各种尺寸纳米金的合成

Seeding Growth for Size Control of5-40nm DiameterGold NanoparticlesNikhil R.Jana,*Latha Gearheart,and Catherine J.Murphy* Department of Chemistry and Biochemistry,University of South Carolina,631Sumter Street,Columbia,South Carolina29208Received March22,2001.In Final Form:August7,2001Following a seeding growth approach,gold nanoparticles of diameters5-40nm were prepared with 10-15%standard deviation in diameter from3.5(0.7nm gold particle seeds.Particle size can be controlled by varying the ratio of seed to metal salt,and thus any size in the range5-40nm can be prepared.The method can also be scaled up to produce10-100mg of gold nanoparticles.IntroductionMetal particles in the nanometer size regime show characteristic size-dependent properties different from bulk metals with the most significant size effects occurring for1-10nm diameters.1-7Consequently,extensive in-vestigations involving metallic,especially gold,nanopar-ticles as building blocks for nanoscale materials and devices are currently underway.8-19To expedite such studies,bulk quantities of particles of uniform size are necessary.There are many synthetic methods to make gold nanoparticles;20-39however,only a few methods produce particles of uniform size.The most common approach involves citrate reduction of a gold salt to produce 12-20nm size gold particles with a relatively narrow size distribution(standard deviation,∼10-16%).20,21The most popular method for producing smaller gold particles was developed by Brust et al.This method utilizes borohydride reduction of gold salt in the presence of an alkanethiol capping agent to produce1-3nm particles.22 By varying the thiol concentration,sizes can be controlled between2and5nm.23Phosphine-stabilized gold clusters (1.4(0.4nm)have also been prepared and further converted to thiol-capped clusters by ligand exchange in order to improve their stability,24-30and recently phos-phine-stabilized monodispersed gold particles(1.5(0.4 nm)were prepared using a similar protocol to the Brust method.30Most of the other methods to make small gold clusters (1-5nm)take advantage of the strong capping action of thiols.31,34,36Capping agents include disulfides,31polymers with mercapto and cyano functional groups,34and den-drimers.32,33Varying the capping agent concentration allows for size control between1and4nm with good monodispersity,but any attempts to make larger size particles(>5nm)with these procedures lead to a wider particle size distribution(standard deviation∼25-100%),23,39and further narrowing of size distribution is required.40-43Occasionally,aging,annealing,or ligand*To whom correspondence should be addressed.E-mail murphy@ and jana@.(1)Creighton,J.A.;Eadon,D.G.J.Chem.Soc.,Faraday Trans. 1991,87,3881.(2)Henglein,A.J.Phys.Chem.1993,97,5457.(3)Belloni,J.Curr.Opin.Colloid Interface.Sci.1996,1,184.(4)Valden,M.;Lai,X.;Goodman,D.W.Science1998,281,1647.(5)Link,S.;Burda,C.;Wang,Z.L.;El-Sayed,M.A.J.Chem.Phys. 1999,111,1255.(6)Link,S.;El-Sayed,M.A.J.Phys.Chem.B1999,103,8410.(7)Hodak,J.H.;Henglein,A.;Hartland,G.V.J.Phys.Chem.B 2000,104,9954.(8)Schmid,G.;Hornyak,G.L.Curr.Opin.Solid State Mater.Sci. 1997,2,204.(9)Whetten,R.L.;Shafigullin,M.N.;Khoury,J.T.;Schaaff,T.G.; Vezmar,I.;Alvarez,M.M.;Wilkinson,A.Acc.Chem.Res.1999,32,397.(10)Dirix,Y.;Bastiaansen,C.;Caseri,W.;Smith,P.J.Mater.Sci. 1999,34,3859.(11)Dirix,Y.;Bastiaansen,C.;Caseri,W.;Smith,P.Adv.Mater. 1999,11,223.(12)Caseri,W.Macromol.Rapid Commun.2000,21,705.(13)Kell,A.J.;Stringle,D.L.B.;Workentin,.Lett.2000, 2,3381.(14)Labande,A.;Astruc,mun.2000,1007.(15)Wu,M.L.;O’Neill,S.A.;Brousseau,L.C.;McConnell,W.P.; Shultz,D.A.;Linderman,R.J.;Feldheim,mun.2000, 775.(16)Hainfeld,J.F.;Powell,R.D.J.Histochem.Cytochem.2000,48, 471.(17)Templeton,A.C.;Wuelfing,M.P.;Murray,R.W.Acc.Chem. 