Selective electrochemical recognition of the α-naphthol isomer and in situ immobilization of naphthoquinones for tunable electrocatalysis

Full text of DOI:10.1002/asia.201201170

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DOI: 10.1002/asia.201201170
Selective Electrochemical Recognition of the a-Naphthol Isomer and In Situ
Immobilization of Naphthoquinones for Tunable Electrocatalysis
Puchakayala Swetha and Annamalai Senthil Kumar*[a]
Due to their small structural differences and similarity in
the physicochemical properties, differentiating structural iso-
mers or stereoisomers of simple organic molecules is a chal-
lenging and intriguing research interest for biotechnology,
chemical and pharmaceutical industry, and also for environ-
mental pollution monitoring. In fact, biological reactivities
of some of the isomeric compounds differ significantly. For
instance, a-naphthol (a-NAP) and b-naphthol (b-NAP) are
different structural isomeric derivatives of naphthalene poly-
aromtic hydrocarbon derivatives, known to have carcinogen-
ic activity,[1] and can permeate into skin and can cause differ-
ent pathological variations once they come in contact; a-
NAP decreases semen quality and reduces testosterone
levels in the male reproductive system,[2a] whereas b-NAP
can produce cataracts in the eyes and cancerous tumors
orally or intravenously (tested with rats and mice).[2b] In
2007, Wu et al. pointed out that these isomers have different
binding properties with protein (bovine serum albumin).[3]
a-NAP can deeply interact with hydrophobic sites, whereas
the b-NAP prefers to be in a basic environment on the pro-
tein.[3] Selective oxidation of the isomers is an important
route to decompose them and reduce the toxicity of the
molecules, and such methodology may also be used as a tool
for selective reorganization of the isomers in a mixture.
Several chemical oxidation methods using catalysts such
as iodosylarene-8G,[4a] hydrogen peroxide/methyltryoxorhe-
nium (CH3ReO3),[4b] copper(I) bisoxazoline,[4c] vanadium
substituted ordered mesoporous silicates (V-OMS),[4d] metal-
loporphyrin catalyst,[4e,f] platinum and palladium catalysts,[4g]
and aluminum hydroxide,[4h] as well as biochemical methods
utilizing enzymes such as phenol oxidase,[5a] pseudomonas
fluorescens TTC1 (NCIMB 40605),[5c] polyphenol oxidase,[5c]
lactase,[5d] peroxidase,[5e] and tyrosinase[5f] were reported in
the literature. Unfortunately, all the catalysts, including the
enzymatic reactions, were nonselective! They can mediate
oxidation of both the isomers, a-NAP and b-NAP![4,5]
Meanwhile, few analytical methods have been reported
for the selective identification of the isomers apart from the
regular separation-based techniques.[6] The kinetically faster
rate of coupling of diazonium salt with a-NAP over b-NAP
was often utilized for the spectroscopic detection of the iso-
mers.[6a] Herein, we report a highly selective recognition of
a-NAP isomer by simple electrochemical oxidation on mul-
tiwalled carbon nanotube (MWCNT)-modified glassy
carbon electrode (GCE/MWCNT) at neutral pH value.
Due to easy polymerization of the naphthol isomers as
tarry polynaphthol products at positive potentials, electro-
chemical techniques were rarely used for oxidation.[7] To
solve this problem, composite systems like gold nanoparti-
cle/hollow nitrogen-doped carbon microsphere/b-cyclodex-
trin[8a] and cationic polymer (poly(acridine orange))[8b]
chemically modified electrodes and specific solution-phase
mediator like Ce4+/Ce3+[8c] were used for electrocatalytic ox-
idation of both naphthols in acidic conditions. Meanwhile,
MWCNTs were often used as a strong adsorbent for effec-
tive adsorption of the naphthol isomers.[9] In this work, we
observed the formation of various naphthoquinones (desig-
nated as np-quinones) upon electro-oxidation of a-NAP
isomer on the GCE/MWCNT. Interestingly, those electro-
chemically oxidized products were immobilized on the
GCE/MWCNT and showed tunable electrocatalytic re-
sponse to an environmental pollutant (hydrazine) at
+0.15Æ0.005 V versus Ag/AgCl and to an important bio-
chemical, (b-nicotinamide adenine dinucleotide (NADH)
phosphate) at À0.1Æ0.005 V versus Ag/AgCl selectively. At
the same time, b-NAP failed to show any such oxidation
and electrocatalytic characteristics!
