We report the synthesis of a gold-carbon dot nanocomposite and its utility as a recyclable catalyst for the reduction of aromatic nitro groups. The presence of carbon dots on gold nanosurfaces enhanced the reduction rate by two-fold

Bera, Abhijit

Bhattacharyya, Swarup Krishna

Das, Mithun

Dwivedi, Jaya

Ganguly, Debabrata

Ghosal, Krishanu

Gupta, Bipin Kumar

Gupta, R. K.

Kumar, Pawan

Maiti, Subhabrata

Mart, Angel A.

Mondal, Pritiranjan

IR@NPL

RSC AdvancesCOMMUNICATIONPublished on 23 May 2014. Downloaded by IIT Indore , Central Library on 30/01/2015 08:01:51. View Article OnlineView Journal  | View IssueaDepartment of Applied Chemistry and IndVidyamandira, Belurmath, Howrah-711 20gmail.com; dganguly93@gmail.combDepartment of Chemistry, Pittsburg State UcDept. of Chemistry and Bioengineering, RicdNational Physical Laboratory (CSIR), DrIndia. E-mail: bipinbhu@yahoo.com† Electronic supplementary informationGNPs and CDs, other experimental detailgraph, see DOI: 10.1039/c4ra02837fCite this: RSC Adv., 2014, 4, 25863Received 31st March 2014Accepted 20th May 2014DOI: 10.1039/c4ra02837fwww.rsc.org/advancesThis journal is © The Royal Society of CFormation of a gold–carbon dot nanocompositewith superior catalytic ability for the reduction ofaromatic nitro groups in water†Pritiranjan Mondal,*a Krishanu Ghosal,a Swarup Krishna Bhattacharyya,a Mithun Das,aAbhijit Bera,a Debabrata Ganguly,a Pawan Kumar,d Jaya Dwivedi,d R. K. Gupta,bAngel A. Mart´ı,c Bipin Kumar Gupta*d and Subhabrata MaitiaWe report the synthesis of a gold–carbon dot nanocomposite and itsutility as a recyclable catalyst for the reduction of aromatic nitrogroups. The presence of carbon dots on gold nanosurfaces enhancedthe reduction rate by two-fold.Nanomaterials have emerged as new prospects across scienticdisciplines owing to their specic physicochemical and opto-electronic properties. In this context, since the pioneering workof Haruta, researchers have found the usefulness of nano-materials for catalyzing chemical reactions.1 At present, noblemetallic nanoparticles like gold and silver are being used incatalyzing diversied class of reactions ranging from carbonmonoxide oxidation, C–C coupling to hydrogenation.1–3 Theadvantages of using gold nanostructures in modulating catal-ysis lie in their straightforward synthesis in diﬀerent forms,stability, and surface functionalization, among others.4,5 Typi-cally, smaller nanoparticles show higher catalytic ability invarious kinds of reactions.5 However, in the case of redoxreactions, anisotropic nanostructures with high electrontransporting ability such as nanocages or nanoboxes have beenobserved to enhance the catalytic rate.4Amalgamation of carbon nanostructure and metal nano-particle is providing some exceptional features in terms of theircatalytic, electronic or photovoltaic properties.6,7 To this end,among the family of nanodimensional allotropes of carbon,carbon dots (CDs) (which are new but exciting) are showinginteresting potential in applications ranging from sensing,ustrial Chemistry, Ramakrishna Mission2, India. E-mail: pritiranjanmondal21@niversity, Pittsburg, KS 66762, USAe University, Houston, TX 77005, USAK S Krishnan Road, New Delhi 110012,(ESI) available: Details of synthesis ofs, TEM images, UV-vis spectra, kineticshemistry 2014biomedical devices to photocatalytic applications owing to theirintrinsic photophysical properties.8 CDs can also exhibit highelectron transporting ability as they have an electron richgraphite core. At this point, a nanocomposite comprising CDsand an anisotropic gold nanospecies (like GNR) is of highinterest for catalyzing redox type reactions. GNR stabilized withcationic cetyltrimethylammonium bromide (CTAB) can beconjugated with carbon dots having anionic carboxylate func-tional groups. In principle, the designed composite might oﬀerseveral advantages: (i) the presence of an anisotropic goldnanospecies (aGNS), which is already known for its high cata-lytic eﬃciency due to the presence of diﬀerent facets; (ii) theavailability of the graphite core of CDs, which can expedite thetransfer rate of electrons; and most importantly, (iii) thehydrophobic domain between aGNS and CDs can help traphydrophobic molecules, which would allow aqueous catalyticreactions of water-insoluble compounds. Thus, an aGNS–CDnanocomposite is an interesting approach for the developmentof a superior nanocatalyst. Therefore, a gold–CD nano-composite is an interesting approach for the development of asuperior nanocatalyst.Initially, we synthesized aGNS with a transverse surfaceplasmon peak (ltrans, SPR) at 520 nm and a longitudinal peak(llong) at 695 nm using standard protocols (Fig. 1a).10 The detailsof the synthetic procedure can be found in the ESI.