DaltonTransactions

PAPER

Cite this: Dalton Trans., 2016, 45,5833

Received 19th February 2016,Accepted 21st February 2016

DOI: 10.1039/c6dt00670a

www.rsc.org/dalton

Non-enzymatic amperometric sensing of glucoseby employing sucrose templated microspheres ofcopper oxide (CuO)†

Mohit Saraf,a Kaushik Natarajana and Shaikh M. Mobin*a,b,c

We report a facile hydrothermal synthesis of copper oxide microspheres (CMS) for the enzymeless

amperometric detection of glucose in an alkaline medium. The crystallinity, morphology and size were

examined by powder X-ray diffraction (PXRD), scanning and transmission electron microscopy (SEM/TEM)

and dynamic light scattering (DLS) techniques, respectively. The fabricated CMS were grafted onto the

working area of a carbon screen printed electrode (CSPE) and covered with a thin Nafion layer (Nafion/

CMS/CSPE), forming a modified carbon screen printed electrode (MCSPE) which acts as a working elec-

trode. Further, the electrochemical behavior of MCSPE was investigated under optimized conditions

through cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), differential pulse

voltammetry (DPV) and chronoamperometry (CA) techniques. The CV results showed a drastic enhancement

of the current response in the presence of glucose. The amperometry results reveal the catalytic ability of

CMS for glucose oxidation with a notable limit of detection (LOD) of 20.6 μM in a wide linear range of

2–9 mM with a high sensitivity of 26.59 μA mM−1 cm−2. Moreover, the anti-interference test confirmed the

selectivity of the fabricated sensor towards glucose in the presence of interfering agents such as uric acid

(UA), ascorbic acid (AA) and dopamine (DA).

Introduction

The design and development of inexpensive, highly sensitive,reliable glucose sensors possessing an excellent selectivity hasbeen a topic of interest for researchers during the last fewdecades.1,2 Due to their high demand, glucose oxidase (GOx)enzyme based sensors were the prime focus and numeroussensors based on different nanomaterials constituting GOxhave been developed.3–8 However, enzymatic glucose sensorssuffer from several drawbacks including an intricate process ofenzyme immobilization, critical operating conditions(optimum temperature, chemical instability and pH) as well ashigh cost.9 Additionally, the activity of enzymes is also affectedby the operating conditions such as temperature, pH, humidityand toxic chemicals. To overcome these issues, many non-enzymatic sensors have been developed consisting of modified

working electrodes by employing metals and metal oxide dis-persed carbon nanotube frameworks,10,11 metal oxides such asNiO,12,13 Cu2O,

14 TiO2,15 CuO,16,17 and ZnO18 and composites

of materials19 for the improvement of electro-catalytic activity,reliability and selectivity toward the oxidation of glucose.20

Among these oxides, cupric oxide (CuO) is a promisingtransition metal oxide which has been explored for a widerange of applications including electrode materials for highperformance lithium ion batteries,21,22 gas sensors,23 high per-formance supercapacitors,24 catalysis,25 H2O2 sensors,26 andphoto-electrochemical sensors.27 CuO of different sizes, struc-tures and morphologies have been reported for glucosesensing applications.16,17,28–30 A non-enzymatic glucose sensorbased on a CuO nanoseed modified electrode has beenreported recently.30 Various methods have been incorporatedfor the fabrication of such morphologies viz. hydrothermal,31

solvothermal,32 template assisted method,33 and reflux con-densation.34 The template mediated process is the mostemployed technique,35–38 where both hard templates likesilica39 and polymer beads40 and soft templates like vesicles41

and micelles42 display remarkable performance in terms ofachieving the desired micro/nanostructure, but the compul-sion of template removal before the product formation makesthe process complex and unfeasible and therefore limits itsadvantages for application purposes.43 In contrast, bio-

†Electronic supplementary information (ESI) available: All other figures. SeeDOI: 10.1039/c6dt00670a

aDiscipline of Material Science and Engineering, Indian Institute of Technology

Indore, Simrol, Indore-452020, IndiabDiscipline of Chemistry, School of Basic Sciences, Indian Institute of Technology

