Borophene Quantum Dots with Strong Photoluminescence for Selective Metal Ion Sensing

Borophene is a relatively unexplored two-dimensional (2D) layered material known for its high carrier mobility and robustness. Being the lightest 2D layered material, borophene quantum dots (BQDs) with small lateral dimensions have triggered a surge of interest in the material research community due to their distinct electronic and optical properties. In this work, we have carried out a contamination-free liquid-phase exfoliation of crystalline boron chunks to synthesize ultrasmall crystalline BQDs of sizes ∼4.1 and ∼9.2 nm. The BQDs are few-layered in nature, as confirmed by atomic force microscopy. The as-synthesized BQDs exhibit extraordinary visible photoluminescence (PL) with a high PL quantum yield (∼40%), and the emission wavelength is independent of the excitation wavelength, unlike many other 2D quantum dots. The PL spectrum could be deconvoluted with four peaks, which are marginally size-dependent. The selective coordination of different metal ions (Fe 3+, Cu 2+, Ag +, Zn 2+, K +, Pb 2+, Mg 2+, Na +, Ni 2+, Hg 2+, Mn 2+, Ce 4+, and Fe 2+ ) with the BQDs has been studied systematically through PL and density functional theory (DFT) analyses. Interestingly, the BQDs show a high PL quenching ratio (>90%) and ultralow limit of detection (∼5 nM) through PL in the presence of Fe 3+ ions. The proposed sensor is successfully implemented, for the first time, for the sensitive detection of Fe 3+ ions at the nanomolar level, and the sensing mechanism, including selectivity, is explored in detail using DFT calculation and phenomenological modeling of the experimental data to account for the sensing quantitatively over a wide dynamic range. Our results indicate that BQDs hold great potential for developing highly sensitive gas, molecular, bio-, and optoelectronic sensors in the future.