An intracellular monitoring device (IMD) that fits completely inside a living cell, and causes no significant impairment, to a cell's normal biological processes. The IMD monitors a cell for its level of a biological substance (e.g., calcium ion concentration) of interest. If the biological substance reaches or exceeds a threshold, the IMD transmits an electromagnetic signal, received by an antenna outside the cell. Each IMD has its electromagnetic signal encoded with a unique frequency. Detection of the frequency components, in the signals received by an antenna, permits identification of the source IMD's. A high calcium ion concentration is indicative of a strongly-activated cerebral cortex neuron. Brain tissue is relatively transparent to near infrared, making it a good frequency band, for the electromagnetic signals from neuron-monitoring IMD's. The near infrared of each IMD can be produced by quantum dots, powered by bioelectric catalysis triggered by high calcium ion concentration.