Nanocomposites

DOI: 10.1002/ange.200504461

Modulation of Viscoelastic Properties of PhysicalGels by Nanoparticle Doping: Influence of theNanoparticle Capping Agent**

Santanu Bhattacharya,* Aasheesh Srivastava, andAsish Pal

Materials in the nanometer regime are of great contemporaryinterest because of their numerous properties and the variousapplications envisaged for them.[1] In real-life or high-endapplications, these materials are often mixed with othersystems to yield new composites that have superior propertiescompared to those of the constituents. For example, novelcomposites engineered from polymers and carbon nanotubes(CNTs) offer the promise of plastics with enhanced thermal,electronic, and mechanical properties.[2] Similarly, there arereports of studies on nanoparticles (NPs) embedded inpolymers and polymeric gels.[3a] These mixtures exhibitinteresting optoelectronic properties, such as temperature-dependent reversible UV/Vis spectral changes, as a result ofthe controlled and reversible aggregation of the NPs inpolymer–NP and dendrimer–NP ensembles.[3b–d] The elec-tron–phonon dynamics of NPs entrapped in polymeric gelshave also been studied. Compared to NPs in the solutionphase, NPs in such confined media display quite differentbehavior.[3e] Thus, the design and investigation of thesematerials is highly important and of significant technologicalrelevance.

Despite the intense interest in composites of polymers andnanotubes, the effects of nanomaterial incorporation into gelsbased on low-molecular-mass organogelators (LMOGs) havenot been explored. Also, the ability to control and quantifyparticle dispersion in such gels is an unresolved issue offundamental importance. A molecular-level understanding ofthe interaction of NPs with molecular gels would be useful forthe design of new materials. Hence, we sought to examinewhether incorporation of NPs into LMOG-based gels isfeasible without disturbing the gelation, and if so, whether theresulting gels have any advantages.

Gels exhibit properties intermediate to those of a New-tonian liquid and a Hookean solid, and hence gels aredescribed as viscoelastic materials. In contrast to polymericgels, where suitable solvents are retained by the polymernetwork, the LMOGs self-assemble into fibrous aggregates,

[*] Prof. Dr. S. Bhattacharya, A. Srivastava, A. PalDepartment of Organic ChemistryIndian Institute of ScienceBangalore 560012 (India)Fax: (+91)80-2360-0529E-mail: sb@orgchem.iisc.ernet.in

[**] This work was supported by DBT and DST. We thank G. Rajesh andDr. A. K. Sood for access to the rheology facility, and the MaterialsResearch Center and the Institute Nanotechnology Initiative for theTEM and SEM experiments, respectively.

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which results in a meshlike organization that holds the solventmolecules by interfacial tension and supramolecular inter-action effects.[4] The self-assembly of fibrillar networks ispredominantly through hydrogen-bonding, p–p stacking, orvan der Waals interactions.[5] In a few cases charge transferand ion pairing are also involved.[6] A large variety of sugarand amino acid derivatives are known to rigidify solvents atvery low concentrations, sometimes gelling both organic andaqueous media.[7] In rare cases, the gelator molecules maycrystallize too, thus giving an insight into the interactionbetween the gelator and the solvent.[8]

Recently, a thiol-containing organogelator has beensynthesized and gold NPs were prepared with this gelator asthe capping agent.[9a] Upon solvent evaporation it sponta-neously gave fibrous NP assemblies. In another reportMcPherson et al. described the spatial compartmentalizationof CdS NPs into strands of an organogel assembly.[9b] Theabove findings indicate that the interesting properties ofLMOGs can be successfully applied toward templating novelNP composites. Herein, we show the utility of supramoleculargel aggregates for assembling Au NPs, and also discover forthe first time the profound influence of the NP capping agenton achieving control over the viscoelastic properties of theresultant composites.

We have previously reported the synthesis and character-ization of an efficient gelator based on a fatty acid amide of l-alanine (1).[10] Gelator 1 produced stable gels in both aromatic

and aliphatic hydrocarbons. It also exhibited importantproperties, such as phase-selective, thermoreversible gelationof a variety of organic solvents, even in the presence of water.We wanted to examine whether monolayer-protected Au NPscan be incorporated in such gels without compromising thegelation manifested by 1.

