Fabrication of FRET based nano sensor from biomass-derived fluorescent carbon quantum dots and naphthalimide for ratiometric detection of nitric oxide: To examine nitrite levels in meat samples

Nitric oxide (NO) is a ubiquitous, gaseous, free radical signaling molecule which plays a key role in physiological and pathological processes. Literature reports revealed that the conventional methods such as colorimetry, electron paramagnetic resonance (EPR), electrochemical etc. to detect NO are costly, time consuming and lack resolution, particularly in aqueous or biological system. Thus, in this context, herein we have developed covalently linked biomass derived carbon quantum dots (CQDs) and naphthalimide based nano sensor system for FRET based ratiometric detection of nitric oxide (NO) in pure aqueous media. The CQDs derived from orange peels were characterized using UV-visible absorption, fluorescence spectroscopy, PXRD, TEM, FT-IR and zeta potential studies. Further, the obtained CQDs were functionalized with amine functionality, and subsequently linked with naphthalimide derivative (5) using terephthaldehyde through covalent bond formation. The conjugation of naphthalimide (5) and functionalized CQDs was studied using DLS, zeta potential, FT-IR and time resolved fluorescence spectroscopy. The excitation of developed nano sensor system at λ_ex 360 nm results in fluorescence emission at λ_em 530 nm which establishes the FRET pair between the CQDs and naphthalimide unit. However, in the presence of NO, the observed FRET pair abolishes due to the cleavage of NO susceptible imine bond. The developed sensor demonstrates high selectivity towards NO with limit of detection (LOD) and limit of quantification (LOQ) of 15 nM and 50 nM respectively. Further, the developed sensor system was also utilized for indirect detection of nitrite (NO₂^-) in food samples for food safety and monitoring.

N-Nitrosation Based Fluorescence Turn-On Nitric Oxide Probe: Kinetic and Cell Imaging Studies

Nitric oxide (NO) is a ubiquitous messenger molecule playing a key role in various physiological and pathological processes. However, producing a selective turn-on fluorescence response to NO is a challenging task due to (a) the very short half-life of NO (typically in the range of 0.1-10 s) in the biological milieu and (b) false positive responses to reactive carbonyl species (RCS) (e.g., dehydroascorbic acid and methylglyoxal etc.) and some other reactive oxygen/nitrogen species (ROS/RNS), especially with o-phenylenediamine (OPD) based fluorosensors. To avoid these limitations, NO sensors should be designed in such a way that they react spontaneously with NO to give turn-on response within the time frame of t1/2 (typically in the range of 0.1-10 s) of NO and λem in the visible wavelength along with good cell permeability to achieve biocompatibility. With these views in mind, a N-nitrosation based fluorescent sensor, NDAQ, has been developed that is highly selective to NO with ∼27-fold fluorescence enhancement at λem = 542 nm with high sensitivity (LOD = 7 ± 0.4 nM) and shorter response time, eliminating the interference of other reactive species (RCS/ROS/RNS). Furthermore, all the photophysical studies with NDAQ have been performed in 98% aqueous medium at physiological pH, indicating its good stability under physiological conditions. The kinetic assay illustrates the second-order dependency with respect to NO concentration and first-order dependency with respect to NDAQ concentration. The biological studies reveal the successful application of the probe to track both endogenous and exogenous NO in living organisms.

Optical/electrochemical methods for detecting mitochondrial energy metabolism

This review highlights the biological importance of mitochondrial energy metabolism and the applications of multiple optical/electrochemical approaches to determine energy metabolites. Mitochondria, the main sites of oxidative phosphorylation and adenosine triphosphate (ATP) biosynthesis, provide the majority of energy required by aerobic cells for maintaining their physiological activity. They also participate in cell growth, differentiation, information transmission, and apoptosis. Multiple mitochondrial diseases, caused by internal or external factors, including oxidative stress, intense fluctuations of the ionic concentration, abnormal oxidative phosphorylation, changes in electron transport chain complex enzymes and mutations in mitochondrial DNA, can occur during mitochondrial energy metabolism. Therefore, developing accurate, sensitive, and specific methods for the in vivo and in vitro detection of mitochondrial energy metabolites is of great importance. In this review, we summarise the mitochondrial structure, functions, and crucial energy metabolic signalling pathways. The mechanism and applications of different optical/electrochemical methods are thoroughly reviewed. Finally, future research directions and challenges are proposed.

