Fabrication of an amyloid fibril-palladium nanocomposite: A sustainable catalyst for C-H activation and the electrooxidation of ethanol

Article abstract of DOI:10.1039/c8ta11134k

Amyloids are highly ordered nanofibrils and their tensile strength is similar to that of steel, which makes them resistant to extreme pH and temperature. Based on this rationale, we demonstrate a facile synthesis of palladium, copper, platinum, gold and silver nanocomposites using α-Synuclein (α-Syn) fibrils as a template. We showed that an α-Syn-fibril-palladium nanoparticles (α-Syn-PdNPs) composite can be used as a heterogeneous catalyst in C-H bond activation and the electrooxidation of ethanol. The study demonstrated α-Syn-PdNPs to be a superior heterogeneous catalyst for the synthesis of pharmaceutically valuable benzofuran, naphthofuran, coumarin and N-arylindole via C-H activation. Further, the electrooxidation of ethanol using α-Syn-PdNPs displayed an electrochemically active surface area of 160.6 m² g^-1, which is much higher than the previously reported values for supported Pd nanocomposites.

Full text of DOI:10.1039/c8ta11134k

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2019, DOI: 10.1039/C8TA11134K.
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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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DOI: 10.1039/C8TA11134K
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Fabrication of amyloid fibril-palladium nanocomposite: A
sustainable catalyst for CH activation and electrooxidation of
ethanol
Received 00th January 20xx,
Accepted 00th January 20xx
Ramasamy Jayarajan,†ab Rakesh Kumar,†b Jagriti Gupta,c Gayathri Dev,a Pradeep Kadu,b Debdeep
Chatterjee,b Dhirendra Bahadur,c Debabrata Maiti*a and Samir K. Maji*b
DOI: 10.1039/x0xx00000x
www.rsc.org/
Amyloids are highly ordered nanofibril and its tensile strength is similar to that of steel, which makes them resistant to
extreme pH and temperature. Based on this rationale, we demonstrate a facile synthesis of palladium (Pd), copper (Cu),
platinum (Pt), gold (Au) and silver (Ag) nanocomposites using α-Synuclein (α-Syn) fibrils as a template. We showed that α-
Syn-fibril palladium nanoparticles (α-Syn-PdNPs) composite can be used as a heterogeneous catalyst in CH bond activation
and electrooxidation of ethanol. The study demonstrated α-Syn-PdNPs as a superior heterogeneous catalyst for the
synthesis of pharmaceutically valuable benzofuran, naphthofuran, coumarin and N-arylindole via CH activation. Further
electrooxidation of ethanol using α-Syn-PdNPs displayed an electrochemical active surface area of 160.6 m2/g, which is much
higher than the reported supported Pd nanocomposites.
applications.12b,13 Amyloids have also been demonstrated to be a
suitable substrate for cell adhesion, drug delivery and tissue
Introduction
Amyloid fibrils are protein/peptide aggregates consist of cross-β
-sheet rich secondary structure.1 Although, amyloids are mostly
associated with human diseases,1-2 recent studies however
suggested that amyloid could do normal physiological functions in
host organisms.2a,3 For example, pMel amyloids are used for
templating the melanin polymerization inside the mammalian
melanosomes.3b Amyloids are highly stable protein fibrils, whose
tensile strength is similar to steel4 and resistant against various harsh
environmental conditions such as extreme pH and temperature.5
These properties caught attention of researchers to explicate
amyloid based novel biomaterials.6 In the recent past, Mezzenga and
co-workers have actively engaged in the development of amyloid
based composites for different applications. These include β-
lactoglobulin amyloid as a template for Au as fluorescence material,7
Au and Pd nanoparticles for continuous flow catalysis,8 hybrid
composite with activated carbon for water purification9 and gold
nanocomposite for aerogel.10 Further, amyloid iron (Fe)
nanoparticles was synthesized and it was shown to be bioavailable.11
In addition, many studies have reported the application of amyloids
and their metal composites, for example building conductive metal
nanowires,12 nanocomposites for optoelectronics and LED
engineering applications.14
Among various transition metals, Pd-catalysts proved to be
privileged in activation of inert CH bonds over the last decades.15
Almost every available form of Pd has been exploited in
homogeneous catalysis, where the catalyst is often expensive with
low recoverability/recyclability.15c,16 In contrast, a heterogeneous
catalytic strategy is inherently promising, since it allows an easier
recovery and reuse of catalyst.17 The supported Pd nanoparticles
(PdNPs) found widespread applications in catalysis mostly for
classical C–C coupling reactions, due to narrow size distribution, large
surface area and high stability.17c,18 However, the heterogeneous
catalysis on CH functionalization is currently on the verge of
exploration.19
a. Dr. R. Jayarajan, G. Dev, Prof. Dr. D. Maiti
Department of Chemistry, Indian Institute of Technology Bombay
Powai, Mumbai-400 076 (India). E-mail: dmaiti@chem.iitb.ac.in
b. Dr. R. Jayarajan, R. Kumar, P. Kadu, D. Chatterjee, Prof. Dr. S. K. Maji, Department
of Biosciences and Bioengineering, IIT Bombay. E-mail: samirmaji@iitb.ac.in
c. J. Gupta, Prof. Dr. D. Bahadur
Fig. 1 Synthesis and application of α-Syn-PdNPs. Two applications are
shown, one for chemical reactions involving CH bond activation and
another for electrooxidation of ethanol.
