Nitrate (NO₃^–) contamination is becoming a major concern due to the negative effects of an excessive NO₃^– presence in water which can have detrimental effects on human health. Sensitive, real-time, low-cost, and portable measurement systems able to detect extremely low concentrations of NO₃^– in water are thus becoming extremely important. In this work, we present a novel method to realize a low-cost and easy to fabricate amperometric sensor capable of detecting small concentrations of NO₃^– in real water samples. The novel fabrication technique combines printing of a silver (Ag) working electrode with subsequent modification of the electrode with electrodeposited copper (Cu) nanoclusters. The process was tuned in order to reach optimized sensor response, with a high catalytic activity toward electroreduction of NO₃^– (sensitivity: 19.578 μA/mM), as well as a low limit of detection (LOD: 0.207 nM or 0.012 μg/L) and a good dynamic linear concentration range (0.05 to 5 mM or 31 to 310 mg/L). The sensors were tested against possible interference analytes (NO₂^–, Cl^–, SO₄^2–, HCO₃^–, CH₃COO^–, Fe²⁺, Fe³⁺, Mn²⁺, Na⁺, and Cu²⁺) yielding only negligible effects [maximum standard deviation (SD) was 3.9 μA]. The proposed sensors were also used to detect NO₃^– in real samples, including tap and river water, through the standard addition method, and the results were compared with the outcomes of high-performance liquid chromatography (HPLC). Temperature stability (maximum SD 3.09 μA), stability over time (maximum SD 3.69 μA), reproducibility (maximum SD 3.20 μA), and repeatability (maximum two-time useable) of this sensor were also investigated.

Aptamers are chemically synthesized single-stranded DNA or RNA oligonucleotides widely used nowadays in sensors and nanoscale devices as highly sensitive biorecognition elements. With proper design, aptamers are able to bind to a specific target molecule with high selectivity. To date, the systematic evolution of ligands by exponential enrichment (SELEX) process is employed to isolate aptamers. Nevertheless, this method requires complex and time-consuming procedures. In silico methods comprising machine learning models have been recently proposed to reduce the time and cost of aptamer design. In this work, we present a new in silico approach allowing the generation of highly sensitive and selective RNA aptamers towards a specific target, here represented by ammonium dissolved in water. By using machine learning and bioinformatics tools, a rational design of aptamers is demonstrated. This “smart” SELEX method is experimentally proved by choosing the best five aptamer candidates obtained from the design process and applying them as functional elements in an electrochemical sensor to detect, as the target molecule, ammonium at different concentrations. We observed that the use of five different aptamers leads to a significant difference in the sensor’s response. This can be explained by considering the aptamers’ conformational change due to their interaction with the target molecule. We studied these conformational changes using a molecular dynamics simulation and suggested a possible explanation of the experimental observations. Finally, electrochemical measurements exposing the same sensors to different molecules were used to confirm the high selectivity of the designed aptamers. The proposed in silico SELEX approach can potentially reduce the cost and the time needed to identify the aptamers and potentially be applied to any target molecule.

In this work, we propose a novel disposable flexible and screen-printed electrochemical aptamer-based sensor (aptasensor) for the rapid detection of chlorpyrifos (CPF). To optimize the process, various characterization procedures were employed, including Fourier transform infrared spectroscopy (FT-IR), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). Initially, the aptasensor was optimized in terms of electrolyte pH, aptamer concentration, and incubation time for chlorpyrifos. Under optimal conditions, the aptasensor showed a wide linear range from 1 to 10⁵ ng/mL with a calculated limit of detection as low as 0.097 ng/mL and sensitivity of 600.9 µA/ng. Additionally, the selectivity of the aptasensor was assessed by identifying any interference from other pesticides, which were found to be negligible (with a maximum standard deviation of 0.31 mA). Further, the stability of the sample was assessed over time, where the reported device showed high stability over a period of two weeks at 4 °C. As the last step, the ability of the aptasensor to detect chlorpyrifos in actual samples was evaluated by testing it on banana and grape extracts. As a result, the device demonstrated sufficient recovery rates, which indicate that it can find application in the food industry.

Furaneol is a widely used flavoring agent, which can be naturally found in different products, such as strawberries or thermally processed foods. This is why it is extremely important to detect furaneol in the food industry using ultra-sensitive, stable, and selective sensors. In this context, electrochemical biosensors are particularly attractive as they provide a cheap and reliable alternative measurement device. Carbon nanotubes (CNTs) and silver nanoparticles (AgNPs) have been extensively investigated as suitable materials to effectively increase the sensitivity of the biosensors. However, a comparison of the performance of biosensors employing CNTs and AgNPs is still missing. Herein, the effect of CNTs and AgNPs on the biosensor performance has been thoughtfully analyzed. Therefore, disposable flexible and screen printed electrochemical aptasensor modified with CNTs (CNT-ME), or AgNPs (AgNP-ME) have been developed. Under optimized conditions, CNT-MEs showed better performance compared to AgNP-ME, yielding a linear range of detection over a dynamic concentration range of 1 fM–35 μM and 2 pM–200 nM, respectively, as well as high selectivity towards furaneol. Finally, our aptasensor was tested in a real sample (strawberry) and validated with high-performance liquid chromatography (HPLC), showing that it could find an application in the food industry.

Electrochemical biosensors are widely investigated as they represent attractive analytical tools for detection of a broad range of bio-molecules, thanks to their simplicity, high sensitivity and short response time. Especially, biosensors employing an electrolyte-gated field-effect transistors (EG-FETs) as electrochemical transduction element have gained increasing interest, due to the signal amplification and the intrinsic low voltage range of operation. In this work we report the fabrication of flexible EG-FETs using spray-deposited semiconducting carbon nanotubes (CNTs), with a specific focus on the optimization of the CNT channel to optimize the performance of the resulting CNT-based EG-FET (EG-CNTFET). The transfer and the output characteristic of different devices with varying spraying parameters were tested, finding out that only devices with source-drain resistance of about ≤10 kΩ showed proper EG-CNTFET operation: for these devices we recorded a typical p-type behavior with an on–off ratio of 214 A/A up to 469 A/A (depending on number of the spray-deposited CNT layers). The fabricated EG-CNTFETs were functionalized with anti-spermidine antibodies to detect polyamine spermidine - a well-known chemical indicator of food quality. To ensure controlled immobilization and at the same time to preserve the electrical properties of the nanotubes, the spray-deposited films were modified with a bifunctional molecule, which attaches to the CNT via non-covalent π - π interactions and leaves a free NHS-ester group for amide coupling of the antibodies. The fabricated EG-CNTFET-based immunosensors showed a linear detection range for spermidine from 10-3 to 102 nM, with the sensitivities ranging from -1.03 to -2.45 μA/decade.