Novel developments of ZnO/SiO2 nanocomposite: a nanotechnological approach towards insect vector control

Recently, there is increasing efforts to develop newer and effective larvicides to control mosquito vectors. This study was carried out to examine the efficacy of ZnO/SiO2 nanocomposite synthesized using Gum Arabic against Culex quinquefasciatus larvae. The elemental composition, morphology, functional groups and surface plasmon resonance of the ZnO/SiO2 nanocomposite was analyzed by Energy dispersive X-ray analysis (EDX), Scanning electron microscope (SEM), FTIR and UV-Visible spectroscopy respectively. In bioassay, larvae were exposed to three different concentrations of synthesized ZnO/SiO2 nanocomposite. The mortality rates at concentrations of 10, 20 and 25 were found to be (70%, 80%, 86%), (56%, 64%, 84%) and (44%, 48%, 76%) for 1st, 2nd, and 3rd instar respectively. This study revealed that the synthesized ZnO/SiO2 nanocomposite exhibit high larvicidal activity; 1st instar (LC50=4.024, LC90= 39.273 mg/l), 2nd instar (LC50=8.767, LC90=51.069 mg/l) and 3rd instar (LC50=13.761.LC90=81.809 mg/l) DOI:10.46481/jnsps.2021.198


Introduction
Application of chemical insecticides in order to kill mosquito larvae or pupae in the water is known as larviciding. Larviciding is generally more effective and target-specific than adulticiding (applying chemicals to kill adult mosquitoes). The most common synthetic chemicals used in controlling mosquitoes' larvae are methoprene, pyrethroids, diflubenzuron, malathion, dichlorodiphenyltrichloroethane (DDT), organophosphate temephos and as well as phytochemicals [1][2][3]. However, synthetic chemicals (insecticides) are known to cause serious environmental problem thereby killing non-target organism and affecting human health. Moreover, continuous application of synthetic chemicals (insecticides) results in control failures due to development of resistance by the mosquitoes (vectors) [4]. Hence, it has been reported that ZnO and SiO 2 nanoparticles provides a lay down of a novel green nanotechnology to control insect pest including mosquitoes' larvae [5][6][7][8]. ZnO/SiO 2 nanocomposites have been synthesized using various techniques such as chemical vapor deposition, sputtering, chemical etching and sol gel process and they are used in different applications such as an-timicrobial, photonic crystals, photocatalysts, gas sensors, vacuum fluorescent display and varistors etc., [9][10][11][12][13][14][15][16][17][18][19]. But based on our search, there was no report on the effect of ZnO/SiO 2 nanocomposites against mosquito larvae except for the individual ZnO and SiO 2 nanoparticles.
Culex quinquefasciatus is a vector of lymphatic filariasis. The breeding site of Culex species includes; gutters and water retention sites having organic matter. Filariasis has been reported to be a public health problem in Africa as well as other part of the world [20][21][22]. Thus, to prevent mosquito bornediseases and improve the quality of public health, it is necessary to control mosquito larvae. In this study, ZnO/SiO 2 nanocomposite was synthesized using Gum Arabic and was tested against the larvae of Culex quinquefasciatus. The formation of ZnO/SiO 2 nanocomposite was confirmed using ultravioletvisible (UV-Vis) spectrophotometry, Scanning electron microscopy (SEM) coupled energy dispersive X-ray (EDX) spectroscopy and Fourier transforms infrared (FTIR) spectroscopy.

Collection of Gum Arabic (Acacia senegalensis)
A fresh A. senegalensis extrudes were collected from Billiri Local Government of Gombe State. The Gum extract were neatly collected and allowed to dry properly under the sun. The Gum Arabic was crushed to powder using pestle and mortar.

Synthesis of ZnO/SiO 2 Nanocomposite
One gram (1 g) of Gum Arabic was poured into a beaker containing 40 ml of distilled water which was magnetically stirred for 10 minutes at 90 • C. Following this, 2 g of Zn(NO 3 ) 6 .6H 2 O and 2 g of silica gel were added and stirred for 30 minutes. Upon addition of Zn(NO 3 ) 6 .6H 2 O and silica gel the aqueous solution changed to milk colour. With time the solution became viscous. A cloudy formation at the bottom of the beaker indicated the formation of resin. The resin obtained was placed in a furnace at 450 o C for two hours to obtain ZnO/SiO 2 nanocomposite [23].

Characterization of the Green synthesized ZnO/SiO 2 nanocomposite
ZnO/SiO 2 Nanocomposite was characterized using Ultravioletvisible Spectroscopy, Fourier transformed infrared spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) coupled with energy dispersed x-ray techniques.

Collection of Culex quinquefasciatus larvae
Culex quinquefasciatus mosquito larvae were collected from different areas of Gombe metropolis. Using ladle and a collection bottle, the ladle was lowered into the water (breeding site) at an angle of about 45 o until one side is just below the surface of the water. While dipping, care was taken not to disturb the larvae which may cause them to swim downward. The larvae were maintained and fed in the laboratory with glucose for the larvicidal bioassay. The collection was done based on previous literature [23].

Larvicidal Activity of ZnO/SiO 2 Nanocomposite
The test was carried out according to our previous work [24]. 0.1g of ZnO/SiO 2 nanocomposite was weighed and diluted with distilled water in a 1000 ml volumetric flask and shaked to obtain 100 mg/L concentration. The bioassay was done by placing different instars (1 st -3 rd ) of the larvae into 200 ml of plastic container with four replicates and a control in each of the instars, each replicate comprised of twenty-five larvae. 100 ml of dechlorinated water was added in each of the replicates. Finally, 10 mg/L, 20 mg/L and 25 mg/L of the ZnO/SiO 2 Nanocomposite concentrations were inoculated into each of the replicates. And percentage mortality was calculated as follows:

Statistical Analysis
Percentage mortality, Probit analysis, Chi square and Correlation analysis were calculated and tabulated using (SPSS, 2016).

Ultraviolet visible analysis
Absorption spectrum of synthesized ZnO/SiO 2 nanocomposite at different wave lengths ranging from 260 to 380 nm revealed the maximum absorption wavelength of 280 nm, (Figure 1). Elsewhere, maximum absorption wavelength of 300 nm was reported for CaO/SiO 2 nanocomposite [25]. This optical property is in the same range with the one in the current.    Figure, 3A depict SEM image, 3B depict elemental data and 3C depict EDX spectrum of ZnO/SiO 2 nanocomposite. The SEM image showed large and dispersed particles of silica coated ZnO nanoparticles. The EDX spectrum showed different element apart from the expected Zn, Si and O. The presence of other element is due to Gum Arabic used in the synthesis because Gum Arabic has been reported to contained; Al, Ba, Ca, Fe, K, Mg, Mn, P, S and Sr [28]. From Figure 3B, percentage composition of Si, Zn and O were 38.02, 37.42 and 7.21 % respectively.

Conclusion
In this research, ZnO/SiO 2 was synthesized using Gum Arabic and characterized by UV-Visible, FTIR, SEM and EDX techniques. The larvicidal activity of ZnO/SiO 2 nanocomposite was tested against Culex quinquefasciatus larvae using desired concentrations which showed significant results. This study showed that, the application of ZnO/SiO 2 nanocomposite can serve as a replacement of insecticide in mosquito vector control.