DOI: 10.1021/la904199f 7469Langmuir 2010, 26(10), 7469–7474 Published on Web 02/11/2010

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© 2010 American Chemical Society

Poly(allylamine)-Stabilized Colloidal Copper Nanoparticles: Synthesis,

Morphology, and Their Surface-Enhanced Raman Scattering Properties

Yanfei Wang†,‡ and Tewodros Asefa*,†,‡

†Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 TaylorRoad, Piscataway New Jersey 08854, and ‡Department of Chemical Engineering and Biochemical Engineering,

Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, New Jersey 08854

Received November 4, 2009. Revised Manuscript Received January 24, 2010

Poly(allylamine)-stabilized spherical- and rod-shaped copper nanoparticles are synthesized by a simple chemicalreaction. The synthesis is performed by the reduction of copper(II) salt with hydrazine in aqueous solution underatmospheric air in the presence of poly(allylamine) (PAAm) capping agent. Noteworthy of the advantages of thesynthetic method includes its production of water dispersible copper nanoparticles at room temperature under no inertatmosphere,making the synthesismore environmentally friendly. The resulting copper nanoparticles are investigated byUV-vis spectroscopy and transmission electronmicroscopy (TEM). The results demonstrate that the amount ofNaOHused is important for the formation of the copper nanoparticles while the reaction time and concentration of PAAmplaykey roles in controlling the size and shape of the nanoparticles, respectively. The resulting colloidal copper nanoparticlesexhibit large surface-enhanced Raman scattering (SERS) signals.

1. Introduction

Noble metal nanoparticles have attracted considerable atten-tion owing to their potential applications in such fields ascatalysis, biology, electronics, and information technology.1-4

In particular, copper nanoparticles are widely used as a catalystfor various chemical transformations including water-gas shift5

and gas detoxification6 reactions, and as an electrocatalyst insolid-oxide fuel cells.7 Another important application for coppernanomaterials is their potential use for surface-enhanced Ramanscattering (SERS), whose enhancement factor is dependent on thestrength of the local electric field and the nature of the material.8

Indeed, it has already been reported that noblemetals such asAu,Ag, and Cu nanostructures generate large SERS. However, theinvestigation of Cu nanoparticles synthesized in solution phaseas SERS substrate has remained largely unexplored. This ismainly because copper nanostructures are prone to fast oxida-tion and, consequently, are hard to synthesize chemically. Thishas inspired us to develop synthetic methods for stable colloi-dal Cu nanoparticles in aqueous solution and explore theirSERS properties.

Recently, a few different types of synthetic methods formaking shaped copper nanoparticles including rods or cubeshave been reported.9-19 The first and most commonly usedmethod involves the chemical reduction of copper ions inaqueous solution in the presence of various capping agents.For instance, using surfactant capping agents such as cetyl-trimethylammonium bromide (CTAB),8,9 ethylenediamine(EDA),10 polyvinylpyrrolidone (PVP),11 and sodium dodecyl-benzene sulphonate (DBS),12 various shaped copper nanopar-ticles were produced by reduction of copper(II) ions. As in thesynthesis of many other metal nanomaterials, adjustment ofthe synthetic parameters, in particular the type of the surfac-tant, the preparation of the seed solution,13 temperatures, andthe ratios of additives in the growth solution14 render obviousshape evolution of the copper nanoparticles. Pileni and herresearch group demonstrated the size and shape controlledsynthesis of copper nanoparticles by using mixed reversemicelles as a template.15 Furthermore, copper nanoparticles,nanorods, and nanodisks with different size and shape weresynthesized by varying the water content, the concentration ofreducing agent, or the salt concentrations in AOT-water-oilreverse micellar systems.16-19 In a second commonly usedmethod, copper nanoparticles were prepared in organic sol-vents by decomposing or reducing various copper complexes inthe presence of capping agents. For instance, copper nanorodsand nanocubes were synthesized by treating Cu(acac)2 in anoctyl ether solvent in the presence of oleic acid and oleyl aminecapping agents.20 In this case, the shapes of the nanoparticleswere tuned by changing the reaction temperature. In the thirdsynthetic method known as the polyol reduction process,ethylene glycol was used both as a solvent and as a reductantfor copper ions while PVP was used as a capping agent for

*To whom correspondence should be addressed. Telephone: 732-445-2970.Fax: 732-445-5312. E-mail: tasefa@rci.rutgers.edu.(1) Xia, Y.; Xiong,Y.; Lim, B.; Skrabalak, S. E.Angew. Chem., Int. Ed. 2008, 47, 2.(2) Tao, A. R.; Habas, S.; Yang, P. Small 2008, 4, 310.(3) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293.(4) Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547.(5) Ressler, T. B.; Kniep, L.; Kasatkin, I.; Schl€ogl, R. Angew. Chem., Int. Ed.

