Foods of animal source, as eggs, milk, honey and meat plays an important role in our ordinary diets and during the last years a rapid increase of their consumption was registered, having been demonstrated to be so useful for human health and well-being [1-3]. In this scenario an ever-increasing importance must be paid to the presence of veterinary medicines in foods, as denounced by the European Food Safety Authority (EFSA) [
3]. Substances as hormones, antibiotics and growth promoters are extensively employed to prevent and treat diseases in animals, to boost their growth and improve feeding [1-3]. Unfortunately, several of them can be transferred as contaminants in food products [
3]. One of the greatest concerns is regulating and controlling food safety, especially monitoring the presence of veterinary medicines in animal-source foods [1-3]. In this scenario, the World Health Organization (WHO) has identified antibiotic resistance as one of the three biggest threats to human health. Clear evidence has been provided of the adverse consequences for human health of the resistance to antimicrobials developed by the organisms because of the continuous exposure to antibiotics for non-human use [4-6]. Antibiotic resistance is a global sanitary emergence: the largest the amount of antibiotics used, the highest the probability for bacteria to develop resistance mechanisms to them. The result of this mechanism is that antibiotics become ineffective, causing substantial failure of antimicrobial treatments. For these reasons, awareness is increasing on the need and urgency to actively protect antibiotics against over-use [
7,
8]. To account all these issues, the development and employment of methods able to detect animal-drugs residues in animal-sources foods, helping to determine if their quantity is lower to the maximum residues limits (MRL), represent the great challenges in many countries to ensure a certain level of food safety [
9,
10]. Moreover, methods are needed that are capable to accurately reveal the presence of antibiotics continuously and on-site even in complex matrices, such as food and biological samples [11-16]. In this regard, in the literature several and different analytical methods are proposed, such as High-Performance liquid chromatography (HPLC) coupled with Mass Spectrometry (MS) and Liquid Chromatography combined with tandem mass spectrometry (LC-MS/MS), to detect antibiotics accurately and simultaneously in different environmental media [
12,
13]. Other determination techniques such as capillary electrophoresis (CE) [
14], Raman Spectroscopy [
15], and enzyme linked immunosorbent assays [
16] were exploited to monitor drugs residues in food products. Even very innovative, many of the bioassay and conventional methods cannot be implemented for on-site routine use, since they need pretreatments, are expensive and not enough sensitive. With the main target to overcome all these limits, biosensors received great interest as effective methods for screening antibiotics residues. Biosensors not only offer high sensitivity and selectivity, but also ensure high degree of automation, combined to cost-effectiveness, real-time measurements and high throughput [17-19]. Basically, in the general scheme of biosensors, a biological recognition element interacts with the target compound inducing a biological response that the physical transducer transforms in a detectable signal proportional to the content of analytes [20-23]. Several biological recognition elements, including cofactors, enzymes, antibodies, organelles, tissues, cells, and whole microorganisms have been used in the design of biosensors [
24]. Among these biological elements, microorganisms (e.g. algae, bacteria and yeast) offer a valid alternative to fabricate biosensors thanks to their easiness of manipulation, better viability and stability in vitro and ability to enhance performance of biosensors [
25]. Nonetheless, further improvements are necessary to develop more effective biosensors, which can be promising tools for the detection and quantification of antibiotics in food products [
11]. One of the most promising type of bio-electrochemical sensors, based on whole microorganisms as recognition elements, is represented by Microbial Fuel Cells (MFCs), which converts the chemical energy, contained in organic compounds, known as fuels, into electrical energy thanks to the metabolic activity of so called exo-electrogenic bacteria [
26,
27]. Since the power output is strictly correlated with the metabolic activity of these microorganisms, the presence of a toxicant can directly affect the overall performance of the device in terms of the output electrical parameters [28-32]. Therefore, in MFC-based biosensors, microbial metabolism enables the conversion of a chemical signal, associated to the energy trapped in the fuel, into an electrical signal, i.e. the output electrical energy of MFCs. Thus, the energy conversion and the signal transduction steps are intimately coupled, avoiding the need of any external transducer and additional power unit. MFC-based biosensors can be successfully used ad fast response early low maintenance detectors, being also cost effective, as they can be built on low-cost carbon-based materials [
33,
34].
In this work, air-cathode Single Chamber Microbial Fuel Cells (a-SCMFCs) are proposed as effective bio-electrochemical devices for the detection of a particular kind of antibiotic, tetracycline, added to a water-based electrolyte and to the honey, previously investigated as effective more complex electron donor [
35]. Based on the results of our past article [
35], optimal proliferation of microorganisms at the anode has been ensured using nanostructured anodes, based of carbon paper decorated by nanofibers made of polyethylene oxide (CP/PEO-NFs). This approach allowed to enhance not only the overall a-SCMFCs performance but also to improve their capability to monitor the presence of antibiotic into the electrolytes. We investigated a-SCMFCs as bio-electrochemical sensors for antibiotic detection by employing and comparing two different electrolytes. The first is a water-based electrolyte used as SCMFCs’ behaviors benchmark when antibiotics are added in traces to water. We selected a very low concentration value of tetracycline, which is close to (3.53 ±0.13) μg⁄kg and refereed to the amount of honey. This value was selected since it results to be one order of magnitude lower than the MRL defined for honey as food matrix, which is close to 10 μg⁄kg. Proven/Demonstrated/Confirmed thus the effectiveness of SCMFCs as bio-electrochemical sensor for tetracycline detection when water-based electrolyte was employed, we investigated, in the present work, the possibility to add the antibiotic traces directly into honey-based electrolyte. Indeed, to this purpose, the second electrolyte was prepared dissolving honey to deionized water in quantities equal to those used for sodium acetate-based electrolyte. Finally, we demonstrated the SCMFCs’ capability to detect the presence of drugs directly into the food-matrix without the necessity to extract the antibiotics before their detection. It was possible to achieve a decreasing of current density values, close to 50%, when tetracycline was added to the water-based electrolyte. Moreover, 96% of decreasing of current density reached when (3.53 ±0.13) μg⁄kg of tetracycline was added to the electrolytes based on honey, demonstrating thus the possibility to apply SCMFCs as sensor for the tetracycline detection, directly using honey as carbon sources for microorganisms. Finally, we evaluated and confirmed the capability of SCMFCs to recover the same current density value when fresh electrolyte, based on only sodium acetate and honey, is employed. Finally, the behavior of all devices is also analyzed evaluating the amount of energy recovered (E
rec) by a unit volume of the electrolyte [36-38]. E
rec obtained for both electrolytes, based onto honey and sodium acetate with and without the presence of antibiotics, was compared, demonstrating the microorganisms’ capability to accurately detect the presence of antibiotics, guaranteeing also their recovery when the antibiotic is not present into electrolytes.