Gold(III) chloride (HAuCl₄), β-cyclodextrin, N’- [3-(trimethoxysilyl) propyl] diethylenetriamine (silane monomer used to prepare SSG), and hexaammineruthenium(III) chloride (Ru(NH₃)₆Cl₃) were obtained from Sigma-Aldrich. Hydrazine hydrate, sodium sulfite, and sodium nitrite were purchased from Merck. All the glassware was thoroughly cleaned with aqua regia (HNO₃/HCl (1:3) v/v) (caution: aqua regia is a powerful oxidizing agent and it should be handled with extreme care) and rinsed extensively with distilled water before use. The scanning electron microscopy (SEM) images were obtained for the electrodeposited films on an indium tin oxide (ITO) substrate using a JEOL JSM-7800F. We have used the ITO electrodes only for SEM measurements and the GC electrode for electrochemical studies. The cyclic voltammograms (CVs) and square wave voltammograms (SWVs) of the modified electrodes were recorded using Ivium Technologies’ electrochemical workstation. The electrochemical experiments were performed using a single-compartment three-electrode cell. The GC electrode (with a geometric area of 0.07 cm²) was used as the working electrode and a platinum wire as the counter electrode. The reference electrode was Ag/AgCl (in 3M NaCl). The electrolyte solution was deaerated by purging nitrogen gas for 30 min prior to each experiment.

A homogeneous SSG-CD composite was prepared [19] by the addition of 10 μL of 1 M silane monomer to 10 mL of 7 mM aqueous CD solution under vigorous stirring and the stirring was continued for another 60 min.

The Au NSs-modified electrodes were prepared using a previously reported procedure [21]. Briefly, a known amount (5 μL) of SSG or SSG-CD composite was drop-casted on the GC electrode and allowed to dry at 37℃ for 1 h. The Au NSs were electrodeposited at the modified electrode by applying a potential of −0.2 V (Ag/AgCl) for 300 s using an electrolyte solution containing 3 mM HAuCl₄ and 0.5 M H₂SO₄. The prepared modified electrodes are represented as GC/Au, GC/SSG-Au, and GC/SSG-CD-Au. For the SEM measurements, ITO electrodes (1 × 2 cm) were used instead of the GC electrode. The drop-casted composite volume was 50 μL. The remaining experimental parameters were the same as in the previously reported procedure.

The experiments were carried out using a 100 μM of each analyte (N₂H₄, SO₃²⁻, and NO₂−). During the measurements, a solution of N₂H₄, SO₃²⁻, and NO₂⁻ was added to a 0.1 M PBS buffer electrolyte solution (pH = 7.2).

To understand the morphology of the different modified electrodes, the SEM analysis was performed and the images are shown in Fig. 1 and 2. The ITO/SSG-CD electrode without any electrodeposited Au NSs showed a clear image of the SSG film (Fig. 1A). The Au NSs at the ITO/SSG-Au electrode (Fig. 1B) showed randomly inter-connected structures. At the ITO/Au electrode, the Au NSs resembled flowerlike structures (Fig. 1C). However, in the presence of CD in the modified surface of the electrode, the Au NSs were deposited in a different manner at the ITO/SSG-CD electrode (Fig. 1D). The electrodeposited NSs were smaller than those at the other two electrodes and were not inter-connected with each other. The growth of such small Au NSs at the ITO/SSG-CD electrode can be envisaged, as the nucleation of atomic gold in the presence of CD forms Au atomic clusters. The SSG-CD composite at the electrode surface stabilizes the Au atomic clusters, aided by the hydrogen bonding interaction between the secondary −OH groups of CD. The Au nucleation centers favor bonding with the other Au nucleation centers, leading to the formation of such small Au NSs [21]. Furthermore, higher magnification images (Fig. 2A-D) revealed the size and shape of the deposited NSs. As Fig. 2C shows, the Au NSs resembled raspberry-like structures, as reported previously [21]. SEM-energy dispersive X-ray (EDX) analysis (Fig. 2E and F) clearly revealed the active presence of CD on the electrode surface and that the NSs were composed of elemental Au.

