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Cite this: Dalton Trans., 2017, 46,15848

Received 16th October 2017,Accepted 19th October 2017

DOI: 10.1039/c7dt03888g

rsc.li/dalton

Small biomolecule sensors based on an innovativeMoS2–rGO heterostructure modified electrodeplatform: a binder-free approach†

Mohit Saraf, a Kaushik Natarajan,a Anoop Kumar Saini b andShaikh M. Mobin *a,b,c

The requirement of sensitive diagnostic chips for small biomolecules has triggered the urgent develop-

ment of versatile nanomaterial based platforms. Therefore, numerous materials have been designed with

fascinating properties. Herein, we report a facile one-pot synthesis of MoS2–rGO nanoflowers grown by

the hydrothermal method and their applicability in the simultaneous sensing of AA, DA and UA. The struc-

ture and morphology of nanoflowers have been probed by various physico-chemical techniques such as

XRD, SEM/TEM, AFM, Raman and XPS. Furthermore, these nanoflowers were used to construct a glassy

carbon based working electrode (MoS2–rGO/GCE), by a facile drop-casting method in the absence of

any commercial binder. The electrochemical investigations revealed high separating potency of the

MoS2–rGO/GCE towards AA, DA and UA with distinguishable oxidation potentials (AA–DA = 204 mV and

DA–UA = 122 mV) and a notable detection limit and reasonable sensitivity for each of these biomolecules.

The charge transfer resistance and capacitive components obtained by electrochemical impedance spec-

troscopy (EIS) were found to be in agreement with the voltammetric observations. The observed synergy

between MoS2 and rGO opens up new possibilities to consider the MoS2–rGO nanostructures as the

cutting edge material for electrochemical sensor development.

1. Introduction

Ascorbic acid (AA), dopamine (DA), and uric acid (UA) usuallyco-exist in human physiological fluids (serum and urine), andimproper levels of these may cause severe diseases.1–12 AA,being a water-soluble antioxidant, helps in protecting livingorganisms from oxidative stress.12–16 DA is a consequentialneurotransmitter,17–20 and UA is a crucial biomolecule presentin urine and serum.21–27 The imbalance of their contents maycause some fatal diseases, such as cancer, Parkinson’s andseveral other cardiovascular diseases.12–27 These moleculesoxidize at almost the same potential over traditional electro-des, resulting in the overlapping of their oxidation peaks andhence their simultaneous detection is a daunting task.3,4

Owing to poor selectivity, reproducibility and pronounced elec-trode fouling,5,6 simultaneous detection, quantification andeasy separation of these three small biomolecules have been apersistent issue.7,8 In this concern, electrochemical techniqueshave been proven to be an effective toolbox over the existingmethods due to their high sensitivity, selectivity, cost-effective-ness and rapid response.9,10 Various electrode platforms havebeen developed to effectively detect these biomolecules withdistinct oxidation potentials, but they could address only sen-sitive and selective detection.1–10 Moreover, such platformssuffer from some serious loopholes such as requirement oftedious and time-consuming preparation,28,29 pre-treat-ments,30 binders like Nafion31 and incorporation of expensivemetals such as Au and Ag during the modification of the elec-trode.32,33 Hence, the proper selection of materials and binder-fee modification of the electrode surface to achieve dis-tinguishable oxidation peaks with reasonable sensitivity isimportant.1–33

In electrochemical applications, reduced graphene oxide(rGO), owing to its high surface area, excellent conductivityand mechanical robustness, has been widely explored as com-posites with various metallic nanostructures and metal oxidesto synergistically improve its properties.34 Although these com-posites have shown remarkable enhanced properties, their

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c7dt03888g

aDiscipline of Metallurgy Engineering and Materials Science, Indian Institute of

Technology Indore, Simrol, Khandwa Road, Indore 453552, India.

