5. Probing Cancer Cells through Intracellular Aggregation Induced
5.3 Experimental Section 118
Copper (II) nitrate hexahydrate (Merck, India), L-cysteine (Sigma Aldrich, USA), chitosan (high molecular weight, deacetylated chitin, viscosity average molecular weight 310-375 kDa, Sigma Aldrich, USA), sodium hydroxide (Merck, India), quinine sulphate (Sigma Aldrich, USA), sulphuric acid (Merck), coumarin 153 (Sigma Aldrich, USA), bovine serum albumin (BSA, Sisco Research Laboratories Pvt. Ltd., India), lysozyme (Sisco Research Laboratories Pvt. Ltd., India), 3-(4,5- dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT, Himedia, India) were purchased and used as received without further purification. Milli-Q grade water (18.2 MΩ cm, Millipore) was used for all experimental works.
5.3.2. Preparation of Chitosan solution (pH= 5.5)
15 mg chitosan was dissolved in 3 mL water by adding a few drops of glacial acetic acid (99-100%) under constant stirring. The pH of the resulting solution was then adjusted to pH= 5.2±0.2 with 1.0 M NaOH.
5.3.3. Synthesis of cysteine stabilized copper nanoclusters (CuNCs)
(a) Red emitting CuNCs. In a typical synthesis, 30 mg L-cysteine was added to 4 mL aqueous solution of chitosan-water mixture (containing 0.5 mL, 5mg/mL as prepared chitosan solution). The resulting mixture was stirred vigorously for 3 h at room temperature. At the end of 3 h of stirring, 75 mM Cu(NO3)2.6H2O solution was added dropwise to the reaction mixture, which was then allowed to stir for 30 min at the same condition. The resulting transparent CuNCs so obtained exhibited bright orange-red emission under 365 nm UV-light illuminations. (pH= 4.5) The so obtained Cu NCs were further purified by dialysis using 1kDa dialysis membrane for 1 h and was stored at 4 °C for further use.
(b) Green emitting CuNCs. The as-synthesized Cu NCs obtained at pH=4.5 was then further treated with 1.0 M NaOH to gradually increase the pH of the medium to 7.4. The clear solution immediately turned into semi-transparent dispersion, which was allowed to age for 1-6 h. During the aging time, the semi-transparent dispersion turned into a yellow-brown dispersion, which exhibited striking green fluorescence under 365 nm UV-light illuminations.
(c) Time-dependent PL study of CuNC dispersion at pH 7.4. 4 mL of the as - synthesized CuNC dispersion was taken and pH of the dispersion was adjusted to 7.4 (using 0.1 M NaOH). PL spectra of the dispersion were then recorded at different intervals of time to monitor the change in emission intensity.
(d) Cysteine stabilized CuNCs in absence of chitosan. Freshly prepared Cu(NO3)2.6H2O (75 mM) was added dropwise to 4 mL aqueous solution containing 30 mg L-cysteine. The mixture was allowed to stir for 30 min when a cloudy dispersion of CuNCs was formed. The solution was allowed to stand for 1 h when
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white precipitate of the as-synthesized CuNCs separated out. Thereafter, the precipitate was collected and redispersed in 4 mL water for further use.
5.3.4. Control experiment
(i) Time dependent AIE study in presence and absence of chitosan
4 mL from each of the dispersions containing CuNCs and synthesized in the presence and absence of chitosan was taken in a vial and the pH of dispersion was slowly adjusted to 7.4 using 0.1 M NaOH. Thereafter, time-dependent kinetic study was continued till 240 min.
