Scientists from Utrecht University have built a new kind of sensor with an unparalleled ability to detect very small amounts of molecules. The unique sensor is approximately 500 times smaller than the width of a strand of human hair.
These sensors can be employed to find and identify minute quantities of substances like molecules or chemical pollutants essential in the field of medicine. The sensors utilize Raman scattering, a phenomenon that delivers such exclusive signals for diverse molecules that it is frequently denoted as “molecular fingerprinting.”
The scientists have presented the preparation and utilization of these miniature sensors in the journal Advanced Functional Materials.
Our design relies on the assembly of gold nanorods, which enhance the Raman scattering of molecules placed close to their tips tens of thousand times, into a larger spherical cluster in which the Raman signals are even further enhanced. A crucial step in the preparation was to first wrap each gold nanorod in its own protective porous coating.
Prof. Alfons van Blaaderen, Lead Researcher, Utrecht University
“By controlling the thickness and porosity of this coating, we were able to control how closely the nanorods could be packed together, and how easy or difficult it is for molecules to enter into the sensor,” Prof. Alfons van Blaaderen added.
Small Water Droplets
A crucial goal for the study’s lead authors Harith Gurunarayanan and Jessi van der Hoeven was to bring the coated rods together in a nano-sensor.
We wanted to controllably form a spherical cluster out of these rods, where so-called ‘hot spots’ for the Raman scattering would overlap and enhance the Raman signals even further. To do so, we put the rods in small water droplets. By slowly evaporating the water, the nanorods were forced to pack together into a spherical assembly.
Jessi van der Hoeven, Study Lead Author, Utrecht University
Using this method, the scientists could prepare an entire range of diversely structured nano-sensors.
Gurunarayanan states, “We were excited to see that these nanorod assemblies were not only pretty structures, but also very good at detecting very small quantities of molecules, better than previous assemblies of gold nanorods.”
Owing to the finger-printing features of the Raman scattering analysis, these supraparticles — particles developed from nanoparticles — are ideal in a number of applications, spanning from analyzing chemical mechanisms in catalysis, to identifying minute quantities of chemical pollutants and molecules essential in the fields of medicine or biology. It is imperative to state that portable Raman scattering equipment, which is comparatively expensive, is available at the lab.
Room for Improvements
Although the newly acquired sensing supraparticles outclassed formerly reported Raman sensing structures of gold nanorods, it is also stimulating that there is still a lot of space for significant enhancements on this preliminary design.
A number of concepts are already being investigated to further enhance the functionality and sensitivity of these assemblies. Both literally and figuratively, these Raman supraparticle sensors have a positive future ahead.
A new type of nano-sensor: a 3D spherical “supraparticle” of gold nanorods. Video Credit: Utrecht University.
Van der Hoeven, E. S., et al. (2022) Silica-Coated Gold Nanorod Supraparticles: A Tunable Platform for Surface Enhanced Raman Spectroscopy. Advanced Functional Materials. doi.org/10.1002/adfm.202200148.