A water-soluble eumelanin polymer with typical polyelectrolyte behaviour by triethyleneglycol N-functionalization

Article abstract of DOI:10.1039/c4tc01997k

N-functionalization of 5,6-dihydroxyindole with a hydrophilic triethyleneglycol (TEG) chain provides access to a new class of water-soluble eumelanin-like materials with relatively high dielectric constant and polyelectrolyte behaviour, reflecting enhanced charge transport by in-depth incorporation of hydration networks. This journal is

Full text of DOI:10.1039/c4tc01997k

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A water-soluble eumelanin polymer with typical
polyelectrolyte behaviour by triethyleneglycol N-
functionalization†
Cite this: DOI: 10.1039/c4tc01997k
Stefania R. Cicco,a Marianna Ambrico,b Paolo F. Ambrico,b
Maurizio Mastropasqua Talamo,c Antonio Cardone,a Teresa Ligonzo,d Rosa Di
e
f
f
ac
Paola Manini,g
*
Mundo, Cinzia Giannini, Teresa Sibillano, Gianluca M. Farinola,
g
g
g
*
Alessandra Napolitano, Valeria Criscuolo and Marco d'Ischia
Received 29th September 2014
Accepted 14th January 2015
N-functionalization of 5,6-dihydroxyindole with a hydrophilic triethyleneglycol (TEG) chain provides access
to a new class of water-soluble eumelanin-like materials with relatively high dielectric constant and
polyelectrolyte behaviour, reflecting enhanced charge transport by in-depth incorporation of hydration
networks.
DOI: 10.1039/c4tc01997k
www.rsc.org/MaterialsC
electronics.9,10 While early theories invoked the amorphous
semiconductor model to explain eumelanin electrical proper-
Introduction
ties, more recent evidences suggested an alternative water-
dependent ionic–electronic hybrid conductor model. In this
model, hydration shis the comproportionation equilibrium
between catechol and o-quinone components and semiquinone
radicals so as to dope electrons and protons into the system.
However, the different hydration dependence of the intrinsic
spin density with respect to the electrical conductivity and the
mSR relaxation rates suggests a complex interplay of mecha-
nisms that still require elucidation.7 Eumelanin pigments have
also been proposed to serve as biologically derived charge
storage materials for application in various types of devices.4
Despite the technological potential of eumelanin-based
materials and hybrids, several issues related to the insolubility,
structural complexity and manifold levels of chemical
disorder,11 account for reported difficulties in the deposition of
eumelanin lms with controllable thickness,12 hindering
rational property optimization and tailoring for organic elec-
tronics applications. Various approaches have been proposed to
prepare smooth melanin-based lms. DMSO has been shown to
be a useful processing solvent, yielding homogeneous and
smooth lms growing in a quasi-layer-by-layer mode, each layer
being composed of nanoaggregates, 1–2 nm high.13 Good
quality synthetic eumelanin (SM) layers were obtained by spin-
coating a commercial eumelanin sample onto indium tin
oxide (ITO) or silicon surfaces modied by a dry plasma
procedure,14–16 in order to improve adhesion. Investigation of
charge carriers and related transport mechanisms across the
SM layers led to important insights into the eumelanin-Si/ITO
interfaces. By the same procedure, electrically tuneable
eumelanin-like polymers from structurally modied
polydopamine via copolymerization with 3-aminotyrosine and
p-phenylenediamine were obtained as components of innovative
Eumelanins, the black insoluble 5,6-dihydroxyindole-based
biopolymers responsible for the dark coloration of human skin,
hair and eyes,1 attract growing interest as so biocompatible
and bioavailable multifunctional materials for a broad range of
applications, including high-tech hybrid devices, bioelectronics
and bionanomedicine.2,3 Within biological systems, eumela-
nins are believed to play a variety of roles, e.g. photoprotection,
antioxidant and metal binding, which depend on a unique
combination of physicochemical properties, such as broadband
absorption spanning the UV-visible range, efficient non-radia-
tive excited state deactivation, ion storage,4 a stable free radical
