Optical and electrochemical behavior of new nano-sized complexes based on gold-colloid and Co-porphyrin derivative in the presence of H2O2

Article abstract of DOI:10.1016/j.jallcom.2016.06.246

A wide band absorption hybrid material, prepared from a metalloporphyrin and gold nanoparticles (nAu) with the purpose to develop optical and electrochemical detection of hydrogen peroxide was investigated. Gold nanoparticles smaller than 20?nm were obtained and functionalized with Co(II) 5,10,15,20-meso-tetra(3-hydroxyphenyl)porphyrin (Co-3OHPP). The dependence between the intensity of absorption of the gold plasmonic band and the increasing concentration of Co-3OHPP proved that nAu is sensitive to porphyrin detection, showing potential in early medical diagnosis. The nAu-porphyrin complex was exposed to increased amounts of H₂O₂to prove its sensing capacity and the changes of the absorption spectra were monitored by UV–vis spectroscopy. The AFM and FT-IR studies showed significant structural and morphological modifications of the hybrid material after H₂O₂exposure. In order to verify if the range of physiologically-relevant H₂O₂concentration levels (5–35?μM) might be detected, three modified glassy carbon (GC) electrodes were realized, by deposition onto their surfaces of nAu and Co-porphyrin (alone and in successive layers) and were tested to evidence the electrochemical response for H₂O₂. The comparative linear and cyclic voltammetry of the bare and modified GC electrodes evidenced an increased electrocatalytic effect on the reduction of H₂O₂onto GC electrode modified with nAu/Co-3OHPP ordered layers.

Full text of DOI:10.1016/j.jallcom.2016.06.246

Accepted Manuscript
Optical and electrochemical behavior of new nano-sized complexes based on gold-
colloid and Co-porphyrin derivative in the presence of H O
2 2
Eugenia Fagadar-Cosma, Iuliana Sebarchievici, Anca Lascu, Ionela Creanga, Anca
Palade, Mihaela Birdeanu, Bogdan Taranu, Gheorghe Fagadar-Cosma
PII:
S0925-8388(16)31969-7
10.1016/j.jallcom.2016.06.246
JALCOM 38115
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 7 April 2016
Revised Date: 26 May 2016
Accepted Date: 24 June 2016
Please cite this article as: E. Fagadar-Cosma, I. Sebarchievici, A. Lascu, I. Creanga, A. Palade, M.
Birdeanu, B. Taranu, G. Fagadar-Cosma, Optical and electrochemical behavior of new nano-sized
complexes based on gold-colloid and Co-porphyrin derivative in the presence of H O , Journal of Alloys
2 2
and Compounds (2016), doi: 10.1016/j.jallcom.2016.06.246.
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ACCEPTED MANUSCRIPT

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OPTICAL AND ELECTROCHEMICAL BEHAVIOR OF NEW NANO-SIZED
COMPLEXES BASED ON GOLD-COLLOID AND Co-PORPHYRIN DERIVATIVE IN
THE PRESENCE OF H2O2
Eugenia FAGADAR-COSMAa*, Iuliana SEBARCHIEVICIb, Anca LASCUa, Ionela
CREANGAa, Anca PALADEa, Mihaela BIRDEANUa,b, Bogdan TARANUb,
Gheorghe FAGADAR-COSMAc
a Institute of Chemistry Timisoara of Romanian Academy, M. Viteazul Ave. 24, 300223-
Timisoara, Romania, efagadar@yahoo.com, Tel: +40256/491818; Fax: +40256/491824
b National Institute for Research and Development in Electrochemistry and Condensed Matter, 1
Plautius Andronescu Street, 300569 Timisoara, Romania
c Politehnica University of Timisoara, Vasile Parvan Street 6, 300223-Timisoara, Romania,
Abstract
A
wide band absorption hybrid material, prepared from a metalloporphyrin and gold
nanoparticles (nAu) with the purpose to develop optical and electrochemical detection of
hydrogen peroxide was investigated.
Gold nanoparticles smaller than 20 nm were obtained and functionalized with Co(II)
5,10,15,20-meso-tetra(3-hydroxyphenyl)porphyrin (Co-3OHPP). The dependence between the
intensity of absorption of the gold plasmonic band and the increasing concentration of Co-
3OHPP proved that nAu is sensitive to porphyrin detection, showing potential in early medical
diagnosis.
