DOI: 10.1002/cssc.201000410

Removal of Organic Compounds from Water by Using aGold Nanoparticle–Poly(dimethylsiloxane) NanocompositeFoamRitu Gupta and Giridhar U. Kulkarni*[a]

Introduction

Water treatment is important for various purposes. Nontrivialpurification of water involves removal of toxic ions, organic im-purities, microbes, and their byproducts, as well as oil spills.The removal of organic contaminants from water is a major in-dustrial concern. The contaminated water from industries usu-ally contains considerable amounts of organic hydrocarbons,such as benzene, toluene, ethylbenzene, and xylene (BTEX),prohibiting direct release into natural water bodies. The dis-solved organic compounds cause bacterial growth, odor gen-eration, and biofouling, which results in limited reuse.[1]

While water treatment is a diverse field, herein we focus onremoval of organic compounds from water. The simultaneousremoval of oil spills, organic pollutants, and toxic chemicalsfrom water demand a combination of material properties, suchas high sorption capacity; longer retention; and a wide rangeof catalytic activity, recoverability, and reusability of the materi-al. The challenging goal here is to detect, decompose, andremove contaminants usually present in low concentrations.[2]

Towards this end, different types of sorbent materials havebeen developed to date, the most common of which is activat-ed carbon.[3] Though the use of activated carbon is still consid-ered to be one of the best methods, the disposal of adsorbedcontaminants along with the adsorbent is a major concern.Thus, the decomposition of the adsorbed organic contami-nants and regeneration of the adsorbent is a critical step to-wards cost-effective use of the porous adsorbents. With world-wide efforts in this direction, the literature abounds with alter-native solutions. Zeolites, butyl rubber, polypropylene, and hy-drophobic cotton fibers have been in use for some time, andmore recently with the advent of nanotechnology, many morematerials have been added to the list of sorbents.[4] These in-clude carbon nanotubes,[5] metal nanoparticles,[6, 7] and porousnanostructured materials,[8] as well as cyclodextrins.[9] For ex-ample, Zuo et al. synthesized a nanocomposite of multiwalled

carbon nanotubes and nanoparticles for catalytic oxidationand removal of phenols.[10] Ichiura et al. prepared a compositeof TiO2 with zeolites for the removal of indoor pollutants usingUV irradiation.[11] The chemistry of noble-metal nanoparticleshas also been utilized for the purification of water from halo-genated organic compounds, pesticides, heavy metals, and mi-croorganisms.[12] Wu et al. prepared carbon nanotube spongesfor the removal of organic pollutants, such as oils and solvents,from the surface of water.[13] Polydimethylsiloxane (PDMS) canbe a robust, efficient, and recoverable adsorbent material, asshown by Park et al. , for the removal of BTEX and oil spillsfrom water.[14] Moreover, it is also highly lucrative, inexpensive,and commercially available.[15]

Recently, we reported the synthesis and characterization ofgold nanoparticle/PDMS nanocomposites (AuPDMS) in theform of gels, foams, and films.[16] The importance of AuPDMSfoam for water treatment was recognized in an earlier reportof a short study that involved the removal of toluene, thiophe-nol, thioanisole, and Na2S.[16] Herein, we report a detailed studyon the kinetics of contaminant removal with thioanisole as amodel system. We have also addressed the efficiency and re-coverability of the material. In our study, the active role of thegold nanoparticle surface for sulfur bonding has been exploit-ed for the effective removal of odorous, sulfur-containing mol-ecules. In addition to odorous molecules, we have studied theremoval of BTEX and oil spills using an AuPDMS composite

A low density, highly compressible, porous foam of poly(di-methylsiloxane) (PDMS) incorporated with Au nanoparticles(10–50 nm) has been synthesized by using a single-step pro-cess with water as a medium. It exhibits high swelling ability (�600 %) against benzene, toluene, ethylbenzene, and xylene(BTEX)—a property that has been exploited in the removal ofoil spills from water. It is resistant to harsh chemical environ-ments. It is also effective against odorous sulfur containing

contaminants such as thioanisole. It works repeatedly and effi-ciently over many cycles. The regeneration of the foam israther simple: heating in air to 300 8C for short time bringsback its original activity. The fascinating properties of Au nano-particles could be mingled with those of PDMS to provide asustainable and practical solution for water treatment. It is alsodemonstrated to work effectively for deodorizing garlic extractwith a promise as a food packaging material.

