Bird-Wing Optical-Reflector Design with Photocatalyst for Low-Glare Mosquito Trapping System with Light-Emitting Diodes

Abstract

Dengue fever is the most serious vector-borne disease in Asia. There are still no dengue vaccines or therapeutic drugs, and vector-mosquito control is the main prevention and treatment method. The prevention and control of dengue-fever vector mosquitoes in Taiwan is still dominated by larval control. However, the removal of the source of mosquito-borne diseases has not been fully implemented, and the removal process of vector-borne mosquitoes cannot keep up with their breeding rate. In addition, chemical agents used in the elimination of pathogenic mosquitoes may cause mosquito resistance and environmental pollution. Therefore, it is important to develop new prevention and control technologies. This study is dedicated to the development of a mosquito trapping optical system with high efficiency and low glare that is safe for humans. The system is mainly equipped with ultraviolet light-emitting diodes (UV-LED), a freeform-surfaced optical reflector, and a photocatalyst. The reflector can lead light downward for the protection of user eyes, and expand the range of trapping mosquitoes to 225 π m². Based on practical experiments, captured mosquitoes increased by about 450% through the proposed system compared to conventional traps using UV LED. In addition, the proposed system is shown to be 45% more enhanced in trapping capability after a photocatalyst (titanium dioxide) coating is applied to its reflector.

1. Introduction

Mosquitoes are the vector of many diseases, especially in environments with high temperatures and humidity, which are more suitable for the growth and reproduction of mosquitoes and diseases transmitted by them, making them more serious in this kind of environment. According to the statistics from the World Health Organization, about one million people on average die each year from infectious diseases spread by mosquitoes [1]. For instance, filariasis and the West Nile virus are transmitted by Culex quinquefasciatus [2], the Japanese encephalitis virus (J.E.V.) is transmitted by Culex annulus, and dengue fever and the Zika virus [3,4] are transmitted by Aedes aegypti and/or Aedes albopictus. Therefore, methods effectively reducing the number of mosquitoes and preventing them from biting human skin are a concerning issue to everyone.

The prevention and control of dengue-fever vector mosquitoes in Taiwan is still dominated by larval control [5]. However, the removal of the source of mosquito-borne diseases has not been fully implemented, and the removal of vector-borne mosquitoes cannot keep up with their breeding rate. In addition, chemical agents used in the elimination of pathogenic mosquitoes may cause mosquito resistance and environmental pollution.

Mosquitoes are highly sensitive to optical intensity, direction, wavelength, and contrast ratio [6,7]. Studies of sensitivity of mosquito eye to ultraviolet (UV) rays or short-wavelength visible light have been conducted [8,9,10]. According to our investigation, current commercialized mosquito trappers are not quite satisfactory with customers with regard to their trapping capability and due to their uncomfortable glare. As a result, a low-glare mosquito trapping system with high trapping capabilities is proposed in this study. The proposed system is equipped with an optimized optical reflector with a freeform surface coated with a titanium dioxide as a photocatalyst. The reflector can transform the LED source flux as appropriate light distribution in space to attract mosquitoes more easily. The photocatalyst coating (titanium dioxide) was adopted on the reflector to increase carbon dioxide concentration and simulate human exhalation like in the enviroment, which is demonstrated to increase the power of the proposed device in trapping mosquitoes.

2. Principles and Methods

It is known that the visible spectrum of most insects falls between 300 and 700 nm (Briscoe and Chittka, 2001) [11]. In this spectrum, ultraviolet, blue, green and other wavelength ranges are more likely to attract mosquitoes. A study by Muir et al. [12] in 1992 pointed out that Aedes albopictus is better attracted at wavelengths of between about 340 and 520 nm. A study by Burkett and Butler in 2005 pointed out that both Aedes aegypti and Aedes albopictus have different degrees of attraction in a wavelength range from 350 to 700 nm [13]. In the study, the physical properties of attractants, including specific light wavelengths, and supplemented with a photocatalyst coating of titanium dioxide on mosquito traps, were used to increase carbon dioxide concentration to simulate human living environments and enhance the lure of dengue-vector mosquitoes.

Ultraviolet light is harmful to the human eye, but conventional mosquito trappers, as shown in Figure 1, arranges the UV light source in their inner space to preventing direct contact with human eyes. In consequence, commercial mosquito trappers have the problems of low light intensity and a strong glare to human eyes. The proposed mosquito trapping device utilizes ultraviolet LED as its light source to attract mosquitoes, and an air-inhalation flow system to trap and kill them, as shown in Figure 2. The airflow vortex inhalation system was designed to trap mosquitoes, preventing them from escaping. The airflow, generated by a fan hidden in the inner space of the mosquito trapper, targets the mosquito habit to move with the airflow, guiding the wandering mosquitoes to gather at and get sucked into the mosquito box in the reflective cavity. The continuous airflow goes into the mosquito-collection area, causing the mosquito to die due to air-drying and dehydration from the inhalation airflow. The safe and hygienic mosquito-collection area can reduce the spread of bacteria carried by mosquitoes, which is superior to the electric shock-type mosquito trap system. Mosquitoes have phototaxis characteristics, attracting surrounding mosquitoes by light trapping. By entering the reflective cavity area, mosquitoes are inadvertently inhaled into the mosquito-collection area with the airflow until they are killed.

