Ibersensor 2010, 9-11 November 2010, Lisbon, Portugal
IB-059
New Potentiometric Chemical Sensors Based on Ligninpolyurethane Composite Polymers
F.A.C. Faria1, A. Rudnitskaya2#, M.T.S.R. Gomes2, J.A.B.P. Oliveira2, M.P.F.Graça3, L. Cadillon
Costa3, D. Evtuguin4
1
Department of Chemistry, University of Aveiro, Aveiro, 3810-193, Portugal
2
CESAM/Department of Chemistry, University of Aveiro, Aveiro, 3810-193, Portugal
3
I3N and Department of Physics, University of Aveiro, Aveiro, 3810-193, Portugal
4
CICECO/Department of Chemistry, University of Aveiro, Aveiro, 3810-193, Portugal
#
Corresponding author; email: [email protected]; tel. +351 234401527 (ext. 24903)
Abstract
The purpose of this study was synthesis and characterization of the co-polymer of polyurethane with
lignin for the fabrication of all-solid-state potentiometric chemical sensors. Polymers based on lignins
derived from different pulping processes namely kraft, organosolv and sulfite were synthesized.
Obtained polymers were characterized using thermogravimetric analysis (TGA), differential scanning
calorimetry (DSC), scanning electron microscopy (SEM), atomic force microscopy (AFM) and
electrochemical impedance spectroscopy (EIS). Polyurethane–lignin co-polymers were doped with
small amount of carbon nanotubes (0.72w/w%) with the aim to increase material conductivity to the
levels suitable for sensor applications. Chemical sensors were prepared by drop casting of liquid
polymer on the surface of carbon glass or platinum electrode. Sensitivity of the sensors towards wide
range of transition metals and pH was evaluated. Redox response was studied in the solutions of two
redox pairs Cr(III)/Cr(VI) and Fe(CN)63-/4-. Low or no sensitivity was observed to all ions except Cr(VI)
at pH 2, to which response of 39, 50 and 53mV/pX was displayed by the sensors based on kraft,
organosolv and lignosulfonate lignins respectively. Redox sensitivity close to theoretical of 20 and 21
mV/pX for organosolv and lignosulfonate sensors respectively was observed in the Cr(III)/Cr(VI)
solutions while very low response was observed in the Fe(CN)63-/4- solutions. Conducting composite
polymers based on the polyurethanes co-polymerized with lignins and doped with carbon nanotubes
were demonstrated to be promising materials for Cr(VI)-sensitive potentiometric sensors.
Keywords: lignin, polyurethane, potentiometric sensors, carbon nanotubes, chromium(VI)
Introduction
Conducting polymers comprise various types of
polymeric materials with electronic and/or ionic
conductivity
including
doped
conjugated
polymers, redox polymers, polymer composites,
and polymer electrolytes [1-2]. Conducting
polymers of different types attract increasing
attention as sensing materials for chemical
sensors. In the field of potentiometric sensors,
conducting polymers are employed primarily as
ion-to-electron transducers allowing fabrication
of all-solid-state polymeric sensors. Conducting
polymer can be used as solid inner contact, can
be mixed with other polymer containing active
substances or functionalized with ion recognition
groups. The latter approach also permits to
diminish the leaching of membrane components
into aqueous phase thus increasing sensor life
time and reproducibility while decreasing
detection limits [1].
The main idea of the present study was
synthesis of conducting polymer incorporating
lignin as an active substance to be used as
sensing
material
for
the
solid-state
potentiometric chemical sensors.
Lignin is one of the most abundant natural
polymers constituting from one quarter to one
third of wood dry weight and it is also one of the
main waste products of pulping industry. Lignin
is formed by phenylpropane units and contains a
variety of functional groups such as hydroxyl,
carbonyl, carboxyl, etc [3]. These groups impart
to lignin a capability of complexing a wide range
of compounds, from transition metals to
pesticides and humic substances. Based on that
numerous application of lignin as sorbent for
bioremediation [4] and an ionofore for chemical
sensors [5-7] have been suggested. Detection of
copper, lead, cadmium and humic substances
using impedimetric or amperometric electrodes
modified by Langmuir–Blodgett lignin films was
Ibersensor 2010, 9-11 November 2010, Lisbon, Portugal
reported in [5-6]. Oxidized lignin film deposited
on gold electrode displayed red-ox behavior,
which allowed application of this lignin modified
electrode to the voltammetric detection of
ascorbic acid [7].
In the present work lignin was copolymerized with isocyanate and resulting
polymer was used for the fabrication of solidstate potentiometric chemical sensors. Copolymerization of lignin allowed fixing it in the
membrane material and avoiding its leaching to
the aqueous phase. Lignin composition and
functional group content depend on the wood
origin as well as employed pulping process,
therefore, lignins resulting from three different
pulping processes, namely kraft, organosolv and
sulfite were used in this study. Characterization
of lignin based polymers and assessment of
electrochemical
properties
of
solid-state
potentiometric sensors fabricated using them
were carried out.
Experimental
Oxidation of kraft lignin.
