Rabiul Alam

Morphology-Directing Synthesis of Rhodamine-Based Fluorophore Microstructures and Application toward Extra- and Intracellular Detection of Hg2+

A new, easily synthesizable rhodamine-based chemosensor with potential N2O2 donor atoms, L(3), has been characterized by single-crystal X-ray diffraction together with (1)H NMR and high-resolution mass spectrometry (HRMS) studies. L(3) was found to bind selectively and reversibly to the highly toxic Hg(2+) ion. The binding stoichiometry and formation constant of the sensor toward Hg(2+) were determined by various techniques, including UV-vis, fluorescence, and Jobs studies, and substantiated by HRMS methods. None of the biologically relevant and toxic heavy metal ions interfered with the detection of Hg(2+) ion. The limit of detection of Hg(2+)calculated by the 3σ method was 1.62 nM. The biocompatibility of L(3) with respect to its good solubility in mixed organic/aqueous media (MeCN/H2O) and cell permeability with no or negligible cytotoxicity provides good opportunities for in vitro/in vivo cell imaging studies. As the probe is poorly soluble in pure water, an attempt was made to frame nano/microstructures in the absence and in the presence of sodium dodecyl sulfate (SDS) as a soft template, which was found to be very useful in synthesizing morphologically interesting L(3) microcrystals. In pure water, micro-organization of L(3) indeed occurred with block-shaped morphology very similar to that in the presence of SDS as a template. However, when we added Hg(2+) to the solution of L(3) under the above two conditions, the morphologies of the microstructures were slightly different; in the first case, a flowerlike structure was observed, and in second case, a simple well-defined spherical microstructure was obtained. Optical microscopy revealed a dotlike microstructure for L(3)-SDS assemblies, which changed to a panicle microstructure in the presence of Hg(2+). UV-vis absorption and steady-state and time-resolved fluorescence studies were also carried out in the absence and presence of Hg(2+), and also the SDS concentration was varied at fixed concentrations of the receptor and guest. The results revealed that the fluorescence intensity increased steadily with [SDS] until it became saturated at ∼7 mM SDS, indicating that the extent of perturbation to the emissive species increases with the increase in [SDS] until it becomes thermodynamically stable. There was also an increase in anisotropy with increasing SDS concentration, which clearly manifests the restriction of movement of the probe in the presence of SDS.

ESIPT blocked CHEF based differential dual sensor for Zn2+ and Al3+in a pseudo-aqueous medium with intracellular bio-imaging applications and computational studies

A novel 3-hydroxymethyl-5-methylsalicylaldehydenaphthyl-hydrazone (H3SAL-NH) exhibits ESIPT behaviour due to proton transfer from the phenolic OH group to the azomethine N atom in the excited state. Through this ESIPT behaviour together with cis–trans isomerization of the azomethine group, the free ligand becomes very weakly fluorescent. However, in the presence of Zn2+ and Al3+ the ESIPT and isomerization are blocked due to coordination to the metal ions thereby causing turn on fluorescence for Al3+ and Zn2+. Moreover, Zn2+ can easily be displaced from the [H2SAL-NH–Zn2+] complex by Al3+ thereby enhancing the differential selectivity for Al3+ over Zn2+. This probe was found to be selective for Al3+ over Zn2+ in the presence of Na2H2EDTA, under both intra- and extracellular conditions. The LODs for Zn2+ and Al3+ were determined by 3σ methods and were found to be 3.1 nM and 0.92 nM, respectively. Thus, the differentially selective turn-on fluorescence behaviour of H3SAL-NH for Zn2+ and Al3+ is based on the combined blocking of ESIPT and CN isomerization, and a chelation-enhanced fluorescence (CHEF) effect. The coordination modes of the complexes were investigated through spectroscopic and computational studies. H3SAL-NH also exhibits good photostability and very low cytotoxicity and is useful for fluorescence imaging of Zn2+ and Al3+ ions in live HepG2 cells.

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 pyrene-2-(pyridin-2-ylmethylsulfanyl)ethylamine based turn-on dual sensor for Al3+: experimental and computational studies

A new pyrene based highly sensitive and selective Al3+ sensor, pyrene-2-(pyridin-2-ylmethylsulfanyl)-ethylamine (PP), was found to exhibit a turn-on fluorescence enhancement (FE) as high as 7.4 fold with Kd (2.55 ± 0.10) × 10−4 M and n = 1. This probe binds reversibly with Al3+ in the presence of H2EDTA2−, both under intra- and exctracellular conditions. LOD determined by 3σ methods was found to be 15 nM while LOQ = 52.8 nM. The tentative coordination environment in the Al3+–PP complex was delineated by DFT calculations both on the free ligand and complex. The TDDFT calculations reveal spectral features comparable to the experimental ones.