character and an unusual water-dependent semiconductor-like
behaviour.5–8
Since its discovery in the mid 1970's the semiconductor
behaviour of eumelanins has raised considerable interest for
the design of innovative functional materials for organic
aIstituto di Chimica dei Composti OrganoMetallici ICCOM di Bari, Consiglio
Nazionale delle Ricerche CNR, via Orabona 4, 70126-Bari, Italy. E-mail:
gianlucamaria.farinola@uniba.it
bIstituto per le Metodologie Inorganiche e dei Plasmi IMIP di Bari, Consiglio Nazionale
delle Ricerche CNR, via Orabona 4, 70126-Bari, Italy
c
`
Dipartimento di Chimica, Universita degli Studi di Bari A. Moro, via Orabona 4,
70126-Bari, Italy
d
`
Dipartimento di Fisica, Universita degli Studi di Bari A. Moro, via Orabona 4, 70126-
Bari, Italy
eTribology LAB, Dipartimento di Meccanica, Matematica e Management, Politecnico di
Bari, v.le Japigia 182, 70126-Bari, Italy
fIstituto di Cristallograa, Consiglio Nazionale delle Ricerche CNR-IC, via Amendola
122/O, 70126-Bari, Italy
g
`
Dipartimento di Scienze Chimiche, Universita di Napoli “Federico II”, Via Cintia 4,
80126-Napoli, Italy. E-mail: dischia@unina.it
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc01997k
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metal-insulator-semiconductor (MIS) devices.17 Very recently, under a reduced pressure over molecular sieves, and stored
cysteine-containing derivatives of the adhesive biopolymer under nitrogen atmosphere.
polydopamine in variable ratios were obtained, showing a
FT-IR spectra were measured on a PerkinElmer 1710 spec-
marked photoimpedance response to light stimuli.18 In all these trophotometer using dry KBr pellets. H and 13C NMR spectra
cases, layer deposition was carried out by extensive dissolution were recorded in CDCl3 on a Bruker AM 500 spectrometer at 500
of the synthetic pigments in 26% aqueous ammonia.19 In spite MHz and 125 MHz, respectively.
of technological convenience, eumelanin deposition under
1
alkaline conditions would expectedly be accompanied by Synthesis of 5,6-bis(benzyloxy)-1-(2-(2-(2-methoxyethoxy)-
structural modication due to oxidative degradation.1 Whether ethoxy)ethyl)-1H-indole (2)
and by what mechanism such structural modications could
affect eumelanin electrical properties is however unclear. A
NaH (0.078 g, 60% w/w in mineral oil, 1.95 mmol, 1.2 eq.) was
introduced in a three-necked round bottom ask under a
central issue in this context is the possible impact of eumelanin
nitrogen atmosphere. The mineral oil was washed away with
degradation on aggregation and hydration since there are still a
hexane, then dry DMF (10 mL) was added. 5,6-Dibenzyloxy-
indole (1) (0.536 g, 1.63 mmol, 1 eq.) dissolved in dry DMF (20
mL) was added to the mixture. The resulting mixture was stirred
number of fundamental gaps concerning the mechanisms by
which the aggregation state and water-binding affect the charge
transport mechanisms. The supramolecular organization in
for 30 min, cooled at 0 ꢀC and then 1-bromo-2-(2-(2-methoxy-
eumelanin-based materials has been the focus of several
ethoxy)ethoxy)ethane (1.541 g, 6.79 mmol, 2.5 eq.) dissolved in
studies, and the contribution of orderly p-stacked indolic
dry DMF (10 mL) was added dropwise. The reaction mixture was
stirred at room temperature for 4 h, then quenched with satu-
rated aqueous NH4Cl, diluted with ethyl acetate and washed
components has been proposed as being critical for several
physicochemical properties.20,21
A convenient alternative to dissolution in ammonia for
eumelanin thin lm processing would be the development of
melanin-derivatives soluble in organic solvents by proper
functionalization.22,23 However, achieving water-soluble eume-
lanins appears more appealing: processing from aqueous
media would allow an efficient permeation of water molecules
many times with water (5 ꢁ 20 mL). The organic layer was dried
over anhydrous Na2SO4 and the solvent removed under a
reduced pressure. The product was puried by silica gel chro-
matography using a mixture of hexane–ethyl acetate (7 : 3) as
the eluent, affording 0.660 g (85% yield) of 2 as a yellowish
dense oil.
