Chain length effect of spiro-ring N-alkylation on photophysical signalling parameters in Fe(III) selective rhodamine probes.

Manifestation of photophysical signalling parameters in rhodamine derivatives exhibiting complexation induced spiro-ring opening is crucial for the realization of selective metal ion detection at trace levels. Substitution of various functional groups, such as alkylation to the core architecture, modulates the physico-chemical properties of such molecular probes. Despite a few studies, relationships between the extent of photophysical signal modulations and the chain lengths of n-alkyl substituents are still elusive. In this investigation, a few molecular probes based on the rhodamine B (1-5) and rhodamine 6G (6-10) platform were synthesized by their derivatization with n-alkyl substituents of varying chain lengths at the amino-donor of their spiro-ring end, which exhibited Fe(III)-selective absorption and fluorescence 'off-on' signal transduction along with colorization of solution. The Fe(III)-selectivity in these probes remained the same despite their structural distinctions through varied n-alkyl chain lengths of the substituents; however, the quantifiable signalling parameters such as spectroscopic enhancement factors, sensitivity, the kinetics of spiro-ring opening and effectiveness of probe-Fe(III) interactions were analyzed. These parameters were also correlated in terms of the influence of different chain lengths of n-alkyl substituents that efficiently contributed to their inter-componential interactive stereo-electronic environment.

Effect of n-alkyl substitution on Cu(ii)-selective chemosensing of rhodamine B derivatives.

Rhodamine B hydrazide-based molecular probes (1-10) were synthesized by derivatization with n-alkyl chains of different lengths at the hydrazide amino end. These probes exhibited selective absorption (A∼557) and fluorescence (I∼580) 'off-on' signal transduction along with a colourless → magenta colour transition in the presence of Cu(ii) ions among all the competitive metal ions investigated. The effective coordination of these probes to Cu(ii) ions under the investigated environment forming [Cu·L]2+ (L = 1-5) and [Cu·L2]2+ (L = 6-10) complexes led to their spiro-ring opening, which in turn was expressed through signatory spectral peaks of ring-opened rhodamine. All these probes exhibited Cu(ii) selectivity in signalling despite structural modifications to the core receptor unit through variation of the nature of the alkyl substituents. However, the sensitivity of the signalling and kinetics of the spiro-ring opening varied and could be correlated with the number of carbon atoms present in the n-alkyl substituents. Structural elucidation with X-ray diffraction and X-ray photoemission spectroscopic analyses provided further insight into the structure-function correlation in their Cu(ii) complexes. These probes with Cu(ii) coordination showed selectivity in signalling, high complexation affinity (log Ka = 4.8-8.8), high sensitivity (LOD = 4.1-80 nM), fast response time (rate = 0.0017-0.0159 s-1) and reversibility with counter anions, which ascertained their potential utility as chemosensors for Cu(ii) ion detection.

Signaling preferences of substituted pyrrole coupled six-membered rhodamine spirocyclic probes for Hg(2+) ion detection.

Two six-membered spirocyclic rhodamine derivatives containing substituted pyrroles (1 and 2) were shown to exhibit preferential dual mode 'turn-on' signaling responses in the presence of Hg(2+) ions through complexation mediated spiro-ring opening. Although the rate of spiro-ring opening mediated through complexation is much slower in these probes in comparison to their five membered analogues, they exhibited a comparable sensitivity in metal ion detection.

pKa Modulation in rhodamine based probes for colorimetric detection of picric acid.

Tuning the pKa in acid sensitive rhodamine spirolactam derivatives as a function of the solvent medium resulted in the selective detection of picric acid from its lower nitro phenolic analogues and a few other carboxylic acids.

Preferences of rhodamine coupled (aminoalkyl)-piperazine probes towards Hg(II) ion and their FRET mediated signaling.