2005, 44, 4704.(6) Vukojevic, S.; Trapp, O.; Grunwaldt, J.; Kiener, C.; Schth, F.Angew. Chem.,

Int. Ed. 2005, 44, 7978.(7) Park, S.; Gorte, R. J.; Vohs, J. M. Appl. Catal., A 2000, 200, 55.(8) Bozzini, B.; D’Urzo, L.; Re,M.; De Riccardis, F. J. Appl. Electrochem. 2008,

38, 1561.(9) Wu, S.; Chen, D. J. Colloid Interface Sci. 2004, 273, 165.(10) Chang, Y.; Lye, M. L.; Zeng, H. C. Langmuir 2005, 21, 3746.(11) Huang, H.; Yan, F.; Kek, Y.; Chew, C.; Xu, G.; Ji, W.; Oh, P.; Tang, S.

Langmuir 1997, 13, 172.(12) (a) Zhou,G.; Lu,M.; Yang, Z.Langmuir 2006, 22, 5900. (b) Liu, Z.; Yang, Y.;

Liang, J.; Hu, Z.; Li, S.; Peng, S.; Qian, Y. J. Phys. Chem. B 2003, 107, 12658.(13) Gole, A.; Murphy, C. J. Chem. Mater. 2004, 16, 3633.(14) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648.(15) Pileni, M. P. Nat. Mater. 2003, 2, 145.

(16) Lisiecki, I.; Pileni, M. P. J. Am. Chem. Soc. 1993, 115, 3887.(17) Salzemann, C.; Urban, J.; Lisiecki, I.; Pileni,M. P.Adv. Funct.Mater. 2005,

15, 1277.(18) Cason, J. P.; Roberts, C. B. J. Phys. Chem. B 2000, 104, 1217.(19) Lisieki, I.; Pileni, M. P. J. Phys. Chem. 1995, 99, 5077.(20) Galkowski, M. J.; Wang, L.; Luo, J.; Zhong, C. Langmuir 2007, 23, 5740.

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making copper nanoparticles.21 Finally, physical vapor deposi-tion,22,23 chemical vapor deposition,24 γ-irradiation,25 UV-lightirradiation,26 sonochemical method,27 and electrochemical app-roach28-30 have been reported as synthetic methods for thepreparation of copper nanostructures. In general, most of themethods reported so far have rarely resulted in copper nanoparticleshaving uniform sizes below 50 nmor in high yield. Furthermore, theprocedures used in previously reported methods are rather compli-cated, usually involving a number of steps or requiring an inert gassuch asArorN2 in order to avoid oxidationof theCunanoparticles.

Herein, we report on a one-step synthetic method to colloidalCu nanoparticles by reducing copper ions with hydrazine usingpoly(allylamine) (PAAm) as a stabilizing agent. The advantagesof themethod include the use of atmospheric air (or nouse of inertatmosphere) for the synthesis of the nanoparticles and its produc-tion of copper nanoparticles with reasonably uniform size andrelatively higher yield.While changing the amount ofNaOHusedin the synthesis was found to help us to produce pure coppernanoparticles as opposed to copper oxide, varying the reactiontime and concentration of PAAm enabled us to control the sizeand shape of the nanoparticles. Investigation of the SERSproperty of the resulting colloidal copper nanoparticles showedthat the nanoparticles gave a 104 times SERS enhancement for4-mercaptopyridine (4-Mpy) adsorbed on the nanoparticles com-pared to bulk 4-Mpy.

2. Experimental Section

2.1. Materials and Reagents. Copper(II) sulfate, hydrazinehydrate solution (78-82%), poly(allylamine) solution (PAAm)(averageMw∼ 17000, 20 wt% inH2O), and 4-mercaptopyridine(4-Mpy) (95%) were obtained from Sigma-Aldrich. Sodiumhydroxide (98.8%) was purchased from Fisher Scientific. Allchemicals were used as received without further treatment.