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Chem.1995,99,7036.6782Langmuir2001,17,6782-678610.1021/la0104323CCC:$20.00©2001American Chemical SocietyPublished on Web10/02/2001exchange improves size dispersion,but at the expense of small particle size.18,28Difficulty in controlling the nucleation and growth steps occurring at intermediate stages of particle formation results in a broad particle size distribution.3For the citrate reduction method,citrate acts both as a reducing agent and as a capping agent,and due to its weak reducing and capping character,the average size is relatively large.In comparison,the Brust method uses a strong reducing agent(NaBH4),causing more nucleation,and a strong capping agent(alkanethiol)which drastically inhibits growth.Even with a strong capping agent,however, Murray et al.have shown that thiol-capped particles slowly grow within the following60h of preparation.43 Another common approach for controlling the size of nanoparticles is seeding growth.44-55In seeding growth methods,small metal particles are prepared first and later used as seeds(nucleation centers)for the preparation of larger size particles.Recently,seeding growth methods were developed for size control of Au,Ag,Ir,Pd,and Pt particles.48-55Providing a controlled number of preformed seeds(as nucleation centers)and a growth condition that inhibits any secondary nucleation,the particle size can be controlled simply by varying the ratio of seed to metal salt.However,difficulty in finding a suitable growth condition that inhibits additional nucleation during the growth stage limits the application of such methods.46,47 In general,these conditions include using a reducing agent too weak to reduce the metal salt(in the growth stage) without the presence of seeds.We have observed in other work that the presence of seeds often induced further nucleation rather than growth when the ratio of seed to metal salt was relatively small,resulting in a broad size distribution.56To avoid additional nucleation,a step by step particle enlargement method was more useful, allowing a large seed to metal salt ratio to be maintained throughout successive growth steps.56Another limitation of seeding growth is the lack of availability of smaller seeds with a narrow size distribution,and thus small particles1-10nm are difficult to synthesize.In this paper,we used3.5nm(standard deviation∼20%) gold particles as seeds to prepare gold nanoparticles in the size range5-40nm that are relatively uniform in size (standard deviation∼10-15%).The gold seeds were prepared by borohydride reduction of gold salt in the presence of citrate,a capping agent.52Secondary nucle-ation during the growth stage was inhibited by carefully controlling the growth conditions using a weak reducing agent,aqueous surfactant,and performing step by step seeding.In addition,the presence of surfactant stabilizes the particles for over1month.Experimental SectionMaterials.Trisodium citrate(Fisher),HAuCl4‚3H2O(Sigma), ascorbic acid(Aldrich),NaBH4(Mallinckrodt),and cetyltrim-ethylamonium bromide(CTAB)(Aldrich)were used as received. Ultrapure deionized water(Continental Water Systems)was used for all solution preparations.All glassware was cleaned with aqua regia.Preparation of3.5(0.7nm Gold Seed.A20mL aqueous solution containing2.5×10-4M HAuCl4and2.5×10-4M trisodium citrate was prepared in a conical flask.Next,0.6mL of ice-cold,freshly prepared0.1M NaBH4solution was added to the solution while stirring.The solution turned pink immediately after adding NaBH4,indicating particle formation.The particles in this solution were used as seeds within2-5h after preparation. Here,citrate serves only as a capping agent since it cannot reduce the gold salt at room temperature(25°C).Preparation of Growth Solution.A200mL aqueous solution of2.5×10-4M HAuCl4was prepared in a conical flask. Next,6g of solid cetyltrimethylammonium bromide(0.08M final concentration)was added to the solution,and the mixture was heated until the solution turned a clear orange color.The solution was cooled to room temperature and used as a stock growth solution.Seeding Growth.Four sets of50mL conical flasks were labeled A,B,C,and D.In set A,7.5mL of growth solution was mixed with0.05mL of freshly prepared0.1M ascorbic acid solution.Next,2.5mL of seed solution was added while stirring. Stirring continued for10min after the solution turned wine red. Particles prepared this way were spherical with a diameter of 5.5(0.6nm.Similarly,9mL of growth solution and0.05mL of0.1M ascorbic acid solution were mixed as set B,and1.0mL of