Department of MEMS, IIT Bombay.
†These authors contributed equally to this work
Benzofuran and indole core structures exhibit a multitude of
Electronic Supplementary Information (ESI) available: DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
pharmaceutical and industrial applications.20
A
simplest
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spacing of 0.224 nm corresponding to the (111) planesViinewmAertticalelliOcnPlinde.
retrosynthetic disconnection path has always caught the attention of
synthetic chemistry community.21 Design of supported PdNPs as a
heterogeneous catalyst with high performance for CH
functionalization still remains a great challenge. Further, developing
Pd-based direct ethanol fuel cells is an attractive greener strategy,
where the support plays a vital role in its enhanced electrocatalytic
activity, and superior stability.22 Choice of supporting material for
PdNPs has always been crucial, since the interaction between
support and Pd can be reflected in their catalytic behavior and
durability.8 Although, synthesis and various applications of amyloid
templated metal composites are well studied,11,13,23 the application
of amyloid templated Pd nanocomposite as a heterogeneous catalyst
in CH activation and electrooxidation of ethanol is not explored. In
this study, we demonstrate the synthesis of palladium (Pd), copper
(Cu), platinum (Pt), gold (Au) and silver (Ag) nanocomposites using α-
Syn-fibrils as a support. Detailed spectroscopic studies on selected
nanocomposite, α-Syn-PdNPs and its application as a heterogeneous
catalyst in CH bond activation and electrooxidation of ethanol have
been described (Fig. 1).
h) Electron diffraction pattern.
DOI: 10.1039/C8TA11134K
The TEM images of α-Syn-fibrils supported Pd, Cu, Pt, Au and
Ag nanoparticles showed highly coated nanoparticles along the fibril
length (Fig. S7).25 Selected area electron diffraction (SAED) pattern of
all metal nanocomposites suggested face-centered cubic (fcc) crystal
structure.26 Size of the nanoparticles were found to be ca. 2.8 nm for
Pd, ca. 3 nm for Cu, ca. 2.5 nm for Pt, ca. 4 nm for Au, and ca. 4.4 nm
for Ag, respectively (Fig. S7).25
Owing to the synthetic and electrochemical applications of Pd-
based materials, we chose to study the structural and functional
properties of α-Syn-PdNPs in detail. However, the detailed study and
possible application of other nanocomposites (Cu, Pt, Au and Ag)
would be our future interest. The TEM of α-Syn-fibril and PdNPs
alone are shown in Fig. 2a and 2b. The reduction of Pd salt in the
absence of fibrils showed a cluster of aggregates (Fig. 2b). A typical
single amyloid fibril decorated with PdNPs is depicted in Fig. 2c,
which showed a uniform diameter of ca. 16 nm. The low-
magnification TEM image reveals abundant 1D α-Syn-PdNPs with
several micrometer lengths and the mean size of PdNPs was found
to be 2.87 nm (Fig. 2d and 2e). Energy dispersive X-ray spectroscopy
Results and Discussion
Synthesis and characterization of amyloid-metal nanocomposite
