Using metal-organic framework (MOF) made of cadmium chloride and synthesised under ambient conditions, a research team has been able to detect organophosphorus pesticides such as azinphos-methyl, chlorpyrifos, and parathion both in water solution and in apples and tomatoes. The MOF is highly sensitive such that the presence of pesticides as low as in parts per billion (ppb) can be detected.
The team led by Dr. Partha Mahata from the Department of Chemistry at Jadavpur University, Kolkata and Dr. Sudip Kumar Mondal of Department of Chemistry, Visva-Bharati University, Santiniketan found that the ability to detect the three pesticides was unaffected by the presence of surfactants. Surfactants are used to dissolve pesticides in water.
Though currently available methods for pesticide detection are efficient in terms of sensitivity, they have other challenges such as complicated procedure to prepare samples, longer time to analyse samples, need for sophisticated instruments and lack of portability for use infield use.
The researchers synthesised the MOF by mixing cadmium chloride and organic ligand in methanol and water at room temperature. After seven days, a single crystal of MOF is formed; such crystals were used for the study.
The MOF was found to emit intense luminescence at 290 nanometre when excited by light at 225 nanometre. “It is the organic ligand that shows the intense luminescence,” says Dr. Mahata. “The organic ligand is tightly bound to the metal framework and forms a crystalline structure. So loss due to vibration gets reduced and luminescence intensity increases.”
In a paper published in the journal Inorganic Chemistry, the researchers say that traces of pesticides in samples were detected by looking for reduction in the intensity of luminescence when the sample containing pesticide was added. At a concentration of 30 micromolar of azinphos-methyl and chlorpyrifos pesticides, the luminescence intensity reduced (or quenched) by 90% and 52% respectively. There was 49% reduction in luminescence in the case of parathion.
Three factors
“A combination of three factors is responsible for luminescence quenching,” says Dr. Mahata. “When the MOF comes in contact with pesticides, there is energy transfer (resonance energy transfer) from MOF to the pesticides. There is also electron transfer from MOF to pesticides. Finally, pesticides also absorb some amount of light used for exciting the MOF.
In the case of azinphos-methyl pesticide, the MOF was able to detect its presence in water even when the concentration of the pesticide was as low as 25 nanomolar (almost 0.008 mg per litre or 8 ppb). “The sensitivity will be a little less in the case of other two pesticides [chlorpyrifos, and parathion] but it will still be in the ppb range,” he says.
The researchers tested the MOF’s ability to detect azinphos-methyl pesticide in apple and tomato extracts. When 30 micromolar of the pesticide was used, the luminescence quenching or reduction was 71% and 80% in the case of apple and tomato extract respectively. “So the cadmium-based MOF can be used as an alternative pesticide sensor in both water solution and sample extracts,” says Dr. Mahata.
“We are also planning to develop MOF-based sensors where we can see colour changes in visible region of light. We are currently studying the detection pesticides in many other fruits and vegetables,” he says.
