Construction Of Bacteriorhodopsin/rare-earth Upconversion Nanoparticles Bionanosystem And Its NIR-triggered Photoelectrical Response

Bacteriorhodopsin (bR) is a retinal-containing chromo-protein that is embedded in purple membrane (PM) of Halobacterium salinaru. As a light-sensitive protein it could create a light-driven pH gradient between intracellular and extracellular parts of the bacteria under an appropriate wavelength light irradiation. Since it was found in 1971, extensive research works have been carried out to investigate the functions of bR. Among them, the photoelectrical response caused by the light-driven proton movement attracts tremendous interests of the researchers. Because bR is only sensitive to the visible light, all research works so far utilize visible light as irradiation source (-480 nm or -570 nm) to trigger its photoelectrical responses. Infra-red (IR) is an important part of the solar electromagnetic radiation that reaches the earth. Therefore, the limitation of bR’s sensitive wavelength greatly hinders its applications as a solar energy conversion unit.In order to achieve IR-triggered photoelectrical response upconversion nanoparticles (UCNPs) were integrated with bR to fabricate a bR/UCNPs photoelectrical system. Moreover, two types of UCNPs with blue and green emissions were combined with bR to explore the mode change of bR based photoelectrical signals. The following parts are contained in the thesis:(1) Culture of Halobacterial halobium S9 strain and isolation of bR. The Halobacterial halobium S9 strain was cultured for the isolation of purple membrane. A typical process including bacteria harvesting, enzymatic hydrolysis, dialysis, membrane meshing, impurity removement, washing and sucrose density gradient centrifugation was conducted to collect the bR-embedded purple membrane. The UV-visible spectrum and atomic force micrograph show the good quality of bR, which can meet the requirement of photoelectrical experiments. The concentration of the isolated bR is calculated to be 1.65 mg/mL based on the Lambert-Beer’s law.(2) Fabrication of a UCNPs-bR integrated system to achieve IR-triggered photoelectrical responses. Water-soluble polyethylenimine modified NaYF4:Yb,Er nanocrystals were synthesized with a one-pot hydrothermal method. Field emission-scanning electron microscope (FESEM) and X-ray powder diffraction (XRD) results indicate the particular shape and excellent crystalline structure of the materials, respectively. More importantly, the emission of the UCNPs matches the absorption of bR very well. A photoelectrical system containing a back-to-back stacked bR/ITO glass electrode/IR cut filter/UCNPs working electrode, a U-shape electrolyte loading chamber and a platinum counter electrode was fabricated to monitor the IR-triggered electrical signals. Transient spikes from the bR photocycle are triggered with IR irradiation for the first time by using the system. The re-absorption of IR-excited UCNPs luminesence by bR molecules may be the mechanism of this phenomenon.(3) Combination of blue and green emission UCNPs with bR to change the pulse mode of its photoelectrical signal. By adjusting the doping elements, the emission of UCNPs were tuned into blue (wavelength range of 450-500 nm) and green (wavelength range of 520-580 nm) wavelength range. FESEM and XRD data indicate the uniform hexagonal and high crystallinity structure. The prepared UCNPs were mixed with bR in an appropriate proportion to form a composite layer. Under an IR exposure the UCNPs in the composite can emit bright green and blue light simultaneously. The former one drives bR to start the photocycle and transform from ground state to the intermediate state "M", while the latter one can promote the transformation of "M" state back to the ground state. During the proton pumping and capture process a sustainable electrochemical gradient change is obtained, leading to a stationary photocurrent signal. The IR-triggered stationary photocurrent signal is obtained for the first time in a bR based system.In summary, UCNPs are integrated with bR to achieve the IR-triggered photoelectrical response for the first time. Moreover, two types of UCNPs with blue and green emissions were combined with bR to change its photoelectrical current from transient spike to stationary mode. The present studies may open new horizon for the applications of both bR and UCNPs.