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An Improved Procedure for Preparing Smaller and Nearly Monodispersed Thiol-Stabilized Platinum Nanoparticles Jun Yang,† Jim Yang Lee,*,†,‡ T. C. Deivaraj,‡ and Heng-Phon Too‡,§ Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore-MIT Alliance, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, and Department of Biochemistry, National University of Singapore, Singapore 119260 Received April 8, 2003. In Final Form: August 15, 2003 An alternative process was developed to prepare smaller and nearly monodispersed thiol-stabilized platinum nanoparticles. The procedure involves the reduction of a Pt(IV) precursor salt by a small (10%) stoichiometric excess of sodium borohydride in the presence of a toluene solution of 1-dodecanethiol. Complete reduction of the Pt precursor was confirmed by elemental analysis and UV-vis spectroscopy. The platinum nanoparticles prepared as such could be transferred directly to the toluene layer without the addition of concentrated hydrochloric acid that was previously reported as essential for the transfer to take place. TEM imaging shows a mean particle diameter of 2.6 nm and a nearly monodispersed particle size distribution (σ ) 0.44 nm). These results were used to formulate a protocol to prepare oligonucleotide-stabilized Pt nanoparticles with narrow particle size distribution and excellent stability in aqueous solutions.
Introduction The phase-transfer method, in which metal nanoparticles are formed in a hydrocarbon phase, or extracted from an aqueous phase to a hydrocarbon phase, is commonly used to prepare organosols of metals including platinum, ruthenium, and gold.1-6 The metal nanoparticles need to be stabilized, and alkanethiols are often used for that purpose.2-3,5 The Brust procedure6 is often considered as the first reported method to prepare thiolstabilized nanoparticles. In this method the metal ions from an aqueous solution are extracted to a hydrocarbon (toluene) layer using tetraoctylammonium bromide as the phase-transfer agent, and reduced by aqueous NaBH4 solution in the presence of an alkanethiol. Here the nucleation and growth of the metal particles and the attachment of the thiol molecules occur simultaneously in a single step. Sarathy2,3 and Zhao5 adopted a different approach in which Pt nanoparticles, instead of the metal ions, were directly transferred to the organic phase for thiolation. A Pt hydrosol was prepared in advance using NaBH4 as the reducing agent, and mixed with a toluene solution of 1-dodecanethiol. Concentrated HCl was then added to the biphasic mixture under stirring to enable the transfer. The thiol-stabilized Pt nanoparticles so prepared had an average diameter of 4.4 nm. The particles in the Pt hydrosol prior to the transfer had not been characterized, and particle agglomeration could have * To whom correspondence should be addressed. Fax: 6567791936. E-mail:
[email protected]. † Department of Chemical and Environmental Engineering. ‡ Singapore-MIT Alliance. § Department of Biochemistry. (1) Viau, G.; Brayner, R.; Poul, L.; Chakroune, N.; Lacaze, E.; FievetVincent, F.; Fievet, F. Chem. Mater. 2003, 15, 486-494. (2) Sarathy K. V.; Kulkarni, G. U.; Rao, C. N. R. Chem. Commun. 1997, 537-538. (3) Sarathy K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876-9880. (4) Fu, X.; Wang, Y.; Wu, N.; Gui, L.; Tang, Y. J. Colloid Interface Sci. 2001, 243, 326-330. (5) Zhao, S.-Y.; Chen, S.-H.; Wang, S.-Y.; Li, D.-G.; Ma, H.-Y. Langmuir 2002, 18, 3315-3318. (6) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802.
