Abu Saleh Musha Islam

A rhodamine based fluorescent trivalent sensor (Fe3+, Al3+, Cr3+) with potential applications for live cell imaging and combinational logic circuits and memory devices

A sensor (HL5) based on rhodamine 6G–en and 3-(3,5-dimethyl-pyrazol-1-ylmethyl)-2-hydroxy-5-methyl-benzaldehyde (HL4) has been developed for a highly sensitive and selective CHEF based recognition of trivalent metal ions M3+ (M = Al, Fe and Cr) over mono- and di-valent and other biologically abundant trivalent metal ions with prominent enhancement in absorption and emission intensities. A large enhancement of fluorescence intensities for Fe3+ (21 fold), Al3+ (14 fold) and Cr3+ (10 fold) was observed upon addition of 1.8 equivalents of these metals into the probe in methanol/H2O (1 : 1, v/v, pH 7.2) with the possibility of naked eye detection. The corresponding Kf values were evaluated to be 6.7 × 104 M−1 (Fe3+); 8.2 × 104 M−1 (Al3+) and 6.0 × 104 M−1 (Cr3+). The quantum yields of HL5 and [HL5–Fe3+] and [HL5–Cr3+] and [HL5–Al3+] complexes in methanol/H2O (1 : 1, v/v, pH 7.2) are found to be 0.013, 0.290, 0.120, and 0.158, respectively, using rhodamine-6G as the standard. The LOD for Fe3+, Al3+ and Cr3+ were determined by 3σ methods with values 0.29, 0.34 and 0.31 μM, respectively. An arsenate ion snatches Al from the Al–HL5 complex and quenches its fluorescence via its ring closed spirolactam form. Advanced level molecular logic devices using the inputs 2 and 4 and memory devices, have been constructed. The low cytotoxicity and large enhancement in fluorescence intensity of HL5 upon complexation with M3+ metal ions make the probe suitable for bio-imaging of M3+ (M = Al, Fe and Cr) in living cells and native cellular iron pools.

A novel 8-hydroxyquinoline-pyrazole based highly sensitive and selective Al(III) sensor in a purely aqueous medium with intracellular application: experimental and computational studies

A new 8-hydroxyquinoline-pyrazole based highly sensitive and selective Al3+ sensor, 8Q-NH-Pyz (H2L3), was found to exhibit a turn-on fluorescence enhancement (FE) as high as 157 fold with Kd = (1.76 ± 0.06) × 10−5 M. The 1:1 binding stoichiometry was revealed from the linear fit of (Fmax − F0)/(F − F0) vs. 1/[Al3+] of the fluorescence titration data which was further substantiated by Jobs method and HRMS studies. The LOD determined by 3σ methods was found to be 4.29 nM and quantum yields were determined to be 0.002 and 0.28 for the ligand and its Al3+ complex, respectively. The tentative coordination environment in the [Al(L3)(H2O)]+ complex was delineated by DFT calculations. TDDFT calculations reveal spectral features comparable to the experimental ones. This constitutes the first report on the fluorescence sensing of Al3+ and hence F− in a purely aqueous medium.

A Smart Molecule for Selective Sensing of Nitric Oxide: Conversion of NO to HSNO; Relevance of Biological HSNO Formation

A smart molecule, QT490, containing thiosemicarbazide moiety acts as a highly selective turn-on in vitro NO sensor through the unprecedented NO-induced transformation of thiosemicarbazide moiety to 1,3,4-oxadiazole heterocycle with the concomitant release of HSNO, thereby eliminating any interference from various endogenous biomolecules including dehydroascorbic acid, ascorbic acid, etc. The kinetic studies of the reactions between QT490 and NO provide a mechanistic insight into formation of HSNO/RSNO from the reaction between H2S/RSH and NO in the biological system. This novel probe is non-cytotoxic, cell permeable, water-soluble, and appropriate for intracellular cytoplasmic NO sensing with the possibilities of in vivo applications.

