Non-aromatic single amino acid, lysine: A simple fluorescence sensor for selective detection of Cu2+ and Co2+ with colorimetric distinction by the virtue of its self-aggregation behaviour

Abstract

Non-aromatic amino acids are mostly considered as non-emissive due to their lack of conventional fluorophores. However, at high concentrations, they can undergo conformational rigidification via self-aggregation and emit in the visible range. Herein, we have exploited that very property of non-aromatic amino acids as a fluorescence sensor. To the best of our knowledge, this is the first report demonstrating a single unmodified non-aromatic amino acid, Lysine (Lys) as a selective and sensitive fluorescence sensor for the detection of Cu2+ and Co2+ along with colorimetric distinction. An as-prepared aqueous solution of Lys (6.8 mM) served as a fluorescence sensor for Cu2+ and Co2+ with a detection limit of 420 nM and 460 nM, respectively in water. The instantaneous appearance of blue and pink color upon adding Cu2+ and Co2+, respectively to the Lys solution makes the system useful for their real-time analysis. The sensing of Cu2+ and Co2+ are also demonstrated through the fabrication of paper strips. Moreover, the sensor could detect Cu2+ and Co2+ in real water samples (lake water and tap water). The exceptionally low cost (<1 U.S. cent/10 mg Lys), reversibility of the sensor, and its ease of use uniquely qualify Lys as a remarkably simple fluorescent sensor.