Mechanistic Aspects of Quantum Dot Based Probing of Cu (II) Ions: Role of Dendrimer in Sensor Efficiency

Abstract

Selective quenching of luminescence of quantum dots (QDs) by Cu2+ ions vis-à-vis other physiologically relevant cations has been reexamined. In view of the contradiction regarding the mechanism, we have attempted to show why Cu2+ ions quench QD-luminescence by taking CdS and CdTe QDs with varying surface groups. A detailed study of the solvent effect and also size dependence on the observed luminescence has been carried out. For a 13% decrease in particle diameter (4.3 nm -->3.7 nm), the quenching constant increased by a factor of 20. It is established that instead of surface ligands of QDs, conduction band potential of the core facilitates the photo-induced reduction of Cu (II) to Cu (I) thereby quenching the photoluminescence. Taking the advantage of biocompatibility of dendrimer and its high affinity towards Cu2+ ions, we have followed interaction of Cu2+-PAMAM and also dendrimer with the CdTe QDs. Nanomolar concentration of PAMAM dendrimer was found to quench the luminescence of CdTe QDs. In contrast, Cu2+-PAMAM enhanced the fluorescence of CdTe QDs and the effect has been attributed to the binding of Cu2+-PAMAM complex to the CdTe particle surface. The linear portion of the enhancement plot due to Cu2+-PAMAM can be used for determination of Cu2+ ions with detection limit of 70 nM.

Luminescence of Polyethylene Glycol Coated CdSeTe/ZnS and InP/ZnS Nanoparticles in the Presence of Copper Cations

“…[22] However, it was rapidly assessed that for many transition metal ions, such as copper, the main mechanism is related to transfer of electrons from the photoexcited QD to the free cations in solution or bound at the nanoparticle surface. [21,28,29] This electron transfer could have different impact, depending on the electronic structure of the nanocrystals. Indeed, the energy levels of the valence and conduction bands of QDs depend on their structure (core, core/ shell), materials, size, and surface oxidation state.…”

“…[17,18] Indeed, transition metal cations such as Hg 2 + , Ag + , and Cu 2 + have been reported to quench the luminescence of quantum dots. [19][20][21][22][23][24][25][26][27] This phenomenon, attributed to redox reactions occurring between the metal cations and the nanocrystals, [28,29] has been used for the development of sensitive and relatively selective luminescent cation sensors. A copper detection limit as low as the nanomolar range can be reached.…”

“…[20][21][22]29] Core/shell polyethylene glycol (PEG)-coated nanoparticles are preferentially used for biological applications. The inorganic shell (usually ZnS) prevents loss of quantum yield in biological buffers, whereas the PEG coating minimizes nonspecific protein adsorption on the QD surface.…”

“…Similar to other metal cations, the reduction of Cu 2 + to Cu + is most probably involved in the mechanism of QD photoluminescence quenching. [21,28,29] To isolate the sole influence of Cu ions on the photoluminescence quenching, we incubated the QDs with Cu 2 + without addition of sodium ascorbate. Figure 2 displays the effect of Cu 2 + cations on the luminescence of CdSeTe/ZnS/PEG 2000 -NH 3 + QDs (sample S1, see Table 1).…”

Amphipol-encapsulated CuInS2/ZnS quantum dots with excellent colloidal stability

“…Similar non-linear quenching is observed in systems where the quencher has limited accessibility to the fluorophore, 53 and can be modeled by considering a modified Stern-Volmer expression, (1) 80 where is the fluorescence intensity in the absence of the ; the core QDs displayed an order of magnitude lower sensitivity to Cu ions than the coreshell QDs. Quenching of QD fluorescence in the presence of Cu 2+ ions has 30 been observed for CIS QDs 54 as well as for other QD systems 55,56 and the quenching mechanism is still under debate. Reports suggest that that the Cu 2+ ions can react with the ZnS surface of core-shell QDs creating Cu x S y , which lead to increased density of surface trap states.…”