Quantum-dot Spectrometer Fits In A Smartphone

MIT scientists have demonstrated they can make spectrometers small enough to fit inside a smartphone camera, utilizing miniscule semiconductor nanoparticles called quantum dots. According to lead study author Jie Bao devices, the devices could be used to diagnose diseases, especially skin conditions, or to detect environmental pollutants and food conditions.

This also is a new application for quantum dots, which have previously been used mostly for labeling cells and biological molecules, as well as in computer and television screens.

Quantum dots, a type of nanocrystals discovered in the early 1980s, are made by combining metals like lead or cadmium with other elements including sulfur, selenium, or arsenic.

Through manipulation of the ratio of the starting materials, the temperature, and the reaction time, scientists can fabricate a nearly unlimited number of dots with differences in an electronic property known as bandgap, which determines the wavelengths of light that each dot will absorb.

The new quantum dot spectrometer includes hundreds of quantum dot materials that each filter a specific set of wavelengths of light. The quantum dot filters are printed into a thin film and placed on top of a photodetector such as the charge-coupled devices (CCDs) found in cellphone cameras.

The more quantum dot materials present, the more wavelengths, and the higher the resolution that can be obtained. Here, researchers used about 200 types of quantum dots spread over a range of about 300 nanometers. With more dots, such spectrometers could be designed to cover an even wider range of light frequencies.

Incorporated into a small handheld device, this kind of spectrometer could be used to diagnose skin conditions or analyze urine samples, Bao says. They could also be used to track vital signs such as pulse and oxygen level, or to measure exposure to different frequencies of ultraviolet light, which vary greatly in their ability to damage skin.

Jie Bao & Moungi G. Bawendi
A colloidal quantum dot spectrometer
Nature 523, 67–70 (02 July 2015) doi:10.1038/nature14576

Illustration: Quantum Dot (QD) spectrometer device printing QD filters — a key fabrication step. Other spectrometer approaches have complicated systems in order to create the optical structures needed. Here in the QD spectrometer approach, the optical structure — QD filters — are generated by printing liquid droplets. This approach is unique and advantageous in terms of flexibility, simplicity, and cost reduction. Credit: MIT