Soft template induced phase selective synthesis of Fe2O3 nanomagnets: One step towards peroxidase-mimic activity allowing colorimetric sensing of thioglycolic acid

Article abstract of DOI:10.1039/c6ra00963h

Different Schiff's base (SB) templates have been obtained to mediate phase selective synthesis of α- or γ-Fe₂O₃ from the same temperature condition. The inherent phase transformation tendency (γ-Fe₂O₃ to α-Fe₂O₃) is found to be inhibited. Again, the phase selective evolution emerges out of the binding modes of SBs which has been illustrated here. The nature of SBs governs the shape and size distribution of Fe₂O₃ NPs. As both the phases are magenetically active, easy magnetic separation of them widens their applicability in catalysis. Magnetically active both α- and γ-Fe₂O₃ mimic interesting peroxidase like activity by oxidising colourless 3,3′,5,5′-tetramethylbenzidine (TMB) to blue coloured oxidised product (Ox-TMB) in aqueous H₂O₂. These results prompted us to improvise further to detect thioglycolic acid (TGA) at a micromolar level which provides commercial applicability.

Full text of DOI:10.1039/c6ra00963h

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2 3
Soft Template Induced Phase Selective Synthesis of Fe O Nanomagnets: One Step
towards Peroxidase-Mimic Activity Rendering Colorimetric Sensing of Thioglycolic
Acid
Anindita Roy, Ramkrishna Sahoo, Chaiti Ray, Soumen Dutta and Tarasankar Pal*
Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India
Keywords: Soft template • Phase selectivity • Magnetisation •Peroxidaseꢀmimicking activity
•
Thioglycolic acid (TGA) sensing
ABSTRACT:
Different Schiff’s base (SB) templates have been obtained to mediate phase selective
synthesis of αꢀ or γꢀFe
transformation tendency (γꢀFe
selective evolution emerges out of the binding modes of SBs which has been illustrated here.
The nature of SBs governs the shape and size distribution of Fe NPs. As both the phases
are magenetically active, easy magnetic separation of them widens their applicability in
catalysis. Magnetically active both αꢀ and γꢀFe mimic interesting peroxidase like activity
by oxidising colourless 3,3׳,5,5׳ꢀtetramethylbenzidine (TMB) to blue coloured oxidised
product (OxꢀTMB) in aqueous H . These results prompted us to improvise further to detect
2
O
3
from the same temperature condition. The inherent phase
2
O
3
to αꢀFe ) is found to be inhibited. Again, the phase
2 3
O
2
O
3
2
O
3
2
O
2
thioglycolic acid (TGA) in micromolar level which renders commercial applicability.
INTRODUCTION:
Iron oxides occur ubiquitously in the environmental, geological, planetary, and technological
2 3
settings. Amongst the polymorphs of ferric oxides, hematite (αꢀFe O ) is the most stable form
and being nꢀtype semiconductor, it attracts great deal of attention for water treatment,
1
catalysis, electrocatalysis, energy storage and gas sensing. However; antiferromagnetic (AF)
1

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2 3
nature of αꢀFe O restricts the application pertaining to its separation with an external
magnet. Importance of magnetically separable catalyst finds innumerable applications in
2
biology, radiology, therapeutics, electronics etc. Hence, there comes the usage of maghemite
i.e. γꢀFe O in different fields. Being cationꢀdeficient spinel (Fd3), γꢀFe O shows
2
3
2
3
ferromagnetism and the charge neutrality is rendered by cation vacancies, with a Curie
temperature (T ) of 928 K. Due to canted AF property, rhombohedral αꢀFe (R3c), below
260 K (Morin temperatureꢀT ), shows weak ferromagnetism (WF) and that is explained by
Dzyloshinsky ꢀMoriya mechanism (T 950 K). Tailoring of magnetic property of
C
2 3
O
~
M
3
4
C
nanocrystals relates to wellꢀdefined size, shape and selfꢀassembly which has become a topic
of intensive research because of the enormous scope for applications of these materials as
storage media, biomedical devices, optical devices, sensors and magnetic fluids.
Transformation of magnetically active metaꢀstable γꢀFe
inactive αꢀFe is a consequence of temperature, time domain and precursor salt variations.
Nowꢀaꢀdays, template assisted synthesis has become very popular for obtaining
monodispersed crystalline iron oxide nanoparticles (NPs). Among these, carbon NPs, carbon
2 3
O into the most stable magnetically
5
2
O
3
6
7
8
nanotubes (CNTs), mesoporous silica etc. are successfully employed as the well suited
templates. Selfꢀaggregation of NPs can be prevented in the templates due to
compartmentalization of the particles within the restricted domain of the template cavity.
5
In case of hard template mediated multistep synthesis, Ohkoshi et al. showed that
template cavity helps the oxidation of the precursor under confinement at high temperature
(~600ꢀ1200°C) and subsequent template dissolution finally set free the desired oxides. In this
2
regard, iron complexes like acetylacetonates, acetates, oleates, iron containing different
2
metalꢀorganic framework, ferritin proteins etc. are used extensively as template which upon
controlled pyrolysis produce template free magnetic NPs. Using specially designed threeꢀ
9
dimensional mesoporous silica (KITꢀ6), Bruce et al. showed the synthesis of different
2

