Sensing – LSPR & SERS
Noble metal nanostructures display localized surface plasmon resonance (LSPR) properties, which originate from the collective oscillation of the conduction electrons of nanostructures when their resonance frequency matches with the incident photons. The unique LSPR properties can be exploited to design and fabricate nanoplasmonic sensors. Metallic nanostructures with sharp tips and edges are particularly important for sensing applications because of their strong electromagnetic (EM) field enhancement at these areas. Our research program is focused on design and fabrication of solid-state sensors for ultrasensitive detection of biomolecules (e.g., proteins and microRNAs) that could be used as disease biomarkers directly in human biofluids. For this quest, we have used chemically synthesized gold triangular nanoprisms (Au TNPs) that possess sharp tips and edges, as plasmonic transducers for solid-state sensors fabrication. Our research goals are to selectively manipulate nanoscale structural parameters such as the sensing volume and decay length of Au TNPs and their surface ligand chemistry in order to achieve high sensitivity. We are able to fabricate Au TNP-based LSPR microRNA sensors to assay various microRNAs at attomolar concentration (10-18 M) in plasma and serum of pancreatic cancer patients. We hypothesize that such unprecedentedly high sensitivity is caused by a novel LSPR-based transduction mechanism involving delocalization of wave-functions inducing electron movement in the duplex DNA helix resulting in a substantial influence on the LSPR properties of our sensors. We are actively pursuing this hypothesis by controlled variation of both DNA and microRNA structures.
Finding an ultrasensitive LSPR response in our Au TNPs in biosensing prompted us to design and fabricate the first solid-state, LSPR-based molecular sensor to characterize the photo-induced isomerization of a molecular machine (e.g., azobenzene). The photoresponsive molecule azobenzene shows promise because it is capable of undergoing a reversible cis to trans photo-isomerization, which can be used in optical information storage and other applications. However, photoswitchable solid-state molecular devices have not yet been designed in which the photo-induced conformational change of azobenzene could be detected by monitoring the LSPR properties of a metal nanostructures. Utilizing the strong EM field enhancement of Au TNPs at their sharp tips and edges, where the near-field is intense (hot spots), we further investigated azobezene conformational change and its photoreversibility through surface-enhanced Raman spectroscopy (SERS). We also demonstrated that SERS enhancement at such field hotspots decays over a distance of few Å, much shorter than the typical decay length reported for SERS. For this investigation, we constructed SERS sensors utilizing Au TNPs with azobenzene reporter molecules linked to the TNP surface using variable chain length alkanethiol spacers allowing additional control.
The prevalence of explosives in terrorist attacks requires availability of inexpensive, swift, accurate, reliable, and reproducible detection ability in field blast samples without extraction or swabbing is extremely important. However devices with these characteristic have remained a big challenge for homeland security. Utilizing the unique SERS characteristics of our Au TNPs, we are developing highly sensitive, SERS-based, self-assembled, inexpensive, reliable, and reproducible sensors for explosive detection. We utilize a “Lift-off” approach to fabricate SERS substrate onto a 3M adhesive tape that serve as a flexible sensor. Importantly, our flexible sensor provides convenient sampling efficiency and obviates the need of swabbing and extraction procedures that are required for the GC-MS method. Moreover, our SERS sensors can be successfully used to detect illicit drugs at parts-per-trillion levels. We are actively pursuing this research to further optimize our sensors response through modification of (i) Au TNPs assembly on the flexible substrate, (ii) surface chemistry, and (iii) and flexible substrate materials for in-field homeland security and forensic applications.