5.2 Interactions between Graphene and Cell membrane
5.2.1 Introduction
and depth levels. Upon examining the details of the photocurrent mapping and the corre- sponding fluorescent image, we can conclude that only the soma and axons can generate action potentials. However, numerous photocurrent signals occurred in the region without an apparent retina structure on top of it. What could be the cause of these photocurrent signals beside the action potentials generated by the RGCs? One possible explanation is that the cell membrane with a weak charge can contribute charges to the graphene and be detected by the photocurrent, which detects local charge changes. Additionally, the mouse strain used in our measurements is Thy1-YFP, and only the RGCs are expressed under green fluorescence. Therefore, the photocurrent signal detected may also originate from other types of cells in the retina, such as muller cells. These cells support and pro- tect neurons in the retina, maintain the correct balance of ions and neurotransmitters in the extracellular space, and buffer changes in pH while regulating glucose uptake and re- lease [139]. Muller cells have been shown to generate electrical signals in response to various stimuli, including light, neurotransmitters, and changes in extracellular potassium concentration [140; 141; 142].
5.2 Interactions between Graphene and Cell membrane
Figure 5.3: (a) Scanning photocurrent image of a typical G-FET before the transfer of the retina on top of the device. (b) Corresponding photocurrent result when the retina is on top of the device and with the negative pressure applied to the microfluidic channel to enhance the physical contact between G-FET and graphene ribbon for 5 mins and (c) after the photocurrent signal reaches a stable state. (d), (e), (f) The corresponding graphene band structure to the photocurrent mapping. (g) The fluorescent image of a retina on top of a G- FET device. The scaler bar is 20µm. (h) Gate-dependent transport curve before the retina with the Dirac point of 0.15 V. (i) The Dirac point G-FET shifts to -0.06 V after the retina on the device.
tivity and biocompatibility, making it sensitive to charge changes in the electrochemical environment [43]. By examining the behavior of individual cells in terms of charge-based interactions, we can gain new perspectives.
The primary approach to investigating individual living cells involves the utilization of antibodies (as depicted in Figure 5.4a). This technique is dependent on the high binding affinity that exists between an antibody and its corresponding antigen [48; 145; 146; 147], which can be used as the recognition elements to detect specific biomolecules of interest.
To use a graphene-based biosensor with the coating of antigen and antibody, the surface of the graphene is first functionalized with a linker molecule that can attach to the antigen or antibody. When a sample containing the complementary biomolecule is introduced to the biosensor, the antigen or antibody will bind to the immobilized biomolecule, resulting in a change in the electrical properties of the graphene. However, these changes in con- ductance are mainly due to interactions between the antibody and antigen, rather than the cell membrane itself. Another challenge in studying individual cells is the manipulation of their movement. Micropipettes or PDMS-based manipulators (Figure 5.4b) have been used to change the location of cells, but these techniques can be cumbersome and difficult to control [47; 148]. An optical tweezer, which uses a highly focused laser beam, pro- vides a non-invasive method to manipulate the cell [149; 150]. It allows for precise control and manipulation of small particles, including individual cells. The optical tweezer setup is a widely used tool in biophysics and biological research for the study of physical and mechanical properties of cells and biomolecules [151].
The optical tweezer is a tool used in scientific research that utilizes a tightly focused laser beam and a high numerical aperture objective lens to manipulate small objects. When an object is placed in the light field created by the laser beam, it experiences two types of forces. The first is the scattering force, which is proportional to the intensity of the light and acts in the direction of the incident light. The second is the gradient force, which is induced by the uneven light field and has a direction along the gradient of the light intensity.
Figure 5.4: 1 (a) Graphene-based biosensor with the employment of antibody and antigen.
This method relies on the strong binding affinity between an antibody and its corresponding antigen. By coating graphene field-effect transistors with antibodies, it is possible to detect changes in electrical signals induced by the binding reaction between a specific antigen and the antibody. (b) A PDMS-based manipulator is used to control the movement of the target cell to a graphene-based sensor.
As a result, the dominant gradient force will push the object towards the position with the strongest light intensity (Figure 5.5). By carefully controlling the intensity and position of the laser beam, researchers can use the optical tweezer to manipulate and study small objects, such as cells, with great precision.
The forces exerted by the OT system are typically in the piconewton range, which is approximately the gravitational force experienced by a red blood cell on Earth. These forces are too small to be felt by hand, but they are sufficient to manipulate many biological objects. In addition to exerting force, the OT system can also be used to measure force.
The system behaves like a spring, with the trapped particle being pulled back towards the center when it is displaced. The force can be determined using Hooke’s law, where F is the force,α is the stiffness of the system, and x is the displacement of the trapped particle.
The displacement of the particle can be detected using a position-sensitive detector (PSD), which records the lateral direction of the particle by detecting the scattering light collected by an objective lens. The stiffness of the OT system can be calibrated using Brownian noise as a test force:
−γx˙=F(t)−α∆x (5.1) where γ is the drag coefficien of the bead. According to the Fourier transformation, the power spectrum can be described as:
S= kBT
π2β(fo2+f2) (5.2)
where fo= 2π βα , and the stiffness of the optical tweezer can be extracted asα=2πfoβ. This allows researchers to accurately measure the forces exerted by the system and to study the effects of different forces on the particles being manipulated. Overall, the optical tweezer is a valuable tool for studying small-scale phenomena in a variety of scientific fields.
Figure 5.5: (a) Schematic of optical tweezer setup, which is equipped with a 1064 nm laser source, a pair of water immersion objectives, a position sensitive detector and so on. (b) Schematic of the mechanism of optical trapping when a particle is exposed to the uneven and highly focused laser beam.
Here we apply a graphene multi-ribbon transistor (gMRFET) in combined with an op- tical tweezer system to explore the interactions between graphene and cell membrane.
Furthermore, we investigate the graphene-cell interactions from the electrical and opto-
electronic measurements by integrating with scanning photocurrent microscopy. This fun- damental research can not only provide a way to study individual cells but also help to understand the mechanisms of 2D material-based biosensors.