(ii) Time dependent AIE study of as - synthesized CuNCs and in presence of BSA and lysozyme at pH 7.4
BSA and lysozyme were chosen as representative proteins for the experiments. A stock solution of 12 mg/mL of BSA and lysozyme were prepared. 4 mL as - synthesized CuNCs was diluted to 10 mL and was kept for three different set of experiment. 3 mL each of diluted dispersions of CuNCs was taken in three different vials. Thereafter, 1 mL as prepared BSA and lysozyme solution (12 mg/mL) each was added to two respective vials and the mixture was allowed to homogenize by gentle shaking. The pH of the dispersion of all three sets was then adjusted to 7.4 using 0.1M NaOH and time-dependent kinetic study was carried out at various time intervals till 240 min. Each set of experiments were carried out in triplicate.The rate of AIE kinetics was obtained from the plot of intensity ratio (I/I0) versus time. For all experiments, intensity at 470 nm was monitored.
Thereafter, rate constant values were obtained from the slope of corresponding integrated rate equation plot i.e., Ln(I/I0) versus time.
5.3.5. Cell Culture Studies
Human breast adenocarcinoma cells (MCF-7) and Human embryonic kidney cells (HEK-293) for cellular study were acquired from the National Centre for Cell Sciences (NCCS), Pune, India. MCF-7 cells and HEK-293 cells were cultured in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% (v/v) fetal bovine serum (PAA Laboratories, Austria), L-glutamine (4mM), penicillin (10000
units), streptomycin (10 mg/mL, Sigma-Aldrich) and maintained in 5% CO2
humidified incubator at 37 °C.
5.3.5.a. Cell viability assay
To carry out cell viability assay, MCF-7 cells and HEK-293 cells (5×103 cells/well) were seeded in a 96 well microplate, containing DMEM growth medium supplemented with 10% (v/v) fetal bovine serum (FBS) under 5% CO2 humidified incubator at 37 °C, overnight. The MCF-7 cells as well as HEK-293 cells were first treated with varying concentrations of CuNCs (0.072−0.362 μg/mL with respect to copper). Copper content in the NC probe was estimated by atomic absorption spectroscopic (AAS) measurements. After 24 h incubation, MTT based cell viability assay was carried out. The absorbance at 570 nm due to formazan formation in DMSO was recorded using a TECAN microplate reader. The assay was carried out in triplicates for each concentration of NC probe mentioned above. Cell viability was calculated as follow:
where A570 corresponds to absorbance of formazan and A690 arises due to background.
5.3.5.b. Intracellular pH adjustment
To carry out live cell imaging at two different pH conditions, pH of culture media (phenol red free) was adjusted to pH 4.5 with 0.1 M HCl and pH 7.4 with 0.1 M NaOH, and measured by pH-meter. For imaging in acidic condition, the intracellular pH was attained by incubating the cells (MCF-7 or HEK-293) in pH adjusted culture media (pH-4.5) and allowing it to homogenize for 10 min before addition of the probe. Whereas for imaging in pH 7.4, the probe was incubated in fresh DMEM (pH 7.4) media followed by repeated washing and fixing.
5.3.5.c. Cell culture and confocal laser scanning microscopic (CLSM) imaging
100 cells control of
) A (A
cells treated of
) A cell (A
viable
%
690 570
690
570
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(i) Fixed cell imaging: For confocal imaging at two different pH conditions in both MCF-7 and HEK-293, cells were seeded onto a microscopic coverslip placed in two 35 mm culture plates and were allowed to grow overnight in DMEM growth medium. Before treating the cells with the NC probe, for imaging at acidic conditions, the cells were washed with PBS twice, which was replaced with phenol red free culture media (pH 4.5). After homogenization for 10 minutes at acidic conditions the probe (0.144 μg/mL) was added and incubated for 1h. In case of imaging at pH 7.4, the cells were incubated in fresh DMEM (phenol red free, pH 7.4) media with the probe for 1 h. After incubation, the cells from both pH conditions were washed with PBS and were fixed with 70% chilled ethanol.
