Reversible Photoswitching of Carbon Dots

Abstract

We present a method of reversible photoswitching in carbon nanodots with red emission. A mechanism of electron transfer is proposed. The cationic dark state, formed by the exposure of red light, is revived back to the bright state with the very short exposure of blue light. Additionally, the natural on-off state of carbon dot fluorescence was tuned using an electron acceptor molecule. Our observation can make the carbon dots as an excellent candidate for the super-resolution imaging of nanoscale biomolecules within the cell.

Figure 1. Photoswitching of carbon dots

(a) The on-off switching observed by alternating 401 nm and 639 nm laser excitation. (b) Photon counts in a hybrid photomultiplier detector show photo-decay and subsequent gain in intensity (photoswitch) using 401 laser. The blue arrow shows the 401 nm pulses (detected in channel 1) and the red bars show the photo-decay with 639 nm Laser (detected in channel 2). The 4th and 6th cycle, with a slightly higher exposure time of blue light, results a better recovery of fluorescence.

Figure 2. Single molecule time trace showing

(a) radical blinking in presence of an electron acceptor (methyl viologen) and (c) single step bleaching in presence of an electron donor (ascorbic acid). (b) A Jablonski diagram and (d) an energy barrier diagram of the favored dark state formation in the red emissive carbon dots. Chemically lowering the energy barrier by an electron acceptor molecule favors the photoswitching, while an electron donor molecule retards the process.

Figure 3. Characteristic single molecule time traces as recorded by EMCCD detection unit.

Figure (a) and (c) represents the fluorescence trace in presence of 5 mM methyl viologen and 5 mM ascorbic acid respectively. (b) represents fluorescence trace without any electron donor/acceptor. Brightness decreases in presence of methyl viologen and increases in the presence of ascorbic acid.

Figure 4. Electron transfer from the neighboring high energy traps can efficiently switch the chromophore back from the dark radical state to a bright fluorescent state.

The high energy non-emissive traps which can effectively absorb the higher frequency of light might have a role at the core of photoswitching process.

Figure 5. Fluorescence recovery after photobleaching within a self-assembled monolayer of carbon dots.

(a) Three different lasers (401, 488 and 639 nm) were focused on a point region of interest to study comparative photobleaching. (b) The blue emission decays slowly and recovers fast, while the red emission decays fast and recovers slowly.