[a] R. Gupta, Prof. G. U. KulkarniChemistry and Physics of Materials Unit, DST Unit on NanoscienceJawaharlal Nehru Centre for Advanced Scientific ResearchJakkur P.O. , 560064 Bangalore (India)Fax: (+ 91) 80 22082766E-mail : kulkarni@jncasr.ac.in

Supporting Information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cssc.201000410.

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foam. We synthesized a series of AuPDMS composite foamsamples with varying physical features, for example, compressi-bility, density, and pore volume, by tuning the reaction condi-tions. Interestingly, we have been able to test the affinity ofthe AuPDMS foam towards garlic to explore its potential appli-cations in the food packaging industry.

Results and Discussion

Synthesis and characterization of the foams

The AuPDMS and PDMS foams were synthesized and charac-terized by spectroscopic and electron microscopy techniquesbefore using them for water purification.

Three samples of AuPDMS foams were prepared by using0.1, 0.5, and 1 mm aqueous solutions of KAuCl4 following theprocedure described in Experimental Section and are designat-ed AuPDMS-1, AuPDMS-2, and AuPDMS-3, respectively. ThePDMS solid becomes red in color within 30 min and begins tofloat on top of the solution due to a lower density (Figure 1 a).The UV/Vis spectra (Figure 1 b) of the foam samples exhibit the

characteristic peak at 534 nm due to surface plasmon excita-tion in Au nanoparticles;[17] the intensity increases in the orderof nanoparticle loading, AuPDMS-1 to AuPDMS-3. The weightpercentage of Au nanoparticles calculated from the weight ofPDMS and precursor concentration (assuming complete reduc-tion of Au3+ ions), amounts to an Au loading (by weight) of0.01, 0.03, and 0.06 %, respectively, for the three samples. At-tempts to incorporate a higher Au content resulted in uncured,sticky AuPDMS gels. It should be noted that there is no exter-nal reducing agent used in this preparation of Au nanoparti-cles. The particles are formed inside the PDMS matrix at the

cross-linking Si�H sites. The IR spectra provide direct evidenceof this process, in which a relative decrease in Si�H and an in-crease in Si�O�Si bond intensity is observed upon formationof Au nanoparticles in PDMS (shown in Figure S1 in the Sup-porting Information). Concomitantly, the formation of Au nano-particles is expected to slow down the cross-linking process ofthe polymer, since the number of available Si�H sites decreas-es. While the elevated temperature during the reaction (80 8C)can effectively keep the cross-linking process going, it produ-ces water vapor that gets trapped inside and around thePDMS matrix. As the trapped vapor escapes, the curing processresults in a porous structure that is highly expanded relative tonormal PDMS. The elastic property of PDMS is largely retained,which provides a compressible nature to the foam (see Fig-ure S2 in the Supporting Information).

Table 1 lists the various physical parameters obtained for thefoams. We observe that higher the nanoparticle loading in thecomposite results in a lower density. Because the PDMS surface

is hydrophobic and its density is comparable to that of water,the foams essentially float on water (Figure S2b in the Support-ing Information). The degree of compressibility of the foamalso varies with the Au nanoparticles loading: the higher theAu content results in a more compressible foam (Figure S2c inthe Supporting Information and Table 1). Importantly, thefoams regained shape following decompression and were notdisintegrated. The pore volume was also much higher thanthat of PDMS, the AuPDMS-2 foam had the largest porevolume of 5.52 cm3g�1 (Table 1). Although AuPDMS-3 containsa higher Au loading, we found it to be highly fragile, perhapsdue to hindered cross-linking. Of the three AuPDMS foams, theAuPDMS-2 foam was chosen for further studies along with theplain PDMS foam for comparison.