Indore, Simrol, Indore-452020, IndiacDiscipline of Bioscience and Biomedical Engineering, Indian Institute of Technology

Indore, Simrol, Indore-452020, India. E-mail: xray@iiti.ac.in; Tel: +91 731 2438 762

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molecules such as glucose, sucrose and tyrosine have emergedas important templates due to their compelling self-assem-bling properties, unique properties and special structureswhich make them suitable for the construction of complexstructures.44 In this respect, tyrosine was used for the greenfabrication of CuO hollow structures for lithium-ion bat-teries,45 and glucose as a template was used for the synthesisof copper micropuzzles for non-enzymatic glucose sensors.44

However, glucose mediated synthesis suffers from complexredox reactions throughout the carbonization step between themetal precursor and glucose. To overcome this problem, non-reducing sucrose can be incorporated as an alternative pre-cursor in place of reducing glucose to avoid such redox reactionsthroughout the carbonization process.46 Although reports areavailable where sucrose has been applied to the fabrication ofcopper or copper oxide nanostructures,46–49 its use as a tem-plate in the hydrothermal synthesis of such structures, particu-larly for glucose sensing applications, has not been reportedso far to the best of our knowledge.

Herein, we demonstrate a facile hydrothermal synthesis ofcopper oxide microspheres (CMS) from copper nitrate tri-hydrate (Cu(NO3)2·3H2O) as a metal precursor and sucrose(C12H22O11) as a template. The newly constructed CMS wereemployed as a modifier to the carbon screen printed electrode(CSPE) and the modified electrode MCSPE was applied as aworking electrode in electrochemical techniques to demon-strate its enhanced electron transfer capability for the non-enzymatic amperometric sensing of glucose. Thus, theobtained results propose the excellent catalytic activity of CMStowards glucose.

ExperimentalChemicals

Copper nitrate tri-hydrate, sucrose, and sodium hydroxidewere purchased from Sigma and used for experiments asreceived. Nafion, glucose, uric acid (UA), ascorbic acid (AA)and dopamine (DA) were procured from Sigma. All the chemi-cals used in our reactions were of analytical grade and used assuch without further purification. Throughout the synthesis,deionized water (18.2 MΩ cm) was used. DropSens carbonscreen printed electrodes (CSPE) with dimensions of 3.4 ×1.0 × 0.05 cm (working area 4 mm diameter) were procuredfrom Metrohm; their reference electrode is made of silver andtheir counter electrode is made of carbon.

Characterization

Powder X-ray diffraction (PXRD) was performed with a RigakuSmartLab X-ray diffractometer using monochromated CuKαradiation (λ = 1.54 Å). The absorption spectrum was recordedat room temperature on a Varian UV-VIS spectrophotometer(Carry 100 Bio). The surface morphology was investigatedusing a field emission scanning electron microscope (FESEM,Hitachi S4700). Dynamic light scattering (DLS) was performedon a particle size and zeta potential analyzer (Micromeritics

NanoPlus 3) to obtain the particle size. The specific surfacearea and other textual parameters were obtained by theBrunauer–Emmett–Teller (BET) method performed on Auto-sorb iQ, version 1.11 (Quantachrome Instruments). TEMresults were recorded on a transmission electron microscopy(TECNAI-120 kV) system.

Electrochemical measurements

All electrochemical measurements were carried out at roomtemperature on a computer controlled Autolab PGSTAT 204using NOVA software version 1.10. Amperometric measure-ments were carried out under magnetically stirred conditions.

Synthesis of CMS

The synthesis of CMS was performed using a hydrothermalreaction of copper nitrate tri-hydrate in the presence of sucrosefollowed by calcination at 550 °C.46 In the typical synthesis,8.2 g of sucrose (5 mmol) was dissolved in 25 mL deionizedwater and 1.205 g of copper nitrate tri-hydrate (1 mmol) wasdissolved in 10 mL deionized water. Both solutions weremixed properly under stirring for 2 h and the resultant homo-geneous solution was then transferred to a Teflon-lined stain-less steel autoclave and placed in a furnace for 24 h at 180 °C.The product was filtered and washed several times with de-ionized water and ethanol. Subsequently, the product wasdried in an oven for 5 h at 60 °C. Further, the obtained blackpowder was calcined at 550 °C for 4 h to obtain the phase andpurity. The final product was referred to as CMS.

