Β-Cyclodextrin Modified Reduced Graphene Oxide Based Chemiresistive Sensor for Detection of Lead (Pb(II)) in Water

Abstract

Reduced graphene oxide or rGO, a much studied 2D material, is a chemical equivalent of graphene. rGO has been extensively studied in the field of wide variety of electrochemical, optical and spectroscopic sensors for accurate and selective detection of heavy metals in water. Lead (Pb (II)) is one of the most toxic heavy metals which causes numerous physiological and neurological hazards. It needs constant and efficient monitoring.Here, we report Chemiresistive sensing device for sensing Pb(II) ions. Chemiresistive sensing technique is derived from the very well-known FET based technique. Ease of fabrication and operation, no necessity of gate electrode and supporting electrolyte are the reasons for choosing Chemiresistive technique as the method for sensing Pb(II) in aqueous solution. Reduced graphene oxide was chosen to be the material for constructing the channel for charge carrier movement because of its excellent conductivity and stability in ambient conditions.In order to achieve this, graphene oxide (17mg/ml) was reduced in chemical route and was subsequently annealed thermally. We were able to make rGO films of dimension 20mmx20mm in this method. Electrodes were coated on the rGO film using an optimized amount of silver paste and they were further annealed at 120ºC. The roughness of the resulting film was around 50nm and the stacking of the rGO sheets was accounted for that. In our measurements, 5µl of Pb(NO3)2 solution of different concentrations were used. Milli Q water was used as the reference. The response was calculated as percentage change in resistivity in the presence of analyte under ambient conditions.The resistivity was found to increase in the presence of lead nitrate salt solution. For rGO, response towards 20 ppm and 50 ppm Pb(NO3)2 solution was around 1.47% and 6.23% respectively as shown in figure 1.a. The residual oxygen containing groups on rGO help in interacting with the lead ions. Doping or electrostatic gating effect is assumed to be the major contributing factor behind increase in resistivity of the sensing material. We incorporated selectivity into our Chemiresistive sensor by addition of β-cyclodextrin. The cavity size of β-cyclodextrin is such that it shows more affinity towards Pb(II) as compared to other smaller cations such as Cd(II). β-cyclodextrin modified rGO films showed up to 13.3% and 53.13% resistivity changes towards 20 ppm and 50 ppm Pb(NO3)2 solutions respectively, which was almost 10 times in comparison with only rGO as shown in figure 1.a. The calibration curve for rGO-β-cyclodextrin in the concentration range of 10 ppm to 50 ppm Pb(NO3)2 solutions is given in figure 1.b. Secondary deprotonation of the β-cyclodextrin molecule leads to the selective binding of the Pb(II) ions. Though the LOD (limit of detection) of our sensor lies in ppm range, this is the first Chemiresistive Pb(II) sensor ever reported. Further work is being carried out in this domain to lower the LOD and determine the uptake of Pb(II) by rGO-β-cyclodextrin. Figure 1