Comparative electrochemical investigations on series of SH-terminated-functional porphyrins

Article abstract of DOI:10.1016/j.electacta.2012.01.049

The electrochemical behaviors of self-assembled substituted porphyrins (SH-terminated, abbreviated as H₂TPPO(CH₂)_nSH, n = 3, 12) on a gold electrode were investigated using the steady-state scanning electrochemical microscopy (SECM). The different electron-transfer (ET) kinetics, including the bimolecular ET between the porphyrin self-assembled monolayers (SAMs) and the redox mediator [K₃Fe(CN)₆], the tunneling ET between the underlying gold electrode and [K₃Fe(CN) ₆], and pinholes or defects, were clearly distinguishable. The SECM strategy was developed to deal with the two types of porphyrin SAMs. First, a model using alkanethiols [(CH₂)_nSH, n = 3, 12] as the functional template was proposed to change the conformation of porphyrin SAMs in a unit area of the electrode. Second, the porphyrin SAMs were directly prepared by inserting a metal (cobalt) into the center of the porphyrin ring. The results show the distinct effect of the presence of alkanethiols on the kinetics of the different-chain length porphyrins. In addition, the rate constants of the bimolecular ET significantly increased after the insertion of cobalt. The results are in agreement with the density functional theory (DFT).

Full text of DOI:10.1016/j.electacta.2012.01.049

Electrochimica Acta 65 (2012) 244–250
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Comparative electrochemical investigations on series of
SH-terminated-functional porphyrins
Wenting Wanga,b, Yaqi Hub, Chunming Wanga,∗, Xiaoquan Lub,∗∗
a College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
b Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province, College of Chemistry & Chemical Engineering, Northwest Normal University,
Lanzhou 730070, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The electrochemical behaviors of self-assembled substituted porphyrins (SH-terminated, abbreviated as
H2TPPO(CH2)nSH, n = 3, 12) on a gold electrode were investigated using the steady-state scanning electro-
chemical microscopy (SECM). The different electron-transfer (ET) kinetics, including the bimolecular ET
between the porphyrin self-assembled monolayers (SAMs) and the redox mediator [K3Fe(CN)6], the tun-
neling ET between the underlying gold electrode and [K3Fe(CN)6], and pinholes or defects, were clearly
distinguishable. The SECM strategy was developed to deal with the two types of porphyrin SAMs. First,
a model using alkanethiols [(CH2)nSH, n = 3, 12] as the functional template was proposed to change the
conformation of porphyrin SAMs in a unit area of the electrode. Second, the porphyrin SAMs were directly
prepared by inserting a metal (cobalt) into the center of the porphyrin ring. The results show the distinct
effect of the presence of alkanethiols on the kinetics of the different-chain length porphyrins. In addition,
the rate constants of the bimolecular ET significantly increased after the insertion of cobalt. The results
are in agreement with the density functional theory (DFT).
Received 30 November 2011
Received in revised form 12 January 2012
Accepted 13 January 2012
Available online 21 January 2012
Keywords:
SH-terminated porphyrins
Alkanethiols
Electron transfer (ET) kinetics
Scanning electrochemical microscopy
(SECM)
Density functional theory (DFT)
© 2012 Elsevier Ltd. All rights reserved.
1. Introduction
long-range electronic properties were also studied through the use
of two kinds of alkanethiol monolayers containing amide bonds
The electron-transfer (ET) process occurs in a wide variety
of chemical and biological systems, and ET kinetics has been
extensively studied in analytical electrochemistry. Porphyrin com-
pounds, which are important biochemical molecules, were used in
numerous applied studies, including those of catalysts, selective
anion sensing and photocurrent catchers [1–4]. Several methods
of the immobilization with porphyrins, such as the formation
of self-assembled monolayers (SAMs) [5,6], covalent appended
monolayers [7,8], and polymerization monolayers with the forma-
tion of an electroactive film [9,10] are available. Of these, SAMs
can spontaneously form highly ordered monolayers and are exten-
sively used [11]. Meanwhile, alkanethiols are widely used in the
fabrication of SAMs, and result in improved stability and sensitivity
of electrochemical biosensors [12]. The electrochemical proper-
ties of surfaces modified by alkanethiols were investigated, and
the functionality of the surface was controlled through the gen-
eration of alkanethiol groups in SAMs [13,14]. The structure and
[15,16]. Given the advantages of alkanethiols in SAMs, their use
as the functional template on a gold electrode was proposed.