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mesoporous iron oxides at different reaction conditions (150ꢀ650°C). A conceptually new
10
2 3
strategy has been adopted by Wei et al. for growing atomically thin αꢀFe O nanomaterial
by bringing in transition metal oxide (TMO) sheet as host material. Again, biological
template assisted synthesis of iron oxide NPs has also found their relevance towards synthesis
2
as well as biocompatibility.
Herein, understanding the great utility of template assisted synthesis, Schiff base (SB), a
condensation product of aldehyde and amine, draws our attention to improvise a prospective
template. Considering the donorꢀacceptor property of SB, iron (II/III) chelation is evident and
then NPs stabilization is realized in the subsequent steps. Thus, four different SBs, generated
from the permutative combination of two different aldehydes (glyoxal and glutaraldehyde)
and two different amines (ethylenediamine and triethylenetetramine) have been employed to
serve the purpose of template for the synthesis of morphologically different iron oxide.
Mohr’s salt was used as iron source and hydrazine as hydrolyzing reagent (Scheme 1) at a
refluxing temperature of 373 K. Under this condition, morphologically different Fe O NPs
2
3
11
were produced in 1 h time as a consequence of dehydration.
The asꢀsynthesised magnetic nanoparticles (MNPs) exhibit property as peroxidaseꢀmimicking
agent catalysing the oxidation of 3,3׳,5,5׳ꢀtetramethylbenzidine (TMB) to blue coloured
2 2
oxidised product (OxꢀTMB) in presence of H O which has similar activity of Horseradish
Peroxidase (HRP), a natural enzyme. Moreover, these enzymeꢀmimicking nanomaterials or
nanozymes show several advantages over natural enzymes, such as cost effectiveness,
tuneable catalytic activities, and high stability against rigorous conditions, thus being used in
1
2, 13
various ways and means.
3 4
Utility of Fe O as magnetic peroxidaseꢀlike nanozyme follows
14
a well known pathway considering Fenton’s catalysis. But Fe
2
O
3
lacks this ability due to
2+
insignificant proportion of Fe ions at the NPs surface. Here the SB template mediated
2+
growth of Fe
2
O
3
MNPs leaves a higher propensity of Fe ions at the surface transcending it
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into an effective peroxidaseꢀlike nanozyme. Meanwhile, because of the magnetic nature the
nanozyme can easily be separated, concentrated and thus reusability of MNPs also popularise
the system towards TMB and H
2 2
O detection. In general, to evade the sophisticated
instrumentation and naked eye detection of different analytes, the colorimetric detection
1
5ꢀ19
technique is well accepted.
colorimetric sensing of thioglycolic acid (TGA) applying the peroxidase mimicking property
of Fe
So, we further extent our TMB oxidation catalysis on
2
3
O .
EXPERIMENTAL SECTION:
Materials used and instrumentation are described in supporting information (SI) as S1 and S2
respectively.
SYNTHESIS:
2 3
Well defined αꢀ and γꢀphases of Fe O NPs were synthesized by impregnation of
2
+
Fe precursor; Mohr’s salt into SB templates followed by hydrolysis of ironꢀSB templates
using hydrazine as hydrolyzing agent. The general strategy can be described as follows. An
ꢀ2
aliquot of the chosen aldehyde and amine (5×10 M) were mixed together in 10 ml of
ethanol. The mixture was stirred for ~ 1 h using a magnetic stirrer to produce SB template
(Figure S2a: FTIR gives the evidence of prepared Schiff base). After 1 h of stirring, 10 ml of
ꢀ2
ethanolic solution was mixed with 90 ml aqueous Mohr’s salt solution (5×10 M) to obtain
an effective reaction mixture. Then 0.5 ml hydrazine hydrate (0.16 M) was added into the
reaction mixture and the solution was refluxed, until (~ 1h) the supernatant became
colourless. TemFe A (from glyoxal and ethylenediamine), TemFe B (from glutaraldehyde
and ethylenediamine), TemFe C (from glyoxal and triethylenetetramine) and TemFe D (from
glutaraldehyde and triethylenetetramine) were prepared by the same procedure. After the
preparation of four sets, the products were thoroughly washed with ethanol, water and finally
4