E-mail: xray@iiti.ac.in; Tel: +91 731 2438 762bDiscipline of Chemistry, Indian Institute of Technology Indore, Simrol, Khandwa

Road, Indore 453552, IndiacCentre of Biosciences and Biomedical Engineering, Indian Institute of Technology

Indore, Simrol, Khandwa Road, Indore 453552, India

15848 | Dalton Trans., 2017, 46, 15848–15858 This journal is © The Royal Society of Chemistry 2017

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tedious and time-consuming synthesis as well as sophisticatedequipment cost restrict their advantages. Additionally, theattachment of these composites on the electrode surfaceusually requires additional binders such as Nafion, PVDF, etc.,which further increases the production cost.30–33 Moreover, thehydrophobicity and poor dispersion even after prolonged ultra-sonication of rGO restricts its practicability. Therefore, thesurface modification of rGO with a suitable candidate toimprove its properties is very essential.35 Molybdenum di-sulphide (MoS2), having a similar structure to rGO, with excep-tional physical, electrical and structural properties, hasemerged as a fascinating candidate for being combined withrGO to form a new hybrid with improved characteristics.36

MoS2 is composed of three atomic layers and forms an S–Mo–Ssandwich type structure, in which the hexagonal Mo plane issandwiched between the layers of S atoms and the as-formedtriple layers are bonded by weak van der Waalsinteractions.37–39 It can be assumed that this layered structurehelps proper insertion of foreign atoms and provides structuralrigidity to the whole structure, which is beneficial for theelectrochemical stability of sensor. MoS2 with higher intrinsicionic conductivity than metal oxides adds its advantages forsensors. However, due to the semi-insulating nature, it is notan immediately attractive electrode material.40 The surfacemodification of rGO with MoS2 improves the dispersion andprocessability to exploit its unique nanostructures. Therefore,the composite of MoS2 and rGO is expected to exhibitimproved sensing performance by fully utilizing the benefitsof both structures.35,41a

With the above aspects in mind, we present a facile one-pothydrothermal technique to fabricate a MoS2–rGO compositeencompassing porous nanostructure, mechanical robustness,high surface area and good conductivity. Benefiting from aunique 2D multi-layered nanostructure and the synergisticeffect between the layered MoS2 and rGO nanosheets, the as-prepared MoS2–rGO hybrid exhibits superior electrochemicalperformance towards the simultaneous electrochemical deter-mination of AA, DA, and UA with distinguishable oxidationpotentials. The excellent results prove the remarkable sensingproperties of MoS2–rGO, which are better than other metal/metal oxide based sensors.

2. Experimental2.1. Materials

The materials used in the present work were procured fromMerck and used without any further purification.

2.2. Characterization

Powder X-ray diffraction (PXRD) spectra were recorded on aRigaku SmartLab X-ray diffractometer system incorporatingmonochromated CuKα radiation (λ = 1.54 Å). SEM images andelemental mapping were performed on a Supra55 Zeiss Field-Emission Scanning Electron Microscope equipped with EDXfacility. TEM analysis was performed on a JEOL (JEM-2100)

system. The N2 isotherm was conducted on an Autosorb iQ,version 1.11 (Quantachrome Instruments) and the corres-ponding pore size distribution curve was obtained by theBrunauer–Emmett–Teller (BET) method. The Raman study wasperformed on a STR500 Confocal Micro-Raman spectrometer(Airix Corporation, Japan). X-ray photoelectron spectroscopy(XPS) data were recorded on a PHI 5000 Versa Prob II system.AFM analysis was performed on a Multimode 8, Brukersystem.

2.3. Electrochemical measurements

The electrochemical testing was performed at room tempera-ture on an Autolab PGSTAT 204N using NOVA software (version1.10). Glassy carbon electrodes (3 mm GCE diameter), plati-num foil and Ag/AgCl were assembled as the working, counterand reference electrodes, respectively.

2.4. Synthesis of the MoS2 and MoS2–rGO composites

Initially, graphite oxide (GO) was synthesized using a modifiedHummers method according to our previous reports.42 TheMoS2–rGO composite was produced by a facile one-step hydro-thermal reaction between Na2MoO4·2H2O and NH2CSNH2 con-taining GO. In the present work, Na2MoO4·2H2O (1 g) andNH2CSNH2 (1.2 g) were dissolved into the GO solution (100 mgGO in 60 mL DI), followed by stirring for half an hour. Afterthis, the as-formed mixture was transferred into a steel auto-clave (Teflon-lined) and heated for 24 h at 180 °C under theoptimized conditions as obtained from Fig. S1–S4.† Duringthe reaction, NH2CSNH2 releases H2S, which reduces the GOinto rGO. The obtained black precipitates were cooled down toRT, and washed properly with DI water and ethanol and sub-sequently dried at RT. Bare MoS2 was synthesized by employ-ing the same method in the absence of GO.41a

2.5. Fabrication of electrodes

Initially, properly cleaned glassy carbon electrodes (GCEs) wererinsed several times with DI water and dried. Subsequently,5 mg of MoS2 and MoS2–rGO composite powders were dis-persed separately in 10 mL DMF by ultra-sonication for30 min. Thereafter, under the optimized conditions, 5 µL ofthese stable suspensions (mass loading of 2.5 µg) were drop-cast onto the exposed active part of the GCE and dried in air.Subsequently, these GCEs were rinsed with DI water and driedbefore incorporating them for sensing experiments. Thus, theprepared electrodes were designated as MoS2/GCE and MoS2–rGO/GCE.

