the ligand signals. But, while the replacement of the QD capping
ligand with the thiol led to a decrease in the WFof only 8%, the
thiolate produced a drastic decrease (39%) of the QD emission. For further details of these and additional experiments regarding the effect on the emission properties of QD-CS4, QD-CS5,
Fig. 6 XPS spectra of Cd 3d and N 1s for QD-CS2, CS2@KP ,
Fig. 7 Top to bottom: XPS spectra of S 2s for QD-CS2, CS2@KPrt,
CS2@KP, and KP-SH.
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Published on 22 December 2011 on http://pubs.rsc.org | doi:10.1039/C1RA01005K
The photostability of nanoparticles is of great importance in their applications; therefore, the photostability of several of the prepared thiol capped QDs was compared with that of their amine capped precursor. The samples were irradiated (400 nm ,
l , 700 nm, lmax at 420 nm, 70 W/m
2
) in toluene or water, depending on the solubility of the corresponding QD, under nitrogen or air atmosphere for ca. five hours.
Surprisingly, the emission maximum of the QDs experienced only a small blue-shift (up to 6 nm) after 5 h irradiation under air
atmosphere (Table 1).19 Blue-shifts up to 40 nm have been
reported for TOPO capped CdSe/ZnS QDs and attributed to
photooxidation of the CdSe core.20For oxidation to take place,
oxygen has to diffuse through the ZnS shell. Therefore, our data demonstrated that the CdSe core was well passivated by the ZnS shell in the QDs studied in this research work. The diminished
WFof the organic-soluble QDs after their irradiation suggested
photooxidation of mainly the ZnS layer,21 since it was
accompanied by only a slight blue-shift of the emission maximum. In addition, the intensity decrease can be caused by
the formation of lattice defects in the core-shell QDs.21
In addition, the photostability in air of the QDs covered with chemisorbed thiol ligands was higher than that of the amine- capped CdSe/ZnS QDs (see comparison between QD-CS3 (toluene) and CS3@MPA (water) in Fig. 8, and between
QD-CS2 (toluene) and CS2@KP (toluene) in Fig. 9.
Moreover, CS3@MPA exhibited an improved fluorescence (10%) after 5 h irradiation. This could be explained by the removal of recombination centers, created during the ligand
exchange, by illumination.22
The fluorescence of the amine-capped QDs also drastically decreased after irradiation under anaerobic conditions (though less than under air atmosphere conditions). However, irradiation of CS2@KP, CS3@MUA, and CS3@MPA in the presence or absence of oxygen made little difference and only slightly affected the QD fluorescence performance (Fig. 10, Fig. S7 and S8, ESI,{ and Table 1).
Table 1 Physical properties of the CdSe/ZnS QDs capped with thiols and their precursors, before and after irradiation
lmaxexciton lmaxemission WF
before irradiation after irradiation under N2Air before irradiation after irradiation under N2Air before irradiation after irradiation under N2Air QD-CS2 521 520 520 539 537 538 0.61 0.31 0.22 CS2@KP 520 515 515 540 535 533 0.70 0.44 0.47 QD-CS3 564 564 563 582 581 581 0.63 0.53 0.42 CS3@MUA 571 572 570 595 594 592 0.54 0.52 0.39 CS3@MPA 566 566 567 586 588 587 0.45 0.46 0.49
Fig. 8 Normalized fluorescence spectra of aerated solutions of
CS3@MPA (water), before (&) and after (
N
) 270 min irradiation at lFig. 9 Normalized fluorescence spectra of aerated toluene solutions of
CS2@KP, before (&) and after (
N
) 270 min irradiation at l . 400 nm.Inset: comparative fluorescence spectra of QD-CS2.
Fig. 10 Normalized fluorescence spectra of deaerated toluene solutions
of CS2@KP, before (&) and after (
N
) 270 min irradiation at l . 400 nm.Downloaded on 04 September 2012
Published on 22 December 2011 on http://pubs.rsc.org | doi:10.1039/C1RA01005K
4. Conclusions
In summary, we have demonstrated that the replacement of amine ligands by thiols to lead to either organic-soluble or water- soluble QDs can be performed under mild conditions, preserving or enhancing not only the emission properties of the nanopar- ticles but also their photostability. The QDs remain stable over the six-month study period. These results may help researchers to design new functional QDs for future applications in which highly fluorescent, photostable, and small-sized QDs are required.
Acknowledgements
We thank MEC (Project CTQ2008-06777-CO2-01, contract granted to J. A-S, and RyC contract granted to R.E.G), GVA (Project ACOMP/2009/334), and UVEG (Project UV-AE-09- 5805) for their support.
References
1 (a) M. G. S. Hyldahl, T. Bailey and B. P. Wittmerhaus, Sol. Energy, 2009, 83, 566; (b) X. Michalet, F. F. Pinaud, L. A. Bentolila, J. M. Tsay, S. Doose, J. J. Li, G. Sundaresan, A. M. Wu, S. S. Gambhir and S. Weiss, Science, 2005, 307, 538; (c) A. P. Alivisatos, Nat. Biotechnol., 2004, 22, 47; (d) F. Chen and D. Gerion, Nano Lett., 2004, 4, 1827.
2 (a) O. Dabbousi, J. F. Rodriguez-Viejo, V. Mikulec, J. R. Heine, H. Mattoussi, R. Ober, K. F. Jensen and M. G. Bawendi, J. Phys. Chem. B, 1997, 101, 9463; (b) B. M. A. Hines and P. Guyot-Sionnest, J. Phys. Chem., 1996, 100, 468.
