The development of novel carbon nanodots (CDs) with excellent optical properties and promising applications is an attractive but a challenging area. Herein, we demonstrate that the fluorescence properties of CDs can be optimized by simple modification of the molecular structure of the carbon source. Significantly, the final optimized CDs 1 exhibit a rare extremely acidic (pH 1−3) sensitivity. A distinguishable red/brown/green fluorescence color change is achieved for visual readout by simply tuning the pH to 1, 2, or 3. The distinguishable color change combined with ratiometric fluorescence changes greatly improves the naked-eye visual resolution up to 0.2 pH units, which is higher than that of commercial precision pH test strips. Furthermore, a smartphone was employed in a portable, low-cost, rapid, and precise pH measurement method, which satisfied the requirement of point-of-care testing. Inspired by the reversible color change of red/brown/green tri-fluorescence on tuning the pH to 1, 2, or 3, a simple yet efficient molecular logic circuit has been designed. Given the unique pH-dependent photoluminescence properties, these CDs have been further applied in the anti-counterfeiting and information encryption field, making the "burn after reading" encryption technology a reality.

Three 4- (benzo[d]thiazol-2-yl)-2,5-dihydroxybenzaldehyde fluorophores were introduced to construct a tripodal multifunctional ESPIT fluorescence probe L. The fluorescent analysis revealed that probe L exhibited excellent recognition capability towards Cr 3+ , Al 3+ , Zn 2+ and Fions with large Stokes shift. Furthermore, under the optimal conditions, the detection limit of probe L towards Cr 3+ , Al 3+ , Zn 2+ and Fwere low, of the order of 10 -8 M, which indicated that probe L was sensitive to these four ions. Interestingly, the fluorescent and 1 H NMR titration experiments revealed that the recognition mechanism of probe L towards the ions Cr 3+ , Al 3+ , Zn 2+ and Fwere different. The presence of Cr 3+ and Al 3+ recovered the ESIPT, but the presence of Zn 2+ trigger a moderate deprotonation of the phenolic OH and induced an ESIPT red shifted (60nm) emission wavelength. Finally, the presence of Fcompletely deprotonated the free phenolic OH and a remarkable red shifted (130 nm) ESPIT emission was observed. In other words, the ESIPT process of probe L is controllable. Furthermore, the utility of probe L as a biosensor in living cells (PC3 cells) towards Cr 3+ , Al 3+ and Zn 2+ ions has been demonstrated.

The tripodal probe L was readily prepared via introducing rhodamine and azobenzene groups in a two-step procedure. Studies of the recognition properties indicated that probe L exhibited high sensitivity and selectivity towards F -, AcOand H2PO4through a ratiometric colorimetric response with low detection limits of the order of 10 -7 M. The complexation behaviour was fully investigated by spectral titration, 1 H NMR spectroscopic titration and mass spectrometry. Probe L not only recognizes F -, AcOand H2PO4 -, but can also distinguish between these three anions via the different ratiometric behaviour in their UV-vis spectra (387/505 nm for L-H2PO4 -, 387/530 nm for L-AcOand 387/575 nm for L-Fcomplex) or via different colour changes (light coral for L-H2PO4 -, light pink for L-AcOand violet for the L-Fcomplex). Additionally, given the presence of NH and OH groups in probe L, which can be protonated and deprotonated, probe L further exhibited an excellent pH response over a wide pH range (pH 3 to pH 12).

We have strategically incorporated three different fluorophores at tren to construct a multi-energy donor/acceptor "smart" probe L. This probe operates by using three-dimensional scales (response time, wavelength and fluorescence intensity) which allows for the selective recognition and discrimination of the Cu, Hg, Fe and F ions.