Light-sheet Raman tweezers for whole-cell biochemical analysis of functional red blood cells.

Micro-Raman spectroscopy has emerged as one of the foremost techniques for analyzing biological cells in recent years due to its non-destructive nature and high spatial resolution. The development of optical tweezers has eased the research on biological cells as they confine living cells and organisms in the optical trap without causing much damage. Combining optical tweezers with Raman spectroscopy has opened a wide range of applications in the biomedical field as it facilitates biochemical analysis of biological samples by maintaining in-vivo conditions. Herein, we developed a light sheet-based optical tweezer that traps red blood cells (RBCs) at a very low power density spread across the whole cell, otherwise impossible with conventional optical tweezers. Furthermore, it is combined with micro-Raman spectroscopy to perform whole-cell biochemical analysis for the first time. Raman spectra of individual RBCs recorded under the line focal spot excitation are of superior quality and lack spectral signatures of photo-oxidation and heme aggregation, which is common in point focal spot excitations.

Surface-Plasmon-Polaritons for Reversible Assembly of Gold Nanoparticles, In Situ Nanogap Tuning, and SERS.

A transportable reversible assembly of gold nanoparticles (AuNPs) in an aqueous environment addresses the need for in situ surafce-enhanced Raman spectroscopy (SERS) hotspot creation for biological applications. Usually, light-directed AuNP assembly methods use higher laser powers and surfactants and are, hence, unsuitable for biological applications. Here, surface plasmon polaritons-assisted dynamic assembly of AuNPs are demonstrated at laser power density as low as 100 nW µm-2 . The AuNP assembly with multiple controllable hotspots is generated in an Au-water interface for solution-based SERS measurements. The major advantage of the method is that the interparticle nanogap is tunable to achieve analyte and AuNP-specific optimum SERS enhancement. The SERS intensity is reproducible on multiple reassembly cycles and assembly attempts, proving repeatability in the produced nanogap pattern. The assembly experiments reveal the influence of AuNP surface charge and the resulting polarizability on the SPP forces. The developed system and method can detect sulforhodamine 101 (SR101) dye molecules at concentrations as low as 10-10  m. Further, the SERS measurements on double-stranded DNA suggest that the molecules are oriented in a fashion to expose adenosine to the enhanced field, leading to its dominance in the recorded spectra.