Colloidal CdSe Nanocrystals Passivated by a Dye‐Labeled Multidentate Polymer: Quantitative Analysis by Size‐Exclusion Chromatography

Abstract

Quantum dots are a class of colloidal semiconductor nanocrystals (NCs) that show unique size-dependent optical properties.[1] These materials are of great interest for applications in light-emitting diodes, [2] photovoltaic devices, [3] and biological labels.[4] High-quality NCs of II-VI semiconductor materials such as CdSe and CdTe can be obtained with controlled sizes and shapes through an organometallic approach.[5] These as-prepared NCs, which are chemically passivated by a layer of organic ligands such as tri-noctylphosphine oxide (TOPO), are dispersible only in nonpolar organic solvents and are typically unstable in solution in the absence of excess free ligands. [1b, 6] These factors limit their use for biological and material applications. As a consequence, ligand-based functionalization of the NC surface is an essential step to control their physical properties as well as to provide the necessary chemical accessibility and biological functionality.For this purpose, various polymeric substances such as organic dendrons, [7] linear [8][9] or hyperbranched [10] polymers, chemically-modified proteins, [11] and amphiphilic polymers [12] have been employed to manipulate the surface properties of NCs through a ligand-exchange or micellar encapsulation process. Analytical methods such as dynamic light scattering, [9c, 10] gel electrophoresis, [7, 9b, 12c] fluorescence resonance energy transfer, [9e, 11] thermogravimetric analysis, [12b] and atomic force microscopy [13] have been used to characterize the polymer binding on NCs. However, the quantitative analysis of polymeric ligands on NCs still remains a considerable challenge. There are two distinct problems that need to be solved. First, one needs a methodology to separate the NC from free polymer. The polymer is normally added in excess to ensure that the NC surfaces are saturated or encapsulated, and this excess polymer is very difficult to remove from the sample. This problem can in principle be solved by centrifugation or dialysis, [12d] but then one encounters the second problem. One needs analytical methods to determine if any free polymer remains in the solution and to quantify the amount of polymer bound to the NC. This paper presents a quantitative analytical method based upon size-exclusion chromatography (SEC) that addresses this need.To facilitate the detection of polymeric ligands on NCs by spectroscopic methods, we synthesized a low-molecularweight sample of poly(dimethylaminoethyl methacrylate) labeled at one end with pyrene as a fluorescent tracer (Pyr-PDMAEMA, M n = 6700) by atom-transfer radical polymerization (ATRP; for details of the synthesis and characterization, see the Supporting Information). Pyr-PDMAEMA can bind to CdSe NCs as a multidentate ligand, as there is a dimethylamino function on each pendant group (Scheme 1). The CdSe NCs were prepared by an established organometallic synthesis with TOPO as ligands.[5] Two samples were used in this study. The first (CdSe 520 nm ) had a diameter of 3.4 AE 0.3 nm as determined by transm...

Size-exclusion chromatography of metal nanoparticles and quantum dots

“…There, it was demonstrated that SEC of CdS QDs stabilized with alkanethiols, in THF containing 1 mmol L −1 Cd(ClO 4 ) 2 + 1 mmol L −1 dodecanethiol, is feasible. Later studies by Wilcoxon and Provencio [37], Wang et al [39], and Kruger et al [36,38] have shown how SEC reveals various characteristics and size-dependent properties of CdSe and CdSe/ZnS core/shell nanocrystals, as described below.…”

“…[39] used SEC for separation of CdSe nanocrystals from the excess polymer (unbound poly(dimethylaminoethyl methacrylate) labeled with pyrene) used as coating material for nanocrystals. They were also able to quantify the amount of polymer that was bound to the nanocrystals.…”

Highly Luminescent Covalently Linked Silicon Nanocrystal/Polystyrene Hybrid Functional Materials: Synthesis, Properties, and Processability

“…[ 26 ] Fortunately, the ionic bonding of ligands with QD surfaces can undergo equilibrating exchange processes that allow introduction of various surface moieties, including polymers. [27][28][29] Another promising method for interfacing QDs with polymers relies upon exploiting the collective contributions of comparatively weak bonding interactions (e.g., van der Waals interactions) between surface groups on QDs and polymers. [ 30,31 ] This approach has provided tailoring QD solubility.…”

Sensing Polymer/DNA Polyplex Dissociation Using Quantum Dot Fluorophores

“…CdSe QDs have been reported to bind molecules having tertiary amines with high affinity. 24 Though not previously demonstrated, hydrophilic Cyscapped CdSeϪZnS QDs were considered as viable binding surfaces for the pentablock copolymers (having similar blocks of tertiary amines) used in this work. Interestingly, pentablock copolymers induced significant quenching of QD fluorescence upon mixing.…”