The microstructure and morphology of the pores in theAuPDMS-2 foam is shown in Figure 2. The presence of openand closed micropores is evident in Figure 2 a and b. Theclosed pores appear like pits on the surface of the foam,whereas open pores go deep inside the sample. The foam con-sists of irregular and interconnected pores distributed all overthe surface in the order of 2–20 mm in size. The variations inthe contrast arise due to the nonplanar sample area inside thepores. The histogram in Figure 2 c shows a bimodal distribu-

Figure 1. a) Photographs showing the synthesis of an AuPDMS foam. Forcompletion of the reaction, the medium was stirred for 2 h. b) UV/Vis spectraof AuPDMS-1, AuPDMS-2, and AuPDMS-3 synthesized by using KAuCl4 pre-cursor concentrations of 0.1, 0.5, and 1 mm, respectively. The spectrum ofPDMS is also shown for comparison. The curves are shifted laterally for clari-ty.

Table 1. Physical properties of the AuPDMS foams along with those ofthe PDMS foam for comparison.

Sample Au[a]

[wt %]Density[b]

[g cm�3]Compressibility[%]

Pore volume[cm3 g�1]

PDMS 0 1.3 48�2 0.97�0.1AuPDMS-1 0.01 1.2 87�4 2.67�0.1AuPDMS-2 0.03 1.1 105�3 5.52�0.1AuPDMS-3 0.06 0.9 127�5 3.12�0.1

[a] Au content [%] is calculated by assuming complete reaction. [b] Thedensity of foam is calculated by taking its weight and volume (by consid-ering the foam to be cylindrical) from geometrical parameters, as shownin Figure S2a in the Supporting Information.

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G. U. Kulkarni and R. Gupta

tion of pores with maxima around 4 and 12 mm for small andlarge pores, respectively.

The Au nanoparticles formed inside PDMS were character-ized by TEM and SEM. The TEM image in Figure 3 a shows thatthe Au nanoparticles are mostly spherical in shape. The ED pat-tern clearly indicates that the particles are crystalline (Fig-ure 3 b). The Au nanoparticles bound to the surface of PDMSare seen in the SEM image shown in Figure 3 c. The particlesize varies over the range of 10–30 nm as revealed by the his-togram shown in Figure 3 d.

Stability of the foams

The robustness of the Au nanoparticles in the AuPDMS-2 foamagainst chemicals was investigated by UV/Vis and IR spectros-copy (Figure S3 in the Supporting Information). As shown inFigure S3a in the Supporting Information, the surface plasmonabsorption band from the Au nanoparticles was hardly affect-ed even after exposure to harsh chemical environments, suchas concentrated ammonium hydroxide and sulfuric acid (6 m),although the polymer underwent slight degradation, as ob-served in the IR spectra (Figure S3b in the Supporting Informa-tion). Furthermore, the intensity of the plasmon peak remainedunaltered when the AuPDMS-2 foam was under a flow ofwater for prolonged time intervals (Figure S4 in the SupportingInformation), indicating insignificant leaching of Au nanoparti-cles from the foam.

