Effect of substrate (ZnO) morphology on enzyme immobilization and its catalytic activity

N A N O E X P R E S S

Open Access

Effect of substrate (ZnO) morphology on enzyme

immobilization and its catalytic activity

Yan Zhang

, Haixia Wu

, Xuelei Huang

, Jingyan Zhang

and Shouwu Guo

Abstract

In this study, zinc oxide (ZnO) nanocrystals with different morphologies were synthesized and used as substrates for enzyme immobilization. The effects of morphology of ZnO nanocrystals on enzyme immobilization and their catalytic activities were investigated. The ZnO nanocrystals were prepared through a hydrothermal procedure using tetramethylammonium hydroxide as a mineralizing agent. The control on the morphology of ZnO nanocrystals was achieved by varying the ratio of CH3OH to H2O, which were used as solvents in the hydrothermal reaction system. The surface of as-prepared ZnO nanoparticles was functionalized with amino groups using

3-aminopropyltriethoxysilane and tetraethyl orthosilicate, and the amino groups on the surface were identified and calculated by FT-IR and the Kaiser assay. Horseradish peroxidase was immobilized on as-modified ZnO

nanostructures with glutaraldehyde as a crosslinker. The results showed that three-dimensional nanomultipod is more appropriate for the immobilization of enzyme used further in catalytic reaction.

Introduction

The composition, size, and surface characteristics of sub-strates have been considered as the important factors that affect the physical/chemical properties of the immobilized enzymes [1-3]. However, for conventional bulk solid sub-strates, such as glass slides [4], polymer monoliths [5,6], and silica beads [3], the control on their morphologies and functionalizing their surfaces are usually laborious. Nanos-caled materials, such as metal [7] and metal oxide nano-spheres [8], carbon nanotubes [9,10], graphene oxide nanosheets [11], have also been utilized as the substrates for enzyme immobilization. It has been demonstrated that the size of nanostructured materials might play significant roles to regulate the catalytic activity of the immobilized enzymes [3]. The preparation of nanostructured materials with controlled chemical composition, size, morphology, and the surface functionalization have witnessed great pro-gressive achievements during the last two decades [12,13]. However, the systematic study of the effects of the mor-phology of the nanoscale substrates on the enzyme immo-bilization remains to be expanded. ZnO nanocrystals have

unique physical/chemical properties and pronounced bio-compatibility, which are beneficial for many practical applications. In addition, a variety of nanostructures of ZnO, such as nanospheres [14], nanowires [15], nanorods [16], nanonails [17], nanotubes [18], nanotetrapods [19], nanotablets [20], and nanoflowers [21], have been pre-pared successfully. Therefore, ZnO nanocrystals are ideal materials to study the effect of the morphology of the sub-strate on the catalytic efficiency of the immobilized enzymes. Horseradish peroxidase (HRP) was used as a model enzyme because it has been wildly studied and used in many fields, such as organic syntheses [22], phenol removal [23], biosensor, and drug delivery [24].

In this study, we report the effects of the morphology of the ZnO nanocrystals on the enzyme immobilization. ZnO nanocrystals with different morphologies, including nanosphere, nanodisk, and nanomultipod, were fabri-cated simply through a hydrothermal procedure. The surface of ZnO nanocrystal was functionalized with the amino groups using 3-aminopropyltriethoxysilane (APTES) and tetraethyl orthosilicate (TEOS) [25]. Glutar-aldehyde was used as a crosslinker to immobilize the HRP enzyme molecules on the surface of as-modified ZnO nanocrystals. Then the enzyme loading and catalytic activity were evaluated.