through the bulk of the lms and a more efficient charge
1H NMR (500 MHz, CDCl3): d ¼ 7.51–7.50 (m, 4H), 7.39–7.36
transport. Furthermore, water is the most suitable biocompat-
(m, 4H), 7.33–7.29 (m, 2H), 7.19 (s, 1H), 7.05 (d, 1H, J ¼ 3.1 Hz),
ible medium for a biodevice design. Three different and effec-
tive methods to obtain largely water-soluble eumelanin-type
6.96 (s, 1H), 6.35 (d, 1H, J ¼ 3.1 Hz), 5.20 (s, 2H), 5.18 (s, 2H),
4.20 (t, 2H, J ¼ 5.8 Hz), 3.75 (t, 2H, J ¼ 5.8 Hz), 3.54 (m, 8H), 3.37
polymers have been reported. The rst method is based on the
(s, 3H) ppm; 13C NMR (125 MHz, CDCl3): d ¼ 146.57, 144.92,
oxidative polymerization of glycated precursors such as 5,6-
137.92, 137.65, 131.27, 128.34, 128.31, 127.64, 127.53, 127.46,
dihydroxyindole (DHI);24 the second consists in the oxidation of
127.43, 122.32, 107.16, 100.79, 97.41, 72.46, 72.39, 71.86, 70.70,
5,6-dihydroxyindole substrates in aqueous buffer containing
70.55, 70.49, 70.10, 58.93, 46.39 ppm; FT-IR: n ¼ 3062, 3030,
1% poly(vinyl alcohol) (PVA) to prevent polymer precipitation;25
2925, 2872, 1505, 1484, 1453, 1420, 1377, 1350, 1302, 1219,
the third involves polymerization of suitable precursors in
1197, 1142, 1061, 1026, 1004, 914, 876, 849, 760, 740, 697, 648,
proteins or biological matrix, such as egg white.26,27 Although
597, 470 cmꢂ1
.
these approaches may be useful for the elucidation of eumela-
nin structural and optical properties, their value for electrical
property optimization and semiconductor implementation in
devices has still to be assessed and the exploration of novel
strategies to improve “solubility-processing approaches”
reported in the literature seemed desirable.
Herein we report the synthesis and electrical properties of a
new water-soluble eumelanin-like material, triethyleneglycol-
melanin (TEGM), obtained by oxidative polymerization of N-
functionalized DHI with a hydrophilic TEG group. The under-
lying rationale is that N-functionalization with TEG chains
would favour processability due to water-solubility and would
allow for a higher degree of hydration to improve electrical
properties.
Synthesis of N-TEG melanin (TEGM)
N-Triethyleneglycol-5,6-dibenzyloxyindole (2) (0.250 g, 0.53
mmol) in ethyl acetate (30 mL) was poured into a hydrogenation
bomb, added with palladium on charcoal (70 mg) and subjected
to a positive pressure of hydrogen gas (30 atm). Aer 18 h the
reaction mixture was ltered to remove the catalyst and the
solvent evaporated under a reduced pressure. The residue
(0.120 g) was taken into methanol (5 mL) and dissolved into an
aqueous ammonia solution (1%). The reaction mixture imme-
diately turned into deep violet and aer 24 h was dark brown in
colour. Aer 48 h the mixture was acidied to pH 4 with 3 M HCl
and evaporated under reduced pressure. The dark residue was
taken up in water and freeze-dried overnight to afford TEGM as
a dark solid (0.100 g).