Contaminant removal by using the foams

We have carried out thermogravimetric analysis (TGA) of thefoams to examine their stability and regeneration. The TGAtrace of the plain PDMS foam shows a gradual weight loss upto 20 %, while heating below 300 8C; this is attributable to thedepolymerization of PDMS chains to give volatile cyclic oligo-mers.[18] Otherwise, both foams are nearly stable till 300 8C; anaspect that is crucial in regeneration after contaminant remov-al. The thermal decomposition of the AuPDMS-2 takes placesharply at 506 8C (Figure 4). This is clearly seen from the deriva-tive curves shown in Figure 4 c. On the other hand, the PDMSfoam decomposes at higher temperature ranging from 490 to590 8C; a property inherent to PDMS. Interestingly, the totalweight loss is higher for the AuPDMS-2 foam (71 %) than theplain PDMS foam (54 %), despite additional weight due to thepresence of Au nanoparticles in the matrix. In the former, thedecomposition became simpler and easier perhaps due topoorer cross-linking of the polymer. The products after theheat treatment have been examined. Au is present in extreme-ly small amounts (0.4 at. wt %; Figure 4 c), as estimated fromEDS spectra of the AuPDMS-2 sintered product (Figure S5 inthe Supporting Information). Furthermore, the FTIR spectrumof the pyrolyzed end products of the AuPDMS-2 and PDMSfoams shown in Figure 4 d became featureless in the range of1500–4000 cm�1 due to the decomposition of C�H, C�C, andSi�H bonds, which are expected around 2800–3000 cm�1 and2165 cm�1, as seen in Figure S1 in the Supporting Informationand also observed in the literature.[19] After heating theAuPDMS-2 and PDMS foam, the FTIR spectrum shows presenceof only three characteristic absorption bands attributed to Si�O�Si, Si�C, and Si�O�C stretching, which indicates completedecomposition. The strong signal at 1105 cm�1 in pyrolyzedend product of PDMS corresponds to Si�O�Si stretching,which is due to the formation of amorphous silica, as also seenin the XRD pattern (Figure S6 in the Supporting Information).Another lower intensity signal at 790 cm�1 is from Si�C stretch-ing due to the terminal CH3 ends of PDMS bonded to Si. Theend product obtained from the decomposition of AuPDMS-2shows the presence of Si�O�C (1210 cm�1) and Si�C

Figure 2. a) and b) SEM images of the AuPDMS-2 foam at different magnifi-cations c) The pore size distribution obtained from a scanning electron mi-crograph of the AuPDMS-2 foam.

Figure 3. a) TEM image, b) electron diffraction (ED) pattern, and c) SEMimage of Au nanoparticles in the PDMS foam. d) Particle size histograms ob-tained from the TEM image by statistical analysis of 75 particles. The largerfeatures observed in TEM are considered to be loose aggregates of nanopar-ticles and are not part of the histogram.

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Gold Nanoparticle–Poly(dimethylsiloxane) Nanocomposite Foam

(790 cm�1) stretching bonds present in a higher ratio than Si�O�Si bonds, indicating the possible presence of silicon oxycar-bides along with amorphous silica. What is noteworthy fromTGA measurements is that the end products are simple andenvironmentally benign. The AuPDMS-2 foam produces lesssolid residues.

The AuPDMS-2 foam was found to have a high swelling ca-pacity for different aromatic hydrocarbons, such as benzene,toluene, ethylbenzene, and xylene, present in contaminatedsolvents, as shown in Figure 5. With different hydrocarbons,the PDMS foam was found to swell between 60 and 160 % ofits weight (Figure 5). Amazingly, the degree of swelling wasfound to be nearly six times greater (300–600 % of its originalmass) with the AuPDMS-2 foam. The uptake was withinminutes.

The AuPDMS-2 foam has also been tested for the removalof oil spills, as demonstrated by using diesel oil (Figure 6). Thediesel oil on the surface of the water was rapidly sucked up by

the AuPDMS-2 foam. The dieseluptake capacity of AuPDMS-2and PDMS was found to be 125and 25 % of its weight, respec-tively. The values are moderatecompared with other adsorbentmaterials (shown in Table S1 inthe Supporting Information). Im-portantly, the foam could be re-generated by heating to 300 8Cfor 30 min, so that the adsorbeddiesel oil was given up. This oilremoval test was repeated 10times with approximately equaldegrees of removal.

The role of Au nanoparticlespresent in the AuPDMS-2 foamagainst odorous contaminantmolecules, such as thiophenol,thioethers, and sulfides, has al-

ready been realized in our previous study.[16] The rate of remov-al, efficiency of the process, the recovery of the foam, and theregeneration method were investigated for the removal ofodorous compounds, such as thioanisole (methyl phenyl sul-fide). Thioanisole is a typical example of an organic sulfide thatis a toxic pollutant and imparts a pungent odor to water.[20, 21]

In fact, the AuPDMS-2 nanocomposite foam shows a high se-lectivity for sulfur-containing molecules. Au nanoparticlesinside the foam may offer a high surface area for binding ofthioanisole molecules and should enable easy extraction of thi-oanisole from contaminated water.