* Correspondence: haixiawu@sjtu.edu.cn; swguo@sjtu.edu.cn 1

National Key Laboratory of Micro/Nano Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China

Full list of author information is available at the end of the article

Results and discussion

ZnO nanocrystals with different morphology

Figure 1 shows the SEM images of ZnO nanocrystals with different morphologies. The nanospheres, with dia-meter of approximately 30 nm, were obtained when pure methanol was used as the hydrothermal reaction solvent as shown in Figure 1a. When the ratio of methanol to water was adjusted to 1:1, ZnO nanodiscs, approximately 85 nm in width and 25 nm in thickness, were acquired as shown in Figure 1b. Figure 1c shows the ZnO nanomulti-pods composed of several rods of 100 nm in diameter and 200 nm in length when the methanol/water ratio reaches 1:9. This illustrates that the ratio of methanol/ water is certainly a crucial factor to control on the mor-phology of ZnO nanocrystals. Although the morpholo-gies of aforementioned ZnO nanocrystals are different, the XRD patterns are all well consistent with the stan-dard wurtzite ZnO structure as shown in Figure 2. Thus, the effect of crystal form on the surface modification and enzyme immobilization can be neglected.

Surface modification of ZnO nanocrystals

In order to immobilize enzyme, the surface of ZnO nano-crystals were functionalized with amino groups using APTES/TEOS. Figure 3a shows a typical image of coated ZnO nanodiscs, where a thin film with uniform thickness of 2 nm formed on the surface. Comparing the Zeta-potentials of the bare ZnO and the as-modified ZnO nanocrystals (Figure 3b), it can be deduced that the sur-face electrostatic state of ZnO nanocrystals was changed. The surface groups of the modified ZnO were further characterized by FT-IR spectra. Through comparing the FT-IR spectra before and after modification in Figure 3c, except for a few peaks at 3430, 1630, and 433 cm-1 corre-sponding to water (moisture) and ZnO nanocrystal, the

peaks at 2936.3 and 2872 cm-1of the C-H stretching

vibration [26], and the peaks at 1330 and 1560 cm-1of the stretching vibration of C-N and bending vibration of N-H can be found. In addition, the strong absorption peaks at 3428.6 and 1633.5 cm-1, assigned to N-H bend-ing vibrations, are overlapped with the bendbend-ing vibration of the absorbed H2O [27]. These results confirmed the

presence of amino groups on the ZnO nanocrystals surface.

The amount of amino groups and the thickness of the coating layer can be controlled by adjusting the ratio of TEOS to APTES. When the ratio of TEOS to APTES was 1:1, a coating layer of approximately 2 nm can be generated on the surface of ZnO nanocrystal, but, at the

same time, lots of isolated SiO2 nanocrystals were

formed, as shown in Figure 4a. When the ratio of TEOS to APTES was 1:4, a layer with uniform thickness of about 2 nm was formed as shown in Figure 4b. When the ratio of TEOS to APTES was decreased to 1:10, no fully covered coating layer can be generated, as shown in Figure 4c. According to the standard curve of glycine obtained by Kaiser Assay, the amount of amino groups on the surface of ZnO nanocrystals was deduced. When the ratios of TEOS to APTES were 1:1, 1:4, and 1:10 used for the surface modification, the amounts of amino groups on the surface of ZnO nanodisks were 0.03, 0.07, and 0.02 mmol/g, respectively. These results show that the ratio of TEOS to APTES used for the surface modi-fication determines the uniformity of the coating layer as well as the amount of amino groups.

The aforementioned procedure was also performed on the surface modifications of ZnO nanospheres and nano-multipods with TEOS to APTES ratio of 1:4. Figure 5 depicts the TEM images of the nanosphere and nanomul-tipod after the modification. Similar to the nanodisks, there are thin coating layers formed both on nanospheres and nanomultipods. The amounts of amino groups on

the surfaces of ZnO nanospheres and nanomultipods modified with TEOS to APTES ratio of 1:4 were also measured, which are 0.127 and 0.044 mmol/g, respec-tively. The specific surface areas of ZnO nanospheres, nanodisks, and nanomultipods are 33.59, 11.99, and

11.85 m2/g, respectively, which were measured using

Brunauer-Emmett-Teller (BET) method. Thus, consider-ing the surface areas of different morphologies, the sur-face densities of amino groups on ZnO nanospheres, nanodisks, and nanomultipods were calculated, which

are determined to be 3.78, 5.94, and 3.71 μmol/m2,

respectively.

