Previous efforts to improve the electrochemical detection of theobromine involved surface modification with nanomaterials such as metal oxides and carbon-based hybrids to enhance surface area and conductivity. Nevertheless, these sensors still relied heavily on alkaline bulk conditions, which introduce matrix effects and impair selectivity. The key innovation proposed here is to engineer localized alkaline microdomains at the electrode interface without altering the bulk solution pH.
Strontium oxide (SrO), positioned between calcium oxide and barium oxide in the alkaline earth oxide group, is selected for its balanced surface basicity and solubility characteristics. When integrated with a conductive carbon nanohybrid of functionalized carbon black and reduced graphene oxide (f-CB@r-GO), the SrO nanoparticles generate localized alkaline zones at the interface that promote proton abstraction and efficient oxidation of theobromine under neutral conditions. In addition, the π−π interactions between r-GO and the electron-transfer capabilities of f-CB synergistically enhance sensor performance.
Design of SrO-Integrated Carbon Nanocomposite Sensor
The sensor fabrication involved incorporating SrO nanoparticles into a functionalized carbon black and reduced graphene oxide matrix, forming a ternary SrO@f-CB@r-GO nanocomposite, which was then drop-cast onto a glassy carbon electrode (GCE). The materials used included strontium acetate as a precursor for SrO, graphite for r-GO synthesis, and commercially available carbon black that was chemically functionalized to introduce oxygen groups.
The modified electrodes were characterized using various techniques, including scanning and transmission electron microscopy for morphology, X-ray diffraction for crystallinity, and X-ray photoelectron spectroscopy for chemical states. Electrochemical measurements used a three-electrode setup with Ag/AgCl as reference and platinum wire as counter electrode. Differential pulse voltammetry (DPV) was employed to optimize the sensing parameters and quantify theobromine in standard solutions and real beverage samples such as tea, coffee, and chocolate milk. The real samples were filtered and diluted appropriately, with minimal to no chemical pre-treatment.
Electrochemical Performance and Detection Sensitivity
Microscopy and elemental mapping confirmed the uniform distribution of nanoscale SrO particles within the carbon nanohybrid matrix, preserving the structural integrity and creating a hierarchical conductive network conducive to electron transfer. Electrochemical tests demonstrated that, unlike sensors needing strongly alkaline bulk electrolytes, the SrO@f-CB@r-GO sensor efficiently oxidized theobromine at neutral pH. The key mechanism was the formation of localized alkaline microdomains at the SrO sites that facilitated interfacial proton abstraction from theobromine molecules adsorbed via π−π interactions on r-GO. This microenvironment enhanced oxidation kinetics while maintaining electrode stability.
The sensor exhibited a remarkably wide linear detection range from 0.03 μM to 1550.5 μM, with an ultralow detection limit of 0.00285 μM and a sensitivity of 0.012 μA μM−1cm−2. Selectivity tests confirmed that the sensor could distinguish theobromine from structurally similar methylxanthines and common interfering substances in beverages.
Repeatability and reproducibility studies yielded relative standard deviations under 5%, demonstrating consistent performance. When applied to real samples - such as barley tea, black tea, coffee, and chocolate milk - the sensor achieved recovery rates between 96.8% and 103.2% without requiring extensive sample preparation or bulk pH adjustments.
A comparative analysis with existing sensors highlighted several advantages of this work. Most previously reported sensors either operated under harsh alkaline conditions or had narrow detection ranges, limiting their practical use. The localized interfacial alkalinity concept introduced here enabled neutral pH sensing with high sensitivity and robustness across complex matrices. This distinction represents a significant departure from conventional bulk electrolyte alkalinization strategies, emphasizing interfacial chemistry control over mere surface area enhancement.
Download the PDF of this page here
Implications for Interfacial Sensor Design and Real-World Applications
In summary, this study successfully engineered a nanocomposite sensor that leverages interfacial alkaline microdomain formation to enable highly sensitive and selective electrochemical detection of theobromine at neutral pH.
The ternary combination of SrO nanoparticles with functionalized carbon black and reduced graphene oxide provides a synergistic platform where localized basic sites promote effective proton abstraction and oxidation, while the carbon materials facilitate strong adsorption and fast electron transfer. The sensor demonstrates exceptional analytical metrics, including low detection limits, wide linear ranges, and excellent selectivity and stability.
This work introduces a transferable design principle focused on nanoscale interfacial chemistry modulation rather than conventional bulk pH alteration or surface area increases. By enabling the direct sensing of weakly electroactive organic molecules under physiologically relevant conditions, the approach promises enhanced practical applicability for food and beverage quality monitoring and other electrochemical biosensing applications. Future efforts could explore direct pH mapping at such interfaces and expand the concept to other analytes and sensing platforms, potentially facilitating portable, onsite analytical solutions.
Journal Reference
Gopakumar G. M.; Selvam, A., et al. (2026). Interfacial Alkaline Microdomain Engineering via SrO-Integrated Carbon-Based Ternary Nanocomposites for Electrochemical Detection of Theobromine. ACS Applied Nano Materials 9, 6849–6863. DOI: 10.1021/acsanm.6c00840, https://pubs.acs.org/doi/pdf/10.1021/acsanm.6c00840?ref=article_openPDF