To quantify the efficiency of AuPDMS-2 for removal of thioa-nisole from contaminated water, water (200 mL) contaminatedwith 0.2 mm thioanisole was treated with the AuPDMS-2 foam(1 g) for set intervals of time (Figure 7). Between the intervals,the foam was heated for 2 h in air at 300 8C to remove the ad-sorbed thioanisole and regenerate the foam. The thioanisole

Figure 5. Swelling degree of AuPDMS-2 foam for BTEX in comparison withthe PDMS foam. The sequential steps of dipping, removing, and weighingwere repeated until the weight reached a constant value. The whole processwas done quickly enough that evaporation of the solvent was avoided. Thesame procedure was adopted for the plain PDMS foam for comparison.

Figure 6. Optical photographs, taken during removal of diesel oil from watersurface, of a) a Petri dish containing water colored with R6G dye for visualaid, b) diesel oil (100 mL) dropped over water (5 mL), c) the AuPDMS-2 foam(0.5 g) brought close to oil drops, d) the oil drops being taken up by thefoam, e) the foam carrying the diesel oil being moved away, and f) the waterin the Petri dish after removal of the oil spill.

Figure 4. a) TGA data, b) first derivative plot of TGA of PDMS and AuPDMS-2 foams, c) Energy dispersive X-rayspectroscopy (EDS) signal distribution, and d) FTIR spectra of decomposition products of PDMS and AuPDMS-2foams.

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G. U. Kulkarni and R. Gupta

shows an absorption signal at 250 nm in the UV/Vis spectra,which decreased in intensity with time (Figure 7 a). It was usedto quantify the percentage removal of thioanisole per gram ofthe AuPDMS-2 foam used. The lower the intensity of the UV/Vis spectrum the higher is the percentage of thioanisole re-moval. The concentration corresponding to each absorption in-tensity value was evaluated by using a calibration curve (Fig-ure S7 in the Supporting Information). The concentrationversus contact time plot followed first-order kinetics (Fig-ure 7 b). The rate constant was estimated to be 0.15 h�1. Thisvalue is comparable to the rate constants from TiO2-based ma-terials (0.18–0.46 min�1).[22, 23] However, the latter refers to cata-lytic conversion rather than removal of contaminants, as in thecase of AuPDMS-2 foam. The adsorbed thioanisole is seen inthe UV/Vis spectrum of the AuPDMS-2 foam as a redshiftedsignal at around 256 nm (see Figure S8 in the Supporting Infor-mation). The AuPDMS-2 foam was further tested for reusabilityby conducting several cycles of thioanisole removal as, shownin Figure 7 c. Several stock solutions of 0.29 mm thioanisole(200 mL) were treated in succession, every 2 h, with a chosenpiece of the AuPDMS-2 foam (1 g), which was regeneratedevery time by heating to 300 8C. The UV/Vis spectra from thesolution, given in Figure 7 d, show a decrease in absorption in-tensity after each removal step. The treatment process was re-stricted only to 2 h and the calibration curve shown in Fig-ure S7 (given in the Supporting Information) was used for esti-

mation. The percentage removalwas estimated to be around20 %, with around 10 % variation(Figure 7 e). This is quantitativelysignificant for understanding thereusability or recycling efficiencyof the material. During the re-generation of AuPDMS-2, thesignal at 256 nm vanished, im-plying complete removal of thio-anisole by desorption (see Fig-ure S8 in the Supporting Infor-mation).

The TGA of AuPDMS andPDMS foams containing thioani-sole has been carried out(Figure 8). The TGA data showsthe weight-loss behavior forAuPDMS and PDMS foams treat-ed with thioanisole. The initialweight losses at around 90–100 8C were attributed to theloss of water trapped insidepores. The water loss was 33.8 %in the case of AuPDMS com-pared with 3.3 % in PDMS; thiswas clearly as a result of ahigher pore volume in theformer. No weight-loss featuredue to the adsorbed thioanisolewas expected because from esti-

mates it would be below the detection limit of TGA (0.1 %).The removal of the trapped thioanisole may take place in therange from 100 to 250 8C.[24,25] The additional weight losses cor-

Figure 7. a) UV/Vis spectra of thioanisole in water after repeated treatment with AuPDMS-2 foam. b) Variation inthe concentration of thioanisole with progressive treatment. c) Schematic illustration of the cyclic removal of con-taminants by using AuPDMS-2 composite foam. d) UV/Vis spectra corresponding to different cycles, every 2 h, ofthioanisole removal. e) Percentage of thioanisole removal from a fresh stock solution by a given AuPDMS-2 foamin repeated cycles.

