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J. Electrochem. Sci. Technol., Epub ahead of print
−
1
−
Carbon Particle-Doped Polymer Layers on Metals as Chemically
and Mechanically Resistant Composite Electrodes for Hot Electron
Electrochemistry
Nur-E-Habiba
1,2
*, Rokon Uddin
2
, Kalle Salminen
2
, Veikko Sariola
1
, and Sakari Kulmala
2
*
Faculty of Medicine and Health Technology, Tampere University, Tampere 33720, FINLAND
Department of Chemistry and Materials Science, Aalto University, Espoo 02150, FINLAND
ABSTRACT
This paper presents a simple and inexpensive method to fabricate chemically and mechanically resistant hot electron-emit-
ting composite electrodes on reusable substrates. In this study, the hot electron emitting composite electrodes were man-
ufactured by doping a polymer, nylon 6,6, with few different brands of carbon particles (graphite, carbon black) and by
coating metal substrates with the aforementioned composite ink layers with different carbon-polymer mass fractions. The
optimal mass fractions in these composite layers allowed to fabricate composite electrodes that can inject hot electrons into
aqueous electrolyte solutions and clearly generate hot electron- induced electrochemiluminescence (HECL). An aromatic
terbium (III) chelate was used as a probe that is known not to be excited on the basis of traditional electrochemistry but
to be efficiently electrically excited in the presence of hydrated electrons and during injection of hot electrons into aqueous
solution. Thus, the presence of hot, pre-hydrated or hydrated electrons at the close vicinity of the composite electrode sur-
face were monitored by HECL. The study shows that the extreme pH conditions could not damage the present composite
electrodes. These low-cost, simplified and robust composite electrodes thus demonstrate that they can be used in HECL
bioaffinity assays and other applications of hot electron electrochemistry.
Keywords: Hot Electron Electrochemistry, Hot Electron-Induced Electrochemiluminescence, Composite Electrodes, Hot
Electron Injection, Hydrated Electrons
Received : 1 July 2021, Accepted : 22 August 2021
1. Introduction
Hot electron electrochemistry, which provides
tools to work beyond the electrochemical window of
water restricting the traditional electrochemistry
aqueous solutions, has been explored already for
quite a long time. The principles of utilizing hot and
hydrated electrons in analytical applications have
been earlier studied by using thin insulating film-
coated electrodes [1-5]. Semiconductor electrodes
and thin-insulating oxide-coated electrodes are par-
ticularly attractive in this field because the electrons
and holes can remain separated in energy in the con-
duction and valence band, respectively. The unusual
ability to directly inject/emit electrons into solutions
forming solvated electrons is the key point in the
properties of these relatively new electrode materials.
Solvated electrons, and hydrated electrons in water,
are the chemist's perfect reducing agent in many
ways. Recently, we made efforts to develop low-cost
replacements [6,7] for chemically quite non-resistant
oxide-coated aluminium electrodes [8-10] and typi-
cally a bit too expensive oxide-coated silicon elec-
trodes [7,8] for hot electron injection into fully
aqueous electrolyte solutions [1-5].
We have earlier shown that by using thin-film man-
ufacturing technologies quite sophisticated electrode
chips can be made on glass chips e.g. by utilizing alu-
minium sputtering and atomic layer deposition of
alumina or from silicon chips [11,13,14]. Such elec-
trodes can be used in disposable manner for e.g. bio-
affinity assays that are important in real-world point
of care testing [12,15,16]. In these assays, the lowest
Research Article
*E-mail address: nur-e-habiba@tuni.fi, sakari.kulmala@aalto.fi
DOI: https://doi.org/10.33961/jecst.2021.00640
This is an open-access article distributed under the terms of the Creative Commons
Attribution Non-Commercial License (http://creativecomm ons.org/licenses/by-nc/4.0)
which permits unrestricted non-commercial use, distribution, and reproduction in any
medium, provided the original work is pr operly cited.
2Nur-E-Habiba et al. / J. Electrochem. Sci. Technol., Epub ahead of print
determination limits are typically obtained by using
aromatic Tb (III) chelates as labels, however many
organic luminophores [5,17-19] or Tris(bipyri-
dine)ruthenium(II) [Ru(bpy)3
2+] -type labels [20,21] can
also be used when lower assay sensitivity is sufficient.
