Introducing Thermal Wave Transport Analysis (TWTA): A Thermal Technique for Dopamine Detection by Screen-Printed Electrodes Functionalized with Molecularly Imprinted Polymer (MIP) Particles

Abstract

A novel procedure is developed for producing bulk modified Molecularly Imprinted Polymer (MIP) screen-printed electrodes (SPEs), which involves the direct mixing of the polymer particles within the screen-printed ink. This allowed reduction of the sample preparation time from 45 min to 1 min, and resulted in higher reproducibility of the electrodes. The samples are measured with a novel detection method, namely, thermal wave transport analysis (TWTA), relying on the analysis of thermal waves through a functional interface. As a first proof-of-principle, MIPs for dopamine are developed and successfully incorporated within a bulk modified MIP SPE. The detection limits of dopamine within buffer solutions for the MIP SPEs are determined via three independent techniques. With cyclic voltammetry this was determined to be 4.7 × 10(-6) M, whereas by using the heat-transfer method (HTM) 0.35 × 10(-6) M was obtained, and with the novel TWTA concept 0.26 × 10(-6) M is possible. This TWTA technique is measured simultaneously with HTM and has the benefits of reducing measurement time to less than 5 min and increasing effect size by nearly a factor of two. The two thermal methods are able to enhance dopamine detection by one order of magnitude compared to the electrochemical method. In previous research, it was not possible to measure neurotransmitters in complex samples with HTM, but with the improved signal-to-noise of TWTA for the first time, spiked dopamine concentrations were determined in a relevant food sample. In summary, novel concepts are presented for both the sensor functionalization side by employing screen-printing technology, and on the sensing side, the novel TWTA thermal technique is reported. The developed bio-sensing platform is cost-effective and suitable for mass-production due to the nature of screen-printing technology, which makes it very interesting for neurotransmitter detection in clinical diagnostic applications.

Keywords: biomimetic sensors; cyclic voltammetry; heat-transfer method (HTM); molecularly imprinted polymers (MIPs); neurotransmitters; screen-printing technology; thermal wave transport analysis (TWTA).

Figure 1

Binding isotherms of MIP (solid squares) and the corresponding NIP (open squares) upon exposure to aqueous dopamine solutions. To test the selectivity, the response of the MIP to serotonin was evaluated (filled circles). Solid lines are based on the allometric fit described in the text (R² = 0.95); the imprinting factor for optimized monomer blend is close to 3. Error bars were calculated over the results of three individual experiments.

Figure 2

The effect of increasing dopamine concentrations (0–50 μM) made into a pH 7.4 PBS using both bare SPE (open squares) and MIP-SPE (filled squares) The average response is shown and the corresponding standard deviation (N = 3) are presented in the error bars.

Figure 3

Time-dependent thermal resistance data (panel A). At constant T₁, the temperature inside the liquid of the flow cell (T₂) clearly decreased with solutions of increasing dopamine concentrations (pH = 7.4) due to blocking of heat-transfer in the MIP. The linear regime of the sensor lies from 0.3 μM to 0.8 μM (B), and the sensor has a maximum effect size of 16.5% ± 1%. Error bars were calculated as the standard deviation over a 500 s window after each addition, measurements performed in threefold.

Figure 4

A thermal wave was applied by the heat sink and the MIP-SPE (with ratio M_P/M_I 30%) were exposed to a dopamine-free PBS solution and subsequently to solutions with dopamine concentrations of 300, 400 and 800 nM in PBS (pH = 7.4). The wave has an amplitude of 0.1 °C and an increasing frequency, ranging from 0.01 to 0.05 Hz (A). The resulting heat wave (T₂) inside the liquid compartment was followed in time with increasing concentrations of dopamine in PBS and normalized to the initial temperature of 37.00 °C (B).

Figure 5

The phase shifts (Hz) for the concentrations of dopamine (0, 300, 400, 800 nM) in PBS solutions (pH = 7.4) as a function of the frequency. The highest response was achieved at 0.03 Hz, demonstrating this would be the optimum frequency to monitor dopamine binding to the MIP particles.

Figure 6

A thermal wave (amplitude 0.1 °C) was applied by the heat sink and the MIP-SPE were exposed to a dopamine-free solution of banana and subsequently to banana fluid with spiked dopamine concentrations of 100, 200, 500, 1000 and 2500 nM. The results on the thermal wave output normalized to the initial temperature of 37.00 °C are shown in (A), while (B) provides the corresponding phase shifts. 0.03 Hz was determined to be the optimal frequency for the measurements, as was observed in previous experiments.

Figure 7

Schematic representation of the thermal wave analysis transport set up, with TCU corresponding to the thermal control unit. The thermal wave (with phase ϕ) is sent through the MIP-SPE and the output signal (ϕ′) is subsequently measured in the liquid.