seed solution was added while vigorously stirring.Stirring continued for10min.The solution’s final color was deep red.Particles prepared this way were spherical with a diameter of8.0(0.8 nm.The particles prepared here were used as seeds in set C30 min after preparation.In set C,9mL of growth solution was mixed with0.05mL of 0.1M ascorbic acid solution,and1.0mL from set B was added while stirring vigorously.Stirring was continued for10min.The final color of the solution was reddish brown.Particles prepared in this way were roughly spherical with a diameter of17(2.5 nm.This solution was used as seed in set D30min after preparation.In set D,9mL of growth solution was mixed with0.05mL of 0.1M ascorbic acid solution,and1.0mL from set C was added while stirring vigorously.Stirring continued for the next10min, and the final color of the solution was brown.Particles prepared this way consisted of a mixture of spheres(37(5nm diameter) and rods(with an average major axis of200nm and minor axis of17nm).The solutions A,B,C,and D were stable for more than a month due the presence of CTAB as a particle stabilizer.Each solution,A-D,contained2.5×10-4M gold(atoms).Thiol Capping and Subsequent Solvent Extraction.First, 10µL of dodecanethiol was added to10mL of particle solution (to make17-fold excess of dodecanethiol compared to gold)and stirred for1h.Next,20mg of KI and0.5mL of toluene were added to the particle solution and shaken for1-2min.The upper toluene portion was red due to particle extraction,and the water phase was colorless.The KI was added to enhance phase transfer. Often salts or acids are added to induce nanoparticle phase transfer by screening the interparticle electrostatic repulsion.57-59 We tested HCl,NaCl,and NaBr;however,the phase separation using these acid and salts was slower(separation after24h)(40)Whetten,R.L.;Khoury,J.T.;Alvarez,M.M.;Murthy,S.;Vezmar,I.;Wang,Z.L.;Stephens,P.W.;Cleveland,C.L.;Luedtke,W.D.; Landman,U.Adv.Mater.1996,8,428.(41)Schaaff,T.G.;Knight,G.;Shafigullin,M.N.;Borkman,R.F.; Whetten,R.L.J.Phys.Chem.B1998,102,10643.(42)Schaaff,T.G.;Whetten,R.L.J.Phys.Chem.B2000,104,2630.(43)Chen,S.W.;Templeton,A.C.;Murray,ngmuir2000, 16,3543.(44)Zsigmondy,R.;Thiessen,P. A.Das kolloide Gold;Akad. Verlagsges:Leipzig,1925.(45)Turkevich,J.;Hillier,J.Anal.Chem.1949,21,475.(46)Overbeek,J.Th.G.Adv.Colloid Interface Sci.1982,15,251.(47)Wiesner,J.;Wokaun,A.Chem.Phys.Lett.1989,157,569.(48)Schneider,S.;Halbig,P.;Grau,H.;Nickel,U.Photochem. Photobiol.1994,60,605.(49)Watzky,M.A.;Finke,R.G.Chem.Mater.1997,9,3083.(50)Brown,K.R.;Natan,ngmuir1998,14,726.(51)Brown,K.R.;Walter,D.G.;Natan,M.J.Chem.Mater.2000, 12,306.(52)Henglein,A.;Giersig,M.J.Phys.Chem.B1999,103,9533.(53)Henglein,ngmuir1999,15,6738.(54)Teranishi,T.;Miyake,M.Chem.Mater.1998,10,594.(55)Teranishi,T.;Hosoe,M.;Tanaka,T.;Miyake,M.J.Phys.Chem. B1999,103,3818.(56)Jana,N.R.;Gearheart,L.;Murphy,C.J.Chem.Mater.2001, 13,2313.(57)Hirai,H.;Aizawa,H.;Shiozaki,H.Chem.Lett.1992,1527.(58)Sarathy,K.V.;Kulkarni,G.U.;Rao,mun. 1997,537.(59)Pal,T.;Sau,T.K.;Jana,N.R.J.Colloid Interface Sci.1998, 202,30.Gold Nanoparticles Langmuir,Vol.17,No.22,20016783compared to using KI,which occurred in5-6h.Similar results were observed for the phase transfer of Ag nanoparticles.59 Procedure for Scaling Up.Seeding growth was performed similar to the procedure outline above for sets A,B,C,and D, but with higher concentrations of seed and gold salt.The procedure for the3.5nm seed preparation was the same.(Seed solutions prepared from concentrated gold salt were unstable.) The growth solution contained the same CTAB concentration but10times more HAuCl4(2.5×10-3M).The volume of growth solution used in each set,labeled A′,B′,C′,and D′,was the same as above.For sets A′and B′,10times more volume of3.5nm seed was used(25and10mL,respectively).1.0mL of B′was used for preparing C′,and1.0mL of C′was used for preparing D′.Ten times more volume of ascorbic acid(0.1M)was used for all sets. The samples were centrifuged or extracted in toluene,depending on the particle size,to concentrate the particles and remove excess surfactant.This procedure was also scaled up by increasing the volume of growth solution to100mL