Green fabrication of 3-dimensional flower-shaped zinc glycerolate and ZnO microstructures for p-nitrophenol sensing

Article abstract of DOI:10.1039/c5ra06286a

The solvent or reaction medium always plays a lead role in synthesis chemistry. Glycerol has been studied as a green solvent for different organic transformations and is also expected to give interesting control in material synthesis. In this study, we use aqueous glycerol to synthesize zinc glycerolate and the corresponding ZnO micro-flower structures with an intention to encourage the utilization of glycerol as a green reaction medium in material synthesis. A zinc ammonium complex is used as a source of zinc, which converts to zinc glycerolate in the presence of glycerol. Glycerol plays a dual role as a reactant to form zinc glycerolate and as a solvent to control the morphology. The unreacted glycerol is recovered after the reaction and reused further. The flower-structured zinc glycerolate and ZnO are then used for the first time to modify a glassy carbon electrode to make a binder-free non-enzymatic amperometric chemical sensor for p-nitrophenol that is a brutal environmental pollutant. The modified electrode is found to be an excellent alternative for the purpose with respect to sensitivity, selectivity and stability.

Full text of DOI:10.1039/c5ra06286a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Green fabrication of 3-dimensional flower-shaped
zinc glycerolate and ZnO microstructures for p-
nitrophenol sensing†
Cite this: RSC Adv., 2015, 5, 37721
*
Apurba Sinhamahapatra, Dhrubajyoti Bhattacharjya and Jong-Sung Yu
The solvent or reaction medium always plays a lead role in synthesis chemistry. Glycerol has been studied as
a green solvent for different organic transformations and is also expected to give interesting control in
material synthesis. In this study, we use aqueous glycerol to synthesize zinc glycerolate and the
corresponding ZnO micro-flower structures with an intention to encourage the utilization of glycerol as
a green reaction medium in material synthesis. A zinc ammonium complex is used as a source of zinc,
which converts to zinc glycerolate in the presence of glycerol. Glycerol plays a dual role as a reactant
to form zinc glycerolate and as a solvent to control the morphology. The unreacted glycerol is
recovered after the reaction and reused further. The flower-structured zinc glycerolate and ZnO are
then used for the first time to modify a glassy carbon electrode to make a binder-free non-enzymatic
amperometric chemical sensor for p-nitrophenol that is a brutal environmental pollutant. The modified
electrode is found to be an excellent alternative for the purpose with respect to sensitivity, selectivity and
stability.
Received 8th April 2015
Accepted 17th April 2015
DOI: 10.1039/c5ra06286a
www.rsc.org/advances
which make it greener and sustainable solvent in comparison to
other organic solvents and polyol (e.g. ethylene glycol) that are
Introduction
oen used in material chemistry. Therefore, use of glycerol as
solvent is an interesting area of research and attracting more
attention to develop greener protocols for material synthesis.
Glycerol also shows good chelating behavior towards transition
metal which can add further interest to the synthesis of mate-
rials with controlled morphology.
Here, it should be also mentioned that the research on the
utilization of glycerol is also provoked by the continuous
growing demand of biodiesel in modern days as it is a major
side product obtained during production of biodiesel. In fact,
almost 1 kg of glycerol is generated during preparation of ꢁ10
kg of biodiesel.10
The solvent plays pivotal roles in deciding the chemical struc-
ture as well as morphology of the nal product during material
preparation.1–9 It also determines the post reaction factors, like
choice of work-up procedures, recycling or disposal strategies.
On the other hand, a large amount of solvent used for chemical
process is a big threat to the environment. Considering the
environmental legislation, the search for green solvents has
been the most important and challenging task for researchers
recently.1,2 However, material chemists are comparatively less
concerned about the use of green solvent although water,
supercritical CO2 and ionic liquids are commonly used as green
solvents in material preparations.1,2
In recent days, besides organic synthesis, the use of glycerol
also started in material synthesis as a novel solvent system. One
of the important recognized uses of glycerol in material chem-
istry is synthesis of metal nanoparticles with high purity.1 High
purity Cu, Ag, Au, Pt and Pd nanoparticles were synthesized in
glycerol with high yield.12–14 Interestingly, glycerol also acts as a
reducing agent. Further, different nanostructured materials
were synthesized using glycerol in the presence of co-solvent
like water, ethanol or any nonpolar solvent.6,15–26 Co-solvent
also plays an important role when glycerol is used as solvent.