occurred before or during the transfer. In addition the effect of NaBH4 on the particle-transfer process was not quantified in these cited works. In this paper we report an alternative process whereby the addition of concentrated HCl is not necessary for the transfer of the metal nanoparticles. This new process is therefore more amenable to particle modifications to be carried out in a biological environment. In this new process the Pt nanoparticles are prepared by the NaBH4 reduction of a platinum precursor salt, and are transferred directly to a toluene solution of 1-dodecanethiol without the need for HCl addition. It was found that the amount of NaBH4 used in the reduction reaction strongly influenced the transfer process and the final particle size. A mechanism is proposed to interpret the experimental results. These results also led to the development of a procedure for the preparation of oligonucleotide-stabilized Pt nanoparticles. Experimental Section Hydrogen hexachloroplatinate(IV) hydrate and 1-dodecanethiol from Aldrich, sodium borohydride (98%) from Fluka, toluene and concentrated hydrochloric acid (36%∼38%) from Baker, PBS buffer from Sigma, and alkanethiol-modified oligonucleotide poly(A) (5′-SH-(CH2)6-AAA-AAA-AAA-AAA-AAA-AAA-AA; 248 µM) from Proligo were used as received. Deionized water was distilled by a Milli-Q water purification system. All glassware and Teflon-coated magnetic stir bars were treated in aqua regia, followed by copious washing with distilled water and drying in an oven. Transmission electron microscopy (TEM) was performed on a Philips CM 300 FEG system operating at 200 kV. For TEM measurements a drop of the metal nanoparticle solution was placed onto a 3 mm copper grid coated with a continuous carbon film. Excess solution was removed by an absorbent paper. The mean particle size and particle size distribution were obtained from a few randomly chosen areas in the TEM image containing approximately 200 nanoparticles each. The Pt hydrosol was centrifuged at 5000 rpm for 5 min to remove the nanoparticles. The resultant supernatant was analyzed for residual Pt(IV) presence by inductively coupled plasma (ICP) spectroscopy using a Perkin-Elmer Optima 3000 spectrometer. UV-vis spectroscopic measurements of the H2PtCl6 solution before and after NaBH4 addition were also taken on a Shimadzu UV-2450 spectrophotometer. X-ray photoelectron spectroscopic (XPS) analysis was
10.1021/la034596q CCC: $25.00 © 2003 American Chemical Society Published on Web 10/15/2003
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carried out on a VG ESCALAB MKII spectrometer uisng the Pt nanoparticles obtained from the hydrosol by centrifugal separation. Samples for XPS were copiously washed with distilled water before the XPS examination. Narrow scan photoelectron spectra were recorded in the Pt 4f region. The X-ray diffraction (XRD) patterns of the Pt hydrosol precipitates were measured by a Rigaku D/Max-3B diffractometer using Cu KR radiation. All samples were vacuum-dried before the measurements. Colloid 1. Under vigorous stirring, 0.2 mL of a freshly prepared 0.11 M aqueous solution of NaBH4 was added dropwise to 10 mL of a 2 mM aqueous solution of H2PtCl6. The mole ratio of BH4to Pt(IV) was 1.1 (10% in excess of the stoichiometric amount needed for reduction). The color of the solution turned from yellow to dark brown, indicating the formation of a Pt hydrosol. A 10 mL sample of toluene containing 100 mg of 1-dodecanethiol was then introduced, and the mixture was stirred for 15 min before it was transferred to a 100 mL separation funnel. After the mixture had settled down to two immiscible layers, only the toluene layer was brown in color, indicating the successful transfer of platinum nanoparticles from the hydrosol via 1-dodecanethiol attachment. The toluene layer was saved as colloid 1. Colloid 2. The procedure for preparing colloid 1 was repeated using 0.8 mL of a 0.11 M NaBH4 solution, corresponding to a BH4- to Pt(IV) ratio of 4.4. A 10 mL sample of toluene containing 100 mg of 1-dodecanethiol was introduced after the formation of the Pt hydrosol. Despite vigorous stirring, no transfer of Pt from the hydrosol took place. Instead, a black precipitate was obtained after the water-toluene mixture was stirred for several minutes. The only way to transfer the Pt nanopartricles was to quickly add 10 mL of concentrated hydrochloric acid after the toluene addition, and to continue stirring for 3 min. The color change in the toluene layer confirmed the transfer of Pt nanoparticles in this case. The mixture was then transferred to a 100 mL separation funnel, and the toluene layer after separation was washed three times with 150 mL of distilled water to remove the excess hydrochloric acid. The washed toluene layer was saved and labeled as colloid 2. Colloid 3. Thiol-stabilized Pt nanoparticles named as colloid 3 were prepared by a slight modification of the procedure used for preparing colloid 1. In this case 10 mL of an aqueous solution of H2PtCl6 was first mixed with 10 mL of toluene containing 100 mg of dissolved 1-dodecanethiol. Then under vigorous stirring, 0.2 mL of a freshly prepared 0.11 M aqueous solution of NaBH4 was added dropwise to the water-toluene mixture. Stirring was continued for 15 min, and the toluene layer after separation was identified as colloid 3.
Results and Discussion Figure 1 is the TEM image of a Pt hydrosol taken immediately after it was formed during the preparation of colloid 1. The histogram resulting from counting 200 well-separated particles (Figure 1) depicts a narrow particle size distribution with a mean particle diameter of 2.5 nm (standard derivation σ ) 0.45 nm). The hydrosol was however unstable, and black precipitates would appear after a few hours of storage. The TEM image of the corresponding colloid 1 is given in Figure 2. The transfer of nanoparticles was successful, but the resulting particle size distribution was rather broad, and the mean particle diameter was also larger (d ) 3.4 nm, σ ) 1.38 nm). The increased inhomogeneity in the particle size distribution is an indication of particle agglomeration during the transfer process. Similar observations were also obtained from colloid 2, the TEM image and the histogram of the size distribution are shown in Figure 3. The hydrosol used in the preparation of colloid 1 was centrifuged to remove the nanoparticles. Analysis of the supernatant by ICP-AES showed no residual presence of Pt. In addition, the UV-vis spectrum of PtCl62- before the addition of a 10% stoichiometric excess of NaBH4 shows an absorption peak at 457 nm (Figure 4), which was
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Figure 1. TEM image of Pt hydrosol taken immediately after its formation, d ) 2.5 nm, σ ) 0.45 nm.
