Structural Contributions to Hydrodynamic Diameter for Quantum Dots Optimized for Live-Cell Single-Molecule Tracking

Area of Science:

• Nanotechnology and Materials Science
• Neuroscience and Cell Biology
• Biophotonics and Imaging

Background:

• Quantum dots (QDs) are fluorescent nanoparticles with tunable emission, useful for biological imaging.
• Conventional QDs often exceed the size of target proteins, leading to steric hindrance and limited access to cellular structures like synapses.
• Understanding the hydrodynamic size contribution of QD components is crucial for optimizing probes for specific biological applications.

Purpose of the Study:

• To analyze a new class of engineered quantum dots with minimized size for imaging receptors in narrow neuronal synaptic junctions.
• To quantify the contributions of core, organic coating, and targeting proteins to the total hydrodynamic diameter of QDs.
• To determine the optimal QD formulation for accessing narrow cellular structures in neuronal labeling.

Main Methods:

• Utilized fluorescence correlation spectroscopy (FCS) and transmission electron microscopy (TEM) to measure QD hydrodynamic diameter.
• Investigated various core materials with emission spanning 545-705 nm.
• Assessed the impact of different organic coatings (amphiphilic polymers vs. compact ligands) and varying numbers of targeting streptavidins.

Main Results:

• Compact ligand coatings (HS-(CH2)11-(OCH2CH2)4-OH) reduced QD diameter by ~2-5 nm compared to standard amphiphilic polymers.
• Minimizing streptavidinylation further reduced QD diameter by ~5-11 nm.
• Optimized QDs achieved hydrodynamic diameters of 13.8-18.4 nm, enabling access to narrow synaptic junctions.

Conclusions:

• The organic coating and protein functionalization significantly influence QD hydrodynamic size, impacting their ability to access narrow cellular spaces.
• Minimized QD size, achieved through compact ligands and reduced streptavidinylation, is critical for effective neuronal labeling in synaptic junctions.
• QDs around 14 nm with minimal streptavidinylation can successfully label narrow cellular structures, unlike larger QDs (>27 nm) with extensive functionalization.