Cells utilize different ions to support various physiological activities. Ca²⁺ participates in neurotransmitters release and muscle contraction. Na⁺ concentration is the highest in the body fluid; thus plays an important role in regulating the blood pressure and maintaining the balance of blood circulation. The increase in the serum Na⁺ concentration elevates the blood pressure. In conjugation with K⁺, Na⁺ helps the transmission of nerve impulses. K⁺ is the most abundant ion inside the cell. K⁺ regulates many cell activities like membrane permeability, growth, etc; most importantly, K⁺ determines the resting membrane potential to modulate the neuron excitability. We are interested in understanding the concentrations of Na⁺ and K⁺ at both sides of the plasma membrane. We first used the Na⁺ specific fluorescence indicators to investigate the change of intracellular Na⁺ concentration ([Na⁺]_i). The basal [Na⁺]_i was 16.5 ± 1.6 mM and elevated by 1.5 fold in cultured neurons stimulated by 10 µM glutamate. To investigate the K⁺ efflux at the plasma surface, we modified a K⁺-aptamer on the silicon nanowire field-effect transistor. The data shows the functionalized device responded to different alkali ions with various affinities; K⁺ (K_d = 7.9 ± 0.4 mM) > Cs⁺ (K_d = 12.1 ± 0.5 mM) > Na⁺ (K_d = 24.8 ± 4.1 mM) > Li⁺ (K_d = 189.0 ± 44.5 mM). We will anchor cell onto the silicon nanowire to investigate the K⁺ concentration at the membrane surface. These measurements of the ion concentration at the micro-environment will provide detail information in understanding the neuron-neuron interactions.