Figure 1 shows the typical continuous cyclic voltammetry
(CV) responses of unmodified GCE and MWCNT modified
GCE (GCE/MWCNT) for 1 mm a-NAP isomer in pH 7
PBS at v=50 mVsÀ1. Prior to the experiment, the mixture
was sonicated for 10Æ2 min at room temperature (25Æ
28C). The unmodified GCE failed to show any redox behav-
ior, whereas the GCE/MWCNT showed continuous growth
of two redox peaks, A1/C1 and A2/C2 at equilibrium poten-
tials (E1/2) À0.1Æ0.002 V and 0.15Æ0.002 V versus Ag/AgCl
and peak-to-peak separation (EpcÀEpa =DEp) 40Æ2 and
15Æ2 mV, respectively. Calculated surface coverage (G)
values are 1.1 and 0.43 nmolcmÀ2, respectively, for the A1
and A2 peaks. After the experiments, both the GCE and
GCE/MWCNT were transferred to blank pH 7 PBS medium
and exposed to 10 CV cycles (Figure 1B). Interestingly, no
[a] P. Swetha, Prof. A. S. Kumar
Environmental and Analytical Chemistry Division
School of Advanced Sciences
Vellore Institute of Technology University
Vellore-632014 (India)
Fax : (+91)416-2243092
E-mail: askumarchem@yahoo.com
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201201170.
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Puchakayala Swetha and Annamalai Senthil Kumar
Figure 1. Continuous CV responses of A) GCE (a) and GCE/MWCNT (b) in 1 mm a-NAP mixed pH 7 PBS, B) the same electrodes after transferred to
blank pH 7 PBS medium, and C) the same electrode but with b-NAP in pH 7 PBS (a) and its medium-transferred response (b).
peak response was observed with GCE whereas the GCE/
MWCNT yielded stable A1/C1 and A2/C2 redox peaks
(RSD=0.84%; n=5) under mixed adsorption- and diffu-
sion-controlled electron transfer (see the Supporting Infor-
mation, Figure S1A). The redox peaks show strong pH-de-
pendence, with Nernstian type proton-coupled electron-
transfer characteristics (Figure S2 in the Supporting Infor-
mation). This medium-transferred electrode is here after
designated as GCE/np-quinones@MWCNT. Furthermore, b-
NAP was also subjected to electrochemical oxidation similar
to a-NAP on the MWCNT electrode, but no such redox re-
sponse was noticed on GCE/MWCNT (Figure 1C). This ob-
servation highlights the selective electrochemical oxidation
of a-NAP isomer on the MWCNT.
cal studies (Figures S4 and S5 in the Supporting Informa-
tion). Interestingly, the graphite-modified electrodes as in
cases (1) and (2) showed appreciable A1/C1 and A2/C2
redox responses qualitatively similar to the electrode system
modified with pristine MWCNTs (Figure 1Ba) but with
about three times lower peak current values, whereas the
electrodes modified with metal oxides failed to show any
such behavior (Figure S5 in the Supporting Information).
Thus, it can be concluded that the graphite units present in
the pristine MWCNTs are responsible for enhanced a-NAP
electrochemical oxidation in this work.
