Mostafa Taghipour

Ultrafine particles (UFPs), defined as aerosols smaller than 100 nm in diameter, are of growing concern due to their ability to penetrate deep into the respiratory system and enter systemic circulation, posing environmental and health risks. However, widely applied monitoring of UFPs is currently hindered by the high cost and lack of portability of available detection methods.
One of the most widely used systems for characterizing UFPs is the Scanning Mobility Particle Sizer (SMPS), which classifies particles by electrical mobility and enlarges them through condensation for detection via light scattering. The high cost and lack of portability of this instrument make it unsuitable for widespread application in air quality monitoring.

Here we present a new approach for the detection and monitoring of UFPs that could potentially be much lower cost than existing methods. In our approach, we employ nanophotonic optical structures integrated onto a fiber tip that sense particles through local changes in refractive index.
This sensor detects single nanoparticles directly, without the need for condensation, and at a lower cost.

While promising for detecting particles down to sizes of 50 nm, the sensor currently faces two main challenges: First, the detection volume is extremely small, which makes it difficult to deliver UFPs into the detection volume and keep them there long enough to be detected. Second, so far we have validated the functioning of the sensor only in a liquid environment rather than in air. 

To address these limitations, our work focuses on the manipulation of UFPs in air in order to efficiently deliver them to the detection volume; this involves separating UFPs from larger particles, concentrating them, and delivering them precisely to the detection region of the fiber-tip sensor. To do so, we employ physical mechanisms that are able to displace the aerosols relative to the background air — namely inertial(Yuen,2014), electrostatic(Kim,2018), and acoustic forces— for displacing and concentrating UFPs in air and liquid.  We asses these methods analytically through scaling laws, by performing COMSOL simulations, and via experiments. We produce components using microfabrication techniques such as photolithography, 3D printing and femtosecond laser machining. As an example, Figure 2 shows the design of a T-junction with a cut-off size of 1 µm and a COMSOL simulation of particles (300 nm diameter) flowing through the geometry; as expected, theparticles largely follow the fluid streamlines. Currently, we are working on designing miniaturized cascade impactors to achieve smaller cut-off sizes. Additionally, we are developing low-cost, compact needle-to-plate chargers for charging UFPs. Electrostatic forces could then provide an effective mechanism for delivering UFPs to our optical sensors.