Detection of Nanosize Biological Threats

with Greg Salamo

Biological cells are the complex mixture of a large number of biomolecules enclosed in cell membrane, including nucleic acids, proteins, polysaccharides, and lipids. Most cells can live, grow, and reproduce in liquid growth media, allowing for continuous changes in the biochemical composition inside the living cell. Identification of biomolecules inside living cells is therefore very important to understand various cellular processes. However, a living cell under study may randomly move away from the “study region” due to Brownian motion and cell mobility and, therefore, living cells have to be immobilized either physically or even chemically. As a result, immobilization will likely change the chemical micro-environment of the living cell and may yield unknown effects on a cell under study. The use of a Raman tweezers permits the capture of a mobile biological particle in solution without physical contact thereby optimizing detection and identification without modify-cation. It also makes possible the study of one DNA molecule that is attacked by a single biologically threatening spore or cell.

Figure 1. The NIR Raman spectra of a single E. Coli bacterium (a) immobilized on a glass cover plate; and (b) optically trapped 15mm above the cover plate in LB solution. Curve A is the spectrum recorded when the E.Coli cell is inside the focused laser beam, curve B is the background without the cell in the beam, and curve C is the calibrated subtraction spectrum (A ­ B). The laser power is 15-mW excitation at 785 nm. The insets are the images of the E. Coli cells.

A Raman tweezers is a confocal microscopy system that combines optical tweezers and Raman spectroscopy. Optical tweezers use a focused near-infrared laser beam to capture and manipulate a single biological particle including a cell, bacterium or virus, without physically touching it. Raman spectroscopy uses the same laser beam to illuminate the cell and analyze the collected optical spectrum from the scattering light, to provide information about the species, structure, or molecular organization within the cell. As a result, the Raman tweezer can provide biochemical composition of a single living cell without chemically interfering with it. Moreover, the measured vibrational energy levels can be used as fingerprints for identification of biological cells or for changes in them. In this REU project, the student will use a Raman Tweezer to trap various cells and examine the enhancement of the collected Raman signal when the cell is brought near a silver surface. The enhancement will be characterized as a function of distance between the cell and the silver surface. In this way, the study will led to a sensitive detection and identification system as well as to a better understanding of the enhancement phenomena