Properties and Uses of Nanoparticles

Surface Area to Volume Ratio

As a particle gets smaller, a fascinating mathematical shift happens: its surface area to volume ratio increases significantly. This means a much higher proportion of its atoms are exposed on the surface rather than being hidden inside the particle. A useful rule to remember is the "Factor of 10 Rule": if you decrease the side length of a cube by a factor of 10, its surface area to volume ratio increases by a factor of 10.

What is the surface area to volume ratio of a cube-shaped nanoparticle with a side length of 5 nm, compared to a larger fine particle with a side length of 50 nm?

Step 1: Calculate the surface area and volume of the larger fine particle.

• $\text{Surface Area} = (50 \times 50) \times 6 = 15,000\text{ nm}^2$
• $\text{Volume} = 50 \times 50 \times 50 = 125,000\text{ nm}^3$
• $\text{SA:V Ratio} = \frac{15,000}{125,000} = \mathbf{0.12 : 1}$

Step 2: Calculate the surface area and volume of the nanoparticle.

• $\text{Surface Area} = (5 \times 5) \times 6 = 150\text{ nm}^2$
• $\text{Volume} = 5 \times 5 \times 5 = 125\text{ nm}^3$

Step 3: Compare the results.

• The side length decreased by a factor of 10 (from 50 nm to 5 nm), and the SA:V ratio increased by a factor of 10 (from 0.12 to 1.2).

Nanoparticles in Sunscreens

Because of their incredibly high surface area to volume ratio, nanoparticles behave very differently from the exact same substance in bulk material form. A perfect example is the use of titanium dioxide ($TiO_2$) in sunscreens. In its bulk form, titanium dioxide is a white, opaque solid that reflects visible light (it is even used as a pigment in house paint). However, when manufactured as nanoparticles, it becomes transparent and invisible on the skin. This occurs because the nanoparticles are physically smaller than the wavelength of visible light ($380\text{--}750\text{ nm}$), meaning they cannot scatter or reflect it.

Despite being invisible, these nanoparticles are still large enough to absorb and scatter harmful ultraviolet (UV) radiation. Their huge surface area to volume ratio provides significantly better skin coverage than bulk materials. They can be spread far more thinly and evenly, leaving no tiny microscopic gaps for UV radiation to penetrate, which provides vastly superior protection against skin cancer.

Other Uses: Catalysts and Antibacterial Agents

The unique properties of nanoparticles also make them exceptional catalysts. Because so many atoms are exposed at the surface and available to react, a much smaller quantity of nanoparticles is required to achieve the exact same catalytic effect as a larger, heavier block of bulk material. Similarly, silver nanoparticles possess strong antibacterial properties that bulk silver does not have. They are frequently woven into wound dressings, plasters, and even socks to prevent infections and bacterial odours.

Risks and Ethical Concerns

While extremely useful, the exceptionally tiny size of nanoparticles introduces unique risks. The most common concern raised by scientists is that the long-term health effects are not yet fully understood. Because they are so incredibly small, there is a risk that nanoparticles in sunscreens or cosmetics could penetrate the skin and enter the bloodstream or individual cells. Once inside the body, they might catalyse harmful internal reactions or cause toxic effects. Furthermore, spray-on sunscreens present a serious inhalation risk, where nanoparticles could be breathed directly into the lungs. Environmentally, nanoparticles washed off in the shower or the ocean can accumulate in water systems, potentially harming aquatic life.