The effects of morphologies of ZnO nanocrystals on HRP immobilization and their activity

The two aldehyde groups (-COH) of glutaraldehyde can bond separately to the amino groups of HRP and as-modi-fied ZnO [28], and thus, glutaraldehyde was used as a crosslinker to immobilize HRP molecules on the modified ZnO nanocrystal surfaces. As shown in Figure 6, the high-est loadings of HRP on the ZnO nanospheres, nanodisks

and nanomultipods were 0.094, 0.275, 0.240 mg/m2,

respectively From Figure 6, we find that the immobiliza-tion of HRP on ZnO nanomultipods can reach the highest loading at the lowest ratio of glutaraldehyde to amino groups. The maximum loading of HRP on ZnO nanomul-tipods was higher than that on the nanospheres, but, as high as that on the nanodisks, even if the surface density of amino groups on ZnO nanomultipods was relatively lower than that of the other two.

The catalytic activity of the HRP immobilized on dif-ferent ZnO nanocrystals was assayed through phenol oxidation reaction. Soluble HRP was also characterized

as a control. Their kinetic parameters were obtained from the Lineweaver-Burk equation, and the data are summarized in Table 1. 1/Km, which express the affinity

of phenol compounds to HRP, and the Kmof the HRP

immobilized on ZnO nanospheres, nanodisks and nano-multipods were tagged as Km(s), Km(d), and Km(m), Figure 2XRD patterns of ZnO nanocryatals.(a)Nanospheres,(b)

nanodisks, and(c)nanomultipods.

respectively. Table 1 shows thatKm(s) was unexpectedly higher than that of the free HRP, whileKm(d) and Km (m) are close to that of free HRP suggesting that the substrate affinity to the HRP was not affected by the immobilization on these two ZnO materials. Thus, we can assume the morphology of the ZnO nanocrystals must be an important fact that affects the affinity of phenol compounds to HRP.

The catalytic property of the immobilized HRP was further characterized by the catalytic efficiency (Kcat/Km). The catalytic efficiency of immobilized HRP was much lower than that of free HRP, which coincide with the report of the literature [29]. As shown in Table 1, the cata-lytic efficiency values of immobilized HRP on the

nanospheres, nanodisks and nanomultipods were 0.78, 1.09, 1.28 mM-1s-1, respectively. Through comparing the

Kcat/Km value of the HRP immobilized on three ZnO

nanocrystals with different morphologies, the nanomulti-pod ZnO nanocrystals are apparently favorable for enzyme immobilization.

Those all may due to the three dimensional structural feature of nanomultipods, which could affect the enzyme interaction with the immobilized substrate and conforma-tion of the enzyme, and result in increasing the enzyme loading on the solid substrate and catalytic efficiency. Because the nanomultipods have several pods, and the spaces among the pods are limited, the glutaraldehyde is difficult to self-polymerize, and the amino groups on the

Figure 4TEM images of amino group modified ZnO nanodisks.(a-c)TEM images of modified ZnO nanodisks with different TEOS:APTES ratios of 1:1, 1:4, and 1:10.

multipods are more efficiency to immobilize HRP. While, nanospheres and nanodisks have opened spaces, glutaral-dehyde tends to polymerize. Thus, HRP loadings on nano-spheres and nanodisks need more glutaraldehyde to reach the maximum loadings. Due to the opened structure and lower surface density of amino groups, the loading of HRP on nanospheres is lowest. Thus, HRP loadings on nano-spheres and nanodisks need more glutaraldehyde to reach the maximum loadings. Compared with nanodisks and nanospheres, the HRP loading on ZnO nanomultipods reached the highest when its glutaraldehyde to amino groups ratio was lower, which indicates that every HRP molecule needs less glutaraldehyde to immobilize HRP on the surface of ZnO nanocrystal and less conformation happened. Therefore, the 3D structure of nanomultipods and the lowest ratio ofNglutaraldehyde/N-NH2may result in

the stabilization of HRP immobilized on nanomultipods, which indicates that the 3D nanomultipod is more appro-priate for the immobilization of enzyme and its catalytic efficiency.