Similarly, for imaging with cytopainter green, the cells incubated with the probe at pH 4.5, were washed twice with PBS followed by incubation with 1.0 mL cytopainter green for 1 h and fixing. Finally, the fixed cells were mounted on a glass microscopic slide, which was sealed from the edges. The fixed samples were then imaged under Zeiss LSM 880 confocal microscope (λex = 405 nm) and (λex = 488 nm) for the probe and cytopainter green, respectively.
(ii) Live cell imaging: For live cell confocal imaging, cells (MCF-7 and HEK-293) were seeded onto a 35 mm live cell culture plate and allowed to grow overnight in DMEM medium. The cells were then washed with PBS twice, which was replaced with phenol red free culture media (pH 4.5) and were allowed to homogenize for 10 min before treating them with NC probe (0.144 µg/mL). After treating the MCF-7 cells for 30 min under CO2 incubation at 37 °C, live cell imaging was carried out in red channel with the diode laser (λex= 405 nm). To observe intracellular behavior of the probe at pH 7.4 and to get insight of AIE therein, the cells (incubated at pH 4.5) were washed with PBS, which was replaced with fresh DMEM media (pH 7.4, phenol red free). That was then followed by time- dependent live cell confocal imaging under Zeiss LSM 880 confocal microscope.
The live cell imaging was recorded for total 8 cycles with a time lag of 5 min and was obtained in green channel with diode laser (λex =405 nm).
5.3.6. FACS Analysis
The uptake of NC probe and their intracellular behavior at pH 4.5 and 7.8 was
monitored by flow cytometry method. For FACS study, MCF-7 cells (5 × 103) grown overnight in DMEM medium were washed with PBS twice before proceeding for uptake study. The overnight grown cells were first homogenized in phenol red free culture medium (pH=4.5) for 10 min followed by incubation with NC probe (0.144 µg/mL with respect to copper) for 1 h before harvesting them. To confirm the uptake, the MCF-7 cells treated with NC probe for 1 h in acidic culture medium (pH 4.5) were collected by trypsinization. The fluorescence was then recorded in PE-H channel (band-pass filter, 585/42 nm), which corresponds to red emission in Cyto FLEX flow cytometer (Beckman Coulter). Similarly, for AIE study, MCF-7 cells (5 × 103) were grown in 6 well plates followed by homogenization in acidic medium and incubation with the probe (0.144 µg/mL with respect to copper) for an hour as mentioned above. Thereafter cells were washed with PBS, which was replaced with fresh culture medium (pH=7.4). The treated cells were then harvested at different time points viz., 30 min, 60 min, 120 min and 180 min each, trypsinized and time - dependent fluorescence was then recorded in FITC channel (530/30nm) corresponding to green emission, using CytoFLEX flow cytometer (Beckman Coulter).
5.3.7. Kinetic assay to measure the intracellular AIE rate
To carry out kinetic assay, MCF-7 cells (5×103 cells/well) and HEK 293 cells (5×103 cells/well) were seeded in 96 well microplates containing DMEM growth medium supplemented with 10% (v/v) fetal bovine serum (FBS) under 5% CO2
humidified incubator at 37 °C overnight. The cells were then washed with PBS twice, which was replaced with acidic culture medium (pH 4.5). After homogenization for 10 min at acidic condition, the respective cell lines were then treated with 0.144 μg/mL NC probe (with respect to copper) and were then incubated for an hour. To study AIE kinetics in the respective cell lines, the culture media of the treated cells at pH 4.5 was replaced with fresh culture media (pH 7.4) after 1 h of incubation. This was then followed by time dependent kinetic study in 96 well microplate reader. The kinetic measurements were monitored by recording the emission intensity at 470 nm for λex = 365 nm at an intervals of 5
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min each. The assay was carried out in triplicates for both the cell lines at the aforementioned concentrations of the NC probe.
The AIE rates were obtained from the plot of ratio of fluorescence intensity at a given time to the fluorescence intensity at initial time (I/I0) with respect to time.
Rate constant values were then obtained from the slope of the integrated rate equation plot of Ln (I/I0) vs time (t).