Toward the Control of the Creation of Mixed Monolayers on Glassy Carbon Surfaces by Amine Oxidation

Article abstract of DOI:10.1002/chem.201503120

A versatile and simple methodology for the creation of mixed monolayers on glassy carbon (GC) surfaces was developed, using an osmium-bipyridyl complex and anthraquinone as model redox probes. The work consisted in the electrochemical grafting on GC of a mixture of mono-protected diamine linkers in varying ratios which, after attachment to the surface, allowed orthogonal deprotection. After optimisation of the deprotection conditions, it was possible to remove one of the protecting groups selectively, couple a suitable osmium complex and cap the residual free amines. The removal of the second protecting group allowed the coupling of anthraquinone. The characterisation of the resulting surfaces by cyclic voltammetry showed the variation of the surface coverage of the two redox centres in relation to the initial ratio of the linking amine in solution.

Full text of DOI:10.1002/chem.201503120

DOI: 10.1002/chem.201503120
Full Paper
&
Mixed Monolayers |Hot Paper|
Toward the Control of the Creation of Mixed Monolayers on
Glassy Carbon Surfaces by Amine Oxidation
[
a]
[b]
[c]
Jessica Groppi, Philip N. Bartlett,* and Jeremy D. Kilburn*
Abstract: A versatile and simple methodology for the crea-
tion of mixed monolayers on glassy carbon (GC) surfaces
was developed, using an osmium–bipyridyl complex and an-
thraquinone as model redox probes. The work consisted in
the electrochemical grafting on GC of a mixture of mono-
protected diamine linkers in varying ratios which, after at-
tachment to the surface, allowed orthogonal deprotection.
After optimisation of the deprotection conditions, it was
possible to remove one of the protecting groups selectively,
couple a suitable osmium complex and cap the residual free
amines. The removal of the second protecting group al-
lowed the coupling of anthraquinone. The characterisation
of the resulting surfaces by cyclic voltammetry showed the
variation of the surface coverage of the two redox centres in
relation to the initial ratio of the linking amine in solution.
Introduction
rently most of the studies focus on mixed self-assembled
monolayers (SAMs) on gold, followed in popularity by the elec-
trografting of aryl diazonium salts.
The covalent modification of electrode surfaces has been one
of the central aspects in the development of electrochemical
devices for the last forty years, since it represents a powerful
tool for improving the stability and lends new properties to
the modified surfaces. Extensive reviews are available, describ-
ing the methodologies for the creation of organic layers cova-
The production of mixed SAMs has been achieved through
several techniques: electrochemical oxidation of thiols fol-
[
6]
lowed by a replacement reaction or selective reduction of ad-
[
7]
[8]
sorbed thiols and electrochemical control, although binary
monolayers of thiols are commonly obtained by co-adsorp-
[1]
[9]
lently bonded to various electrode materials. Amongst these
techniques, the electrooxidation of primary diamines at glassy
carbon (GC) surfaces has received considerable attention in
tion. Usually, the purpose of mixed SAMs is the creation of
surfaces where a linker is surrounded by a diluting compound.
Gooding et al. published a series of papers describing mixed
SAMs in which ferrocene- or anthraquinone-derived norbornyl-
ogous bridges, along with alkyl chains of different length as
diluents, were used to study the behaviour of redox probes
[
2]
recent years and, applied in conjunction with solid-phase
synthesis techniques, allows the linking of different redox cen-
[3]
tres to the electrodes. A step forward in the modification of
electrodes is represented by the creation of mixed monolayers.
The development of methodologies for the creation of mixed
monolayers has been the focus of research in various fields,
[
10]
within the electrical double layer. Lee et al. recently reported
the modification of gold surfaces with mixed SAMs of 1-unde-
canethiol and 2-bromoisobutylate-terminated undecanethiol as
initiator platforms for the polymerisation of pOEGMA brushes
[
4]
from semiconductors to organic electronics and sensors. The
appeal of mixed monolayers resides in the possibility of assem-
bling multiple components in a bottom-up approach, with
[
11]
modified with biotin for the binding of proteins.
The use of diazonium salts extends the possibility of apply-
ing mixed monolayers to a wide range of materials, including
carbon. Gooding et al. first reported the electrografting of mix-
[
5]
control over their organisation to the molecular level. Cur-
[
12]
tures of diazonium salts on gold in 2005, and since then
they have studied the mechanism of grafting and the proper-
[
a] J. Groppi
School of Biological and Chemical Sciences
[
13]
Queen Mary University of London
Mile End Road, London, E1 4NS (UK)
ties of the mixed layers both on gold and glassy carbon.
