The incorporation of CNTs onto the sensing substrate enhances sensor sensitivity by leveraging their high surface area, which provides additional sites for the interaction of analyte molecules. In this investigation, a straightforward cast and pull-back technique was employed to deposit CNTs on the GCE and IDE sensing substrates using CNT aerogel film. Simultaneously, a drop-casting technique was applied to deposit CNT aerogel film directly onto the working electrode site of the SPE (Fig. 1a). As seen from Supporting Figure S2a and b, Due to the exceptional flexibility of the synthesized freestanding CNT aerogel film, it can be effortlessly positioned on a sensor substrate and relocated with ease. The mechanical integrity of CNT aerogel electrode post-exposure to liquids and sonication is shown in Supporting Figure S2c and d. It is confirmed that liquid wetting or sonication does not disintegrate CNT aerogel film. The SEM analysis of the CNT-deposited sensor substrates is presented in Fig. 2. Observations from Fig. 2b reveal the deposition of a forest of long CNTs (~ 50 µm) on the GCE. In the case of SPE (Fig. 2d), the CNT aerogel film is directly deposited on the centered working electrode area, constituting an entangled CNT network (Fig. 2e). The IDE surface characteristics demonstrate a forest of CNTs deposited between the interdigitated gold electrodes, providing additional sites for analyte molecule interaction (Fig. 2f, g). The deposited CNT network between the two gold electrodes is depicted in Fig. 2h. Furthermore, Fig. 2i illustrates the use of CNT aerogel film as an electrode, revealing a multi-directional tenuous network of clustered CNTs embedded in the CNT aerogel electrode, as shown in Fig. 2j and k. The CNT aerogel film comprises a condensed and arbitrarily distributed network of long CNTs, imparting its free-standing and flexible characteristics. When wetted with ethanol or acetone, the bare CNT aerogel film normally adheres to glass/silicon and other substrates (Supporting Figure S2e and f). This stickiness is attributed to the presence of polyaromatic hydrocarbons, a residue resulting from the decomposition of the carbon precursor 'ethanol' at 1200°C during the synthesis of CNT aerogel film[26]. The CNT aerogel film thus consists of polyaromatic hydrocarbons along with a network of CNTs, contributing to its sticky nature. This stickiness was utilized to advantageously transfer CNT arrays/forests/bundles onto the sensing substrates.
To evaluate the applicability of the CNT aerogel electrode in nanoelectronics, the electronic transport characteristics of the electrodes were assessed, as depicted in the Supporting Figure S1. The current-voltage (I-V) curves displayed a linear response across the 0 to 1.0 V range, signifying the ohmic behavior of the CNT aerogel film. Notably, the CNT aerogel electrode exhibited nearly isotropic conductivity in x,y axis direction of film, likely due to the continuous and densely packed formation of CNT bundles in all directions. The electrical resistance of the electrode sheet was quantified at 41.5 ± 3.0 Ω. The observed ohmic and nearly isotropic electronic conductivity in the CNT aerogel electrode can be ascribed to the distinctive entangled and dense arrangement of CNTs in various directions, as evident in the scanning electron microscopy images (Fig. 2j and k). The entanglement of CNTs within the film imparts autonomous electrical properties, rendering the electrode well-suited for utilization in electrochemical reactions.
The electrochemical performance of the bare and CNT incorporated GCE, SPE, alongwith CNT aerogel electrode was comparatively evaluated through CV, DPV, and SWV analyses for the oxidation/reduction of the 1 mM potassium ferricyanide/potassium ferrocyanide redox couple [Fe(CN)6]3−/[Fe(CN)6]4− in deionized water, as illustrated in Fig. 3. The nature of the redox reaction of the potassium ferricyanide/potassium ferrocyanide remained consistent across all electrodes (Fig. 3a). The comparative CV plot revealed a distinct pattern in the oxidative/reductive peak current of the potassium ferricyanide/potassium ferrocyanide: CNT aerogel electrode > CNT-SPE > CNT-GCE > SPE > GCE. This pattern was consistent across sensitive electrochemical oxidative DPV (Fig. 3b), reductive DPV (Fig. 3c), oxidative SWV (Fig. 3d), and reductive SWV (Fig. 3e) techniques for the oxidation and reduction of potassium ferricyanide/potassium ferrocyanide. This highlights the capacity of introducing CNTs on the sensing platform to offer additional absorption sites for analyte ions/molecules, thus significantly improving the performance of CNT-incorporated GCE and SPE. Furthermore, as shown in Supporting Figure S3, it was noted that the CV pattern of GCE and SPE exhibited negligible changes even after 200 cycles. This validates that the CNT forest/bundles deposited on the sensor platform remained undisturbed even after numerous electrochemical measurements, ensuring consistent performance. This resilience may be attributed to the presence of long CNTs in the CNT aerogel film, with the transferred longer CNTs having a higher likelihood of remaining intact on the sensing substrate surface. Additionally, the CNT aerogel electrode excelled among all other electrodes, attributed to the substantial network of CNTs present in the electrode. Moreover, the mechanical integrity of the CNT aerogel electrode remained unaffected after exposure to liquid during electrochemical measurements and subsequent sonication in aqueous/non-aqueous solutions. According to literature reports[27], the narrowed difference in peak potential (ΔEp) of the redox couple probe [Fe(CN)6]3−/[Fe(CN)6]4− with an increase in peak current indicate faster electron transfer kinetics on electrode. Further, electrodes with larger peak currents (oxidative/reductive) exhibit higher sensitivity in sensors. Consequently, it can be concluded that the CNT aerogel electrode, followed by CNT network-incorporated conventional GCE and SPE electrodes, may yield a superior response for biomedical sensing applications.