Experimental
General information
TEGM/SM layers deposition
All chemicals were purchased from Aldrich, Alfa Aesar and A TEGM solution for thin lm deposition was prepared by dis-
Acros and used without further purication. DMF was distilled solving 70 mg of TEGM in deionized water (3 mL). Synthetic
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Melanin SM solution for thin lm deposition was prepared by
dissolving SM powder (70 mg, Sigma-Aldrich) in water (1 mL)
and 28% aq ammonia solution (2 mL). The TEGM and SM layers
of same thickness were deposited by drop casting on plasma-
treated Indium Tin Oxide (ITO)/glass supports. The TEGM and
SM layer thickness were measured by an Alpha Step prol-
ometer and both found 400 nm thick (D120, Tencor Instru-
ments). All samples were dried at room temperature under
vacuum at a pressure of 2 ꢁ 10ꢂ3 mbar for 12 h.
Electrical characterization of TEGM-based MIM devices
Current–voltage (IV) and impedance spectroscopy (IS)
measurements have been performed on TEGM-based MIM
devices and compared with those collected on SM-based MIM
devices. All measurements were performed at room tempera-
ture under ambient air at a relative humidity degree of 50%.
Current–voltage hysteresis loops on TEGM-based MIM
structures were carried out under dark and by using a Keithley
Model 617 electrometer. The loop read outs were performed
starting from zero up to a maximum positive value of the
continuous bias, dening the hysteresis loop amplitude +VL,
and then alternating back and forth between +VL and ꢂVL. The
loop amplitudes VL were in the range 1.0–3.0 V and the
hystereses were acquired at different voltage sweep speeds.
Impedance spectroscopy (IS) measurements were made by
using a NOVOCONTROL impedance analyzer in the frequency
range 0.1 Hz to 1 MHz of the sine wave voltage signal with a
signal amplitude VAC ¼ 5.0 mV and at zero DC bias voltage.
AFM characterization
Surface topography has been characterized by means of an
NTEGRA, NT-MDT Atomic Force Microscope in non-contact
mode by using high accuracy non-contact (HA-NC) “etalon”
probes (high aspect ratio tip, cantilever frequency 140 kHz).
Several images at different scan size were acquired on the
samples.
Results and discussion
TEGM synthesis
XRD characterization
X-ray diffraction (XRD) analysis of TEGM powders was per-
formed with a Rigaku RINT2500 rotating anode laboratory
diffractometer (50 kV, 200 mA) equipped with a silicon strip
Rigaku D/teX Ultra detector, using Cu Ka1 monochromatic
TEGM was prepared according to the procedure illustrated in
Scheme 1. Briey, 5,6-dibenzyloxyindole (1)28 was N-function-
alized with a triethyleneglycol (TEG) chain to give N-TEGylated
5,6-dibenzyloxyindole (2). Deprotection of 2 by catalytic hydro-
genation followed by oxidative polymerization in aqueous
ammonia solution led to the formation of a water-soluble
˚
radiation (l ¼ 1.5405 A). Moreover, a diffraction pattern has
been also acquired from a sample of synthetic melanin (SM) in
powder (SIGMA-Aldrich) and on DHI–melanin in powder, in
order to compare their structures with that of TEGM (see ESI†).
MIM device fabrication
A series of metal-insulator-metal (MIM) devices were tailored,
for statistical purposes, by thermally evaporating a set of gold
(Au) dots (Ø ¼ 500 mm, 150 nm thick) on top of the structures
consisting of casted TEGM and SM layers.14,15 A simplied
sketch of the typical device conguration and measurement set
up adopted in MIM structure characterizations is displayed in
Fig. 1.
Scheme 1 Synthesis of N-functionalized 5,6-dibenzyloxyindole (2)
Fig. 1 Scheme of the metal-insulator-metal (MIM) device and of the and its conversion to TEGM by aerial polymerization after deprotection
electrical measurement set up.
from benzyl group.
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eumelanin-like polymer, which was recovered by evaporation of
the aqueous solvent. Black TEGM proved to be freely soluble in
water, as veried by ltration on nylon membrane (0.45 mm).
Thin lms of TEGM were deposited by drop casting from an
aqueous solution onto ITO supports under very mild non-
alkaline conditions. UV-vis spectrum of cast TEGM lm is
reported in the ESI, Fig. S3.†
TEGM and SM layer characterizations
AFM and XRD data analyses. Fig. 2 reports a 10 mm ꢁ 10 mm
AFM image of the surface of the TEGM layer deposited on ITO.