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separated simply by employing an external door magnet. Thorough washing of the product
removed all the adhering remnants from the surface making it pure enough for further
characterisation and application in catalysis.
Catalytic Oxidation of TMB:
The peroxidaseꢀlike catalytic activities of both αꢀ or γꢀFe O are described out of the
2
3
ꢀ2
oxidation of TMB in presence of H
2
O
2
. In this study, 25 ꢁL (10 M) ethanolic solution of
TMB was oxidised by 100 ꢁL (30%) H
2
O
2
in presence of 0.1 mg of dispersed catalyst in
acetate buffer medium (pH = 4.0). The steady state kinetic study has been carried out at room
temperature (RT). The UVꢀvis spectra have been recorded at 5 minute interval at RT for
performing the kinetic study. With completion of the reaction, colourless TMB solution has
been found to be oxidised to blue coloured oxidised product OxꢀTMB which generated a
broad peak at 652 nm. Thus, formation of OxꢀTMB leads to study kinetically following
spectroscopic observation. The MichaelisꢀMenten parameter has been determined from
double reciprocal LineweaverꢀBurk plot.
1
/V = 1/V + K /Vmax[S0]
(1)
0
max
m
where V
concentration, and K
oxidation along with the variation in TMB concentration (0ꢀ0.32 mM) and H
0
and Vmax are initial and maximum velocity respectively, [S
0
] is substrate
m
represents MichaelisꢀMenten constant. We have carried out TMB
2
O
2
concentration (0ꢀ80 mM). We have also performed a control experiment of TMB oxidation in
presence of SB templates only. But absence of any peak at 652 nm (Figure S8b) ensures the
catalytic inability of SB templates for TMB oxidation catalysis.
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Detection of thioglycolic acid (TGA) concentration:
In a general procedure, freshly prepared aqueous thioglycolic acid (TGA) solution (0.033ꢀ
.67 mM) was introduced into the previously mentioned reaction mixture containing TMB.
0
Keeping other reagents unaltered, we only varied the TGA solution in the reaction mixture
maintaining the final volume (3 ml) constant. Successive colour variation, arising from the
concentration gradient of TGA solution, was studied thoroughly by UVꢀvis spectral
measurements.
RESULTS AND DISCUSSION:
The crystal structure, shape, size and size distribution of NPs is the resultant effect of
controlled crystallisation process. The theoretical approach to understand the mechanism of
crystal formation has been explained. Lowering the high Gibbs’ free energy in supersaturated
solution thermodynamically favours the formation of nucleation and growth of crystal. The
Gibbs’ free energy per unit volume of the solid phase (ꢂG
concentration of the solute and is represented as,
v
) is dependent on the
ꢃꢄ
ꢅ
ꢃꢄ
ꢂꢀ =ꢂ− ln( ) =ꢂ− ꢂln ꢂ( 1 +ꢂσ)
(2)
ꢁ
ꢃ
ꢅ
ꢃ
ꢆ
0
where, C and C are the concentrations of the solute and the equilibrium concentration or
solubility respectively, σ is supersaturation concentration (defined by C ꢀ C / C ) and ꢃ is the
0
0
atomic volume. So, for spontaneous nucleation, the solute concentration must exceed than
that of equilibrium in reaction medium. Again the nucleus becomes stable only when the
radius size exceeds the critical size r*; which can be explained by
ꢇ∗ =ꢂ−2ꢈ/∆ꢀꢁ
(3)
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where γ denotes surface energy per unit area. If the nucleus radius is less than r* then it will
solubilise in medium; whereas for greater value of radius, it will be stable enough to grow
bigger. When the particle radius is little smaller than r* then the crystallisation process
become faster, forming larger particles due to more negative Gibbs’ free energy of
crystallisation. During the growth of particles, there is a tendency of smaller particle to
coalesce together for bigger particles, bringing agglomerated products.
Scheme 1. Schematic illustration of Schiff base template mediated synthesis of various
2 3
phases of Fe O NPs using Mohr’s salt as precursor salt.
20
Nucleation is the key in progress of crystallisation. Under refluxing condition, large
numbers of nuclei are formed and remain homogeneously distributed throughout the bulk
solution. Cao et al. also enriched the crystallisation mechanism by proposing the stronger
effect of bubble formed during this refluxing condition in nucleation process, thus crystal
21
formation takes place. So, refluxing condition leads to highꢀyielding route to synthesis NPs.
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In our case, the refluxing condition for an hour and the N bubble formed from the
2
decomposition of hydrazine effects the crystallisation process by formation of large number
of nucleation sites in the reaction medium. This facile synthesis leads to gram level
production of phase pure crystalline γꢀFe
2
O
3
or αꢀFe
2
O
3
, depending upon the variable SB
2
+
3+
templatesʹ nature. Oxidation of Fe to Fe is a spontaneous process in the ambient condition.
2+
Considering Mohr’s salt as precursor, we wished to control the transformation of Fe ions,
which further act as catalyst showing peroxidaseꢀlike activity. The effect of SB templates in
the phase specific formation of Fe
SBs accelerates the hydrolysis of the metal hydroxide. This supports the benefit of SB
template for the synthesis of Fe nanoparticles under refluxing condition. The PXRD
2 3
O has been elaborated here. Advantageous imine group in
2
2
2
O
3
patterns of TemFe A, B, C (See SI, PXRD pattern of TemFe B and C; Figure S1) were
indexed to γꢀFe O justified with JCPDS file no. 04ꢀ0755. For TemFe D, the PXRD pattern
2
3
suggests it as ɑꢀFe
2
O3, supported by JCPDS file no. 84ꢀ0311.
18000
22000
TemFeA
TemFeD
JCPDSno. - 04-0755
20000
8000
16000
4000
2000
10000
000
20
JCPDSno. - 84-0311
16000
1
14000
1
12000
1
10000
8
8
000
2
0
40
60
80
40
60
80
2θ
(degree)
2θ(degree)
Figure 1. PXRD patterns of TemFe A and TemFe D, represent existence of γꢀFe
Fe respectively.
2 3
O and αꢀ
2
O
3
2 3
The sharp peaks indicate the crystallinity of ɑꢀ or γꢀFe O . Absence of other peaks confirm
the phase purity in the system. In the present investigation, we find that the nature of the SB
governs the formation of specific phases along with definite size distribution of particles. The
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preferential adsorption of four different SBs with different iminic bonding motif could alter
the surface energy and restricts the cystal growth in certain orientation by attachment of ions,
introducing formation of different phases. There is a natural tendency of greater interaction of
metal ion with tridentate imine ligands. Thus, the hyrolysis is acceletared locally in presence
of the imine group attached to the iron ion. So, facile formation of maghemite occurs in case
of TemFe A, B and C. But in case of TemFe D, due to the larger chain length of both
aldehyde and amine used in SB (TemD), particle size increases which leads to the formation
of most stable hematite phase. With increase in particle size preferential nucleation to form
thermodynamically stable hematite phase occur due to the nanomization of the free surface
2
3
energy. Also tenacious hold of water on the surface of nanoparticle leads to the formation of
24
hematite. Increasing the size of template, depending upon the chain length of aldehydes and
amines in the SB skeletons, the particle size increases which is confirmed from the FESEM
(
Fig S3) and TEM (Figure 2) images. The average particle size of TemFe A and B lies within
0ꢀ50 nm, whereas for TemFe C it varies from 100ꢀ120 nm and for nanorod (TemFe D), the
length is about 400ꢀ500 nm with an aspect ratio ~ 20. Larger available free space of the
2
25
templates support surface energy driven oriented attachment of the evolved Fe
nanoparticles. Fringe spacing in figure 2 shows that TemFe A has plane orientation along
311) and that of TemFe D is (116) proven by presence of fringe spacing 0.25 nm and 0.16
nm respectively. The crystalline nature of αꢀFe O is also supported by the hexagonal bright
2 3
O
(
2
3
spot in selected area electron diffraction (SAED) pattern (Figure 2). Formation of rings in the
SAED patterns (Figure 2) of the γꢀFe signifies the polycrystalline nature.
2
O
3
FTIR analysis (Figure S2b) was performed to confirm the formation of different phases of
iron oxide NPs illustrated by the vibrational modes. Generally in maghemite, being inverse
3+
3+
spinel, half of the Fe ions occupy the tetrahedral interstices and remaining half of Fe in
3+
octahedral sites whereas in hematite each Fe occupy the octahedral coordination site.
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3
+
Vibration of Fe ions in tetrahedral positions was attributed to the high frequency band and
3+
26
low frequency band was attributed to the vibration of Fe ions in octahedral positions.
Figure 2. a1ꢀa4 represent HRTEM images of TemFe A, B, C and D respectively; b1ꢀb4
represent fringe spacing and corresponding crystal planes of TemFe A, B, C and D
1
0