3 H. Borchert, D. V. Talapin, C. McGiney, S. Adam, A. Lobo, A. R. B. Castro, T. Mo¨ller and H. Weller, J. Chem. Phys., 2003, 119, 1800. 4 B. K. Pong, B.L. Trout and J. Y. Lee, Langmuir, 2008, 24, 5270. 5 (a) K. Susumu, E. Oh, J. B. Delehanty, J. B. Blanco-Canosa, B. J.
Johnson, V. Jain, W. J. Hervey IV, W. R. Algar, K. Boeneman and P. E. Dawson, J. Am. Chem. Soc., 2011, 133, 9480; (b) I. L. Medintz; N. D. Abazovic, J. Z. Kuljanin-Jakovljevic and M. I. Comor, Russ. J. Phys. Chem. A, 2009, 83, 1511; (c) W. Liu, H. S. Choi, J. P. Zimmer, E. Tanaka, J. V. Frangioni and M. Bawendi, J. Am. Chem. Soc., 2007, 129, 14530; (d) V. V. Breus, C. D. Heyes and G. U. Nienhaus, J. Phys. Chem. C, 2007, 111, 18589; (e) R. Gill, I. Wilner, I. Shweky and U. Banin, J. Phys. Chem. B, 2005, 109, 23715. 6 J. Aldana, Y. A. Wang and X. Peng, J. Am. Chem. Soc., 2001, 123,
8844.
7 KP-SH was synthesized following the Yamamoto procedure: K. Ishihara, M. Nakayama, S. Ohara, H. Yamamoto, Synlett. 2001, 7,
1117. See ESI for further details.{ KP-SH was chosen to prepare
thiol-capped QDs since it exhibits1H-NMR signals well separated
from those of ligands of the commercial or home-made QDs used for these studies. In addition, the benzophenone chromophore of KP-SH does not absorb at 400 nm . l . 700 nm used for studies of the QD photostability . In addition, as the fluorescence studies show, the benzophenone did not significantly quench the QD emission.
8 Z. A. Peng and X. Peng, J. Am. Chem. Soc., 2001, 123, 183. 9 (a) X. Ji, D. Copenhaver, C. Sichmeller and X. Peng, J. Am. Chem.
Soc., 2008, 130, 5726; (b) J. S. Owen, J. Park, P. E. Trudeau and A. P. Alivisatos, J. Am. Chem. Soc., 2008, 130, 12279.
10 Yoon. et al. have reported an enhanced emission after binding of cysteine to CdSe/ZnS QDs, but they found indications that cysteine was actually bound to the QD-surface by both its thiol and its amine group. In addition, the enhancement disappeared at high cysteine
concentration10.
11 C. Park and T. H. Yoon, Colloids Surf., B, 2010, 75, 472.
12 For comparison, a chloroform solution (25 mL) of QD-CS1 and KP- SH ([thiol]/[QD] = 5000 molar ratio) was maintained at room temperature under nitrogen atmosphere for 48 h. After almost total solvent evaporation (2 mL) and the addition of methanol (30 mL),
the samples were centrifugated at 8000 rpm for 20 min at 25uC and
the supernatant was decanted. The nanoparticles (CS1@KPrt) were
dissolved in toluene (1 mL) and the purification process was repeated
three times. The QDs exhibited a lower WFthan the amine capped
precursor. See also,J. Aguilera-Sigalat, S. Rocton, R. E. Galian and J. Pe´rez-Prieto, Can. J. Chem., 2011, 89, 359.
13 C. A. Leatherdale and M. G. Bawendi, Phys. Rev. B: Condens. Matter, 2001, 63, 165315.
14 G. Zundel, J. Mol. Struct., 1982, 84, 205.
15 A. Lobo, T. Mo¨ller, M. Nagel, H. Borchert, S. G. Hickey and H. J. Weller, J. Phys. Chem. B, 2005, 109, 17422.
16 (a) G. K. Olivier, D. Shin, J. B. Gilbert, L. M. A. Monzon and J. Frechette, Langmuir, 2009, 25, 2159; (b) A. Adenier, M. M. Chehimi, I. Gallardo J. Pinson and N. Vila`, Langmuir, 2004, 20, 8243; (c) E. Uchida and Y. Ikada, J. Polymer Sci., 1996, 61, 1365.
17 J. J. Park, SHDP. Lacerda, S. K. Stanley, B. M. Vogel, S. Kim, J. F. Douglas, D. Raghavan and A. Karim, Langmuir, 2009, 25, 443.
18 This peak was also present in the S 2s spectrum of CS2@KPrt. By
contrast, CS2@KP showed the signal of the disulfide..
19 For one example of a slight hypsochromic shift upon photooxidation seeL.I. Gurinovich, M.V. Arteme´v and A.A. Lyutich, J. Appl. Spectrosc., 2006, 73, 572.
20 W. G. J. H. M. Van Sark, P. L. T. M. Frederix, D. J. Van den Heuvel, H. C. Gerritsen, A. A. Bol, J. N. J. Van Lingen, C. de Mello Donega´ and A. Meijerink, J. Phys. Chem. B, 2001, 105, 8281.
21 W. G. J. H. M. Van Sark, P. L. T. M. Frederix, A. A. Bol, H. C. Gerritsen and A. Meijerink, ChemPhysChem, 2002, 3, 871. 22 C. Carrillo-Carrio´n, S. Ca´rdenas, B. M. Simonet and M. Valca´rcel,
Chem. Commun., 2009, 5214.
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Published on 22 December 2011 on http://pubs.rsc.org | doi:10.1039/C1RA01005K
Highly fluorescent and photostable organic- and water-soluble
CdSe/ZnS core-shell quantum dots capped with thiols
Jordi Aguilera-Sigalat, Simon Rocton, Juan F. Sánchez-Royo, Raquel E. Galian
*and
Julia Pérez-Prieto
SupportingInformation
*