To investigate the gel–NP interaction, we selected differ-ent kinds of Au NPs. The first set was based on cappingagents, for example, n-alkanethiols (AuCm+2) where m= 4, 6,and 10. The other NPs were coated with either a cholesterol-based thiol (AuChol) or p-thiocresol (AuPhMe). These NPswere prepared by a two-phase method.[11] The resulting NPshad an average diameter of (4.5� 0.5) to (5.5� 0.5) nm forAuCm+2, (6� 1) nm for AuPhMe, and (3� 0.8) nm for

AuChol, as observed by TEM (not shown). Thus, there wasa small variation of the NP size with the molecular structureand bulkiness of the capping agent under comparableconditions of NP synthesis. The capping agents also gavestructural diversity to the NPs.

We found that even in the presence of the Au NPs, 1 stillformed stable gels in both toluene and heptane. It wasobserved that even at a NP/gelator ratio of 1:1 (w/w), gelationwas achievable in a thermoreversible manner. Presumably,this is because the NPs are very small as compared to the gelfibers. X-ray diffraction studies on the gel and gel–NPcomposites indicate that the gel arrangement is not affectedby NP incorporation. The diffraction peaks overlap in allcases (not shown).

TEM images of the Au NPs in toluene (Figure 1a) and inthe gel–NP composite indicate that 1 forms onion-likeaggregates in toluene. The NPs are embedded on the surfaceof the gel assembly (Figure 1b), which is clear evidence ofNP incorporation into the aggregates formed by 1.

Scanning electron microscopy (SEM) of the xerogel andthe corresponding gel–NP composites, however, revealed thatthere was a striking morphological transformation of the gelmicrostructure on incorporating NPs. Interestingly, the micro-structures of the gel–NP composites were dependent on thecapping agent on the NPs. The gel in its native state hadfibrous assemblies (Figure 2a). When AuC12 (2.17 wt%) wasincorporated into it, the fibers appeared to coalesce (Fig-ure 2b). This finding is probably a result of interdigitation ofthe long chains on the NP surface with the gelator moleculeswithin the gel assemblies, which brings the fibers together.Incorporation of AuChol resulted in the formation of “rolled-tubular”-type aggregates (Figure 2c). These aggregates weremuch larger than the fibers of the native gel, which indicatesthat the NPs induce further aggregation of the gel fibers.Incorporation of AuPhMe into gels of 1 led to the formationof platelet-like aggregates with a thickness in the range of 1–2 mm (Figure 2d). This result suggests that the interactions of1 with AuPhMe are not as strong as they are in the case of1–AuChol or 1–AuC12 composites.

Long hydrocarbon and cholesteryl moieties are known tostabilize supramolecular aggregates by interdigitationthrough van der Waals interactions.[12] However, this type ofstabilization is not effective when a small, rigid aromaticcapping agent is anchored on the NP surface. This is reflected

Figure 1. TEM images of AuC12 a) as a solution in toluene and b) uponincorporation into gels of 1 in toluene. The black arrow shows the gelaggregate and the white arrow indicates the localization of AuC12 onthe gel assembly.

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in the lower rigidity of the 1–AuPhMe composite as comparedto the other two composites (see below).

The results obtained from TEM and SEM clearly indicatethat NPs interact intimately with the gelator molecules insidegels. This finding prompted us to investigate whether therheology of the gel–NP composites would also be influencedby NP incorporation. Such studies give information about theflow behavior and the rigidity of gels.[13,14] The rheology alsoindicates the yield stress sy, which is the critical applied stressafter which the gel starts to flow.[14] Accordingly, theviscoelastic behavior of the native gel and the gel–NPcomposites was measured. We observed that the gel madein toluene succumbs to the applied stress and begins to flow atabout 160 Pa (Figure 3, inset). Incorporation of various NPsinto the gel results in the rigidification of the resultingcomposites (Figure 3). For AuC12 the yield stress of the gel–

NP composite reaches a maximum at 2.17 wt% of NPs. Themaximum yield stress obtained in this case was about 430 Pa.Further incorporation of NPs lowered the yield stress of thecomposite. Thus, an optimum level of NP incorporation givesthe gel maximum rigidity, after which the gel organizationbecomes perturbed in the accommodation of NPs, and gelrigidity decreases.

Incorporation of NPs coated with smaller chain (hexaneand octane) thiols also rigidified the gel, albeit with yieldstresses considerably lower than that in the AuC12 case. Also,the rigidification achieved by AuC8 incorporation was greaterthan that for AuC6. Clearly the gel rigidity depends criticallyon the chain length of the alkanethiols attached to the goldsurface. In the homologous series of n-alkanethiol-basedcapping agents, the rigidity of AuCm+2–NP compositesincreases steadily as the chain lengths are increased. Thisfinding suggests chain interdigitation as a possible mechanismof attaining mechanical stability of the resulting composites.In the case of AuChol incorporation, the composite possessedthe highest rigidity (ca. 450 Pa) amongst the five 1–Au NPcomposites examined. This is probably a consequence of thegreater van der Waals interaction area of the inflexiblecholesteryl group of AuChol as compared to the n-alkylgroups of AuCm+2. This composite also achieved maximalrigidity at a lower weight percentage of NPs (1.74 wt%).