A rhodamine based turn-on chemosensor for Fe3+ in aqueous medium and interactions of its Fe3+ complex with HSA

A novel di-coordinating rhodamine-based chemosensor, HL with NO donor atoms, selectively and rapidly recognizes Fe3+ in the presence of all biologically relevant as well as toxic metal ions and numerous anions and also with other reactive oxygen and nitrogen species. It exhibits a lower detection limit (0.17 μM) and comparatively higher formation constant (Kf = 1.72 × 104 M−1). The DNA-binding properties of [LFe(NO3)2]+ complex have been comprehensively studied by using UV-Vis, fluorescence, and optical melting studies and circular dichroism, which clearly indicate that [LFe(NO3)2]+ interacts with DNA via a groove binding mode. In particular, competition experiments with Hoechst and DAPI constitute firm evidence for this binding mode, and clearly rule out intercalation. The negative ΔG0 and positive ΔS0 values obtained from a calorimetric technique confirm the spontaneity of the binding of [LFe(NO3)2]+ with DNA. The resulting [LFe(NO3)2]+/DNA composite material could be a valuable candidate for future photonics and/or biological applications.

Domain-Specific Association of a Phenanthrene–Pyrene-Based Synthetic Fluorescent Probe with Bovine Serum Albumin: Spectroscopic and Molecular Docking Analysis

In this report, the interaction between a phenanthrene–pyrene-based fluorescent probe (PPI) and bovine serum albumin (BSA), a transport protein, has been explored by steady-state emission spectroscopy, fluorescence anisotropy, far-ultraviolet circular dichroism (CD), time-resolved spectral measurements, and molecular docking simulation study. The blue shift along with emission enhancement indicates the interaction between PPI and BSA. The binding of the probe causes quenching of BSA fluorescence through both static and dynamic quenching mechanisms, revealing a 1:1 interaction, as delineated from Benesi–Hildebrand plot, with a binding constant of ∼105 M–1, which is in excellent agreement with the binding constant extracted from fluorescence anisotropy measurements. The thermodynamic parameters, ΔH°, ΔS°, and ΔG°, as determined from van’t Hoff relationship indicate the predominance of van der Waals/extensive hydrogen-bonding interactions for the binding phenomenon. The molecular docking and site-selective binding studies reveal the predominant binding of PPI in subdomain IIA of BSA. From the fluorescence resonance energy transfer study, the average distance between tryptophan 213 of the BSA donor and the PPI acceptor is found to be 3.04 nm. CD study demonstrates the reduction of α-helical content of BSA protein on binding with PPI, clearly indicating the change of conformation of BSA.

A turn-on fluorogenic chemosensor for Fe3+ and a Schottky barrier diode with frequency-switching device applications

A novel highly sensitive and selective fluorescent chemosensor L has been synthesized and characterized by various physicochemical techniques. In 3 : 7 water : MeCN (v/v) at pH 7.2 (10 mM HEPES buffer, μ = 0.05 M LiCl), it selectively recognizes Fe3+ through 1 : 1 complexation resulting in a 106-fold fluorescence enhancement and a binding constant of 8.10 × 104 M-1. The otherwise non-fluorescent spirolactam form of the probe results a dual-channel (absorbance and fluorescence) recognition of Fe3+via CHEF (chelation enhanced fluorescence) through the opening of the spirolactam ring. We have also carried out fluorescence titration and anisotropy (r) studies in pure water in the presence of SDS (sodium dodecyl sulphate). Based on the dependence of FI (fluorescence intensity) and r on [SDS] it was proposed that the probe is trapped between two SDS monolayers which again interact among themselves by ππ stacking. As a result, there is an increase in FI up to [SDS] ∼ 7 mM - a phenomenon reminiscent of aggregation-induced enhancement of emission (AIEE). Beyond this concentration of SDS (7 mM), micelle formation takes place and the ππ stacked polymer now becomes a monomer and is trapped inside the micellar cavity. As a result, there is a decrease in FI at [SDS] > 7 mM. But for anisotropy, it increases with [SDS] beyond 7 mM. Ligand, metal, and SDS interactions are well established through different optical and morphological studies. [L-Fe(NO3)]2+ thin films on FTO (Fluorine-doped Tin Oxide) glass substrates have been designed with the help of the spin-coating deposition technique. The deposited film of thickness 1.6 × 10-5 cm is well characterized by optical band gap calculation with a direct band gap, εg ∼ 1.6 eV. FESEM was also performed for the [L-Fe(NO3)]2+/FTO film. The current-voltage characteristics were measured by the two-probe technique. Light-dependent exciton generation was carried out by taking the top and bottom contacts with graphite paste on FTO and on the [L-Fe(NO3)]2+ films for the measurement of switching behavior. The response ratio curve for the light-induced frequency-switching phenomena has been obtained. The frequency taped here is the oscillation frequency of the photo-generated electron and the hole in an exiton. Thus, the light-induced frequency-switching behavior and Schottky barrier diode characteristics of the material were established.

