Nanoparticles are remarkably small particles that fall within the nanoscale, a measurement scale commonly utilized in the field of nanotechnology. They typically range in size from $1$ to $100$ nanometres. To provide some context, a nanometre is equivalent to one billionth of a metre, which means that nanoparticles are thousands of times smaller than the width of a human hair.

The unique properties of nanoparticles arise primarily from their size. Due to their diminutive dimensions, they possess a high surface area-to-volume ratio. This characteristic results in a greater proportion of their atoms being located at the surface compared to larger particles. Consequently, nanoparticles can exhibit distinct chemical and physical properties, such as enhanced reactivity or alterations in color. For instance, gold nanoparticles can appear red or purple, despite bulk gold being yellow.

The size of nanoparticles can be precisely controlled during their synthesis. This is accomplished by adjusting various parameters of the synthesis process, such as temperature, pressure, and the concentration of reactants. By manipulating these conditions, scientists can tailor the properties of nanoparticles for specific applications. For example, smaller nanoparticles may be utilized in medicine for targeted drug delivery, while larger nanoparticles may be employed in electronics due to their conductive properties.

In summary, nanoparticles are exceptionally small entities that typically range in size from $1$ to $100$ nanometres. Their small size endows them with unique properties, and these properties can be fine-tuned during synthesis to meet the demands of various applications.