The metal ion induced absorption and emission signaling pattern of rhodamine coupled bis-(aminopropyl)-piperazine (1-3) and (aminoethyl)-piperazine (4) based probes evaluated in MeCN as well as in an MeCN-H2O binary mixture medium revealed that these probes exhibit optical signaling perturbations to a varying extent in MeCN, however, their complexation induced signaling could be tuned selectively towards Hg(II) in the presence of an aqueous component in the solvent medium where competitive interactions such as metal-probe interactions and hydration of metal ions play the determining factor to induce aqueous promoted Hg(II) selectivity. Attachment of another fluorophore (anthracene and nitrobenzofurazan moieties in 2 and 3 respectively) at the other end of the rhodamine coupled bis-(aminopropyl)-piperazine receptor enabled these probes to facilitate a complexation induced fluorescence resonance energy transfer (FRET) from the excited fluorophore to the ring-opened rhodamine along with contributions through operative PET inhibition and rhodamine delactonization processes. The enhancement in absorption transition of these probes at ~557 nm upon selective Hg(II)-complexation and consequent colourless to pink colour change in the solution imply a chromogenic signaling pattern whereas simultaneous fluorescence amplification and/or FRET initiation lead to fluorogenic signaling to facilitate detection at lower concentration. The Hg(II)-selective photo-physical spectral modulation in the presence of other competitive metal ions, and their reversible dual channel signaling pattern under the action of counter anions or chelating agents such as EDTA or ethylenediamine establish the potential of these probes for highly selective, sensitive and reversible 'OFF-ON-OFF' detection of Hg(II). The complexation induced optical signaling pattern of probes with a propyl-linker in their receptor (1-3) in comparison with that of 4 consisting of an ethyl-spacer indicate that signaling probe design with a substituted 'aminoalkyl-lactonized-rhodamine' subunit preferentially exhibit Hg(II) selective and sensitive dual mode signaling in an organic-aqueous mixture medium irrespective of carbon-length of the flexible alkyl spacer.

Anti-cancer potential of substituted "amino-alkyl-rhodamine" derivatives against MCF-7 human breast cancer cell line.

Breast cancer is the most prevalent diagnosed cancer among women and the main cause of morbidity and mortality. As for breast cancer, MCF-7 cells are an important candidate since they are widely utilized in research for estrogen receptor (ER)-positive breast cancer cell assays, and various sub-clones have been identified to reflect different classes of ER-positive tumors with varied levels of nuclear receptor expression. Rhodamines and its derivatives have shown a great interest over the past two decades due to their excellent structural and spectroscopic properties. Rhodamine derivatives have been widely investigated for their mitochondrial targeting and chemotherapeutic properties. Rhodamine derivatives, in particular, have been widely investigated for their therapeutic properties. In this regard, several studies have shown that rhodamine dye derivatives have promising in vitro and in vivo therapeutic efficacy. The present study deals with potential anticancer activity of few synthesized rhodamine derivatives against MCF-7 cell lines.

Alteration of selectivity in rhodamine based probes for Fe(III) and Hg(II) ion induced dual mode signalling responses.

The probes for metal ion induced chromo- and fluorogenic signalling responses alter their selectivity depending upon the nature of substituent as well as a function of solvent medium. 2 has shown selectivity towards Fe(III) ion, 4 towards Hg(II) ion while 3 is responsive towards both Fe(III) and Hg(II) ions.

A Pyrene-Rhodamine FRET couple as a chemosensor for selective detection of picric acid.

Selective detection of nitroaromatic compounds such as Picric acid (PA), those being explosive materials and hazardous pollutants of environmental and biological concern is highly desirable. With the operational advantages of the chemosensing approach, a pyrene-rhodamine-B couple (1) was explored in this investigation as a ratiometric molecular probe for selective and sensitive detection of picric acid. The bi-fluorophoric probe displayed absorption and fluorescence enhancements along with colourless to reddish-brown colour transition as signaling responses in the selective presence of PA among all the nitro aromatic analyte investigated. The signaling module relies on PA- mediated modulation of various operational photo-physical processes such as (a) inhibition of photo-induced electron transfer (PET) operative from amino-donor to excited pyrene (b) a conformational translation through spiro-ring opening of rhodamine-B segment, and (c) initiation of Fluorescence Resonance Energy Transfer (FRET) between excited pyrene donor and ring-opened rhodamine acceptor. The ratio of fluorescence from both fluorophores (pyrene and Rhodamine) as output channel displayed sensitive signaling performance (LOD = 13.8 nM) in the detection of PA. The investigation that inferred to the PA-induced selectivity in signalling, higher binding affinity (log Ka≈11), a faster response time, and reversibility in signalling with a counter analyte and an operational pH range established the probe's efficacy as a chemosensor for PA detection.

