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Layer-by-layer growth of polymer/quantum dot composite multilayers by nucleophilic substitution in organic media.

Layer-by-layer (LbL) self-assembly is a versatile and simple methodology for growing polymer and polymer/inorganic nanoparticle hybrid multilayer thin films with controlled chemical composition and thickness on the nanometer scale. Traditional LbL assembly is carried out in aqueous media and is based on the electrostatic attraction between two oppositely charged materials, such as polycations and polyanions. The recent progress in utilizing hydrogen bonding, click chemistry, disulfide bonding, silanization, esterification, urethane linking, amidation, and so forth, for LbL self-assembly has allowed the growth of multilayer thin films in polar solvents, mainly water and/or alcohols. To our knowledge, LbL self-assembly for functional organic/inorganic nanocomposites has not yet been implemented in nonpolar solvents. Herein we report the first success in using a nucleophilic substitution reaction for LbL self-assembly of organic/inorganic multilayers in nonpolar solvents. Based on a nucleophilic substitution reaction between Br and NH2, alternating layers of highly hydrophobic CdSe@ZnS quantum dots (QDs) capped with 2-bromo-2-methylpropionic acid (BMPA) in toluene or hexane and poly(amidoamine) dendrimer (PAMA) in ethanol were deposited to form QD/ PAMA composite multilayer thin films. The resulting thin films exhibited more robust photoluminescence (PL) in air (oxidation) and in the presence of moisture (hydrolysis) than those obtained by electrostatic LbL self-assembly. These results also demonstrate the possibility of LbL growth of patterned films based on nucleophilic substitution with the aid of microcontact printing. Photoluminescent (and electroluminescent) polymer/QD nanocomposite films are quite important in technical applications and may be used as functional components in electronic devices, such as optical thin films, or for biomedical imaging. Nevertheless, there has been limited success in fabricating polymer/QD composite thin films using the LbL self-assembly techniques developed to date, because the PL properties of the embedded QDs are usually poor. Conventional LbL self-assembly techniques are carried out in aqueous or polar media, which means that the QDs, which are produced either directly in aqueous or polar media or obtained through ligand exchange or phase transfer, have poor surface passivation, which makes the PL of the resulting QDs vulnerable either during LbL self-assembly or during the thin film storage. Recent studies have shown that a high packing density of small and hydrophilic thiol ligands reduces the quantum yield of QDs significantly. Kotov et al. reported that the PL intensity of composite multilayer thin films of polyelectrolyte and citrate-stabilized CdSe@CdS QDs was increased by 50–500 times after ambient light irradiation for several days owing to surface oxidization on the QDs with ambient oxygen for 3 days, which was accompanied by a notable blue shift in the PL bands with exposure time. To date, the growth of polymer/QD multilayer thin films that preserve the original PL behavior of the QDs is a significant challenge for LbL self-assembly. To circumvent this challenge, we prepared LbL-assembled highly hydrophobic BMPA-stabilized CdSe@ZnS QDs in nonpolar solvents based on nucleophilic substitution of the terminal Br groups of a BMPA coating with the NH2 groups of PAMA. CdSe@ZnS QDs consisting of 4 nm CdSe cores and 1 nm ZnS shells stabilized by oleic acid were prepared in hexane or toluene according to a reported method. The original oleic acid stabilizer ligands were replaced with BMPA through ligand exchange, leading to BMPA-stabilized CdSe@ZnS QDs, denoted BMPA-QDs (see the Supporting Information, Figure S1). Ligand exchange reduced the quantum yield of the QDs (relative to coumarin 545) from 59 % to 30%. For comparison, the oleic acid stabilizers of the QDs were also replaced with mercapto acetic acid (MAA) through phase transfer (from toluene to aqueous media) to form negatively charged MAA-coated QDs, denoted MAA-QDs (see experimental details in the Supporting Information). In this case, the relative quantum yield of the resulting MAA-QDs at pH 9 was approximately 9 %, which shows that using hydrophilic thiol ligands to replace the original hydrophobic ligands caused a dramatic decrease in the PL quantum yield. [*] B. Lee, Y. Kim, S. Lee, Prof. J. Cho School of Advanced Materials Engineering Kookmin University, Seoul 136-702 (Korea) E-mail: jinhan@kookmin.ac.kr