Highly Sensitive Ratiometric Chemosensor and Biomarker for Cyanide Ions in the Aqueous Medium.

A newly designed cyanide-selective chemosensor based on chromone containing benzothiazole groups [3-(2,3-dihydro-benzothiazol-2-yl)-chromen-4-one (DBTC)] was synthesized and structurally characterized by physico-chemical, spectroscopic, and single-crystal X-ray diffraction analyses. The compound DBTC can detect cyanide anions based on nucleophilic addition as low as 5.76 nM in dimethyl sulfoxide-N-(2-hydroxyethyl)piperazine-N'-ethanesulfonic acid buffer (20 mM, pH 7.4) (v/v = 1:3). The binding mode between receptor DBTC and cyanide nucleophile has also been demonstrated by experimental studies using various spectroscopic tools and theoretical studies, and the experimental work has also been verified by characterizing one supporting compound of similar probable structure of the final product formed between DBTC and cyanide ion (DBTC-CN compound) by single-crystal X-ray analysis for detailed structural analyses. In theoretical study, density functional theory procedures have been used to calculate the molecular structure and the calculation of the Fukui function for evaluation of the electrophilic properties of each individual acceptor atom. Furthermore, the efficacy of the probe (DBTC) to detect the distribution of CN- ions in living cells has been checked by acquiring the fluorescence image using a confocal microscope. Notably, the paper strips with DBTC were prepared, and these could serve as efficient and suitable CN- test kits successfully.

Introduction

The detection and recognition of anionic analytes have become a field of a huge awareness in recent years.[1−8] Among a wide range of anions, n class="Chemical">cyanide (CN–) ion has been paid considerable attention because of its acutely toxicity; existence in many natural sources such as apple seeds, cassava, and peach kernel and production by certain microbes such as fungus and bacteria.[9−11] In human body, this cyanide ion binds with iron within the protein strongly, which inhibit the enzyme cytochrome c oxidase process and hinder the electron transport; and in turn, this results in vomiting and convulsion and then loss of consciousness and eventual death.[12,13] However, in numerous industrial processes, cyanide as an important chemical anion plays an imperative role in many areas, for example, electroplating and case-hardening of metals, gold mining and extraction, and synthesis of resins, fibers, pharmaceuticals, pesticides, and intermediates, and so on.[14−18] As a result, different international, national, and local regulations and guidelines regularize the level of cyanide in air, water, and other media.[19] The maximum contaminant level for cyanide set by the US EPA in drinking water is 200 g L–1 where as it is 70 g L–1 by the World Health Organization limit in drinking water.[20,21] In spite of the acute toxicity, lower limits of cyanide ion is required for ecosystems and it is as much as 4 and 2 g L–1 set by the Australian and New Zealand Environmental and Conservation Council, respectively, to protect the 99% of the species in the freshwater and marine water.[22] Thus, it has been an utmost crucial task to exploit effective ways for monitoring the presence of cyanide anion, and ultimately, the development of highly selective chemosensor for cyanide ion has become the cynosure to the chemists.

Conventional techniques such as potentiometry,[23] electrochemical,[24] polarography,[25] simple titrations,[26] and flow injection amperometric[27] are time-consuming. Therefore, there is an increasing demand of development of more efficient and sensitive methods to measure cyanide ions directly at the microgram/liter level in different matrices. In this context, fluorescent chemosensors for n class="Chemical">cyanide ions are significantly attractive because of low cost and present numerous advantages, including high sensitivity and easy operation.[28,29]

CN– selective receptors based on the mechanism of n class="Gene">nucleophilic addition reactions,[30−34] hydrogen-bonding interactions,[35,36] coordination,[37] sol–gel technique,[38] ion recognition,[39−44] and metal-cyanide affinity (displacement approach)[45−47] have been reported. In this regard, various organic compounds have been employed as the fluorophore moiety was exploited as sensors for cyanide, hitherto, naphthalene,[48,49] naphthalimide,[50] coumarin,[51] indole,[52] BODIPY,[53] phenothiazine,[54] phenazine[55] but chromone-based selective CN– anion chemosensors are still unexplored.