A rhodamine embedded bio-compatible smart molecule mimicking a combinatorial logic circuit and 'key-pad lock' memory device for defending against information risk

Organic molecules with the possibility of logic operations are highly useful building blocks for the development of molecule-based “intelligent” devices for information processing applications. We have designed herein a very simple bio-friendly chemosensor (LC) equipped with a rhodamine fluorophore moiety. This probe showed a chromo-fluorescence response profile for Al3+ but a colorimetric response for Cu2+ metal. The absorption responses of LC caused by these metal ions along with the “OFF–ON–OFF” fluorescence behavior of an LC–Al3+ complex towards EDTA were employed for the development of a three-input and one output combinatorial molecular system. Interactions of the mentioned metal ions with LC in controlled sequential experiments gave fluorescence responses, enabling us to fabricate a ‘key-pad-logic’ function. So, a single molecular system performing such multiple ‘Boolean’ operations not only simplifies the complexity of a chemical driven ‘Intelligence’ device but also enriches the security of such a device against information invasion due to the sequence controlled sensor–analyte interactions and may find potential applications in biocompatible molecular logic platforms.

A thiosemicarbazone based chemo and fluorogenic sensor for Zn2+ with CHEF and ESIPT behaviour: computational studies and cell imaging application

We report herein the development of a novel, diformyl-p-cresol (DFC)–thiosemicarbazide (TS) based sensor (DFC–TS) that selectively and sensitively recognizes Zn2+ by both UV-Vis and fluorescence methods. The gradual addition of Zn2+ to a solution of the ligand developed a new absorption band at 430 nm, while the bands at 370 and 316 nm gradually decrease generating one well defined isosbestic point at 390 nm exhibiting ∼17 fold turn-on fluorescent enhancement (FE). When we plot absorbance (at 430 nm) vs. [Zn2+] there is a gradual increase in absorption with [Zn2+], becoming saturated at ∼1 equivalent of Zn2+ and then again it increases with the increase in [Zn2+] and ultimately becomes saturated at ∼2 equivalents of added Zn2+. This clearly demonstrates that the Zn2+ binding event to the ligand occurs in two steps, one at a time. Non-linear least-squares computer-fitting of these data gives the parameters: K′f1 = (9.70 ± 5.51) × 105 M−1, n = (1.28 ± 0.05) for the first step and K′f2 = (1.11 ± 0.65) × 105 M−1 and n = (1.01 ± 0.06) for the second step. So far, this study provides the opportunity where we have successfully, for the first time, determined the stepwise formation constants; though they have values of the same order of magnitude. The ground state geometries of DFC–TS, both enol and keto forms and [Zn(DFC–TS)(OAc)], [Zn(DFC–TS)(OAc)]−, and [Zn2(DFC–TS)(OAc)2] were optimized using the Gaussian-03 suit program and bond distances of all species are in reasonable agreement with the reported values.

Dual channel selective fluorescent detection of Al3+ and PPi in mixed aqueous media: DFT studies and cell imaging applications

A new, easily synthesizable chemosensor, DFC-EN-p-Ph-NO2, derived by the Schiff base condensation between 2,6-diformyl-p-cresol and N-(4-nitrophenyl)ethylenediamine, with potential N4O donor atoms was found to act as a dual channel (colori- and fluorimetric) sensor towards Al3+ and PPi emitting at 486 nm (blue region) and 534 nm (green region), respectively in MeOH–H2O (8 : 2, v/v) at pH 7.2 (10 mM HEPES buffer), μ = 0.05 M (LiCl), temperature 25 °C. The binding stoichiometries and formation constants of the sensor towards both Al3+ and PPi were determined by the combined UV-Vis and fluorescence titrations and Jobs method, and confirmed by MS (m/z) studies. The corresponding detection limits as calculated by the 3σ method are: 7.55 μM and 3.34 μM. The most interesting part of this study is that on addition of 230 μM PPi to an ensemble of DFC-EN-p-Ph-NO2−Al3+ (20 μM Ligand and 380 μM Al3+) the fluorescence is totally quenched but on further addition of PPi a new emission peak appears at 534 nm. All biologically relevant metal ions and toxic heavy metals did not interfere with the detection of Al3+ ion. Its bio-compatibility with respect to its good solubility in mixed organo-aqueous media (MeOH–H2O) and cell permeability with no or negligible cytotoxicity provide good opportunities towards in vitro cell imaging studies of these ions. In particular, the fluorescent detection of PPi was not interfered by the presence of 400 μM of ATP or Pi although most reported PPi sensors that work in aqueous solution displayed cross-sensitivities toward ATP or Pi. The obvious excellent sensing capability of DFC-EN-p-Ph-NO2 towards PPi and Al3+ was further scrutinized in HCT116 cell lines without much cytotoxicity. The modes of 1 : 1 binding of DFC-EN-p-Ph-NO2 towards Al3+ and PPi were delineated by DFT calculations.