Nanoparticles are tiny particles with dimensions typically between 1 and 100 nanometers, where materials exhibit unusual physical and chemical properties compared to their bulk form. At this scale, surface area is extremely high relative to volume, leading to enhanced reactivity, altered electrical behavior, and distinct optical effects such as vivid colors from quantum size effects in metals and semiconductors.

Synthesis methods fall into two broad categories: top down and bottom up. Top down approaches, such as milling and lithography, start from larger solids and break them into nanoscale pieces, but can introduce defects and size irregularities. Bottom up methods, including chemical precipitation, sol gel processing, vapor deposition, and self assembly, build nanoparticles atom by atom or molecule by molecule, often giving better control over size, shape, and composition.

Metal nanoparticles like gold and silver are widely studied for catalysis, sensing, and medical imaging. Semiconductor quantum dots are used in displays and bioimaging because their emission color depends sensitively on particle size. Magnetic nanoparticles, typically iron oxides, enable targeted drug delivery, magnetic resonance contrast, and hyperthermia cancer treatments. Carbon based nanomaterials, including fullerenes and carbon nanotubes, provide exceptional mechanical strength and unusual electronic properties.

Applications span medicine, where nanoparticles can deliver drugs, genes, or imaging agents with high specificity; energy technologies, such as improved batteries, fuel cells, and solar cells; and environmental uses, including pollutant removal and contaminant detection. At the same time, there is active research on toxicity, environmental fate, and safe design, as nanoscale materials may interact with cells, tissues, and ecosystems in ways not predicted from their bulk counterparts.