Initially, we expressed and purified α-Syn monomer from E.coli showed the presence of Pd as a major constituent, whereas oxygen,
and it was used for the synthesis of fibrils. The fibril formation was
confirmed by high thioflavin T binding (amyloid fibril specific dye),24
β-sheet structural transition by circular dichroism (CD) spectroscopy
(a negative peak at 218nm)25 and fibrillar morphology by
transmission electron microscopy (TEM). To explore the capability of
amyloid fibrils as a template in nanoparticle synthesis, we tested five
different metal salts such as Pd(OAc)2, CuSO4.5H2O, PtCl4,
HAuCl4.3H2O and AgNO3 in presence of α-Syn-fibrils.25
carbon, and nitrogen represent the key elements of amyloid fibrils
(Fig. 2f). The HR-TEM lattice spacing of PdNPs is found to be 0.224
nm, which is consistent with (111) plane of metallic Pd (Fig. 2g).27 The
SAED pattern of PdNPs also confirmed the fcc structure (Fig. 2h). Line
mapping of the composite revealed the presence of PdNPs on
surface of the fibril (Fig. S8).25
Fig. 3 Spectroscopic characterization of α-Syn-PdNPs. a) XPS resolved
spectrum shows two peaks at 334.8 eV and 340.02 eV corresponding
to Pd3d5/2 and Pd3d3/2 respectively. b) Four peaks in powder-XRD
pattern reveal the fcc lattice structure of Pd. c) FTIR spectra of α-Syn-
fibril, Pd(OAc)2 and α-Syn-PdNPs. d) Thermogravimetric analysis in
the temperature range of 25 °C to 200 °C.
Fig. 2 TEM characterizations of α-Syn-PdNPs. a) Morphology of α-
Syn-fibrils. b) Cluster of aggregates observed, when reduction of Pd
salt in the absence of fibrils. c) Single amyloid fibril uniformly
decorated with PdNPs. d, e) Morphology in different magnification
showing highly coated PdNPs along with amyloid fibrils. f) Energy
dispersive X-ray spectroscopy profile. g) The lattice fringes showing
The chemical state of PdNPs in the composite was studied by X-
ray photoelectron spectroscopy (Fig. S9a).25 Resolved XPS spectrum
showed two peaks; one at 334.8 eV and another at 340.02 eV
corresponding to Pd3d5/2 and Pd3d3/2 respectively (Fig. 3a), that
confirms the zero oxidation state of Pd.17a The X-ray diffraction
pattern clearly displayed four peaks at (111), (200), (220) and (311)
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reacting 4-nitro/methoxy phenol with methyl acrylate (4s and 4t,
representing the fcc lattice structure of Pd (Fig. 3b, JCPDS 00–005-
0681).28 To study the interaction between the α-Syn-fibril and PdNPs,
FTIR analysis was carried out (Fig. 3c). The peak at ~3451 cm-1 in the
α-Syn-fibril can be assigned for NH stretching present in the
peptide. The broadening of that peak in α-Syn-PdNPs is due to the
interaction of NH with PdNPs.28 Thermal stability of composite was
analyzed by thermogravimetric analysis (Fig. S9b).25 The spectra
recorded from 25 °C to 200 °C temperature is presented, as the
chemical reaction was carried out at 110 °C and 130 °C for CH
activation by using this nanocomposite. The results showed the
thermal durability of α-Syn-PdNPs, however a slight weight loss has
been observed (Fig. 3d).
