Chapter 2. Materials and Methods

Transcription

1 Chapter 2 Materials and Methods

2 2. Materials and Methods This chapter describes the chemicals, reagents and instruments used for carrying out this study. A brief discussion of the methods used for the analysis and the common experimental procedure is given here. The specific procedure for each experiment is given at the beginning of the respective chapters. 2.1 Chemicals and reagents Titanium foil (Ti, 0.25 mm thick), tantalum foil (0.25 mm, 99.9% trace metals basis), chloroplatinic acid (ACS reagent), glucose (Glu, ACS reagent), ascorbic acid (AA, reagent grade), dopamine (DA,), uric acid (UA, 99.0% crystalline), acetamidophenol (AP, 98% analytical standard), fructose ( 99.9%), galactose ( 99.9%) maltose (from potato, grade 1, 98.0%), lactose ( 99.9%), urea and creatinine (anhydrous, 99.9%) were purchased from Sigma Aldrich. Conducting inks of carbon, silver and Ag/AgCl were purchased from Dupont Pte.Ltd. Singapore. Nafion 117 polymer solution was purchased from Fluka and diluted with absolute alcohol for use. CoCl 2. 6H 2 O, CuSO 4.5H 2 O, NiSO 4.6H 2 O, boric acid, KCl and NaOH were of analytical grade and purchased from FINAR chemicals, India and used without further purification. All stock solutions except uric acid were prepared in Millipore water (15.2 MΩ cm, Millipore, Germany) and stored at 4 C when not in use. Uric acid stock solution was prepared in 0.01 M NaOH. 2.2 Instrumentation and electrodes All electrochemical measurements were carried out using CHI 660C electrochemical workstation and CHI 608D electrochemical analyzer (CH Instruments, TX, USA) with a three electrode cell. All potentials were either referred to conventional Ag/AgCl (saturated KCl) or to saturated calomel electrode. In the case of screen printed electrodes AgCl ink printed electrode acts as the pseudo reference electrode. The modified electrodes were used as the working electrode and platinum wire as the counter electrode. Screen printed electrodes were prepared indigenously by manually printing different inks. Surface morphology of the modified electrodes was studied using Variable Pressure Field Emission Scanning Electron 31

3 Microscope (FESEM, Hitachi SU6600) and Atomic Force Microscope (Model XE-70, Park Systems, Korea) in noncontact mode. The crystallinity and surface composition of the electrodes were established using X-ray powder diffractometer (Bruker AXS D8). 2.3 Experimental Fabrication of TiO 2 nanotube arrays TiO 2 nanotube arrays (TDNT) were synthesized by electrochemical anodization of titanium foil from a bath containing 0.15 M ammonium fluoride, glycerol and water in a ratio 90:10 (v/v %) [245,247]. Prior to the anodization titanium plate was cut into 2x50 mm strips and washed successively with detergent, acetone and water. The electrode is covered with Teflon tape such that the electrode area was 0.09 cm 2. A platinum electrode was used as the cathode on both the sides of the strips and a potential of 20 V was applied from a DC power supply. The electrolyte was constantly stirred for 5 h for the efficient anodization. The anodized samples were washed and annealed at 500 C for 3 h to get increase in crystallinity and electrochemical stability [209] Fabrication of screen printed carbon electrodes Screen printed carbon electrodes were prepared by serially printing silver, AgCl and carbon ink on a PET substrate. The schematic representation of stepwise process presented in Fig Stencils were designed separately for each ink and printed manually. One stencil could be used for printing 105 electrodes at a time. The photograph of the screen used for printing carbon is given in Fig Determination of detection limit The detection limit of an electrochemical method for determination of concentration of an analyte was calculated based on the reported method. According to this method, the minimum distinguishable analytical signal, S m, is taken as the sum of the mean blank signal, S bl, plus a multiple 3 of the standard deviation of the blank signal, s bl. ie., S m = S bl + 3s bl 32

4 For this, 30 replicate measurements were performed in the blank solution and the resulting data were then treated statistically to obtain S bl and s bl. Finally, the slope of the calibration plot (m) was used to convert S m to C m, which is the minimum detection limit with signal to noise ratio (S/N) equals to 3. Cm = (S m -S bl )/m Figure 2.1 Stepwise procedures for the fabrication of SPCE. Figure 2.2. The screen or stencil used for the printing of carbon ink. 33

5 2.3.4 Analysis of serum Serum was extracted from human blood samples by centrifugation at g (12000 rpm) for 30 minutes. 50 μl of the serum samples were injected to 4 ml of the testing solution under constant stirring. The amperometric measurements were carried out at the optimized detection potential. The concentration of analyte in serum was estimated by standard addition technique. A standard solution of the analyte is added to the testing solution and the response is compared with unknown serum samples. The concentration of glucose in blood by the sensor was compared with that obtained using Johnson and Johnson One Touch Select test strips Extraction of cholesterol from chicken egg yolk Cholesterol is extracted from egg yolk by the method proposed by Zhang et.al., without saponification [349]. Egg yolk is separated from white and washed several times with distilled water, rolled over a filter paper to remove as much albumin as possible. The yolk membrane is then punctured and 5 g of yolk was diluted with 20 ml distilled water. 5 ml of this solution is mixed with 5 ml of IPA followed by 12.5 ml diethyl ether and 12.5 ml petroleum ether, shaken well and the organic and aqueous layer formed is allowed to separate. The organic layer containing cholesterol is separated by evaporating the solvent. The cholesterol extracted is dissolved in Triton X-100 and diluted with PBS is used for the analysis Extraction of skin tissue cholesterol Skin cholesterol is extracted by adopting the procedure reported previously by various authors [350,351]. Briefly, 0.5 ml of IPA is taken in a 10 ml plastic cup and placed in contact with the palm for 60 s. The solvent in the extract was evaporated to dryness. The extracted cholesterol is dissolved in Triton X-100 and diluted with PBS and used for the analysis Composition of phosphate buffer saline PBS of ph 7.4 is prepared using KH 2 PO 4, Na 2 HPO 4, KCl and NaCl in the composition given in the Table

6 Table 2.1. Composition of phosphate buffer saline (PBS). Sl. No. Chemicals Amount (g L -1 ) 1 Sodium chloride Potassium chloride Potassium dihydrogen phosphate Disodium hydrogen phosphate