Surface-Enhanced Raman Spectroscopy-Based Label-Free Insulin Detection at Physiological Concentrations for Analysis of Islet Performance.

Label-free optical detection of insulin would allow in vitro assessment of pancreatic cell functions in their natural state and expedite diabetes-related clinical research and treatment; however, no existing method has met these criteria at physiological concentrations. Using spatially uniform 3D gold-nanoparticle sensors, we have demonstrated surface-enhanced Raman sensing of insulin in the secretions from human pancreatic islets under low and high glucose environments without the use of labels such as antibodies or aptamers. Label-free measurements of the islet secretions showed excellent correlation among the ambient glucose levels, secreted insulin concentrations, and measured Raman-emission intensities. When excited at 785 nm, plasmonic hotspots of the densely arranged 3D gold-nanoparticle pillars as well as strong interaction between sulfide linkages of the insulin molecules and the gold nanoparticles produced highly sensitive and reliable insulin measurements down to 100 pM. The sensors exhibited a dynamic range of 100 pM to 50 nM with an estimated detection limit of 35 pM, which covers the reported concentration range of insulin observed in pancreatic cell secretions. The sensitivity of this approach is approximately 4 orders of magnitude greater than previously reported results using label-free optical approaches, and it is much more cost-effective than immunoassay-based insulin detection widely used in clinics and laboratories. These promising results may open up new opportunities for insulin sensing in research and clinical applications.

Simple, Large-Scale Fabrication of Uniform Raman-Enhancing Substrate with Enhancement Saturation.

It is well-known that gold nanoparticle (AuNP) clusters generate strong surface-enhanced Raman scattering (SERS). In order to produce spatially uniform Raman-enhancing substrates at a large scale, we synthesized vertically perforated three-dimensional (3D) AuNP stacks. The 3D stacks were fabricated by first hydrothermally synthesizing ZnO nanowires perpendicular to silicon wafers followed by repetitively performing liquid-phase deposition of AuNPs on the tops and side surfaces of the nanowires. During the deposition process, the nanowires were shown to gradually dissolve away, leaving hollow vestiges or perforations surrounded by stacks of AuNPs. Simulation studies and experimental measurements reveal these nanoscale perforations serve as light paths that allow the excitation light to excite deeper regions of the 3D stacks for stronger overall Raman emission. Combined with properly sized nanoparticles, this feature maximizes and saturates the Raman enhancement at 1-pM sensitivity across the entire wafer-scale substrate, and the saturation improves the wafer-scale uniformity by a factor of 6 when compared to nanoparticle layers deposited directly on a silicon wafer substrate. Using the 3D-stacked substrates, quantitative sensing of adenine molecules yielded concentrations measurements within 10% of the known value. Understanding the enhancing mechanisms and engineering the 3D stacks have opened a new method of harnessing the intense SERS observed in nanoparticle clusters and realize practical SERS substrates with significantly improved uniformity suitable for quantitative chemical sensing.