Monitoring the binding affinities and kinetics of protein interactions is important in clinical diagnostics and drug development because such information is used to identify new therapeutic candidates. Surface plasmon resonance is at present the standard method used for such analysis, but this is limited by low sensitivity and low-throughput analysis. Here, we show that silicon nanowire field-effect transistors can be used as biosensors to measure protein–ligand binding affinities and kinetics with sensitivities down to femtomolar concentrations. Based on this sensing mechanism, we develop an analytical model to calibrate the sensor response and quantify the molecular binding affinities of two representative protein–ligand binding pairs. The rate constant of the association and dissociation of the protein–ligand pair is determined by monitoring the reaction kinetics, demonstrating that silicon nanowire field-effect transistors can be readily used as high-throughput biosensors to quantify protein interactions.

Label-free nanosensors can detect disease markers to provide point-of-care diagnosis that is low-cost, rapid, specific and sensitive.1-13 However, detecting these biomarkers in physiological fluid samples is difficult because of problems like biofouling and nonspecific binding, and the resulting need to use purified buffers greatly reduces the clinical relevance of these sensors. Here, we overcome this limitation by using distinct components within the sensor to perform purification and detection. A microfluidic purification chip captures multiple biomarkers simultaneously from blood samples and releases them, after washing, into purified buffer for sensing by a silicon nanoribbon detector. This two-stage approach isolates the detector from the complex environment of whole blood, and reduces its minimum required sensitivity by effectively pre-concentrating the biomarkers. We show specific and quantitative detection of two model cancer antigens from a 10 uL sample of whole blood in less than 20 minutes. This study marks the first use of label-free nanosensors with physiologic solutions, positioning this technology for rapid translation to clinical settings.

Carrier transport characteristics in high-efficiency single-walled carbon nanotubes (SWNTs)/silicon (Si) hybrid solar cells are presented. The solar cells were fabricated by depositing intrinsic p-type SWNT thin-films on n-type Si wafers without involving any high-temperature process for p-n junction formation. The optimized cells showed a device ideality factor close to unity and a record-high power-conversion-efficiency of >11%. By investigating the dark forward current density characteristics with varying temperature, we have identified that the temperature-dependent current rectification originates from the thermally activated band-to-band transition of carriers in Si, and the role of the SWNT thin films is to establish a built-in potential for carrier separation/collection. We have also established that the dominant carrier transport mechanism is diffusion, with minimal interface recombination. This is further supported by the observation of a long minority carrier lifetime of ~34 μs, determined by the transient recovery method. This study suggests that these hybrid solar cells operate in the same manner as single crystalline p-n homojunction Si solar cells.

Label-free nanosensors can detect disease markers to provide point-of-care diagnosis that is low-cost, rapid, specific and sensitive. However, detecting these biomarkers in physiological fluid samples is difficult because of ionic screening. Here, we overcome this limitation by using distinct components within the sensor to perform purification and detection. 1 A microfluidic purification chip captures multiple biomarkers simultaneously from blood samples and releases them, after washing, into purified buffer for sensing by a silicon nanoribbon detector. This two-stage approach isolates the detector from the complex environment of whole blood, and reduces its minimum required sensitivity by effectively pre-concentrating the biomarkers. We show specific and quantitative detection of two model cancer antigens from a 10 uL sample of whole blood in less than 20 minutes.

The signal-to-noise ratio (SNR) for silicon nanowire field-effect transistors operated in an electrolyte environment is an essential figure-of-merit to characterize and compare the detection limit of such devices when used in an exposed channel configuration as biochemical sensors. We employ low frequency noise measurements to determine the regime for optimal SNR. We find that SNR is not significantly affected by the electrolyte concentration, composition, or pH, leading us to conclude that the major contributions to the SNR come from the intrinsic device quality. The results presented here show that SNR is maximized at the peak transconductance. Silicon nanowire field-effect transistors (SiNW-FET) have shown great sensitivity when employed as biological/ chemical sensors (bioFETs). [1][2][3] The principle of operation is that a charged species bound to the nanowire (NW) surface (modified with some receptor molecules) induces a change in surface potential at the NW surface, which translates into a change in drain-to-source current (DI) which is then measured. It is well-known that the sensitivity (defined as DI/I for a current based sensing experiment) is maximized in the subthreshold regime. 4-6 However, it is also known that the normalized current noise power amplitude (S I /I 2 ) reaches a plateau and is highest in the subthreshold regime for siliconsilicon oxide devices 7,8 and concerns have been expressed that signal-to-noise ratio (SNR) would be impacted for measurements carried out in subthreshold. 4,9 On the other hand, S I /I 2 is lower in the linear regime but the sensitivity is also lower. In order to determine the ideal regime for optimal SNR, we carry out both I-V and noise measurements for solution gated devices. Our measurements indicate that the current noise is independent of electrolyte concentration, composition, or pH, leading us to conclude that the intrinsic electronic properties of the SiNW bioFETs determine the optimal SNR achievable by these sensors. We also find that SNR is maximized in the linear regime at the point where the transconductance is largest.The SiNWs were fabricated from SOI wafers (Soitec) with a high resistivity boron doped active layer (>2000 ohm cm) as described previously. 10 Devices used for the experiments were nominally 100 nm wide and 5 lm long. The devices were covered with a passivation layer of SU-8 (an epoxy based negative photoresist) with windows opened for the NW channel and the contact pads. An optical micrograph of the devices is shown in Fig. 1(b). The NW surfaces were functionalized with a monolayer of APTES (3-aminopropyltriethoxysilane) using the protocol described earlier, 11 which increases device stability and reduces gate leakage current in solution. A fluidic well was then glued on top of each chip for the sensing experiments, with a platinum wire used as the solution gate electrode. The device structure and experimental setup is shown schematically in Fig. 1(a). For all noise and transfer characteristics measurements, the back-gate contact was l...