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A Gel-Based Approach to Design Hierarchical CuS Decorated Reduced Graphene Oxide Nanosheets for Enhanced PeroxidaseLike Activity Leading to Colorimetric Detection of Dopamine Soumen Dutta, Chaiti Ray, Sourav Mallick, Sougata Sarkar, Ramkrishna Sahoo, Yuichi Negishi, and Tarasankar Pal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08421 • Publication Date (Web): 28 Sep 2015 Downloaded from http://pubs.acs.org on October 3, 2015

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The Journal of Physical Chemistry

A Gel-Based Approach to Design Hierarchical CuS Decorated Reduced Graphene Oxide Nanosheets for Enhanced Peroxidase-like Activity Leading to Colorimetric Detection of Dopamine Soumen Dutta,† Chaiti Ray,† Sourav Mallick,† Sougata Sarkar,§ Ramkrishna Sahoo,† Yuichi Negishi,‡ and Tarasankar Pal†* †

§

Department of Chemistry, Indian Institute of Technology, Kharagpur – 721302, India

Department of Chemistry, Ramakrishna Mission Vivekananda Centenary College, Rahara, Kolkata - 700118, India

‡

Department of Applied Chemistry, Tokyo University of Science, Tokyo-1628601, Japan E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Supramolecular colorless copper(I)-thiourea hydrogel (Cu-TU gel) has been made a mechanically strong functional hybrid material in graphene oxide (GO) framework. Mild heat treatment (85 ºC) of the hybrid material fetches black colored hierarchical copper sulfide decorated reduced graphene oxide nanosheets (CuS-rGO) through an obvious inhouse redox transformation reaction between Cu(I) and GO without any additive. The assynthesized CuS-rGO nanocomposite exhibits impressive peroxidase-like activity where oxidation of colorless 3, 3’, 5, 5’-tetramethylbenzidine (TMB) to blue colored product is observed in solution phase with H2O2. Systematic control experiments suggest that strong covalent interaction between CuS and rGO synergistically enhances the catalytic activity of CuS-rGO in comparison to its individual counterparts. Furthermore, important biomolecule, dopamine has been found to selectively inhibit, in succession, the oxidizing action of H2O2 for TMB oxidation reaction. Thus dopamine dependent successive inhibition reaction creates a one-pot reporter platform to sense dopamine down to 0.48 µM concentration level by UVvis spectrophotometry. Keywords: Hydrogel, Hybrid nanostructure, Reduced graphene oxide/CuS nanocomposite, Peroxidase-like activity, Dopamine sensing


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INTRODUCTION Enzymes are extensively used in various fields mainly as biosensor in pharmaceutical and food industries owing to their outstanding selective catalytic activities towards a specific substrate.1 However tedious purification steps during their preparation, high preservation expenditure, and extremely sensitive nature towards environmental conditions are major concerns for practical applications of these native enzymes. Horseradish peroxidase (HRP) is an important biocatalyst and used for the oxidation of 3, 3ʼ, 5, 5ʼ-tetramethylbenzydine (TMB), a chromogenic substrate by hydrogen peroxide (H2O2). However, the stability, storage, limited availability of this natural peroxidase enzyme and also the reproducibility, recyclability by this catalyst are the foremost and common issues and concerns for the use of this native enzyme.2 In order to overcome these obstacles numerous research activities have been directed leading to the discovery of effective alternatives of this expensive substance which can really mimic the activity of HRP but are certainly beneficial due to their low production cost, improved stability and so on without losing analytical novelty.3, 4 Over the last few years a variety of nanomaterials have been employed as suitable catalysts to show intrinsic peroxidase-like activity.5 Nanostructures of various metal oxides or metal sulphides,6-10 monometallic or bimetallic metal nanoparticles,11-14 carbon based nanomaterials15-18 and numerous composites19-24 have been utilized as TMB oxidation catalyst where the colourless solution changes to blue coloured product in presence of H2O2. This transformation can easily be confirmed even by naked eye detection and does not require any sophisticated instrumentation or techniques.

25, 26

This colorimetric assay further

executed as a simple, low cost and highly effective platform for biosensing application.25 Over the years this protocol has been adopted for colorimetric determination of various molecules and ions such as glucose,25 nucleotides,27 H2O2,8 ascorbic acid,23 amino acids,8 melamine,28 and Hg2+,29 SO32-, 30.