2.2. Synthesis of Copper Nanoparticles. In a typical synth-esis of copper nanoparticles, CuSO4 (0.05 mmol) and variousamounts (0.2-0.3 mL) of PAAm were completely dissolved inMillipore H2O (10 mL) under vigorous stirring at 60 �C for 20min, forming a transparent light-blue solution. Then, 0.6-0.8mLof NaOH (0.5 M) was added dropwise into the above solution.After stirring for 20 min, 1.0 mmol N2H4 3H2O solution wasdropped into the above solution under constant stirring. Thereactor was kept in a water bath at 60 �C for 40-90min. It shouldbe pointed out that N2H4 3H2O can also increase the pH of thesolution. The reaction was monitored by UV-vis spectroscopyuntil no change of the absorbance spectrum was observed. Themetallic copper nanoparticles in this workwere obtained from theredox reaction between Cu2þ and hydrazine in basic solution inthe presence of PAAm capping agent.

2.3. Characterization. The UV-vis absorbance spectrawere measured with a Lambda 950 spectrophotometer(PerkinElmer). Transmission electron microscopy (TEM) sam-ples were prepared by casting a drop of the as-prepared copper

nanoparticle suspension on a carbon-coated copper grid and thendrying them in air. The dried grid was then placed under a JEOL1200 EX transmission electron microscope. The images weretaken at an acceleration voltage of 120 kV. Surface-enhancedRaman scattering (SERS) spectra were obtained on a micro-Raman instrument (Renishaw 1000, Gloucestershire, U.K.)equipped with a He/Ne laser (632.8 nm) and a CCD detectoroperated at room temperature. The laser power at the sampleposition was typically 2.0 mW for SERS spectra and normalRaman spectra.

3. Results and Discussion

3.1. Synthesis of Copper Nanoparticles and Formation of

Copper versus Copper Oxide Nanoparticles in Aqueous

Solution. As in the synthesis of many types of colloidal nano-materials, copper nanoparticles also require organic ligands toprevent them from irreversible aggregation in solution. Here, weused PAAm to prepare and stabilize small Cu nanoparticles.Besides providing long-term stability to the nanoparticles bypreventing particle agglomeration, polymer capping agents suchas PAAmmake the particles dispersible in aqueous solution.31 Inour study, we found that several factors, including the amount ofNaOH solution, concentration of PAAm, and reaction time,affect the composition, size, morphology, and degree of agglom-eration of the resulting copper nanoparticles.

Figure 1 shows the UV-vis absorption spectra for PAAm-capped copper nanoparticles and their absorptionmaxima versusmolar concentration of NaOH used. Figure 1a displays theUV-vis spectra of colloidal Cu nanoparticles recorded 30 minafter addition of hydrazine intoCu2þ solution in various amountsof 0.5MNaOH in the presence of 0.22 mL of PAAm. The results

Figure 1. (A) UV-vis absorbance spectra of PAAm-capped cop-per nanoparticles that were synthesized in aqueous solution with0.22mLof PAAmand different amounts (ormoles) ofNaOH. (B)Absorptionmaximaof the copper nanoparticles versus amount (ormoles) of NaOH used.

(21) Cha, S. I.; Mo, C. B.; Kim, K. T.; Jeong, Y. J.; Hong, S. H. J. Mater. Res.2006, 21, 2371.(22) Wang, J.; Huang, H.; Kesapragada, S. V.; Gall, D.NanoLett. 2005, 5, 2505.(23) Vitulli, G.; Bernini, M.; Bertozzi, S.; Pitzalis, E.; Salvadori, P.; Coluccia, S.;

Martra, G. Chem. Mater. 2002, 14, 1183.(24) (a) Wang, J.; Yang, T.; Wu, W.; Chen, L.; Chen, C.; Chu, C.Nanotechnology

2006, 17, 719. (b) Kim, C.; Gu, W.; Briceno, M.; Robertson, I. M.; Choi, H.; Kim, K. Adv.Mater. 2008, 20, 1859. (c) Choi, H.; Park, S. H. J. Am. Chem. Soc. 2004, 126, 6248.(25) Henglein, A. J. Phys. Chem. B 2000, 104, 1206.(26) Kapoor, S.; Palit, D. K.; Mukherjee, T. Chem. Phys. Lett. 2002, 355, 383.(27) Vijaya Kumar, R.; Mastai, Y.; Diamanta, Y.; Gedanken, A. J. Mater.

Chem. 2001, 11, 1209.(28) Bozzini, B.; D’Urzo, L.; Mele, C. Trans. IMF 2008, 86, 267.(29) Huang, L.; Lee, E.; Kim, K. Colloids Surf., A 2005, 262, 125.(30) (a)Wang, T.; Hu, J.; Yang, W.; Zhang, H. Electrochem. Commun. 2008, 10,

814. (b) Zhang, D. W.; Chen, C. H.; Zhang, J.; Ren, F. Chem. Mater. 2005, 17, 5242–5245. (31) Sardar, R.; Park, J. W.; Shumaker-Parry, J. S. Langmuir 2007, 23, 11883.