A freeform-surfaced optical reflector was designed to generate effective light spatial distribution and provide enough light-soure excitation to the photocatalyst coating for producing CO₂, creating a simulation of human exhalation and increasing attractiveness to mosquitoes. The freeform-surfaced optical reflector is like a bird wing. Each wing has three segmented reflecting portions with length D_i and tilt angle θ_i, as shown in Figure 3. A UV light source is located under the center of the bird-wing reflector. The height from the source to the center and the end of the reflector is H₁ and H₂, respectively. In the design processes, Hi, Di, and θ_i are used as variables to accomplish the targeted reflected light distribution. The reflector is optimized to concentrate light downward and prevent it from meeting users’ eyes, enhancing light intensity and expanding its reach much further than conventional reflectors. The reflector-design workflow with the application of TiO₂ to the LED mosquito trapping system is shown in Figure 4.

Most of the photocatalyst is made of semiconducting material, having an energy-band structure, as shown in Figure 5a. When excited by light, their valence electrons are in the lower energy-valence band, where electrons are trapped around atoms and are not active [14]. The pair of electron holes are excited by the photocatalyst after being illuminated, and nearby molecules (H₂O, O₂-) are ionized to form positive or negative ions, or free radicals (OH-, OH*).

TiO₂ + hv → h⁺ + e⁻

(1)

OH⁻ + h⁺ → OH*

(2)

O_2ads + e⁻ → O₂⁻_ads

(3)

O₂⁻, H+, OH⁻, OH*, and HO₂·plasmas and free radicals are produced. O₂⁻ and HO₂·radicals have a strong reducing ability, and OH⁻ and OH* radicals have a strong oxidizing ability, which can decompose other organic compounds on the surface of the photocatalyst into water and carbon dioxide [15].

OH^* + pollutant + O₂ → products (CO₂, H₂O, …)

(4)

I the stimuli of these valence electrons are given enough energy to overcome the energy gap and transition to the conduction band, however, they become active free electrons (represented by e-). At this time, the valence band leaves due to the electrons, relatively producing a positively charged active hole (indicated by h+). Free electrons and holes are generated in pairs, so the number is the same. The electrons and holes generated by the photocatalyst can move freely. They may recombine to release energy, or they may move to the surface of the photocatalyst to decompose other organic compounds into water and CO₂ [16], as shown in Figure 5b.

3. Experiments and Results

The UV LED strip light shown in Figure 6 has six chips, and each chip was derived in 0.5 W to emit UV light with a wavelength of 395 nm, packaged in epoxy n: 1.5, to act as the source of the proposed system in the following experiments. The light distribution curve of the strip light was measured by ProMetric near-field measurement system (PM-NFMS), developed by Radiant Vision Systems Co., shown in Figure 6.

In the reflector-design procedure, the Di and θ_i of the bird-wing reflector shown in Figure 3 were used as the variables to accomplish the targeted reflected light distribution. The light is located under the center of the reflector, about 50 mm (H₁). The targets of the light-intensity distribution in optics design include Peak angle: 160° ± 5° and Divergent angle referring the peak angle: ≤ 30°. For rapidly finding solutions in the optimization process, θ₁ and θ₂ were set as constants of 45 and 170 degrees, respectively. After optimization through SolidWorks, an optics design software with optimization functions, three different reflectors and their light distribution curves were prepared, as shown in Figure 7. The parameters of the optimized reflectors are listed in Figure 8.

Type A: Angle θ₁ is 45 degrees, angle θ₂ is 170 degrees, angle θ₃ is 165 degrees, projection distance D₁ is 40 mm, projection distance D₂ is 40 mm, and projection distance D₃ is 140 mm.

Type B: Angle θ₁ is 45 degrees, angle θ₂ is 170 degrees, angle θ₃ is 160 degrees, projection distance D₁ is 40 mm, projection distance D₂ is 40 mm, and projection distance D₃ is 130 mm.

Type C: Angle θ₁ is 45 degrees, angle θ₂ is 170 degrees, angle θ₃ is 150 degrees, projection distance D₁ is 40 mm, projection distance D₂ is 40 mm, and projection distance D₃ is 140 mm.