Not only plummeting the viscosity of glycerol, but it also helps
to form emulsion which has a denite impact on the
morphology of the nal product.24 Glycerol was also used as a
medium for synthesis of polypyrrole nanospheres and carbon
dots.27–29
In this regard, glycerol can be good choice as green
solvent.1,3,10 Nowadays, the use of glycerol has already gained
interest in organic synthesis due to its unique properties like
remarkable solubility, very high boiling point (290 ꢀC), low
vapor pressure.1,3–6,10,11 In addition, glycerol is nontoxic, bio-
degradable, non-ammable, safe to use and largely available,
Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science &
Technology (DGIST), Daegu, 711-873, Republic of Korea. E-mail: jsyu@dgist.ac.kr;
Fax: +82-53-785-6409; Tel: +82-53-785-6443
† Electronic supplementary information (ESI) available: Comparison of present
and earlier reported methodology of zinc glycerolate prepared with different
synthesis conditions, synthesis condition in details, thermal analysis, probable
chemical reactions involved during synthesis of materials and results of
temperature dependency, interference and stability study of the modied
electrode. See DOI: 10.1039/c5ra06286a
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 37721–37728 | 37721

View Article Online
RSC Advances
Paper
In this context, we would like to report the use of glycerol– very difficult to degrade.43 Among these, PNP is the most atro-
water as a green solvent for the fabrication of novel ower-like cious and listed as priority pollutant by the United States
zinc glycerolate and corresponding zinc oxide (ZnO) micro- Environmental Protection Agency (USEPA). Therefore, it is
structures from zinc ammonium complex in reux method. highly desired to develop efficient, reliable, robust and highly
Present study also recognizes glycerol–water system as greener sensitive sensor for the detection of PNP.50 In this study, the
reaction medium for material synthesis which is rarely dis- synthesized zinc glycerolate and corresponding ZnO ower
cussed before. The study also discusses role of water and glyc- structures are used to modify glassy carbon electrode (GCE) for
erol for the formation of ower-like morphology. The reusability PNP sensing for the rst time.
of the glycerol is also studied as one of the key features of green
solvent. Successful reuse in the present work has special
meaning as the reuse of glycerol for material synthesis rarely
been approached earlier. ZnO is a potential and commonly
Experimental
Materials
studied n-type semiconductor having a large band gap of 3.37
eV. It has been investigated for wide range of applications such
as in sensors, piezoelectric and photoelectric devices, solar
cells, light emitting diodes and catalysis.29 Due to its applica-
bility and unique properties, fabrication of nanoscale ZnO is all
time favorite area of research and several shapes for nanoscale
ZnO have been introduced.24,28–34 However, three-dimensional
(3D) hierarchical assembled porous structures are always gain-
ing some special attention due to some specic applications
like sensors, lithium-ion batteries, solar cells and catalysis.29,30
Therefore, development of new synthetic methodology for 3D
nanostructured ZnO is always desired. Additionally, zinc glyc-
erolate is also an important compound in bio-medical eld. It is
one of the important slow-release nontoxic sources of thera-
peutic zinc with antiarthritic, antiulcer, and antiinammatory
activities.35 Recently, zinc glycerolate was also found to play as
heterogeneous catalyst for the synthesis of fatty acid methyl
esters and formation of glycerol carbonate.36,37
All the chemicals were obtained from either Sigma-Aldrich or
Samchun chemicals (South Korea) and used as received.
Synthesis of ower shaped zinc glycerolate and ZnO
microstructure
In a typical procedure, 0.3 mM of zinc nitrate was dissolved in
20 ml water, and excess amount of 30% (v/v) ammonium
hydroxide was added drop-wise with continuous stirring to get a
clear solution of metal ammonium complex (pH ꢁ 9). 40 ml of
glycerol was added to the clear solution and stirred for 30 min.
The nal solution was either stirred at room temperature or
ꢀ
reuxed or kept in autoclaved at 100 C for 12 h. Aer that, a
white precipitate was obtained, which was ltered and washed
thoroughly with water and ethanol. The material was dried at
80 ꢀC for 12 h to afford ower-shaped zinc glycerolate, and
ꢀ
on subsequent calcination at 500 C for 4 h in aerobic condi-
tion, ZnO was obtained with identical morphology. The as-
synthesized zinc glycerolate samples are denoted as ZG-X
(where X = 1, 2, 3......). To study the formation of ower shape,
the synthesis was performed in different time intervals, keeping
other parameters constant.
Here, it should be mentioned that polyol-mediated synthesis
of ZnO was reported earlier.25,26,36–42 However, these reports have
rarely discussed the sustainability of the develop protocols and
also oen used glycol or poly glycol as solvent which is more
hazardous compared to glycerol. Furthermore, most reports on
synthesis on zinc glycerolate and corresponding ZnO with
Characterization
different morphology did not recognized the use of glycerol as The Scanning Electron Microscope (SEM) images of the
green medium for material synthesis.25,35,38–40 One of the main synthesized zinc glycerolate and zinc oxide samples were
intension of the present work is to establish and point out obtained using Hitachi Scanning Electron Microscope (SEM) S-
glycerol as an efficient green solvent system for synthesis of 4700, operated at an acceleration voltage of 10 kV. Transmission
materials with controlled morphology. It is also noticed that the Electron Microscopy (TEM) images were collected using JEOL
earlier process affords specied morphology of zinc glycerolate FE-2010, operated at 200 kV. Powder X-ray diffraction (XRD)
at much higher temperature and differs from the present data were recorded using a Rigaku Smartlab diffractometer with
developed methodology in respect of synthesis conditions Cu-Ka (0.15406 nm) operated at 40 kV and 30 mA at a scan rate
(Table S1, ESI†).
