Electrochemical Immunosensor based on Nanostructured Lanthanum oxide substituted reduced graphene oxide Interface for Ultralow Ciprofloxacin Detection in Spiked Milk Samples

In the present work, we have reported nanostructured lanthanum oxide nanoparticles decorated reduced graphene oxide nanocomposite (nLa2O3 NPs@rGO) based biosensing platform for efficient and label-free determination of ciprofloxacin (CPX) antibiotic....


EIS Studies:
The electrochemical impedance spectroscopic studies of ITO (curve i), APTES/nLa 2 O 3 @rGO/ITO (curve ii), anti-CPX/APTES/nLa 2 O 3 @rGO/ITO (curve iii) and BSA/anti-CPX/APTES/nLa 2 O 3 @rGO/ITO (curve iv) have been done in PBS solution containing 5 mM [Fe(CN) 6 ] 3-/4-in the frequency range of 100 kHz to 10 Hz [Fig. 5(d)].The semicircle of Nyquist plot gives the charge transfer resistance (R ct ) of electrode that depends on the dielectric features of electrode surface and electrolyte interface.The value of R ct was obtained as 249.35Ω for ITO glass substrate (curve i), which is the highest.However there is significant decrease in the value of R ct 205.2 Ω after the deposition of APTES/nLa 2 O 3 @rGO nanocomposite onto ITO surface (curve ii), indicates the high electrical conductivity of nLa 2 O 3 @rGO nanocomposite and fast diffusion of redox species [Fe(CN) 6 ] 3-/4- to the electrode surface.However, after the covalent immobilization of anti-CPX antibodies to the free -NH 2 sites of APTES functionalized nLa 2 O 3 @rGO nanocomposite, the value of R ct drastically decreases to 165.58 Ω for anti-CPX/APTES/nLa 2 O 3 @rGO/ITO (curve iii).This decrease in the value of R ct can be assigned to the spatial orientation of anti-CPX molecule onto electrode surface that facilitate increased charge transfer process between redox species [Fe(CN) 6 ] 3-/4-and free available functional group (NH 2 ) of anti-CPX molecule (Kumar et al.

2015)
. The value of R ct increases (176.42Ω; curve iv) after the BSA immobilization on the anti-CPX/APTES/nLa 2 O 3 @rGO/ITO immunoelectrode due to the macromolecular structure and insulating structure of BSA that hinder the charge transfer between redox species [Fe(CN) 6 ] and electrode surface.
The heterogeneous electron transfer rate constant (K ct ) for all respective electrodes was calculated using Eq.
Where R is gas constant, T is the absolute temperature, n is the no. of transferred electrons per molecule of the redox probe, F is Faraday constant, A is the surface area of the electrode (cm 2 ) and [S] is the concentration of redox probe.The K ct value of ITO electrode was found as 2.006 × 10 -6 while in the case of APTES/nLa 2 O 3 @rGO/ITO the value of K ct increases to 3.044 × 10 -6 exhibiting the faster electron transfer between the APTES/nLa 2 O 3 @rGO/ITO electrode and redox species.After immobilization of anti-CPX, the value of K ct again increases to 4.329 × 10 -6 shows the fast electron exchange between immunoelectrode and redox species.However, after the BSA immobilization on anti-CPX/APTES/nLa 2 O 3 @rGO/ITO immunoelectrode, the value of K ct decreases to 2.160 × 10 -6 , due to its insulating nature which resist the electron transfer.
The Time constant (τ) for each electrode has been calculated using the given equation as below After the EPD of APTES/nLa 2 O 3 @rGO on ITO substrate there is abrupt decrease in value of time constant was found as compared to ITO glass substrate (8.938x 10 -4 s).It reveals the fast diffusion of [Fe(CN) 6 ] 3-/4-ions at the interface of electrolyte and APTES/nLa 2 O 3 @rGO/ITO electrode.The immobilization of anti-CPX on to the APTES/nLa 2 O 3 @rGO/ITO result in decrease of τ value 2.716 x 10 -4 s due to the fast diffusion of [Fe(CN) 6 ] 3-/4-ions.After the BSA immobilization the value of τ again increases (2.724 x 10 -4 s) due the insulating behavior of BSA.

Table S1 .
EIS features of various electrodes