K. K. Upadhyay

Pyrimidine based highly sensitive fluorescent receptor for Al3+ showing dual signalling mechanism

A new fluorescent probe (5-[(4-diethylamino-2-hydroxy-benzylidene)-amino]-1H-pyrimidine-2, 4-dione) (Receptor 1) has been synthesized by the Schiff base condensation of 5-aminouracil with 4-(diethylamino)salicylaldehyde. The receptor 1 exhibits high selectively for Al(3+) in DMSO as well as in aqueous solution even in the presence of biologically relevant cations such as Na(+), K(+), Ca(2+), Mg(2+), Pb(2+) and several transition metal ions. The lowest detection limit for the receptor 1 was found to be 1.62 × 10(-10) M with its linear response towards Al(3+) in the concentration range of 1.75 × 10(-9) to 3.3 × 10(-8) M in DMSO. Receptor 1 is the first ever example where a single molecular probe is able to show imine (C=N) isomerization inhibition along with twisted intramolecular charge transfer (TICT) in combinatorial fashion.

A coumarin based ICT probe for fluoride in aqueous medium with its real application.

A new coumarin based hydrazone (receptor 1) synthesized by modifying one of our earlier reported receptor detected fluoride ion selectively through naked eye in aq. DMSO (5:95, v/v). It was also able to detect fluoride through naked eye in a toothpaste sample. The addition of 1 equiv. of fluoride as its tetrabutylammonium salt to the 5 x 10(-5) M aq. DMSO solution of the receptor 1 produced red color while the similar addition of acetate produced faint pink color. The dihydrogenphosphate and a variety of other anions were not able to produce any significant color change with receptor 1 under similar experimental conditions. The corresponding UV-vis measurements showed a bathochromic shifting of 455 nm band of receptor 1 to 514 and 484 nm for fluoride and acetate, respectively. The non-linear fittings of corresponding UV-vis titration data in 1:1 binding equation yielded association constants in 10(5):1 ratio for fluoride and acetate, respectively. The (1)H NMR titrations studies shade further light on their mode of binding with receptor 1. The quantum mechanical calculations through time dependant density functional theory (TD-DFT) using basis set b3lyp/6-311g** supported our experimental findings nicely.

Naked-eye recognition of CuII, ZnII and acetate ion by the first guanine-based difunctional chromoinophore

The first guanine-based difunctional colorimetric receptor 8-[(4-nitrophenyl)azo]guanine (receptor 1) was designed, synthesized and characterized by IR, UV-vis, (1)H NMR and mass spectrometric analysis. The synthesized receptor 1 was able to recognize Cu(II), Zn(II) and acetate ion through visible color changes in its DMSO and DMSO-H(2)O (95:5, v/v) solution. The binding affinity of the receptor 1 with the chosen analytes has been estimated in terms of binding constants through non-linear fittings of corresponding UV-vis titration data while the binding mode of the same was studied through (1)H NMR spectral studies.

A Zn2+-responsive highly sensitive fluorescent probe and 1D coordination polymer based on a coumarin platform

A coumarin-based Schiff base (receptor 1) exhibited fluorescence enhancement selectively with Zn(2+) at a nanomolar level in near-aqueous medium (EtOH-H2O; 1:1, v/v). The response was instantaneous with a detection limit of 3.26 × 10(-9) M. The sensing event is supposed to incorporate a combinational effect of intramolecular charge transfer (ICT), chelation-enhanced fluorescence (CHEF) and C[double bond, length as m-dash]N isomerization mechanisms. Various spectroscopic methods, viz. IR, UV-visible, fluorescence and NMR in association with single crystal XRD studies, were used for thorough investigation of the structure of receptor 1 as well as of the sensing event. The Zn(2+) complex of receptor 1 exhibited a very nice 1D chain coordination polymeric framework in its single crystal XRD.

Reversible colorimetric switching of thiophene hydrazone based on complementary IMP/INH logic functions

We have synthesized a thiophene hydrazone (receptor 1) that acts as a colorimetric receptor showing a reversible color switching (ON and OFF) function induced by anion and cation recognition. This remarkable color switching is based on two-input complementary IMP/INH logic functions. The (ON and OFF) process of this molecular switch is triggered by an anion (CH3COO−, C6H5COO−, HCOO−, H2PO4− or F−), while it is quenched by any one MII ion of 3d (d5–d10) as well as CdII and HgII. A DMSO solution of receptor 1 underwent a bathochromic shift from 407 to 505 nm upon adding any one of the above-mentioned anions, while switch OFF was achieved by the addition of any one of the above-mentioned cations to regenerate a band at 407 nm.

Al3+ selective an efficient colorimetric receptor derived from 5-aminouracil.