Preparation of MCSPE

Initially, a fresh CSPE was cleaned properly and its workingarea was covered with a smooth film of the prepared CMS/ethanol (5 mg mL−1) suspension. After drying the electrode,Nafion solution (5 μL, 1%) was cast onto the electrode anddried for 24 h to form a homogeneous film on the electrode.The modified electrode is now referred to as Nafion/CMS/CSPE(MCSPE).

Results and discussion

CMS were prepared by a facile hydrothermal reaction of coppernitrate tri-hydrate in the presence of sucrose. Scheme 1 showsthe schematic representation of stepwise formation of CMS.This final product (CMS) was characterized by PXRD, SEM,TEM, BET and DLS techniques. The synthesized CMS weresubjected as a modifier to the carbon screen printed electrode(CSPE) by applying a smooth layer of Nafion, which acts as anadhesive for the CMS film in MCSPE. Nafion plays animportant role as a protective membrane for the MCSPE andalso reduces the effect of interfering species for achieving abetter sensor response. This modified electrode (MCSPE) wasemployed as a working electrode in the electrochemicalworkstation to study the efficient catalytic activity of CMStowards glucose oxidation by using CV, DPV, EIS and CAtechniques (Scheme 2).

Paper Dalton Transactions

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Characterization of CMS

The crystallinity and phase purity of CMS were analyzed byusing a powder X-ray diffractometer (XRD) equipped with Cu-Kα radiation (1.54 Å) in the range of 10–80° as presented in

Fig. 1(a). The diffraction peaks obtained by the PXRD spec-trum can be readily indexed to the monoclinic phase of CuO(JCPDS no. 01-089-5897) with the lattice parameters a =4.6870 Å, b = 3.4220 Å, c = 5.1300 Å, α = γ = 90°, and β = 99.50°.The presence of sharp peaks without any impurity confirmsthe high phase purity and crystallinity of CMS. The averagecrystallite size was found to be 27.4 nm using the Debye–Scherrerformula. The UV-visible absorption spectrum shows anabsorption peak around 280 nm as demonstrated in Fig. 1(b),which is consistent with many other reports.50,51 The FESEMimages of CMS at different magnifications as shown inFig. 1(c and d) reveal the presence of smooth and uniformspheres with diameter ranging from 1 to 6 µm throughout themass. The TEM image further confirms the presence ofsmooth microspheres (Fig. 1(e)). These microspheres arecomposed of aggregated CuO nanoparticles which provideporosity to the material.46 Further, a N2 adsorption–desorptionisotherm was obtained (Fig. S1†) from which the material wasdetermined to be mesoporous in nature with a high surfacearea (294.808 m2 g−1), which is suitable for achieving a betterelectrochemical performance. A Dynamic Light Scattering(DLS) experiment was also performed to ascertain the averagesize of the produced spheres. Fig. S2† shows the sizedistribution curve of CMS from which the average size of thefabricated CMS was found to be 2.2 µm, which is consistentwith the FESEM results.

Characterization of MCSPE

In order to confirm the presence of CMS over MCSPE, XRDspectra of both plane CSPE and MCSPE were recorded(Fig. S3†). To easily distinguish the characteristic peaks ofCMS in MCSPE, the diffraction peak intensities of CMS wereshown to be decreased as presented in Fig. S3(a).† Conse-quently, the XRD spectrum of MCSPE shown in Fig. S3(b)†reveals the presence of CMS along with CSPE, which confirmsthat CMS have been properly grafted onto the CSPE surface.

