Strontium nitrate [Sr(NO₃)₂], zirconium nitrate [Zr(NO₃)₄], and tannic acid (C₇₆H₅₂O₄₆) were used as starting materials. All chemicals were of analytical grade and procured from standard commercial suppliers without any further purification. Distilled water was used as the solvent throughout the synthesis process. Tannic acid served both as a fuel and a green reducing agent in the combustion reaction.

SrO₂ nanoparticles were synthesized via a low-temperature solution combustion method. Precisely 1.242 g of Sr(NO₃)₂ and 1.0 g of tannic acid were dissolved in a minimal amount of distilled water under continuous stirring to form a homogeneous solution. The resulting mixture was transferred into a ceramic crucible and placed in a preheated muffle furnace maintained at 500 °C. Upon heating, the solution underwent dehydration and decomposition, releasing gaseous by-products. At a critical temperature, spontaneous ignition occurred, and a self-sustaining combustion reaction ensued. This highly exothermic process lasted for approximately 5 minutes, resulting in the rapid formation of a dry, foamy residue. The greyish-black product, presumed to be SrO₂, was allowed to cool to room temperature, then finely ground using an agate mortar and pestle and stored in an airtight container for subsequent analysis.

For the synthesis of Zr-SrO₂ nanoparticles, 1.242 g of Sr(NO₃)₂, 1.465 g of Zr(NO₃)₄, and 2.0 g of tannic acid were dissolved in distilled water with constant stirring until a clear solution was obtained. The prepared solution was poured into a crucible and introduced into a muffle furnace preheated to 500 °C. As with the SrO₂ synthesis, the solution underwent a sequence of boiling, dehydration, and thermal decomposition steps. The combustion reaction was spontaneously triggered at the ignition temperature, resulting in a rapid exothermic reaction that completed within 5 minutes. The resulting black powder, attributed to Zr-doped SrO₂, was cooled, ground into a uniform fine powder, and stored for further characterization.

Working electrodes were fabricated by mixing 0.25 g of the synthesized active material with 0.2 g of graphite and 3–5 drops of polytetrafluoroethylene (PTFE) binder to form a homogenous paste. This mixture was uniformly coated onto a nickel mesh (area = 1 cm²) and allowed to dry under ambient conditions. Electrochemical measurements were performed in a typical three-electrode system, using the prepared electrode as the working electrode, a platinum wire as the counter electrode, and an Ag/AgCl reference electrode in 2 M KOH aqueous electrolyte.

The synthesis of SrO₂ and Zr-SrO₂ nanoparticles was examined using an XRD Maxima-7000 instrument from Shimadzu to analyze their structural features. Morphology and EDX analyses were performed with a JEOL JSM-5600LV and a SU-1500 from HITACHI. The UV-Vis absorption spectrum was obtained using an SL 159 ELICO UV-Visible spectrophotometer. Electrochemical activity was assessed using an ACHI608E potentiostat in a three-electrode setup, with a modified electrode, a platinum wire, and Ag/AgCl as the working, counter, and reference electrodes, respectively. A 0.1 M HCl solution served as the electrolyte.

The structural properties of the synthesized SrO₂ and Zr–SrO₂ nanoparticles were investigated using powder XRD, as shown in Figs. 1a and 1b. The analysis offers important details about the phase composition, crystallinity, and the influence of Zr⁴⁺ substitution on the SrO₂ crystal lattice, which are discussed. This study examines the influence of Zr⁴⁺ substitution on the SrO₂ crystal lattice. The XRD pattern of pure SrO₂ (Fig. 1a) exhibits sharp and intense peaks at 2θ values of 22.8°, 26.6°, 29.7°, 32.1°, 39.5°, 46.5°, 52.7°, 57.4°, 62.6°, and 68.5°, which correspond to the crystal planes (100), (110), (200), (111), (200), (210), (211), (220), and (221), respectively. These reflections are consistent with the standard JCPDS card for orthorhombic SrO₂, confirming the formation of a single-phase orthorhombic structure with high crystallinity ³⁶. The most intense peak at 22.8° (assigned to the (100) plane) indicates a preferred growth orientation along this plane. The narrow full width at half maximum (FWHM) of the peaks suggests that SrO₂ particles have larger crystallite sizes and minimal microstrain, indicative of well-developed crystalline domains.

