Optical Biosensing and Bioimaging with Porous Silicon and Silicon Quantum Dots ( Invited Review )

Silicon is the second most abundant element in Earth’s crust, and it is considered one of the most important materials for the world. Crystalline silicon has continued to serve as the foundational building block for the microelectronic industry, and new forms of silicon materials have promised an even brighter future with emerging applications from optoelectronic devices, energy and environment technologies and new therapeutics [1–3]. Many of these promises are often associated with reduction of the physical size of the material to the micro/nano scale which yields novel physical properties. For this reason, understanding and learning how to control these features is of high importance, and unsurprisingly, low dimension silicon structures have drawn broad research interests from physicists, chemists, materials engineers and medical scientists. At the new frontiers of nanostructure silicon research, biomedical applications are very appealing because silicon is highly biocompatible [4]. With the small sized silicon materials suitable for these applications, two distinct structures are porous silicon, and silicon nanocrystals which are also called quantum dots. Porous silicon is a form of crystalline silicon where the surface is embedded with nanometer sized pores [5], while silicon quantum dots are ultrasmall crystals of only a few nanometers in size [6]. They both exhibit unique optical features suitable for sensing and imaging, which can be tuned via comparable surface engineering methods. For this reason, this review combines the two subjects in one article, with the scope of advancing th fields through a comparative approach. Since both porous silicon and silicon quantum dots have been actively researched in the past two decades and multiple excellent reviews have been published [3, 5, 7], this paper will only highlight recent progresses in the past several years.