🧬 biology

A study of particle behaviour in dynamic extracellular environments using nanoparticles as model nanoscale probes

This study utilizes a label-free optical caustics-based microscopy technique to demonstrate that the dynamic extracellular environment of human mesenchymal stem cells significantly alters the transport behavior of gold nanoparticles, highlighting the critical influence of local biological conditions such as serum exposure, surface charge, and protein content on nanoscale mobility.

Original authors: Genevieve Schleyer, Eann A. Patterson, Judith M. Curran

Published 2026-10-30

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Genevieve Schleyer, Eann A. Patterson, Judith M. Curran

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

In the microscopic world of living tissue, the space between cells is far from empty. It is a bustling, crowded highway filled with proteins, sugars, and other molecules that constantly shift and change. For scientists trying to understand how tiny particles move through this environment—whether they are natural cellular components or man-made drugs designed to deliver medicine—this dynamic landscape presents a major puzzle. If a particle moves too slowly, it may never reach its target; if it moves too fast, it might pass right by. Traditionally, researchers have watched these particles by attaching bright fluorescent tags to them, like putting a tiny light on a car to track its speed. However, these lights can change how the car behaves, altering its weight or how it interacts with the road, which means the data collected might not reflect reality. To see the true nature of movement in these complex biological systems, scientists need a way to watch without touching, a method that observes the particles exactly as they are.

A team of researchers at the University of Liverpool has developed a new way to watch these tiny travelers using a technique that relies on light itself rather than artificial markers. They focused on gold nanoparticles, tiny spheres of metal just 100 nanometers in diameter, which serve as excellent models for understanding how nanoscale objects behave. The researchers placed these gold spheres into a fluid environment containing human stem cells and used a specialized microscope setup to create optical patterns, known as caustics, around the particles. These patterns appear as bright spots surrounded by dark rings, allowing the team to track the precise position of each particle as it drifted through the fluid above the living cells. By observing these movements over time, the scientists could measure how fast the particles moved and how far they traveled without ever altering the particles with chemical tags.

The study revealed that the environment created by living cells is not a static fluid but a constantly evolving medium that significantly slows down particle movement. When the gold nanoparticles were first introduced, they moved at a certain speed, but as time passed, their motion became noticeably slower. After about an hour and a half, the particles were moving much more sluggishly than they had at the start, regardless of how far they were from the cell surface. This suggests that the cells are actively changing the chemistry of their surroundings, perhaps by releasing new proteins or rearranging existing ones, which creates a thicker or more crowded path for the particles to navigate. The researchers found that this slowing effect was not due to the particles sticking directly to the cells, as the slowdown occurred even at distances where the particles were not touching the cell layer.

The behavior of the particles also depended heavily on their electrical charge and the presence of proteins in the fluid. The team tested two types of gold particles: one with a negative charge and one with a positive charge. When the fluid contained serum proteins, which are common in blood and cell culture, the positively charged particles were shielded by a layer of these proteins, allowing them to continue moving relatively freely. However, when the proteins were removed, the positively charged particles quickly stuck to the glass surface of the container, unable to move at all. This happened because the positive charge of the particles was attracted to the negative charge of the glass, a force that the protein layer normally prevents. In contrast, the negatively charged particles did not stick to the glass but still slowed down over time in the presence of cells, indicating that the cells were changing the fluid itself rather than just acting as a sticky trap.

One of the most striking findings was that while the speed of the particles changed dramatically, the fundamental nature of their movement remained surprisingly consistent. The particles did not start moving in erratic, unpredictable bursts or get stuck in a single spot; instead, they continued to drift in a random, natural pattern, just at a slower pace. This suggests that the living environment acts like a thickening fluid that resists motion, rather than a chaotic maze that traps the particles in unusual ways. The researchers also observed that the area covered by the particles' movement shrank over time, meaning they were exploring a smaller and smaller territory as they became more hindered by their surroundings.

This work provides a clear, label-free window into how nanoscale objects navigate the complex reality of living tissue. By showing that the local environment around cells is dynamic and constantly changing, the study highlights that the journey of a particle is not just about its own properties, but about the ever-shifting conditions of the world it travels through. The ability to track these movements without interference offers a powerful new tool for understanding how drugs and other nanoscale agents might behave inside the human body, ensuring that future medical designs are based on how things actually move in life, not just how they move in a simple test tube.

Technical Summary: A Study of Particle Behaviour in Dynamic Extracellular Environments Using Nanoparticles as Model Nanoscale Probes

Problem Statement
Quantifying the transport and behavior of nanoscale entities within complex, heterogeneous biological environments remains a significant challenge. While fluorescent labeling is a standard method for visualizing nanoparticles, the attachment of fluorophores can alter surface properties and obscure native interactions, particularly in dynamic physiological conditions. Furthermore, many label-free techniques (e.g., interferometry, SPRM) require specialized, often inaccessible instrumentation. There is a critical need for low-cost, label-free methods capable of real-time, quantitative tracking of nanoparticles in living systems without perturbing their native behavior. Additionally, the extent to which living cell populations alter local transport conditions and influence stochastic motion in the extracellular space requires further experimental evidence.

