DNA-Functionalized Nanoparticles for Multicolor Cathodoluminescence Imaging

This study demonstrates a DNA-based functionalization strategy that renders hydrophobic lanthanide nanoparticles hydrophilic and stable under electron microscopy preparation protocols, enabling multicolor cathodoluminescence imaging for the simultaneous visualization of specific proteins and cellular ultrastructure.

The Gist

Imagine you are trying to take a photo of a bustling city at night. You want to see two things at once: the intricate layout of the streets and buildings (the ultrastructure) and the specific people walking around (the proteins).

Usually, you have to use two different cameras. One camera (like a standard microscope) can see the people clearly if they are wearing bright neon vests, but it can't see the details of the buildings. The other camera (an electron microscope) can see the tiny cracks in the pavement and the texture of the bricks, but it can't tell you who the people are unless they are wearing heavy, dark uniforms that blend into the background.

Scientists have tried to combine these cameras, but it's like trying to take two photos at different times and then trying to glue them together perfectly. It's messy, and the details often get lost.

This paper introduces a brilliant new tool: Glow-in-the-Dark Nanoparticles that work with the "building camera" (the electron microscope) to show you the people and the city at the exact same time.

Here is the story of how they made it happen, using some simple analogies:

1. The Problem: The "Greasy" Nanoparticles

The scientists wanted to use special tiny crystals called Lanthanide Nanoparticles (LNPs). Think of these as microscopic lightbulbs. When you shine an electron beam on them (like a super-powerful flashlight), they glow in very specific, distinct colors (red, green, blue) depending on what's inside them. This allows you to tag different proteins with different colors.

However, there was a catch. These lightbulbs were made in a factory using oil and grease. They were hydrophobic, meaning they hated water.

• The Analogy: Imagine trying to drop a greasy pizza slice into a glass of water. It won't mix; it will clump together and sink.
• The Issue: Biological samples (like human cells) are mostly water. You can't put greasy nanoparticles into a cell without ruining the cell or the particles clumping up into a useless blob.

2. The Solution: The "DNA Life Jacket"

To fix this, the scientists needed to give these greasy nanoparticles a "life jacket" that would let them float in water without losing their glow.

They used DNA (the molecule that holds our genetic code) as this life jacket.

• The Analogy: Imagine the greasy nanoparticles are wearing a heavy, oily raincoat. The scientists took that raincoat off and replaced it with a suit made of DNA. DNA loves water (it's hydrophilic), so suddenly, the nanoparticles could swim happily in the cell's watery environment.
• The Magic: Because DNA is like a Lego brick, they could attach other things to it later. This means they could eventually use these nanoparticles to grab onto specific proteins, like a magnet finding a specific piece of metal in a junkyard.

3. The Test: Surviving the "Extreme Makeover"

Before these nanoparticles can be used to look at cells, the cells have to go through a very harsh preparation process to be seen under an electron microscope. It involves:

• Staining: Soaking the sample in chemicals (like osmium tetroxide) to make the cell structures visible. This is like using a harsh bleach on a photo; usually, it kills the glow of regular fluorescent dyes.
• Drying: Removing all the water. This is like freeze-drying a flower.

The scientists were worried: Will our DNA-coated nanoparticles survive this harsh treatment, or will they stop glowing?

The Result: They survived! Even after being soaked in harsh chemicals and dried out, the nanoparticles kept glowing brightly. They were tough enough to handle the "extreme makeover" of the electron microscope.

4. The Grand Finale: Multicolor Imaging

Finally, they tested the system on real human cells (HEK293 cells).

• They put two or three different types of nanoparticles on the cell surface. Each type glowed a different color (e.g., one glowed red, one green, one blue).
• They took a picture with the electron microscope.
• The Result: In a single image, they could see the detailed texture of the cell's skin (the ultrastructure) and they could see the specific glowing dots (the nanoparticles) in their distinct colors, all perfectly aligned.

Why This Matters

This is a huge step forward because it solves the "glue problem."

• Before: You had to take a blurry photo of the people, take a sharp photo of the buildings, and hope you could line them up perfectly.
• Now: You can take one photo that shows the sharp buildings and the glowing people in their exact locations.

This technology opens the door to seeing how tiny biological machines (proteins) move and interact within the complex city of a cell, all at a scale we've never been able to see so clearly before. It's like finally getting a map of the city that also shows exactly where every single person is standing, in real-time.

Technical Summary

1. Problem Statement

Current biological imaging faces a fundamental trade-off: Electron Microscopy (EM) offers nanoscale spatial resolution of cellular ultrastructure (membranes, cytoskeleton) but lacks intrinsic molecular specificity. Conversely, fluorescence microscopy provides molecular specificity but is limited by the optical diffraction limit and cannot resolve unlabeled ultrastructure.

• Correlative Light and Electron Microscopy (CLEM) attempts to bridge this gap but suffers from complex sample preparation incompatibilities, registration errors between modalities, and sequential imaging steps.
• Cathodoluminescence (CL) microscopy offers a solution by generating optical emission via electron beam excitation, allowing simultaneous acquisition of ultrastructural (Secondary Electron, SE) and molecular (CL) data with intrinsic spatial registration.
• The Specific Challenge: While Lanthanide-doped Nanoparticles (LNPs) are promising CL probes due to their stability under electron beams and distinct emission spectra, they are typically synthesized with hydrophobic surface ligands (e.g., oleic acid). This prevents their use in aqueous biological environments and standard EM sample preparation protocols (which involve osmium tetroxide staining and dehydration). Furthermore, existing functionalization methods often require multi-step processes that lead to particle aggregation.

