ICFO nanosensor improves biomolecule detection

31 May 2023

Hybrid gold and aluminum structures maintain sensitivity in the presence of background signals.

A project at Spain's ICFO research center has developed a new hybrid nanosensor composed of gold and aluminum, intended to enhance the fluorescence detection of single molecules.

Biosensors based on the new nanoscale design should provide improved sensitivity beyond the capabilities of current nanophotonic sensors, according to ICFO.

As reported in ACS Nano, the sensors exploit the design principle termed antenna-in-a-box (AiB), in which a nanoantenna is located within a nanoaperture in the structure.

When the light stimulating a response from a sample molecule is also confined in this small location, only fluorescent molecules in the "hotspot" are strongly excited and others remain dark, effectively enhancing the fluorescent signal.

"Biosensors based on metallic nanostructures are particularly useful for detecting fluorescent labels with high sensitivity," commented the ICFO team.

"But when trying to detect individual biomolecules in environments with many surrounding molecules, the fluorescence background generated by these labels can decrease the fluorescence detection sensitivity."

ICFO has already studied the use of AiB nanostructures made of gold as a means of enhancing a fluorescence signal, but gold is an inefficient material when it comes to screening out background signals, so AiB nanosensors based on that material are not ideal for the task.

The new study tackled this hurdle through a AiB design using a hybrid material system of gold and aluminum, specifically having a gold bowtie-shaped nanostructure inside a circular aluminum nanoscale aperture. Numerical modelling predicts that this architecture can maximise the signal enhancement, and ICFO also designed a manufacturing process for these novel structures based on a two-step electron beam lithography overlay operation.

A plethora of biosensing applications

ICFO modelling demonstrates that the new biosensors with hybrid AiB nanostructures can improve the single-molecule detection sensitivity through better background reduction and higher fluorescence emission of the fluorescent label. This combination of properties represents "a major gain in the detection of single biomolecules in the presence of strong background signals, which is the common scenario of biological samples."

Having designed the gold-aluminum structures used in this project, ICFO anticipates testing other material combinations suited to particular bioimaging tasks. Composite nanostructures also including silver/aluminum sensors could allow the study of multiple species via multicolor labelling, for example, especially if the different sensor types are combined in large arrays.

ICFO believes its platform also holds great potential for label-free sensing applications such as surfaced-enhanced Raman spectroscopy, which could benefit not only from reduced background signals but also from the architecture's strong enhancement factors; an advantage for techniques based on nonlinear responses and low scattering cross sections.

"We foresee that biosensors based on hybrid AiBs will provide improved sensitivity beyond the capabilities of current nanophotonic sensors for a plethora of biosensing applications, ranging from multicolor fluorescence detection to label-free vibrational spectroscopy," commented ICFO in its published paper.