of 4ꢀ minꢂ1. The nitrogen adsorption–desorption isotherms of
ꢀ
Furthermore, the synthesized ower-shaped zinc glycerolate the samples were measured at ꢂ196 C using a Micromeritics
and ZnO microstructures are used to make binder-free glassy ASAP 2460 accelerated surface area and porosity analyzer aer
carbon electrode (GCE), which can be used as an electro- the samples were degassed at 150 ꢀC to 20 mTorr for 12 h. The
chemical sensor for p-nitrophenol (PNP). Nitrophenols are specic surface area was determined based on Brunauer–
found in both terrestrial and aquatic environments and Emmett–Teller (BET) method from nitrogen adsorption data in
considered as serious environmental contaminants, which are the relative pressure range from 0.05 to 0.2. Pore size distribu-
mainly comes from the industrial sectors like pesticides, dyes, tion was derived from desorption branches by the Barrett–Joy-
and pharmaceuticals.43–50 Convulsion, cyanosis, coma, and even ner–Halenda (BJH) method. Thermal Gravimetric Analysis
death for human beings can happen on contact to these (TGA) was carried out on a Bruker TGꢂ-D1 TA3000SA thermal
compounds by inhalation, ingestion, eye or skin.43,44 These are analyzer at a heating rate of 10 ꢀC min under owing N2,
ꢀ
mostly highly toxic in nature, highly soluble in water, showing increasing from room temperature to 600 C to investigate the
carcinogenicity, remain for a long time in environment and are decomposition of zinc glycerolate materials.
37722 | RSC Adv., 2015, 5, 37721–37728
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Fabrication of modied electrode for PNP sensing and
electrochemical measurements
For the detection of PNP, glassy carbon electrode (GCE) having
surface area 0.0707 cm2 was modied with resultant nano-
structured materials and the modied electrode was designated
as sample name/GCE. Before modication, the surface of GCE
was polished with 0.05 mm alumina slurry followed by rinsing
with water thoroughly, and dried at room temperature. 5.0 mg
of ower-shaped ZnO was dispersed in 1.0 ml water and soni-
cated for 10 min. 5 ml of suspension was dropped on the cleaned
GCE surface and dried at room temperature for 24 h. All the
electrochemical experiments were performed at room temper-
ature with
a conventional three-electrode system, which
consists of modied GCE as a working electrode, a Pt wire as a
counter electrode and a Ag/AgCl (saturated KCl) electrode as a
reference electrode, using an electrochemical workstation
(VMP3, Biologic) in phosphate buffer solution (PBS) of pH 7.
Results and discussion
Synthesis and characterization of the materials
In the present study, glycerol–water was used as a reaction
medium for the synthesis of ower-shaped zinc glycerolate and
ZnO with an aim to encourage the utilization of glycerol for the
fabrication of nanostructured materials as a green solvent. The
previous studies,15–28,36–41 which uses glycerol as solvent for
material synthesis, rarely explore or deliberate it as a green
reaction medium for nanostructured material synthesis.
Besides, as a reaction medium, glycerol also acts as a reagent to
form zinc glycerolate in this work. A clear solution of zinc
ammonium complex was prepared from Zn(NO3)$6H2O and
ammonium hydroxide and used as a source of zinc. The freshly
prepared clear solution of zinc ammonium complex was mixed
with water–glycerol mixture and kept in different conditions to
obtain a white precipitate of zinc glycerolate which produced
ZnO on subsequent calcination.
Fig. 1 SEM images of synthesized zinc glycerolate microstructures.
that the required temperature is comparatively lower than that
of the previously reported process.25,35,38–40 Besides, it was also
observed that the yield of obtained material was quite low for
the synthesis performed at room temperature.
Aer that, different samples were synthesized by varying the
ratio of glycerol and water using reux (ZG-2, ZG-4, ZG-5, ZG-6,
ZG-7) and solvothermal (ZG-3, ZG-8, ZG-9, ZG-10, ZG-11) treat-
ments. The SEM images of the synthesized materials clearly
show that different denite 3D ower-like morphologies are
obtained with different ratios of glycerol and water in reux
condition. Only the sample ZG-2 and ZG-6 look somewhat
similar with uniform ower-structure. This dissimilarity in
morphology of the synthesized samples proves the dependency
of glycerol–water ratio. It can be also realized that glycerol and
water both inuences on the obtained morphology. In case of
solvothermal synthesis, somewhat different 3D morphologies
are also observed with different glycerol water ratios. However,
uniform denite morphology is obtained only for the sample
ZG-3 and ZG-10. These used the same ratios (glycerol : water
ratios of 1 : 0.5 and 1 : 1) that were used for ZG-2 and ZG-6,
respectively. These results clearly indicate that for the present
study, the above mentioned ratio of glycerol and water is able to
produce uniform denite 3D ower morphology of zinc glyc-
erolate. It also suggests that in the present case, reux condition
is comparatively better than solvothermal treatment. Further
study was carried out using different zinc sources like ZnCl2,
Different experiments are designed by varying the parame-
ters to synthesize zinc glycerolate. SEM images of different
synthesized zinc glycerolate microstructures are represented in
the Fig. 1 and specic reaction conditions with obtained
morphology are summarized in Table S2 (ESI†). At the earlier
stage, three different sets of reaction were performed using
simple stirring at room temperature, reux and solvothermal
treatment at 100 ꢀC, respectively with other parameters and