In the case of Zr-SrO₂ (Fig. 1b), the XRD pattern retains the fundamental peaks of SrO₂, indicating the preservation of the orthorhombic crystal structure even after Zr incorporation. There is, however, significant peak broadening and minor changes to elevated 2θ angles, particularly everywhere. ever. Significant peak broadening and minor changes to elevated 2θ angles are apparent, especially around the (100), (110), and (200) planes. These changes are attributed to the substitution of Sr²⁺ (ionic radius 1.18 Å) by the smaller Zr⁴⁺ ions (ionic radius 0.72 Å) ³⁷³⁸, which introduce compressive strain and lattice distortion into the crystal matrix. Additionally, a modest reduction in the relative peak intensities for Zr-SrO₂ suggests defect formation or decreased ordering due to dopant-induced strain. The lack of secondary peaks in both patterns confirms that the samples are single-phase and supports the idea that Zr can form a solid solution within the SrO₂ lattice.

To quantify the influence of doping on the crystallite size, the average crystallite dimensions were estimated using the Debye–Scherrer equation. A comparison of peak positions, Miller indices, and calculated crystallite sizes is provided in Table 2.

Fig. 2 presents the SEM micrographs of (a) SrO₂ and (b) Zr-doped SrO₂ (Zr–SrO₂) synthesized materials. These images reveal significant morphological changes resulting from Zr doping in the SrO₂ matrix. The SEM image of pristine SrO₂ (Fig. 2a) exhibits irregular, agglomerated particles with a rough surface texture. The particles are relatively large and unevenly distributed, showing a tendency to cluster into micron-sized aggregates. This morphology is characteristic of oxide materials synthesized via combustion or precipitation methods, where rapid reaction kinetics often lead to uncontrolled grain growth and poor dispersion ³⁹. The loosely packed, porous structure of SrO₂ may mean that it has a small surface area, which could affect how well it works in applications like sensing or catalysis.

Notable changes in particle morphology occur upon incorporating Zr into the SrO₂ matrix (Fig. 2b). The Zr–SrO₂ sample displays a more refined microstructure, with reduced particle size and enhanced uniformity. The dispersion of particles appears significantly improved, and the agglomeration is comparatively less pronounced. The red-circled region highlights a layered or flake-like surface texture, likely resulting from structural distortion introduced by Zr doping. Furthermore, the area labelled “Zr” suggests localized Zr-rich domains or surface modifications associated with Zr integration that contribute to the observed microstructural refinement.

The morphological change that happens when Zr is added is because Zr⁴⁺ ions stop SrO₂ grains from growing during synthesis. Zirconium is known to act as a grain refiner, promoting homogeneous nucleation while suppressing particle coalescence and excessive crystallite growth, leading to enhanced textural properties and increased active surface area ⁴⁰. Such morphological characteristics are often beneficial in materials intended for heterogeneous catalysis, electrochemical sensing, or solid-state devices, where surface activity and uniformity are key variables that determine functional efficiency.

The elemental composition of SrO₂ and Zr–SrO₂ nanoparticles was examined using EDX, as shown in Figs. 3(a) and 3(b). The EDX spectrum of pristine SrO₂ exhibits characteristic peaks corresponding to Sr and O, confirming the formation of strontium oxide without detectable impurity phases. A minor carbon signal is observed, which can be attributed to surface contamination or the carbon tape used during sample preparation. The presence of a small Au peak is associated with the gold coating applied to improve conductivity during SEM analysis.