3. Results and discussion

The MoS2 and MoS2–rGO composites were synthesized by asimple one-pot hydrothermal reaction between Na2MoO4·2H2Oand NH2CSNH2 in the absence and presence of graphite oxidesolution, respectively, at 180 °C (Scheme 1). Though abottom-up approach, such as chemical vapor deposition(CVD), has been used to produce MoS2 structures or films with

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a high surface area, their production rate is very poor andrequires sophisticated and expensive equipment. In contrast,hydrothermal and solvothermal techniques employed in thiswork have shown remarkable potential owing to their facilenature and cost-effectiveness.43 The synthesized materials werecharacterized by XRD, Raman, SEM/TEM, EDX, XPS, BET andAFM.

3.1. Characterization

The MoS2 and MoS2–rGO composites were firstly investigatedby XRD. In the diffraction pattern of MoS2, the major peakslocated at 14.15°, 33.77° and 58.49° can be attributed to the(002), (100), and (110) planes of MoS2 (JCPDS card no. 37-1492). The peak labelled as (002) at 2θ = 14.15° with a calcu-lated lattice spacing d = 6.26 Å signifies the layered MoS2.

41a Incontrast, the MoS2–rGO composite exhibits two distinct peaksat 2θ = 9.23° and 18.45° with d values of 9.625 and 4.8125 Å,respectively. Such a type of diploid relationship between thed spacings in the composite represents the formation of a newlamellar structure with a broadened interlayer spacing com-pared with that of MoS2.

41b,c Hence, the XRD spectrum provesthat the composite is composed of MoS2 (with an increasedspacing of the 002 plane) and rGO (Fig. 1a).44,45 It can beobserved that the peaks of the composite are comparatively

weaker, indicating the successful incorporation of rGO in thecomposite.41a Additionally, no peaks corresponding to anyimpurity ions were detected, further indicating the phasepurity of the composite. The Raman spectrum of MoS2 exhibitstwo characteristic distinct peaks at 377 cm−1 and 407 cm−1,respectively, due to an in-plane (E2g) mode and an out-of-plane(A1g) mode (Fig. 1b). It is known that E2g belongs to the in-layer displacements of Mo and S atoms, whereas A1g representsthe out-of-layer symmetric displacements of S atoms along thec axis.41a In the MoS2–rGO composite, the characteristic bandsdue to the E2g and A1g modes of MoS2 are clearly observed.46

In addition, D and G bands were also observed confirming thepresence of rGO in the composite. The G band (at 1582 cm−1)arises due to the vibration of sp2 carbon atoms, while the Dband (at 1349 cm−1) emerges due to the defects and disorderof rGO.47,48 The morphology and structure of the preparedmaterials were further probed by SEM and TEM. Fig. 2a–cpresent the SEM images of the bare MoS2 exhibiting a spheri-cal morphology composed of flowerlike nanoflakes grown inhigh density. Fig. 2d–f show the TEM images of MoS2, whichfurther confirms the nanoflake type structure and the aggre-gated nature of MoS2. The SEM images of the composite asshown in Fig. 2g–i suggest that the spherical structures havebeen markedly increased compared to pristine MoS2, which

Scheme 1 Schematic formation of the MoS2–rGO composite.

Fig. 1 (a) XRD and (b) Raman graphs of the MoS2 and MoS2–rGO composites.

