PersL's luminous nanoparticles will make it easier to track the route of drugs in the body

Instead of glowing only under the lamp, PersL nanoparticles glow long after the lamp is turned off—in infrared light, which penetrates tissue better. Research by physicists from Wrocław shows that their signal remains readable when in contact with blood proteins, and a carefully selected surface coating allows for safer drug tracking.

Bioimaging involves taking pictures of cells and tissues in the laboratory and in preclinical studies. It is used to observe where drugs are being delivered, how cells are behaving, and whether inflammation is occurring. Most often, luminous "labels" (probes) are used, which are attached to molecules or cells. There are two challenges: the tissues themselves glow slightly (autofluorescence), and they also scatter light, causing the image to lose contrast. Ideally, we would like the marker ("label") to produce a strong, clear signal and act predictably in the presence of blood proteins, as these are the first proteins it comes into contact with.

This is where the PersL phenomenon comes into play – the glow of nanoparticles (ZnGa2O4:Cr3+), which, like fluorescent stickers, persists after the backlight is turned off. First, the marker is charged with light, and then the image is collected in the dark, when the tissue background is minimal. Substances that produce this radiation in the so-called biological window, i.e., infrared (approx. 700–950 nm), have an additional advantage: such light passes more easily through tissue. One of the most promising PersL materials are chromium-doped ZnGa2O4 nanoparticles (ZGO:Cr3+), which glow in this region and are also very chemically stable. They can also be read by re-exciting the tissue with weak light (optically stimulated luminescence), which extends the marker's lifespan.

The study's authors, from the Institute of Low Temperature and Structural Research in Wrocław, in collaboration with teams from France and Belgium, examined how ZGO:Cr3+ behaves when exposed to albumin, the most common blood protein (the model used was bovine albumin, BSA). They prepared three versions of the nanoparticles: as synthesized, after firing at 650°C (providing a more ordered structure), and with an oleic acid coating added to the surface (providing a strong negative charge and improved dispersion in water). All had diameters of approximately 10–20 nm, and under a microscope, the version with the coating had a visible "halo" (glow) around the particles.

When illuminated with violet (405 nm), ZGO:Cr3+ nanoparticles glowed with typical chromium lines—around 685/694/707 nm. After the material was baked, these lines were more clearly separated, meaning the signal became purer. Importantly, the glow persisted even when mixed with albumin (a model of a blood protein), which supports high-contrast clinical images. The protein itself did not release energy to the nanoparticles noticeably, as its average illumination time changed minimally—from approximately 6.05 to 6.35 ns.

The effect on albumin shape distortion was assessed using Raman spectroscopy, examining the spectrum sensitive to the number of "alpha helices"—ordered spirals in the protein (the so-called amide I band). After burning, the particles slightly increased the proportion of these helices, indicating a slight stabilization of the protein. Particles with an oleic acid shell decreased the proportion of helices, indicating partial unfolding. The conclusion is simple: the strength of the effect is determined by the surface chemistry—the type of coating and its charge.

In practice, the albumin-particle mixture turned cloudy after baking and remained cloudy, indicating larger aggregations of these particles and potentially problematic. The coated particles, on the other hand, formed a stable suspension (a good thing), but significantly modified the protein's shape (a bad thing). This is a classic compromise between stability in water and gentleness towards proteins.

According to an article published by the study authors in the Journal of Molecular Structure (doi: 10.1016/j.molstruc.2025.144081), the project is in the pre-implementation phase. So far, pure water and model proteins have been used, and measurement accuracy could only be ensured in laboratory conditions. However, we have a clear indication that ZGO:Cr3+ provides a stable infrared signal even in the presence of proteins. The particle surface must be selected to limit changes in their structure (appropriate coating and charge). Next steps include tests with other serum proteins, monitoring the natural "protein corona" that forms on the particles, and studies in more complex biological systems.

From an application perspective, this could result in clearer diagnostic images (less background light), shorter sample exposure times, more accurate tracking of drug carriers, and in the long run, better treatment planning and faster detection of disease lesions. (PAP)

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