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The nAu-porphyrin complex was exposed to increased amounts of H2O2 to prove its
sensing capacity and the changes of the absorption spectra were monitored by UV-vis
spectroscopy. The AFM and FT-IR studies showed significant structural and morphological
modifications of the hybrid material after H2O2 exposure.
In order to verify if the range of physiologically-relevant H2O2 concentration levels (5–35
µM) might be detected, three modified glassy carbon (GC) electrodes were realized, by
deposition onto their surfaces of nAu and Co-porphyrin (alone and in successive layers) and
were tested to evidence the electrochemical response for H2O2. The comparative linear and
cyclic voltammetry of the bare and modified GC electrodes evidenced an increased
electrocatalytic effect on the reduction of H2O2 onto GC electrode modified with nAu/ Co-
3OHPP ordered layers.
Keywords: Co-porphyrin, gold colloid, UV-vis spectroscopy, modified GC-electrodes,
voltammetry, electrocatalysis.
1. Introduction
The capacity of metalloporphyrins to produce, detect, transport and even store gases and
to undergo facile reduction and oxidation is known since the discovery of hemoglobin and
chlorophyll [1]. Obtaining of porphyrin based hybrid systems that are capable to preserve the
porphyrin amazing optical and catalytic properties [2] by incorporation into a matrix, organic or
inorganic, in order to develop new multifunctional materials, represents the actual approach in
research. Among the metalloporphyrins, Co(II)-porphyrins are active in catalytic aziridination of
styrene [3] and possess electrocatalytic properties for transformation of H2O into H2 and O2,
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respectively [4]. Besides, cobalt-porphyrin based organic matrices have been reported as new
kind of electrocatalysts for oxygen reduction reaction [5]. Moreover, the mechanism for
molecular oxygen reduction, in basic environment, was established to undergo via a two-electron
process to H2O2 or by a direct four-electron reaction. The most important finding is that the
cobalt catalytic centre in the porphyrin moiety preserves its electrocatalytic property even after
the self-assembling process took place [6].
Another complex between Co(II) porphyrin and lipoic acid, deposited on gold electrodes,
exhibited catalytic activity towards the reduction of molecular oxygen, this time in acidic
solution [7]. The same catalytic performance for oxygen reduction in high acidic media, at room
temperature, was obtained by grafting a Co(II) porphyrin onto multiwalled carbon nanotubes [8]
and onto the network of three-dimensionally distributed gold (polyoxometallate-modified)
nanoparticles producing cobalt porphyrin-hybrid multi-layered film with high activity toward
electroreduction of hydrogen peroxide [9]. Electroactive hybrid films of gold colloid
nanoparticles and cobalt porphyrins fabricated with layer-by-layer assembly method showed
catalytic activity toward the oxygen reduction. The catalytic activity decreases when bilayer
number increases due to aggregation of the [AuNP/cobalt porphyrin] hybrid [10].
Spintronic molecular device applications were started after the demonstration that spin
states of Co–porphyrin on Au(III) can be reversibly switched over by binding and unbinding of
the nitrogen oxide (NO) molecule. After NO exposure, a molecular complex NO–Co–porphyrin
was generated [11]. The Kondo effect explained this switching phenomenon by scattering of
conduction electrons in cobalt due to magnetic impurities and as a consequence changing of
electrical resistivity function of temperature.
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Metal nanosized particles are of constant interest due to their controllable surface-to-volume
ratio and their hybrids with porphyrins have properties that can be adjusted depending on the
obtaining method [12]. For example, 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt-platinum-
functionalized reduced graphene oxide was used for sensitive electrochemical detection of
Aflatoxin [13].
Porphyrins modified gold nanoparticles of 5-20 nm have low cytotoxicity, fair
fluorescence emission and reliable cell uptake ability [14]. Gold nanoparticles of this size are
ideal carriers to deliver the porphyrin molecules into cells, thus providing an improved agent in
PDT and molecular imaging. Gold nanoparticles of 1–2 nm in diameter could be more toxic due
to the possibility of irreversible binding to the biopolymers in cells [15]. Functionalization of
gold nanoparticles with organic moieties containing nitrogen represents a source of successful
materials for sensor formulations. In this respect, N-methylimidazole-functionalized gold
nanoparticles were reported to recognize bis- and tris-Zn-porphyrins [16].
