⚛️ quantum physics

Simultaneous nanorheometry and nanothermometry using intracellular diamond quantum sensors

This paper presents a dual-mode quantum sensor based on nitrogen-vacancy centers in nanodiamonds that enables the simultaneous, high-resolution measurement of intracellular temperature and viscoelasticity to investigate their interplay in live cells.

Original authors: Qiushi Gu, Louise Shanahan, Jack W. Hart, Sophia Belser, Noah Shofer, Mete Atature, Helena S. Knowles

Published 2026-03-02

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CC BY 4.0

Original authors: Qiushi Gu, Louise Shanahan, Jack W. Hart, Sophia Belser, Noah Shofer, Mete Atature, Helena S. Knowles

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a cell as a bustling, microscopic city. Inside this city, there are roads (cytoskeleton), vehicles (molecular motors), and a thick, gooey atmosphere (the cytoplasm). For the city to function—moving supplies, dividing into two, or changing shape—the "traffic" needs to flow smoothly, and the "weather" (temperature) needs to be just right.

For a long time, scientists had a problem: they could measure the traffic flow, or they could measure the temperature, but they couldn't do both at the exact same time with the same tiny tool. It was like trying to measure the speed of a car and the temperature of the air it's driving through, but your thermometer was too big to fit in the car, and your speedometer couldn't tell you the temperature.

This paper introduces a superhero tool that solves this problem: a tiny, diamond-based sensor that acts as both a thermometer and a traffic monitor simultaneously.

Here is how it works, broken down into simple concepts:

1. The Tiny Diamond Detective

The researchers use nanodiamonds—diamonds so small they are invisible to the naked eye, about the size of a virus. Inside these diamonds are tiny defects called Nitrogen-Vacancy (NV) centers. Think of these centers as tiny, magical compass needles made of atoms.

The Magic Trick: These atomic compass needles are sensitive to their surroundings. If the temperature changes, the needle wobbles in a specific way. If the "goo" around it gets thicker or stickier, the diamond's movement changes.
The Superpower: Because they are diamonds, they are tough, non-toxic, and don't fade away like other glowing dyes. They can survive inside a living cell for a long time.

2. The "Orbiting Satellite" Tracking System

How do you follow a speck of dust moving inside a dark, crowded room? You don't just stare at it; you orbit it.

The scientists built a custom microscope that acts like a satellite. Instead of shining a laser directly on the diamond, the laser beam dances in a tiny circle (an orbit) around the diamond.

The Feedback Loop: The microscope listens to the light coming from the diamond. If the diamond drifts slightly to the left, the light pattern changes. The computer instantly says, "Oh, it moved left!" and steers the laser back to the center.
The Result: This happens thousands of times a second. The system locks onto the diamond, tracking its every move with incredible precision (down to 3.7 nanometers—thousands of times smaller than a human hair).

3. Two Jobs, One Tool

This is where the magic happens. The tool does two things at once:

Job A: Nanothermometry (The Thermometer)
The researchers zap the diamond with microwaves. The diamond glows, but the color or frequency of that glow shifts slightly depending on how hot it is. By measuring this shift, they know the exact temperature inside the cell, right next to the diamond.
Job B: Nanorheometry (The Traffic Monitor)
While measuring the temperature, they also watch how the diamond jiggles.
- If the diamond jiggles wildly and quickly, the cell's interior is like water (low viscosity).
- If the diamond moves slowly and sluggishly, the interior is like honey or jelly (high viscosity/elasticity).
- By analyzing the "jiggling pattern," they can calculate how stiff or fluid the cell's interior is.