and accordingly increased the volume of the seed solutions and ascorbic acid solution.Thus, we can prepare10-100mg of solid particles(including CTAB) that can be redispersed.Instrumentation.Absorption spectra of the prepared solu-tions were measured using a CARY500Scan UV-vis-NIR spectrophotometer.Transmission electron microscopy(TEM) images were acquired with a JEOL JEM-100CXII electron microscope.TEM grids were prepared by placing1µL of the particle solution on a carbon-coated copper grid and drying at room temperature.TEM images were made from different parts of the grid and with different magnifications.Approximately200 particles were counted and measured for size distribution using AMT Kodak digital camera software.Results and DiscussionThe orange color of the gold salt in the CTAB solution disappeared when ascorbic acid was added.We have attributed this color change to the reduction of Au3+to Au+according to the reduction potential of Au3+/Au+(+0.8 V vs NHE in the presence of Br-)and ascorbic acid(+0.13 V vs NHE).However,the reduction of Au+to Au0does not occur,and we do not observe the gold plasmon band indicative of Au0nanoparticles even after24h.This indicates that ascorbic acid is too weak to reduce Au+under our experimental conditions.However,the gold salt reduction to elemental gold nanoparticles occurs in the presence of3.5nm seed particles.This is consistent with our earlier work in which we observed that the rate of gold salt reduction by ascorbic acid was enhanced in the presence of12nm size gold particle seeds.56Seeding growth methods,involving the growth of small particles(seeds)into larger particles,were used to prepare different sized gold nanoparticles.We followed either one step or step by step seeding growth depending on the target particle size.Step by step seeding employs subsequently prepared larger particles as seeds for later stage particle enlargement.Three sets of spherical particles corresponding to sets A,B,and C having diameters of5.5(0.6,8.0(0.8and 17(2.5nm,respectively,were prepared from the3.5( 0.7nm seed by this seeding growth method(Table1).The 5.5and8.0nm particles were prepared by one-step seeding but by varying the ratio of seed to metal salt.The17nm size particles were prepared by step by step seeding where 3.5and8nm particles were used as seeds in the first and second steps of the growth processes,respectively.When 17nm particles were used as seed for further size enlargement,a fraction of particles were nonspherical (rodlike,oblate,and platelike)in shape.For example,in set D we prepared37(5nm spheres with a significant amount(∼10%)of rods(∼200nm major axis and17nm minor axis).By centrifugation,the rods were removed from the solutions of spheres.When one-step seeding was used to make particles20-50nm in size using3.5nm seeds,short rods(with∼60nm major axis and16nm short axis)and plates were formed.60The UV-vis spectra,TEM images,and histograms for different size particles are shown in Figures1,2,and3, respectively.The particle plasmon absorption band at524 nm becomes sharp and intense with increasing particle size from3.5to37nm.The plasmon band red shifts from 520to530nm for particle sizes between3.5and37nm. The absorbance increase for37nm size particles was not prominent(∼2%)because some of the gold salt reduced to form rods(∼10%in number)which were subsequently separated.The calculated molar extinction coefficients for the different size gold particles are given in Table1. These values increase linearly with the particle volume and are on the order of or greater than the absorptivity of organic dyes.The extinction coefficient values of our particles are consistent with other results.6The relative standard deviation of the average particle size decreased (compared to seed)for5.5and8.0nm size particles (standard deviation11%and10%,respectively)but became wider for17nm diameter particles(standard deviation15%)due to the formation of nonspherical shapes.For37nm particles polydispersity was very high due to the formation of rods and plates.When spheres(60)Jana,N.R.;Gearheart,L.;Murphy,C.J.J.Phys.Chem.B2001, 105,4065.Table1.Data for Gold Nanoparticles sample,measdparticle