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Table 1) under slightly modified conditions.25
DOI: 10.1039/C8TA11134K
Table 1. Synthesis of benzofuran, naphthofuran, coumarin and N-
arylindole via CH activation using amyloid supported PdNPs as a
heterogeneous catalyst
H
H
R1
-Syn-PdNPs

1
R1
or
+
R
reaction condition
R
NPh
R
4
5
, X = O
, X = NPh
OH
X
1
2
3
isolated
Benzofuran[a]
4a
O2N
O2N
cycle 1: 91%[d]
cycle 2: 87%[d]
cycle 3: 86%[d]
cycle 4: 83%[d]
4b,
R=4-OMe,
78%
t
4c,
R=4- Bu,
R=3-Me,
62%
4d,
86%
O
4a
O
4e
R=3-OMe, , 75%
Heterogeneous catalysis on CH activation using α-Syn-PdNPs
R
Me
NC
Br
4f
After unambiguous characterization, we explored the activity of
α-Syn-PdNPs as heterogeneous catalyst in the organic reactions
involving CC, CO/CN bond formation via triple CH bond
activation, which is previously performed through homogeneous
fashion with Pd(OAc)2.21b The amount of Pd in the composite was
estimated to be 20 wt% by inductively coupled plasma atomic
emission spectroscopy (ICP-AES) measurement.25 During initial
exploration, the reaction was performed between 4-nitrophenol and
styrene using 5 mol% of α-Syn-PdNPs as a catalyst, 10 mol% of 1,10-
phenanthroline, 1 equiv of Cu(OAc)2.H2O and 3 equiv of NaOAc at
110 °C in dichloroethane (DCE) as a solvent.21b The reaction produced
only 12% of benzofuran product (Table S3).25 The low yield is likely
due to the inefficiency of Cu(OAc)2.H2O in the oxidation of Pd(0) to
Pd(II) in the initial step under reaction conditions. Use of 1,4-
benzoquinone (1 equiv) as an oxidant, however, provided slightly
improved yield (28%) (Table S3).25 Among various oxidants, V2O5
resulted in the improved yield of 48%. After an extensive
optimization of reaction conditions, excellent yield of product 5-
nitro-2-phenylbenzofuran 4a (91%) was achieved from 4-nitrophenol
and styrene, using α-Syn-PdNPs (7.5 mol%), 1,10-phenanthroline (15
mol%), V2O5 (2.5 equiv), and NaOAc (1.5 equiv) in DCE at 110 °C for
24 hours (Table S3-S10).25 This reaction, therefore, suggests that
along with catalyst, the use of base and ligand plays a significant role
in the reaction.
R=H, , 88%
4g
R=3-CHO, , 66%
t
4h,
R=4- Bu,
60%
O
O
O
Cl
Me
4i,
R=4-Cl,
55%
4j,
4k,
66%
54%
R
Br
Br
4n
R=4-F, , 79%
R=2,4-Me, , 87%
4o
O
O
O
R
4l,
4m,
90%
76%
Coumarin[b]
Br
MeO
O2N
O2N
Ph
O
O
O
Me
Me
O
O
O
b
4s,
82%
4p
4q,
4r,
61%
, 65%
N-arylindole[c]
58%
5a
,
cycle 1: 87%[d]
cycle 2: 83%[d]
cycle 3: 81%[d]
O
b
4t
, 65%
N
N
R
Ph
5a
Ph
Cl
5b,
R=Me,
72%
Me
5c
R=F, , 65%
N
N
5d,
R=Cl,
67%
Ph
N
5e,
R=CN,
59%
Me
t
5g,
Ph
67%
5f
Me
R= Bu, , 61%
5h,
5i
, 69%
65%
Me
Me
N
N
Me
F
Proceeding further with the optimized condition, we probed
scope of the reaction with various substituted phenols and styrenes
(Table 1). Meta or para substituted styrenes were also well reacted
with 4-nitrophenol and yielded the product in 62-86% (4b-4e, Table
1). Differently substituted styrenes such as carbonyl (4g), sterically
encumbered tert-butyl (4h) and halogen (4i) were all well tolerated
and delivered the desired product. Switching the electronic nature of
the phenol by varying substitution was also tested (4j). Halogen
substituent at both phenol and styrene also produced benzofuran
(4k) with 66% yield. Subsequently, we focused on the synthesis of
pharmacologically important naphthofuran derivatives.29 We tested
2-naphthol derivatives with electron withdrawing and donating
substituent on the styrene. All reactions successfully yielded 2-
arylnaphthofuran derivatives 4l-4o (Table 1).