Water induced chromogenic and fluorogenic signal modulation in a bi-fluorophore appended acyclic amino-receptor system.

An acyclic amino-receptor based bi-fluorophoric signaling system 3 exhibits water-induced simultaneous dual channel chromogenic and fluorogenic signal modulation. Its micromolar solution in various organic solvents exhibits an enhancement in absorption in the presence of water in trace amounts through the water-induced delactonization of rhodamine dye, rendering a visual perception as a function of colour change. The presence of water molecules also facilitates a fluorescence resonance energy transfer (FRET) from the excited nitro-benz-oxa-diazole fluorophore to rhodamine dye of 3 and leads to an enhancement of emission up to a second order of magnitude. The failure of the reference compounds 2 and 4 to respond under similar conditions reveals the role of structural variation of the receptor on the perturbation of photophysical processes.

Rhodamine-based probes for metal ion-induced chromo-/fluorogenic dual signaling and their selectivity towards Hg(II) ion.

The new signaling probes 2-6, rhodamine-B derivatives of various receptors which contain different donor atoms for effective metal ion coordination, were synthesized and their absorption as well as fluorescence spectral responses were evaluated in the presence of various metal ions. All these probes along with the reference probe 1 have exhibited optimal metal ion-induced absorption and fluorescence enhancement with Hg(II) ion in the longer wavelength region (>500 nm) in MeCN, exploiting the spectral characteristics of metal ion-induced structural transformation of rhodamine. The selectivity and sensitivity towards Hg(II) ion were better pronounced in MeCN-H(2)O (1 : 1 v/v) medium, implying the role of the solvent molecules, water in particular, in the preferential Hg(II) coordination environment. Complexation of Hg(II) to 1-6 not only enhanced the absorption at ~560 nm, which turned the colourless solution into pink to facilitate a naked eye detection, but also amplified the fluorescence intensity simultaneously to offer high sensitivity of detection at lower concentration. The Hg(II)-induced photo-physical spectral responses of 1-6 in presence of other competitive metal ions rendered their high selectivity towards Hg(II). Further, their reversible dual channel signaling pattern under the action of counter anions, exploiting coordination tendency of anions towards Hg(II), which compete with probe-metal interaction, implied the reversibility in their Hg(II) coordination. The selectivity, sensitivity and reversibility, in principle, establishes the potential of these probes as chemosensors for Hg(II) ion detection.

Selective DCP detection with xanthene derivatives by carbonyl phosphorylation.

Rhodamine derivatives (1-2) exhibited dual channel 'turn-on' photophysical signalling selectively with diethyl chlorophosphate (DCP) among various organophosphates (OPs), where the spiro-ring opening corresponds to the adduct formation through phosphorylation at their carbonyl O-spiro nucleophilic centres.

Two-step FRET mediated metal ion induced signalling responses in a probe appended with three fluorophores.