These labels are typically excited with sequential one-
electron reduction and oxidation steps either by red-ox,
or ox-red routes depending on the (1) redox properties
of the luminophores or ligands of the complexes, and
(2) the stability of luminophore or ligand radicals in the
aqueous solution [4,18,19,22,23].
We have previously shown that hydrated electrons
can be obtained by hot electron injection into aque-
ous solutions. These conclusions were based on the
measurements using various hydrated electron scav-
engers with the known reaction rate constants
obtained from pulse radiolysis studies. The hot- and
hydrated electrons allow to carry out difficult one-
electron reduction reactions in aqueous solutions that
are usually not obtainable using traditional electro-
chemistry and active electrodes. These highly reduc-
ing intermediates also enable efficient production of
strongly oxidizing radicals by one-electron reduction
from precursors such as hydrogen peroxide
(hydroxyl radical), peroxydisphosphate (phosphate
radicals) and peroxydisulfate (sulfate radicals)
[1,17,23]. Thus, strongly reducing, but also simulta-
neously strong oxidizing conditions can be created by
the hot- and hydrated electrons.
Our group has utilized hot electron injection into
aqueous solution only in generating hot electron-
induced electrochemiluminescence (HECL) of our
labels for bioaffinity assays, but there are many other
application areas for hot electron electrochemistry. Sol-
vated electrons can be utilized in organic chemistry [24]
and in inorganic chemistry [25,26] and e.g. in disinfec-
tion of potable water and treatment of waste waters [27-
29]. Most common methods for generation of hydrated
electrons have so far been (1) high energy irradiation of
water (either high energy electrons or photons [25], (2)
photoemission of electrons from electrodes [30], (3)
photoionization and photodetachment of solutes [31-
33], (4) dissolution of wide-band gap inorganic crystals
containing trapped electrons [34].
Our recent developments, composite electrodes,
have been so far made by using polymer materials,
such as polystyrene [6,7] and ethyl cellulose [35] as a
matrix that can easily be dissolved into common
organic solvents and can then immediately be doped
with suitable conducting particles by simply mixing
and sonicating with ultrasound. The doped polymer
is finally spin-coated upon a conductive substrate
such as on a metal or a strongly doped semiconductor
disc. Thus, the final structure of C/CPDP (conductor/
conducting particle doped polymer) composite elec-
trode has been created [6,7,35]. In these composite
electrodes, the final electron injection to solution is
typically occurring through an ultrathin polymer layer
naturally formed on top of the conducting particles
during manufacture process [7]. However, it is possible
that those conducting particles in direct contact with the
electrolyte on the surface may in addition inject hot elec-
trons by field emission into electrolyte solution, either as
such, or through a hydrogen gas barrier generated by
hydrogen evolution [36].
This time we made efforts to manufacture composite
electrodes in a very simple way that could produce com-
posite electrodes usable also in many organic solvents,
and mixtures of aqueous electrolyte solutions and sol-
vents miscible in water, to inject hot electrons into these
solvents or solvent mixture solutions. Nylon 6,6 was
chosen as a polymer since it is known to have high
mechanical strength, rigidity, good stability under heat
and high chemical resistance [37].
Nylon 6,6 is one of the most commonly used poly-
amides in industries and in 1938, it was first used in
toothbrush filaments production [38]. It consists of
two monomers, containing hexamethylenediamine
and adipic acid. As the melting point is related to the
degree of hydrogen bonding between the chains,
therefore, nylon 6,6 has a sharp melting point of
264oC due to the density of amide groups leading to
hydrogen bond formation. In addition, the symmetri-
cal structure of even-even monomers of nylon 6,6
and its amide groups allow the hydrogen bonds to be
formed in any direction that the chains are facing, pil-
ing up on top of each other. This phenomenon leads
to faster crystallization rate and processing window.
Though characteristically, nylon has the ability to
absorb significant amount of water in general [37],
the higher crystallinity of nylon 6,6 also helps its
lower moisture absorption, and thus affects its modu-
lus and tensile strength.
Nylon 6,6 also shows much higher fatigue resis-
tance, advantageous abrasion resistance and coeffi-
cient of friction. Usually, the volume resistivities of
dry nylon are between 1014-1015 Ω·cm. Dry nylon 6,6
has dielectric strength of 24 kV/mm (short time) and
Nur-E-Habiba et al. / J. Electrochem. Sci. Technol., Epub ahead of print 3
11 kV/mm (step- by-step) [37]. In terms of chemical
resistance, nylons have been proven as an excellent
material, as polyamides are known to be particularly
resistant towards nonpolar materials e.g. hydrocar-
bons. However, strong acids and phenols can disrupt
the hydrogen bonding and even can dissolve nylons
and nylon 6,6 can be dissolved e.g. in ethylene glycol
above 160oC. Nylon 6,6 is being industrially synthe-
sized, and it is currently an easily obtainable low-cost
material for the present applications.