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respectively; (c) c1ꢀc4 represent SAED patterns of TemFe A, B, C and D respectively
representing the polycrystalline nature of the samples.
26
ꢀ1
According to Waldron, the band appeared at 350–450 cm is attributed to the stretching
+3
ꢀ2
ꢀ1
vibration of Fe ꢀ O in the octahedral complexes and at 500–600 cm to that of bending
vibrations in tetrahedral complexes. The presence of intense peak at 500ꢀ600 regions
+
3
confirms the presence of Fe ions in tetrahedral position which supports the formation of γꢀ
Fe in case of TemFe A, B and C. Again absence of such intense peak at that region in
TemFe D also suggests the evolution of αꢀFe . This association of SB template together
with Fe O was established from the shift of imine streching frequency (Figure S2b) towards
2
O
3
2
O
3
2
3
ꢀ1
lower frequency region (1556 and 1628 cm ) which is due to the lowering in bond order in
3
+
C=N because of the drifted electronic cloud towards Fe ions. This suggests the formation of
3+
coordination bond between imine ᴨꢀelectron cloud and Fe ion. The tenacious adsorption of
SBs as protective layer stabilises the meta stable γꢀFe . All these stretching vibrations in
2
O
3
FTIR analysis (Figure S2b) are attributed to fingerprint vibrational modes of SBs, indicating
their adherence at the surface of the NPs.
The XPS spectra (Figure S4) have been measured considering adventitious carbon (284.0 eV)
as reference material. The XPS peak position at 711.0 eV (Fe2p3/2), 724.0 eV (Fe2p1/2) and
3+
2ꢀ
5
29.0 eV (O 1s) confirms the presence of Fe and O in Fe
2
3
O . We also observed the
2
+
presence of Fe (709.6 eV) ions in asꢀsynthesised TemFes after deconvulation of the XPS
2
+
3+
spectra. The peak area ratio of Fe and Fe has also been obtained from the deconvulated
2
+
3+
XPS peak (Figure 3) of iron. The [Fe ]/[Fe ] of TemFe A, B, C and D are calculated from
2+
the peak area respectively. Thus, the surface attached Fe ions and their relative
concentration helps to establish the mechanism and increasing order of the catalysis. The data
related to deconvulated XPS peaks for TemFe A, B, C and D are given in SI, table S1.
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1
1
1
1
6
4
2
0
8
6
4
2
16
+
2
+
708.9 eV- Fe
+
2
TemFe A
TemFe C
709.88 eV - Fe+
3
710.86 eV- Fe
3
1
1
1
4
2
0
8
6
4
2
711.39 eV - Fe
7
04
706
708
710
712
714
716
Binding Energy (eV)
704
706
708
710
712
714
716
Binding Energy (a.u.)
1
4
2
0
8
6
4
2
8
7
6
5
4
3
2
+
2
TemFe B
709.39 eV- Fe
TemFe D
+2
710.0 eV - Fe
+
3
7
11.29 eV- Fe
1
1
+3
711.86 eV - Fe
7
04
706
708
710
712
714
716
7
06
708
710
712
714
716
Binding Energy (eV)
Binding Energy (eV)
Figure 3: Deconvulation of XPS spectra of Fe2p3/2for all TemFes were done. From the ratio
+
2
+3
of the deconvulated peak area of Fe and Fe , the concentration ratios have been
+
2
+3
determined. The [Fe ]/ [Fe ] of TemFe A, B, C and D are 1.743, 1.374, 1.112, and 1.37
respectively.
+
2
+3
Though the [Fe ]/ [Fe ] of TemFe B and D were similar; but the critical observation of
+2
deconvulated peak position of TemFe D denotes that the peak corresponds to Fe (709.0) is
+3
shifted to 710.0 eV which reflects the formation of Fe ion. So, the order of the catalysis is
+
2
+
justified. Because the peak position denotes that conversion from Fe to Fe took place. So,
+
2
absence of Fe ions lowers the catalytic activity of TemFe D as peroxidaseꢀmimic catalyst.
1
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MAGNETIC PROPERTY:
Presence of magnetic domain in material is responsible for magnetic property in them. As the
particle size decreases to nanoregime, tinier than the magnetic domain; NPs are considered to
2
7
be completely magnetised. Because in nanoregime the multiꢀmagneticꢀdomain organization
28
is energetically unfavourable, thus particles with singleꢀmagneticꢀdomain are formed. The
term “superparamagnetism” is not very accurate but is commonly used to emphasis the
magnetic behaviour for paramagnetic nanoparticles which arises from the coupling of several
thousands of atoms in comparison to the paramagnetism of a single atom. This magnetization
can reach nearly the saturation magnetization of ferromagnetic iron oxide, but in contrast to
ferromagnetic iron the particles no longer show magnetic interactions after elimination of the
magnetic field. According to the theory of magnetism for exhibiting superparamagnetic
P
behaviour, required minimum NP volume (V ) is
ꢋꢌꢍꢎꢏꢂ
ꢉꢊ =
(4)
ꢐ
where, K is the anisotropic constant, T is the room temperature (300 K) and k
B
is the
2
9
Boltzmann constant. Thus, spherical NP size below 26 nm exhibits superparamagnetic
30
behaviour. This shows the control over magnetic nature by particle size variation at
nanoregime. The magnetic data of all these TemFes are examined and discussed thoroughly
to decipher the linear relationship between the particle size and size dependent magnetism.
Early studies also established the unusual magnetic property as the intrinsic nature in
3
1
polyimineꢀiron materials. Figure 4 shows the appealing hysteresis of vacuum dried four
TemFe samples obtained by SQUID magnetometer with an applied magnetic field +/ꢀ 2T at
both low (5K) and high (300K) temperature. The comparative magnetic property study at
both temperatures is presented in Table 1.
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+3
In rhombohedral hexagonal crystal of hematite, Fe occupy the octahedral coordination site,
+3
making it magnetically inactive, whereas in maghemite, the unequal distribution of Fe in
octahedral and tetrahedral voids in inverse spinel structure arouse net magnetisation. The
natural tendency of magnetisation in nanoparticle is less than that of their massive parent
materials. The reason behind such reduction in magnetization in NPs is often attributed to the
finite size effects such as surface spin disorder, spin canting as well as magnetically dead
32
layer on the surface of synthesized NPs. But here in nanoregime, the inherited magnetic
property of γꢀFe is enhanced by the presence of uncompensated surface spin at grain
2
O
3
boundary region. The uncompensated surface spins can overcome the AF property of the core
part, so αꢀFe O NPs can also exhibit a net magnetisation which is reported here. The
2
3
concentration of unsaturated spins on the surface of NPs depends on the size and shape of
NPs which is modulated by the variable SB templates.
Table 1. Brief comparison of magnetic nature of all TemFes
Sample
Saturation
Remnant
Coercive Field
(Oe)
Squareness
Magnetisation
(2 T)
Magnetisation
R S
(M / M )
(
S
M )emu/g
(
M
R
)emu/g
Nature
5
K
300 K
5 K
300K
5 K
300 K
5 K
300 K
TemFe A
TemFe B
TemFe C
TemFe D
90.44
97.32
60.62
3.73
83.63
19.6
ꢀ
ꢀ
ꢀ
ꢀ
255
ꢀ
0.217
0.208
0.274
0.238
ꢀ
γꢀ Fe
γꢀ Fe
γꢀ Fe
2
2
2
O
3
89.2
20.3
16.6
0.89
234
182
402
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
O
3
55.64
3.74
O
3
2 3
αꢀ Fe O
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4