5j,
5k,
60%
63%
Me
Me
[a]Reaction conditions: 1 (1 mmol) and 3 (0.5 mmol), -Syn-PdNPs
(7.5 mol%), 1,10-phenanthroline (15 mol%), V2O5 (2.5 equiv), NaOAc
(1.5 equiv), ClCH2CH2Cl at 110 oC for 24 h;
1 (1 mmol) and
[b]
ethylacrylate (0.5 mmol), -Syn-PdNPs (10 mol%), 1,10-
phenanthroline (20 mol%), V2O5 (2.5 equiv), NaOAc (1.5 equiv), MS
(4Å, 130 mg), ClCH2CH2Cl at 110 oC for 24 h; [c] 2 (1 mmol) and 3 (0.25
mmol), -Syn-PdNPs (10 mol%), 1,10-phenanthroline (20 mol%),
CuOAc (2 equiv), ClCH2CH2Cl at 130 C for 24 h; [d]Refer Supporting
o
Information.25
We further tested α-Syn-PdNPs in the synthesis of N-arylindoles
by direct reaction between N,N-diarylamines and olefins (Table 1).21a
Initial reaction between diphenylamine and styrene in acetic acid
gave 30% yield of the desired product (Table S11).25 Considering the
recoverability of the catalyst, we intended to find an alternate
solvent. Thus, the reaction condition was extensively optimized to
obtain the desired N-arylindole in synthetically useful yield (Table
S11-S16).25 The highest synthetic yield of product, 1,2-diphenyl-1H-
indole 5a (87%) was obtained under optimized condition of
Use of aliphatic olefins such as cyclohexene or allyl chloride as a
coupling partner with 4-nitrophenol provided fused benzofuran
derivative (4p) or 2-methyl-7-arylbenzofuran (4q), respectively with
good yields. Allyl bromide with 6-bromonaphthol gave desired
naphthofuran derivative (4r). Next, we tested coumarin synthesis by
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benzofuran 4. A similar type of catalytic cycle can be followed for the
diphenylamine (1 equiv), styrene (0.25 equiv), α-Syn-PdNPs (10
mol%), 1,10-phenanthroline (20 mol%) and CuOAc (2 equiv) in DCE
at 130 °C for 24 h (Table S15).25 To explore the feasibility of the
optimized reaction conditions, substituted diphenylamines and
styrenes were exploited to get the desired 2-aryindole products (5b-
5k, Table 1).
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synthesis of N-arylindole 5 (Scheme S1).25
DOI: 10.1039/C8TA11134K
Table 2. Screening of different catalysts[a]
Catalyst
O2N
O2N
H
+
1,10-Phenanthroline
OH
V2O5, NaOAc
ClCH2CH2Cl, 110 ºC, 24h
O
Entry
Catalyst
α-Syn monomer-PdNPs
Yield[b]
50
Since amyloid fibrils are insoluble, we hypothesized that α-Syn-
PdNPs can repeatedly be isolated by centrifugation after each
reaction cycle. The recyclability of the catalyst was examined for both
benzofuran and N-arylindole synthesis under standard reaction
conditions.25 The calculated turnover number (TON) with the use of
7.5 mol% catalyst (α-Syn-PdNPs) loading for the benzofuran (4a)
synthesis is ~13 (12.67). Catalytic activity remained nearly unaffected
up to four reaction cycles in benzofuran (4a, Table 1) and three cycles
in N-arylindole (5a, Table 1) synthesis, respectively. TEM was
recorded for the recovered composite up to four cycles in
benzofuran and after the third cycle in N-arylindole synthesis (Fig. S6
and S11). A slight change in morphology of amyloid fibril was
observed, however, PdNPs were intact on the fibril surface.
Interestingly the size of the Pd nanoparticle size distributions is
mostly unchanged even after four cycle of the reaction suggesting
durability and integrity of the catalyst (Table S1 and Fig. S6).25 Further
CD spectra for both -Syn fibril as well as the nanocomposite (-Syn-
PdNPs) showed a negative peak at 218 nm, which can be assigned for
the -sheet structure of the fibril.30a-c Similar trend has been
observed for a recovered -Syn-PdNPs (after four cycle) with a
slightly reduced intensity. This data suggest that -Syn fibril structure
is mostly unaffected even after four cycle of chemical reaction.
1
2
3
4
Sonicated α-Syn-PdNPs
Proteinase k digested α-Syn-PdNPs
PdNPs supported on fibrous silica, (KCC-1-
PEI/Pd)30 (10 mol%)
PdNPs supported on fibrous silica, (KCC-1-
PEI/Pd)30 (20 mol%)
PdNPs alone (unsupported)
α-Syn fibril alone
PdNPs (7.5 mol%) + α-Syn fibril (5 mg)
PdNPs (7.5 mol%) + α-Syn fibril (10 mg)
α-Syn-PdNPs
65
30
20
5
35
6
7
8
9
10
57
0
58
55
95
[a]Reaction conditions: 4-nitrophenol (1 mmol) and styrene (0.5
mmol), catalyst (7.5 mol%), 1,10-phenanthroline (15 mol%), V2O5 (2.5
equiv), NaOAc (1.5 equiv), ClCH2CH2Cl at 110 oC for 24 h; [b]Yields
were calculated by GC using n-decane as internal standard.