Electrocatalytic oxidation of glucose on MCSPE

The modified electrode (MCSPE) was employed as a workingelectrode and subjected to electrolytic oxidation of glucose inan alkaline medium (50 mM NaOH solution) by the cyclicvoltammetry technique at a scan rate of 100 mV s−1 as presentedin Fig. 2(a). The MCSPE shows no characteristic peak in theabsence of glucose in Fig. 2(a1), whereas Fig. 2(a2) reveals awell-defined broad oxidation peak at 0.41 V due to the pres-ence of glucose. The aggregated copper oxide nanocrystalsyield an inherent porosity of the shell and provide more activeabsorption sites leading to enhanced catalytic activity whichfacilitates the charge transfer process in glucose sensing andresults in an enhanced current in the presence of glucose.46

The enhanced current in the presence of glucose showsgood electrocatalytic properties of MCSPE, improved sensingperformance and electron transfer capability of CMS towardsglucose oxidation. Fig. 2(b) shows the effect of the scan rate(20–200 mV s−1) of MCSPE in NaOH (50 mM) solution in thepresence of 2 mM glucose. The current of the oxidation peak

Scheme 1 Schematic of the fabrication of CMS.

Scheme 2 Schematic representation of the fabrication of MCSPE.

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Fig. 1 (a) PXRD pattern of CMS, (b) UV-visible absorption spectrum of CMS, (c, d) FESEM images of CMS at different magnifications and (e) TEMimage of CMS.

Fig. 2 (a) CV curves for MCSPE in the absence (a1) and presence of 2 mM glucose (a2); (b) CV of MCSPE at different scan rates from 20 to 200 mV s−1

in the presence of 2 mM glucose, a plot of the current and square root of the scan rate (left inset); (c) DPV curve of MCSPE; (d) schematicrepresentation of glucose oxidation over MCSPE.

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increases linearly with the square root of scan rates, whichsuggests a diffusion-controlled process at the MCSPEsurface.31 Further, DPV was also performed to confirm the oxi-dation potential of MCSPE towards glucose oxidation. Fromthe DPV curve as presented in Fig. 2(c), a broad oxidation peakat around 0.41 V can be observed which is quite well in agree-ment with the results of CV.

The MCSPE represented in Fig. 2(d) was employed forglucose sensing in an alkaline medium. The expected trans-formations on the oxidation of glucose are gluconic acid andother intermediates. Until now, the exact mechanism ofglucose oxidation in an alkaline medium is not known.However, the probable mechanism for glucose oxidation onMCSPE can be explained by the following equation:

CuOþ 2OH� þH2O ! CuðOHÞ4� þ e�

The electrons released during the oxidation of glucosemigrate to MCSPE. In the electro-oxidation process, Cu(II)would be first oxidized to Cu(III) on MCSPE. Thereafter, thisCu(III) catalyzes glucose oxidation to produce glucolactonewhich ultimately transforms to gluconic acid. The mechanismsuggests that the electrochemical analysis of glucose ismediated by the Cu(II)/Cu(III) redox couple.17,52

Electrochemical impedance spectroscopy (EIS) study

The charge transfer capability of MCSPE was further probed byan impedance study. In EIS, the data obtained are generallyexpressed by means of Nyquist plots, where the imaginaryimpedance (Z″, out-of-phase) is plotted against the real impe-dance (Z′, in-phase) at each excitation frequency.53 Generally,Nyquist plots are represented by a series of one or more semi-circles. The diameter of the semicircle observed at higher fre-quencies represents charge transfer resistance (Rct) andindicates the electron transfer limited mechanism. Fig. 3

shows the Nyquist plots of bare CSPE (a, b) and MCSPE (c, d)with an input frequency ranging from 0.01 Hz to 100 kHz inNaOH solution under an open circuit potential. As expected,the bare electrode exhibits very high resistance both in theabsence and presence of glucose; however in the presence ofglucose the resistance decreases, perhaps due to the sensitivityof the electrode towards the redox process in the presence ofglucose. In contrast, the EIS spectra of MCSPE exhibit very lowresistance and better charge transfer efficiency compared tobare EIS as shown in Fig. 3(c and d), where the semicircle con-firms that CMS has been attached onto the CSPE surface. Asexpected, the addition of glucose reduces the resistance due tothe redox process that increases the number of free ions andelectrons in the electrochemical system (which is consistentwith an increase in current as observed in amperometry). Anequivalent circuit obtained by fitting the EIS data using theSimplex method is shown in the inset of Fig. 3. The resultsconfirm that CMS is favorable for charge transfer between theelectrode and the electrolyte medium. Further, capacitancevalues (Cdl) increase markedly with MCSPE compared to thebare electrode, indicating that a space charge region has beenformed, which is typical of crystalline semiconductingmaterials. It is also observed that Cdl increases with theaddition of glucose for the modified electrodes, which indi-cates the formation of an ionic atmosphere at the electrode/electrolyte interface, possibly due to deposition of a thin ioniclayer on the surface. Table S1† summarizes the data obtainedafter fitting EIS data to the presented circuit.

