Fluorescent nanodiamonds release a small molecule inside breast cancer cells and use quantum sensing to watch how the drug changes local cell chemistry in real time.
(Nanowerk Spotlight) When a therapeutic molecule enters a living cell, its path becomes almost impossible to follow. Chemical reactions unfold on nanometer scales that remain invisible even to advanced imaging. Drug efficacy often depends on events that cannot be directly observed, including how a compound moves through intracellular compartments and how the surrounding chemistry responds.
Conventional imaging can trace location but not the molecular activity that determines whether a treatment succeeds. Bridging that gap between delivery and observation is one of the central technical challenges in precision medicine.
The problem is particularly pressing in cancer therapy. Many anticancer drugs reach tumor cells but behave unpredictably once inside them. They may react with unintended molecules or trigger defence mechanisms that reduce their effectiveness. Understanding these intracellular processes in real time could change how therapies are designed, yet most tools capable of sensing chemical change cannot also deliver drugs or tolerate the conditions inside living cells.
Progress in nanomaterials and quantum sensing now makes it possible to combine these functions. Nanodiamonds, which are carbon crystals less than 100 nm wide, can carry therapeutic molecules while hosting atomic-scale defects called nitrogen vacancy centers. These defects emit light and respond to nearby magnetic and chemical fluctuations, enabling them to act as quantum sensors.
Chemical linkers that detach in mildly acidic environments can control where a drug is released. Tumor tissue and cellular organelles such as lysosomes have lower pH than healthy tissue, so such linkers allow targeted delivery. The combination of pH sensitivity and quantum sensing opens a path to materials that both administer treatment and monitor its effects.
In this work, a research team from the University of Groningen in The Neterlands, describe fluorescent nanodiamonds that can transport a small molecule drug, release it selectively in acidic compartments, and measure local redox changes through optical signals arising from their quantum properties.
Schematic illustration of a pH-responsive nanodiamond platform (FND-HPG-DMA-DZX) for investigating diazoxide-triggered redox modulation in MDA-MB-231 cells via quantum diamond relaxometry. a) Stepwise surface functionalization of fluorescent nanodiamonds (FNDs) with hyperbranched polyglycerol (HPG), 3-(4-methyl-2,5-dioxo-2,5-dihydrofuran-3-yl)propanoic acid (DMA), and diazoxide (DZX), yielding the final FND-HPGDMA-DZX construct. b) Intracellular mechanism of redox sensing: FND-HPG-DMA-DZX accumulates in endo-/lysosomal compartments, where the mildly acidic environment induces cleavage of the pH-labile amide-oxobutenoic acid linkage, releasing DZX. The liberated DZX activates mitochondrial ATP-sensitive potassium (mitoK_ATP) channels, promoting mitochondrial ROS production (①). The resulting changes in intracellular redox balance (②) are monitored in situ at the endo-/lysosomal level via diamond quantum relaxometry (③). c) pH-sensitive release mechanism of DZX from the DMA linker under acidic conditions, yielding 2-methylmaleic acid (MMA) and free DZX. d) Chemical structures of HPG, DMA, and DZX used in the nanodiamond construct. (Image: Reprinted from DOI10.1002/adfm.202514294:, CC BY) (click on image to enlarge)
The system begins with fluorescent nanodiamonds containing nitrogen vacancy centers. A nitrogen vacancy center is a missing carbon atom adjacent to a nitrogen atom in the diamond lattice. When illuminated, it emits light whose intensity depends on the spin state of its electrons. That spin state changes in the presence of unpaired electrons such as those in free radicals, making it a sensitive probe of local chemical activity.
To improve compatibility with biological fluids, the researchers coated the nanodiamonds with hyperbranched polyglycerol, a water-soluble polymer that prevents aggregation and provides reactive sites for chemical attachment. Spectroscopic and light-scattering measurements confirm the polymer coating and improved stability in salt solutions similar to physiological conditions.
A pH-sensitive linker is then added. This small molecule forms a bond that remains stable at neutral pH but breaks in mildly acidic environments. The design allows the attached drug to remain bound in circulation and detach only in tumor-like conditions. The chosen drug, diazoxide, is clinically known for controlling low blood sugar and hypertension by opening potassium channels in mitochondria.
