A multimodal fluorescent probe for portable colorimetric detection of pH and it's application in mitochondrial bioimaging

A single excitation dual emission semi-salamo type multi-functional probe for sensitive pH and Cu2+ detection

pH and Cu2+ ion concentration changes are linked to disorders like Alzheimer's and cancer. Rapid detection of pH and Cu2+ ions is critical for public health and environmental concerns. The semi-salamo-type probe (E)-2-hydroxy-1-naphthaldehyde O-(2-(aminooxy)ethyl) oxime (NSS) demonstrated substantial dual-functional performance, sensing pH change and Cu2+ ions. A single excitation and double emission characteristic on the probe NSS made it distinctive. Probe NSS exhibits pH-dependent excited state intramolecular proton transfer (ESIPT), and its optical properties vary based on the pH environment. Probe NSS detects pH changes from 2 to 11 by changing the "off-on-off" of the excited state intra-molecular proton transfer (ESIPT) mechanism, exhibiting rapid, reversible, and selective responses. In addition, the luminescent salamo-like naphthalene-based probe NSS can coordinate with Cu2+ ions, achieving great selectivity and sensitivity to identify Cu2+ ions with a detection limit of 0.84 ppb (13.2 nM) Probe NSS can detect Cu2+ ions in actual water samples such as tap water and yellow river water. The test strip loaded with probe NSS enabled quick and accurate detection of Cu2+ ions in water samples. Consequently, the versatile salamo-type probe NSS lays the foundation for developing high sensitivity and fast-response dual-mode pH meters as well as Cu2+ sensing.

Ratiometric fluorescent probes for pH mapping in cellular organelles

The intracellular pH (pHi) in organelles, including mitochondria, endoplasmic reticulum, lysosomes, and nuclei, differs from the cytoplasmic pH, and thus maintaining the pH of these organelles is crucial for cellular homeostasis. Alterations in the intracellular pH (ΔpHi) in organelles lead to the disruption of cell proliferation, ion transportation, cellular homeostasis, and even cell death. Hence, accurately mapping the pH of organelles is crucial. Accordingly, the development of fluorescence imaging probes for targeting specific organelles and monitoring their dynamics at the molecular level has become the forefront of research in the last three decades. Among them, ratiometric fluorescent probes minimize the interference from the excitation wavelength of light, auto-fluorescence from probe concentration, environmental fluctuations, and instrument sensitivity through self-correction compared to monochromatic fluorescent probes, which are known as turn-on/off fluorescent probes. Small-molecular ratiometric fluorescent probes for detecting ΔpHi are challenging yet demanding. To date, sixty-two ratiometric pH probes have been reported for monitoring internal pH alterations in cellular organelles. However, a critical review on organelle-specific ratiometric probes for pH mapping is still lacking. Thus, in the present review, we report the most recent advances in ratiometric pH probes and the previous data on the role of mapping the ΔpHi of cellular organelles. The development strategy, including ratiometric fluorescence with one reference signal (RFRS) and ratiometric fluorescence with two reversible signals (RFRvS), is systematically illustrated. Finally, we emphasize the major challenges in developing ratiometric probes that merit further research in the future.

Chemometric-Assisted Litmus Test: One Single Sensing Platform Adapted from 1-13 to Narrow pH Ranges

A novel 3 × 4 colorimetric sensing platform, named the chemometric-assisted litmus test (CLT), has been developed by covalently anchoring commercial pH indicators to ethylene vinyl alcohol (EVOH). The proposed device can be exploited for pH determinations in a wide range from 1 to 13 and in specific narrow ranges, achieving sufficient accuracy and errors below 0.5 pH units. The experimental procedure is simple, quick and reliable; equilibration is reached in less than 2 h, CLT pictures are acquired by a camera, and data treatment is performed applying chemometric techniques such as principal component analysis (PCA) and partial least square regression (PLS) to RGB indices.

The detection of multiple analytes by using visual colorimetric and fluorometric multimodal chemosensor based on the azo dye

In recent decades, researchers have conducted in-depth studies of the design and synthesis of colorimetric/fluorometric probes and the application of such probes to biological and practical samples. The multifunctional colorimetric and fluorescent azo benzene-based probe (4′-hydroxyl-2,4-diaminoazobenzene, MP) was designed to detect Al³⁺, Fe³⁺, Cu²⁺ and F¯. Based on the distinct redshift of the absorption band and a significant color change (yellow → purple), MP was utilized for both naked-eyed and quantitative detection of Al³⁺ and Fe³⁺ after formation of the 1:1 complex. Test paper coated with MP and used in conjunction with a cell phone was used for colorimetric detection of Al³⁺ and Fe³⁺ ions (20 μM–2.0 mM) in water samples through naked-eye and digital image colorimetry. The “MP-Fe³⁺” coordination shift that occurs in the presence of the competitive ligand F¯ was used in the colorimetric measurement of F¯ in toothpaste. In the presence of Cu²⁺ ion, the non-emissive MP has transformed into fluorescent benzotriazole product PMP (Φ = 0.53) through the bimolecular rate-limiting step, and the second-order rate constant k is calculated as 31 ± 2 M⁻¹ s⁻¹. MP exhibits a “turn-on” fluorescence response in the presence of Cu²⁺ that is greater than its response in the presence of competitive species such as Fe³⁺, Al³⁺, Co²⁺, Fe²⁺, Zn²⁺, Cd²⁺, Mg²⁺, Mn²⁺, Ni²⁺ and Ag⁺. MP was shown to have low toxicity to living HeLa cells and to present good imaging characteristics for tracking of Cu²⁺ in vivo.