螢光鑽石之光學檢測及效能增益研究

Abstract

螢光奈米鑽石(FND)，其螢光特色是寬波段且非常穩定，不會有漂白和光爍的現象，非常適合於長時間的觀測研究極具有類似的光譜。FND的製作方法，首先是將含有微量氮雜質的人工奈米鑽石當原料，再將其置於真空環境中以高能加速器產生的電子(貝他射線)、離子或質子束進行轟擊，於奈米鑽石內部生成晶格空缺(V)，最後以攝氏八百度的高溫促使內部空缺進行移動，加熱約1 ~ 2小時使得空缺(V)與鑽石晶體中之氮原子(N)結合後形成氮-空缺中心(NV)，再進行最後的退火。NV以帶電荷的形式分為二類:一類是中性的NV，其能隙為2.156電子伏特;另一類是帶負電性的NV，其能隙為1.945電子伏特。當含有NV的FND受到黃綠色光照射時，擁有NV的FND會發出波長為550奈米–900 奈米螢光。 FND有個缺點:其螢光的強度會隨尺寸變小而驟減。回顧FND在高溫加熱二小時的製作過程，靠近FND表面的空缺(Vs)在移動過程中，若不幸的沒有被鄰近的氮原子(Ns)所捕捉生成氮-空缺中心(NVs)，遷移到表面的空缺就會形成表面孔洞而消失，根據先前文獻研究螢光鑽石表面36點的NVs深度數據，在此論文中進而提出有效NV區間的假設：假設在不同尺寸的FND，其粒子的表面至表面底下深度達D的殼層區間是都不易形成NVs，是NV空乏區間；基於此假設:距離表面D的NV 空乏區間，佔小尺寸FND之體積比例相較於佔大尺寸FND的體積比例會大許多，於此就可以解釋:小尺度FND之螢光的強度驟減的原因。因此，我們在本論文推導了一個有效NV區間的方程式，若假定FND其NV空乏區的深度D距離表面殼層為10奈米，經此有效NV區間的方程式的推導，100奈米尺寸的FND之有效區間為51.2%;依據推論當球型螢光鑽石的直徑大到1微米，其有效區間可高達94.1%。 近年來，科學家利用金屬表面侷域電漿共振方式，將周遭的電場聚集於表面間產生侷域熱點，並很成功的運用產生熱點來增強二維材料在侷域間的螢光或拉曼訊號，但是對於尺度較高的的立體三維FND，螢光增益的幅度很有限，這與本論文的有效NV區間假設是相互呼應的，因為三維FND之有效NV區間是在FND內部而非表面，表面侷域電漿所產生的熱點很侷域且涵蓋範圍有限，因此無法有效對於直徑100奈米的FND之螢光進行增益。於此，本論文的第一部分，重點在於有效增益FND之螢光強度：因為FND具有寬的放光波段，我們選擇具有高涵蓋率的金屬/介電材料共振腔結構之二維材料，來進行FND之螢光增益，經由實驗結果，我們成功地藉由調整介電材料的厚度，來調控表面空間電場之位置，有效增強FND之有效NV區間之電場，來增益其螢光強度。於實驗過程中我們發現:增加介電材料的厚度，不僅能增益螢光強度，還能使FND所放出的螢光產生紅位移。經由實驗結果，我們證實532奈米波段光源會激發中性NV發光，而影響螢光效率；若採用633奈米的雷射光源，並結合適合的金屬/介電材料共振腔結構，能有效將帶有負電荷NV的螢光增益並不改變其光譜形貌，將100奈米的FND之螢光增益值達到目前最高的11.2倍，是過往採用表面侷域電漿共振的技術所不能達到的。 在過去十年，螢光鑽石的研究在生物體系中確實扮演重要的角色，然而研究都在僅侷限於奈米尺度的FND，對於具有高含量NVs且會發出極明亮螢光的微米尺寸之螢光微米鑽石(FMD)，少有相關的研究報告。本論文的第二部分，利用非破壞性且侷域檢測的方式來進行FMD的研究:在此實驗中我們發現，使用一般的532奈米光源，可以使微米鑽石(MD)在同一張光譜能產生螢光與拉曼的訊號，於此可以得到NV的相對密度；但是對於維米尺度螢光鑽石FMD僅產生非常強的螢光，並不易取得的拉曼訊號了；偶然間發現，當我們使用785奈米的光源，以二次光子來激發FMD內的NVs，結果在同一張光譜能同時產生FMD之螢光與拉曼訊號，而沒有經過螢光鑽石製程的MD，只會有拉曼訊號而沒有螢光;基於此光學特性，我們將FMD拉曼訊號強度固定來偵測其螢光強度，用此來代表FMD侷域間被激發的NV之相對數量，亦是此侷域間的相對NV密度。此非破壞性光學檢測方法能快速檢測FMD品質。 本論文的第三部分是FMD的應用；<應用一>；運用FMD具有很明亮的螢光，當我們於深度達2–3釐米的雞肉組織中，置入二顆FMD且相距約200微米，並採用637奈米的光源來照射雞肉組織，仍可獲得具有相當辨識度的螢光訊號，這訊號可用來作為生物的淺層組織之標定；<應用二>: 運用鑽石有極高的導熱係數，我們將FMD表面進行中空金奈米粒子(HGN)的修飾，此HGN-FMD複合材料在785奈米、2.4 W/cm2的光源密度下，僅十分之一覆蓋率的HGNs即能吸收大部分的光能並轉換成熱能、而且在此雷射光源的激發下30分鐘，螢光強度沒有衰退，HGN-FMD複合材料優異的再現性與次釐米的解析度，很適合來進行生物的組織實驗。當我們在雞肉組織下深度2釐米處植入5顆HGN-FMDs，在785nm 光源下、我們再用熱像儀來量測雞肉組織表面溫度，並搭配COMSOL溫度模擬軟體的估算，於此深度的雞肉組織內可以產生20度以上的溫差，此HGN-FMD的優異光轉熱效應，有利於未來在淺層肌膚下之熱療及監測應用。

Fluorescent nanodiamond (FND) possesses nitrogen vacancy centers NVs, can fluoresces in broad wavelength in the regime from 550 nm to 900 nm after excited by a green yellow light. The fluorescence generated from the NVs of the FND is very suitable for long time observation because it is stable and immune to both photo-bleaching and photo-blinking issues. Fluorescent diamond (FD) fabrication contains three processes. Firstly, the type Ib diamond containing 100 to 200 ppm nitrogen atoms is widely used to be the starting material. Secondly, it is through high energy electron (e−, beta particle), proton (H+) or helium (He+) irradiation to