An interference-free naked-eye recognition of Al(3+) at its micromolar level has been done in 5% aqueous DMSO solution employing a Schiff base 5-[(2-hydroxy-5-nitro-benzylidene)-amino]-1H-pyrimidine-2,4-dione (receptor 1) which is an intramolecular charge transfer (ICT) probe. The pyrimidine and nitrophenyl groups serve as electron rich (donor) and deficient (acceptor) pockets in receptor 1 exhibiting a broad ICT band at 434 nm (olive green). The concomitant additions of Al(3+) as its chloride salt to the 5x10(-5) M aqueous DMSO solution of the receptor 1 lead hypsochromic shifting of its ICT band to 395 nm (colorless). The same ICT band undergoes a marginal bathochromic shifting (6 nm) along with a hyperchromic shift on separate additions of a basic anion like F(-), CH(3)COO(-) and H(2)PO(4)(-) to the receptor 1 and faced almost similar fate on concomitant additions of Al(3+) as mentioned above.

A highly sensitive naphthaoxazole-based cell-permeable ratiometric chemodosimeter for hydrazine

The environmental toxicity, detonable characteristics and widespread usage of hydrazine in industrial activities, coupled with the fact that it is a leading candidate as a hydrogen reservoir, mean that selective methods for the detection of trace levels of hydrazine are much needed. We report herein a ratiometric chemodosimeter (P1) for the highly efficient detection of hydrazine at a lowest level of 1.79 × 10−9 M. The P1 probe was designed by the judicious anchoring of a naphthaoxazole skeleton to malononitrile. The same chemodosimeter also enables the bioimaging of hydrazine in live cells. The performance of P1 was tested in the form of test paper strips, as well as in the solid state for the identification of hydrazine vapours. The sensing mechanism was established through spectroscopic techniques and was further fortified through theoretical calculations using density functional theory.

Brightening Quinolineimines by Al3+ and Subsequent Quenching by PPi/PA in Aqueous Medium: Synthesis, Crystal Structures, Binding Behavior, Theoretical and Cell Imaging Studies

Recent years have witnessed an upsurge of Al3+ selective optical sensors involving simple Schiff bases to other complex organic frameworks. However, more than ∼95% of such reports lack crystallographic evidence, and proposals of binding sites for Al3+ are based upon spectroscopic evidence only. We herein synthesized and fully characterized a quinolineimine derivative (CMO) and explored its potential toward efficient detection of Al3+ with crystallographic evidence. The ongoing nonradiative photoinduced electron transfer (PET) and excited state intramolecular proton transfer (ESIPT) processes in CMO got inhibited via the chelation enhanced fluorescence (CHEF) effects induced by Al3+, and consequently turn-on fluorescence response was observed with 18-fold emission enhancements. The theoretical calculations performed were in good consonance with experimental results. We also explored further the applicability of the CMO·Al3+ complex toward highly sensitive and selective detection of inorganic phosphate (PPi) and an explosive picric acid (PA) via fluorescence quenching processes through two different chemical routes. The bioimaging of Al3+ and PPi were carried out in the living human cancer cells (MCF-7).

Design-specific mechanistic regulation of the sensing phenomena of two Schiff bases towards Al3+

We report herein two optical probes (R1 and R2) for the fluorogenic detection of Al3+ at the level of 10−8 M. R1 and R2 were synthesized by simple Schiff base condensation of 4-amino-3-hydroxy-1-naphthalene sulfonic acid with 5-bromosalicaldehyde and 2-hydroxy-1-naphthaldehyde, respectively. The same were characterized by various spectroscopic techniques. R1 and R2 both underwent fluorescence emission upon their respective interactions with Al3+ in an ethanol:water mixture (4:1, v/v). The binding modes of the receptors with Al3+ were studied through 1H NMR spectroscopy, Job plots, and HR-MS, as well as through binding constant determination involving fluorescence titration data. The quenching of –CN isomerization and of photoinduced electron transfer (PET) seem to be responsible for the fluorogenic switch-on situation of R1 and R2 with Al3+. At the same time, excited state intramolecular proton transfer (ESIPT) also plays an important role in the ratiometric fluorescence response of R2, which is a consequence of a minor structural variation in R1 where the bromophenyl moiety is replaced with a naphthalene moiety. The mechanistic aspects of the sensing phenomenon are discussed in terms of 1H NMR titration as well as theoretical calculations at the density functional level.

Cysteine, homocysteine and glutathione guided hierarchical self-assemblies of spherical silver nanoparticles paving the way for their naked eye discrimination in human serum

Herein, we unravel the naked eye discrimination of three important biothiols viz., cysteine, homocysteine and glutathione through silver nanoparticles immobilized over an azonaphthol. This visual detection is an outcome of three hierarchical self-assemblies in the form of flowers, thorns and petals guided by cysteine, homocysteine and glutathione respectively. The bright yellow silver nanoprobe displayed bluish gray colour with cysteine, purple pink colour with homocysteine and orange colour with glutathione with lowest detection limits of 260 nM, 10 nM and 4.11 μM respectively. The same nanoprobe was also successful in detecting the above mentioned biothiols in human serum.