Photophysics of quantum dots and relationship to labeling of living cells

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1 Photophysics of quantum dots and relationship to labeling of living cells Hicham Chibli, Department of Biomedical Engineering &

2 Colloidal nanocrystals of different materials

3 And different geometries From: Science January 28; 307(5709): 538 ミ 544.

4 Medieval Nanotechnology! The colors in some stainedglass windows from medieval cathedrals are probably due to nanocrystals of compouds of Zn, Cd, S, and Se.

5 History of nanoparticles 1980 Ekimov observed quantum confinement on a sample of glass containing PbS Brus L. s group conducted CdS colloid preparation and investigation of band-edge luminescence properties Murray C., Norris D., Bawendi M., Synthesis and Characterization of Nearly MonodisperseCdE (E=S, Se, Te) Semiconductor Nano- crystallites Hines M., Guyot-Sionnest P., reported synthesis and Characterization of Strongly Luminescent ZnS-Capped CdSe Nanocrystals 1998 Alivisatos and Nie independently reported Bio-application of core shell dots Nie s group described Quantum dot-tagged microbeads for multiplexed optical coding of biomolecules.

6 Quantum Dots How to make QDs Quantum mechanics Optical properties What is it good for? Interesting physics Applications in optoelectronics Applications in biology

7 Synthesis

8 A semiconductor has a forbidden zone or band gap between the conduction and valence band When an electron is excited into the conduction band, there is a hole left in the valence band; this pair is an exciton pair When the size of the crystal is comparable to the exciton Bohr radius, the confinement energy becomes signficant at this point we have a quantum dot Quick review of semiconductors

9 Quantum mechanics of QDs Energy 0 e- h+ E e = h2 2 nl 2m e * E h = h2 2 nl 2m* h E CB = m h * E VB m* e + E gap ( ) = 3.2 (wurzite) Bulk CdSe Q dot Because of these quantized energy levels, QDs are more like atoms than like bulk materials--earning them the name artificial atoms

10 This is an oversimplification Box wells are not infinite Particles aren t spherical Boundary conditions must be considered We assume only a single electron However--the approximation is surprisingly good!

11 A Size-dependent spectra Temporal evolution of CdSe nanocrystals nm (5 s) 2.6 nm (20 s) 3.0 nm ( 1 min) 3.3 nm (1.5 min) 3.6 nm (2 min) 4.2 nm (30min,rt) WL/nm

12 Intensity 250 Emission CdSe nanocrystals nm 3.0 nm 3.2 nm 3.6 nm Wavelength (nm)

13 Characterization AFM image of a cluster of CdSe nanocrystals (3.3 nm). Image size 70nm x70 nm

14 High resolution TEM

15 Normalized intensities So what is it good for? 3 to 10 nm CdSe, CdS, ZnS,CdTe, etc Emission wavelength is related to the size of the crystal Slow to photobleach and radiation resistant 1 Emission can be quenched/modulated by attaching electron donors or acceptors to the surface Absorption Emission Can be suspended in aqueous and nonaqueous environments Many colors obtained with a single UV excitation source (nm) Surface can be conjugated to chemically and biologically important molecules

16 Interesting physics! Trap states Stokes shift Stark Effect Blinking

17 The importance of surface states More than half the atoms are at the surface

18 How to probe surface states Transient absorption spectroscopy Electron and hole acceptors quench PL ==> PL results from exciton recombination Burda et al, J. Phys. Chem. B, 105 (49), , 2001

19 What causes the Stokes shift? Exciton fine structure Independent of surface Norris and Bawendi, JOURNAL OF CHEMICAL PHYSICS 103 (13): OCT

20 Blinking

21 Two Models Fluctuating distribution of electron traps in the immediate vicinity of, but external to, the QD. Tunneling of the electron out of the QD results in a charged particle, quenching emission (Kuno et al. 2003, Phys. Rev. B 67, ). Internal hole traps, presumably at surface states or crystal imperfection sites. Energetic diffusion of the electronic states results in a time-dependent resonance condition in which Auger-assisted trapping of the hole results in an off state (Frantsuzov and Marcus 2005, Phys. Rev. B 72, )

22 Stark Effect Shift in energy with electric field Permanent dipole moment: dependence as E Polarizability: as E 2 QDs show both aspects, but E dependence is only seen in singledot studies (not ensembles) Empedocles and Bawendi, Science 19 December 1997: Vol no. 5346, p 2114