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implies that more number of MoS2 participate and that activesubstances can be used more efficiently in electrochemicalprocesses. The MoS2 in the composite mainly consists oflimited-layer MoS2 structures and are tightly coupled with rGOsheets.41a During the hydrothermal process, GO reduces torGO, and due to partial overlapping or coalescing, rGO self-assembles into a flexible architecture. Such a type of overlap-ping or coalescing of the rGO forms an interconnected con-ducting network, and facilitates rapid electron transport,which is beneficial for sensors.35,39 Furthermore, the structureof the MoS2–rGO composites also improves the stability due tothe high mechanical robustness of rGO.4 Such a type of archi-tecture can greatly expand the interfacial contact areas at theelectrolyte/electrode interface and is favourable for improvingthe sensitivity of the electrochemical sensor.4,49 Fig. 2j–l

present the TEM images of the MoS2–rGO composite, exhibit-ing the nanoscaled MoS2 sheets in the form of loose and softagglomerates. Moreover, the TEM images at high magnifi-cation unveil a mild appearance of MoS2 grown on the rGOsheets, which can be distinguished by their brightness.44

Fig. S5† presents the EDX spectra and elemental mappingimages of the MoS2 and MoS2–rGO composite. The SEM imageand the corresponding EDX spectrum of MoS2 are shown inFig. S5a and b,† which shows the presence of molybdenum(Mo) and sulfur (S) atoms. Furthermore, Fig. S5c† presents thered colored mapping image of MoS2 and Fig. S5d and e†demonstrate the elemental mapping images with an extensivedistribution of Mo and S, respectively. Similarly, Fig. S5f andg† show the green colored mapping image of the MoS2–rGOcomposite and the corresponding EDX spectrum. The pres-

Fig. 2 SEM (a–c) and TEM (d–f ) images of MoS2 at different magnifications; SEM (g–i) and TEM ( j–l) images of the MoS2–rGO composite atdifferent magnifications, respectively.

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ence of only Mo, S and C atoms in the EDX spectrum verifiesthe phase purity of the hybrid. Additionally, Fig. S5h–j† showthe elemental mapping images of C, Mo and S, respectively,further confirming the proper distribution of all threeelements in the composite. The chemical states of the MoS2–rGO composites were investigated by XPS analysis. As shownin Fig. 3a, the Mo, S, C, and O elements were detected in thesurvey spectrum, and the atomic ratio of the Mo to S elementwas found to be 1 : 2.202, which was very close to the theore-tical value of MoS2, and also consistent with the EDS results(1 : 2.243). The C (1s) peak of the composite is composed ofC–C, C–O and CvO species at 284.5, 285.86 and 288 eV, respect-ively (Fig. 3b), indicating the existence of the epoxy, hydroxyland carboxyl groups in the rGO. The peaks at 227.12 and231.05 eV can be assigned to the Mo4+ (3d5/2) and Mo4+ (3d3/2),respectively, and the higher energy peak located at ∼235.02 eVshould be attributed to the oxidic Mo, and the lower energypeak at 224.5 eV can be attributed to S (2s) (Fig. 3c). The peakscentered at 161.25 and 162.4 eV in Fig. 3d can be indexed to S(2p3/2) and S (2p1/2), respectively, representing the existence ofS in the MoS2–rGO composites.51 These results clearly demon-strate the possible interactions between MoS2 and rGO in thecomposite as a result of electron transfer between the MoS2and rGO sheets. The XPS results also imply that the formationof the MoS2–rGO composite is not a result of simple physicalmixing of MoS2 and rGO powders. Hence, the electron inter-actions between MoS2 and rGO can markedly enhance compo-site’s conductivity.41a,50,51 The N2 adsorption–desorption iso-

therms of the pristine MoS2 and MoS2/rGO compositemeasured at 77 K are shown in Fig. S6a and c.† The BETsurface areas and average pore diameters of the pristine MoS2and MoS2–rGO composite are 16.5, 18.38 m2 g−1 and 2.02,3.96 nm, respectively. The pore size distributions of the pris-tine MoS2 and MoS2–rGO composite derived using the BJHmethod are given in Fig. S6b and d,† respectively, whichreveals that both the pristine MoS2 and MoS2–rGO compositesare mesoporous in nature. The mesopores of the pristine MoS2may originate from the void spaces among the interlacedflakes.38 It is to be noted that even if these pores may not con-tribute significantly to enhance the surface area of the compo-site, they can provide more shortened paths for ion diffusion,and also promote the easy access of the electrolyte. Inaddition, the increased electrode–electrolyte interfacial areamay result in a great improvement in the sensing performanceof the MoS2–rGO composite.38 Both the MoS2 and MoS2–rGOsamples were further analysed using AFM considering a100 µm × 100 µm surface area (Fig. S7†). The results of thisanalysis are presented in Table S1.† It is found that the compo-site electrode displays a higher effective surface area, which isalso confirmed by the BET analysis earlier. Furthermore, thehigher values of roughness parameters obtained for the MoS2–rGO films imply a higher conductivity and better attachmentof the rGO to MoS2, as was found in previous studies involvingother semiconductors.52,53 The entropy deficit for the MoS2–rGO structures confirms the presence of two phases in thecomposite electrode (as seen in the attached ESI†).

