Langmuir 2007, 23, 10891-10892
10891
Fine-Tuning Nanoparticle Size by Oligo(guanine)n Templated Synthesis of CdS: An AFM Study Lorenzo Berti,*,† Andrea Alessandrini,†,‡ Manuele Bellesia,† and Paolo Facci† National Research Center on nanoStructures and bioSystems at Surfaces S3, INFM-CNR, Via Campi 213/A, 41100 Modena, Italy, and Department of Physics, UniVersita’ di Modena e Reggio Emilia, Via Campi 213/A, 41100 Modena, Italy ReceiVed June 22, 2007. In Final Form: July 31, 2007 We are presenting a method for modulating the size of CdS nanoparticles by templating their formation with oligo(guanine)n oligomers where n varied from 5 to 20. The variation in template length resulted in observable changes in the size distribution of the CdS nanoparticles. Statistical analysis of AFM images showed a general trend whereby the CdS average height decreased for longer oligoGn and increased for shorter oligoGn. Concomitantly, shorter oligoGn yielded more dispersed populations, while longer oligoGn gave less dispersed populations. This synthetic methodology could be extended to the synthesis of other nanoparticles and even to mixed-metal nanoparticles resulting in a powerful method for fine-tuning size-dependent properties.
The use of nucleic acids as templates for the growth of inorganic materials is a topic of current interest, and recent studies have shown that DNA and RNA can be effective scaffolds for positioning1,2 or synthesizing3,4 inorganic nanocomponents. The templating nature of nucleic acids stems from preferential interactions between individual nucleotides and an inorganic precursor (i.e., a metal ion or a nanoparticle) and is therefore dependent on the oligonucleotide sequence. Each nucleotide contributes differently to the outcome of the templated process, although an understanding of their role is a topic still in need of much investigation. The nucleotide’s heterocyclic ring and phosphate are both known to form stable complexes with metal ions, and both moieties have been explored as ligands for controlling the growth of inorganic nanoparticles. For instance, recent reports have shown that PbS5 and CdS6-9 nanoparticles are formed in the presence of nucleotides as capping agents, and it was also found that the size of the resulting nanoparticles was dependent on the nucleotide employed.5,6 RNA10-13 and DNA3,14-16 have also been exploited as templates for the synthesis * Corresponding author:
[email protected]. † National Research Center on nanoStructures and bioSystems at Surfaces S3, INFM-CNR. ‡ Department of Physics, Universita’ di Modena e Reggio Emilia. (1) Le, J. D.; Pinto, Y.; Seeman, N. C.; Musier-Forsyth, K.; Taton, T. A.; Kiehl, R. A. Nano Lett. 2004, 4, 2343-2347. (2) Li, H. Y.; Park, S. H.; Reif, J. H.; LaBean, T. H.; Yan, H. J. Am. Chem. Soc. 2004, 126, 418-419. (3) Dittmer, W. U.; Simmel, F. C. Appl. Phys. Lett. 2004, 85, 633-635. (4) Berti, L.; Alessandrini, A.; Facci, P. J. Am. Chem. Soc. 2005, 127, 1121611217. (5) Hinds, S.; Taft, B. J.; Levina, L.; Sukhovatkin, V.; Dooley, C. J.; Roy, M. D.; MacNeil, D. D.; Sargent, E. H.; Kelley, S. O. J. Am. Chem. Soc. 2006, 128, 64-65. (6) Green, M.; Smith-Boyle, D.; Harries, J.; Taylor, R. Chem. Commun. 2005, 4830-4832. (7) Kumar, A.; Mital, S. Photochem. Photobiol. 2002, 1, 737-741. (8) Kumar, A.; Mital, S. J. Colloid Interface Sci. 2001, 240, 459-466. (9) Dooley, C. J.; Rouge, J.; Ma, N.; Invernale, M.; Kelley, S. O. J. Mater. Chem. 2007, 17, 1687-1691. (10) Ma, N.; Dooley, C. J.; Kelley, S. O. J. Am. Chem. Soc. 2006, 128, 1259812599. (11) Gugliotti, L. A.; Feldheim, D. L.; Eaton, B. E. J. Am. Chem. Soc. 2005, 127, 17814-17818. (12) Gugliotti, L. A.; Feldheim, D. L.; Eaton, B. E. Science 2004, 304, 850852. (13) Kumar, A.; Jakhmola, A. Langmuir 2007, 23, 2915-2918. (14) Jin, J.; Jiang, L.; Chen, X.; Yang, W. S.; Li, T. J. Chin. J. Chem. 2003, 21, 208-210. (15) Levina, L.; Sukhovatkin, W.; Musikhin, S.; Cauchi, S.; Nisman, R.; BazettJones, D. P.; Sargent, E. H. AdV. Mater. 2005, 17, 1854-+. (16) Ritchie, C. M.; Johnsen, K. R.; Kiser, J. R.; Antoku, Y.; Dickson, R. M.; Petty, J. T. J. Phys. Chem. C 2007, 111, 175-181.