In the present study, nylon 6,6 was first dissolved
in hot formic acid and then doped either with graphite
particles or carbon black particles. We used two types
of graphite particles and two types of carbon black
particles and first examined the optimal composition
of each of the DP (doped polymer) layers of C/DP
electrodes. The DP mixture was then dispensed on a
round metal substrate and very simply cured on a hot
plate at a suitable temperature.
Finally, we studied the analytical performance of
the present composite electrodes for detecting Tb
(III) chelate and Ru(bpy)3
2+ chelate on the basis of
their cathodic HECL. Our aromatic Tb (III) chelates
cannot be excited on the basis of traditional electro-
chemistry at active metal electrodes due to the insuf-
ficient electrochemical window available because of
hydrogen and oxygen evolution reactions, but are
showing strong chemiluminescence in the presence
of hydrated electrons and highly oxidizing radicals
[3,4]. The excitation of Ru(bpy)32+ chelate by
hydrated electrons and oxidizing radicals [39] as well
as by hot electron injection into aqueous solution has
been studied in detail earlier [23].
2. Experimental
2.1 Chemical and reagents
Nylon 6,6 pellets from Sigma-Aldrich was used as a
matrix polymer of composite ink layers and four types of
individual conducting particles were studied (Table 1).
The solvent for making the composite material i.e.
ink was 100 % formic acid (Fisher Scientific). The
composite material mixtures were made by using
Cole-Parmer ultrasonic homogenizer.
The measurements solutions were made from dis-
tilled water and appropriate salts. 0.20 M borate buf-
fer containing 0.100 M of sodium sulfate (Sigma-
Aldrich) was made in 0.05 M sodium tetraborate
decahydrate solution (Na2B4O7·10 H2O, Merck). The
weighed amount of sodium tetraborate was calcu-
lated to yield 0.05 M sodium tetraborate solution,
since each mole of tetraborate produces two moles of
boric acid and two moles of borate ions upon dissolu-
tion. The stock solution (0.05 M) of peroxydisulfate
was made from the product of Merck. Stock solution
of terbium (III) chelate (chelate ligand was terbium
(III)-4-(phenylethyl) (1- hydroxybenzene)-2,6diyl)
bis-(methylenenitrilo) tetrakis (acetic acid), synthe-
sized in University of Turku) (abbreviated as Tb (III)-
L hereafter) was 0.01 M. For pH sensitivity study in
section 3.5.2, two additional luminophores: Tris (2,2’
-bipyridyl)dichloro-ruthenium (II) hexahydrate) sol-
vated as Ru(bpy)3
2+ chelate in the solutions, and fluo-
rescein from Sigma- Aldrich were used.
2.2 Fabrication of electrodes
Round brass substrates were made with a diameter
of 12 mm from technical brass sheet (soft brass 0.002
thick 44 gauge 12” × 30”) purchased from K&S pre-
cision metals. Then masks made of Teflon adhesives
(Irpola Oy, Turku) were added on top of the sub-
strates as masks revealing a round area of 0.64 cm2 in
the middle of the substrate for dispensing the com-
posite ink. Each of the composite inks were then dis-
pensed in the wells surrounded by teflon and the
electrode was dried on a hot plate at a temperature of
90oC for two hours (Fig. 1a). The electrodes were let
to cool before use. After the use, these composite
inks can be easily washed off or removed using com-
mon organic solvent, and then the substrates can be
Table 1. Characteristics of carbon-based conducting particles
Carbon black 1 Carbon black 2 Graphite 1 Graphite 2
Manufacturer
(Brand)
Cabot, Latvia
(Vulcan XC-72R)
Timcal, Belgium
(C 65)
Timrex, Belgium
(SLP 30)
Forward looking
solutions, UK
(unknown)
Particle size 50 nm 20 nm 32 µm 4-10 µm
Impurities ~40 ppm ~20 ppm ~60 ppm ~100 ppm
4Nur-E-Habiba et al. / J. Electrochem. Sci. Technol., Epub ahead of print
reused for fabricating new composite electrodes. In
this study, oxide-covered aluminium discs as elec-
trodes, cut from 99.9% pure aluminium (Merc art.