Fig. 3 XPS spectra of the MoS2–rGO composite (a) survey spectrum, and (b) C 1s, (c) Mo 3d and (d) S 2p.

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After detailed characterization, the MoS2–rGO compositewas used as a modifier to the glassy carbon electrode (GCE) toconstruct the MoS2–rGO/GCE, which was then subjected as aworking electrode for the AA, DA and UA sensors(Scheme S1†). The SEM image of the MoS2–rGO/GCE shows aclear appearance of the MoS2–rGO nanostructure on the elec-trode surface (Fig. S8†) signifying the potency of the proposedmethod of electrode fabrication.

3.2. MoS2–rGO/GCE as the electrochemical sensor of AA, DA,UA

The working electrodes (MoS2/GCE and MoS2–rGO/GCE) forthe electrochemical sensing experiments were prepared bysimple drop-casting of materials on the working area of theglassy carbon electrode (GCE) as described earlier in theExperimental section. Unlike most of the previous reports,Nafion or any other binder was not used in the present study,which reduces the production cost of the designed sensor. Thesensing results were obtained by cyclic voltammetry (CV) anddifferential pulse voltammetry (DPV) techniques by employingPBS solution (0.1 M, pH = 7). The results were compared withthe bare GCE and MoS2/GCE.

3.2.1. Electrochemical properties of the MoS2–rGO/GCE.The sensing behavior of the bare GCE, MoS2/GCE and MoS2–rGO/GCE towards the AA, DA and UA was firstly indicated bythe cyclic voltammetry (CV) technique (Fig. 4). It can beobserved that the bare GCE and MoS2/GCE exhibit extremely

poor current response towards the individual electro-oxidationof AA, DA, and UA, indicating a lethargic electron transferprocess. The poor response can be attributed to the electrodefouling as a result of the formation of the oxidation product ofthese biomolecules. In contrast, a well-defined response wasobserved at the MoS2–rGO/GCE and clearly visible distinct oxi-dation peaks were observed for each of the analytes, i.e. AA, DAand UA, respectively due to the high electronic conductivityand electrocatalytic activity of the MoS2–rGO composite, whichaccelerates the charge transfer process at the electrode surface(Fig. 4a–c).13

On simultaneous addition of AA, DA, and UA, three distinctoxidation peaks emerged at the MoS2–rGO/GCE, while no sig-nificant peaks were observed for the rest of the electrodes(Fig. 4d). This observation demonstrates that the MoS2–rGO/GCE can easily and effectively distinguish these three bio-molecules by their oxidation potentials, while the bare GCEand MoS2/GCE do not show any separating potency of thesethree biomolecules and exhibit mainly the backgroundcurrent. Furthermore, the effect of scan rates (10–500 mV s−1)on the oxidation peak current for each analyte was also ana-lysed for the MoS2–rGO/GCE (Fig. 5a–c). A linear response wasobserved on the oxidation peak currents with the increasingscan rates corresponding to each biomolecule (Fig. 5d–f ). Thelinear relationship between the current response and squareroot of scan rates advocates a surface-controlled process hap-pening at the MoS2–rGO/GCE.

54 To clearly distinguish the oxi-

Fig. 4 CV profiles of the bare GCE, MoS2/GCE and MoS2–rGO/GCE in the presence of (a) 300 µM AA, (b) 100 µM DA, (c) 200 µM UA, and (d) themixture of 300 µM AA, 100 µM DA and 200 µM UA, respectively, at a scan rate of 100 mV s−1.

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dation peaks, the CV profiles of the MoS2–rGO/GCE are shownat a scan rate of 10 mV s−1 on injection of all three bio-molecules (AA, DA and UA) separately as well as simul-taneously (Fig. S9†). Moreover, the current responses corres-ponding to the oxidation peaks for all three analytes injectedsimultaneously were also recorded, and the linear incrementsin the current responses were observed (Fig. S10†), suggestingthe potential capability of the MoS2–rGO hybrid as a sensingmaterial for AA, DA and UA.