Further, the np-quinones@MWCNT material was subject-
ed to characterization by Raman spectroscopy using a laser
source at 532 nm (Figure 2B), FTIR spectroscopy (Fig-
ure 2C), and transmission electron microscopy (TEM, Fig-
ure S6 in the Supporting Information). In the TEM image,
intense black spots were noticed around the multiwalls of
the CNTs along with twist- and clusterlike structures. It is
expected that the electrified GCE/MWCNT can act as elec-
trophoresis system and can allow transport of small charged
species inside the walls of the CNTs. In the present case the
electro-oxidized species (np-quinones) might have trans-
ferred inside the MWCNT and settled as nanoaggregates in
between the walls of CNT as encapsulated species.
The Raman spectra of the graphitic materials show D and
G bands at approximately 1330 and 1570 cmÀ1, respectively
due to the sp3 (defect structure or disordered graphitic struc-
ture) and sp2 bonding. The intensity ratio of D and G bands
(ID/IG) can be taken as a measure of the disorder in the
graphitic structure.[11a,b] The GCE/MWCNT and GCE/np-
quinones@MWCNT systems yielded ID/IG ratio values of
0.53 and 0.71 (before and after a-NAP modification), re-
spectively. Oxygen-rich functional groups such as quinones
and quinols formed on the modified electrode by electro-
chemical oxidation of naphthol might be responsible for the
disordered graphitic structure (D band) and the increased
ratio. FTIR spectra of np-quinones@MWCNT and of
MWCNTs and a-NAP as controls (Figure 2C) support the
Electrodes modified with different carbonaceous materi-
als, such as purified MWCNTs (p-MWCNTs), functionalized
MWCNTs (f-MWCNTs), single-walled CNTs (SWCNTs),
and graphite nanoparticles (GNPs) were examined for the
electrochemical oxidation of a-NAP (Figures S3 and S4 in
the Supporting Information). With respect to the G value of
the A1 peak the order of np-quinones@CNT oxidation is
pristine
MWCNT
(1.1 nmolcmÀ2) >p-MWCNT
(0.63 nmolcmÀ2) >GNP
(0.71 nmolcmÀ2) >f-MWCNT
(0.5 nmolcmÀ2) >SWCNT (0.32 nmolcm2, Figure 2A). Of
all these systems, the pristine MWCNT modified electrode
yielded the most strongly enhanced electrochemical oxida-
tion response for a-NAP oxidation. Since the pristine
MWCNTs contain 0.48% graphite and 5.2% metal oxide
(Fe2O3, NiO and Co2O3) impurities,[10a] those species may
have some positive effect on the electrochemical oxidation
reaction. Note that the groups of Compton and Pumera re-
ported detailed studies regarding the contribution of impuri-
ties towards various electrochemical oxidations,[10] in partic-
ularly graphite impurity to phenol oxidation.[10d] In order to
pin-point what kind of impurity is responsible for the a-
NAP electrochemical oxidation, GCEs were modified with
1) graphite, 2) graphite+MWCNT mixture ({MWCNT+
GNP}), 3) NiO, and 4) Fe2O3 and subjected to electrochemi-
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Puchakayala Swetha and Annamalai Senthil Kumar
Figure 2. A) Plot of G (nmolcmÀ2) versus various GCE/np-quinones with different carbonaceous materials. B) Comparative Raman spectroscopy respons-
es of MWCNTs (a) and np-quinones@MWCNT (b). C) FTIR/KBr responses of MWCNTs (a), a-NAP (b) and np-quinones@MWCNT (c). Plots of Dm
versus Q of D) EQCM-Au/np-quinones@MWCNT and E) b-NAP on EQCM-Au/MWCNT systems.
existence of quinone (1615 cmÀ1) and quinol (1382 cmÀ1)
functional groups within the modified electrode. However,
at this stage it is difficult to conclude what kind of quinone
and hydroxy groups formed after the electrochemical oxida-
tion on the GCE/MWCNT.
