Complex impedance plane representation and circuit model development
(a) In-vivo studies
Impedance was recorded as a function of frequency for PEDOT: PSS-based biosensor integrated in plant, before and after subjecting the plant to salt stress with 1 M NaCl solution. Nyquist plots obtained showed one semi-circular arc with an offset from origin at high frequencies and another semi-circular arc at low frequencies as shown in Fig. 3. Best fit for the Nyquist plots in Fig. 3 is given by equivalent electric circuit shown in Fig. 4. Values of the circuit elements are tabulated in Table 1.
In the equivalent circuit shown in Fig. 4, R1 can be attributed to bulk sap resistance. R2|CPE2 corresponds to the depressed semicircle at high frequencies and represents impedance of functionalized thread where R2 and CPE2 are thread resistance and Constant Phase Element (CPE) of thread respectively. R3|CPE3 corresponds to the depressed semicircle in low frequency range and represents impedance of sap/thread interface where R3 and CPE3 are interfacial resistance and CPE of interface respectively. CPE is used to represent non-ideal capacitor whose impedance is given by the 1/ (jω) n C where C and n are capacitance and exponent with value between 0 and 1 respectively [14].
Table 1
Values of circuit elements of equivalent electrical circuit shown in Fig. 4 derived from Fig. 3
Before subjecting plant to salt stress
|
After subjecting plant to salt stress
| |
---|
R1 (kΩ)
|
0.0992
|
1.617
|
---|
R2 (kΩ)
|
0.637
|
4.057
|
---|
CPE2
|
n2 = 0.8369
C2 = 9.36*10− 8 F
|
n2 = 0.5777
C2 = 5.148*10-7F
|
R3 (kΩ)
|
1.429
|
9.821
|
CPE3
|
n3 = 0.3018
C3 = 1.046*10− 5 F
|
n3 = 0.5951
C3 = 1.129*10− 4 F
|
Resistance values of the biosensor device, R1, R2 and R3 increased after subjecting plant to salt stress as shown in Table 1. This is due to decrease in overall ionic concentration in sap of a plant after being subjected to salt stress. Value of resistance has a strong effect on amplitude of real and imaginary parts of impedance [15].
(b) In-vitro studies
Impedance was recorded as a function of frequency for PEDOT: PSS-based biosensor integrated in vial filled with extracted plant sap, before and after addition of NaCl solution to the sap. Impedance spectra were recorded for 1 M NaCl solution. Nyquist plots obtained showed one semi-circular arc with an offset from origin at high frequencies and another semi-circular arc at low frequencies as shown in Fig. 5. Best fit for the plots in Fig. 5 is given by equivalent electric circuit shown in Fig. 6.
In the equivalent circuit shown in Fig. 6, R1 can be attributed to bulk sap resistance. R2|CPE2 corresponds to the depressed semicircle at high frequencies and represents impedance of functionalized thread where R2 and CPE2 are thread resistance and Constant Phase Element (CPE) of thread respectively. R3|CPE3 corresponds to the depressed semicircle in low frequency range and represents impedance of sap/thread interface where R3 and CPE3 are interfacial resistance and CPE of interface respectively.
Figure 5 Nyquist plots obtained from impedance data for in-vitro PEDOT: PSS - based biosensing of salt stress for 1 M NaCl solution: (A) Before addition of NaCl solution and (B) After addition of NaCl solution
Table 2
Values of circuit elements of equivalent electrical circuit shown in Fig. 6 derived from Fig. 5
Before addition of NaCl solution
|
After addition of NaCl solution
| |
---|
R2 (kΩ)
|
235.2
|
65.57
|
CPE2
|
n1 = 0.3188
C1 = 7.016*10− 6 F
|
n1 = 0.3262
C1 = 1.7*10− 5 F
|
R3 (kΩ)
|
20.47
|
6.53
|
CPE3
|
n3 = 0.61
C3 = 2.086*10− 8 F
|
n3 = 0.5785
C3 = 4.927*10− 8 F
|
Values of circuit elements for the circuit shown in Fig. 6 derived from Fig. 5 are tabulated in Table 2. Bulk sap resistance value was found to be negligible for Nyquist plots in Fig. 5 and hence, not included in Table 2. Resistance values of the biosensor device, R2 and R3 decreased after addition of NaCl solution to extracted sap as shown in Table 2. This is due to increase in overall ionic concentration in sap after the addition of NaCl solution. Resistance value has a strong effect on magnitude of real and imaginary parts of impedance [15].
Impedance representation
Spectra of real and imaginary components of impedance for in-vivo biosensing are presented in Figs. 7 and 8 respectively. The plot of Z’ vs. log f shown in Fig. 7 clearly showed an increase in resistance of the OECT device at all frequencies, after the plant was subjected to salt stress. This was also implied by Nyquist plot shown in Fig. 3 and the corresponding circuit element values derived in Table 1. In the Z” vs. log f plot, each R|CPE component of the equivalent circuit is characterized by a relaxation frequency and appears as a resonance peak with height of the peak being proportional to the resistance value of R|C component [16]. In Fig. 8(A), only one broad resonance peak near 104 Hz corresponding to R2|CPE2 component was observed and resonance peak corresponding to R3|CPE3 component was completely suppressed. In Fig. 8(B), the resonance peak corresponding to R2|CPE2 component shifted to lower frequencies with a fivefold increase in height of the peak; this can be attributed to increase in resistance which was also implied by Nyquist plot shown in Fig. 3.
Spectra of real and imaginary components of impedance for in-vitro biosensing are presented in Figs. 9 and 10 respectively. The plot of Z’ vs. log f shown in Fig. 9 clearly showed a decrease in the resistance of OECT device at all frequencies, after addition of NaCl solution to plant sap. This was also implied by Nyquist plot shown in Fig. 5 and the corresponding circuit element values derived in Table 2. In Fig. 10(A), only one broad resonance peak near 105 Hz corresponding to R1|CPE1 component was observed and resonance peak corresponding to R3|CPE3 component was suppressed. In Fig. 10(B), the resonance peak corresponding to R3|CPE3 component shifted to higher frequencies with a decrease in height of the peak; this can be attributed to decrease in resistance which was also implied by Nyquist plot shown in Fig. 5.
Comparison between in-vivo and in-vitro PEDOT: PSS - based biosensing of salt stress
Plants growing in soils with high sodium concentration have high concentration of Na+ ion in sap. This increase in concentration of Na+ ion in plant sap leads to decrease in uptake of K+, Ca2+, Mg2+ ions by the plant thus, decreasing the overall concentration of mobile ions in the sap [17–23]. Hence, in the case of in-vivo biosensing of salt stress, with overall concentration of ions in sap decreasing after a plant was subjected to salt stress, impedance of the OECT device increased as shown in Fig. 3. Decrease in ionic concentration in sap and increase in hole concentration in the channel of OECT after plant was subjected to salt stress is illustrated in Fig. 1(a). Hole concentration in channel increases because of decrease in ions from sap entering the channel which alter the electrostatic effect of sulphonate anions on the holes [24–26]. However, in the case of in-vitro biosensing of salt stress, impedance of the OECT device decreased after addition of NaCl solution to plant sap as shown in Fig. 5, since overall concentration of ions increases with increase in Na+ ions in the sap. Increase in ionic concentration in sap and decrease in hole concentration in the channel of OECT after the addition of NaCl solution is illustrated in Fig. 2(a).