They used this technique for the creation of surfaces modified
with oligo(phenylethynylene) molecular wires diluted with
poly(ethylene-glycol). The molecular wire provided a rigid an-
choring group for the covalent attachment of horseradish per-
[
b] Prof. P. N. Bartlett
Chemistry, Faculty of Natural and Environmental Sciences
University of Southampton
Southampton, SO17 1BJ (UK)
[
14]
E-mail: P.N.Bartlett@soton.ac.uk
oxidase, with good electron transfer kinetics. Further work
involved the use of mixtures of molecular wires alternated
with aryl carboxylic acid groups, the role of which was both to
anchor and stabilise a glucose oxidase enzyme and orient the
[
c] Prof. J. D. Kilburn
King’s College, The University of Aberdeen
Aberdeen, AB24 3FX (UK)
E-mail: j.kilburn@abdn.ac.uk
[
15]
active site toward the electrode surface. In both cases the
direct electron transfer between enzyme and electrode was in-
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/chem.201503120.
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[
20]
vestigated. Most recently the combination molecular wires/
poly(ethyleneglycol) was applied to the creation of an electro-
avoid leakage of the species. Recently the focus has shifted
to the development of methods for the covalent attachment
of the complexes through SAMs or reduction of diazonium
[16]
chemical immuno-biosensor.
In separate studies, Downard
[
21]
et al. reported the formation of mixed monolayers by sequen-
tial electrografting of diazonium salts whereby the use of
a bulky protecting group on the first modifier allowed enough
salts.
[17]
space for the second group to attach to the surface.
Results and Discussion
Our group reported a different approach to the creation of
mixed monolayers. A monolayer of N-Boc-ethylenediamine was
formed by oxidation of the free primary amine and removal of
the protecting group. The resulting surface was modified with
three different redox probes by sequentially dipping the elec-
trode in three different coupling solutions each containing
Synthesis and characterisation of bis-(2,2’-bipyridyl)[4-(pyri-
din-4-yl)butanoic acid](chlorido)osmium(II) hexafluophos-
phate (5)
The synthetic pathway for osmium complex 5 is presented in
Scheme 1. The precursor [Os(bpy) Cl ] (3) was synthesised fol-
2
2
[3b]
[19]
a single component. Alternatively, mixed monolayers of di-
amines could be obtained by simultaneous electrografting of
lowing a literature procedure. Pyridine ligand 1 functional-
ised with a butanoic acid moiety at C4 was synthesised by re-
acting 4-vinylpyridine with diethylmalonate, followed by hy-
[18]
two amines in solution in varying relative ratios.
[
22]
The use of mixed monolayers with two or more components
presents many advantages in the development of biosensors:
enzymes can be anchored to the surface and surrounded by
redox mediators to improve the mediated electron transfer,
and their orientation, density and distance to the surface can
be regulated, leading an optimal response from the system.
Moreover the approach could be used to create models that
mimic the environment of the active site of enzymes by sur-
rounding redox mediators with functionalities that could affect
their physical and electrochemical properties. However, in
order to create such carefully engineered surfaces it is crucial
that the mixed monolayers can be created both reliably and
predictably, and although it is possible to anticipate a number
of practical issues that might need to be overcome in order
that such reliability and predictability are achieved, to date no
studies have been undertaken to examine the practical difficul-
ties or to provide solutions for them. In this paper we describe
fundamental studies directed at just such an objective and
demonstrate that with careful experimental techniques pre-
dictability and reliability is achievable.
drolysis and decarboxylation under acidic conditions.
In
II
order to avoid chelation of the Os by the carboxylate group,
compound 1 was converted to the methyl ester derivative 2.
Complex 4 was obtained following an adapted literature pro-
[
19]
[23]
cedure. The hydrolysis of the ester gave complex 5, suit-
able for coupling to a layer of amines on the electrode surface.
Here we report the development of a methodology to
create mixed monolayers at glassy carbon surfaces in a reliable
and reproducible way, using the oxidation of primary amines
and solid-phase synthesis as the main tools. In the model stud-
ies reported here we used two different redox mediators, an
osmium–bipyridyl complex and an anthraquinone, to measure
the surface coverage electrochemically. These two redox
groups are of significantly different size, a common situation in
the design of mixed layers for particular applications, and a sit-
uation that creates its own problems as we shall see.
Osmium complexes have found wide application as redox
mediators in biosensing, given the easily tunable potential of
Scheme 1. Synthesis of bis-(2,2’-bipyridyl)[4-(pyridin-4-yl)butanoic acid]-
(chlorido)osmium(II) hexafluophosphate (5).