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showed that the intensity and position of the absorption bandchanged with the amount of NaOH used. When the NaOHamount was between 0.6 and 0.75 mL, the absorbance maximumwas between 560 and 568 nm, a characteristic plasmon absorptionband for copper nanoparticles.32 The intensity of the plasmonpeak reached its maximum when the NaOH amount was in-creased to 0.7 mL. This corresponds to the formation of thehighest possible yield of copper nanoparticles. When the amountof NaOH was adjusted above 0.8 mL, a peak at around 575 nm,whose intensity became stronger with increasing amount ofNaOH solution, started to appear. Since a peak at g575 nmwas reported to correspond to metallic copper nanoparticles thatare surrounded by a copper(I) oxide shell,33 our result hereindicated that the synthesis yields a mixture of Cu and Cu2Oparticles or Cu2O-coated Cu nanoparticles when>0.8 mL of 0.5M NaOH solution (or 0.4 mmol OH-) was used. The studydemonstrated thatwater-soluble andpolymer-coated pure coppernanoparticles in the presence of PAAm capping ligand could besynthesized by using an appropriate amount, 0.60-0.75 mL of0.5 M NaOH (or 0.30-0.38 mmol OH-). These results alsoclearly implied that adjusting the molar concentration of NaOHin the reaction was crucial for the synthesis to result in either purecopper or copper-oxide-containing copper nanoparticles and toproduce the highest possible yield of copper nanoparticles.3.2. Growth of PAAm-Stabilized Copper Nanoparticles

in Aqueous Solution. The growth of colloidal Cu nanoparticlesduring the reactionwent through several steps as observed from aset of color changes, from light yellow to wine red (Scheme 1). Abrown colored solution was first formed instantly as soon as anaqueous NaOH was added into a mixture of CuSO4 and PAAm.Further addition of hydrazine into this reactionmixture produceda yellow solution containing Cu2O seed particles within 30 min.The presence of Cu2O nanoparticles was proved by UV-visabsorption spectroscopy (please see below). These Cu2O nano-particles were further reduced by excess N2H4 3H2O and yielded ared colored solution containing colloidal copper nanoparticleswithin 35min of reaction. The color of the solution turned to deepred after stirring the reaction mixture at 60 �C for ∼90 min,indicating the growth of the colloidal Cu nanoparticles.

The intermediate products during the growth process of thenanoparticles were characterized at intervals of 10 min byUV-vis absorption spectroscopy. Figure 2 displays the UV-visabsorption spectra that were taken at different stages of thecontinuous transformation of copper ions into PAAm-coatedcopper nanoparticles. After 30 min of reaction, a yellow solu-tion, which displayed a very weak absorption peak at 578 nmcorresponding to the plasmon resonance of Cu2O nanopar-ticles, was observed (Figure 2). However, after 40 min of overall

reaction time, the solution turned red and showed a well-definedabsorbance band at ∼560 nm corresponding to the plasmonresonance of Cu nanoparticles. This clearly indicated that theCu nanoparticles began to form between 30 and 40 min ofreaction time. Between 40 and 90 min reaction time, the absorp-tion spectra exhibited sharp plasmon resonance bands, whichwere slightly red-shifted from 560 to 568 nm, suggesting thegrowth of nanoparticle size. Furthermore, the intensity of theabsorption peak continued to increase upon increasing the reac-tion time up to 90 min. However, after 90 min, no furtherevolution of the absorption band or growth of the nanoparticleswas observed, indicating the reduction of copper ions came tocompletion. These results suggest that size controlled synthesis ofcopper nanoparticles with narrow size distribution would bepossible with this approach.

TEM images further confirmed that the nucleation andgrowth of copper nanoparticles progressedwith the reaction time.Figure 3 shows the TEM images of the copper nanoparticles atdifferent reaction times after injecting hydrazine into the Cu2þ/PAAm solution. As described above, nucleation of the nanopar-ticles began at ∼30 min after injection of hydrazine, as indicatedby the sudden change of the solution color from brown to yellow.As the reaction proceeded for an additional 5 min, the solutioncolor rapidly changed from yellow to red. Samples collected rightafter 35 min of reaction time showed an average size of 40 nmcopper nanoparticles (Figure 3a). The rate of further growth ofthe size of the nanoparticles became slower as evidenced by thestable red color of the solution. This was further confirmed byUV-vis absorption and TEM. For instance, as shown inFigure 3b, the average size of the nanoparticles after 60 minreaction time became only 50 nm. Nevertheless, this method wasstill proven toproduce copper nanoparticleswith variable averagesizes, at least between 40 to 50 nm, simply by changing thereaction time.

