利用共聚焦顯微鏡研究Oxazine1單分子在二氧化鈦(TiO2)奈米薄膜的動力學現象

Abstract

在單分子光譜（single molecule spectroscopy）探測技術發展以前，大多數的實驗是探測分子的平均效應，為一個整體所表現出的平均值(ensemble average)，此測量結果會損失掉因為個別環境差異所造成的資訊，而單分子探測可針對系統中的單個分子進行研究，能即時瞭解分子構象變化的資訊，或是探測環境對分子所造成的變異性。許多方式都可以用來研究單一分子，如原子力顯微鏡、近場光學顯微鏡等，然而，此種技術對研究分子有輕微的干擾，若使用光學的方式來探測，如:螢光，不僅對研究分子所造成的干擾最小，再加上螢光的靈敏度高，所以利用螢光方式來研究單分子，將比其他方式更準確可靠。因此本實驗利用共聚焦顯微鏡，藉由量測螢光的方式，來偵測單一分子。 本論文研究單分子Oxazine 1在二氧化鈦奈米粒子的動力學過程，發現當Oxazine 1吸附在二氧化鈦薄膜上時，此分子除了以放螢光與intersystem crossing到三重態再回到基態的方式外，其激發態與二氧化鈦的conduction band有相當程度的重疊，使得電子可以由激發態進入二氧化鈦的傳導帶，再以非放光形式回到基態。電子停留在二氧化鈦的時間經過計算後約為數百個毫秒(millisecond)，且不同分子間也因為環境不同而有所差異，這是在平均效應下所探測不到的訊息，經由單分子光譜的偵測可以很輕易地被解析出來。

For single molecule spectroscopy (SMS), the molecules are typically studied one at a time by focusing the laser to a diffraction-limited spot and centering the molecule of interest at the laser focal volume. The fluorescence from the molecule is collected, the intensity, and spectrum can be studied for each particular molecule. Notch and band-pass filters are commonly used to prevent the excitation light from reaching the detector. The main advantage of SMS is the ability to explore of heterogeneity in complex materials (like polymer films and glasses) as well as direct observation of dynamical state changes arising from photophysics and photochemistry. Here we describe a detailed investigation of the fluorescence intensity dynamics before photobleaching. We report on single-molecule studies of photosensitized interfacial electron transfer process in Oxazine 1 - TiO2 nanoparticles (NPs) system. We characterized the triplet-state blinking dynamics and electron transfer dynamics by analyzing the autocorrelation functions. The blinking time due to the triplet state is clearly distinguished from the fluorescence intensity fluctuation time of subseconds to seconds due to electron transfer (ET) process. We observed that the interfacial ET of single molecules on the surface of TiO2 NPs are statically inhomogeneous, varying the rate of the ET reactivity fluctuations from molecule to molecule. Furthermore, dynamic inhomogeneity is associated with the ET fluctuations from time to time for the same individual molecule. These inhomogeneities may be attributed to the difference of Franck-Condon coupling and molecule-surface vibronic coupling caused by different environment.