2
+/3+
the Os
couple by changing the nature of the ligands and
[
19]
their higher stability compared to other metal complexes.
Moreover the steric bulk of the metal complex appears to pre-
vent its adsorption on graphite surfaces, a process that is often
unavoidable with other redox mediators such as organic dyes
and quinones. Most of the literature concerning the use of
osmium complexes as redox mediators involves their embed-
ding in polymeric matrices or hydrogels, where one of the li-
gands is cross-linked in the polymeric backbone in order to
The osmium–bipyridyl complex 5 was fully characterised
and recrystallised by slow evaporation from a DCM/MeOH so-
lution. The structure of the complex was confirmed by X-ray
diffraction. The coordinates of the atoms, determined through
the crystal structure of complex 5, were transferred to the soft-
ware Gaussian9.0. Gaussian9.0 was used to analyse the crystal-
lographic coordinates, by means of the DFT hybrid method
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B3LYP with the standard 6-31G basis set for the organic com-
ponents of the complex, while the basis set given for osmium
was LanL2DZ, to create a computational model of the complex
Mixed monolayers: diamine linker/monoamine
To investigate the possibility of forming mixed layers at the
electrode surface by amine oxidation, a model experiment
(Scheme 2) was carried out using a mixture of a linker, mono-
N-Boc-1,6-hexanediamine (HDA-Boc), and 1-butylamine (1-BA).
[24]
that could be used to measure its dimensions. Thus consid-
ering the pyridyl complex as a sphere, it has a diameter of
11.5 Š, with a chain giving the overall length of the complex as
1
6.4 Š (Figure 1).
Mixtures of solutions of these two amines in CH CN (each
3
2
0 mm with 0.1m TBATFB as supporting electrolyte) were pre-
pared in different ratios starting from 100% HDA-Boc and
gradually increasing the fraction of 1-BA up to 99.9%. Given
the different electrooxidation potential of the components, the
grafting of the mixed layer on the GC electrodes was per-
formed by chronoamperometry, holding the potential of the
working electrode at 2.1 V to try to minimise the effects of the
different rates of formation of the radical cations of the two
amines. Removal of the Boc-protecting group under acidic
conditions yielded free amino groups available for coupling
with complex
Scheme 2.
5 under solid-phase synthesis conditions,
Figure 1. Computational model of complex 5 obtained by analysing the
crystallographic coordinates with Gaussian 9.0.
Scheme 2. Synthetic steps for the covalent modification of the electrode sur-
face with complex 5.
These measurements allowed us to calculate the theoretical
surface coverage of a monolayer of complex 5. Three assump-
tions were made: 1) the monolayer of linkers on the GC sur-
face is tightly packed, 2) the amines have no mobility and 3)
no repulsion occurs between positively charged osmium com-
plexes. In these conditions two limiting situations could occur
at the electrode surface: 1) the molecules are standing perpen-
dicular to the surface and can be approximated to circles with
Figure 2 presents the results obtained: the cyclic voltammo-
2
+/3+
grams (CV) show the redox peaks for the Os
covalently
bound to the surface at about 0.25 V versus SCE. The decrease
in surface coverage (G) of the osmium complex in relation to
the fraction of HDA-Boc in the starting solutions is represented
in the bar-plot. The coverages of osmium complex were deter-
mined by integration of the charge after background subtrac-
tion (see Supporting Information for details) and were correct-
ed by the corresponding controls obtained by dipping the
amine modified electrodes in a solution of complex 5 in DMF
with no coupling agents added.
radius r =5.75 Š; 2) since complex 5 is attached to the linker
1
through a four-carbon chain, the molecules could bend
toward the electrode surface and with free rotation could
sweep out an average area corresponding to a circle with
radius r =16.5 Š. In both situations the approximated circles
2
were assumed to organise according to the hexagonal close-
packing model, since it represents the densest form of organi-
It is clear from comparison of G values in Figure 2b that the
[25]
3
sation of molecules in a monolayer. According to this model
the area occupied by each molecule can be calculated using
Equation (1).
coverage of osmium complex varies only 3-fold for a 10 -fold
variation in concentration of the HDA-linker. Clearly the relative
coverages of the two amines on the surface do not follow the
relative concentrations of the two amines in solution. This is
presumably due to differences in the rates of attachment of
the two amines to the GC surface: attachment of HDA-Boc is
presumably favoured over 1-BA, hence the unexpectedly high
amount of osmium complex even at low percentages of HDA-
pffiffiffiffiffiffi
2
A ¼ 2 3r
ð1Þ
[
26]
Boc in solution. The surface coverages measured for com-
plex 5 fall within the range expected given the size of the
complex determined by theoretical calculation with a coverage
The surface coverage values for a monolayer of complex 5,
À2
calculated knowing the surface area of the GC electrode
of 16–18 pmolcm consistent with the second limiting situa-
2
(
0.071 cm ), resulted in
a
range between 134 and
tion described above in which once coupled to the surface the
complex hinders subsequent coupling to amines within its
radius of gyration. This also explains why a significant amount
À2
1
7 pmolcm , these theoretical values provide a reference to
explain the behaviour of complex 5 at the surface.
