Atomic defects in nanodiamonds enable both precise heating of lysosomes inside macrophages and nanoscale temperature measurement, revealing that localized thermal spikes trigger immune cell reprogramming.
(Nanowerk Spotlight) The human body maintains a core temperature of about 37 °C, but the interiors of individual cells tell a different story. Some studies suggest that mitochondria, the organelles that generate cellular energy, can run as hot as 15 °C above the surrounding cytoplasm. This observation, confirmed through increasingly sophisticated measurement techniques, has raised a provocative question: do cells use localized heat as a signaling mechanism, much as they use chemical messengers?
The idea is not entirely new. Fever, after all, is a whole-body thermal response that enhances immune function. Clinical hyperthermia, in which tumors are heated to 40–43 °C, has been used for decades to sensitize cancerous tissue to radiation and chemotherapy. Thermal ablation, which destroys tissue at temperatures above 50 °C, is a standard tool in surgical oncology.
Yet these approaches operate at the scale of tissues and organs. What happens when heat is delivered to a single organelle inside a single cell has remained largely mysterious, partly because the tools to both generate and measure such localized temperature changes did not exist.
Conventional temperature probes for biological systems rely on fluorescent dyes whose emission intensity changes with heat. These molecular thermometers suffer from photobleaching, meaning they degrade under the very light used to read them. They also respond to environmental factors other than temperature, such as pH and ion concentration, making their readings ambiguous. And because many of these probes absorb light in the same spectral region as photothermal agents, using them to monitor heating while simultaneously generating that heat has been impractical.
A study reported in the journal Advanced Materials (“Single‐Cell Hyperthermia: Diamond Quantum Thermometry Reveals Thermal Control of Macrophage Polarization”) introduces a dual-function nanodiamond platform that sidesteps these limitations. The platform combines quantum sensing with photothermal heating in a single nanoparticle. This enables researchers to both raise the temperature inside specific cellular compartments and measure that temperature change with sub-degree precision.
The work, conducted by an international team based primarily at the Max Planck Institute for Polymer Research in Mainz and Ulm University, applies this system to macrophages, a type of immune cell central to inflammation and tissue repair. The results suggest that localized heating of lysosomes, the digestive compartments inside cells, can trigger a cascade of events that reprograms macrophages into a pro-inflammatory state, independent of the classical heat-shock response.
This schematic illustrates how croconium-coated fluorescent nanodiamonds function as both nanoscale heaters and quantum thermometers inside macrophages. (a) The nanodiamond core contains nitrogen-vacancy centers, atomic defects that enable two types of quantum sensing: temperature measurement through a technique called optically detected magnetic resonance, which detects shifts in electron spin resonance frequencies, and radical detection through spin relaxation dynamics between quantum states. The outer shell consists of a nanogel coating embedded with croconium dye molecules and polyethylene glycol chains, which absorb near-infrared light at 810 nm and convert it into localized heat. (b) When macrophages take up these particles, they accumulate in endo-lysosomal compartments, the cell’s digestive vesicles. Upon near-infrared irradiation, the croconium dyes generate heat within these compartments, triggering oxidative stress and lysosomal membrane damage. This activates the tumor necrosis factor signaling pathway, including key regulators such as Tpl2 and PI3K, which in turn drive expression of pro-inflammatory mediators including interleukins and chemokines. (c) The cumulative effect is polarization of the macrophage from a resting M0 state toward a pro-inflammatory M1 phenotype. A temperature rise of approximately 23–26 °C triggers this reprogramming, while smaller increases below 15 °C do not. Classical heat-shock proteins remain unchanged throughout, indicating that the thermal effect stays confined to the targeted subcellular compartment rather than stressing the entire cell. (Image: Reproduced from DOI:10.1002/adma.202517076, CC BY) (click on image to enlarge)
The core of the technology is a fluorescent nanodiamond containing nitrogen-vacancy centers, atomic-scale defects in the diamond crystal lattice where a nitrogen atom sits adjacent to an empty lattice site. These defects behave as quantum sensors. When illuminated with green laser light, they fluoresce red.
