| Abstract [eng] |
In this thesis, a caffeine sensitive molecularly imprinted polymer (MIP) sensor was developed and characterised, with the recognition layer formed from a commercial 405 nm UV-curable urethane-acrylate photopolymer resin deposited on a 5 MHz AT cut gold coated quartz crystal microbalance (QCM) electrode. Previously reported caffeine MIP QCM sensors have relied either on the classical MAA/EDMA acrylate system or on electropolymerised polypyrrole; the novelty of this work lies in being the first to demonstrate that a commercial 3D printing photopolymer resin can serve as a functional MIP matrix for gravimetric caffeine detection. The solubility of the resin and caffeine was evaluated in several organic solvent systems, and the optimal composition was identified as acetone/ethanol (3:7 v/v) with 50 µL of resin and 8 mg of caffeine. MIP film formation and subsequent template removal by a deionised water flow (0,5 mL/min) were monitored in real time by QCM. It was established that films cured in air for 10 hours behaved viscoelastically and could not be interpreted by the Sauerbrey equation, whereas a 30 h curing period produced a sufficiently rigid film yielding a reproducible Sauerbrey compatible negative frequency shift (Δf = –7,5±0,5 Hz upon exposure to 4 mmol/L caffeine). An additional 10 % methanol wash did not produce a statistically significant increase in sensitivity and posed a risk of structural damage to the film. Comparison of the MIP sensor with a non-imprinted polymer (NIP) of identical composition yielded an imprinting factor of IF ≈ 4,0. The MIP frequency response was linearly dependent on caffeine concentration in the range of 1–4 mmol/L (Δf = –1,5·c – 1,84; R² = 0,98), with a sensitivity approximately twice that of the NIP. Kinetic and thermodynamic parameters derived from the association and dissociation curves (kₐ = 0,298 M⁻¹s⁻¹, kd = 7,21·10⁻³ s⁻¹, KD = 2,42·10⁻² M, ΔG = –9.06 kJ/mol) indicate a thermodynamically favourable, spontaneous complex formation, consistent with a recognition mechanism dominated by hydrogen bonding and electrostatic interactions. A four-day stability study revealed a gradual decrease in signal (~19 % over the first 24 h, ~32 % over the full period), attributable to swelling of the polymer matrix in aqueous medium; nevertheless, the sensor remained functional throughout the observation period. The work confirms that readily available commercial UV curable urethane-acrylate resins can be used as a functional MIP matrix in gravimetric sensors for small molecules. Selectivity towards structural caffeine analogues was not assessed in this work and remains a subject for further studies. |