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The anisotropy factor in eqn. 5 also explains the enhanced magnetic property.
ꢑ = ꢓ ꢉ
(5)
ꢒ
ꢒꢂ
a a
Where, E is anisotropy energy, K is the anisotropy constant and V is the crystal volume. So,
3
both the volume and shape play a noticeable role in magnetism.
In the present investigation for all synthesized NPs, magnetic aggregation is almost absent
which is in agreement with the individual particle coating and the pseudo selfꢀassembly due
to the presence of the various SB templates. So, ceasing the spin alignment with variation of
temperature is observed. Thus, there is no distinct change in saturation magnetisation occurs
amongst any of these TemFes at 5 and 300 K respectively. Presence of greater anisotropy in
both magnetic and morphological nature of nanorod than in sphere causes spin alignment to
generate net magnetism. The convergent nature of zeroꢀfieldꢀcooling (ZFC) and fieldꢀcooling
(FC) curves (Figure S7) were obtained under 100 Oe magnetic fields within the temperature
window 5ꢀ300 K.
The soft paramagnetic nature of all templateꢀdirected NPs has been explained both by MꢀH
curve and ZFCꢀFC curve. Being soft magnetic materials, it is easy to align the magnetic
domains in a definite direction. We observed that AF hematite (TemFe D) also shows
unnatural saturation magnetisation (M
TemFe D) αꢀFe can be explained by firstꢀorder transition observed at T
Figure S7, which is much less than that of bulk αꢀFe (263 K) in Dzyaloshinskyꢀ
S
) 3.74 emu/g at RT. The WF property associated with
(
2
O
3
M
~110° C, in
2
O
3
3
,4
Moriya mechanisms. In this transition, the spinꢀflip occurs from the spin orientation along
the symmetry axis to perpendicular direction with a canted angle giving rise to weak
ferromagnetism. Decrease in T
M
with rise in temperature attributed to the anisotropy effect,
33
lattice strain, crystal defect etc. occurred in αꢀFe
O .
2 3
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5