Methodology
The study employed an optical caustics-based microscopy technique to track the motion of 100 nm gold nanoparticles in the extracellular space above human mesenchymal stem cell (hMSC) monolayers. This label-free approach utilizes a standard inverted optical microscope equipped with a reduced condenser aperture and a narrow-band filter to generate distinct caustic signatures (bright spots surrounded by dark rings) for individual nanoparticles.

Experimental Setup: hMSCs were cultured on glass coverslips to 90–95% confluence. Nanoparticles (negatively charged citrate-coated and positively charged BPEI-coated) were dispersed in Dulbecco's Modified Eagle Medium (DMEM), with and without 10% fetal bovine serum (FBS).
Tracking Protocol: Coverslips with cell monolayers were placed onto microscope slides containing the nanoparticle solution. Particles were tracked at discrete axial ranges (20–25 µm, 45–50 µm, 70–75 µm, and 95–100 µm) above the cell surface. Videos were recorded for 2 seconds at 50 fps.
Data Analysis: Particle trajectories were analyzed to calculate:
1. Diffusion Coefficients ( $D$ ): Derived from the Mean Square Displacement (MSD).
2. Anomalous Diffusion Exponents ( $\alpha$ ): Determined from the slope of the log-log plot of MSD versus time lag to distinguish between normal ( $\alpha \approx 1$ ), superdiffusive ( $\alpha > 1$ ), and subdiffusive ( $\alpha < 1$ ) transport.
3. Convex Hull Metrics: Calculated to quantify the spatial extent of particle movement.
Controls: Experiments included cell-free glass surfaces and comparisons between media with and without serum proteins. Cell viability was confirmed via LIVE/DEAD assays, showing negligible cell death after 2 hours of exposure.

Key Results

Impact of Living Cells: The presence of living hMSCs significantly reduced nanoparticle diffusion rates compared to cell-free conditions. Negatively charged nanoparticles in DMEM with FBS showed a diffusion coefficient reduction of 40–65% relative to Stokes-Einstein predictions within 15 minutes of exposure.
Temporal Evolution: Nanoparticle dynamics were time-dependent. After 105 minutes of exposure to the cell layer, diffusion rates decreased significantly across all measured heights (25–100 µm), suggesting the extracellular environment evolves over time due to cellular activity (e.g., secretion of biomolecules or protein corona rearrangement).
Role of Surface Charge and Serum:
- Positively Charged Particles: In the absence of FBS, positively charged nanoparticles exhibited significantly lower diffusion coefficients and eventually adhered to the glass surface, likely due to electrostatic attraction. The presence of FBS (forming a protein corona) shielded the surface charge, preventing adhesion and allowing faster diffusion.
- Negatively Charged Particles: These showed less sensitivity to the absence of FBS regarding adhesion but still experienced diffusion slowdowns in the presence of cells.
Diffusion Regime: Despite the reduction in diffusion coefficients, the ensemble-averaged anomalous diffusion exponent ( $\alpha$ ) remained close to 1 for all conditions. This indicates that while the rate of transport is modified by the biological environment, the fundamental mode of transport remains consistent with normal Brownian diffusion at the ensemble level.
Spatial Heterogeneity: Convex hull analysis revealed a wide distribution of movement areas, particularly for positively charged particles in serum-containing media. This suggests a heterogeneous environment where some particles interact locally with biological components (trapped) while others diffuse freely.

Significance and Claims
The paper establishes that the living extracellular environment acts as a dynamic and evolving transport medium that modifies nanoparticle movement beyond direct particle-cell contact. The study demonstrates that:

Label-Free Viability: Optical caustics microscopy is a robust, accessible, and label-free method for tracking nanoparticles in complex, living biological systems without the perturbations associated with fluorescent labeling.
Environmental Sensitivity: Nanoparticle transport is highly sensitive to local biological conditions, including cell proximity, time, surface charge, and protein content. The "hard" and "soft" protein corona dynamics likely contribute to the observed temporal changes in mobility.
Deviation from Ideality: Experimental diffusion in biological media deviates substantially from idealized Stokes-Einstein predictions, highlighting the necessity of empirical characterization in physiologically relevant environments.

The authors conclude that this framework provides a reproducible baseline for investigating nanoparticle dynamics in heterogeneous biological systems, offering insights into how cellular activity influences the transport of nanoscale entities in the extracellular space.

Technical Summary: A Study of Particle Behaviour in Dynamic Extracellular Environments Using Nanoparticles as Model Nanoscale Probes

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