reactant concentration kept the same. The respective samples
are ZG-1, ZG-2 and ZG-3. Short rod-like structures without any
denite 3D morphology were obtained at room temperature,
while nice ower-like morphology with uniform size and shape
was observed for the sample synthesized by reux condition. In
solvothermal condition, comparatively bigger and compact
ower-like morphology was obtained. It looks like rod-like
primary structures assemble to form ower-like morphology
upon increasing the reaction temperature. So, to obtain denite
3D controlled morphology of zinc glycerolate from zinc
ammonium complex and glycerol, it seems that temperature
ꢀ
(100 C) treatment is necessary. Here, it should be mentioned
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 37721–37728 | 37723

View Article Online
RSC Advances
Paper
Zn(O2CCH3)2(H2O)2 and ZnO instead of Zn(NO3)$6H2O in reux (DTG) analysis shows a peak at 383 ꢀC. This value conrms the
condition. The study reveals that with other zinc salt, somewhat formation of zinc monoglycerolate.36
deformed ower like morphology (ZG-12, ZG-13) was obtained,
The corresponding TEM images of the calcined ZG-2 sample
or no specied uniform morphology was observed from ZnO are presented in Fig. 3, while TEM image of corresponding ZG-2
(ZG-14). This suggests that the presence of other anions in the precursor is represented in the inset of Fig. 3a. The low reso-
system during the formation of material has some inuence on lution TEM image (Fig. 3a) of calcined ZG-2 shows the ower-
the nal morphology of materials. The presence of different shaped morphology resembling the corresponding SEM image
anions may inuence the formation of zinc glycerol complex in Fig. 2b. The arrangement of petals are clearly perceptible in
and consequently nal morphology of the obtained materials. higher resolution image (Fig. 3b). It also indicates that the
This can be strongly supported by no specied morphology petals are single crystalline rather consist of smaller crystallite,
obtained when ZnO is used as starting material as there is no which further proved by the continuous lattice fringe observed
extra anion (like, NO3–, Cl– or CH3 COO–) present in the system. in the petals (Fig. S2, ESI†). Presence of distinct lattice fringes in
The powder X-ray diffraction (XRD) patterns of ZG-2 and the high-resolution TEM image conrms high crystallinity of
calcined ZG-2 are presented in Fig. 2a. The bottom XRD pattern ZnO structure. The lattice fringe spacing of 0.26 nm is in good
reveals specic characteristic peaks of zinc glycerolate for the agreement with the d-spacing between the (0 0 2) planes of
sample before calcination. The characteristic diffraction peaks hexagonal wurtzite ZnO (Fig. 3c) in support of XRD pattern. The
at 2q ¼ 10.9ꢀ, 20.67ꢀ, 17.12ꢀ, and 27.53ꢀ can be indexed to (1 selected area electron diffraction ring pattern indicates poly-
0 0), (1 1 1), (0 0 1) and (2 1 1), respectively, according to the crystalline nature of the ZnO ower (Fig. 3d).
ICDD no. 23-1975. This strongly suggests the formation of zinc
The N2 adsorption–desorption analysis of ZG-2 and calcined
mono glycerolate (Zn(C3H6O3)). Relative high intensity of the ZG-2 samples results in BET surface areas of 1.3 and 27 m2 gꢂ1
,
signal at 2q ¼ 10.9ꢀ also suggest crystallographic preferred respectively. The low surface area of zinc glycerolate was also
orientation along [1 0 0] axis.36 This phenomenon can be previously reported and this may be due to the presence of
correlated to directional growth of petals for formation of organic species and low porosity. During heat treatment in
ower-like microstructure as seen in SEM image. To obtain the aerobic condition, the huge weight loss occurs due to glycerol
ower-shaped ZnO, zinc glycerolate precursor was calcined at decomposition to CO2 to produce ZnO and the ower structure
500 ꢀC for 4 h in aerobic condition. The powder XRD pattern of is more opened up like blossoming of owers as observed in
the calcined material shows formation of ZnO with pure zincite TEM (Fig. 3a) and SEM (Fig. 2b) image which leads to higher
phase of hexagonal system (ICDD no. 36-1451). The well- surface area. This phenomena can be supported by observed
resolved peaks with high intensity are indexed to (1 0 0), (0 hysteresis loop in the N2 isotherms with desorption average
0 2), (1 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), (0 pore width (4V/A) of 5.3 nm (the isotherm gure not shown).
0 4), and (2 0 2) planes of the hexagonal structure of ZnO. No
To understand the formation of above presented ower-
peaks corresponding to impurities like zinc hydroxide and zinc shaped morphology in glycerol–water medium, SEM analysis
glycerolate are observed, which indicates the formation of pure was carried out for different samples obtained in different
ZnO crystal phase. The SEM image of calcined ZG-2 material is
presented in Fig. 2b. It is clearly observed that the ower
morphology is retained aer calcination, but the ower is more
opened up in comparison to zinc glycerolate structure. This
phenomenon can be stated as the blooming of owers. The zinc
glycerolate (ZG-2) was further characterized by TGA (Fig. S1,
ESI†). The observed weight loss (ꢁ47%) in the temperature
range of 280 to 480 ꢀC is attributed to the removal of chemically
bonded glycerol moiety. The differential thermal gravimetric
Fig. 3 TEM images of calcined ZG-2 (a and b) and TEM image of ZG-2
Fig. 2 (a) Powder XRD patterns of ZG-2 and calcined ZG-2 and (b) (inset of (a)). Lattice fringe pattern (c) and electron diffraction pattern
SEM image of calcined ZG-2.
