# Advances in Graphene Field Effect Transistors (FETs) for Annine Neurotransmitter Sensing
## 4 Challenges and Limitations
### Ionic Strength and Electrolyte Concentration  
One of the key challenges in improving the detection limit of GFET-based biosensors is controlling the molecular charge shielding by counter ions in a solution (known as Debye shielding). The Debye length is the distance at which charge interactions are effective, and it is \(<\)1 nm, which determines shielding and effective sensing distance [212]. In physiological fluids with high salt content (e.g., blood or cerebrospinal fluid), the shortened Debye length can impair the sensor's capacity to detect NTs at low concentrations unless the sensor design accounts for this. In other words, an increase in Debye length minimises the screening effect and enables more sensitive electrical detection of charged biomolecules. Small target molecules with few or no charges have little effect on graphene transconductance unless they trigger conformational changes in charged receptors at or near the Debye length or adjust surface potentials [216]. Using aptamers and polymers will help with reducing the screening effect [212; 259]. No research has been carried out to address this limitation when conducting amine NT detection; however, the approaches below can be replicated to mitigate Debye length challenges when investigating amine NT biosensing.  
* Electrolytes near concave surfaces have increased Debye length compared to those near flat surfaces. Crumpled graphene, which has nanoscale bends and deformations that create concave regions, improves the Debye length in GFET-based nucleic acids biosensing [216]. In comparison to flat graphene, these concave regions effectively allow counter ions to diffuse further away from the surface. Modulating local dielectrics reduces charge-screening effects and allows real-time sensing in high-ionic-strength physiological fluids without preprocessing.  
Figure 5: (a) \(\mathrm{I_{D}}\)–\(\mathrm{V_{DS}}\) characteristic of TGN-based electrochemical sensor for different pH values (5–9). (b) Conductance–voltage characteristic of TGN-based electrochemical sensor for different pH values (5–10). (Reprinted with permission from [257]. 2019, IET).  
* PEG-based hydrogels are especially effective at altering the local dielectric microenvironment [176], increasing the effective Debye screening length, which enables the precise identification of biomolecules in biofluids via GFETs. The silk fibroin (SF) hydrogel significantly enhanced the Debye screening length in the GFET biosensor by lowering the concentration of mobile ions near the graphene surface [260].
* The Debye screening length can be modified by controlling the PBS concentration. A GFET biosensor using a low concentration of PBS (0.001X) achieved a Debye screening length of approximately 24 nm [261]. This length was specifically chosen to adequately encompass the binding points between exosomes and antibodies on the sensor surface. The sensor incorporated a PBASE linker and antibodies, creating a dielectric layer of about 12 nm between the exosomes and graphene, which enabled the effective detection of negatively charged exosomes, evidenced by a positive shift in the I-V curve upon exosome introduction. The sensor demonstrated the capability to detect exosomes at concentrations as low as 0.1 ug/mL, confirming that the selected Debye screening length was appropriate for sensitive detection.
* Debye length limitation can be addressed by working within the constraints of Debye screening and sense within the electric double layer (EDL). A graphene-based chemiresistive platform monitored experimental screening lengths in aqueous solutions indirectly [262] and presented EDL components, including the Stern layer, diffuse layer, and bulk solution, with an emphasis on the influence of dissolved oxygen. Through electrostatic charge screening, cations in the Stern layer n-dope graphene are neutralised by the surface adsorption of dissolved oxygen. The experimental screening length increases with salt concentration, contrary to the expectations of Debye-Huckel theory.