For the copper nanoparticles synthesized with 0.22 mL ofPAAm after 60 min of reaction time, the pH of the solution was∼10.8 and the absorptionmaximumwas at 561 nm.When the pHwas increased above 12.0, the UV-vis absorption remained tohave only one absorption peak but the absorptionmaxima shiftedfrom 561 to 578 nm. This result suggests that metallic coppernanoparticleswere oxidized toCu2Oor amixture ofCu andCu2Onanoparticles at higher pHs. This is corroborated by previousreports which indicate that an absorption maximum atg575 nmcorresponds tometallic copper nanoparticles that are surroundedby a copper(I) oxide shell.33 Although the copper nanoparticlesunderwent oxidation, it is interesting to note that their shape

Scheme 1. Schematic Illustration of the Procedure Used for the

Synthesis of PAAm-Capped Colloidal Cu Nanoparticles

Figure 2. UV-vis spectral evolution during the formation ofPAAm-capped colloidal Cu nanoparticles from the reduction ofCu2þ ions with hydrazine in water at 60 �C in the presence of 0.22mL of PAAm.

(32) Hambrock, J.; Becker, R.; Birkner, A.; Weiss, J.; Fischer, R. A. Chem.Commun. 2002, 68.(33) Yanase, A.; Komiyama, H. Surf. Sci. 1991, 248, 11.

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remained almost unchanged and the size appeared slightly morepolydisperse after increasing the pH to 12.5 (Supporting Informa-tion Figure S2).3.3. Concentration of PAAm and Synthesis of Copper

Nanospheres or Nanorods. It has been widely reported that thesize and shape of noble metal nanoparticles can be controlled bychanging the types and concentration of capping agent.10,15,34-36

Here also, as the TEM image in Figure 4 shows, nanorods mixedwith nanoparticles were formed with our synthetic method, byincreasing the relative concentration of PAAm in the solutioncompared to the one used above. The UV-vis absorption,however, showed barely any shift on the absorption maxima(Supporting InformationFigure S3). The significant change in theabsorption maxima was observed only when the metal coppernanoparticles were oxidized into Cu2O nanoparticles. The slightincrease of PAAm concentration was favorable for the formationof Cu nanorods. This proves that the use of a different amount ofPAAm leads to nanoparticles with significantly different mor-phologies. However, our attempt, by further increasing thePAAm concentration, to enlarge the aspect ratio of the nanorods

or increase their yield with respect to the nanoparticles did notprove to be successful. It is worth noting that Cu nanorods havebeen demonstrated to form in AOT-water-oil systems bychanging the water-to-oil or salt concentration of the reaction.15

In another case, capping agent EDA concentration played animportant role in determining Cu nanowire and disklike mor-phologies. An appropriate amount of EDA resulted in theformation of Cu nanowires. When EDA was overused, however,the axial 1D growth of nanowires could be switched totally todisklike morphologies.10 However, this growth mechanism ofsuch Cu nanowires dependent on the concentration of cappingagents has not yet been well explored.

A number of capping reagents have been examined to controlthe growth rates of metal surfaces kinetically and thus achieve1D growth.34-36 For instance, uniform gold nanorods with con-trollable aspect ratios are prepared by using CTAB as the cappingreagent. In general, the 1D nanostructures synthesized withCTAB as the capping reagent are twinned in crystal structure.Johnson et al.34 and Gai and Harmer35 have proposed a mechan-ism inwhich gold nanorodswere assumed to evolve frommultiplytwinned particles (MTPs) with a decahedral shape. In anotherexample, silver nanorods and nanowires are grown with PVP ascapping reagent.36 It has been believed that each silver nanowireor nanorod evolved from a MTP of silver with the assistance ofPVP at the initial stage of the Ostwald ripening process. TheMTPs of silver consist of 10 {111} facets. As with the singlytwinned seeds, anisotropic growth of decahedral Ag seeds can beinduced to facilitate the formation of 1D nanocrystals such asnanorods and nanowires. The anisotropic growthwasmaintainedby selectively covering the {100} facets with PVPwhile leaving the{111} facets largely uncovered byPVPand thus highly reactive. Inaddition toMTPs, another important factor for the formation of1D nanorods is selective binding of capping agent to the differentcrystallographic planes of the metal. In general, the presence of acapping agent can change the order of free energies for differentcrystallographic planes and thus their relative growth rates. Theplane with a lower addition rate will be exposed more on thenanocrystal surface. For example, PVP is a polymeric cappingagent whose oxygen atoms bindmost strongly to the {100} facetsofAg.36a The role of PAAmon the formation ofCunanorodswassimilar to that of CTAB on the formation of gold nanorods34,35

