🔬 mesoscale physics

Method for real-time monitoring of paramagnetic reactions using spin relaxometry with fluorescent nanodiamonds

This paper presents a cost-effective, real-time monitoring method for paramagnetic reactions using fluorescent nanodiamonds and an optimized FPGA-based system, achieving a two-orders-of-magnitude speedup over traditional techniques to track chemical kinetics such as Cu(II) reduction within 15 seconds.

Original authors: Trent Ralph, Erin S. Grant, Lianne Lay, Sepehr Ahmadi, David A. Simpson

Published 2026-02-23

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CC BY 4.0

Original authors: Trent Ralph, Erin S. Grant, Lianne Lay, Sepehr Ahmadi, David A. Simpson

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

The Big Idea: A "Smart Watch" for Chemical Reactions

Imagine you are trying to watch a magic trick happen in a jar of water. You want to see exactly when a magician (a chemical) turns a red ball (a copper ion) into a blue one.

In the past, scientists had a very sensitive camera (a single crystal diamond sensor) to watch this, but it was so slow that by the time the photo developed, the magic trick was already over. They had to wait 50 minutes just to take one picture of the reaction.

This paper introduces a new, super-fast method using Fluorescent Nanodiamonds (FNDs). Think of these as millions of tiny, glowing fireflies suspended in the water. The researchers built a new system that can watch these fireflies blink in real-time, allowing them to see the chemical reaction happen as it occurs, not just after the fact.

How It Works: The "Glowing Fireflies" and the "Noise"

1. The Fireflies (Nanodiamonds)
Inside these tiny diamonds are defects called "Nitrogen-Vacancy" centers. Think of them as tiny, glowing lightbulbs that change their brightness based on their "mood" (spin state).

The Trick: When you shine a green laser on them, they glow red.
The Sensitivity: If a "noisy" magnetic neighbor (like a paramagnetic ion) gets too close, the firefly gets distracted and stops glowing as brightly or for as long.

2. The Noise (Paramagnetic Ions)
The scientists are looking for specific chemical "noise-makers," like Copper ions. These ions have unpaired electrons that act like tiny magnets, creating a magnetic "static" that disturbs the fireflies.

The Goal: By measuring how quickly the fireflies stop glowing (a process called T1 relaxometry), the scientists can tell how much "noise" (Copper ions) is in the jar.

The Old Way vs. The New Way

The Old Way (The Slow Turtle):
Previously, scientists used a highly sensitive detector called a SPAD (Single Photon Avalanche Diode).

Analogy: Imagine trying to count raindrops falling into a bucket, but you can only catch one drop at a time with a tiny thimble. It takes forever to get a good count.
Result: It took 50 minutes to get a clear reading. You couldn't watch a fast reaction; you could only look at the aftermath.

The New Way (The Speedy Squirrel):
The team built a new system using two main upgrades:

A Linear Avalanche Photodiode (PD): Instead of a tiny thimble, they used a wide-open bucket. It can catch millions of light photons at once without getting "full" (saturated).
An FPGA (The Brain): They used a fast, programmable computer chip (FPGA) to process the data instantly, like a super-fast accountant tallying the raindrops the moment they hit the bucket.

The Result:

Speed: They went from 50 minutes down to 15 seconds (a 200x speed-up!).
Cost: They replaced expensive, custom lab equipment with off-the-shelf parts, cutting the cost by 10 times.

The Experiment: Watching Copper Change Colors

To prove their system worked, the scientists performed a chemical reaction:

The Setup: They put the glowing nanodiamonds in water.
The Intruder: They added Copper (II) ions. These are magnetic "noise-makers." The fireflies immediately got distracted, and their glow changed speed.
The Magic: They added Ascorbic Acid (Vitamin C). This acts like a "silencer," turning the noisy Copper (II) into quiet Copper (I).
The Observation: As the Vitamin C did its work, the "noise" disappeared, and the fireflies' glow returned to normal.

Because their system was so fast, they could watch the "silencing" happen in real-time. They saw the reaction progress over 20 minutes, whereas the old system would have missed the whole process.

Why This Matters

Real-Time Chemistry: Scientists can now watch chemical reactions happen second-by-second, not hour-by-hour. This is huge for understanding how drugs work or how pollutants behave.
Accessibility: Because the system is cheap (using a $500 computer board and a standard light detector), any university lab or even a high school with a good budget can build one. It democratizes high-tech sensing.
Future Potential: The authors believe that with a few more tweaks (like better lenses), they could eventually watch these reactions in milliseconds, opening the door to studying the fastest chemical processes in biology and physics.

In a Nutshell

The researchers took a slow, expensive, and finicky way of sensing chemicals and turned it into a fast, cheap, and robust tool. They replaced a "thimble" with a "bucket" and a "sloth" with a "squirrel," allowing us to finally watch the invisible world of chemical reactions dance in real-time.