However, there are a very few “turn-on” and “ratiometric” cyanide sensing probes in the literature.[56,57] As the molecular systems of the enhanced/ratiometric fluorescence signal response with the addition of the target analyte are generally superior in response to those systems of the “turn-off” or quenching fluorescence signals, the design of n class="Chemical">cyanide-selective chemosensors based on off-on/ratiometric signaling pathway is a very meaningful and demanding task. Herein, we have designed, synthesized, and structurally characterized a new chromone-based ratiometric chemosensor probe [3-(2,3-dihydro-benzothiazol-2-yl)-chromen-4-one (DBTC)] which selectively senses CN– ions as low as 5.76 nM in dimethyl sulfoxide–N-(2-hydroxyethyl)piperazine-N′-ethanesulfonic acid (DMSO–HEPES) buffer (20 mM, pH 7.4) (v/v = 1:3). The probe (DBTC) is competent of detecting and sensing the distribution of CN– ions in living cells, which was confirmed by a confocal microscope. Moreover, the DBTC-based paper strips can also be used as CN– ion test kits as these strips remarkably senses CN– ions.

Results and Discussion

Synthesis of the DBTC

The organic moiety DBTC was synthesized by condensation of n class="Chemical">3-formylchromone with 2-aminothiophenol (1:1 mole ratio) in anhydrous ethanol (Scheme ). After cooling, the yellow-colored precipitate was obtained, washed with hot ethanol two times, and then dried giving a yellow-colored shiny crystalline compound. Its chemical structure was well characterized by electrospray ionization (ESI) mass spectrometry, 1H NMR, 13C NMR, and infrared (IR) spectra (Figures S1–S4, Supporting Information) and elemental analysis. Finally, the confirmation of the structure of the organic moiety (DBTC) was established by single-crystal X-ray crystallographic analysis (Figure ).

Scheme 1

Synthetic Procedure of the Probe DBTC and DBTC–CN Complex

Figure 1

Structural representation and atom numbering scheme of DBTC.

Light yellow block-shaped crystals of DBTC suitable for single-crystal X-ray diffraction analysis were obtained after 15 days of slow evaporation of an n class="Chemical">ethanol solution. DBTC crystallized into the monoclinic space group P21/n with Z = 4 and cell volume 1232.19 (Table S1). The organic moiety of DBTC has excellent planarity between chromone and benzothiazole ring with the dihedral angle 3.65°, which is beneficial to green fluorescence properties. A schematic view of the DBTC motif is shown in Figure ; the selected bond lengths and angles are tabulated in Tables S1 and S2 in the Supporting Information.

Synthesis of the DBTC–CN Complex

The ethanolic solution of n class="Chemical">DBTC and aqueous solution of NaCN is mixed in equimolar ratio, and the resulting mixture was stirred for 2 h (Scheme ). The clear resulting solution was filtered, and after three weeks, a reddish black color precipitate was obtained. The DBTC–CN complex was characterized by ESI mass and 1H NMR, 13C NMR, and IR spectra (Figures S5–S8).

The emission spectra of DBTC studied in different solvents of varying polarity (Figure ) show the largely red-shifted fluorescence spectra of n class="Chemical">DBTC with the increase of the solvent polarity. This trend of the characteristic peaks is in support of the ICT pathway as in polar solvents (CH3CN, DMSO, EtOH, and MeOH) the charge-transfer emission is very significant.

Figure 2

Emission spectra of DBTC (λex = 370 nm) in toluene, dichloromethane, CHCl3, dimethylformamide, CH3CN, DMSO, EtOH, and MeOH solvents.

Emission spectra of DBTC (λex = 370 nm) in n class="Chemical">toluene, dichloromethane, CHCl3, dimethylformamide, CH3CN, DMSO, EtOH, and MeOH solvents.

UV–Visible Studies

The absorption spectrophotometric titration in HEPES (20 mM) buffer at pH 7.4 was carried out at 37 °C to understand the mode of interaction of n class="Chemical">DBTC with CN– ions, and it is depicted in Figure which demonstrates the UV–vis titration curve of DBTC with added CN– ion.

Figure 3

Absorption spectra of DBTC (10 μM) in DMSO–HEPES buffer (20 mM, pH 7.4) (v/v = 1:3) upon the titration with CN– ion solution.

Absorption spectra of DBTC (10 μM) in n class="Chemical">DMSO–HEPES buffer (20 mM, pH 7.4) (v/v = 1:3) upon the titration with CN– ion solution.

