Label-free rapid electrochemical detection of DNA hybridization using ultrasensitive standalone CNT aerogel biosensor.

We report the development of an ultrasensitive label-free DNA biosensor device with fully integrated standalone carbon nanotube (CNT) aerogel electrode. The multi-directional tenuous network of clustered CNT embedding into the CNT aerogel electrode demonstrates linear ohmic and near isotropic electrical properties, thereby providing high sensitivity for nucleic acid detection. Using this device, the target DNA hybridization is detected by a quantifiable change in the electrochemical impedance, with a distinct response to the single-stranded probe alone or double-stranded target-probe complex. The target DNA is specifically detected with limit of detection (LoD) of 1 pM with a turnaround time of less than 20 min, which is unprecedented for a miniaturized CNT aerogel sensor and impedance spectroscopy without an intermediate DNA amplification step. Moreover, this system is able to differentiate between the closely related target sequences by the distinct impedance response rendering it highly specific. To the best of our knowledge, this is the first report showing the use of standalone bare CNT aerogel electrode without any substrate support, coupled with electrochemical impedance spectroscopy, for the detection of DNA hybridization. Altogether, the results show that our system is fast, sensitive and specific for label-free rapid direct DNA detection, promising a novel avenue for bio-sensing.

Combinatorial Large-Area MoS2/Anatase-TiO2 Interface: A Pathway to Emergent Optical and Optoelectronic Functionalities.

The interface of transition-metal dichalcogenides (TMDCs) and high-k dielectric transition-metal oxides (TMOs) had triggered umpteen discourses because of the indubitable impact of TMOs in reducing the contact resistances and restraining the Fermi-level pinning for the metal-TMDC contacts. In the present work, we focus on the unresolved tumults of large-area TMDC/TMO interfaces, grown by adopting different techniques. Here, on a pulsed laser-deposited MoS2 thin film, a layer of TiO2 is grown by atomic layer deposition (ALD) and pulsed laser deposition (PLD). These two different techniques emanate the layer of TiO2 with different crystallinities, thicknesses, and interfacial morphologies, subsequently influencing the electronic and optical properties of the interfaces. Contrasting the earlier reports of n-type doping at the exfoliated MoS2/TiO2 interfaces, the large-area MoS2/anatase-TiO2 films had realized a p-type doping of the underneath MoS2, manifesting a boost in the extent of p-type doping with increasing thickness of TiO2, as emerged from the X-ray photoelectron spectra. Density functional analysis of the MoS2/anatase-TiO2 interfaces, with pristine and interfacial defect configurations, could correlate the interdependence of doping and the terminating atomic surface of TiO2 on MoS2. The optical properties of the interface, encompassing photoluminescence, transient absorption and z-scan two-photon absorption, indicate the presence of defect-induced localized midgap levels in MoS2/TiO2 (PLD) and a relatively defect-free interface in MoS2/TiO2 (ALD), corroborating nicely with the corresponding theoretical analysis. From the investigation of optical properties, we indicate that the MoS2/TiO2 (PLD) interface may act as a promising saturable absorber, having a significant nonlinear response for the sub-band-gap excitations. Moreover, the MoS2/TiO2 (PLD) interface had exemplified better phototransport properties. A potential application of MoS2/TiO2 (PLD) is demonstrated by the fabrication of a p-type phototransistor with the ionic-gel top gate. This endeavor to analyze and perceive the MoS2/TiO2 interface establishes the prospectives of large-area interfaces in the field of optics and optoelectronics.

Photoelectrochemical Solar Water Splitting: The Role of the Carbon Nanomaterials in Bismuth Vanadate Composite Photoanodes toward Efficient Charge Separation and Transport.

Photoelectrochemical performance of bismuth vanadate (BiVO4) photoanode is limited by poor charge separation and transport properties. The roles of carbon nanotube, reduced graphene oxide, or graphitic carbon nitride in BiVO4 composite photoanode were investigated toward enhancing light absorption and reducing overall impedance during photoelectrochemical water oxidation process. X-ray diffraction and Tauc analysis showed that BiVO4 retains its monoclinic phase, n-type semiconductor nature, and band gap in all carbon nanomaterials-incorporated composite photoanodes. It was observed that the carbon nanomaterials incorporation in BiVO4 film increases its surface porosity, ultimately leading to enhanced light absorption. The BiVO4 photoanode with reduced graphene oxide and graphitic carbon nitride showed same bulk charge separation efficiency, whereas the latter showed better charge transfer. It was found that the graphitic carbon nitride formed composite with BiVO4 to provide enhanced light absorption efficiency, i.e., 89% in 350-505 nm range. The BiVO4 with graphitic carbon nitride photoanode showed the best performance with a photocurrent of 2.2 mA cm-2, charge separation efficiency of 67%, and photocurrent of 4.0 mA cm-2 with cobalt-phosphate surface catalyst at 1.23 VRHE for water oxidation under 1 sun illumination. The Mott-Schottky and impedance measurements confirmed the shift of conduction band position toward hydrogen reduction potential and reduction in film resistance, respectively, with carbon nanomaterials addition, and the shift was most significant for graphitic carbon nitride. It is concluded that by concomitant formation of junction during photoanode fabrication between carbon nanomaterials, BiVO4, and fluorine-doped tin oxide glass substrate, better charge separation, transport, and light absorption can be achieved.