The structural investigation of the sample prepared is performed first by X-ray Diffraction spectroscopy (XRD) (Bruker D8 Advance Diffractometer). The 2Ɵ value for the sample is varied from 100 to 850, at a scan speed of 0.1s and step size of 0.020. A source of CuKα (λ=1.5418Å) is used for the purpose. Generator parameters were fixed at 40KV and 40mA. Fig(2) shows a sharp peak at ~69.90 which is characteristic for silicon based sample, PSi sample in this case. The morphological investigation of the sample was performed by FESEM (FEI INSPECT F50). Fig 3(a) confirms the development of PSi sample with uniformly distributed pores, of diameter ranging from 780 to 1073nm visible on the silicon surface. Fig 3(b) shows the detector surface after surface-functionalization. CaM can be seen clearly attached to the PSisurface.
The schematic diagram of the final detecting platform structure is shown in the fig (4). The electrical response of the detector is tested with and without Ca2+ ion incubation of the detecting platform. The current (I) response of the detecting platform is calibrated by simultaneous variation of the applied voltage (V) across it, within a fixed voltage range. As shown in fig 5(a), the detector’s current response is seen to be almost exponential, when the platform is incubated with Ca2+ ion, in the voltage range of 2.5-5V, clearly visible by the red line in the graph, in comparison to the response for the detector without ion incubation, depicted by the black line in the same graph. The substantial increment in the current is attributed to the binding of the Ca2+ ion to CaM functionalized PSi detecting platform. Due to the binding affinity of CaM towards Ca2+ ion, Ca2+ ion gets attached to the detecting platform, which creates conduction pathways in the detector platform, causing resistance to decrease drastically resulting in the exponential rise in the current value. The detector’s selectivity towards Ca2+ ion is further tested by the calibration of the current versus voltage characteristics of the detector for other bi-valent and mono-valent cations such as Mg2+, Mn2+, Na+ and K+ ions (those which are commonly present in human body). Fig 5(a) shows no drastic current change of the detector for other ions. For the mixture ion solution, the detector’s response closely mimics its response in the case of Ca2+ ion, as shown by the turquoise line in fig 5(a). The detector thus prepared shows high discrimination towards Ca2+ ion within all the other ions, which is evident from the detector’s response for the mixture ion solution. Fig 5(a) shows the exponential increment of the current response in the case for detector incubated with Ca2+ ion, in comparison to the other ions. The detector’s highly discriminative response for Ca2+ ion is also demonstrated form the closely mimicking graphs of current response for Ca2+ ion and mixture ion incubation.
The change in capacitance value for changing input voltage frequency is calibrated for the detector. As shown in fig 5(b), the detector shows high capacitive response for Ca2+ ion incubation at low input voltage frequency, in comparisons to other ions. The Capacitance value however diminishes with increase in voltage frequency. For the mixture salt incubation the capacitance response of the detector closely mimics the response for Ca2+ ion, once again demonstrating the high selectivity of the detector towards Ca2+ ion among all the other ions under consideration.
The change in capacitance value in lower frequency range (10-105 Hz) can be attributed to differential polarization effect [36-37]. In lower frequency range of the input voltage the space-charge polarization and orientational polarization results in the change of dielectric constant, characteristics fornano-structured materials that leads to the change in the capacitance value [37, 38]. A nano-structured material like PSiposses a large number of interfaces and numerous defects within these interfaces. On application of external electric field, the change in the positive and negative space charge region that occurs through these defects produces dipole moment. The developed dipole moment is termed as space-charge polarization. The rotation of these dipole on application of external electric field results in orientaional polarization, effecting the dielectric constant of the material, hence the capacitance value gets effected [36, 37, 39]. In frequencies greater than 105, the dipoles are unable to orient themselves as rapidly as the externally applied electric field changes, resulting in the nullification of the orientational polarization effect. At higher frequency only space charge polarization effect is present, which too saturates at even higher frequencies that explains the behavior of the frequency versus capacitance graph in higher input voltage frequency range.
Comparative Study:-
The electrical response of the detector developed in this work is that of the I-V response and the C-F response of the detector for different cations. Mathematically the I-V response of the detector is obtained by using the following formula:
Ri= (Ii– Iwi) / (Iwi).
For the C-F response of the detector:
Rc= (Ci– Cwi) /(Cwi).
Where, Ii= Current value at 5V with ion incubation
Iwi= Current value at 5V without ion incubation
Ci= Capacitance value at min frequency with ion incubation
Cwi= Capacitance value at min frequency without ion incubation
In the Optical calcium detector [35], developed previously the fall in reflection peak intensity, absorption loss and the scattering loss of the detector platform were under taken as parameters to study the response of the detector platform [35]. The detector showed appreciable response and selectivity towards calcium ion. In the mentioned optical detector the optical response, namely reflection peak intensity fall, scattering loss and absorption loss of the detecting platform was foundto be 8.49%, 13.09% and 2.16% for calcium ion, 8.65%, 17.88% and 3.60% for mixture ion solution, 1.25%, 2.49% and 0.10% for Magnesium, 0%, 0.80% and 0.26% for Manganese, 1.40%, 2.25% and 0.13%for Potassium and 0.76%, 0.32% and 0.07% for sodium [35]. In this work, the electrical response of the detector is obtained by calibrating the change in the I Vs V response of the detector as well as the C Vs F change of the detecting platform. The I-V response of the detector is found to be 303.84% for calcium ion, 253.84% for mixture ion solution, 7.69% for magnesium, 100% for manganese, 69.3% for potassium and 42.3% for sodium. The C Vs F response of the detector shows a response of 178% for calcium ion, 175% for mixture ion solution, 37.5% for magnesium, 115% for manganese, 92.55% for potassium and 95% for sodium.Thus both the electrical response is observed to be highest for calcium ion and mixture ion solution in comparison to all the ions. It is clearly visible from the response obtained from the electrical parameters that the detector developed in this work shows appreciable discrimination towards calcium ion, that is more evidently established when the electrical calcium detecting platform is tested with mixture ion solution and the response being consistent with the response of the electrical detector for calcium ion only, as depicted in fig 6. Both the electrical parameter response of the detector shows highest response for calcium ion and for mixture ion solution, showing evidence of the selectivity of the detector towards calcium ion.
Comparing the optical response [35] with the electrical response of the detector surface developed in this work, it is found that the response of the electrical detector is more profound and appreciably consistent with the response of the optical detector, as clearly visible in fig 6.The electrical detector developed shows considerably selective towards calcium ion as in the case of the previously developed optical detector. This may find use in the designing of multi-parametric calcium sensor with better selectivity and sensitivity.