Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

Detection of chemical and biological agents plays a fundamental role in biomedical, forensic and environmental sciences1–4 as well as in anti bioterrorism applications.5–7 The development of highly sensitive, cost effective, miniature sensors is therefore in high demand which requires advanced technology coupled with fundamental knowledge in chemistry, biology and material sciences.8–13

In general, sensors feature two functional components: a recognition element to provide selective/specific binding with the target analytes and a transducer component for signaling the binding event. An efficient sensor relies heavily on these two essential components for the recognition process in terms of response time, signal to noise (S/N) ratio, selectivity and limits of detection (LOD).14,15 Therefore, designing sensors with higher efficacy depends on the development of novel materials to improve both the recognition and transduction processes. Nanomaterials feature unique physicochemical properties that can be of great utility in creating new recognition and transduction processes for chemical and biological sensors15–27 as well as improving the S/N ratio by miniaturization of the sensor elements.28

Gold nanoparticles (AuNPs) possess distinct physical and chemical attributes that make them excellent scaffolds for the fabrication of novel chemical and biological sensors (Figure 1).29–36 First, AuNPs can be synthesized in a straightforward manner and can be made highly stable. Second, they possess unique optoelectronic properties. Third, they provide high surface-to-volume ratio with excellent biocompatibility using appropriate ligands.30 Fourth, these properties of AuNPs can be readily tuned varying their size, shape and the surrounding chemical environment. For example, the binding event between recognition element and the analyte can alter physicochemical properties of transducer AuNPs, such as plasmon resonance absorption, conductivity, redox behavior, etc. that in turn can generate a detectable response signal. Finally, AuNPs offer a suitable platform for multi-functionalization with a wide range of organic or biological ligands for the selective binding and detection of small molecules and biological targets.30–32,36 Each of these attributes of AuNPs has allowed researchers to develop novel sensing strategies with improved sensitivity, stability and selectivity. In the last decade of research, the advent of AuNP as a sensory element provided us a broad spectrum of innovative approaches for the detection of metal ions, small molecules, proteins, nucleic acids, malignant cells, etc. in a rapid and efficient manner.37



Figure 1

Physical properties of AuNPs and schematic illustration of an AuNP-based detection system.



In this current review, we have highlighted the several synthetic routes and properties of AuNPs that make them excellent probes for different sensing strategies. Furthermore, we will discuss various sensing strategies and major advances in the last two decades of research utilizing AuNPs in the detection of variety of target analytes including metal ions, organic molecules, proteins, nucleic acids, and microorganisms.

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Gold nanoparticles in chemical and biological sensing.

PATTERNING matter on the nanometre scale is an important objective of current materials chemistry and physics. It is driven by both the need to further miniaturize electronic components and the fact that at the nanometre scale, materials properties are strongly size-dependent and thus can be tuned sensitively1. In nanoscale crystals, quantum size effects and the large number of surface atoms influence the, chemical, electronic, magnetic and optical behaviour2—4. 'Top-down' (for example, lithographic) methods for nanoscale manipulation reach only to the upper end of the nanometre regime5; but whereas 'bottom-up' wet chemical techniques allow for the preparation of mono-disperse, defect-free crystallites just 1–10 nm in size6–10, ways to control the structure of nanocrystal assemblies are scarce. Here we describe a strategy for the synthesis of'nanocrystal molecules', in which discrete numbers of gold nanocrystals are organized into spatially defined structures based on Watson-Crick base-pairing interactions. We attach single-stranded DNA oligonucleotides of defined length and sequence to individual nanocrystals, and these assemble into dimers and trimers on addition of a complementary single-stranded DNA template. We anticipate that this approach should allow the construction of more complex two-and three-dimensional assemblies.

Organization of 'nanocrystal molecules' using DNA

Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

Nanomaterials, such as metal or semiconductor nanoparticles and nanorods, exhibit similar dimensions to those of biomolecules, such as proteins (enzymes, antigens, antibodies) or DNA. The integration of nanoparticles, which exhibit unique electronic, photonic, and catalytic properties, with biomaterials, which display unique recognition, catalytic, and inhibition properties, yields novel hybrid nanobiomaterials of synergetic properties and functions. This review describes recent advances in the synthesis of biomolecule-nanoparticle/nanorod hybrid systems and the application of such assemblies in the generation of 2D and 3D ordered structures in solutions and on surfaces. Particular emphasis is directed to the use of biomolecule-nanoparticle (metallic or semiconductive) assemblies for bioanalytical applications and for the fabrication of bioelectronic devices.

Integrated Nanoparticle–Biomolecule Hybrid Systems: Synthesis, Properties, and Applications

Polyurethane/polybutyl-methacrylate interpenetrating polymer networks (IPNs) film was formed on n-Si substrate by the dropping technique. When aluminum (Al) was vacuum deposited on the top of the film, the Al/IPNs/n-Si (metal–insulator–semiconductor) structure was fabricated successfully. With the aid of the high-frequency capacitance–voltage (C-V) characteristics at room temperature, the dielectric constant of IPNs was obtained. In the C-V curves, an increased hysteresis at high sweep voltage and a plateau in the depletion region were observed. This plateau indicates that the unsaturated bonds beyond IPNs film could act as electron well at the applied voltage above 10 V. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 721–727, 2000

Study of the dielectric constant of polyurethane/polybutyl‐methacrylate interpenetrating polymer networks using metal–insulator–semiconductor structure

Horseradish Peroxidase-Catalyzed Preparation and Optoelectronic Property of Poly(1,5-dihydroxynaphthalene) Composite in Porous Silicon Nanohosta

Size-dependent FTIR spectroscopy of nanoparticulate α-Fe2O3-stearate alternating Langmuir-Blodgett films

Effect of surface state properties of TiO2 nanoparticle film on its photocatalytic activity

The morphology of self-assembly formed by amphiphilic complementary components, 5-(4-dodecyloxybenzylidene)-2,4,6-(1H,3H)-pyrimidinetrione and 4-amino-2,6-didodecylamino-1,3,5-triazine, was changed from square and rectangle sheets into nanocolumns and helixes corresponding to a recognition pattern change from a linear tape to a cyclic hexamer or a helically grown tape by varying the polarity of the solvent.

Tiejin Li

Papers

Study of the dielectric constant of polyurethane/polybutyl‐methacrylate interpenetrating polymer networks using metal–insulator–semiconductor structure

Horseradish Peroxidase-Catalyzed Preparation and Optoelectronic Property of Poly(1,5-dihydroxynaphthalene) Composite in Porous Silicon Nanohosta

Size-dependent FTIR spectroscopy of nanoparticulate α-Fe2O3-stearate alternating Langmuir-Blodgett films

Effect of surface state properties of TiO2 nanoparticle film on its photocatalytic activity

Formation of nanocolumn self-assembly by solvent polarity control