Porphyrins and metalloporphyrin derivatives such as snowflake-
shaped porphyrin dendrites, porphyrins with axial carbazole-based
dendritic substituents, and long-lived porphyrins can therefore
be successfully constructed [17–19]. Co porphyrins, which are
metalloporphyrin derivatives, were widely used as oxygen car-
riers to investigate the facilitated transport of oxygen [20]. The
photo-electrochemical properties of electrodes modified using an
ordered supramolecular assembly of porphyrins and the effect of
the porphyrin substituent groups were investigated [21,22]. The
steady-state scanning electrochemical microscopy (SECM), which
provides a rapid identification of new dyes as well as the electro-
catalytic activity of the metalloporphyrin-modified substrates with
regard to the oxygen reduction reaction [23,24], has been exten-
sively described [25–28]. Given the many advantages of SECM in
observing the electrochemical reactions at the interfaces, it was
widely used to mimic and study the chemical nature of interfaces
[29–31]. The structure of porphyrin SAMs on the substrate affected
the electron transfer process, and the alkyl chain length also influ-
enced the ET kinetics of porphyrins via SECM [32,33]. Furthermore,
the tilt-angles of porphyrins with different alkyl chains on the elec-
trode were investigated [34]. The results showed that porphyrins
∗
Corresponding author. Tel.: +86 931 8912582; fax: +86 931 8912582.
Corresponding author. Tel.: +86 931 7971276; fax: +86 931 7971276.
E-mail addresses: wangcm@lzu.cn (C. Wang), luxq@nwnu.edu.cn,
∗∗
taaluxq@gmail.com (X. Lu).
0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2012.01.049

W. Wang et al. / Electrochimica Acta 65 (2012) 244–250
245
Scheme 1. The scheme of sulfhydryl porphyrins compounds (H2TPPO(CH2)nSH and Co H2TPPO(CH2)nSH).
with longer alkyl chains (n = 12) can be almost perpendicular to
the surface of the electrode, however, porphyrins with short alkyl
chains (n = 3) are evenly distributed on the surface of the electrode.
Based on the previous work, two types of systems were designed
to investigate the factors affecting the ET kinetics of porphyrins
on the electrode. First, alkanethiols [(CH2)nSH, n = 3, 12] as a func-
tional template was proposed to change the porphyrin SAMs in a
unit area on the electrode. Second, porphyrin SAMs without alka-
nethiols were metalized with Co ions in the center of the porphyrin
ring. Using the SECM technique, the three ET pathways, which were
bimolecular ET between porphyrin SAMs and the redox mediator
[ferricyanide, K3Fe(CN)6], the tunneling ET between the underly-
ing gold electrode and [K3Fe(CN)6] and the pinholes or defects were
clearly distinguishable. The corresponding ET rate constants were
then obtained. In addition, the density functional theory (DFT) was
applied to support the experimental results.
careful rinsing with deionized (DI) water, sonication in absolute
ethanol and DI water for 10 min in turn, and then dried with nitro-
gen gas. SAMs was prepared with a solution of 1 mM alkanethiols
in ethanol, and then put in 1 mM sulfhydryl porphyrins com-
pounds in chloroform for 24 h. SAMs with sulfhydryl Co-porphyrins
compounds were prepared with a solution of 1 mM sulfhydryl por-
phyrins compounds in chloroform for 24 h and then metalized
with cobalt ion. During the self-assembled process, the electrode
was disposed in solution of chloroform or ethanol and DI water in
turn with ultrasonic instrument every 3 h to remove the physical
adsorption.