create vacancies inside diamonds. Finally, an annealing process at typically elevated 800 oC for two hours is applied to facilitate the created mobile vacancies to be effectively captured by neighboring nitrogen atoms to perform nitrogen vacancy centers (NVs). In addition, the NVs can be classified into a neutral type NV0 with the energy gap 2.156 eV, or a negatively charged type NV− with the energy gap 1.945 eV. Furthermore, the artificial FND, In recent studies, the fluorescence intensity of the fluorescent diamonds always drops dramatically as the size of the fluorescent diamonds is decreased, especially in nanometer scale. Actually, the vacancies embed carbon lattices site will migrate during the vacuum annealing process of the FND fabrication. Not every vacancy in the outermost diamond shell layer will be captured by the neighboring nitrogen atoms to perform the NV. Once the migratory vacancy reaches to the diamond surface, the migratory vacancy will become a surface void or a part of ambient in the air. In this dissertation, we are the first one to propose an effective NV volume ratio (Veff) of FND hypothesis. If the outermost shell layer of the FND in various sizes is lack of NVs and this shell layer has a thickness D from its surface, the fluorescence intensity of the FND would drop dramatically as the FND size is decreased. In this dissertation, an equation is developed to perform the Veff of the FND in various sizes. If the thickness D of the FND is 10 nm, the Veff of 100 nm FND is 51.2%. When the FD diameter is 1 µm, the Veff would be up to 94.1%. In the past decades, localized surface plasmon resonance (LSPR) has been widely employed to enhance Raman signals or imaging contrast. LSPR is a powerful technique for concentrating an electromagnetic field at localized hot spots; the generated hot spots are frequently used for both metal-enhanced fluorescence (MEF) and surface-enhanced Raman scattering (SERS). However, the coverage of the MEF method is not larger enough to cover the three dimensional FND particle, and the enhancement of the fluorescence intensity is limited. This is coherent with our Veff of the FND hypothesis. Because a simple two–layer structural nanocavity is promising for easily available, enlarging the coverage, and manipulating the space electric-field power density above the nanocavity. The first part of this dissertation, we present a simple nanocavity structure that provide a large region for efficient enhancement of fluorescence that can cover most 100–nm FNDs. By tuning the thickness of the capping SiO2 layer of the Al/SiO2 nanocavities, the distributions of both the spatial and spectral electric field intensities of the FNDs could be controlled and manipulated. To enhance the fluorescence intensity from the NV− centers of the FNDs, we designed