PEG-Coated Carbon Nanotubes Slip Into the Rat Brain Cortex After a Simple Injection

For decades, the blood–brain barrier has stood as the body’s most formidable gatekeeper, a dense wall of tightly joined endothelial cells that shields the central nervous system from blood-borne threats while simultaneously blocking roughly ninety-eight percent of potential therapeutic molecules from reaching the brain tissue they might heal. Now, a team of Brazilian researchers has reported that single-walled carbon nanotubes coated with polyethylene glycol, injected directly into the bloodstream of rats, can breach this barrier and accumulate in a specific region of the brain — the cerebral cortex — without triggering any detectable acute toxicity. The findings, published in Frontiers in Carbon, represent one of the clearest demonstrations to date that PEGylated carbon nanotubes can cross the blood–brain barrier following systemic administration and lodge themselves in defined brain parenchyma.

The research, led by Gisele Eva Bruch and Daniela M. Barros at the Federal University of Rio Grande, alongside collaborators at the Federal University of Minas Gerais and the Nuclear Technology Development Center, set out to answer two intertwined questions: where do intravenously delivered PEGylated single-walled carbon nanotubes (SWCNT-PEG) actually go in the body, and do they cause harm along the way? To answer the first question, the team turned to Raman spectroscopy, a technique exquisitely sensitive to the unique vibrational fingerprint of carbon nanotubes. To answer the second, they deployed a battery of biochemical, histological, hematological, and behavioral assays.

The choice of Raman spectroscopy was no accident. Single-walled carbon nanotubes possess a characteristic Raman spectrum dominated by the radial breathing mode near 180 wavenumbers and the tangential graphite-like phonon mode, or G band, near 1,600 wavenumbers. By exciting the samples with a 1,064-nanometer laser — an energy below the self-fluorescence range of biological tissue — the researchers could obtain background-free spectra from lyophilized tissue samples, allowing them to detect nanotube signatures with remarkable specificity. The excitation energy sat in resonance with the second optical transitions of semiconducting nanotubes, further amplifying the signal.

Twenty-four hours after injection, the Raman signatures of SWCNT-PEG appeared in the liver, bone, blood, cerebral cortex, lung, and spleen of the treated rats. Notably, the nanotubes were not detected in other brain structures examined, suggesting a region-specific pattern of accumulation that the authors describe as unprecedented. While a considerable portion of the administered material was sequestered by the liver and spleen — organs known for clearing particulate matter from circulation — a measurable fraction had evidently traversed the blood–brain barrier and embedded itself within the cortical parenchyma.

The toxicological picture was equally striking. Biochemical markers of liver and kidney function, including creatinine, alanine aminotransferase, and aspartate aminotransferase, all remained within normal limits across all treatment groups. Histological examination of the brain, lung, spleen, and kidney revealed no evidence of extensive cell injury, even in tissues where nanotubes were clearly present. Necropsy findings and overall health indicators such as body weight and activity levels showed no abnormalities.

Hematological analysis, however, told a more nuanced story. Rats receiving the highest dose of SWCNT-PEG — 1.25 milligrams per kilogram — exhibited a significant decrease in platelet count, dropping from approximately 703,000 per microliter in controls to roughly 373,000 per microliter. The same high-dose group also showed monocytosis and eosinophilia, elevations in monocytes and eosinophils that the researchers interpret as markers of an acute inflammatory response. The team suggests that these changes likely reflect platelet activation triggered by contact between blood cells and the nanotubes, possibly mediated by complement system activation — a known phenomenon even for PEG-coated nanomaterials. Uncoated patches on the nanotube surface, they propose, may have been rapidly enveloped by a protein corona composed of naturally circulating blood proteins, which in turn recruited the immune system’s first-line cellular defenders.

