Hansen Matthew, Clogston Jeffrey D.
Nanotechnology Characterization Lab, Cancer Research Technology Program, Frederick National Laboratory for Cancer Research sponsored by the National Cancer Institute, Frederick, MD 21702
This protocol describes how to measure the size and concentration of individual metallic nanoparticles using ICP-MS (inductively coupled plasma - mass spectrometry) in single particle (SP) mode. Accurately determining the size of individual nanoparticles on a per particle basis both quickly and accurately is an ever-increasing need within nanoparticle characterization. ICP-MS is capable of measuring a broad range of nanoparticle sizes with high resolution, thus allowing the measurement of multiple particle populations for the quality assessment of nanoformulations. Additionally, SP-ICP-MS can accurately determine particle concentrations without the need for concentration standards. ICP-MS is an ultrasensitive technique which can determine the concentration of trace elements in solution down to the pg/g level. Delguard et al first demonstrated in 2006 that ICP-MS could accurately measure the mass of individual metallic nanoparticles in what has come to be known as SP-ICP-MS [1]. By introducing a nanoparticle solution into the ICP, the individual nanoparticles are ionized into an ion cloud which then is carried as an intact unit to the detector. After a brief rapid expansion of the ion cloud, the duration of this cloud, defined as the time it first hits the detector to the time the last ion hits, is approximately 100-650 μs for 30-150 nm particles, respectively. This allows for two methods for measuring nanoparticles in SP mode, both of which will be outlined in the protocols below. In method 1, a large integration time (10 ms) with respect to the duration of the ion cloud is used in combination with a sufficiently dilute solution to measure individual nanoparticles. Since the integration time is much larger than the particle cloud duration, this method measures the entire particle cloud as a single event. Due to this, it is extremely important that the nanoparticle solution is sufficiently dilute to prevent multiple particles from hitting the detector during the integration window. This method has the advantage of being usable for many modern ICP-MS instruments that do not have a dedicated single particle counting mode since most of these instruments are capable of integrations times as low as 10 ms. Additionally, the long integration times allow for a longer time period for the detector to recover after a nanoparticle event, leading to a lower occurrence of detector saturation and small particle events. Method 2 uses a shorter integration time than the duration of the ion cloud which allows for the measurement of nanoparticle events by rastering multiple points across the detection event. The 50 μs integration time is generally available in instruments with a dedicated single particle mode and provides a few advantages over 10 ms integration measurements. The faster integration time allows for the collection of more information than the 10 ms integration which is limited to the total or summed intensity of the nanoparticle event. Three types of information are available due to the ability to raster across the peak. Like 10 ms integrations, the total or summed intensity is available, however, also available are the max height intensity of the peak and the duration of the ion cloud. Additionally, due to the faster integration time, samples about ten times higher in concentration than the 10 ms method can be accurately analyzed simplifying the sample preparation process.
本方案描述了如何在单颗粒（SP）模式下使用电感耦合等离子体质谱仪（ICP-MS）测量单个金属纳米颗粒的尺寸和浓度。在纳米颗粒表征领域，快速且准确地逐个确定单个纳米颗粒的尺寸需求日益增长。ICP-MS能够高分辨率地测量广泛的纳米颗粒尺寸，从而能够测量多种颗粒群体以进行纳米制剂的质量评估。此外，单颗粒ICP-MS无需浓度标准即可准确测定颗粒浓度。ICP-MS是一种超灵敏技术，能够测定溶液中低至pg/g水平的微量元素浓度。德尔加德等人在2006年首次证明，ICP-MS能够在所谓的单颗粒ICP-MS中准确测量单个金属纳米颗粒的质量[1]。通过将纳米颗粒溶液引入ICP，单个纳米颗粒被电离成离子云，然后作为一个完整单元被输送到检测器。离子云经过短暂快速膨胀后，该云的持续时间（定义为第一个离子撞击检测器到最后一个离子撞击的时间）对于30 - 150 nm的颗粒分别约为100 - 650 μs。这使得在SP模式下有两种测量纳米颗粒的方法，以下方案中将对这两种方法进行概述。在方法1中，相对于离子云的持续时间使用较长的积分时间（10 ms），并结合足够稀释的溶液来测量单个纳米颗粒。由于积分时间远大于颗粒云的持续时间，此方法将整个颗粒云作为单个事件进行测量。因此，纳米颗粒溶液充分稀释以防止在积分窗口期间多个颗粒撞击检测器极为重要。该方法的优点是可用于许多没有专用单颗粒计数模式的现代ICP-MS仪器，因为这些仪器中的大多数能够进行低至10 ms的积分时间。此外，较长的积分时间使检测器在纳米颗粒事件后有更长的恢复时间，从而降低检测器饱和和小颗粒事件的发生率。方法2使用比离子云持续时间更短的积分时间，通过在检测事件上扫描多个点来测量纳米颗粒事件。50 μs的积分时间通常在具有专用单颗粒模式的仪器中可用，并且与10 ms积分测量相比具有一些优势。更快的积分时间允许收集比10 ms积分更多的信息，后者仅限于纳米颗粒事件的总强度或求和强度。由于能够在峰上扫描，可获得三种类型的信息。与10 ms积分一样，可以获得总强度或求和强度，但是还可以获得峰的最大高度强度和离子云的持续时间。此外，由于积分时间更快，可以准确分析浓度比10 ms方法高约十倍的样品，从而简化了样品制备过程。