4. What They Discovered in the "City"

The team tested this on living human cancer cells (HeLa cells) and found some fascinating things:

The "Active" City: Cells aren't just passive blobs of goo. They are alive and active. The researchers saw the diamonds suddenly zooming in straight lines. This wasn't random jiggling; it was active trafficking. It's like seeing a delivery truck being driven by a molecular motor (like a tiny forklift) moving cargo around the cell.
The "Elastic" Gel: When they stopped the molecular motors (by using a drug called nocodazole), the "traffic" stopped. The diamonds went back to slow, random jiggling. This revealed that without the active motors, the cell's interior acts more like a weak elastic gel (like Jell-O) rather than a flowing liquid.
The Temperature Mystery: They heated the outside of the cell and watched the inside. Surprisingly, the cell didn't seem to have its own internal thermostat that fought against the heat. The inside temperature rose exactly as fast as the outside temperature. The cell didn't try to "cool down" the specific spot where the diamond was.

Why Does This Matter?

Imagine trying to understand why a car engine is failing. If you only look at the speedometer, you don't know if the engine is overheating. If you only look at the temperature gauge, you don't know if the gears are slipping.

This new tool lets scientists see both the temperature and the "mechanical health" of a cell at the exact same moment. This is huge for understanding:

Disease: How cancer cells move and divide.
Metabolism: How cells generate heat and energy.
Drug Testing: How new medicines change the physical structure of a cell.

In short, the researchers built a tiny, diamond-encrusted spy that can sneak into a cell, tell us how hot it is, and report on how the cell's internal "traffic" is flowing, all at the same time. This gives us a clearer picture of the microscopic world that keeps us alive.

1. Problem Statement

Understanding the interdependence of intracellular temperature and viscoelasticity is critical for deciphering cellular metabolism, homeostasis, and disease progression (e.g., cancer). However, existing biosensing tools face significant limitations:

Scale and Noise: Probing phenomena at the nanoscale within complex, heterogeneous cellular environments often suffers from poor signal-to-noise ratios.
Optical Artifacts: Traditional optical methods are susceptible to autofluorescence, spectral transmission variations, and refractive index changes in biochemical environments.
Coupled Variables: A change in one physical property (e.g., temperature) often alters others (e.g., viscosity or reaction rates). Without independent, simultaneous measurement of these variables, it is difficult to distinguish cause from effect or to accurately model the system's response to external perturbations.
Lack of Multimodal Sensors: There is a need for a single probe capable of measuring both local temperature and mechanical rheology (viscoelasticity) simultaneously without measurement dead time.

2. Methodology

The authors developed a dual-mode quantum sensing platform using Nitrogen-Vacancy (NV) centers in nanodiamonds (NDs).

Hardware and Setup:

Sensor: Nanodiamonds (~25 nm radius) containing an ensemble of 100–300 NV centers.
Custom Sensing Chip: A gold-patterned glass coverslip featuring a coplanar waveguide (CPW) for microwave delivery, resistive heaters, and a Resistive Temperature Detector (RTD) for precise substrate temperature control.
Microscopy: A home-built confocal microscope equipped with a 532 nm laser for excitation and two avalanche photodiodes (APDs) for fluorescence collection.

Key Techniques:

Double-Plane Orbital Tracking: To track the rapidly diffusing nanodiamonds in real-time, the excitation laser performs a circular orbit (radius 50 nm, period 9.6 ms) around the particle. Two confocal collection planes are axially offset by ~50 nm. Asymmetries in the photoluminescence (PL) intensity between the orbit and the two planes provide real-time feedback to correct the laser position in $x, y,$ and $z$ . This achieves a spatial resolution of 3.7 nm with a 9.6 ms update rate.
Optically Detected Magnetic Resonance (ODMR): While tracking, the system performs continuous-wave ODMR. The ground-state spin transition of the NV center ( $m_s=0$ to $m_s=\pm1$ ) is driven by microwaves. The resonance frequency of this transition shifts linearly with temperature. By sweeping the microwave frequency and monitoring PL dips, the central frequency is extracted to determine local temperature.
Passive Nanorheometry: The mean square displacement (MSD) of the tracked nanodiamond is analyzed. Using the Stokes-Einstein relation (for viscous fluids) or generalized Stokes-Einstein equations (for viscoelastic media), the diffusion coefficient is converted into viscosity or complex modulus ( $G^*$ ).

