Nanostructured Titanium Dioxide Surfaces for Electrochemical Biosensing

Abstract

TiO₂ electrochemical biosensors represent an option for biomolecules recognition associated with diseases, food or environmental contaminants, drug interactions and related topics. The relevance of TiO₂ biosensors is due to the high selectivity and sensitivity that can be achieved. The development of electrochemical biosensors based on nanostructured TiO₂ surfaces requires knowing the signal extracted from them and its relationship with the properties of the transducer, such as the crystalline phase, the roughness and the morphology of the TiO₂ nanostructures. Using relevant literature published in the last decade, an overview of TiO₂ based biosensors is here provided. First, the principal fabrication methods of nanostructured TiO₂ surfaces are presented and their properties are briefly described. Secondly, the different detection techniques and representative examples of their applications are provided. Finally, the functionalization strategies with biomolecules are discussed. This work could contribute as a reference for the design of electrochemical biosensors based on nanostructured TiO₂ surfaces, considering the detection technique and the experimental electrochemical conditions needed for a specific analyte.

Keywords: electrochemical biosensors; functionalization with biomolecules; nanostructured surfaces; titanium dioxide.

Figure 1

The electrochemical anodization process for obtaining TiO₂ NTs.

Figure 2

Sol-gel process for obtaining TiO₂ thin films. The process begins with the formation of the sol by hydrolysis and condensation of titanium alkoxides mixed with alcohol and catalytic agents. After the deposition, by dip-coating, the film is formed. Next, a thermal treatment is used for the preparation of nanocrystalline thin films.

Figure 3

Hydrothermal method for obtaining TiO₂ NWs.

Figure 4

SP configuration for obtaining TiO₂ thin films. A static ultrasonic nebulizer generates a uniform distribution of the aerosols in the diameter range of 1 to 3 µm. The aerosols generated are transported to the heated substrate (between 300 and 550 °C), evaporate and react (on the substrate) to form the TiO₂ films.

Figure 5

Scheme showing the preparation of TiO₂ thin films using ALD. TDMATi reacts with the –OH terminated surface of the Ti substrates (with TiO₂ superficial), leaving a methyl terminated surface. N₂ subsequently cleans the surface of compounds not adhering to the substrate. Water later reacts with the methyl groups, creating a surface with –OH groups. Next, N₂ cleans the surface again. This process is repeated over several cycles to obtain a film with the desired thickness.

Figure 6

Schematic representation of the reactive sputter deposition process with OAD configuration. Oxygen atoms, originating from O₂ gas molecules, can react with sputtered Ti atoms on the substrate to form a TiO₂ film. The magnetic field (generated by the magnets) produces a force on the electrons, which keeps them on a helical path close to the target for relatively long times. In this way, very low sputtering gas pressures can be used together with an OAD configuration to allow the ejection of atoms to reach the substrate in an oblique direction.

Figure 7

Electron-beam physical vapor deposition with OAD configuration for obtaining TiO₂ NCs. A target anode (contained in a crucible) is bombarded with an electron beam emitted by a charged tungsten filament (electron gun) under high vacuum. The electron beam causes the evaporation of the target material: the ejected atoms react with oxygen atoms from O₂ gas molecules and precipitate into solid form, coating the substrate with TiO₂ nanocolumns.

Figure 8

Typical signals to be measured from the different electrochemical biosensors (for any detector, support electrolyte and redox molecule used). When an analyte is detected, (a) the characteristic I-V curve for a voltammetric biosensor, (b) I-t for an amperometric biosensor, (c) V-t for a potentiometric biosensor, (d) I-V for a conductometric biosensor, (e) Z”-Z’ for an impedimetric biosensor or (f) drain current-drain voltage for a FET biosensor is obtained.

Figure 9

Two typical bioreceptor-analyte reactions of biosensors with a surface consisting of TiO₂ NTs. Reduction of H₂O₂ catalyzed by the immobilized hemoglobin on the biosensor (a) and oxidation of glucose catalyzed by GOD immobilized on the biosensor (b).

Figure 10

The enzymatic catalytic hydrolysis reaction in the vicinity of the urease functionalized Ti/TiO₂ electrode.

Figure 11

Schematic representation of the configuration of the microfluidic biosensor with 3D porous GF electrode modified with carbon-doped TiO₂ NFs for the detection of breast cancer biomarkers.

Figure 12

Schematic representation of the configuration of the BioFET with a back-gate.

Figure 13

Schematic representation of the preparation of the functionalized WE for the detection of MCF-7 cells. Initially, a blockade of non-specific sites of WE is performed with BSA (a), and then WE are treated with the cells for detection (b).