1057, batch 721 k4164557) were used for compari-
son with the composite electrodes.
The most significant part of the electrode fabrica-
tion process is the preparation of the composite ink.
In order to prepare the composite inks, carbon parti-
cles and nylon 6,6 were weighed in the vials and kept
at 90oC for one week together with concentrated for-
mic acid. Prior to the dispensing each composite ink
variant containing different mass ratios of dry matters
and solvent were sonicated for 10 minutes (amplitude
40%, 45s on-off-cycle; Cole-Parmer ultrasonic
homogenizer). The dry matter consisted of nylon 6,6
and carbon particles where the weight of nylon 6,6
was always 80% of that of total dry matter. Different
mass ratio of dry matter (50, 75, 100 and 150 mg/mL
of solvent) were tested in order to find the optimum
consistency of the ink based on the formulated ink’s
viscosity and ability of injection of hot electrons.
Finally, the total dry material concentration was
selected to be 75 mg/mL in the study experiments.
Interestingly, the replacement of carbon particles by
~100nm particle sized silver nano powder (Sigma-
Aldrich) produced composite electrodes that could
not produce a measurable amount of HECL.
The measuring cell was composed of two parts that
could be screwed together. The lower part provided
electrical contact to the brass substrate discs and
upper part provided teflon vial for dispensing the
electrolyte solution onto the working/composite elec-
trode area, and a platinum wire as a counter electrode
(Fig. 1b). The electrolyte solution volume in the mea-
surements was always 150 μL.
2.3 Measuring instruments and measurement
procedure
The instrument setup consisted of an in-laboratory
built coulostatic pulse generator, a photomultiplier
tube module (Perkin Elmer MH1993 1364-H-064) for
optical detection, a photon counter (Stanford Research
Systems SR-400), a preamplifier (Stanford Research
Systems SR-445), Nucleus Inc MCS-II multiscaler
card and two PC units. For DC excitation a laboratory
DC voltage source was used instead of coulostatic
pulse generator measurements. The sheet resistivity of
different composite electrodes was measured with Jan-
del RM3000 test unit equipped with a cylindrical four-
point probe head to avoid contact resistance.
As mentioned in section 2.1, all the measurements
were conducted in 0.20 M borate buffer containing
0.10 M of sodium sulfate as supporting electrolyte,
using a solution volume of 150 μL. The HECL mea-
surements begun with measuring reference value
using 99.9% pure aluminium discs, where the
cathodic excitation pulses were generated using the
pulse generator when constant charge voltage pulses
of -41.8V was delivered with a pulse charge of
31.5 μC and a pulse frequency of 50 Hz. Simultane-
ously, optical detection was performed using the
aforementioned photomultiplier, assisted with the
amplifier, through an interference filter (i.e. for the
electrochemiluminescent labels Tb (III), fluorescein,
Fig. 1. a) Fabrication of composite electrode using nylon-carbon black composite ink b) Measuring cell during experiment,
containing both working electrode (composite ink on brass substrate) and counter electrode (platinum wire); the presence of
luminophore in this experimental electrolyte solution causes green light emission.
Nur-E-Habiba et al. / J. Electrochem. Sci. Technol., Epub ahead of print 5
and Ru(bpy)3
2+ - a 545±40 nm, 550±40 nm and
600±40 nm respectively) passing the label specific
spectral line. Besides, the amplified photon pulses
were also recorded in the multiscaler card. Similarly,
following the reference measurement, then all the
composite electrodes were individually tested replac-
ing the aluminium, repeating the same measurement
procedure. A schematic figure of the overall reaction
pathways (a. ox-red pathway, and b. red-ox pathway)
in composite electrodes during the HECL measure-
ment is depicted in Fig. 2.
3. Results and Discussion
3.1 The effect of the weight fraction of conducting
particles
When the weight fraction of conductive particles of
the total solids of the composite coatings was studied, it
was observed that the coatings with higher weight frac-
tion than 40% became too brittle to be usable in case of
carbon black particles, but graphite particles allowed the
experiments up to 70% of carbon particles (Fig. 3).
It was observed that, when the starting point was
10% of conductive particles of the total mass of the
compositions, all the electrodes were working for the
purpose, but about two orders of higher magnitude
HECL intensity could be obtained with graphite par-
ticles with much higher weight fraction of the con-
ductive particles. In case of graphite 1, the optimum
was at about 60% of graphite and with graphite 2 con-
sists of smaller particle size, the optimum weight frac-
tion was already at about 45% of graphite of total solids.