Kraft lignin was subjected to oxidative treatment
prior to be used for sensor material preparation.
Optimization of the oxidation conditions aimed at
maximizing content of functional groups and
yield while avoiding lignin decomposition was
reported in [8]. The following optimized
procedure was used. Weighed amount of lignin
was placed in the stainless steel reactor
together with the solution of sodium
phosphovanadomolibdate Na4[PMo10V2O40] with
the ratio of 1mM of POM 1g of lignin. Oxygen
pressure of 5 bar was secured and reactor was
heated to 50ºC and maintained at this
temperature during 2 hours. Oxidized lignin was
separated from the liquid by centrifugation,
repeatedly washed with distilled water and dried
at 40ºC.
Polymer synthesis and sensor preparation.
Three types of lignin, eucalypt kraft lignin (K),
spruce
organosolv
(OS)
and
eucalypt
lignosulfonate (S), were used for polymer
synthesis. Polycondensation reaction of lignin
with isocyanate using method suggested in [9]
was employed. Lignin powder or mixture of
lignin powder with carbon nanotubes was placed
in the glass reactor with poly(propylene glycol),
tolylene 2,4-diisocyanate terminated and stirred
for 40 minutes at 40ºC in order to obtain
homogenous
viscous
solution.
Then
temperature was increased to 60ºC and liquid
catalyst (dibutyltin dilaurate) was added.
Reaction was carried under the nitrogen.
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Mixture was stirred for further 10 minutes until it
started to thicken after which it was removed
from the reactor and used for the fabrication of
the sensor membranes and polymeric films.
Films for the polymer characterization were
prepared by pouring polymer into the flat mould.
Sensors were prepared by placing a drop of
liquid polymer on the tip of the glass carbon or
platinum electrode. At least two parallel sensors
of the same composition were prepared.
Electrode surface was polished with abrasive
paper followed by 6μm diamond polish and
thoroughly washed with ethanol and distilled
water prior to membrane deposition. Both
sensor membranes and polymeric films were
cured for 4 hours at 60ºC.
Polymer characterization.
Obtained polymers were characterized using
thermogravimetric analysis (TGA), differential
scanning calorimetry (DSC), scanning electron
microscopy (SEM), atomic force microscopy
(AFM)
and
electrochemical
impedance
spectroscopy (EIS).
Potentiometric measurements.
Electrochemical measurements were carried out
in the following galvanic cell: Cu|Ag|AgCl,
KClsat |sample|membrane|Pt/GC|Cu
Emf values were measured vs. Ag/AgCl
reference electrode with precision of 0.1mV
using custom made multichannel voltmeter high
input impedance connected to the PC for data
acquisition and processing. Besides studied
sensors, pH glass electrodes and Pt electrodes
were used when necessary. Sensitivity of the
sensors was evaluated by means of calibration
measurements in the solutions of nitrates and
chlorides of sodium, calcium, zinc, cadmium,
lead, copper, mercury, chromium(III), iron (III)
and potassium chromate. Concentration ranges
were from 10-7 to 10-3 for all studied ions. Redox
response was studied in the solutions of redox
pairs Cr(III)/Cr(VI) and Fe(CN)63-/4-. Total
concentration was 1mM for both pairs with ratio
of oxidized to reduced form changing from 0.01
to 100. Measurements in the solutions of
chromium(III), iron (III), chromate and redox
pairs were made at pH 2 on the background of
0.01molL-1 nitric acid. Sensitivity to chromate
was also studied at pH 3, 4 and 6 on the
background of 1mM nitric acid and 0.05mM of
acetate and phosphate buffers respectively.
Measurements in other ions’ solutions were
made on the background of phosphate buffer at
pH 6. Response to pH was measured in the
range from 2 to 9. At least 3 replicated
calibrations were made for each ion. Parameters
of Nernst equation i.e. slope of the electrode
function and standard potential were calculated
Ibersensor 2010, 9-11 November 2010, Lisbon, Portugal
using linear regression and averaged over
replicated
calibration
runs
and
sensor
compositions.
Results and Discussion
Synthesized polyurethane-lignin co-polymers
were characterized using a range of techniques.
Impedance measurements revealed that
polyurethane-lignin co-polymers were dielectric
with conductivity of about 10-10 – 10-9 Sm-1
(Table 1).
Table 1. Conductivity of the lignin based polyurethanes with
and without carbon nanotubes.
Lignin
Kraft
Lignosulfonate
Organosolv
CNT
concentration,
w/w %
0
0.72
0
0.72
0
0.72
Conductivity,
S m-1
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along lignin clusters as was evidenced by SEM
of the mixture of kraft lignin with CNTs (Fig 1).
Distribution of CNTs in the lignin
based polymer was studied using AFM (Fig.2). It
was observed that distribution of CNTs inside
the polymer was less homogenious compared to
the mixture of the former with lignin. Zones with
agglomeration of CNTs and with their even
distribution were found on the surface of the
polymer.