In the case of Zr–SrO₂, additional peaks corresponding to Zr are clearly observed along with Sr and O, confirming the successful incorporation of zirconium into the SrO₂ matrix. No secondary elemental peaks were detected, indicating high chemical purity and the absence of unwanted phases. The compositional results are consistent with XRD findings and verify that Zr doping was achieved without altering the overall phase purity of the host lattice. The confirmed elemental incorporation supports the enhanced structural and electrochemical properties observed for the doped sample.

Fig. 4 illustrates the optical absorption behavior and estimated band gap energies of (a) SrO₂ and (b) Zr–SrO₂ based on UV–Vis diffuse reflectance spectroscopy (DRS) data. The inset in both graphs represents the absorbance vs. wavelength curves, while the main graphs show Tauc’s plots, constructed to determine the optical band gap energy. The UV–vis absorbance spectrum of pristine SrO₂ (inset of Fig. 4a) reveals a moderate absorption in the UV region (200–900 nm), with a steep onset of absorption, suggesting that it is a semiconductor with a wide band gap.

The corresponding Tauc’s plot, drawn using the Kubelka–Munk function and assuming an indirect allowed transition, (αhv)² vs. hv, displays a linear region extrapolated to the energy axis to estimate the optical band gap. The calculated band gap for SrO₂ is approximately 3.47 eV, consistent with previous reports for alkaline earth metal oxides possessing wide band gaps ⁴¹. This wide band gap implies that SrO₂ is optically transparent in the visible region, which can be advantageous in optoelectronic, photocatalytic, and UV shielding applications ⁴².

However, such a wide band gap may limit its photocatalytic efficiency under visible light unless modified via doping or surface engineering. Upon doping with Zr, the optical properties of the material exhibit notable changes, as shown in Fig. 4b. The UV–vis absorbance spectrum (inset) of Zr–SrO₂ shows an enhanced absorption tail extending into the visible region, indicating the generation of sub-bandgap states or a narrowing of the band gap. The corresponding Tauc’s plot shows a reduced band gap energy of 3.2 eV. The decrease in band gap upon Zr doping may be attributed to lattice distortion and the introduction of defect states or oxygen vacancies, which allow electronic transitions at lower photon energies ⁴³. Such band gap engineering enhances the potential applicability of Zr–SrO₂ in photocatalytic degradation, photoelectrochemical water splitting, and light-assisted sensing.

The electrochemical behavior, proton diffusion efficiency, and energy storage capability of SrO₂ and Zr–SrO₂ were evaluated using CV and are presented in Fig. 5(a–d). The CV measurements were carried out within a potential window of −1.0 to +1.0 V at scan rates ranging from 10 to 100 mV/s. Fig. 5 (a, c) shows the CV behavior of SrO₂ and Zr–SrO₂. The strong redox features are due to the fact that zirconium is electroactive in the host lattice. behavior of SrO₂ and Zr–SrO₂, where the pronounced redox features can be attributed to the electroactive role of zirconium within the host lattice. The incorporation of Zr⁴⁺ ions into the SrO₂ matrix induces lattice distortion and creates additional defect sites, particularly oxygen vacancies, which facilitate enhanced charge transfer kinetics. The observed cathodic and anodic peaks correspond to the reversible redox transition between Zr⁴⁺/Zr³⁺ states, indicating the active participation of zirconium in the electron transfer process. Compared to pristine SrO₂, the Zr–SrO₂ electrode exhibits higher peak currents and improved peak definition, suggesting increased electrochemical activity and conductivity. Furthermore, the systematic increase in peak current with scan rate confirms a diffusion-controlled redox mechanism, while the slight shift in peak potential indicates quasi-reversible behavior. The strong interaction between Zr ions and the SrO₂ lattice enhances electron mobility and provides additional active sites, thereby improving the overall redox performance. This enhanced electrochemical behavior highlights the significant role of Zr doping in tuning the redox properties of SrO₂, making it a promising candidate for high-performance electrochemical sensing applications. The closer spacing of the oxidation (EO) and reduction (ER) peaks indicates better electron movement in Zr–SrO₂ ⁴⁴.