With the stepwise increase of the concentration of added CN– ions, the absorption intensity of n class="Chemical">DBTC at 350 nm gradually decreases with the appearance of new peaks of weak absorption intensity at 305 and 460 nm. The phenomenon of increase of new peaks with the increase of cyanide ions is due to the interaction of the probe (DBTC) with CN– ion.

pH Studies

The role of CN– ions in biological systems is significantly noteworthy. Therefore, it is imperative of having an efficient sensing probe applicable for detection of n class="Chemical">cyanide ions over a wide range of pH. To verify the pH effect, the emission spectra (at λem = 430 nm) of the probe (DBTC) (10 μM) without and with CN– (1 equiv) were recorded over a wide range of pH 4–12 (Figure S9). This study clearly indicates that the detection of CN– ions is possible at the biological pH 7.4 in DMSO–HEPES buffered solution (1:3 v/v).

Fluorescence Studies

The fluorescence profiles of DBTC and their potential n class="Chemical">CN– reaction products are even more distinct than their corresponding UV–vis spectra. The fluorescence intensity dramatically enhances at λem = 430 nm with the progress of time (Figure S10). A considerable effect on the emission profile of DBTC was observed because of the incremental addition of CN– ions. Here, a huge blue shift of the fluorescence maximum from 540 to 430 nm through an isoemissive point at 505 nm was recorded, and it is of about 110 nm (Figure ). It is noteworthy to mention that the fluorescence intensity at 540 nm was gradually decreased with concomitant increase of the newly appeared fluorescence maxima at 430 nm. Interestingly, a plateau was obtained after 10 equiv addition of CN– ions. This emission profile clearly suggests the formation of a well-defined DBTC–CN compound.

Figure 4

Fluorescence spectra of DBTC (10 μM) upon the addition of 0–10 equiv CN– anion in DMSO–HEPES buffer solution (20 mM, pH 7.4) (v/v = 1:3).

Fluorescence spectra of DBTC (10 μM) upon the addition of 0–10 equiv n class="Chemical">CN– anion in DMSO–HEPES buffer solution (20 mM, pH 7.4) (v/v = 1:3).

In addition, from this study, it is observed that the ratio of the emission intensities at 430 and 540 nm (I430/I540) increased to 9.9 from 0.24 (∼41-fold enhancement) upon the gn class="Gene">radual addition of CN– ions (inset of Figure ). This result significantly designates the probe (DBTC) as a ratiometric fluorescent chemosensor for CN– ions. This spectroscopic change has been supported by the visual color change of the DBTC solution of DBTC from green to blue because of the addition of 10 equiv CN– ions upon excitation by a hand held UV lamp of 370 nm (Figure , inset).

These fluorescence spectral data were used to draw the Job’s plot (Figure S11) curves to calculate the stoichiometry of the interaction of the DBTC receptors with n class="Chemical">cyanide ions. This curve with the maxima at ∼0.5 mole fraction dictates the formation of 1:1 (receptor/cyanide ion) ensemble/compound. From the fluorescence titration data, the binding constant toward the formation of the DBTC–CN compound was calculated to be 4.25 × 105 M–1 using the modified Benesi–Hildebrand equation[66] corresponding to 1:1 stoichiometry.where F0, Fx, and F∞ are the emission intensities of DBTC in the absence of CN– ions, at an intermediate CN– ion concentration, and at a concentration of complete interaction, respectively, and where K is the binding constant and [C] is the CN– ion concentration. From the intercept/slope of the plot of (F∞ – F0)/(Fx – F0) against [C]−1 (Figure ), the K value was determined to be 4.25 × 105 M–1 which clearly indicates that DBTC has a significantly strong binding affinity toward the CN– ions.

Figure 5

Plot of (F∞ – F0)/(Fx – F0) against 1/[CN]−1: Binding constant (K) of 4.25 × 105 M–1 for CN– ions with DBTC.

Plot of (F∞ – F0)/(Fx – F0) against 1/[CN]−1: Binding constant (K) of 4.25 × 105 M–1 for n class="Chemical">CN– ions with DBTC.

To find the limit of detection of CN– by the probe, that is, how lower amount (minin class="Gene">mum concentration) of CN– anion can be estimated by the probe (DBTC), from the fluorescence titration study, it was found that minimum 5.76 nM CN– anion can develop the fluorescence intensity ratio (I430/I540) of DBTC (Figure ).