Encouraged by our previous results in H2O2 and CO2 detection using porphyrins in
combination with various inorganic/polymeric matrices [17, 18], the next step of this research
was to create a new sensitive material, that can optically or electrochemically detect minute
quantities of H2O2 (in the range of 5–35 µM, representing relevant levels in human physiology)
and which might easily be incorporated in a detection device. Gold nanoparticles smaller than
20 nm in which total extinction is nearly all contributed by absorption [19] have been
functionalized with Co(II) 5,10,15,20-meso-tetra(3-hydroxyphenyl) porphyrin (Fig. 1) in order to
achieve a better nanomaterial for H2O2 monitoring.
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HO
OH
N
N
N
N
Co
HO
OH
Fig. 1. Structure of Co(II) 5,10,15,20-meso-tetra(3-hydroxyphenyl) porphyrin
(Co-3OHPP)
In order to extend the possibility of application of the nAu/ Co-3OHPP complex, three
modified glassy carbon (GC) electrodes were realized by deposition onto their surfaces of nAu
and Co-3OHPP alone and of ordered layers (nAu- Co-3OHPP) and were comparatively tested
by linear and cyclic voltammetry to evidence the best electrochemical response for H2O2.
2. Experimental
2.1. Apparatus
A JASCO UV-visible spectrometer, model V-650, was used for recording UV-visible spectra
in various solvents (THF, THF-aqueous system) using quartz cuvettes of 1 cm length. A
Nanosurf® Easy Scan 2 Advanced Research AFM was used for AFM imaging, carried out in
ambient conditions. The samples were obtained by drop casting on silica plates. FT-IR spectra
were carried out in ATR mode, in the 4000-400 cm-1 range on a JASCO 430 apparatus. TEM and
STEM analysis of the samples was performed using a Titan G2 80-200 TEM/STEM microscope
(FEI Company, the Netherlands) operating at 80kV (image from Fig. 3) 200 kV (image from Fig.
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6). Samples for TEM analysis were prepared on carbon-coated copper grids by placing 2 µL of
diluted (1:3 v/v) solution of gold colloid. The water was evaporated under vacuum. In order to
study the interaction between Co-3OHPP and nAu, the porphyrin was dissolved in THF and the
resulting solution (0.15 mM) was drop-casted together with the nAu solution (diluted 1:4 with
double-distilled water) on 200 mesh copper grids covered with carbon film.
Electrochemical measurements were performed on a VoltaLab PGZ 301 potentiostat
(Radiometer Analytical). A conventional three-electrode system was used for linear/cyclic
voltammetry studies. A platinum wire was used as auxiliary electrode, saturated calomel
electrode (SCE) as reference electrode and glassy carbon (GC, 0.07 cm2) modified with the
synthesized Co-3OHPP and gold nanoparticles (nAu) as working electrodes. All potentials were
measured with respect to the SCE electrode. The electrocatalysis of hydrogen peroxide reduction
was carried out in 0.05 M phosphate buffer solution (PBS)/0.05M KCl, pH 5.8, at a scan rate of
50 mVs−1. All the experiments were carried out at room temperature and the cell was open to the
atmosphere. The parameters for the square wave voltammetry (SWV) were: step potential 50
mV; amplitude 5 mV and duration 0.1 sec, scan rate 50 mV/s.
2.2. Materials
All reagents were acquired of highest purity from Merck and Fluka and used without further
purification. Doubly distilled water was used for the solutions. Buffer solutions (0.05M KCl,
pH=5.8) were prepared from NaH2PO4, Na2HPO4 and H3PO4. The porphyrin base, 5,10,15,20-
meso-tetra(3-hydroxyphenyl) porphyrin, was obtained by Adler method, according to [20].
2.2.1. Synthesis of Co(II) 5,10,15,20-meso-tetra(3-hydroxyphenyl) porphyrin (Co-3OHPP). The
synthesis was done by adapting the previous reported method [21]. A solution of 0.2 g (0.29×10-
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3
mole) 5,10,15,20-meso-tetra(3-hydroxyphenyl) porphyrin in 60 mL THF is brought to reflux
under intense stirring in a round bottom three necked flask equipped with magnetic stirrer and
refrigerator. Another solution comprised of 0.72 g (3.02×10-3 mole) CoCl2×6H2O dissolved in 40
mL CH3OH is heated and then dropwise added to the porphyrin solution (final molar ratio
porphyrin: metal salt = 1:10). The reaction is monitored by registering the UV-vis spectra every
10 minutes (Fig. 2). The refluxing and stirring of the mixture are maintained for 45 minutes. The
solvent mixture is evaporated then the dry product is repeatedly washed with distilled water until
the water is clear and all the excess cobalt salt is removed. The product is then dried and
recrystallized from CHCl3.