Based on the three different designs above, three sets of bird-wing reflectors were prototyped and installed with the airflow inhalation, respectively, for the experiments, as shown in Figure 9. A 3-W commercial mosquito trapper was also used in the experiments for comparison, as shown in Figure 10. After the experiments, the amount of captured mosquitoes for each testing day by the commercial trapper and the bird-wing trapper were all shown in Figure 11. Based on the results, the number of trapped mosquitos can be calculated such that the trapped mosquito numbers were up by 450% ((526/116) × 100%) by using the proposed system with Type C reflector when compared with the conventional system. It can be found that Type C was the best of all the experimental systems.

In order to verify the effect of the photocatalytic coating on the proposed system, a TiO₂ gel was applied on the center portion of the bird-wing optical reflector, as shown in Figure 12. The TiO₂ gel was prepared by fully mixing 80% TiO₂ powder with 20% clean water in a beaker. Through a CO₂-detecting instrument, CO₂ concentration was measured besides, below, and away from the bird-wing reflector, respectively, as shown in Figure 13. The data were recoded for 12 h and are shown in Figure 13, which demonstrates that photocatalytic coating can really generate more CO₂ around the system.

When light meets TiO₂, it becomes a photocatalyst. CO₂ is generated by the photocatalyst process. Insects know where a human being is by detecting the CO₂ of people exhaling. Insect-killing devices imitate the respiration of human beings. TiO₂ photocatalysts detect and record changes in CO₂ concentration with a CO₂ detection device (12 h), as shown in Figure 14.

Since TiO₂ application is effective for CO₂ production that mimics human exhalation, the proposed system with Type C reflector was used as the testing trapper to evaluate the difference before and after coating TiO₂ on Type C reflector in mosquito trapping efficiency. After the experiments, the amount of captured mosquitoes for each testing day by the Type C bird-wing trapper and that with TiO₂ coating were all shown in Figure 11. It can be figured out that the trapped amount could be further elevated by 45% ((771 − 556)/556 × 100%) after applying the coating, as shown in Figure 15.

4. Discussion and Conclusions

Dengue fever is the most serious vector-borne disease in Taiwan—there is still no vaccine or therapeutic drug. At present, the prevention and control strategy of dengue fever is still based on the principle of eliminating the source of vector mosquitoes and spraying chemical agents to kill vector-mosquito larvae. Although this method can quickly kill vector mosquitoes, they gradually gradually develop resistance. Modern technologies are dedicated to the development of chemicals that eliminate pests such as mosquitoes. After a long period of time, organism resistance becomes stronger, and the high concentration of pesticides directly or indirectly harming humans through the food chain is becoming increasingly serious.

This study is dedicated to the development of a new mosquito trapper that target the physical trapping factors of mosquitos including lighting, color and evaluation tests in parks, primary schools and hospitals of southen Taiwan. Currently, LEDs with 395 nm or 365 nm are both the mainstream products in the UV LED market. But, so far, 365 nm LED is still more expensive and much less efficient than a 395 nm LED. In consequence, 395 nm LED is adopted as the source in the experiments instead of 365 nm LED, although 365 nm is usually the UV peak wavelength of conventional mosquito trapper using fluorescent light tubes. The light-intensity distribution peak angle and the divergent angle centering the peak angle of the new trapper are targeted as 160° ± 5° and ≤ 30°, respectively in the reflector optimization process. The 160° peak angle is for covering the trapping area as far as possible and the limited 30° divergent angle is for restricting upward UV rays generation to decrease the glare to human eye. The study‘s experiment results show that the equal-intensity curve (red line) with a radiation intensity of 135 w/sr has a lightlike curve, similar to the shape of a butterfly wing, indicating that the range of light divergence is wider. This study demonstrates that the optimization of a bird-wing optical reflector for outdoor UV LED mosquito systems can better attract mosquitoes (as shown in Figure 7c and Figure 11). Compared with a conventional mosquito trapping device (shown in Figure 1), the outdoor trapping range can be extended to 225 πm², and the number of captured mosquitoes increased by about 450%.

In addition, applying TiO₂ to the bird-wing optical reflector of the UV LED mosquito trapping system can not only imitate human breath, but also attract mosquitoes to enter the mosquito trapping system. Compared with the anti-mosquito effect of each group of bird-wing optical reflectors of the UV LED mosquito-removing system, it can increase mosquito-killing efficiency by more than 45%.

The UV LED mosquito trapping system with a photocatalyst optical reflector operates for about 24 h. It begins to catch mosquitoes immediately when there is activity in the area. Female mosquitoes looking for blood are drawn to the trapping device and eliminated. The trap gradually reduces the number of mosquitoes as it attracts and destroys them. It may take 6–8 weeks of use to reduce the number of mosquitoes to the point where the proposed device has successfully interrupted the mosquito breeding cycle [17].

Therefore, the bird-wing optical reflector with a photocatalyst for a low-glare mosquito trapping system with light-emitting diodes has been scientifically proven to be safe and effective in reducing the number of mosquitoes, and to be the best auxiliary solution for dengue fever.