A metal-organic framework showing selective and sensitive detection of exogenous and endogenous
4.2 Experimental section
15 SPCPtEs screen-printed carbon electrodes
60-460 µM L−1 60 µM L−1 56
16 Pd NW arrays nanowire 2 µM - 1 mM 0.5 µM 57
17 Pd-modified TiO2 electrode
TiO2 electrode 0 - 17.7 mM 0.015 mM 58 18 AgPd/Ch–IL nanoparticles 0.060 - 20 mM 0.022 mM 59 19 PdSPE screen-printed
macroelectrodes
19.68 - 27.84 μM 2 μM 60
20 screen-printed carbon
electrodes modified with gold clusters
gold clusters 1 - 10 mM 0.9 mM 61
C. Enzymatic sensors 21 thin-film
planar
electrodes and immobilised alcohol oxidase
conductometric enzyme biosensor
0.05 - 500 mM 0.05 mM 62
22 electrodes- formaldehyde- dehydrogenase
Dehydrogenase enzyme-based sensor
30 ng mL-1 - 1.5 µg mL-1
30 ng mL-1 63
scanning electron microscope. Dynamic light scattering (DLS) measurements were performed by a Zetasizer Nano ZS90 (model no. ZEN3690) instrument.
Thermogravimetric analyses (TGA) were collected under air atmosphere at a heating rate of 10 °C min−1 in a temperature region of 25-800 °C by employing a Netzsch STA-409CD thermal analyzer. Fluorescence emission behavior was analyzed by a HORIBA JOBIN YVON Fluoromax-4 spectrofluorometer. The excitation wavelength (λex) was 330 nm for all the fluorescence experiments. The nitrogen sorption isotherms were performed employing a Quantachrome Autosorb iQ-MP gas sorption analyzer at -196 °C. Prior to the sorption measurement, degassing of the material was performed at 130 °C for 12 h under dynamic vacuum. A Bruker Avance III 600 spectrometer was used for recording 1H-NMR and 13C-NMR spectra at 600 MHz. Before the NMR measurements, the ligand (5 mg) was dissolved in 500 μL of DMSO-d6. The mass spectrum (in ESI mode) was measured with an Agilent 6520 Q-TOF high-resolution mass spectrometer. Prior to the ESI-MS measurement, 15 mg of 3′ and FA-treated 3′ were separately digested in 2 mL of HPLC grade methanol by the treatment with 200 µL of 48% HF (caution!). For FA-treated 3′, required amount of NaHCO3 was added to the medium to attain neutral pH (as imine bond is unstable in acute acidic medium), filtered and then FA was introduced in the filtrate. For both samples, the organic layer was collected by filtration after 15 min of sonication. Then, the samples were further diluted by adding HPLC grade methanol for the ESI-MS measurements. Fluorescence lifetime measurements were carried out using time correlated single-photon counting (TCSPC) method by an Edinburgh Instrument Life-Spec II instrument.
4.2.2 Preparation of H2BDC-N2H3 ligand
In a 100 mL round-bottom flask, a mixture of 2-amino-1,4-bezenedicarboxylic acid (1.81 g, 10 mmol) and 20 mL conc. HCl was placed. This mixture is cooled to 0 °C and stirred at this temperature for full dissolution. Then, an ice-cold solution of NaNO2 (0.7 g) dissolved in water (5 mL) was introduced. The resulting mixture was stirred for 2 h, maintaining the temperature at 0 °C. Afterward, an ice-cold mixture containing SnCl2∙2H2O(4.5 g) and conc. HCl (15 mL) was introduced. The stirring was continued for another 3 h at ambient conditions. A light yellow colored precipitate was obtained. It was filtered off and washing was carried out several times with water until neutral pH is attained. Finally, drying of the precipitate was conducted in an air oven at 60 °C for 6 h.
The yield was 1.2 g (6.1 mmol, 61 %). 1H-NMR (600 MHz, DMSO-d6): δ = 7.50 (d, 1H),
7.69 (s, 1H), 7.99 (d, 1H), 9.15 (s, 1H) ppm. 13C NMR (150 MHz, DMSO-d6): δ = 168.36, 166.58, 147.26, 135.79, 131.91, 120.42, 117.07, 114.63 ppm. ESI-MS (m/z): 195.0331 for (M-H)- ion (M = mass of H2BDC-N2H3 ligand).In Figures 4.1-4.3 the NMR and mass spectra of the H2BDC-N2H3 ligand are shown.
Figure 4.1 1H NMR spectrum of H2BDC-N2H3 ligand measured in DMSO-d6.
Figure 4.2 13C NMR spectrum of H2BDC-N2H3 ligand measured in DMSO-d6.
Figure 4.3 ESI-MS spectrum of H2BDC-N2H3 ligand measured in methanol. The spectrum shows m/z peak at 195.0331, which corresponds to (M-H)− ion (M = mass of H2BDC- N2H3 ligand).
