Spatially and spectrally resolved plasmonic fluorescence enhancement

尺寸小于100纳米（1/10000毫米）的银和金颗粒，大约是绿光波长的1/5，与较大的这些金属块表现出非常不同的光学特性。特别是，这些纳米颗粒可以非常有效地吸收和散射电磁波谱中可见部分特定波长的光，这取决于纳米颗粒的大小、形状和材料。这是金属中电子振荡的共振激发的结果，被称为局域表面等离子体，它在纳米粒子周围产生非常强烈的电磁场。近年来，制造形状和尺寸明确的金属纳米颗粒的能力催生了等离子体学的蓬勃发展，等离子体学利用了这些材料的新型光学特性。在现代生物学和生物医学中，使用荧光标记（或“荧光团”）作为特定生物分子的标记或标记特定过程的发生已成为标准做法。这些标签是分子或纳米颗粒，在照射时发出明确颜色的光，广泛用于成像，从整个生物体到单个细胞和分子，在DNA测序，药物发现和生物传感器中。本项目研究了当荧光团与银或金纳米粒子非常接近（小于100纳米）时，荧光团的发射被显著改变的方式。观察到的主要影响是荧光强度的增强和荧光团量子效率的增强（即它发射的光子数与它吸收的光子数的比率）。这种效应具有高灵敏度荧光检测的潜力，可以检测到更低的分子浓度。然而，为了为新技术应用的开发提供信息，我们需要首先了解发生的基本物理现象！特别是，我们将在广泛的激发和发射波长范围内进行测量，以将金属纳米颗粒的局部表面等离子体共振光谱的行为变化联系起来。我们还将详细研究荧光发射对金属纳米颗粒和荧光团的纳米分离的依赖。为了做到这一点，我们将先进的光学显微镜技术与扫描探针显微镜相结合（在扫描探针显微镜中，一根锋利的针绘制出一个表面）来提供完整的定位信息。

Silver and gold particles with dimensions less than 100 nanometres (1/10000th of a millimetre), which is about 1/5th of the wave-length of green light, exhibit very different optical properties from larger pieces of these metals. In particular, these nanoparticles can absorb and scatter light very efficiently at specific wavelengths in the visible part of the electromagnetic spectrum, which depend on the size, shape and material of the nanoparticle. This occurs as a result of the resonant excitation of an oscillation of the electrons in the metal, called a localised surface plasmon, which results in very intense electromagnetic field around the nanoparticle. The ability in recent years to fabricate metal nanoparticles of well-defined shape and size has spawned the burgeoning area of plasmonics, which exploits the novel optical properties of these materials.In modern biology and biomedicine it has now become standard practice to use fluorescent tags (or 'fluorophores') which are used as labels for particular biological molecules, or to flag the occurrence of specific processes. These tags are molecules or nanoparticles which emit light of a well-defined colour when illuminated and are employed extensively in imaging, from the level of whole organisms down to individual cells and molecules, in DNA sequencing, in drug discovery and in biosensors. This project is concerned with a study of the ways in which the emission from fluorophores is modified significantly, when a fluorophore is in close proximity (less than 100 nm) to a silver or gold nanoparticle. The main effects observed are an enhancement of the fluorescence intensity and an enhancement in the quantum efficiency of a fluorophore (i.e. the ratio of the number of photons it emits, to the number it absorbs). Such effects have potential for high sensitivity fluorescence detection, enabling detection to much lower molecular concentrations. However, to inform exploitation for new technological applications, we need to first understand the basic physics of what happening! In particular, we will perform measurements over a wide range of excitation and emission wavelengths, to tie in changes in behaviour with the localised surface plasmon resonance spectrum of the metal nanoparticles. We will also investigate in detail the dependence of fluorescence emission on the nanometric separation of the metal nanoparticle and the fluorophore. To do this, we will combine advance optical microscopy techniques with scanning probe microscopy (in which a sharp needle maps out a surface) to provide full positioning information.