(d) of calcined ZG-2.
37724 | RSC Adv., 2015, 5, 37721–37728
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
reaction times with maintenance of reaction parameters used in Here, it should be mentioned that the present nitrate ions may
sample ZG-2 except reaction time. Based on the different SEM participate in stabilization/destabilization process of different
images obtained for zinc glycerolate, a pictorial diagram of step metal complexes involved during synthesis and also help in the
by step formation path is represented in Fig. 4 for the corre- formation of nal morphology as discussed earlier.
sponding ower-shaped morphology. As depicted in Fig. 4, the
Reusability is the one of the important features of green
clear solution of zinc ammonium complex in aqueous glycerol solvent. The reuse of glycerol as solvent for material synthesis
becomes turbid within 0.2 to 0.3 h. No specied morphology is has been rarely approached earlier. Here, it should be
observed at this stage. It looks like small pieces of amorphous mentioned, although glycerol is one of the reactants to form
bean-like sludge. In the next stage around 1 h of reaction, the zinc glycerolate but it is also used as a solvent to obtain the
growth in different directions of the bean-shaped structure is present ower-like morphology. So, we have tried to reuse the
observed, just like petals oriented in different directions of a used glycerol as solvent for further synthesis. For this, aer
ower, which gradually turns into the nal ower-shaped collection of the white precipitate by ltration, the ltrate was
morphology of zinc glycerolate due to crystallographically collected, and the water was removed from the mixture using a
preferred orientation growth in around twelve hour of reaction. rotary-evaporator. The water-free glycerol was then again used
The nucleation and growth can be resembled to the La Mer for synthesis of ZG-2 sample following the same procedure and
concept.51 At the initial stage, the clear solution of zinc as expected, identical morphology was obtained as shown in the
ammonium complex releases ammonia on heating and SEM image of ZG-15 in Fig. 1. The recovered glycerol also was
produces Zn2+ ion, which further reacts with glycerol to produce analyzed by 1-H NMR spectroscopy, and no signicant change
zinc glycerolate. The concentration of zinc glycerolate is was observed. It shows only glycerol protons, and the spectrum
increased steadily with time and form a nuclei via homoge- was similar to commercial pure glycerol that was used in this
neous self-nucleation as concentration reached to super- study.
saturation levels. With continuous supply of monomers, the
nuclei grow in a preferred direction to form the petals of the
owers in glycerol medium. The directional growth to form
Application of the synthesized materials
As a result, interesting 3D ower-shaped zinc glycerolate and
corresponding ZnO are successfully fabricated using aqueous
glycerol. The synthesized ower-shaped zinc glycerolate (ZG-2)
and corresponding ZnO were further investigated for the
fabrication of electrode to sense PNP electrochemically. The
samples were dispersed in water and drop-casted on the clean
surface of GCE to achieve binder-free modied electrodes. The
modied GCE was studied as PNP amperometric chemical
sensor in PBS of pH 7 using cyclic voltammetry. The obtained
cyclic voltammograms in presence and absence of PNP are
presented in Fig. 5.
petals can be supported by the XRD pattern of zinc glycerolate
(Fig. 2a) and its single crystalline nature as observed in the HR-
TEM analysis (Fig. 3 and S2, ESI†). On calcination in air, the
ZnO ower structure is more opened up.
To understand the role of glycerol, the present adopted
synthesis method is analyzed step by step by chemical reactions
involved (Scheme S1, ESI†). In the rst step, zinc nitrate and
ammonium hydroxide produces a clear solution of zinc tetra-
ammonium complex. Aer that, the clear solution was added
to aqueous glycerol. On heating, the zinc tetra-ammonium
complex produces the mono zinc glycerolate due to ligand
exchange reaction with the release of ammonia gas, and a solid
product starts to be generated, which produces ZnO upon
calcination. The formation of mono zinc glycerolate complex is
conrmed by the XRD result of the as-synthesized material.
In Fig. 5a, presented CVs were recorded in absence of PNP. It
is clearly seen that modied electrode ZnO/GCE shows highest
current compared to ZG-2/GCE and bare GCE, indicating that
modication of GCE improves the performance. Representative
Fig. 5 CVs of bare GCE and modified GCEs (a) in the absence and (b)
in the presence of 0.5 mM of PNP in PBS of pH 7 at a scan rate of 50 mV
Fig. 4 Schematic representation for step-wise formation of flower-
s
ꢂ1. (c) Probable electrochemical redox reactions involved for PNP
shaped zinc glycerolate and corresponding ZnO microstructure.
sensing.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 37721–37728 | 37725

View Article Online
RSC Advances
Paper
CVs of different electrodes in the presence of 0.5 mM PNP in The observation clearly indicates that the reaction of PNP over
Fig. 5b clearly show the difference in the current trend, similar the modied electrode is diffusion-controlled process.