Conclusions

ZnO nanocrystals with different morphologies, including nanosphere, nanodisk, and nanomultipod, were prepared via hydrothermal reactions using the mixtures of metha-nol/water with different volume ratios. The surfaces of

ZnO nanocrystals were modified with amino groups using TEOS and APTES to study the effect of the mor-phology of the materials to the enzyme immobilization. The surface density of the amino groups and the thick-ness of coating were controlled by tuning the ratio of TEOS to APTES. It was demonstrated when the ratio of TEOS to APTES was 1:4, a layer of uniform thickness of approximately 2 nm can be generated on surface of the ZnO nanocrystals.

HRP molecules were immobilized on the modified ZnO nanocrystal surfaces using glutaraldehyde as a crosslinker. It was illustrated that the enzyme loading on the ZnO nanostructure was in the order of nanospheres < nano-multipod < nanodisks, while the nanonano-multipod reached the highest loading when its glutaraldehyde to amino groups ratio was lower than the other two, causing less conformation change of HRP on the ZnO surface, leading to a higher catalytic efficiency. In brief, the 3D nanomulti-pod is more appropriate for the immobilization of enzyme and for being used in catalytic reaction than the other two, which has great implications for the many ongoing studies of enzyme immobilization and applications of the immobi-lized enzymes.

Methods

Materials

Zn(Ac)2·2H2O, (CH3)4NOH (25%), Glutaraldehyde (25%),

phenol (99%), 4-aminoantipyrine (4-AAP), methanol, and

H2O2(30%) were purchased from Sinopharm Chemical

Reagent Company, Shanghai, China. APTES (≥98.0%) was

bought from Sigma-Aldrich, USA. TEOS was obtained from Linfeng Chemical Reagent Company, Shanghai, China. HRP was purchased from Majorbio Biotech Com-pany, USA. All reagents were used as-received.

Synthesis of ZnO nanocrystals

The ZnO nanocrystals were prepared through a thermal procedure using tetramethylammonium hydro-xide as a mineralizing agent. The control on the morphology of ZnO nanocrystals was achieved by

vary-ing the ratio of CH3OH to H2O, which were used as

solvents in the hydrothermal reaction system. In a typi-cal experiment, Zn(Ac)2·2H2O (5 mmol) was dissolved

in 15 mL of the mixture of methanol and water with different volume ratios in a flask under vigorous stirring. Then, 15 mL of (CH3)4NOH was dropped into the flask Figure 6 The enzyme loadings on different morphologies of

ZnO nanocrystals. The loadings of HRP with different ratios of glutaraldehyde and amine groups on the surface of the modified ZnO nanocrystals.

Table 1 The loadings and kinetic properties of the HRP immobilized on ZnO nanocrystals

sample Enzyme loading (mg/m2_{) K}

m(mM) Kcat/Km(mM-1s-1)

Free HRP - 6.24 80.13

HRP on ZnO nanospheres 0.094 23.06 0.78

HRP on ZnO nanodisks 0.275 3.95 1.09

as mineralizing agent. The as-obtained turbid suspen-sion was transferred into a Teflon inner reactor of stain-less-steel autoclave. The autoclave was heated at 200°C for 24 h for the hydrothermal reaction. The reaction solution was then cooled down to room temperature and the solid product was separated from the reaction mixture by centrifugation (4000 rpm), and was washed with ethanol and water alternately, each for three times.

Characterization of the ZnO nanocrystals

The size, morphology, BET surface area, and crystallinity of the ZnO nanocrystals before and after the surface modification were characterized using scanning electron microscope (Zeiss ultra 55, Germany), transmission elec-tron microscopy (TEM) (JEM-2100, Japan), accelerated surface area, porosimetry system (Micromeritics ASAP 2010 M+C, USA), and X-ray powder diffraction (XRD) (BRUKER-AXS, Germany). The surface functionalities of the ZnO nanocrystals after the modification were stu-died using FT-IR with a Nicolet 5700 Fourier transform infrared spectrometer (Thermo Electron, USA), and Zeta potentials obtained on Nicomp 380/ZLS (America).

Surface functionalization of ZnO nanocrystals

A typical surface functionalization process was as follows.

In general, 100 mg of ZnO nanocrystals was suspended into 20 mL of ethanol (pH = 10.8) under sonication.