Altering those channels can change mitochondrial function and the production of reactive oxygen species, which are chemically active oxygen-containing molecules that play key roles in cell stress and signalling. The ability to monitor these species makes diazoxide an appropriate test case for a system that measures redox changes near its own site of release.
Each stage of construction modifies the nanoparticle’s characteristics in predictable ways. The hydrodynamic diameter increases from about one 120 to about two 220 nanometres as coatings and drug molecules accumulate, and the surface charge becomes less negative, which can enhance cell uptake. Thermal analysis indicates that the final construct is approximately three quarters diamond by mass, with the remainder consisting of polymer, linker, and drug.
To verify controlled release, the researchers first used a fluorescent dye instead of diazoxide. When the construct was placed in a mildly acidic buffer, most of the dye detached within several hours, while at neutral pH almost none was released. Substituting diazoxide produced similar behaviour. About eighty percent of the drug was released at pH 6.5 within eight hours, compared with roughly one quarter at pH 7.4. The rate followed first order kinetics, confirming systematic rather than random release.
When the nanodiamonds were added to cultures of triple negative breast cancer cells, they accumulated mainly inside lysosomes, the cellular compartments responsible for digestion. Over two days, the overlap between nanodiamond fluorescence and lysosomal markers declined, suggesting that after drug release the particles redistributed within the cell.
The applied dose, equivalent to 2.75 micromolar diazoxide, was selected to allow observation of biochemical effects without causing significant toxicity. Tests of metabolic activity confirmed that cell viability remained high during the observation period.
The same nitrogen vacancy centers that give the nanodiamonds fluorescence also serve as quantum sensors through a mechanism known as T1 relaxometry. When excited by green light, the centers’ electron spins are polarized and then allowed to return to equilibrium. The relaxation time, called T1, shortens when unpaired electrons such as those in free radicals are nearby because they introduce magnetic noise. Measuring changes in T1 therefore provides a direct, non-invasive readout of local radical concentrations.
After diazoxide release, T1 measurements revealed distinct shifts across cellular regions. At 24 hours, T1 increased in lysosomes and the surrounding cytoplasm, indicating a reduction in radicals near those particles. Meanwhile, fluorescence assays targeting mitochondrial superoxide showed elevated radical levels inside mitochondria. By 48 hours, the cytoplasmic and lysosomal readings returned toward baseline while mitochondrial stress persisted. The pattern suggests that diazoxide enhances radical generation within mitochondria, while other parts of the cell transiently boost antioxidant activity to restore balance.
Control experiments confirmed that these results did not arise from pH or temperature effects. The nanodiamonds showed stable T1 values across a range of pH conditions and remained responsive to known magnetic noise from paramagnetic ions, demonstrating that the sensing signal reflected genuine redox changes near the particles.
The findings outline a sequence of events within the cell. The nanodiamond enters an acidic compartment, the linker cleaves, and diazoxide is released. The drug reaches mitochondria, modifies potassium channel function, and increases reactive oxygen species production. The cytoplasm initially experiences reduced radical levels, likely due to temporary activation of antioxidant responses, and later returns to equilibrium. The process unfolds without significant loss of cellular viability at the tested dose.
The study’s significance lies in the integration of three capabilities within a single nanoscale platform: pH-triggered drug release, optical fluorescence for tracking, and quantum sensing for monitoring redox dynamics. Together these functions allow researchers to observe the chemical consequences of drug action precisely where they occur, using the same structure that delivered the compound.
The platform provides quantitative, spatially resolved data on how therapeutic molecules alter their microenvironment, an insight that conventional bulk assays cannot offer.
Several technical challenges remain before such systems can be applied beyond cell cultures. The enlarged particle size and less negative surface charge may influence circulation and tissue distribution in living organisms. The formulation becomes less stable at high concentrations, which could limit dosage. The sensing method still relies on confocal microscopy and precise alignment to monitor individual particles, making its extension to thicker tissues or animal models nontrivial. Addressing these issues will be necessary before the approach can be tested in vivo.
Despite these limitations, this study demonstrates a clear proof of concept. By combining a controlled delivery mechanism with built-in quantum sensing, fluorescent nanodiamonds provide a way to monitor chemical stress at the exact site of drug action inside living cells. The approach brings experimental drug studies closer to direct observation of intracellular dynamics, offering a foundation for therapies guided not only by where drugs go but by what they do once they arrive.
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