C: conv-time Time (hour)
CỐ ĐỊNH GALACTOSE OXIDASE LÊN THANH NANO ZN O ỨNG DỤNG TRONG CẢM BIẾN SINH HỌC
TRONG CẢM BIẾN SINH HỌC
La Phan Phƣơng Hạ, Ngơ Văn Chí Quang, Trần Quang Trung
Khoa Khoa học Vật liệu, Trường ĐH KHTN, ĐHQG-HCM
Email: lppha@hcmus.edụvn
TĨM TẮT
ZnO cấu trúc nano, với định hướng tinh thể ưu tiên phát triển theo trục c, cĩ nhiều ứng dụng trong màng dẫn điện trong suốt, pin mặt trời, cảm biến khí,...; đặc biệt thanh nano ZnO cĩ cố định enzime sẽ được ứng dụng trong cảm biến sinh học. Trong đề tài này, nhĩm chúng tơi mơ tả quá trình cố định enzime galactose oxidase lên bề mặt thanh nano ZnO bằng chất tạo liên kết ngang glutaraldehydẹ Cấu trúc tinh thể và bề mặt của thanh nano ZnO được phân tích thơng qua XRD, SEM. Hiệu suất của quá trình cố định enzime được tính thơng qua hàm lượng enzime cố định lên thanh nano ZnO (hàm lượng enzime được tính bằng phương pháp Bradford). Kết quả phân tích cho thấy enzime galactose oxidase được cố định lên thanh nano ZnO với hiệu suất khoảng 36% với thời gian cố định 2h.
INTRODUCTION
For many applications of nanorods, such as field emission and polymer-inorganic solar cell, catalyst, gas/bio sensor, their1D structures play an important role [1,2]. ZnO nanorods have some advantages such as nontoxic, fast electron communication, high surface to volume, low cost and it has high ionic bonding characteristic so it is stabile for a long time at biological pH. Furthermore, the isoelectric point (IEP) of ZnO is high (about 9.5) and thus it is a better material to immobilize low IEP such as enzyme or protein. With these characteristic, ZnO nanorods structure is applied in biosensor. In this report, we have presented the solution method to grow ZnO nanorod and the immobilization process of galactose oxidase enzyme on ZnO nanorods surfacẹ The solution method is used to grow ZnO nanorods because of some advantages as simple method, reaction to form rods is occurred at low temperature (about 50oC – 120oC)…Our experiment results show that ZnO nanorods have about 40nm -50 nm diameter, aligned vertically on ZnO seed coated FTO substrate and have a preferential orientation of (002) planẹ Galactose oxidase enzyme (2mg/mL concentration) was immobilized on ZnO nanorod surface with different time immobilization, the efficiency of this process was about 36.5% with 2hours immobilization timẹ
EXPERIMENTAL
The growth of ZnO nanorods
The seed ZnO is important in growth process of ZnO nanorods by solution method. Reagent grade (RG) zinc acatate dehydrate (Zn(CH3COO)2.2H2O) and monoethanolamine (MEA) was first dissolved in an ethanol solvent with Zn2+ concentration 0.75 M to form sol solution. This solution was magnetic stirred in 2 hours at room temperaturẹ After that, FTO substrate was coated from sol solution by spin coating and then the annealling process was performanced at 5000C to form ZnO crystal. At the end, this product was cool down at room temperature to have ZnO seed layer.
ZnO nanorods to grow by solution method from seed ZnO coated FTO substratẹ Precursor zinc nitrate dehydrate (Zn(NO3)2.2H2O) and hexamethylenetetramine (HMTA, C6H12N4 ) was first dissolved in an aqueous solvent with Zn2+ concentration 0.02 M. This solution was magnetic stirred in 2 hours at room temperaturẹ After that, substrate coating ZnO seed was dipped in that solution and kept at 5 hours, 80oC to growth ZnO nanorods following some below chemical reaction. Then, an obtained products were cool down at room temperaturẹ The processing stages are outlined in Fig. 1
ISBN: 978-604-82-1375-6 106
Fig 1.Flow chart for the preparation of ZnO nanorods
The immobilization galactose oxidase enzyme on ZnO nanorods surfacẹ
In this part of the experiment work, we prepared two kinds of solution: 2.5% Glutaraldehyde (GA) in 0.1 mM phosphate buffer solution (PBS) as well as galactose oxidase solution in PBS having a concentration of 2mg/mL of enzymẹ These GA and enzyme solutions are mixed in one bottle, then ZnO nanorods substrate is dipped into it for different times in order to investigate the saturation of the ZnO nanorods surface with the enzymẹ We calculated the efficiency of this immobilization by Braford method based on the enzyme content adhered to ZnO nanorods surfacẹ With Braford method, to define the enzyme content immobilized on ZnO, we measure OD (optical density) of enzyme solution at 559nm. The process of immobilization was illustrated in figure 2.