Then, 15μL of TEOS and 60μL of APTES were added

into the ethanol solution, and the mixture was stirred for 5 h. The solid product was filtered and washed with etha-nol, and then dried at room temperature. The amounts of amino groups on the surface of ZnO nanocrystals were measured by Kaiser Assay. At first, the standard curve of glycine obtained by Kaiser Assay. Glycine were mixed with 2 mL of acetate buffer (0.6 M, pH = 4.5) and 2 mL of 10 mg/mL ninhydrin ethanol solution. The mixture was treated at 90°C for 20 min. After centrifugation at

10,000 rpm for 3 min, the amino groups’concentration

in the supernatant was monitored spectrophotometrically 565 nm, and then, 10 mg of modified ZnO was utilized to monitor spectrophotometrically as glycine. Then, the amount of amino groups was calculated based on the standard curve of glycine.

100 mg modified ZnO was suspended into 20 mL pH

7.0 0.1 M phosphate buffers, and then 750 μL of 25%

glutaraldehyde solution was added to the mixture. The mixture was incubated at room temperature, 200 rpm for 2 h. After filtration and washing with the phosphate buffer, the samples were utilized to immobilize HRP.

HRP immobilization

For HRP immobilization, 100 mg of functionalized ZnO nanocrystals was added into 1 mL, 0.1 M, and pH 7.0 of

potassium phosphate buffer containing 100 μg of HRP.

The mixture was incubated at 4°C for 2 h with 240 rpm shaking, and then centrifuged at 10,000 rpm for 3 min. The supernatant was collected. The sediments were cen-trifuged and rinsed alternately three times with 0.1 M, pH 7.0 phosphate buffer solution to remove non-specifi-cally adsorbed enzymes. The solid was stored at 4°C for further measurements. The supernatants were employed to determine the enzyme loading on modified ZnO.

Characterization of the immobilized HRP

The enzyme loading is obtained by subtracting the amount of the left HRP in the supernatant from the total HRP added. Free HRP and the immobilized HRP activity was assayed by colorimetric method using 4-AAP as pre-viously described [2]. In brief, the immobilized enzyme was added into 1 mL of 0.1 M, pH 7.0, phosphate buffer which contained 60 mM of phenol, 14.38 mM of 4-AAP, and 1.21 mM of hydrogen peroxide, and then reacted at 30°C for 3 min. The initial catalytic reaction rates of the enzyme in the supernatant and the immobilized enzyme were determined by measuring the UV absorbance of the reaction mixture at 510 nm. The double reciprocal plots of the rates and substrate concentrations were plotted to

obtainKmand Kcat according to the Lineweaver-Burk

equation.

Abbreviations

4-AAP: 4-aminoantipyrine; APTES: aminopropyltriethoxysilane; BET: Brunauer-Emmett-Teller; HRP: horseradish peroxidase; TEM: transmission electron microscopy; TEOS: tetraethyl orthosilicate; XRD: X-ray powder diffraction; ZnO: zinc oxide.

Acknowledgements

This study was financially supported by the National_“973_”Program (Nos. 2007CB936000 and 2010CB933900), the NSFC (No. 20774029 and No. 20906055) of China, the State key laboratory of bioreactor engineering (No. 2060204), and China postdoctoral science foundation

(No. 20100470131).

Author details 1

National Key Laboratory of Micro/Nano Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China2_{State Key Laboratory of} Bioreactor Engineering, School of Pharmacy, East China University of Science and Technology, Shanghai, 200237, PR China

Authors’contributions

YZ conducted the experiments and performed data analysis. XH helped in designing the immobilized enzyme test. JZ participated in the design and helped in compiling the data of immobilization enzyme interpretation. HW helped during the most of operation and data interpretation of analytic equipments used. SG conceived basic idea of this technique and supported the organization of this article.

Competing interests

The authors declare that they have no competing interests.

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doi:10.1186/1556-276X-6-450

Cite this article as:Zhanget al.:Effect of substrate (ZnO) morphology on enzyme immobilization and its catalytic activity.Nanoscale Research Letters20116:450.

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