Ex Tenebris Lux: Illuminating Reactive Oxygen and Nitrogen Species with Small Molecule Probes

Reactive oxygen and nitrogen species are small reactive molecules derived from elements in the air─oxygen and nitrogen. They are produced in biological systems to mediate fundamental aspects of cellular signaling but must be very tightly balanced to prevent indiscriminate damage to biological molecules. Small molecule probes can transmute the specific nature of each reactive oxygen and nitrogen species into an observable luminescent signal (or even an acoustic wave) to offer sensitive and selective imaging in living cells and whole animals. This review focuses specifically on small molecule probes for superoxide, hydrogen peroxide, hypochlorite, nitric oxide, and peroxynitrite that provide a luminescent or photoacoustic signal. Important background information on general photophysical phenomena, common probe designs, mechanisms, and imaging modalities will be provided, and then, probes for each analyte will be thoroughly evaluated. A discussion of the successes of the field will be presented, followed by recommendations for improvement and a future outlook of emerging trends. Our objectives are to provide an informative, useful, and thorough field guide to small molecule probes for reactive oxygen and nitrogen species as well as important context to compare the ecosystem of chemistries and molecular scaffolds that has manifested within the field.

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.

A novel dual-channel fluorescent probe for selectively and sensitively imaging endogenous nitric oxide in living cells and zebrafish

Nitric oxide (NO) plays various physiological and pathological roles in lots of biological processes. It is crucial to detect NO sensitively and selectively in vivo and in vitro as homeostasis of NO is closely related to various diseases. Herein, a novel dual-channel fluorescent dye (ENNH₂) based on dicarboxyimide anthracene was developed as a highly sensitive and selective probe to detect NO in living systems using the dual-channel fluorescence. ENNH₂ can emit bright red fluorescence due to the intramolecular charge transfer (ICT) from the amino group at the 6-position of 1,2-dicarboxyimide anthracene to the conjugated aromatic ring, and the ICT is effectively inhibited by the reductive deamination of the amino in the presence of NO to obtain the remarkable strong green emission with the excellent sensitivity (5.52 nM). Promisingly, ENNH₂ exhibits an excellent performance in endogenous NO dual-channel fluorescence imaging of RAW 264.7 cells and zebrafish.

Recent progress on the organic and metal complex-based fluorescent probes for monitoring nitric oxide in living biological systems

Nitric oxide (NO) is an important gaseous signaling molecule related to various human diseases. To investigate the biological functions of NO, many strategies have been developed for real-time monitoring the NO levels in biological systems. Among these strategies, fluorescent probes are considered to be one of the most efficient and applicable methods owing to their excellent sensitivity and selectivity, high spatiotemporal resolution, noninvasiveness, and experimental convenience. Therefore, great efforts have been paid to the design, synthesis, and fluorescence investigation of novel NO fluorescent probes in the past several years. However, few of them exhibit practical applications owing to the low concentration, short half-life, and rapid diffusion characteristics of NO in biological systems. Rational design of NO fluorescent probes with excellent selectivity and sensitivity, low cytotoxicity, long-lived fluorescent emission, and low background interference is still a challenge for scientists all over the word. To provide spatial-temporal information, this article focuses on the progress made in the organic and metal complex-based NO fluorescent probes during the past five years. The key structural elements and sensing mechanisms of NO fluorescent probes are discussed. Some novel ratiometric, luminescence, and photoacoustic probes with low background interference and deep tissue penetrating ability are mentioned. All these probes have been used for imaging exogenous and endogenous NO in cells and animal models. More importantly, this article also describes the development of multi-functional NO fluorescent probes, such as organelle targeting probes, dual-analysis probes, and probe-drug conjugates, which will inspire the design of various functional fluorescent probes.

Effect of n-alkyl substitution on Cu(ii)-selective chemosensing of rhodamine B derivatives

Rhodamine B hydrazide-based molecular probes (1-10) were synthesized by derivatization with n-alkyl chains of different lengths at the hydrazide amino end. These probes exhibited selective absorption (A∼557) and fluorescence (I∼580) 'off-on' signal transduction along with a colourless → magenta colour transition in the presence of Cu(ii) ions among all the competitive metal ions investigated. The effective coordination of these probes to Cu(ii) ions under the investigated environment forming [Cu·L]2+ (L = 1-5) and [Cu·L2]2+ (L = 6-10) complexes led to their spiro-ring opening, which in turn was expressed through signatory spectral peaks of ring-opened rhodamine. All these probes exhibited Cu(ii) selectivity in signalling despite structural modifications to the core receptor unit through variation of the nature of the alkyl substituents. However, the sensitivity of the signalling and kinetics of the spiro-ring opening varied and could be correlated with the number of carbon atoms present in the n-alkyl substituents. Structural elucidation with X-ray diffraction and X-ray photoemission spectroscopic analyses provided further insight into the structure-function correlation in their Cu(ii) complexes. These probes with Cu(ii) coordination showed selectivity in signalling, high complexation affinity (log Ka = 4.8-8.8), high sensitivity (LOD = 4.1-80 nM), fast response time (rate = 0.0017-0.0159 s-1) and reversibility with counter anions, which ascertained their potential utility as chemosensors for Cu(ii) ion detection.

Smart dual T1 MRI-optical imaging agent based on a rhodamine appended Fe(iii)-catecholate complex

The high spin Fe(iii) complex Fe(RhoCat)3 is reported as a smart dual-modal T₁ MRI-optical imaging probe to visualize the NO molecule and an acidic pH environment.