Figure 6

Plot for the estimation of LOD for cyanide ions in the DMSO–HEPES buffer (20 mM) solution (v/v = 1:3) at pH 7.4.

Plot for the estimation of LOD for cyanide ions in the n class="Chemical">DMSO–HEPES buffer (20 mM) solution (v/v = 1:3) at pH 7.4.

Therefore, the detection limit (LOD) of DBTC for n class="Chemical">CN– ions was calculated to be 5.76 nM by the use of the equation 3σ/S (where S = gradient of the calibration curve and σ is the standard deviation at zero level).[67] This LOD value (5.76 nM) of DBTC for CN– ions calculated from this method is significantly lower than that of MCL.

To evaluate the sensing selectivity of DBTC, fluorescence changes toward n class="Chemical">CN–, various anions were verified in replicate experiments under the same conditions. Among various competing anions, only cyanide ions incredible response of the fluorescence intensity ratio (I430/I540), whereas the other anions such as SCN–, F–, Cl–, Br–, I–, ClO–, ClO4–, N3–, NO2–, NO3–, H2PO4–, H2AsO4–, HCO3–, HS–, S2–, HSO3–, HSO4–, and AcO– did not bring any significant change in the emission intensity ratio (I430/I540) plot (viz., Figure S12). This is reasonable considering the stronger nucleophilicity of cyanide compared with other anions. In addition, to verify the interference, we evaluate the fluorescence response of DBTC with CN– in the presence of other competitive anions and cations (viz., Figures S13 and S14), which clearly showed that the signal response of DBTC induced by CN– was not hampered at any extent by the presence of other different anions and cations.

Density Functional Theoretical Studies

To investigate the mechanism of the ratiometric response of probe DBTC to n class="Chemical">cyanide ion, density functional theory (DFT) calculations were carried out for the probe DBTC and DBTC–CN compound with the B3LYP/6-31G(d) method. The optimized geometries of DBTC and DBTC–CN compound are shown in Figure . DBTC was almost planar, with a dihedral angle of 3.65° between the chromone and benzothiazole moiety which make this two groups in good conjugation, whereas the DBTC–CN compound reveals a tilted conformation with dihedral angle 19.80°, indicating a hampered conjugation. Therefore, the ICT process between chromone and benzothiazole group could proceed efficiently in probe DBTC but be inhibited in the DBTC–CN compound. This reduced ICT character of the probe is due to the disruption of the π-conjugation in the DBTC–CN compound, and in turn, the significant blue shift in the emission spectrum is occurred. It is also well supported by considering the comparison of HOMO–LUMO energy band gaps of DBTC–CN compound which is lower than DBTC.

Figure 7

HOMO–LUMO energy calculation and optimized structure of DBTC and DBTC–CN compound.

HOMO–LUMO energy calculation and optimized structure of DBTC and n class="Chemical">DBTC–CN compound.

The time-dependent DFT (TDDFT) calculations show two important peaks in the 252.44 and 331.65 nm UV spectra of DBTC in n class="Chemical">water. The band around 331.65 nm is governed by the HOMO → LUMO excitations, and the band around 252.44 nm is essentially due to HOMO – 8 → LUMO + 1; HOMO – 7 → LUMO + 1; HOMO – 7 →LUMO + 4; HOMO – 7 → LUMO + 6; HOMO – 4 → LUMO + 1; HOMO – 4 → LUMO + 4; and HOMO – 4 → LUMO + 6 transitions (Figure S15). In the case of DBTC–CN compound, the calculated band around 467.87 nm is attributable to the HOMO – 2 → LUMO; HOMO – 2 → LUMO + 2; HOMO – 1 → LUMO; HOMO – 1 → LUMO + 1; HOMO → LUMO; and HOMO → LUMO + 2 transitions (Figure S16). All of the detailed vertical excitation energies, oscillator strengths, and salient transitions are tabulated in Tables S4 and S5.

To recognize the most suitable atoms for nucleophilic attack,[68] the condensed Fukui function values f+ may be used. These values were calculated to examine the electrophilicity properties of the acceptor atoms of the probe n class="Chemical">DBTC. Selected f+ values are given in Table , considering the atom numbering scheme, viz., Figure .

