Neurotransmitter levels in the brain can indicate brain health and neurodegenerative diseases like Alzheimer's. However, the protective blood-brain barrier (BBB) makes delivering fluorescent sensors that can detect these small molecules to the brain difficult. Now, researchers demonstrate a way of packaging these sensors for easy passage across the BBB in mice, allowing for improved brain imaging. With further development, the technology could help advance Alzheimer's disease diagnosis and treatment.