Cystic fibrosis (CF) is one of the most lethal autosomal recessive disorder caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR). It is estimated that about 60–70% of CF patients develop Pseudomonas aeruginosa chronic infection, with progressive loss of lung function, as well as increased antibiotic resistance and mortality.

Tobramycin has become an important contributor to CF as a first-line treatment for the eradication of early acquisition of P. aeruginosa and means to control the chronic infection. In order to improve the efficacy and safety of treatments, Tobramycin dosage needs to be adjusted to the actual needs of each patient in a truly personalized medicine approach. The therapeutic range of tobramycin varies between ~ 2 to 12 μg/mL or between ~ 4 to 26 μM in serum.

Regular evaluation is key for timely and accurate treatment of CF, and therefore, it is essential to develop biosensor field effect transistors (FETs) with high sensitivity towards tobramycin. Aptamers with their inherent advantage to sustain multiple regeneration steps and high chemical stability allow them to be an attractive option for biosensors. This is because they can be easily immobolised on the surfaces due to the ready reaction with different chemical groups.

Taking advantage of aptamers and enhanced sensitivity of electrochemical biosensors such as FET, Barua et al. reported an approach to sense tobramycin through aptamer functionalized zinc oxide (ZnO) ultra thin film transistors (TFTs). These TFTs were fabricated and functionalized with a 26 nucleotide thiolated tobramycin aptamer through wet etch process and tested to show good functionalization with aptamer based biosensing approaches (Figure 1).

Figure 1. (A) ZnO surface functionalization with (1) Amine terminated silane coupling agent APDMES, (2) Sulfo-MBS linker molecule, and (3) Thiolated tobramycin aptamer. (B) Transfer characteristics at different stages of functionalization: bare ZnO (black), after APDMES silane treatment (red), and after tobramycin aptamer functionalization (blue)

Signals were transduced from the secondary conformational change of the aptamer structure in contact with a target, to the surface of the ZnO TFT, which showed to function as both a transducer as well as an amplifier (Figure 2A). The TFTs were successfully used to sense different concentrations of tobramycin from 1 nM to 100 nM which has practical applications in cystic fibrosis and bacterial infections in cancer patients (Figure 2B). Adsorption isotherm simulations enabled the extraction of the lower detection limit (LDL) of 0.1 nM for the device between the aptamer and the target. This clearly proved the compatibility of ZnO based TFT with aptamer-based approaches which can be extended to other biomolecules.

Figure 2. (A) Tobramycin aptamer structure with 5’ end attached to the thiol group through the phosphate backbone (a), primary configuration on ZnO (b), and secondary conformational change after target binding event (c). (B) Modulated current response of tobramycin aptamer functionalized TFTs on target binding with 1 nM, 10 nM and 100 nM tobramycin target at time steps of 30 seconds.

Overall, the above analysis demonstrated that tobramycin aptamer-functionalized ZnO TFTs can be used to detect tobramycin at high sensitivity in a mono-target solution. Taking account of aptamers uncomplicated construction and compact size, along with the remarkable performances, represent a leap forward toward effective point-of-care devices for therapeutic drug concentration monitoring.

At Aptamer Group, we are involved in continuous development of similar biosensors for small molecule detection and even protein biomarkers using our high affinity aptamers. If you would like more information on such platforms, please contact us using the form below.

Reference: A. Barua, T. H. Nguyen, Y. Wu, V. M. Jain, R. J. White and R. Jha, “Ultrasensitive label-free tobramycin detection with aptamer-functionalized ZnO TFT biosensor,” NAECON 2018 – IEEE National Aerospace and Electronics Conference, Dayton, OH, 2018, pp. 331-338.

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