Black Phosphorus-Based Reusable Biosensor Platforms for the Ultrasensitive Detection of Cortisol in Saliva.

A black phosphorus (BP)-based reusable biosensor platform is developed for the repeated and real-time detection of cortisol using antibody-conjugated magnetic particle (MP) structures as a refreshable receptor. Here, we took advantage of the low-noise characteristics of a mechanically exfoliated BP-based field-effect transistor (FET) and hybridized it with anti-cortisol antibody-functionalized MPs to build a highly sensitive cortisol sensor. This strategy allowed us to detect cortisol down to 1 aM in real time and discriminate cortisol from other hormones. In this case, we could easily remove MPs with used antibodies from the surface of a BP-FET and reuse the chip for up to eight repeated sensing operations. Moreover, since our platform could be fabricated using conventional photolithography techniques and the sensor can be reused multiple times, one should be able to significantly reduce operation costs for practical applications. Furthermore, this method could be utilized to detect different hormones with high sensitivity and selectivity in complex environments such as artificial saliva solutions. In this respect, our reusable BP-FET biosensing platform can be a powerful tool for versatile applications such as clinical diagnosis and basic biological analysis by conjugating various antibodies.

Nanoscale mapping of edge-state conductivity and charge-trap activity in topological insulators.

We report the nanoscale mapping of topological edge-state conductivity and the effects of charge-traps on conductivity in a Bi2Se3 multilayer film under ambient conditions. In this strategy, we applied an electric field perpendicular to the surface plane of Bi2Se3via a conducting probe to directly map the charge-trap densities and conductivities with a nanoscale resolution. The results showed that edge regions had one-dimensional characteristics with higher conductivities (two orders) and lower charge-trap densities (four orders) than those of flat surface regions where their conductivities and charge-traps were dominated by bulk effects. Additionally, edges showed an enhanced conductivity with an elevated electric field, possibly due to the creation of new topological states by stronger spin-Hall effects. Importantly, we observed ultra-high photoconductivity predominantly on edge regions compared with that of flat surface regions, which was attributed to the excitation of edge-state carriers by light. Since our method provides an important insight into the charge transport in topological insulators, it could be a significant advancement in the development of error-tolerant topotronic devices.

Nanoscale Mapping of Carrier Mobilities in the Ballistic Transports of Carbon Nanotube Networks.

Much progress has been made in the nanoscale analysis of nanostructures, while the mapping of key charge transport properties such as a carrier mobility remains a challenge, especially for one-dimensional systems. Here, we report the nanoscale mapping of carrier mobilities in carbon nanotube (CNT) networks and show that charge transport behaviors varied depending on network structures. In this work, the spatial distribution of localized charge transport properties such as mobilities and charge trap densities in CNT networks were mapped via a scanning noise microscopy. The mobility map was obtained from the conductivity maps measured at different back-gate biases, showing up to two orders of mobility variations depending on localized network structures. Furthermore, from the maps, correlations between mobility/conductivity and charge trap density were analyzed to determine charge transport mechanisms. In metallic CNT networks, the regions with rather high (low) or low (high) charge trap densities (mobilities) exhibited a diffusive or ballistic transport behavior, respectively. Interestingly, semiconducting CNT networks also exhibited a gradual transition from a diffusive to a ballistic transport behavior as the CNT mobility was increased by reaching the on-state with negative gate biases. The mapping of the cross-patterned CNT network showed that metallic CNT electrodes could achieve a good electrical contact with semiconducting CNTs without high contact resistance regions. Since this method allowed one to map versatile charge transport properties such as mobility, conductivity, and charge trap density, it can be a powerful tool for basic research about charge transport phenomena and practical device applications.