A tri-fluorophoric molecular probe (1) with three different derivatized fluorophores, i.e. anthracene (An), 7-nitrobenz-2-oxa-1,3-diazole (NBD) and rhodamine-B (Rh) appended on to a Tren [tris-(2-aminoethyl)amine] receptor was demonstrated to exhibit metal ion induced ratiometric fluorescence signalling through the initiation of a two-step fluorescence resonance energy transfer (FRET) process owing to a compatible and substantial spectral overlap of electronic absorption and fluorescence of initial donor-intermediate donor/acceptor-final acceptor pairs. The steady-state and time-resolved fluorescence spectral investigation of probes (1a-1f) with individual excitation energy donor (D)/acceptor (A) fluorophores as well as different bi-fluorophoric D-A pairs appended to the same receptor, and comparison with those of 1 revealed a higher energy transfer efficiency between the initial donor (FAn) and the final acceptor (FRh) in the presence of an intermediate (FNBD) fluorophore. The determination of FRET efficiency involving rhodamine-B derivatized systems is complicated as the dynamics and equilibria of their complexation induced spirolactam to ring-opened transformations are driven by various influential parameters; despite this, the FRET efficiencies in 1 and other intermediates were determined and compared under similar conditions for a better comprehension of individual fluorophoric contribution in two-step FRET-based sensing. The selectivity in chelation induced two-step FRET mediated fluorescence signalling in 1 is controlled by a coordination environment and solvent medium; it exhibited Hg2+ ion selective signalling responses in aqueous-MeCN medium while rendering discriminatory responses towards Fe3+/Fe2+ redox couples in MeCN medium.

Good's buffer derived highly emissive carbon quantum dots: excellent biocompatible anticancer drug carrier.

Here, a facile one-step approach has been developed for the synthesis of carbon quantum dots (CQDs) from Good's buffer. The as-synthesized CQDs emit a bright greenish blue coloured fluorescence after exposure to a 365 nm UV-lamp illumination. The bright fluorescence nature of the CQDs has proven them to be excellent probes for cellular imaging. The CQDs are highly biocompatible in nature, which has been validated by an MTT assay test. The in vitro MTT assay demonstrates a more than 95% survival rate when HEK293 (human embryonic kidney) and H357 (human oral squamous carcinoma) cells were treated with CQDs. The low cytotoxicity of Good's buffer derived CQDs opens the door to biomedical applications. The anticancer drug doxorubicin (DOX) was successfully loaded on the CQDs and their delivery efficiency to the target cells via in vitro treatment of cancerous cells was explored. The CQDs supported DOX showed a higher killing rate of the cancer cells compared to bare DOX due to its ease of internalization and efficient pH-triggered release inside the cells.

Attachment of electron-withdrawing 2,4-dinitrobenzene groups to a cryptand-based receptor for Cu(II)/H+ specific exciplex and monomer emissions.

[reaction: see text] In a cryptand-based fluorescent signaling system with the configuration "fluorophore-spacer-receptor", attachment of the electron-withdrawing 2,4-dinitrobenzene groups to the cryptand receptor renders it highly selective for Cu(II). The system exhibits dual monomer and exciplex emissions in the presence of either of Cu(II) and H(+) as input with concomitant movements of one of the 2,4-dinitrobenzene groups.

Perturbation of the PET process in fluorophore-spacer-receptor systems through structural modification: transition metal induced fluorescence enhancement and selectivity.

Several fluorescent signaling systems are built in the format fluorophore-spacer-receptor with ethylenediamine or N,N-dimethylethylenediamine as the receptor, anthracene as the fluorophore, and a methylene group as the spacer. The receptors are derivatized with different electron-withdrawing groups such as 4-nitrobenzene, 4-nitro-2-pyridine, and 2,4-dinitrobenzene, to perturb the photoinduced intramolecular electron transfer (PET) process from the nitrogen lone-pair to the fluorophore. The photophysical properties of these supramolecular systems and their fluorescence responses toward a number of quenching transition metal ions are reported. It is shown that the PET is highly efficient in the absence of a metal ion. With a metal ion input, the fluorescence can be recovered to a different extent depending on the nature of the metal and on the overall architecture of the system as well. Despite the possibility of strong interaction between the fluorophore and the metal ion, significant fluorescence enhancement is observed with quenching of paramagnetic transition metal ions. The complex stability data show that the stability constants for the metal ions showing fluorescence enhancement are of the order of 10(4) M(-1). This study shows that structurally simple fluorescent signaling systems for quenching transition metal ions can be built by maximizing the PET. It is also shown here that simple structural modification can make these systems highly specific for particular transition metal ions for potential applications in several contemporary areas of research.

