Determination of Fluorogen-Labeled Neurotransmitters at the Zeptomole Level Using Two-Photon Excited Fluorescence with Capillary Electrophoresis

Abstract

Two-photon excited (2PE) fluorescence detection is demonstrated to be a highly sensitive means for analyzing fluorogen-labeled neurotransmitters fractionated in submicrometer capillary electrophoresis channels. In this approach, fluorescamine-labeled neurotransmitters that have been electrophoretically separated in 620-nm-i.d. capillaries intersect the focused output from a near-infrared mode-locked titanium−sapphire laser positioned at the capillary outlet. Extremely high peak laser intensities (∼1011−1012 W cm-2) facilitate the nearly simultaneous absorption of two near-IR photons (λex ≈ 780 nm) to excite fluorescamine derivatives ordinarily excited with a single, near-ultraviolet photon (λex ≈ 390 nm). Rapid cycling of analytes through the fluorescent excited state and low background from scatter and out-of-sample luminescence combine to make 2PE fluorescence a highly sensitive approach for detecting minute quantities of neurotransmitters. In these studies, mixtures of the fluorescamine derivatives of ...