2. Methodology

The authors developed a streamlined strategy to render LNPs hydrophilic and compatible with biological imaging while preserving their CL properties.

• Synthesis:
  • LNPs (specifically $\text{NaHo}_{0.8}\text{Lu}_{0.2}\text{F}_4$, $\text{NaDy}_{0.8}\text{Lu}_{0.2}\text{F}_4$, and $\text{NaTbF}_4$) were synthesized via coprecipitation in oleic acid and 1-octadecene, yielding monodisperse, hydrophobic nanoparticles (~13 nm diameter).
• Surface Functionalization (DNA Ligand Exchange):
  • Instead of multi-step coating, the authors employed a one-step DNA ligand exchange.
  • Hydrophobic LNPs were transferred from hexane to chloroform, then vigorously mixed with an aqueous solution of 30-thymine single-stranded DNA (T30 ssDNA).
  • The phosphate groups of the DNA coordinate with positively charged ions on the LNP surface, replacing the hydrophobic ligands and rendering the particles hydrophilic and colloidally stable.
• Sample Preparation & Testing:
  • The DNA-functionalized LNPs (LNPs@DNA) were subjected to standard EM preparation steps:
    1. Osmium Tetroxide ($\text{OsO}_4$) staining: Used for membrane contrast.
    2. Drying protocols: Hexamethyldisilazane (HMDS) and Critical Point Drying (CPD).
  • Biological samples were prepared using HEK293T cells: fixed, incubated with LNPs@DNA, air-dried, and sputter-coated with Pt/Pd.
• Imaging Setup:
  • A custom-modified ZEISS SUPRA 55VP FESEM equipped with a parabolic mirror and a three-channel photomultiplier tube (PMT) system was used.
  • Spectral channels were tuned to the characteristic emission peaks of $\text{Ho}^{3+}$ (647 nm), $\text{Dy}^{3+}$ (572 nm), and $\text{Tb}^{3+}$ (545 nm).

3. Key Contributions

1. Development of LNPs@DNA: A robust, one-step method to convert hydrophobic, inorganic CL probes into water-soluble, non-aggregating nanoparticles suitable for biological environments.
2. Validation of CL Stability: Demonstration that LNPs retain their single-particle CL emission and spectral distinctness after rigorous EM sample preparation, including $\text{OsO}_4$ treatment and various drying methods.
3. Multicolor Biological Imaging: Successful implementation of simultaneous, multicolor CL imaging on mammalian cell surfaces, distinguishing three distinct spectral channels ($\text{Ho}^{3+}$, $\text{Dy}^{3+}$, $\text{Tb}^{3+}$) against the backdrop of cellular ultrastructure.

4. Key Results

• Hydrophilicity and Stability:
  • Dynamic Light Scattering (DLS) confirmed that DNA-functionalized LNPs remained suspended in water for at least 60 minutes.
  • The hydrodynamic diameter increased from ~13 nm (as-synthesized) to ~55–57 nm, attributed to the DNA layer and potential small aggregates, but the particles remained dispersed.
  • SEM imaging showed significantly less aggregation in the aqueous phase for DNA-functionalized LNPs compared to controls without DNA.
• CL Brightness Retention:
  • Baseline: LNPs@DNA in water exhibited CL brightness comparable to hydrophobic LNPs in hexane (e.g., ~726 photons/s for $\text{Ho}^{3+}$, ~266 photons/s for $\text{Dy}^{3+}$, ~570 photons/s for $\text{Tb}^{3+}$).
  • $\text{OsO}_4$ Treatment: A reduction in brightness was observed (e.g., ~418 photons/s for $\text{Ho}^{3+}$), attributed to oxidative crosslinking of surface thymines introducing non-radiative loss pathways. However, the signal remained sufficient for reliable single-particle detection.
  • Drying: HMDS and CPD did not quench the signal; in some cases, brightness increased (e.g., ~979 photons/s for $\text{Ho}^{3+}$ after CPD), likely due to improved particle dispersion or reduced background.
• Biological Imaging:
  • The authors successfully imaged HEK293T cells with LNPs@DNA.
  • Simultaneous Acquisition: Secondary Electron (SE) images revealed plasma membrane topography, while CL images simultaneously identified specific LNPs based on their color channel.
  • Multiplexing: Three distinct colors were resolved on the same cell surface without the need for complex post-acquisition image registration or particle classification algorithms.

5. Significance

This work represents a critical step toward nanoscale, multiplexed protein localization in electron microscopy.

• Overcoming CLEM Limitations: By enabling simultaneous acquisition of ultrastructure and molecular identity, this approach eliminates the registration errors and workflow complexities inherent in CLEM.
• Robustness: The demonstration that LNPs survive harsh EM preparation (fixation, staining, drying) makes them viable for standard biological workflows.
• Future Applications: The DNA-functionalized surface serves as a versatile platform. The authors propose that these LNPs can be further conjugated with antibodies, nanobodies, or aptamers to target specific proteins or nucleic acids. This would allow for the visualization of complex biological processes (e.g., endocytosis, cell signaling, neural circuit function) with molecular specificity at the nanoscale, bridging the gap between structural biology and molecular cell biology.