Imprinted Polydimethylsiloxane-Graphene Oxide Composite Receptor for the Biomimetic Thermal Sensing of Escherichia coli

Abstract

This work presents an imprinted polymer-based thermal biomimetic sensor for the detection of Escherichia coli. A novel and facile bacteria imprinting protocol for polydimethylsiloxane (PDMS) films was investigated, and these receptor layers were functionalized with graphene oxide (GO) in order to improve the overall sensitivity of the sensor. Upon the recognition and binding of the target to the densely imprinted polymers, a concentration-dependent measurable change in temperature was observed. The limit of detection attained for the sensor employing PDMS-GO imprints was 80 ± 10 CFU/mL, a full order lower than neat PDMS imprints (670 ± 140 CFU/mL), illustrating the beneficial effect of the dopant on the thermo-dynamical properties of the interfacial layer. A parallel benchmarking of the thermal sensor with a commercial impedance analyzer was performed in order to prove the possibility of using the developed PDMS-GO receptors with multiple readout platforms. Moreover, S. aureus, C. sakazakii and an additional E. coli strain were employed as analogue species for the assessment of the selectivity of the device. Finally, because of the potential that this biomimetic platform possesses as a low-cost, rapid, and on-site tool for monitoring E. coli contamination in food safety applications, spiked fruit juice was analyzed as a real sample. Reproducible and sensitive results fulfill the limit requirements of the applicable European microbiological regulation.

Keywords: Biomimetic sensing; cell imprinting; food safety; graphene oxide; imprinted polymers.

Figure 1

Schematic representation of the interfacial surface-imprinting process of PDMS. (A) Brigthfield microscopy of E. coli safranin-stained on imprinted polymer after curing. (B) Brigthfield microscopy of empty bacteria cavities on polymer. (C) Scanning electron microscopy of bacteria imprints after the removal of the template.

Figure 2

(A) Schematic representation of the thermal recognition of E. coli. (B) Fluorescence microscopy image visually depicting the rebinding of stained target to the empty cavities on the polymer (PDMS-GO SIP). After exposition to the target, the layers were rinsed with PBS in order to remove unbound cells. (C) Real-time temperature response of the sensor employing surface-imprinted PDMS SIP, PDMS-GO SIP receptors as well as the nonimprinted PDMS layers (NSIP). (D) Dose–response curve of the sensor employing the different receptor layers. The dashed line indicates the limit of detection, defined as three times the average of the error on the data. Error bars are calculated, making use of the noise of the signal of the sensor.

Figure 3

Effect size curves for selectivity of the PDMS-GO imprints. Receptors imprinted for a specific strain of E. coli were exposed to S. aureus, C. sakazakii and an additional E. coli strain. Bacteria concentrations employed were 0, 1 × 10², 1 × 10³, 1 × 10⁴, 1 × 10⁶, and 1 × 10⁷ CFU/mL. Error bars are calculated making use of the noise of the signal of the sensor. The dashed line indicates the limit of detection, defined as 3 times the average of the error on the data.

Figure 4

Effect size curve for detection of E. coli in juice. Bacteria concentrations employed were 0, 1 × 10², 1 × 10³, 1 × 10⁴, 1 × 10⁶, and 1 × 10⁷ CFU/mL. Error bars are calculated making use of the noise of the signal of the sensor. The dashed line indicates the limit of detection, defined as 3 times the average of the error on the data