Sensitive detection of strong acidic condition by a novel rhodamine-based fluorescent pH chemosensor

Three Novel Rhodamine 6G-Based Colorimetric and Fluorescent pH Switches

Three rhodamine 6G derivatives (REHA, RETA and REDA) were designed and synthesized by connecting rhodamine 6G and 3-methyl-2-thiophenal with hydrazine hydrate, ethylenediamine and diethylenetriamine, respectively. In CH3CN/H2O (50/50, v/v), the absorbance of REHA, RETA and REDA at 528 nm was suddenly enhanced by 3.2, 3.8 and 7.2 times within the pH range of 3.03-2.31, 3.05-2.32 and 3.06-2.34, respectively, and the solution changed from colorless to pink. Meanwhile, the maximal fluorescence intensity sharply increased by 53.9, 26.6 and 24.9 times in the pH range of 3.86-3.46, 3.88-3.47 and 3.89-3.48, respectively, and the solution changed from dark to bright yellow-green fluorescence. REHA, RETA and REDA can act as highly selective and sensitive colorimetric and fluorescent pH switches with good recyclability and anti-interference ability. The response mechanism of REHA, RETA and REDA to pH was studied by 1H NMR spectroscopy, and their application in indicating small pH changes in dyeing wastewater was investigated.

A Study of Small Molecule-Based Rhodamine-Derived Chemosensors and their Implications in Environmental and Biological Systems from 2012 to 2021: Latest Advancement and Future Prospects

Rhodamine-based chemosensors have sparked considerable interest in recent years due to their remarkable photophysical properties, which include high absorption coefficients, exceptional quantum yields, improved photostability, and significant red shifts. This article presents an overview of the diverse fluorometric, and colorimetric sensors produced from rhodamine, as well as their applications in a wide range of fields. The ability of rhodamine-based chemosensors to detect a wide range of metal ions, including Hg+2, Al3+, Cr3+, Cu2+, Fe3+, Fe2+, Cd2+, Sn4+, Zn2+, and Pb2+, is one of their major advantages. Other applications of these sensors include dual analytes, multianalytes, and relay recognition of dual analytes. Rhodamine-based probes can also detect noble metal ions such as Au3+, Ag+, and Pt2+. They have been used to detect pH, biological species, reactive oxygen and nitrogen species, anions, and nerve agents in addition to metal ions. The probes have been engineered to undergo colorimetric or fluorometric changes upon binding to specific analytes, rendering them highly selective and sensitive by ring-opening via different mechanisms such as Photoinduced Electron Transfer (PET), Chelation Enhanced Fluorescence (CHEF), Intramolecular Charge Transfer (ICT), and Fluorescence Resonance Energy Transfer (FRET). For improved sensing performance, light-harvesting dendritic systems based on rhodamine conjugates has also been explored for enhanced sensing performance. These dendritic arrangements permit the incorporation of numerous rhodamine units, resulting in an improvement in signal amplification and sensitivity. The probes have been utilised extensively for imaging biological samples, including imaging of living cells, and for environmental research. Moreover, they have been combined into logic gates for the construction of molecular computing systems. The usage of rhodamine-based chemosensors has created significant potential in a range of disciplines, including biological and environmental sensing as well as logic gate applications. This study focuses on the work published between 2012 and 2021 and emphasises the enormous research and development potential of these probes.

One-step microwave synthesis of red-emissive carbon dots for cell imaging in extreme acidity and light emitting diodes

Red emissive carbon dots (R-CDs) have received great attention in biological fields due to their deep tissue penetrability, great bioimaging capability, low interference from auto-fluorescence, and potential for optoelectronic applications. Herein, excitation-independent, highly acid-sensitive R-CDs were successfully obtained via one-step microwave treatment of o-phenylenediamine (o-PD) and phosphoric acid and carefully purified by column chromatography. The relationship between the fluorescence emission and surface groups of the R-CDs was studied in detail using XPS, NMR, and fluorescence spectroscopy, and the different mechanisms of action of the R-CDs and acid in H₂O and ethanol were determined. The excellent anti-interference ability and biocompatibility of the R-CDs were confirmed, and the probes were successfully used for imaging A549 and Escherichia coli (E. coli) cells in extreme acidity. Finally, based on their relatively high quantum yield and long wavelength emission, the application potential of the R-CDs in the fabrication of red light-emitting diodes (LEDs) was investigated.

Highly acid-sensitive R-CDs were obtained via a microwave method. The relationship between the FL emission and the surface groups of the R-CDs was studied in detail. The R-CDs were used for cell imaging in extreme acidity and fabrication of red LEDs.

4-Methyl-2,6-diformylphenol based biocompatible chemosensors for pH: discrimination between normal cells and cancer cells

Two compounds, namely, 2-hydroxy-5-methyl-3-((pyridin-2-ylimino)methyl)benzaldehyde (HM-2py-B) and 2-hydroxy-5-methyl-3-((pyridin-3-ylimino)methyl)benzaldehyde (HM-3py-B), have been explored as fluorescent chemosensors for pH. HM-2py-B and HM-3py-B were synthesized by single step condensation reaction between 4-methyl-2,6-diformylphenol and the appropriate aminopyridine. These compounds have been characterized by elemental analysis, FT-IR, ¹H NMR, ¹³C NMR, ESI mass spectrometry, and absorption and fluorescence spectroscopy. Their structures have been confirmed by single crystal X-ray diffraction analysis. Both of the compounds show low emission at 530 nm at low pH. Fluorescence intensity increases with the increase in pH. With the alteration in pH of the medium from 4.0 to 10.0, the fluorescence intensity at 530 nm enhances by 66 and 195 fold for HM-2py-B and HM-3py-B, respectively. pK_a values of HM-2py-B and HM-3py-B have been determined to be 7.15 and 6.57, respectively. Fluorescence increase occurs mainly due to deprotonation of the phenolic OH group. Several cations and anions could not induce significant change in fluorescence behavior for both of the probes. The quantum yield and life-time enhance significantly when the pH of the medium is changed from 5.0 to 9.0. Naked eye identification of different pH environments is possible by using these compounds. Some theoretical calculations have been carried out to support experimentally obtained spectral transitions. As cancer cell has a pH in the range of 5.5–7.0 in comparison to normal cell pH of 7.4, these probes have been used effectively to discriminate between normal cells and cancer cells.

Two 4-methyl-2,6-diformylphenol based compounds with pyridylamine have been established as chemosensors for pH. The probes are able to differentiate between normal cells and cancer cells.

A turn-off fluorescence probe based on terpyridine for pH monitoring

A new pH-dependent fluorescence probe 2,8-bis((E)-4-([2,2':6',2″-terpyridin]-4'-yl)styryl)-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (TBPTP) based on Tröger's base (TB) bound to terpyridine was designed and synthesized. Photophysical properties and titration experiments of TBPTP were investigated by absorption and fluorescence spectroscopy. TBPTP exhibited high sensitivity in an acidic environment with the working pH range 7.2-2.5, especially having a good liner response to pH changes in the range 2.5-4.3, which suggested that TBPTP is a good candidate for pH monitoring.