To investigate the interaction behavior of various sensor substrates, we analyzed the ECSA (Fig. 4) performance of the electrodes. The electrochemical capacitance was determined for all electrodes by conducting CV within different potential ranges, depending on the non-faradic zone specific to each electrode. Capacitive currents were measured in a potential range where no faradic processes were observed. The recorded capacitive currents are depicted as a function of scan rate in Fig. 4a, c, d, f, and g, and a linear fit facilitated the determination of specific capacitance, as illustrated in the respective Fig. 4b, e, and h. The specific capacitance can be translated into ECSA using the specific capacitance value for a flat surface area. The ECSA of the working electrode can be estimated from the slope of the current density versus scan rate curves, employing Eq. 1.
\(ECSA=\frac{{C}_{DL}}{{C}_{s}}\) −− (1)
where \({C}_{DL}\) is the electrochemical capacitance, and \({C}_{s}\)is the specific capacitance for a flat surface and its value is generally found to be in the range of 20–60 µF cm− 2 [28]. In our calculation, we have assumed specific capacitance 40 µF cm− 2. The calculated ECSA is shown in Table 1.
Table 1
Calculated ECSA for electrodes.
Electrode
|
ECSA
|
CNT aerogel electrode
|
40
|
SPE
|
0.78
|
SPE-CNT
|
9.7
|
GCE
|
0.045
|
GCE-CNT
|
0.073
|
Notably, the CNT aerogel electrode, featuring a dense array of entangled CNTs, exhibited the highest ECSA, with a value around 40. Following the incorporation of CNTs, the SPE and GCE demonstrated approximately 12 and 2 times increases in ECSA, respectively. This underscores that the straightforward incorporation of CNTs using our technique can significantly enhance the electrochemical activity of sensing substrates multiple times.
The additional performance of the IDE both with and without CNTs is investigated by evaluating the current-voltage (I-V) characteristics (Fig. 5a) and two-electrode resistance (Fig. 5b) in the presence of a redox solution of potassium ferricyanide/potassium ferrocyanide on the sensor surface, as depicted in Fig. 1b. Figure 5a reveals that the potential window for the bare IDE testing is approximately 0-0.4V (linear region), where the current between the two interdigitated gold electrodes saturates after 0.4V. In contrast, with the incorporation of CNTs, the linear region can be extended up to 0-0.55V (linear region), and even beyond 0.55V, the current does not saturate, making this extended potential applicable for sensing purposes. The observed increase in current in the IDE-CNT sensor, even after 0.55V, indicates that the extended CNTs between the interdigitated gold electrodes can function as charge carriers and remain effective within a higher potential window. As observed in Fig. 5b, the change in resistance of IDE-CNT in the presence of the potassium ferricyanide/potassium ferrocyanide redox solution is approximately 1700 times, while for the bare IDE, it is only 200 times. This highlights that the incorporation of CNTs in IDE provides significantly improved sensitivity.