an Al/70–nm SiO2 nanocavity to function at excitation and emission wavelengths of 633 and 710 nm, respectively, allowing the NV− centers to be excited efficiently; as a result, we achieved an enhancement in fluorescence intensity of 11.2–fold. In the past decades, FND has been widely investigated and applied in the optical and biological field. Comparing to FND, there are few studies about the brilliant FMDs. The second part of this dissertation, we adopt a non-destructive optical method to inspect the localized quality of an individual FMD particle. While using a 532–nm excitation laser source, the fluorescence intensity of FMD is tremendous and it’s hard to distinguish fluorescence and Raman signals simultaneously. Occasionally, we adopted a 785–nm laser to excite the NVs− abundant FMD, and then first obtained both the broadening fluorescent signals in the regime from 620 to 900 nm as well as diamond Raman peak at 1332 cm−1 in one spectrum. To address this issue with precision, in this study, we first utilized the fluorescence intensity of the FMD to reference its’ Raman peak at 1332–cm−1 intensity, and the localized surface fluorescent quality of an individual FMD could be fast inspected. This dissertation also characterized the optical properties of the brilliant fluorescent microdiamonds (FMDs, diameter ~ 400 µm) and explored their potential use as fiducial markers for image-guided photothermal therapy. Although, the strong light scattering generated by the tissue with incident light may result in the image of FMD blurred, and deteriorated markedly with the increase of the phantom thickness. We first applied the FMDs for deep-tissue imaging with a 637–nm laser for the excitation and obtained spatially resolved fluorescence images of the individual particles in chicken breast tissue of ~3 mm in thickness. To well use the superior thermal conductivity property of diamond for image-guided therapy, the surface-functionalized FMDs were then decorated with hollow gold nanoparticles (HGNs). HGNs performed outstanding light-to-heat conversion. Through Mie theory calculation, one–tenth quantities of HGNs could absorb most of the 798-nm light. Through SEM inspection, up to 4 × 107 HGNs were estimated to be attached to a single FMD particle. A temperature rises of more than 20 °C was achievable when five HGN-FMDs particles were embedded 2 mm deep in chicken breast and irradiated by a 785–nm laser at a power density of 2.4 W/cm2. No fluorescence decay was observed for the FMDs over 30–min excitation, allowing repetitive and precise localization of the hybrid photothermal agents in tissue with submillimeter resolution. Our results highlight the promising use of NV-containing diamond microcrystals in conjugation with gold nanoparticles for local hyperthermia and inspection applications.