completely obliterated after the reduction reaction. These experimental observations were taken to indicate that the 10% stoichiometric excess of NaBH4 was sufficient to react the Pt(IV) precursor away. The composition of the nanoparticles from the Pt hydrosol used in the preparation of colloid 1 was analyzed by XPS. Figure 5 shows that the Pt 4f region of the spectrum can be deconvoluted into two pairs of doublets. The more intense doublet (at 70.9 and 74.2 eV) is a signature of Pt in the zerovalent state. The second and weaker doublet (at 72.3 and 75.1 eV), with binding energies 1.4 eV higher than that of Pt(0), could be assigned to the Pt(II) oxidation state as in PtO and Pt(OH)2.7,8 In addition, XRD measurements were also carried out on the same Pt sample used in the XPS analysis (Figure 6). The lines of (111) (d ) 0.2255 nm), (200) (d ) 0.1955 nm), (220) (d ) 0.1380 nm), (311) (d ) 0.1179 nm), and (222) (d ) 0.1129 nm) diffractions of metallic Pt are clearly identifiable, indicating that the Pt nanoclusters have an FCC lattice structure.9 The average Pt particle size calculated from the half-width of the (220) diffraction line is ∼3.1 nm, which agrees well with the TEM results shown in Figure 1. In the preparation of colloid 2, particle transfer could not take place without the addition of concentrated (7) Liu, Z.; Lee, J. Y.; Han, M.; Chen, W. X.; L. Gan, L. M. J. Mater. Chem. 2002, 2453-2458. (8) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell, C. J.; Rumble, J. R., Jr. NIST Standard Reference Database 20, Version 3.2 (Web Version). (9) McClune, W. F. Powder Diffraction File Alphabetical Index Inorganic Phase; JCPDS: Swarthmore, PA, 1980.
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Figure 3. TEM image of colloid 2, d ) 4.7 nm, σ ) 1.12 nm. Figure 2. TEM image of colloid 1, d ) 3.4 nm, σ ) 1.38 nm.
hydrochloric acid. Prolonged stirring of the Pt hydrosol in this case would eventually result in the formation of a black precipitate. It is hypothesized that it was BH4- that prevented the transfer from occurring. BH4- therefore functions not only as a reducing agent but also as a stabilizer against particle agglomeration. Specifically adsorbed anions can be used as stabilizing agents in the preparation of metal nanoparticles. For example, adsorbed OH- is responsible for the stability of noble metal particles prepared by the polyol process.10,11 The nonuniform transfer of Pt nanoparticles in a shortage of NaBH4 may be rationalized by this hypothesis of BH4- adsorption as follows: On the basis of stability consideration particles with a low surface coverage of BH4- were less protected and hence would transfer faster than those with a higher surface coverage of the reducing agent. In addition, incomplete surface coverage by BH4- could not completely inhibit particle agglomeration, and aggregates of different sizes were formed and transferred, resulting in the formation of an inhomogeneous Pt nanosystem in the toluene layer. When excess NaBH4 was present, the surface of Pt nanoparticles in the hydrosol was well protected by BH4- initially, thereby blocking the access of 1-dodecanethiol to the nascent metal surface, and suppressing the transfer of nanoparticles from the aqueous phase to toluene. However, BH4- is not a solution-stable species, and its slow decomposition in the aqueous phase would eventually lead to the partial exposure of the Pt (10) Wang, Y.; Ren, J.-W.; Deng, K.; Gui, L.-L.; Tang, Y.-Q. Chem. Mater. 2000, 12, 1622-1627. (11) Komarneni, S.; Li, D.-Sh.; Newalkar, B.; Katsuki, H.; Bhalla, A. S. Langmuir 2002, 18, 5959-5962.
Figure 4. UV-vis spectra of H2PtCl6 before and after the addition of a small stoichiometric excess of NaBH4.
Figure 5. X-ray photoelectron spectrum of the Pt nanoparticles used to produce colloid 1.
nanoparticle surface, establishing the anchor points for particle agglomeration. Hence, prolonged stirring of the Pt hydrosol would cause precipitation to occur even without the addition of other reagents. A low pH value brought about by concentrated hydrochloric acid addition accelerated the decomposition of NaBH4, making the nanoparticles more susceptive to 1-dodecanethiol attach-
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Figure 6. XRD patterns of the Pt nanoparticles used in the preparation of colloid 1.
