Articulo

Anal. Chem. 2005, 77, 3511-3515

Characterization of Nanocrystalline CdSe by Size Exclusion Chromatography
Karl M. Krueger, Ali M. Al-Somali, Joshua C. Falkner, and Vicki L. Colvin*

Department of Chemistry, Rice University, MS-60 6100 Main Street, Houston, Texas 77005

High-performance size exclusion chromatography (HPSEC) is a powerful tool for probing the size and size distribution ofcomplex materials. Here we report its application to the analysis of cadmium selenide nanocrystals produced in organic solvents. If nanocrystal-column interactions are minimized, this method provides an accurate measure of nanocrystal hydrodynamic diameter directly in solution; such information is complementary to TEM in that it can measure the thickness of various capping agents. While theresolution of single-pass HPSEC is limited to 1 nm, we show here that recycling size exclusion chromatography can be applied to assess the fine details of a sample’s distribution. Finally, semiconductor nanocrystals can be made a variety of shapes whose optical characteristics are difficult to distinguish. HPSEC can be applied to the general problem of shape separations which we demonstrate with atetrapod material.
Nanocrystalline CdSe quantum dots are a widely studied nanomaterial whose size-dependent optical and electronic properties have inspired both fundamental research and novel applications.1-6 Control over the nanocrystal diameter and size distribution is critical in this field, and this requires accurate and rapid determination of particle size and size distribution in solution. Thispaper demonstrates the application of high-performance size exclusion chromatography (HPSEC) to this problem. Unlike transmission electron microscopy (TEM), HPSEC provides a quantitative assessment of the particle diameter directly in the solution phase.7 In particular, it measures the hydrodynamic diameter of a nanoparticle that includes information about both the core and its surface coating.HPSEC is also an important complement to optical absorption in that chromatographic elution times can be directly related to hydrodynamic size. This permits
* To whom correspondence should be addressed. E-mail: colvin@rice.edu. Phone: 713-348-5741. Fax: 713-348-2578. (1) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 24252427. (2) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P.Nature 1994, 370, 354357. (3) Gao, X. H.; Chan, W. C. W.; Nie, S. M. J. Biomed. Opt. 2002, 7, 532-537. (4) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016-2018. (5) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (6) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science2000, 290, 314317. (7) Yu, W. W.; Qu, L. H.; Guo, W. Z.; Peng, X. G. Chem. Mater. 2003, 15, 2854-2860. 10.1021/ac0481912 CCC: $30.25 Published on Web 04/16/2005 © 2005 American Chemical Society

particle sizing without any reliance on models of nanocrystal optical properties. The application of size exclusion chromatography to the problem of nanocrystal analysis has been limited, though it is awidely used technique for measuring the properties of complex macromolecules such as polymers and proteins.8-14 Fischer et al. first evaluated chromatography for separating aqueous colloidal CdS particles over a decade ago.15 Even though these nanomaterials were not stable for long times in solution, under the appropriate conditions, elution time and particle size were found to be relatedqualitatively as expected for a size exclusion process.16-19 This work highlighted the persistent problem of enthalpic particle-column interactions, which could be minimized but not removed by the use of additives in the running phase.20 More recently, the application of HPSEC to gold nanocrystals has been reported.19,21-25 This work extends past efforts on colloidal CdS by illustrating methods for...