Crucially, the frequency at which their electron spins resonate with an applied microwave field shifts predictably with temperature. This phenomenon, called optically detected magnetic resonance, allows researchers to infer temperature changes at the nanometer scale without the degradation problems that plague fluorescent dyes.
To turn the sensor into a heater, the team coated the nanodiamonds with croconium dyes. These organic molecules absorb strongly in the near-infrared and convert that light into heat with high efficiency, outperforming conventional agents such as indocyanine green in both photothermal conversion and photostability.
The resulting hybrid particles measure roughly 55 nm in diameter and localize to endo-lysosomal compartments after being taken up by macrophages. When irradiated with 810 nm light at a power density of 0.6 W cm⁻², individual particles raised local temperatures by 27.5 ± 6.9 °C within 15 min on a glass substrate, and by 24.6 ± 3.9 °C inside living cells.
Beyond temperature, the nitrogen-vacancy centers can detect paramagnetic species, molecules with unpaired electrons such as superoxide and nitric oxide radicals. This capability relies on measuring how quickly the electron spins in the defect relax to equilibrium, a property called longitudinal relaxation time. Radicals accelerate this process.
After the researchers heated the lysosomes, relaxation times dropped significantly, indicating a surge in reactive oxygen species. A conventional assay using a fluorescent dye confirmed the result.
The transcriptional consequences proved striking. Whole-transcriptome analysis of the heated macrophages revealed that classical heat-shock proteins did not increase. This finding supports the notion that only the lysosomal compartment, not the entire cytoplasm, experienced elevated temperatures.
Instead, the cells activated genes associated with vesicular repair, membrane trafficking, and oxidative stress responses. Pathway analysis pointed to activation of tumor necrosis factor-alpha signaling, a key driver of inflammation.
These gene expression changes translated directly into altered cell behavior. The heated macrophages shifted toward the M1 state, a pro-inflammatory phenotype characterized by increased display of CD80 and CD86, surface proteins that indicate immune activation. Flow cytometry confirmed a statistically significant increase in these markers after heating at 0.6 W cm⁻².
Reducing the laser power by half limited the temperature rise to about 11 °C and produced no significant polarization. This result suggests a thermal threshold for the effect.
When the researchers scavenged reactive oxygen species with N-acetyl-L-cysteine, the increase in CD80 and CD86 was partly attenuated. This indicates that oxidative stress plays a critical role in the polarization process, though additional temperature-sensitive mechanisms likely contribute.
Experiments on primary bone marrow-derived macrophages showed a similar trend but with greater variability. Under identical irradiation conditions, these cells achieved temperature rises 8–12 °C lower than the immortalized cell line. Only two of five independent experiments showed pronounced polarization, underscoring the need for optimized photothermal coupling in physiologically relevant systems and eventual in vivo applications.
The significance of this work lies in its demonstration that intracellular temperature can be both precisely controlled and linked to specific cellular outcomes. The platform provides a method to study how heat, delivered to defined subcellular locations, influences signaling and gene expression.
It also enables therapeutic strategies that exploit localized hyperthermia to modulate immune responses without raising whole-body temperature. The ability to polarize macrophages toward a pro-inflammatory state has potential relevance in cancer immunotherapy, where reprogramming tumor-associated macrophages remains an active area of research.
The researchers envision extending the platform to include additional sensing modalities, such as detection of paramagnetic ions and pH changes, for multiplexed mapping of subcellular environments. Portable quantum diamond microscopes, now entering the commercial market from companies including Qnami and QDT, are already making such measurements more accessible. Nanodiamonds equipped with nitrogen-vacancy centers may soon become standard tools for probing the thermal and chemical landscape inside living cells.
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Tanja Weil (Max Planck Institute for Polymer Research)
, 0000-0002-5906-7205 corresponding author
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