To study the role of amyloid fibril as a support in catalytic
performance, we carried out the control experiments by varying
morphology of the α-Syn-fibril (Fig. S10),25 with previously reported
fibrous silica supported PdNPs (KCC-1-PEI/Pd),30d unsupported
PdNPs, α-Syn-fibril alone and mixture of both α-Syn-fibril and PdNPs
(entries 1-9, Table 2). Excellent yield of the product was observed
only when intact α-Syn-fibril was used as support compared to others
(entry 10, Table 2). The relatively low yields were obtained when the
reaction carried out with a physical mixture of PdNPs and -Syn
fibrils (entries 8-9, Table 2). These results suggest that catalyst
synthesized with -Syn fibril template possess higher catalytic
efficiency.
OH
+
OH
-Syn-PdNPs
O
NaOAc
I
R
R
PdII--Syn
V
O
OH
PdIILsOAc
PdII--Syn
[O]
II
R
OAc
IV
migratory
insertion
Electrooxidation of ethanol using α-Syn-PdNPs
O
O
PdIILs
R
R
O
Previous reports demonstrated electrooxidation of alcohol using
Pd or Pt nanoparticles with different supporting materials such as
metals, metal alloys, reduced graphene oxide and insulin amyloid
fibrils.17a,22c,26,32 We explored the electrocatalytic activity of α-Syn-
PdNPs towards electrooxidation of ethanol. Initially the cyclic
voltammetry (CV) measurements were carried out for α-Syn-PdNPs
in 1 M KOH solution (without ethanol) at the scan rate of 50 mVs-1,
to examine the electrochemical property and stability (Fig. 5a-c). The
Fig. 5a shows one peak in the anodic scan (forward scan) at potential
−0.59 V, which is associated with the formation of adsorbed OH and
desorption of hydrogen on the Pd surface.32a In the backward scan,
two peaks were observed; one peak in the cathodic scan at potential
−0.38 V, which is associated with the reduction of PdO to metallic Pd
and another peak at potential −0.83 V is related to hydrogen
adsorption.17a Important to note that the reduction of PdO to
metallic Pd is not only an electron transfer event, but might also be
a bond breaking reaction that involves in the release of OH. The
plausible reaction mechanism is discussed in Supporting
Information.33
R
PdIILsOAc
VII
VI
(Ls= -Syn)
III
R
Scheme 1. Plausible catalytic cycle for α-Syn-PdNPs catalyzed
synthesis of benzofuran
The plausible catalytic cycle for α-Syn-PdNPs catalyzed
formation of benzofuran have been described in Scheme 1.21b In the
first step, palladium catalyst I (α-Syn-PdNPs) inserts at the ortho
position of phenol and can provide the quinone-type intermediate
(II). The Pd(0) coordination with oxygen atom of phenol is unlikely
according to hard and soft acids and bases (HSAB) principle.31a The
most likely conjugate base of phenol (hard base) might not
coordinate to the Pd(0); whereas the ortho carbon of phenol-a soft
base can apparently form a bond with Pd(II), which is a soft acid.
Olefin coordination at Pd center of intermediate II leads to
intermediate III and migratory insertion of olefin via sterically less
hindered carbon can lead to intermediate IV.31b Subsequent steps
can provide intermediate V or VI, which can be converted into the
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DOI: 10.1039/C8TA11134K
Fig. 5 a) Cyclic voltammograms (CVs) of α-Syn-fibril and α-Syn-PdNPs on GCE in 1 M KOH at the scan rate of 50 mV s-1. b) CVs of α-Syn-PdNPs
in 1 M KOH with varying scan rate. c) CVs of day wise experiment with α-Syn-PdNPs in 1 M KOH at the scan rate of 50 mV s-1 d) CVs of α-Syn-
fibril and α-Syn-PdNPs in 1 M KOH + 1 M ethanol at the scan rate of 50 mV s-1 up to 1000 cycles. e) Current density changes with varying scan
rate in 1 M KOH + 1 M ethanol. f) CVs of α-Syn-PdNPs in 1 M KOH with various concentration of ethanol at the scan rate of 50 mV s-1.