3. Results and discussion
In the current paper, the electron-transfer (ET) kinetics of the
porphyrin self-assembled monolayers (SAMs) using the steady-
state SECM in unbiased conditions was based on the heterogeneous
rate constant (keff). SECM is an electrochemical approach to scan-
ning probe microscopy with a good resolution. In steady-state
SECM, an electrochemical redox cycle between the substrate (elec-
trode) and the 25-␮m diameter ultramicroeletrode (UME) tip
occurs. The tip currents (iT), are all divided by the steady-state cur-
rent (iT,∞). iT/iT,∞ is called normalized current (Inorm), and iT,∞ is
expressed in the following equation:
2. Experimental
2.1. Chemicals and reagents
Sulfhydryl
porphyrins
compounds
(abbreviated
as
H2TPPO(CH2)nSH, n = 3, 12) were synthesized by the previous
reported methodology [35] and characterized by UV–Vis, IR and
1H NMR (see Supporting Information), and Co-porphyrins was
prepared using the methods reported by Nishimura et al. (the
structure is shown in Scheme 1) [36]. Alkanethiols ((CH2)nSH,
n = 3, 12) were used as received from Alfa Aesar (China(Tianjin)
Co., Ltd). All other reagents were analytical-reagent grade, unless
otherwise specified. Solutions were prepared from water that had
been purified through an Ultra-pure water system Milli-Q Plus
(Millipore).
i = 4nFDca
(1)
where n is the number of electrons transferred per molecule, F is the
Faraday constant, D is the diffusion coefficient of the electroactive
molecule, c is the bulk concentration of the electroactive molecule,
and a is the radius of the tip [37]. The tip current (iT) is expressed
as follows:
ꢀ
ꢁ
ITins
ITk − ISk 1 −
+ ITins
(2)
(3)
2.2. Apparatus
ITc
Electrochemical experiments were carried out using a CHI 900
scanning electrochemical microscope (CH Instruments Co. Ltd.,
Austin, USA) with a four-electrode cell. The gold (or modified gold)
electrode, a platinum wire, and a KCl saturated Ag/AgCl electrode
were used as working, counter and reference electrode, respec-
tively. The SECM tip was a 25-␮m diameter Pt ultramicroelectrode
(UME). Before each experiment, the tip was polished with 0.3-␮m
alumina and rinsed with DI water, and the solutions were purged
with pure nitrogen for 15 min before each run.
0.78377
L(1 + 1/ꢀ)
0.68 + 0.3315 exp(−1.0672/L)
1 + (11/ꢀ + 7.3)/(110 − 40L)
ISk
=
+
where ITc and ITins are the tip currents of the conductive and insulat-
ing substrates, respectively. In the current study, the substrate with
the porphyrin compounds is a conductor, and the ITc is expressed in
the following equation:
ꢂ
ꢃ
0.78377
1.0672
ITc = 0.68 +
+ 0.3315 exp
−
(4)
L
L
where ꢀ = keffd/D (keff is equal to the apparent heterogeneous rate
constant (cm s−1) obtained using the experimental probe approach
curves that fit the theoretical data, and D is the diffusion coefficient).
L = d/a, where L is the distance between the normalized tip electrode
2.3. Preparation of substrate and SAMs
A gold electrode was used as the substrate. The electrode was
polished to a mirror finish using 0.05 ␮m alumina powder before

246
W. Wang et al. / Electrochimica Acta 65 (2012) 244–250
ogy [40,41]. The values of kox for the short-chain SAMs (n = 3) could
be calculated using Eq. (8), as follows:
1 + exp[−F(Es − Eaods)/RT]
ꢁ ∗
keff − kꢀ
co
kb
=
+
(8)
kox
where Eaods is the formal potential of the SAMs-bound redox species,
ꢁ * is the surface concentrations of oxidized and reduced forms of
redox centers in the monolayers. Additionally, the values of kox can
be calculated using the following equation for the long-chain SAMs
(n = 12):
keff = kox
ꢁ
∗ + kꢀ
(9)
where Es is much more negative than Eo . For kb, the value for the
ads
rate constant can be calculated using the following equation:
kbꢁ ∗
co
keff
=
+ kꢀ
(10)
where Es is relatively low and equal to Eo
.