2.2.2. Synthesis of gold nanoparticles. The modified method of synthesis belongs to sustainable
chemistry [22]. An amount of 0.035 g HAuCl4×3H2O (0.088×10-3 mole) is solved in 116.2 mL
doubly distilled water and brought to reflux in a 150 mL three necked flask equipped with
magnetic stirrer and refrigerator. Then 12.25 mL (1 wt %) solution of trisodium citrate (0.122 g,
0.41×10-3 mole) in doubly distilled water is added at once and the mixture is vigorously stirred
and refluxed for 15 minutes. The molar ratio gold salt:sodium citrate = 1:5. The solution turnes
from yellow to black and then to dark red.
2.2.3. The complex Co-3OHPP-nAu formation. The experiment was conducted as follows: gold
colloid solution, 3 mL (0.47×10-3 M) was treated with portions of 50 µL Co-3OHPP solution in
THF (10-5 M). The mixture was stirred and then the UV-vis spectrum was recorded for each step.
2.2.4. Preparation of nAu/Co-3OHPP composite modified electrodes. Prior to each experiment
the glassy carbon (GC) surface was mechanically polished to a mirror using fine grade emery
papers (5 and 0.3 ꢀm), then washed ultrasonically in bi-distilled water and acetone for three
minutes respectively. After that, the electrode was electrochemically cleaned in sulfuric acid
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solution (0.5 M) and the surface was again washed with bi-distilled water and ethanol and
allowed to dry.
The nAu film was formed by drop-casting (5 ꢀL) of the colloidal solution (0.68 mM),
onto the well cleaned GC surface and left to dry, followed by rinsing with water. The Co-
porphyrin film was formed by drop-casting (3 ꢀL) from 1.1 mM methanol solution onto the GC
surface, or GC/nAu surface and left to dry. Then the surface was rinsed with methanol and water
and left to dry in the dark.
3. Results and discussion
The continuous monitoring of the UV-vis changes during the obtaining of Co-3OHPP by
the metalation reaction put into evidence a significant red shifting of the Soret band with 9 nm in
comparison to the porphyrin base (Fig. 2, line 1), accompanied by the decrease in intensity of
absorption. Regarding the Q bands, a change in shape can be observed along with a lesser
resolution and their number is reduced (Fig. 2, lines 2-6). After purification, the Co-3OHPP
metalloporphyrin absorption bands are large and increased in intensity (Fig. 2, line 7) in
comparison with the porphyrin base, at the same concentration. The Soret band is broadened and
red shifted from 418 nm in bare porphyrin to 427 nm in Co-3OHPP metalloporphyrin and
accompanied by a single well defined Q band located at 543 nm.
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Fig. 2. UV-vis monitoring of Co-3OHPP synthesis: porphyrin base (line 1) and reaction mixture
after: 5 minutes reflux (line 2): 15 minutes reflux (line 3): 25 minutes reflux (line 4); 35 minutes
reflux (line 5); 45 minutes reflux (line 6) and the Co-3OHPP recrystallized product (line 7)
The presence of only one Q band in the metalloporphyrin absorption spectrum confirms
the increase in symmetry due to the metal insertion into the porphyrin core.
The UV-vis spectrum of the gold colloid displays a plasmonic band located at 522 nm
and spherical/ovoid shaped nonaggregated particles having diameters in the range 15-20 nm
(Fig. 3).
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Fig. 3. UV-vis spectrum and TEM image of the gold colloid solution (c=0.47×10-3 mole/L)
The obtaining of Co-3OHPP-nAu hybrid was realized by adding of small amounts of
Co-3OHPP compound into the gold colloid solution and was continuously monitored by UV-vis
spectroscopy (Fig. 4). An isosbestic point located at 484 nm proved the complex formation
between the Co-porphyrin and the colloidal gold. The complex generation and some expected
aggregation phenomena are sustained by the significant broadening and shifting of the gold
plasmonic band from 522 nm up to 600 nm. All these changes occur with noticeable increases in
absorption intensity. These types of nanomaterials, with wide band absorption covering all
visible spectrum, are considered to have great potential as multifunctional hybrid materials with
applications as photosensitizers both in PDT and in formulation of photovoltaic cells. Another
isosbestic point (located at 610 nm) helps us to determine the stability of the Co-3OHPP-nAu
complex that suffers consecutive changes after a certain concentration in porphyrin is reached.