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Graphene, a two-dimensional (2D) sp2 hybridized thin atomic layer of carbon has become an interesting area of research due to its various intriguing properties such as high thermal and electrical conductivity, outstanding mechanical strength and remarkable surface area etc.31,

32

Graphene oxide (GO) is generally used as a starting material for the wet

chemical synthesis of graphene and its numerous derivatives.32 Incorporation of metal, metal chalcogenides, conducting polymer etc. into the reduced form of GO i.e. rGO matrix has been done from solution phase GO based straightforward method with plentiful applications.32-35 Copper sulfide (CuS)36 and GO18 are individually reported to exhibit intrinsic peroxidase-like activity. So combination of CuS and rGO might exhibit improved catalytic efficiency where the problem of nanoparticles aggregation, sluggish electron transfer process and poor dispersion in solution have been resolved.37 Recently, our group has become successful in fabrication of porous tubular copper sulfide nanostructures from Cu(I)thioacetamide metal organic complex through auto-degenerative hallowing.38 Another interesting study has been reported again from our group involving aqueous solution of CuCl2 and thiourea to form copper(I)-thiourea supramolecular hydrogel (Cu-TU gel).39 All these ideas motivate us to design a hierarchical porous CuS nanostructure on rGO nanosheets for improved peroxidase activity through a new supramolecular gel based synthetic strategy. Firstly, GO encapsulated Cu(I)-thiourea supramolecular gel has been prepared at room temperature and then in-situ reaction between Cu(I)-TU centre and GO skeleton under heating in water bath provides our desired composite in gram quantity. Finally, the assynthesized CuS-rGO composite has been employed for the oxidation of TMB to evolve blue coloration in presence of H2O2. The facile oxidation process is further found to be arrested by dopamine in succession and thus becomes an intriguing platform for colorimetric detection of this important neurotransmitter, dopamine.


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EXPERIMENTAL SECTION Materials. All the reagents used were of AR grade and used as received without further purification. Double distilled water was used throughout the experiments. Synthesis of graphene oxide (GO) integrated Cu(I)-TU hydrogel (GO-gel). First, graphene oxide (GO) was prepared from pristine graphite according to Hummers’ method.40 It has been previously reported from our group that copper chloride and thiourea (TU) form supramolecular hydrogel at room temperature.39 Graphene oxide (GO) integrated Cu(I)-TU hydrogel (GO-gel) was prepared following the above procedure but in presence of GO dispersion. In a typical process, 10 mL highly dispersed aqueous GO solution (2 mg/mL) was stirred with 0.4 mmol solid copper chloride (CuCl2. 2H2O) for 30 minutes. Then the resultant mixture was added to 10 mL aqueous thiourea (0.4 mmol) solution under sonication for 2 minutes which consequences its transformation into a gel matrix. Synthesis of CuS-rGO Nanocomposite: The as-obtained gel matrix was washed thoroughly in order to remove adsorbed CuCl2 or TU from the gel matrix. The whole mixture was then heated on a water bath at 85 ⁰C (~3 h) for redox transformation reaction [(Cu(I)/GO) to (CuS-rGO)]. It is interesting to note that the GO incorporated grey colored supramolecular gel gradually transformed into a black mass. After 3 h of heating, the black mass was washed several times first with distilled water followed by ethanol. Finally the isolated black product was dried under vacuum and used for further characterizations and employed as a robust catalyst. Catalytic Oxidation of TMB by CuS-rGO Nanocomposite: In order to study the peroxidase-like activity of our synthesized nanocomposite, 25 µL of 3, 3´, 5, 5´tetramethylbenzidine i.e. TMB (0.01 M) was oxidized by 100 µL H2O2 (1 M) in 0.1 M acetate buffer (pH = 4.4) in presence of 0.1 mg CuS-rGO catalyst. In our study total volume of the solution was made to 2.5 mL. The UV-vis spectra of the as-prepared reaction mixture


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were recorded at a definite time interval of one minute at 30 ⁰C. Here the kinetics of the catalysis was monitored with respect to the absorbance at 652 nm against reaction time as deep blue colour arises during the oxidation of TMB. The Michaelis-Menten parameter was calculated from Lineweaver–Burk plots 1/V0= 1/ Vmax + Km/Vmax [S0] Where V0 and Vmax is initial and maximum velocity respectively, [S0] is substrate concentration and Km represents the Michaelis constant. Dopamine Detection. In a typical process, variable amount of freshly prepared aqueous dopamine hydrochloride (DA) solution (2-100 µM) was introduced into the above indicated reaction mixture. Here the amount of TMB, H2O2, and catalyst are kept unaltered as described in the preceding section maintaining the volume of the reaction mixture same. The successive change of blue color of the reaction mixture with variable DA concentration was evaluated from UV-vis spectral measurements. In order to investigate the selectivity of our process for DA detection, we carried out the same experiment but in presence of other related specimens such as glucose, ascorbic acid, phenylalanine, uric acid, tryptophan, tyrosine, LDOPA in lieu of DA.