The separating efficiency of the MoS2–rGO/GCE for the oxi-dation of AA, DA, and UA was further probed by differentialpulse voltammetry (DPV) in their mixtures. Three well-resolvedDPV peaks at the oxidation potentials of −26 mV (AA), 178 mV(DA), and 300 mV (UA), respectively, were observed (Fig. S11†).Thus, the peak to peak separating potentials for AA–DA andDA–UA at the MoS2–rGO/GCE are 204 and 122 mV, respectively,which are far enough to distinguish these biomolecules simul-taneously, compared to most of the earlier reports.5–12

3.2.2. Simultaneous determination of AA, DA, and UA atthe MoS2–rGO/GCE. The simultaneous determination of AA,DA, or UA over the MoS2–rGO/GCE was further verified byDPV. In these experiments, the concentration of one analytewas varied while the concentrations of other two analyteswere kept constant (Fig. 6). The AA peak current was foundto be linear to the AA concentrations in the range from 300to 3300 μM with a regression equation of Ip,AA (μA) =0.00205CAA (μM) + 6.828 (R2 = 0.99562) as shown in Fig. 6a.The detection limit (LOD) of AA was calculated to be0.09 μM. The sensitivity was calculated to be 8.2 μA mM−1

cm−2 by dividing the slope of the generated regressionequation with the electrochemically active surface area(ECSA) of the MoS2–rGO/GCE.

Herein, the electroactive surface areas (ECSAs) of the bareGCE, MoS2/GCE and MoS2–rGO/GCE were calculated by per-forming CVs in standard K3[Fe(CN)6] solution (Fig. S12†) andusing the Randles–Sevcik equation as follows:13

Ip ¼ 2:69� 105AD 1=2n3=2γ 1=2C:

In this equation, Ip is the peak current, A is the electroche-mically active surface area ECSA (cm2) to be calculated, γ is thescan rate (V s−1), n is the number of transferred electrons forthe [Fe(CN)6]

3−/4− redox couple (n = 1), and D and C are thediffusion coefficient (6.7 × 10−6 cm2 s−1) and the concentration(mol L−1) of K3[Fe(CN)6], respectively. Accordingly, the calcu-lated ECSA was found be the highest for the MoS2–rGO/GCE(0.2511 cm2) followed by the MoS2/GCE (0.0959 cm2) and bareGCE (0.0707 cm2). This observation suggests that the MoS2–rGO nanocomposite markedly enlarges the specific surfacearea of the electrode due to its porosity and a positive synergybetween MoS2 and rGO.

Similarly, the peak currents for DA and UA were linearlyproportional to the DA and UA concentrations in the range of100 to 1000 and 200 to 3400 μM, respectively (Fig. 6b and c).Thus, the linear regression equations for DA and UA can beexpressed as: Ip,DA (μA) = 0.01299CDA (μM) + 13.810 (R2 =0.99293) and Ip,UA (μA) = 0.00636CUA (μM) + 11.37896 (R2 =0.98751), respectively. The corresponding detection sensi-tivities and the LODs for DA and UA were calculated to be51.9 and 25.4 μA mM−1 cm−2 and 0.126 and 0.106 μM,respectively.

3.2.3. EIS analysis towards AA, DA and UA sensors. EISanalysis was carried out between 0.01 Hz and 100 000 Hz. Theresults were fitted based on a model proposed by Lario-Garcia

Fig. 5 Effect of scan rate on the CV profiles (10–500 mV s−1) of the MoS2–rGO/GCE with injections of (a) 300 µM AA, (b) 100 µM DA, and (c)200 µM UA, respectively. The plots of peak current vs. square root of scan rates for (d) AA, (e) DA, and (f ) UA, respectively.

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and Pallàs-Areny55 for conductometric sensors. The model isdescribed by a “series” resistance (Rs), which is a combinationof resistances from the experimental setup, as well as theresistance added from the electrolyte solution, and a set of par-

allel impedances, represented by RF (which is indicative ofresistance to Faradaic processes), and ZCPE, which represents acomplex element arising from the non-ideal dielectric behav-iour of the material under aqueous media. The complex

Fig. 6 DPVs of the MoS2–rGO/GCE containing (a) 100 µM DA, 200 μM UA and different concentrations of AA, (c) 300 µM AA, 200 μM UA anddifferent concentrations of DA, and (e) 300 µM AA, 100 μM DA and different concentrations of UA, (b, d, f ) show the corresponding calibration plotsbetween the injected concentrations vs. peak current for AA, DA and UA, respectively.

