A Novel Molecularly Imprinted 4-Vinyl Pyridine Electrode for Detection and Quantification of Cinnamic Acid in Food Samples
Cinnamic acid (CA) is a naturally occurring aromatic carboxylic acid widely found in cinnamon, black tea, and fruits. It has a crucial role in plant metabolism and offers numerous health benefits in humans, including antioxidant, anti-inflammatory, antimicrobial, and potential anticancer properties. However, unregulated or excessive use can have adverse effects, underscoring the need for precise quantification methods across food items rich in CA.
Conventional analytical techniques like high-performance liquid chromatography (HPLC), chemiluminescence, and mass spectrometry are accurate but have downsides, like being time-consuming, costly, and unsuitable for on-site detection. On the other hand, electrochemical sensors are another alternative due to their simplicity, portability, and low cost, but traditional graphite paste electrodes suffer from poor selectivity.
To address this challenge, a graphite-based electrode is developed using the molecularly imprinted polymer (MIP) technique, providing high specificity for CA while retaining its excellent electrochemical performance. The electrode, termed 4-VP@G, was fabricated by polymerising 4-vinyl pyridine (4-VP) as the monomer with ethylene glycol dimethacrylate (EGDMA) as the cross-linker in the presence of CA as a template. After polymerisation, CA was extracted, leaving complementary cavities capable of selectively rebinding CA molecules as in the MIP technique. For reference, its non-imprinted polymer (NIP) counterpart was also fabricated in the same method, except for the CA addition step.
Various techniques were used to characterise the MIP and the NIP electrodes. X-ray diffraction confirmed that the MIP possessed higher crystallinity than the NIP, indicating successful imprinting. Scanning electron microscopy images showed distinct nanopores on the MIP surface, serving as CA-specific binding sites, which were absent on the NIP. Energy-dispersive spectroscopy analysis revealed a lower oxygen content, confirming successful extraction of CA, and UV–Vis spectra showed the disappearance of the characteristic absorption peak at 250 nm after washing, verifying complete template elimination. In cyclic voltammetry, the MIP-based electrode showed a fivefold higher oxidation peak (250 µA) than the NIP (50 µA), confirming its specificity for CA.
Differential pulse voltammetry (DPV) analysis revealed a linear detection range of 1–1000 µM and a remarkably low limit of detection (LOD) of 8.2 nM, which is significantly lower than those previously reported methods. The sensors underwent repeatability, reproducibility, and long-term stability tests, and the responses showed very low relative standard deviation values of 1.01%, 1.14%, and 1.013%, respectively, confirming high reliability and robustness. The electrode maintained stable performance even after 40 days, indicating excellent reusability.
Selectivity tests were performed using common interfering species such as Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, and structurally similar phytochemicals, including gallic acid, cinnamyl acetate, and p-coumaric acid. The sensor displayed minimal signal deviation (±5%), confirming its high selectivity for CA. Further, the sensor’s real-world applicability was demonstrated using commercially available three black tea and two cinnamon powder samples. The DPV results showed excellent agreement, with detection accuracy ranging from 94.2% to 98.8% when compared to HPLC measurements.
The electrode’s simple fabrication, low LOD, reusability, and portability make it a practical alternative for measuring cinnamic acid in food products, which is suitable for laboratories and small-scale industries.