Sensitive and selective target capture, recognition, and signal transduction in detection of chemical and biological molecules is essential for fundamental biomedical studies, disease diagnosis, and drug screening. To achieve fast, sensitive, large-scale, and low-cost molecular analysis, a wide variety of detection technologies such as fluorescent, spectroscopic, electrical, magnetic and mechanical methods have been developed.

The photoelectrochemical (PEC) approach is a recently developed one for biomolecular analysis. The basic principle of PEC is the photo-to-electric conversion of a semiconducting material, phot-excited electrons or holes are transferred to proper sites to initiate redox reactions. This photon-to-charge conversion process is highly sensitive to the surface chemistry and microenvironment fluctuation. When the semiconductor surface is functionalized with a receptor layer that can specifically recognize and bind to bio analyte in the solution, the photocurrent density of the semiconductor is changed. For the electron transfer, the driving force is the energy difference between the conduction band of the semiconductor and the reduction potential of the acceptor redox couple. Compared to conventional electrochemical methods, the PEC detection usually benefits from advantages like high sensitivity, simple instrumentation and low cost. In addition, in PEC detection, two separate forms of energy are employed for signal production in which irradiated light excites the photoactive species and the electrical signal is subsequently transduced and detected leading to reduced background and potentially higher sensitivity. To learn more about principles and state of the art in the subject, these papers are strongly suggested:

1- Tang et al. Solar-Energy-Driven Photoelectrochemical Biosensing Using TiO₂ Nanowires, DOI : 10.1002/chem.201406643

2- Bai et al. Titanium Dioxide Nanomaterials for Sensor Applications, DOI: 10.1021/cr400625j

A particularly alarming issue in world health today is the rise and prevalence of antibiotic-resistant bacteria, which significantly increases death rates and costs of treatment; and a group of pathogens responsible for the majority of hospital acquired infections – commonly referred to as the ‘ESKAPE’ pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) – have been named as one of the biggest threats to health as a result of their multidrug resistance. On 27 February 2017, WHO published list of bacteria for which new antibiotics are urgently needed. Although the Gram-positive bacteria in the ESKAPE group, including the methicillin-resistant Staphylococcus aureus, have rightly drawn attention over the past decade, infections caused by the Gram-negative microbes have recently been recognized as a more critical healthcare issue. Despite the fact that many Gram-negative bacteria have acquired antibiotic resistance, the pipeline for the development of new antimicrobials that target Gram-negative bacteria remains empty. The dearth of drug candidates against Gram-negative bacteria is attributed to the fact that they might be harder to kill than Gram-positive bacteria, largely due to the presence of an outer membrane (OM) that serves as a highly impermeable barrier, as well as additional defense mechanisms that might be absent in Gram-positive bacteria. Therefore, it is imperative that as these antibiotic-resistant bacteria evolve, so must the medicines that are utilized to treat them.

Nanomaterials are an alternative approach to treating and mitigating infections caused by resistant bacteria. Microbial cells are unlikely to develop resistance to nanomaterials, because they exert toxicity through different mechanisms than conventional antibiotics. For example, as a novel functional nanostructure material, graphene oxide is currently the subject of intense research due to the unique optical, electronic, and mechanical properties that result from its two-dimensional sp²-hybridized carbon structure. As graphene nanostructures have been found to exhibit limited toxicity towards eukaryotic cells, the utilization of graphene oxide for biological applications has been attracting significant attention from the scientific community.

Generally, there are three layers of complexity that are interconnected and need to be considered carefully in the development of graphene oxide for use in biomedical applications: material characteristics; interactions with biological components (tissues, cells, and proteins); and biological activity outcomes. To understand and follow antibacterial mechanisms of this family of nanomaterials, it is critical to know how graphene oxide properties are determinant in their bactericidal performances. The most important factors are the sheets size, concentration, surface area, surface roughness, dispersibility, hydrophilicity and surface functional groups. To know more about fundumentals and details of the subject, some useful review papers are published like:

1- Hegab et al. The controversial antibacterial activity of graphene-based materials. 

2- Shi et al. The Antibacterial Applications of Graphene and Its Derivatives

Supercapacitor devices, also known as electrical double-layer capacitors (EDLCs), store charge by adsorption of electrolyte ions onto the surface of electrode. No redox reactions are required, so the response to changes in potential without diffusion limitations is rapid and leads to high power. However, the charge is confined to the surface, so the energy density of EDLCs is less than that of batteries. In the 1970s, Conway and others recognized that reversible redox reactions occurring at or near the surface of an appropriate electrode material lead to EDLC-like electrochemical features but the redox processes lead to much greater charge storage. This pseudocapacitance represents a second mechanism for capacitive energy storage.

It seems now well-established that the term “hybrid” supercapacitor should be used when pairing two electrode with different charge storage behavior, i.e., one capacitive and one faradaic, and the resulting device is in-between a supercapacitor and a battery. “Asymmetric” supercapacitor covers a wider range of electrode combinations because it can be used for supercapacitors using electrodes of the same nature but with different mass loading, or two electrodes using different materials. We suggest the term “asymmetric” should be used only when capacitive or pseudocapacitive electrodes are involved (such as activated carbon//MnO₂ asymmetric supercapacitor) in order to avoid confusion with true “hybrid” devices.

More useful information on this subject can be found in informative published papers like:

1- Gogotsi et al. Where Do Batteries End and Supercapacitors Begin? Science, 2014.

2- Brousse et al. To Be or Not To Be Pseudocapacitive? journal of the Electrochemical Society, 2015.

The 2D materials are generally composed of strong covalent bonds leading to in-plane stability and weak van der Waals bonds, which sustain the stacked layer structure. Following the discovery of graphene in 2004, a new horizon has opened up for exploring other 2D layered materials such as transition metal dichalcogenides (TMD), transition metal oxides, graphitic carbon nitride (GCN), and hexagonal boron nitride (h-BN). These 2D materials can be integrated with a three dimensional (3D) SC material as a new building block to fabricate interfacial heterostructures. The electrocatalytic activity of this material strongly depends on its quality and morphology. For instance, defects and oxygen-containing groups may increase graphene electrocatalytic properties, while simultaneously reducing its electrical conductivity if they damage the conjugated Π structure.

In order to store sustainable energy, such as solar energy, and transform it into current power, photocatalytic or photoelectrochemical reactions in water splitting for hydrogen generation are promising candidates. Numerous informative papers on nanostructures for oxygen evolution reactions (OER) have been published. In addition, significant reports on applying 2D materials as counter electrodes in solar cells and/or water splitting devices, providing a thorough comparison concerning various synthesis and characterization methods of this class of new materials as well as their performance as a CE.

However, in many cases, the 2D materials themselves are neither photocatalysts nor photoelectrodes; however, these materials have been successfully applied as sensitizers, electron mediators, co-catalysts, and protective layers in combination with other SC materials. The 2D/SC hybrid materials can induce synergetic effects and ultimately improve the electrical, optical, and PEC properties of 2D/SC electrodes. Here, review of recent trends in this subject is available:

1- Irani et al. A review of 2D-based counter electrodes applied in solar-assisted devices, Coordination Chemistry Review, 2016.

2-  Faraji et al. Two-dimensional materials in semiconductor photoelectrocatalytic systems for water splitting, Energy & Environmental Science, 2018.