1. Problem Statement

Fluorescent nanodiamonds (FNDs) containing nitrogen-vacancy (NV) defects are powerful sensors for paramagnetic species (e.g., free radicals, metalloproteins) via $T_1$ spin relaxometry. However, traditional measurement methods face significant limitations:

Slow Acquisition Speed: Conventional setups using Single Photon Avalanche Diodes (SPADs) and time-correlated single-photon counting (TCSPC) require long integration times (often ~50 minutes) to achieve sufficient signal-to-noise ratios (SNR). This makes real-time monitoring of dynamic chemical reactions impractical.
High Cost: High-end SPADs and TCSPC electronics are expensive, limiting the accessibility of this sensing technology.
Saturation Limits: SPAD detectors have low saturation limits (~1 MHz), preventing the use of high FND concentrations to boost signal strength.

2. Methodology

The authors developed a streamlined, low-cost, high-speed system for in-solution $T_1$ relaxometry. Key components and protocols include:

Hardware Architecture:
- Detector: Replaced SPADs with a linear-mode Avalanche Photodiode (APD) (Hamamatsu C10508-01). This allows for much higher photon flux handling and higher saturation limits.
- Data Acquisition: Utilized a low-cost, open-source Field-Programmable Gate Array (FPGA) board (Red Pitaya STEMLab 125-14) capable of sampling at 125 MS/s. The FPGA performs real-time filtering, decimation, and averaging of the voltage signal.
- Optical Setup: A 532 nm laser excites the NV centers. The system uses a custom 3D-printed cuvette holder that integrates the APD module directly against the sample. This design minimizes the stand-off distance to maximize collection efficiency and includes an aperture to block high-angle scattered excitation light.
Measurement Protocol:
- Pulse Sequence: An all-optical pulse sequence is used where laser pulses (100 µs) polarize the NV spins, followed by variable dark evolution times ( $\tau$ ).
- Signal Normalization: To correct for laser intensity fluctuations and NV charge state changes, the signal is normalized by comparing the photoluminescence (PL) collected in the first 10 µs (spin-dependent) against the last 10 µs (spin-independent plateau) of each pulse.
- Data Efficiency: Only the relevant data segments are transferred from the FPGA, reducing data transfer overhead by an order of magnitude.
- Fitting: Data is fitted to a stretched exponential function to extract the $T_1$ relaxation time.

3. Key Contributions

Speed Improvement: Achieved a two-orders-of-magnitude (100x) increase in measurement speed compared to previous SPAD-based methods.
Cost Reduction: Reduced the system cost by one order of magnitude by replacing expensive TCSPC electronics with off-the-shelf APDs and FPGA boards.
High Concentration Capability: The linear-mode APD and optimized optical geometry allow for FND concentrations up to 200 µg/mL (previously limited to ~20 µg/mL due to SPAD saturation), significantly boosting signal intensity.
Real-Time Chemical Monitoring: Demonstrated the ability to monitor chemical kinetics with a time resolution of 15 seconds using a 3-point $T_1$ acquisition sequence.

4. Results

Benchmarking:
- The new APD/FPGA system yielded a $T_1$ time of 290 ± 10 µs for FNDs in water, consistent with the SPAD system (290 ± 20 µs).
- SNR Comparison: The APD system reached a 5% fit error in under 10 minutes, whereas the SPAD system required the full 90-minute window to reach the same error level.
- Concentration Scaling: At 200 µg/mL, the APD system achieved a 5% fit error in under 60 seconds.
Chemical Reaction Demonstration:
- Reaction: The reduction of paramagnetic Cu(II) (magnetic) to diamagnetic Cu(I) (non-magnetic) using ascorbic acid (AA) as a reductant.
- Observation: Upon adding Cu(II), the $T_1$ time dropped rapidly from a baseline of ~131 µs to ~76 µs due to magnetic noise. Upon adding AA, the $T_1$ time recovered over 20 minutes as Cu(II) was reduced to Cu(I).
- Temporal Resolution: While full curve acquisition took 2 minutes, reprocessing the data using a 3-point sequence (measuring only specific time points) maintained the trend and improved resolution. The authors project that with further FPGA optimization (direct TCP data transfer), a resolution of 15 seconds is achievable, and potentially <500 ms with improved optics and higher NV-doped FNDs.

5. Significance

This work transitions $T_1$ spin relaxometry from a static, high-cost characterization tool into a dynamic, real-time sensing platform.

Accessibility: By utilizing low-cost, commercially available components, the method makes paramagnetic sensing accessible to a broader range of researchers in physics, chemistry, and biology.
Kinetic Studies: The ability to resolve sub-minute (and potentially sub-second) kinetics opens new avenues for studying fast paramagnetic processes, such as free radical generation and enzymatic reactions, in solution.
Material Characterization: The rapid measurement capability allows for efficient quality control and characterization of FND batches, which is crucial for the development of quantum sensing materials.

In conclusion, the authors have successfully engineered a robust, low-cost system that overcomes the speed and cost barriers of traditional NV-diamond sensing, enabling the real-time monitoring of paramagnetic chemical reactions in solution.