The detection of specific biomolecular interactions with micro-Hall magnetic sensors.

The detection of reagent-free specific biomolecular interactions through sensing of nanoscopic magnetic labels provides one of the most promising routes to biosensing with solid-state devices. In particular, Hall sensors based on semiconductor heterostructures have shown exceptional magnetic moment sensitivity over a large dynamic field range suitable for magnetic biosensing using superparamagnetic labels. Here we demonstrate the capability of such micro-Hall sensors to detect specific molecular binding using biotin-streptavidin as a model system. We apply dip-pen nanolithography to selectively biotinylate the active areas of InAs micro-Hall devices with nanoscale precision. Specific binding of complementarily functionalized streptavidin-coated superparamagnetic beads to the Hall crosses occurs via molecular recognition, and magnetic detection of the assembled beads is achieved at room temperature using phase sensitive micro-Hall magnetometry. The experiment constitutes the first unambiguous demonstration of magnetic detection of specific biomolecular interactions with semiconductor micro-Hall sensors, and the selective molecular functionalization and resulting localized bead assembly demonstrate the possibility of multiplexed sensing of multiple target molecules using a single device with an array of micro-Hall sensors.

Mechanism and optimization of pH sensing using SnO2 nanobelt field effect transistors.

We report a systematic investigation about the mechanism of pH sensing using SnO2 nanobelt field effect transistors (FETs). The FETs, based on single SnO2 nanobelts, are channel-limited and with proper contact passivation; the pH sensing was conducted with sodium phosphate solutions through integrated microfluidics. The responses of the FET channel conductance to pH were measured at different gate voltages: a linear pH dependence was observed in the linear transport "on" state, while an exponential dependence was observed in the subthreshold regime. Measurements at the same pH but different ion concentrations demonstrated that the FET's pH sensitivity decreases logarithmically with the ion concentration. The effect of APTES-functionalization was evaluated by comparing the pH responses of the same device with and without the surface modification. The APTES functionalization results in a slight enhancement of the pH sensitivity and a large suppression of the noise level, leading to marked improvement in the signal-to-noise ratio. The results indicate that the pH sensing is based on a screened field-effect response of the FETs to the surface protonation/deprotonation on the nanobelt. This study provides several useful guidelines for optimizing the sensor performance for chemical and biomolecular detection.

Photothermal melting and energy migration in conjugated oligomer films doped with CdSe quantum dots.

The effect of thin film morphology on energy transfer and migration in host-guest systems involving a phenylene-ethynylene oligomer matrix doped with colloidally prepared CdSe quantum dots is studied. Using correlated spectroscopy techniques including DSC, Raman, and temperature-dependent photoluminescence, we find that annealing the film produces continuous domain structures that enhance excitation migration by extending the excitation diffusion length. Under optical excitation, the thin films exhibit rapid melting of the host lattice, followed by resonant energy transfer to the CdSe QD guests. The ability to optically manipulate the structure and subsequently optically detect this change makes this material an important candidate for an all-optical read-write memory system.

Enzymatic manipulation of DNA-nanomaterial constructs.

The demonstration and control of biofunction between inorganic nanomaterials and biological scaffolding is crucial to the development of the field of biomaterials. Although unique hierarchical structures can be generated, the impact of nanosized materials on the biological activity of DNA-protein interactions is relatively unknown. Using highly selective proteins that induce sequence-specific conformational perturbations within DNA, we demonstrate the absolute maintenance of biofunction for biomaterials composed of duplex DNA appended with 1.4-nm Au particles. Enzyme activity and DNA binding affinities (K(d)) are unaltered by the nanoparticle-DNA conjugates. Our results provide a foundation for interfacing more complex and diverse protein-DNA-systems.