A lipid droplet-targeted fluorescence probe for visualizing exogenous copper (II) based on LLCT and LMCT

Novel Fluorescence Probe toward Cu2+ Based on Fluorescein Derivatives and Its Bioimaging in Cells

Copper is an important trace element that plays a crucial role in various physiological and biochemical processes in the body. The level of copper content is significantly related to many diseases, so it is very important to establish effective and sensitive methods for copper detection in vitro and vivo. Copper-selective probes have attracted considerable interest in environmental testing and life-process research, but fewer investigations have focused on the luminescence mechanism and bioimaging for Cu²⁺ detection. In the current study, a novel fluorescein-based A5 fluorescence probe is synthesized and characterized, and the bioimaging performance of the probe is also tested. We observed that the A5 displayed extraordinary selectivity and sensitivity properties to Cu²⁺ in contrast to other cations in solution. The reaction between A5 and Cu²⁺ could accelerate the ring-opening process, resulting in a new band at 525 nm during a larger pH range. A good linearity between the fluorescence intensity and concentrations of Cu²⁺, ranging from 0.1 to 1.5 equivalent, was observed, and the limit detection of A5 to Cu²⁺ was 0.11 μM. In addition, the Job’s plot and mass spectrum showed that A5 complexed Cu²⁺ in a 1:1 manner. The apparent color change in the A5–Cu²⁺ complex under ultraviolet light at low molar concentrations revealed that A5 is a suitable probe for the detection of Cu²⁺. The biological test results show that the A5 probe has good biocompatibility and can be used for the cell imaging of Cu²⁺.

A Highly Sensitive and Selective Fluorescein-Based Cu2+ Probe and Its Bioimaging in Cell

Copper is a vital trace metal in human body, which plays the significant roles in amounts of physiological and pathological processes. The application of copper-selective probe has attracted great interests from environmental tests to life process research, yet a few of sensitive Cu²⁺ tests based on on-site analysis have been reported. In this paper, a novel fluorescein-based fluorescent probe N4 was designed, synthesized, and characterized, which exhibited high selectivity and sensitivity to Cu²⁺ comparing with other metal ions in ethanol–water (1/1, v/v) solution. The probe N4 bonded with Cu²⁺ to facilitate the ring-opening, and an obvious new band at 525 nm in the fluorescence spectroscopy appeared, which could be used for naked-eye detection of Cu²⁺ within a broad pH range of 6–9. Meanwhile, a good linearity between the fluorescence intensity and the concentrations of Cu²⁺ ranged 0.1–1.5 eq. was observed, and the limit of detection of N4 to Cu²⁺ was calculated to be as low as 1.20 μm. In addition, the interaction mode between N4 and Cu²⁺ was found to be 1:1 by the Job's plot and mass experiment. Biological experiments showed that the probe N4 exhibited low biological toxicity and could be applied for Cu²⁺ imaging in living cells. The significant color shift associated with the production of the N4-Cu²⁺ complex at low micromolar concentrations under UV light endows N4 with a promising probe for field testing of trace Cu²⁺ ions.

Triphenylamine Schiff base as a lipid droplet-targeted fluorescent probe using Si-O-Si as a bridge for the detection of Cr6+ applied in bio-imaging

Lipid droplets are known to play an important role in many cellular activities, as revealed by recent studies. Additionally, hexavalent chromium is considered extremely toxic because it readily passes through cellular membranes and easily accumulates in living cells. In this study, a novel lipid droplet-targeted fluorescent probe (Si-LDS) for recognition of Cr6+ in living cells was designed and synthesized using triphenylamine derivatives and organosiloxane. Si-LDS detected Cr6+ with high selectivity and sensitivity. The novel probe was successfully applied to cell imaging of exogenous Cr6+ in HeLa cells, and Si-LDS was able to localize mainly in the lipid droplets of HeLa cells. Si-LDS is the first lipid droplet-targeted fluorescent probe for monitoring Cr6+.

An On-Off-On Fluorescence Probe Based on Coumarin for Cu2+, Cysteine, and Histidine Detections

A simple coumarin-based Schiff base (probe L) was successfully developed. It showed strong green fluorescence emission at 527 nm with a 70.3% of fluorescence quantum efficiency (Φ_F). However, after the addition of common metal ions, probe L can only combine with Cu²⁺ ions and displayed significant fluorescence quenching of > 96.2% (Φ_F = 2.7%) due to the paramagnetic quenching action from Cu²⁺. Conversely, by the coordination action of cysteine (Cys) and histidine (His), the quenching fluorescence of the complex (L-Cu²⁺) between probe L and Cu²⁺ ions was recovered mostly because the Cys and His can usurp Cu²⁺ of L-Cu²⁺ and led to the liberation of probe L. Based on the fluorescence changes of probe L with the actions of Cu²⁺, Cys, and His, an on-off-on reversible fluorescence probe for sensitive and specific monitoring Cu²⁺, Cys, and His has been prepared. More importantly, the probe L and L-Cu²⁺ ensemble can be used, respectively, to test Cu²⁺ and Cys/His in live cells and human urine samples with great reliability.

A two-photon fluorescent probe for detecting lipid droplet viscosity in living cells and zebra fish

A fluorescent probe CBA with high photostability to measure the viscosity changes of lipid drops.