This second isosbestic point is associated with the decrease of the plasmon absorption capacity
so that we can conclude that the first complex is stable in the range of 1-19 µM porphyrin
concentration.
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Fig. 4. UV-vis spectra recorded by adding Co-3OHPP to the gold colloid solution. The
dependence between the concentration of Co-3OHPP and the intensity of the plasmon
absorption.
With increasing the concentration of Co-porphyrin, the Soret band of the Co-3OHPP-
nAu hybrid suffers a hyperchromic effect. Besides, the absorption band of the gold plasmon
increases in intensity in an almost perfect linear fashion (Fig. 4) with increasing the amounts of
Co(II)-porphyrin, meaning that gold colloid is suitable as optical sensor for small amounts of
porphyrin detection. This result might find valuable medical applications due to the importance
in detecting porphyrin derivatives and metabolites present in the human body in early diagnosis
of different diseases [23].
The generation of the complex between the Co-porphyrin and the colloidal gold is also
accompanied by changes in color. An unexpected violet-blue color of the complex was obtained
from the starting materials that are colored in red and respectively yellow (Fig. 5) and that was
proven by TEM images to be given by newly formed complex aggregates (Fig. 6).
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Fig. 5. Color change of solutions during the formation of the Co-3OHPP-nAu complex. Red
gold colloid (1); yellow Co-3OHPP (2); violet-blue gold-porphyrin complex (3).
The dendritic structure observed in Fig. 6a is comprised mainly of Co-3OHPP H- and J-
type self-aggregates due to their interaction with gold nanoparticles. The disk shaped structures
from Fig. 6c can be attributed to the metalloporphyrin associated by sandwich type process, that
was previously reported in our papers [24, 25].
On the other hand, the gold nanoparticles are associated in chains of 3-4 round particles
and in higher disordered associations as can be seen in Figs. 6b and 6d.
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Fig. 6. Images obtained in STEM mode during the study of the interaction between Co-3OHPP
and nAu deposited on TEM grids.
In order to test the capacity to detect H2O2, a mixture composed of 3 mL gold colloid
solution (0.47×10-3 M) and 1 mL Co-3OHPP (10-5 M) in THF solution (the mixture that
provided Co-3OHPP-nAu complex with the highest intensity absorption in UV-vis analysis -
see Fig. 4) was used. To this mixture portions of 50 ꢀL H2O2 solution (30 wt %) were
sucessively added, stirred and UV-vis monitored (Fig. 7).
The absorption intensity of the gold-porphyrin complex is gradually decreasing with the
increasing of H2O2 concentration. There is a linear dependence between the concentration of
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hydrogen peroxide and the intensity of absorption of the plasmonic complex, characterized by
an excellent correlation coefficient of 0.992 (Fig. 7).
Fig. 7. UV-vis spectra after succesive H2O2 adding. Dependence between the concentration of
hydrogen peroxide and the intensity of absorption of the plasmon.
AFM analysis showed that the behavior of the cobalt porphyrin is in accordance with our
previous studies [17, 26] regarding the morphology and aggregation architectures of porphyrin
derivatives from different solvents. The nanosized particles of Co-3OHPP are evenly oriented,
have the shape of uniform triangular lamels and are displayed at different depths (Fig. 8A). The
gold colloid as deposited on silica plate is associated in larger spherical aggregates (Fig. 8B). In
case of the hybrid complex, the triangular shaped, well oriented porphyrin particles are
embedded in the gold colloid matrix and the particle H-aggregation process into sandwich type
geometry is more visible than in the case of cobalt porphyrin alone (Fig. 8C). After H2O2 adding
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the whole material is completely restructured and the J- and H-aggregation processes are
amplified creating a more uniform and compacted surface (Fig. 8D).
Fig. 8. AFM images in noncontact mode for Co-porphyrin (A), gold colloid (B), gold-porphyrin
hybrid complex (C) and after H2O2 treatment of the complex (D).
