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We present a systematic study of low-frequency noise in Au nanoparticle chemosensors. All the sensors we have studied exhibit 1∕f-type noise at low frequencies. The magnitude of the 1∕f noise was smaller in devices with a larger device area, indicating that the 1∕f noise is caused by intrinsic processes. The noise amplitude was found to be strongly temperature dependent between 40–300K, with a local peak at around 100K, and weakly dependent below 40K. The noise data could not be fit by a single activated process indicating that multiple noise processes must be present in our sensors.

With solvent evaporation, polystyrene fine particles are found to exhibit an unusual fluorescence change from possessing a typical excimer emission band centered at 330 nm to a variety of strong structured bands at longer wavelength with maxima at 356 nm, 365 nm, 372 nm, 383 nm, 404 nm and 426 nm. Similar fluorescence emissions are also observed in polystyrene film after solvent removal. The excitation spectra observed by monitoring these fluorescence emissions at the wavelengths of maxima reveal fluorescence is emitted from different fluorophores. The folding of polystyrene chains during the preparation of polystyrene fine particles through reprecipitation and polystyrene chain relaxation during solvent evaporation are believed to be prerequisites for the formation of different fluorophores via phenyl aggregations, while the aromaticaromatic interactions between the pendant phenyl groups on polystyrene chains are considered to bring a powerful driving force for the formation of fluorophorous phenyl aggregations responsible for the observed fluorescence phenomena. This discovery can contribute to understanding and control of molecular interactions in aromatic systems, especially aromatic macromolecule systems. The luminescence can be used to manufacture light devices and other hybrid materials, as well as to probe local environment in aromatic systems.

The fluorescence of polystyrenes solutions are reported in three different concentrations with one in semi-diluted regime and two in diluted regime. With the excitation energy of 260 nm, only the widely studied monomer fluorescence emission near 285 nm and excimer fluorescence emission near 330 nm are observed. With the excitation energies at 277nm and longer wavelengths, the unexpected emissions in the longer wavelength range 350-480 nm that was recently discovered in polystyrene fine particles are also found to appear in polystyrene solutions, but with fewer structures and major peaks in different wavelengths. The major emissions shift from the region of 320-390 nm in polystyrene fine particles to the region of 380-450 nm in solutions. The excitation spectrum of the emissions at longer wavelength reveals much broader absorption bands with fewer resolved structures in polystyrene solutions than the absorption bands recorded in fine particles. Considering the simplicity of polystyrene molecules and previous studies, aromatic-aromatic (or π-stacking) interactions are believed to be the major driving forces for the formation of phenyl dimers and multimers from which the new emissions in longer wavelengths originate. The mechanism of the formation of these phenyl multimers in polystyrene solutions remains unsolved in this study, however the new fluorescence emissions discovered will contribute to the study of π-stacking interactions, and can also be a powerful probe for monitoring molecular motions and ordered structures of polystyrene molecules or other aromatic macromolecules in solutions.

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