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were chosen: mono-N-Boc-ethylenediamine (EDA-Boc) and
mono-N-(6-aminohexyl)-2,2,2-trifluoroacetamide (HDA-tfa): the
Boc group can be easily removed in acidic conditions, while tfa
requires mild basic conditions. To test this approach initially,
grafting of the two linkers to form a monolayer was performed
by chronoamperometry from mixtures of the two components
(
each 20 mm) in CH CN in different ratios, starting from 100%
3
of HDA-tfa and increasing the fraction of EDA-Boc up to 99%.
After the removal of tfa, leaving the EDA-Boc unaltered, com-
plex 5 was coupled to the free amine groups on the HDA link-
ers CVs were then recorded and the corresponding surface
coverage for the osmium complex calculated. Next the the Boc
group was removed, and the free EDA then coupled to anthra-
quinone-2-carboxylic acid. CVs were recorded for anthraqui-
none and the surface coverages calculated, see Supporting In-
formation for details.
These experiments showed that, although the approach is
broadly successful with the coverage of osmium complex in-
creasing and the coverage of anthraquinone decreasing as the
ratio of HDA-tfa to EDA-Boc in the initial grafting solution in-
creases there was still a significant amount of anthraquinone
attached to the surface even when using 100% HDA-tfa. This
clearly illustrates the problem when trying to attach groups of
different steric bulk to the surface. Thus although, as shown in
the experiments described above the osmium complex blocks
attachment of further osmium complexes to the HDA amine it
does not prevent the smaller anthraquinone groups from at-
taching to the same HDA amines.
To overcome this problem we introduce a further step in
which acylation is used to cap any residual HDA amino groups
before the removal of the Boc group, Scheme 3. We also
found, during the course of these experiments that much
more reproducible results could be obtained using glassy
carbon electrodes with a meniscus contact to the solution,
which eliminates the insulating material around the GC rod,
rather than conventional GC electrodes potted in glass (see
Supporting Information). The meniscus contacting method to
the solution was therefore used in all of the following studies.
Figure 2. a) Comparison of the CVs recorded in 0.1m PBS solution pH 7
versus SCE, electrode area 0.071 cm2, at 50 mVs scan rate for different
À1
HDA-Boc linker solution ratios. b) Bar-plot for the variation of for complex 5
according to the ratio of HDA-Boc linker in solution. Coverages were calcu-
lated by averaging the values obtained for two replicates and subtracting
the control value obtained by dipping amine modified electrodes in
a 10 mm solution of complex 5 in DMF without coupling reagent.
Acetyl chloride (AcCl) and acetic anhydride (Ac O) were
2
of immobilised complex 5 is found even at the lowest (0.1%)
tested as possible capping reagents. Overall the acylation step
led to a significant reduction of anthraquinone at the surface
(Figure 3) compared to the previous experiments (Supporting
Information Figure S3), both on modified electrodes, where
the anthraquinone surface coverage decreased from
linker ratio in solution. The coverage for a monolayer of HDA,
À2 [3]
determined by XPS, is reported to be G=1 nmolcm
; this is
5
0 times bigger than the calculated surface coverage for
a monolayer of complex 5, and means that even at 10% HDA
on the surface, if uniformly distributed, a full monolayer of
complex 5 can still be achieved.
À2
À2
55 pmolcm to 5 pmolcm , and control electrodes, where
À2
the surface coverages went from an average of 30 pmolcm
À2
to 2 pmolcm . This could have meant that the non-covalent
attachment of anthraquinone at the surface depended not
only on adsorption processes but also on interactions with the
residual free amines, such as hydrogen bonding. Acyl chloride
gave the lowest values of surface coverage for anthraquinone,
so it was chosen as acylating agent.