RESULTS AND DISCUSSION Supramolecular gels are highly important and today they are often used as a support material for metal/metal oxide nanostructure synthesis through a simple and greener pathway. Gel networks direct the growth and stability of the as-synthesized materials and the resultant composites exhibit various interesting properties such as catalysis, improved mechanical strength, stability, and so forth.41, 42 The importance of the in-situ preparation procedure relies on atom economy as no extra reducing agents or stabilizers are introduced to complete the


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reaction.43,

44

Our previous study has revealed that aqueous solution of Cu2+ and thiourea

(TU) proceeds through spontaneous one electron transfer protocol involving TU and Cu2+ which results in Cu(I) species and oxidized form of TU.39 The presence of Cl- and water molecules in the solution are very important as they participate in weak force interaction (i.e. H-bonding) that evolves Cu(I) metallogel (Cu-TU).39 In the present case, incorporation of GO into Cu-TU hydrogel not only increases the mechanical strength of the supramolecular hydrogel but also provides a platform to fabricate copper sulfide-reduced graphene oxide nanocomposite (CuS-rGO) under mild heating condition at 85 ºC without the need of any foreign agents. Thus a new gel based approach has been established for the first time where Cu(I) helps GO reduction with its automatic conversion into Cu(II) to generate stable CuSrGO nanocomposite. Scheme 1 represents the adopted synthetic technique for CuS-rGO nanocomposite. Figure 1a shows the digital images of Cu-TU gel, GO-gel, and CuS-rGO nanocomposites. Stable gel formation has been confirmed for Cu-TU gel and GO-gel through ‘vial inversion technique’. Addition of GO during gel formation leads to gray coloration of the supramolecular gel. Mechanical strength of both the gel materials was measured from rheological experiments (Figure S1). In both the cases the storage modulus (G’) and loss modulus (G’’) are found to be independent with respect to the change in frequency and G' is higher than G" which indicates the formation of typical gel materials. The gap between G’ and G’’ has been significantly increased from Cu-TU to GO-gel suggesting improved mechanical strength through the incorporation of GO. The material is completely disrupted its gelatinous nature on heating and transform into black product (Figure 1a), but reversibility was not observed because of the oxidation of Cu(I) to Cu(II). This observation suggests that our protocol is highly effective towards the conversation of gel to CuS-rGO powder. FESEM image in Figure 1b describes the fibre like morphology of Cu-TU gel whereas in GO-gel the


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lamellar morphology of GO can be identified along with ultra long fibres (Figure 1c). After complete conversion of GO-gel to CuS-rGO upon heating at 85 °C for 3 h, numerous one dimensional CuS nanostructures with hierarchical morphology of ~200 nm width have been found to be attached on rGO nanosheets (Figure 1d, e). It is worthwhile that heating at lower temperature e.g. 60 °C fails to complete the conversion of GO-gel to CuS-rGO composite (Figure S2). The morphologies of the GO-gel and CuS-rGO were further characterized by TEM analysis. GO-gel consists of two types of nanostructures as shown in Figure 2a-c. Here, a plenty of nanofibres for Cu(I)-gel are observed to be attached with two dimensional GO nanosheets which makes a hybrid system for GO-gel. The fibres are very long with variable width dimension (may be due to their wrapping by GO sheets) and their entangled nature through GO sheets results in a network structure. In the EDAX spectra of GO-gel various elements such as Cu, Cl, N, S, C, and O are found (Figure S3). It is worth mentioning that GO-gel completely losses its fibril network on heating at 85 ⁰C for 3 h and it transforms into a stacked form of CuS nanoflakes embedded on rGO sheets (Figure 2d-f). Strong ultrasonication might cause the slight loosening of its one dimensional nature, but its wrinkled thin nanoflakes (~8-10 nm) at the outer surface are rearranged to resemble like hierarchical flower. HRTEM image of the flakes demonstrates a fringe spacing of 0.32 nm which corresponds to the standard interlayer for (101) plane of the hexagonal CuS (Figure 2g). EDAX spectra of the material also confirm the formation of CuS in the nanocomposite (Figure 2h). Phase purity of the synthesized composite was first characterised by XRD analysis. In case of GO-gel, one strong XRD peak arises at 2θ = 8.54⁰ (may be from common low angle XRD peak for metal organic framework, Figure S4) which is found to be vanished completely on heating. Additionally, new peaks have been appeared which reveal the


