Sensors & Transducers Journal, Vol. 113, Issue 2, February 2010, pp. 1-17
3.2. Porous Silicon Biosensors
Porous silicon (PS) and Si-nanocrystal have amazing properties that are particularly suitable for applications [57, 59] in the biosensor industry. Both PS and Si-nanocrystals have potential applications to optical biosensors, DNA detection sensors, or photodetectors [32, 46-51]. Sensors based on PS offer enhanced sensitivity, reduced power demands and low cost. A review article explaining various applications of PS as a transducer material has been reported recently by Andrew Jane, et al. . The interesting features about PS are its high surface area and reactive surface chemistry. Si-nanocrystals can also be obtained from PS [54-55] in aqueous form.
Porous Silicon is an electrochemically derived nanostructured material consisting of nanometer-sized silicon regions surrounded by empty space, and can be prepared as quantum wires or quantum dots. The quantum confinement of Si atoms in PS leads to interesting optical, chemical, and electronic properties. The visible room temperature photoluminescence (PL) and the electroluminescence properties of PS, along with the simplicity of its fabrication process, make it extremely convenient and useful material for several opto-electronic and sensor applications. The wavelength of the photoluminescent light can be changed by simply increasing or decreasing the porosity of the material. For example, a highly porous sample (70-80% porosity) will emit green/blue light while a less porous sample (40%) will emit red light. The most acceptable theory about this photoluminescence (PL) property is the quantum confinement effect where by confining the matter in the nanoscale dimension, the interaction between matter and light can be limited in nano dimension (as described in Section 3.1).
PS can be divided into three main categories based on their pore size: 1) for microporous porous silicon the pore width is less than 2 nm, 2) for mesoporous the pore width is in between 2nm to 50 nm, 3) for macroporous the pore width is greater than 50 nm. With appropriate modification of the electrochemical process, PS can also be fabricated to behave as 1-D photonic crystals . The intensity and wavelength of the reflected light is determined by the nanostructure, and these optical properties can be deployed in sensing of chemical and biological agents like viruses and bacteria . Because of their non-invasive and non-radioactive nature, they promise versatile applications to medical diagnostics, pathogen detection, gene identification, and DNA sequencing [12-13, 39].
The standard procedure for fabrication of nanostructured porous silicon is the electrochemical etching method in hydrofluoric (HF) acid solution. The etching resulted in a system of disordered pores with nanocrystals remaining in the inter-pore region. The pores propagate primarily in the 100 direction of the crystal. Almost all properties of PS, such as porosity, porous layer thickness, pore size and shape, as well as microstructure, strongly depend on the fabrication conditions. In the case of anodization, these conditions include HF concentration, chemical composition of electrolyte, current density (and potential), wafer type and resistivity, crystallographic orientation, temperature, time, electrolyte stirring, illumination intensity, and wavelength, etc. Thus, a complete control of the fabrication is complicated and all possible parameters should be taken into account. Some of these parameters also depend on each other.
The average diameter of the pores can be tuned from a few nanometers to several micrometers. Tuning the pore diameters and chemically modifying the surface allow developers to control the size and type of molecules adsorbed [60, 111]. The large surface area enables bio-organic molecules to adhere to the surface of the PS [81, 85]. Aqueous HF is suitable for the etching process because the silicon surface is hydrophobic. The porous layer can be made more structurally uniform if an ethanoic solution is used – this increases the wettability of the silicon and allows more surface penetration by the acid. Fig. 3 shows scanning electron microscope (SEM) images of varying n-type doped PS with different etching current densities [106, 107].