and PVP on the formation of silver nanowires.36a Thus, theabsence of Cu nanorods synthesized with 0.27 mL of PAAm

Figure 3. TEM images of PAAm-capped copper nanoparticles synthesized with 0.22 mL of PAAm after (a) 35 min and (b) 60 min of reac-tion time.

Figure 4. TEM image of PAAm-capped copper nanorods synthe-sized with 0.27 mL of PAAm.

(34) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S.J. Mater. Chem. 2002, 12, 1765.(35) Gai, P. L.; Harmer, M. A. Nano Lett. 2002, 2, 771.(36) (a) Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. Nano Lett. 2003, 3, 955. (b)

Sun, Y.; Gates, B.; Mayers, B.; Xia, Y.Nano Lett. 2002, 2, 165. (c) Sun, Y.; Xia, Y.Adv.Mater. 2002, 14, 833. (d) Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem.Mater. 2002, 14, 4736.

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in the final product could be attributed to two possibilities:(i) PAAmwith this concentrationwas favorable for the formationof MTPs and (ii) PAAm might bind more strongly to the {100}facets of Cu nanorods. This preferential capping can drive theaddition of Cu atoms to the other crystal facets to form Cunanorods when seeds are enough.3.4. Effect of Temperature on the Synthesis of Copper

Nanospheres/Nanorods. In addition to the synthesis at 60 �Cabove, experiments at lower and higher temperatures to deter-mine the effect of temperature on the synthesis and structure ofcopper nanomaterials were conducted. The results suggested thatthe reaction at room temperature (23 �C) could also lead tocopper nanomaterials. However, the yield of the copper nano-particles was very low at 23 �Cwhen 0.22mL of PAAmwas used.On the other hand, when 0.27 mL of PAAm was used, thesynthesis did not form nanorods at 23 �C as it did at 60 �C. Thisis probably because the relatively lower reaction temperaturecould not provide enough energy required for the activation ofspecific faces of the copper nanoparticles for anisotropic growthinto nanorods. For the synthesis at a higher temperature of 80 �C,the reaction took place quickly and the nucleation appeared tohappen in 5 min. However, the size distribution of the resultingcopper nanoparticles was rather more polydisperse compared tothose synthesized at lower temperature. So, we used an optimizedreaction temperature of between 50 and 60 �C in order to obtainmore uniformly sized Cu nanoparticles in a higher yield.3.5. SERS Activity. Noble metal nanostructures have been

demonstrated to be effective SERS-active substrates. According tothe electromagnetic theory of SERS, SERS enhancements dependon the excitation of the localized surface plasmon resonance, whichis influenced by several significant parameters, such as the size,shape, and nature of aggregates of the nanomaterials.37-39 There-fore, it is important to develop monodisperse size metal nanopar-ticles in order to control or optimize the factors influencing thelocalized surface plasmon resonance and subsequently maximizetheir SERS signals. The colloidal Cu nanoparticles we havesynthesized could provide an ideal substrate for SERS and tostudy the compositional dependence of chemical adsorption andreactions on the surface of Cu nanoparticles. Here we studied and

evaluated the SERS activity of our as-prepared Cu nanoparticlesusing 4-Mpy as the SERS reporter molecule (Figure 5).

Figure 5a demonstrates a normal Raman (NR) spectrum ofneat 4-Mpy. Figure 5b shows a typical SERS spectrum of 4-Mpyadsorbed on colloidal Cu nanoparticles. The SERS spectraconsist of several observable bands at 1594, 1477, 1222, 1098,1006, and 710 cm-1, which are intrinsic to 4-Mpy. Detailedpeak frequency assignments are given in Table 1. The peak at1594 cm-1 can be attributed to the ring stretchmode of the 4-Mpymolecule with deprotonated nitrogen. A previous SERS study of4-Mpy on a silver substrate at different pH values40 showed thatthis peak at 1594 cm-1 was particularly sensitive to the environ-ment of the molecule. Generally, 4-Mpy on Ag substrate gives apeak at 1623 cm-1 at pH<1.00. However, when the pH is raisedabove 1.00, a newbandnear 1580 cm-1 also appears and increasesin intensity with pH. At pH = 12.00, the peak at 1623 cm-1

