis illed wa e and cyclic ol ame ic cu es we e eco ded o e i y
ha any ma e ials ha we e physically o chemically adso bed on he elec ode su ace we e
emo ed. The modi ied elec ode was p epa ed by mixing 32 mg o he hyb id ma e ial
(o i s componen s, i.e., sL and MWCNTs), ca bon black, and PTFE in an
8:1:1 weigh
a io
in 10 mL o e hanol. The mix u e was ul asonica ed o 10 min, a e which 40
µ
L we e
d op-cas on o he GCE and d ied a 50 ◦C o 5 min unde ai .
De e mina ion o Elec o-Ac i e Su ace A ea
The de e mina ion o he elec oac i e su ace a ea was de e mined by using cyclic
ol amme y a a ying scan a es (200 mV/s, 100 mV/s, 70 mV/s, 50 mV/s, 20 mV/s, and
10 mV/s). The Randles–Še ˇcík Equa ion (5) was applied o he calcula ion o he su ace
a ea o he wo king elec ode [35,36]:
IP=2.69 ×105A D1/2 n3/2 1/2C (5)
whe e A is he elec oac i e su ace a ea (cm
2
), D is he di usion coe icien (cm
2
s
−1
), n is
he numbe o ans e ed elec ons (which is 1 o he Fe
3+
/Fe
2+
edox couple),
ν
is he
scan a e, and C is he concen a ion o he elec oac i e species (mol cm
−3
). By plo ing
he peak cu en (I
pa
and I
pc
) agains he squa e oo o scan a e (
½
), he a e age slope o
he esul ing Ipa and Ipc lines was used o calcula e he EASA o each elec ode.
3. Resul s and Discussion
3.1. Pa icle Size and Su ace Cha ge o Hyb ids
The e ec i eness o US in dispe sing he indi idual ma e ials, soda lignin, and MWC-
NTs as well as hei co esponding hyb ids (10 and 20% MWCNTs–sL) we e in es iga ed.
The DLS esul s showed signi ican ly smalle HDD alues o he hyb id ma e ials, indica -
ing a syne gis ic e ec upon hyb id o ma ion.
As shown in Figu e 1a, a g adual educ ion in HDD o soda lignin om 1866 nm o 684
nm wi hin 120 min o US was obse ed. By con as , US did no educe he HDD o CNTs
o he sub-mic on ange, as alues emained abo e 1
µ
m e en a e ex ended ea men
(Figu e 1b) a 120 min o US. US exposu e o up o 60 min esul ed in a s eady dec ease
in HDD, eaching a pla eau ha pe sis ed o up o 120 min. P olonged US may induce
s uc u al deg ada ion o sho ening o nano ubes [
37
–
39
]. The e o e, 60 min was selec ed
as he op imal US ime o achie e e ec i e dispe sion in wa e while a oiding po en ial
MWCNT damage.
Bo h MWCNTs–sL hyb ids exhibi ed be e dispe sion p ope ies. As abula ed in
Table 1, bo h 10% and 20% hyb id sys ems possessed signi ican ly lowe HDD alues a e
US ea men . Mo e speci ically, a 60 min o US, he 10% and 20% hyb ids exhibi ed an
HDD o 645 nm and 661 nm, espec i ely. These alues we e sligh ly lowe compa ed o
he ones obse ed o sL (HDD = 690 nm) and signi ican ly lowe compa ed o p is ine
MWCNTs (HDD = 1185 nm). This educ ion was e en g ea e a 120 min o US, wi h he
10% and 20% hyb ids exhibi ing an HDD o 384 nm and 470 nm, espec i ely, compa ed
wi h 684 nm o sL and 1225 nm o MWCNTs. The ze a po en ial alues (
−
20 o
−
25 mV)
indica ed mode a e elec os a ic s abiliza ion, wi h sligh ly highe absolu e alues obse ed
a lowe MWCNT loading. This esul is consis en wi h smalle HDD and na owe size
dis ibu ion in he case o 10% MWCNTs–sL hyb id (Figu e 2), indica ing a mo e e icien
s abiliza ion by lignin mac omolecules. A lowe CNT loading, lignin can mo e e ec i ely
Elec onics 2025,14, 4539 7 o 20
adso b on o indi idual nano ube su aces h ough
π
–
π
in e ac ions and hyd ogen bonding,
enhancing elec os a ic and s e ic s abiliza ion, whe eas pa ial e-agglome a ion a highe
CNT con en educes he a ailable su ace a ea o lignin adso p ion and sligh ly lowe s
he absolu e ZP alues [40].
