A gold nanoparticle (diameter 10 nm) and a gold sphere (diameter 10 mm) are placed in identical chemical environments. The nanoparticle reacts far more rapidly. What fundamental property difference explains this?

Gold at the nanoscale is a different allotrope with a different crystal structure The nanoparticle has an enormously higher surface-area-to-volume ratio, so a much greater fraction of its atoms are at the surface and available for reaction The nanoparticle is closer in size to individual molecules, enabling direct bonding Nanoparticles have a higher melting point, which increases reaction rate

A single-walled carbon nanotube (SWCNT) has a measured tensile strength of approximately 100 GPa. High-strength steel has a tensile strength of about 1.8 GPa. How many times stronger is the SWCNT?

Approximately 10 times stronger Approximately 55 times stronger Approximately 200 times stronger Approximately 1,000 times stronger

Titanium dioxide (TiO₂) nanoparticles in modern sunscreen are transparent to visible light yet block UV radiation, while bulk TiO₂ powder is visibly white and opaque. What accounts for this difference?

The nanoparticle manufacturing process changes TiO₂ into a different chemical compound that is UV-absorbing but visible-transparent Nanoparticles are smaller than visible wavelengths (400–700 nm) so they scatter visible light negligibly, yet their electronic structure still absorbs UV photons Nanoparticles absorb visible light internally and re-emit it as heat, making them appear transparent The nano form of TiO₂ has a higher refractive index, bending visible light around the particles

A smartphone's MEMS accelerometer (such as the ADXL345) detects motion and orientation. How is it manufactured, and what enables its mechanical and electronic functions to coexist in a single chip?

Laser sintering fuses metal powder into tiny gears that drive a piezoelectric crystal, generating a voltage proportional to acceleration Photolithography etches tiny suspended mass-spring structures from silicon; the same wafer process integrates readout electronics, producing a monolithic electromechanical device The accelerometer is a miniaturised version of a macro gyroscope, assembled by pick-and-place robotics at the micron scale Self-assembled DNA scaffolds position carbon nanotube sensors that produce a current when bent by inertial forces

ASML holds a near-monopoly on Extreme Ultraviolet (EUV) lithography machines. An EUV system uses 13.5 nm wavelength light to print chip features as small as 2–3 nm. Why does EUV require a near-vacuum environment and reflective mirrors rather than glass lenses?

13.5 nm light refracts too strongly through glass to form sharp images Air and glass both absorb EUV radiation almost completely, so lenses are opaque at 13.5 nm and air must be evacuated from the beam path Mirrors are used because EUV light causes glass lenses to shatter from thermal expansion Vacuum is needed to prevent oxidation of the silicon wafer surface during UV exposure

A research team wants to build a nanoscale sensor using a bottom-up fabrication approach. They design DNA strands that fold into a precise 3D scaffold, positioning gold nanoparticle antennas at exact locations. How does this differ fundamentally from photolithography?

DNA origami uses biological rather than semiconductor materials, but the patterning mechanism is the same as photolithography Photolithography is top-down (patterns etched into existing material from above), while DNA origami is bottom-up (molecular components self-assemble into the target structure) Both approaches use masks; DNA origami simply uses biological templates instead of chrome masks Photolithography is bottom-up because it builds up photoresist layers, while DNA origami removes material to reveal structures

Multi-walled carbon nanotubes (MWCNTs) show promise as additives in polymer composites for aerospace structures. A manufacturer mixes MWCNTs at 2% by weight into an epoxy matrix. Despite the exceptional MWCNT tensile strength (~60–150 GPa), the composite's strength improvement is modest. What is the most likely reason?

2% by weight is far above the percolation threshold; excess nanotubes act as stress concentrators Poor interfacial bonding between MWCNTs and the epoxy matrix means load is not effectively transferred to the nanotubes MWCNTs are metallic conductors and therefore cannot reinforce insulating epoxy matrices At nanoscale dimensions, quantum effects reduce the effective stiffness below the macro-scale carbon fibre value

A hospital is evaluating silver nanoparticle coatings on door handles and bed rails to reduce pathogen transmission. A nanotoxicologist raises a concern about the coating. What legitimate concern does nanotoxicology introduce that would not apply to a bulk silver surface?

Silver nanoparticles are far more expensive, making coating hospital surfaces economically unviable Silver nanoparticles can be inhaled or absorbed through skin and cross biological barriers (blood-brain barrier, cell membranes) that bulk silver cannot penetrate, with incompletely understood toxicity Nanoparticle coatings always wear off faster than bulk metal coatings, reducing long-term antibacterial efficacy Silver nanoparticles are too reactive to remain stable on surfaces at room temperature

Iron nanoparticles are injected into contaminated groundwater at a polluted industrial site for in-situ remediation. What reaction mechanism makes nanoparticles more effective than injecting bulk iron filings?

Nanoparticles are magnetic and can be steered to the exact contamination zone using external magnetic fields The vastly higher surface area of nanoparticles provides far more reactive sites, enabling rapid chemical reduction of chlorinated pollutants throughout the aquifer Iron nanoparticles produce more heat per gram than bulk iron, thermally degrading the contaminants Nanoparticle surfaces generate UV light that photocatalyses breakdown of chlorinated compounds underground

A graphene membrane is proposed for seawater desalination. Single-atom-thick graphene with nanometre-scale pores can pass water molecules while blocking hydrated salt ions. What property of graphene makes an ultra-thin membrane structurally viable for industrial-scale pressure-driven filtration?

Graphene's zero electrical resistance means salt ions are repelled by the induced electric field Graphene's extraordinary in-plane strength (~130 GPa) allows a single-atom-thick sheet to withstand the applied hydraulic pressure without rupturing Graphene absorbs 2.3% of visible light, producing photocatalytic energy that drives salt ion separation Graphene's hydrophobic surface repels seawater, so only deionised water vapour permeates through defect pores

The photolithography process for making a MEMS pressure sensor involves spin-coating photoresist, exposing through a mask, developing, and etching. A process engineer introduces a small vibration during the UV exposure step. What defect is most likely to result?

The photoresist develops unevenly across the wafer due to temperature gradients from the vibration Blurred pattern edges on the photoresist, causing etched feature dimensions to deviate from design, reducing sensor accuracy The silicon wafer cracks from mechanical shock, destroying the substrate Vibration causes the UV lamp intensity to fluctuate, underexposing some regions and overexposing others

Carbon nanotubes can be either metallic or semiconducting depending on chirality — the angle at which the graphene sheet is rolled. A researcher wants to build a CNT-based transistor. Why is chirality control a critical manufacturing challenge, and how does it differ from conventional silicon transistor production?

CNT chirality controls diameter, and transistors require an exact 2 nm diameter; deviations cause current leakage Current synthesis produces a mixture of metallic and semiconducting CNTs; there is no reliable method to grow only semiconducting tubes, whereas silicon doping is precisely controllable Chirality affects CNT colour, and optical alignment in photolithography requires transparent nanotubes Metallic CNTs overheat and destroy the transistor gate oxide; only semiconducting CNTs can survive CMOS process temperatures