Hi Laboni.... Alcohol can not prolong the effect of essential oils. When you apply the spray on skin, the alcohol containing volatile oil will spread and after some time, only volatile oil will retain. As both alcohol and essential oils are volatile in nature, this combination will not work for your purpose. What you need to do is to sustain the effect of evaporation of essential oil. For this purpose you can entrap the essential oil in any semisolid formulations (cream, gel, lotion) or make encapsulated system.

Citronella oil is well know for its insect repellent properties, but it is associated with very strong odor (may lead to patient incompliance). So you need to use its in low concentration in combination with other oils. I am not sure about the mosquito repellent properties of eucalyptus, and peppermint oil rather these have antimicrobial action. Other essential oils you can choose are Basil oil, Vetiver oil, and Lavender oil.    

Best.....

You can use alcohol with lang chain is possible or other solvent that can solubilyze the fragrance and/or essential oil.

you can try basil oil for spray or creams, and neem oil in formulations

I thought two strategies to increase the effectiveness. 1. More effective essential oil and 2. Longer alcohol with low volatility. Thanks for the suggestion of essential oils. I have already ordered those. If anybody has idea what long chain is suitable or usually what is using in perfume for staying longer plz share with me. 

@Jay, Thanks a lot for your reply. I know it is very difficult to keep it active in spray form, but I have no choice right now. If I change phase state I need to buy another production and packaging equipment. People like spray as it is easier to use. 

You must have pure oil of those essential oil so that it will stay longer.