of inorganic nanoparticles. Here, the role of each nucleotide is less clear, although it can be speculated that preferential precomplexation of the metal ion by specific nucleotides plays an important role and that the polymeric and electrostatic nature of the nucleic acid limits the growth of the nanoparticles. When compared to other templates and capping agents, nucleic acids present a major advantage as an exquisite control over their length and sequence can be easily achieved by chemical and biochemical methods. Length and sequence are both important properties of the template that could be exploited to fine-tune the shape and size of the resulting stabilized nanoparticles. In this communication, we wish to demonstrate that the size distribution of CdS nanoparticles can be fine-tuned by controlling the length of a templating homogeneous oligonucleotide sequence. Our approach was based on the following assumptions: (1) a strong templating effect can be achieved by quantitative precomplexation of Cd2+ by a homogeneous, strongly binding nucleotide sequence; (2) a variation in the oligo length will yield oligo-Cd2+ complexes (precursors) of different sizes; (3) the size of the oligo-metal ion precursor will define the average size of the resulting CdS nanoparticles. For our proof-of-concept study, we employed oligo(guanine)n (oligoGn) as the template, where n varied between 5 and 20 nucleobases. The rationale for using such sequences was given by the well-known complexing nature of G toward several metal ions, including Cd2+ 17,18 and by other studies6,9,19 indicating that G is a very effective nucleotide in mediating the synthesis of CdS nanocrystals. The oligoGntemplated synthesis of CdS nanoparticles was accomplished by premixing oligoGn with Cd2+ at a 2 to 1 G:Cd2+ ratio, as this ensured that all of Cd2+ was complexed to DNA. For the formation of CdS, a preliminary titration experiment determined that a 7-fold amount of sulfide was necessary to completely convert Cd2+ to CdS. This finding is not unexpected, as an excess of S2is usually required to drive the ionization equilibrium toward the insoluble CdS.20,21 For all subsequent experiments, a 10-fold excess of sulfide was employed. The formation of CdS was followed by UV-vis spectrophotometry. CdS formed instantly upon sulfide addition as witnessed by the appearance of its characteristic absorption profile. UV-vis spectra also showed (17) Izatt, R. M.; Christensen, J. J.; Rytting, J. H. Chem. ReV. 1971, 71, 440481. (18) Burda, J. V.; Sponer, J.; Leszczynski, J.; Hobza, P. J. Phys. Chem. B 1997, 101, 9670-9677. (19) Jiang, L.; Zhuang, J.; Ma, Y.; Yang, B.; Yang, W.; Li, T. New J. Chem. 2003, 27, 823-826.
