Interview with Jacek Fiutowski in interviews Jacek Fiutowski in an article about the save use of nanoparticles. He mentions some of the advantages of nanoparticles and some possibly hazardous effects of nanoparticles on cells. He points out that the research in a laboratory is different from the conditions in an open environment. In the project CheckNano, we observed that silver nanoparticles aggregate or dissolve in a complex environment like food within a short time.
It is also important to know how many particles enter the body. It is crucial to know the dose when the particles are hazardous for the body. But it is unknown how many particles enter the body from packaging and how long they survive inside the body.
The interview is complemented by research done at the national centre for the work environment. They studied Titaniumdioxid nanoparticles in the air because they are widely used in paints. It can be said that it is dangerous to work with nanoparticles in spray form.
Read the articles in Danish here.

Lab-on-chip (LoC) nanoparticle sorting

The analysis of nanoparticles (NPs) in biological matrices poses many challenges when trying to sample specific particles. We explored various Lab-on-chip (LoC) filtering components, with a particular focus on Field-flow fractionation (FFF) coupled with 3D dynamic light scattering (DLS) to provide size-related information. The LoC allows us to work on a small scale and with small amounts of sample solvents.

At SDU-NanoSYD we want to optimize nanoparticle detection efficiency. The particles that we want to detect are in a food matrix or other matrix of solvents. For sensitive detection, it is crucial to extract the particles from the matrix and preferably sort them by size.

We explored Field-flow fractionation (FFF) coupled with 3D dynamic light scattering (DLS) to provide size related information. The NPs, to be detected from food or other matrices of solvents, are mechanically pre-filtered by commercial syringe filters. Afterwards, based on hydrodynamic size, the NPs ranging from a few nanometers to an undefined level of micrometres, are separated. In the FFF technique, the NPs are resolved by a hydrodynamic pressure gradient which pushes the particles of different sizes into different channels.

To ensure high experimental throughput and keep the time between a theoretical concept and a real prototype as short as possible, we introduced stereolithography (SL) 3D printing as a simple prototyping method. The technique proposed here offers a low entry barrier for the rapid prototyping of microfluidics (LoC), enabling iterative design for laboratories without access to conventional soft-lithography carried on at cleanroom facilities. We have explored the diversity of photocurable resins to create a fully 3D printed, biocompatible and modular microfluidic platform.

Figure 1a shows an example of a first fully 3D printed LoC filtering unit. To check the performance, a mixture of 50 µL of concentrated Ag nanoparticles (5 mg/mL) of 50 nm and 200 nm diameter, diluted with 200 µL DI water, was processed through the filtering units and examined by a DLS method.
b) Experimental setup with a microfluidic filter in yellow.
c) shows the results of FFF filtering, where the larger particles, namely 200 nm diameter NPs where removed from the solution.

De-polarized dynamic light scattering by silver nanoparticles

Dynamic light scattering (DLS) (also known as photon correlation spectroscopy) is one of the most widely used techniques for the study of colloidal systems. It is a fast, convenient and relatively simple technique, enabling absolute estimates of particle size for a wide range of colloidal suspensions including suspended metal nanoparticles. In this technique a laser beam is passing through a small volume of solution which contains colloidal particles, and the scattered light is collected over a small solid angle, in a direction that can be moved around the sample. Figure 1 on the top displays the setup at NanoOpitcs in Odense.

The phase and polarization of the scattered light is determined by the size, shape, and composition of scatterers. The random Brownian motion of the nanoparticles causes time-dependent fluctuations of the total intensity at the detector, with the time scale of the fluctuation being determined by the time scale of the Brownian motion of the particles. In a DLS measurement, an autocorrelation function is generated from the intensity fluctuations, and this function contains information also about the particle size distribution, which is obtained after mathematical treatment of the autocorrelation function.

Owing to their polycrystalline nature, round metallic nanoparticles have an inhomogeneous internal structure and are not perfectly spherical. These imperfections are strong enough to result in a small but highly relevant optical anisotropy.

On the other hand, when excited by electromagnetic waves, silver nanoparticles support coherent oscillations of the surface conduction electrons. This phenomenon, i.e., the confined oscillations of the charge density, is referred to as localized surface plasmon resonance. It has been shown that upon scattering, as localized surface plasmon resonance along with such optical anisotropy, it results in a depolarized speckle pattern. Investigating depolarization effect on the temporal fluctuation within the DLS measurements will highly increase information on the Ag NP size and aggregation. Initially, the commercial setup from the Brookhaven Instruments we owed was working at wavelength of 660nm (red light), and first experiments for 50 and 100 nm particles, and mixed 50 and 100 nm, were carried out at this wavelength (see figure 2).

Fig. 2 DLS signals for 100 nm, 50 nm and a mixture of both sizes in solution.

Nevertheless, the scattering from nanoparticles depends on the wavelength, for example for spherical nanoparticles the scattering cross section is inversely proportional to the fourth order of the wavelength, which means that shorter wavelengths will provide larger scattered signals.

On the other hand, the localized surface plasmon resonance for silver nanoparticles is met at wavelengths near 400nm. With all this in mind there was a need to change the laser and shift to blue light, which allows us to work with smaller nanoparticles as well as doing upgrade with depolarization measurements.