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. 2017 May 1;8(5):3879-3884.
doi: 10.1039/c7sc00592j. Epub 2017 Mar 13.

Electrostatically driven resonance energy transfer in "cationic" biocompatible indium phosphide quantum dots

Gayathri Devatha  1 , Soumendu Roy  1 , Anish Rao  1 , Abhik Mallick  1 , Sudipta Basu  1 , Pramod P Pillai  1
Affiliations

Electrostatically driven resonance energy transfer in "cationic" biocompatible indium phosphide quantum dots

Gayathri Devatha et al. Chem Sci. .

Abstract

Indium Phosphide Quantum Dots (InP QDs) have emerged as an alternative to toxic metal ion based QDs in nanobiotechnology. The ability to generate cationic surface charge, without compromising stability and biocompatibility, is essential in realizing the full potential of InP QDs in biological applications. We have addressed this challenge by developing a place exchange protocol for the preparation of cationic InP/ZnS QDs. The quaternary ammonium group provides the much required permanent positive charge and stability to InP/ZnS QDs in biofluids. The two important properties of QDs, namely bioimaging and light induced resonance energy transfer, are successfully demonstrated in cationic InP/ZnS QDs. The low cytotoxicity and stable photoluminescence of cationic InP/ZnS QDs inside cells make them ideal candidates as optical probes for cellular imaging. An efficient resonance energy transfer (E ∼ 60%) is observed, under physiological conditions, between the cationic InP/ZnS QD donor and anionic dye acceptor. A large bimolecular quenching constant along with a linear Stern-Volmer plot confirms the formation of a strong ground state complex between the cationic InP/ZnS QDs and the anionic dye. Control experiments prove the role of electrostatic attraction in driving the light induced interactions, which can rightfully form the basis for future nano-bio studies between cationic InP/ZnS QDs and anionic biomolecules.

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Figures

Scheme 1
Scheme 1. Schematics for the synthesis of [+] InP/ZnS QDs. The place exchange reaction between the myristic acid capped InP/ZnS QDs and the [+] TMA ligand is represented. The photographs of the vials show the successful transfer of [+] InP/ZnS QDs into the aqueous layer.
Fig. 1
Fig. 1. Spectroscopic and microscopic characterization of [+] InP/ZnS QDs. (a) The normalized absorption and photoluminescence spectra of InP/ZnS QDs before and after the place exchange reaction. The unnormalized photoluminescence spectra in the inset show that the [+] InP/ZnS QDs retained ∼80% of their photoluminescence after the place exchange reaction. (b) The photoluminescence decay profiles of InP/ZnS QDs before and after the place exchange reaction. (c) A representative HRTEM image of 2.8 ± 0.8 nm sized [+] InP/ZnS QDs. The inset shows the lattice fringes with an interplanar distance of 0.287 nm, corresponding to the zincblende phase of bulk InP. (d) A typical zeta potential plot (measured at pH ∼ 7) confirming the cationic charge on the surface of the InP/ZnS QDs.
Fig. 2
Fig. 2. Biocompatibility of [+] InP/ZnS QDs. (a) The viability of MCF-7 cells incubated with different concentrations of [+] InP/ZnS QDs and [+] CdSe/ZnS QDs for 24 h. (b) A representative confocal image showing the fluorescence of [+] InP/ZnS QDs inside MCF-7 cells.
Fig. 3
Fig. 3. Steady state resonance energy transfer studies. (a) The spectral overlap (shaded portion) between the emission of [+] InP/ZnS QDs and the absorption of [–] MC dye. (b) A bathochromic shift of ∼30 nm was observed in the absorption of MC dye upon complexation with [+] InP/ZnS QDs. (c) Spectral changes in the emission of the [+] InP/ZnS QDs on the addition of varying concentrations of [–] MC dye. The inset is the Stern–Volmer plot showing the relative changes in the emission intensity of [+] InP/ZnS QDs as a function of [–] MC dye concentration. (d) A plot showing the saturation of the relative QD emission decay and FRET efficiency vs. the concentration of the MC dye.
Fig. 4
Fig. 4. Time resolved energy transfer studies. (a) The photoluminescence decay profiles of [+] InP/ZnS QDs in the absence and presence of 2 μM [–] MC dye. (b) The photoluminescence decay profiles of the [+] InP/ZnS:::[–] MC complex collected at the emission maxima of the donor (525 nm) and acceptor (585 nm), on a 50 ns time scale. The inset shows the TRES of the [+] InP/ZnS:::[–] MC complex recorded immediately and after a time delay of 450 ps.
Fig. 5
Fig. 5. Proof of electrostatically driven resonance energy transfer. The changes in (a) the steady state and (b) the time-resolved photoluminescence of [+] InP/ZnS QDs on the addition of [–] MC dye in PBS. The inset of (a) is the Stern–Volmer plot of the [+] InP/ZnS:::[–] MC complex in PBS. (c) The steady state and (d) the time-resolved photoluminescence of [+] InP/ZnS QDs on addition of [+] CY dye in water. The inset of (c) is the Stern–Volmer plot showing negligible changes in the emission intensity of [+] InP/ZnS QDs as a function of [+] CY dye concentration.
Scheme 2
Scheme 2. Electrostatically driven resonance energy transfer in [+] InP/ZnS QDs. (a) Efficient resonance energy transfer was observed between [+] InP/ZnS QDs and the [–] MC dye in water. (b) The presence of high concentration of salt in PBS screened the charges on the QDs and the dye, thereby decreasing the FRET efficiency. (c) No appreciable energy transfer was observed when the charges on the donor and the acceptor were the same, confirming the role of electrostatics in FRET. The steady state emission plots and optical images of the samples corresponding to each of the three conditions are shown on the right. The concentration of [+] InP/ZnS QDs and dyes was maintained at ∼0.8 μM and ∼2 μM respectively, in all of the three conditions.
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