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complete conversion of GO and metal organic framework to their corresponding products. Systematic XRD analysis suggests the presence of pure hexagonal CuS (JCPDS: 78-2121) in the nanocomposite along with reduced graphene oxide (rGO) as one broad peak appears in the range of 2θ ~ 25º for the diffraction from (002) plane of the restored graphitic moiety (Figure 3a). Detail chemical states of C and Cu in GO-gel and resultant CuS-rGO were further compared through XPS analyses. Deconvulated C 1s XPS spectrum of GO-gel indicates the presence of various functionalities such as C=C (284.9 eV), C-O (286.7 eV), C=O and C-N (288.7 eV) as shown in Figure 3b. Moreover two peaks in the high resolution XPS spectrum of Cu 2p have been observed at 932.1 and 952.1 eV for Cu(I) 2p3/2 and Cu(I) 2p1/2 electronic state (Figure 3c). These results authenticate the presence of highly functionalized carbon element as well as Cu(I) in the gel matrix. On the other hand deconvulated narrow range C 1s XPS spectrum for CuS-rGO is found to lose the peak intensities corresponding to various functional groups and ultimately C=C group appears as main peak (Figure 3d). This suggests the regeneration of the sp2 hybridized graphitic domains during heat treatment. Similarly the doublet of Cu 2p spectrum emerge at higher binding energy compared to the previous one (Cu 2p3/2 = 933.1 eV, Cu 2p1/2 = 953.1 eV) confirming the presence of Cu(II) in the nanocomposite (Figure 3e). A strong peak is also observed at around 944 eV in the Cu 2p XPS spectrum which can be ascribed as the characteristic shakeup satellite peaks for Cu(II). It is important to note that Cu 2p XPS spectra of GO-gel also contains shakeup satellite peak (may be due to aerial oxidation of the surface to some extent), but has less significant contribution compared to its resultant product CuS-rGO. Furthermore the S 2p spectrum of the composite exhibits typical doublet peaks at about 161163 eV for S 2p3/2 and S 2p1/2 which illustrates the presence of S2- in the composite (Figure 3f).45 This study authenticates the regeneration of the sp2 hybridized graphitic domains and the conversion of Cu(I) to Cu(II) through a in-situ reaction by the mild heat treatment.


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In the FTIR spectra of GO (Figure 4a), numerous peaks are appeared due to the stretching and bending modes of various functional groups such as O–H stretching mode (~3417 cm-1), C=O stretching mode (~1720 cm-1), O–H bending mode (~1393 cm-1), C–O (epoxy) stretching mode (1218 cm-1) and C–O (alkoxy) stretching mode (1060 cm-1). The peak at 1620 cm-1 is presumably resulted in from the unoxidized graphitic domains. For CuTU xerogel, signal at 3000-3500 and 1616 cm-1 correspond to N-H stretching and bending modes respectively. The vibrational modes for C–N and C=S group come at 1440 and 1106 cm-1 respectively. Now in GO-gel the peaks in 3000-3500 cm-1 wavenumber range get broadened which supports the extended hydrogen bonding in GO-gel through the oxygen containing functional groups of GO and N-H bond present in Cu-TU gel. On heating the GOgel material the resultant composite looses the peak intensity of various oxygen containing groups present in GO and the peaks for C–N, C=S and N–H from the Cu-TU gel component are observed to be vanished completely. The present peaks are due to restored graphitic C=C (~ 1575 cm-1), remaining C–Oepoxy (1220 cm-1), C–S (1092 cm-1), and resultant Cu–S (618 cm-1)37 groups. Raman spectra further revealed that the chemical reduction of GO and supports the formation of CuS-rGO nanocomposite through our proposed pathway (Figure 4b). In GO two bands near 1356 and 1595 cm-1 were found which are ascribed as D and Gband respectively. The former band arises from a breathing mode of κ-point photons of A1g symmetry whereas the latter comes from the first order scattering of the E2g phonon of sp2 C atoms.33, 34 In the resultant composite the above indicated two peaks arises at 1355 and 1584 cm-1 respectively. The observed significant red shift of the G band (~11 cm-1) can be explained in terms of the restoration of the hexagonal carbon network. On the other hand the ratio of the intensity of the D and G band (ID/IG) increases from 0.86 to 1.04 which also suggests the formation of new graphitic moieties after the composite formation. Another band around ~463 cm-1 arises for CuS in the as-synthesized composite.46


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Mechanistic Insight of GO-Gel Formation and Its Transformation to Hierarchical CuS-rGO Material. It is evident from the FTIR and XPS measurement that GO consists of numerous oxygen containing functional groups which help the attachment of Cu2+ ion on its surface through electrostatic interaction. It is well known that TU reduces Cu2+ to Cu+ through one electron transfer process and itself transforms into α,α′dithiobisformidinium dichloride.39, 47-49 Related crystal structure study discloses the formation of ‘CuS4’ unit where Cu+ centre experiences tetrahedral geometry through sulfur donors of TU and the oxidized dithio product.39, 50 Here it is important to note that each ‘CuS4’ motif held together through sharing dithio product and also with extended hydrogen bonding. Now the present –NH2 group of the coordinating ligands play the important role for various types of hydrogen bonding interaction such as N-H.....Cl and N-H....S in the Cu-TU system as described previously.39 Water molecules and present Cl- also participate in this supramolecular interaction and form the hydrogel. Present case deals with additional GO which consists of numerous oxygen functionalities such as carboxyl, hydroxyl, epoxy groups etc. So, GO surface extends the weak force interactions through hydrogen bonding. Furthermore,

there

is

a

possibility

of

electrostatic

attraction

between

α,

α′-

dithiobisformidinium cation and the negatively charged GO surface to form such supramolecular hybrid gel assembly. The average zeta potential values of GO and α, α′dithiobisformidinium cation (prepared by mixing Cu2+ and TU) are -14.2 and 11.3 mV respectively (Table S1). Addition of GO into α, α′-dithiobisformidinium cation results in a mixture with -0.08 mV zeta potential which authenticates electrostatic force directed assembly formation in the GO-gel matrix. After thorough washing of the gel material followed by mild heating opens up the Cu(I) mediated reduction of GO with subsequent oxidation of Cu(I) in the gel skeleton. On the other hand the oxidized TU moiety produces CO2 and free S2- ion in the aqueous solution