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1
20
120
@
5 K
TemFe A
TemFe B
TemFe C
TemFe D
@ 300 K
TemFe A
TemFe B
TemFe C
TemFe D
8
0
0
0
80
40
4
0
-
40
80
-40
-80
-120
-
a
b
-
120
-10000
0
10000
-10000
0
10000
Magnetic Field (Oe)
Magnetic Field (Oe)
Figure 4. Comparison of saturation magnetization (M
S
) data from MꢀH curves of asꢀ
synthesised TemFe A, B, C and D at both 5 K and 300 K.
The decrease in M
fluctuations, resulting disturbance in spin alignment. But the unchanged M
be supported by change in spinꢀaxis from cꢀaxis to cꢀplane due to increase in thermal
fluctuations. There is also a possibility of selfꢀgrown oxygen vacancies, destroying the AF
S
of γꢀFe
2
O
3
with temperature increase can be illustrated due to thermal
S
of αꢀFe can
2 3
O
3
4
interactions between the sublattices, results in net magnetism for αꢀFe O . Increased
2
3
temperature facilitates the transition of large fraction of clusters from ferromagnetic to
superparamagnetic. The soft nature of the entire template stabilized Fe at RT can also be
explained by the squareness value. In absence of template, the asꢀsynthesised γꢀFe showed
as usual saturation magnetisation of ~ 68 and ~ 60 emu/g respectively at 5 K and 300 K
Figure S5), which draws special attention emphasising the template mediated synthesis
2
O
3
2
O
3
(
technique.
Peroxidase-like activity:
To observe the natural enzyme mimic property of catalyst, TMB is used as a chromogenic
substance. In presence of H
2 2
O , our asꢀsynthesised TemFes catalyse the oxidation of TMB to
produce blueꢀcoloured OxꢀTMB at RT. This crucial finding makes the system very important
1
6

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for analytical purposes which are enhanced by the magnetic separation technique. We have
performed the catalysis with all of the TemFes and SB template free γꢀFe . We found that
2
O
3
amongst all catalyst variation, TemFe A act as best catalyst towards TMB oxidation reaction.
This can be explained by the size variation of the catalysts. With the smallest size, the boon
of high surface to volume ratio comes into the picture and the phenomenon is also true in this
2+
case, which is observed from the rate of TMB oxidation. Generally Fe ions are responsible
for the Fenton’s reaction with H , which here drives the catalysis of TMB oxidation. The
2
O
2
+2
enhanced presence of surfaceꢀattached Fe ions in our synthesised catalyst surface due to the
SB template which is directly related with the small size of catalyst, widen the probability of
catalysis. Amongst all TemFes, the order of catalysis was found; TemFe A> TemFe B>
TemFe C> TemFe D (Figure S8).
2 2 2
In presence of 0.1 mg catalyst (1 mg dispersed in 1 ml H O), 0.1 ml (30%) H O in acetate
ꢀ2
buffer medium (pH = 4.0) efficiently oxidises (0.025 ml, 10 M) TMB to blueꢀcoloured Oxꢀ
TMB generating a peak at 652 nm at RT. The oxidised TMB products become stable at acidic
pH condition. The partially oxidised, one electron, TMB produces cation radical species
which is in equilibrium with the diamineꢀdiamine species forming blue coloured charge
transfer complex (~ 652 nm), is stable at pH=4.0. Upon increasing the pH, the stability of the
spectroscopic observable peak (~652 nm) decreases and thus we observe a decrease in
3
5
intensity. The mechanism of oxidation of TMB is given below.
2 2
Kinetic study with the variation of both TMB concentration (80ꢀ320 ꢁM) and H O
concentration (40ꢀ80 mM) has been performed with TemFe A, relatively better catalyst than
other, establishing its peroxidaseꢀlike property.
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7

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Figure 5: Schematic representation of step wise TMB oxidation.
Mechanism of Peroxidase-like activity:
The peroxidaseꢀlike catalytic activity in TemFe may be generated from its surfaceꢀattached
+
2
Fe ions. The mechanism may follow Fenton’s reaction. The reaction can be written as (6)
and (7), where k and k are reaction rate constants.
1
2
+
2
+3
ꢀ
ꢀ1 ꢀ1
Fe + H O = Fe + ȮH + OH ; k = 76 L mol S
(6)
(7)
2
2
1
+
3
+2
+
ꢀ1 ꢀ1
2 2 2
Fe + H O = Fe + ȮOH + H ; k = 0.002 L mol S
Step (7) has a low rate constant and thus become a rateꢀlimiting reaction process. So, the
possible reaction mechanism can be elaborated following two cascade steps. At first, H
get attached to the surface of the catalyst. Then Fenton’s reaction takes place at the NPs
2
O
2
+2
surface. The Fe ion generates the highly active ȮH (hydroxyl radical) species which
36
catalyses the oxidation of TMB. After oxidation, the blueꢀcoloured OxꢀTMB has been
thoroughly investigated spectroscopically. To justify the proposed mechanistic pathway, we
have also performed oxidation of terephthalic acid (TA) by H under same experimental
condition in presence of catalyst. Though TA is a nonꢀfluorescent compound, but reaction
2
O
2
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8

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with highly reactive ȮH radical, it generates 2ꢀhydroxyterephthalic acid (HTA) which is a
3
7
highly fluorescent compound. In presence of catalyst, we observed enhanced fluorescence
due to HTA which justifies the enhanced production of ȮH radical in presence of catalyst
(Figure S9). Thus, ȮH radical mediated mechanism for the oxidation of TMB with catalyst
+2
has been justified unequivocally. A direct surface attached Fe concentration dependent
reaction mechanism for TMB oxidation catalysis can be explained explicitly from the XPS
+2
+3
spectra (Figure 3). The [Fe ]/[Fe ] ratio give the conclusive explanation of higher catalytic
activity of TemFe A with respect to others when the TMB oxidation reaction goes according
to Fenton’s reaction as proposed. In presence of H O and TemFe catalyst in acetate buffer
2
2
(
pH= 4.0) medium, TMB catalytically oxidises into blueꢀcoloured OxꢀTMB arising a peak at
652 nm in 20 minutes time duration. It is important that presence of both catalyst and H
can oxidise TMB in this reaction condition. In absence of any of these, oxidation of TMB
~
2 2
O
into OxꢀTMB has become futile. It was also observed that rate of oxidation is dependent upon
the concentration of H O . So, we can also estimate spectroscopically the concentration of
2
2
H O observing the catalytic oxidation of TMB.
2
2
Steady State Kinetic Study:
The steady state kinetics of TMB oxidation has thoroughly done by using TemFe A as
catalyst at RT keeping the concentration of one substrate constant during the variation of the
concentration of other substrate. The kinetic study of this nanozymatic TMB oxidation
reaction has been studied thoroughly using UVꢀvis spectrophotometer. Following the
LambertꢀBeer’s law, the concentration of the OxꢀTMB product with the molar extinction
ꢀ1
ꢀ1
coefficient of 39000 M cm is calculated from the absorbance value obtained from the UVꢀ
vis spectra with time variation. The obtained data was observed to wellꢀfit into Michaelisꢀ
2 2
Menten equation for both the substrate, TMB and H O , (Figure S10).
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9