However, with smaller carbon particle sizes a cou-
ple of orders of higher magnitude HECL intensity
was obtained. Carbon black 2 having the smallest
particle size, i.e. 20 nm (Table 1), appeared to be the
best conductive particle source for these composite
films upon the metal electrodes, as a whole. How-
ever, both carbon black 1 and carbon black 2 showed
Fig. 2. A schematic of two possible reaction pathways (ox-red and red-ox) during HECL measurement using
electrochemiluminescent label in composite electrodes.
Fig. 3. The effect of weight fraction of carbon particles on
the TR-HECL. Each point represents a mean of 6 replicates.
Solution: 0.2 M borate buffer at pH 9.2, 0.1 M Na SO , 0.01 M
Tb (III)-L, 1 mM K S O . Interference filter: 545±40 nm.
Excitation parameters: Pulse voltage -41.8 V, pulse charge
31.5
μ
C, pulse frequency 50 Hz, 2000 excitation cycles.
6Nur-E-Habiba et al. / J. Electrochem. Sci. Technol., Epub ahead of print
the optimal weight fraction of carbon particles to be
at about 30% where they gave the same intensity
(photon count) as the best result of all.
3.2 Measurement of sheet resistance in composite
films
Sheet resistance measurements of the composite
films were carried out using 4-point probe head from
the composite films fabricated on top of the non-con-
ductive polyester film. The sheet resistance decreased
as a function of increasing weight fraction of carbon
as expected. The best performance in electrical con-
ductivity was obtained with compositions having a
sheet resistivity of about 100 Ω/ □ (Fig. 4). The com-
posites containing more than 50% of carbon black
cracked upon drying and could not be used in these
measurements.
There are three important areas for current transport
in the composite film. First, charge transfer from the
metal substrate to the carbon particles at the metal-
composite interface. Secondly, electron transport
through the composite film via resonance tunneling
between the carbon particles in the film, and finally
electron transfer or emission into the electrolyte solu-
tion at the composite film-electrolyte interface. From
Fig. 4, in terms of sheet resistance, the overall perfor-
mance of carbon black 1 composite film seems to be
the best, compared to the other three composite films.
3.3 Characterization of the composite electrodes
The composite electrodes were characterized by
different techniques such as scanning electron
microscopy (SEM, Zeiss Supra 40) for surface topol-
ogy and composition imaging, 2D stylus profilome-
try (Bruker Dektak XTL) and 3D optical profiling
(Filmetrics Profilm 3D/ Model: 205-0792) for ana-
lyzing the surface measurements or textures i.e. sur-
face roughness, height variations. In Fig. 5, the
surface characterization results of carbon black 1
composite electrode are presented, where the com-
posite ink contains 70% nylon 6,6 and 30% of carbon
black 1 particles.
Fig. 4. Sheet resistances of composite films on top of a
polyester film, measured by using a 4-point probe.
Fig. 5. Surface structure analysis of nylon 6,6 composite electrodes by SEM (Zeiss Supra 40) at different magnifications
and locations.
Nur-E-Habiba et al. / J. Electrochem. Sci. Technol., Epub ahead of print 7
The 2D step height of the nylon electrodes from 6
measurements is on an average 23.66 ± 1.20 μm.
As shown in Fig. 5 and Fig. 6, the surface of the
electrode in different locations and at different mag-
nifications seems to be quite homogeneous, with very
small variation.
Based on the characterization analysis of the nylon
composite electrodes, it can be said that despite hav-
ing trivial uneven area at micro-level, the electrode is
pretty good for the HECL experimentation as proven
in Fig. 3 and Fig. 4, and also in the following sec-
tions.
3.4 Blank emission from composite electrodes and
the effect of peroxydisulfate as a co- reactant
When hot electron injection into aqueous electro-
lyte solution is used to excite luminescent labels in
bioaffinity assays, it is often beneficial to add peroxy-
disulfate (S2O82−) as a co-reactant [1-5,23]. Peroxydi-
sulfate reacts near diffusion-controlled rate,
producing highly oxidizing sulfate radical in reaction
with the hydrated electrons [25]:
e aq− + S2O82− →SO
42− + SO4−
k = 1.0 × 1010 mol-1 dm3 s-1
The reduction potential of sulfate radical has been
reported to be as high as 3.4 V vs. standard hydrogen
electrode (SHE) [25]. Thus, when both hydrated elec-
trons and sulfate radicals are present, highly reduc-
ing and oxidizing conditions are simultaneously
created.
