A photoelectrochemical (PEC) DNA biosensor is developed using surface‐modified TiO2 nanoparticles (NPs) as a sensitive transducer. Different catecholates and gallates are used as sensitizers for TiO2 NPs. The molecules are adsorbed on TiO2 via the catecholate type bonding mechanism to enhance light absorption in the visible range. The adsorbed molecules act as charge transfer mediators and enhance photocurrent. Despite the similar bonding mechanism of the molecules, the TiO2 NPs exhibit significant differences in photocurrent. The modified TiO2 films showed photocurrent increase in the order: 3,4‐dihydroxy‐L‐phenylalanine < 2,3,4‐trihydroxybenzoic acid < 3,4‐dihydroxybenzoic acid < 2,3,4‐trihydroxybenzaldehyde < 3,4‐dihydroxyphenylacetic acid < 3,4‐dihydroxybenzaldehyde < caffeic acid. Testing results provide an insight into the influence of the structure and properties of the organic molecules on their adsorption and photocurrents of modified TiO2 films. The TiO2 NPs modified with caffeic acid are used for the fabrication of PEC DNA biosensor by forming photoelectrodes and immobilizing probe single‐stranded DNA on their surface. The caffeic acid‐modified TiO2‐based photoelectrodes offer the required signal magnitude to distinguish between complementary and non‐complementary DNA sequences in the 100 nM–1 pM DNA concentration range and with a limit of detection of 1.38 pM, paving the way towards PEC DNA sensing.

Metal oxide nanostructures are increasingly important materials for various emerging photocatalytic, photovoltaic and photoelectrochemical (PEC) applications. They are commonly used as photoelectrode materials due to their unique functional properties such as wide bandgap, reactive electronic transitions, and high stability. To increase the effectiveness of semiconductor metal oxides photoelectrodes, researchers seek to use various photoabsorption amplification and colloidal stability enhancement strategies. An effective method for achieving this is the surface functionalization of metal oxide semiconductors with catechol‐type ligands. Catechol‐type ligands are a family of organic molecules that adsorb very strongly onto metal oxides by forming complexes with metal atoms through adjacent phenolic —OH groups. Once adsorbed, catechol‐type ligands facilitate improved particle dispersion by inhibiting agglomeration and enhance photoexcitation in metal oxide semiconductors by improving visible light absorption. Herein, the surface complexation of catechol‐type ligand onto metal oxide semiconductor surfaces and their photoabsorption enhancement mechanisms is described. In addition, recent advances and trends in this area are described by presenting recent advancements made in applications of catechol‐modified metal oxide systems in photocatalysis, PEC biosensing, and solar cells.

There is an increasing interest in using photoelectrochemistry for enhancing the signal-to-noise ratio and sensitivity of electrochemical biosensors. Nevertheless, it remains challenging to create photoelectrochemical biosensors founded on stable material systems that are also easily biofunctionalized for sensing applications. Herein, a photoelectrochemical immunosensor is reported, in which the concentration of the target protein directly correlates to a change in the measured photocurrent. The material system for the photoelectrode signal transducer involves using catecholate ligands to modify the properties of TiO2 nanostructures in a three-pronged approach of morphology tuning, photoabsorption enhancement, and facilitating bioconjugation. The catecholate-modified TiO2 photoelectrode is combined with a signal-off direct immunoassay to detect interleukin-6 (IL-6), a key biomarker for diagnosing and monitoring various diseases. Catecholate ligands are added during hydrothermal synthesis of TiO2 to enable the growth of three-dimensional nanostructures to form highly porous photoelectrodes that provide a three-dimensional scaffold for immobilizing capture antibodies. Surface modification by catecholate ligands greatly enhances photocurrent generation of the TiO2 photoelectrodes by improving photoabsorption in the visible range. Additionally, catecholate molecules facilitate bioconjugation and probe immobilization by forming a Schiff-base between their −COH group and the −NH2 group of the capture antibodies. The highest photocurrent achieved herein is 8.89 μA cm–2, which represents an enhancement by a factor of 87 from unmodified TiO2. The fabricated immunosensor shows a limit-of-detection of 3.6 pg mL–1 and a log–linear dynamic range of 2–2000 pg mL–1 for IL-6 in human blood plasma.

Photoactive electrodes with high photon-to-electron conversion efficiency are key to achieving sensitive photoelectrochemical sensors. Among all the photoactive materials, titanium dioxide (TiO2) nanoparticles have attracted much attention due to their unique electronic and optical properties. However, the large bandgap of TiO2 results in limited photocurrent signal generation under visible irradiation, which is important for its use in many applications including sensing. Herein, we modified TiO2 nanoparticles with both pyrocatechol violet and graphene quantum dots to obtain high photocurrents at visible light excitation while also improving TiO2 nanoparticle dispersion and film forming properties. This material system enhances photocurrent five-fold compared to TiO2 nanoparticles that are modified with only pyrocatechol violet and 60 times compared to TiO2 nanoparticles modified with graphene quantum dots. Additionally, the optimized photoelectrodes were used to detect hexavalent chromium (Cr(VI)), which has been reported as a toxic carcinogen. Under visible light irradiation, the fabricated sensor offered a low limit-of-detection of 0.04 µM for Cr(VI), with selectivity against Na, Mg, Cu, and Cr (III) ions, paving the route toward photoelectrochemical Cr(VI) sensing.

A dip-coating technique is designed for deposition of poly(methyl methacrylate) (PMMA) from water/2-propanol mixture, avoiding the use of traditional toxic solvents. Solutions of PMMA macromolecules with high molecular weight (MW) are obtained for a water/2-propanol ratio of 0.15–0.33 and the solubilization mechanism is discussed. The ability to use concentrated PMMA solutions and high MW of the polymer are the key factors for the successful dip coating deposition. The coating mass for 10 g L−1 polymer solutions shows a maximum at a water/2-propanol ratio of 0.25. The deposition yield increases with the polymer concentration increase and with an increasing number of the deposited layers. PMMA deposits protect stainless steel from aqueous corrosion. The coating technique allows for the fabrication of composite coatings, containing flame-retardant materials (FRMs), such as commercial halloysite, huntite, hydrotalcite, and synthesized Al(OH)3, in the PMMA matrix. The FRM content in the coatings is modified by variation of the FRM content in colloidal suspensions. A fundamentally new method is developed, which is based on the salting out aided dispersive extraction of Al(OH)3 from the aqueous synthesis medium to 2-propanol. It is based on the use of hexadecylphosphonic acid molecules as extractors. The method offers advantages of reduced agglomeration.