Amplified fluorescence of Mg 2+ selective red-light emitting carbon dot in water and direct evaluation of creatine kinas
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BSTRACT Carbon quantum dot/carbon dot (CD) exhibiting sustained photoluminescence at longer wavelengths in aqueous solution is difficult to prepare, but has enormous potential in biomedical applications. For the first time, we report the magnesium(II) selective fluorescence enhancement of a red-light emitting anthrarufin and boric acid-derived CD in aqueous solution for direct evaluation of creatine kinase (CK) enzyme activity. The CD displayed visually detectable, intense red fluorescence only in the presence of magnesium ion (Mg2+) at physiological pH value when irradiated with an ultraviolet (UV) source. Concurrently, a significant increase in steady-state fluorescence intensity and fluorescence lifetime was documented. A time-dependent density functional theory (TD-DFT) analysis displayed a bathochromic shift in UV–visible (vis) absorption, and increased oscillator strength of transition resulting from the selective chelation of Mg2+ with β-hydroxy keto functionality on the surface of the CD. The CD–Mg2+ assembly was subsequently used to conceptualize the detection of CK directly through the exploration of the differential binding affinity of Mg2+ with adenosine triphosphate (ATP), adenosine diphophate (ADP), and CD that is otherwise not possible with commercially available kits as of today. Thus, the report delineated here usher grandeur potential of CD for biological explorations related to Mg2+ or ATP sensing and monitoring of Mg2+-dependent enzymatic activity through a clear understanding of the chemistry.
KEYWORDS carbon dot (CD), Mg2+ indicator, fluorescence amplification, creatine kinase (CK), adenosine triphosphate (ATP)
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Introduction
Luminescent carbon quantum dots/carbon dots (CDs) are an emerging subset of nanoparticle that has attracted stupendous attention owing to its potential applications in energy harvesting, optoelectronics, metal ion, bio-sensing, drug/gene delivery, bioimaging, and others [1–3]. Particularly for metal sensing with CD, the quenching or recovery of the fluorescence of CD in the presence of the metal ions through a host of the possible mechanism is being reported [4, 5]. However, to the best of our knowledge, no report cites an abrupt fluorescence enhancement of CD in the presence of an undoped metal ion, that too, in the red region of the visible spectrum in aqueous media. Herein, we present anthrarufin, and boric acid-derived CD, whose red-emission intensity is highly amplified in the presence of magnesium ion (Mg2+) at physiologically relevant pH value, so as to become visually detectable. Concomitantly, this Mg2+-dependent emission of CD was studied from theoretical perspectives and also monitored for its response towards relevant enzymatic pathways that generate intriguing glimpses towards its potential biological applications. CDs have been derived from a plethora of carbonaceous sources and mostly emit in the blue to green region of the spectrum [6]. CDs with red fluorescence have also been reported to exhibit remarkable solvent dependent photoluminescence (PL) [7, 8]
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