disappears completely and the peak at 1580 cm-1 becomes verystrong. In our study of the SERS spectra of 4-Mpy with colloidalCu nanoparticles, the pH value was set above 12.00 as thenanoparticles are not stable or oxidize below that pH. Thus, weobserved only a single peak at 1594 cm-1. However, a fewremarkable spectral changes were observed upon adsorption of4-Mpy on the colloidal Cu nanoparticles. For example, as shownin Figure 5b-d, a marked downshift of the ν(C-S) mode at710 cm-1 and a dramatic increase in intensity of the ν(C-S)modeat 1006 cm-1 in comparison to the Raman spectrum of bulk4-Mpy were exhibited. A similar downward shift and enhance-ment have been observed for 4-Mpy adsorbed on other metalsubstrates such as Au,41 Ag,42-45 and Pt,46 which has beeninterpreted to be due to the coordination of 4-Mpy with the metalsurface through its sulfur atom. This further suggests that4-Mpy is chemisorbed on the colloidal Cu nanoparticles alsovia its S atom.

To evaluate the SERSactivity of as-preparedCunanoparticles,it is useful to obtain the enhancement factor (EF). The EF can becalculated according to the following expression (eq 1):47

EF ¼ ISERS=IRamanð Þ Nbulk=Nadsð Þ ð1Þwhere ISERS and IRaman are the intensity of a vibrational mode inthe SERS spectrum and normal Raman spectrum, respectively.Nbulk is the number of 4-Mpy molecules in the bulk, and Nads is

Figure 5. (a) Normal Raman (NR) spectra of neat 4-Mpy with0.1 M concentration. SERS spectra for 1 � 10-5 M 4-Mpy thatare adsorbed on colloidal copper nanoparticles synthesized from0.22 mL of PAAm after (b) 40 min, (c) 60 min, and (d) 90 minreaction time.

Table 1. Raman Shifts (cm-1) and Assignments for 4-Mpya

NR (cm-1) SERS (cm-1) assignment EF

718 710 β(CC)/ν(C-S) 2.0� 103

1003 1006 ring breathing 1.2� 104

1120 1098 ring breathing/C-S 3.0� 104

1214 1222 β(CH)/δ(NH) 5.1 � 104

1475 1477 ν(CdC/CdN) 1.0 � 104

aNRpeaks are from 0.1M4-Mpy and SERSpeaks are from 1� 10-5

M of 4-Mpy on colloidal Cu nanoparticles synthesized from 0.22 mL ofPAAm after 40 min reaction time. Assignments were obtained fromrefs 42-44.

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7474 DOI: 10.1021/la904199f Langmuir 2010, 26(10), 7469–7474

Article Wang and Asefa

the number of 4-Mpy molecules adsorbed on the Cu nanoparti-cles. Thus, the determination of the EF requires that the spectrafromthe adsorbedand freemolecules bemeasuredunder identicalconditions. Raman spectra of a 10-5 M concentration of 4-Mpyadsorbed on colloidal Cu nanoparticles and a 0.1 M concentra-tion of 4-Mpy solution were measured to obtain information onthe band intensities of adsorbedandbulkmolecules directly underthe same experimental conditions. Both experiments were withinthe laser focal volume (1 μm in diameter). Assuming the theore-tical surface areas of 4-Mpy 40 nm size copper nanoparticles, weestimated that the theoretical maximum number of 4-Mpymolecules that could be adsorbed on our colloidal Cu nanopar-ticles in 1 mLwould be 2.19� 1017 (see detailed calculation in theSupporting Information). However, since we have, in fact, usedonly 1mLof 10-5M4-Mpy in the colloidal Cu solution for SERSdetection, the actual maximum number of adsorbed 4-Mpy is6.02 � 1015 assuming complete adsorption of 4-Mpy on copper.On the other hand, the theoretical maximum adsorbed numberwas bigger than the actual numberwe used in the experiment. It is,therefore, obvious that the copper nanoparticle surface was largeenough to adsorb all of the 1 mL of 4-Mpy (10-5 M). Here,assuming that all 4-Mpy molecules from the concentration of10-5 M were adsorbed on the Cu nanoparticles, we used theconcentration 10-5M as the adsorbed species to calculate the EF.Since the concentration of bulk 4-Mpy solution was 0.10 M, weobtained an Nbulk/Nads ratio of 104. Combined with the intensityratio, this makes the SERS EF for the peak at 1006 cm-1 of4-Mpy to be 1.2 � 104. Thus, the average SERS EF of ourpolymer-capped colloidal Cu nanoparticles is estimated to be inthe order of 104 (Table 1). These results indicate that Cunanostructures, especially Cu nanoparticle colloidal systems,are attractive substrates for SERS detection of molecular species.However, the SERS spectrum of the copper nanorods exhibitedstrong fluorescence, which covered the SERS signals of 4-Mpymolecules (Supporting Information Figure S4).