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ꢔꢕꢖꢗ[ꢘ]
V = ꢐꢕ
(8)
ꢙ[ꢘ]
The Michaelis–Menten equation describes the relationship between the rate of substrate
conversion by an enzyme and the concentration of the substrate. In this equation, V is the
velocity or rate of conversion, Vmax is the maximum rate of conversion, [S] is the substrate
m
concentration, and K is the MichaelisꢀMenten constant.
V
max is the direct parameter for ensuring the enzymatic catalytic activity. K
important parameter in enzymatic reaction to define the binding affinity of substrate and
enzyme. Lower the value of K , higher is the binding affinity of substrate towards
corresponding enzyme and vice versa. We have also determined the K and Vmax values of
both individual substrate, H and TMB, considering TemFe A as enzyme mimic catalyst
from double reciprocal LineweaverꢀBurk plot (Figure 6a and b). Comparison of the K
m
is also an
m
m
2
O
2
m
38
values between Horseradish Peroxidase (HRP) and TemFe A has been tabled (Table 2) out.
Table 2. Comparison between Apparent Michaelis-Menten Constant (K ) And Maximum Reaction Rate
m
3
8
(
Vmax) Of TemFe A And HRP
TMB
2 2
H O
Catalyst
Vmax
Vmax
K
m (mM)
K
m (mM)
-
1
-1
(
M s )
(M s )
ꢀ
8
6
ꢀ8
ꢀ6
TemFe A
HRP
0.0887
5.90
0.97×10
1.83×10
157.19
0.63
1.284×10
0.225×10
ꢀ
This value helped us for discussing about peroxidase mimicking activity of asꢀsynthesised
catalyst.
m
The K value calculated for TMB as substrate using TemFe A 0.0887 is much less than that
of HRP 5.9. This shows the higher affinity of TemFe A towards TMB with respect to HRP, a
natural enzyme. This result can be explained by the presence of more number of active sites
2
0