The surface was fairly smooth except for the presence of micro-
scale spaced bumps, not exceeding 90 nm heights. Images
acquired at this scan size indicated an average root mean
square (RMS) roughness of 7 nm. No other type of structuring
could be distinguished at higher resolution scans.
Fig. 3 X-ray diffraction spectra of TEGM (black curve) and commercial
melanin SM (red curve).
The effect of the TEG chains on eumelanin solid state
packing was investigated by X-ray diffraction analysis of TEGM
powder in comparison with a commercial synthetic melanin
(SM) powder sample as reference. Typically, melanin-like
amorphous compounds produce broad peaks in the diffraction
spectrum.23 In the case of TEGM powder, the position of the
amorphous peak was found at around 2q ꢃ 21.5ꢀ, suggesting a
correlation distance d ¼ 0.42 nm, whereas in the case of SM the
same peak registered at around 2q ꢃ 25ꢀ indicated a distance d
¼ 0.35 nm (Fig. 3), the former being 20% larger compared to the
latter.
A correlation distance very similar to that measured for SM
was obtained from the XRD spectra of 5,6-dihydroxy-indole-
melanin (DHI-Mel). This is a melanin-like material equivalent
to TEGM, without the TEG chains, ad hoc synthesized for this
comparison (Fig. S4, ESI†).
This observation suggests that increased distance is likely
due to the presence of the TEG chains on the TEGM polymers.
Current–voltage and impedance results. Fig. 4a–c displays
the results of current–voltage (I–V) hysteresis loops collected in
Fig. 4 (a) I–V hysteresis loops collected on TEGM-based MIM struc-
tures at different VL. For clarity, the points corresponding to Vocꢄ and
ꢄ
Isc have been also indicated. The inset shows the comparison
ꢄ
between Voc vs. VL extracted from TEGM-based and SM-based MIM
devices. (b) Loop areas extracted by integrating the I–V hysteresis
loops of SM (red curves) and TEGM (blue curves) at different VL values
and fixed voltage sweep speed s ¼ 12 mV sꢂ1. The inset shows the
comparison between representative loops (at VL ¼ 1 V and s ¼ 12 mV
s
ꢂ1) of SM- and TEGM-based MIM devices (VL ¼ 1 V and s ¼ 12 mV sꢂ1);
(c) I–V hysteresis loops collected on TEGM-based MIM device at
different voltage sweep speeds s (see legend). Vocꢄ and Iscꢄ meanings
are the same as in (a). In the inset the extracted values of Isc vs. s.
ꢄ
Fig. 2 10 mm ꢁ 10 mm AFM image of the surface of the TEGM layer
deposited on ITO.
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ambient air on TEGM-based MIM devices for different voltage TEGM-based (blue squares) and on SM-based (red squares)
loop amplitudes, VL (VL ¼ 1.0, 2.0 and 3.0 V), while keeping MIM devices (see inset in Fig. 4b).
constant the voltage-sweep speed, s, (a and b) and at different
Furthermore, the hysteresis curves showed an increase of the
voltage-sweep speeds s by keeping constant the voltage loop loop area with voltage sweep speed s at a xed voltage loop.