Mixed monolayers: diamine linker/diamine linker
The modification of the GC surface with two redox compo-
nents, requires a step-wise approach: two linkers must be at-
tached to the surface and selectively reacted with the mole-
cules of interest. It is well known from peptide synthesis that
the use of orthogonal protecting groups allows the control of
the site of elongation of a peptidic chain, by controlling the
conditions, basic or acidic, of removal of the protecting
Once the optimisation of the synthetic steps was completed
(see Supporting Information), the whole procedure for the cre-
ation of the mixed monolayers was repeated (Scheme 4).
Figure 4 summarises the results obtained: the new condi-
tions gave a better control over the variation of surface cover-
[
27]
groups.
Considering this, two mono-protected diamines
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Scheme 3. Synthetic steps for the test of the acylating agents.
two factors: HDA-tfa oxidises more easily than EDA-Boc and,
although chronoamperometry was applied for the grafting of
the amine mixtures to overcome this problem by applying
a potential that should generate both amine radicals at the
same time, the formation of the HDA radical is probably fav-
oured kinetically, so its concentration at the surface will always
be higher than that in solution. Moreover the bulkiness of
complex 5 limits the range of G values achievable as explained
above. Anthraquinone shows a much more significant variation
consistent with our original expectations and suggesting that
the acylation step was successful and necessary. The methodol-
ogy developed proved to be reproducible and reliable, the
total surface coverage of the replicates for each amine mixture
tested remained constant within the experimental error, show-
ing that the oxidation of mixture of amines presenting orthog-
onal protecting groups is a good tool for the creation of
mixed monolayers.
Figure 3. Bar-plot for the variation of AQ according to the acylating agent,
calculated by averaging the values obtained for two replicates.
Conclusions
age of the redox species, and the non-covalent attachment of
material to the electrode surface was limited to a large extent.
The controls for complex 5 obtained by exposing the amine
modified electrodes to a neat solution of the complex in DMF,
presented no adsorbed material after 20 h washing in acetoni-
trile, while the anthraquinone controls required longer wash-
ing time in acetonitrile, 48 h, to achieve a stable signal for
which no desorption was detected, but still some material re-
We have described a methodology for the creation of mixed
monolayers on GC with a sequential approach, using the elec-
trochemical oxidation of amines and solid-phase synthesis
techniques. We have shown that it was possible to control the
relative amount of two redox probes at the surface by varying
the relative amount of the respective amine linkers in solution.
The optimisation of the method revealed that many factors
have to be considered in order to obtain reproducible and reli-
able results. The structure of the redox probes has a great
impact on the surface coverage values, since they can hinder
the attachment of similar molecules (osmium complex) or de-
À2
mained (typically 2–4 pmolcm ). The values of surface cover-
age reflected what was expected from previous considerations:
at lower concentrations of HDA linker attachment of complex
5
still presents a significant surface coverage. This was due to
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Scheme 4. Optimised synthetic steps for the difunctionalisation of the GC electrodes with complex 5 and anthraquinone.
face of the F-moc group, characterised by a flat aromatic struc-
ture. The deprotection conditions were optimised and through
the addition of an acylation step—residual free amines left
after the first coupling step was capped, a procedure routinely
applied in solid phase peptide synthesis. The methodology
here presented is a simple approach to the controlled creation
of mixed monolayers and could be applied to different carbon
materials to create much more complex systems for the devel-
opment of biosensors and biofuel cells.
Experimental Section
Materials, experimental procedures for solution and solid-phase
synthesis and characterisation of the compounds are reported in
the Supporting Information.
CCDC 1439213 (3) and 1439214 (5) contain the supplementary
crystallographic data. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre.
Figure 4. Bar-plot comparing the variation of of complex 5 and anthraqui-
none according to the ratio of HDA-tfa linker in solution corrected by the
corresponding controls.
termine non-covalent adsorption (anthraquinone). The setup
of the working electrode is an important factor to consider: Acknowledgements
since the material can adsorb on the sides of the GC rod
giving an overestimation of the surface coverage, using the
meniscus configuration allowed us to characterise only the de-
sired modified surface. The solid-phase synthesis approach ap-
plied had to be adapted to the system developed: instead of
the classical Boc/F-moc orthogonal protecting groups, the
combination Boc/tfa was adopted, in order to avoid interfer-
ences by the possible non covalent adsorption at the GC sur-
High resolution mass spectroscopy was carried out by the
EPSRC National Mass Spectrometry Service, University of Wales,
Swansea. We thank Majid Montevalli for assistance with the X-
ray crystal structures of complexes 3 and 5 and Dr. Devis Di
Tommaso for assistance with the computational model of com-
plex 5. P.N.B. acknowledges receipt of a Wolfson Research
Merit Award.
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