It was observed that the mass current density increases with have carried out GC and NMR studies to identify the products or
increased scan rate and shape of CVs exhibit a relatively reversible intermediate species formed during electrooxidation of ethanol
electrochemical reaction (Fig. 5b). This makes a sufficient fast (after 1000 cycles). Although the accurate quantification of products
electron transfer rate for good electrochemical performance. The α- or intermediate is tricky, the analyzed spectral data revealed the
Syn-PdNPs showed excellent electrochemical performance at all scan presence of compounds such as acetic acid and polymeric species of
rate and higher stability up to 1000 cycles as well as in day wise study acetaldehyde; paraldehyde (Fig. S2-S4).25 This confirms the oxidation
(Fig. 5c). The calculated electrochemical active area (ECSA) of the α- of ethanol occurred during CV experiments. Further, the plausible
Syn-PdNPs composite was found to be 160.6 m2/g, which is higher intermediate formation during electrooxidation of ethanol has been
than the other reported Pd-based nanocomposite (Table S2).25 discoursed in Supporting Information.34
Higher ECSA indicates that composite have higher active catalytic
The effect of ethanol concentration on EOR was further analyzed
surface area with higher number of catalytically active sites, which
(Fig. 5e). The results show that the mass current densities increase
are essential for electrochemical oxidation of ethanol.
with ethanol concentration from 5 to 500 μM as well as peak
The electrocatalytic activity of α-Syn-PdNPs for electrooxidation potential (Ep) shifted towards more positive potentials. Therefore, it
was presumed that OH adsorption is independent of ethanol
concentration. Moreover, when the ethanol concentration increases,
the surface area of Pd nanocomposite covered by ethoxy species
gradually increased, which leads to higher current densities.
Furthermore, to compute the tolerance of α-Syn-PdNPs towards EOR,
chronoamperometry at different potentials were performed in KOH
of ethanol (EOR) was investigated in KOH (1 M) and ethanol (1 M) at
scan rate of 50 mV s-1 (Fig. 5d), which shows two well-defined
characteristic current peaks in the forward and reverse potential
windows. The first peak in the forward direction (positive scan)
corresponds to the ethanol oxidation; while the second peak in the
reverse scan attributes to the oxidation of freshly adsorbed ethanol
and adsorbed carbonaceous species.22c The current in the reverse
scan is quite sharp, that reveals a significant oxidation of adsorbed
intermediate species on the highly active α-Syn-PdNPs composite.
This implies that the ethanol oxidation could not be occurring directly
through a total oxidation to CO2.22b,32a However, no current density
peak is observed for α-Syn-fibril alone indicating that fibril is inactive
for ethanol oxidation. Long-term stability of α-Syn-PdNPs towards
ethanol oxidation was observed up to 1000 cycles, wherein only 8%
loss occurs in the mass current density (Fig. 5d). The mass peak
current density of forward scan of the composite is 9.4 A/mgPd cm2,
which is much higher than the reported Pd-based nanocomposites
(Table S2).25 Further, investigation by varying scan rate showed an
increase in current density with increased scan rate (Fig. 5e). We
(1 M)
+ ethanol (1 M) up to 1000 seconds (Fig. 6). The
chronoamperometry results showed that α-Syn-PdNPs exhibits a
rapid decrease in the current density at the beginning, indicating the
formation of carbonaceous intermediates during the ethanol
oxidation, which then remains constant (Fig. 6).25 The higher current
density was observed with α-Syn-PdNPs at the start and end of the
tests, signifying its superior catalytic activity and durability against
the poisoning. Overall, the enhanced activity of the present Pd
nanocomposite is due to the number of abundant Pd nanoparticle
and its activity improvement on the active sites, which can be
ascribed to the strong interactions between the amyloid and Pd
nanoparticles.
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The amino acid sequences that present in the amyloid might
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Conclusions
The present work revealed the use of newly developed
nanostructured amyloid templated Pd nanoparticles (α-Syn-PdNPs)
as an excellent heterogeneous catalyst for the synthesis of
biologically valuable heterocycles such as benzofuran, naphthofuran,
coumarin and N-arylindole via CH bond activation strategy. The
superior electrocatalytic performance of α-Syn-PdNPs on the
electrooxidation of ethanol further enhances its utility as a catalyst
for the development of potential ethanol fuel cells. These enhanced
activities can be explained based on the unique structural properties
and stability of amyloid fibrils, which is essential to grasp the metal
nanoparticle under various conditions. Overall merging biomolecular
frameworks with metal nanoparticle can further provide unique
applications in material science as well as nanobiotechnology.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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This research activity was supported by DST Nano Mission
(SR/NM/NS-1065/2015) India. Financial support from DST-India (R.J.)
and MHRD (R.K.) is gratefully acknowledged.
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