amplitude VL (c). Interestingly, in this case the open circuit voltage values kept
An increase in the voltage loop or in the voltage-sweep speed approximately the same values, while the short circuit currents
was found to produce larger hystereses (Fig. 4a–c). Similar increased with speed in both hysteresis branches. Based on the
results were previously observed in a SM-based MIM structure, expression (A) the zero bias short circuit current Isc when
conrming that TEGM melanin shows a typical behaviour in varying the voltage sweep speed by keeping constant the voltage
space charge storage mechanisms. More specically, the space loop magnitude, could be ascribed to the dependence of the
charges accumulate at the metal/eumelanin interface, shielding charge-trapping/detrapping mechanisms on the voltage sweep
the applied electric eld and limiting further carrier injection in rate. In this case:
ꢀ
ꢁ
the TEGM layer. Based on this hypothesis, the injection occurs
during the voltage sweep from 0 V up to VL followed by the
accumulation at one of the electrodes (Au or ITO); aerwards
space charges distribute in the structure. During the reverse
sweep and at a certain value of the applied voltage, called open-
circuit voltage Voc+ a balance is established between the external
eld and that due to space charge so that no current is owing
any longer. By further decreasing the voltage, the current ows
again and at zero bias a non-zero current (negative short circuit
current, Iscꢂ) is observable due to the stored charges owing
back through the external circuit. Moving toward a more
negative voltage (from 0 to ꢂVL), charge storage occurs again
and in the subsequent forward sweep direction from (ꢂVL up to
+VL) an open circuit voltage (Vocꢂ) and a positive short circuit
current (Isc+) can be detected. The magnitude of Iscꢄ and Vocꢄ in
both sweep directions depends on the amount of stored charges
at electrode/polymer interface and may differ in the two
hysteresis branches due to the different electrode (Au and ITO)
work functions.29
dV
dt
Isc ꢃ C
¼ Cs
(2)
ꢄ
i.e. a linear dependence of Isc on s would be expected. The Isc
vs. s data displayed in the inset of Fig. 4c suggested that two
linear regimes were present, one at lower voltage sweep speed
(up to s # 60 mV sꢂ1) the other up to s ¼ 160 mV sꢂ1. By linearly
tting Isc+ vs. s values the capacitance varied from 14 nF down to
4 nF as resulting for the low (i.e. s # 60 mV sꢂ1) and high s
values range, respectively. A similar capacitance reduction (i.e.
from 22 nF to 6 nF going from the low to the high s values range)
ꢂ
was observed by linearly tting Isc vs. s values. This variation
was ascribed to the distribution and density of charge trapping
sites contributing to the capacitance depending on the specic
trapping/detrapping characteristic times. The estimate of the
SM capacitance resulting from the data collected on SM-based
MIM devices indicated that the layer capacitance was lower
than that of the TEGM layer (55 pF for the low s range i.e.
s ¼ 1.0–12 mV sꢂ1).
Impedance measurements results. The electrical transport
properties of both TEGM and SM appeared to depend on the
water-ion content, so that they can both be assigned to the class
of water-based polyelectrolytes.31 The analysis of the AC
impedance frequency dispersion can provide further insight
into the relative extent of the different relaxation mechanisms
contributing to the spectra as a result of electrode polarization,
ionic motion and possible dipolar contributions.
To detail better on the observed charge storage effect and
hysteretic behaviour, we start considering the general expres-
sion of the displacement current Id as a function of the voltage
owing across a capacitive medium:29,30
dQ
dt
dðCVÞ
dt
dC
dt
dV
dt
dC
dt
Id ¼
¼
¼ V
þ C
¼ V
þ Cs
(1)
Fig. 5 illustrates the real (30, dielectric constant) and imagi-
nary (300, dielectric loss) part of dielectric permittivity frequency
dispersion, as derived from the impedance data collected in
ambient air on TEGM- and SM-based MIM structures. In the
inset the loss factor tan d vs. f dispersion, i.e. the ratio between
the dielectric loss and dielectric constant, is also shown.
Interestingly, different behaviours of TEGM and SM permittivity
were observed.
The real part of the dielectric permittivity spectra of TEGM
revealed the presence of a plateau in the frequency region up to
10 Hz, assigned to the double layer capacitive contribution.
Relevant to note, the estimated dielectric constant is two orders
of magnitude higher than that of SM, evidencing an increased
interfacial polarization effect at the electrode. In the frequency
region 10 Hz to 10 kHz the TEGM dielectric constant shows a
behaviour typically observed in polyelectrolytes and its value
was consistently higher than that of SM. This can be attributed
to the recognized ability of TEG to increase ionic conductivity
Such a current adds to conduction current component Ir. It
can be deduced that the displacement current can be more or
less enhanced depending on the voltage loop amplitudes and
on the voltage sweep speeds. The term (dC/dt) represents the
increase of capacitance during the voltage sweep.
