The biosensor is an analysis tool or system composed of a biomaterial, a transducing device, and a signal amplification device that are immobilized and have a chemical molecular recognition function.
1 Fundamentals of Biosensors 1.1 Basics of Reactions Biosensor reaction bases are based on four common types of biological reactions: physical changes that accompany the enzymatic, immunological, microbial, and biological responses.
1.2 Working principle The selectivity of a biosensor depends entirely on its molecular recognition element, while other properties are related to its overall composition. The information generated by the biological reaction process is diversified. Selecting different converters to convert the information is very important for the design of biosensors. The basic converters for common biosensors are electrochemical and optical.
2 Application of biosensors in water monitoring The biometric sensors used in water quality monitoring mainly include enzymes, microorganisms, and organelles. The main applications for water quality monitoring include determination of BOD, total bacteria, sulfides, organic pesticides, phenols, and oxygen enrichment in water bodies.
2.1 Biosensors for monitoring BODs BOD is an important indicator for measuring the degree of organic pollution in water bodies. The traditional standard dilution method for determining BOD requires a long time, cumbersome operation, and poor accuracy. BOD sensors not only meet the requirements of actual monitoring, but also have fast and sensitive characteristics. The working principle of the BOD sensor [, ]: The single or mixed population of microorganisms is used as the electrode of the BOD microorganism. Due to the addition or degradation of BOD substances in the water body, the internal and external microbial respiration patterns in the water are changed or transformed. In conjunction with changes in the signal strength of the current, the current output of the sensor is linearly related to the concentration of BOD under certain conditions. The microorganisms used to make BOD biosensors include yeasts, Pseudomonas, Bacillus, luminescent bacteria, and thermophiles.
The BOD tester developed by Zhang Yue et al. uses a polyvinyl alcohol gel-embedding method to immobilize yeast, and the immobilized yeast is directly dispersed and suspended in the solution. The BOD is measured in the DO probe insertion solution. Experiments show that the best measurement conditions are Temperature of 30'C, pH 5.0, 15g of immobilized cells can achieve rapid determination of BOD within 20 minutes. The BOD is. There is a good linearity in the range of -200 mg/L with good accuracy. However, there is still a considerable distance from the actual application and further research is needed.
Abroad, two new yeast strains, SPT1 and SPT2, were isolated and immobilized on glassy carbon poles to form a microbial sensor for measuring BOD. The error is 10% of soil. The sensor was used to measure the BOD concentration in the wastewater from pulp mills, with a minimum value of 2 m g/L and a time of only 5 m in (3).
2.2 Biosensors for Rapid Determination of Total Bacteria The total number of bacteria is one of the important pollution indicators in water quality samples. At present, the plate colony counting method is widely used, the measurement period is long, the accuracy is not high, and the subjective error is large. The rapid determination of biosensors has caused great interest. Han Shubo et al. (4) developed a new type of voltammetric bacterial total biosensor (see Fig. 2). Through the design of the electrode and its auxiliary measuring device, the lower limit of determination was 3x104 cells, and the measurement period was about 0.5 h. The bacterial suspension membrane was filtered by bacterial suspension and immediately attached to a modified electrode in a sterile hood. The electrode and the membrane were fixed to the bottom of the elastic cell using a filter positioning device to record the voltammetric scan. The curves, the peak current values ​​obtained are compared with the calibration curves of the corresponding samples, and the total number of bacteria is calculated. The linear response ranges of Escherichia coli, Bacillus subtilis, Staphylococcus aureus, Salmonella, and Saccharomyces cerevisiae were 2x 104 to 4.8x107, 2x104 to 4.60x107, 3x104 to 9.16x107, and 2x104 to 9.60x107, respectively. 2x 104 to 9.60x 107 cells.
2.3 Biosensors for Monitoring Sulfides The determination of sulfides plays an important role in environmental monitoring. Methylene blue colorimetry, iodometric titration, potentiometric titration, etc. (5) are commonly used in the determination methods. These methods often require the pretreatment of samples, which not only consumes large quantities of drugs but also causes measurement errors. Bai Zhihui and others used sulphide bacilli to make sulphide sensors for the determination of sulphides in domestic sewage, industrial wastewater, and complex samples containing HZS gas (6). The method is to separate and screen the Thiobacillus thiooxidans from the acidic soil of pyrite, and immobilize the Thiobacillus thiooxidans, prepare a microbial membrane, and then assemble a microbial sensor with an oxygen electrode for the determination of trace sulfides in the sample. Experimental studies show that the linear range of the sensor response to S2-mass concentration is 0.06 to 1.50 mg/L, the response time is 3-6 m in, and more than 500 measurements are made within 30 days. The sensitivity remains unchanged.