Table 1

Selected f+ Values Calculated for DBTC

atom	C7	C8	C9	C10
fk+	0.0299	–0.0169	0.1244	0.0524

On the basis of the calculated values of the Fukui functions (viz., Table ), it may be concluded that the C9 acceptor atoms may be predictable to be much stronger acceptor atoms than the C7, C8, and C9 atoms. Hence, n class="Chemical">CN– anion (nucleophile) can attack the C9 carbon atom which is the most electrophilic center in the DBTC probe. Unfortunately, we failed to get the single crystals of the product, the DBTC–CN compound diffractable for single-crystal X-ray diffraction analysis although the probable structure of the DBTC–CN compound has been established by various spectroscopic analyses (ESI mass: Figure S5, 1H NMR: Figure S6, 13C NMR: Figure S7 and IR: Figure S8). In addition, the analogue compound (DBTC-OCH (1)) having the methoxy group (similar nucleophile as cyanide) has been isolated from the same chemical environment using methanol to establish the fact of the formation of the DBTC–CN compound in the ethanol–cyanide mixture. Fortunately, the detailed structure analysis of the compound, DBTC-OCH (1), has been carried out by single-crystal X-ray diffraction analysis (Figure ). This structural analysis (Table S3) dictates the possibility of nucleophilic addition of cyanide ion to the C9 carbon atom of the chromone moiety of DBTC to produce the DBTC–CN compound.

Figure 8

Crystal structures of the compound DBTC-OCH(1).

On the basis of all spectroscopic and theoretical observations, the plausible sensing pathway has been depicted in Scheme .

Scheme 2

Proposed DBTC–CN Formation from Reaction of Receptor DBTC and Cyanide

Cell Imaging Studies

To validate the efficacy of this probe having significant fluorescence color change in solution, some test strips were prepared by immersing filter papers in the DMSO/n class="Chemical">H2O solution of DBTC (1.0 mM) and then dried in air. The DBTC-based test strips were dipped in several aqueous media of CN– ions of different concentrations, and the fluorescence color change from green to blue was observed with gradually increasing concentration of cyanide ions (Figure ). Therefore, this interesting experiment reflects that the DBTC-based test strips can suitably detect CN– in solutions without any other tools.

Figure 9

Photographs of only DBTC (1.0 mM) and after immersion into water solutions with increasing CN– anion on test strips at room temperature and irradiation under UV light at 370 nm.

Photographs of only DBTC (1.0 mM) and after immersion into n class="Chemical">water solutions with increasing CN– anion on test strips at room temperature and irradiation under UV light at 370 nm.

To examine the effectiveness of the probe for CN– ion detection in biological systems, it was performed using n class="Species">human breast adenocarcinoma cell line MDA. In these experiments, both the DBTC and CN– ions were allowed to uptake by the cells of interest and the images of the cells were recorded by the fluorescence microscopy, following excitation at ∼370 nm. Incubation of DBTC alone showed green fluorescence uptake even after 90 min. Uptake into MDA cells was boosted upon using cyanide anion showed blue fluorescence (Figure ).

Figure 10

Confocal fluorescence microscopy images of MDA cells with DBTC only and DBTC plus cyanide ions at 37 °C.

Confocal fluorescence microscopy images of MDA cells with n class="Chemical">DBTC only and DBTC plus cyanide ions at 37 °C.

To verify the compatibility of uses of this probe in the living cells, the cytotoxic effect of the probe (DBTC) was tested against n class="Disease">breast adenocarcinoma cells MDA using a regular method, the MTT assay.[69] The probe (DBTC) showed about 80% cell survivability at their working concentration [100 μg/mL for probe (DBTC)] after 1 and 24 h of incubation both the cases toward cancer cells. Therefore, the probe (DBTC) at a concentration of 100 μg/mL can be used safely for the purpose of fluorescence imaging of cancer cells (Figure S17).