The blank emission in the buffer solution was mea-
sured using composite electrodes with maximum
emission with Tb (III)-L from Fig. 3. Ten replicates
were measured for each of the compositions without
any wavelength discrimination and by using 545-nm
interference filter (Fig. 7). The blank emission is
based on either high field solid state electrochemilu-
minescence in the insulating nylon 6,6 layer or elec-
trogenerated chemiluminescence at the solid-
electrolyte interface. The carbon black 2 as conduct-
ing dopant particles produced less blank emission
than carbon black 1, when no wavelength discrimina-
tion was utilized. However, in the case the composite
electrode containing 30 weight % carbon black 1 and
70% nylon 6,6 the blank emission seemed to differ
from presently interesting 545 nm more than in case
carbon black 2 that allowed the most efficient HECL
generation from our Tb (III) chelate beacon. For the
sake of comparison, oxide-coated aluminium
(0.3 mm thickness; 30 mm width) electrodes were
also used in the measurements. Fig. 8 displays that
oxide-covered 99.9% aluminium (Merck) shows
much stronger background emission during cathodic
pulses than any of the presently fabricated composite
ink coated electrodes. Thus, if the present electrodes
would be used for detection of short-lived emission-
displaying organic luminophores the present alumin-
ium brand electrodes would have much poorer per-
formance than composites containing optimal
amount of carbon black particles.
Fig. 6. 3D surface profile of nylon composite electrode and
its topographical analysis, generated on Filmetrics Profilm
3D.
Fig. 7. Blank emission from different electrodes in time-
resolved measurements, with and without interference filter.
Confidence bars calculated with 95% confidence level (n=10).
Solution: 0.2 M borate buffer, 0.1 M Na SO , 0.001 M
KS O . Interference filter specification: 545±40 nm.
Excitation parameters: Pulse voltage, -41.8 V, pulse charge
31.5
μ
C, pulse frequency 50 Hz, 2000 excitation cycles.
8Nur-E-Habiba et al. / J. Electrochem. Sci. Technol., Epub ahead of print
3.5 The durability of the electrodes
3.5.1 Stability of composite electrodes during
pulse polarization experiments
The TR-HECL intensity was followed for 10000
excitation cycles with all the composite electrode
types and during this time no destruction of the elec-
trodes were observed and the TR- HECL remained at
practically constant level during this time Fig. 9 dis-
plays TR-HECL as a function of only the first 4000
ordinal number of excitation pulses for visualization
purpose.
From Fig. 9, it can be seen that the performance of
the carbon black 1 composite electrode in terms of
HECL intensity (photon counts) is the best during
cathodic pulse polarization experiments. The durabil-
ity for graphite 1 and graphite 2 composite electrodes
are quite similar. On the other hand, the carbon black
2 composite electrode started to crack.
3.5.2 The effect of pH on TR-HECL in
composite electrodes
The pH of buffer solutions was adjusted either with
sulfuric acid or sodium hydroxide and the HECL
intensity was measured as a function of pH of the
electrolyte solution. All our multidentate aromatic
Tb(III) chelates are decomposed at low pH due to the
protonation of the chelating side arms (oxygen and
nitrogen in the side arms). Thus, TR-HECL at low
pH is due to two reasons. First, decomposition of the
chelate, and secondly, conversion of hydrated elec-
trons to atomic hydrogen which is not sufficiently
good one-electron reductant for the excitation reac-
tions [4].
On the other hand, at high pH the formation of
hydroxo complexes start to decompose the chelate
[4]. When the same electrodes used at certain pH
were measured again in Tb (III)-L solution at pH 9.2
(borate buffer), each electrode exhibited the same
TR-HECL intensity within the reproducibility mar-
gins as those electrodes not exposed to the more
acidic/alkaline conditions. This indicates that the
electrodes tolerated the use of both very low and very
high pH solutions without destroying the composite
ink coating (Fig. 10).