The changes in structure of the investigated materials were comparatively studied also by
FT-IR spectroscopy. In the FT-IR spectrum of bare hybrid gold colloid-porphyrin and after
H2O2 exposure (Fig. 9, lines 2, 3) the large bands around 3350 cm-1 and novel high intensity
bands from 1644 cm-1 can be attributed to the stretching mode and bending mode of H2O
molecules, respectively [27]. A number of low intensity IR bands are displayed both in bare Co
II-porphyrin and in the hybrids of gold colloid-porphyrin in the region 900- 1050 cm-1, due to in
plane ring vibrations of porphyrin. The most important band, from around 1650 cm-1, that is
differently shifted in FT-IR spectra to lower or higher wavenumbers, could be attributed to out-
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of-plane breathing of pyrrole rings, due to changes in the cobalt coordination after exposure to
H2O2 (Figure 9, line 2).
Fig. 9. Overlapped FT-IR spectra: Co-3OHPP (1); gold-porphyrin complex treated with H2O2
(2); gold-porphyrin complex (3).
Electrocatalysis of H2O2 at nAu/Co-3OHPP composite film. From the representations of
linear voltammograms for all three modified GC electrodes in the presence of H2O2 (Fig. 10) it
can be observed that the process of reduction begins around: -100 mV for GC/nAu/Co-
3OHPP, -200 mV for GC/nAu and -300 mV for GC/Co-3OHPP respectivelly. Besides, the
reduction currents for all three modified electrodes are higher than that of the GC electrode, that
confirms the electrocatalytic character of the modified electrodes. The increase of the intensity of
catodic currents and the shift of the potential for the H2O2 reduction current peak towards more
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posive potentials might be caused by a sinergic effect produced on one hand by nAu, that might
increase the electrode active surface area and can favour a fast direct electron transfer from the
layered Co-porphyrin and on the other hand, because of the favouring electron transfer effect of
the Co-porphyrin activated electroactive centers (enhanced due to electron-donating properties of
the hydroxyl groups). This type of behaviour was previously reported by other researchers
regarding other types of substituted porphyrins or phtalocianines, such as: hemoglobin [28, 29],
Co-diverse substituted porphyrins [30], cobalt tetracarboxylic acid phthalocyanine [31].
Fig. 10. Linear voltammograms showing the electrocatalytic reduction of hydrogen
peroxide with bare GC; GC modified with Co-3OHPP, nAu and nAu/Co-3OHPP, in
phosphate buffer electrolyte solution (pH 5.8) with 1.16 µM H2O2.
The reaction mechanism for the reduction of H2O2 to H2O through the active centers of
the Au nanoparticles and the electrocatalytic effect of the metalloporphyrin involving the
CoII/CoI system is presented according with eqs. (1)-(8) and discussed by similitude with
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reported electrocatalytic systems [31-33]. The mechanism presumes the adsorption of H2O2 on
the electrode surface followed by a two electron transfer in the presence of H+ ions and the
formation of the final product, H2O. These assumptions are also sustained by our experimental
results. On the GC/nAu electrode (Fig 11) the supposed reaction steps are presented by the
equations (1)- (4). The porphyrin layer from the electrode acts as an electron transfer mediator
involving the redox couple Co(II)/Co(I) for the cathodic branch and the Co(II)/Co(III) system for
the anodic reaction (Fig. 12), eqs. (5)-(8).
Au0 + H2O2 (bulk) → AuH2O2(ads)
AuH2O2(ads) + e → AuOH(ads) + OH─
AuOH(ads) + e → Au0 + OH─
(1)
(2)
(3)
(4)
(5)
(6)
(7)
2OH─ + 2H+ → 2H2O
Co(II)-3OHPP + e + H+ → Co(I)-3OHPP(red)
2Co(I)-3OHPP(red) + H2O2 → 2H2O + 2Co(II)-3OHPP
Co(II)-3OHPP → Co(III)-3OHPP(ox) + e
2Co(III)-3OHPP(ox) + H2O2 → O2 + 2H+ + 2Co(II)-3OHPP (8)
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Fig. 11. Cyclic voltammograms of GC modified with nAu in 0.05M PBS solution and 0.58 µM
H2O2.
Fig. 12. Cyclic voltammograms of GC modified with Co-3OHPP in 0.05M PBS solution and
1.16 µM H2O2.
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The influence of the H2O2 concentration on the cathodic currents for the GC modified
with nAu/Co-3OHPP is presented in Fig. 13, where the square-wave voltammetric response is
given. The increase of the intensity of cathodic current can be noticed as the concentration of
H2O2 becomes higher (from 17.6 µM up to 1.7 mM), these results offering the premises for
further investigations of the possibility to use this electrode as sensor for hydrogen peroxide. The
linear dependence between the ꢁic and the H2O2 concentration is characterized by an excellent
correlation coefficient.
