A ratiometric nanosensor based on fluorescent carbon dots for label-free and highly selective recognition of DNA

A ratiometric nanosensor for label-free and highly selective recognition of DNA was reported in this work, by employing fluorescent carbon dots (CDs) as the reference fluorophore and ethidium bromide (EB), a specific organic fluorescent dye toward DNA, playing the role of both a specific recognition element and response signal. Fluorescent CDs were synthesized through a microwave irradiation technique. When EB was present, the fluorescence of CDs was quenched effectively due to the electron transfer process between CDs and EB, while the fluorescence of EB was increased partially without fluorescence resonance energy transfer under same excitation wavelength. Upon the addition of DNA, the fluorescence of EB was enhanced dramatically but the fluorescence intensity of CDs stayed almost constant, leading to a ratiometric detection of DNA. This fluorescent nanosensor exhibited good sensitivity, a broad dynamic linear range of 1.0 mu M-100 mu M, and low detection limit down to 0.47 mu M. The relative standard deviation for 30 mu M DNA was 0.2% (n = 5). The present ratiometric nanosensor also showed high accuracy and excellent selectivity for DNA over some chemical substances, such as amino acids, nucleotides, proteins, and RNA. The proposed method was applied to the determination of DNA in synthetic samples with satisfactory results. The proposed DNA detection method was quite simple, rapid and convenient due to the elimination of the modification and separation procedures. The possible fluorescence quenching mechanism was further investigated.