Field-Effect Transistor-Based Biosensors for Environmental and Agricultural Monitoring

Abstract

The precise monitoring of environmental contaminants and agricultural plant stress factors, respectively responsible for damages to our ecosystems and crop losses, has nowadays become a topic of uttermost importance. This is also highlighted by the recent introduction of the so-called "Sustainable Development Goals" of the United Nations, which aim at reducing pollutants while implementing more sustainable food production practices, leading to a reduced impact on all ecosystems. In this context, the standard methods currently used in these fields represent a sub-optimal solution, being expensive, laboratory-based techniques, and typically requiring trained personnel with high expertise. Recent advances in both biotechnology and material science have led to the emergence of new sensing (and biosensing) technologies, enabling low-cost, precise, and real-time detection. An especially interesting category of biosensors is represented by field-effect transistor-based biosensors (bio-FETs), which enable the possibility of performing in situ, continuous, selective, and sensitive measurements of a wide palette of different parameters of interest. Furthermore, bio-FETs offer the possibility of being fabricated using innovative and sustainable materials, employing various device configurations, each customized for a specific application. In the specific field of environmental and agricultural monitoring, the exploitation of these devices is particularly attractive as it paves the way to early detection and intervention strategies useful to limit, or even completely avoid negative outcomes (such as diseases to animals or ecosystems losses). This review focuses exactly on bio-FETs for environmental and agricultural monitoring, highlighting the recent and most relevant studies. First, bio-FET technology is introduced, followed by a detailed description of the the most commonly employed configurations, the available device fabrication techniques, as well as the specific materials and recognition elements. Then, examples of studies employing bio-FETs for environmental and agricultural monitoring are presented, highlighting in detail advantages and disadvantages of available examples. Finally, in the discussion, the major challenges to be overcome (e.g., short device lifetime, small sensitivity and selectivity in complex media) are critically presented. Despite the current limitations and challenges, this review clearly shows that bio-FETs are extremely promising for new and disruptive innovations in these areas and others.

Keywords: bio-FETs; environmental pollutants; flexible electronics; plant stresses; sensors; thin-film fabrication; transistors.

Figure 1

(a) MOSFET with the main p-Si body and two n-Si regions. (b) Top-gate TFT on a substrate that could be many type of materials (both rigid or flexible) and the other layers deposited on top of it. (c) Transfer curve of a FET presenting a p-type behavior; I/W is the measured current divided by the channel width. ON/OFF ratio is I_ON/I_OFF; threshold voltage (V_TH) is the V_GS at which FET turns on. (d) Output curves of a p-type FET-device; curves were obtained at different fixed V_GS.

Figure 2

Some of the most common structures of FET devices that are used as biosensors, i.e., as bio-FETs. All structures have source and drain electrodes (in yellow), a semiconducting channel (in green), and a substrate (in gray). (a) Bottom-gate FET; the substrate is Si, while the dielectric material is SiO₂. (b) EG-FET (planar configuration); gate is in-plane with source/drain and is insulated through the electrolyte solution. (c) ECT; the gate is an external electrode and is dipped in the electrolyte solution. (d) ISFET; the gate electrode is replaced by a reference electrode (very often an Ag/AgCl electrode). (e) ChemFET; the gate electrode is separated from the source and drain by an electrolytic solution, and a semi-permeable membrane is present at the gate interface.

Figure 3

(a) General configuration of a bio-FET: the semiconducting channel is functionalized with a recognition element that binds to its specific analyte. In this example, the analyte of interest is present in a liquid solution; the recognition element is drawn in a Y shape typical of antibodies, which are just one example of recognition elements. (b) Four examples of the most used recognition elements, which are antibodies (immunological proteins), aptamers (single or double stranded DNA or RNA), enzymes (biological proteins), and whole cells. Image re-adapted from [134] (part (b)).

Figure 4

Examples of bio-FETs developed for environmental monitoring. (a) Bottom-gate FET to detect atrazine (ATZ). Top: cross and top view of device (CNTs were functionalized on top, not shown in the figure). Bottom: analytical response obtained at different concentrations of ATZ. The change in current decreased with increasing concentrations of ATZ because of the immunocomplex formed between ATZ and antibodies; the device showed a change in current also in the nanomolar range, and the LOD was lower than the legal limits for ATZ in food and drinking water ($0.1$ ng/mL). Re-adapted with permission from [88]. Copyright 2022, Elsevier. (b) Bottom-gate FET to detect Salmonella. Top: cross view of final bio-FET. Bottom: Transfer curves taken after each step of the functionalization. The net decrease in I_DS for the last curve (yellow line) was due to the exposure to ${10}^7$ cfu/mL of Salmonella, which specifically interacted with the antibodies present on the device. Re-adapted with permission from [92]. (c) Bottom-gate FET to detect botulinum neurotoxin (BoNT/E-Lc). Top: illustration of BoNT/E-Lc binding with Anti-BoNT/E-Lc. Bottom: real-time conductance measurement with varying concentrations of BoNT/E-Lc (ranging fom 52 to 500 fM). With increasing concentration of the analyte, the measured conductance decreased, and it reached a saturation after around 40 min. Re-adapted with permission from [102]. (d) OECT to detect Cu²⁺ ions. Top: schematic images of Cu²⁺ ions before and after coordinating with TCA (recognition element). Bottom: Shifts in V_DP (Dirac-point Voltage) with time for various Cu²⁺ ion concentrations. A small shift could be seen starting from 30 $\mathsf{\mu }$M, while the shift started to be evident at a concentration of 100 $\mathsf{\mu }$M and higher. Re-adapted with permission from [52].

Figure 5

Examples of bio-FETs developed for plant monitoring. (a) OECT developed to monitor plant xylem metabolites. Top: device setup; the sensor was inserted into the xylem of the tree stem. Bottom: Instantaneous monitoring of sucrose (orange), glucose (cyan), and control (black) for 2 days. Bright areas correspond to daytime, while dark areas are related to nighttime. Re-adapted with permission from [122]. Copyright 2022, Elsevier. (b) EG-FET for the detection of plant viruses. Top: Cross-view of final device. Bottom: Transfer characteristics of the sensor after being exposed to various plum pox virus (PPV) concentrations (from 0 to 50 $\mathsf{\mu }$g/mL); a decrease in current is shown with increasing concentrations of PPV. Re-adapted with permission from [53]. Copyright 2022, Elsevier. (c) OECT for the detection of ions in olive trees. Top: a representation of the sensor setup inserted into the stem of an olive tree. Bottom: diurnal fluctuations of the sensor response R (o) and plant transpiration (E) (cyan line) sensed for 48 h. Re-adapted with permission from [107]. (d) Bottom-gate FET for monitoring microalgae membrane depolarization. Top: representation of the sensor functionalized with algae cells and infected with viruses introduced with syringe. Bottom: the current changes upon virus infection. Reprinted with permission from [120]. Copyright 2022 American Chemical Society.