Fig. 13. The square-wave voltammetric response (SWV) of GC modified with nAu/Co-3OHPP
films in the absence or presence of different concentrations of H2O2: 0.017÷1.76 mM, in 0.05M
PBS/0.05M KCl solution. Dependence between cathodic current (ꢁic) and H2O2 concentration
(at E= -750 mV).
Conclusions
Gold nanoparticles smaller than 20 nm have been functionalized with Co(II) 5,10,15,20-
meso-tetra(3-hydroxyphenyl) porphyrin (Co-3OHPP). The porphyrin-gold hybrid complex
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showed wide band absorption in the UV-vis spectrum. The gold colloid proved its sensitivity to
small amounts of porphyrin detection showing an excellent linear dependence between the
intensity of absorption of the gold plasmonic band and the increasing concentration in Co-
3OHPP. This result might find valuable medical applications because abnormal levels of
porphyrin derivatives (porphobilinogen, aminolevulinic acid, uroporphyrin, coproporphyrin,
protoporphyrin) present in human body are indicative for many metabolic diseases (diabetes,
porphyrya, kidney diseases).
The nAu-porphyrin complex was exposed to increased amounts of H2O2 and the changes
of the absorption spectra were monitored by UV-vis spectroscopy, proving its sensing capacity.
The absorption intensity of the gold-porphyrin complex is gradually decreasing with the
increasing of H2O2 concentration. The process is reproducible and there is a linear dependence
between the concentration of hydrogen peroxide and the intensity of absorption of the plasmonic
complex. The same experiment performed on bare gold colloidal solution led to chaotic response
of the plasmon intensity function of H2O2 concentration, proving that the presence of
metalloporphyrin is indispensable for detection.
The FT-IR and AFM studies showed significant structural and morphological
modifications of the hybrid material after H2O2 exposure. The AFM investigations show the
morphological changes that take place for each procedure. Thus, in case of the complex
formation between Co-3OHPP and gold nanoparticles, porphyrin particles are embedded in the
gold colloid matrix and the particle H-aggregation process is more visible than in the case of
cobalt porphyrin alone. STEM investigation of the complex between Co-3OHPP and gold
nanoparticles revealed some dendritic structures comprised mainly of Co-3OHPP H- and J-type
self-aggregates due to their interaction with gold nanoparticles.
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After H2O2 adding the whole material is completely restructured and the J- and H-
aggregation processes are amplified creating a more uniform and compacted surface. The FT-IR
structure investigation shows that the Co coordination sphere changes after the exposure of the
hybrid material to H2O2.
In order to enlarge the possibility of electroanalytical application of the nAu/ Co-3OHPP
complex, modified glassy carbon electrodes were realized by deposition of nAu and Co-3OHPP
alone and of ordered layers nAu-Co-3OHPP and were comparatively tested by linear and cyclic
voltammetry to evidence their response to H2O2. The electrocatalytic effect of the nAu/ Co-
3OHPP hybrid system for the electrochemical reduction of hydrogen peroxide in the range of
physiologically-relevant levels creates the opportunity to use modified GC electrodes with
ordered layers for H2O2 analyses.
The further incorporation of nAu/Co-3OHPP hybrid material into miniaturized
electrochemical detecting devices is our future goal. Other promising applications of this
material could be in medicine, where the transport of porphyrins into cells by means of nontoxic
gold nanoparticles is of great interest for imaging [34] and PDT of cancer diseases [35] and in
formulation of photovoltaic cells [36].
Acknowledgements: The authors from Institute of Chemistry Timisoara of Romanian
Academy are kindly acknowledging the support from Program 3-Porphyrins/2015 and STAR
Programme- SAFEAIR Project 76/2013. This work is dedicated to Anniversary of 150 years
since the establishment of Romanian Academy.
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Highlights
► Wide UV-vis absorption band gold-metalloporphyrin hybrid was obtained.
► Gold nanoparticles are sensitive to trace amounts of Co-porphyrin.
► Gold-Co-porphyrin complex is sensitive to H2O2 detection by UV-vis spectroscopy.
► Co-porphyrin/gold nanoparticle-modified glassy carbon electrodes were tested.
► Modified electrodes show electrocatalytic effect on the reduction of H2O2.