For over 30 years, porous silicon as a material has been leveraged for its usefulness in biomedical and sensing applications. Its tunable structural features, low toxicity profile and readily modifiable surface render this material extremely useful for a wide variety of applications. By chemically modifying surface species, such as silicon hydrides and silicon oxides with silanes, the properties of porous silicon can be enhanced for its use for chemical sensing, biomedical imaging, and drug delivery. After a brief introduction to porous silicon materials, the first part of this dissertation details surface-modified porous silicon photonic crystals for the chemical sensing of toxic vapors and nerve agents. Chapter 2 utilizes a dual-peak porous silicon photonic crystal embedded with specific for the selective detection of hydrogen fluoride (HF), hydrogen cyanide (HCN), and the chemical nerve agent diisopropyl fluorophosphate (DFP). The pore walls are rendered hydrophobic with octadecylsilane to aid with the loading of the colorimetric molecules while being insensitive to humidity fluctuations. This provides a robust means to develop a remote detection system for chemical agents. Chapter 3 employs the same photonic crystal, modified, however, with a specialized protein-based gatekeeper that is rendered semi-permeable only in the presence of HCN. This is one of the first novel designs of a bio-inorganic sensor capable of detecting chemical agents with high specificity and precision. The second portion of the dissertation describes how surface-modified porous silicon nanoparticles can be applied in biomedical applications. The first project details the use of Anti-KIT protein DNA-aptamers decorated onto a fluorescently labelled porous silicon nanoparticle for the in vitro and in vivo imaging of gastrointestinal stromal tumors. This work provides an effective platform in which aptamer-conjugated porous silicon nanoparticle constructs can be used for the targeted imaging of KIT-expressing cancers. The final project utilizes hydrophobic porous silicon nanoparticles for the delivery of erucamide, a highly hydrophobic fatty acid amide, within the retina. By harnessing the versatility of porous silicon, erucamide's target cells and mechanism of neurotrophic action can be identified.