Dual-excitation red-emissive carbon dots excited by ultraviolet light for the mitochondria-targetable imaging and monitoring of biological process in living cells

The bifunctional carbon dots (CA-p-CDs) with truly red-emission under short-wavelength light excitation (such as ultraviolet light, blue light or cyan light) were easily and successfully fabricated by attaching citric acid (CA) molecules onto the pristine p-CDs using an ultrasonic-triggered surface re-engineering approach under room conditions. Especially, the CA-p-CDs show truly red emission and excitation-wavelength-independent properties when excited by ultraviolet-light with a wavelength in the range of 360–400 nm or excited by progressively longer excitation wavelengths in the range of 440–500 nm. The CA-p-CDs were verified to be very low cytotoxicity. Importantly, the CA-p-CDs were further used for the targeted mitochondria imaging or direct evaluation on the intracellular biological process in living cells through variable PL emission colors with the gradually depletion of attached CA molecules. Furthermore, the strongest PL peaks of CA-p-CDs showed fluorescence enhancement response toward Cr3+ ions. The CA-p-CDs were also used for the highly sensitive and selective detection of Cr3+ ions in tap water with a limit of detection as low as 12 nM. This work presented designable fabrication of bifunctional CDs by linking molecules as an excellent nanoprobes for the mitochondria-targetable imaging and chemical sensing of heavy metal ions in water.