To confirm the nature of the electro-oxidized species on
the GCE/np-quinones@MWCNT, an ethanolic extract of the
modified electrode was characterized by GC-MS. The chro-
matographic pattern (Figure 3A) yielded peaks at retention
(RT) times of 10.5, 11.52, 12.83, 14.09, 15.14, 15.82, 17.23,
19.12, 20.66, and 22.33 min, and the corresponding m/z
values are 192, 176, 126, 228, 154, 158, 168, 154, 146, and
204. By considering m/z=144.06 for a-NAP, it can be under-
stood that two to four atoms of oxygen were added to a-
NAP upon electrochemical oxidation. Identified organic
fragments are displayed in Figure 3A.
Separate in situ CV and electrochemical quartz crystal mi-
crobalance (EQCM) experiments (Figures S8 and S9 in the
Supporting Information) were also performed with
MWCNT-modified EQCM-Au electrode similar to that used
in Figure 1Ab. The EQCM data were further subjected to
molecular mass calculation as per our reported procedur-
e.[12a] Interestingly, molar mass values 173Æ2, 190Æ1, and
204Æ2 gmolÀ1 were identified during the CV cycling experi-
ments, and the obtained m/z values closely matched some of
the GC-MS m/z values corresponding to the species 5-hy-
droxy naphthalene-1-one (m/z=173Æ2), 5,8-dihydroxy
naphthalene-1,4-dione (m/z=192Æ2), and 4-dihydroxy
naphthalene-1,2,5,8-tetrone (m/z=204Æ2, Figure 2). The
rest of the m/z values obtained in the GC-MS experiment,
which are missing in the EQCM, may all be either redox-in-
active species or not measurable by EQCM. Interestingly,
parallel EQCM experiment with the b-NAP isomer on an
MWCNT-modified electrode showed molar masses of 3Æ1,
3.4Æ1, 8Æ2 and 8Æ2 gmolÀ1, which may be due to involve-
ment of oxygen and a few proton in the process (Fig-
ure 2D). There are two possibilities for the A1/C1 and A2/
C2 redox peaks on the GCE/np-quinones@MWCNT
system: 1) consecutive electron-transfer reactions (same
compound with multiple redox sites) or 2) discreet electron-
transfer reaction (different compound functional groups).[12b]
Since, the approximate charges under the A1 (15Æ1 mC)
and A2 (5.7Æ0.2 mC) redox peaks are not same, discreet
functional groups may have major responsibility for the
redox activities in this work.[12b] Considering the above re-
sults and control experiments with 1,4- and 1,2-naphthoqui-
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Puchakayala Swetha and Annamalai Senthil Kumar
Regarding the mechanism for
the naphthoquinone formation,
it is expected that the phenolic
functional group of naphthol is
electro-oxidized and subse-
quently forms polyphenolic or
polyquinone derivatives. Previ-
ously, for the phenol oxidation
on the GCE/MWCNT elec-
trode, we predicted either the
electro-generated phenoxy radi-
cal at a positive potential of ap-
proximately 0.45 V versus Ag/
AgCl (case I) or in situ genera-
tion of hydrogen peroxide radi-
cal in the oxygen reduction re-
action (ORR; case II) at
À0.45 V versus Ag/AgCl, which
favors selective formation of
1,4-hydroquinone products and
immobilization as nanoaggrega-
tes.[12a,d] Similarly, studies were
performed with the present
system and revealed that the
oxidation reaction and immobi-
lization of the 1,4-hydroquinone
products take place through the
case II mechanism (Figure S11
Figure 3. A) Gas chromatogram responses of ethanol extract of the np-quinones@MWCNT, and its respective
mass spectrum at the retention times 10.2 (B) and 22.3 min (C). Plausible molecular species are shown.
in the Supporting Information
and Scheme 1A).[12d] Since the
para position in b-NAP is
blocked by the aromatic ring,
none derivatives,[12c] it can be concluded that the naphthol
derivatives having 1,4-naphthoquinone groups are responsi-
ble for A1/C1 and 1,2-naphthoquinone groups are responsi-
ble for A2/C2 redox couples.
the b-naphthoxy radical is not active enough to be further
involved in any chemical reaction, and thus only the a-NAP
isomer is electro-active.