(a)
(b)
Figu e 1. Pa icle size dis ibu ion cu es o (a) soda lignin in wa e du ing 120 min o US ea men
and he co esponding HDD alues s. US ime, (b) MWCNTs in wa e du ing 120 min o US
ea men and he co esponding HDD s. US ime.
Table 1. HDD and ZP alues o 10% and 20% MWCNTs–sL hyb ids du ing 120 min o US ea men .
Sample US Time (min) HDD (nm) ZP (mV)
10% MWCNTs–sL 60 645 −23
10% MWCNTs–sL 120 384 −25
20% MWCNTs–sL 60 661 −20
20% MWCNTs–sL 120 470 −21
Figu e 2. Pa icle size dis ibu ion cu es o 10% and 20% MWCNTs–sL hyb ids du ing 120 min o
US ea men .
Elec onics 2025,14, 4539 8 o 20
Mo eo e , size dis ibu ion p o iles (Figu e 2) showed ha o bo h hyb ids, wo
pa icle size popula ions we e p esen a e 60 min o US. P olonging he US o 120 min led
o a single popula ion, e i ying he imp o ed dispe sion. This highligh ed he need o
ex ended US (120 min) o he op imum in eg a ion o MWCNTs wi hin he lignin ma ix,
e ec i ely elimina ing agg ega es [
40
]. The be e pe o mance in hyb id sys ems can be
a ibu ed o he in e ac ion be ween lignin mac omolecules and MWCNT su aces, likely
by
π
–
π
s acking and hyd ogen bonding. Lignin can also ac as a s e ic and elec os a ic
s abilize , p e en ing MWCNTs om ebundling du ing US [19,41].
3.2. S abili y Tes
To assess he long- e m colloidal s abili y o he 10% MWCNTs–sL dispe sion, HDD
and ZP measu emen s we e pe o med using DLS o e a 90-day pe iod. As depic ed in
Figu e 3, he dispe sion emained isually homogeneous h oughou he s o age ime. No
signi ican agg ega ion o phase sepa a ion was obse ed, al hough minimal sedimen a ion
o la ge pa icles appea ed a he bo om o he con aine a e se e al weeks.
Figu e 3. S abili y es o 10% MWCNTs–sL hyb id suspended in wa e o e a 90-day pe iod.
Figu e 4 u he con i ms he long- e m colloidal s abili y o he dispe sion. HDD
alues exhibi ed mino luc ua ions, anging om 353 o 384 nm o e he 90-day pe iod,
sugges ing a s able pa icle size dis ibu ion. The ZP alues anged om
−
24 mV o
−25.3 mV
, indica ing ha su icien elec os a ic epulsion was main ained o p e en
signi ican agg ega ion. The esul s con i med ha lignin e ec i ely s abilizes MWCNTs
in aqueous media wi hou he need o addi ional syn he ic su ac an s. A simila 10-day
s abili y s udy was also pe o med o he 20 w % MWCNTs–sL hyb id dispe sion, and he
esul s a e p esen ed in Figu e S1 o he Supplemen a y In o ma ion.
Figu e 4. HDD and ZP alues o 10% MWCNTs–sL du ing s abili y es s o e 90-day pe iod.