We have used the ITO electrodes only for SEM measurements because the conventional GC electrodes cannot be used for SEM. The electrochemical studies were carried out using the GC electrode. Besides, switching between the ITO and GC substrates would have a negligible impact on the results of this study. Furthermore, the modified surface film (SSG-CD) contributed majorly to the nucleation and growth of the NSs. The Au NSs-deposited electrodes were electrochemically characterized by recording the CVs in the deaerated PBS buffer (pH = 7.2) (Fig. 3A). The oxidation peaks observed around 0.93 V (Fig. 3A(a-c)) were ascribed to the formation of Au oxide and the corresponding reduction peaks were observed around 0.46 V (Fig. 3A(a-c)) because of the reduction of Au oxides [21,22]. Among the different modified electrodes, an increase in the anodic and cathodic peak currents was noticed for the GC/SSG-CD-Au electrode because the highly active Au surface rabidly formed the Au oxide layer at this particular electrode surface. Since the formation of the Au surface oxide and its subsequent reduction are pH dependent [15], a negative shift in the oxidation and reduction peaks (0.927 and 0.455 V, respectively) was observed in the PBS buffer (pH = 7.2), compared to those in an acidic medium (0.5 M H₂SO₄) for all the Au NSs-modified electrodes [23].

The redox behavior of [Ru(NH₃)₆]³⁺ is a valuable tool for testing the kinetic barrier of the interface because the electron transfer between the species in solution and the electrode must occur by electron tunneling through the defects or pinholes in the modified electrode [24]. Fig. 3B(a-d) shows the cyclic voltammetric responses of the bare GC, GC/SSG, GC/SSG-CD, and GC/SSG-CD-Au electrodes, respectively, toward the [Ru(NH₃)₆]³⁺ redox marker. As expected, [Ru(NH₃)₆]³⁺ exhibited reversible behavior at the bare GC electrode (Fig. 3B(a)) with a peak-to-peak separation (ΔEp) of 83 mV at a scan rate of 50 mV/s in 0.1 M PBS and 0.1 M KCl. The GC/SSG electrode (Fig. 3B(b)) showed a quasi-reversible voltammetric response with a very low peak current and a peak-to-peak separation of 123 mV. The higher peak potential difference (ΔEp) observed at the GC/SSG electrode compared to the bare GC electrode is a measure of the kinetic hindrance/repulsion exerted by SSG toward the electron-transfer process at the electrode surface for the [Ru(NH₃)₆]³⁺ ions. This observation (Fig. 3B(b)) revealed that a small amount of the [Ru(NH₃)₆]³⁺ ions might diffuse through the pinholes present in SSG. Such pinholes formations are expected during the swelling of SSG. The GC/SSG-CD electrode (Fig. 3B(c)) showed a quasi-reversible voltammetric response with a peak current higher than that of the GC/SSG electrode (Fig. 3B(b)). Furthermore, the response was almost equal to that observed at the bare GC (Fig. 3B(a)) and the peak-topeak separation was 116 mV. Since the CD molecule is electrochemically inert, one cannot expect any electrocatalytic effect of CD on the [Ru(NH₃)₆]³⁺ ions. The basic pH (i.e., 9.84) [21] of the SSG-CD composite caused the deprotonation of the −OH groups in the CD molecules, thereby making the electrode negatively charged and showing permselective behavior toward the positively charged [Ru(NH₃)₆]³⁺ ions. Each peak in the CV corresponds to a particular electroactive species in the test solution, and the height of the peak is proportional to the concentration of the analyte. In this case, the deprotonation of −OH groups at the GC/SSG-CD electrode made it negatively charged, and hence, the electrode exhibited permselective behavior toward the positively charged [Ru(NH₃)₆]³⁺ ions. Due to this preconcentration effect, the [Ru(NH₃)₆]³⁺ ions both from the solution and those adsorbed on the electrode collectively get reduced by Au, leading to an increased reduction current. During the reverse cycle, such preconcentrating effect of the corresponding species was not observed. Hence, there is asymmetry in the behavior of the redox peaks. The GC/SSG-CD-Au electrode (Fig. 3B(d)) also showed a quasi-reversible voltammetric response with a peak current higher than that of the other modified electrodes (Fig. 3B(a-c)), and the peak-to-peak separation was measured to be 133 mV. This observation clearly revealed that the GC/SSG-CD-Au electrode acts as a new electrode with increased electrode area [21,25] and that the Au NSs at the electrode achieve good electrical communication with the underlying electrode surface. It is evident from the higher peak current that the presence of Au NSs at the SSG-CD film shows a marked influence on the barrier property of the GC/SSG-modified electrode.