When xing the voltage sweep speed s, (see Fig. 4a) the
variation of the amount of stored charges vs. VL, can be obtained
by integrating expr. (1) i.e. by calculating the loop area values
(see Fig. 4b). The results indicate that the stored charges are
almost linearly dependent on the magnitude of the voltage loop
amplitude. The comparison between results on TEGM-based
and SM-based MIM devices evidences that in the former the
stored charge is two order of magnitude higher than in the
latter, thus indicating a more effective space charge accumula-
tion mechanisms under a voltage stimulus. This is also evi-
denced by the comparison between two typical hysteresis loops
collected at the same voltage sweep speed s and for VL ¼ 1.0 V on
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Data in Fig. 4 and 5 clearly showed that the inclusion of the
highly hydrophilic TEG chain results in a profound alteration of
typical eumelanin electrical properties with a marked increase
in the dielectric constant and a distinct polyelectrolyte behav-
iour. It is suggested that the TEG chains allow for the incorpo-
ration of water molecules into the bulk of the eumelanin lms,
with the formation of a hydrogen-bond network enabling effi-
cient charge transport in the solid state. It is tempting to
assume, in this context, that the oligoether chains play the dual
role of favouring eumelanin aggregation in a relatively regular
manner and keeping the water molecules tightly bound
between the stacked indolic layers (Fig. 6).
Fig. 5 Dispersion spectra of the real (30, left axis) and imaginary (300,
right axis) part of the dielectric permittivity as resulting from imped-
ance spectra collected on TEGM (blue curves) and SM (red curves) MIM
devices. The inset shows the calculated tan d loss dispersion spectra.
Conclusions
A water soluble eumelanin biopolymer TEGM is described,
which was obtained by oxidative polymerization of a DHI
derivative N-substituted with a hydrophilic TEG chain. Current–
voltage measurements in MIM devices indicated markedly
higher open circuit values and hysteresis loop areas in the case
of TEGM compared to the commercial melanin SM, suggesting
in the former a more effective space charge mechanism.
Impedance data consistently indicated for TEGM a double layer
contribution much higher than for the reference SM and a
higher dielectric constant. These results point to DHI func-
tionalization with the solubilizing and highly hydrophilic TEG
chain as an effective approach to confer polyelectrolyte behav-
iour to eumelanin materials, possibly via the formation of a
hydration shell around oligomer molecular components.
and/or dipolar effects, as observed in other polymers. Further-
more, a major water molecules enclosure should be expected
due to the TEG major hydrophilicity.32 The relaxation process
then extends up to 1 MHz and the corresponding relaxation
time was extracted by the peak frequency in the loss tan d vs. f
spectra and found to be s ¼ 2.56 ms.32
Compared to TEGM, the 30 of SM showed a dispersion
spectrum whose behaviour was similar to those commonly
observed in biological items. In particular, the two typical
regions generally encountered in biological items 30 spectra,
were distinguishable. The rst one was constituted by a relax-
ation region generally extending from 1 kHz up to 1 MHz
and whose magnitude is strictly related to the embedded
water content. Such a region is commonly referred to as the b
region (see ESI†). The second one, at a relaxation frequency
above 1 MHz, evidenced the presence of the water-molecule
dipole reorientation processes and was commonly dened as
the g-region (see ESI†).33
Acknowledgements
Prof. Giuseppe Carbone is gratefully acknowledged for allowing
to use the Atomic Force Microscope of the Tribology Lab
(DMMM, Politecnico di Bari). This work was supported in part
by grants from MIUR, PRIN 2010–2011 (PROxi project) and 2012
(AQUASOL Project PRIN2012 A4Z2RY), by the FIRB 2009/2010
project “Rete integrata per la Nano Medicina (RINAME)” –
RBAP114AMK_006 and in part by European Commission (Pol-
yMed project in the FP7-PEOPLE-2013-IRSES frame, r.n.PIRSES-
GA-2013-612538) and was carried out in the frame of the activ-
ities of the EuMelaNet special interest group. Giuseppe Chita
is acknowledged for technical support in the RINT2500
laboratory.
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