Figure 8. TEM image of alkanethiol-capped oligonucleotidestabilized Pt nanoparticles, d ) 2.5 nm, σ ) 0.70 nm.
Figure 7. TEM image of colloid 3, d ) 2.6 nm, σ ) 0.44 nm.
ment and their subsequent transfer to the toluene layer. The larger average diameter for the particles of colloid 2 in the TEM image of Figure 2 can then be attributed to particle agglomeration that occurred because of the progressive decomposition of NaBH4. On the basis of the above analysis, concentrated hydrochloric acid addition is only a means to release the borohydride-covered Pt surface for 1-dodecanethiol attachment. The preparative procedure was therefore slightly modified to enable the phase transfer to occur as soon as the particles were formed. This was accomplished by mixing the chloroplatinate solution and the toluene solution of 1-dodecanethiol before the dropwise addition of the reducing agent NaBH4. This procedure is different from that used by Brust and co-workers in that the reduction in this work was carried out in the aqueous environment, whereas Brust and co-workers carried out the reduction by microcontacting two immiscible phases. Figure 7 is the TEM image of the resulting colloid (colloid
3). The smaller mean diameter of 2.6 nm and the nearly monodispersed distribution of particle size (σ ) 0.44 nm) are evidence in support of our previous conclusion. Hence, Pt nanoparticles were only temporarily protected by BH4- against particle agglomeration. We demonstrated that the transfer of Pt nanoparticles from the aqueous solution to the toluene layer could actually take place without the addition of concentrated hydrochloric acid. This is a welcome feature for this technique to be used in a biological environment where pH has to be controlled within a small window to prevent denaturizing the biological substrates. As an example, we have successfully prepared alkanethiol-modified oligonucleotide-stabilized Pt nanoparticles by replacing the toluene solution of 1-dodecanethiol with a PBS buffer solution of alkanethiolmodified oligonucleotides. Briefly, 50 µL of the Pt hydrosol used in the preparation of colloid 1 was diluted with 50 µL of distilled water, and 100 µL of 1 × PBS buffer containing 2.5 nmol of alkanethiol-modified oligonucleotide (poly(A)) was introduced. After thorough mixing, a brown-colored Pt nanosystem in which the particles were stabilized by poly(A) was obtained, which could be stored for several weeks without changes. The benefit of capping the nanoparticles with the oligonucleotides is the increased stability of the nanoparticles in high salt concentrations. In a solution of 1 M NaCl, pristine Pt nanoparticles would easily undergo irreversible particle-growth reactions that resulted in their precipitation. The poly(A)-stabilized Pt nanoparticles, on the other hand, were stable in 1 M NaCl for days. Presumably their oligonucleotide-modified surfaces had prevented the particles from getting close enough for the particle growth to occur. Figure 8 shows the TEM image of the oligonucleotide-stabilized Pt nanoparticles.
Thiol-Stabilized Platinum Nanoparticles
The details on the use of oligonucleotide-stabilized Pt nanoparticles will be reported later. Conclusions Particle agglomeration and its control in the thiolfacilitated transfer of platinum from a hydrophilic phase to a hydrophobic phase were investigated in this work. When the metal nanoparticles were prepared from the NaBH4 reduction of the metal precursor salt, and were transferred from the hydrosol to toluene, the amount of NaBH4 used had a strong impact on the transfer process. When only a small stoichiometric excess of NaBH4 was present, the transfer was fast and Pt nanoparticles 3.4 nm in diameter with a broad particle size distribution could be obtained. In a large stoichiometric excess of NaBH4, the transfer was inhibited unless concentrated hydrochloric acid was added. It is hypothesized that NaBH4 not only acted as a reducing agent, but also as a stabilizer against particle agglomeration. The addition of
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concentrated hydrochloric acid decomposed NaBH4 homogeneously, thereby releasing the nascent nanoparticle surface for thiolation, and enabling the subsequent transfer of the nanoparticles to the toluene phase. The hypothesis forms the basis of a modified procedure for preparing even smaller (d ) 2.6 nm) and nearly monodispersed thiol-stablizied Pt nanoparticles (σ ) 0.44 nm). In addition, we have developed a technique to prepare oligonucleotide-stabilized Pt nanoparticles, which could be used to initiate self-assembly of the metal nanoparticles based on Watson-Crick base pairing of complementary single-stranded DNA sequences. Acknowledgment. We acknowledge general financial support from the Singapore-MIT Alliance. J.Y. acknowledges the National University of Singapore for his research scholarship. LA034596Q
