New Six-Membered pH-Insensitive Rhodamine Spirocycle in Selective Sensing of Cu²⁺ through C–C Bond Cleavage and Its Application in Cell Imaging

A new rhodamine-based chemosensor <b>1</b> with a six-membered spirocyclic ring has been synthesized, which exhibits excellent pH stability and shows selective “turn-on” fluorescent detection of Cu²⁺ ions over a series of other metal ions including Cu⁺ ions. The expansion of spirocycle improves the stability and selectivity of the chemosensors in sensing of metal ions. Till today only few rhodamine structures <b>R1</b>–<b>R5</b> with thiourea-, hydrazine amide-, or pyrrole-decorated six-membered spirocyclic rings are known that exhibit metal-ion sensing via C–N bond cleavage of the spiro ring. In this context, rhodamine compound that responds to the metal ion through C–C bond cleavage of the six-membered spiro ring is completely unknown. The present example is a first-time report that demonstrates selective sensing of Cu²⁺ ions through C–C bond cleavage over the conventional existing systems in the literature. The chemosensor <b>1</b> is cell permeable and can detect Cu²⁺ in live cells using confocal microscopy in the biologically relevant pH range with high photostability

Highly Selective and Sensitive Two-Photon Fluorescence Probe for Endogenous Peroxynitrite Detection and Its Applications in Living Cells and Tissues

A new two-photon fluorescence probe for endogenous peroxynitrite (ONOO^–) detection was designed and synthesized. The probe exhibits good selectivity and sensitivity for ONOO^– in phosphate-buffered saline solution with a low detection limit (3.5 × 10^–8 M). Furthermore, the probe displays good performance in detecting endogenous ONOO^–, not only in RAW 264.7 cells but also in rat hippocampal tissue, with a high two-photon cross-section value (δ ≈ 100 GM) at a deep depth of 120 μm

Carboxylate-Containing Two-Photon Probe for the Simultaneous Detection of Extra- and Intracellular pH Values in Colon Cancer Tissue

Acidified extracellular pH (pHe) is directly related to various disorders such as tumor invasion and the resistance to drugs. In this study, we developed two-photon-excitable emission ratiometric probes (<b>XBH1–3</b>) for the in situ measurement of pHe. These probes, based on benzimidazole and polar solubilizing groups, exhibited a strong two-photon-induced fluorescence and sensitive blue-to-green emission color changes with p<i>K</i>_a values of 5.1–5.7. <b>XBH1</b>, containing a carboxylic acid, stained the extracellular region in neutral media; it entered the cell under acidic media, thereby allowing a precise measurement of the extra- and intra-cellular pH values in the acidified tissue. <b>XBH2</b>, containing the sulfonate peripheral unit, facilitated the monitoring of the pHe value only. Ratiometric two-photon microscopy imaging revealed that <b>XBH1</b> can directly monitor the pH values both inside and outside the cells in colon cancer tissue; there is also the morphological aspect. This could be useful for cancer analyses and drug development

Two-Photon Tracer for Human Epidermal Growth Factor Receptor-2: Detection of Breast Cancer in a Live Tissue

We have developed a two-photon fluorescent tracer (Pyr-affibody) that shows high selectivity for human epidermal growth factor receptor-2 (HER-2). Pyr-affibody showed absorption and emission maxima at 439 and 574 nm, respectively, with a two-photon absorption cross-section value of 40 × 10^–50 cm⁴s/photon (GM) at 750 nm in aqueous buffer solution. The effective two-photon action cross-section value measured in HeLa cells was 600 GM at 730 nm, a value sufficient to obtain bright two-photon microscopy (TPM) images. Using Pyr-affibody, it was possible to detect HER-2 overexpressing cells and breast cancers at a depth of 90–130 μm in live mouse tissue by TPM

A Two-Photon Fluorescent Probe for Imaging Endogenous ONOO^– near NMDA Receptors in Neuronal Cells and Hippocampal Tissues

In this study, we developed a two-photon fluorescent probe for detection of peroxynitrite (ONOO^–) near the <i>N</i>-methyl-d-aspartate (NMDA) receptor. This naphthalimide-based probe contains a boronic acid reactive group and an ifenprodil-like tail, which serves as an NMDA receptor targeting unit. The probe displays high sensitivity and selectivity, along with a fast response time in aqueous solution. More importantly, the probe can be employed along with two-photon fluorescence microscopy to detect endogenous ONOO^– near NMDA receptors in neuronal cells as well as in hippocampal tissues. The results suggest that the probe has the potential of serving as a useful imaging tool for studying ONOO^– related diseases in the nervous system