Scientists are trying a new technique by stimulating the cortical neurons with natural light conjugated with QDs. This is a very promising technique, mostly because of its non-invasive properties, which do not include any genetically or chemical manipulation. It is proven that optically excited QDs can perturb the electrochemical equilibrium of a cell membrane . Aß aggregates in AD induce the process of depolarization of the cell membrane . By using CdSe QDs we could inhibit this event, because of their unique optoelectronic features mentioned above. QDs size, about 3-4 nm, is appropriate for crossing the cell membrane and it has also a protein range size. They can be activated and deactivated by turning on and off the excitation light .
Similar research was done in the field of motor dysfunctions in dementia. It was the first study to introduce a novel type of Aß-induced cytotoxicity. A reversible depolarization besides the influence of oligomers aggregates, originates from increased Na+-influx to muscle fibers trough TTX (Tetrodotoxin)-sensitive Na+-
channels . Aß concentration from 10-6
lead to slow significant and reversible depolarization of the skeletal muscle membrane in the resting potential state. These concentrations were derived from the experiment with the frog skeletal muscle fibers According to these findings, some scientists are referring to AD as a disorder of plasma membrane [10,13].
It is important to say that Aß-induced depolarization is concentration dependent and reversible .
Aß oligomers lead to hyperexcitability, due to the induced membrane depolarization and this hyperexcitability causes cognitive deficits in early and mid stages of AD. This state of neurons, influenced by the event of hyperexcitability, probably occurs before the cell death, that Ca-influx induces .
Together with the usage of quantum dot films it is possible to govern the opening/closing of ion channels, by which the events such as depolarization and hyper polarization can be controlled. Quantum dots provide us with better understanding of neural behavior, because the same cells function and communicate through electrical signaling. With the excitation of dots through light, we actually excite electrons inside the QDs. And if the voltage change of the membrane within this event is high enough, action potential is generated. Action potential is the signal of communication between neurons. This is the leading premise of all experiments made in the field of Photostimulation [12,14].
Excited QDs produce a temporary electric dipole. This dipole moment triggers an electric field and with enough strength it can stimulate the opening of ion channels. Dipole moment occurs because of the charge separation between the electron and hole in the exciton of the QD (Figure 3) .Figure 3:
Process of induced depolarization using quantum dots and light energy. The light excites electrons inside the quantum dots and because of that they are producing electric dipole moment, which opens the voltage-gated ion channel and lets the positively charged ions flow into the cell and change the action potential of the neural cell .
In one of these experiments, the mentioned statements above were confirmed, together with the usage of CdSe films and probes. Events such as hyperpolarization or depolarization were observed through patch clamp recording with the results of Na+
activated channels .
The delivery of QDs to the human brain tissue wouldn't be a problem, thanks to the possibility of the modification of QDs surface and attachment of many molecules that can selectively bind the dots on the specific neural cells . The selectivity here is a very important feature, due to the binding to the proteins and receptors on the surface of the cell. They are ideal for the changing of membrane action potential, because all the receptors and proteins on its surface are in a nanometer range scale .
Nanoparticles were firstly used only for imaging, but their unique optoelectronic features are far beyond this type of their usage. Today they are widely used in photovoltaics, lasers and light emitting diodes .
When it comes to the component of light and its delivery to the brain, the solution would be the retina because it naturally absorbs light .
In order to get the most convenient and satisfied results when going through this type of QD application, we have to come across few limitations: the strength of the dipole moment, the size of the nanocrystals and its separation distance from the ion channel .