† Trans-mission electron microscopy (TEM) images revealed that thewidth of the aGNS (among some other gold nanospecies likenanocubes) is 10  2 nm with an aspect ratio of 2.8  0.3(Fig. 1b and c and S1a, ESI†). The synthesized aGNS are highlystable as they are stabilized by the quaternary ammoniumgroup of cetyltrimethylammonium bromide (CTAB).10In order to produce the aGNS–CD nanocomposite, wedesigned carbon dots functionalized with a long chain anioniccarboxylate (Fig. 1d). CDs were synthesized by thermal couplingbetween sodium-salt of 11-aminoundecanoic acid and citricacid as discussed elsewhere (Fig. 1d).11 Here citric acid acts asthe source of the carbon core and 11-aminoundecanoic acid iscovalently linked with the carbon nanoparticle via amide bonds.RSC Adv., 2014, 4, 25863–25866 | 25863Fig. 1 (a) UV-vis spectra, (b and c) TEM images of synthesized aGNS–CD nanocomposite ([Au] ¼ 100 mM, [CD] ¼ 100 mg mL1) and (d)schematic representation of the formation of the aGNS–CDnanocomposite. Fig. 2 Fluorescence spectra of CD in absence and presence of aGNSof varying concentration (Inset: Visual ﬂuorescent image of CD).RSC Advances CommunicationPublished on 23 May 2014. Downloaded by IIT Indore , Central Library on 30/01/2015 08:01:51. View Article OnlineThe size of the CDs was 3  1 nm as found from TEM images(Fig. S1b, ESI†). Moreover, a negative zeta potential (z) value of23.8  2 mV conrmed the presence of a carboxylate moietyon the surface of the CDs.The synthesized aGNS solution was centrifuged at 14 000rpm for 90 min to remove excess CTAB, and the residualmaterial was redispersed in aqueous phosphate buﬀer (pH7.0, 25 mM). This buﬀered aGNS solution was then used tomake the conjugate with CDs functionalized with a carboxy-late moiety. The nanoconjugate was prepared by mixing aGNSand CD solutions in phosphate buﬀer at diﬀerent ratios. Theformation of the aGNS–CD nanoconjugates was conrmedboth by spectroscopic and microscopic methods. The strongphotoluminescence properties of the CDs (emission andexcitation wavelength maxima ¼ 420 and 340 nm) werequenched in the presence of aGNS, as gold nanomaterials areknown to quench the emission of uorophores in closeproximity (Fig. 2).5 The blue-shi in emission maxima (byalmost 20 nm, from 420 nm to 400 nm) in the presence ofaGNS is likely due to the interaction between nanogold andCDs. In addition, the TEM images (Fig. 1b and c) show theoccurrence of CDs over the gold surface, which is consistentwith the formation of the gold–CD composite. We proposethat the electrostatic attractive force between cationic aGNSand anionic CDs, as well as the hydrophobic attraction of thelong carbon chain, play crucial roles in nanocompositeformation (Fig. 1d).To test the catalytic ability of the synthesized nano-composite, we chose the reduction of p-nitrophenol as themodel reaction.12 The conversion of p-aminophenol from itsnitro analogue is a reaction of versatile applications thatincludes antipyretic or analgesic drug development, dyeindustry, and corrosion removal.13,14 Moreover, the reactiondoes not proceed in the absence of any catalyst. The progressionof the reaction can be easily monitored by observing the rate atwhich the characteristic 400 nm peak of p-nitrophenoldecreases as it converts to p-aminophenol with the generation25864 | RSC Adv., 2014, 4, 25863–25866of a new peak at 300 nm (Fig. S2 and S3a, ESI†). The reactionfollowed pseudo rst order kinetics as the concentration ofNaBH4 is in excess (almost 1000 times higher) of that of p-nitrophenol.Initially, we monitored the apparent rate constant (kapp) ofthe reaction in the presence of varying concentrations of aGNS([Au] ¼ 10–100 mM). We have observed an increase in kapp (from0.05 to 0.072 s1) with increasing Au concentration up to 50 mM,and then it remained almost constant (Table S1, ESI†). At thispoint, the gold–CD composite was fabricated by varying CDconcentration from 12.5 to 100 mg mL1, while the concentra-tion of aGNS was kept constant (50 mM). Increasing the aGNSconcentration resulted in colloidal instability of the composite.Interestingly, an increase in the CD concentration in the gold–CD composite (up to 50 mg mL1) led to a 2-fold increase in thekapp from 0.072 to 0.143 s1, while a further increase in the CDconcentration (100 mg mL1) had almost no eﬀect on the kapp at0.135 s1 (Fig. 3 and S3, ESI†). However, the presence of onlyCDs led to an almost negligible eﬀect towards catalyzing thereaction (Fig. S3, ESI†).Furthermore, we also monitored the recyclability of thegold–CD nanocomposite in catalyzing the reduction. Ourexperiments showed that the catalytic ability remained atabout 80% even aer completing the fourth reduction cycle(Fig. 4). In fact, the TEM image of the gold–CD nano-composite revealed the encirclement of the CDs on the goldnanosurface even aer the fourth cycle (Fig. S4, ESI†).However, the gradual loss in the catalytic ability might be dueto detachment of parts of CDs (probably during puricationof the composite by centrifugation aer each run) from thegold nanosurface and the corresponding decrease in electrontransfer eﬃciency by the CDs as the number