Nitric Oxide Sensing through 1,2,3,4-Oxatriazole Formation from Acylhydrazide: A Kinetic Study

A simple molecular probe displays highly selective turn-on response toward NO by the unprecedented NO-induced formation of a 1,2,3,4-oxatriazole ring exhibiting no interference from various endogenous biomolecules including DHA, AA, etc. Kinetics of the reactions between NO and the probe provide a mechanistic insight into the formation of 1,2,3,4-oxatriazole which showed that, though initially 1,2,3,4-oxatriazole is formed and extractable in solid form, it exists in equilibrium with the ring opened azide form which ultimately hydrolyzed and converted to carboxylic acid and nitrate. The reaction displays second-order dependence on [NO] and first-order on [Probe]. The probe is water-soluble, cell permeable, and noncytotoxic and appropriates for live cell imaging. This constitutes the first report where there is a direct evidence of NO-induced ring closing reaction of an acyl hydrazide moiety leading to the formation of 1,2,3,4-oxatriazole.

A rhodamine hydrazide–4-nitroindole-3-carboxaldehyde based turn on Hg2+ chemosensor: Cytoplasmic live cell imaging, logic gate and memory device applications and computational studies

A new, highly sensitive probe L2 for the selective detection of Hg2+ in organo-aqueous (H2O:CH3CN, 1:1, v/v, HEPES buffer, pH 7.2) medium has been synthesized from rhodamine 6G-hydrazide and 4-nitroindole-3-carboxaldehyde. It was thoroughly characterized by physicochemical techniques including single crystal X-ray diffraction studies. The reaction of L2 with Hg2+ gives a 1:1 stoichiometry resulting in a 146 fold fluorescence enhancement and a binding constant (Kf) of 3 × 104 M−1. The spirolactam form of the probe is non-fluorescent; however, it shows dual channel (absorbance and fluorescence) recognition of Hg2+ via CHEF effect through the opening of the spirolactam ring. The quantum yields of L2 (0.00045) and L2-Hg2+ (0.29) show the higher stability of complex in the excited state over the free ligand. The 44.5 nM LOD value demonstrates the detection of Hg2+ at a very low concentration range. Cell imaging studies show the cytoplasmic recognition of Hg2+ by L2. Experimental results are comparable with theoretical values obtained by DFT studies. The fluorescence emission of the complex was completely quenched by I- and from the reversibility studies an advance level INHIBIT logic gate and memory device can be framed. Graphical Abstract

Design of a Pyrene Scaffold Multifunctional Material: Real-Time Turn-On Chemosensor for Nitric Oxide, AIEE Behavior, and Detection of TNP Explosive

A dual-emission pyrene-based new fluorescent probe (N-(4-nitro-phenyl)-N′-pyren-1-ylmethyl-ene-ethane-1,2-diamine (PyDA-NP)) displays green fluorescence for nitric oxide (NO) sensing, whereas it exhibits blue emission in the aggregated state. The mechanism of nitric oxide (NO/NO+) sensing is based on N-nitrosation of aromatic secondary amine, which was not interfered by reactive oxygen species and reactive nitrogen species. The aggregation-induced enhancement of emission (AIEE) behaviors of the PyDA-NP could be attributed to the restriction of intramolecular rotation and vibration, resulting in rigidity enhancement of the molecules. The AIEE behavior of the probe was well established from fluorescence, dynamic light scattering, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, optical fluorescence microscopy, and time-resolved photoluminescence studies. In a H2O/CH3CN binary mixture (8:2 v/v), the probe showed maximum aggregation with extensive (833-fold) increases in fluorescence intensity and high quantum yield (0.79). The aggregated state of the probe was further applied for the detection of nitroexplosives. It displayed efficient sensing of 2,4,6-trinitrophenol (TNP), corroborating mainly the charge-transfer process from pyrene to a highly electron-deficient TNP moiety. Furthermore, for the on-site practical application of the proposed analytical system, a contact-mode analysis was performed.