Carbon Dots Detect Water-to-Ice Phase Transition and Act as Alcohol Sensors via Fluorescence Turn-Off/On Mechanism

Abstract

Highly fluorescent carbon nanoparticles called carbon dots (CDs) have been the focus of intense research due to their simple chemical synthesis, nontoxic nature, and broad application potential including optoelectronics, photocatalysis, biomedicine, and energy-related technologies. Although a detailed elucidation of the mechanism of their photoluminescence (PL) remains an unmet challenge, the CDs exhibit robust, reproducible, and environment-sensitive PL signals, enabling us to monitor selected chemical phenomena including phase transitions or detection of ultralow concentrations of molecular species in solution. Herein, we report the PL turn-off/on behavior of aqueous CDs allowing the reversible monitoring of the water–ice phase transition. The bright PL attributable to molecular fluorophores present on the CD surface was quenched by changing the liquid aqueous environment to solid phase (ice). Based on light-induced electron paramagnetic resonance (LEPR) measurements and density functional theory (DFT) calculations, the proposed kinetic model assuming the presence of charge-separated trap states rationalized the observed sensitivity of PL lifetimes to the environment. Importantly, the PL quenching induced by freezing could be suppressed by adding a small amount of alcohols. This was attributed to a high tendency of alcohol to increase its concentration at the CD/solvent interface, as revealed by all-atom molecular dynamics simulations. Based on this behavior, a fluorescence “turn-on” alcohol sensor for exhaled breath condensate (EBC) analysis has been developed. This provided an easy method to detect alcohols among other common interferents in EBC with a low detection limit (100 ppm), which has a potential to become an inexpensive and noninvasive clinically useful diagnostic tool for early stage lung cancer screening.

Aggregation and luminescence in carbonized polymer dots

“…But in the solid phase (ice), its emission was severely quenched, so that phase transition can be monitored. [ 108 ]…”

“…But in the solid phase (ice), its emission was severely quenched, so that phase transition can be monitored. [108] Similarly, in the conjugated π-domain produced from carbonization, there also exists severe quenching in the solid state due to their polycyclic aromatic structure. Accordingly, controlling the degree of carbonization can achieve solid luminescence.…”

Ionic Liquid‐Assisted Fast Synthesis of Carbon Dots with Strong Fluorescence and Their Tunable Multicolor Emission

“…[33] The strong absorption at 575 nm of R-CDs may be assigned for CN absorption. [52,53] Photographs of B-CDs, G-CDs, Y-CDs, and R-CDs placed under natural light and under 365 nm UV light, are shown in Figure 5D. The carbon dot solution shows an apparent fluorescence (from blue to red emission).…”

Carbon dots for virus detection and therapy

“…As we have recently shown, CDs are already used in a number of areas including optical applications (sensors [45], anticounterfeiting [46]), energy applications (light-emitting diodes, photovoltaics, supercapacitors), and catalytic applications [47,48]. Moreover, CDs have attracted a tremendous interest in biomedicine, particularly in bioimaging (one-photon and two-photon imaging, live cell imaging, and tracking [49][50][51][52][53] and microbial [54] and plant tissue imaging), as well as in therapeutic applications including phototherapy, photodynamic therapy, and drug or gene delivery [20].…”