Abstract
Many types of nanoparticles are produced by liquid colloidal chemistry. In order to observe single nanoobjects, suspensions must be deposited on cleaned non-luminescent substrates and dry from the solution or they may be mixed in polymers and spincoated.
In both cases the particles may cluster together and change their properties compared to the original liquid environment. Is there any way to observe single nanoparticles directly in liquid? We decided to investigate the application of microfluidic chips for optical microscopy and spectroscopy of various nanoparticles: fluorescent quantum dots and gold nanorods.
The tested chips were purchased from Dolomite Ltd. - we found that their glass chips produce strong fluorescence background over the whole visible spectrum, therefore the fused silica microfluidic chip was used. Another obstacle is quite large thickness of the chip (4 mm), where the microchannels are in the center.
Therefore, we have to use objective lenses with correction to a substrate of 2 mm (Olympus 20x/0.45, Nikon 40x/0.6). The microfluidic system was fixed inside our home-build system based on an inverted microscope with the wide-field epifluorescent excitation.
Due to the relatively fast movement of objects, only the very efficiently emitting nanoparticles with high emission rate can be observed (using the strongest possible non-bleaching intensity of excitation). We also investigated imaging of gold nanorods in the dark-field scattering mode.
The linear polarization character of scattering can be exploited to obtain information on the orientation of a nanorod using the Wollaston prism inserted into the detection path.