Figure 8. a) TGA and b) first-derivative curve for PDMS and AuPDMS-2 foamsafter treatment with thioanisole to test its removal from water.

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Gold Nanoparticle–Poly(dimethylsiloxane) Nanocomposite Foam

respond to decomposition of the foams much in the sameway as without thioanisole adsorption (see Figure 4 a).

The ability of Au nanoparticles to remove odorous mole-cules from water was tested by taking a natural source, garlicextract, in water. Garlic is known for its odor and flavor, whichis derived from the sulfur-containing molecule allicin. Allicinhas thiosulfinate linkages and, at room temperature, it givesrise to 2-propenesulfenic acid and thioacrolien, both of whichcontain sulfur and thus give odor.[26] A few garlic bulbs (10 g)were crushed and ground in water (10 mL). The filtrate wasused as a stock solution of garlic extract. The UV/Vis spectrawere recorded for garlic solutions of different concentrationsto understand the changes in the spectral features (Figure S9in the Supporting Information). The AuPDMS-2 and PDMScomposites were treated with 2 % (v/v) garlic solution for 6 hand absorption spectra from the film were recorded, as seen inFigure 9. As can be seen from Figure 9 a, the PDMS foam ex-

hibits a broad feature at around 258 nm that can be ascribedto an n!p* transition taking place in the allicin molecule. Thecollection of garlic by the PDMS foam is evident, as also con-firmed by the smell. In the case of AuPDMS-2 foam, the signifi-cance of Au nanoparticles could be realized because no appre-ciable change could be seen in the spectrum after garlic treat-ment (Figure 9 b). Moreover, the garlic solution was deodorizedalmost completely on treatment with AuPDMS-2, which couldbe detected by smell. The complex nature of the interaction ofgarlic constituents with Au nanoparticles in PDMS deserves aseparate study. It appears that sulfenic acid and thioacroleinare chemisorbed onto the surface of the Au nanoparticlesthrough sulfur linkages and are possibly converted into mole-cules that possess less or no odor. Thus, the foam could act asbarrier for spreading odorous molecules and may therefore beof interest for food packaging. Interestingly, with the plainPDMS foam, the malodor continued to emerge even after amonth.

Conclusions

This study has dealt with the issue of water purification froman environmental perspective by using an AuPDMS nanocom-posite in the form of a foam. With this nanocomposite, wehave been able to bring together the unique properties of Aunanoparticles and PDMS elastomer. The synthesis is a single-step, room-temperature process involving the reduction ofAu3+ ions by the KAuCl4 precursor in the PDMS matrix in thepresence of water, thus eliminating the need for external re-ducing and capping agents. It is essentially a green process.The Au nanoparticle loading in the PDMS foam has been opti-mized (0.03 wt %) to attain a low density (�1.1 g cm�3), highcompressibility (�105 %), as well as pore volume(�5.45 cm3 g�1) to make it a high-performance material. Theperformance of AuPDMS-2 nanocomposite has been testedagainst BTEX for comparison, where the sorbent capacity was

found to be nearly six timeshigher than that of the PDMSfoam without Au nanoparticles.The material is ideally suited forremoval of organic compoundsand oil spills because of its highand fast swelling capacity withlonger retention, as demonstrat-ed for diesel oil on water. TheAuPDMS-2 foam has also beenutilized for the highly selectiveremoval of an organic sulfide(thioanisole) from water withconsistently good activity; this israrely found in literature.[27] Itsability was also tested to elimi-nate odor from water that con-tained natural garlic extract. Ad-ditionally, the AuPDMS-2 foam

could be recycled and reused repeatedly for many cycles byheating in air at 300 8C.