Fig 2.Flow chart for the immobilization galactose oxidase on ZnO nanorods surfacẹ
RESULTS AND DISCUSSION
The growth of ZnO nanorods by solution method.
Fig 3 shows the X-ray diffraction pattern of ZnO nanorods that grew on seed ZnO coated substrate showed the significantly higher intensity from the (002) peak, indicated that the nanorods were preferentially orientated along c axis direction. This obtained result was confirmed that the seed layer ZnO has an important role in alignment growth of ZnO nanorods on ZnO seed layer having c axis orientation.
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Fig 3. XRD of ZnO nanorods
The SEM images of these samples are observed (in Fig 4) the hexagonal structure of ZnO nanorods with the higher vertical alignment on ZnO seed coated substratẹ The average diameter of nanorods in this situation was 40-50 nm. The reaction in solution by the hydrolysis of anhydrous zinc acetate to ZnO nanorods was summarized by the following [5]:
C6H12N4 + 6 H2O 6 HCHO + 4 NH3 NH3 + H2O NH4+ + OH-
Zn2+ + 2 OH- Zn(OH)2 Zn(OH)2 ZnO + H2O
Fig 4. SEM of ZnO nanorods
These results show that we successfully grow ZnO nanorods with good orientation, high surface areạ This structure can be used to immobilize enzyme on it to apply on biosensor.
The immobilization galactose oxidase enzyme on ZnO nanorods surfacẹ
We immobilized galactose oxidase enzyme on ZnO nanorods in 5 minutes, 1 hour, 2 hours timẹ The OD value of enzyme solutions after the immobilization was measured. From these OD value, we calculated the enzyme content immobilized on ZnO, so that we can find out the efficiency of the immobilization (H%) base on standard curve of Braford method (figure 5). The result of OD measurement with different immobilization time is shown in table 1.
𝑯% =𝒃𝒆𝒈𝒊𝒏𝒏𝒊𝒏𝒈 𝒆𝒏𝒛𝒚𝒎𝒆 𝒄𝒐𝒏𝒕𝒆𝒏𝒕 − 𝒆𝒏𝒛𝒚𝒎𝒆 𝒄𝒐𝒏𝒕𝒆𝒏𝒕 𝒂𝒇𝒕𝒆𝒓 𝒕𝒉𝒆 𝒊𝒎𝒎𝒐𝒃𝒊𝒍𝒊𝒛𝒂𝒕𝒊𝒐𝒏
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Fig 5. The standard chart of Braford method
Table 1. OD value and the efficiency of the immobilization with different immobilization timẹ Immobilization time 5 minutes 1 hour 2 hours
OD value 0.095 0.091 0.086
ΔOD 0.034 0.03 0.025
Enzyme concentration (find out from Braford standard graph ) (μg)
34.5 30.45 25.38
Enzyme content after the immobilization (μg) 1725.5 1522.5 1269 Immobilized enzyme content (μg) 274.5 477.5 731 H% 13.8 24 36.5
From some values in table 1, we draw the curve of the efficiency of the immobilization follow different immobilization time and shown in figure 6. The efficiency of the immobilization is 13.8%, 24% and 36.5% corresponding to 5 minutes, 1 hour, 2 hours immobilization timẹ
Fig 6. The chart of the efficiency of the immobilization follows different immobilization timẹ
With these achieved results above, it shows that we successfully immobilized galactose oxidase on ZnO nanorods with the efficiency of the immobilization is 36.5% corresponding to 2 hours immobilization timẹ
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CONCLUSION
We have successfully grown up the ZnO nanorods on seed ZnO coated FTO substrate by solution method, the nanorods were preferentially orientated along c axis direction, the average diameter of nanorods in this situation was 40-50 nm. We also successfully immobilized galactose oxidase on ZnO nanorods with the efficiency of the immobilization is 36.5% corresponding to 2 hours immobilization timẹ These results are basic for galactose sensor application based on ZnO nanorods in the next step.
Acknowledgement: We thank for Solid State Physic Department, University of Natural Sciences, VNU-HCM.
THE PROCESS OF IMMOBILIZATION OF ZNO NANORODS SURFACE WITH GALACTOSE OXIDASE – APPLY TO GALACTOSE BIOSENSOR