4.2.3 Preparation of [Al(OH)(BDC-N2H3)]·0.85DEF·1.0H2O (Al-MIL-53-N2H3;3) In a Pyrex tube, Al(NO3)3·9H2O (57 mg, 0.15 mmol) and H2BDC-N2H3 (30 mg, 0.15 mmol) ligand was placed. Then, 3 mL of DEF/H2O (1 mL/2 mL) mixture was poured into the tube, which was sealed and sonicated for 15 min. Afterward, it was heated at 120
°C for 24 h using a block heater. The resulting yellow colored material was obtained by filtration and dried inside an air oven at 70 °C for 6 h. Yield: 30 mg (0.08 mmol, 53%).
The yield was calculated on the basis of the aluminum salt. FT-IR (KBr, cm−1): 3426 (br), 2968 (sh), 2934 (w), 2851 (w), 1657 (m), 1628 (sh), 1580 (vs), 1505 (w), 1430 (s), 1381 (sh), 1313 (w), 1265 (m), 1216 (w), 1098 (m), 1003 (m), 853 (w), 804 (m), 768 (m), 678 (m), 578 (br), 481 (br).
4.2.4 Activation of the material
At first, heating of the as-synthesized compound (100 mg) was conducted at 150
°C for overnight in 20 mL DMF by using a Teflon-lined stainless steel autoclave. Later, stirring of the filtered compound was carried out in acetone (50 mL) for overnight at room temperature. In the third step, heating of the filtered material at 130 °C for overnight was performed using high vacuum pump. The guest-free material is named as 3′.
4.2.5 Fluorescence sensing experiments in aqueous medium
The probe (5 mg) was placed in a glass vial and water or 10 mM HEPES buffer (5 mL each) was added. The preparation protocol for the HEPES buffer solution has been reported by us previously.47 The mixture was sonicated for 1 h. After keeping for one day at room temperature, a stable suspension was obtained.
The above-mentioned suspension (200 μL) of the probe (in water or HEPES buffer) was placed inside a quartz cuvette. Then, we have added 2800 μL of deionized water or HEPES buffer to the suspension. All the fluorescence titration measurements were carried out using the resulting suspension. These experiments involved addition of the solutions of various aldehydes (20 mM) to the suspension (3 mL) of the probe. Formaldehyde, oxaldehyde, acetaldehyde and 4-chlorobenzaldehyde are readily soluble in water.
Therefore, 20 mM aqueous or HEPES buffer solutions of these aldehydes were prepared in pure deionized water. The other aldehydes (butyraldehyde, propionaldehyde, valeraldehyde, crotonaldehyde, benzaldehyde and anisaldehyde) are insoluble in water.
For them, the required amount of an aldehyde (for 20 mM solution) was dissolved in minimum amount of distilled ethanol. Then, deionized water or HEPES buffer was added to fulfil the required volume.
4.2.6 Fluorescence sensing experiments in vapor phase
To examine the fluorescence detection performance of 3′ towards FA vapor, time- dependent fluorescence titration experiments were carried out. Prior to the experiment, a thin film of 3′ was fabricated on the quartz slide. At first, a suspension of 3′ was prepared by dispersing 50 mg of 3′ in 5 mL of ethanol. Before film deposition, the dispersion was sonicated for 30 min in an ultrasonic bath to get a homogeneous suspension of 3′. The spin-coating method was applied to prepare the thin film of 3′ on the quartz slide. By maintaining the spin-coating speed at 2000 rpm for 1 min, the suspension of 3′ was dropped on the quartz slide which was fixed on a spin-coater. This spin-coating process was repeated for several times until a homogeneous coating was obtained. Afterward, the as-prepared film of 3′ on the quartz slide was dried at room temperature.
The film-coated quartz slide was fixed inside of a quartz cuvette in a suitable position (Figure 4.4). The solid-state emission spectra of the spin-coated sample were measured in the beginning. Then, 100 µL of FA was carefully introduced at the bottom of the cuvette by a pipette. After that the cuvette was quickly sealed by cap. After the introduction of FA, the emission spectra were recorded with 1 min interval until saturation
in intensity of the emission spectra was obtained. To ensure the selectivity of 3′ towards FA in the vapor phase, similar experiments were repeated with different liquid aldehydes (oxaldehyde, acetaldehyde, butyraldehyde, propionaldehyde, valeraldehyde, crotonaldehyde, benzaldehyde and anisaldehyde). It is worth to note that 4- chlorobenzaldehyde was excluded from the vapor phase sensing experiment as it is a solid compound.
Figure 4.4 Custom-designed sensor set-up used for the vapor-phase sensing of FA.
4.2.7 Cell culture and imaging experiments
The culture of the MDAMB-231 breast cancer cells was performed in DMEM:F12 media consisting of 10% fetal bovine serum (FBS) and 1% antibiotic cocktail as described formerly.64 Cells were loaded with 3′ (0.5 mg/mL) for 10 h in serum-containing media. To remove excess probe, the washing of the cells was carried out with phosphate buffered saline (PBS) thrice. Then, the cells were treated with FA (50 µM) for 60 min at 37 °C in PBS. The cancer cells were observed in the bright field and the blue channel (λex = 390 nm, λem = 430 nm) using Cytell cell imaging system (GE Healthcare) and images were captured from randomly selected fields.