Elec onics 2025,14, 4539 9 o 20
3.3. Mic oscopic Imaging
3.3.1. SEM Imaging
Figu e 5depic s he SEM mic og aphs o he s a ing ma e ials a di e en magni i-
ca ions: sL (Figu e 5a,b) and MWCNTs (Figu e 5c,d). The pa icles showed a wide size
dis ibu ion, anging om ~1 o 40
µ
m. The p esence o la ge s uc u es indica ed a
endency o lignin pa icles o sel -agg ega e. MWCNTs mic og aphs (Figu e 5c,d) e-
ealed an en angled ne wo k o cylind ical nano ubes wi h a high aspec a io. The dense,
angled mo phology is ypical o MWCNTs and is sui able o he o ma ion o conduc i e
pe cola ion pa hs in composi es/hyb ids.
(a)(b)
(c)(d)
Figu e 5. SEM images o s a ing ma e ials: soda lignin (a,b) and MWCNTs (c,d).
Figu es 6and S2 p esen he mic og aphs o he wo syn hesized hyb ids. In bo h
cases, he mic og aphs indica e ha he MWCNTs we e en angled a ound he lignin pa icles
wi h in e connec ed ne wo ks being c ea ed. Lignin appea ed o ac as a ma ix, wi h he
MWCNTs en angled on i s su ace, indica ing an in e ac ion be ween he wo ma e ials. A
10% MWCNT loading (Figu e 6a,b), he hyb ids displayed a mo e uni o m s uc u e, whe eas
a 20% MWCNT loading (Figu e 6c,d), he p esence o agg ega es became mo e p onounced.
3.3.2. TEM Imaging
Figu e 7displays TEM mic og aphs o aw MWCNTs (Figu e 7a), 10% MWCNTs–sL
hyb id (Figu e 7b,c), and 20% MWCNTs–sL hyb id (Figu e 7d). In Figu e 7a, he aw
MWCNTs exhibi ed hei cha ac e is ic long and ubula s uc u es wi h ela i ely uni o m
diame e . In he 10% MWCNTs–sL hyb id (Figu e 7b,c), a dense ne wo k was obse ed
in which he MWCNTs we e isibly in eg a ed wi hin he lignin ma ix. The in e ac ion
appea ed o p omo e he o ma ion o lignin nanopa icle agg ega es in ce ain egions.
Fo he 20% MWCNTs–sL (Figu e 7d,e), he s uc u e became dense and mo e compac .
The da k a eas indica ed agg ega ion, and MWCNTs we e no longe easily dis inguishable.
This could be a ibu ed o highe MWCNTs loading and educed dispe sion e iciency
due o an de Waals in e ac ions [
42
]. Ne e heless, MWCNTs can s ill be iden i ied as
en angled wi h he lignin ma ix, implying he o ma ion o conduc i e pa hways. The
TEM obse a ions a e consis en wi h he DLS and SEM esul s, con i ming ha he
10 w %
Elec onics 2025,14, 4539 16 o 20
owed o he esul ing elec ical conduc i i y o he hyb id, due o he soda lignin ma ix
embedding he MWCNTs.
