Analyzing Engineered Nanoparticles using Photothermal Infrared Spectroscopy

Nanoparticles, particles having one or more dimensions smaller than 100 nm, show novel properties and functions that differ significantly from those of their corresponding bulk counterpart. Due to their small size and large surface to volume ratio they can e.g. diffuse easily and be highly reactive.

To facilitate occupational safety and health there is a need to develop instruments to monitor and analyze nanoparticles in the industry, research and urban environments. The aim of this Ph.D. project was to develop new sensors that can analyze engineered nanoparticles. Two sensors were studied: (i) a miniaturized toxicity sensor based on electrochemistry and (ii) a photothermal spectrometer based on tensile-stressed mechanical resonators (string resonators).

Miniaturization of toxicity sensor targeting engineered nanoparticles was explored. This concept was based on the results of the biodurability test using redox activity measurements. With a new setup adapted to miniaturization, stable pH was achieved, platinum was found to be more suitable than gold for open circuit potential-time measurements, miniaturized platinum working electrodes and quasi silver/silver chloride reference electrodes were fabricated, and Gambles solution with dispersed iron oxide nanoparticles showed lowered potential as expected.

Despite the potential of this concept instability and lack of reproducibility continued to be an unneglectable issue. The concept of utilizing string resonators for photothermal spectroscopy was, for the first time, studied in details both theoretically and experimentally. The string-based photothermal spectrometer consists of a string resonator on which an analyte is collected.

Wavelength-dependent absorption by the analyte leads to heating of the string which is reflected in its resonance frequency. This setup allows for spectroscopic measurement of the analyte. An analytical model describing the resonance frequency of a string resonator locally heated was developed. The developed model was in agreement with FEM simulations and experimental results.

Theoretical and experimental work lead to a set of design rules for the responsivity of the string-based photothermal spectrometer. Responsivity is maximized for a thin, narrow and long string irradiated by high power radiation. Various types of nanoparticles and binary mixtures of them were successfully detected and analyzed.

Detection of copper-chelation of the antidiabetic drug metformin was demonstrated as well. The estimated detection limit for the developed system is an analyte with a mass of ∼150 ag (1 ag = 10-18 g). In short, it has been demonstrated that the string-based photothermal spectrometer is a promising technique for nanoparticle detection and analysis.