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under heating condition. These are the species very much responsible for porous/blister surface structure of the hybrid. Finally the in-situ generated active S2- and Cu(II) leads to the production of hierarchical CuS nanostructures decorated on rGO nanosheets.45 Porous nature of the as-synthesized CuS-rGO materials has been investigated by Brunauer-Emmett-Teller (BET) surface area measurement. The calculated BET surface area is found to be 148.2 m2/g with Barrett-Joyner-Halenda (BJH) desorption pore diameter of 4.1 nm suggesting its mesoporous nature. Nitrogen adsorption-desorption isotherm and pore size distribution plot of our synthesized CuS-rGO have been shown in Figure S5. In order to understand the importance of our adopted supramolecular gel based strategy to fabricate such interesting morphology of CuS attached on rGO, we have performed control reaction with copper sulphate (CuSO4. 5H2O) instead of CuCl2. In absence of Cl- the supramolecular gel formation fails to happen,39 which consequences spherical CuS particles decoration on rGO sheets unlike our described hierarchical CuS-rGO case. XRD pattern, FESEM image and EDAX spectra of this material have been shown in Figure S6. This study reveals that initially formed one dimensional metal organic framework intermediate actually acts as soft template for such an interesting hierarchical CuS nanostructures attached on rGO sheets. Peroxidase-like Activity. In order to explore the catalytic activities of our synthesized CuS-rGO nanocomposite, we have performed the catalytic oxidation of 3, 3ʼ, 5, 5ʼ-tetramethylbenzidine (TMB), a well known peroxidase substrate in presence of H2O2 at room temperature. CuS-rGO efficiently catalyzes the transformation of colorless TMB to its blue colored oxidized product depicting absorbance maxima at 652 nm. Time dependent UVvis spectra of oxidation kinetics catalyzed by CuS-rGO have been shown in Figure 5a. Here pH of the reaction mixture influences the enzyme-like catalytic activity of CuS-rGO and it is found that pH=4.4 stands out to be the optimum condition for the excellent catalytic activity of CuS-rGO as shown in Figure 5b. Here the absorbance at 652 nm after 15 minutes of


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reaction has been considered as the optimum wavelength and time, respectively to monitor the corresponding catalytic activity under different reaction conditions. Additionally, various related materials such as GO, rGO, CuS and spherical CuS decorated rGO (Figure S6) have been tested as catalyst in the above reaction to estimate the significance of the our synthesized composite to mimic natural peroxidase enzyme. We have also varied the added GO content (1, 2, 3 mg/mL aqueous dispersion) during the formation of GO-gel and the derived differently exfoliated CuS-rGO nanocomposites (CG-1, CG-2 and CG-3) are eventually investigated to examine their catalytic activities. Comparative catalytic activity study in Figure 5c reveals that CG-2 exhibits highest catalytic performances among the others under the experimental conditions ({TMB: 25 µL; 10-2 M}, {H2O2: 100 µL; 1M}, {catalyst: 100 µg}, {pH : 4.4}). It is important to note that the prepared material (i.e. CG-2) displays almost 6, 2.5, 2 times higher activity than rGO, free CuS and spherical CuS decorated rGO composites. Here strong covalent interaction between hierarchical CuS nanostructures and two dimensional rGO sheets creates synergistic coupling between them to show such improvement in catalytic activity. However, maximum performance can be obtained from the respective nanocomposite with a desired ratio of the two nanocomponents.36,

37

Increasing GO content during gel formation leads to CuS-rGO

nanocomposite with higher rGO amount and eventually less CuS quantity, which helps to increase the surface area but reduces the dispersibility of the catalyst in aqueous medium. It is important to note that CG-2 material is highly dispersible in water (3 mg/mL) which helps to improve its catalytic performance. Meanwhile composite with lower rGO and higher CuS content i.e. CG-1 might have experienced sluggish electron transfer, lower accessible surface area as well as aggregation of CuS nanostructures during catalysis, which result in lower catalytic activity in contrast to CG-2. These studies actually help to conclude the optimized amount of GO i.e. 2 mg/mL for the production of most efficient catalyst (CG-2 in our case).


