Objective Recently, researchers increasingly focused on air quality. The air microorganisms cause severe harm to human health and severe effects on some industrial production. The latest national standard “Public Places Hygiene Indicators and Limits Requirements” stipulates the total number of air bacteria. Microbial detection based on fluorescence technology is widely used in medicine, pharmaceuticals, food, and environmental monitoring. The detection technologies include real-time fluorescent quantitative polymerase chain reaction (PCR), adenosine triphosphate (ATP) fluorescence detection, and microbial particle fluorescence-sensing system. The fluorescent microbial particle counter can directly count microbial particles, which has some advantages of good real-time performance, a high degree of automation, and simple operation. Besides, it is suitable for real-time online monitoring of the concentration of air microorganisms. This study designs a optical system based on the fluorescence detection principle in microbial particle counter to realize the online monitoring of microbial particles.
Methods The optical system is designed based on the fluorescence detection principle. In this study, the fluorescence characteristics of riboflavin, nicotinamide adenine dinucleotide, and other substances contained in microbial particles were tested and analyzed at first. Then, a 405 nm wavelength semiconductor laser was determined as the excitation light source. To reduce the output stray light of the laser, the light source shaping and fluorescence detection optical paths based on the combined diaphragm and lens were designed to obtain a high-quality flat rectangular line spot. Subsequently, consisting of oxidized and blackened aluminum alloy, the optical detection cavity structure were designed in the shape of hexahedron, and the production and assembly of the inspection structure were completed. Finally, the performance of the optical detection system was tested and analyzed using fluorescent microspheres in different particle sizes.
Results and Discussions In this study, the fluorescence detection and analysis of two fluorescent substances, riboflavin and nicotinamide adenine dinucleotide, were first conducted. Under the excitation of blue light of the same wavelength, the fluorescence peak position of riboflavin is 541 nm [Fig. 1(a)]. The peak position of the fluorescence spectrum of adenine dinucleotide is 492 nm [Fig.1(b)]. Then, fluorescent microspheres were used to test the performance of the designed optical detection system. The results showed that the final output noise of the system was is about 20 mV [Fig. 8(c)], and it could achieve graded detection of 10, 5, 2, and 1 μm fluorescent microspheres. The test voltage signal of 10, 5, 2, and 1 μm fluorescent microspheres has a signal amplitude of 350--380 mV (Fig. 9), 250--290 mV (Fig. 10), 130--140 mV (Fig. 11), and 78--90 mV (Fig.12), respectively. The test results showed that the optical detection system can effectively detect the signals of fluorescent microspheres of different particle sizes and has the characteristics of high signal-to-noise ratio and high detection sensitivity, which is of great significance for the further development of aerosol microbial particle counting instruments.
Conclusions In this study, an aerosol microbial particle counting instrument optical system based on fluorescence detection technology was designed. The overall structure of the instrument optical system was proposed. Besides, the design and structure manufacturing of the optical system were completed. By designing optical denoising optical path, optical noise is effectively suppressed, and the system signal-to-noise ratio is improved. Combined with the second-order RC low-pass filter circuit, the final noise of the system is only about 20 mV. The performance of the detection system was preliminarily tested with 10, 5, 2, and 1 μm fluorescent microspheres. The measured pulse signal amplitudes were 350--380 mV, 250--290 mV, 130--140 mV and 78--90 mV; system detection resolution is better. Next, some microbial samples, such as Staphylococcus aureus and Escherichia coil will be tested, and further the structure will be optimized to complete the overall design of the instrument.