Highly Sensitive D-A-D-Type Near-Infrared Fluorescent Probe for Nitric Oxide Real-Time Imaging in Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a common gastrointestinal inflammatory disease, affecting a huge number of people worldwide with increasing morbidity each year. Although the etiology of IBD has not been fully elucidated, it is understood to be closely related to upregulation of the production of NO. Herein, we first report a donor-acceptor-donor (D-A-D)-type near-infrared (NIR) fluorescent probe LS-NO for real-time detection of NO in IBD by harnessing the enhanced intramolecular charge transfer mechanism. LS-NO exhibited good water solubility, high photostability, and excellent NIR absorbance and emission at 700 and 750/800 nm, respectively. Moreover, it was able to sensitively and specifically detect exogenous and endogenous NO in the lysosomes of living cells. Notably, LS-NO enabled to noninvasively visualize NO generation in a lipopolysaccharide-induced IBD mouse model for 30 h, showing a two- to threefold higher NIR fluorescence intensity in the intestines and feces of IBD mice than normal mice. This work demonstrates that LS-NO is promising as a diagnosis agent for real-time detection of NO in IBD and may promote inflammatory stool examination simultaneously.

A coumarin embedded highly sensitive nitric oxide fluorescent sensor: kinetic assay and bio-imaging applications

Fluorescence spectroscopy is a significant bio-analytical technique for specific detection of nitric oxide (NO) and for broadcasting the in vitro and in vivo biological activities of this gasotransmitter. Herein, a benzo-coumarin embedded smart molecular probe (BCM) is employed for NO sensing through detailed fluorescence studies in purely aqueous medium. All the spectroscopic analysis and literature reports clearly validate the mechanistic insight of this sensing strategy i.e., the initial formation of 1,2,3,4-oxatriazole on treatment of the probe with NO which finally converted to its carboxylic acid derivative. This oxatriazole formation results in a drastic enhancement in fluoroscence intensity due to the photoinduced electron transfer (PET) effect. The kinetic investigation unveils the second and first-order dependency on [NO] and [BCM] respectively. The very low detection limit (16 nM), high fluorescence enhancement (123 fold) in aqueous medium and good formation constant (K_f = (4.33 ± 0.48) × 10⁴ M^-1) along with pH invariability, non-cytotoxicity, biocompatibility and cell permeability make this probe a very effective one for tracking NO intracellularly.

A FRET-based supramolecular nanoprobe with switch on red fluorescence to detect SO2 derivatives in living cells

A fluorescent nanoprobe for detection of SO₂, an important gasotransmitter, is reported.

Differentiation of Intracellular Hyaluronidase Isoform by Degradable Nanoassembly Coupled with RNA-Binding Fluorescence Amplification

Hyaluronidase has two cruical isoforms, hyaluronidase-1 (Hyal-1) and hyaluronidase-2 (Hyal-2), which are essential for cellular hyaluronic acid (HA) catabolism to generate different-sized oligosaccharide fragments for performing different physiological functions. In particular, Hyal-1 is the major tumor-derived hyaluronidase. Thus, specific detection of one hyaluronidase isoform, especially Hyal-1, in live cells is of scientific significance but remains challenging. Herein, by use of differentiated tolerance capability of an amphiphilic HA-based nanoassembly to Hyal-1 and Hyal-2, we rationally design a Hyal-1 specific nanosensor, consisting of cholesterylamine-modified HA nanoassembly (CHA) and RNA-binding fluorophores (RBF). The RBF molecules were entrapped in CHA to switch off their fluorescence via aggregation caused quenching. However, CHA can be disassembled by Hyal-1 to release RBF, resulting in fluorescence activation. Moreover, the fluorescence of the released RBF is further enhanced by cytoplasm RNA. Owing to this cascade signal amplification, this nanosensor RBF@CHA displays a significant change of signal-to-background-noise ratio (120-fold) toward 16 μg/mL Hyal-1 in cellular lysates. In contrast, it is resistant to Hyal-2. By virtue of its selective and sensitive characteristics under a complicated matrix, RBF@CHA had been successfully applied for specifically visualizing Hyal-1 over Hyal-2 inside live cells for the first time, detecting a low level of intracellular Hyal-1 and distinguishing normal and cancer cells with different expressions of Hyal-1. This approach would be useful to better understand biological functions and related diseases of intracellular Hyal-1.