吾人合成邊長為45 nm的奈米金方，並利用二氧化矽作為間隙物(spacer)，並以螢光素(Fluorescein, FITC)作為染料接在二氧化矽殼層的外圍，再藉由控制二氧化矽的厚度，定義染料與奈米金方之間的距離，吾人總共合成五種厚度之包覆二氧化矽的奈米金方，厚度(Ls/LA)分別為12/9、23/20、30/27、36/34、40/37，並利用自架式的靜態螢光光譜及時間解析之螢光光譜法研究中性及鹼性下奈米金方存在時染料之增強倍數(enhancement factor)及螢光生命期的改變。而由靜態螢光光譜可知，隨染料與奈米金方之間距離增加，其增強倍數也會隨之減少；由時間解析的螢光光譜可知，當染料與奈米金方之間距離下降，染料之螢光生命週期也會隨之減少。由實驗得到樣品之增強倍數與螢光生命期後，吾人利用本實驗組林等人所建立的動力學模型:在染料由基態被激發至激發態濃度設為([FiTC*, GNC]ex)之後，在激發態之染料可藉由以下三種路徑將能量釋出，在激發態之染料可直接藉由放出螢光(kr,m)或者以非放光(knr)的形式將能量釋出，此為第一種路徑，將此路徑之速率常數總和定義為k1，即k1 = kr,m + knr，而除了放光至基態之外，激發態之染料也可以將能量傳遞至奈米金方的明亮模式(dipole mode, 亦為bright mode , [FiTC , GNC*(bright)])，此過程之能量傳遞速率為k2，在將能量傳遞至明亮模式的奈米金方後，能量也可能回傳至激發態之染料，此過程之速率為k－2，除了回傳至激發態染料之外，能量也可藉由放光的形式將光子傳遞至遠場，此過程之能量傳遞速率為k3，但若是染料將能量傳遞至奈米金方的暗模式(dark mode, [FiTC , GNC*(dark)])，則能量不會以放光的形式釋出，而是以熱即聲子－聲子(phonon－phonon coupling)耦合等非放光的路徑將能量釋出，而能量由激發態之染料傳遞至暗模式的速率為k4，在藉由動力學模型解出鹼性下k1 = 4.5–0.77  109 s–1, k2 = 4.0–2.3  109 s–1, k-2 = 1.0–0.80  109 s–1,k4 = 20–2  109 s–1；中性下k1 = 5.1–0.84  109 s–1, k2 = 5.3–2.1  109 s–1, k-2 = 1.1–0.91  109 s–1,k4 = 29–2.3  109，比較各個速率常數可以看到k4相較於其他速率常數大，吾人認為這是因為奈米金方之表面電漿共振峰和染料分子放光峰之重疊程度高，造成奈米金方與染料之間能量傳遞速率快，且由於吾人合成之奈米金方邊長僅有45 nm，因此螢光增強效應較小，而在各速率常數中除k-2之大小，其他速率常數皆隨二氧化矽殼層厚度之上升而下降，造成此趨勢不符合預期的原因，吾人認為是因為當二氧化矽厚度上升時，染料與奈米金屬粒子間之能量傳遞速率下降，導致k3無法被忽略，且相較奈米金球，由於奈米金方之尖端也會與染料進行作用，因此即使在染料與奈米金方之間的距離到達40 nm仍可將能量傳遞至奈米金方之明亮模式。

We study the effects of metal-enhanced fluorescence (MEF) on fluorescein dye molecule by gold nanocubes. The silica shell was used as spacer to define the distance between fluorescein and gold nanocube (AuNC). Self-build steady state fluorescence spectroscopy and time-correlated single photon counting (TSCPC) technique were used to detect the weak intensities and interaction between gold nanocube and dye. AuNCs length 40-50 nm were synthesized with different thicknesses of silica shells. As the thickness of silica shell decreased, the enhancement factor increased due to the strong induced electric field by the nanoparticle. Besides, the fluorescence decay curves showed biexponential decay, indicating multiple pathways for relaxation of the excited-state fluorophore. We derived a kinetic model to explain the biexponential behavior to solve the rate constants and to interpret the processes of energy transfer. We obtained the rate constant k1 = 4.5–0.77  109 s–1, k2 = 4.0–2.3  109 s–1, k-2 = 1.0–0.80  109 s–1, and k4 = 20–2  109 s–1 in basic solution and k1 = 5.1–0.84  109 s–1, k2 = 5.3–2.1  109 s–1, k-2 = 1.1–0.91  109 s–1, and k4 = 29–2.3  109 s–1 in neutral solution. The values of k4 were the greatest among the other rate constants because the surface plasma resonance of the gold nanocubes and the emission of the dye molecule have a high degree of overlap resulting in rapid energy transfer rate between dye and AuNC. Since the side lengths of the nanocubes were only 45 nm, the fluorescence enhancement effect was small. All rate constants decreased with the increase in the thickness of the silica shell except for k-2. The energy transfer rate between the dye and nanoparticles decreased with distance, hence, the rate constant k-2 from nanocubes back to FITC became small. As a result, the radiation from nanocube k3 cannot be ignored at large distances. Compared with gold nanospheres, the tips of the AuNC have sharp edge electric field that interacts with the dye. Therefore, even if the distance between the dye and the nano-gold cube reaches 40 nm, the energy can be transferred to the bright modes of AuNC.

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