Xiao‐Yang Zhu

We report the direct time domain observation of a many-body process in a two-dimensional system: small polaron formation from the localization of a conduction band electron in NaCl thin films of unit cell thickness. Contrary to theoretical prediction for bulk NaCl crystal where an electron polaron does not exist, time-resolved two-photon photoemission reveals small polaron formation from delocalized conduction band electrons in crystalline NaCl thin films. The increased deformability and the reduced electronic bandwidth of a crystalline lattice in the thin film format are both responsible for the formation of small polarons that are absent in bulk solids.

Keeping protein molecules in the active state on a solid surface is essential to protein microarrays and other protein‐based biosensors. Here, we show that the 2‐D chemical environment controls the refolding of the denatured green fluorescent proteins tethered to solid surfaces. Refolding occurs readily on the repulsive PEG functionalized surface but is inhibited on the attractive –NH₂ functionalized surface. This result shows the critical importance of the 2‐D chemical environment in the maintenance and revival of protein activity on surfaces and opens the door to designing 2‐D molecular chaperones for protein folding.

We compare the catalytic activities of enzymes immobilized on silicon surfaces with and without orientation. While oriented sulfotransferases selectively immobilized on an otherwise zero‐background surface via 6×His tags faithfully reflect activities of solution phase enzymes, those with random orientation on the surface do not. This finding demonstrates that controlling the orientation of immobilized protein molecules and designing an ideal local chemical environment on the solid surface are both essential if protein microarrays are to be used as quantitative tools in biomedical research.

A high‐density poly(ethylene glycol) (PEG)‐coated Si(111) surface is used for the immobilization of polyhistidine‐tagged protein molecules. This process features a number of properties that are highly desirable for protein microarray technology: (i) minimal nonspecific protein adsorption; (ii) highly uniform surface functionality; (iii) controlled protein orientation; and (iv) highly specific immobilization reaction without the need of protein purification. The high‐density PEG‐coated silicon surface is obtained from the reaction of a multi‐arm PEG (mPEG) molecule with a chlorine terminated Si(111) surface to give a mPEG film with thickness of 5.2 nm. Four out of the eight arms on each immobilized mPEG molecule are accessible for linking to the chelating iminodiacetic acid (IDA) groups for the binding of Cu ²⁺ ions. The resulting Cu ²⁺ ‐IDA‐mPEG‐Si(111) surface is shown to specifically bind 6x histidine‐tagged protein molecules, including green fluorescent protein (GFP) and sulfotransferase (ST), but otherwise retains its inertness towards nonspecific protein adsorption. We demonstrate a particular advantage of this strategy: the possibility of protein immobilization without the need of prepurification. Surface concentrations of relevant chemical species are quantitatively characterized at each reaction step by X‐ray photoelectron spectroscopy (XPS). This kind of quantitative analysis is essential in tuning surface concentration and chemical environment for optimal sensitivity in probe‐target interaction.