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Mechanism of Peroxidase-like Activity. The catalytic activity of CuS-rGO may come from Cu2+ centre which acts like Fenton’s reagent. The Cu2+ in the hybrid helps the production of highly reactive hydroxyl species (·OH) from H2O2 via Cu+ (Scheme S1).45, 51 This newly generated ·OH actually oxidizes TMB to its blue coloured form. The oxidation mechanism has been further confirmed from terephthalic acid (TA) oxidation experiment performed under similar reaction condition.23, 52 Although TA is a nonfluorescent molecule, it selectively transforms into highly fluorescent 2-hydroxy terephthalic acid (HTA) on ·OH attack. Control experiments indicate the importance of CuS-rGO catalyst and also its amount for the decomposition of H2O2 to produce ·OH (Figure S7). Introduction of higher catalyst dose results in elevated fluorescence intensity i.e. more HTA formation from same set of reaction. Thus ·OH mediated TMB oxidation in our peroxidase-like activity study has been confirmed. The hierarchical porous structure of CuS provides more accessible surface area which in turn helps the catalytic oxidation of TMB by H2O2. On the other hand rGO provides large surface area and better electron conduction channel to facilitate the oxidation process. Here it is important to note that the catalytic reaction occurs in presence of catalyst, H2O2 and TMB. Absence of any component fails to exhibit blue coloration. Catalytic activity is found to vary with respect to the concentration of the added H2O2 which provides us a technique for the colorimetric detection of H2O2.18,

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Plot of absorbance of the blue colored oxidized

product of TMB (λmax = 652 nm) with respect to introduced H2O2 concentration (10-120 mM) has been shown in Figure 5d and its inset represents the calibration curve for the strength of H2O2 with respect to the absorbance of the product. The calculated limit of detection (LOD) for H2O2 is found to be 0.43 mM with a linear range from 10-40 mM (R2= 0.99). The relative standard deviation (RSD) is calculated to be ~2% for 20 mM H2O2 with three parallel measurements (n = 3).


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Kinetics of the TMB Oxidation. The TMB oxidation kinetics was further studied through steady-state kinetics measurement where the experiments were performed by changing one substrate concentration while keeping the strength of other component constant. The obtained absorbance values were converted to the concentration of the blue colored product by applying molar attenuation coefficient value of 39000 M-1cm-1. It has been found that the typical Michaelis–Menten model has been followed over a certain concentration range for both H2O2 and TMB (Figure 6). The Michaelis constant (Km) is an important parameter to estimate the binding affinity between substrate and enzyme. From the Linewaver–Burk plot (inset of Figure 6), Km for TMB and H2O2 are found to be 0.02 and 3.1 mM respectively. The reported Km values with HRP enzyme are 0.062 and 3.7 mM for TMB and H2O2 respectively (Table S2).53 This result leads to the conclusion that TMB has better affinity towards our synthesized CuS-rGO than HRP enzyme. Table S3 presents the comparative account on Michaelis–Menten constants with various previously reported nanomaterials which implies the high performance of our synthesized hierarchical CuS decorated rGO material for peroxidase mimicking activity.2, 16, 52, 54 Dopamine (DA) Detection. Dopamine (DA) is an important neurotransmitter which has various important roles in mammalians brain and body. It helps to send the signal from one nerve cell to other nerve cells.55 Deficiency of DA in human brain causes huge problems such as Perkinson’s diseases, attention deficit hyperactivity disorder, senile dementia etc.55-57 DA is a member of catecholamine and phenethylamine families. The electroactive nature of DA encourages researchers for its determination through electrochemistry, but unfortunately uric acid (UA) and ascorbic acid (AA) interferes strongly due to their close voltammetric response.58 Moreover, various methods such as fluorescence,59 colorimetric,60,

61

liquid

chromatography-mass spectrometry62 etc. are also adopted for the detection of DA. In our


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case the abovementioned peroxidase study has been implemented for the detection of DA though simple UV-vis spectroscopy without any significant interference from AA and UA. Here the blue coloration of the oxidized TMB gets quenched in presence of DA which indicates the inhibition effect of DA on the oxidizing nature of H2O2. Increasing the amount of DA concentration in the reaction mixture leads the diminishing blue color intensity which provoked us for a tempting colorimetric application of our hybrid platform for DA sensing. Under the optimized condition the changes of absorbance at ~652 nm wavelength were monitored with the introduction of DA solution. Figure 7a illustrates the UV-vis spectra of TMB oxidation reaction with varying DA concentration. It has been found that a linear relationship could be established from the absorbance vs. concentration of DA graph in 2-100 µM concentration range with R= 0.989 (Figure 7b). The calculated limit of detection (LOD) value from this regression analysis is 0.48 µM which is superior than the other reported colorimetric methods as compared in Table S4.63-66 The reproducibility of the sensing protocol was tested by five parallel measurements (n = 5) with 30 µM DA and the relative standard deviation is found to be 3.7%. The actual reason behind such inhibition effect from DA is due to its suitable electroactive nature which favours its oxidation into dopamine-oquinine by H2O2 over TMB.67 To investigate the selectivity of our designed sensing tool, we have performed the TMB oxidation study in presence of other related organic molecules such as glucose (Glu), ascorbic acid (AA), phenylalanine (Phe), uric acid (UA), tryptophan (Tryp), tyrosine (Tyro), L-DOPA. Figure 7c represents the UV-vis graph of TMB oxidation reaction in presence of 100 µM each of the indicated compounds including DA. Here it can be concluded that only DA can quench the oxidation process successively and effectively. The difference in absorbance value at 652 nm in absence and presence of investigated molecules has been shown in Figure 7d which reflects the high selectivity of our adopted procedure towards DA.