a
to the results in the absence of PNP, but with much higher Further, the modied electrode is studied for PNP detection as a
current response and new signals upon addition of PNP. The function of temperature (Fig. S3, ESI†). The CVs at different
CVs taken in the presence of PNP also clearly exhibit a new temperatures show that the current response of oxidation peak
irreversible reduction peak (R) around ꢂ0.74 V and a pair of new O1 and O2 increases with the decrease of oxidation potential
reversible oxidation (O1) and reduction (R1) peak observed at upon increase of temperature. This phenomenon indicated
around 0.145 V and 0.113 V, respectively for the electrode faster oxidation reaction at high temperature. The irreversible
modied by the developed ower-shaped ZnO microstructure reduction peak (R) current also increases similarly, whereas the
(ZnO/GCE). The obtained results are similar to previously reversible R1 peak current decreases with increasing tempera-
reported results using a-MnO2 nanotube modied GCE.44 The ture. The phenomena indicate the decrease of reversibility by
responses towards PNP of synthesized zinc glycerolate and ZnO- accelerating the oxidation step (O1) and decelerating the
modied GCE are quite impressive and make them promising reversible (R1) step with the increase of temperature.
candidates for the amperometric electrochemical PNP sensor.
To further understand the PNP sensing performance, the
ZnO shows better performance compared to zinc glycerolate as developed ower-shaped ZnO-modied electrode is subjected
ZnO is a semiconductor and possesses electron communication to different concentrations of PNP from 0.01 mM to 1.0 mM and
ability, which leads to higher current response.43 The probable CV is recorded in Fig. 6b. From the series of CVs presented in
electrochemical redox reactions44 taking place during sensing Fig. 6b, it is clearly seen that peak currents of the corresponding
are presented in Fig. 5c. The rst step is irreversible reduction redox reaction are increasing with the increase of PNP
(R) of p-nitrophenol to p-hydroxyaminophenol with involvement concentration. The Fig. 6b inset shows the linear relationship of
of 4 electrons and release of one water molecule. The next step the peak current of O1 peak with PNP concentration with a
is reversible 2-electron redox (O1 and R1) transformation in correlation coefficient of 0.994. This observation can be the
between p-hydroxyaminophenol and p-nitrosophenol. Here, it clear evidence of that the present modied electrode ZnO/GCE
should be mentioned that a further oxidation (O2) of phenolic can detect PNP in the linear range of 0.01 to 1 mM. The sensi-
group to a radical species is also occurred, and the irreversible tivity is 404.35 mA mMꢂ1 cmꢂ2, and LOD (limit of detection) is
oxidation peak around 1.1 V can be attributed to this calculated to be 0.013 mM considering initial four points with a
phenomenon.52,53
correlation coefficient of 0.994. Here, it should be mentioned
The performance of ZnO/GCE-modied electrode is further that on further increase in PNP concentration beyond 1.0 mM,
studied in different scan rates from 10 to 500 mV sꢂ1 in the no further increase in current response of O1 peak is observed
presence of 0.5 mM PNP, and the CVs are presented in Fig. 6a. and the linear relation between current and concentration
The values of peak current corresponding to oxidation and (Fig. S4, ESI†) also destroyed (Fig. S5, ESI†). The R1 peak posi-
reduction, increase linearly with the increasing scan rate and tion also shied towards lower potential with large decrease in
exhibit a linear relation with the root of scan rate (Fig. 6a inset). current response. This phenomenon may indicate surface
The peak potential difference (DEp) is founded ꢁ45 mV which is contamination of the modied electrode due to continuous
higher than the expected value (59/2 mV) for a perfectly decomposition of PNP.
reversible reaction. This phenomenon indicated that the elec-
On further investigation, the performance of ZnO/GCE
trochemical reaction at electrode surface is quasi-reversible electrode is studied in the presence of different interferents,
reaction. However, there is no signicant change observed in and no signicant inuence is found in the current of O1 peak
the peak potential difference (DEp) with respect to the scan rate. (Fig. S6a, ESI†), demonstrating remarkable selectivity towards
PNP in presence of similar nitrophenol derivatives. The stability
of the fabricated electrode is also studied and the results are
presented in Fig. S6b (ESI†). It is founded that the peak current
value of O1 peak is decreased gradually aer each consecutive
usage for 7 days, and aer 1 month, it still maintains around
60% of initial peak current. Interestingly, 95% of peak current is
observed for O1 peak when the electrode is stored without use.
For comparative study, the commercial nanocrystalline ZnO
(surface area ¼ 11 m2 gꢂ1, no denite morphology and size,
Fig. S7, ESI†) was also used to modify GCE and studied for PNP
detection under the same conditions. The results show lower
current response (O1 Peak) than that of the present synthesized
ower-shaped ZnO (Fig. S8, ESI†). The enhancement of the
current responses can be attributed to the enhanced electron
exchange ability of well-interconnected networks formed by the
3D ower-shaped ZnO and higher surface area of ower-shaped
ZnO compared to commercial random-shaped nanocrystalline
ZnO. The above results for the present fabricated electrode ZnO/
Fig. 6 CVs obtained (a) in different scan rates (inset: plots of O1 peak
current and R1 peak current vs. square root of scan rate with linear
fitting) and (b) in the presence of different PNP concentrations at 50
mV sꢂ1 scan rate (inset: plot of O1 peak current vs. PNP concentration
with linear fitting) for ZnO/GCE.