Amperometric detection of glucose

The chronoamperometry technique was employed to deter-mine the detection of glucose by employing MCSPE with suc-cessive injections of 1 mM glucose in a 50 mM NaOH solutionafter every ∼30 s at an applied voltage of 0.41 V (Fig. 4). Priorto this, in order to obtain the best amperometric response, the

Fig. 3 Nyquist plots of (a, b) bare CSPE and (c, d) MCSPE in theabsence and presence of glucose, respectively, the left inset shows thefitted circuit and the right inset shows a magnified view of MCSPE (c, d)at higher frequencies.

Fig. 4 Amperometric response of MCSPE towards glucose in the sup-porting solution of NaOH (50 mM), calibration curve between theamperometric responses and the glucose concentrations (left inset).

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NaOH concentration was optimized for the CMS towardsglucose oxidation by employing three different concentrationsi.e. 30 mM, 50 mM and 100 mM (Fig. S4†). From the compari-son of amperometric responses, it is clear that MCSPE showeda stable and sensitive response towards successive glucoseinjections for 50 mM NaOH solution as presented in Fig. S4(b)† and an insensitive and unstable response was observedfor other concentrations, which is clearly visible in Fig. S4(a and c),† and therefore 50 mM NaOH was found to be theoptimum for CMS towards glucose oxidation. As expected,MCSPE showed a highly enhanced response to successiveinjections of glucose. The time required to obtain the stableamperometric response was less than 5 s, which signifies thefaster and sensitive response. The calibration curve of thefabricated glucose sensor clearly indicates a linear increase incurrent with glucose concentrations up to 9 mM (inset ofFig. 4); however beyond this concentration the sensor responsedeteriorates, perhaps due to the saturation of active sitespresent on the MCSPE surface. Further, the MCSPE reveals ahigh sensitivity of 26.59 μA mM−1 cm−2 in a wide linear rangeof 2–9 mM and a low limit of detection of 20.6 μM for glucosesensing.

The comparison of MCSPE with some recently reportedglucose sensors based on different metal/metal oxides includ-ing Cu/CuO nanomaterials reveals that the CMS based MCSPEelectrode employed in the present study has an edge over theexisting reported electrodes (Table 1).

Reproducibility and storage stability tests

The amperometric response of four newly prepared MCSPEtowards 2 mM glucose in NaOH (50 mM) solution was testedat 0.41 V applied potential. It was observed that these MCSPEshow negligible variations in amperometric response, whichsuggests that these electrodes can be revised. Similarly, the

electrodes were stored in air tight desiccators and the ampero-metric response was checked twice a week for about 30 daysand negligible variation in response was observed. Thus, theseMCSPE were very stable.

Selectivity test

Selectivity is also a consequential factor for an enzymelessglucose sensor. The test was performed to evaluate the selecti-vity of the fabricated glucose sensor towards some easilyoxidizable, potentially interfering agents such as dopamine(DA), uric acid (UA) and aspartic acid (AA) which generallyco-exist in blood plasma with glucose. According to previousreports, the concentration of glucose in the human blood isconsidered to be 30 times that of these interfering species.58

Although these interfering molecules are present at low con-centration with glucose, they affect the glucose detection sig-nificantly. Therefore, in the present work, the amperometricresponse toward the successive additions of 2 mM glucose,0.1 mM UA, 0.1 mM AA and 0.1 mM DA was tested in NaOHsolution (50 mM) at an applied potential of 0.41 V. A well-defined response was obtained for successive glucoseadditions and an insignificant response was observed frominterfering species (Fig. 5). The anti-interference efficiency ofMCSPE can be assigned to the synergistic effect of CMS, whichprovides a better electron transfer efficiency between glucoseand the working electrode as well as the protective Nafionmembrane present over the modified working electrode. Thenegatively charged Nafion which was used to entrap the CMSover the working area of CSPE plays a pivotal role to reduce theeffects of the interference agents. The Nafion film repelled theinterfering species such as uric acid and ascorbic acid easily.Hence, the selectivity test confirmed that the MCSPE is selec-tive towards glucose and the interfering species do not affectthe sensing activity.