4. Conclusions
In his s udy, sus ainable lignin–MWCNT hyb id ma e ials o conduc i e and elec o-
chemical applica ions, using a simple, wa e -based ul asonica ion p ocess, we e success-
ully de eloped and cha ac e ized. This en i onmen ally benign syn hesis ou e a oided
he use o o ganic sol en s and haza dous eagen s in acco dance wi h g een chemis y
p inciples. Soda lignin se ed no only as a biobased ma ix bu also as an e icien na u-
al dispe san o MWCNTs h ough
π
–
π
in e ac ions and hyd ogen bonding. This dual
unc ionali y enabled homogeneous dispe sion o MWCNTs in wa e , as e idenced by he
signi ican educ ion in HDD and s able ZP alues o e ex ended s o age, con i ming long-
e m colloidal s abili y wi hou he need o syn he ic su ac an s. SEM and TEM imaging
con i med he o ma ion o dense conduc i e ne wo ks, while Raman spec oscopy indi-
ca ed he imp o ed s uc u al o de o MWCNTs upon hyb id o ma ion. These indings
indica e an in e acial compa ibili y be ween lignin and MWCNTs, which plays a decisi e
ole in p omo ing e ec i e cha ge anspo pa hways. Despi e con aining up o 90 w %
o insula ing lignin, he hyb ids exhibi ed adequa e elec ical pe o mance. BDS analysis
e ealed conduc i i ies up o 6
×
10
−2
S/cm, while di ec 4PP measu emen s showed
e en highe alues o ~2 S/cm o 10 w % MWCNTs and ~5 S/cm o 20 w %, con i ming
he o ma ion o e icien pe cola ed MWCNT ne wo ks. Such alues shows ha lignin
e ec i ely con ibu es o ne wo k o ma ion in combina ion wi h MWCNTs, a he han
ac ing as a passi e insula o . Elec ochemical cha ac e iza ion u he demons a ed he
high unc ional pe o mance o he hyb ids. The modi ied GCEs exhibi ed subs an ially
inc eased cu en esponses, imp o ed e e sibili y, and signi ican ly enhanced elec on
ans e kine ics compa ed o ba e o lignin-modi ied elec odes. The elec oac i e su ace
a ea inc eased up o 21 imes ela i e o he unmodi ied GCE, unde sco ing he hyb ids’
po en ial as e icien elec ochemical in e aces. This pe o mance is a ibu ed o he syne -
gis ic e ec be ween he conduc i e MWCNT ne wo k and he a oma ic lignin s uc u e,
which acili a es as e elec on ans e kine ics a he elec ode–elec oly e in e ace.
O e all, his wo k showed ha he combina ion o enewable lignin and conduc-
i e MWCNTs p o ides an e ec i e s a egy o p oducing cos -e ec i e, scalable, and
en i onmen ally esponsible ma e ials o g een elec onics, senso s, and ene gy- ela ed
applica ions. The app oach o eplacing syn he ic ossil-based su ac an s and polyme ic
ma ices wi h a ully biobased componen o e s a p ac ical pa hway owa d he sus ainable
p oduc ion o unc ional conduc i e ma e ials ha align wi h he ci cula economy and
eco-design p inciples.
Supplemen a y Ma e ials: The ollowing suppo ing in o ma ion can be downloaded a h ps://www.
mdpi.com/a icle/10.3390/elec onics14224539/s1, Figu e S1: S abili y es o 20%
MWCNTs–sL
hyb id suspended in wa e o e a 10-day pe iod; Figu e S2: SEM images o he wo hyb ids: 10%
MWCNTs–soda lignin (a & b) and 20% MWCNTs–soda lignin (c & d) hyb ids; Figu e S3: Raman
spec a o 110D elec ode wi h soda Lignin; Figu e S4: Cyclic ol ame ic cu es o ba e GCE,
MWCNTs/GCE, sL/GCE, 10% MWCNTs–sL/GCE and 20% MWCNTs–sL/GCE hyb ids, eco ded
in 1mM K
3
[Fe(CN)
6
] in 0.1 M KCl aqueous suppo ing elec oly e, a a scan a e o 100 mV/s and
20 mV/s: (a) & (b) sL/GCE and ba e GCE a an expanded cu en scale and (c) & (d) all elec odes
plo ed oge he o compa ison; Figu e S5: Cyclic ol ame ic cu es o Ba e GCE eco ded in
1mM K3[Fe(CN)6]
in 0.1 M KCl aqueous suppo ing elec oly e, a a scan a e o 200 mV/s, 100 mV/s,
70 mV/s, 50 mV/s, 20 mV/s and 10 mV/s (a). Linea i o I
pa
and I
pc
s. scan a e
1/2
o ba e
elec ode o es ima e EASA (b).