In contrast, when the capping agent on the NPs waschanged to a rigid aromatic system like thiocresol, weobserved a completely different behavior. The incorporationof even a small amount (0.8 wt%) of AuPhMe made thecomposite less rigid (140 Pa) than the native gel. An increasein the number of NPs endowed some rigidity to thecomposite, although even at its maximal rigidity, this compo-site was still less rigid than the native gel itself. This increase inrigidity is probably a result of incorporation of the hardnanoparticulate material in the sample. A further increase ofthe NP loading led to a decreased rigidity of the composite,similar to the decrease observed in other cases. In controlexperiments, inclusion of the corresponding thiols (up to2.7 wt%) did not lead to any noticeable increase in the gelrigidity. This could be because of random distribution of thethiols in the gel matrix. The chemisorption of thiols on theAu NP surface, however, impacts the gel rigidity quiteremarkably. Thus, the capping agent present on the NP sur-face dictates the interactions of the NPs with the gelatoraggregates. Enhanced van der Waals interactions of the gelfibers with sufficiently long alkyl-chain or cholesteryl units onthe surface of the NPs collated the gel fibers into close-packedaggregates, which resulted in their greater rigidity as com-pared to that of the native gel.

It is clear that Au NPs can be very effectively incorpo-rated into LMOG-based gels of 1 without disturbing thegelation properties. The interaction of NPs with 1 is intimatelyrelated to the molecular structure of the capping agentanchored on the NP surface. Incorporation of NPs altered themicrostructure of the gel aggregate, although gelation was notcompromised. The gel fibers became more compact uponincorporation of Au NPs coated with capping agents capableof interdigitation (Scheme 1). This was reflected in theincreased mechanical strength of the gel assembly. The

Figure 2. SEM images of xerogels of a) 1, b) 1–AuC12 composite, c) 1–AuChol composite, and d) 1–AuPhMe composite.

Figure 3. Plots of yield stress versus NP concentration. Inset: plots ofstorage modulus G’ and loss modulus G’’ versus stress for a gel of 1in toluene and a 1–AuC12 composite containing 2.17 wt% of NPs.

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changes exhibited by these NP additives for relatively lowNP concentrations (� 2 wt%) are impressive. We were ableto define a relationship between materials and molecularproperties by manipulation of the molecular structures of theNP capping agents. Therefore, this method opens up aconvenient new path to modulating the properties of suchcomposites.

Experimental SectionGelation: The resistance of the solvent–gelator mixture toward flowunder gravity was used as the test of gelation. Gelator 1 was heated ina water bath at 60 8C in a test tube containing a toluene solution ofNPs to form a clear sol, which, upon cooling under ambient conditionsfor 30 min, formed a gel that was stable to inversion of the test tube.

TEM: A toluene solution of NPs (0.1 mgmL�1) was drop-coatedon a carbon-coated Cu grid and allowed to dry under ambientconditions. The coating was analyzed in a JEOL 200CX TEMoperated at 120 kV. For gel–NP composites, the sample also contained1 (1 mgmL�1) along with the NPs at same concentration.

SEM: The gel–NP composites were carefully scooped on thebrass stubs and were allowed to dry overnight in air. The sampleswere further dried in a vacuum for 1 h, then coated with gold vapor tomake them conducting and analyzed on a Quanta 200 SEM operatedat 20 kV.

Rheology: An Anton Paar 100 rheometer with a cone–plategeometry (CP 25-2) was used. The gap distance between the cone andthe plate was fixed at 0.05 mm. The native gel in toluene (preparedfrom 20 mgmL�1 of 1) or composite (containing the requisite numberof NPs in addition to 1) was scooped on the plate of the rheometer. Astress–amplitude sweep experiment was performed at a constantoscillation frequency of 1 Hz for the strain range 0.001 to 100 at 20 8C.The rheometer had a built-in computer which converted the torquemeasurements into eitherG’ (storage modulus) or G’’ (loss modulus)in oscillatory shear experiments.

Received: December 15, 2005Revised: January 20, 2006

.Keywords: gels · gold · nanostructures · rheology ·supramolecular chemistry

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Scheme 1. Schematic illustration of the interaction of AuC12 with thegelator assembly.

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