Buddhadeb Sen

A water soluble FRET-based ratiometric chemosensor for Hg(II) and S2− applicable in living cell staining

A new highly sensitive and selective Hg(II) probe, 2-(rhodamine-b-hydrazido)-N-(quinolin-8-yl)acetamide (L1) was developed and characterized. L1 specifically binds to Hg(II) in the presence of a large excess of other competing ions with visually observable changes in both electronic and fluorescence spectral behaviour to make possible the naked eye detection of Hg(II) at a very low level (up to 4.5 × 10−7 M) through a fluorescence resonance energy transfer (FRET) process in HEPES buffer (1 mM, pH 7.4; 2% EtOH) at 25 °C. The theoretical and experimental kinetic study also support the binding of Hg(II) ion to induce the opening of the spirolactam ring in L1 for enabling the FRET process. Further studies reveal that the selective dissociation of the L–Hg complex in the presence of sulphide anions to restore the native structure of L1 is also useful in the detection of sulfide anions with a detection limit of a submicromolar range in the same medium of HEPES buffer (1 mM, pH 7.4; 2% EtOH) at 25 °C. L1 could be employed as a FRET based time dependent reversible chemosensor for imaging Hg(II) in living cells and whole bodies, and also could be used as an imaging probe for the detection of sulfide anions in HeLa cells.

A FRET-based ‘off–on’ molecular switch: an effective design strategy for the selective detection of nanomolar Al3+ ions in aqueous media

A new water-soluble rhodamine-based Al3+ ion-selective probe (L1) was synthesised and characterized by physico-chemico and spectroscopic tools. In the presence of a large excess of other competing ions, L1 specifically binds Al3+ ions with a concurrent visually observable change from colorless to pink in electronic spectral behavior, making it possible to detect the presence of Al3+ ions with the naked eye. The addition of Al3+ ions to a solution of L1 in HEPES buffer (1 mM, pH 7.4, 2% EtOH) at 25 °C, results in a decrease in the weak fluorescence intensity at λem = 470 nm, while a new peak (at λem = 588 nm) increases gradually through a fluorescence resonance energy transfer process. This ratiometric enhancement helps to detect Al3+ ions at a very low concentration of 33 nM. The detection limit of L1 for Al3+ ions was estimated to be 6.19 × 10−9 M using the 3σ method. This probe is also useful for imaging Al3+ ions in HeLa cells.

A cell permeable Cr3+ selective chemosensor and its application in living cell imaging

An efficient fluorescent Cr3+ receptor, 2-(5,6-dihydro-benzo-[4,5]imidazo[1,2-c]quinazolin-6-yl)-quinolin-8-ol (H2L1) was synthesized and characterized by physico–chemico and spectroscopic tools along with single crystal X-ray crystallography. This probe (H2L1) behaves as a highly selective fluorescent sensor for Cr3+ ions at biological pH in ethanol–water (1 : 5, v/v) HEPES buffer (0.1 M, pH 7.4) at 27 °C. Metal ions, viz. alkali (Na+, K+), alkaline earth (Mg2+, Ca2+), and transition-metal ions ((Mn2+, Fe3+, Co3+, Ni2+, Cu2+, Zn2+) and Pb2+, Ag+ did not interfere. The lowest detection limit for Cr3+ was calculated to be 3.6 × 10−7 mol L−1 within a very short responsive time (15–20 s) in ethanol–water (1 : 5, v/v) HEPES buffer (0.1 M, pH 7.4) at 27 °C. The sensor is efficient for detection of Cr3+ in vitro, developing a good image of the biological organelles.

Development of a rhodamine–benzimidazol hybrid derivative as a novel FRET based chemosensor selective for trace level water

A newly designed rhodamine–benzimidazol hybrid molecule has been developed as a FRET-based chemosensor for the selective detection of trace level water in both polar protic and aprotic organic solvents.

Selective and Sensitive Turn-on Chemosensor for Arsenite Ion at the ppb Level in Aqueous Media Applicable in Cell Staining

A newly designed and structurally characterized cell permeable diformyl-p-cresol based receptor (HL) selectively senses the AsO3(3-) ion up to ca. 4.1 ppb in aqueous media over the other competitive ions at biological pH through an intermolecular H-bonding induced CHEF (chelation-enhanced fluorescence) process, established by detailed experimental and theoretical studies. This biofriendly probe is highly competent in recognizing the existence of AsO3(3-) ions in a living organism by developing an image under a fluorescence microscope and useful to estimate the amount of arsenite ions in various water samples.

Development of a cell permeable ratiometric chemosensor and biomarker for hydrogen sulphate ions in aqueous solution

A newly designed organic moiety, 5H-5,7a,12-triaza-dibenzo[a,e]azulen-6-one (L) containing a seven membered ring behaves as a hydrogen sulphate ion selective ratiometric chemosensor. The formulation and detailed structural characterisation of L have been established using physico-chemical, spectroscopic tools and single crystal X-ray diffraction study. On additions of hydrogen sulphate ions to the solution of L in HEPES buffer (1 mM; water : ethanol (v/v), 98 : 2) at 25 °C at biological pH, a new fluorescence peak generated at 483 nm was increased with concomitant decrease of the weak fluorescence of L at 430 nm through an isoemissive point at 449 nm due to the selective binding of HSO4− ions with L in a 1 : 1 ratio with a binding constant (K) of 4.13 × 106 M−1, and detects HSO4− ions as low as 5.5 × 10−7 M. The ratiometric enhancement of the fluorescence is based on intermolecular hydrogen bonding assisted chelation enhanced fluorescence (CHEF) process which has been evidenced by 1HNMR titration and supported by theoretical (DFT) calculations. The probe (L) having no cytotoxic effect is also useful for the detection of intracellular HSO4− ion concentrations under a fluorescence microscope.

