Boron Precursor-Dependent Evolution of Differently Emitting Carbon Dots

Abstract

Attention has been directed toward electron-deficient boron doping in carbon dots (CDs) with the expectation of revealing new photophysical aspects in accordance with varying amounts of boron content. It has been emphatically shown that boron uptake in CDs varies with different boron precursors evolving altered emissive CDs. Boron doping in CDs causes definite surface defect due to the generation of electron-deficient states. Modified hydrothermal treatment of a mixture of ascorbic acid (AA) and different boron precursor compounds (borax/boric acid/sodium borate/sodium borohydride) produces different kinds of boron-doped CDs (BCDs). These BCDs (<6 nm) differ in size, emission maxima (∼15 nm), and fluorescence intensity but carry unchanged excitation maxima (365 nm). These differences are related to the nature of boron precursor compounds. The most fluorescing BCD (quantum yield ≈ 5%) is identified from the borax-mediated reaction and is used for the detection of Fe(III) on a nanomolar level in water via the fluorescence "Turn Off" phenomenon. Again, Fe(III)-infested CD solution regains its lost fluorescence, with AA paving the way for nanomolar level AA detection from the same pot. The proposed method has been tactfully made interference free for the quantitative measure of Fe(III) and AA in real samples. Furthermore, new photophysical properties of the CDs with variable boron contents supplement information that is hitherto unknown. Theoretical calculations also justify the observed optical behavior of the as-synthesized BCDs. The calculations describe the variable amount of boron doping-related huge charge polarization within the carbon surface, leading to the formation of surface defects. Thus, subsequent electronic transition-related red shift in the absorption spectrum authenticates experimental findings.