37726 | RSC Adv., 2015, 5, 37721–37728
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
´
´
´
GCE are quite impressive compared to the previously reported 10 J. I. Garcıa, H. Garcıa-Marın and E. Pires, Green Chem., 2014,
GCE-based modied electrodes44–48 for PNP sensing, and thus,
16, 1007–1010.
the present ZnO/GCE can be a promising alternative.
11 G. Cravotto, L. Orio, E. C. Gaudino, K. Martina and D. Tavor,
ChemSusChem, 2011, 4, 1130–1134.
12 A. Sinha and B. P. Sharma, Mater. Res. Bull., 2002, 37, 407–
416.
Conclusions
13 A. Sinha and B. P. Sharma, Bull. Mater. Sci., 2005, 28, 213–
217.
14 J. Kou, C. Bennett-Stamper and R. S. Varma, ACS Sustainable
Chem. Eng., 2013, 1, 810–816.
15 G. Galgali, E. Schlangen and S. van der Zwaag, Mater. Res.
Bull., 2011, 46, 2445–2449.
16 J. Yin and H. Cao, Inorg. Chem., 2012, 51, 6529–6536.
17 Z. Zhang, J. Hao, W. Yang, B. Lu, X. Ke, B. Zhang and J. Tang,
ACS Appl. Mater. Interfaces, 2013, 5, 3809–3815.
18 S. Chen, F. Liu, Q. Xiang, X. Feng and G. Qiu, Electrochim.
Acta, 2013, 106, 360–371.
19 Y. Li, X. Yan, W. Yan, X. Lai, N. Li, Y. Chi, Y. Wei and X. Li,
Chem. Eng. J., 2013, 232, 356–363.
20 H. Li, Z. Bian, J. Zhu, D. Zhang, G. Li, Y. Huo, H. Li and Y. Lu,
J. Am. Chem. Soc., 2007, 129, 8406–8407.
In conclusion, ower-shaped zinc glycerolate microstructures
were synthesized using aqueous glycerol as a green reaction
medium for the rst time from zinc ammonium complex. Zinc
ammonium complex afforded zinc glycerolate micro-ower in
the presence glycerol at 100 ꢀC, which is relatively lower
temperature than that of earlier reports. Corresponding ZnO
was obtained upon calcination in aerobic condition. The glyc-
erol–water ratio has inuence on the nal morphology. The
glycerol can be easily recovered and it can be used again. So the
present process is simple, economical and sustainable to ach-
ieve zinc glycerolate and ZnO microstructures. The synthesized
ower-shaped zinc glycerolate and ZnO microstructures were
used for the fabrication of amperometric electrochemical PNP
sensor for the rst time. The modied electrode showed
promising results for PNP sensing with high selectivity and
relatively long-term stability and can be an excellent candidate
for the determination PNP in polluted water. Thus, glycerol as
green medium for material synthesis can nd more attention in
future to develop a sustainable chemical process for safe envi-
ronment. In addition, developed zinc glycerolate and ZnO-
based sensors can also nd interest for monitoring other envi-
ronmental pollutants in sustainable way for the healthy
environment.
˜
21 A. Escudero, E. Moretti and M. Ocanaa, CrystEngComm,
2014, 16, 3274–3283.
22 Z. Ma, G. Shao, Y. Fan, G. Wang, J. Song and T. Liu, ACS Appl.
Mater. Interfaces, 2014, 6, 9236–9244.
23 Y. Yan, Z. Zhou, Y. Cheng, L. Qiu, C. Gao and J. Zhou, J. Alloys
Compd., 2014, 605, 102–108.
24 B. Wang, J. S. Chen, H. B. Wu, Z. Wang and X. W. Lou, J. Am.
Chem. Soc., 2011, 133, 17146–17148.
25 S. Zhang, P. Yang, A. Zhang, R. Shi and Y. Zhu,
CrystEngComm, 2013, 15, 9090–9096.
Acknowledgements
26 R. Moleski, E. Leontidis and F. Krumeich, J. Colloid Interface
Sci., 2006, 302, 246–253.
The work was supported by NRF grant (NRF-2010-0029245) and
Global Frontier R&D Program at Center for Multiscale Energy
System (NRF-2011-0031571) funded by Korean government.
Authors also thank KBSI at Jeonju and Daejeon for SEM and
TEM measurements.
`
27 F. Ghamouss, A. Brugere, A. C. Anbalagan, B. Schmaltz,
E. Luais and F. Tran-Van, Synth. Met., 2013, 168, 9–15.
28 L.-H. Mao, W.-Q. Tang, Z.-Y. Deng, S.-S. Liu, C.-F. Wang and
S. Chen, Ind. Eng. Chem. Res., 2014, 53, 6417–6425.
29 S. K. Arya, S. Sahab, J. E. Ramirez-Vickc, V. Guptab,
S. Bhansali and S. P. Singhe, Anal. Chim. Acta, 2012, 737,
1–21.
Notes and references
30 A. Sinhamahapatra, A. K. Giri, P. Pal, S. K. Pahari, H. C. Bajaj
and A. B. Panda, J. Mater. Chem., 2012, 22, 17227–17235.