Perhaps the most intriguing result emerged from behavioral testing. Using the Morris water maze — a gold-standard assay for spatial learning and memory in rodents — the researchers trained rats for five days to locate a submerged platform, then injected them with SWCNT-PEG or saline after the final training session. When tested twenty-four hours later, the treated animals showed no impairment whatsoever in their ability to recall the platform’s location. In fact, rats receiving the two lowest doses spent significantly more time in the target quadrant during the test session, indicating enhanced memory evocation rather than any deficit. The authors caution that the mechanism underlying this improvement remains unknown, and that the lowest dose — 0.05 milligrams per kilogram — produced no detectable Raman signal in the cortex, possibly because the concentration fell below the detection limit of the technique.

The physical characterization of the nanotube dispersion offers important context for interpreting these results. Transmission electron microscopy revealed that the commercial SWCNT-PEG sample used in the study formed bundles of two to five tubes, occasionally connected by polymeric masses that gave rise to larger aggregates. Even prolonged bath sonication could not fully disperse them. Zeta potential measurements of approximately negative 60 millivolts indicated the presence of numerous unreacted carboxylic acid groups on the nanotube surface, evidence that the PEGylation was incomplete and unstable. The researchers attribute this to steric hindrance during the functionalization reaction and to the relatively short length of the PEG chains employed — molecular weight 600 daltons — which were insufficient to completely cloak the nanotube surface.

This incomplete coverage has consequences. Hydrophobic regions and free carboxyl groups on the nanotube surface can interact promiscuously with biological molecules, forming protein coronas that alter biodistribution and immune recognition. Prior work by Liu and colleagues demonstrated that longer PEG chains and more complete surface coverage extend blood circulation time and reduce uptake by the liver and spleen. The Brazilian team’s findings are consistent with this framework: the substantial hepatic and splenic accumulation they observed likely reflects opsonization and scavenging by the mononuclear phagocyte system, the body’s standard disposal route for foreign particulates.

Despite these caveats, the study’s central finding — that systemically administered PEGylated carbon nanotubes can reach the brain cortex without acute toxicity — represents a meaningful step forward for the field of nanoneuroscience. The blood–brain barrier remains the single greatest obstacle to treating Alzheimer’s disease, Parkinson’s disease, brain tumors, and stroke, conditions in which even the most potent pharmacological agents fail to reach their targets in therapeutically meaningful concentrations. If carbon nanotubes can be engineered to cross this barrier reliably and safely, they could serve as versatile platforms for drug delivery, biosensing, and even direct neural interfacing. The Brazilian team emphasizes that their results are preliminary, based on a single twenty-four-hour time point and a limited dose range. Long-term studies examining chronic exposure, biodegradation, clearance kinetics, and behavioral outcomes across extended timeframes will be essential before any clinical translation. Nevertheless, the demonstration that these cylindrical carbon lattices — each one tens of thousands of times thinner than a human hair — can navigate the bloodstream, slip past one of biology’s most selective barriers, and settle into the brain’s outermost layer without leaving a trace of damage opens a tantalizing window onto the future of neurological medicine.

Subject of Research: Biodistribution and safety of PEGylated single-walled carbon nanotubes crossing the blood–brain barrier in rats

Article Title: Biodistribution of intravenously delivered PEGylated carbon nanotubes to the rat brain cortex

Article References: Bruch, G. E., Dal Bosco, L., Cordeiro, A. P., Cordeiro, M. F., Sahoo, S. K., Peixoto, C., Klosterhoff, M. C., Romano, L. A., Fantini, C., Santos, A. P., & Barros, D. M. (2025). Biodistribution of intravenously delivered PEGylated carbon nanotubes to the rat brain cortex. Frontiers in Carbon, 3, Article 1363919. https://doi.org/10.3389/frcrb.2024.1363919

Image Credits: AI Generated

DOI: 10.3389/frcrb.2024.1363919

Keywords: carbon nanotubes, PEGylation, blood-brain barrier, Raman spectroscopy, biodistribution, nanomedicine, neuroscience, Morris water maze, drug delivery, nanotoxicology, cerebral cortex, single-walled carbon nanotubes