Comparison of the properties of polyurethanelignin co-polymers was carried out using
thermogravimetric
analysis
(TGA)
and
differential scanning calorimetry (DSC). DSC
revealed the same behavior for polyurethanelignin polymers with and without CNTs, glass
transition temperature being -49C for all six
polymers studied.
3.13*10-9
6.0*10-4
1.43*10-9
2.29*10-3
4.50*10-10
3.32*10-4
Such low conductivity does not allow using
these polymers in potentiometric sensors.
Lignin-based co-polymers were doped with
multiwall carbon nanotubes with the aim to
increase conductivity to the levels suitable for
sensor applications. It was found that the
addition of only 0.72 w/w% of carbon nanotubes
(CNTs) led to the conductivity increase of 5 to 6
orders of magnitude (Table 1).
Fig.2. AFM image of kraft-lignin based polyurethane.
Fig. 1. SEM micrograph of kraft lignin mixed with carbon
nanotubes.
Significant increase of conductivity of polymer
after addition of small amounts of CNTs can be
explained by the even distribution of the CNTs
TGA curves have shown that thermal
degradation of polymers both with and without
CNTs begin at 250ºC. However, degradation of
polymers based on kraft and organosolv and
doped by CNTs proceeded faster compared to
the same polymers without CNTs. Difference in
the degradation rate between doped and pure
polymers was more pronounced in the case of
polymers
based
on
organosolv
lignin.
Presumably, CNTs affect structure of ligninbased polymers making it looser and thus
facilitating their thermal degradation.
Therefore, doping with low concentration of
CNTs did not affect significantly polymer
properties but conductivity. Higher rate of
thermal degradation observed for polymers
based on kraft and organosolv lignins should not
have any implication of the sensor properties of
the respective polymers.
Ibersensor 2010, 9-11 November 2010, Lisbon, Portugal
K
OS
S
30
Slope, mV/pX
25
lignin sensors includes both specific interaction
between chromate and lignin (ion exchange) as
well as a redox process. Further experiments
are necessary for elucidation of the mechanism
of the response of the lignin based sensors
towards Cr(VI).
50
40
30
20
20
10
15
0
Cr(VI)
Cr(VI)/Cr(III)
3-/4-
Fe(CN)6
Fig. 4. Slopes of the electrode function together with
standard deviations in the solutions of Cr(VI) and of redox
pairs at pH 2.
10
5
0
K
OS
S
Pt
Chalc. glass
60
Slope, mV/pX
All three polymer composition has shown similar
electrode behavior in the studied solutions.
Sensors did not display no response to sodium,
calcium,
zinc,
cadmium,
mercury(II),
chromium(III) and iron (III) and very low
response of about 12 mV/pM to copper and lead
ions. pH sensitivity was also quite low but linear
practically in all studied range: 11 mV/pH for K
sensors in the pH range from 2 to 9 and 14
mV/pH for OS and S sensors in the pH range
from 3 to 9. High sensitivity was found towards
chromate at pH 2 (Fig. 3).
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Zn
Cd
Pb
Cu
pH
Fig. 3. Slopes of the electrode function together with
standard deviation of lignin sensors in the solutions of
transition metals and pH sensitivity.
Sensors’ response to Cr(VI) was not Nersntian
and cationic though chromate is an anion.
Moreover, response to Cr(VI) was observed only
at pH 2 while low anionic or no response at all
were found at pH from 3 to 6. Similar behavior
was reported earlier for the Cr(VI)-selective
chalcogenide glass electrode [10], which also
displays cationic response to chromium(VI) at
pH 2. This effect was attributed to the partly
redox mechanism of the generation of the
sensor potential though no studies on this
subject have been published. Furthermore,
redox sensitivity of lignin based sensors was
evaluated in the solutions of two redox pairs –
Cr(III)/Cr(VI) and Fe(CN)63-/4-. Responses of the
studied polymer sensors and Pt redox sensor
are shown in the Fig. 4. Pt electrode was used
as a reference displaying theoretical or close to
theoretical response in the solutions of redox
pairs. In the case of Cr(III)/Cr(VI) and Fe(CN)63-/4systems, response of Pt electrode was 17 and
55 mV/pX respectively.Sensors based on
organosolv and lignosulfonate lignins displayed
theoretical redox response in the Cr(III)/Cr(VI)
solutions of 20 and 21 mV/pX respectively while
very low sensitivity was observed in the
Fe(CN)63-/4- solutions. Therefore, it is plausible
to suggest that response mechanism of the
Conclusions
Conducting composite polymers based on the
polyurethanes co-polymerized with lignins and
doped
with
carbon
nanotubes
were
demonstrated to be promising materials for
sensing application. High conductivity of these
materials allows easy preparation of the allsolid-state chemical sensors while high chemical
stability and low leaching of the membrane
components, which yield reproducibility of
sensor characteristics. High sensitivity and
selectivity of the sensors based on organosolv
lignin and lignosulfonate towards Cr(VI) in the
acid media were observed. Further studies are
necessary for optimization of the polymer
compositions
and
elucidation
of
the
potentiometric response generation mechanism.
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