Elec onics 2025,14, 4539 17 o 20
Au ho Con ibu ions: Concep ualiza ion, A.Z.-K., I.D. and A.G.; me hodology, A.Z.-K., I.D., A.G.,
S.P.M. and S.K.; alida ion, A.Z.-K., I.D., A.G., S.P.M. and S.K.; o mal analysis, S.P.M., S.K., K.B. and
A.G.; in es iga ion, S.P.M., S.K., K.B. and M.K.; esou ces, A.Z.-K. and I.D.; da a cu a ion, S.P.M.,
S.K., K.B. and M.K.; w i ing—o iginal d a p epa a ion, S.P.M., S.K. and A.G.; w i ing— e iew and
edi ing, all au ho s; supe ision, A.Z.-K., I.D. and A.G.; unding acquisi ion, A.Z.-K. All au ho s ha e
ead and ag eed o he published e sion o he manusc ip .
Funding: This esea ch was unded by he Eu opean Communi y’s Ho izon Eu ope F amewo k
P og am (g an numbe : 101070556; p ojec : Sus ain-a-P in , Sus ainable ma e ials and p ocess o
g een p in ed elec onics; h ps://www.sus ainap in .eu/, accessed on 15 Oc obe 2025).
Da a A ailabili y S a emen : All he da a o his s udy a e included in he manusc ip and Supple-
men a y Ma e ial.
Acknowledgmen s: The au ho s would like o exp ess hei g a i ude o Panagio is Klonos and
Apos olos Ky i sis, Depa men o Physics, Na ional Technical Uni e si y o A hens (NTUA), o he
BDS analysis; Pe os Tsaki idis, School o Mining and Me allu gical Enginee ing, Na ional Technical
Uni e si y o A hens (NTUA), o TEM obse a ions; Ma ía Begoña González Ga cía and Daniel
Izquie do Bo e, Me ohm D opSens, S.L., o Raman analysis; and G igo is Kal sas, Depa men o
Elec ical and Elec onics Enginee ing—Head o he mic oSENSES Lab, Uni e si y o Wes A ica, o
4PP measu emen s.
Con lic s o In e es : Se e al au ho s (S.P.M., S.K., A.G., K.B., M.K., A.Z.-K.) a e employees o C ea i e
Nano PC, and one au ho (I.D.) is an employee o AXIA Inno a ion GmbH. The companies had no
comme cial o inancial in e es in he ou comes o his esea ch, which was conduc ed wi hin he
amewo k o he EU- unded p ojec Sus ain-a-P in (g an numbe : 101070556). No o he con lic s o
in e es a e decla ed.
Abb e ia ions
The ollowing abb e ia ions a e used in his manusc ip :
BDS B oadband dielec ic spec oscopy
CNTs Ca bon nano ubes
CTAB Ce imonium b omide
CGA Chlo ogenic acid
CE Coun e elec ode
DI Deionized
DLS Dynamic ligh sca e ing
DMF Dime hyl o mamide
DMSO Dime hylsul oxide
EASA Elec oac i e su ace a ea
EUG Eucommia ulmoides gum
GCE Glassy ca bon elec ode
HDD Hyd odynamic diame e
I-V Cu en – ol age
MTMS Me hyl ime hoxysilane
MWCNTs Mul i-walled ca bon nano ubes
PAN Polyac yloni ile
PDMS Polydime hylsiloxane
PLA Polylac ic acid
PP-g-MA Polyp opylene-g a ed-maleic anhyd ide
RE Re e ence elec ode
RsShee esis ance
SDS Sodium dodecyl sul a e
SEM Scanning elec on mic oscopy
SHE S anda d hyd ogen elec ode
Elec onics 2025,14, 4539 18 o 20
sL Soda lignin
TEM T ansmission elec on mic oscopy
T i on X-100 Oc yl phenol e hoxyla e
US Ul asonica ion
WE Wo king elec ode
ZP Ze a po en ial
4PP Fou -poin p obe
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