Selective Quantification of Carnitine Enantiomers Using Chiral Cysteine-Capped CdSe(ZnS) Quantum Dots Carolina Carrillo-Carrio ´ n, Soledad Ca ´ rdenas, Bartolome ´ M. Simonet,* and Miguel Valca ´ rcel Department of Analytical Chemistry, University of Co ´ rdoba, E-14071 Co ´ rdoba, Spain We report the first observation of selective and specific recognition of chiral L-cysteine (L-Cys)- or D-cysteine (D-Cys)-capped CdSe(ZnS) quantum dots (QDs) with carnitine enantiomers in aqueous solution. The intensity fluorescence of L-Cys-capped QDs decay in the presence of D-carnitine but are not affected by L-carnitine. On the other hand, the fluorescence of D-Cys-capped QDs was only affected by L-carnitine. The applicability of chiral Cys- capped QDs for the analysis of chiral mixtures on enan- tiomers has been demonstrated for 1:100 mixtures, and the results that were obtained had high precision (<2.3%) and low error (<2.7%). Recently, semiconductor quantum dots (QDs) have emerged as fluorophores, because of their high quantum efficiencies, narrow emission peaks, size-dependent wavelength tunability, and excellent chemical stability. 1-5 These properties have created many nanotechnological applications, including biological and chemical sensing. 4-9 However, the development of chiral QDs for chiral analysis has been scarcely applied, despite its importance in biology and toxicology. To facilitate applications in aqueous systems, various ligand exchange methods have been developed recently to form stable water-soluble QDs. 10-12 Most of those are based on the bond of ligands to the surface of QDs through thiol groups. 12-14 One example is cysteine-capped QDs, the fluorescence of which has been demonstrated to be affected by the presence of metal cations and proteins. 15,16 In this work chiral, cysteine-capped CdSe(ZnS) QDs has been synthesized, using L- or D-cysteine. Cysteine-capped QDs have been previously synthesized di- rectly from aqueous solution. 17 Recently, cysteine-capped Cd- Se(ZnCdS) QDs have been synthesized by transferring the QDs from an organic phase to an aqueous phase of cysteine in phosphate buffered saline solution. 18 This procedure resulted in stable QDs for only 24 h due to the oxidation of cysteine to cystine. Through the introduction of sodium borohydride, the stability was increased to a week. 18 Chiral QDs have been scarcely synthesized and studied in the literature. One example is D- or L-penicillamine-capped QDs. 19 However, to date, the selective interaction of chiral QDs with enantiomers has not been described. Therefore, we also per- formed experiments to test the selectivity of the interaction of carnitine enantiomers with the chiral QDs. EXPERIMENTAL SECTION Materials. All chemical reagents were of analytical grade and used as purchased, with no additional purification. Cadmium oxide (99.99%), trioctylphosphine oxide (TOPO, 99%), trioctylphosphine (TOP, 90%), selenium (powder, 100 mesh, 99.99%), diethylzinc solution (ZnEt 2 , ∼1 M in hexane), bis(trimethylsilyl) sulfide ((TMS) 2 S), L-cysteine (L-Cys, 99.5%), D-cysteine (D-Cys, 99%), anhydrous methanol, and anhydrous chloroform were pur- chased from Sigma-Aldrich (Madrid, Spain). Hexylphosphonic acid (HPA) was obtained from Alfa Aesar (Karlsruhe, Ger- many). Equipment. Absorption and fluorescence emission spectra were measured on a PTI QuantaMaster spectrofluorometer (Photon Technology International) that was equipped with a 75-W xenon short arc lamp and a Model 814 P detection system. FeliX32 * To whom correspondence should be addressed. Tel./Fax: +34 957 218616. E-mail: qa2sisub@uco.es. (1) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. J. Am. Chem. Soc. 1995, 115, 8706–8715. (2) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, D.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307, 538–544. (3) Wang, Q.; Liu, Y.; Ke, Y.; Yan, H. Angew. Chem., Int. Ed. 2008, 47, 316– 319. (4) Michalet, X.; Pinaud, F. F.; Bentolita, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307, 538–544. (5) Alivisatos, P. Nat. Biotechnol. 2004, 22, 47–52. (6) Tacar, R.; Chem, Y.; Snee, P. T. Nano Lett. 2007, 7, 3429–3432. (7) Yao, H.; Zhang, Y.; Xiao, F.; Xia, Z.; Rao, J. Angew. Chem., Int. Ed. 2007, 46, 4346–4349. (8) Medintz, I. L.; Clapp, A. R.; Brunel, F. M.; Tiefenbrunn, T.; Uyeda, H. T.; Chang, E. L.; Deschamps, J. R.; Dawson, P. E.; Mattoussi, H. Nat. Mater. 2006, 5, 581–588. (9) Shi, L.; De Paoli, V.; Rosenzweig, N.; Rosenzweig, Z. J. Am. Chem. Soc. 2006, 128, 10378–10379. (10) Guo, W.; Li, J. J.; Wang, Y. A.; Peng, X. J. J. Am. Chem. Soc. 2003, 125, 3901–3907. (11) Liedl, T.; Sobey, T. L.; Simmel, F. C. Nano Today 2007, 2, 36–41. (12) Liu, W.; Choi, H. S.; Zimmer, J. P.; Tanaka, E.; Bawendi, M. J. Am. Chem. Soc. 2007, 129, 14530–14531. (13) Jiang, W.; Mardyani, S.; Fischer, H.; Chan, W. C. W. Chem. Mater. 2006, 18, 872–878. (14) Bolt, K.; Bruns, O. T.; Gaponil, N.; Eychumuller, A. J. Phys. Chem. B 2006, 110, 1959–1963. (15) Chen, Y.; Rosenzweig, Z. Anal. Chem. 2002, 74, 5132–5138. (16) Shi, L.; Rosenzweig, N.; Rosenzweig, Z. Anal. Chem. 2007, 79, 208–214. (17) Chen, J. L.; Zhu, C. Q. Anal. Chim. Acta 2005, 546, 147–153. (18) Liu, W.; Choi, H. S.; Zimmer, J. P.; Tanaka, E.; Frangioni, J. V.; Bawendi, M. J. Am. Chem. Soc. 2007, 129, 14530–14531. (19) Moloney, M. P.; Gun’ko, Y. K.; Kelly, J. M. Chem. Commun. 2007, 38, 3900–3902. Anal. Chem. 2009, 81, 4730–4733 10.1021/ac900034h CCC: $40.75  2009 American Chemical Society 4730 Analytical Chemistry, Vol. 81, No. 12, June 15, 2009 Published on Web 05/22/2009