A water soluble copper(II) complex as a HSO4− ion selective turn-on fluorescent sensor applicable in living cell imaging

A water soluble non-fluorescent copper(II) complex (1) of a quinazoline derivative formulated as [Cu(L′)(Cl)] (1) has been synthesized via a facile synthetic method and characterized by physico-chemical and spectroscopic tools along with the single crystal X-ray crystallography for detailed structural analysis. 1 behaves as a highly selective and sensitive for HSO4− ions through the enhancement of fluorescence of the system based on intermolecular hydrogen bonding assisted chelation enhanced fluorescence (CHEF) process in ‘turn-on’ style, which has been confirmed by systematic optical techniques and electrochemical studies. This mode of sensing pathway and binding of HSO4− ions with the receptor 1 has also been validated by optimizing the structures of [Cu(L′)(Cl)] (1) and [Cu(L′)(Cl)]·HSO4− adduct (2) with the help of theoretical calculations. This non-cytotoxic probe senses HSO4− ions as low as 3.18 × 10−7 M in water:DMSO (9:1, v/v) at biological pH (using 1 mM HEPES buffer) and it is also useful for the detection of intracellular HSO4− ions under a fluorescence microscope.

Substituent effect on fluorescence signaling of the cell permeable HSO4- receptors through single point to ratiometric response in green solvent

Two new 2-(2-aminophenyl)benzimidazole-based HSO4− ion selective receptors, 6-(4-nitrophenyl)-5,6-dihydrobenzo[4,5]imidazo[1,2-c]quinazoline (L1H) and 6-(4-methoxyphenyl)-5,6-dihydrobenzo[4,5]imidazo[1,2-c]quinazoline (L2H), and their 1 : 1 molecular complexes with HSO4− were prepared in a facile synthetic method and characterized by physicochemical and spectroscopic techniques along with the detailed structural analysis of L1H by single crystal X-ray crystallography. Both receptors (L1H and L2H) behave as highly selective chemosensor for HSO4− ions at biological pH in ethanol–water HEPES buffer (1/5) (v/v) medium over other anions such as F−, CI−, Br−, I−, AcO−, H2PO4−, N3− and ClO4−. Theoretical and experimental studies showed that the emission efficiency of the receptors (L1H and L2H) was tuned successfully through single point to ratiometric detection by employing the substituent effects. Using 3σ method the LOD for HSO4− ions were found to be 18.08 nM and 14.11 nM for L1H and L2H, respectively, within a very short responsive time (15–20 s) in 100 mM HEPES buffer (ethanol–water: 1/5, v/v). Comparison of the utility of the probes (L1H and L2H) as biomarkers for the detection of intracellular HSO4− ions concentrations under a fluorescence microscope has also been included and both probes showed no cytotoxic effect.

A quinazoline derivative as quick-response red-shifted reporter for nanomolar Al3+ and applicable to living cell staining

A newly synthesized and structurally characterized quinazoline derivative (L) has been shown to act as a quick-response chemosensor for Al3+ with a high selectivity over other metal ions in water-DMSO. In the presence of Al3+, L shows a red-shifted ratiometric enhancement in fluorescence as a result of internal charge transfer and chelation-enhanced fluorescence through the inhibition of a photo-induced electron transfer mechanism. This probe detects Al3+ at concentrations as low as 1.48 nM in 100 mM HEPES buffer (DMSO-water, 1 : 9 v/v) at biological pH with a very short response time (15-20 s). L was applied to biological imaging to validate its utility as a fluorescent probe for monitoring Al3+ ions in living cells, illustrating its value in practical environmental and biological systems.

Selective and sensitive turn-on chemosensor for Al(III) ions applicable in living organisms: nanomolar detection in aqueous medium

A highly sensitive and selective fluorescent reporter L for Al(III) ions was synthesized and characterized by physicochemical and spectroscopic tools along with single crystal X-ray crystallographic study. This is so far the first report of a crystallographically established fluorescence probe having two rhodamine units which make this probe highly sensitive towards Al(III) ions. L, with a high binding affinity towards Al(III) ions of 3.33 × 108 M−2, selectively detects Al(III) ions with almost no interference among various competitive, biologically relevant ions by a strong fluorescence (250 times) as well as colour change in HEPES buffer (1 mM, pH 7.4; acetonitrile/water: 1/3, v/v). The quantum yields (Ф) and molar extinction coefficient (e) of [Al2(L)2(CH3CN)(H2O)(NO3)4](NO3)2 (complex-1) were significantly greater than the sensor L, meaning this probe (L) can detect Al(III) ions at concentrations as low as 3.26 nM, which is comparable with the lowest LOD available in the literature. This non-cytotoxic probe (L) is also an efficient candidate to detect the intercellular distribution of aluminium ions in human lung cancer cells (A549) and Al(III) ions in matured tea leaves.