Semiconductor-graphene-metal (SGM) architectures for contaminant detection

The inexpensive detection of organic contaminants at very low concentrations is vital for those living in areas without easy access to clean water. The creation of a customizable semiconductor-graphene-metal (SGM) framework allows for the low-cost detection of a broad range of trace environmental contaminants.¹ The SGM film architecture is a structural design that enables control over both the size of silver nanoparticles and their relative concentration ('loading') when grown on graphene oxide. This film can then be employed for the detection of low-level contaminants via surface enhanced Raman spectroscopy (SERS, see Figure 1).

The SGM film is constructed from a thin membrane of TiO₂ (semiconductor), a single-layer of reduced graphene oxide (graphene), and silver nanoparticles (metal). Besides allowing the designer to select the size and loading of metal nanoparticles, the SGM films also have the advantage that graphene can adsorb certain organic molecules from its surface, thereby increasing the concentration of contaminant molecules adjacent to the metal nanoparticles. This combination of structural control and contaminant concentration provides enhanced sensitivity.

The silver nanoparticles support a large local electric field near their surface when resonant light excites their conduction electrons, causing them to oscillate in a coherent fashion.² The enhancement of light-induced processes, like Raman scattering, occurs for molecules near the surface of the nanoparticle. This type of enhanced Raman scattering leads to routine signal enhancements of 10⁶ or greater.³ Controlling the metal nanoparticle architecture in SGM films allows for the fabrication of versatile SERS substrates. The SGM films created in this work showed excellent SERS enhancement of the target molecule TAPP (tetra (4-aminophenyl) porphyrin): target concentrations as low as 5nM were detectable with a signal to noise ratio of ∼15 (see Figure 3A).

To achieve optimum sensitivity for SERS detection, control over both size and loading is essential. The semiconductor component in SGM films allows for the photosensitization of the film, enabling the illumination-controlled growth of silver nanoparticles. The electrons in TiO₂ are excited by UV-light and this excitation results in their transfer to, and subsequent transmission through, the reduced graphene oxide. Excess electrons that reside on the graphene surface, opposite the TiO_2, then become available for the reduction of silver ions to silver nanoparticles (see Figure 2). As an added benefit, the many defects inherent in reduced graphene oxide serve as seeding sites to achieve high loading of metal nanoparticles. Using this method, one can control both the size and loading by simply adjusting illumination time and metal ion concentration.

Figure 1. Semiconductor-graphene-metal films enable detection of low-level organic contaminants.

Figure 2. (A) Schematic depicting the structure and electron transfer events of SGM films: TiO₂electrons excited by UV-light are transferred to the reduced graphene oxide layer. Excess electrons that have passed through the graphene and are located on its surface are then available for the reduction of silver ions to silver particles. (B) Scanning electron micrograph of silver metal nanoparticles loaded onto the reduced graphene oxide surface of a SGM film. Reprinted¹ with permission. Copyright 2012, American Chemical Society.

Figure 3. (A) SGM films used as SERS sensors display strong signal to noise detection of 5nM concentrations of TAPP (tetra (4-aminophenyl) porphyrin), the target molecule. (B) Schematic showing the electron transfer from TiO₂(titanium dioxide) to graphene; the electron hops to the opposite side of the graphene film and reduces silver ions to nanoparticles. Reprinted with permission from ref. 1. Copyright 2012. American Chemical Society.

In this work, the electron transfer events necessary for the formation of SGM films have been demonstrated by Lightcap et. al.; electrons hop from one side of graphene to the other (Figure 3B) and this process allows for the separation of metal from semiconductor nanoparticles by a single layer of graphene. Architectures such as these show potential for application in a number of fields where control over composition, size and loading of semiconductor and metal nanoparticles is paramount: for example, in catalytic applications such as the photocatalytic generation of solar fuels. An SGM architecture used in this role, with the semiconductor and metal nanoparticles located on different sides of a graphene sheet, may be useful in creating hybrid materials with the ability to split water into oxygen and hydrogen. These architectures are currently being adapted using various metals to achieve different types of functionality for selective and versatile sensing applications.