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The corresponding digital images have been shown in the inset of Figure 7d which also corroborates the above fact.

CONCLUSIONS In short, a new gel based synthetic technique has been adopted for morphologically important CuS nanostructure decoration on reduced graphene oxide nanocomposite from a GO encapsulated Cu(I)-gel. The protocol is devoid of any surfactant or foreign agents, rather in-situ redox reaction between GO and Cu(I) centre under mild reaction condition produces the desired product in gram quantity. The supramolecular gel gets disrupted when the backbone of the gel Cu(I) is oxidized, but one dimensional morphology of the Cu(I) precursor helps autogeneration of hierarchical nanostructures of CuS. The as-synthesized product exhibits excellent peroxidase-like activity by oxidizing colourless 3, 3ʼ, 5, 5ʼtetramethylbenzidine (TMB) to blue colored product in presence of H2O2 in the solution phase. The oxidation phenomenon is found to be inhibited by dopamine which actually constructs a colorimetric sensing platform for dopamine concentration dependent study by using a simple UV-vis spectrophotometer. The limit of detection value is found to be 0.48 µM with a wide linear range of 2-100 µM. Thus a gel based redox reaction in graphene skeleton proclaims functionalized nanohybrid material for multifaceted applications.

Acknowledgements: The authors are thankful to DST and CSIR, New Delhi, India for financial assistance, and the IIT Kharagpur for instrumental support.

Supporting Information: Detailed instrumentation; Rheological study; FESEM, EDAX, XRD of GO-gel and CuS-rGO; BET and BJH analyses of CuS-rGO. Mechanism of


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peroxidase-like activity of CuS-rGO. Table for zeta potential, comparative account on catalytic activities of various materials, comparative account on various dopamine detection assays. This material is available free of charge via the Internet at http://pubs.acs.org.


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Figures and Schemes Scheme 1: Schematic representation of the synthetic protocol adopted during the formation of CuS-rGO nanocomposites.

Solid CuCl2 Stirring, RT

GO dispersion in water

Aqueous thiourea solution

85°C, 3 h

Sonication, RT

GO-gel


CuS-rGO

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Cu-TU gel

a GO- gel

b

CuS-rGO

c

e

d

Figure 1. (a) Digital images of Cu-TU gel, GO-gel, and CuS-rGO nanocomposites. (b) FESEM images of (c) Cu-TU gel, (d) GO-gel, and (e, f) CuS-rGO nanocomposites. Yellow arrows in Figure 1d indicate the rGO support.


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a

GO

b

d

c

GO

Fibers

Page 30 of 35

GO

Fibers

e

f CuS

rGO

g

0.32 nm

h

0.32 nm

Element

Weight%

Atomic%

CK

63.93

79.11

OK

15.44

14.34

SK

7.48

3.47

Cu K

13.16

3.08

Totals

100.00

Figure 2. (a-c) TEM images of GO-gel in different magnifications, (d-f) TEM images of CuS-rGO nanocomposites in different magnifications, (g) HRTEM and (h) EDAX spectra with elemental content of CuS-rGO nanocomposites.


30

GO-gel

a

285 eV

20

30

40

116

108

50

288.6 eV

275

60

280

285 eV

Intensity (a.u.)

Intensity (a.u.)

d

287 eV 288.8 eV

280

285

285

290

930

295

Binding Energy (eV)

2θ (degree)

275

Cu(I)2p3/2

290

940

S 2p3/2

e

Cu (II) 2p3/2

940

950

156

960

Binding Energy (eV)

Binding Energy (eV)

950

960

Binding Energy (eV)

Cu (II) 2p1/2

930

295

Cu(I)2p1/2

S 2p1/2

Intensity (a.u.)

10

106

100 101 102 103 006

110

JCPDS 78-2121 (CuS)

c

b

286.8 eV

Intensity (a.u.)

Intensity (a.u.)