´ ˆ
1 Y. Gu and F. Jerome, Green Chem., 2010, 12, 1127–1138.
2 Y. Medina-Gonzalez, S. Camy and J.-S. Condoret, ACS 31 A. K. Giri, A. Sinhamahapatra, S. Prakash, J. Chaudhari,
Sustainable Chem. Eng., 2014, 2, 2623–2636.
V. K. Shahi and A. B. Panda, J. Mater. Chem. A, 2013, 1,
814–822.
´
´
3 A. E. Dıaz-Alvarez and V. Cadierno, Appl. Sci., 2013, 3, 55–69.
4 J. H. Kim, D. Bhattacharjya and J.-S. Yu, J. Mater. Chem. A, 32 S. G. Kumar and K. S. R. Koteswara Rao, RSC Adv., 2015, 5,
2014, 2, 11472–11479. 3306–3351.
5 W. Yanga, Z. Gaoa, J. Maa, J. Wanga, B. Wanga and L. Liu, 33 P.-P. Wang, Q. Qi, R.-F. Xuan, J. Zhao, L.-J. Zhoua and
Electrochim. Acta, 2013, 112, 378–385. G.-D. Li, RSC Adv., 2013, 3, 19853–19856.
6 B. Fang, N. K. Chaudhari, M.-S. Kim, J. H. Kim and J.-S. Yu, 34 X. Dong, Y. Cao, J. Wang, M. B. Chan-Park, L. Wang,
J. Am. Chem. Soc., 2009, 131, 15330–15338.
W. Huanga and P. Chen, RSC Adv., 2012, 2, 4364–4369.
7 N. K. Chaudhari, H. C. Kim, C. S. Kim, J. Park and J.-S. Yu, 35 R. Remias, A. Kukovecz, M. Daranyi, G. Kozma, S. Varga,
´
´
´
´
´
Z. Konya and I. Kiricsi, Eur. J. Inorg. Chem., 2009, 3622–3627.
CrystEngComm, 2012, 14, 2024–2031.
8 N. K. Chaudhari, S. Chaudhari and J.-S. Yu, ChemSusChem, 36 M. Reinoso, D. E. Damiani and G. M. Tonetto, Appl. Catal., B,
2014, 7, 3102–3111. 2014, 144, 308–316.
9 N. K. Chaudhari, M.-S. Kim, T.-S. Bae and J.-S. Yu, 37 T. W. Turney, A. Patti, W. Gates, U. Shaheena and
Electrochim. Acta, 2013, 114, 60–67.
S. Kulasegaram, Green Chem., 2013, 15, 1925–1931.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 37721–37728 | 37727

View Article Online
RSC Advances
Paper
38 H. Dong and C. Feldmann, J. Alloys Compd., 2012, 513, 125– 47 M. A. El Mhammedi, M. Achak, M. Bakasse and A. Chtainia,
129. J. Hazard. Mater., 2009, 163, 323–328.
39 A. Mezni, F. Kouki, S. Romdhane, B. Warot-Fonrose, 48 G. Bharath, V. Veeramani, S.-M. Chen, R. Madhu,
´
S. Joulie, A. Mlayah and L. S. Smiri, Mater. Lett., 2012, 86,
M. M. Raja, A. Balamurugan, D. Mangalaraj,
C. Viswanathan and N. Ponpandian, RSC Adv., 2015, 5,
13392–13401.
153–156.
40 N. Mira, M. S. Niasaria and F. Davarc, Chem. Eng. J., 2012,
181–182, 779–789.
41 I. Trenque, S. Mornet, E. Duguet and M. Gaudon, Inorg.
Chem., 2013, 52, 12811–12817.
49 R. Panigrahia and S. K. Srivastava, RSC Adv., 2013, 3, 7808–
7815.
50 M. M. Rahman, G. Gruner, M. S. Al-Ghamdi, M. A. Daous,
S. B. Khan and A. M. Asiri, Chem. Cent. J., 2013, 7:60, 1–12.
42 B. W. Chieng and Y. Y. Loo, Mater. Lett., 2012, 73, 78–82.
43 F. Karim and A. N. M. Fakhruddin, Rev Environ Sci 51 V. K. LaMer and R. H. Dinegar, J. Am. Chem. Soc., 1950, 72,
Biotechnol, 2012, 11, 261–274. 4847–4854.
44 J. Wu, Q. Wang, A. Umar, S. Sun, L. Huang, J. Wanga and 52 T. Alizadeha, M. R. Ganjalib, P. Norouzib, M. Zareb and
Y. Gao, New J. Chem., 2014, 38, 4420–4426. A. Zeraatkar, Talanta, 2009, 79, 1197–1203.
45 Y. Xu, Y. Wang, Y. Ding, L. Luo, X. Liu and Y. Zhang, J. Appl. 53 K. C. Honeychurch and J. P. Hart, Electroanalysis, 2007, 19,
Electrochem., 2013, 43, 679–687.
2176–2184.
46 Y. Tang, R. Huang, C. Liu, S. Yang, Z. Lua and S. Luo, Anal.
Methods, 2013, 5, 5508–5514.
37728 | RSC Adv., 2015, 5, 37721–37728
This journal is © The Royal Society of Chemistry 2015