Quantitative probing of Cu2+ ions naturally present in single living cells is realized by developing a quantum-dot-embedded nanowire-waveguide probe. The intracellular Cu2+ ion concentration is quantified by direct monitoring of photo luminescence quenching during the insertion of the nanowire in a living neuron. The measured intracellular Cu2+ ion concentration is 3.34 +/- 1.04 x 10(-6) M (mean +/- s.e.m.) in single hippocampal neurons.