10.1021/la701867f CCC: $37.00 © 2007 American Chemical Society Published on Web 09/19/2007
10892 Langmuir, Vol. 23, No. 22, 2007
Figure 1. AFM images showing the effect of varying n of the oligoGn template (1 µm × 1 µm, z-scale ) 5 nm). In the absence of a template, no nanoparticle formation was observed (top left), whereas the presence of oligoGn induced nanoparticle formation. Increasing the length of oligoGn resulted in the formation of nanoparticles of progressively smaller average height.
that a 24 h aging period was necessary in order to obtain stable spectral characteristics. In the absence of oligoGn, the absorption profile was typical of bulk CdS, while the spectra for oligoGntemplated CdS showed a steep band edge absorption typical of quantum confinement22 (Figure S1, Supporting Information). Varying the length of oligoGn did not result in significant spectral differences between the different CdS populations limiting the usefulness of this technique for assessing average sizes according to the method of Brus.20,21 The lack of significant spectral differences could be attributed to the different capping for each CdS population and to aggregation in solution. The use of AFM as a sizing tool for nanoparticles has been reported before as an alternative to more conventional methods such as TEM.4,23 AFM images were obtained after 24 h from the initial CdS formation, to study the effect of oligoGn length over the average height of the nanoparticles. Samples for AFM imaging were prepared by air-drying a diluted CdS suspension on mica, and the resulting images are shown in Figure 1. In the absence of oligoGn, nanoparticle formation was not observed, whereas addition of oligoGn resulted exclusively in nanoparticle formation. Qualitative analysis of the images also seemed to indicate that their size distribution was dependent on oligoGn length. In order to rule out the possibility that the objects observed by AFM imaging are simply artifacts, TEM images were obtained for CdS NPs templated by G5, and the images are shown in Figure 2. In order to establish quantitatively the effect of oligoGn length over nanoparticle size, the AFM images were analyzed statistically. Histograms representing nanoparticle’s height distribution for each oligoGn were obtained and a Gaussian curve was fitted (Figure 3). The observed trend indicates that nanoparticles obtained for shorter oligoGn are larger than those obtained for longer oligoGn. Furthermore, the Gaussian curve width narrows as the average height decreases, indicating that longer oligoGn are able to template smaller and less dispersed nanoparticle populations. Figure S2 (Supporting information) summarizes the observed trend and compares the Gaussian fits. Both graphs provide a significant example of nanoparticle size and distribution modulation through template length. (20) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 82, 552-559. (21) Rossetti, R.; Ellison, J. L.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1984, 80, 4464-4469. (22) Qi, L. M.; Colfen, H.; Antonietti, M. Nano Lett. 2001, 1, 61-65. (23) Ebenstein, Y.; Nahum, E.; Banin, U. Nano Lett. 2002, 2, 945-950.
Letters
Figure 2. TEM images for a CdS sample, templated by G5. Inset: magnification of a 3 nm CdS nanoparticle.
Figure 3. Count frequency vs height distribution histograms for CdS nanoparticles obtained by varying the length of oligoGn template after 24 h of aging. Shorter oligoGn stabilize larger particles and more dispersed CdS populations, while longer oligoGn produce smaller particles and less dispersed CdS populations.
In conclusion, we have shown that oligonucleotide length can be effectively exploited as a fine-tuning variable in controlling the size of CdS nanoparticles. This concept could be extended to other homogeneous oligo sequences and even to mixed oligo sequences consisting of alternating complexing and noncomplexing nucleotides. Furthermore, controlling the stoichiometric ratio between metal ion precursor and nucleotide could be a parameter worth investigating, as this too could lead to a dimensional tuning over the synthesis of nanoparticles. This latter hypothesis is particularly interesting, as the stoichiometric introduction of different metal ion precursors could lead to mixedmetal nanoparticles with tunable optical properties defined by the ratio between the two metals. Acknowledgment. The authors wish to thank Mauro Zapparoli of the Centro Interdipardimentale Grandi Strumenti (CIGS) of the University of modena and Reggio Emilia for expert assistance in obtaining TEM images. Supporting Information Available: Materials and methods; UV-vis spectra of oligoGn templated CdS nanoparticles, comparison of Gaussian curves and oligo length vs NPs size graph, TEM microanalysis, table illustrating the results of the statistical analysis. This material is available free of charge via the Internet at http://pubs.acs.org. LA701867F
