Quantum-Dot Complex
4.3 Experimental Section
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(ii) Preparation of 8-hydroxyquinoline (HQ) solution: 7.3 mg of solid HQ was dissolved in 10.0 mL of methanol by sonication at room temperature to prepare 5.0 mM methanolic solution of HQ.
(iii) Fabrication of WLE-QDC: The WLE-QDC was prepared by adding 15.0 µL of 5.0 mM methanolic solution of HQ into 3.0 mL of as-prepared aqueous dispersion of Qdots (with absorbance of 0.04 at 357 nm). Then, the aforementioned mixture of HQ and Qdots was centrifuged at 20,000 rpm for 10 min. The pellet obtained from centrifugation was redispersed into same volume of water and centrifuged under same condition and then the pellet was washed as before. Finally, the pellet was dispersed into same volume of water, which was then used for further experiments. For the preparation of WLE-QDC, the optimum concentration of HQ used was calculated to be 25.0 µM.
4.3.3 Sensing of LCUFAs: The sensing of LCUFA was performed by adding 2.5 mM aqueous solution of sodium oleate sequentially to 3.0 mL of aqueous dispersion of WLE QDC (with absorbance of 0.06 at 357 nm) followed by recording of their photoluminescence spectra and other optical properties. The experiment was carried out in triplicate. The intensity ratio (I480/I590) of WLE QDC against concentration of oleate was plotted to find out the linear range and to calculate the limit of detection (LOD).
Equation 3σ/k was used for the calculation of LOD. Similarly, the sensing of other LCUFAs (like linoleate and erucate) by WLE-QDC was also carried out.
4.3.4 Recognition of long chain unsaturated fatty acid (LCUFA) from their saturated form: The different responses of WLE-QDC towards LCUFA (e.g., oleic acid) and their corresponding saturated form (i.e., stearic acid) were examined by adding mixture of sodium salt of oleic acid and stearic acid with different ratio of concentrations (with keeping total concentration of fatty acids to a fixed amount) into the solution of WLE-QDC. Accordingly, five different ratiometric mixtures were prepared with oleate and stearate ratio of 0:1, 1:3, 1:1, 3:1 and 1:0 (keeping the total fatty acid concentration equal to 2.5 mM). The highest amount of oleate added to WLE-QDC (with absorbance of 0.06 at 357 nm) for this experiment was 80.6 µM and the photoluminescence properties of oleate added WLE-QDC were monitored. Similar experiment was performed for stearate (of 80.6 µM). The experiments were performed in triplicate.
4.3.5 Selectivity of Sensing by WLE-QDC towards LCUFAs: Selectivity of sensing by WLE-QDC towards LCUFAs was examined by adding sodium salt of different
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saturated and unsaturated fatty acids (like oleic acid, linoleic acid, erucic acid, palmitic acid, lauric acid, geranic acid and stearic acid), anions (like citrate tartrate, oxalate and fluoride) and metal ions (like Ca2+, Mg2+, Na+ and K+). The mentioned interfering substances were added to 3.0 mL of aqueous dispersion of WLE QDC (with absorbance of 0.06 at 357 nm) separately such that their concentration became 80.6 µM (same as the highest concentration used in sensing experiment as in Fig. 4.2 of the Chapter 4). The experiments were carried out executed in triplicate. The change in intensity ratio (Δ (I480/I590) = (I480/I590) after addition - (I480/I590) before addition) of WLE-QDC after addition of each substance was plotted in bar diagram for comparative study. It is to be noted here that linoleic acid, erucic acid, palmitic acid, lauric acid and geranic acid were converted to their corresponding sodium salts by following a previously reported method.6 In short, excess amount of solid sodium carbonate was added to 5.0 mL of 100 mM methanolic solution of above mentioned fatty acids separately and stirred for 20 min at 40 °C. The reaction mixtures were then filtered to remove unreacted Na2CO3 and were dried in vacuum to get sodium salt of corresponding fatty acids.6
4.3.6 Commercial Vegetable Oil Analysis. For real sample analysis commercial sunflower, edible and soybean oils were purchased from local market and saponified with 20% aqueous NaOH solution following a well -known procedure.6 In short, 5.0 mL of 20% aqueous NaOH was added to 5.0 mL o f each of above-mentioned oils separately and stirred for 30 min at 80 °C. The formed sodium salts of fatty acids were precipitated out by cooling the reaction mixture in an ice cold condition. Then the precipitate was filtered and washed with ice cold water followed by drying in vacuum. Dried solid salt of sunflower, edible and soybean oil were dissolved in water to make 0.2, 0.5 and 0.3 mg/mL solutions, respectively. LCUFAs in those solutions were determined following above-mentioned sensing procedure by adding 20.0 µL of each solution to 3.0 mL of WLE QDC (with absorbance of 0.06 at 357 nm) separately and monitoring the photoluminescence properties. Notably, a linear relationship between I480/I590 of WLE- QDC and concentrations of equivalent mixture of (i) oleate, (ii) linoleate and (ii) erucate was measured (Fig. A.4.15, Appendix) and used for the quantification of LCUFAs in commercial vegetable oils such as sunflower, edible and soybean oils (Table 4.1 of Chapter 4).
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4.3.7 Instruments: To characterize the samples and to probe the recognition and sensing phenomena, HORIBA Jobin Yvon FluoroMax-4 spectrofluorimeter was mainly used to record photoluminescence. Digital photographs were taken using Realme Mobile (Realme 5) under spectrofluorimeter excitation source. OSRAM color calculator (CIE- 1931) software was used to calculate chromaticity coordinates from photoluminescence spectra and Image-J software was used to calculate hue from digital photograph.
PerkinElmer Lambda 35 UV–vis spectrophotometer was used to record the absorption spectra of the samples. Transmission electron microscopy (TEM; Model: JEOL JEM 2100F, maximum accelerating voltage: 200 kV) was used to analyze the size and lattice parameters of the nanoparticles. Gatan Digital Micrograph software was used to analyze TEM image, High resolution TEM image and corresponding inverse fast Fourier transformation. X-ray diffraction patterns of the samples were recorded by using Rigaku TTRAX-III X-ray diffractometer.