Scheme 2. The mode on SECM system of porphyrins with the templates of alka-
nethiols on the gold electrode.
ads
3.1. Electrochemical characterization of the formation of the
self-assembled monolayers (SAMs)
and the substrate, d is the distance between the tip electrode and
the substrate and a is the radius of the tip electrode. ITk, ITc and ITins are
normalized by the tip current at an infinite tip–substrate distance
[38,39].
In Scheme 2, K3Fe(CN)6 was chosen as the reversible redox cou-
ple, the substrate was the porphyrin SAMs, and alkanethiols were
used as the template on the gold electrode in the SECM electro-
chemical cell.
Electrochemical measurements can provide valuable insights
into the structure of a film on a substrate surface [5,42]. Cyclic
voltammetry (CV) was used to determine the surface coverage
(ꢁ ) of porphyrin self-assembled monolayers (SAMs) through redox
stripping at the negative potential (−0.9 V) according to the follow-
ing equation:
By controlling the substrate potential (Es), redox reactions occur
at the interfaces with porphyrins (TPP) as follows:
Q
ꢁ =
nFA
Fe(CN)64− − e → Fe(CN)63−(tipoftheferricyanide)
(5)
(6)
where Q is the peak area in coulombs, A is the electrode surface area,
F is the Faraday constant and n is the number of electrons involved
in the electrode interaction [43]. To further confirm the results, the
hindrance (ꢂ) of the electrode as a qualitative parameter of SAMs
density was obtained using the following equation:
[TPP]+ + Fe(CN)64− → [TPP] + Fe(CN)6
3−
Given that the reaction in Eq. (6) is not reversible and that in Eq. (5)
is quasi-reversible, keff is expressed as
ꢄ
ꢅ
ipf (TPP)
ipf (Au)
k
oxkbꢁ ∗
+ kꢀ
(7)
ꢂ = 1 −
keff
=
k
oxco + kb + kf
where co is the concentration of the redox mediator, kox is the
bimolecular oxidation rate constant, kb is the tunneling rate con-
stant, and kꢀ is the defects and pinholes rate constant. For SAMs with
different chain lengths, the ET between the substrate and the redox
species was measured using the newly developed SECM methodol-
where ipf (TPP) and ipf (Au) are the forward (reduction of ferricyanide)
peak currents at the modified and bare electrodes, respectively.
As representatives of the short-chain and long-chain SAMs, the
porphyrins [H2TPPO(CH2)nSH, n = 3, 12] and the corresponding
alkanethiols (as the functional template) were provided. Table 1
8
A
5
B
6
5
7
5
4
6
4
4
3
2
5
4
3
3
1
3
0.2
0.4
0.6
0.8
1.0
1
2
3
4
5
C ,10 M
10 1/C,mol cm
2
1
2
1
0.0
0.5
1.0
1.5
0.0
0.5
1.0
1.5
(L=d/a)
(L=d/a)
Fig. 1. (A) Probe approach curves (PACs) of H2TPPO(CH2)nSH (n = 3) with alkanethiols as the template (from bottom to top the concentration of K3Fe(CN)6 are 0.2, 0.4, 0.6, 0.8,
1.0 mM). Insert: Dependence of the effective heterogeneous rate constants in 0.1 M KCl solution. (B) PACs of H2TPPO(CH2)nSH (n = 12) with alkanethiol as the template (from
bottom to top the concentration of K3Fe(CN)6 are 0.2, 0.4, 0.6, 0.8, 1.0 mM). Insert: Dependence of the effective heterogeneous rate constants. The diamonds are experimental
PACs, lines are theoretical PACs.

