There is a constant search for innovative tools for diagnostics and bioanalytics: They should be able to detect molecules of interest with sufficient sensitivity, preferably label-free and multiplexed, to be usable outside of dedicated laboratories and with less qualified personnel, at minimal costs.
Plasmonic nanostructures promise to provide sensing capabilities with the potential for ultrasensitive and robust assays in a high parallelization and miniaturization, and without the need for markers. When functionalized with certain biomolecules (such as single-stranded DNA or antibodies) which bind the target molecule of interest (e.g., a biomarker like DNA or a protein), they should capture this target molecule with high specificity. Upon binding of target molecules, the localized surface plasmon resonance (LSPR) of these structures is changed, and can be used as sensoric readout. This is possible even on a single nanostructure level, using optical darkfield detection introduced more than 100 years ago, as demonstrated for DNA detection. In contrast to SPR, LSPR senses only in a very thin layer (on the scale of the particle diameter), resulting in an efficient background suppression. Only molecules bound directly at the surface contribute significantly to the signal, but much less molecules farer away.
In order to multiplex this approach, imaging spectrometer setups, e.g. based on a Michelson interferometer or multiple LEDs have been developed, able to readout a whole array of sensors in one step. On the sensor side, microarrays of gold nanoparticle spots were fabricated using spotting of pre-synthesized gold nanoparticles. Such chemically synthesized particles allow for a cost-efficient generation of highly crystalline particles as nanosensors; by using microfluidic approaches, a higher quality and reproducibility can be achieved. Using this microarray approach, a multiplex DNA-targeting detection of fungal pathogens involved in sepsis could be demonstrated. DNA-based signal amplification, e.g. by hybridization chain reaction, improves the sensitivity. Beyond DNA detection, LSPR sensing is also applicable for the detection of protein targets, such as CRP.
In conclusion, the results demonstrate the potential of this LSPR-based array platform for molecular detection of biomolecules of interest, with possible applications in bioanalytics and diagnostics.
Funding by BMBF (02WIL1521, 01DR20010A and 13N15717) and TAB (2018 VF 0015; 2018 FE 9039).