Glassy carbon in GCE, also referred to as polymeric carbon or vitreous carbon, constitutes a non-graphitic sp2 bonded carbon allotrope, and its microstructure manifests as discrete fragments of curved carbon planes, exhibiting both glassy and ceramic characteristics[29]. Notably, GCE presents sluggish electron transfer kinetics for the majority of electrochemical processes, and most species do not readily adsorb onto its surface. In the case of SPEs, the typical method involves the printing of the working electrode using carbon paste/ink. Micrographs of the working electrode reveal the presence of discrete carbon particles distributed across the surface[30]. The inherent porosity and absorptive nature of the deposited carbon contribute to the superior performance of SPEs compared to GCEs. Conversely, in IDEs, the absence of absorption sites between printed gold electrodes results in poor sensitivity and selectivity. To enhance the sensitivity of these sensor substrates, surface modification is often employed using CNTs, as extensively reported in the literature (Table 2). As shown in Table 2, in the conventional drop casting/screen printing process, the utilization of CNT ink involves the preparation of a suspended solution of CNTs, requiring functionalization to mitigate the hydrophobic nature of CNTs through several methods. The multi-step process complicates CNT deposition on sensor films. In contrast, our study showed a simple binderless CNT deposition onto sensing substrates employing a cast and pull-back technique along with drop-casting using CNT aerogel film. This simple method streamlines the entire process, offering a more straightforward solution. Furthermore, as per comparative compilation shown in Table 2 for different CNT based sensor system, the electrochemical activity assessment of the freestanding CNT aerogel electrode in our study reveals exceptionally high electron transfer kinetics and the ability to adsorb most species due to its porous nature and the substantial surface area provided by the CNTs, making it an optimal choice for ultra-sensitive applications either film as such or after deposition on different sensing platforms.
Table 2
Literature survey on the CNT based bio sensors.
Reference
|
Sensor system
|
Method of CNT introduction on sensor
|
Electrochemical Performance (oxidative peak current in CV)
|
M.M. Rao, et al. [31]
|
MWCNT modified SPE
|
Drop casting of f-MWCNT suspension
|
~ 5*10− 3 mA @ 50 mV/sec (0.05M PB solution)
|
P. Pasakon et al. [32]
|
MWCNT-graphene modified SPE
|
Screen printing MWCNT-graphene paste
|
~ 5*10− 3 mA @ 100 mV/sec (0.1 mM ferri/ferro solution)
|
K. Park et al. [33]
|
SWCNT modified SPE
|
Drop casting
|
~ 12*10− 3 mA @ 100 mV/sec (0.05 mM ferri/ferro solution)
|
B. J. Brownlee et al. [34]
|
Vertically aligned CNT on gold IDE
|
CVD method
|
~ 0.3mA @ 150 mV/sec in ferri/ferrocynide
|
H. Sugime et al. [16]
|
vertically aligned dense CNT forests on IDE
|
UV lithography and a low temperature CVD process (470°C)
|
~ 0.06*10− 3 mA @ 10 mV/sec in ferri/ferrocynide
|
X. Wei et al. [35]
|
MOF modified carbon cloth
|
Dip coating of MOF on carbon cloth
|
~ 10*10− 3 mA @ 100 mV/sec (0.1 M NaOH solution)
|
H. Yang et al. [36]
|
3D graphene/ carbon fiber paper electrode
|
--
|
~ 0.1 mA @ 50 mV/sec (1 mM Glucose in 50 mM NaOH solution)
|
A. M. Abdel-Aziz et al. [37]
|
Activated GCE
|
repetitive CV between − 1.5 and + 2.5 V
@ 100 mV/s in 0.1 M PBS
|
~ 30*10− 3 mA @ 100 mV/sec (0.1M PB solution)
|
J. Amarnath et al. [38]
|
g-C3N4/CNT@ AgNPs/GCE
|
Drop casting of aliquot
|
~ 25*10− 3 mA @ 100 mV/sec (0.1M PB solution)
|
Prashu Jain et al. [39]
|
Functionalized MWCNT-GCE
|
Drop casting
|
~ 20*10− 3 mA @ 50 mV/sec (5 mM [Fe (CN)6]3−/4- (1:1) in 0.1 M KCl solution)
|
Fabiana A. Gutierrez
et al. [40]
|
GCE/CNT-polyethylenimine-Cu
|
Drop casting
|
~ 25*10− 3 mA @ 100 mV/sec in 0.050 M phosphate buffer solution
|
E. Pradeepa et al. [41]
|
Pencil graphite electrode/CNT
|
Drop Casting
|
~ 1*10− 3 mA @ 100 mV/sec in 1.0 mM K3[Fe(CN)6] in 0.1 M KCl solution
|
Present study
|
CNT aerogel/ GCE-CNT/SPE-CNT/ IDE-CNT
|
a cast and pull-back technique for GCE, IDE and drop-casting for SPE
|
0.5mA/ 0.42 mA/ 0.26 mA (@ 100 mV/sec) in 1 mM [Fe(CN)6]3−/[Fe(CN)6]4− solution in DI water.
1700-fold increase in resistance of IDE-CNT.
|