J. Phys. Chem. C 2010, 114, 7007–7013
7007
Structural Implications on the Electrochemical and Spectroscopic Signature of CdSe-ZnS Core-Shell Quantum Dots Stefania Impellizzeri,† Simone Monaco,‡ Ibrahim Yildiz,† Matteo Amelia,‡ Alberto Credi,*,‡ and Franc¸isco M. Raymo*,† Department of Chemistry, UniVersity of Miami, 1301 Memorial DriVe, Coral Gables, Florida, 33146-0431, and Dipartimento di Chimica “G. Ciamician”, UniVersita` di Bologna, Via Selmi 2, 40126 Bologna, Italy ReceiVed: March 8, 2010
We investigated the influence of the core diameter, shell thickness, and ligand length on the spectroscopic and electrochemical signature of CdSe-ZnS core-shell quantum dots and on the ability of these nanoparticles to exchange electrons with complementary acceptors or donors upon excitation. Our studies demonstrate that the core diameter controls the absorption and emission wavelengths of the quantum dots as well as the potentials for their oxidation and reduction. Both wavelengths increase monotonically and both redox potentials shift in the negative direction with an increase in diameter. The presence of a ZnS shell enhances significantly the luminescence quantum yield and shifts both reduction potentials in the positive direction. Interestingly, the shell thickness has negligible influence of the position of the absorption and emission wavelengths, but controls the electrochemical band gap energy. Specifically, an increase in thickness translates into a decrease in the electrochemical band gap energy, but does not affect the optical band gap energy. Similarly, the length of the oligomethylene chains of the alkanethiols adsorbed on the nanoparticles surface has negligible influence on the spectroscopic signature, but regulates the electrochemical response. Indeed, the elongation of the organic ligands increases the electrochemical band gap energy. The optical band gap energy and redox potentials of the quantum dots suggest that the transfer of an electron to methyl viologen or from ferrocene upon excitation is exoergonic. However, only methyl viologen quenches the luminescence of the nanoparticles. Specifically, this electron acceptor adsorbs on the surface of the quantum dots in the ground state and quenches statically their excited state. Nonetheless, an increase in shell thickness and the elongation of the organic ligands have a depressive effect on the stability of the complex and quenching rate constants. In summary, our experimental observations provide valuable insights on the structural factors dictating the spectroscopic and electrochemical behavior of CdSe-ZnS core-shell quantum dots and can facilitate the rational design of luminescent chemosensors based on these nanoparticles and photoinduced electron transfer. Introduction Semiconductor quantum dots are inorganic nanoparticles with unique photophysical properties.1-6 In particular, the huge absorption cross sections, tunable emission bands, long luminescence lifetimes, and outstanding photobleaching resistances associated with these nanostructured constructs offer the opportunity to develop luminescent probes with unprecedented performance. Indeed, quantum dots are gradually replacing conventional organic dyes in a diversity of biomedical applications.7-12 The subtle stereoelectronic factors regulating the photophysical properties of organic dyes, however, have been the subject of intense investigations for decades. These studies have eventually led to valuable strategies to transduce recognition events into significant changes in fluorescence intensity and, as a result, convenient chemosensors for the detection of a diversity of analytes.13-18 Specifically, photoinduced electron transfer has emerged as a versatile mechanism to signal supramolecular association with a luminescent enhancement.19,20 In these systems, a receptor unit is generally designed to accept or donate an electron from or to a fluorescent unit, quenching its emission. Nonetheless, the binding of a * To whom correspondence should be addressed. E-mail: alberto.credi@ unibo.it and
[email protected]. † University of Miami. ‡ Universita` di Bologna.
complementary substrate is engineered to alter the redox potentials of the receptor in order to suppress the electron transfer process and, hence, turn the emission of the fluorescent unit on. Under these conditions, the presence of a target analyte is transduced into a luminescence signal. In principle, the very same operating mechanisms can be extended from organic dyes to quantum dots and, in fact, representative examples of chemosensors based on these nanoparticles and photoinduced electron transfer have already been realized.21-23 The rational design of chemosensors based on photoinduced electron transfer requires prior knowledge of the redox potentials and spectroscopic signature of the receptor and fluorescent components.19,20 The fine-tuning of these parameters, together with the physical separation of the functional components, then offers the opportunity to optimize the response of the chemosensing assembly. In this context, the vast amount of information available on the structural and electronic properties of organic compounds is invaluable in the assembly of these functional constructs. The same level of understanding on the interplay between the structural and electronic properties of semiconductor quantum dots, however, has not yet been achieved. In particular, the influence of the diameter of their luminescent core, thickness of their protective shell, and nature of their passivating ligands on their redox potentials and electron transfer kinetics is still rather unclear,24 and systematic investigations on the structural
10.1021/jp1021032 2010 American Chemical Society Published on Web 03/29/2010
7008
J. Phys. Chem. C, Vol. 114, No. 15, 2010
Impellizzeri et al.
Figure 1. Absorption and emission spectra (1.5 µM, THF, 20 °C, λEx ) 420 nm) of n-decanethiol-coated CdSe quantum dots with core diameter of 2.1 (a and d), 2.3 (b and e), and 2.5 nm (c and f).
TABLE 1: Electrochemical and Spectroscopic Parameters of n-Decanethiol-Coated CdSe Quantum Dots with Different Core Diameters in THF at 20 °C dCo (nm)a λAb (nm)b ∆EOp (eV)c λEm (nm)d φe EOx (V)f ERed (V)g ∆EEl (eV)h
2.1 467 2.66 493
