Microbial BOD sensors based on Zr (IV)-loaded collagen fiber

Highlights

• ZrCF is a kind of excellent carriers for the immobilization of Saccharomyces cerevisiae and Escherichia coli.

• The ZrCF-immobilized Saccharomyces cerevisiae and Escherichia coli could be used as the sensitive biofilm of BOD sensor.

• The performance of microbial BOD sensors based on ZrCF, such as sensitivity and linaer response range, were greatly affected by the amount of immobilized microorganisms and density of the biofilm.

Abstract

Biochemical oxygen demand (BOD) sensors based on Zr (IV)-loaded collagen fiber (ZrCF), a novel material with great porous structure, were developed. This novel material shows adsorbability by microorganisms. Saccharomyces cerevisiae and Escherichia coli were used for the construction of BOD sensors. Factors affecting BOD sensor performance were examined. The ZrCF-based BOD sensor showed different sensitivities and linear response ranges with different biofilm densities. The amount of microorganisms strongly affected the performance of the BOD sensor. Poor permeability of previously reported immobilization carriers were greatly circumvented by ZrCF. The service life of the ZrCF-based BOD sensor was more than 42 days. The immobilized microorganisms can be stored for more than 6 months under 4 °C in PB solution. There was good correlation between the results of the sensor method and the standard 5-day BOD method in the determination of pure organic substrates and real water samples.

Introduction

Biochemical oxygen demand (BOD) is a widely used parameter for indicating the biodegradable organic pollutant content of wastewater. The conventional method for measuring BOD (5 days BOD, BOD₅) [1] is time-consuming, exhibits low reproducibility, involves tedious steps, and cannot provide an indication of pollution in a timely manner. Development of microbial BOD sensors has received considerable attention lately [2], [3], [4] because it can provide quick measurements, requires minimal labour, and can be controlled online.

The microbial film (immobilized microorganisms) is the key component of a microbial BOD sensor that greatly affects the sensor’s performance, in terms of sensitivity, linear response range, response time, and service life. A range of methods (i.e., cross-linking, sandwich, absorbing, and embedding) have been applied for the immobilization of microorganisms [3], [4]. Different carriers, including polycarbonate [5], agarose [6], Ca-alginate [7], PCS [8], Al₂O₃ sol-gel [9], are involved in these processes. Almost all reported immobilization carriers possess an important feature (porous structure) that allows the substrates (e.g., dissolved oxygen, and organic compounds) to permeate the biofilm and react with the microorganisms, after which the signals can reach the electrode [3], [4]. However, most of these materials are not sufficiently porous [10]; they show different degrees of compactness, which limits the spreading of the substrates in the biofilms, and restricts the thickness of the biofilms (i.e., the amount of immobilized microorganisms). Consequently, biosensor characteristics, such as sensitivity, response range, and response time are limited. To solve this problem, researchers have tried various approaches such as modification of the carriers [11], [12] and changing the structure of biosensors [10]. However, these methods have showed drawbacks of involving complicated procedure, high cost and low utilization of microorganisms.

Collagen fiber (CF) is a natural polymeric material extracted from animal skins (e.g., pig hide, and cattle hide). It has porous structure, excellent water binding capacity, and good bio-compatibility [13]. Zr (IV)-loaded collagen fiber (ZrCF) has many advantages, including excellent mechanical property, physical and chemical stability, anti-biodegradability, and adsorbability toward various biomaterials (proteins, enzymes, and microorganisms) [14], [15]. Therefore, ZrCF can be used as an immobilization carrier in biosensors.

Various microorganisms [3], [4] have been used for the construction of BOD sensors. Fungi and bacteria are different in terms of their cytoderm structure. Saccharomyces cerevisiae (S. cerevisiae) and Escherichia coli (E. coli) are often used as the representatives of fungus and bacteria, respectively.

In this study, S. cerevisiae and E. coli were immobilized on ZrCF to prepare BOD sensors. The main purpose of this research was to investigate the effects of various factors, including temperature, pH, concentration of NaCl, density of the biofilm, and the amount of immobilized microorganisms on the ZrCF-based BOD sensor. Pure organic substrates and real water samples were determined by the sensor method and the 5-day BOD (BOD₅) method, respectively.