Surface Modification on Substrates and Their Applications in Sensing and Separation

• Main Authors: Wen-Lung Lee, 李文龍
• Other Authors: Shih-Ming Lai
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/15655332362299345731

博士 === 雲林科技大學 === 化學工程與材料工程研究所 === 97 === The purpose of surface modification on substrates is to give some special functionalities on their surfaces and therefore to enhance their surface properties. The surface modified substrates can then be applied in various application fields. In this study, the techniques of surface modification on gold electrodes and silica adsorbents were investigated and the surface modified substrates were used in sensing and separation, respectively. The first part of this study is “Preparation and Characterization of Glucose Biosensor Using Self-Assembled Monolayers of Alkanethiols.” Glucose biosensors were prepared by immobilizing glucose oxidase (GOD) on gold electrodes modified with a self-assembled monolayer (SAM) of 3-mercaptopropionic acid (MPA). The fabrication process includes the following three steps: (1) chemisorption of MPA at gold electrode surface to produce a carboxylic acid terminated SAM, (2) activation by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) to generate a stable NHS ester intermediate, (3) replacement of the NHS ester by the amino group of glucose oxidase. The important parameters in this fabrication process were investigated and the optimal fabrication conditions were selected. Using the glucose biosensors manufactured by the optimal procedure, the glucose concentration measurements were performed by chronoamperometry, in which the optimal operating potential and time were selected. Also, the effect of mediator concentration on the sensitivity and the reproducibility of the SAMs based glucose biosensors were investigated. The results of the first part of this study can be summarized as follows: (1) In the monolayer formation step, the MPA-SAM modified electrodes were characterized by qualitative analysis by cyclic voltammetry (CV) in Fe(CN)64−/3− solution and surface coverage determination by reductive desorption in KOH solution. The results proved that the MPA-SAM was almost completely formed right after 5 min of immersion. The surface coverage was calculated to be about 7.3×10-10 mol/cm2 under the immersion conditions of 30℃, 5 min, and 10 mM MPA concentration in ethanol solution. (2) In the EDC/NHS activation and enzyme immobilization steps, the optimal operating conditions were 90 min activation time in 0.004 M EDC and 0.01 M NHS solution, and 90 min and 40℃ immobilization time and temperature, respectively, in 0.96 mg/mL of GOD solution, at which the amount of immobilized GOD of 3.96×10-14 mol cm-2 and sensitivity of 0.058 μA mM-1 cm-2 were reached. (3) The glucose concentration measurements were performed by chronoamperometry, in which the optimal operating potential and time were selected as +0.25 V and 120 s, respectively. The sensitivity increased from 0.028 to 0.083 μA mM-1 cm-2 as the mediator concentration increased from 0.05 to 0.5 mM. 0.1 mM ferrocenemethanol was selected to be an optimal mediator concentration. It is also proved that the reproducibility of the SAMs based glusoce biosensors was reasonably good. The second part of this study is “Preparation of Silica Adsorbents Containing Cyclodextrins and Their Use in the Separation and Purification of Catechins and Caffeine from Green Tea Extracts.” The preparation of silica adsorbents containing β-cyclodextrins (β-CD) includes the following two steps: (1) modification of 3-glycidoxypropyl-trimethoxysilane (GOPS) on silica particles to produce epoxy terminated GOPS_silica particles, (2) chemical grafting of Na-β-CD, obtained from the reaction of β-CD with sodium hydride, on GOPS_silica particles to produce β-CD_GOPS_silica adsorbents. First, the important parameters in this preparation process were investigated and the optimal preparation conditions were selected. Then, the batch adsorption experiments were performed using the green tea extracts, concentrated by the liquid-liquid extraction method, as the raw materials and β-CD_GOPS_silica, commercial silica as well as XAD-7 resin as the adsorbents. The adsorption capacities of caffeine (CA) and the four major catechins, including epigallocatechin (EGC), epicatechin (EC), epigallocatechin gallate (EGCG), and epicatechin gallate (ECG), on these three adsorbents were investigated and compared. The results of the second part of this study can be summarized as follows: (1) The optimal preparation conditions for β-CD_GOPS_silica adsorbents were in the modification step of GOPS using a concentration of 0.485 M and a time of 6 hr and then in the grafting step of β-CD using a temperature of 140℃. The capacities of GOPS and β-CD on the silica surface were 0.1534 mmol GOPS/g silica 0.0530 mmol β-CD/g silica, respectively, by the element analysis (EA) measurement, in contrast to 0.0987 mmol GOPS/g silica 0.0359 mmol β-CD/g silica, respectively, by the thermogravimetric analysis (TGA) measurement. (2) By transmission absorption infrared (ATR) spectroscopy measurement, the absorption peak of the asymmetric C–H stretching band (2928 cm-1) for the β-CD_GOPS_silica adsorbents was increased with the grafting temperature till 140℃, which reveals that 140℃ was an optimal grafting temperature. (3) CA and catechins in the green tea powders were concentrated by the liquid-liquid extraction method under the following conditions: ethyl acetate as the single solvent, feed concentration of 50 mg/mL, water/oil ratio of 1:1, extraction temperature of 30oC, no pH adjustment for the feed solution, and extraction time of 2 hours. The purity of the four catechins was raised from 14.0% to about 63.0%, the concentration ratio was about 4.5, and the recovery of the four catechins was about 67.0%. (4) Under a polar aqueous solution, the β-CD_GOPS_silica adsorbent revealed a good adsorptivity and selectivity with respect to EGCG, compared with CA and the other catechins. This is due to the hydrophobic cavity interaction between the β-CD grafting silica adsorbents and EGCG. It is then concluded that the β-CD_GOPS_silica adsorbents can be further applied in the separation and purification of catechins from green tea extracts.