Fluorescence enhancement of a signaling system in the simultaneous presence of transition and alkali metal ions: a potential AND logic gate.

Attachment of a laterally non-symmetric cryptand and a macrocycle at the 9- and 10-positions of anthracene leads to a fluorescent signaling system L1 which gives fluorescence enhancement in the simultaneous presence of alkali and transition metal ions.

Dual mode signaling responses of a rhodamine based probe and its immobilization onto a silica gel surface for specific mercury ion detection.

A 3-aminomethyl-(2-amino-1-pyridyl) coupled amino-ethyl-rhodamine-B based probe (2) exhibited simultaneous chromogenic and fluorogenic dual mode signaling responses in the presence of Hg(II) ions only among all the metal ions investigated in an organic aqueous medium. The spiro-cyclic rhodamine signaling subunit undergoes complexation induced structural transformation to result in absorption and fluorescence modulation. Its complexation induced signaling exhibited reversibility with various contrasting reagents having higher affinity towards Hg(II) ions, such as anions (AcO(-)) and competing chelating agents (En). It also exhibited Hg(II)-specific photophysical signaling responses when immobilized onto a silica gel surface attached through its amino-ethyl-receptor end, owing to its structure-conformational advantages for effective coordination. The surface modified silica appended with 2 (SiR-1), as evaluated through the FTIR spectral pattern, thermogravimetric analysis, FESEM images, elemental analysis, X-ray diffraction, surface area determination and particle size analysis, also exhibited reversible Hg(II)-specific signaling in its suspension state in an aqueous medium, enhancing the probe's utility for practical applications such as the detection, isolation and extraction of Hg(II) ions in the presence of other competitive metal ions.

Transition-metal-induced fluorescence resonance energy transfer in a cryptand derivatized with two different fluorophores.

A laterally nonsymmetric aza cryptand has been derivatized with one 7-nitrobenz-2-oxa-1,3-diazole (fluorophore(1)) and one/two anthracenes (fluorophore(2)) to obtain 1 and 2. Their emission characteristics are probed in the presence of a number of transition metals and proton. In the case of 1, Cu(II), Zn(II), Cd(II), and proton afford a large enhancement of fluorescence, whereas Fe(II) and Ag(I) exhibit one order of magnitude less enhancement. In contrast, 2 gives a large enhancement with Cu(II), Ag(I), and proton. The enhancement is observed in the diazole moiety even when the anthracene fluorophore is excited because of substantial fluorescence resonance energy transfer from anthracene to the diazole moiety. Compounds 1 and 2 can be termed as the second-generation fluorescence signaling systems.

Attachment of an electron-withdrawing fluorophore to a cryptand for modulation of fluorescence signaling.

The laterally nonsymmetric aza cryptand synthesized by condensing tris(2-aminoethyl)amine (tren) with tris[2-[(3-(oxomethyl)phenyl)oxy]ethyl]mine readily forms mononuclear inclusion complexes with both transition- and main-group-metal ions. The fluorophore 7-nitrobenz-2-oxa-1,3-diazole is attached to one of the secondary amines, to give an integrated fluorophore-receptor configuration. The fluorophoric system does not show any appreciable emission when excited due to an efficient photoinduced intramolecular electron transfer (PET) from the nitrogen lone pair. When a metal ion enters the cavity, the PET is blocked, causing recovery of fluorescence; Cd(II) gives the highest quantum yield. The fluorophore, with pi-accepting ability, drastically alters the binding property of the cryptand. With perchlorate or tetrafluoroborate salts of Cd(II), the metal ion enters the cavity, causing recovery of fluorescence. However, in the presence of coordinating ions such as Cl-, N3-, and SCN-, the metal ion comes out of the cavity, causing PET to take place once again, and the fluorescence is lost. Thus, translocation of Cd(II) between the inside and outside of the cryptand cavity can lead to a reversible fluorescence on/off situation.