W. Wang et al. / Electrochimica Acta 65 (2012) 244–250
247
NH
N
N
O(CH2)nSH
HN
S S
S
s
M
A
n
i
S
r
y
)
h
a
(
p
r
o
p
-
H
S
Au
(
b
)
S
A
S
M
H
s
-
a
l
k
SAMs
a
n
e
t
h
i
o
l
S S S S
S S S S S
porphyrin
Scheme 3. The scheme (a) porphyrins directly self-assembled on the substrate; (b) porphyrins self-assembled on the substrate of alkanethiols as the templates.
Table 1
ꢁ , ꢂ and molecular area value of the functional porphyrins (H2TPPO(CH2)nSH (n = 3,
12)) at alkanethiols ((CH2)nSH, n = 3, 12)) of the same chain length as the template
with different self-assembled time.
6
5
4
3
2
1
0.95V
0.9V
0.8V
0.7V
0.6V
0.5V
System
Parameter
Time (h)
2
4
6
8
ꢁ (pmol/cm2)
ꢂ (%)
Molecular area
(Å2 molecule−1
ꢁ (pmol/cm2)
ꢂ (%)
68
34.36
245
190
38.69
84.75
273
29.02
74.49
223
6.6
60.85
n = 3
)
)
262
51.9
63.4
452
25.44
36.75
274
15.37
60.63
64
1.81
259.56
n = 12
Molecular area
(Å2 molecule−1
0.0
0.5
1.0
1.5
L=d/a
shows the ꢁ , ꢂ and molecular area of the porphyrins with alka-
nethiols of the same alkyl chain length as the functional template
at different self-assembly times. The calculated ꢁ is consistent with
previous results [44,45]. When a porphyrin ring was considered a
Fig. 2. Probe approach curves of totally Co porphyrins (n = 12) at different substrate
potential in 0.1 M KCl solution. Etip = −0.1 V vs Ag/AgCl, ꢁ = 305 pmol/cm2.
cube and vertically arranged on the electrode, the molecular area
was 200 A and ꢁ was 330 pmol/cm2 [6,32]. Table 1 shows that
2
˚
of long-chain alkanethiol SAMs was easier than that of the short-
chain ones. Thus, the long-chain porphyrins obtained the maximum
ꢂ at the initial stage of the self-assembly adsorption (t = 2 h),
whereas the short-chain porphyrins obtained the maximum ꢂ at
the middle stage of the adsorption (t = 4 h). In addition, for the long-
chain porphyrin SAMs (n = 12), the minimum molecular area and
the molecular area and ꢁ were all lower than the aforementioned
values. Therefore, alkanethiols of the same chain length can play
supporting roles for the porphyrin ring and can be used as templates
for porphyrin SAMs. For the porphyrins with short-chain lengths
(n = 3), ꢂ decreased with increasing ꢁ . Meanwhile, the molecular
area also decreased with the self-assembly time of the alkanethiols
when the template was increased. However, for porphyrins with
long-chain lengths (n = 12), the change trend of ꢁ and the molec-
ular area caused by the decreasing hindrance ꢂ was not obvious.
Comparing the two cases, the self-assembly adsorption of the long-
chain alkanethiols was more difficult than that of the short-chain
ones at the initial stage of adsorption. By contrast, the formation
8
4
0.54
0.70
0.26
0
Table 2
ET rate constants of porphyrins (H2TPPO(CH2)nSH (n = 3, 12)) with alkanethiols
((CH2)nSH, n = 3, 12)) of the same chain length as the template.
