Surface Enhanced Raman Probes for Targeted Melanoma Detection

Abstract

Melanoma is the deadliest form of skin cancer, and rates of melanoma in the UK appear to be rising. Despite this increase, current diagnostic methods remain slow, invasive and speculative. In this work, surface enhanced Raman spectroscopy (SERS) is proposed as an alternative diagnostic method, with the potential to provide rapid, point-of-care testing for early-stage melanoma skin cancer.
Gold nanostars decorated with red and near infrared-absorbing dyes were designed and synthesised as probes for surface enhanced resonance Raman spectroscopy (SERRS). Various dyes were evaluated as potential reporter molecules; methylene blue was chosen for the final probe structure due to its clear SERS signals and superior stability when bound to the nanostar surface. The probe was coated with a protective silica layer and functionalised with a peptide for the selective targeting of melanoma, before testing with various mammalian cell lines.
The results show that human melanoma cells dosed with the SERS probe retained SERS signals after rinsing with water, whereas the probe appeared to ‘rinse out’ of other cell lines, giving significantly diminished SERS signals. Confocal and fluorescence microscopy images also show the adhesion of the probe to human melanoma cells.
Following the success of this method, alternative reporter molecules were investigated. Namely, the use of molecular wires as potential Raman reporters was explored, as it was theorised that the conductive properties of these molecules could augment the SERS effect. The wires were synthesised and adhered to the surface of gold nanostars, in order to evaluate the effect of wire length on SERS intensity. The SERS spectra of the molecular wire-functionalised gold nanostars did not display improved signal intensity compared to gold nanostars functionalised with shorter molecules. These experiments were largely hindered due to instability of the wire-functionalised gold nanostars in solution, and so to fully rule out the possibility of molecular wire augmented SERS, work would need to be done to find a wire-functionalised substrate less prone to aggregation.
The observations herein demonstrate that a selective probe for the detection of melanoma, based on methylene blue decorated gold nanostars, was successfully created. While the preliminary results of this study indicate that the SERS method is a viable approach for melanoma detection, further optimisation of the probe is required for clinical applications. In particular, strategies to improve the SERS signal intensity are discussed.