23

24 Uses of Stark Effect

25 Interesting applications! Biological labels Photodynamic therapy Single-particle tracking Biosensors Solar cells

26 Biological labeling: neurons and glia Pathak, S. et al. J. Neurosci. 2006;26: Copyright 2006 Society for Neuroscience

27 What is Photodynamic Therapy? Photodynamic therapy (PDT) is a treatment in which photosensitizers destroy malignant cells upon irradiation. Image from Centre for Photochemical Sciences

28 Single-particle tracking From: Science January 28; 307(5709): 538 ミ 544.

29 QDs as biosensors Doxorubicin (adriamycin) Dopamine

30 QD-dopamine as a redox sensor Energy CB h O, R h O R VB Dopamine is an excellent electron donor

31 Normal conditions

32 Reducing conditions

33 Uptake into cells

34 With antioxidants

35 Redox dependence

36

37

38 Summary QDs allow us to observe atomic physics at the almost macroscopic scale However, there are always complications due to surface states, solvent interactions, etc that make them more than a particle in a box A lot has been done, but a lot more remains to be done before we understand these particles and can use them in complex media

39 Les incontournables 1. Aldana, J., Wang, Y.A. & Peng, X.G. Photochemical instability of CdSe nanocrystals coated by hydrophilic thiols. Journal of the American Chemical Society 123, (2001). 2. Burda, C., Green, T.C., Link, S. & El-Sayed, M.A. Electron shuttling across the interface of CdSe nanoparticles monitored by femtosecond laser spectroscopy. Journal of Physical Chemistry B 103, (1999). 3. Chan, W.C. & Nie, S. Quantum dot bioconjugates for ultrasensitive nonisotopic detection. Science 281, (1998). 4. Cho, S.J. et al. Long-term exposure to CdTe quantum dots causes functional impairments in live cells. Langmuir 23, (2007). 5. Derfus, A.M., Chan, W.C.W. & Bhatia, S.N. Probing the cytotoxicity of semiconductor quantum dots. Nano Letters 4, (2004). 6. Empedocles, S.A. & Bawendi, M.G. Quantum-confined stark effect in single CdSe nanocrystallite quantum dots. Science 278, (1997). 7. Empedocles, S.A., Norris, D.J. & Bawendi, M.G. Photoluminescence Spectroscopy of Single CdSe Nanocrystallite Quantum Dots. Physical Review Letters 77, (1996). 8. Hagfeldt, A. & Gratzel, M. Light-Induced Redox Reactions in Nanocrystalline Systems. Chemical Reviews 95, (1995). 9. Haram, S.K., Quinn, B.M. & Bard, A.J. Electrochemistry of CdS nanoparticles: A correlation between optical and electrochemical band gaps. Journal of the American Chemical Society 123, (2001). 10. Bruchez, M., Jr., Moronne, M., Gin, P., Weiss, S. & Alivisatos, A.P. Semiconductor nanocrystals as fluorescent biological labels. Science 281, (1998). 11. Klimov, V.I. et al. Optical gain and stimulated emission in nanocrystal quantum dots. Science 290, (2000). 12. Murray, C.B., Norris, D.J. & Bawendi, M.G. Synthesis and Characterization of Nearly Monodisperse Cde (E = S, Se, Te) Semiconductor Nanocrystallites. Journal of the American Chemical Society 115, (1993). 13. Dabbousi, B.O. et al. (CdSe)ZnS core-shell quantum dots: Synthesis and characterization of a size series of highly luminescent nanocrystallites. Journal of Physical Chemistry B 101, (1997). 14. Leatherdale, C.A. & Bawendi, M.G. Observation of solvatochromism in CdSe colloidal quantum dots. Physical Review B 6316, art. no (2001). 15. Nirmal, M. et al. Observation of the Dark Exciton in Cdse Quantum Dots. Physical Review Letters 75, (1995). 16. Shimizu, K.T. et al. Blinking statistics in single semiconductor nanocrystal quantum dots. Physical Review B 63, (2001). 17. Kuno, M., Fromm, D.P., Hammann, H.F., Gallagher, A. & Nesbitt, D.J. Nonexponential "blinking" kinetics of single CdSe quantum dots: A universal power law behavior. Journal of Chemical Physics 112, (2000).

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