CuS-rGO

159

162

f

165

168

Binding Energy (eV)

Figure 3. (a) XRD pattern of GO-gel and CuS-rGO, (b) C 1s XPS spectra of GO-gel, (c) Cu 2p XPS spectra of GO-gel, (d) C 1s XPS spectra of CuS-rGO, (e) Cu 2p XPS spectra of CuS-rGO, (f) S 2p XPS spectra of CuS-rGO .

100

618 cm

-1

(Cu-S stretching)

800

CuS-rGO Dry GO-gel

a

Intensity (a.u.)

% Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity (a.u.)

Page 31 of 35

Cu-TU

75

GO

50

25

0

b 600

400

200

CuS-rGO GO

0

1000

2000

3000

4000

-1

Wavenumber (cm )

Figure 4.

600

900

1200

1500

1800

-1

Raman shift (cm )

(a) Comparative FTIR spectra of GO, Cu-TU, GO-gel and CuS-rGO. (b)

Comparative Raman spectra of GO and CuS-rGO.


31


15 minutes

-2

0.6

0.25 mL TMB (10

b

a

M)

0.1 mL H2O2 (1 M) 0.1 mg catalyst

Absorbance

Absorbance

0.6

0.3

0.0

0.4

0.2

0.0 550

600

650

700

750

4

Wavelength (nm) 0.6

6

0.56

c

CG-1

5

7

8

pH

CG-2 CG-3

d

Spherical CuS -rGO CuS rGO GO NO Catalyst

0.0

0.6

0.48

y = 0.414 + 0.00383 x 2

R = 0.9946

Absorbance

0.3

Absorbance

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 35

0.40

0.5

0.4

10

20

30

40

Concentration of H2O2 (mM)

550

600

650

700

750

10

Wavelength (nm)

20

30

40

Concentration of H2O2

Figure 5. (a) Time dependent absorbance spectra of TMB oxidation catalyzed by CuS-rGO in presence of H2O2 {TMB (0.01 M) = 25 µL; H2O2 (1 M) = 100 µL; catalyst = 0.1 mg; pH = 4.4}, (b) pH dependent peroxidase-like activity study for CuS-rGO catalyst, (c) comparative catalytic activity of the various catalysts at pH 4.4, (d) Dependence of peroxidase-like activity of CuS-rGO under variable concentration of H2O2. Inset shows the linear calibration curve for H2O2. Error bars represent for the standard deviation for three repetitive measurements.


32


-1

1.25 y = 0.48 + 0.01308x

1.0

R = 0.9960

8

0.5

1.00

0.75

0.50

0.0

0

10

20

30

40

50

-1

1/S0 (mM )

0.0

0.1

0.2

TMB concentration (mM)

0.3

1.5

b 0.9 y = 0.6024 + 1.88x

1.0

R = 0.9831

8

a 1.5

2.0

1/V0 (10 )

-8

Velocity (V0) (Ms ) (10 )

2.0

1/V0 (10 )

-8 -1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Velocity (V0) (Ms ) (10 )

Page 33 of 35

0.5

0.8

0.7

0.6

0.0

0.04

0.08

0.12

0.16

-1

1/S0 (mM )

0

10

20

30

40

H2O2 concentration (mM)

Figure 6. Steady-state kinetic assays using Michaelis–Menten and Linewaver–Burk models (inset) of the CuS-rGO nanocomposites. (a) The concentration of TMB was varied from 0-25 mM with 40 mM H2O2 concentration and (b) the concentration of H2O2 was varied from 0-40 mM in presence of 0.1 mM TMB solution. All the experiments were performed with 0.1 mg catalyst at pH= 4.4.


33


0.6

y = 0.47125-0.004x

b

2 µM 5 µM

0.4

.

Absorbance

Absorbance

0.6

0 µM

a

. .

0.2

. . .

0.0

.

0.4

0.2

. 100 µM

-0.2 550

600

650

700

0.0

750

0

Wavelength (nm)

50

75

100

DA concentration (µM)

c

Glu AA Phe Tryp Tyro

0.2

UA DOPA DA

0.0

600

650

700

750

A-ABlank at λ max = 652 nm

Blank

0.4

-0.2 550

25

0.50

0.6

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 35

d

0.25

0.00

Wavelength (nm)

Glu

AA

Phe

Tryp

Tyro

UA

DA

DOPA

Figure 7. (a) UV-vis spectra of TMB oxidation in presence of varied amount of DA, (b) The linear calibration graph for DA; Error bars represent the standard deviation for three parallel measurements, (c) UV-vis spectra of TMB oxidation in presence of 100 µM various organic species, (d) selectivity comparison of CuS-rGO based catalytic system among the tested species and inset shows corresponding digital image.


34

Page 35 of 35

TOC

85°C, 3 h

GO-integrated Cu(I)-TU Gel

A-ABlank at λ max = 652 nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TMB + H2O2

Glu

AA

Phe Tryp Tyro UA

DA DOPA


35

A Gel-Based Approach To Design Hierarchical ... - ACS Publications

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