M.R.R. de Planque

Effect of subthreshold slope on the sensitivity of nanoribbon sensors

In this work, we investigate how the sensitivity of a nanowire or nanoribbon sensor is influenced by the subthreshold slope of the sensing transistor. Polysilicon nanoribbon sensors are fabricated with a wide range of subthreshold slopes and the sensitivity is characterized using pH measurements. It is shown that there is a strong relationship between the sensitivity and the device subthreshold slope. The sensitivity is characterized using the current sensitivity per pH, which is shown to increase from 1.2% ph(-1) to 33.6% ph(-1) as the subthreshold slope improves from 6.2 V dec(-1) to 0.23 V dec(-1) respectively. We propose a model that relates current sensitivity per pH to the subthreshold slope of the sensing transistor. The model shows that sensitivity is determined only on the subthreshold slope of the sensing transistor and the choice of gate insulator. The model fully explains the values of current sensitivity per pH for the broad range of subthreshold slopes obtained in our fabricated nanoribbon devices. It is also able to explain values of sensitivity reported in the literature, which range from 2.5% pH(-1) to 650% pH(-1) for a variety of nanoribbon and nanowire sensors. Furthermore, it shows that aggressive device scaling is not the key to high sensitivity. For the first time, a figure-of-merit is proposed to compare the performance of nanoscale field effect transistor sensors fabricated using different materials and technologies.

Self-assembly of vesicle nanoarrays on Si: A potential route to high-density functional protein arrays

The authors show that 100nm unilamellar thiol-tagged vesicles bind discretely and specifically to Au nanodots formed on a Si surface. An array of such dots, consisting of 20nm Au–Si three-dimensional islands, is formed by self-assembly on terraces of small-angle-miscut Si(111) after Au deposition. Consequently, both the formation of the nanopattern and the subsequent attachment of the vesicles are self-organized and occur without the need for any “top-down” lithographic processes. This approach has the potential to provide the basis of a low-cost, high-density nanoarray for use in proteomics and drug discovery.

Dual-gate polysilicon nanoribbon biosensors enable high sensitivity detection of proteins

We demonstrate the advantages of dual-gate polysilicon nanoribbon biosensors with a comprehensive evaluation of different measurement schemes for pH and protein sensing. In particular, we compare the detection of voltage and current changes when top- and bottom-gate bias is applied. Measurements of pH show that a large voltage shift of 491 mV pH(-1) is obtained in the subthreshold region when the top-gate is kept at a fixed potential and the bottom-gate is varied (voltage sweep). This is an improvement of 16 times over the 30 mV pH(-1) measured using a top-gate sweep with the bottom-gate at a fixed potential. A similar large voltage shift of 175 mV is obtained when the protein avidin is sensed using a bottom-gate sweep. This is an improvement of 20 times compared with the 8.8 mV achieved from a top-gate sweep. Current measurements using bottom-gate sweeps do not deliver the same signal amplification as when using bottom-gate sweeps to measure voltage shifts. Thus, for detecting a small signal change on protein binding, it is advantageous to employ a double-gate transistor and to measure a voltage shift using a bottom-gate sweep. For top-gate sweeps, the use of a dual-gate transistor enables the current sensitivity to be enhanced by applying a negative bias to the bottom-gate to reduce the carrier concentration in the nanoribbon. For pH measurements, the current sensitivity increases from 65% to 149% and for avidin sensing it increases from 1.4% to 2.5%.

Top-down fabricated ZnO nanowire transistors for application in biosensors

Top-down ZnO nanowire FETs have been fabricated using mature photolithography, ZnO atomic layer deposition (ALD) and plasma etching. This paper investigates the effects of oxygen adsorption by measuring FET characteristics at different gate bias sweep rates and by characterizing hysteresis effects. Unpassivated devices exhibit a low threshold voltage shift of 5.4 V when the gate bias sweep rate is varied from 2500 V/s to 1.2 V/s and a low hysteresis width of less than 1.5 V. These results are considerably better than the state of the art for bottom-up as-fabricated ZnO nanowire FETs and demonstrate the suitability of this top-down technology for biosensor applications.

ZnO nanowire-FET for charge-based sensing of protein biomolecules

Top-down method was used to fabricate zinc oxide (ZnO) nano wire field effect transistor (NWFET) biosensor. The nanosensor was used to measure the electrical characteristics of lysozyme (LYSO) and bovine serum albumin (BSA) protein solutions in phosphate buffered saline (PBS). The LYSO and BSA proteins are oppositely charged at measurement pH of 7.4. Subthreshold voltage shift of 340 mV and 700 mV due to surface charge effect on the device channel is obtained for the LYSO and BSA solutions respectively. A NWFET sensitivity of 72 % is achieved for the LYSO proteins while the BSA proteins resulted in a sensitivity of 98%.