-4
-8
0.14
0.48
System
Time (h)
kox (mol−1 cm3 s−1
)
kb (s−1
)
kꢀ (cm s−1
)
4.76 × 108
1.27 × 108
8.80 × 108
2.35 × 108
1.60 × 1010
3.72 × 1010
2.97 × 1010
2.41 × 1010
2.24 × 105
1.22 × 105
1.3 × 105
6.41 × 10−4
4.66 × 10−4
4.44 × 10−4
2.71 × 10−4
3.06 × 10−5
3.01 × 10−5
3.70 × 10−5
4.94 × 10−5
0.68
2
4
6
8
2
4
6
8
n = 3
0.7 × 105
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
0.72 × 103
0.73 × 103
1.06 × 103
0.47 × 103
E/V vs. Ag/AgCl
n = 12
Fig. 3. The CVs of Co porphyrins (n = 12) in 1 mM K3Fe(CN)6 and 0.1 M KCl solution
with scan rate of 100 mV/s.

248
W. Wang et al. / Electrochimica Acta 65 (2012) 244–250
Fig. 4. Molecular electron cloud density and orbital energy calculation: left, H2TPPO(CH2)nSH (n = 3), right, TPPO(CH2)nSH Co (n = 3).
maximum ꢁ were obtained at the middle stage of the self-assembly
adsorption (t = 4 h), whereas for the short-chain porphyrins, the
parameters varied with increasing self-assembly time. This phe-
nomenon further demonstrates the effect of the chain length on the
formation of SAMs at different stages of the self-assembly adsorp-
tion.
3.2. ET constants of porphyrins with alkanethiol templates
Sulfhydryl compounds were firmly bound to the gold electrode
through the Au S bond, and the high-order monolayer can sponta-
neously be formed on the electrode surface [46–50]. By considering
porphyrin rings as the terminal group in SAMs as well as the

W. Wang et al. / Electrochimica Acta 65 (2012) 244–250
249
-1.4000
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
-2.0972
-2.2444
-2.2920
LUMO+1
LUMO
HOMO
HOMO-1
-4.8793
-4.9988
-5.2127
-5.3038
SH-CoTPP
SH-TPP
Fig. 4. (Continued).
presence of pinholes and defects on the surface of the electrode, the
corresponding alkanethiol compounds with the same alkyl chain
lengths were used as the templates on a substrate. In Scheme 3, two
cases are presented: (a) porphyrins directly self-assembling on the
substrate and (b) porphyrins self-assembling after the alkanethiol
template has self-assembled on the substrate. The formation of por-
phyrin SAMs was disordered without the templates, but became
highly order when alkanethiols were used as the templates. The
values of the ET rate constant for the porphyrins with alkanethi-
ols whose chain lengths are similar to those of the templates were
calculated (Fig. 1). Table 2 shows the different ET rate constants
obtained for the two cases. For the long-chain porphyrins (n = 12),
kox reached a maximum value when the self-assembly time of the
alkanethiols was short (t = 4 h). However, the kox for the short-
chain porphyrins (n = 3) obtained its maximum value when the
self-assembly time of the alkanethiols was increased (t = 6 h). This
phenomenon strongly demonstrates the effect of the chain length
on the structure of alkanethiols as the templates, and further affects
the electron transport of porphyrin SAMs in the bimolecular reac-
tion. The change trend may be attributed to the following reasons:
(a) in the initial phase of the self-assembly adsorption, the short-
chain alkanethiols can be adsorbed on the gold electrode better
than the long-chain alkanethiols. The temporary disorders in the
formation of the short-chain alkanethiols inadequately support the
porphyrin ring at the short self-assembly adsorption time; (b) com-
pared with the short-chain alkanethiols, the spacers containing
long-chain alkanethiols exhibited better order on the gold elec-
trode in the initial phase of the self-assembly adsorption. Under
the same conditions, porphyrin compounds can insert the alka-
nethiol templates better. When the assembly time of alkanethiols is
short, porphyrins with long-chain alkanethiols can absorbed at the
absorptive site via self-assembly much easier than the short-chain
ones; (c) the morphologies of the porphyrin rings with different
alkyl chain lengths are different. That is, the orientation is parallel
(n = 3) or perpendicular (n = 12) to the gold electrode, depending on
the surface coverage (ꢁ ) and the molecular areas [32,44,45]. In the
presence of alkanethiols as the template, the arrangement of the
porphyrin ring in SAMs can be perpendicular (n = 12) to the gold
electrode, resulting in the accumulation of more porphyrin com-
pounds on the electrode. In summary, as the template on the gold
electrode, the alkanethiols significantly affected the ET process of
the porphyrin compounds.