Table 1 Comparison of the amperometric performance of the present sensor (MCSPE) with recently reported glucose sensors based on differentmetal/metal oxides including Cu/CuO

MaterialLinearity(mM)

Sensitivity(μA mM−1 cm−2)

Detectionlimit (μM)

Responsetime (s) Reference

CMS 2–9 26.59 20.6 <5 Present workCuO nanoseeds on Au sputtered glass 0.1–13.3 1101 50 ∼2 30Helical TiO2 nanotube arrays modified by Cu2O 3.0–9.0 14.56 62 3 15Cu doped N2 doped graphene 0.004–4.5 48.13 1.3 <5 54Nanoporous CuO layer modified Cu electrode 0.1–2.04 1066 2 ∼2 55Cu–CuO nanowires 0.1–12 — 50 5 56CuxO/Cu nanoparticles 0–6 1620 49 ∼5 14Cu/CuO/ZnO hybrid 0.1–1 408 18 — 57Chitosan/Au/CuO — 628.34 7.4 — 58Au–zinc oxide 1–20 1.409 20 — 59Ag-reduced graphene oxide 0.5–12.5 0.27 160 — 60Mesocellular graphene 1.0–12 2.87 250 — 61Porous CoOOH nanosheet 0.003–1.109 526.8 1.37 — 62Carbon nanotube–Ni nanocomposites 0.005–2 1384.1 2 3 63Platinum nanoparticle decorated graphene oxide 5–20 137.4 — 5 64Nickel ion implanted-modified indium tin oxide 0.001–0.350 189.5 0.5 — 65DNA-dispersed graphene/NiO hybrid 0.001–8 14.3 2.5 8 66NiO hollow microsphere 0.0167–6.87 2390 0.53 3 67Fe2O3 nanowire arrays 0.015–8 726.9 6 <6 68

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Conclusions

In summary, we have reported a facile synthesis of copperoxide microspheres (CMS) by acquiring copper nitrate tri-hydrate as a metal precursor and sucrose as a template. Fur-thermore, the obtained CMS were used for the modification ofthe CSPE with the protective Nafion membrane and the thusfabricated MCSPE was employed for glucose detection. Theapplicability of the fabricated electrode in enzymeless glucosedetection was examined by the CV, DPV, EIS and CAtechniques in an alkaline medium. Under the optimized con-ditions, the fabricated sensor can detect glucose in a widedynamic linear range of 2–9 mM with a good sensitivity of26.59 μA mM−1 cm−2 and a low detection limit of 20.6 μM withquick response characteristics (<5 s). The storage stability andreproducibility confirmed the robustness and repeatability ofthe developed sensor. Furthermore, the distinguished selectivitywas also observed for the MCSPE towards glucose over otherinterfering compounds such as DA, AA, and UA. Finally, thesensing performance of the present sensor was compared withmany other copper as well as other metals based sensors. Fromthe comparison, it can be concluded that in the absence of anyother conducting material as composites, CMS alone can be apromising candidate for non-enzymatic glucose sensors.

Acknowledgements

S. M. M. would like to acknowledge CSIR, New Delhi and IITIndore for funding and the Sophisticated InstrumentationCentre (SIC), IIT Indore for providing the characterizationfacility. M. S. would like to thank MHRD, New Delhi, India forproviding a fellowship. We thankfully acknowledge the AdvanceImaging Center, IIT Kanpur for providing the TEM facility. M. S.also thanks all the group members for their support. We thankDr. Tridib Kumar Sarma for providing DLS facility.

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Fig. 5 Amperometric response of MCSPE with successive injections of2 mM glucose, 0.1 mM UA, 0.1 mM AA, and 0.1 mM DA in NaOH solution(50 mM) at an applied potential of 0.41 V.

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Paper Dalton Transactions

5840 | Dalton Trans., 2016, 45, 5833–5840 This journal is © The Royal Society of Chemistry 2016

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