Finally, we observed electrocatalytic oxidation of NADH
and hydrazine on the GCE/np-quinones@MWCNT in pH 7
Scheme 1. Schematic illustrations for the A) GCE/np-quinones@MWCNT modified electrode preparation by selective a-NAP electrochemical reaction
on GCE/MWCNT at oxygen reduction reaction potential (ORR) and B) plausible electrocatalytic mechanism for the tunable NADH and hydrazine oxi-
dation reactions on the working electrode.
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Puchakayala Swetha and Annamalai Senthil Kumar
PBS. The presence of various
naphthoquinones within the op-
timal working electrode was
identified using Raman spec-
troscopy, FTIR spectroscopy,
GC-MS and in situ CV and
EQCM characterization. The
GCE/MWCNT electrode with
immobilized naphthoquinones
showed excellent electrocatalyt-
ic activities towards hydrazine
and NADH at discreet oxida-
tion potentials without any in-
terference from other biochem-
ical, such as AA, citrate, CySH,
À
and NO3 . This study 1) de-
scribed the first observation of
the selective recognition of the
a-NAP isomer by an electro-
chemical method without any
separation procedure, 2) dem-
onstrates an easy way to pre-
pare an electrode chemically
modified with naphthoquinones
that is highly useful for the
electrocatalytic oxidation and
sensing of hydrazine and
NADH, and 3) suggests that
Figure 4. A,B) Electrochemical responses of GCE/MWCNT (a,c) and GCE/np-quinones@MWCNT (b) with
(a,c) and without (b) 100 mm of NADH (A) and hydrazine (B) at v=10 mVsÀ1. C,D) Amperometric i–t re-
sponses of GCE/np-quinones@MWCNT (a), GCE/MWCNT (b), and GCE (c) with continuous spikes of
NADH (C) and hydrazine (D) at Eapp =À0.1 V and 0.15 V vs Ag/AgCl, respectively.
disposable screen-printed elec-
trodes can be used for specific
analytical applications.
PBS. The unmodified electrodes, GCE/MWCNT and GCE,
failed to showed any NADH and hydrazine oxidation sig-
nals in a potential range of À0.25 to +0.25 V versus Ag/
AgCl (Figure 4A,B), whereas the GCE/np-quino-
nes@MWCNT electrode resulted in a huge NADH and hy-
drazine oxidation peak current signal at À0.1 and +0.15 V
vs Ag/AgCl respectively, corresponding to the para-quinone
(A1/C1) and ortho-quinone (A2/C2) redox peaks
(Scheme 1B). Amperometric current–time (i–t) sensing of
Acknowledgements
The authors gratefully acknowledge the Department of Science and
Technology (DST) under the SERC scheme for financial support. P.S.
thanks the Council of Scientific and Industrial Research (CSIR) for
a Senior Research Fellowship.
Keywords: electrocatalysis · electrochemistry · isomers ·
nanostructures · oxidation
NADH (Eapp =À0.1 V vs Ag/AgCl) and hydrazine (Eapp
=
0.15 V vs Ag/AgCl) discreetly at GCE/np-quino-
nes@MWCNT yielded linear calibration plots in the concen-
tration range 20–380 mm, and the respective detection limits
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NAP isomer on pristine MWCNT electrode in physiological
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COMMUNICATION
Fits like a glove: Separationless and
selective electrochemical oxidation of
the a-naphthol (a-NAP) isomer yields
naphthoquinone species on the surface
of multiwalled carbon nanotubes,
which can further catalyze the electro-
oxidation of NADH and hydrazine at
different potentials. The b-NAP isomer
failed to show any such electro-oxida-
tion.
Electrocatalysis
Puchakayala Swetha, Annamalai
Senthil Kumar*
&&&& — &&&&
Selective Electrochemical Recognition
of the a-Naphthol Isomer and In Situ
Immobilization of Naphthoquinones
for Tunable Electrocatalysis
7
&
&
Chem. Asian J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
These are not the final page numbers! ÞÞ