(magnesium porphyrins) can undergo reversible redox interactions
on the porphyrin rings or the central metal ion in natural pro-
cesses [51]. The Co porphyrins have been chosen in the current
study on a gold electrode because Co metal ion has the highest
percentage of coordination under the same experimental condi-
tions [52]. Fig. 2 shows the feedback curves of the metal (Co)
porphyrins at the gold electrode with the substrate potential rang-
ing from 0.5 V to 0.9 V. The experimental curves closely follow the
theoretical ones, with a positive feedback at the substrate poten-
tial below 750 mV. These findings indicate that SAMs with metals
exhibit no apparent blocking with the redox mediator, when the
metal (Co) is inserted into the center of the porphyrin ring. The
CVs of the metal porphyrins in 1 mM K3Fe(CN)6 and 0.1 M KCl
solution are shown in Fig. 3. The three pairs of quasi-reversible
peaks of Co porphyrins appeared, in with the first correspond-
ing to the porphyrin ring, the second to the Co(II)–Co(III) bond
and the third to Fe(CN)63−/Fe(CN)64−. The corresponding formal
potentials were 700, 540 and 260 mV, respectively. Compared
with the formal potential (700 mV) of porphyrins, the peak of the
porphyrin rings in the metalloporphyrin shows a negative shift.
Furthermore, the molecular electron cloud density and orbital
energy of the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital LUMO were obtained using
the DFT calculation shown in Fig. 4 . The energy change in the
HOMO and LUMO can cause the formal potential of porphyrin
rings to shift negatively. Table 3 shows the ET rate constant of
the Co porphyrin compounds with different alkyl chain lengths.
Compared with those of the porphyrin compounds [32], the ET
rate constants of the Co porphyrins clearly increased. The results
are attributed to the following: (a) the changes in the orbital
energy of HOMO and LUMO show that the insertion of the Co
ion enhanced the electrochemical activity of the porphyrin rings
to some extent; (b) the porphyrin ring underwent a small distor-
tion from the coordination with the Co ion in its center, and the
arrangement of porphyrins in SAMs became as loose as possible
to facilitate the ET; (c) a comparison between the porphyrins and
metalloporphyrins shows that an ET channel was added to pro-
vide for the insertion of the metal and dominate the ET pathway of
metalloporphyrins [53].
Table 3
3.3. ET constants of porphyrins with Co metal inserted in the
center of the porphyrin rings
ET rate constants of Co-porphyrins with different alkyl chain lengths.
System
kox (mol−1 cm3 s−1
)
kb (s−1
)
kꢀ (cm s−1
)
n = 3
n = 12
6.37 × 109
1.65 × 1011
1.07 × 106
4.77 × 104
7.81 × 10−3
9.65 × 10−4
Metalloporphyrin coordination compounds, such as vitamin B12
(Co porphyrins), haemachrome (Fe porphyrins), and chlorophyll

250
W. Wang et al. / Electrochimica Acta 65 (2012) 244–250
4. Conclusions
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Kinetic investigations on the two kinds of functional porphyrin
SAMs on a gold electrode using SECM were completed. In both
cases, the porphyrins were the focus of the study. Alkanethiols were
prepared and used as the template on the gold electrode. The opti-
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Acknowledgments
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This work was supported by the Natural Science Foundation
of China (Nos. 20775060, 20875077 and 20927004), the Natural
Science Foundation of Gansu (No. 0701RJZA109) and Key Projects
of Scientific Research Base of Department of Education, Gansu
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