diam(nm),%SDcalcdparticle size(nm)aplasmonband max(nm),M-1cm-1(molesparticles)b seed,3.5(0.7,20%5200.4×107 A,5.5(0.6,11% 5.6524 1.8×107 B,8.0(0.8,10%7.55247.0×107 C,17(2.5,15%1652478×107 D,37(5,14%35530540×107a See text for formula.b We estimated the concentration of nanoparticles produced from knowing the total amount of Au atoms in solution for seed plus gold from the added salt and then determining how many atoms would fit in each particle.The volume of a sphere corresponding to the seed or the resultant particle was calculated,and the total number of atoms fitting into each volume was determined using the crystal structure of gold(cubic unit cell )4.0786Åon edge,4gold atoms/unit cell),giving us the average number of atoms/particle.The number of particles in each sample was calculated by dividing the total number of gold atoms by the atoms/particle.This was converted to number of moles of particles/liter.Figure1.UV-vis spectra of3.5(0.7nm gold seed and the larger gold particles prepared from seed.The spectra for the 37(5nm sample was taken after separation of rods.The gold concentration(in terms of gold salt)was10-4M for all sizes.6784Langmuir,Vol.17,No.22,2001Jana et al.were separated from rods and plates,the standard deviation of the 37nm size particles was 14%.The utility of our synthetic method is that the desired size particles are prepared by varying the seed to metal salt ratio.The expected size can be calculated from a simple theoretical equation:r )r seed {([M added ]+[M seed ])/[M seed ]}1/3where r s and r indicate particle radius for seed and larger particle,respectively,and [M seed ]and [M added ]indicate metal concentrations in seed and added ion,respec-tively.45,46,48Table 1compares the calculated and experi-mental results.The measured diameters closely match the calculated diameters (the calculation assumes only growth occurs to give spherical particles)and provides support for the notion that only growth occurs in our experiments.Surface derivatization of nanoparticles is often neces-sary to control their self-assembly and physical proper-ties.17,61Successful surface derivatization has been achieved by ligand exchange reactions of alkanethiol-capped gold particles with other functionalized thiol derivatives.17Such derivatization processes often lead to particle aggrega-tion.29Our surfactant stabilized gold nanoparticles have two advantages in this respect.First,the particle stabi-lization does not involve the formation of a strong metal -ligand bond (as in gold particle -thiol composite),and the surfactant can be easily removed.Second,the presence of a surfactant can solubilize the organic functional com-pounds that are otherwise insoluble in water.We have surface derivatized our nanoparticles with dodecanethiol,a widely used surface derivatizing agent,17,61to make the particles hydrophobic and extractable in organic solvent.Particles of 3.5-8nm in size were extracted into organic solvent after thiol capping.Dodecanethiol capped 8(0.8nm particles showed a tendency to form a superlatice viaself-assembly (Figure 4).Particles >8nm were not extracted into the organic solvent even after thiol capping and formed particle films at the aqueous -organic inter-face.Sastry et al.observed a similar film formation during the solvent extraction of carboxylic acid-derivatized 13nm gold particles in the presence of octadecylamine.62They also observed a size dependence for the particle partition-ing process.63,64Pileni et al.observed the influence of particle size and alkyl chain length on the extraction of alkanethiol-capped Ag 2S into organic phase.65Particles 2-5nm in size can be extracted into organic solvent but 7.6nm particles cannot,even after varying the alkyl chain length.65This size-dependent solvent extraction behavior has been used for separation of different sized particles(61)Collier,C.P.;Vossmeyer,T.;Heath,J.R.Annu.Rev.Phys.Chem.1998,49,371.(62)Mayya,K.S.;Sastry,ngmuir 1999,15,1902.(63)Mayya,K.S.;Sastry,M.J.Phys.Chem.B 1997,101,9790.(64)Gole,A.M.;Sathivel,chke,A.;Sastry,M.J.Chromatogr.A 1999,848,485.