There is great interest in using single-walled carbon nanotubes (SWNTs) as nanoscale probes and sensors in biological electronics and optical devices because the electronic and optical properties of SWNTs are extremely sensitive to the surrounding environments. [1][2][3][4][5] For the applications of SWNTs-based sensors in a biological environment, an immediate question is how the sensors respond to the biological conditions such as pH, 5c glucose, various ions, and proteins. This study requires a well-controlled modification of SWNT surfaces to obtain interfaces that are sensitive to these variables. 6 The exploration in this exciting area is still not in full blossom, partially due to the difficulty in preparing water-soluble SWNTs while maintaining the SWNT electronic structure intact. 4 In light of recent great progress in solubilization of SWNTs in various solvents by polymer wrapping and sidewall functionalization, 3a,4,5b,7-10 a better controlled modification of SWNT surfaces may be realized soon. In this work, we report a facile chemical routine to prepare water-soluble SWNTs that still retain their van Hove singularities after oxidative treatment. 7 The solubility in water for as-treated SWNTs with modified surfaces provides us with a unique opportunity to reveal the relationship of their electronic and optical properties with pH. Here we observe that after surface modification with carboxylate groups, the optical absorption of asprepared water-soluble semiconducting SWNTs (Tube@Rice and HiPco) reversibly responds to the pH change.Purified pristine Tube@Rice SWNTs suspended in toluene were purchased from Rice University. Raw HiPco SWNTs were purchased from Carbon Nanotechnologies, Inc., and were purified by the method described in ref 11. Because similar results were obtained with these two types of SWNTs, we reported the results here only for Tube@Rice SWNTs for the sake of clarity.The facile routine for preparation of water-soluble SWNTs was a modification of the acid oxidative method developed in Smalley's group. 7 In a typical experiment, 14 mg of SWNTs were added into 5 mL of a 9:1 concentrated H 2 SO 4 /30% H 2 O 2 aqueous solution. The mixture was stirred for 30 min. After the reaction, 15 mL of the 9:1 concentrated H 2 SO 4 /30% H 2 O 2 solution was added into the mixture. Then the mixture was divided into six aliquots in test tubes. Each aliquot was placed in an ultrasonic bath (Branson model 1510) and was sonicated for a different period of time, ranging from 0 to 5.0 min. Each resulting SWNT dispersion was diluted using 250 mL of distilled water and then was filtered through a 0.4 µm Millipore polycarbonate filter membrane. The resulting six SWNT mats were continuously washed using 10 mM NaOH solution and distilled water until the pH of the filtrates was 7. Then the wet SWNT mats were separated from the filters by dispersing them in distilled water. Six aqueous solutions of the SWNTs (0.03 mg/ mL) were prepared by sonication for 1-2 min. No tube precipitation was observed from these solutio...

The fabrication of porous coordination frameworks in thin-film forms has been investigated intensively with a view to using their structural response to external stimuli and guests for potential nanotechnological applications, for example as membranes for gas separation. Here we report a coordination framework that exhibits a dynamic guest-sorption behaviour in a nanometre-sized thin-film form (16 nm thick), yet shows no guest uptake in the bulk. Highly oriented crystalline thin films of this coordination framework--which consists of interdigitated two-dimensional layers of {Fe(py)2[Pt(CN)4]} (py, pyridine)--were fabricated through liquid-phase layer-by-layer synthesis. The resulting thin film exhibited a clear guest uptake with a structural transformation of the gate-opening type as characterized by in situ X-ray diffraction. Increasing the film's thickness markedly suppressed this behaviour. We envisage that such a crystal-downsizing effect may be observed with other coordination frameworks, and may be of use to develop functional materials, for example, for switching or sensing devices.