In order to evaluate the electrocatalytic ability of the different fabricated Au NSs, the electrooxidation of N₂H₄, SO₃²⁻, and NO₂⁻ was studied both individually and concurrently by cyclic voltammetry. The concurrent electrooxidation of the target analytes at the GC/SSG-CD-Au electrode is schematically represented in Fig. 4. Fig. 5(A-C) displays the CVs observed for the electrooxidation of 100 μM each of N₂H₄, SO₃²⁻, and NO₂⁻, respectively, at the different modified electrodes. Fig. 5D represents the concurrent electrooxidation of a mixture of N₂H₄, SO₃²⁻, and NO₂⁻ (100 μM each) at the different modified electrodes. It was observed that N₂H₄, SO₃²⁻, and NO₂⁻ cannot be electrooxidized at the bare GC electrode (Fig. 5(A-D)(a)) within the chosen potential window. The GC/Au electrode exhibited poor electrooxidation behavior for all the three analytes and the mixture (Fig. 5(A-D)(b)). Under the same experimental conditions, the GC/SSG-Au electrode exhibited improved catalytic activity toward all the analytes (Fig. 5(A-D)(c)). However, the GC/SSGCD-Au electrode demonstrated an increase in the anodic peak currents (Fig. 5(A-D)(d)) and also successfully resolved the electrooxidation peaks of all the three analytes (Fig. 5D(d)). In the absence of the analytes, electrochemical responses were not observed at the GC/SSG-CD-Au electrode (Fig. 5(A-D)(e)). The CVs obtained at the GC/SSG-CD-Au electrode demonstrated a decreased overpotential (E_pa) and an enhanced peak current (I_pa). Furthermore, the cathodic peak observed around 0.5 V in Fig. 5C could be attributed to the reduction of the Au surface oxides [21].

The experimental results showed well-resolved oxidation peaks for the mixture of 100 μM each of N₂H₄, SO₃²⁻, and NO₂⁻ at the GC/SSG-CD-Au electrode (Fig. 5D(d)) in 0.1 M PBS at 0.100, 0.375, and 0.757 V, respectively, compared to those at the bare GC, GC/Au and GC/SSG-Au electrodes, which were in good agreement with the previous report [15]. The electrochemical responses observed for the mixture were irreversible and no cathodic current was observed during the reverse cycle (Fig. 5D(d)). The faster electron transfer lead to well-resolved peaks and a considerable increase in the peak currents at the GC/SSG-CD-Au electrode. Hence, the N₂H₄, SO₃²⁻, and NO₂⁻ species in a mixture can be monitored effectively and simultaneously at the GC/SSG-CD-Au electrode, without any redox mediator or enzyme immobilization.

Individual SWVs of N₂H₄, SO₃²⁻, and NO₂⁻ were studied (Figs. 6, 7, and 8) at the different modified electrodes. Similar to the electrooxidation behavior, a better electrochemical sensing ability was observed at the GC/SSG-CD-Au electrode for the mixture of analytes with linearly increasing peak currents corresponding to the increasing concentrations of analytes. Furthermore, the sensitivities of the modified electrode for the oxidation of N₂H₄, SO₃²⁻, and NO₂⁻ were found to be 0.012, 0.028, and 0.055 μA/μM, respectively, and the respective correlation coefficients (n = 8) were found to be 0.9970, 0.9890, and 0.9880 (Fig. 10A). The sensitivities of the GC/SSG-CD-Au electrode for NO₂⁻ and SO₃²⁻ were 4.6 and 2.3 times higher than that for N₂H₄, respectively.

Fig. 9(A-C) illustrates the SWVs observed for the mixture of N₂H₄, SO₃²⁻, and NO₂⁻ at the GC/SSG-Au (A), GC/Au (B), and GC/SSG-CD-Au (C) electrodes with the successive addition of 100 μM of the analytes and their corresponding calibration plots (Fig. 9(D-F)). Among the three different electrodes, the GC/SSG-Au electrode (Fig. 9A) failed to resolve and sense the analytes, whereas the GC/Au (Fig. 9B) and GC/SSG-CD-Au (Fig. 9C) electrodes showed well-resolved oxidation peaks. The peak currents observed at the GC/SSG-CD-Au electrode for the mixture of analytes increased linearly with increasing analyte concentrations (Fig. 10B). The sensitivities of the electrode for N₂H₄, SO₃²⁻, and NO₂⁻ were found to be 0.0312, 0.0493, and 0.0870 μA/μM, respectively, and the corresponding correlation coefficients (n = 8) were found to be 0.9830, 0.9810, and 0.9900 (Fig. 10B), respectively. The sensitivities of the GC/SSG-CD-Au electrode for NO₂⁻ and SO₃²⁻ were 2.7 and 1.5 times higher, respectively, than that for N₂H₄. The SWVs observed at the GC/SSG-CD-Au electrode were highly stable. Additionally, nearly 40% decrease was observed in the sensing ability of the GC/SSG-CD-Au electrode when it was applied to detect the analytes from the mixture. The catalytic activity and sensing ability of the Au NSs at the GC/SSG-CD-Au electrode can be correlated to the larger electrochemically active surface area resulting from the small-sized Au NPs [21].