2.4 Biosensors for the Determination of Phenols Phenol is one of the "five poisons" in water systems and it is of great significance to carry out effective monitoring. Biosensors for the determination of phenols include enzyme electrodes, microbial electrodes, and plant tissue electrodes, all of which are based on the following reactions:
Phenol ten O2+2 H++ tyrosinase-catechol catechol ten O2+tyrosinase phthalocyanine two-step reaction both require large amounts of oxygen, according to which tyrosinase or tyrosine rich The biosensors of biosensors such as fresh mushrooms, potatoes and bananas of acidase were combined with dissolved oxygen electrodes to determine phenol. Mu Dongyan et al. studied the method of using a maltodextrin-modified tyrosinase carbon paste electrode to construct a current-based biosensor for the determination of phenolic contaminants in water. The applied voltage was 100 mV (vs. SCE) and p H was 5.40. In the phosphate buffer solution, there is a good linear relationship between the electrode voltage and the phenol concentration in the range of 2.0 x 10-7 to 1.0 x 10-5 mol/L. The detection limit is 1.0 x 10-7moUL and the response time is 2 minutes. This electrode has a good response to other phenolic substances such as catechol, p-chlorophenol, o-cresol and so on. This electrode can be used to detect the concentration of phenols in industrial wastewater [7].
Feng Zhiping et al. [8] Extracted crude catechol from mushroom tissue. Using silk fibroin under the action of methanol, the molecular structure changes from a soluble arbitrary coiled structure to an insoluble knife-sheet, thus converting catechol into catechol. The crude enzyme was immobilized on the silk fibroin membrane to obtain a catecholenzyme sensor. The sensor had good response characteristics in the working medium of KH2PO4, -Na2HPO4 with a pH of 6.0, and the working linear range was 1.0x10-5. -2.5x 1 0-4m ol/L, detection limit 5.0x 10-6mol/L, response time 2 min, the enzyme has high heat resistance after fixed by silk fibroin, and can keep enzyme for a long time active. The sensor is stored in KH2PO4, -N a2HPO4, buffer solution, and its service life is up to two months.
2.5 Biosensors for the determination of organic pesticides With the development of agricultural technology, the use of herbicides has exceeded that of insecticides and fungicides in many countries (9). Li Jianping used herbicides to catalyze the decomposition of hydrogen peroxide by plant thylakoid enzymes, developed an electrochemical biosensor called a rapid detection of trace herbicides, and achieved on-site monitoring.
2.6 Other biosensors T. C ha rle s P aul et al. [11] studied a microbiological sensor composed of a porous gas permeable membrane, immobilized denitrifying bacteria, and an oxygen electrode to determine the concentration of nitrate in a sample. Since the denitrifying bacteria use nitrate as their sole source of energy, their selectivity and anti-interference are quite high. They are not affected by volatile substances (such as acetic acid, ethanol, amines) or non-volatile substances (such as glucose, amino acids, K+, Na+). The effect of the above equation is based on the linear relationship between the oxygen electrode current and the denitrifying bacterial oxygen consumption. Min Qing et al. [12] Utilizing N3-[(3-dimethylamino)]-N'-ethylcarbodiimide hydrochloride (EDC) and mesitylphthalimide (NHS) pair 11 Monothiol undecanoic acid monolayer modified quartz crystal electrode surface activation covalently binds polymylin (PMB) to the surface of the electrode, establishing a quartz crystal microbalance organism that can be used to detect bacterial endotoxins sensor. Li Baixiang et al developed a rapid, sensitive, and simple biosensor for detecting acute toxicants in water [13]. The sensor uses cell immobilization technology to immobilize luminescent bacteria as a sensitive element, which is tightly combined with a high-sensitivity silicon photodiode, and utilizes the luminescence of photoluminescent bacillus as an indicator of toxicity. The cell immobilization technology, biosensor technology, and luminescent bacteria toxicity detection technology are organically combined to construct a flow-through acute poison fast tester.
An optical biosensor is currently available for the determination of nutrient concentrations in water samples (143. The sensor is based on cytochrome nitrite reductase immobilized on controlled capillary glass. Its working principle is that when nitrite is present, Enzymatic oxidation measures the change in the spectrophotometric value of nitrite reductase to determine the nutrient concentration.
3 Insufficient (1) Reproducibility problems. In the course of working of some sensors, there are often cases in which an irreversible chemical reaction occurs between the identification element and the substance to be measured, which inevitably reduces the recognition ability of the identification element and thus affects the sensitivity of the sensor.
(2) Miniaturization. Miniaturization of the instrument will reduce the sample volume, reagent consumption and production costs.
4 Prospects The development trend of biosensors in water quality testing includes: accelerating the process from the laboratory to the commercialization; the application in water quality monitoring will be further broadened; the development of single-function biosensors to multifunctional biosensors; and others Precision instruments combine to complement each other; biosensors are becoming smaller, more integrated, and more intelligent.
In summary, after several decades of research accumulation and technological improvement, biosensors have entered a new era of development and application. Various new biosensors are constantly being developed and are dizzying. In addition, due to the advantages of fast, low-cost, high selectivity, high sensitivity, easy operation, and online or on-site detection, biosensors will have unlimited application prospects in water monitoring. (end)