Reversible chemical tuning of charge carriers for enhanced photoelectrochemical conversion and probing of living cells

Reversible Charge Transfer Doping in Atomically Thin In2O3 by Viologens

Atomically thin oxide semiconductors are emerging as potential materials for their potentiality in monolithic 3D integration and sensor applications. In this study, a charge transfer method employing viologen, an organic compound with exceptional reduction potential among n-type organics, is presented to modulate the carrier concentration in atomically thin In2O3 without the need of annealing. This study highlights the critical role of channel thickness on doping efficiency, revealing that viologen charge transfer doping is increasingly pronounced in thinner channels owing to their increased surface-to-volume ratio. Upon viologen doping, an electron sheet density of 6.8 × 1012 cm-2 is achieved in 2 nm In2O3 back gate device while preserving carrier mobility. Moreover, by the modification of the functional groups, viologens can be conveniently removed with acetone and an ultrasonic cleaner, making the viologen treatment a reversible process. Based on this doping scheme, we demonstrate an n-type metal oxide semiconductor inverter with viologen-doped In2O3, exhibiting a voltage gain of 26 at VD = 5 V. This complementary pairing of viologen and In2O3 offers ease of control over the carrier concentration, making it suitable for the next-generation electronic applications.

Affinity-Based Detection of Biomolecules Using Photo-Electrochemical Readout

Detection and quantification of biologically-relevant analytes using handheld platforms are important for point-of-care diagnostics, real-time health monitoring, and treatment monitoring. Among the various signal transduction methods used in portable biosensors, photoelectrochemcial (PEC) readout has emerged as a promising approach due to its low limit-of-detection and high sensitivity. For this readout method to be applicable to analyzing native samples, performance requirements beyond sensitivity such as specificity, stability, and ease of operation are critical. These performance requirements are governed by the properties of the photoactive materials and signal transduction mechanisms that are used in PEC biosensing. In this review, we categorize PEC biosensors into five areas based on their signal transduction strategy: (a) introduction of photoactive species, (b) generation of electron/hole donors, (c) use of steric hinderance, (d) in situ induction of light, and (e) resonance energy transfer. We discuss the combination of strengths and weaknesses that these signal transduction systems and their material building blocks offer by reviewing the recent progress in this area. Developing the appropriate PEC biosensor starts with defining the application case followed by choosing the materials and signal transduction strategies that meet the application-based specifications.

TiO2 nanowires as an effective sensing platform for rapid fluorescence detection of single-stranded DNA and double-stranded DNA

Numerous nanomaterials have been utilized for novel biosensors with sensitivity and selectivity in the last decades due to their intrinsic unique properties. Herein, a facile fluorescence method for nucleic acid detection was developed by employing TiO₂ nanowires (NWs) as the sensing platform. The quenching effect of TiO₂ NWs to fluorophore-labelled single-stranded DNA (ssDNA) was found to be more significant than that to fluorophore-labelled double-stranded DNA (dsDNA) or triplex DNA probes. More importantly, the whole quenching process was also fast since it just took about ten minutes to reach the equilibrium. Based on the different affinities of TiO₂ NWs to ssDNA, dsDNA and triplex DNA probes, the sequence-specific nucleic acids were detected with sensitivity and specificity. Further investigation has demonstrated that the quenching efficiency of TiO₂ NWs to long ssDNA was apparently superior than that to short ssDNA. Moreover, the fluorescence from various ssDNA probes labelled with a wide spectrum of fluorescent dyes could also be quenched by TiO₂ NWs. These inspiring results reveal that TiO₂ NWs could be an excellent universal nanoquencher used in the next-generation biosensors.

Interfacial Engineering of Hierarchical Transition Metal Oxide Heterostructures for Highly Sensitive Sensing of Hydrogen Peroxide

Hydrogen peroxide (H₂ O₂ ) is a major messenger molecule in cellular signal transduction. Direct detection of H₂ O₂ in complex environments provides the capability to illuminate its various biological functions. With this in mind, a novel electrochemical approach is here proposed by integrating a series of CoO nanostructures on CuO backbone at electrode interfaces. High-resolution transmission electron microscopy (HRTEM), X-ray diffraction, and X-ray photoelectron spectroscopy demonstrate successful formation of core-shell CuO-CoO hetero-nanostructures. Theoretical calculations further confirm energy-favorable adsorption of H₂ O₂ on surface sites of CuO-CoO heterostructures. Contributing to the efficient electron transfer path and enhanced capture of H₂ O₂ in the unique leaf-like CuO-CoO hierarchical 3D interface, an optimal biosensor-based CuO-CoO-2.5 h electrode exhibits an ultrahigh sensitivity (6349 µA m m^-1 cm^-2 ), excellent selectivity, and a wide detection range for H₂ O₂ , and is capable of monitoring endogenous H₂ O₂ derived from human lung carcinoma cells A549. The synergistic effects for enhanced H₂ O₂ adsorption in integrated CuO-CoO nanostructures and performance of the sensor suggest a potential for exploring pathological and physiological roles of reactive oxygen species like H₂ O₂ in biological systems.

WO₃ nanoflakes for enhanced photoelectrochemical conversion

We developed a postgrowth modification method of two-dimensional WO3 nanoflakes by a simultaneous solution etching and reducing process in a weakly acidic condition. The obtained dual etched and reduced WO3 nanoflakes have a much rougher surface, in which oxygen vacancies are created during the simultaneous etching/reducing process for optimized photoelectrochemical performance. The obtained photoanodes show an enhanced photocurrent density of ∼1.10 mA/cm(2) at 1.0 V vs Ag/AgCl (∼1.23 V vs reversible hydrogen electrode), compared to 0.62 mA/cm(2) of pristine WO3 nanoflakes. The electrochemical impedance spectroscopy measurement and the density functional theory calculation demonstrate that this improved performance of dual etched and reduced WO3 nanoflakes is attributed to the increase of charge carrier density as a result of the synergetic effect of etching and reducing.