The AuPDMS composite is indeed an attractive material forwater-based pollutants and odorous molecules. It is a uniquematerial among its kind and can selectively remove odorousmolecules and act as an adsorbent for BTEX and oil spills. Itcan be molded into any shape during or after synthesis and fitinto water channels or installed in water bodies for practicalapplications. This is clearly an advantage compared with sorb-ents in the form of particulates, along with the low cost andclean processing of the material with no byproducts after use.This can have further potential to act as a sensing material forsulfur impurities usually present in aromatic solvents. Its usemay be extended to the removal of malodorous chemicalspresent in air ; this constitutes our future studies.

Experimental Section

Synthesis of PDMS and AuPDMS foams: The AuPDMS foam sam-ples were prepared by using a reported procedure with somemodifications (Figure 1 a).[16] Three foam samples with different

Figure 9. UV/Vis spectra of a) PDMS and b) AuPDMS-2 before and after garlic adsorption. The absorbance valuesare on the same relative scale. The inset shows a schematic of the garlic adsorption process in the PDMS andAuPDMS-2 foams. The spectra are shifted laterally for clarity.

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G. U. Kulkarni and R. Gupta

loading of Au nanoparticles were prepared by using PDMS mixture(2 g; elastomer and curing agent in the ratio 10:1, Dow Corning,Sylgard-184) by stirring for 2 h along with aqueous KAuCl4 (4 mL)of concentrations 0.1, 0.5, and 1 mm at 80 8C. A PDMS foamsample (without Au) was also prepared by stirring in plain water.At the end of preparation, the samples were obtained in the formof foams floating on water. For a model study on removal of or-ganic compounds, a stock solution of thioanisole (Aldrich) in water(0.2 mm) was prepared and kept in tightly closed bottles andstirred for 24 h prior to use. Other solvents used in the study (tolu-ene, benzene, ethylbenzene, and xylene) were purchased from SD-Fine Chemicals (AR grade).Characterization: Scanning electron microscopy (SEM) measure-ments were carried out by using a Nova NanoSEM 600 instrument(FEI Co. , The Netherlands). Transmission electron microscopy (TEM)measurements were carried out with a JEOL-3010 instrument oper-ating at 300 kV (l= 0.0196 �) and electron diffraction (ED) patternswere collected at a camera length of 20 cm (calibrated with re-spect to the standard polycrystalline Au thin film). The samples forTEM and ED were prepared by dissolving the nanocomposite(taken during synthesis) in toluene and drop-casting the solutionon a carbon-coated grid. The UV/Vis absorption spectra were re-corded on a Perkin–Elmer model Lambda 900 UV/Vis/NIR spec-trometer. The FTIR spectra were recorded by using a BrukerIFS66 V/S spectrometer. The samples were pelletized by manuallygrinding the nanocomposite with dry KBr. The TGA measurementswere carried out by using Mettler TGA-850 equipment with 15–20 mg of samples in air at a heating rate of 10 8C min�1 from 25 to800 8C. The compressibility of the sponge was quantified by plac-ing the foam between two rectangular blocks fitted with a screwgauge. The density of the foam was estimated from its weight andcylindrical shape. The pore volume, V, of different foam sampleswas calculated by using water. The different foam samples weredipped in water as a nonsolvent for 5 min until all pores werecompletely filled and measurements were taken when the weightreached a constant value. The pore volume was taken as the differ-ence between the initial and final weight divided by the initialweight. Water was employed because it only fills the pores withoutcausing swelling of PDMS.

Acknowledgements

We thank Professor C. N. R. Rao, FRS, for his constant encourage-ment. Support from the Department of Science and Technology,Government of India is gratefully acknowledged. R.G. thanks Dr.Abhay Sagade for assistance.

Keywords: adsorption · gold · nanoparticles · oil spills ·thioanisole

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Received: November 30, 2010Revised: January 30, 2011Published online on May 12, 2011

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Gold Nanoparticle–Poly(dimethylsiloxane) Nanocomposite Foam