Development of Semiconductor-Based Membrane Potential Nanosensors For Neuron Research
Visualizing neural activities is an important step towards studying and understanding the human brain. Since neural signals are transmitted through modulations of their membrane potentials, a membrane potential sensor capable of translating such signals into experimental observables is essential for recording neural signals. Moreover, the capability of observing membrane potential in the functional structures of a neuron, which can be as small as femto-liters in volume, is crucial for studying important brain functions such as memory. For this reason, we developed inorganic nanosensors based on semiconductor nanorods as membrane potential sensors. These nanorods, which operate via the quantum confined Stark effect, display large voltage sensitivities by changes in fluorescence intensity, spectra and lifetime, allowing non-invasive observation of minute fluctuations in the membrane of live cells. The extreme brightness of these nanorods also allow single particle recordings, which will enable studies of membrane potentials in tiny neuronal structure such as synapses. In this thesis, we will first introduce the background, review the existing membrane potential sensors and introduce the relevant literatures regarding solid-state membrane potential sensors. For our development, we will first describe the development of a spectrally-resolved microscope for measuring the spectra of single nanoparticles in Chapter 2. Then, the characterizations of the quantum confined Stark effect in a variety of nanoparticles will be described in Chapter 3. The temporal response, long-term stability and the capability of recording electric field modulation at 1 kHz using a single nanorod will be reported and discussed. In Chapter 4, we will demonstrate a surface functionalization approach utilizing designed alpha-helical peptides and zwitterionic ligands for facilitating insertion of the nanorods into lipid membranes. These functionalized nanorods were shown to spontaneously insert into cell membranes and report membrane potential in live cells. The detailed characterization of the functionalized nanorods in the membrane and the capability of recording the membrane potential using a single functionalized nanorod will be demonstrated in Chapter 5.