Minimizing the Hydrodynamic Size of Quantum Dots with Multifunctional Multidentate Polymer Ligands

Abstract

Semiconductor quantum dots (QDs) are light-emitting nanocrystals with unique optical and electronic properties that are not available from organic dyes or fluorescent proteins. 1 In the near term, one of the most promising applications of these particles is for use as fluorescent probes for molecular, cellular, and in vivo imaging. 2 However, a major problem is the large size of conventional QD probes, which adversely affects their molecular binding and in vivo biodistribution. This bulkiness is not an intrinsic problem of QD nanocrystals, but arises mainly from organic surface coatings used for encapsulation and stabilization. In fact, small 4−7 nm QDs have been shown to have hydrodynamic sizes (diameters) of 20−40 nm when coated with amphiphilic polymers. 3 Small hydrophilic and cross-linked ligands such as thioglycerol and dihydrolipoic acid have been used to reduce the coating layer thickness, 4-7 but the resulting dots often suffer from low colloidal stability, photobleaching, or low quantum yield problems. Consequently, these size-reduced QDs have found only limited utility in live cell and in vivo applications. 2Here we report a new class of multifunctional multidentate polymer ligands not only for minimizing the hydrodynamic size of QDs but also for overcoming the colloidal stability and photobleaching/signal brightness problems encountered in previous research. A major finding is that a mixed composition of thiol (-SH) and amine (-NH 2 ) groups grafted to a linear polymer chain can lead to a highly compact QD with long-term colloidal stability, strong resistance to photobleaching, and high fluorescence quantum yield. In contrast to the standing brush-like conformations of PEGylated dihydrolipoic acid ligands and monovalent thiols, we believe that these multidentate polymer ligands can wrap around the QD in a closed "loops-and-trains" conformation. 8 This structure is highly stable from a thermodynamic perspective, and is thus responsible for the excellent colloidal and optical properties observed. As a result, we have prepared a new generation of bright and stable CdTe QDs with small hydrodynamic diameters between 5.6 nm to 9.7 nm, with fluorescence emission tunable from the visible (515 nm) to the near infrared (720 nm). In addition to CdTe nanocrystals, we find that this new class of multidentate polymers is applicable to a broad range of core nanocrystals as well as core/shell nanostructures including CdS, ZnSe, CdSe/ZnS, and CdTe/CdS, more versatile and robust than amphiphilic phytochelatin peptides. 9As depicted in Scheme 1, the multidentate polymer was synthesized by covalently modifying about 35% of the carboxylic acids of polyacrylic acid (PAA, MW ∼1800) with cysteamine and N-Fmocethylenediamine using diisopropylcarbodiimide (DIC) and N-hydroxysuccinimide (NHS). After deprotection of the amine with piperidine and purification, each polymer molecule contained approximately 3.5 active thiols and 3.0 active amines, as determined via E-mail: E-mail: snie@emory.edu. Supporting Information Available: ...

Toxico-/biokinetics of nanomaterials

“…The particle size distribution can be examined in samples suspended (e.g., in 0.05 % ethanol) and dispersed using a mixer, vortex, and/or ultrasonic treatment (e.g., 35 kHz for 10 min) by dynamic light scattering measurements with an ultrafine particle analyzer measuring the light that scatters back from the sample, calculating the Doppler shift to determine particle size (range 0.003-6.5 lm) using as a light source (e.g., 780-nm) a semiconductor laser directed at the sample through a fiber optic cable and a measurement angle of 180° (Ma-Hock et al 2007b). References on method and/or caveats Transmission electron microscopy (TEM) Landsiedel et al (2010a) Dynamic light scattering Ma-Hock et al (2007a, b) Gel electrophoresis Bücking and Nann (2006), Hanauer et al (2007), Park and Hamad-Schifferli (2008) Size exclusion chromatography in organic solvents Al-Somali et al (2004), Krueger et al (2005), Wang et al (2004) Size exclusion chromatography in aqueous phase Pinaud et al (2004), Carion et al (2007), Smith and Nie (2008) Analytical ultracentrifugation Calabretta et al (2005), Jamison et al (2008), Lees et al (2008), Schulze et al (2008), Landsiedel et al (2010a, b) Magnetic sedimentation Berret et al (2007) The particle size distribution and agglomeration state can be determined by analytical ultracentrifugation, an adequate method for the determination of particle size distribution Landsiedel et al 2010b). Up to 300,000 g, solutes and NP sediment into fractions that are separated according to their size in the range 0.5-10,000 nm.…”

Quantum Dots and Their Ligand Passivation

“…Thus, an effective passivation is a critical step to avoid any detrimental effect on the host biosystem and to stabilize the chemical and structural integrity of the QD [38,39]. A spherical structure of (CdSe) 13 and a large surface-to-volume ratio facilitate a high density coating with various organic ligands [24][25][26][27]. Among many available ligands coupled to CdSe QDs [40][41][42][43][44][45], trimethyl phosphine oxide (TMPO) and its derivatives (i.e., OPMe 2 (CH 2 ) n Me, n = 0, 1-3) stand out due to their high ligand exchange capacity on the CdSe surface [46].…”

Fluorescence Tracking of Motor Proteins In Vitro

“…Unfortunately, commercially available QDs are large (~15À20 nm diameter), which can hinder the dynamics of many motor proteins, especially when two QDs are used. Several groups have developed smaller QDs for in vivo labeling [33,34], but reduction in the thickness of the polymer coat around the semiconductor core of a QD reduces their solubility and photostability over extended periods of time. In addition, QDs "blink" on and off frequently.…”