Fig. 7 (a) DPV curves for the MoS2–rGO/GCE in the presence of 300 µM AA, 100 μM DA and 200 μM UA only and 1 mM of various interfering sub-stances such as glucose, citric acid, CaCl2, Na2SO4, ZnCl2, galactose and sucrose, and (b) the peak current ratio of the MoS2–rGO/GCE to 300 µMAA, 100 μM DA and 200 μM UA in the presence of 1 mM of various interfering substances.

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element, also known as the constant phase element (CPE), isdescribed by using the following equation:56

ZCPE ¼ A�1�s�α;

wherein, α is a value between 0 and 1, with 1 indicating aperfect capacitor, and 0 indicating a perfect resistor, and s rep-resents the Laplace transform of the frequency dependentcomponent of the impedance. The EIS results are displayed in(Fig. S13†), and the table corresponding to the values of theparameters obtained by the fitting is shown as Table S2.† It isobserved that the highest faradaic resistance is obtained forthe bare GCE, indicating a high resistance to redox processes,which is reflected in the poor response to the analyte in com-parison with the MoS2–rGO/GCE. Furthermore, the lowestresistance is found for the solution with DA as the analyte,with the cumulative effect of DA, AA and UA causing a drop inresistance and a rise in current, as is also seen in voltammetricresults. Overall, it is clear from the EIS results that the MoS2–rGO/GCE is sensitive towards AA, UA and DA.

3.2.4. Selectivity, stability and reproducibility. The selecti-vity of the MoS2–rGO/GCE was evaluated by comparing thepeak current ratio as shown in Fig. 7. The DPV profiles afterthe injection of only AA, DA and UA and after the addition ofdifferent interfering species have been shown (Fig. 7a). Thehistograms of the peak current ratio are also shown fordifferent interfering species such as glucose, citric acid CaCl2,Na2SO4, ZnCl2, galactose and sucrose (Fig. 7b). Herein, I1, I2,and I3 represent the induced current response for 300 μM AA,100 μM DA, and 200 μM UA, respectively, and I0 represents thecurrent response for 1 mM of different interferential agents. Itcan be observed that the ratio of peak current change is ±4%,indicating noteworthy selectivity of the proposed biosensortowards the simultaneous detection of AA, DA and UA. Thestability of the MoS2–rGO/GCE was also investigated byrunning 50 continuous CV cycles in 5 mM [Fe(CN)6]

3−/4−

aqueous solution containing 0.1 M KCl (Fig. S14†). A negli-gible decay in the voltammetric current was observed duringthe complete scan, suggesting good stability of the MoS2–rGO/GCE. The reproducibility of the proposed sensor was tested bypreparing four independent modified electrodes by the sameprocedure. Negligible variations were found in the CV currentresponses of all four electrodes with the addition of 300 μMAA, 100 μM DA and 200 μM UA, respectively, suggesting goodreproducibility (Fig. S15†). Thus, the above sensor character-istics prove MoS2–rGO as an ideal electrode modifier for sensi-tive, selective, stable and reproducible detection of AA, DA andUA.57

The sensing performance of the present sensor has beencompared with that of the previously reported sensors(Table 1). Interestingly, the MoS2–rGO/GCE possesses a fasci-nating electrocatalytic activity as it delivers a remarkable peakseparating efficiency with notable sensitivity, a wider linearrange and a lower detection limit. In addition to this, it exhi-bits easily distinguishable and wider peak to peak separationoxidation potentials (AA–DA = 204 and DA–UA = 122), high- T

able

1Comparisonofthesensingperform

ance

oftheMoS 2–rG

O/G

CEwithpreviouslyreportedsenso

rs

Electrode

Peak

tope

aksepa

ration

(mV)

Linearrange

(μM)

LOD(μ

M)

Sensitivity

(μAμM

−1cm

−2)

Ref.

