Volume 24 Issue 3 - June 21, 2013 PDF
Synthesis of an imprinted hybrid organic-inorganic polymeric sol-gel matrix toward the specific binding and isotherm kinetics investigation of creatinine
Yong-Sheun Chang, Ting-Hsien Ko, Ting-Jung Hsu, and Mei-Jywan Syu*
Department of Chemical Engineering, College of Engineering, National Cheng Kung University
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Creatinine is a metabolite from creatine and is the final product from kidney metabolism. Its concentration in blood serum and urinary excretion is less affected by dietary change. Therefore, the creatinine is used as a reliable indicator to evaluate renal, thyroid, and muscular functions. It was then chosen as the target molecule in this work.

Molecular imprinting has become an available method for the preparation of polymeric materials with ability to bind specifically to the target analyte. Preparation of MIP (molecularly imprinted polymer) based on the sol-gel process has already been developed by researchers. In recent years, the hybrid sol-gel method was further utilized for the preparation. By introducing the organosilicon precursor into the sol-gel-based MIP, the silica gel could be grafted with certain functional groups which could then interact with the imprinted molecules. MIP prepared from the sol-gel process was proposed in order to form a confined and rigid structure so that superior binding specificity toward the target molecule could be achieved from such materials.

An inorganic precursor, tetraethoxysilane (TEOS), was mixed with an organic functional monomer, 2-acrylamido-2-methylpropane-sulfonic acid (AMPS), and the creatinine template to form a hybrid organic-inorganic imprinted polymer. The preparation scheme is shown in Fig. 1. The chemical functionality was achieved as well as a confined matrix via the polymerization and the hydrolysis-condensation of the sol-gel.

The imprinting effect from the hybrid organic-inorganic imprinted materials against the corresponding non-imprinted was confirmed to be the ratio of 3.42. Additionally, the affinity distribution of the hybrid imprinted materials derived from the allosteric model was analyzed from the adsorption isotherm data.
Fig. 1  Schematic preparation of the hybrid organic-inorganic sol-gel creatinine-imprinted polymer.
The adsorption isotherms of hybrid MIP and hybrid NIP are plotted in Fig. 2. An allosteric model for multilayer binding and also considering a stereo effect was proposed to illustrate the binding kinetics. Allosteric means “other site”. The solutes other than creatinine are assumed to bind to the nonspecific sites, whereas creatinine molecule binds to the specific site. Therefore, an allosteric model is appropriate to describe the specific binding behavior of the hybrid organic sol-gel imprinted polymer toward creatinine in this work. The fitting of the allosteric model with the experimental data for the adsorption isotherm is given in Fig. 2. The allosteric equation is expressed as follows:


where B and F are defined the same as above. m is an index. qs is the maximum specific bound amount of creatinine. Ks is the binding constant.
Fig. 2 Adsorption isotherms of creatinine analyte by the imprinted (●) and the nonimprinted (○) hybrid organic-inorganic polymer.

Most MIPs were amorphous which readily made their binding sites not identical. Therefore, each binding site could have an individual association constant of its own. Therefore, an affinity distribution function derived from the binding kinetic model was reported for MIPs. Heterogeneous binding sites were assumed in the literature. Similarly, the affinity distribution function from the allosteric model is investigated in this work. The reported affinity spectrum was used in this work to characterize the binding of the hybrid imprinted polymer. The number of the binding sites, Ni, could be expressed by the following function:


where Ni is the number of binding sites. B is the bound analyte. F is the unbound analyte. Ki is the association constant. R is a constant.
The values of B against F was calculated and Eq. (2) is solved and is expressed as


where m = 1.072; Ntqs = 31.819 mg/g MIP; a ≡ 1/Ks = 4.505. The affinity spectrum is a semilog plot of Ni versus log Ki. The affinity distribution profile fitted with data for the hybrid organic-inorganic creatinine-imprinted polymer is shown in Fig. 3. The affinity distribution profile shows a continuously normal distribution that represents the relationship between the binding site and its association constant in this case. Maximum number of binding sites, 34 mg/g MIP, was obtained at the association constant of 3.55. At the region of lower values of association constant, the number of binding sites, Ni increased as Ki increased. On the contrary, at the region of higher values of association constant, Ni decreased as Ki increased.
Fig. 3 Affinity distribution of hybrid organic-inorganic creatinineimprinted sol-gel polymer derived from the allosteric model.
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