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at the surface of TemFe A whereas only one iron site is present in natural enzyme, HRP.
Magnetically active TemFe A can also be separated from the reaction medium using simply
an external magnet which also increases the efficiency. But we observed significantly higher
K value for H O using TemFe A with respect to HRP. This describes that higher
m
2
2
concentration of H O is required to achieve maximal catalytic activity. We have calculated
2
2
the limit of detection (LOD) for both TMB and H
2
O
2
by varying TMB concentration from 0ꢀ
0
.32 mM and H O concentration from 0ꢀ80 mM respectively.
2 2
0
.9
TMB
H
O
2
0
.22
.20
.18
.16
.14
.12
2
y = 0.00914x + 0.10375
0.8 y = 12.23x + 0.07785
0
a
0.7
b
0.6
0.5
0.4
0.3
0.2
0
0
0
0
0
.01 0.02 0.03 0.04 0.05 0.06 0.07
2
4
6
8
10
12
14
-
1
-1
1/[S0] (mM
)
1
/[S ] (mM
)
0
0.24
y = 0.0028x + 0.00598
y = 0.60416x + 0.18119
H O = 15 mM
2 2 (LOD)
0.35
TMB
= 95
LOD)
ꢀ
M
0.20
(
c
d
0.16
0.12
0.08
0.04
0.30
0.25
0.20
10
20
30
40
50
60
70
80
90
0.05
0.10
0.15
0.20
0.25
0.30
0.35
H O concentration (mM)
2
2
TMB concentration (mM)
Figure 6. Representation of steady state kinetic assay using LineweaverꢀBurk model for TMB
oxidation by TemFe A catalyst. (a) The concentration of TMB varied from 0ꢀ0.32 mM with
2 2 2 2
80mM concentration of H O . (b) The concentration of H O varied from 0ꢀ80 mM with
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TMB concentration at 0.08 mM. In both cases, 0.1 mg TemFe A is used as catalyst at pH =
2
4
.0 buffer medium. (c) and (d) denotes the limit of detection (LOD) of TMB as 95 ꢁM (R =
2
0
.96) and H
2
O
2
as 15 Mm (R = 0.96) respectively.
Thioglycolic acid (TGA) Detection:
Thioglycolic acid (TGA) or mercaptoacetic acid (MAA) has commercial importance from
drug to cosmetic industries due its applications as chemical depilatory, hair perming agent
39
etc. Among various compounds which are applicable as relaxers for hard hair (e.g. cysteine,
cysteamine etc.), TGA was found to possess greater efficiency in terms of reduction potential
at all pH values. The chemistry behind its activity is the tendency of breaking of disulfide
bond easily. The whole hardening and softening process of hair has been elaborated as
follows:
KꢀSꢀSꢀK + 2RꢀSH
2KꢀSH + RꢀSꢀSꢀR
(9)
KꢀSH + O
2
KꢀSꢀSꢀK + H
2
O
(10)
where KꢀSꢀSꢀK and KꢀSH denote keratin and reduced keratin respectively. Eqn. (9) and (10)
concisely describe the mechanism of TGA as relaxer. However, exposure of TGA present in
40
cosmetic products towards human health has potential adverse effect. The safety level of
4
1
TGA and thioglycolate salts in cosmetic products is 5%. Thus, the easy and effective
method for detection of TGA is needed for supervising purpose. Among different detecting
methods of TGA, recently employed methods are capillary electrophoresis (CE), high
4
2
performance liquid chromatography (HPLC), ion chromatography etc. In this study, we have
introduced colorimetric detection of TGA using the peroxidase mimic property of TemFe A.
Actually in presence of TGA, hindrance in the formation of blueꢀcoloured OxꢀTMB from
2 2
TMB by H O and catalyst was observed. Practically, TGA restricts the oxidising ability of
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2 2
H O to oxidise TMB. Increasing the amount of the TGA concentration, a linear decrease in
the absorbance of OxꢀTMB was observed which motivated us to go for the colorimetric
detection of TGA using TMB oxidation catalysis. Observation of the change in absorbance at
peak position 652 nm for OxꢀTMB helps us to determine TGA quantitatively by simple UVꢀ
vis spectroscopic study. It has been demonstrated that a linear absorbance vs. concentration
of TGA relationship can be established (Figure 7). From the data, we have determined the
2
limit of detection (LOD) of TGA as 50 ꢁM (R =0.98).
y
= 0.24945 -0.30799x
0
0
0
0
0
0
.25
.20
.15
.10
.05
.00
TGA(LOD)= 50
ꢀM
0
.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
TGA concentration (mM)
Figure 7. Limit of detection (LOD) of thioglycolic acid (TGA) determined as 50 ꢁL by
TemFe A.
Mechanism:
TGA has a redox active thiol side linkage which is prone to oxidation to form disulphide
linkage spontaneously. In this study, the presence of TGA hinders the usual oxidation of
2 2
TMB by H O to blue coloured OxꢀTMB and leads to a path for sensing TGA. The hindrance
in blue colour evolution occurs due to the thermodynamically more spontaneous oxidation of
TGA by H O instead of TMB. The proclamation of feasibility of the oxidation of TGA
2
2
43
instead of TMB by H O is explained by the redox potential values, shown in table 3.
2
2
Thus, thermodynamics drives the oxidation of TGA to generate disulphide linkage by H
presence of TMB. The phenomenon is TGA concentration dependent.
2 2
O in
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Table 3. Reduction potentials of TGA, TMB and H
Reaction
2 2
O
Compound
Reduction
Potential
(
V)
~ ꢀ0.34
.22
HSꢀCH
2
ꢀCOOH (TGA)
2 HSꢀCH ꢀCOOH
2 HOOCꢀCH ꢀSꢀSꢀCH ꢀCOOH
2e
2
2
2
NH
2
NH2
0
NH
H
3
C
3
H C
3
H C
CH
3
CH3
CH
3
2
H
2e
3
H C
3
H C
3
H C
CH
3
CH
3
2
H N
CH
3
2
H N
HN
(TMB)
H
2
O
2
1.76
H O + 2e + 2H
2H O
2
2
2
Thus, with increase in concentration of TGA, tendency of formation of blue coloured Oxꢀ
TMB decreases. So, concentration gradient of TGA is responsible for the UVꢀvis spectra
variation and a calibration curve can easily be obtained. Thus, leads to its application for
colorimetric sensing of TGA.
CONCLUSION:
In conclusion, different magnetically active phase pure αꢀ and γꢀFe O synthesis at gram level
2
3
has been illustrated without entertaining temperature change and tedious hard template
dissolution strategies. The main attraction of this work lies in the fact that the synthesis of
2 3
varying morphology, phase and magnetism, of all the Fe O are described simply as the
exposition of the nature of SB templates with variable chelating imine donor sites and chain
length variation keeping other factors constant. Again SBꢀmodified αꢀ and γꢀFe O driven
2
3
peroxidase mimicking activity leads to the catalysis of oxidation of TMB. This fact leads to
the importance of TemFes in analytical usage of colorimetric TGA sensing with much
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4

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improved magnetic character. Thus, transcending the traditional barrier of templates, SBs as
soft template will play an important role in near future.
ASSOCIATED CONTENT:
Supporting Information:
XRD, FTIR, FESEM, XPS spectra, UVꢀvis spectra, ZFCꢀFC, MꢀH curve of TemFes and
2 3
template free γꢀ Fe O are given.
Author Information:
Department of Chemistry
Indian Institute of Technology
Kharagpur 721302, India
Email: tpal@chem.iitkgp.ernet.in
ACKNOWLEDGEMENT:
CSIR and DST, New Delhi and Indian Institute of Technology, Kharagpur are greatly
acknowledged for financial and instrumental support. We are grateful to Prof. A. Sundaresan
and Mr. Chandan Dey (JNCSAR) for their generous support in the magnetic data
measurement.
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Graphical Abstract
Soft Template Induced Phase Selective Synthesis of Fe O Nanomagnets: One Step
2
3
towards Peroxidase-Mimic Activity Rendering Colorimetric Sensing of Thioglycolic
Acid
Anindita Roy, Ramkrishna Sahoo, Chaiti Ray, Soumen Dutta and Tarasankar Pal*
Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India
2 3
Peroxidase-mimicking property of soft template mediated phase selective α- and γ-Fe O
magnetic nanoparticles are capable of colorimetric sensing for thioglycolic acid (TGA) down
to μM level.