(65)Motte,L.;Pileni,M.P.J.Phys.Chem.B 1998,102,4104.Figure 2.TEM image of larger gold particles prepared from seed:(a)5.5(0.6,(b)8.0(0.8,(c)17(2.5,and (d)37(5nm after separation of rods.The 5.5(0.6nm particles were extracted into toluene after thiol capping for TEM in order to remove excess surfactant.The other size particles were separated from excess surfactant bycentrifugation.Figure 3.Histograms of gold particles grown from 3.5nm seed:(a)5.5(0.6,(b)8.0(0.8,(c)17(2.5,and (d)37(5nm after separation of rods.Gold Nanoparticles Langmuir,Vol.17,No.22,20016785and to narrow the particle size distribution.19,61The exact mechanism of the phase transfer is not well established;however,it is related to the formation of a stable hydrophobic monolayer of thiols on the particle surface,where the stability depends on particle size.65Many studies involving nanoparticles need appreciable amounts of particles in the solid phase.The Brust method of making gold nanoparticles allows for this,but few other methods are conducive to scaling up.22Accordingly,we prepared more concentrated solutions of nanoparticles.The method works well for a 10-fold increase in reagent concentrations but with a compromise in standard devia-tion (∼20%)for gold nanoparticles 5.5nm in size.However,for our 8,17,and 37nm size particles,the standard deviations were 10-15%.For 37nm size particles,formation of rods are suppressed,possibly due to less availability of rodlike micelles at high reagent concentra-tions.66Seeding growth methods have been employed to prepare gold particles of varied sizes,from 20to 50nm.50-53Unfortunately,the preparation of particles less than 10nm by seeding growth has been problematic because of the difficulty in preparing small,monodisperse seeds and because of the tendency for both particles and seeds toaggregate.51,67In this work we used citrate-capped,borohydride-reduced 3.5nm (standard deviation 20%)size gold particles as the seed.We have used an aqueous surfactant (CTAB)media for a growth environment which inhibited particle aggregation during growth and stabi-lized the particles after they formed.Earlier,Natan et ed citrate-capped,borohydride-reduced 2.6nm and citrate-reduced 12nm gold particles for making particles >20nm and observed a decrease,or focusing,of the percent relative standard deviation of the average particle size as the particles grew.51Henglein et ed ∼2nm particles as seed (prepared by γirradiation in the presence of polymer)to make 5-50nm particles with particle size distribution focused for 53nm (standard deviation ∼13%)compared to 5nm (standard deviation ∼44%)particles.53Our results indicate a similar focusing of the size distribution with increasing particle size from 3.5to 8nm.However,for particles larger than 8nm,the appear-ance of nonspherical faceted shapes,rods,and plates contribute to the size polydispersity.For diffusion-controlled growth,the particle size distribution should be focused with increasing size since small particles will grow faster than larger particles.68-70The particle mediated growth of metal nanoclusters has been extensively stud-ied.7,71The presence of particles strongly catalyzes the reduction and growth processes,and hence a diffusional growth mechanism may be contributing to growth.ConclusionWe have developed a seeding growth approach to prepare gold nanoparticles 5-40nm in diameter with a very narrow size distribution (SD ∼10-15%).The particle sizes can be easily manipulated by varying the ratio of seed to metal salt.To avoid any secondary nucleation,a step by step particle enlargement is more effective than a one-step seeding method.In addition,this method can be used for larger scale synthesis of gold nanoparticles.Acknowledgment.We thank the National Science Foundation for funding this research.LA0104323(66)Tornblom,M.;Henriksson,U.J.Phys.Chem.B 1997,101,6028.(67)Mallick,K.;Wang,Z.L.;Pal,T.J.Photochem.Photobiol.A 2001,140,75.(68)Reiss,H.J.Chem.Phys.1951,19,482.(69)Peng,X.G.;Wickham,J.;Alivisatos,A.P.J.Am.Chem.Soc.1998,120,5343.(70)Liu,H.;Penner,R.M.J.Phys.Chem.B 2000,104,9131.(71)Gachard,E.;Remita,H.;Khatouri,J.;Keita,B.;Nadjo,L.;Belloni,J.New J.Chem.1998,22,1257.Figure 4.TEM image of the self-assembly of dodecanethiol-capped 8.0(0.8nm gold particles.6786Langmuir,Vol.17,No.22,2001Jana et al.。