Label-Free Nanoscale ZnO Tetrapod-Based Transducers for Tetracycline Detection

Abstract

Antibiotic pollution of freshwaters and even food products has become an important concern worldwide. Hence, it is of utmost importance to develop cost-effective and reliable devices that can provide information on the presence of such contaminants to the general population. In the present work, zinc oxide (ZnO) nanotetrapods (NTP) produced via a high yield laser processing approach were used as transducers in a luminescent-based immunosensor to detect tetracycline (TC). These tetrapodal structures present needleshaped branches with a high aspect ratio, exhibiting lengths from hundreds of nanometers to a few micrometers and an average thickness of tens of nanometers, providing a high surface area for bioreceptor immobilization and analyte reaction, which is quite desirable in a transducer material. Besides, these ZnO NTP display intense photoluminescence (PL) at room temperature, making such a signal rather promising for transduction. Indeed, the intensity of the ZnO PL signal was seen to correlate with the TC concentration. The PL quenching with increasing analyte concentration is explained considering the rise in the bending of the electronic bands of the semiconductor near its surface due to increased charge density at this region, induced by the interaction between the bioreceptor (anti-TC antibodies) and the TC molecules. As a larger depletion width (and potential barrier) is promoted near the surface, the excitonic recombination probability is reduced and, consequently, the PL intensity in the ultraviolet spectral region, allowing us to use this relationship as a sensing mechanism. This information enabled us to define a calibration curve for TC quantification in the 0.001 to 1 μg L −1 range, which is the range of interest of this antibiotic in freshwaters. A limit of detection (LOD) of ∼1.2 ng L −1 is reported, corresponding to one of the lowest LOD found in the literature for this antibiotic, indicating that the present ZnO NTPbased biosensors rival the current state-of-the-art ones.

ZnO Transducers for Photoluminescence-Based Biosensors: A Review

“…Higher TC concentrations were tested (up to 500 µg L −1 ), however, the signal was seen to reach a plateau in the high concentration regime (above ~10 µg L −1 ), which suggests that the majority of the antigenbinding sites were already engaged with TC antigens at such concentrations. Indeed, TC concentrations usually found in freshwaters are in the range of 0.008-1 µg L −1 [108], which is in agreement with the one obtained for our sensors [107]. A LOD of ~1.2 ng L −1 was estimated in this case, corresponding to one of the lowest LOD values reported so far for TC.…”

“…where I UV 0 stands for the integrated intensity of the NBE for the blank/reference (without any analyte) and I UV Analyte corresponds to the integrated intensity of the same emission after analyte immobilization for each concentration C [9,107]. Therefore, the data demonstrated that the developed biosensor can be employed in hCG quantification, presenting fairly results in the range between 5 and 50 ng mL −1 [9].…”

“…Recently, our research group has also devoted great efforts studying ZnO-based transducers to be explored into optical biosensors [9,107]. In 2020, we investigated a PLbased immunosensor for the detection of the human chorionic gonadotropin (hCG) hor- In 2021, Myndrul et al [106] used polyacrylonitrile (PAN)/ZnO nanofibres as the transducer integrated into a microfluidic cell to detect Aflatoxin B1 (AFB1), which is a toxin with a harmful effect in human health.…”

“…Recently, our research group has also devoted great efforts studying ZnO-based transducers to be explored into optical biosensors [9,107]. In 2020, we investigated a PL-based immunosensor for the detection of the human chorionic gonadotropin (hCG) hormone [9].…”

“…Similar ZnO tetrapodal structures were also employed in the exploitation of a biosensing platform for the detection of tetracycline (TC) [107], which is a broad-spectrum antibiotic that can sometimes be present in freshwaters or animal food products [108]. Here, the anti-TC AB were covalently immobilized on the ZnO transducer.…”