of cycleincreases. This cyclic stability and retainment of the catalyticactivity of the nanocomposite is of high signicance in termsof its use as a potential catalyst.This journal is © The Royal Society of Chemistry 2014Fig. 3 Variation in kapp of p-nitrophenol reduction by NaBH4 in thepresence of aGNS–CD nanocomposite in aqueous phosphate buﬀer(pH 7, 25 mM) at 25 C ([Au] ¼ 50 mM in all aGNS–CD composites).Fig. 4 Recyclability of the gold–CD nanocomposite as a catalyst forthe reduction of p-nitrophenol.Communication RSC AdvancesPublished on 23 May 2014. Downloaded by IIT Indore , Central Library on 30/01/2015 08:01:51. View Article OnlineIt is obvious that the presence of CDs resulted in a two-foldincrease in the catalytic rate of the reduction of p-nitrophenolwhen combined with aGNS, which itself is known as a goodcatalyst for this reaction.9 This might be due to the accelerationof the electron transport through the graphitic carbonaceouscore of the CDs.15 In addition, p-nitrophenol is rmly placedwithin close proximity to the gold surface aided by the hydro-phobic carbon chain of the nanocomposite. In fact, the pres-ence of p-nitrophenol in the hydrophobic domain wasconsistent with a slight blue-shi in the UV spectra of thecompound in the presence of the aGNS–CD composite inaqueous buﬀer (Fig. S5, ESI†). Thus, it is likely that an enhancedcapacity for transferring electrons leads to an increase in thecatalytic rate of the reduction. However, beyond a certainconcentration of CDs in the aGNS–CD nanocomposite, the rateThis journal is © The Royal Society of Chemistry 2014of catalysis remained the same probably due to the lack ofpenetration space for the reactant molecules to get close to thegold nanosurface.To this end, we have also observed the ability of this gold–CDnanocomposite to catalyze the reduction of o-nitrophenol too-aminophenol. The concentration of Au and CDs in thenanocomposite were maintained at 50 mM and 50 mg mL1,respectively. In this case, the progression of this pseudo rstorder reaction was monitored spectrophotometrically byobserving the rate of decrease in the absorbance at 420 nm(characteristic UV peak of o-nitrophenol) (Fig. S6, ESI†). Herealso, the rate of the reduction was increased by 1.6-fold in thepresence of CDs. The observed kapp was 0.064 s1 in only aGNSsolution, which increased to 0.104 s1 in the gold–CD nano-composite in aqueous buﬀer (Fig. S6, ESI†); however, in theabsence of the catalyst, almost no reduction occurred.We also used one water-insoluble substrate (m-dinitroben-zene) to check the eﬃcacy of our designed gold–CD nano-composite towards its reduction. m-Dinitrobenzene wasreduced to m-phenylenediamine in the presence of NaBH4 andour nanocomposite in aqueous phosphate buﬀer.14 The corre-sponding change in the UV-vis spectra also demonstrated this fact(Fig. S7, ESI†). In addition, the gradual blue shi in the UV-visspectral maxima ofm-dinitrobenzene (244 to 241 nm) in the aGNSand gold–CD composite validates the presence of the compoundin the hydrophobic domain of the nanocomposite (Fig. S8, ESI†).In this case also, the pseudo rst order rate constant of the reac-tion was found to be increased by two-fold in the presence of CDs(from 0.14 s1 to 0.28 s1) (Fig. S9, ESI†).In conclusion, we have developed a new class of gold–CDnanocomposites, which show high eﬃciency in catalyzing thereduction of diﬀerent aromatic nitro groups. Importantly, to thebest of our knowledge, this is the rst report of the catalyticproperties of carbon dots in conjugation with a gold nano-species for the reduction of both hydrophilic and hydrophobicnitroarenes. We believe that the presence of CDs increased therate of electron transfer, as well as secures the localization of thereactant molecules within the catalyst; thus, a two-foldenhancement in catalytic activity in comparison to only aGNSwas found in almost every case. This novel catalyst is stable inwater and has shown good recyclable eﬃciency towards catal-ysis. Most interestingly, the catalysis can also be eﬃcientlyperformed for water-insoluble compounds, like m-dinitroben-zene. Therefore, the presented nanocomposite has potentialapplications in the design of various industrially importantcatalysts.AcknowledgementsWe are thankful to the University Grant Commission (UGC) andthe Department of Higher Education, West Bengal for fundingthis study. We are truly grateful to our Principal, Vice-PrincipalMaharaj and faculty members of the Chemistry Department.Notes and references1 M. Haruta, Catal. 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Formation of a gold-carbon dot nanocomposite with superior catalytic ability for the reduction of aromatic nitro groups in water

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Formation of a gold-carbon dot nanocomposite with superior catalytic ability for the reduction of aromatic nitro groups in water

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