In addition, the effect of pH was also studied by
using fluorescein and Ru(bpy)3
2+ luminophores as
depicted in Fig. 11 and Fig. 12 respectively. In princi-
ple, both red-ox and red-ox excitation routes are pos-
sible for fluorescein on thermodynamic grounds, but
we have earlier found evidence that the ox-red route
is again the predominating excitation pathway for
this luminophore [19]. The results with fluorescein
clearly showed that the present composite electrodes
work well at highly basic solutions with lumino-
phores able to emit HECL at highly alkaline condi-
tions.
In the case of Ru(bpy)32+, the measurement with
Fig. 8. Blank emission in the absence of luminophores
during and after the cathodic pulse with individual
disposable electrodes. Solution: 0.2 M borate buffer, 0.1 M
Na2SO4. Interference filter: 545 ± 40 nm. Excitation
parameters: pulse charge 31.5
μ
C, pulse voltage -41.8 V,
frequency 50 Hz, 2000 pulses.
Fig. 9. Photon count over the ordinal number of excitation
pulse was measured, using a 545±40 nm interference filter.
Solution: 10 M Tb (III)-L, 10 M K2S2O in 0.2 M
borate buffer at pH 9.2. Excitation parameters: pulse
charge 31.5
μ
C, pulse voltage -41.8 V, frequency 50 Hz,
4000 pulses.
Nur-E-Habiba et al. / J. Electrochem. Sci. Technol., Epub ahead of print 9
the same electrode was again repeated at pH 9.2
(borate buffer) after measurement at certain specific
pH (Fig. 12). Ru(bpy)3
2+ is excited almost solely by
ox-red mechanism since its one-electron reduced
form is very unstable in aqueous solution and is dis-
integrated. On the other hand, one-electron oxidized
form of the chelate has a long lifetime in aqueous
solution and therefore gives a good steppingstone for
excitation reaction by hot/hydrated electrons [23].
Thus, all the molecular probes/beacons presently
applied in the absence of peroxydisulfate ions are
first one-electron oxidized by hydroxyl radicals gen-
erated from dissolved molecular oxygen followed by
the excitation step by hot/hydrated electron [5].
The metal-to-ligand excited triplet state is always
the emitting species in case of Ru(bpy)3
2+, regardless
whether the excited state first formed in electron
transfer excitation step is metal to ligand excited sin-
glet state (1MLCT*) or metal to ligand charge trans-
fer excited triplet state (3MLCT*). If the 1MLCT* is
initially formed intersystem crossing occurs and
3MLCT* is obtained which finally emits light
[23,39]. Unfortunately, the luminescence lifetime of
3MLCT* emission is too short to be utilized for TR-
HECL measurements due to the time constants of the
present instrumentation and the present electrolytic
cells.
The Ru(bpy)3
2+ measurements also showed that the
present composite electrodes did tolerate the use both
in highly acidic, or basic conditions. However, the
repeated measurements in normal buffer solution (pH
9.2) following the pH treatment/exposure showed
that the composite electrodes were still usable after
the use at extreme conditions.
Fig. 10. The effect of pH on the TR-HECL of Tb (III)-L.
Solution: 10 M Tb (III)-L, 10 M K S O in 0.1 M Na SO .
Interference filter: 545±40 nm. Excitation parameters: pulse
charge 31.5
μ
C, voltage -41.8 V, frequency 50 Hz, 2000 pulses.
Fig. 11. The effect of pH on the HECL of fluorescein.
Solution: 10 M fluorescein and 10 M K SO in 0.1 M
Na SO . Interference filter: 550±40 nm. Excitation parameters:
pulse charge 31.5
μ
C, pulse voltage -35 V, frequency 50
Hz, 2000 excitation cycles.
Fig. 12. Effect of pH on the HECL intensity of Ru(bpy)
chelate (black squares). The intensity presented with red
circles was obtained when the electrode originally measured in
certain pH was measured again in pH 9.2 borate buffer solution.
Solution: 10 M Ru(bpy) and 10 M K SO in 0.1 M
Na SO . Interference filter: 600±40 nm. Excitation parameters:
pulse charge 31.5
μ
C, pulse voltage -35 V, frequency 50 Hz,
1000 excitation cycles.
10 Nur-E-Habiba et al. / J. Electrochem. Sci. Technol., Epub ahead of print
3.6 Analytical applicability of the nylon 6,6 com-
posite electrodes
The composite film containing 30% of carbon
black 2 exhibited the highest TR-HECL intensity
(Fig. 3). Thus, the analytical applicability of nylon
6,6, composite electrodes doped by carbon black 2
particles was studied by measuring TR-HECL cali-
bration curves of Tb (III)-L, both in the absence and
presence of peroxydisulfate ions. Linear calibration
plots spanning over several orders of magnitude of
Tb (III)-L concentration were obtained (Fig. 13).