AA–D

ADA–U

AAA

DA

UA

AA

DA

UA

AA

DA

UA

Ag-Pt/pCNFs/GCE

——

—10

–500

——

0.11

——

2.24

—5

P 2W

16V2-AuP

d/PE

I 8/ITO

——

1.2–16

102.1–20

60—

0.43

0.83

——

——

6Pd

NPs

-GO/GCE

——

20–2

280

——

——

—0.08

7—

—7

Pd3Pt

1/PDDA-RGO/GCE

184

116

40–1

200

4–20

04–40

00.61

0.04

0.10

0.07

70.64

0.49

8Au/RGO/GCE

200

110

240–15

006.8–41

08.8–53

051

1.4

1.8

0.00

20.31

0.15

93D

GH-AuN

Ps/GCE

180

120

1.0–70

00.2–30

1–60

0.02

80.00

260.00

5—

——

10Au@

Pd-RGO/GCE

178

164

1–80

00.1–10

00.1–35

00.02

0.00

20.00

50.31

6.08

1.22

11PG

/GCE

194.4

155.8

9–23

145–71

06–13

306.45

±0.23

2.00

±0.25

4.82

±0.33

0.06

674±0.00

464

0.11

25±0.01

790.10

29±0.00

8312

3DGLC

Fs/GCE

230

130

12.5–400

0.05

–10

0.05

–15

20.01

0.01

——

—15

PImox–G

O/GCE

140

6075

–227

512

–278

3.6–24

9.6

180.63

0.59

——

—21

Graph

ene/Pt-m

odifiedGC

185

144

0.15

–34.4

0.03

–8.13

0.05

–11.85

0.15

0.03

0.05

0.34

570.96

950.41

1926

Fe3O4/r-GO/GC

150

170

160–72

27.1

0.4–3.5

4–20

200.08

0.5

0.03

3538

.84.50

273.5–16

020

–212

12.9

1.22

Fe3O4@Au–

S–Fc

/GS-

chitosan

/GCE

180

906–35

00.5–50

1–90

10.1

0.2

——

—28

Ni/C/GCE

——

20–2

400

1–55

5–18

05

0.05

0.1

0.35

245

.571

11.873

30HNP-PtTia

lloy/GCE

200

140

200–10

04–50

010

0–10

0024

.23.2

5.3

——

—32

MoS

2–rGO/GCE

204

122

300–33

0010

0–10

0020

0–34

000.09

0.12

0.10

0.00

80.05

10.02

5This

Work

Paper Dalton Transactions

15856 | Dalton Trans., 2017, 46, 15848–15858 This journal is © The Royal Society of Chemistry 2017

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lighting its future potential over most of the earlier reportedsensors. Hence, the MoS2–rGO composite is an emerging andcutting edge candidate for sensitive, selective and simul-taneous detection of AA, DA, and UA.

4. Conclusions

In summary, a simple two-step method was employed forthe fabrication of MoS2–rGO nano-flowers. These nano-flowers were grafted on the working area of the glassycarbon electrode in the absence of any commercial binder.Thus the designed electrode (MoS2–rGO/GCE) was employedfor the simultaneous detection of AA, DA and UA. Theelectrochemical investigations reveal that the MoS2–rGO/GCE can selectively distinguish AA, DA and UA with goodpeak separating efficiency (AA–DA = 204 mV and DA–UA =122 mV) and noteworthy detection limit and sensitivities.The good sensor performance was attributed to the note-worthy large surface area, suitable porosity, and superiorconductivity of the MoS2–rGO composite. Furthermore, thecharge transfer and capacitive components of the electrodewere probed by electrochemical impedance spectroscopy,which were found to be in correlation with the voltammetricresults. The present work is of significance because thefacile preparation of a smart MoS2–rGO heterostructure andthe corresponding binder-free electrode preparation mayopen new possibilities in the development of cost-effectiveand reliable biosensors. However, the major challenge is theselection of appropriate materials and to combine their pro-perties into one structure, which is possible by stackingmonolayers of these materials together using scalable self-assembly processes. Moreover, the issue of the high cost ofproducing these heterostructures can be addressed by inno-vative facile synthesis and manufacturing processes to makethem affordable.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We thank the Sophisticated Instrumentation Centre (SIC), IITIndore for all the characterization facilities. M. S. andK. N. thank MHRD and A. K. S. thanks UGC, New Delhi, India,respectively, for providing fellowships. We are grateful toProf. G. Hundal and Dr Sanyog Sharma for providing TEMimages. M. S. thanks the Materials Research Center (MRC,MNIT, Jaipur) for Raman and AFM analyses and ACMS, IITKanpur for providing the XPS facility. S. M. M. thanksSERB-DST (Project No. EMR/2016/001113), New Delhi and IITIndore for financial support.

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Paper Dalton Transactions

15858 | Dalton Trans., 2017, 46, 15848–15858 This journal is © The Royal Society of Chemistry 2017

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