Calibration:

Thermometry: The temperature coefficient ( $\kappa$ ) was calibrated for each nanodiamond, yielding values around $-60.0$ kHz/°C. The system achieved a temperature sensitivity of 2.3 °C/ $\sqrt{\text{Hz}}$ .
Rheometry: Tracking accuracy was verified by mimicking Brownian motion on a stationary substrate, confirming the system could track particles with diffusion coefficients up to $5 \times 10^4$ nm $^2$ /s.

3. Key Contributions

Simultaneous Dual-Modal Sensing: The first demonstration of simultaneous, independent nanoscale thermometry and rheometry in a dynamic environment using a single quantum sensor.
High Spatiotemporal Resolution: Achieved 3.7 nm spatial resolution and millisecond-scale updates, allowing the tracking of fast intracellular dynamics without dead time.
Decoupling of Variables: The ability to measure temperature and mechanical properties independently allows for the disentanglement of thermal effects from mechanical responses in complex media.
Active vs. Passive Discrimination: Developed a method to distinguish between passive Brownian motion and active, directed transport (e.g., motor-driven trafficking) within live cells by analyzing trajectory directionality and power-law exponents.

4. Key Results

A. Validation in Controlled Media:

Glycerol (Viscous): In a purely viscous medium, the system successfully measured the temperature-dependent viscosity. The extracted hydrodynamic radius of the nanodiamond (28 nm) matched the supplier's nominal value, and the viscosity changes correlated perfectly with the independently measured temperature.
GCX (Viscoelastic): In a glycerol-crosslinked xanthan gel, the sensor detected the decrease in both elastic ( $G'$ ) and viscous ( $G''$ ) moduli as temperature increased, consistent with the time-temperature superposition principle.

B. Intracellular Measurements in Live HeLa Cells:

Temperature Regulation: The study found that HeLa cells do not actively regulate their internal temperature against external thermal perturbations (within the measurement sensitivity). The intracellular temperature tracked the external substrate temperature linearly.
Active Trafficking: Analysis of nanodiamond trajectories revealed two distinct motion regimes:
- Non-directed motion: Exhibited Brownian-like behavior (power-law exponent $\alpha \approx 0.97$ ).
- Directed motion: Exhibited superdiffusive behavior ( $\alpha \approx 1.65$ ), indicative of active motor-driven transport.
Role of Microtubules: Upon treatment with nocodazole (a microtubule destabilizer), directed motion ceased. The remaining motion became subdiffusive ( $\alpha \approx 0.3$ ), indicating that in the absence of active forces, the cytoplasm behaves as an elasticity-dominated weak gel.
Viscoelasticity: The system successfully mapped the complex modulus of the cytoplasm, revealing that while active forces dominate particle motion, the underlying passive material is viscoelastic.

5. Significance

This work represents a major advancement in intracellular biophysics:

Metabolic Insights: By correlating local temperature with rheology, researchers can now study how metabolic heat generation (thermogenesis) influences local mechanical properties and vice versa.
Disease Mechanisms: The ability to probe the interplay between temperature and viscoelasticity offers new avenues for studying cancer progression, where both thermal dysregulation and cytoskeletal remodeling are hallmarks.
Methodological Breakthrough: The orbital tracking combined with ODMR overcomes the trade-off between spatial resolution and temporal resolution, enabling the study of fast, transient cellular events.
Future Applications: The platform is compatible with super-resolution imaging, optical tweezers, and targeted delivery to specific organelles (e.g., mitochondria), paving the way for investigating nanoscale temperature gradients and local biochemical events in live cells.

In summary, the authors have created a robust, multimodal quantum sensor that resolves the long-standing challenge of simultaneously measuring temperature and mechanical properties at the nanoscale, providing a powerful tool for understanding the physical biology of living cells.