Thus, the electrodes can be used e.g. in bioaffinity
assays utilizing Tb (III) chelate labels.
3.7 Electron transfer through the composite film
We assume that current transport through the com-
posite layers has three steps: (1) current injection
from metal to composite layer, (2) current transport
in the composite layer by resonance tunneling and (3)
current injection to the electrolyte solution (Fig. 14).
The electron injection to the composite film can be
based on the direct contact of carbon particles to the
base metal, but part of the current is in all likelihood
produced by tunneling mechanism variants to the
particles inside of the polymer matrix but in close
proximity of the metal interface.
Inside the composite film the electrons are trans-
ported via carbon particles mainly by resonance tun-
neling [40,41] and field-assisted direct tunneling [42]
and, finally, electrons are injected into the aqueous
electrolyte solution by field-assisted direct tunneling
or field emission [43] from the carbon particles
located at the surface of composite films. This step is
still under investigation in our lab, but for many prac-
tical purposes it is not important whether the hot elec-
tron injection is based on the direct field-assisted
tunnel emission or field emission from the composite
electrode.
Usable cell for HECL measurements can be achieved
simply by dispensing a suitable volume of studied
solution inside the hydrophobic ring (Fig. 14) and
putting e.g. an ITO-glass anode or alternatively a
plastic sheet coated with carbon nanotubes as an
anode on top of the hydrophobic ring acting as a
spacer that defines the volume of the cell with its
thickness together with the circle area inside the
hydrophobic ring. In this way the light emission can
be measured through optically transparent anodes.
4. Conclusions
The novel field of hot electron electrochemistry is
presently largely unexplored. The most significant
differences with traditional aqueous electrochemis-
try at active electrodes are: (1) the stability limits of
water can be exceeded, (2) one electron reductions
can be made in situations where traditional electro-
chemistry leads to concerted two-electron transfers,
Fig. 13. Calibration curves of Tb (III)-L in the presence of 1
mM K S O (black squares) and in the absence of
peroxodisulfate ions. Electrodes: 30% carbon black 2; 95%
confidence intervals. Solution: 0.2 M borate buffer at pH 9.2.
Interference filter: 545±40 nm. Excitation parameters: Pulse
voltage -41.8 V, pulse charge 31.5 µC, pulse frequency 50 Hz,
2000 excitation cycles.
Fig. 14. Electron transport through the composite film. (1)
Current injection from metal to composite layer, (2)
Current transport in the composite layer and (3) Current
injection to the electrolyte solution. Pulse voltage is
typically -20 - 50 V.
Nur-E-Habiba et al. / J. Electrochem. Sci. Technol., Epub ahead of print 11
and (3) the reductions can be made of the distance at
least several tens of nanometers away from the elec-
trode thus allowing e.g. efficient excitation of labels
in immunometric immunoassays. The present com-
posite material withstanding a very wide pH range
and many types of solvents allows the production of
robust composite electrodes for injection of hot elec-
trons (i) into aqueous solutions, and (ii) also into
many other solvents or solvent mixtures due to the
good chemical durability of nylon 6,6. Hot- and
hydrated/solvated electrons can be easily obtained
with these electrodes in any laboratory to induce var-
ious one-electron reductions in aqueous solutions not
obtainable at active metal electrodes on the basis of tra-
ditional electrochemistry. For instance, metal ions at
unusual oxidation states not obtainable at active metal
electrodes can be created at the present composite elec-
trodes as in the case in pulse radiolysis [24], many types
of luminophores can be excited by reaction cycles
involving one-electron redox steps [1- 5]. Different
types of organic pollutants can be disintegrated by hot
and hydrated electrons [5,32,44-46]; also some specific
organic synthesis can be carried out with hydrated elec-
trons [47]. Moreover, bacteria can be exterminated by
introducing hot and hydrated electrons in an aqueous
media needing disinfection [27,29,48]. All in all, several
promising application fields seem to exist where the
present electrodes could be utilized.
Acknowledgements
This work was financially supported by Aalto Uni-
versity and the Academy of Finland (grant #311415).
N.E.H. would like to acknowledge Panu Rautiainen
and Félix Sari Doré for contributing in some of the
measurements.
Conflicts of Interest
The authors declare that there is no conflict of interest.
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