The proton diffusion coefficient (D) was estimated from the scan rate-dependent CV data, as shown in Fig. 5 (b, d). The calculated D values were 0.16 cm²/s for SrO₂ and 0.26 cm²/s for Zr–SrO₂, indicating that Zr doping significantly enhances ion diffusion and charge transport within the electrode matrix (Table 3 and Table 4). A higher diffusion coefficient, coupled with improved redox reversibility, clearly demonstrates the superior electrochemical kinetics of the Zr–SrO₂ system ⁴⁵.

In addition to electrochemical reversibility and ion transport, the specific capacitance (Cₛₚ) of the electrodes was calculated using the following Eq. (1):

....... (1)

where A - area under the CV curve, m - mass of active material, s - scan rate, and V₂ – V₁ - potential window.

This parameter quantifies the electrode material's energy storage capacity. The specific capacitance was found to be approximately 72.68 F/g for SrO₂ and significantly enhanced to 201.99 F/g for Zr–SrO₂. The more than two-fold increase in capacitance is attributed to increased surface area, improved electronic conductivity, and higher density of electroactive sites resulting from Zr doping ⁴⁶. The electrochemical analysis demonstrates that Zr doping in SrO₂ significantly improves its redox reversibility, enhances proton diffusion, and markedly increases its energy storage capability. These enhancements render Zr–SrO₂ a promising candidate for applications in electrochemical sensors, supercapacitors, and bio-electronic interfaces.

The histidine-sensing behavior of the synthesized SrO₂ and Zr–SrO₂ materials was investigated using CV and is presented in Fig. 6(a, b). The measurements were carried out at varying histidine concentrations (1–8 mM) with a scan rate of 30 mV (2 M KOH) to evaluate the electrochemical response, sensitivity, and redox behavior of both sensing platforms. Histidine is an essential amino acid with biological significance, and its electrochemical detection is of growing interest for biosensing, medical diagnostics, and food quality monitoring ⁴⁷.

In Fig. 6a, the SrO₂ electrode exhibits a pair of redox peaks in response to histidine addition within the potential window. As the histidine concentration increases from 1 to 8 mM, both the anodic and cathodic peak currents increase, indicating that the redox reaction is concentration-dependent. This current enhancement suggests an efficient electrocatalytic interaction between SrO₂ and histidine, enabling effective electron transfer and signal amplification ⁴⁸. However, the peak separation remains relatively broad, and the redox features are less defined, which may be attributed to the limited conductivity and slower electron mobility of pristine SrO₂. Additionally, the relatively lower peak currents at higher concentrations suggest that the SrO₂-based sensor may have moderate sensitivity to histidine.

Fig. 6b shows the corresponding CV curves for the Zr–SrO₂ electrode under the same histidine concentrations. A marked improvement in the current response is observed when compared to SrO₂. The anodic peak currents increase significantly with histidine concentration, and the redox peaks appear sharper and more symmetric, indicating better electrochemical reversibility and faster charge transfer kinetics. The distinct separation of peaks and higher signal amplitude even at lower concentrations implies that the Zr–SrO₂ electrode has a higher sensitivity and electrocatalytic activity toward histidine oxidation. These results strongly support that Zr doping facilitates better charge transfer and adsorption kinetics, improving histidine oxidation and overall sensor performance ⁴⁹.

The electrochemical sensing data confirm that Zr–SrO₂ exhibits superior electrocatalytic properties and higher sensitivity toward histidine detection compared to pristine SrO₂. The improved performance is due to the combined effects of zirconium doping, which makes conductivity, active surface sites, and redox reversibility better. These findings demonstrate the potential of Zr-SrO₂-based materials for amino acid biosensing applications in biomedical and environmental analysis.