The observation of strong fluorescence in case of colloidal Cunanorods and strong SERS in case of colloidal Cu nanoparticlescan be explained on the basis of possible differences in the degreeof interactions of the organicmolecules with the different facets ofthe shaped copper nanostructures. For instance, for silver nano-wires capped with PVP,36a it has been demonstrated that PVPinteractsmore stronglywith the {100} facets (i.e., the side surfacesof a silver nanowire) than with the {111} facets (i.e., the ends of asilver nanowire). Attaching 1,12-dodecanedithiolmolecules to thesurfaces of a silver nanowire verified such a difference in interac-tion strength. The results demonstrated that the sides werecompletely passivated by PVP while the nanowire ends were onlypartially passivated (or essentially uncovered) by PVP.36a In lightof these previous results, it is inconceivable that the sides of thecopper nanorods could also be tightly passivated by PAAmwhilethe nanorod ends were largely uncovered and remained moreattractive (or reactive) toward molecules such as 4-Mpy. Thus,4-Mpy molecules are expected to be preferentially adsorbed onthe {111} facets (i.e., the ends) of Cu nanorods. This results in astrong laser excited fluorescence from the {100} facets of Cunanorods, which overwhelms the SERS signals. However, for Cu

nanoparticles, 4-Mpy molecules could cover up nearly all thecrystal faces of Cu by replacing PAAm. This results in thefluorescence energy being able to transfer between the 4-Mpymolecules and the Cu nanoparticles, leading to the quenching ofthe fluorescence intensity and the amplification of the Ramanenhancement factors.48 In addition, previous reports showed thatthe morphologies of as-obtained Au49 and Ag50 substrates couldhave a great effect on the SERS activities of the nanomaterialsfor organic molecules. For instance, Ag nanoparticles showedstronger SERS signals than Ag nanowires did.50 SERS en-hancement from spherical Ag nanoparticles relies on inter-particle coupling or aggregations, where the junctions of theadjacent two particles render the SERS-active sites.51 Fromthis viewpoint, the nanorods with the least junctions have thesmaller enhancement ability. Our observation that Cu nano-particles gave rise to stronger SERS enhancement than Cunanorods did is also consistent with this observation for Agnanoparticles and nanorods.50 The SERS enhancement forthe Cu nanoparticles compared to Cu nanorods could also bedue to the aggregation of the Cu nanoparticles as shown inFigure 3. As mentioned above, stronger SERS enhancementdue to nanoparticle aggregation has indeed been well-recog-nized for nanomaterials such as Ag nanoparticles.52

4. Conclusions

In conclusion, we have successfully developed a facile, aqueous-phase procedure for the synthesis of stable, polymer-coated coppernanoparticles with well-controlled size using PAAm as a cappingagent. The size and shapeof theCunanoparticleswere controlled bychanging the relative concentration of PAAm in the solution. Weemployed UV-vis spectroscopy to monitor the growth process ofthe nanoparticles. The results showed that yellow Cu2O seednanoparticles were first formed at the beginning of the chemicalreaction, which were then converted to Cu nanoparticles as thereactionprogressed.TheTEMstudies indicated that the average sizeof the nanoparticles increased from 40 to 50 nm when the reactiontime was increased from 30 to 90 min. The as-synthesized colloidalcopper nanoparticles were proved to serve as effective SERS-activesubstrates with SERS enhancement factors in the order of 104.

Acknowledgment. We gratefully acknowledge the financialassistance by the U.S. National Science Foundation (NSF),CAREER Grant No. CHE-0645348 and NSF DMR-0804846for this work. Supporting Information is available online or fromthe authors.

Supporting Information Available: UV-vis absorptionspectra of Cu NPs synthesized using different PAAm con-centrations; SERS spectra for 1 � 10-5 M 4-Mpy absorbedon copper nanorods. This material is available free of chargevia the Internet at http://pubs.acs.org.

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