# DBT JRF 2022 — Question 44

Exam: DBT JRF 2022 · Section: B · Topic: Cell Biology › Fluorescence Microscopy & Quantum Dots · Difficulty: Medium

## Question

The color of emitted light of Quantum dots used in fluorescence microscopy depend on:

## Options

- **A.** The antibody they are coupled with
- **B.** The size of the nanocrystal
- **C.** Time at which they are visualized
- **D.** The material that coats them

## Answer

Option B

## Explanation

Quantum dot emission and particle size: Quantum dots (semiconductor nanocrystals, e.g., CdSe/ZnS) exploit quantum confinement: • As particle size DECREASES, the band gap INCREASES (quantum confinement effect). • Larger band gap → higher energy photons emitted → BLUE/shorter wavelength emission. • Smaller band gap (larger QD) → lower energy photons → RED/longer wavelength emission. Practical range: • ~2 nm QD → blue emission (~450 nm) • ~5 nm QD → green emission (~530 nm) • ~7 nm QD → red/orange emission (~620 nm) The emission wavelength is determined by PARTICLE SIZE (not chemical composition alone). Answer: Quantum dot emission wavelength is determined by its size; larger dots emit at longer (red) wavelengths.

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