Conclusions

Herein, we have developed an easy to synthesize and structurally characterized potential ratiometric probe (DBTC) which detects n class="Chemical">CN– ions fluorimetrically as low as 5.76 nM anions based on nucleophilic addition in the DMSO–HEPES buffer (20 mM, pH 7.4) (v/v = 1:3). Interestingly, the probe (DBTC) shows the incredible response for cyanide ions specifically over other common anions. This significantly enhanced fluorescence intensity is due to the nucleophilic addition of the nucleophile cyanide ion to the C9 carbon atom of the chromone moiety to form DBTC–CN compound, and it is well supported by the isolation of DBTC-OCH (1), an analogue of DBTC–CN compound. The possibility of DBTC–CN compound is confirmed by the detailed spectroscopic techniques and theoretical studies. The structural analysis of DBTC-OCH (1) also helps us to conclude the feasibility of nucleophilic addition of potential nucleophile (cyanide ion is more potential nucleophile than methoxide ion) to the C9 carbon atom of the chromone moiety. This noncytotoxic probe (DBTC) is competent of acquiring the images of the distribution of cyanide ions in living bodies, and it was confirmed by the confocal microscopic imaging. Again, it is also noteworthy that the DBTC-based paper strips can also be used as CN– ion test kits as these strips sense CN– ions significantly. This developed method of cyanide ion detection in the aqueous medium is superior to the earlier reports (viz., Table S6) in terms of the detection limit and the solvent medium. This ratiometric probe (DBTC) can sense as low as 5.76 nM CN– ions fluorimetrically, which is also lower than the lowest value (30 nM CN– ions) of the reported literature tabulated in the comparative table (Table S6).

Experimental Section

Materials and Methods

Standard procedures have been performed to purify and dry the solvents used for spectroscopic studies, but all other chemicals and reagents purchased from Sigma-Aldrich were used without any purification. Solutions of the anions were prepared from their sodium salts of n class="Chemical">cyanide, thiocyanate, arsenate, azide, nitrite, nitrate, bicarbonate, hydrogen sulfide, hydrogen sulfite, hydrogen sulfate, hypochlorite, chlorate, and tetrabutylammonium salts of halides (F–, Cl–, Br–, and I–), acetate, and dihydrogen phosphate. In HEPES (20 mM) buffered water at pH 7.4, all of the experiments for titration were carried out at 25 °C.

Mass spectra were recorded on a micrOTOF-Q mass spectrometer and XeVOG2QTof HRMS spectrometer in methanol. H NMR was carried out using the Bruker AVANCE DPX 500 MHz spectrometer. Elemental analyses (C, H, and N) were carried out with a n class="Disease">PerkinElmer CHN analyzer 2400. A Systronics pH meter (model 335) was used for the pH measurements. Fourier transform IR (FTIR) spectra were done as KBr pellets using a Prestige-21 FTIR spectrophotometers.

A yellow-colored crystal of DBTC was mounted on a glass tip, and data were collected in a Bruker’s APEX-II CCD diffractometer using MoKα (λ = 0.71069); necessary corrections were applied using SADABS from Bruker.[58] Fourier full-matrix least-squares refinement methods based on F2, using SHELX-97 was used to refine all of the nonn class="Chemical">hydrogen atoms anisotropically in order to solve the structure.[59] WinGX package was also utilized for all of the calculations.[60,61] As per the deposition of the cif files with the Cambridge Crystallographic Data Centre, the codes were allocated as CCDC—1835425 and 1835426 of DBTC and 1, respectively.

Theoretical Calculation

The DFT method at the B3LYP level[62] with Gaussian-09 software over a Red Hat Linux IBM cluster[63] was used for full geometry optimizations of DBTC and their corresponding complexes. The 6-31G(d) basis set was employed for all of the elements. Computation of vertical electronic excitations based on B3LYP optimized geometry was performed using the time-dependent DFT (TD-DFT)[64] formalism in n class="Chemical">water with the conductor-like polarizable continuum model[65] to calculate the fractional contributions of various groups to each molecular orbital.

General Procedure for Fluorescence and Absorption Study

Path length of the cells used for absorption and emission studies was 1 cm. The fluorescence properties of the sensor were investigated in DMSO–n class="Chemical">HEPES buffer solution (20 mM, pH 7.4) (v/v = 1:3). Fluorescence measurements were performed using 5 × 5 nm slit width. All of the fluorescence and absorbance spectra were taken after 30 min of mixing of CN– and DBTC.

Biological Methods

Cell Culture and Platting

Human n class="Disease">breast adenocarcinoma cell line MDA was maintained in Dulbecco’s modified Eagle’s medium supplemented with 10% fetal bovine serum and 1% penicillin–streptomycin antibiotic solution in a humidified CO2 incubator (5% CO2). The cells were detached from a culture flask using trypsin–EDTA (ethylenediaminetetraacetic acid) solution and washed in the growth medium. After final wash, cells were plated into 96-well microtiter plates (4 × 103 cells per well) for the purpose of MTT assay and permitted 24 h to adhere. For the purpose of imaging, cells were seeded on 14 mm glass cover slips into a 6-well microtiter plate and allowed 24 h for complete adherence.

Imaging

Cells growing on glass cover slips were incubated for 1.5 h with 100 μg/mL aqueous solution of DBTC in the presence and absence of n class="Chemical">cyanide ion solution (20 μL), containing 1% DMSO (v/v) to assist solubilization of the compound. Followed by incubation, the cells were washed with phosphate-buffered saline and mounted on grease free glass slide and observed under a Leica TCS SP8, a confocal laser scanning fluorescence microscope.

MTT Assay for the Assessment Cytotoxicity

The cytotoxicity of the compound n class="Chemical">DBTC against human breast adenocarcinoma cells MDA was assessed by exploring the colorimetric and quantitative MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] reduction assay. After complete adherence cells were treated for 1 and 24 h separately in preoptimized doses (100 and 200 μg/mL) of DBTC with or without cyanide ion solution (20 μL). After completion of treatment, the cells were washed with complete media and resuspended in MTT solution (0.5 mg/1 mL) and incubated at 37 °C for 4 h at dark. Dark violet formazan crystals were formed as a result of reaction between MTT and mitochondrial succinate dehydrogenase enzyme (present only in living cells). The resulting crystals were dissolved in DMSO. Cellular viability was determined by quantifying the intensity of formazan crystal using a microplate reader (Bio-Rad).

Characterization of DBTC

DBTC (C16H9NO2S)

Calcd C, 68.80; H, 3.25, N 5.01, O 11.46; Anal. Found: C, 68.47; H, 3.08; N, 4.81. mp 224 ± 2 °C. ESI-MS: [n class="Chemical">DBTC + H+] m/z: 280.00 (observed) [(calcd: m/z: 279.03; where DBTC = molecular weight of organic moiety]; 1H NMR (DMSO-d6, 400 MHz, δ ppm): 7.491–7.453 (t, 1H), 7.590–7.552 (t, 1H), 7.669–7.631 (t, 1H), 7.861–7.840 (d, 1H), 7.965–7.926 (t, 1H), 8.072–8.051 (d, 1H), 8.210–8.191 (d, 1H), 8.289–8.269 (d, 1H) and 9.542 (s, 1H); 13C NMR (400 Hz DMSO-d6): δ 180.38, 155.69, 143.30, 141.56, 136.81, 134.62, 132.33, 132.30, 125.99, 123.51, 122.56, 121.86, 117.99, 114.83, 104.92 and 100.01; IR data νC–S: 750.06, νC=N: 1588.80, νC=O: 1652.77.

Characterization of DBTC–CN Complex

DBTC–CN (C34H18N4O4S2)

Calcd C, 66.87; H, 2.97; N, 9.17; O, 10.48; Anal. Found: C, 66.56; H, 3.21; N, 9.41. ESI-MS: [DBTC–n class="Chemical">CN] m/z: found 307.0522 and 611.0818 [calcd for [M + H]+ 307.0536, where M formulated as C17H10N2O2S and [P + H]+ 611.0842, where P formulated as C34H18N4O4S2]; 1H NMR (DMSO-d6, 400 MHz, δ ppm): 7.068–7.086 (d, 2H), 7.140–7.198 (m, 4H), 7.329–7.365 (d, 2H), 7.404–7.422 (d, 2H), 7.623–7.643 (d, 2H), 7.832–7.889 (m, 4H) and 8.461 (s, 2H); 13C NMR (400 Hz DMSO-d6)): δ 167.85, 165.24, 141.87, 133.97, 133.07, 127.21, 125.80, 125.63, 125.34, 124.62, 122.77, 121.91, 121.63, 121.25, 118.64, 118.41 and 117.97; IR data: νS–S: 668.32, νC–S: 753.62, νC=N: 1531.94, νC=O: 1595.91, νCN: 2201.86.